Accoya, wood made to last 50 years outdoors and 25 years in soil or water

Although it was and is one of the most popular materials used in construction and landscaping, wood is sometimes overlooked because of its low outdoor resistance. With the exception of a few exotic species with very high resistance to decay and which, due to intensive exploitation, have become endangered, wood needs to be protected to withstand weathering and UV radiation. The best outdoor wood protection materials do not last for more than 12-14 years, which is why new methods have been sought to increase its durability, and this has led to its impregnation with various substances. At first, biocides were used, which were effective against mold and insects that affected the wood and lowered its resistance, but were dangerous to humans. That's why the search began for impregnation products that would leave 100% wood natural. We recently presented KebonyThe process of impregnating pine wood with a liquid made from plant debris in Norway increases its external resistance to over 30 years. Now it will be Accoya, a resinous wood treated by an acetylation process, resulting in a natural material with an external resistance of up to 50 years.

What is Accoya

Accoya is a resinous wood treated under special conditions with acetic anhydride. What is acetic anhydride? A substance obtained by dehydrating acetic acid, an acid we all know as vinegar when its concentration is 6 or 9% in water. The treatment makes the wood much more dimensionally stable and greatly reduces its water repellency. The material is produced by Accsys Technologies PLC, based in Arnhem, the Netherlands, which also owns the brand Accoya ®. The wood used for acetylation comes from pine (Pinus Radiata) plantations in New Zealand.

As in the case of Kebony, the treatment transforms a resinous wood, which is considered softer and faster growing, into a much harder and more resistant wood, comparable to that from exotic species with very good outdoor resistance, such as sapeli or ipe. The transformation takes place at the cellular level, but the modification does not affect the properties of the wood as a natural material, only improves them. Accoya is classified as resistance class 1 according to EN 350-2, EN 113, EN 252 and EN 807.

Accoya was first showcased in May 2008 in the UK in a TV show featuring the construction of a house. In the same year it was used to build several road bridges in the Netherlands, the largest of which has two lanes in each direction, a length of 30 m and an estimated lifespan of 80 years.

How the idea of acetylation came about

Now for a bit of chemistry. Don't be scared, I promise it will be short and I will try to be as self-explanatory as possible, as less of a chemist. 🙂 Wood is a non-uniform material, it is not made of simple molecules like water (H2O), say. Its main components are cellulose, a uniform substance of well-determined structure and lignincomplex and non-uniform. These large molecules (chemists call them macromolecules) have many -OH groups (hydroxyl groups), which are very friendly to water. It is they that cause water to be absorbed, and it is water that is responsible for wood's dimensional instability, cracking, warping and rotting.

Hydroxyl groups react readily with acetate groups (syn acetic anhydride), something that has long been known. This is how cellulose acetate, the material used for photographic film in old cameras, was obtained by the reaction of cellulose, which is very rich in hydroxyl groups, with acetic anhydride. This gave rise to the idea of acetylating wood to obtain a less water-friendly product.

To obtain Accoya, the wood is treated with acetic anhydride in autoclaves under controlled conditions of temperature and pressure. In the process, most of the hydroxyl groups are replaced by acetyl groups, resulting in acetylated wood and acetic acid which re-enters the process (from which acetic anhydride is obtained.) The acetyl groups are not water-loving and so the wood no longer absorbs water, becomes more stable, more resistant to cracking and rotting, and therefore more durable. Basically, what happens can be summarized as in the picture below.

Advantages and disadvantages of Accoya wood

First of all, the advantages of Accoya are that it is a material 100% natural and non-toxic, acetyl groups are typically part of wood. Let us not forget that one of the methods of obtaining vinegar is the dry distillation of wood, mhe method also used here many years ago. Accoya scraps can be recycled, burned or composted with absolutely no problem.

Its appearance is similar to untreated wood and it can be mechanically processed like any other wood. Acetylation takes place in the depth, not just on the surface, so there is no need to protect fiber ends resulting from mechanical processing or wood splitting.

The dimensional stability of the material is greatly improved, considered to be up to 3 times higher than that of ordinary wood. The tendency to cracking is also greatly reduced, studies showing that water absorption in wood and cracks are reduced by 75%.

It is resistant to fungal and insect attack. The altered material is not recognized by insects (including termites) as wood and therefore they do not use it as food. Mushrooms also do not do well on Accoya because it does not provide the moisture they need to thrive.

The biggest disadvantage is the high price of the product. The fact that the wood is sourced from controlled plantations in New Zealand (not by clearing forests) and the processing is done remotely in the Netherlands makes the product not an easily affordable one. However, its impressive durability - 50 years above ground and 25 years in soil and clean water - somehow justifies the price.

Although I didn't find this mentioned, I think it is quite difficult to finish. It can be painted, which is also mentioned in website, but I guess not with every product. The low water absorption means that adhesion relies solely on surface mechanical processing, not on the penetration of the material into the wood. Again, this is just my opinion and I would love to hear from someone who has worked with this product, painted or varnished it, how it performed compared to other finished woods.

Where it is used

Accoya is recommended wherever increased resistance to environmental factors and durability is needed. It is used for doors and windows, house cladding, decking or landscaping. It is also used successfully in boat building. Not being a chemically impregnated wood, it can also be used indoors and has no restrictions on use in spaces where children are present.

I hope you find the above information interesting. If you have others or have worked with Accoya, please share your experience with us. And if you have any questions or queries, leave them below in the dedicated space. I will definitely reply.