Facile fabrication of superhydrophobic polysiloxane/magnetite nanocomposite coatings with electromagnetic shielding property.
Keywords Superhydrophobic coatings, Nanocomposite coatings, Polysiloxane, [Fe.sub.3][O.sub.4] nanoparticles, Electromagnetic shielding property
"Lotus effect" superhydrophobic coatings, with a water contact angle (WCA) above 150[degrees](1,2) and contact angle hysteresis below 10[degrees], have attracted special interests due to their wide applications in anti-adhesion,(3) antifouling,(4) self-cleaning,(5-8) and microfluidic systems.(9) Until now, versatile approaches have been developed for the preparation of superhydrophobic coatings, including template-based techniques,(l0-12) plasma-enhanced chemical vapor deposition (PECVD),(13-16) electrochemical deposition (ECD),(17-18) layer-by-layer (LBL) deposition,(19) sol-gel process,(20), (21) colloidal assembly,(22-26) and so on. However, these methods suffer from their own inherent drawbacks, e.g., template approach is limited to the attainable geometry; PECVD, ECD, and LBL involve sophisticated equipment, multistep procedures, and sometimes harsh conditions; sol-gel process has to undergo a calcination step at high temperatures; colloidal assembly needs further coverage of a layer of low surface free energy chemicals, and even treatment with high temperature to improve the mechanical properties and the adhesive force with the substrate.(23-26) All these drawbacks make them difficult in large-scale applications, especially on the surfaces of big objects such as buildings, bridges, and outdoor apparatus.
To overcome the shortcomings as mentioned above, a facile "one-pot coating" method was developed to acquire the superhydrophobic coatings in recent years. (27-30) Namely, organic (or organic-inorganic hybrid) binder was blended with micro/nanoparticles and solvents, and then cast on substrates. After drying, superhydrophobic surface was readily achieved through the self-organization of the coating components. The method has great potential for practical use, since its features are similar to "conventional organic coatings." Until now, several one-pot superhydrophobic coatings have been exploited. For example, Yang et al. prepared superhydrophobic coating by modification of micro-[CaCO.sub.3]/nano-Si[O.sub.2] composite particles with hexamethyldisilazane and further blending with a mixed polysiloxane binder that was composed of polydimethylsiloxane (PDMS), vinyltriethoxysilane, and [gamma]-aminopropyltriethoxysilane.(27) Cao et al. prepared anti-icing superhydrophobic coatings from orga-nosilane-modified nano-(or micro-)Si[O.sub.2] particles, acrylic polymer and silicone resin (Dow Corning 840).(28) Xu et al.(29) combined perfluorooctanoic acid (PFOA)-modified Ti[O.sub.2] nanoparticles with polystyrene (PS) while Harton et al.(30) blended PDMS-modified Si[O.sub.2] with PS to get the superhydrophobic coatings. Nevertheless, among these reports, micro/nanopanicles were usually modified with organosilane or fluorocarbon chemicals in advance, complicating the preparation process of coatings, while preparation of one-pot superhydrophobic coatings directly from unmodified micro/nanoparticles was seldom reported. Hikita et al. fabricated the superhydrophobic coating through direct mixing of colloidal silica particles (10-20 nm) with sol-gel derived silica binder and the hydrophobe (heptadecafluoro-1,1,2,2-tetrahydrodecyl) triethoxysilane.(31) Hsiang et al. prepared the superhydrophobic coatings by mixing boehmite nanopowder (20 nm) and anatase nanopowder (6 nm) with perfluroalkyl methacrylic copolymer (Dupont Zonyl 8740). However, the former case was limited to thin thickness (500 nm) of the dried film while the latter one employed a thermal-cured process (120[degrees]C, 2 h).(32)
In this article, one-pot superhydrophobic coatings were prepared through direct mixing of unmodified magnetite ([Fe.sub.3][O.sub.4]) nanoparticles with the silicone resin and the curing agent, aminopropyltriethoxysilane (APS). Compared to the previous one-pot process for superhydrophobic coatings, this preparation process does not need to carry out the modification of nanoparticles and is rather simple. The coatings can be thick (60 [micro]m) and cured at ambient temperature to reach the desired mechanical strength. Moreover, because of the magnetic property of [Fe.sub.3][O.sub.4] nanoparticles, the as-obtained superhydrophobic coatings have excellent electromagnetic interference shielding capability, and thus particularly match those self-cleaning applications in communication base station and outdoor electronic/electrical equipments.
