GB2061980A - Method for the preparation of vinyl chloride resins by suspension polymerization - Google Patents

Abstract

The invention provides means for preventing polymer scale deposition on the reactor walls during the suspension polymerization of vinyl chloride monomer in an aqueous medium.The method of the invention comprises (a) coating of the reactor walls, prior to polymerization, with an aqueous coating solution containing an organic dye having sulfonic acid groups or carboxylic acid groups in the alkali metal or ammonium salt form or an alkali metal or ammonium salt of an organic sulfonic acid or carboxylic acid having at least one pair of conjugated double bonds per molecule, followed by drying, and (b) adding a water-soluble thiocyanate to the polymerization mixture. Previously, the use of an organic peroxide with a relatively large solubility in water as initiator in the reaction had led to large scale deposition on the reactor walls but this is avoided under the conditions specified above. Use of such an organic peroxide is advantageous in improving the thermal stability and initial colouring of the resin products.

Description

SPECIFICATION Method for the preparation of vinyl chloride resins by suspension polymerization The present invention relates to the preparation of a polyvinyl chloride resin by suspension polymerization.

At least in its preferred forms, it provides a method for the preparation of a polyvinyl chloride resin, having excellent heat stability as well as remarkably decreased initial coloring in molding, by the suspension polymerization of the vinyl chloride monomer in an aqueous medium, in which the efficiency of the polymerization can be markedly improved owing to the substantially reduced amount of polymer scale build-up on the walls of the polymerization reactor.

Needless to say, polyvinyl chloride resins are in most cases prepared by the suspension polymerization of vinyl chloride monomer in an aqueous medium. One of the greatest technical problems in the suspension polymerization of vinyl chloride monomer in an aqueous polymerization medium is the polymer scale build-up on the walls of the polymerization reactor. In the suspension polymerization of vinyl chloride monomer in an aqueous medium contained in a polymerization reactor of stainless steel or other materials, polymer scale deposits on the inner walls of the reactor or other surfaces coming into contact with the monomer such as the stirrer installed in the reactor result in not only a decrease of the polymer yield but also a reduction of the cooling capacity of the reactor with consequent decrease of the overall production capacity.In addition, the polymer scale often comes off from the reactor walls and enters the resin product leading to lowered product quality due to the increased amount of so-called fish-eyes, which is a problem in the molding fabrication of polyvinyl chloride resins. Even worse, removal of the polymer scale after each run of the polymerization takes much labor and time causing decreased productivity as well as the serious problem of the danger to workers' health brought about by the toxicity of the vinyl chloride monomer which is contained in the polymer scale in large amounts.

On the other hand, it is also known in the art that some of the factors determining the product quality of the polyvinyl chloride resins are influenced by several parameters in the polymerization reaction. For example, heat resistance and initial coloring in molding fabrication of the resin are substantially affected by the kind of polymerization initiator used in the polymerization reaction. As is well known, the polymerization initiators used in the suspension polymerization of vinyl chloride monomer are always monomer-soluble ones. These monomer-soluble polymerization initiators are almost insoluble or hardly soluble in water but it should be noted that even these monomer-soluble polymerization initiators have a solubility in water even though the solubility may be very small.It has come to be known in the art that the solubility of the polymerization initiator in water is a parameter affecting the heat resistance and initial coloring of the polyvinyl chloride resin products polymerized therewith and, in this respect, an organic peroxide initiator having a relatively large solubility in water, say 0.2 % by weight or more at 20"C, is advantageous in improving the heat resistance and initial coloring of the resin products in comparison with an organic peroxide having a lower solubility in water.

Despite the above described advantages in the thermal stability of the resin products polymerized therewith, such organic peroxides having a relatively large solubility in water are, in practice, not used widely in the industrial production of polyvinyl chloride resins. This is partly due to the undesirable fact that the use of such an organic peroxide results in much larger amounts of polymer scale deposition on the reactor walls than is the case with less water-soluble organic peroxides.

Of course, various attempts have been made to decrease the amount of polymer scale deposition in the suspension polymerization of vinyl chloride monomer in an aqueous medum. Generally, there have been two approaches, i.e. that of providing the reactor walls with a coating layer of a material less susceptible to deposition of polymer scale prior to polymerization and that of adding certain specific additives to the polymerization mixture under reaction.

