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Coatings 2018, 8(3), 100; https://doi.org/10.3390/coatings8030100
The Synthesis and Morphology of a Perfluoroalkyl [email protected]2 Resin and Its Performance in Anti-Fingerprint Coating
Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Shaanxi University of Science and Technology, Xi’an 710021, China
Author to whom correspondence should be addressed.
Received: 27 December 2017 / Accepted: 8 March 2018 / Published: 9 March 2018
In order to improve the hydro- and oleo-phobic properties of anti-fingerprint coating, novel oligosiloxane intermediate bearing perfluorodecyl/octyl and triethoxy silylethylene groups were synthesized; then, a series of nano-hybrid perfluoroalkyl oligosiloxane resins ([email protected]2) were synthesized using the hydrolysis and condensation of FVPS with tetraethylorthosilicate. The chemical structure, morphology, and performance of [email protected]2 were investigated. The results indicate that the [email protected]2 is a nano hybrid fluorinated polysiloxane resin with mean particle sizes of 200–400 nm. And under nanoparticles and perfluoroalkyl groups bonded in the resin, [email protected]2 not only showed a micro rough morphology in atomic force microscopy observation but also could provide the treated substrates with excellent hydro- and oleo-phobicity. As a result, the water and oil contact angles reached 120.3° and 87.5° on the treated glass, respectively; meanwhile, fingerprints were easily cleaned without any stains.
Keywords:morphology; nano-hybrid; perfluoroalkyl oligosiloxane; hydrophobic; oleophobic; transparent; touchscreen
As we all know, when a finger touches a glass surface, a fingerprint contamination layer remains. This undesired residue is predominantly water but also contains skin oils, sweat, and cosmetics, resulting in an oily liquid . Once deposited on a transparent surface, such as the touchscreen of a mobile phone, iPad, or pocket computer, it decreases optical transmittance and hinders visibility [2,3]. Therefore, a study on how to keep touchscreens free of fingerprints and make them easier to clean should probably focus on developing transparent hydrophobic and oleophobic coatings; this has become an urgent and primary problem that needs to be solved , especially as electronic devices and wearable electronics have developed rapidly. Thus, the demand for anti-fingerprint materials is expected to increase greatly in the next few years .
Super-hydrophobic phenomena in nature have provided human beings with effective and feasible models (such as lotus leaves and water striders) for the creation of anti-fingerprint coatings. The lotus leaf’s self-cleaning effect is due to the unique geometrical structure and hydrophobic waxy materials of the surface, which offer promising applications in hydro- and oleo-phobicity, self-cleaning, and anti-fouling . Thus, combining nano-particles with low-surface-energy materials such as fluorinated or organosilicon compounds via physical blending or chemical bonding could mimic the lotus leaf structure in hydro- and oleo-phobic coating fabrication . A number of reports have been developed on the preparation of nanoparticles with SiO2 [8,9,10]. However, among the low-surface-energy materials, only perfluoroalkyl compounds and polymers are ideal candidates for improving both dirt resistance and hydrophobic and oleophobic (i.e., amphiphobic) properties, due to their characteristic low surface tension, low wettability, and low adhesiveness of stains to the coating [11,12,13,14]. Zhang et al.  successfully formed superhydrophobic coatings by introducing fluorinated alkyl silane functionalized silica nanoparticles into polydimethylsiloxane resin. Ling et al.  used amino-silane-treated glass as the substrate and perfluoro-decyltrichloro-silane for a subsequent vacuum evaporation process, and transparent superhydrophobicity was achieved. Wang et al.  published a study describing the production of durable superhydrophobic coated surfaces based on tridecafluoro-octyl-triethoxy-silane and a modification of SiO2 nanoparticles with polydimethylsiloxane.
Using nanoparticles plus low-surface-energy materials via blending could rapidly create a superhydrophobic coating with a water contact angle >150° and a sliding angle <10° [18,19,20]. However, the nanoparticles contained in the system easily fall off the coating surface. Besides, cracking, less transmittance, and poor durability to rubbing are still challenges that are related to the practical use of such fabricated coatings. Chemically bonding or robustly combining nanoparticles with perfluoropolymer or perfluorosilsesquioxane may solve the problem and provide better performance (such as rigidness and transparency) for the fabricated coating.
