coatings

Article Effect of Different Color Paste on Properties of /Aluminum Infrared Low Emissivity Coating

Xiaoxing Yan 1,2 1 Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China; [email protected] or [email protected] 2 College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China

 Received: 28 November 2019; Accepted: 8 January 2020; Published: 13 January 2020 

Abstract: The effect of the four kinds of red, dark yellow, purple, and black pastes on the properties of fluorine resin/aluminum low emissivity coating was studied. The infrared emissivity coatings with red and black pastes were higher than the coatings with dark yellow and purple pastes. The hardness of the coatings with red, dark yellow, and purple color pastes was 6H, and that with black pastes was 6B. The adhesion and impact resistance of dark yellow coating was better, followed by red and purple, and the adhesion and impact resistance of black coating was the worst. Electrochemical polarization curves indicated that fluorine resin coatings with purple paste had better corrosion resistance. After the salt resistance test, there was no obvious loss of light in the coatings with the four kinds of color pastes. The purple paste coating had no obvious loss of light and less bubble, suggesting that the fluorine resin/aluminum low emissivity coating with purple paste had better performance. The results of this study provide a new prospect for the application of infrared low emissivity coating in infrared stealth and compatibility with visible light.

Keywords: color paste; aluminum powder; fluorine resin; coating property

1. Introduction At present, the research area related to infrared light is applied more and more in the military field. As a result, infrared low emissivity coating (IRLEC) has attracted more and more attention because infrared emissivity is an important index [1]. IRLEC is part of the infrared stealth technology [2]. Some universities, research institutes and so on have made relatively thorough research on IRLEC. Dong et al. [3] studied the reflection characteristics of infrared low emissivity camouflage coating in terahertz wave. Li et al. [4] prepared multilayer AlCrN/Cr/AlCrN coating on nickel matrix by cathodic arc ion plating, which is used in low infrared emissivity applications below 750 ◦C. Wang et al. [5] used polyethylene glycol (PEG) 400 and inorganic silicate binder to modify the CeO2 filler with different content to prepare the coating with outstanding thermal shock resistance and low-temperature infrared emissivity. Corrosion is a problem because it limits the application of the coating. Corrosion protection is usually carried out by coating. The above researchers mainly focus on the reflection characteristics, thermal shock resistance and infrared emissivity, but less on the corrosion resistance, color [6] and compatibility with visible light. Infrared coating is required to have the following properties at the same time: low emissivity, environmental protection, good corrosion resistance, low gloss, so as to have a good effect on visible light compatibility. Fluorine resin is environmentally acceptable, not only has the high surface hardness, good adhesion, and corrosion resistance [7,8], but also can increase the water contact angle of the coating to achieve superhydrophobic properties [9–11]. Coating color is one of the important indicators of coating decoration performance, but different colors can resist different visible light detection [12]. For stealth coatings, besides low infrared emissivity,

Coatings 2020, 10, 70; doi:10.3390/coatings10010070 www.mdpi.com/journal/coatings Coatings 2020, 10, 70 2 of 11 it is also necessary to consider the compatibility with visible light stealth [13]. Therefore, it is necessary to reduce the brightness of the coatings and tone them [14]. Different color pastes can change the coating color of IRLEC; however, there are few relevant reports for IRLEC [15]. This paper mainly focused on the color matching problem, aiming to study the effect of quantitatively changing the dosage of different color pastes of red, dark yellow, purple, and black color pastes on the infrared emissivity, gloss, brightness, adhesion, impact resistance, corrosion resistance and roughness of fluorine resin/aluminum coatings, so as to obtain the optimum color of low gloss and IRLEC, laying a foundation for the application of IRLEC and compatibility with visible light in engineering.

