Influence of rheological properties on the sagging of polypropylene and ABS sheet for thermoforming applications

The isothermal sagging resistance of different grades of conventional and a high melt: strength (HMS) PP has been correlated with the rheological characteristics of the polymers, such as dynamic shear properties, melt: strength, and zero shear viscosity. A thermoforming grade of acrylonitrile-butadiene-styrene (ABS) was used as a reference material. At 190 degrees C, ABS had the highest viscosity and elastic modulus in the frequency range measured, showing that this polymer is highly elastic. HMS PP had a greater shear thinning behavior than conventional PP because of its broader molecular weight distribution. The tan delta of the polymers shaved that conventional PP had a higher tendency to flow than HMS PP and ABS when heated above 172 degrees C. This was confirmed with sagging experiments performed in an air circulating oven, where the rate of sagging decreased as the melt strength and the zero shear viscosity of the polymer increased.