Treatments to Loosen Bark

Shear strength of bark attachment to wood from young trees of Douglas-fir [Pseudotsuga menziesii (Mirb.) France], western hemlock [Tsuga heterophylla (Raf.) Sarg., lodgepole pine [Pinus contorta Dougl.] and ponderosa pine (Pinus ponderosa Laws) was inversely related to moisture content of the material. Attachment strength to small specimens may increase more than 5-fold from < 6 kg/cm2 to 17-22 kg/cm2 as fresh material air-dries to a moisture content of 14 percent. The relationship is reversible and bark attachment may be weakened additionally by heating in saturated steam. Steaming wet specimens at 100?C for 3, 20, and 40 minutes reduced attachment strength from 4-5 kg/cm2 to 2-3, 1-2, and 1 kg/cm2, respectively. Attachment became unusually strong (24-26 kg/cm2) if steamed specimens became air-dry. Strongly attached bark tended to shear in phloem, rather than in cambium, and left some bark attached to wood. Presteaming of chips was beneficial to a compression-debarking process. Processed chips usually had least bark (2-5 percent) if presteamed at elevated pressure (2.1 kg/cm2 for 5 minutes); chips steamed at atmospheric pressure for 20 minutes contained 4-6 percent of bark. Bark attachment to Douglas-fir trees killed by injection of monosodium methanearsonate was very weak; bark fell freely from sticks split for chipping. Wood of killed trees left standing for a year dried to 50-60 percent moisture content with no visible degrade.

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