Silicone resin (Dow Corning[R] 3037, dimethyl phenyl siloxane methoxy-terminated) was supplied by Dow Corning. Aminopropyltriethoxysilane (APS, KH550) and xylene (chemical grade) were purchased from Shanghai Yaohua Chemical Plant of China and Sinopharm Chemical Reagent Co., Ltd., respectively. [Fe.sub.3][O.sub.4] nanoparticles (cubic crystal, primary particle size: 20 nm) were the product of Beijing Nachen S&T Ltd, China. Dispersant (DISPERBYK[R]-162, solution of a high molecular weight block copolymer with pigment affinic groups) was obtained from BYK. All the materials were used as received.
Preparation of polysiloxane/[Fe.sub.3][O.sub.4] nanocomposite coatings
A certain amount of dispersant (12 wt% relative to the weight of nano-[Fe.sub.3][O.sub.4] particles) and 40 g of xylene were added to a 250 mL plastic beaker and stirred for 10 min. Afterward, 6 g of silicone resin and a certain amount of [Fe.sub.3][O.sub.4] nanoparticles were added to the plastic beaker and ball-milled for 2 h at a rotation speed of 2000 rpm using zirconia beads of 0.5 mm in diameter. Just before application, APS (12% based on the weight of silicone resin in the coatings) was mixed thoroughly into the dispersion solution. The coatings were cast on glass slides or polycarbonate (PC) plates using a drawdown rod and dried at ambient temperature (15-25[degrees]C) at RH of 60-80% for 1 week. The thickness of the dried coatings was about 60 [micro]m.
FTIR spectra were collected over the range of 4000-400 c[m.sup.-1] using a NICOLET Nexus 470 FTIR spectrum instrument (Nicolet, USA) with a resolution of 0.5 c[m.sup.-1] and 32 scans. The surface morphology of the coatings was observed with a field emission scanning electron microscope (FESEM, JSM-6701F, JEOL Co., Ltd, Japan) at an accelerated voltage of 10 kV. The specimens were sputtering-coated with gold prior to FESEM imaging. AFM topography of the typical coatings was performed by a SPM-9500J3 microscope (SHIMADZ, Inc., Japan) using tapping mode at ambient condition. Silicon tips with a radius of 10 nm, a spring constant of 5.5-22.5 N [m.sup.-1] and frequency of 190-325 kHz were used. Root-mean-square roughness (RMS) of the images was calculated using commercial software. The surface composition of the coatings was measured by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ECSA) using Al K[alpha]. radiation at a 90[degrees] take-off angle. All the binding energy values were calibrated using the reference peak of C1S at 284.6 eV.
Static WCA at equilibrium and sliding angle were determined with an OCA 15 contact angle analyzer (Dataphysics, Germany) using a 5 [micro]L water droplet and obtained from the average value of five parallel measurements. The long-term durability of the dried polysiloxane/[Fe.sub.3][O.sub.4] nanocomposite coatings was tested in a QUV accelerated weathering tester (QUV/se, Q-Panel Co., Ltd., USA). UV lamps with wavelength of 340 nm were used and the accelerated weathering cycle was set as follows: UV-irradiation for 8 h at 50[degrees]C and irradiation intensity of 0.68 W/[m.sup.2], and condensation for 4 h at 40[degrees]C. Pencil hardness was determined using Zhonghua[R] pencils (China) with hardness varying from 6B (the softest) to 9H (the hardest). The grade of the hardest pencil that does not cause any marring of the surface was adopted. The coatings coated on PC plates were used to measure electromagnetic shielding effectiveness (SE) according to the reference standard SJ20524-1995 using a E8257D signal generator (Agilent, USA) and a E447A spectrum analyzer (Agilent, USA).
