PROCESS FOR PREPARATION OF POLYARYLENE THIOETHERS HAVING HIGH MELT CRYSTALLIZATION TEMPERATURE

Abstract: Polyarylene thioethers having increased melt crystallization temperature are produced by subjecting a polyarylene thioether recovered from a conventional polycondensation of dichlorobenzene with sodium sulfide to a treatment with an aqueous non-oxidative strong acid such as HCl, H₂SO₄ or H₃PO₄ or with an aqueous salt of the non-oxidative strong acid with a weak base such as ammonia.

Description: The present invention relates to a process for increasing the melt crystallization temperature of poly­ arylene thioethers, which polyarylene thioethers are advantageously used in, among others, injection molding. Among thermoplastic resins, a polyarylene thioether is a heat-resistant resin having a relatively high crystallization speed. Accordingly, it is expected that this polymer will be applied to injection molding for which a high crystallization speed is required. Most heretofore marketed polyarylene thioethers are products in which the apparent melt viscosity is increased by subjecting a product having a relatively low melt viscosity (that is, a low molecular weight) to curing, that is, a high temperature treatment in the presence of oxygen or the like to carry out such a reaction as crosslinking or branching. In a resin having a melt viscosity increased by curing, in general, the melt crystallization temperature (Tmc) (the temperature at which crystallization is caused when the molten resin is cooled) is much higher than the melt crystallization temperature of the untreated resin, and therefore, the crystallization speed is greatly increased. Accordingly, only in view of the crystallization behavior, it can be said that a polyarylene thioether in its cured state is suitable for injection molding. However, although the crystallization behavior is appropriate, the cured polyarylene thioether is defective in that its coloring is extreme, and, since it is highly crosslinked and branched, its mechanical strength is insufficient. EP-A-87038 is dealing with this technical problem of discoloration originating from the thermoplastic pro­ cessing which gives a brownish-black hue. It is stated that this coloring can be decreased by carrying out an after­ treatment with a strong acid solution on the PAS polymer before it is undergoing a thermoplastic processing. In order to free the treated polymer slurry from any electro­ lytes it is recommended to subject it to a thorough water wash, thereby achieving neutrality of the polymer. No mention is made in this prior art of any effect this acid wash treatment would have on the melt crystal­ lization temperature. In practice a great volume of wash water is needed and the washing step must be repeated several times in order to get a polymer free of electrolytes. The Applicants have previously proposed a polymerization process in which a large amount of water is added midway in the polymerization reaction, and, simultaneously, the polymerization temperature is elevated (a linear polyarylene thioether having a high molecular weight is economically advantageously prepared by water-adding two-­ staged polymerization). See Japanese Patent Application No. 126725/84, which is now Japanese Patent Laid-Open Application No. 7332/1986. Since a polyarylene thioether prepared according to this process ordinarily has a sufficiently high melt viscosity, the resin can be directly applied to melt processing such as extrusion molding or compression molding without curing, and, since curing is not carried out, coloring is substantially controlled. Moreover, this resin has excellent mechanical strength because it has a linear structure. However, it was found that this high-molecular-weight polyarylene thioether developed by the Applicants is still inadequate as a resin material for injection molding. More specifically, the Tmc is somewhat too low for application to injection molding, and under certain processing conditions for solidifying the molten resin, it sometimes happens that crystallization is insufficient. It is desirable that polyarylene thioethers for injection molding have a Tmc of at least 220°C, preferably at least 225°C. Generally, the higher the Tmc is, the higher is the crystallization speed when the molten resin is cooled and solidified. SUMMARY OF THE INVENTION The applicants carried out studies with a view to developing a process capable of increasing the Tmc of a high -molecular-­ weight linear polyarylene thioether, and as a result they established a hypothesis that the Tmc would be elevated by substantially neutralizing or acidifying basic residues (presumed to be -SNa and the like) bonded to the terminals of the polyarylene thioether. On the basis of this assumption the present invention was arrived at, which solves the technical problem of increasing the melt crystallization temperature to at least 220°C of polyarylene thioethers prepared by subjecting an alkali metal sulfide and a dihalo-aromatic compound to dehalogenation/sulfidation in a polar organic solvent. The process of the invention is characterized in that the polymer is separated from the polymerization reaction mixture and is subjected to a treatment with a solution containing 0.1 to 30% by weight of an ammonium salt of a non-oxidative strong acid at 0 to 100°C for 5 to 500 minutes. According to the process of the present invention, it has become possible to obtain a polyarylene thioether having a