As the coating materials used in the first approach, there have been proposed, for example, polar organic compounds, organic dyes and pigments including amine compounds, quinone compounds, aldehyde compounds and the like (see, for example, Japanese Patent Publication 45-30343, 45-30835 and so on), polar organic compounds or organic dyes treated with a metal salt (see Japanese Patent Publication 52-24953), mixtures of an electron donor compound and an electron acceptor compound (see Japanese Patent Publication 53-28347) and certain inorganic salts and inorganic complex compounds (see Japanese Patent ,Publication 52-24070).

The above mentioned coating methods are effective when the polymerization initiator used in the reaction is an azo compound or an organic peroxide having long-chain alkyl groups and consequently a very small solubility in water. Unfortunately, these methods are almost ineffective in decreasing the polymer scale deposition when the polymerization is carried out by use of an organic peroxide having a relatively large solubility in water. Thus, no effective means have been developed for polymer scale prevention in the polymerization of vinyl chloride monomer using an organic peroxide having a solubility in water of 0.2 % by weight or larger as the polymerization initiator.

The present invention consists in an improvement in the suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride in an aqueous polymerization medium by use of a monomer-soluble organic peroxide having a solubility in water of at least 0.2 % by weight at 20"C as a polymerization initiator, which improvement comprises (a) prior to the introduction of the polymerization mixture into the polymerization reactor, coating the inner walls of the polymerization reactor with an aqueous coating solution containing an organic dye having a sulfonic acid group -SO3H or carboxylic acid group -COOH in the molecule in the form of an alkali metal or ammonium salt or an alkali metal salt or ammonium salt of an organic sulfonic acid or carboxylic acid having at least one pair of conjugated double bonds in the molecule, followed by drying, and (b) adding a water-soluble thiocyanate into the polymerization mixture in an amount of at least 1 p.p.m. based on the amount of the monomer or monomers.

Preferred embodiments of the invention will now be described.

The suspension polymerization of vinyl chloride monomer according to the invention is applicable not only to the homopolymerization of vinyl chloride monomer but also to the copolymerization of vinyl chloride monomer with one or more copolymerizable ethylenically unsaturated monomers provided that the main component, say, 50 % by weight or more, is vinyl chloride. The comonomers copolymerizable with vinyl chloride are exemplified by vinyl esters such as vinyl acetate, vinyl propionate and the like, vinyl ethers, acrylic and methacylic acids and esters thereof, acrylonitrile, vinylketones, vinylidene halides such as vinylidene chloride, vinylidene fluoride and the like and olefins such as ethylene, propylene and the like.

The procedure for the polymerization may be conventional and the same as the prior art method excepting the coating of the reactor walls and the addition of the thiocyanate as mentioned above. For example, the dispersing agents used for suspending the monomer or monomer mixture in the aqueous medium include water-soluble synthetic and natural polymeric substances such as completely or partially saponified polyvinyl alcohol, cellulose ethers, e.g. methylcellulose, hydroxypropylmethylcellulose and the like, polyacrylic acid, copolymers of vinyl acetate and maleic anhydride, gelatin and starch, nonionic surface active agents such as sorbitan monolaurate, sorbitan trioleate and the like and dispersion aids such as carboxymethylcellulose and the like.

As has been mentioned before, the method of the invention is specifically applicable to the suspension polymerization of vinyl chloride in an aqueous medium in which the polymerization initiator used is a monomer-soluble organic peroxide having a solubility in water of at least 0.2 % by weight at 200C although no adverse effects are produced even when the method is applied to the polymerization of vinyl chloride with a polymerization initiator which is an azo compound or an organic peroxide having a greater solubility in water.

Examples of the above mentioned relatively water-soluble organic peroxides are di-2-ethoxyethylperoxy dicarbonate, bis-3-methoxybutylperoxy dicarbonate and dibutoxyethylperoxy dicarbonate, referred to hereinafter as EEP, MC and BEP, respectively, for brevity, although the applicability of the method is not limited to these three initiators.

The coating solution to be applied to the inner walls ofthe polymerization reactor and, according to need, to the surfaces of other parts such as stirrer of the reactor coming into contact with the monomer or monomers during polymerization, is prepared by dissolving the above mentioned alkali or ammonium salts of organic dyes or acids in water.

