Physico-chemical aspects of aqueous gloss emulsion paints · this research and the writing of this...
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Physico-chemical aspects ofaqueous gloss emulsion
paints
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• A Doctoral Thesis. Submitted in partial ful�lment of the requirements forthe award of Doctor of Philosophy of Loughborough University.
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PHYSlCO _. CHF)o{ICAL ASPECTS OF AQUEOUS
GLOSS EMULSICW PAIHTS
by Clr1st1ne Slack
A Doctoral Thes1s
&Ibmitted in partial Mfilment of the requirements for the award of
Doctor of Philosophy of the Loughborough lhiversity of Technology,
June 1987
&Ipervisor: M.J. Jaycock, Ph.D(Nottm.), Ph.D(Cantab.) Department of OIemistry .
© by Clr1st1ne Slack 1987
~""roc!,~' :Jnl • .....,.
., T ",~""; ..• :;, !.~ ~---- ...... '" .. _----~_O...s!.c.~7 _ ...
Cl;.!:"!!
Ace. OI3,,~ 11o-z... ~. ,
"I believe that a colloid chemist, if asked
today to explain the coagulation of a lyophobic
hydrosol by electrolytes will make a rather
unhappy face. Most presumably, when explaining
simple and well investigated cases as, for
example, the flocculation of arsenious
trisulphide sols with neutral salts, a
conscientious colloid chemist will even voice
a warning to ·the effect that this matter is
not as simple as it looks."
~ ....... _\
- i -
W. Ostwald (1938)
>,..: it ,.
., " ,
TO
KlH. DAD
HARIAlf AND JAHET
AND KEVIN
- ii -
Initially I would like to give my utmost thanks to Mike, whose
never ending support enabled me to complete this research •
. 1 would also like to thank the staff of Loughborough University
and the Research and Technical Research Laboratory of Iobbil Oil Company
Limited, for their time, knowledge, faith and encouragement both during
this research and the writing of this thesis.
Thanks also go to the backers of this research, Tioxide U.K. Ltd.
for providing financial support, time and experimental equipment,
particularly L.A. Simpson and K.A. Elliott for their practical help and
assistance.
Last, but certainly not least, thanks and appreciation must go to
my friends, particularly Beverley and Sarah, and especially
Kevin whose inexhaustable
this thesis possible.
love
- iii -
, patience and understanding made
DECLARATION OF ORIGIHALm
The work in this thesis is the original work of the
author, except where otherwise acknowledged, and has not
been presented previously, wholly or in part, for the
award of any degree in any University.
- iv -
c. Slack June 1987
SUIfo1ARY
The formulation of an aqueous paint involves the mixing of a very
complex chemical system. The objective of this study was to investigate
the colloidal and surface properties of this type of system. From this
an overall picture of the physico-chemical interactions occurring
between the pigment and polymer, and the effect of the soluble
components, could be obtained. By a greater appreciation of the basic
chemistry of the paint system, improvements of the final commercial
products cculd be possible.
A simple paint system was designed which contained industrial
pigment, poly(vinylacetate) polymer and soluble ccnstituents including
sodium hexametaphosphate (dispersant), and sodium dodecylsulphate
(emulsifier). A non ionic thickener, sodium hydroxyethylcellulose, and
an ionic thickener, carboxymethyl cellulose was also included.
The effect of the alumina coating was investigated by testing both
alumina coated pigment, RTC90, uncoated ccnventional rutile, RD rutile
and alumina pigment, Hydral. The effect of these soluble aluminium
species on the pclymer was also demonstrated.
The interaction of the soluble components on the polymer and
pigment WoS investigated using microelectrophoresis and adsorption
techniques. The distribution of mcbilities in colloidal samples was
obtained for both ccated and uncoated pigments in a variety of solutes.
The effect of thickener concentration on paint formulations and
millbases was measured at constant temperature in precision bore glass
columns. The data was analysed using an interactive computer programme
based on the work of Carstensen & Su (1970a 1970b). The effect of
thickener with respect to flocculation, film formation and gloss was
also estimated using optical and infrared techniques.
This study has given an idea _of. the -~~-complexity - -of-paint-
formulations and the need for further model studies on the individual
components of the system.
Galgon
O1C
O1C F75
CTAB
DLVO Theory
DTA
EDTA
HEC-
HEC 250GR
GLOSSARY AND ABBREVIATIONS
- Commercially produced sodiumhexametaphosphate
- Sodium carboxymethylcellulose
- Purified cO!llllercial grade of sodium carboxy
methylcellulose with a defined viscosity (1%w/w solution = 6-9 x 10-1g.cm-1 s-1@ 250 C)
- Cetyltrimethylammonium bromide
Classical theory describing the stability of
lyophobic dispersions
- Differential thermal analysis
1, 2 - diaminoethanetetra-acetic acid
Hydroxyethylcellulose
Commercial grade of hydroxyethylcellulose designed
for easy dissolution with a defined viscosity
(1%w/w solution = 1-3.5 x 10-2g.cm- 1s- 1)
i. e. p. Isoelectric point
Laser Zee Meter - Instrument for determining the zeta-potential of
particles dispersed in water. Consists of a
microelectrophoresis cell in which the particles
are illuminated with a helium-neon laser and
observed with a microscope.
LZM - Laser Zee meter
MEP - Microelectrophoresis
PVAc - Poly(vinylacetate)
- vi
pzc
RD rutlle
RTC90
S3000
SDS
SE}1
SHMP
SP
TE}1
1MP
- Point of zero charge
- Basic 'drymilled' uncoated rutl1e pigment
- Alumina coated rutile pigment
- Automated version of the Laser Zee Meter
- Sodium dodecylsulphate
- Scanning electron microscopy
- Sodium hexametaphosphate
- Streaming potential
- Transmission electron microscopy
- TrimethylQlpropane
- vii -
Acknowledgements
Declaration of Originality
Summary
Glossary and Abbreviations
CHAPTER 1 INTRODUCTION
1 • 1 Aqueous Gloss Emulsion Paints
1 . 1 . 1 Constituents of Commercial
Gloss Emulsion Paints
1.1.2 Purpose of this Project
1.1.2.1 Pigment
1.1.2.2 Polymer
1.1.2.3 SUrfactants
1.1.2.4 Dispersants
1.1.2.5 Thickeners
1.1.2.6 pH
1.2 Related work . 1.3 Aims of the present work
CHAPTER 2 THFDRY
Aqueous
2.1 Electrical Distribution at the Solid-Liquid Interface
2.1.1 The Electrical Double Layer
2.1.2 Charge Development at the Interface
2.1.3 Stern Theory Application to Specific Adsorption at the Solid-Liquid Interface
2.1.4 The Zeta-Potential
2.1.4.1 Movement of the Shear Plane
- viii -
Hi
iv
v
vi
1
2
3
4
5
6
6
7
7
8
10
10
11
12
15
21
2.2 Interaction between Particles 21
2.3
2.2.1 Stability of Aqueous Dispersions - DLVO Theory 21
2.2.2 Repulsion between Particles - Interaction of Spherical Double Layers 22
2.2.2.1 Interaction of Similar Double Layers at Constant Potential 23
2.2.2.2 Interaction of Dissimilar Double Layers at Constant Potential 24
2.2.2.3 Interaction at Constant Charge 25
2.2.3 Attraction between Particles - Van der Waals Forces 25
2.2.3.1 Unretarded Attraction between Spheres 26
2.2.3.2 Retarded Attraction between Spheres
2.2.3.3 calculation of Hamaker Constants
2.2.3.4 ~Interaction between Spheres with Adsorbed
27
29
Layers 33
2.2.4 Total Energy of Interaction 34
2.2.5 Other Factors Affecting Flocculation
2.2.5.1 Polymer Systems
2.2.5.2 Hydration Forces
2.2.5.3 Brownian Flocculation
2.2.6 Zeta-Potential and Colloid Stability
Sedimentation Theory
2.3.1 Introduction
2.3.2 Dilute Dispersions
2.3.3 Concentrated Dispersions
2.3.4 Comprehensive Treatments of Sedimentation
2.3.4.1 The Ini tial Region
2.3.4.2 The Final Region
- ix -
35
35
36
36
37
38
38
39
42
44
45
47
CHAPTER 3 EXPERIKENTAL
3.1 Chemicals 52
3. 1 . 1 Tridistilled Water 52
3.1.2 pH Variants 52
3.1.3 Sodium Dodecylsulphate (SDS) 52
3.1.4 Sodium Hexametaphosphate (SHMP) 53
3.1.5 Thickeners 53
3.1.5.1 Natrosol 250GR (HEC) 53
3.1.5.2 Courlose F75G (CMC) 54
3.1.6 Butylcarbitolacetate 54
3.1.7 Aluminium Sulphate 54
3.1.8 Tital 54
3.1.9 Analyses 55
3.1.9.1 Aluminium Analysis 55
3.1.9.2 Analysis of Sodium Hexametaphosphate 56
3.1.9.3 Analysis of Sodium Dodecylsulphate 57
3.1.9.4 Analysis of Thickeners 58
3.2 Poly(vinylacetate) La~ex 58
3.2.1 Latex Preparation 58
3.2.1.1 Polymerization Reagents 59
3.1.2.1 Method 60
3.2.2 Theory of Emulsion Polymerization 62
3.2.2.1 General Theory 62
3.2.2.2 The Effect of Monomer Solubility 64
3.2.3 Application of the Theory 64
3.2.3.1 Seeded Polymerization 65
3.2.3.2 Molecular Weight 65
- x -
3.2.4 Poly(vinylacetate) Latex Characterization
3.2.4.1 Particle Size
3.2.4.2 Surface Charge
3.2~4.3 Glass Transition Temperature
3.2.4.4 Infrared Spectrum
3.2.4.5 Molecular Weight
3.3 Pigments
3.3.1 Alumina Coated Rutile - RTC90
3.3.2 Uncoated Rutile - RD Rutile
3.3.3 Alumina - Hydral
3.4 General Equipment
3.4.1 Glassware
3.4.2 Centrifuge
3.4.3 pH Measurement
3.4.4 Ultrasonic DiSintegration
3.5 Electrokinetic Measurements
3.5.1 Mobility
3.5.2 Laser Zee Meter Model 400
3.5.2.1 Cleaning of the Cell
3.5.2.2 Electrode Plating
3.5.2.3 Cell Parabola
66
66
66
67
67
67
68
70
70
71
72
72
72
73
73
74
74
75
76
76
76
3.5.2.4 Measurement of the Laser Zee Meter Cell (LZM) 77
3.5.2.5 Comparison of Cell Length Ratio to Potential Ratio 77
3.5.2.6 Comparison of LZM (timing) to LZM (Prism) 78
3.5.2.7 Experimental Procedure
3.5.3 Laser Zee Meter Model S3000
- xi -
79
79
3.6 Mobility Studies - Pigments 81
3.6.1 Mobility Studies on RD Rutile and Hydral 81
3.6.1.1 Mobility of RD Rutile and Hydral as a Function of pH 81
3.6.1.2 Mobility of RD Rutile in Aluminium Sulphate 81
3.6.1.3 Mobility of RD Rutile and Hydral in RTC90 Supernatant 81
3.6.1.4 Mobility of RD Rutile as a Function of Dispersant Concentration 81
3.6.1.5 Mobility of RD Rutile as a Function of Thickener Solution 82
3.6.2 Mobility Studies on RTC90
3.6.2.1 Mobility as a Function of pH
3.6.2.2 One Component Systems
3.6.2.3 Two Component Systems
3.6.2.4 Three Component Systems
3.7 Mobility Studies on Poly(vinylacetate)
3.7.1 As a Function of pH
3.7.2 One Component Systems
3.7.3 Two Component Systems
3.7.4 Three Component Systems
3.8 Adsorption onto Pigments
3.8.1 Sample Preparation
3.8.2 Analysis
3.8.2.1 SDS
3.8.2.2 SHMP
3.8.2.3 Thickeners
3.8.3 Mixed Adsorption Isotherms
- xii -
82
82
83
83
84
84
84
84
85
85
85
85
85
85
85
3.9 Adsorption onto Poly(vinylacetate)
3.9.1 Single Component Adsorption Isotherms
3.9.2 Mixed Adsorption Isotherms
3.10 Sedimentation
3.10.1 Sedimentation of Paint Formulations
Sedimentation of Millbases
3.11 Film Performance
3.11.1 Dry Film Flocculation
3.11.2 Wet Film Flocculation
3.11.3 Gloss Measurements
3.11.4 Scanning Electron Microscopy (SEM)
CHAPTER 4 RFSULTS
86
86
86
86
87
87
88
88
88
88
89
4.1 Introduction 90
4.2 Electrokinetic and Adsorptive Properties of Pigments 90
4.2.1 Pigment Surface
4.2.2 Solubility of Aluminium Hydroxide
90
90
4.2.3 Solubility Diagram for the Alumina Coating of RTC90 92
4.2.4 Desorption of Aluminium from RTC90 Alumina Coating 92
4.2.4 1 Desorption at pH5
4.2.4.2 Desorption at pH9
93
93
4.2.5 Electrokinetic Properties of Pigments in Water 94
4.2.6 Mobility of RD Rutile in Aluminium Sulphate 98 Solutions
4.2.7 Mobility of Hydral and RD Rutile in RTC90 Supernatant Solution
4.2.8 Adsorption of SHMP on Pigments
4.2.9 Mobility of Pigments in SHMP Solution
4.2.10 Adsorption of SDS on Pigments
- xiii -
98
100
101
106
109
4.2.11 Mobility of Pigments in SDS Solution
4.2.12 Adsorption of Thickeners on Pigments
4.2.12.1 Adsorption of HEC
4.2.12.2 Adsorption of CMC
4.2.13 Mobility of Pigments in Thickener Solution
4.2.13.1 Mobility in HEC Solution
4.2.13.2 Mobility in CMC Solution
4.2.14 Adsorption on Pigments in Mixed Solutions
4.2.15 Mobility of RTC90 with respect to Thickener Concentration at Constant Concentration of SHMP
4.2.15.1 HEC
114
116
118
119
121
121
123
124
125
125
4.2.15.2 CMC 126
4.2.16 Mobility of RTC90 in Solutions containing SHMP and SDS with respect to Thickener Concentration 127
4.3 Electrokinetic and Adsorptive Properties of Poly(vinylacetate)
4.3.1 Mobility of PVAc in Water
4.3.2 Adsorption of SDS on PVAc Latex
4.3.3 Mobility of PVAc in SDS Solution
4.3.4 Mobility of PVAc in Butylcarbitolacetate
4.3.5 Mobility of PVAc in RTC90 Supernatant
4.3.6 Adsorption of SHMP on PVAc.
4.3.7 Mobility of PVAc Latex in SHMP Solution
4.3.8 Adsorption of Thickeners on PVAc
4.3.9 Mobility of PVAc in Thickener Solution
(BCA)
128
128
129
131
133
133
134
135
135
137
4.3.10 Adsorption onto PVAc in Mixed Solutions 137
4.3.11 Mobility of PVAc with respect to SHMP 138 Concentration of SDS
4.3.12 Mobility of PVAc with respect to Thickener Concentration at Constant Concentration of SDS 138
- xiv -
~ 11.11 Mobility Distribution in Electrophoresis Samples 139
4.4.1 Histograms of Pigments in Aqueous Solution 141
4.4.2 Histograms in SHHP Solution 143
4.4.3 Histograms in SDS Solution 145
4.4.4 Histograms in Mixed SHHP/SDS Solutions 145
4.4.5 Summary 145
4.5 Film Performance 147
lQ7
148
148
4.6
4.5.1 General
4.5.2 Wet Film Flocculation
4.5.2.1 HEC
4.5.2.2 OlC
4.5.3 Dry Film Flocculation
4.5.3~1 HEC
4.5.3.2 O1C
4.5.4 Gloss Measurements
151
151
153
153
154
4.5.4.1 Gloss Measurements of Films containing HEC 156
4.5.4.2 Gloss Measurements of Films containing O1C 156
4.5.5 SEM Photographs of Etched Paint Films
4.5.6 Surmnary
Sedimentation Analysis
4.6.1 The Computer Model
4.6.1.1 Critical Time
4.6.1.2 Short Time Data
4.6.1.3 Long Time Data
- xv -
156
157
159
159
159
160
161
4.6.2 Sedimentation of Paint Formulations
4.6.3 Sedimentation of Millbases
4.6.3.1 CMC
4.6.3.2 HEC
CHAPTER 5 SUIfoIARY AND CONCLUSIONS
5.1 General Conclusions
5.2 Future Work
- xvi -
~
162
165
165
166
169
176
CHAPTER 1
INTRODUCTION
1.1. AQIJEX)US GLOSS EMlJLSI<W PAIHTS
Aqueous gloss emulsions, sometimes known as lat.ex paints, have
one dist.inct advant.age over alkyd or 'spirit I based paints. The
solvent, i.e. water, is cheaper, toxicologically more safe, and easier
to handle than solvents normally used in the glossy alkyd coatings.
However, the formulation of emulsion paints has been more of an art
than a science. Although emUlsion paints are very prevelant in the
consumer market, problems have been encount.ered wit.h t.heir storage
st.ability and the distribut.ion of the pigment in the final paint film.
As a result of the lat.ter the gloss of latex paints has never become
as good as their alkyd counterparts.
1.1.1. Constituents of Commercial Aqueous Emulsion Paints
An emulsion paint is essentially made from two component.s, a
synthetic resin emulsion and a pigment dispersion. These are blended
together to form an even composition. The first emulsion paints used
vinylalcohol and vinylacetate polymers. These were followed
by styrene-butadiene latices and later by acrylate esters.
The major constituent of the pigment composition is nowadays the
rutile form of titanium dioxide. This is normally 60-70 percent of
the pigment composition.· The balance consists of extenders such as
silica, talc, clays, mica and calcium carbonate. The pigments are
ground in an aqueous solution containing a dispersant, normally a
polyphosphate. Cellulosicthickeners can be added into the pigment
mix ,or added later. These compounds may also behave as dispersants.
The pigment paste is blended with a surfactant stabilized.
emulsion. The surfactant can be ionic, such as an alkyl-sulphonate
or non-ionic such as poly(vinylalcohol) or poly (ethylene oxide). In
. -
certain cases a coalescing agent is added to improve film formation.
This is particularly necessary where un-plasticized polymer latices are
used. Additional additives include antimicrobi~l agents, antifreeze,
anti foaming and thixotropic agents, and rust inhibitors. The final
paint· formulation is normally alkaline, being in a pH range of
approximately 7 - 9.
Typical properties required by an emulsion paint include can
stabili ty, satisfactory viscosity for application, and stability to
extremes of temperature. They are also required to give good covering
power and film formation, and freedom from foaming and microbial attack.
1.1.2. Purpose of this Project
Very little fundamental experimentation has been carried out to
explain the major factors involved in particle flocculation and
stability during paint formulation and film formation. This project is
an extension of the work carried out by Kayem (1978). He investigated
a very simple paint system containing latex and pigment, complete with
individual stabilizers. This present work was designed to answer
questions raised by Kayem, and to investigate the effect of adding
cellulosic thickeners to the paint system. Additional experiments
investigating flocculation and dispersion of the pigment, and the
effect on gloss, in wet and dry films, were also included.
In order to perform these investigations, the model emulsion paint
again had to be defined. It was necessary to ottain a formulation that
was:-
a) simple enough to be able to conduct fundamental
experiments without incurring difficulties
arising from a multitude of components, and
- 2 -
b) fully representative of an industrial paint
system which is chemically complex.
Components to be defined were therefore:-
a) pigment and .polyme.r
b) pigment stabilizer (dispersant)
c) polymer stabilizer (surfactant)
d) thickener
These model components need to be representative of commercial practice.
"1.1.2.1. Pigment
Titanium dioxide is an excellent pigment for paints having the
following properties:-
a) good chemical stability
b) durability
c) non toxicity
d) opacity
e) good reflectance
Titanium dioxide pigments can be manufactured by two methods, the
'sulphate' process or the 'chloride' process. The chloride process is
more recent and is now predominantly used.
diagramatically in Figure 1.1.
It is explained
In this process, the raw pigment, is reacted with chlorine to form
liquid titanium tetrachloride, purified by distillation and then burnt
to produce titanium dioxide pigment.
Two different crystal structures of titanium dioxide, rutile and
anatase, are available. The different structures are shown in Figure
1.2. Rutile has a mere compact crystal structure than anatase, which
accounts for its higher refractive index, greater stability and higher
- 3 -
A-COke~ - . ruti le s tore chlorine
chlorine
ftl tratio n
FIGURE 1.1.
titanium tetrachloride production
cooli n 9
~.,... water ID dispersion
purifi ca tion
ox idati 0 n
W pigment coating .. o filtration & washing
drying
final milling
packing
The Chloride Process.
Rutile
I---i 1~
Anatase
FIGURE 1.2.
Projections of the Crystal Structures of Rutile and Anatase
- Titanium atoms shown as small circles, Oxygen atoms as large ones.
density. It is more economic and durable and is widely used in the
paint industry.
Two pigments were investigated in this project:-
i) RD rutile - basic 'drymilled' or untreated pigment.
ii) RTC90
Rutile pigment is dry-ground after calcination
to give the desired particle size.
- RD rutile with a modified surface.
A predominently alumina coating is precipitated
onto the surface by dispersing RD rutile
pigment in a mixture of alumina, titania and
concentrated sulphuric acid. The coating
constitutes 4% of the total pigment but is
likely to contain impurities such as silica
and titanate.
The particle size of both pigments is approximately 0.2/Lm.
This is optimum for the uniform scattering of light.
The outer coating gives the final paint films mere resistance to
chalking. This is release of pigment by breakdown of polymer due to
liberation of hydroxy radicals from UV reduction of titanitDII (TiIV to
TiIII ). The advantages of coated pigments has been described by
Kempfer (1973).
1.1.2.2. Polymer
Previous work by Kayelli (1978) used a poly(vinylacetate) latex
which had been polymerized in the laboratory. Poly (vinylacetate) was
chosen again as it is used as the basis of the majority of ccmmercial
latices. It· was attempted to obtain a suitable cOlllDercial latex to
- 4 -
maximise project timE for colloidal experiments. However, it was
impossible to obtain a commercial product containing one emulsifier and
no additives. Also because of commercial constraints, complete
knowledge of the latex systems was not available. It was therefore
necessary to prepare the polymer in the laboratory. Again a final
latex particle size of approximately 0.2 ~m and a solids content of 4~
was chosen.
1.1.2.3. Surfactants
Surfactants are necessary to stabilize'the polymer emulsion phase
by preventing coagulation or coalescence. They orient" . with the
hydrophobic entity adsorbed onto the emulsion particle and the
hydrophilic entity in the aquecus phase. This results in a reduction
in the interfacial tension which improves the stability of small
particles. If the surfactant is also ionic, the adsor~ed charge helps
the particles to repel each other, rather than coalesce.
Sodium dodecylsulphate was ctosen for the vinylacetate
polymerization and to stabilize the poly(vinylacetate). It is an
anionic surfactant, st.able over a wide pH range, and has well defined
characteristics. A very pure sample of sodium dodecylsulphate was
available for this project.
Some very high molecular weight, water soluble, products may also
act as surfactants or emulsifiers. In contrast some water soluble
products such as sodium hexametaphosphate and some organic products,
which can strongly adsorb on many particles (such as pigments), have
no surfactant properties and are known as dispersants. These are of
use in pigmenting eoculsions to formulate paints.
5 -
1 .• 1 .• 2 .4. Dispersants
Many surfactants may also act as dispersants for solid pigments.
Dispersion is a similar action to emulsification but the principal
difference is that no interfacial tension reduction between the
particles is necessary. The function of the dispersant is to ensure
that the agglomerates of pigment particles are broken up and separated.
It is desirable to have a dispersant containing an active group which
is adsorbed on (or attracted to) the surface, and a water soluble
portion which gives electrostatic stability.
A number of polyphosphates can be used as dispersing agents. One
of the most efficient, sodium hexametaphosphate (SHMP) , known ®
commercially as 'Galgon', was used in this project.
® Most dispersants, such as Galgon, are of low molecular weight
compared to polymeric materials. They achieve dispersion by giving
the particle charge and electrostatic stability. Because of their
molecular weight there is little evidence for protective colloid action
or steric forces.
1.1.2.5. Thickeners
Thickeners are added to paint formulations to increase the
viscosity of the system and improve the flow and application. They may
also act as protective colloids, ar.d help to disperse pigments by
rendering hydrophobic particles hydrophilic.
Two cellulose thickeners were chosen for this project, one ionic -
sodium carboxymethylcellulose (CMC) and one non-ionic - hydroxyethyl-
cellulose (HEC). These are bc,th ccmmon paint additives. They give
good thickening power and hence good flow and levelling. However,
® -C~\~o-;::-- \~ "- -1:: • .,':\" "''''K 0<;' \-\"-~'-'" 6 -
hydroxethylcellulose paints tend to thicken on storage whilst sodium
carboxymethylcellulose tend to thin. As it is ionic, CMe may be
precipitated by cations or leached out of the paint by water.
1.1.2~6. .E!!
Most commercial ereulsion paints are formulated at alkaline pH. As
it was decided to investigate the effect of pH, two pH levels, one
alkaline (pH 9) and one acidic (pH 5) were chosen. pH 9 is typical of
paint formulations and pH 5 is considered to be the lowest pH that an
emulsion paint would attain during storage or on drying.
1.2. REl.ATED WORK
As already stated, this project is an extension of work by Kayem
(1978) into the stability of a simple paint system. All previous work
was aimed primarily at one of the two solid components, latex or
pigment.
Kayem showed that both SHMP and SDS could disperse pigment but
only SDS stabilized the polymer. Increasing amounts of SHMP destablized
poly(vinylacetate). He made theoretical calculations based on DLVO
(Derjaguin & Landau, Verwey & OVerbeek) theory or colloid stability
and showed that the pigment was very sensitive to flocculation on
increase in ionic strength. Although DLVO theory predicted high
stability for paint formulations, breakdown or polyphosphate and
hydrolysis of the polymer reduced the pot lire.
In drying paint films, the preferred order or flocculation was ;
shown to be in the order:-
i) pigment pigment homoflocculation
7 -
11) pi~nt
11i) latex
latex
latex
heteroflocculation
homoflocculation
Investigation of dried latex and pi!!PIented films showed that SHMP
caused poor film fonnation whilst SDS improved films by solubilizing
the latex, Jaycock & Kayem (1982, 1983).
Balfour & Hird (1975) and Balfour (1977) used infrared scattering
to investigate the effect of flocculation on paint properties. They
showed that increase in flocculation resulted in a reduction in opacity
and gloss, an increase in surface roughness and paint degradation and a
tendency to yellowing.
Zeta-potential, pH and conductivity were measured by Cremer (1977)
to characterize changes on the surface of pi!!PIents. He showed that
zeta-potential could provide significant information on the scattering
power of pi~nt, in water based paints. However, zeta-potential alone
was not sufficient. Sedimentation and adsorption experiments were also
carried out to investigate the effect of surface treatment, particle
size and polymer adsorption.
On the polymer side, work was carried out by', Mercurio et al
( 1982) to substitute aqueous gloss enamels in sol vent. based alkyd
applications. They used high molecular weight aqueous, latex polymers,
with coalescing solvents and aqueous thickeners, to produce faster
drying, non yellowing and toxicologically more safe enamels.
1 .3. AIMS OF THE PRESENT WORK
This project was intended to take into account certain factors
which were not investigated by Kayem. These include the effect of
solubilization of the alumina coating on the pi!!PIent and the reason Why
- 8 -
alumina coated pigment behaves electrophoretically more similar to
rutile pigment than to alumina.
As well as confirmation of similar electrophoretic trends on a
different type of pigment, it was hoped to extend the model paint
systein to include ionic and non-ionic thickeners. In addition to
electrophoretic and adsorption data, flocculation in wet and dry films
and the effect on gloss and sedimentation was to be investigated.
Due to the availability of an automated microelectrophoresis
system in the Pen Kern S3000, the effect of aqueous components on
the mobility distribution in an electrophoresis sample was also
possible.
- 9-
CHAPTER 2
TIlEORY
2.1. El.ECTRICAL DISTRIBUTION AT WE SOLID-LIQUID INTERFACE
2.1.1. The Electrical Double-Layer
When a charged solid is dispersed in aqueous media, it usually
acquires a surface charge. The surface charge causes a redistribution
in the relative concentration of positive and negative ions in the
surrounding solution. Since the overall system is electrically
neutral, the total charge of the medium must be equal', but opposite in
sign to that of the solid surface. The total system is known as a
"double layer'.
The overall structure and description of the potential
distribution across the double layer has been developed over the years.
The accepted classical theory is that developed by Grahame (1941) based
on the initial assumptions and prinCiples of Helmholtz (1819), Perrin
(1904), Gouy (1910, 1911), Chapman (1913) and Stern (1924). These
theories have been extensively reviewed by Overbeek (1952), Parsons
(1954), Bockris et al (1963), Sparnaay (1912) and Hunter (1981) and
will not be fully detailed here.
The model describes a solid particle surrounded by electrolyte
solution. The area surrounding the particle is divided into two layers
shown diagramatically in Figure 2.1. The inner, or Stern layer, is
separated from the outer, or diffuse layer, by a hypothetical boundary
known as the Stern surface. This is drawn through ions adsorbed on the
charged wall and allows for their finite size. The charge or potential
distribution is determined by the adsorbed ion concentration and will
approach a constant value as the surface becomes saturated. The Stern
layer is subdivided into the Inner Helmholtz plane (locus of chemisorbed
anion centres) and the Outer Helmholtz plane (locus of cation centres
closest to the wall), Grahame (1941). Outside the Stern layer is the
- 10 -
FIGURE 2.1.
r
~--jc:==== solvated cation
~~~~l-__ ~==:::::::....!p~r~im~ary wate r ...: layer
--p~~--t=======:..specifica"y adsorbed anion
.,-I secondary I water layer I I
I I STERN PLANE
: ~ OHP (Outer Helmholtz Plane)
Xl X2 I HP (Inner Helmholtz Plane)
Structure of the Double Layer according to
Bockris et al (1963).
diffuse part of the double layer where the potential distribution is
described by the Poisson-Boltzmann equation. For metallic oxides, the
majority of the countercharge appears to lie between the solid surface
and the Stern layer.
2.1.2. Charge Development at the Interface
When solid particles are dispersed in an aqueous medium various
mechanisms can occur which result in a surface charge. These include:-
i) Adsorption of ionic species from the aqueous solution,
or unequal adsorption of ions of opposing charge.
This can also include negative adsorption.
ii) Dissociation of any ionogenic groups present in the
particle surface, e.g. - COOH groups which give a
negative charge at high pH.
iiil Unequal dissolution of oppositely charged ions of
which the particle may be composed.
Paint pigment, both coated and uncoated, may be considered as a
metallic oxide. In aqueous solution metallic oxides acquire charge by
. + -adsorptl.on of H or OH • Parks & de Bruyn ( 1962) have proposed an
alternative mechanism for an oxide such as Fe2D3
, where metallic -
hydroxo complexes e.g. + Fe(OH)2 and FeD; are fonned in solution and
readsorbed. This process is also pH dependent. Thenoodynamically it
is impossible to distinguish between the two methods of charge
acquisition, but the concentration of these complexes in solution can
be determined.
Levine & Smith (1971) derived a modified Nernst equation for oxide
surfaces in aqueous solution with
- 11 -
+ Hand as the potential
detennining ions. This took into account the chemical potentia Is of
the positively and negatively charged surface groups. The other solid
+ -component of the paint system, the latex, does not adsorb H or OH
ions from aqueous solution. Latex polymerization involves the use of
ion radicals (Section 3.2.2.) which are present on the resulting latex
particles. The latex therefore acquires charge through mechanism (ii)
by dissociation of the ionogenic surface groups (S04 -, S03 - or CO2 -) .
Stabilizers of colloidal particles act by specifically adsorbing
onto the particle surface. A theory put forward by Stern ( 1924 )
enabled extension of the double layer theory to incJ,ude specific
adsorption.
2.1.3. Stern theory application to specific adsorption at the solid-liquid interface.
The theory proposed by Stern (1924) allowed f'or specific inter-
action of the first layer of adsorbed ions with the particle surface
USing the specific chemical adsorption potential.
The diffuse double layer starts at distant x2 (Figure 2.1.) from
the wall (Stern plane) and for x;:. x2 specific' interaction forces are
negligible. The distance of closest approach to the wall is x 1 : so
that the region ° <!;; x <: X 1 is free of charge. The number of ions in
the Stern plane is calculated using a Langmuir'isotherm modified with
a Boltzmann factor. He obtained the following expression for the
charge density in the 'Stern plane:-
(2.1)
- 12 -
where (11 : surface charge density
NI : number of available sites for adsorption per cm 2
Z : valencies of ions of concentration n t
1/;d : Stern potential
cJ>t. : specific adsorption potential of the ions
M : molecular weight of the solvent
NA: Avogadro number
When there is only one specifically adsorbing ion (2.1 ) simplifies
to:-
(1j = njez = NI ez (2.2 )
1+ expl~Go/KT)
x
where nj = number of ions adsorbed per cm2
x = mole fraction
The free energy of adsorption can be split into two parts:-
where
(2.3 )
~G = the chemical adsorption potential
ze1/;d = the electrical component
Parsons (1954) has questioned the breakdown into
chemical and electrical contributions. However, for interpreting
ionic adsorption in practice these uncertainties are acceptable.
From equation (2.3.), it is predicted that where the adsorbing ion
and the surface have the same sign, adsorption may still occur if the
chemical term ~G , is negative·-andlarge.·-If·~Gis-positivei-··only· -
adsorption of an ion opPOSitely charged to the surface is possible.
- 13 -
Specific adsorption of anions is more c<XlJDOn that cations. This
is because they are less strongly hydrated. (In Figure 2.1. the ions
were shown unhydrated).
Al though the Stern theory is often used to interpret ionic
adsorption, various criticisms of the approach have been made. These
include the omission of:-
i) effects of lateral repulsion between adsorbed ions
ii) discreteness of charge effects
iii) influence of adsorbed ions on the dielectric constant of
the Stern layer and
iv) the use of the Langmuir equation which assumes that
adsorbed ions are immobile
Models to calculate the permittivity of the inner layer have been
developed by Levine et al (1969), Parsons (1975), Oldham & Parsons
(1977), Salel!1 (1976), Damaskin (1977) and Robinson & Levine (1973).
Using this type of model, the capacitance of the compact layer can be
calculated.
Taking into account the discreteness of charge has permitted the
explanation of a variety of effects. These include the observation of
Grahame & Soderberg (1954) that the specific adsorption of anions can
cause the potential in the OHP to pass through a maximum as the surface
potential, ~o , i~made increasingly negative. Smith (1973) also showed
a maximum in Vtd as ~o increased. The discreteness of charge effect can
therefore be used to indicate specific adsorption, as shown by Lyklema
(1972). However, of more importance is that the true chemical
adsorption potential is less negative after application of the
correction, and particularly that it is independent of surface charge,
Levine & Smith (1971).
- 14 -
These authors c~bined a model of the inner double layer with the
discreteness of charge effect and described the ion adsorption
isotherms. Resulting theoreti,cal differential capacities and surface {
charge densities were in agreement with experimental data for titanium
dioxide, Berobe & de Broyn (1968a , b) silica, Abendroth (1970), zinc
oxide, Blok & de Bruyn (1970) and tin oxide, Ahmed (1969).
For the diffuse layer, the major approximations and assumptions
have been demonstrated by Haydon (1964) to only correct the Poisson
Boltzmann theory by less than 2% for typical values.
2. 1 .4. The zeta ..potential
Double layer theory, as given in Section 2.1.1. only took into
account the surface potential,1/;o , of the particles. Although of use
theoretically, this parameter is not accessible in practical
experiments because of specific adsorption. When adsorption takes
place, the potential 1/;0 will be effectively 'masked' by the adsorbance"
and in most cases will not be the potential observed by the surrounding
particles.
As electrokinetic effects involve the relative movement between two
phases, (solid and liquid for dispersions) the mcst useful potential
will be at the boundary between the two phases. Any adsorbate which
moves, with the solid component will be regarded as being part of the
solid phase. The potential at this boundary, commonly known as the
shear or slipping plane, is known as the zeta-potential, r As the
Stern layer is considered to be immobile, the slipping plane is located
in the diffuse part of the double layer (the Gouy-Chapman layer).
The difference between the zeta-potential and the Stern potential,
- 15 -
I/;d ' depends on the relative positions of the Stern plane and the
slipping plane. They are rarely thought to coincide, but there is
considerable experimental evidence to suggest that- they are not
significantly different.
The zeta-potential is calculated from available formulae, using an
experimentally available parameter, the electrophoretic mobility VE •
The experimental procedure to obtain the mobility is described in
Section 3.5.1.
Various reviews on the calculation of zeta-potential from the
electrophoretic mobility are available, both from early work, Overbeek
(195 cl and Booth (1948, 1953), and modern theory, Overbeek & Wiersma
(1961), Dukhin & Derjaguin (1914), Overbeek & Bijisterbosch (1919) and
O'Brien & White (1918). Hunter (1981) has also produced a detailed
summary in monograph form.
The first equation for calculating the zeta-potential, r ,from
electrophoretic mobility, vE ' was proposed by Helmholtz (1819) but
was later modified by Smoluchowski (1903, 1921). It equates viscous
and electrical forces and is ... \mown _ as the Smoluchowski equation:-
( 2.4 )
where 0 = the dimensionless dielectric constant (0 = e I eo )
1/ = the viscosity of the medium
e = ,,-el~drlC: permittivity
eo = perm! ttivity of -free'-'space'"
This equation can be applied to impervious particles of definite
shape and orientation. Haydon (1964) proposed that if counterions can
penetrate into the particles, or the particles are of no definite shape,
then the calculation of the zeta-potential becomes more inaccurate.
