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Architects memo no. 77: September 2004
t i m b e r r r !!


Wood is certainly one of our most beautiful building materials and one which inspires quite strong emotions. People talk of it as a "living" material that needs care and nourishment and it evokes all sorts of warm fuzzy feelings.

To the cynical chemist however, wood is a very imperfect material, with its composition varying dramatically, not only species to species but within the same tree and even the same stick! It is a mixture of cellulose, hemicellulose, lignin, wood sugars and variously gums, anti-oxidants, natural dyes and Uncle Tom Cobbly and all - of which only cellulose has any real exterior durability. Many of the constituents will dissolve in water, or at least swell in the presence of water and also provide a great source of nutrients for moulds and fungi.

Yeah, yeah, I know - it is still a lovely material! That's why such a lot of effort is put into trying to preserve its warmth and good looks.

In the interior environment - free from the twin aggressors, water and UV radiation - this is a reasonably facile task and a wide range of stains, sealers, oils, waxes and varnishes are available to provide colour, finish and cleanability.

Outside is quite a different story. Any piece of timber exposed to the elements will quickly lose its natural colouring and its soluble sugars. The hemicellulose and lignin go next leaving exposed cellulose fibres on the surface. The rate of degradation is inversely proportional to the density of the timber with light woods such as Cedar eroding the fastest.

If this occurs in an arid climate the cellulose fibres take on a pleasant silvery grey appearance with the exposed fibres protecting the rest of the timber beneath. In humid, temperate environments, such as are found in New Zealand and Australia, mould and fungi thrive on such a surface and regular applications of fungicidal wash are needed to achieve a pleasant appearance.

Trying to maintain timber in a pristine condition outside is a major challenge and one which is rarely met.

It is tempting to use varnishes to achieve this but the clarity that enhances the grain so beautifully also lets through a portion of the damaging UV light. This radiation damages the sensitive wood surface beneath the film until the first signs of degradation, a slight colour change on the timber surface, herald the beginning of the end.

Even though the paint technologist has some very powerful UV absorbers at their disposal, including nano-pigmentation, Nature, with time on her side, always wins.

If it could be arranged that at the same moment that the timber surface started degrading the varnish could be made to fall away completely, all would be well. The reality is patchy peeling occurs, which makes preparing a new uniform surface a nightmare.

Only the boaties, who are prepared to sand down and re-coat every season, can successfully use such systems.

If such film-forming materials are not a practical answer, how about non film-forming penetrative materials?

Penetrative materials can and do perform a very valuable role with an ability to carry into the timber useful stabilising chemicals such as fungicides, UV absorbers and water-repellents. Because they do not form significant surface films, there is no danger of the dreaded sunburnt peeling that makes film former degradation such a disaster.

The major drawback with such systems is that, because they penetrate into the timber there is little to no protection on the surface. With well formulated products in this class you can get "Defence in Depth" but need to titivate the surface regularly.

It is something of a misconception that vegetable oils, such as linseed, "nourish" the timber. Being an excellent food source for fungi and mould, the only things that they nourish are these surface defacing "greebies".

Between the fully film forming varnishes and the fully penetrating preservatives there is a large fertile area for coating chemists to ply their craft. The result will always be a compromise between surface protection and ease of maintenance; UV protection and clarity of grain; fungal protection and environmental impact.

In such an area of compromise one will always find a "snake oil" merchant with a better, revolutionary solution. Snake oil! Now there's a thought - I wonder how that would go in a timber treatment! I must get back to the lab.

Resene Paints Ltd

Resene Paints Ltd   – www.resene.com

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