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Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials
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CERAMICS INTERNATIONAL
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Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials Yonglin Lein, Qiuru Wang, Jichuan Huo State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, PR China Received 20 December 2013; received in revised form 20 February 2014; accepted 17 March 2014
Abstract In order to provide an effective damp-proof protection of inorganic radome materials, we have developed a new method to fabricate superhydrophobic coatings, which uses Polyvinylidene fluoride (PVDF) and SiO2 micron particles to seal porous surfaces of inorganic radome materials and hydrophobized silica colloid to form superhydrophobic surface. The coatings were characterized by moisture absorption rate analysis, dielectric constant and loss tangent measurements, static contact angle and sliding angle measurements, SEM, AFM, TG, FTIR and wear tests. All of the characterizations revealed the coatings had a micro–nano hierarchical structure with a static contact angle of 157.51 and sliding angle of 91, and possessed high thermal stability and excellent mechanical durability. These coatings have potential uses in engineering applications of inorganic porous materials requiring damp-proof protection and high durability. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Super-hydrophobic; Coatings; Micro–nano hierarchical structure; Mechanical durability; Radome
1. Introduction An aircraft is generally equipped with radar at the front and a radome is used as a protecting cover for radar, so that a radome to be used in an aircraft must have excellent aerodynamic characteristics, and must, of course, withstand the severe atmospheric conditions. In particular, a radome to be used at the front of an aircraft is generally made of the inorganic radome materials under the necessity of enhancing the heat resistance. However, inorganic radome materials such as quartz ceramics, nitride ceramics and phosphate chrome–alumina matrixes are hydrophilic by nature. The free hydroxyl groups and pores on the surfaces of inorganic radome materials promote water absorption, which causes a major effect on applications [1]. So, to fabricate a uniform, well adhered, insoluble, hydrophobic coating provides an efficient method of controlling water penetration within inorganic radome materials. Up to now, many various techniques have been developed to improve damp-proof properties of n
Corresponding author. Tel.: þ86 816 2419209. E-mail address: [email protected] (Y. Lei).
inorganic radome materials such as organic or inorganic coatings, surface treatment, and so on [2]. Recently, owing to remarkable features, superhydrophobic surfaces show potential in a variety of applications ranging from antisticking, anti-contamination, and self-cleaning to anti-corrosion and low friction coatings [3,4]. The superhydrophobic coatings with Polytetrafluoroethylene (PTFE)/silica sol composites on surfaces of inorganic radome materials possess good properties for damp-proof [5]. Polytetrafluoroethylene/Polyvinylidene fluoride (PTFE/PVDF) composites can also obtain superhydrophobic coatings with excellent mechanical durability [6]. However, the adhesion between the substrates and the fluorine-based hydrophobic coatings is via van der Waals force and, thus, is weak. Some early reports indicate that superhydrophobic coatings combining with a lowsurface energy and complementary role of microstructure (pressure dissipation) and nanostructure (high capillary pressure) can further increase pressure tolerance critical for robustness of such coatings [7–9]. However, they still fail under pressure, at high or low temperatures, or in a humid environment [10]. Acrylic and polyurethane mixing with organoclay can make obtained superhydrophobic coatings high adhesion [11–13].
http://dx.doi.org/10.1016/j.ceramint.2014.03.087 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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However, non-excellent thermal stability of such coatings limits its application in the surface of the radome materials. Thus, the polymers with excellent thermal stability, such as, silicone rubber and epoxy resins, are chosen to enhance the adhesion of superhydrophobic coatings on inorganic radome materials. Incorporating nanoparticles into silicone rubber can increase hardness and the wear resistance of nanoparticles or agglomerates of nanoparticles [14–18]. However, the poor adhesion of silicone rubber to the substrates and micro agglomerates results in damage to micro–nano structures and easy peeling from substrates. Epoxy resins which have excellent adhesion can act as cross-linking agent in hierarchical structured coatings to improve the mechanical durability of the coatings [19–22]. However, due to their high surface energy, epoxy resins being blended with nanoparticles in nano layer may lead to poor superhydrophobic properties [23]. Herein, the combinations of incorporating nanoparticles into silicone rubber and blending microparticles with epoxy resins may be a good method to enhance the wear resistance of modified nanoparticles and the mechanical durability of micrometer-sized agglomerates. However, as if being mixed together, epoxy resins and silicone rubber are not compatible. The pre-cured epoxy resins and silicone rubber being separately added into different layers of the coatings can solve this problem. But, very few studies have been conducted on the fabrication of such structured superhydrophobic coatings on the surfaces, especially on the surfaces of inorganic radome materials. The hydrophobic silica particles of different sizes have been used for creating micro–nano scale structures to enhance mechanical durability. There are many reports available on the fabrication of hierarchical structured superhydrophobic coatings using silica particles [24–29]. Among the various methodologies in creating the superhydrophobic coatings bases on silica particles with hierarchical structures, the colloidal self-assembly is a simple, fast, and inexpensive technique [30,31]. With this in mind, we fabricated hierarchical structured coatings with double damp-proof protection using a colloidal self-assembly process (as shown in Fig. 1). The first protection was from the PVDF and SiO2 micron particles, which could reduce porosity to limit the accessibility of moisture to the surfaces of inorganic radome materials and increase micron roughness for the subsequent fabrication of hierarchical micro/ nanostructure. The second one was the hydrophobized colloidal silica, which increased nano roughness to repel the moisture and further reduced the water media absorption on the surfaces. In addition, epoxy resin (EP) was added into the first layer to strengthen the adhesion between the layers. Polydimethylsiloxane (PDMS) was added into the second layer to make nano silica particles embed in PDMS chain and improve the wear resistance of second coating. 2. Experimental 2.1. Materials Tetraethoxysilane (TEOS, 98%), methyltriethoxysilane (MTES, 98%) and Polydimethylsiloxane (PDMS) were purchased from
Fig. 1. Fabrication scheme of hierarchical structured coatings.
