Analysis of shrinkage in kiln-dried Pinus caribaea sawn wood at Busoga Forest company, Mayuge district in Uganda

Abstract

Wood is a heterogeneous, hygroscopic, cellular, and anisotropic material that consists of cells of various types depending on the species. Pinus caribaea wood species consists mostly of tracheid cells which are water conducting vessels and when trees are cut down and logs sawn to the required dimension, the loss of water from green moisture content towards equilibrium moisture content starts. During kiln-drying, shrinkage does not occur until the moisture content drops below the fiber saturation point (Bergman, 2010) which is taken as 30 percent moisture content (Eckelman, 1998). The problem addressed by this study was insufficient knowledge about shrinkage extent of kiln-dried P. caribaea sawn timber. The objectives of the study were: i) to determine the tangential and radial shrinkages of kiln-dried P. caribaea sawn timber; ii) to determine the volumetric shrinkage of kiln-dried P. caribaea sawn timber; and iii) to assess the effect of sawn timber size on radial and tangential shrinkage. A total of 304 cant sawn sawn timber pieces of 4000mm uniform length from 18-year-old P. caribaea logs were sampled for kiln-drying from green to 12% moisture content (MC) in a convectional kiln at Busoga Forest Company (BFC) with not less than 40 samples selected from six commonly sawn sawn timber size classes that is; 100mm × 50mm, 75mm × 50mm, 150mm × 50mm, 150mm × 25mm, 75mm × 25mm, and 100mm × 25mm. The results obtained showed a mean shrinkage value of 1.70% in the radial direction, 3.41% in the tangential direction, and 7.67% as the volumetric shrinkage with tangential shrinkage being twice as much as the radial shrinkage. Results further showed that volumetric shrinkage varied from one size class to another and that both tangential and radial shrinkages were positively correlated with the size of sawn timber in that larger pieces shrink more than smaller pieces. It is therefore recommended that machine operators study the different sawing patterns for easy identification of grain direction and use different shrinkage allowances when sawing timber to different sizes for kiln-drying