GB2029844A - Polyethyleneimine derivative for suppressing build-up in vinyl halide polymerization reactors - Google Patents

Abstract

Process for the production of a build-up suppressant coating product which comprises reactively admixing at least polyethyleneimine and one or both of hydroquinone and catechol under oxidative conditions in the presence of an iodate of formula MIO3 where M is a group I alkali metal. An o-aldehyde-substituted phenol or phenate (eg. salicylaldehyde or a phenate derived therefrom) may be included in the reaction. The product is used for coating internal surfaces of a vinyl halide polymerisation reactor and vinyl halide polymerisation is carried out in a reactor so coated.

Description

SPECIFICATION Production of build-up suppressant product, its use for coating a vinyl halide polymerisation reactor and vinyl halide polymerisation in a reactor so coated The present invention relates to a process for the production of a build-up suppressant coating product, to the use of the product for coating internal surfaces of a vinyl halide polymerisation reactor, and to vinyl halide polymerisation carried out in a reactor so coated.

When vinyl halide monomers, particularly vinyl chloride, are polymerised in aqueous dispersion in a reactor it is well known that a problem arises in that surfaces inside the reactor become coated with tenaciously adhering polymeric material known as build-up. By polyermisation in aqueous dispersion is meant polymerisation in aqueous emulsion or aqueous suspension (including aqueous microsuspension). The formation of tenaciously adhering build-up is undesirable from the point of view of achieving efficient heat transfer for cooling and heating the reactor contents, effective usage of monomer, effective control and monitoring of the polymerisation reaction, and acceptable polymer quality (in view of contamination by dislodged particles of the build-up).

Because of the problems in respect of heat transfer, polymerisation control and polymer quality, it is necessary to clean the reactor between each polymerisaion cycle wherein the deposited material is removed as completely as possible, e.g. by scraping or chipping by hand, solvent cleaning or pressure-washing.

This is wasteful in terms of the expense of the equipment and manpower required to carry out such cleaning and also in terms of the loss of productivity for a given reactor arising from the time taken to effect the cleaning operation. Hand cleaning is additionally undesirable in that it may constitute a health hazard to the cleaner in view of the harmful properties of certain monomers, particularly vinyl chloride. Indeed, opening up the reactor, between polymerisation cycles for cleaning is itself undesirable in this respect since it releases residual monomer in the reactor to the surrounding atmosphere.

It has been proposed in our British patent 1 444360 to coat the internal surfaces of a reactor used for vinyl chloride polymerisation with polyethyleneimine in order to prevent or reduce the formation of build-up therein. Our British patent 1 439 339 is directed towards the same object but employs, inter alia, polyethyleneimine which has been cross-linked with an aldehyde such as formaldehyde. We have found that these techniques, while effective, are not wholly satisfactory in plant-scale polymer production because the reactor surface needs to be polished and even then there is still some build-up produced, particularly above the liquid level of the polymerisation medium, which necessitates fairly frequent cleaning of the reactor.

A technique has been disclosed in our Belgian patent 862023 which is a distinct improvement in terms of build-up suppression over those of British patents 1 444 360 and 1 439 339. In this, the internal surface(s) of the polymerisation reactor for the vinyl halide polymerisation is coated with an adherent coating of a product derived from the admixture of at least polyethyleneimine and one or more of hydroquinone, p-benzoquinone, catechol and o-benzoquinone and then the vinyl halide polymerisation is carried out in the reactor so coated. A preferred coating product for use in this technique described in Belgian patent 862023 is one derived by admixing polyethyleneimine, hydroquinone and optionally a small quantity of pyrogallol.The coating product in this technique is applied to the internal surfaces of the reactor as a dispersion or solution in a liquid carrier medium which must be caused by heating to evaporate so as to leave an adherent coating in a reasonably short time.

In a further improvement upon the technique described in Belgian patent 862023, an o-aldehyde-substituted phenol (or phenate derived therefrom), particularly salicylaldehyde, is included as one of the constituents from which the coating product is derived; this results in the formation of a product of enhanced adhesive properties allowing the product to be used more effectively as a buildup suppressant. Thus this product may be applied to the reactor internal surfaces as a dispersion or solution in a liquid carrier medium without there being any necessity to evaporate off the liquid carrier medium. All that is required e.g. is to allow the excess of the dispersion or solution to drain from the reactor followed if necessary by rinsing with water.

