1. Introduction
Infrared low emissivity coatings can not only benefit infrared stealth militarily, but also reduce the harm of infrared light to the human body in daily life by virtue of its ability to reflect more and absorb less infrared light [
1]. As a special functional coating, it is widely used in military investigation and other aspects [
2]. Lv et al. [
3] fabricated a polydimethylsiloxane (PDMS)-modified polyurethane (PU)/Ag composite coating with super-hydrophobicity and low infrared emissivity via the glass rod scraping method. The results indicated that at the 10 μm particle size of flaky Ag powder and a 50% concentration of Ag powder in the coating, the coating had a dense surface structure and the lowest infrared emissivity. He et al. [
4] fabricated a low infrared emissivity coating using graphene surface-modified flaky aluminum complex powders (rGO-FAl) as fillers. Compared with pure flaky aluminum, the Vis-NIR diffuse reflectance of the rGO-FAl complex powders was significantly decreased, which was good for the low gloss of the coating. In addition, the coating maintained low infrared emissivity. Voti et al. [
5] designed and simulated VO
2/metal multilayers to obtain a large tunability of the thermal emissivity of the infrared (IR) filters in the typical mid wave IR window of many infrared cameras. The multilayer structure was optimized to realize a low emissivity filter at high temperatures, useful for military purposes.
With strict national control of the environmental protection industry, waterborne coatings have been vigorously promoted [
6,
7]. Among them, waterborne UV-curable coatings have gradually become the research hotspot of domestic and foreign scholars due to their advantages of easy adjustment of concentration, fast drying speed [
8], harmless volatiles and wide application range [
9]. However, at the same time, it also has the disadvantages of poor corrosion resistance [
10] and high gloss [
11], which is less propitious regarding compatibility with visible light, thus limiting its domain in infrared low-emission materials. Therefore, an effective solution for achieving compatibility between low infrared emissivity and visible light is the only way to expand the application field of waterborne UV coatings.
Currently, most of the infrared low emissivity coatings are produced by giving the metal a strong infrared reflection ability, via a filler added into the coating [
12]. For instance, Qi et al. [
13] prepared low infrared emissivity coatings with outstanding thermal stability utilizing silicone resin as a binder, flake Al powder as a pigment and a low-melting glass powder as a functional filler. The results investigated that the low emissivity coating displayed favorable thermal stability at a 1.5 Al/polysiloxane ratio and the coating infrared emissivity was about 0.25 at 600 °C, which can show a good application prospect. Zhang et al. [
14] prepared a polydimethylsiloxane (PDMS)-modified polyurethane (PU)/Al composite paint coat with super-hydrophobicity and low infrared emissivity. Research suggested that the paint coat possessed the lowest infrared emissivity and gloss performances at an 80 °C curing temperature. Hu et al. [
15] made low infrared emissivity paint coats owning excellent heat resistance, choosing epoxy-siloxane and Al as the binder together with pigment. Detailed conclusions indicated that the cured composite paint coats still possess low emissivity, profitable heat ageing and anti-thermal shock after the stability test. From the above research, we see that the metal Al powder stands out among numerous metal fillers on account of its low-cost, accessibility and satisfactory manifestation. However, these studies are based on silicone resin, polydimethylsiloxane-modified polyurethane, and epoxy-siloxane, but less based on waterborne UV coatings.
In a preliminary study [
16], the influence of infrared emissivity, chromaticity distortion and corrosion resistance of Al/waterborne acrylic coatings were investigated. For infrared emissivity coatings, the drying speed of the waterborne coatings is relatively slow [
17], while the waterborne UV-curable coatings prepared in this paper are environmentally friendly and fast in drying. Waterborne UV-curable coatings not only eliminate volatile organic compounds (VOC) pollution, but also have the characteristics of rapid curing and energy saving.
As a crosslinking agent, a silane coupling agent can promote the interface combination between a coating and resin and improve the corrosion resistance of the coating [
18]. In addition, a nano silica slurry can effectively reduce the coating gloss [
19]. In this paper, an orthogonal experiment was utilized to optimize the process parameters, in order to obtain the waterborne UV coating possessing a low infrared emissivity, low gloss, low brightness as well as favorable corrosion resistance and mechanical properties. We also probe the influence of the Al powder concentration on the coating, so as to lay the foundation for the technical production of UV-curable, infrared and low emissivity coatings.
2. Materials and Methods
2.1. Experimental Materials
Isophorone diisocyanate (IPDI, resin, Mw: 222.28 g/mol, CAS No.: 4098-7-19), polyether diols (PED, resin, Mw: 2000 g/mol, CAS No.: 25322-69-4), hydroxyethyl methacrylate (HM, resin, Mw: 130.14, CAS No.: 868-77-9), 2,2-dihydroxymethylpropionic acid (DMPA, Chain extender, Mw: 134.13 g/mol, CAS No.: 4767-03-7), tripropylene glycol diacrylate (TGD, Active diluent, Mw: 300.35 g/mol, CAS No.: 42978-66-5) and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TBDPO, Photoinitiator, Mw: 348.4 g/mol, CAS No.: 75980-60-8) were offered by Linyi Dongyi Chemical Co., Ltd., Shandong, China. Filler: Al powder 4017 (piece diameter: 10 μm, thickness: 220 nm, solid concentration: 65.0%, 65.0% Al powder mixed with 35.0% oleic acid) was offered by Xuhui New Material Technology Co., Ltd., Shandong, China. Dispersant: KH560 was offered by Zhongjie Chemical Technology Co., Ltd., Guangzhou, China. Other auxiliary materials: nano silica slurry (particle size: 150 nm, solid concentration: 15.0%) was offered by Nanjing Tianxing New Material Co., Ltd., Jiangsu, China. Anhydrous ethanol was offered by Zhongji Chemical Co., Ltd., Shandong, China. Al substrate (100 mm × 50 mm × 1 mm) was provided by Wuxi Dongyue metal material Co., Ltd. All reagents were pure without further treatment.
