Wax and oil emulsion additives: How effective are they at improving the performance of preservative-treated wood?

Wax or oil emulsion additives are incorporated into aqueous wood preservatives to improve the appearance of treated wood exposed outdoors. But, which type of additive is more effective? How is the performance of wood treated with an oil-modified preservative influenced by the quantity of oil emulsion additive in the treatment solution? This study attempted to answer these questions. The checking of radiata pine which had been treated with a chromated copper arsenate preservative containing oil (7% or 14% w/w) or wax emulsion additive (2.5%) and exposed outdoors in Australia for 1 year was compared. Water absorption and swelling of the treated wood was examined before and after weathering. The wax and oil emulsion additives were equally effective at reducing both the checking of treated wood exposed to weathering and the water absorption and swelling of treated wood before the wood was weathered. The oil emulsion additive, however, was more effective at reducing water absorption and swelling of treated wood after it was weathered. Doubling the concentration of oil emulsion additive in the preservative from 7 to 14 percent reduced water absorption of treated wood, both before and after the wood was weathered, and had small, positive, effects on reducing swelling and the size of checks. The findings from this study help explain the current interest in wax emulsion additives for carbon-based and micronized metal preservatives; but, they also suggest that oil emulsion additives may be better at dimensionally stabilizing treated wood exposed outdoors. Hence, there may be merit in adding oil to the emulsion additives used to improve the performance of wood treated with aqueous preservatives.