PLASTICIZATION OR MELTING - A CRITICAL PROCESS FOR FREE-RADICAL GRAFTING IN SCREW EXTRUDERS

This paper shows for the first time that when a monomer is to be graft ed onto a polymer backbone by a free radical mechanism in a twin screw extruder, the grafting process occurs mainly, if not exclusively, in the plastification (melting) zone. For this purpose, the free radical grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) and po lyethylene (PE) was chosen as model systems. A co-rotating self-wiping twin screw extruder of type Werner Pfleiderer ZSK-30 (L = 42D) was us ed to process the grafting. Owing to its modular character in terms of barrel arrangement, screw element combination and barrel temperature, the position and length of the plastification zone can be adjusted vi rtually at will. This allowed us to follow up the grafting not only at the die exit, but also in the plastification zone under different gra fting conditions. Our results clearly show that it is in the plastific ation (melting) zone that the entire grafting process occurs. This len gth is usually very short in a co-rotating twin screw extruder like ou rs. Under the grafting conditions, it varied from 1D to 5D. Thus, any relevant analysis or model of a free radical grafting process carried out in a screw extruder must be based on detailed information generate d not only at the die exit, but also and most importantly in the plast ification zone. Otherwise, it may lead to incomplete and/or wrong conc lusions.