EP0000083A1 - Use of vinylchloride polymer powders in the manufacture of battery separators - Google Patents

Abstract

Process for the aqueous suspension polymerization of vinyl chloride in the presence of a dispersing system comprising a cellulose derivative and an anionic emulsifier in which the polymerization is initiated by the intervention of an oil-soluble initiator and a water-soluble initiator is added during polymerization. The polyvinyl chloride powders thus obtained are particularly suitable for the manufacture of battery separators.

Description

The present invention relates to a process for the aqueous suspension polymerization of vinyl chloride and to the use of the polyvinyl chloride powders obtained according to this process for the manufacture of separation plates for batteries and accumulators (battery separators).

Among the qualities required of powdered resins intended for the manufacture of battery separators, mention may be made of good wettability and a small average particle size capable of ensuring the production of separators with small pores. It is for this reason that it is usually recommended to use, for this particular application, polyvinyl chloride powders obtained by polymerization in aqueous emulsion. It turns out that the powdered polymers obtained in aqueous emulsion generally have good processing properties: they are sinterable and can be easily applied with a doctor blade or a roller. The wettability of the resulting separators is also generally satisfactory. These are nevertheless deficient in terms of electrical and mechanical resistance and they tend to cause foaming due to their high content of emulsifiers.

In French patent 71.20132 filed on June 3, 1971 in the name of SUMI-TOMO CHEMICAL CO LTD, the manufacture of polyvinyl chloride powders is described by polymerization in aqueous suspension of vinyl chloride using an oil-soluble initiator and d a dispersing system comprising a cellulose derivative and an anionic emulsifier. It is stated therein that such a polyvinyl chloride powder has good mechanical and electrical properties, but that used alone, it is nevertheless not suitable not for the manufacture of battery separators because of its poor properties when applied with a doctor blade or roller and in particular because of its poor sinterability.

In this same patent, it is recommended to use for the manufacture of battery separators compositions comprising a mixture of 95 to 50% by weight of such a powdered polyvinyl chloride prepared by polymerization in aqueous suspension and from 5 to 50% by weight of polyvinyl chloride powder prepared by aqueous emulsion polymerization, as well as from 0.1 to 3 parts by weight per 100 parts of said mixture of a nonionic antistatic agent. The addition of polyvinyl chloride powder obtained in aqueous emulsion and of a nonionic antistatic agent, in the proportions specified above, certainly improves the sinterability and the machining properties of the polyvinyl chloride powder obtained in suspension aqueous. However, the mechanical properties of the resulting sintered separators are reduced and they still show a tendency to cause the formation of foams.

It therefore appears that the qualities of the battery separators obtained from the compositions of the prior art are not optimal. The preparation of such compositions is further complicated since it requires the manufacture of polyvinyl chloride powders according to two different recipes and the final mixing with an antistatic agent.

The Applicant has now found a particularly simple process for the direct production of polyvinyl chloride in the form of sinterable powders suitable for the manufacture of good quality battery separators which do not have the abovementioned drawbacks.

The present invention therefore relates to a process for the polymerization in aqueous suspension of vinyl chloride using an oil-soluble initiator of radical polymerization and in the presence of a dispersing system comprising a cellulose derivative and an anionic emulsifier, according to which initially carries out the polymerization with the intervention of the oil-soluble initiator and a water-soluble initiator of the radical polymerization is added during the polymerization.

The Applicant has discovered in fact that when the polymerization in aqueous suspension of vinyl chloride is initiated by means of an oil-soluble initiator, in the presence of a cellulosic dispersing agent and of an anionic emulsifier, and that an initiator is added water-soluble during polymerization, it is possible to obtain polymers directly by sinterable vinyl chloride powder, suitable for the manufacture of battery separators with good electrical and mechanical properties.

The time at which the addition of the water-soluble initiator is carried out is not particularly critical, provided, however, that this takes place during polymerization. It can therefore be carried out at any time during the polymerization, that is to say after the start but before the end of the polymerization. By end of polymerization is meant the moment when, the desired conversion rate having been reached, the polymerization reactor is degassed by lowering the pressure. Usually, the degassing polymerization is interrupted when the conversion rate reaches approximately 85 to 95%. As a general rule, the water-soluble initiator is therefore added before the conversion rate has reached 85%. It is particularly advantageous to add it before the conversion rate exceeds 80%.

