FI62319B - FOERFARANDE FOER POLYMERISERING AV VINYLKLORID I VATTENDISPERSION - Google Patents

Description

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Patent- och reglsterstyreleen 'AnaMun utlagtf odi utl ^ krMtwi pubUcarad 31.08.82 (32) (33) (31) Pyrd * «y« cuoikew. ii «rd photh ·» 06.09 · 76

Luxembourg (LU) 75733 (71) Solvay & Cie, 33, rue du .Prince Albert, B-1050 Brussels, Belgium-Belgium (BE) (72) St ^ phane Noäl, Grimbergen, Belgium-Belgium (BE) (fk) Oy Kolster Ab (5 ^) Method for polymerizing vinyl chloride in aqueous dispersion -Förfarande för polymerisering av vinylklorid i vattendispersion

The invention relates to a process for the polymerization of vinyl chloride in an aqueous dispersion.

Halogenated vinyl monomers are usually polymerized in small droplets which are dispersed in water with mechanical stirring in the presence of emulsifiers or dispersants, using fat-soluble polymerization initiators in suspension polymerization and water-soluble initiators in emulsion polymerization. The polymerization is generally carried out by applying a batch process in steel tank reactors with a paddle stirrer and possibly baffles.

In the application of these conventional polymerization methods, and in particular in suspension polymerization, solid deposits of the polymer are formed during polymerization, commonly referred to as "karstes", which adhere firmly to the inner surfaces of the reactors (tanks, stirrers, baffles).

Scaling is very harmful. In fact, the scales covering the interior of the tank reduce the amount of heat that can be removed by the coolant circulating in the double jacket with which such reactors are generally equipped. As a result, productivity is reduced because lower polymerization rates are forced than in the absence of scaling.

In addition, it often happens that during the polymerization the karsts are partially detached and contaminate the formed polymers, which then contain insoluble spherical particles from the karsts. This phenomenon is commonly referred to as "fisheyes".

Finally, carding, which always occurs in an irregular manner, makes it difficult to carry out the polymerization and to adjust its stages.

However, the inner surfaces of the reactor are usually cleaned after each polymerization cycle. This cleaning is often done by hand, but there are also more advanced methods that use hot solvents or equipment that eject water jets at high speeds. However, this purification is always an embarrassing task that involves the cost of craftsmanship, power, and substance, and forces the polymerization reactor to be shut down for long periods of time.

To this end, efforts have been made to prevent the occurrence of scaling by adding various additives to the polymerization mixture which have an anti-scaling effect. Accordingly, DE patent application 2 518 260, filed on 4 March 1976 in the name of the business name Chisso Corp., proposes the use of oxalic acid or a salt of oxalic acid for this purpose.

However, the inhibitory effect of oxalic acid and its salts on scaling is not perfect. As a result, these inhibitors do not allow very long series of polymerization cycles without any intermediate purification.

A new class of anti-scaling agents has now been invented which is perfectly effective and has no detrimental side effect.

More particularly, the invention relates to a process for polymerizing vinyl chloride in an reactor as an aqueous dispersion using oil-soluble radical polymerization initiators in the presence of an anti-scaling agent. The invention is characterized in that the anti-scaling agent is an anion of an organic hydroxylated carboxylic acid having 3 to 8 carbon atoms, used at least 1 ppm and less than 200 ppm based on the amount of water in the polymerization mixture. The best results are obtained with anions derived from organics in which 3 6231 9 have 4 to 6 carbon atoms.

According to the invention, anions derived from organic acids having at least two carboxyl groups per molecule are preferably used as anti-scaling agents, and the best results are obtained with anions derived from organic acids having at least two carboxyl groups and at least two hydroxyl groups.

Examples of suitable anions for use in the invention are those derived from the following acids: hydroxyacetic (glycol), hydroxyphenylacetic, hydroxybenzoic, hydroxybutane, hydroxybutylene, hydroxycyclohexanecarboxylic, hydroxyhexane, hydroxypropane (milk), hydroxymalonic (tartrane), hydroxyoctane, hydroxypentanedione, hydroxyphthalic, hydroxypropane, citric and especially dihydroxybutane (tartaric), dihydroxymaleic, gluconic and citric acid.

