CA1161024A - Process for the polymerization of ethylenically unsaturated monomers - Google Patents

Abstract

Abstract A process for the polymerization of ethylenically unsaturated monomers wherein a solid, monomer-soluble, free-radical forming compound is used as initiator. The initiator is charged to the polymerization system in the form of an aqueous dispersion comprising an emulsifier combination which consists of an ethoxylated nonionic emulsifier with an HLB-value above 15, which does not con-tain cyclic inner ether bonds, and a non-ethoxylated nonionic emulsifier with an HLB-value below 9. The emulsifier combination has a resulting HLB-value within the interval 11-17 and the combination is used in an amount of at least 1 % by weight based on the initiator. Polymerizates produced according to the method have good electric insulation properties. A means suitable for the polymeriza-tion of ethylenically unsaturated monomers consists of an aqueous dispersion of a free-radical forming initiator, which dispersion comprises the mentioned emulsi-fier combination in an amount of at least 1 per cent by weight, based on the initiator. The dispersion preferably also contains a protective colloid such as polyvinyl alcohol. The dispersions have good stability and are easy to handle.

Description

A PROCESS FOR THE POLYMERIZATION ~F ET~LENICALL'~
UNSATURATED MONOMERS
The present invention relates to a process for the polymerization of ethylenically unsaturated monomers where-in solid, monomer-soluble~ free-radical forming compounds are used as initiators. More particularly the invention re-lates to a process wherein the initiator is charged to the polymerization system in the form of an aqueous dispersion, comprising a specific emulsifier system. The invention also 10. relates to a means suitable for carrying out the polymeri-zation process.
Polymers of ethylenically unsaturated monomers, such as polyvinyl chloride and copolymers based on vinyl chloride, are to a great extent used for insulation of cab-15. les and other electric materials. It is, in this connection,essential that the electric insulating power, ~he volume resistivity, is as high as possible, in order to avoid electric losses. At polymerization of e.g. vinyl chloride in aqueous systems, protective colloids and/or emulsifiers 20. are used to obtain the desired particle size of the poly-merizate, and free-radical forming initiators are used to carry out the polymerization. When these chemicals are used, their influence on the properties of the finished polymerizate must be carefully considered. Above all, 25. certain types of these additives have a de~rimental effect on the volume resistivity of the polymerizate.
As solid, monomer-soluble, free-radical initiators use can be made of organic peroxides such as aromatic and aliphatic diacyl peroxides, diaralkyl peroxides and dial-30. kyl peroxydicarbonates, mixed anhydrides of organic sul-phoperacids and organic acids, and also azo compounds.
It is of course possible to charge initiators of this type to the polymerization system in solid form or dissolved in organic solvents. However, these methods of ~eeding are 35. not satisfactory ~ith respect to handling and occupatio-nal environment. ~herefore, development has, in the last few years, been centred on the preparation of aqueous dis~
persions of the solid initiators, which dispersions can be used in closed systems.
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U.S. Patent Specification No. 3,825,509 describes a process for suspen-sion polymerization of v myl chloride in which process the initiator is charged in the form of an emLlsion, containing an emulsifier of the ethoxylated polyol-ester type, and more specifically polyoxyethylene sorbitan m~nolaurate, and poly-vinyl alcohol. Further, dispersions of solid peroxides wherein the emulsifier system is a combination of two or more emulsifiers are known from the U.S. Patent Specification No. 4,039,475. This patent specification describes initiator sus-pensions which are suitable for use in the polymerization of vinyl chloride and which suspensions contain a ccmbination of a nonionic emulsifier with an HLB-value not higher than 12.5 and a nonionic emulsifier with an HLB-value not lower than 12~5. Alternatively, combinations of nonianic em~lsifiers with an ~DLB-value not hi~her than 12.5 and anionic emulsifiers can be used according to the patent. Ethoxylated emulsifiers have mainly been used as the nonionic emulsi-fiers. It is also known to use different cationic and anionic emulsifiers, as described for example in the German patent application 26 29 467.
m e dispersions mentioned above have varying degrees of stability and most of them do not give stable initiator dispersions, but only dispersions which are pumpable or which are stable only for a shorter period of time. Ano~her dis-advantage of hereto kno~n dispersions is that the emulsifiers used, to a varying extent, often impair the electric proFerties of the produced polymerizate, and the produ~ts can then only to a minor extent be used for example in the cable industry. Furthermore, many of the suggested emulsifiers will have a negative effect on the emLlsifiers and protective colloids which are most commonly used for polymerization so that it, when using the initiator dispersions, will not be possible to use conventional polymerization recipes without mcdifications or with-out impairing the properties of the polymerizate.
It has now been found that a combination of certain types of emulsi-z~

