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Preparation and characterization of a greenish yellow lackluster coating with low infrared emissivity based on Prussian blue modified aluminum
Progress in Organic Coatings 77 (2014) 1163–1168
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and characterization of a greenish yellow lackluster coating with low infrared emissivity based on Prussian blue modified aluminum Wei-min Tan a,∗ , Liu-fang Wang b , Fei Yu a , Ning Huang c , Li-jun Wang a , Wei-liang Ni a , Jun-zhi Zhang a a National Engineering Research Center for Coatings, CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd, Changzhou 213016, PR China b Specialty Coatings Department, CNOOC Changzhou EP Coating Co., Ltd, Changzhou 213013, PR China c National Quality Supervision and Test Center for Coatings, CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd, Changzhou 213016, PR China
a r t i c l e
i n f o
Article history: Received 30 December 2013 Received in revised form 4 March 2014 Accepted 3 April 2014 Available online 4 May 2014 Keywords: Infrared emissivity Optical properties Mechanical properties Prussian blue Al powder Composite coatings
a b s t r a c t Greenish yellow lackluster coatings with low infrared emissivity were prepared by Prussian blue (PB) surface modified Al powders and polyurethanes. The morphology and component of PB/Al powder were characterized by scanning electron microscopy and X-ray diffractometer. The infrared emissivity, surface gloss and visible light color of PB/Al composite coating were investigated by an infrared emissometer, a glossmeter and a colorimeter, respectively. Mechanical properties of PB/Al composite coatings were studied by using adhesion test and impact strength test. The results indicate that PB/Al powder decreases not only the gloss of the coating, but also its emissivity within the wavelength range of 8–14 m. The composite coatings have good adherence and impact strength at PB/Al content below 50 wt.%, and then the mechanical properties decrease in the PB/Al content range from 50 wt.% to 60 wt.%. By comparing PB/Al composite coating and Al powder tinting coating with the same color and surface gloss, PB/Al composite coating exhibits significant lower infrared emissivity, which is attributed to closer inter-powder distances of metallic fillers and higher electrical conductivity in the coating. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It is known that infrared emissivity is an important factor for electromagnetic radiation, where higher emissivity can result in higher radiation energy. Thus, infrared emissivity of some materials is expected to be controlled to fulfill special requirements. For example, low infrared emissivity materials are required for vehicles and aircrafts, which could decrease the radiation energy to cloak these equipments from detection by electromagnetic waves. As we know, coating exterior surface of objects with low emissivity materials is the most convenient method to achieve this requirement. In the past decade, several studies focused on decreasing coating infrared emissivity have been reported, such as nanocomposite films [1,2], multilayer structures [3,4], transparent conductive
∗ Corresponding author at: CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd, No. 22 Middle Longjiang Road, Changzhou 213016, PR China. Tel.: +86 519 83299300; fax: +86 519 83299300. E-mail address: [email protected] (W.-m. Tan). http://dx.doi.org/10.1016/j.porgcoat.2014.04.003 0300-9440/© 2014 Elsevier B.V. All rights reserved.
oxide compounds [3–6], organic/inorganic composite coatings [7,8], as well as Ag and Cu [9,10]. Especially, organic/inorganic composite coatings are promising with advantages of low cost and excellent performance for engineering applications. In general, organic/inorganic composite coatings are composed of organic adhesives and inorganic pigments. Among organic adhesives, polyurethanes are widely used for their excellent physical and durability properties, as well as high tensile, impact strengths, and resistance to chemicals, corrosion, scratches, abrasion [11–16]. However, polyurethanes contain some strong photoabsorptive groups, which are sensitive in the wavelength range of 8–14 m [17]. Therefore, special metallic pigments such as Al powders are usually used as fillers to decrease the coating emissivity due to their high spectral reflectance and low infrared emissivity [18,19]. However, the high glossiness of Al powders inevitably leads to compatibility problems with visible and near-infrared light, which constrain their application ranges [20–22]. For this reason, superficial pretreatment is thought to be one way to reduce the gloss and maintain low emissivity of Al power. But to our knowledge, there are very few studies carried out on this approach.
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Fig. 1. SEM images of (a and b) Al powders and (c and d) PB surface modified Al composites.
Herein, we demonstrated a convenient method of using PB surface modified Al powders and polyurethanes to prepare low infrared emissivity coatings. The influences of modified Al powder content on the infrared emissivity, surface gloss and mechanical properties were systematically investigated. Furthermore, comparing to Al powder tinting coatings, the effects of PB/Al powder on the infrared emissivity and visible light performances of the composite coatings were also discussed.
the mixture was maintained at 80 ◦ C for 4 h to yield greenish yellow PB surface modified Al powders. The solid product was then centrifuged and washed with deionized water for several times. Finally, the product was dried at 80 ◦ C for 4 h in an oven. 2.3. Preparation of PB/Al composite coating and Al tinting coating
All solvents and chemicals were of analytical grade and used without further purification. Potassium ferricyanide (K3 Fe(CN)6 ), ferric chloride (FeCl3 ·6H2 O), acetic acid and other chemicals were obtained from Shanghai Chemical Reagent Co., Ltd, China. Aluminum powders (particle size is 30–50 m) were purchased from Zhangqiu Metallic Pigment Co., Ltd, China. (Cr,Sb,Ti)O2 , (Co,Ni,Zn)2 (Ti,Al)O4 and Fe(Fe,Cr)2 O4 pigments for tinting were purchased from Heubach Ltd, Germany. The polyurethane resin and universal tinting colorants were kindly supplied by CNOOC Changzhou EP Coating Co., Ltd, China. Deionized water was used throughout the experiments.
In the process, tinplate sheets with dimensions of 12 cm × 5 cm × 0.3 cm were used as substrates. The sheets were rubbed using fine abrasive paper and rinsed with acetone. PB/Al composite coating and Al tinting coating were prepared by the following method. First, fixed amounts of PB/Al composites and polyurethane were mixed under continuous stirring for 1 h, or calculated ratios of uniformly dispersed tinting colorants and Al powder were mixed with polyurethane under continuous stirring with the same time. Then the mixture was painted onto tinplate substrates by the sputtering method using an accurate speed motor and appropriate pressure, the spray rate could be adjusted by controlling the rotation speed of the spinner. The distance between substrate and spray gun is about 25 cm and the spray gun should be perpendicular to the substrate during spraying. The coating thickness was controlled about 40 m. Finally, the coatings were cured in an oven at 80 ◦ C for 2 h or at room temperature for 7 days and kept for further analysis.