ZheJiang QuZhou ZhengBang Organosilicon CO. LTD, aqueous ammonia (25%), silica particle (SiO2 50–80 μm), Polyvinylidene fluoride (PVDF, 2–15 μm), absolute ethanol (99.5%) and triethanolamine (TEA) were obtained from ChengDu KeLong Chemical Co. Ltd, and epoxy resin (EP, E51) was supplied by SiChuan HaiNa Resin Liability Company. Inorganic radome materials (phosphate chrome–alumina composite plates with the length of 3 cm and the width of 1.5 cm) were prepared in the laboratory. 2.2. Preparation of sealing emulsion The sealing emulsion was made by mixing SiO2 (particle size 50–80 μm), PVDF (particle size 2–15 μm) and ethanol in a mass ratio of 25: 5: 2 (C2H5OH: PVDF: SiO2), then the mixture was heated to 45 1C under stirring and maintained at this temperature for 2 h. Finally, the obtained emulsion was mixed with 10 wt% EP and 1.2 wt% triethanolamine (TEA). 2.3. Preparation of superhydrophobic emulsion Ethanol, ammonia and deionized water were mixed at 30 1C for 30 min. Then, ethanol solution of TEOS was added and stirred for 90 min. Subsequently, PDMS, MTES and ethanol were added at 60 1C. After 19 h of stirring, the solution was placed at room temperature for 3 days to obtain superhydrophobic emulsion containing nano-SiO2 particles. The final molar ratios of ethanol, ammonia, water, TEOS, MTES, and PDMS were 61.7: 4.3: 16.67: 1: 1: 1. 2.4. Preparation of hierarchical superhydrophobic coatings on phosphate chrome–alumina composite plates The hierarchical superhydrophobic coatings were made as follows: firstly, the phosphate chrome–alumina composite plates were polished smoothly with sandpaper, and then were coated with sealing emulsion. Then, the samples were dried at room temperature (21 1C) and cured at 120 1C. Secondly, the pretreated samples were dipped in superhydrophobic emulsion for about 5 min, and dried at 90 1C for 15 min. This procedure was repeated for 3 times to get enough thickness of the
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coatings. After that, the composite plates were annealed at 180 1C for 2 h and the final products were obtained. 2.5. Characterization Moisture absorption rate studies of the coatings were analyzed using an artificial wetting system that the composites were put in constant temperature and humidity controlling box at 25 1C and 70% of humidity for 7 days. And their weight change was recorded. Dielectric constant and loss tangent were measured by an ENA vector network analyzer (Agilent Technologies). Static contact angle and sliding angle measurements were performed using a SL200B instrument (KRÜSS Company, Ltd., Germany). Static contact angle (CA) was determined in sessile drop mode. The sliding angle (SA) was measured by a tilting stage method, as the tilt angle of the stage at which a water drop rolled off the surface. Surface morphological studies of the coatings were evaluated using scanning electron microscopy (Philips XL30 SFEG SEM) and atomic force microscopy (AFM, SPI3800N). Thermogravimetry (SDT Q600 synchronous thermal analyzer) was used for thermal behavior characterization. The samples were heated in air at a rate of 20 1C/min up to 700 1C. The chemical structure changes of surface at different temperatures were investigated by a Fourier transform infrared spectroscope (FTIR, Nicolet 380, US). The abrasion property of the coatings was evaluated by wear test. A sheet of M40 emery paper with a heavy object (100 g) was put on the surfaces of coatings and the emery paper was dragged back and forth three times. 3. Results and discussion 3.1. Superhydrophobic properties of the hierarchical structured coatings on phosphate chrome–alumina composite plates It is well known that water, which is the fatal weakness of radome materials, has a great effect on dielectric properties of phosphate chrome–alumina substrates. The uncoated phosphate chrome–alumina composite plates increased by 10.68% in weight (seen in Table 1), their dielectric constant and loss tangent were all serious influenced. The phosphate chrome– alumina composite plates coated with sealant could decrease the porosity and moisture absorption from environment (the
