Synthesis of phenolic resins by substituting phenol with modified spruce kraft lignin

In this study, a commercial kraft lignin (KL) was used as the raw material to prepare phenolic resin (PF) based on a detailed analysis of its molecular structure. To further increase the viscosity of the phenolic resins and improve their environmental performance, demethylated and hydroxymethylated KL (DKL and HKL) were also prepared and used to produce DKLPF and HKLPF. Compared with the methoxy content of KL, that of DKL was lower by 39.34%, and the alcoholic hydroxyl contents of HKL were higher by 59.39%. Therefore, the increased active sites on the benzene ring of DKL promote the hydroxymethylation reaction, and HKL adds more methylol groups, which is more conducive to the condensation reaction to produce PF. The synthesis process and properties of the lignin-based PF with high phenol substitution rates were investigated. The results showed that the maximum substitution rates of KL, DKL, and HKL to replace phenol to prepare phenolic resin were 70%, 50%, and 30% (w/w). The viscosity, free formaldehyde content, and other important indices of LPF meet the requirements of the GB/T14732-2017 standard. Besides, as the substitution rate of phenol by lignin increases, the viscosity of DKLPF and HKLPF performance is higher. When the 30% (w/w) phenol was replaced by lignin, the adhesive strengths of KLPF, DKLPF, and HKLPF all reached the maximum, and the trend of adhesive strengths was KLPF < DKLPF < HKLPF.