FI61499C - FOERFARANDE FOER POLYMERISATION AV VINYLKLORID I EN VATTEN SUSPENSION UNDER ANVAENDNING AV DIALKYLPEROXIDIKARBONATER OCH GENOM BEHANDLING AV POLYMEREN MED EN HYDROXID - Google Patents

Description

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Patent · och regieterstyrelsen Amdkan utligd och utUkriften pubiicmd U4 · 0e; (32) (33) (31) Privilege claimed —Begird prlertut 26.05-75

France-France (FR) 7516579 (71) Solvay & Cie, 33, rue du Prince Albert, B-1050 Brussels, Belgium-Belgium (BE) (72) St £ phane Noel, Vilvoorde, Belgium-Belgium (BE) (7) ^) Oy Kolster Ab (5M Process for the polymerization of vinyl chloride in aqueous suspension using dialkyl peroxide carbonates and treatment of the polymer with hydroxide

The invention relates to a process for the polymerization of vinyl chloride in aqueous suspension using dialkyl peroxide carbonates and treating the polymer with hydroxide to obtain polymers with improved thermal stability.

Dialkyl peroxide carbonates are known to be particularly effective accelerators. These have the advantage that they improve the productivity of the polymerization units. Unfortunately, the resulting polymers result in moldable products (tubes, liners, bottles) with poor initial thermal stability.

By initial thermal stability is meant the tendency of the polymer to resist degradation as the temperature to which the polymer must be brought in order to mix various additives and use the polymer. Poor initial durability manifests itself as a change in the original color of 2 61 499, which is more noticeable the worse the persistence.

It has previously been proposed to improve this stability by post-polymerization post-treatment in which the polymers are contacted with an aqueous alkaline solution which preferably contains a phenol antioxidant (CA Patent 865,802, dated February 9, 1970 in the name of Esso Research and Engineering). Vinyl chloride polymers treated in this way have slightly improved stability as specific compositions, especially containing compositions. However, in many compositions, the initial thermal stability is generally inferior to that of untreated polymers. The effectiveness and applicability of this treatment is therefore very limited.

The present invention relates to the polymerization of vinyl chloride in aqueous suspension using dialkyl peroxide carbonates. The object of the invention is to improve the initial thermal stability of polyvinyl chlorides obtained by polymerization in aqueous suspension using specific initiators. For this purpose, the polymer is treated with a monovalent hydroxide (without any other hydroxide present) at the end of the polymerization, i.e. when the desired degree of conversion is reached and before the unpolymerized vinyl chloride is removed, i.e. before degassing and / or removal of residual monomer. The present invention results from the surprising finding that when treated with a monovalent hydroxide and at a given time, the polymer is significantly better in color than a polymer having undergone a similar treatment after degassing, i.e. after some of the non-polymerizable monomers have been removed. In the process of CA Patent 865,802, the polymer is subjected to post-treatment after polymerization. In this process, the polymer is first separated from the polymerization medium and then contacted with an aqueous alkaline solution, preferably an antioxidant consisting of a phenolic compound. in the atmosphere precludes the treatment of PVC in an aqueous suspension from the polymerization which has not been degassed 61499 3. However, it is under these specific conditions that it is possible to clearly improve the thermal stability of PVC with a monovalent hydroxide.

The invention is characterized in that the polymer is treated with a monovalent hydroxide at the end of the polymerization when the desired degree of conversion is reached and before the unpolymerized vinyl chloride is removed,

The polymerization is generally stopped when the conversion rate of the vinyl chloride in the reactor is about 90-95%, thus currently 5-10% of unpolymerized vinyl chloride remains. A significant portion of the unpolymerized vinyl chloride is removed from the aqueous suspension by evaporation under reduced pressure. This operation is called dry distillation. After dry distillation, the reaction mixture contains only about 10 g of non-polymerizable vinyl chloride per kg of polymer. The majority of this residual vinyl chloride is in the form of inclusions in the polymer particles. It has now been found that using hydroxide before the end of dry distillation significantly improves the treatment efficiency. The treatment is preferably carried out when the aqueous suspension still contains at least 25 g of non-polymerizable vinyl chloride per kg of polymer. The best results are obtained when this concentration is at least 50 g of vinyl chloride / kg.

