CH623833A5 - Process for aqueous dispersion polymerisation of halogenated vinyl monomers - Google Patents

Description

The present invention relates to a process for the aqueous dispersion polymerization of halogenated vinyl monomers such as vinyl chloride.

Usually, the halogenated vinyl monomers are polymerized in the form of droplets dispersed in water thanks to a mechanical stirring and to the presence of emulsifying or dispersing agents, to the intervention of liposoluble (polymerization in suspension) or water-soluble polymerization initiators (emulsion polymerization). The polymerization is generally carried out according to a batch process in steel tank reactors provided with a paddle stirrer and possibly with deflectors.

In these traditional polymerization techniques, and more particularly in suspension polymerization, solid deposits of polymer are formed during polymerization, generally called crusts, which adhere strongly to the internal surfaces of the reactors (tanks, agitators, deflectors). . This phenomenon is generally referred to as crusting.

Crusting is extremely harmful. Indeed, the crusts covering the inside of the tank reduce the amount of calories that it is possible to evacuate by the heat transfer fluid circulating in the double jacket with which the reactors are generally provided. As a result, the productivity is reduced, because one is forced to use lower polymerization rates than if there was no crusting.

In addition, it frequently happens that during polymerization the crusts partially detach and contaminate the polymers obtained which then contain infusible nodules originating from the crusts, commonly called fish-eyes.

Finally, crusting, which always occurs irregularly, makes it more difficult to conduct and control the polymerization cycles.

Admittedly, it is usual to clean the internal surfaces of the reactors after each polymerization cycle. This cleaning is often done manually, but there are also more advanced techniques using hot solvents or devices projecting jets of water at high speed. However, this cleaning is always a painful operation, costly in terms of labor, energy and material, which immobilizes the polymerization reactor for a long time.

This is why an attempt has been made to prevent crusting from occurring by adding to the polymerization medium various additives having an inhibiting effect on crusting. Thus, in German patent application No. 2518260 filed March 4, 1976 in the name of Chisso Corp., it is proposed to use for this purpose oxalic acid or a salt of oxalic acid.

However, the inhibitory effect on the crusting of oxalic acid and its salts is not total. Consequently, this inhibitor does not allow very long series of polymerization cycles without any intermediate cleaning.

We have now found a new class of crust inhibitors which is completely effective and which has no harmful side effects.

The present invention therefore relates to a process for the polymerization in aqueous dispersion of halogenated vinyl monomers by the intervention of radical polymerization initiators in the presence of a crusting inhibitor, in which the crusting inhibitor is chosen among the anions derived from hydroxylated organic carboxylic acids.

The anions used as crust inhibitors according to the present invention can be derived from any organic acid comprising in its molecule at least one carboxylic group - COOH and a hydroxyl group --OH substituting for carbon atoms. In general, anions derived from organic acids comprising from 2 to 20 carbon atoms in total are used. Preferably, anions derived from organic acids comprising from 3 to 8 carbon atoms are used, the best results being obtained with anions derived from acids comprising from 4 to 6 carbon atoms.

Preferably, anions derived from organic acids comprising at least two carboxylic groups per molecule are used as crust inhibitors according to the invention, the best results being obtained with anions derived from organic acids comprising at least two carboxylic groups. and at least two hydroxyl groups.

Examples of anions which can be used according to the present invention are those derived from hydroxyacetic (glycolic), hydroxyphenylacetic, hydroxybenzoic, hydroxybutanoic, hydroxybutenoic, hydroxycinnamic (coumaric), hydroxycyclohexanecarboxylic, hydroxyhexanoic, hydroxypropanoic (lactic), leucic, hydroxymalonic (hydroxy) , hydroxypentanedioic, hydroxyphthalic, hydroxypropanoic, hydroxyphenylpropanoic, citric and, more particularly, dihydroxy-butanedioic (tartaric), dihydroxymaleic, gluconic and citric.

Of all the anions which can be used as crust inhibitors according to the present invention, the most effective are those derived from tartaric acid and citric acid. Very particularly preferred anions are those derived from tartaric acid. These anions can derive indifferently from all varieties

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optics of tartaric acid. We can therefore generate anions from dextrorotatory tartaric acid (d-tartaric or L (+) tartaric), levorotatory tartaric acid (1-tartaric or D (-) tartaric), mesotartaric acid, optically inactive by internal compensation, or racemic tartaric acid (dl-tartaric). Advantageously, natural tartaric acid (d-tar-tric or L (+) tartaric) is used.

