Dispersion of particulate additives in rubber using the ... · additives in rubber using the batch...

Loughborough UniversityInstitutional Repository

Dispersion of particulateadditives in rubber using the

batch operated internalmixer : a study of flow

behaviour and properties ofrubber mixes

This item was submitted to Loughborough University's Institutional Repositoryby the/an author.

Additional Information:

• A Doctoral Thesis. Submitted in partial fulfilment of the requirementsfor the award of Doctor of Philosophy of Loughborough University.

Metadata Record: https://dspace.lboro.ac.uk/2134/10466

Publisher: c© Wan Idris Wan Yaacob

Please cite the published version.

https://dspace.lboro.ac.uk/2134/10466

This item was submitted to Loughborough University as a PhD thesis by the author and is made available in the Institutional Repository

(https://dspace.lboro.ac.uk/) under the following Creative Commons Licence conditions.

For the full text of this licence, please go to: http://creativecommons.org/licenses/by-nc-nd/2.5/

- -- -- -- -- -- - --_. - -

LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY

LIBRARY

AUTHOR/FILING TITLE

_____ ~~~ __ LQBJ §-' __ ~ ___________________ .

-- - - - - -- --- --- ------------- -- - -- --- ----- - - --" -~- ---ACCESSION/COPY NO.

IS:lOS5/0:l ---------- ------- ---- --- --- - -- --- --- ---- -- - - -- - - --VOL. NO. CLASS MARK

Lo~N Copy

26 JUN 1998 - 8 0 C:I.-M~-

25 JUN 1999 24 NOV 1993

-1 JUl

30 JUN 199

- 3 aCT 1997

,------------------------ ~- .

o 0 C <l 0

',./ '

DISPERSION OF PARTICUlJl.1E JlDDITIVES IN PJJBBER

USING THE BATGI (ffM1ED IN1ERNAL MIXER -

A STIIDY OF FLOW BEHAVIaJR .£IM) P~PERTIES OF PJJBBER MIXES

by

WJlN IDRIS \'iJ.IN Y MCOB Chem. Ing., M.Sc.

A Doctoral Thesis

.. ,

Submitted in partial fulfilment of the requirements for

the award of Doctor of Philosophy of the

Loughborough University of Technology.

Loughborough, September 1978

Director: Professor A. W. Birley

Supervisor: Mr. P. K. Freakley . Institute of Polymer Technology

(§} by WAN IDRIS WAN YAACOB (197~)

Loughborough Unl.erslty

of Technolo;;~f Library

Date O.t.]" " Class

Acc. I S2.0~~ I~l, No,

/ To my wife, JamaUah, and my daughters, AUza and

Azlin, for'their support and patience while I was

preparing this thesis.

" ~ .

,

ACKNOAl.E)GEl"8IT

I would like to express ll\Ygt:atitude to Mr. P; K. Freakley for the

supervision, guidance and encouragement throughout Il\Y work.

My thanks are also'due to Professor A. W. Birley, director of the

Institute ,of Polymer Technology, and all the members' of academic and

technical staff for their general advice and co-operation.

Notleast I would like to mention the help given to me' by external

organisations; in particular staff of the Special Projects Group of

the Avon Rubber Company and the staff of M.R.P.R.A.

Lastly, Il\Y thanks to Miss Linda Malins for typing this thesis.

ABSTRACT

As an aid to understanding the mechanics of mixing 1n an internal

mixer laboratory scale trials have been carried out using a mixer having

a transparent plastics chamber. The use of a transparent rubber and

coloured 'markers' then permitted direct viewing of the characteristic

flow patterns deriving from the use of three fill factors.

These mixing trials. have indicated the rheological properties

which must be measured in order to predict the mixing behaviour of a

rubber. Also considerable information is contained in the visualisations

which will aid further work into control and instrumentation strategies

and into fundamental design/mathematical modelling studies.

A labor~tory Banbury mixer and Brabender Plastograph are used to

prepare the rubber compounds which are then characterised for the

dispersion ofoompounding ingredients. For carbon black dispersion

studies several techniques are employed. Capillary rheometry is used

to·study their stress-strain rate relationships and to obtain die swell,

. shear and tensile properties. Creep and elongational tests are also

carried out on uncured mixes. In addition measurements on Mooney

viscometer, Mons ant 0 rheometer and analysis of bound rubber are made.

These tests for filler dispersion are supported by microscopic exam-

ination of microtomed sections. Work is also geared to examine properties

that are not only sensitive to changes in levels. of carbon black

dispersion but also that which are readily measured and can be used in

industry • Measur~ments of mechanical phase angle and electrical

resistivity are considered. Dispersion of non-black compounding ingred

ients is studied by X-ray microradiographic technique and the analysis

of vulcanisate properties.

To relate the performance of the Plastograph and Banbury mix the

concept of mixing energy per unit volume of material is used •

•

" '" ~.

CNIDITS

CHAPTER 1: INTRODUCTION ,

1.1 RUBBER PROCESSING

1.1.1 MIXING SYSTEMS

1.1.2 MIXING MACHINERY

1.1.2.1 MILLS

1.1.2.2 INTERNAL MIXERS

1.1.2.3 CONTINUOUS MIXERS .. 1.2' PROBLEMS ASSOCIATED WITH THE MIXING PROCESS AND

APPROACHES TO THEIR SOLUTION

1. 3 AIM OF WORK

REFERENCES

CHAPTER 2:" DISPERSION OF PARTICULATE ADDITIVES IN 'THE

INTERNAL MIXER ••

2.1 SIMPLE AND DISPERSIVE MIXING

2.2 DEFINITION OF 'DISPERSION' AND 'MIXEDNESS'

2.3 REVIEW OF SOME HYDRODYNAMIC ANALYSIS

2.3.1 MIXER GEOMETRY - ROTOR DESIGN

2.3.2 HYDRODYNAMIC ANALYSIS

2.4 FACTORS AFFECTING DISPERSION OF PARTICULATE ADDITIVES

IN THE INTERNAL MIXER .. 2.4.1 SHEAR STRESS

2.4.2 SHEAR STRA'IN RATE

2.4.3. SHEAR STRAIN

2.4.4 RAM PRESSURE

2.l.f.5 ROTOR SPEED .. 2.4.6 COOLING EFFICIENCY

2.5 MIXING TECHNIQUE •. .. 2.5.1 CONVENTIONAL AND UPSIDE-DOWN MIXING TECHNIQUE

•

Page No.

1

1

2

5

5

6

8

11

16

17

20

20

23

25

25/

28

36

37

39

39

40

41

42

44

44

2.5.2 MASTERBATCHING

2.5.3 HOT MIXING

2.5.4 DEGREE OF FILLING - FILL FACTOR .. , 2.6 MATERIAL MODIFICATION TO'IMPROVE DISPERSION

2.6.1 FILLERS .. 2.6.2 VULCANISATION INGREDIENTS ' .. 2.7 SURFACE TREATMENTS

REFERENCES .. CHAPTER 3: TECHNIQUES OF ASSESSING DISPERSION AND MIX

PROPERTIES

3.1 MICROSCOPY

,3.1.1 OPTICAL MICROSCOPY

3.1.1.1 EXAMINATION OF VULCANISATE SURFACES ••

3.1.1.2 EXAMINATION OF MICROTOMED SECTIONS

3.1.2 ELECTRON MICROSCOPY .. 3.1.3 RADIOGRAPHY

3.2 EFFECT OF DISPERSION ON COMPOUND PROPERTIES

3.2.1 PROCESSING PROPERTIES

3.2.1.1 VISCOSITY

3.2.1.2 VISCOELASTICITY

3.2.1.2.1 CREEP . . 3.2.1.2.2 STRESS RELAXATION

3.2.1.2.3 DIE SWELL •• r

3.2.1.2.4 DYNAMIC RESPONSE

3.2.2 RESISTIVITY

3.2.3 VULCANISATE PROPERTIES

~FERENCES ..

.. '

'"

..

..

Paeie No.

45

47

48

49

49

51

52

54

57.

57

57

57

58

61

62

63

63

64

68

. . 72

73

76

77

77

78

80

;0',,' CHAPTER 4: VISUALISATION OF FLOW WRING TIiE PROCESSING

OF RUBBER IN AN INTERNAL MIXER

4.1 INTRODUCTION

4.2 EXPERIMENTAL , .. 4.3 RESULTS AND DISCUSSION

4.3.1 FLOW VISUALISATION ••

. . ,.

. . ..

4.3.2 PRESSURE VARIATIONS IN THE MIXING CHAMBER

4.3.3 DEPENDENCE OF MIX UNIFORMITY ON FILL FACTOR •.

REFERENCES .' .

Page No.

83

83

84

87

87

103

108

111

~TER 5: TIiE MIXING PROCESS AND CARBON BLACK DISPERSION STUDIES112

5.1 INTRODUCTION .. , 112

5.2 COMPOUND PREPARATION 113

5.2.1 EXPERIMENTAL .. .. 113

5.2.2 RESULTS AND DISCUSSION .. 114

" 5.3 OPTICAL MICROSCOPY 120

5.3.1 EXPERIMENTAL 120

5.3.2 RESULrS AND DISCUSSION .. 121

5.4 CAPILLARY RHEOMETRY 126

5.4.1 EXPERIMENTAL .. 132

5.4.2 RESULTS AND DISCUSSION 132

5.4.2.1 FLOW CURVES .. 132

5.4.2.2 PROPERTIES UNDER SIMPLE SHEAR AND. TENSION 154

5.5 DIE SWELL" .. 163 L


,5.5.2 RESULTS AND DISCUSSION 164

5.6 CREEP MEASUREMENT .. 169

5.6.1 EXPERIMENTAL . . ' .. 169

5.6.2 RESULTS AND DISCUSSION .. . . . . 171

,

<'"

. Page No.·

5.7 .ELONGATION TESTING · . 179



5.8 MECHANICAL PHASE ANGLE MEASUREMENTS 188


5.8.2 RESULTS AND DISCUSSION ... 191

5.9 MOONEY VISCOSITY .. 191 r.

u

5.9.1 EXPERIMENTAL .. . . 192

5.9.2 RESULTS AND DISCUSSION . .. 192

5.10 WALLACE PLASTICITY . . . ... . . 198



5.11 CURE CHARACTERISTICS 204



5.12 BOUND RUBBER MEASUREMENTS 207


5.12.2 .RESULTS AND DISCUSSION 208

5.13 ELECTRICAL RESISTIVITY 212

5.13.1 EXPERIMENTAL .. · . 213


5.14 VULCANISATE PROPERTIES . . .. 217

5.14.1 EXPERIMENTAL . . · . .. ... 217


5.15 MIXING IN DEFINABLE SHEAR FIELD .. 220


5.15.2 RESULTS ·AND DISCUSSION 222

REFERENCES .. 228

•

Page No.

CHAPTER 6: DISPERSION OF NON-BLACK COMPOUNDING

INGREDIENTS 229

6.1 INTRODUCTION .. 229

6.2 EXPERIMENTAL .. 232

6.3 RESULTS AND DISCUSSION 233

REFERENCES .. 238

CHAPTER 7: GENERAL DISCUSSION · . 239

REFERENCES 245

CHAPTER 8: CONCLUSIONS · . 246

APPENDIX I 248

APPENDIX 11 .. 249

APPENDIX I I I 256

,. APPENDIX IV . . · . . . . . 258

.J

..

CHAPTER 1

HITRODUcr I ON

In the early period of the rubber industry it was realised that

raw rubber had limited applications because of its poor properties.

The discovery of vulcanisation, which greatly improved the general

properties of the end product by the addition of sulphur, marked the

beginning of the use of additives in rubber compounds. Today

commercially produced rubber compounds are mostly complex mixtures of

a blend of two or more elastomers and ten or more organlc and in-

organic particulate of li~uid additives l . These compounding ingredients

include:

(i) Filler

(ii) Vulcanising agent

(iii) Accelerator \

(iv) Activator

(v) Softener

(vi) Antidegradent

(vii) Peptising agent

(viii)' Special agent.

The dispersion of these additives significantly influences the

final properties of the vulcanisate - the optimum properties being

achieved at a high level .of dispersion. The process of mixing has the

primary objective of ·incorporating and dispersing the compounding

ingredients .

. 1.1 RUBBER PROCESSING

Processing is the term applied to the variety of operations, to

which the raw elastomer is subjected, to convert it into finished

1

products. The basic steps in rubber processes are:

(i) the mixing of compounding ingredients into the elastomer

or elastomer blend

(ii) shaping of the mixed compound into a semi-finished shape

(iii) the final forming and curing.

1.1.1 MIXING SYSTEMS.

Each one of the three steps mentioned above consists of several

processing operations2 (unit operations) shown schematically in Fig. 1.1.

The first step, which is the preparation of the rubber mix, is by

far the most important due to its influence ori successive processing

operations. It comprises blending, mastication, compounding and mixing

of compounding ingredients, generally in an internal mixer. At the end

of the mix·ing cycle the mix, called at this stage carbon black master

batch (CBMB), is dumped on a mill or a screw extruder followed by a

cooling system and a wig-wag piling3. In the next step, in which a

similar set up is· used, curatives are added to form the final mlX,

Fig. 1.2. This method is called a two-stage mixing technique and is

commonly used in the tyre industry; however multiple-stage systems are

not uncommon.

In contrast, the single stage technique, in which all the mixing

is carried out to completion ln a·single mixer, is also common as it

requires less handling. Variable speed mixing is sometimes used so

that the temperature rise can be checked by the use of lower speed.

Cycle time on this process is similar to the total cycle time on a two

stage process 4• The dump could be on a two-roll mill or extruder.

This system is such that the unit can also be used as a masterbatch

unit and a final mix unit; and any special compound needing two or more

stages can be mixed in this way.

2

w

SACK 1t TIPPER

FlEXICONE BIN ON PALLET

EMERGENCY SUPPLIES OF CARBON BLACK

PACKET FEEDER

BLACKS WHITES SMALL POWDER BINS

~~r ~~D AA ---.,., _____ _ SCALE ON RAILS MANUALLY WEIGHED POWDERS (IN BINS WITH SACK TIPPERS)

r-- _. - o· ',MINERAL OIL

: i :~~Kp T:~~':~~~LG '-- •••• _J IV CUSTOMER

~~ PUMP

lOlLS IN : HEATED TANKS , , Lo ••

~ ~ -RAM UNiTS

LJ(a=~t4o~ .-"--'v..- ,--~I;;E~~ 2.ADOITIO~N"S=_-6)-_' AU!O. WEIGHSCALES CONVEYOR INTUIMIX \ INJECTORS INJECT RAM DOWN PUMP FEt:DER (IF REOO.) ./

.. ~m:ll' OR DUMP feSTOON ER SHEET OR WIG.WAG EXTRUDER COOLER STACKER

•

MIMIC ~ e V 'OWE

INDICATING" RECORDINQ

Aura CONTROL OF WEIGHERS" MIXER fUNCTION·

PANIL IN CONTROL ROOM

Oil SCALE

G (2) TIME TEMPERATE (0- C.NDrCA.tlNG)

10 MIN.)

MANUAL CONTROL OF WEIGHER DISCHARGE" MIXER fUNCTION

PANEL AT MIXER

FIG, 1.1: FLOW DIAGRAM FOR A SINGLE STAGE MIXING SYSTE~l (FRANCIS SHAW TECHNICAL LITERATURE)

, MASTICATION 2a MASTERBATCH 2b FINAL MIX

natural rUb~Vr synthetic additives --:::::.. rubber (number

as required) n :: ciF':i j~ J~ 1·3 17tft\Jl!1IT1 2a.1 ~M\lfw1 fl2b'1 ~~m1 1-4 ·Ffu$F4'il['Ul1.. ~-1li1 2b·2 r~tfiJf;ra

2a·2 J ; 2b3r}j~Q 2 ~: f1Yl a·3/La b!_=<=L-=L..

2a'3b~ . ,r.1 2b4~\,m.,

additives (number as required)

FIG. 1.2: FLOW DIAGRAM FOR A MULTIPLE STAGE MIXING SYSTEM4

Many mixing studies have been made using a single stage system,

though there may be a small number of mixes which·cannot be prepared

by this?method; there are also some difficulties in obtaining adequate

dispersion.

After the mixing stage the mix is then cut, extruded or calendered

into a semi-final shape. The building operation may follow where the

shaped rubber component is assembled with other types of components

produced from other rubber m~xes. Final shaping and curing is carried

out during vulcanisation.

1.1.2 MIXING MACHINERY

Reviews of current mixing and other process~ng equipment have been

made by Peakman3 and Ottenheimer5 •

1.1.2.1 MILLS.

The mill.is the oldest of all the rubber. processing machinery

remaining substantially unchanged to the present day. It consists of

two horizontal rolls parallel to each other. The distance between them,·

the nip, is adjustable. The speed of the two rolls are often adjusted

to run with a friction ratio, the magnitude~of which depends upon the

flow properties of the rubber. 'cooling of the rolls is achieved by --either having the coolant flowing through the core axially drilled or

through a labyrinth of passages drilled on the periphery of the rolls

(drilled rolls) ..

In the milling operation rubber is dropped onto the top of the n~p

and is allowed to band on the front roll; at the same time the nip ~s

adjusted so that a rolling bank of rubber is formed above the nip.

Compounding ingredients are added to the rolling bank in the desired

5

order. The bands of rubber compounds are then cut with knives and

cross-blended. Finally it is refined with a narrow nip - a process

aimed at breaking down the remaining agglomerates and further increasing

the level of dispersion by increasing the shear stress.

The mill is also used to handle a masterbatch discharged from an

internal mixer or to feed a rubber compound to a screw extruder.

Problems inherent in mill mixing are manifold and are related to

the behaviour of the elastomers on the mill. Crumbling of raw rubber

or masterbatch will not allow the formation of a smooth band around

the mill,' Instead, the stock will drop to the" floor under the mill.

Sometimes the compound tends to 'bag'. Certain masterbatches are

difficult to pass through the nip thereby delaying production. with

oily and sticky masterbatches, removal from the rolls will be difficult.

1.1.2.2 INTERNAL MIXERS

Since the intrOduction of Thomas Hancock's pickle in 1830 the

internal mixer has been used not only for masticating rubber but also

for mixing compounding ing"redients. Today it is acknowledged as the

most versatile and rapid mixer with large throughput. There are a

number of internal mixers commercially available, notably the Banbury

mixer, the Shaw Intermix and the Werner & Pfleiderer mixer.

The basic design of an internal mixer is schematically shown in

"Fig. 1.3. It consists of principallY two horizontal contra-rotating

rotors with w1ngs or nogs enclosed in a mixing chamber. The Banbury

mixer rotors run at a-small speed differential (1:1.2) and the working

zone is" mainly between the rotor surface and the chamber wall. The

high shear region between the rotor tip" and the wall is responsible

for most of the dispersive ~ction. The Francis-Shaw Intermix, however,

has the rotors running at even speeds with the nogs designed to produce

6

WEIGHT CYLINDER··--.-

rEED MODULE

FLOATING WEIGHT---~IJ,,~

rEED HOI'PE;R---+l-JI.--'-:"-

DRILLED

ROTORS ---i---t;H;<;?;/t'\

DOORTOP --+r-;;r~~~~~~ DOORTOP ~ SUPPORT

LOCKING ----=~~~~rT=::::!~.-l[-~;i MECHANISM-

Cross Section of F270 Mixer

FARREL-BRIDGE

BANBURY

FRANCIS SHAW INTERMIX

FIG. 1.3: SCHEMATIC DIAGRAMS OF INTERNAL MIXERS

7

a friction ratio between themselves. The material to be mixed 1S con

tained within the mixing chamber by pressure applied through the ram.

A swing or drop door is positio~ed at the bottom of the chamber to

permit, rapid discharge of ,the mi",ed batch.

Internal mixers are available",in a variety of sizes which are

classified according to th'e', v;ol1iJiJ.e of the 'mixing chambers. A useful

tabulation of the characteristlcs of a number of ,internal mixers of

various sizeS has been made" by' Palnigren6 .

The rotor speed and ram thrust can be varied. The rotors and the

chamber wall are cooled by wat~r. In larger mixers the ram and the

discharge door are also cooled,. 'Cooling efficiency has recently been

improved significantly by the use of spray, jet, and drilled sides7 .

The choice of mixing condit,ions, such as rotor speed, ram pressure,

mixing cycle and batch size depend upon the rheological properties of

the raw rubber and the additives used; and determine the power require

ments. High speed mixing is limited by the problem of excessive heat

build-up, heat dissipation and a possible scorch of stock. Large batch

size of stiff stock can, coupled with high rotor speed and ram pressure,

cause overloading of power supply and excessive stresses on m1x1ng

equipment, possibly rupturing the m1x1ng chamber. Gross heterogeneity

of rubber blends and mixes from batch to batch and poor incorporation,

, dispersion and non-uniform distribution of ingredients are common

problems encountered in internal mixing.

1.1.2.3 CONTINUOUS MIXERS

Early attempts to replace the batch m1x1ng by a continuous mDang

process employed internal mixers and a series of mills. Currently there

are only a'few continuous mixers available. The Farrel continuous

mixer 1S the first of the series to be introduced. Basically it is a

8

lubrication tor dust seolS

\._./

Fud

IStotk cuLler~

Discharge orillce

FIG. 1.4: FARREL CONTINUOUS MIXER

FIG.-1. 5: DIAGRAMATIC SKETCH OF TRANSFERMIX

9

tvin-screv extruder vith the screvs lying side by side 1n a single

barrel (Fig. 1.4)2.

The Transfermix or the Shearmix vas later introduced to both m1X

and extrude'_rubber compounds. It is similar to extruders to the extent

that there is a single rotating screv inside-a barrel (Fig. 1.5). The

smooth barrel of the extruder produces some relative motion of adjacent

parts of the stock mass - e.g., vortexing or -rolling in flights, and

shear betveen the stock adjacent -to the barrel and that in the screv -

vhile a large part of the material receives little or no vorking.

Extruders are, therefore, mainly used to convey and re-form the stock.

The barrel of a Shearmix, hovever, is not a smooth bore and the

screv design is quite different from that of a conventional extruder -

i.e., the-grooves diminish along a portion of the screv's length until

they disappear completely. As this occurs, an opposite hand thread

develops in the barrel. Thus the stock, still mixing forward, is

transferred from the screv to the barrel. In the next stage the barrel

thread gradually diminishes and disappears and the thread again develops

in the screv so that the stock transfers back from the barrel to the

screv. This transfer can be repeated for any desired number of stages.

In the narrov cylindrical gap betveen the screv and the barrel,

the stock - as it advances from the hopper to the discharge end of the

extruder - is forced to pass through this cylinder or shear-surface

each time the screv or barrel thread disappears. In so doing, the

-stock is subjected to a highly intensive shearing action. At the same

time, there is a less drastic shear in both the screv and the barrel

grooves, together vith- some rolling and bending.

The capital cost for such equipment is very high but very high

outputs can be achieved and automation is possible, coupled vith clean

operation. Hovever, lack of versatility due to their non-self cleaning

characteristics betveen successive batches presents problems.

10

1.2 PROBLEMS ASSOCIATED WITH THE MIXING PROCESS AND APPROACHES TO THEIR.

SOLUTION

The mixing of particulate additives into a matrix material of

high vi~cosity, such as rubber, is a complex mechanical operation. To ., .

break down the particulate aggregates to. a degree of subdivision

necessary to confer the desired physical properties on the finished

product it ·is necessary to subject the mixture to very high shear

forces. During the elastomer processing stages the dispersion of

additives, particularly flllers, is the most important single factor

for the level and consistency of the general properties of any particular

compound; mixing must, therefore, be considered as an extremely

critical process. It is the rate-determining step, as the level of

filler dispersion, the amount of work done on:: the rubber mix, the

molecular breakdown and the change of molecular weight distribution

occuring during the m~x~ng process greatly influence the flow properties

of the rubber compound and hence the intermediate processing stages.

Being the most complex, variable and'intangible in rubber technology

the subject of mixing is then the least understood of all. Although

some attention has been given to this subject progress has been very

·slow, to the extent that the mills or internal mixers are still used as

they were when they were first introduced about a century ago. Initially

fillers were used indiscriminately, primarily to cheapen the product

with little consideration to the properties of the mixed compound and

the vulcanisate. Later work has been confined to the development of

chemicals and balancing the physical and chemical properties of the

final product, while neglecting the flow behaviour of the intermediate

'compound. To some extent the state of mixing of today is still more o:f

an art than a science.

It ~s only in recent years that noticeable attempts have been made

to systematically study and elucidate the complex mixing process. The

changes seen in the rubber world in the past few decades can be con

sidered to be one of the main reasons for this new interest. The range

of various types of rubbers, fillers and other additives has widened

considerably - each with differing processing characteristics. The

introduction of new or modified synthetic rubbers present greater prob

lems in mixing and compounding than do natural or the earlier synthetics.

The new carbon blacks of low structure were found to incorporate less

readily than the normal and high structure blacks.

In addition the rubber industry has increased In importance and

size. As evident from the ever increasing world consumption of raw

rubber its use and application has greatly widened and has penetrated

into almost every industry. In such a highly competitive industry with

high capital investment rubber processors are under constant pressure

to optimise production. This can be achieved by the development of com

pounds obtained by using the relevent data from rheological studies

combined with better understanding of mixing parameters, which would lead

to trouble-free mixing and efficient production of mixes with minimum

rej ects.

The incorporation and dispersion of the various types of additives

In rubbers of varying flow properties and further processing of the raw

stocks involve the interrelationships betweenthe properties of the

polymer, filler, softeners and conditions of mixing. Any attempt to

tackle the problems involved in converting raW rubber into finished

products, therefore, requires a basic study of rheology, viscoelasticity

and the mixing processB•

A good and comprehensive review of rheology of elastomers and their

processing was given nearly a decade ago by White9 while Studebaker and

BeattylO,ll recently discussed the raw stock properties that affect

12

-

-

mixing, and their measurements. The flow behaviour of a rubber m1X

which is highly non-Newtonian 1n character, greatly influences the

suitability or ease of mixing (it also affects the other processing

'stages). Power consumption and heat build-up and transfer are the

variables which are governed by the rheological properties. But most

important of all the molecular breakdown, the change in molecular weight

distribution and the state of dispersion of additives in the compound

significantly affect its rheological behaviour. Thus, the Mooney

viscometer has been widely used to measure 'the viscosity of the raw

stock but the measurements are empirical and limited to low shear rates.

It is then unable to predict factory processing conditions where higher

shear rates (10 - 10000 s-l) are involved. A more versatile instrument,

from which viscosities and shear stresses over a wide range of shear

rates can be obtained, is the capillary r~eometer. Since viscosity is

dependent on shear rate it is a powerful tool for rheological studies.

However, the flow behaviour of a rubber mix is governed not only

by the viscous but also by the elastic component 12. The resultant of

these components create a shear stress which can be related to a viscosity

at a partic~ar shear rate. The elastic component is such an important

factor in processing that the Williams recovery test is sometimes found

to give more useful results than the Mooney viscosity tests, which

reflects primarily the viscous component only l3.

The multiple phenomena occuring during mixing either singlY or

concurrently make a proper definition for a "satisfactory" mix difficult.

Rubber processors are left with nO complete solution to the question as

to when mixing is complete or what properties or criteria are associated

with a satisfactorily mixed compound. The limited capability of Mooney

viscometer and Wallace plastimeter by providing only with empirical data

emphasise the need for a more relaible test method 14-19

13

It is important that testing of compounds must be carried out

under the same conditions as they will encounter in processing. For

this purpose Brabender Plastograph (Plasticorder), which is a miniature

internal mixer, is, therefore, a useful instrument to compare rubbers

and mixing procedures; it does not, however, measure rational rheol

ogical properties mainly because the material is deformed at a spectrum

of shear rates 20 •

Rheological instruments to accurately measure processibility of

rubbers must be capable of measuring their dynamic flow properties in

terms of viscosity and elasticity and their dependence on a wide range

of shear rates and temperatures. Of recent developments are the

Mechanical Spectrometer and Viscoelastic Tester 21,22, and the Dynamic

Stress Relaxometer 23 which are capable of measuring the polymer viscosity

and elasticity; they, however possess the same inherent deficiency as

the Mooney viscometer in that they both operate at low shear rates.

More promising are the R.A.P.R.A. Processibility Tester 18,24 and the

Monsanto Processibility Tester 19 ,25, both of which measure the stress

relaxation characteristics of the material; the latter also measures

the die swell. Work in this area is also being carried out by the

Avon Rubber Co., U.K., in conjunction with Sondes Place Instruments.

To explain the visco'elastic behaviour of uncured rubber compounds

various models were presented, namely the Maxweli and the Voigt-Kelvin

models. Dove et a126 and Turner et a1 27 have recently used a power law

viscosity Maxwell element to describe the viscous and elastic behaviour

of unvulcanised rubber and, with some success, correlated them with

stress relaxation and creep measurements. This has further been

developed into a model consisting of two power-law Maxwell elements in

parallel in order to predict transient and recovery characteristics.

One of, the major problems confronting rubber processors is' the

14

scale-up process. Many of the laboratory trials to test formulations

and choose mixing variables ar'e carried out on a mill or in a small

internal mixer. Seve~al workers 28 - 31 suggest the use of power

integrator and energy-controlled mixing to scale-up the'mixing process.

However, evidence to support the validity of this method is still

lacking and mixing time or temperature r,ise are still used as 'dump'

criteria.

Towards fundamental understanding of the dispersion process con-

siderably less effort has been expended. A few hydrodynamic analyses

of internal mixers have been presented by Bergen32 , Bolen and Colwcl1 33 ,

Guber34 , Udal'tsov35 , and Funt 36 • All of the, analyses suggested are

based on many assumptions, due to the complexity of the mixing mechanism.

Bergen assumed that Newton's law is valid and that isothermal conditions

persist. In addition to these assumptions Bolen and Colwell considered

the drag and pressure flows around the rotor tip to dominate; while

Guber used the whole 'sickle-shaped' zone in front of the rotor wing

and treated it as a region of high shear. Whil~ certain assumptions

being made here are essential to simplify the analysis)the flow

mechanism prevailing in an internal mixer needs a mor'e detailed

practical study before a more valid analysis can be made.

Empirical information regarding mixing and dispersing of ingred-

ients in rubber however have been 'widely studied10 ,11,28,37,38,39 , ,

although the conclusions drawn from most of this work are restricted

to the particular systems under investigation.

Without undermining the importance of fundamental research it

must be recognised here that rubber industry's main need is for that

work which is directly relevant to processing and which can be used to

'significantly' improve a particula.rprocess or ,machine. Since large

scale production is always associated with high capital investment

15

','

sound economic justification is needed before any change in the current

process can be made. Economics rather than improvement alone is the

decisive factor.

1.3 AIM OF WORK

The aim of this work is, therefore, to try and rationalise some

of the complex problems associated with mixing of rubber in an internal

mixer and study the pertinent rheological properties of the filled-

. compound. Filler incorporation and the mixing mechanism in an internal

mixer are studied by.making direct observatio~ of the flow patterns of

a model compound in a transparent chamber. Tlie various modes of

deformation occurring, in the mixer are examined and are correlated "ith

the related fundamental rheological properties. The effect of batch

size on mixing efficiency is analysed.

The study of the dispersion of particulate additives are conducted

by preparing a series of rubber mixes, by a single stage process, based

on OESBR and OEHR and HAF carbon black in a Banbury mixer and Brabender

Plastograph with different mixing times. The rheological, optical

electrical' and mechanical properties of the rubber mi~ are also studied.

These.properties are correlated with each other and with a mixing

parameter. The possibility of intrOducing new and rapid methods of

testing the quality of the mixes is also examined.

16

REFERENCES

1. Windspear, G. G. (Ed.); "The Vanderbilt Rubber Handbook", pub. by

R. T. Vanderbilt Co. Inc., New York, 196B.

~ParShal, C. M., and Saulino, A. J., Rubber World, 156 (2), 7B

(1967).

3. Peakman, M. G., J. LR.L, .4 (1), 35 (Feb. 1970).

).7 Whitaker, P., J. LR.I., !!. (4), 153 (Aug. 1970).

5 .. Ottenheimer, R. J., A;C.S. Rubb. Div. Meeting, Chicago, U.S.A.,

May 1977.

6. Palmgren, H., Rubb. Chem. Tech., 42 (1), 257 (1969).

7. Ellwood, H., I.R.I. Rubb. Proc., 2nd Ann. Conf., Blackpool,

U.K., May 1974.

B. Johnson, P. s.; A.C.S. Rubb. Div. Meeting, Chicago, U.S.A., May

1977 .

9. White, J. L., Rubb. Chem. Tech., .42 (1), 257 (1969).

10. Studebaker, M. L., and Beatty, J. H., Rubb. Age .1Q!l,(5), pt. I,

21 (May 1976). loB (6), pt. II, 21 (June 1976).

11. Beatty, J. R., and Studebaker M. L., Rubb.Age loB (11), pt. I,

21 (Nov. 1967). loB (12), pt. II, 27 (Dec. 1967). ..

12. Borzenski, F. J., A.C;S. Rubb. Div. Meeting, Chicago, U.S.A.,

May 1977.

13. Nanomiya, K., and Yasuda, G., Rubb. Chem. & Tech., 42 (3), 714 ;;,.'

(1969).

14. Wood, E. C. F., Inter. Rubb. Conf. Brighton, May 1972.

15. Kontos, E. G. Rubb. Chem. Tecn., 43 (5), 10B2 (1970).

16. Tokita, N., and Pliskin, I., Rubb. Chem. Tech., 46 (5), 1166 (1973).

17. White, J. L., Rubb. Chem. Tech.,.2Q.(l), 163 (1977).

lB. Berry, J. P., and Sambrook, R. W., I.R.I. Inter. Rubb. Conf.,

Brighton, U.K., May 1977.

