Polymer encapsulation of titanium dioxide : efficiency, stabilityand compatibilityCitation for published version (APA):Janssen, R. Q. F. (1995). Polymer encapsulation of titanium dioxide : efficiency, stability and compatibility.Technische Universiteit Eindhoven. https://doi.org/10.6100/IR428840

DOI:10.6100/IR428840

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POLYMER ENCAPSULATIONOF TITANIUM DIOXIDE

CIP-DATA KONINKLUKE BIBLIOTHEEK, DEN HAAG

Janssen, Rodericus Quintus Franciscus

Polymer encapsulation of titanium dioxide: efficiency,stability and compatibility / Rodericus QuintusFranciscus Janssen. - [S.l. : s.n.]Thesis Eindhoven. - With ref. - With summary in Dutch.ISBN 90-386-0254-5Subject headings: polymer encapsulation / emulsionpolymerisation

@1994 R.Q.F. Janssen, Helmond

Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaargemaakt door middel van druk, fotokopie, microfilm of op welkeandere wijze dan ook zonder voorafgaande schrifielijke toestemmingvan de auteur.

No part of this publication may be reproduced or transmitted in anyform or by any means, electronic or mechanical, includingphotocopy, recording, or any information storage and retrievalsystem, without permission of the copyright owner.

POLYMER ENCAPSULATIONOF TITANIUM DIOXIDE:

EFFICIENCY, STABILITY AND COMPATIBILITY

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan deTechnische Universiteit Eindhoven, op gezag vande Rector Magnificus, prof. dr. J.H. van Lint,voor een commissie aangewezen door het Collegevan Dekanen in het openbaar te verdedigen op

vrijdag 6 januari 1995 om 16.00 uur

door

Rodericus Quintus Franciscus Janssen

geboren te Roermond

druk: wibro dissertatiedrukkerij, helmond.

Dit proefschrift is goedgekeurd door

de promotoren:

en de copromotor:

prof.dr.ir. A.L. German

prof.ir. E.L.J. Bancken

dr. A.M. van Herk

The author is indebted to Akzo Nobel Corporate Research bv Arnhem (NL) , for

financially supporting this work.

SUMMARY

The polymer encapsulation of inorganic pigments can offer great advantages

with respect to the dispersability of these pigments in polymer matrices. Besides, a

number of (mechanical) properties of the ftnal product (matrix with pigment) can be

improved substantially by this encapsulation.

The encapsulation of the pigment (titanium dioxide) is achieved by means of

an emulsion-like polymerization reaction, after the pigment has been hydrophobized

with a titanate. The present research mainly has been focused on the factors

determining the efftciency of the encapsulation reaction and the colloidal stability of

the reaction product. On-line conductivity measurements have been used in order to

gain more insight in these phenomena (chapter 4).

With the aid of conductivity measurements the strong influence of monomer

on the critical micelle concentration (CMC) of a surfactant can be visualized. It was

determined that, under the influence of styrene (Sty) and sodium 4,4'-azo-bis~(4­

cyanopentanoate) (SACPA), the CMC of sodium dodecyl sulfate (SDS) at 60°C

drops from 10.16 mmol/L to 7 mmol/L.

A number of important events can be observed from the conductivity signal,

both during an emulsion polymerization and during an encapsulation reaction. After

all components have been added, inhibition, marked by a constant conductivity,

sometimes occurs. The moment of initiation is marked by a strong decrease in the

conductivity. This decrease takes place because surfactant is adsorbed onto the new

surface area that is created as a result of the polymerization reaction: upon

adsorption the mobility of surfactant molecules and thus the overall conductivity

decreases. A decrease in surface area, as a result of shrinkage or coagulation of

particles, will lead to a release of surfactant and, therefore, to an increase in the

conductivity.

Summary

The influence of the monomer concentration in the aqueous phase on the

number of micelles is also reflected in the conductivity signal. After the monomer

droplets have disappeared at the beginning of Smith-Ewart interval III the monomer

concentration in the aqueous phase decreases which causes an increase in the

conductivity as a result of the break-up of micelles. This break-up will certainly take

place in case of a (moderately) water soluble monomer at low monomer-water ratios,

where interval III begins before the stage of particle formation has ended, especially

if homogeneous nucleation plays an important role, like in the case of methyl

methacrylate (MMA).

The efficiency of an encapsulation reaction can be strongly increased by

adding the monomer semi-continuously, instead of pre-charging all monomer. In the

former case the extra number of micelles caused by the presence of monomer is

minimized as a result of the low monomer concentration in the aqueous phase

('monomer starved conditions'). The use of a redox initiator (cumene hydroperoxide,

in combination with iron(II)sulfate and sodium formaldehydesulfoxylate) instead of

SACPA also led to higher efficiencies (chapter 5). This is partly because the redox

initiator does not lower the CMC as much as SACPA, and partly because it shifts the

polymerization reaction towards the interface between water and pigment, especially

with monomers that preferentially undergo homogeneous nucleation (like MMA).

The use of a sulfosuccinate (OT-lOO) instead of SDS contributes to the

stability of the system, especially in the case of styrene (chapter 6). The semi­

continuous addition of SDS during an encapsulation reaction increases the stability as

well, as compared with a reaction without surfactant addition. Pre-charged non-ionic

surfactants and an 'inisurf' were tested also but did not provide sufficient stability.

Finally, it has been found that both the composition (distribution) and the glass

transition temperature of the surface polymer can be controlled (chapter 7). This has

been demonstrated for two monomer combinations: MMA- Sty and MMA-BMA

(Qutyl methacrylate).

SAMENVATTING

Ret omhullen van anorganische pigmenten met polymeer kan veel voordelen

bieden met betrekking tot de dispergeerbaarheid van deze pigmenten in een polymere

matrix. Bovendien kunnen door de omhulling een aantal (mechanische) eigenschap­

pen van het uiteindelijke produkt (matrix met pigment) aanzienlijk verbeterd worden.

De omhulling van het pigment (titaandioxide) geschiedt door middel van een

emulsiepolymerisatie, die uitgevoerd wordt nadat het pigment met behulp van een

titanaat hydrofoob gemaakt is. Onderzocht zijn voornamelijk de factoren die het

rendement van de omhullingsreactie en de collo"idale stabiliteit van het reactieprodukt

bepalen. On-line geleidbaarheidsmetingen zijn gebruikt om hierin meer inzicht te

verschaffen (hoofdstuk 4).

De geleidbaarheidsmetingen hebben onder meer aan het licht gebracht dat

monomeer een sterke invloed heeft op de kritische micel concentratie (CMC) van .een

zeep. Bepaald kan ~orden dat de CMC van natriumdodecylsulfaat (SDS) onder

invloed van styreen (Sty) en de radicaalinitiator natrium 4,4'-azo-bis-(4-cyanopenta­

noaat) (SACPA) bij 60°C daalt van 10,16 mmol/L tot 7 mmol/L.

Tijdens een emulsiepolymerisatie of een omhullingsreactie kunnen een aantal

zaken uit het geleidbaarheidssignaal bepaald worden. Een constante waarde van de

geleidbaarheid na het toevoegen van alle componenten duidt op inhibitie, terwijl de

start van de reactie gekenmerkt wordt door een snelle daling in de geleidbaarheid.

Deze daling wordt veroorzaakt doordat ten gevolge van de polymeervorming het

totale (deeltjes-)oppervlak toeneemt, hetgeen gepaard gaat met de adsorptie van zeep­

moleculen die gedurende het adsorptieproces aan mobiliteit inboeten. Analoog

hieraan leidt een oppervlakte-afname, ten gevolge van krimp of coagulatie van

deeltjes, tot een vrijkomen van zeep en tot een toename in de geleidbaarheid.

Samenvatting

De invloed van de monomeerconcentratie in de waterfase op het aantal

micellen is ook terug te vinden in het geleidbaarheidssignaal. Na het verdwijnen van

de monomeerdruppels aan het begin van Smith-Ewart interval III neemt de

monomeerconcentratie in de waterfase af hetgeen zich manifesteert middels een

toename in de geleidbaarheid ten gevolge van het opbreken van micellen. Dit treedt

zeker op wanneer in het geval van een redelijk wateroplosbaar monomeer en bij lage

monomeer-water-verhoudingen interval III begint v66rdat het deeltjesvormings­

stadium voorbij is, en indien homogene nucleatie een belangrijke rol speelt, zoals

bijvoorbeeld in het geval van methylmethacrylaat (MMA).

Het rendement van een omhullingsreactie kan sterk verhoogd worden door het

monomeer semi-continue toe te voegen, in plaats van alle monomeer voor te leggen.

In het eerste geval wordt door de lage monomeerconcentratie in de waterfase

('monomer starved conditions') de toename van het aantal micellen ten gevolge van

de aanwezigheid van monomeer beperkt. Ook het gebruik van een redoxinitiator

(cumeenhydroperoxide in combinatie met ijzer(II)sulfaat en natriumformaldehydesul­

foxylaat) in plaats van SACPA of natriumpersulfaat verhoogt het rendement

(hoofdstuk 5), deels omdat het eerstgenoemde initiatorsysteem de CMC minder

verlaagt, deels omdat diezelfde initiator de reactie aan het grensvlak tussen waterfase

en pigmentoppervlak bevordert, vooral bij monomeren die bij voorkeur homogene

nucleatie vertonen (zoals MMA).

Het gebruik van een sulfosuccinaat (OT-lOO) in plaats van SDS verhoogt de

stabiliteit met name bij reacties met Sty (hoofdstuk 6). De semi-continue additie van

SDS gedurende een omhullingsreactie verschaft ook meer stabiliteit in vergelijking

met reacties zonder zeepadditie. Non-ionische zepen en een 'inisurf' (voorgelegd)

zijn ook getest, maar boden onvoldoende stabiliteit.

Tenslotte is het aan de hand van twee monomeercombinaties (MMA-Sty, en

MMA-butylmethacrylaat) mogelijk gebleken om de samenstelling(-sverdeling) en de

glasovergangstemperatuur van het oppervlaktepolymeer te sturen (hoofdstuk 7).

CONTENTS

SUMMARY

SAMENVA'ITING

CONTENTS

Chapter 1

INTRODUCTION

1.1 General introduction

1.2 Polymer encapsulation of inorganic pigments

1.3 Aim and justification of this investigation

1.4 Outline of this investigation

Chapter 2

BACKGROUND OF PIGMENT ENCAPSULATION

AND EMULSION POLYMERIZATION

1

2

5

8

2.1 Introduction 11

2.2 Overview of polymer encapsulation methods for inorganic particles 11

2.2.1 Polymer encapsulation in organic solvent or in bulk 11

2.2.2 Polymer encapsulation in aqueous systems 12

2.2.3 Polymer encapsulation in aqueous systems preceded by

pigment modification 15

2.3 Emulsion polymerization 17

2.3.1 Harkins I theory (micellar nucleation) 17

2.3.2 Alternative nucleation mechanisms 18

2.3.3 Exceptional behaviour in intervals II and II 19

Contents

Chapter 3

EXPERIMENTAL

3.1 Purification and modification oftitanium dioxide

3.2 Determination of surfactant adsorption by pigments

3.3 Polymerization and encapsulation reactions

3.3.1 Ingredients

3.3.2 Experimental setup

3.3.3 Reaction conditions

3.4 Separation and analysis of reaction products

3.4.1 Experimental

3.4.2 Formulas and calculations

Chapter 4

EMULSION (-LIKE) POLYMERIZATION REACTIONS

MONITORED WITH ON-LINE CONDUCTIVITY

21

23

23

23

25

26

27

27

28

4.1 Introduction 31

4.2 The formation of (free) micelles 33

4.2.1 Effect of counterions on the apparent CMC 33

4.2.2 Effect of monomer on micellization 34

4.2.3 Combined effects of reaction components on the CMC 38

4.3 Emulsion (-like) polymerization reactions and on-line conductivity

measurements 42

4.3.1 Interpretation of on-line conductivity measurements 42

4.3.2 Batch reactions with MMA 43

4.3.3 Batch reactions with styrene 51

4.3.4 Semi-continuous reactions with MMA 55

4.3.5 Semi-continuous reactions with styrene 60

4.4 Concluding remarks 62

Appendix 4.1

Appendix 4.2

Chapter 5

THE INFLUENCE OF THE TYPE OF INITIATOR ON

ENCAPSULATION REACTIONS

5.1 Introduction

5.2 Comparison of initiating systems

5.2.1 Reactions with SACPA

5.2.2 Reactions with sodium persulfate

5.2.3 Reactions with cumene hydroperoxide

5.3 Concluding remarks

Contents

64

66

69

71

71

74

77

81

Chapter 6

THE ROLE OF SURFACTANTS IN ENCAPSULATION REACTIONS

6.1 Introduction 83

6.2 Experimental 84

6.3 Comparison of surfactants 85

6.3.1 Non-ionic surfactants 85

6.3.2 Inisurfs 87

6.3.3 Sulfosuccinates 88

6.4 Surfactant addition during encapsulation reactions 91

6.5 Pigment modification and concentration 95

6.5.1 The influence of pigment modification on encapsulation reactions 95

6.5.2 The influence of the pigment concentration on encapsulation

reactions 97

6.6 Concluding remarks 97

Contents

Chapter 7

COPOLYMER ENCAPSULATION REACTIONS

7.1 Introduction 99

7.2 Experimental procedures 100

7.2.1 Encapsulation reactions 100

7.2.2 Extraction of surface (co-)polymers 102

7.2.3 Preparation of standard (calibration) copolymers for HPLC 102

7.2.4 Characterization of (surface) copolymers: lH NMR and HPLC 103

7.3 Copolymer encapsulation of Ti02 106

7.3.1 (Co-)polymerization of MMA and styrene 106

7.3.2 (Co-)polymerization of MMA and BMA 113

7.4 Concluding remarks 117

EPILOGUE

REFERENCES

Dankwoord

Curriculum vitae

List of symbols and abbreviations

119

125

Chapter 1

INTRODUCTION

1.1 General introduction

Polymer encapsulated materials have an extensive number of (possible)

applications. The process of polymer encapsulation is used e.g. in pharmaceutical

and agricultural products as well as in cosmetics and coatings. It is often applied for

reasons of toxicity, taste and odour masking, or to facilitate storage or transport of

the encapsulated product. A good example is the encapsulation of pesticides in which

case the capsule may offer the possibility of controlled release. Depending on the

polymer, either the dosage (diffusion controlled) and/or the moment of release can be

controlled e.g. through the effect of weather or other environmental conditions.

When applied to drugs, polymer encapsulation can be used for instance to mask the

(bad) taste or for so-called drug targeting, which again is a form of controlled

release. In this case the capsule does not dissolve or is imperm~able to the medicine

inside until it is in contact with for example gastric acid. These and other

applications that involve the (temporary) protection of an environment from the

encapsulated material, or vice versa, are mentioned by Finch,l along with the

production methods of polymer capsules, and treated more specifically by others in

'Encapsulation and Controlled Release'. 2

Next to materials that are at some point released from their capsule, polymer

encapsulation is applied to a separate class of materials: inorganic (sub-)micron

particles. Here, the general goal is to improve both the interaction between the

inorganic material and a polymer matrix and, by doing so, the (mechanical)

properties of the composite system.3 Examples of inorganic materials that can be

encapsulated with polymer are carbon and graphite (electric applications, catalysts

and toners)/ magnetite (immunoassay, cell labelling, affinity chromatography),4

2 Chapter 1

silicium dioxide/,6,l,8,9, 10, II,12 calcium carbonate,13,14 di-alumina tri_oxide12,15,16 and

titanium dioxide (in e.g. paints and other high performance plastics).17,18,19,20,21,22 The

properties or variables that can be improved by encapsulating inorganic particles

include flow, dispersability,23 gloss, mouldability/ modulus, loss-tangent, mechanical

strength,24 or the degradation due to photo activity or chemical reactions.

1.2 Polymer encapsulation of inorganic pigments

In this thesis the encapsulation of titanium pigments will be focused on. The

opacifying properties of this pigment are so superior that titanium dioxide is the most

widely used pigment in paint systems, despite its radical producing character which

in paint systems leads to the deterioration and degradation of the surrounding binder

material. This problem of radical production is induced by light but is met by coating

the pigment with inorganic shells of e,g. aluminium, zirconium and/or silicium

oxides.

Recently, water-borne paints have gained much importance. Because of their

low organic solvent content these paints are believed to contribute less to ozone

forming in the troposphere, while generally speaking also the working and health

conditions of professional (house) painters will improve. However, as far as the paint

system itself (application, durability) is concerned, the disadvantages of water-borne

paints might outnumber their advantages in some cases.

The basic materials for a paint are the binder (a polymer that forms a coherent

film with all the components in the system), the pigment (colour) and in most cases a

solvent or dispersing medium (to allow application). The other components may

include: siccatives, wetting agents, thickeners, co-solvents, surfactants, fungicides,

extenders and perfume.

One of the (major) problems introduced by the use of water instead of organic

solvents is caused by the difference in surface properties of the pigment and the

binder material. Because in oil based paints the binder is dissolved in the organic

solvent a smooth, coherent paint film is more or less guaranteed. However, in a

Introduction 3

water based system wetting of the pigment surface by the polymer is poor and the

hydrophobic binder is present in the form of small particles that are dispersed in the

aqueous phase. Because the pigment particles are hydrophilic, in contrast with the

binder material, agglomeration is likely to take place during the drying and during

the film formation process. The pigment agglomerates thus formed less efficiently

reflect light, which reduces the hiding power, while the gloss can be reduced as well.

These agglomerates also introduce hydrophilic zones which can enhance water

diffusion to the substrate surface. In other words: the mechanical and protecting

properties of the paint are negatively affected.

The problem of pigment agglomeration may be overcome by encapsulating the

pigment with a polymer prior to the paint production. In this way it is in principle

possible to (partially) adjust the pigment surface properties to the binder material

present. During drying and film formation processes this would lead to a more even

distribution of the pigment in the paint film while the final film should have

improved durability and weather resistance, and improved mechanical properties

(figure 1.1).

The methods of polymer encapsulation of pigments are numerous, but can be

divided into three main categories:

- encapsulation in an organic solvent (solution or bulk polymerization, polymer

adsorption)

encapsulation in water (dispersion or emulsion polymerization, polymer

adsorption)

encapsulation In water by means of an emulsion polymerization after a

modification reaction in an (organic) solvent.

These categories in turn can be divided in systems where either the pigment surface,

or the polymer or both are activated to enhance the encapsulation. Finally a division

is possible between chemi- and physisorbed surface polymer. The relevant

encapsulation methods will be discussed in more detail in chapter 2.

4 Chapter 1

WATER-BORNE PAINT'NannaI' pigment Encapsulated pigment

••••~ ••@.e;. °e·...~ ·e··~.~••~ • Ti02 ., • TiD2 •• • Ti02 ·1• Ti02 .. -. e· ...• -.. ·0. TXT ..... TiD2 • e~• • • ~ • \f!!!3 rr et paint ••• .,:. Ti02 :

··-6-."1!!!7··.~~ .6)*.. 4!O-S-· ...::.~. Q. "1!!!7. ~ Ti02 .~. Ti02 •••... .~.. ~. .. .•• ••••• ••• • w. ••

I Drying andt fUm formation

Agglomeration andpoor adhesion

Dispersability andadhesion improved

Figure 1.1 Compatibility between pigment and binder rTUJterial is improved by polymerencapsulation of the pigment. The binder (or polymer) is represented by the dark areas

The encapsulation method consisting of an organic modification of the pigment

surface followed by an emulsion-like polymerization process was adopted in this

research (see figure 1.2 for the process scheme): after modification of the pigment

with titanates the pigment particles become hydrophobic. These particles are then

dispersed in water with the aid of a surfactant. An emulsion polymerization reaction

is carried out in the presence of the dispersed pigment particles and (part of the)

polymer is formed at or migrates towards the hydrophobic pigment surface.

Introduction 5

IPRODUCT I

/ = surfactant'VV = titanate groups

~~" ~"l~:~DIFICATlO*NI

+n Ti :lO T"O~C / \ 1H6-o 0-9-<1~35

I 0

~c ~

/,SIFICATlON i

\! IPOLYMERIZATION I \ !~~~ + monomer ~~....~TIO . .. .. TI02 ~~ ~ + lDlt1ator ~~ ,

! \ '----=JA 1 \oligomers

H

H~OOHHO TiO OH

HO 1l OH

Figure 1.2 The principle of polymer encapsulation of pigments as utilized in this thesis. Themodification with a titanate is carried out in heptane. The hydrophobized pigment particles are firstdispersed in water with the aid of a surfactant and subsequently encapsulated with polymer l7y meansof an emulsion-like polymerization process

1.3 Aim and justification of this investigation

The major aim of this investigation is to unravel the parameters that

determine the efficiency of the reaction and the stability of the reaction system. The

efficiency, compatibility and stability of an encapsulation reaction and its end product

are closely related, as will become clear in this thesis. A key feature determining

efficiency and stability is the number of free micelles that is formed during an

encapsulation reaction or at least the number that is initiated.

6

free polymer ---4.

Chapter 1

polymer type 2

Figure 1.3 The basic criteria for polymer encapsulation of pigments in a nut-shell: efficiency,compatibility and stability

The efficiency can be defined as the fraction of polymer that is located on the

pigment surface in relation to the total amount of monomer that was added. This

efficiency is strongly influenced by the number of free micelles that can and will

compete as a site of polymerization. If free micelles are initiated this will lead to the

formation of free polymer particles (figure 1.3). Therefore, it is advantageous to

minimize the number of free micelles not only for economical reasons (in many cases

the encapsulated pigment and the free polymer will have to be separated), but also

for reasons of stability.

Introduction 7

The stability of the reaction mixture strongly depends on the amount of

surfactant available for stabilization. If this amount diminishes quickly during the

reaction, coagulation will occur. This will be the case especially if a large number of

free polymer particles are formed during the reaction (low efficiency! I). Then the

surface area that needs to be stabilized increases very rapidly and depletion of the

emulsifier(s) will occur, leading to the already mentioned coagulation and to a very

polydisperse reaction mixture of (partly) encapsulated pigment agglomerates (figure

1.3). Surfactant depletion will not take place in the case where most or all of the

polymer is formed at the pigment surface, because here the same volume of newly

formed polymer is located in a relatively thin shell around the relatively large

pigment particles. From the above one can conclude that the absence of free micelles

will favour both the efficiency and the stability of an encapsulation reaction.

In section 1.2 it was already mentioned that in an environment of hydrophobic

polymer particles the aggregation of hydrophilic pigment particles can take place, and

that the complete coverage of the pigment surface with polymer must be pursued for

an improved compatibility between pigment and binder. Of course a high

encapsulation efficiency will help, because then the thickness of the polymer layer at

the inorganic surface can be controlled optimally. However, also the type of

(surface) polymer is important for the pigment-binder compatibility (figure 1.3).

Therefore, a second aim of this investigation is to determine which (type of) (co-)

polymers can be formed at the pigment surface, and whether it is possible to form

sequential layers of different (co-)polymers at that surface. The monomers used in

this investigation were chosen on the basis of their difference in hydrophobicity

(methyl methilcrylate, MMA, versus ~rene, Sty, and Qutyl methilcrylate, BMA), in

glass transition temperature (BMA versus Sty and MMA), and because they are

frequently used both in practical applications and in research.

8 ~~ul

1.4 Outline of this investigation

In chapter 2 the background and theory of polymer encapsulation of pigments

will be given by means of a short overview of the possible encapsulation methods. A

further justification will be given for the choice of the two-step encapsulation method

which was adopted in this investigation. Because the second step in this method

involves an emulsion polymerization reaction a short survey will be given of the

basic principles of this polymerization technique as well. More specific theoretical

issues will be discussed in the other chapters where needed.

The experimental details and procedures will be discussed in chapter 3. This

includes among other things reactor schemes, recipes, analytical procedures, as well

as some general calculations (efficiency, particle number, etc.). More specific

experiments will be discussed in chapter 6 and 7 concerned.

The possibilities of using (on-line) conductivity measurements in monitoring

and controlling emulsion polymerization reactions are introduced in chapter 4. The

principle is based on detection of the mobility of surfactant molecules, which changes

when these molecules migrate from one phase to another. This technique can be used

to determine the influence of e.g. initiators, monomers, or the monomer addition

method on the number of free micelles and on the course of the reaction.

Furthermore, some of the reaction mechanisms and events occurring during

encapsulation and emulsion polymerization reactions will be enlightened. This

knowledge forms the basis for the development of surfactant addition profiles on

which preliminary experiments are reported in chapter 6.

With the insights gained in chapter 4 several initiating systems have been

tested and the results will be compared in chapter 5. The initiators used include a

persulfate, an azo-initiator and a redox initiator. The pros and cons of each system

will be discussed in terms of both the encapsulation efficiency of the reaction and the

stability of the reaction mixture.

Introduction 9

The influence of the surfactant type on encapsulation reactions will be

discussed in chapter 6. Non-ionic and anionic emulsifiers were tested as well as a

so-called inisurf (an azo-initiator with emulsifying properties). Preliminary

experiments on surfactant addition profiles, and experiments in which the influence

of the pigment modification and concentration were determined will be discussed as

well.

Chapter 7 deals with the possibility of forming co-polymers at the pigment

surface. The formation of successive polymer layers of different composition at the

pigment surface, and any possible differences between surface (co-)polymer and free

(co-)polymer will be discussed as well. This chapter contains an experimental section

in which the extraction and analysis of copolymers are described.

In the epilogue the present results will be discussed against the background of

the aims of this investigation. Furthermore, promising topics of future research on

polymer encapsulation of pigments will be indicated.

Symbols, abbreviations and the most important definitions are listed at the end

of this thesis.

Chapter 2

BACKGROUND OF PIGMENT ENCAPSULATION

AND EMULSION POLYMERIZATION

2.1 Introduction

In chapter 1 it was mentioned that polymer encapsulation reactions can be

divided into three main categories based on the solvents used:

- encapsulation in an organic solvent (solution or bulk polymerization,

polymer adsorption),

- encapsulation in water (emulsion polymerization, polymer adsorption),

- encapsulation in water by means of an emulsion-like polymerization

preceded by a modification reaction in an (organic) solvent (emulsion

polymerization, polymer adsorption).

An overview of various encapsulation methods will be given in the next

section, along with a summary of the basic parameters known to influence

encapsulation reactions. This section is followed by an overview of the basic

principles of emulsion polymerization (section 2.3).

2.2 Overview of polymer encapsulation methods for inorganic particles

2.2.1 Polymer encapsulation in organic solvent or in bulk

The first class of polymer encapsulation reactions comprises processes

performed in organic solvents. By far the easiest method of encapsulation is the

adsorption of a polymer dissolved in an organic solvent onto the pigment surface.

12 Chapter 2

However, one should pay close attention to the polarities of the components

involved. 21,15,26 Encapsulation is most likely to take place if polymer and pigment are

of comparable polarity. The solvent polarity must differ (strongly) from that of both

other components because otherwise the encapsulated particles may suffer from

severe polymer desorption.

An alternative to polymer adsorption is the adsorption of monomer vapour by

the pigment surface,15,16,27 followed by y-radiation induced polymerization. However,

this seems to be inappropriate to large scale processes.

