GB1588991A

GB1588991A - Vinyl chloride resin slurry purification

Info

Publication number: GB1588991A
Application number: GB11431/78A
Authority: GB
Original assignee: Tenneco Chemicals Inc
Current assignee: Tenneco Chemicals Inc
Priority date: 1977-04-06
Filing date: 1978-03-22
Publication date: 1981-05-07
Also published as: SE8301491D0; SE8301491L; FR2386560A1; JPS53125454A; DE2814731A1; SE7803839L

Description

(54) IMPROVEMENTS IN OR RELATING TO VINYL CHLORIDE RESIN SLURRY PURIFICATION (71) We, TENNECO CHEMICALS, INC. of Park 80 Plaza West-l, Saddle Brook, New Jersey 07662, United States of America, a corporation organised and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for the colour stabilization of vinyl chloride resins. More particularly, it relates to a procedure for minimizing or preventing discolouration of the resins during the removal of residual vinyl chloride from slurries of vinyl chloride homopolymers or copolymers by heating at temperatures above 70"C.

When vinyl chloride or a mixture of vinyl chloride and a comonomer is polymerized in an aqueous medium by suspension or emulsion polymerization techniques, there is obtained a slurry which contains from 5% to 50% by weight of vinyl chloride resin and up to 5%'by weight of residual vinyl chloride. Most of the unreacted monomer is usually removed by heating the slurry under vacuum to about 65"C. As it is ordinarily practised, this stripping procedure reduces the monomer content of the slurry to about 100 ppm to 20,000 ppm.

Further processing yields dry products which may contain 100 ppm or more monomer.

In view of the recently-developed safety standards which require the amount of vinyl chloride in vinyl chloride resins and in the atmosphere to be maintained at very low levels, it is necessary for the monomer content of vinyl chloride resin slurries to be sufficiently reduced so that these requirements can be met.

Among the most effective procedures which have been developed for the removal of residual monomer from vinyl chloride resin slurries are those that involve heating the slurries at temperatures above 70"C. until the unreacted monomer has been removed. One such process is disclosed in British Patent Specification 1,490,152. In this process, a vinyl chloride resin slurry or latex which contains from 100 ppm to 15,000 ppm by weight of vinyl chloride is heated at 70"C to 1250C., and the evolved vinyl chloride vapour is removed until the slurry or latex contains less than 50 ppm and preferably less than 10 ppm of monomer. While this heat treatment effectively removes vinyl chloride from the slurry or latex, it often causes some deterioration of the resin as is evidenced by its development of colour. Although the slight decomposition that the resin undergoes during heating to reduce its monomer content does not detract to any appreciable extent from its physical and mechanical properties, the discolouration seriously restricts its use in many applications.

German Patent Application 2,723,377, which was published on December 8, 1977, and GB Application 17986/77 disclose a process in which the discolouration which vinyl chloride resins undergo, when slurries containing them are heated at temperatures above 70"C. for a sufficient time to reduce their monomer content to the required very low levels, can be reduced by carrying out the heating step in the presence of a water-soluble oxidizing agent, such as oxygen, hydrogen peroxide, sodium perborate and potassium persulphate.

It has now been found that the discolouration of vinyl chloride resins which usually occurs when slurries containing the resins are heated at a temperature above 70"C until their monomer content has been reduced to less than 10 ppm can be minimized or eliminated. This treatment results in the recovery of a lighter and brighter resin.

According to the present invention, a process is provided for the removal of vinyl chloride from an aqueous slurry containing 5%to 50% by weight of a vinyl chloride resin and 100 to 15,000 ppm of vinyl chloride, which comprises heating the slurry at a temperature in the range of 70" to 100"C and removing the evolved monomer from it until it contains less than 10 ppm of vinyl chloride, while inhibiting discolouration of the vinyl chloride resin by contacting the vinyl chloride resin in the slurry during the heat treatment with 0.2 to 5 parts by weight per 100 parts by weight of vinyl chloride resin in the slurry of a monomer-miscible colour stablizer selected from: a) an organic peroxide having the structural formula:

wherein R and R' are the same or different and each represents an alkyl group having 2 to 12 carbon atoms, a haloalkyl group having 2 to 12 carbon atoms, an aryl group having 6 to 10 carbon atoms, or a haloaryl group having 6 to 10 carbon atoms; b) an organic azo compound having the structural formula: R" - N = N- R" ' wherein R" and R" ' are the same or different and each represents an aliphatic hydrocarbon group having 1 to 18 carbon atoms, a substituted aliphatic hydrocarbon group having 1 to 18 carbon atoms and one or more nitrile, ester, ether, carboxyl, or thioether groups or halogen atoms as substituents, a cycloaliphatic hydrocarbon group having 3 to 18 carbon atoms, or a substituted cycloaliphatic hydrocarbon group having 3 to 18 carbon atoms and one or more of the aforementioned substituents; c) mixtures thereof; and d) mixtures of a), b) and c) with an epoxide having 2 to 22 carbon atoms.

