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The infrared stealth coatings were prepared using water-based techniques composed of aluminum powder with different particles sizes (325 mesh, 500 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh) embedded in different matrix resins (acrylic resin, epoxy resin and polyurethane resin). The relationship between the type and the viscosity of resins and the infrared emissivity of different coatings was discussed. The effects of the particles size and content on the infrared emissivity of coatings were also studied. The results showed that the polyurethane resin (PR)-Al coatings demonstrated the lowest infrared emissivity, suggesting that they were suitable to be used as infrared stealth coatings. The infrared emissivity of coatings gradually decreased in an almost linear relationship with the increase of matrix resin viscosity. The optimum aluminum particles size was 325 mesh. The infrared emissivity of coatings decreased with the increase of the aluminum powder content.
© 2016 Optical Society of America
1. Introduction
In diverse areas, such as building energy efficiency and military equipment, it is highly desired to develop surface coatings that can reduce the infrared radiation of the objects (e.g., weapons), as a response to the improvement of electronic warfare technology and infrared detection technology [1]. Developing the infrared stealthy technology could increase the survivability of weapons [2]. The most widely studied infrared stealth coatings were generally composed of matrix resin, metal filler and additives which have important effects on the infrared reflection of coatings [3]. Although great achievements have been made in designing infrared stealth coatings, there are still several challenges waiting to be resolved [4,5]. First, most of the coatings technologies are solvent-borne systems that will induce pollutions, while those relying on environmentally benign waterborne systems are scarcely investigated [6,7]. Second, the application of floating metal filler is not compatible with visible light stealth [8,9], which will increase the glossiness of coatings. At last, the infrared stealth performance of matrix resin is usually neglected in previous studies, only considering the infrared transmittance of matrix resin [10–12].
Considering the above challenges during designing infrared stealth coatings, we developed a water-based technology to create infrared stealth coatings using non-floating aluminum powder embedded in different matrix resins (acrylic resin, epoxy resin and polyurethane resin). The influence of the type and viscosity of different matrix resins on the infrared emissivity was systematically studied. The effects of different particles sizes and filling contents on the infrared emissivity of coatings were also investigated.
2. Experiment
2.1 Experiment reagents
Aluminum powder (325 mesh(45 μm), 500 mesh(25 μm), 800 mesh(18 μm), 1000 mesh(13 μm), 1200 mesh(10 μm), 1500 mesh(8 μm)) was purchased from Dongguan city, Chang’an Debang Arts & Crafts accessories. Acrylic resin (AR), epoxy resin(ER) and polyurethane resin (PR) were bought from Zhuhai Jili Chemical Industry Enterprise Co., Ltd. Carboxy Methyl Cellulose Acetate Butyrate (CMCAB) as the thickening agent was obtained from Eastman Chemical Company. The dispersing agent (BYK-377) was purchased from Nanjing Daoning Chemical Industry Enterprise Co., Ltd. The defoaming agent (BX-399) was supplied by Yitong High Polymer Material Co., Ltd. In Guangzhou. All chemicals were analytical reagent (A.R.) grade and used without further purification.
2.2 Coating preparation
The stainless steel was first treated by sanding, then degreased by acetone and cleaned by deionized water. Consequently, 200 g matrix resin and aluminum powder of different contents were mixed thoroughly after 0.1 wt.% thickening agents and 0.2 wt.% defoaming agents were added. After spinning coatings at 400 r/min for 10 minutes, the coatings composed of aluminum power of different contents embedded in different resins were obtained named as acrylic resin (AR)-Al, epoxy resin (ER)-Al and polyurethane resin (PR)-Al. Setting an appropriate spraying pressure (0.1 MPa), the coatings could be evenly sprayed onto the substrate surface. The spray gun and the substrate were kept vertical with a distance of 30 cm. The spraying time was 3 s. AR-Al coatings and PR-Al coatings were cured for 24 h at room temperature. ER-Al coatings were cured at 60 °C in a furnace for 5 h.
2.3 Characterization and optical performance of the coatings
The infrared emissivity in the wavelength range of 8~14 μm was measured using the HWF-2 infrared emissivity testing instrument manufactured by North Chihong Photoelectric Co., Ltd which measurement error was less than or equal to 0.02. The absorption spectrum of coatings was measured using a Fourier transform infrared spectrometer (Nicolet 6700 provided by the Thermo Fisher Company). The viscosity of different resins was determined according to GB/T1723-93.Scanning electron microscope (S-3700N Rili Company) was used to characterize the microstructure of coatings.
