Commercially available polymer films used in structural applications, such as biaxially oriented poly(ethylene terephthalate), exhibit pronounced anisotropy in mechanical properties, thermal expansion, and long-term dimensional stability. Films with more nearly isotropic in-plane properties have been produced by laminating plies of PET film at various angles to one another. In addition, composite films have been made with nearly isotropic properties and with significantly reduced coefficients of expansion compared to those of commercially available polymer films. A laminate with PET faces bonded to a low expansion alloy foil core had the best dimensional stability and the least anisotropy of the films studied.
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