fiers give dispersions of solid, monomer-soluble free-radical formm g initia-tors, which dispersions are stable on storage, easy to handle and do not give rise to any appreciable negative effects on the electric insulation pro~erty of -the pro-duced polymerizate. Neither do the dispersions seem to have any considerable in-fluen oe on the conventionally used protective colloids or em~lsifiers, and modi-fications of the polymerization recipes can thus be avoided when the present dis-persions are used.
The present invention thus relates to a process at the polymerization of ethylenically unsaturated monomers wherein a solid, free-radical forming comr pound is used as initiator and is charged in the form of an aqueous dispersion.
The initiator dispersion which is charged to the polymerization system comprlses an emulsifier system which consists of a) an etho~ylated nonionic emulsifier with an HIE~value above 15, ~hich emulsifier does not contain cyclic inner ether-bon~s, and b) a non-ethoxylated nonionic emulsifier with an HIB-value kelow 9, whereby the total amount of emulsifiers is at least l per oent by weight based on the amount of the solid initiator, and whereby the resulting HLB-val~le of the emulsi-fier combination is within the range of 11-17.
By HIB-value is to ke understood the hydrophilic-lipophilic balance as described in "The Atlas HIB-System, a time saving guide to emulsifier selection"
published by Atlas Chemical Industries Inc., 1363, and by resulting HLB-value is to ke understcod the FTR-value for blends of emLlsifiers, which value Gan be cal-culated with respect to the weight ratio of the components, as likewise described in the mentioned publication.
The present method can be used in current known methods for the poly-merization of ethylenically unsaturated monomers in aqueous sys~ems, wherein monomer soluble initiators are used. As e~amples of ethylenically unsaturated polymerizable ccmpounds can be mentioned: vinyl arcmatic compounds, e.g. styrene I "

and substituted styrenes such as p-chlorostyrene, esters of aliphatic alpha~
methylene carbom c aci~, preferably lower alkyl esters, e.g. methylacrylate, ethylacrylate, methyl methacrylate, ethyl methacrylate, acrylic acid nitrile, vinyl esters, e.g. vinyl acetate, vinyl halides, vinyl ethers, e.g. vinyl methyl-ether, vinylidene chloride and lcwer aIkenes, e.g. butadien. me present method is preferably used at polymerization of vinyl chloride or vinyl chloride and up to 20 per cent by weight, based on the vinyl chloride, of copolymerizable mono-mers, such as aIkenes, vinyl acetate, vinylidene chloride, acrylic acid and meth-acrylic acid, acrylates and methacrylates, acryl nitrile and methacryl nitrile, vinyl esters etc., by known suspension and micro-suspension polymerization methods.
It has surprisingly been found that the particular emulsifier system which is used for the initiator dispersion doe s not have any substantial negative effect on the electric insulation pcwer of the produced polymer, and the produoe d polymer satisfies the requirements which can be set for the use thereof in electric insulating materials. The values for the v~lume resistivity, which are obtained when an initiator dispersion according to the invention is used, are fully cx~mparable with those obtained when the initiator is charged in pcwder fo~n, i.e. without addition of extra chemicals.
To carry out polymerization using initiator dispersion, it is also re-quired that the dispersion has a suitable viscosity, both with respect to easy handling for the ch æging and to the distribution in the polymerization system.
m e dispersions of the invention fulfil these requirements and they also shcw a very satisfactory stability.
It has been found that it, in order to avoid unacceptable changes in the electric insulating pcwer of the polymer, is essential not only to avoid theuse of ionic emulsifiers in the initiator dispersions, but also that specific de~
mands must be made on the used nonionic emulsifiers.

