Epoxy resins constitute an important class of thermosets which are used extensively in the field of composites, adhesives surface coatings, etc. They are of aliphatic, cycloaliphatic or aromatic backbones. Epoxy resins based on bisphenol A are commercially available and they have good thermal and mechanical properties. They are cured by a variety of curatives, such as amines and anhydrides, and the mechanism of curing largely depends on the type of curing agent used. They show comparatively low cure shrinkage. Cured epoxies are brittle with poor resistance to crack initiation and growth. Their mechanical properties, in particular toughness, can be modified by incorporating a rubbery phase into the resin matrix. The epoxy matrix can also be modified by various other methods such as incorporation of thermoplastics and particulate fillers. The toughening process leads to improvement in fracture toughness and impact resistance. This review examines the various options and the state-of-the-art vis-à-vis epoxy modification.

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Cardanol is the main ingredient of cashew nut shell liquid, an agro-byproduct. Its comparative merits vis-a-vis aromatic oil for plasticizing HAF-black-filled natural rubber (NR) is the subject of this article. Aromatic oil is the conventional plasticizer employed for such compounding requirements. The mechanical properties, ageing behavior, and torque time curves during cure of NR plasticized by both these materials are compared under identical conditions. It has been established that cardanol, when used as plasticizer, gives rise to similar tensile properties, shorter cure times, and superior ageing behavior compared to aromatic oil.

The use of cardanol, a renewable natural resource, in place of a phenol or diphenol, in the synthesis of two different epoxy systems has been studied in this work. Cardanol, prepared by the distillation of cashew nut shell liquid (CNSL), was epoxidised using epichlorohydrin in presence of caustic soda catalyst at different reaction intervals and the products were characterized by spectroscopy, gel-permeation chromatography and by determining the epoxide equivalent. The epoxide equivalent increased sharply with time of epoxidation and reached almost a limiting value around 9 h. Cardanol, being less reactive, undergoes epoxidation to a smaller extent than phenol or bisphenol. Although these epoxy compounds do not form cross-linked networks due to being mono-functional, they are found to be effective in producing relatively flexible systems when reactive blended with commercial DGEBA. Co-epoxides containing bisphenol A and cardanol in varying molar compositions were also synthesised and characterised. Introduction of 20 mol% cardanol into bisphenol A resulted in a resin having reduced tensile, impact and compressive strengths upon curing by a polyamine hardener. However, the resin showed considerable improvement in elongation-at-break without much decrease in energy absorption.

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