In the technique described in Belgian patent 862 023, and in the refinement using o- aldehyde-substituted phenol, the preparation of the coating product is best effected in the presence of oxygen. Normally this is supplied by exposing the constituents from which the product is derived to the surrounding air; such exposure may be during or after admixture.

However, this method of supplying oxygen is disadvantageous if it is desired to prepare the coating product in-situ on the reactor walls or from a spray of a preformed dispersion of the coating product without opening up the reactor to the surrounding atmosphere--as would be the case when carrying out production runs where the reactor is not opened between polymerisations so that the reactor space does not contain air when the material which is to be or form the coating product is inside the reactor. Furthermore, the uptake of oxygen derived from the surrounding air, or from injected air or oxygen, tends to be rather nonquantitative and somewhat non-controllable even when the coating product is not formed in situ.

We have now devised an excellent alternative to the provision of oxygen from the surrounding air or from injected air or oxygen when preparing the coating product.

According to the present invention there is provided a process for the production of a build-up suppressant coating product which comprises reactively admixing at least polyethyleneimine and one or both of hydroquinone and catechol under oxidative conditions in the presence of an iodate of formula Ml03 where M is a group I alkali metal.

Preferably Ml03 is potassium or sodium iodate.

There is also provided according to the invention the use of a build-up suppressant coating product prepared by the process defined herein, for coating internal surfaces of a vinyl halide polymerisation reactor.

There is further provided according to the invention a process for the polymerisation of vinyl halide in aqueous dispersion wherein polymerisation is carried out in a reactor having adherently deposited on internal surfaces thereof a build-up suppressant coating product prepared by the process defined herein.

The preparation of the coating product may be effected in the complete absence of gaseous oxygen or air. However, it is possible for the preparation to be carried out in a reaction vessel without positively excluding air from the vessel although of course air will not be injected through the reaction mixture as oxidation will be effected substantially by the iodate.

Other organic oxygen-containing compounds may be incorporated into the mixture from which the coating product is derived, such as polyhydric phenols having the formula

where R1, R2, R3 and R4, which may be the same or different, are selected from H, halogen (fluorine, chlorine, bromine or iodine), hydroxyl, alkoxyl, aryl (including substituted aryl), nitro, alkyl (including substituted alkyl)-preferably lower alkyl, alkenyl (including substituted alkenyl)-preferably lower alkenyl, esterified carboxyl and acyl, provided that the above formula excludes hydroquinone and catechol. The polyhydric phenol preferably contains not more than three nuclear hydroxyl groups and preferably has at least two of R1, R2,R3 and R4 as H.

Also, as mentioned above, o-aldehyde-substituted phenols in particular may advantageously be incorporated into the mixture from which the coating product is desired. For the sake of clarity, by an o-aldehyde-substituted phenol is meant a phenol (i.e. a mono- or polyhydroxyl nuclear substituted aromatic compound such as one based on a benzene ring) having an aldehyde group -CHO as a nuclear substitutent ortho to a nuclear hydroxyl group. Preferably the o-aldehyde-substituted phenol has the formula

where R5, Re, R7 and R8, which may be the same or different are selected from H, halogen, hydroxyl, alkoxyl, aryl (including substituted aryl), alkyl (including substituted alkyl and preferably lower alkyl), alkenyl (including substituted alkenyl and preferably lower alkenyl), carboxyl, esterified carboxyl and acyl.

The o-aldehyde-substituted phenol preferably has at least two of R5, Re, R7 and R8 as H.

More preferably at least three of R5, Re, R7 and R8 are H, with the other substituents being e.g. alkyl (e.g. methyl), OH or H. Examples of suitable o-aldehyde-substituted phenols include salicylaldehyde and gentisaldehyde.

The most preferred o-aldehyde-substituted phenol is salicylaldehyde.

Where the mixture from which the coating is derived includes an o-aldehyde-substituted phenol (like salicylaldehyde), it is preferable not to include other oxygen-containing compounds (like pyrogallol) in the mixture.

The preparation of the coating product may be undertaken under a variety of different conditions.

For example the preparation may be carried out under a wide range of temperature, vary ing from preparations with temperatures of the order of 15-100"C, to preparations at higher temperatures e.g. using temperatures of the order of 100-1 50"C and above. The usual range of preparation temperature is 15-150 C, typical heat-treatment ranges being 15-100"C and 50-150"C.

