Whiter than white: options for the compounder

c Whiter than white: options

for the compounder A number of white pigments can be employed in plastic compounds to achieve strong white colours and high reflectance, as well as for modifying other colours and increasing brightness.Titanium dioxide is the most important of these pigments for the plastics industry, but there are some alternatives as well. Plastics Additives & Compounding summarizes some of the issues at stake for compounders when selecting a suitable white pigmentation system.

Titanium dioxide Titanium dioxide is the most important white pigment used in the plastics industry, offering a higher refractive index than other white pigments and good chemical stability. Titanium dioxide is non-haz- ardous and has good dispersability and thermal stability. There are two forms used: rutile and anatase. Rutile titanium dioxide has higher opacity and is signifi- cantly less photocatalytically active than

anatase and also has a slightly higher refractive index, which gives better light- scattering power. Rutile titanium dioxide also accepts surface treatments more read- ily, bonding better than anatase. Anatase is used mainly in paper and elastomers. In thermosetting resins systems, it retards gel time and may prevent cure altogether. Titanium dioxide absorbs much of the UV radiation that would otherwise degrade a polymer matrix and also pro- tects the polymer photochemically.

However, if it is untreated then the pigment is also photocatalytic. Although it converts most of the UV energy to heat, the remaining energy creates active or rad- ical sites on the surface of the pigment particle. The reaction at these sites accel- erates the breakdown of the surrounding polymer, which leads to chaulking and loss of gloss. Nearly all the titanium dioxide pigments used in plastics are surface treated. In

PVC, the main purpose is to minimize photocatalytic activity. However, high lev- els of surface treatment reduce the pro- portion of titanium dioxide in the pigments, which reduces opacity and tinting strength. By modifying the interfacial interactions between polymer and pigment, the treatment may also aid dispersion and reduce the requirements of power and shear when mixing.

Particularly for PVC, most titanium dioxide pigments also have an organic coating,

Table I : Meit flow index of polycarbonate, pigmented with 5% titanium dioxide showing the effect of various surface

treatments. Source: Kronos

Virgin resin Il.4

Rutile titanium dioxide with modified siloxane 13.2 Rutile titanium dioxide with 0.5% alumina/O.5% polyol 21.5 Rutile titanium dioxide with I% alumina/0.5% polyol 25.3

Rutile titanium dioxide with I% alumina/O.5% siloxane 23.7 Rutile titanium dioxide with 3.25% alumina/ 1.5% silica/O.5% siloxane 25.2

which aids the development of an optimal state of pigment dispersion and ensures that the maximum opacity and durability potential of the pigment is achieved. The combination of inorganic and organic treatments determines whether the pigment is best suited to giving resistance to weathering and discoloration in uPVC window profile applications, for example, or providing viscosity and opacity (for example, for use in plastisols).

Property differences arise from the type, thickness, combination and method of application of surface treatment. Table 1

shows how the melt flow index of polycarbonate is affected by different surface treatments. The level of photocatalytic activity may be reduced by surface treatment of the base pigment with suitable inorganic compounds. For use in plastics, these are usually alumina or a combination of alumina, silica, siloxane, and polyol, or sometimes zirconia. These treatments mainly hmction by creating a physical barrier between the pigment surface and the polymer matrix, which blocks the active sites and minimizes degradation. The effectiveness of a particular surface treatment depends on the type of compounds used, the amount applied, unifor- mity of the treatment and density of coating on the pigment particle. Treatment with alumina remains the common surface treatment. Other inorganic surface treatments include the less widely

Plastics Additives & Compounding October 2002

White pigments

3 used silica and zirconium. Inorganics are

usually added to improve properties of the

end product, but they also enhance the

dispersion characteristics. They also inhib-

it undesirable chemical reactions by form-

ing a barrier between the titanium dioxide

and the resin.

Polyols are the most common form of

organic surface treatments. Amines, silox-

anes and phosphated fatty acids are also

used. Organics usually act as aids to pro-

cessing and dispersion, which promotes

de-agglomeration, wetting and dispersion

of the pigment. All the major effects of

organic treatments are concentrated on

the surface of the particle and the figure

for percentage composition usually indi-

cates the amount on the surface.