2. Experimental Section

2.1. Experimental Materials Fluorine resin (JF100) and curing agent: Juhua Group Co. Ltd., Quzhou, China. 4017 Al powder (Mw: 26.9815 g/mol, CAS No.: 7429-90-5, diameter: 10 µm, thickness: 220 nm, 65.0% Al powder mixed with 35.0% oleic acid): Zhangqiu Metal Pigment Co. Ltd., Zhangqiu, China. Red 8011 (solid concentration: 25.0%), dark yellow 8032 (solid concentration: 25.0%), purple 8005 (solid concentration: 15.0%), black 9927 (solid concentration: 20.0%): Suzhou Shiming Technology Co. Ltd., Suzhou, China. Technical aluminum plate [16]: 100 mm 50 mm 1 mm, Nanjing Chengxiang Aluminum Co. Ltd., Nanjing, China. × × 2.2. Preparation of Coating Sandpapers were used to polish the pretreated substrate and to remove oil, and then they were dried in the oven. According to the previous research experience, the coating has good properties when the color paste content is 40% [17]. The solid content of aluminum powder and color pastes respectively referred to the quality of aluminum powder and color pastes respectively proportional to the quality of original aluminum powder and color pastes under the condition of vacuum (105 2) ± ◦C, where the aluminum powder and the color pastes were heated for 30 min. The solid content of fluorine resin and solid content of curing agent were measured according to the same method. The coating was prepared according to the formula of Table1. After mixing, the coating was coated on the surface of the flat technical aluminum plate. They were solidified at 40 ◦C for 2 h in a blast dryer. The thickness of the dry coating was about 60 µm.

Table 1. The composition of different color pastes.

Color Paste Coating Samples Red Dark Yellow Purple Black No Paste Quality of fluorine resin (g) 3.0 3.0 3.0 3.0 3.0 Solid content of fluorine resin (%) 50.0 50.0 50.0 50.0 50.0 Quality of curing agent (g) 0.3 0.3 0.3 0.3 0.3 Solid content of curing agent (%) 90.0 90.0 90.0 90.0 90.0 Quality of aluminum powder (g) 1.815 1.815 1.815 1.815 1.815 Solid content of aluminum powder (%) 65.0 65.0 65.0 65.0 65.0 Quality of color paste (g) 2.83 2.83 4.72 3.54 0 Solid content of color paste (%) 25.0 25.0 15.0 20.0 0

quality of aluminum powder solid content of aluminum powder × quality of aluminum powder solid content of aluminum powder+quality of fluorine resin solid content of fluorine resin+quality of curing agent solid content of curing agent 100% = 40% Note: quality× of color paste solid content of color paste × × × . × quality of fluorine resin solid content of fluorine resin+quality of curing agent solid content of fluororesin 100% = 40% × × ×

2.3. Test and Characterization The specific information of the experimental test equipment is presented in Table2. The infrared emissivity of the coating was tested in the range of 8~14 µm. In the test of corrosion resistance, a three-electrode battery structure was used for electrochemical tests. The saturated calomel electrode was used as the reference electrode and the platinum electrode as the counter electrode. The samples were immersed in 3.5% NaCl solution for 30 min and the open-circuit potential (OCP) was monitored Coatings 2020, 10, 70 3 of 11 until constant. After the open-circuit potential was stabilized, the polarization tests were carried out. The test range was open-circuit potential 0.25 V. The coatings were immersed in 3.5% NaCl solution ± for 7 days, and the salt water resistance test was carried out. All the experiments were repeated four times with an error of less than 5.0%.

Table 2. Experimental test equipment.

Property Standard Equipment Producer JKTH-1250B digital coating Tianjin Jingke Material Testing Machine thickness - thickness gauge Factory, Tianjin, China. surface structure JSM-5610LV scanning FEI Company in the United States, - and morphology electron microscope Hillsboro, USA. Chinese Academy of Sciences Shanghai Bruker Vertex 70 infrared infrared emissivity - Institute of Technological Physics, emissivity tester Shanghai, China. Shenzhen Sanenshi Technology Co. Ltd., gloss GB/T 9754-2007 BGD512-60 vancometer Shenzhen, China. Tianjin Precision Material Experiment impact resistance GB/T 20624.2-2006 QCG paint film impact tester Machine Factory, Tianjin, China. Shenzhen Hanpu Testing Instrument adhesion GB/T 4893.4-2013 QFH coating adhesion meter Co. Ltd., Shenzhen, China. QHQ-A coating Quzhou Aipu Measuring Instrument hardness GB/T 6739-2006 hardness meter Co. Ltd., Quzhou, China. Guangzhou Biaogeda Experimental color difference GB/T 3181-1995 SEGT-J colorimeter Instruments Co. Ltd., Guangzhou, China. JB-4C precision Shanghai Taiming Optical Instrument roughness GB/T 13288.5-2009 roughness tester Co. Ltd., Shanghai, China. CHI660E Beijing Huakoputian Technology Co. Ltd., corrosion resistance GB/T 15748-2013 electrochemical workstation Beijing, China. Beaker for 3.5% Beijing Huakoputian Technology Co. Ltd., salt water resistance GB/T 15748-2013 NaCl solution Beijing, China.

