Kiln Time and Temperature Affect Shrinkage, Warp and Mechanical Properties of Southern Pine Lumber

Four hundred and eighty No. 2 Dense southern pine 2 by 6’s, 95 inches long, were kiln-dried in 4-foot-wide loads with a 3,000-pound top load restraint. The kiln-drying regimes consisted of dry-bulb temperatures of 180? 240?, and 270?F, with a wet-bulb temperature of 160?F and kiln times of 120 hours at 180? F; 36 and 120 hours at 240?F; and 9, 36, and 120 hours at 270?F. After kiln-drying and a 1-year conditioning period, boards were loaded to failure in edgewise bending. From undamaged sections, small clear specimens were removed for evaluation of several properties. MC of loads on emergence from the kiln ranged from 0.2 to 11.9 percent. After 1 year of conditioning, boards dried at high temperature for a prescribed number of hours had lower EMC than boards dried an equal time at low temperature. Shrinkage was least in wood dried at 270?F for 9 hours. Boards dried at 240?F and equilibrated had less average crook, bow, and twist, and less maximum crook and bow than boards dried for 120 hours at 180?F. Boards dried by schedules approximating commercial practice (180?F for 120 hr., 240?F for 36 hr., and 270?F for 9 hr.)did not differ significantly in MOR, proportional limit, MOE, compression strength parallel to the grain, shear strength parallel to the grain, hardness, and toughness. Regression relationships of MOR to MOE were also similar for the three drying treatments. Boards dried 120 hours at 240?F or 270?F had reduced MOR and toughness; also, the regression relationships of MOR to MOE were different from those observed for wood dried on the shorter schedules.

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