Beside polymer and monomer adsorption it is also possible to encapsulate

pigments by activating the inorganic surface, the polymer or both. Laible et at. have

mentioned three different kinds of surface activation. 26,28 The activating group can

cause either a ring-opening or a radical polymerization reaction (initiation from the

surface), but also a co-polymerizable group can be attached to the surface

(propagation) or a group that causes grafting of a polymer through termination.

Activated polymers can also react with the hydroxyl groups present at the pigment

surface. Often a silane is used to activate the polymer. 29

The idea of surface and/or polymer activation is to obtain a covalent bond

between pigment surface and polymer. The methods of activation are described in

more detail elsewhere.6,Il,l4,27,28,29,30,31 In all these cases the experimental routes are

rather laborious. Although the graftivities of the encapsulated pigment (= amount of

polymer per gram of bare pigment) can reach high values (up to 0.6 gram polymer

per gram bare pigment), the efficiency often is low (for the latter example no more

than 25 wt % of the total amount of polymer formed is located at the pigment

surface).3l Besides, the amount of actually covalently bound polymer (i.e. polymer

that cannot be extracted by means of a good solvent) is often very low (0.3-3 wt%).6

2.2.2 Polymer encapsulation in aqueous systems

Given the number of disadvantages of encapsulation processes in organic

solvents (laborious synthesis, low efficiencies and extensive use of organic solvents)

Background ofpigment encapsulation and emulsion polymerization 13

it is desirable to find alternative encapsulation routes. Water has been used as a

reaction and/or dispersion medium instead of organic solvents by a number of

researchers.

Polymer adsorption from the aqueous phase onto the pigment surface was

investigated for instance by Heijman. 32 He tried to deposit p'oly(acrylic acid) (PAA)

on TiOz by adjustment of the pH.

A similar method was adopted by Meguro et al. for p'oly(s.tyrene) (PS) and

TiOz.33 In this case polymer particles instead of chains were adsorbed. The surface

charge of the PS particles, like that of the pigment particles, depended on the pH,

and an iso-electric point was found for both types of particles.

The polymer layer deposited on a pigment surface by means of adsorption is

not very uniform, while desorption can take place rather easily under changing

'environmental' conditions (pH, electrolyte concentration). Processes in which the

pigment surface is an active site for polymer formation seem to be more promising,

because then the encapsulation becomes (more) irreversible of nature.

Emulsion polymerization in the presence of inorganic particles has been

frequently applied as a process of polymer encapsulation of pigments. Emulsifier free

emulsion polymerization reactions were performed by e.g. Nagai et al..10 They used

a cationic surface active monomer that was adsorbed onto the surface of silica, after

which an initiator was added to start the polymerization reaction. Under certain

conditions, especially at monomer over pigment ratios less than or equal to 0.13, all

polymer is formed at the pigment surface. The maximum attainable polymer content

of the pigment, however, is rather low, unlike the reactions described by Hergeth et

a[.34 who polymerized yinyl acetate (VA), methyl methacrylate (MMA) or ~rene

(Sty) in the presence of silica particles. The mechanism of encapsulation is similar to

that described by Nagai et al., 10 but the polymer layer in the vicinity of the pigment

surface was thick as compared with the diameter of the silica particles (diameter SiOz

particles is 26 nm; the polymer layer was found to be 2-9 nm thick).34

14 Chapter 2

Haga et at. 18 performed emulsifier free reactions also, but they first adsorbed

the initiator onto the pigment surface. Like in the case of polymer adsorption, the pH

and the iso-electric point played a dominant role in the efficiency of the encapsulation

reactions. Pigment surface and initiator preferably should carry opposite charges.

Hasegawa et at. /5,36 found that large filler particles could be encapsulated quite

easily in emulsifier free reactions. However, the surface polymer existed of adsorbed

polymer particles.

The influence of surfactants on the encapsulation reactions has been studied as

well. Hergeth et at. 8 found that a minimal pigment surface area is necessary for

encapsulation reactions to take place. Therefore, these authors suggest that for good

results pigment particles should be well under 100 nm in diameter. However,

Hasegawa et at. 35,36 mentioned that large filler particles could be encapsulated quite

easily in emulsifier free reactions (see previous paragraph), although a uniform

polymer film was formed only upon the addition of small amounts of surfactant prior

to reaction. Under these circumstances a surfactant bilayer is thought to be present

around the inorganic particles, creating a preferential locus for polymerization at the

pigment surface. This method of surfactant adsorption was also used by Meguro et

at. 20 who found that beside single particles also aggregates were encapsulated, which

suggests that maintaining the stability of the reaction mixture can be or can become a

problem during reaction.

Templeton-Knight et at. 22,37 also used low surfactant concentrations in

combination with ultrasound. The use of ultrasound had a promoting effect on the

polymer formation at the pigment surface. Surfactant concentrations below the

~ritical micelle ~oncentration (CMC) gave the best encapsulation results. Because of

its higher CMC an ionic surfactant was more efficient than a non-ionic one.

Background ofpigment encapsulation and emulsion polymeriZiltion

2.2.3 Polymer encapsulation in aqueous systems preceded by pigment modification

15

From the above it can be concluded that the existence of a (hydrophobic)

surfactant layer around the inorganic particles enhances polymerization at the

pigment surface. Furusawa et al. 7 adsorbed hydroxyl propyl cellulose (HPC) at the

surface of SiOz' This adsorption was more or less irreversible, at least under the

reaction conditions employed. Surfactant ~odium Qodecyl ~ulfate, SDS) was added to

stabilize the inorganic particles. At SDS concentrations well below the CMC

polymerization mainly took place in the HPC layer, but above the CMC free

polymer particles were formed that became adsorbed to (partly encapsulated) SiOz

particles to form raspberry-shaped composite structures.

Smith and Hoy8.39 created a hydrophobic bilayer by adsorbing an amphiphilic

polymer together with a 'companion surfactant' onto the pigment surface. The

extremely high efficiencies found (sometimes higher than l00%!?) suggest that

coagulation or adsorption of free polymer particles onto the pigment surface may

have occurred.

The surface modifications mentioned above are physical in nature and "the

modifying agents therefore are prone to desorption. Reaction conditions like pH,

(electrolyte) concentration and temperature may play an important role. In order to

circumvent this possible problem of desorption and to work with well-defined

surfaces, in the present investigation a method was chosen described earlier by Caris

et al.. 40 They described the modification of the pigment surface (TiOz) with titanates

in an a-protic medium (di-chloro methane). The titanate reacts with the hydroxyl

groups at the pigment surface and forms a hydrophobic layer around the inorganic

particle. Next the hydrophobic pigment particles are dispersed in water with the aid

of a surfactant (SDS), which leads to the formation of a bilayer at the pigment

surface. Finally an emulsion polymerization is carried out in the presence of the

dispersed pigment particles (see chapter 1, figure 1.2 and chapter 3 also). Several

titanates were tested ranging from merely hydrophobic (hydrophobic interaction of

the polymer with the titanates) to co-polymerizable titanates (with a vinyl group) and

16 Chapter 2

titanates with a reactive (amine) group to which an azo-initiator was chemically

bound (in these cases the polymer may become chemically bound to the surface).

A complication for the use of the vinyl-group-containing titanate was that it

was sensitive to hydrolysis in an aqueous environment, and therefore had to be used

in combination with a merely hydrophobic titanate. Because it stabilizes radicals

originating from the pigment surface, which can lead to instantaneous termination

reactions, the amine-containing titanate, onto which an initiator could be attached,

caused some inhibition problems in the presence of (UV) light. In order to prevent

this, the pigment had to be doped with ZnO and light had to be excluded.

Several experimental routes were tested for their usefulness in attaching an

initiator to the amine containing titanate. Most of these methods were quite laborious

while only part of the initiator was bound to the pigment surface. 40,41,42

Given the complications with the vinyl-group-containing and with the amine­

containing titanate, and in order to have the best-defined surface, the merely

hydrophobic titanate was chosen in the present investigation on polymer

encapsulation. Caris41 found that pigments modified with this titanate did have an

influence on the conversion-time behaviour of an emulsion polymerization reaction

whereas non-modified pigment did not: a plateau in the conversion-time curve was

observed, which could be related to coagulation phenomena. Coagulation seemed to

be worse in case of low surfactant concentrations or high pigment contents. Neither

the pH nor the electrolyte concentration were found to have a large effect on

coagulative behaviour. The plateau and (part of) the coagulation disappeared under

improved mixing conditions. As was found by other researchers the encapsulation

efficiency (= the fraction of the amount of monomer added to the system that is

transformed into surface polymer) seemed to benefit from low surfactant

concentrations7•2o,22,35,36,37 and a large pigment surface area. 8,l0 Low monomer

concentrations also helped to improve the encapsulation efficiency, but (in batch

processes) the absolute amount of polymer per gram pigment simultaneously

decreased.

Background ofpigment encapsulation and emulsion polymerization 17

In the following chapters a number of the phenomena mentioned above will be

discussed in more detail. Experiments were conducted to obtain more insight in the

encapsulation kinetics as well as to improve the encapsulation efficiency and the

stability against coagulation. In the next section a short overview of the basic

principles of emulsion polymerization will be given. More specific theoretical

backgrounds will be given in the text where needed.

2.3 Emulsion polymerization

Emulsion polymerization in aqueous systems is a widely applied

polymerization technique. The basic ingredients are water, monomer, an initiator and

mostly a surfactant. The emulsion polymerization process allows a good temperature

control, without the risk of hot spots or runaways. Therefore, it is easy to control,

quite safe, and reasonably friendly towards the environment as well. It is also a very

complex process, and many parameters can influence the composition and the

properties of the final product or latex.

The kinetics of emulsion polymerization have been described by several

authors. A qualitative model was proposed by Harkins43 and was later quantified by

Smith and Ewart. 44 According to their theory an emulsion polymerization can be

divided in three intervals. Crucial to their theory is the presence of micelles, which

can be formed if the surfactant concentration exceeds the CMC.

2.3.1 Harkins' theory (micellar nucleation)

During interval I radicals formed upon decomposition of the initiator will

enter monomer swollen micelles. These radicals react with the monomer to form

polymer particles (micellar nucleation). Monomer is transported from monomer

droplets through the aqueous phase towards the initiated (= oligomeric radicals

containing) micelles. As the reaction continues the polymer will be swollen with

monomer and the initiated particles will grow. Stability of the growing particles is

18 Chapter 2

provided for by adsorption of surfactant from the non-reacting micelles. The moment

all micelles have disappeared and only polymer particles remain (beside monomer

droplets) marks the beginning of interval II.

In interval n the total number of particles is constant in the ideal case. The

monomer concentration in the particles is practically constant too and so is the poly­

merization rate. The monomer concentration can be kept constant because the mono­

mer droplets still serve as a reservoir. Furthermore, within the swollen latex particles

there is a thermodynamic equilibrium between the creation of new surface area and

the mixing of polymer and monomer. The moment the monomer droplets disappear

interval ill is entered and the monomer concentration within the particles and in the

aqueous phase starts to decrease. Because the polymerization rate is a function of the

monomer concentration it will drop also. The particles will shrink somewhat upon

the conversion of monomer into polymer because of the higher density of the latter.

2.3.2 Alternative nucleation mechanisms

Although useful, the micellar nucleation theory is unable to describe all

emulsion polymerization systems. If no surfactant is present, or if the CMC of the

surfactant involved is too high, micellar nucleation is less likely to occur. Both in the

case of emulsifier free reactions and in the case where the monomer is (relatively)

water soluble, homogeneous nucleation is likely to take place (HUFT theory, after

Hansen and Jjgelstad, and £itch and Isal). 45,46,47 According to this theory initiation

and growth of oligomers takes place in the aqueous phase until the polymer chain

reaches a length at which it becomes surface active. If a surface is present these

oligomers will precipitate onto that surface, otherwise they will grow until they reach

the critical length at which they become water insoluble, after which they will

co-precipitate to form unstable precursor particles. These precursor particles in turn

will coalesce to form stable polymer particles. If little or no surfactant is present,

some initiators, like persulfates, give rise to oligomeric radicals that provide

additional stability, mostly because of electrostatic repulsion. In principle these

charged oligomers can be regarded as in-situ surfactant molecules.

Background ofpigment encapsulation and emulsion polymerization 19

An extension to this theory of homogeneous nucleation was given by Gilbert,

Napper, and co-workers: homogeneous/ coagulative nucleation. 48,49,50 The particles

formed during the early stages of initiation and precipitation are called precursor

particles and are believed to coagulate among themselves until finally stable latex

particles are obtained. Like in the case of 'regular' homogeneous nucleation the

surfactant determines the final number of particles. The presence of surfactant is less

essential if the oligomeric radicals can provide stability in the way mentioned above.

2.3.3 Exceptional behaviour in intervals II and III

In many reactions an interval occurs during which both the number of

particles and the reaction rate are constant (interval 11). The more water soluble the

monomer and the polymer are the more likely it is that new particles are formed

throughout the reaction until or even beyond the beginning of interval m. In other

words: interval n does not always exist (see chapter 4 also).

In some cases the so-called gel or TrommsdoTjf iffect will lead to an increase

in the reaction rate during interval m despite the decreasing monomer

concentration. Many monomers obey the zero-one system5l which means that a latex

particle can contain at most one active radical: if a second one enters termination

instantaneously occurs and no active radicals remain within that particle (the average

number of radicals per particle is 0.5). However, after the monomer droplets have

disappeared the monomer concentration in the particles decreases. Sometimes this

will lead to a strong viscosity increase within the particles. Biradical termination

reactions may become strongly diffusion controlled and the average number of

radicals per particle may become larger than 0.5. Because the reaction rate is not

only a function of the monomer concentration but of the number of radicals within a

particle as well, the reaction rate may increase despite the decreasing monomer

concentration.

20 Chapter 2

For the emulsion polymerization of methyl methacrylate it was reported

earlier52 that a gel-effect will take place after the swollen particles have reached a

certain size (51 nm) and it may be enhanced if monomer starvation occurs: if the

polymerization rate is sufficiently rapid, the rate of monomer transport into the

polymer particles may be insufficiently high leading to a rapid increase in viscosity.52

This may also play an important role in the case of encapsulation reactions. The core

of encapsulated particles can not be penetrated by radicals, which means that

instantaneous termination may be hindered. Therefore, the average number of

radicals per particle may exceed 0.5. This in turn can lead to an enormous increase

in the reaction rate and to the already mentioned monomer starvation effect and

viscosity increase. This means that the possible occurrence of a gel-effect in

encapsulation reactions certainly has to be taken into account.

Chapter 3

Experimental

3.1 Purification and modification of titanium dioxide

The TiOz pigments used in this investigation were RLK and KR2190 (both

rutile) supplied by Kronos. RLK is a titanium pigment that has not been stabilized

with other oxides and therefore consists only of TiOz. RLK is produced according to

the sulfate process, it has a density of 4.2 g/cm3, a weight average diameter of

approximately 250 nm (determined with a disc centrifuge5J; polydispersity between 2

and 2.4) and a specific surface area41 of approximately 8 mZ/g. Traces of sulfates

(mainly KZS04) were removed by washing the pigment with de-ionized water

(Millipore Super Q). The pigment was subsequently dried under vacuum at BO°C to

remove (crystal) water.

KR2190 is a pigment which has been treated with 3 wt% Alz0 3 , 0.3 wt%

ZrOz, and 0.25 wt% of an organic compound (tri-methylol propane: 2-ethyl-2­

(hydroxymethyl)-1,3-propanediol). The pigment surface was found to consist of

approximately 54% Alz0 3, 41 % TiOz and 4% ZrOz. These values were derived from

ESCA data (Electron Scattering for Chemical Analysis) which were reported by

Caris. 41 Some carbon was found at the surface also. This partly originates from the

organic compound, but during the analysis some carbon monoxide was formed too.

Unlike RLK, KR2190 does not contain any salts and, therefore, was not washed

prior to the drying under vacuum at 130°C. KR2190 has a density of 4.1 g/cm3 and

a weight average diameter of approximately 200 nm with a broad distribution, as was

determined with a !lisc ~entrifuge equipped with a Uhoto sedimentometer (DCP,

Brookhaven Instruments). The specific surface area41 was approximately 14.9 mZ/g.

22 Chapter 3

ILC 0

\rL ,...OH Hb-o 0-1-c171fu

Hl ~! CH3+ '» HboH

::1-/ \-C-CI7IfuT1 +2

/ \-C-CI7ffi5b3

OH ~ g o gFigure 3.1 Modification of inorganic pigments with a titanate (CAlO). The hydroxyl groups at thesurface react with the isopropoxy groups of the titanate (alcoholysis). Iso-propanol is the by-product

The TiOz was modified with a titanate in n-heptane (Merck, p.a.). The titanate

used was di-isopropoxy titanium di-isostearate (TILCOM CAlO, supplied by

Tioxide), which reacts with the hydroxyl groups at the pigment surface (figure 3.1).

Both heptane and CAlO were used without further purification. The amount of

titanate needed for the modification of the pigment was determined by means of

surfactant adsorption experiments: above a certain titanate content (monolayer

coverage) the amount of adsorbed surfactant no longer increases with the titanate

content (plateau value).41 The titanate content at which this plateau is reached can

also be determined by plotting the amount of added titanate against the weight loss of

the modified pigment after heating to 700°C (weight loss determined after washing

and drying of the pigment). Based on the amount of pigment, either 1.5 wt% (in the

case of RLK) or 3.6 wt% CA10 (in the case of KR2190) was dissolved in 500 mL

heptane and was added to the pigment (400 g) in a polyethylene bottle of 1 L. After

the addition of approximately 600 g of glass beads (diameter: 2 mm) the bottle was

placed on a roller-bench for at least one hour, which is sufficient for a good

dispersion and modification of the pigment.

After completion of the modification the glass beads were separated from the

TiOz-dispersion by means of filtration. Next the dispersion was centrifuged (with a

MSE Mistral 3000E) at 3200 rpm for 8 minutes to separate the pigment from the

heptane. The pigment was re-dispersed in fresh heptane and centrifuged again.

Re-dispersion and centrifugation were repeated twice. Finally the modified pigment

(KR2190-CAlO or RLK-CAlO) was dried under vacuum at room temperature.

Experimental

3.2 Determination of surfactant adsorption by pigments

23

The amount of surfactant that adsorbs to the surface of (modified) pigments

was determined by means of a two-phase titration as described by Reid et af..54 A

known amount of pigment (4 g) was dispersed in 60 mL of an aqueous solution of

~odium godecyl ~ulfate (SDS, Fluka p.a.) under high shear stirring (Ystral

ultraturrax). The surfactant concentration applied was (slightly) above the CMC of

SDS.

After centrifugation the surfactant concentration in the liquid phase was

determined. For this purpose 10 mL of the aqueous liquid was pipetted and mixed

with 7.5 mL of chloroform (Merck, p.a.), 5 mL of indicator solution and 5 mL of

distilled water. This mixture was titrated with a 0.004 M aqueous solution of

hyamine (C27H42CINO, Merck). The indicator was a mixture of dimidium bromide

and disulfine blue in an aqueous 0.2 M H2S04 solution.

The difference between the initial SDS concentration and the concentration in

the liquid phase after the pigment had been dispersed and centrifugation (Phywe

centrifuge, 8000 rpm, 8 minutes) had taken place, is the amount of SDS that had

been adsorbed onto the pigment. For RLK-CAIO an adsorption of 1.6*10-5 mol

SDS/g pigment (variation: 0.3*10-5 mol/g) was found. KR2190-CAIO adsorbed

3.4*10-5 mol SDS/g pigment (variation: 0.4*10-5 mol/g).

3.3 Polymerization and encapsulation reactions

3.3.1 Ingredients

Two types of reactions were performed: 'regular' emulsion polymerization

reactions, i.e. un-pigmented reactions, and encapsulation reactions. The materials and

typical concentrations are listed in table 3.1.

24 Chapter 3

Table 3.1 Materials and typical quantities used in emulsion polymerization and in encapsulationreactions. Concentrations are based on the amount of water unless stated otherwise

.... ·i 1>.····/..../ii\Material Quantity............

Ti02-CAlO 0-55.5 giL

SDS 3.8 - 11.5 mmol/L

water 900 mL

monomer 140 - 555 mmol/L

initiator:

1) SACPA 1.3 - 1.5 mmol/L

2) SPS 1.5 - 5.0 mmol/L

NaHC03 4.2 - 8.0 mmol/L

3) CHP 5 - 10 mglg monomer

FeS04 1.9 - 3.2 *10.5 mol/L

EDTA 1.9 - 3.2 *10-5 mol/L

SFS 0.7 - 1.3 mmol/L

The pigment used in the encapsulation reactions was treated as described in

section 3.1. De-ionized water (Millipore Super Q) was purged with nitrogen in order

to remove oxygen. Sodium godecyl liulfate (SDS, Fluka p.a.) was the most

frequently applied surfactant and was used without further purification. Other

surfactants used (see chapter 6) were: Antarox CO-880 and CO-990 (non-ionic

nonylphenoxypoly(ethyleneoxy)ethanols; GAF Corporation) and Aerosol OT-IOO

(sodium dioctyl sulfosuccinate; Cyanamid). These surfactants were supplied as wax

and were used without further purification. Their structure formulas are given in

chapter 6.

Experimental 25

The monomers used in this investigation were either ~rene (Sty, Merck,

p.a.), methyl methilcrylate (MMA, Merck, p.a.) or Qutyl methacrylate (BMA,

Merck, p.a.). Inhibitors were removed from the monomer by vacuum distillation or

by means of commercial inhibitor removers (Aldrich). After inhibitor removal the

monomers were stored at 4°C in order to avoid thermal polymerization.

The initiator used was either ~odium ner~ulfate (SPS, Fluka p.a.) in

combination with NaHC03 (Merck p.a.), ~odium 4,4'-azo-bis-(4-~yano-nentanoate)

(SACPA) or a redox system containing ~umene llydroneroxide (CHP, Fluka, 80%

pure) in combination with FezS04.7HzO (Merck p.a.), ~thylene giamine tetra-acetic

acid (EDTA, Merck p.a.) and ~odium formaldehyde ~ulfoxylate (SFS, Fluka p.a.).

All materials, except for SACPA, were used as received.

~odium 4,4'-azo-bis-(4-~yanonentanoate) (SACPA) was prepared from

metallic sodium and 4,4'-azo-bis-(4-~yanonentanoic acid) (ACPA, Eastman Kodak

Chemicals). The sodium (0.5 g) was dissolved in 15 mL of methanol (Merck, p.a.)

to form sodium methanolate. This solution was added to a dispersion of ACPA (3 g)

in 20 mL of methanol, resulting in a clear solution of SACPA in methanol. The salt

was precipitated by slowly adding the SACPA solution to a large excess of diethyl

ether (Merck, p.a.). After decantation the salt was dried.

3.3.2 Experimental setup

Emulsion polymerization and encapsulation reactions were carried out in an all

glass I L reactor with four baffles (figure 3.2).. A six-blade stainless steel turbine

stirrer was used to ensure good mixing conditions. Under these mixing conditions a

plateau in the conversion-time plot, as observed by Caris41, was not found.

The reactor was thermostatted at 60°C by means of a water bath (MGW

Lauda, CS6). The change in conductivity was measured on-line with a PW9571160

four point electrode cell in combination with a PW9527 digital conductivity meter

(both produced by Philips) at 4000 Hz, or with a PPI042 two point electrode cell in

combination with a CDM80 conductivity meter (both produced by Radiometer) at

26 Chapter 3

2000 Hz. The reference temperature of the conductivity meter was set to 25°C.

Because only changes in conductivity were measured the electrode cells were not

calibrated.

r---......i-----::J:""""-----4----N,

E'!!l--'f---- itJiIiatr BDIuIiDn'---~

I -++---1IDH1tI6

........r--t-H---tuI'IJintt 8fimIr

r LL.-~• ..-----.Lr-r---nIf1CfDr WDI

IIrdr inItIt (iJIMmoaM,IHI)

IFRONT I

Figure 3.2 Experimental setup used for emulsion polymerization (and encapsulation) reactions

3.3.3 Reaction conditions

In the case of encapsulation reactions first the modified pigment had to be

dispersed in the surfactant solution by means of a high-shear stirrer (ultraturrax,

Ystral). The stirring time was approximately 45 minutes. In some cases (see below)

the initiator (solution) was added five minutes before the mixing was ended.

After the reaction mixture had been poured into the reactor, the temperature

was raised to the reaction temperature (60°C). A slight overpressure of nitrogen (0.2

bar) was maintained to keep the reactor free of oxygen. Prior to its addition the

monomer was purged with nitrogen, also to remove traces of oxygen.

Experimental 27

The monomer was added either in one step at the beginning of the reaction

('batch' -operation), or it was added at a certain rate throughout the reaction (' semi­

continuous' operation; monomer addition with a Metrohm 665 dosimat). Batch

reactions were started either by adding an initiator solution to the reaction mixture

already containing the monomer, or by adding the monomer to the reaction mixture

in which the initiator was already present. In the case of semi-continuous operation,

of course, the initiator was added to the reaction mixture prior to the monomer.

However, in the case of semi-continuous reactions with a redox initiator one of the

initiator components (CHP) was dissolved in the monomer that was added semi­

continuously.

During the reaction the conductivity was measured on-line (chapter 4), and

samples were taken from which the monomer-polymer conversion (x) was

determined gravimetrically. Other samples were checked with dark-field

microscopy,55 with transmission electron microscopy (TEM) , with a disc ~ntrifuge

equipped with a llhoto sedimentometer (DCP, Brookhaven Instruments Corporation)

or with dynamic light ~cattering (DLS, Malvern autosizer or Malvern S4700 PCS),

to determine whether coagulation had occurred, or to measure the particle size. For

DCP measurements on Ti02 samples a spin fluid was used consisting of 12 mL

sucrose solution (10 wt% in distilled water). During the spinning of the disc 2 mL

distilled water was injected. 'Boosting' (acceleration of the disc) was used to create a

gradient. Finally the sample (0.25 mL) was injected.

3.4 Separation and analysis of reaction prodncts

3.4.1 Experimental

After completion of the reaction the (encapsulated) pigment was separated

from the free polymer by means of centrifugation at 8000 rpm for eight minutes. The

particle size of the free polymer was determined by means of DLS-measurements.

28 Chapter 3

The solid pigment phase was re-dispersed in an SDS solution (3 g SDS/L) and

centrifuged again to remove any possibly remaining free polymer particles. This

procedure was repeated twice. Next the pigment was re-dispersed in distilled water

and centrifuged to remove the surfactant. This procedure was repeated once. Finally

the pigment was dried under vacuum at room temperature.

After the pigment had been dried, !hermogravimetrical analysis (TGA, Perkin

Elmer TGA7) was used to determine both the polymer content of the encapsulated

pigment, as well as the encapsulation efficiency of the reaction (see section 3.4.2).

In some reactions two monomers were used instead of one. In these cases, an

additional characterization was carried out (chapter 7). After the encapsulated

pigment and the free polymer had been separated, the major part of the surface

polymer was removed from the pigment surface by means of extraction with

tetrahydrofuran. Subsequently, both the free polymer and the surface polymer were

analyzed by means of NMR and/or HPLC to determine the chemical composition and

the chemical composition distribution. The experimental conditions of

copolymerization reactions are described in more detail in chapter 7.

3.4.2 Formulas and calculations

In encapsulation reactions two parameters determine whether or not the

reaction has been successful: the polymer content of the encapsulated pigment and

the encapsulation efficiency. In addition, the number of free polymer particles can be

important.