19. Barker, R. I., Hanna, G,.L., and Dodger, E. L., A.C.S. Rubb.

17

Div. Meeting, San Francisco, U.S.A., Oct. 1976.

20. Harwood, J. A. C., in "Rubber Chemistry and Technology", ed. by

Blow, C. M., Newnes-Butterworths, London, 1971, Chapter 3.

21. Willey, S. J., Davis, W. M., and Macosko, C. W., Trans. Soc.

Rheo.,18 (4), 515 (1974).

22. Mukherjee, D. P., Poly. Eng. Sci., 11 (11), 788 (Nov. 1977).

23. Moghe, S. R., Rubb. Chem. Tech., .!±2. (2), 249 (1976).

24. Watson,W. F., Rubb. Ind., 74 (April 1975).

25. Hanha, G. L., Barker, R. I., and Rodger, E. R., A.C.S. Rubb. Div.

Meeting, Chicago, Illinois, May 1977.

26. Dove, R. A., Turner, D. M., and Martin," T., LR.L Inter. Rubb.

Conf., Brighton, U.K., May 1977.

28. Bourne, J. R., New Scientist, 11, 334 (1967).

29. Van Buskirk, P. R., Turetzky, S. B._, Gunberg, P. F., Rubb. Chem.

Tech., 48 (4), 577 (1976).

40.. Turetsky, S. B., Van Buskirk,. P. R., Gunberg, P. F., Rubb. Chem.

Tech., 49 (1), 1 (1976).

31. Dizon, E. S., Rubb. Chem. Tech., J±2 (1), 12 (1976).

32. Bergen, J. T., in ·"Processing of Thermoplastic Materials", ed. by

Bernhardt, E. C.; Van Nostrand Reinhold Co.·, New York (1959),

Chapter 7. tlf '

33. Bolen, W. R., and Colwell, R. F., S.P.E. Tech. Papers, IV, 1004

(1958).

34. Guber, F. B., SOY. Rubb. Tech., 26 (1), 23 (1967).

35. Udal' tsov, V. V., Vostroknutov,· E. G., and Novikov, M. L, Sov.

Rubb. Tech., 31 (6), 10 (1972).

36. Punt, J. M. "Mixing of RUbbers", Pub. by R.A.P.R.A., Shawbury, U.K.,

1977.

37.' Jones, H. C., and Snyder, E. G., Ind. Eng. Chem. 43, 2602 (1951).

18

38. Drogin, I., Rubb. Age, ~ (5), 791 (1961).

39. Boonstra, B. B., and Meda1ia, A. l., Rubb. Chem. Tech., 36 (1),

115 (1963).

.19

-CHAPTER 2

, , DISPERSION OF PARTICULATE ADDITIVES IN TIiE INTERNAL f:1IXER

Having discussed the various types of rubber m~xers and the

mixing systems 'it is essential to define the modes of mixing prevail-

ing in the mixer, what is re~uired of a mixing system and the various

factors which affect dispersion.

2.1 SIMPLE AND DISPERSIVE MIXING

A mixture is composed of a major component, which has the higher

over-all concentration, and a minor component. The objective of mixing

is to disperse the particles of the minor component in the medium of

the major component.

The term simple'mixing ~s used to des,cribe any operation which

'results in an increase in the randomness of the spatial distribution

of the minor component particles without reducing their size. The

term dispersive mixing is applied to those mixing processes in which

the s~ze 'Of the minor component is reduced to its ultimate particles

and their position randomized by a certain magnitude of force.

Many simple mixing processes occur spontaneously or naturally

by molecular diffusion process, such as with gases Or miscible li~uids,

although in the latter case a longer period is re~uired. In non-

diffusive mixing, as with high polymers, the components must be set in

motion by some external force. With'a fluid or gas as the maJor

component turbulent mixing is the usual mode of operation. In systems

with high viscosity fluid as a major component, which is the case with

rubber compounds, it is not possible to achieve turbulence; only

laminar mixing, which proceeds at a slower rate, can be obtained l .

20

In mixing particulate additives in rubber a number of elementary

steps are involved2 , as illustrated in Fig. 2.1.

(i) Subdivision of large lumps, pellets or aggregates to smaller

ones , suitable for easy incorporation in the, rubber. This

first step takes place as the additives are compacted and

sheared by forces generated in the rubber.

(ii) Incorporation of the powdered (or liquid materials) into the

rubber to form soft agglomerates which contain the elastomers

and the additives in about the proportion of the void ratios as

determined by oilabsorption3• The reduction of voids in

particulate additives minimises compressibility of the stock

resulting in higher force being applied to it. The incorpor

ation stage is important for effective mixing to take place.

Loose ,ingredients would only be tumbled around.

(iii) Dispersion finally takes place as the incorporated agglomerates

are further broken down to their ultimate'sizes, i.e. dispersive

or intensive mixing.

(iv) Simple mixing may take place at the same time as dispersive

mixing and continue throughout the mixing process. It is also

called extensive mixing.

(v) ,Viscosity reduction by mechano-chemical breakdown of the

polymer and transforming it to a state where flow can take

place more easily with less elastic recovery. Although this

step is not necessarily linked with the mixing itself it is

significant for the properties of the mixed material.

21

-

C0J .. . ::_ ..... : ... . .. . . . . . . . . . . .

~Ot) Q~O . 000" . Q INCORPORAT ON ODOo~ ODCJO 000 000 00(/ 0(1

~ ______________ J~

o

,

DISPERSION

-+

SIMPLE -MIXING

l> 0

. FIG. 2.1: STEPS ,IN RUBBER MIXING PROCESS , ,

22

o D

o 0

o 0

o 0 0

o o o

..

2.2 DEFINITION OF 'DISPERSION' AND 'MIXEDNESS'

The first requirement of any mixing system is output, which ~s

determined by the size or the batch weight of the mixer and the

cycle time. Other requirements are that the system will work with

minimum of labour and supervision and that it has a minimum capital

cost subject to providing the required mixing performance. But the

most important parameter of all is satisfactory mixing, which is

usually defined by carbon black dispersion.

Like many other technological terms 'dispersion'is vaguely defined

and the concept is not well understood. In its strictest sense

dispersion is the homogenous distribution of a minor component in a

major component. According to ASTM D-2663 it is defined as the state

of separation of agglomerates into discrete aggregates. Carbon black

agglomerates are loosely held clusters of aggregates, which are a more

tightly bound carbon black unit. While the level of dispersion,

particularly of fillers, influences the properties of the final

vulcanisate, increasing dispersion does not always result in improved prop

erties. Although many optimum properties are. achi'eved at high degree of,

dispersion of the additives some desired properties are also obtained

at less than perfect homogeneity.

The degree of 'mixedness' must be defined in order to evaluate

the quality of the mix or the 'efficiency of a mixing process. Because

of the large number of ultimate particles a mixture is composed of

statistics has been used in describing the state of the mixture.

The different states of a mix containing two components are

illustrated in Fig. 2.2 a-d. Fig. 2.2a shows a completely unmixed

state. Fig. 2.2d shows a perfect mix while in Fig. 2.2b and c can be

seen a varying degree of mixedness. A mix is perfect when one com

ponent is randomly distributed in the other. However random

23

Q)

rI.l

~

'tl I !il ~

~

~ I

r"I

0 ~ ()

~ 0 H rI.l

~ ~ I

~ .. N

N

<!>

~

24

distribution is not un1que as can be seen in Fig. 2.2e which is also

a perfect m1X. The degree of m1X 1S rated by calculating the mean

and variance l . If N samples are taken from a mixture consisting of a

particulate minor component, whose volume concentration is X, and a

fluid major component the

Mean, X =

and the

Variance, S2 = N

1 E

N-l i=l

(2.1 )

(X· - X)2 _1 (2.2)

The experimental results can be compared with the theoretical

variance (a 2 ) of samples taken from an ideally perfect mix.

= p(l - p)

n (2.3)

where p 1S the fraction of the minor component,

n 1S the total number of particles in all.

The ratio m = a2 is the index of degree of mix. The variance, s2, ;z-

approaches a limiting minimum value, a2 , as the mixture-approaches a

random state.

2.3 REVIEW OF SOME HYDRODYNAMIC ANALYSES

2.3.1 MIXER GEOMETRY - ROTOR DESIGN

~ The most -important components of -an internal m1xer are the rotors)

The principle of their design was inherited from the traditional two

roll mill.~When mixing on a mill it is necessary to cut -and turn the

25

1 • ~ ( t

a) DANBURY 2-WING

r

b) BANBURY 4-WING

FIG. 2.3: ROTOR GEOMETRY OF A FEW INTERNAL MIXERS

26

c) WERNER PFLEIDERER

d) SHAW INTERMIX

27

rubber since the lateral shearing produces a m1x1ng action 1n the

circumferential direction only, with no mixing through the thickness

of the rubber sheet. Rotors of the internal mixers are provided with

wings or nogs to perform the task of the mill operator ..

Fig. 2.3 shows examples of a few rotor designs. The Farrel

Bridge Banbury type non-intermeshing rotors have two basic designs:

the two-wing and the four-wing, the choice of which is governed by

the polymer being mixed and the power available. Four~wing design is

claimed to give more mixing work, and hence more. throughput, and

reduce energy cons·umption per volume of mix4. The rotor blades are

elliptical in cross-section. As the rotors contra-rotate·the material

in front of the tip is forced into a steadily decreasing space. The

material flowing between the tip of the rotor and the chamber wall is

subjected to high shear deformation and undergoes dispersive mixing.

The wings 'pump' the material from the ends to the centre section,

ostensibly important for extensive or simple mixing.

The Francis Shaw Intermix have their rotors featured with heavy

projections,· one large and two smaller, on each contra-rotating rotorS.

These projections or·nogs are designed to intermesh as the rotors

contra-rotate at even speeds, creating high shear zone not only in

the narrow clearances between the rotor surface and the chamber wall

but also at the point of intermeshing. Thenogs are also part helical

in shape and opposite·handed so that the mix is constantly passed back

and forth along the length of the mixing chamber. This design creates

a larger rotor surface area making more efficient cooling possible.

2.3.2 HYDRODYNAMIC ANALYSIS

Mixing of rubber in an.internal mixer is undoubtedly an extremely

complex process, so that a complete hydrodynamic analysis is still

28

unfeasible. In the batch operated internal mixer conditions are very

rarely constant and not easily defined. The few studies reviewed

recently by Funt 6, that have been carried out· so far are therefore

made using several conditions and assUmptions to simplify the model

and the mathematical treatment.

The complicated construction of the rotors contributes to a

large degree to the complex conditions prevailing in the mixing

chamber. In the nip region of intensive shear, the·approximation of

an ideal viscous fluid is relatively acceptable, i.e. Newton's Law is

obeyed. In other reg~ons, where a spectrum of shear rates and shear

stresses exist, whose .magnitudes al'e much lower than those in the nip

region, the viscous and elastic properties of real materials dominate.

Isothermal condi~ions can only be assumed though it is not true.

The viscosity of the rubber is high enough to generate heat during its

defol'mation process and this significantly influences the rheological

behaviour of the mixing compound.

All of the analyses are restricted only to the small n~p region

or the sickle-shaped zone in the mixing chamber. Other regions, which

are also responsible for other modes of deformation and contribute

significantly to the· mixing action, are not considered. Furthermore

axiai flow is also ignored. Complete filling of the mixing chamber is

also assumed. This is seldom true as voids in the mixing chambers are

important for efficient mixing process.

One of the first investigations into the hydrodynamics of

shearing, published in Bernhardt's book7, restricm the analysis to

the nip region of the Banbury mixer, ~.e. between the rotor tip and

the. chamber periphery, where most of the defol'mation and dispersive

process is assumed to occur (Fig. 2.4a). Other working regions are

neglected. Assuming that Newton's Law and isothermal conditions are

29

a

b

c

SICKLESHAPED ZONE

I v- I

j--b--l

FIG. 2.4: INTERNAL MIXER·MODELS USED FOR HYDRODYNAMIC ANALYSIS

a: Bergen7 , b: Bolen & Colvel18 , c: Guber9

30

valid, the volumetric drag flow rate Q lS glven by

where h = the heisht of the constant depth channel and

u = peripheral speed.

The shear', str'ess T is

T =

where n = viscosity of rubber

(2.4)

(2.5)

The power transmitted to the blade surface of length L and blade-tip

width b lS

(2.6)

The shear output is directly proportional to the blade speed and blade

tip width. As the ,shear output acts per unit time on a volume of

material Q,the specific shear output 0 (shear output per unit volume)

lS more useful:

Similar treatment is also used for a tapered channel.

Bolen and Colwel1 8 used an idealized dispersive mixer (Fig.

2.4b) to approach the problem in a different way. In front of the

rotor tip pressure is developed as the rotor blade pushes the

material forward. Behind the rotor tip exists a low pressure zone as

the material is dragged away. The nip offers a region for the material

to flow from the high to low pressure zones - this tip flow is the

resultant of the pressure and drag flows. Assuming that the material

is incompressible and neglecting leak flow in the nip, the material,

balance is given

Ql + Q3 = Qz + Q" (2.8)

Where Ql = drag flow in channel %

Q2 = drag flow in nlp

31

Q3 = pressure flo", ln channel ) ) ~

Q4 = pressure flo", at tip )

Each of the drag and pressure flo",s can be

Ql = 1TDc shN/2

Q2 = 1TDtsgN/2

Q3 = -sh311p/121TDc )lc

~ = sg 311p/12e)lt

Where g = tip clearance

h = channel clearance

e = tip ",idth

s = length of rotor

N = rotor speed

Dc = diameter at shaft

Dt = diameter at tip

)lc = viscosity of fluid in channel

)It = viscosity of fluid at tip

tJ' = pressure drop across flight'

calculated:

(2.9)

(2.10)

(2.11)

(2.12)

Other models have also been discussed ln the literature.Guber9

considered the ",hole sickle-shaped zone in front of the rotor "'ing.

From the pressure investigations he suggested that high pressures are

not only restricted to the tip region alone but over the ",hole of this

sickle-shaped zone (Fig. 2.4c). Dividing this zone in small, elements

the shear rates'y can be calculated from the velocitiesvofthe points

of the elements on the rotor surface and the varying clearances h., \

i.e. assuming no slippage.

= (2.13)

The shear stress can be obtained from the po",er la'" equation.

, 32

.-

While acknowledging that the deformation of an anomalous-

viscosity material in a rotor-type internal mixer with very complex

geometry of working elements does not permit complete hydrodynamic

analysis Udal'tsov et allO proposed to examine the mechanics of

deformation of mixes in the surface layer adjacent to the periphery

of the mixing chamber (Fig.- 2.5)' Thus the flow profile (Fig. 2.6),

composing of drag and pressure flows, In the sickle-shaped zone was

considered. Neglecting the effect of material resilience, compress-

ibility and non-isothermal conditions it was assumed that in the flow

of a mix the hydrostatic pressure p changes only in direction x,

while the rate ;s changes only in direction y.. They then obtained

the equation:

= 21TRn

h + h E.l?

211 al<. (2.14)

so that multiplying both parts of Equation (2.14) by 11 Equation (2.15)

is obtained:

=

or =

~II h

+

+ h E.l? 2 ax (2.15)

(2.16)

where Tw = the shear stress in the layer adjacent to the chamber wall

Td = the shear stress due to drag flow

Tp = the shear stress due to pressure flow

From Equation (2.15i:it can be seen that although Td increases,

with decreasing value of h along angle ~p' Tp' with a constant

pressure gradient (ap/ax), decreases. The net result is that Tw

remains almost constant and seems independent of the angle of rotation

around the rotor. Their experimental measurements of shear stress

confirmed this argument, which is analogous with another experimental

33

y

+---t> x

FIG. 2.5: DIAGRAM OF DEFORMATION OF A MIX IN AN INTERNAL MIXER WITH

OVAL ROTORS. The surface of the .chamber with radius R1 has the centre

at 01 while the working surface of the rotor blade with the shape of

an arc has a radius R2 and centre 02. h is the varying nip clearance

in the sickle-shaped zone; the maximum size of the nip is H and the

minimum is ho . ~p is the angle subtended at the centre by the zone·

under consideration with the pressure oscillating between the minimum

Po and the maximum Pmax. R· =

34

ho

2

w Vl

GRAn P t>-

v P

y :::: 2rrRn <l

GRAD P -=:1>-------

CHAMBER WALL

t

ROTOR BLADE

FIG. 2.6: PROFILES OF THE RATES OF FLOH OF MATERIAL;rN THE SICKLE-SHAPED ZONE NEAR THE CHAMBER WALL (OPENED-OUT PRESENTATION)

.c

observation that shear stress is independent of rotor speed.

The design parameters of the mixer and the shear stress pre-

vailing in the material are related to the torque on the rotor shafts

from which the energy consumption can be obtained.

The torque on the rotor shaft is

M = (2.17)

in which = (2.18)

where (L 1 + LZ ) 1.S the length of the proj ections of the short and

long rotor blades along the axis of the rotor,

Z is the number of pairs of·blades on the rotor,

; is "the coefficient of utilisation of the working surface of the

chamber without the area of the space Sp between the half-chambers.

SK - Sp 3 ;. = % '4 SK (2.19)

The work A performed in one revolution of the rotor will be

A = 21rM = 21rTw . SK R . (2.20)

and the power input in the rotor shaft will then be

Np = 21rM n = 2Tw . SK . R . n . (2.21)

2.4 FACTORS AFFECTING DISPERSION OF PARTICULATE ADDITIVES IN THE

INTERNAL MIXER

Factors that affect the dispersion of the particulate additives

and the raw stock are related to the basic steps in the mixing process,

mentioned in section 2.1. Each of these steps is governed by the various

properties of the material to be mixed, and the characteristics of

the mixer. PalmgrenZ has made a detailed survey of some process

variables associated with internal mixing.

36

-

2.4.1 SHEAR STRESS

Compounding ingredients, such as carbon black, 'possess high

surface area and the agglomerates are made up of clusters of aggre-

gates which exhibit a mutual attraction for each other through Van

der Waal's forces. During dispersive mixing these cohesive forces

must be overcome. Dizon et alII used a simple theory developed by

McKelveyl to illustrate the balance of fbrces that are involved in the

dispersion of carbon black. The application of a shearing strain on

the soft agglomerates will generate a hydrodynamic drag force acting

on the agglomerate tending to separate at its weakest link l2 (Fig.

2.7). The magnitude of the force acting against the cohesive force

of the aggregates depends on the effective diameter of the agglomerate,

the viscosity of the matrix, and the strain rate and can be described

by Stakes' Law. The force balance has been quantified by McKelveyl

who characterised the progress of mixing by a parameter K; the dis-

persibility factor.

6rrRa)JY 6rrR,;,'T K = = (2.22) C C

where Ra is the agglomerate radius

)J ~s the viscosity of the matrix

y ~s the strain rate

, and C is the cohesive force of aggregates

)J is the viscosity of the matrix assuming the Newtonian

relationship T/ . = y

From Equation (2.22) it can be seen that in the agglomerate

stage when Ra is large compared to C, 'the hydrodynamic drag force

exceeds the interaggregate cohesive force resulting in the breakdown

of agglomerates and dispersion of aggre£ates. This balance of forces

becomes less favourable for dispersion as Ra diminishes. The process

SHEAR FIELD

FIG. 2.7: INTERACTIVE FORCES IN THE DISPERSIVE MIXING OF CARBON BLACK12

of agglomerate breakdpwn proceeds until an e~uilibrium is achieved

between the two oppos1ng forces. At this stage continued mixing does

not improve the dispersion and the Ra approximates the dimensions of

individual aggregates. Experience with various types of carbon blacks I

. confirm the above interpretation. High-structure blacks exhibit a

higher shear force during the dispersion stage while low-surface area

blacks possess low cohesiveness.

The approximate magnitude of the values of shear stress, T, for

the various m1xers can be calculated using the· power law e~uation,

T = (2.23)

where k and n can be obtained from the flow curve and

y, the shear rate, is calculated from the rotor speed.

Knowledge of the level of shear stress and its relationship with rotor

-

speed is essential in anticipating the process~ng problems.

2.4.2 SHEAR STRAIN RATE

A rubber mix ~n an internal m~xer is subjected to a spectrum of

shear rates. In the areas far away from the rotor tip the shear rates

are minimum, increasing gradually till a maximum is reached in the

nip region. This maximum shear rate, y, above the tip of the rotor

wings can be calculated using equation (2.13). This relationship

holds if pressure flow is ignored and that no slippage prevails.

Typical range of shear rates 2 for low intensity mixing is 100 - 250 s-l

and' for high intensity mixing is 200 - 600 s-l.

From Equation (2.13) mear strain rate ~s directly proportional

to the rotor speed and inversely proportional to rotor tip clearance.

It is important to know at what shear rate a mixer is operating as

optimum dispersion is obtained only within a limited range of rotor

speeds, with decreasing degrees of dispe~sion on both sides of it.

This is attributed ,to the power law type of relationship between shear

strain rate and shear stress.

2.4.3 SHEAR STRAIN

The shear strain,y, determines the extent of mixing and batch

uniformity. Assuming that rubber sticks to both the c~ber wall and

the rotor tip Bergen7 found that the shear strain varies between the

ratio of width, b, of the rotor blade to the clearance, h, and maximum

at the rotor tip., It is best characterised by the shear output, s,

which is the shear per unit volume of the material and unit time:

s = (2. 24)

"

39

2.4.4 RAM PRESSURE

The importance of ram pressure on mixing efficiency has been

acknowledged4 ,13. It influences several mixing mechanisms ·and material

behaviour in the mixer.

The effect of ram pressure is fully exploited only when the

chamber is full. As such the early stages of the· mixing cycle will be

greatly influenced by ram pressure. Due to the low bulk density and.

fluffy nature of most of the additives ram pressure helps.to reduce

the incorporation time by decreasing voids and increasing the area of

contact. In addition the resultant increase in hydrostatic pressure

reduces slippage on the working surfaces of the mixer. Slippage must

MIXING TIME (MIN)

10.

8

6

4

2

OL-----~----~--~~----~----o 25 50 75 100

RAM PRESSURE (PSI)

FIG. 2.8: EFFECT OF RAM PRESSURE ON MIXING TIME

40

.be reduced to a minimum as it not only affects the shear rate and the

dispersive action at the rotor tips but also reduces the cooling

efficiency. Ram pressure has more effect· in m1xers with two-wing than

four-wing rotors.

Capillary rheometry has shown the effect of hydrostatic pressure

on polymer melt viscosity14. This explains the increase of shear stress

with ram pressure and the 1ncrease 1n power consumption accompanying

it. Again this effect is particularly more pronounced in soft stocks

than stiff ones.

The resultant effect of increasing ram pressure is, therefore, the

overall reduction in mixing time 15 (Fig. 2.8 J and therefore improved

·efficiency16. Imp~oved dispersion17 and reduction in variability of

the mix16 with increasing ram pressure are also reported.

2.4 • 5 ROTOR SPEED

Up to a certain limit rotor speed determines the m1x1ng time and

the output rate. The straight line relationship between rotor speed and

mixing time and power consumption is well-known16 : a constant number of

rotor revolutions.is required to obtain the same end result.

The theoretical optimum range of shear rates can be' obtained from

Equation (2.13).

With soft stocks the optimum rotor speed for shear rate must be

seiected to achieve the essential minimum shear stress level which may

not be attained at low or normal speed. Equation (2.23) also reveals

the power law relationship between shear stress and shear rate. As

such after a certain point further increase in rotor speed may not

significantly change the shear stress magnitude. Increasing rotor

speed also results in higher stock temperature which counteracts the

increase in shear stress. Therefore, since dispersion can be efficient

41

only if the shear stress exceeds a critical value, which depends on the

cohesive strength of the aggregates, mixing efficiency of a mixer is

reduced if the rotor speed is too high16 ,18.

The above phenomena is exploited for variable. speed m~x~ng

techniques 13 .

·2.4.6. COOLING EFFICIENCY

The me'chanical shearing action imparted onto the .filled rubber

m~x produces considerable frictional heat due to the dry .. particulate

additives, and the relative motion of the moving material against the

rotor surfaces and the chamber wall. In addition, the shear deformation

that rubber is subjected to during the mixing process results in

hysteritic heat loss. It is a well-known fact that rubber mixing

process is an extremely inefficient operation15 • As such much of the

energy put into the mix is inevitably converted to heat.

The mixing temperature exerts very appreciable effect on the

rheological properties of the mixing stock. The temperature dependence

of the viscosity, n, liquids is usually exponential and can be des

cribed by an Arrhenius type of equation 19:

n = Aexp (E/RT) (2.25 )

where A ~s a constant, R is the gas constant, E is the activation

energy and T is the absolute temperature. Because of the inverse

.relationship between the viscosity and temperature and the effect on

the shear stress level mixing temperature must be effectively controlled.

Further, excessive heat build-up increases the risk of scorch.

Typical relationship between the apparent viscosity of a rubber compound

and temperature is represented in Fig. 2.9 •

. 42

LOG APPARENT VISCOSITY 11 a

.! T

FIG. 2.9: TYPICAL VISCOSITY-TEMPERATURE RELATIONSHIP OR A RUBBER MIX

It has been estilIla.ted that as much as 45-55% of the total energy

input is absorbed as heat in the coolant. The significance of cooling"

efficiency can be seen from the Heat Balance Equation16 :

w = H + C (2.26)

(Hork done by rotor) (Heat into rubber) (Heat into Cooling System)

The work done by the rotor, W, is the principal term which determines

the amount of dispersion. For efficient mixing C must be big so as to

keep H small. The rotors of the Shaw Intermix are claimed to con-

siderably increase cooling efficiency because of the larger working

surface area. Farrel-Bridge Banbury mixing chambers are now equipped

with drilled sides which significantly improve the cooling system. In

spite of these developments the amount of heat developed in the mixing

stock still hinders the attainment of maximum efficiency of the mixing

43

'.

process of an internal mlxer.

Excessive cooling, particularly at the beginning of the mixing

cycle, has a peculiar effect20 . Mixing becomes poorer due to slippage

of rubber on the shell of the mixing chamber. This is overcome by

increasing inlet water temperature slightly (e.g. from 250 to 30oC) and

has led to the design and development of water-tempering systems.

In order to have a check on the workability and quality of a batch

of stock it is necessary to have information on the mixing cycle temp-

eratures. While a watt-meter and mixing time is widely used to judge

the dump period the temperature of the compound at the end of the mix-

lng cycle is required. These emphasise the significance of cooling.

2.5 MIXING TECHNIQUE

2.5.1 CONVENTIONAL AND UPSIDE-DOWN MIXING TECHNIQUE

The basis for all the techniques employed in the mixing process

lies on the assumption that a high level of shear stress is an important

prerequisite for efficient dispersion of additives - similar to that

when the mixing temperature, ram pressure, rotor speed and cooling

efficiency are considered.l V·iscosity of rubber is known to increase

by the addition of dry additives. Therefore most of the powdery ,,/

ingredients are added in stages at the beginning of the mixing cycle

when the rubber is still cool. Compounding ingredients, such as

plasticisers and processing oils, that tend to reduce the viscosity of

the mixing stock are added towards the end of the mixing period when

most of the additives are already incorporated and dispersed. Further-

more processing oil tends to cool the hot compound slightly. This

method is commonly.known as the conventional mixing technique.

It was, however, reported21 that polychloroprenes mix better with

early addition of oil. This may be true due to their pronounced

44

I

u

thermoplastic properties. While some oil-extended rubbers, such as

natural rubber, are more difficult to mix, dispersion of additives in

others is often equal and occassionally better .than the non-extended

grades 22 • No reason for this has yet been postulated but it may be

due to the higher molecular weights of therubber used in the oil

extended grades. Perhaps a better understanding of this approach lS

still required.

The extreme of this conventional method is the upside-down mix

lng technique. It is the procedure by which all the ingredients of a

batch are introduced into the mixer at the same time, and the ram is

immediately applied, thus developing a high energy input to the batch

at the start of the mixing cycle. Improved dispersion of additives is

the result of the high power input required by the stiff consistency

of the batch.

When all the ingredients of the batch are thrown together in the

mixer at one time the rubber is 'broken up' and is heavily coated with

the pigments which are then quickly incorporated. The rapidly formed

stiff stock demands the maximum work and hence maximum power is

required.

The high energy input at the beginning of the mixing cycle rapidly

heats up the cool chamber surface so that the rubber adheres more

readily to it and promotes greater shearing action. A high level of

dispersion of additives is obtained by this technique23 though it lS

accompanied by higher power consumption and the mixer·is subjected to

higher strain.

2.5.2 MASTERBATCHING

Earlier extensive studies of dispersion methods have been centered

around straight or direct method of compounding involving the dry

45

mixing of loose blacks into rubber. Later latex mast erbatching24 , 25,26·

was developed, which involves the incorporation and, to some extent,

dispersion of carbon black into the rubber in the latex stage, with or

without a dispersing agent, followed by recovery of black-rubber

mixture by coprecipitation to produce the black masterbatch. Sometimes

processing oil is also added together with the black to give oil-black

masterbatch. However, the term 'masterbatch '. is also used to describe

the first stage preparation in a multiple mixing process.

The primary objective of latex masterbatching was· not to eliminate

intensive mixing, but rather to better utilize manufacturing and ship-

ping facilities and make compounding a cleaner and more convenient

operation. Above all, shorter mixing times per batch is observed with

the masterbatch method, although over a long period of time and with a

large volume of stock being processed it shows a higher mixing cost

compared to the direct method. La Porte27 claimed a saving of approx-

imately 40 percent in overall mixing time, where a minimum cycle time

of six minutes on a Number 11 Banbury batch is a negotiated labour

practice, by replacing the two-stage dry mix by single stage mast er batch

mix. A further advantage of this method is that significant variability

in black content of stocks is eliminated. Power consumption is, on an

overall as·well as a peak power basis, also lower.

The increase in consumption of oil-black-SBR mast er batch in recent

years reflects the virtues of using this material.· Particularly with

the dispersant-free mast er batches, dispersions are generally superior

in tread stocks when compared to a dry mix of an oil-extended polymer.

The improved dispersion shows up in better tread wear ratings from 20

t6 to 30% compared to dry mix counterpart. The excellent dispersion of

all carbon blacks obtained in dispersant free masterbatches has made

possible, for the first time, a much greater utilization of the higher

46

reinforcing properties of finer ISAF and SAF blacks than was possible

by 'dry mixing. This development could help to solve the problems in

the use o.f new types of blacks of specially superior reinforcing

properties that could not be exploited through other mixing techni~ues.

The most significant development in this area is the introduction

of the hydrosolution masterbatch, HSMB. Its production involves the

use of an a~ueous dispersion of carbon black mixed with a water based

dispersion of a polymer solution under conditions of high agitation.

The high degree of agitation is re~uired so that the polymer solution

sweeps out the carbon black particles to form a polymer-black mixture

without reagglomeration of the carbon black. ,Compounding with HSMB

produce vulcanisates of physical properties at least as good as, and

generally technically superior to, those which can be obtained by dry

mixing in an internal mixer.

2.5.3 HOT MIXING,

The unsatisfactory mixing of butyl rubber with carbon black by

conventional mixing techni~ue led Gassler 28 to investigate heat treat

ment to solve the problem. He proposed a techni~ue of cyclic heat

treatment whereby the carbon black-butyl rubber masterbatch was sub

jected to a series of milling and heating cycles. For Banbury mlx~ng

he found that a temperature range of 200-26ooc was re~uired to obtain

a reasonable mixing cycle. A prere~uisite for effective heat treatment

of rubber-black mixtures is a high degree of unsaturation of rubber

molecules. Another conse~uence of heat treatment is the increase in

the number of carbon black-polymer bonds through some mechanisms

involving the isoprene units present in butyl rubber29 •

It later became apparent that heat treatment alone ~s impractical.

Heat treatment together with chemical promo'tion30 is found to speed up

the rubber-black, interaction at lower mixing temperature (150-200oC)

and improve the dispersion of ingredients. The principle of this

method are the assumptions that: a) the increased material stiffness

would increase. the level of shear stress and 'hence improves dispersion.

b)'the increase in rubber-black interaction produces some improvement

in properties 29 .

The use of chemical promoters and their effect on rubbers have

been investigated by several workers, among them Doak et a131 , Walker

and Kerwood 3 2 •

2.5.4 DEGREE OF FILLING - FILL FACTOR

There are several reasons why the degree of filling or,the batch

Slze has to be predetermined:

i) Due to the low bulk density and fluffy nature of the particulate

compounding ingredients, their total volume at the beginning of

the mixing cycle is about 50-80% larger than when they are mixed2 •

Although ram pressure reduces the excess volume to some extent

overfilling the mixing chamber results in inefficient incorpor

ation process.

ii) Oversize batches allow material to be retained underneath the ram

and cause sample heterogeneity.

iii) On~he other hand undersize batches markedly reduces the effect

of ram pressure and the shear stress level. Consequently the

,problem of slippage and long mixing time are confronted.

iv) The formation of voids in the mixing chamber during mixing lS

required in order to promote extensive mixing ana to produce

the complex flow paths that give efficient cooling. This is

equivalent to laminar versus turbulent flow in cooling water

pipes.

48

2.6 MATERIAL MODIFICATION TO IMPROVE DISPERSION

2.6.1 FILLERS

The principal characteristics of reinforcing fillers in general,

are their particle size and chemical activity. For reinforcing carbon

blacks there are several primary properties which largely determine

their effects on the properties of elastomer systems in which they are

present. Gessler et a133 discusses these properties, which are

presented in Tables 2.1.

