As pressures on traditional wood protection treatments increase, the industry has become constantly involved in developing viable alternatives, such as the copper-based products like Tanalith E, successfully introduced as alternatives to CCA. Further research is investigating metal-free products based on new non-persistent targeted organic biocides. Non-biocidal treatments, known as ‘wood modification’ technologies, are also being developed.
Wood can be modified in a variety of ways to improve properties like durability, dimensional stability, permeability and strength (notably hardness). Three systems that lead to durability improvements are now commercially available or are at pilot plant stage in Europe.
Acetylation
One way of modifying wood is by impregnating it with a chemical that reacts with the wood. The most advanced of this type is acetylation. Acetic anhydride is pressure impregnated into the wood and heated to produce reaction, leaving acetic acid as a by-product. Process time is estimated at four to six hours.
Acetylated material retains a natural appearance, albeit slightly bleached, and the bulking effect of the chemical causes the wood to swell irreversibly. Durability can be greatly improved and higher levels of modification can protect wood in ground contact. Exactly how acetylation protects wood is unknown, but possibilities are that it reduces moisture content below that required by fungi, that the bulking blocks the passage of fungal enzymes, or that the reaction makes the wood ‘unrecognisable’ to the fungi.
Dimensional stability is enhanced, with high levels of modification reducing swelling and shrinkage by up to 75%. Resistance to insects and UV degradation is also improved. But treatment can cause deformation in material containing refractory heartwood as the sapwood is swollen, while untreated heartwood is not. Untreated portions of refractory species or heartwood are also unprotected as fungi can grow through the treated surface, causing decay. Residual acetic acid is also difficult to remove wholly and can leave a vinegary smell, though processes are being developed to eliminate this. Another drawback is plant cost; the corrosiveness of acetic anhydride means stainless steel has to be used, leading to cost estimates of £15m for a full-size plant. As 100-200 litres of acetic anhydride are needed per cubic metre of wood, processing plants must also be near the chemical supplier. Treatment cost is put at £200-450/m3.
Two pilot plants are currently in existence in Europe, operated by AKBV in the Netherlands and A-Cell in Sweden. Both are planning full-scale plants.
Impregnation treatment
Wood Polymer Technologies AS (WPT) in Norway is working on a process known as furfurylation to improve wood durability. Wood is impregnated with dilute furfuryl alcohol resin in a pressure treatment vessel and then steamed to cure the resin.
Furfurylated wood is dark brown and, depending on the extent of treatment (from 75-250kg/m3), is resistant to decay fungi, insects and marine borers. At higher retentions dimensional stability is also improved.
The disadvantages of furfurylation include a slight residual odour, a lack of impregnation into refractory species or heartwood possibly causing difficulties similar to those for acetylation, and a loss of impact bending strength (toughness) at higher treatment levels.
WPT is building a full-scale plant (10,000m3 per year) estimating lower level treatment costs at £100-160/m3.
Heat treatment
Heat treatment is the only modification process listed here that is fully commercialised. The wood is subjected to high temperatures in the absence of oxygen, causing a permanent change in chemical composition and improving dimensional stability up to 50% for higher temperature treatments. Durability is also enhanced, possibly due to lower moisture content or changes in chemical structure rendering the wood indigestible to fungi. The improvement is less than that achieved by other modifications, but decay susceptible species can attain natural durability class 3 (eg heartwood of pitch pine, Douglas fir, keruing) or class 2 (eg heartwood of western red cedar, European oak). Heat-treated wood is generally not suitable for ground contact and, while insect resistance is improved, it remains susceptible to termites.
An advantage is that treatment occurs throughout the cross-section of the timber, even in the most refractory species. The dimensional stability imparted, plus the removal or setting of resin by the process, improves coating lifetime. The treated wood turns brown in colour, but is not resistant to weathering.
The main disadvantage is strength loss, which rises with increasing treatment/temperature, so the wood is not advised for use in load-bearing situations.
Several technologies have been developed, mainly differing in temperature (160-260OC) and the atmosphere in which the wood is treated (eg nitrogen, steam, oil). Treatment times vary between three and five days for green timber, but can be lower (approximately eight hours) for initially dry timber. Existing processes include Thermowood (licensed by VTT, Finland), Retification (SA NOW, France), Bois Perdure (Bci-MBS, France), PLATO (the Netherlands), and Oil-Heat Treatment (Menz-Holz, Germany).
Heat treatment costs depend on the process, timber section size and extent of treatment, but most estimates are around £60–100/m3.
The alternative treatments listed above offer possibilities for the application of wood products in a wider range of end uses. Some of the disadvantages of modification mean that many markets for modified wood are unlikely to be identical to those for preserved wood. But, the added benefits of modified wood, most notably improved dimensional stability, offer possibilities beyond those of preserved wood and into potential competition with durable hardwoods and non-wood materials.