The polymer content

The polymer content (PC) can be defined in three ways:

1) the amount of polymer per gram of encapsulated pigment (PC,),

2) the amount of polymer per gram of bare pigment (pez),

3) the amount of polymer per gram of modified pigment (PC3).

Any polymer content mentioned in the text is based on the last definition (PC3) ,

unless stated otherwise. It can be derived that (equations 3.1 through 3.3):

Experimental

w-wPC ,,"__01 100

3.1 3.2 3.3

29

with: PC", = the polymer content in (g polymer/ g pigment) according to thedefinition ad x

w the weight loss (= polymer + titanate + crystal water) of theencapsulated pigment as determined by means of TGA [wt%]

Wo = the weight loss (= titanate + crystal water) of the modified pigmentas determined by means of TGA [wt%]

A maximum relative error of approximately 5% can occur, because wand Wo are not

based on exactly the same amount of bare (unmodified) pigment. The solid content of

CAlO was not taken into account either: if CAlO is heated the organic groups are

burned off and TiOz remains behind. Furthermore, the amount of crystal water could

not be determined accurately: upon heating the weight loss of modified pigment

seemed to be less than that of the pigment before modification.

The encapsulation dficiency

The encapsulation efficiency (11) can be defined as: 100% times the total

amount of surface polymer [g] divided by the total amount of monomer added [g]. It

can be derived that (equation 3.4):

3.4

with: 11PC]

pm

PC PT] =_3_ ·100%

m

the encapsulation efficiency [wt %]the polymer content of the modified pigment [g polymer/ g modifiedpigment]the total amount of modified pigment added during the reaction [g]the total amount of monomer added during the reaction [g]

Again the relative error amounts to maximum 5%, for the same reason as mentioned

for the polymer content. This error will be larger if a pigment or filler is used that

looses a substantial amount of crystal water upon heating, like e.g. Mg(OH}z or pure

Alz0 3 •

30 Chapter 3

305

The number of free polymer particles

In the case of 'regular' emulsion polymerization reactions the number of

polymer particles simply can be calculated from the conversion and the particle size.

It can be derived that (equation 3.5):

xomol000 xmNp PPO,.vpoW 4 d °10-9

P °1<f°_°1t°(_P_-)30Wpol 3 2

and for encapsulation reactions (equation 3.6):

(x-_TJ -)'m·1000100%

(x-_TJ _) om100%

3.6

with: Np = the number of free polymer particles in the reaction mixture per litrewater [L-l

]

x = the fractional (end) conversion [-]m = the amount of monomer added [g]Ppol = the density of the polymer [g/cm3

]

vp = the average volume of a polymer particle [cm3]

W = the volume of water added to the reactor [cm3]

dp = the average diameter of the polymer particles [nm]" = the encapsulation efficiency [wt%]

The fractional conversion was derived from the solid content of the reaction mixture.

The average diameter of the polymer particles was measured by means of dynamic

light scattering.

Chapter 4

EMULSION (-LIKE) POLYMERIZATION

REACTIONS MONITORED WITH ON-LINE

CONDUCTIVITY

4.1 Introduction

The success of an encapsulation reaction is largely determined by the

encapsulation efficiency of such a reaction and by the stability of the reaction system,

as was mentioned in chapter 1. The most disturbing events frequently occurring

during encapsulation reactions are firstly the formation of free polymer particles, and

secondly the (partial) coagulation of (partially encapsulated) pigment particles.

Naturally, the formation of free polymer reduces the encapsulation efficiency, but it

can also be the cause of (hetero) coagulation.56,57

During an encapsulation reaction the formation of free polymer particles is

enhanced by the presence of free micelles, especially for those monomers that

undergo micellar nucleation as a primary nucleation mechanism like for instance

styrene (Sty). For these reactions the surfactant concentration will be especially

critical.58 For monomers that primarily undergo homogeneous nucleation, like methyl

methacrylate (MMA), other factors may playa more dominant role, as there are: the

water solubility of the monomer, the overall monomer concentration, the pigment

concentration and the initiator efficiency. These parameters can influence the number

of (surface active) oligomeric radicals and the number of precursor particles (see

section 2.3 also). The latter may enhance micelle formation in the form of acting as

a co-surfactant or may even form micelles themselves.

32 Chapter 4

Both (precursor) particles and initiated micelles will compete with the pigment

particles as a site for polymerization. The formation of polymer at the pigment

surface will not lead to a strong increase in surface area, at least not as compared to

the increase that is caused by the growth of the newly formed free polymer particles:

the surface-area-over-volume-increase is small if polymer is formed at the outside of

existing particles. The larger the surface area increase, the more surfactant is needed

for stabilization. According to this scenario, at some point the surfactant

concentration drops below the minimum concentration that is needed for total

stability and coagulation occurS.56,57

1 micelles I

" I~~j;'

/1.

'>'.::l0

~5u

CMC[Surfactant] •

Figure 4.1 Schematic representation of the determination of a CMC and the concentration at whichthe first micelles are actually formed (indicated by the arrow)

The first micelles are formed just before the surfactant concentration exceeds

the CMC of a reaction system: the CMC is obtained from the intercept of two

tangents (of e.g. the conductivity signal versus the surfactant concentration), which

means that micelles start to form below that concentration (see fIgUl'e 4.1). Besides,

in practice the CMC has to be corrected for the interaction of the various reaction

components with the surfactant molecules. This interaction often leads to an

enhancement of micelle formation. The corrected CMC is often referred to as the

apparent CMC or CMCapp (see section 4.2).

Emulsion (-like) polymerization reactions monitored with on-line conductivity 33

The change in surface properties when going from a modified pigment surface

to a surface covered with polymer may have an additional effect on the surfactant

migration. On poly(methyl methacrylate) (PMMA) the surface area of one surfactant

molecule (0.79 nm2)59 is approximately equal to that on modified pigments (0.73- 0.8

nm2 per SDS molecule, as calculated from data in sections 3.1 and 3.2). On

polystyrene (PS) this surface area (0.5 nm2 per SDS moleculei9 is lower than on

modified pigments. This may lead to an enhanced adsorption of surfactant the

moment the pigment is encapsulated with PS, more than in the case where PMMA is

the encapsulating polymer. Therefore, reactions with Sty will suffer more from

coagulation phenomena as will become clear in section 4.3.5.

In this chapter the effects of various relevant components on micelle formation

are discussed. Conductivity measurements were used to visualize the effect of

monomers on the aggregation behaviour of surfactant molecules. The use of on-line

conductivity measurements during emulsion polymerization and during encapsulation

reactions will be discussed as well. The results of these conductivity measurements

will be interpreted in terms of the changing mobility of surfactant molecules when

the latter are adsorbed from the aqueous phase by micelles or by (hydrophobic)

surfaces.

4.2 The fonnation of (free) micelles

4.2.1 Effect of counterions on the apparent CMC

In fIgure 4.1 the relation between the conductivity, which is proportional to

the mobility of ions in a system, and the surfactant concentration is given and it was

shown that the CMC is obtained from the intercept of two tangents. Above the CMC

the slope is lower, because the mobility of a micelle is smaller than that of the sum

of the separate surfactant molecules it is composed of.

34 Chapter 4

Counterions, especially small ones like Na+, reduce the repulsive forces that

exist between the charged head groups of the surfactant molecules in a micelle. This

means that micellization is enhanced. Equation 4.160,61 describes the semi-empirical

relation between the concentration of counterions and the CMCapp'

log(CMCapp) = -a 'log(EC;) +b 4.1

In this equation: a and b are constants for a given ionic head at a particulartemperaturea a constant61 with a value of approximately 0.65Lei = the sum of the concentration of all monovalent counterions

(from the surfactant and from any other salt)(I)b a constant with a value of approximately -3.29, calculated

from reference 61

In the present thesis LCi equals the concentration of the Na+ ions. To calculate LCi ,

the Na+ ions of buffer and initiator salts are added to the Na+ ions from the

surfactant (SDS) at the apparent CMC. The latter is initially based on the regular

CMC of SDS at the reaction temperature of 60°C (10.16 mmollLt2 and is adjusted

iteratively according to the calculated CMCapp until no further differences occur

between successive iterations.

Example: at an initiator concentration of 1.3*10"3 moUL (with SACPA, which has two sodiumions, as the initiator), the initial LCi at 60°C is 12.76*10.3 mollL (= 2*[SACPAI + CMC) and theCMCapp of SDS becomes 9.2*10.3 mol/L,

4.2.2 Effect of monomer on micellization

The addition of monomer to a surfactant solution in principle can be regarded

as the addition of a non-conducting and water insoluble phase (the monomer) to a

conducting phase (the surfactant solution). Maxwell's law describes the change in the

conductivity of an ionic solution resulting from the addition of an insoluble phase. 63

For non-conducting monomers and small monomer fractions this law can be

simplified to equation 4.2:63

(I) Note that l:Ci contains the monovalent counterions belonging to the initial CMC of the surfactantwhen the iterations start

Emulsion (-like) polymerization reactions monitored with on-line conductivity 35

1200

1100

WOO

] 900j

800.q"E 700.g= 6000u

500

fA): addition of MMA

-A-A-A _

water solubility MMA:

Maxwell's law

becomes valid

- --0--0-0--

-~-~~~---------

1200

1100

WOO

i 900--enJ

800.~.~ 700g]

6000u

500

(B): addition of Sty

-A-A_--Sl·-\l__'v. v __~:~:::~::

:~:u:~:::::~---~-­water solubility Sty: --- --

axwell's law

mes valid

-~-~~~---------

4OOe~~~~0.00 0.01 0.02 0.03 0.04 0.05

volume fraction MMA (-)

400 -'r--'r--r-,..--,---.----r--.---r-,..--,0.00 0.01 0.02 0.03 0.04 0.05

volume fraction Sty (-)

[SDSl: 0 3.8 mM <> 5.8 mM 0 8.1 mM

X 8.4 mM \l 10.5 mM h. 11.5 mM IFigure 4.2 Effect of monomer addition on the conductivity of suifactant solutions of variousconcentrations at 60°C. Figure A: methyl methacrylate, figure B: styrene. Dashed lines representMaxwell's law; in equation 4.2 the Kc used was fitted from the last data point of each curve. Thevertical arrows indicate the literature values of the water solubility of MMA and Sty, respectively

K =K .2(1-lp)x c 2+lp

4.2

the conductivity of the emulsion [ILS/cm]the initial conductivity of the continuous phase (being the surfactantsolution) [ILS/cm]. For this research Kc was fitted from conductivitiesat higher volume fractions of monomer, because at low surfactantconcentrations surfactant and monomer interacted (see below)the volume fraction of the dispersed phase (being the monomer) [-]

If Maxwell's law does apply then the addition of monomer to a surfactant solution

should lead to an almost linear decrease in the conductivity. A deviation from

Maxwell's law might indicate an interaction between monomer and surfactant

molecules.

36 Chapter 4

In figures 4.2A and B the effect of the addition of MMA or Sty on the con­

ductivity of SDS solutions of various concentrations is shown. Clearly, in the range

of the horizontal arrows Maxwell's law does not always apply, which means that in­

teraction between monomer and SDS takes place under certain conditions. The value

for Kc as needed in equation 4.2 was fitted from the last data point of each curve.

At very low SDS concentrations ([SDS]= 3.8 mmollL for MMA or [SDS]=

5.8 mmollL for Sty) the conductivity behaviour is correctly described by Maxwell's

law, even though at low volume concentrations the monomer is dissolved, not

dispersed. Apparently dilution (with monomer) of the conducting (SDS) solution

causes a decrease in conductivity also following Maxwell's law. At these low SDS

concentrations the addition of monomer does not have an effect on the aggregation

behaviour of the SDS molecules. However, a surfactant will cause an increase in the

water solubility of the monomer, even if only a small amount of the former is

present,64 although in this case the solubility increase may be very small as well.60

At higher SDS concentrations, equation 4.2 becomes valid only after a certain

volume fraction of monomer is exceeded. In figures 4.3A and B these volume

fractions are indicated with vertical arrows which coincide with or lie slightly above

the solubility of the respective monomers in pure water (the water solubility of

MMA52 is 150 mmollL, which corresponds to a volume fraction of 0.0167, while

that of Sty59 is 3 mmollL, which corresponds to a volume fraction of 0.0004). Above

the solubility limit all the conditions for equation 4.2 are met: the monomer is non­

conducting and, besides, its concentration in the aqueous phase no longer increases,

while the total volume fraction of monomer still is very small.

At SDS concentrations higher than 3.8 mmollL (for MMA) or 5.8 mmollL

(for Sty) there seems to be interaction between the monomer and the SDS molecules.

Micelles are formed more easily in the presence of monomers, as was mentioned by

Rosen6O also. Micelle formation leads to a decrease in mobility and consequently to a

decrease in Kx ' which would explain why in f"IgUreS 4.2 and 4.3 the conductivity

initially (at the lowest volume fractions) decreases stronger (almost exponentially)

than could be expected from dispersion effects as described by equation 4.2 (almost

linearly).

Emulsion (-like) polymerization reactions monitored with on-line conductivity 37

volume fraction MMA (-)

CAl: addition of MMA..

solubility

Sty

.' .. ~~:::~:::C5:::~

-~~AA8···········-n-·

···v-··v············-····

(B): addition of Sty

••~OO(~.._-_.._....~.

600 ..J,-.,r-r-r'T-''rJ-r-T"'T"T""T""''T""T-r-r.,...,

0.000 0.002 0.004 0.006 0.008

volume fraction Sty (-)

700

1200

1100

S 1000~en..E>

900.~.~

i 800

u

0.03

-G.Q'-G-(

solubility

MMA

0.02

1200

1100

S 1000~en..E>

900.~.~-0

800.gI::0u

700

6000.00 0.01

[SOS]: <> 5.8 mM 0 8.1 mM X 8.4 mM I'V 10.5 mM 6. 11.5 mM----

Figure 4.3 Some data from figure 4.2, x-axis expanded. Transition period marked with 'A'

Once they are present, micelles can be swollen with monomer (solubilization)

until the total solubility of monomer in the water and the micelles is exceeded and

droplets are formed. In systems with an initial SDS concentration higher than the

'regular' CMC this solubilization effect will probably prevail over the effect of

enhanced micelle formation. Upon swelling of micelles the total surface area

increases and more surfactant is adsorbed from the aqueous phase onto the micelles,

leading to a decrease in the conductivity. The more micelles are present (high initial

SDS concentration), the stronger the initial conductivity decrease (at low volume

fractions of monomer) will be. The maximum amount of monomer solubilized will

depend both on the type of monomer (size, po!arity)64.65 and on the surfactant

concentration.

At the highest surfactant concentrations ([SDS]~ 8.1 mmol/L) in the case of

MMA a transition stage (marked 'A' in figure 4.3A) can be observed during which

the conductivity increases. This transition stage is likely to be the result of a

38 Chapter 4

reduction of the total surface area, which can be caused by coalescence of swollen

micelles and the first monomer droplets, which will be only a fraction larger than the

swollen micelles. This coalescence will be in favour of the larger species, and thus

the size distribution of micelles and droplets is altered to a distribution with a total

surface area that is smaller. A smaller surface area is in need of less surfactant for

stabilization and a release of surfactant will cause a slight increase in the conductivity

again as can be observed at the higher surfactant concentrations in Ugure 4.3,

especially in the case of the more hydrophilic monomer MMA (Ugure 4.3A,

[SDS]= 11.5 mmol/L).

Another explanation for the temporary conductivity increase was given by

Grimm et at. 66 and by Capek: 67 the increase was thought to be the result of monomer

absorption in the outer layer of the micelles, causing a release of counterions.

However, the latter explanation is not likely to be true because this effect is thought

to be a much more gradual change which cannot explain the discontinuity.

From the above it can be concluded that the addition of monomer has an

effect on the aggregation behaviour of surfactant molecules, which can be adequately

detected by conductivity measurements.

4.2.3 Combined effects of reaction components on the CMC

The determination of the combined effect of the reaction components on the

CMCapp is very problematic. 68,69 Especially at the reaction temperature (60°C) the

combined effect of monomer and initiator on the CMCapp is hard to determine and is

therefore seldom accounted for. 22,41 During the determination of these effects

polymerization reactions must be prevented. This is only possible if extreme

inhibition takes place, or if a non-reactive substitute is used, either for the monomer

or for the initiator. However, these substitutes may interact differently with the

surfactant molecules.

Emulsion (-like) polymerization reactions monitored with on-line conductivity 39

(A): monomer addition of Sty1700

118 9 10

[SDS] (mmoI/L)

7

(B): difference in conductivity140

120

100

]: 80

~ 60~

<I 40

20

00.01 0.02

"'0 0······0·····...........",,,,"'0

....EEE....E}••E}.-.•EI-....

~·"A... A

~"'A"'n

volume fraction Sty (-)

0.00

1200

1600

e~V)

.J 1500

.~

.~<J.g8 1300

[SDS]: 0 6.93 mM 0 7.33 mM IA 1O.8mM

'-_.......Figure 4.4 Figure A: effect of monomer addition (Sty) on the conductivity (Kx ) of an inhibitedreaction mixture containing a surfactant (SDS) and an initiator (SA CPA) at 60·C. The differencebetween the conductivities at the beginning of the addition of styrene and at the moment thatMaxwell's law (dashed lines) becomes valid is indicated as tlKx and is drawn as afunction of the SDSconcentration in figure B

Figure 4.4A shows the conductivity signal as function of the monomer

concentration for a reaction mixture that contained both an initiator and a surfactant.

Polymerization did not occur because of the presence of oxygen (an inhibitor). If one

compares figure 4.4A with ftgures 4.2B and 4.3B it becomes clear that the change

in conductivity as a result of the addition of Sty to an SDS solution is not influenced

qualitatively by the presence of an initiator (SACPA), at least not if polymerization is

prevented through inhibition. However, compared to experiments without SACPA the

volume fraction at which Maxwell's law becomes valid, and consequently the total

amount of monomer solubilized, has shifted to higher values. This can only be the

result of an increase in total micellar volume, which means that either the number of

micelles or the number of surfactant molecules per micelle has increased. In either

case the apparent CMC is lowered by the presence of SACPA.

40 Chapter 4

In order to minimize the number of micelles present in a reaction mixture (as

necessary e.g. in encapsulation reactions), one should keep the surfactant

concentration in this mixture (also containing the initiator) below the value at which

equation 4.2 just becomes valid for the entire range of monomer volume fractions

(thus below CMCapp). At this surfactant concentration no micelles can be formed

regardless the concentration of the added monomer.64 This means that A~ (f'IgUre

4.4A), which is defined as the difference between the conductivity at the start of the

monomer addition and the conductivity at the moment that Maxwell's law starts to

apply, should be zero. In f'Igure 4.4 this becomes true at an SDS concentration of

approximately 7 mmol/L, which therefore can be regarded as the apparent CMC of

SDS in the presence of monomer and initiator. This can be explained as follows. The

linear part in f'Igure 4.4B is comparable to the linear part of a conductometric

titration curve like the one in f'Igure 4.1 (above the CMC). At one volume fraction

of Sty and at different SDS concentrations the conductivity values in f'Igure 4.4B

would actually form a conductometric titration like the one shown in f'IgUre 4.1.

It can be concluded that the method introduced here is an alternative way to

determine the CMC in the presence of initiator and monomer (for MMA and Sty the

influence of the monomer alone on the CMCapp can be determined from f'Igure 4.2 in

a similar way rendering approximate values of 5 mmol/L and 7.5 mmol/L,

respectively). Furthermore, this method probably is not hampered by large

experimental inaccuracies caused by impurities or by the presence of monomer,

unlike e.g. surface tension measurements, provided that no polymerization reaction

takes place. An inhibitor or a non-reactive substitute for either the monomer or the

initiator will therefore be required.

In an encapsulation reaction the hydrophobic pigment has an effect on the

CMCapp as well, although opposite to that of the monomer and the initiator.

Surfactant molecules are adsorbed by the pigment particles, leading to an increase in

CMCapp ' Unfortunately, the amount of adsorbed surfactant can only be determined at

room temperature, because some of the steps in the analytical procedure, especially

Emulsion (-like) polymerization reactions monitored with on-line conductivity 41

the separation of the pigment, could not be performed under accurate temperature

control (section 3.2). An impression of the combined effect of initiator and pigment

on the apparent CMC can be obtained from equation 4.3.

CMC::; = CMC."" + Surf_ 4.3

with: CMc::,; =CMCappSUrfads

the apparent CMC corrected for surfactant adsorptionthe apparent CMC as it is calculated according to equation 4.1the amount of surfactant adsorbed by the amount of pigment thatwas added to 1 L of water.

In equation 4.3, neither the effect of the counterions on the surfactant

adsorption, nor the fact that the amount of adsorbed surfactant has been determined

at a different temperature has been taken into account. Furthermore, it is likely that

monomer, beside influencing the number or size of the free micelles, also has an

influence on surfactant adsorption ('monomer enhanced surfactant adsorption'), for

example by acting as a co-surfactant as was suggested earlier.56•57 Because the

pigment surface is hydrophobic it is likely that the monomer concentration at this

surface is higher than that in the aqueous phase. This means that the 'monomer

enhanced surfactant adsorption' will be important especially in the case of

encapsulation reactions.

In principle, the combined effect of all three components (initiator, monomer

and pigment) can be determined according to the method of figure 4.4. However,

the reaction mixture may not be sufficiently homogeneous at low surfactant

concentrations as a result of (limited) pigment agglomeration. Hydrophobic monomer

droplets (Sty) will enhance this problem. Although applicable, the method of figure

4.4 will therefore be somewhat less accurate in the case of pigment dispersions,

because an extrapolation from higher SDS concentrations has to be made.

n O~4

4.3 Emulsion(-Iike) polymerization reactions and on-line conductivity

measurements

4.3.1 Interpretation of on-line conductivity measurements

It has been shown above that the conductivity of a reaction mixture (KJ can

be influenced by a number of factors. In the case of an emulsion polymerization,

before the reaction actually starts, the initiator, the surfactant and the monomer

determine the initial value of Kx• The surfactant and an ionic initiator contribute most

to the conductivity. Because the monomer and the counterions of the initiator

enhance micellization, the overall conductivity will be lower than that of the sum of

the solutions of the separate components. 69

During the reaction a number of· events can influence the conductivity

behaviour (see figure 4.7 also). Dissociation of the initiator leads to an increase in

Kx because the formed radicals are smaller than the molecule they originate from. On

the other hand: the charge of the radicals is smaller than that of the original

molecule, which leads to a conductivity decrease. As was determined experimentally

(without the presence of monomer), the effect of initiator decomposition on Kx is

negligible, which also is a result of the half-life of the initiators used (approximately

30 to 40 hours at 60°C, both for SACPA and for SPS): during the time of reaction

(up to 3 hours) no more than 3 to 10% of the initiator is dissociated.

The immobilization of the small charged initiator fragments which initiate a

polymer chain will most certainly give a decrease in Kx• However, even when 100%

initiator efficiency is assumed this still only will result in a change in Kx from 12 to

40 I-tS/cm.

The strongest contribution to changes in the conductivity will be given by the

surfactant molecules. It was already explained that micellization causes a decrease in

the conductivity. When surfactant is adsorbed onto surfaces a similar decrease in Kx

is expected, because adsorption, like micellization, reduces the mobility of the

surfactant molecules. Especially during ab initio emulsion polymerization reactions

Emulsion (-like) polymerization reactions monitored with on-line conductivity 43

much surface area is created, because of the formation and growth of a large number

of latex particles. Since these particles have to be stabilized, much surfactant will be

withdrawn from the aqueous phase, leading to a strong decrease in the conductivity.

Other events that can cause a change in Kx are the release of counter­

ions,64,66,67,70 phase inversion/1 coagulation or shrinkage of particles, or the disappea­

rance of monomer droplets.56,57 All four phenomena cause an increase in Kx• The

release of counterions was described e.g. by Eiworthy et ai. 64 for the swelling of

micelles, but is unlikely to take place during an emulsion polymerization reaction

(see section 4.3.2). Phase inversion of a water-in-oil emulsion to an oil-in-water

emulsion was described by Jain and Pii171Ul71, but is not expected in these systems

either.

Coagulation, however, can take place, especially in reactions where titanium

dioxide is present, and where many new polymer particles are formed during the

reaction. The surface area increase can cause a surfactant deficiency resulting in

(hetero) coagulation which, if massive, causes the release of surfactant. 41,56,57

An increase in Kx due to the disappearance of monomer (droplets) is also

likely to occur during a polymerization reaction. Beside a very small increase in Kx

caused by the decrease of the surface area of the monomer droplets, the main effect

will be the influence of the aqueous monomer concentration on the CMC. In

principle the curves of fIgures 4.2 and 4.3 will be followed, but in opposite

direction: from 'high' volume fractions of monomer to low fractions.

4.3.2 Batch reactions with MMA

The changes in conductivity, as they take place during a batch emulsion

polymerization of MMA are shown in figure 4.5. In figure A the conductivity and

the conversion are drawn as a function of time, while in figure B the conductivity is

drawn as a function of the conversion. The conversion was determined gravimetri­

cally. The conductivity curve in figure 4.5A can be divided into four regions, of

which the latter three can be related to the emulsion polymerization intervals I

through III (section 2.3), as will be explained below:

44 Chapter 4

~transition to interval ill

at c nversion of 0.238

0.0 0.2 0.4 0.6 0.8 1.0

1000

1050

~::5

950.~

"B.g8 900u

900

;8'1'

.7';;o

.6·~II)

.51;

.48CiI.3 8

'-'.{~-r,--.LIJ 2'P. ~

O. 14::

.0850 +-,ri-!-,l,r-'T""'-r--1,....-r..,...,-r-T"'+-Q.1

0102030405060

950

1000

1150

1200

~ 1100

~::5 1050

time (min) fractional conversion (-)

-0-conductivity (left axis)

-D-fractional conversion (right axis)I o conductivity

--(jib order polynomal fit

Figure 4.5 Conductivity (q and conversion (q as a function of time (figure A) and theconductivity (0) as a junction of conversion (figure B) for a batch emulsion polymerization ofMMA at60°C. The theoretical moment of monomer droplet disappearance (transition to Smith-Ewart intervalIII) is indicated by the arrows. [SDS]= 8.05 mmol/L. [SACPA]= 1.30 mmol/L and [MMA]= 0.454M (ep = 0.046)

(0) a decrease in the conductivity (reactant addition stage: no reaction),

(I, II) a decrease in the conductivity (interval I and sometimes II),

(IlIa) an increase in the conductivity (the beginning of interval lID,

(IIIb) a decrease in the conductivity (the end of interval III).

These changes can be explained as follows:

ad ree:ion (0):

In this region reaction components are added and no reaction takes place.

Kx decreases if monomer is added to a reaction mixture already containing the

surfactant (SDS) and the initiator (SACPA). This decrease is caused by the

solubilization, micellization and dispersion effects described in section 4.2.

Emulsion (-like) polymerization reactions monitored with on-line conductivity 45

In some reactions (see e.g. figure 4.9A) region (0) is marked by an

increase in Kx which is caused by the fact that there the initiator was added to

a reaction mixture already containing the surfactant (SDS) and the monomer

(Sty).