Results and discussion
Preparation of polysiloxane/ [Fe.sub.3][O.sub.4] nanocomposite coatings
The silicone resin was adopted as the polymer binder of the nanocomposite coatings because it has both methyl and methoxyl groups connecting to the Si atoms. The methyl groups offer the hydrophobicity of the coatings while the pendant methoxyl groups endow the coatings with the crosslinking capability via hydrolysis/condensation reactions with the help of moisture in the air. APS was used as the alkali catalyst for the hydrolysis/condensation reactions of the silicone resin, and meanwhile, participated in the hydrolysis/condensation reactions.
Figure 1 shows the FTIR spectra of APS, silicone resin, and the dried coatings. A sharp peak at 2840 c[m.sup.-1], attributed to the stretching vibration of-C-H bond in methoxyl groups, disappears in the FTIR spectra of the dried coatings. This directly indicates the occurrence of the hydrolysis reaction during drying. Since Si-O-Si bonds inherently exist in the silicone resin, the newly formed Si-O-Si bond by condensation cannot be easily demonstrated from the absorption band at 1000-1100 c[m.sup.-1]. Nevertheless, the band for the dried coating is broader in comparison with that for the silicone resin, which may imply condensation.
Figure 2 shows the WCA and SA of polysiloxane/ [Fe.sub.3][O.sub.4] nanocomposite coatings as a function of [Fe.sub.3][O.sub.4] nanoparticle content. WCA steadily increases at [Fe.sub.3][O.sub.4] nanoparticle content ranging from 0 to 40 wt%, and reaches a plateau at [Fe.sub.3][O.sub.4] nanoparticle content beyond 48 wt%. Meanwhile, SA gradually decreases as [Fe.sub.3][O.sub.4] nanoparticle content increases. WCA of K46.4[degrees] and 158.3[degrees], and SA of 6.5[degrees] and 4.3[degrees] are obtained at 48 and 56 wt% of [Fe.sub.3][O.sub.4] content, respectively. This indicates that "lotus effect" superhydrophobic coatings are successfully fabricated at high [Fe.sub.3][O.sub.4] nanoparticle content, and also further confirms that "one-pot coating" method is feasible for acquiring the superhydrophobic coatings. The above percolation threshold for superhydrophobicity is close to that reported for polysiloxane/CaC[O.sub.3]/Si[O.sub.2] (40%) (27) and PS/nano-PFOA-Ti[O.sub.2] (50%) (29) coatings, however, much higher than that of fluorinated polysiloxane/nano-Ti[O.sub.2] coatings (25%) (33) in which the low percolation threshold may be due to the high porosity of Ti[O.sub.2] nanoparticles aggregates.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
The surface morphology of the coatings was examined using FESEM as shown in Fig. 3. It displays the distinctive topographical features of the coatings with various [Fe.sub.3][O.sub.4] contents. The coating without nano- [Fe.sub.3][O.sub.4] particles is very smooth (Fig. 3a). However, when 24 wt% [Fe.sub.3][O.sub.4] nanoparticles were introduced, some small protrusions with micron size (1-3 [micro]m) embedded in the coatings are visible. These protrusions are caused by the aggregates of [Fe.sub.3][O.sub.4] nanoparticles. Rough surface with small holes is observed for the coating with 40 wt% of [Fe.sub.3][O.sub.4] nanoparticles content (Fig. 3c). Micronano binary structures, namely irregular micropapillae (1-5 [micro]m) and nanoapophysis at the surface of the micropapillae, are clearly exhibited for the coating with 56 wt% [Fe.sub.3][O.sub.4] nanoparticles (Fig. 3d). These micronano binary structures are actually constituted from the aggregates of [Fe.sub.3][O.sub.4] nanoparticles that were insufficiently de-agglomerated.