3. Results and discussion

3.1. Infrared Spectrum of Color Paste Corresponding to Coating The infrared spectra of the four kinds of color paste corresponding to the coating are shown in 1 Figure1. The attribution of the peak is shown in Table3. 3434 cm − had an obvious -OH absorption 1 1 1 peak. 2943 cm− was -CH3 absorption peak. 2874 cm− was -CH2 absorption peak. 1750 cm− was 1 -C=O- absorption [18,19]. 1441 cm− was the -COO absorption peak [20]. Obvious CF3 absorption 1 peak appeared at 1110 cm− [21]. IR absorption peaks indicated that fluorine resin formed covalent bond. According to Kirchhoff’s law, in the state of heat balance, the infrared energy absorbed and emitted by an object is equal [22]. As can be seen in Figure1, the infrared peaks of the purple coating, the dark yellow coating and the coating without paste were relatively flat, the infrared absorption was less, and the emittance of the corresponding coating was the lower (Figure1), which was conducive to reduce the surface emissivity [23]. The black coating had more infrared absorption, so the surface emissivity was the highest, which was not conducive to the preparation of low infrared emissivity coating. Coatings 2020, 10, x FOR PEER REVIEW 4 of 11 Coatings 20202020,, 1010,, 70x FOR PEER REVIEW 44 of of 11

Figure 1. Infrared spectrum of the coatings with different color pastes. Figure 1. Infrared spectrum of the coatings with different color pastes. Figure 1. Infrared spectrum of the coatings with different color pastes. Table 3. Assignment of peaks. Table 3. Assignment of peaks. Table 13. Assignment of peaks. Peak (cm− ) Assignment Peak (cm−1) Assignment Peak (cm−1) Assignment 3434 3434 absorption absorption of -OH of -OH 3434 2943 absorption absorption of -CH of 3-OH 2943 absorption of -CH3 2943 2874 absorption absorption of -CH of -CH2 3 2874 absorption of -CH2 2874 1750 absorption absorption of -C of=O- -CH2 1750 absorption of -C=O- 1750 1441 absorption absorption of -COO of -C=O- 1441 absorption of -COO 1441 1110 absorption absorption of -CFof -COO3 1110 absorption of -CF3 1110 absorption of -CF3 3.2. Influence of Color Paste on Infrared Emissivity of Coating 3.2. Influence of Color Paste on Infrared Emissivity of Coating 3.2. Influence of Color Paste on Infrared Emissivity of Coating The influence of different color pastes on the infrared emissivity of fluorine resin coating is shown The influence of different color pastes on the infrared emissivity of fluorine resin coating is in FigureThe 2influence. The addition of different of red andcolor black pastes pastes on madethe in fluorinefrared emissivity resin coating of fluorine possess higherresin coating infrared is shown in Figure 2. The addition of red and black pastes made fluorine resin coating possess higher emissivityshown in Figure (>0.3). 2. Dark The addition yellow and of red purple and pastesblack pastes could made make fluorine fluorine resin resin coating coating possess possess higher lower infrared emissivity (>0.3). Dark yellow and purple pastes could make fluorine resin coating possess infraredinfrared emissivity (0.10~0.25).(>0.3). Dark yellow Dark yellow and purple paste hadpastes lower could infrared make fluorine emissivity resin than coating purple possess paste. lower infrared emissivity (0.10~0.25). Dark yellow paste had lower infrared emissivity than purple Thislower is infrared due to the emissivity dark yellow (0.10~0.25). paste had Dark the yellow lowest paste infrared had absorption lower infrared (Figure emissivity1) and the than strongest purple paste. This is due to the dark yellow paste had the lowest infrared absorption (Figure 1) and the reflectivitypaste. This comparedis due to withthe dark the other yellow color paste pastes had used the inlowest the experiment. infrared absorption The infrared (Figure emissivity 1) and of the strongest reflectivity compared with the other color pastes used in the experiment. The infrared darkstrongest yellow reflectivity paste was compared the lowest with [24]. the other color pastes used in the experiment. The infrared emissivity of the dark yellow paste was the lowest [24]. emissivity of the dark yellow paste was the lowest [24].