It has thus been found that it is essential that ethoxylated nonionic emulsifiers have ccmparatively high HIB-values i.e. that they have distinct hydro-philic tendencies. In order to obtain dispersions which fulfil all the require~
ments, it is also essential that this -type of e~lsifier does not contain cyclic inner ether-bonds, i.e. that they are not anhydro derivatives of polyhydric alcohols.
According to the present invention a nonionic ethoxylated emulsifier with an FT~-value above 15 is used, and this emulsifier will hereinafter be re-ferred to as the hydrophilic emulsifier. The hydrophilic emulsifier must not con-tain cyclic inner ether bonds and it is suitably selected from the groups con-sisting of etho~ylated alkylphenols, ethoxylated fatty acids~ ethoxylated glycol-and glycerol-fatty esters, aIkylene oxide block-copolymers. The H~B-value for the hydrophilic emulsifier should preferably exceed 17. A suitable upper limit for the value is 25. Preferred hydrophilic emulsifiers are the ethoxylated alkylphenols, fatty alcohols and fatty acids and particularly preferred ar~ the ethoxylated fatty acids. By ethoxylated nonionic en~llsifiers is to be understood, as far as derivatives of acids and different alcohols are concerned, that they contain predGminantly ethylene oxide groups, but other lcwer alkylene oxide groups, such as propylene oxide, may be present. Preferably all of the aIkylene oxide groups are ethylene oxide groups.
m e nonionic emulsifier with a comparatively low ~B-value, below 9, in the dispersions have lip~philic tendencies and will hereinafter be referred to as the lipophilic e~lsifier. mis e~lsifier must not contain ethoxy groups as it has been found that it will th n have a negative effect on the elac*ric proper-ties. Suitable lipophilic emulsifiers are the partial fatty acid esters of poly-hydric alcohols having 2 to 8 carbon atcms, or anhydro deri~atives thereof, i.eO
fatty acid esters containing at least one free hydroxyl group. Suitable pol~-,24 valent alcohols, or anhydro derivatives thereof, ccmprise glycols such asethylene glycol, propylene glycol, glycerol, diglycerol and higher glyaerols, polyols such as xylitol, pentaerythritol, sorbitol and anhydro derivatives of these and similar polyols. It is preferred that the lipophilic em~lsifier does not contain cyclic inner ether bonds and that it aonsists of a partial fatty acid ester of alcohols which originally have three hydroxyl groups at the most. Parti-cularly suitable are the partial fatty acid esters of glycerol. The lipophilic emulsifier preferably has an HLB-value belcw 7. A suitable lower l;mit for the HIE-value is 0.5.
Instead of one em~lsifier from the respective group it is of course possible to use two or more, as long as the above m~ntioned requirements are met, and as long as the resulting HIB-value for a blend of two or more hydrophilic or two or more lipophilic emMlsifiers is above 15 or is below 9, respectively.
In the method of the invention solid, oil-soluble radical-initiators are used and suitable such initiators æe different types of solid organic per-oxides, mixed anhydrides of organic sulfoperacids and organic acids, and azo com-pounds. Known azo compounds for initiation of polymerization are e.g. azovalero-nitrile and azobisiscbutyronitrile. Preferably solid organic peroxides are us2d and hereby is to be understood peroxides which are stable at a temperature of about 20 &. As examples of groups of such peroxides can be mentioned arom~tic and aliphatic diacyl peroxides, e.g. dibenzoyl peroxide and dilauryl peroxide, diaIkyl and diaraIkyl peroxides such as dic~myl peroxide, hydroperaxides, per-esters, perketals, ketone peroxides, peroxydicarbonates. At the polymerization solid dialkyl peroxydicarbonates are preferably used and as e~amples of such can be mentioned d~myristyl-, dicetyl-, distearyl-, dicyclohexyl- and di-4-tert.
butylcyclohexyl-peroxydicarbonate.
m e dispersion may have a solid content of initiator of up to about 40 per cent by weight and the solid content sui-tably exceeds 15 per cent by weight.
The total amount of the emulsifier combination should be at least 1 per cent by weight, based on the initiator. The total emulsifier amount should not exceed 15 Fer cent by weight. Protective colloids or thickening agents may of course also be present in the dispersions.
For polymerization using the present initiator dispersions the amount of initiator is calculated in the same m~nner as for polymerization using solid initiators which are not in the form of dispersions, and this means that the amDunt of initiator generally is within the range of 0.01 to 2 Fer cent by weight based on the weight of the monorfer. In polymerization with ~he present disper-sions conventionally used polymerization recipes can be used, and other poly-merization agents can be used in accustomed amounts.
The invention also relates to a means, suitable for use in polym~eriza-tion of ethylenically unsaturated moncmers in aqueous systems. m e m~eans is an a~ueous dispersion of a solid, monomer-soluble, free-radical forming compound, suitably a solid organic peroxide and preferably a solid diaIkyl peroxydicarbon-ate, which dispersion contains an emulsifier system consisting of at le æt one hydrophilic emulsifier and at least one lipophilic em~lsifier as hereinbefore respectively defined, whereby the emulsifier combination has a resulting ~E~
value within the range 11-17 and is present in the dispersion in an amount of at least 1 per cent by weight, based on the free-radical forming initi~tor. The total am~unt of emulsifiers should not exceed 15 per cent. me amount is suit-ably within the range of 1 to lO per cent by weight and preferably within the range of 1 to 5 per cent by weight The h~drophilic emulsifier in the dispersions have an HLB-value above 15 and preferably above 17. m is emulsifier must not contain cyclic inner ether bonds, i.e. it must not be of the ethoxylated sorbitan-derivati~e type. The