- 16 -
Huckel (1924) also put forward.a similar type of equation but with
a different result:-
VE .. (411" Eo )·21 611"71
= El. (2.5 ) 3 11
The difference between the two equations was investigated by Henry
(1931). He observed that a different account of the electrical field
in the neighbourhood of the Pa:rticle had been taken by the two authors.
Huckel disregarded the deformation of the applied field by the particle
whereas Smoluchowski asslUDed the field to be uniform and parallel to
the particle surface. These assumptions are applicable to the extreme
situation as follows:-
K8 «1 - Huckel
K8 .:>.:> 1 Smol uchowski
where a is the particle radius and K, is known as the Debye-Huckel
parameter, the reciprocal thickness of the double layer. This is used
in the Debye-Huckel approximation for solutions of the Poisson
Boltzmann equation in the theory of strong electrolytes. It is defined
in the following equation -
2 L 2~ 0 2 K = pe"""1 zl (2.6)
EKT
If the electrical force on· a particle is balanced against the Stokes
frictional reSistance, the follOwing equation is obtained:-
VE .. PE = a (2.7) Ez 611"8TJ
where 8 = particle radius
a = particle charge
Ez = electrical field
PE = particle velocity
- 17 -
Substituting for the particle charge gives:-
= (411"fO).~ 611"1]
(1 + Ka) (2.8)
which reduces to Huckel's equation, (2.6.) for very small values of K8.
Henry demonstrated that if the external field was superimposed on
the local field around the particle, the following equation could be
used to describe the mobility:-'
liE = (411"fo). 0 r 611"1]
" (1<8) = ur. " (Ka) 31]
The function " Ka is dependent on the particle shape. For a
sphere this is given by:-
',(K8)= , + (Ka )2 5(K8)3 (K8)4 (Ka)5 (2.10) 16 48 96 96
tK:r K8
'Ko)'J." J .t 01 a
96 I
for K8 <,
When Ka «1 this reduces to a value of 1, i.e. the Huckel equation.
When K8 >1
3
2
9
2K8
330 K38 3
(2.11)
and .When K8»1 this reduces to a value of. 3/2, i.e. the Smoluchowski
equation.
Values of the experimental integral are given in tables of
mathematical functions, for example Jahncke & Emde (1945), and values
of " (K8) are available from Abramson et al (1942). In this treatment,
- 18 -
the assumption that the external field can be superimposed on the
particle field (described by the Poisson-Boltzmann equation) means it
is only valid for low potential particles (<25mV).
In the region between the Huckel and Smolochowski equations Le.
1< Ka < 100, electrical forces, frictional forces and electrophoretic
effects must be taken into account. The Henry equation however, fails
to take into account relaxation effects. On movement of the particles,
a finite relaxation time is required for the initially distorted ion
atmosphere surrounding the particle to regain its original sY)DDletry.
This decreases the mobility of the particle and has greatest influence
in the region r >lOmV and 1<Ka<10.
Later work to consider the relaxation and retardation effects was
undertaken by Overbeek (1943). These results were later confirmed by
Booth (1950). They used power series to obtain more reliable results
for higher values of r and found that for intermediate values of
Ka the relaxation effect is very important. Wiersem.. (1966)
developed a computer programme to calculate zeta~tentials up to 15OmV.
This indicated that Overbeek (1943) and Booth (1950) had overestimated
the relaxation correction for Ka values between 0.2 and 50.
These correction procedures are not Simple to calculate particularly
for systems other than thos"e containing simple electrolytes. A more
successful numerical and computerised solution to this problem was
provided by O'Brien & White (1978). They used similar types of t-h e c\\(I""! ",SiC" \~'Ss t\ .. ~(\ct. \01'\
equations and plotted mobilit:,: againstJ,,(r Ei 10) for a large number of
Ka values. The results of O'Brien & White complement and agree with
those of WierS2mQ(1966). An observation from this more recent work is
that for values of Ka ;;;. 3, the mobility function has a maximum value.
The aspects of this development will be further discussed with respect
- 19 -
to the paint system in Chapter 4.
As most I( 8 values incurred in this paint system were in the
region where relaxation effects are high (1-100) it is important that
they are taken into consideration on converting mobility values,
otherwis.e large errors could be made. However, in complex systems,
precise identification of 1(8 values is not an easy task. The theory
requires precise knowledge of all the individual ionic concentrations
and ionic conductances for every conducting species. In a system, such
as emulsion paint, which contains, for example SHMP and aluminium
species in the range pH 5 - 9, this information is not available. At
present it is not even clear how it might be obtained.
Expressions to account for the effect of conductivity on r were
developed by Henry (1948) and Booth (1948). These are not very often
used, having already been taken into account, in the modern theories,
on treatment of the relaxation effect. However, when the more simple
Huckel and Smoluchowski equations are used, significant errors (>5%) may
be incurred when 0.1< I( 8 > 500 if the correction is not applied
particularly at large values of r Of more consequence is the effect of surface charge on the
permittivity and viscosity in the double layer, due to the increased
field strengths. Both effects will decrease the particle mobility at
constant zeta-potential. The effect of. a decreasing permittivity as
the potential increases has been investigated by Grahame (1950) and
Conway et al (1951). Lyklema & Overbeek (1961) have indicated that the
variation due to the permittivity change can be neglected. The
significant rise in viscosity however, on increase in potential due to
ordering of the solvent dipoles, is expected to have some influence on
the zeta -potential. This change in. viscosity is known as the
viscoelectric effect.
- 20 -
Lyklema & Overbeek (1961) described the viscosity change due to
the viscoelectric effect as follows:-
~(x) - ~o [1 + f(~t)2] (2.12)
where ~o = the viscosity of the bulk liquid at zero field
f = the viscoelectric constant
This equation was previously suggested by Andrade & Dodd (1951).
However, the precise value of the viscoelectric constant is assumed to
be different for each liquid and is not very precisely known. Because
of this, viscoelectric effects are widely neglected for practical
purposes.
2.1.4.1. Movement of the shear plane
When macromolecules are adsorbed on colloidal particles they may
act in such a way as to move the shear plane away from the particle
surface. At high surface coverage, the adsorbing chains no longer lie
across the surface but are attached in loops which protrude out into
the aqueous media. This effectively moves the shear plane away from
the poSition at zero adsorption.
Koopal & Lyklema (1975) for
This effect has been explained by
silver iodide sols with adsorbed
poly(vinylalcohol), and will be fUrther considered in Chapter 4.
2.2. INTERACI'ION·~ PARTICLES
2.2.1. Stability of Aqueous Dispersions - DLVO Theory
The stability of lyophobic collOids, or the non-tendency of the
particles to flocculate, is governed-_by· the. balance between the· . forces
of attraction and repulsion. The classical theory, known as DLVO theory
- 21 -
was developed by Derjaguin & Landau (1941) and .Verwey & Overbeek
(1948) to predict the stability of aqueous dispersions of lyophobic
colloids in the presence of simple electrolytes. DLVO theory is
extremely useful in colloid chemistry in attempting to explain in broad
terms, . very complex observations. It is only however, applicable to
static systems and does not take into account any time dependent
processes.
The basis of DLVO theory is that the total energy of interaction
between two particles, Vr , is made up of an attraction term V A' and a
repulsion term VR, such that:-
= (2.13)
The usual convention is to associate attraction with negative
potential energies and repulsion with positive potential energies. The
sign of Vr will, therefore, determine whether there is a net attraction
(negative) or repulsion (positive) interaction between the particles.
2.2.2. Repulsion between particles - Interaction of spherical double layers.
The main repulsion between electrically charged colloidal
particles is due to the overlap of their respective diffuse double
layers. Mathematical solutions are available for certain regular
shapes (sphere, flatplate and cylinder) and for particles with either
similar or dissimilar double layers. An assumption of a constant
charge or constant potential system also needs to be taken into account.
For the purpose of this work the colloidal particles are assumed
to be spherical. As the particle interactions may be similar
(pigment - pigment or latex - latex interaction) or dissimilar
(pigment - latex), both systems will be considered. Both constant
- 22 -
potential and the less common constant charge solutions will be briefly
discussed.
2.2.2.1. Interaction of similar double layers at constant potential.
An expression for the interaction, VR, of two spherical double
layers of equal potential was first obtained by Derjaguin (1934, 1940).
This is applicable when the range of interaction is small compared to
the radius of the particle.
Der jaguin considered two spheres of radius a and distance H apart.
The spheres are divided into rings of radius. h. and width • 15 h.
The interaction energy of two rings will be 2 'lrh V (H)5 h where V( H) is
equal to the interaction energy per cm2 of two flat plates. By
integration, from h = 0 to h = ex> • the energy of interaction of the
spheres. vi is given by:
ex>
vI/; R = 'lraJ V (H) GH
Ho
(2.14)
where Ho is the minimum distance of separation between the spheres.
For small potentlals,KH»l andKa»l equation (2.14) reduces to:-
= 2
Ea 1/;0 In (2.15) 2
- 23 -
2.2.2.2. Interaction of Dissimilar Double Layers at Constant Potential
It was again Der jaguin ( 1954) who first considered the
interactions of dissimilar double layers. Attempts since then require
graphical or numerical integration and are not easily used for
practical purposes, for example Devereaux and de Bruyn (1963). For
heterodisperse systems Hogg et a1 ( 1966), using the basis of the
theory in Section 2.2.2.1. have shown that:-
j V (H) oH
Ho
Where a 1 and a2 are the radii of the two particles.
(2.16)
This again is only valid when Ka»1, KH»1, and H «a1and a2'
Hogg et al simplified the calculation of VR using the Debye-Huckel
approximation for low surface potentials (<25mV) giving:-
2. 2. (411" Eo). D 81 82' (~01 + ¥t02 )
4 (111+ 82 )
Where ~01 and ~02 are the potentia1s of the two spheres .
(2.17)
Verriey &
Overbeek (1948) have shown that the basic theory of Derjaguin (1934,
1940) gives reasonable results at Ka>5 and is Significantly improved
at Ka >10.
More recently, Oshima et al (1982) have developed the work of
Hogg et al to extend the calculation of VR to moderate potentials.
- 24 -
2.2.2.3. Interaction at Constan~ Charge
Weise & Healy (1970), using a method developed by Frens (1968)
developed an equation for calculating the repulsive interaction at
constant charge, v~ , from the interaction at constant potential:-
v~ = vl-(J.-r;Eo'Pa,a.J, ¥o~+,p~) In[ 1-exp(-2KH)]
2 (a1+a2)
(2.18)
From this, the difference between V ~ and vi tends to decrease
with increasing surface potential. However, at ,/(H <1 and small
potential, the difference between v~ and vt becomes very significant.
At large separations they become equal. Gregory (1969) has discussed
the ~ituation between constant charge and constant potential, .but this
will not be further expanded here.
Jones & Levine (1969) have shown' that neither the constant
potential nor the constant charge approach is applicable when KH<O. 5.
2.2.3. Attraction between Particles - van der Waals Forces
The attractive part of the total interactive energy, VA, originates
from the interactions of the following types:-
(1) Permanent dipole - induced dipole interaction - Debye forces (1921)
(2) Permanent dipole - permanent dipole interaction - Keesom forces
( 1921)
(3) Induced dipole - induced dipole interaction - London - van der
Waals forces (1930).
These interactions all involve an inverse sixth power law and are
almost always negative (Le.' attractive forces). There is also an
inverse seventh power law which is a special case of the induced
- 25 -
•
dipole - induced dipole interaction~simir & Polder) which applies to
the case of large separations. This set of attractive forces are known
collectively as van der Waals attraction.
As summarised by London (1937) the interaction of two neutral atoms
can be expressed as - ~/H where H is the distance between the particles
and ~ is a constant to which the three types of forces contribute.
The main . contribution to the interaction comes from the induced
dipole - induced dipole forces.
Hamaker (1937) developed a method of calculating the interactive
attraction energy, V A' sUlIlIling all the possible interactions between
pairs of molecules in different particles. These interactions were
thought to be of electromagnetic origin. However, at large separations
there can be a retardation (or time lag) between emission and reception
of waves, reducing the magnitude of the attractive energy. This leads
to the inverse seventh power observed by Gasimir & Polder compared to
the inverse sixth power proposed by London.
There exists therefore a Hamaker solution for small particle
separations of the order H < }../2 (where A is the electromagnetic
wavelength) • Solutions for both retarded and unretarded attraction
are available.
2.2.3.1. Unretarded attraction between Spheres
Hamaker (1937) used an integration to develop analytical solutions
for unretarded attraction between two spheres of radii a1 and a2
separated by distance H, as follows:-
- 26 -
where )( D
y = A
+
H / (81 + 82 )
82/81 ; 82> 81
+
the Hamaker constant, ( Section 2.2.3'.3)
When 81 D 82 . this reduces to
[
282
H(H+4a)
+ + In H( H+ 48 ) ] 2
(H+ 2 a)
(2.19)
(2.20)
When R»H, this equation can be further reduced, as shown by Verwey &
OVerbeek (1948):
- Aa 12H
(2.21)
These equations only apply for small separations where H «A!2. In the
model emulsion paint system, it is believed thaj; H :> A .
Therefore the retarded equations are more applicable for this
project. The effect of adsorbed layers on the interacting particles
must also be taken into account.
2.2.3.2. Retarded attraction between spheres
Equations were developed by Schenkel & Kitchener (1960) to allow
for retardation. This solution to the problem was very simple to apply
practically. However, inaccuracies in the method have been pointed out
by Vincent (1973) who indicated, that for small particles, this type of
solution is more inaccurate than using the unretarded equation.
- 27 -
Clayfield et al (1910) also produced retarded equations but these are
more complex.
Osmond et al (1913) and Vincent (1913) have developed ideas put
forward by VoId (1961) producing equations which are easier to use.
These equations are of the form:-
" -"2 L I(Ai)Gj (2.22)
where I( Ai) is a function of the Hamaker constant, and G j is a function
of the geometl'Y of the system. Vincent split the geometry, Gj' into
two functions, one for the long range interaction, G L ,and the other
for the short range interaction, GS . He derived the equations given
below for G Sand G L. In order to accommodate interaction of particles
W1 th adsorbed layers, r, and r 2 are defined as -the radius of the
particles with adsorbed layers and A is the separation of these
particles (i.e. when
Gs = b[ uY+.-!+ U+Y
+ c' [2 - 2 60rl u+y u
2 In (~) J ' U+Y
and
8c;l[ 2y +(2u+y)
u u+y
y~y+' _\ y(,+y)2_
(utY)2 u 3
, .1.= H ).
In( u~J ] (2.23)
(u;y) ]
y ( 1- Y )32
]
(u + y)
, (2.24)
I
The constants b, b' • c and c have the following values:-
where
b " 1.01 ti = 2.45( hI27f)
u = d " x "
y =
x2+ xy+x rl H 2 TA
.1./2,., r 2/ r,
- 28 -
c = 0.14 r 27f/A J c' = 2.04 ( hi 27f)
At a critical separation, I::i: , there is a transition from one to
the other, where for ~ < Ij':, G s is used and for ./::i. >./::i.., G L is used.
Vincent has given a polynomial for the rapid calculation of
./::i.. but Nazir (1977) has shown that this can be grossly in error. For
this reason, an iterative technique has been used to locate ./::i..
Examples of the transition from Gs to GL are shown in Figures 2.2 to
2.10. Figures 2.2 to 2.6 show the effect of increasing the radii of
equal size particles. The smoothness of'the transition from GS to
G L decreases as the radii of the particles increase and is poor for
particle radii >1J!m (Figure 2.5). If one of the interacting particles
is kept small, a smooth transaction is maintained (Figures 2.7 to 2.10).
This implies that these equations are suitable for calculating VR for
sphere - sphere interactions and for approximating' sphere-plate
interactions. They are not sui table, however, for calculating the
interaction between two plates when the radii would be assumed to be
infinitely large. For the particle radii used in this project no
problems would occur.
2.2.3.3. Calculation of Hamaker Constants
This has been extensively reviewed by Gregory (1969), Visser (1972)
and Nir (1979) and will therefore only be briefly outlined here.
There are two methods of calculating Hamaker constants:-
(1) Microscopic approach - based on the summation of interactions
of the individual molecules.
(2) Macroscopic approach - based on the electrodynamics of the"---'
bulk material.
- 29 -
2 __ ------------------------------~ ... , ... " ... , ••• A. o.UtI""'.~ "
1
o
"" t!) .... -1 ... trl" "" tlI Q - -2
-3
-4
-s+-----r---~----~----~----,_--~ o 1 2 3 4 6
FIGURE 2.2.
The behaviour of Vincent's short and long range geometric
fUnctions for small particles of equal radii.
..
)(10-1 30~ ________________________ ~~
• • .. , .... .&. -, .... .&. , 0.1 f.4-s3.:I$ A
25
20
15
10
5
o L
s
-5+-----r---~----~~--~--~ o 5 10 15 20 25
Figure 2.3.
- )(10 1
H/Angsf.rom
The behaviour of Vincents' short and long range geometric
functions for particles of equal radii.
4~ ________________________________ -, s
·3
2
1
o •
s
#1.-'''' .• J. _,_ .• J. o.lu-s7.$' J.
-1+-----~--~--._----~------r_----~ o 5 10 15 20 25
Figure 2.4.
Xl0 1
H/).ngsf.rom
The behaviour of Vincents' short and long_range geometric. ___ _
functions·for particles of equal radii.
""\
)(10- 1 50~ ________________________________ _
45
40
5
... ,_ .. ,..
... ,_ •• A 0.1 c..~.~, ,..
t!).... 35 '"
30
25
L
20 L
15+-______ ~s~----~------_,------~~----~ o 5 10
Figure 2.5.
15 20 25 )(10 1
H/).ngstrom
The behaviour of Vincents' short and long range geometric .
functions for large particles of equal radii.
"'\
)(10-1
60 __ --------------------------------~ •
55
50
• .. "ussr .• J. ... 0 sr .• '" o.lc..~ .• .,'A
(,0..... 45 ..
-... 40
35
30 L
25+-____ ~------~----~------~----~ o 5 10 15 20 25
Figure 2.6.
)(10 1
H/Angsf.rotrl
The behaviour of Vincents' short and long range geometric
fUnctions for large particles of equal radii.
...
3~ ________________________ ~--,
... , .... 1>.
... , .... 1>. I»lC .. _'Sf. ,.1>.
• 2
1
o
-1
L
L
-2+-----r---~------r_---~---~~~~~ o 5 10 15 20 25 30 35
Figure 2.7 /
)(10 1
H/Angstrom
The behaviour of Vincents' short and long range geometric
functions for particles of unequal radii.
3~ ____________________________ -,
I
2
1 ..
o
-1
L
-2~-----r----___ r-----'-----~----~ o 5 10 15 20 25
)(10 1
H/Angstrom
Figure 2.8
The behaviour of Vincents I short and long range geometric
functions for particles of unequal radii.
...
3~ __________________________ -.
... , .... ,..
2
1
o
-1
L
-2 0 5 10 .
Figure 2.9
15
-, .. ,.. 0.",,-'f2.f9'"
L S
20 25 X10l
H/).ngstrom
The behaviour of Vincents' short and long range geometric
fUnctions for particles of unequal radii.
..
-...
4~ ______________________________ ,
3
2
1
o
-1
-2
s
,
... , .... A ... , .. ,.. o.l", .. ~.67 ,..
L
-3+---~----r---'----'----.---~--~ o 5 10 15 20 25 30 35 Xl0l
H/Angsf.rom
Figure 2.10
The behaviour of Vincents' short and long range geometric
functions for particles of unequal radii.
(1) Microscopic Approach
According to Hamaker ( 1931) the Hamaker constant, A , for two
bodies of material 1 acting across a vacuum is given by:-
(2.25)
where Nl = the number of atoms or molecules per C1D3 ~11= the constant in the equation developed by London (1930).
and (2.26)
where V" = the attractive energy between two molecules distance
r apart.
For dissimilar materials, 1 and 2, the Hamaker constant, A12,is
given by:-
(2.21)
where {312 is a composite London constant,and providing that:-
.- (2.28)
then: -
(2.29)
the Hamaker constant, A131 , for two bodies of similar material in a
medium 3 is given by:-
(2.30)
If the two bodies in medium 3 are different i.e. 1 and 2 then:-
(2.31 )
- 30 -
Using (2.29), (2.30),and (2.31) these can be reduced to:
t t 2 A131 "-' (All - A33 )
and
(2.32)
(2.33)
Schenkel & Kitchener (1960) debated that the right hand side of
the equations should be divided by the dielectric constant of medium 3.
However, the results of Krupp et al (1972) indicate that it should be
division by a constant of 1.6 when material 3 is water .
. _ .. Other methods of estimating Hamaker constants include those of
Gregory (1969) and ·Tabor & Winterton (1969) who used the static
dielectric constant,E10 ,to give:-
27 h 1'1' 64
where E10 is the static dielectric constant obtained from the square
of the refractive index at the characteristic frequency of the atom, Pp
Fowkes (1964, 1967, 1968) related the Hamaker constant to the
interfacial tension to give:-
and
where
All = 12.2 0 2 'Y0 I>" Al31 = 12.2 [0, ('Yf )~2 03( 'Yf )1/2 I
E3 >"13
(2.35)
(2.36)
are the dispersion contributions to the surface
tension of materials 1 and 3, 0 is the separation of atomic centres
at contact (O.4mm for inorganic material and o.43mm for water), >"13
" 0.9 and E3 is the dielectric constant of lIBterial 3. When 3 is
water 1:3>"13 = 1.6.
Hamaker theory has been criticised by Parsegian & Ninham (1971)
- 31 -
for the following reasons:-
i) as liquid water is highly polar, most of the dispersion
force originates from polarization at infrared and
microwave frequencies rather than the temperature
independent ultraviolet, as assumed in Hamaker theory.
ii) pairwise summation of individual interatomic interactions
in condensed media is incorrect, and
iii) the liquid between the interacting particles cannot be
dealt with by the insertion of an arbitrary dielectric
constant at a single frequency.
An alternative method for calculating attractive forces between
particles was given by Lifshitz (1955, 1956) and Dzyaloshinskii et al
(1961). They produced a solution for interaction of any combination of
dielectric media from measurement of their spectral properties.
As this method" uses bulk properties, it is known as a macroscopic
approach.
(2) Macroscopic Lifshitz Theory
The dispersion energy V132 (H, T) between flat media of material
1 and 2 in a medium 3, separated by distance, H, is given by Lifshitz
(1956) as:-
(2.37)
where A 13 2 ( H , T ) represents the summation of contributions from
fluctuations
where
over the entire frequency range. It is defined as:-00' 00
Al32(H,T) = -3;TL Iox x 1n[1-A12 A32 e-xJ n-O 0
Aki = ek ( i ~ - e i (i ~n )
ek(i~n) + edi~n)
- 32 -
(2.38)
(2.39)
tn = n211"kT and x = 2 k H (2.40)
h
where: = Planck constant
k = Boltzmann constant
. . x Hough & White (1980) have demonstrated that as ~13 ~23e- c:: 1 :-
CD' CD
~132(H,T) '" -3~TL L(~12~32)S (2.41)
n-O 5.0 5
where s is the number of terms used in the summation
has been developed by Ninham & Parsegian (1910) in terms of
experimental quantities Le. relaxation frequencies, "'j. and oscillator
strengths, f j , . such that:
n
f(i~) = 1 + L (2.42)
i .1
and:
A similar solution obtained for unretarded interaction by Mitchell &
Ninham (1912) does not significantly resolve the complexity of the
calculation.
2.2.3.4. Interaction between Spheres with Adsorbed Layers
VoId (1961) developed an equation, based on (2.24) and (2.29) for
the interaction of spheres with adsorbed layers. For a system of two
. particles of radius a, and a2 with Hamaker constants A'l and, A22 ,
adsorbed layers of thickness 01 and 02' with ·Hamaker constants A44 and
ASS' in a medium of Hamaker constant A33
, the attraction force V A is
given by:
- 33 -
(2.44)
= A55 (2.44) reduces to the interaction of
two spheres without adsorbed layers.
2.2.4. Total Energy of Interaction
As previously indicated DLVO (Derjaguin & Landau' (1941) and
Verwey & Overbeek (1948) theory states that the total energy of
interaction, VT' is given by the addition of the repulsive energy, VR,
and the attraction energy, V A"
The resultant potential energy-distance curve will be dependent on
the relative magnitudes of the two forces. A typical variation of VT
with distance is shown in Figure 2.11.
VR decreases approximately exponentially with distance and has a
range of the order of thickness of the double layer. VA decreases as
an inverse power of the separation i.e. it is proportional to H-x
where x will vary from 1 to 7, and has a range comparable to the
particle size. V A dominates at close separations and there exists a
deep primary minimum. At intermediate distances, the value of VT
depends on the magnitude of VR. The potential energy barrier to
- 34 -
+
>. 01 L
" C
" o ... c " ... o Q.
\
\
\
",total potential energy
H--t--'f--~------~~~S:~==::~~dd'~,s~t~a~n~ee betvveen surfaces
,/
/ secondary minimum
/ I primary minimum
FIGURE 2.11.
The potential energy of interaction as a function of
the separation of two particles.
flocculation when VR > VA depends on the surface potential and
electrolyte concentration.
It is the potential barrier that causes a system to remain stable.
However, it is possible for the -attractive-term- of -large particles to
dominate at large distances of separatipn. This results in a
secondary minimum. In practise an energy barrier of VT > 5kT is
necessary to prevent flocculation.
At 0 < VT "" 5kT a diffusion controlled flocculation can occur. As
VR is increased, the dispersion becomes more stable.
However, if a secondary minimum is present, the particles may
form a loosely structured floc. The computer programme, DLVO,
Kayem (1978), was written to rapidly calculate values of VT for
different Hamaker constants and Stern potentials. It must be
remembered though, that DLVO theory is limited in describing precisely
the interaction between particles as it neglects viscosity effects -
Spielman (1970), hydrodynamic factors - Honig et al (1971) and
solvation layers - Ottewill (1977). All these effects tend to slow
down flocculation.
2.2.5. Other Factors Affecting Flocculation
2.2.5.1. Polymer Systems
The major effect of macromolecules present in the continuous
phase is a hydrodynamic one. The rate of flocculation is reduced as
the viscosity of the solution increases. When the concentration of
polymer is high, it has been reported, Vrij (1976), Cowell et al (1978)
and Vincent et al (1980) that weak reversible flocculation can occur in
a similar way to phase separation. Sperry et al (1982) - have also
rePorted flocculation by polymers due to volume restriction.
- 35 -
Alternatively, Feigin & Napper (1980), have discussed a depletion
tloc.c.u..\a.bon. of particles by free nonionic polymers.
When polymers are adsorbed onto particles, other repulsive forces
occur. As the separation between two particles is reduced to the sum
of the adsorbed layers, there arise repulsion forces due to:
i) the entropy rise when the adsorbed macromolecules lose
configurational freedom,
ii) the increase in osmotic pressure due to a higher segment
density of macromolecules, and
iii) if the macromolecule is also a polyelectrolyte, there
will be additional double layer repulsive forces.
However, the steric stabilization of particles by macromolecules
is still regarded as complex and not completely resolved. Reports on
the subject include Vincent & Whittington (1982) and Napper (1977).
In addition, polymer bridging on free adsorption sites of two
particles can occur if the macromolecules are large enough to give a
polymer layer greater than twice the double layer thickness.
2.2.5.2. Hydration Forces
Israelachvilli & Adams (1978) have reported short range repulsive
forces at a hydrophilic mica/water interface which prevented adhesion
contact. However, Pashley & Israelachvilli (1981) have measured
attractive forces on hydrophobic surfaces e.g. CTAB coated mica.
Hydration forces are known to exist but little is known .about their
magnitude, effect and range.
2.3.5.3. Brownian Flocculation
Time dependent forces such as Brownian motion and bulk medium flow
- 36 -
are not accounted for in DLVO theory, but they do affect flocculation
and stability of colloidal dispersions.
Smoluchowski (1917) observed that coagulation of spheres followed
second order kinetics as given by:
Where
= - 4kT
31/
number concentration of particles
1/ = viscosity of the medium
(2.45)
The kinetics of the slow flocculation of a system, under Brownian
motion, where there is an energy barrier was developed by Fuchs (1934).
This was applied to aqueous dispersions by Der jaguin ( 1940) • He
described a stability factor,W,which is related to VT by the following
equation:-co
W = 2J exp ( VT/kT as s2
2
(2.46)
s = 2(a1+ a2+~)/ (a1+ a 2) (2.47)
As the energy barrier is reduced to zero, W tends to unity.
2.2.6. Zeta Potential and Colloid Stability
The barrier to flocculation is mostly governed by the value of
VR and the value of VR is dependent on the potential of the double
layer. It is therefore logical that the stability of a system is
related to the zeta-potential of the particles. Indeed it has long
been recognised that measurement of zeta-potential can indicate good or
bad stability and the value of r is commonly used in this manner
e.g. Riddick (1968). Hunter & Alexander (1963) measured the
- 37 -
zeta-potential and produced potential energy curves for kaolinite
particles in varying concentrations of indifferent electrolyte. As
the concentration increased, the zeta-potential and the barrier
opposing coagulation was reduced .... The point - when the barrier just
disappeared, was called the critical coagulation concentration
(c.c.c.).
The zeta-potential has also been correlated with the stability
ratio, W. The behaviour of moderately stable sols, (W = 1-20) has
been investigated by Wiese & Healy (1975a) for Ti02 and Al203
dispersions. They observed that rapid coagulation occurred in all
cases at values less than r = 14 + 4mV, or A = 13kT. Contradiction to
this type of behaviour has been observed by Ottewill & Rastogi (1960)
but in general the zeta-potential, especially for aqueous systems, is
a valuable guide to stability behaviour.
2.3. SEDIMENTATION THEORY
2.3.1. Introduction
Sedimentation experiments on paint formulations yield information
on the processes occurring in storage. However, of more importance, by
detailed analysis of the data, information about flocculation of the
particles in the wet state can be obtained. Sedimentation data can
therefore complement electrophoretic data in understanding the
stability of the solid components before the aqueous layer is finally
evaporated to form the final dry paint film.
Sedimentation theory includes the primarily developed solutions
for dilute and concentrated, unflocculated systems. Of more importance
are the treatments of the complete process which are required in this
project.
- 38 -
2.3.2. Dilute Dispersions
Initial studies on the sedimentation velocity, Uo , of a single
rigid sphere of radius, a, in an infinite Newtonian fluid of viscosity,
~ , in the absence of other forces is given by Stokes (1856):-
2 = 2 a (Ps - pm ) 9 (2.48)
9~
where Ps = density of the particle
Pm = denSity of the medium
9 = acceleration due to gravity
As the concentration of the suspension increases, the volume
fraction, lP, is reduced and hydrodynamic interactions will reduce the
settling velocity. This occurs even in the absence of particle
agglomeration as reported by Kaye & Boardman (1962).
The concentration effect on sedimentation velocity has been
reviewed by Happel &' Brenner (1965). The theory falls into three
schools of thought wi th differing assumptions about the spacial
distribution of the spheres and the nature of their interaction.
One group considered the centres of the spheres to be in a regular
cubic array. A second group used a cell model where the hydrodynamic
effect surrounds the sphere in a spherical boundary. Both of these
theories predict that sedimentation rate reduction due to hydrodynamic
interactions is proportional to the volume fraction.
The third group (Burgers ( 1942) , Puyn & Fixman ( 1964) and
Batchelor (1912, 1916» assumed the particles to be randomly distributed.
They used statistical analysis to show that the velocity was given by:-
u (2.49 )
- 39 -
where K = sedimentation coefficient
Uo = the initial sedimentation velocity
The value of K was given as 6.55 (Batchelor (1972, 1976», 6.88
(Burgers (1942» and 7.16 (Puyn & Fixman (1964». The difference
between the values was reported by Batchelor (1972) as being due to
the statistical method used.
Development of sedimentation theory to include attractive and
repulsive interactions was considered by Dickinson (1980). He expanded
Batchelor's work to include the effect of double layer forces on the
sedimentation rate of monodisperse particles. The sedimentation
constant,K , of the hard sphere model was defined as:-
2 -1 1.5ao + 3.75ao - 1.32 (2.50)
where a o = the radius of the repulsive barrier around
the particle ( representative of the radius
of the double layer).
Due to double layer repulsion, the descending particles cannot
approach each other as closely as uncharged particles. Because of
this each particle cannot take advantage of the maximum flux downwards
which occurs near the surface of other particles. The upward flux
opposing the downward current therefore has more of an effect on each
descending particle. A limi ting value for K of 5.8 was obtained by
Reed & Anderson (1976, 1980).
An additional correction for the reduction in velocity due to the
ionic atmospheres, known as the Oom (1876) effect was defined by
Booth (1954). The corrected velocity, liD' is given by:-
- 40 -
where e = the proton le charge
q; = an electrolyte perameter
~'= the geometry of the suspension
The reduction in sedimentation velocity imparted by double layers
and ionic atmospheres are comparable, being approxllately 5 - 10% for
a dispersion of 0.03 volume fraction.
Sedimentation velocity is dependent on the size of particles. In
a coagulating or flocculating system, floc formation will increase the
polydispersity. This effectively increases the sedimentation velocity.
Cornell et al (1979) have shown that the effect of a secondary minimum
can increase the sedimentation velocity even before the onset of
coagulation.
Maude & Whitmore (1958) have studied early experimental data on the
boundary of a cloud of sedimenting particles. They obtained the
equation for the settling velocity, U , of identical spheres:-
u Uo(1 - cp )~ (2.52)
• Which approximately reduces to (2.49) when ~ is approximately 5.
Similar values of K have been obtained by Cheng & Schachman (1955)
(who obtained K = 5.1 for monodisperse polystyrene latices) and Buscall
et al (1982) (who obtained K = 5.4 for stable sediments with cp<0.085).
This refutes the value of K = 6.55 obtained by Batchelor (1972, 1976),
where the non-hydrodynamic forces were not taken into account.
It must be noted that the aforementioned theories are only valid
for dilute, monodisperse model systems. They are not applicable for
non-ideal practical formulations such as paint mixes which have
irregular polydispersed particles with adsorbed layers.
- 41 -
2.3.3. Concentrated Dispersions
As the concentration of a stable monodisperse system increases.
the theoretical solutions of Section 2.3.2. break down. Buscall et al
(1982) have shown that for ~ >0.085. plots ofU/Ubversus ~ are no longer
linear. and tend to zero at high volume fractions. as the particles
become closely packed.
When the system reaches the region of non-linearity. any solution
must contain the interparticle forces that exist when particles come
close together. This is a complex problem.
Several modifications of Stokes' equation (2.48) have been attempted.
Steinour (1944) obtained the definition:-
(2.53)
Where € = the proportion of the total volume of the suspension
occupied by the solution. Steinour demonstrated that this equation is
valid for 0.5 <€< 0.7.
Other equations include:-
obtained by Richardson & Zaki (1954) and
where Vo = the instaneous volume
Vr» = ultimate volume
k = rate constant
obtained by Garner et al (1953) and Bischoff (1964).
(2.54)
No account was
taken of interparticle interaction in these solutions.
For flocculated systems. Steinour (1944) developed an equation
- 42 -
•
using ,VVj, the volume of liquid considered immobile in the flocs:-
u = U o O.123(E-VVj) (2.56)
(1- VVj)2(1 - E)
Two types of sedimentation characteristics were ideritified by
Smellie & La Mer (1956) for phosphate slimes. The two types of curves,
shown in Figure 2.12, were dependent on the initial flocculation being
fast or slow. For slow flocculation (Type I) the sedimentation rate
is initially slow and gradually increases. This indicates that the
particle size is gradually increasing until a size is reached where
the sedimentation rate is constant with time. If fast flocculation
occurs, the steady rate is reached almost instantly (Type 11) and the
rate can be defined by:-
t = ex + {3t (2.57) Ho- H
where Ho : sediment height at t·: 0
H : sediment height at time t
ex : constant
{3 : constant
Type II behaviour has also been identified for intermediate
concentrations of non flocculated suspensions. The transi tion from
Type I to 11 sedimentation rate has been used by Sadowski & Laskowski
(1980) to determine the isoelectric pOints of minerals.
Michaels & Bolger (1962), in studying the sedimentation of
flocculated kaolin dispersions, postulated an initial loose floc
structure Which then developed into aggregates and finally settled
into a more compact structure. A theoretical explanation for the floc
- 43 -
Time
FIGURE 2.12.
The two types of sedimentation pattern observed by
Smellie & La Mer for sedimenting phosphate slimes.
fonnation was given by Sutherland (1967>. This model agreed with the
experimental results of Michaels & Bolger. A general observation is
that flocculation due to polymers produces flocs with a more open
structure than flocculation due to electrostatic forces.
The theories given in Sections 3.4.2. and
applicable to the initial sedimentation of a system.
3.4.3. are only
As the volume of
the sediment decreases, the gravitational and buoyancy forces acting on
the particles give way to interparticle interaction. The way that the
particles sediment after this volume is reached can therefore yield
infonnation about the system.
Non flocculated systems result in a closely packed bed whilst the
compaction rate of an electrostatically flocculated suspension will be
slower. This is due to the rearrangement and collapse of the
aggregates formed on flocculation.