Table 2.1: PROPERTIES OF CARBON BLACKS

1. Particle size, surface area, porosity.

2. Aggregate structure (bulkiness).

3. Amount of carbon per aggregate.

4. Surface activity:

5. Trace constituents.

Problems in dispersion are generally encountered with blacks of /

very fine particle size, low·structure ,and high surface area so that

the high reinforcing SAP and ,ISAF (Tabie 2.2)35,are extremely difficult

to mix.

Table 2.2: PARTICLE SIZES OF FILLERS

Filler Average particle diameter ( EM) in nm

Black: SAF N - 110 11 - 19

ISAF N - 220 20 - 25

RAF N - 330 26 - 30

FEP N - 550 40 -48

Silicas 20 - 30

Silicates 30

ICont 'd •..

,

------

Black: GPH N - 660 49 - 60

HMF N - 601 46 - 66

SRF N - 770 61 - 100

FT N - 880· 101 - 200

MT N - 990 201 - 500

Kaolin clays 500 - 3000

Calcium carbonates 1500 - 4000

Silicates 10000 - 40000

Bolt and Dannenberg's observation34 that these blacks possess

high packing ability so that they tend to form dense agglomerates which

are not readily dispersed,led them to suggest the pour or bulk density

as a suitable dispersibility index.

Compaction and pelletization of the extremely fine and low.bulk

density carbon blacks are found to increase the ease of dispersion

considerably. Optimum bulk handling, dispersibility and purity depend

upon pellet properties such as pellet size distribution, hardness,

strength and fines content 35 •

However Voet 36 argued that pelletization causes irreversible

changes in the black, possibly due to chemical interaction of free

radicals present on the surface of the black. Vulcanisates reinforced

with such blacks possess , to some extent, deteriorating properties,

such as lower modulus and reduced abrasion resistance. To overcome

this problem Voet 37 suggested a new system of pelletization of carbon

black using oil, whereby strong chemical bonds between free radicals

present in the particles were avoided. He claimed that the oil

pelletized blacks can be advantageously used in dry mixing process with

rapid dispersion of not only the black but also the oil, without

causing any deterioration of vulcanisate properties;

50

2.6.2 VULCANISATION INGREDIENTS

Vulcanisation ingredients are used only in small-quantities,

generally 1-5 parts per hundred rubber and must therefore be well

dispersed. Problems involving incorporation and dispersion of com

pounding ingredients have been investigated by' Drogin 35 . Studies by

Grove 38 show that both ease of inco,poration and dispersion differ widely

with variations of zinc. oxides and processing factors. With powdered

Z1nc oxides particle size, shape and nature of surface influence the

mixing behaviour in the same manner as with carbon black. As such

coarse. particle size zinc oxides are easier to disperse. However greater

activity is found in zinc oxides of fine particle size ranging from

0.1'-0.7 1139. American process zinc oxides are "needle-like" while

French process types are generally finer and spherical-shaped. The

presence of occluded or absorbed gases and the degree of compatibility

with the polymer are other factors affecting the ease of mixing.

Neoprene is found to mix with zinc oxide very poorly, forming soft

sponge stocks. When added at the beginning of the mixing cycle the ·st.ock

tends to scorch. Magnesia-buffered zinc oxide (MBZ) 39, an oil paste

co-dispersion complex of MgO-Mg(OH)2-lnO, is suggested to eliminate the

above problem. -

The problem of the addition and dispersion of sulphur without

risking the danger of scorch is still confronting the rubber indust~y.

Due to excessive heat build-up in the internal mixer sulphur is added

towards the end of the mixing cycle or in the second stage of the n,ix

ing process. But regular.sulphur, used in this way, tends cO agglo~erate

in the raw stock and cause hard lumps in cured product and because ef

its solubility in rubber presents another problem oi\ blooming. Inscl.~I"';.~_~.2

sulphur will not bloom; however it is more difficult to incorporate

and disperse.

51

Accelerators are presenting fewer problems ln mixing. The varlOUS

physical forms, such as briquettes, flakes and rods reduce dusti~g

while aiding the mixing process.

2.7 SURFACE TREATMENTS

In analysing the significance of electrical contact potential to

the reinforcement of natural rubber and SBR. Havenhill et al40 ,41,

provided a theory ln which

"the strong electrostatic attractive forces between the

positive (+) pigments and the negative (-) rubbers"

are supposed to form the basis of reinforcement. All the materials

can be arranged in 'an electrostatic contact potential series: at the

extreme of the negative scale is rubber while the carbon black rein

forcing fillers an'd zinc oxides are at the more positive end.

The role of the contact potenti~ must therefore be important

during mastication, incorporation and dispersion processes41 . Proper

sequence of the addition of ingredients in the mixing operation can be

determined by the electrostatic curves. Based upon this theory

Havenhil142 explained:

'In general, those softeners which make the rubber more

negative (-), such as lauric acid, should be added prior

to the addition of the positive (+) reinforcing pigment

in order to 'take advantage of the strong electrostatic

attractive forces present. Softeners which make the

rubber positive (+), should be added after the positive

(+) reinforcing agent so as not to encounter the repulsive

action of the positive fields. On this basis, mineral

oil should be added after the pigment, because it does

made the stock slightly less negative.'

52

It was also shown that optimum processing conditions are obtained

by mixing materials from both ends of the electrical contact potential

series. The charges of the materials can be made either more positive

or negative by surface treatment.

Zinc oxide can be effectively treated or coated by a short-chain

organic acid38 - propionic acid - which reacts to form z~nc propionate.

It separates aggregates into their individual particles due to the more

positive charge, reducing the times for incorporation and dispersion.

The resulting compound is organophilic and thus markedly improves

wettability by elastomers. Another processing advantages obtained is

the elimination of peak electrical loads demanded by the mixer motor.

Surface treatment also prevents the formation of hard zinc oxide flakes

caused by some sticky zinc oxide predensified ~n the mixer. During

manufacture some negative charges may be formed which cause some

cohesive agglomeration. These negative charges an re removed by surface

treatment.

A further claim made by Havenhill is that by inserting a probe

designed to measure electrostatic charges in the mixing chamber the

completeness of filler or oil incorporation can be judged from the

reading of the contact potentials; which are produced only when unlike

materials contact and are then separated from the probe. Any free oil

or pigment will coat the probe and the potential drops to zero. When

the oil is completely incorporated the normal contact potential of the

oil-rubber system is abruptly obtained.

To ease the dispersion of sulphur, for example ~n nitrile rubber,

it is coated with magnesium carbonate, carbon black or oil. Surface

treated sulphur is non-caking, free-flowing and easily dispersed.

53

REFERENCES

1. McKelvey, J. M., "Polymer Proc'e'ssing", John Wiley& Sons, Inc.,

N. York, 1962.

2. Palmgren, H., Rubb. Chem. Tech(., 48 (3), 462 (1975).

3. Boonstra, B. B., and Medailia, A. 1., Rubb. Age, .2£, 892 (~larch

1963).

4. Cargal, J. M., I.R.I. Proc. i~. 232 (Dec. 1966).

5. Anon., Europ. Rubb. J., 160 (3), 39 (Apr. 1978).

6. Funt, J., "Mixing of Rubbers", RAPRA, Shrewsbury, '1977.

7. Bergen, J. T., in "Processing of Thermoplastic Materials", ed. by

Bernhardt; E. C., Vari,Nostrand Reinhold Co., N. York (1959),

Chapter 7.

8. Bolen, W. R., and Colwell, R. E., S. P. E. Tech. Paper,s, IV, 1004

(1958) .

9. Gub'er, F. B., Sov. Rubb. Tech., 26 (1), 23 (1967).

10. Udal'tsov, V. V., Vostroknutov, E. G., and Novikov, M. I., SOy.

Rubb. Tech., 31 (6), 10 (1972).

11. Dizon, E. S., Micek, E. J., ,and Scott, C. E., A.C.S. Rubb. Div.

Meeting, Philadelphia, U.S.A., Oct. 1974,.

12. Dizon, E. S., Rubb. Chem. Tech., ~ (1), 12 (1976).

13, Perlberg, S. E., Rubb. World, 150 (2),27 (1964).

14. Cogswell, F. N., Poly; Eng. Sci. ~ (1), 64 (Jan. 1972).

15. Anon., Rubb. J., 151 (10), 48 (1969).

16. Whittaker, P., J. 1.R.1., 4 (4), 153 (Aug. 1970).

17. Comes, D. A., Rubb. Age, 78 (3), 395 (1956).

18. Bourne, J. R., New Scientist, 33, 334 (1967).

19. Bestul, A. B., and Belcher,H. V., J. Appl Phys., 24, 696 (1953).

20. Ellwood,.H., I.R.I. Rubb. Proc., 2nd Ann. Conf., Blackpool, U.K.,

May 1974.

21. Bament, J. C., S. G. F. Conference, Stockholm, Sweden, 1964.

22.. Bussemaker, O.K.F., S.G.F. Conference, Stockholm, Sweden, 1964.

23. Comes, D. A., Ind. Rubb. World, 122 (2),180 (May 1950).

24. Carrol, J. H. and Cooper R. N., ~n "Reinforcement of Elastomers",

Kraus, G., (Ed), Intersc. Pub., N. York, 1965.

25. Forrester, R. A., Rubber Age, 86 (4), 446 (Jan 1960).

26. Samuels, M. E., Rubber Age, 86 (4),649 (Jan. 1960).

27.' La Porte, R. T., Rubber Age, 86 (4), 653 (Jan 1960).

28. Gessler, A. M., Rubber Age, .I!±. (1), 59 (1953).

29. Braendle, H. A., Rubber Age, 72 (2), 205 (1952).

30. Gessler, A. M., and Ford, F. P .. , Rubber Age, 74, 397 (1953).

31. Doak, K. W., Ganzhorn, G. H., and Barton, B. C., Rubb. Chem.

Tech., 28 (3), 895 (1955).

32. Walker, L. A. and Kerwood, J. E., Rubber Age, 90 (6), 925 (1962).

33. Gessler, A. M., Hess, W. M., and Medalia, A. I., Plast. and Rubb.

Proc.,l (1), Pt. 1, 1 (Mar. 1978).

34. Bolt, T. D., and Dannenberg, E. M., Rubb. Chem. Tech., 34 (~),

. 43 (1961).

35. Drogin, I., Rubb. Age, 89 (5), 791 (1961).

36. Voet, A., Ind. Eng.Chem., Prod. R 80, 1., 195 (1962).

37. Voet, A., Rubb. World, 150 (3), 33 (June 1964).

38. Grove, G. E., Rubb. Age, 93 (3), 405 (June 1963).

39. Vickery, G. C., and Snyder, J. E., Rubb. Age, 106 (5), 33 (May

1974) •

40. Havenhill, R. S., Carlson, L. E., Emergy, H. F., and Rankin, u. J.,

Trans. Inst. Rubb. Ind., n, 339 (1951).

55

.41. Havenhill, R. S., Carlson, L. E., Emery, H. F., and Rankin, J. J.,

Trans. Inst. Rubb. Ind., !±2., 1128 (1953).

42. Havenhill, R.S. Carlson, L. E., and· Rankin, J. J., Trans. Inst.

Rubb. Ind. 79 (1), 75 (1956).

56

C\LI\!'TER :3

TECHNHJES OF !1SSESSING DISPERSHIJ NlD r·1IX PROPERTIES

The determination of sue and dispersion of ingredients, particularly

reinforcing fillers and curati ves is indispensable to any studY of

mixing and reinforcement. Microscopy and microradiography are the

principal techniques essential for such analysis. However, in addition

to the fact that microscopic examination is generally cumbersome and

tedious, ·high degree of dispersion may not be the only criteria to

adjudge the basic purpose of mixing. A proper balance between viscosity

and elasticity in a compound at the end of the mixing cycle is also

essential. Nevertheless, in most cases when this requirement is met

all other requirements, such as degree of dispersion, are also

attained1 . Filler dispersion and compound properties can thus be

correlated with one an·other:

3.1 MICROSCOPY

3.1.1 OPTICAL .NICROSCOPY

3.1.1.1 EXAMINATION OF VULCANISATE SURFACES

Early methods of assessing the quality of mixing involved "primitive"

methods. For example, a swollen rubber mix·was pressed between two

microscopic slides and the size of. undispersed.agglomerates analysed

at low magnification. : Qualitative estimation of black dispersion can

also be made by a technique devised at NRPRA (now NRPRA) in which a de

Nattia flex test piece is .bent and the groove examined in the stretched

state.

Stumpe and Railsback2 suggested the use of razor cut surfaces.

for comparison with a set of ten standards. This method, howevel", is

not suitable for·p:llymer blends and mixes containing reclaim or crumb 3 .

57

Examination of torn surfaces rather than cut surfaces and refer-

lng to standard dispersion with numerical ratings permits a number to

be quoted for degree of dispersion. The ,basic procedure of evaluating

the "torn surfaces" with five ,standards is outlined in ASTM D 2663.

Medalia ,and 'Walker4 have also described a modified rating chart with

the use of ten standards (Fig. 3.1).

All the above techniques described above are designed to be used

as rapid factory control tests' to assess the degree of carbon black

dispersion. Apart from some limitations imposed on such methods the

information obta:tned is useful but does not reveal the interior of

the specimen, and can be regarded as complementary to those obtained

by examination of microtomed sections.

3.1.1.2 EXAMINATION OF MICROTOMED SECTIONS

The fundamental techniques of analysis of ,carbon black dispersion

by optical microscopy using thin sections have been well described by

Leigh-Dugmore 5,6. Basically the rubber sample is frozen on a micro-

tome stage and sectioned by a glass knife. Carbon dioxide or sometimes

liquid nitrogen is used to freeze the sample. The opaque nature of

rubber makes it necessary to obtain specimen sections of about 2-3 )l

thick. The specimen is then mounted on a microscope slide. A liquid

such as petroleum, naptha or xylene, is used to swell the specimen

sl:tghtly, which makes it transparent and easier to unfold.

Quantitative measurement of dispersion of carbon black in the

microtomed section is made with an eyepiece graticule with 10,000 Equares

, whose sides correspond to about 15 )lm in the swollen specimen. A total

magnification of between 70X and 150X will be needed depending on the • graticule and the combination of lenses used. By counting the number

of squares covered by the aggregates Leigh-Dugmore suggested that the

58

fliumerl".1 ratin"

•• ' .. ' .. ' .. -~. . ""!

- ,.," , , "; , .- ., , ')l . .

. -1. \~

43

4.0

. I' (1 I,·." . • , "., • ' .. ~' ;." ,", I ,;". ",,' .• - .t "'. ."

.}.,~ ... " • \ 1 ~ •• : ' " ,". '" I ... ' • " \' ~ ~ 4.;; '"'., '. 'A' • ,:' , ' .' :' ',;-:" .,' ,'. J.

"'., ~:i;' l \. • I;" , ... ' , 01<\. ---..., ..... i. - •• • 'r-!"~ + ..... t. . ;. " ,"~I ' I .: I," ," ",.- . .'

30

2.5

2.0

'.0

FIG. 3,1: CABOT TORN SURFACE RATING CHART

59

0\ o

Cabot disp"" classificatioo

'a i11 1

2

3

4

6

A B C

ILI-"" tl.7-"'1

.'-11.8

... ,-.. ..2

FIG. 3. 2

&4.4 - 71.4 ..... '" 71.4 - 48.1i1

U7-IU

48.9 -10

SUI -10.0 10 -.

00.0 -.

percentage of the carbon black that lS dispersed as individual

particles and as aggregates smaller than those counted can be calculated.

However, Medalia pointed out that the Leigh-Dugmore's method of

calculation contains implicit . errors since the agglomerates do not

swell and contain certain amount of rubber thereby altering their

density.

To avoid these errors Medalia7 ·revised Leigh-Dugmore's formula,

thus:

D = 100 vUs AI,

wpere v = volume fraction of carbon in the agglomerates

U = average of the number of squares in each field

s = areal swelling factor of entire section

A = areal .swelling factor of agglomerates

L = carbon black loading (by volume percent) in the mix.

(3.1)

He also estimated the value of v to be equal to 0.4 and that A = s.

Equation (3.1) thus becomes:

D = 100 0.4 U

L

As an alternative to the above method,which involves the tedious job

of counting the squares and agglomerates,Medalia8 suggested the use of

a classification chart (Fig. 3.2) to estimate the dispersion ratings.

A di~tinct advantage in this method is that the· percentage of dispersed

black and the size of the undispersed agglomer,,-:-.:;s can be estimated

simultaneously. . I

3.1.2 ELECTRON MICROSCOPY

Particles of reinforcing fillers, which are usually below 50 r.~

in mean particle diameter, are well below the resolving power of light

,

microscopy. 'The higher resolution of an electron mlcroscopy is' re~uired

for accurate size, determination. For dispersion analysis electron

microscopy is not re~uired and recommended at al~ unless such a study

is warranted, such as when the ultimate dispersion of the filler

particles' is important and all other methods' fail to show any

difference9 •

Transmission electron microscope (TEM) produce lmages of extremely

small areas to the extent that it is' impractical to assess dispersion.

Variations of specimen thickness can be confused with large black

agglomerates.

3.1.3 RADIOGRAPHY

'Carbon black agglomerates are extremely opa~ue to light and, hence,

can be resolved very well by light microscopy. The dispersion of

inorganic pigments or curative~, such as silica and Zlnc oxide,

cannot be.followed by similar techni~ues. In such cases microradio

graphy lS utilised. Microradiographic analysis involves the use of

soft X-radiation; fillers such as clay, zinc oxide and silica show

considerably higher opacity to soft X-rays than most elastomers and

their presence can be detected even in the presence of carbon black.

This is because the absorption differential between carbon black and

the ~olymer hydrocarbon is small and hence, th~ contrast is poor.

Another advantage microradiography has over light microscopy is that

relatively thicker specimens can be used.

Using this techni~ue Hess 9 studied the dispersion of Zlnc oxide

and titanium dioxide in SBR white tyre sidewall mixes. In his work he

used a Phillips Contact Microradiography Unit which is capable of

producing extremely soft X-rays in the 1-5 K.V. range (0.6-0.8 nm wave

length). With the rubber 'Pecimen mounted directly on to the emulsion

1

of a fine grain recording film; exposure times between 4-25 minutes

had to be used10 . Magnification on the film is the same as the specimen

and the microradiograms have to be enlarged optically.

3.2 EFFECT OF DISPERSION ON COMPOUND PROPERTIES

3.2.1 PROCESSING PROPERTIES

The rubber industry of today is involved with numerous types of

rubber compounds based on the wide variety of rubbers and compounding

ingredients available and prepared in large batch- operated internal

mixers under various mixing conditions. One of the major processing

problems is understandably the formulation and execution of tests which

will, at least sufficiently, characterise current and proposed future

materials and their compounds to be utilised in production processes.

Much of the work carried out so far to study processing properties

has been empirical and conducted under conditions different from those

actually encountered with industrial processing equipment. Increasing

degree of dispersion of fillers has long been associated with improved

milling, extrusion, calendering and building (tack) 11 so that rubber

properties are justifiabl~ regarded to closely reflect the level of

dispersion. However much of the published work reported is

applicable only to the systems studied. Fundamental studies to explain

the effect of dispersion on the vital processing properties are meagre, , .

due to the lack of proper rheological equipment.

One of the early comprehensive studies was made by Dannenberg12

quiterome time ago in which he investigated the effect of different

mixing times on carbon black dispersion. He reported that extremely

short mixing time suffices for the development of reasonably good

mechanical properties. However, Drogin 13 pointed out that although this

is true longer mixing cycle may be desirable to improve process~ng by

63

reducing the various effects of high viscosity and elasticity (nerve).

Studebaker and Beatty14,15 investigated the changes in molecular

weight, molecular weight distribution and properties of the raw stock

prepared using diff.erent mixing times and emphasised the need for the

reduction of the elastic component while adequately dispersing the

compounding ingredients during the mixing process. Vulcanisate

properties were compared with the compound characteristics and level of

dispersion but the effects of dispersion on processing properties and

polymer breakdown were not differentiated.

Turetzky et al 16 described a method of examining the m~x~ng

operations through the construction of a "processing profile". A

series of curves describing the changes in compound properties, such as

Mooney viscosity, die swell, bound rubber and carbon black dispersion

level, which take place during the mixing operation are plotted against

a mixing parameter. With increasing dispersion bound rubber increases,

and Mooney viscosity decreases to reach an assymptotic level, while

die swell.first increases but subsequently drops slightly. These

observations have been echoed by earlier workers 17- 19 as well.

The process of dispersion in ·the mixing operation and the down

stream processes that . are affected by the state of carbon black dis

persion, all involve deformation and flow of material and wherever

these occur rheological properties are important. The pertinent

rheological parameters are viscosity and elasticity and in both shear

and tensile flow modes.

3.2.1.1 VISCOSITY

The viscosity under simple shear is the most commoLly used

rheological measurement. For empirical values Mooney viscometer20 is

usually used where shear rates of 1.3 or 1.6 s-1 are used depending on

64

the rotor slze. As rubber is highly non-Newtonian and its viscosity

is shear rate dependent this instrument, although reliable, has severe

limitations. More fundamental and useful measurement of viscosities

.is made with a capillary rheometer21 from which the relationship

between shear stress and viscosity and shear rates can be obtained.

Viscosity is basically a measure of the degree of disentanglement

and re-entanglement of the long and flexible polymer molecules as they

rotate in the shear planes on application of a shear deformation force.

As the shear rate is increased the rate of entanglement drops behind

that of disentanglement so that viscosity decreases. The effects of

molecular weight, filler and plasticiser loadings, pressure and

temperature on viscosity are well-known. However, the effect of dis

persion on viscosity is not ade~uately elucidated.

The respective power-time and tor~ue-time profiles of the Banbury

~nd Brabender Plastograph clearly show a sharp maximum at the start of

the mixing cycle but gradually level off .on further mixing. Ford and

Gessler22 , while noting the reduction in Mooney viscosity with lncreas

lng mlxlng time or.dispersion, claimed that this reduction was far

greater than might have been expected from polymer breakdown alone.

Dannenberg12 reported that the Mooney viscosity·measurement shows a

sharp drop during the first four minutes of mixing time followed by

markedly small change on continued mixing. Two phenomena may con

tribute to this behaviour. Firstly, at the start of the mixing cycle

the incorporation of the various particulate fillers or additives

results in a steep increase in the viscosity of the mixture. This lS

because the large agglomerates within the polymer network increase the

stress necessary for molecular motion. During the dispersive sheari~g

process there is also high frictional resistance within the dry black

agglomerates. The resistance to flow is also contributed by the

-

relatively high interparticle cohesive forces within the agglomerates.

Secondly, on further mixing the incorporation process is accompanied by

the dispersion process during which deagglomeration of the black takes

place resulting in a reduction of the frictional and cohesive forces, .

and thus the viscosity is reduced. Large agglomerates form hinderances

to the flow paths of the moving molecules - the smaller the aggregates

the less is the obstacle, and hence, the lesser the viscosity. As the

rate of deagglomeration follows an exponential pattern,so does the change

in viscosity.

To explain the variation of Mooney torque of a given compound with

mixing time Kraus 23 and Medalia24 also proposed the concept rubber

occluded by the carbon black. Using a simple spherical model of filler

(Fig. 3. ~ carbon black agglomerates, in the undispersed state after

the wetting process, can be considered to contain not only filler

particles but also the rubber occluded between the particles. The

r/J (t) = r/J e +

FILLER OCCLUDED RUBBER ~.

AGGLOMERATED FILLER

DISPERSED FILLER

FIG. 3.3: SCHEMATIC DIAGRAM OF COMPOSITE MORPHOLOGY

66

entire filler agglomerate together with its occluded rubber behaves as

a single filler particle when a stress is applied. The occluded rubber,

being trapped by the irregular convolutions of the filler aggregate,

must be considered as part" of the· volume of the aggregate. Accordingly

the effective volume ~e in the early stages of mixing is always larger

than that of the well mixed volume. Thus with a filler loading of ~

the effective volume ~ at any time t progressively decreases as the . e.

volume fraction of occluded rubber ~ decreases with increasing disor

persion, so that

= +

, In a poorly dispersed compound the effective volumes are very

large, hence the high viscosity; Well dispersed mixes possess only

small amounts of·occluded rubber and hence the least effective volume

and lower viscosity.

The major 'factors influencing viscosity can be summarised as shown

in Fig. 3.4.

LOe, VISCOSITY

MOLECULAR WEIGHT

PRESSURE

LOe, SHEAR RATE

FIG. 3.4: FACTORS INFLUENCING VISCOSITY OF A FILLED RUBBER COMPOUND

61

3.2.1.2 VISCOELASTICITY

Rubber is a \!i.scoelastic material in which the elastic element.

forms the continuous phase but encompasses a frictional viscous element.

If it is subjected to a fixed stress the deformation-time curve will

'generally show an initial rapid deformation, due to the elastic com-

ponent, followed by a steady continuous flow, due to the viscous com-

ponent.The relative importance of elasticity and viscosity depends

on the time scale of the deformation.' At short times elasticity

dominates while at long times the flow is purely viscous. The measure-

ment of viscosity inVOlves, essentially, the ratio of a shear stress to

a shear rate, and does not describe the elast1c and viscous components, ,

the vectoriai resultant of which >ields the shear stress25 . The ratio

of the viseous modulus to elastic modulus, obtained during a sinusoidal

deformation dynamic test, is termed the ~echanical phase angle, loss

factor or tan o. Hence

.!. (EZ + yZ 11 '" =

y t (3.4)

tan 0 = Y E (3.5)

where 11 = viscosity (apparent)

T = shear stress

y = shear l'ate

E = Elastic stress

v = Viscous stress

A polymer system having a high viscous modulus and low elastic

modulus will possess a relatively higher loss factor while another with

a greater tendency towards elastic behaviour will have a larger elastic

modulus and lower loss factor. .>

Elasticity pl~s a larger part in extensional flows than in simple

68

shear flows of the same magnitude26 ,27. In rubber processing extensional

flows are more concealed than shear flows. Wherever convergent flow

takes place there occurs extensional,flow, for example, at the entrance to

,the nip of a two-roll mill and calender, the sickle-shaped zone in front

of the Banbury rotor where the curved surfaces of the rotor and the

cylindrical wall of the mixing chamber converges to a minimum clearance at

the rotor tip and the die entry' region of an extruder or a capillary

rheometer.' ,Practical significance of elasticity to processing are:

i) Incorporation of fillers

ii) Mill swell, which controls the bagging characteristics, on a mill

iii) Feeding of extruders, where elastic recovery reduces the feeding

efficiency

iv) Die swell

v) Surface appearance of extrudates

vi) Flow patterns and forces in flow in regions of changing cross

sectional area.

Average molecular weight, temperature and pressure have but small

influence on elasticity., The main controlling factors are molecular

weight distribution (MWD) and filier structure. Materials with narrow

MWD will exhibit higher elastic modulus than those with wider distri

bution. The effect of MWD persists throughout the process and is one

of the significant causes of problems in mixing14 ,15. During the

mixing process transient structures of the filler are continuously

broken down28 by the dispersive,forces and the rheological behaviour

of the mix will be governed by the ultimate structure left at the end

of the mixing cycle. Payne29 ,30 showed that compounds with the shortest,

mixing times, i.e. the poorest filler dispersion, possess the highest

dy'namic eiastic modulus but also the largest decrease in modulus with

increase of amplitude. As the large agglomerates are broken down the

elastic modulus decreases, reaching a minimum value at maximum dispersion.

69

The decrease in modulus with increasing mixing time or dis

persion can also be explained using the concept of occluded rubber23,24.

Large agglomerates with high volume of occluded rubber tend to reduce

the molecular orientation by absorbing some of the force~through

_ formation of bridge structures between particles 31 and thus reducing

the effect of elasticity. As the agglomerates, are broken down the

magnitude of polymer orientation increases resulting in increasing

elastic behaviour. Continued mixing after the attainment of a certain

degree of dispersion reduces the elasticity due to molecular breakdown.

Viscoelastic response manifests itself during elongational

deformation, creep, stress relaxation, die swell, shear recovery, and

dynamic deformations. Appropriate analyses of the data obtained from

such phenomena will yield the viscous ,and elastic forces Bithin the

material.

Data on viscoelastic behaviour of uncured rubber compounds have

been meagre. Many of the test methods currently available to character

ise the processing behaviour of rubber mixes are either too tedious

to perform or limited to a narrow range of processing conditions. The

reason for the lack of signiticant progress is that the viscous and

elastic effects in the highly non-Newtonian rubber mixes 'are distributed

in a dual network which makes the isolation of the fundamental para

meters more elusive.

An interesting contribution has been made in this field recently

by Turner and coworkers 32 ,33. To overcome the deficiencies of single

point viscosity measurements", such: as'Mooney viscosity or Wallace

Plasticity, ,and to avoid the, tedious and inadequate characterisation

by capillary rheometer, they proposed a mechanical model to explain the

viscoelastic,behaviour'of uncured rubber compounds and obtain the

viscoelastic parameters.

E o F - FAILURE, -

/' "

K J

FIG. 3.5: THE T.M.S. (TURNER, MOORE,' SMITH) MODEL

, -" 'I

71

The proposed T.M.S. model consists of basically two Maxwell net

works in parallel (Fig. 3.5). The use of dual elements helps to

broaden the relaxation times attributed to the model and produce time

dependent recovery effects due to the interaction between the two

elements. The elastic components are represented by the Hookean springs

of moduli D and E - D is normally between 5 and 20 times the magnitude

of E. The viscous elements are characterised by constants J, K and

power law index n. F represents the limit of stress at which the spring

E is 'fractured'.

Although the model is not to.be taken as literal representation of·

rubb'er it can be used as a guide to the arrangement of the terms in

the constitutive equations (Appendix 1:0) which can be used to obtain

the six parameters and predict the behaviour of unvulcanise·d rubber in

a range of common test and processing conditions. Using suitable

instruments these six parameters can also be obtained to sufficiently

characterise rubber compounds. The T.M.S.· instrument, to measure the

properties·· under shear (up to 60 s':'1), and the Elongation Tes.ter have·

been found to reliably yield these parameters. Data gathered so far

seem to indicate.that this new approach could make possible the

"frustrating exercise" of correlating the ·various viscoelastic responses 34 •

3.2.1.2.1 CREEP

Creep can be defined as a progressive increase in strain, observed

over an exten·ded period, in a .polymer system subjected to a constant

stress. The Voigt-Kelvin ~odel, a parallel comb~nation of a spring

and dashpot or a combination of a four-element model arranged in series

and parallel (Fig. 3.6) have been used to characterise the creep

behaviour of a vulcanised rubber.

72

-

FIG. 3.6: VOIGT~KELVIN MODEL

On applying a constant stress to the system there follows aprogres-

sive increruEin strain which can be described approximately by:

dt) = (aO/E2) {l - exp C-t/TR) } (3.6)

"

c(t) ,

where = Observed strain at any tiine t

ao = Applied stress

E2 = Elastic component of the element

TR = Retardation time

3,2.1.2.2 STRESS RELAXATION

The significance of stress relaxation in determining the flow and

deformation of polymers was first 'emphasised by Ninomiya and Yasuda35

and by Toki ta and White 36 , 37 • Stress relaxation experiment's, conducted

73

in extension or shear, involve the measurement of force required to

maintain the deformation produced initially by an applied stress as a

function of time. The rate and degree of' decay of the observed stress

reflect the elastic and viscous forces within the material.

Models based on the original Maxwell model consisting of a spring

and a dashpot arranged in series (Fig. 3.7) have been used to describe

the stress relaxation behaviour so that

ott) = 00 exp (- t/TR )

where 00 = Original applied stress

ott) = Observed stress at any time t

TR = l1/E = Relaxation time

E = Elastic component

" = Viscous component

E

FIG. 3.7: MAXWELL MODEL

74

STRESS

- \ -

1-- t c

INITIAL ,STRESS

'0-

% DECAY

FINAL STRESS

'F - - - - ===-==--.:

TIME! •

- FIG. 3.8: ' iSCIlEMATIC OF S~S RELAXATION CURVE

'. j . -

\ . I I

75

Typical relaxation curve is shown in Fig. 3.8. The stress relax

ation time constant, t c ' is the parameter normally measured and

represents the time for the stress to dec~ a specified percentage of

the initial stress, TO (Fig. 3.8). It is a measure of the forces

involved in the viscous flow. The value of the final (relaxed) stress

has been shown to provide a good estimate of elasticity and bagging

behaviour during the milling operation. Low TF value is associated

with lower elasticity and poorer milling characteristics3B •

3.2.1.2.3 DIE SWELL

Die swell, observed after extrusion through a die, is defined as

the ratio of the cross-sectional area of the extrudate to that of the

die. The phenomenom is attributed to polymer orientation during the

flow and slippage of molecular entanglements; on exit the elastic

component of the polymer system undergoes elastic recovery resulting

in a shrinkage in length and lateral expansion.

A standard method of measuring die swell or extrusion shrinkage

was developed originally by Dannenberg and Stokes 39 to evaluate carbon

black structure and is adopted by ASTM (D-2230, method B). Cotten31

reported that the measurement of die swell at constant stress eliminates

those transient effects which are dependent upon die geometry.

With increasing mixing time several authors 14- 19 have reported

that extrusion shrinkage rapidly reaches a maximum upon attainment of

a high carbon. black dispersion, after which decreases gradually as

mixing progresses. This decrease in elasticity on continued mixing was

suggested by Tokita and Pliskin40 to be due to molecular degradation

while Cotten41 associated it to the formation of bound rubber.