Examples of these organic peroxides are diacetyl peroxide, dipropionyl peroxide, di-nbutyryl peroxide. diisobutyryl peroxide, diisovaleryl peroxide, bis-(2-ethylhexanoyl)peroxide, dipelargonyl peroxide, dilauroyl peroxide, bis(3,5 5- trimethylhexanoyl)peroxide, acetyl pelargonyl peroxide, chloroacetyl chlorocaproyl peroxide, caproyl lauroyl peroxide, bis(2-chlorolauroyl)peroxide, 2-chlorolauroyl lauroyl peroxide, 2-ethylhexanoyl 2-bromolauroyl peroxide, bis-(2-bromopelargonyl)peroxide, dibenzoyl peroxide, bis(o-methoxybenzoyl)peroxide, bis(o-methylbenzoyl)peroxide, bis(tert. butylbenzoyl)peroxide, bis(2-chloro-4-tert.butylbenzoyl)peroxide, bis (2,4dibromobenzoyl)peroxide, bis (2,4,6-trichlorobenzoyl)peroxide, acetyl benzoyl peroxide, butyryl 2,4-dichlorobenzoyl peroxide, lauroyl benzoyl peroxide, benzoyl o-chlorobenzoyl peroxide, 2-chlorolauroyl o-methylbenzoyl peroxide, and mixtures thereof.

The organic azo compounds which are useful as colour stabilizers in the process of this invention, having the above-defined structural formula - - N = N preferably are azo bis alkyl compounds, that is compounds having the aforementioned structure wherein R" and R" ' each represents an alkyl group having 2 to 8 carbon atoms or a substituted alkyl group having 2 to 8 carbon atoms. Especially good results have been obtained in the process by using as the colour stabilizer an azo bis(alkylnitrile). Examples of organic azo compounds which can be used in the process of this invention include 2cyano-2-propyl-azo-formamide, 2,2'-azo-bis-isobutyronitrile, 2-(tert.butyl-azo )isobutyronitrile, 2-ter.butyl-azo-2-thiocyanate- propane, 1,1 '-azo-bis- 1 -cyclobutanenitrile, 2 ,2'-azobis(2-methylbutyronitrile) ,4-tert.butylazocyanovaleric acid), 4,4'-azo-bis(4cyanopentanoic acid), 1,l'-azo-bis(t- cyclopentant nitrile) 2,2'-azo-bit (2-cyclopropylproionitrile), 2,2'-azo-bis-(2 ,3-dimethylbutyronitrile), 2.2'-azo-bis-(2- methylvaleronitrile), 2.2'-azo-bis(2-isopropyl-3-methylbutyronitrile), 1,1 '-azo-bis(1-cyclohexanenitrile), 2,2'azo-bis-(2,3,3- trimethylbutyronitrile), 2,2'-azo-bis(2- methylhexylnitrile), 2,2'-azo-bis (2,4-dimethylvaleronitrile), 2 ,2'-azo-bis(2-isopropylvaleronitrile), 2 ,2'-azo-bis-(2- cyclopentylpropionitrile), 2 ,2'-azo-bis(2-isopropyl-3- methylvaleronitrile), 1,1'-azo-bis(1- cyclooctanenitrile), 2,2'-azo-bis-(2-isobutyl-4- methylvaleronitrile), tert.butylazoformamide, azo-bis-isobutyramidine, 2,2'-azo-bis-(methyl-2- methylpropionate), 2tert.butyl-azo-2,4-dimethylpentane, 1-tert.butyl-azo- 1-methoxycyclohexane, azo-bis(1- carbomethoxy-3- methylpropane), 2,2'-azobis-(2-cyclopropylpropionamide), 2,2' azodiisobutyrate, dimethyl-2,2'azodiisobutyrate, and 2,2'-azo-bis(2,3-dimethyl-3 methox yvaleronitrile).