The treatment according to the invention can be carried out simply by adding hydroxide directly to the aqueous suspension while it is still in the polymerization reactor,

The amount of hydroxide added to the aqueous suspension is not critical. The addition of even small amounts of hydroxide improves the initial thermal stability of the polymer. However, it is recommended to use an amount of hydroxide sufficient to bring the pH of the aqueous suspension to at least 7. It is also recommended that the pH should not rise above 12 during processing to avoid any risk of polymer degradation, for example through hydrogen chloride removal. Preferably, an amount of alkali hydroxide sufficient to change the pH of the aqueous suspension to 9-10.5 is used. The type of monovalent hydroxide used is not critical.

It can be selected from alkali metal hydroxides such as sodium, potassium, lithium or ammonium hydroxide. However, water-soluble hydroxides of alkaline earth metals are not suitable for this process of the invention.

61499 4

A very preferred alkali metal hydroxide is ammonium hydroxide. Due to its high volatility, this hydroxide does not remain in the finished polymer. Its use guarantees the availability of polymers with several highly desirable properties, in this case not only very good initial thermal stability, but also improved transparency and good electrical properties.

The method of adding hydroxide is by no means critical. For example, it may be administered in a single dose, in small doses or continuously. The hydroxide can be added solid or dissolved in a liquid that is inert under the polymerization conditions. It can even be added as gaseous ammonia. The hydroxide is preferably added to the aqueous suspension as a solution and especially as an aqueous solution. The concentration of the aqueous hydroxide solution can be any. However, for practical reasons, solutions with a concentration of 0.1 to 10 N, preferably 5 N, are generally used.

According to a highly preferred embodiment, the aqueous polymer suspension is treated with an amount of ammonium hydroxide sufficient to bring the pH to 9-10.5 before the dry distillation is completed. ammonia per 100 parts of vinyl chloride originally used. The duration of treatment is not critical to the invention. In practice, results are achieved in just a few minutes. By extending the treatment time, the results can be improved. However, in order not to unnecessarily prolong the full cycle of polymer production, the polymer is generally kept in contact with the alkali hydroxide for 5-120 min, preferably 5-20 min, before the end of the dry distillation.

The temperature at the beginning of the treatment according to the invention is usually the same as the temperature of the aqueous suspension at the end of the polymerization, since the treatment can be carried out as soon as the desired degree of conversion has been reached. However, it is not necessary to keep the temperature constant during processing. The temperature can thus be lowered or raised and is generally 20 to 140 ° C, preferably 20 to 80 ° C.

It is not necessary to carry out the treatment with stirring. However, in order to ensure rapid and efficient dispersion of the hydroxide in the aqueous polymer suspension and to reduce the treatment time as much as possible, it is preferable to carry out the treatment with stirring. In practice, the mixing device used in the polymerization is kept running during the treatment according to the invention when the treatment according to the invention is carried out in a polymerization reactor.

According to a preferred embodiment of the invention, the polymer is stripped with an inert gas after treatment with a monovalent hydroxide.

The corresponding treatment is described in FI patent publication 60880.

It has now been found that combining the treatment according to the invention with inert gas stripping further improves the initial thermal stability of the polymer.

Stripping with an inert gas is usually performed in an aqueous suspension of the polymer at a temperature at least equal to the polymerization temperature. This stripping is preferably performed at least at the glass transition temperature of the polymer. When combining stripping with the treatment according to the invention, dry distillation of the aqueous suspension is preferably carried out before stripping in order to remove most of the vinyl chloride remaining in the aqueous suspension at the end of the hydroxide treatment. The main purpose of stripping is to remove vinyl chloride cracks trapped in the polymer particles.

Stripping, like the hydroxide treatment, can be performed in a polymerization reactor. It is also possible to use a specially designed vessel, which is preferably equipped with a stirrer. The polymer is transferred to it as an aqueous suspension, for example after dry distillation in a reactor. The concentration of polymer in the aqueous suspension can also be changed before stripping. However, it is recommended that the treatment according to the invention be carried out in a polymerization reactor, after which the aqueous suspension of polymer is transferred to a special vessel, where it is possible to carry out degassing and stripping with an inert gas.

