Facile preparation of self-healing waterborne superhydrophobic coatings based on fluoroalkyl silane-loaded microcapsules

Abstract

Superhydrophobic surfaces have attracted tremendous attraction because of their novel aspects of surface physics and important applications ranging from self-cleaning materials to microfluidic devices. However, most artificial superhydrophobic surfaces easily lose their superhydrophobicity under natural sunlight irradiation, physical rubbing or organic contamination. Here, fluoroalkyl silane (FAS)-loaded microcapsules, photocatalytic TiO₂ nanoparticles and FAS modified SiO₂ nanoparticles were mixed with waterborne polysiloxane resins to obtain waterborne self-healing superhydrophobic coatings. Superhydrophobic surfaces were formed by casting the coatings on the substrates after UV-irradiation and could sustain their superhydrophobicity even after 360 h accelerated weathering test. Meanwhile, the coating was durable enough to withstand water blasting and the attacks of strong acid or basic solutions without apparently changing its superhydrophobicity. More importantly, after being mechanically damaged or contaminated with organics, these coatings could restore their superhydrophobicity under UV light. All these characteristics ensure that the coatings have excellent long-term superhydrophobicity for outdoor service.

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1. Introduction

Superhydrophobic surfaces have been extensively studied during the past decade and drawn a great deal of research interest due to their potential industrial and biomedical applications,^1–5 including self-cleaning, non-wetting fabrics, anti-corrosion and anti-fouling.^6–10 Unfortunately, most of these artificial special surfaces easily lose their non-wettability in service due to mechanical abrasion, sunlight irradiation and organic contamination, which seriously hampers their practical applications.^11–14

Two strategies are widely used to create durable superhydrophobic surfaces. One involves mechanically-durable rough surface. For example, Jung et al. fabricated mechanically durable structured superhydrophobic surfaces by deposition of carbon nanotubes (CNTs) over a molded microstructure epoxy resin.¹⁵ These surfaces showed high mechanical strength and wear resistance because of their hierarchical structure and strong bonding between CNTs with epoxy resin. Xue et al. prepared superhydrophobic surfaces through chemical etching of fiber surfaces and subsequent diffusion of fluoroalkylsilane into the roughened fibers at high temperature.¹⁶ The superhydrophobicity survived after severe abrasion and long-time laundering because the rough surface created directly from bulk materials was inherently superior in mechanical durability. Besides mechanical durability, endowing the superhydrophobic surfaces with photocatalytic self-cleaning performance is another way to extend their durability, especially for the application in heavy-polluted environment. Nakajima et al. ever fabricated a superhydrophobic inorganic coating containing 2% photocatalytic TiO₂ nanoparticles via a sol–gel process.¹⁷ This coating could sustain high contact angle of water after 1800 h outdoor exposure. Zhou et al. also reported a long-term superhydrophobic self-cleaning coating based on an ambient-curable fluorinated polysiloxane binder and TiO₂ nanoparticles.^18,19 The TiO₂ nanoparticles acted as both the building blocks for the construction of the micro/nano-structured surface and the photocatalyst for the decomposition of organic contaminants.

Nevertheless, fabrication of the superhydrophobic surfaces with self-healing ability would be the best strategy to realize their long-term durability. Li et al. fabricated self-healing superhydrophobic coatings containing fluoroalkyl silane (FAS) in their porous structure.¹¹ Once the top layer was decomposed, the preserved FAS migrated to the surface to heal the damaged surfaces, and the repair process was humidity-dependent, with a more accelerated self-healing process under a more humid environment and vice versa. Wang et al. prepared a superhydrophobic and superoleophobic surface via deposition of a mixture of fluorinated-decyl polyhedral oligomeric silsesquioxane (FD-POSS) and hydrolyzed FAS on fabrics.²⁰ Due to the movement of the FD-POSS molecules to the surface under heating, the fabrics could maintain the superhydrophobicity even after 100 cycles of the plasma-and-heat treatment, suggesting the self-healing ability of the surface. Recently, we have successfully prepared the first all-water-based self-healing superhydrophobic coatings based on UV-responsive capsules.²¹ Due to the release of hydrophobic FAS molecules from capsules under UV irradiation, the surface could recover its superhydrophobic and self-cleaning ability under UV light even after it was mechanically damaged or contaminated with organics. Indeed, light-inducing self-healing strategy seems to mimic natures more efficiently and hence is recognized to be more feasible for artificial superhydrophobic surfaces in outdoor service. Additionally, many artificial superhydrophobic coatings contain pungent or volatile solvents, such as ethanol, toluene and acetone, for the purpose of dissolution of hydrophobic substances. This will inevitably cause environmental and safe issues during the process of large-scale preparation and application of these coatings.^22–26

