Graduate Thesis Or Dissertation
 

Improving the durability of second growth timbers of naturally durable species

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  • While the heartwood of many wood species exhibits excellent resistance to fungal and insect attack, this resistance is sometimes diminished in second-growth material of the same species. The reasons for the reduced durability are unclear, but they may reflect a combination of both higher proportions of sapwood as well as reduced levels of heartwood extractives. Second growth timbers are often faster grown and this accelerated growth may affect the production of toxic extractives present in the heartwood. Reduced durability may affect the reputation, quality, and values of the final products. One approach for maintaining the quality of naturally durable second-growth timbers is to supplementarily treat the wood with low concentrations of preservatives. In this research, we evaluated the ability of two commercially-available and environmental-friendly preservatives to protect both the sapwood and heartwood of teak, western red cedar, and coastal redwood. Cubes of these timbers and southern pine, a decay susceptible control, were vacuum-pressure treated with varying retentions of didecyldimethylammonium chloride (DDAC), and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOI), or alkaline copper quaternary (ACQ). Preservatives performance was assessed first in a leaching test. Preservative concentrations in the wood changed little after leaching. Durability improvement was assessed by exposing treated cubes to two decay fungi (Trametes versicolor and Postia placenta) in a soil block test. The results suggested that DCOI (0.6 kg/m3) and DDAC (4 kg/m3) improved sapwood durability of teak, redwood, and western red cedar. While the performance of all treated heartwood was better than the performance of the untreated samples, these differences were too small to delineate differences. These results provided weak evidence that supplemental treatment may enhance heartwood durability. Finally, we evaluated possible interactions between heartwood extractives and DDAC and DCOI, using a bioassay with Trametes versicolor and Postia placenta on agar medium. Sawdust of teak, coastal redwood and western red cedar were extracted with solvents of various polarities and impregnated on filter paper disks. The disks were placed on Petri dish surfaces previously inoculated with fungal suspensions. Western red cedar and teak extractives inhibited growth of the fungi, while coastal redwood extractives had no antifungal activity. Combinations of these extractives and chemical preservatives were no more efficient than the individual chemical. The results suggest that supplemental treatments do not synergistically interact with the heartwood extractives.
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