Figure 2. Influence of different color pastes on the infrared emissivity of the coating. Figure 2. Influence of different color pastes on the infrared emissivity of the coating. Figure 2. Influence of different color pastes on the infrared emissivity of the coating.

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The SEM images of different color coatings are shown in Figure 3. There were more particles in red andThe black SEM coatings, images while of diff obviouserent color particles coatings from are microscopic shown in Figure view3 .could There hardly were more be noticed particles in in darkred yellow and black and coatings,purple pastes, while and obvious the coatings particles were from more microscopic uniform, view thereby could reducing hardly bethe noticed absorption in dark of yellowinfrared and light purple and lowering pastes, and the the infrared coatings emissi werevity more [25]. uniform, More particles thereby and reducing voids the increased absorption the of infraredinfrared absorption light and loweringof coatings the infrared(Figure emissivity1), so the [infrared25]. More emissivity particles and of voidsred and increased black thecoatings infrared increased.absorption of coatings (Figure1), so the infrared emissivity of red and black coatings increased.

FigureFigure 3. SEM 3. SEM images images of different of different color color coatings: coatings: (A) (red,A) red, (B) (darkB) dark yellow, yellow, (C) ( purpleC) purple and and (D) ( Dblack.) black.

3.3.3.3. Influence Influence of Color of Color Paste Paste on Properties on Properties of Coatings of Coatings TheThe effect effect of different of different color color pastes pastes on onthe the properti propertieses of the of the coating coating is shown is shown in Table in Table 4. 4As. As shown shown in Tablein Table 4, 4compared, compared with with dark dark yellow yellow and and purple purple pastes, pastes, red red and and black black pastes pastes could could make make fluorine fluorine coatingscoatings possess possess lower lower luster. luster. The The gloss gloss of black of black coating coating was was the the lowest. lowest. This This is because is because the the coatings coatings withwith red red and and black black pastes pastes had had a stronger a stronger ability ability to absorb to absorb and and emit emit light light and and a weaker a weaker ability ability to reflect to reflect light,light, and and therefore therefore to pr toesent present a lower a lower gloss gloss [26]. [26 ]. Different color pastes have different effects on the hardness of coatings. The hardness of red, Table 4. Effect of color pastes on performance of the coatings. dark yellow, purple coating and the coating without paste was 6H, while the hardness of black coating was the lowest (6B). This is because black paste is mainlyCoatings composed of carbon black particles. Samples Compared with the other three pastes,Red carbon Dark black Yellow particles Purple are more Black loose No affecting Paste the compactness of the coating [27]. For low infrared emissivity coatings, an important role is to protect Gloss (%) 11.4 12.4 17.5 4.3 23.0 the covering, so the Hardnesshardness cannot be too 6H low [28]. 6HIt can be seen 6H that the hardness 6B of 6H the coating added with blackAdhesion paste does (level) not meet the basic 2 hardness 1 index (2H) 2 [29]. For black 3 coatings, 0 in order to ensure thatImpact the coating resistance can (kg meetcm) basic 35hardness index, 50 the hardness 35 can be 30 enhanced 50 by adding · modifiers [30]. Roughness (µm) 1.29 0.947 0.625 5.214 0.500 Table 4 indicated that the adhesion of the coating without paste was the best. The dark yellow coating Dihadfferent better color adhesion pastes haveand the diff erentblack ecoatingffects on had the the hardness worst ofadhesion. coatings. This The hardnessis because of from red, darka microscopicyellow, purple point coating of view, and the the dark coating yellow without coating paste is more was uniform 6H, while and the denser, hardness and of the black components coating was arethe easy lowest to mix (6B). evenly, This while is because the interaction black paste between is mainly carbon composed black ofparticles carbon of black black particles. paste and Compared other componentswith the other of coating three pastes,is less, carbonso it is blackdifficult particles to mix are with more them loose uniformly affecting [31]. the compactnessThe influence of of the differentcoating color [27]. pastes For low on infrared impact emissivityresistance coatings,of the coating an important was different. role is toThe protect black thecoating covering, had sothe the lowesthardness impact cannot resistance, be too lowwhile [28 ].the It candark be yello seenw that coating the hardness and the of coating the coating without added paste with blackhad the paste highest.does notThis meet is because the basic the hardness dark yellow index (2H)coating [29 ].and For the black coating coatings, without in order paste to ensurehas high that adhesion. the coating Goodcan adhesion meet basic can hardness ensure index,good theimpact hardness resistance. can be The enhanced adhesion by addingof coatings modifiers with [black30]. pastes is relatively small, and it is easy to fall off when impacted [32].

