Synthesis of bio based low temperature curable liquid epoxy, benzoxazine monomer system from cardanol: Thermal and viscoelastic properties

Abstract

This paper deals with the synthesis and characterization of cardanol based pre-polymers, one containing epoxy group and the other with a benzoxazine unit, obtained from cashew nut shell liquid which is a by product of cashew industry. Using these monomers a new curing system has been developed by varying the proportion of the monomers in the copolymer mixture. Differential scanning calorimeter was used to study and monitor the cure process of monomers as well as their copolymers. Decrease of onset of reaction and the peak exothermic temperature with increase of epoxy content in the copolymers was observed due to the catalytic effect of free phenolic group in the cardanol epoxy. The tertiary amine formed during benzoxazine ring opening found to catalyze the cross linking reactions. The viscoelastic properties, evaluated by dynamic mechanical thermal analysis indicated an increase in storage modulus and tanδ values with incorporation of benzoxazine in the copolymers. Thermal stabilities of the cured polymers were studied by thermo gravimetric analysis. Two stage decomposition was observed for all the copolymers owing to cardanol side chain degradation at low temperature and main cross linked polymer structure at high temperature. The copolymers displayed lower char yields compared to the pure homopolymers because of less ordered structure due to copolymerization. The cardanol derived functional monomers can be used with epoxy and benzoxazine resins to impart flexibility to the cross linked networks.