This invention relates to polyester compositions. Particularly, the invention relates to polyethylene terephthalate compositions containing certain phthalimide compounds, as plasticizers, and more particularly to compositions for molding engineering thermoplastic parts.
Polyalkylene terephthalates have long been used for the manufacture of various molded articles. Such polymers, particularly when combined with reinforcing materials and fillers such as glass fibers, are suitable for the manufacture of engineered thermoplastic molded articles due to their high wear resistance, durability and high dimensional accuracy. These good physical properties are most readily achieved by molding a composition containing reinforcing material, under conditions under which the polyalkylene terephthalates become partially crystalline.
Of the polyalkylene terephthalates, polyethylene terephthalate imparts preferred physical properties to the molded article. However, polyethylene terephthalate is often not the material of choice for injection molding usage because relatively high mold temperatures, e.g., 120.degree. C. to 140.degree. C., must be utilized to ensure good moldability and to obtain the desired crystallinity. Because of the high temperatures required, relatively long molding times are necessary. These stringent processing conditions often prevent the use of polyethylene terephthalate for injection molding in spite of its high rigidity and good heat distortion temperature. Other polyester polymers, particularly polybutylene terephthalate require shorter molding times and lower molding temperatures because of their higher inherent rate of crystallization. However, these polymers are inferior to polyethylene terephthalate in their physical properties, particularly in their heat distortion temperature.
Thus, it is desirable to produce a polyethylene terephthalate molding composition that will crystallize at lower temperatures. This lowering of crystallization temperature must be accomplished without unduly adversely affecting the physical properties of the molded article. Additionally, any additive that improves crystallization must have sufficiently low volatility so that it does not volatilize out of the composition at elevated processing temperatures, and so that it does not cause deposits on the mold surfaces. The phthalimide compounds of the invention reduce the molding time and temperatures of polyethylene terephthalate compositions by improving and reducing the lower temperature at which crystallization occurs during cooling of the melt. Furthermore, such phthalimide compounds also result in polyethylene terephthalate compositions with low volatility, good physical properties, and high gloss.