Modified wood is a term used to describe solid timber that has undergone a non-biocidal physical or chemical process to enhance its durability (resistance to decay and insect attack) or to provide specific performance characteristics like dimensional stability – a key factor in joinery coating performance.
The modification processes change the source species such as pine or spruce into a higher value commodity, effectively creating a new species with different performance properties. As such, modified woods are not easily compared with naturally durable or preservative-treated wood because performance testing may not be carried out in the same way.
The WPA has resolved this dilemma by developing a scheme for establishing performance of modified wood products and their suitability for use in a range of applications. All the major producers of modified wood have collaborated in the development of this scheme which is published in the WPA Modified Wood Specification Manual.
Modified Wood Specification Manual
This WPA manual provides guidance on the performance of modified wood products and identifies performance criteria, suitable end uses and any limitations to be taken into account prior to their specification.
This is free to download publication
Wood Selection Guide
This WPA publication provides a basic guide to choosing wood and panel products for different BS EN 335 use class applications
This is a free to download publication
Types of wood modification
There are basically three different processes by which commercially available modified wood is produced – physical, chemical and combination.
Thermal modification is the only commercialised example of this type of modified wood available in the UK.
Thermal modification induces a permanent change in the polysaccharides (e.g. starch and cellulose) of the wood to enhance its performance. Wood is heated to a temperature in excess of 160 °C in an environment in which oxygen is restricted or eliminated to achieve the desired performance characteristic without charring of the surface.
The chemical modification of wood induces a reaction between an introduced molecule and the wood polysaccharides creating a permanent change throughout an entire section of wood. Current commercial processes include acetylation (full sapwood and heartwood impregnation with an acetic solution) and furfurylation (full sapwood and heartwood penetration with a solution of furfuryl alchohol derived from plant waste).
The only commercially available example of wood modification by a combination of physical and chemical processes is densification. Solid wood is impregnated with plant polymer extracts in a water-borne solution that contains polymerisation catalysts. It is then kiln dried resulting in in-situ polymerisation which turns the timber into a dense hardwearing product for internal applications.
Photograph: Accoya, factory finished joinery
Photograph: Thermowood Cladding