Effects of Drying Severity on Properties of Western Hemlock

Eight 2 by 4 by 18-inch western hemlock (Tsuga heterophylla (Raf.) Sarg.) specimens were cut from the heartwood of an 80-year-old tree to include the same growth increments. The matched specimens were dried individually, two specimens to each of four drying schedules: air-dried; conventionally kiln-dried; kiln-dried at a constant temperature of 225?F; and kiln-dried with a combined schedule of conventional and high (230?F dry bulb and 200?F wet bulb after reaching fiber saturation point) temperature. Strength properties were measured; to eliminate effects of variation in density all strength values were adjusted to a common specific gravity of 0.342. In static bending there were no significant differences between the high-low and the air-dried specimens for fiber stress at proportional limit, modulus of rupture, and work-to-maximum load. There was a highly significant increase in the modulus of elasticity in lumber of the low-high temperature schedule. The constant-high-temperature schedule produced lumber with lower values for fiber stress at proportional limit and modulus of rupture than those air-dried. Modulus of elasticity and work to maximum load showed no significant difference. There were no significant differences in strength properties of samples dried by the low-high temperature schedule and those dried conventionally. Except for work-to-maximum load where there were no significant differences, samples dried by the constant high temperature schedule had strength properties that were significantly lower at the 99 percent confidence level than samples either air-dried or conventionally dried. The modulus of elasticity of conventionally dried hemlock was lower at the 99 percent level than that air dried; there were no differences for the other properties. The low-high-temperature-dried wood had significantly higher maximum crushing strength than did air-dried samples. The values from the other two schedules were imtermediate without significant differences. The drying schedule had a pronounced effect on the equilibrium moisture content. Ranked from highest to lowest were: 1) air-dried: 2) conventionally dried; 3) constant-high-temperature dried; and 4) low-high-temperature dried. When swelling of the dried wood was measured, there were no significant differences between means of the different drying treatments at the 99 percent confidence level. Holocellulose, hemicellulose soluble in 1.0 percent sodium hydroxide, and alpha-cellulose values showed a slight decrease with drying severity, while hemicellulose soluble in 5 percent and 17.5 percent NaOH showed a slight increase. There was no change in the apparent Klason lignin content.

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