It is furthermore not advisable to introduce the water-soluble initiator into the polymerization medium when the conversion rate is still relatively low, generally less than approximately 60%. In order to obtain a polyvinyl chloride powder whose properties are optimal, it is preferred to introduce the water-soluble initiator after the conversion rate has reached at least 70%.

According to a preferred embodiment of the present invention, a water-soluble initiator of the radical polymerization is therefore added to the polymerization medium when the conversion rate is between approximately 70 and 80% and, moreover, polymerization is continued until the conversion rate has reached a value of 85 to 95%. Usually, the polymerization is continued at the same temperature.

The nature of the water-soluble initiator is not critical and, in general, all of the initiators of radical polymerization in aqueous emulsion are suitable. As examples of such initiators, mention may be made of persalts, peracids, and hydrogen peroxide (hydrogen peroxide). Preferably, inorganic persalts are used, more particularly still alkali metal and ammonium persulfates, or hydrogen peroxide.

In the case where an inorganic persalt, such as for example potassium or ammonium persulfate, is used, it may be desirable to also add a basic substance (for example, an alkali metal or ammonium hydroxide) at the initial polymerization charge if it is desired to obtain separators whose thermal stability is optimal. The amount of basic substance to be used can be easily determined experimentally. mental in each particular case. To fix the idea, an amount of about 0.1 to 1 part by weight per 1000 parts of vinyl chloride is generally suitable for improving the thermal stability of polyvinyl chloride.

One can just as easily implement a single water-soluble initiator as several. When several water-soluble initiators are used, they can be used at different times. They can also be added in fractions or continuously.

The amount of water-soluble initiator to be introduced into the aqueous suspension is not critical. It depends to a certain extent on the polymerization temperature, as well as on the chemical nature of the water-soluble initiator. To fix the ideas, the introduction of very small quantities of water-soluble initiator, of 0.1% 0 approximately by weight relative to the vinyl chloride used generally brings already an improvement of the qualities of the polyvinyl chloride powders and in particular their sinterability. The introduction of higher amounts of water-soluble initiator reinforces this improvement. However, it is generally preferred not to exceed an amount of 3 ‰ by weight. In the case where the water-soluble initiator is an inorganic persalt, the preferred concentration of water-soluble initiator is generally between 0.15 and 1 ‰ approximately. In the case where it is hydrogen peroxide, the zone of the preferred concentrations is between 0.5 and 2.5% _o approximately.

The nature of the oil-soluble initiator used at the start of the polymerization in aqueous suspension is not critical. In general, all the oil-soluble initiators usually used for this type of polymerization are suitable. However, the best results are obtained with peroxydicarbonates of higher dialkyls and azo-bis-nitriles.

The term “higher dialkyl peroxydicarbonate” is intended to denote dialkyl peroxydicarbonates in which the alkyl chains, different or identical and optionally substituted, contain at least 7 carbon atoms and, preferably at least 12 carbon atoms. Advantageously, peroxydicarbonates are used whose identical alkyl chains each contain from 12 to 20 carbon atoms. As examples of such peroxydicarbonates, mention may be made of peridicarbonates of didecyl, dilauryl, dimyristyl, diketyl and distearyl. A very particularly preferred peroxydicarbonate is diketyl peroxydicarbonate.

The term “azo-bis-nitrile” is intended to denote the compounds of general formula CN-RN = NR-CN in which R represents an alkylene chain, straight or branched containing from 2 to 10 carbon atoms, and preferably from 4 to 8 carbon atoms. As examples of such azo-bis-nitriles, mention may be made of azo-bis-isobutyronitrile, azo-bis-valeronitrile, azo-bis-2,4-dimethylvaleronitrile. A very particularly preferred azo-bis-nitrile is azo-bis-2,4-dimethylvalerronitrile.

According to a preferred embodiment of the present invention, the polymerization in aqueous suspension of vinyl chloride is therefore initiated by means of an oil-soluble initiator chosen from peroxydicarbonates of higher dialkyls and azo-bis-nitriles (or of a system catalytic including). Preferably, a higher dialkyl peroxydicarbonate is used.

The quantity of oil-soluble initiator used at the start of the polymerization is not critical. It depends in particular on the half-life of the initiator under the polymerization conditions, as well as on the heat exchange capacity of the installation. Usually, 0.01 to 0.5% by weight of oil-soluble initiator (or of a mixture of oil-soluble initiators) is used relative to vinyl chloride. Preferably, about 0.1% by weight of oil-soluble initiator is used.

