Quantitative measurements of capillary coating penetration in relation to wood and coating properties

In order to study quantitatively the relations between the properties of waterborne coatings and their penetration, two techniques have been developed and applied to five different pigmented paints and unpigmented binders. The first technique measures the depth of penetration into the axial direction, the second one measures the change of contact-angle and volume of a coating droplet as a function of time. The low-shear viscosity of the pigmented paint has a strong influence on the degree of penetration; surface tension seems to be less important as long as the coating wets the cell capillary fairly well. The diameter of the wood cell also has an influence on the degree of penetration part although both negative and positive correlations were found depending on the coating type. From the impact of wood moisture content and the differences between binders with comparable viscosities it can be assumed that the rate of transfer of water from liquid coating to cell wall and the subsequent increase in viscosity has a strong impact on penetration. Experimental results have been compared with calculations based on theoretical models for capillary penetration. In some cases the predictions were fairly adequate, but strong deviations were also obeserved.