A study of the factors affecting pigment distribution in latex paints

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A Study of the Factors Affecting

Pigment Distribution in Latex

Paints

Roger Francis Graham Brown

A thesis submitted to University College London

for the degree of Doctor of Philosophy.

Work carried out at the laboratories of

Paint Research Association, Teddington, UK.

UCL

A u gust 1997

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Ac k n o w l e d g e m e n t s.

G ratitu d e is expressed to the E uropean U nion and th e consortium of

in d u stria l sponsors providing funding for th e BRITE-EURAM project

“H igh Perform ance W aterborne Coatings”.

The a u th o r also w ishes to th a n k the following people: Professor David

W illiams for his supervision and guidance; D r Chris C arr for

overseeing th e work on a day to day basis; M r Jo h n Bernie for

arran g in g funding for postg rad u ate study; M r Mike Taylor for the

com puter program and m any stim u latin g discussions; M r C hris Seah

for help w ith statistics and analysis of results; and D r Yvonne

K ershaw w ithout whom it is doubtful th a t th is th esis would have been

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The distrib u tio n of pigm ent particles w ithin a decorative gloss p a in t is

of g reat im portance in providing opacity an d high gloss. L atex p ain ts

differ from solventborne p a in ts in th e w et sta te in a n u m b er of ways.

One fu n d am en tal difference is th a t th e volume occupied by th e latex

particles cannot be occupied by th e pigm ent particles and th e shape of

th is excluded volume varies as the size ratio of pigm ent an d latex

changes. Also, th ere is com petition betw een latex an d pigm ent

surfaces to adsorb surface active species added to provide colloidal

stability an d rheological control in a latex p ain t. The geometric effect

of varying th e particle size ratio w as modelled using b in ary m ixtures

of chem ically sim ilar latices stabilised w ith th e sam e su rfactan t. It

w as found th a t dispersion of the pigm ent m odelling latex particles

w as im proved in th e dry film as the size ratio of latex particles to

m odel pigm ent particles decreased. Sim ultaneous com puter

sim ulation of bin ary dispersions using h a rd sphere po ten tials

generated th e sam e conclusions. T here w as good num erical agreem ent

betw een th e two sets of resu lts once th e effect of volume concentration

of th e model pigm ent particles h ad been m athem atically corrected for.

Study of system s containing real pigm ent particles showed th a t

geometric effects predicted by modelling were still valid w ithin the

size range studied. Physico-chemical effects associated w ith varying

surface a re a ratio s were not observed. Some chem ical effects were also

evaluated. D ifferent pigm ent grades were studied w ith differing

surface tre a tm e n ts. The type and levels of additives w ere also varied.

I t w as found th a t pigm ent distribution changed radically from grade

to grade in otherw ise sim ilar form ulations. The types and levels of

additives required to prom ote good pigm ent dispersion w as very

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Ab b r e v ia t io n s.

A bbreviations commonly used th roughout th is th esis are listed below.

%SS P ercentage of single spheres.

AA Acryhc acid.

ANoVA A nalysis of variance.

AOT Aerosol OT (sodium dioctylsulfosuccinate).

APS Am m onium persulfate.

BA B utyl acrylate.

cmc C ritical m icellar concentration.

CPVC C ritical pigm ent volume concentration.

CTR Controlled tem p eratu re room.

DLVO D eryaguin, L andau, Verwey and Overbeek.

HEC H ydroxyethyl cellulose.

HLB Hydrophilic-lipophilic balance.

HSVC H ard solids volume concentration.

k-B ar W ire wound apphcator bar.

MEK M ethyl ethyl ketone (butanone).

MEET M inim um tem p eratu re of film form ation.

MMA M ethyl m ethacrylate.

NAD Non aqueous dispersion.

PCS Photon correlation spectroscopy.

PMMA Poly (m ethyl m ethacrylate).

PRA P a in t R esearch Association.

PS Poly (styrene).

PVC Pigm ent volume concentration.

RH R elative hum idity.

SANS Sm all angle n eu tro n scattering.

SEM Scanning electron microscope/microscopy.

TEM T ransm ission electron microscope/microscopy.

Tg G lass tran sitio n tem p eratu re.

UI U niversal indicator.

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Symbols used w ithin th is th esis are listed below.

r S h ear stress.

To Yield stress.

6 Angle betw een incident and observed radiation.

(f) Volume fraction of spheres.

p.. E stim ates of coefficients for effects of variables x-, and

interactions

y Viscosity.

Viscosity of th e continuous m edium .

y^ Reduced viscosity, /

y ^ Viscosity a t infinite sh ear rate.

X W avelength of rad iatio n scattered.

77 Refractive index.

rj^ Relative refractive index, T]Jrj^.

cTp„ Interfacial tension betw een polymer and w ater.

p D ensity of polymer.

^ S hear rate.

% o Solids content, assum ing no conversion of m onom er to

polymer, expressed as a percentage.

E xperlm entalty determ ined solids content, expressed as a percentage.

%„„„ M ass of m onomer in th e in itial recipe, expressed as a

percentage.

% SS Single sphere fraction, expressed as a percentage.

b N um ber of confoundings to be perform ed.

d Particle diam eter.

d f D esired final latex particle diam eter.

ds D iam eter of latex seed particles.

dop, O ptim um particle diam eter for scatter.

Ef S cattering efficiency.

F Reflectivity ratio.

Fa M agnitude of capillary forces prom oting coalescence.

F g M agnitude of forces resistin g deform ation.

/ Single sphere fraction.

h HSVC, volume concentration of h a rd spheres.

/ S cattered in ten sity of radiation.

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It T otal in ten sity of scattered light.

j Time dependent creep compliance function.

M f F in al m ass of latex particles.

m„, M ass of m onomer required.

rris M ass of seed particles required.

Ne N um ber of experim ents.

N Is Average density of discs in two dim ensions.

n N um ber of particles already p resen t in q u a d ra t i.

n, N um ber of levels of variables.

n. N um ber of q u a d ra ts considered.

n ('•) R adial distribution function w ith distance from th e particle

centre.

p PVC, pigm ent volume concentration, expressed as a fraction.

p (y ) Probability of an additional particle, y, being placed in

q u a d ra t i.

Po(o-> ^-) Probability of no particle centres being located w ith in a distance r of a specific particle centre.

PFC.W PVC in the w et state.

R Particle radius.

R, Reflectance from film i.

Roo Reflectance from a film of infinite thickness.

r D istance from a particle centre.

S O verall sc atter coefficient.

Sc C alculated sc atter coefficient.

S„ Flooding corrected (nominal) sc atter coefficient.

Si S catter coefficient for film i.

Tg^ G lass tran sitio n tem p eratu re of a copolymer.

u Com positional fraction of a polym er w ith in a copolymer.

V PVC, pigm ent volume concentration, expressed as a

percentage.

Vp Volume of a particle.

