Melt strength of polypropylene: Its relevance to thermoforming

Abstract

AbstractThe melt strength of homopolymer, copolymer and high melt strength (HMS) grades of polypropylene (PP) was measured to assess the sagging resistance of PP for thermoforming applications. Acrylonitrile‐butadiene‐styrene (ABS), which can be easily processed during thermoforming, was also studied in this work. A polymer with high melt strength is considered to have a better sagging resistance. The melt strength measurements were carried out using a Gottfert “Rheotens” melt strength tester. The melt strength of the polymers increased with decreasing temperature and increasing extrusion rate. ABS generally had the highest melt strength in the low extrusion temperature region approaching the thermoforming region, indicating that it has a good sagging resistance during thermoforming. The HMS PP had significantly higher melt strength than conventional PP grades; therefore HMS PP is expected to have an improved sagging resistance. For conventional PP, melt strength was higher for the lower melt flow index (MFI) grades. A sharp increase in the melt strength was observed for conventional PP at low extrusion temperature, probably due to flow‐induced crystallization of the PP. These results indicate that to minimise the sagging problem with conventional PP, low MFI grades should be used and thermoforming should be performed at temperatures close to the melting point of PP.