Abstract:
A process for preparing a polyarylene sulfide which comprises reacting an alkali metal sulfide compound and a polyhalo-substituted aromatic compound in an organic polar solvent in the presence of 
     (A) an alkali metal salt of a phenolic aromatic carboxylic acid of the formula: 
     
       (HO).sub.m --Ar--(COOM).sub.n 
     
     wherein Ar is a di- to tetravalent aromatic group having 6 to 24 carbon atoms; M is an alkali metal; and m and n are each 1 or 2, 
     (B) an alkali metal hydroxide and/or an alkali metal alcoholate, and optionally, 
     (C) an alkali metal salt of an inorganic acid.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for preparing a polyarylene sulfide. More particularly, it relates to a process for preparing a polyarylene sulfide having high melt viscosity, great whiteness, excellent heat stability and controlled crystallization rate. 
     BACKGROUND OF THE INVENTION 
     Recently, polyarylene sulfides have been noted as plastics having melt molding properties and heat resistance, and have been widely utilized in various fields. For example, they are utilized in the production of various molding articles by subjecting them to injection molding or extrusion molding, and further, fibers and films are produced by subjecting them to melt extrusion and then, optionally, orienting and heat setting the extruded products. Furthermore, they are utilized in protective films of metals such as sealing compounds or coatings of IC and transistors by subjecting them to injection molding. 
     As a process for preparing polyarylene sulfides (hereinafter referred to as PPS), Japanese Patent Publication No. 3368/1970 discloses a reaction of an alkali sulfide and a dihalo-substituted aromatic compound in an organic polar solvent such as N-methylpyrrolidone. A polymer prepared by such a process has low melt viscosity and, therefore, it is difficult to extrude by melt extrusion. Even if it is extruded, further difficulty is encountered in the production of fiber or film having high strength by orienting it after melt extrusion. Accordingly, a procedure for increasing melt viscosity has been employed, wherein a somewhat larger amount of the polyhalo-substituted aromatic compound is added to the polymerization reaction, or heating is effected after the polymerization reaction to proceed branching and cross linking reaction. 
     On the other hand, in order to increase melt viscosity or intrinsic viscosity without employing the above procedure, the use of various polymerization catalysts has been investigated. For example, the use of alkali metal carboxylates is proposed in Japanese Patent Publication No. 12240/1977 and the use of fatty acid alkaline earth metal salts is proposed in Japanese Patent Laid Open Publication No. 40738/1980. U.S. Pat. No. 4,038,260 discloses alkali metal salts of organic sulfonic acids; Japanese Patent Laid Open Publication No. 43139/1980 discloses alkali metal salts of specific aromatic sulfonic acids; Japanese Patent Laid Open Publication No. 20030/1981 discloses tri-alkali metal salts of phosphoric acid; and Japanese Patent Laid Open Publication No. 20031/1981 discloses di-alkali metal salts of phosphonic acid. Further, Japanese Patent Laid Open Publication No. 22113/1983 discloses metal salts of organic carboxylic acids or organic sulfonic acids; and Japanese Patent Laid Open Publication No. 206632/1983 discloses combination of organic carboxylic acids or sulfonic acids and alkali hydroxides. In addition, Japanese Patent Laid Open Publication No. 25822/1984 discloses the use of alkali metal salts of organic carboxylic acids together with a polyoxyethylene ether to reduce a content of electrolytes. 
     By using these catalysts, it is possible to increase melt viscosity or intrinsic viscosity, or to reduce a content of electrolytes. 
     However, in order to obtain PPS having high melt viscosity by using these catalysts, it is required to use the catalysts in a relatively larger amount such as about 0.2 to 1 mole based on the alkali sulfide. Further, when these catalysts are used, it is difficult to reduce a metal content in a polymer by employing a conventional washing with hot water, acetone and the like. 
     Usually, PPS bubbles slightly or, depending upon conditions, vigorously during melt extrusion and injection steps. Further, sometimes, it increases in melt viscosity and forms gel due to crosslinking reaction, and it is remarkably colored. This is caused by low heat resistance of the polymer and, generally, it is considered that this is resulted from oxidation of sulfur atom in the polymer to generate sulfur dioxide, or removal of sulfur atom in the polymer due to formation of hydrogen sulfide with hydrogen in the aromatic ring by heating together with crosslinking reaction of the molecules to each other. In order to prevent this, addition of various additives has been also proposed. 
