Reactive silica nanoparticles turn epoxy coating from hydrophilic to super-robust superhydrophobic

Superhydrophobic organic–inorganic hybrid nanocomposite coatings have received much attention because they possess the advantages of both inorganic and organic materials. Nevertheless, it is difficult to achieve strong bonding of inorganic nanoparticles to the polymer matrix while maintaining a sufficiently rough structure to impart superhydrophobicity. In this study, we fabricated silica nanoparticles with surface reactive groups that can further react with the epoxy resin. Thus, the hydrophobic silica nanoparticles were stably anchored and stabilized in the cured resin matrix while forming nanometer-scale roughness structures. The obtained silica-decorated epoxy resin coating shows great durability and water-repellency after mechanical abrasions and has superior adhesion to the substrate.


Introduction
Coating is an essential step in adjusting the surface properties of materials. 1 Superhydrophobic coatings have received much attention because of their applications in elds such as selfcleaning, 2-4 non-wetting liquids, 5 anti-icing 6-8 and oil-water separation. 7,8 A superhydrophobic coating surface is usually constituted by a combination of low surface energy substances and micro/nanometer scale roughness structures. [9][10][11] The latter, however, always have poor mechanical strength and weak adhesion to the substrates, seriously hindering the large-scale preparation and industrial application of superhydrophobic surfaces. [12][13][14] Introducing nanomaterials into polymers to increase the surface nano-scale roughness has been demonstrated as an excellent strategy for producing low-cost, durable, superhydrophobic nanocomposite coatings. [15][16][17][18] However, the inorganic nanoparticles generally have poor compatibility with organic polymers or solutions, which leads to phase separation during fabrication and tends to diminish the quality of the coating through cracks or weak adhesion to the substrates. 19,20 To overcome this problem, Barron et al. prepared superhydrophobic surfaces by combining the surface roughness of nanoparticle-derived lms with the low surface energy properties of highly branched alkyl chains. 21 Bayer et al. fabricated wear-and abrasion-resistant high-impact polystyrene/silica nanocomposite coatings for metal surfaces by a spray method. 22,23 In our previous work, we have also fabricated superhydrophobic epoxy resin coatings by adding hydrophobic SiO 2 nanoparticles modied by hexamethyldisilazane. 24 Though surface modication can improve the compatibility between the inorganic nanoparticles and organic material to a certain extent, nanoparticles typically still need strong bonds with polymers to construct a structure with durable roughness. If the nanoparticles are just slightly adhered to or sputtered on the layer surface, roughness can be obtained but it is easily damaged or removed under mechanical abrasion. 25 At present, an important approach is to introduce functional groups on the nanoparticle surfaces and endow the nanoparticles with reactive properties, which enable the nanoparticles to take part in chemical reactions and anchor on to the polymer matrix. 26 In past researches, epoxy resin has been widely used to strengthen the adhesion of nanoparticles, especially silica nanoparticles, which are easy to prepare, have syntheticallycontrolled diameters, and possess a high concentration of Si-OH groups suitable for modication. These have been widely introduced into the resin matrix as hydrophobic llers to fabricate superhydrophobic surfaces. [27][28][29][30] Ramakrishna et al. reported the preparation of a superhydrophobic coating using oligomer-wrapped silica nanoparticles obtained through reactions between silanol and isocyanate groups. 31 In this study, we rst fabricated surface functional silica nanoparticles and used them as a SiO 2 /epoxy resin solution that was sprayed on hard or so substrates to fabricate superhydrophobic surfaces. By introducing surface functional silica nanoparticles, the inherently hydrophilic epoxy resin coating can be turned into a superhydrophobic one with mechanical abrasion resistance. There are two advantages in this method: (1) by virtue of the interaction between the epoxy groups on the functionalized silica nanoparticle surface and the primary amine group of the curing agent, silica nanoparticles were stably anchored onto the cured epoxy resin matrix while constructing micrometer-scale roughness structures on the coating surface; (2) epoxy resin is an adhesive itself and using epoxy resin as a coating material can resolve the problem of poor adhesion between the coating and substrates.