2.2. Preparation of PB/Al powders
2.4. Characterization
Firstly, 5 g Al powders were dispersed in 500 ml deionized water. Then, this dispersion was reacted with 2.5 mM FeCl3 aqueous solution (pH 4, adjusted with acetic acid) by ultrasonic agitation for 1 h. After that, 25 mL aqueous solution of K3 Fe(CN)6 (0.1 mol/L)was added drop by drop to this mixture with continuous stirring, and
The surface morphology and structure of the samples were directly inspected by JSM-5610LV scanning electron microscopy system (Japan). The phase formations of samples were analyzed by Shimadzu-3000 X-ray diffractometer (Japan) with Cu K˛ radiation source ( = 0.154056) operated at 40 kV and 35 mA, and the
2. Experimental 2.1. Materials
W.-m. Tan et al. / Progress in Organic Coatings 77 (2014) 1163–1168
Fig. 2. X-ray diffraction patterns of (a) Al powders and (b) PB surface modified Al composites.
scan rate (2) of 2◦ min−1 was applied to record the pattern in the 2 range of 10–85◦ by means of a solid detector and a scintillation counter. The infrared emissivities of the samples within 3–14 m were measured by Bruker Vertex 70 Infrared Emissometer (Germany) at 80 ◦ C, with a standard temperature error of ±0.1 ◦ C. The surface gloss of the coating was observed by MG268-F2 glossimeter (China), at an incidence angle of 60◦ . The coating thickness was measured using a digital magnetism thickness instrument (China), with a standard instrumental error of ±1 m. The electrical conductivity of the composite coatings was measured using the conventional four-probe DC method. The adhesion property of the coatings was determined using QFZ-II circle-cut tester (China) according to China National Standard GB 1720-79(89). The impact strength of the coating was evaluated by QCJ impact instrument (China) according to China National Standard GB/T 1732-93. For the characterization of color measurement, the spectral reflectance and the CIE L*a*b* of all the coating samples under 10◦ were performed by Spectraflash SF450 colorimeter (D65 illuminant, USA). 3. Results and discussion 3.1. Characterization of PB/Al powders 3.1.1. SEM observation The SEM images of Al powders and PB modified Al composites are shown in Fig. 1. From Fig. 1(a) and (b), it can be found that Al powder pigments have irregular and sheet-shaped structure with smooth surface. Fig. 1(c) and (d) shows the SEM micrographs of PB surface modified Al powders, Fig. 1(d) is the high-magnification SEM image of modified powder surface in Fig. 1(c). After being coated, a massive particulate matters emerge on the Al powder surface, which leads rough surface. It can be seen from Fig. 1(d) that these particulate matters are composed of nanocubes with an average edge length about 50 nm, which densely gather on the surface of Al powder pigments, serving as a dense covering film. The difference of these SEM images indicates that the coating process has changed the surface morphology a lot and PB has been generated on the surface of Al powder pigment with the shape of nanocube. 3.1.2. XRD analysis X-ray powder diffraction was used to characterize the Al and PB/Al powders (shown in Fig. 2). A typical XRD pattern of Al is
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Fig. 3. Relationship between PB/Al powder content and the surface gloss of PB/Al composite coatings.
shown in Fig. 2(a). For Al powders, the 2 values of 38.49◦ and 44.75◦ correspond to the (1 1 1) and (2 0 0) lattice planes, respectively. The structure of PB/Al composites can also be confirmed by the XRD pattern. As shown in Fig. 2(b), the intense peaks corresponding to 2 = 17.48 (2 0 0), 2 = 24.68 (2 2 0), 2 = 35.28 (4 0 0) and 2 = 39.48 (4 2 0), Bragg reflections of Fe4 [Fe(CN)6 ]3 are in good agreement with those reported for Fe4 [Fe(CN)6 ]3 crystal particles [23]. This result indicates that the PB/Al powders contain Prussian blue crystal particles. An estimation of mean size of Prussian blue particles is performed from the width of the (2 2 0) Bragg reflection using the Debye–Scherrer equation. The mean size of the Prussian blue particles is about 43 nm, this result basically accord with the size of the Prussian blue nanocubes, which is viewed from the SEM image of PB surface modified Al powder in Fig. 1(d). 3.2. Effect of PB/Al powder content on the infrared emissivity and surface gloss of composite coatings In order to study the influence of PB modified Al powders on the emissivity and surface gloss of the composite coatings, experiments were carried out with different PB/Al powder concentrations under the same processing condition. The results are shown in Figs. 3 and 4. Firstly, The glosses of PB/Al coatings were determined. Fig. 3 reveals that the gloss of modified Al powder coating decreases from 9.7 to 4.4 with increased PB/Al powder content from 30 wt.% to 60 wt.%. According to the accredited standard, the specular gloss of a lackluster coating film should be less than 6 when the detected angle is 60◦ [20]. Fig. 3 implies that when the coating contains more than 40 wt.% of PB modified Al powder, the specular gloss will almost reduce to under 6. Therefore, a composite coating gloss lower than 6 can be obtained when the minimum PB/Al powder content exceeds 40 wt.%. Secondly, the relationship between PB/Al content and infrared emissivity of composite coatings is shown in Fig. 4. It can be seen that the emissivities of composite coatings are significantly reduced with increasing PB/Al content. The average emissivity approaches the lowest value of 0.426 when the PB/Al content is about 60 wt.% in the wavelength between 8 and 14 m, where we are mostly caring about. On the other hand, the peaks within range of 8–14 m in Fig. 4(a) are assigned to the vibration absorptions of polyurethane groups, and these peaks are weakened by increasing PB/Al content in the composite coating as shown in Fig. 4(b–e). So the emissivities
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Kirchhoff’s law gives the relationship between the refractive index (n) and reflectivity (): =
n − 1 2 n+1
=
1−
2 n+1
2 (2)
Also Kirchhoff’s law and Principle of Conservation of Energy give the relationship of absorptivity (˛), reflectivity (), and transmittivity ( ) [25,26]. ˛++ =1
(3)
According to Eqs. (1) and (2), it can be concluded that the increasing content of PB/Al powder in the coatings would leads to higher electrical conductivity and thus make higher refractive index and reflectivity, which in turn caused lower emissivity. Eq. (3) indicates that the emissivity of any body is equal to its absorbance (ε = ˛). For an opaque surface, = 0; consequently, ε = 1 − . Therefore, the infrared emissivities of the composite coatings decrease with increased PB/Al content, due to the increase of the electrical conductivity. Fig. 4. Relationship between PB/Al powder content and the infrared emissivity of PB/Al composite coatings: (a) pure polyurethane, (b) 30% PB/Al, (c) 40% PB/Al, (d) 50% PB/Al, and (e) 60% PB/Al.
