EP0112346A1

EP0112346A1 - Aqueous autoxidisable polymer dispersions

Info

Publication number: EP0112346A1
Application number: EP83901886A
Authority: EP
Inventors: David Vincent Gibson
Original assignee: Dulux Australia Ltd
Current assignee: Dulux Australia Ltd
Priority date: 1982-06-29
Filing date: 1983-06-24
Publication date: 1984-07-04
Also published as: ZA834769B; EP0112346A4; WO1984000169A1; JPS59501165A; ZW14783A1

Abstract

Dispersions aqueuses auto-oxydables d'un polymère formant une pellicule. Un matériau à support aqueux auto-oxydable et formant une pellicule, exempt de la plupart des défauts des matériaux antérieurs, comprend une dispersion aqueuse de particules d'un polymère formant une pellicule, les particules comprenant un mélange d'un polymère d'addition et d'un composé chimique comprenant au moins deux groupes de formule CH2 = CR-CH2-O-, où R est H ou CH3. Le composé chimique peut être polymère ou non polymère, et les particules peuvent également comprendre d'autres polymères. Un procédé préférentiel de préparation consiste à mélanger un monomère éthyléniquement insaturé, un composé chimique, un composé stabilisateur et de l'eau pour obtenir une dispersion aqueuse stable de particules de monomère-composé chimique, après quoi on provoque la polymérisation du monomère. Lorsque le composé stabilisateur est un composé amphipathique, il doit être exempt d'insaturation éthylénique. Les dispersions décrites par la présente invention sont utiles dans la préparation de compositions de revêtements et d'adhésifs séchant à l'air.Self-oxidizable aqueous dispersions of a film-forming polymer. A self-oxidizing and film-forming aqueous support material, free from most of the defects of prior art, comprises an aqueous dispersion of particles of a film-forming polymer, the particles comprising a mixture of an addition polymer and of a chemical compound comprising at least two groups of formula CH2 = CR-CH2-O-, where R is H or CH3. The chemical compound may be polymeric or non-polymeric, and the particles may also include other polymers. A preferred method of preparation consists in mixing an ethylenically unsaturated monomer, a chemical compound, a stabilizing compound and water to obtain a stable aqueous dispersion of particles of monomer-chemical compound, after which the polymerization of the monomer is brought about. When the stabilizing compound is an amphipathic compound, it must be free from ethylenic unsaturation. The dispersions described by the present invention are useful in the preparation of air-drying coating compositions and adhesives.

Description

AQUEOUS AUTOXIDISABLE POLYMER DISPERSIONS

This invention relates to aqueous dispersions of polymer for use in surface coatings and adhesives and more particularly to such dispersions wherein the particles comprise autoxidisable polymers. Autoxidisable polymers, that is polymers which will crosslink on exposure to oxygen, have been known for many years and have been used extensively in the surface coatings industry. The autoxidisable species which have almost universally been used are the long chain unsaturated fatty acids derived from naturally-occurring triglyceride drying oils; there are other autoxidisable species such as vinyl dioxolane, dicyclopentadiene, certain poly(butadienes) and certain allyl group-containing entities, but the drying oil-derived species have hitherto remained the most widely used.

A goal long pursued by the surface coatings industry is the water-borne autoxidisable coating. There have been many attempts to achieve this. One typical approach has been to prepare a high acid value alkyd resin which can then be water solubilised by means of a base. However this system is prone to hydrolysis and it has never realised its initial promise. A more recent method is the dispersion of a conventional alkyd resin in water with the assistance of a non-ionic surfactant which possesses ethylenic double bonds capable of participating in an autoxidation reaction. Thus, when the film is formed, the surfactant is combined with the alkyd and does not confer water sensitivity on the film. Such an approach is described in European Published Application No. 0 002 252 wherein the ethylenic double bonds are allylic.

An alternative approach to achieving an aqueous autoxidisable coating composition has been to utilise an autoxidisable polymer as one component of a multi-polymer particle. By "multi-polymer particles" I mean particles of film-forming polymer comprising at least two polymers, at least one of these polymers (the "in situ polymer") being formed in the presence of at least one other polymer (the "pre-formed polymer"). This results in a blend of polymers which, depending on the materials used, may be an intimate physical mixture of several different polymers or a blend wherein there has been a degree of reaction between the polymers. Such dispersions promise considerable versatility in the formulation of coating compositions - it may often be possible to obtain coating films with the advantageous properties of several polymers. The general method of preparation of such particles is to dissolve or disperse at least one pre-formed polymer in ethylenically unsaturated monomer, disperse the solution thus formed in water and then polymerise the monomer to form an aqueous dispersion of multi-polymer particles. Examples of such multi-polymer particles can be found in, for example, British patents 1,417,713, 1,421,114 and

1,515,723 and European Published Application 0 010 424.

