1. Introduction
Mobile phones are common communication tools. Because of its frequent use, the screen of a mobile phone is easily polluted by other substances. Thus it is necessary to develop a self-cleaning, wear-resistant and transparent mobile phone screen, and super-wetting materials are one of the choices of mobile phone screen coatings. Super-wetting materials have good anti-current conduction, anti-corrosion, waterproof, anti-fog, anti-mold, anti-snow, anti-freeze, anti-sticking, anti-pollution, and resistance reduction qualities, and other functions [1,2,3,4,5,6] which have a wide range of application prospects in the fields of transportation, clothing and textiles, liquid transportation, biomedicine, daily necessities and packaging, construction, and microanalysis [7,8,9,10,11]. Transparent superhydrophobic materials can be used in field lenses, car glass, curtain walls, mobile phone screens, and other occasions, and have anti-fouling, anti-fog, and waterproof properties [12,13,14,15], which can increase the safety and reliability of equipment, and is a special coating that is widely used. The light transmittance of this coating is one of the very important parameters. Only by improving surface hydrophobicity and abrasion resistance while taking into account light transmittance can we successfully construct a transparent superhydrophobic coating that meets people’s needs. However, the hydrophobicity and transparency of the coating are often a pair of mutually restrictive characteristics [16,17]. On the one hand, the superhydrophobicity of the surface increases with increasing roughness; on the other hand, the increase in roughness increases the scattering effect of light in the propagation process, greatly reducing the surface transparency, and also reducing the wear resistance. This characteristic becomes one of the important issues of transparent superhydrophobic coatings. At present, the mobile phone screen is mainly coated with anti-fingerprint oil and fluororesin [18,19], and although these kinds of materials have high transparency, their wear resistance and long-term usability are insufficient. In this paper, the successful research methods of coatings and films in other papers are used for reference [20,21,22,23]. Taking the transparent coating of mobile phone screen as the research object, we use high hardness (9H) inorganic silicon resin crystal coating as the main material, and adopt organic silicon resin as the binder, with the addition of the nano silica particles hydrophobically modified with heptafluorodecyltriethoxysilane. A high-transparent, wear-resistant, superhydrophobic coating with good comprehensive performance is prepared on the glass surface by spraying, which provides an idea for solving the above problems.
2. Materials and Methods 2.1. Coating Preparation
In this experiment, inorganic silicone resin SJ-32F was used as the main material and SJ-32F resin as a special silicone resin. The film is highly transparent, with a hardness of up to 9H, can withstand high temperatures of 450 °C for a short period of time (2–3 min), and can be used at a temperature of 50–300 °C for a long time without yellowing. Silicone resin 9825 was the binder. The composition ratio of SJ-32F and 9825 was changed to find the relationship between the surface hardness of the coating and the resin composition, as in Table 1.
The leveling agent and defoaming agent were organic modified polysiloxane and silane coupling agent A187, respectively, mixed with heptafluorodecyltriethoxysilane (FAS-17) hydrophobically modified nano silica particles (average particle size 30 nm) to produce a rough surface structure. The ratio of various materials is shown in Table 2.
The above-mentioned materials were put into a beaker and stirred with a magnetic stirrer for 15 min to prepare an inorganic/organic silicon anti-stick coating mixed dispersion. Using the Iwata W-71 top pot spray gun produced by ANEST IWATA Corporation, Yokohama, Japan, we poured the prepared non-silicone anti-sticking paint mixed dispersion into a spray gun and sprayed evenly on the glass slide; the air pressure of the air pump was controlled at about 30kPa and the sprayed sample was cured at room temperature for 6 h, pending test.
In anhydrous ethanol solution, the chemical bond between –CH3 and –Si– of FAS-17 was broken, and the chemical bond between –H and –O– of hydroxyl group on the surface of silica was also broken, so that a large number of vacancies are formed on the surface of silica, which promoted the combination of –Si– and –O– and was surrounded by FAS-17, while the other end of FAS-17 had –CF3 and –CF2 low surface energy groups. The modified nano silica with low surface energy was formed, shown as Figure 1.
2.2. Coating Performance Test
2.2.1. Coating Hardness Test
ISO-15184-2020 [24] Paint and Varnish Pencil Method for Determination of Paint Film Hardness was used for testing.
2.2.2. Wetting Test of Coating
A contact angle measuring instrument was used to characterize the water contact angle (CA) and rolling angle (SA) of the coating, with a sampler used to draw 5 µL of deionized water droplets onto the coating surface, and the contact angle measuring instrument used for testing. Each sample was repeatedly tested at 5 points on different parts of the coating surface, with the average value taken.
2.2.3. Coating Transparency and Anti-Reflection Test
The reflectance spectrum and the light transmittance of the slide before and after coating were tested by a LRMS-LP1 differential spectrophotometer (Japan Tsushima Company, Kyoto, Japan). For the application of mobile phones in visible light, the wavelength range of visible light was 390–780 nm. In this paper, 400–800 nm is taken as the measurement range. The coating thickness of the sample was 15 μm.