of modified Al composite coatings are obvious lower than the emissivity of pure polyurethane at the region of 8–14 m, which means Al powder has a greater influence on decreasing the coating emissivity after treatment with Prussian blue, and this result proves that PB modified Al powder can be used to overcome the high emissivity of polyurethane. Therefore, PB/Al powder drastically decreases not only the coating gloss, but also the coating emissivity. These trends that based on the relationship between the structures and properties of PB/Al composites can be explained as following: on one hand, PB is an inorganic wave-scatter agent that can lower the Al coating surface gloss, thereby, improving visible/near-infrared light compatibility. On the other hand, as Al powders are coated with PB nanocubes, the covering film is not enough thick to shade the high reflectance of Al powder. As we know, the increased PB/Al content would increase the amount of the Al powders in the polyurethane adhesives, resulting in the closer packing of the powders, which leads to the increase of electrical conductivity of the coatings [24]. The effect of the PB/Al content on the electrical conductivity of the composite coatings is summarized in Table 1. The electrical conductivity of the composite coatings depends upon the amount of PB/Al powder in the polyurethane. At higher PB/Al loads, the conductivity increases, suggesting that PB/Al powder act as electrically conductive bridges. According to optical basic theory, the refractive index (n) can be denoted from the equation: n = 2 2
ε2
+
4 2
2
+ε
(1)
where is the electrical conductivity, ε is the dielectric constant, is the magnetism conductivity and is the Planck’s constant. And Table 1 Electrical conductivity and mechanical properties of PB/Al composite coatings with different PB/Al content. PB/Al content (wt.%)
Electrical conductivity (S cm−1 )
Adhesion strength (grade)
Impact strength (kg cm)
30 40 50 60
225 340 382 418
1 1 1 2
50 50 50 40
3.3. Mechanical properties of PB/Al composite coatings In order to evaluate the adhesion between coating layer and substrate and coatings fatigue behaviors, the adhesion strength and impact strength of the coatings were systematically investigated, the results are also listed in Table 1. The adhesion strength of the coatings measured by circle-cut test determines the extent to which they stick to the substrates under defined loads. Table 1 reveals that the adhesion strength keeps unchanged by increasing the PB/Al content up to 50 wt.%, which can be attributed to the high cohesive strength and mechanical interlocking of the composite coating [27]. However, further increase of PB/Al content from 50 wt.% to 60 wt.% leads to fracture easily with the adherence of 2 grade, perhaps due to fillers accumulation at higher content and difficulty in their dispersion. In this case, the increase of PB/Al content reduces the chance of energy dissipation upon an external force, and result in the decrease in the coating flexibility [28]. The impact strength test implies dynamic loading of the coating surface. As seen in Table 1, the impact strength is 50 kg cm and unchanged when the PB/Al content increased from 30 wt.% to 50 wt.%, but the impact strength is decreased to 40 kg cm with further increase of the PB/Al content from 50 wt.% to 60 wt.%. The change trend is in agreement with the above section for adhesion, indicating that the excellent adhesion strength improved the impact resistance of the composite coatings [29]. Furthermore, it is considered that the outstanding adherence and impact resistance may be attributed to prominent adhesion property of polyurethane which leads to preferable combination between filler and polymer resin and less interstices in the composite coating [27]. Therefore, the great adherence and impact resistance of a coating denotes that it is less susceptible to fracture, which is one of the most important performances of a successful coating. 3.4. Comparison of infrared emissivities between PB/Al composite coating and Al powder tinting coating To investigate the practicability of PB/Al powder in colorful lackluster coating which required low infrared emissivity, the properties of PB/Al composite coating and Al powder tinted coating were compared. For the characterization of visible light optical properties, CIE L*a*b* is an important parameter. Among them, L* is the lightness, a* is red or green, and b* is yellow or blue. From the different CIE L*a*b* values, it indicates the samples have different colors. The CIE L*a*b* values and surface glosses of PB/Al composite coating and Al
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Table 2 The CIE L*a*b* values and surface glosses of PB/Al composite coating and Al powder tinted coating. Samples
L*
a*
b*
Surface gloss (Gs)
PB/Al composite coating Al tinting coating
54.41 54.42
−7.32 −7.21
17.98 18.73
5.8 5.5
Fig. 7. The schematic of electromagnetic wave through (a) PB/Al composite coating and (b) Al powder tinting coating.
Fig. 5. Comparison of (a) PB/Al composite coating and (b) Al powder tinting coating on the infrared emissivity of coating.
powder tinted coating were measured, the results are shown in Table 2. From Table 2, it can be seen that samples have similar CIE L*a*b* values and surface glosses at 60◦ , which reveals they are lackluster coatings with same color at visible light region. Subsequently, the infrared emissivities of these two coatings were measured as shown in Fig. 5. It can be seen that the emissivity of PB/Al coating is significant lower than the emissivity of Al powder tinted coating. The average emissivity of the first one reaches 0.53 when the latter one is about 0.78 at the region of 8–14 m, which illuminates that PB modified Al powder is very useful to overcome the high emissivity of tinted coating. To study the influence of fillers on the emissivity of the composite coatings, the morphologies of coatings were inspected as shown in Fig. 6. The morphology of Fig. 6(a) displays a good visual distribution that PB/Al powders are well dispersed into polyurethane, and the obtained coating is well homogeneous, which shows a uniformly surface. At the same time, the good dispersion may
cause the closer packing of PB/Al powders and thus increase powder-to-powder contact, therefore lead to much higher electrical conductivity, which in turn results in lower emissivity. It is noted that an obvious difference is observed when Al powder was used for tinted coating. Fig. 6(b) clearly shows that Al powders are discretely distributed under other pigments in the composite coating, with relatively large inter-powder distances, therefore, there are many interspaces between the Al powders, which leads to lower electrical conductivity and higher emissivity. Finally, The schematic of electromagnetic wave acting on coatings surface is depicted in Fig. 7. As PB/Al composite coating possesses powders contact each other closer (Fig. 7a), most of the infrared radiation is absolutely reflected due to the presence of many PB modified metallic powders, and only little radiation is absorbed. However, for the Al powder tinted coating with large inter-powder distance of metallic powders (Fig. 7b), a significant absorptance increase and reflectance decrease can be observed from the coating due to the presence of many interspaces in the coating. According to Kirchhoff’s law, when a system is isothermal and in thermal equilibrium, the absorbance is strictly equal to the emissivity [25]. Therefore, compared with Al powder tinted coating, the PB/Al composite coating has more metallic filler contact in the adhesive and thus causes higher reflectance and lower absorptance, which results in lower emissivity.