high melt crystallizatin temperature Tmc without a curing treatment accompanied by crosslinking and branching which are likely to degrade the inherent properties of the polymer. At the present, it is not known whether or not the above-mentioned hypothesis is correct, but the present invention is not limited by the above-mentioned hypothesis. Irrespectively, a polyarylene thioether prepared according to the process of the present invention is less colored and has high Tmc and hence, it has a high crystallization speed and is suitable for injection molding. Moreover, since the polymer need not be subjected to a curing treatment, the mechanical properties of the polymer are excellent. Furthermore, when a polyarylene thioether as polymerized is subjected to the treatment of this invention for elevating Tmc, a considerable increase of melt flow rate or, in other words, a significant decrease of melt viscosity, occurs as an incidental effect without any substantial change in the molecular weight. This incidental effect also enhances the flow property and the melt processability of the polymer. DETAILED DESCRIPTION OF THE INVENTION Formation of Basic Structure of Polyarylene Thioether A basic process for preparing a polyarylene thioether as the premise of the present invention will now be described. The basic structure of the polyarylene thioether is formed by dehalogenation/sulfidation of a dihaloaromatic compound by an alkali metal sulfide in a polar organic solvent. In the present invention, it is preferable to prepare a substantially linear polymer having such a high molecular weight that a curing treatment need not be carried out, and from this viewpoint, the above-mentioned water-adding two-staged polymerization process developed by the Applicant is especially preferred. Whether or not the water-adding two-staged polymerization is adopted, as the metal sulfide (one of the main starting material of the polymerization reaction) sulfides of alkali metals such as Na, Li, K and Rb can be used (hydrous sulfides can also be used). As the other starting material, any of dihalo-aromatic compounds can be used provided that it has two halogen substituents on the aromatic nucleus and can be polymerized through dehalogenation/sulfidation by an alkali metal or alkaline earth metal sulfide. Accordingly, the aromatic nucleus includes not only an aromatic nucleus composed solely of an aromatic hydrocarbon but also an aromatic nucleus containing substituents not inhibiting dehalogenation/ sulfidation. Examples of the dihalo-aromatic compound used in the present invention are compounds represented by the following formulae: Substituents of the respective formulae will now be explained. X is a halogen atom selected from Cl, Br, I and F, especially Cl and Br. Y is selected from _R, _OR and _COOH (R is H, an alkyl group, a cycloalkyl group, an aryl group or an aralkyl group). Ordinarily, the alkyl group or alkyl portion has 1 to about 18 carbon atoms, and the aryl group or aryl portion has 6 to about 18 carbon atoms. V is selected from the group consisting of _0_,_CO_, (R′ and R" are selected from the group consisting of H, an alkyl group, a cycloalkyl group, an aryl group and an aralkyl group). The alkyl group or alkyl portion and the aryl group or aryl portion are as defined above. In the formula (A), m is 2 and n is an integer of from 0 to 4. In the formula (B), a is 2 and b is an integer from 0 to 6. In the formula (C), c is an integer of from 0 to 2, d is an integer of from 0 to 2, the sum of c and d is 2, e and f are each an integer of from 0 to 4. In the formula (D), g is an integer of from 0 to 2, h is an integer of from 0 to 2, the sum of g and h is 2, i and j are each an integer of from 0 to 4. Specific examples of the di-halogen-substituted aromatic compounds represented by the above general formulae are p-dichlorobenzene, m-dichlorobenzene, 2,5-­ dichlorotoluene, p-dibromobenzene, 1,4-­ dichloronaphthalene, 1-methoxy-2,5-dichlorobenzene, 4,4′-­ dichlorobiphenyl, 3,5-dichlorobenzoic acid, 4,4′-­ dichlorodiphenyl ether, 4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfoxide, 4,4′-­ dichlorodiphenylketone, and 4,4′-dichlorodiphenyl sulfide. Among these compounds, p-dichlorobenzene, m-­ dichlorobenzene and 4,4′-dichlorodiphenyl sulfone are especially preferred. A copolymer comprising at least 2 different kinds of reaction units can be obtained by using at least two appropriate dihalo-aromatic compounds in combination, as is apparent from the foregoing description. For example, if p-dichlorobenzene is used in combination with m-­ dichlorobenzene or 4,4′-dichlorodiphenyl sulfone, a co­ polymer comprising units and units or unit can be obtained Also, a small amount of tri-or more halo-substituted aromatic compounds is permitted to be used provided that it does not substantially change the characteristics of a polyarylene thioether of a linear molecular structure. An aprotic solvent is preferred as the polar organic solvent used for the polymerization as the premise of the present invention. Amides or organic ureas such as N-­ methylpyrrolidone, hexamethylphosphoryl triamide, 1,3-­ dimethyl-2-imidazolinone and tetramethylurea are especially preferred because they have high stability. If dehalogenation/sulfidation of the dihaloaromatic compound by the alkali metal sulfide is carried out in a polar organic solvent as described above, a polyarylene thioether can be obtained. Various modifications of this polymerization