The anions can be used in the process according to the present invention in the form of any compound which is soluble in water and which dissociates therefrom to form the anions defined above. In particular, the anions can be generated from the corresponding hydroxylated organic carboxylic acids and their salts. Among these, preference is given to alkaline earth salts and even more to alkaline salts of potassium or sodium for example, and of ammonium. When using salts, it is advisable to choose salts whose cation does not have any harmful effect for the polymerization reaction and the properties of the polymer. The best results are however obtained starting from the hydroxylated organic carboxylic acid itself.

The process according to the invention applies to the polymerization of halogenated vinyl monomers. The term “halogenated vinyl monomers” is intended to denote all the monomers polymerizable by radical polymerization having a terminal olefinic unsaturation and substituted with at least one halogen. Preferably, these monomers are chosen from substituted derivatives of ethylene and contain only two carbon atoms. As examples of such monomers, mention may be made of vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene chloride, vinylidene fluoride, chlorotrifluoroethylene and tetrafluoroethylene. The invention is preferably applied to the polymerization of vinyl fluorinated and chlorinated monomers. It is particularly suitable for polymerizing chlorinated monomers and especially vinyl chloride.

The term “polymerization” is intended to denote both the homopoly-merization of the halogenated vinyl monomers and their copoly-merization with one another or with other monomers copolymerizable with them. As examples of the latter, mention may be made of vinyl esters such as vinyl acetate, acrylic esters such as methyl acrylate and glycidyl methacrylate, unsaturated nitriles such as acrylonitrile and methacrylonitrile, unsaturated diesters such as dibutyl maleate, allyl esters such as allyl acetate, unsaturated amides such as acrylamide, styrenic derivatives and α-olefins such as ethylene and propylene.

However, the invention preferably applies to the manufacture of polymers containing at least 50 mol%, and more particularly at least 80 mol%, of units derived from halogenated vinyl monomers in their molecule.

The invention applies as well to the manufacture of random copolymers as to that of block or graft copolymers.

The process according to the invention can be used in all polymerization techniques in which the halogenated vinyl monomers are dispersed in the form of droplets in a liquid aqueous phase. In particular, it can be used in aqueous emulsion polymerization. In this case, any emulsifying agent can be used and, in particular, anionic emulsifying agents such as sodium benzylsulfonate or sodium dodecylbenzenesulfonate, and nonionic emulsifying agents. It is also possible to use any initiator of water-soluble radical polymerization and in particular the persulfates.

However, the crusting problem is particularly acute in the techniques of polymerization in aqueous suspension and in microsuspension, in which liposoluble polymerization initiators are used.

In suspension polymerization, traditional dispersing agents such as finely solids are generally used

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dispersed, gelatins, water-soluble cellulose ethers, synthetic polymers such as polyvinylpyrrolidone, and vinyl acetate / maleic anhydride copolymers, and mixtures thereof. It is also possible, at the same time as the dispersing agents, to use surfactants. The amount of dispersing agent used generally varies between 0.5 and 1.5% by weight relative to the water.

In the microsuspension polymerization, also sometimes called a homogenized aqueous dispersion, an emulsion of droplets of monomers is produced, by means of powerful mechanical stirring and usually in the presence of emulsifiers of the same kind as those mentioned above, and one realizes polymerization with the intervention of liposoluble initiators.

Any fat-soluble initiator can be used in suspension or microsuspension polymerization. By way of examples, mention may be made of peroxides such as ditertiobutyl peroxide, lauroyl peroxide and acetyl-cyclohexylsulfonyl peroxide, lauroyl peroxide and acetylcyclohexylsulfonyl peroxide, azo compounds such as azo -bisisobutyronitrile and azobis-2,4-dimethylvaleronitrile, dialkyl peroxydicarbonates such as diethyl, diisopropyl, dicyclohexyl and ditertio-butylcyclohexyl peroxydicarbonates, and alkylbores. In general, these initiators are used in an amount of 0.01 to 1% by weight relative to the monomers.

In addition to dispersing or emulsifying agents and initiators, the polymerization medium can also comprise various additives normally used in traditional methods of polymerization in aqueous dispersion. As examples of such additives, mention may be made of buffers, agents which regulate the diameter of the polymer particles, such as polyphosphates of alkali metals, agents which regulate molecular weight, stabilizers, plasticizers and dyes. as well as reinforcing agents or facilitating the use of polymers. It is also possible to incorporate into the polymerization medium, in addition to the compounds generating anions derived from hydroxylated organic carboxylic acids, other crusting inhibitors.