The cellulosic dispersing agent used in the process of the invention can be chosen indifferently from all the water-soluble cellulose derivatives usually used for the polymerization in aqueous suspension of vinyl chloride. Examples of such derivatives include alkylcelluloses such as methyl-, ethyl-, and propylcellulose, hydroxyalkylcelluloses such as hydroxyethyl-, hydroxypropyl- and hydroxybutylcellulose and mixed ethers. cellulose such as 1 '(hydroxypropyl) (methyl) cellulose. Preferably, alkylcelluloses are used. A very particularly preferred alkylcellulose is methylcellulose.

The amount of cellulosic dispersant is usually between 1 and 5 ‰ by weight relative to the vinyl chloride initially used. The best results are obtained when used from 2 to 2.5% by weight _0.

The nature of the anionic emulsifier used in the process of the invention is not critical. However, the best results are obtained with sulfonated emulsifiers. As examples of such emulsifiers, there may be mentioned the alkali metal salts, such as sodium, alkylsulfonic, arylsulfonic and alkylarenesulfonic acids. The best results are obtained with alkali metal alkylsulfonates and alkylene sulfonates and more particularly those in which the alkyl radicals contain from 8 to 16 carbon atoms. Very particularly preferred emulsifiers are the alkali metal alkylarenesulfonates. Among these, preference is given to sodium dodecylbenzenesulfonate.

Although the amount of anionic emulsifier to be used is not particularly critical, it is recommended to use it in very small amounts, generally much less than the amount of cellulose derivative used. Advantageously, it is therefore used in an amount ranging from 0.1 to 2 ‰ by weight relative to the vinyl chloride initially used. The best results are obtained when using from 0.50 to 1.5 ‰ by weight of anionic emulsifier.

According to a very particularly preferred embodiment of the present invention, the polymerization in aqueous suspension of vinyl chloride is therefore started in the presence of diketyl peroxydicarbonate and of a dispersing system based on methylcellulose and sodium dodecylbenzenesulfonate and added hydrogen peroxide and / or an alkali metal or ammonium persulfate during polymerization when the conversion rate is between 70 and 80%.

The polymerization temperature is not critical and is generally between 50 and 75 ° C, advantageously between 55 and 65 ° C. The polymers obtained generally have K values (measured at 25 ° C. in cyclohexanone) of between 53 and 74, preferably between 60 and 69.

The amount of water used is usually such that the total weight of the monomers represents from 20 to 50% of the total water plus monomers, advantageously from 40 to 50%. In some cases, and in particular when using a persalt as a water-soluble initiator, it may be advantageous to start the polymerization with a weight ratio between the monomers and the total charge of approximately 50% and to precede the injection of the water-soluble initiator of the addition of an additional quantity of water, so as to obtain a weight ratio of approximately 40%.

The polyvinyl chloride powder obtained according to the process of the invention is conventionally isolated from the polymerization medium, for example by filtration, and it is not necessary to subject it to special treatments or to add to it special additives. . It can then be dried, with hot air for example, after which it is ready to be used.

The process which is the subject of the present invention applies to the homopolymerization of vinyl chloride and to its copolymerization with up to 20% by weight of a copolymerizable ethylenically unsaturated comonomer such as vinyl acetate, ethylene, propylene and butylene. It is particularly applicable to the homopolymerization of vinyl chloride.

The method according to the invention leads to the production of sinterable polyvinyl chloride powders, reduced content, generally less than 2% ₀ by weight, dispersing agent having a narrow particle size distribution and an average particle diameter of 30 to 45 approximately microns, as well as a low electrical resistance, generally less than 2 milliohm / dm ² .

These polyvinyl chloride powders are particularly suitable for the manufacture of separators for wettable batteries having excellent electrical and mechanical properties. To do this, conventional methods of manufacturing battery separators from polyvinyl chloride powders can be used.

The examples which follow are intended to illustrate the present invention without however limiting its scope.

Examples 1, 2 and 3 illustrate the process of the invention. Example 4R, for reference, relates to polymerization without the addition of a water-soluble initiator during polymerization.

