Effect of different coupling agents on the browning of cellulose-polypropylene composites during melt processing

The effects of two coupling agents on the discoloration of cellulose/polypropylene composites during melt processing have been investigated. Composites of polypropylene, coupling agent and bleached eucalyptus Kraft pulp were produced by compounding in a twin-screw extruder. The coupling agents were maleic anhydride-grafted polypropylene (MAPP) and polyethylenimine (PEI). The discoloration was measured using standard colorimetry and the discoloration was investigated using diffuse reflectance (FTIR and UV-Visible) spectroscopy. Both MAPP and PEI increase the browning, but the two mechanisms are clearly different. PEI reacts with carbonyl compounds produced in the cellulose degradation to form new chromophores; however, no specific discoloration processes were detected in composites made with MAPP. In this case, the detrimental effect on the discolouration was related to the increased frictional degradation.