The molar ratio of the polyethyleneimine repeat unit (-CH2CH2NH-) to the other component(s) (selected from one or both of hydroquinone and catechol and optionally other components such as polyhydric phenols or oaldehyde-substituted phenols as defined) is not critical although the polyethyleneimine (repeat unit) is preferably present in a molar excess over each of the other categories of constituent. For products derived from polyethyleneimine and hydroquinone the preferred molar ratio of polyethyleneimine (repeat unit)/ hydroquinone is 6/0.5 to 1/1, typically 4/1 to 1/1 and particularly 2.5/1 to 1/1.

The molar ratio of polyethyleneimine (repeat unit)/o-aldehyde-substituted phenol (if used) is preferably 6/1 to 1/1, more preferably 4/1 to 1/1, particularly 4/1 to 2/1.

The amount of Ml03 to be used need only be very small in comparison to the amount of polyethyleneimine used, and an effective amount to use is readily determined by experiment. Generally speaking, an amount of MlO3 of 0.2 to 1.0 mole %, based on the molar repeat unit of polyethyleneimine (-CH2CH2NH-), is effective under nearly all circumstances. The particularly preferred iodate to employ is KlO3 on account of its ready commercial availability in a highly purified state. The iodate may be conveniently added as an aqueous solution.

The preparation of the coating product should preferably be undertaken under alkaline conditions, i.e. the medium in which the oxidative admixture takes place having pH > 7. Under such circumstances, the polyhydric phenol or o-aldehyde-substituted phenol (if used) are of course present as phenate ions. To achieve this it is advantageous to employ an added inorganic alkali such as a metal or ammonium hydroxide (which may be added before, during or after admixture) to ensure an alkaline medium; preferably the inorganic alkali is a caustic alkali such as sodium hydroxide or potassium hydroxide.

In a preferred embodiment of the invention, the coating product is derived from the admixture of hydroquinone, polyethyleneimine, potassium iodate and salicylaldehyde under alkaline conditions (pH > 7). In another useful embodiment of the invention, the coating product is derived from the admixture of polyethyleneimine, hydroquinone, potassium iodate and optionally a small quantity of pyrogallol, the amount of pyrrogallol (if used) typically being < 0.3 molar, preferably < 0.1 molar, relative to 1 mole of hydroquinone.

The admixture of the constituents for the coating product may be effected in situ on internal surfaces of the reactor, or in a separate operation away from the internal surfaces to form a product which is then applied to the internal surfaces. In both methods the coating product is applied to the surfaces as a dispersion or solution in a liquid carrier medium.

Where the admixture is effected on interior surfaces of the reactor, such admixture is preferably carried out at a low temperature.

This method of admixture is particularly applicable to the formation of a coating product derived from polyethyleneimine, hydroquinone and optionally salicylaldehyde and may be conveniently effected by applying separate solutions or dispersions of the polyethyleneimine, hydroquinone, Ml03 and salicylaldehyde (if used) sequentially or concurrently to the surface or surfaces to be coated (before adding the ingredients for the vinyl halide polymerisation reaction) whereupon interaction takes place rapidly to form a coating product carried in a liquid medium.

Where admixture is effected in a separate operation before application of the coating product to an internal surface, a solution or dispersion of the product in a carrier medium is prepared and this solution or dispersion is applied to the reactor surface e.g. by spraying, painting, dipping or flooding. In this embodiment, the temperature during the admixing operation is not critical; for example the constituents may be admixed at ambient temperatures, although higher temperatures (e.g. 50-150"C may be used. The order in which the constituents are mixed and optionally heat-treated is not critical.

The product obtained from the mixing operation may vary from a very viscous substance to a glass-like substance and sometimes renders the medium in which the admixture takes place too viscous to be applicable directly to a reactor internal surface, even in cases where the medium has not been removed (e.g. by distillation). Consequently the product resulting from the admixture (whether still in the presence of the admixing medium or not) may sometimes need to be diluted with a solvent or dispersant which will then act as a liquid carrier medium for application to reactor internal surfaces. However, if the medium in which the admixture has taken place is present in sufficient quantity, it may not be necessary to dilute it further and the dispersion of solution of the product can be used as it stands.The coating product dissolves in very few liquids; however ethanol (and to a lesser extent methanol) has been found to be an adequate solvent. The coating product will also dissolve in aqueous alkaline solutions (of e.g.