After the addition of either the monomer or the initiator sometimes

inhibition occurs. During this period of inhibition Kx remains constant, while

no conversion is observed either (see e.g. figures 4.8 and 4.9).

ad region (I,m:

In conductivity region (I,ll) the corresponding Smith-Ewart intervals I and

II can be present. The moment the reaction starts, particles are formed which

will grow throughout the intervals I and II. Both the formation and the growth

of the particles will cause an increase in surface area. Surfactant is adsorbed

from the aqueous phase onto this newly formed surface to ensure stability of

the particles. During this adsorption process the surfactant molecules loose

mobility leading to a decrease in conductivity, as was explained in section

4.3.1. Compared to this decrease in Kx caused by surfactant adsorption the

increase in the conductivity caused by the shrinking of the monomer droplets

is negligible. This minor increase follows the part of the curves in figures 4.2

and 4.3 that is described by Maxwell's law, but in opposite direction: from

'high' volume fractions of monomer to low volume fractions.

ad region <rna):

The conductivity signal in the regions (IlIa) and (llIb) (corresponding to

Smith-Ewart interval III) is the most difficult to explain. However, it is

striking that the onset of region (IlIa) coincides with the beginning of Smith­

Ewart interval III where the monomer droplets just have disappeared (see

section 2.3; for the calculation of the transition interval II~III see appendix

4.1).56,57,72 This was also observed by Fontenot et al.. 7]

46

FwreA

1.40

1.30

2 1.20zo.*....b 1.10.....

1.00

90

80

70

60! 8'50 B '0

Q) 140.~0""d ..

30..!:! ~.~

20 ~A

10

0

0.16

0.12

0.08

0.04

0.00

FWreB

)~. ~, .\! ,.: interval ill~-'

Chapter 4

2800

26006'5

2400 ~~

82200~

'"2000 ~

1800o

0.0 0.2 0.4 0.6 0.8 1.0

fractional conversion (-)--6-mnnber of particles (left axis)

-<>- swollen diameter (right axis)

---------- conductivity signal (qualitative)

0.0 0.2 0.4 0.6 0.8 1.0

fractional conversion (-)

-D-IMlaq (left axis)

-o--total surface area (right axis)

----------conductivity signal (qualitative)

Figure 4.6 The number of particles (left axis,.1) and the swollen diameter (right axis, 0) as ajunction of the conversion (figure A). The concentration of monomer (MMA) in the aqueous phase(left axis, 0 and the total surface area (right axis, 0) as afunction of the conversion (figure B). Theparticle size at the beginning of the reaction (figure A) is an estimote for the size of swollen micelles.The dashed curves represent the conductivity signal (qualitatively), while the horizontal arrowsindicate interval III. Concentrations are as in figure 4.5. Temperature: 60°C

Three effects would explain the increase in Kx : (1) massive coagulation,

(2) the release of surfactant molecules and/or counterions from the particle

surface due to the shrinking of particles, or (3) a decreasing monomer

concentration in the aqueous phase.

Looking at figure 4.6 A and B one can see clearly that the increase in Kx

cannot be caused by either coagulation or by shrinking of the particles.

Coagulation would be accompanied by an increase in the (swollen) particle

diameter, and therefor by a strong decrease in the number of particles and a

decrease in the total surface area. From figure 4.6A it can be seen that

particle formation for MMA (Smith-Ewart interval l) has not yet ended while

entering interval III. Figure 4.6B clearly shows that the total surface area

Emulsion (-like) polymerization reactions monitored with on-line conductivity 47

constantly increases as well. The temporary shrinkage of the swollen particles

is also too small in order to explain the conductivity increase (fIgure 4.6A).

An attempt was made to measure the concentration of (free) Na+ ions in

the aqueous phase on-line. This was done with a sodium specific electrode and

a calomel electrode as reference cell. However, the results are not very clear.

The concentration of free Na+-ions seems to increase somewhat throughout

the reaction. This increase also takes place in conductivity region (I,ll), which

seems illogical, because here the strongest decrease in Kx is observed.

Therefore it must be concluded that this effect of sodium ion migration is

negligible.

The only explanation for the conductivity increase that remains is the

decrease of the monomer concentration in the aqueous phase ([M]aq, appendix

4.2). Indeed, the increase in K. can be the result of effects opposite to those

in region (0). There (region (0» the added monomer interacted with the

surfactant molecules causing a decrease in the CMC. The enhanced

micellization and solubilization led to a decrease in the mobility and,

consequently, to a decrease in Kx• Because the monomer concentration in the

aqueous phase decreases in region (IlIa), micelles (if present) will break 'up

again causing an increase in mobility and in Kx • Therefore, it can be expected

that in region IlIa those parts of the curves of fIgures 4.2 and 4.3 will be

followed that precede the part described by Maxwell's law, but from high to

low volume fractions of monomer. In addition to this effect, in interval III of

emulsion polymerization the surface coverage with surfactant of the polymer

particles can also be influenced by the drastic decrease of the monomer

concentration in the polymer particles as well as in the aqueous phase, Jeading

to a release of surfactant.

Supportive evidence for the presence of micelles in interval III is found in

the fact that the number of particles increases continuously. This also means

that interval II (a constant number of particles) is never reached in the case of

MMA under the reaction conditions as described in fIgure 4.5. Micelles can

still be present at the beginning of interval III, because MMA is partially

48 Chapter 4

initiated through homogeneous nucleation. In other words: the point where

droplets disappear can be reached before the nucleation stage has ceased.

ad region <mb):

The conductivity decrease in region (IIIb) can be explained by the increase

in the total surface area as it takes place during the entire reaction. In region

(IlIa) the decrease in [MJaq dominated the change in Kx; in region (IIIb) this

decrease in [MJaq, although still taking place, becomes less important than the

surface area increase. Because of this surface area increase, surfactant is

continuously adsorbed from the aqueous phase, leading, of course, to a

decrease in Kx • The effect of shrinkage of latex particles, if it occurs at all, is

too small to compensate for the conductivity decrease caused by the surface

area increase. However, the behaviour in interval III depends on the monomer

used, as will be discussed in section 4.3.3.

The most important factors determining the appearance of a conductivity signal are

summarized in figure 4.7.

(0) (I,ll) (rna) (nIb)

initiator decomposition

change in monomer concen­tration in the aqueous Dhase_....i-----~-

release of surfactant due to coagu-1 ~ lation or shrinking of particles

._.-.-.-._~_._.•._._._._.-~::: '-'-'-'.'_:'::.:'_'_'_'_'_'_'_~'" surfaetan! adsomtion due

~ to increase in surface area

>­<.>::>"0co<)

c

CD."cca.<::<)

t---+----+---...;...---......;~I immobilization of radicals

time

Figure 4.7 The most important factors determining the appearance of a conductivity signal of apolymerization reaction (jor the e;r;perimental signal see for instance figure 4.5)

Emulsion (-like) polymerization reactions monitored with on-line conductivity 49

Table 4.1 The effect of the monomer concentration and the SDS concentration on the increase inconductivity (MQ in region (II!') and the decrease in region (IIi') of seeded emulsion polymerizationswith MMA at 60°C. Seed: 15 g PMMA (dp''' 64 nm) per litre water; [SACPAJ= 1.30 mmol/L

i) Ill' .1$111< .........................

••• <...

!...)II

l )

40 8.2 24 40 98

60 9.8 58 56 96

80 9.8 128 68 119

80 3.5 89 25 130

120 15.8 91 128 -

150 19.6 182 230 -

The conductivity increase in region (lIla) (dKx.nrJ and the decrease in region

(IIIb) (dKx.Illb) depend on the reaction conditions. Seeded reactions with MMA (table

4.1) have shown that both dKx.IJIa and dKx.Illb become more pronounced if the

monomer and/or the surfactant concentration are increased. The reaction mixtures

used contained 15 g of seed polymer (PMMA) per litre water.

Only the reactions with 120 and 150 g of monomer (and high SDS

concentrations!) showed a decrease in Kx in region (I,ll) comparable to that in fIgUre

4.5. The other reactions may have been started in interval III of emulsion

polymerization (no monomer droplets present), although it can be calculated with the

use of literature values for the maximum swellability of PMMA with MMA (c~at=

6.3 mol/L swollen latex)52 and for the water solubility of MMA (150 mmol/L)52 that

monomer droplets must be present if more than approximately 45 g of MMA is

present. More accurate values for the swellability and the water solubility can be

obtained by determining them under the exact reaction conditions. Noel et aI. 74

describe how the water solubility can be derived from a conductivity curve like the

one in fIgure 4.5. In principle this method can be extended to the determination of

swellability values. However, the determination of exact values for the swellability of

polymer with monomer, or for the water solubility lies beyond the scope of this

investigation.

50 Chapter 4

0.0 0.2 0.4 0.6 0.8 1.0

1000

1100

1150

1050.~

.~

"8o()

20o

1.1

M 1.00.90.8c /""',

~

0.7 = e0.6·~ c;5

t J0.5 E;0.4 80.3 1

.u:'1r+--+-------L '.;:10.2 ~

0.1 <l::

0.0

900 ..J....-.---'...,....-.L.,-----'-,.-----rlf-.".+-0.140 10020

1000

1200

1300

]::3;.

1100.~.~

I()

time (min) fractional conversion (-)

-o-conductivity (left axis)--a-fractional conversion (right axis)

o fractional conversion

--6'h order polynomal fit

Figure 4.8 Conductivity (q and conversion (q as a function of time (figure A) and theconductivity (q as a function of conversion (figure B) for a batch encapsulation reaction with MMAat 60°C. The theoretical rrwment of rrwnomer droplet disappearance (transition to Smith-Ewartinterval III) is indicated by the arrows. Pigment: RLK-CAlO, 55.6 gIL water; [SDS]= 9.41mrrwIIL;[SACPA]= 1.30 mrrwl/L; MMA, 0.454 M (cp= 0.046)

On-line conductivity measurements during encapsulation reactions

Figure 4.8 shows the changes in conductivity and conversion during an

encapsulation reaction (RLK-CAlO: 55.6 giL, [SDS]= 9.41 mmol/L). It is obvious

that there are hardly any differences with a 'regular' emulsion polymerization. This

was not expected either, because in principle an encapsulation reaction is very similar

to a regular emulsion polymerization reaction. 41 It can be expected only at high

encapsulation efficiencies that the decrease in Kx is less strong than in similar

emulsion polymerization reactions, because then the surface area increase is smaller.

However, in the reaction of figure 4.8 the efficiency was very low (11= 3 wt%).

The only observable difference with figure 4.5 is a short period of inhibition in

conductivity region (0).

Emulsion (-like) polymerization reactions monitored with on-line conductivity 51

FpeA Fi~eB1400

(0) 1(1,11) ~1lI.)1.1 1400

.. I··· 1.01300 I . 0.9 1300

]:0.8 2

E1200 = 12000.7 .S:: ~

~ '" en0.6

....Q) J

.~ 1100 .5;; ..... 11000 .-=:

.~C)

.~1:) 0.4 ca 1:).g = .g= 1000 0.3.g § 10000 ~(.) 0.2 <l:: C)

900 0.1 900

0.0800 -0.1 800

0 50 100 280300 0.0 0.2 0.4 0.6 0.8 1.0

time (min) fractional conversion (-)

-o-conductivity (left axis) I 0 conductivity I-{}-fractional conversion (right axis) --(ith order polynomal fit

Figure 4.9 Conductivity (q and conversion (q as a function of time (figure A) and theconductivity (q as a function of conversion (figure B) for a batch emulsion polymerization ofSty. Thetheoretical moment of monomer droplet disappearance (transition to Smith-Ewart interval III) is indi­cated by the arrows. [SDS]= 8.42 mmol/L; [Sty]= 0.419 mmol/L (cp= 0.046); [SACPA]= 1.3mmol/L; temperature: 60°C

4.3.3 Batch reactions with styrene

Batch emulsion polymerization reactions with styrene have a conductivity

signal comparable to that of reactions with MMA. However, reactions with styrene

show only three conductivity regions as can be seen in fIgure 4.9A and B. Indeed,

region (0) and region (I,ll) still are present, but at a conversion of approximately 0.4

a behaviour different from that of MMA can be observed. Region (IlIa) in a reaction

with Sty is marked by an increase in conductivity, like in the case of MMA, but the

increase is relatively small. The fact that this increase in conductivity is not followed

by a region of a decrease in Kx ' region (IIIb) for MMA, is the most striking

difference between reactions with MMA and reactions with Sty.

52 Chapter 4

Three important factors are responsible for the difference in the conductivity

signal of Sty and MMA. Firstly, the water solubility of Stf9 is much smaller than

that of MMA52 (3 mmol/L instead of 150 mmol/L). In the figures 4.2 and 4.3 it can

be seen that the addition of Sty to a solution with a low SDS concentration ([SDS]=

8.1 mmol/L) has a different effect on the conductivity than the addition of MMA: in

case of the latter the decrease in Kx is stronger (until the solubility is exceeded) and a

transition stage can be observed which is not present with Sty.

If no micelles are present at the beginning of interval III the decrease of the

monomer concentration in the aqueous phase will at most have a slight influence on

the adsorbed amount of surfactant on the particles, but breakup of micelles

accompanied by a large increase in Kx will not occur. The presence of micelles in

interval III is unlikely in the case of Sty, which is a second factor causing the

conductivity behaviour to be different from that of MMA. Unlike MMA, Sty is

primarily subject to micellar nucleation. Styrene oligomers become water insoluble at

a chain length of 2 or 3 monomer units75 and will be captured by micelles or

particles almost instantaneously. Besides, the surface area covered by one surfactant

(SDS) molecule59 is smaller for polystyrene (0.5 nm2) than for (P)MMA (0.79 nm2

).

This means that polystyrene (PS) will withdraw more surfactant from the aqueous

phase in order to maintain a stable latex. Furthermore, in the case of styrene

monomer droplets are present for a longer time, for which reason more surfactant

will be extracted before interval III is entered as well. In the case of Sty interval III

starts at a conversion between 0.4 and 0.5; in the case of MMA droplets disappear at

a conversion of approximately 0.25 (appendix 4.1).

The third factor causing a different conductivity behaviour, is the change in

the total surface area during the reaction. Figure 4.10A and B clearly show that the

total surface area decreases slightly towards the end of the reaction in the case of

Sty, whereas it constantly increased in the case of MMA (figure 4.6B). The decrease

in surface area at most can cause a slight increase in Kx ' but only if surfactant is

released as a result of the shrinking or coagulation of the particles. This is the most

striking difference with MMA, explaining the absence of a decrease in the

conductivity in the case of styrene (region (IIIb) in reactions with MMA).

Emulsion (-like) polymerization reactions monitored with on-line conductivity 53

7.00

6.50

2zo. 6.00

*'"b- 5.50

5.00

120110

10090",",

80 !70~

60~50 :e40~30 .~

20 c..

10

+-'-~rr-+-r..,...,--r-T""T'"+O0.0 0.2 0.4 0.6 0.8 1.0

fractional conversion (-)

3.0

2.5

;J' 2.0:a~ 1.5'-"

g'

~ 1.0

0.5

FimeB

0.0 0.2 0.4 0.6 0.8 1.0

fractional conversion (-)

2400

2200N'5

2000 '"~u

1800~

~1600'3

B

1400

----6-munber of particles (left axis)

~wollendiameter (right axis)

-------- conductivity signal (qualitative)

-O-IMlaq (left axis)

----o---totaI surface area (right axis)---------conductivity signal (qualitative)

Figure 4.10 Batch emulsion polymerization of Sty, conditions as in figure 4.9. Figure A: number ofparticles (left axis, .4) and the swollen diameter (right axis, 0). Figure B: the monomerconcentration in the aqueous phase ([MJaq, 0, left axis) and the total surface area (0, right axis). Theconductivity signal is (qualitatively) represented by the dashed curve

The conductivity behaviour in region (IlIa) depends on both the surfactant and

the monomer concentration, as observed earlier in seeded reactions with PMMA.

Clearly, during ab initio reactions with styrene interval I has ended before interval

III starts and the disappearance of the monomer droplets from the aqueous phase no

longer has such a pronounced effect on the conductivity as in the case of PMMA: the

conductivity increase is negligible until a conversion of 0.75 is exceeded. The

conductivity increase observed thereafter may be the result of limited coagulation or

shrinkage of particles, accompanied by some release of surfactant.

High surfactant concentrations will lead to a somewhat stronger increase in K.

at the beginning of region (IlIa), as shown in figure 4.11. It is possible that here the

monomer-surfactant interaction plays a role. It is unlikely that in the case of Sty

micelles are still present at the beginning of interval III of emulsion polymerization,

as was mentioned earlier in this section, because for this monomer micellar

54 Chapter 4

nucleation is the primary nucleation mechanism, unlike MMA, which primarily

undergoes homogeneous nucleation. Therefore, with styrene a break-up of micelles is

not expected, but it is likely that the presence of monomer in the aqueous phase also

influences the adsorption of surfactant at the polymer surface. Disappearance of that

monomer may lead to a decrease in the amount of surfactant adsorbed at the surface

and thus to a release of surfactant from the surface. If the overall surfactant

concentration is higher of course the surface coverage with surfactant will be higher:

more surfactant can be released in this case, leading to a stronger increase in the

conductivity.

17001 11600'high' [SDS]

....... 15005-- 1100~'-'

1000a....> 900...=(.).g 800 'low' [SDS] and [Sty]§(.) 700

6000.0 0.2 0.4 0.6 0.8 1.0

fractional conversion (-)o [SDS)= 8.1 mM; [Sty)= 0.455 M 0 [SDS)= 8.1 mM; [Sty)= 0.904 M

I::i. [SDS)= 24.3 mM; [Sty)= 0.455 M __6th order polynomal fit

Figure 4.11 Effect of[SDSjand [styj on the conductivity signal ofa batch emulsion polymerization ofstyrene. [SACPAj= 1.3 mmol/L; temperature: 60·C

Finally, also in the case of batch experiments with styrene the concentration of

free sodium ions during the emulsion polymerization reaction was investigated with a

sodium specific electrode and a calomel reference cell. The signal indicated that only

minor migration of sodium ions took place during the reaction. Like in the case of

MMA an increase in the concentration of mobile Na+ ions was found over the entire

Emulsion (-like) polymerization reactions monitored with on-line conductivity 55

conversion range, also in interval I and II (conductivity region (I,ll)), during which

the conductivity signal shows the strongest decrease. However, with styrene the

increase in the concentration of mobile Na+ ions seemed to be a little bit stronger

during interval III (conductivity region IlIa) as compared with MMA, especially at

higher surfactant concentrations ([SDS] = 24.3 instead of 8.05 mmol/L). Still, the

effect seems to be too little to cause a change in the conductivity signal.

4.3.4 Semi-continuous reactions with MMA

The effect the monomer has on micelle formation (section 4.2.2) accounts for

more than the sudden increase in Kx during region (IlIa) of batch emulsion

polymerization and encapsulation reactions. 57 It also explains why low monomer

concentrations or actually 'monomer starved' conditions were found to offer great

advantages in trying to encapsulate modified Ti02• In figure 4.12 is shown how both

the polymer content of the pigment and the encapsulation efficiency were found to

depend on the monomer concentration (Caris)41. A decrease in the monomer

concentration also resulted in a more stable reaction mixture.

5 25

,-..bO ,-..

ba ~

,g ~'-"a ~

~ 50 '<:)0

~....;0Q.

00.15

[MMA] (moJjL)

Figure 4.12 Effect of monomer concentration on the polymer content of the pigment and on theefficiency ofa batch encapsulation reaction (qualitatively reproduced from reference 41)

56 Chapter 4

These effects were attributed to the fact that no SDS was needed for the

stabilization of monomer droplets, while at the same time the initially formed layer

of surface polymer was supposed to be sufficiently swollen with monomer, thus

enhancing further polymerization at the pigment surface. Bridging flocculation was

thought to occur after a certain amount of (surface) polymer had been formed. 41 The

monomer present after this flocculation had taken place was thought to form mainly

free polymer (in batch experiments). The influence of monomer on the CMCapp was

not taken into account.

The amount of surfactant needed for the stabilization of monomer droplets, as

can be calculated, is relatively small, because the droplets do not represent a large

surface area. As was shown in section 4.2 the addition of initiator and, importantly,

monomer, can cause a decrease in CMC, a fact which is of greater importance,

because the free micelles compete with the pigment particles as sites for

polymerization. Correction of the SDS concentration for the amount of initiator and

monomer is only partly possible, as explained in section 4.2.3, which makes it

essential to minimize the concentration of the latter two components.

Semi-continuous addition of monomer throughout the reaction, at very low

rates (monomer starved conditions), offers the opportunity to stay close to the

maximum of the CMC. By adding the monomer at a rate below the (maximal

attainable) rate of reaction the lowering of the CMC due to monomer addition will be

negligible because excess monomer will not be present. In this case SACPA (or any

other initiator) is the only component present at the beginning of the reaction capable

of lowering the CMC. This decrease can be accounted for in accordance with

eqnation 4.1. Following this strategy the amount of free polymer should be kept to a

minimum.

Emulsion (-like) polymerization reactions monitored with on-line conductivity 57

1250

fm: encapsulation reaction1300 1.1

1.00.9

fA): emulsion polymerization1300 1.1

1.00.9

1250

1200 1200 ..--0.82 0.8 ....:...

a 1150 0.7 § § 1150d

0.7.S~ 0.6·~ Vi '"~ 1100 -":;1100 0.6 ~

0.5 § d» .~ 1050

0.58.~ 1050 0.4~ 0.4 c;l·fi '.;:l

~ u §.g 1000 0.3 0 .g 1000 0.3 ..;:l'.;:l S ald 0.2 ~0 950 u 950 0.2<l::u

0.1 <l:: 0.1900 0.0 900 0.0850 -0.1 850 -0.1

0 50 100 150200 250 300 0 50 100 150 200 250 300

time (min) time (min)-0- conductivity (left axis)----b- instantaneous conversion (right axis)

-0- overall conversion (right axis)

Figure 4.13 Semi-continuous emulsion polymerization (figure A) and encapsulation reactions (figureB) with MMA. Temperature: 60°C; [SACPAJ= 1.3 mrrwl/L; MMA was added at a rate of 0.2 mL/minto a final concentration of 0.454 M. Emulsion polymerization: [SDSJ= 8.05 mrrwl/L; encapsulationreaction: [SDSJ= 9.4 mrrwl/L; 55.6 g RLK-CAJO was added to one litre of distilled water

In fIgure 4.13 the conductivity and conversion versus time plots are displayed,

both of a semi-continuous emulsion polymerization reaction (fIgure 4.13A) and of a

semi-continuous encapsulation reaction (fIgure 4.13B). The latter has a higher SDS

concentration to compensate for the adsorption of SDS by the pigment. The monomer

used was MMA, which was added at a rate of 0.2 ml/min. The instantaneous

conversion (xmsJ is based on the amount of monomer added until the moment of

sampling, while the overall conversion (xaver) is based on the total amount of

monomer to be added during the entire reaction.

The conductivity curve of a semi-continuous reaction (fIgure 4.13) differs

from that of a batch reaction (fIgure 4.5) but, again, there is no essential difference

between a 'regular' emulsion polymerization reaction (fIgure 4.13A) and an

encapsulation reaction (fIgure 4.13B). Again only part of the reaction is an

58 Chapter 4

encapsulation process, because 'normal' emulsion polymerization takes place

simultaneously ('11 = 9 wt%). Of course the decrease in K.: in a semi-continuous

reaction is much more gradual than in a batch reaction (figure 4.5), simply because

in the former case the overall rate of reaction (which is a function of the monomer

concentration) and, consequently, the rate of surface area increase are much lower.

Mark that, unlike in batch reactions, no regions (IlIa) and (IIIb) can be distinguished

in the conductivity signal of semi-continuous reactions. This is the best proof that no

build-up of monomer has taken place, contrary to the reaction displayed in tlgure

4.14. In the initial phase of the latter reaction inhibition has caused an accumulation

of monomer, which is reflected in a sudden increase in K.: when the conversion

'catches up' with the monomer addition. After a maximum in the conductivity the

monomer starved conditions are met (again).

1300

1250

W1200

---~ 1150'-'».-=: 1100.~u 1050.gt:0 1000u

950

moment of initiation; approxi-mately 2lmL of MMA added,'------ --. "I '

100

time (min)

250

-:-O--conductivity (left axis)

l:!. instantaneous conversion (right axis)

-Q-overall conversion (right axis)

------- amount of monomer added (qualitatively)

Figure 4.14 Build-up of monomer during semi-continuous emulsion polymeriwtion reactions leads tobatch-like conditions

Emulsion (-like) polymerization reactions monitored with on-line conductivity 59

The semi-continuous encapsulation reactions were checked for coagulation by

means of dark field microscopf5 and disc centrifuge experiments. The reaction

mixture remained stable throughout the reaction, although some minor agglomerates

were observed, in general not larger than two or three primary particles. Not only

the coagulation characteristics of the encapsulation reaction were improved, the

polymer content of the pigment and the encapsulation efficiency were higher as well,

like expected: changing the monomer addition method from batch to semi-continuous

under otherwise unchanged conditions resulted in a fivefold increase in the polymer

content of the encapsulated pigment: from 16 mg polymer (11 = 2 to 3 wt%) to 80

mg polymer per gram Ti02 (11= 9 wt%).57

Two factors are responsible for the improved encapsulation success:

1) due to the low monomer concentration the number of free micelles is kept

to a minimum in semi-continuous reactions, as was mentioned earlier in

this section, and

2) because of the lower monomer concentration in the aqueous phase less

oligomers are formed and therefore the chance of secondary

(homogeneous) nucleation is lowered.

The second reason seems contradictory, because oligomeric radicals normally

provide electrostatic stability (like in emulsifier free emulsion polymerizations), but

can be explained in terms of initiator efficiency and monomer concentration. MMA,

being a moderately water soluble monomer, is initiated through homogeneous

nucleation and has a high initiator efficiency.52 The rate of nucleation is a function of

the monomer concentration. If the latter is high, many oligomeric radicals will be

formed: more than can be captured by the (pigment) surface present, unless the

pigment concentration is very high (section 6.5). The excess oligomers can co­

precipitate to form (instable) precursor particles, as mentioned in section 2.3, which

eventually will become free polymer particles. If the number of oligomers is

restricted by working under monomer starved conditions the chance on secondary

nucleation is lowered.

60 Chapter 4

4.3.5 Semi-continuous reactions with styrene

Styrene is a much more hydrophobic monomer than MMA, while the initiator

efficiency is substantially lower. 76 Because the water solubility of Sty is so low, the

monomer addition rate during semi-continuous reactions is more critical than in the

case of MMA. In figures 4.15 the conductivity and conversion versus time plots are

displayed, both of a regular emulsion polymerization (figure 4.15A) and of an

encapsulation reaction (figure 4.15B). The initial monomer addition rate was 0.05

mL/min to prevent the build-up of monomer and the formation of monomer droplets.

Once particles (regular emulsion polymerization) or surface polymer (encapsulation

reactions) have been formed, the polymer present can be swollen with monomer and

the addition rate becomes less critical. Therefore, in order to speed up the reaction

the addition rate was increased to 0.2 mL/min after 90 minutes.

