Water uptake of radiata pine wood after microwave and superheated steam treatments: sapwood vs. heartwood

Resin canals and ray cells in pine wood connect the interior to the exterior. The blockage of resin canals and ray cells by the resin, directly determines the water uptake of the wood. However, research on water uptake by resinous wood is lacking. Hence, this study aimed to explain how microwave (MW) or superheated steam (SS) treatment improves wood water uptake from the perspective of resin content (RC). Therefore, radiata pine sapwood and heartwood were treated with MW, SS, or microwave-superheated steam (MS) for different times. The results showed that MW and SS treatments expelled the internal resin of radiata pine. Owing to the low RC and high moisture content (MC) of sapwood, the effect of the MW treatment on the RC of radiata sapwood was insignificant. Heartwood has a markedly higher RC and lower MC than sapwood; therefore, MW treatment can drive part of the resin to migrate out of the wood. With increasing SS treatment time, the RC of the sapwood and heartwood decreased to varying degrees; however, the core layer resin could not be completely removed. In addition, MW treatment can accelerate the degreasing process as it discharges some part of the resin and opens some parts of the resin canals. Moreover, MS treatment can increase the porosity of sapwood and heartwood due to the expulsion of resin. MS treatment had no significant effect on sapwood water uptake; however, it increased that of heartwood by more than three times. The dye solution enters the wood through the open resin canals or ray cells and communicates with the axial resin canals. Therefore, the water uptake of heartwood is mainly related to the resin content and the blockage of the resin canals by the resin determines the entry of the dye solution to the interior of the wood.