Vp,g Volume concentration of pigm ent.

Vrcs^sot. Volume concentration of resin and solvent.

V^cer Volume concentration of w ater.

V N um ber of variables.

Xi Particle size variable.

HSVC, b a rd solids volume concentration variable.

%4 S u rfactan t concentration variable.

Xi T hickener concentration variable.

Zi Film thickness of film i.

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Page

Title Page. 1

Acknowledgements. 2

A bstract. 3

Abbreviations. 4

Symbols. 5

Table of C ontents. 7

1). Introduction. 12

1.1). Background. 1 2

1.1.1). W hat is a Latex? 14

1.2). Problem s Associated w ith L atex Coatings. 16

1.2.1). Incom plete Coalescence. 17

1.2.2). Rheology Control. 21

1.2.3). Pigm ent Flocculation. 24

1.3). Aim of th e Study. 31

1.4). Notes on the Project 'H igh Perform ance W aterborne 33

Coatings'.

1.5). A B rief Guide to This Thesis. 33

1.6). References for C hapter 1. 34

2). Control of Particle Size in Em ulsion Polym erisation. 40

2.1). Introduction. 40

2.1.1). Theory of Particle Form ation. 41

2.2). Polym erisation Strategy. 45

2.3). G eneral Procedure. 47

2.4). R esults and Discussion. 48

2.4.1). C hanging Particle Size by Inducing Secondary 48

Nucléation.

2.4.2). S u rfactan t Policy Investigation. 53

2.4.3). Two Step Polym erisation. 55

2.4.4). R efinem ent of Polym erisation Techniques. 58

2.4.4.1). G reater Control of Particle Size. 58

2.4.4.2). Control of L atex Solids Content. 59

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3). E valuation of D istinguishable Latices. 64

3.2). System s Considered. 65

3.2.1). Mixed C hlorinated and Non C hlorinated Particles. 6 6

3.2.2). Mixed Conducting and Non Conducting Particles. 6 6

3.2.3). Mixed Stained and U n stain ed Particles. 70

3.2.3.1). P rep aratio n of Stainable Particles. 70

3.2.3.1.1). Redox In itiate d Em ulsion Polym erisation. 70

3.2.3.1.2). Studies of th e R ate of Seed 73

Polym erisation.

3.2.3.1.3). P rep aratio n of Latices C ontaining 40% 77

Solids.

3.2.3.1.4). P rep aratio n of Latices C ontaining 79

Isoprene.

3.2.3.2). Scanning E lectron Microscopy E xperim ents. 84

3.2.3.3) Prelim inary T ransm ission Electron Microscopy 87

Experim ents.

3.2.4). Mixed Film Form ing and Non Film Form ing 90

Particles.

3.2.4.1). P rep aratio n of Film Form ing Latices. 90

3.2.4.2). Prelim inary Experim ents. 92

3.3). F u rth e r Developm ent of Selected System . 94

3.3.1). M anual E nhancem ent of Im ages. 98

3.4). Problem s Observed w ith U nthickened L atex M ixtures. 101

3.4.1). Investigation of the Problem Observed w ith 104

U nthickened M ixtures.

3.4.1.1). A ssessm ent of C oncentration of P articles a t 105

A ir an d S u b strate Interfaces.

3.4.1.2). A ssessm ent of G ravitational Effects. 107

3.4.1.3). Film Thickness Effects. 108

3.4.1.4). A ssessm ent of Cross-Sections. 109

3.4.1.5). Conclusions. 1 1 0

3.5). P relim in ary E xperim ents - Thickened L atex M ixtures. I l l

3.6). O verall Conclusions for D istinguishable Latices. 113

4). Studies of Physical Model System s. 118

4.2). A nalysis of S p atial Co-ordinates. 1 2 0

4.2.1). C alculation of Percentage of Non C lustered 1 2 1

Spheres.

4.2.2). C luster Allocation. 1 2 2

4.2.3). V erification th a t Random D istributions w ere 126

O btained.

4.3). S tudies of the D istribution of Non Film Form ing 129

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4.3.1.1.1). Effects of V ariables on P ercentage Single 133 Spheres.

4.3.1.1.2). A ssessm ent of R andom ness of 140

D istributions.

4.3.2). E valuation of V ariables for Film s C ast From 141

Thickened L atex M ixtures.

4.3.2.1). E xperim ental Design. 143

4.3.2.2). R esults and Analysis. 145

4.3.2.3). Discussion of Significance. 149

4.3.2.4). Conclusions. 156

4.4). Studies of the D istribution of Non Film Form ing 156

Particles in Cross Sections of Coalesced Films.

4.4.1). Prelim inary Experim ents. 157

4.4.2). E xperim ental Design. 161

4.4.3). R esults for Studies of Film Cross-Sections. 163

4.4.3.1). Film s C ast from Thickened L atex M ixtures. 163

4.4.3.2). Film s C ast from Non Thickened L atex 164

M ixtures.

4.4.4). A nalysis and Discussion. 165

4.4.4.1). Film s C ast from Thickened L atex M ixtures. 166

4.4.4.2). Film s C ast from Non Thickened L atex 169

M ixtures.

4.4.5). Com parison of R esults for Sections W ith R esults 173

for Surfaces.

4.4.6). Conclusions. 174

4.5). Effects of V ariables on T heoretical S catter Efficiency. 174

4.5.1). Surfaces of Film s C ast from U nthickened L atex 176

M ixtures.

4.5.2). Surfaces of Film s C ast from Thickened L atex 182

M ixtures.

4.5.3). Sections of Film s C ast from Thickened L atex 185

M ixtures.

4.5.4). Sections of Film s C ast from U nthickened L atex 188

M ixtures.

4.6). O verall Conclusions for Physical Models. 191

5). C om puter Sim ulation Studies. 197

5.2). Theory of O peration of the Com puter Model. 198

5.2.1). In itia l G eneration of Packings - Random P arking. 198

5.2.2). Increasing the Packing Fraction - Scaling Down 199

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5.2.3). Removing Overlaps G enerated by th e Scaling 199 Process.

5.2.4). Moving Tow ards a Random ised E quilibrium S tate 200

- The Monte Carlo Shaker.

5.3). G eneration of Dense Random Packings. 201

5.4). O btaining R esults from the C om puter Model. 203

5.4.1). Effect o f ‘C ritical G ap’. 205

5.5). E valuation of th e Effect of L atex Particle Size on 207

Pigm ent Single C rystal Fraction.

5.5.1). R esults and Discussion. 208

5.5.1.1). Com parison of Com puter S im ulation and 210

Physical Model Results.

5.5.1.1.1). D erivation of a Correction F actor for 211

P igm ent Volume Concentration.

5.5.1.1.2). C orrelation Betw een C om puter 214

Sim ulation and Physical Models.

