Graduate Thesis Or Dissertation
 

Potential for Using Boron Compounds with Differing Water Solubilities in Pastes for Remedial Treatment of Utility Poles

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  • Boron and boron compounds have been long used for pressure treatment, dip-diffusion, and remedial treatments of wood because of their proven efficacy against fungi and insects. These materials are especially attractive because their high water solubility promotes deeper penetration into wood. Other useful properties of boron include its neutral pH, non-corrosiveness, lack of color or odor, and little or no effect on wood strength. Boron also has low-mammalian toxicity and minimal environmental effects. However, high water solubility limits outdoor applications because the boron will leach from the wood when it is exposed to rain or is in direct soil contact. Boron is widely used as a component in external remedial pastes, primarily as disodium octaborate tetrahydrate (DOT). This boron compound has higher water solubility and only remains in wood 3 to 7 years after application. Using differing degrees of water solubilities in the paste system might result in longer-term protection. To test this hypothesis, the ability of sodium octaborate tetrahydrate (Timbor), sodium tetraborate decahydrate (Borax Decahydrate), sodium tetraborate pentahydrate (Etibor48), sodium-calcium pentaborate octahydrate (Ulexite), zinc borate, and di-calcium hexaborate pentahydrate (Colemanite) mixtures to move into Douglas-fir sapwood block conditioned to 40% or 60% moisture content (MC) was assessed over 3 or 6 weeks in small block tests. The objective was to examine the ability of borate mixtures with differing degrees of solubility to deliver threshold levels of boron into wood samples. Boron levels were affected by MC and incubation time. Boron levels in the outer zone were much higher than those in the next zone. All paste formulations delivered more than the required amount of boron for protection against internal decay (0.10% BAE: Boric acid equivalent) in the outer zone at both MC and incubation times. Timbor paste formulations distributed the highest amount of boron (0.25% BAE at 40% MC and 0.5% BAE at 60% MC) in the next zone in wood blocks 6 weeks after treatment and also exceeded the minimum threshold to arrest internal decay. However, boron levels in the inner zone in blocks treated with most of the other paste formulations were below 0.1% BAE 6 weeks after treatment, although they increased with increasing incubation time and moisture content. Differences between the paste formulations containing DOT and those treated with other paste formulations suggest that boron levels in wood were also affected by the solubility of each boron compound and the boron concentrations in the paste formulations.
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