Water uptake by scots pine sapwood, and its restriction by the use of water repellents

Summary

Water in contact with wood surfaces is able to penetrate into the cellular structure by three routes: 1. As liquid water into cell lumena, by capillarity. — 2. As water vapour, by diffusion into cell lumena. — 3. As bound water, by diffusion within the cell wall.

Transport from cell lumena into adjacent cell walls occurs rapidly by diffusion. In Scots pine (Pinus sylvestris) sapwood, it is shown that transport over short distances occurs much more rapidly by capillarity than by either of the diffusion processes. Treatment of wood with resin/wax water repellent formulations greatly reduces the rate of water flow due to capillarity and hence significantly cuts down the rate of dimensional change of specimens exposed to wet conditions.

Swelling rates due to vapour phase and bound water diffusion were measured experimentally, and these data were used to predict the water sorption rates for specimens treated with a theoretically perfect water repellent, viz. that which excludes all liquid water.

It was found experimentally that specimens immersed in water, after treatment with resin/wax water repellents, swelled more rapidly than predicted by the above procedure. This more rapid swelling is probably due to a certain amount of liquid water flow made possible by displacement of the hydrophobic film from cell wall surfaces (preferential wetting). It is suggested that the use of hydrophobic agents bonded chemically to the cell wall may be necessary to attain optimum water repellent effectiveness.