An example of an attempt to utilise the multi-polymer particle approach of forming a polymer in situ in the presence of a pre-formed polymer to make an autoxidisable dispersion may be found in United States Patent 3,620,989 wherein an alkyd resin is blended with ethylenically unsaturated monomer. In this patent, the alkyd resin has ethylenic unsaturation and is deliberately copolymerised with the monomer. This polymerisation has, however, a poor conversion rate, thus giving the resulting coating compositions an excessively and unacceptably high level of free monomer. I have now found that it is possible to prepare an aqueous dispersion of particles suitable for use as the film-forming component of an autoxidisable coating composition with highly satisfactory performance. I therefore provide, according to the present invention, an autoxidisable film-forming material which comprises an aqueous dispersion of particles of film-forming polymer, the particles comprising at least one chemical compound which comprises at least two autoxidisable groups and polymer formed by the addition polymerisation of ethylenically unsaturated monomer, the dispersion additionally comprising at least one stabilising compound for the particles, characterised in that

(a) at least two of the autoxidisable groups are groups of the formula CH₂ = CR-CH₂-O- where R is H or CH3;

(b) the material of the disperse particles comprises from 0.2 - 35% by weight of the said autoxidisable groups;

(c) the said groups comprise at least 20% of the total number of autoxidisable groups present in the material of the particles; and

(d) when a stabilising compound is amphipathic it is free from ethylenic unsaturation. 1 further provide a process of preparing an autoxidisable film-forming material by the steps of

(a) forming a mixture of monomer, chemical compound comprising at least 2 autoxidisable groups, water and stabilising compound, and

(b) polymerising the monomer to give an aqueous dispersion of particles of film-forming polymer characterised in that

(c) at least two of the autoxidisable groups are groups of the formula

CH₂ = CR-CH₂-O- where R is H or CH₃;

(d) the material of the disperse particles comprises from 0.2 - 35% by weight of the said autoxidisable groups;

(e) the abovementioned groups comprise at least 20% of the total number of autoxidisable groups present in the material of the particles; and (f) where the stabilising compound is amphipathic, it is free of ethylenic unsaturation. It is a surprising result that films of compositions according to my invention touch and through dry remarkably quickly to give tough, hard films - in some cases they can dry quicker than films of conventional solvent-borne alkyd resins. A further surprising fact is that these advantageous properties can be attained even at relatively low concentrations of the groups mentioned hereinabove.

I place no restrictions on the nature of the chemical compound other than it must comprise at least two autoxidisable groups of the formula H₂C=CR-CH₂-O- where R is H or CH₃ (hereinafter referred to as " (meth)allyloxy groups"). These groups may be already present on the compound or they can be added thereto by a suitable chemical reaction. The compound itself may be monomeric, that is non-polymeric and non-oligomeric in nature, for example, pentaerythritol triallyl ether or trimethylol-propane diallyl ether, or it may be oligomeric or polymeric in nature, for example, poly(allyl glycidyl ether). When I refer to an "oligomer", I use the word in its art-recognised sense, that is, "a polymer compound of molecules containing only two, three or a few mer" ("Paint/Coatings Dictionary" by the Federation of Societies for Coatings Technology, 1978). I have found that if the chemical compound is oligomeric or polymeric in nature, there are substantial advantages to be gained and for this reason I prefer to use such chemical compounds.

More than one (meth)allyloxy group-containing chemical compound may be present. Such chemical compounds may also comprise reactive groups capable of reaction with, for example, a cross-linking agent, the polymer formed in situ or other polymer.

It is permissible for the chemical compound to comprise autoxidisable groups other than the (meth)allyloxy groups, provided that at least 20% of the total number of autoxidisable groups shall be (meth)allyloxy groups and that such groups shall comprise from 0.2 - 35% (preferably 1-10%) by weight of the material of the disperse particles.