3. Results and discussion
3.1 Study on matrix resins
3.1.1 The effects of the matrix resin type on the infrared emissivity of coatings
Figure 1 showed the absorption spectra of AR, ER, and PR in the wavelength range of 8-14 μm, which exhibited that all of the three resins had strong absorption in the wavelength range of 8-10 μm and good transmissivity in the wavelength range of 10-14 μm. The absorption peaks of the three resins were analyzed. For AR, the 1165 cm−1 peak was the characteristic peak of acrylate copolymer. The absorption peak at 1073 cm−1 originated from the C-N stretching vibration mode. The COO angle variable vibration absorption peak contributed to the formation of the peak located at 761 cm−1. As to ER, the 1041 cm−1 can be attributed to the C-O-C vibration modes. The peak located at 830 cm−1 could be assigned to the vibration absorption of the C-O-C bonds. The CH2 vibrations gave rise to the formation of the peak situated at 756 cm−1.As to PR, the peaks located at 1242 cm−1 and 1149 cm−1 were the characteristic symmetric stretching vibrations of C-O-C in ammonia ester group. The C-O-C stretching vibration rendered the appearance of the peak at 1045 cm−1.
In most of the previous literatures, the infrared transmittance of matrix resin was used as the exclusive parameter to select the appropriate matrix resin. That is, the lower the absorbance coefficient is, the better the infrared transmittance of matrix resin will be, then the lower infrared emissivity could be obtained. However, this selection rule was insufficient. Figure 2 demonstrated the evolution of the infrared emissivity as a function of the content of aluminum powder in different resins in the coatings. As shown in Fig. 2, the infrared emissivity of PR-Al coatings was lower than that of AR-Al coatings and ER-Al coatings with the content of aluminum powder increased. In contrast, the average absorption coefficients of AR, ER and PR in the wavelength range of 8-14 μm were 0.0293, 0.048 and 0.0368, as shown in Fig. 1. Therefore, the infrared transmittance cannot be used as the exclusive screen parameter in selecting appropriate matrix resin to create optimized infrared stealth coatings.
Fig. 2 The infrared emissivity of different coatings created with different resins as a function of the Al content.
The morphology of coatings which were created by different resins (AR, ER and PR) composed of 15 wt.% aluminum powder was showed in Fig. 3. The red arrows pointed to aluminum powder. Because of the different molecular structures and the relative molecular weights of AR, ER and PR, the distributions of aluminum powder in the three kinds of matrix resin coatings were quite different. Compared with AR-Al coatings and ER-Al coatings, aluminum powder was dispersed more evenly in PR-Al coatings, and most of aluminum powder was concentrated at the coatings surface (Fig. 3(c)), which reduced the coating thickness. The even distribution of aluminum powder was beneficial to enhance the infrared radiation and reduce the infrared absorption by matrix resin. Taken together, the infrared emissivity of coatings was reduced.
Fig. 3 SEM images of coatings created by different resins composed of 15 wt.% aluminum powder. (a) AR-Al, (b)ER-Al, (c) PR-Al.
3.1.2 The effects of different viscosities of PR on the infrared emissivity of coatings
The 325 mesh aluminum powder in the content of 15 wt.% was dispersed in PR. The effects of different viscosities of PR on the infrared emissivity of coatings were systematically studied, as shown in Fig. 4. Meanwhile, Fig. 5 showed the infrared emissivity of coatings with different viscosities. It was found that the infrared emissivity could decrease by 0.17 from 0.56 to 0.39 with the increase of the PR viscosity. A linear relationship was revealed between the decrease of the infrared emissivity and the increase of PR viscosity.
According to the solidifying process of coatings, the influence of the viscosity of matrix resin on the infrared emissivity of coatings was investigated. During the solidifying process of coatings, aluminum powder would be settled at different levels according to the particles size. Sedimentation rate would directly influence the distribution of aluminum powder in low emissivity coatings. The sedimentation rate of aluminum powder produced prominent differences on the structure of coatings with different viscosities. The relationship between the sedimentation rate and the viscosity of matrix resin could be described by Stokes law:
where, was sedimentation rate, was the size of filling particles, and were the density of filling particles and matrix resin, respectively. While, was the viscosity of solution. As shown in Eq. (1), ignoring the effect of the weight of thickening agent on the density of matrix resin, the viscosity of solution increased gradually with the increasing content of thickening agent, which in turn decreased the sedimentation rate of aluminum powder. When the sedimentation rate of aluminum powder was low, the matrix resin membrane on the surface of aluminum powder was thin after the coatings solidified. Therefore, the infrared emissivity of such coatings will decrease, while the infrared reflection of coatings increased.