1400

fB): encapsulation reaction

0.05 mLImin ---> 0.2 mL/min1500 t

o 50 100 150200250300350

1000

ien 1300-3~

."l::

.~ 1200u.g8 1100u

o

0.820.780.6·~

0 51=. 0

O.4~0.380.2·~0.14::

0.0

+-..,...-....--'r--r--r--r--r---r-r -0.1100 200 300 400

fA): emulsion polymerization1400 0.05 mLImin > 0.2 mL/min 1.1

t 1.00.9

1200

600

]:~ 1000o:~I 800

u

time (mr=in~) --,::time (min)

-0- conductivity (left axis)

-fr- instantaneous conversion (right axis)

~-O- overall conversion (right axis)

Figure 4.15 Semi-continuous emulsion polymerization and encapsulation reaction with Sty.Coagulation during encapsulation reactions will lead to oscillations in the conductivity signal

Emulsion (-like) polymerization reactions monitored with on-line conductivity 61

As one can see in figure 4.15A the increase in the monomer addition rate is

marked by a temporary backsliding of the instantaneous conversion and by a stronger

decrease in the conductivity because now the overall reaction rate and the rate of

formation of new surface area increases. If the monomer is added semi-continuously

there are no (qualitative) differences between the conductivity behaviour of an

emulsion polymerization with Sty or a reaction with MMA. However, the

encapsulation reaction with Sty shows a different conductivity behaviour, as can be

seen in figure 4.15B. The conductivity signal is less stable and oscillations can be

observed. These oscillations are the result of severe coagulation, the latter of which

led to non-representative samples for determination of the conversion. The final latex

contained a large amount of coagulated material (about 50 wt% of the solid

materials).

The relation between the conductivity signal and the coagulation can be

explained as follows. During the process of massive coagulation surfactant is released

from the pigment surface leading to an increase in Kx • As the reaction continues this

surfactant is re-adsorbed leading to a decrease in Kx • Because the amount of

surfactant will become insufficient again the process of partial coagulation and

subsequent continuation of the reaction is repeated, leading to the conductivity

oscillations observed. These oscillations are not caused by fouling of the electrode.

Furthermore, although coagulation occurs, the inhomogeneities in the reactor are

much smaller than the size of the electrode and therefore are in itself not causing the

oscillations. If fouling becomes so severe that the entire electrode is covered with

coagulated material that can not be removed under normal mixing conditions the

conductivity signal is marked by fluctuations over the entire conductivity range.

Probably on-line conductivity measurements can also be utilized to develop surfactant

addition profiles that may help to prevent coagulation as will be discussed in section

6.4.

62

4.4 Concluding remarks

Chapter 4

On-line conductivity measurements offer very good perspectives in unravelling

emulsion polymerization and encapsulation mechanisms. The most critical parameters

determining the course of both emulsion polymerization and encapsulation reactions

are the apparent CMC and the type of prevailing nucleation mechanism connected

with the applied monomer.

The apparent CMC is strongly influenced by both the initiator and the

monomer, as well as by their concentrations. The apparent CMC of a surfactant in

the presence of monomer can be determined by plotting the conductivity as a

function of the volume fraction of monomer for a series of surfactant solutions of

different concentrations. The apparent CMC is reached when the entire conductivity

curve of a solution with a certain surfactant concentration can be described with

Maxwell's law, while any higher concentration will lead to enhanced interaction

between monomer and surfactant in the form of micelle formation and solubilization.

With this method even combined effects of monomer and initiator on CMC""" can be

determined, provided polymerization reactions are inhibited, e.g. by the presence of

oxygen. For a mixture containing SACPA and Sty the apparent CMC of SDS at

60°C was found to be 7 mmol/L, substantially lower than the 10.16 mmol/L for SDS

alone,62 or the 9.2 mmol/L calculated taking into account the effect of counterions.

On-line conductivity measurements provide valuable information about

emulsion polymerization and encapsulation reactions. The start of the reaction can be

visualized as can the moment of droplet disappearance in batch reactions (the

beginning of interval III of emulsion polymerization).

'Monomer starved' conditions, obtained by adding the monomer semi­

continuously, improve the encapsulation efficiency because in this case the CMC is

not notably influenced by the addition of monomer as it is during batch reactions.

The absence of monomer starved conditions becomes manifest unambiguously in the

Emulsion (-like) polymerization reactions monitored with on-line conductivity 63

occurrence of one maximum in the conductivity curve. During an encapsulation

reaction with styrene coagulation was revealed by an oscillation in the conductivity

signal showing a number of maxima and minima.

The information obtained from the conductivity measurements can be used to

improve the encapsulation efficiency and can form the basis of surfactant addition

profiles. For example it is clear that in batch encapsulation reactions the addition of

surfactant should be complementary to the conductivity curve. The conductivity

signal in itself is directly related to the concentration of the 'free' surfactant

molecules and indirectly to the surface coverage of the particles with surfactant. To

prevent coagulation an addition profile of surfactant can be designed, based on

conductivity measurements, just as to keep the surfactant concentration below the

apparent CMC and just above the critical surface coverage of the particles. It is clear

from the conductivity measurements that especially in region (I,ll) surfactant has to

be added, whereas in region IlIa and IIIb, depending on the monomer, less surfactant

is needed or surfactant is even released by the system. In chapter 6, on the basis of

these findings, some initial experiments with surfactant addition profiles are

discussed.

64

Appendix 4.1: Calculation of the beginning of interval ill

Chapter 4

The beginning of interval III of emulsion polymerization can be calculated

from a simple mass balance (equation 4.4).

4.4

with: MoPMp =

Md

Maq

the amount of monomer added at the beginning of the reaction [g]the total amount of polymer formed at a certain time or conversion [g]the total amount of monomer in the swollen polymer phase at thattime or conversion [g]the amount of monomer present in monomer droplets at that time orconversion [g]the amount of monomer in the aqueous phase at that time orconversion [g]

Of course, at the beginning of interval III monomer droplets are no longer present,

which means that Md is zero. The amount of polymer simply can be calculated from

the conversion (P= x • Mo, with x = the conversion).

The moment the monomer droplets disappear the monomer concentration still

has it's maximum value both in the swollen polymer phase and in the aqueous phase.

This means that the saturation data for the amount of monomer in the swollen

polymer phase (Mp,saJ and in the aqueous phase (Mag,saJ can be used. Equation 4.4

now becomes (equation 4.5):

4.5

with: Mox111

Mp,sar

the amount of monomer added at the beginning of the reaction [g]fractional conversion at which monomer droplets disappear [-]the amount of monomer per gram polymer in the swollen polymerphase at saturation swelling [g monomer/g polymer]the amount of monomer in the aqueous phase at saturation [g]

Rearrangement of equation 4.5 will give the conversion at which the monomer

droplets disappear (equation 4.6):

Emulsion (-like) polymerization reactions monitored with on-line conductivity 65

4.6

In this investigation Maq•sat and Mp•sat have been determined from literature

values for [Ml::t, which is the concentration of monomer in the aqueous phase at

saturation (in mmol/L water), and C:.rat, which is the concentration of monomer in

the swollen polymer phase at saturation (in moles of monomer per litre swollen

polymer phase). [Ml::t was also determined from figures 4.2 and 4.3 at the volume

fraction at which Maxwell's law becomes valid. These and other data needed for the

calculation of xm are given in table 4.2. The batch experiments with MMA had an

Mo of 45.5 g per litre water; for reactions with Sty an Mo of 43.6 g per litre water

was used.

Table 4.2 Conversion at which monomer droplets disappear (xlII)' for MMA and Sty, and the datanecessary for the calculation. MfllbSlIl was calculated for one litre water

t=lj)......

M..-t ~••atPpOI XmI,. "..(g/CD13) (g) (gig pol) (-),"'. ":7

MMA 150 6.3 0.897 1.19 15.02 1.81 0.24

Sty 3 5.2 0.870 1.12 0.31 1.28 0.43

66

Appendix 4.2: Determination of [M).q in interval ill

Chapter 4

After the monomer droplets have disappeared at the beginning of interval III

the monomer concentration will decrease both in the swollen polymer particles and in

the aqueous phase. Both concentrations ([M)p and [M].J are interrelated and can be

determined from two equations: the Vanzo equation77 and a mass balance. Instead of

the concentration or amount of monomer in the swollen polymer phase often the

volume fraction of monomer in the swollen polymer phase (VJ is used. The relation

between Vm and ~ and P is given in equation 4.7.

Mp

VmPmoll 4.7

Mp P--+-Pmoll Ppol

Pmon

Ppol

volume fraction of monomer in the swollen polymer particles [-]the total amount of monomer in the swollen polymer particles [g]the amount of polymer at a certain time or conversion [g]the density of the monomer [g/cm3

]

the density of the polymer [g/cm3]

As mentioned in appendix 4.1 P can be calculated from the conversion (x)

and the amount of monomer at the beginning of the reaction (Mo): P= (xeMo). After

re-arranging, Mp can be written in terms of Vm (equation 4.8):

4.8

the total amount of monomer in the swollen polymer phase at thattime or conversion [g]the amount of monomer added at the beginning of the reaction [g]

Emulsion (-like) polymerization reactions monitored with on-line conductivity 67

Combination of the mass balance equation 4.4 with equation 4.8, bearing in

mind the fact that Md is zero in interval III and P== (xeMo), will lead to equation

4.9, which describes the amount of monomer in the aqueous phase (Maq, in grams) as

a function of Vm:

x'M'VM =M -x'M _ ° m

aq ° ° Ppol (l-Vm

)

Pmoll

4.9

with: Maq the amount of monomer in the aqueous phase at that time orconversion [g]

M o the amount of monomer added at the beginning of the reaction [g]x the fractional conversion [-]Vm volume fraction of monomer in the swollen polymer particles [-]Pmon the density of the monomer [g/cm3

]

Ppol == the density of the polymer [g/cm3]

From this equation the concentration of monomer in the aqueous phase ([M]a.J can

be derived (equation 4.10):

10Q0-M--_o=---(l-x

MM (W+~)

m Pmon

X'Vm'P moll )

PpoP -Vm) 4.10

with: [MJaq

Mm

WM aq

monomer concentration in the aqueous phase [mol/L]molecular mass of the monomer [g/mol]volume of water in the reactor [cm3

]

the amount of monomer in the aqueous phase at that time orconversion [g]the amount of monomer added at the beginning of the reaction [g]the fractional conversion [-]volume fraction of monomer in the swollen polymer particles [-]the density of the monomer [g/cm3

]

the density of the polymer [g/cm3]

The other relation between [M]aq and Vm is found in the Vanzo equation77

(equation 4.11):

68

[M]In( aq )=In(I-VhV +corr.=ln(V )+I-V +CO".[M] rpm m

aq,1/lt

Chapter 4

4.11

with: fM]aq.sar the concentration of monomer in the aqueous phase at saturation[mol/L]

lj, = volume fraction of polymer in the swollen polymer phase [g];Vp = I-Vm

corr. a correction term equal to: -[In(1-VP,..J+VP...J. 77 This term canalso be written as -[In(Vm,..J+ I-Vm...J. In these terms VP...t andVm,..t represent the volume fractions in the swollen latex particlesat saturation of the polymer and the monomer, respectively.

The actual value for [M].q and Vm at a certain conversion in interval III is

found by plotting [M].q as a function of Vm based on both the mass balance

(equation 4.10) and the Morton equation (equation 4.11). The intersection of both

curves gives the desired values both for the monomer concentration in the aqueous

phase and for the volume fraction of monomer in the swollen polymer phase. This

procedure must be repeated for each conversion data point in interval III in order to

obtain curves like the ones shown in fIgures 4.6B and 4.10B ([M].q as a function of

the conversion).

Chapter 5

THE INFLUENCE OF THE TYPE OF

INITIATOR ON ENCAPSULATION REACTIONS

5.1 Introduction

In the preceding chapter the effects of various reaction components on the

CMC were discussed. It was then indicated that both the presence of monomers and

salts (e.g. the initiator) will lower the CMC, which is unfavourable because then at a

constant surfactant concentration the amount of free polymer is increased, and

subsequently the encapsulation efficiency is decreased. One way to circumvent this

problem is to choose an initiator or initiating system that encourages the initiation

and propagation reactions to take place at the pigment surface. 41,42

In literature various initiators are described in relation to encapsulation

reactions. Some of them involve water soluble initiators like potassium persulfate for

non-modified pigments,38 or azo-compounds, both for modified and non-modified

pigments. 18,19,41,57,72 Caris41 described a method that in principle will contribute to a

higher efficiency despite the presence of free micelles. The method is based on an

initiator that is chemically bonded to the pigment surface via a titanate. 41. 78 In this

manner polymerization should mainly take place at the particle surface.

Unfortunately, only part of the formed radicals stays bonded to the pigment

surface, and the method to establish this bond is very laborious (see also section

2.2). Most initiator fragments will only be attached to the surface with one end,

while, upon radical formation, the other end will migrate towards the aqueous phase.

This, of course, decreases the encapsulation efficiency. Cage effects79 can become a

problem also, ironically enough especially in those cases where both initiator

fragments are attached to the pigment surface. In this case the radicals cannot diffuse

away from each other rapidly enough and instant termination will take place.

70 Chapter 5

Table 5.1 Materials used in encapsulation reactions with various initiating systems. Concentrationsare based on the amount of water unless stated otherwise

Material.............. 2· .................. .......... i ....... ••••••••••••••... ...... ...... <

Ti02- CAlO 55.5 giL

SDS 7.7 - 10.2 mmol/L

water 900 mL

monomer 140 - 555 mmol/L

initiator:1) SACPA 1.3 - 1.5 mmol/L2) SPS 1.5 - 5.0 mmol/L

NaHC03 4.2 - 8 mmol/L3) CHP 5 - 10 mg/g monomer

FeS04 1.9 - 3.2 *10,5 mol/LEDTA 1.9 - 3.2 *10,5 mol/LSFS 0.7 - 1.3 mmol/L

An alternative for the chemically bonded initiator, but without the

disadvantage of a cage-effect,79 might be a redox initiator system. One system

described in literature involves potassium persulfate and sodium metabisulfite in the

presence of non-modified pigments.37 Hoy and Smith used t-butyl hydroperoxide and

sodium formaldehyde sulfoxylate (SFS) in combination with a bilayer containing

pigment.38,39 The encapsulation efficiencies they claim to have found are very high

(up to 100 wt%).

In this chapter three different initiating systems will be discussed: (I) sodium

4,4'-azo-bis-(4-cyanopentanoate), (m sodium persulfate (with sodium bi-carbonate),

and (III) cumene hydroperoxidel FeS04-EDTAI sodium formaldehyde sulfoxylate.

Materials and quantities are listed in table 5.1. The monomer was added semi­

continuously in all reactions. On-line conductivity measurements (chapter 4) were

used to check whether coagulation occurred and whether monomer starved conditions

were maintained. Cumene hydroperoxide (CHP) was added dissolved in the

monomer. Other experimental details and procedures are discussed in chapter 3.

The influence of the type of initiator on encapsulation reactions

5.2 Comparison of initiating systems

5.2.1 Reactions with SACPA

71

So far, encapsulation reactions with TiOz-CAlO have almost exclusively been

carried out with sodium 4,4'-azo-bis-(4-cyanopentanoate) (SACPA). This initiator,

although water soluble, is somewhat hydrophobic and is believed to be able to

migrate rapidly towards a hydrophobic (pigment) surface. If the initiator itself will

not go to the surface, then at least the oligomers it forms with the monomer present

should have an enhanced tendency to do so. However, this tendency to migrate

towards the particle surface strongly depends on the degree of dissociation of the

carboxylic acid groups and thus on the pH (PKa~ 4.3).

500 150

Cl 400~Ci

.§. ;UPIIC\ 100 !300C >-~ ()

c c:200 II)

0 '0() 50 ;:

'0 100 a;Q.

0 07.25 9.25 9.25 9.55

[SOS] (mmoI/L)

content _ efficiency

Figure 5.1 Effect of the SDS concentration on the polymer content and on the encapsulationefficiency. Initiator: [SACPA}= 1.3 mmol/L. Monomer: 0.193 mol (20 mL) styrene was added at arate of 0.05 mL/min; T= 60°C

The effect of the surfactant concentration on encapsulation reactions with

styrene and SACPA is shown in figure 5.1: the higher the surfactant concentration

the lower both the encapsulation efficiency and the polymer content of the pigment.

Comparable behaviour was found by Caris4/,78 for batch reactions with MMA. Also

for semi-continuous reactions with MMA it was found that the efficiency changes

with the surfactant concentration, albeit less pronounced.

72 Chapter 5

Most of the experiments described in this section were carried out at SDS

concentrations of 9.2 mmol/L. Substantially lower surfactant concentrations of 7.25

mmol/L in the case of a reaction with Sty resulted in unrealistic values for both the

encapsulation efficiency and the conversion (see figure 5.1). The material that had

coagulated during the reaction amounted to over 25 wt % of the total amount of

pigment and monomer (now polymer) added. A substantial part of the free polymer

must have been included in the aggregates leading to inhomogeneous samples, which

explains the high (apparent) efficiency.

The amount of coagulate of course decreases with increasing SDS concentra­

tion (~17 wt% if [SDS] = 9.25 mmol/L and 8 wt% if [SDS] = 9.55 mmol/L), which

is also the case for the efficiency (57 wt% and 37 wt% of the monomer added is

located at the pigment surface, respectively). TEM revealed the presence of polymer

at various locations: free polymer, surface polymer (also adsorbed particles) and

polymer trapped in aggregates. In the case of reactions with MMA, compared with

Sty, both the amount of coagulate (5-12 wt%) and" (13-17 wt%) were lower

([SDS] = 9.25 mmol/L). However, the polymer layer at the surface was smoother.

The decrease in the polymer content and in the encapsulation efficiency with

increasing surfactant concentration is of course the result of the increasing number of

free micelles that is introduced with an increasing excess of surfactant: only part of

the surfactant is needed for the stabilization of the pigment, the rest will form

micelles and will compete with pigment particles as a site of polymerization (section

4.1). Too Iowa surfactant concentration, however, will lead to instability of the

pigment dispersion and subsequently to coagulation (see section 6.4 also).

Decisive for stability on the one hand and for high efficiencies on the other, is

the 'apparent' critical micelle concentration (CMCapp). As was mentioned in section

4.2.1 counterions have a large effect on the CMCapp. This effect can be calculated

with equation 4.1. For the reactions with SACPA this leads to a CMC~pp of 9.2

mmol/L. If we also compensate for the surfactant adsorption by the pigment

(approximately 1.04*10-5 mol SDS/g Ti01-CAIO) the CMC:; (section 4.2.3)

becomes 9.77 mmol/L. The effect of (counter-) ions on the adsorption of SDS by the

pigment was not taken into account.

The influence of the type of initiator on encapsulation reactions 73

(A): styrene (B): methyl metacrylate400 70 120 20

q..'~###.350 60 \~ 0-

"~"100 ..- 16

300 l:!. ••••

//·······fi-c:':__50,..... ,..... 80 0 ~,.....~ ><~ bll250

4<! ~ 12!g ----.r-l'~

>. '-' 60 ~/ l:!. ------11. >.

-=200

30~ -=g11)

~ 6 8 'G-=0 150 S 040 S<.> <.> --V'.. 2011) 11)

0 100 .. 0A A 4

20 ..50 10

0 0 0 00 100 200 300 400 500 600 0 100 200 300 400 500 600

monomer added (mmollL) monomer added (mmol/L)

~0 polymer content l:!. efficiency

Figure 5.2 Effect of the amount of monomer added (semi-continuously) on the polymer content (0,left axis) and on the encapsulation efficiency (A, right axis). [SACPA]= 1,3 mmollL, [SDS]= 9.2mmollL. Monomer: styrene (figure A) or MMA (figure B), T= 60°C. The dashed lines represent thetrends as expected from figure 4.12 (batch experiments), although they may be different for semi­continuous reactions

In the figures 5.2A and B one can see that an increase in the total amount of

monomer added will lead to an increase in the polymer content of the pigment (left

axis). The encapsulation efficiency (right axis), however, is decreased. Both effects

are obvious in the case of styrene (figure 5.2A). [n the case of MMA (figure 5.2B)

the relative changes are even more drastic, probably because in this case

homogeneous nucleation is more important than micellar nucleation (see e.g. section

2.3 and 4.1). Apparently, surface polymer is formed mainly in the beginning of the

reaction. After a certain period the competitive surface areas of the free micelles and

of the polymer particles have become so large that the amount of polymer formed at

those sites exceeds the amount formed at the pigment surface by far.

74 Chapter 5

The differences in efficiency and polymer content as a result of the use of dif­

ferent monomers are shown in tlgure 5.3. The volume of monomer added was kept

constant, which means that, due to differences in the densities and molecular weights

of the three monomers, the absolute amount of monomer added in terms of moles

increases as follows: BMA (0.140 mol) < Sty (0.193 mol) < MMA (0.209 mol). It

is clear that the differences in the polymer content and efficiency partly lie in the

difference in water solubility of the monomers.

300 70

bO 60~

co 50 Ill!

! 200...!.

i:l 40 >..'" ui:l 30 ='"0 100 '"u

20 l;:"0 ...

'"c:>. 10

0 0MMA STY BMA

monomer

cooteot _ efficieocy

Figure 5.3 Effect of the monomer on the polymer content and on the encapsulation efficiency.[SACPAJ= 1.3 mmollL. [SDSJ= 9.25 mmollL. T= 60°C. The monomer (20 mL) was added at a rateof O. 05 mL/min

5.2.2 Reactions with sodium persu/fate

In regular emulsion polymerizations the use of sodium persulfate (SPS) is far

more common than SACPA. The former is often used in surfactant free emulsion

polymerizations with Sty. In these reactions, stability of the polymer particles is

provided for by the sulfate oligomers formed during the polymerization reaction. In

case of Sty those oligomers become surface active at a length of 2 or 3 monomer

units and insoluble at a length of 3 or 4 units. On the other hand, for MMA surface

activity is reached at a length of about 4 or 5 units and insolubility at a length of 10

or 11 units. 75 Also during encapsulation reactions the sulfate oligomers might provide

extra stability, which would be desirable, especially in the case of the coagulation

The influence of the type of initiator on encapsulation reactions 75

sensitive reactions with Sty. On the other hand, because SPS is more water soluble

than SACPA the former may direct the initiation and propagation towards the

aqueous phase even stronger.

One of the problems with SPS is the need to buffer the system in order to

prevent side-reactions, which would lower the efficiency at low pH. 80 For this

purpose sodium bicarbonate (NaHC03) was added. As a consequence, the CMCapp

varied from 6.3 to 7.8 mmol/L (instead of 9.2 mmol/L for SACPA), while the

CMC::; varied from 6.87 to 8.37 mmol/L (instead of 9.77 mmol/L for SACPA),

both substantially lower than for SACPA reactions. Adjustment of the surfactant

concentration is of course possible (section 4.2.3), but a decrease in the surfactant

concentration, like in the case of reactions with SACPA, will lead to more instability

(especially if Sty is used), as will be discussed further on.

75 20

--. 0l>O <0--0· --.-- 15 ~l>O,g 50 :§,... »= 10., y

d =.,0 25 !l~ 'ut> !l .~

cJ 5 ::::.,r:>.

o 0o 100 200 300 400 500 600

monomer added (mmol/L)

o content !l efficiency

Figure 5.4 Effect of the amount of monomer added (semi-continuously) on the polymer content (0,left axis) and on the encapsulation efficiency (A, right axis). Initiator: [SPS]= 1.5 mmol/L,[NaHCOJ= 4.25 mmol/L, [SDS]= 9.2 mmol/L, T= 60°C. Monomer: methyl methacrylate.

As is shown for MMA in figure 5.4, the presence of NaHC03 has no effect

on the trends of the efficiency and the polymer content as a function of the amount of

monomer added. As compared with reactions with SACPA at the same surfactant and

initiator concentration, in the case of SPS the level of efficiency and polymer content

are, however, much lower as can be seen in figure 5.5. This decrease in both the

76 Chapter 5

polymer content and the efficiency, when using SPS instead of SACPA, most likely

is a result of the fact that the difference between the SDS concentration and the

actual CMCapp and CMC: is larger in the case of SPS (because of the presence of

NaHC03). In fact, this means that the free micelle concentration is higher also,

although the surfactant concentration is the same for both reactions ([SDS] = 9.2

mmol/L). Another, less likely, explanation for the higher efficiencies would be that

the hydrophobicity of the SACPA radicals causes (surface active) oligomers to

migrate towards the pigment surface somewhat sooner than in the case of SPS.

400 SO

'00300

40 ,....

] ~..30 it.. '-'

c:l 200 >..G) 0

d c:l20 G)

0 ..,0

5"0 10010 G)

a.

0 0SPS SACPA SPS SACPA

monomer added (mmol/L)

content _ efficiency

Figure 5.5 Effect of the initiator system on the polymer content and on the encapsulation efficiency.[SDSJ= 9.2 mmollL. Reactions with SACPA: [SACPAJ= 1.3 mmollL. Reactions with SPS: [SPSJ=1.5 mmollL. [NaHCOJ= 4.25 mmollL. The amount of monomer added to one litre water was 0.42mol. The addition rate was changed from 0.05 to 0.2 mL/min after 90 minutes. T= 60°C

Although the efficiencies are not satisfactory, the use of SPS and NaHC03 do

have the advantage of a more stable reaction mixture. Only a small fraction of the

pigment, up to a maximum of approximately 3 wt % of the total of solids (polymer

included), was found to agglomerate during the reaction. However, under more

'extreme' conditions ([SDS]= 7.7 mmol/L, [SPS]= 2.8 mmol/L, [NaHC03]= 7.9

mmol/L) reactions with Sty showed coagulation up to 7 wt%, based on the total

amount of solids, or up to 30 wt% at even higher SPS concentrations ([SPS]= 5.0

mmol/L, [NaHC03]= 5.0 mmol/L). All this implies that persulfate initiated

encapsulation reactions do have an improved stability over the azo initiated reactions,

The influence of the type of initiator on encapsulation reactions 77

but simultaneously micelle formation, homogeneous nucleation and reactions in the

free micelles are enhanced unintentionally. Therefore, the oligomers with persulfate

groups add more to the stability of the system8] than the SACPA containing

oligomers, as can be expected.

5.2.3 Reactions with cumene hydroperoxide

A second alternative for SACPA would be the use of a redox initiator.

Initiators of this type frequently have been used in grafting reactions. 82 Redox

initiators have also been used in encapsulation reactions before/7,38,39 although the

pigments used in those cases had not been modified with titanates. In case of

modified pigments especially the combination of a hydrophobic (hydro-) peroxide

with a water soluble iron(II)-salt could be interesting, because this could direct

initiation towards an interface between a hydrophobic particle (modified pigment) and

the aqueous phase.

In the present investigation a combination of cumene hydroperoxide (CHP)

and iron(II)sulfate (FeS04) was used. Ethylene diamine tetra-acetic acid (EDTA) was

used to complex FeH; sodium formaldehyde sulfoxylate (SFS) was used to transform

Fe3+ back into FeH (see figure 5.6).80,82

Figure 5.6 Reaction-diagram of the electron transfer between cumene hydroperoxide (ROOH) andFe2+, and between sodium formaldehyde sulfoxylate (FS) and Fe'+

78 Chapter 5

One of the major advantages of this system is that in the hydrophobic

environment only one radical is formed, instead of two, which means the cage effect

is unlikely to take place. From literature remains uncertain whether a second radical

is formed at all, because it is not entirely clear whether the formaldehyde sulfoxylate

ion forms an ion or a radical (formula: SOz-.(HCHO».83 Based on conversion data

of reactions initiated solely by SFS it seems that the latter is the case. 84 However, the

occurrence of the cage effect remains unlikely, because SOz-.(HCHO) is water

soluble (unlike the CHP originated radical) and will stay in or diffuse into the

aqueous phase quite easily, thus preventing termination with the CHP radical.