5.6). F u rth e r Functionality of the C om puter Model. 216

5.6.1). Polydisperse Pigm ent Particles. 217

5.6.2). Non H ard Sphere Potentials. 217

5.7). Conclusions. 218

6). Studies on Pigm ented System s. 2 2 1

6.1.1). Possible Factors Affecting Pigm ent D ispersion. 2 2 2

6.2). Selection and C haracterisation of M aterials. 224

6.2.1). Pigm ents. 224

6.2.2). Latices. 226

6.2.3). Additives. 228

6.2.4). M aterials Not Selected. 229

6.3) E valuation of th e Effects of V ariables. 230

6.3.1). E xperim ents Perform ed. 230

6.3.2). R esults. 236

6.3.2.1). T reatm en t of Results. 236

6.3.3). Discussion. 237

6.3.3.1). C o n trast Ratio. 238

6.3.3.1.1). Pigm ent RCL-6. 238

6.3.3.1.2). Pigm ent TR92. 239

6.3.3.1.3). Pigm ent RCL-628. 240

0.3.3.2). Gloss. 240

6.3.3.2.1). Pigm ent RCL-6. 240

6.3.3.2.2). Pigm ent TR92. 241

6.3.3.2.3). Pigm ent RCL-628. 242

6.3.3.3). Flocculation G radient. 243

6.3.3.3.1). Pigm ent RCL-6. 243

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6.3.3.4.2). Pigm ent TR92. 245

6.3.3.4.3). Pigm ent RCL-628. 246

6.5). Recom mendations. 247

7). O verall Conclusions and Recom m endations. 251

7.1). Sum m ary. 251

7.3). Com parison of D ifferent System s Studied. 254

7.4). Recom mendations. 254

Appendix A). D etails of E xperim ental In stru m en tatio n an d 256

M aterials.

Appendix B). E xperim ental D etails of L atex Syntheses. 259

Appendix C). E xperim ental D etails of P rep aratio n of Film s 282

From Latex M ixtures.

Appendix D). E xperim ental D etails of P rep aratio n of 293

Pigm ented Films.

Appendix E). Tables of R esults for Pigm ented System s. 297

T hroughout th is thesis, the term ‘resin ’ is used to imply any polym eric

of th e latex form ulations and th e paints, w h eth er they be cross-linked

or otherw ise. The tru e definition of ‘resin ’ is th a t which im plies a

cross-linked m atrix.

T hroughout th is thesis, the term ‘concentration’ is used to imply

loadings in the specific system. Thus, the correct u n its (mol dm-3) are

not alw ays used. This is in line w ith conventional practice in the

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1). In t r o d u c t io n,

1.1). Background.

From w hen early m an p ain ted pictures on th e w alls of caves, h u m an s

have used p a in ts to decorate, protect and inform. In th e m odern

world a diverse range of p a in ts is now available w ith properties

tailored to su it the application for which th e p a rtic u la r p a in t is used.

Figures for W est E uropean p a in t consum ption in th e late 1980's are

broken down in Table 1.1k From Table 1.1 it can be seen th a t

decorative p a in ts account for nearly 50% of to ta l p a in t consum ption.

In 1989 in terio r gloss p a in ts accounted for approxim ately 19%^ of the

to ta l dem and for decorative paints.

Y ear 1985 1987 1989

T otal consum ption ('000 tonnes)^ 4,366 4,913 5,224

M arket share of types of p a in t (%)

Decorative p a in ts (professional) 31.83 32.00 33.70

Decorative p a in ts (retail) 16.37 16.50 16.80

T ran sp o rt coatings 8.45 9.21 8.92

Wood finishes 8.53 7.26 7.05

Coil coatings 6 . 0 0 6.15 6.25

C an coatings 7.35 7.38 7.42

O ther production line p ain ts 1 2 . 1 1 12.83 1 1 . 0 0

Pow der coatings 2.25 2.35 2.60

A nti corrosives 3.25 3.07 3.16

M arine p ain ts 2.40 2.17 2.06

All other 1.36 1.08 1.04

T a b le 1.1). W est E uropean p a in t consum ptionk

High gloss p a in ts have trad itio n ally been form ulated using organic

solvents. The resin is dissolved in th e solvent which evaporates to

leave a continuous film. W ith environm ental considerations coming

to the fore th e acceptability of using solvent based p a in ts is being

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organic compound (VOC) content of such p ain ts. As well as w ider

environm ental consequences, the h e alth h a za rd posed to w orkers by

organic solvents is u n d er scrutiny. There is therefore a large am ount

of ex tern al p ressu re on the coatings in d u stry to develop altern ativ es

which are environm entally and toxicologically more acceptable th a n

the solventborne system s currently available. A n u m ber of

technologies are em erging which m arkedly reduce th e volume of

organic solvents which evaporate during drying. Exam ples of these

technologies can be found in high solids coatings^, powder coatings^

and radiation-cured coatings®. A fu rth er possible altern ativ e is to use

an 'environm entally friendly' solvent as the continuous m edium , in

o ther words coatings should be form ulated using w ater as the

'solvent'. As well as m inim ising environm ental im pact and

decreasing toxicity th ere are fringe benefits from using w ater based

paints. These include the ease of cleaning up equipm ent after use

and low fiam m ability of the vapour during application.

In w aterborne coatings the resin m ay eith er be dissolved in, or be

p resen t as a discrete phase dispersed in w ater. W ater soluble resins

generally suffer from resid u al w ater sensitivity after drying"^. As

p ain ts are often used for protection as well as decoration it is

im p o rtan t th a t the w a ter sensitivity of the final film is kept to a

m inim um . In latex system s th e resin is p resen t as a dispersion of

tiny beads suspended in the aqueous phase an d can be essentially

hydrophobic in n atu re. L atex system s therefore ap p ear to offer an

attractiv e route to the form ation of tough w a ter re s ista n t films from

w ater based form ulations. However, latex system s are not w ithout

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RFG B ro w n P h D T h e sis

1.1.1 ). What is a Latex?

A latex is sim ilar to a n em ulsion in th a t th e dispersed phase is

p resen t as tin y particles suspended in th e continuous phase.

However, while an em ulsion is a m ixture of immiscible liquids, a

latex is a colloidal dispersion of a solid w ithin a liquid. Initially, latex

w as used as a term for the sap from th e ru b b er p la n t Hevea

Brasiliensis^. The sap consists of a dispersion of n a tu ra l ru b b er in

w ater. Strictly the m edium m u st be w a ter an d latex can th u s be

defined as a colloidal dispersion of polym er particles in w ater.

Colloidal dispersions of polym er particles in organic solvents are

classified as non aqueous dispersions^ (NAD's) instead.

In itial synthetic latices in the 1920's were redispersions of n a tu ra l

ru b b er in wateri®. By the late 1930's, latices of synthetic polym ers

w ere being p rep ared as replacem ents for n a tu ra l ru b b er on a large

scale by em ulsion polym erisation^i. Typically diam eters of resin

particles in latices are of the order of 0.1pm. A commercially

p rep ared latex will contain approxim ately 50% by w eight of resin.