3.2 Study on aluminum powder
3.2.1 The effect of the particles size on the infrared emissivity of coatings
To study the influence of aluminum powder size on the infrared emissivity of coatings, aluminum powder above 200 mesh were used. According to previous exploration experiments [11], the aluminum powder less than 325 mesh was too large to disperse in the resin uniformly, which resulted in uneven coatings prepared by spraying process and affected the infrared emission performance of the coatings, seriously. On the other hand, aluminum powder more than 1500 mesh demonstrated the poor infrared emission performance [11]. SEM images of aluminum powder with different particles sizes (325 mesh, 500 mesh, 800 mesh, 1000 mesh, 1200 mesh, 1500 mesh) were shown in Fig. 6. All the aluminum powder showed scale-like morphology (Fig. 6). The surface of 325 mesh aluminum powder was smooth, and 325 mesh aluminum powder was not agglomerated in PR-Al coatings compared with other aluminum powders (500 mesh, 800 mesh, 1000 mesh, 1200 mesh,1500 mesh) as shown in Fig. 6. The PR membrane on the surface of the 325 mesh aluminum powder was thin because of its larger superficial area and smaller settling displacement, which resulted in lower infrared emissivity of coatings (Fig. 6(a)).
Fig. 6 SEM images of aluminum powder with different particles sizes dispersed in PR. (a) 325 mesh, (b) 500 mesh, (c) 800 mesh, (d)1000 mesh, (e) 1200 mesh, (f) 1500 mesh.
Figure 7 showed the effect of aluminum particles sizes on the infrared emissivity of coatings with different filling contents (from 5 wt.% to 30 wt.%). With the aluminum particles size decreased, the infrared emissivity first increased, then decreased and at last increased again. It was because the infrared emissivity was related to the infrared absorption and the scattering coefficient of coatings. The aluminum particles size in the coatings determined the scattering coefficient. On the other hand, the scattering coefficient was inversely proportional to the infrared emissivity of coatings. Therefore, the aluminum particles size in the coatings also determined the infrared emissivity of coatings.
The particles size range of 325-800 mesh belonged to large particles scattering range. In this range, the scattering coefficient decreased with the decrease of the particles size, and the infrared emissivity of coatings increased gradually. Comparatively, the aluminum particles in the size range of 800-1000 mesh located in the scope of Mie scattering range, where the scattering coefficient was inversely proportional to particles size and the infrared emissivity was diametrically opposite. In the size range of 1000-1500 mesh, the Rayleigh scattering was the predominate issue. In this case, the decrease of particles size led to the decrease of the scattering coefficient, and the increase of infrared emissivity.
In the area of military equipment, the perfect infrared stealth coatings should be equipped with low absorption coefficient and high scattering coefficient. Taken all together, the optimum aluminum particles size was 325 mesh because of the lower infrared emissivity (in other words, the high scattering coefficient). When the content of aluminum powder in the coatings was 30 wt.%, the infrared emissivity of coatings was the lowest with the value of 0.24.
3.2.2 The effect of the aluminum powder content on the infrared emissivity of coatings
As afore mentioned, the coatings with aluminum powder of 325 mesh performed lower infrared emissivity. Furthermore, the effect of the aluminum powder (325 mesh) content on the infrared emissivity of coatings was studied. According to previous exploration experiments [12], the dispersion performance of aluminum powder was poor in the resin with more than 30 wt.% content, coatings composed of aluminum power in different contents was shown in Fig. 8. With the increase of the aluminum powder content, the infrared emissivity of coatings gradually decreased.