In this paper, we report a novel and feasible method to fabricate self-healing waterborne superhydrophobic coatings, which contains polysiloxane latex, FAS-loaded microcapsules, fluorinated SiO₂ nanoparticles (FMS), and photocatalytic titania nanoparticles (P25). These coatings can be large-scale spayed on the substrates to form superhydrophobic surfaces after UV-irradiation. Compared with other self-healing superhydrophobic coatings. These new self-healing coatings have following advantages: (i) the preparation process of self-healing superhydrophobic coatings is facile, and the triggering mechanism for self-healing is based on the UV light or sunlight, which is very promising for the large-scale practical applications; (ii) due to a large amount of healing agents (FAS) in the microcapsules, this coating can quickly recover its superhydrophobicity by UV light after mechanical damages or oily contaminations and can sustain the superhydrophobicity for a long time in outdoor; (iii) this coating is waterborne and environmentally-friendly, and it can be easily produced and applied on a large-scale.

2. Material and methods

2.1. Materials

Polystyrene (PS, M_w = 2 × 10⁵ g mol⁻¹) and sodium dodecylbenzene sulfonate (SDBS, ≥95%) were purchased from Aladdin Chemical Reagent Co. (China). Dichloromethane (≥99.5%), ammonia solution (NH₃·H₂O, 25 wt%), acetic acid (≥99.7%) and ethanol (≥99.7%) were purchased from Sinopharm Chemical Reagent Co. (China). All these reagents were used as received. Perfluorooctyltriethoxysilane (FAS13) was purchased from Xeogia Fluorine-Silicon Chemical Co., Ltd. (China). Polysiloxane latex (BS45, solid content: 50 wt%) was purchased from Wacker Chemicals (Germany). Titania nanoparticles (P25, primary size: 27 nm) was purchased from Degussa (Germany). Fumed silica (hydrophilic, primary size: 20 nm) was purchased from Shanghai Cabot Chemical Co., Ltd. Deionized water was used throughout the whole experiments.

2.2. Syntheses of microcapsules

6 g PS and 18 g dichloromethane were mixed with 4.5 g FAS13 in a 200 mL glass beaker to form a homogeneous solution. Then 100 mL deionized water and 0.9 g SDBS were added to the solution and ultrasonically emulsified for 10 min with ultrasonic cell disruptor (JY 98-111DN, Ningbo Scientz Biotechnology Co., Ltd, China) in the presence of an ice-bath. The miniemulsion was kept stirring at room temperature overnight to evaporate dichloromethane. The as-obtained microcapsules were separated by centrifugation at 10 000 rpm for 10 min and then re-dispersed in the water with a solid content of 30 wt%.

2.3. Modification of SiO₂ nanoparticles

Briefly, 10 g ethanol and 0.5 g FAS13 were added to 250 mL glass beaker and stirred uniformly. Afterwards, 150 g water, 5 g fumed silica and 0.3 g ammonia was added and stirred at 600 rpm for 30 min. Subsequently, the mixed solution was kept stirring at 65 °C for 20 h to obtain FMS. The FAS13-modified silica nanoparticles were then collected by centrifugation (8000 rpm, 10 min), and then re-dispersion in distilled water for further use.

2.4. Fabrication of waterborne superhydrophobic coatings

The microcapsule aqueous dispersion (solid content: 30 wt%), FAS13-modified SiO₂ aqueous solution (solid content: 18 wt%), polysiloxane latex and P25 were mixed in a plastic beaker and stirred at 500 rpm for 10 min. Then the coating was cast on an aluminum plate with a drawdown rod (120 μm) and dried at 80 °C for 10 min. After that, this coating was placed in the QUV accelerated weathering tester for 36 h and the surface of coating became superhydrophobic. For comparison, three control coatings were prepared following the same procedure. Table 1 summarizes the formulations for the preparation of the coatings.