NaOH, KOH and Ca(OH)2) which can therefore also be used as solvents. Solvent mixtures such as ethanol (or methanol) with aqueous alkali can also be used. A suitable non-solubilising dispersant is non-alkaline water, although care may sometimes be necessary to ensure that the product is evenly dispersed in the water.

In addition, it may sometimes be advantageous to incorporate a wetting agent into the solution or dispersion of the coating product which is applied to the reactor surfaces in order to ensure the best possible coverage thereof; this particularly applies where a reactor, e.g. a large plant-scale reactor made of steel, has surfaces that are rather rough or blemished from many years of operation.

The chemical nature of the coating product is by no means properly understood. However, since the product is invariably deeply coloured (e.g. red, brown or black) it is believed that the product may at least to some extent include aminoquinonoid structures containing groups of the following type

(and/or the corresponding ortho structures) with the nitrogen atoms being derived from the polyethyleneimine chain. It is thought that this type of structure may be rather more significant when admixture takes place at a low temperature.

It is also thought that the coating product consists at least to some extent of a condensation reaction product formed as a result of condensation between the amino groups of the polyethyleneimine and the hydroxy groups of e.g. the hydroquinone (and/or catechol).

It is further though that the coating product consists at least to some extent of a stabilised form of the radical anion structurally derived from p-benzoquinone, i.e. the radical anion of the formula

(and/or the corresponding radical structurally derived from o-benzoquinone) the stabilisation being effected by the dispersal of the radical anion in the polyethyleneimine matrix.

The effect of the o-aldehyde-substituted phenol (if used) is not properly understood but it is believed to react with the primary amino groups in the polyethyleneimine (which are present because of a certain amount of chain branching) to form Schiff's bases along the polymer chain which interact with the reactor surface thereby enhancing the adhesion of the coating product to the reactor surface.

The amount of the coating product employed is not too critical (although the coating should not be too thick). Generally speaking, an amount of 1 to 100 ppm, preferably 2 to 25 ppm (by weight on the monomer to be charged), coated evenly over internal surfaces of the reactor is sufficient for many sizes and shapes of reactor. Of course, the surface area/volume ratio of reactors will vary considerably according to the sizes of the reactors.

The coating may be formed on any surface inside the reactor which is liable to suffer the formation of build-up thereon. For example, it may be applied to the interior surface of the main body of the reactor, and to the interior surface of the roof of the reactor which is often above the liquid level of the polymerisation medium and usually suffers tenacious build-up thereon. It may also be applied to the surfaces of the agitation equipment inside the reactor. If a condenser is installed in a part of the reactor that is in contact with the gaseous phase during polymerisation or if it is installed outside the reactor and connected thereto by conduit piping, the condenser and conduit piping may be similarly coated.

It is to be appreciated that for the best results, a surface to be coated should be as clean and as smooth as possible to begin with. If the surface is of somewhat dubious quality in this respect, it may be advisable to coat it with two or more successively applied layers of the coating product.

Similarly if it is desired to carry out many successive polymerisations in the same reactor (e.g. up to five or more) without opening or cleaning or recoating the reactor between polymerisations, it is preferable to use a multiple coating (e.g. three coatings) before starting the sequence of polymerisations.

The coating may be formed on a surface in combination with one or more other materials, e.g. materials which also have a suppressing effect on polymerisation build-up.

A reactor having coated internal surfaces according to the invention may be used for the polymerisation of vinyl halide monomers particularly vinyl chloride, wherein the formation of build-up is eliminated or very much suppressed. The polymerisation reaction may be carried out in the presence of a basic substance such as NaHCO3 or a suitable buffering system to ensure an adequately high pH for the reaction medium (e.g. pH > 4) as such an expedient can further enhance the build-up suppressant effect of the coating product.

By "vinyl halide monomers" is meant those monomers polymerisable by free-radical polymerisation which are olefinically unsaturated in the a-position and substituted by at least one halogen atom. These monomers are preferably selected from substituted derivatives of ethylene and contain only two carbon atoms.

Examples of such monomers include vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene fluoride, chlorotrifluoroethylene and tetrafluoroethylene. The invention is preferably applied to the polymerisation of fluorine- or fluorine-containing vinyl monomers, especially vinyl chloride.

