Self-curable epoxide resins based on cardanol for use in surface coatings

Purpose - The purpose of this paper is to develop eco-friendly coatings based on low-cost epoxide resins prepared by using a natural phenolic material such as cardanol (non-toxic), in place of ordinary phenol (toxic), which can be self-curable at an optimum temperature. Design/methodology/approach - Cardanol-formaldehyde novolac resins (CNs) were prepared by reacting cardanol with formaldehyde in different molar ratios varying from 1:0.6 to 1:0.9. Prepared CNs were epoxidised by reacting with epichlorohydrin to produce epoxide resins, which would be called as epoxidised CNs (ECNs). Further, ECNs were modified by reacting with diethanolamine (DEtOA), a secondary amine to introduce tertiary amino group(s) into the molecules, required for self-curability of ECNs. These modified ECNs are referred to as MECNs. The molar ratio of ECNs to DEtOA was taken in accordance with epoxy functionality of epoxide resins (ECNs) which ranged from 0.5 to 2.9. Nine numbers of MECNs (MECN1 to MECN9) were prepared by Wing four epoxide resins. These resins viz. CNs, ECNs and MECNs were characterized by H-1 NMR and FTIR spectroscopic methods for their structure elucidatation, and by gel permeation chromatography for determining their molecular weights. Findings - The most suitable molar ratio of ECN:DEtOA for the preparation of MECNs was found to be 1:1. The CN prepared by using cardanol and formaldehyde in the molar ratio of 1:0.7 was used for the preparation of ECN2 and MECN2. Applied films of epoxide resins, designated as MECN2, had reasonably good physical and chemical resistance properties. With a wide cure window, the films of MECN2 were found to be self-curable at an optimum cure schedule of 160 degrees C/30 min. Owing to self-curability of the developed epoxide resins, the coatings based on them did not require any additional/external crosslinker to be incorporated in the coating composition. Research limitations/implications - The prepared epoxide resins (MECNs) had good physical and chemical resistance properties, but demonstrated low stability and low resistance to xylene, in particular. Originality/value - The paper shows how the epoxide resins were prepared by using a low-cost phenolic material (cardanol) which is obtained from natural renewable resources, instead of petroleum, and is non-toxic. These developed coatings can be applied as primer coat and top coat on metallic substrates. True self-curability of the coating films has been achieved via anionic polymerization.