Untersuchungen zum Einfluss der Feuchtigkeit auf die Haftfestigkeit von Oberflächenbeschichtungen auf Holz

Investigations on the influence of moisture on the adhesion strength of surface coating of wood.

Four different commercially available coating systems have been applied on spruce boards. After application the prepared samples have been conditioned in 3 different environments: standard climate corresponding to 20 °C/ 65 % relative humidity, application of fluid water at the transition zone between wood and coating and in a constant water vapour flow caused by a vapour gradient in the system. Regarding the conditioning procedure a clear dependency with the adhesion strength could be analysed. General increasing humidity causes a reduction of adhesion. The humidity based reduction is related to the loss of material strength of the coating and the reduced adhesion between wood and coating. Further local swelling differences between late and early wood in spruce can induce new forces in the system which creates a shear load in the transition zone wood and coating. The composition of the systems as well as thickness and the number of layers influence the adhesion strength too. Systems with thicknesses of about 130 µm seem to be less strength than thinner ones. But based on practical tests the adhesion strength does not always correspond to the performance and durability of the system.