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water water everywhere - but the boards didn't shrink?

Architect's memo 63: May 2000

The drive to move from solventborne paints to their waterborne analogues remains as strong as ever. If the rate of change has slowed, it is indicative that all of the easier changes have been accomplished; and the remaining solventborne systems are simply more difficult to match technically.

Penetrating wood stains are not only technically difficult to match with water, they have also an emotional hurdle to overcome. "Everyone knows that timber is a living material which needs oils to feed and nourish it, don't they?" The reality is, that the tree died when the chainsaw went through, and it lost its appetite at that point.

What timber is, however, is a unique fibre-reinforced polymer system which is adapted to an internal aquatic food delivery system. These channels for sap, render the timber continually open to ingress by water, especially through the end-grain. Fluctuations in moisture levels cause changes in the dimensions of the timber (possibly fuelling the living concept).

Obviously then, it is beneficial to have a treatment that excludes water. Typical waterborne acrylics are good at this. Their particulate form blocks the pores of the timber, and forms a tough durable film over the surface. Their very durability, however, leads to problems in semi-transparent stains.

Even the best semi-transparent stain allows some UV light to percolate through. The natural polymers of the timber degrade under UV light at a rate, far faster than the acrylic stain. The degradation of the timber, which is faster for the less dense timbers, leads to a failure at the stain/timber interface with the dreaded "flaking" scenario ensuing.

Having flirted with film-forming stains, the industry realises that penetrating stains, with little to no film forming ability, are the best compromise for long term ease of maintenance. Penetration is best accomplished using solutions of low molecular weight polymers and waxes and solvent systems are king.

To design a waterborne penetrating system obviously needed a new paradigm as materials that dissolve in water can often get washed-off with water.

The approach Resene took, was to hybridise the desired oil component so that it behaved like a waterborne material in the can and during application. Following a period of exposure to the air, a phase change occurs which sees the product take on a completely hydrophobic nature. It penetrates deeply into the timber, at the same time conferring water-repellent properties.

The product, Resene Waterborne Woodsman, dries to a pleasant low gloss waxy finish. It develops water-resistance in minutes and develops full film properties after a month.

So does it form a film? Well yes, sort of - where it hasn't penetrated the latewood bands. But it is designed to have low cohesive strength so that preparation for recoating is straightforward.

It is available in the Resene Woodsman colour range and in testpots.

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Architects memos
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.

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