Hence the interfacial a re a is extrem ely large an d a m echanism for

stabilising th is interface m u st exist. The latex can be ren d ered stable

e ith er th ro u g h charge repulsion, steric stabilisation or depletion

effects.

C harged groups m ay be p resen t 'on' th e latex particle surface arising

from in itiato r residues eith er d i r e c t l y o r indirectly th ro u g h

subsequent reactions (e.g. hydrolysis, oxidation) of th ese residues^^.

C harged groups a t th e surface m ay also arise from sim ilar reactions

w ith groups along the polym er backbone^^ A lternatively th e charge

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such as surface active agents^^ (surfactants) or more generally

adsorbing charged p o l y m e r s(dispersants).

Steric stabilisation m ay be provided by the incorporation of non ionic

su rfactan ts Steric stab ilisers contain a hydrophobe an d a non ionic

bydropbibc group such as a poly(etbylene oxide) chain. Close

approach of th e particles is hindered by the steric bulk of the

hydrophilic chaini^. C hain length of each com ponent in the

su rfactan t is im p o rtan t so as to m ain tain th e correct hydrophilic-

lipophilic balance 19 (HLB) while also gen eratin g the desired steric

effect. If th e len g th of th e hydrophilic chain is too short th e n th e

steric hindrance is not g reat enough to p rev en t close approach of the

particles. If th e chain is too long th en chain en tan g lem en ts become

possible betw een adjacent particles^o.

Depletion stabilisation^i is achieved th ro u g h th e addition of high

m olecular w eight w ater soluble polym er a t high enough

concentration to yield th e correct osmotic effect. As two particles

come together, the polym er in the gap becomes concentrated w ith

respect to th e bulk m edium . An osmotic tendency for th e m edium to

diffuse into th e gap is therefore generated an d the particles become

separated. If th e concentration of the soluble polym er is too low, the

probability of a polym er chain residing in th e gap betw een two

approaching p articles becomes slight. The polym er in th e gap

therefore becomes dilute w ith respect to the bulk m edium , the

osmotic tendency is for the m edium to diffuse out of th e gap and

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RFG B ro w n P h D T h esis

1.2h Problems Associated with Latex Coatings.

There are a num ber of problem s associated w ith latex c o a t i n g s ^ ^

which are listed below.

i. Coalescence during drying m ay not occur completely,

which can lead to film defects.

ii. In the absence of additives, rheology in th e w et sta te is

inapt. This is because th e form ulation exhibits a sim ilar

rheological profile to th a t of th e continuous m edium , i.e. w ater.

iii. Pigm ent flocculation can lead to loss of gloss an d opacity

and can possibly cause film defects.

iv. S u rfactan ts used for stabilising th e pigm ent dispersion

or latex can lead to foaming. This can lead to film defects in the form o f ’pin-holes'.

V . The kinetics of drying are affected by th e relative

hum idity of the environm ent. Hence in hum id clim ates the coating m ay tak e days or even w eeks to dry.

vi. 'F lash ru stin g ' of m etallic su b stra te s m ay occur due to

w a ter being in contact w ith the su b stra te during drying.

For in terio r gloss p ain ts, the m ost im p o rtan t of these problem s are

the problem s associated w ith coalescence, rheology an d pigm ent

flocculation, which are discussed in g reater d etail below. Foam ing

rem ains a problem b u t can be overcome by th e incorporation of a

defoaming agent^^ to reduce the surface tension of any foam formed

th u s m aking it less stable. Defoaming agents are p ro p rietary

products often based on hydrocarbon blends. The effect of relative

hum idity on drying is also a problem b u t th is is an in h ere n t property

of w aterborne p ain ts an d can only be modified by th e u tilisatio n of

c o s o l v e n t s ^ s . This compromises the environm ental benefits of using

w aterborne technology. It is hoped th a t for in terio r p a in ts the

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1.2.1). Incomplete Coalescence.

For conventional solventborne coatings, the continuous m edium in

the w et sta te is a solution of the resin in th e solvent. Hence,

providing th ere is enough resin to bind th e coating together, if th e

w et film is continuous the dry film left behind after evaporation of

the solvent will also be continuous. For latex coatings, in the w et

state, the resin is dispersed in the continuous phase as opposed to

being p a rt of it. Hence as the w ater evaporates and th e to ta l volume

of the system decreases, the packing fraction of the resin particles

will increase u n til critical packing occurs. On evaporation of fu rth e r

w ater, a continuous film will only be formed if the resin particles flow

out to fill th e voids which th e w ater leaves behind.

The m echanism of coalescence w as first studied by B radford et. al.^^

who described the process m athem atically as th e sin terin g of

particles in th e dry state. Brown^^ was th e first w orker to realise the

im portance of polym er-w ater interfacial tension in th e coalescence

process. Brown^? also specified a simple criterion (1.1) upon which

film form ation could be based:

(1.1)

Where:

Fcis th e m agnitude of capillary forces prom oting coalescence;

F g is th e m agnitude of forces resisting deform ation of the

polymer.

This leads to th e im p o rtan t realisation th a t as the coating dries

eith er the polym er particles flow out to form a film or th ey do not.

W hether coalescence occurs or not depends on th e m agnitude of the

(19)

m odulus of th e resin which will depend upon th e tem p era tu re of

drying and th e glass tran sitio n tem p eratu re (Tg) of th e polymer.

Mason^s reports th a t for a given tem p eratu re, Tg is th e m ost

im p o rtan t p a ra m e te r in determ ining w h eth er film form ation will

occur. Conversely, for a given resin, tem p eratu re is th e m ost

im p o rtan t p a ra m e te r governing film form ation. This im plies the

concept of a m inim um film form ing tem p era tu re (MFFT)^^ for latices,

which is now widely accepted. M FFT can be defined as th a t

tem p eratu re above which a latex dries to give a tra n s p a re n t film free

of defects^®.