The epoxides which can be used in the colour stabilizers of this invention preferably are vicinal epoxides having 2 to 22 carbon atoms. A preferred group consist of epoxides having 2 to 8 carbon atoms, such as ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, epoxyethylbenzene, allyl glycidyl ether, vinyl cyclohexene oxide, glycide, styrene oxide, glycidyl acrylate and glycidyl methacrylate. Another preferred group of epoxides consists of epoxidized fatty acids having 16 to 18 carbon atoms, esters of these epoxidized fatty acids, and mixtures thereof. Illustrative of this group of epoxides are epoxidized cottonseed oil, epoxidized soybean oil, epoxidized linseed oil, epoxidized tall oil, methyl epoxystearate, butyl epoxystearate, tridecyl epoxystearate, and butyl epoxytallate. A single monomer-miscible epoxide or a mixture of two or more of these compounds may be used.

The colour stabilizer may be an organic peroxide, an organic azo compound, a mixture of organic peroxides, a mixture or organic azo compounds, or a mixture of at least one organic peroxide and at least one organic azo compound. Alternatively, it may be a mixture of at least one organic peroxide and/or organic azo compound with an epoxide. Such a mixture generally contains from 50% to 95%by weight of an epoxide and 5%to 50% by weight of an organic peroxide and/or an organic azo compound. It preferably contains 75% to 90% by weight of an epoxide and 10% to 25% by weight of an organic peroxide.

The amount of the colour stabilizer used is that which will prevent discolouration of the vinyl chloride resin when a slurry which contains the resin is heated at a temperature above 70"C. for a time sufficient to reduce its vinyl chloride content to less than 10 ppm without causing foaming or other handling problems. It is dependent upon such factors as the temperature at which the slurry is heated, the length of the heating period, and the monomer content of the slurry. 0.2 part to 5 parts by weight of the colour stabilizer per 100 parts by weight of vinyl chloride resin in the slurry will prevent discolouration of the resin during the heating treatment. Larger amounts of the colour stabilizer could be used, but they generally do not provide additional improvement in the colour of the product. Particularly advantageous results have been obtained when the slurry was heated in the presence of from 0.3 part to 3 parts by weight of the colour stabilizer per 100 parts by weight of vinyl chloride resin. The colour stabilizer or a solution of the colour stabilizer in a carrier solvent, such as vinyl acetate, may be added to the slurry which contains the vinyl chloride resin before the heating of the slurry is begun, or it may be added either portionwise or continuously during the heating period.

As used herein, the term "vinyl chloride resin" includes both the high molecular weight homopolymers of vinyl chloride and the high molecular weight copolymers formed by the copolymerization of vinyl chloride with an essentially water-insoluble ethylenicallyunsaturated monomer that is copolymerizable therewith. Suitable comonomers include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl stearate, vinyl benzoate, ethylene, propylene, ethyl acrylate, acrylic acid, acrylamide, acrylonitrile, methacrylonitrile, vinylidene chloride, dialkyl furmarates and maleates, vinyl ethers, and the like. When one or more of these comonomers are used, the monomer component contains at least 70 % and preferably 80 % to 90% of vinyl chloride.

The vinyl chloride resin slurrires that are treated in accordance with the process of this invention to prevent discolouration of the polymer during processing to remove residual vinyl chloride from them may be prepared by the well-known suspension or emulsion polymerization processes. In the suspension polymerization processes, the monomer is suspended in water by a suspending agent and agitation. The polymerization is initiated with a freeradical-generating polymerization initiator, such as dilauroyl peroxide, dibenzoyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, tertiary butyl peroxypivalate, azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, and mixtures thereof.

Suspending agents which may be used include methylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydrolyzed polyvinyl acetate, gelatin and methyl vinyl ether-maleic anhydride copolymers. In emulsion polymerization processes, the polymerization initiator may be hydrogen peroxide, an organic peroxide, a persulphate or a redox system. Surfaceactive agents, such as alkyl sulphates, alkane sulphonates, alkyl aryl sulphonates and fatty acid soaps are used as emulsifiers in these processes. The reaction mixtures produced by these processes are usually heated under vacuum to about 60"C to 650C. to remove most of the unreacted monomer from them. The resulting stripped slurries generally contain 5 % to 50% and preferably 15% to 40% by weight of vinyl chloride resin as particles that range in size from about 5 microns to 200 microns and from about 100 ppm to 15,000 ppm of vinyl chloride. Following the removal of vinyl chloride from them by the process of this invention, the slurries may be dewatered, for example, on a rotary drum filter, and then dried, or they may be spray dried. The vinyl chloride resins prepared in thisray have excellent brightness and colour and usually contain less than 1 ppm of vinyl chloride.

