EFFECT OF BIAXIAL STRETCHING ON THICKNESS UNIFORMITY AND SURFACE-ROUGHNESS OF PET AND PPS FILMS

Surface roughness and thickness uniformity are two of the most important parameters to control in the manufacture of polymeric films. In this paper, we present the results of a detailed experimental study on the effect of biaxial stretching variables on these parameters in polyethylene terephthalate and poly p-phenylene sulfide films. Decreasing processing temperature and increasing stretch ratio were found to improve surface smoothness and thickness uniformities of PET films. However, in the PPS films, biaxial stretching resulted in rougher surfaces. The main mechanism to improve these variables especially the thickness uniformity was found to be the strain hardening. In PET stretching this mechanism was present and in PPS it was found to be absent in the processing range investigated. This resulted in the improvement in surface roughness and thickness uniformities in the PET films with the increase of stretch ratio and with the decrease of stretching temperature. In PPS, increase of stretch ratios had an opposite effect on the surface roughness. This may be due to branching in the PPS.