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Table4 indicated that the adhesion of the coating without paste was the best. The dark yellow coating had better adhesion and the black coating had the worst adhesion. This is because from a microscopic point of view, the dark yellow coating is more uniform and denser, and the components are easy to mix evenly, while the interaction between carbon black particles of black paste and other components of coating is less, so it is difficult to mix with them uniformly [31]. The influence of different color pastes on impact resistance of the coating was different. The black coating had the lowest impact resistance, while the dark yellow coating and the coating without paste had the highest. This is because the dark yellow coating and the coating without paste has high adhesion. Good adhesion can ensure good impact resistance. The adhesion of coatings with black pastes is relatively small, and it is easy to fall off when impacted [32]. As can be seen in Table4, the roughness of dark yellow and purple coatings was lower, and the surface was smoother. The roughness of black coatings was the highest. This may be because the smaller particle size of the dark yellow and purple pastes, which can fill the gap between the aluminum powder in the coating and increase the compactness of the coating. The carbon black particles of the black paste aggregate together to form large particles, thus increasing the roughness [33]. Table5 is the e ffect of four kinds of color pastes on the chromatic aberration of coatings. The color difference refers to the difference of the overall brightness and hue of the coating [34]. L, a and b represent the brightness, red-green and yellow-blue value of different color coating respectively. L’, a’ and b’ represent these values of the substrate itself. The difference ∆L, ∆a and ∆b are obtained by subtracting them, which are respectively expressed as lightness difference, red-green index difference and yellow-blue index difference. Thus, the color difference ∆E can be obtained according to the formula [35].

Table 5. Effect of color pastes on ∆E of the coatings.

Coatings Sample Red Dark Yellow Purple Black No Paste L 53.8 65.2 30.9 71.6 67.8 a 28.7 7.8 22.1 5.4 1.1 b 4.6 35.2 32.2 1.7 2.3 − − − L’ 25.9 25.9 25.9 25.9 25.9 a’ 5.2 5.2 5.2 5.2 5.2 b’ 3.1 3.1 3.1 3.1 3.1 − − − − − ∆L 27.9 39.3 5.0 45.7 41.9 ∆a 23.5 2.6 16.9 0.2 4.1 − ∆b 7.7 38.3 29.1 1.4 0.8 − ∆E 37.3 54.9 34.0 45.7 42.1

According to Table5, the e ffect of different color pastes on color difference of the coatings was different. The color difference of purple paste was the smallest compared with that of red, dark yellow and black pastes. At the same time, the L value of purple coating was the lowest, which made the coating darker. The other three coatings had larger L value and were brighter. From the above study, it can be found that the coatings with purple, red, and dark yellow pastes have better comprehensive performance. Therefore, the electrochemical corrosion resistance of these three coatings was tested. The test results are shown in Figure4 and Table6. The βa and βc are respectively the anode Tafel slope and the cathode Tafel slope [36]. The unit is mv/dec, mv refers to the voltage, and dec refers to the logarithmic current. The higher the potential Ecorr, the greater the resistance Rp, and the smaller the current Icorr and the better the corrosion resistance. Coatings 2020, 10, 70 7 of 11 Coatings 2020, 10, x FOR PEER REVIEW 7 of 11