A number of reactions were performed with the CHP/FeS04/EDTAISFS

initiator system. The encapsulation results and the concentrations of the important

reaction components are listed in table 5.2. All components were added to the

reactor at the beginning of the reaction, except for the CHP, which was dissolved

into the monomer, which was added semi-continuously to the reactor at a constant

addition rate of 0.05 mLimin.

Table 5.2 Experimental data and results of encapsulation experiments with cumene hydroperoxideand iron(II)sulfate as the redox initiator. T= 60°C

EXp.#

REDI

RED2

RED3

RED5

RED6

monomer [SDS] [SFS] [CHP]U.[mon] CMC::; PC3 t'l(-) (mmoI/L) (mmoIIL) (mg/g) (uioIIL) (mmoI/L)(mg/g) (wt%)

MMA 7.9 1.3 10.0 0.190 10.2 98 26

MMA 8.4 1.3 10.0 0.190 10.2 18 19

MMA 9.2 0.6 4.9 0.209 10.5 175 58

MMA 10.2 1.3 10.0 0.190 10.7 75 39

Sty 9.5 0.6 4.9 0.193 10.5 155 42

BMA 9.8 0.6 4.9 0.140 10.5 127 35

1) CHP was added together with the monomer; 'concentration' in Img CHP/ g monomer)Z) The amount of TiOz-CAIO used was 100 instead of 50 g

The influence of the type of initiator on encapsulation reactions 79

It immediately becomes clear from table 5.2 that especially encapsulation

reactions with MMA benefit from the redox initiator system. Dependent on the CHP

concentration, the efficiency and polymer content are brought to or even exceed

levels that are common for reactions with Sty in combination with SACPA or SPS.

The amount of coagulated material based on the total amount of solids, polymer

included, was 0 to 7 wt% for reactions with MMA, and 8 to 15 wt% for the

reactions with Sty and BMA, respectively. Given the additional fact that all reactions

were carried out at surfactant concentrations below CMC::; (like the reactions with

SACPA but unlike the reactions with SPS), the use of a redox initiator has led to

substantial improvements on all fronts.

The question remains why the redox system leads to such a substantial

improvement for reactions with MMA, while the effects are not so clear for the

reactions with BMA and Sty. It becomes evident that the water solubility of the

monomer is not the only factor determining for instance the efficiency: the

mechanism of both initiation and encapsulation must play an important role as well.

This necessarily leads to the conclusion that for MMA initiation takes place in the

aqueous phase regardless whether SPS or SACPA are used, and that the oligomers

strongly contribute to the formation of new latex particles. If, on the other hand,

MMA is used in combination with the redox initiator described above, initiation is

directed towards the interface between the aqueous phase and the hydrophobic

pigment. In the case of BMA and Sty homogeneous nucleation does not play an

important role, regardless of the initiator used. Therefore, for these monomers a

change in the location of initiation will not have a (large) effect on the efficiency of

the encapsulation process.

The latter becomes even more clear when we look at figure 5.7. Here the

three initiators are compared with respect to the polymer content of the pigment and

the encapsulation efficiency. NaHC03 is present in all of these reactions to obtain

comparable counterion concentrations. Still the redox reaction shows the highest

efficiency, followed by SACPA and SPS respectively, but it is obvious that the

addition of NaHC03 has a negative effect on the encapsulation efficiency. It is

80 Chapter 5

important to note that the SDS concentration exceeds CMC::; in all three cases,

although the excess amount of SDS (= [SDS] - CMC::;) is the lowest for the reac­

tion with CHP, where CMC::; is 9.0 mmol/L as opposed to the other two reactions

where CMC::; is 8.3 mmol/L. This may account for the remaining differences in the

polymer content and the encapsulation efficiency.

100 50

bO 80 40"- """bD

~!i$

60 30 l>-0

~l'l.,

0 40 20 'u0 5'0

.,'" 20 10

0 0SPS SACPA CHP

monomer added (mmol/L)

content • efficiency

Figure 5.7 Effect of the initiator system on the polymer content and on the encapsulation efficiencyfor three different initiators and in the presence of NaHC03• {NaHCOJ= 4.25 mmollL, {SDS]= 9.2mmollL. Reactions with SPS or SACPA were performed at the same initiator concentration:{SACPA]= {SPSJ= 1.5 mmollL. Reaction with CHP: 0.05 g CHP/ per g MMA, {SFS]= 0.7 mmollL,{FeSOJ= {EDTA]= 0.02 mmollL. Monomer: 0.209 mol MMA was added at a rate of 0.05 mL/min.T= 60°C

The fact that the encapsulation efficiency of the redox initiated reaction

strongly decreases under the experimental circumstances of figure 5.7 indicates that

the apparent CMC plays a very important role, even in the case of a redox initiator

system. It still can be true that with a redox initiator the initiation is directed towards

an interface between a hydrophobic and a hydrophillic phase, but in the presence of

NaHC03 (or more accurate: in the presence of free micelles) this means the initiation

is directed to the micelles rather than to a pigment/water interface.

Beside the negative effect of decreasing the encapsulation efficiency, NaHC03

seems to have a positive effect on the stability of the reaction mixture, as already

mentioned for reactions with SPS. This seems somewhat contradictory, since an

The influence of the type of initiator on encapsulation reactions 81

increase in the ion concentration would lead to a compression of the electrical

double-layer and through ion association to a lower charge in the particles. It is not

clear if steric stabilization, possibly by small polymer particles, becomes an

important factor. Further investigation of these phenomena will be needed.

5.3 Concluding remarks

Both the polymer content of encapsulated Ti02-CAlO and the encapsulation

efficiency are strongly influenced by the initiator system used. SACPA is more

suitable for obtaining higher polymer contents and encapsulation efficiencies than

SPS, mainly because in the latter case the difference between the actual SDS

concentration and the CMC::; is large. In general the stability of the reaction product

was better when SPS was used.

The best results on both the subjects of stability and efficiency were obtained

by using a redox initiator basically consisting of CHP and Fe2+. Especially with

MMA good results were obtained: an efficiency of 50 wt% and a fairly stable

product. It is believed that the use of CHP and Fe2+ directs the initiation towards the

interface between water and hydrophobic species. If the number of free micelles is

very large, e.g. because of the presence of salts like NaHC03, this means that

initiation is directed towards the micelles rather than to the pigment/water interface.

Upon the addition of NaHC03 the efficiency strongly decreased to 23 wt%, whereas

the stability was improved.

Chapter 6

THE ROLE OF SURFACTANTS IN

ENCAPSULATION REACTIONS

6.1 Introduction

Until now two methods have been described to minimize the amount of free

polymer in encapsulation reactions: firstly the semi-continuous addition of the

monomer (chapter 4), and secondly the choice of a proper initiating system (chapter

5). In both cases the influence on the apparent CMC is reduced by decreasing the

concentration of the components that contribute to the decrease in the CMCapp : the

monomer and the initiator salts, respectively.

In this chapter the effect of the surfactant type on the encapsulation efficiency

and on the stability of the reaction mixture will be discussed. Most of the

encapsulation reactions with modified Ti02 particles in aqueous emulsion systems

have been carried out with sodium dodecyl sulfate (SDS). This anionic surfactant and

its properties are well known. Besides, commercial SDS is available in high degrees

of chemical purity (> 99%), and the CMC of SDS is relatively high.

In this chapter two non-ionic surfactants (Antharox CO-880 and CO-990), a

two-tailed surfactant (Aerosol OT-loo) and an inisurf (a surface active initiator) are

compared to check whether they are able to provide more stability and higher

encapsulation efficiencies than SDS (in table 6.3 the optimal combination of initiator

and surfactant will be given, both for encapsulation reactions with MMA and with

Sty). Furthermore, some preliminary surfactant addition experiments will be

discussed of which the surfactant addition profiles are based on the conductometric

experiments in chapter 4. Finally, in section 6.5 the effect of the pigment surface

modification and of the pigment concentration on the encapsulation efficiency will be

discussed.

6.2 Experimental

The experimental set-up and procedures of the experiments mentioned in this

chapter are the same as those described in chapter 3. The only difference is the type

of surfactant used. The surfactants tested were the non-ionic Antarox CO-880 and

Antarox CO-990, both of which are nonylphenoxypoly(ethyleneoxy)ethanols (Ugure

6.1), and the anionic sodium dioctyl sulfosuccinate (Ugure 6.2, Aerosol OT-lOO).

The difference between Antarox CO-880 and CO-990 lies in the average number of

ethyleneoxide moles per mole nonylphenol, 29 (CO-880; HLB= 17) and 99 (CO­

990; HLB= 19), respectively.

Figure 6.1 Structure of nonylphenoxypoly(etheleneoxy)ethanol; '(n-I) , is the average number ofmoles of ethelene oxide per mole nonylphenol. Antarox C0-880: n=30; CO-990: n=IOO

I<1gure 6.2 Structure of sodium dioctyl suifosuccinate (Aerosol OT-lOO)

All the reactions mentioned above were initiated by means of the redox system

described in section 3.3.1 and 5.2.3. The concentration of cumene hydroperoxide

(CHP) was 0.005 g per gram of monomer, the EDTA concentration equalled the

FeS04 concentration and was 1.6*10.5 moltL, while the sodium formaldehyde

sulfoxylate (SFS) concentration was 4.4*10-4 mol/L. During each reaction 15 mL of

monomer (MMA or Sty) was added semi-continuously.

The role of the surfactants in encapsulation reactions 85

Two experiments were not conducted with the redox initiator, but with a so­

called inisurf: a surface active initiator, kindly prepared and supplied by M. van den

Enden. The inisurf was prepared by attaching an azo-initiator to Antarox CO-880

according to a method described by Kusters. 79 The structure of this inisurf is shown

in figure 6.3.

Figure 6.3 Structure of the inisurf, with n= 30, used in this investigation (AC0-880 according tothe notation of Kusters) 79

6.3 Comparison of surfactants

6.3.1 Non-ionic surfactants

One of the advantages of non-ionic surfactants is that they are less sensitive to

ions than ionic surfactants. The CMCapp is hardly affected by ionic strength, because

the ions do not influence the aggregation behaviour of the surfactant molecules. In

the case of anionic surfactants cations will cause a compression of the electrical

double-layer of micelles, thus enhancing micelle formation.

A second advantage of non-ionic surfactants is that anionic initiator fragments

are not repelled by an electrical charge at the (pigment) surface because adsorbed

surfactant itself carries no charge. Caris even tried to use cationic surfactants to

attract more anionic radicals towards the pigment surface. 41, 78 However, the opposite

charges of surfactant and initiator made the surfactant concentration even a more

critical parameter than it was already: at or just above the CMC the number of free

micelles becomes so large that most of the initiator is drawn towards these micelles,

instead of towards the pigment surface. Then, of course, 11 decreases dramatically.

86 Chapter 6

Another benefit of non-ionic surfactants is that their water solubility and CMC

can be adjusted by changing the length of the hydrophilic part, in this case by

changing the number of ethylene oxide groups (EO-groups) of the Antarox

surfactants. In this investigation two non-ionic surfactants were tested: Antarox CO­

880, with 29 EO-groups, and CO-990, with 99 EO-groups (fIgure 6.1).

Table 6.1 Results of reactions with non-ionic surfactants. CO-880 has 29 EO-groups, CO-990 has99 EO-groups

.

massive coagulation; monomeraddition was stopped

11 g (16 wt% of total amount ofsolids)

Exp. surfactant [iturfJ ~ffit# (-) "rmollI.NONI CO-88O 0.63 -

NON2 CO-88O 2.1 49 - 56

NON3 CO-990 1.0 - 32 g (48 wt% of the total amountof solids

All reactions with non-ionic surfactants (table 6.1) were carried out at

surfactant concentrations above the CMC, which increases with the number of EO­

groups~,85 (CO-880 has a CMC of 0.25 mmol/L, CO-990 has a CMC of 1 mmol/L,

both at 25°C), but decreases with increasing temperature. Furthermore, the reaction

temperature (60°C) lies well below the cloud point (a 1 wt% solution of either of the

two surfactants is clear at 100°C). 86

During the reactions with non-ionic surfactants severe coagulation took place

in all cases (table 6.1). During experiment NON1, with CO-880, the monomer

addition even had to be stopped after 1 hour, because the presence of coagulates was

clearly visible. The other reactions also suffered from severe coagulation,· although

the agglomerates were not so large as in the case of NONl. Coagulates lead to non­

representative samples, as can be seen e.g. from the spreading in the encapsulation

efficiency (T\) of reaction NON2 (ll was determined twice with different results), and

from irregularities in conversion-time plots.

The role of the surfactants in encapsulation reactions 87

During the reaction with CO-990 severe coagulation took place as well. The

CMC of CO-990 is higher than that of CO-880, as mentioned in the previous

paragraph. The latter surfactant showed that a concentration of more than eight times

the CMC did not result in a stable reaction mixture. Besides, a lot of free polymer is

formed above the CMC, although the concentration dependency seems to be less than

in the case of an anionic surfactant like SDS. This can be caused by the fact that the

non-ionic surfactants have a stronger affinity to the modified pigment surface than

SDS: in general non-ionic surfactants have a strong affinity to hydrophobic

surfaces. 8S Surprisingly, this positive effect apparently is cancelled by the low CMC

values of non-ionic surfactants, or because the stabilizing properties of the latter are

insufficient which may be a result of the fact that the stabilization is merely steric of

nature. The latter explanation seems more likely.

6.3.2 Inisurfs

Inisurfs are surface active initiators.79.87.88 The advantage of such a compound

is that the locus of initiation can be directed towards the surface the inisurf adsorbs

to. The inisurf used in this investigation was built up by combining an Antarox CO­

880 surfactant with an azo-initiator (figure 6.3). The antarox surfactant has a rather

high affinity with the hydrophobic pigment surface, as was mentioned in 6.3.1. The

inisurf is asymmetrical in order to avoid cage-effects, and has a CMC of 0.63

mmol/L at room temperature. 79

Like the reactions with Antarox CO-880 and CO-990 the reaction mixture did

not remain stable if an inisurf was used. Severe coagulation was found both at inisurf

concentrations of 1.3 mmol/L (which was based on the initiator concentration of

reactions with SACPA as described in chapter 4 and 5), and of 6.0 mmol/L. In the

latter reaction coagulation started already when the reaction mixture was heated to

the reaction temperature of 60°C. Here the solubility of the inisurf was exceeded

when the cloud point (47.5 0 C)89 was reached which apparently led to an instable

system.

88

6.3.3 Sulfosuccinates

Chapter 6

The use of non-ionic (based) surfactants clearly does not contribute to a stable

reaction mixture during encapsulation reactions. However, a surfactant with an

affinity towards the hydrophobic pigment surface which is comparable to or greater

than that of the non-ionic surfactants may offer good perspectives, if it can provide

sufficient stability.

In this line of thought OT-lOO (fIgure 6.2) was investigated. This compound

has a CMC of 0.68 mmol/L at room temperature,90 which is considerably lower than

that of SDS (8.1 mmol/L at 25°C). The fact that OT-loo is anionic (electrostatic sta­

bilization), and that it has two hydrophobic tails (high affinity; 2.8*10-5 mol OT-lOO

is adsorbed per gram KR2l90-CAlO: 0.88 nmz per OT-IOO molecule) might contri­

bute to the desired combination of good stability and a high encapsulation efficiency.

The first conclusion that can be drawn from table 6.2 is that the use of OT-100 in

general provides stable reaction mixtures. Only at very low surfactant concentrations

substantial coagulation occurs.

Table 6.2 Results of reactions with OT-100 at 60°C. Monomer addition rate: 0.05 mL/min. Totalamount of monomer: 15.15 mL per litre water

."-...: .......EXp. monomer lOT-loo]

lDJDol/L (~) •••••••• ........

OTt Sty 2.3 - surfactant concentration too low;experiment aborted

OTI Sty 4.1 53 2 g (::=; 3 wt% of solids)

OT3 Sty 5.1 35 1.2 g (::=; 2 wt% of solids)

OT4 Sty 6.8 27 1 g (::=; 1.5 wt% of solids)

OT5 MMA 3.2 39 11.8 g (17.5 wt% of solids)

OT6 MMA 4.1 20 no coagulation

OT6d MMA 4.1 20 no coagulation (duplicate reactionof OT6)

The role of the surfactants in encapsulation reactions 89

As with SDS (figure 5.1), in the case of OT-loo the encapsulation efficiency

decreases with an increasing surfactant concentration (fIgure 6.4), while the stability

increases simultaneously. The improved stability (especially with Sty) of the reaction

mixture of experiments conducted with OT-loo and a smoother surface polymer

layer, as compared with SDS experiments (see chapter 5), are important advantages.

Another important conclusion to be drawn from table 6.2 is the fact that

reactions with MMA have much lower efficiencies than reactions with Sty, despite

the fact that a redox initiator system, as described in section 5.2.3, is used. Figure

6.5 clearly shows that the efficiency of reactions with MMA is much lower if OT­

100 (exp. OT6) is used instead of SDS, at least at the same initiator concentration

(exp. RED3, table 5.2). In the case of OT-lOO it is likely that the surfactant

concentration needed for a stable system (between 3.2 and 4.1 mmol/L) lies too far

above the apparent CMC. The efficiency of a reaction with SDS (figure 6.5 and

table 5.2, expo RED2) at twice the initiator concentration of experiment OT6 is

comparable to that of OT6. This increase in the initiator concentration lowers the

apparent CMC while, at the same time, more oligomers are formed. Both these

factors can lead to a decrease in the encapsulation efficiency (see chapter 5).

55

50----~ 451 40~5 35....()

fE 30Q.)

25.................................... l1

4.0 4.5 5.0 5.5 6.0 6.5 7.0

[OT-IOO] (mmol/L)

.. .. l1 .. encapsulation efficiency IFigure 6.4 Relation between the encapsulation efficiency and the concentration dioctylsulfosuccinate (monomer: Sty; T= 60·C)

90

200~

150 f

100

50

o

70

60 ,....50 ~..40 ~

;..,

30 ~II>....u

20 5II>

10

0

Chapter 6

OT6 RED2

experimentRED3

1"'::i:"iA COD'tm' _ ..mcitmcy

Figure 6.5 Encapsulation reactions with OT-1OO (OT6, table 6.3; 15 mL MMA) and with SDS(RED2 and RED3: experimental details in table 5.2; 20 mL MMA)

From the results in the chapters 5 and 6 (up to this point) the conclusion can

be drawn that the highest encapsulation efficiency and the best stability is obtained if

one of the following combinations of initiator and surfactant is used: a redox initiator

(main compound: cumene hydroperoxide) and SDS when MMA is the monomer, or

a redox initiator and OT-loo when Sty is the monomer (table 6.3). Because of its

nucleation mechanism (primarily micellar), for Sty especially the type of initiator is

less critical than for MMA (primarily homogeneous nucleation, see chapter 5 also).

The monomer should always be added under 'monomer starved conditions' (see

chapter 4).

Table 6.3 Optimal encapsulation systems for MMA and Sty. For Sty the initiator system is lesscritical (see chapter 5). The manomer should always be added semi-continuously (under 'starvedconditions', see chapter 4)

Monomer:

MMA

Sty

Initiator:

redox system

redox system

.......Surfactant: -

sodium dodecyl sulfate

sodium dioctyl sulfosuccinate

The role of the surfactants in encapsulation reactions 91

Note: One factor that makes the use of sulfosuccinates less attractive than the use ofe.g. SDS, is the fact that little is known about the former. Literature valuesfor the CMC at room temperature vary from 0.18 mmol/L to 6.8 mmol/L, thelatter of which seems to be a misprint of the original data (0.68 mmol/L)given by Williams. 90 The temperature dependency of the CMC of sulfo­succinates is not described well either. Furthermore, in literature the structuralformula of sodium dioctyl sulfosuccinate is sometimes presented as that ofsodium di-2-ethylhexyl sulfosuccinate, an isomer which has a stronglydifferent CMC of 2.5 mmol/L,65 Possibly OT-loo is a mixture of these twocompounds.

6.4 Surfactant addition during encapsulation reactions

Both earlier batch (Carisll and the present semi-continuous experiments

(Janssen)56,57,91 have shown that the stabilizing properties of SDS are not always

sufficient, especially with styrene. In previous chapters it was shown that the

surfactant concentration has to be well balanced. Figure 6.6 shows the course of the

surfactant concentration in the aqueous phase during an encapsulation reaction. Most

reactions will be started at or above CMCapp in order to prevent agglomeration of the

pigment at the beginning of the reaction. As the reaction continues the surface area

increases, especially if free polymer particles are formed: the amount of surfactant

available for stabilization decreases and, at a certain moment, will go below a critical

concentration, after which coagulation occurs (figure 6.6, curve I). If the reaction

can be started at surfactant concentrations below the CMC, at a concentration where

the initial dispersion is stable, the amount of free polymer will be kept to a

minimum.

In order to prevent coagulation one should either use a surfactant that can

provide more stability to the pigment at concentrations at which no or very few

micelles are being formed, or one should add surfactant during the reaction at such a

rate as to just compensate for the increase in surface area (figure 6.6, curve 2). If

the surfactant is added too slowly, coagulation will still take place, although it may

occur later on in the reaction (curve 3).

92

Free polymer~ 4

Ideal situation

timelIOrmal reaction (without surfactant addition)

2 weU-rontroUed surfactant addition3,4 i1uuJequate surfactant addition

Chapter 6

Figure 6.6 The course of the surfactant concentration in the aqueous phase during an encapsulationreaction and the influence ofsurfactant addition. Curve 1: no addition; curve 2: surface area increaseis just compensated for by surfactant addition; curve 3: too little surfactant is added; curve 4: toomuch surfactant is added

If the surfactant is added too quickly, or if too much surfactant is added

(curve 4), more micelles will be formed leading to a decrease in the encapsulation

efficiency. This risk is large, especially in interval III of a batch encapsulation

reaction with a moderately water soluble monomer like MMA, because there some

surfactant is released (as can be seen from the conductivity measurements in chapter

4).

One batch experiment with styrene has been conducted to see if the addition of

surfactant, in this case SDS, could lead to an improvement in the stability and the

encapsulation efficiency. The SDS concentration was raised from 8 mmol/L at the

beginning of the reaction to 9 mmol/L after 35 minutes, the time where (in batch

reactions without surfactant addition) the decrease in conductivity has (almost) ceased

(chapter 4). The initiator system was the same as used in the experiments described

in the sections 6.2 and 6.3. The CHP was added with the monomer. The reaction

was started by adding a solution of FeS04 , EDTA and SFS to the reaction mixture.

The results were compared with a batch reaction without surfactant addition. The

course of the conversion and the conductivity of both reaction systems are displayed

in rIgure 6.7.

The role of the surfactants in encapsulation reactions 93

time (min)

0.820.7 §0.6·~

0.5 §0.4~0.3 §0.2·~0.1 <./::

interval III 0.0

........-+---.----.--o---.----,----r-+-o. 150 100 150 200

1000

(B): without surfactant addition1200 1.1

1.00.9

CAl: with surfactant addition

Hgg:k 1.11.0

900 0.9

8000.82

e 0.7 §~ 700 0.6·~rnJ

0.5 §.~ 600.~ 0.4~~ 500 0.3 §~ '.00 400 0.2 ~u

0.1 <./::300 interval III 0.0200 -0.1

0 50 100 150 300

time (min)

-o-conductivity (left axis)

conversion (right axis)

Figure 6.7 Comparison of a batch encapsulation reaction with surfactant addition (figure ..4:[SDSJ= 8.0 mmol/L ~ 9 mmol/L), and a reaction without SDS addition (figure B: [SDSJ= 9mmol/L). The surfactant was added approximately until interval III started

The shape of the conductivity curves of both the reaction with and that without

surfactant addition are alike, apart from a small decrease at the end of the former

reaction. The main difference lies in the total conductivity decrease in both reactions.

With surfactant addition this decrease is not nearly as strong. This is partly the result

of a lower initial surfactant concentration, which means that there are fewer, if any,

free micelles, leading to a smaller increase in surface area in the first part of the

reaction and consequently to a decrease in the amount of surfactant that is adsorbed.

Of course the continuous addition of surfactant (= ions) in the first 35 minutes of the

reaction means that during this period the conductivity is constantly increased,

although the resultant of the surface area increase and the surfactant addition still

yields a net decrease in conductivity.

94 Chapter 6

The increase in the conductivity after approximately 30 minutes of reaction is

about the same for both reactions: 200 p.S/cm. Of course the relative increase is

much stronger in the case of the surfactant addition reaction. Probably coagulation

plays a role here, beside the effects already mentioned in sections 4.3.2 and 4.3.3.

The conversion data indicate coagulation also, especially in the reaction were

surfactant was added: instead of a gradual increase in conversion with time, a

slowdown in the reaction rate seems to occur (the 'plateau' as mentioned by Caris).41

Because the coagulation was more severe in the case of the reaction with surfactant

addition (10-11 g, or 11 wt % of coagulate was collected from the bottom of the

reactor at the end of the reaction versus 2 g, or 2 wt% in the case of the reaction

without surfactant addition), apparently the initial surfactant concentration (8

mmol/L) was too low to maintain stability of the initial dispersion. This more

massive coagulation may also explain why the conductivity can decrease again

towards the end of the 'addition' reaction: severe coagulation can cause a release of

surfactant which, at the end of the reactiQn, can be adsorbed·again onto new or

growing particles.

If the surfactant addition is applied to a batch reaction with a higher initial

surfactant concentration (~ 9 mmol/L), or to an encapsulation reaction during which

the monomer is added semi-continuously as well, then the stability is likely to be

improved. One reaction was performed under 'monomer starved' conditions (semi­

continuous addition of monomer) with an initial SDS concentration of 9.25 mmol/L.

The moment the conductivity started to decrease (= the moment of initiation, 30

minutes after the monomer addition was started) the addition of surfactant was started

as well. In 4.5 hours the overall surfactant concentration was raised to 9.7 mmol/L

(by adding 13.5 mL of a 40 mM SDS solution at a rate of 0.005 mL/min). At the

end of the reaction only 2.9 of coagulate could be collected (4 wt% based on the

total amount of solids). This is only half the amount that was found after a

comparable reaction without surfactant addition which, besides, was started at a

higher SDS concentration ([SDS] = 9.55 mmol/L, experiment REDS, table 5.2).

Therefore, the stability indeed can be improved by controlled surfactant addition.

The role of the surfactants in encapsulation reactions

6.5 Pigment modification and concentration

6.5.1 The influence ofpigment modification on encapsulation reactions

95

Batch encapsulation reactions with pure rutile Ti02 (Kronos RLK) performed

by Caris41 already pointed out that with this pigment modification is necessary in

order to achieve reasonable encapsulation efficiencies. In the present investigation

this necessity of an organic pigment modification was tested in semi-continuous

reactions. The reaction conditions were the same as for the reactions with

sulfosuccinates described in section 6.3.3: a redox initiator and an OT-1oo

concentration of 4.1 mmol/L. Under these conditions the modification was found to

have a negligible effect on the encapsulation efficiency (table 6.4): 39 wt% with

modified and 41 wt% with non-modified pigment. However, the latter value is not

very reliable, because severe coagulation took place: all pigment sagged the moment

stirring was ceased. In contrast with this, the reaction mixture with modified pigment

remained completely stable.