Further compact ion processes are observed with polymer flocculated
systems. This is due to the repulsive force from the compaction of the
'polymeric material between the particles. However, anomalous results
have been reported. Fleer (1971) showed that the sedimentation volume
of silver iodide sols, flocculated with poly(vinylalcohol) and
electrolyte was less than with electrolyte only. No explanation was
given for this observation.
2.3.4. Comprehensive Treatments of Sedimentation
As already indicated, full information about the interaction of
particles in a suspension, requires a more complex sedimentation theory.
Kynch (1952) described the sedimentation of monodisperse particles
throughout the entire process. He assumed that the sedimentation
- 44 -
veloci ty, U , of a particle was dependent only on the number of
particles, n, surrounding it. He derived expressions for the reduction
in height of the solid component of a suspension with time. This was
divided into three phases as shown in Figure 2.13 and characterised as
follows:-
Phase 1
Phase 2
Phase 3
AOB, where n is the same as the initial
concentration no.
OCD, where n is at a maximum concentration nu.
OBC, where the concentration is riSing rapidly
between no and nu.
For non-flocculated suspensions Kynch predicted that the settling
rate was constant in Phase 1 and logarithmic in Phase 3. Phase 2 was
the transition period betwen the two.
However, this analysis did not include hydrodynamic interactions
and the effect of a sediment fOrming at the base of the vessel during
Phase 1 was not taken into account.
An improved treatment was given by Carstenson & SU (1970a, b) for
the analysis of sedimenting flocculated suspensions. This allows a
complete analysis of sedimentation data. The analysis uses a plug of
height, H, moving downwards with increasing velocity, U , until a
critical height is reached where the rate of change of height is
significantly decreased. These two regions, the initial and final
region were considered separately.
2.3.4.1. The Initial Region
The analYSis of the initial region was similar to that put forward
by Michaels & Bolger (1962), and Gaudin & Fuerstenau (1959). At time,t,
..: 45 -
A
FIGURE 2.13
lime
C 7""'___ 0
Fall of the Surface of a Dispersion according
to Kynch (1952).
the height, H, of the sediment is given by the sum of the height of a
plug, x , containing the initial volume fraction of solids, lP, and the
height of a cake, y, of volume fraction,lPc • The following equation was
shown to describe the rate of fall of the plug:-
x = Ho eXP (- kt) (2.58 )
where Ho = the height at t = 0
Gaudin & Fuerstenau (1959) showed that the height of the cake
decreases exponentially such that:-
(2.59)
where W is the exponential decay constant
As the plug descends it contributes to the cake. Carstensen & Su
(1970a) developed an equation for the volume fraction of solids in the
cake:-
IPc = (Ho - x) IP
y
The total change in y wi th time is therefore:-
= k IPo x
IPc
- wy
(2.60)
(2.61)
Substituting (2.58) and (2.60) into (2.61) and solving for y gives:-
y = c[ 1 - exp (-k t) 1 exp (-wt) (2.62)
Combining (2.58) and (2.62) gives an expression for the height of the
initial sediment at time t:-
H = Ho exp(-kt) + c [1 - eXP(-kt)Jexp(-wt} (2.63)
- 46 -
which can be rearranged to give:-
In (
H - Ho exp (- k t ) ) = 1 _ exp (- k t )
-Wt+lnc (2.64)
An iterative analysis can obtain the optimum value of k which
yields values of wand e from the slope and the intercept of the
straight line.
This analysis assumes that the cake is of uniform concentration.
Attempts to overcome this by expressing~ as a function of H were not
found possible. However, Gaudin & Fuerstenau (1959) showed that the
approximation worked well in practise. They found that equation (2.58)
described the height of the plug, x , for a dilute calcium oxide
suspension. It is also found to apply to more concentrated systems.
One detectable limitation of the model is the deviation from
linearity near the critical height due to other rate determining
processes.
2.3.4.2. The Final Region
The" analysis of the final region stems from the observation that
the decrease in height follows a combination'of two exponential decay
curves. Several authors have implied this type of pattern, including
Robinson (1926), Haines & Martin (1961) and Michaels & Bolger (1962).
In order to analyse the final region, the time axis needs to be
redefined so that zero time is at the end of the initial region. The
intermediate point between the initial and final region, i.e. zero
time for the final region, 7 = 0, is defined as the critical time,
te. The height of the sediment at the critical time is known as the
critical height, He.
- 47 -
The new time axis is, therefore:-
T = l - le (2.65)
The critical time and height are determined by a graphical interpolation
of the sedimentation curve between the two sedimentation regions.
There are three types of forces being exerted on the compacting
sediment, gravitational, reactional and electrical. The graVitational
forces work in a downward direction. The frictional forces contain two
components, one viscosity dependent and one viscosity independent.
These both act in an upward direction. The electrical forces are
assumed to be repulsive and will, therefore, have a component in the
upward direction, the magnitude increasing as the interparticle
distances are reduced. The sum of these forces is equal to the
sediment·mass times the forces due to gravity as follows:-
M [ 1 - (Pm / Ps) J 9 - (3 (1],R)~- ~R)
where
gravitational force
aT viscosity dependent and independent frictional forces
Ps: density of the solid
Pm: denSity of the liquid
M : mass of the sediment
9 : acceleration due to gravity
R : radius of the tube
Equation (2.66) can be rewritten as:-
- Oy =
electrical force
+ +§.L M
= [1 - (Pm / Ps) 1 9 - 1/t ( R )
M
From (2.66) and (2.67) the solution to the differential equation is
given by:-
- 48 -
.(2.66)
(2.67)
J • = M [ 1- IPm/Ps )1 9 - ,pI R)
()
(2.68)
Other solutions are obtained by inserting an expected solution of
the fonn y=_AeWt into the homogenous equation corresponding to (2.66).
This results in:
-Wt - Ae
(2.69)
When the bracketed expression is equal to zero (2.69) has the following
roots:-
w =,BIT/,R) 2M
If ,BIT/ ,R)/M » ()/,BIT/,R)
then w1 = ,BIT/,R) M
and w 2 = ()
,BIT/,R)
(2.70)
(2.71)
(2.72)
The complete solution of equations (2.66) and (2.67) in terms of y is:-
(2.73)
From (2~73). , • y = M [I-(Pm/Psl]g -,pI R) / () corresponds
to the value of y at infinite time, Y f/J ,and therefore can be related
to the ultimate height, Hu , of the sediment by:-
(2.74)
- 49 -
If (2.73) is inserted into (2.74) and the equations rearranged, the
solution for the rate of change in H with time, in terms of the forces
considered is:-
H - Hu (2.75)
where A, = 2A', and A2 = 2A'2
-wiT -W2T As w,»W2 • A, e should predominate at small values Qf T and A2 e
at high values of T.
Estimates ofHuand consequentlYW2andA2can be obtained from long tau
data and therefore (2.75) reduces to:-
An interation procedure is used to solve for H u'
With estimates of A2 and w2 •
obtained from the following:-
the value of
This can also be solved by an iterative technique.
Thus analysis of the long time data gives:
(i) the exponential decay constants,
(ii) the pre-exponential factions in the descent of the
sedimentation boundary, ·A, and A2 , and
(iii) the ultimate height of the sediment, Hu.
(2.76)
(2.77)
The change in these values for different systems can yield
information on the type of flocculation occurring, if any, of the
solid component, and the type of floes formed. The computer programme
SEDIMENT Dunlop-Jones (1982) was written to enable rapid analysis of
~ 50 -
the data. The behaviour of the model when applied to the paint
formulations containing various concentrations of thickener is
described in Chapter 4.
- 51 -
•
CHAPTER 3
EXPERIMENTAL
The following is an outline of the main compounds, materials,
equipment and methods used in this project.
3.1. ClID{[CALS
These are the soluble components of the paint system, including
pH variants, dispersants and thickeners, together with reagents used
in the analyses.
3.1.1. Tri-distilled water
All water used in the experiments was tri-distilled through
quartz apparatus.
-1 mho. cm •
3.1.2. pH variants
-6 The average value of conductance was 1.2 x 10
Hydrochloric acid (AR grade) and aJIIDOnium hydroxide (AR grade)
were used to adjust the pH of the system.
3.1.3. Sodium dodecylsulphate (SDS)
This was obtained from Cambrian Chemicals Ltd. Very pure samples
. of SOS are very difficult to obtain as reported in the literature by
Smith (1978) and Vijayendran (1976).
$u. ~o c<2 ten'S. \0(\ - 4 1 This sample gave a smallJlminimum of approximately 5 x 10- Nm-
at the critical micelle concentration, indicative of high purity.
- 52 -
3.1.4. Sodium hexametaphosphate (SHMP)
This was obtained from Fisons Ltd. and was described as being
This is not strictly correct since industrially
produced SHMP (known as Calgon) consists of mainly high molecular
polyphosphate chains plus a small percentage of metaphosphate. rings
(Corbridge (1978». These polyphosphates are of the formula (Na3P06)n
The value of n, for SHMP has been reported by Toy (1973) to be
15-20. The molecular weight is, therefore, between 1700 and 2100. It
was decided to adopt 1700 as the relative molecular mass. It takes
the form of a polymeric glass with a degree of dissociation of about
30~, as given by Toy (1973) and SUch (1971). This is similar to the
dissociation of polyphosphoric acids. SHMP therefore acts as
something between a 1:3 and 1:5 electrolyte.
3.1.5. Thickeners
The two thickeners used were Natrosol 25OGR, obtained from the
Hercules Powder Company Inc., and Courlose F15G, from Courtaulds Ltd.
3.1.5.1. Natrosol 250GR (HEC)
This is essentially hydroxyethylcellulose (HEC) produced by
reacting ethylene oxide with cellulose. The average number of moles
of ethylene oxide attached to each anhydrogluconic unit of cellulose
is 2.5, see Figure 3.1. This gives the optimum solubility in water.
R grade Natrosol is a specially treated type of' HEC for fast
dissolution. G grade Natrosol is medium molecular weight HEC which
-2 - 1 -1 has a viscosity of 1 - 3.5 x 10 g.cm .s for a 1,. w/w aqueous
solution at 250C.
- 53 -
/
,- - CH20H - -, 1 1
1 0 1
1 o .-!... 1 1
1
H 1 , 1 1_- H OH - -I
Anhydrogluconic Unit
/ CH2CH2"
H OH H2
O OH
° 0
° CH2 H ° '" '" /0 CH2 CH2 I \ CH2 CH2 ,/ "- 0 0
"" / CH2 lH2 I C!l.z CH2
./ OH HO
FIGURE 3.1.
Natrosol 250 GR - Hydroxyethylcellulose
3.1.5.2. Courlose F75G (CMC)
This is a purified grade of sodium carboxymethylcellulose (CMC)
made by reacting cellulose with alkali, followed by addition of
monochloracetic acid. The water soluble CMC is obtained when the
degree of substitution of the three hydroxyl groups per anhydro-
gluconic unit, available for etherification, is 0.4 - 1.5, see Figure
3.2. The approximate viscosity of a 1% w/w aqueous solution of
-1 -1 -1 at 250 C. Courlose F75G is 6 - 9 x 10 g.cm .s
3.1.6. Butyl Carbitol Acetate (BCA)
C4H9 (CH2)20(CH2)20~CH3 o
This is the acetate form of the diether of ethylene glycol used
as a coalescing agent for paint films. The sample used in this
project was obtained from Fisons Ltd.
3.1.7. Aluminium Sulphate
This was used in the electrophoresis experiments and was of AR
grade obtained from BDH Chemicals Ltd.
3.1.8. Tital
This was obtainect from Tioxide Uk Ltd. It consists of a mixture
of concentrated sulphuric acid, alumina, and· titanium dioxide in the
proportions stated below:-
76.7 gdm-3
328.0 gdm-3
56.8 gd!!r3
- 54 -
,- - CH OH , 1 1 , 0 0-'-,
1
, --I ._- H OH
Anhydrogluconic Unit
OCH COONa
o o
H H OH
FIGURE 3.2.
Courlose F75G - Carboxymethylcellulose
3.1.9. Analyses
Three colorimetric analyses were used during the course of this
project. All chemicals were of Analytical grade unless otherwise
stated.
3.1.9.1. Aluminium Analysis
The method used to determine the aluminium content in aqueous
solution was the spectrophotometric analysis using catechol Violet dye
and cetyltrimethylammonium bromide (CTAB) as described by Chester et
al (1970). This analysis utilises the increased photometric sensitivity
of a ternary complex of aluminium, catechol violet and CTAB at pH
10-12.
Method
The following solutions were used in the analysis:-
1. Reagent solution - Catechol violet (0.1546g) and CTAB (1.458g)
dissolved in 500 cm3 water and diluted to 2dm3•
2. EDTA - EDTA (7.5g) was dissolved in 200 cm3 of water by addition
of the minimum volume of concentrated ammonia, the pH adjusted to
9.5 and the solution diluted to 25Ocm3.
3. Buffer solution - concentrated ammonia (62.5g) was diluted to
375cm3 and the pH adjusted to 10.2 with concentrated hydrochloric
acid. This solution was then diluted to 500 cm3•
The sample for analysis was diluted to an aluminium content
within the range of detection (1 - 50 mgdm-3). This solution (1 - 10g)
was transferred to a 100 cm3 flask together with the reagent solution
(25cm3), ascorbic acid solution (5cm3 of 5% w/w) and buffer solution
The flask was placed in a water bath at 2SOC for 20 minutes ,
- 55 -
EDTA solution (5cm3) added and the mixture made up; to 100cm3 with
water. The absorbance at 670 run, when measured against a reagent
blank was compared with a calibration curve to obtain the .aluminium
content.
3.1.9.2. Analysis of Sodium Hexametaphosphate
SHMP was analysed using the molybdenum blue analysis for
phosphate described by Murphy & Riley (1962). A blue complex is
formed on mixing ammonium molybdate with antimonypotassium tartrate in
the presence of phosphate. As the determination was only possible for
orthophosphate, a calibration curve was obtained for hexametaphosphate
which had been heated in concentrated sulphuric acid. This converted
the phosphate present to orthophosphate.
Sufficient dilution of the resulting solution was necessary as
concentrated acid interfered with the colour formation.
Method
The follOwing solution was used in the analysis:-
Mixed reagent - Sulphuric acid (125cm3 of 2.5 mole dm-3) and
ammonium molybdate solution (37.5cm3 of 4% w/w) were mixed
together. Ascorbic acid solution (75cm3 of 1.76% w/w) and
potassium antimonyl tartrate solution (12.5cm3 of 0.2743% w/w)
were then added. This solution was prepared as required and
not stored for greater than 12 hours.
40g of the sample to be analysed was transferred into a
calibrated flask, 8cm3 of the mixed . reagent. added and the
solution made up to 50cm3• The absorbance of the solution at
882nm . versus a reagent blank' was ; measured and the amount
of sodium hexametaphosphate present obtained from a calibration
curve.
- 56 -
3.1.9.3. Analysis of Sodiumdodecylsulphate
Analysis of SDS in aqueous solution was by the Longwell & Maniece
(1955) method for analysis of anionic surface active agents. This
comprises of the colorimetric analysis of a complex of SDS with
methylene blue after extraction with chlorofonn. Use of the Slack
(1959) modification simplified the extraction stage in which only one
chlorofonn extraction was used.
Method
The following solutions were used in the analysis:-
1. Alkaline phosphate solution - Disodium hydrogen phosphate
(lOg) was dissolved, adjusted to pH 10 with sodium hydroxide
and made up to ldm3•
2. Neutral methylene blue solution - Methylene blue (0.35g)
made up to 1dm3 with water.
3. Acid methylene blue solution - Methylene blue (0.35g) was
dissolved in 500cm3 of water, sulphuric acid (6.Scm3) added
and made up to ldm3•
A sample containing approximately 1mg.dm-3 of anionic surface
active agent was diluted to 100cm3• Alkaline phosphate solution
(10cm3) and neutral methylene blue solution (5cm3) was added and the
mixture shaken with chloroform (50cm3). The chlorofonn layer was
then run into a separating funnel and shaken with acid methylene blue
solution (5cm3) in water (110cm3). It was then run through a small
funnel plugged with cotton wool. The first 5cm3 of filtrate was
reje'cted and the next volume collected for adsorption analysis at
650 nm.
- 57 -
3.1.9.4. Analysis of Thickeners
Due to the lack of a sui table colorimetric analysis for HEC and
CMC the amounts of thickener in aqueous solution was determined
gravimetrically. HEC or CMC solution (50g approximately) was . 0
evaporated at 70 C and the residue weighed to find the amount of
thickener present.
3.2. POLY(vnm.ACETATE) LATEX
The method of latex polymerization was based on previous work
with poly("'~~lacetate), by Kayem (1978), the original method being
modified in order to satisfy practical considerations.
As previously described in Chapter1 the following properties of
the polymer latex were required:-
1. a particle size of approximately 0.2 p,m,
2. a high molecular weight, and
3. a solids content of approximately 40% w/w
These constraints were imposed in order to study a paint system
which closely resembled a commercial gloss emulsion paint giving high
reflectant, low chalking films.
3.2.1. Latex Preparation
Emulsion polymerization was originally chosen to produce the
poly(vinylacetate) in order to obtain a high molecular weight polymer,
not possible from bulk polymerization.
In previous work by Kayem, the emulsion polymerization method
used an anionic surfactant, SDS, as the stabilizer and an initiator
- 58 -
system of ammonium per sulphate and sodium metabisulphite. The
.polymer was prepared from unstabilized vinyl acetate from an industrial
source, the preparation consisting of three stages. A polymer seed
was prepared in the first stage and subsequently grown in the
following two stages. The polymerizations were all carried out at 350 C.
In this project unstabilized vinylacetate was not available.
The monomer used was stabilized by hydroquinone which was removed by
distillation. Using this method, polymerization did not occur in the
time allowed, at 350 C and the initiator concentrations quoted.
Therefore the initiator concentration was dcubled and the temperature
raised to 500 C resulting in 100% conversion of monomer. Lack of
polymerization at the original conditions would have been caused by
the presence of little or no initiator radicals in the system or by
the inhibition of the reaction. The higher temperature and greater
initiator concentration increased the radical concentration and
overcame slight traces of acetaldehyde in the monomer (detected by
gas chromatography). Acetaldehyde can act as a transfer agent in
vinylacetate polymerization (Lindemann (1967)) and a higher initiator
concentration would be needed to complete the reaction.
3.2. 1 • 1. :Polymeriza tion Reagents
Analar grade vinylacetate was obtained from Fluorochem Ltd. The
hydroquinone stabilizer present (0.0015% w/w) was removed by
distillation at atmospheric pressure through a Vigreux column. The
fraction collected for polymerization distilled at 720_ 730 C. Ammonium
persulphate «NH4)2S20a) in conjunction with sodium metabisulphite
(Na2S205
) were used as the initiator system. Both were of analar grade
and obtained from Fisons Ltd.
- 59 -
Sodium dodecylsulphate, used as the emulsion stabilizer, was as
previously described.
3.2.1.2. Method
A number of attempts were made to produce. a stable emulsion of
the correct concentration and particle size. These involved changes
in polymerIzation times, temperatures and concentrations from those
used by Kayem. The final method is described below:-
The method consisted of three stages:-
Preparation of the initial seed
Growth of the initial seed
Stage 1
Stage 2
Stage 3 Further growth of the seed to the required
particle size of 0.2 p,m diameter
The chemical concentrations used in each stage are shown in
Table 3.1.
Stage 1
Sos was dissolved in the water and placed in a 1dm3 reaction
vessel fitted with a 7 cm paddle stirrer, a water cooled condenser
and a nitrogen gas inlet. o This was immersed in a 50 ± 0.2 C water
bath, the solution stirred at 100 rpm and purged with nitrogen for 15
minutes. Sodium metabisulphite solution was then added dropwise via
a pipette, the nitrogen stream reduced to a slOW bubble rate and the
vinylacetate dripped into the mixture. Ammonium persulphate solution o
was heated to 70 C and added to the vessel dropwise over 1 minute via
a pipette. The polymerization contents were stirred at 300 rpm at
- 60 -
Table 3.1
Formulae for Latex Preparation
Stage 1
Vinylacetate monomer
Water
SDS
Ammonium persulphate
Sodium metabis~lphite
Stage 2
86.0g
800.0g
O.4g
O.6g in 15 cm3 of water (4% w/v)
O.36g in 9cm3 of water (4% w/v)
Stage 1 seed (dialysed) - 10% solids 100.0g
Vinylacetate monomer 86.0g
Water 300.0g
Ammonium persulphate .O.48g in 12cm3 of water (4% w/v)
Sodium metabisulphite o.6g in 3cm3 of water (2% w/v)
SDS 2.1g
Stage 3
Stage 11 seed (dialysed) - 20% solids 100.0g
Vinylacetate monomer
Water
Ammonium per sulphate
Sodium metabisulphite
SDS
251.6g
300.0g
o.6g in 15cm3 of water (4% w/v)
O.4g in 2cm3 of water (2% w/w)
3.3g
50t 0.2°C for 1 hour, after which the temperature was raised to 80 t
2°C for 15 minutes to complete the reaction. After cooling the latex
was exhaustively dialyzed against tri-distilled water, through a
Visking dialysis tube until the conductivity of the water did not
change over 24 hours.
Stage 2
Dialyzed latex from Stage 1 and water (150g) were placed in the
0.7dm3 reaction vessel at 50t 0.2o
C, stirred at 100 rpm and purged with
nitrogen gas for 15 minutes. Sodium metabisulphite solution was added
dropwise via a pipette, the nitrogen stream reduced to a slow rate and
the vinylacetate dripped into the reaction. After a further 10
minutes, SOS dissolved in water (150g) was added and the stirrer speed
increased to 300 rpm. Ammonium persulphate solution was heated to
70°C and added to the reaction over 1 minute via a pipette. The
polymerization contents were stirred for 6 hours and the temperature
then raised to 80:t2oC for 30 minutes. The resulting latex was
dialyzed as in Stage 1.
Stage 3
Dialyzed latex from Stage 2 and water (150g) were placed in the
0.7dm3 reaction vessel at 50 ± 0.2oC, stirred at 100 rpm and purged
with nitrogen gas for 15 minutes. Sodium metabisulphite solution was
added, the nitrogen stream reduced to a slow rate and the stirrer
speed increased to 300 rpm. Vinylacetate was added dropwise along
with SOS (1.1g) in water (50g). After 10 minutes, the remaining SDS
(2.2g). dissolved in water (100g) was added, followed after a further
15 minutes by ammonium persulphate solution added over 1 minute via a
pipette. The polymerization contents were stirred for 4 hours after
- 61 -
o which the temperature was raised to 80! 2 C for 30 minutes. Any
coagulant was removed by filtration and the latex dialyzed as in
Stage 1.
3.2.2. Theory of Emulsion Polymerization
As many reviews on emulsion polymerization ·are available,
Lindemann (1967), Alexander & Napper (1971), Blackley (1975) and Dunn
(1982), only a basic theory will be covered here.
3.2.2.1. General Theory
The qualitative basis .of emulsion polymerization theory was
first put forward by Harkins (1947, 1950). This theory was then put
into quantitative terms by Smith & Ewart (1948) and Haward (1949).
They described the emulsion polymerization system conSisting of water,
a water insoluble monomer, an emulsifier and a water soluble initiator.
The emulsifier solubilizes the monomer in micelles. The initiator
decomposes in the water and forms radicals which diffuse into the
micelles to initiate the polymerization. There are a greater number
of micelles than monomer particles ·( 1018cm-3 compared to 1010cm-3)
and as the monomer in the micelles is used up, it is replenished by
further diffusion from the monomer particles. Eventually the monomer
is used up and the micelles become polymer. particles .
. Smi th & Ewart, and Haward proposed· three cases for the
polymerization:-
Case 1 the entry of radicals into the particle is low and
the rate of exit is high.
- 62 -
Case 3 - the rate of exit or termination of radicals is much
slower than the rate of entry and the kinetics are
similar to those of bulk polymerization.
Case 2 - the rate of termination is much faster than the
rate of entry or exit. The rate of polymerization
is then similar to the propagation step in any
polymerization.
Later modifications of the Smith-Ewart theory have been proposed
by Stockmeyer (1957), O'Toole (1965) and Gardon (1970).
As several monomers were seen to deviate from the Smith-Ewart
system, another theory was put forward by Sheinker & Medvedev (1954).
The basic difference between the theories is the position of the
locus of polymerization after the initiation stage. Smith-Ewart
theory predicts the locus of polymerization to be the monomer swollen
micelles whereas Scheinker-Medvedev predict the organic/aqueous
interface. Thus the two theories propose different polymerization
kinetics.
Recent investigation into the emulsion polymerization of vinyl
acetate include the work of Friis & Nyhagen (1973), Litt et al (1970),
Dunn & Tay lor (1965) and Dunn & Chong (1970). The results are wide
in variation but agree with the following:-
(i) the rate of reaction is zero order with respect to
monomer concentration at 20-85~ conversion,
(ii) no new particles are produced after 30% conversion,
(iii) ,polymerization rate is dependent on the particle
concentration to the power of 0.2 approximately, and
- 63 -
(iv) dependence of the polymerization rate on the
emulsifier concentration is small.
3.2.2.2. The Effect of Monomer Solubility
The major theories of emulsion polymerization, found to be very
satisfactory when applied to styrene rnonomer, were not suitable when
applied to the data for vinylacetate. This is attributable to the
greater water solubility of vinylacetate, as shown below:-
Solubilization of Monomer in Solution
Vinylacetate
Styrene
2.3
0.03
Solubility in NaSDS @ 300 C,%
4.0
0.12
Solubility in water @ 600 c,%
2.8
0.54
The greater solubility of vinylacetate in water was shown to be
important by the work of Okamura and Motoyama (1962). This work
compared the order of reaction of vinyl hexanoate and styrene. Vinyl
hexanoate has a solubility in water similar to styrene, but a
reactivity of radicals (transfer constant to monomer) similar to that
of vinylacetate. The order of reaction of vinyl hexanoate was shown
to be similar to that of styrene which follows the Smith-Ewart
equation. When the solubility of styrene in the continuous phase was
increased by the addition of alcohol, the order of reaction was
reduced to a value, similar to vinylacetate.
3.2.3. Application of the Theory
In order to obtain the finished polymer latex with the required
properties, the following considerations were taken into account:-
- 64 -
3.2.3.1. Seeded Polymerization
The required particle size (0.2 ~m) was only obtainable using a
seeded reaction. Emulsion polymerization using no emulsifier can
produce particles of the required size, 0.16 - 0.42 ~m, (Priest 1952)
which are stabilized by the surface groups obtained from the initiator.
Any small particles produced are unstable and coalesce with the larger
particles. At the high solids content required in this project,
these latices are unstable. Therefore an emulsifier was necessary.
However, the average particle size produced using SDS is smaller due
to the increased stabilization during polymerization, the average size
being of the order of 0.02 ~m. This has been observed by Napper &
Alexander (1962) and Dunn & Taylor (\<\I>S).
It has been shown by Litt et al (1910) that particles can be
grown to the size required. It was thought that no new particles
were formed, but the range of particle sizes produced indicate that
this was not the case. The monomer added in the seed growth stages
is absorbed into the polymer seeds and polymerizes therein. If excess
surfactant is present further initiation in the water phase can occur.
3.2.3.2. Molecular Weight
High molecular weight polymer formation is favoured by keeping
the locus of polymerization in the organic phase. In homogenous
emulsion polymerization the initial polymerization occurs in the
aqueous phase and it is ·not until new particles are no longer formed
that the particles <;ibsorb monomer and the polymerization moves into
the organic phase. A seeded reaction therefore favours high molecular
weight polymers.
- 65 -
High molecular weights are also favoured by low reaction
temperatures. At lower temperatures, termination is mainly by
combination, rather than disproportionation, which can produce
molecular weights of twice the value. Reactions were tried at various
temperatures, 500 C being the lowest temperature at which the vinyl
acetate used would polymerize. At lower temperatures, polymerization
did not occur indicating that insufficient radicals had been
generated.
3.2.4. Poly(vinylacetate) Latex Characterization
3.2.4.1. Particle Size
This was determined from scanning electron microscopy (SEM)
photographs shown in Figure 3.3. The particle mean diaineter was
0.2/Lm. The particle size distribution of the PVA latex is shown in
Figure 3.4. It is quite wide but still centred near the required size
of 0.2 /Lm. Initiation of new particles in Stage 2 and 3 account for
the smaller particles and subsequent coalescence of unstable small
particles could give rise to the particles of greatest diameter.
In obtaining the SEM photographs the particles have started to
coalesce. This problem has been observed in the literature by
Bradford & Vanderhoff(1959).
3.2.4.2. Surface Charge
An ion exchange resin Dowex (50W-XB) from BDH Chemicals was made
cationic as suggested by Vanderhoff et al (1970). Dialysed latex
(100cm3) of 0.05gdm-3 concentration was added to the cationic exchange
resin (10g). These were shaken together for 12 hours and the resin
- 66 -
-w .,-
• '- .. -_. -
le 32,000
FIGURE 3.3.
S.E.H. Photograph of PVAc Latex
20
~ o
,,10
z
1 6 24 32 /
x 10-8 dn m FIGURE 3.4.
Number fraction, N , of PVAc Particles as a Function of Diameter, d n
allowed to settle. The cation exchanged latex (4Ocm3) was placed in
a Shedlovsky conductance cell at 20oC. This was conductimetrically
titrated against carbonate free sodiwn hydroxide (0.025 mol dm-3).
The graph of conductance versus the volume of added sodium
hydroxide is shown in Figure 3.5. There are two changes in gradient
in the curve. The first, from negative to positive 1s due to the
strong acid, strong base titration of sulphate (S04-) and sulphonate
(SOi) which are conductimetrically indistinguishable. The second
change in gradient, to a slightly steeper positive slope, is due to
the weak aCid, strong base titration of carboxyl ions (C02-).
3.2.4.3. Glass Transition Temperature
The glass transition temperature, Tg, was obtained using a Du
Pont 900 DTA instrument and found to be 31.50 C. This is comparable to
the Tg value of 28-31oC for pure lineal" poly<.vinylacetate) given by
the Encyclopaedia of Polymer Science and Technology (1971).
3.2.4.4. Infrared Spectrum
The infrared spectrum of the polymer was obtained using a Pye
Unicarn infrared spectrophotometer on a thin polymer film cast from an
acetone solution. This is shown in Figure 3.6. A c~parison with the
sample spec"trum for pure poly(vinylacetate) confirms that the sample
W<l,S virtually pure, and negligible amounts of hydrolysis to Poly
(viny lalcohol) had occurred.
3.2.4.5. Molecular Weight
The number average molecular weight, Mn. of the polymer was
- 67 -
150
co 0 ...... )(
0 ~
E
"" u
50
5 15 25
M / mol x 10.6
FIGURE 3.5.
Surface Charge of PVAc Particles - Conductance of Latex,
C ,as a Function of Sodium Hydroxide Addition, M.
r ! ·F
~I: ~-1 ! . l' I 1: I -1i
::~ zi!. I 0.1 ' ~I
~F -1; , .
o_J. ' "l_i:
.I,
:j: '
t'I" o •• .,
"
. 0 i' .q-;-
!
I ...,'
I~ :
i .
,"., -,", , ., . ., .,
,_ ..••• : I
. I.' .. ... ;- ...
" "
I ,g ,1-I
I I?,
.,:. j':~ ....
1 I ,
I
! i ! I I III ! ! I j I i
8 ., 0 7-
,,' ., ..
) ,
, i i 1
,I ; i) i I I .
, , '
obtained fran high pressure membrane osmometry using the Hewlett
Packard 502 HP membrane osmometer. The polymer solutions were made
from degassed, filtered, Analar grade toluene. Mnwas in excess of
the upper limit of the instrument which was quoted as 1 x 106.
The viscosity average molecular weight, My , was measured using a
o Ubbelohde viscaneter thennostated to 25! 0.2 C. The polymer solutions
were made up in dry acetone. My, was calculated from the Staudinger
equation. and was in excess of 2 x 10 6•
A high molecular weight polymer was required for good film
formation. The molecular weights obtained were considered sufficient
for this project.
3.3. PIGMENTS
Pigments were provided courtesy of Tioxide UK Ltd. In total
three pigments were used. The majority of the experiments were
performed on RTC90, a commerCial alumina coated rutile. In order to
understand the role of the coating, experiments were also performed
on the uncoated rutile, (known as RD rutile). used to prepare RTC90
and on an alumina pigment, Hydral. The surface area of each pigment
was determined by Brunauer, EDlnett and Teller (1938) (BET) nitrogen
adsorption taking the molecular area of nitrogen to be 16.2 x 10-20m2 •
Particle diameters were obtained using Transmission electron microscopy
(TEM) and X-ray sedimentation, provided by Tioxide UK. Ltd. The
chemical specifications of the pigments are as specified' by the
manufacturers.
- 68 -
3.3.1. Alumina Coated Rutile RTC90 (Ref.No: TS 38406)
Surface area = 2 -1 15.0 m .g (BET)
Particle diameter dn = 0.17 ~m (TEM)
dw = 0.2 ~m (X-ray)
Chemical composition % w/w
Cao 0.003
2nO <0.002
Fe 0.003
A1 203
3.960
Si02 0.120
P205
0.088
S03 0.080
Zr02 <0.002
Pb 0.002
K20 0.002
Nb205 0.003
Sb203
0.002
Cl 0.011
Organic addition - Trimethylolpropane (TMP)
CH20H I
C2H5 - C - CH20H I
CH20H
The alumina coating is added to the rutile to improve its adsorption
properties and reduce chalking of the final paint film. The alumina is
- 69 -
added as a mixture of sulphuric acid, titanium dioxide and alumina.
This mixture is known as Tital, (see 3.1.8). All other inorganic
substances present in the pigment analysis are generated in the
processing of the rutile pigment and therefore are properties of the
bulk pigment. TMP is added after production to ease grinding of the
pigment and hence is a surface additive along with the al\JllJina. All
other constituents are considered to be part of the inner structure of
the particles. The only compound present to a significant level, except
from the added alumina, is silica, (0.1~). This value has significantly
increased from the silica content of the uncoated pigment (see Section
3.3.2) and therefore must be added when the pigment is coated. Silica
present on the surface of the pigment will significantly affect the
pigment charge. rEl1 photographs of the coated pigment are shown in
Figure 3.7, the alumina coating seen as a grey outer edge around the
denser rutile. The pigment is polydispersed, as seen from the particle
size distribution given in Figure 3.8. The X-ray sedimentation graph is
shown in Figure 3.9.
3.3.2. Uncoated Rutile - RD Rutile (Ref.No: rs 38772)
Surface area = 8.am2g-1
Particle diameter dn = 0.17 m (rrn)
Cao
ZnO
Fe
d = 0.20 m (X-ray) W
0.002
<0.002
0.001
1.330
- 70 -
x 20,000
FIGURE 3.7.
S.E.H. Photograph of RTC90 Pigment
FIGURE 3.8
Number Fraction, N ,of RTC90 particles as a Function of
diameter, dn.
----_. ------ -- ------------ - -- ------ ------ -
cD I 0 ..-x
E
""'-:a
't:I
6-0
• 2-0
0·8
0'4
0·2
5 50
FIGURE 3.9
Weight Percentage ,Md' of RTC90 Particles of Diameter
Greater Than dw - fran X-ray Sedimentation.
9S
Si02 0.013
P205
<0.010
S03 0.021
Zr02 <0.002
Pb <0.001
K20 0.002
Nb205
<0.001
Sb203
<0.002
Cl 0.192
The X-ray sedimentation curve for RD rutile is shown in Figure 3.10.
This pigment is the bare rutile used to prepare RTC 90. Although
the pigment has not been coated, the alumina content is still approx-
mately one third of the coated pigment value. The silica content is
correspondingly lower than the RTC90 pigment. All of these components
will be contained within the bulk of the pigment. The RD rutile was
washed with water before use to remove any chloride present left over
from the production process.
3.3.3. Alumina (Hydral) ®
6 2-1 Surface area = 13. m g
Particle diameter dn = 0.3 ~m
dw = 0.28 ~m
- 71 -
6·0
2·0
CD I
o r-
x
E
,0'8 ;)
"0
0·4
0·2
FIGURE 3.10
S 50
,Weight Percentage, Md' of RD Rutile Particles of Diameter
Greater than d w - from X-ray Sedimentation.
9S
Chemical comRQsition ~ w/w
A1 203
64.1
Si02 0.04
Fe203
0.01
Na20 (total) 0.60 \
Na20 (soluble) 0.22
This pigment was supplied by the Aluminium Company of America. No
indication was given by the manufacturers as to the impurities residing
in the bulk or on the surface of the pigment.
3.4. GENERAL EXlUIPHEliT
3.4.1. Glassware
All glassware used was cleaned by:-
1. Washing with Quadralene (phosphate free) detergent and water,
to clear the surface of all soluble organic material and any
dispersible solids.
2. Immersion in chromic acid, to oxidise residual organic
material and strip off the outer layer of the glass.
3. Washing with distilled and tri-distilled water.
4. Steam cleaning to remove any residual inorganic material.
3.4.2. Centrifuge
For mobility and adsorption studies, pigment was spun down from the
supernatant liquid by use of a MSE 25 centrifuge operating at 75,400g.
- 72 -
The centrifuge tubes used were made of polypropylene and were replaced
when worn too deeply by the pigment to be properly cleaned. Pigment
was removed from the tubes USing a stiff brush and water but no
detergent. They were then steam cleaned.
3.4.3. pH Measurement
pH was measured using a Radiometer PHM52 pH meter with a combined
glass electrode, GK240C, which had an Ag/AgI internal reference. The
meter was calibrated USing three buffers:-
( kg-l) 1. Potassium hydrogen pthalate 0.05 mol
pH 4.008 at 25°C Bates (1973).
