Farm Forestry New Zealand

Accoya wood An industry made for New Zealand

Denis Hocking, New Zealand Tree Grower November 2020.

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 Radiata pine plantations have long been the mainstay of our forest industry and have provided a very productive source of a versatile timber. While many may argue that we are over-dependant on radiata, and that includes me, there is no denying its value to the country.  However, the wood does have deficiencies − most notably its lack of natural durability, limited stiffness and in younger wood, potential lack of stability.

Durability has been cured by using various chemical treatments, most notably copper, chrome and arsenic commonly referred to by the acronym CCA or the trade name Tanalith. However, the use of such toxic chemicals creates other problems, notably disposal of such treated timber at end of life. Stability and stiffness are more difficult to correct but are greatly improved by growing trees on longer rotations of 30 years or more.

Durability and improved stability

Recently there have been significant advances in a technology which offers excellent durability and improved stability for softwoods using low environmental impact chemical treatment. This may sound counter-intuitive – how do you stop bugs with benign chemicals? The answer, or at least one very useful answer, is acetylation.

Chemists have been acetylating wood in various ways for 100 years, including work at Scion, but recent advances in cheaper synthesis of the acetylating agent, acetic anhydride, has made this a very competitive process. They do not seem to be sure exactly why acetylating the hydroxyl groups of the cellulose, hemicellulose and lignin in the wood cell walls makes wood so durable. It could possibly be because it makes these polymers inaccessible to the normal degradation enzymes or perhaps because it excludes water from the wood structures, or both. It does turn radiata pine and other softwoods into class one, top durability timbers with greatly improved stability.

Substitute for hardwoods

Meanwhile the acetyl molecule, the key component of vinegar and a universal metabolic intermediate in all living cells, presents nothing like the threat of copper, chrome and arsenic. This new acetylation technology
is owned by the British Accsys Group and the product, known by the trade name Accoya wood, is now being used quite extensively in Europe.

Uses range from canal linings to motorway bridges, decking, exterior joinery and numerous others. In many of these roles Accoya is substituting for tropical hardwoods and in others it is replacing carbon dioxide
intensive steel and concrete. Another good point for Accoya wood is its stability. It is very stable even when painted black and exposed to full sun.

Pruned pine is best

There are plans, or perhaps hopes, to expand into the United States market. The reservations centre on the question about whether there is a suitable wood resource to support such expansion. This is where it gets very interesting for New Zealand’s forest industry.  Radiata pine is regarded as the best timber in the world for this process, although not just any radiata pine. They want clear, knot-free radiata sapwood. Finger-jointed
material is not an acceptable substitute.

In other words, they want pruned, well-grown radiata pine logs. The best place for pruned radiata pine is right here in New Zealand where pruning has been very much part of forest management for more than 40
years. At present a number of New Zealand sawmills are supplying significant volumes of clearwood to Accsys and these mills are paying top dollar for good pruned logs.

As you would expect, there are problems with this very attractive prospect. The majority of corporate forest growers have virtually given up pruning because the accountants who control them decided the premium for
pruned over unpruned logs had dropped below the $60 a tonne required to justify pruning. There has been a general move to quantity over quality for forestry, driven largely by the currently buoyant Chinese market for low grade logs.

A long-term industry seems to have been overtaken by short-term thinking. However, the $60 premium for pruned over unpruned logs has now returned, and over the last three years pruned log prices have been very
stable, whereas export log prices have fluctuated widely.  Looking to the future we need to remember logs exported to China are mostly used for concrete boxing, and probably have limited long term prospects. By
contrast, value added clearwood Accoya with a myriad of uses should have much better prospects in a ‘post concrete pouring economy’.

There will be an inevitable hesitation in investing in wood acetylation if Accsys is not confident that an adequate resource of suitable pruned logs will be available. We can provide such a resource if we get out and carry on pruning and thinning trees. In the process we can help ensure better more diversified returns for land owners, good employment prospects and more vibrant rural communities. Forestry is already a much
better export earner than sheep and beef farming on a per hectare basis and the extra value offered by an Accoya chain should further lift forestry returns.

Denis Hocking is a member of Middle Districts branch.