The polymer particles may also comprise at least one polymer which is distinct from both the in situ polymer and the chemical compound and which does not comprise (meth)allyloxy groups. I have found that when the chemical compound is non-polymeric or non- oligomeric in nature, it is preferable that the particles include such polymer. The polymer may be chosen from a wide range of polymers known to the art and the choice is restricted only by the extent to which such polymer can contribute desirable properties to the final dispersions. The polymer may be convertible or non-convertible; it may comprise, for example, reactive groups for crosslinking by reaction with a cross-linking agent, the chemical compound, other polymer or the in situ polymer.

It is essential in my invention that most of the autoxidative drying capacity be contributed by the (meth)allyloxy groups. Such a concept is difficult to quantify as it depends not only on the number of units of a given autoxidisable species present per unit of compound but also on the ability of such species to take part in an autoxidation reaction. One method of ascertaining the appropriate level of (meth)allyloxy groups in a given system having at least one other autoxidisable species is to test the system with all but one of the autoxidisable species removed, repeating this for each species. The time and degree of drying can then be ascertained and assessed. Such a method can be long and laborious and I have found that a convenient measure is that (meth)allyloxy groups must comprise at least 20% of the number of autoxidisable groups present in the material of the disperse particles.

The calculation of the proportion of (meth)allyloxy groups in the total number of autoxidisable groups is generally straightforward. However, when there are present autoxidisable polymers, either as independent polymers or as components of other molecules, allowance must be made for the higher autoxidative capacity of such polymers. The generally understood meaning of "autoxidisable group" is a molecular entity with the capacity to undergo autoxidation. Thus, in the case of a drying oil or alkyd resin, each fatty acid chain is considered to be a single autoxidisable group even though it may comprise up to three ethylenic double bonds. However, this cannot realistically be applied to an autoxidisable polymeric species such as poly(butadiene) which may have a molecular weight of several thousand and the autoxidative capacity of several autoxidisable species. I have found that this difficulty can be overcome by applying a "rule of thumb" in which I consider that each 300 units of molecular weight of the autoxidisable polymeric species is equivalent to one autoxidisable group. Thus, a polymeric autoxidisable species with a molecular weight of 600 is considered to constitute 2 autoxidisable groups, 750 2 autoxidisable groups, 900 3 autoxidisable groups, and so on. I find that this "rule of thumb" works very well for autoxidisable polymers such as poly(butadienes) and poly(vinyl ethers).

The monomer from which the in situ polymer is formed may be selected from one or more the wide range of α , β -ethylenically unsaturated monomers known to the art. Typical examples of monomers which can be used in our invention are methyl, ethyl, propyl, butyl, allyl, lauryl and stearyl acrylates and methacrylates, styrene, the mixed isomers of methyl styrene known as "vinyl toluene", vinyl chloride and vinyl acetate and di-alkyl maleates. Functional monomers such as hydroxy-propyl methacrylate and acrylic and methacrylic acids may also be used if, for example, it is desired to provide the ability for the in situ polymer to react with another entity such as pre-formed polymer, a cross-linking agent or a chemical compound as hereinabove described.

When the film-forming disperse polymer particles of my invention are formed, they comprise a blend of in situ polymer, chemical compound and, if present, other polymer. However, the particles may not be just simple mixtures of ingredients; it is possible (and indeed often desirable) that during the addition polymerisation reaction in which the in situ polymer is formed, there is also a degree of reaction of the (meth)allyloxy groups of the chemical compound with the monomer forming the in situ polymer. However, I have found that this reaction never occurs to such a degree that it removes the autoxidation capacity from the dispersions. I consider that the chemical compound present in such a system to be within the scope of my invention although it may not be present in its original form.

The choice of stabilising compound is not critical to the performance of my invention and a suitable compound can be freely selected from the wide variety known to the art, with the exception that when the stabilising compound is amphipathic, it must also be free from ethylenic unsaturation. Stabilising compounds comprising ethylenic unsaturation have been long known and widely used in connection with non- aqueous dispersion systems, but have relatively recently been used in aqueous sytems. Examples of such stabilising compounds may be found in European Published Application No. 0 002 252. However, it is a requirement of my invention that the amphipathic stabilising compounds for use therein be entirely free of ethylenic unsaturation. A stabilising compound suitable for use in my invention can be selected from a wide variety of commercially-available materials. It may be, for example, an amphipathic compound lacking ethylenic unsaturation as hereinabove described. Typical examples include commmercially-available ionic and non-ionic surfactants. Non-ionic surfactants Include compounds such as octyl and nonylphenol ethoxylates, for example, the "Teric" (trade mark) and "Triton" (trade mark) series. Ionic surfactants may be cationic (such as various commercially-available quaternary ammonium compounds) or anionic (such as sulphated natural oils, long chain fatty acid soaps and various isopropyl naphthalene sulphonates, sulphosuccinates and sulphosuccinamates).