Preparation of superhydrophobic silica powder
The surface functionalized silica nanoparticles were prepared by an in situ surface modication method and Fig. 1 schematically shows the preparation processes. Briey, sodium metasilicate and hydrochloric acid were dissolved in deionized water, separately, and the g-(2,3-epoxypropoxy)propyltrimethoxysilane (KH560) was dissolved in absolute alcohol as a modier. Then the solution of hydrochloric acid and KH560 were added into the sodium metasilicate solution dropwise. Gradually, an amount of foam appeared on the surface of this aqueous solution and the suspension was heated to 60 C and was stirred for 4 h at this temperature. The suspension separated into two layers quickly, and a layer of white oc oated on the top. The oc was collected by ltration and washed clearly. Finally, the ltered cake was re-dispersed into a quantity of mixed solution to form an emulsion, and the emulsion was spray-dried at 140 C to get the white SiO 2 nanoparticles. A typical formula was used as follows: 0.3 mol L À1 sodium metasilicate, 0.72 mol L À1 hydrochloric acid, and 0.05 mol L À1 modier. The detailed information about surface functionalized SiO 2 nanoparticles are shown in S1 in the ESI. †

Preparation of silica/epoxy resin superhydrophobic coatings
Briey, 10 g of epoxy resin, 3 g of the triethylenetetramine as curing agent and 4 g of hydrophobic silica nanoparticles were dispersed in 150 g of xylene to make a uniform suspension. Then, the obtained suspension was spin-coated (2500 rpm, 20 s) on hard or so substrates and cured at 140 C for 1.5 h to get the silica/epoxy resin superhydrophobic coatings.

Characterization
The surface morphology of the samples was examined with a eld emission scanning electron microscope (FESEM, JSM-6701F). The surface components of the samples were analyzed with an X-ray photoelectron spectrometer (XPS, AXIS ULTRA). Thermogravimetric (TG) measurements were performed with a NETZSCH STA 449 C at a dynamic heating rate of 10 C min À1 . The contact angles (CAs) and sliding angles (SA) of water were measured using a DSA-100 optical contact angle meter (Kruss Company, Ltd, Germany) at room temperature (22 C). The average CA value was determined by measuring the same sample at ve different positions. The volume of all liquids was 5-8 mL when the contact angles and sliding angles were measured. The image of the droplet was obtained with a digital camera (NIKON, P600). The transparency of the coatings was tested using a UV-vis spectrometer (UV-1800PCS, Mapada Instruments). The transmittance of each sample was recorded at 340-800 nm and collected at an ambient temperature.

Abrasion test
Abrasion tests were carried out on the SiO 2 /epoxy resin coating as well as the purchased commercial Never Wet coating. In this test, the coating was placed face-down on the sandpaper (standard glass paper, Grit No. 800) and moved 23 cm, then rotated by 90 (face to the sandpaper) and moved 20 cm. This process is dened as one abrasion cycle, which guarantees the surface abrasion longitudinally and transversely in each cycle, even if it is moved in a single direction.

