The Effect of Molecular Weight Distribution on the Physical Properties and Morphology of Polypropylene Stretched Tape

Abstract

Stretched tape and film were made from isotactic polypropylene (iPP) homopoly mer produced with SHAC* catalyst (Shell High Activity Catalyst). The resins used were nominal 3.5 and 8.0 melt flow (MF) resins with narrow, medium, and broad molecular weight distributions (MWD). Measurements were made of tape and film physical properties as a function of MWD and process heat history. Small angle x-ray two-theta scans through the meridional direction were done to obtain Long Period Spacing (LPS) Long period spacing for an idealized semicrystalline model is the combined distance through both a crystalline lamella and the amorphous region between the lamellae. LPS was correlated to resin melt flow and MWD. Wide angle x-ray scattering (WAXS) techniques using pole figures and Hermans orientation func tions were used to determine molecular orientation.

Density and physical properties (percent elongation at break, Young's modulus, and tenacity) did not change significantly as a function of MWD for the 3.5 and 8 MF tapes. The tenacities of the 8 MF tapes averaged 0.8 gm/denier less than those of the 3.5 MF tapes. The density of the 8 MF tapes was higher than that of the 3.5 MF tape density (0.9039 and 0.8998 gm/cm³, respectively) indicating a higher percent crystallinity for the 8 MF tapes.

The lower molecular weights of the 8 MF resins apparently contribute to a more highly ordered structure; the latter keeps the physical properties of the tape from deteriorating as molecular weight decreases. The net result is that lower molecular weight resins (higher MF) can produce stretched tape with acceptable physical prop erties at high processing speeds.