Since the anti-fingerprint coating must have the characteristics of smoothness, rigidity, transparency, and thinness (usually 2–3 μm) in industry applications, and in order to obtain the expected amphiphobic properties in coating fabrication, a novel intermediate perfluoro-1H,1H,2H,2H-decyl/octyl and trimethoxysilylethylene cyclotetrasiloxane (FVPS) and nano SiO2 bonded with perfluorodecyl/octyl oligosiloxane ([email protected]2) resin was designed and synthesized. On the basis of this, a strong hydrophobic and oleophobic coating with easy-to-clean properties was built.
2.1. Materials and Reagents
1H,1H,2H-perfluoro-1-decene (PFDE) and 1H,1H,2H-perfluoro-1-octene (PFOE) were purchased from Fuxin Hengtong Chemical Co., Fuxin, China. Vinyl triethoxysilane (VTES) was supplied by Qufu Chenguang Chemical Co., Qufu, China. 1,3,5,7-tetramethylcyclotetrasiloxane (D4H) and methyl-silicone (MQ) resin were provided by China National Bluestar (Group) Co., Beijing, China. Platinum complex catalyst (KP22) and trifluoroethanol (TFE) were obtained from Shanghai Zhongzixing Chemical Co. (Shanghai, China) and Shanghai Haiqu Chemical Co. (Shanghai, China), respectively. Dibutyl tin diacetate was supplied by Hangzhou Guibao Chemical Co., Hangzhou, China. Tetraethylorthosilicate (TEOS), petroleum ether, acetone, and hydrochloric acid were purchased from Xi′an Chemical and Reagent Co., Xi′an, China. All reagents were used as received.
2.2. Synthesis of Hydro- and Oleo-Phobic Intermediate
In a three-neck flask equipped with a thermometer, a mechanical stirrer, and a reflux condenser, PFDE or PFOE, VTES, and D4H were added in a mole ratio of PFDE/PFOE:VTES:D4H of about 3:1:1. After being stirred for 10 min, the mixture was heated to 80 °C under nitrogen protection, and then a catalytic amount of platinum complex catalyst KP22 was added. The mixture was maintained at 80 °C for 10 h, and the low-boiling-point substances were removed at reduced pressure. Finally, a transparent to slightly brown fluid, the intermediate, 1,3,5,7-tetramethyl-3,5,7-tri(perfluorodecyl/octyl)-1-triethoxylsilylethylene cyclotetrasiloxane, was obtained and noted as FVPS.
2.3. Synthesis of Nano SiO2 Hybrid Fluorinated Alkyl Oligosiloxane Resin
In another three-neck flask equipped with a thermometer, a mechanical stirrer, and a reflux condenser, FVPS and TEOS were added in turn and mixed in a mole ratio of FVPS:TEOS of 1:3, then the solvent TFE was added in a mass ratio of TFE to (FVPS + TEOS) of about 10:1. The mixture was stirred and heated to 40 °C, then hydrochloric acid and deionized water were added in dropwise fashion. After the reaction was carried out for 10 h, the solvent and low-boiling-point substances were removed via reduced pressure distillation at 85–90 °C and 260 mm Hg until the resin mass concentration reached 60%–70%. Finally, a clear solution containing nano SiO2 hybrid fluorinated alkyl oligosiloxane ([email protected]2) resin was obtained (illustrated as Scheme 1).
In addition, the SiO2 sol prepared from TEOS, following the same route as above, was used as a reference.
2.4. Preparation of the [email protected]2 Anti-Fingerprint Coating
Pretreatment of the glass or mobile touchscreen: Prior to the coating treatment, pieces of glass or mobile touchscreen were washed with detergent solution and deionized water, then ultrasonically in acetone for 30 min to remove oil residue and dirt. After being dried at 100–105 °C for 20 min, the cleaned glass or touchscreen was etched by plasma in air atmosphere for 20–30 s, while the gas flow rate was controlled at about 1.69 × 10−1 Pa·m3/s.