Brown27 actually postu lated th ree possible events depending upon

th e m odulus of the resin. H ard resins have a Tg which is

significantly higher th a n am bient tem p eratu re. Upon drying h a rd

resins will not coalesce and the final coating will be discontinuous,

consisting of m any tiny polymer spheres. Soft resins have a Tg which

is th e sam e or lower th a n am bient tem p eratu re. These resins

coalesce completely upon drying to give a continuous film. The

in term ed iate sta te involves resins which have a Tg close to though

slightly higher th a n the am bient tem p eratu re. A lthough the particles

undergo deform ation during drying, electron m ic ro sc o p y sh o w s the

film as a packing of hexagonal particles. Hence th e film form ed is not

tru ly continuous and poor b a rrie r properties can arise due to poor

adhesion (autohésion) betw een particles. It m u st however be noted

th a t the hom ogeneity of such p artially coalesced films im proves w ith

tim e^2 beyond w hen drying is complete. Hence th e case p o stu lated by

B radford et. al.^^ can still be of use in considering th is fu rth er

g rad u al coalescence. A more rigorous tre a tm e n t of th e levelling of

viscoelastic films after drying is provided by K eunings and

(20)

polymer air interfacial tension is reta rd ed by elastic behaviour of the

polymer. Hence polym ers of high elastic m odulus will exhibit poorer

film form ing properties and will also undergo g rad u al coalescence

more slowly. Hence th e theories of Brown^^ an d Bradford^® reinforce

each other as proposed by V a n d e r h o f f ^ ^

A nother phenom enon arising from using resins of in term ed iate

h ard n ess is th a t of m ud cracking. Brown^? sta te d th a t if the

deform ation of the particles is g reat enough to allow p a rtia l

coalescence b u t not g reat enough to dissipate the stresses arising

during drying th en m ud cracking will occur. The stresses occur

because of th e volume contraction of th e to ta l coating as the w ater

evaporates beyond critical packing of th e resin. A more detailed

account of m ud cracking is provided by B i e r w a g e n ^ ^

The m ost recent criterion for film form ation (1.2) is th a t of Eckersley

and Rudin^^. This is based on a viscoelastic model for th e polym er

spheres.

1 34 (T

< p w _(1.2)

R

Where:

j^ |/jis th e tim e dependent creep compliance of th e resin;

(T^„is th e polymer w a ter interfacial tension;

R is th e rad iu s of the particles.

From a qualitative point of view, the inequality expressed above (1.2)

will be more easily satisfied if the resin exhibits a high creep

compliance. In other words, film form ation will occur if stress on the

(21)

function of th e m odulus of th e polymer and will be more easily

satisfied by th e use of 'soft' resins.

All theories are in agreem ent th a t soft resin s will show b e tte r film

form ing properties. U nfortunately, the final film is usually required

to be tough an d reasonably durable. This im plies th a t the resin used

m u st not be too soft or the film will be too easily dam aged an d will

not re ta in its properties for long. If h a rd resin s are to be used th is

will jeopardise the film form ing ability of th e latex.

One way around th is is to use a h a rd e r resin an d to use a

p l a s t i c i s e r ^ G to lower the Tg of the polym er so th a t film form ation can

occur effectively. One problem associated w ith plasticisers is th a t the

Tg of the resin is p erm an en tly reduced so th a t the toughness of the

film is compromised. A fu rth er problem is th a t if the p lasticiser is not

totally compatible w ith the polym er th e n surface defects m ay occur

as the plasticiser is exuded to th e surface^^.

An effective m ethod of using h a rd e r resins to produce tougher films

while reta in in g good film form ing properties is to use coalescing

agents^"^. These can be considered as 'tra n sie n t plasticisers'.

Coalescing agents show lim ited volatility and will have left th e

coating after a prolonged period of tim e. Hence the m echanical

properties of the film after th e coalescing ag en t h as evaporated

depend upon the m echanical properties of th e unplasticised resin.

The im provem ent in film form ing quality will depend upon th e

m agnitude of the plasticising action which th e coalescing agent has

and also on th e p artitio n in g of the ag en t betw een w a te r and th e

polymer. By correct form ulation it is possible to achieve the film

form ing properties of a soft polym er while th e m echanical properties

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m ajor draw back of using coalescing agents is th a t th ey contribute

strongly to th e VOC content of w ater-based coatings. Also, coalescing

ag en ts can som etim es rem ain in the film in significant

concentrations for a y ear or more^^>^^. This re ta rd s th e block

resistance of th e coating: heavy objects cannot be stood on shelves

coated w ith such p ain ts u n til th e coalescent h a s left th e film^o.

1.2.2). Rheology Control.

In the absence of additives, the rheology of latices can be modelled as

a dispersion of h a rd spheres. A num ber of models have been proposed

over the years, the first being th a t of Einstein^i as show n in

E quation 1.3.

y = / o ( l +2.5(g)) (1.3)

Where:

y is the viscosity;

Yq is th e viscosity of the continuous m edium ;

(j) is th e volume fraction of h a rd spheres.

Strictly speaking, E instein's model is only valid for extrem ely dilute

dispersions though it is possible to model certain system s up to 60%

solids by a rb itra ry ad ju stm en t of the volume fraction c o e f f i c i e n t ^ ^

O th er models have been proposed. For exam ple. Mooneyes obtained

a n em pirical equation (1.4) based on rheological studies of relatively

concentrated dispersions.

(23)

A liquid is defined as exhibiting N ew tonian flow if the sh e ar stress

across th e liquid is directly proportional to th e ra te a t which the

liquid is sheared. The constant of proportionality is th e viscosity of

the liquid. This is expressed m athem atically below (1.5).

® = - (1.5)

y

Where:

CO is the sh ear rate;

r is the sh ear stress;

Y is the viscosity.

Asbeck and Casson^^ plotted the square root of viscosity versus the

reciprocal of the square root of sh ear rate. A stra ig h t line is obtained

the gradient of which is the yield stress w ith th e in tercep t being the

viscosity a t infinite sh e ar rate. This is shown in E quation 1.6. Hence,

for N ew tonian flow, a plot of vs co~^'^ will produce a stra ig h t line

of zero gradient indicating th a t viscosity is co n stan t an d th a t th ere is

no yield point.

(1.6)

Where:

Y is the viscosity;

Y^ is the viscosity a t infinite sh e ar rate;

Fq is the yield stress;

CO is the sh ear rate.

The rheological models shown above in E quations 1.3 an d 1.4 show

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assum e th a t, in the absence of any rheology modifiers, latex

dispersions w ill show rheological properties sim ilar to a N ew tonian

liquid. In reality th is is not the easels as sh e ar can cause the

particles to pack together more efficiently th u s increasing th e free

volume of th e system and decreasing th e viscosity. However, th is

effect is not g reat enough to be commercially useful. D ifferent

chem ical groups on th e surface of the latex particles can also cause

deviations from N ew tonian b e h a v i o u r i ^ fu rth e r com plicating the

issue.

Commercial p a in ts require a very specific rheological profile. The

general req u irem en t is for a p a in t to exhibit considerable

pseudoplasticity or sh e ar th in n in g behaviour. E xplained more

simply, th is m eans th a t p ain ts m u st have low viscosity a t high sh ear

rate s to allow easy application while a t low sh e ar th e viscosity m ust

be high to prevent 'sagging' before th e p a in t is dry. I t is im p o rtan t

th a t th e viscosity a t low shear ra te s is not too high so th a t a certain

am ount of levelling can occur prior to drying th u s rem oving brush-

m arks. In practice p ain ts are often thixotropic in n a tu re as well

m eaning th a t the viscosity decreases to a certain lim iting value w ith

tim e a t a constant sh ear rate. The viscosity th e n increases again

w hen th e source of sh e ar is removed. Gillespie"*® h a s developed a

model (1.7) for dispersions exhibiting pseudoplasticity b u t not

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RFG B r o w n P h D T h e sis

= (1-7)

W here:

/ is th e ratio of the N ew tonian viscosity of th e dispersion to

th e viscosity of the continuous medium;

(j) is th e volume concentration of dispersed spheres;

Argis a constant experim entally found to alw ays be g reater

th a n th e E in stein coefficient of 2.5;

kpi^ a constant which depends on th e m ean dynam ic

aggregation.