The stabilising compound need not be an amphipathic compound but may be selected from the group of known suspending agents and colloid stabilisers. Examples of these include poly(vinyl alcohols), various watersoluble cellulose derivatives such as the hydroxyalkyl celluloses and complex polysaccharides such as gum tragacanth. The stabilising compound is typically present to the extent of 0.1 - 20.0% by weight of the total weight of the material of the particles. The dispersions of my invention may be prepared by any convenient means. One method which may be used is the preparation of a suitable dispersion of polymer particles, followed by the addition thereto of chemical compound. A further method is the preparation of a bulk polymer incorporating chemical compound followed by the dispersion thereof in water by any suitable means. However, my preferred method is the method used by the art to form multi-polymer particles, that Is, the polymerisation of unsaturated monomer in the presence of another component. I find this to be the best method because of its convenience and its versatility; it may be used with a very wide range of materials.

One preferred process for preparing the dispersions according to my invention comprises the steps of mixing chemical compound, monomer, stabilising compound and water so as to form a dispersion of particles wherein the individual particles comprise a blend of chemical compound and monomer, and then polymerising the monomer. The formation of the initial dispersion of monomer-chemical compound particles may be achieved simply by adding the chemical compound and monomer separately to water whilst stirring. However, my preferred method is to blend the chemical compound with the monomer and then to disperse the blend in water. It is preferable that the chemical compound and the monomer be compatible but this is not essential and in some of my systems they are relatively Incompatible. The stabilising compound may be added either to the water or to the chemical compound and/or monomer. A useful variant of these methods is the addition of the chemical compound and a portion of the monomer to the water and the initiation of polymerisation, the remainder of the monomer being fed into the mixture during the course of the polymerisation. All of the foregoing is also applicable to cases where it is desired to add other polymer.

Free radical addition polymerisation may be initiated by any of the means well known to the art. For example, I can use a free radical initiator such as azobisisobutyronitrile. I can also use a redox initiation system and polymerisation can be conveniently initiated at room temperature. The nature of the redox initiation system to be used depends to some extent on the natures of the polymer and monomer but selection of a suitable system is within the knowledge of the art. Typical examples of suitable systems are t-butyl perbenzoate/sodium ascorbate and cumene hydroperoxide/sodium ascorbate.

The initiation systems may be oil-soluble systems, and the result will be a suspension (mini-bulk) polymerisation. It is also possible to use watersoluble initiation systems. The nature of the polymerisation involved in this case is not completely known to me but I believe without restricting my invention in any way that the chemical compound is transported to the forming particles by monomer which is migrating to micelles formed by the stabilising compound in the manner of a conventional emulsion polymerisation. The cumene hydroperoxide/sodium ascorbate system hereinabove described can be used in this fashion as cumene hydroperoxide is partially soluble in water, but I prefer to use a persulphate salt such as ammonium persulphate; such salts are widely used in emulsion polymerisations.

The aqueous dispersions prepared by the process of this invention may be used in a number of applications. They may, for example, be used as the film-forming components of coating compositions. Conventional additives such as pigments, extenders, thickening agents and fungicides may be added in art-recognised quantities.

The invention is further illustrated by the following examples In which all parts are expressed by weight.

EXAMPLE 1 Preparation and testing of an aqueous polymeric dispersion wherein the chemical compound was a polyester theoretically comprising on average 8 allyloxy groups. In this example the only autoxidisable groups present were allyoxy groups and the allyloxy groups comprised 7.5% by weight of the polymeric dispersion. polyester¹ 17.4 parts A methyl methacrylate 25.5 " butyl acrylate 14.5 " stabilising compound² 4.3 "

azo-bis-isobutyronitrile 0.2 B

deminerallsed water 9.1 "

deminerallsed water 29.7 " D

drier solution³ 0.7 " E

1. A polyester resin prepared from allyl glycidyl ether, adipic acid and isophthalic acid in the molar ratio of 1.4:0.5:0.5 polymerised to an acid value of 8 mg KOH/g.