Of all the anions used as anti-scaling agents according to the invention, the most effective are anions derived from tartaric acid and citric acid. Particularly popular are anions derived from tartaric acid. These anions can be derived equally well from all optical alternatives to tartaric acid. Thus, anions can be obtained using dextrorotatory tartaric acid (d-tartaric acid or L (+) - tartaric acid), dextrorotatory tartaric acid (1-tartaric acid or D (-) - tartaric acid), mesotartaric acid which is optically inactive by internal compensation, or racemic acid (dl-tartaric acid). Naturally occurring tartaric acid (d-tartaric acid or L (+) - tartaric acid) is suitably used.

The anions can be used in the process of the invention as an arbitrary form of the compound which is soluble in water and dissociates therein to form the anions defined above. In particular, anions can be obtained using the corresponding organic hydroxylated carboxylic acids and their salts. Of these, alkaline earth metal salts are best used, and especially alkali metal salts of, for example, potassium or sodium and ammonia. When these salts are used, they are preferably selected so that their cation has no detrimental effect on the polymerization reaction and the properties of the polymer. However, the best results are obtained by using the organic hydroxylated carboxylic acid itself as the starting material.

The process according to the invention can be applied to all polymerization processes in which the vinyl chloride monomer is dispersed in small droplets in the liquid aqueous phase.

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The invention is particularly applicable to aqueous emulsion polymerization. In this case, any suitable emulsifier can be used, and in particular anionic emulsifiers such as sodium benzylsulfonate or sodium dodecylbenzenesulfonate, and nonionic emulsifiers.

The scaling problem is particularly pronounced in polymerization processes using aqueous suspension and aqueous microsuspension and using fat-soluble polymerization initiators.

Conventional dispersants such as dispersed finely divided solids, gelatins, water-soluble cellulose ethers, synthetic polymers such as polyvinylpyrrolidone, and copolymers of vinyl acetate and maleic anhydride are usually used in suspension polymerization. In addition to dispersants, surfactants can also be used at the same time. The amount of dispersant used generally ranges from 0.5 to 1.5% by weight, based on water.

Micro-suspension polymerization, sometimes also referred to as homogenized aqueous suspension polymerization, provides an emulsion of small droplets of monomers by mechanical vigorous stirring and usually in the presence of an emulsifier of the same nature as mentioned above, wherein the polymerization is performed with fat-soluble initiators.

Any fat-soluble catalyst can be used in the polymerization in suspension or microsuspension. Examples which may be mentioned are peroxides, such as di-tert-butyl peroxide, lauryl peroxide and cetylcyclohexylsulphonyl peroxide, azo compounds, such as azo-bis-isobutyronitrile and azobis-2,4-dimethylvaleronitrile, dialkyl peroxide, dialkyl peroxide, dicarbonate dicarbonate dicarbonate, dicarbonate. -terthio-butylcyclohexyl, and alkyl borides. In general, these catalysts are used in an amount of 0.01 to 1% by weight, based on the monomers.

In addition to dispersing or emulsifying agents and catalysts, the polymerization mixture may also contain various additives conventionally used in known polymerization processes carried out in aqueous dispersion. Examples of such additives are buffers, polymer particle diameter adjusters such as alkali metal polyphosphates, molecular weight regulators, stabilizers, plasticizers, colorants as well as reinforcing agents or agents which facilitate the use of polymers.

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Other anti-scaling agents may also be included in the polymerization mixture in addition to the compounds which form anions derived from organic hydroxylated carboxylic acids.

The operating conditions of the polymerization process according to the invention do not differ from the conditions generally used. The temperature is usually in the range of 35-80 ° C. The pressure is usually less than 1.47 MPa. The pH is generally acidic, for example in the range 2-7. The amount of water used is generally such that the total weight of the monomers is 20-50% of the total weight of water and monomers.