S hear th in n in g behaviour is induced by the addition of rheology

m odifiers called associative thickeners because of the m echanism by

which they work. A lthough th e flow properties of m icellar solutions

of thickeners are Newtonian^'^, w hen added to a latex or pigm ent

dispersion a loose netw ork is formed th ro u g h hydrophobic

associations'^®. On sh e ar th is loose stru ctu re is broken down an d the

form ulation is m arkedly pseudoplastic. The loose stru c tu re is quickly

reform ed once sh e ar is removed. Hence the correct rheology can be

im p arted to th e coating.

A problem connected w ith associative thickeners is th a t by forcing

pigm ent particles to in teract th ere m ay be a deleterious effect on

pigm ent dispersion^^. This effect m ay be p artially offset by the

increase in viscosity m aking it more difficult for pigm ent to settle out

over time^®.

1.2.3). Pigment Flocculation.

P a in ts are applied in stead of varn ish es if the role is to hide as well as

to protect. I t is therefore im p o rtan t th a t a decorative p a in t is capable

(26)

p a in t to m ask the layer u n d e rn e ath is know n as opacity. The opacity

of a p a in t is determ ined by how m uch of a n incident light beam

p e n etrates to th e su b stra te and is subsequently reflected to the

observer. This is governed by the ability of th e coating to absorb light

or to scatter it before it can be reflected from th e su b strate. In w hite

p a in ts pigm ented w ith tita n iu m dioxide very little h g h t is absorbed

by th e pigm ent and the opacity is a function of the in ten sity of

scatter. The raw scatterin g power of a pigm ent can be calculated

from refractive indices using Fresnel's^i equation (1.8).

F

(1.8)

Where:

F is th e reflectivity ratio;

77 is refractive index. The subscripts 1 and 2 refer to the

continuous m edium an d the pigm ent respectively.

Hence the higher the refractive index of the scatterin g particles, the

greater the scatterin g will be. This explains th e dom inance of

tita n iu m dioxide as a w hite pigm ent. Rutile tita n iu m dioxide h as a

refractive index of 2.70. This is higher th a n th a t of th e a n atase

crystal form (ti=2.55) an d m uch higher th a n th a t of zirconium oxide

(r|=2.40) w hich is the next strongest scatterin g w hite pigm ent.

Scattering is also a function of particle size an d particle s p a c i n g ^ ^

theory for scatterin g of light by sm all particles w as developed by

S tru tt^3,54 (later Lord Rayleigh). For Rayleigh scattering, th e size of

the particles h ad to be sm all com pared to the w avelength of light

(27)

sam e size as th e w avelength of scattered light provided th a t the

refractive index of these larger particles w as sim ilar to th a t of the

m edium in which th ey were dispersed. Rayleigh scatterin g is

expressed in E quation 1.9.

Where:

/ is th e scattered intensity;

/ois th e incident intensity;

Km is constant;

Fpis th e volume of a scattering particle;

is th e relative refractive index of the particles com pared to

the medium;

X is th e w avelength of the incident light;

0 is th e angle betw een the incident an d observed beam s.

Hence for a given angle of viewing, a given relative refractive index

and a given w avelength of incident rad iatio n th e light scattered by a

single particle is proportional to the square of the volume of the

particle (1.1 0):

W here:

R is th e rad iu s of the particles.

The scatter by u n it volume of m ateria l will be proportional to the

intensity scattered by each particle m ultiplied by the num ber of

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scatterin g species the num ber per u n it volume is inversely

proportional to the volume of each particle. Hence th e to ta l scattered

light will be proportional to th e cube of the particle rad iu s (1.1 1).

(

1 1 1

)

^ P

W h ere/r is th e to ta l in ten sity of scattered light.

In practice, above a certain concentration of scatterin g bodies the

increase in opacity w ith increasing volume concentration becomes

non linearis, a ttrib u te d to dependent scatterings^.

As scatterin g is a feature of interfaces, for macroscopic sc a tte r (i.e.

scatterin g by large particles) th e am ount of light scattered by a large

particle will be a function of its surface areals (1.1 2). Hence,

analogous to th e arg u m en t above, th e sc atter per u n it volume will be

dependent on th e num ber of particles per u n it volume. For a given

volume concentration th is will be a function of the reciprocal volume

of each particle. It therefore follows th a t the to ta l am ount of light

scattered will be proportional to th e reciprocal rad iu s of the particles

(1.13).

I

(

1

.

12

)

(1.13)

Hence absolute scatterin g from dispersed particles will alw ays

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volume concentration of scatterin g particles, if the particles are

sufficiently large th e n th e to ta l scatter from u n it volume of sam ple

will decrease as the particle size increases. At sm all p article size, the

to tal scatter will increase w ith increasing particle size. It

qualitatively follows th a t th ere will be an optim um particle size

w here the two effects are m ost effectively balanced. T his h^^pens-to^

occur w hen th e diam eter of the scatterin g particles is approxim ately

h a lf the w avelength of the incident l i g h t R e l a t i v e refractive indices

also have an effect and E quation 1.14 m ay be used^o.^i to determ ine

the optim um particle diam eter for m axim um sc atter from a given

volume concentration of particles.

Where:

dop, is th e optim um particle d iam eter for efficient scattering;

and ri - \ Tj

OC = —Y---, w ith ( 7 7 = - f ) •

The subscripts 1 and 2 denote th e m edium an d pigm ent respectively.

The pigm ent in a p a in t is a large proportion of the raw m aterials cost

in m anufacture. It is therefore im portant, purely from a comm ercial

standpoint, to m axim ise the efficiency of pigm ent utilisation. For

m axim um sc atter from the m inim um am ount of pigm ent it is

im p o rtan t th a t th e size of the pigm ent particles in th e film is

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The size of pigm ent particles in a final film is governed by two

factors. The p rim ary factor is the crystal size of th e pigm ent. C rystal

size is determ ined by the m anufacturing process conditions and is

therefore out of the control of the p a in t form ulator. Pigm ent

m an u factu rers aim to supply pigm ents w ith m ean cry stal sizes of the

order of 0.25pm, th is being approxim ately h a lf the w avelength of

green light (555nm), the w avelength a t which the eye is m ost

sensitive^2 The second factor affecting p igm ent particle size is the

degree of dispersion. As the crystal size of the pigm ent is optim ised

by the pigm ent m anufacturer, the degree of dispersion becomes the

m ost im p o rtan t p a ra m eter in determ ining the particle size

distribution in the final film. As pigm ent particles flocculate the

m ean size will rise an d to ta l scatterin g from the film will decrease.

