Acetylation

[edit] What is acetylation?

Acetylation is a chemical process, a reaction which introduces an acetyl functional group into an organic chemical compound ie substituting acetyl for a hydrogen atom. Deacetylation involves the removal of an acetyl group from an organic chemical compound.

In construction acetylation is most commonly associated with modified wood products, where the structure of wood is permanently modified to improve its resistance to decay and movement. This is different to timber treatment which generally refers to surface treatment with some soaking in.

[edit] Acetylated wood

Wood contains chemical groups called free hydroxyls, these absorb or release water which make timber hygroscopic, which is why wood expands in winter and shrinks in summer and also why it can be prone to decay. Accetylation transforms the free hydroxyls into acetyl groups by reaction with acetic anhydride (acetic acid), which is why timber modified in this way often smells vinegary, as vinegar contains 4% acetic acid. Once transformed the cell wall is filled with chemically bonded acetyl groups that fill the voids, so the potential for water absorption is reduced, which means it is more stable and less prone to decay.

The images above show a community raised bed in North London built by RoWCA community group and sponsored by Accoya timber. The left images show the newly installed bed made from acetylated wood in 2009 and the right images show the same bed 13 years later in 2022. The structure of the wood remains in tact and solid, with no signs of rotting, the grain of the wood is more exposed and the colour has greyed as with most timbers.

[edit] Characteristics of acetylated wood

The process does however make the wood more corrosive to metals because of the acid, which means fixings can corrode unless they are stainless steel. The wood is normally darker and only sap wood should be used (not the heartwood), the vinegary smell is noticeable when it is cut. As moisture is not absorbed and released into the material cells it can lie on the surface and in some situations create surface mould, though it does not compromise the integrity of the wood and can normally be cleaned away. The lifespan of most softwoods treated in this way is up to 50 years above ground and 25 years below ground or in moist conditions, which is significantly higher than it would be otherwise.

[edit] History of acetylated wood

In the late 1920's the first experiments in the acetylation of wood were carried out by Fuchs in Germany using a sulphuric acid as a catalyst, Horn at the same time acetylated beech wood. The first scientist to described the process as a way to stabilise wood was Tarkow. Since then various approaches with different catalysts and wood species have been trialled, along with commercialisation attempts in the USA, Russia and Japan in the 60's, 70's and 80's, but high costs remained restrictive (Rowell 1983, 1984, 2012). In the 90's Professor Miltz gained some success with a semi-industrialised process in the Netherlands but it was not until 2007 that a well-known company made its first commercial batch of treated wood in the Netherlands. Today most acetylation reactions are carried out without the use of a catalyst (Rowell 2012; Larsson-Brelid 2013).

[edit] Related articled on Designing Buildings

• Furfurylation.
• Hardwood.
• Petrification.
• Softwood.
• Sustainable timber.
• The differences between hardwood and softwood.
• Timber.
• Timber treatment.
• Timber vs wood.
• Types of timber.

[edit] External references

• Mantanis, G. I. (2017). "Chemical modification of wood by acetylation or furfurylation: A review of the present scaled-up technologies," BioRes. 12(2), 4478-4489.
• https://www.accoya.com/uk/why-accoya/

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