The standard method of measuring die swell is based upon the

volumetric method. The MQnsanto Processability Tester42 , introduced

76

recently, involves the use of laser for rapid determination of die

swell. The two methods of measurement appear to be comparable with

one another.

3.2.1.2.4 DYNAMIC RESPONSE

Dynamic properties of raw rubber compounds are usually carried

out using orthogonal type rheometers. It is convenient to consider

the dynamic properties of. rubbers as a complex sum of their elastic

and viscous properties. The complex shear moduius G* is composed of a

truly elastic or in-phase component G' and a viscous or out-of-phase

component G" so that

G* = G' + iG"

where i signifies a component 900 out-of-phase. Phase angle tan 0 can

be obtained by taking the ratio G" /G' •

3.2.2 RESISTIVITY

The degree of dispersion, apart from ultimate particle size and

structure, of carbon black in a rubber vulcanisate has long.been known

to influence its electrical resistivity12,19. Generally, resistivity

increases with increasing degree of dispersion although it goes through

a minimum at extremely low states. of distribution of black. Boonstra

and Medalia19 suggested that this minimum arises from two opposing . .

tendencies: the incorporation .and gradual disappearance of large black

agglomerates, which have low resistivity, and the appearance of colloid

ally dispersed black in the matrix, which increases the' resistivity.

Test conditions and the state of the sample greatly influence the

magnitude of the resistivity values.. As such cured and uncured mixes

possess different resistivity although the effect of relative degree of

dispersion remains basically unchanged. Samples'can also be conditioned

7T

before their resistivities are measured.

However, although the measurement of resistivity and the degree

of dispersion correlate quite well it is 'still not possible to develop ~

instruments, based on that principle, to ~be used on the factory floor,

due to the contact resistance problem.

Boonstra43 ,recently developed a novel method of measuring the

resistivity of uncured rubber compounds by using a coaxial electrode.

The sample is formed in these electrodes under 'pressure and the

resistivity is read after the sample has reached the state of equil

ibrium after a short period. Coefficients of variation of the results

obtained were reported to be between 4-8%. Such an instrument could

provide a useful rapid test for carbon black dispersion.

3.2.3 VULCANISATE PROPERTIES

The attainment of maximum reinforcing capacity of fine fillers is

directly related to their fine distribution throughout the rubber matrix.

However, the improvement in properties, with increasing dispersion tends

to' level off when about 90 percent of the fillers are well dispersed.

Further dispersion beyond this level is not only commercially

uneconomic but also rarely leads to significant improvement of the

desired properties.

Relationships between carbon black dispersion and the various

vulcanisate properties have been investigated by several workers. One

of the early works in this area was conducted by Dannenberg12 who

'reported that extremely short mixing times are sufficient to produce

vulcanisate properties usually possessed by well dispersed vulcanisates;

thus mixing HAF black in natural rubber and cold SBR in a laboratory

(Model B) Banbury for 1.1 and '1.25 minutes respectively at 77 r.p.m.

yield vulcanisate properties which correspond to good dispersion.

78

Maximum tensile strength, modulus, elongation, hardness, tear strength,

and abrasion resistance are attained after about 3 minutes of mixing.

Boonstra and Medalia19 also showed similar observations except that

the tear strength appeared to be insensitive to mixing time. Stude

baker and Beatty14,lS, however, reported that during the period following

the maximum Troque or power consUIl1ption, the physical properties of

the corresponding vulcanisates follow a rather rugged pattern but

smooths out on further mixing'whereby the level of dispersion reaches

a plateau. Static compression, permanent set and hysteresis decreases

with increasing dispersion. Improved tread wear resistance has always

been assoCiated with high level of dispersion.

79

REFERENCES

. 1. Jacobs, H. L., A.C.S. Rubb. Div. Meeting, Los Angeles, California,

May 1969.

2. Stumpe Jr., N. A., and'Railsback, H. G., Rubb. World, 151 (3), 41. ~

(1964) •

3. Bussemaker, O. K. F., Rev. Gen. Cout. Plast., ~ (11),1455 (1965).

4. Medalia, A. I., and Walker, D. F., Cabot Corp. Tech. Report,

RG-124.

5. Leigh-Dugmore, C.H., Rubb. Chem. Tech., 29 (4),1303 (1956).

6. Leigh-Dugmore, C. H., "Microscopy of Rubber", W. Heffer 80 Sons,

Cambridge, England, 1961.

7. Medalia, A. I., Rubb. Chem. Tech., 34 (1), 1134 (1961).

8 .. Medalia, A. I., Rubb. Age, 21. (1), 82 (1965).

9. Hess, W. M., Rubb. Chem. Tech., ]2 (1), 228 (1962).

10. Cosslet, V. E;, and Nixon, W ~ C., J. App. Phys. 24, 616 (1960).

11. Ford, F. P.,and Mofflau, A. Y., Rubb. Age, 70 (4), 457 (1952).

12. Dannenberg, E. M., Ind. Eng. Chem., 44 (4), 813 (1952).

13. Drogin, I., Rubb. Age 80 (3), 457 (1956).

14. Studebaker, M. L., and Beatty, J. R., Rubb. Age, 108 (5), Pt. i.,

21 (1976), 108 (6), pt. 11,21 (1976).

15. Beatty, J. R., and Studebaker, M. L., Rubb. Age, 1Q§ (11), Pt. I,

2l (1967), 108 (12), pt. II, 27 (1967) •.

16. Turetzky, S. B., Van Buskirk, P. R., and Gunberg, P.F., Rubb. Chem.

Tech., .!!2. (1), 1 (1976).

17. Tokita, N., and Pliskin, I., Rubb. Chem. Tech., 46 (5), 1166 (1973).

18. Pliskin, 1., Rubb. Chem. Tech., ~ (6), 1218 (1973)

19. Boonstra, B. B., and Medalia, A.' 1.. Rubb. Chem. Tech .• 36 (1), . -115 (1963).

20. Mooney, M •• Ind. Eng. Chem. Anal. Ed. §,. 147 (1934).

80

21. Brydson, J. A., "Flow Properties of Polymer Melts", Butterworth &

Co., London, 1970.

22. Ford, F. P., and Gessler, A. M., Ind. Eng. Chem., 44 (4), 819 (1952).

23. Kraus, G., Poly. Letters, §., 601 (1970).

24. Medali~, A. 1., Rubb. Chem. Tech., !t2., 1171 (1972).

25. Borzenski, ·F. J., A.C.S. Rubb. Div. Meeting,. Chicago, Illinois,

May 1977.

26. Cogswell, F. N., and Lamb, P., Plast. and Poly., 38, 331 (Oct.

1970) •

27. Cogswell, F. N.; Plast. and Poly., 38, 391 (Dec. 1970).

28. Gessler, A. M., Proc. Int. Rubb. Conf., .Brighton, U.K., 249 (May

1967) .

29. Payne, A. R. , Rubb. Chem. Tech. , 39, 365 (1966) .

30. Payne, A. R., Rubb. Chem. Tech. , ]2., 915 (1966) .

31. ". Cottpn, G. R., Rubb. Age, 100 (11), 51 (1968).

r 32. Turner, D. M. , Moore, M. D., and Smith, R. A., Bob Payne Memorial

Symp;, Univ. Loughborough, U.K., April 1978.

33. Dove, R. A., Turner, D. M., and Martin, T., I.R.I. Inter. Rubber

Conf., Brighton, U.K., May 1977.

34. Lim, I. C.,. and Maxwell, B, 47th Meeting Soc. Rheol., NYC, U.S.A.,

March 1977.

35. Ninomiya, K., and Yasuda, G., Rubb. Chem. Tech., 40, 493 (1967).

36. Tokita, N., and White, J. L., J. App. ·Poly. ScL, .2. 1929 (1965).

37. Tokita, N., and White, J. L., J. App. Poly. Sei., 11, 321 (1967).

38. Snyder, R. H., and.Nichols, P. M., Natural Rubber Research Conf.,

Kuala Lumpur, Malaysia, Sept. 1960.

39. Dannenberg, E. M., and Stokes, C. A., Ind. Eng. Chem., 41,812

40. Tokita, N., and Pliskin, Rubb. Chem. Tech., 46 (5), 1166 (1973).

41.' Cotten, G. R., Rubb. Chem. Tech., 48, 548 (1975).

42. Hanna, G. L., Barker, R. I., and Rodger, E. R., A.C.S. Rubb. Div.

Meeting, Chicago, Illinois, Ma;y 1977,.

43. ~onstra. B. B., Rubb. Chem. Tech., .2Q (1). 194 (1977).

-

'82 '

-

QIAPiER LI

VISUALISATION OF FLa·, DURING TIlE P~CESSING

OF PJJBBER IN AN IN1ERWl.L MIXER

4.1 INTRODUCTION

The mechanics of flow in the chamber of an internal mixer are com

plex and poorly understood. The most effective investigations to date

have been simple optimisation studies 1 ,2, relating effective mixing

times to the basic variables of rotor speed, ram pressure ·and cooling

water temperature, by measurement of some property of the mixed rubber.

Although these techniques are useful they do not provide the engineer

or technologist faced with process development or trouble-shooting

problems·with the insight necessary for effective optimisation or

problem solving.'

Simplified hydrodynamic models of flow in the region of the rotor

tip have been presented by Bernhardt 3 , Bolen and Colwel14 , GuberS ,6,

Udal'tsov7 and Stupachenko 8• While providing some fundamental insight

. into the shearing action in the region of the rotor tip the very con

siderable simplifications of both rotor configuration and rheological

properties severely limit.the practical applications of these analyses.

It can be seen that a very considerable gap exists between the

requirements of the processor and the capabilities of current mathemat

ical tlow analyses to fulfill these requirements. The fundamental

problems are those of the complexity of the rheological behaviour of

rubber and the intentionally imposed 'disorder' of flow in the internal

mixer. Boundary conditions and justifiable assumptions are difficult

to determine due to non-steady state·conditions.

The flow visualisation studies. described here were conceived as an

alternative, pragmatic, approach to an· understanding of the mechanics

of mixing. In adopting this. approach the difficulties of boundary con-

83

ditions and uncertain assumptions can be largely circumvented. However,,'

the technique should not be seen as standing in isolation. A very good

comparison can be drawn with hydrodynamic. studies of estuaries and

tideways. Here models have been used 'for many years but recently, with

accumulating data and improvements in techniques of mathematical

analysis"it is found that in many cases a viable mathematical model

can be constructed. The mathematical model then allow,s the effects of

changes to be computed far more quickly and economically than does the

alternative physiCal model.

4.2 EXPERIMENTAL

The work was 'carried out using a Brabender Plastograph having a

'cam type' mixing head. ,The transparent plastics mixing chamber took

the same form as the steel chamber also used in these trials; the clear

ances between the rotor tips and the' chamber wall .. ere in the order of

0.3 mm to give shear rates comparable with the B Banbury used in later

correlation trials. For the visualisation experiments a rotor speed

of 15 rev/min was used to limit the stresses in the plastics chamber.

The elastomer used was a silicone gum (ICI grade SE33), .. hich gave the

desired transparency and 'rubbery' behaviour while yielding the

relatively 10 .. shear stresses required with the plastics chamber.

The technique of flow visualisation is essentially a dynamic one,

since the phenomena observed during mixing are very dependent upon time

and rate. The ideal method of recording experimental data is to use a

cine camera or a high speed camera. During cinematography it is also

possible to place an inclined mirror underneath the mixing chamber to

: record the axial flow characteristics simultaneously with the circum

ferential flow. Problems of focussipg and image size make this

84

-

PRESSURE

!

\, \,

\

FI,G. 4.1 CRqSS-SECTION OF BRABENDER PLASTOGRAPH MIXING CHAMBER. LETTERS REFER TO REGIONS ITEMISED IN THE DISCUSSION;

i

difficult with normal still photography. Some aspects of the discussion

of flow visualisation are therefore drawn from cine film data which

cannot be presented here.

,Photographs presented in this chapter to record the various flow

patterns were taken with an 'Olympus' camera fitted with an extension

ring. The film used was a high speed Ektachrome which gives good colour

saturation at high speed essential for the work and produce slides from

which colour prints were obtained. The camera was positioned directly

in front of the mixing chamber,about 1 metre away. Two photographic

lights were also used; they were positioned by the sides in such a way

that they form an angle of about 450 with the. plane of the mixing

chamber. Under such conditions a shutter speed of 1/250 s was

used with an'aperture of 5.6.

Parallel studies of mixing oil-extended natural rubber, (OENR) and

oil-extended styrene-butadiene rubbe, (SBR 1712t, with 50 parts by.

weight of RAF carbon black in the steel chamber were carried out for

: the purpos'e of correlating the behaviour of the 'model' rubber mix with

a more practical one under similar mixing conditions. A Dynisco

pressure transducer (0-1500 psi) let into the wall of the mixing

chamber in the path of the rear rotor tip (Fig. 4.1) was used in con

junction with a U. V. recorder to provide the necessary data for comparison

of the flow characteristics.

Finally, the effect of fill factor on extensive mixing (the uniformity

of distribution of additives, sometimes called simple mixing) 'was

investigated using a Banbury size B mixer. In these trials fill

factors of 0.5, 0.6, 0.7, 0.8 0.9 and 1.0 were used while other mixing

conditions 'were held,.'constant'. The mixing cycle used was as follows:

1) Load all ingredients except for accelerator, and sulphur and mix

for 7 minutes 15 seconds.'

86

2) Add accelerator and sulphur then mix for' a further 45 seconds.

3)' Dump and sheet off on two-roll mill.

For each mix the rotor speed was 77 rev/min, starting temperature 250 C

and the ram pressure 0.28 MPa. The mix is given in Appendix I.

By taking samples from different regions of the mix, vulcanising

the samples in sheets and carrying out tensile tests, it was hoped that

a coefficient of variation could be established from the results which

would reflect the dispersion of the curative system and the efficiency

of mixing. '

4.3 RESULTS AND DISCUSSION

4.3.1 FLOW VISUALISATION

A detailed examination of the flow patterns can best be made by

considering separate regions of the mixing chamber and their inter

action with each other, namely,

A. in front of the'rotor tip

B. the tip region

C. behind the rotor tip

D. 'on the bridge

E. between two adjacent wings of'a rotor (axial flow).

A. IN FRONT OF THE ROTOR TIP

The region in front of the rotor tip is sickle-shaped in positions

well away, from the bridge. This sickle shape, formed by the curved

surfaceS of the rotor and the cylindrical wall of 1I1e mixing chamber,

converges to a minimum clearance at the rotor tip. Due to the motion

of the rotor this region is constantly filled with material provided

that the fill factor is in the order of 0.7 dr greater (Figs. 4.2, 4.3

and 4.4).

87

Due to the .converging form of the region the flow is complex. For

a simple Newtonian liquid the situation can be adequately described by

the wedge term of Reynolds equation9 , indeed, this i; the approach used

by some investigators. However, Cogswell1 0., 11, shows that in the

converging flow of a polymeric material the tensile properties contrib-

ute to the overall behaviour, in addition to the shear properties.

If the re'gion in front of the rotor tip is continuously filled then

streamline flow occurs. This is a summation of pressure flOW, due to

the convergence of the flow path, and drag flow, due to the motion of

the rotor relative to the chamber wall. The flow profile in this region

is best illustrated by the markers inserted in' front of the tip of the

left-hand rotor in Figs. 4.5a and b. The shape'of this profile is a

result of the shear and tensile flow properties, which produce a con-

siderable distortion as the markers move in a rapidly decreasing

aperture. It should be realised that a velocity profile for the

initial position of the markers cannot be drawn directly from Figsi}.5a

. and b. However, the separate components of flow and the summated

velocity profile areehown schematically in Fig. 4.7. The relative and

absolute values of the two components will vary continuously with

position but the configuration of the flow. profiles of Fig. 4.5b

indicates a considerable velocity differential between 'adjacent stream-

tines'. This points to high stresses and an effective mechanism for

breaking down and dispersing filler aggregates.

This region of flow is the most amenable to mathematical analysis .,

but it must be remembered that the rotor blade or wing is set at an

angle to the direction of rotation and that axial 'flow is an important

factor. Also considerable extensional flow will occur as a result of

the convergence of rotor and chamber wall.

88

B. THE TIP REGION

The flow over the rotor tip can be considered to be equivalent to

the 'leakage flow' occurlng over the flight tips in an extruder. This

region is the most difficult to deal with by visualisation due to the

very small size and intense stresses. However, the flow behaviour can

be deduced from the regions immediately in front of and behind the tip.

Consideration of the region behind -the rotor tip (C) shows that the

pressure is lower than in front of the tip. The pressure flow is

therefore reversed and opposes the drag flow. It should be possible to

determine the magnitudes of these components of flow from considerations

of geometry and from pressure transducer readings. A scaled velocity

. profile could then be drawn as a direct aid to understanding the

behaviour in this region. Since the rotor tip area is considered to

provide the major mechanism for breakdown of filler aggregates it is

important to quantify both the amount of material that actually passes

under the rotor tip and the stress distribution, in order to assess the

overall contribution to the characteristics of the final mix.

C. BEHIND THE ROTOR TIP

The material behaviour behind the rotor tip is extremely dependent

upon time and rate, requiring-that reference is made to cine film studies.

However, Figs. 4.2, 4.3 and 4.4 show clearly the voiding behind the

rotor tip and its dependence on fill factor. Even at a fill factor of

unity (Fig. 4.4) slight voiding is still detectable, although this is

probably analogous to a cavitation effect and does not play ·a signifi

cant role in mixing.

The extremely well ordered flow of Fig. 4.4, as indicated by the

tracer materials, shows clearly the very poor extensive and dispersive

mixing obtained unless some mechanism for 'disordering' of the flow

regimes is included. Reducing the fill factor and thus increasing the

voiding behind the rotor tip provides this mechanism. Referring to the

cine film studies, it is found that the leakage flow under the rotor

tip causes a layer of material to be 'sheeted out' onto·the wall of

the mixing chamber. This layer then separates from the wall and

becomes sharply folded as it undergoes elastic recovery (shear and

tensile recovery). ~nis material isthen swept up by and incorporated

into the now front of the material being 'pumped' round by the action

of the rotors. .This flo)( front, which delineates the boundary of the

void, is shown clearly in both the left and right lobes of the mixing

chamber in Fig. 4. 3b •

The visco-elastic properties of the material undergoing mixing are

extremely important for the effectiveness of this mechanism. The main

requirement is for the sheet to retain its integrity after. leaving· . r

the chamber wall and until it is incorporated into the following ·flow

front. A rubber having a low extensibility before fracture would

undoubtably break up here and create the problem of mixing failure, due

to crumbing of the rubber and filler separation, which is familiar to

most processors.

Fracture will be dependent upon time, since the properties of the

material passing under the rotor tip change rapidly with mixing time.

Rate (rotor speed), temperature and fill factor can also be expected

to significantly influence the fracture behaviour.

The possibility of fracture behind the rotor tip does explain why,

with some rubbers, e.g. butyl, EPDM and some grades of NBR, it is

generally believed that better mixing is. obtained at higher fill

factors. If the presence of a void results in the reduction of a mix

to crumb form it is necessary, given the present state of the· science

of mixing, to ensure that there are no discontinuities in the flow.

However, a longer mixing cycle is indicated in these cases, to offset

90

the poor extensive mixing obtained at high fill factors.

D. ON THE BRIDGE

The flow in the region of the bridge is required to perform two

critical functions:

a) to rapidly incorporate materials fed to the mixer into the bulk of

the rubber mix in the chamber.

b) to provide a .mechanism of exchange of material between the two lobes

of the mixing chamber.

If the addition of materials to the empty mixer is being considered

function (a) can be viewed as a requirement for rapid engagement of the

materials with the rotors.

The motion of particulate pigments added under the ram is highly

dependent upon fill factor. Fig. 4.4 shows that with a fill factor of

unity the flow streamlines create an area just under the ram in which

there is'no apparent movement. However, this may be a result of the

flat bottomed ram used. From Fig. 4.3 it can be seen that a fill factor

of 0.7 results in the bottom of.the ram being swept by flows in alter

nating directions, dependent upon the rotor wing which happens to be

adjacent to the bridge. Pigments added under the ram are immediately

subjected to flow and swept into the sickle shaped zone of region A, in

either lobe of the mixing chamber.

These observations indicate that incorporation is efficient with

the type of rotor used, provided that a suitable fill factor is specified.

It was not possible in these trials to add the large volumes of part

iculate material used in a conventional rubber mix, which would confirm

;.the validity of these results for practical mixing. However, with the

exception of the acceptance of bulk particulate materials the observations

are considered to be valid for industrial scale mixing.

91

The mechanism of material exchange between the lobes of 'the mixing

chamber is dependent upon ,the relative position of the rotors. The .~

\ rotors of the Brabencie:t'" Plastograph run at' a ratio of 3: 2, so that a

'I

favourable configuration between a pair of ·opposing wings can occur

every three revolutions of the faster rotor and ~very second revolution

of the slower one •• It consists essentially of one rotor wing pumping

material across the bridge into path of the wing on the other rotor,

which then carries the material gained into the sickle shaped zone of

region A. This is also the mechanism for incorporation, material placed

under the ram will be taken in by this flow. For effective exchange it

·wouid ,appear to be necessary fbr one rotor to pump material directly into

the void behind the ti~ of the opposing rotor, otherwise the balance of

pressure inhibits the exchange. This again emphasises the importance

of fill factor.

The sequence illustI'ated in Fig. 4.6 is taken at intervals of

approximately three revolutions of the slower rotor. The fill factor

is 0,7. Fig~ 4.5a being deceptive due to the method required for loading.

These may be compared with Fig. 4.4a, taken after approximately 6

revolutions and Fig. 4.4b after approximately 12 revolutions. Very

little exchange between the lobes ·of the chamber is apparent with a fill

factor of unity.

E. BETWEEN TWO ADJACENT WINGS OF A.ROTOR (AXIAL FLOW)

Each rotor has. four wings. the tips of which are ,set at 900

intervals •

• a centre section carried two wings at 1800 to each other while each end

section carries one rotor tip. 'The rotor tips are angled to give an

'axial component of flow in addition to the cfrcumfer,ential component'.

This results in material being transferred from the high pressure zone

in front of one wing into the low pressure zone behind the leading

92

•

adjacent wing. Some transfer is also effected into the lower pressure

zone in front of the trailing w1ng. At the ends of the mixing chamber

transfer can only occur towards the centre. Cine film studies show

that a 'bow wave' occurs in front of a rotor tip and that axial flow

is dependent more upon pressure than on the angle of the rotor tip, for

the rotor design used.

A further consequence of the transfer of material out of the paths

of the rotor tips is an unequal distribution of 'shear history' in the

\ mix. This is particularly true when a high fill factor is used and the

l extensive mixing mechanism' due to the void behind the rotor tip is not

formed. The result is a corresp:niliIl>1YunequaJ, degree of breakdown of

filler aggregates, hence a poor dispersion. The cam type rotor is

particularly inadequate from this point of view due'to the very short

length of the rotor tips. The material in the mix can probably be

separated into that which has passed under the rotor tips and that

which has not. The latter, in a practical mix, would present more

resistance to flow (higher apparent viscosity) and would probably be

less likely to pass under the rotor tip than the material already

sheared.

This is partly 'confirmed b~ trials using a steel m1x1ng chamber

- with mixes of OE NR and OE SBR including 50 parts by we;i.ght of RAF

carbon black. At a fill factor of unity large undispersed aggregates

of carbon black were observed even after ten minutes of mixing. However,

the additional contributions to poor dispersion from the lack of

exchange of material between the lobes of the mixing chamber, the 'dead

area' under the ram and the absence of the voids behind the rot'or tips

must also be considered.

93

a

b

FIG. 4 .2: MIXING WITH FILL FACTOR = 0.5

94

a

b

FIG . 4.3 : MIXING WITH FILL FACTOR = 0 . 7

95

a

b

FIG . 4. 4: MIXING WITH FILL FACTOR = 1. 0

96

a

b

FIG . 4 . 5 : FLOW PROFILE IN FRONT OF ROTOR TIP

97

c

d

98

e

f

99

FIG . 4. 6: MATERIAL EXCHANGE BETWEEN THE TWO LOBES OF THE MIXING CHAMBER

100

a

b

c

d

101

A

ROTOR •

MIXING CHAMBER WALL

DRAG FLOW

FIG. 4.7: DEVELOPED SCHEMATIC OF SHEAR FLOW VELOCITY PROFILE AT HYPOTHETICAL SECTION A-A IN FRONT OF THE ROTOR TIP

102

4.3.2 PRESSURE VARIATIONS IN THE MIXING CHAMBER

The pressure, in the mixing chamber was recorded at fill factors

of 0.5, 0.7 and unity using a DYnisco meit pressure transducer located

~n the path of the rear rotor tip at the position sliown in Fig. 4.l.

The peaks in the traces of Figs. 4.8, 4.9 and q.10 correspond to the

passage of the rotor tip over the transducer.

The shape of the major peak and the maximum pressure value is, in

part, dependent upon the size of the pressure transducer tip (8 mm

diameter) in comparison with the mixing chamber size (each lobe is 40

mm in diameter) and the practically knife edge of the rotor tip.

Although the exact position of the rotor tip'could not be determined in

relation to the pressure traces a number of points can be made by

deduction:

(i) the maximum pressure reading is likely to occur as the rotor tip

is just starting its traverse across the pressure transducer tip.

(ii) the fall-off in pressure recorded after the passage of the rotor

tip is less abrupt than the actual pressure drop.

At the point when the rotor wing is about to start its traverse

across the pressure transducer the whole of the transducer tip is in

the sickle-shape'd zone fonned in front of the rotor. The pressures in

this region change ~rkedly with position, resulting in an average

pressure reading being taken due to the finite size of the transducer

tip. If it'is assumed that the pressure changes linearly with position

the peak pressure recorded will be equivalent to the actual pressure

at a point 4 mm in front of the rotor tip.

As the rotor tip sweeps over the pressure transducer the transducer

,. tip will ,be progressively exposed to the low pressure region behind

the rotor. ,This results in a progressive fall in pressure which must

be discounted in order to obtain the truepressure drop characteristics.

103

PRESSURE (MPa)

0 • .5

0.25

'0

, ONE. REVOLUTION I,

c 0

FIG. 4.8 TYPICAL PRESSURE TRACE FOR ONE ROTOR REVOLUTION ':! AT A FILL FACTOR OF 0.5 ",

, ,

104 •

'"

1.0

PRESSURE (MPa)

0.5

o ONE REVOLUTION

\ .

FIG. 4.9: TYPICAL PRESSURE TRACE FOR ONE; ROTOR REVOLUTION AT A FILL FACTOR OF 0.7

105

•

•

-

•

PRESSURE (MPa)

4

3

·2

1

O~------~---------------------ONE REVOLUTION

/

, FIG. 4.10: TYPICAL PRESSURE TRACE FOR ONE ROTOR

REVOLUTION AT A FILL FACTOR OF UNITY

106 ,.

..

'.

In order to obtain an accurate direct' measure of pressure

variations in the region of the rotor tip it is necessary for the

pressure transducer tip to ·be small in co.mparison with the region,

Given the size of current commercial pressure transducers this requires

the instrumentation"of a larger mixer.

The general form of each peak on the pressure traces is assymetric,

with a shoulder on the leading edge due to axial flow effects. This.

is the 'bow wave' effect discussed in the visualisation section.

The low and variable pressure peaks encountered using a fill factor

I of 0.5 confirms that th~ zone in front of the rotor tip is not contin

uously or sufficiently filled. Also the lack of a shoulder in the

trace, except in a few isolated cases, (where it appears as a separate

but smaller peak leading the primary) indicates that aXial material

transfer is inadequate to achieve good mix uniformity.

fhe higher pressures· and more regular peaks recorded using a fill

factor·of 0.7 indicat~ a consistent and adequate filling of the zone

in front of the rotor tip, coupled with a regular axial pressure com-

ponent.· However, it is possible that the shoulder of the pressure

peak is due to a pressure wave in the material rather than an axial

transfer of material, although the presence of the shoulder does confirm

the existence of a pressure differential which should be adequate to

transfer material into the void behind the leading rotor tip.

• The presence of the void behind the rotor tip fs confirmed by the

fact that the pressure goes to zero during each revolution of the rotor.

That it does not drop immediately to zero is attributed to the effect

of transducer size already discussed, although· the material sheeted out

onto the wall.of the mixing chamber may exert a positive pressure

immediately after the passage of the rotor tip.

High pressures and a continuously maintained pressure in the mixing

.

chamber are characteristic of a fill factor of unity. The very pro-

nounced shoulder with its two humps is attributed to pressure wave

effects rather than to material transfer.· The first and larger hump

is due· to the passage of one of the wings in the c~ntre'section of the

rotor past, the pressure transducer. The second and minor hump is due

to a pressure wave transfered from the wing at the front (opposite end)

of the chamber •

4.3.3 DEPENDENCE OF MIX UNIFORMITY ON FILL FACTOR

For convenience the tensile test waS chosen as a measure of the

,- dependence of:'mix variation on fill factor, deriving from B Banbury

•

trials. The· dependence of mix uniformity on fill factor was determined

·by calculating the coefficients of variation, as governed by the

distribution of sulphur and accelerator. The variance, S2. , was first

obtained: N

S2 = E (X, ..: X )2 (4.1 )

i=l l.

N - 1

Where S = Standard deviation

N = Number of samples

X, = Value of sample i l.

X = Mean value of N samples.

The coefficient 'of variation, ,a, is then derived:

§. x 100 (4.2) a =

X

The data obtained are presented in Table 4.1 and 'show some definite

\ systematic effects. For the recipe used a fill factor of 0.7 was

l found to exhibit the minimum coefficients of variation. The very marked

dependence of these coefficients of variation on fill factor is con-

sistent with the observation made during·the viSUalisation studies that

'1'08

voids behind the rotor tips are essential to material transfer between

the lobes of the chamber and to transfer in an axial direction vital

to extensive mixing. This also confirms that observations made from

the Brabender cam head system apply· equaliy to the commercial Banbury

rotor type.

TABLE 4.1: EFFECT OF FILL FACTOR ON MIX UNIFORMITY

SBR MIXES

Fill Tensile Stress at Stress at lFactor Elongation Strength 300% 500%

a~ &f'eAk

0.5 4.5 4.5 8.6 8.3

0.6 5.0 5 3.0 1.7

0.7 4.6 3.1 2.1 1.7

0.8 12.0 15.0 10.7 4.2

0.9 10.4 16.7 10.5 5.4

1.0 6.9 16.0 17.2 15.3

NR MIXES

Fill Tensile Stress at Stress at ""actor Elongation Strength 300% 500%

0.5 5.0 7.7 8.6 5.1 . 0.6 5.5 6.8 3.2 1.8

0.7 4.5 3.5 2.2 1. 5 .

0.8 10.2 12.3 8.3 6.5

0.9 11.3 14.1 12.2 10.1

1.0 15.1 16.3 16.2 15.1 ..

109

The level o~ mix homogeneity can also be analysed ~rom the

Oscillating Disk Rheometer curves. The best consistency o~ minimum

and maximum torque readings are seen to be possessed by the mixes

prepared using lower ~ill ~actors •. The slopes o~ the portion of the

curves, which correspond to the cure rates, seem to indicate that the

highest rates of cure are exhibited when a fill factor of 0.6 is used.

110

REFERENCES

1. M. G. Peakman, Inst. Rubber Ind. Conf., Leamington Spa, U.K., 1972.

2. P. Whitaker, J. Inst. Rubber Ind. ~,153 (1970).

3. E. C. Bernhardt, 'Processing of Thermoplastic Materials', Van

Nostrand Reinhold, New York, (1959), pp. 424-446.

4. w. R. Bolen and R. E. Colwell,.Soc. Plast. Eng. Technical Paper

No. 98, 4, 1004 (1958). - .

5. F. B. Guber, SOy. Rubber Technol. ~ (9), 30 (1966).

6. F. B. Guber, Sov. Rubber Techno1. 26 (1), 23 (1967).

7; V. c. Udal'tsov, E. G. Vostroknutov, and.M. I. Novikov, SOy.

Rubber Technol. 31 (6), 10 (1972).

8. 0; G. Stupachenko,·A. P. Pukhov, and K. D. Bebris, SOy. Rubber

Technol. 30 (7), 17 (1971).

9. D. F. Moore, 'The Friction and Lubrication of Elastomers', Pergamon,

Oxford, (1972).

10. F. N. Cogswell, Plast. and Polymers, 38, 291 (1970).

11. F. N. Cogswe11, Polymer Engng. and Sci., 12, 1 (1972).

111

QillPTER 5

TIlE r·lIXING PROCESS AND CARB(Jl BLACK DISPERSIO:~ STUDIES , , .

5.1 INTRODUCTION

While the objective of the 'mixing process is primarily to incorp-

orate and disperse carbon black subsequent trouble-free downstream

'operations require that the mixed compound possess certain consistent . .

flow properties. With batch operated mixers, batch to batch uniformity

must also be maintained while most efficient use of equipment time and

power must be utilized.

Lack of uniformity from batch to batch is attributed to ,variation

of raw m~terials, incorrect weighing' of ingredients and, most important

of'all, inadequate control of the mixing cycle., Errors in weighing can

be eliminated by 'the use of automation and computer. Variation of raw

materials can be minimised by blending several lots of rubber. However,

raw materials still need to 'be rapidly tested'to adequately characterise

,and 'predict their behaviour during processing - a suitable instrument

for such testing is still unavailable. Control of the mixing cycle,

which largely influences the quality of the final mix, is still

unspecific. The traditional methods using mixing time and temperature

are basically empirical in principle and are largely dependent on mixer

type, size, mixing technique' and conditions. It has been reported

recently that energy or work input can be used as a mixing parameter l - 3,

which enables a more precise prediction of compound properties to be

made, irrespective' of mixer type and mixing conditions used, so that

batch to ,batch uniformity can be readily attained.