It h as been observed th a t in conventional gloss p a in ts a m axim um of

35% of pigm ent is p resen t as single discrete c r y s t a l s ® ^ This is

optimistic, it being suggested th a t th e sectioning technique used in

p rep arin g sam ples for assessm ent created artificially good

dispersion. A more realistic figure for the percentage of pigm ent

existing as single crystals would be 20-25%^®. In latex p ain ts, it is

generally o b s e r v e d ® ^ th a t th e degree of pigm ent clustering is m uch

g reater th a n in a solvent based gloss paint. This m ay be p artly

a ttrib u ta b le to excluded volume effects. In a conventional

solventborne coating, in the w et state, the pigm ent volume

concentration (PVC) is expressed in E quation 1.15. The analogous

PVC for a latex based p a in t in the w et sta te is expressed in E quation

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RFG B r o w n P h D T h e sis

y .

For solvent borne: PVC^^,=—---- ^ --- (1-15)

p ig resi-solv

V .

For latex based: PVC^^,=—— ^ — (1.16)

p ig water

W here the subscript: p ig indicates pigm ent; res indicates resin;

solv indicates organic solvent; and water is obviously w ater.

From E quations 1.15 and 1.16 it is a p p aren t th a t th e pigm ent is

dispersed in a sm aller volume in the latex based system . As drying

proceeds, th e PVC of pigm ent in w ater in a la te x h a se d system

increases very sharply. As fu rth er w a ter evaporates, th e pigm ent-

w ater interfaces are eith er replaced by pigm ent-resin interfaces or

pigm ent-air interfaces. In a solvent borne coating, th e m edium

surrounding the pigm ent gradually becomes richer in resin as the

solvent evaporates. Hence both the m echanism of drying and th e

geometry of the system are different up u n til th e point w hen all the

solvent or w a ter h as evaporated. It is obvious th a t any flocculation in

the w et sta te will be passed on to the dry film, however it is

suggested®^ th a t th e g reatest p a rt of flocculation in th e final film

arises during the drying process. It is therefore hoped th a t by

studying system s w ith different geometries, it will be possible to

m inim ise th e effects of excluded volume.

There are o th er problem s associated w ith pigm ent flocculation.

Flocculation can lead to m arked heterogeneities u n d er th e film

surface, which can cause a loss of gloss®^. Large flocculates can b reak

the surface of a film th u s m aking th e film less effective as a barrières

and leading to a fu rth er loss of gloss. Finally, if flocculation is severe

(32)

the critical pigm ent volume concentration (CPVC)®® m ay be breached

on a localised basis. This leads to areas of th e film being powdery and

non continuous, an d these areas will show practically no b a rrie r

properties a t all.

1.3). Aim of the Study.

The aim of th is study w as to evaluate factors affecting the

distribution of pigm ent particles in latex based p ain ts.

L atex p a in ts an d solvent based p ain ts are sim ilar in th a t th e volume

occupied by the resin is excluded to the pigm ent. A m ajor difference,

however, is th a t the to tal num ber of possible conform ations which

the excluded volume can take is very m uch more lim ited in th e w et

sta te for latex p ain ts w hen compared to solvent based p ain ts. This

effect is aggravated during drying as the w et film approaches critical

packing and pigm ent particles m ay be forced to g eth er into clusters.

T his imposes a fun d am en tal geometric co n strain t on achievable

levels of pigm ent dispersion in latex p ain ts which is not p resen t in

solvent based paints.

It w as intended to evaluate th is fundam ental geom etric co n strain t in

model system s w here competitive interactions have been m inim ised

and by com puter sim ulation using b ard spheres. I t w as hoped th a t by

m inim ising competitive interactions it would be possible to

determ ine th e b est practical distribution achievable w ith in the

geometric constraints. Knowledge gained would th e n be teste d in

model p a in ts containing latex polymer, tita n iu m dioxide pigm ent,

d isp ersan ts and rbeology modifiers. F u rth e r physico-chemical effects

(33)

The study w ith pigm ents w as confined to tita n iu m dioxide as th is is

by far th e m ost commonly used. A range of grades w ith different

surface tre a tm e n ts were considered however.

It w as intended to establish guidelines for achieving im proved levels

of pigm ent dispersion in latex paints.

W ork reported in this thesis uses m ixtures of latices to model a

m ixture of latex and pigm ent particles. S patial co-ordinates of non

coalesced particles w ithin a coalesced film are obtained. A single

sphere fraction is calculated analogous to single cry stal fraction used

in the lite ra tu re as a m easure of dispersion®® ®^. The effect of varying

size ratios and concentration ratios on th is property is evaluated.

To the best of the au th o r’s knowledge, evaluation of th e fu n d am en tal

physical constraints on pigm ent dispersion imposed by particle

packing effects h as not been reported previously.

L atex m ixtures have previously been studied to determ ine stru ctu re

fa c to rs ® ^ e ffe c ts on film f o r m a t i o n ' l l and effects on rbeology^®.

Blends of latices containing different sized particles have also been

used to yield fast drying high solids d i s p e r s i o n s ' ^ ^ while h a rd latex

particles have been mixed into films cast from soft latex particles to

improve the m echanical properties'^®'^®. A ttem pts were also m ade to

characterise clustering of the h a rd spheres by tu rb id ity

m easurem ents in these systems'^®'^® b u t not by analysis of sp atial co

ordinates an d not as a specific function of latex size ratio.

To the b est of the au th o r’s knowledge, previous use of latex m ixtures

outlined above®^'^®, much of which w as published subsequent to th is

work being performed, h as not extended as far as the study described

(34)

1.4). Notes on the Project *Hiah Performance Waterborne

Coatings'.

The above nam ed project w as p artially funded by th e E uropean

U nion u n d er th e 'BRITE' scheme (Basic R esearch in In d u stria l

Technologies, Europe). The rem aining funds were provided by

in d u stria l sponsors, the w ishes of whom were considered in the

choice of m aterials for study. The research project w as a collaborative

venture betw een the P a in t R esearch Association (FRA) and four

other research institutes'^'^ ®® in different E uropean countries. The

problem s of film form ation an d correct rbeology for application were

studied e l s e w h e r e A detailed study of com petitive adsorption

effects w as also m ade elsewhere®®. The role of FRA w as to study the

problem s related to pigm entation of latex coatings. As the work was

carried out w ithin a research group, not all th e work reported w as

physically carried out by th e author. W here th is w as th e case it is

indicated by reference.

1.5). A Brief Guide to This Thesis.

C hapter 1 h as provided a review of some of th e fu n d am en tal

problem s associated w ith latex p ain ts. The aim s of th e study and

how it w as intended to achieve these w as described in Section 1.3.