Fig. 6. SEM images of (a) PB/Al composite coating and (b) Al powder tinting coating.
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4. Conclusions In summary, we have demonstrated a simple method of preparing low infrared emissivity lackluster coatings with PB/Al powders and polyurethanes. SEM and XRD analyses indicated that Al powders were successfully coated with Prussian blue nanocubes by a traditional wet chemical method. The PB modified Al powders decreased the gloss and infrared emissivity of the composite coating with increased PB/Al powder content from 30 wt.% to 60 wt.%. The adhesion and impact strength reached to 1 grade and 50 kg cm, respectively, at PB/Al powder content below 50 wt.%. In addition, the infrared emissivity of PB/Al coating was significantly lower than the emissivity of Al powder tinted coating with the same greenish yellow color and surface gloss. The achievement of low infrared emissivity is thought to be generated by closer inter-powder distances of metallic powders, which are effective to higher electrical conductivity. The lackluster composite coatings with low infrared emissivity may have great potential applications in cloaking equipments from detection by infrared waves. Acknowledgments This work was supported financially by CNOOC Technology Development Program (CNOOC-KJ 125 00 00 000 00 CTY/201104) and Changzhou Research Program on Applied Fundamentals (20120363). References [1] X. Bu, Y. Zhou, M. He, Z. Chen, T. Zhang, Appl. Surf. Sci. 288 (2014) 444–451. [2] T. Wang, J. He, J. Zhou, J. Tang, Y. Guo, X. Ding, S. Wu, J. Zhao, J. Solid State Chem. 183 (2010) 2797–2804.
[3] L. Kang, Y. Gao, Z. Chen, J. Du, Z. Zhang, H. Luo, Sol. Energ. Mater. Sol. C 94 (2010) 2078–2084. [4] X. Chu, H. Tao, Y. Liu, J. Ni, J. Bao, X. Zhao, J. Non-Cryst. Solids 383 (2014) 121–125. [5] Z. Zhang, Y. Gao, H. Luo, L. Kang, Z. Chen, J. Du, M. Kanehira, Y. Zhang, Z.L. Wang, Energy Environ. Sci. 4 (2011) 4290–4297. [6] W. Wang, L. Zhang, Y. Liu, H. Xu, Y. Zhong, Z. Mao, Ind. Eng. Chem. Res. 52 (2013) 15066–15074. [7] X. Yan, G. Xu, Prog. Org. Coat. 73 (2012) 232–238. [8] H.A. Babrekar, J.P. Jog, V.L. Mathe, D.K. Avasthi, S. Ojha, S.V. Bhoraskar, Nucl. Instrum. Meth. B 287 (2012) 135–140. [9] W. Zhang, G. Xu, R. Ding, J. Qiao, K. Duan, Physica B 422 (2013) 36–39. [10] W. Zhang, G. Xu, R. Ding, K. Duan, J. Qiao, Mater. Sci. Eng. C 33 (2013) 99–102. [11] A.K. Mishra, R. Narayan, K.V.S.N. Raju, T.M. Aminabhavi, Prog. Org. Coat. 74 (2012) 134–141. [12] G. Markevicius, S. Chaudhuri, C. Bajracharya, R. Rastogi, J. Xiao, C. Burnett, T.Q. Chastek, Prog. Org. Coat. 75 (2012) 319–327. [13] S. Thakur, N. Karak, Prog. Org. Coat. 76 (2013) 157–164. [14] X. Kong, G. Liu, H. Qi, J.M. Curtis, Prog. Org. Coat. 76 (2013) 1151–1160. [15] D. Ren, C.E. Frazier, Int. J. Adhes. Adhes. 45 (2013) 77–83. [16] Y. Zhang, J. Maxted, A. Barber, C. Lowe, R. Smith, Polym. Degrad. Stabil. 98 (2013) 527–534. [17] Y. Yang, Y. Zhou, J. Ge, X. Yang, React. Funct. Polym. 72 (2012) 574–579. [18] Y. Wang, G. Xu, H. Yu, C. Hu, X. Yan, T. Guo, J. Li, Appl. Surf. Sci. 257 (2011) 4743–4748. [19] C. Hu, G. Xu, X. Shen, J. Alloy Compd. 486 (2009) 371–375. [20] G. Wu, D. Yu, Prog. Org. Coat. 76 (2013) 107–112. [21] L. Yuan, X. Weng, J. Xie, W. Du, L. Deng, J. Alloy Compd. 580 (2013) 108–113. [22] L. Yuan, X. Weng, W. Du, J. Xie, L. Deng, J. Alloy Compd. 583 (2014) 492–497. [23] L. Wang, Y. Ye, X. Lu, Y. Wu, L. Sun, H. Tan, F. Xu, Y. Song, Electrochim. Acta 114 (2013) 223–232. [24] H. Yu, G. Xu, X. Shen, X. Yan, C. Shao, C. Hu, Prog. Org. Coat. 66 (2009) 161–166. [25] M. Auslender, S. Hava, Infrared Phys. Tech. 36 (1995) 1077–1088. [26] S. Enoch, J.J. Simon, L. Escoubas, Z. Elalmy, F. Lemarquis, P. Torchio, G. Albrand, Appl. Phys. Lett. 86 (2005) 261101. [27] C.J. Patel, A. Dighe, Prog. Org. Coat. 60 (2007) 219–223. [28] S.M. Mirabedini, M. Mohseni, Sh. PazokiFard, M. Esfandeh, Colloid Surf. A 317 (2008) 80–86. [29] P.W. Shuma, Z.F. Zhoua, K.Y. Lia, C.Y. Chan, Thin Solid Films 458 (2004) 203–211.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation and characterization of a greenish yellow lackluster coating with low infrared emissivity based on Prussian blue modified aluminum Wei-min Tan a,∗ , Liu-fang Wang b , Fei Yu a , Ning Huang c , Li-jun Wang a , Wei-liang Ni a , Jun-zhi Zhang a a National Engineering Research Center for Coatings, CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd, Changzhou 213016, PR China b Specialty Coatings Department, CNOOC Changzhou EP Coating Co., Ltd, Changzhou 213013, PR China c National Quality Supervision and Test Center for Coatings, CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd, Changzhou 213016, PR China
a r t i c l e
i n f o
Article history: Received 30 December 2013 Received in revised form 4 March 2014 Accepted 3 April 2014 Available online 4 May 2014 Keywords: Infrared emissivity Optical properties Mechanical properties Prussian blue Al powder Composite coatings