process have been proposed to increase the molecular weight or to attain other objects. In the present invention, these modifications can also be adopted, as long as a substantially linear resin is obtained, and attainment of the intended objects of the present invention is not inhibited. A polyarylene thioether to which the treatment of the present invention is preferably applied is one prepared according to the above-mentioned process of Japanese Patent Laid-Open Application No. 7332/1986 or a block copolymerization process disclosed in Japanese Patent Application Nos. 134633/1984, 178016/1984 and 178017/1984. Improvement of Tmc by the Present Invention Summary of Treatment Injection molding may require that polyarylene thioethers have Tmc of at least 220°C, preferably 225°C. According to the present invention, the above described process for preparing a polyarylene thioether is modified and a process capable of preparing a polymer having high Tmc is provided. This modified process of the present invention will now be described. According to the modified process of the present invention, a polymer formed is separated from the polymerization reaction mixture liquid after completion of the polymerization conducted in the above described manner, and the polymer is treated with a solution of an ammonium salt of the strong acid. It is assumed that by this treatment, terminal basic residues of the polymer will be neutralized or acidified (of course, the present invention is not restricted by this assumption, as pointed out hereinbefore). In the treatment process of the present invention, a solid polymer separated from the polymerization reaction mixture liquid is used. The solid polymer can be a solid polymer which is separated from the mixture liquid by filtration or sieving, and to which the organic polar solvent used for the polymerization still adheres, or a solid polymer obtained by removing the adhering polar organic solvent by washing with a solvent in which the polar organic solvent is soluble, such as methanol or water. When the particle size of the polymer is coarse, it is referred that the polymer be pulverized and finely divided by a mill. It is generally preferred that the solid polymer to be treated according to the present invention be in the form of such a wet cake. However, if necessary, this wet cake can be dried to obtain a dry solid from which the adhering solvent (the polymerization solvent and/or washing solvent ) is substantially completely removed. This polymer is treated with a solution of an ammonium salt of the strong acid. It is preferred that the concen­ tration of the polymer in the treating solution be 2 to 50% by weight. Strong acids as those having a dissociation constant K of at least 10 _3 at 25°C in an aqueous solution are preferable. Suitable non-oxidative acids are hydrochloric acid, sulfuric acid, phosphoric acid, formic acid and halogenacetic acids. Among the ammonium salts, NH₄Cl, (NH₄)₂SO₄ and (NH₄)₃PO₄ are especially pre­ ferred because their effect is high. As the solvent of the salt solution, water and mixed solvents comprising water as the main component and an organic water-soluble solvent such as an alcohol, ketone or ether are used. It is preferred that the alcohol, ketone or ether have compatibility with water and salt dissolving capability sufficient to form an aqueos solution as the solvent. In view of the high salt dissolving capability, water and/or an alco­ hol (especially a lower alcohol) is preferred. The concentration of the ammonium salt in the treating solution is 0.1 to 30% by weight, preferably 0.2 to 20% by weight. If the salt concentration is lower than 0.1% by weight, the effect is insufficient, and if the salt concentration is higher than 30% by weight, the treatment is economically disadvantageous because the effect is not substantially enhanced even by increasing the salt concentration. The treatment temperature is 0 to 100°C, preferably 20 to 100°C, especially preferably 20 to 80°C, and the treatment time is 5 to 500 minutes, especially preferably 10 to 300 minutes. If the treatment temperature is lower than 0°C, the solution of the salt hardly permeates into the core portion of the solid polymer (ordinarily granular or powdery). On the other hand, if the treatment temperature is higher than 100°C, there is a risk of deterioration or de­ composition of the polymer. If the treatment time is shorter than 5 minutes, the reaction is insufficient, and if the treatment time is longer than 500 minutes, the process be­ comes economically disadvantageous because the treatment efficiency is not substantially increased by prolongation of the treatment time. The atmosphere in which the salt treatment is con­ ducted can be air since no oxidation nor curing of the polymer will take place at such a moderate temperature as 0 to 100°C. It is preferable, however, that the salt treatment be conducted in a non-oxidative atmosphere. If the polymer treated is washed with water after the treatment with the salt solution, a cleaned stable polymer can be simply obtained. Polymer of the Invention The so-obtained polyarylene thioether has very high Tmc and hence, a very high crystallization speed. Accordingly, when this polyarylene thioether is used for injection molding, the polymer is sufficiently crystallized while the molten polymer is cooled and solidified, and therefore, the resulting molded article need not be subjected to a high-temperature treatment (so-called annealing) again. Furthermore, the flow property and melt processability are considerably enhanced owing to the significant increase of melt flow rate. Accordingly, the arylene thioether polymer obtained according to the process of the present invention is especially suitable for injection molding. Moreover, since the polymer is not subjected to curing, the polymer has excellent hue, and since the polymer has a substantially linear molecular structure, a molded article of high mechanical strength can be easily obtained. Since the arylene thioether polymer according to the present invention is included in the category of a thermoplastic polymer, the polymer of the present invention can be subjected to various modifications applicable to thermoplastic polymers. For example, the polymer of the present invention can be used in a state wherein it is mixed with a powdery filler such as carbon black, calcium carbonate, silica, titanium oxide, alumina, calcium silicate, talc, mica, kaolin, calcium sulfate, iron oxide, or glass powder or a fibrous filler such as carbon fiber, glass fiber, asbestos, polyaramide fiber, alumina fiber, silica fiber, potassium titanate fiber, calcium sulfate fiber, zirconia fiber or wollastonite. Moreover, the polymer can be used in a state wherein, it is mixed with at least one synthetic resin selected from polycarbonates, polyphenylene oxides, polysulfones, polyarylenes, polyacetals, polyimides, polyamides, polyesters, polystyrenes, ABS resins and the like and/or at least one elastomer selected from the group of olefin elastomers, silicone elastomers, fluor-­ rubbers, hydrogenated SBRs, butyl rubbers, copolyester elastomers and copolyamide elastomers. Experiments Synthesis Example A titanium-lined polymerization vessel was charged with 371 Kg of hydrous sodium sulfide (having 46.1% assay) and 800 Kg of N-methylpyrrolidone, and the temperature was elevated to about 203°C to distil water. Then, the polymerization vessel was after-charged with a liquid mixture comprising 318 Kg of p-dichlorobenzene, 2 Kg of m-dichlorobenzene and 280 Kg of N-­ methylpyrrolidone. Then, the temperature was elevated to 220°C and polymerization was carried out for 4 hours. Then, 115 Kg of water was after-charged into the polymerization vessel, and the temperature was elevated to 260°C and polymerization was continued for 5 hours. The polymer was recovered from the reaction liquid by sieving, and the polymer was washed with methanol and then with water and the slurry was treated by a mill to pulverize the polymer. Then, the slurry was filtered to obtain a wet cake Wl of the polymer (water content = 48%). This wet cake Wl was subjected to a treatment with a solution of a buffer solution containing an ammonium salt of a strong acid. Examples A-1 through A-3 and Comparative Example A-4 (Buffer Solution Treatment) To the ammonium salt was added a specific amount of water to form an aqueous solution having a specific concentration, and the wet cake Wl was added to the solution in such an amount that the polymer concentration was 8% by weight. The treatment was conducted at a specific temperature for a predetermined time. After the treatment, the polymer was recovered by filtration, washing with water and drying in vacuo at 80°C overnight. Thus, polymers A-1 through A-4 were obtained. The melting crystallization temperature (Tmc) was measured according to the following method. The polymer sample was pre-heated in a nitrogen stream at 320°C for 2 minutes and then melt-pressed by a hot press at 320°C for 1 minute and quenched by water to prepare an amorphous sheet having a thickness of about 0.25 mm. By using a differential scanning calorimeter (Model DSC-30 supplied by Shimadzu Seisakusho, Japan), 10 mg of the sheet sample was made molten at 300°C and then cooled from 300°C at a cooling speed of 10°C/min. in a nitrogen stream, and the exothermic peak observed was designated as Tmc. The results obtained are collectively shown in Table 1.

Claims: 1. A process for increasing the melt crystallization tempe­ rature of polyarylene thioethers, prepared by subjecting an alkali metal sulfide and a dihalo-aromatic compound to dehalogenation/sulfidation in a polar organic solvent, to at least 220°C, characterized in that the polymer is separated from the polymerization reaction mixture and is subjected to a treatment with a solution containing 0.1 to 30% by weight of an ammonium salt of a non-oxidative strong acid at 0 to 100°C for 5 to 500 minutes. 2. A process for the preparation of polyarylene thioethers according to claim 1, wherein the ammonium salt of the strong acid is an ammonium salt of hydrochloric acid, sulfuric acid or phosphoric acid. 3. A process for the preparation of polyarylene thioethers according to claim 1 wherein the strong acid has a disso­ ciation constant K of at least 10 _3 at 25°C in an aqueous solution. 4. A process for the preparation of polyarylene thioethers according to any one or the preceding claims 1 to 3 wherein the polymer separated from the polymerization mixture with the polar organic solvent adhering thereto is washed with a solvent in which the polar organic solvent dissolves, and the polymer thus washed with the washing solvent adhering thereto is subjected to the treatment.

Inventors: IIZUKA, YO, IWASAKI, TAKAO, KOUYAMA, TOSHITAKA, SHIIKI, ZENYA
Applicants: KUREHA KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1986
Main classification: C08G75
IPC codes: C08G75