2.2.4. Coating Adhesion Test
The cross-cut method was used to detect the adhesion of the coating. A rectangular grid cut line was made on the surface of the coating and the test tape was used to adhere to the cut mark; the tape was pulled up quickly at an angle of 90° and the tightness of the coating according to the peeling off of the edge coating was judged. The judgment standard is shown in Table 3. The Accutester ACT-15 adhesion pull tester (Beijing Hongou Chengyun Instrument Equipment Co., Ltd, Beijing, China) was used to test the coating adhesion.
2.2.5. Characterization of Coating Wear Resistance
The coating friction test method is shown in Figure 2. A piece of 800 mesh sandpaper was put on the prepared coating surface, with 20 g of standard weight put on the sandpaper; the sandpaper was pulled to move 100 mm at a constant speed on the coating surface, and the process was cycled. The contact angles of the water droplets on the surface of the coating with rubbing times of 20, 40, 60, 80, and 100 were respectively measured. The data of five positions were taken for each contact angle measurement, with the average value taken as the final result.
3. Results 3.1. Coating Hardness Test Results
The test was carried out using ISO-15184-2012 Paint and Varnish Pencil Method for Measuring the Hardness of Paint Films, and the test results of coating hardness of different resin compositions are shown in Table 4. According to the results of communication between the author of this article and Chinese mobile phone manufacturers, the surface hardness of mobile phone screen coatings is 3H–4H, which meets the technical requirements for mobile phone use. Therefore, the coating with 9825 resin content of 10% was selected as the main research object in this article.
The molecular formula of silicone resin 9825 and inorganic silicone resin SJ-32F are shown in Figure 3. When the two resins are mixed, the silanol group in the organic silicon molecular structure is dehydrated and crosslinked with the silanol group in the inorganic silicon molecular structure to form a low surface energy composite material; the modified nano silica particles were uniformly dispersed into the composite coating to form the superhydrophobic coating shown in Figure 4.
3.2. Coating Wettability
Figure 5a–f show the inorganic/silicone anti-stick coatings; the mass ratios of modified nano silica were 0%, 1.5%, 1.7%, 1.9%, 2.1%, 2.3% of contact angle images, respectively, and the contact angle values are shown in Table 5.
It can be seen from Figure 5 and Table 5 that the hydrophobicity of the inorganic/organic silicon hybrid coating itself is not good. After the modified nano silica is incorporated, the contact angle is significantly increased. When the mass ratio is above 1.7%, the superhydrophobic effect can be achieved: the contact angle is greater than 150° and the rolling angle is less than 10°. When the modified nano silica particles account for 1.9%, 2.1%, and 2.3% by mass, it can be found that the contact angle of the coating is basically unchanged, and that the rolling angle is slightly reduced. As the silica content increases, the surface roughness of the coating will increase and the light transmittance of the coating will decrease; therefore, in this paper, 1.7% is selected as the mass percentage of the modified nano silica.
3.3. Coating Transparency Test Results
The light transmittance results of the test slide before and after coating are shown in Figure 6. Figure 6a is the light transmittance of the uncoated slide. As shown in the figure, the light transmittance of the slide before coating is close to 100%. After coating, Figure 6b displays light transmittance being about 91%–95%, showing high transparency. Figure 6c shows the transparency of the coating.
As a coating for a mobile phone display, its reflectivity will affect the display effect of the screen. Figure 7 shows that the reflectivity of the coating is less than 1% and the anti-reflection effect is better.
3.4. Coating Adhesion Test The cross-cut method was used to detect the adhesion of the coating and a rectangular grid cutting line was made on the surface of the coating; the test tape was used to adhere to the cut marks and the tape was quickly pulled up at a 90° angle to determine the tightness of the coating based on the peeling off of the edge coating. The test showed that the prepared coating had smooth edges, that no peeling off was at the edges and intersections, and that the bonding force reached 5B. The test results using Accutester ACT-15 adhesion pull tester were 5.2, 5.6 and 5.3 MPa, and the average value was about 5.4 MPa, indicating that the bonding force was good. 3.5. Characterization of Coating Wear Resistance
The coating friction test method is shown in Figure 2. We put 800 mesh sandpaper on the surface of the prepared coating, then put 20 g of standard weight on the sandpaper, pulled the sandpaper to move 100 mm on the coating surface at a uniform speed, and cycled the process. The contact angle and light transmittance of the water droplets on the coating surface with the rubbing times of 20, 40, 60, 80, and 100 were respectively measured. The test results are shown in Table 6. The test results show that as the number of rubs increased, the contact angle of the coating remained basically unchanged, while the light transmittance of the coating decreased; when the number of rubs reached 100 times, the light transmittance remained above 80%, shown in Figure 8. Due to the high hardness of the coating, during the friction process, the surface of the coating only showed the peeling of individual nano silica particles and the deformation of micro-protrusions. The frictional surface roughness did not change significantly, which resulted in small changes in the contact angle and light transmittance of the coating, indicating that the coating had good friction resistance.