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hydrophilic emulsifie.r is suitably selected frcm the groups oonsisting of ethoxy-lated alkylphenols, ethoxylated fatty alcohols, ethoxylated fatty acids, ethoxy-lated glycol- and glyoerol fatty esters and aIkylene oxide block-ccpoly~.ers. In -the five first-mentioned groups alkyl and fatty rests, respectively, are present and hereby is to be understood alkyl groups containing at least 8 carbon atoms and fatty rests containing at least 8 carbon atoms. The latter preferably con-tain between 8 and 22 carbon atoms and can be saturated or unsaturated. The last-mentioned group, the block copolym.ers and can more spe d fically be defined as con-densates of ethylene oxide and a hydrophobic base, fo~med by condensation of pro-pylen~ oxide and propylene glycol. At least 40 per cent by ~eight of the mole-cular weight of the block-copolymers should be made up from ethylene oxide units.
Preferred hydrophilic emulsifiers are ethoxylated alkylphenols, ethoxylated fatty alcohols and ethoxylated fatty acids. Particularly preferred are the ethoxylated fatty acids. This group does of course comprise as well reaction pro-ducts of fatty acid and ethylene oxide, as of fatty acid and polyethylene glycol, the last-mentioned reaction products are often referred to as polyethylene glycol esters, and both m~no- and di-fatty acid esters æe included.
The no.n-ethoxylated emulsifier with an ~LB-value below 9, and prefer~
ably below 7, is a partial fatty acid ester of polyvalent alcohols having 2 to 8 carbon atcms, or anhydro-derivatives of these. The non-ethoxylated emNlsifier thus contains at least one free hydroxyl group. The fatty acid rest can be saturated or unsaturated and contains, as above, at least 8 car~on atoms and pre-ferably bet~een 8 and 22 carbon atoms. Suitable polyvalent alcohols, or anhydro derivatives thereof, co~prise ethylene glycol, diglycerol and higher glycerols, propylene glycol, butylene glycol, diethylene glycol, triethylene glycol, xylitol, erythritol, pentaerythritol, sorbitol and sorbitan. It is preferred that the lipophilic emLlsifier, like the hydrophilic em~lsifier, d oe s not CQntain cyclic inner ether bcnds. m e lipophilic emulsifier is preferably a partial fatty acid ester of an alcohol originally having not more than three hydroxyl groups and particularly suitable are the partial fatty acid esters of glycerol.
The resulting HIB-value for the em~lsifier cc~bination should be within the interval 11-17 and the lipophilic and hydrcphilic emulsifiers are thus used in such weight ratios that values within this interval are obtained. A suitable interval for the weight ~atio of hydrophilic emulsifier to lipophilic emulsifier is 1:10 to -8a-'~
,,~

