Structure–property-processing investigations of the tenter-frame process for making biaxially oriented HDPE film. I. Base sheet and draw along the MD

Abstract

We discuss here the first two stages of the tenter-frame film manufacturing process for HDPE, viz. (a) the extrusion of the polymer onto a cooled cast roll (base sheet) and (b) drawing along the machine direction (MD) using several sets of rolls (referred to as machine draw orientation (MDO) sheet). The structure–property relationships are analyzed for a single resin when the cast-roll temperature, MDO temperature and MD draw ratio are systematically varied. The thick base sheets were made by extrusion onto a cast roll at three different temperatures of Tb, Tb−11°C and Tb+11°C (Tb being a reference temperature for the base-sheet draw). While the WAXD indicated an unoriented system, atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed spherulites of ca. 3–5μm diameter across the thickness of this base sheet. DSC indicated that the base sheet possessed a broad distribution of lamellar thickness and had a degree of crystallinity (Xc) of ca. 75%. After drawing 6X along the MD at three different drawing temperatures, of TMDO, TMDO−8°C and TMDO+8°C (TMDO being a reference temperature for the MDO-sheet draw), both AFM and TEM revealed a complete change in morphology to well-developed lamellar stacks oriented perpendicular to the MD. The MD draw ratio was then systematically varied from 1.5X, 2.25X, 3X, 4.5X to 6X, while keeping the other processing variables constant to understand how transformation of the spherulites to a stacked lamellar morphology took place. A very systematic change in morphology and orientation of the MDO sheet was observed with changing draw ratios, while the degree of crystallinity did not change. For the conditions utilized, the draw ratio of 2.25X was enough to destroy the general features of the bulk spherulitic superstructure although the broken and deformed lamellar remnants of the spherulites remained. With higher draw ratios, well-stacked lamellar structures oriented perpendicular to MD were gradually introduced at the expense of the deformed lamellae. At a draw ratio of 6X, only stacked lamellar structures were present. We discuss the causes of this transformation from primarily three standpoints: (a) localized melting and recrystallization followed by microfibrillation, (b) bulk melting followed by row nucleation and (c) a combination of the above two mechanisms. Interestingly, both X-ray and refractive index measurements revealed that the MDO process resulted in some level of biaxial orientation of the crystal phase. This arises due to the nature of the MDO process, which places constraints on the narrowing of the transverse dimensions during drawing.