Building Green With Modified Wood: Using acetylated wood to take sustainability to new levels

Photos courtesy Accoya Wood

By Pablo van der Lugt, PhD
Wood’s renewability, ability to sequester carbon (CO2), and natural origins are among the features making it an attractive alternative to synthetic materials. At the same time, new technologies such as cross-laminated timber (CLT) have expanded the number of applications with which wood can be used.

Within the category itself, there have emerged even more sustainable options—wood products that improve even further on existing materials without sacrificing performance or longevity. Among those is acetylated wood, which offers the performance and protection of chemically treated wood products without anything toxic added, making it a sustainable choice for windows, siding, decks, and nearly any other exterior application, in-ground or above.

One of several methods for ‘modifying’ wood, acetylation has been studied for more than 90 years, becoming available as a commercial product during the last decade. The acetylation process targets two different elements already contained in the wood—free hydroxyls and acetyl molecules. Free hydroxyl chemical groups readily absorb and release water; they are what causes wood to swell and shrink as temperatures and humidity levels rise and fall. Acetyl groups, also naturally present in the wood, are more stable and do not bond with water.

The acetylation process replaces the free hydroxyls with the acetyl groups. The modified wood, now with a higher level of acetyl molecules, does not respond to moisture like traditional wood does, thereby improving the shrink/swell cycle by 75 per cent or more. The nontoxic modification also makes the wood unrecognizable as a food source, protecting it against rot, decay, and insects. The process changes wood at its molecular level by increasing the natural compounds already found there—nothing is added to the wood that is not naturally present.

The acetylation process alters the wood through to its core, so it can be cut, routed, or otherwise exposed without requiring sealing or other protection. Acetylation is nonreversible, so there is no risk of leaching or loss.

Once the wood is modified, it can be used in a range of exterior applications without treatments, sealants, stains, or paints (though it can be painted or stained if desired for appearance purposes). It is suitable for applications including decking, cladding, windows, doors, and various outdoor furnishings.

Acetylated wood siding helped the Tashjian Bee Pollinator Center at the University of Minnesota achieve a modern take on the barn-like esthetic while contributing to the structure’s various sustainability-related attributes.

Sustainability across the life cycle
By significantly enhancing the durability and dimensional stability of fast-growing, abundantly available certified wood species—such as radiata pine—calling for acetylated wood can reduce environmental impacts associated with scarce, slow-growing hardwoods or forest products treated with chemicals, as well as nonrenewable carbon-intensive materials such as plastics, steel, and concrete.

When evaluated per life cycle phase, acetylated wood demonstrates the following characteristics from environmental, performance, and human health standpoints.

Production phase
Besides forming a natural ecological habitat, forests are actually an important carbon sink, filtering CO2 out of the air and absorbing it in the biomass of trees. In the case of responsible management, it is actually advised to harvest as much wood as is sustainable for production of durable products such as furniture or construction products, which act as additional carbon stores while providing the forest the opportunity to regenerate and produce new biomass.

However, supplies of certified slow-growing tropical hardwoods suitable for outdoor use are limited, resulting in illegal logging, which in turn leads to the deforestation of tropical rainforests. In tropical regions, deforestation is still continuing, but in temperate regions such as Europe and North America, the net forest area has been increasing steadily for several decades thanks to a focus on sustainable forestry and timberland management.

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