- ' ~

10:1, preferably l:l to 7:1.
The dry content of initiator in the dispersions can reach about 40 per cent by weight and is generally above 15 per cent by weight. The amount o~ emulsifier 5. is of course suited to the amount of initiator, and it should generally not exceed 15 per cent by weight, as it has been found that the viscosity otherwise might be negatively influenced.
The dispersions according to the invention should lO. have a comparatively low viscosity and thereby be easy to handle. The viscosity should not exceed 1000 cp (measured with Brookfield*viscosimeter RVT, spindle 3, 100 r.p.m., a-t a temperature of 20C) and the viscosity is generally below 150 cp. The dispersions are further very stable and this means that 15. the viscosity of the dispersions does not increase sub-stantially during a longèr period of storing.Neither do the dispersions, after a longer period o~ storing, show phase-separation of such a kind that they cannot be used for polymerization.
20. It is suitable to incorporate substances which function as thlckeners and/or protective colloids in the dispersions, in order to prevent that the initiators in the aqueous dispersion can no longer be homogenously dis-tributed in the continuous phase, should phase separation 25. have occurred. These substances are preferably incorpora ted in most cases, and in such amounts as to prevent phase-separation to the greatest possible extent.
Examples of substances which are suitable for this purpose are synthetic and natural polymers such as 30. polyvinyl alcohol, which may be partially hydrolyzed, polyvinylpyrrolidone, polyacrylic acid, and acrylic acid copolymerizates with acrylic amide or acrylic esters 3 water-soluble cellulose derivatives such as hydroxyethyl and hydroxypropyl cellulose, methyl cellulose and carboxy 35. methyl cellulose, gelatin, starch etc. The amount of pro-tective colloid is chosen with respect to the desired viscosity of the dispersion, with respect to the emulsi-fiers and the initiator. The amount is suitably within the range of 0.05-lO per cent by weight, based on the dispersion,and *Tracle mark preferably within the range of 0~2-5 per cent by weight. Polyvinyl alcohol or water soluble cellulose derivatives æe preferably used.
The dispersions of the invention are prepared in a kncwn manner by mix-ing and homogenizing the co~ponents by means of known equipment, such as colloidmills, pressure homogenizers, ball mills, ultrasonic homogenizers e-tc. As per-oxides are susceptible to higher tem eratures, the temperature at mixing and homogenizing thes_ should not exceed 20C to prevent deccmposition.
The inventian is further illustrated in the following examples which, however, are not intended to limit the same. Parts and per cent relates to partsby weight and per cent by weight unless otherwise stated.
Example 1 20 g of dicetyl peroxydicaxbonate were added to 80 g of a water solu-tion containing 0.6 g of polyvinyl alcohol, 0.5 g of ethoxylated oleic ad d withan HIE-value of 18.4 and 0.4 g of glycerol dioleate with an ~LB-value of 2.9.
The mixture was dispersed by means of an Ultraturrax ~at 10000 r.p.m.
and was then homogenized by means of an Ultrasonic homogenizer at a temperature of about 20 & and at a pressure of about 1000 kPa. The viscosity of the disper-sion was 68 cp, measured with a Brcokfield visoosimeter typo ~VT, measuring spindle 3, at 100 r.p.m.
Only a slight increase in viscosity and a slight separation could be noticed after 2 mDnths storage and this means that the dispersion ha~ a ~ery good storage stability.
Example 2 30 g of laurylpero~ide were added to 70 g of water solution containing 0.4 g of hydroxypropyl methylcellulose, 0.6 g of ethoxylated lauric acid with anHLB-value of 18.8 and 0.2 g of glycerol monolaurate with an HLB-value of 3Ø
~ dispersion was prepared in the same manner as in Example 1. The vis-~s~

