| Abstract: | Sheets of two molding grades of polyethylene terephthalate (PET) with weight average molecular weight of 57,000 and 52,000 were drawn uniaxially or biaxially to ratios of up to 3 in a compression mold at 200°C. Tensile modulus measurements were done for axial and transverse strain. Thermal conductivity measurements were done with a transient‐state surface probe for thermal conductivity in the normal‐to‐the surface direction (Kn), as well as the axial (Ka) and transverse (Kt) directions in the plane of the sheet. Values are presented as normalized by the tensile modulus (Mo) or the normal conductivity (Kno), respectively, of undrawn isotropic specimens. For the uniaxial experiments, both M/Mo and Ka/Kno were linearly related to draw ratio. A regression analysis on results for both grades of PET for Ka/Kno in terms of M/Mo gave Y = 0.804*X + 0.286; std. err. of Y est. equal to 0.17. Kn/Kno decreased slightly. This non‐intrusive conductivity measurement provides a convenient means for monitoring the development of increased modulus in hot‐forging processes. Results on biaxially oriented specimens showed lesser modulus increases with draw ratio, consistent with orientation distribution in a plane, rather than in a single direction. Thermal conductivity (Ka/Kno as well as Kt/Kno), however, increased to a greater degree than for the case of uniaxial draw for the higher molecular weight material, suggesting the possibility of tilting of crystalline aggregates in the deformation process. |
