Conventional phenolic resins are often composed of petroleum-derived phenol and formaldehyde, which can lead to serious environmental and health hazards. In this work, a bio-based phenolic varnish resin was synthesized by partially replacing phenol and formaldehyde with cardanol and 5-hydroxymethylfurfural (HMF). When 50 wt% of phenol was replaced by cardanol, the resulting resin showed the best overall performance: its bonding strength was 7.68 MPa, free formaldehyde content was as low as 0.0015%, and free phenol content was 1.98%, which was significantly better than that of commercial adhesives (bonding strength of 0.7 MPa and free formaldehyde of 0.3%). In addition, the cured resin’s water contact angle was 88.14°, indicating its good hydrophobicity. In conclusion, our study innovatively used the synergistic impact of cardanol and HMF to provide a new and more sustainable alternative for the synthesis of more environmentally friendly adhesives with satisfied performance. In particular, the application prospects are quite promising.
Tang et al. (Sat,) studied this question.