Dynamic adhesive wettability of wood

Adhesive wettability of wood is usually evaluated by contact angle measurem ent. Because of liquid penetration and spreading on the wood surface, the c ontact angle changes as a function of time. In this study, a wetting model was developed to describe the dynamic contact angle process in which a para meter (K) can be used to quantify the adhesive penetration and spreading du ring the adhesive wetting process. By applying the wetting model, the adhes ive wettability of sapwood and heartwood of southern pine and Douglas-fir w as studied. Liquid wettability along and across the wood grain direction wa s also compared. Two resin systems, polymeric diphenylmethane diisocyanate (PMDI) and phenol-formaldehyde (PF), were evaluated. It was learned from th is study that the wetting model could accurately describe the dynamic adhes ive wetting process on wood surfaces. Through applying this model, it is sh own that PMDI resin exhibited a better wettability on wood than PF resin. T he adhesive is more easily wetted along the grain direction than across the grain direction. Species and drop location have no significant effect on t he spreading and penetration rate (K-value). However, the interaction term between species and resin type shows a significant effect for the K-value. PMDI exhibits a greater K-value on the Douglas-fir surface, while PF resin shows a greater K-value on the southern pine surface. Heartwood shows a low er instantaneous contact angle than sapwood. Douglas fir has a greater inst antaneous contact angle than southern pine. The effect of species on the eq uilibrium contact angle is strongly dependent on the location of the drop o n the wood surface. The equilibrium contact angle of Douglas-fir is smaller than that of southern pine for sapwood, but is greater for heartwood.