Investigation of properties of the PET film dependent on the biaxial stretching

To investigate the properties of PET films, PET films were extruded at various temperature above T_m and quenched at 18 °C for amorphous sheet, and stretched along a direction defined as the machine direction (MD) with a transverse direction (TD) above T_g at various stretching ratios and then annealed at various temperatures produced by SKC PET line. Thermal shrinkage of MD and TD increased with decreasing annealing temperature and extruding temperature, and increasing stretching ratio. The degree of crystallinity, density, heat of fusion (△H) and pre-melting point (Tm') increased with increasing annealing temperature and extruding temperature. Number average molecular weight (M_n) and intrinsic viscosity decreased with increasing extruding temperature. Tensile strength and modulus increased with increasing stretching ratio, however decreased with increasing annealing temperature. Reflective index of both stretching and thickness direction increased with increasing stretching ratio and annealing temperature.