112

Using a Banbury internal mixer and Brabender Plastograph two series

of mixes, based on styrene butadiene rubber and on natural rubber, were.

prepared at different mixing times ·and hence varying energy inputs.

Apart from the standard laboratory and factory test methods new measure

ment techniques have been employed to study the more relevant fundamental

rheological properties. These properties are correlated with the energy

input while the mixing performances of the two mixers are analysed.

Capillary rheometer is used to study mixing in a definable shear

field since material passing through the die is subjected to stresses

and strains. the magnitude of which can be readily calculated. A

preliminary experiment is carried out to observe the effect of the

. shear. stress on carbon black· dispersion.

5·.2 ·COMPOUND PREPARATION

5.2.1 EXPERIMENTAL

A Farrel-Bridge laboratory Banbury (Model B) internal mixer and

Brabender Plastograph with a cam type mixing· head were used in this

work. Two series of carbon black-filled rubber mixes were prepared,

based on an oil-extended styrene butadiene rubber (SBR 1712) and an

oil-extended natural rubber and HAF carbon black (Vulcan 3). The

formulations of the mixes prepared are given in Appendix I. An upside

down mixing technique was employed. As determined in Chapter 4 a fill

factor of 0.7 was used for all mixes prepared in both mixers. The

mixing cycle used was as follows:

1) Load all ingredients,.except for sulphur and accelerator and mix

for the specified mixing time.

2) Two minutes before the end of the relevant mixing time add the

curatives, sulphur and accelerator.

3) Dump and sheet. off on two-roll mill at 8ooc.

n3

Mixing time~ used range from 2 to 13 minutes. Conditions of mixing

used were:

Rotor speed

Ram pressure

=

=

77 rev/min

0.28 MPa

Starting temperature =

No cooling was used during the whole mixing cycle.'

The rotor speed of the Brabender Plastograph was adjusted to

impose a maximum shear rate of 170 s-l, which was the maximum shear

rate achieved in the nip region of the Banbury at 77 r.p.m. With a

rotor clearance of 0.4 mm a rotor speed of 35 r.p.m. was required. At

this rotor speed NR and SBR mixes were, prepared using the same cycles

and techniques as with the Banbury mixer. However with NR compounds a

higher starting ,temperature (50oC) had to be used because of the higher

1jorque ~ Several batches of· the compound at each mixing time were

prepared to provide sufficient material for the various tests.

, 5.2.2" 'RESULTS AND DISCUSSION

The temperature, power and torque mixing profile obtained on the

Banbury mixer and Brabender Plastograph are shown in Figs. '5.1 and 5.2.

The unit work (Wu )' which is defined as the energy input per unit volume

of mix, was calc'ulated from the area under the energy-time trace, which

was measured by a planimeter. 'Results, are given in Tables 5.1 and 5.2

and their relationship to mixing times is shown in Figs. 5.3 and 5.4.

) ,

114

t:: V1

~

----. .~ o Po.

.-,

;rl----------------------~----------------------------_,

.,

,rt ,

~

.- ." ·tJ\~ L

'r I ., J ./ - - -- -- -J

/

/ .. /

---..... - . ' ...... . _-- -

-~~----- ----. - -~~ ...... _---/

//

SBR

--- NR

/' /

2 4 6 8 10 12

MIXING TIME (MINS)

FIG. 5.1: POWER AND TEMPERATURE BANBURY MIXING PROFILE

100

80

60

40

20

u o

; i

! ::;: I

0

i 0 E-t

2000

.. i

I

I(

. . ,

2. 4

r - .~ -- - --

L

---"-------

--SBR

---NR

6 8 10 12

MIXINT TIME (MINS) ... FIG. 5.2:. TORQUE AND TEMPERATURE BRABENDER MIXING PROFILE

116

100

u 0

§ ~ "" ~

60

40

20

Table 5.1: MIXING TIME: AND UNIT WORK FOR BANBURY MIXER

MIXING TIMES OESBR OENR' - ~~- (min) . (ID/m3 ) (ID 1m3 )

2 568 521

3 867 679

5 1263 995

7 1706 1236

10 2187 1600

13 2700 1927

Table 5.2: MIXING TIME AND WORK UNIT FOR BRABENDER PLASTOGRAPH

MIXING TIME OESBR OENR (min) (ID 1m3 ) (ID/m 3 ) ,

1.5 290

2 450

3- 650 530

5 1000 750

7 1312 970

10 ' 1885 1215

13 2270

15 1550

The power-time and temperature-time traces (Fig. 5.1) of the

mixes prepared in the Banbury mixer are typical for SBR and NR compounds.

The higher viscosity ofSBR mixes results in higher dump temperature.

Although the initial 'torques of both mixes are almost the same the NR

117

3000.

r UNIT WORK (MJm- 3 )

2000

1000

.. -0- SBR MIXES ! I I -~- NR MIXES

2 4 6 8 10 13

MIXING TIME (MIN) •

1-1

hG. 5.3: RELATIONSHIP BETWEEN MIXING TIME AND UNIT WORK FOR SIZE B BANBURY

2400

1 UNIT WORK

" (MJm- 3 )

1600 "

800 '

-0- SBR MIXES

-~- llR MIXES

3 6 9 12 15

MIXING TIME (MIN) •

.I

FIG. 5.4: RELATIONSHIP BETWEEN UNIT WORK AND MIXING TIME FOR ! BRABENDER PLASTOGRAPH .

119

mix undergoes a more rapid reduction in viscosity due to the mastication

e~£ect. This can be seen more readily £rom the torque-time curves o£

the Brabender Plastograph.

SBR and NR mixes prepared on Brabender Plastograph show slightly

di££erent behaviour (Fig. 5.2). The high initial torque, due to the

ram £orcing the material down into the chamber, is common to both

mixes. With SBR mixes an initial sharp torque dropoff is observed.as

the bulk volume decreases due to filler incorporation. Shortly after

that the torque increases to a narrow maximum as the bulk volume

decreases, accompanied by increased viscosity. The time to reach this

maximum (BIT)4 is surprisingly quite shQrt (less than 1 minute). After

the maximum is reac.hed a' progressive reduction in the torque is observed.

This'is attributed to polymer breakdown and increasing filler dispersion.

NR mixes, however, do not exhibit any maximum after the high

initial torque. This might, be due to the difference in the starting

temperature used. Instead it ,only shows a more rapid'torque reduction

right £rom the beginning of the mixing cycle.

5.3 OPTICAL MICROSCOPY

. Optical microscopy presents the most convenient, albeit tedious,

method of.direct assessment of carbon black dispersion, 'which can be

used to correlate with the various rheological; vulcanisate, physico

chemical, mechanical, and electrical properties o£ the mix.

5.3.1 EXPERIMh~TAL

Sectioning of samples using a microtome was carried out using·the

technique suggested by Leigh-Dugmorel~ Basically a small piece of

vulcanisate was placed on the microtome stage. A tiny drop of glue was

120

used to strengthen the position of the sample on the stage, t'hen carbon

dioxide was used to freeze it. Sections of between 2-3 ~ thick were

mounted on a microscopy slide, using xylene to swell and spread the

sectioned speCimen.·~

5.3.2 RESULTS AND DISCUSSION

The light micrographs of the microtomed sections are presented in

Figs. 5.5 to 5.8. Neither qualitative nor quantitative evaluation of

dispersion in each mix is made. From Figs. 5.5 and 5.6 rapid and

progressive increase in the level of carbon black dispersion can be seen

with SBR and NR Banbury mixes. During the initial stages of the mixing

cycle (less than 2 mins.) the process of black incorporation dominates

the mixing process. This is revealed by the high concentration of

large black agglomerates distributed in the bright background of rubber.

A closer examination of the large agglomerates also shows that they are

devoid of large holes suggesting that high degree of wetting has already'

been achieved. The presence of only a small amount of loose black when

the mixed is dumped indicates that the incorporation stage is almost

completed after 2 minutes.

As mixing progresses the incorporation process is brought to com

pletion. At the same time dispersion of the large agglomerates dominates

the process whereby the agglomerates are reduced in size to colloidal

blacks, which results in the rubber phase becoming darker in intensity.

After 7 or 10 minutes of mixing time the concentration of large

agglomerates is very small and that of the dispersed colloidal black

particles is high. Rubber compounds for most high technology products

are required to possess at least this level of black dispersion. It

must be emphasised here that the 7 minutes mixing time used is without

cooling water. With cooling water, as in commercial processes, the

/ 121

2 MINS - 568 MJm- 3

13 MINS 2700 MJm- 3

FIG. 5.5: LIGHT MICROGRAPH OF SBR MIXES PREPARED IN BANBURY MIXER (150X)

2 MIN - 450 MJm- 3

• ,._ - J _''"-- --....

.... -111l::::::: :<... ~ - ~ ,""' '. -~ ... -. '.

5 MIN - 1000 MJm-3

.-....

-- , ", ~ -- --. ,.. .-- • .-- - ,

3 MIN 650 MJm- 3 . ~ .-. ..... . • -• • . ' :., .

" -- ... ...... ~ , , " ..... ........... ...- "

.... ... . -... .... u .• , ....

" -- ,'-... .--. .-7 MIN - 1312 MJm- 3

13 MIN - 2270 MJm- 3

-

.'

FIG . 5.6: LIGHT MICROGRAPH OF SBR MIXES PREPARED IN BRABENDER PLASTOGRAPH (150X)

.. - •

..

-.. .,

5 MIN - 995 MJm- 3 7 MIN l 236 MJm- 3

•

lO MIN - l600 MJm- 3 l3 MIN - 1927 MJm- 3

Fr:> . 5 . 7 : LIGHT MICROGRAPH OF NR MIXES PREPARED IN BANBURY MIXER (l50X)

124

• . . . -"',:4e

10 MIN - 1215 MJm- 3

7 MIN - 970 MJm- 3

I

15 MIN - 1550 MJm- 3

FIG. 5.8: LIGHT MICROGRAPH OF NR MIXES PREPARED IN BRABENDER PLASTOGRAPH (150X)

125

time required for the compound to achieve the same level of dispersion

would be considerably shortened.

After 13 minutes-.the degree of dispersion in SBR and NR mixes is

very high. The level of dispersion can also be achieved if the mix is

dumped after 5 minutes_and worked on a two roll mill, such as for the

addition of curatives.

F0!n ° 0

~g. 5.6 presents m~crographs of SBR ~xes prepared on Brabender

Plastograph. Evidence- of low rate of black dispersion during mixing can be

clearly seen. Even after long mixing time and high energy input the

amount of large unbroken agglomerates are still present. It appears

that with SBR the rate of_incorporation of b~ack, under the conditions _

used, is very slow so that even after a long mixing period the incorp-

oration process is still not completed.

Mixing efficiency of Brabender Plastograph is much improved when

NR mixes are prepared at-a higher starting temperature (500 C). Almost

identical levels of carbon black dispersion are obtained at correspond-

ing energy input (Fig. 5.8).

5.4 CAPILLARY RHEOMETRY

Capillary rheometry has been widely used to study the fundamental

rheological properties of polymers under conditions close to practical

or factory processing sit,uations.- Thus properties, such as- viscosity

or shear stress, over a wide range of shear rates and temperatures are

obtained.

The basic rheometer, such as the Davenport capillary rheometer,

consists of an extrusion assembly equipped with a heated barrel and a

motor driven plunger, the speed of-which can be regulated. The material

is extruded through a die and a pressure transducer is let into the

barrel just above the die (Fig. 5.9).

If the applied pressure is assumed to be h~ld in ~quilibrium only

PISTON

BARREL

RUBBER

PRESSURE -TRANSDUCER Lr-;"77"rrPl "TT..-r.l

DIE

FIG. 5.9: PRINCIPLE OF DAVENPORT CAPILLARY RHEOMETER

. PRESSURE = o ,.

I

R I~'

\

~

Fl

P I

F3

.

F3

dZ .

L'

LIP

t d •

.. F2 t~

..

J I 4 Z DIRECTIO~

FIG. 5.10:. BALANCE OF FORCES ONA FLUID ELEMENT MOVING ALONG WITH THE FLOW IN A TUBE. EF=0=Fl+F2+F3' ··.,sothat ~lIrL [p + (6P / 6Z) dZ] lIr2 + 211rdZT = 0, thus T=r §. . - '-

2 6Z

"

127 "

by the viscous loss in the shearing layers of the material in the

capillary, that ·is, if the exterior pressure is counterbalanced (Fig. 5.10)

exclusively by the viscous loss inside the tube, the shear stress, TR'

at the rim of the capillary can be calculated from:

R6P =

2L

where R = Radius of capillary

L = Length of capillary

6P = Pressure drop across the ends of the capillary.

This assumption is an over-simplification. While there is an

extensive ·flow in the die it has been shown and confirmed by visual

studies that there is intense activity in the die entry region where

the material is subjected to a converging flow into the die orifice.

This led BagleyS to introduce the empirical· entrance effect

correction factor, n~which expresses the effective increase in the

capillary length due to the viscous drag occuring in this region. Then

the corrected shear stress TC·at the wall capillary is given by:

6PR 6P = =

Using a number of dies of constant diameter but of varying L/R, ~can

be read from the negative intercept of the pressure-shear rate plot.

Bagley's work with polyethylene melts yield linear relationships for

such curves.

The entrance correction factor, "E' has been suggested by several

workers6- 9 to be the sum of the separable viscous and elastic components.

Assuming that Hooke's Law is applicable for the elastic component

nB = J + \l SR (5.3)

nB = J + (hd T (5.4)

where J = viscous component /

128

SR = Recoverable shear

~ = viscosity.

The end correction c",!.also be made ·by using two dies of the same

diameter but different lengths. The corrected shear stress will be

= (PI - P2 ) R

(LI - L2) 2

where PI and P2 - Pressure drops across the two capillaries.

LI and L2 = Lengths of the two capillaries.

Alternatively Po' the intercept on the pressure versus LID plot,

can be used. It represents the pressure drop across the surface of

the die in the die entry region for a die of zero length. Thus

= (p - po) R

2L (5.6)

Plots of n versus T have been found to be linear for some polyethylenes

at shear stresses below that corresponding to melt fracture. From

these linear plots ~, n and SR can be obtained.

The calculated wall shear stress is related to the corresponding

. . shear rate, YN, at the wall of the capillary, which, assuming that the

fluid is Newtonian can be calculated from the equation:

=

where Q = Output rate from the capillary

R = Capillary radius.

For non-Newtonian materials the shear rate calculated from the

above equation is only an approximation and is thus commonly termed as

the apparent shear rate.

The true apparent or the non-Newtonian shear rate, Yta' is obtain

able from the Rabinowitsch equation

= (5.8)

129 "

where n' d log TC • = represents the slope at shear rate YN

on the log Ya d log Ya

versus log T curve. The magnitude of n' is a measure of deviation c • from Newtonian behaviour.

The viscosity is obtained from the knowledge of the calculated

shear stress and shear rate. •

or =

where' na = apparent viscosity

= true apparent viscosity

With a series of shear stresses obtained over a range of shear

rates a flow curve is obtained. Since, according to the power law:

T =

= -log K (l-n) log Ya

where K = flow constant

n = power law index.

" Hence from the log na - log Ya plot the parameters, K and n, can be

obtained from the intercept and the slope respectively.

Apart from the shear flow, upon which the above analysis is made,

Cogswell~ointed out that elongationalor tensile flow occurs in any

case of converging flow. He suggested that converging flows may be

analysed in terms of their extensional and simple shear components and

derived equations from which the viscosities and moduli under these two

modes of deformation can,be obtained. Using a mathematical model lO

he obtained a relationship between the die swell ratio B and recoverable,

shear' YR, TO obtain the equations for the analysis of properties under

simple tension Cogswell considered a coni-cylindrical die l !.

The relevant equations presented by Cogswell'to calculate the

fundamental properties are as follows:

130

/

Viscosity under simple shear, n a

Elasticity under simple shear, G

=

where YR = Recoverable shear obtained from

B 2 ·L =

1 3/2 + - 2) Y . R

BL is swell ratio with capillary of length L.

(5.13)

Swell ratio is the ratio of the extrudate to die diameter,.

Viscosity under simple tension, A

where n = power law index

= Pressure drop with zero

Modulus under simple tension, E =

where ER = Recoverable strain

= In B 2 0

Bo is swell ratio with

at shear rate Yo·

=

2 9(n + 1)

32 n ,a

length die at shear rate Yo

,3 (n + 1) Po

8 ER (5.15)

capillary of zero length measured

General assumptions used are that:

a) the rubber is incompressible

b) the simple shear flow properties are adequately described by the

power law, T = kyn

c) the viscosity under simple tension is independent of stress.

Capillary rheometer is used in this work to study the change in

the flow curves and rheological properties with respect to work input

and test conditions. The validity of the analysis presented by Cogswell

to obtain the viscosities and moduli under simple shear and simple

tension is investigated.

131 -'

5.4.1 EXPERIMENTAL

The capillary rheometer used in this work is manufactured by

Davenport. A ?ye-Ether pressure transducer (O~5000'psi or 0-10000 psi)

situated just above the die was used to measure the pressure drop

across the capillary and was recorded simultaneously onto a pen recorder

and a digital millivolt meter. Shear rate range of 20-600 s-1 was used.

The extrusion chamber was adjusted to maintain a constant temperature

of 100 ±loC. The material to be tested was first cut into small pieces

and after being loaded into 'the chamber it was left to stand for 10

minutes for it to attain the test temperature.

2 mm diam. dies with L/D equal to a, 20, and 30 were used.

For Cogswell's analysis extrudates obtained using dies of zero length

and 40 mm long at shear rate of 100s-1 were collected on a horizontal

platform to avoid thinning and allowed to stand overnight after which

the weight of a specific length of the extrudate was measured.


5.4.2.1 FLOW CURVES

Shear stress and shear rate dataare obtained on each of the mixes

prepared. End corrections are made on the calculated wall shear stress

by using the data obtained from a pair of dies 'and applying Eqn. 5.5

and 5.6.

Flow curves for all the mixes, calculated from the data obtained

from different pairs of dies, were. given in Fig. 5.11-5.18 in which

the results are expressed as log/log plots of shear stress versus shear

rate and viscosity versus shear rate. (Appendix IV)

Examination of log shear· stress - log shear rate curves reveals

that within a range of shear rates along each of the curves there

132

-

FIG. 5.11a: LOG STRESS-LOG STRAIN RATE RELATIONSHIP (F'SBR BANBURY MIXES

(DIE DIMENSION: D ~ 2 mm, LI = 60 mm, L2 = 0 mm.)

FIG. 5.11b: LOG APPARENT VISCOSITY-LOG STRAIN RATE RELATIONSHIP OF SBR

BANBURY MIXES

(DIE DIMENSION: D = 2 mm, LI = 60 mm, L2 = 0 mm.)

FIG. 5.12a: LOG STRESS-LOG STRAIN RATE RELATIONSHIP OF SBR BANBURY MIXES


FIG. 5.12b: LOG APPARENT VISCOSITY-LOG STRAIN RATE RELATIONSHIP OF

SBR BANBURY MIXES


FIG. 5.13a: LOG STRESS-LOG STRAIN RATE RELATIONSHIP OF SBR BANBURY

MIXES

(DIE ,DIMENSION: D = 2 mm, LI = 60 mm, L2 = 40 mm.)


SBR BANBURY MIXES


FIG. 5.14~: LOG STRESS-LOG STRAIN RATE RELATIONSHIP OF SBR BRABENDER

MIXES

,(DIE DIMENSION: D = 2 mm, LI = 40 mm, L2 = 0 mm.)

FIG. 5.14b: LOG APPARENT VISCOSITY-LOG STRAIN RATE RELATIONSHIP'OF

SBR BRABENDER MIXES

(DIE DIMENSION: D = 2 mm, LI = 40 mm L2 = 0 mm.)

FIG. 5.15a: LOG STRESS-LOG STRAIN RELATIONSHIP OF NR BANBURY MIXES



NR BANBURY MIXES '

(DIE DIMENSION: D = 2~, LI = 60 mm, L2 = 0 mm.)

FIG. 5.16a: LOG STRESS-LOG STRAIN RATE RELATIONSHIP OF NR BANBURY

MIXES

(DIE DIMENSION; D = 2 mm, Ll = 40 mm, L2 = 0 mm.) ,

133


NR BAN BURY MIXES

(DIE DIMENSION: D = 2 mm, Ll' = 40 mm; L2 = 0 mm.)

FIG. 5.17a: LOG STRESS-LOG STRAIN RATE RELATIONSHIP OF HR BANBURY

MIXES

(DIE DIMENSION: D = 2 mm, Ll = 60 ~, L2 = 40 mm.)


NR BANBURY MIXES

(DIE DIMENSION: D = 2 mm, Ll = 60 mm, L2 = '40 mm.)

FIG. 5.18a: LOG STRESS-LOG STRAIN RATE RELATIONSHIP OF NR BRABENDER

MIXES

(DIE DIMENSION: D = 2mm, Ll = 40 mm, L2 = 0 mm.)


NR BRABENDER MIXES

(DIE DIMENSION: D = 2 mm', Ll = 40 mm, L2 = 0 mm.)

134

- I LOG STRESS

55

54

53 KEY

A 568 MJm- 3

V 867 MJm- 3

+ 1263 MJm- 3

x 1706 MJm- 3

o 2187 MJm- 3

52 <> 2700 MJm- 3

/

51 1-0 15 20 25 ')0 , .. ,_I

I LOG STRAIN RATE X 10'-1 ,

I

I FIG. 5.11a , , , • -1

" 135

X 10-1 40

r 38

OG APPARENT VISCOSITY

36

34

32

30

28

, ''1\

, \ \ \ \t, \ \

, \' , , \~ . , , '

KEY

REFER TO FIG. 5.11a

',\ \ '~ \ \

\ \'

\. ' " \

\. ,

\ \ \ \\

\ , ,~,

, \ " .\~\

" \0 \ \ \

\ \ ',0 \ , \

26,.l.-__ ~_----r-------'r-----' l' 0 15 20 25 • 30

LOG STRAIN RATE X 1 0 - 1 FIG. 5.llb

136

.- j

X10- 1

56

1 (>

LOG STRESS

55

54

53 KEY

REFER TO FIG. 5.11a

, //" ,..> . . ,

ST , .

./ .

I /. /

5'·.-.~.--------~--------~---------,--------~ 1015 20 25 • 30

LOG STRAIN RATE X10- 1 FIG. 5.12a

137

I 38

LOG 'APPARENT

VISCOSIT

36

34 :i . ,

32 . '! I

30

28

\ z\ , ~ \

t:\ \ \ ~\ '

KEY \ \ \ \

\~ ~ \ ,

REFER TO FIG. 5.11a

\ \.

\ \ \ \ \ \ \ ,. \~\\

'," ',\ \ \\ \ \ '

\ '

, \t\

'\

\ \

\ )\ \ x \ , \[1

, \ \ \ ~\ '

\D \ \ \ \

\ ~ \ ' \ ,~

26~ ____ ~~ ____ ~ ______ ~ ____ ~ ,

10 , 15 20 25. 30 LOG STRAIN RATE X 1 0 - 1

FIG. 5.12b

',-

\

! LOG

STRESS

55

54

53

52

51

"-

,

1!0

1 , !

KEY .

REFER TO FIG. 5.11a

15 20 25 • 30 LOG STRAIN RATE X10- 1

FIG. 5.13a

i39

38 LOG

APPARENT . VISCOSITY

35

32

30

28

\.

KEY

REFER TO FIG. 5.11a

, ...

o

26.1, ----,----"'!"20~---:2!;5------;. Jli \ 0

1 5

LOG STRAIN RATE X 1 (r 1 . ! FIG. 5 .• 13b () "i

140

X18-2 550

1 LOG STRESS

L4S J,

CJ' 'lr~

.. J . .)

530

, , ' , I

KEY

A 521 MJm- 3

V 679 MJm- 3

+ 995 MJm- 3

¥ 1236 MJm- 3

C 1600'MJm- 3

~ 1927 MJm- 3

S20:L'~~~-L ______ ~~ ____ ~~ ____ ~~ ____ ~ 11_2 14 1618 28 __ " 22 i LOG STRAIN RAT~ 1 0- 1

FIG. 5.14a

,

141

I LOG

APPARENT VISCOSITY

REFER TO FIG. 5.14a -',,"- -------- _. ~ -

'l' , jl

34

r)rj

JJL·~----.J~---~1 b;::"'" ---~'I~).i\.i-~· ---~2nv,---~n 112 1 4 u .. ,_ ,_ , i ! LOG STRAIN RATE X 1 0 - I

FIG. 5.14b

;,

'- 142

X,, n-')

, I /' c. , ."

I LOG STRESS

SIr: L.J

515

510

-0~ JJ /

KEY

L> 450 MJm- 3

V 650 MJm- 3

+ 1000 MJm- 3

/ le 1312 MJm- 3

c 1885 MJm- 3

9 2270 MJm- 3

500_'--_~ __ -r-____ --r _____ ,.... ____ ...,

15

i I ,

20 25 LOG STRAIN RATE

FIG. 5.15a

• ., !") " , ---.... ~"

\

X1T I

i

Xl !;r- l \ v

1 LOG

APPARENT VISCOSITY

3[,

31

32 1

30

26

~ Q , \' \

\ \ \ \

\\ . \ \

\~ ~,

\' \' , \

\ . .. ,. \

·····'f ,\

KEy

. REFER TOFIG. 5.15a

\ \ , \

\

\.

\

\\ \~ \~ \ .\ \

\ i.

.. \ \ ~\

\ ~ \ ~, \~ \ \ \ \\

\.~ '..J

..

r4 ! 1-L...---~----r--__ ~.---__ ----,

15 28· 25 LOG STRAIN RATE

FIG. 5.15b

,

144

Xi 8-- 2 530

r LOG STRESS

C2'J J

520

i 5 1~ ,,)

·518

505

,

KEY

REFER TO FIG. 5.15a

I.

FIG. 5.16a

. 1 LOG STRAIN RATE ·X 1 r~ -- I

, I \...

145

LOG APPARENT

\ ,',\ ~ \

,0 \ , VISCOSITY ' \ ' \

\ , KEY

')h J,J

')4 J "

Ji

38

25

\

\ \ REFER TO FIG. 5.15a

, "

\ \ \ \

\ \

\ ~\\ , \v \

\ \ \ \

\ \\ \ :-;,

~ \v \\ '\ \

\ \ , \ \

\ \ ~\ \\' \

, "

\ \ .\ \~\ " \ \

\ 'L\ \~ \ \ \\ \ +,

\i" h.:

\

4 I ~-":"'----C--~-:------;"-'Cl L ,~.L f1 ")~ •

'~I~ 15 2\a L0.. ~c. v LOG STRAIN RATE X 1 8 - 1

FIG. 5.16b

I 1 .

146

-

r

5i5

KEY

518 REFER TO FIG. 5.15a

.....

505

28 , .

LOG STRAIN RATE·

FIG. 5.17a

147

-'1

X 1 ~- I

401 : l,

I LOG

APPARENT VISCOSITY

]1;

31

32

30

28

25

r l: \ ~j \

\ \ \ , \ \

\ \ \\ \, $' ,* \ , '

\ \

\ '\

\11\ .

. ~.\

.. \ \ . \, \

KEY

'REFER TO FIG. 5.15a

\ \ \, ),. v \ .

~\ . \, ..

v,\

'\\ \.~\

\ \ \ \ \ '\ .~

l~------~------~-;~--------75~====~~~ 'M .00 • Jo Ll_ 20 LJ , 18 , ~ . I v

LOG STRAIN RATE '. X 1 8- I

FIG. 5.1Tb

148

X10-2 530

I LOG STRES

'le:; SLv

520

c;.1c; 'Jlv

510

505

KEY

6. 290 MJm- 3

V 530 MJm- 3

+ 750 MJm- 3

'x 970MJm- 3

[J 1215 M.Jm- 3

5001~~--~~----i1I6-----j188-----:L~)0f'===:.~22 12 14 LOG """" AA'" X16- 1

FIG. 5.18a

149

X10- 1

t 'Xi -J~,

38 LOG

APPARENT VISCOSITY

J7

3r -'- J

33

32

,,' JI

~ '1\

- \ ~\

- \ \ \

\ \ - \ - ~-

- \ r _\x\

- ~ \--\ \,

\ t\ \~ \

\ \

KEY

REFER TO FIG. 5.18a

\\ - \ '

\,

\

-, \ , \ \

\ ~\ \~ \

- \ \ \ ). \~\ ) \ , \ -\ ,

\ \ \ \

\ ~\

, 30 ~12~---~1~4----~----L-----~--~

,c-,', fj:':

15 18 20 22 • LOG STRAIN _ RAX 1 0 - 1

FIG. 5.18b

150

f-' VI f-'

.TABLE 5.3: RHEOLOGICAL PARAMETERS FOR BANBURY SBR MIXES

-L60 - LO L"O - LO L60 - L"O

I UNIT WORK - .-

n ' log K n x 10- 3 IlP X 10-7 (MJm- 3) n log K n X 10-3 IlP X 10-7 n log K n X 10-3 IlP X 10-6 . a . a a ,

(NBm-2 ) (Pa) (Nsm-2 ) , (Pa) (Nsm-2 ) (Pa) !

568 0.15 5.15 2.96 3.55 0.34 4.84 3.27 2.62 0.02 5.42 2.34 9.35 I

I

867 0.17 5.1() 2.95 3.54 . 0.32 4.87 3.21 2.57 0.03 5.27 2.21 8.84

1263 0.17 5.10 2.83 3.39 0.25 4.99 3.27 2.65 0.06 5.19 1.93 7.72

1706 0.13 5.15 2.76 3.31 . 0.20 5.05 3.08 2.47 0.12 5.08 2.11 8.43

2187 0.17 5.06 2.62 3.15 0.20 5.05 3.02 2.41 0.14 4.98 1.84 7.34

2700 0.17 5.03 2.57 3.08 0.18 5.08 2.95 2.36 0.15 4.94 1.79 7·17 -------- -- -- -

N.B.: L60 = Die of length 60 mm. L40 = Die of length 40 mm. Lo = Die of 'zero' length.

,

..... VI

'"

"

UNIT WORK . , (MJm- 3 ) , '

n log K

.-- .

521 0.12 4.99

679 0.11 5.00

995 0.11 . 5.00

1236 0.11 5.00

1600 0.12 4.93

1927 0.12 4.94

L60 - LO

na x 10-3

(Nsm-2 )

1.74

1.64

1.58

1.56

1.54

1.51

TABLE 5.4: RHEOLOGICAL PARAMETERS FOR BANBURY NR MIXES

,L4 0 - LO L60 - L40

'. -- -

liP x 10-7 n log K 11 . x 10-3 liP x 10-7 n . log K Tla x 10-3 'liP x 10-6 a '

(Pa) (Nsm-2) (Pa) , (Nsm-2) (Pa) , . ,

2.09 0.14 4.96 1.70 1.36 0.1 5.06 1.83 7.31 . , 1.97 0.12 4.96 1.58 1.26 0.9 5.06 1.77 7.07

. _1.90 0.12 4.94 1.55 1.24 0.09 5.03 1.65 6.6

1.87 0.12 4.95 1.54 1.23 0.09 5.03 1.62 6.46

. 1.85 0.12 4.94 1.50 1.20 0.14 4.91 1.63 6.52

1.82 0.12 4.89 1.43 1.14 0.10 5.02 1.68 6.73 ---- --- -- --

· .TABLE 5.5: RllEOLOGICAL PARAMETERS FOR SBR BRABENDER MIXES

W ·n· log K na liP u x 10-7

x 10-3 (MJm- 3

.(Ns.m-2 ) (Pa)

452 0.27 5.0 3.15 2.51

650 0.25 5.0 3.23 3.58

1000 0.26 5.0 3.11 2.49

1312 0.26 5.0 3.10 2.48

1885 0.25 5.0 3.06 2.44

2271 0.25 5.0 3.05 2.44

TABLE 5.6: RllEOLOGI CAL PARAMETER FOR NR BRABENDER MIXER

W n log K· "a lIP

.U x 10-7 x. .10- 3

(MJm- 3 ) ; (Ns .m-2 ) (Pa)

290 0.11 . 4.99 1.55 1.24

530 0.10 4.98 1.52 1.21

750 0.13 4.92 1.51 1.21

970 0.12. 4.92 1.38 1.11

1215 0.15 4.87 1.47 1.17

-I.

, I

153

exists an inflexion, irrespective of the die dimensions and level of

carbon black dispersion •. With SBR mixes the inflexion occurs at a

shear rate of about 100 s-1 while with NR· mixes at a higher shear rate

-of about 150 s-l. --Turner et -al 12 Bug-gest that' such an inflexion could

be a fundamental characteristic of an unvulcanised rubbe~ compound,

although wall slip and fracture might also contriQute to this behaviour.

The viscosity-shear rate relationships for all the mixes seemed

to be well described by Equation 5.11, i.e.

log 11 = log K (l-n) log y

Using the least squares method of analysis the data points appear to

possess a high degree of correlation factor (r = 0.99). The constant,

K, and the flow index, n, are als"o determined from the analysis. For

the purpose of studying the change in the flow properties of the various

mixes with varying energy input it is more convenient to analyse the

properties at a reference shear rate. Tabl~ 5.3-5.6 summarises these

properties' at a reference shear rate of 100 s-l. The values of n, K

and 11 appear to be dependent on the dies used. This. may be attributed a

to thixotropy, the effect of which is very pronounced in mixes of low

energy input, thus agreeing with the work of Payne on the dependence of

structure breakdown on mixing time I3 •· With increasing energy input n

and K do not change very much, however~ the values of apparent viscosity,

as ~xpect.ed, decrease.

