Description of the Foaming Process During the Extrusion of Foams Based on Renewable Resources

Abstract

Some polymers based on renewable resources like starch can be plasticated by extrusion processing. Foams based on starch have an increasing importance in nonfood applications. Starch contains water, which works as a physical blowing agent for expansion of molten starch.

The aim of this work is to investigate how cell nucleation and foaming are affected by process parameters in the case of starch materials. Therefore, the cell nucleation has been investigated by a slit die with transparent inserts in the flow channel. It could be shown that a higher pressure gradient in the die leads to a later cell nucleation. Contrary to this, an earlier nucleation is supported by high shear rates. A later nucleation leads to a high cross-sectional expansion ratio.

Furthermore, the rheological properties also have an impact on the foaming behavior of molten starch. It was observed that, besides water content and temperature, the screw speed had a significant impact on the viscosity. In this case, the major effect is the specific mechanical energy input (SME) which increases with increasing screw speed.