Effects of Heat Post-Treatment on Dimensional Stability and Water Absorption Behaviours of Mechanically Densified Uludağ Fir and Black Poplar Woods

One of the most persistent problems with mechanically densified wood is its inherent dimensional instability. The effects of heat post-treatment on the changes in spring-back (SB), compression ratio recovery (CRR), thickness swelling (TS), and water absorption (WA) of newly-tested Uludağ fir (Abies bornmuelleriana Mattf.) and black poplar (Populus nigra L.) wood samples that had been thermo-mechanically densified were investigated. Samples were densified with compression ratios of 25% and 50% with temperatures of 100 and 140 °C, respectively. Then, the heat post-treatment was applied to the samples at 185 and 212 °C for 2 h. For the two newly-tested wood types, results of the preliminary study show that SB and TS were higher at a 50% compression ratio compared with 25%. Regarding densification temperature, TS was lower in samples densified at 140 °C, while SB was higher. WA values were lower in compressed samples (50%) at high rates. The effect of the densification temperature on WA was insignificant. Heat post-treatment had a considerable effect on the dimensional stability and hygroscopicity of the densified Uludağ fir and black poplar samples. With an increase in heat treatment temperature, the dimensional stability increased, while the hygroscopicity of densified samples decreased. As a result of heat posttreatment applied at 212 °C, SB, CRR, TS, and WA decreased by 88%, 85%, 79%, and 53%, respectively.