Wetting behaviors of phenol- and urea-formaldehyde resins as compatiblizers

Understanding wetting behavior and surface coverage of resins on a wood surface is important to obtain satisfactory adhesion and optimize adhesive application for wood composite manufacturing. Sessile and micro-droplets of urea- and phenol-formaldehyde (UF and PF) resins were generated on wood surfaces to observe wetting behaviors using three directional image generation system and atomic force microscopy (AFM). The generated micro-droplet sizes varied in diameter from 1-100 μm and showed differing wetting behavior based on droplet size and surface conditions. Rougher wood surfaces prevented micro-droplet spreading and resulted in higher contact angles. Contact angles along the fiber direction of the earlywood significantly differed from the angles collected across the fiber direction. Sessile droplet models and dimensionless droplet shape factors (DSF) were used to develop the parameters governing the droplet shape changes from a spherical droplet to an enclosing hemispherical droplet for early- and latewood surfaces. Droplet dispersing areas with earlywood showed 111% with UF and 42% for PF faster changes along the fiber surface as compared to across the fiber surface.