2. Mixed phosphate buffer - equimolal solutions (0.25 mol kg-l)
of disodium hydrogen phosphate and potassium dihydrogen phosphate.
° pH 6.865 at 25 C Bates (1973).
3. Buffer tablets supplied by Cambridge Instruments Company Ltd.
pH 9.27 at 15°C.
Buffer tablets were used for the high pH buffer . as sui table
reagents were not available. All buffer solutions were carefully stored
and protected against moisture, carbon dioxide and damage.
3.4.4. Ultrasonic DiSintegration
Aqueous dispersions of the pigments- used were prepared by ul tra
sonic disintegration. The instrtmlent used was' the Rapidis model A180G
of Ultrasonics Ltd. It consists of two units, a generator which can
produce up to 180 watts at 20 KHz and a disintegrator head. This
consists of a magneto strictive transducer coupled to a stainless steel
- 73 -
probe with a titanium tip. This unit was housed 1n a sound insulating
cabinet. A 9 mm probe was used to disperse the pigment.
3.5. . ELECTROKINETIC MEASUREMElITS
As a full explanation of microelectrophoresis theory and apparatus
has been given by Kayem (1978) Nazir (1977) and Dunlop Jones (1982)
and a comprehensive monograph written by Hunter (1981), only a summary
is given below.
3.5.1. Mobility
Mobility measurements are made at the stationary layer of the cell.
For a rectangular cross section cell the stationary layer has been
shown by Komagata (1933) to be located at
y = b(2(O.S+192b/7r S a l/3t2
(3.1)
where 2a is the breadth of the cell
2b is the depth of the cell (see Figure 3.11)
Conventional microelectrophoresis apparatus normally consists of a
silica cell immersed in a water bath and viewed down a microscope. A
voltage is applied across the cell electrodes and with the microscope
focused at the stationary layer the time taken for a particle to cross
a measured distance is obtained. Particles are timed in both directions
by reversing the electrode polarity. A large number of particles need
to be measured in order to obtain a good average particle velocity.
The electrophoretic mobility v~2s-1v-1 at the stationary layer is given
by:-
( 3.2 )
- 74 -
I i
A- 2bx2a
Flat cell
__ r ________ ~b}~ cell constant
r yL ~ 2 b C - - - X - -- / ""ti"""", layem
1-- ____ ~ _a ~ cell cross section
i' 2a .
FIGURE 3.11 Stationary Layer Determination for a Flat Cell
Inner electrodes
HI> anode (+)
Pt cathode (-)
Outer electrodes
FIGURE 3.12 Diagram of LZM cell
where nD = distance travelled
(n is the number of graticule squares
of side D)
t = time
V = voltage applied
L = distance between electrodes
3.5.2. Laser Zee Meter Model 400 (LZM)
The majority of the electrokinetic measurements were made using a
modified Pen Kern Inc. model 400 Laser Zee Meter (LZM). A diagram of
the LZM is shown in Figure 3.12. This equipment used the basis of
microelectrophoresis but the time needed for mobility measurements is
very much reduced. The particles are illuminated by a red helium laser.
A cubic prism, situated between the optics, rotates (the rotation speed
being controlled by the operator) causing a translation of the particle
field image as viewed down the microscope. The rate of rotation is
controlled so that on application of a voltage the movement of the
particles is cancelled out. The particle cloud then appears stationary
except for Brownian motion. The speed of rotation of the prism is then
proportional to the velocity of the particles. An electronic unit
digitally displays the magnitude and Sign of the zeta-potential assuming
the Smoluchowski equation is applicable. Thus one average measurement
can be obtained from all the particles in the field of view rather than
taking the average from numerous particles.
The zeta-potential, as measured by the LZM, is calculated electron
cally and assumes that the voltage drop across the liquid between the
electrodes is· linear. If polarization occurs, this is not the case.
Steps were therefore taken to detect if polarization was occurring to
- 75 -
prevent errors in measurement. The LZM cell was modified to include a
set of platinum blacked stainless steel inner electrodes located
approximately 5cm apart between the main outer electrodes. If polari
zation does not occur the voltage ratio across·the outer, Vo, and inner
Vi' electrodes will be constant. This ratio, Vo/Vi , will be equal to
the ratio of the "electrical" lengths between the outer and inner
electrodes, Lo/Li. This can be checked during LZM measurements.
3.5.2.1. Cleaning of the Cell
Before any measurements could· be made the LZM cell had to be
cleaned and shown to be in good working order.
The perspex cell was cleaned using the soap solution provided by
the manufacturer. The platinum plated cathode needed no cleaning, but
the molybdenum anode.needed the oxide layer periodically removing using
an abrasive powder.
3.5.2.2. Electrode Plating
The inner set of stainless steel electrodes were plated using a
platinizing solution (stannic chloride and lead acetate) and a DC
source using a current of 0.2 mA. Lead, which was simul taneously
plated onto the electrode was removed by soaking in dilute hydrochloric
acid.
3.5.2.3. Cell Parabola
The derivation of equations to obtain mobility values depend upon
the cell walls in the y direction having equal charges and the cell
having a parabolic velocity profile for the return flow symmetrical
- 76 -
about the line x = 0, y = O. For a flat cell a plot of mobility versus
l/b2 should give two straight lines intersecting at x = 0, y = O.
The parabolae obtained for the cell were found to be satisfactory.
Two dispersions were tested, a negative AgI sol, Figure 3.13 and
RTC90, Figure 3.14.
3.5.2.4. Measurement of the Dimensions of the LZM Cell
These were both measured using a travelling microscope.
From these values the axial ratio and hence the pOSition
of the stationary layer could be found.
2. The outer electrodes.
The cell was filled with potassium chloride solution
(0.01 Demal) and the conductance measured at 25°C with
a Wayne Kerr bridge. From this value the observed length,
Lo , between the outer electrodes could be calculated.
3. The inner electrodes.
The circuit in Figure 3.15 was used to obtain the length
LI between the inner electrodes. The Wayne Kerr bridge
could not be used as it would cause distortion of the
conductance lines. From the two measurements Lo and
Lj , the cell length ratio, Lo IL j , of the outer to the
inner electrodes could be calculated.
3.5.2.5. Comparison of Cell Length Ratio to Potential Ratio
The electrical diagram in Figure 3.16 was used to determine the
ratio of potential at the outer electrodes to the potential at the inner
- 77 -
o~-----------------------------------,
-1
CD I
-2 0 ..... )(
... , > ... -3 'Ill
N E
"" ~-4 -5
-6
-7+-------~--------1r------_,._------_;
-1
FIGURE 3.13
o 2 b-2 y.
Parabola of LZM cell using AgI Solution - Mobility,
V E , as a Function of' Height, y, from the Cell Centre
1
2~------------------------------~
1
Cl)
I 0 ~
x ... • 0 > ... 'Ill
N E
"-w
::> -1
-2
-3+--------r-------.--------r-------~ -1 1
FIGURE 3.14
Parabola of LZM cell using RTC90 Dispersion - Mobility.
v E • as a Function of Height. y. from the Cell Centre
Outer electrodes
FIGURE 3.15
I v I r
(
v· - 1-
LZH cell
Inner elec1rodes
Vo
Resistor
Power source
Determination of the Electrical Length, Lj, Between the
Inner Electrodes
Outer electrodes
FIGURE 3.16
r- v i-
I-- Vo -
Power sourc e
LZHcell
Inner electrodes
Determination of the Cell Potential Ratio, V IV .• o I
electrodes. The LZM only gave a stable voltage above 10 - 15 volts.
Below this a stable output was verified with a Solartron digital volt
meter. Thus values of potential across the outer electrodes, Vo , were
obtained for values of potential across the inner electrodes, . Vi. A
plot of Vo /V i versus applied voltage was obtained. From this plot,
Figure 3.11, the ratio increases with applied voltage until it reaches a
constant value of 2.1 in the range of 15 - 300 volts. This compares
favourably with the cell voltage ratio obtained above. For mobility
measurements, voltages of greater than 15 volts were used.
3.5.2.6. Comparison of LZM (timing) to LZM (prism).
In order to check that the speed of the rotating prism was correctly
matched to the speed of the particles, mobilities obtained using the
prism were compared with those obtained from USing LZM as a conventional
microelectrophoresis apparatus. Mobilities of RTC90 in water were
obtained USing the LZM as conventional microelectrophoresis equipment.
Particles were timed over a distance under an applied field, and an
average of the times obtained. The LZM prism was then used to determine
the mobility value and the two results compared.
The following results were obtained:-
Mobility from timing:
-8 2 -1 -1 = 1.009 x 10 m s v
Mobility from LZM:
The difference in the two methods was found on average to be approx-
mately 4%, similar to the value obtained by Dunlop-Jones (1982). The
LZM was therefore working within acceptable limits of accuracy.
- 78 -
3
2
1
10 20 30
Vo / val ts
FIGURE 3.17
The Ratio of Voltages at the Inner and Outer Electrodes,
Vo/Vj' Versus the Applied Voltage at the Outer Electrodes, Vo.
3.5.2.1. Experimental Procedure.
The LZM was rinsed twice with the test sample and filled using
a glass syringe. This was then mounted on the microscope stage and
focused at the stationary layer. A suitable potential was applied
across the electrodes and the prism speed adjusted until the particle
field appeared stationary. The mobility was obtained by dividing the
readout of the LZM by 14.2, the constant used by the manufacturers.
During the reading the potential' across the inner electrodes was
measured to ensure that the ratio of Vo IV i was correct and that the
cell was not polarizing.
3.5.3 Automated Laser Zee Meter' - 53000
As already mentioned in Chapter 1, an automated Laser Zee Meter,
the Pen Kern 53000, was available for a limited period of time, to make
some of the electrophoresis measurements.
The 53000 uses a precision made, cylindrical, silica tube for the
electrophoresis chamber. The tube, of 1mm internal diameter, has a
slightly enlarged portion at either end in which there is a palladium
electrode bonded to the silica surface. This chamber is mounted
liorizontally in a thermostatically controlled water bath. A vertically
mounted 2mW helium-neon laser provides the illumination for the
electrophoresis measurements. The computer system controlling the
equipment calculates the stationary layer of the cylindrical chamber
and positions the laser so that illumination only occurs at that point.
The cell parabola can also be checked by a computer programme.
The image of the illuminated particles is projected onto the
surface of a rotating glass disc containing ,a precision grating. The
movement of the particle image with respect to the grating causes a
- 79 -
fluctuation in the intensity of the transmitted light. This is then
converted into an electrical signal by the detector. The comparison of
the signal with a reference detector determines the electrophoretic
velocity. Many particles are measured simultaneously, the data being
interpreted by a Fast Fourier Transform Analyzer.
The average mobility of each sample is printed out by the computer,
but also the histogram showing the mobility distribution of the sample
can be numerically obtained and shown graphically.
Each electrophoresis sample is automatically fed into the chamber
by a computer controlled pump. Cleaning solutions are also fed into
the chamber in this manner.
Because of the time constraints, the S3000 was only used to
investigate the mobility of RTC90, RD rutile and Hydral pigments in
aqueous solution, and solutions of SHMP, SDS aluminium sulphate, and
RTC90 supernatent. The results are summarised in Section 4.4.
- 80 -
3.6 MJBILITY STUDIES - PI<HmS
3.6.1 Mobility Studies on RD rutile and Hydral
Studies on Hydral and RD rutile required the sample concentration
of piBJllent to be O.05g.kg-1. Experiments were carried out at pH 9 and
5 for dispersant systems and also as a function of pH. All samples for
mobility studies were thermostated at 25 ± O.20 C before measurement.
3.6.1.1 Mobility of RD rutile and Hydral as a Function of pH
PiBJllent (O.05g.kg- 1) was dispersed in water. Portions of the
dispersion were adjusted to various pH levels before measurement.
3.6.1.2 Mobility of RD rutile'in Aluminium Sulphate
RD rutile (O.05g.kg-1) was dispersed in various concentrations of
aluminium sulphate solution. Portions of each concentration were
adjusted to various pH levels, before measurement. If any aluminium
salt precipitated on adjustment of the pH value, this was removed prior
to measurement.
3.6.1.3 Mobility of RD rutile and Hydral in RTC90 SUpernatant
RTC90 supernatant was obtained at various pH levels by
equilibrating the pigment in water at various pH levels. the RTC90.
pigment was removed by centrifugation and a small amount of RD rutile
or Hydral redispersed in the liquid. The samples were thermostated and
the mobility measured.
3,6.1.4 Mobility as a Function of Dispersant Concentration
RD rutile dispersion was mixed with dispersant solution
- 81 -
(0.025g,kg-1
) and water to give a pigment concentration of 0.05g.kg- 1
and a varying dispersant concentration from 10-5 _10-2 mol. g-1. The
pH was adjusted to the required value. After thermostating, the sample
mobility was measured. The dispersants tested were SHMP and SDS.
3.6.1.5 Mobility of RD rutile as a Function of Thickener Solution
RD rutile dispersion was mixed with water and thickener solution
(3% w/w) to give a pigment concentration of 0.05g.kg-1 and a thickener
concentration of 0 - 1. Q'X, solution. The pH was adjusted to the
required value, the sample thermostated,and the mobility measured.
3.6.2 Mobility Studies on RTC90
Systems containing RTC90 required a different approach due to the
coating of alumina on the surface. It was necessary to ensure that
enough pigment was present to enable the solution to become saturated
with aluminium ions without removing a considerable amount of the
coating. Therefore excess pigment (6g/100g of solution) was used in
the sample preparation. After equilibrium had been reached the excess
pigment was centrifuged and a small amount redispersed in the super-
natant liquid to give the correct concentration for mobility
measurement.
3.6.2.1 Mobility as a Function of pH
Equilibrated RTC90 pigment was redispersed in its supernatant
liquid at various pH levels and the mobility measured.
3 .. 6.2.2. One Component Systems
Equilibrated RTC90 pigment was redispersed in the various
- 82 -
concentrations of SDS, SHMP, CMC,or HEC,at pH 5 and 9. The mobility
was measured as a function of the concentration of the component.
3.6.2.3 Two Component Systems
The effect of thickener on the mobility of pigment in the presence
of SHMP was inVestigated. This method was similar to that of Section
3.6.2.2. RTC90 was dispersed in a known concentration of SHMP and the
pH adjusted to the required value. Thickener solution was added and
the pH readjusted if necessary.
For each set of results the SHMP concentration was kept constant
and the thickener concentration varied.
The effect of both HEC and CMC on the mobility of RTC90 in SHMP
solution was investigated.
3.6.2.4 Three Component Systems
This was an extension of Section 3.6.2.3 and investigated the
effect of adding SDS solution, as a third component, to the system.
The procedure was the same as in Section 3.6.2.3 with the SDS added
last.
For each set of results, both SHMP and SDS concentration remained
constant, and the thickener concentration varied.
3.7 K>BILITY S'l1JDIES - POLY(VINYLACETATE)
Mobility studies were carried out on poly(vinylacetate) which had
been exhaustively dialysed through Visking dialysis membrane until the
conductance of the water remained unchanged. All samples used a latex
concentration of 0.05g.kg-1 and were thermostated for 24 hours at
o 25:t 0.2 C before measurement. Mobility was measured as a function of
- 83 -
dispersant or thickener concentration' at pH 9 and 5.
3.7.1 As a Function of pH
Portions of latex dispersion were adjusted to various pH values,
thermostated and the mobility measured. Measurements were also
. obtained as a function of pH in RTC90 supernatant.
3.7.2 One Component Systems
Latex dispersion was mixed with individual solutions of SDS, SHMP,
HEC and CMC. Samples were adjusted to the required pH value, allowed
to equilibrate and,the mobility measured.
Mobility measurements were also obtained for various
concentrations of BCA.
3.7.3 Two Component Systems
In the two component system, the effect of varying the
concentration of a second component (SHMP or thickener) in the presence
of a constant concentration of SDS was examined. Three different
concentrations of SDS were studied. The latex was prestabilized in ,SDS
and the pH adjusted to the required value. The SHMP or thickener was
then added and the pH readjusted if necessary. After equili bra tion ,
the mobility of the latex was measured.
3.7.4 Three Component Systems
The final mobility study on the latex was with all three paint
additives present, SDS, thickener and phosphate. This was an extension
of Section 3.7.3, the effect of SHMP addition .to the two component
mixtures being investigated at various concentrations.
- 84 -
3.8 ADSORPTION ONTO PIQ1Elll'S
Adsorption isotherms of SOS, SIIMP, !lEC and (}lC were obtained on
RD rutile and RTC90. The isotherms were obtained at 25:tO.2° C and pH
values of 5 and 9.
3.8.1 Sample Preparation
6g of pigment was dispersed in water. Dispersant or thickener was
added from a stock solution and the sample made up to 106g with water.
Each sample was adjusted to pH 5 or 9, thermostated in a water bath,
and shaken regularly. The pH was checked at various intervals and
readjusted if necessary. When the pH had remained constant for 12
hours the pigment was centrifuged and the supernatant analysed.
3.8.2 Analysis
3.8.2.1 SDS
SDS was analysed using the methylene blue complexation method in
Section 3.1.9.3, Longwell & Maniece (1955).
3.8.2.2 SHMP
SHMP was analysed using the molybdenum blue complex analysis in
Section 3.1.9.2, Murphy & Riley (1962).
3.8.2.3 Thickeners
!lEC and CMC were analysed gravimetrically, as described in Section
3.1.9.4.
3.8.3 Mixed Adsorption Isotherms
k mixed adsorption isotherm involving all the main additives. of
the paint system was carried out on RTC90. The concentrations used
- 85 -
were an approximation of those used in the paint fonnulation (see
Section 3.10). Slight deviations from those values were also
investigated. Analysis was carried out as described in Section 3.8.2.
3.9 ADSORPTION ONTO POLY(vnm.ACETATE)
This method was similar to that described for adsorption onto
pigments, Section 3.8.1.
3.9.1 Single Component Adsorption Isotherms
30g of latex was mixed in varying individual concentrations of
SDS, SHMP, HEC and CMC solution and the samples adjusted to pH 9 or 5.
After equilibration the latex was centrifuged, trying to avoid too firm
a deposit of solid in the tube. The adsorbents were analysed as in
Section 3.8.2.
Adsorption of SHMP onto PVAC was only possible at low
concentrations due to flocculation of the latex.
3.9.2 Mixed Adsorption Isotherms
These were carried out to investigate the effect of thickener or
SHMP solution on SDS adsorption. The method was similar to Section
3.9.1 but the second component (SHMP or thickener) was added to a
mixture of latex prestabilized in SDS at pH 5 or 9 and the pH
readjusted to the required value if necessary.
, 3.10 SEDIHEm'ATION
Sedimentation experiments were. performed. on . paint. f01"lllY!~tiol)~
using the specifications advised· by Tioxide UK Ltd. These samples
contained polymer, pigment, SDS, SHMP and thickener. Resul ts were
- 86 -
obtained for the corresponding millbases which were analogous to paint
formulations but containing no polymer.
3.10.1 Sedimentation of Paint Formulations
The formulations were blended to the following specifications:-
1. 4~ solids
2. Polymer to pigment ratio of 1.5
3. sos concentration of 0.015~ w/w on polymer
4. SHMP concentration of O.04~ w/w on pigment
5. Thickener concentration of 0 - 1~ w/w on total solids
The pigment was dispersed in the SHMP and thickener solutions to
form the millbase and adjusted to pH 9. The millbase was added to the
SDS stabilized latex, the pH adjusted if necessary and the mixture
stirred for one hour. The sample was then placed in' a lOmm diameter
constant bore sedimentation tube, stoppered, and placed vertically in a
thermostated cabinet at 25° C. The level of the solid component was
monitored over time and marked with a fine felt pen. When sufficient
time had elapsed the height measurements were obtained using a
cathetometer.
3.10.2 Sedimentation of Millbase
Sedimentation experiments were ~arried out on the mill bases
corresponding to the paint formulations in 3.10.1.
The millbases were made up as in 3.10.1 but no latex was added to
the samples.
3.11 FIIJ4 PERFORMANCE
Paint formulations were made up as in 3. 10. 1. Samples were
adjusted to pH 9 or left at their original ... value. Formulations
- 87 -
containing butyl carbitol acetate at 51> w/w on polymer were also
blended. The following experiments were performed on each sample:-
3.11.1 Dry Film Flocculation
The procedure adopted in this section was that developed by
Dr. L.A. Simpson and Mr. D. Rutherford of Tioxide OK PLC (1982). Paint
films were drawn down on mel inex sheets using wire wound applicator
bars to give nominal wet film thicknesses of values between 24 and
60pm. The paint films were allowed to dry and a 36cm2 square cut. out
of each film. The paint square was placed in the infrared beam of a
modified Beckmann spectrophotometer. Backscatter from the film
compared to that of a disc of compressed barium sulphate (reference
giving 100% backscatter) was measured for each film. Fach square was
then weighed, the paint film removed using sol vent and the square
reweighed. From this measurement and the density of the dry paint, the
film thickness could be calculated. A plot of backscatter against film
thickness for each sample results in a line, from which the
flocculation gradient was obtained.
3.11.2 Wet Film Flocculation
Measurements were made using the Beckmann infrared spectrophoto-
meter in Section 3.11.1. The paint formulation sample was put into a
wet cell which enabled a film thickness of 40/Lm to be placed in the
infrared beam. The backscatter from the film compared to the reference
enabled the degree of flocculation of the wet paint films to be
evaluated.
• 3.11.3 Gloss Measurements
Paint films were drawn down onto glass plates using a wire wound
- 88 -
applicator bar. These were allowed to dry. Gloss measurements were
nade at 40 and 60 degrees using Byk Mallinckrodt gloss meters. These
measured the percentage of light reflected from the film at the angles
stated.
3.11.4 Scanning Electron Microscopy(SEM)
SEM pictures of the paint films used in the flocculation gradient
measurements were taken in order to give a Visual appreciation of the
meaning of this unit. The paint film was etched on the surface using
excited oxygen to remove the organic binder. This leaves the pigment
particles exposed and undisturbed. Gold was then deposited on the
etched surface in order to improve the conductivity. Examination of
the surface was carried out using an ISI Super IlIA scanning electron
microscope.
- 89 -
CHAPTER 4
RESULTS AND DISCUSSION
4.1. INTRODUCTION
This Chapter contains a sW!lDary of the resul ts obtained from
experiments described in Chapter 3. It includes electrokinetic and
adsorption data,and sedimentation and flocculation results. Paint film
studies are also reported both in the wet and dry state.
4.2. ELECTROKINETIC AND ADSORPTIVE PROPERTIES OF PIGMFlITS
4.2.1. Pigment Surface
As described in 3.3.1, the major surface components of the coated
pigment, RTC90, are Al203, Si02 ,and traces of the organic milling agent,
TMP. Since the pigment coating is precipitated from a strong acid
solution containing titania, it is probable that there will be residual
titanate ions present on the surface. This could not be confirmed from
the compositional data. These ions present on the surface will have an
influence on both the electrokinetic and adsorptive properties when the
pigment is dispersed in aqueous media.
From the analysis of the uncoated RD rutile, the major surface
properties will originate from the bulk titanium dioxide with some ,. influence from a small percentage of silica and alumina.
A consideration of the formation and solubility of aluminium ions
in aqueous solution was therefore necessary.
4.2.2. Solubility of Aluminium Hydroxide
Aluminium hydroxide is a very slightly soluble compound in water.
The hydrolysis of aluminium species produced in aqueous solution has been
extensively investigated, but only pertinent aspects will be considered
here. For a more detailed review, the reader should refer to papers
- 90 -
such as Johansen (1960), Aveston (1965), Sillen (1959), Matijevic &
Tezak (1953), Matijevic et al (1961, 1966), Parks (1965) and Patterson
& Tyrie (1973). It is generally thought that a large number of
hydrolyzed ions are possible. Among those reported are [A1 2 (OH2 )24+],
[A1 6(OH)153+], [A17
(OH)174+] [A1 S(OH)204+] and [A1 13(OH)345+].
From thermodynamic data, in the literature, solubility diagrams
for aluminium species can be constructed. figure 4. 1. shows such
diagrams obtained by Morgan (1967) using sets of thermodynamic data
from Sillen (1964) and Biedermann (1964). Figure 4.1 (a) represents
the region of stability for freshly precipitated Al(OH)3 using only the
simple monohydroxo species and the cationic dimer. When the solubility
diagram is expanded to include some of the more complex species, the
area of precipitated Al(OH)3 is reduced as in figure 4 .1(b). These
diagrams represent the 'short time' equilibrium behaviour of aluminium
which will be appropriate for processes involving alumium salts or
conditions such as in short-term coagulation experiments. Figure
4.1 (c) represents a similar solubility diagram for gibbsite. Here,
the simple species account for the solubility, the polymeric forms
having negligible influence;; This will represent the ' long time'
equilibrium behaviour. From these diagrams, time, will be very
important· in analyzing the behaviour of aluminium species in aqueous
media. This will be further complicated by the presence of other
compounds such as silica.
The difference between theoretical and experimentally produced
data has been demonstrated by Dezelic et al (1971) and supports
Morgan's emphasis on the limitations of thermodynamic data. Dezelic et
al produced a polynomial from experimental data describing the total
solubility of aluminium. This takes the form:-
- 91 -
I
0 0
-2
-4
~
'::'-6 ... .. <!) -8 0
(a) ...J
-'0
-'2
-.4
0 0
-2
-4
~ - -6
(b) ~
~ .. ~
<!) -8 0 ...J
-'0
-'2
-14
00
-2
. -4
~ -6
(c) ... :i -8
<!)
0 ...J -10
-12
-14
2 4
2 4
6 pH 8 10
FRESHLY PRECIPITATED
pH 6 8 '0
FRESHLY PRECIPITATED
12 ••
12 14
----- [A' 'OHI.'o.'J?
2 4
.4 ''T (OHI"
·0 AI'3 (OH)34
AI (OHt 2
.. AI7 10H)'7
6
GIBBsrTE
AI2 0;,-3 H2 0
10 12 14
FIGURE 11_ 1 Aluminium solubility diagram
'6
16
'6
= -4 + 1.24 x 10 + 3.87 x
+ 2.51 x 1011[H+]3 (4.1)
Using this equation the solubility curve in figure 4.2. was obtained.
for comparison, the plots obtained by Morgan in figure 4.1 are
also shown. Good agreement is obtained in the alkaline region, but
differences in the slope and position of the curve in the acid region
results in a variation of the minimum solubility.
4.2.3. Solubility Diagram for the Alumina Coating of RTC90
An experimental solubility diagram for the alumina coating on
RTC90 was obtained by analysis of the aluminium content of the
supernatant liquid from pigment slurries, equilibrated at various pH
valves. This gave the plot in figure 4.3. The results obtained,
compare most closely to the data in figure 4.1(a), but with the minimum
solubility moved to a directionally higher pH. The immediate conclusion
from this data that the polymerized aluminium species do not exist is
unlikely due to the evidence in the literature. Impurities such as
silica may alter the solubility of aluminium by the formation of
aluminosilicates. This in addition to time dependent processes
therefore, make this data inconclusive.
4.2.4. Desorption of Aluminium from RTC90 Alumina Coating
from 1"'3u.res 4.2 and 4.3, the alumina coating has a minimum
solubility at pH 6.3. As the pH is moved to the values used in this
project, 5 and 9, the potential for greater solubility of the coating
is raised. The solubility of the coating was ~hought to be a time
dependent process. Therefore, the variation of aluminium in solution
of an aqueous Slurry of RTC90 was measured with respect to time. ' The
pH of the slurry was maintained at a constant~valu.e, ,the' experiment
- 92 -
<? 1 \ A I (OH)3 I
E
\ '0
( s ) 0 - \ E
\ " 3 \ ... .
~
\ \
« \ ~ , ,
; , --I' Cl \ 0 -J
5 \ f \ I clear
solutIon . / It-
7
3 7 11
I: pH
- - -- Dehllc
Horga n ( Figure 4.1(a»)
-'-' " ( Figure 4.1(b) )
FIGURE 4.2.
AlumiM.I!\ solubility diagram according to Dezelic (1971)
and Morgan (1967) •
-2
T
.. Cl - 4 ~ I-
g' - 6 ....J
-8
2
FIGURE 4.3.
6 pH
RTC90 Alumina Coating'Solubility Diagram.
10
carried out at pH 5 and 9. The results are shown in Figure 4.4.
4.2.4.1. Desorption at p.H 5
After an initial rise with respect to time, the aluminium content
of the aqueous solution dropped to a constant value. This corresponds
to the [Al]T value for pH 5 shown in Figure 4.3. The excess of
aluminium ions during the early stage of the experiment can be
explained by initial supersaturation of the aqueous medium by aluminium
species followed by one of two processes:-
i) adsorption of polymerized alumimommolecules, formed by
hydrolysis of alumililVmions in solution (described in 4.2.2),·
on to the pigment.
ii) re-precipitation of the excess alum~\~ions onto the pigment
surface.
The ability of hydrolyzed alumirl1~mspecies to adsorb onto the
surface of AgBr and AgI sols has been reported by Matijevic et an 1964).
This results in either coagulation or stabilization of the sol (due to
charge reversal) dependent on the pH and aluminium ion concentration.
Work done by Erikson ,et al (1973) has also demonstrated the
adsorption and desorption of hydrolyzed aluminium ions. They showed
that the rate of desorption is slow compared to adsorption (although
this could have been due to the presence of surface active agents).
4.2.4.2 Desorption at pH 9
At pH 9 a. similar shaped desorption ct:rve was obtained but at a
higher alumi~\umion concentration. However, the only aluminium species
thought to be present in solution at this pH is [Al(OH) 4 -]. This
species would be unlikely to adsorb back onto the pigment. Therefore
supersaturation of the aqueous solution followed by precipitation of
- 93 -
III I
24
o 16
..,X I E
"'C
C7'I
............ 12 u
6
FIGURE 4.4.
pH 9
• • • •
•
5 12 28 40 66 T / hours
Concentration of Aluminium Ions (C) Desorbed From RTC90
Coating as a Function of Time (T).
pH 5
•
aluminium hydroxide was the only explanation. The mechanism for such a
process however, is obscure.
Figure 4.4. indicates that a constant aluminium concentration was
reached after approximately 24 hours. Stirring the solution however,
would affect this time. In practice, a constant pH was used to /"
indicate that equilibrium had been reached.
Cornell et al (1983) have reported dissolution of alumina from the
surface of commercial alumina coated rutile. They suggested that the
aluminium released came from two different surfaces:-
i) A readily soluble surface alumina that dissolved very
quickly during their titrations.
ii) a bulk coating which dissolved more slowly.
The rate of dissolution depended upon the solution pH, the surface
area, and the nature of the production process of the pigment.
Dissolution of alumina has also been studied at various pH values
by Wiese and Healy (1975 d). However, they investigated the variation in
aqueous aluminium content after a colloidal dispersion of alumin".a
had been further diluted. Their experiments were therefore not carried '"
out from zero aqueous aluminium content and any supersaturation of the
solution would not be observed. Similar aluminium ion concentrations
in solution were reported at pH9 as were observed in Section 4.2.3.
4.2.5. Electrokinetic Properties of Pigments in Water
Solid oxides suspended in water are electrically charged due to
one of two mechanisms:-
i) amphoteric dissociation of surface M -OH groups (M = metal), or
ii) adsorption of metal hydroxo complexes originating from
hydrolysis of dissolved material from the oxide.
- 94 -
A review of the surface charge of oxides, including titanium
dioxide and alumina, in aqueous solution is given by Parks (1965) and
Robinson et al (1964). The surface charge is dependent on pH of the
aqueous media and the presence of potential determining ions, p.d.i.,
(hydroxyl and hydrogen ions). These ions can react electrochemically
at the surface. For alumina this can be represented by Figure 4.5.
This diagram is a two dimensional representation of octahedrally
co-ordinated aluminium ions with close packed oxygen ions. Each A1-0
represents a half bond.
For each oxide there exists a bulk concentration of p.d.i. at
which the immersed solid surface has zero net charge. This is known as
the point of zero charge (p.z.c.) and can be determined by a direct
measurement of the surface charge as a function of p.d.i. concentration.
There is another point which mayor may not be coincident with the
p.z.c. This is the point at which the p.d.L concentration has been
adjusted to make the zeta potential zero. This is called the
isoelectric point (Le.p.), and is measured by electrokinetic
experiments. When H+ and OH- are p.d.i:s as in the case of A1203
and Ti02 , pH will have a drainatic effect on the Le.p. In addition
various important changes may occur at this point such as the reversal
of charge which can result in the flocculation of suspensions.
The variation of surface charge, representated in this project by
the mobility was measured with respect to pH for the three pigments
RTC90, RD rutile and Hydral. The mobility curves given in Figure 4.6.
show distinct differences.
Hydral, the alumina pigment has a positive surface charge across
the majority of the pH range with an Le.p. at pH 9.7. Both RD rutile
and RTC90 undergo reversal of charge as the pH is raised with i.e.p.s
of 6.2 and 7.1 respectively.
- 95 -
\1/ \1/ Al Al
\ //\ /H \ /1\ 0 0 1+1 ---'"
0 ~ 0 0 OH
/ \ 1/ \H H,O· / \1/
Al Al
/1\ /1\
H OH-
\1/ Al
\ //\ H20 + 0 0 o I-I
/ \// Al
/1\
FIGURE 4.5.
Electrochemical Reaction at the Surface of Aluminium
Oxide in Acidic and Alkaline Solution.
Cl) I o .... x
i >
i \11
N
e
"-..
3 o
o
1
------------
w -1 ;:,
• Rn rutile
• RTC90
o Hydral
-3
5 7
pH
FIGURE 4.6.
Mobility (Ve) of pigments in aqueous solution as a
function of pH.
o
•
9
The value obtained for RD rutile is consistent with results
obtained from the literature given in Table 4.1. Variation in Le.p.
is generally acknowledged to be due to impurities in the samples.
Major impurities in the RD rutile sample, given by the chemical
composition (Section 3.3.2.), are alumina and, to a lesser extent,
silica. These have opposing affects, the silica tending to raise and
the alumina tending to lower the Le.p. Cornell et al (1975) have
reported an Le.p. value of 6.0 for hydrous rutile that contains no
detectable lmpurities. The effect of chloride and silanol groups on the
properties of the surface of rutile has been investigated by Parfitt et
al (1972). They demonstrated that 'silanol groups formed on rutile by
treatment with silicon tetrachloride and water vapour would only
affect the i.e.p. of rutile when there was a substantial amount present
on the surface. At low coverage the hydroxyl groups on rutile were
present in sufficient quantities to dominate the surface charge.
Surface and bulk chloride ions lowered the i.e.p. of rutile due to the
lower basicity of the chloride/hydroxyl surface compared to hydroxyl
groups alone.
Hydration of rutile pigment is possible on exposure to water, but
there is no thermodynamic data available for the aqueous chemistry of
titanium in equilibrium with rutile. Weise & Healy (1975d), have
reported that no dissolved titanium was detected in a supernatant
solution obtained from dispersing Ti02 in potassium nitrate at pH 9 for
four days. The limit of detection for the analysis method was
approximately -6 -3 10 mol dm •
It was therefore assumed that the concentration of titanium ions
in equilibrum with a rutile sol would be negligible.
Many values for the Le.p. of alumina have been quoted in the
- 96 -
TABLE 4.1.
Isoelectric Points of Titanium Dioxide
Sample
RDrutile
RDrutile
Anatase
Natural Rutile
Natural Rutile
Natural Rutile
Natural Rutile
High Purity Synthetic Rutile
Hydrous Synthetic Rutile
Hydrous Synthetic Rutile
Hydrous Synthetic Rutile
Rutile
Rutile
Rutile
Treatment
a,w,d
ground,WlC
c,W
Method
MEP
MEP
MEP
MEP
SP
c,w,d(120oC) MEP
c,w,d(120oC) SP
c,w,a.
x,w,a.
w,a.
w,a,doig (1000 C)
SP
MEP
MEP
MEP
MEP
MEP
MEP
MEP microelectrophoresis
SP streaming potential
lEP Author
+ 6.2-0.2 This work
5.2 Kayem (1978)
3.8:0.1 Pearson (1973)
4.7:0.2 Feaney (1965)
3.5:0.2 Graham (1962)
4.8
5.5
Johansen(1957(a»
Johansen(1957(b»
6.7:0.1 Purcell (1963)
6.0:0.3 Feemiy (1965)
6.0
4.7
5.9
5.5
6.0
Johansen (1957(a»
Johansen (1957(B»
Wiese (1975)
Furlong (1978)
Cornell (1975)
a aged or stored in distilled water
c cleaned by leaching in concentrated acid
d dried
ig ignited
w washed in water until free of ions, e.g. a-, S04-2-
x sample identified by x-ray diffraction
literature. A summary of these are given in Table ~.2. There is a
wide variation in these values depending on the type of alumina,
£X- A1203
, -y:.. A1203
, £X- AIOOH, alumina hydroxides etc. Heating and
grinding of the sample can also have an effect. Robinson et al (196~)
demonstrated that the p.z.c. of £X- A1203
, having a value of 9.2 when
the surface was fully hydroxylated was reduced to pH 6.7 by calcination
at temperatures above 10000C. Ageing the calcined alumina in water at
room temperature for a period of weeks raised the p.,z.c. by
rehydroxylating the surface. In general the values given in Table 4.2.
indicate that all of the oxides and hydroxides of aluminium slowly
hydrate until' they are covered in a film which has the properties of
Al(OH)3' with an Le.p.in the region of pH 9.2. Hydral would appear
to have a hydroxylated surface, and high purity with little effect
shown by the small percentage of silica present in the composition
(0.04%).
