## 4 Conclusions  
In this paper, a polyformaldehyde method using phenol and formaldehyde as raw materials with the catalyst potassium borate was proposed to prepared B-containing PR. The critical process parameters for preparation of B-containing PR, such as the molar ratios of formaldehyde/phenol and potassium borate/phenol, and reaction time, was optimized. The measurement of surface tension of B-containing PR and wettability between B-containing PR solution and carbon fibers was taken, providing useful theory and experimental data for the preparation of B-containing PR-based composites.  
The influence of formaldehyde/phenol molar ratio within 1.0-2.0 on the char yield of B-containing PR was investigated, with the molar ratio of potassium borate/phenol kept at 0.2 and reaction time at 13 h. The char yields were found to be between 60% and 71%, and maximum char yield was acquired when the formaldehyde/phenol molar ratio was 1.8. The effects of potassium borate/phenol molar ratio ranging between 0.1 to 0.4 and reaction time on char yield of B-containing PR was evaluated in the condition of formaldehyde/phenol molar ratio = 1.8. The char yield of B-containing PR in the condition of formaldehyde/phenol = 0.1 was distinctly lower than those of products prepared when the formaldehyde/phenol = 0.2 or 0.4. When the molar ratio of potassium borate/phenol was 0.2 or 0.4, the B-containing PR possessed a high char yield ranging from 60% to 71%, and the char yield increased first and then stabilized with the extension of reaction time. By optimization of critical process parameters, the highest char yield of B-containing PR could be as high as 71%, with molar ratios of formaldehyde/phenol = 1.8 and potassium borate/phenol = 0.2, and reaction time = 13 h, respectively. In addition, the surface tension of B-containing PR solution was measured under different concentrations and tested temperatures; its value was found to decrease with increase in concentration and tested temperature. Finally, the wettability between B-containing PR solution and carbon fibers was investigated for the first time, and B-containing PR solution with 0.8 g/mL at 35 \({}^{\circ}\)C was found to spread on to the surface of carbon fibers within 7.5 s to reach an equilibrium state, along with contact angle of 0\({}^{\circ}\), indicating good wettability between B-containing PR solution and carbon fibers.  
In conclusion, the results of this study indicate that the high char yield of B-containing PR with good wettability between B-containing PR solution and carbon fibers could be used to prepare B-containing PR-based composites, which can be used as thermal insulation materials. Our next research will concentrate on the design and fabrication of B-containing PR-based composites in an effort to characterize their microstructures and properties.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/4/1408/s1](http://www.mdpi.com/2076-3417/10/4/1408/s1)]([http://www.mdpi.com/2076-3417/10/4/1408/s1](http://www.mdpi.com/2076-3417/10/4/1408/s1)).  
[PERSON]. ([PERSON]) performed the experiments and wrote the paper, and [PERSON] ([PERSON]) designed the experiments and revised the paper. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
This work was supported by the National Natural Science Foundation of China (No. 11972135).  
The authors declare no conflict of interest.