Bio-based thermosetting resins composed of cardanol novolac and bismaleimide

Abstract

Cardanol novolac (CDN) was synthesized by the reaction of cardanol (CD) and paraformaldehyde in the presence of oxalic acid. The prepolymerized compounds of CD/4,4′-bismaleimidediphenylmethane (BMI) and CDN/BMI with CD/maleimide unit ratios 1/2, 1/4 and 1/6 at 200 °C were finally compression-molded at 250 °C for 5 h to produce cured CD/BMI (cCD/BMI) and cured CDN/BMI (cCDN/BMI) resins. Although the proton nuclear magnetic resonance (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) analyses of the model reaction product of CD and N-phenylmaleimide (PMI) at 200 °C for 8 h suggested the occurrence of the ene reaction and subsequent Diels–Alder reaction, the FTIR analysis of cCD/BMI and cCDN/BMI suggested the occurrence of the ene reaction and addition copolymerization. The cCD/BMI and cCDN/BMI with CD/maleimide ratio lower than 1/2 did not show glass transition until 300 °C and had a 5% weight loss temperature higher than 450 °C. The cCD/BMI and cCDN/BMI with CD/maleimide ratio 1/4 showed the most balanced flexural properties (flexural strength 60–80 MPa, flexural modulus 2.0–2.5 GPa). 4,4′-Bismaleimidediphenylmethane (BMI) was cured finally at 250 °C with cardanol (CD) or CD novolac (CDN) prepared by the reaction of CD and paraformaldehyde with CD/maleimide unit ratios 1/2, 1/4 and 1/6. The obtained cured resins with CD/maleimide ratio lower than 1/2 did not show glass transition until 300°C and had 5% weight loss temperature higher than 450°C. The curing mechanism was elucidated by means of the proton nuclear magnetic resonance (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) analyses of the model reaction products of CD and N-phenylmaleimide (PMI) and the cured resins.