As RTC90 is a rutile pigment coated with alumina, it was expected
that the i.e.p. would be similar to that of Hydra!. However, from
Figure 4.6. the i.e.p. has been moved nearer to that of RD rutile. In
an earlier study, of similar pigments Kayem (1978) it was predicted that
this could be due to:-
i) the surface treatment of the pigment giving an incomplete
alumina coating, or
ii) the concentration of the pigment used in the electrokinetic
experiments not being sufficient to create a saturated
solution of aluminium ions without dissolving the majority
of the alumina coating.
From TEM photographs of the pigment, Figure 3.7, (1) was thought
unlikely as the pigment coating was intact. The problem outlined in
(2) had been overcome by preparing slurries of RTC90 with excess
- 97 -
TABLE 4.2.
lsoelectric Points of Aluminium Oxides and Hydroxides
Sample Treatment Method IEP Authors
-A1 203 MEP 9.9 This work
-A'203 MEP 9.35 Kayem (' 978)
Synthetic x-A12(OH)3 ig(1~ SP 9.1-9.2 Koz' mina (1963) 1350 C)
x,w,si,a
-A1 203 19(~ SP 6.4.-6.7 Robinson (1964)
1400 C) a( 1 day)
-A1 203 ig( l000oC) SP 9.1-9.5 Robinson (1964) a(7days) X,C,W
-A1 203 19(14oooC) SP 7.7 Robinson (1964) a(7days) x,c,w
-A1 203 x,c,w,a (7days) T1 9.1 Yopps (1964)
x,c,w,a SUB 9.0-9.1 (7days)
x,c,w,a MEP 9.1 (7days)
Synthetic ig a MEP 7.5-8.2 Holmes (1965) Sapphire (1day)
Olromatographic A 10.0 Herczynska (1962)
A1 203
0 9 - 10.0
T1 pJtenUonet.r1c tltraticm
SUB 8Ub:sldenee rate or eoagulated su:spensicm
.l adsorpt.ion of indifferent poslt1w: lcm = net;ltbe Ions
o no pH drift
a aged or stored in distilled wa ter
c cleaned by leac.h1ng in concentrated 8(lid
d dried
w washed in water until free or ions e.g. C,- 3:)42-
x sample identified by x-ray diffraction
•
pigment present. This gave a saturated solution of aluminium species
wi thout dissolving away a large percentage of the coating. The
supernatant liquid was centrifuged and a small amount of plgroent
redispersed.
As the result could not be explained, further mobility studies
were carried out.
4.2.6. Mobility of RD Rutile in Aluminium Sulphate Solutions
The variation in mobility with pH for RD rutile in varying
-6 -5 concentrations of a1umint.!'f\.su1phate (0,10 , 10 ,
dm-3) is shown in Figure 4.7. As the concentration of a1uminlunlions in
solution is increased, the i.e.p. of the ruti1e increases until at 10-4
and 10-3 mol dm-3, it reaches the literature values of pH 9.2 (Parks
1965). This is comparable to the experimental value of Hydra1 of pH 9.~
A- similar effect has been demonstrated by Wiese & Healy (1975c) for
Ti02 sols in aluminium nitrate and by Pearson (1973) for anatase in
aluminium sulphate. Weise et al (1973) and Weise & Healy (1975b) have
also described a A1203ITi02 mixed system in coagulation experiments.
As no soluble titanium species is dissolved from the Ti02 , whereas
hydro1yzed aluminium complexes are released from A 12°3
, the system
behaves as A 12°3 after 24 hours equili bra tion. This is due to the
adsorption of aluminium species on the Ti02 particles.
The i.e.p. corresponding to alumina was reached at lower aluminium
ion concentration when using aluminium nitrate than with . the sulphate.
This may be due to the differences observed between the hydrolysis
products of different aluminIum sa.1ts. Over the higher pH range sulphate
ions can act as "penetrators", Stryker & Matijevic (1969). replacing
hydroxyl groups in hydrolyzed alum:il\lllN\ complexes. Mattscn (1930) found
that "sulphated~ aluminium hydroxides are isoelectric at lower pH· values·
- 98 -
than their "chloridated" counterparts. This could account for the
lower Le.p. values obtained for adsorption of aluminium' species from
sulphate solutions compared to nitrate solutions of similar
concentration. Evidence for the formation of these sulphated species
has been demonstrated in coagulation/reversal of charge experiments by
Matijevic et al (1964) and Matijevic & Stryker (1966) from differences
in stabilization boundary diagrams obtained from aluminium sulphate and
nitrate.
The general ability of hydrolyzed alum:iniocn species to reverse the
charge of colloids by adsorption onto the surface is demonstrated by
Figure 4.7. Other observations include the effect of increasing
alumifllum sulphate concentration on the mobility at low pH. As the
concentration is increased at constant pH, the mobility is decreased.
This is due to the increase in ionic strength causing the,
'potential to fall more rapidly from the surface as predicted from a
simple Gouy-Chapman model of the double layer. This is a similar
effect to the reduction in zeta-potential of Ti02 as a function of pH
with increasing concentration of an indifferent electrolyte, KN03
,
Wiese & Healy (1975c). This effect is seen more clearly at higher
values of surface charge but will occur whenever ~o \ O. There is an
indication that this also happens at high pH in Figure 4.7 but it is
not so clearly demonstrated.
Comparison of the 0, 10-6 and 10-5 mol dm-3 aluminium sulphate
plots indicate that some adsorption does take place at the low
concentration of aluminlufI\ ions, but the surface coverage of the adsorbed
layer is not sufficient to emulate an alumina ccating.
This effect of mimicking the surface properties of alumina, by the
adsorbance of aluminium species on the surface of RD rutile does not
explain why the i.e.p. of the alumina coated pigment RTC90 is not of a
similar value. However, industrial coating processes may introduce
- 99 -
other parameters which have a significant effect on the surface
properties. The effect of impurity on electrophoresis experiments can
be very large due to the low surface area of sample under test, over
which the impurity is sprea.d. Although no experimental data was
obtained, silicate or titanate ions were thought to be responsible due
to their higher bulk concentration in the pigment.
4.2.7. Mobility of Hydral and RD Rutile in RTC90 Supernatant Solution
To explain the unpredicted Le.p. of RTC90 pigment, a similar
experiment to Section 4.2.6 was performed. RD rutile and Hydral were
both dispersed in samples of supernatant liquid obtained from
equilibrating RTC90 in water at various pH levels. In this way, any
difference between ionic species from RTC90 coating and an aluminIum salt
would be observed. The resulting mobility versus pH curves are given
in Figure 4.8. The solid curve corresponds to the RTC90 plot given in
Figure 4.7. The majority of the points now lie along this curve. The
Le.p. of Hydral has been reduced, and that of RD rutile raised, to
emulate the plot of RTC90. The curve however does not show the charge
reduction seen in Figure
in Figure 4.8 resembles
sulphate in Figure 4.7.
4.7 due to high ionic concentration. The plot
-6 -3 that of the 10 mol dm curve of aluminIum
This indicates that the surfaces of Hydral and
RD are covered with adsorbed species from RTC90 supernatant liquid.
This must have sufficient coverage to make each surface behave
similarly, and contain species other than aluminIum.
Schwarz et al (1984) have reported that the p.z.c. of mixed silica/
alumina supensions, were a function of the percentage of each component
present i.e. the p.z.c. of a 50/50 mixture would be halfway between
p.z.c. (silica) and p.z.c. (alumina). This was in agreement with a
- 100 -
3
co I 0 ....
)C
I 1 > T
III
'" E
"'-w :::. -1
-3
•
---- --- -----
• Hydral
• RTC90
o RD rutile
5
pH
FIGURE 4.8.
• • •
7
It>bility (VE) of pigments in RTC90 Supernatant
as a Function of pH.
o
9
model presented by Parks (1965) for calculating the p.z.c. for any
composition of a mixed oxide system. It is in contrast to the
difference observed by Parfitt et al (1972) between a mixed oxide and
an oxide modified by adsorption. They noted that sUanol groups on
rutile gave a surface that was similar electrophoretically to rutile
onto which silica had been precipitated. These were both different to
a mixed silica/titania surface. Similar composite plots, confirming
this effect were given by James & Healy (1972). They showed that the
electrophoretic mobility of rutile as a function of pH in 10-4
mol dm- 3
cobalt (Il) salts, behaved as rutile from pH 3 to 5. At pH>6 the
adsorbance of the cobalt reversed the surface charge and at pH>8 the
electrophoretic behaviour was similar to cobalt (11) hydroxide.
The situation perceived in this project for RTC90 and RD rutile is
complex. It is obvious that the pigment coating on RTC90 is not pure
alumina, but probably a mixture of alumina, titania and silica. It
thus behaves as a mixed oxide.
When dispersed in aqueous media, the species dissolved from the
surface are able to transmit their properties onto other pigments .
Because these species, i.e. 'aluminium • silicate, and titanate
ions have opposing effects and are also composi te materials of the
pigments being studied, it is difficult to qualitatively or
quantitatively determine precisely the reactions which are occurring
without further experimentation.
As this project was primarily concerned with a mul ti component
paint system and not just directed towards an oxide - water interface,
no further investigation was pursued.
4.2.8. Adsorption of SHMP on Pigments.... _
As previously stated, the pigment dispersant used in this project
- 101 -
was SHMP. It was therefore necessary to investigate the adsorption of
SHMP on RD rutile and RTC90 at pH 5 and 9. The adsorption isotherms
obtained are shown in Figure 4.9 (pH 5) and 4.10 (pH 9). The following
observations were made from these isotherms:-
i) the alumina coated pigment, RTC90, adsorbs a greater
amount of SHMP (when the adsorption is expressed as
mol m-2) than the corresponding RD rutile,
H) both pigments adsorb a greater amount of SHMP at pH 5
than at pH 9, and
Hi) the isotherms at pH 9 reach a plateau at high equilibrum
concentration of SHMP whereas at pH 5 the amount adsorbed
appears to be increasing across the whole of the
equilibrum concentration range studied.
The units that have been used to express the adsorption isotherms
(mol m-2) depend directly on the surface area of the pigments. These
surface areas were obtained using the B.E.T. gas adsorption technique.
As surface areas from gas adsorption measurements are being used for
measurements of adsorption from solution, practical error cannot be
ruled out. The difference in adsorption observed between RD rutile and
RTC90 therefore, may not be a real effect. Alternative surface area
measurements such as electron microscopy and adsorption from solution
also contain significant potential error. Problems in surface area
determination by electron microscopy are caused by lack of ideality of
the shape and particle size distribution of pigment particles.
Solution methods of obtaining surface area rely on an accurate value of
the area occupied by the adsorbing molecule. This is commonly obtained
by using a standard solid whose surface area has been predetermined
using the B.E.T. method. Thus the determined value has the uncertainty
of both solution and B. E. T. methods. For practical purposes, the
surface area of RD rutile and RTC90, as seen by gaseous and aqueous
- 102 -
CD I
~ 1·5 x
C\I
e
01·0 e
0·5
• RTC90
• RD rutile
0-4 0'8 1·2
( / mol _ kg-I X 10-2
FIGURE 4.9.
Amount of SHMP Adsorbed per unit __ .lI:-~ (A) _~t pHS as a
Function of the total concentration (C).
1'6
0-4 CD • 0 .-
)(
N
• E 0-3
-0 E
" 0-2 ""
0-1
• RTC90
• RD rutile
• • •
0-4 O-B 1-2
C / mol. kg -I
le 10-2
FIGURE 4.10.
Amount of SHMP Adsorbed per unit area (A) at pH9 as a
Function of the total concentration- (C).---- ---------- -
/
• • •
1-6
media were assumed to be identical, or at least any deviation,
comparable for e2ch pigment.
The isotherms at pH 5 and 9 follow a modified LangmuirlStern
adsorption profile, indicating adsorption of phosphate within the Stern
layer. At pH 5 in particular, the isotherm is of the high affinity
class, as described by Giles et al (1960, 1974a, 1974b) where strong
adsorption at very low concentration gives an apparent intercept on the
ordinate axis. The value of kL (the Langmuir ccnstant) is higher at
pH 5 than at pH 9 indicating a larger free energy of adsorption at the
acid pH.
From the basis of Stern theory, as explained by James & Healy
( 1972) , there are three major forces controlling adsorption,
electrostatic (coulombic), chemical and solvation. The overall free
energy of adsorption, AG,:,ds ,of a species ,is equal to the sum of l.
these forces i.e:
AG,:,ds 1
= + (4.2)
A elec where Gi ' AG7hem and AG~olv are the free energies of electro-1 1
static force, specific chemical potential, and solvation respectively.
For adsorption to occur AG,:,ds must be negative. . l. .
As the phosphate ions are relatively large compared with water
molecules, the effect of solvation energy is small and can be
considered negligible compared to the other two energy terms. At pH 5,
both RTC90 and RD rutile have a positively charged surface and A G~lec
will give a negative contribution to AG,:,ds. At pH 9, RD rutile has a 1
negatively charged surface. This will impart an electrostatic barrier
to adsorption and AG~lec will be positive. This is verified by the
lesser amount of SHMP adsorbed at pH 9.
In addition, the relative amount adsorbed at the two pH values,
being of the same order, implies that the electrochemical free energy,
- 103 -
.~
~ GCrem, is of reasonable proportion. This will overcome the positive
value of ~Gelec and allow adsorption to occur.
There is 8 similar reduction in phosphate adsorption for RTC90 1n
raising the pH from 5 to 9. It is probable that a similar effect is
occurring as with RD rutile and that the surface of RTC90 is also
negati ve at this pH. Kayem (1978) investigated the adsorption of SHMP
on alumina pigment at pH 5 and 8. He showed that this pigment had an
Le.p. of 9.35 being positively charged at both pH 5 and 8. The
difference in phosphate adsorption on increaSing the pH between. 5 and 8
rW -6 -2 was small, only lvl" of the amount adsorbed at pH 5 (10 mol m ).
In-Figure 4.9 and 4.10 .the difference in SHMP adsorption on moving
from pH 5 to pH 9 is much more Significant. This, in addition to RTC90
and RD rutile adsorbing similar amounts, is attributable to the
adsorbing molecule and the surface of RTC90 having the same charge at
pH 9, i.e. negative.
Anderson & Malotky (1979) investigated the adsorption of phosphate
on a variety of hydrous oxides ( 'Y - A1 203
, amorphous A1(OH)3 and
anatase) at the Le.p. of the systems. From the results he postulated
that the chemical adsorption potential did not vary with pH.
Differences in ~G~dS at pH 5 and 9 were therefore only attributed to
A elec changes in ~Gi .
The slight difference in adsorption between RTC90 and RD rutile
can be explained by the greater negative contribution to
A elec . . ~ G i due to the greater positlve charge on the surface
~G~dS from
of RTC90 at
pH '5. At pH 9, the lower positive contribution to the free energy (a
lower electrostatic barrier), due to the less negative surface charge
of RTC90 will have a similar effect.
A mechanIsm for the adsorption of phosphate at pH values above the
- 104 -
i.e.p of the solid oxide (including alumina and titania) was postulated
by Morrison (1984) based on work by Hingston et al (1967, 1972). This
involves hydrogen bonding of the residual hydroxy .groups on the
phosphate ions, to the negatively charged surface oxide ions.
The adsorption of ortho-phosphate on a- aluminia has been studied
by Chen et al (1973). They observed a similar trend in phosphate
adsorption with pH. Higher values of the constant related to the heat
of adsorption, (I).) for Langmuir isotherms were obtained as the pH was
reduced. This was accompanied by a decrease in the surface area
occupied by a phosphate ion, resulting from a higher adsorption
concentration. Direct comparison of the amount of phosphate adsorbed
is not possible due to the different form of phosphate studied. Also,
as previously stated, the RTC90 coating may have somewhat different
properties to a pure alumina surface.
Huang (1975) has also reported the adsorption of ortho-phosphate
on 'Y - al umina . He investigated variation of KL' total phosphate
adsorbed, and surface area occupied per phosphate ion with pH. Again,
(although the values are greater for 'Y - alumina) there is a reduction
in KL as pH is raised. which reflects the effect of (oulombic forces.
Muljadi et al (1966) characterised three different adsorption
regions for phosphate adsorption on kaolinite, gibbsite and pseudo-
boehmite. These corresponded to:-
i) phosphate concentration < 10-4 mol dm-3
- a steep part of a Langmuir isotherm, essentially irreversible
ii) 10-4 mol dm-3 < total phosphate < 10-2 mol dm-3
- a less steep portion of a Langmuir isotherm reversible on a
time scale of days
iii) -2 -3 total phosphate > 10 mol dm
- a linear reversible isotherm
- 105 -
The differences in adsorption isotherm was attributed to the
existence of different surface sites. The concentration studied in
Figures 4.9 and 4.10 fall into regions which correspond to (i) and (ii)
and therefore the Lar,gmuir fit would appear to be correct •
. A final consideration is the effect on pH of the solubility of
phosphate in the presence of aluminium. Chen et al (1973) calculated a
solubility diagram for total aluminium and total phosphate, in solution,
in the presence of The model predicts the
formation of variscite (AIP04• 2H20) due to a large negative energy
change at low pH, (approximately pH 4). The Langmuir isotherm was
obeyed even at pH 4, indicating that the formation of a crystalline
phase would be kinetically slow. However, the practical limitation of
the solubility diagram as stated by Chen et al must also be conSidered.
Studies by Van Riemsdijk & Lyklema (1980a, 1980b) have suggested that
at high phosphate concentration and low pH, a phosphate precipitate can
occur onto the initial adsorbate. This is dependent on a long reaction
time, of the order of days. The shorter reaction times used in this
project could account for the non-plateau formation seen in the pH 5
isotherm. The system may not have been completely at equilibrium due
to the beginning of phosphate precipitation. In practice, it was
observed that the samples at pH 5 did take longer to "equilibrate" than
at pH 9.
4.2.9 Mobility of Pigments in SHMP Solution
As SHMP is ads or bed onto the pigment surface, it will have an
effect on the electrokinetic properties. Figures 4. 11 and 4.12 show
the mobility of RD rutile and RTC90 in solutions of varying
concentration of SHMP at pH 9 and 5 respectively. All mobility values
are negative indicating that the i.e.p. of both RD rutile and RTC90 has
- 106 -
co I
0 .-
)(
i >
i III
N
E
"-W
::.
-s
-3
•
-1 • RTC90
• RDrutile
---.~------_r------_,--------r_~~ 10- 2
C / mol.kg-'
FIGURE 4.11
Mobility (VE) of P~ents in Solution at pHS as a
Function of SHMP concentration (C).
00 1
0 ~
x
i >
1 III
C\J
e
"-W ::.
-6
-4
• RTC90
-2 • RD rutile
----,---------~--------_r--------_r----~ o
( / mol. kg-I
FIGURE 4.12.
Mobility (VE) of Pigments in Solution at pH9 as a
Function of SHMP concentration (C).
been lowered below pH 5. This occurs by acquisition of negative charge "
fran adsorption of the negat1ve phosphate °10ns, even at low SHMP
concentration.
The four curves obtained in Figure 4.11 and 4.12 are of similar
-4 Shape with a maximum mobility occurring for both pH values at 5 x 10
mol kg -1 The adsorption isotherms of SHMP on RTC90 and RD ruti le
obtained in Section 4.2.8 indicate that maximum phosphate adsorption
occurs at 4 x 10-3 mol kg-'. This means that the increase in
-4 -1 phosphate adsorption in the reg10n of 5 x 10 mol kg < SHMP
concentration < 4 x 10-3 mol kg -1 does not result in an increase in
mobili ty, for both pigments at both pH values. The value of K from
the expression:-
reduces to:-
K = 12000~ )IF \ foDRT
-1 m
where F =_ Faraday °of charge
R = gas constant
I = ionic strength
for water at 25oC. (4.4)
The value of K increases significantly as the concentration
increases. Taking the pigment particle radius as 1 x 10-7 m the
value of Ka rises rrom 2 to 100
1 x 10-2 mol kg-1.
-5 for SHMP concentrations of 10 to
O'Brien & White (1978) have described a computer programme for the
conversion of mobility measurements to zeta-potential for spherical
colloidal particles. This mcdel is far more flexible than the previous
graphs' proposed by Wiersma et a1 (1966) where convergence at
- 107 -
high zeta ..potential limited the calculations to moderate values of r. There was however no disagreement between results which were obtained
by both authors. The method described by O'Brien & White proposed
that for Ita values greater than 2.75 there is a maximum in the
mobility/zeta-potential plots, Le. there exists a maximum mobility
value dependent on the value of Ita. Detailed calculation using the
0' Brien & White programme was not possible however, since precise
values for the ionic charge and conductivity were not known.
The mobility curves for RTC90 and RD rutile in SHMP solutions
appear to corroborate this theory. Increase in the zeta-potential
-4 -3 as more phosphate is adsorbed above 5 x 10 mol dm does not give
the expected increase in mobility. ~is is because there is
insufficient increase in mobility allowed, due to the increase in Ita,
as the zeta-potential is raised. Figure 4.11 and 4.12 therefore show
a maximum at a lower concentration than where the maximum adsorption
occurs.
This maximum in the mobility fUnction is thought to arise because
of the electrophoretic retarding forces, acting on the particle,
increasing at a faster rate with zeta-potential than the driving force
to motion
i.e. dri ving force = qE Cl( r . r2
retard~ng force Cl( ~
(4.5)
(4.6)
The theories of Booth (1950) and Overbeek (1943) based on a
perturbation expansion in terms of r also indicate a mobility
maximum.
- 108 -
The values of /Ca for the p1gment/SHMP system lie in the region
where Henry (1931) predicts that there is a transition between the
.. applicability of - the Debye Huckel equation IKa - «1) and the
Smoluchowski equation (Ka »1), (Section 2.1.4.). Neither theory, will
therefore apply directly, and the increasing -significance of the
frictional force and the electrophoretic retardation must also be
taken into account. A . satisfactory analysis will require a
simultaneous treatment of the relaxation effects whic!1 are at their
greatest in this region of Ka values. The work by O'Brien & White
greatly simplifies the problem of conversion of mobility values to
zeta-potential when the necessary ionic and conductivity dataare known.
An additional problem is the correct assignment of K for the SHMP
solutions. Insufficient knowledge about the dissociation of SHMP in
water, results in the possibility of SHMP being a 1 :3, 1:4 or 1:5
electrolyte. The value of K is dependent on which. of these relations
is used.
3 -1 Above an SHMP concentration of 4 x 10 mol kg , no further
adsorption takes place on either pi~ent at pH 9 or 5. The reduction
in mobility as the concentration is raised is due to further ionic
effects similar to that of increase in indifferent electrolyte. The
solution viscosity of SHMP relative to water, starts to significantly
increase in this concentration area, and may also reduce the mobility.
POSSible anomolous behaviour of RTC90 at pH 5 may be due to
the high amounts of acid necessary to equilibrate the pigment slurry
to produce electrophoresis samples.
4.2.10 Adsorption of SOS on Pigments
SOS 1s used 1n the paint system as a dispersant for the latex.
- 109 -
It was therefore neessary to investigate the adsorption of SOS on
the pigments at pH 5 and 9. The resulting isotherms are given in
Figures 4.13 and 4.14 Again certain characteristics. can be observed
from the plots:-
i j more SOS is adsorbed on both pigments at pH 5
ii) the adsorption isotherm is S shaped for both pigments
at pH 9. and
iii) RTC90 adsorbs more SDS than RD rutile at both pH levels.
The type of analysis used in Section 4.2.8 for SHMP adsorption
isotherms cannot be directly applied to surfactants. Use of the
Stern/Langmuir equation is restricted to situations where lateral
interactions between adsorbed molecules is inSignificant relative to
the interaction between solute and adsorbent.
S shaped isotherms are typical of the adsorption of surfactants
on mineral oxide surfaces as shown by Wakamatsu & Fuerstenau (1968).
Dick et al (1971) and Rosen & Nakamura (1977). They occur where
lateral interaction forces come into effect. The Stern/Lan~"'u.\"
equation can therefore be considered as:-
~<?ds 1 = + (4.7)
where ~~dS and ~G~lec are as previously described in Section 4.2.8
d A Gs.pec . . . . an ~ 1S a spec1f1C adsorpt1on term which contains all 1
contributions to the adsorption free energy that are dependent on the
non electrical nature of the system.
This can be described as:-
= + + ~cf.s (4.8) 1
where ~ G~c = free energy due to cohesive chain-chain interaction
between the hydrophobic alkyl chains
- 110 -
1·2 • RTC90
• RD rutile
In 1·0
I 0 ..-
X N 0·8 ,
E
~
0 E
0·6 '-.. <{
0'4
0·2
0·4 0·8 1·2 1·6
( / mol. kg-' x 10- 2
FIGURE 4.13.
Amount of SOS Adsorbed per unit area (A) at pH5 as a
Function of the total concentration (C).
0'6
CD , 0 .-
N)(
0·4 I e
0 e
"" ~
0·2
• RTC90
_. HO rutile
0-4 0-8 1· 2
FIGURE 4.14.
Amount of SDS Adsorbed per unit area (A) at pH9 as a
Function of the total concentration (C).
• •
= free energy due to interactions between the
chain and the solid
A~ = free energy due to interaction between
the head group and the solid
AG~c accounts for the lateral interaction between
the SOS molecules
The chain solid interation 1.e. AG~s is not expected to have a
significant contribution to AG~dS. It tends to have a greater
significance with less polar surfaces which do not cause structuring
of interfacial water beyond the first layer of water molecules. It
has greatest Significance at low concentration of solute, which will
cause a high initial slope in the isotherm. This is not observed with
the highly polar oxide surfaces, but occurs, for example, with the
adsorption of alkyl' quaternary ammonium ions on negatively charged
poly(styrene) latex particles investigated by Conner & Ottewill (1971).
A~s, the headgroup-solid interaction is used to cover all other
interactions, i.e. chemical and solvation terms.
Somasun~~a." <& Fuerstenau (1966) have described the S shaped
isotherms as being split into four 'regions , shown in-Figure 4.15.-
Region I occurs at low concentrations of surfactant coverage.
The contribution to AG~pec is low, as can be seen by
literature examples showing that ionic surfactants do not adsorb very
highly on surfaces of like charge. This has been shown by Somasundaran
et al (1971) for the adsorption of a sodium, dodeyclsulphonate on
alumina as a function' of pH. He' showed a decrease in the amount
adsorbed as the pH increased which became extremely low above the
p. z . c . (pH 9. 1). It was also shown that the r - potential curves for
various concentrations of solute coincided at pH values above p.z.c.
- 111 -
c: o ~
0-L. o VI
"'CJ ..:(
---------Region
---------
Region II
erne
(oncen tration
FIGURE 4.15.
Region IV
Adsorption plateau
Regions of Surfactant Adsorption on Mineral Oxide
Surfaces According to Somasundaran & Fuerstenau (1966).
indicating zero adsorption. As the adsorption concentration is low,
lateral interaction between adsorbed molecules will be negligible and
Henry's law will apply.
Region 11 is characterised by a rapid increase in adsorption which
then .decreases in rate in Region Ill. A concept describing this
observation was postulated initially by Gaudin & Fuerstenau (1955) and
later developed by Fuerstenau (1910, 1911), Fuerstenau & Healy (1912)
and Somasundaran & Goddard (1919). This is known as hemi-micell1zation.
As the surfactant adsorption increases, the dominant contribution to
~G~pec is given by ~G~c. Ths is due to removal of hydrophobic chains
from the aqueous media to form two dimensional aggregates on the
particle surface. These are described as 'hemi-micelles'. The driving
force for formation of the hemi-micelle is thought to be similar to
that for the bulk micellar units. Data to defend this theory was
obtained from the adsorption of sodium dodecyl sulphonate on alumina by
Wakamatsu & Fuerstenau (1968). The initial low rise in adsorption of
the solute was accompanied by no increase in r -potential. At the
hemi-micelle concentration the adsorption rate dramatically increased
with a similar rise in r -potential. At the solute concentration
between Region Il and Ill, where the adsorption rate decreased, the
Le.p. was exceeded and the adsorption hindered by the positive
electrical term, forming an electrostatic barrier to adsorption.
Further support for this theory is given by Fuerstenau (1910). He
identified the absence of sharp changes in the J -potential and
adsorption of surfactants, with a chain length of C8 and less. The
formation of hemi-micelles will be heavily dependent on chain length,
becoming less likely as the chain length decreases.
evidence for hemi-micellization is not conclusive.
- 112 -
However, the
In Region IV, at concentrations above the c.m.c, the adsorption
isotherm levels off to reach a plateau. This is another feature of
surfactant adsorption. Adsorption above the c.m.c may.occur in certain
circumstances even though micelles do not adsorb significantly. This
is due to an equilibrum between micelles and adsorbing monaner above
the c.m.c illustrated in Figure 4.16. However, the equilibrum constant
will determine how far above the c.m.c the plateau will be reached.
Bilayer adsorption is also possible when the adsorbent and solute
are of opposite charge. The charged head of the first layer are on
the solid surface, and those of the second layer exposed to solution.
The hydrocarbon tails of both layers form a hydrophobic core between
the heads. The resulting "lamellar micelle" is illustrated in Figure
4.11. This mechanism will not occur when the headgroup and surface
charge are similar, due to the opposite orientation of the first layer.
Application of the theory to Figures 4.13 and 4.14 gives the
following conclusions. The difference in adsorption at pH 5 and 9 is
due to the electrostatic barrier produced at pH 9 due to the positive
contribution to .6.G~ds by .6.G~lec. The large difference between the J. J.
amounts adsorbed at the two pH levels indicates that the chemical
interaction between SOS and the oxide surface is not so great as in the
adsorption of SHMP. This rapid increase in adsorption in the S shaped
isotherms at pH 9 cannot be precisely identified, as hemi-micellization,
but was observed. At pH 5, an S shaped isotherm may have been present
but Region I and II would be masked by the equilibrium concentration
being too high, the adsorption at this pH being very much greater.
Comparison of the adsorption data with mobility data is discussed in
Section 4.2.11.
The area occupied by the headgroup of an SDS molecule in the
- 113 -
<: 0--
il
FIGURE 4.16.
Equilibrium Between Surfactant Micelles and Adsorbed Monomer.
- -/<D;7~~~~~
FIGURE 4.17.
Multilayer Adsorption of Anionic Surfactant.
surface of a micelle is S8A2 , Tartar (1955). Using the plateau
adsorption values for the isotherms, the area occupied by an SDS
molecule at pH. 5 is 35A2 and 22 A2 for RD rutlle and RTC90
This definitely ,indicates at least bilayer, if not multi-layer
adsorption.
The slight differences .between RTC90 and RD rutile adsorption for
SHMP were discussed in Section 4.2.8. A similar adsorption
differential was obtained for SDS, and a similar explanation can only
be accounted for here.
4.2.11 Mobility of Pigments in SOS Solution
The mobility of RTC90 and RD rutile in SOS solution at pH 5 and 9
is shown in Figures 4.18 and 4.19. At pH 5 the mobility of RD rut:He
and RTC90 follow a similar pattern, but RD rutile has a greate..., negative
charge. This does not correspond to the adsorption isothenns, where
RTC90 adsorbed a greater amount of SOS per unit surface -area, than
RD rutile. It would, therefore, be expected to have a higher charge.
This anomalous behaviour of RTC90 at pH 5 could be explained by the
method of preparation of the electrophoresis samples. Large amounts
of acid were required to equilibrate the slurries at pH 5, before a
small amount of pigment was redispersed in the supernatant. The excess
concentration of acid in the electrophoresis samples may therefore give
erroneous values due to the high ionic concentration.
At this acidic pH, where both pigments are positively charged in
the abs'ence of SDS, the 1.e.p. of the system is reached at a very low
-5 -1 concentration, less than 10 mol kg • This indicates a large
- 114 -
-5 • RTC90
Cl) • RD rutile , 0 .... x
'i >
I
\11 -3 C\I
E
'" w ;:,
-1
c / mol. kg-'
FIGURE 4.18.
H::>bility (vEl of Pigments in Solution at pHS as a
FUnction of SOS concentration (C).
Cl)
I
0 .-)(
, >
T \11
N
E
'" w :::.
-5 • RTC90
• RD rutile
-3
-1
-----~------~------~--------T_--~
( / mol. kg-'
FIGURE 4.19.
Mobility (VE) of Pigments in Solution at pH9 as a
Function of SDS concentration (C).
10-2
negative free adsorption energy,
contributions from d Ge1ec and i
A~ds L.1 i ' resulting from negative
The shape of the mobility
curves are similar to those produced by Fuerstenau· (1971) for the
adsorption of sodium dodecy1 sulphonate on a1um1na. At a concentration -4 -1 .
of 5 x 10 mol kg , the mobility rises very rapidly to a plateau
value at 5 x 10-3 mol kg-1. This corresponds to the region in which
the adsorption isotherms reach' the plateau described as Region HI.
Above 5 x 10-3 mol kg-1, the mobility decreases for both pigments. As
indicated in Section 4.2.10 this type of behaviour does not
conclusively prove the hemi-micellization theory.
As SOS is a 1: 1 electrolyte the values of K for SOS are lower than
for SHMP.
This results in lower values for Ka rising fran 1 to 50 for
SOS concentrations of 10-5 to 1 x 10-2 mol kg-1. The restriction on
maximum mobility postulated by O'Brien & White (1978) and discussed in
Section 4.1.9 will therefore possibly not affect the pigJDent colloids
in SOS solution as much as their SHMP counterparts. Fran Figure 4.18
the mobility maximum occurs at an SOS concentration of 5 x 10-3 mol
kg-1 at pH 5 for bothRTC90 and RD rutile. Increasing the
concentration further resulted in a decrease in mobility indicating a
possible restriction as predicted by O'Brien & White.
At pH 9 the mobility of RTC90 and RD rutile in SOS solution follow
a similar path. As both pigJDents are negatively charged at zero SOS
concentration, no charge reversal occurs, and an i.e.p is not observed.
The mobility at low SDS concentration reuains constant due to simple
/ exchange between OS- and OH- ions on the pigment surface. At 5 x 10-4
-1 mol kg concentration, the mobility rises (at a lower rate than at
pH 5), to reach a maximum at 7 x 10-3 mol kg -1 • This corresponds to
the plateau in the adsorption isothenn. This behaViour is similar to
- 115 -
the concept of hemi-micellization.
Fran a practical point of view, pH only has an effect on the
-4 -1 _ pigl!lent mobility at low concentrations of SOS below 5 x 10 mol kg ,
where adsorption is comparatively low. However, in the paint mixture
SOS is only added to the system after the pigl!lent has been stabilized
in SHMP solution. The realistic si tuation is therefore more
canplicated and required further investigation. This is discussed in
Section 4.2.16.
4.2.12 Adsorption of Thickeners on Pigments
In order to canplete the investigation of the adsorption of the
individual soluble paint system canponents onto the pigments, the
adsorption of thickener onto RTC90 and RD rutile was carried out. Two
thickeners were used, sodium carboxymethyl cellulose (CMC) and
hydroxyethyl cellulose (HEC). These have been previously described
in Section 3.1.5. Again the adsorption isotherms were studied at
pH 5 and 9.
Complications involved in the investigation of adsorption of
thickeners result fran insufficient knowledge of the mean molecular
weight and distribution of the thickeners. Other errors are involved
in the method used to obtain the amount adsorbed. Gravimetric analysis
at these addition levels incurs greater error than colorimetric
analysis. Also, due to the viscosity of the dispersions, increased
centrifugation was necessary to make sure that all pigment particles
were removed fran the supernatant liquid.
A complete discussion of the adsorption of polymers has been given
by the reviews of Ash (1973), Miller & Bach ( 1973) , Vincent &
Whi ttington (1981) and Fleer & Lyklema. ( 1981 ). Trends arising fran
- 116 -
adsorption data in the literature can be summarised as fo11ows:-
i) A critical adsorptive free energy per segment is
required for adsorption to occur. Due to the many
segments present on the adsorbent, once the critical
value is exceeded the adsorbed amount will increase
rapidly,
ii) The entropy loss per molecule on adsorption is greater
for polymers than for small molecules. However, the
decrease in energy on adsorption is correspondingly
higher due to the many possible attachments per chain,
iii) The adsorption isotherms are generally of the high
affinity type due to the strong adsorption
characteristics. This is generally predicted by
modern theories such as Hoeve (1966, 1970, 1971),
Silberberg (1968), Roe (1974) and Scheutjens & Fleer
(1979, 1980, 1982).
iv) adsorption is greater from bad than from good solvents
v) adsorption increases with molecular weight in a poor
sol vent. Good sol vents do not show this effect.
vi) the effect of temperature variation on adsorption is
small.
vii) polymer desorption by dilution is difficult, but exchange
with other polymers or low molecular weight solutes is
possible. This is partly due to the high affinity
character of the adsorption.
viii) polymer adsorption is slower than with lower molecular
weight substances especially if the molecular weight
distribution is wide.
When macromolecules are adsorbed, three types of segment sequences
- 117 -
are possible, trains, loops and tails. These are illustrated in
Figure ~.20. This description was proposed by Jenkel & Rumbach (1951).
The proportion of tails, loops and trains is dependent on the solvent,
molecular weight, free adsorption per segment and concentration. The
amount of each can be predicted from adsorption theory.
~.2.12.1 Adsorption of HEC
The adsorption isotherms of HEC are given in Figure ~ . 21 (pH 5 )
and ~. 22 (pH 9). Both plots indicate the thickener adsorptions are .. of
the high affinity type. Distinct plateau regions do not occur in any
of the isotherms.