Coating preparation: A flow chart of the preparation of anti-fingerprint coating is displayed in Scheme 2. First, the 60%–70% [email protected]2 resin was dissolved and diluted with acetone to form a mass concentration of 0.8% treating solution, then the 30% methyl-MQ silicon resin solution, which was dissolved by petroleum ether (17 wt % in treating solution), and a catalytic quantity of dibutyl tin diacetate (about 2–4 drops in 100 mL solution) were added and fully stirred. Then, the etched glass was fixed on an experimental platform and sprayed with the above treating solution to form a coating. After being kept at room atmosphere for 2–3 min, the coated glass was placed in an oven and cured at 130 °C for 25–30 min. Finally, the [email protected]2 coating anchored on the glass substrate was obtained and kept in a desiccator until it was used.
Fourier-transform infrared spectroscopy (FT-IR) of the intermediate FVPS and the target [email protected]2 was recorded on a Bruker VECTOR-22 spectrometer (Bruker, Ettlingen, Baden-Wurttemberg, Germany), KBr liquid film method. The nanoparticle sizes in the [email protected]2 solution were measured by Malvern Nano-ZS particle sizer (Malvern Instruments Co., Malvern, Worcestershire, UK). Transmission electron microscopy (TEM, Hitachi, Tokyo, Japan) of the [email protected]2 resin was performed on an H-600 transmission electronic microscope with an acceleration voltage of 200 kV. The samples were stained with 2% phosphotungstic acid solution. SEM measurement was carried out on a TM-1000 scanning electron microscope (Hitachi, Tokyo, Japan), and morphologic photographs were taken at a magnification of 1000× Atomic force microscopy (AFM) images were obtained with a Nanoscope IIIA AFM (Agilent Technologies, Palo Alto, CA, USA) in tapping mode. All scanning was performed at 22 °C in air with relative humidity of 48%. The chemical structure of the [email protected]2 film on the treated glass was investigated by an Axis Ultra X-ray photoelectron spectrum (XPS) made in Kratos, Manchester, UK. Static water contact angles (WCAs) on the glass treated with [email protected]2 were determined via the sessile drop method on a JC2000A contact angle goniometer (Shanghai Zhong Chen Digital Co., Shanghai, China) at 20 °C. The liquid volume was 5 μL, and the average of 5 readings from different regions of the same sample was used as the final contact angle of each sample. Spectral transmittances between 300 and 800 nm wavelength were measured using a Cary 5000 ultraviolet-visible spectrophotometer (Agilent Co., Palo Alto, CA, USA), with blank glass as a control. Abrasion resistance analysis was done using LKY-II wet abrasion scrub tester (Hongwei Co., Dongguan, Guangdong, China) with 500 gram weight; cotton cloth was used as abrasion material.
3. Results and Discussion
3.1. About Design and Synthesis of [email protected]2
In synthesizing the intermediate and target products, bonding perfluoroalkyl and trialkoxyl silyl groups into the oligosiloxane structure could impart the intermediate with excellent amphiphobicity and reactivity similar to perfluoroalkyl trialkoxylsilane. Introducing the rigid Q (quaternary siloxane) segments or nano-rough structures into the fluorinated polysilsesquioxane/polysiloxane could make the resin generate an inorganic-organic hybrid and improve the rigidity and transmittance of the [email protected]2. Meanwhile, the more fluorinated groups introduced in the oligosiloxane, the stronger the hydro- and oleo-phobic properties of the intermediate and target [email protected]2. In view of this, the mole ratio of PFDE/PFOE:VTES:D4H was designed to be 3:1:1 in reaction. To ensure that the Si–H residue from D4H was completely consumed in the synthesis of FVPS, slightly excessive PFDE/PFOE and VTES were replenished before the end of hydrosilylation.
An effective method of producing nano-hybrid is to hydrolyze perfluoroalkyl silane with 4 to 6 functional silanes, the nano source material in acidic, or alkali conditions. Therefore, in this experiment, the intermediate FVPS was hydrolyzed with TEOS in a mole ratio of 1:3 (FVPS:TEOS) at a pH of about 4–5. However, to avoid the cyclotetrasiloxane oligomer opening the ring in hydrolysis, the reaction temperature was maintained at around 40 °C, which may be suitable for the alkoxy silyl to convert into Si–OHs and condense into the target [email protected]2 resin.