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weight increase was 0.81%). However, due to the poor hydrophobic properties of the substrates after being sealed (CA o 831, SA 4 451), the covered water of surfaces would slowly immerse into substrates through the micro channel of sealed layer, which could result in unsatisfactory dielectric properties of the substrates (dielectric constant is 7.217(7 d), loss tangent is 2.638(7 d)). In contrast, the weight increase of substrates treated with sealant and hydrophobic emulsion was only 0.02%. And their dielectric properties were constant enough to meet the requirement of radome materials. These results indicated the formation of superhydrophobic coatings (CA 4 157.51, SA o 91) on the surfaces of sealed layer was beneficial to improve the performance of phosphate chrome– alumina composite plates. 3.2. Surface morphology of the hierarchical structured coatings The surface morphology study for uncoated phosphate chrome–alumina composite plates, phosphate chrome–alumina composite plates coated with sealant and phosphate chrome– alumina composite plates coated with sealant and hydrophobic emulsion was carried out using the SEM analysis. The SEM image of uncoated phosphate chrome–alumina composite plates as shown in Fig. 2a shows that there are many micro holes, crevices and cracks on their surfaces. These defects are result of the escape of water and gas in the cured process of phosphate chrome–alumina composite plates, which leads to moisture absorption and low CA (651). As for phosphate chrome–alumina composite plates coated with sealant, the SEM image depicted in Fig. 2b shows that the micro holes, crevices and cracks of the surfaces are reduced significantly and many micro scale protrusions on their surface. These micro scale protrusions make the surface microroughness increase. Since the surfaces coated with the hydrophilic silica powder, theirs hydrophobicity has not been significantly improved. However, micro scale protrusions on their surface provide a surface basis for preparing superhydrophobic coatings. The SEM image of phosphate chrome–alumina composite plates coated with sealant and hydrophobic emulsion shows a nano-porous surface morphology with many 200 nm silica particles (seen in Fig. 2c). These particles tend to trap the air in the pores of the coatings contributing to the higher water contact angle (157.51), which causes water drop easy rolling off on the surface.
Table 1 Superhydrophobic properties of the hierarchical structured coatings on phosphate chrome–alumina composite plates. Samples of phosphate chrome–alumina composite plates
Moisture absorption rate (%)
Dielectric constant/ 10 GHz
Loss tangent/ 10 GHz
Uncoated sample
10.68
3.864(0 d) 17.136(7 d) 3.912(0 d) 7.217(7 d) 3.952(0 d) 3.985(7 d)
0.0381(0 d) 4.023(7 d) 0.0397(0 d) 2.638(7 d) 0.0398(0 d) 0.0411(7 d)
Sample coated with sealant
0.81
Sample coated with sealant and hydrophobic emulsion
0.02
Contact angle (Deg)
Sliding angle (Deg)
65
–
83
445
157.5
o9
Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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Fig. 2. SEM images and the CA on each surface: (a) uncoated phosphate chrome–alumina composite plates, (b) phosphate chrome–alumina composite plates coated with sealant, and (c) phosphate chrome–alumina composite plates coated with sealant and hydrophobic emulsion.
3.3. Thermal stability of the hierarchical structured coatings Fig. 3 is TG profiles of the hierarchical structured coatings. The TG profiles reveal that the weight loss of coatings is 2.41% at 30–420 1C and 77.14% at 420–600 1C. These results suggest the thermal stability of the coatings is very excellent during 30–420 1C, and the major decomposition of the coatings, which attributed to the decomposition of PVDF, PDMS and EP in the coatings, starts at about 420 1C. Fig. 4 shows the FTIR absorption spectra of the hierarchical structured coatings at different temperatures (25 1C, 200 1C, 300 1C and 400 1C). For all the hierarchical structured coatings at different temperatures, the Si–O–Si group gives a strong absorption band in the spectral region near 1106 cm 1, 772 cm 1 and 470 cm 1. The presence of those peaks confirms the formation of a network structure inside the coatings. The absorption peaks at 3026 cm 1 and 1274 cm 1, which begin to appear at 200 1C, are due to the ¼ CH2 bending vibration of PVDF in coatings. The characteristic bands of the –CH3 group are found at 841 cm 1 (strong band, swinging vibration of CH2), and at 1402 cm 1 (symmetrical deformation vibration of C–H bond). For the hierarchical structured coatings at 400 1C,
Fig. 3. TG profiles of the hierarchical structured coatings.
the peaks at 841 cm 1 and 1402 cm 1 disappear, which indicates the –CH3 groups of coatings are oxidized. The peaks at 3432 cm 1 and 1631 cm 1 correspond to the O–H absorption bond, which is caused by physically adsorbed water. After
Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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sintered at 400 1C for 2 h, the peaks of O–H (3432 cm 1 and 1631 cm 1) become more obvious than the other three samples because of the serious phenomenon of moisture absorption on surface of materials.
