Long Lasting Lumber

Wood is perhaps the world's most familiar building material. It's been used for ages because it is readily available, easy to cut and shape, and is relatively strong (depending on the type of wood). Unfortunately it doesn't nearly have the strength or durability of metals, and has swelling, rotting, and flammability problems that other building materials do not. That's why, when cost is no obstacle, wood is not typically the first choice.

But a well understood chemical reaction called acetylation could help to change this and may solve many of wood's apparent shortcomings.

Most types of wood modifications are physical treatments that involve infusing materials into the wood or lacing the surfaces with coatings. Acetylation on the other hand actually changes the wood's chemical composition. In the acetylation process, acetic anhydride reacts with the hydroxyl groups on large molecules in the wood's plant cell walls (e.g. lignin and hemicellulose). The reaction replaces hydroxyl groups with acetyl groups and produces the by-product acetic acid. This same process has been used through the last century for making cellulose acetate (acetylated wood pulp) as material for many different products such as photographic film, wedding dress fabric, playing cards, and cigarette filters.

(Chemical outline of the acetylation process. Credit: C&EN)

The result of acetylation on wood is impressive. Most freshly-cut wood will lose about 10% of its volume when it is dried, and unfortunately it has the potential to regain this size if in contact with enough water. Through the acetylation process, acetyl groups actually spread out the cell wall and restore the wood to its original volume. Acetylated wood is as big as it's going to get; in other words, no swelling. Because of its resistance to moisture, the wood is also quite a bit stronger and more durable. This subsequently helps it resist termite infestation because of its increased hardness.

Strength, durability, decay resistance - all this is great, but what's the catch? Well, acetylated wood treatment has been around for a long time, but various technical and economic limitations have prevented it from taking off.

The technical difficulties of acetylation mainly arise when treating whole pieces of wood rather than fiber or pulp. For starters, there is little uniformity between different pieces of wood, since (thanks to the beauty of nature) every piece is unique. In addition, the surface area for treatment on a block of wood is also minimal, making deep penetration harder to accomplish. In addition to these problems, chemists and engineers have to deal with the acetic acid produced by the acetylation process, which if left untreated can corrode regular steel fasteners and make the wood smell like vinegar.

On the money side, acetylated wood is much more expensive than your typical lumber. For example, a 16-foot board of decking, pressure treated wood might go for ~$15, composite would be around $45, and Perennial Wood (Eastman Chemical Company's brand of acetylated wood) would top nearly $52. That's a pretty penny for quality, and the largely equivocal synthetic materials have proved dominant on the market so far.

However, in a future where consumers and architects begin to move away from non-renewable materials and carbon-intensive industries, acetylated wood may be able to compete. That's what Eastman Chemical Company and a few other firms are hoping for as they focus on particular niches including decking, windows, doors, and cladding. Certainly there is something to be said for using real wood, and it will be interesting to see how this initiative affects the future of building and construction.

(Eastman is focusing on outdoor furniture and decking for its Perennial Wood brand. Credit: Eastman -->)

References

Making Wood Last Forever With Acetylation - C&EN