Particle flow and sintering processes in extrusion

Abstract The flow mechanism of particulate polymers during processing is reviewed. The extrusion of starch‐g‐poly(methyl acrylate) polymers is a good example of this type of processing, since graft copolymers retain their granular appearance and do not melt, even at high temperature, due to the rigid starch matrix. The formation of a continuous plastic by extrusion is accomplished by the sintering of heat‐deformed granules in the high‐pressure zone of the extruder die. Photomicrographs confirm this mode of formation. Plots of flow rate through the extruder die versus pressure in the die are presented. Tensile strengths and percent elongation of extruded specimens are not greatly affected by variations in flow rate. The equilibrium moisture content (about 5 percent) of starch‐g‐poly(methyl acrylate) plasticizes the starch component and thus reduces the operating pressure in the die. Tensile strengths, however, are unaffected by the presence of equilibrium moisture. Final traces of residual acid (resulting from initiator solution) are difficult to remove from starch‐g‐poly(methyl acrylate), and prolonged heating of the graft copolymer in the presence of this acid results in polymer degradation.