Alumina (aluminium oxide): Applied

during manufacturing at a level of 0.5-

3.5%, alumina is compatible with all

main resin systems. However, it can give

problems by outgassing its water of crys-

tallization at elevated temperatures, with

the water vapour remaining as a pocket in

the melt and finished product. In thin

films this causes voids or lacing. When

processing at above 250-300°C it is rec-

ommended to use particles with 0.5%

alumina or below.

Silica (silicon dioxide): Used with alumi-

na to improve the weathering resistance of

certain grades of titanium dioxide - for

rigid PVC compounds for outdoor appli-

cations. The alumina and silica form a

surface impermeable to UV light, which

prevents pigment/matrix reactions that

are started by exposure to Uv light.

Polyols and amines: Wetting and disper-

sion are enhanced in almost all polymer

systems.

Siloxanes: Generally give around the same

performance as polyols and special grades

are effective in retarding undesirable reac-

tions in polycarbonates. However, in

amounts above 1% siloxanes can separate

from the titanium dioxide and migrate to

the surface.

A typical range of titanium dioxide grades

includes:

l Stabilized, surface-treated micronized

rutile, chloride process - this gives the

highest weather stability in resin systems,

particularly PVC and polyolefins. This

grade also offers good dispersability,

brightness and tinting strength.

Stabilized, surface-treated micronized

rutile titanium dioxide grades offer

good weather stability and very good

dispersability, particularly in PVC.

Good photochemical stability and dis-

persability in aqueous systems is

achieved. These grades are good for

urea and melamine moulding com-

pounds and rigid PVC.

Surface-treated micronized rutile grades

provide brilliant shades in colour com-

pounds, high tinting strength and good

dispersabiltiy. These grades provide

good economic value, but the weather

stability is limited in PVC with lead sta-

bilizers.

Surface-treated micronized anatase tita-

nium dioxide grades offer high rindng

strength, bluish colour tone, outstand-

ing brightness and dispersability. It is

less abrasive than rutile pigments, but

are not recommended for outdoor

apphcations.

Untreated micronized anatase titanium

dioxide grades have very good bright-

ness, bluish colour tone, high tinting

strength and good dispersability in

aqueous systems.

Important criteria for all pigments and

whites in particular are opacity and tint-

ing strength.

Opacity is particularly important in thin-

section applications, where a highly

opaque pigment is useful even at low con-

centrations. However, lower concentra-

tions of titanium dioxide pigment can

detract from durability.

Tinting strength measures how well an

amount of pigment affects the overall

colouring of the moulded product. With

titanium dioxide this means how well it

lightens a coloured compound, or adds

whiteness and brightness to a white sys-

tem. This is important not only in white

compounds, but also in compounds where

the colouring influence of other additives

must be masked. With titanium dioxide,

the development of opacity and tinting

strength depends on light-scattering

power. This is governed by refractive

index, particle size distribution, titanium

dioxide content and dispersion in the poly-

mer. The greater the difference between

the refractive indexes of the pigment and

its surrounding medium, the higher the

light-scattering power, and therefore the

opacity and tinting strength.

If the titanium dioxide is to achieve its

maximum opacity, then control of parti-

cle size distribution is critical. For the

most efficient light scattering, the particle

diameter should be about 50% of the

wavelength of the light to be scattered.

Consequently, some grades of titanium

dioxide pigments are produced to maxi-

mize the number of particles in the range

0.20-0.40 microns, which is approximate-

ly half the 400-700 nm range of the visi-

ble light spectrum. A particle size distri-

bution of 0.2-0.4 microns develops

opacity in titanium dioxide, while a parti-

cle size distribution of 0.4-1.0 microns

affects durability. The titanium dioxide

content of a pigment is also an important

factor in the opacity developed by a spe-

cific pigment. Typical grades have a con-

tent of 88-97%.

Good dispersion is also important for

developing high opacity and tinting

strength. Maximum values will be devel-

oped only if the number of aggregates and

agglomerates is few, and those that are

present are well distributed throughout

the polymer matrix.