The organic dyes having a sulfonic acid group or carboxylic acid group in the molecule in the form of an alkali metal salt or ammonium salt are exemplified by: C.I. Direct Yellow 1; C.I. Acid Yellow 38; C.l. Acid Yellow 3; C.I. Reactive Yellow 3; C.I. Direct orange 2; C.I. Direct Orange 10; C.l. Direct Red 1; C.l. Acid Red 2; C.i. Acid Red 18; C.l. Acid Red 52; C.l. Acid Red 73; C.l. Direct Red 186; C.l. Direct Red 92; C.l. Basic Red 2; C.l.

Direct Violet 1; C.l. Direct Violet 22; C.l. Basic Violet 10; C.l. Acid Violet 11; C.l. Acid Violet 78; C.I. Mordant Violet 5; C.l. Direct Blue 6; C.I. Direct Blue 71; C.l. Direct Blue 106; C.I. Reactive Blue 2; C.l. Reactive Blue 4; 0.1 Reactive Blue 18; C.I. Acid Blue 116; C.l. Acid Blue 158; C.I. Mordant Blue 1;C.I. Mordant Black 1; C.l.

Mordant Black 5; C.l. Acid Black 2; C.l. Direct Black 38; C.I. Solubilized Vat Black 1; C.l. Fluorescent Brightening Agent 30; C.l. Fluorescent Brightening Agent 32; C.I. Azoic Brown 2 and the like.

The organic sulfonic acids or carboxylic acids having at least one part of conjugated double bonds in the molecule to be used as the solute in the coating solution in the form of an alkali metal or ammonium salt are exemplified by : toluidines; a-naphthalenesulfonic acid, anthraquinonesulfonic acids; dodecylbenzenesulfonic acid; p-toluenesulfonic acid; abietic acid; isonicotinic acid; benzoic acid; phenylglycine; 3-hydroxy-2naphthoic acid and the like.

The coating solution is prepared by dissolving one or more of the above specified organic dyes or organic sulfonic acids or carboxylic acids, all in the form of the salts, in water. The concentration of the coating solution is not important in so far as the coating can be performed without inconvenience to give a suitable coating amount on the reactor walls.

The coating solution prepared as above is usually more or less alkaline but it is preferable that the coating solution is used after adjusting the pH value of the solution so that it does not exceed 7 by adding a small amount of an acid after dissolution of the salts of the organic dyes or organic sulfonic acids or carboxylic acids. Suitable acids include sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, carbonic acid, perchloric acid, molybdic acid, tungstic acid, formic acid, acetic acid, oxalic acid, lactic acid, maleic acid, glycolic acid, thioglycolic acid, phytic acid and the like. It is convenient to add these acids to the coating solution as an aqueous solution prepared in advance.

The aqueous coating solution is applied to the inner walls of the polymerization reactor and other surfaces coming into contact with the monomer or monomers by a suitable method, e.g. spraying, brush coating and the like, and the thus coated surface is then thoroughly dried, if necessary, by heating. The surface to be coated with the coating solution may be heated in advance at a temperature of 40 to 100"C before coating. In any case, the coated and dried surface is washed with water, if necessary.

The aqueous coating solution may be prepared by use of a mixed solvent of water and a water-miscible organic solvent, if desired. An advantage is obtained in this way in that the drying of the coated surface is accelerated necessitating no heating for drying in some cases. Suitable organic solvents include alcoholic solvents, ester solvents and ketone solvents.

The coating amount with the coating solution is preferably such that the amount as dried is at least 0.001 g/m2 on the coated surface, as in the prior art coating methods for polymer scale prevention.

After completion of the coating of the reactor walls in the above described manner, the polymerization reaction is started in the usual manner by introducing water for the polymerization medium, dispersing or suspending agent, polymerization initiator and the vinyl chloride monomer or monomers as well as other optional ingredients to form the polymerization mixture. It is essential that at least water-soluble thiocyanate is added to the polymerization mixture.

The water-soluble thiocyanates suitable for use in the method of the invention are exemplified by thiocyanates of an alkali metal such as sodium, potassium and cesium, ammonium thiocyanate and thiocyanates of alkaline earth metals such as magnesium, calcium, strontium and barium. These thiocyanates may be added directly to the polymerization mixture to be dissolved in the aqueous medium, preferably before the start of the polymerization run, although about the same degree of the desired effects can be obtained even when the thiocyanate is added after the polymerization has been started (if not too late).