a b s t r a c t Greenish yellow lackluster coatings with low infrared emissivity were prepared by Prussian blue (PB) surface modified Al powders and polyurethanes. The morphology and component of PB/Al powder were characterized by scanning electron microscopy and X-ray diffractometer. The infrared emissivity, surface gloss and visible light color of PB/Al composite coating were investigated by an infrared emissometer, a glossmeter and a colorimeter, respectively. Mechanical properties of PB/Al composite coatings were studied by using adhesion test and impact strength test. The results indicate that PB/Al powder decreases not only the gloss of the coating, but also its emissivity within the wavelength range of 8–14 m. The composite coatings have good adherence and impact strength at PB/Al content below 50 wt.%, and then the mechanical properties decrease in the PB/Al content range from 50 wt.% to 60 wt.%. By comparing PB/Al composite coating and Al powder tinting coating with the same color and surface gloss, PB/Al composite coating exhibits significant lower infrared emissivity, which is attributed to closer inter-powder distances of metallic fillers and higher electrical conductivity in the coating. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It is known that infrared emissivity is an important factor for electromagnetic radiation, where higher emissivity can result in higher radiation energy. Thus, infrared emissivity of some materials is expected to be controlled to fulfill special requirements. For example, low infrared emissivity materials are required for vehicles and aircrafts, which could decrease the radiation energy to cloak these equipments from detection by electromagnetic waves. As we know, coating exterior surface of objects with low emissivity materials is the most convenient method to achieve this requirement. In the past decade, several studies focused on decreasing coating infrared emissivity have been reported, such as nanocomposite films [1,2], multilayer structures [3,4], transparent conductive
∗ Corresponding author at: CNOOC Changzhou Paint and Coatings Industry Research Institute Co., Ltd, No. 22 Middle Longjiang Road, Changzhou 213016, PR China. Tel.: +86 519 83299300; fax: +86 519 83299300. E-mail address: [email protected] (W.-m. Tan). http://dx.doi.org/10.1016/j.porgcoat.2014.04.003 0300-9440/© 2014 Elsevier B.V. All rights reserved.
oxide compounds [3–6], organic/inorganic composite coatings [7,8], as well as Ag and Cu [9,10]. Especially, organic/inorganic composite coatings are promising with advantages of low cost and excellent performance for engineering applications. In general, organic/inorganic composite coatings are composed of organic adhesives and inorganic pigments. Among organic adhesives, polyurethanes are widely used for their excellent physical and durability properties, as well as high tensile, impact strengths, and resistance to chemicals, corrosion, scratches, abrasion [11–16]. However, polyurethanes contain some strong photoabsorptive groups, which are sensitive in the wavelength range of 8–14 m [17]. Therefore, special metallic pigments such as Al powders are usually used as fillers to decrease the coating emissivity due to their high spectral reflectance and low infrared emissivity [18,19]. However, the high glossiness of Al powders inevitably leads to compatibility problems with visible and near-infrared light, which constrain their application ranges [20–22]. For this reason, superficial pretreatment is thought to be one way to reduce the gloss and maintain low emissivity of Al power. But to our knowledge, there are very few studies carried out on this approach.
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Fig. 1. SEM images of (a and b) Al powders and (c and d) PB surface modified Al composites.
Herein, we demonstrated a convenient method of using PB surface modified Al powders and polyurethanes to prepare low infrared emissivity coatings. The influences of modified Al powder content on the infrared emissivity, surface gloss and mechanical properties were systematically investigated. Furthermore, comparing to Al powder tinting coatings, the effects of PB/Al powder on the infrared emissivity and visible light performances of the composite coatings were also discussed.
the mixture was maintained at 80 ◦ C for 4 h to yield greenish yellow PB surface modified Al powders. The solid product was then centrifuged and washed with deionized water for several times. Finally, the product was dried at 80 ◦ C for 4 h in an oven. 2.3. Preparation of PB/Al composite coating and Al tinting coating
All solvents and chemicals were of analytical grade and used without further purification. Potassium ferricyanide (K3 Fe(CN)6 ), ferric chloride (FeCl3 ·6H2 O), acetic acid and other chemicals were obtained from Shanghai Chemical Reagent Co., Ltd, China. Aluminum powders (particle size is 30–50 m) were purchased from Zhangqiu Metallic Pigment Co., Ltd, China. (Cr,Sb,Ti)O2 , (Co,Ni,Zn)2 (Ti,Al)O4 and Fe(Fe,Cr)2 O4 pigments for tinting were purchased from Heubach Ltd, Germany. The polyurethane resin and universal tinting colorants were kindly supplied by CNOOC Changzhou EP Coating Co., Ltd, China. Deionized water was used throughout the experiments.
In the process, tinplate sheets with dimensions of 12 cm × 5 cm × 0.3 cm were used as substrates. The sheets were rubbed using fine abrasive paper and rinsed with acetone. PB/Al composite coating and Al tinting coating were prepared by the following method. First, fixed amounts of PB/Al composites and polyurethane were mixed under continuous stirring for 1 h, or calculated ratios of uniformly dispersed tinting colorants and Al powder were mixed with polyurethane under continuous stirring with the same time. Then the mixture was painted onto tinplate substrates by the sputtering method using an accurate speed motor and appropriate pressure, the spray rate could be adjusted by controlling the rotation speed of the spinner. The distance between substrate and spray gun is about 25 cm and the spray gun should be perpendicular to the substrate during spraying. The coating thickness was controlled about 40 m. Finally, the coatings were cured in an oven at 80 ◦ C for 2 h or at room temperature for 7 days and kept for further analysis.
