Hydroxymethylation of softwood kraft lignin and phenol with paraformaldehyde

Abstract

Phenol-formaldehyde (PF) resins are the most commonly used adhesives in particleboard and plywood production. Lignin, the second most abundant biopolymer, has the potential to be a valuable resource for phenolic compounds; therefore, lignin could serve as an inexpensive and nontoxic candidate to partly replace phenol in resin preparation. The use of lignin saves fossil resources, but the difficulty in lignin utilization is the low reactivity compared to that of phenol. Furthermore, the complicated structure of lignin can cause problems, for example, in the condensation of methylol groups. In this study, the progress of hydroxymethylation of mixtures of softwood kraft lignin and phenol at molar ratios of 1:1 and 3:1 was studied in the temperature range of 80–120 °C. The formaldehyde source used was paraformaldehyde due to its simple methylene glycol composition and absence of methanol. The reaction products were analyzed by using the lignin-specific 31P and 1H–13C HSQC methods together with the quantitative 13C NMR method. The NMR results indicated that methylolation and condensation of methylols to methylene bridges occurred as a function of the increasing reaction temperature. The reaction at 120 °C using the L:P molar ratio of 1:1 showed hydroxymethylation of both phenol and lignin. However, the same reaction at the 3:1 L:P molar ratio gave fewer methylol and methylene bridge products due to the lower reactivity of lignin. The 13C and 1H–13C HSQC NMR results indicated that the methylene linkages are formed mainly between the methylolated phenols through condensation in the temperature range studied.