Elongational Viscosity as a Tool to Predict the Foamability of Polyolefins

Abstract

The foaming of polyolefins (POs) by direct gas injection is a delicate balance between the melt strength of the expanding polymer and the pressure of the blowing gas in the growing cells. Important parameters in this process are: melting and crystallization temperature of the polyolefin grade, viscosity in the crystallization temperature range and elongational viscosity in the crystallization temperature range. In particular, the increase in elongational viscosity or so-called strain hardening is vital for a successful foam structure. This study shows how the elongational viscosity is measured according to the rheological melt extension (RME) method. The experimental conditions for LDPE, LLDPE, HDPE, mPE plastomers, and PP are defined and used for a series of 25 PO grades. These experiments show that the strain hardening ratio (SHR) is influenced by the average molecular length, molecular weight distribution (MWD) and the long chain branching (LCB).