Factors influencing the determination of wood contact angles and wettability ― extractives contamination, wicking, and bulk sorption effects

摘要

Studies of wetting phenomena on wood generate important information about the capacity of wood to interact with other materials. Such information may be used to improve wood-polymer interactions and bonding. The objective of this study was to consolidate some experience and issues regarding measurements of wetting properties of wood, and to present a 'reliable' methodology for the determination of wood wettability. A review and discussion about the complexities of the wood material related to wetting studies of wood is presented. This mainly involves the recognition of important factors that specifically influence and complicate wetting measurements on wood. The following three factors have been recognized to strongly influence the determination of pine and spruce wood wettability: 1) the first factor concerns the risk of contamination of the probe liquids by wood extractives. This contamination is caused by the presence or dissolution of extractives at the wood-liquid interface which, in most cases, noticeably decreases the liquid surface tension. It is also indicated that a reorientation of the functional groups of some wood extractives may decrease the apparent surface tension of the liquid; 2) the second factor concerns wicking (or capillary transport) of the probe liquids into void spaces of wood and along the surface structure in the fiber direction, which results in a pronounced non-equilibrium state. It is also possible that an initial inertia effect is present at the moment when a liquid contacts a wood specimen; 3) the third factor concerns the risk of an extensive bulk sorption in the wood substance and resulting swelling effects, especially during wetting analyses of wood particles. To address the difficulties in wood wetting measurements caused by these factors, a modified Wilhelmy method is presented. This new wetting measurement technique is suitable for hygroscopic, porous, and rough materials like wood, and seems to provide an efficient means of determining and controlling the contamination, wicking, and sorption effects.