US EPA

4408403 

Journal Article 

Adhesive Wettability of hydroxymethyl resorcinol (HMR) treated wood 

Gardner, DJ; Tze, WT; Shi, SQ; FPP; FPP 

2001 

321-327 

WOS:000186182900046 

The objective of this study was to examine the adhesive wettability of wood treated with hydroxymethyl resorcinol. (HMR) coupling agent which is known to enhance wood bonding. Adhesive wettability was determined by measuring contact angle changes as a function of time for phenol formaldehyde (PF) and polymeric diphenylmethane diisocyanate (pMDI) on southern pine and Douglas-fir treated with HMR. The contact angles of the adhesive on the wood were measured at different time intervals to provide data for the calculation of the adhesive penetration/spreading parameter simply termed the K-value. The surface free energy of HMR treated and untreated wood was determined from the contact-angle data of probe liquids: water, formamide, ethylene glycol, and methylene iodide. The results of the study reveal that HMR treatment decreases the adhesive wettability (contact angles) and the rate of wettability (K-values) of wood. Southern pine adhesive wettability is affected to a greater extent (both contact angles and K-values) than Douglas-fir. The pMDI adhesive, with its lower surface tension, exhibited better wettability than PF resin. The wood surface characterization study indicates that HMR treatment reduces dispersive (non-polar): surface free energy, but increases the polar surface free energy. The decreased adhesive wettability as evidenced by increased contact angles and lower K-values was correlated to the dispersive surface free energy which decreased as a result of the HMR treatment. The results of this study imply that the bond enhancement of HMR treatment can not be attributed to improved adhesive wettability. It is likely that the increase in polar surface free energy promotes improved interaction between the HMR treated surface and the adhesive to form strong secondary bonding interactions and the possible formation of covalent bonds during adhesive curing.