By "polymerisation" is meant both the homopolymerisation of the vinyl halide monomers and the copolymerisation with each other or with other comonomers copolymerisable therewith. Examples of the latter include vinyl esters such as vinyl acetate, acrylic esters such as methyl acrylate and butyl methacrylate, acrylic nitriles such as acrylonitrile and methacrylonitrile, unsaturated diesters such as diethyl maleate, allyl esters such as allyl acetate, a-olefines such as ethylene and propylene, vinyl ethers and styrene compounds.

We prefer to apply the invention to the production of polymers containing at least 50% molar and more particularly at least 80% molar, of units derived from vinyl halide monomers, particularly vinyl chloride.

The present invention may be particularly employed in any polymerisation technique where a monomer(s) is dispersed in the form of droplets in a liquid aqueous phase. For example it may be used for polymerisation in aqueous emulsion in which case any suitable emulsifying agent can be used, and in particular an ionic emulsifying agent such as sodium lauryl sulphonate or sodium dodecyl benzene sulphonate, and non-ionic emulsifying agents.

It is also possible to use any water-soluble free-radical polymerisation initiator and particularly a persulphate.

The invention is also very applicable to polymerisation in aqueous suspension and microsuspension.

Any suitable dispersing agent may be used for polymerisation in aqueous suspension, and particularly finely dispersed solids, gelatin, polyvinyl acetates of various degrees of hydrolysis, water-soluble cellulosic ethers and polyvinyl pyrrolidones. These dispersing agents can be used together with other surface-active agents if desired. The amount employed may vary widely and is generally between 0.05 and 1.5% by weight calculated on the water used.

Any suitable free-radical polymerisation initiator that is monomer-soluble may be used for polymerisation in aqueous suspension. Examples of these include peroxy compounds such as di-tertiarybutyl peroxide, lauroyl peroxide and acetyl cyclohexyl sulphonyl peroxide, azo compounds such as azo-bis-isobutyronitrile and 2,2'-azo-bis-2,4-dimethyl-valeronitrile and boron alkyls.Monomer-soluble free-radical polymerisation initiators that are particularly suitable for use in a polymerisation process according t6 the invention are the dialkyl peroxydicarbonates whose alkyl radicals contain up to 20 carbon atoms, such as diethyl peroxydicarbonate, diisopropyl peroxydicarbonate and d icetyl peroxydicarbonate, d icycloal kyl peroxydicarbonate such as bis(4-tertiarybutylcyclohexyl) peroxydicarbonate, and azo compounds such as 2,2'-azo-bis-2,4-dimethyl-valeronitrile and azo-bis-isobutyronitrile. These initiators may be used in conventional quantitiesgen- erally speaking from 0.01 to 1% by weight calculated on monomer.

Polymerisation in homogenised aqueous dispersion, sometimes known as polymerisation in microsuspension, comprises mechanically homogenising an aqueous dispersion of the monomer or monomers in the presence of a surface-active agent (for example by subjecting it to a violent shearing action in a colloid mill), and polymerising the homogenised dispersion in the presence of an initiator that is monomer-soluble.

Conventional emulsifying agents and monomer-soluble initiators can be used for polymerisation in microsuspension such as for example a cationic emulsifying agent like sodium dodecylbenzene sulphonate, and peroxide initiators of the dialkanoyl peroxide type, e.g.

lauroyl peroxide.

In addition to the emulsifying or dispersing agents and initiators, the aqueous dispersions (i.e. suspensions, microsuspensions and emulsions) may contain one or more additives that are sometimes employed in conventional processes for polymerisation in aqueous dispersion. Examples of such additives include particle size regulators, molecular weight regulators, stabilisers, plasticisers, colouring agents, reinforcing agents, and processing aids.

The operating conditions for polymerisation according to the present invention may be those customarily used. For example, in the case of vinyl chloride polymerisation, the polymerisation temperature is generally between 40 and 75"C and the pressure generally below 15 kg/cm2.

The present invention is now illustrated by the following Examples. Unless otherwise specified all parts and percentages are by weight.

EXAMPLE 1 100 ml of a 20% aqueous solution of a commercially available polyethyleneimine (i.e.

the solution containing 20 g or 0.47 mole of polyethyleneimine repeat unit) were added to 100 ml water under nitrogen in a blending machine. After mixing well at ambient temperature, 25.5 g of hydroquinone (0.23 mole) were blended in followed by 4.6 g (0.115 mole) NaOH dissolved in 100 ml water. 10 minutes later 0.5 g KlO3 (0.002 mole) dissolved in 100 ml water were added and the mixture left stirring at ambient temperature for 2.5 hours under a nitrogen blanket. The caustic solution of the coating product so formed was found to have a solids content of about 11% and was used as such.

