6 August 2012
Alexander H. Tullo
No building material is as familiar as wood. Most everyone on Earth has walked on it or been sheltered by it. Even the least handy among us has had occasion to form it, cut it, or drive a nail through it. The shortcomings of wood are also familiar. Who hasn’t treaded gingerly on rotting planks or struggled with a window swelled by dampness?
Acetylation, a chemical reaction first employed more than a century ago, may solve these problems and make wood a building material of the future as well as of the past. After many attempts over the years to commercialize acetylated wood, a host of firms believe the material’s time has come.
Unlike pressure treatment, in which preservatives such as ammoniacal copper quaternary compounds are infused into wood, acetylation chemically modifies wood. In the process, acetic anhydride reacts with the hydroxyl groups on large molecules such as lignin and hemicellulose in the plant cell wall. The reaction replaces hydroxyl groups with acetyl groups and yields acetic acid as a by-product.
It’s the same chemistry employed since the early part of the last century for making cellulose acetate—acetylated wood pulp—for photographic film, wedding dress fabric, cigarette filters, and playing cards.
Several firms are ramping up output of acetylated wood and are seeding the market with product. Eastman Chemical, one of the world’s major cellulose acetate makers, is acetylating wood at its Kingsport, Tenn., chemical complex. The lumber is being sold under the Perennial Wood brand name at 50 Lowe’s stores in New England. Accsys Technologies develops acetylation technology, which it is licensing to third parties, and also acetylates wood in Arnhem, the Netherlands.
These companies are betting that consumers are willing to pay for the improved wood properties—namely better dimensional stability and decay resistance.
Roger Rowell, professor emeritus of biological systems engineering at the University of Wisconsin, Madison, explains that newly harvested “green” wood will lose about 10% of its volume when it is dried before being cut into lumber. Introducing acetyl groups stretches out the wood’s cell walls again, locking the wood back into its original volume. “It is about as big as it can get,” he says. “Water doesn’t swell it.”
Acetylated wood has the strength of regular wood that is very dry, Rowell says. For every 1% gain in moisture, wood loses about 5% of its modulus of rupture, a measurement of how a material bears a large load. “Is acetylated wood stronger?” he asks. “Yes, by quite a bit, because it is drier.” Similarly, acetylated wood resists termites partially because it is harder than unmodified wood.
Acetylated wood isn’t hard to work with, its backers say. “All the construction techniques that you can use with wood, you can use on acetylated wood,” says Jimmy Dickerson, Eastman’s technology manager for acetylated wood.
However, Eastman recommends using stainless steel fasteners with acetylated wood because residual acetic acid would corrode regular steel. The acetic acid can also make the wood smell like vinegar, something that acetylated wood producers can reduce by wicking out excess acid and applying a finish to the wood.
There are two main schools of thought as to why acetylated wood resists decay. One is that acetylation changes the conformation of the substrate that fungi are trying to eat. The fungal enzymes that break down the wood don’t recognize it as food.
Rowell is a proponent of another theory based on research he did in Sweden 20 years ago. Again, dryness is the key. “Acetylation reduces moisture in the cell wall to the point where there just isn’t enough to support fungal degradation,” he says. “In other words, in order to hydrolyze a glycosidic bond, you have to have a molecule of water. And if you don’t have water at the glycosidic bond, hydrolysis can’t take place.” Glycosidic bonds link carbohydrate molecules in polysaccharides such as cellulose.
If acetylation imparts such benefits, why didn’t it take off decades ago? Rowell notes that patents covering acetylated wood do date back to the 1930s, but the handful of companies that subsequently attempted to introduce it had little luck. For example, Koppers tried launching a product in the 1960s. Japan’s Daiken introduced acetylated wood flooring in the 1980s. And firms involved in acetyl chemistry such as Eastman and BP have long dabbled in the technology.
Observers cite technical and economic reasons for why acetylation hasn’t been viable until now. Although the process involves chemistry similar to that used to make cellulose acetate, there are subtleties to using whole wood, Eastman’s Dickerson notes.
“One of the beautiful things about wood is its uniqueness. Every piece of wood is different,” Dickerson says. “That same character is a nightmare for a chemist.” Additionally, producing cellulose acetate involves acetylating a well-dispersed fiber, he says. A block of wood, on the other hand, has much less surface area. For aceytlation to be beneficial, the reaction needs to penetrate deep into the wood.
Another difference, Rowell points out, is that cellulose acetate is made with a catalyst such as sulfuric acid. But acetylating cellulose in wood is undesirable because cellulose is the source of the wood’s strength. In order to relegate the reaction to the lignin and hemicellulose, commercial wood acetylation processes tend not to use catalysts, Rowell says.
Acetylated wood also isn’t cheap. At Lowe’s, a 16-foot board of premium pressure-treated decking goes for $14.57. The same board of Trex plastic composite decking can cost up to $44.97. A Perennial Wood board will set the consumer back $51.74.
Eastman’s Dickerson says consumers are becoming used to paying more for materials such as composite lumber because of their durability. “The synthetic materials that are out there have largely proved a price point, particularly in decking,” he says.
Paul Clegg, chief executive officer of Accsys, says alternative materials have raised the bar in other applications as well. For example, wood was once the dominant material for building windows. Today in the U.K., some 90% of the windows installed in new structures are made of aluminum or polyvinyl chloride, which don’t swell as wood does. “The PVC and aluminum industries have done a fantastic job in raising the level of expectation,” he notes.
Yet architects and consumers increasingly want renewable construction materials, Clegg points out. They are also shying away from using materials from carbon-intensive industries, such as PVC and aluminum.
Eastman and Accsys are approaching the acetylated wood market differently. Eastman is initially focusing on decking materials. Dickerson says the company is also gaining traction in outdoor furniture and porch flooring markets.
Rather than license the technology to a wood company, Eastman has decided to manufacture and market acetylated lumber itself. Dickerson says it is easier to bring wood onto a chemical site than it would be to duplicate the chemical infrastructure in a wood processing plant.
Since 2007, Accsys has run a plant in Arnhem, where 12.5-meter-long tube-shaped pressurized reactors can create up to 40,000 m3 of its Accoya acetylated wood per year. But the company’s long-term business model is to license its technology to third parties.
Last month it agreed to license the technology to Solvay’s Rhodia Acetow unit. By the end of 2014, Solvay expects to have built a 63,000-m3 Accoya plant, likely in Freiburg, Germany, where it already makes cellulose acetate. Accsys is also forming a joint venture with chemical maker Ineos to further develop and license its recently commercialized Tricoya technology for making boards from acetylated wood chips.
Although Accsys, like Eastman, is looking at decking, Clegg says windows, doors, and cladding are higher priority applications, largely because raw materials are a small part of the cost of such products. “For a window, wood is 15% of the cost, but it is 90% of the performance,” he says.
Clegg believes that consumers will eventually pay for the performance, but he also recognizes that the construction market is conservative. “The wood industry is not the semiconductor industry,” he remarks. “We don’t go from invention to obsolescence in 18 months.”