One of the major advances in wood technology in recent years has been the development of modified wood. Various new products are now widely available, offering enhanced durability or improved performance in unique ways. While these products are frequently grouped together as ‘modified wood’ their attributes vary, so each of the main technologies is discussed separately below.
This is a chemical modification technique. The commercial brand name is Accoya, and an acetylated MDF product named Tricoya is also available. The wood is reacted with small molecules of acetic anhydride, which react with the wood to leave its surface polarity altered. The new acetyl groups on the surface effectively block access to the surface by water molecules, which reduces the wood’s tendency to take up water or moisture from the atmosphere. One major benefit is a reduction in the level of swelling seen in damp weather, another benefit is a great increase in durability. Initially the majority of acetylated wood on the market was radiata pine (a softwood), however new products based on hardwoods have been developed recently.
There are many brands of thermally modified wood available, and there are some differences between brands, and in some brands there are two or more products. For this reason it is difficult to generalise about the properties achieved – some products have increased dimensional stability, others have this and a level of durability in exterior environments (but generally not in ground contact). All the products are similar in that heat has been used as a method to alter the chemistry of the wood just sufficiently to reduce the affinity for water molecules. The level of this change is governed by the temperature and other treatment conditions used; with higher temperatures tending to have a greater degree of durability than the lower temperature treatments. Thermally modified timber is also darker than the original wood, and is said to weather relatively rapidly to a silver grey when used in products such as cladding.
Impregnation with resins and polymers
Various treatments have been developed in which a resin or polymer system is pumped into the timber, then cured or cross-linked under controlled conditions to leave a solid polymer or resin within the cell spaces of the wood. Some treatments leave this polymer as a coating on the cell wall, while others tend to be more restricted to the heart of the spaces in the cells. One product is Kebony, in which furfuryl alcohol is polymerised, providing a reduction in the quantity of water taken up by the wood, and an increase in durability. This product is available in different grades, and further information should be sought from the manufacturer. Other products based on phenolic resins or melamine resins have been trialled, and may be established in the market soon. Other techniques such as polymerising acrylic in the wood are also possible. These impregnation treatments frequently increase the hardness of the timber, as well as offering some stability to moisture, and may offer other benefits in terms of durability, depending on product type.