EXAMPLE 2 In order to test the build-up suppressant of Example 1, three rectangular stainless steel plates of increasing roughness were painted on one side with the caustic suppressant solution from Example 1 and allowed to dry in the atmosphere at ambient temperature. Part of the painted surface area of each plate was then painted with another coat of the suppressant solution and again allowed to dry. The plates were mounted on a trident and suspended within the stirred aqueous reaction medium of a typical polymerisation run to make granular vinyl chloride homopolymer using diethylperoxydicarbonate as initiator and partially hydrolysed polyvinyl acetate as suspension agent. No retardation of the polymerisation rate occurred.

At the end of the polymerisaion the trident was removed and the plates rinsed with water. No build-up was observed on the areas of the plates treated with the build-up suppressant of Example 1 (whether treated with one or two coats). However the untreated surfaces on the other sides of the plates were covered with a thick skin of build-up.

EXAMPLE 3 The following components were mixed at ambient temperature (18"C) in a closed flask from which the air had not been removed: 44 g (0.40 mole) hydroquinone washed in with 75 ml water; 26.4 g (0.22 mole) salicylaldehyde washed in with 75 ml water; 1 72 ml of a 20% aqueous solution of a commercially available polyethyleneimine (i.e. the solution containing 34.4 g or 0.80 mole of polyethyleneimine repeat unit) washed in with 90 ml water; and 4 ml concentrated HCl washed in with 1 3 ml water. the bright yellow mixture was heated to 60"C to dissolve the hydroquinone and then cooled to 30"C. 450 ml of this preparation were transferred to a caustic solution containing 60.3 g (1.51 mole) NaOH dissolved in 1.349 ml water to which was added 8.56 Kl03 (0.04 mole) dissolved in 100 ml water over 1 5 minutes. The caustic solution of the coating product so formed was found to have a density of 1.043 and pH 13.1.

EXAMPLE 4 In this example, a conventional vinyl chloride suspension polymerisaion was carried out in a stainless steel reactor (capacity 5 litres) provided with a paddle stirrer using a polymerisation temperature of 57"C, diethylperoxydicarbonate as initiator and partially hydrolysed polyvinyl acetate as suspension agent.

Before charging the reactor, the interior surfaces (having been cleaned) were painted once with a sample of the solution of coating product from Example 3 diluted by 50% with water (no rinsing). Inspection of the reactor interior surfaces after the termination of polymerisation (in which no retardation occurred) and removal of the polymer slurry showed that no build-up had been formed.

EXAMPLE 5 Example 4 was repeated but using bis(4tertiarybutylcyclohexyl) peroxydicarbonate as initiator. No build-up was formed on the reactor interior surfaces.

EXAMPLE 6 Example 4 was repeated by employing a larger reactor (capacity 1 60 litres). No buildup was formed on the reactor interior surfaces.

Claims (9)

1. A process for the production of a buildup suppressant coating product which comprises reactively admixing at least polyethyleneimine and one or both of hydroquinone and catechol under oxidative conditions in the presence of an iodate of formula MlO3 where M is a group I alkali metal.

2. A process according to claim 1 wherein the reactive admixture is effected in the absence of gaseous oxygen or air.

3. A process according to either claim 1 or claim 2 wherein the iodate used is potassium iodate or sodium iodate.

4. A process according to any one of the preceding claims wherein the reactive mixture to form the coating product includes an oaldehyde-substituted phenol (or the phenate derived therefrom).

5. A process according to claim 4 which comprises reactively admixing polyethyleneimine, hydroquinone, salicylaldehyde (or the phenate derived therefrom) under oxidative conditions in the presence of potassium iodate or sodium iodate.

6. A process according to any one of the preceding claims wherein the production is carried out under alkaline conditions (pH > 7).

7. A build-up suppressant coating product prepared by a process according to any one of the preceding claims.

8. The use of a build-up suppressant coating product, prepared by a process according to any one of claims 1 to 6, for coating internal surfaces of a vinyl halide polymerisation reactor.

9. A process for the polymerisation of vinyl halide in aqueous dispersion wherein polymerisation is carried out in a reactor having adherently deposited on internal surfaces thereof a build-up suppressant coating product prepared by a process according to any one of claims 1 to 6.