     Further, generally, PPS is obtained as a slightly colored material and, according to a particular polymerization catalyst used, it is colored in pale brown to pale off-white. Heretofore, it has not been succeeded in the production of PPS in pure white because coloring itself occurs during polymerization and it is difficult to increase whiteness after the polymerization, while heat resistance can be improved, and progress of coloring in the later stages can be controlled by addition of an additive. When whiteness of a polymer is inferior, fibers and films as well as molded articles produced therefrom can hardly be marketed and, therefore, it has been requested to produce a polymer which has great whiteness after the polymerization reaction. 
     As is seen from the above, in a conventional process, it has been very difficult to obtain PPS which meets all the requirements such as high melt viscosity or intrinsic viscosity, a low content of electrolytes, great whiteness and high heat stability, and can be polymerized stably and economically by using a smaller amount of a polymerization catalyst. 
     Under these circumstances, the present inventors have intensively studied to obtain PPS which can meets these requirements. As the result, it has been found that desired PPS can be obtained by using certain polymerization catalysts. assignee discloses a different polymerization catalyst suitable for this purpose, which contains an alkyl metal salt of a phenolic aromatic sulfonic acid. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide a novel process for preparing PPS having high melt viscosity, great whiteness, excellent heat stability and controlled crystallization rate. 
     This object as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description. 
     According to the present invention, there is provided a process for preparing a polyarylene sulfide which comprises reacting an alkali metal sulfide compound and a polyhalo-substituted aromatic compound in an organic polar solvent in the presence of 
     (A) an alkali metal salt of a phenolic aromatic carboxylic acid of the formula: 
     
         (HO).sub.m --Ar--(COOM).sub.n                              (I) 
    
     wherein Ar is a di- to tetravalent aromatic group having 6 to 24 carbon atoms; M is an alkali metal; and m and n are each 1 or 2, and 
     (B) an alkali metal hydroxide and/or an alkali metal alcoholate. 
     Further, in another aspect of the present invention, a polyarylene sulfide is prepared by carrying out the above reaction in the presence of 
     (C) an alkali metal salt of an inorganic acid, in addition to the above catalysts of (A) and (B). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The alkali metal sulfide compound used in the process of the present invention is lithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide, cesium sulfide or a mixture thereof. Further, a hydrate or an aqueous solution thereof can be also used. In case of using the hydrate or the aqueous solution, it can be dehydrated prior to addition of the polyhalo-substituted aromatic compound. By the way, the alkali metal sulfide compound can be prepared from an aqueous solution of an alkali bisulfide and an aqueous solution of an alkali hydroxide. Further, a trace amount of an alkali bisulfide and an alkali metal thiosulfate contained in the alkali metal sulfide compound can be neutralized by a small amount of an alkali hydroxide. 
     The polyhalo-substituted aromatic compound used in the present invention includes di-, tri- and tetrahalo-substituted aromatic compound. As the monomer to be reacted with the alkali metal sulfide compound, the dihalo-substituted aromatic compound is mainly used. However, the tri- or teterahalo-substituted aromatic compound can be copolymerized in order to introduce a branched structure into the polymer to increase melt viscosity, to proceed the polymerization reaction efficiently, or to control crystallization rate. 
     Examples of the dihalo-substituted aromatic compound include p-dihalo-substituted compounds such as p-dichlorobenzene, 2,5-dichloro-p-xylene, 4,4-dichlorodiphenyl sulfone, 2,5-dichlorotoluene, 4,4-dichlorodiphenyl ether, dichloronaphthalene, dibromonaphthalene, dichlorobenzophenone and dibromobenzophenone; m-dihalo-substituted compounds such as m-dichlorobenzene; and o-dihalo-substituted compounds such as o-dichlorobenzene. 
     Examples of the trihalo- and tetrahalo-substituted aromatic compounds include 1,2,4-trichlorobenzene, 1,2,3-trichlorobenzene, 2,4,6-trichlorobenzene, 2,4,6-trichlorotoluene, 1,2,4-tribromobenzene, 1,2,3-trichloronapthalene, 1,2,4-trichloronaphthalene, and 1,2,3,4-tetrachloronaphthalene. 
