Effects of coupling agents on the oxidation and darkening of cellulosic materials used as reinforcements for thermoplastic matrices in composites

Oxidation and darkening occur during the processing of composites made from thermoplastic matrices and cellulosic reinforcements. We have studied the effects of several coupling agents on both the oxidation and darkening of c ellulosic materials at temperatures close to those used in the processing o f cellulose-reinforced thermoplastics. A maleated polypropylene wax (Epolen e E-43(TM)) and two silanes (N-2-aminoethyl-3-aminopropyltrimethoxysilane a nd methyltrimethoxysilane) were used as coupling agents. Oxidation was meas ured by Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy . Standard colorimetry was used to measure darkening. Coupling agent effect s depend on the nature and extent of cellulose modification achieved by tre atments and the nature of the coupling agent. Epolene wax E-43 produced sca rce effects on both the oxidation and darkening of cellulosic materials at 200 degrees C. Only for longer oxidation times was an increase in oxidation and darkening observed in E-43-treated samples. Silane coupling agents inh ibited the formation of carbonyl and carboxyl groups for shorter oxidation times. The diaminosilane produced a stronger darkening, probably due to a c hemical reaction that generated new chromophores containing C=N bonds.