4. Conclusions
In this article, we took the transparent coating of a mobile phone screen as the research object, used high-hardness (9H) inorganic silicon resin crystal coating as the main material, and used organic silicon resin as the binder, with nano silica particles hydrophobically modifying with heptafluorodecyltriethoxysilane; a high-transparent, wear-resistant, superhydrophobic coating with good comprehensive performance was prepared on the glass surface by spraying. What the research shows is the following (Table 7):
Enlarge this image.Figure 3. Molecular formula of silicone resin 9825 (a) and inorganic silicone resin SJ-32F (b).
Enlarge this image.Figure 5. Coating surface contact angles of different mass ratios of nano silica (a) 0%, (b) 1.5%, (c) 1.7%, (d) 1.9%, (e) 2.1%, and (f) 2.3%.
Enlarge this image.Figure 8. Transmittance of glass slide before and after friction test (test number 100 times, line ① before friction test, line ② before friction test).
| 9825 resin mass fraction (wt.%) | 0 | 5 | 10 | 15 | 20 | 25 |
| Resin SJ-32F mass fraction (wt.%) | 100 | 95 | 90 | 85 | 80 | 75 |
| Material Name(Producer) | Function | Quality Score (wt.%) |
|---|---|---|
| Inorganic silicone resin SJ-32F (Zongyang Sanjin Pigment Co., Ltd., Zongyang, China) | Ingredients | See Table 1 |
| Silicone resin 9825 (Zongyang Sanjin Pigment Co., Ltd., Zongyang, China) | Glue | See Table 1 |
| Organically modified polysiloxane (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) | Leveling agent, defoamer | 0.3 |
| Silane coupling agent A187 (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) | Coupling agent | 0.5 |
| Heptafluorodecyltriethoxysilane (FAS-17) modified nano silica (Sinopharm Chemical Reagent Co., Ltd., Beijing, China) | Hydrophobic modifier | 1.5%, 1.7%, 1.9%, 2.1%, 2.3% |
| Qualified Adhesion Requirements: Adhesion ≥ 4B | |
|---|---|
| 5B | The edge of the scribe line is smooth, and there is no fall off at the edge and intersection. |
| 4B | Small pieces fall off at the edge and intersection of the scribe line, and the total area of fall off is less than 5%. |
| 3B | Small pieces fall off at the edge and intersection of the scribe line, and the total area of fall off is between 5% and 15%. |
| 2B | Pieces fall off at the edge and intersection of the scribe line, and the total area of fall off is between 15% and 35%. |
| 1B | Pieces fall off at the edge and intersection of the scribe line, and the total area of fall off is between 35% and 65%. |
| 0B | Pieces fall off at the edge and intersection of the scribe line, and the total fall off area is greater than 65%. |
| 9825 resin mass fraction (wt.%) | 0 | 5 | 10 | 15 | 20 | 25 |
| hardness | 9H | 5H–6H | 3H–4H | 2H | F | HB |
| Addition of modified nano silica (wt.%) | 0% | 1.5% | 1.7% | 1.9% | 2.1% | 2.3% |
| Contact angle | 103° | 144° | 151° | 150.5° | 151° | 150° |
| Roll angle | 74° | 20° | 9° | 5° | 4° | 2.5° |
| Number of friction | 0 | 20 | 40 | 60 | 80 | 100 |
| Mean contact angle | 151° | 151° | 150° | 151° | 151° | 149° |
| Transmittance | 85% | 85% | 84% | 83% | 82% | 82% |
| Serial Number | Data & Conclusions |
|---|---|
| 1 | When the composition mass ratio of SJ-32F resin to 9825 resin is 9:1, the coating has a hardness of 3H–4H suitable for mobile phone screens. |
| 2 | When the mass ratio of modified nano silica is 1.7%, the coating contact angle can reach more than 150°, the rolling angle is less than 10°, and the coating light transmittance remains high at 91–95%. |
| 3 | The cross-cut method is used to detect the adhesion of the coating; the prepared coating has a smooth scribe edge, no peeling off is at the edge and intersection, and the bonding force reaches 5B. The average bonding force tested by the adhesion pull tester is about 5.4 MPa, and the bonding force is good. |
| 4 | As the number of rubbing increases, the contact angle of the coating is basically unchanged, and the light transmittance of the coating is reduced; when the rubbing number reaches 100, the light transmittance remains above 80%. |
| 5 | Compared with the existing research results about transparent coatings of mobile phone screens, the transparency of the transparent superhydrophobic coating can be increased by 5–8% and the wear resistance can be increased by 50% under the condition of maintaining superhydrophobic properties. |
Funding
This study was funded by China National Natural Science Foundation, Grant No. 51775545.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study are available in article.
Acknowledgments
The experimental process was guided by experts from China University of Mining and Technology. We are very grateful for their help.
Conflicts of Interest
The authors declare no conflict of interest.
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Yanze Liu
School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 016040, China
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