     As the organic polar solvent in which the polymerization reaction of the present invention is carried out, there can be used, for example, N-methylpyrrolidone, hexamethylphosphoric triamide, N-methylformamide, N,N&#39;-dimethylacetamide, caprolactam, N-methyl-ε-caprolactam, tetramethylurea, dimethylacetamide and the like. 
     In the present invention, the above compounds (A), (B) and, optionally, (C) are used as polymerization catalysts. 
     The polymerization catalyst (A) is an alkali metal salt of a phenolic aromatic carboxylic acid of the formula (I). In the formula (I), the group Ar includes, for example, ##STR1## and the like. Among them, phenylene is preferred. Examples of M include sodium, lithium, potassium and the like. 
     Particularly, examples of the alkali metal salt of the phenolic aromatic carboxylic acid (A) include the alkali metal salts of p-hydroxybenzoic acid, m-hydroxybenzoic acid, 2,3-dihydroxybenzoic acid, 2,4-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid, 3,4,5-trihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid, 3-methylsalicylic acid, 4,6-dihydroxy-2-methylbenzoic acid, 3-hydroxyphthalic acid, 2-hydroxyisophthalic acid, 5-hydroxyisophthalic acid, 2,5-dihydroxyterephthalic acid, 1-hydroxy-2-naphthoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, o-, m- and p-hydroxyphenyl acetic acid, 4&#39;-hydroxybiphenyl-4-carboxylic acid, 4,4&#39;-dihydroxydiphenylmethane-3,3&#39;-dicarboxylic acid and the like. They may be used in any form such as non-hydrates, hydrates and aqueous solutions. These can be used alone or in combination thereof. The catalyst (A) can be used in an amount of 0.02 to 1.0 mole, preferably, 0.05 to 0.20 mole per 1 mole of the alkali metal sulfide compound. 
     The catalyst (B) which is used together with the catalyst (A) is an alkali metal hydroxide and/or an alkali metal alcoholate. Examples of the catalyst (B) include sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium methylate, sodium ethylate, potassium methylate, potassium ethylate, lithium methylate, lithium ethylate and the like. These can be used alone or in combination thereof. The catalyst (B) can be used in the molar ratio to hydroxy group of the catalyst (A) of 0.5 to 1.5, preferably 0.8 to 1.5, more preferably 0.9 to 1.1. When the molar ratio is not within this range, inefficient progress of the polymerization reaction, or decomposition of the polymer once formed in the reaction system is resulted and it is undesirable. 
     The catalyst (C) which is optionally used together with the catalysts (A) and (B) is an alkali metal salt of an inorganic acid. Examples of the catalyst include sodium, potassium and lithium salts of inorganic acids selected from the group consisting of sulfuric acid, nitric acid, carbonic acid, phosphoric acid and boric acid; a hydrate thereof; and an aqueous solution thereof. These can be used alone or in combination thereof. Water contained in these compounds may be dehydrated prior to addition of the polyhalo-substituted aromatic compound. The catalyst (C) can be used in the molar ratio to hydroxy group of the catalyst (A) of 0.2 to 10, preferably 0.5 to 5. When the molar ratio is not within this range, heat stability during the polymerization reaction becomes inferior, which occasionally results in an insufficient polymerization degree. 
     The catalysts (A), (B) and (C) can be added to the reaction system simultaneously, or either the catalysts (A) and (B) or all of them can be previously mixed and then added to the reaction system. 
     In a preferred embodiment, the process for preparing PPS of the present invention is carried out as follows. 
     Firstly, the alkali metal sulfide compound is added to the organic polar solvent and, under nitrogen atmosphere, preferably, with bubbling nitrogen gas through the resulting mixture, temperature is gradually raised to 210° C. with stirring. During this step, the alkali metal salt of phenolic aromatic carboxylic acid (A), the alkali metal hydroxide and/or the alkali metal alcoholate (B), and optionally, the alkali metal salt of inorganic acid (C) are added to the mixture in one portion or by small portions. Alternatively, these catalysts may be present from the beginning. When the catalyst (C) is in a non-hydrated form, it can be added prior to the polymerization step. 
