The production of a new partially biodegradable starch plastic by reactiveextrusion

The grafting of polystyrene onto dissolved starch in a twin screw extruder has been studied. This copolymerization was initiated using the thermal ini tiators benzoyl peroxide and K2S2O8. As end product a mixture containing po lystyrene-grafted starch, homopolymer of polystyrene and starch was obtaine d. Parameters like screw rotation rate, fully filled length of the extruder , wall temperature and throughput have been varied in order to obtain infor mation about their influence on conversion, graft percentages and molecular weight of the materials formed. To increase the amount of graft points, ma leic acid anhydride (MAH) was added resulting in an increased grafting of p olystyrene onto starch. Graft percentages of 60% could be achieved. The tot al conversion of styrene could be controlled by adjusting extruder paramete rs like barrel temperature, fully filled length and initiator type. Convers ions of 95% were found. Molecular weights of the polystyrene formed could b e controlled by adding a chain transfer agent (dodecylmercaptan) to the sty rene phase. In this way the molecular weight (M-n) of the styrenic part cou ld be varied from 20,000 to 140,000. The experiments with the chain transfe r agent showed that the grafting of polystyrene onto starch is a process oc curring at the interface between the dissolved starch and the styrene phase .