The difference between the red and blue

reflectance, normalized against the green

reflectance, as measured in a grey tint is

known as the undertone. The more nega-

tive that this value is, the bluer will be the

undertone of the pigment. The undertone

can also be expressed as a CIE b-value

measurement in a standard grey flexible

compound, and compared with the b-

value of a standard titanium dioxide in

the same system.

Undertone is a function of pigment parti-

cle size. Titanium dioxide pigments with

an average particle size of close to 0.20

microns impart a bluer undertone to

thick sections than do pigments of a larg-

er particle size. But for thin, translucent

items the appearance of colour arises from

the transmitted light, and the effect of

particle size is reversed: for bluer cransmit-

ted light, larger particle sizes, neutral or

yellow undertone products, are selected.

When colour matching tinted com-

pounds, it is usual to use a titanium

dioxide with the correct undertone -


E White pigments

although compensation for small differences can sometimes be made by adjust- ing the tinting system. Pure titanium dioxide scatters all wave- lengths of visible light uniformly, and therefore appears as brilliant white in a colourless plastic. Pigment colour is essen- tially dependent on purity, so maximizing the potential for producing a brilliant white.

Zinc sulphide

Zinc sulphide is a good alternative to titanium dioxide pigments, where these can

cause technical problems. Pigments have various concentrations of barium sulphate, which is characterized by high brightness

and very good light stability. Zinc sulphide fibre-reinforced plastics (unlike abrasive pigments absorb less UV radiation than pigments such as titanium dioxide). Main

titanium dioxide pigments. This means application areas include thermosetting that they have a wider UV ‘window’, giv- compounds, glass-fibre-reinforced ther- ing the highest efficiency of optical bright- mosets and thermoplastics, and polyolefins. ener: fluorescent additives also retain their Compared with rutile titanium dioxide,

effectiveness, giving a brilliant appearance, zinc sulphide offers white pigments for and photoinitiators in UV-curable paints plastics that are non-abrasive, catalytically remain effective when pigmented with zinc inactive, dry lubricants. They can reduce

sulphide. Tinting strength depends on the friction, polymer decomposition and the zinc sulphide content. Micronized grades wear of processing equipment. Uncoated are easily and homogeneously dispersed in and surface-treated grades are available to

plastic compounds. Particle size is 0.3 improve dispersion. In particular, microns, which is regarded as optimum for micronized grades are easy to disperse a sophisticated white pigment. and suitable for dissolver grinding. Low Due to low Moh’s hardness, zinc sulphide oil absorption allows higher loadings, pigments cause little wear on moulds and while maintaining good rheological do not affect the mechanical strength of behaviour and improved flow properties

Table 2: Properties of white pigments and fillers Source: Sachtleben

Trpe

Titanium dioxides Rutile chloride Rutile Rutile

Rutile Anatase Anatase

Zinc sulphates Micronized ZnS normal 60% 30% Micronized

30%

Barium sulphates I .O-3.0 microns Micronized

Other materials Barytes Kaolin Talc

Wollastonite

‘In PVC according to DIN 53 775, with 0.025% carbon black and 3% pigment (r?f pigment standard - TiO, = 100)

2Depends on sy stem: very good in polyamides and melamine/formaldehyde moulding compounds.

Surface modified

Relative tint reduction’

(approx.)

AI,O, SiO, org. AI,O, SiO, org. Al,O, SiO,

AI,O, org. AI,O, org.

102 8 4.0 Very good Very good

90 8 4.0 Good Very good

90 7 4.0 Good Very good

102 8 4.1 Moderate Good 87 8 3.9 Fair Moderate 85 8 3.9 Fair Fair

org. 62 7 4.0 Fai$ Good2

D: org. 55 6-7 4.0 Fair2 Good2

D: org. 37 7 4.2 Fair2 Good2 org. 24 8 4.3 Fair2 Good2

D: org 22

org.

pH value

(app=)

7

9 9

7-10 5 9

IO

Density Weather Light resistance fastness

4.3 Fair2 Good2

4.4 Very good Very good 4.4 Very good Very good

4.0-4.3 Very good Very good 2.6 Very good Very good 2.8 Very good Very good

2.9 Good Good


White pigments

3 of pigment pastes. It also offers good

dielectric properties - particularly suitable

for electrical and insulation applications.