The other pigment used in this investigation was Kronos KR2190, which has

several inorganic oxides at its surface and an organic compound as well. This

compound is added by the manufacturer to improve the dispersability of the pigment.

Three experiments were performed to investigate the influence of the organic

compound. For the first experiment the pigment was used as received, for the second

the organic compound was removed by heating the pigment to 400°C for 24 hours,

and for the third experiment the pigment was treated and modified as described in

chapter 3. The other reaction components and conditions were the same as described

above for RLK.

It was found (table 6.4) that KR2190, which was used as supplied (i.e. with

the tri-methylol-propane added by the manufacturer, but without a modification with

titanates) gave the best results, both in terms of efficiency and stability. The

efficiency was 88.5 wt% and only 0.3 g of coagulate (0.5 wt%) could be collected

from the bottom of the reactor.

96 Chapter 6

Table 6.4 Influence of the modification and of the pigment type on the efficiency and on thestability of encapsulation reactions peiformed with OT-100 (4.1 mmollL) and Sty (15.15 mL per litrewater). Temperature: 60°C

pigDlent mOdificiitionI ,...

(wt%)

RLK none 41 complete

RLK-CA1O titanate 39 negligible

KR2190 none (heated) 66 28 wt % of solids

KR2190 commercial 88.5 0.5 wt% of solids

KR2190-CA10 titanate 53 3 wt % of solids

The pigment modified with CAW came in second: an efficiency of 53 wt%

and 2 g of coagulate (3 wt%). Actually the efficiency of the third experiment, where

the organic compound had been removed and no modification with titanate had been

applied, was higher (66 wt%), but this value is somewhat obscure due to the large

amount of coagulate that was found: 19.5 g (28 wt%).

Apparently, in the case of KR2190, the organic 'modification' applied by the

manufacturer is more adequate than a modification with titanates, which was also

revealed by TEM-photographs. Of course this makes the process of pigment

encapsulation commercially more attractive, because it means that the modification

and subsequent purification steps described in this thesis can be omitted. In order to

be able to compare the experiments in which KR2190 was used with those performed

earlier in which RLK was used, in this investigation KR2190 was always modified

with titanates, apart from the ones mentioned in this section.

Furthermore, it seems that the modification primarily serves to improve

interaction with the surfactant, and thus improves the stability, and that the

mechanism of encapsulation itself is not through swelling of the hydrophobic layer,

but by capturing oligomers from the aqueous phase.

The role of the surfactants in encapsulation reactions

6.5.2 The influence of the pigment concentration on encapsulation reactions

97

In literature34,41,42 and in previous chapters it has already been mentioned that

the pigment surface area plays an important role in encapsulation reactions. The

number of pigment particles preferably exceeds the number of free micelles. Most

reactions in this investigation have been performed at a pigment concentration of ap­

proximately 55 g per litre water. Under optimal conditions (a redox initiator and an

OT-100 concentration of 4.1 mmol/L) the maximum efficiency obtained in reac-tions

with KR2190-CAlO and Sty was 53 wt%, with only 2 g of coagulate (3 wt%).

Reactions with KR2190, like the ones described in literature, benefit from an

increase in surface area. A reaction with 108 g KR2190-CA10 in one litre of water

was conducted at the same optimal reaction conditions that earlier led to an efficiency

of 53 wt%. Merely the surfactant concentration (OT-1OO) was raised to 5.6 mmol/L

to compensate for the larger surface area. Thus the efficiency was increased to 63

wt%, but at the cost of the stability of the reaction mixture: 13.8 g of coagulate

could be collected from the reactor bottom (11 wt %). However, at a concentration of

396 g KR2190-CAlO in one litre water ([OT-1OO]= 13.8 mmol/L) the amount of

coagulate found was negligible. The efficiency found was 108 wt %. This value

indicates that some coagulation must have taken place, albeit that the error introduced

with the calculation of the efficiency on basis of the equation for PC3 described in

section 3.4.2 almost fully explains the supernumerary 8 wt %. The fact that repeated

centrifugation and mixing with surfactant did not result in a separation of free poly­

mer particles further justifies the finding that all polymer was present at the pigment

surface. If coagulation did take place it can only have been of minor importance.

6.6 Concluding remarks

The stabilizing properties of the non-ionic surfactants Antarox CO-880 and

CO-990 are insufficient for encapsulation reactions. The concentrations needed to

keep the reaction mixture stable are so high that the advantage of the high affinity of

98 Chapter 6

these surfactants to the modified pigment surface is cancelled by the large number of

free micelles formed at these concentrations. An Antarox CO-880 based inisurf

displays similar effects, which are probably enhanced by a low CMC and by a low

cloud point.

Despite a low CMC value the use of sulfosuccinate Aerosol OT-loo leads to

satisfactory encapsulation results. During reactions with styrene the stability is

strongly improved as compared with reactions with SDS, while the encapsulation

efficiency can reach the same or slightly higher values. In the case of MMA indeed

the stability is improved or comparable with reactions conducted with SDS, but the

efficiency is substantially lower.

The addition of SDS during batch encapsulation reactions with styrene may

offer additional stability if the initial surfactant concentration is high enough. A

reaction where the initial SDS concentration was too low led to more coagulation as

compared with a reaction without surfactant addition where the initial SDS

concentration was higher. In a reaction where, beside the surfactant, the monomer

was added semi-continuously as well (under monomer starved conditions) the stability

was greatly improved as compared with a reaction without surfactant addition (but

with a higher initial SDS concentration). Even better results are expected when a

surfactant like OT-100 is used, both initially and for the controlled addition.

The surface of the commercially available KR2190 pigment seems to be of

such a nature that a modification with titanates is unnecessary. In fact during

experiments with styrene better results were obtained with the unmodified pigment,

both in terms of stability and of efficiency. However, in case of RLK the

modification is necessary in order to obtain a stable reaction mixture and high

encapsulation efficiencies.

Finally the efficiency of an encapsulation reaction can be improved by

increasing the pigment concentration. If the concentration is high enough all polymer

(PS) will be located at the pigment surface, without the problem of massive

coagulation.

Chapter 7

COPOLYMER ENCAPSULATION REACTIONS

7.1 Introduction

The success of encapsulation and the resulting compatibility of encapsulated

pigment particles with a polymer matrix is determined by the three parameters

already mentioned in chapter 1: the efficiency of the encapsulation reaction, the

stability of the reaction mixture during and after the reaction, and, finally, the

chemical composition of the encapsulating polymer. In the previous chapters the

possibilities of achieving high encapsulation efficiencies and stable reaction mixtures

have been discussed. In this chapter the possibility of forming copolymers and/or

multi-layered polymer shells at the pigment surface will be focused on.

The use of copolymers, instead of or in combination with homopolymers,

offers the opportunity to improve polymer-polymer interactions (between the surface

and the matrix polymer), or to adjust the (mechanical) properties of the polymer at

the pigment surface. One can think of properties like the hydrophobicity, elasticity or

glass transition temperature (Tg).

In some cases it can be advantageous to create multi-layered polymer shells

around the pigment particles, especially when a polymer required for good

interaction with the polymer matrix can not be formed directly at the pigment

surface. In such a case one can start with a monomer that can be polymerized at the

pigment surface, followed by the monomer that interacts well with the polymer

matrix. In order to obtain the best interaction between the first and the second

polymer at the pigment surface, a stage of copolymerization may be inserted, during

which both monomers are added simultaneously (see figure 7.1). Power-feed

techniques92,93 may also be used to go from one monomer (mixture) to another.

100 Chapter 7

~polymer 1

~polymer2

copolymer

Figure 7.1 Optimal adjustment of the surface properties of encapsulated pigments bycopolymerization or by sequential.addition of monomers or monomer mixtures

7.2 Experimental procedures

7.2.1 Encapsulation reactions

The experimental procedures for the pigment modification and for the

subsequent polymer encapsulation are described in chapter 3. The materials and

quantities used for copolymerization reactions are described in table 7.1. The

monomers were added semi-continuously. The reactions COl to C04 were initiated

with SACPA, while the chain transfer agentl-dodecyl mercaptan (NOM) was added

in order to reduce the molecular weight of the polymer. The other reactions were

initiated by a redox system while no NOM was added.

In the reactions C04, COS and COS a monomer mixture (table 7.1: MS:

MMA and Sty, or MB: MMA and BMA) was added with one dosimat. In the other

reactions the monomers were added by means of two dosimats, including the

'monomer mixture' (MS or MB: the monomers were added simultaneously, but by

separate adding devices). The monomer fractions (fMMA> fsly and fBMJ were based on

the total feed. The addition rates (in mLimin) of the monomers were as follows:

COl: MS: 0.1 (for 90 minutes), Sty: 0.2 (60 min.) and MMA: 0.2 (142 min.)

C02: Sty: 0.05 (90 min.), MS: 0.2 (90 min.) and MMA: 0.2 (104 min.)

C03: MMA: 0.2 (90 min.), MS: 0.2~.1 (100 min.) and Sty: 0.1~.2 (60 min.)

C04: MS: 0.05 (90 min.), MS: 0.15 (60 min.), MS: 0.2 (120 min.)

C06, 7, 9 and COlO: 0.05 mLimin for each monomer; this means that the 'mix'

(MS or MB) is added at an overall rate of 0.1 mLimin.

COS and COS: 0.05 mLimin (overall)

d hb d hclaTable 7.1 Materials and quantities used in copolymer encapsu tion reactions. oncentratlons are ase on t e amount of water un ess state ot erwzse................

...........

..·······.····.······· ..·.i.......................................

•••••••••••

........... i ....· .. · ..........i........ ............... ....> .·.......iii.... .....

Exp:....

••• EiP: liiP: :E:!P:I ........

Material COl CO2 IC03. <:;04 .. ,?I.J ............... COS C09 COlO·.·••··I. ...

TiOz 55 giLwater: 900mL

SDS: 12.9 12.9 11.4 12.9 9.3 9.3 9.3 9.3 9.3 9.3 mM

initiatior :-SACPA 1.3 1.3 1.3 1.3 - - - - - - mM- CHP - - - - 5 5 5 5 5 5 mg/g- FeS04 - - - - 0.017 0.017 0.017 0.017 0.017 0.017 mM- EDTA - - - - 0.017 0.017 0.017 0.017 0.017 0.017 mM- SFS - - - - 0.64 0.64 0.64 0.64 0.64 0.64 mM

chain transferagent 2.08 2.01 2.13 2.14 - - - - - - pphm

monomer:- fMMA 0.77 0.71 0.56 0.52 0.51 0.52 0.52 0.50 0.60 0.60 (-)- fsty 0.23 0.29 0.44 0.48 0.49 0.48 0.48 - - - (-)- fBMA - - - - - - - 0.50 0.40 0.40 . (-)

1 MS Sty MMA MS MS Sty MMA MB BMA MMAsequence: 2 Sty MS MS MS MS MS MS MB MB MB

3 MMA MMA Sty MS MS MMA Sty MB MMA BMA

total amount ofmonomer added: 43.4 43.3 42.4 43.5 20 20 20 20 20 20 mL

102

7.2.2 Extraction of surface (co-)polymers

Chapter 7

After the encapsulated pigment had been separated from the free polymer (see

the procedure in section 3.4.1) the surface polymer was removed from the TiOz­

surface by means of extraction. The extractions of the surface copolymers of

reactions COl to C04 were performed in glass 100 mL reactors. To the reactor 5 g

of encapsulated pigment was added together with 50 mL of tetrahydrofuran (THF).

The mixture was stirred for 24 hours with a magnetic stirring device at 25°C. Nz

was led over the reaction mixture to prevent the formation of explosive mixtures of

peroxides. The (soxlett) extraction of the products of the other reactions were

performed with THF in a Soxtec System HTI (Tecator AB) at 120°C for 24 hours.

Here 3 g of encapsulated material was dispersed in 80 mL of THF.

After extraction, the polymer, now dissolved in THF, was separated from the

pigment by means of centrifugation (at 7000 rpm for 8 minutes). Thereafter, the

efficiency of the extraction procedure was determined (by means of thermo­

gravimetrical analysis of the, dried, pigment), which was found to vary from 80 to

100%. The dry polymer was obtained by evaporating the THF (reactions COl to

C04). Another method is to add the polymer solution to a tenfold excess of

methanol, after the pigment has been removed, to allow precipitation of the polymer

(reactions C05 to COlO). If no precipitation took place 200 mL of distilled water

was added under vigorous stirring to enhance precipitation. Next, the polymer was

dried under vacuum at room temperature. Finally, the efficiency of the precipitation

procedure was determined, which was found to vary from 46 to 82 %.

7.2.3 Preparation of standard (calibration) copolymers for HPLC

In order to be able to analyze the surface (co-)polymers with High

Performance Liquid Chromatography (HPLC) standard copolymers had to be made.

A sequence of 7 (co-)polymers per monomer combination (MMA-BMA or MMA-

Copolymer encapsulation reactions 103

Sty) was prepared. The mole fraction MMA in these (co-)polymers ranged from 0 to

1: 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0.

The copolymers were produced by adding the pertaining monomer mixture

semi-continuously to a mixture of 0.07 g SDS, 0.013 g potassium persulfate and

0.005 g NaHC03 in 60 g distilled water (oxygen free). The monomer (mixture) (20

mL) was added at a rate of 0.014 mL/min (for a period of 24 hours).

7.2.4 Characterization of (suiface) copolymers: lH NMR and HPLC

Analysis of copolymers with lH NMR

The overall chemical composition of the surface and the free polymer from

reactions C05 to COlO, and of the standard (co-)polymers mentioned in 7.2.3, were

determined by means of lH NMR. The polymer was dissolved in deuterated

chloroform (CDCI3) and analyzed at 300 MHz and 25°C with a Varian Gemini-3oo.

The lH NMR spectra of MMA-Sty and of MMA-BMA copolymers are

displayed in figures 7.2 and 7.3, respectively. The fraction MMA (FMMJ in the

MMA-Sty copolymer is calculated according to equation 7.1:

(B-~*A)/85 7.1

with: A= surface area of the signal caused by the aromatic protons (Sty) at 6 to 7.6ppm in figure 7.2,

B= surface area of the signals caused by the other protons (Sty and MMA)

FMMA of the MMA-BMA copolymers was calculated according to equation 7.2:

F (B-6*A)/8MMA A

- +(B-6*A)/82

7.2

with: A = surface area of the signal belonging to the O-CH2 protons of BMA infigure 7.3,

B= surface area of the signals caused by the other protons (MMA and BMA).

104 Chapter 7

rTTf'T,T,,' 1-'-I-T'I-T,·rrT"'-,,-rn-'T,rr-, T' '-"-r"'--'-'- r r' r,-rr"r ,"rTTTrTTTTT,rT,-'(Trrrl-r'"TTTT1"j ',-TIT","r 1Tl r rrrr .---rrT-III ~J B 7 ti :J ~ 3 ,-! I PPM

A

Figure 7.2 1H NMR spectrum of an MMA -sty copolymer in deuterated chloroform at 300 MHz and25°C.

A B

Figure 7.3 IH NMR spectrum of an MMA-BMA copolymer in deuterated chloroform at 300 MHzand 25°C.

Copolymer encapsulation reactions 105

Analysis of copolymers with HPLC

The ~hemical ~omposition distribution (CCD) of copolymers can be

determined by means of gradient high Qerformance liquid ~hromatography (gradient

HPLC) as described e.g. by van Doremaele et at. .94,95 For the reactions COl to C04

a 5 J-tm silica column (150 x 4.6 mm) was used. The solvents were n-heptane,

tetrahydrofuran (THF) and methanol. The eluent gradient applied was multi-linear

and is given in table 7.2a. The injection volume was 30 J-tL and the temperature was

90°C. Prior to injection the polymer samples were dissolved in a mixture of 90 vol %

THF and 10 vol % methanol.

For the other reactions the column used was a C18 Novopak Guard Pack,

which actually is a pre-column (length: 10 mm). The solvents used for reactions

COS to C07 (MMA-Sty) were THF and water (table 7.2b), or acrylonitrile (ACN,

Merck HPLC-grade) and THF (table 7.2e). For reactions COS to COlO (MMA­

BMA) THF and water were used (table 7.2d).

Table 7.2a Eluent gradient applied for the analysis of the MMA-Sty (co-)polymers of reactions COlto C04

.... flow Il~heptane THF methanol.tillle(min·) (tnI>/iniD) ... vol% vol% vol%

0 0.6 70 30 01.99 0.6 35 59 62.00 0.3 35 59 613 0.3 0 90 10

Table 7.2b Eluent gradient applied for the analysis of the MMA -Sty (co-)polymers of reactions costo C07

time Dow water THF(JIlin.) (JIlL/min) vol% vol%

0 1 100 030 1 50 5033 1 0 10035 1 100 0

106 Chapter 7

Table 7.2e Eluent gradient applied for the analysis of the MMA-Sty (co-)polymers of reactions C05to C07

time(min.)

o303335

flow(mL/mi!l)

1111

10040o

100

_~_. II

TlFvo ~

o60

100o

Table 7.2d Eluent gradient applied for the analysis of the MMA-BMA (co-)polymers of reactionscos to COlO

time(min.)

o303335

10070o

100

o30

100o

Of the gradients for the MMA-Sty (co-)polymers those of table 7.2a and c

gave a good separation. The gradient used for the MMA-BMA (co-)polymers gave

satisfactory results, but other gradients are currently under investigation in order to

improve the separation even further.

7.3 Copolymer encapsulation of Ti02

7.3.1 (Co-)polymerization of MMA and styrene

The influence of a chain transfer agent

The use of a chain transfer agent (eTA) like 1-dodecyl mercaptan (NDM) in

'regular' emulsion polymerization reactions typically will lead to a lowering of the

molecular weight of the polymer. It was found,72 that the weight average molecular

weight (Mw) of the PMMA formed dropped from 1980 kg/mole, when no NDM was

Copolymer encapsulation reactions 107

used, to 40.5 kg/mole at the highest concentration of NDM applied (2.125 parts of

NDM per hundred parts of monomer = 2.125 pphm). At this NDM concentration in

batch reactions (homopolymerizations) inhibition occurred and the initiation period

(interval I) was relatively long, but the reaction rate was hardly influenced by the

presence of CTA. During semi-continuous experiments of the /NDM did not have an

effect on the reaction behaviour (inhibition, initiation, reaction rate) at all.

Table 7.3 The influence of a chain transfer agent on encapsulation products (monomer: MMA)

[NDM](PPbtn)

1.002.12

PC3

(mg/g TiOJ

155.471.472.9

M....urf(kg/mole)

65.6 2.89

M..,free(kg/mole)

113 1.72

The addition of NDM during encapsulation reactions (monomer: MMA) has

one very important effect beside the ones mentioned above: the amount of surface

polymer (PC3) is reduced (table 7.3). It was also found that the molecular weight of

the surface polymer (Mw,surf) is much lower, while its polydispersity «MjMJ,urf) is

much higher than that of the free polymer (Mw,rr.J.

These effects will probably be the result of different polymerization kinetics at

the surface and in the free polymer particles. Possible explanations include a higher

concentration of NDM or a higher number of free radicals at the pigment surface.

The first situation would result in the increased occurrence of chain transfer reactions

which in turn could lead to a lower molecular weight and more exit of radicals. The

latter would decrease the polymer content of the pigment.

The second possible situation (high number of free radicals) may lead to an

increase in termination reactions which, in the case of MMA, often means

disproportionation.52 This of course can happen both in the presence and in the

absence of NDM, but probably in the presence of NDM the chains remain shorter,

thus enhancing exit. However, further research is needed for conclusive explanations

of these phenomena.

108 Chapter 7

The use of NDM in copolymer encapsulation reactions also resulted in rather

low polymer contents (table 7.4), compared with those found for homopolymers

(chapters 4 and 5). The amount of surface polymer tends to be slightly enhanced by

low initial monomer addition rates and by starting with the addition of a monomer

mixture rather than a single monomer.

Table 7.4 Copolymerization of MMA and Sty at the surface of TiOz-CAlO in the presence of achain transfer agent. Other conditions in table 7.1. Reaction temperature: 60°C

.....6·••• .< .••...•••.•••. .•••••••••••.•••••••.. <ii <i ···fiB·.·. ................. i

Exp. [NOM] [8D8] ...• ...;<-") •. -/ <~

........ >'t# (ppbn:1) mlllollL .... :~) ........•••• ....

COl 2.08 12.9 0.77 MS ..Sty..MMA 64.3 8.7

CO2 2.01 12.9 0.71 Sty.. MS ..MMA 45.6 6.2

cm 2.13 11.4 0.56 MMA.. MS ..Sty 57.4 8.0

C04 2.42 12.9 0.52 MS 73.5 10.1

Initially, the major reason for using a chain transfer agent was to allow

extraction of the surface polymer. Based on experimental results of Caris41 it was

suggested that without NDM the molecular weight would be to high. 72 In the present

investigation the latter assumption could not be confirmed: the surface polymer could

be separated from the pigment almost completely, regardless whether NDM was used

or not.

Chemical composition (distributions) ofMMA-Sty copolymers

The encapsulation efficiency and the overall chemical composition of both free

and surface copolymers of encapsulation reactions with MMA and Sty are displayed

in table 7.5.

Copolymer encapsulation reactions 109

Table 7.5 Encapsulation efficiencies, and the overall chemical composition of sulface and freepolymers (MMA-Sty) as determined with HPLC and NMR

.... HPLC NMR..

Exp. fMMA 11 FMMA,HPLC,s F MMA,HPLC,f FMMA,NMR,s FMMA,NMR,f

# (-) (wt%) surface free surface free

COl 0.77 8.7 0.93 -- -- --

CO2 0.71 6.2 0.91 0.65 -- --

C03 0.56 8.0 -- 0.54 -- --

C04 0.52 10.1 0.40 0.40 -- --

C05 0.51 35 0.48 0.49 0.49 0.59

C06 0.52 42 0.56 0.61 0.55 0.57

C07 0.52 23 0.59 0.60 0.61 0.58

The large difference between the dficiency of the first four and the last three

reactions clearly is the result of the fact that in the latter case the reaction conditions

were much better: a low SDS concentration (9.3 mmol/L instead of 11.4 or 12.9

mmol/L), a better initiator system (redox instead of SACPA), and lower monomer

addition rates. In obtaining high encapsulation efficiencies it seems to be favourable

to have Sty present at the beginning of the reaction, either alone (C06) or in a

mixture with MMA (COl, 4 and 5). In the case of C02 the efficiency was

extremely low, but here the overall fraction of styrene in the feed was rather low as

well (0.29 based on the entire reaction), while the addition rate in the second

addition step (a mixture of MMA and Sty) may have been too high (0.2 mLimin.).

The overall copolymer compositions in table 7.5 have been determined by

means of NMR and HPLC, which give comparable results. In general the mole

fraction of MMA in the free polymer did not differ much from that in the surface

polymer, unless the amount MMA added was much larger than the amount of Sty

(COl and 2).

110 Chapter 7

It seems that sequential addition of monomers (instead of just a mixture of

monomers) leads to higher fractions of PMMA at the pigment surface, especially if

. MMA is the first monomer added (C03 and C07). This could be an indication that

most of the surface polymer is formed in the beginning of the reaction, as was

observed for batch encapsulation reactions with one monomer also. 41

Reactions C04 to C07 all were conducted at an overall feed composition

approximately equal to the azeotropic composition (fMMA = 0.51). The azeotropic

mixture which was fed in COS in fact resulted in a copolymer of the same overall

composition (within the experimental error).

In all reactions other than COS either the fraction MMA in the free polymer

(FMMA,f) or in the surface polymer (FMMA,.) should be lower, given the composition of

the feed. The difference that is not accounted for by the experimental error is

probably caused by the fact that the monomer was not fully converted into polymer,

especially in those reactions where styrene was the last monomer added. However,

the experimental procedure of obtaining the surface polymer can playa role also: in

most cases 80 to 100% of the polymer is removed from the pigment surface, but

between 18 and 54% of the removed polymer is lost during precipitation. Of course

this problem does not occur when the polymer is obtained by evaporation of the

solvent, but in this case contaminants (traces of surfactant for instance) that could not

be removed during (earlier) purification steps will remain in the polymer.

The chemical cOmpOSitiOn distribution (CCD) was found to be influenced

rather strongly by the order of monomer addition. Addition of a monomer mixture

resulted in a copolymer (COS) with an F MMA of approximately 0.45. Only a slight

difference was found between the HPLC signals of the free polymer and the surface

polymer (the CCD of the surface polymer is shown in figure 7.4). This was also the

case in reaction C07 where the monomer addition was started with MMA, followed

by a mixture of MMA and Sty, and where finally pure Sty was added. In this case a

negligible amount of homopolymer (PMMA) was found, next to two clearly visible

copolymers with an F MMA of approximately 0.22 and 0.65, respectively (CCD of the

surface polymer: figure 7.5).

Copolymer encapsulation reactions

0.008 ..---------------- 1.0/ ..~-

0.006!~ 0.8

0.60.004

3~~

0.4~

0.0020.2

0.000 0.00.0 0.2 0.4 0.6 0.8 1.0

FMMA

111

Figure 7.4 CCD of the surface polymer of reaction C05, where an azeotropic monomer mixture ofMMA and Sty was added

0.004 .,.------- 1.0..!~

0.80.003

0.60.002 3~~

0.4~

0.0010.2

0.000 0.00.0 0.2 0.4 0.6 0.8 1.0

FMMA

Figure 7.5 CCD of the surface polymer of reaction C07, where MMA was added firstly, followedby a mixture of MMA and Sty, and finally by pure MMA

Finally, for reaction C06 where Sty was added firstly, followed by a mixture

of MMA and Sty and which was ended with the addition of MMA, a slight

difference was found between the CCD of the free polymer and the surface polymer

112 Chapter 7

(see figure 7.6 for the CCD of the surface polymer). Beside some homopolymer one

copolymer was found (FMMA~ 0.47), instead of the two that were found in

.experiment C07. Furthermore, the copolymer is rich in Sty. The CCD of the

surface polymer was broader than that of the free polymer, a result of inhibition

causing the build-up of monomer (Sty) at the beginning of the reaction (monomer

starved conditions not fully obtained).

0.004 .-~_.-._-_.---- 1.0, ...-.

0.003It- 0.8

0.60.002 S~~

0.4~

0.0010.2

0.000 0.00.0 0.2 0.4 0.6 0.8 1.0

FMMA

Figure 7.6 CCD of the suiface polymer of reaction C06, where MMA was added firstly, followedby a mixture of MMA and Sty, and finally by Sty alone

From the above one can conclude that it is possible to adjust the composition

of the surface polymer. Even layers of different composition seem to be feasible.

However, no conclusive evidence can be given as to whether the various (00-)

polymers actually form layers and that they are formed in the same order as the

monomers were added. If polymers are formed in the pursued order it still remains

unsure whether the resulting morphology will be maintained in time. It is possible

that the more hydrophilic polymers migrate towards the aqueous phase, while the

hydrophobic polymers become situated closer to the pigment surface. 95 More

research is needed in order to be able to answer these questions.

Copolymer encapsulation reactions 113

Table 7.6 Encapsulation efficiencies of MMA-BMA copolymer encapsulation reactions, and theoverall chemical composition of surface and free polymers as determined with HPLC and NMR

> ....... ....

