## 4 Conclusions  
_Pterocarpus macrocarpus_ Kurz wood was thermally modified at 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, and subsequently moderate temperature (150 \({}^{\circ}\)C) TM specimens were impregnated with wax. The effect of TM and TM + WI on the wood properties related to water, mechanical strength, and the micro-structure of the wood were studied. Conclusions were drawn as below:  
1. Mass loss (_ML_) of wood at the moderate temperature condition was small, and became obvious at the 200 \({}^{\circ}\)C condition. The weight percentage gain (_WPG_) of the 150 \({}^{\circ}\)C/6 h TM group was greater than that of the 150 \({}^{\circ}\)C/2 h TM group.
2. TM time affected the amount of wax impregnation; _EMC_ of 150 \({}^{\circ}\)C TM + WI group was the lowest among the control and modified groups. Moderate TM decreased the _EMC_ of the wood, but the WI played a key role in the reduction of _EMC_, which was 5.38 times and 3.34 times the contribution of TM at 150 \({}^{\circ}\)C/2 h and 150 \({}^{\circ}\)C/6 h, respectively; In contrast to the 150 \({}^{\circ}\)C TM group, adsorption swellings decreased maximally by 53.57% and 51.62% in the tangential and radial directions, respectively, after WI. WI played a dominate role for reduction of wood adsorption swelling, and the WI improvement on dimensional stability was bigger than TM. The absorption swelling of the modified groups presented the same tendency as the adsorption swelling; the 150 \({}^{\circ}\)C TM + WI group had the lowest absorption swelling value.
3. The MOR of the thermally modified groups were all decreased, however, the MOR of the 150 \({}^{\circ}\)C TM group was increased after WI, presenting almost the same value as the control group. The MOE of all modified groups were increased in this study; wax was successfully impregnated into the wood, and presented a uniform distribution on the internal surfaces of the wood cells.
4. The chemical composition and micro-structure of the wood were modified after TM and WI treatment. TM and TM + WI decreased the relative content of -OH and C=O in the wood, which decreased hygroscopicity and improved the dimensional stability of the wood. WI improved the wood mechanical strength, but decreased the lightness and deepened the color of the wood. The combination of moderate temperature thermal modification combined with subsequent wax impregnation is a practical approach for improving wood properties.  
Conceptualization, H.-H.L. and L.Y.; methodology, J.-W.Z., T.-Q.H. and Q.Y.; validation, J.-W.Z., T.-Q.H. and Q.Y.; formal analysis, J.-W.Z. and T.-Q.H.; investigation, J.-W.Z. and T.-Q.H.; resources, H.-H.L. and L.Y.; data curation, J.-W.Z., T.-Q.H. and Q.Y.; writing--original draft preparation, J.-W.Z. and T.-Q.H.; writing--review and editing, H.-H.L. and L.Y.; visualization, J.-W.Z., T.-Q.H. and Q.Y.; supervision, H.-H.L. and L.Y.; project administration, H.-H.L. and L.Y.; funding acquisition, H.-H.L. and L.Y. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant No. 31870545) and Key Laboratory of Bio-based Material Science and Technology (Northeast Forestry University), Ministry of Education (SWZ-MS201903).  
The authors declare no conflict of interest.