Characteristics of Preservative Solutions Which Influence Their Penetration Into Wood

The effect of preservative chemicals, wood extractives, chemical reaction products, and type of carrier on penetration into Douglas-fir heartwood was studied. It was found that the water-borne preservative FCAP undergoes an aging process as it is continuously recycled in the treating operation. This aging results in a significant reduction in the penetrability of the treating solution. There appear to be three factors that contribute to the aging process. These are: 1) chemicals used in formulating the preservative, 2) progressive build-up of wood extractives, and 3) progressive build-up of chemical reaction products. Of the preservative chemicals in FCAP, sodium fluoride has no effect, whereas sodium arsenate, sodium chromate, and sodium pentachlorophenate have a deleterious effect on penetrability. Sodium pentachlorophenate appears to have the most significant effect of these three chemicals. Of all the chemicals tested, borax had the greatest effect on penetration. The build-up of extractives in the treating solution is affected by pH. More extractives are leached from the wood in an alkaline solution which results in a greater decrease in penetrability. Non-polar solvents penetrate wood much more rapidly that polar solvents. Based on this and previous studies, it is hypothesized that this difference in penetrability is attributable to hydrogen bond forces in polar liquids.

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