2.2. Preparation of PB/Al powders
2.4. Characterization
Firstly, 5 g Al powders were dispersed in 500 ml deionized water. Then, this dispersion was reacted with 2.5 mM FeCl3 aqueous solution (pH 4, adjusted with acetic acid) by ultrasonic agitation for 1 h. After that, 25 mL aqueous solution of K3 Fe(CN)6 (0.1 mol/L)was added drop by drop to this mixture with continuous stirring, and
The surface morphology and structure of the samples were directly inspected by JSM-5610LV scanning electron microscopy system (Japan). The phase formations of samples were analyzed by Shimadzu-3000 X-ray diffractometer (Japan) with Cu K˛ radiation source ( = 0.154056) operated at 40 kV and 35 mA, and the
2. Experimental 2.1. Materials
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Fig. 2. X-ray diffraction patterns of (a) Al powders and (b) PB surface modified Al composites.
scan rate (2) of 2◦ min−1 was applied to record the pattern in the 2 range of 10–85◦ by means of a solid detector and a scintillation counter. The infrared emissivities of the samples within 3–14 m were measured by Bruker Vertex 70 Infrared Emissometer (Germany) at 80 ◦ C, with a standard temperature error of ±0.1 ◦ C. The surface gloss of the coating was observed by MG268-F2 glossimeter (China), at an incidence angle of 60◦ . The coating thickness was measured using a digital magnetism thickness instrument (China), with a standard instrumental error of ±1 m. The electrical conductivity of the composite coatings was measured using the conventional four-probe DC method. The adhesion property of the coatings was determined using QFZ-II circle-cut tester (China) according to China National Standard GB 1720-79(89). The impact strength of the coating was evaluated by QCJ impact instrument (China) according to China National Standard GB/T 1732-93. For the characterization of color measurement, the spectral reflectance and the CIE L*a*b* of all the coating samples under 10◦ were performed by Spectraflash SF450 colorimeter (D65 illuminant, USA). 3. Results and discussion 3.1. Characterization of PB/Al powders 3.1.1. SEM observation The SEM images of Al powders and PB modified Al composites are shown in Fig. 1. From Fig. 1(a) and (b), it can be found that Al powder pigments have irregular and sheet-shaped structure with smooth surface. Fig. 1(c) and (d) shows the SEM micrographs of PB surface modified Al powders, Fig. 1(d) is the high-magnification SEM image of modified powder surface in Fig. 1(c). After being coated, a massive particulate matters emerge on the Al powder surface, which leads rough surface. It can be seen from Fig. 1(d) that these particulate matters are composed of nanocubes with an average edge length about 50 nm, which densely gather on the surface of Al powder pigments, serving as a dense covering film. The difference of these SEM images indicates that the coating process has changed the surface morphology a lot and PB has been generated on the surface of Al powder pigment with the shape of nanocube. 3.1.2. XRD analysis X-ray powder diffraction was used to characterize the Al and PB/Al powders (shown in Fig. 2). A typical XRD pattern of Al is
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Fig. 3. Relationship between PB/Al powder content and the surface gloss of PB/Al composite coatings.
shown in Fig. 2(a). For Al powders, the 2 values of 38.49◦ and 44.75◦ correspond to the (1 1 1) and (2 0 0) lattice planes, respectively. The structure of PB/Al composites can also be confirmed by the XRD pattern. As shown in Fig. 2(b), the intense peaks corresponding to 2 = 17.48 (2 0 0), 2 = 24.68 (2 2 0), 2 = 35.28 (4 0 0) and 2 = 39.48 (4 2 0), Bragg reflections of Fe4 [Fe(CN)6 ]3 are in good agreement with those reported for Fe4 [Fe(CN)6 ]3 crystal particles [23]. This result indicates that the PB/Al powders contain Prussian blue crystal particles. An estimation of mean size of Prussian blue particles is performed from the width of the (2 2 0) Bragg reflection using the Debye–Scherrer equation. The mean size of the Prussian blue particles is about 43 nm, this result basically accord with the size of the Prussian blue nanocubes, which is viewed from the SEM image of PB surface modified Al powder in Fig. 1(d). 3.2. Effect of PB/Al powder content on the infrared emissivity and surface gloss of composite coatings In order to study the influence of PB modified Al powders on the emissivity and surface gloss of the composite coatings, experiments were carried out with different PB/Al powder concentrations under the same processing condition. The results are shown in Figs. 3 and 4. Firstly, The glosses of PB/Al coatings were determined. Fig. 3 reveals that the gloss of modified Al powder coating decreases from 9.7 to 4.4 with increased PB/Al powder content from 30 wt.% to 60 wt.%. According to the accredited standard, the specular gloss of a lackluster coating film should be less than 6 when the detected angle is 60◦ [20]. Fig. 3 implies that when the coating contains more than 40 wt.% of PB modified Al powder, the specular gloss will almost reduce to under 6. Therefore, a composite coating gloss lower than 6 can be obtained when the minimum PB/Al powder content exceeds 40 wt.%. Secondly, the relationship between PB/Al content and infrared emissivity of composite coatings is shown in Fig. 4. It can be seen that the emissivities of composite coatings are significantly reduced with increasing PB/Al content. The average emissivity approaches the lowest value of 0.426 when the PB/Al content is about 60 wt.% in the wavelength between 8 and 14 m, where we are mostly caring about. On the other hand, the peaks within range of 8–14 m in Fig. 4(a) are assigned to the vibration absorptions of polyurethane groups, and these peaks are weakened by increasing PB/Al content in the composite coating as shown in Fig. 4(b–e). So the emissivities
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Kirchhoff’s law gives the relationship between the refractive index (n) and reflectivity (): =
n − 1 2 n+1
=
1−
2 n+1
2 (2)
Also Kirchhoff’s law and Principle of Conservation of Energy give the relationship of absorptivity (˛), reflectivity (), and transmittivity ( ) [25,26]. ˛++ =1
(3)
According to Eqs. (1) and (2), it can be concluded that the increasing content of PB/Al powder in the coatings would leads to higher electrical conductivity and thus make higher refractive index and reflectivity, which in turn caused lower emissivity. Eq. (3) indicates that the emissivity of any body is equal to its absorbance (ε = ˛). For an opaque surface, = 0; consequently, ε = 1 − . Therefore, the infrared emissivities of the composite coatings decrease with increased PB/Al content, due to the increase of the electrical conductivity. Fig. 4. Relationship between PB/Al powder content and the infrared emissivity of PB/Al composite coatings: (a) pure polyurethane, (b) 30% PB/Al, (c) 40% PB/Al, (d) 50% PB/Al, and (e) 60% PB/Al.
