The effects of degree of stretching on the properties of heat set biaxially stretched naphthalate-containing polyester films

The thermal and gas barriers of PET may be enhanced by heat setting. Those properties also may be improved by modifying PET with PEN either as a copolymer or a blend. The effects of degree of stretching and heat setting on density shrinkage, and oxygen permeability of biaxially stretched films of PET, PET-N8 copolymer (PET modified with 8 mole percent naphthalate), and 10.6 and 25 wt % blends of PEN-TB (PEN modified with 8 mole percent terephthalate) with PET have been studied. For all compositions, density increased with increasing degree of stretching and with increasing heat set temperature. At the highest heat set temperature (180 degrees C) the density reached was almost independent of the starting stretch ratio. Shrinkage and oxygen permeability decreased with increasing naphthalate content and increasing heat set temperature with the 3.5 x biaxially oriented 25% blend composition (non-heat set or heat set) having oxygen permeability about 40% less than similarly oriented PET.