Rapid construction of superhydrophobic fabric coating assisted with ‘Tyndall Effect’

Introduction

Superhydrophobic fabric coating has great potential in the application of civil, military, medical and other industrial fields [1,2] because of the excellent waterproofness, self-cleaning performance, antimicrobial properties, good oil and water separation capability and stain resistance [3-5]. Therefore, the preparation of superhydrophobic fabric coating has become the focus of research all over the world.

Inspired by ‘lotus effect’, the construction of concave–convex micro-nano structures is the key to the superhydrophobicity of coating. At present, various methods are available to prepare superhydrophobic coatings [6-12], such as self-assembly, template method, phase separation, nano-doping technique [13], electrochemical spinning, sol–gel method [14] and chemical etching. However, some of these methods are complicated in operation, some are restricted by external environment (temperature, humidity or air flow rate), some requires harsh experimental conditions, and some are even not suitable for fabric materials, which limit the development and application of superhydrophobic fabric coating [15-20]. Therefore, it is still of great significance and challenge to establish a simple, accurate and rapid method for the construction of superhydrophobic fabric coating.

It is known that polymer has different solubility in different solvent systems [21,22], which are characterized by showing a linear extension state in good solvents and curled state in poor solvents. Therefore, a homogeneous polymer solution system can produce nanoparticles by adding poor solvent to induce the crystallization of polymers in it. If there is another film-forming polymer in the system, a superhydrophobic fabric coating with micro-nano structure will be constructed by casting it on the surface of fabric. In this process, ‘Tyndall effect’ [23,24] can be used to construct the nanocrystals solution accurately. If a bright light channel appeared in the solution by being irradiated with a laser pointer, it indicates that nanocrystals have been formed, which will be used to fabricate the micro-nano structures of the coating. However, it should be noted that the nanocrystals must have a high glass temperature. Otherwise, it will be melted into a film at room temperature after being coated on the substrate surface. Considering that PMMA with high glass temperature tends to crystallize in the mixed solvent of DMF and ethanol, and silicone resin with low surface energy is soluble in the solvent, a nanocrystals solution can be constructed with the mixture of DMF and ethanol as solvent, PMMA as nanocrystals and silicone resin as film forming material, as follows (Figure 1). OPEN IN VIEWER

Compared with other methods, this way is more accurate and fast, less affected by external environmental factors, and the whole process does not involve sophisticated instruments and complex operations. More interestingly, so far there have been few researches for preparing superhydrophobic coatings assisted with ‘Tyndall effect’.

In this paper, a nanocrystals solution system was constructed assisted with ‘Tyndall effect’ by adding ethanol to a solution of silicone resin and PMMA dissolved in DMF. With silicone resin as film forming material and PMMA as nanoparticles, a superhydrophobic fabric coating was prepared by casting the solution on the surface of cotton cloth. The solution system and the coating were characterized by ‘Tyndall effect’, SEM, XPS, CA and self-cleaning test, respectively.

Materials and methods

Results and discussion

The result of construction of nanocrystals solution and superhydrophobic fabric coating is recorded in Figure 2. OPEN IN VIEWER

Figure 2(a) is the homogeneous solution configured first by dissolving PMMA and silicone resin in DMF. As the solute size in the solution is less than 1 nm, the scattered light is very weak when irradiated by laser pointer, thus no light channel appears in the system. With the dripping of ethanol, the solubility of PMMA in the system is getting worse and worse. When the amount of ethanol added to the system is 28.0 g, PMMA is precipitated from the solution as nanocrystals. As nanoparticles have good ability of light scattering, a bright light channel is generated when the system is irradiated with laser (Figure 2(b)). Figure 2(c) is the fabric coating obtained by casting the nanocrystals solution on the surface of cotton cloth. Clearly, the cuprous chloride solution droplets roll on the surface of the coating like pearls, which demonstrate that the coating has obvious hydrophobicity.

As shown in Figure 3(a–d) represents the surface microstructure of the coating at different magnification, respectively. OPEN IN VIEWER

Obviously, there are concave–convex micro-nano structures as expected. When the nanocrystals solution is poured into the fabric, the PMMA nanocrystals and silicone resin are transported to everywhere under the action of capillary force. With the volatilization of DMF and ethanol, the nanocrystals can be reassembled and glued together by silicone resin to form micro-nano structures with the overall size ranging from several microns to tens of microns. As shown in Figure 3(d), the height of the micro-nano structure in the square box is about 6 μm, and the diameters of the convex on the surface are hundreds of nanometres.

The surface chemical composition of the micro-nano structures is tested by XPS, and the result is shown in Figure 4. OPEN IN VIEWER

As can be seen from the figure, peaks at 532, 285, 102 eV in the curve are corresponding to O(1s), C(1s), Si(2p), respectively. These elements are derived from PMMA and silicone resin. It is shown that the micro-nano structures of the coating are composed of PMMA nanocrystals and silicone resin, which is in agreement with the theoretical analysis.

As shown in Figure 5, the water CAs at different positions of the coating are 152.1°, 154.2° and 152.9° respectively, and their average value reaches 153.1°, indicating that the fabric coating has superhydrophobicity. OPEN IN VIEWER

The concave–convex micro-nano structure on the surface has a strong ability to hold air, thus forming an air layer to lift the water droplets. So, the water cannot penetrate into the micro-nano structure and tends to condense into spherical droplets on the surface of the coating. Besides, the silicone resin with low surface energy has weak interaction with the water molecules, which also promotes the hydrophobicity of the coating.

To test the self-cleaning performance of the fabric coating, a certain amount of cuprous chloride powders is wiped on its surface, as shown in Figure 6. The coating is placed with an inclined angle, and water droplets can roll down to take away the powders. OPEN IN VIEWER

As can be seen from the figure, the powders are cleaned within a few seconds, indicating that the coating has excellent self-cleaning properties. That is because the powders have a small contact area with the micro-nano structures, the interactive force between them is weak. So the powders are affixed to the surface of water droplets and then slip off when the droplets roll on the coating, thus achieving the self-cleaning effect.

Conclusions

A solution system contained PMMA nanocrystals was constructed assisted with ‘Tyndall effect’ by adding ethanol to the solution of silicone resin and PMMA dissolved in DMF.