The invention is further illustrated by the following examples. In these examples, all parts are parts by weight and all percentages are percentages by weight.

Example 1 A. An aqueous slurry of a vinyl chloride-vinyl acetate copolymer was prepared by heating in an autoclave at 650C for 18 hours a polymerization system that contained 85 parts of vinyl chloride, 15 parts of vinyl acetate, 185 parts of deionized water, 0.125 part of dilauroyl peroxide, 1.75 parts of gelatin, and 0.69 part of trichloroethylene. The slurry was heated under vacuum at a temperature below 65"C to reduce its vinyl chloride content to less than 5000 ppm.

B. Sixteen hundred grams of the slurry, which contained 32.5% of the vinyl chloride/vinyl acetate copolymer, was charged together with 2 grams of dilauroyl peroxide into a 2000-ml. reaction flask that was equipped with a thermometer, a stirrer, a condenser, and a nitrogen inlet tube. The reaction flask was immersed in a water bath. The slurry was stirred and gradually heated to 700-750C. while nitrogen was passed over its surface at the rate of about 0.5 cubic feet per hour. A 100 ml. sample of the slurry was taken when the treated slurry reached 70"-75"C. and additional 100 ml. samples were taken at hourly intervals. The samples were filtered, and the wet solids were dried for 12 hours at 400-450C. in a forced air drying oven and then crushed. The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride.

The colours of the initial sample and the final sample of the dry, crushed copolymer were measured by reflectance using a Hunter Colour Meter, Model D-25, which gives readings directly from the polymer surface. The results obtained are given in Table I. In this table, the L value relates to the brightness of the polymer, that is, to the percentage of the light that is reflected, with a perfectly white polymer having an L value of 100. The a and b values relate to hue. Positive a denotes red, negative a denotes green, positive b denotes yellow, and negative b denotes blue.

Example 2 The procedure described in Example 1B was repeated except that instead of dilauroyl peroxide dibenzoyl peroxide was used as the colour stabilizer. The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colours of the initial and final samples of the dry, crushed copolymer. as measured by reflectance, are given in Table I.

Example 3 The procedure described in Example 1B was repeated except that the slurry that contained dilauroyl peroxide was heated at 90"-92"C. for one hour. The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colours of the initial and final samples of the dry, crushed copolymer, as measured by reflectance, are given in Table I.

Example 4 The procedure described in Example 3 was repeated except that instead of dilauroyl peroxide dibenzoyl peroxide was used as the colour stabilizer. The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colours of the initial and final samples of the dry, crushed copolymer as measured by reflectance, are given in Table I.

Example 5 The procedure described in Example 3 was repeated except that instead of dilauroyl peroxide 2,2'-azo-bis-(2,4-dimethylvaleronitrile) (Vazo 52) was used as the colour stabilizer.

The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colours of the initial and final samples of the dry, crushed copolymer, as measured by reflectance, are given in Table I.

Example 6 The procedure described in Example 3 was repeated except that instead of dilauroyl peroxide 2,2'-azo-bis-(2,4-dimethyl-4- methoxyvaleronitrile) (Vazo 33) was used as the colour stabilizer. The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colours of the initial and final samples of the dry, crushed copolymer as measured by reflectance, are given in Table I.

Comparative Example The procedure described in Example 3 was repeated except that no colour stabilizer was added to the slurry before it was heated to reduce its monomer content. The copolymer obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colours of the initial and final samples of the dry. crushed copolymer, as measured by reflectance, are given in Table I.

TABLE I Amount Reflectance of Vinyl Chloride Vinyl of Colour Acetate Copolymers Stabilizer Ex. Colour Stabilizer Added Slurry Initial Sample Final Sample No. Added (PHR) Treatment L a b L a b 1 Dilauroyl Peroxide 0.35 3 Hrs. at 96.60 +0.72 +2.50 96.00 +1.40 +2.90 70 -75 C 2 Dibenzoyl Peroxide 0.35 3 Hrs. at 97.60 0.00 +2.40 98.00 -0.20 +2.85 70 -75 C.

