Product Review from Environmental Building News
March 1, 2008
PureWood:
Chemical-Free Preserved Wood
The treated-wood industry has been evolving rapidly in the past few years, leaving behind more toxic chemicals like chromated copper arsenate (CCA) and replacing them with a variety of preservatives, some more environmentally friendly than others (see EBN
Vol. 15, No. 8). While all of the other treatment options involve adding chemicals to the wood, a new preserved-wood product, which will be shipped starting in March 2008, relies instead on heat and steam. The product, PureWood, is manufactured by Bay Tree Technologies using a sophisticated thermal-modification process.
In a multistep process lasting 50–60 hours, Bay Tree first heats lumber to about 210°F (100°C) for preconditioning and drying. A second stage of heating boosts the temperature to between 370°F and 480°F (190°C–250°C), which chemically transforms sugars in the wood into forms that cannot be digested by insects or eaten by decay organisms. Then steam conditioning restores some moisture to the wood. By comparison, conventional lumber kilns operate at about 165°F (74°C).
Along with protecting the wood from decay and insect attack, the thermal-modification process makes the wood more stable. Company president Ron Long explained that the heating process removes hydroxyl (–OH) groups from the wood. Water normally adheres to hydroxyl groups, and with these removed, water doesn’t reside in the wood in the same way. “Without the water having a place to go, [the wood] doesn’t swell,” he told
EBN. The process reduces shrinking and swelling by 80%, according to Long—which affects how the product is installed. In decking—the initial market for PureWood—if gaps are desired between decking boards, the wood has to be installed with those gaps in place; shrinkage can’t be depended on to create them. These chemical changes also dramatically reduce checking, according to Long.
Thermally modified wood has been used in Europe since the mid-1990s and is commonly used there for decking, outdoor furniture, highway barriers, and saunas. Thermally modified wood has also come to Canada, where PCI Industries introduced Perdure in 2007 (though the Perdure process is slightly different and involves heat only, not steam). The PureWood technology was developed by the Finnish company Stellac Oy, with which Bay Tree Technologies has a partnership. Stellac Oy also manufactures the high-tech, stainless-steel kilns used in the process.
Bay Tree Technologies is producing PureWood at its first plant, located in Terra Haute, Indiana, using southern yellow pine. Other wood species can readily be treated with the process, according to Long, and a planned plant in the Pacific Northwest will treat Douglas fir. In Europe, aspen is often treated with this process, along with certain hardwoods.
The samples received by EBN show that the wood is significantly darkened, like a hardwood, even though the grain pattern still looks like southern yellow pine. (The wood also has a pleasant toasted aroma, which diminishes over time.) The traditional decking boards (called Sahara by Bay Tree Technologies) are 1¼" (32 mm) thick by 5" (127 mm) wide. The company also produces a European-style decking (Linea) with narrow ribbing on one side and wider ribbing on the reverse; its dimensions are identical to those of Sahara. Both products are produced in two-foot increments from 8' (2.4 m) to 20' (6 m). The Linea line’s ribbing offers a more comfortable feel underfoot and provides a unique appearance (see photo).
After the thermal-modification process, all of the boards go through a high-speed molder that produces the finished dimensions and the ribbed profile on the Linea product. All PureWood boards are suitable for outdoor aboveground and ground- contact applications and carry a 25-year warranty—even in ground-contact. The product can be painted, stained, and sanded just like conventional wood. Because there are no chemicals in the wood, scrap can be burned (depending on local regulations) without creating hazardous air pollution or toxic ash.
PureWood’s lack of chemical additives is an attractive environmental feature, but the high temperature requirements for an extended period of time point to a high energy intensity. Bay Tree Technologies operates five high-temperature kilns at its Terra Haute plant, each able to treat approximately 2,500 board feet (60 m
3) in a batch. These kilns are heated with oil boilers supplemented by a wood boiler that burns milling waste. The company hopes eventually to get 80% of its energy from renewable sources.
PureWood is fairly expensive, likely because of the high energy costs involved. Bay Tree Technologies is positioning PureWood as a high-end decking product, similar in price to the “tropical” composite products, such as Trex Brasilia and TimberTech Earthwood, or to clear redwood, according to Long. This price is significantly higher than that of conventional copper-treated decking. Long says that costs may come down somewhat with competition (as other thermally modified wood products enter the market), but he doesn’t think it will ever be cost-competitive with chemically infused wood.
Bay Tree Technologies recommends that PureWood decks be protected with a UV inhibitor, such as products made by Cabot Stain. Without the UV protection, the wood will weather to a silver-gray color over time. Because of the differences between PureWood and conventional treated-wood products, the company recommends training for installers and is setting up a program to certify PureWood deck builders.
Bay Tree will distribute PureWood through a conventional two-step distribution network; those distributor relationships are currently being established. Stocking dealers—primarily building-supply yards serving professionals—will carry the product. PureWood will also go through a code-listing process with the International Code Council that Long expects will take 18 to 24 months.
PureWood was first announced at the national Remodeling Show in Las Vegas in October 2007 and introduced at the International Builders’ Show in Orlando in February 2008. According to Long, many people are excited for a real wood alternative for decking that is environmentally friendly, and we’re excited to start delivering product to them.” Because Bay Tree Technologies had not begun shipping PureWood by press time,
EBN was unable to reach users about installation.
– Alex Wilson
For more information:
Bay Tree Technologies USA, LLC
Memphis, Tennessee
888-575-4180
www.purewood.net
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Photo: Bay Tree Technologies USA, LLC
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