Anisotropy of the Coefficients of Linear Thermal Expansion and Other Properties of Six Biaxially Oriented Films of Poly (Ethylene Terephthalate)

The anisotropy of the coefficient of linear thermal expansion and of the shrinkage affected by heating specimens to 190°C was determined on five commercial biaxially oriented films (ca. 3 mils thick) of poly(ethylene terephthalate) (PET). Expansion coefficients were evaluated near 25° and also near 130°C for specimens heated the second time to 190°C. (Specimens contract above 70-80°C during the first heating period.) The coefficients conform to where and are the coefficients, at a particular temperature, parallel and perpendicular, respectively, to the slow optic axis. The minimum expansion coefficients near 25°C and 130°C are and , respectively. The data show that JTS-118 (an experimental film from ICI) and Melinex-O are the most nearly isotropic near 25° and 130°C, respectively, and that the anisotropy of all films, JTS-118, is somewhat less at 130° than at 25°C. At 25°C, is related semiquantitatively to the birefringence. The maximum shrinkage (first heating tends to be perpendicular to the slow optic axis. The effect of themal hist and tensile stress on the expansion and shrinkage of Mylar D were innvestigated briefly . A special PET film, prepared by stretching radially a disc of an amorphous PET film, was found to be isotropic in the plane of the film. DSC measurements were made on the commercial film.