Figure 4. Electrochemistry diagram of different color pastes. Figure 4. Electrochemistry diagram of different color pastes. Table 6. Effect of different color pastes content on corrosion resistance. Table 6. Effect of different color pastes content on corrosion resistance. Coatings Sample Coatings Sample Red Dark Yellow Purple No Paste Red Dark Yellow Purple No Paste Ecorr (V) 0.75 0.76 0.71 0.80 Ecorr (V) 2 −0.75− − −0.76 − −0.71 − −0.80 Rp (Ω/cm ) 415.5 343.1 1301.1 670.2 Rp (Ω/cm2) 2 415.5 5 343.14 5 1301.1 5 670.2 Icorr (A/cm ) 8.78 10− 1.00 10− 2.76 10− 2.79 10− 2 × −5 × −4 × −5 × −5 Icorr (A/cmβa (mV) /dec) 8.78 × 16310 1.00 144 × 10 124 2.76 × 10 73 2.79 × 10 βa (mV/dec)βc (mV /dec) 163 172 175 144 248 124 105 73 βc (mV/dec) 172 175 248 105 The salt water resistance of coatings with four color pastes immersed in 3.5% NaCl solution for 7 daysThe was salt tested water to resistance observe the of change coatings of with surface four condition color pastes after immersed a certain corrosion in 3.5% NaCl time. solution The loss for of light,7 days discoloration, was tested to and observe foaming the density change in of the surfac actuale condition corrosion processafter a certain of the paint corrosion film were time. evaluated. The loss Whenof light, the discoloration, loss of light was and<3%, foaming 4–15%, density 16–30%, in 31–50%, the actual 51–80% corrosion and > 80%,process respectively, of the paint the film degree were of lossevaluated. of light When of coatings the loss were of light at the was 0, 1,<3%, 2, 3,4–15 4, and%, 16–30%, 5 level, 31–50%, respectively. 51–80% When and the >80%, change respectively, of color dithefference degree before of loss and of light after of salt coatings water were resistance at the was0, 1, <2,1.5, 3, 4, 1.6–3.0, and 5 level, 3.1–6.0, respectively. 6.1–9.0, 9.1–12.0 When andthe change>12.0, respectively,of color difference the degree before of and loss ofafter light salt of water coatings resist wasance non-discoloration, was <1.5, 1.6–3.0, very 3.1–6.0, slight 6.1–9.0, discoloration, 9.1–12.0 slightand >12.0, discoloration, respectively, obvious the discoloration,degree of loss severe of light discoloration of coatings and was complete non-discoloration, discoloration, very and slight the gradesdiscoloration, were 0, slight 1, 2, 3,discoloration, 4 and 5 level, obvious respectively. discoloration, The test resultssevere arediscoloration shown in Tablesand 7complete and8, anddiscoloration, Figure5. and the grades were 0, 1, 2, 3, 4 and 5 level, respectively. The test results are shown in Tables 7 and 8, and Figure 5. As can be seen in Table 7, the coatingsTable 7. Change had no of obvious loss of light. loss of light. The loss of light grade was 0 or 1, indicating that fluorine resin coatings with different color pastes had good corrosion resistance. Coatings As can be seen in SampleTable 8, there was no obvious discoloration of the purple coating. The discoloration grade was 0, indicating that the purpleRed coating Darkhad better Yellow corrosion Purple resistance, Black which No was Paste consistent with theGloss results before of electrochemical immersion (%) testing 11.4 (Figure 12.44). Under the 17.5 same experimental 4.3 23.0conditions, the dark yellowGloss coating after immersion and the black (%) coating 11.4 changed 12.0color obviously, 13.0 and the 2.0 color change 21.7 grade was at level 3 or above.Loss of light (%) 0 0.4 4.5 2.3 1.3 Grade of loss of light (level) 0 0 1 0 0

Table 7. Change of loss of light.

Coatings Sample Red Dark Yellow Purple Black No Paste Gloss before immersion (%) 11.4 12.4 17.5 4.3 23.0 Gloss after immersion (%) 11.4 12.0 13.0 2.0 21.7 Loss of light (%) 0 0.4 4.5 2.3 1.3 Grade of loss of light (level) 0 0 1 0 0

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Table 8. Color difference change after salt solution test.