2.2. 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 °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 40 °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. After the coating was cured to a constant weight, the coating was burnished via #1000 sandpaper, the dust was wiped off using dry cloth, and the aforementioned coating steps were repeated 3 times.
2.3. Testing and Characterization
The coating impact resistance was determined using a QCG-50 coating impact tester (Ningbo Yinzhou Rijin testing equipment Co., Ltd., Ningbo, China). The coating of the sample was positioned horizontally on the iron plate, and the weight hammer was fastened on a certain height of the sliding cylinder through the control device. As the control button was pressed, the heavy hammer would fall freely on the press. When lifting the heavy hammer to take out the experimentation plate, the deformation degree of the coating surface was observed, and the height of the heavy hammer falling on the experimentation plate was recorded. Impact resistance referred to the maximum height on the test plate without damaging the coating.
The adhesion was tested according to ISO 2409-2007 [
20]. The coating adhesion was determined with a QFH-HG600 coating scriber (Zhongxi Yuanda Technology Co., Ltd., Beijing, China). The handle was held and a multiblade tool was used on the plate. The coating was cut at 20–50 mm/s under uniform pressure. Then, the manipulate was repeated by rotating it 90° to form a 1 mm × 1 mm grid pattern. The 3M brand 600-1PK transparent pressure-sensitive adhesive tape (19 mm wide) was applied to the entire grid and revealed at a minimum angle. The coating damage was observed. The adhesion level was judged in line with the damage degree. The adhesion of the coating was 0, which made known that the coating adhesion was the best. Coating hardness was determined via a QHQ-A coating hardness tester (Flora Automation Technology Co., Ltd., Tianjin, China).
Coating gloss was determined with a BGD512-60 gloss meter (3NH Technology Co., Ltd., Shenzhen, China). The color of the coating shall be tested using a SEGT-J colorimeter (Shenyang Guoti Precision Testing Instrument, Liaoning, China). L, a* and b*, respectively, stand for the lightness, red-green axis color index and yellow-blue axis color index of the Al plate, while L’, a*’ and b*’ represent the surface coating of Al plate. ΔL, Δa* and Δb* represent the lightness difference, red-green index difference and yellow-blue index difference of the two samples, respectively. According to the color difference Formula (1), the color difference ΔE was computed:
Coating microstructure was characterized with SEM. A Vertex 80V infrared spectrometer was used to measure the influence of different Al powder concentrations on the waterborne UV-curable coating. Far-infrared emissivity of the coating in the range of 8–14 μm was determined using an IR-2 far-infrared emissivity tester (Chinese Academy of Sciences Shanghai Institute of Technological Physics, Shanghai, China). The emissivity measurement instrument uses an active blackbody radiation source to measure the normal reflectivity of the object to be measured, and then measures its normal emissivity in the 8–14 μm infrared band. First, the temperature was set to 250 °C, the power was 43% and the calibration started after the value was stable. Then the copper reference plate was placed so that the emissivity was 0.45. The aluminum-coated compensation reference plate was placed on the bracket directly below the test head, and the mirror reflection (MR) was adjusted to make the emissivity reach 0.05. After adjustment, the infrared emissivity of the coating was tested. The corrosion resistance of the waterborne UV-curable coating was determined by a CHI660E electrochemical workstation (Beijing Jinyang Wanda Technology Co., Ltd., Beijing, China) in line with GB/T 15748-2013 [
21], and the NaCl concentration was 3.5% at room temperature. A three-electrode cell structure was utilized for the electrochemical experiment. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum electrode as the counter electrode. Each sample was soaked in a 3.5% NaCl solution and the open circuit potential (OCP) was monitored until constant. The polarization experiment began from scanning with respect to OCP-0.3V and stopped with respect to OCP+0.3V, and the scanning rate was 10 mV/s. All the experiments were repeated four times with an error of less than 5.0%.
4. Conclusions
Among the three factors, the most significant one was the concentration of the Al powder. By changing the concentration of the Al powder, the influence of the Al powder on the infrared emissivity, gloss, color difference, hardness, corrosion resistance as well as other properties of the waterborne UV-curable coating was studied. The optimal concentration of the Al powder was explored when the original good performance was ensured, and the low infrared emissivity, low sub light and low brightness values were simultaneously possessed. The results demonstrated that at the 25.0% Al powder concentration, the hardness, adhesion, impact resistance and corrosion resistance of the waterborne UV-curable coating were higher; the infrared emissivity and gloss were also lower. Conclusively, it is acknowledged that the value of the paper lies in the methodology adopted to fabricate waterborne UV/Al low infrared emissivity coatings.