a) Field of the Invention
The present invention relates to molding resins, specifically to rubber-modified styrene copolymers that exhibit excellent external appearance, rigidity, impact strength and moldability. More specifically, the present invention relates to styrene copolymers modified by block copolymerized rubber having a star like radial structure.
b) Description of the Prior Art
Rubber-modified styrene copolymers are well known that are prepared by polymerizing a monomer such as styrene, in which a rubbery polymer is dissolved, and which consist of dispersed phases and a continuous phase. Such copolymers have been used extensively as molding materials, for example, in components of electrical equipment, automobiles and the like. Due to the increased applications for such copolymers, the external appearance of the molded articles has gained importance. Further, rigidity and improved impact strength for machine components is strongly desired. Rigidity improves with lower rubber content, however at lower rubber contents, impact strength decreases. Therefore it is desirable to improve and balance both rigidity and impact strength. In addition, high gloss or a matte appearance may be desired for some applications.
On the other hand, a demand for higher precision molding and improved molding cycles lead to a demand for resin materials having high fluidity. Usually, in high fluidization of resin materials (1) decreased molecular weight and decreased rubber content or (2) addition of plasticizers for high fluidization is effective. However, impact strength decreases when the molecular weight and rubber content are decreased and heat resistance or rigidity decreases when plasticizers are added. Thus, it is desirable to improve fluidity and achieve a balance with impact strength, heat resistance and rigidity.
Japanese Patent Publication No. 58-4934 discloses the use of a specific rubbery polymer. Although several proposals have been made to improve gloss, rigidity, external appearance of molded articles, impact strength of such rubber-modified styrene polymers, fluidity, and further improvements are desired. U.S. Pat. No. 4,421,895 discloses the use of a particular organic peroxide in a specific amount, a specific rubbery polymer and a solvent to control the particle size of the rubbery polymer. However, this patent does not address the problem that properties such as gloss, rigidity, external appearance, impact strength and fluidity must be both maintained at a high level and be well-balanced.