The amount of the thiocyanate to be added to the polymerization mixture is desirably at least 1 p.p.m. by weight based on the amount of the vinyl chloride monomer or monomer mixture. The upper limit of the amount of the thiocyanate is 500 p.p.m. by weight or, preferably, 100 p.p.m. by weight based on the monomer or monomer mixture. This upper limit is determined by the adverse effects on the quality of the resin products with an excessively large amount of the thiocyanate whereas no particular additional effects can be obtained with regard to polymer scale prevention.

Although the method of the invention is applicable to the suspension polymerization of vinyl chloride not only with an organic peroxide initiator having a solubility in water of at least 0.2 % by weight but also with such an initiator having a solubility in water smaller than 0.2 % by weight at 20"C, the organic peroxide cannot have an excessively large solubility in water since the polymerization in such a mixture containing an excessively water-soluble organic peroxide initiator proceeds rather like an emulsion polymerization so that the desired suspension polymerizate of the polyvinyl chloride resin is not obtained.In this regard, the polymerization according to the present invention should be carried out by use of an organic peroxide polymerization initiator having a solubility in water not exceeding 10 % by weight or, preferably, not exceeding 5 % by weight at 20"C.

The method of the present invention wll now be illustrated in further detail by way of examples.

Example 1 (Experiments No. 1 to No. 29).

Four series of polymerization tests were undertaken each using 7 kinds of monomer-soluble organic peroxides as the polymerization initiator. The organic peroxides were: di-2-ethylhexylperoxy dicarbonate, lauroyl peroxide, 3,5,5-trimethyl hexanoyl peroxide and a-cumylperoxy neodecanoate, referred to hereinafter as OPP, LPO, L-355 and L-188, respectively, for brevity, in addition to the aforementioned EEP, MC and BEP.

The solubility of these peroxides in water was determined by dispersing an excess amount of each of the peroxides in water at 20"C and, after establishment of the equilibrium, iodometrically titrating the aqueous solution obtained by centrifugal separation of the dispersion. The results were: OPP 0.04 % by weight; LPO 0.003 % by weight; L-355 0.05 % by weight; L-1 88 0.01 % by weight; EEP 0.41 % by weight; MC 0.24 % by weight; and BEP 0.20 % by weight.

The procedure for each of the polymerization runs was as follows. In the first place, three kinds of coating solutions were prepared with the formulations given below and the inner walls of a 1000-liter capacity polymerization reactor made of stainless steel and the surface of the stirrer installed in the reactor were coated with either one of the coating solutions followed by drying of the coated surfaces by heating for 10 minutes at 90"C. The formulations of the coating solutions were: .Coating solution A : 0.1 % by weight of methyl alcohol solution of C.l. Solvent Black 7; Coating solution B : 0.1 % by weight methyl alcohol solution of sodium thiocyanate; and Coating solution C : 0.1 % by weight aqueous solution of C.l. Acid Black 2 having a pH value of 3.0 as adjusted by adding sulfuric acid.

The coating amount with each of the coating solutions was 0.1 gim2 as dried.

Into the thus surface-treated polymerization reactor were introduced 500 kg of water containing 150 g of a partially saponified polyvinyl alcohol, one of the above named organic peroxides in an amount of 500 g for LPO or 100 g for each of the other peroxides and 250 kg of vinyl chloride monomer together with or without the addition of 2.5 g of ammonium thiocyanate and polymerization run was carried out by heating at 50"C for 10 hours in the usual manner. After completion of the polymerization reaction, the polymerizate slurry was discharged out of the polymerization reactor and the amount of the polymer scale deposited on the reactor walls was examined to give the results set out in Table 1 below. For comparison, an additional polymerization run was undertaken without the coating treatment of the reactor walls to give the results also given in the table.

TABLE 1 (Polymer scale deposition g/m2) Coating Ammonium Organic peroxide solution thiocyanate OPP LPO L-355 L-188 EEP,,,. MC BEP A Yes 1 2 1 3 450 400 250 B Yes 12 18 25 33 550 600 320 C Yes 1 2 1 4 0 0 0 C No 1 3 3 8 300 100 85 None No - - - - 1500 - As is clear from the results shown in Table 1, of which the experiments initiated by EEP, MC or BEP and with the coating solution C for the coating treatment and addition of ammonium thiocyanate are the examples in accordance with the invention, the polymer scale deposition in the polymerization with EEP, MC or BEP can be almost completely eliminated only when both the two measures of coating and thiocyanate addition are takeri in accordance with the invention.

