Case hardening and equilibrium moisture content of European aspen and silver birch after industrial scale thermo-mechanical timber modification

Abstract

Case hardening (CH) is an unwanted feature, which causes dried wood to deform after sawing and conditioning the moisture content. The tendency for CH depends on the drying parameters. Its probability increases due to increased internal stresses in rapid kiln drying, thus conditioning and steaming after the cooling minimises the problem. A process of thermo-mechanical timber modification (TMTM™), enables drying, compression and thermal modification of sawn timber in a single treatment unit. This process is a potential method for reducing CH. It also reduces the equilibrium moisture content (EMC) of wood. The aim of this study is to analyse the effect of initial moisture content, compression degree and thermal modification of the CH and EMC of European aspen (Populus tremula L.) and silver birch (Betula pendula Roth). A total of 252 clear wood CH specimens were manufactured from TMTM™ modified boards that had green dimensions of 40 × 100 × 2,700 mm. Modifications were carried out in one of two ways: the compression was started either on green state or after pre-drying to approximately 20% MC. After the compression phase, half of the boards were thermally modified at 190 °C. After storage of four years, specimens were conditioned and analysed according to the standard CEN/TS 14464. Compression increases the CH substantially, but subsequent thermal modification practically eliminates it in both studied hardwood species. Tangentially and radially compressed specimens did not differ from each other in terms of CH. An increase in process temperature also reduced the EMC of birch and especially aspen.