The operating conditions of the polymerization process according to the invention do not differ from those usually used. The temperature is generally between 35 and 80 ° C. The absolute pressure is generally less than 15 kg / cm2. The pH is generally acidic, for example between 2 and 7. The amount of water used is generally such that the total weight of the monomers represents 20 to 50% of the total weight of the water and the monomers.

In general, the polymerization of halogenated vinyl monomers * is carried out batchwise in cycles in which we first introduce water, then the various components of the reaction medium (dispersing or emulsifying agents, initiators, monomers, etc.). According to a preferred embodiment of the invention, the crusting inhibitor is introduced into the polymerization medium before any halogenated vinyl monomer, the best results being obtained when it is introduced before any initiator also. To do this, simply add the crusting inhibitor after the water has been introduced, possibly at the same time as the dispersing or emulsifying agents.

The crusting inhibitor can also be introduced into the polymerization medium several times during the polymerization or continuously.

To cause the start of the polymerization, the polymerization medium is heated, for example by means of the heat transfer fluid circulating in the double jacket with which the reactors are usually provided. Preferably, the crusting inhibitor is introduced into the polymerization medium before starting the heating.

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When the polymerization is carried out continuously, the crusting inhibitor is preferably also introduced continuously into the polymerization medium.

The amounts of crusting inhibitor to be used can vary to a very large extent, in particular depending on the nature of the monomers and the state of the internal surfaces of the reactors used. In general, an amount of crusting inhibitor is used such that the polymerization medium contains at least 1 ppm relative to water. Preferably, the polymerization medium contains at least 5 ppm of inhibitor relative to the water. The best results are obtained when the polymerization medium contains at least 10 ppm of crust inhibitor relative to water.

There is no disadvantage in using high amounts of crust inhibitor. However, it is generally unnecessary to exceed in the polymerization medium a content equal to 10,000 ppm of crusting inhibitor relative to water. Most often, a content equal to 1000 ppm is sufficient. In almost all cases, a content of at most 200 ppm is entirely satisfactory.

It has been observed that a certain amount of crusting inhibitor is adsorbed on the internal metal surfaces of the polymerization reactors. This is why it is preferred that the polymerization medium contains an amount of crusting inhibitor at least equal to 1 mg / m2 of internal metallic surface, and preferably to at least 5 mg / m2. Also because of this, a certain amount of crusting inhibitor can remain adsorbed on the internal metal surfaces of the reactors at the end of a polymerization cycle and exert its effects during the following cycle. We can take this amount into account and reduce the amount added during the next cycle.

When starting to use the process according to the invention in a specific reactor, it is advantageous to carry out a particularly careful cleaning of the internal surfaces beforehand. To do this, it is possible to use particularly effective solvents for the polymers forming the crusts, pyrolysis of the walls, pickling or even repolishing.

The new class of crust inhibitors which is the subject of the present invention has remarkable efficiency. In fact, crust inhibitors known to be effective in the known technique, such as oxalic acid and its salts which are described in the aforementioned German patent application No. 2518260,

must be used at a rate of approximately 50 ppm relative to water so that their effect is marked in the homopolymerization of vinyl chloride in suspension. The crusting inhibitors according to the invention are already fully effective when approximately 15 ppm are used with respect to water. In addition, they include a very large number of compounds which have no harmful side effects with regard to the appearance of the polymer, its odor, its heat and light stability and its suitability for use in packaging. food.

The polymers obtained according to the present invention can be used for all the usual applications of this type of product and in particular for the manufacture of objects such as bottles or profiles according to traditional extrusion-blow molding and extrusion techniques for example .

The examples which follow are intended to illustrate the invention and are in no way limiting.

Examples 1,2,4 and 5 are carried out according to the invention. Example 3 (R) is given for comparison.

Examples 1 to 3 (R):

A laboratory reactor made of stainless steel with a capacity of 3 1 is used, provided with a double jacket in which a heat transfer fluid circulates and with a traditional paddle stirrer made of stainless steel. The internal surfaces of the reactor are cleaned by washing with tetrahydrofuran, followed by pyrolysis carried out by heating the wall at 400 ° C. for

30 mins. The reactor is then pickled by means of a fluorometric bath, then rinsed 5 times with demineralized water.

1500 g of demineralized water and 10 mg of a hydroxylated organic carboxylic acid are successively introduced into the reactor. 1.65 g of polyvinyl alcohol are then introduced. We start the agitator. 500 mg of ditertiobutylcyclohexyl peroxy-dicarbonate are then introduced in powder. Vacuum is then made twice in the reactor (at 100 mm of absolute mercury) and, between the two operations, the reactor is swept with technical nitrogen at the pressure of 1360 mm of absolute mercury. 1000 g of vinyl chloride are then introduced. The polymerization medium is heated to 61 ° C at the rate of 1 ° C / min.

