PURPOSE: To provide a primary coated film layer of optical fiber, which is composed of a polybutylene terephthalate resin composition having excellent mechanical strength under a high temp. and sufficient elastic modulus.
CONSTITUTION: The primary coated film layer of optical fiber is formed from the polybutylene terephthalate resin composition containing 0.05-3 pts.wt. cyclic organic phosphate metallic salt expressed by the formula per 100 pts.wt. polybutylene terephthalate having relative viscosity of 2.2-3.3. In the formula, each of R1 and R2 represents hydrogen atom, alkyl group, arylalkyl group, aryl group or alkylaryl group, X represents direct bond hand, alkylidene group or sulfur atom, M represents metallic atom, (n) represents the valency of metal. The polybutylene terephthalate is one kind of a crystalline thermoplastic polyester and is made worse in moldability and is not suitable for the primary coated film layer if the relative viscosity is out of the range. And if the quantity of the cyclic organic phosphate to be blended is <0.05, the effect is not shown and even if the quantity to be blended is over 3 pts.wt., the effect is not changed.
YAMAMOTO KOICHI
HASHIMOTO AKIO
