Heat durability of biaxially oriented polyester films

A biaxially oriented copolymer film of ethylene terephthalate and isophthal ate was annealed at a temperature between 160 degrees C and 220 degrees C. The molecular weight decreased and crystallization progressed with increasi ng annealing time, and then tensile strength of the film was decreased by t hese changes. Additionally, we investigated the effect of molality of ethyl ene isophthalate on heat durability, Although the heat durability of the co polymer films got shorter with increasing the molality of ethylene isophtha late at a fixed temperature, it became longer with increasing the molality of ethylene isophthalate at the temperature which was 5 degrees C lower tha n the melting point of each copolymer.