How techniques and chemicals can significantly affect the lifespan of wood

Materials World magazine
1 Feb 2014

Wood preservation techniques and products can have a significant impact on the lifespan of a product. Dr Chris Coggins of the Wood Protection Association outlines the associated legislation.

The approval of wood preservative products has recently been through a transition. The national system of approvals – called the Control of Pesticides Regulations (CoPR) moved via the EU Biocidal Products Directive (BPD) and culminated in the introduction, in September 2013, of the Biocidal Products Regulation (BPR).

The new system involves approval of active ingredients at EU level and subsequent approval of wood preservative formulations at national level, with the possibility of mutual recognition of approvals between member states of the EU. A number of wood preservatives have already gone through the EU system and are on the UK market. While the BPR is current and entirely replaces the BPD, there are still a few CoPR products on the market as the system catches up.

One of the impacts of the raft of legislative changes relates to the approval of chromated copper arsenate (CCA), which ceased in September 2007. Wood already on the market treated with CCA could be sold and used as permitted (for example, not inside buildings), but it eventually vanished from the supply chain. The BPR introduces controls over treated wood – for example, it is now illegal to import CCA-treated wood into the EU, even for those uses that are, in theory, still approved. Wood treated with CCA can remain in place and second-hand CCA-treated wood could be supplied for permitted end uses, but this rarely happens and eventually, CCA-treated wood will disappear as it comes to the end of its useful life.

In the run-up to September 2007, preservative manufacturers converted many customers to copperand chromium-based preservatives that could be used in CCA plants without compatibility problems. These included copper/chromium (CC), copper/chromium/ boron (CCB) and copper/chromium/phosphorous (CCP) types and were used for about a year while the changeover to copper-organic preservatives was completed. Copper-organic preservatives combine copper with organic biocides such as propiconazole, tebuconazole and quaternary ammonium compounds, and some have boron added. Boron continues to be approved for wood preservation despite the recent change in classification. Other types of copper and organic fungicide water-based preservatives have reached the UK market within the last year or so, and all have been tested and CoPR/BPR approved.

Chromium-based preservatives disappeared from the UK market by early 2008 because chromium was not supported through the BPD process. The UK took FEATURE the view that chromium was an active ingredient, so preservatives containing it had to be withdrawn. Germany, France, the Netherlands, Latvia, Spain, Poland and Sweden continued to allow the use of chromium-containing preservatives, arguing that it is a fixative rather than an active ingredient.

Correct treatment
About four years ago, The Wood Protection Association began to get reports of early failure of treated wood in ground contact, mostly fence posts. These all turned out to be due to poor treatment, particularly on resistant species such as spruce. Examples included CCA, the CC types and the newer copper-organic types, and this triggered quite a crisis of confidence in the market. Investigation into a number of failures showed that all cases were due to inadequate treatment (poor penetration and low loadings). Although the jury is still out on this, there is a feeling that CCA was so effective that it was forgiving of poor practice, and even badly treated posts performed reasonably well. While other preservatives work, as has been proven by laboratory and field tests, it is vital that treatment complies with the requirements for penetration and retention set out in BS 8417. If the treatment process is carried out correctly, treated wood will deliver the performance expected of it.

To help overcome this crisis of confidence, WPA introduced a new scheme – called Benchmark – where independent auditors check that plants are treating the wood properly. The auditors examine treated wood as well as areas such as moisture control, solution concentration checking and traceability of treated packs as part of the overall quality check. This goes beyond what is covered in ISO 9001 audits and is being well received in the industry and the markets. Some companies have brought in new incising equipment and have started incising their ground contact wood so that the required penetration can be easily achieved. Benchmarked treated spruce is now on the market for the use class four, 15-year service life category.

For use in construction, wood may be treated in pressure plants (vacuum/high pressure) and in double-vacuum plants. Pressure treatment is used for many outdoor and all in-ground uses. In both cases, it is important that the use class from BS 8417 is specified when treated wood is ordered or purchased. For repair work in buildings, use class two (interior, risk of wetting) is probably the best, and this will include effectiveness against fungi and insects. Exterior, above-ground work is use class three (coated/uncoated). Almost all class three and four preservatives provide protection against both fungi and insects. Buying treated wood with Benchmark certification provides a high level of confidence in performance.

Users of treated wood products should be sure that the correct level of treatment has been supplied. Unfortunately, some still regard green wood as suitable for any application, including ground contact. Furthermore, during fixing and installation, untreated wood may be exposed by cross-cutting. The treated envelope must be maintained to keep up the correct level of protection by applying preservative to the cut surfaces, but all too often this is ignored. Specifying the appropriate level of treatment required and understanding the need for best practice in its use are the keys to obtaining the required performance.