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2.6.4. Rupture of Wood Tissue When being dried, wood shrinks unevenly along each of the three directions (tangential, longitudinal and radial) because of its anisotropy. The result of differential shrinkage is drying defects due to rupture of wood tissue. Defects that belong in this category are: surface checks, end checks, collapse, internal checks, and checked knots (Ward & Simpson, 1997). Surface checks occur because the board interior shrinks differently from its surface, or because radial and tangential shrinkage is not the same. Also, the presence of reaction wood can account for surface checking since normal wood shrinks differently. These checks commonly occur at the early stages of drying and mostly with boards that are wide and thick. During the latter drying stages, the checks can close but will possibly open again when the wood is in use. To reduce the occurrence of surface checks, high humidity in the early stages of drying is recommended (Bramhall & Wellwood, 1976). End checks occur on the end-grain surfaces at the first stages of the drying process. Checks follow the direction of the rays. Faster movement of moisture along the grain (10 to 15 times greater than by radial or tangential surfaces) results in board ends becoming drier than the rest of the board. Stresses then develop since differential shrinkage is enhanced and this causes checking when the stresses are higher than the strength of the board (Bramhall & Wellwood, 1976). Coating the board ends by substances that reduce moisture movement helps in minimizing this defect. Rice (1995) revealed that despite the fact that coatings were thin they reduced moisture transport from wood by one to two orders of magnitude in comparison with uncoated boards. Another way to deal with this problem is to have high relative humidity early in the drying process. The shape of the board under collapse appears in grooves (as shown on Figure 5) or there may be extreme shrinkage. During collapse, the fibres flatten because of tensions that occur when saturated lumens lose 22PDF Image | KILN DRYING PROBLEMS AND ISSUES IN CANADA
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