Irrespective of the wood preservative used or specified, when working with pressure-treated wood, it is advisable to follow the safe practices listed on the end tag or refer to proper association guidelines.
Modified wood undergoes a “non-biocidal modification to improve its properties, such as strength and hardness and resistance to water, swelling, and biological agents,” as described by Evans.
The two chemically modified wood processes that have been commercialized in the last 15 years are furfurylation and acetylation. Where preservatives add a biocide to wood, furfurylation and acetylation chemically change the molecular structure of the material. Both of the options take fast-growing softwood or hardwood and through a proprietary process, the wood becomes harder, decay resistant, and in some instances, insect resistant and dimensionally stable. Even more attractive is that both processes are non-toxic and offer sustainable benefits.
Furfurylation has been studied for more than 50 years and is beginning to achieve market acceptance. The process uses furfuryl alcohol and catalyst(s) to penetrate and fill the wood cell walls or lumens with a resin-like plastic. The wood is then heated to just above 100 C (212 F) to harden the resin and finally dried.
While it can be used for interior applications, wood treated by furfurylation is typically employed for exterior applications and one manufacturer offers a 30-year warranty for decking and cladding products only. ICC has approved it for above-ground use only in decking applications. Canadian Standards Association (CAN/CSA) 080, Wood Preservation, is used to evaluate the durability of wood. Currently, there are no Canadian standards for chemically modified wood.
Acetylation, studied for more than 90 years, was first commercialized in 1997 in Europe. The process chemically alters the wood’s free hydroxyls into stable acetyl groups. Acetyl groups are naturally present in all wood species, which means nothing toxic is added. The altered cell structure of the wood makes it an unrecognizable food source for insects, including termites, and prevents fungal decay.
Only one manufacturer has successfully commercialized acetylated solid wood. It is warrantied for 50 years above ground and 25 years below ground, and swelling and shrinkage are reduced by 75 per cent or more. The manufacturer also meets ICC requirements for above-ground or ground-contact applications requiring protection against fungal decay or termites as described by the code.
The pressure treatment method with preservatives only penetrates the envelope or perimeter of the board, often only a few millimetres deep. These treatments do not modify the cells of the wood like chemically modified wood processes.
Acetylated wood has been used on a number of projects in some of the most extreme applications, such as a water-submerged pedestrian bridge and marine docks including canal linings in the Netherlands, as well as coastal siding, outdoor furniture and cabinetry, windows, doors, and cladding on commercial buildings.
Chemically modified wood uses raw material sourced from forests certified by the Forest Stewardship Council (FSC). It is a sustainable alternative to tropical hardwood.
Future of wood technology
Several new wood product technologies are currently being explored. It is, however, a long process to create new products. For example, acetylation of wood was first attempted in the 1920s. The United States FPL studied and worked with it in the 1940s. It was not commercialized until the late 1990s.