Cardanol based matrix biocomposites reinforced with natural fibres

In this work, the synthesis, formulation and characterization of a thermosetting resin based on cardanol, suitable as composite matrix, is presented. The polycondensation reaction between cardanol, a phenolic based by-product of the cashew nut industry, and formaldehyde in presence of a basic catalyst, in different molar ratios, has been applied to the synthesis of a resole type pre-polymer. A thermosetting resin, containing approximately 40% of cardanol by weight, has been obtained adding an epoxy monomer and an acid based catalyst to the resole compound. The reactivity of different functional groups of the mixture has been studied by FTIR (Fourier Transformed Infrared spectroscopy) while the calorimetric analysis has been applied to the definition of the curing cycle, also taking into account the mechanical performances of the matrix. The glass transition temperature (Tg) of the resin, in the range 42–56 °C for different curing cycles and formulations, has been determined by dynamic-mechanical analysis. Finally, the formulation characterized by adequate properties and curing temperatures has been reinforced with short ramiè, flax, hemp fibres, and with a juta fabric. For comparison purposes a standard polyester resin has been also used as matrix for the same juta fabric. In this last case, two different surface treatments of the fibre surface have been also attempted in order to improve the adhesion characteristics. Coupling natural fibres with a natural based matrix has led to an amount of natural components in the composite that can reach 73% by weight. Starting from the new cardanol based matrix and the above mentioned fibres, flat plane samples have been prepared and tested in tension and bending.