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Fig. 5 represents the surface morphology of phosphate chrome–alumina composite plates with the hierarchical structured coatings after sintered. In the process of thermal treatment, the amount of the holes increased and the pore size was enlarged and the micro/nanostructures were destroyed. After heating at 200 1C and 300 1C, the CA decreased to 138.471 (Fig. 5a) and 129.351 (Fig. 5b), respectively. When the heating temperature reached 400 1C, the coatings lost the effect of damp proof. Water droplets could spread out completely on the surface, and the CA approached zero. The TG curve and FTIR absorption spectra of coatings indicated that the hydrophobic methyl groups were burned off when heated to 400 1C. It is the reason why the CA of coatings fell from 157.5 1C down to zero. 3.4. Mechanical durability of hierarchical structured coatings
Fig. 4. FTIR absorption spectra of the hierarchical structured coatings at different temperatures.
To improve mechanical durability of the hierarchical structured coatings, epoxy resin (EP) and Polydimethylsiloxane (PDMS) were added into coatings. The SEM image (Fig. 6a) of coatings in which there are no added EP and PDMS reveal the surface of coatings has a large number of cracks. And it
Fig. 5. SEM images and the CA of the hierarchical structured coatings: (a) heated at 200 1C, (b) heated at 300 1C, and (c) heated at 400 1C. Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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Fig. 6. SEM images of coatings: (a) without adding EP and PDMS, and (b) adding EP and PDMS.
Fig. 7. AFM images of the hierarchical structured coatings: (a) no abrasion (CA 157.51), average roughness ¼ 32.74 nm, (b) without EP and PDMS after abrasion (CA 127.031), average roughness ¼175.2 nm, and (c) with EP and PDMS after abrasion (CA 1421), average roughness ¼ 49.3 nm.
can be seen from Fig. 6b that adding EP and PDMS into coatings promotes the formation of dense structure, fine and no micro-crack film. To evaluate mechanical durability of the hierarchical structured coatings, wear test process was employed which method was similar with the method reported by other papers [32]. After the wear experiment, the wetting ability and surface roughness changes of the samples were evaluated by contact angle measurement and AFM images respectively, and the results are as shown in Fig. 8. After the wear experiment, the morphology and average roughness of the surface with adding EP and PDMS were not significantly changed (32.74 nm after compared with 49.3 nm before) (Fig. 7c). And the wetting ability of those coatings was stable, the CA was kept about 1421. However, the after-image for no added EP and PDMS reveals the micro/nanostructures of coatings are destroyed (Fig. 7b). In addition, the average surface roughness for no added EP and PDMS increased drastically from 32.74 nm to 175.2 nm. And the corresponding CA was down to 127.031. These results demonstrate that the mechanical durability of the coatings with added EP and PDMS is superior to that of the coatings with no added EP and PDMS, which attributes to the
cross-linking effect of EP and PDMS (as shown in Fig. 8). Adding EP in first layer of the coatings can wrap and enclose a part of particles (microparticles, nanoparticles) and substrates. These particles attach to substrates by the cross-linking between epoxy groups of EP and hydroxyl groups of particles and substrates. That is the reasons why the adhesion strength between layers is enhanced after EP addition. Adding PDMS into second layer of the coatings can make methyltriethoxysilane-modified nano SiO2 particles embed in PDMS chain. The Si–OH groups from the embedded SiO2 and the O of Si–O– groups in PDMS form the hydrogen bond, which can result in the wear resistance enhancement of silane-modified nano SiO2 particles. 4. Conclusions Mechanically durable, thermally stable and micro–nano hierarchically structured coatings on surfaces of inorganic radome materials were fabricated by colloidal self-assembly method. This method simply involved sealing porous inorganic radome surface with PVDF and SiO2 micron powder and precipitating low surface energy PVDF and hydrophobized silica colloid. The as-formed coatings showed excellent superhydrophobic properties (CA 4 157.51, SA o 91). The weight
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Fig. 8. The mechanism of mechanically durable enhancement.
increase of inorganic radome materials with these coatings was only 0.02%. And the dielectric properties of inorganic radome materials were constant enough to meet the requirement of radome materials. Thermal analysis showed that these superhydrophobic coatings began to lose hydrophobicity when heated above 400 1C. In addition, the wear tests demonstrated the mechanical durability of the coatings with adding EP and PDMS was excellent and superior to that of the coatings with no added EP and PDMS. The excellent superhydrophobic properties and mechanical durability have promising applications in damp-proof protection of other inorganic porous materials.
Acknowledgments This work was supported by the State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, China (Grant No. 11zxfk25) and the research grant of the 863 National Projects (No. 2009AA035002).