5 • 4 • 2 • 2 PROPERTIES UNDER SIMPLE SHEAR AND TENSION

Using the equations derived by Cogswell viscosities and moduli

under simple shear and tension are calculated. Results obtained are

summarised in Table 5.7-5.10 and FigJo5J9&;'20. The general behaviour

of the mixes is that ail the magnitudes of the properties measured

154

I-' V1 V1

.--

-

-MIXING

TIME

(MINS)

2

3

5

1

lO

l3

-UNIT WORK

(MJ.m- 3 )

568

861 -

l263

l106

2181

2100

•

TABLE 5.1: PROPERTIES UNDER SIMPLE SHEAR AND SIMPLE TENSION - SBRBANBURY MIXES

. - . --. .-~

B BL ' n APO .10-6 YR n .10-3 G.lO-5

0 a '

(Pa) (Nsm-2) , (Pa) .-

1.50 1.2l 0.34 5.00 :\..52 3;21 2.l5 .--. -

1.62 1.26 0.32 4.93 1.80 3.2l 1.18

1.12 1.32 0.25 4.16 2.08 3.21 1.49 .-.-

1.15 1.34 0.20 4.59 2.11 3.08 1.42

1.15 1.33 0.20 4.40 2.l3 3.02 1.42

1.15 1.33 0.l9 4.25 2.l3 2.95 1.39 -

>-'10-: 5 E.1O-6

(Nsm-2) (Pa)

3.81 3.ll

3.6l 2.5l

3.20 2.01

2.19 1.86

2.63 1.19

2.44 1.1l

I-' \J1 0\

-

MIXING TIME

(MINS)

2

3

5

7

10

13

UNIT. WORK

(MJ .m-3)

452

650

1000

1312

1885

2270

TABLE 5.8; PROPERTIES UNDER SIMPLE SHEAR AND SIMPLE TENSION -SBR BRABENDER MIXES

....-... -

BO BL n llPO.10-6 YR "a .10-3 G.1O- 5

-(Pa) (Nsm-2 ) (Pa)

- .

1.48 1.17 0.27 4.35 1.34 3.15 2.35. -

1. 54 1.21 0.25 4.39 1. 55 3.23 2.08 -

1.62 1.25 0.27 4.18 1. 76 3.11 . 1.77

1.63 1.26 0.26. 3.84. 1.82 3.10 1.76

1.64 1.27 0.26 3.67 1.83 3.06 1.67 . --

1.64 1.27 0.25 3.5 1.83 3.05 1.66

A .10-5 E.l0-6

(Nsm-2 ) (Pa)

2.73 2.64

2.62 2.37

2.55 2.06

2.12 1.85

1.97 1. 75

1.77 1.65

« .--

MIXING UNIT· TIME WORK

(MINSl (MJ .m- 3 l

2 521

3 679

5 995

7 1236

10 1600

TABLE 5.9: PROPERTIES UNDER SIMPLE SHEAR AND SIMPLE TENSION - NR BANBURY MIXES

B BL n . 6PO .10-6 YR 11 .10-3 G.1O-5

0 a

. (Pal. (Nsm-2 l (Pal

1.81 1.20 0.14 5.10 1.48 1.70 1.15

1.81 1.19 0.12 3.46 1.42 .1.58 loll

1. 78 1.20 0.12 2.45 1.48 1.55 1.05

1. 76 1.20 0.12 2.38 1.48· 1.54 1.04

1.67 1.19 0.1 2.16 1.44 1. 50 1.05

>..10- 5 E.l0-6

(Nsm-2 l . (Pal.

3.28 1.50

2.38 1.15 .

1.22 0.85

1.16 0.83

.0.98 0.84

I-' V1 0>

MIXING TIME .

(MINS)

1.5

3

5

7

10

WORK UNIT

-:.,-- -. ,-. ----- -:-

TABLE 5.10: PROPERTIES UNDER SIMPLE SHEAR AND SIMPLE TENSION .;. NR BRABENDER MIXES

.- - .

BO BL n llPO·1O-6 YR "a· 10-3 G.1O-5

(ID .m- 3 ) . (p!,) (Nsm-2) (Nm-2 )

290 1.57 1.13 0.10 3.23 1.14 1.55 1.36 ..

530 1.68 1.20 0.10 2.48 1.48 1. 52 1.03 ._ ..

750 1.65 1.19 0.13 2.11 1.41 1.51 1.07 -

970 1.65 1.19 0.12 1.87 1.40 1.38 . 0.99

1215 1.61 1.8 0.15 1.63 1. 39 1.47 0.98 -

Ll0-5 E.I0-6

(Nsm-2 ) (Pa)

2.29 1.48

1.38 0.98

1.06 0.89

0.89 0.78

0.67 0.74

'i

3.2 VISCOSITY,

3.0

2.9

2.2

MODULUS, G

(Pa) 2.0

1.8

1.4

700 \

x - X - BANBURY MIXES

- 0- BRABENDER MIXES

o

1400 2100 2800 . i UNIT WORK (MJm- 3) ,

, . FIG. 5.19a: VISCOSITY AND MODULUS UNDER SIMPLE SIn!:AR

OF SBR MIXES

159

x1.0S

3.7

[,.2 VISCOSITY ,A

,(Nsm-2 )

2.8

2.3 '

1.8

x106

i , ! 3.0 ,

, MODULUSE !

(Paj .

2.4 !

2.1

1.8

1.5

- x- BANBURY MIXES

-0 - BRABENDER MIXES.

o

700 1400 2100 2800

UNIT, WORK (MJm-3) •

FIG. '5.19b: VISCOSITY AND MODULUS UNDER SIMPLE TENSION FOR SBR MIXES

. ' \-,

VISCOSITY n .(Nsm-2 ) a,

1.6

1.5

1.4

x10

r 1.3 MODULUS G

(Pa)

1.2

-1.1

1.0

f- '.

I,

x -X-BANBURY MIXES

- 0 -BRABENDER MIXES

0

.

0

x

400 1200 1600 '

•

FIG. 5.20:, VISCOSITY AND MODULUS UNDER SIMPLE SHEAR 'FOR NR MIXES

4.0

L 2.0

1.0

1.5

, . ~.3 MODULUS E

(Pa) 1.1

0.9

I

i, \

x106 . '

"

1 .\'

, , .,

\

800

'"

-x- BANBURY MIXES

~O-BRABENDER MIXES

1200 1 00

UNIT WORK •

FIG. 5.20b: 'VISCOSITY AND MODULUS UNDER SIMPLE TENSION FOR NR MIXES

162

,-

exhibit an initial rapid rate of reduction after which they tend to

approach more or less constant value. Shear ~d extensional viscosities

and moduli are thus shown to decrease with energy input.

--~However ,- while the values-of-shear"viscosityand-mo-dulliseither

show occasional scatter or small change with increasing energy input

extensional viscosity and modulus, measured in this way, appear to be'

more sensitive to variation of mixing time.

Although the properties calculated appear to correlate well with

the mixing parameter used there is some doubt as to the validity of

the analysis for rubber. The calculation of the Poisson's ratio, v,

yields a value greater than' 0.5. A possible xeason·for this might be

due to thixotropy which can account for up to 20% increase in apparent

viscosityl".. Further work is therefore required in this area.

5.5 DIE SWELL _

Die swell measurements were performed on the mixes since the

phenomenon is known to be governed by the elastic behaviour of the

rubber mixes. While the Garvey Die tests for determining the extrusion

characteristics of rubber compounds are standard methods the die swell

measurements here were made. as part of viscosity characterisation by a

c,apillary rheometer.

5.5.1 EXPERIMENTAL


Die swell was computed ,from the measurements of weight, length and

density of the extrudates, so that

Percentage Die Swell A-A· (, il) x 100

A =

o

where Ao = Cross-sectional area of the capillary die

A = Cross-sectional' area of the swollen ext'rudate.

The effect of mixing time on die swell can be clearlY,seen with a

single die length and a reference shear, rate. Table 5.11 gives the

values of the die swell measurements made using dies of varying length

to diameter ratio, but constant diameter, and a reference shear rate

of 100 s-1 at 1000C. Figs 5.21 - 5.24 illustrate the relationship

between die swell and energy input. In all cases die swell seems to

increase significantly in the initial stages of the mixing process and

after a maximum is reached the values tend to either level off, as with

~ mixes" or gradually decrease, as with NR mixes.

Upon analysing the curves in Figs. 5.21 - 5.24 their shapes appear

to be determined principally ,by the base polymer, when the filler type

and content are unchanged. Irrespective of the die dimension used the

maximum die swell values are reached at around a specific amount of unit

work - at approximately 1100 MJ.m~3 for SBR mixes and 650 MJ.m- 3 for NR

mixes. Further mixing beyond these points results in extrudates with

more or less constant die swell in ,the' case ofSBR.

Mixes prepared in Brabender Plastograph also produce similar

'shaped c'urves, with the values tending to level .off at about the same

unit work as the mixes prepared in Ban~ury mixer. In all cases, however,

Brabender mixes possess lower values especially at, higher energy in-

put.

Long capillaries appear to dampen the extrudate die swell, so

164

-

TABLE 5.11: DIE SWELL MEASUREMENTS ON NR AND SBR MIXES PREPARED IN A BANBURY MIXER AND BRABENDER PLASTOGRAPH MEASURED AT 100s-1

AND 1000C 'USING' 2.0 mm DIAMETER DIES

, L .

DIE LID RATIO MIXING UNIT WORK TIME MJm-:3

(MINS) 0 20

2 568 124 46

Cl) 3 '867 161 59 ><f:<l

5 1263 194 74 !5!;;!

~!il 7 1706 205 80 Cl) 10 2187 . 205 75

13 2700 205 80

2 450 119 37

P:;CI) 3 650 138 47 f:lf:<l 5 1000 162 57 fil!;;!

~!il 7 1310 166 60 I 1885 169 61 IXlCl) 10 I

13 2275 170 61

2 521 225 40

3 ' ,579 228 45 >fCl)

I !5f:<l 5 995 ' 216 44 "

1Xl!;;! 7 1236 211 44 ~P:; .. 10 1600 189 42

L. 13 19?7' 176 35

1.5 290 146 28

P:; 3 530 183 44

f:l~ 5 750 172 41 fil!;;!

7 970 172 40 ~P:; 1Xl .. 10 1215 158 38

15 1550 157 37

, ,

165

90

1 DIE SWELL (%)

70

50

.30

-' --.- - ---.: - -

700 1400

o

o

-0- BANBURY

--4-- BRABENDER

2100 2800

..

FIG. 5.21: DIE SWELL AS A FUNCTION OF UNIT WORK FOR SBR MIXES MEASURED AT 100 a-1 , 100oC, USING A DIE WITH LiD = .20

_ .. ,-.

166

240

l~o DIE SWELL

200

180

160

i 1140

120· -0- BAN BURY MIXES

-b.- BRABENDER MIXES

700 1400 2100 2800

UNIT WORK .(MJm- 3)

FIG. 5.22: DIE SWELL AS A FUNCTION OF UNIT WORK FOR SBR MIXES MEASURED AT 100 a-I, 100oC, USING A DIE WITH L/D = 0

I1 , . " ,

167

50

r "I DIE SWELL (%)

30

20

FIG. 5.23:

-, ,I r

i

240 I

500 1000

o

_ 0- BANBURY MIXES

-/l- BRABENDER MIXES

1500 2000

UNIT WORK (MJm- 3 ) •

DIE SWELL AB A FUNCTION OF, UNIT WORK FOR NR MIXES MEASURED AT 100s-1. '100oC. USING DIE WITH LID = 20

I

, I

, ' / .

I , " , l

1 J-

220

1.-' DIE SWELL (%)

200

180

160

\

,140

'b

'.-, --,"-

500 1000

',-0- BANBURY MIXES

-/l- BRABENDER MIXES

1500 2000

UNIT WORK (MJm-3 ) • FIG. 5.24: DIE SWELL AS A FUNCTION OF UNIT WORK FOR NR MIXES

MEASURED AT 100 s-l. 100oC. USING A DIE WITH LID = 0

168

that a knife-edge die (L/D = 0) produce values about 2~ larger than

that with L/D equal to 40. The effect of varying energy input also 1S

more conveniently differentiated by the exaggerated values obtained from

extremely short dies. This is particularly significant for NR compounds

which exhibit much 'reduced die swell response with long dies so that

the effect of energy input on die swell appear to be less prominent.

NR mixes also show a substantial drop in die swell after the maximum

has been attained, indicating ,the dominant effect of reduced viscosity

due to molecular breakdown.

The phenomenon of die swell or post extrusion shrinkage has been

associated with the elastic component of the rubber mix. Measurement

of die swell can therefore be used to qualitatively estimate the

" elastic behaviour of rubber and its compound. The maximum of the die

swell-unit work curve can be correlated with a specific stage of the

,- mixing process if other similar processing profiles are analysed.

Although the absolute values of the die swell are not comparable the

series of mixes prepared in the Banbury and Brabender Plastograph indicate

the significance of 'energy input as a mixing parameter.

5.6 CREEP MEASUREMENT

Creep is one of the viscoelastic responses of rubber. Since, in

the course of the, mixing process, the magnitudes of the viscous and

.elastic components are continuously altered it would be useful to

'investigate the change in creep behaviour of the various mixes. In

conjunction with the Avon Rubber Company, U.K., the TMS model was used "

to explain and quantify the results obtained,

" . 5.6.1 EXPERIMENTAL"

The two series of mixes were extruded on the capillary rheometer

169

,

us1ng a die 7 mm diameter and 5 mm long. Test specimens, each about

100 mm long were cut from the extrudates and their cross-sectional

areas were obtained from their dimensions and density.

The apparatus used to measure creep at constant stress is the

uniaxial extensometer fitted with a cam head (Fig. 5.25).

Measurements were only able to be made at room temperature. On

one mix a series of creep measurements were made at various stress

, levels . Measurements on all the mixes were conducted using a stress

f .' of 50 KPa.

FIG. 5.25: EXTENSOMETER WITH A CAM HEAD

170

].6.2 RESULTS AND DISCUSSIONS

The application of the TMS model to some of the results obtained

was carried out by Turner and·co-workers at the Avon Rubber Co.

laboratories.and is described in Appendix Ill.

By varying the magnitude of the stress fora single mix a series

of curves were obtained, as shown in Fig. 5.26. To account for the

various creep curves it is justifiable to introduce a systematic

change in the parameter J. All the parameters would remain constant

and only J is stress dependent (Table 5.12).

The creep curves for the two series of mixes at a constant stress

are shown in Figs. 5.27 and 5.28. The TMS parameters for some of the

mixes are given in Tables 5.13 and 5.i4; in both ·cases the values of K )

do not appear to vary ·significantly to change the shape of the curve \

and are·assumed to be constant. Other parameters decrease quite

significantly with increasing energy input. For example, increasing

th~ energy. input on the NR mixes. by a factor of 2.7 causes the values

of E and D to reduce to 21.l of the original values. With SBR mixes D

decreases to a a and E to about ~ of their initial· values as the energy

input is increased by a factor of 3.2. These reductiomin viscous and

elastic components are expected with increased mixing. The power law

indices, although they are slightly greater. in magnitude compared to

the· values obtained from the ~apillary rheometer, also exhibit a

gradual increase with increase in energy input.

The series of creep curves obtained. show that at short mixing times,

up to 7 minutes, or at relatively small energy inputs/the mixes possess

markedly ·different viscoelastic properties. As the energy input is

fUrther increased ~he rate of change in.these·properties tend to level

off.

The curves derived from the experimental points and the equations

171

t STRAIN (%)

50

38

"

22

Symbols: Experimental points

Line: Model

5 18

L.

~ C lu

100 KPa

"2 L

TIME (MIN)

FIG. 5.26: CREEP CURVES AT VARIOUS STRESSES FOR SBR MIXES •

172

'Jr, '- .......

,\

85 KPa

75 KPa

50 KPa

25 KPa

lr" ,,1 J _

JC1 (J

25

'12 L

STRAI~ l~J

18

18

n ~)

~~i c G l I I

FIG. 5.27:

Symbols: Experimental

Line·: Model

·0 \\

':. .~ v

\'

~,

points

, L.1

t, . 695 MJm- 3

V 939 .f- 1321

1 8' I '

" "

')2 L

TIME (MIN)

\ V

KEY

" D

~

25

A V "

::x:

'-

1705 MJm- 3

2226 "

2574 "

CREEP CURVEs FOR SBR BANBURY MIXES (CONSTANT S~RESS)

30 0 0 x

r x

x x x

X STRAI~ X

[7(J 4 X Y Y Y x r;;

r;; x v

'I 20 X V

XX r;;

X r;; 0 'i/

16 v 'i/ X r;;

'i/

12

8 Symbols: Experimental· points·-

Line: . Model

KEY

11 870 IDm- 3 " 1600 IDm- 3

V 1078 " 0 1774 " ... 1322 " ~ 2365 " ,

6 10 1 4 18 22 25 30 • TIME (MIN)

FIG. 5.28: CREEP CtlRVES FOR NR BANBURY MIXES (CONSTANT STRESS)

174

-TABLE 5 .12: MODEL P AMMETERS FOR SBR MIXES

StressKNm-2 E kNM"1.J

K rKJ{",·~~ r ... !!·') J r." ..... ' n

50 100 185 930 61 0.365

75 100 185 930 80 0.365

85 100 185 930 81 0.365 '100 100 185 930 92 0.365

TABLE 5.13: MODEL P AMMETERS FOR SBR MIXES

Work Unit I4Jm-3 E [101 .. ··)

K 1(" "".1" tIJ

D r ~ .... 'J J r.·_· .. ' n

870 .. 140. 200 .1000 60 0.39

'1322 100 200 800 55 0.38

2365 90 200 600 50 0.37

TABLE 5.14: MODEL PARAMETERS FOR NR MIXES

Work Unit I4Jm- 3 ['N~·'J K r ..... · .... 3 D

KN .... -1S""] r.;L~. n

695 275 200 . 4000 61 0.42

939 180 200 2800 55 0.41

1321 142 200 . 1150 50 0.40

2226 . 125 . 200 1000 43 0.39 .

of the TMS model superimpose upon one another very well. The model,

theF"efore, 'can be used to adequately describe the creep behaviour of

uncured rubber mixes, since it can quantifY the various viscoelastic

parameters essential for. the characterisation ·of rubber mixes. In

spite of the small elongation rates used the results show ~ definite

change of viscoelastic properties with'increasing energy input.

An alternative for the analysis of the creep data is to assume

the strain-time relationship to obey the power law. With this

175 ~.

--.:: - . - .• _-.c... "'- _ X Cl,_1 h. I

15

f 1 ,! f\,' ."\ . \~~~ ~' 0 ~ ~~' .'

" ~ ". ~ .\

fZ X . +++ ·LOG STRAIN " ~ y ., QX IXl ++ V

'v + + + 12 ., \ n ~ + v x . + .\ .,

Y x + t.t (\ .it! v

+ V ~ + v7'i;V /, .., , >< + r.

'1 Cl V ~ .+ r;s { <., ~ + r; v , i + " ~ ! + V

r-V V /- . + v r; ~ ,

G .\ .LI '1 ", t L .'-"-

r. ~ L,

V . ~ • t cl

V t. ", ~ 17 , t.. 1,;., V L, 5 v \

/:, r; ", i ~. v w

6 A ~

4 d A '-' ,

~ KEY "-.

t REFER TO FIG. 5.27

'), L

0, i 0 2 I.l 6 8 1~ 12 1/2 15 I

• LOp TIME 1 X18- 1

FIG. 5.29: CREEP 'MEASUREMENT FOR SBR MIXES'"" LOG STRAIN VERSUS '\ . LOG TIME CURVE . .

,I

r· 4 I .

LOG STRAIN

13

12

'11 I , ,

, I ," I '

10

\> \> 0

o 0

0'0 x

o x x +

'x + 'V

+ 'V 'i/

2

+ 'V

o 0 o

KEY

\

REFER TO FIG. 5.28

6 8 . 10 12 _1 ...... 4__ 16 LOG TIME

FIG. 2.30: CREEp MEASUREMENT FOR NR MIXES - LOG STRAIN VERSUS LOG TIME· CURVE

X10- 1

177

assumption the strain can be related to the time on a log-log plot

for easy data treatment. Thus if

y = ~ -- -

log Y = log K. + n log t

where y = strain

t = time

K = constant

n = power·iaw index.

Results are given in Tables 5.15 and 5.16 and the log-log relation-

ship in Figs. 5.29 and 5.30. The relationship between strain and time

on logarithmic scales appear to be linear, though an inflexion is

observed after a certain point on the curve. After the point of

inflexion the rate of strain .is reduced. With increasing energy input

K increases substantially, particularly at lower energy input. This

. agrees very well with other properties measured: the. rate of change in

flow properties diminishes with increasing energy input.

TABLE 5.15: CREEP PARAMETERS FOR SBR MIXES

Unit Work (MJm-3) n log K

695 0.423 0.252 939 0.445 0.451

1321 . 0.366 0.798 1705 0.340 0.927 2226 0.345 0.933 2574 0.331 0.974

TABLE 5.16: CREEP PARAMETERS FOR NR MIXES

Work Unit (MJm- 3) n log K

870 0.334 0.759 1078 0;349 0.883 1322 0.349 0.926 1600 0.340 0.967 1774 0.343 1.033 2365 0.319 1.077

178

5.7. ELONGATION TESTING

The dependence of properties in extensional flow is a measure of

the viscoelastic characteristics of the material. Turner and his

coworkers 12 have recognised the practical significance of elongational

testing and, in conjunction with the TMS model, utilised the test results

to obtain the parameters related to the viscoelastic properties of

unvulcanised rubber mixes. As with the creep measurement, an analysis

is made using the TMS model parameters to characterise the series of

mixes used in this work.

Elongation testing has. received little attention, one of the reasons

_ . for this has been the absence of a suitable instrument. While shear

strain rate can readily be attained elongational strain rate is less

straigh1l-forward. Elongational strain, £, is defined as

£

so that elongational strain rate, E, is

= dR. .1 dt 1. \

For constant elongational strain rate

=

where 1.0 = Initial length

R.t = Lenttb of a given element at time t

It follows from Eqn. 5.2Oathat.

R.t = 1.0 • exp (Et)

. This means that in. order to maintain a constant elongational strain

rate the elongation must proceed at an exponentl.al rate. .Standard

tensile instruments cannot, therefore, be used. However, if an instru-

ment is fitted with a cam head, a limited range of use can be found.

To circumvent this problem Meissner1S ,16 found an ingenious method in

179

which two sets of rollers, fixed at a distance 1 apart,replace th~

two clamps. !Jnder these conditions a constant elongational strain rate

is achieved and can be calculated from

=

where R = Radius of the rollers with angular velocities, wland W2'

The tension between the nips is monitored. as a function of time and,

with the knowledge of the cross-sectional area at that time, the stress

is· calculated.

The elongation tester developed by Turner and coworkers at the Avon

Rubber Company, U.K., uses two pulleys, one of which is fixed to a force

transducer. This design enables the stress to be easily measured and

the problemcf clamping the specimens is eliminated by passing an

extruded cord of rubber from one pulley around the other and back to

the first pulley. Since only one pulley is rotated at an angular

velocity w, elongation rate is calculated from

= Rw/1

= 21TRn/1

where n = rotational speed.

5.7.1 EXPEIlIMENTAL

The cord specimen was prepared by extruding it on the Davenport

capillary rheometer using a 2 mm x 2 mm die and a sh~ar rate of 300 s-I

at lOOoC. The extrudate was collected and allowed to stand for one

hour in a trough containing water maintained at 500 C .to allow gradual

elastic recovery. It was next left to stand and dry for a further 12

hours after which 45 mm of the strand was cut and weighed. .

The measurement was carried out on the Elongation Tester at the

laboratory of the Avon Rubber Company. The schematic diagram of the

instrument is shown'in Fig. 5.31. After the cord had been placed

,.

180

SIDE VIEW FRONT VIEW

" .... ~'4' ., ~ ... 1st ,,"

PULLEY •• __ 1'1 (ROTATING) ,

I 0, a

- SPECIMEN CORD

2nd,

-

PULLEY _

FORCE I---< .. ~ TRANSDUCER ....

~

TO OSCILLOSCOPE, TRANSIENT RECORDER AND, MINI C0!'1PU';l'ER"

FIG. 5.31: AVON ELONGATION TESTER

181

"

.,.

around the pulleys it was allowed to attain the test temperature (lOOoC)

before the clutch was engaged· to rotate the pulley. A rotor speed of

35 r.p.m., corresponding to an elongation 'rate £ = 3.7 s-I, was used.

An oscilloscope connected to the instrument registered the force versus

elongation curve. A transient recorder and a mini computer calculator

were also.connected to the instrument .to print out the true stresses ·at

'-'-'various elongat ion rat ios •.


ay ~lotting the true stress as a fUnction of elongatio~.ratio and

applying the constitutive equations of the'TMS model the model para-

meters can be obtained. Equations describing the test resUlts on the

basis of the model' are given in'Appendix 11. The parameters are obtained

by curve fitting, the computer progrwmne for which is given in the

Appendix IV. The absence of a distinct region of constant slope

necessitates the use of an arbitrary point from which the intercept

. giving elastic modulus E can be calculated. For this purpose a

reference point· is selected at an elongation ratio of 2.0.: To obtain

the slope ,wh~ch repr~sents the sum of the two elastic moduli D and E

an elongation ratio of 1.02 is selected.

Figs •. 5.32 and 5.33 show the true· stress-elongation ratio relation

ships of SBR and NR mixes respectively. Reproducibility of experimental

results are good. SBR specimens break almost at a constant elongation

ratio. With NR mixes the elongation ratio corresponding to maximum stress

decreases progressively with increasing energy input. This behaviour

may be attributed to polymer breakdown - long molecules possess more

molecular entanglement, which produces crosslinking effects. A few NR

specimens, particularly those prepared at short mixing times do exhibit

a sudden rise in stress at high elongation. This is tentatively

,.

182

, x ~ 81

8 ,

I 7 TRUE STRESS (KPa) .

5

5

", t.

v

v -

+ + x

+ x

v '

+

x

., LJ

+ x + x

+ X 0

o o

X 0 o

o

KEY

'\l 876"

+ 1263"

X' 2187 "

P 2700"

+ x

o

x

0~,' __ ~ __ ~~~ __ ~~~~~~~~ 10 1520' 25 30 35 40 45 50

I L,

!

ELONGATION RATIO X1C1 - 1 \ , G

FIG. 5.32: PLOT OF TRUE STRESS VERSUS ELONGATION RATIO FOR SBR BANBURY MIXES

•

x

C,: . vi

I TRUE STRESS (KPa)

8

7

4

') .J

2

1

~ t

t. v v

to V+++ + !J. V +

t. 'V + V + +

x x x x

'. KEY

521 MJrn:- 3

679 n

+ 995 n

X 1600 n

o 1927·n

0~~~~~~~~~~~~~~~~ 1 2 , ,

i

• , .

6 7 3 4 I

ELONGATION ,RATIO

FIG. 5.33: PLOT OF TRUE STRESS VERSUS ELONGATION RATIO FOR NR BANBURY MIXES

,

184 . ,"

• Q v

r;j

1

X10' 8

f MODEL 7

. . PARAMETERS

5

4

3

2

1

-

- - - _.- - 0..0:,.-

J [I(I/,.;·s"]

UNIT WORK (MJm- 3)

FIG. 5.34: RELATIONSHIP BETWEEN TMS MODEL PARAMETERS WITH UNIT WORK WITH SBR BANBURY MIXES

..

+

17 MODEL

PARAMETERS

.-

" .,

6

j 3: ,

2

.A.

t

0L-~~ __ ~~ __ ~~ __ ~~ __ ~~~ lA 2 5 8 lA '2 14 16 1'(1 "'(1 72" u- 1 () i .' ' I G Lt! ~ I " ~

X18 L • . UNIT WORK (MJm- 3)

FIG. 5.35: RELATIONSHIP BETWEEN TMS MODEL PARAMETERS WITH UNIT WORK "FOR .NR BANBURY MIXES , .

186

'~'.- --- ~

attributed to domains of raw rubber where poor carbon black dispersion

prevails and strain induced crystallisation can take place. From this

observation it can be suggested that strain induced crystallisation

~coc~urs, dUl:"ing mixing,- particularly, in-,the,early stages~,--~-

Computer analysis indicates that high degree polynomial equations

(between 9-10) are required to fit the experimental data. The values

of the parameters of the TMS model are given'in Tables 5.17 and 5.18

and their relationship with energy input is shown in Figs. 5.34 and

5.35.

TABLE 5.17: MODEL PARAMETERS FOR SBR BANBURY MIXES

,UNIT WORK (MTm-3 ) E IOI"",..&J

K K"",'~

D [tetrl",·:t.j 1.:1.-'1';

568 27 63 72 17

867 20 66 54 16

1263 12 56 50 15

1706 11 54 52 15

2187 10 47 27 14

TABLE 5 .18: MODEL PARAMETERS FOR NR BANBURY MIXES

UNIT WORK (MTm-3 ) E r.,,,,-IJ K 1UrI ... -Li'

D' r~"M-':l Ir~~ •. ~ ...

521 17 68 43 13

995 14 56 38, 11

" 1236 12 37 33 11

1600 12 31 11 11

With SBR mixes all of the model parameters, and hence ~he viscous

; and elastic components, decrease progressively with increasing mixing

time and black dispersion. The elastic moduli D and E show the most

substantial reduction as mixing progresses but while the viscous .' 187

constants J and K also decrease ~he latter appears to be the least

sensitive.

The model parameters for NR mixes also show a progressive

reduction with energy input, except'J, ·which~seems' to be-more·or less

unaffected. The reduction In viscosity and elasticity in NR mixes

with increasing energy input are shown by the substantial decreases in

D and K.

5.8, MECHANICAL PHASE ANGLE MEASUREMENTS

Efficient operation in rubber processing requ1res a reliable and

rapid test to assess the quality of a rubber mix or consistency of

compounds prepared in batch operated mixers. An attempt is made here

to investigate the possibility of using currently available instruments

- and to exploit the principles upon which certain compound properties

are based, to correlate specific parameters with varying levels of

carbon black dispersion.

In this respect the Monsanto Oscillating Disc Rheometer is used to

determine the mechanical phase angle, which is a measure of the ratio

of the viscous to the elastic stress components resulting from the

application of a sinusoidallY varying ,strain. The oscillation of the

disc of the Monsanto Rheometer produces a sinusoidal, strain input and

stress output from which the phase angle 0 can be obtained. However,

it is more convenient to obtain the phase angle' by using only ,the

sinusoidally varying stress output and a singl'e point reference signal.

5.8.1 EXPERIMENTAL

A Monsanto Oscillating Disc Rheometer Model operating at 100 c.p.m.

and a KEMO Digital Phase Meter were used. Phase reference signal was

188

obtained by placing a coil near the path of the rotating magnet which

was used for the rectification of the strain gauge signal output. The

unrectified sinusoidal stress output, which is measured by the strain

gauge, was extracted from the terminals at the re-corder. The two

signals were fed into the phase meter from which the phase angle can

be directly read out. The ratio of the input voltages fed into the

phase meter was adjusted to be within the range of 10:1 by using an

attenuator potentiometer network. The circuit diagram of the system

used is show in Fig. 5.36. Phase angle measurement of rubber mixes

was carried out at 600 c and, using the small dies and rotor 10 g of

rubber sample was taken to fill the die cavity. With the disc in

oscillation 2 minutes were allowed for the material to reach temper-

ature equilibrium before the final constant reading on the phase meter

was taken.

PHASE METER

r----l ~-------------------I~ I ____ -1

STRAIN GAUGE.

POWER SUPPLY UNIT

FIG. 5.36: CIRCUIT DIAGRAM FOR PHASE ANGLE MEASUREMENT

, .

TABLE 5.19: MECHANICAL PHASE ANGLE MEASUREMENTS FOR SBR MIXES

Unit Work (MJm- 3 ) Phase Angle

568 38

867 35 Banpury 1263 33 , .

Mixes 1706 31

2187 25 . 2700 24

450 29

650 23

Brabender 1000 13 Mixes 1310 9

1885 7 2270 6

TABLE 5.20: MECHANICAL PHASE ANGLE MEASUREMENTS FOR NR MIXES

Unit Work (MJm-3 ) Phase Angle

521 6

679 6

Banbury 995 6 Mixes 1236 4

1600 3

290 6 •

530 4

Brabender 750 3 Mixes 910 3

1215 3

190

5.8.2 RESllLTS AND DISCUSSION

As the temperature of the specimen in the mould increases the

reading on the phase meter decreases continuously. When an e'l.uilibrium

in the specimen is reached the reading remains practically constant.

Tables 5.19.and 5.20 show. the phase angle 6 for each of the mixes

prepared in the Banbury mixer.

The magnitude of the phase angle 6 depends on the r~lative

magnitude of the viscous and elastic components. Materials with high

viscous component will possess greater phase angle. The higher values

obtained with SBR mixes compared to NR mixes indicate that SBR mixes

possess higher viscous and low elastic component while the reverse ~s

true for NR mixes. But while SBR mixes show a sUbstantial change in

phase angle with varying energy input or dispersion of carbon black,

no clear discrimination can be made with NR mixes due to the low

sensitivity of the phase meter used (±lo).

