Thermal behavior of hydroxymethylated resorcinol (HMR)-treated maple veneer

The objective of this research was to study the effect of hydroxymethylated resorcinol (HMR) treatment on the thermal and dynamic mechanical properties of maple veneer. The veneers were soaked in HMR solution for either 1, 15, or 30 min, and subsequently dried for either 1, 12, or 24 h at 20°C and 65% relative humidity. Dynamic mechanical thermal analysis (DMTA) tests were performed at a controlled heating rate of 5°C/min using a 3-point bending mode at an oscillatory frequency of 1Hz and an oscillating dynamic strain of 0.01%. Differential scanning calorimetry (DSC) was performed from -40 to 150°C at a heating rate of 10°C/min. Depending on the amount of drying, the storage moduli of wood can be unaltered or reduced as a response to HMR soaking time. Overall, there was no evidence that HMR treatments reinforce wood. The lignin glass-transition temperature of HMR-treated maple veneer decreased with an increase in treatment time. The lowering of T_g by HMR treatments was confirmed by DSC results. Both DMTA and DSC data showed a glass-transition shift of wood hemicellulose that was subtle or none in responding to HMR treatments. HMR was theoretically determined to have a closer solubility parameter match (better compatibility) with lignin compared to the other wood cell-wall polymers (i.e., cellulose and hemicellulose). Based on these findings, HMR is postulated to act as a lignin plasticizer. This study provides new insights into the interactions of HMR with wood and is expected to stimulate further investigations that lead to a better understanding of the wood bond durability enhancement of HMR treatment.