Coatings Sample Red Dark Yellow Purple Black No Paste L 55.2 68.3 34.5 30.0 74.5 a 34.3 5.2 17.4 3.3 1.4 b 5.3 44.0 −32.4 4.4 -2.2 Coatings 2020, 10, 70 L’ 25.9 25.9 25.9 25.9 25.9 8 of 11 a’ 5.2 5.2 5.2 5.2 5.2 b’ Table 8. Color di−ff3.1erence change−3.1 after salt solution−3.1 test. −3.1 −3.1 ΔL 29.3 42.4 8.6 4.1 48.6 Coatings - ΔSamplea 29.1 0 12.2 −1.9 3.8 Δb Red8.4 Dark Yellow47.1 Purple−29.3 Black7.5 No Paste 0.9 ΔE L 55.242.1 68.363.4 34.532.9 30.08.8 74.5 48.8 Chromatic aberrationa before and 34.3 5.2 17.4 3.3 1.4 b 5.34.8 44.08.5 32.4 1.1 4.436.9 -2.2 6.7 after salt water resistance − L’ 25.9 25.9 25.9 25.9 25.9 Color change grade (level) 2 3 0 5 3 a’ 5.2 5.2 5.2 5.2 5.2 b’ 3.1 3.1 3.1 3.1 3.1 − − − − − As can be seen∆ Lin Figure 5, the coating29.3 was 42.4destroyed after 8.6 7 days 4.1 of 3.5% NaCl 48.6 solution immersion for the corrosion∆a resistance 29.1test, and coatings 0 with color 12.2 pastes 1.9had a certain -3.8 degree of − ∆b 8.4 47.1 29.3 7.5 0.9 foaming. The foaming density of dark yellow and purple coatings− was lower, and black coating had ∆E 42.1 63.4 32.9 8.8 48.8 a small amount of foaming, while red foaming was more serious. The compact coating and little pores Chromatic aberration before 4.8 8.5 1.1 36.9 6.7 (Figure 3B,C)and after can salt prevent water resistancethe NaCl solution from entering, improve the corrosion resistance of the coating, andColor not change easily grade cause (level) the phenomenon 2 of light 3 loss, discoloration, 0 and 5 bubbling. 3

Figure 5. Foaming profile of fluorine resin coating for red, dark yellow, purple and black pastes. Figure 5. Foaming profile of fluorine resin coating for red, dark yellow, purple and black pastes. As can be seen in Table7, the coatings had no obvious loss of light. The loss of light grade was 0 Based on the above analysis, it can be concluded that purple 8005 can be used to prepare low or 1, indicating that fluorine resin coatings with different color pastes had good corrosion resistance. infrared emissivity fluorine resin coatings with better performance. The structure of purple 8005 As can be seen in Table8, there was no obvious discoloration of the purple coating. The discoloration paste was characterized. SEM image of purple 8005 is shown in Figure 6, and the infrared spectrum grade was 0, indicating that the purple coating had better corrosion resistance, which was consistent is shown in Figure 7. with the results of electrochemical testing (Figure4). Under the same experimental conditions, the dark Figure 6 showed that the size of purple 8005 paste was small. As can be seen in Figure 7, purple yellow coating and the black coating changed color obviously, and the color change grade was at level 8005 paste contains abundant organic functional groups [37]. 3314 cm−1, 2963 cm−1, 1638 cm−1 and 1560 3 or above. cm−1 corresponded to the characteristic absorption peaks of N-H, C-H, C=C and C-N respectively, so As can be seen in Figure5, the coating was destroyed after 7 days of 3.5% NaCl solution immersion they were easy to disperse in fluorine resin coatings and had good compatibility with fluorine resin for the corrosion resistance test, and coatings with color pastes had a certain degree of foaming. coatings. The foaming density of dark yellow and purple coatings was lower, and black coating had a small amount of foaming, while red foaming was more serious. The compact coating and little pores (Figure3B,C) can prevent the NaCl solution from entering, improve the corrosion resistance of the coating, and not easily cause the phenomenon of light loss, discoloration, and bubbling. Based on the above analysis, it can be concluded that purple 8005 can be used to prepare low infrared emissivity fluorine resin coatings with better performance. The structure of purple 8005 paste was characterized. SEM image of purple 8005 is shown in Figure6, and the infrared spectrum is shown in Figure7. Figure6 showed that the size of purple 8005 paste was small. As can be seen in Figure7, purple 1 1 1 8005 paste contains abundant organic functional groups [37]. 3314 cm− , 2963 cm− , 1638 cm− and 1 1560 cm− corresponded to the characteristic absorption peaks of N-H, C-H, C=C and C-N respectively, so they were easy to disperse in fluorine resin coatings and had good compatibility with fluorine resin coatings. CoatingsCoatings 20202020,, 1010,, 70x FOR PEER REVIEW 99 of 1111 Coatings 2020, 10, x FOR PEER REVIEW 9 of 11

Figure 6. SEM image of purple paste: (A) low and (B) high magnification. Figure 6. SEM image of purple paste: (A) (A) low and (B) (B) high high magnification. magnification.