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CERAMICS INTERNATIONAL
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Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials Yonglin Lein, Qiuru Wang, Jichuan Huo State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, School of Material Science and Engineering, Southwest University of Science and Technology, Mianyang 621010, PR China Received 20 December 2013; received in revised form 20 February 2014; accepted 17 March 2014
Abstract In order to provide an effective damp-proof protection of inorganic radome materials, we have developed a new method to fabricate superhydrophobic coatings, which uses Polyvinylidene fluoride (PVDF) and SiO2 micron particles to seal porous surfaces of inorganic radome materials and hydrophobized silica colloid to form superhydrophobic surface. The coatings were characterized by moisture absorption rate analysis, dielectric constant and loss tangent measurements, static contact angle and sliding angle measurements, SEM, AFM, TG, FTIR and wear tests. All of the characterizations revealed the coatings had a micro–nano hierarchical structure with a static contact angle of 157.51 and sliding angle of 91, and possessed high thermal stability and excellent mechanical durability. These coatings have potential uses in engineering applications of inorganic porous materials requiring damp-proof protection and high durability. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Super-hydrophobic; Coatings; Micro–nano hierarchical structure; Mechanical durability; Radome
1. Introduction An aircraft is generally equipped with radar at the front and a radome is used as a protecting cover for radar, so that a radome to be used in an aircraft must have excellent aerodynamic characteristics, and must, of course, withstand the severe atmospheric conditions. In particular, a radome to be used at the front of an aircraft is generally made of the inorganic radome materials under the necessity of enhancing the heat resistance. However, inorganic radome materials such as quartz ceramics, nitride ceramics and phosphate chrome–alumina matrixes are hydrophilic by nature. The free hydroxyl groups and pores on the surfaces of inorganic radome materials promote water absorption, which causes a major effect on applications [1]. So, to fabricate a uniform, well adhered, insoluble, hydrophobic coating provides an efficient method of controlling water penetration within inorganic radome materials. Up to now, many various techniques have been developed to improve damp-proof properties of n
Corresponding author. Tel.: þ86 816 2419209. E-mail address: [email protected] (Y. Lei).
inorganic radome materials such as organic or inorganic coatings, surface treatment, and so on [2]. Recently, owing to remarkable features, superhydrophobic surfaces show potential in a variety of applications ranging from antisticking, anti-contamination, and self-cleaning to anti-corrosion and low friction coatings [3,4]. The superhydrophobic coatings with Polytetrafluoroethylene (PTFE)/silica sol composites on surfaces of inorganic radome materials possess good properties for damp-proof [5]. Polytetrafluoroethylene/Polyvinylidene fluoride (PTFE/PVDF) composites can also obtain superhydrophobic coatings with excellent mechanical durability [6]. However, the adhesion between the substrates and the fluorine-based hydrophobic coatings is via van der Waals force and, thus, is weak. Some early reports indicate that superhydrophobic coatings combining with a lowsurface energy and complementary role of microstructure (pressure dissipation) and nanostructure (high capillary pressure) can further increase pressure tolerance critical for robustness of such coatings [7–9]. However, they still fail under pressure, at high or low temperatures, or in a humid environment [10]. Acrylic and polyurethane mixing with organoclay can make obtained superhydrophobic coatings high adhesion [11–13].
http://dx.doi.org/10.1016/j.ceramint.2014.03.087 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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However, non-excellent thermal stability of such coatings limits its application in the surface of the radome materials. Thus, the polymers with excellent thermal stability, such as, silicone rubber and epoxy resins, are chosen to enhance the adhesion of superhydrophobic coatings on inorganic radome materials. Incorporating nanoparticles into silicone rubber can increase hardness and the wear resistance of nanoparticles or agglomerates of nanoparticles [14–18]. However, the poor adhesion of silicone rubber to the substrates and micro agglomerates results in damage to micro–nano structures and easy peeling from substrates. Epoxy resins which have excellent adhesion can act as cross-linking agent in hierarchical structured coatings to improve the mechanical durability of the coatings [19–22]. However, due to their high surface energy, epoxy resins being blended with nanoparticles in nano layer may lead to poor superhydrophobic properties [23]. Herein, the combinations of incorporating nanoparticles into silicone rubber and blending microparticles with epoxy resins may be a good method to enhance the wear resistance of modified nanoparticles and the mechanical durability of micrometer-sized agglomerates. However, as if being mixed together, epoxy resins and silicone rubber are not compatible. The pre-cured epoxy resins and silicone rubber being separately added into different layers of the coatings can solve this problem. But, very few studies have been conducted on the fabrication of such structured superhydrophobic coatings on the surfaces, especially on the surfaces of inorganic radome materials. The hydrophobic silica particles of different sizes have been used for creating micro–nano scale structures to enhance mechanical durability. There are many reports available on the fabrication of hierarchical structured superhydrophobic coatings using silica particles [24–29]. Among the various methodologies in creating the superhydrophobic coatings bases on silica particles with hierarchical structures, the colloidal self-assembly is a simple, fast, and inexpensive technique [30,31]. With this in mind, we fabricated hierarchical structured coatings with double damp-proof protection using a colloidal self-assembly process (as shown in Fig. 1). The first protection was from the PVDF and SiO2 micron particles, which could reduce porosity to limit the accessibility of moisture to the surfaces of inorganic radome materials and increase micron roughness for the subsequent fabrication of hierarchical micro/ nanostructure. The second one was the hydrophobized colloidal silica, which increased nano roughness to repel the moisture and further reduced the water media absorption on the surfaces. In addition, epoxy resin (EP) was added into the first layer to strengthen the adhesion between the layers. Polydimethylsiloxane (PDMS) was added into the second layer to make nano silica particles embed in PDMS chain and improve the wear resistance of second coating. 2. Experimental 2.1. Materials Tetraethoxysilane (TEOS, 98%), methyltriethoxysilane (MTES, 98%) and Polydimethylsiloxane (PDMS) were purchased from
Fig. 1. Fabrication scheme of hierarchical structured coatings.
