Surface deactivation of wood strands due to drying at elevated temperatures was evaluated by different techniques. The ability of resin to spread over the strands was estimated by measuring the contact angle between the resin droplet and the strand surface. Contact angles were measured by the Wilhelmy tensiometry balance and by the Sessile drop goniometer methods. Electron spectroscopy for chemical analysis (ESCA) was also used to follow chemical changes of the strand surface in terms of carbon and oxygen content and in terms of oxygen to carbon (o/c) atomic ratio as a function of the drying conditions. Three series of strands were used in this study: multiple-pass dried strands, conveyor-dried strands, and lab-made strands. The first two series of strands were industrially produced ready and dried for panel production. Drying schedules varied from freeze-drying to temperatures of 200°C. Wilhelmy advancing contact angles (2a) as measured on the mill dried strands increased with the increase of post drying temperature while the receding angle (2r) decreased with the first series of strands but increased with the second series. ESCA analysis of these same samples showed a transportation and deposition of extractives from the inner layers to the outer surface of strands. Surface oxidation due to the high temperature drying conditions could also contribute to this surface chemical composition variation.
Sessile drop results from the lab-made strands showed excellent correlation with resin viscosity, contact angles for any wood type increased in the order of the resin viscosity i.e. PF face (110 cP)< PF core (170 cP)< UF (320 cP). Contact angles also increased in the following order for any given resin: southern yellow pine