Recently heat-resistant thermoplastic resins have been used as engineering plastics for machine materials or electronic parts. In molding a thermoplastic resin, in order to improve productivity, it has been proposed to blend, for the purpose of reducing melt viscosity, promoting crystallization and improving mold releasability, various resin modifiers, e.g., fatty acids such as stearic acid, metal salts of fatty acids, ester derivatives prepared from a fatty acid with a polyhydric alcohol such as pentaerythritol, polyethylene glycol or the like, aliphatic amides such as ethylenebisstearamide, etc. (Japanese Unexamined Patent Publications (Kokai) No. 44547/1985 and No. 250049/1991).
However, since heat-resistant thermoplastic resins generally have a high fusing temperature, said resin additives may thermally decompose or boil when melted, thereby tending to stain a mold or form void in the resin. For this reason, it has been desired to develop heat-resistant resin modifiers.
In view of the recent increasing need for plastic materials of high performance, polyarylene sulfide (PAS) excellent in heat resistance, mechanical characteristics and chemical resistance is attracting attention, and is now used for extending applications, for example, as automotive parts, parts for precision machines, electric and electronic parts, etc.
PAS is a crystalline engineering plastic but has low crystallization rate. Therefore, in order to obtain a resin of sufficient mechanical strength by injection molding, a high mold temperature of about 130.degree. C. and a prolonged cooling time were necessary. This defect posed a serious problem in extending the applications of PAS, and it is desired to mold PAS in a short time at a mold temperature of not higher than 100.degree. C. which is among the molding conditions for resin-molding apparatus commonly used.
To solve this problem, there have been proposed a method of increasing the mold releasability by adding a fatty acid ester (Japanese Unexamined Patent Publication No.154867/1992), an aromatic sulfonamide (Japanese Unexamined Patent Publication No.59279/1993) or the like, and a method of promoting the crystallization by adding a modified polyalkylene glycol (Japanese Unexamined Patent Publication No.250049/1991) or an organic phosphoric acid metal salt (Japanese Unexamined Patent Publication No.142854/1990) or the like.
However, because of insufficient heat resistance of the resulting resin compositions, these methods entail the problem of failing to achieve the contemplated results and the problem of exhibiting reduced mechanical strength and heat resistance even if the mold releasability or appearance of the molded products may be improved.