Closeup of the wood sample from IWF.
How do you get the properties of a hardwood with a softwood? The people at Accoya Technologies think they have an answer.
Accoya wood is made from sustainable softwood, but the finished product is designed to be as durable and stable as tropical hardwoods. The technology behind Accoya wood is based on acetylation, a process that has been studied in a number of countries for dimensional stability, durability, UV resistance, paint retention and in a variety of extreme cold and wet environments, including below ground and in water. According to Accoya, the wood has retained its appearance and has required less maintenance than other materials.
The wood product can be used for windows, doors, decking, cladding and glulam structural beams. Its properties have even led to its use in a heavy traffic road bridge with a special design, and even as a canal siding in Holland. It was removed after 10 years’ exposure to water without showing signs of degradation.
By enhancing the durability and dimensional stability of fast-growing and available certified wood such as radiata pine, Accoya wood also provides environmental advantages over hardwoods, woods treated with toxic chemicals, and non-renewable materials such as plastics, steel and concrete.
Wood acetylation process
The technology behind Accoya is based on wood acetylation. According to Accoya, the process essentially alters the actual cell structure of wood by transforming free hydroxyl groups into acetyl groups. Acetyl groups simply consist of hydrogen, oxygen and carbon and are already present in all wood (ranging from 1 to 8 percent by weight) and can be derived independently from acetic acid (vinegar).
During the Accoya production process, the free hydroxyl groups within the wood are changed into acetyl groups and this reduces the ability of the wood’s cell walls to absorb water by approximately 80 percent, greatly improving the wood’s dimensional stability and resulting in reduced maintenance frequencies for coatings.
In addition, the change in cell structure means that fungi do not recognize Accoya as wood and do not attack it. Insects that attempt to eat the wood die of starvation because the modified structure makes it indigestible, according to Accoya.
Unusual bridge design in Holland employed the treated softwood product.
The wood needs to be kiln dried before acetylation, because the acetic anhydride used reacts with water as well as with hydroxyl groups in the wood. The lower the moisture content of the wood, the more efficient and cost effective the process.
Machining and paintability
Accoya wood is modified all the way through, not just at the surface layer. When Accoya is cut or jointed there are no exposed unacetylated surfaces.
Accoya can be machined in the same way as unmodified wood, and its gluing parameters are the same as for hardwoods. Most commercially available glues can be used with Accoya wood.
Acetylated wood can also be glue laminated and fingerjointed. Wood cannot, however, be acetylated after it has been laminated or fingerjointed as the process would damage glue lines and result in delamination.
Due to the improved dimensional stability of Accoya wood (reduced swelling and shrinking), maintenance of the coating system can be increased, according to Accoya. Collaboration with Sikkens, part of Akzo Nobel, has resulted in a package of guarantees for windows and doors. In addition, tests have shown better coatings adhesion and reduced costs in applying coatings in a manufacturing environment. Teknos, Sherwin Williams and Guard Industrie are testing their systems in conjunction with Accoya.
Acetylation causes no meaningful change to the color of the source species. There is a slight bleaching of red colors and a slight darkening of the outer surface of other wood. This is reported to be insignificant compared to other processes such as thermal modification which deepens the color of wood to dark brown.
Overall, this wood product offers new features for a number of applications. It is available from Royal Plywood, Rex Lumber Co., Atlas Lumber, Universal Lumber Products, Sierra Forest Products, Upper Canada Forest Products.
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