Substituting phenol in phenol-formaldehyde resins for wood modification by phenolic cleavage products from vacuum low-temperature microwave-assisted pyrolysis of softwood kraft lignin

Wood modification using phenol-formaldehyde resins (PF resins) is a well-known and commercialized method to improve, amongst others, the dimensional stability and the durability of wood. However, phenol is mainly obtained from non-renewable resources, and the substitution of phenol by renewable resources has been a topic of research interest for years. Due to the high availability of technical lignins, lignin-derived cleavage products are promising candidates. In this study, organic cleavage products obtained from vacuum low-temperature microwave-assisted pyrolysis of softwood kraft lignin were used to substitute up to 45% of phenol in PF resins. The curing behavior and the free formaldehyde content of the resins was studied. Pine sapwood was treated with these resins by a vacuum pressure treatment, and the weight percent gain, leaching, and dimensional stability of the species were examined. Selected samples were analyzed with microscopy. The results indicate that up to 30% substitution of phenol by lignin-based organic products does not lead to significantly inferior wood modifications. At a substitution of 45%, the performance decreases, most likely due to the lower reactivity of the lignin cleavage products compared to pure phenol. The method described in this study could be a way to improve the environmental footprint of wood modification by PF resins.