Example 2 fExperiments No. 30 to No. 41).

Aqueous coating solutions were prepared each by dissolving one of the coating compounds indicated in Table 2 below in a concentration of 1 % by weight with subsequent adjustment of the value of pH to 4.0 with addition of acetic acid. The inner walls of a 1000-liter capacity polymerization reactor made of stainless steel and the surface ofthe stirrer installed in the reactor were coated with the coating solutions in a coating amount of 0.1 g/m2 as dried followed by drying by heating at 90"C for 10 minutes and washing with water.

Into the thus treated polymerization reactor were introduced 400 kg of water, 250 g of a partially saponified polyvinyl alcohol, 25 g of a hydroxypropylmethylcellulose, 75 g of EEP and 200 kg of vinyl chloride monomer without or with addition of ammonium thiocyanate in an amount as indicated in Table 2 and the polymerization was carried out by heating the polymerization mixture at 570C for 12 hours.

After completion of the polymerization reaction, the polymerizate slurry was discharged out of the reactor and the amount of the polymer scale deposition on the reactor walls was examined to give the results set out in the table. In parallel, the dried polyvinyl chloride resin product obtained in each of the polymerization runs was examined for the amount of fish-eyes and the thermal stability in the test procedures given below to give the results shown in Table 2. The thermal stability of the resin product obtained in the last-given experiment was somewhat inferior to those obtained in the other polymerization runs indicating that an excessively large amount of the thiocyanate was disadvantageous to the thermal stability of the resin product.

Testing of fish-eyes: a resin compound composed of 100 parts by weight of the resin product, 50 parts by weight of dioctyl phthalate, 1 part by weight of dibutyltin dilaurate, 1 part by weight of cetyl alcohol, 0.25 part by weight of titanium dioxide and 0.05 part by weight of carbon black was kneaded in a two-roller mill at 1500for 7 minutes and then shaped into a sheet of 0.2 mm thickness, of which the number of pieces of fish-eyes was counted for 100 cm2 area of the sheet under illumination with transmitted light.

Testing of thermal stability: a resin compound composed of 100 parts by weight of the resin product, 1 part by weight of stearic acid and 1 part by weight of dibutyltin maleate was kneaded in a two-roller mill at 17000 with a roll gap of 0.7 mm for 10 minutes and then shaped into a sheet of 0.7 mm thickness. The test pieces taken from this sheet were heated in a Geer's oven at 180 C and the time to the blackening of the test piece in minutes was taken as a measure of the thermal stability of the resin product.

TABLE 2 Ammonium Fish- Thermal Polymer Coating compound thiocyanate eyes, stabil- scale added, p.p.m. pieces ity, deposi by weight minutes tion, per monomer g/m2 None None 60 120 1600 None 10 55 120 1500 C.l.AcidBlue116 None 40 120 350 C.l.AcidBlue116 10 8 120 0 Ammonium abietate 10 10 120 3 Sodium a-napthalenesulfonate 10 10 120 2 C.l. Direct blue 6 20 6 120 0 0.1. Acid Violet 78 1 10 120 3 C.l. Acid Violet 78 10 6 120 1 C.l. Acid Violet 78 50 4 120 0 0.1. Acid Violet78 100 4 120 0 0.1. Acid Violet 78 1000 4 80 0 Example 3 (Experiments No. 42 to No. 48).

Aqueous coating solutions were prepared each by dissolving one of the coating compounds indicated in Table 3 below in a concentration of 1 % by weight with subsequent adjustment of the value of pH to 2.5 with addition of phosphoric acid. The inner walls of a 100-liter capacity polymerization reactor made of stainless steel and the surface of the stirrer installed in the reactor were coated with the thus prepared coating solutions in a coating amount of 0.1 g/m2 as dried followed by drying with heating at 900C for 10 minutes and washing with water.

Into the thus coated polymerization reactor were introduced 40 kg of water, 17 kg of vinyl chloride monomer, 3 kg of vinyl acetate monomer, 12 g of a partially saponified polyvinyl alcohol, 4 g of a hydroxypropylmethylcellulose, 6 g of MC and 200 g oftrichloroethylene without or with addition of a thiocyanate of the kind and in an amount as indicated in Table 3 and, after 15 minutes of preliminary agitation, polymerization was carried out by heating the polymerization mixture to a temperature of 58"C for 12 hours.

