Improving on nature?11 February 2010
Dr Andy Pitman, TRADA Technology’s head of construction technical services, reviews some of the advantages of using modified wood products over standard timber
One of the main drivers for commercialising wood modification was to improve decay resistance of low durability timbers. These timber species normally require preservative treatment to achieve desired service life for outdoor applications.
However, when wood is modified, other beneficial changes to its properties may result, including improved dimensional stability and hardness, improving both the performance of wood products and the range of applications possible.
Wood modification is carried out using three types of treatment – chemical, physical or biological, the first two being most commercially important. It excludes wood treated with preservatives. The most important physical modification process is thermal and various types have been practised over the centuries. Chemical modification is more recent, though laboratory modification was undertaken as early as the 1920s. The scaling up of these modification processes to supply timbers of dimensions suitable for commercial applications has only taken place over the past 10 years, with modified wood products available commercially in the UK for around five years.
The range of commercial products and their availability in the UK are outlined in the table. The properties of these modified timbers will vary, depending on:
For this reason it is important to deal with product suppliers to understand performance in specific applications.
All thermal modification processes involve the controlled heating of wood in an atmosphere with reduced oxygen content, or protected by steam, or under oil. In all thermal modification processes, wood is heated to temperatures of 160-230ºC for several hours to several days. One of the benefits of thermal modification is the reduction in wood resin content that reduces the risk of knot bleeding and the problems this causes for coatings.
Since modification occurs throughout, the resultant product can be processed without the risk of exposing unmodified wood. Thermally modified wood tends to be darker than air or kiln dried wood and lighter coloured softwoods can be made to resemble tropical hardwoods.
Chemical modification involves the impregnation of permeable wood species with chemicals that react with the water binding sites in the wood cell wall. In unmodified wood it is the binding of water at these sites that results in movement (shrinkage and swelling in service) and that plays an important role in decay.
Chemical modification processes use pressure impregnation techniques similar to those used for preservative pre-treatment to achieve a good level of penetration throughout the wood structure. Wood is then heated, causing a reaction between the water binding sites in the wood cell wall and the chemicals. The water binding sites are then permanently substituted by chemicals that prevent the binding of water.
Impregnation modification reacts small compounds impregnated into the wood together to fill voids normally filled with air in dry wood. This increases wood density and hardness. Dyes may be added to formulations to change wood colour throughout its section, which may be used to enhance the aesthetics of softwoods.
For wood, durability is its inherent resistance to attack by wood destroying organisms with the heartwood of timbers classified into one of five classes based on durability. Since decay fungi are the most significant pests of timber in service in the UK, testing the resistance of modified woods against these has received most attention. Some modified species have also been tested against resistance to wood boring insects and marine borers.
Most modification processes improve durability of wood against fungi, thus offering an alternative to preservative treatment. A number of the thermal treatments improve durability, although the level of performance (durability class) achieved varies depending on modification process and treatment conditions.
Several modification processes reduce water absorption by the wood cell wall. This results in improved dimensional stability, ie reduced movement in service. For this reason some modified wood products have a reduced risk of cupping when used for cladding, floor and decking boards, a reduced risk of gapping developing between floorboards, or avoid ridging in floors as a result of expansion of floor elements. Splitting/checking around fixings over time are also reduced.
An additional benefit of reduced movement is improved performance of wood coatings. Reduced movement allows for application of greater film thicknesses and reduces maintenance frequencies for coatings compared with unmodified wood, which may significantly reduce the whole life costing of exterior joinery.
Many wood modification processes bring about changes to the mechanical properties of wood – but this may mean in some cases a reduction in strength properties. Hardness, the resistance of timber to indentation, is another important property. It provides useful information about how timber is likely to wear in service and is important for applications such as flooring and decking.
Interestingly, the reduction in water binding under atmospheric conditions (equilibrium moisture contents) through chemical and thermal modification has been shown to increase some strength properties relative to unmodified woods of the same species.
Environmentally, producers of modified wood products have identified a number of benefits. These include sustainable sourcing of the ‘raw materials’ used for the processes. Most wood species used are obtained from managed plantations in temperate countries, while the chemicals for chemical and impregnation modification are derived form natural materials, often agricultural wastes. They are designed to remain fixed within the wood in use and therefore have low ecotoxicity in service. And end of life disposal is no different for wood modified using most processes from that which is unmodified.
To ensure product consistency, a number of quality assessment standards have been established for modified wood products at national level – for example, the Finnish Thermowood Association, the Dutch KOMO product certificate for modified wood and the German TMT quality label.
How the modification process affects the inherent character of the wood will vary from process to process and species to species, so it is important for specifiers to study producers’ technical literature, to ensure performance will meet requirements for different applications.
Modified wood products offer specifiers additional choice. As with unmodified timber, however, they must understand the properties of their chosen product to ensure that it will perform as they would wish throughout its service life.