cosity of the dispersion was 71 cp. A slight separation was noticed after 2 months storage but the peroxide could easily be homogenously distributed in the aqueous phase again by shaking.
Example 3 15 g of dicetyl peroxydicarbonate were added to 85 g of a water solu-tion containing 0.4 g of ethylhydroxy ethylcellulose, 0.4 g of sorbitan mono-stearate with an HLB-value of 4.7 and 1.0 g of ethoxylated stearic acid with an HIB-value of 18.8.
A dispersion was prepared in the same n~nner as in Example 1. The vis-cosity of the dispersion was 55 cp. The storage stability of the dispersion wasgood, i.e. only a slight separation and no increase in viscosity could be noticed after 2 months storage.
Example 4 20 g of dicet~l peroxydic~rb~nate were added to 80 g of a water solu-- tion cont~ming 0.5 g of hydroxypropyl methylcellulose, 0.5 g of gly oe rol mono-stearate with an HIB-value of 2.7 and 1.0 g of ethoxylated stearic acid with an HIB-value of 17.9.
A dispersion was prepared in the same m~nner as in Example 1. The vis-cosity of the dispersion was 75 cp and it showed good stabillty on storage.

Æxample 5 30 g of dicetyl peroxydicarbonate were added to 70 g of a water solu-tion containing 0.4 g of ethylhydroxy ethyl oe llulose, 0.3 g of glycerol dioleate with an HLB-value of 2.9 and 0.9 g of ethoxylated ole~l alcohol with r~n HLB-value of 17.7.
~ dispersion was prepared in the same m~nner as in Example 1. rme vis-cosity of the dispersion was 89 cp. The storage stability of the dispersion was good, i.e. only slight separation and no increase in viscosity could be noticed after 2 months storage.

".

Example 6 6300 g of water, 5 g of sorbitan monolaurate, 4 g of hydroxypropyl methylcellulose and different initiator dispersions according to the invention, an initiator dispersion for comparison and initiator in solid form, respectively, were introduced into a 15 1 steel reactor. The initiator dispersions were charged in amounts corresponding to 3 g of dry initiator. The agitation speed in the system was 415 r/min. and the temperature was 40C. 5500 g of vinyl chloride were charged after evacuation, and the system was heated to 55 & .
I~e polymerization was stopped after about 8 hours, when the pressure was 500 kPa. Unreacted vinyl chloride was blown off and the polymerizate was de-watered and dried.
200 g of the produ oe d poly~erizate were mixed with 16 g of tribasic lead sulphate and 100 g of dioctyl phthalate. The mixtures were rolled to a fell for 10 munutes and thereafter pressed into plates at 160& and oonditioned on a water bath at 23& for 4 hours. The vol~me resistivity was then measured by means of a Radiometer IM6 megohmmeter.
The following dispersions were used in the tests described above:
a) A dispersion acoording to Example 1 b) A dispersion accor~ing to Exa~ple 2 c) A dispersion according to Example 5 d) Comparative example. A dispersion of 17 g of dicetyl peroxydicarbonate, 0.5 g of hydroxypropyl methyl oe llulose and 0.5 g of ethoxylated nonylphenol, with an ~DLB-value of 13.3, in 82 g of water, the dispersion having been prepared in the same manner as i Example 1.
e) Comparative example. A dispersion of 15 g of di oe tyl peroxydicarbonate, 0.4 g of ethylhydroxy ethylcellulose, 0.3 g of sorbitan monolaurate with an HLB-value of 8.6 and 0.4 g of ethoxylated sorbitan monolaurate with an FTR-value of 16.7, in 83.9 g of water, the dispersion having been prepared in the same manner as ln Example l.
A polymerizate, produced and treated in the same manner c~s above but for which the same amount of dicetyl pero~ydicarbonate had been added to the poly-merization system in powder form, was used as a reference in each test.
The measurements were made both on polymerizates prepared ~ith fresh dispersions and with the same dispersions after they had been stored for 2 months.
In the table below, the dispersions which had been stored are indicated with index l. m e values for the volume resistivity which c~re shcwn in the table, are mean values of five measurements.