By inert gas is meant shielding gases such as nitrogen gas and carbon dioxide, or in addition, at the stripping temperature, vapors of inert and volatile liquids, such as, for example, pentane, hexane, chloroform, carbon tetrachloride and water vapor. It is particularly advantageous to use steam as an inert stripping gas. At least a portion of the water vapor used for stripping can be generated in situ by boiling the aqueous suspension. However, it is preferred to introduce the water vapor, preferably overheated, into the aqueous suspension. The temperature at which stripping with an inert gas is carried out may be of the same order of magnitude as the hydroxide treatment temperature. However, it has been particularly preferred, as previously stated, that the treatment with an inert gas takes place at the glass transition temperature of the polymer. In order to determine the glass transition temperature, a differential thermal analysis is preferably used which gives a sufficiently accurate measurement result. The aqueous suspension from the hydroxide treatment can be heated to achieve at least the glass transition temperature of the polymer before the suspension is stripped. The heating of the aqueous suspension can take place by any suitable means, such as by circulating the hot fluid in the double jacket of the boiler containing the aqueous suspension and / or by spraying the suspension directly with a hot inert gas such as air, nitrogen or water vapor. However, it is recommended that steam be blown into the suspension. The use of steam makes it possible to reach the desired temperature in a very short time,

The maximum temperature at which stripping with an inert gas can be performed is determined by the decomposition temperature of the polymer, in which case a conventional thermostabilizer may optionally be added to the polymer.

It should be noted, however, that heating to a higher temperature, well above 100 ° C, causes unnecessary energy loss. The high temperature treated aqueous dispersion must be subsequently cooled before it can be centrifuged, which prolongs the cooling time and / or increases the amount of coolant required.

For these reasons, it is recommended that the temperature of the aqueous suspension be kept below 140 ° C during stripping, and preferably below 120 ° C. The temperature of the inert substance used for stripping the residual monomer and possibly for heating the aqueous suspension may be any. It is obvious that the higher the temperature of the liquid used for heating, the shorter the total treatment time of the aqueous suspensions,

The duration of the stripping is also not a critical parameter.

Because of this, it can be easily prescribed experimentally on a case-by-case basis. For example, a time of a few minutes to 2 hours, and especially 5-45 minutes, is generally sufficient to reduce the vinyl chloride residue to a few tens of ppm by weight of the amount of polymer.

7 61499 After centrifugation and drying under normal conditions, these suspensions give vinyl chloride polymers which, at best, contain less than 2 ppm by weight of residual monomer,

It may be advantageous to complete the stripping by bringing the aqueous suspension to a boil under reduced pressure.

The polymerization conditions of the polymers to be treated according to the invention are those conventionally used in the polymerization of vinyl chloride in aqueous suspension using dialkyl peroxide carbonates,

Polymerization of vinyl chloride refers to the homopolymerization and copolymerization of vinyl chloride with comonomers. The content of vinyl chloride is preferably more than 50 mol%, and preferably 80 mol%. The comonomers can be selected from monomers copolymerizable with vinyl chloride, such as vinyl acetate, propylene and ethylene.

The polymerization of vinyl chloride is carried out in the presence of additives conventionally used in aqueous suspension polymerization, namely suspending agents and, if appropriate, additives added to a polymerization step, such as stabilizers (e.g. epoxidized soybean oil), plasticizers (e.g. butadiene-styrene resins), processing aids (for example stearic acid or fatty alcohols) or antioxidants such as thioethers, organic phosphites or phenolic compounds. Examples of such compounds are phenol, bisphenol A, hydroquinone, tertiary p-butyl catechol, tertiary p-butyl cresol, tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane, n-octadecyl-3 ( 3,5-Ditertier butyl 4-hydroxy-phenyl) propionate, etc.

Because antioxidants inhibit polymerization, they are preferably added to the aqueous suspension at the end of polymerization and before the unpolymerized vinyl chloride is removed. They are added to the polymerizable mixture before the treatment with the monovalent hydroxide according to the invention. The suspending agent can be selected from conventional suspending agents such as polyvinyl alcohols, gelatin, alkylcelluloses and water-soluble carboxymethylcelluloses, as well as 8 61,499 various copolymers and condensation products such as the condensation products of polyalkylene glycols and polyamines. Mixtures of suspending agents can, of course, be used and the method of their addition is not critical.