3 Dilauroyl Peroxide 0.30 1 Hr . at 96.80 -0.10 +2.65 95.50 +1.20 +5.65 90 -92 C.

4 Dibenzoyl Peroxide 0.35 1 Hr . at 96.60 +0.85 +3.00 97.05 -0.30 +4.15 90 -92 C.

5 2,2'-Azobis(2.4- 0.35 1 Hr . at 96.65 +0.85 +3.10 95.95 +1.10 +4.75 dimethylvalero- 90 -92 C. nitrile) 6 2,2'-Azobis(2.4- 0.30 1 Hr . at 96.60 +0.35 +2.75 94.80 +1.45 +4.70 dimethyl-4-methoxy- 90 -92 C. valeronitrile) Comp. None - 1 Hr . at 96.70 +1.25 +3.30 94.35 +2.15 +5.00 Ex. A 90 -92 C.

The data in Table I deomonstrate that the addition of a monomer-miscible colour stabilizer to a vinyl chloride/vinyl acetate copolymer slurry that is to be maintained at an elevated temperature until its monomer content has fallen below 1 ppm resulted in the recovery of a lighter-coloured and brighter product than was obtained in the absence of a colour stabilizer.

Particularly bright products were obtained when the colour stabilizer was dibenzoyl peroxide.

Examples 7-9 The procedure described in Example 1B was repeated except that other colour stabilizers were used instead of dilauroyl peroxide. In each case, the copolymer that was obtained by drying the final sample contained less than 1 ppm of vinyl chloride. The colour stabilizers and the amounts of each that were used and the results obtained are given in Table II.

TABLE II Amount Reflectance of Vinyl Chloride/Vinyl of Colour Acetate Copolymers Stabilizer Ex. Colour Stabilizer Added Slurry Initial Sample Final Sample No. Added (PHR) Treatment L a b L a b 7 Dibenzoyl Peroxide 0.33 1 Hr. at 96.70 +0.70 +3.00 97.35 -0.45 +4.10 Dilauroyl Peroxide 0.33 90 -92 C.

8 Dibenzoyl Peroxide 0.33 1 Hr. at 96.65 +1.10 +3.10 97.20 -0.55 +4.20 2,2'-Azobis(2.4- 0.33 90 -92 C. dimethylvaleronitrile) 9 Dibenzoyl Peroxide 0.38 1 Hr. at 96.60 +0.65 +2.85 97.65 -0.20 +3.40 Epichlorohydrin 2.00 90 -92 C.

Comp. - 1 Hr. at 96.70 +1.25 +3.30 94.35 +2.15 +5.00 Ex. A None 90 -92 C.

From the data in Table II, it will be seen that the use of colour stabilizers that were mixtures of dibenzoyl peroxide with either another organic peroxide, an organic azo compound, or an epoxide resulted in products that were lighter-coloured and brighter than that obtained in the absence of a colour stabilizer.

Examples 10-12 Sixteen hundred grams of a slurry that contained about 32% of polyvinyl chloride was charged together with an epoxide and an organic peroxide into a 2000 ml. reaction flask equipped with a thermomometer, a stirrer, a condenser, and a nitrogen inlet tube. The flask was immersed in a water bath. The slurry was stirred and gradually heated to 950C. while nitrogen was passed over its surface at the rate of 0.5 cubic feet per hour. After the slurry had been heated at 95 C. for two hours, a 100 ml. sample was taken. The sample was filtered, and the wet solids were dried for 12 hours at 40"-450C. in a forced air drying oven and then crushed. The dry polyvinyl chloride contained less than 1 ppm of vinyl chloride.

The colours of the samples of dry crushed polyvinyl chloride was measured by the procedure described in Example 1. The colour stabilizers used and the results obtained are given in Table III.

Comparative Examples B and C The procedure described in Examples 10-12 was repeated except that no colour stabilizer was added to the slurry. In Comparative Example B, the slurry was not heated; in Comparative Example C, it was heated at 95"C. for 2 hours. The colours of the samples of dry crushed polyvinyl chloride, as measured by reflectance, are given in Table III.

Example 13 The procedure described in Examples 10-12 was repeated except that the slurry was heated for 2 hours at 95 C. under a vacuum of 3 inches. The results obtained are given in Table III.

Comparative Example D The procedure described in Example 13 was repeated except that no colour stabilizer was added to the slurry. The results obtained are given in Table III.

From the data in Table III, it will be seen that the addition of a monomer-miscible colour stabilizer that contains an epoxide and an organic peroxide to a vinyl chloride resin slurry that is to be maintained at an elevated temperature until its monomer content has fallen below 1 ppm resulted in the recovery of a lighter-coloured and brighter product than was obtained in the absence of a colour stabilizer.

