Determination of Wettability of Wood and Its Significance in Wood Science and Technology: A Critical Review

Determination of contact angles on surfaces of wood is an important part of investigations in the field of wood science and technology. Some examples of the significance of wood wettability measurements are given in this review. For instance, without inclusion of wettability studies, it is almost impossible to make serious investigation of wood adhesion and adhesives, of surface finishes, of weathering and of wood surface hydrophobisation. Some basic information on structure of softwoods and hardwoods and their chemistry is given, in order to be able to understand the most common and important problems that appear during measurements of contact angles on extremely non-ideal surfaces of wood. Subsequently, the most common methods that are applied to determine contact angles on wood are listed and discussed. Most commonly used techniques are the sessile drop and the Wilhelmy plate methods. Although, there are much more results on the sessile drop measurements in the literature, it seems that maybe the Wilhelmy plate method could be a better choice for wood as a substrate. In the third part of the paper an overview of various properties and treatment processes, with some specific examples, that influence contact angle values on wood are presented. There is an extremely high diversity of contact angle data in the literature and the apparent contact angles of various liquids on wood have only a relative value when measured within the same set of experiments. It is shown in this review that in most cases it is useless to compare contact angle data on wood, obtained in different laboratories with different wood specimens of the same species.