Reactive processing of polymers: mechanisms of grafting reactions of functional antioxidants on polyolefins in the presence of a coagent

The antioxidant 3,5-di- tert-butyl-4-hydroxy benzyl acrylate (DBBA) reacts with the trifunctional coagent trimethylol propane triacrylate (Tris) in the presence of a small concentration of free radical initiator in PP melt during processing to give very highly grafted DBBA. The nature of the grafting achieved by this special processing method (referred to as novel reactive processing, NRP) has been examined. The nature and extent of the competing reactions was investigated. Very high extent of antioxidant homopolymerisation, about 60% (the major competing reaction) was observed in the case of reactive processing carried out in the absence of Tris led to low grafting levels. The use of NRP method in the presence of Tris, however, has drastically reduced the level of competing reactions (mainly homopolymerisation, to less than 10%) giving rise to over 90% grafting efficiency of DBBA on the polymer. A mechanism of the grafting reaction is proposed based on spectroscopic (solid-state and solution NMR and FTIR) investigations of high temperature reactions of DBBA with PP and with decalin, used as a hydrocarbon model, in the presence and absence of Tris. The mechanism comprises three main steps involving finally the grafting of DBBA-Tris units on the polymer chains without causing adverse modification to the physical characteristics of the polymer, in terms of its molar mass and solubility.