After completion of the polymerization reaction, the polymerizate slurry was discharged out of the polymerization reactor and the amount of the polymer scale deposition on the reactor walls was examined to give the results shown in Table 3.

TABLE 3 Thiocyanate added Polymer Coating compound (p.p.m. by weight per scale monomer) deposi tion, glum2 None None 1800 None Ammonium thiocyanate 1600 (10) 0.1. Acid Black2 None 1300 C.l. Acid Black 2 Ammonium thiocyanate 3 (10) C.l. Acid Red 2 Sodium thiocyanate 2 (20) C.l. Acid Yellow 38 Calcium thiocyanate 1 (20) C.l. Direct Red 1 Potassium thiocyanate 1 (10)

Claims (8)

1. A method for the preparation of a polyvinyl chloride resin by the suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride monomer in an aqueous polymerization medium containing a suspending agent and a monomer-soluble polymerization initiator which is an organic peroxide having a solubility in water of at least 0.2 % by weight at 20"C in a polymerization reactor, which comprises (a) coating, priorto polymerization, the inner wall surfaces of the polymerization reactor with an aqueous coating solution containing, dissolved therein, at least one compound selected from (i) organic dyes having at least one sulfonic acid group or carboxylic acid group per molecule in the form of an alkali metal salt or ammonium salt, and (ii) alkali metal salts or ammonium salts of organic sulfonic acids and carboxylic acids having at least one pair of conjugated double bonds per molecule, followed by drying, and (b) including at least one water-soluble thiocyanate in the polymerization mixture in the polymerization reactor.

2. The method as claimed in claim 1 wherein acid is added if necessary to adjust the pH of the aqueous coating solution to a pH value not exceeding 7.

3. The method as claimed in claim 1 or 2 wherein the coating on the inner wall surfaces of the polymerization reactor deposited from the aqueous coating solution is at least 0.001 g/m2 as dried.

4. The method as claimed in claim 1, 2 or 3 wherein the water-soluble thiocyanate is selected from thiocyanates of alkali metals, ammonium thiocyanate and thiocyanates of alkaline earth metals.

5. The method as claimed in any preceding claim wherein the water-soluble thiocyanate is included in the polymerization mixture in an amount in the range from 1 to 500 p.p.m. by weight based on the vinyl chloride monomer or the monomer mixture.

6. The method as claimed in any preceding claim wherein the organic dye is selected from C.l. Direct Yellow 1, C.l. Acid Yellow 38, C.l. Acid Orange 2, C.l. Direct Orange 10, C.l. Direct Red 1, C.l. Acid Red 2, C.l.

Acid Red 18, C.l. Acid Red 52, C.l. Acid Red 73, C.l. Direct Red 186, C.l. Direct Red, 92, C.l. Basic Red 2, C.l.

Direct Violet 1, C.l. Direct Violet 22, C.l. Basic Violet 10, C.l. Acid Violet 11,0.1. Acid Violet 78, C.l. Mordant Violet 5, C.l. Direct Blue 6, C.l. Direct Blue 71, C.l. Direct Blue 106, C.l. Reactive Blue 2, C.l. Reactive Blue 4, C.l.

Reactive Blue 18, C.l. Acid Blue 116, C.l. Acid Blue 158, C.i. Mordant Black 1, C.l. Mordant Black 5, C.l. Acid Black 2, C.l. Direct Black 38, C.l. Solubilized Vat Black 1, C.l. Fluorescent Brightening Agent 30, C.l. Fluorescent Brightening Agent 32 and C.l. Azoic Brown 2 and the organic sulfonic acid or carboxylic acid having at least one pair of conjugated double bonds per molecule is selected from toluidene, a-naphthalenesulfonic acid, anthraquinonesulfonic acids, dodecylbenzenesulfonic acids, p-toluenesulfonic acid, abietic acid, isonicotinic acid, benzoic acid, phenylglycine and 3-hydroxy-2-naphthoic acid.

7. The method as claimed in claim 1, substantially as hereinbefore described in any of the Examples.

8. A polyvinyl chloride resin when prepared by a method according to any preceding claim.