ZheJiang QuZhou ZhengBang Organosilicon CO. LTD, aqueous ammonia (25%), silica particle (SiO2 50–80 μm), Polyvinylidene fluoride (PVDF, 2–15 μm), absolute ethanol (99.5%) and triethanolamine (TEA) were obtained from ChengDu KeLong Chemical Co. Ltd, and epoxy resin (EP, E51) was supplied by SiChuan HaiNa Resin Liability Company. Inorganic radome materials (phosphate chrome–alumina composite plates with the length of 3 cm and the width of 1.5 cm) were prepared in the laboratory. 2.2. Preparation of sealing emulsion The sealing emulsion was made by mixing SiO2 (particle size 50–80 μm), PVDF (particle size 2–15 μm) and ethanol in a mass ratio of 25: 5: 2 (C2H5OH: PVDF: SiO2), then the mixture was heated to 45 1C under stirring and maintained at this temperature for 2 h. Finally, the obtained emulsion was mixed with 10 wt% EP and 1.2 wt% triethanolamine (TEA). 2.3. Preparation of superhydrophobic emulsion Ethanol, ammonia and deionized water were mixed at 30 1C for 30 min. Then, ethanol solution of TEOS was added and stirred for 90 min. Subsequently, PDMS, MTES and ethanol were added at 60 1C. After 19 h of stirring, the solution was placed at room temperature for 3 days to obtain superhydrophobic emulsion containing nano-SiO2 particles. The final molar ratios of ethanol, ammonia, water, TEOS, MTES, and PDMS were 61.7: 4.3: 16.67: 1: 1: 1. 2.4. Preparation of hierarchical superhydrophobic coatings on phosphate chrome–alumina composite plates The hierarchical superhydrophobic coatings were made as follows: firstly, the phosphate chrome–alumina composite plates were polished smoothly with sandpaper, and then were coated with sealing emulsion. Then, the samples were dried at room temperature (21 1C) and cured at 120 1C. Secondly, the pretreated samples were dipped in superhydrophobic emulsion for about 5 min, and dried at 90 1C for 15 min. This procedure was repeated for 3 times to get enough thickness of the
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coatings. After that, the composite plates were annealed at 180 1C for 2 h and the final products were obtained. 2.5. Characterization Moisture absorption rate studies of the coatings were analyzed using an artificial wetting system that the composites were put in constant temperature and humidity controlling box at 25 1C and 70% of humidity for 7 days. And their weight change was recorded. Dielectric constant and loss tangent were measured by an ENA vector network analyzer (Agilent Technologies). Static contact angle and sliding angle measurements were performed using a SL200B instrument (KRÜSS Company, Ltd., Germany). Static contact angle (CA) was determined in sessile drop mode. The sliding angle (SA) was measured by a tilting stage method, as the tilt angle of the stage at which a water drop rolled off the surface. Surface morphological studies of the coatings were evaluated using scanning electron microscopy (Philips XL30 SFEG SEM) and atomic force microscopy (AFM, SPI3800N). Thermogravimetry (SDT Q600 synchronous thermal analyzer) was used for thermal behavior characterization. The samples were heated in air at a rate of 20 1C/min up to 700 1C. The chemical structure changes of surface at different temperatures were investigated by a Fourier transform infrared spectroscope (FTIR, Nicolet 380, US). The abrasion property of the coatings was evaluated by wear test. A sheet of M40 emery paper with a heavy object (100 g) was put on the surfaces of coatings and the emery paper was dragged back and forth three times. 3. Results and discussion 3.1. Superhydrophobic properties of the hierarchical structured coatings on phosphate chrome–alumina composite plates It is well known that water, which is the fatal weakness of radome materials, has a great effect on dielectric properties of phosphate chrome–alumina substrates. The uncoated phosphate chrome–alumina composite plates increased by 10.68% in weight (seen in Table 1), their dielectric constant and loss tangent were all serious influenced. The phosphate chrome– alumina composite plates coated with sealant could decrease the porosity and moisture absorption from environment (the