2. A non-ionic stabiliser prepared by reacting bisphenol A with 16 moles of propylene oxide which is in turn reacted with 54 moles of ethylene oxide.

3. Solution of cobalt naphthenate (8% Co metal).

The materials A were mixed and warmed to dissolve the stabilising compound. A was then cooled to room temperature and B was added. When B was dissolved the mixture was added to C under high speed mixing and diluted with D. The dispersion had a maximum particle diameter of 1.5 μm.

Polymerisation was carried out by heating the dispersion to 60°C for 4 hours and to 80°C for a further 2 hours, the dispersion being stirred continuously to dissipate the heat evolved.

E was added to this white dispersion which on drawing down formed a film at room temperature. After ageing for four hours at 25°C this film was found to be solvent (xylene) resistant. EXAMPLE 2

Preparation and testing of an aqueous polymeric dispersion wherein the (meth)allyloxy groups comprised 2.5% by weight of the polymeric dispersion. The procedure of Example 1 was repeated except that 5.8 parts of polyester, 32.8 parts of methyl methacrylate and 18.6 parts of butyl acrylate were used in place of the quantities of Example 1.

Polymerisation was carried out as for Example 1 to give a white dispersion. This gave a film which was xylene solvent resistant after the film was aged for 4 hours at 50°C.

EXAMPLE 3 Preparation and testing of an aqueous polymeric dispersion wherein the (meth) allyloxy groups comprised 15% by weight of the polymeric dispersion.

The procedure of Example 1 was repeated except that 34.8 parts of polyester, 12.7 parts of methyl methacrylate, 7.3 parts of butyl acrylate and 0.1 parts of azo-bls-iso-butyronitrile were used in place in the quantities of Example 1.

Polymerisation was carried out as for Example 1 to give a white dispersion. This formed a film which was xylene solvent resistant after 3 hours at 25°C.

EXAMPLE 4

Preparation and testing of an aqueous polymeric dispersion wherein the chemical compound was monomeric comprising on average 3 allyloxy groups. In this example the only autoxidisable groups present were allyloxy groups and the allyloxy groups comprised 20% by weight of the polymeric dispersion. The reactants and procedure of Example 1 were repeated except that the 17.4 parts of polyester were replaced with 17.4 parts of pentaerythritol triallyl ether.

Polymerisation was carried out as for Example 1 to give a white dispersion. This produced a film which was xylene solvent resistant after the film had aged for 3 hours at 50°C, or 48 hours at 25°C The time to develop xylene solvent resistance at 25°C is more than 10 times longer than in Example 3 where the chemical compound was polymeric, even though the allyloxy content in Example 4 was higher than in Example 3.

EXAMPLE 5 Preparation and testing of an aqueous polymeric dispersion wherein the chemical compound is monomeric and the particles also comprise a polymer which is free from meth(allyloxy) autoxidisable groups.

polyester¹ 12.5 A methyl methacrylate 16.0 butyl acrylate 13.1 nonyl phenol ethoxylate 3.1 stabilising compound

demineralised water 16.8 B

demineralised water 34.5 C

cumene hydroperoxide 0.2 D demineralised water 2.2 sodium erythorbate 0.1

pentaerythritol triallyl ether 0.9 E

cobalt acetate (8% cobalt in water) 0.6 F Polyester composition: 1,6 hexane diol (2.5 moles); fumaric acid (1.15 moles) and adipic acid (0.85 moles); condensed to an acid value of 5 mg KOH/g of polyester. The materials A are premixed and added to B under high speed mixing to form a fine oil-in-water emulsion which is then diluted with C. D is premixed and added to the fine emulsion resulting in polymerisation of the emulsion. After polymerisation E is added slowly with stirring, followed by the addition of F. This dispersion was then drawn down to form a film which developed xylene resistance in one hour at 50°C. In the absence of either the polyester or the monomeric pentaerythritol triallyl ether, xylene resistance had not been attained after 3 hours at 50°C.