._ _ v Dispersion ~olume Resistivity x 101 ohm.m a 68 referen oe 58 b 63 referen oe 55 c 49 reference 52 d, comparative 15 reference 55 e, comparative 33 referen oe 55 al 62 reference 55 bl 63 reference 56 dl, CQmparative 12 re~erence 56 _ ~ . _

Claims (10)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   l. A method in the polymerization of ethylenically unsaturated monomers in aqueous systems wherein a solid, monomer-soluble, free-radical forming compound is used as initiator and this is added to the polymerization system in the form of an aqueous dispersion, characterized in that the dispersion comprises an emulsifier system consisting essentially of a) an ethoxylated nonionic emulsifier with an HLB-value above 15, which emulsifier does not con-tain cyclic inner ether bonds, and b) a non-ethoxylated nonionic emulsifier with an HLB-value below 9, whereby the resulting HLB-value of the emulsifier blend is within the range of 11-17 and whereby the total amount of emulsifier is at least 1 per cent by weight, based on the initiator.
2. 2. A method according to claim 1, characterized in that the non-ethoxy-lated emulsifier in the dispersion is a partial fatty acid ester of an alcohol having not more than three hydroxyl groups.
3. 3. A method according to claim 1, characterized in that the ethoxylated emulsifier is an ethoxylated alkylphenol, an ethoxylated fatty alcohol or an ethoxylated fatty acid.
4. 4. A method according to claims 1,2 or 3 characterized in that the solid initiator is an organic peroxide.
5. 5. An aqueous dispersion of a solid, monomer-soluble, free-radical forming initator, which dispersion is suitable for the polymerization of ethy-lenically unsaturated monomers in aqueous systems, characterized in that it comprises an emulsifier system consisting essentially of a) an ethoxylated nonionic emulsifier having an HLB-value above 15, which emulsifier does not contain cyclic inner ether bonds, and b) a non-ethoxylated nonionic emulsifier with an HLB-value below 9, whereby the resulting HLB-value of the emulsifier blend is within the range of 11-17 and whereby the total amount of emulsifier is at least 1 per cent by weight based on the initiator.
6. 6. A dispersion according to claim 5, characterized in that the ethoxy-lated emulsifier is an ethoxylated alkylphenol, an ethoxylated fatty alcohol or an ethoxylated fatty acid.
7. 7. A dispersion according to claim 5 or 6, characterized in that HIB-value of the ethoxylated emulsifier is above 17.
8. 8. A dispersion according to claim 5, characterized in that the non-ethoxylated emulsifier is a partial fatty acid ester of an alcohol having not more than three hydroxyl groups.
9. 9. A dispersion according to claim 5 or 8, characterized in that the non-ethoxylated emulsifier has an HLB-value below 7.
10. 10. A dispersion according to claim 5, characterized in that it contains 0.05 to 10 per cent by weight, based on the dispersion, of a thickener.