1. Field of the Invention
The present invention relates to a heat stable thermoplastic resin composition. More particularly, it relates to a thermoplastic resin having improved heat stability which can be molded even at high temperatures to give a molded article having few or no silver streaks on the article.
2. Description of the Prior Art
It is known that .alpha.-methylstyrene base copolymer with good heat resistance is produced by replacing a part or the whole of styrene in a styrene base resin such as a styrene-acrylonitrile copolymer with .alpha.-methylstyrene and that blend of the .alpha.-methylstyrene base copolymer with other resin such as an ABS graft polymer provides a resin composition which has improved heat resistance. The .alpha.-methylstyrene base copolymer and the blend of said copolymer with the ABS graft polymer are in practical applications used. Such copolymer or blend may be colored by an inorganic or organic colorant.
As an .alpha.-methylstyrene content in the copolymer or the blend increases, heat resistance of the copolymer or blend is improved but its processability deteriorates. Therefore, he copolymer or the blend having the higher .alpha.-methylstyrene content should be processed at the higher temperature. But, at the higher processing temperature, silver streaks often appear on the surface of the molded article and do not disappear in subsequent fabrication steps such as coating. When the processing temperature is lowered to prevent the formation of silver streaks, processability is decreased and problems such as short molding are caused.
It is known that, in general, the silver streaks appear more frequently when the resin contains a larger amount of volatile components. To remove the volatile components, the resin is intensively dried before molding. Since the presence of volatile components in the resin is not the sole cause for the silver streaking and there may be some unknown causes, intensive drying of the resin does not necessarily prevent the formation of silver streaks. Addition of an antioxidant cannot prevent the formation of silver streaks either.
In some cases, a colorant is added to the .alpha.-methylstyrene base copolymer and its resin composition before or during molding as in the case of other thermoplastic resin. Particularly, the addition of an organo-metallic colorant causes the more frequent occurrence of the silver streaks than other colorants.