Influence of Thermal Treatments of Short Duration on the Toughness and Chemical Composition of Wood

Specimens 5/8 by 5/8 by 12 inches of red oak, longleaf pine, and Douglas-fir were prepared. Five specimens of each species were assigned to each of 18 treatment groups. Two samples, 3/4 inch long, were taken from the ends of each specimen and stored in jars for analysis. A span of 10 inches was marked, and the specimens dried for 16 hours at l05?C. After the period, specimens were weighed, the cross-sectional dimensions measured to 1/1000 inch and stored in polytheylene bags. Thermal treatments were applied by both heating in an oven and steaming in a retort. Nine time-temperature levels were employed with each method. Temperatures of 150?C., 175?C., and 200?C. and time intervals of 20, 40, and 60 minutes were used in oven heating. Steam treatments were conducted at pressures of 20, 35, and 50 psi for exposure periods of 30, 60, and 120 minutes. Experimental results indicated significant reductions in toughness and changes in carbohydrate content occurred in all species. In general, red oak was more sensitive to the thermal treatments than longleaf pine and Douglas-fir. Steaming had a more deleterious effect on both mechanical and chemical properties than oven heating. Reduction in toughness in all three species varied in an approximately linear fashion with temperature and exposure times, and was highly correlated with change in carbohydrate content. Most of the variation in toughness was accounted for by variations in hemicellulose content.

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