> < ....... .. HPLC NMR

Ex I)••••.•••• fMMA t1 FMMA,HPLC,s FMMA,HPLC.r FMMA,NMR,s FMMA,NMR,r······h> i····................ H (Wt~) surface free surface free

C08 0.50 57 0.49 0.56 0.50 0.52

C09 0.60 53 0.67 0.64 0.57 0.59

COlO 0.60 42 0.58 0.64 0.55 0.63

7.3.2 (Co-)polymerization ofMMA and BMA

Copolymer encapsulation reactions with the monomers MMA and BMA give

results that are comparable with the MMA-Sty copolymerization described in 7.3.1.

Here also the encapsulation dficienc;y is the lowest if MMA is added first (COlO,

table 7.6). Addition of a monomer mixture (COS) resulted in the highest efficiency,

but here the overall fraction of MMA in the feed (fMM.J is much lower than in the

other two reactions. Again the more hydrophobic monomer, in this case BMA seems

to determine the efficiency. Further, copolymer encapsulation reactions with BMA

suffered somewhat more from coagulation (7.5 wt% based on the total amount of

solids) than comparable reactions with Sty (4.5 wt%). This may be the result of the

low Tg of PBMA.

Like in the case of MMA and Sty, the differences between the overall

chemical compositions as obtained from NMR or HPLC is small in the reactions with

MMA and BMA, and lie within the experimental error. In general the difference

between FMMA in the surface polymer and in the free polymer is not very large

either. The FMMA seems to depend only little on the monomer addition sequence.

114

0.006 1.0

0.8

0.0040.6

~~ B0.002

0.4~

0.2

0.000 0.00.0 0.2 0.4 0.6 0.8 1.0

FMMA

Chapter 7

Figure 7.7 CCD of the suiface polymer of reaction COB, where a monomer mixture of MMA andBMA was added

0.003

0.002

0.001

0.6

B0.4~

0.2

0.000 0.00.0 0.2 0.4 0.6 0.8 1.0

FMMA

Figure 7.8 CCD of the suiface polymer of reaction C09, where BMA was added firstly, followed bya mixture of MMA and BMA, and finally by MMA alone

The chemical composition distributions of the MMA-BMA experiments are

similar to those obtained from MMA-Sty experiments as well. Again the addition of

a monomer mixture (COS) resulted in one copolymer. both at the pigment surface

Copolymer encapsulation reactions 115

(figure 7.7) and in the free polymer, both with an MMA fraction of approximately

0.5. Some PBMA homopolymer was present, although it was hard to determine how

much because a 'ghost-peak' has the same retention time as the PBMA-peak.

The second reaction (C09) which started with the addition of BMA and ended

with the addition of MMA, resulted in more copolymers or at least in a much

broader distribution (especially of the surface polymer, figure 7.8), beside some

homopolymer.

The last experiment (COlO), which started with the addition of MMA and

ended with the addition of BMA, gave two co-polymers, one of which was slightly

different in the free polymer (FMMA = 0.70) as compared with the surface polymer

(FMMA= 0.62). At the pigment surface (for the CCD see figure 7.9) hardly any

MMA homopolymer was found, which is surprising, and which can only be

explained if inhibition has taken place.

0.004 1.0

0.003 0.8

0.60.002

S~~

0.4~

0.0010.2

0.000 0.00.0 0.2 0.4 0.6 0.8 1.0

FMMA

Figure 7.9 eCD of the surface polymer of reaction COlO where MMA was added firstly, followedby a mixture of MMA and BMA, and finally by BMA alone

116 Chapter 7

Differential scanning calorimetry

An indication of the copolymer composition can be obtained not only from

.HPLC or NMR results, but also from gifferential ~canning ~alorimetry (DSC)

measurements, as can be seen in table. 7.7. The glass transition temperature (Tg) of

copolymers as calculated either with equation 7.3 or with 7.4 lies around 61°C

based on the Tg's of BMA (approximately 30°C) and MMA (l05°C), combined with

an FMMA of 0.5 (which gives a weight fraction MMA of 0.413, and a weight fraction

BMA of 0.587):

7.3

7.4

with W MMA and wBMA as the weight fractions of MMA and BMA, respectively.

A Tg of 61°C corresponds well with the experimental values found for

reaction COS (table 7.7). Judging from the Tg's the fraction MMA in the free

polymer is slightly higher than that in the surface polymer, as was indicated by the

NMR and HPLC measurements.

Table 7.7 Glass transition temperatures of MMA-BMA (suiface) copolymers as determined withDSC

Exp. fMMA sequence It>'1',< ,I.··········.·

>,•..••••••.•••,•.

<II (-) H (Q( ., ... »....\

C08 0.50 MMA/BMA (mixture) 62.1 64.7

C09 0.60 BMA..MMA/BMA..MMA 64.5 77.5

COlO 0.60 MMA..MMA/BMA..BMA 63.1 90.2

Copolymer encapsulation reactions 117

The DSC data indicate that the order of monomer addition has only little

influence on the composition of the surface polymer, since the respective Tg's at the

surface differ only slightly. However, it is striking that the Tg of the free polymer

strongly depends on the order of monomer addition. One would expect a reaction

ended with the addition of MMA (C09) to exhibit the highest Tg for the free

polymer, since normally the surface polymer is formed during the first period of the

reaction, while the remaining monomer forms free polymer. Apparently, this is not

what happened during experiment COlO, that yields the highest Tg of the free

polymer. It is possible that inhibition has taken place during the first addition step (of

MMA, COlO), or that BMA oligomers formed during the second or third addition

step have replaced the (P)MMA already present at the surface, although the latter

seems less likely. Inhibition, however, would also explain why the encapsulation

efficiency of this particular reaction is rather low, and why the fraction MMA in the

surface polymer is lower in reaction COlO as compared with reaction C09 (table

7.6).

Multiple Tg's as expected with sequential monomer addition were not clearly

observed. However, the Tg's mentioned in table 7.7 are determined from relatively

broad ranges and therefore refer to broad CCD ranges as well, especially of

reactions C09 and COlO: after some build-up of the firstly added monomer has

occurred then mainly copolymer will be formed. The broadness of the Tg-range is in

correspondence with the HPLC data: the broader the CCD, the broader the Tg-range.

Tg's of homopolymers (especially PBMA) were sometimes observed during a first

DSC run, which may have been amplified by traces of monomer still present: during

a second run these homopolymer Tg's disappeared almost completely.

7.4 Concluding remarks

In an encapsulation reaction the addition of a chain transfer agent reduces the

molecular weight of both surface and free polymers. However, for the removal of

surface polymer from the pigment surface, the addition of a CTA is not necessary.

118 Chapter 7

The surface polymer could well be extracted with tetrahydrofuran regardless whether

a CTA was used or not. On the other hand, the encapsulation efficiency was

substantially lower when a CTA was used.

Formation of copolymers of MMA with either styrene or BMA at the pigment

surface was found to be possible. HPLC and DSC measurements indicated that

multiple polymer shells can be formed at the pigment surface. However, inhibition

occurring during the first addition step may strongly affect the formation of the

(homo-)polymer pursued in that first addition step.

EPILOGUE

The principle of encapsulating inorganic particles with a layer of polymer

through an emulsion polymerization has been a promising approach for more than a

decade now. Although several companies and universities have followed this

approach in the past, only a few successful applications can be recorded. One of the

main benefits encapsulation would bring is improved compatibility between the

polymeric matrix and the inorganic particles contained in that matrix. Usually

'maximum' properties are obtained when the inorganic particles are distributed

evenly and as single (primary) particles in the matrix. This means that in the various

steps towards obtaining the final product dispersing the inorganic particles is of major

importance. Also in that respect encapsulated particles would improve dispersability

during the different stages of the production process (as well as in the final matrix).

The major problem of dispersability is therefore shifted towards the

encapsulation process itself. Initially the 'bare' particles should be well dispersed in

the aqueous phase and (partial) coagulation during the polymer encapsulation step

must be avoided at all times. In this first step (partial) coagulation followed by

encapsulation would lead to irreversible adherence of the coagulates.

On this criterion many ambitious programs have failed. During previous work

on encapsulation in our group we also encountered these problems in the form of

'plateaus' occurring in conversion-time plots caused by partial coagulation. This

problem is greatly diminished in this work by improving the mixing conditions.

In order to enable dispersion of the inorganic particles in a reaction medium

typical of emulsion polymerization with similar surfactants, and to create an

environment that enhances polymerization at the pigment surface, in advance the

inorganic particles should be more or less hydrophobized. In this work the particles

120 Epilogue

were hydrophobized with titanates, but it is shown in chapter 6 of this thesis that

commercially available hydrophobized Ti02 particles can be equally suitable,

although their surface is not as well defined.

After these more or less hydrophobic particles have been dispersed with

conventional surfactants like SDS an emulsion polymerization is performed. During

this process a delicate balance has to be maintained between keeping the reaction

mixture (colloidally) stable on the one hand, and preventing formation of free

polymer particles on the other hand. Although an optimum in this balance can be

approximated by adjusting the initial conditions (like the surfactant concentration) in

a batch process, it is evident that dynamically maintaining this balance during the

entire course of the encapsulation reaction must give better results.

In our opinion, dynamic control of the surfactant concentration during an

encapsulation process, is the best solution to a major problem that has been

hampering the commercial production of encapsulated particles with a high efficiency

and stability, for many years.

As a contribution to this solution a method has been developed in this work

that gives useful information on the surfactant migration through conductivity

measurements. The conductivity signal is mainly determined by the amount of

molecularly dissolved surfactant and thus gives information on surfactant migration.

For example, adsorption of surfactant molecules onto particles causes a decrease in

the conductivity because the mobility of adsorbed molecules. is lower than that of

molecularly dissolved molecules. In this line of thought the release of surfactant from

a surface (caused by coagulation or shrinkage of particles) and the subsequent

migration to the aqueous phase will cause an increase in the conductivity.

The conductivity measurements have given more insight in several aspects of

both 'normal' emulsion polymerizations and encapsulation reactions. For instance, it

was found that the monomer influences micelle formation to a great extent, resulting

in a strong decrease in the CMC, which is an important finding that was not well

recognized in (encapsulation) literature so far and which helps to better understand

Epilogue 121

the conductivity signal during an entire emulsion polymerization. Because the number

of micelles is influenced very much by the monomer concentration in the aqueous

phase, conductivity measurements can also monitor the appearance or disappearance

of monomer droplets, which is accompanied by a discontinuity in the conductivity

signal.

From the conductivity measurements it can also be deducted that with the

more water soluble monomers (at low solid contents) it is very well possible that

Smith-Ewart (SE) interval III (the disappearance of the monomer droplets) can occur

before SE interval I has ended, implying that new particles can be formed even

during interval III. The beginning of both interval I and III can be detected by means

of on-line conductivity.

The overall understanding of the conductivity signal during both 'normal'

emulsion polymerization and encapsulation reactions forms the basis of using the

conductivity signal in adjusting monomer addition profiles (semi-continuous

reactions) and surfactant addition profiles.

It was found that during an emulsion polymerization, after addition of the

reaction components, the conductivity signal will initially be constant. Depending on

whether inhibition occurs, this period of constancy can be almost invisible or last for

minutes or longer. As soon as the reaction starts (SE interval I), surface area is

created through formation of new particles or growth of a polymer layer on titanium

dioxide. The increase in surface area will continue during a large part of the reaction

and, with no other effects present, would give a continuous decrease in conductivity

that becomes less pronounced at higher conversion. In case shrinkage of the particles

occurs at high conversion (as is the case of e.g. styrene) even a small decrease in

surface area will be reflected in a significant increase in conductivity.

Superimposed on this curve is the increase in the concentration of molecularly

dissolved surfactant as soon as the monomer concentration in the aqueous phase

decreases at the beginning of SE interval III. Depending on the number of micelles

122 Epilogue

left at this stage of the reaction, a small or a larger increase in conductivity is

observed. For the more water soluble monomer MMA at relatively low solid

contents of the reaction mixture, the monomer droplets disappear early in the

reaction and at this point SE interval. I has not finished yet so a relatively large

conductivity increase is observed. With styrene this effect has almost disappeared,

because in this case a regular SE interval II occurs and no micelles are present at the

beginning of SE interval III.

In the case of a semi-continuous addition of monomer (especially MMA) only

the continuous decrease in conductivity is observed, unless the addition of monomer

is too fast and monomer droplets are formed. In the latter case the normal interval 11­

interval III transition can be observed in the conductivity signal. So conductivity

measurements are also very useful in semi-continuous experiments to check whether

starved conditions are maintained or whether inhibition/retardation occurs. Monitor­

ing of semi-continuous reactions in this sense is especially profitable in emulsion

copolymerization reactions where the so called 'optimal monomer addition profiles'

are utilized.

All of the above findings are also applicable to the encapsulation reactions,

either batch or semi continuous. Because under starved conditions the efficiency of

the encapsulation reactions is higher, most of the encapsulation reactions are

performed semi-continuously. The insight in the influence of monomer, surfactant

type and ion concentration on the apparent CMC greatly improved the prevention of

secondary nucleation and thus improved the efficiency of the encapsulation reactions.

A (preliminary) series of surfactant addition experiments makes use of the

conductivity measurements and indicate that indeed the efficiency and the stability in

encapsulation reactions can be improved by controlled addition of surfactant.

However, further optimization of the surfactant addition has to be performed. In this

respect also surfactants other than SDS have to be tested, like sulfosuccinates. The

latter improved the stability of the system during regular encapsulation reactions with

Sty (without semi-continuous surfactant addition).

Epilogue 123

In those cases where nucleation is not micellar, e.g. in case of homogeneous

nucleation, control of the surfactant concentration will only partly solve the problem

of secondary nucleation. In general, in this thesis, it is found that the more

hydrophilic monomers give a lower efficiency in encapsulation reactions. The

efficiency is somewhat improved by directing radical formation towards the particle

surface e.g. by selecting more hydrophobic or interface-bound initiator systems. The

encapsulation efficiency can also be improved by offering more surface area of

titanium dioxide particles to the oligomers formed in the aqueous phase that might

otherwise form new particles.

The final step in a route towards application of particle encapsulation lies in

the improvement of compatibility of the polymer layer and the polymer matrix. For

this purpose it was shown that different (co-)monomer addition sequences can lead to

various core-shell structures and that it is possible to adjust the compositional build­

up, for example the Tg of the polymer layer, in a predetermined manner.

It was beyond the scope of this thesis to offer a 'ready to go' recipe for large

scale production of encapsulated particles, but rather to gain fundamental insight in

those aspects that so far have been hampering this large scale production. In that

. sense, especially the conductivity measurements already have been very valuable in

gaining basic insight in encapsulation and emulsion polymerization reactions, and are

expected to play an important role in the control of large scale processes leading to

well-defined polymer encapsulated products.

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DANKWOORD

Langs deze weg wil ik graag iedereen bedanken die op enigerlei wijze heeft

bijgedragen tot de totstandkoming van dit proefschrift. Speciaal wi! ik allereerst Akzo

Nobel Corporate Research bv Arnhem (NL) noemen, voor de financiele sponsoring

van dit project. Dr. Nico Heijboer en dr.ir. Roberta Hofman-Caris (beide werkzaam

bij voornoemd bedrijf) wi! ik bedanken voor hun discussies.

Verder gaat mijn dank uit naar aIle (ex-)afstudeerders, (ex-)promovendi en

(ex-)medewerkers van de vakgroep TPK van de Technische Universiteit Eindhoven

voor hun interesse, steun, discussies en soms ook hun experimentele ondersteuning

(HPLC, TEM etc.). Om te voorkomen dat ik iemand vergeet zal ik van deze groep

niemand bij naam noemen, met uitzondering van mijn 'eigen' afstudeerders en

stagiaires: Geert-Jan Derks, Onno van Looy, Rian HoI, Carla Verbruggen, Bart

Cruysberg en Laurens Odekerken.

Mijn vrouw Nicole en mijn ouders wil ik bedanken voor hun algemene

(morele) steun. Als laatste, maar zeker niet als minste wi! ik Ton German en Alex

van Herk bedanken voor hun discussies, ideeen en hun morele ondersteuning.

CURRICULUM VITAE

Roy Janssen werd geboren op· 13 september 1966 te Roermond. In 1984

behaalde hij het diploma Gymnasium-~ aan de scholengemeenschap St. Ursula te

Horn. In datzelfde jaar begon hij aan de studie Scheikundige Technologie aan de

Technische Universiteit te Eindhoven. Het doctoraalexamen werd afgelegd op 20 juni

1990. Vanaf 1 augustus 1990 was hij werkzaam als assistent-in-opleiding in de

vakgroep Polymeerchemie en Kunststofteehnologie, onder leiding van prof.dr.ir.

A.L. German, van de Technische Universiteit te Eindhoven. Vanaf 1 november 1994

zal hij werkzaam zijn voor Oce-van de Grinten te Venlo, binnen de afdeling

Research.

LIST OF SYMBOLS AND ABBREVIATIONS

Symbols

a

b

fMMA

constant used in equation 4.1 (value: approximately 0.65) [-]

constant used in equation 4.1 (value: approximately -3.29) [-]

concentration of counterion "in (in this investigation: Na+) [moIlL]

monomer concentration in the swollen latex particles [moIlL]

monomer concentration in the swollen latex particles at saturation [mollL]

the average diameter of the polymer particles [nm]

overall fraction butyl methacrylate in the monomer feed [-]

overall fraction methyl methacrylate in the monomer feed [-]

overall fraction styrene in the monomer feed [-]

mole fraction methyl methacrylate in copolymers [-]

initial conductivity of the continuous phase (mostly a surfactant solution) or,if interaction occurs between monomer and surfactant, the fitted value of thisconductivity (now the continuous phase is a surfactant solution saturated withmonomer) as used in Maxwell's law (equation 4.2) [",S/cm]

conductivity [",S/cm]

difference between the initial conductivity of a surfactant/initiator solutionand the conductivity at the point from whereon the conductivity signal canbe described with Maxwell's law [",S/cm]

the increase in conductivity at the beginning of interval III (conductivityregion (IlIa» [",S/cm]

~Kx.IIIb the decrease in conductivity at the end of interval III (conductivity region(IIIb» £!-,S/cm]

m total amount of monomer added during the reaction [g]

Mo amount of monomer aaded at the beginning of a reaction [g]

[M]aq monomer concentration in the aqueous phase [moIlL]

Maq amount of monomer in the aqueous phase at a certain time or conversion [g]

[M]::t monomer concentration in the aqueous phase at saturation [moIlL]

Maq,sat amount of monomer in the aqueous phase at saturation [g]

List of symbols and abbreviations

~,sat

w

W

WfDl

amount of monomer present in monomer droplets at a certain time orconversion [g]

monomer concentration in the swollen latex particles [moUL]

amount of monomer in the swollen latex particles at a certain time orconversion [g]

total amount of monomer per gram polymer in the swollen polymer phase atsaturation swelling [gig]

weight average molecular weight [g/mol]

polydispersity [-]

the number of free polymer particles in the reaction mixture per litre water[L-1]

total amount of (modified) pigment added during the reaction [g]

total amount of polymer formed at a certain time or conversion [g]

polymer content [gig] (see "defInitions" as well)

amount of polymer per gram encapsulated pigment [gig]

amount of polymer per gram bare pigment [gig]

amount of polymer per gram of modified pigment [g]

temperature [K] or [0C]

glass transition temperature [K] or [0C]

the average volume of a polymer particle [cm3]

volume fraction of monomer in the swollen latex particles [-]

volume fraction of monomer in the swollen latex particles at saturation [-]

volume fraction of polymer in the swollen latex particles [-]

volume fraction of polymer in the swollen latex particles at saturation [-]

weight loss (= polymer + titanate + crystal water) of the encapsulated pig­ment as determined by means of TGA [wt%]

weight loss (= titanate + crystal water) of the modified pigment as deter­mined by means of TGA [wt%]

weight fraction butyl methacrylate in a copolymer [-]

weight fraction of a copolymer with FMMA = i [-]

weight fraction methyl methacrylate in a copolymer [-]

the volume of water added to the reactor [cm3]

cumulative of Wi [-]

List of symbols and abbreviations

x fractional conversion of monomer into polymer [-]

XIII fractional conversion at which monomer droplets disappear [-]

11 encapsulation efficiency [wt%] (see "defmitions" as well)

Pmon density of the monomer [g/cm3]

Ppol density of the polymer [g/cm3]

Cjl volume fraction of the dispersed phase in an emulsion [-]

Abbreviations

ACN

ACPA

BMA

CAW

CCD

CHP

CMC

CO-880

CO-990

CTA

DCP

DLS

DSC

EDTA

ESCA

HLB

HPC

acrylonitrile

4,4' -azo-bis-(4-cyanopentanoic acid)

butyl methacrylate

di-isopropoxy titanium di-isostearate

chemical composition distribution

cumene hydroperoxide

critical micelle concentration [mol/L]

apparent critical micelle concentration: CMC corrected for the pre­sence of monomer and/or counterions [mol/L]

the apparent CMC corrected for surfactant adsorption by pigments[mol/L]

nonylphenoxypoly(ethyleneoxY)n-lethanol, with n= 30

nonylphenoxypoly(ethyleneoxY)n-lethanol, with n= 100

chain transfer agent

disc centrifuge equipped with a photo sedimentometer

dynamic light scattering

differential scanning calorimetry

ethylene diamine tetra-acetic acid

electron scattering for chemical analysis

hydrophilic-lipophilic balance

hydroxyl propyl cellulose

List of symbols and abbreviations

HPLC

KR2190

KR2190-CAlO

MB

MMA

MS

NDM

NMR

OT-loo

PC

PBMA

PMMA

PS

RLK

RLK-CAlO

SACPA

SDS

SE

SFS

SPS

Sty

Surf.d,

TEM

TGA

THF

Ti02-CAlO

high performance liquid chromatography

commercial pigment: rutile titanium dioxide treated with Al20 3 ,

Zr02 and tri-methylol propane (= 2-ethyl-2-(hydroxymethyl)-1,3­propanediol)

KR2190 modified withdi-isopropoxy titanium di-isostearate

monomer mixture of methyl methacrylate and butyl methacrylate

methyl methacrylate

monomer mixture of methyl methacrylate and styrene

l-dodecyl mercaptan

nuclear magnetic resonance spectroscopy

sodium dioctyl sulfosuccinate

polymer content (see "defmitions" and "symbols" as well)

poly(butyl methacrylate)

poly(methyl methacrylate)

polystyrene

commercial pigment: untreated rutile titanium dioxide

RLK modified with di-isopropoxy titanium di-isostearate

sodium 4,4' -azo-bis-(4-cyanopentanoate)

sodium dodecyl sulfate

Smith-Ewart

sodium formaldehyde suifoxylate

sodium persulfate

styrene

amount of surfactant adsorbed by the amount of pigment added toone litre of water [moIlL]

transmission electron microscopy

thermogravimetrical analysis

tetrahydrofuran

titanium dioxide modified with di-isopropoxy titanium di-isostearate

List of symbols and abbreviations

Dermitions

encapsulation efficiency (tV: fraction of polymer at the pigment surface in relation to

the total amount of monomer added [wt%]

graftivity:

polymer content (PC):

amount of polymer per gram pigment (synonymous to

"polymer content") [g polymer/g pigment]

amount of polymer per gram pigment (synonymous to

"graftivity") [g polymer/g pigment]

STELLINGEN

Behorende bij het proefschrift

POLYMER ENCAPSULATION OF TITANIUM DIOXIDE:

EFFICIENCY, STABILITY AND COMPATIBILITY

van R.Q.F. Janssen

1) Het vermelden van een schier eindeloze reeks (kinetische) vergelijkingen in

een artikel dient vermeden te worden, daar deze niet bijdraagt tot een beter

begrip van de materie doch eerder een bron van fouten en misverstanden

vormt.

R.C. Wilson, 2(1" FATIPEC Conference Book (1990), 155.M.A. Jafarizadeh, S. Kama1a1din Seyed-Yagoobi, Iranian J. Polym. Sci. Techn., 3(1) (1994), 38.

2) De bewering van Jain et af. dat de verandering in polymerisatiesnelheid van

een reactie met styreen gepaard gaat met een overgang van een water-in-olie­

emulsie naar een olie-in-water-emulsie zal serieus heroverwogen moeten

worden, daar het moment van de verandering samenvalt met het moment dat

volgens berekeningen de monomeerdruppels verdwijnen.

M. Jain, I. Piinna, Polym. Mater. Sci. Eng., S4 (1986), 358.

3) 'Inisurfs' zullen waarschijnlijk pas echt goed toepasbaar zijn wanneer de

initiatorgroep zich aan het hydrophobe deel van de surfactant bevindt, en

wanneer slechts een radicaal gevormd wordt door bijvoorbeeld een

redoxreactie.

I.M.H. Kusters, "Inisuifs: suiface-aetive initiators", Ph.D. dissertation, Eindhoven University ofTechnology (1993).

4) De verzadigingsconcentratie van monomeer in gezwollen latexdeeltjes kan,

zelfs onder reactiecondities, bepaald worden aan de hand van het minimum in

de geleidbaarheidscurve van die (batch-)reactie.

B.F.I. Noel, R.Q.F. Janssen, W,J.M. van Well, A.M. van Herk, A.L. German, in preparation.

5) Het feit dat de niet lineaire kleinste-kwadraten-methode zo wemlg gebruikt

wordt en men vaak een toevlucht neemt tot het gebruik van gelineariseerde

vergelijkingen, is een gevolg van het feit dat de eenvoud van deze methode

niet onderkend wordt.

A.M. van Herk, accepted by J. Chern. Education (1995).

6) Stadium III van emulsiepolymerisatie (Smith-Ewart) kan een aanvang nemen

(het verdwijnen van de monomeerdruppels) v66rdat stadium I (de deeltjesvor­

mingsfase) beeindigd is.

w.v. Smith, R.H. Ewart, J. Chern. Phys., 16(6) (1948), 592.Dit proefschrift, hoofdstuk 4.

7) De bewering in het proefschrift van Caris dat de reactiesnelheid bij 85 % con­

versie afneemt ten gevolge van het verdwijnen van de monomeerdruppels is in

strijd met het feit dat bepaaId kan worden dat deze druppels in het geval van

methylmethacrylaat bij een conversie van ten hoogste 30% verdwenen zijn.

C.H.M. Caris, "Polymer encapsulation of inorganic submicron particles in aqueous dispersion",Ph.D. dissertation, Eindhoven University of Technology (1990), 107.

8) Weinig termen zijn ongelukkiger gekozen dan de term 'houtvrij papier'

aangezien voor de produktie van deze papiersoort eerder mrer dan minder

bomen gebruikt worden.

9) Het grote aantal misdaden dat gepleegd is en wordt in de naam van (een) God

kan, al dan niet terecht, de indruk wekken dat een 'goddeloze' maatschappij

(zeer) vreedzaam is.

10) Men kan zich afvragen of de terughoudendheid van Groot-Brittannie tegen de

opening van de kanaaltunnel, o.a. ingegeven door een angst voor immigratie

van hondsdolle vossen vanaf het vasteland, op het vasteland van Europa zou

kunnen leiden tot eenzelfde angst voor immigratie vanuit Groot-Brittannie van

koeien besmet met de 'gekke koeienziekte' .

Nieuwsblad, 21 september 1993.Eindhovens Dagblad, 8 februari 1994.