3.2. Characterization of FVPS and [email protected]2
In the following procedures, FVPS prepared from PFDE and [email protected]2 prepared from FVPS:TEOS in a mole ratio of 1:3 were used as samples to investigate the structure, morphology, and performance of the intermediate and target resins.
Figure 1 and Figure 2 show the FT-IR and proton nuclear magnetic resonance (1H-NMR) spectra of the intermediate FVPS and target [email protected]2 resins. As is obvious in Figure 1, the stretching and distortion vibration peaks due to Si–O and C–F bonding, resulting from the skeleton and side groups of FVPS, respectively, occurred at 1099 (VSi–O), 1212 (VC–F), and 780 (VC–F) cm–1. Adsorption at 3400 cm–1 that belonged to Si–OH groups appeared in a weak peak in [email protected]2 spectrum (Figure 1b), indicating that a large number of Si–OH groups originate from the hydrolysis of FVPS and TEOS reacted with each other and condensed in the target product. In addition, FVPS exhibited characteristic stretching peaks at 2991 and 2878 cm–1, belonging to the C–H groups.
Figure 2 shows the 1H-NMR spectra of [email protected]2 and its intermediate FVPS. The chemical shift signal due to Si–CH3 (aH) appeared at δH 0.15, and bH and b′H belonging to Si–CH2 occurred at δH about 0.74. The peak signal at δH 2.05 should be derived from the cH of the methylene bonded with perfluorodecyl/octyl (–CH2Rf, Rf = C8F17 or C6F13). Meanwhile, the strong signals at δH 1.22 and 3.83 in FVPS that belonged to SiOCH2CH3 clearly vanished from the spectrum of [email protected]2 after the hydrolysis.
3.3. State and Particle Size Distribution of the [email protected]2 Resin Solution
In synthesizing [email protected]2, the bigger the nanoparticles combined in the resin, the rougher of the coating fabricated from the [email protected]2 . However, the big nano particles have an unfavorable impact on the transmittance and smoothness of the coating, especially as the big particles load on the perfluoro oligosiloxane coating surface. Thus, the nano particle size distribution of the [email protected]2 solution was investigated.
Figure 3 shows the TEM photograph, state, and particle size distribution (PSD) of the [email protected]2 resin solution. Figure 3a shows the TEM of FVPS resin. Compared with the FVPS and [email protected]2, obviously, [email protected]2 was a nano-hybrid resin (Figure 3b), although it was clear in appearance (Figure 3c). The nanoparticles in the [email protected]2 solution were about 200–400 nm (Figure 3c) and seemed to have a quasi-spherical core-shell structure. In addition, the average particle was about 321.3 nm in diameter, which accorded well with the TEM result.
The XRD spectra are illustrated in Figure 4. The spectra of SiO2 sol and [email protected]2 did not show the characteristic crystal peak of SiO2; instead, they all displayed significant dispersive peaks. Thus, SiO2 sol and [email protected]2 all have an amorphous structure. In Figure 4b,c, evidently, at 2θ = 24.6°, the dispersive peaks signals were stronger than those of the FVPS. It is easy to see that a large number of amorphous nanoparticles were formed in the SiO2 hybrid perfluoro oligosiloxane resin.
3.4. Morphology and Hydrophobic Properties of the [email protected]2 Anti-Fingerprint Coating on Substrates
Hydrophobicity, oleophobicity, and other performance properties of a solid surface are determined by chemical composition and morphology of the solid surface [22,23,24]. Moreover, micro-/nano-rough structure is an essential and primary factor in preparation of super-hydrophobic surfaces . Therefore, exploring the macro- and micro-morphology of the [email protected]2 resin  will help us to better understand why the coating exhibits strong hydro- and oleo-phobicity and reveals the anti-fingerprint mechanism of [email protected]2.
SEM and AFM provide two effective methods for observing the morphology of a coating on the μm-to-nanometer scale . As shown as in Figure 5b and Figure 6c,d, the [email protected]2 formed a rugged and uneven coating on a toughened glass and silicon wafer substrate. As the data scale was 81.69 nm and the scanning field was 4 × 4 μm2, a lot of nanoparticles emerged in the [email protected]2 coating. Additionally, under the influence of these particles, the root mean square roughness of the [email protected]2 reached about 1.190 nm, which should have a positive contribution to the amphiphobicity of the fabricated coating. However, as shown in the SEM and AFM images of the FVPS (Figure 5a and Figure 6a,b), a relatively smooth coating was formed on the wafer.
