In Europe, acetylated wood carries a Class 1 durability rating under EN 113:1997, Wood Preservatives: Test Method for Determining the Protective Effectiveness Against Wood-destroying Basidiomycetes. This is the highest available and comparable to the best tropical hardwoods. In fact, a recent durability test in New Zealand showed the wood product to be more durable than even old-growth teak.
“We’ve converted a number of different projects from mahogany to acetylated radiate pine, because this is a material that will last like the existing windows,” Gentleman says. “This is a 100-year product.”
The durability and, particularly, stability provide a much more stable substrate for wood coatings. Numerous outdoor and accelerated coating tests demonstrate wood coatings last significantly longer on acetylated wood—film-forming opaque coatings, for example, have been found to endure up to three times longer. Therefore, maintenance needs are dramatically reduced.
“It is a stable substrate for coatings and has good resistance to weathering,” says Roger Rowell, professor emeritus at University of Wisconsin-Madison. “These properties give the acetylated wood a much longer usable lifespan without the use of toxic preservatives.”
Sustainable attributes
Acetylation is an environmentally benign process—no toxins are added to the wood, and the chief byproduct is acetic acid, which is nontoxic and biodegradable. This byproduct is either recycled back into the compound used for the acetylation (i.e. acetic anhydride) or sold into the substantial acetic acid merchant market. Besides a very small amount of natural resins isolated from the wood, there is very little waste, and greenhouse gas emissions (GHG) are minimized.
A recent lifecycle assessment (LCA), conducted by Joost Vogtlander of Delft University of Technology in the Netherlands (and verified by Andrew Norton or Renuables UK), concluded acetylated wood has a much lower environmental impact compared to many commonly used building products, including metals, concrete, and tropical hardwood.
The radiata pine used to make the material can be sourced from Forest Stewardship Council (FSC)- or Programme for the Endorsement of Forest Certification (PEFC)-certified forests. Further, at least one manufacturer has achieved Cradle-to-Cradle Gold certification—a first for wood products under the McDonough Braungart Design Chemistry (MBDC) program.
Acetylated wood can also contribute to multiple credits under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) v4 program, including:
- Materials & Resources (MR) Credit 1, Building Life Cycle Impact Reduction;
- MR Credit 2, Building Product Disclosure and Optimization−Environmental Product Declarations;
- MR Credit 3, Building Disclosure and Optimization−Sourcing Raw Materials; and
- MR Credit 4, Building Product Disclosure and Optimization−Material Ingredient.
Acetylated wood and window replacements
A look at two projects making use of acetylated wood will provide a good sense of how the material can be employed in applications. With respect to window retrofits, the venerable Minnesota State Capitol is undergoing a $272 million, four-phase restoration plan. The project includes restoring the regal exterior designer Cass Gilbert envisioned when the building was completed in 1905.
A major part of the project was the restoration of the capitol’s original wood windows, which had been covered over with aluminum windows 30 years ago. The aging units—242 in total—suffered from fogging, failing glass, air leakage, and broken balances that rendered the windows inoperable, said Gentleman, whose firm was selected to restore the windows.
“They were boxy, and they didn’t look like the original. They didn’t have the same kind of detailing as the original windows. Plus, the glass was failing and the windows were leaking air because the weatherstripping had failed,” he explains.
The aluminum replacements were removed, revealing the original wood window frames, which were then restored using restoration epoxies and replicated wood parts. Gentleman and company used an original complete wood window that still remained in the building as the basis of design for replicating new sashes with acetylated wood, which will last well beyond the glass.