Example 1

Into a 4.5 liter reactor fitted with a double jacket and an agitator with vertical blades, are successively introduced at 20 ° C. with stirring (200 rpm) 1000 g of demineralized water, 0.75 g of sodium dodecylbenzenesulfonate in the form of an aqueous solution at 10% by weight, 2 g of a commercial methylcellulose powder sold under the name METHOCEL A 4C and 1 g of diketyl peroxydicarbonate. The reactor is closed and stirring is maintained at 200 rpm for 5 minutes. We stop the agitation. The reactor is placed under partial vacuum (100 mm Hga), then it is subjected to a sweep with nitrogen (1360 mm Hga), before being put back under the same partial vacuum. 1000 g of vinyl chloride are then introduced into it. The stirring is restarted (500 rpm) and the stirring is continued for 30 minutes. The medium is heated to 58 ° C. When the conversion rate reaches 75%, 0.25 g of ammonium persulfate is introduced into the reaction medium and the polymerization is allowed to continue until the conversion rate reaches 92%. The aqueous suspension is degassed (by lowering the pressure to 360 mm Hga), the polymer is filtered off and dried under the usual conditions (2 h 30 min at 70 ° C).

Example 2

This example is in accordance with Example 1, except that 0.3 g of sodium hydroxide is introduced into the aqueous charge after the introduction of demineralized water.

Example 3

This example is identical to Example 1 except that the ammonium persulfate is replaced by 0.6 g of hydrogen peroxide (100%).

Example 4R

This example is identical to Example 1, except that the introduction during the polymerization of ammonium persulfate is omitted.

In Table I, in the appendix, the results of the particle size analysis of the polyvinyl chlorides obtained according to Examples 1, 2, 3 and 4R are recorded.

The comparison of the results shows that the delayed introduction of a water-soluble initiator has no marked effect on the particle size of the polyvinyl chloride obtained by polymerization with the intervention of an oil-soluble initiator in the presence of a cellulose derivative. and an anionic emulsifier.

The sintering ability of the polyvinyl chlorides obtained according to Examples 1 to 4R was also evaluated. To do this, 0.4 mm thick battery separator elements were made by roller coating the polyvinyl chloride powder with laboratory equipment.

The results of the evaluation are shown in Table II.

The comparison of the results of Examples 1, 2 and 3 with those of Example 4R indicates that the delayed introduction of a water-soluble initiator has a very marked effect on the sinterability of the polyvinyl chloride powders obtained. Thus, the polyvinyl chloride powder produced according to Example 4R adheres strongly to the metal support during coating and leads to sintering of very poor quality, so much so that it has not been possible to '' assess the breaking tension of the sintered sample On the other hand, the polyvinyl chloride powders obtained according to Examples 1, 2 and 3 adhere little, if at all, to the roller and lead to the production of good-quality sintered samples having a tensile strength greater than 50 kg / cm ² .

Claims (10)

1 - Process for the aqueous suspension polymerization of vinyl chloride using an oil-soluble initiator of radical polymerization and in the presence of a dispersing system comprising a cellulose derivative and an anionic emulsifying agent, characterized in that initially carries out the polymerization with the intervention of the oil-soluble initiator and in that, during the polymerization, a water-soluble initiator of the radical polymerization is added. 2 - Process according to claim 1, characterized in that the water-soluble initiator is added before the conversion rate exceeds 80%. 3 - Process according to claim 1 or 2, characterized in that the water-soluble initiator is added after the conversion rate has reached 70%. 4 - Process according to any one of claims 1 to 3, characterized in that the polymerization is interrupted when the conversion rate reaches from 85 to 95 X. 5 - Process according to any one of Claims 1 to 4, characterized in that the water-soluble initiator is chosen from alkali metal or ammonium persulfates and hydrogen peroxide and is used in an amount ranging from 0, 1 to 3 ‰ by weight relative to the vinyl chloride used. 6 - Process according to any one of claims 1 to 5, characterized in that the cellulose derivative is chosen from alkylcelluloses, hydroxyalkylcelluloses and (hydroxyalkyl) (alkyl) celluloses and is used in an amount between 1 and 5 ‰ by weight relative to the vinyl chloride used. 7 - Process according to claim 6, characterized in that the cellulose derivative is an alkylcellulose. 8 - Process according to any one of claims 1 to 7, characterized in that the anionic emulsifier is chosen from the alkali metal salts of alkylsulfonic, alkylarenesulfonic and arylsulfonic acids and is used in an amount ranging from 0.1 to 2 ‰ by weight relative to the vinyl chloride used. 9 - Process according to claim 8, characterized in that the anionic emulsifier is an alkali metal salt of an alkylarene sulfonic acid. 10 - Use of the powders of polymers of vinyl chloride obtained according to the method of any one of claims 1 to 9 for the manufacture of separation plates for batteries and accumulators.