The Effects of Recycling and Degradation on Parison Extrusion

Abstract Finished goods manufactured by the extrusion blow moulding process are strongly dependent on the parison formation stage, since it is the primary step in the process. The magnitude of the die swell and the degree of sag exhibited by the parison, after being extruded from an annular die, are dependent on the material's molecular structure and strongly affect the performance of the final product. Repassing or recycling of polymeric materials results in degradation reactions that modify the molecular structure, and therefore of parison swell and sag. Polyethylene simultaneously undergoes crosslinking and chain scission. Polypropylene undergoes predominately chain scission. The extent of these degradation reactions can be minimized with the use of appropriate stabilizers. This work involves the study of the effects of repassing on the parison formation stage of extrusion blow moulding. The repassing of the same material gives a representation of what occurs when recycling is undertaken. The effect of stabilizer addition is also studied. Particular emphasis is placed on parison swell and sag.