The polymerization medium is kept at 61 ° C. with stirring until the pressure drops from 3.5 kg / cm 2 absolute. The polymerization is then stopped by expansion of the pressure in the reactor and vaporization of the unpolymerized vinyl chloride. The mixture is cooled and the polymer is collected by wringing followed by drying.

Table I specifies the nature of the organic acid used as well as the result of the inspection of the internal surfaces of the reactor at the end of the polymerization cycle.

Table I

25 Acid Example Result of Inspection

Organic number

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Tartaric acid

No crust

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L (+)

Citric acid

Crust on a part of the periphery at the interface between the liquid medium and the gaseous sky covering approximately 1% of the surfaces exposed to crusting

35 3 (R)

Oxalic acid

Crust all around the interior

face between the liquid medium and the gaseous sky covering approximately 2% of the surfaces exposed to crusting

Example 4:

Successively introduced into a reactor identical to that used for the preceding examples 1500 g of demineralized water, 45 10 mg of (L (+) tartaric acid and 10 mg of potassium iodide. polyvinyl alcohol. The agitator is started up. 200 mg of diethyl peroxydicarbonate are then introduced. The reactor is then evacuated twice (to 100 mm of absolute mercury) and, between the two operations, 50 sweeps the reactor with technical nitrogen at a pressure of 1360 mm of absolute mercury, then 1000 g of vinyl chloride are introduced and the polymerization medium is heated to 61 ° C. at a rate of 1 ° C./min. .

The polymerization medium is kept at 61 ° C. with stirring until the pressure drops from 3.5 kg / cm 2 absolute. The polymerization is then stopped by expansion of the pressure in the reactor and vaporization of the unpolymerized vinyl chloride. It is cooled and the polymer is collected by wringing followed by drying.

Inspection of the internal surfaces of the reactor after polymerization reveals that they are free of any crust.

A similar polymerization could be repeated 3 times before thorough cleaning of the reactor was necessary.

Example 5:

65 This example conforms in all points to Example 4 except that, before the introduction of polyvinyl alcohol, successively introduced into demineralized water 10 mg of (L (+) tartaric acid, 10 mg d potassium iodide and 25 mg poly-

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sodium metaphosphate (sold under the name Meta-gon).

Inspection of the internal surfaces of the reactor after polymerization reveals that they are also devoid of any crust.

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A similar polymerization could be repeated 10 times before further cleaning of the reactor was required.

These examples show the superiority of the crusting inhibitors according to the invention, and more particularly that of the anions derived from tartaric acid, over oxalic acid.

R

Claims (10)

623,833 1. A method for the polymerization in aqueous dispersion of halogenated vinyl monomers by the intervention of radical polymerization initiators in the presence of a crust inhibitor, characterized in that the crust inhibitor is chosen from anions derived from hydroxylated organic carboxylic acids. 2. Method according to claim 1, characterized in that the anions are derived from organic acids comprising from 3 to 2 3. Method according to one of claims 1 or 2, characterized in that the anions are derived from organic acids comprising at least two carboxylic groups in their molecule. 4. Method according to claim 1, characterized in that the anions are derived from tartaric acid. 5. Method according to claim 1, characterized in that the anions are derived from citric acid. 6. Method according to one of claims 1 to 5, characterized in that the anions are introduced into the polymerization medium in the form of the corresponding organic acid or of alkali or ammonium salts of the corresponding organic acid. 7. Method according to one of claims 1 to 6, characterized in that the anions are introduced into the polymerization medium before the halogenated vinyl monomers and before the radical polymerization initiators. 7. Method according to one of claims 1 to 6, characterized in that the anions are introduced into the polymerization medium in the form of the corresponding organic acid or alkali or ammonium salts of the corresponding organic acid. 8. Method according to one of claims 1 to 7, characterized in that the anions are used at a rate of at least 5 ppm and less than 200 ppm relative to the water present in the polymerization medium. 8 carbon atoms in their molecule. 9. Method according to one of claims 1 to 8, characterized in that the anions are used at a rate of at least 5 mg per square meter of internal metallic surface of the reactors. 10. Method according to one of claims 1 to 9, characterized in that the polymerization is used in aqueous suspension or in aqueous microsuspension and in that the halogenated vinyl monomer is vinyl chloride.