Dynamic adhesive wettability of wood

Adhesive wettability of wood is usually evaluated by contact angle measurement. Because of liquid penetration and spreading on the wood surface, the contact 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 parameter (K) can be used to quantify the adhesive penetration and spreading during the adhesive wetting process. By applying the wetting model, the adhesive wettability of sapwood and heartwood of southern pine and Douglas-fir was studied. Liquid wettability along and across the wood grain direction was also compared. Two resin systems, polymeric diphenylmethane diisocyanate (PMDI) and phenol-formaldehyde (PF), were evaluated. It was learned from this study that the wetting model could accurately describe the dynamic adhesive wetting process on wood surfaces. Through applying this model, it is shown that PMDI resin exhibited a better wettability on wood than PF resin. The adhesive is more easily wetted along the grain direction than across the grain direction. Species and drop location have no significant effect on the 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 lower instantaneous contact angle than sapwood. Douglas-fir has a greater instantaneous contact angle than southern pine. The effect of species on the equilibrium contact angle is strongly dependent on the location of the drop on the wood surface. The equilibrium contact angle of Douglas-fir is smaller than that of southern pine for sapwood, but is greater for heartwood.