## 2 Materials and Methods
### Preparation of Coatings  
The compositions are based on mass/mass% values. The pre weighed IPDI, HM, DMPA, TGD and TBDPO were mixed with PED, and then added into a four-neck flask with a condensator, mixer and thermometer for mixing. The rotating speed of this system was set to 100 r/min, and it reacted at 75 \({}^{\circ}\)C for 4 h, and afterwards the temperature at a slow speed fell to room temperature to acquire the waterborne UV-curable coating. The pH value and solid concentration of the waterborne UV-curable coating were approximately 7.0 and 26.5%, respectively. Among them, the IPDI concentration was 35.0%, PED concentration was 15.2%, DPA concentration was 5.8%, HM concentration was 30.0%, TGD concentration was 10.0% and TBDPO concentration was 4.0%. TBDPO in waterborne UV-curable coatings can generate active radicals or cations afterwards, absorbing the high-intensity UV light produced by the Fumax 650 UV curing apparatus (Kunshan Honest Electronic Technology Co., Ltd., Kunshan, China), which can stimulate polymerization, cross-linking and a graft reaction, causing the coating to cure at a certain time. An orthogonal experiment is a method to study multi factors and multi levels. It selects some representative points from the overall test according to the orthogonality. These representative points have the characteristics of uniform dispersion, integrity, and comparability. Orthogonal experimental design is the main method of fractional factorial design, and is a high-efficiency, fast and economic experimental design method. Table 1 showed the configuration information of the experimental samples, among which Samples 1-4 were the preparation samples of the 3-factor and 2-level orthogonal experiments. It was determined that the Al powder concentration was 20.0-40.0%, and the silica slurry concentration was 2.0-10.0%. The Al powder as well as silica slurry was put into the waterborne UV-curable coating, and afterwards 2.0% KH560 was added, then mixed and stirred at room temperature for 1 h. Samples 5-11 were an independent test based on the orthogonal test. The fixed silica slurry concentration was 2.0% and the drying time was 6.0 min, and then 2.0% KH560 was added. The concentration of the Al powder was changed. First, the substrate was pretreated; that is, sanded, degreased, and dried. The waterborne UV/Al low infrared emissivity coating was applied on the Al substrate by an SZQ tetrahedral fabricator (Beijing Zhongjian road industry instrument and Equipment Co., Ltd., Beijing, China). The coating was dried in a blast dryer (Wujiang Tongfeng Oven Electric Furnace Co., Ltd., Wujiang, China) at \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**Sample** & **IPDI** & **HM** & **DMPA** & **TGD** & **TBDPO** & **PED** & **KH560** & **Al** & **Nano** & **Drying** \\
**(\#)** & **(g)** & **(g)** & **(g)** & **(g)** & **(g)** & **(g)** & **(g)** & **(g)** & **Size** & **Time** \\ \hline
1 & 26.6 & 22.8 & 4.4 & 7.6 & 3.04 & 11.6 & 2.0 & 20.0 & 2.0 & 2.0 \\
2 & 23.8 & 20.4 & 3.9 & 6.8 & 2.72 & 10.3 & 2.0 & 20.0 & 10.0 & 6.0 \\
3 & 19.6 & 16.8 & 3.2 & 5.6 & 2.24 & 8.5 & 2.0 & 40.0 & 2.0 & 6.0 \\
4 & 16.8 & 14.4 & 2.8 & 4.8 & 1.92 & 7.3 & 2.0 & 40.0 & 10.0 & 2.0 \\
5 & 30.1 & 25.8 & 5.0 & 8.6 & 3.44 & 13.1 & 2.0 & 10.0 & 2.0 & 6.0 \\
6 & 28.3 & 24.3 & 4.7 & 8.1 & 3.24 & 12.3 & 2.0 & 15.0 & 2.0 & 6.0 \\
7 & 26.6 & 22.8 & 4.4 & 7.6 & 3.04 & 11.6 & 2.0 & 20.0 & 2.0 & 6.0 \\
8 & 24.8 & 21.3 & 4.1 & 7.1 & 2.84 & 10.8 & 2.0 & 25.0 & 2.0 & 6.0 \\
9 & 23.1 & 19.8 & 3.8 & 6.6 & 2.64 & 10.0 & 2.0 & 30.0 & 2.0 & 6.0 \\
10 & 21.3 & 18.3 & 3.5 & 6.1 & 2.44 & 9.3 & 2.0 & 35.0 & 2.0 & 6.0 \\
11 & 19.6 & 16.8 & 3.2 & 5.6 & 2.24 & 8.5 & 2.0 & 40.0 & 2.0 & 6.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Configuration information of the waterborne UV/Al low infrared emissivity coating. 40 \({}^{\circ}\)C for 10 min, and afterwards cured by UV radiation of 365 nm medium pressure mercury lamp (400W) in a Fumax 650 UV curing instrument. Radiation curing control and drying curing time control was 2.0-6.0 min.