Figm ent clustering in latex p ain ts w as modelled by com puter

sim ulation of bin ary packings of h a rd spheres and by physical

m ixtures of distinguishable particles. A theoretical correction for

volume concentration effects w as derived and resu lts from both types

of model w ere compared. In th is thesis, physical model system s are

described first followed by com puter model results. A pplication to

pigm ented form ulations of th e resu lts obtained from m odelling is

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C hapters 2 an d 3 describe development of th e physical model

system s. C hapter 2 provides details of a m ethod for controlling

particle size in em ulsion polym erisation. C hapter 3 provides details

of evaluation of latex m ixtures where different types of latex particle

could be distinguished by electron microscopy. C h ap ter 4 th e n

describes the p rep aratio n and evaluation of films cast from latex

m ixtures. R esults obtained from such films are presented.

C hapter 5 describes the operation of the com puter model an d resu lts

obtained by sim ulation. A theoretical correction factor for volume

concentration effects is derived. R esults from sim ulation are th en

com pared to resu lts from C hapter 4.

C hapter 6 describes the p rep aratio n of simple p ain ts containing

tita n iu m dioxide pigm ent. The validity of resu lts obtained in

C hapters 4 and 5 was tested.

C hapter 7 sum m arises th e w ork perform ed, conclusions m ade and

offers recom m endations for the form ulation of latex p a in ts w ith

improved pigm ent distribution.

E xperim ental details of latex synthesis and p rep a ratio n of m ixtures

are supplied in Appendices A-D. T abulated resu lts obtained for

pigm ented system s are p resen ted in Appendix E.

1,6). References for Chapter 1.

^ "A profile of the West European paint industry." 9th Edn. Pub.

Information Research Ltd. (London, 1990). p.67.

(36)

3 Ref. 1, p.62.

4 RF Storey in "Surface Coatings 1". Eds. AD Wilson, JW Nicholson, H J

Prosser. Pub. Elsevier Applied Science. (London, 1987). Chapter 3, pp69-70.

® TA Misev. "Powder Coatings: Chemistry and Technology". Pub. John

Wiley and Sons Ltd. (Chichester, 1991). Chapter 1, ppl-3.

3 E Levine. Modern Paint & Coatings. 1983, 73(8), 26.

CA Finch. Chem. Ind. 1981(22), 800.

3 F Marchionna, "Latex and its industrial apphcations". Pub. Rubber Age

Publishing Co. (New York, 1933). Preface, p.xxi.

9 JV Dawkins, SA Shakir in "Polymer Latexes". Eds. ES Daniels, ED Sudol,

MS El-Aasser. ACS Symp. Ser. 492. (1992). Chapter 27, p432.

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2). C o n t r o l o f P a r t i c l e S ize in E m u ls io n

P o ly m e r is a tio n .

The objective of work described in th is ch ap ter w as to develop a

simple technique for p rep arin g latices of different, p redeterm ined

particle size.

2,1). Introduction,

To study th e effect th a t the size ratio of latex to pigm ent particles

has on dispersion quality, it w as necessary to p rep are a series of

latices w ith different particle sizes. There are m any exam ples in the

lite ra tu re describing techniques for controlling particle size in

em ulsion polymerisation^-®. U nfortunately, solids contents of the

system s described are generally less th a n 20%. Latices used in

form ulating p a in ts typically have solids contents of 40% or greater^.

Hence, while th e inform ation available in the lite ra tu re aids

u n d erstan d in g of th e em ulsion polym erisation system , it w as

necessary to develop a process for controlling particle size in more

concentrated system s. A m ethod w as developed which allows the

simple p rep aratio n of concentrated (up to 50%) latices of

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2.1.1). Theory of Particle Formation.

Following th e w ork of H arkins^ ^ io, S m ith an d Ewart^^ published the

first widely accepted theory of em ulsion polym erisation in 1948

based on experim ents perform ed w ith styrene. The th ree cases

postu lated are listed below.

Case I: N um ber of radicals p er particle is m uch sm aller

th a n unity.

Case II: N um ber of radicals p er particle is approxim ately

0.5.

Case III: N um ber of radicals per particle is m uch larg er th a n

unity.

It w as found^i th a t the rate of em ulsion polym erisation of styrene

was dependent on the su rfactan t concentration (3/5^^® power). This

was related to the num ber of micelles p resen t a t the s ta r t of th e

reaction and also the num ber of particles subsequently formed. At

higher soap concentrations, more micelles will be p resen t a t th e s ta rt

of the reaction leading to the form ation of more p rim ary polym er

particles. For th is to affect th e rate, th e n u m ber of polym erisation

loci m u st depend on the num ber of particles present. This p resen ts a

good arg u m en t for micelles being the locus of polym erisation initially

and particles swollen w ith adsorbed m onom er being th e locus

subsequently. The locus of polym erisation will tra n sfe r from being in

micelles to being in particles w hen all the soap th a t w as p resen t as

micelles h a s been adsorbed on th e surface of th e particles. Harkins^^

calculated th a t th is occurred a t some point betw een 10-20%

conversion for a 5% soap concentration. The H ark in s description is

modelled by Sm ith-Ew art^i Case II w here th e num ber of

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the num ber of particles present, the figure of 0.5 being a rb itra ry to a

certain extent. Hence the ideas of H ark in s were verified for styrene

and a m echanism for em ulsion polym erisation w as form ulated.

The basic concepts of classical em ulsion polym erisation as envisaged

by H ark in s are th a t th ere are initially th ree p h ases present:

bulk monomer: th is is immiscible w ith w a ter an d provides a reservoir of m onomer for subsequent polym erisation;

w ater: th is is the reaction m edium , and th e in itiato r compound is p resen t dissolved in the aqueous phase. A sm all am ount of m onomer will also be dissolved in the aqueous phase;

m onom er solubilised in su rfa ctan t micelles: above th e "critical m icellar concentration" (cmc) the su rfa ctan t will he p resen t as micelles providing lipophilic m icrophase regions w ith in the micelle for which the m onomer will have high affinity. Hence m onom er will be tak e n up from the bulk p hase by the

su rfa ctan t micelles.

On heating, the in itiato r (containing some form of peroxide bond, e.g.

potassium persulfate) dissociates to form radicals. These radicals

in itiate polym erisation w ithin a micelle. As th e polym er chain grows

it becomes solid and the su rfactan t becomes adsorbed a t the polymer-

w ater interface. This continues u n til th ere are no more micelles

p resen t an d a large num ber of very sm all particles are p resen t w ith

su rfactan t adsorbed a t the surface. A num ber of these sm all p articles

may aggregate so as to decrease the surface a re a to volume ratio of

the system . This stage is term ed nucléation an d will continue u n til

the surface charge density on the particles is g reat enough to

m ain tain stability. It is possible th a t th e su rfa ctan t concentration