Studies on the modification of commercial epoxy resin using cardanol-based phenolic resins

Bisphenol A epoxy resin is blended with different phenolic novolac resins for property modification. The phenolic resins employed are (a) novolac resins derived from phenol/formaldehyde (PN) and cardanol/formaldehyde (CDN) condensation and (b) condensation products of cardanol-phenol mixtures with formaldehyde (CPNs). Cardanol is the main constituent of cashew nut shell liquid (CNSL), a renewable resource. Epoxy resins containing 2.5-20 wt% of the above novolacs are prepared and tested for mechanical properties, water absorption, etc. Blends containing 5-10 wt% of novolacs show substantial improvement in properties. These properties show a declining trend with higher novolac loading. The stoichiometric ratio between phenolic material and formaldehyde in the novolacs is optimized (1 : 0.8) for maximum property enhancement. Among the novolacs studied, those prepared from cardanol-phenol mixtures of different molar ratios (CPNs) yield comparatively higher energy absorption at break and this property increases with increase in cardanol content in the modifier resin up to 40%. The property profiles of these epoxy-novolac blends imply a toughening action by novolac resins, especially phenol-cardanol-formaldehyde resins.