Summary
• Wood modification has been defined as the alteration of timber at the molecular level to achieve a desired property change.
• Timber can be modified using heat or chemicals.
• Densification involves hardening of the timber, but there is no molecular change.
• Modified timber is particularly suitable for outdoor and joinery applications.

The term modified timber has been around for many years but in-depth discussion of the wood science involved in the development of new products under this banner has really been for consenting adults only.

Times change, however, and so too has the economic viability of processes previously considered too costly to justify commercial production and modified timber is now one of the fastest moving product developments in the industry. Several processes are being actively promoted in the UK, and specifiers need more information to ensure a better understanding of the relative properties and actual availability of the many new products.

Definition

Wood modification has been defined by Professor Callum Hill of Napier University’s Centre for Timber Engineering as the alteration of timber at the molecular level to achieve a desired property change. The products on offer have all been produced in one of two ways, either by thermal or chemical modification, although a third category, ‘densification’, is listed in Digest 504, the BRE’s concise introduction to the subject.

The first group contains products such as Platowood from the Netherlands and Finnforest’s Thermowood. The type of thermal modification processes used for each are similar and involve heating the wood at temperatures of 180-240ºC in the absence of air. This can be done either by using a steam blanket (hygrothermal modification), under nitrogen or in a vacuum. Thermally modified wood is usually dark brown in colour and has low abrasion resistance but is also brittle and tends to split easily. It does, however, have improved dimensional stability together with slightly improved decay resistance. While not really suitable for engineered or ground contact applications, these products are nevertheless very suitable for external use and cladding purposes.

Another type of thermal modification – oleothermic – is used in the production of Osmose Royale and Menzholz from Germany. This process uses vegetable oils at temperatures in excess of 160ºC, with the oil aiding heat transfer into the timber and to exclude oxygen.

As regards chemically modified timber, the most readily available product currently on the market is Accoya, by Titanwood. Distributed in the UK by BSW, Accoya is produced by acetylation, which increases the natural acetyl content of the timber. Acetic acid (vinegar) occurs as a by-product of the process and leaves a slight odour. The benefits are an increase in the dimensional stability of the raw material (in the case of Accoya, radiata pine), an improvement in decay resistance, reduced hygroscopicity, enhanced performance under clear coatings and the possibility of use in engineered applications.

Densification

The third category – densification – remains the subject of some debate amongst wood scientists as to whether it qualifies as ‘modification’ since the impregnation methods used to create products such as Osmose’s Indurite, BASF’s Belmadur, Kurawood, Kebony and Visorwood don’t actually cause any changes in the molecular structure of the cell walls. The latter two products (produced in Norway by Kebony ASA) use different loadings of furfuryl alcohol to impregnate the timber while the others use resin impregnation to create hardened, dimensionally-stable wood. And now another player has come on the scene, Lignia, from Fibre7, an operation set up by Pacific Hardwoods of New Zealand. This is also a resin impregnation product and, according to a spokesperson, is about to be put into use by a UK furniture producer.

But why modify wood at all? Current interest in the subject has unquestionably been stimulated by the need to identify alternatives to non-sustainable tropical hardwoods and conventional preservative treated timber, but the new generation of modified products claim substantially enhanced performance characteristics that make them – despite the sometimes higher initial cost – very attractive to specifiers. Durability is a critical factor, and many of the latest variants are marketed especially for outdoor applications such as cladding and decking. Dimensional stability, too, is of immense benefit to manufacturers of products such as doors and windows since not only can any swelling due to changing moisture content or relative humidity have a profound effect on their ability to open or close, but any paints and varnishes also last significantly longer. Also, most modified timbers available in the UK are non-toxic under service conditions and do not release any toxic substances at end of life following disposal or recycling.

Innovative examples of modified timber in use – such as the motorway bridge constructed from Accoya at Sneek in the Netherlands – can be seen throughout Europe; it’s unlikely to be too long before new projects in the UK begin to emulate them.