Table 1 The formulations for the preparation of the coatings

Components	The mass percent of different coatings (wt%)
	BS45/P25	BS45/P25/FMS	BS45/P25/microcapsules	BS45/P25/FMS/microcapsules
Microcapsules	0	0	43	25
FMS	0	43	0	18
P25	3	3	3	3
BS45	97	54	54	54

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2.5. Characterization

The morphologies of the microcapsules were characterized by a scanning electron microscope (SEM, Sigma HD field, Zeiss, Germany) at an accelerating voltage of 5 kV and a transmission electron microscope (TEM, JEM-2100, JEOL Ltd., Japan) at an accelerating voltage of 20 kV. The particle size distribution of microcapsules was measured by dynamic light scattering (DLS) method using Nano-ZS 90 (Malvern, UK). The water contact angle (WCA) was measured by optical contact angle measuring instrument (Dataphysics, Germany), using a 5 μm deionized water droplet. The final numerical value of the measured coatings was the average value for four parallel measurements on different sites of the same coating. The accelerated weathering tests were tested in a QUV accelerated weathering tester (QUV/se, Q-Panel Co., Ltd., USA), using UV lamps with a wavelength of 310 nm. The accelerated weathering cycle was set as follows: UV-irradiation at 60 °C for 4 h and irradiation intensity of 0.71 W m⁻², and condensation at 50 °C for 4 h.

The durability of the coatings in water was examined by immersing the coatings in water for a week. The WCAs were measured after 24 h interval.

The fluid impact test as following: the sample was fixed on a substrate tilted at 45° and placed 5 cm below a water pipe, and then jetted for 3 min at 25 kPa pressure.

The acid–alkali resistance of the superhydrophobic surfaces was evaluated by dripping the different pH value solution on the surface of the coatings. The different pH values of solution were adjusted by mixing acetic acid solution and ammonia solution, and the hydrophilic dyes were used to distinguish the different pH value droplets.

The abrasion resistance of the superhydrophobic surfaces was evaluated by dragging a piece of 1500-mesh sandpaper under 1 kg weight in one direction with a speed of 1 cm s⁻¹. The contact area between the sandpaper and the underlying superhydrophobic coating was 2.25 × 2.25 cm².

The oily resistance of the superhydrophobic surfaces was evaluated by using oleic acid as the model pollutant. A thin layer of oleic acid was casting on the coatings and then was put into the QUV accelerated weathering tester. The WCAs were measured after 12 h interval.

3. Results and discussion

3.1. Preparation of waterborne superhydrophobic coatings

Fig. 1a clearly shows the morphology of the FAS-loaded microcapsules. As shown in the pictures, the microcapsules were quite round and uniform, and the size of microcapsules was about 300 nm. Moreover, as shown in the Fig. 1b, these microcapsules synthesized by solvent evaporation were the core–shell structure, illustrating the microcapsules was formed by PS as shell material and FAS as core material. DLS measurement showed that the mean diameter of nanoparticles was 300 nm (Fig. 1c), consistent with SEM and TEM images.

Fig. 1 (a and b) SEM and TEM micrographs of microcapsules. (c) Particle size distribution of microcapsules measured by DLS.

The FAS-loaded microcapsules together with P25 and FMS can be well dispersed into the waterborne polysiloxane resins and their corresponding coating (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt) surface shows hydrophilicity (WCA: 89°). Further increasing microcapsules or FMS amount did not cause any obvious increase in WCA. An appropriate amount of P25 (i.e., 3 wt%) is very critical. At this content, the coatings can not only quickly achieve the superhydrophobicity but also keep the superhydrophobicity for a long time under UV-irradiation. Interestingly, the hydrophilic surface could turn to be superhydrophobic (WCA: 157°) in the QUV accelerated weathering tester for 36 h, as shown in Fig. 2a. Due to the degradation of hydrophilic components of coatings (composition: P25/BS45 = 3/97 wt/wt) by the photocatalysis of P25, the WCA of this coating was increased to 95°, but it could not reach the superhydrophobicity. Although nanocomposite coating (composition: P25/FMS/BS45 = 43/3/54 wt/wt) with P25 and modified silica showed high hydrophobicity, WCA of the surface was decreased after UV irradiation, which could be the result of the degradation of surface hydrophobic components. The WCA of nanocomposite coating (composition: microcapsules/P25/BS45 = 43/3/54 wt/wt) with P25 and FAS-loaded microcapsules can increase to 132° after accelerated weathering for 36 h. However, the surface cannot reach the superhydrophobicity because of their lower initial contact angle (Fig. 2b). This illustrates that the forming of the self-healing superhydrophobic surface is originated from the proper proportion of FAS-loaded microcapsules, P25 and FMS in the waterborne polysiloxane latex. Moreover, these P25 can photo-catalytically degrade PS to release the self-healing agents FAS, which is essential for the durability of the superhydrophobicity of coatings. In addition, the surface morphologies of the coating before and after UV irradiation were further examined by SEM. As shown in Fig. 2c, the surface of the coating had a dual-scale roughness with large number of microscale aggregates and nanoscale protuberances, which was mainly composed of the microcapsules and FMS. While the morphology of the coating surface did not changed obviously after UV irradiation, further suggesting that the superhydrophobicity of the surface was resulted from the releasing of FAS13 in the microcapsules.