EXAMPLE 6

Preparation and testing of an aqueous polymeric dispersion wherein the chemical compound comprised a mixture of an allyloxy polyester and drying-oil alkyd. The proportions of these reactants was such that the allyloxy groups represented 35% by number of total number of autoxidisable groups. The allyloxy groups comprised 3.7% by weight of the polymeric dispersion.

17.4 parts of the polyester of Example 1 were replaced with 8.7 parts of the same polyester and 8.7 parts of a 70% oil length soys bean oil/pentaerythritol/ phthalic anhydride alkyd.

Polymerisation was carried out as in Example 1 to give a white dispersion. This produced a film which was xylene solvent resistant after the film had aged for 16 hours at 50°C. EXAMPLE 7 Preparation and testing of a dispersion using an emulsion polymerisation process

demineralised water 153.0 parts A polyester* 31.6 methyl methacrylate 157.7

2-ethyl hexyl acrylate 123.9 methacrylic acid 2.6

"Aerosol" (trade mark) OT 1.5 surfactant

"Ethylan" (trade mark) A4 12.9 B surfactant

"Triton" (trade mark) X-200 110 . 0 C surfactant demineralised water 100 . 0

deminerallsed water 22.44 D ammonium persulphate 0 . 8 borax 0 . 6

*Reaction product of allyl glycidyl ether/diethylene glycol/adipic acid in the molar ratio of 1.0:0.5:1.0

B was premixed, D was added to C, and then these were separately fed into A at 85°C over two hours.

The result was a white dispersion which produced a xylene solvent resistant film after ageing for 48 hours at 50°C.

Claims

1. An autoxidisable film-forming material which comprises an aqueous dispersion of particles of film-forming polymer, the particles comprising at least one chemical compound which comprises at least two autoxidisable groups and polymer formed by the addition polymerisation of ethylenically unsaturated monomer, the dispersion additionally comprising at least one stabilising compound for the particles, characterised in that

(a) at least two of the autoxidisable groups are groups of the formula

CH₂ = CR-CH₂-O- where R is a moiety selected from the group consisting of H and CH₃;

(d) when a stabilising compound is amphipathic it is free from ethylenic unsaturation.

2. An autoxidisable film-forming material according to claim 1, characterised in that there is present in the particles at least one polymer or oligomer other than the polymer formed from the ethylenically unsaturated monomer.

3. An autoxidisable film-forming material according to claim 2, characterised in that at least one chemical compound is oligomeric or polymeric.

4. An autoxidisable film-forming material according to claim 2, characterised in that in addition to the polymer formed from the monomer, the particles comprise polymer which is free from the said autoxidisable groups.

5. An autoxidisable film-forming material according to claim 1, characterised in that the particles are formed by the polymerisation of ethylenically unsaturated monomer in the presence of the chemical compound.

6. An autoxidisable film-forming material according to claim 1, characterised in that the material of the particles comprises from 1-10% by weight of the said autoxidative groups.

7. A process of preparing an autoxidisable film- forming material according to claim 1 by the steps of (a) forming a mixture which comprises monomer, chemical compound comprising at least 2 autoxidisable groups, water and stabilising compound, and (b) polymerising the monomer to give an aqueous dispersion of particles of film-forming polymer characterised in that

(c) at least two of the autoxidisable groups are groups of the formula CH₂ = CR-CH₂-O- where R is a moiety selected from the group consisting of H and CH₃;

(d) the material of the disperse particles comprises from 0.2 - 35% by weight of the said autoxidisable groups; (e) the abovementioned groups comprise at least 20% of the total number of autoxidisable groups present in the material of the particles; and (f) when the stabilising compound is amphipathic, it is free from ethylenic unsaturation.

8. A process according to claim 7, characterised in that at least one constituent of the mixture prior to polymerisation of the monomer is polymeric.

9. A process according to claim 8, characterised in that at least one chemical compound is oligomeric or polymeric.

10. A process according to claim 8, characterised in that the mixture prior to polymerisation of the monomer comprises polymer which is free from the said autoxidisable groups.

11. A process according to claim 7, characterised in that the material of the disperse particles comprises from 1-10% by weight of the said autoxidisable groups.

12. A process according to claim 7, characterised in that the chemical compound is blended with the monomer prior to dispersion in water.

13. A process according to claim 7, characterised in that polymerisation is initiated by an oil-soluble initiator.

14. A process according to claim 7, characterised in that polymerisation is initiated by a water-soluble initiator.