Source: {"pile_set_name": "USPTO Backgrounds"}

High molecular weight linear thermoplastic polyesters such as polyethylene terephthalate and poly(1,4-butylene terephthalate) are well known film, molded article and fiber formers. See, for example, U.S. Pat. Nos. 2,465,319 and 3,047,539. Polyethylene terephthalate has not been widely exploited as a molding and extrusion resin due to its unacceptably slow rate of crystallization. The prior art has overcome, to some extent, this problem by providing careful molecular weight control in the production of polyethylene terephthalate; by including nucleating agents to hasten crystallization; or by using unconventional, long, two-step molding cycles.
For example, U.K. patent application Ser. No. 2,015,013, published Sept. 5, 1979, discloses the use of a sodium or potassium salt of a selected organic polymer containing pendant carboxyl groups together with a selected low molecular weight organic ester of an aliphatic carboxylic acid. U.S. Pat. No. 4,212,791 discloses polyalkylene terephthalate molding compositions comprising an inert nucleating agent, an oligomeric polyester crystallization promoter and a segmented thermoplastic copolyester-ether elastomer. U.S. Pat. No. 3,843,615 discloses a thermoplastic molding composition comprise,d of a cross-linked polyester as a nucleating agent. U.S. Pat. No. 3,761,450 is directed to linear saturated polyester molding compositions containing lithium and/or sodium salts of polycarboxylic acids. U.S. Pat. No. 3,575,931 discloses polyethylene terephthalate molding compositions which contain a nucleating agent such as metal salts of carboxylic acids, metal salts of inorganic acids and highly polar organic compounds.
By way of contrast, poly(1,4-butylene terephthalate) resins crystallize very rapidly. These resins provide excellent molding compositions because they can be fabricated with moderate stock temperatures, low mold temperatures and rapid cycle times. Because of their highly crystalline nature, these resins exhibit desirable chemical resistance, thermal stability and product appearance (i.e., they exhibit a smooth, glossy finish). Such resins also exhibit superior strength, stiffness, low friction and wear properties and good resistance to brittle fracture. Because of their rapid crystallization times the poly(1,4-butylene terephthalate) resins do not normally require the presence of nucleating agents during the injection molding process. However, one disadvantage of the poly(1,4-butylene terephthalate) resins resides in their significantly higher cost of manufacture in comparison to polyethylene terephthalate.
It is therefore desirable from an economic point of view to substitute polyethylene terephthalate for polybutylene terephthalate. However, the crystallization rate of polyethylene terephthalate constitutes an impediment to such a substitution.