Anisotropy in mechanical properties, expansion coefficients, and long-term shrinkage of PET films was determined. The films ranged in thickness from 0.92 to 14 mils. Young's modulus, ultimate strength, ultimate elongation, and coefficient of thermal expansion were strongly anisotropic. This anisotropy was related to orientation in the films. Yield stress and coefficient of hygroscopic expansion were more nearly isotropic. In general, the thin films showed more anistropy than the thick films due to better in-plane orientation in the thin films.
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Bruce F. Blumentritt (1979) studied this question.
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