Relaxation behavior of biaxially stretched PLA film during the heat setting stage

Abstract

In this paper, the relaxation behavior of polylactic acid (PLA) film in the heat-setting stage of biaxial stretching was studied. Firstly, the polylactic acid casting films were stretched synchronously in different ratios. We found that the Machine direction (MD) and Transverse direction (TD) stress relaxation curves exhibited a separation trend with the increase in the stretching ratio, and the relaxation amplitude increased gradually. Then, the stress relaxation curves were fitted by the expansion exponential equation (KWW equation). The results showed that the coefficient used to characterize the homogeneity of stress relaxation increased with the increase in the stretching ratio, and the homogeneity in Machine direction was better than that in Transverse direction. Finally, we analyzed the evolution of rheological units and the activation energy spectrum during stress relaxation. We found that the volume of rheological units gradually decreased with the increase in the stretching ratio. The activation energy spectrum exhibited a Gaussian distribution, and the symmetry axis of distribution curves shifted to the high energy. The above results would be of great significance in further understanding the deformation mechanism of polylactic acid film during biaxial stretching and providing theoretical guidance for the preparation of high-performance BOPLA films.