The amplitude at which the above measurements are made is very

--=---=

low. Phase angle difference and other dynamic properties are strongly

dependent upon the amplitude. Hence further work using higher amplitude

is re'l.uired, in addition to resolving the viscous and elastic com-

. ponents. The effect of thixotropy alsomedtobe investigated, as this

might contribute to the large differences in the phase angles between

the Banbury and Brabender SBR mixes. At the same level of energy input

SBR mixes prepared on the Brabender Plastograph, however, possess

lower levels of carbon black dispersion.

5.9 MOONEY VISCoSITY

The conventional Mooney viscometer still remains the most widely

used processibility testing instrument in the rubber industry, despite

the obvious deficiencies mentioned earlier. In studying the processing

properties of the rubber compounds it would therefore be very useful

to obtain their Mooney viscosities and discuss their significance and

relationship with the data'obtained from other instruments.

5.9.1 EXPERIMENTAL

Mooney viscosity measurements (ML 1 + 4) 1000 C on the samples

were made according to ASTM Standard Method D-1646.

5.9.2 RESULTS AND DISCUSSION.

The Mooney viscosity data obtained are plotted against unit work

on both linear and double logarithmic.scales. Buskirk et all suggest

that such treatment of the data would be usefUl for qualitative

characterisation of material processibi1ity. From the log-log plot

the relationship can be described by the equation:

log VIDD = a log Wu , + log b (5.23)

where VIDD = Mooney viscosity; ML (1 + 4) 100oC.

Wu = Unit Work

.a = Slope of the log-log curve ')

b = Intercept

The slope a is called the Viscosity Work Index (VWI). Buskirk et

all relate poor-processing during the initial stages of Banbury mixing

to high VWI value, which indicates high masterbatch viscosity at low

Wu'

Results of Mooney viscosity measurements are given in ~ab1es 5.21

" and 5.22 and Figs. .5. 37 and 5. 38. The log-log plots are linear (r = 0.99)

within' the range of unit work used. 'The VWI values 'for SBR and NR

mixes are found to be about 0.28 and 0.34 respectively, the later being

, 192

TABLE 5.21: MOONEY VISCOSITY OF SBR MIXES

Unit Work (MJm- 3) (ML 1 + 4) 100°C VWI

568 73

867 62 Banbury

1263 56 0.284 Mixes 1706 52

2187 48

450 68

650 60

Brabender 1000 54 0.265 . Mixes

1310 50

1885 '46

2270 44

TABLE 5.22: MOONEY VISCOSITY OF NR MIXES

Unit Work (MJm-3) ML (1 + 4) 100°C VWI

521 44

679 39 Banbury 995 35 0.339

Mixes 1236 32

1600 30

290 44

530 36

Brabender 750 32 0.325 Mixes

970 29 .. . 1215 27

1550 26

-

193

Gt;J u\..!

MOONEY VISCOSITY

S5

58

,

I " I ,

, .'.

5 18 , "

11:; IJ

- tF- BANBURY

- V - BRABENDER

':i": __ .., ... L,-:

UNIT WORK (MJm- 3) X 102

FIG.·5.37a: ,'MOONEY VISCOSITY-UNIT WORK RELATIONSHIP . OF SBR MIXES

194

I IOC; I U'",

, LOG ML(l,-4)

i80

i75

178

; SC I J

.. .

- " - BANBURY

- v - BRABENDER

150_~ __ -.. ___ --r ___ --r ___ .....,

25 38 32 , LOG UNIT WORK

FIG. 5. 37b: PLOT OF LOG MOONEY VISCOSITY-LOG UNIT WORK FeR SBR MIXES

195

.. r, , ! .• JI

50

t - - ~- - -

45 MOONEY

VISCOSITY

,-

- /; - BAN BURY

- v - BRABENDER

40

35

30

25

20-.L-----~------r_~--_r----~ o 5 10 15 • 20

.'

FIG. 5.38a: MOONEY VISCOSITY-UNIT WORK RELATIONSHIP OF NR MIXES

.196

X10-2 170

t 165_

LOG ML(1+4)

160

'. 55_

150

145

140

135

- /:, - BANBURY

- v - BRABENDER

r30_'~ ____ ~ ______ ~ ____ ~ ______ -r ____ ~ ,

24 26 28

..

30 LOGW

u

32 • 34 . X 10- 1

FIG. 5.38b: PLOT OF LOG MOONEY VISCOSITY-LOG UNIT WORK FOR NR MIXES

197 . .

higher due to rapid rate of molecular breakdown of NR.

Again the curves for Banbury and Brabender mixes do not super-

impose but run almost parallel to one another although VWI values, to =-""=- .-=~ ---

a good approximation, tally.

5.10 WALLACE PLASTlMETER

Plasticity tests are frequently conducted for process control

purposes due to its sensitivity to minor processing variations and

rapidity. The instrument used for this work is of the type which

measures the thickness after the specimen has been deformed for a

specific period by a known force.

5 .10.1 EXPERIMENTAL

Measurements are conducted on Wall ace Plastimeter according to

BS 1673, Part Ill, 1969.


As with the Mooney viscosity the Wallace plasticity data were

also plotted on both linear and double logarithmic scales and the

relationship is described by the similar equation:

log PlOO = ,a log Wu + log b

where PlOO is Wallace.plasticity at 1000C

and a 1S the slope of the log-log plot.

The slope can be termed as the Plasticity Work Index.

Results are summarised in Tables 5.23 and 5.24 and Figs. 5.39 and

5.40 and appear to exhibit similar behaviour as that observed for

Mooney viscosity measurements. These results, however, are only

198

TABLE 5.23: WALLACE PLASTICITY OF SBR MIXES

Unit Work_ Wal1a.ce Plasticity Number PWI (MJm- 3 )

568 35

Banbury 867 30 0.381

Mixes 1263 26 _ .. I 1706

, 23

2187 21

450 32

Brabender 650 26

Mixes 1000 23 0.394

1310 19

1885 18.5

TABLE 5.24: WALLACE PLASTICITY OF NR MIXES

! .L { " . Unit Work Wal1ace Plasticity Number PWI

(MJm- 3 )

521 23

679 I 20

Banbury 995 16 0.448 Mixes 1236 15

1600 14

1927 13

290 22 . 530 16.5 0.45

Brabender 750 14 Mixes

970 13

1215 11.5

1550 11

199

36

31 WALLACE PLASTICITY

30

- 6- BANBURY

- v - BRABENDER

· 16_t-, ---r---i; --:,-__ /;--_-;-__ ....,.. ·0 510 15 20 30

FIG. 5.39a: WALLACE PLASTICITY-UNIT WORK RELATIONSHIP OS SBR MIXES

200

LOG WALLACE PLASTICITY

155

110_1

135. I

! , I 130_ i

125.1 ,

- fl -' BANBURY

. - v - . BRABENDER

,

\, 'i/

\

\ \

120_.t",! -r----r, -r-~-.,-~, -"---;-i --r---..,

25 27 29 31 33

X10- 1 , I LOG UNIT WORK

FIG. 5. 39b: PLOT OF LOG WALLACE PLASTICITY-UNIT WORK RELATIONSHIP FOR SBR.MIXES .

201

35

t WALLACE PLASTICITY

28

, c IJ

18

5

~ t. - BANBURY

- V - BRABENDER

~.

.0_L.-_~_r-'---_r--___ r--__ ---, 9 5 i8

, UNIT WORK

FIG. 5.40a: WALLACE PLASTICITY-UNIT WORK RELATIONSHIP FOR SBR MIXES

202

- x" ~-) I'll -IL

WALLACE PLASTICLTY

I'"l}

11 0

"4 L, 26 28

- 11 - BANBURY

- v - BRABENDER

30 UNIT WORK (MJm- 3) i

Xl fl,,-I i C

FIG. 5.40b: WALLACE PLASTICITY-UNIT WORK RELATIONSHIP OF NR MIXES ' . .

203

,-

empirical and as such are only useful for process' control purposes in

vhich results of the test are compared vith standard values of knovn

compounds.

~.ll CURE CHARACTERISTICS

Apart from yielding the optimum vulcanisation parameters the

assessment of cure characteristics of a rubber mix is the only technique

whereby both 'the rheological and mechanical properties, in the uncured

and cured states respectively, can be obtained from a single test.

The use of curometers for studying the effects of curative contents

and controlling the batch to batch'variations is well knovn. B,y deter

mining the vulcanisation behaviour of the two series of mixes available

tqe extent of the usefulness of a curometer can be analysed.

5.11.1 EXPERIMENTAL

" Cure tests were carried out on the two series of the Banbury

mixes using Monsanto Rheometer Model 100-FR operating at an oscillat

ing frequency,;of 100 cycles per minute and a 30 arc. 'A single test

temperature of l600 c was adopted since the data obtained are to be used

only' for qualitative comparison.

5.11.2 ,RESULTS AND DISCUSSION

Cure traces obtained are given in Figs. 5.4.1 :and 5.42. A summary of the

various properties is shovn in Tables 5.25 and 5.26.

From the results obtained it can be seen that the type of the base

polymer significantly influences the c~re characteristics, particularly

the cure time. Of all the parameters evaluated only the minimum and

maximum torques exhibit any noticeable change vith increasing mixing

204

I\)

'0

'"

%

~~ . Monsanto Rheograph ~~ KO> e2 AC O~2

100

o 2

CHART MOTOR .. 3Q ........ mln. STOCK ..........•••••..•...............

RANGE SEL. . ...... !:?P ........ . PREHEAT .............. -::-........... ~: :P~ ::::::j~~L::::::::::~

l' .. DATE ....•...•.••.••.••.. .: ................... ~

~ OPER. ..•••........•••••.•.•.•.. ~

PRO.!. No ...•.......•.................•..•.

..., l INClfASING ENOOf INPlIT

J: ~

80

40

30

FIG. S.LQ: a:;CIUATIr6DISC

3 4 5 6 7 8 TIME. MINUTES

9

I*!J£TER TRACES OF 1120

NR BANBURY MIXES

10 11 100

12 ....

. .1;~ .' '.

'of'" . . f"

.---;..,

TABLE 5.25: VULCANISATION PARAMETERS OF SBR MIXES

MIXING TIME

ML ~F 'T ss ' TC90

2 16.5 54 5 13.25

3 14 52 5 14.0

5 12 48 .' 5.05 13.0

7 11,.5 47.5 5.05 13.25

10 11 47 5.1 13.5 13 10.5 46 5.1 14.5

TABLE 5.26: VULCANISATION PARAMETERS OF NR MIXES

MIXING ML MHF TS5 TC90 TIME

2 10 42.5 4.8 8

3 9 41. 5 4.8 7.3

5 8.5 40.5 5.0 7.5

7 8.5 41 5.0 8.5

10 8.0 41 5.0 8.5

ML = Minimum torque (in lbs.)

MHF = Equilibrium torque (in lbs. )

: TS5 = Scorch time to 5 units o£ torque increase above minimum torque

TC90 = Cure time to 90 percent of maximum torque development

time or quality of the mix, although they become less discriminating

as the level of carbon black dispersion increases.

5'.12· BOUND RUBBER MEASUREMENTS

The formation o£ bound rubber is attributed to the £ormation of

and reaction between the free radicals at the newly £ormed chain ends

207

of the polymer and the reactive site on the fresh carbon black surfaces.

Since dispersion is a result of the breakdown of large carbon black

agglomerates, thereby producing the active site, a measurement of

bound rubber would gi v~~a ;:;'seful--{iiforniatfon-on -the -quality -of a, mix.

5.12.1 EXPERIMENTAL

Estimation of bound rubber was made by immersing about 0.3 grammes

of the compound in toluene and allowed to stand for 3 days at room

temperature. The toluene was then decanted and the swollen gel was

carefully dried in a vacuum oven at 500 C overnight. The dried residue

was next weighed.


The percentage of bound rubber was calculated with the correction

for the filler and other insoluble contents:

% Bound Rubber x 100 (5.25)

where WRES = Weight of dried residue

WINS = Weight of insolubles

WRH = Weight of rubber hydrocarbon

The results of the bound rubber measurements are given in Tables

5.27 and 5.28. The corresponding Figs. 5.43 and 5.44 show the relation-

ship between the bound rubber formation and extent of mixing. For SBR

percentage bound rubber exhibits a substantial increase at first but

tends to reach a plateau on further mixing. However, the NR mixes

possess a different pattern on fUrther mixing. After the rapid increase

, in bound rubber formation at the beginning of the mixing cycle a max

imum is reached after which it decreases very significantly. This is a

result of polymer breakdown, Which become,s the dominating factor. Since

208

,

bound rubber is associated with improved physical properties, excessive

mixing of NR compound will inevitably result in,reduced vulcanisate

properties, while SBR might show a reverse effect.

As with die swell, when the two mixem are co~ared; the shape of

the curves are similar, although mixes prepared on the Banbury possess

overall higher values than'that of those prepared on the Brabender

Plastograph.

TABLE 5.27: ,BOUND RUBBER MEASUREMENTS FOR SBR MIXER

Unit Work (MJ .m-3) Bound Rubber (%)

568 15

867 21

Banbury 1263 ' , 26 Mixe's

1706 28

2187 30

2700 31

450 10

650 15

Brabender 1000 18.5 Mixes 1310 23.5

1885 23.5

2275 25

209

, 40

1 BOUND RUBBER

30 (%) ,

20

10

o

-0- BANBURY MIXES

~A- BRABENDER MIXES

,,'

700 1400 2100 2800, UNIT WORK (MJm- 3 ) •

FoIG. 5.43: BOUND RUBBER AS A FUNCTION OF UNIT WORK . ·U/j/l...)

210

r BOUND RUBBER

(%)

40

30

500

-0- BANBURY MIXES

-A,- BRABENDER MIXES

1000 1500 2000

UNIT WORK (MJm- 3) •

FIG. 5.44: BOUND RUBBER AS A FUNCTION OF UNIT WORK ( fl/IL l

211

, ,

TABLE ;;.28: BOUND RUBBER MEASUREMENT FOR NR MIXES

Unit Work (l4J.m-3)' Bound Rubber (%) - -0

.

521 47

679 49 Banbury 995 51 Mixes 1236 47

1600 43

1927 42

290 20

530 30.5

Brabender 750 42.5 Mixes 970 41.4

1215 38.5

1550 33.5

5.13 ELECTRICAL RESISTIVITY

In spite of several practical difficulties, such as contact

resistance and variation in test conditions, the measurement of

electrical resistivity is claimed17 ,18 to be a reliable method of

determining the degree of carbon black dispersion in vulcanisates.

Around the region of extremely poor level of black dispersion the

resistivity shows a minimum, after which,it rapidly increases with

increasing dispersion. On this basis a few attempts have been made to

develop simple but rapid test methods for measuring electrical

resistivity, to monitor mixing and de~ree of dispersion in uncured

rubber compounds. Asmall degree of success is seen in this area

especially in the work by Boonstra18 . He suggested the use of a coaxial

eletrode probe which is pressed into a sheet of unvulcanised rubber'

compound to form a ring-shaped sample. The resistivity was then

measured on the specimen under a small pressure at 600 c.

212

UPPER PLATEN

\ : L~-ILL==========I~,==:L~"'_~PTFE INSULATOR

r---,------.~' TEST SPECIMEN

...:j ______ .~ ELECTRODE

I--~;r-,---....... PTFE INSULATOR

1 __ •• LOWER PLATEN

FIG. 5.45: SCHEMATIC DIAGRAM OF COAXIAL ELECTRODE

I 5.13.1 EXPERIMENTAL

In this work the platen and the pressure~stem of the Mooney

viscometer were used.' , The moulds were replaced'by specially con-

stru~ted moulds fitted with stainless steel coaxial electrodes and \

PTFE insulators (Fig. 5.45). 'The outer ring with a 24.0 mm diameter

formed the negative electrode while the positive electrode waS a 6.0 mm

diameter steel rod in the centre of the cavity. The depth of the

cavity formed by the electrodes was 2.0 mm. The electrodes and the

cavity were heated by the two platens., The electrodes were connected

to a digital ohmmeter.

To measure the resistivity of a mix a ring-shaped specimen of

about the same radii as the cavity was cut from a 4 mm thick sheet.

It was then prewarmed in an oven at the test temperature before being

transferred into the caVity ,heated to 60oc. The top platen with the

213

PTFE disc was then lowered to mould the specimen and maintain it at a

constant pressure in the cavity. With an air pressure of 0.5 kg/cm3

the effective pressure on the sample was 4, MPa. The reading was taken

two minutes after the top platen was lowered.


Resistivity is calculated by integrating the total resistance

across the radius of the ring specimen.

!. r2~

T H

..1.

rrt4' ! I

I i

I FIG. 5.46: CROSS-SECTION OF RING RUBBER SPECIMEN

Resistance R

dR

R

Resistivity P

With H -, 2 mm

r2 = 12 mm

= Pr A

=

= P .dr 2nrH

P

dir

R.n (~) '·1

R

214

Resistivityp 2 .2 x 10-3 =

R.n(12/ 3 )

where R = Measured Resistance

p =

r =

=

=

Resistivity

r2 :... rl

Internal radius

External radius

x R

H = Height of specimen or depth of cavity

After the specimen is moulded and held at a constant pressure

in the cavity the reading on the ohmmeter shows a rapid droproff initially

but becomes ,substantially less rapid after the specimen equilibrates.

Results are summarised in Tables 5.30 and 5.31. They appear quite

different from the results obtained by Boonstra and other workers, in

which the resistivity increases in an exponential manner with increas-

ing mixing time. Figs. 5.47 and 5.48 show that 'resistivity of SBR

mi,xes increases nearly linearly ,with energy input while the resistiyity

of NR mixes appear to be less sensitive to energy input.

It seems that more improvements in the technique are needed before

electrical resistivity and energy input or carbon ,black dispersion can

be correlated. Also, the lack of, reasonable correlation with the work

by Boonstra and others suggests an extreme dependence of results on

test method.

215

1 RESISTIVITY

(Om)

2.0

·1.0

1 .RESISTIVITY

( Om)

0.5

700 1400 2100

FIG. 5.47: RESISTIVITY AS A FUNCTION OF UNIT WORK FOR SBR BANBURY MIXES

•

500 1000 1500 2000 .

UNIT WORK (MJm-3 )

FIG. 5.48: RESISTIVITY AS A FUNCTION OF'UNIT WORK. FOR NR MIXES

216

•

2800

TABLE 5.30: ELECTRICAL RESISTIVITY MEASUREMENT FOR SBR MIXES

Unit Work (MJm- 3) Resistivity (nm)

568 c- 0.7- - ~'" - -

867 1.2

1263 2.5

1706 3.5

2187 5.1

2700 5.9

TABLE 5.31: ELECTRICAL RESISTIVITY MEASUREMENT FOR NR MIXES

Unit Work (MJm -3) Resistivity (nm)

521 0.26

679 0.28

995 0.30

1236 0.33

1469 0.60

1927 LOO

.5.14 VULCANISATE PROPERTIES

The ultimate goal of the mixing process is to produce compounds

with optimum and repeatable vulcanisate properties. Reinforcing

properties are particularly dependent upon the extent of mixing,

whioh governs·the level of dispersion of fillers and other compounding .-ingredients.

5.14.1 EXPERIMENTAL

Measurements were conducted on MRPRA Automatic Tensile Tester at

MRPRA laboratorY in Hertford. Small dumb bell-shaped specimens were

. 217

TABLE 5.32: VULCANISATE PROPERTIES OF SBR MIXES

·Unit Work Modulus at Tensile Strength Elongation (MJ.m- 3 ) 100% 300% 500% (MPa) at Break

568 1.88 8.08 15.8 15.9 504

867 1.47 6.98 14.7 18.9 606

Banbury 1263 1.32 6.95 15.2 20 620 Mixes 1706 1.28 6.92 15.1 21.5 635

2187 1.28 6.84 . 15.0 23.6 675

2700 1.24 6.93 14.9 24.0 685 , 450 1.65 6.79 - 10.9 425

650 1.26 5.59 12.5 16.4 607 Brabender 1000 1.05 5.47 12.5 19.0 649

Mixes 1312 1.08 4.99 12.0 19.4 669

1885 1.00 4.68 11.9 22.7 745 2270 1.01 5.10 12.5 22.1 705

.. , TABLE 5.33: VULCANISATE PROPERTIES OF NR MIXES

Unit Work Modulus at Tensile Strength Elongation (MJm- 3 ) 100% 300% 500% (MPa) at Break

521 1. 31 6.50 15.2 21.5 617

679 1.25 6.08 14.8 22.5 639 Banbury 995 1.10 5.89 13.8 22.9 669

mixes 1236 1.30 6.62 15.2 20.0 . 584

1600 1.16 6.40 14.7 23.6 .' .657

1927 1.20 6.24 14.6 22.7 653

290 1.34 5.00 12.3 13.8 540

.530 1.43 6.90 20.0 22.1 610

Brabender 750 1.45 7.13 19.1 24.8 587 mixes 970 1.22 6.68 16.2 22.7 612

1215 1.27 6.72 16.0 23'.7 640

1550 1.36 7.12 17.1 22.6 602

used and the results were computed and printed out by the mini computer

connected to the instrument.

, -'5.14; 2 RESULTS AND DISCUSSION

The results obtained are summarised in Tables 5.32 and 5.33. With

SBR mixes a systematic change in properties are observed. Moduli at

100% and 200% show a decrease while Tensile Strength and elongation at

break increase systematically with increasing energy input and black

dispersion. The tensile properties of NR mixes do not appear to change

much as mixing progresses except at the initial stages with very low

energy input. This may be attributed to self-reinforcing behaviour of

natural rubber. Nonetheless tensile properties are' a good measure of

mix quality, at least where strain-induced crystallisation is absent.

5.15' MIXING IN 'DEFINABLE SHEAR FIELD

" With the complex geometry of the rotors of rubber mixers only the

shear rates and shear stress at the rotor tips can be' calculated. The

contributions by other regions in the mixing chamber to the mixing

process, though believed to be substantial, are not considered because

the conditions are less well defined.

Shear rates and shear stresses in the capillary rheometer dies

are well defined and can be easily calculated. If rubber is subjected

to these definable shear rates and shear stresses in the capillary

rheometer a certain degree of mixing may be expected to occur, resulting

in changes in flow, behaviour.

In this work the effects of specific amount of shear, obtained

through repeated passes through the die, on polymer breakdown (mastication)

and carbon black dispersion is examined. Bagley corrections for

entrance effects are also examined.

220

5.15.1 EXPERIMENTAL

The mastication and mixing processes of' natural rubber (SMR 5CV)

and its carbon black-f'illed mixes were investigated. The normal,

commercially available black masterbatch materials possess a good level

of' black dispersion already, due to the high shearing f'orces used

during.their manuf'acture. Consequently the rubber-black masterbatch

used f'or this study was prepared in solution so that the.black is

incorporated, but not dispersed, without any signif'icant shearing.

Masticated rubber was obtained by passing the raw rubber twice

through a die and swollen in toluene overnight. HAF carbon black

(Vulcan 3) was then added to the swollen gel in the proportion of' 35

parts per hundred rubber and mixed f'.or ~ hour in' a Z-blade mixer. The

thick viscous mixture was dried by spreading on a glass plate and lef't

in a vacuum chamber overnight. The prepared samples were cut, diced

and extruded through a. die (1 nun diameter and 12.5 mm length). The

extrusion was repeated up to six times on the same sample. Corrections

f'or entrance ef'f'ects were carried out using several dies of' the same

diameter but varying lengths. The ef'f'ects of' varying shear rates were

also examined.

Thin sections of' the extrudates were obtained on a cold stage

microtome .f'or optical microscopic examination of' carbon 'black dispersion.

Uncured rubber-black mixes were f'ound extremely dif'f'icult, if' not

impossible, to section. Consequently the specimens were vulcanised

bef'ore sectioning by cutting them into small pieces, dusted with dicumyl

. 0 • peroxide and cured at 100 C f'or 30 IIll.nutes.

221


The amount of shear y is defined as

y = =

o

where Ya = apparent shear rate

ta = apparent residence time of the material in the capillary •

Since Ya = .!±R TfR3 (5.32)

t = Tfr2L ,a Q (5.33 )

4L = (5.34) a R

where Q = volumetric flow rate

R = radius of capillary

'L = length of capillary.

The parabolic velocity profile necessitates the use of an apparent

or mean residence time and shear rate since they are actually infinite

at the capillary wall and minimum at the centre of the die.

Bagley5 suggests that the entrance effects can be considered as

increasing the effective length of the.die by an empirical factor nB so

.True shear stress T . lIPR (5.35)

=

nB can be obtained from the negative intercept on the horizontal axis

when lIP is plotted against L/R• Any increase in shear rate normally

increases the value of nB from which, it has been claimed6- 9, that the

elastic and viscous components can be determined.

Using the Bagley correction· Equation 5.34 becomes

y' = a

222

I\) I\) w

« ","

60

f 50

llP (KPa) ",

40

-10 o

FIG. 5.49: TYPICAL PLOT OF llP VERSUS L/R FOR RAW NR (SMR 5CV)

400 5- 1

300 5- 1

200 5- 1

'.

-20 -lO

60

f,P (KPa)

40

30

10

o lO

FIG. 5.50: TYPICAL PLOT OF f,P VERSUS L/R FOR CARBON BLACK MASTERBATCH

20 L/R ...

200 s-1

30

I\) I\) V1

0.5

0.4 \ SHEAR STRESS T

(KPa)

0.2

400

I I 1\

\ \

\ \

\ \

. \

800

TOTAL SHEAR y a

1200

FIG. 5.51: RELATIONSHIP BETIIEEN SHEAR STRESS AND TOTAL OF SHEAR FOR RAW RUBBER AND

RUBBER-BLACK MASTERBATCH

1600

•

FIG. 5 . 52a: LIGHT MICROGRAPH OF CARBON BLACK MASTERBATCH AFTER ONE PASS (18ox)

.. ~.-~ \I 'J ..

I'" .. ,~" .,. - ' .. . - k · ...

FIG. 5 . 52b: LIGHT MICROGRAPH OF CARBON BLACK MASTERBATCH AFTER FIVE PASSES (18ox)

226

TYpical plots of 4P versus L/R for raw rubber and carbon black

masterbatch are given in Figs. 5.49 and 5.50. with increasing shear

rates nB appears to increase but in a scattered manner and does not

.increas.e. linearly with shear stress. ~ Hence -the.·elastic and viscous

effects could not be separated as desc~ibed by Bagley6. The values of

nB are high, ranging from 15 to 25. The curves produced by raw rubber

_and black masterbatch are generally linear. However, with black MB

at high L/R ratios the curves tend to form a plateau at low shear rates

while at high shear rates the appears a sharp rise. The reason for

this is not clear; however wall slip and melt fracture may contribute

to this observation.

The flow properties of the mixes after each·pass are obtained by

calculating the shear stress at 100 s-I using Equation 5.35 and the

amount of shear is derived from Equation 5.36. Their relationship is

illustrated in Fig. 5.51. The shear stress decreases substantially

after each pass but tends to level off after several consecutive passes

through the die. However, in spite of the considerabl:e change in the

flow properties optical microscopic examination of microtomed sections

(Figs. 5.52a & b ) reveal poor levels of carbon black dispersion even

after six passes. Banbury mixes with varying levels of black dispersion,

as described earlier, are shown to possess distinct flow properties.

Therefore characterisation of flow behaviour on the rheometer may provide

no specific information on carbon black dispersion. Extrusion through

the die of the capillary rheometer does not seem to produce the conditions

conducive to proper mixing and dispersion of carbon black •

. 227.

REFERENCES

1. Van Buskirk, P. R., Turetzky, S. B., Gunberg, P .. F., Rubb. Chem;

Tech., 48 (4), 577 (1976). _. __ - -_---:-c::.-

--.. :=:.--=- - """"--. -- . - - . --' -- -:..... .--- --::-.- -- - --.

2. Turetzky, S. B., Van Buskirk, P. R., and Gunberg, P. F., Rubb.

3.

4.

5.

6.

7.

8.

10.

Chem. Tech., .!±.2. (1), 1 (1976).

Dizon, E. S., Rubb. Chem. Tech., 49 (I), 12 (1976).

Palmgren, H., Rubb. Chem. Tech., 42 (I), 257 (1969).

Bagley, E. B., J. App. Phys., 28, 624 (1957).

Bagley, E. B.,. Trans. Soc. Rheol., 5, 355 (1961).

Philippof'f', W., md Gaskins, F. H., Trans. Soc. Rheol., ,g, 263 (1958).

Arai, T. and Aoyama, H., Trans. Soc. Rheol., I, 333 (1963).

Ram, A., and Narkis, M., J. App. Poly. Sci., 10 361 (1966). . !

Cogswe11, F. N., Plast. and Poly., 38, 391 (1970).

11. Cogswe11, F. N., Poly. Eng. and Sci., 12 (I), 64 (1972).

12. Turner, D. M., Moore, M. D., Smith, R. A., Bob Payne Memorial

Symp., Uni. of' Loughborough, U.K., April 1978.

13. Davey , , and Payne, A. R., "Rubber in Engineering Practice""

14.

15.

16.

Maclaren & Sons, London 1964.

Freakley, P. K., private communications.

Meissner, J., Trans. Soc. Rheol., 16, 405 (1972).

Meissner, J., Rheol. Acta., .!1., 78. (1969) .

Boonstra, B. B., and Medalia, A. 1., Rubb. Chem. Tech., 36 (1),

115 (1963).

18. Boonstra, B. B., Rubb. Chem. Tech., 2Q (I), 194 (1977).

19. Leigh-Dugmore, C. H., "Microscopy of' Rubber", W. Hef'f'er & Sons Ltd.,

Cambridge, England, 1961.

228

. CHAPTER 6

DISPERSION OF NQ\}-BLACK CQ'>1POUtIDING INGREDIENTS

6.1 INTRODUCTION

The importance of soft X-radiation microradiography for the

dispersion study of inorganic fillers and· compounding ingredients in

rubber and its technique is well established1 ,2. Pugh and West 3 dis

cussed the basis of a method of adapting a scanning electron microscope

(SEM) for X-ray microradiography. Recently Hemsley·and Hayles4 of

Loughborough University suggested an improved version whereby the

standard stage of·S~M is replaced by a new ·device which allows monitor

ing of the X-ray intensity reaching the photographic emulsion. An

important advantage of this· new technique is that much thicker specimens

can be examined without the loss of contrast and reasonably short

exposure times can be used.

The operation principle of the device is illustrated in Fig. 6.1.

with the final aperture of the SEM replaced by one of 400 ~m diameter

the electron beam is focussed on to the metal target foil 4 ~m thick,

to provide a spot source of soft X-rays. The unfiltered X-rays passing

through the ~pecimen are recorded by a 1 cm square silicon solar cell

(Plessey se 4); alternatively the image is projected on a photographic

film. Contrast is generated by the differential absorption of X-rays

in the specimen. Fig. 6.2 shows the X-ray device ready for insertion

into the S2A stereos can.

. . . 229

.'

400 nun APERTURE

(SEM) ELECTRON BEAM

• TARGET

c=====~==t;====~I~-SM@~

FILM

I

INCIDENT RAY

1 ADHESIVE

TAPE

FIG. 6.1: OPERATION PRINCIP~ OF THE DEVICE

230

FIG. 6 .2: X-RAY MICRORADIOGRAPHY UNIT

. '

6 .2' EXPERIMENTAL

The following compounds were prepared on the Brabender Plastograph

. Using~thecam type "mixingohead"at05 r.p.m. ---_~ -

•

(i) SBR 1712 + 50 pphr RAF with 10 minutes mixing time. (Mix 1)

(ii) Same as (i)' with 5 pphr ZnO added 5 minutes before end of mixing

cycle. (Mix 2)

(iii) Same as (i) with 2 pphr S' added 5 minutes before'end of mixing

cycle., (Mix 3)

(i v) SBR 1112 + 50 pphr RAF mixed for 20 minutes with 5 pphr ZnO and

2 pphr S added 2 minutes before the end of mi~ing cycle. (Mix 4)

(v) Same as (iv) with ZnO and S added at the beginning of mixing

cycle. (Mix 5)

Together with raw SBR 1712 thin specimen sheets were prepared by

compressing about 2 g of material sandwiched between two sheets of

cellophane in a hYdraulic press. By using the cellophane sheets smooth

clean specimens were obtained which were easily separated. The thickness

of the specimens obtained was about 300~. About 1 cm2 of the specimen

was removed from the cellophane sheets and placed on the specimen

holder and positioned in the path of the X-ray beam.

The device was used on a scanning electron microscope mode S2A

stereoscan. The operating conditions were:

Accelerating Voltage

Beam Current

Solar Cell olP

Photographic Plate

30 KV

160 ~A

3 mV

Kodak Industrex MX

Under these conditions the exposure time was 2 minutes. The film was

developed for 4 minutes in Kodak DX80.

, 232

--==

6.3 RESULTS AND DISCUSSION

Figs. 6.3 - 6.8 are the X-ray microradiographs of the rubber

. :mixes~j.Ilvestige.ted. _: c

,

Fig;' 6.3 represents the control sample which isihe rawmbber

(SBR 1712). A few small dark ~pots of very low contrast can be seen.

Their presence is unexpected although they may be due to traces of

impurities or additives introduced during the manufacture. The micro

graph of SBR containing 50 pphr (Mix 1) is given in Fig •. 6.4. A sparse

distribution of dark areas can be seen. These areas vary ~n size and

their poor contrast suggests that they are the impurities or large

unbroken agglomerates of carbon 'black. ' Optical microscopic examination

of microtomed sections of the same sample also reveal the low level of

black dispersion. The dispersion of zinc oxide (Mix 2)' and sulphur

(Mix 3) are shown in Figs. 6.5 and 6.6 respectively. The images of the

former are less sharp and dark than that or the latter.