Zinc sulphide pigments can be regarded

as having low binder requirements, good

rheological properties, resistance to floc-

culation and suppression of floating. In

addition, they have good anti-corrosion

properties and can be compared with zinc

phosphate pigments. Ageing resistance

can be improved in some instances and

weather stabiliry is good in polyamides

and melamine compounds, but limited in

most other plastics. Partially substituting

titanium dioxide pigments with double

the quantity of zinc sulphide does not

impair the brightness, pigment dispersion

or melt flow index of a masterbatch.

However, it can reduce friction and abra-

sion and increase temperature stability,

which means that it can act as a process-

ing aid, particularly in linear LDPE. For

all practical purposes, the pigments are

free of heavy metals.

There are a number of other fine-particle

minerals that offer good whiteness proper-

ties and dispersability and can often be used

as extenders to titanium dioxide, which can

reduce the overall costs. These include alu-

minium silicate, barium sulphate, calcium

silicate and magnesium silicate.

Aluminium silicate

Fine-particle aluminium silicates (kaolin)

have a high specific surface and low grit

content. They are easily dispersed in

plastics, increasing the hardness and

elasticity. Generally, mechanical proper-

ties are lower than for coarser fillers.

Thermo-optic treated grades have high

whiteness and can be used to extend tita-

nium dioxide and other more expensive

pigments. Calcined kaolins have good

electrical properties, low compression set

and low water absorption, for use in

cable formulations.

Barium sulphate

Precipitated barium sulphate or ‘blanc

fixe’ is an inert white filler that is resistant

to acid and alkalis and has good weather-

ing resistance. It does not absorb light

from the ultraviolet to the infra-red range

and therefore has no effect on the bril-

liance of colour pigments. The particle

size range is 0.7-3.0 microns.

Dispersability and lack of grit are high:

hardness and stiffness of plastics are

improved without effect on surface quali-

ty, particularly gloss and colour brilliance.

Barium sulphate is also used to increase

density and X-ray opacity - particularly

for toys and medical articles - and

improves sound insulation values. Special

grades can increase light scattering, with-

out absorption in semi-opaque com-

pounds. Ultrafine particle grades have

been developed as nucleating agents for

partially crystallized thermoplastics.

Natural barium sulphates or barytes are

inert and allow high loadings. Fine parti-

cle grades are generally used to increase

the density of a plastics compound, while

coarse particles can be used in acoustic

applications, for example in automobiles.

Blank fixe micro is a white inorganic

powder for plastics and coatings, compris-

ing barium and sulphate, and is virtually

insoluble in water, organic solvents and

acids and alkalis. Produced from barytes

with the removal of impurities, a narrowly

defined particle distribution can be

achieved.

Its advantages include low binder replace-

ment, ready dispersability, extreme fine-

ness, low agglomerate content and high

gloss in coatings. It can also act as a ‘spac-

er’ between white and coloured pigments,

which can reduce titanium dioxide costs

by 5- 15%, or reducing pigment costs or

raising solids content. Costs can be

reduced by around 5% without affecting

the properties.

Calcium silicate

Needle-like, non-toxic calcium silicate or

wollastonite can be used to reinforce ther-

moplastic and thermosetting resins.

Magnesium silicate

Pure, white platelet magnesium silicate

(talc) is used to reinforce and nucleate

partially crystalline thermoplastics, partic-

ularly polypropylene and polyamide. It is

used more as a reinforcement, giving good

stiffness and dimensional stability.

Further reading: Additives for Plastics

Handbook, Second Edition, by john Murphy.

Published by Eheuier Advanced Technoiog.

winning Polymers with TOG%REXgraph/te powders

0 Electrical Conductivity

0 "C Thermal Conductivity

RES No.014 - USE THE FAST NEW ENQUIRY SERVICE @ www.addcomp.com


Whiter than white: options for the compounder

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