Fig. 2 (a) Photographs of water droplets and contact angles on the superhydrophobic surfaces before and after UV irradiation in accelerated weathering tester for 36 h. (b) Changes of WCA on various coatings before and after UV irradiation in the accelerated weathering tester for 36 h. (c and d) SEM micrographs of superhydrophobic surface before and after 36 h accelerated weathering test (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt).

3.2. Durability performance of waterborne superhydrophobic coatings

The hydrophobic substances of superhydrophobic surfaces could be easily degraded by sunlight in the outdoors and lost their superhydrophobicity.^11,13 As a consequence, the superhydrophobic coatings (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt) were carried out for the accelerated weathering tests. As shown in Fig. 3a, the WCA of the surface gradually rose up from hydrophilicity to superhydrophobicity (89.1° → 160.3°) after the accelerated weathering test, indicating that the hydrophobic molecules FAS13 released from the microcapsules, which was attributed to the photocatalytic degradation of P25 under UV irradiation and then reduced the surface energy of surface. Although the WCA of this coating decreased after 144 h accelerated weathering test due to the loss of FAS13, the coating could sustain the superhydrophobicity after 360 h acceleration test, suggesting that this coating had good durability under UV irradiation and could serve in the outdoors for a long time.

Fig. 3 (a) The WCA of the coatings as a function of accelerated weathering time. (b) The WCA of superhydrophobic coating after immersed in water for a week and after UV irradiation. (c) Photograph of the water flow impacting the superhydrophobic surface. (d) Photograph of water droplets with different pH on the superhydrophobic surfaces (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt).

However, waterborne coatings often have issues of stability in water. We immersed a waterborne superhydrophobic coating in the water for a week and found that this coating lost its superhydrophobicity (155.2° → 130.3°), which may be resulted from the enrichment of hydrophilic molecules and the loss of hydrophobic FAS13 molecules on the surface. Fortunately, the surface topography did not change, and when re-exposed to UV light, the surface of this coating restored the superhydrophobicity (153.1°), illustrating the release of the hydrophobic FAS13 molecules from the microcapsules again (Fig. 3b). Furthermore, the coating could maintain water resistant ability even under fluid impact (Fig. 3c). This coating could also withstand the attacks of strong acid solution (pH = 2) and strong basic solution (pH = 12), as shown in Fig. 3d, these results showed that these waterborne superhydrophobic coatings may have anticorrosion performance.

3.3. Self-regeneration of waterborne superhydrophobic coatings upon mechanical damage

The artificial superhydrophobic coatings are inevitably impacted by the physical abrasion when they serve in the outdoors. These abrasions will not only destroy the micro- and nanostructures but also remove the hydrophobic substances of the surfaces, which results in permanent loss of superhydrophobicity.^12,14 To evaluate the influence of mechanical abrasion and the self-healing ability of the waterborne superhydrophobic coating (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt), we used a piece of sandpaper to polish the surface of coating under a load of 20 kPa (Fig. 4a). As shown in Fig. 4c, it is found that the WCA of the damaged surface declined from 160° to 141°. Nevertheless, after 40 h acceleration test, the WCA of damaged surface gradually increased and finally restored its superhydrophobicity (158°), demonstrating that the damaged surface was covered again with the hydrophobic FAS13 molecules. Moreover, the damaged coating can recover its superhydrophobicity even after six abrading-accelerate weathering cycles (Fig. 4d), and the repaired surface showed excellent self-cleaning ability (Fig. 4e), meaning that the coating can use for a long time in the outdoors due to its wear self-healing ability.

