Capillarity in hardwood: An important factor for hardwood market expansion

The Swedish hardwood sawmills are today experiencing problems of finding profitable markets for products containing for instance knots and red or brown heartwood. By using hardwood outdoors, it would be possible to expand the market of hardwood with new products that may set other requirements on the wood than the present use. In Sweden today, hardwood is normally used in furniture, joinery, floors and kitchens where esthetical requirements are dominant. When considering outdoor use, technical requirements such as durability become more important and especially the capillary characteristics of the wood are of interest. The capillarity determines the ability of the material to absorb and desorb moisture, characteristics that affect the resistance of degrading. In this paper, the capillary characteristics were modelled and studied by exposing the cross section of wood to a free water surface. The study compares two Swedish hardwood species, aspen and oak with pine, a softwood species that is common in Sweden and often used outdoors. The results show that the capillary transport mechanism is affected by the microstructure of the wood and the natural ability of the material to close the transport paths in the structure with for instance extractives, tyloses and closed pits. Based on these factors, a characteristic capillary mean radius is introduced to describe the longitudinal capillary water absorption in the studied wood species.