Properties and Film Applications of Metallocene-Based Isotactic Polypropylenes

Abstract

When using a stepwise isothermal segregation technique (SIST) by DSC, multiple melting endotherms are observed for metallocene-based isotactic polypropylenes (miPPs), whereas only a single melting peak is obtained for standard iPPs made by the Ziegler-Natta (Z-N) catalyst system. The multiple endotherms suggest discrete morphological populations of segregation effects due to heterogeneity in stereo defect distribution among chains. A distinct peak at about 150TC may be attributable to the presence of 2-1 insertion defect, which is detected only in the miPPs using ¹³C NMR. This lower melting family of polymer species may result in the unique stress-strain profile observed when using a laboratory scale biaxial film stretcher. The miPPs require lower draw temperatures and show a similar yielding behavior to that of high crystallinity iPPs at an equivalent yield stress; however, they show an earlier onset and sharp increase of stress due to strain hardening. The insertion defects inherent to the miPPs as well as the high isotacticity may thus control the final film properties which result in superior optical, mechanical, barrier and COF (coefficient of friction) properties in the cast and BOPP (biaxially oriented) film applications.