. The shape of the isotherms can be affected by incomplete
equilibration times. At high concentrations of adsorbate, the
adsorption becomes more sensitive to time. This is due to the lower
coefficient of diffusion of· the higher molecular weight fractions.
Variation of isotherm shape with time has been reported by Koopal
(1981), Lankveld & Lyklema (1972) and Grant et al (1979). This could
be one reason for the less than "ideal" shape of the isotherms obtained.
The effect of time on the adsorption of the thickeners was not possible
due to time constraints.
Polydispersity can also effect the shape of the adsorption
isotherm. Cohen Stuart et al (1980) have investigated this effect.
Adsorption is enhanced by increasing chain length, especially in a poor
solvent. There is evidence in the literature for this, both
theoretical, Roe (1980), Scheutjens and Fleer (1982) and experimental,
Felter & Ray (1970), Felter et al (1969), Howard & Woods (1972) and
Sadakne & White (1973). Where adsorbed polymer is in equilibrium with
polymer in solution, the long chains will displace shorter ones on the
surface. As the equilibrium polymer concentration increases, there
- 118 -
\
loop
\
tail
FIGURE 4.20.
Model of Adsorbed Polymer Chains with Loops, Trains and Tails.
C')
I
o
)(
C\I I
E
4
Cl 2 .......... ~
• RTC90
• RO
o·s 1·0 1·5
c / % solution
FIGURE 4.21.
Amount of !lEC Adsorbed per Unit Area (A) at pHS as a
Function of the Total Concentration (C).
•
M I o
N I
x
E
4
Cl 2 "<
• RTC 90
• R 0
0'5 1'0 1 ·5
( / % solution
FIGUR 4.22.
Amount of HEC Adsorbed per Unit Area (A) at pH9 as a
Function of the Total Concentration (C).
exists a greater number of long chains which can preferentially adsorb.
This will increase the adsorbed amount and produce a rounded isotherm.
The degree of polydispersity of the thickeners used in this project was
unknown, but it is unlikely that this will not have an affect on
practical experimentat,ion. The adsorbed amounts were expressed as
-2 g.m as the precise molecular weights were unknown.
A theoretical description of the adsorption of uncharged polymers
has been described by Hoeve (1966, 1970, 1971). The free energy of
adsorption was considered to contain the following contributions:-
1. The configurational change from a random coil in
solution into a sequence of trains on the surface,
plus the interaction of the trains with the surface.
2. The interaction of the adsorbed trains with each other
and with the solvent within the adsorbed layer.
3. The interactions between the dangling loops and tails,
and the solvent.
It is therefore not expected that there would be a pH effect on
HEC adsorbance. From Figures 4.1.21 and 4.1.22 there is more HEC
adsorbed at pH 5 than pH 9. As there is no electrostatic contributions
to the free energy, only non-ionic contributions should be contained in
the adsorption energy. Also greater adsorption occurs with RD rutile
than with RTC90.
4.2.12.2 Adsorption of CMC
The train loop and tail model of polymer adsorption given in Figure
4.20 is similar for a polyelectrolyte. For CMC the negative charges
will be incorporated into the polymer chain, the positive sodium ions
occupying the surrounding aqueous solution. Hesselink (1977) has
extended the work of Hoeve (1971) to the adsorption of
- 119 -
pclyelectrolytes, which he described as resembling the adsorption of
non-ionic macromolecules -at high ionic strength, or at low charge
density of the polyelectrolyte. The free energy of adsorption of the
ionic polymer molecules will contain the contributions which apply to
non-ionic adsorbents given in Section 4.2. 12. 1.
also be the following electrical contributions:-
However, there will
4. The change in free energy of the electrical double
layer,on adsorption of charged trains on a charged
interface with charged loops dangling into the solution.
5. The adsorption energy of uncharged segments on a charged
interface or of a charged segment on an uncharged surface.
This is due to dipole and polarizability effects.
The dependence of the adsorbence of CMC on pH can be seen in
Figures 4.23 (pH 5) and 4.24 (pH 9). More CMC is adsorbed at the
acidic pH. At this pH the surfaces of both RTC90 and RD rutile are
positive. As CMC is an anionic polymer, this term will favour
adsorption. On increasing the pH to 9 the surface of the pigment
becomes negative for RD rutile and either negative or at least less
positive in the case of RTC90. This will give a positive contribution
to the free adsorption energy, 1. e. an electrostatic barrier.
Therefore, there will be a less negative free energy of adsorption and
less CMC will be absorbed. The fact that adsorption still occurs at
pH 9 indicates that the non-ionic contributions are large enough to
overcome the electrostatic barrier and still favour adsorption.
CMC does not undergo any viscosity change at alkaline pH. At
acidic pH it is possible to form the free acid of CMC which is
insoluble in water and can separate as a gelatinous precipitate. Any
such formation would be centrifuged out of the supernatant liquid. In
addition the intra- and inter-molecular interactions of the highly
- 120 .,.
4 • RH 90 • • RD
.., • I 0 ..-)( •
N • I E
Cl 2 "-« •
0-5 1-0 1 ·5
(/ % so I uti 0 n
FIGUPl-: 4.23.
Amount of CMC Adsorbed per Unit Area (A) at pHS as a
Funetion of the Total Concentration (C).
C') I
o
C\J I
)(
E
4
C'I 2
"" «
• RTC90
• RO
• 0·5 1· 0 1 ·5
( / % solution
FIGURE 4.24.
Amount of (;MC Adsorbed per Unit Area (A) at pH9 as a
Function of the Total Concentration (C).
.'
polar carboxymethyl group are of a greater order than the less polar
hydroxy groups present in HEC. There is therefore a higher likelihood
of the formation of water insoluble salts in CMC solutions either
immediately or over a long period of time.
In very general . terms, for CMC, monovalent cations form water
soluble salts, divalent cations form partly soluble salts and trivalent
cations form water insoluble salts. This however depends on:-
i) The grade type of CMC
ii) The solution concentration
iii) The salt concentration
iv) Temperature
Salt formation will be more pertinent in the case of RTC90 where
there is the possibility of aluminium salts of CMC being formed. This
will depend on the relative kinetics of the adsorption of CMC and the
solubility of the aluminitiin coating. At pH 5 however, more CMC has
adsorbed onto RO rutile than RTC90 and it is unlikely that extensive
salt formation has occurred. The greater adsorbance onto RO rutile
than RTC90 is the reverse situation than was seen for the adsorption of
SHMP or SOS. Joppien & Hamimn (1977) have reported stronger infrared
interactions for the adsorption of polyester onto titanium dioxide
than onto silica or alumiria surfaces, which supports this result.
From the isotherms it would appear that there is a similar amount
of CMC adsorbed on HEC. However, the molecular weight of HEC was
approximately three times that of CMC resulting in a higher molar
adsorption of the ionic thickener .
4.2.13 Mobility of Pigments in Thickener Solution
4.2.13.1 Mobility in HEC Solution
The mobility curves for RTC90 and RO rutile are given in Figure
1":>1 _
4.25 (pH 5) and 4.26 (pH 9). The mobility characteristics of both
pigments are similar at acidic and alkaline pH. The mobility reduces
to zero as the polymer concentration is increased.
When non-ionic polymers are adsorbed on a substrate, they can
alter the observed mobility or zeta-potential in the following ways:-
1. By altering the adsorption characteristics of the ions
in solution, both inert electrolyte and potential
determining ions.
2. By moving the plane of shear away from the particle surface ...
3. By the influence of the excluded volume of the polymer on
the free energy of the double -layer ions.
Competition between adsorbing polymer and potential determining
ions will only have an effect at higher potymer concentration. It is
likely that this effect will be masked by the effect described in 2.
At low concentrations the polymer will be adsorbed mainly as trains,
the long chains lying close to the surface, as in Figure 4.27 (a) .
As the polymer concentration increases, competition for the pigment
surface causes a pronounced formation of loops and tails which forces
the shear plane out from the surface to a distance which increases with
the concentration Figure 4.27 (b) and (c). Fleer et al (1972)
-8 obtained distances of the shear plane from the surface up to 10 m for
the adsorption of poly(vinylalcohol) (PVA) on 'silver iodide. Koopal &
Lyklema (1979), studying the same system, investigated mobility as a
function of pAg at various concentrations of polymer. As the polymer
concentration was initially raised the iep of the system increased
wi thout affecting the maximum mobility. At higher concentrations
however, the maximum was significantly reduced. They also correlated
an increase in the fractional coverage of the surface with a shift in
the shear plane, which was independent of molecular weight.
- 122 -
-5
CD I '0 ..-
)(
., >
I -2 <11 N
E
'" w ::>
+ 1
-
0-2
FIGURE 4_25.
-
In (H(
• RT (90
o RO
- - - - - -
In HE (
• RTC90
o RO
0-4 0-6
( / % soluti on
-
Mobility (UE) of Pigments in Solution at pH5 as a
Function of CMC Concentration (C).
•
0-6
Cl) I 0 .-
x ., >
I III
'" E
""-
-4
In C HC
• R T( 90
o RD
In HEC
0 RTC 90
• RD W·
;:,
o
0·2 0'4 0·6
C / % solution
FIGURE 4.26.
Mobility (Ve) of Pigments in Solution at pH9 as a
Function of CHC Concentration (C).
/
• •
0-8
o (a) Low Polymer Concentration
(b) Increasing Concentration (c) High Concentration
FIGURE 4.21.
Variation in Conformation of Polymer Chains on a
Particle Surface with Increase in Polymer Concentration.
•
The effect of the phenomenon described in 3. above was believed
to increase the zeta-potential with polymer concentration due to the
change in the ion distribution in the double layer. The extension of
the double layer is increased which increases the surface potential of
the particle. This, model was put forward by Brooks (1973) who
investigated polymer adsorption on red blood cells, polystyrene and
silica surfaces.
The reduction in mobility, as seen by the adsorption of HEC on
RTC90 and RD rutile, is almost certainly dcminated by the shift~ng
position of the shear plane described in 2. Any change in the iep of
the systems was not investigated but it is possible that the adsorbed
HEC will affect both the adsorption of ions as in 1 . and the
distribution of ions in the double layer as in 3.
4.2.13.2 Mobility in CMC Solution
The effect of adsorbed polymer described in Section 4.2.13.1 will
also apply to the adsorption of CMC. However, as CMC is a polyelectro
lyte, the electrical interactions must be taken into account. The
mobility curves for RTC90 and RD rutile in CMC solution are shown in
Figures 4. 25 (pH 5) and 4. 26 (pH 9). Again, as in Section 4.2. 1 3. 1 ,
the variation in mobility is similar for both pigments and pH value.
Jones (1979) has predicted the mobility characteristics of
negatively charged particles coated with negatively charged poly
electrolyte. He identified two effects governing the mobility. These
were, the electrical effect and the hydrodynamic resistance of the
polyelectrolyte to the electrolyte flowing through it. He observed
that for low surface concentration of adsorbent, the electrical effect
was the dominant feature. As this concentration increased the
hydrodynamic effect took over.
- 123 -
At pH 5, charge reversal of both pigments occurred at very low
concentration of CMC confirming the high affinity adsorption
characteristics of the thickener. As the CMC concentration is
increased there is very little charge in the mobility. For RD rutile
there is some evidence of a mobility maximum. At higher CMC
concentration the mobility reduces slightly to a plateau as predicted
by Jones. This occurs as the adsorption isotherm reaches the critical
value where competition for the surface transforms the orientation of,
adsorbing polyelectrolyte from predominantly trains to predominantly
loops. Jones observed that at this value the zeta-potential was
independent of the fraction of segments in contact with the particle.
Also as the asymptotic value of r is reached, it becomes independent
of the surface charge density of the particle. At this point, the
particles can be thought to be assuming the colloidal properties of
the adsorbent, in this case CMC. As CMC has a negative charge, the
resulting mobility at high concentration is negative. In the case of
HEC there is no resulting surface charge as the polymer is non-ionic.
4.2.14 Adsorption onto Pigments in Mixed Solutions
In paint formulation preparation, the pigment is initially
dispersed in SHMP solution. This is then mixed with thickener solution.
The latex, prestabilized in SDS solution is then combined with the
pigment dispersion.
It was of interest therefore to investigate any competition for
adsorption onto the surface of the pigment by the individual components.
However, in order to do this there would have to be no interference by
any of the compounds on the analysis methods,' It was found that SHMP
could not be analysed in the presence of HEC or CMC. Therefore as the
thickeners were analysed by gravimetric analysis, mixed ,adsorption
- 124 -
isotherms were not possible. Any information about the species present
at the surface and the competition between them was therefore only
possible from mobility data. This was conducted on the coated pigment,
RTC90, used in paint formulations.
4.2.15 Mobility of RTC90 with Respect to Thickener Concentration at Constant Concentrations of SHMP
In order to emulate the colloidal characteristics of pifgllent in
paint formulations, the samples used in mobility experiments were
prepared in a similar fashion, using a similar order of addition.
RTC90 was dispersed in various concentrations of SHMP solution. These
were allowed to equilibrate at pH 5 or 9. The thickener solution was
then added and the pH readjusted if necessary. The electrophoresis
samples were then prepared as usual by centrifugation, followed by
redispersion of a small amount of the solid in the supernatant.
4.2.15.1 HEC
Figures 4.28 and 4.29 show the change in mobility with HEC
concentration in SHMP solution at pH 5 and 9 respectively. Mobility
curves are given for SHMP concentrations ranging from 0 to 10-2
-1 mol. kg
At pH 5 it can be seen that up to an SHMP concentration of
-4 -1 10 mol.kg the mobility curves resemble that of RTCgO in HEC solution
alone. SHMP has no effect on the mobility. The SHMP ions are not
adsorbed to a sufficient extent to reverse the surface charge of RTC90.
Above 1O-4mOl.kg- 1 SHMP, the dispersant is presen~ in sufficient
quantity to successfully adsorb on the surface and purvey a negative
charge onto the particles. At higher HEC concentrations, the
- 125 -
Cl) -2 1 o
)(
i >
I III
C\I
E
." W
=>0
+2
.
-I SHMP 0 o mol. k 9
+ 10- 5 •
0 10.4 •
• 10- 3 •
• 10-2 •
0·2 0·4 0·6 o·e ( / % solution
FIGURE 4.28.
Iobbility (UE) of RTC90 in Solution at pHS as a
Function of HEC Concentration (C) after Prestabilization
in SHMP.
• • • •
-4
co • 0 ....
x
j > - 2 j VI
N
E
'" w :::.
o
+ 0 mol. k g-' SHMP
0 10- 5 .1 • 0 10- 4
" •
• 10-3 " "
• 10-2 " u
0·2 0'4 0·6 0·8
( / % solution
FIGURE 4.29.
Mobility (UE) of RTC90 in Solution at pH9 as a Function of
HEC Concentration (C) after Prestabilization in SHMP.
reduction in mobility indicates that HEC is being adsorbed and the
double layer is extended.· Again this results in a mobility plateau.
An increase in SHMP to 10-2mol kg- 1 does not prevent a reduction
in mobility· with an increase in HEC concentration. There must
therefore exist, competition for the surface between thickener and
SHMP. Because the SHMP is added before the HEC it is probable that
the polymer chains can exchange with the phosphate. The extent to
which this occurs will be dependent on the equilibrium concentration
of SHMP and HEC.
At pH 9, a similar effect is observed. However, a slightly lower
-4 -1) concentration of SHMP is required (10 mol kg to make a
significant negative contribution to the surface of the pigment. This
can be due to either the lower mobility of RTC90 in SHMP at pH 5 than
at pH 9, or due to greater competition for the surface between the two
adsorbents in the acidic environment.
4.2.15.2 CMC
Wi th CMC thickener, it is not so easy to correlate competi ti ve
adsorption with mobility as both CMC and SHMP will confer a negative
charge onto the particles. However, direct comparison can be made
between the mobility curves obtained at various SHMP concentrations.
These are given in Figures 4.30 (pH 5) and 4.31 (pH 9).
Again at pH 5 it is not until the SHMP concentration rises to
1O-3mol kg- 1, that there is a significant change in the mobility
characteristics of the particles. At this concentration there is a
sufficient adsorption of SHMP to increase the surface charge. However
as the CMC is increased, more of the thickener is adsorbed, to
- 126 -
Cl)
1 o .-)(
1
-4
> -3 T
\11
C\J
E
-2
-I -o 0 mol. k 9 SHMP
o 10-4 • •
• 10-3 • •
• •
o
0-2 0-4 0·6 0·8
( / % solution
FIGURE 4.30.
Mobility (vEl oLRTC90 in Solution at pHS as a Function of
CMC Concentration (Cl after Prestabilization in SIIMP.
-4
•
-I
0 mol. k 9 SHHP 10-4
• 10-3
• 10- 2
•
0·6 0·8
( / % solution
FIGURE 4.31.
Mobility (VE) of RTC90 in Solutions at pH9 as a Function of
CMC Concentration (C) after Prestabilization in SIIMP.
• • ..
sufficiently extend the dcuble layer and reduce the mobility.
-2 -1 At a concentration of 10 mol.kg SHMP, the mobility curve is
reduced to a lower value than RTC90 in CMC alone. . Ionic effects,
which are not present in the case of HEC are probably responsible for
this result.
At pH 9, similar characteristics can be observed, but at higher
overall mobility values. Again, as with HEC, a lower concentration of
-4 -1 SHMP (10 mol.kg ) is required at alkaline pH to Significantly change
the mobility from the base curve (0 mol.kg-l SHMP). However, the
-4 -1 curve at 10 mol.kg SHMP undergoes greater mobility reduction with
increase of CMC concentration than the curve at 10-3mol.kg -1 SHMP.
The greater amount of SHMP in solution forces greater competition,for
the surface, with CMC chains. As the SHMP is raised further to
-2 -1 10 mol.kg the mobility is again suppressed as at pH 5 due to ionic
effects.
4.2.16 Mobility of RTC90 in Solutions Containing SHMP, and SOS.with Respect to Thickener Concentration
As seen in Section 4.2·.15, the mobility of RTC90 in solution is
influenced by both thickener and SHMP concentration. In the finished
paint formulations, SOS is added as the latex dispersant. As there
will be an equilibrium between the SOS adsorbed on the latex particles
and the aqueous media, any effect of the solubilized surfactant on the
mobility of the pigment was needed to be investigated. Therefore,
samples were prepared as in Section 4.2.15, SOS solution was then
added to give a final SOS concentration of 1O-3mol.kg-1·. Results were
obtained for SHMP concentrations of 0, 10-4 and 10-3mol.kg-1.
No significant difference in mobility was obtained on addition of
-3 -1 4 10 mol.kg SOS, to' the solutions originally dispersed in 10- or
- 127 -
10-3mol.kg-1 SHMP solution. This occurred for both thickeners at both
pH values.
When no SHMP was present, there was a directional rise in
mobility as shown in Figures 4.32 (pH 5) and 4.33 (pH 9). The rise
however was not comparable to the effect observed for phosphate and
thickener dispersions in Section 4.2.15.
From electrophoresis data, therefore, it can be concluded that
the major effect on the mobility of the pigment particles will rise
from the SHMP and thickener concentration. In the final paint
formulation, the equilibrium concentration of SDS is unlikely to reach
-4 -1 10 mol.kg and will therefore have little effect on the pigment
particles.
4.3. ELECTROKINETIC AND ADSORPTIVE PROPERTIES OF POLY(VINYLACETATE) LATEX
As previously indicated electrokinetic and adsorptive stUdies in
mixed colloids was not practically possible. The mobility and
adsorptive experiments performed on RTC90 and RD rutile, described in
Section 4.2 were therefore also performed on the PVAc latex. Sample
preparation for these tests is detailed in Sections 3.7 and 3.9.
4.3.1. Mobility of PVAc Latex in Water
The mobility of PVAc latex in water in the range pH 3-11 is given
in Figure 4.34. All the mobility measurements are negative, and no
. iep is observed. This· is a result of the sulphate, sulphonate and
carboxyl groups on the latex particle surface. These groups were
investigated in the conductimetric titration in Section 3.2.4.2.
The negative mobility increases from pH 3 to 5. At pH 5 a
plateau value is reached which does not change across the alkaline pH
- 128 -
-5
co ~ 1
0 ..--x
T >-2 I
III C\J
E
"-w
:::.
+1
~
CMC o = 0 mol.
-3 • = 10 •
. -. kg SOS
• •
---- -------------
0·2
FIGURE 4.32.
HEC
o = 0 mol • = 10- 3
•
0·4
-I
kg SOS • •
0·6
C / % so lution
o·e
Mobility (VE) of RTC90 in Solution at pH5 as a FUnction of
Thickener Concentration (C) after Prestabilization in SDS.
-4
co 1
0 or-
)(
T> -2 T
III N E
"-w
:::.
0
C HC -I
o 0 mo I . k g SOS • = 10-3 • •
•
HEC -I
Cl = o mol kg SOS
• = 10-3 • •
0·2 0·4 0'6 0'8
C / % solution
FIGURE 4.33.
Mobility (Ve) of RTC90 in Solution at pH9 as a Function of
Thickener Concentration (C) after Prestabilization in SDS.
-5 Cl) I 0 .-x
i >
'Ill
(\J
-3 E
"-w
:::.
-1
3 ·5 7 9
pH
FIGURE 4.34.
Mobility (VE) of PVAc Latex in Solution as a Function of pH.
range investigated. The negative mobility above pH 5 is due to the
total negative charges of the three types of surface groups. However,
the pKa of carboxyl ions occurs between pH 4 and 6. ~erefore, as the
aqueous media becomes more acidic below pH 5, the carboxyl groups
become protonated and ,do not contribute to the negative charge.
The surface charge of PVAc latex was therefore identical at pH 5
and 9. It was predicted that similar results would be obtained in the
electrophoresis and adsorption experiments carried out at pH 5 and 9.
This was found to be the case and in the following results, values
obtained at pH 5 and 9 are plotted on the same curve.
4.3.2. Adsorption of SDS on PVAc Latex
The adsorption isotherm for SDS on PVAc latex was obtained in a
similar method to RTC90 and RD rutile. The resulting curve for
adsorption at pH 5 and 9 are given in Figure 4.35. No significant
difference was observed between the two sets of values.
As the latex is negatively charged, there will be an electro-
static barrier to adsorption of the dodecylsulphate anion. The
adsorption is analogous to ·adsorption of SDS onto pigment at pH 9.
The physical adsorption of the 0$- anion is sufficient to overcome the
electrostatic barrier and results in a negative free energy of -
adsorption. The driving force of physical adsorption is the reduction
in contact area between the hydrophobic surfactant chain and the water
molecules. Hydrocarbon, hydrocarbon interaction between the chains
and the latex surface will also favour adsorption and will orientate
the molecules with the sulphate groups in the aqueous phase. This
driving force results in an adsorption isotherm,again of the Langmuir
type,where adsorption occurs at low concentration of SDS.
The maximum amount of surfactant adsorbed on the polymer will be
- 129 -
~ 8 o
le
I
c:n
-o E 4
" « • pH 9
• pH 5
eme
0·5 1 ·0 1· 5
( / mol. kg -I
x10- 2
FIGURE 4.35.
Amount of SDS Adsorbed per Unit Area (A) as a Function
of the Total Concentration (C).
dependent on many factors. Pirma & Chen (1980) have shown that for
the adsorption of alkylsulphonates on polymer the limiting adsorption
was increased with the following:-
i) A decrease in temperature - this is due to thermal forces
offsetting the hydrophobic interations. Coulombic
interactions are affected very little by temperature changes.
ii) An increase in the alkyl chain length - the free energy
change of hydrophobic adsorption and orientation becomes
more negative as the chain length increases.
iii) An increase in the electrolyte concentration - electrolyte
will reduce the hydration of the surfactant and the
electrostatic repulsion between the polar heads. Therefore
more surfactant molecules can be adsorbed and packed closer
with less energy. For similar reasons the cmc of the
surfactant will decrease as the electrolyte concentration
is raised. Similar observations have been made by Paxton
(1969) for the adsorption of sodium dodecylbenzensulphonate
(SDBS) on polystyrene and by Connor & Ottewill (1971) for
the adsorption of cationic ammonium soap on polystyrene.
iv) A decrease in the polarity of the polymer - as the polarity
of the polymer increases, the affinity between the surfactant
and the particle surface becomes less than the affinity
between the surfactant molecules themselves. This trend has
also been observed by Paxton (1969) and Sutterlin et al
(1976).
The amount of SDS adsorbed at the isotherm plateau in Figure
4.31 will represent monolayer coverage with the polar portion pointing
into the aqueous solution. The area occupied by one molecule was
calculated as being 0.55 nm2 This is in comparison to the molecular
- 130 -
areas for the following:-
i) 0.585 ru/ SDS in a micelle, Tartar (1955)
E) 0.585 nm2 SDS on polystyrene latex, Brodnyan &
Kelly (1969)
iii) 0.47 nm2 SDS on polystyrene latex Piirma &
Chen (1980)
iv) 1.51 nm2 SDS on poly(methylmethacrylate) latex
Piirma & Chen (1980)
In Figure 4.35 the adsorption plateau was reached just before the
equilibrium concentration of SDS reached the cmc. A similar effect
has been seen for SDS adsorption on polystyrene, by Saunders (1968)
and by Brodnyan & Kelly (1969) for the adsorption on poly(n-butyl
methacrylate).
As with pigment adsorption isotherms, adsorption of the cationic
species will raise the surface charge density of the latex. Therefore
the electrostatic barrier to adsorption will increase with SDS
adsorption. This could be the reason for the reduction in adsorption
before the cmc was reached. The Langmuir assumption, that the
activation energy for adsorption on the surface is independent of the
fractional coverage, will therefore be violated, and the curve cannot
be analysed to fit the Langmuir equation.
4.3.3. Mobility of PVAc in SDS Solution
As SDS is significantly adsorbed onto the PVAc latex it will have
an effect on the particle mobility. The mobility of PVAc as a
function of SDS concentration is given in Figure 4.36. As the SDS
equilibrium concentration is raised, the mobility rises from the value
obtained in aqueous solution until at 10-3mol.kg -1 a plateau value is
reached. Further concentration of SDS in the supernatant slightly
reduces the mobility. Similar trends have also been observed by
- 131 -
co • 0 ~
x
"'j >
i \11
C\J
E
"-w :::.
-6
latex solubil ization
-4
-2
----,-------~--------~------~--~ o
C / mol. kg-'
FIGURE 4.36.
Mobility (UE) of PVAc Latex in Solution as a Function
of SOS Concentration (C).
Siegleff and Mazur (1962) for polystyrene in SDS solution.
The increase in mobility is due to the adsorption of OS- ions
described in Section 4.3.2. The adsorption of an~onic molecules
increases the surface charge of the latex particles. The maximum
mobility occurs at a' lower concentration (10-3mol.kg -1) , than the
value at which the adsorption plateau is reached (6 x 10-3mol.kg-1).
This is a similar si tuation to that ot-served in Section 4.2. 1 1 for
the mobility of RTC90 and RD rutile in SDS solution. The mobility
restriction observed by O'Brien & White (1978) may account for this
phenomenon.
As the concentration of SOS was raised to above 7.5 x 1O-3mol.
kg-1, the latex particle concentration appeared to be reduced without
-2 -1 any flocculation occurring. At an SOS ccncentration of 10 mol.kg ,
the electrophoresis sample became transparent. This process has been
described as "latex solubilization" and only occurs above the cmc of
surfactants. This effect has been investigated by authors such as
Issacs & Edelhauser (1966) for PVAc stabilized by SOBS, Isemura &
Kimura (1955) and Isemura & Imanishi (1958) for poly(vinylformal),
poly(vinylbutyral) • and PVAc in SOS, and Sata & Saito (1952) and
Jaycock & Kayem (1982) for PVAc in SOS. As the amount of adsorbed
SOS increased, the water molecules around the latex particles are
gradually pushed away from the surface. This results in the polymer
particle residing in a hydrocarbon core, Le. in the form of a
micelle. As the organic tails of the SOS are compatible with PVAc,
the polymer is effectively dissolved in a similar manner to polymer
dissolution in an organic solvent. The polymer is thus held in the
micelle. On solubilization, PVAc will behave as a polyelectrolyte
and can be adsorbed onto undissolved particles. Polymer bridging can
thus occur, resulting in flocculation. Free SOS concentrations
- 132 -
approaching the cmc will therefore need to be avoided in paint
formulations.
4.3.4. Mobility of PVAc in Butylcarbitol Acetate (BCA)
, BCA is a coalescing agent, commonly used in paint manufacture.
Because of the number of additives used in this study, it was not
possible to investigate the interaction of BCA with the other
constituents. However, as BCA was used in the paint formulations in
Section 4.5.1 the mobility of PVAc with BCA in solution was
investigated. The mobility of PVAc dispersion in varying
concentrations of BCA is given in Figure 4.37. From this curve,
there is no significant effect on mobility by the addition of BCA.
Coalescing agents act by cutting down the interparticle forces
of the latex particles when the polymer spheres move together on
evaporation of the solvent. This results in better film formation
and improved gloss properties of the dry paint film.
4.2.5 Mobility of PVAc in RTC90 Supernatant Liquid
As polymer and pigment would be ultimately mixed in aqueous
solution, any effect of the desorbing ions from the pigment coating
on the polymer stability was investigated. PVAc was dispersed in
RTC90 supernatant obtained at various pH levels. The mobility values
obtained for the samples above pH 5 were unchanged from the values
obtained in aqueous solution in Section 4.2.1. At pH 5 and below,
the ions present in the supernatant liquid caused the polymer
particles to flocculate. This is due to the electrostatic attraction
between the negatively charged latex and the positively charged
aluminium ions. It is possible that the positive ions can be
- 133-
co I 0 -5 --x
I
> ,. 1/1
N E • • "" -3 • w
;::,
-1
----r---"'T""----.---~--....,...--~ o
( / % solution
FIGURE 4.37.
Mobility (VE) of PVAc Latex in Solution as a Function
of BCA Concentration (C).
adsorbed into the inner-double layer of the latex-solution interface.
This will reduce the negative charge on the latex surface, reduce the
mobility, and hence the stability of the PVAc particles.
However, this experiment was carried out at zero SOS
concentration. In the final paint formulation the latex will have
added stabilization from adsorbed SOS. Also, it is unlikely, due to
the adsorbed phosphate ions, that sufficient aluminium ions will be
present in the final paint formulation at the concentration in the·
electrophoresis samples. Paint formulations are normally blended at
alkaline pH where the hydrolyzed ions do not occur.
4.3.6 Adsorption of SHMP on PVAc
As SOS (latex dispersant) was found to adsorb onto RTC90 and RO
rutile, the possibility of SHMP (pigment dispersant) adsorbing onto
the latex was investigated. It was found that adsorption above a
-4 -1 SHMP concentration of 5 x 10 mol kg was not possible due to
flocculation of the latex. Below this concentration, no adsorption
of SHMP onto the PVAc latex was observed.
Adsorption of the highly negatively charged polyphosphate ions
onto the negatively charged latex will not be electrostatically
favourable. . Any possible adsorption would require sufficient
physical interaction between the adsorbant and' substrate to overcome
the electrostatic barrier and promote adsorption. Such physical
interactions would not be expected between the highly polar phosphate
ions and the relatively less polar hydrocarbons contained within the
latex. However, it is electrostatically favourable for adsorption of
sodium ions into the inner regions of the double layer of the latex-
solution interface. The effect of this was investigated by
electrophoresis of the PVAc in SHMP solution.
- 134 -
4.3.7 Mobility of PVAc Latex in SHMP Solutions
The mobility of PVAc latex in aqueous solution with respect to
SHMP ccncentration is given in Figure 4.38. As the SHMP concentration
is increased from
obtained in aqueous
zero, the mobility is reduced from the value
-4 . -1 solution until at approximately 5 x 10 mol.kg
the electrophoresis samples begin to flocculate. This cannot be due
to ionic effects alone because of the low concentrations and therefore
relatively low K values involved. Adsorption of the phosphate ions
was not detected in Section 4.3.6, but sodium ion adsorption into the
double layer would result in a reduction of the observed mobility of
the latex as seen in Figure 4.38.
4.3.8 Adsorption of Thickeners onto PVAc
The adsorption of HEC and CMC onto PVAc was investigated in a
Similar:. way to the adsorption onto pigment in Section 4.2.12. The
isotherms obtained are given in Figure 4.39.
It was found that both HEC and CMC are not adsorbed on PVAc to a
similar extent as was obtained with RTC90 and RD rutile. However, a
reduction in the equilibrium thickener concentration was obtained in
the adsorption samples indicating that some adsorption had taken place.
PVAc latex particles are negatively charged. Adsorption of non-ionic
HEC will be due to physical interaction only. Similarly, adsorption
of CMC will be due to physical interactions but as CMC is an anionic
thickener, any adsorptive free energy will need to overcome the
electrostatic ba.rrier occurring between the two negative entities.
This is an electrically analogous situation to the adsorption onto
pi~nt at pH 9.
As the adsorbed amounts are Significantly lower in this case, the
physical interactions between cellulosic polymers and PVAc will be
- 135 -
co I 0 -4 e-
x , >
'\11 (\J
E
" -2 w
:::.
o
o
FIGURE 4.38.
( / % solution
latex floccula t ion
Mobility (UE) of PVAc Latex in Solution as a Function
of SHMP Concentration (C).
15
• (M(
• HEC
v • • I '
010 • • • ..-)(
'" • I • • E
• Cl
""-~ 5
0'4 0-8 1 -2
( / % solution
, FIGURE 4.39.
Amount of Thickener Adsorbed (A) per Unit Area as a
Function of the Total Concentration (C).
considerably lower than with metallic oxides.
However, certain aspects of the experimental procedure must be
taken into account. Centrifugation was needed to obtain the clear
supernatant for thickener content analysis. The PVAc latex coalesced
into·a solid portion which effectively decreases the surface area and
could produce a desorption of adsorbent. It is also possible that
some thickener, loosely associated with the latex could be removed by
centrifugation. These considerations cast serious doubts upon the
validity of the adsorption isotherms. One other aspect of the
adsorption experiment was that all SDS (from PVAc preparation) must be
removed from the latex by dialysis. Any residual surfactant present
on the surface would cause a significant reduction in adsorption. The
effect on the adsorption isotherm by adsorbed emulsifier has been
reported by Saunders & Sanders (1956) and Saunders (1968) for the
adsorption of methylcellulose onto polystyrene latex and by Brodnyan &
Kelly (1969) for the adsorption of HEC onto poly(n-butylmethacrylate).
The latter authors also showed that the adsorption of HEC depended on
the type of polymer adsorbate.
One further observation during this experiment was that the
adsorption samples containing greater than a 0.6% solution of CMC the
caused the viscosity of~mixture to increase dramatically and rendered
them useless. The solid resembled a flocculated latex. This may have
been caused by the presence of sodium ions adsorbing into the double
layer as with SHMP. The effective reducti ve in surface charge will
result in the removal of the ele.ctrical stability of the particles and
hence flocculation.
There is evidence in the literature Sperry et al (1981) for the
flocculation of latex due to volume restriction by water soluble
polymers. They described flocculation of polystyrene and polyacrylate
- 136 -
latex by HEC under conditions where no thickener had been adsorbed
onto the latices.
4.3.9 Mobility of PVAc in Thickener Solution
-The mobility of PVAc in HEC and CMC solution is given in Figure
4.40. As with SHMP, the mobility is reduced for both thickeners as
the concentration increases. The rate of decrease, however, is less.
Because of restrictions on the electrophoresis of viscous liquids,
the thickener concentration was not raised above 0.8% solution. The
mobility reduction is greater for (}IIC than for HEC solutions. As
shown in Section 4.2.8 very little adsorption of HEC or CMC onto the
PVAc latex occurred. It is therefore surprising that the mobility
was significantly reduced. The introduction of sodium ions into the
system in the solutions containing CMC may account for some of the
difference observed between the ionic and non-ionic thickener.
Adsorption of the thickener onto the PVAc surface could also account
for the drop in mobility, analogous to the adsorption onto RTC90 and
RD rutile (Section 4.2.12) but some discrepancy between the
adsorption isotherms and the electrophoresis results must be accepted.
4.3.10 Adsorption onto PVAc in Mixed Solutions
As discussed in Section 4.2.14, for adsorption onto pigments in
mixed solutions, the analyses becomes more difficult as a greater
number of species are introduced into the aqueous solution. For the
adsorption onto PVAc of one component the following conclusions have
been drawn:-
i) SDS adsorbs onto PVAc latex
ii) SHMP does not adsorb onto PVAc latex
iii) HEC and (MC do not adsorb onto PVAc latex as strongly
- 137 -
• HEC
-5 • (MC
<0 I
0 or-
)C
I >
I -3 VI
N
e
" w :::.
-1
0·2 0·4 0·6 o·e
c / % soluti on
FIGURE 4.40.
Mobility (VE) of PVAc Latex in Solution as a Function
of Thickener Concentration (C).
1· 0
as the adsorption onto pigments.
In a paint formulation, the PVAc latex is predispersed in SOS
and added to the pigment/SHMP/thickener system. Evidence in the
literature suggests that when a polymer has adsorbed surfactant on the
surface, a reduction in thickener adsorption is observed, Section
4.3.8. Although the addition of SHMP and thickener may slightly
alter the adsorption characteristics of SOS it is unlikely that the
SOS is removed from.the surface. Further investigation of the mixed
systems containing PVAc comprised of mobility experiments.
