In situ polymerization of vinyl monomers during compressive deformation of wood treated with boric acid to delay boron leaching

Boron compounds are useful in wood preservation owing to their environmentally safe characteristics and relatively low costs in addition to their well-known high bioactive and fire-resistant properties. Despite their unique properties, however, these compounds are readily leachable from treated wood under humid conditions. Therefore, they are of limited market value for exterior applications. A supplementary treatment with vinyl monomers (styrene and methyl methacrylate) was applied to extend the service life of boron-treated wood. Sapwood specimens of Japanese cedar (Cryptomeria japonica D. Don) were first treated with boric acid (BA) at 1.00 percent aqueous solution concentration, then after air-drying at ambient temperatures, they were impregnated with vinyl monomers. Polymerization was performed during radial compression of monomer-impregnated wood to a 50 to 70 percent dry compression set dimension under a hot-press heated to the polymerization temperatures of 60? and 90?C required by the selected catalysts VAZO and benzoyl peroxide, respectively. The resulting compressed-wood polymer composites (CWPCs) exhibited remarkably high dimensional stability and moisture exclusion efficiency with minimum water-holding capacity as determined by submerging test specimens in tap water, boiling water, and exposing them to 10 cycles of accelerated severe weathering. As a result, the rate of boron leaching from CWPCs pretreated with BA was considerably slower than that from wood-polymer composites (WPCs). Scanning electron microscope observations suggested how controlled-but-continuous boron leaching might have occurred. An “enveloping” treatment of boron deposited in cell walls by an effective bulking process was found essential for better immobility.

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