TABLE III Colour Stabilization of Polyvinyl Chloride During Heating Amount of Colour Ex. Colour Stabilizer Stabilizer Slurry Reflectance of Polyvinyl Chloride No. Added Added Treatment (PHR) L a b 10 Epichlorohydrin 1.00 2 Hrs. at 95 C 98.12 +0.40 +0.94 Dibenzoyl peroxide 0.25 at atmospheric pressure 11 Epoxidized 2 Hrs. at 95 C. 98.18 +0.30 +0.96 soybean oil* 1.00 at atmospheric Dibenzoyl peroxide 0.25 pressure 12 Epoxidized 2 Hrs. at 95 C. 98.06 -0.06 +1.26 soybean oil* 0.35 at atmospheric Dibenzoyl peroxide 0.10 pressure Comp.

Ex. B. None ---- None 98.36 +0.22 +0.72 Comp. None ---- 2 Hrs. at 95 C. 97.06 +0.48 +1.09 Ex. C at atmospheric pressure 13 Epichlorohydrin 1.00 2 Hrs. at 95 C. 96.80 +0.40 +1.70 Dibenzoyl peroxide 0.25 under 3" vacuum Comp. None ---- 2 Hrs. at 95 C. 95.36 +1.12 +1.62 Ex. D under 3" vacuum *NUOPLAZ # 849 (Tenneco Chemicals Inc.)

Claims

WHAT WE CLAIM IS:

1. A process for the removal of vinyl chloride from an aqueous slurry containing 5% to 50% by weight of a vinyl chloride resin and 100 to 15,000 ppm of vinyl chloride, which comprises heating the slurry at a temperature in the range of 70" to 100"C and removing the evolved monomer from it until it contains less than 10 ppm of vinyl chloride, while inhibiting discolouration of the vinyl chloride resin by contacting the vinyl chloride resin in the slurry during the heat treatment with 0.2 to 5 parts by weight per 100 parts by weight of vinyl chloride resin in the slurry of a monomer-miscible colour stabilizer selected from: a) an organic peroxide having the structural formula:

wherein R and R' are the same or different and each represents an alkyl group having 2 to 12 carbon atoms, a haloalkyl group having 2 to 12 carbon atoms, an aryl group having 6 to 10 carbon atoms, or a haloaryl group having 6 to 10 carbon atoms; b) an organic azo compound having the structural formula: - - N = N wherein R" and R" ' are the same or different and each represents an aliphatic hydrocarbon group having 1 to 18 carbon atoms and one or more nitrile, ester, ether, carboxyl, or thioether groups or halogen atoms as substituents, a cycloaliphatic hydrocarbon group having 3 to 18 carbon atoms, or a substituted cycloaliphatic hydrocarbon group having 3 to 18 carbon atoms and one or more of the aforementioned substituents; c) mixtures thereof; and d) mixtures of a), b) and c) with an epoxide having 2 to 22 carbon atoms.

2. A process according to claim 1, wherein the colour stabilizer comprises dibenzoyl peroxide.

3. A process according to claim 1 or 2, wherein the colour stabilizer comprises dilauroyl peroxide.

4. A process according to claim 1,2 or 3, wherein the colour stabilizer comprises 2,2'-azo bis(2,4-dimethyl-valeronitrile) .

5. A process according to any preceding claim, wherein the colour stabilizer comprises 2,2'-azo bis(2,4-dimethyl-4- methoxyvaleronitrile).

6. A process according to any preceding claim, wherein the colour stabilizer contains an organic peroxide and a vicinal epoxide having 2 to 8 carbon atoms.

7. A process according to any preceding claim, wherein the colour stabilizer contains 50% to 95% by weight of the epoxide and 5% to 50% by weight of the organic peroxide.

8. A process according to claim 7, wherein the colour stabilizer contains 75%to 90%by weight of the epoxide and 10% to 25% by weight of the organic peroxide.

9. A process according to claim 7 or 8, wherein the epoxide in the colour stabilizer is selected from epoxidized fatty acids having 16-18 carbon atoms, esters of such acids and mixtures thereof.

10. A process according to any preceding claim, wherein 0.3 to 3 parts by weight of the colour stabilizer is added per 100 parts by weight of vinyl chloride resin in the slurry.

11. A process according to claim 1, substantially as herein described with reference to the foregoing Examples.

12. A vinyl chloride resin composition, when obtained by a process according to any preceding claim.