Figure 7. Infrared spectrum of purple paste. 4. Conclusions Figure 7. Infrared spectrum of purple paste. Figure 7. Infrared spectrum of purple paste. 4. ConclusionsThe four kinds of red, dark yellow, purple, and black color pastes were added into the fluorine resin coating system, so that the fluorine resin coating has low infrared emissivity, low gloss, low 4. ConclusionsThe four kinds of red, dark yellow, purple, and black color pastes were added into the fluorine brightness, good adhesion, and impact resistance, as well as low roughness. Purple paste can make resinThe coating four kindssystem, of sored, that dark the yellow, fluorine purple, resin coandating black has color low pastesinfrared were emissivity, added into low the gloss, fluorine low fluorine resin coating possess low infrared emissivity and better comprehensive properties. The salt resinbrightness, coating good system, adhesion, so that and the impact fluorine resistance, resin coating as well has as low low infrared roughness. emissivity, Purple pastelow gloss, can make low water resistance test showed that the coatings with red, dark yellow, purple, and black color pastes brightness,fluorine resin good coating adhesion, possess and low impact infrared resistance, emissivi asty well and asbetter low comprehensiveroughness. Purple properties. paste can The make salt had no obvious loss of light. The purple coating had no obvious discoloration, and the dark yellow fluorinewater resistance resin coating test showed possess thatlow theinfrared coatings emissivi withty red, and dark better yellow, comprehensive purple, and properties. black color The pastes salt and black coatings had obvious discoloration under the same experimental conditions. The foaming waterhad no resistance obvious losstest showedof light. thatThe purplethe coatings coating with ha dred, no darkobvious yellow, discoloration, purple, and and black the colordark yellowpastes densities of the dark yellow and purple coatings were less, i.e., the fluorine resin coating with the hadand noblack obvious coatings loss had of light.obvious The discoloration purple coating under had the no sameobvious experimental discoloration, conditions. and the Thedark foaming yellow purple paste has the better performance. This work provides a new prospect for the compatibility with anddensities black ofcoatings the dark had yellow obvious and discoloration purple coatings under we there sameless, i.e., experimental the fluorine conditions. resin coating The foamingwith the visible light stealth of low infrared emissive coatings in engineering. densitiespurple paste of the has dark the yellowbetter perfor and purplemance. coatings This work we reprovides less, i.e., a nethew fluorine prospect resin for thecoating compatibility with the purpleFunding:with visible pasteThis light has research thestealth receivedbetter of lowperfor no infrar externalmance.ed funding.emissive This work coatings provides in engineering. a new prospect for the compatibility Acknowledgments:with visible light stealthThis project of low is supportedinfrared emissive by the Youth coatings Science in andengineering. Technology Innovation Fund of Nanjing ForestryFunding: University This research (CX2016018). received no external funding. Funding: This research received no external funding. ConflictsAcknowledgments: of Interest: ThisThe project authors is declare supported no conflict by the Youth of interest. Science and Technology Innovation Fund of Nanjing Acknowledgments:Forestry University (CX2016018).This project is supported by the Youth Science and Technology Innovation Fund of Nanjing ForestryReferencesConflicts University of Interest: (CX2016018). The authors declare no conflict of interest. Conflicts1. Liang, of J.;Interest: Li, W.; Xu,The G.Y.;authors Meng, declare X.; Liu, no K.;conflict Tan, of S.J. interest. Preparation and characterization of the colored coating Referenceswith low infrared emissivity based on nanometer pigment. Prog. Org. Coat. 2018, 115, 74–78. [CrossRef] References2.1. Fang,Liang, K.Y.; J.; Li, Fang, W.; Xu, F. 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