4.3.11 Mobility of PVAc with Respect to SHMP Concentration at Constant Concentrations of SOS
Figure 4.41 shows the mobility of PVAc as a function of SHMP
concentration after prestabilization in a variety of SOS
concentrations. This was an attempt to investigate the effect of
pigment dispersant concentration, SHMP, present in the final paint
formulation. All the mobility curves follow the general trend
observed in Section 4.3.7 (i.e. Figure 4.41, zero SOS concentration).
As the SHMP concentration is increased, the mobility is decreased.
However, the effect of pre-dispersion .in SOS is to reduce the
flocculating effect of the SHMP, by raiSing the surface charge of the
particles. The orientation of the adsorbed 05- ions with the ionic
head pointing out into the aqueous region will serve as an electro-
static barrier to flocculation. Thus the latex will be able to
tolerate a greater concentration of SHMP in solution when SOS is
adsorbed on its surface.
4.3.12 Mobility of PVAc With Respect to Thickener Concentration at Constant Concentration of SOS
A similar investigation to Section 4.3.11 was also carried
- 138 -
co I
0 .-x
'j >
i III
C\I
's
'" w ;;;,
-6
• -4
-2
o
( /
FIGURE 4.41.
0
0
• •
o mol, kg-I SOS
10-5 • •
•
•
latex flocculated
-.
•
•
mol., kg
Mobility (11:) of PVAc Latex in Solution as a Function
of SHMP Concentration (C) after Prestabilization in SDS.
out to investigate the effect of the added thickeoer in solution on
the electrophoretic stability of the PVAc particles with adsorbed
SOS. The results are given in Figure 4.42 (HEC) and 4,.43 ( Q1C) . It
was shown in Section 4.3.11 that the mobility of PVAc was gradually
reduced as the concentration of thickener in solution was raised.
Again this trend was observed after the latex had been prestabilized
in SOS, but again, the presence of the detergent formed a barrier to
electrophoretic instability, by its adsorption on the latex surface.
The mechanism for mobility reduction in thickener solution is not
fully comprehended, but sufficient SOS must be present to preserve
the latex stability during the paint formulation when high thickener
concentration occurs.
4.4 MOBll.ITY DISTRIBIITION IN ELECTROPHOR&SIS SAMPLES
As described in Section 3.5.3. a S3000 Laser Zee Meter was
available for a limited period of time for electrophoresis
measurements. The S3000 is a computer controlled automated
development from the conventional model 400 Laser Zee Meter. It has
the advantage that the precise mobility of an extremely high number
of particles can be electronically measured and the mean value
obtained .. Also, the distribution of mobilities in a sample can be
obtained in histogram form, very quickly. Thus the effect of the
solution components on the deviation about the mean mobility can be
investigated. Due to the limited availability of the S3000 the
investigation was concentrated on pi81llent mobilities and the effect
of SHMP, SOS,and soluble aluminium species.
The stability of a colloid, evaluated by the stability ratio,
W, is affected by both particle size and Stern potential. The
- 139 -
-I 0 0 mol. -k 9
-6 0 10-5 •
• 10-4 • co • 10- 3 1 •
0 .... IC
T >
1 VI -4 N E
" w :::.
-2
0·2 0·6 0·8 1·0
c / % solu tion
FIGURE 4.42.
Mobility (Ve) of PVAc Latex in Solution as a Function
of HEC Concentration (C) after Prestabilization in SDS.
SOS
• • •
mol -I
0 = 0 kg SOS
• = 10-5 01 • -6 10-4 0 = • 11
Cl)
0 • = 10 -3 • 11
0 .- -5 x
T >
i -4 VI
C\I
E
'\ -3 w
;:,
-2
-1
0·2 0'4 0'6 0'8
c / % sol ution
FIGURE 4.43.
Mobility (VE) of SOS Prestabilized PVAc as a Functi9n of
CMC Concentration (C).
1·0
average stability ratio of a homogenous colloid will therefore be
different than for a heterogenous system with the same mean size or
mobili ty value. Cooper (1912) has demonstrated that the non
uniformity in either particle size or Stern potential reduces the
average stability ratio. Obtaining homogenous systems particularly
in industrial processes is extremely difficult due to large batch
production and possible contamination by impurities. The
distribution in particle size for PVAc latex and RTC90 has been shown
in Section 3.2.4.1 and 3.3.1 to be relatively wide. Model latices
such as those produced by Ottewill & Shaw (1972) also have
significant standard particle size deViations, typically 5 - 15% of
the mean.
Studies of the electrophoretic mobility of polymer latices by
Uzgiris & Winterton (1969) and Schmut (1965) have shown a typical
standard deviation of 10 - 2~~ of the mean mobility. This implies a
distribution in the Stern potential.
The effect of the distribution in surface properties has been
studied by Prieve & Mitchell (1982). Stability ratios of so Is having
a distribution -of either -particle size or Stern potential were
computed as a function of ionic strength (for both constant charge
and constant potential surfaces). At low ionic strengths, a
distribution in the Stern potential had much more of an effect in
reducing the stability ratio of the system than a comparable
distribution in the particle size. The sensitivity cif the sol to
salt concentration was also enhanced to a greater extent by a
distribution in charge (or potential) than in particle size. These
effects were increased with the size of the particles.
Mobility values of electrophoresis samples obtained wi th the
S3000, gave essentially similar trends and results to those contained
- 140 -
in Section 4.2. A mobility distribution histogram was obtained from
every electrophoretic measurement. Due to the numbers obtained, only
limited examples of the histograms will be given in this Section.
A complete set of histograms is given on microfiche in Appendix I.
All .histograms show mobility, UE' on the abscisscae and number of
counted particles, N, on the ordinate axes.
Any change in particle size distribution in the various solution
components was not investigated. Changes in particle size
distribution will have an effect on the mobility distribution.
Changes in particle sizes would ultimately need to be taken into
account for a complete analysiS.
4.4.1 Histograms of Pigments in Aqueous Solution
Figures 4.44 and 4.45 show examples of the mobility distribution
of RTC90 and RD rutile in aqueous solution at various pH values. At
acidic pH, the distribution of RTC90 appear directionally narrower
than RD rutile when comparing similar mobility values. This does not
occur at higher pH. This could be due to the adsorption of positive
hydrolyzed aluminium ions back onto the RTC90 surface. This would
gi ve a more homogenous surface layer which would impart a lower
distribution of potentials, and hence mobilities. RTC90 is
manufactured from RD rutile and would therefore tend to have a
similar particle size distribution assuming that the coating is
evenly distributed over the individual particles. Further
investigation of the effect of coating the pigment particles was
obtained from the series of histograms of RD rutile in:-
a) Aqueous solution (Figure 4.45)
b) 10-6 mol. kg -1 aluminium sulphate solution
c) 10-5 mol.kg-1 aluminium sulphate solution
- 141 -
•• o~· ______________________________________ __
I 72 .s .s ~
"" ~ 3> U :!>< 11 .2
. .~
;~1 / l 2 2 .40 .~ '" "- "- "-• • •
.. ~ e~ ) "'~ ~ '8
e • • •
:>2 40 ... ".; >6 2. ;>2 .., -~ . -'6 -06 o -,.. -.., -'" .,.:c VI.' ..... 10 / . . -.
VI.".10 / '" -. V,,/.· .... ,. ... ,0·· V, / .......... ,0·'
pH 4-8 pH 6-5 pH 7-6 pH 7-9
FIGURE 4.44.
Mobility Distribution of RTC90 in Aqueous Solution.
- Number of Particles (N) as a Function of Mobility (VE).
,J •• ., , . ••
'" 30 .2 i 1 UI .2
"
10
: )°1 r ~~ ~
• 30
"-• •
'8 l .8
-. - ., "- J
~u I~
•
"j , , < ~ e e
2-0 2 )6 -0"" .0-. .. ,'2 12'0 -'-e -1-0 - 0-2 +06 -) -2:8 _t~2
/ '" -. VI •• .,. '0 V, • / .... ," • 10 -. / '" -. 'VI ...,. 10 v., / ............ .
pH 4-3 pH 6-0 pH a-o pH 9-6
FIGURE 4.45.
Mobility Distribution or RD rutile in Aqueous Solution.
- Number of Particles (N) as a Function of Mobility (VE).
"
d) 10-4 mo1.kg-1 aluminium sulphate solution
e) RTC90 supernatant solution
__ .Examples of.-.(~) to·· (e) are· given in Figures· 4.46 - 4.49.
Addition of 10-6 mo1.kg-1 aluminium sulphate solution to RD rutile
dispersion tended to widen the mobility distribution. From' Figures
. -6 -1 4.1 and 4.3, 10 mo1.kg aluminium ions will be in solution across
the complete range of pH .values. It is therefore likely that
insufficient hydrolyzed aluminium ions are available to adsorb onto
the pigment to cause a monolayer of positive ions. This will cause
the individual particle surfaces to vary according to the number of
ions adsorbed and create a wider distribution. At a higher
concentration of aluminium sulphate 10-5 mol.kg-1 more ions are
available to adsorb, causing an increase in the iep of the pigment
and creating a more homogenous surface. This is seen in the
significant narrowing of the histograms in Figure 4.47 (pH 7.4 and
. 7.7). As the pH is raised, the histograms widen, Figure 4.47 (pH 8.5
and 10.0), due to the reduction in available positive ions for
adsorption. A similar effect is observed as the aluminium sulphate
concentration is raised further to 10-4 mol.kg-1 (Figure 4.48). The
histograms are again narrower particularly at intermediate pH where
the solubility of aluminium ions is at the lowest values.
RTC90 supernatant causes the mobility distribution of RO rutile
to narrow (Figure 4.49), but not to as great an extent as the pure
aluminium sulphate. This, along with the fact that RTC90 does not
have a mobility curve, with respect to pH, with an iep similar. to
that of aluminia indicates that other ions apart from aluminium are
present in the surface coating. Figures 4.50 and 4.51 show
histograms of Hydral (alumina pigment) in aqueous solution and RTC90
supernatant respectively. Again the ions in the supernatant causes
- 142 -
45
35
-~ 2.
"-o. '!>
5
O~ ':'4 22
._/ '.. .. "'. ..".10
pH 6·7
:>0
4' '4
,. ~
42 U
~ ~ -~
" 30
"- "- "-• • •
I. 18
• 6
-2:8 -2·0 ·"·2 -0·" -3~ -2=8 '20 -1-2
v ... / ..... " ... '0" v,./ .......... ,0'·
pH 7·0 pH 7·2
FIGURE 4.46.
Mobility Distribution of RD rutile in Aluminium Sulphate (10-6mol . kg -1)
- Number of Particles (N) as a Function of Mobility (VE).
4'
.. 2'
.. •
-4:4 -):e ·if) 40
v" •• ". to / '.. .. pH 7·5
~.
.2
o
.", , •
,a
r 6 Af 1
o·e, ,~ 22 3
V,,/ ........ -'0'·
pH N
.~ ., ., IIJ
35 3, J>
~ o ~ ., " 2> ,
• ., •
, •
u
" .,
"
'~ 5
/'-
'·0 2-41 oe o -H -1'G
~ -0" o
5
VII .a ........ ,0'· / '" .. v. • .... '0
pH 7·7 pH 8·5
FIGURE 4.47.
Mobility of Distribution of RD !'Utile in Aluminium SUlphate (10-5mol . kg-1)
- Number of Particles (N) as a Function of Mobility {vE'.
-... -. .J2
/ '.. .. v# .".10
pH 10·0
.~ ,
1 I
.~ 72 ~
l~ ~. " .2
. 0 ·0
2 2 2~ ,
•
4() 20
"-, • •
,Xl •
'" 2' 1~ ~ 1.
~ ~ ~ ~ .0;.
, .>l) ·'"2 v. / ...... w" • 10 ..
~ , ~ • ~ \
'211 fO:. .,.) ..QO ·2~ ·2-2 .,.. -':>6 v .. / ......... '0'·
'\r" / .", ...... 10'·
'0'
6
·fe '·0 v .. ~.~ ...... : .,0..,,:4
pH 6'2 pH 7'3 P I-l 8'9 pH 10·3
FIGURE 4.48.
Mobility Distribution of RD rutile in Aluminium Sulphate (10-4mol. kg -1)
- Number of Particles (N) as a Function of Mobility (VE)'
72
!16 f
~ ·0
- Cl
"-•
2·
• J
'·2 20 ... V, ........ 10 / ' . ..
pH 6'2
3'6
. o
72
~.
• 4()
"-•
2.
O<J '\ .1.'6 .~
v" / •.•. .,1 .,0"
pH 6'9
FIGURE 4.49.
o .0"
72
!16
o
Cl
"-•
2.
• ~.
'3. -30
v" / ........ 10"
pH 7·9
Mobility Distribution of RD rutile in RTC90 Supernatant
- Number of Particles (N) as a Function of Mobility (VE)'
~
.2
o 30
"-• ••
• -22 .. NI ..... -:>8 -30 -,
/ '" .. v.. • ••• 10
pH 9·3
.~. ,. ~e ,.
"1 r ~ '2 U .2
! -!! -!! a JO JO
18l (~ -!!
20
, " " • • • t~ ~ f l t .. ~ ,... In t ..
~ e e
} u~ r
«~ \.l ,:2 ':0 ,.-.. 2'2 0-0 t· , -02 ~
, , .,.'8 ·to
..... • ." •• 10 I · .. .. \Qr / ..... w" • to·' \0 / ..... w" .,0·' "'-/.· ....... '0 ..
pH 7-1 pH 8·3 pH g., pH 11-0
FIGURE 4.50.
Mobility Distribution of Hydral Pi~ent in Aqueous Solution
- Number of Particles (N) as a Function of Mobility (VE).
72
••
-~
"" "-•
,.
• ~ ~ ... :., 40 .e \0/ ._._.4 .....
pH 5'1
72 72
.. ~ 11 .. "l
; ""1 In -:!!
40
"-• .. ~ ,.
• • "" -H. ""0 -0"2 -0"0 "to
\4 / ......... ,0·· '*/ •.. - .... ,. .. pH 7·0 pH 7'3
FIGURE 4.51.
Mobility Distribution of Hydral Pi~ent in RTC90 Supernatant.
- Number of Particles (N) as a Function of Mobility (VE).
"
.. 11 ~ rl
"" "-•
.. 1 r
• ......
-0:2 • )8 . .., fl ••• '*/ •· ..... ·.to··
pH 8·0
the histograms to narrow and the iep to be moved to a more acidic
pH.
4.4.2 Histograms in SHMP Solution
Figures 4.52 and 4.53 show the variation in mobility
distribution of RTC90 at pH 9 and 5 as the SHMP concentration is
increased.
No significant difference was observed until the SHMP
concentration reached 5 x 10-3 mol.kg-l. This corresponds to the
plateau of the adsorption isotherm being reached, as seen in Section
4.2.8, Figures 4.9 and 4.10. At this concentration, a homogenous
layer of negative ions is present on the surface, purveying a
similar potential on a greater number of particles.
Por HD rutile, Figure 4.54, as the concentration of SHMP is
increased, the mobility distribution widens significantly, giving
very wide histograms. The reason is not immediately obvious but
must be due to the difference between the two adsorbing surfaces,
alumina and titania. A well formed layer of ions cannot be forming
on the titania surface perhaps due to the inherent impurities
arising from pigment manufacture. Another factor could also be the
presence of aluminium ions, in the RTC90 system, which lower the
SHMP solubility by forming alumina-phospho complexes. However, no
indication of this was oetained from the mobility of HTC9C and HD
rutile in SHMP solution from electrophoresis measurements on the
model 400 although less SHMP was found to adsorb onto the uncoated
pigment.
- 143 -
•• •• 12 .. '2 ,.
" ~
. 'Ol ~ 0
••
~
30 ,. • '0 ·30
"- "-• • "-• "-•
'8 '. 2' '. e • •• J ~I •
. ". ~ -4'8 . .., .•. '8 , -:io -2:2 "11 ., . .,
\f) / .", ...... ,0" v,./ ........ -'0·' v .. / ......... to·· v" / ......... '0··
e. sxIO-Smol.kg-' e" S ~ IO-4mol.kg-1 e. S" 10-3 mol. kg-' e • 10-2 mol.kg-I
FIGURE 4.52.
Mobility Distribution of RTC90 in SHMP Concentrations (C) at pH9
- Number of Particles (N) as a Function of Mobility (VE).
• s
3S
.. ~ 25
"-•
'"
~
~ -3& -2'8 ->0 -,.
v .. / ..... , •• , •. ,
-4 -I C. 5 IC 10 mol.kg
72 s •
~ .,
. 0
- <0 ':
lO
"- "-• • ..J ( ,. 8~ ) &
~ -.~ ., ->8 -.. ~ -,~ -2'4
v., / ..... w· .,0·· V.. • I .... '0 / . . .. Ca 10- 3 mOl.kg-' 3 -I
CAS .. 10 mol.kg
FIGURE 4.53.
Mobility Distribution of RTC90 in SHMP Concentrations (c) at pH 5
- Number of Particles (N) as a Function of Mobility (VE).
72
~
'0 -""
"-• .. e
\ ~ -,~ -H ->0 -.', . ,,-.
/ . . .. \I) •• oe. .,'
-2 -I
C " 10 mol.kg
)6j 11 11 )6, 11 )a.) l 36
1
leJ I· I 11 2. 2·i ~~ 28
•
>/ .~
I "~\" ~
\ .
0 20 • 20 20 • "- " ~
z " .. '2 '2 '2
• • • • -). -!>. -... -!>8 -.2 -~ -8:2 -!>4 -~ -J~
--I ..... .• - •• ".10 Cc 10-5 mol kg~1
'0/ ............ . C = 5~10-5 mol kg-I
"'" I ......... '0'· -3 -I
C·IO mol kg
-!>O . "·2 .,. ... -2
"" / ..... ,," • to'·
C .5. 10-3 mol kg-I
FIGURE 4.54.
Mobility Distribution of HO rutile in SHMP Concentrations (c) at pH 9
- Number of Particles (N) as a Function of Mobility (VE)'
4.4.3 Histograms of Pigments in SDS Solution
SDS was not found to have an effect on the mobility distribution
of RTC90 within the concentration range studied (Figure 4.55).
However, as with SHMP, the distribution of RD rutile was significantly
widened (Figure 4.56). This would tend to have a detrimental effect on
the stability of the system, particularly at low potential, where
interparticle repulsi ve forces are not high enough to prevent
flocculation.
4.4.4 Histograms of RTC90 in Mixed SHMP/SDS Solution
Two sets of samples were studied:-
(a) Constant concentration of SHMP (5· x 10-5 mol kg- 1)
and varying concentration of SDS - Figure 4.57.
(b) Increasing concentration of total dispersant with
a 1 : 1 ratio of SHMP to SDS - Figure 4.58.
Samples prepared as in (a) were not found to vary as the SDS
concentration increased. Samples prepared as in (b) were found to
narrow in mobility distribution at a total dispersant concentration
-2 -1 of 10 mol kg . These results indicate that although SDS aDd SHMP
both individually adsorb onto RTC90 pigment, SHMP will replace SDS
on the surface.
4.4.5 Summary
The S3000 Laser Zee Meter is a useful piece of equipment for
measuring mobility values quickly. Mean mcbility values from the S3000
were very close to those obtained from the non-automatic-Model-400.· ----
- 144 -
n
". ~.
•• ••
~ '"
-!! 30
~ \ ~
•• .. • • ~
-~ -lb -H ~. -» -,., '':2 -.' '*/ ............ . -../ ._.4 ...... .
-5 -I C:2-S ~ 10 mol. kg -4 -I
C = 10 mol.kg
FIGURE 4.55.
Mobility Distribution of RTC90 in SOS Concentrations (c) at pH 9
_ Number of Particles (N) as a Function of Mobility (VE)-
.,
"j ,-
-~ "1 } ) ~
,~ j I L._
,
·n '000 . j, .",
-../ ..... " ... ,. .. C '" 10-5 mOI,kg-
1
., .,
,,~ r'lIl~ ]I!
~ U •
-~ .,i ) \ ~
" } }
,~~ ,~
~ ,
·.·e ·40 -]:~ -~ ... - .... . ,., \40 / ..... ,," ",,"
\lit / ..... ,,- " ,.'.
-4 -I C • 5" I 0 mol. kg -3 -.
C 2 2·5" 10 mol.kg
FIGURE 4.56.
Mobility Distribution of RD rutile in SOS Concentrations (C) at pH 9
- Number of Particles (N) as a Function of Mobility (VE).
,.
••
-~ )0
}
le
.. - !t:2 -.;. .,. .• 41
\It/ .'.4 ...... . -2 -I
C - 10 mol. kg
72
... . l!
4()
~
...
•
....
.. ,. J
., j ~ .,
"" 2 Xl
"-•
1>
} 'Iq I -: Xll J
}
... ,. j
) G j ~i ·i J /
.)08 ·28 -aeJ '\40 / ._.4.4 . ,.'. C = 10-5 moLkg-1
-4<) ')-2 .~ ,
-40 -~ -H -,.., '\4/ ........ to· a
\4' • ..... • '0 / . - _. C. 10-4 mOLkg-1 3 -I
C = 10· mo'-kg
FIGURE 4.57.
Mobility Distribution of RTC90 in Varying SOS Concentrations (C) After
Prestabilization in SHMP (5xl0-5mol. kg-l) - Ntunber of Particles (N)
as a Function of Mobility (VE)·
.,
.,
"
..
• -401 -le -)0 -H
'tit / ............. .
C = 10.2 mOLkg- 1
.~
16 I~
'"
I~
, --. . .,. ••• • \6 ·oe
\Gr / ......... to'·
-s -I C " 10 mol.kg
7> ~
:>0. 42i I I
- • U ~ ~ .<0
.' Xl
"- ~ • .. .. e ) ~ e
'4<1 . .., .» .>0 .... . .. 'M ·M
'*/ ............ . \(r ........ to / . .. -4 -I
C:2·5x 10 mol.kg 3 -I C=2·S x 10' mol. kg
FIGURE 4.58.
Mobility Distribution of RTC90 in Dispersant Concentrations (C)
at a SHHP : SOS Molar Ratio of 1: 1. - Number of Particles (N)
~s a Function of Mobility (UE). ".
"
7>
~
• ~
40 ,.
~
.. •
.:>of ·M ... . . ., "-/ .......... ~.
C .. 2·5 .. 10-2 mol. kg -I
The histograms obtained were used to ol:;serve general trends in
the mobility distributions as follows:-
(1) The mobility distribution of RTC90 is narrower than
RD rutile, in aqueous solution at acidic pH; this
effect is less noticeable at alkaline pH.
(2) An increase in the concentration of aluminium ions
in solution decreases the width of mobility distribution
of RD rutile, particularly at intermediate pH where the
aluminium solubility is lowest.
(3) RTC90 supernatent does not narrow the mobility
distribution of RD rutile to as great an extent as
pure aluminium sulphate.
(4) SHMP narrows the mobility distribution of RTC90 at a
concentration which corresponds to the adsorption
plateau. Conversely, the distrlbution is widened
significantly for the RD rutile.
(5) SDS did not appear to have an effect on the mobility
distribution of RTC90; but, as with SHMP, the distribution
of RD rutile was increased.
(6) In mixed SHMP/SDS solutions, distributions of RTC90 did
not vary with increase in SDS. When both SHMP and SDS
concentrations were increased the distribution narrowed,
indicating preferential adsorption of SHMP.
One criticism of the S3000 was that it appeared to use the same
portion of particles in the suspension to build up the histogram. The , " -
polarity reversal mechanism of the S3000 was such "that the'· p;iirticfe·s
observed on one sweep were reversed and swept across the detector time
and time again. In this way, any non-representative particles appeared
- 145 -
to be anomalously counted giving a non representative mobility value.
An example of this is given in Figure 4.59 where a large peak is
observed outside of the main distribution. This problem would not
normally be encountered with a manual Model 400 histogram (which would
take a considerable time) as large non-representative particles would
be avoided. However, the S3000 still counts a comparatively large
number of particles in order to obtain the average mobility value. The
correlation of mean mobility with the Model 400 indicates the particles
counted were typical of the sample tested.
4.5 FILM PERFORMANCE
4.5.1 General
The mechanism for the film formation of simple paint films has
been detailed by Kayem (1978) and will therefore not be repeated here.
He discussed the work of Dillon et al (1951), Brown (1956) and
Vanderhoff (1970). The conclusions from the work were as follows:-
(1) Increase in SDS concentration improved the film formation.
This was thought to be due to improved pigment distribution
because of latex solubilization.
(2) Increase in SHMP concentration caused poor film formation.
When both SHMP and SDS were present in the paint, the
quality of the film formed improved as the ratio of SDS
to SHMP increased, the effect of SHMP being completely
removed when the 'ratio of SDS to SHMP was high.
(3) Theoretical calculations based on DLVO theory indicated
that the pigment was very sensitive to flocculation, by
increasing ionic strength. It was shown that the preferred
order of flocculation during drying in the absence of latex
- 146 -
.. o x
54
42
"" 30 z
18
G
Figure 4.59:
1·8 4·2
The Effect of Non-Representative Particles
on the Mobility Distr'ibution Histogram of
the 83000
solubilization was pigment - pigment homoflocculation
» pigment - latex heteroflocculation > latex - latex
homoflocculation. No flocculation experiments were
carried out.
All flocculation experiments in this project were carried out at
constant concentrations of SDS and SHMP. Only thickener, i.e. HEC or
CMC ccncentrations were varied to investigate any effect.
4.5.2 Wet Film Flocculation
As described in Section 3.11.2, the extent of flocculation in a
wet paint film was measured by infrared backscatter at a film thickness
of 40 I'm. No direct information can be obtained from the individual
figures, but trends can be observed. Plots of backscatter versus
thickener concentration are given in Figure 4.60. From these plots,
addition of a low concentration of thickener has a detrimental effect
on flocculation, indicated by the higher backscatter reading. The
flocculation for the system containing CMC was greater than for the
corresponding HEC formulations. However, on raising the concentration,
the polymers showed more similar resul ts, the HEC films increasing in
flocculation. A minimum was seen in the CMC curve Where decrease in
flocculation was followed by an increase, on increase in thickener
concentration. Addition of BCA had little effect on the CMC system but
appeared to decrease the backscatter of the HEC films.
As the concentration of thickener was the only variable, the
differences in flocculation would be due to this change only.
4.5.2.1 HEC
From Figure 4.60, the backscatter of the wet paint film increased
- 147 -
FIGURE 4.60
0·2 0·1t 0'6 0·8 1·0
c / % solution
Flocculation of Paint Films
- Backscatter of Wet Films (Bf ) with
Respect to Thickener Concentration, (C).
on addition of HEC, indicating an increase in the flocculation, or
decrease in the dispersion, of the system.
As HEC is a non-ionic polymer, any reduction in dispersion would
be due to either steric destabilization of the colloidal particles by
the non-ionic macromolecules or by the effect of HEC on the electro
static stability of the pigment or latex.
The stability of colloidal dispersions in the presence of
interfacial polymer layers has been reviewed by Vincent (1974) and
Napper (1977). The stability and instability of disperse systems with
adsorbed polymer has also been discussed by Ottewill (1977). However,
for such stabilization to occur, sufficient coverage of the particle
surface, by the polymer, is required in order to provide the steric
barrier. This is less likely at low concentrations of HEC.
If the length of the polymeric molecule is sufficiently long it
can span the distance across the energy barrier of two particles, and
adsorb onto the two surfaces. This phenomena, known as polymer
bridging, has been reported in the literature by Ash & Clayfield (1976),
Brown & Garrett (1959), Gillespie (1960), Gregory (1969) and La Mer &
Healy (1963). The latter authors, along with DiMarzio & Rubin (1971),
Hall (1974) and Vincent (1974), have given a theoretical basis for this
mechanism.
An alternate hypothesis has been considered theoretically by
Asakura & Oosawa (1954, 1958), Richmond & Lal (1974), Vrij (1976),
Joanny et al (1973), Sato (1979) and Feigin & Napper (1980). The
hypothesis, known as depletion flocculation, is that soluble polymer
can flocculate a dispersion in the absence of adsorption. It is based
on the proposal that there is a contribution to the attractive force
between proximate colloidal particles suspended in a solution of
- 148 -
macromolecules when the effective diameter of the macromolecular cOils
exceeds the distance between the particles. The macromolecules are
excluded from the interparticle region, since their insertion in the
restricted space would result in a loss of configuration entropy and a
free energy increase. This "volume restriction" attractive force
augments the van der Waals attractive forces between the particles.
Vrij (1976), Joanny et al (1973) and Vincent et al (1980) have
also proposed a third mechanism which may be seen as a hybrid of the
two. This involves incompatible interaction of free dissolved polymer
wi th strongly adsorbed polymer. Experimental support for the volume
restriction mechanism has been given by Sperry et al (1971) for the
flocculation of latices by HEC.
It is not obvious if one or all three of these processes are
occurring during. the formulation of the simple paint mixtures
containing HEC. From the adsorption isotherms and mobility
measurements of HEC with RTC90 and PVAc, it is proposed that HEC is
adsorbed on RTC90 to a reasonable extent, even in the presence of SHMP,
but is not ad sorted on PVAc to any great extent, particularly in the
presence of SDS. It is possible that polymer bridging of pigment
particles occurs in the mixing of the paint formulations when the
concentration of HEC is low. Volume restriction flocculation of the
latex is less likely due to the stec ic barrier provided by the SDS,' but
will depend on the magnitude of the attractive force. The third
mechanism for the flocculation of pigment particles with adsorbed HEC,
at higher concentrations of the water soluble polymer, will also depend
on this attractive force. The backscatter results indicate that as the
HEC concentration is increased, the flocculation is also increased.
This would tend to back up the volume restriction theories, al though
bridging may also occur. However, as HEC is further adsorbed, the
- 149 -
---------------------- ------------ ---------------
electrostatic barrier to flocculation will be reduced which will also
promote flocculation.
4.5.2.2 CMC
. As CMC is also a macromolecular species, the bridging and volume
restriction, flocculation theories will apply. However, as CMC is an
ionic polymer, an electrostatic barrier will still be present on
adsorption. This will enhance, or compensate for any change in the
steric stabilization. From the backscatter data, low concentrations of
CMC significantly increase the flocculation of the system shown by the
dotted line in Figure 4.60. However, an increase in concentration
appears to improve the dispersion of the system, which is reversed on
further concentration increase. The flocculation at low concentration
is greater than with HEC, possibly due to a higher molecular weight and
hence longer chain length. This would give a gre~ter tendency for both
volume restriction flocculation or bridging. The relatively' lower
change in flocculation with concentration may possibly be due to the
sustained steric barrier imparted by the ionic thickener.
4.5.3 Dry Film Flocculation
Flocculation gradient, Simpson & Rutherford (1982), is determined
by plotting film backscatter, Bf , against film thickness, Tf , which
should give a straight line. As described in Section 3.11.1 the
extent of flocculation in dry paint films, as measured by the
flocculation gradient, was obtained as a function of thickener
concentration. Concentrations of SDS and SHMP were constant. An
example of a plot used to determine the flocculation gradient is given
in Figure 4.61. From these diagrarr.s the effect, of increasing the
150 -
2S
~ 1S '-
S
Flocculation = Sf / T f gradient
8 1 6 -6
Tf /m x 10
Bf
FIGURE 4.61 Calculation of Flocculation Gradient
- Change in Backscatter, (Bf) with respect
to Film Thickness, (Tf ) .
24
thickener cc,ncentration, or the folocculation gradient of a paint film
could be determined. The results for HEC and CP.C with and without BCA,
at pH 9, are shown in Figure 4.62.
It was initially obvious that the films are more flocculated than
a wEll dispersed commercial alkyd gloss paint, which has a flocculation
gradient in the region of 0.25 (Rutherford & Simpson (1982)). Also,
some points appear to have an excessively high gradient. This was
due to the points, used in the flocculation gradient plot, appearing to
form a cur've. Curved plots normally occur when the film thickness is
too great. However, it was recommended that if there was a tendency
for the plot to curve, then the gradient should be drawn at the lower
values of film thickness. This will result however in the flocculation
gradient being dependent on a fewer number of points which will give a
larger degree of error. If these points are disregarded, then the
effect of thickener concentration is denoted by the continuous line.
Sufficient time was not available to determine the repeatability or
reproducibility of the method for this type of simple paint system, but
some general conclusions were drawn from the data, as follows:-
(1) Addition of low concentrations of either thickener
significantly increased the flocculation gradient.
(2) The flocculation gradient of the CMC system decreased as
the CMC concentration was increased. A minimum value was
then reached and further increase in concentration resulted
in a slight increase in flocculation.
(3) -The flocculation' gradient of the system containing HEC was
greater than the comparable CMC system at the higher
concentrations. A directional increase in flocculation
gradient was observed with increase in HEC concentration.
- 151 -
• 0 (HC D
1-2 • (HC + S(A
D HEC
1'0 • HEC + BCA 0 .... )(
7 E
L-- ~ ~ 0·8
0 D
"-~ - 0·6 I!:l
i • 0
O·lt
0·2
(}2 O-lt 0·6 0·8 1'0
( / % solution
FIGURE 4.62 Flocculation of Paint Films
- ---- - -- - --
- Flocculation gradient of dry films,CGf),
with respect to thickener concentration, CC).
(4) No conclusion about the eff~ct of BCA could be made.
As in Section 4.5.2, the difference in flocculation gradient was
due to the concentration of thickener. Any flocculation occurring in
the wet state will impact on flocculation in the dry films. As the
water evaporates from the films these flocculated particles will not be
redispersed. In addition flocculation forces, occurring as the solid
particles approach each other, will have an effect on the backscatter
results.
4.5.3.1 HEC
From the dry film backscatter results, the effect of HEC on
flocculation is negligible, across the ccncentration range used. This
is in contrast to the results obtained for the wet film system in
Section 4.5.2.1 where the HEC concentration increased the flocculation.
As it is unlikely that flocculation is decreased on drying of paint
films, the data suggests that the formulations containing lower levels
of HEC flocculate to a greater extent on drying, as the effective
concentration of all the compcnents is dramatically increased. This
could result from steric effects physically preventing further approach
of pigment particles, due to increased adsorption of the macromolecular
chains. The increased viscosity with HEC increase may also account for
this effect.
4.5.3.2 CMC
A minimum in the degree of flocculation with increasing CMC
content which was seen in the wet films is also observed in the dry
state. As CMC is an ionic thickener, it will provide electrostatic and
steric stabilization on adsorption. It will therefore impart a greater
- 152 -
degree of stabilization at lower ~oncentrations.
4.5.4 Gloss Measurements
Specular gloss measurements were made of each paint, drawn down on
glass plate, with an 850 gloss meter. The angle of measurement was
large as the films were of relatively low gloss compared to the high
gloss of oil based paints.
When a beam of light of intensity Ii is incident on an optically
smooth film, a percentage of the light is refracted into the body. of
the film, It,and the remainder is reflected as I r . The magnitude of Ir
is defined by Fresnels equation for unpolarised incident light as
follows:-
1 = +
2
where i = angle of incidence
r = angle of refraction
As sin i/sin r is the refractive index, n, of the film for a given
wavelength, it can be shown that:-
1
= 2
and I/Ii is referred to as the specular reflectance, R.
If paint films were optically smooth, R would be directly related
to the refractive index of the surface of the film. Paint films
however have some degree of roughness. They contain pigment which
promotes opacity by light scattering. Reflected light therefore, will
- 153 -
contain a diffuse component due to pigment particles. This will
result in a decrease in intensity in the reflected image, i.e. a
reduction in gloss. This decrease is particularly noticeable. unlike
that of an alkyd paint where a clear layer may exist. in an emulsion
paint where pigment is present at the surface, Murley & Smith (1970).
The gloss of an emulsion paint will depend on the degree of roughness
of the surface. This roughness can be due to micro-roughness Figure
4. 63(a). or macro-roughness Figure 4. 63(b) . Micro-roughness is the
result of small surface defects comparable te the size of emulsion or
pigment particles of the order of 0.2 - 0.6~~. Macro-roughness is the
result of larger defects of the order of 0.8 - 1.0 ~m or mere.
As surface roughness of emulsion paint films will affect reflected
light intensity, any parameter increasing the particle size at the
surface will reduce the gloss. As the pigment concentration was kept
constant in this project, the gloss was dependent on pigment dispersion.
An increase in flocculation would increase the effective particle size
and potential surface roughness. This would increase diffraction of
light and hence reduce the gloss. Floccula:ion is also detrimental
to opacity and cclour. However, results to the contrary have been
reported by Bal four (1977). He found that an alkyd paint, flocculated
to a low extent, had a gloss greater than that of a well dispersed
system. On increasing the flocculation the gloss decreased. This was
explained by the result of interference effects arising from variations
in the thickness of the clear layer which may occur at the sur· face of
the film. Murley & Smith (1970) have shown that the thickness of the
clear layer is proportional to the mean pigment particle size.
Therefore in a paint film, slight flocculation may increase the clear
layer and hence the gloss. Further increase in flocculation will
eventually have a detrimental effect on the surface, increasing the
- 154 -
---:a- Scattered Light
Reflected Light
• • • • • • • • •
(a) Paint Film with Micro-roughness
.-. - • •
•
(b) Paint Film with Macro-roughness
•
•
FIGURE 4.63 Light Scattering and Reflectance after Incidence
on Different Film Surfaces.
roughness and decreasing the gloss-
4.5.4.1 Gloss Measurements of Films Containing HEC
The gloss measurements of paint films containing HEC, drawn down
on gloss are shown graphically in Figure 4.6~. Initial increase in HEC
concentration decreases gloss in line with increased flocculation
of the dry films shown in Figure 4.62. However, further increase in
concentration results in higher gloss readings with correspondingly
little change in flocculation. Disregarding the results of Balfour,
which would normally only occur with alkyd paints, this conflict of
data may- be due to difficulty in flocculation gradient measurement,
discussed in Section 4.5.3 Further tests would be needed to confirm
these results.