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weight increase was 0.81%). However, due to the poor hydrophobic properties of the substrates after being sealed (CA o 831, SA 4 451), the covered water of surfaces would slowly immerse into substrates through the micro channel of sealed layer, which could result in unsatisfactory dielectric properties of the substrates (dielectric constant is 7.217(7 d), loss tangent is 2.638(7 d)). In contrast, the weight increase of substrates treated with sealant and hydrophobic emulsion was only 0.02%. And their dielectric properties were constant enough to meet the requirement of radome materials. These results indicated the formation of superhydrophobic coatings (CA 4 157.51, SA o 91) on the surfaces of sealed layer was beneficial to improve the performance of phosphate chrome– alumina composite plates. 3.2. Surface morphology of the hierarchical structured coatings The surface morphology study for uncoated phosphate chrome–alumina composite plates, phosphate chrome–alumina composite plates coated with sealant and phosphate chrome– alumina composite plates coated with sealant and hydrophobic emulsion was carried out using the SEM analysis. The SEM image of uncoated phosphate chrome–alumina composite plates as shown in Fig. 2a shows that there are many micro holes, crevices and cracks on their surfaces. These defects are result of the escape of water and gas in the cured process of phosphate chrome–alumina composite plates, which leads to moisture absorption and low CA (651). As for phosphate chrome–alumina composite plates coated with sealant, the SEM image depicted in Fig. 2b shows that the micro holes, crevices and cracks of the surfaces are reduced significantly and many micro scale protrusions on their surface. These micro scale protrusions make the surface microroughness increase. Since the surfaces coated with the hydrophilic silica powder, theirs hydrophobicity has not been significantly improved. However, micro scale protrusions on their surface provide a surface basis for preparing superhydrophobic coatings. The SEM image of phosphate chrome–alumina composite plates coated with sealant and hydrophobic emulsion shows a nano-porous surface morphology with many 200 nm silica particles (seen in Fig. 2c). These particles tend to trap the air in the pores of the coatings contributing to the higher water contact angle (157.51), which causes water drop easy rolling off on the surface.
Table 1 Superhydrophobic properties of the hierarchical structured coatings on phosphate chrome–alumina composite plates. Samples of phosphate chrome–alumina composite plates
Moisture absorption rate (%)
Dielectric constant/ 10 GHz
Loss tangent/ 10 GHz
Uncoated sample
10.68
3.864(0 d) 17.136(7 d) 3.912(0 d) 7.217(7 d) 3.952(0 d) 3.985(7 d)
0.0381(0 d) 4.023(7 d) 0.0397(0 d) 2.638(7 d) 0.0398(0 d) 0.0411(7 d)
Sample coated with sealant
0.81
Sample coated with sealant and hydrophobic emulsion
0.02
Contact angle (Deg)
Sliding angle (Deg)
65
–
83
445
157.5
o9
Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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Fig. 2. SEM images and the CA on each surface: (a) uncoated phosphate chrome–alumina composite plates, (b) phosphate chrome–alumina composite plates coated with sealant, and (c) phosphate chrome–alumina composite plates coated with sealant and hydrophobic emulsion.
3.3. Thermal stability of the hierarchical structured coatings Fig. 3 is TG profiles of the hierarchical structured coatings. The TG profiles reveal that the weight loss of coatings is 2.41% at 30–420 1C and 77.14% at 420–600 1C. These results suggest the thermal stability of the coatings is very excellent during 30–420 1C, and the major decomposition of the coatings, which attributed to the decomposition of PVDF, PDMS and EP in the coatings, starts at about 420 1C. Fig. 4 shows the FTIR absorption spectra of the hierarchical structured coatings at different temperatures (25 1C, 200 1C, 300 1C and 400 1C). For all the hierarchical structured coatings at different temperatures, the Si–O–Si group gives a strong absorption band in the spectral region near 1106 cm 1, 772 cm 1 and 470 cm 1. The presence of those peaks confirms the formation of a network structure inside the coatings. The absorption peaks at 3026 cm 1 and 1274 cm 1, which begin to appear at 200 1C, are due to the ¼ CH2 bending vibration of PVDF in coatings. The characteristic bands of the –CH3 group are found at 841 cm 1 (strong band, swinging vibration of CH2), and at 1402 cm 1 (symmetrical deformation vibration of C–H bond). For the hierarchical structured coatings at 400 1C,
Fig. 3. TG profiles of the hierarchical structured coatings.
the peaks at 841 cm 1 and 1402 cm 1 disappear, which indicates the –CH3 groups of coatings are oxidized. The peaks at 3432 cm 1 and 1631 cm 1 correspond to the O–H absorption bond, which is caused by physically adsorbed water. After
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sintered at 400 1C for 2 h, the peaks of O–H (3432 cm 1 and 1631 cm 1) become more obvious than the other three samples because of the serious phenomenon of moisture absorption on surface of materials.