Figure 7a–d exhibits the wide-scanning XPS and high-resolution F1s, C1s, Si2p spectra of [email protected]2.
Apparently, four strong characteristic peaks, belonging to fluorine, oxygen, carbon, and silicon, occurred at binding energies of about 687.00 (F1s), 530.00 (O1s), 283.00 (C1s), and 100.00 (Si2p) eV, respectively. It is clear that the C1s spectra could be curve-fitted into four peak components at binding energies of about 287.10, 285.80, 289.03, and 292.80 eV, respectively, which were attributable to the C–C/C–H, C–Si, C–F(CF2), and C–F(CF3) and consistent with those reported in . The Si2p spectra in Figure 7c illustrated the peaks at 99.80 eV for Si and 103.32 eV for organo/inorgano Si–O bonds. Also, the strong feature signals duo to F1s split into two peaks at 688.00 and 687.43 eV (shown in Figure 7d), indicating a large amount of organo C–F bondage in the [email protected]2 coating.
Figure 8a–d exhibits the wide–scanning XPS and high–resolution F1s, C1s, Si2p spectra of FVPS. Compared with the [email protected]2, spectra are not very different.
The presence of a large amount of C–Fs cooperating with nanoparticles, as well as their aggregates, provides the [email protected]2 with a good opportunity not only to have a transparent appearance (Figure 9) but also to possess strong hydrophobicity in coating fabrication, though the thickness of the [email protected]2 coating was 3 μm. As a result, a strong hydro- and oleo-phobic coating prepared from the [email protected]2 resin was obtained in our experiment. As shown in Figure 9, the water droplets not only could stand on the treated touchscreen but could also easily slide from the treated surface.
3.5. Performance of the FVPS and [email protected]2
The [email protected]2 treating solution was prepared from FVPS bearing C8F17 groups and TEOS in a mole ratio of 1:3. The mass concentration of treating solutions was 0.8 wt %. The temperature and curing time of the coating was 130 °C × 25 min, and the coating method was spraying. The result is shown in Table 1.
Table 1 indicates that the WCA and OCA of the [email protected]2 coating became bigger than the control, the untreated glass. In addition, the WCA and OCA of the [email protected]2 coating (thickness of about 3 μm) reached 120.3° and 87.5°, respectively, which were significantly lower than the water contact angle of the same resin coated on the etched wafer by the dipping method (WCA = 150.3°; Figure 10). Obviously, the layer thickness has some passive effect on the hydrophobicity. Moreover, the abrasion resistance is determined by evaluating the change in a static WCA. The glass treated with FVPS without TEOS hydrolysis also showed durable hydro- and oleo-phobicity with slightly lower WCA and OCA compared with that treated with [email protected]2. These control tests indicate that the high superhydrophobicity is a result of the silica nanoscale roughness imparted by the silica nanoparticles, rather than just the perfluoroalkyl (FVPS) alone. As can be seen from Figure 11, after the 5000 abrasion cycles resistance test, the WCA of FVPS and [email protected]2 coating changed slightly, and the WCA decreased 2.9° and 0.8°, respectively. The abrasion resistance simulates actual damage. Although the WCA reduced with a further increase of the abrasion cycles, the treated glasses withstood at least 5000 cycles of abrasion damage without losing their hydrophobicity. Compared with the Dow Corning® (Midland, MI, USA) 2604 Coating, via the same test method, the Dow Corning new anti-fingerprint coating after 5000 abrasion cycles, the WCA decreased 1°, but the [email protected]2 coating only decreased 0.8° , which manifests the [email protected]2 coating durability meet the basic practical applications. Apparently, chemically bonding nanoparticles with perfluorosilsesquioxane can endow glasses with highly durable coating. Furthermore, from Figure 12, it can be seen that the [email protected]2 and FVPS resin coatings both have good transmittance (>90% as compared with the control—the blank touchscreen glass). In addition, for the fingerprint test, Figure 13 clearly shows that the fingerprint on blank glass is much more conspicuous than on the [email protected]2 coating. Also, the fingerprints on blank glass were difficult to clean, but the fingerprints on [email protected]2 coating were easily wiped off. Therefore, [email protected]2 treating solution has good anti-fingerprint properties.