The polymerization of halogenated vinyl monomers is generally carried out in batches, starting with the addition of water first and then the various components of the reaction mixture (dispersants or emulsifiers, catalysts, monomers, etc.). According to a preferred embodiment of the process according to the invention, the anti-scaling agent is added to the polymerization mixture before any halogenated vinyl monomer, and the best results are obtained if the addition is also carried out before the addition of any catalyst. To this end, the anti-scaling agent is added after the water has been added, possibly simultaneously with the dispersing or emulsifying agents.

The anti-scaling agent can also be added to the polymerization mixture several times as the polymerization continues, or continuously.

To initiate polymerization, the polymerization mixture is heated, for example, by a heating medium circulating in the double jacket with which such reactors are usually equipped. The anti-scaling agent is suitably added to the polymerization mixture before the heating is started.

When continuous polymerization is applied, the anti-scaling agent is also suitably added continuously to the polymerization mixture.

The amounts of anti-scaling agent used can vary within very wide limits, depending in particular on the nature of the monomers used and the condition of the inner surfaces of the reactors. Generally, an anti-scaling agent is used to such an extent that the polymerization mixture contains at least one part per million, based on water. Suitably the polymerization mixture contains at least 5 ppm by weight of inhibitor, based on water, and the best results are obtained if the polymerization mixture contains at least 10 ppm by weight of anti-scaling agent, based on water.

There is nothing to stop you from using the anti-scaling agent to the fullest. In general, however, it is unnecessary to exceed a concentration of 10,000 ppm of anti-scaling agent in the polymerization mixture, based on 6,621,99 of water. In most cases, a concentration of up to 1,000 ppm is sufficient. In almost all cases, a concentration of up to 200 ppm is completely satisfactory.

It has been found that a certain amount of anti-scaling agent is adsorbed on the metallic inner surfaces of the polymerization reactors. For this purpose, the polymerization mixture must suitably contain at least 1 mg / m of anti-scaling agent and suitably at least 5 mg / m of metallic inner surface. Likewise, a certain amount of anti-scaling agent may remain adsorbed on the metallic inner surfaces of the reactors at the end of the polymerization cycle and then act during the next cycle. This amount may be taken into account and reduced during the following period.

When starting to apply the process according to the invention in a given reactor, it is advantageous to first clean the inner surface of the reactor particularly carefully. For this purpose, solvents can be used which are particularly effective in dissolving the polymers from which the scales are formed, of heating the walls, of coating them or even of polishing them.

The new class of anti-scaling agents which are the subject of the invention has a remarkably high efficiency. In fact, anti-scaling agents which have been found to be particularly effective in the prior art, for example oxalic acid and its salts described in the aforementioned DE patent application 2,518,260, must be used in an amount of about 50 ppm by weight of water to have a clear effect on vinyl chloride suspension. . The anti-scaling agents according to the invention are already fully effective if they are used in an amount of about 15 ppm, based on water. In addition, they include a very large number of compounds which do not have any detrimental side effect on the appearance, odor, thermal stability and light stability of the polymer, nor on its suitability as a food coating.

The polymers obtained according to the invention can be used for all the conventional uses of the products in this field, and in particular for the production of articles such as bottles or profiles by applying conventional methods, for example injection molding and injection molding.

The following examples illustrate the invention.

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

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Examples 1-3 (R) A 3-liter stainless steel laboratory reactor with a double jacket with a circulating heat medium and a conventional stainless steel vane stirrer is used. The inner surfaces of the reactor are cleaned by washing with tetrahydrofuran, after which the wall is heated at 400 ° C for 30 minutes. The reactor is then covered in a fluorobutyric acid bath and rinsed five times with demineralized water.

1,500 g of demineralized water and 10 mg of organic hydroxylated carboxylic acid are added alternately to the reactor. 1.65 g of polyvinyl alcohol are then added. The mixer starts. 500 mg of powdered di-tert-butylcyclohexyl peroxydicarbonate are then added. A vacuum of 13.3 KPa is then generated twice in the reactor, and between these two treatments the reactor is blown clean with technical nitrogen at a pressure of 180 KPa. Finally, 100 g of vinyl chloride are added. The polymerization mixture is heated to 61 ° C at a rate of 1 ° C / minute.