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Fig. 5 represents the surface morphology of phosphate chrome–alumina composite plates with the hierarchical structured coatings after sintered. In the process of thermal treatment, the amount of the holes increased and the pore size was enlarged and the micro/nanostructures were destroyed. After heating at 200 1C and 300 1C, the CA decreased to 138.471 (Fig. 5a) and 129.351 (Fig. 5b), respectively. When the heating temperature reached 400 1C, the coatings lost the effect of damp proof. Water droplets could spread out completely on the surface, and the CA approached zero. The TG curve and FTIR absorption spectra of coatings indicated that the hydrophobic methyl groups were burned off when heated to 400 1C. It is the reason why the CA of coatings fell from 157.5 1C down to zero. 3.4. Mechanical durability of hierarchical structured coatings
Fig. 4. FTIR absorption spectra of the hierarchical structured coatings at different temperatures.
To improve mechanical durability of the hierarchical structured coatings, epoxy resin (EP) and Polydimethylsiloxane (PDMS) were added into coatings. The SEM image (Fig. 6a) of coatings in which there are no added EP and PDMS reveal the surface of coatings has a large number of cracks. And it
Fig. 5. SEM images and the CA of the hierarchical structured coatings: (a) heated at 200 1C, (b) heated at 300 1C, and (c) heated at 400 1C. Please cite this article as: Y. Lei, et al., Fabrication of durable superhydrophobic coatings with hierarchical structure on inorganic radome materials, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.03.087
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Fig. 6. SEM images of coatings: (a) without adding EP and PDMS, and (b) adding EP and PDMS.
Fig. 7. AFM images of the hierarchical structured coatings: (a) no abrasion (CA 157.51), average roughness ¼ 32.74 nm, (b) without EP and PDMS after abrasion (CA 127.031), average roughness ¼175.2 nm, and (c) with EP and PDMS after abrasion (CA 1421), average roughness ¼ 49.3 nm.
can be seen from Fig. 6b that adding EP and PDMS into coatings promotes the formation of dense structure, fine and no micro-crack film. To evaluate mechanical durability of the hierarchical structured coatings, wear test process was employed which method was similar with the method reported by other papers [32]. After the wear experiment, the wetting ability and surface roughness changes of the samples were evaluated by contact angle measurement and AFM images respectively, and the results are as shown in Fig. 8. After the wear experiment, the morphology and average roughness of the surface with adding EP and PDMS were not significantly changed (32.74 nm after compared with 49.3 nm before) (Fig. 7c). And the wetting ability of those coatings was stable, the CA was kept about 1421. However, the after-image for no added EP and PDMS reveals the micro/nanostructures of coatings are destroyed (Fig. 7b). In addition, the average surface roughness for no added EP and PDMS increased drastically from 32.74 nm to 175.2 nm. And the corresponding CA was down to 127.031. These results demonstrate that the mechanical durability of the coatings with added EP and PDMS is superior to that of the coatings with no added EP and PDMS, which attributes to the
cross-linking effect of EP and PDMS (as shown in Fig. 8). Adding EP in first layer of the coatings can wrap and enclose a part of particles (microparticles, nanoparticles) and substrates. These particles attach to substrates by the cross-linking between epoxy groups of EP and hydroxyl groups of particles and substrates. That is the reasons why the adhesion strength between layers is enhanced after EP addition. Adding PDMS into second layer of the coatings can make methyltriethoxysilane-modified nano SiO2 particles embed in PDMS chain. The Si–OH groups from the embedded SiO2 and the O of Si–O– groups in PDMS form the hydrogen bond, which can result in the wear resistance enhancement of silane-modified nano SiO2 particles. 4. Conclusions Mechanically durable, thermally stable and micro–nano hierarchically structured coatings on surfaces of inorganic radome materials were fabricated by colloidal self-assembly method. This method simply involved sealing porous inorganic radome surface with PVDF and SiO2 micron powder and precipitating low surface energy PVDF and hydrophobized silica colloid. The as-formed coatings showed excellent superhydrophobic properties (CA 4 157.51, SA o 91). The weight
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Fig. 8. The mechanism of mechanically durable enhancement.
increase of inorganic radome materials with these coatings was only 0.02%. And the dielectric properties of inorganic radome materials were constant enough to meet the requirement of radome materials. Thermal analysis showed that these superhydrophobic coatings began to lose hydrophobicity when heated above 400 1C. In addition, the wear tests demonstrated the mechanical durability of the coatings with adding EP and PDMS was excellent and superior to that of the coatings with no added EP and PDMS. The excellent superhydrophobic properties and mechanical durability have promising applications in damp-proof protection of other inorganic porous materials.
Acknowledgments This work was supported by the State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, China (Grant No. 11zxfk25) and the research grant of the 863 National Projects (No. 2009AA035002).
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