By hydrosilylation of D4H with PFDE/PFOE and VTES, an intermediate cyclotetrasiloxane oligomer bearing perfluoroalkyl and trimethoxy silylethylene groups was synthesized. Based on FVPS, a series of perfluoro nano-SiO2 hybrid polysiloxane oligomer resins were synthesized by the hydrolysis and condensation of FVPS with TEOS. An investigation of TEM and AFM indicated that nanoparticles were combined in the [email protected]2 resin. Under the influence of nano particles and their aggregates, as well as the perfluorodecyl/octyl, [email protected]2 not only showed relatively rough morphology but also good hydro- and oleo-phobic properties on the treated substrate. As a result, the WCA on the [email protected]2 coating reached 120.3°, and the OCA reached 87.5°, since the low-surface-energy perfluorodecyl/octyl groups aggregated on the coating surface. Contact angle and visual analysis of artificial fingerprints, both before and after wiping, most readily demonstrated that the [email protected]2 treating solution had good hydrophobicity and oleophobicity, and fingerprints were easily cleaned without any stains. A fingerprint mainly contains water and oils; therefore, the transparent [email protected]2 coating showed good anti-fingerprint properties, and such good durability may be useful for various functional applications.
This work was supported by a major project of the Ministry of Science and Technology of China [2017YFB0307700], a major project of scientific and technological coordination from Shaanxi Province [2015KTCL01-14], and an industrialization cultivation project of Shaanxi Provincial Education Department [16JF004].
Qiufeng An, Zhujun Lyu, Wenchao Shangguan, and Bianli Qiao conceived and designed the experiments; Zhujun Lyu, Wenchao Shangguan, and Bianli Qiao performed the experiments; Zhujun Lyu and Wenchao Shangguan analyzed the data; Zhujun Lyu and Pengwei Qin contributed reagents/materials/analysis tools (Origin 8.0, Nanoscope analysis, XPS peak 4.1, MestReNova, JC2000A contact angle goniometer and Excel); and Qiufeng An and Zhujun Lyu wrote the paper.
Conflicts of Interest
The authors declare no conflict of interest.
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Scheme 1. Schematic illustration of synthetic procedure of [email protected]2 resin.
Scheme 2. Flow chart of preparation of anti-fingerprint coating.
Figure 1. FT-IR spectra of (a) FVPS; (b) [email protected]2; and (c) SiO2 sol.
Figure 2. 1H-NMR spectra of [email protected]2 (a) and intermediate FVPS (b).
Figure 4. XRD spectra: (a) SiO2 sol; (b) FVPS; and (c) [email protected]2.
Figure 7. XPS and high-resolution spectra of [email protected]2 on the silicon wafer: (a) wide scanning; (b) C1s; (c) Si2p; and (d) F1s.
Figure 8. XPS and high-resolution spectra of FVPS on the silicon wafer: (a) wide scanning; (b) C1s; (c) Si2p; and (d) F1s.
Figure 9. Hydrophobicity of [email protected]2 coating on mobile phone screen.
Figure 10. Silicon wafer treated with [email protected]2 in dipping method, WCA = 150.3°.
Figure 11. Impact of Rubbing Durability on Water Contact Angles.
Figure 12. Transmittance of the coatings: (a) blank; (b) [email protected]2; (c) FVPS.
Figure 13. Fingerprint test: (a) coating with [email protected]2 treating solution; (b) blank.
Table 1. Performance of the [email protected]2 coating.
|Treating Solutions||wt %||WCA (°)||OCA (°)||T %||Appearance||Abrasion Resistant/Cycles||AF property|
Notes: The mass of [email protected]2 resin and FVPS resin per 100 g acetone solvent; OCA: oil contact angle (sunflower oil); T: transmittance; AF: anti-fingerprint; ◎: easily cleaned. WCA and OCA of the coating prepared from TEOS and FVPS bearing C6F13 were 120.1° and 84.9°, respectively.
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