AFM was also used to probe the surface topography of the nanocomposite coatings. The 3D height images as well as the RMS values are shown in Fig. 4. It is clearly seen from the images that the roughness of the coatings increases with increasing [Fe.sub.3][O.sub.4]content. Accordingly, the RMS rises from 2 to 540 nm as the [Fe.sub.3][O.sub.4] content increases from 0 to 56 wt%. This result is well consistent with the FESEM results.
[FIGURE 3 OMITTED]
Since the silicone resin is composed of dimethylsiloxane units and methylphenylsiloxane units, it is flexible and has low surface free energy. During drying at room temperature, the polysiloxane chains tend to transfer to the outmost surface of the coatings. The low surface free energy polysiloxane and the micro-nano binary structure are responsible for the "lotus effect" superhydrophobicity of the coatings with high [Fe.sub.3][O.sub.4] nanoparticle content.
The hardness is extremely important for practical application of superhydrophobic coatings. Table 1 shows the pencil hardness of the polysiloxane/ [Fe.sub.3][O.sub.4] nanocomposite coatings. The pure polysiloxane coating has very low pencil hardness. Interestingly, the pencil hardness increases from 4B to B as the [Fe.sub.3][O.sub.4] nanoparticle content increases from 0 to 40 wt%, being attriuted to the reinforcing role of [Fe.sub.3][O.sub.4] nanoparticles for organic coatings. For these superhydrophobic coatings with extremely high [Fe.sub.3][O.sub.4] content (48 and 56 wt%), pencil hardness of B is still preserved. This hardness is quite desirable. It suggests that the superhydrophobic coatings are resistant to mechanical damage.
Figure 5 shows the change of the WCAs of the nanocomposite coatings under QUV accelerated weathering tests. After 960 h of QUV tests, the WCAs of the coatings do not show obvious decline, despite the [Fe.sub.3][O.sub.4] content. The superhydrophobic coating still keeps the hydrophobicity. The pencil hardness of the coatings after 960 h of QUV tests was determined and summarized in Table 1. No decrease in pencil hardness is observed for the nanocomposite coatings including the superhydrophobic nanocomposite coatings. A little improvement is even found for the soit polysiloxane coatings with 0 and 8 wt% of [Fe.sub.3][O.sub.4] nanoparticles, which may be attributed to the further crosslinking of polysiloxane in QUV tests.
[FIGURE 4 OMITTED]
XPS analysis was conducted for the superhydrophobic coating with 56 wt% of [Fe.sub.3][O.sub.4] nanoparticles. The atomic composition is shown in Table 2. Comparing the coatings before and after QUV test, no obvious variation of the atomic composition is distinguished. The slight difference in the atomic composition may be a result of a measuring error. In addition, it can be seen that atomic fractions of Si and C are much higher than that of Fe, suggesting that the outmost surface of the coating is indeed dominated by polysiloxane chains.
Table 1: The pencil hardness of polysiloxane/[Fe.sub.3][O.sub.4] nanocomposite coatings before and after 960 h QUV accelerated weathering test [Fe.sub.3][O.sub.4] content (wt%) 0 8 16 24 32 40 48 56 Before QUV tests 4B 4B 3B 3B 2B B B B After QUV tests 3B 3B 3B 3B 2B B B B Table 2: The surface composition (at%) of the super-hydrophobic coatings with 56% [Fe.sub.3][O.sub.4] content before and after 960 h of QUV accelerated test Element Before QUV test After QUV test Si 16.78 16.73 O 28.21 26.91 C 53.44 55.03 N 1.09 1.04 Fe 0.49 0.30
These results indicate that the superhydrophobic nanocomposite coatings have better weatherability, and their superhydrophobicity can be preserved for a long time during service. The better weather resistance of the coatings is attributed to the polysiloxane matrix of the coatings. The polysiloxane binder with total silicon-oxo backbone is extremely durable to UV-irradiation because the bonding energy of Si-O bond is high, up to 460 kJ/mol, being far beyond the energy of UV light (314-419 kJ/mol).