All dialkyl peroxide carbonates are suitable as accelerators for initiating the polymerization of vinyl chloride in the process of the invention. The general formula of these initiators is: R-O-C-O-O-C-O-R '

Il M

Wherein R and R'f, which may be the same or different, denote an aliphatic radical optionally substituted by, for example, a halogen atom. However, the use of long-chain and usually solid peroxide carbonates can lead to dosage! Therefore, it is preferable to use dialkyl peroxydicarbonates whose alkyl chains do not contain more than 9 carbon atoms, preferably 1 to 4 carbon atoms. A particularly preferred peroxydicarbonate is diethyl peroxydicarbonate. When applying the process of the invention to the polymerization of vinyl chloride in the presence of dialkyl peroxide carbonates containing 1 to 9 carbon atoms, and preferably 1 to 4 carbon atoms, the improvement in the thermal stability of vinyl chloride polymers is most significant.

The vinyl chloride polymers treated in accordance with the invention can be used for all conventional uses of these polymers. They are particularly suitable for use in compositions which are critical for thermal stability and harsh conditions of use. Inert gas stripped polymers, which practically no longer contain vinyl chloride residues, are very well suited for the production of food packaging materials,

The following examples of the polymerization of vinyl chloride in aqueous suspension in the presence of diethyl peroxide carbonate as an accelerator illustrate the invention.

The thermal stability of these polymers over a short period of time (initial discoloration) is determined by a test piece obtained by beating at 180 ° C for about two minutes the three mixtures mentioned below (in a cylinder mixer).

61499 9

Composition ABC

Dry resin

Dioctyl phthalate 40 30 -

Tri-base lead phthalate 2 «

Lead stearate

Barium Cadmium Stabilizer -

Alkaline lead carbonate - · - 1.03

Calcium carbonate - ~ 0.6

Polyethylene wax 0.5 to 0.2

Stearic acid

Example 1

To an autoclave having a volume of 3 liters and equipped with a stirrer and a double jacket were added 1500 g of deionized water, 1.2 g of polyvinyl alcohol and 0.35 g of diethyl peroxydicarbonate at room temperature and with stirring (200 n / min). The autoclave was sealed, the stirring was stopped and the autoclave was evacuated (100 mm Hg), then the contents were purged with nitrogen (600 mm Hg) before being evacuated to the same vacuum. Then 1000 g of vinyl chloride was added thereto. Stirring was started (500 rpm) and the contents were heated to 60 ° C. This moment when the polymerization started was considered the beginning of the whole polymerization cycle.

After 3 hours, 300 g of deionized water were added and the polymerization was continued until the pressure drop in the autoclave reached 3 kg / cm 2. At this time, 172 cm -1 of 0.1 N ammonium hydroxide solution was added to the reactor and stirring was continued for 10 minutes. After this time, the aqueous suspension was dry distilled by reducing the pressure to 500 mm Hg absolute, the polymer was centrifuged and dried in the usual manner (65 ° C for two hours). The initial thermal stability of the obtained polyvinyl chloride with compositions A, B and C is shown in the table above. Its glass transition temperature, determined by differential thermal analysis, is 80 ° C.

Example 2 (Comparative) This example relates to the polymerization of vinyl chloride under conditions similar to those of Example 1. However, since the pressure drop in the reactor reached 3 kg / cm 3, the aqueous suspension was dry-distilled, the polymer was centrifuged and dried at 65 ° C for two hours.

10 61499

The thermal stability of the obtained polyvinyl chloride when the composition is Ar B and C is evident from the following.

Example 3

The polymerization conditions and treatment of the aqueous suspension with ammonium hydroxide corresponded to the conditions of Example 1. However, after dry distillation, the aqueous suspension was transferred to a 7.5 liter vessel equipped with a steam supply tube extending to a vessel maintained at normal pressure; the vinyl chloride content was 12,000 mg per kg of polyvinyl chloride. Water vapor superheated to 152 ° C was introduced into the aqueous suspension via a steam supply pipe. The aqueous suspension, which was originally only at 60 ° C, was rapidly heated in contact with the water vapor condensing thereon. When the aqueous suspension reached a temperature of 100 ° C, (boiling), the water vapor practically no longer condensed, but disappeared with the inclusion of the vinyl chloride residue. A second sample of the aqueous suspension was taken before cooling, centrifuging and drying the polymer,

After steam distillation, the vinyl chloride content of the aqueous suspension was 10 mg of vinyl chloride per kg of polyvinyl chloride and the dry polymer contained less than 1 mg of vinyl chloride per kg of dry polymer.

The initial thermal stability of polyvinyl chloride at compositions A, B and C is shown in the table below.

