Solution‐state NMR analysis of hydroxymethylated resorcinol cured in the presence of crude milled‐wood lignin from Acer saccharum

Abstract

ABSTRACTResorcinol‐formaldehyde adhesives can reinforce stress fractures that appear from wood surface preparation. Researchers have found that applying the resorcinol‐formaldehyde prepolymer, hydroxymethylated resorcinol, to the surface of wood improves the bond strength of epoxy and polyurethane adhesives to wood. Hydroxymethylated resorcinol is thought to plasticize lignin components and stabilize stress fractures through reactions with lignin subunits and hemicelluloses in wood. In this study, a dilute solution of hydroxymethylated resorcinol (HMR) is cured in the presence of a crude milled‐wood lignin (cMWL) from Acer saccharum and subsequently dissolved in dimethylsulfoxide‐d6 to delineate reactivity with lignin and O‐acetyl‐(4‐O‐methylglucurono)xylan using solution‐state NMR spectroscopy. 1H–13C single‐bond correlation NMR experiments revealed that the HMR only formed 4,4′‐diarylmethane structures with itself in the presence of the cMWL; the 2‐methylols that formed remained free and did not crosslink with resorcinol. Cured HMR resin formed both 4,4′‐ and 2,4‐diarylmethane structures, confirming that the presence of lignin and O‐acetyl‐(4‐O‐methylglucurono)xylan hinders crosslinking at the C‐2 position. No evidence of reactivity between HMR and lignin subunits was found. New peaks consistent with ester linkages were observed in 13C‐NMR spectra of the cMWL sample treated with HMR that may be attributable to HMR moieties condensing with glucuronic acid substituents. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45398.