Blocked isocyanates as coupling agents for cellulose-based composites

In this work, the stability of blocked isocyanates has been evaluated and the reactivity of different urethanes with the hydroxyl groups of natural cellulose fibres has been studied during the preparation of composites. The thermal dissociation of carbamate groups studied by TGA was function of the nature of the carbamate bond. According to this, the elimination of alcanol groups took place between 280 and 300 °C, while the elimination of phenolic groups was found at 170–180 °C. Because this last range of temperatures is the commonly used for polypropylene and polystyrene processing, aryl blocked isocyanates were chosen as reactive isocyanate groups for the preparation of natural fibre composites. The reaction of microcrystalline cellulose with such derivatives was evaluated resulting in the formation of new carbamate bonds assessed by FTIR and elemental analysis. Therefore, isocyanates and blocked isocyanates were used as coupling agents during the preparation of pine fibre/polystyrene composites. The addition of MDI or phenol blocked MDI derivative to pine reinforced polystyrene composites gave similar improvements of the mechanical properties of their composites. Consequently, it was stated that these coupling agents could be used for blending natural fibres with other matrices by means of a reactive mixing process.