of modified Al composite coatings are obvious lower than the emissivity of pure polyurethane at the region of 8–14 m, which means Al powder has a greater influence on decreasing the coating emissivity after treatment with Prussian blue, and this result proves that PB modified Al powder can be used to overcome the high emissivity of polyurethane. Therefore, PB/Al powder drastically decreases not only the coating gloss, but also the coating emissivity. These trends that based on the relationship between the structures and properties of PB/Al composites can be explained as following: on one hand, PB is an inorganic wave-scatter agent that can lower the Al coating surface gloss, thereby, improving visible/near-infrared light compatibility. On the other hand, as Al powders are coated with PB nanocubes, the covering film is not enough thick to shade the high reflectance of Al powder. As we know, the increased PB/Al content would increase the amount of the Al powders in the polyurethane adhesives, resulting in the closer packing of the powders, which leads to the increase of electrical conductivity of the coatings [24]. The effect of the PB/Al content on the electrical conductivity of the composite coatings is summarized in Table 1. The electrical conductivity of the composite coatings depends upon the amount of PB/Al powder in the polyurethane. At higher PB/Al loads, the conductivity increases, suggesting that PB/Al powder act as electrically conductive bridges. According to optical basic theory, the refractive index (n) can be denoted from the equation: n = 2 2
ε2
+
4 2
2
+ε
(1)
where is the electrical conductivity, ε is the dielectric constant, is the magnetism conductivity and is the Planck’s constant. And Table 1 Electrical conductivity and mechanical properties of PB/Al composite coatings with different PB/Al content. PB/Al content (wt.%)
Electrical conductivity (S cm−1 )
Adhesion strength (grade)
Impact strength (kg cm)
30 40 50 60
225 340 382 418
1 1 1 2
50 50 50 40
3.3. Mechanical properties of PB/Al composite coatings In order to evaluate the adhesion between coating layer and substrate and coatings fatigue behaviors, the adhesion strength and impact strength of the coatings were systematically investigated, the results are also listed in Table 1. The adhesion strength of the coatings measured by circle-cut test determines the extent to which they stick to the substrates under defined loads. Table 1 reveals that the adhesion strength keeps unchanged by increasing the PB/Al content up to 50 wt.%, which can be attributed to the high cohesive strength and mechanical interlocking of the composite coating [27]. However, further increase of PB/Al content from 50 wt.% to 60 wt.% leads to fracture easily with the adherence of 2 grade, perhaps due to fillers accumulation at higher content and difficulty in their dispersion. In this case, the increase of PB/Al content reduces the chance of energy dissipation upon an external force, and result in the decrease in the coating flexibility [28]. The impact strength test implies dynamic loading of the coating surface. As seen in Table 1, the impact strength is 50 kg cm and unchanged when the PB/Al content increased from 30 wt.% to 50 wt.%, but the impact strength is decreased to 40 kg cm with further increase of the PB/Al content from 50 wt.% to 60 wt.%. The change trend is in agreement with the above section for adhesion, indicating that the excellent adhesion strength improved the impact resistance of the composite coatings [29]. Furthermore, it is considered that the outstanding adherence and impact resistance may be attributed to prominent adhesion property of polyurethane which leads to preferable combination between filler and polymer resin and less interstices in the composite coating [27]. Therefore, the great adherence and impact resistance of a coating denotes that it is less susceptible to fracture, which is one of the most important performances of a successful coating. 3.4. Comparison of infrared emissivities between PB/Al composite coating and Al powder tinting coating To investigate the practicability of PB/Al powder in colorful lackluster coating which required low infrared emissivity, the properties of PB/Al composite coating and Al powder tinted coating were compared. For the characterization of visible light optical properties, CIE L*a*b* is an important parameter. Among them, L* is the lightness, a* is red or green, and b* is yellow or blue. From the different CIE L*a*b* values, it indicates the samples have different colors. The CIE L*a*b* values and surface glosses of PB/Al composite coating and Al
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Table 2 The CIE L*a*b* values and surface glosses of PB/Al composite coating and Al powder tinted coating. Samples
L*
a*
b*
Surface gloss (Gs)
PB/Al composite coating Al tinting coating
54.41 54.42
−7.32 −7.21
17.98 18.73
5.8 5.5
Fig. 7. The schematic of electromagnetic wave through (a) PB/Al composite coating and (b) Al powder tinting coating.
Fig. 5. Comparison of (a) PB/Al composite coating and (b) Al powder tinting coating on the infrared emissivity of coating.
powder tinted coating were measured, the results are shown in Table 2. From Table 2, it can be seen that samples have similar CIE L*a*b* values and surface glosses at 60◦ , which reveals they are lackluster coatings with same color at visible light region. Subsequently, the infrared emissivities of these two coatings were measured as shown in Fig. 5. It can be seen that the emissivity of PB/Al coating is significant lower than the emissivity of Al powder tinted coating. The average emissivity of the first one reaches 0.53 when the latter one is about 0.78 at the region of 8–14 m, which illuminates that PB modified Al powder is very useful to overcome the high emissivity of tinted coating. To study the influence of fillers on the emissivity of the composite coatings, the morphologies of coatings were inspected as shown in Fig. 6. The morphology of Fig. 6(a) displays a good visual distribution that PB/Al powders are well dispersed into polyurethane, and the obtained coating is well homogeneous, which shows a uniformly surface. At the same time, the good dispersion may
cause the closer packing of PB/Al powders and thus increase powder-to-powder contact, therefore lead to much higher electrical conductivity, which in turn results in lower emissivity. It is noted that an obvious difference is observed when Al powder was used for tinted coating. Fig. 6(b) clearly shows that Al powders are discretely distributed under other pigments in the composite coating, with relatively large inter-powder distances, therefore, there are many interspaces between the Al powders, which leads to lower electrical conductivity and higher emissivity. Finally, The schematic of electromagnetic wave acting on coatings surface is depicted in Fig. 7. As PB/Al composite coating possesses powders contact each other closer (Fig. 7a), most of the infrared radiation is absolutely reflected due to the presence of many PB modified metallic powders, and only little radiation is absorbed. However, for the Al powder tinted coating with large inter-powder distance of metallic powders (Fig. 7b), a significant absorptance increase and reflectance decrease can be observed from the coating due to the presence of many interspaces in the coating. According to Kirchhoff’s law, when a system is isothermal and in thermal equilibrium, the absorbance is strictly equal to the emissivity [25]. Therefore, compared with Al powder tinted coating, the PB/Al composite coating has more metallic filler contact in the adhesive and thus causes higher reflectance and lower absorptance, which results in lower emissivity.