Fig. 4 (a) Schematic illustration of abrading experiment, (b) SEM image of sandpaper surface. (c) Changes of WCA for the superhydrophobic coating with accelerated weathering time. (d) Changes of WCA for the superhydrophobic coatings as a function of repeated abrading and accelerated weathering cycles. (e) Self-cleaning ability of the superhydrophobic surface after abrading (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt).

3.4. Self-repairability of waterborne superhydrophobic coatings after oil contamination

The inorganic dusts on the superhydrophobic surfaces are easily eliminated by the self-cleaning effect. However, oily dusts can directly adhere to the surfaces resulting in hard to clean with water, which ultimately influences their superhydrophobicity.^12,17 Herein, oleic acid was used as model oily dust to evaluate oil-resistant ability of the waterborne superhydrophobic coating (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt). As shown in Fig. 5a, the contaminated surface became hydrophilicity (65.5°) when the oleic acid was casted on the surface of the superhydrophobic coating. Nevertheless, the contaminated surface recovered its superhydrophobicity after 80 h accelerated weathering test. Furthermore, after five casting-accelerate weathering cycles, the surface of coating still maintained its superhydrophobic state, which could be vividly watched with our naked eyes (Fig. 5b and c). All results indicate that the TiO₂ nanoparticles in the coating can not only photocatalytically decompose the oleic acid but also degrade the microcapsules to release FAS13 molecules for repairing the contaminated surface, and thus endow the coating with the resistance ability to oily contaminants.

Fig. 5 (a) Changes of WCA for the superhydrophobic coating with accelerated weathering time. (b) Changes of WCA for the super-hydrophobic surfaces as a function of repeated casting and accelerated weathering cycles. (c) Photographs of water droplets on the contaminated surfaces before and after UV light irradiation (composition: microcapsules/P25/FMS/BS45 = 25/3/18/54 wt/wt).

3.5. Self-healing mechanism

In natural superhydrophobic plants, the problems caused by cumulative damage and fouling are minimized by their ability to excrete hydrophobic substances to surfaces or grow new structures.^11,12,14 Similar to this self-healing process, the self-healing mechanism of the waterborne superhydrophobic coating is schematically depicted in Fig. 6. In the waterborne superhydrophobic coatings, the fluoroalkyl silane-loaded microcapsules, worked as the reservoir of healing agents. When the damaged surface was irradiated by sunlight or UV, the polymer shell of microcapsules in the top layer of coatings was decomposed first by the photocatalysis of TiO₂ nanoparticles, and then healing agents FAS13 molecules released from the microcapsules to migrate onto the surface to heal the damaged areas driven by the minimization of surface free energy. In addition, the surface microstructure after mechanical abrasion had no obvious difference because of high pigment/binder ratio of the coatings. Both the micro- and nanoscale roughness features were similar to the interior. Moreover, FAS13 as a coupling agent can react with hydroxyl groups of FMS and P25 nanoparticles so that the surface has enough hydrophobe to keep the superhydrophobicity.^27,28 Therefore, these healing agents can assure the repaired surface with long-term superhydrophobicity.

Fig. 6 The working principle of self-healing waterborne superhydrophobic coatings.

4. Conclusions

The self-healing waterborne superhydrophobic coatings prepared by simply mixture of polysiloxane latex, photocatalytic TiO₂ nanoparticles, hydrophobic SiO₂ nanoparticles and FAS-loaded microcapsules. The as-obtained coatings could be readily sprayed on substrates to form superhydrophobic surfaces after UV-irradiation due to the release of FAS13 of ruptured microcapsules. The formed superhydrophobic surfaces exhibited good durability under UV irradiation and resistance to strong acid or basic solution. Above all, the superhydrophobicity of the damaged surface can be regenerated after mechanical abrasion or contamination with oily dust under UV light irradiation. Furthermore, these coatings were waterborne and environmentally-friendly. Therefore, this waterborne superhydrophobic coating reported here could be easily scaled-up for practical application, especially on big outdoor objects.

Acknowledgements

Financial supports from the Foundation of Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, Jiangnan University (JDSJ2015-03), the Fundamental Research Funds for the Central Universities (JUSRP116017) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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