Electromagnetic interference shielding properties
It is frequently mentioned that [Fe.sub.3][O.sub.4] nanoparticles can be used as electromagnetic shielding materials. (34-36) Nevertheless, previous reports focused on the magnetic Properties (34-36) and seldom on the electromagnetic interference shielding properties of [Fe.sub.3][O.sub.4] nanoparticles. Herein, the obtained superhydrophobic coating has high [Fe.sub.3][O.sub.4] content, and hence would possibly possess electromagnetic shielding capability. In order to understand how high the electromagnetic shielding is, electromagnetic shielding effectiveness (S[E.sub.%]) was measured. The S[E.sub.%] is the ratio of the field strength before and after attenuation and can be calculated according to the following equations:
S[E.sub.dB] =P1-P2 (1)
S[E.sub.%] = (1-[10.sup.-S[E.sub.dB]/10]) x 100% (2)
where S[E.sub.dB] is the logarithmic shielding effectiveness (unit: dB). P1 and P2 correspond to the spectrum analyzer readings (unit: dBm) of the cases without and with shielding materials on test fixture, respectively. The higher the S[E.sub.%] value is, the less energy passes through the shielding material.
Figure 6 illustrates the variation of S[E.sub.%] against the frequency from 10 to 3000 MHz for the pure polysiloxane coating and the nanocomposite coating containing 56 wt% of [Fe.sub.3][O.sub.4] nanoparticles. It can be seen that the S[E.sub.%] of the pure polysiloxane coating is <3% in the measured frequency range, indicating that the polysiloxane coating does not possess electromagnetic shielding ability. In contrast, the superhydrophobic nanocomposite coating has more than 60% electromagnetic shielding effectiveness over the frequency range (10-3000 Hz), suggesting the good electromagnetic shielding ability. This confirms the electromagnetic shielding capability of [Fe.sub.3][O.sub.4] nanoparticles. The electromagnetic shielding should mainly result from the electromagnetic absorption of [Fe.sub.3][O.sub.4] nanoparticles. Perhaps the enhanced reflection due to the rough surface of the superhydrophobic coatings also contributes to the electromagnetic shielding. The good electromagnetic shielding property makes the superhydrophobic coatings useful as self-cleaning coatings in communication base stations, outdoor electronic/electrical equipment, and so on.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
Polysiloxane/ [Fe.sub.3][O.sub.4] nanocomposite coatings were readily prepared by mixing silicone resin with [Fe.sub.3][O.sub.4] nanoparticles and subsequently curing at ambient temperature using APS as the catalyst and curing agent. The WCA and SA of the nanocomposite coatings strongly depend on the [Fe.sub.3][O.sub.4] nanoparticle content. "Lotus effect" superhydrophobic coatings can be achieved when the [Fe.sub.3][O.sub.4] content exceeds 48 wt%. The as-obtained superhydrophobic coatings have good mechanical strength, long-term superhydrophobicity, and electromagnetic shielding effectiveness over 60% in the frequency range of 10-3000 MHz. The superhydrophobic nanocomposite coatings have potential applications as self-cleaning coatings in communication base stations, outdoor electronic/electrical equipment, and so on.
Acknowledgments This work was supported by Shanghai Shuguang Scholar Program (09SG06), Nature Science Foundation of China (51073038), the Foundation of Science and Technology of Shanghai (09DJ1400205), and the innovative team of Ministry of Education of China (IRT0911).
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X. Ding, S. Zhou, G. Gu (El), L. Wu Department of Materials Science and Advanced Coatings Research Center of Ministry of Education, Fudan University, Shanghai 200433, China e-mail: email@example.com
DOI 10.1007/s1 1998-011-9358-6
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|Author:||Ding, Xiaofeng; Zhou, Shuxue; Gu, Guangxin; Wu, Limin|
|Date:||Nov 1, 2011|
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