     After removing water derived from the reactants used such as those in the form of the hydrate, the aqueous solution, etc. or an alcohol derived from the alcoholate used from the mixture, the reaction mixture is cooled to a temperature of about 150° to 180° C. At this stage, the polyhalo-substituted aromatic compound is added to the mixture and, if necessary, additional solvent is also added. By the way, when a mixture of the catalysts (A) and (B) is previously dehydrated, or alcohol is previously removed from the mixture of (A) and (B) by heating, they may be added to the reaction mixture at this stage. Further, the catalyst (C) may be also added this stage. 
     Then, the reaction system is closed, and the reaction is carried out for 1 to 2 hours, while the temperature is gradually raised from about 210° C. to 280° C. under 10 to 30 kg/cm 2 . The reaction is further continued for 1 to 3 hours after the mixture reached to the maximum temperature. Since this is an exothermic reaction, the reaction temperature should be cautiously raised without rapid heating. 
     After completion of the reaction, the reaction system is cooled and the solvent is filtered off from the reaction mixture. The residue is washed with hot water, water, acetone, etc. to remove the organic solvent, salts and the polymerization catalysts, and then it is dried. During or after cooling the reaction system, the reaction mixture may be introduced into hot water or water and then, subjected to the above work up procedure. After drying, the desired white polymer powder is isolated. 
     Although the effect of the alkali metal salt of phenolic aromatic compound (A) used in the present invention has not yet been fully made clear, it is considered that the phenolic hydroxy group thereof attributes to improvement of heat stability and whiteness as well as to prevent crosslinking reaction. 
     Further, it is assumed that the phenolic hydroxy group effects on solubility and precipitation of the polymer in the reaction system to assist precipitation of the polymer in fine powder, and also effects on separation of itself and other salts from the polymer during washing and purfication. 
     In addition, it is considered that the alkali metal salt of inorganic acid (C) enhances heat stability of the polymer formed in the organic solvent and after precipitation. 
     The following Examples and Comparative Examples further illustrate the present invention in detail but are not to be construed to limit the scope thereof. 
     Each physical data in the Examples and Comparative Examples are determined as follows. 
     Intrinsic Viscosity 
     Intrinsic Viscosity was expressed by the value obtained by extrapolating an intrinsic viscosity, [η]=η rel  /polymer conc., at 206° C. in α-chloronaphthalene to that where the concentration of polymer is 0. 
     Melt Viscosity 
     Melt viscosity (poise) was determined by using a melt indexer type flow tester (CFT 500 Model A manufactured by Shimazu Seisaku-sho, Japan) at 300° C. under the following conditions: 
     nozzle diameter: 1.0 mm; 
     L/D=10; and 
     load: 50 kg/cm 2 . 
     Whiteness 
     A tablet of 3.5 mm in thickness and 18 mm in diameter was prepared and color thereof was determined by using a colorimeter (Model CR-100 manufactured by Minolta Camera, Japan). The result was expressed by L value. 
     Na Content 
     Na content (ppm) was determined by dry ashing the polymer at 550° C., preparing a solution of the resulting ash in hydrochloric acid and determining Na by atomic-absorption spectroscopy. 
     EXAMPLE 1 
     N-Methylpyrrolidone (900 ml), sodium sulfide nonahydrate (960.7 g, 4 moles), sodium 3,4-dihydroxybenzoate (35.2 g, 0.2 mole) and sodium hydroxide (16.0 g, 0.4 mole) were placed in a 5 liter stainless steel reaction vessel equipped with a condenser. Under nitrogen atmosphere, the temperature of the mixture was raised to 205° C. with stirring over 2.5 hours. After water (620 ml) was distilled off, the reaction system was cooled to 180° C. 
     The reaction mixture was introduced into a 5 liter autoclave equipped with a stainless stirrer which was previously adjusted at 180° C. under nitrogen atmosphere. To the reaction system were added p-dichlorobenzene (588 g, 4 moles), 1,2,4-trichlorobenzene (2.18 g, 0.012 mole), N-methylpyrrolidone (350 ml) and sodium sulfate (284 g, 2 moles). The autoclave was closed and polymerization reaction was carried out at 180° to 240° C. for 1 hour, at 240° to 270° C. for 1.5 hours and at 270° C. for 2.5 hours, with stirring under nitrogen atmosphere at 2 kg/cm 2 . At the end of the polymerization, the inner pressure was raised to 17 kg/cm 2 . 