4.5.4.2 Gloss Measurements of Films Containing CMC
Gloss measurements similar to those obtained in Section 4.5.4.1
but containing CMC were also sho~~ in Figure 4.64. This set of results
show a decrease in gloss as the CMC concentration is increased. These
results correlate with the dry film flocculation results in Figure 4.62,
if the O.~/o data point is disregarded. As in Section 4.5.5.1 this may
be again due to problems in accurately measuring flocculation gradient.
In addition, the higher gloss measurements obtained with CMC,
compared to HEC, correspond with the lower flocculation gradient
measurements, in Figure 4.62.
4.5.5 SEM Photographs of Etched Paint Films
As described in Section 3.11.1, SEM photographs of a limited
nwnber of the etched paint films were made. These photographs are
- 155 -
\
"
100 ,--"i-__ ___
80
0 ;;.::: 60 .......
..... er
40 0 CHC
• CHC + BCA
20 c HEC
• HEC +BCA
0·2 0'6 o·s
( / % solution
FIGURE 4.6 4 The Effect of Thickener on Gloss
- Reflectance of Paint Films (Rf ), as a
Function of Concentration, (C).
1·0
shown in Figure 4.65 ([email protected]%a.nd 0.6%) and Figure 4.66 (HEC @ 0.2%
and 0.6%).
The photographs of the formulations containing CMC bear out the
trend observed in the dry film data. The electron micrograph at 0.2%
CMC shows more flocculation than at the higher concentration. This
does not agree wi th the gloss measurements as discussed in Section
4.5.4 and again needs further data to confirm the results.
The electron micrograph of the film at 0.6% HEC also supports the
gloss data, and the pigment appears more flocculated than the .electron
micrograph of 0.6%CMC.
The micrograph at 0.2% HEC however, shows a greater degree of
flocculation than the numerical data suggests. One explanation is
that the area used for the backscatter measurement was not
representative of the photographed portion.
that the electron micrograph appears
Another possible reason is
so flocculated that the
-corresponding bilckscatter measurements may break down because- of the
extreme difference in the surface. Insufficient data however was
available to investigate this observation. Measurements of the
backscatter of infrared radiation by Balfour & Hird (1975) and Balfour
(1977) do not mention any similar observations.
4.5.6 Summary
In this Section, practical measurements pertaining to flocculation
were taken, both in the wet and dry state, and the corresponding effect
. on gloss were made. The data obtained was limited due to lack of time
/ and availability of equipment, but general trends can be summarised as
below:-
(1) Flocculation in wet films containing HEC increases
significantly as the concentration is raised. A
- 156 -
(a) 0.21> CMC
(b) 0.6% CMC
FIGURE 4.65 SEM Photographs of Etched Paint Films @ pH 9 "
Containing CMC
------ ---------------- -- -
(a) 0.21> HEC
(b) 0.6% HEC
'. FIGURE 4.6~) SEM Photographs of Etched Paint Films @ pH 9
Containing HEC.
------------------- ----------~-
similar trend is observed when using CMC as the
thickener, but a significant
occurs at 0.2% CMC.
rise in flocculation
(2) In the dry state, the flocculation trend with
concentration is similar to the wet state for the
CMC system. Little variability is seen for
flocculation in the HEC system when the concentration
is raised.
Flocculation as measured by infrared backscatter is
lower when using CMC, indicating improved .pigment
dispersion.
Problems with measuring flocculation gradient may
result from curved plots of backscatter versus film
thickness. Further data is required to confirm these
results.
(3) Gloss measurements of the CMC system correlate
reasonably with the dry flocculation data. They
confirm the reduced flocculation, i.e. improved
dispersion compared to the HEC system, resulting in
higher gloss values.
A gloss maximum occurs for the HEC system at 0.6
0.8% thickener concentration.
(4) SEM photographs of etched paint films can be of great
use in determining flocculation characteristics of
paints. A reasonable correlation of SEM evidence and
flocculation gradient was obtained with the limited
data available.
- 157 -
4.6 SEDIMENTATION ANALYSIS
4.6.1 The Computer Model
The computer programme SEDIMENT, Dunlop-Jor.es 1983 was written to
analyse sedimentation data. It is coded in Prime Fortran IV and is
used with a combination of a high resolution graphics terminal and
GINO graphics software (Corrputer Aided Design Centre, Cambridge).
The ccmputer model is based on the theory of Carstensen & Su
(1970a, 1970b) described in Section 2.3.4 and comprises of three
stages - First, the critical time, t c is identified. The short time
data (t < t c ) is analyzed and fitted to a curve. The long time data
(t > t ) is then analyzed in two stages, in a similar manner, to fit c
two exponentials to the data. The shape of these fitted curves will
indicate the sedimentation characteristics of the system under analysis.
By studying systems which differ frorr each other by certain parameters,
the effect of the differences on sedimentation, and hence particle
interaction, can be quantified. An example of the shape of a typical
sedimentation curve is given in Figure 4.67.
For a more comprehensive description of the computer model, the
reader is directed to the work of Dunlop-Jones (1982).
4.6.1.2 Critical Time
The critical time, t c ' is determined from the solution to a pair
of simultaneous equations fitted by a least squares plot to points
either side of the creak in the sedimentation curve. This is in the
region BC in Figure 4.67. The height of the sedimenting particles at
the critical time is kno~~ as the critical height, H. The accuracy-of c
tc and hence Hc is determined by the number Gf points used in the least
squares plot. An example of the determination from an ideal set of
points is given in Figure 4.68.
- 158 -
A
Time
FIGURE 4.67 Diagram of the Sedimentation CUrve Predicted by
Carstensen & Su (1910a, b) Model.
Xl 0 - 1 455~ ____ ~ __________________________ 1
450
445
440
430
425
420
o
Te'l:98.·S tic .. 43.18
75 o
415+-----r-----r----,----~~--~~--~ o 5 10 15 20 25 30
Time / Hour
FIGURE 4.68 Determination of the Critical Time, (T c), and
Critical Heignt, (He)'
Xl0l
4.6.1.3 Short Time Data
This analysis involves fitting equation 2.64 to the region AB in
Figure 4.67,
In [ H - Ho exp (-kt) J = -wt + lnC 1 - exp (kt)
(2.64)
for a value cf k that gives a straight line by a f'eathering technique.
k is first estimated and then the value giving the lowest residual SUIT,
of square is found. This is done using a half step iteration. At
large k, 2.64 approximates to a simple exponential:
l~ (H) = -wt + In (C) (4.9)
and the computer model fails. In this case the variation of H as
f(t) is a simple exponential. H = C €Xp (-t).
Once k, W, and C have been determined, a theoretical curve can be
drawn through the data as shown in Figure 4.69. Dunlop-Jones has
observed an artificial maximum in the curve at short time, with some
values of k. Heights greater than Ho have no physical meaning and were
not discussed in the basic mcdel. He also noted a need, in most of the
data studied (biological systerr.s), to change the time origin. This was
due to the data not following the simple exponential pattern predicted
by f'hchaels & BoIger (1962). It was thought this was due to either
residual tunbulence within the tube at the start of the experiment, or
the rate of floc formation, causing an apparent shift in time origin.
This part bf the analysis generates the exponential rate constants
. for the decrease in height of the constant density plug, and the
corr.paction of the ted forming at the base of the column. The values
obtained in the systems studied in this project are discussed in
Section 4.6.2.
- 159 -
Xl0l 10+-~ __________________________________ ~
9
8
6
5
k-O.O 0.'8.-0.0919 Ln[C)-4.60 A ••. E,.,.o,. In H-O.IO T ,.. 0,. I" In· J. 00
o
4+-------,--------r-------r-------r------~ o 2 4 6 8 10
Time /Hour
FIGURE ~.69 Analysis of Short Time Sedimentation Data.
4.6.1.4 Long Time Data
Two processes are believed to cccur dl.!ring compaction of the
sediment. Firstly the flocs rearrange to give a more compact bed. The
individual floes then collapse. The latter effect is more predominant
at longer times.
The computer analysis of long time data after the'critical time is
carried out in two stages. Two exponentials are fitted to the
experimental data a", described in Section 2.3.4.2. If it is assumed
that the value cf the constants are sufficier.tly different for the very
long time data to be represented by a single exponential, then a first
estimate cf the A2e- Wi exponential in (2.75) can be obtained by
applying the linear equation:-
= + (2.76)
to a range of terminal data points. The value of Hu is found by
feathering using a half-step iteration technique. The analysis can be
repeated with different point ranges to give the least error possible.
Extens"ive data is not always required as SEDIMENT does provide an
estimate ef H before it actually occurs, and estimated values are u
always to within 0.55mm, Carstensen & Su (1970b). However, insufficient
long time data will give a value of Hu<O. This wa~ a pertinent feature
in the analysis of paint and millbase mixtures as the sedimentation
time was in the region of 3 - 4 years.
With the estimates of A2 andW 2 , the linear equation:-
] InA1 (2.77)
is fitted to a range of points after the critical time Where the
dorr:inant factor is the rearrangement of the flocs. Again the best fit
is achieved by feathering using a half-step iteration technique, this
- 160 -
· time en A,. The region of points used can again be changed to reduce
the error.
Thus values of A" w" A2 , W 2 and Hu can be obtai n;.<I" for eacr.
sedimer:tation system. If the systems follow the Carstensen & Su model
then:-
(') Increasing the concentration of the suspensions will
decrease or mask A, and increase Hu' At a critical
concentration the initial censtant density plug will
disappear and at a high~,' concentration
exponential. will disappear .
(2) The exponential ccmpaction ccnstant for the rearrar:gement
mecr.ani srn, W" is inversely proportional to the
concentration of the sediment, and proportional to
the radius of the sedimer.tation cc·lumn and viscosi ty
of thE. medium.
(3) The exponential compaction constant for the collapse
of thE, fl ocs , W2 ' is inversely proportional to the
viscosity and radiu~ of the tube. It is also prcpor·tional
to () which is a general term acce·untiJ'lg fer the
interparticle forces. Ch.nges inW2
will therefore
indicate chan&es in the flocculation mechanism.
- ,6' -
4.6.2 Sedimentation of Paint Formulations
Sedimentation experiments with paint formulations involved
investigating the effect of thickener, both type and concentration.
SDS, SHMP, PVAc and pigment concentration were held constant, as
described in Section 3.10.1.
Plots of the sedimentation data describing the change in height
with respect to time for paint formulations containing 0.2 - 1.0% w/w
CMC are shown in Figures 4.70 - 4.74; A set of graphs outlining an
example of a complete sedimentation analysis is also shown in Figures
4.75 - 4.78.
A -similar analysis for --formulations containing HEC was not
possible because of their sedimentation characteristics. No interface
between solid and supernatant was observed in tubes containing up to
0.8% w/w HEC. At 1.0% w/w HEC a sediment was just visible after
approximately two weeks, but the supernatant layer was not clear, and
contained a substantial amount of dispersed pigment. This pigment
was not contained within the flocculates of the main cake but still
remained fully dispersed in the liquid phase. This indicates a much
reduced degree of flocculation for the HEC system. under these
conditions. The bluish hue of the particles in suspension suggests
that only the smaller particles remain dispersed. The reduced rate of
floc sedimentation also indicates a smaller structure than with CMC.
Table 4.3 shows the variation of the sedimentation parameters
with respect to C~C concentration. The critical height, Hc increases
with concentration along with the critical time Tc ' up to 1.0 'kw/wo
This is accompanied by al) increase in the time origin required to fit
the data. A shift in the time origin' has been observed by Dunlop
Jones and described as being due to turbulenc(e in the column at the
- 162 -
EO
~ ..., -& ..... ~
Xl0 1 10~ ______________________________ -.
9
18
8 28
7 38
6 18
58
5 68
78
~, 88 4_ o 0 0 0 0 0
3~ __ ~~ __ ~ ____ .-____ .-__ -. ____ ~ o 2 4 6 8 io 12
Xl0 3 Time / Hour
FIGURE 4.70 Results for the Sedimentation of the Paint
Formulation Containing O.~ w/w CMC.
XIO I 10@-__________________________________ ~_.
9 18
28
8
G 7 38
\ ~ -,..)
-& ~ ....
~ 6 .18
58
5" 68
~ 88
4 ~o 0
0 0 0 0
3~-----_,----_r----~----~~----~--~ o 2 4 6 8 10 12
Time/Hour XI03
FIGURE 11.71 Results for the Sedimentation of the Paint
Formulation Containing 0.11% w/w CMC.
Xl0 1 10~ __________________________________ ~
If}
2f}
9
3a
8 1f}
12 Sf} U
\ ...., 7 "& ..., 6f}
~ 7f}
6
8a
0
5 0
00 Sf} 0
0
0 0 0
0 <1 '-; , , , 0 2 4 6 8 10 12
Time/Hour Xl0 3
/ FIGURE 4.72 Results for the Sedimentation of the Paint
Formulation Containing O.6~ w/w CMC.
£'
~ ..., "& ..... ~
XIOI 10
18
9 28
38
8
18
7 58
68
6 j
78
5 88
4 0
98 o 0
0 0
I I
2 4 6 8
Time / Hour
FIGURE 4.13 Results for the Sedimentation of the Paint
Formulation Containing 0.8 w/w CHC
0
10 12 Xl03
Xl0 1 10e---------____________________________ ~
18
9 28
) 38
8. ~.
I> ~18 ~ ...,
7. I' ~ ... 58
~
6 58
~ 0
0 00
0
5_ 0 0
0 71.
0
4 0 2 4 6 8 10 12
Time / Hour XI03
FIGURE 4.74 Results for the Sedimentation of the Paint
Fonnulation Containing 1.0% wfw <:MC.
e:: ~ ..., .c:: tr. ... ~
72
0 15 I.=18.IJ
<> H.=61.JI
70 0
0
0
68 0
58 <>
66
64
62
60
6
58~---+---'r---'---~----~--~--~26 12 14 16 18 20 22 24
TIme/ Hour
FIGURE 4.75 Detennination of the Critical Time and Height for
the Sedimentation of the Paint Formulation Containing
0.8% w/w CHe.
E:
~ -....l
t . ...,
~
Xl0' 12,-------------------------------------,
'0
8
6
4
2
0.,;0=0.0564 Ln[C]=4.96 Ave. Error In H=O.20 Tlfle orlgln=O.O
<><><> .<>
•
O+----r----.---.----.----.----r----.---~ o 5 10 1 5 20 25 30 35 40
Time / Hour
FIGURE 4.76 Short Time Analysis .for_the .Sedimentation of.t!!~ . .!'~:i,I1~._ .. ___ . ___ _
Fonnulation Containing 0.8 w/w tMe at Zero Time Or:tgin. -
Xl0l 11,-__________________________________ __
10
9
8
7
6
0"g.-0.046 Ln[CJ-4·6 Ave. Error In H=O.16 Tit ... orlgln=5.50
°0 <>.
OOQ:,
<0 0 0 0 0
5+----,----r----.---,----~--_r----~--~ o 2 4 6 8 10 12 1 4 16
Time / Hour
FIGURE 4.17 Short Time Analysis for the Sedimentation of the
Paint Formulation Containing 0.8 w/w CHC after a
Change in the Time Origin.
65,-________________________________ -.
60
55,
45_
Hu-~1.30
0 •• gel -0.0027 fI, -g. 57~ O ... g .. =0.000202 11, =~.48S
40+-____ .-____ .-____ -, ____ -, ____ -, ____ ~ o 2 4 6 8 10 12
Xl0 3 Time / Hour
FIGURE 4.78 Long Time Analysis for the Sedimentation of the
Paint Formulation Containing O.8~ w/w CMC.
[CMC]~ w/w k/hr- l Wlhr-,l H Icm tc
/hr- l Time Wl
l W 2 1 Hu /cm c Origin/hr (hr- lxl0-2) (hr- lxl0-3)
0.2 0·302 0.0627 53.58 9.07 1.9 0.523 0.0095 19.25
0.11 0.236 0.0919 56.97 10.63 3.0 0.391 0.190 39.10
0.6 0.257 0.01107 68.111 18.0 3.5 0.154 0.2115 41.21
0.8 3.617 0.01160 61.31 18.13 5.5 0.272 0.202 41.3
,
1.0 1.211 0.0253 90.51 15.68 ' 12.1 0.5311 0.3111 115.81
TABLE 4.3 Results of the Analysis of Sedimentation Data from Paint Formulations
at Various CHC COncentrations
beginning of the experiment. This data however, was collected over a
much shorter period of time. The difference in time origin observed
in these experiments may be due to the variation in rate of floc
formation after this initial turbulence on filling the sedimentation
tubes. The change in time origin greatly increases the fit of the
data to the Carstensen & Su model. As floc formation would be a
viscosi ty dependent process, the increase, wi th increase in
concentration, would be expected.
The constant, k, for the initial rate of sedimentation, of the
constant density plug appeared initially to decrease with
concentration and hence viscosity as predicted by the model. At
higher concentrations completely unrelated values of k emerged.
Although this is a relatively slow sedimenting system where reduced
error would be expected, Carstensen & Su . (1970a) have expressed
significantly reduced confidence in k as the solid concentration is
increased.
The compaction constant, W, of the initial stage increased up to
0.4% w/w CMC but decreases at higher concentrations. The reduction in
wis predicted with increase in viscosity.
As previously discussed, at time greater than the critical time
the forces exerted on the sediment are gravitational, frictional,
reactional, and electrical. Gravitational forces are in a downward
direction and will be proportional to the sediment mass, and the
densi ty of the solid and liquid. The frictional and reactional
forces are in an upward direction. They are relative to viscosity and
to wall and bottom effects. As all of the tubes were of a constant
geometry, the latter forces would not affect any trends observed.
They would however influence the numerical value of the constants.
- 163 -
Electrical forces will be repulsive and increase aE the distance
between part icle s decrease. Electrophoretic measurements (Sections
~.2.'6 and 4.3.'2) showed that, at constant values of SHMP and SDS,
the mobility of polymer and pigment was negative throughOut the
increase in CMC concentration. A decrease in the negative mobility
occurred on increase in concentration but as CMC is anionic, no
charge reversal took place. A possible reduction in electrokinetic
barrier to flocculation was therefore possible.
Other mechanisms also need to be taken into consideration as CMC
is a macromolecular structure. This will contribute a polymer
flocculation or stabilization term into the inter-particle forces,
and will affect the relative values of w1
, and W2
depending on the
floc strength.
From the long time data· in. Table ~. 3 the following trends were
- observed on increase in CMC concentration:-
(i) The ultimate height, Hu ' increased significantly
from 0.2 - O.~% w/w CMC and then more gradually
to 1. 0% w/w.
(ii) The rearrangement, constant, w,;- decreased to a
minimum at 0.6% w/w CMC and then increased.
(iii) The compaction constant, W 2' increased, again
significantly in tpe region of 0.2 - 0.4 w/w CMC.
The large values of H indicate a stronger floc structure with u
a greater concentration of CMC. The simultaneous increase in w2
does
not support the increase in flocculation strength as the cake appears to
suffer more compaction.
contribution of e-W2.
This _ results .from ._ the .. Ir.::reosed. relative.
- 164 -
An interesting trend is observed with the rearrangement constant,
"', which goes through a minimum at 0.6% w/w CMC. Ini tially , the
increase in "'2 is accompanied by a decrease in "'" but above 0.8% w/w
CMC "', starts to increase. This results in al\ ,,,creased change in height
with time a~ the contribution from e-"" becomes smaller.
~.6.3 Sedimentation of Millbases
As described in Section 3.'0.2, sedimentation experiments were
carried out on CMC and HEC millbases which are similar in every
respect to the paint formulations without the PVAc polymer. Although
the concentration of the sedimenting solid will alter the values of
the constants, simple trends and comparisons can be observed. Short
time data was obtained from the HEC mill bases , but unfortunately
insufficient long time was available to obtain values of "'" "'2' and
Hu for this system.
Sedimentation plots for the data obtained from HEC and CMC
millbases at 0.6%,0.8 and 1.0% w/w thickener are given- in Figures
4.19 - 4.84. Experiments at 0.2 and O.~% w/w ~oere not possible
because of the lack of sedimentation definition in the tubes.
The constants determined, from SEDIMENT, for the data are given
in Table 4.~.
4.6.3.' CMC
In the initial region various observations can be made from
comparing the data in Tables ~. 4 and 4.3. No change in the time
origin was necessary to analyse the data, as was required _ for the
paint formulations. This could be -- because- of'~ _ a reduction in
flocculation or because the presence of PVAc polymer somehow reduces
the rate of floc formation after filling the sedimentation tubes .
.,. 165 -
XIOI IO~ __________________________________ ,
8
/9
6
G 29
\ -,.0
"& ....
~ )
4 ~39
I r~
2_ 59
~OOO 69 0 0 0 0 0
O+,-----,,-----,-----,-----,~----r_--~ o 2 4 6 8 12
Time / Hour
Figure 4.79: Results for the Sedimentation of the Millbase
Containing 0.6% w/w CMC
/
Xl0 1 10,-__________________________________ -,
O+----,,----.----,,------r----~--~, 6 2 4 6 8 10 12
Time / Hour Xl0 3
Figure 4.80: Results for the Sedimentation of the Hillbase
Containing O.8~ w/w CMC
X10l 10m-__________________________________ ,
18
/ 28
8.
38
18
6 58 ~ 68 l.J \ ....,
178 .c: C)) -..
~ 4 188
98
I, 118 o 0
2 0 0 0
O_i.--------,--------.--------,------~ o 5 10 15 20
Time/Hour
Figure 4.81: Results for the Sedimentation of the Millbase
Containing 1.0% w/w (MC
, ,
30.-____________________________________ ~
25
D D
P
~ ~18 ...., 20 P -& -
1 5 ) 28 )
\ o o o
38 o
104,-------r------.-------r-,-----,-------r-,----~ o 2 4 6 8 10 12
Time / Hour Xl0 3
Figure 4.82: Results for the Sedimentation of the Millbase
Containing-O.6% HEC
G \ .,., -& ...,
~
Xl 0 1 lOs-----------________________________ ,
8
6
4
2
18
28
38
18 L 58 ~Q:tJ 00 0 o
O+-______ ,-______ .-______ -. ______ ~ o 5 10 15 20
Hours
Figure 4.83: Results for the Sedimentation of the Millbase
Containing 0.8% HEe
Xl 0 1 10~ __________________________ ~ ______ ,
8 18
28
38
6J
6 \
D
-,..)
-& .... 1
18 ~ 4 (8
~
2, ~68
~, 88 0::00 00 0 0 0
O+---------~------_.--------_r------~ o 5 10 15 20
Hours
Figure 4.84: Results for the Sedimentation of the Millbase
Containing 1.0% HEC
',-
[CMC]/'XM/w k/hr- ' w/hr- l Hc /cm t Ihr- '
Time w , 1 wz' Hu/cm c Origin/hr (hr-1X10-2) (hr-'x'0-3)
0.6 0.061 0.0183 17.34 87.45 0 .538 .532 13.24
0.8 0.033 0.0118 15.18 105.05 0 .340 .4189 14.99
1.0 0.101 0.0063 32.54 151.99 0 .210 .146 11.82 ,
(HEel I
" w/w
0.6 0.966 0.103 13.42 19.41 - - - -
0.8 0.63 0.031 13.35 54.11 - - - -
1.0 0.266 0.022 15.03 81.84 - - - -
TABLE 4.4. Analysis of Sedimentation Data from Hillbases at Various Thickener Concentrations
However, unrelated values of the rate constant, k, and the
height, Hc were also obtained.
The cake compaction constant, w, for the initial region is
reduced ccmpared to the paint formulations, as expected, and follows
the same trend.
At longer times however, different trends occur. Values of W1
decrease with concentration with no indication of a minimum. Values
of W 2 show the opposi te trend to the paint formulations and decrease
with concentration indicating a str0"':ll2.r floc formation. This is
.~onsistent with the value of Hu which again shows an increase with
concentration. -,- .. ~ ,
4.6.3.2 HEC
As no long term data ""e.r.e. obtained from the HEC millbases only
the short time data \Ner" available for comparison to the CMC results.
Although only 3 sets of data points were available differences are
apparent.
Values of k and Ware Significantly higher and values of Hc and
Tc Significantly lower, as expected from the fast drop in
sedimentation height with time depicted by the data plots.
These results are indicative of a reduced floc structure. The
viscosity of this system is lower than for CMC, which must also be
taken into account. More data are required to fully explain these ,x
figures.
Both k and Ware inversely proportional to viscosity as predicted
by the model.
- 166 -
In conclusion this technique appears valuble when used in
conjunction with other measurements, for giving qualitative
information about the flocculation processes occurring in paint
formulations. This system however, contains inherent difficulties
when performing sedimentation experiments. Because of the extremely
lengthy sedimentation period, sufficient time is necessary to obtain
all of the data necessary. This period would be of the order of years.
In addition, the sedimentation tubes require a location where no
disturbance will occur and the ambient temperature is constant. This
may account for some of the inconsistent results obtained,
particularly in the initial region.
These data are the initial results from the sedimentation of
simple paint films containing thickener and dispersant. Additional
data are required to explain and confirm these trends, particularly the
use of varying tube diameters:
The major observations from the data can be summarised as
follows:-
(i) Flocculation strength of paint formulations increases
with CMC concentration, resulting in a low compaction
at long times. This shows a similar trend to the
film flocculation results and the dry film gloss
measurements reported in Sections 4.5.2 and 4.5.4.
(ii) Shifts in the time origin are required to fit the
initial data to the model, for the paint formulations.
This can be explained by a reduced rate of floc
formation after turbulence on filling the sedimentation
tubes.
- 167 -
(iii) A peculiar trend is observed for the paint
formulations containing CMC where the long time
rearrangement constant w, goes through a minimum.
(iv) Interparticle electrostatic forces would predict
a decrease inw2 with CMC concentration. The
millbases demonstrate this trend, whereas the
paint formulations show an increase inw2. This
can only be explained by polymer flocculation.
- 168 -
CHAPTER 5
SUMMARY AND CONCLUSIONS
- - - - - _. - - -- - - -- - - - - ------
5. 1 GENERAL CONCLUSIONS
The aim of this project was to investigpte the colloidal
properties of a simple aqueous paint formulation and to attempt to use
this data to explain their effects on the practical aspects such as
flocculation and sedimentation. From this, recommendations could be
made for 'the use of components which would provide commercially
favourable properties such as gloss.
An initial investigation involving a very simple paint system,
containing pigment, latex, SHMP and SDS, had been previously
investigated by Kayem (1978). The present project was to involve a
more ccmplex system including water soluble thickeners, required to
give the necessary viscosity required for paint application.
The starting point of the project was with one of the two major
components of the paint· system, the pigment.
The pigment is a very impcrtant component of the paint system, as
it imparts opacity on the paint film. However, it can also promote
stability problems within the systems, by forming flocculates which
may reduce the film gloss. The main types of pigment used in current
commercial paints new have outer coatings of alumina or silica. This
project used the former. Initial tests concluded that the alumina
coating on RTC90 had the potential to dissolve in aqueous solution, to
a concentration which was dependent on the pH of the system. A
solubility curve for the alumina coating was obtained that was a
similar shape to solubility diagrams in the literature. The desorption
of aluminium ions was found to be a time dependent process; also
dependent on the amount of agitation of the mixture. In a commercial
process, the amount of aluminium ions in solution will depend on the
amount of aqueous media, other compenents in solution, and the amount
- 169 -
of time taken to prepare the mix. As a considerable amount of water
is required for the latex, only a minimum amount of water will be used
to prepare the millbase. Dispersing agents (usually phosphates) are
added to the solution before pigment addition. Adsorption of pROsphate
is thought to reduce the dissolution of aluminium ions due to the
formation of alumina-phospho complexes which are less soluble than
alumina.
One question raised by Kayem was the motility characteristics of
alumina coated rutile. The pigment characteristics were observed to
be intermediate between alumina and titanium dioxide pigment. One
possible reason for this, arising from sample preparation, by partial
or complete dissolution of the coating was removed by ensuring the
presence of excess pigment. Mobili ty measurements using uncoated
rutile indicated that sufficient aluminium ions in solution could make
titanium dioxide demonstrate similar mobility characteristics to
alumina pigment. The supernatant solution obtained from soaking
coated pigment at various pH levels changed the mobility
characteristics of alumina pigment and rutile to be very similar to
that of the coated pigment. Possible ions causing this effect are
silica and titanate. No evidence, in the literature, was found for
the preserice of soluble titanate ions using a detection limit of
-6 3 10 mel dID-, but either ions may be the cause. No further
investigation into this aspect was warranted at this time, but this
could be a source of possible research for the future.
The latex was polymerized in the laboratory, from vinyl acetate
monomer. Initial attempts to purchase an industrially polymerized
latex with complete information of components, stabilizers and
physical properties had proved extremely difficult because of
secretive commerCial practices. Because cclloidal experiments demand
- 170 -
meticulous knowledge of even the lowest concentration of component,
it was decided to produce the polymer "in house".
Initial problems with the procedure recommended by Kayem (1978)
required changes in both reaction temperature and initiator
concentration. This may have been because of the change in monomer
source. However, no literature polymerization of this type was found,
with lower reaction temperatures. Latex supply was consistently
renewed throughout the project.
Subsequent to the basic analysis of pigment and latex, mobility
and adsorption experiments were carried out to attempt to form a
picture of what occurs chemically when paints are formulated.
The main problem in investigating basic colloidal properties of a
commercially oriented complex system is the number of components in
solution. Each component will have an effect on the others but to a
different extent. Information can be obtained by testing individual
components, but it is then difficult to confirm the total picture.
Adsorption experiments, for example, require the ability to quantify
the amount of adsorbing species either by physical or chemical means.
For isotherms in mixed components it is necessary that the additional
material does not interfere with the quantitative test for adsorption.
As the number of components is increased, the problem becomes more
complex.
Electrophoresis experiments do not have this problem. However,
only one solid component, i.e. pigment or latex can be investigated at
one time. In addition, . the ionic strength of the solution may also
impose a restriction. At high ionic strength, polarizing of the Laser
Zee Meter can result in distortion of mobility values.
Using the available analytical techniques from the literature,
- 171 -
described in Section 3, the following adsorption trends were observed.
These were backed up by corresponding mobility measure~ents:-
1. SHMP, SDS, HEC and CMC adsorb onto both coated and uncoated
pigment. Directional differences were observed in adsorif~d
amounts, between RTC90 and RD rutile, possibly due to
thermodynamic differences in the system, or inaccurate surface
area measurements.
Adsorption in mixed component solutions was not possible
because of interference in the analytical methods used.
A greater amount of adsorption occurred at acidic pH rather
than alkaline, because of the more favourable electrostatic
conditions.
Future work in this area would require alternative methods
of analysis, possibly using radioactive carbon labelling;
2. SHMP and SDS improved the mobility of coated and uncoated
pigment at both pH 5 and g. HEC reduced the mobility because
of the reduction in surface charge. Adsorption of CMC
initially raised the mobility but was reduced at higher
concentration. This may have been due to viscometric reasons
as CMC is an anionic polymer and would, therefore still
impart charge onto a surface on adsorption.
Mobility reduction by HEC and CMC adsorption was reduced on
addition of SHMP into the system. Further addition of SDS
had no effect because of preferential adsorption of SHMP.
3. PVAc was negatively charged in aqueous solution, across the
pH range because of its negative surface groups. The latex
adsorbed SDS but not SHMP. SDS increased the negative charge
- 172 -
and thus the stabilization of the latex. High concentrations
of SHMP flocculated the latex, as did higher concentrations
of aluminium ions from RTC90 supernatent.
Thickener adsorbed onto latex, but not to as great an extent
as onto pigment. From the literature the presence of surfactant
was likely to reduce the adsorption.
4. Mobility values confirmed stabilization of the latex by
adsorbed SDS with increased negative surface charge.
Destabilization by SHMP, HEC and CMC were observed with a
drop in mobility.
The mobility reduction by SHMP, HEC and CMC is masked by the
presence of increased concentration of SDS, confirming the
adsorption theory.
Paint formulations will therefore need a· compromise of components.
Pigment is required for opacity and latex is required as a carrier for
good film formation. SDS and SHMP are used as their respective
stabilizers. Thickeners improve the viscosity characteristics for
application but they have the capacity to destabilize the system when
incorrect concentration of stabilizer is present. As SHMP and SDS have
opposing effects, in terms of the latex, their respective concentration
will also need to be correctly balanced as excess of ei ther component
will have a detrimental effect.
One consideration studied, which is not normally taken into
account in colloidal studies, was the mobility distribution in the
electrophoresis samples.
The mobility distribution of a sample can be important in the
stability aspects of a system. The mean mobility may indicate
- 173 -
sufficient surface charge for stability, but if the distribution is
wide certain particles will be potentially unstable. From the
histograms measured, RD rutile has a wide distribution that is
significantly improved when aluminium ions are in solution with the
potential to adsorb., This is confirmed by the narrow distribution
observed with alumina coated RTC90. SHMP further reduced the
distribution of RTC90, but SDS showed no effect. In mixtures of SHMP
and SDS a decrease in distribution was seen as the total concentration
was raised. This indicates that SHMP is preferentially adsorbed over
SDS when present at high concentrations, as seen by Kayem (1978).
However, in the final paint formulation a significant amount of SDS
will be in equilibrium with the adsorbed surface of the latex and may
not even be in contention for the pigment surface. As long as
sufficient SHMP is present to combat the adsorbance of thickener, the
distribution of the pigment will be narrow enough with the surface
charge involved to stabilize the particles. No effect of thickener on
mobility distribution was measured. This is data that needs to be
generated in future projects. Both SHMP and SDS widened the
distribution of RD rutile confirming that coated pigments would improve
paint formulations even though similar mean mobility values were
obtained for both ccated and uncoated pigments. No reason for this
could be confirmed.
Having obtained a basic colloidal picture of the paint system, for
individual components, the more commercial aspects were considered
using fully formulated "simple" paints. Data was obtained from
flocculation and sedimentation experiments. These properties will
ultimately have an effect on the gloss of the final product.
Flocculation measurements were determined at Tioxide UK., thus limited
time was available to gather data. Further measurements are required
- 174 -
to confirm and expand this data but initial trends were observed.
Both thickeners increased the flocculation of paint films in the
wet and dry state. Further increase of CMC appeared to reduce
flocculation in dry films. This effect was not seen wi th ~'. Gloss
measurements gave conflicting results showing a decrease in gloss with
increase in CMC concentration. Also the initial increase in HEC
increased gloss even though the dry film measurements showed increased
flocculation. Further measurements are required to confirm and expand
these initial tests.
Sedimentation experiments on paint formulations were initiated as
soon as latex was available. Prior to this, measurements on pigment
millbases were set under way. The sedimentation measurements created
problems in a different way as they took an extremely long time to
obtain, (up to four years).
The model of Carstensen & Su ( 1970a, b) was used to analyse
sedimentation data from paint formulations and their corresponding
millbases. These contained varying amounts of thickener, both CMC and
HEC, with constant concentrations of SDS and SHMP. The sedimentation
theory was developed on simple model systems and their applicability
to paint systems, even relatively simple ones, was not known. In this
type of system additional interparticle forces such as polymer
flocculation and steric stabilization come into play in addition
to viscosity and electrokine~ic effects.
The time period to complete the sedimentation experiments was
greater than anticipated, taking the order of years to obtain
sufficient long time data. This was required to investigate floc
structure effects. The model was-'reasohably successful in aesorihing
the sedimentation of the formulations, but did give some unrelated
- 175 -
constant values. The long time data followed a combination of two
exponential decays.
Trends observed from the data include the increase in floc
strength at high me concentrations. This concurs with the'·wet film
floccu1ation data and with the gloss results fran dry films. The
paint formulations differed fran the mill bases requiring a change in
the time origin to fill the initial data. This can be explained by
a decrease in the rate of floc formation after turbulence in the
mixture on filling the tubes. The paint formulations also showed an
opposing trend as W2
increased with concentration probably because of
the contribution from polymer floccu1ation rather than decrease in
electrostatic forces.
5.2 FUTURE WORK
Various aspects of the emulsion paint system have been taken into
account in this project, and it has opened up the comp1exi ty and
problems into researching this type of system. Unless more
sophisticated analytical techniques are available, less basic colloidal
information, such as adsorption and mobility, will be possible as the
system becomes more complex in nature. At present, detailed analysis
is possible on simple systems or simple analysis on ccmplex systems.
It may be more advantageous for future workers to concentrate on
one aspect of this project, either m0bility and adsorption of multi
component systems, or the more commercially biased, floccu1ation, gloss
and sedimentation measurements. For the latter research, it would be
beneficial to obtain a commercial latex and accept the restriction on
precise information about the product. Other ··possible work would
include the effect of different latices and copolymers on film
properties. Also the effect of various ions on aluminium solubility,
- 176 -
pigment (both rutile and alumina) mobility, adsorption and desorption
will be advantageous. Additional research into the mobility
distribution and the effect of thickener, would require the
availability of S3000 equipment, but could be investigated using the
model 400.
Future emulsion paints may be available via the route described
by Princen et al (1968), where the pi@llent is encapsulated by hexanel
linseed oil and emulsified in water. Alternati vely, if the
encapsulation of pigment particles in actual latex particles could be
achieved, the resulting film would not suffer the flocculation effects
from the current binary system. This would increase the possibility of
glossy films.
- 177 -
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