Example 5 (comparison) - · · 'l— | This example relates to the polymerization of vinyl chloride under the conditions of Example 2, in which, however, the degassed aqueous suspension was treated under the conditions of steam distillation shown in Example 3.

The initial thermal stability of polyvinyl chloride is shown in the table below.

Comparing Example 1 to Examples 2 and 4, it is found that the alkaline treatment of the aqueous suspension before degassing significantly improves the initial staining of the polyvinyl chlorine resin (Example 2), and in a much better ratio than in the known method (Example 4), 61499 11

From Example 3, it can be seen that the initial thermal stability can be further improved by stripping the aqueous suspension treated according to the invention with an inert gas,

Comparative Example 5 is presented as proof that the alkaline treatment according to the kek invention and the combined inert gas stripping (Example 3) are expected to have a greater improvement in initial thermal stability (Examples 1 and 5).

Alkali treatment of the aqueous suspension prior to dry distillation is thus a very effective and simple means of improving the initial discoloration of vinyl chlorides.

When combined with stripping the monomer residue with an inert gas to the treatment according to the invention, vinyl chloride polymers are obtained which have a further improved initial coloration and which are practically free of residual monomer.

Example 6 This example corresponds to Example 1, except that 0.5 g of tris (2-methyl-4-hydroxy-5-tert-butylphenyl) butane dissolved in 0.5 g of methylene chloride was added before the addition of the ammonium hydroxide solution, (initial) Thermal stability for the resulting polyvinyl chloride in formulations A, B and C is shown in the adjacent table.

Table

Initial thermal stability of polynyl chloride

Example, „„ composition no

ABC

1 white very little light brown rose red 2 (comparison) rose, + dark rose brown a few reds. + multiple nen point red dots 3 white colorless bright light brown 4 (comparison) reddish + dark rose-brown red pis · ^ red you do 5 (comparison) rose-red rose-brown brownish 6 white very slightly bright pink-pink brown

Claims (18)

A process for polymerizing vinyl chloride in aqueous suspension using dialkyl peroxide carbonates and treating the polymer with a hydroxide, characterized in that the polymer is treated with a monovalent hydroxide at the end of the polymerization when the desired degree of conversion is achieved and before the unpolymerized vinyl chloride is removed. Process according to Claim 1, characterized in that a monovalent hydroxide is used in such an amount that the pH of the aqueous suspension is at least 7, Process according to Claim 1, characterized in that the monovalent hydroxide is used in such an amount that the pH of the aqueous suspension does not exceed 12. Process according to one of Claims 1 to 3, characterized in that a monovalent hydroxide is used in an amount such that the pH of the aqueous suspension is 9 to 10.5, Process according to Claim 1, characterized in that the monovalent hydroxide is ammonium, lithium, potassium or sodium hydroxide, Process according to Claim 5, characterized in that the monovalent hydroxide is ammonium hydroxide. Process according to Claim 1, characterized in that the monovalent hydroxide is used as an aqueous solution. Process according to one of Claims 1 to 7, characterized in that an aqueous suspension containing at least 10 g of vinyl chloride per kg of polymer is treated. Process according to one of Claims 1 to 8, characterized in that the treatment time of the aqueous suspension with the monovalent hydroxide is 5 to 120 minutes. Process according to one of Claims 1 to 9, characterized in that an antioxidant is added to the aqueous suspension. Process according to Claim 10, characterized in that the antioxidant is added at the end of the polymerization before the unpolymerized vinyl chloride is removed. Process according to one of Claims 1 to 11, characterized in that the antioxidant is added before the polymer is treated with a monovalent hydroxide. 12 61499 Process according to Claim 10, characterized in that the antioxidant is a phenolic compound. Process according to Claim 1, characterized in that the aqueous suspension treated with a monovalent hydroxide is stripped with an inert fluid. Process according to Claim 14, characterized in that the inert fluid is water vapor. Process according to Claim 14, characterized in that the treated aqueous suspension is subjected to degassing before stripping, A method according to claim 14, characterized in that the treated aqueous suspension is heated to a temperature at least corresponding to the glass transition point of the polymer and that it is maintained at a temperature at least corresponding to the glass transition point of the polymer during stripping, Process according to Claim 1, characterized in that the polymer is treated with an amount of ammonium hydroxide such that the pH of the suspension is 9 to 10.5, while the aqueous suspension still contains at least 25 g of vinyl chloride per kg of polymer. 61499 14