The micrographs '(Figs. 6.7 - 6.8) clearly show the different

levels of the distribution of zinc oxide and sulphur in Mixes 4 and 5.

In Fig. 6.7 two distinct types of particles appear to be present, one

being considerably more absorbent of X-rays, and hence darker the

images, whil~ the other produces lighter images which appear to be also

rhombohedral in shape. Surrounding some of thesc images are a light

halo which may indicate the trapped air due to the large size of the

particles in the rubber. These two types of images may represent the

two added inorganic ingredients, that is, zinc oxide and sulphur. At

low level of dispersion these additives can therefore be identified

from one another. As mixing is prolonged the progressive breakdown

,and dispersion of these particles can be clearly seen from Fig. 6.8

where only small dark spots are seen to be evenly distributed. An

interesting observation here is the less intense, areas of low contrast

, 233

FIG . 6 . 3 : X-RAY MICROGRAPH OF SBR 1712 (250X)

FIG . 6.4 : X- RAY MICROGRAPH OF MIX 1 (250X)

234

FIG. 6. 5: X- RAY MICRORADIOGRAPH OF MIX 2 (250X)

FIG. 6 .6 : X-RAY MICRORADIOGRAPH OF MIX 3 (250X)

235

• I

.. •

FIG . 6 . 7: X-RAY MICRORADIOGRAPH OF MIX 4 (250X)

• •

• • •

•

#

FIG. 6.8: X-RAY MICRORADIOGRAPH OF MIX 5 (250X)

236

, .

seen in carbon black masterbatch. This 1S due to the excellent level

of black dispersion in those mixes with long mixing times as analysed

by optical microscopy.

With the device developed X-ray microradiography can be utilized

with greater efficiency and much reduced time for dispersion studies

of inorganic additives.

237

REFERENCES

1. Coss1et-, V. E., and Nixon, W. C., J. App. Phys., 24, 616 (1960).

2. Waterfie1d, C. G., Peacock, J" S.l".E. Tech. Papers, 19 393 (1973).

3. Pugh, D. J. and West, P. D., J. Mic., 103, Pt.Il, 227 (1975).

4.- Hems1ey, D. A., and Hay1es, M., Inst. Phys. Conf. Ser. No. 36, 53_

,

238

::........::. ::.---,;:~=

. QlPPTER 7

GENEML DISCUSS~ON

A study of the mixing process requires a systematic ·analysis of

the mechanism and· factors controlling mixing efficiency and investiga

tion of relevant rheological properties of rubber mixes.

Flow visualisation, coupled with pressure variation studies and

detailed examination of the mixing activities in the various zones in

the m1xer, reveal some interesting facets·of the mixing process. The

. tip region is the zone of maximum shear stress., which is responsible

for most of the dispersive mixing and corresponds to the peak on the

pressure trace. The level of shear stress in this region increases

with shear rate or rotor speed and batch size.

In front of the rotor tip is a region of high pressure which

influences the shear stress level at the rotor tip. Since the material

in rront of rotor tip converges into the nip region, due to the oval

shaped rotor, a considerable tensile deformation is observed.·

Substantial elastic deformation also· occurs in the low pressure region

behind the rotor tip as the material, after being sheared and sheete.d,

is pulled away from the chamber wall, folded, compacted and mixed with

the rest of the compound •. For the compounding material to be subjected

to all the above deformations correct batch size must be used. Under

filling the mixer results in occasional absence or lack of material in

front of the rotor tip, low shear stress level in the nip region and

inefficient use of the mixer. Excessive batch size reduces the voids

and mixing activities behind the rotor tip while. extensive mixing

becomes poorer.

The viscoelastic behaviour of rubber. and the modes of deformation,

. , 239

"

namely shear and extensional,.prevailing in various parts of an internal

mixer make the study of viscoelasticity in rubber most relevant and

pertinent. Capillary rheometry has been successfully used for plastics

,,~and-its application to rubberise investigat'ed: - Aftliougll tIle flow"

curves of the rubber mixes are readily obtained a few surprising

phenomena are observed. The inflexions seen on the log stress-log strain

rate curves appear to be related to certain specific properties of a rubber-

black system and are independent of test conditions used, thus corrobor

ating the work by Turner et all, who suggest that the shear rate at

which the inflexion occurs is governed by the fracture stress F and

the viscous constant K of the TMS model, and that wall slip and fracture

are related.

Using a pair of dies to ,correct the entrance or end effects

rheological parameters can be obtained .. However, work using different

pairs of dies of varying LID ratios yield incomparable results.

Data obtained from capillary rheometer are also analysed to obtain

viscosities and moduli under' simple shear and tension according to the

analysis presented by Cog~wel12. From the results obtained a Poisson's

ratio very much greater than 0.5 is obtained. This renders the present

analysis invalid for rubber and reducing the results obtained to

empirical status.

These anomalous and peculiar results mB¥ be attributed to, among

others, thixotropy. Freakley3 suggests that thixotropY can account for

about 20% of the total shear stress applied to a specimen having no

significant shear history. The total shear that "the material under

test' receives, is insufficient to achieve a reasonable degree of thixo-

tropic breakdown and the pressure drop across the die can therefore be

expected to be substantially affected by thixotropy. Elimination of

thixotropic effect' can be achieved by having the·material in the barrel

, 240

- . - -

subjected te seme ferm 'Of shear werk at lew shear rates befere it enters

the die. Unless such measures are taken capillary rheemeter data,

witheut undermining the impertance 'Of the 'instrument, cannet be treated

as fundamental. Nenetheless the results 'Obtained witheut 'the abeve

cerrectiens shew a definite trend - with increasing energy input the

viscesities and meduli under simple shearSld tensien' decrease teward a

censtant value.

The use 'Of ether instruments te explere the behavieur 'Of unvulcan

ised rubber mixes is feund te be ~seful when the,results are analysed

with the help 'Of a mechanical medel, namely the TMS visceela,stic medel.

The prepesed medel seems te describe the behavieur 'Of the mixes during

the creep and elengatienal testing. The medel parameters are feund te

systematically change with increasing mixing time. Thus these tests,

when used in cenjuncti-on with the prepesed medel, ceuld 'Offer new metheds

fer assessing, interpreting and predicting the quality 'Of mixing.

Rapid and reliable metheds te test rubber cempeunds fer the

purpese 'Of precess and quality centrel yield 'Only empirical results.

Standard Meeney Viscesity and Wallace Plasticity instruments still

remain as the mest suitable because 'Of their ease ef,measurement and

sensitivity to miner changes in the mix. Mixes with different levels

'Of energy input and level 'Of black dispersien can be discriminated

fairly well. Altheugh the test cenditions en these instruments are

very much different frem precessing situatiens seme qualitative measure

'Of precessibility can be 'Obtained.

The Mens ant '0 Rheemeter dees previde seme infermatien en the varia

t'ien in cempeund preperties 'Of mixes prepared at varying energy input

altheugh the,lack 'Of sensitivity restricts its usefulness. With a

relatively.simple medificatien 'Of the rheemeter it can be usedte

measure the mechanical phase angle 'Of rubber mixes. This ceuld previde

241

a suitable method for characterising the viscoelastic properties by

resolving the viscous and elastic components.

Die swell and bound rubber of SBR mixes are found to progressively

increase with increasing energy input suggesting their direct reiation-

ship with the quality of carbon black dispersion. With NR mixes die

swell and bound rubber decrease with increasing energy input - this is

attributed to polymer·breakdown. Similar behaviour· is obtained with

tensile properties.

Measurement of electrical resistivity of incured rubber mixes ~s

found to be unsatisfactory and sensitive to test technique. The

apparatus requires some modifications to incr~ase the ease of measure-

ment.

Energy input or unit work appear to be a useful mixing parameter

to correlate with the fundamental and empirical properties. However,

unlike the claims made by Van Buskirk and his coworkers 4 , the

properties of the rubber mixes prepared on the Banbury and Brabender

Plastograph at identical levels of energy input ·rarely superimpose upon

one another. The properties of the mixes obtained from Brabender

~~astOgraph appear to be, in general, about 10 - 15% lower than those

of Banbury mixes prepared at corresponding energy input,. although the

property - unit work curves run approximately parallel to each other

the Banbury curves appear to be shifted along the unit work axis by a

'shift factor'. The cause of the shift is uncertain although it ma;y

be that the calculated energy inputs for the Banbury mixes are higher

than the actual values.[ Di-ff~re~~es-i~ ~otor d~~i-gn ~ust :;~~-~~. ~~~jI

However, the distinction between the mixes P1"hlSd~bT.·consi<j.EE:ed:

mixers emphasise the need for more specific mixing conditions. Also,

·optical microscopic examination of microtomed sections reveals the

poor level of carbon black dispersion in SBR Brabender mixes which were

242

-

prepared under identical miXing conditions, such as shear rate and

initial mixing temperature; on the other hand Brabender NR mixes

prepared at a higher starting temperature, possess comparable levels of

black dispersion to the Banbury: mixes at equal energy' input. A closer

look at the temperature-time traces shows that those of NR mixes are

more approximately comparable to one another than those of SBR mixes.

It would iherefore be reasonable to conclude that when various mixers

are used harmonising of conditions are essential if any comparable

results are to be obtained. The criteria for harmonising the mixing

conditions may therefore be:

i) Shear rate and shear stress in the pip reg10n.

ii) Heat history of the mix or the temperature-time profile.

The first criterion can be readily achieved through the adjustment

of rotor speed. This is important because of the dependence of mix

'viscosity on shear rate and shear stress. The second criterion is

obtained through the proper selection of starting temperature and

,control of temperature rise of the mix. Viscosity is strongly

influenced by temperature and largely influences the rate of incorp-

, oration and dispersion of carbon black. The differences in the

properties of the mixes obtained from the two mixers may be narrowed

if the above factors are more closely' controlled. With a more accurate

power consumption integrating recorder unit work would be a suitable

mixing parameter for scale-up of processes.

While light microscopy is a useful tool for the study of carbon

black dispersion X-ray microradiography is found to be an important

technique for the eValuation of non-black inorganic compounding ingre

dients, such as zinc oxide, sulphur, silica and clay. The'use of the

recently developed X-ray unit in a standard scanning electron microscope

has considerably shortened the time required to perform the test and

•

243

also permits the use of thicker test spec~mens. The technique can

therefore be used for rapid tests on factory mixes. The potential

application of this technique may also be £ound in the study of the

blooming process of zinc oxide and sulphur.

, ,

244 ,

c

REFERENCES .

1. Turner, D. M., Moore, M. D., Smith, R. A., Bob Payne Memorial

Symp., Uni. of Loughborough, U.K., April 1978.

2. Cogswell, F. N., Poly. Eng. and Sci., 12 (1),64 (1972).

3. Freakley,. P. K., private communications.

4. Van Buskirk, P. R. TuretzkY, S. B., Gunberg, P. F., Rubb. Chem.

Tech., 48 (4), 557(1976).

,

245

QlAPTER 8

CONQUSIONS

This comprehensive study using the various techniques, old and

new, has indicated that the mixing' process can now be regarded more as

a science rather than an art.

The flow visualisation method of studying internal mixing has

been shown to be a powerful tool for determination of the critical

factors influencing flow and mixing efficiency. Correlation of

visualisations with results obtained for the mixing of 'practical'

rubber mixes confirms the validity of the method ,for prediction of

flow in conventional mixing operations. Fill factor is identified as

one of the key variables influencing mixing uniformity.

The study of fundamental rheological behaviour of rubber mixes has

identified the various viscoelastic properties that can be used to

characterise and predict their processing behaviour during the subsequent

downstream operations. In conjunction with Standard Mooney viscosity

and Wallace Plasticity, measurements evaluation of viscoelastic response

'by elongation, creep and dynamic testings, and capillary rheometry are

found to yield precise rheological parameters related to practical

processing. These, properties are found, generally, to correlate very

well with unit work - this further corroborated the suggestion by

general workers that unit work is a useful mixing parameter to quantifY

and scale-up the mixing process. Occasional irregular and 'abnormal'

values, however, are still obtained and are thought to be due to thixo-

, tropy. ) Future work is suggested in the following fields:

1) Thixotropy - Most of the results obtained show systematic changes

, 246

in properties with increasing energy input. However, the effect

of thixotropy cannot be neglected and must be investigated in more

detail. This is most relevant to capillary rheometry and other low

total shear techniques used to investigate rheological properties.

2) Mixing in internal mixers - The 'flow' visualisation'technique

using the Brabender Plastograph can be used to ease the inter-

pretation of results obtained from the study of flow patterns in

practical internal mixers. Further studies using commercial rotor

types are required.

3) Characterisation techniques' '- 'Refinement of measuring techniques

is still required '~o as to obtain, rapid and reproducible values.

This is a prerequisite before any of the suggested test methods, can

be used on the factory floor.

, 247

APPENDIX I

PARTS BY WEIGHT

Oil-extended styrene butadiene rubber (SBR 1712) 137.5

Oil-extended natur·al rubber (OENR) Sf1e.scJ 133.3

. Carbon Black ~ 'SRF' . ,+0 ~5if 50

Zinc Oxide 5 5

Stearic Acid 2 2

CBS (Accelerator) 1 1

Sulphur 2 2

Antioxidant (Flectol H) 1 1

248

APPENDIX II

Equations derived £rom the T.M.S. model (Fig. 3.5 )

Symbols used:

E Modulus o£ elasticity £or .E/K network

D Modulus.o£ elasticity £or D/J network

K 'Viscous constant £or ElK network

J Viscous constant £or D/J network

n Power law index

F Fracture stress

S Extension . S Extension rate

6t Time increment

0E Stress on ElK network

oD Stress on D/J network

R Recovery

Ro Instantaneous recovery

6R Recovery in time increment 6t

L Extrusion shrinkage

DA Area die swell

Numerical methods

Most o£ the equations described here cannot be solved analytically.

Numerical methods were developed to allow the solutions to be computed

by a desk top programmable calculator.

The notation used in the £ollowing fescriptions uses a dash (X')

to represent the new value o£ a parameter X in a recurring loop.

Original length o£ model . is taken as 1 and at any time the

relaxed length o£ the model is (1 +Y) .

249

a) Elongation

Considering the E:K network

,the 'stress. 0E =

=

Initially after a small time 8~

ES . 1+Y

K [1'( 1+S)] n

= (no yielding)

after a second interval 8t the stress increases by an amount

80E =

Y =

hence 0E ' =

E(S-Y) 8t l+Y

°E K

0' E

l/n'

+

(y = yield rate)

l+S

1". l/n 9. S - ( O~ ) (l+S~

1+Y

similar equations apply to the D:J network.

at .

See section h) of paper for further discussion of (1)

b) Creep

o =

or 0 =

Instantaneous

then SK

S' J

=

=

o constant stress test' o

(1+S)O o constant load test.

extension So' = o/(E+D)

(l+S) 8t + l~n ( O~) ,

+ (O~ )

l/n (l+S) 8t

Effective spring stiffnesses also change:

E' = E

250

(

,

D' = D

Total extens.ion is then

cl Shear Flow

When a fracture occurs

strain on D:J = J(yln D

on E:K = F E

s = a E' +D'

The difference ih strain is E. - J(yln

= FD - E J(yln ED

E D.

After fracture the difference in strain is divided between the two

. networks

[FD-EJ(y)n ] D ED E+D strain on E:K =

on D:J = [ FD - E~J(y)n ] E E+D

then aEo . = E x strain = FD- EJ(y)n E+D

aDo = -0 Eo

The stresses then build up again

°E I = 0E +. E(y - Y l lit K

oD I = oD + D(y - YJl lit

l

,

251

d) Shear recovery

z

J,n _ cJ __ I I I I I I

.'

a_

K,n -' --~---=~-

I~ the external stress is removed the point z,moves to the le~ an

amount

= aD + D

x

·where X is the distance to bring the two networks into force balance.

DX

then R 0

=

= E [a~ =

....L E+D

....!.. E+D

aD X] - D

( a~ a~ ) + aD

D

If the point Z now moves a distance lIR to the le~ in a time interval

lit then the adjustment in the strains in the springs must be such that

D (lIR - LIS) = r

where.lISD

. l/n

= (=f) lit

l/n

= ( ~ ) lit

252

[ I/n hence lIR = lit D (-:D ) E( O~) E+D

Then S I = S - LIS - lIR E E E

S I = SD + LIS - lIR D D

e) Stress relaxation in time t

m =

°E(t) m

°D(t)m

n-l n

=

=

= relaxation constant

°E(o)m I/n r( ~) t

°D(o)m Dm lint

(%) . m

I/nJ

When computing residual stress resulting from the elongation rate at a

die entrance, the values used for the parameters must be those for

'tension; not for shear.

f) .' Die swell and extrusion shrinkage

We can use the' equations gi.ven in section d) for extensional

recovery so long as the shrinkages Ro and R are related to the extended

length of the network in the die. We have used the value of (l+oE/E)

as this length, valid for D»E, where 0E is the stress on the ElK network

at the die exit.

Extrusion shrinkage is then -

L = R r

for D»E

Area die swell DA is related to extrusion shrinkage by -

= ..l!.... l-L

These expressions can be readily evaluated on desk' top calculators such

as the HP 9815A. ' , 253

g) Effects of high extensions on flow formulae

The formula given in section a) is valid for one interpretation

of the model. A number of options are possible, which arise from

changes in the base dimensions of the sample as it becomes deformed.

These considerations are not too important when elongation ratios are

less than 2. Thus in the case of stress relaxation the simplest case

was used. Here as l.n applied stress situations the two networks act

independently so only a single network need be considered.

The basic case is -

a = E(S - Y) =

Y is the yield of the dashpot and removal of the external stress gives

Y = S, which constitutes the definition of Y when the original length

is 1.

The first refinement is that the extension rate applicable to the

generation of stress by viscous flow should be related to the instant an-

eous' length of the sample.

a = K 6Y/(l+S)6t n (2)

This expression was used to derive the equations in section a). An

,alternative which deserves some consideration is that

a K 6Y/(1+Y)6t n (3)

Yielding of the sample creates, extra length 'in the spring element, and

so

a = E(S-Y)/(l+Y)

Y relates to the unstrained length, but the quantity of flow, may be

concerned with the strained length, consequently'

= 6Y2(1+S-Y)/(1+Y) r

6Y1 replaces 6Y in equation (2)

254

(4)

The model cannot be interpreted literally and it is difficult to

decide whether equations (3) and (5) should be used. Equation (5)

should only be used in conjunction with e'quations (2) and (4) but not

";'

Similar expressions apply to the D/J network.

L

,

255

-

APPENDIX III

CREEP MEASUREMENT ON RUBBER MIXES

(by A. Bickley; Avon Rubber Co., Melksham, U.K.)

Introduction

'This'is an attempt to use the TMS model for unvulcanised rubber

to explain the results obtained by P. K. Freakley and W. r. Wan Yaacob

of Loughborough University in their experiment to measure creep at

constant stress.

Theory

On the initial applicat~on of the stress an instantaneous extension

occurs which is solely dependent on the spring'stiffness of the elements.

The initial extension is -

llS o

a = E+D

where a is the applied stress.

The stress ia the two arms is given by -

Ea aE = E+D

OD = ~ E+D

The time dependent extension is affected by both the elastic and

viscous components. The flow in 'a dashpot is found by considering the

stress applied -

= x 1 llt

(4)

where llSK is the incremental extension of the daShpot and EllS is the

total extension of the Maxwell element.

,

256

Thus the increment is

aE l/n

aSK = (l+EaS) at (6) K

aD l/n

Similarly aSJ

= (l+Eas) at (7) J

The flow in the dashpot causes a change in the effective stiffness

of ,the springs -

E' E (8)

D' = D

which leads to a new extension of the model -

Eas a = E' + D' (10)

The stress is the two arms is now given by -

, E'a aE = E'+D' (ll)

and , = D'a aD E'+D' (12)

The new values of aE, aD and EaS can be put back into equations (6)

and (7) to determine the next yielding in the dashpots and so·the next

incremental extension. '

,

257

c-."'O.-""'

APPENDIX IV '

a) COMPUTER PROGRAMME FOR THE ANALYSIS OF ELONGATION TESTING DATA USING

THE TMS MODEL

C FLONGATION TESTING OF RUB~ER MIXES USING TMS MODEL OIMENSION X( 30),V(30),XX(30i30)iVV(30,30),NSTORE(30).

1 AI' ( 1 0 ) • A E ( 1 \) ) • A D ( , 0 ) • A K ( , 0 ) , A J ( 1 0 ) , 1 PO L (30 ) , C ( 1 (; ) • ER ( 1 0 )

INTEG[R AM RU L .1 V, ~V

.q7 FORI'oAT (l0) i\ k FOR ~I A T (1 H () ) 119 FORMAT (2FO.U)

1 110 FOR HAT (r 0 , 'I F 0 . 0 ) 10, FORMAT (30FO.O) 1 I) 6 FOR "1 A T (1 H , 5 X , J 2 , 4 F 1 2 • 2 ) 110 FORMAT (. ENERGY INPUT = '.F7." 1~n FORMAT (' ELASTIC MOOULUS E = , ,F5.2) '~1 FORMAT (t E~ASTIC MODULUS D = , ,F5.2) 122 FORMAT (' VISCOUS CONSTANT K = , ,F5.2) , 123 FORMAT (' VISCOUS CONSTANT J = , ,F5,2)

Ir.OUNT=O 11) REA" (l,10a) N,EJI"PUT

IF (N.En.O), GOTO 999 RFAD (1,99) (ER(.J),J=1.2) ICOUNT'= !COUNT + t RF.A~ (1,101) <X(Il, 1=1.N) RrAO (1,101)(Y(I)'1=1,N)

C CURVE FITTli~G

1·1= 10 CALL EOZACF (x.Y,~,POL,M+1'REF)

?O(l FORMAT (lH, SX, f1~.4) 201 FORMAT (lH ,F5.2,3E12.4) 203 FORMAT (1HO, 6X, 19HCURVE.FIT BY E02ACF/1HO, 4x1 9HPOLYNOM1A,

114 HL COEFFICIENTSi) 2,)(. FOR~IAT (1HO. 2x, lHX, 7x. 4Hf(X),'8X.3HFIT,7X, 10HPERC RESI[)/) 205 FOQMAT(/,5X"DEGREE OF BEST POLYNOMIALI,12)

WRITE (2,110) EINPUT WRITE (2,20,) M WRITE (;>,205) WRITE (2,200) (POL(I) , I=1,M+1) WRIH (?'i204) 00 210 J = 1,N ' .. Z r; XO)

, 258 ,

s = POL(I~+1) I = M

40 S = S *Z + POLII) IF (1-1)80,80,60

60 I = I - 1 (joTD 40

ill) T = V(J) H. = ( l S - T ) / Tl ;, 1 0 0 • WRITE (2.201) Z,S,T,H CONTl NUF Dei 235 ,i=1.2

- . - ~

TERM = M*PDL(M+1)*ER(J)+(M-1)*PDL(M) 1'0 23(1 1=2,M-1

235

11=1'1-1-1+2 TERM = TERM * ER(J) + (11-1) * POl(I!) CONIINUF. C(J) = TERM CONTINUE TEMP=O.Q DO 231> K = 1,M+1 TEMP = TEMP + POL(K) *2.0**(~-1). CONT! N U[

J V = T E J.' P - C ( 2 ) * 1 • 0 KV= YeN) - Jv t = c(2) 0=C(1)-c(2) WRITE 12,Kb) IJRITF. 12",12v)E WRITE ,(?,nnD WRITE (?,1U)KV WRITE (2,12~)JV WRITE (;>,88) NSTClRE (I(.OliNT) =N DO 300 1=1,N ~M(.lCOUNT)=M

A F. ( I en U 1I T ) = E ADIIC()U~T)=r)

AKClCQUNT)=t:v A J e I CO U N T ) =.J V XXII,ICOUN1)=XCI)

I.

259

J

VVII,ICOUNTl=YII) 300 ,COl'l'I fJUE

GOrO 10 9')9 C[lI(rINUf' 2 'I Q H) R 1'1 A T I'M E D K J I

J.lRIH 1~,29~) " ~ I T E ( ? , Ri!) DO '1.0 l.;l,ICOUNT ~IRIn: 12,100) AM(l) ,AF.(L> ,AD(l) ,AK(l) ,AJ(l)

140 C(1~TJ "UF 400 Fn~i~AT (SX ,'GRApH OF TRUE STRESS VS ELONGATION RATIO.',I)

!,J~ITf, (;>,4 00 ) Co\I.L LU1934 CALL ~ E V PAP (2 10. , 3, O. , " CALL IJINOOW(2) C~LL AXjPOS (1,50.,]O."~n.,,) CALL AXjPQS (1,SO.,30.,200 .. Z) C ALL A X I S C A '( 1 ,Il, 1 •• 5 •• 1 ) , CALL AXjSCA (1,Il,O .. 8() .. 2) CALL ,\XjORA (-1,1,,, CALL AXIDRA (1,-1,2) DO 410 .1=1, J COUNT N=NSTO~E(J)

D n 420 I = 1 , rJ X(O=XX(i,J) Y(J);yY(J ,J)

420 CONjINUF. CAL~ GRASyM (X,Y,N,J,n)

/'10 CONTINUE CALL pleCLE CALL OEvEND STOP ~

F.NO ~J·NiSH C

260 ,

b) COMPUTER PROGRAMME FOR THE ANALYSIS OF CAPILLARY RHEOMETER DATA

C PROGRAMME FOR THE ANALVSIS OF CApplLARV RHEOMETER DATA o I MEN S ION R <1 0) , E ( 1 0) , F ( 1 0) , S (1 0 ) , V ( 1 0) , X (1 0) , Y ( 1 0) , T ( 1 0) , B (1 0) ,

1 \J ( 10) , Po L (1 0) , S 1.C1 0) , C (10) , P (10) , Q (10) , U (10) , U L( 10) , A ( 10) ,0 (10) , 1 S L 1 (1 0) , V I 1 (1 0 ) , SO 1 ( 1 0) , CO 1 (1 0) , EX 1 (1 0) , 1 W U ( 1 0 ) , S L ('I 0 ) , Y I ( , 0 ) , S 0 (1 0) , CO ( 1 0 ) , Ex ( 1 0 )

DIMENSIoN VV(10,10),YV(10,10),TTC10,10),EEX(10),EEINPUT(10), 1 PP ( 1 0 , 1 0) , X'; ( 1 0 , , 0) , Q Q (1 0 , 1 0) , U U (1 0 , 1 0) , E E X 1 (1 0)

DIMENSION NSTORE(10) 87 FORMAT (10)

100 FORMAT (FO.O,IO,FO.O) 101 FORMAT (10FO.O) 105 FORMAT (1H ,3(F10.2),S(E15.6» 106 FORMAT C1H ,6F15.5) 112 FORMAT (I RATE VOLT1 VOLT2 PRESSURE1

1PRESSURE2 PRESSURE STRESS APPARENT VISI) 110 FORMAT('OENERGV INPUT = I,F7. 1 ) 111 FORMAT(1 HO)

113 FORMAT ('-~.--------------------------------------~---------- ____ • ,-~-------~-------------------------------------~--~-- ____ wee') 102 FORMAT C' LOG RATE LOG STRESS LOG APPARENT VIS') 103 FORMAT (' ________ - ________ • ___ • ____________ -~ ___________ t)

104 FORMAT C1H , 3CE1~.6» 115 FORMATC' SLOPE "',F10.5) 116 FORMAT(' ylNT =I,F10.5) '17 FORMAT(' SD£V =',nO.5)

,118 FORMATC' CO~R "',F10.S)' 119 FORMATC' POWER =I,F10.5) 130 FORMAT C' ENERGy INPUT SLOPE INTERCEPT

1STD-DEV CORRELN POWER I)

131 FORMAT ('---------------------.----------------~------------- ----.' 1~~--------------~--~------·---·-~-·~-·-~··1) CALL LU1934

READ (1,87)NN DO 998 JJ=1,NN ICOUNT " 0

1'0 READ <1,100) EINPUT,N,OELTA L IFCN.EQ.O)GOTO 999 ICOUNT = ICOUNT + 1 READ C1,101) CRC!),I=1,N) READ C1 ,101 HE(l) .1 .. 1 ,N)

J

261 •

READ (1,101)CSCl)d=1,N) RADIUS = 1. FACTOR' e. 3.4 * 10.**6. 00'1=1,N A(I)=E(I)*~ACTOR' O(I)=S(J)·FACTOR.' F ( I ) =A ( I ) ~ 0 (I) 5(1)= FCI)· RADIUS/(2. * DELTA L) U ( I ) = 5 ( I ) I R ( I ) V(I) = ALOG10{UCI» TCI)=A LoG10(RCI» P(I)=ALOG10(5(11)

, CONTI NUE WRITE (2,1101 EINPUT· WRITE(2",11 WRrTE(2,1,Z) WRrTE(2,1131 DO 250 1"', N WRITE (2,1051 R(lI,E(!),S(!),ACl),0(1),F(I),5(1),U(l)

250 CONTINUE WRITE (2,,111 WRITE (2,10~) WRITE (2,103) DO 260 1=1,t, WRITE (2,104ITCII,P(!),VCI)

260 CONTINUE CALL LEAST (T,V'N'SLOPE,YINT~CORR,SDEV,POWERI' WRITE (2,1111 WRITEC2,11S)5LOPE· WRITECZ,116IYINT WRITE(2.1'7)SOEV . WRITEC l ,11!1)CORR WRITE ~2,11YIPOWER WRITE (2,1111 WU(ICOUNTI=EINPUT S L (I COUNT) =5 LOpE Y I ( IC 0 U N T> = Y I N T SO(ICOUNTI=SDEV CocICOUNT)=CORR EX(ICOUNT)=POWER ~

262 •

120

999

140

400

420

410

800

WRITE(2.111l NSTORE(ICOUNT)=N, DO 120 1=1,1'1 TT(I,ICOUNT)=T(I) VV(I,ICOUNT)=V(I) XXCI, ICOUNT) :: XCI) EEX(ICOUNT)=POWER PP(I,ICOUNT) :: PCI) CONTINUE GOrO 10 CONTINUE WRITE (2,130) DO 140 L=1,ICOUNT

"

WRITEC2.106) WU(L),SL(L),yICL),SO(L),CO(L),EX(L) CONTINUE WRITE (2,400) • FORMAT(SX, 'GRAPH OF LOG APP VISCOSITY VS· LOG APP SHEAR ,RATE.',f) CALL OEVPAP (210. ,310'.,1> CA L L W r'N 0 OW (2) CALL AXIPOS (1.50. ,30. ,140 .. 1> CALl. AXIPOS (1,50.,30 •• 210 .. 2) CALl. AXISCA (1,4.1.2,2.8,1) CALL AXISCA (1,8.2.4,4.0,2) CALl. AXIDRA (-1,,.,1) CALL 'AXIORA (1.-1,2) DO 410 J=1,ICOUNT N=NSTOREeJ) DO 420 I=1,N T(I) :: TTCI,J) vcr) = VV(!,J) CONTII/UE CALL GRASYM eT,V,N,J,O) CONTINUE CALL plCCLE WRITE (2,800) FORMAT CSX,'GRAPH OF LOG SHEAR STRESS ,VS LOG SHEAR RATE.',I) CALl. OEvPAP (210.,310 •• 1> CALL wiNDOW (2) CALl. AXIPOS (1,50 •• 30.,140 .. 1) CALl. AXIPOS (1,50.,30.,210 .. 2)

,

263 ,

\

---~-'-- -

820

810

998

CALL AXISCA (1,4,1.2,2.8,1) CALL AXISCA (1,6,5 .• 5,3,2) CALL AXloRA (-1,1,1) CA L ~ A X I D R A <1, -1 , 2 1 DO 810 J=1,ICOUNT' N=NSTORE(J) , 00 820 I=1,N TO) = TTO,J) P<I) ;;PP(! ,J) CONTINUE CALL GRASVM (T,P~N,J,O) CONTINUE CALL pICCLE CONTINUE CALL OEVEND STOP ENO SUBRoutINE LEAST (T,V,N,SLOPE,VINT~CORR~SDEV,POWER) DIMENSION T(10),Vl10) SUMERR=O, SUMT=O SUMV;:O SUMT2=O suMV2=O. SUMrV=O DO 1 1=1,N SUMT=SUMT+T(I) SUMV;:SUMV+V(!) SUMT2=SUMT2+T(I)**2 SUMV2;:SUMV2+V(I)**2 SUMTV=SUMTV+T(I).V(I)

"

1 CONTINUE A=FLOAT(N) SLOPE=(SUMT*SUMV-A*SUMTV)/(SUMT**2-A*SUMT2) Y!NT=(SUMT2*SUMV.SUMT*SUMTV)/(A*SUMT2~SUMT**2) CORR=(A*SUMTV-SUMT.SUMV)/SQRT«A.SUMT2~SUMT*·2).(A.SUMVZ-SUMV**2» POWER = 1 ,0 + SLOPE DO 2'1""N ERR=V(I)-YINT.SLOPe*T(I)' SUMERR c SUMERR+ERR**2-

264 ,

o

2 CONTINUE" SDEV·SQRT(SUMERRJFLOAT(N~1» RETURN END FINISH

. .

'.

-- +-,

•

Dispersion of particulate additives in rubber using the ... · additives in rubber using the batch...

Documents

Transcript of Dispersion of particulate additives in rubber using the ... · additives in rubber using the batch...