Fig. 6. SEM images of (a) PB/Al composite coating and (b) Al powder tinting coating.
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4. Conclusions In summary, we have demonstrated a simple method of preparing low infrared emissivity lackluster coatings with PB/Al powders and polyurethanes. SEM and XRD analyses indicated that Al powders were successfully coated with Prussian blue nanocubes by a traditional wet chemical method. The PB modified Al powders decreased the gloss and infrared emissivity of the composite coating with increased PB/Al powder content from 30 wt.% to 60 wt.%. The adhesion and impact strength reached to 1 grade and 50 kg cm, respectively, at PB/Al powder content below 50 wt.%. In addition, the infrared emissivity of PB/Al coating was significantly lower than the emissivity of Al powder tinted coating with the same greenish yellow color and surface gloss. The achievement of low infrared emissivity is thought to be generated by closer inter-powder distances of metallic powders, which are effective to higher electrical conductivity. The lackluster composite coatings with low infrared emissivity may have great potential applications in cloaking equipments from detection by infrared waves. Acknowledgments This work was supported financially by CNOOC Technology Development Program (CNOOC-KJ 125 00 00 000 00 CTY/201104) and Changzhou Research Program on Applied Fundamentals (20120363). References [1] X. Bu, Y. Zhou, M. He, Z. Chen, T. Zhang, Appl. Surf. Sci. 288 (2014) 444–451. [2] T. Wang, J. He, J. Zhou, J. Tang, Y. Guo, X. Ding, S. Wu, J. Zhao, J. Solid State Chem. 183 (2010) 2797–2804.
[3] L. Kang, Y. Gao, Z. Chen, J. Du, Z. Zhang, H. Luo, Sol. Energ. Mater. Sol. C 94 (2010) 2078–2084. [4] X. Chu, H. Tao, Y. Liu, J. Ni, J. Bao, X. Zhao, J. Non-Cryst. Solids 383 (2014) 121–125. [5] Z. Zhang, Y. Gao, H. Luo, L. Kang, Z. Chen, J. Du, M. Kanehira, Y. Zhang, Z.L. Wang, Energy Environ. Sci. 4 (2011) 4290–4297. [6] W. Wang, L. Zhang, Y. Liu, H. Xu, Y. Zhong, Z. Mao, Ind. Eng. Chem. Res. 52 (2013) 15066–15074. [7] X. Yan, G. Xu, Prog. Org. Coat. 73 (2012) 232–238. [8] H.A. Babrekar, J.P. Jog, V.L. Mathe, D.K. Avasthi, S. Ojha, S.V. Bhoraskar, Nucl. Instrum. Meth. B 287 (2012) 135–140. [9] W. Zhang, G. Xu, R. Ding, J. Qiao, K. Duan, Physica B 422 (2013) 36–39. [10] W. Zhang, G. Xu, R. Ding, K. Duan, J. Qiao, Mater. Sci. Eng. C 33 (2013) 99–102. [11] A.K. Mishra, R. Narayan, K.V.S.N. Raju, T.M. Aminabhavi, Prog. Org. Coat. 74 (2012) 134–141. [12] G. Markevicius, S. Chaudhuri, C. Bajracharya, R. Rastogi, J. Xiao, C. Burnett, T.Q. Chastek, Prog. Org. Coat. 75 (2012) 319–327. [13] S. Thakur, N. Karak, Prog. Org. Coat. 76 (2013) 157–164. [14] X. Kong, G. Liu, H. Qi, J.M. Curtis, Prog. Org. Coat. 76 (2013) 1151–1160. [15] D. Ren, C.E. Frazier, Int. J. Adhes. Adhes. 45 (2013) 77–83. [16] Y. Zhang, J. Maxted, A. Barber, C. Lowe, R. Smith, Polym. Degrad. Stabil. 98 (2013) 527–534. [17] Y. Yang, Y. Zhou, J. Ge, X. Yang, React. Funct. Polym. 72 (2012) 574–579. [18] Y. Wang, G. Xu, H. Yu, C. Hu, X. Yan, T. Guo, J. Li, Appl. Surf. Sci. 257 (2011) 4743–4748. [19] C. Hu, G. Xu, X. Shen, J. Alloy Compd. 486 (2009) 371–375. [20] G. Wu, D. Yu, Prog. Org. Coat. 76 (2013) 107–112. [21] L. Yuan, X. Weng, J. Xie, W. Du, L. Deng, J. Alloy Compd. 580 (2013) 108–113. [22] L. Yuan, X. Weng, W. Du, J. Xie, L. Deng, J. Alloy Compd. 583 (2014) 492–497. [23] L. Wang, Y. Ye, X. Lu, Y. Wu, L. Sun, H. Tan, F. Xu, Y. Song, Electrochim. Acta 114 (2013) 223–232. [24] H. Yu, G. Xu, X. Shen, X. Yan, C. Shao, C. Hu, Prog. Org. Coat. 66 (2009) 161–166. [25] M. Auslender, S. Hava, Infrared Phys. Tech. 36 (1995) 1077–1088. [26] S. Enoch, J.J. Simon, L. Escoubas, Z. Elalmy, F. Lemarquis, P. Torchio, G. Albrand, Appl. Phys. Lett. 86 (2005) 261101. [27] C.J. Patel, A. Dighe, Prog. Org. Coat. 60 (2007) 219–223. [28] S.M. Mirabedini, M. Mohseni, Sh. PazokiFard, M. Esfandeh, Colloid Surf. A 317 (2008) 80–86. [29] P.W. Shuma, Z.F. Zhoua, K.Y. Lia, C.Y. Chan, Thin Solid Films 458 (2004) 203–211.