The polymerization mixture is maintained at 61 ° C with stirring until the pressure drops to 343 KPa. In this case, the polymerization is stopped by removing the pressure in the reactor and evaporating the unpolymerized vinyl chloride. Cool and recover the polymer by filtration, then dry.

The following table shows the organic acid used as well as the results obtained by inspecting the inner surfaces of the reactor at the end of the polymerization cycle.

table 1

Example No. Organic Acid Inspection Product 1 Tartaric Acid L (+) No Scale, 2 Citric Acid Scaling a portion of the periphery of the inner surface between the liquid reaction mixture and the gas space that covers about 1% of the surfaces prone to scaling.

3 (R) Oxalic acid Scalds the entire inner surface between the liquid medium and the gas space, which covers about 2% of the surfaces prone to scaling.

- '"' -" '- · - .......-' '' ...... - "8 6231 9

Example 4

To a reactor similar to that used in the previous examples, 1,500 g of demineralized water, 10 mg of L (+) - tartaric acid and 10 mg of potassium iodide are added in turn. 1.65 g of polyvinyl alcohol are then added. The mixer starts. 200 mg of diethyl peroxydicarbonate are then added. A vacuum of about 13 KPa is now generated twice in the reactor, and between these two treatments the reactor is purged with technical nitrogen at a pressure of 180 KPa. 1000 g of vinyl chloride are then added. The polymerization mixture is heated to 61 ° C at a rate of 1 ° C / minute.

The polymerization mixture is kept at 61 ° C with stirring until the pressure drops to about 340 KPa. In this case, the polymerization is stopped by lowering the pressure in the reactor and evaporating the unpolymerized vinyl chloride. Cool and recover the polymer by filtration, then dry.

When inspecting the inner surfaces of the reactor at the end of the polymerization, it is found that they do not contain any scab.

Similar polymerization may have been repeated three times before the reactor had to be cleaned.

Example 5 This example corresponds in all respects to Example 4, except that 10 mg of L (+) - tartaric acid, 10 mg of potassium iodide and 25 mg of sodium polymethaphosphate (sold under the trade name "Metagon") are added alternately with demineralized water before the addition of polyvinyl alcohol.

When inspecting the inner surfaces of the reactor at the end of the polymerization, it is found that they do not contain any scab.

The corresponding polymerization may have been repeated 10 times before reactor cleaning has been necessary.

These examples illustrate the superiority of the anti-scaling agents according to the invention and in particular the superiority of anions derived from tartaric acid over oxalic acid.

Claims (10)

A process for polymerizing vinyl chloride in a reactor as an aqueous dispersion using oil-soluble radical polymerization initiators in the presence of an anti-scaling agent, characterized in that the anti-scaling agent is an anion of an organic hydroxylated carboxylic acid having 3 to 8 carbon atoms in a polymer of at least 1 ppm and less than 200 ppm in the order of. Process according to Claim 1, characterized in that the anions are derived from organic acids having at least 2 carboxyl groups in the molecule. Process according to Claim 1 or 2, characterized in that the anions are derived from tartaric acid. Process according to Claim 1 or 2, characterized in that the anions are derived from citric acid. Process according to one of Claims 1 to 4, characterized in that the anions are added to the polymerization mixture as the corresponding organic acid or as the alkali metal or ammonium salt of the corresponding organic acid. Process according to one of Claims 1 to 5, characterized in that the anions are added to the polymerization mixture before the vinyl chloride and before the oil-soluble radical polymerization initiators. Process according to one of Claims 1 to 6, characterized in that the anions are used in an amount of at least 5 ppm, based on the amount of water in the polymerization mixture. Process according to one of Claims 1 to 7, characterized in that the anions are used in an amount of at least 2 to 5 mg per m metallic inner surface of the reactor. Process according to one of Claims 1 to 8, characterized in that the anions are added to the polymerization mixture before it is heated to initiate the polymerization reaction. 9 6231 9 Process according to one of Claims 1 to 9, characterized in that it is applied to the polymerization in aqueous suspension.