Figure 9 demonstrated the morphology of coatings created with PR composed of 15 wt.% and 30 wt.% aluminum powder. The aluminum powder at the content of 15 wt.%, was sparsely dispersed in matrix resin and showed lager interspace in Fig. 9(a). Therefore, the aluminum powder cannot well concentrated on the surface of the coatings. Under this condition, the underneath structure of coatings was not conducive to reduce the absorption of the infrared radiation in matrix resin. As the content of aluminum powder increased to 30 wt.%, a contact coating with no obvious interspace among adjacent aluminum particles was formed, as shown in Fig. 9(b). Because of the mutual accumulation, the thickness of coating was decreased, which was benefit to increase the infrared reflection of aluminum powder and reduce the infrared emissivity of coatings.
4. Conclusion
The aluminum powder could be more uniformly distributed in PR-Al coatings compared with AR-Al coatings and ER-Al coatings. The aluminum powder was concentrated on the outmost surface of coatings and the infrared emissivity of PR-Al coatings was the lowest. Therefore, PR was suitable to be used as a matrix resin to prepare infrared stealth coatings using the water-based techniques.
The viscosity of matrix resin could be adjusted by adding thickening agent. With the increase of the viscosity of matrix resin from 9.7 mm2/s to 7.7 mm2/s, the infrared emissivity of coatings was decreased by 0.17 from 0.56 to 0.39, with a linear relationship between the infrared emissivity and the viscosity.
By the study of the effects of aluminum particles size and content in the infrared stealth coatings, the optimum particles size of aluminum powder was 325 mesh, and the infrared emissivity of coatings decreased with the increase of the aluminum powder content. The infrared emissivity of coatings was 0.24 when the aluminum powder content was 30 wt.%.
References and links
1. H. Y. Li, S. Z. Zhang, C. L. Sun, and D. Y. Guan, “The present situation and prospect of stealthy coatings,” J. Functional Materials. 44(B06), 36–40 (2013).
2. S. Y. Cheng, Z. H. Liu, Z. P. Deng, and S. T. Ye, “Research progress on infrared characteristic of military target,” Infrared Technology. 36(7), 577–581 (2014).
3. W. Fu, “Principle and application technology of IR stealth,” Hongwai Yu Jiguang Gongcheng 31(1), 88–93 (2002).
4. Y. Ren, P. Hua, and R. Z. Gong, “Materials with low infrared emissivity based on epoxy resin,” Ordnance Material Science and Engineering. 34(2), 45–47 (2011).
5. C. Shao, G. Xu, X. Shen, H. Yu, and X. Yan, “Infrared emissivity and corrosion-resistant property of maleic anhydride grafted ethylene-propylene-diene terpolymer (EPDM-g-MAH)/Cu coatings,” Surf. Coat. Tech. 204(24), 4075–4080 (2010). [CrossRef]
6. G. Wu and D. Yu, “Preparation and characterization of a new low infrared-emissivity coating based on modified aluminum,” Prog. Org. Coat. 76(1), 107–112 (2013). [CrossRef]
7. L. Yuan, X. Weng, and L. Deng, “Influence of binder viscosity on the control of infrared emissivity in low emissivity coating,” Infrared Phys. Technol. 56(1), 25–29 (2013). [CrossRef]
8. H. Yu, G. Xu, X. Shen, X. Yan, C. Shao, and C. Hu, “Effects of size, shape and floatage of Cu particles on the low infrared emissivity coatings,” Prog. Org. Coat. 66(2), 161–166 (2009). [CrossRef]
9. H. A. Babrekar, N. V. Kulkarni, J. P. Jog, V. L. Mathe, and S. V. Bhoraskar, “Influence of filler size and morphology in controlling the thermal emissivity of aluminum /polymer composites for space applications,” Mater. Sci. Eng. B 168(1), 40–44 (2010). [CrossRef]
10. S. T. Ye, S. Y. Cheng, Z. H. Liu, F. Wang, and Y. F. Jia, “Application of Cold Pigments in Infrared Stealth Coatings,” Surf. Technol. 45(2), 139–143 (2016).
11. S. T. Ye, S. Y. Cheng, Z. H. Liu, F. Wang, and Y. F. Jia, “Water-based infrared stealth coating in 8-14 μm wavebands,” Hongwai Yu Jiguang Gongcheng 45(2), 0204004 (2016). [CrossRef]
12. S. Y. Cheng, Z. H. Liu, S. T. Ye, S. Yan, Z. Ruan, and G. D. Ban, “Investigation for Preparation Process Optimizing of Water-based Infrared Stealth Coatings,” Surf. Technol. 44(8), 71–75 (2015).
