Thermally modified timber (TMT) is increasingly being used for many outdoor applications such as cladding and decking and garden furniture. This technology has been used for some years to improve the service life of non-durable softwoods and is now being applied to North American hardwoods such as tulipwood and ash.
But like unmodified timber, if it’s left without the application of a finish, its surface will undergo surface degradation by weathering. If left to weather naturally on an external facade without any application of a surface finish then timber will weather to silvery-grey appearance.
However, this natural process will tend to occur more uniformly on TMT since colonisation by disfiguring stain and mould is reduced because the modification process removes some of the nutrients in the timber that allow these organisms to colonise.
If the natural dark colour of TMT is to be maintained or enhanced then a translucent surface finish such as a film-forming stain or a penetrating oil needs to be applied. With any coating the success and longevity depend on the quality of the surface to which it is being applied, and good design principles such as avoiding sharp edges on cladding profiles that will compromise coating adhesion.
Thermal modification may improve the ability of the timber surface to take a microporous stain or paint finish as it increases the dimensional stability of the timber and gives a higher surface quality. The species of timber will also influence the coating performance. Evidence shows that successful coating adhesion can be achieved with thermally-modified pine and spruce as these materials have been commercially used for cladding for several years. However, hardwoods will always tend to perform better because of the absence of less pronounced density differences between the earlywood and the latewood. The denser latewood generally shrinks and swells more than the less dense earlywood, causing stresses at the growth interface and in the coating above it.
So it is possible that the coating maintenance intervals can be extended on thermally modified hardwoods, particularly close-grained species such as soft maple and tulipwood, because of its improved dimensional stability and lower tendency to stress cracking.
"Thermally-modified timbers are now established in the UK market," announced Teknos UK’s technical director Mark Hubbert at a seminar this year. "So it is important that we understand how to optimise coating systems for these products, not just on cladding and decks but also on high-performance joinery products such as the laminated Carna window."
Teknos’s initial trials confirmed its Aquatop translucent and opaque paint systems perform equally well on thermally-modified North American tulipwood and on unmodified timber.
The trials included checking paint adhesion after exposure at 40°C and 100% relative humidity, a severe challenge of the coatings’ performance.
"We have more work to do but these initial trials are very promising and more testing is planned for 2014, including accelerated weathering and long-term exposure trials," said Mr Hubbert. It is hoped that these long-term trials will include exposure to the severe climate of India where thermally modified tulipwood is starting to get recognition as a cladding material.
Other coatings companies have also expressed an interest in seeing how their coatings perform on thermally-modified hardwoods. Generally the same products that are used on unmodified timber are suitable for use on TMT, although some companies have developed wood stains or oils that have been pigmented to match the colour of TMT.
Penetrating oils, usually with the addition of a UV inhibitor, also offer an option when it comes to finishing thermally-modified hardwoods. These penetrate into the timber without forming a film and are particularly suitable for horizontal surfaces which, because of full exposure to sun, rain and wear from foot traffic, require a product that is easier to maintain on a more regular basis.
For the London Design Festival Infinity bench project Adam Kershaw used the Biofa weather protection oil, a blend of wood and linseed oil, which helps protect the timber from weathering and enhance the colour of the five thermally-modified hardwoods ash, tulipwood, red oak, soft maple and yellow birch.
Another developing market for thermally modified hardwoods is internal flooring, either as solid timber or top layers for engineered flooring. Species such as ash and pecan look great when a hard wax oil or lacquer is applied to highlight the richness and warmth of the colour. These species also benefit from the dimensional stability and increased surface hardness that the process adds.
With the right finish, market opportunities that are already being exploited in Continental Europe for thermally-modified hardwoods will soon begin to develop more widely in the UK.
