*3.3. Decay Resistance Performances*

The appearances of untreated and heat-treated wood after a 50-day soil block decay resistance test is shown in Figure 5. The untreated woods showed an uneven color on the surface; there were several dark spots in the untreated *A. confusa*. However, the heat-treated wood had a uniform color after the decay resistance test. The weight loss and color changes of untreated and heat-treated *C. japonica* and *A. confusa* wood after the resistance test are drawn in Figure 6. The weight loss of untreated *C. japonica* and *A. confusa* were 9.3 and 1.6%, while that of the heat-treated woods were 3.0 and 0.3%, respectively. In a study by Vidholdová and Reinprecht [32], 21 tropical woods were attacked intentionally for six weeks by brown-rot fungus, resulting in 0.08% to 6.48% weight loss of the wood. However, our results showed higher values (1.6% and 9.3%). This pheromone can be explained by multiple factors in our soil block test—moisture vibration changes, water flow, temperature cycles, and microorganisms. Nonetheless, the medium difference shows that the brown-rot fungus is a key factor in the decay. The results showed that the heat-treated wood had superior decay resistance properties due to the increase of hydrophobicity and decrease of water absorbency [17,25]. There also was a decrease in OH group content, which affected enzyme recognition blocking of the fungi [33], and reduced the hemicellulose content of the wood, therefore cutting down on the main nutritive source for the fungi [3,14,34]. In addition, the extractives of wood and pyrolysis products such as acidic or phenolic compounds, which have fungicidal efficiency, are deposited on the wood surface, suppressing the growth of fungi [20,27,33,35]. The color changes on Δ*L\** of untreated *C. japonica* and *A. confusa* were −11.3 and −15.8 after the 50-day block decay test, meaning a darkening color was observed on the untreated wood surface; the Δ*L\** was 4.0 and 3.5, respectively for the heat-treated wood, indicating a slight bright color obtained after the decay test. The Δ*E\** of the untreated woods

were 11.5 and 20.9, which was greater than the values of 5.9–0.5 from a previous study [32]. The Δ*E\** values of heat-treated wood were also lower than that of the untreated wood. The best photostability of heat-treated wood color could be partially explained by the increase of lignin stability by condensation during the heat-treatment, restricting light from attacking the lignin. In addition, the phenol content increased after heating and the antioxidant compounds can limit degradation caused by oxygen and radicals [31].

**Figure 5.** Appearances of untreated and heat-treated Japanese cedar (*Cryptomeria japonica*) (**a**) and Formosa acacia (*Acacia confusa*) (**b**) wood after the 50-day decay resistance test.

**Figure 6.** Weight loss (**a**) and color changes (**b**,**c**) of untreated and heat-treated Japanese cedar (*Cryptomeria japonica*) and Formosa acacia (*Acacia confusa*) wood after the 50-day decay resistance test.

#### **4. Conclusions**

The results showed that the normal shear strength of heat-treated *C. japonica* and *A. confusa* with UF and PVAc adhesives was lower than that of untreated wood (the decrease is around 99% to 72%), but the wood failure of heat-treated wood was higher than that of the untreated one. Furthermore, the adhesion and impact resistance of heat-treated *C. japonica* and *A. confusa* finished with PU and NC coatings showed no difference with untreated wood. The finished heat-treated wood also showed superior durability and had better gloss retention and lightfastness than those of the untreated wood. The heat-treated wood also had better decay-resistance than untreated wood, ascertained by a soil block test. The weight loss of heat-treated *C. japonica* and *A. confusa* were 3.0% and 0.3%, respectively. Both values of mass decrease were only 32% and 19% compared to the untreated wood.

**Author Contributions:** Conceptualization, C.-W.C. and K.-T.L.; methodology, W.-L.K. and C.-W.C.; formal analysis, W.-L.K., C.-W.C. and K.-T.L.; investigation, W.-L.K.; data curation, C.-W.C. and W.-L.K.; writing—original draft preparation, K.-T.L.; writing—review and editing, C.-W.C. and K.T. Lu; supervision, K.-T.L.; project administration, K.-T.L.

**Funding:** This research was funded by Forestry Bureau, Council of Agriculture, Executive Yuan, grant number tfbp-990506.

**Conflicts of Interest:** The authors declare no conflict of interest.