     After cooling, the reaction mixture was taken out of the autoclave and N-methylpyrrolidone was filtered off. The residue was washed several times with hot water and acetone. After drying, PPS (392 g) was obtained as white powder. PPS thus obtained had intrinsic viscosity: 0.24; melt viscosity: 2,700 poises; L value: 87; and Na content; 520 ppm. 
     EXAMPLE 2 
     N-Methylpyrrolidone (450 ml), sodium sulfide nonahydrate (480.4 g, 2 moles), sodium 3,5-dihydroxybenzoate (35.2 g, 0.2 mole) and sodium hydroxide (16.0 g, 0.4 mole) were placed in a 5 liter stainless steel reaction vessel equipped with a condenser. Under nitrogen atmosphere, the temperature of the mixture was raised to 205° C. with stirring over 2 hours. After water (320 ml) was distilled off, the reaction system was cooled to 180° C. 
     The reaction mixture was introduced into a 5 liter autoclave equipped with a stainless stirrer which was previously adjusted at 180° C. under nitrogen atmosphere. To the reaction system were added p-dichlorobenzene (294 g, 2 moles), 1,2,4-trichlorobenzene (1.09 g, 0.006 mole) and N-methylpyrrolidone (200 ml). The autoclave was closed and polymerization reaction was carried out at 180° to 240° C. for 1 hour, at 240° to 270° C. for 1 hour and at 270° C. for 2 hours, with stirring under nitrogen atmosphere at 2 kg/cm 2 . 
     After cooling, the reaction mixture was taken out of the autoclave and N-methylpyrrolidone was filtered off. The residue was washed several times with hot water and acetone. After drying, PPS (180 g) was obtained as white powder. PPS thus obtained had intrinsic viscosity: 0.23; melt viscosity: 2,300 poises; L value: 83; and Na content: 450 ppm. 
     EXAMPLE 3 
     According to the same manner as in Example 2, the reaction was carried out except that sodium p-hydroxybenzoate is used instead of sodium 3,5-dihydroxybenzoate. 
     That is, the initial dehydration reaction was carried out by using N-methylpyrrolidone (450 ml), sodium sulfide nonahydrate (480.4 g), sodium p-hydroxybenzoate (32.0 g, 0.2 mole) and sodium hydroxide (8.0 g, 0.2 mole). After completion of the reaction, the polymerization reaction was carried out by addition of p-dichlorobenzene (294 g, 2 moles), 1,2,4-trichlorobenzene (1.09 g, 0.006 mole), N-methylpyrrolidone (200 ml) and sodium sulfate (85.2 g, 0.6 mole). After completion of the polymerization, the resulting polymer was taken out of the autoclave, filtered, washed with hot water and acetone and dried to obtain PPS (165 g) as white powder. PPS thus obtained had intrinsic viscosity: 0.21; melt viscosity: 2,000 poises; L value: 82; and Na content: 620 ppm. 
     COMPARATIVE EXAMPLE 1 
     According to the same manner as in Example 1, the dehydration reaction was carried out except that sodium 3,5-dihydroxybenzoate, sodium hydroxide and sodium sulfate were not added. The polymerization reaction was carried out by using p-dichlorobenzene, 1,2,4-trichlorobenzene and N-methylpyrrolidone according to the same manner as in Example 1 to obtain PPS (320 g) as white powder. PPS thus obtained had intrinsic viscosity: 0.14; melt viscosity: 200 poises; and Na content: 280 ppm. 
     PPS thus prepared by the process of the present invention has great whiteness and high heat stability in addition to excellent properties possessed by conventional PPS such as excellent fire resistance, heat resistance, chemical resistance, electrical properties and the like. Further, in the present invention, because PPS is obtained in the form of powder after polymerization reaction, the solvent, salts and the polymerization catalysts can be readily removed by a simple washing and thereby PPS containing a less amount of electrolytes can be obtained. Furthermore, because, in comparison with a conventional process, PPS having less branching with high melt viscosity and high intrinsic viscosity can be obtained by a less amount of the polymerization catalysts, the resulting PPS is suitable for melt molding and is very useful for engineering plastics, films, fibers, sealing compounds and the like. 
     In addition, PPS obtained by the process of the present invention has a desired crystallization rate and thereby, it is advantageous for orientation in the production of films and fibers as well as for providing great strength and modulus to products.