Superhydrophobic silica-decorated nanocomposite coating
The reactive surface-functionalized SiO 2 nanoparticles can take part in the curing process along with the epoxy resin and are anchored to the coating. The reaction process is shown in Fig. 2. Fig. 3 shows the infrared spectrum of the obtained surfacefunctionalized SiO 2 nanoparticles and the SiO 2 /epoxy resin composite coating. As can be seen from Fig. 3(a), the peaks at 1087 cm À1 , 806 cm À1 and 472 cm À1 are characteristic peaks for SiO 2 . The weak peak at 1261 cm À1 is attributed to the epoxy group's stretching vibration, which indicates that the epoxy groups have successfully been modied on the SiO 2 nanoparticle surface. [32][33][34] Fig. 3(b) shows the obtained SiO 2 / epoxy resin composite coating aer completing the curing process, in which an epoxide-ring opening occurs between the epoxy ring of the epoxy resin and the primary amine group of the triethylenetetramine. This can be conrmed from the diminished intensity of the characteristic bands of the epoxide ring (i.e., 915 cm À1 and 831 cm À1 ) and primary amine (i.e., 3206/ 3316 cm À1 ), as well as the increased intensity of the -OH band (i.e., 3380 cm À1 ) and secondary amine groups (i.e., 1573 cm À1 ). [35][36][37] Fig . 4(a) shows the optical images of glass before and aer being coated with a SiO 2 /epoxy resin coating with a thickness of about 7 mm, and all the letters show good readability under the coated glass. The translucent coatings demonstrate excellent superhydrophobic properties and water droplets dropped on the coating maintain nearly spherical shapes with a contact angle of 156 and sliding angle of about 3 . The transmittance spectrum of the coated glass in the visible region (300-800 nm) is a rational characterization of transparency, as shown in Fig. 4(b).
In order to investigate the thermal stability of the coating, thermogravimetry (TG) was performed, as shown in Fig. 4(c). It is clear that almost no weight loss occurs before 200 C, and between 200 C and 500 C, the SiO 2 /epoxy resin begins to decompose and the weight loss reaches 67%. As far as the surface functionalized SiO 2 nanoparticles are concerned, there is a weight loss of about 10% between 200 C and 380 C, which is due to thermal decomposition of the surface functional groups. To further investigate the thermal coating stability, we also analyzed how the contact angle and sliding angle changed with the curing temperature. It was found that when the curing temperature was higher than 320 C, the superhydrophobicity was destroyed and the sliding contact angle increases sharply (Fig. S2 †), which further indicates that the surface functional reagents that are necessary for superhydrophobicity undergo thermal decomposition. From the SEM observation, it can be seen that SiO 2 nanoparticles have been constructed with micrometer-scale roughness structures with lots of cavities on the surface. In addition, the coating can be easily fabricated on a large scale by a spray method as shown in Fig. 4(f), which is very important for SiO 2 /epoxy resin coatings in a wide range of applications in the real world environment. Note that the coating (Fig. 4(f)) becomes non-transparent because the transmittance of the coating decreases with the increase in the coating thickness. Detailed information is shown in Fig. S3, † where it was found that when the coating thickness is 30.37 mm, the optical transmittance is only 20%, indicating that the coated glass can turn almost opaque.
Herein, the anchoring of SiO 2 nanoparticles plays an important role in adjusting the coating wetting behavior. Through surface modication, hydrophilic SiO 2 converts to hydrophobic SiO 2 , and the aggregations of SiO 2 nanoparticles affect the coating surface roughness as well as reduce the  Paper surface energy that are responsible for inducing the superhydrophobic behavior. To investigate the inuence of SiO 2 nanoparticle content on the wettability of the coating, different SiO 2 nanoparticles were added, as shown in Fig. S4. † The coating wettability gradually switches from hydrophilic to hydrophobic with an increase in the SiO 2 content. From the SEM images, it is clear that the coating has a rough surface and the surface roughness increases with an increase in the content of SiO 2 nanoparticles (Fig. S5 †), which is very important for producing superhydrophobicity.

Mechanical durability
To give an overview of the mechanical and chemical durability of the SiO 2 /epoxy coating, and to compare it with commercial superhydrophobic surfaces, we used radar diagrams to evaluate the experimental data as shown in Fig. 5. In the radar diagrams, we included water contact angles and sliding angles before and aer sandpaper abrasion (SiC, 8000 Cw, 2 kPa pressure, 4000 cm abrasion, and the thickness of both the obtained coating and commercial coating was 75-80 mm), retention ratio aer 4000 cm abrasion, and contact angle aer the "droplet tests" for 60 min; Table 1 shows the rating system of the radar diagram according to the performance of the samples, and their data sheets are shown in Table 2. The larger overall area in the plot indicates that the material can perform better. Compared with the commercial superhydrophobic coating (Never Wet), the obtained SiO 2 /epoxy resin coating shows superior chemical and mechanical abrasion resistance.
The chemical durability of the SiO 2 /epoxy resin coatings was studied by two independent experiments and the detailed   5 Radar diagrams of SiO 2 /epoxy coatings (a), and the "Never Wet" coatings (b) a commercial superhydrophobic paint. Here, "WCA initial" and "WSA initial" refer to the water contact angles and water sliding angles of the samples without any mechanical and chemical tests. "WCA after abrasion" and "WSA after abrasion" refer to the water contact angles and water sliding angles that were measured after the samples being abraded for 4000 cm (2 kPa pressure, SiC, 800 Cw sandpaper). "pH ¼ 1" and "pH ¼ 14" refer to the contact angles that were measured after 60 min "droplet test". information is shown in Fig. 6. In the "droplet test", strong acid (pH ¼ 1) and alkali (pH ¼ 14) droplets were placed on the SiO 2 / epoxy coating for 60 min as shown in Fig. 6(a). The water droplets became smaller as time passed because of evaporation. As the acid/alkali contact time increased, the CAs of acid/alkali droplets slightly decreased but still remained about 150 . In a more aggressive test, the samples were immersed into acid (pH ¼ 1) and alkali (pH ¼ 14) baths for 60 min, and CAs were measured for every 10 min of soak time as shown in Fig. 6(b). Although the WCAs decreased from 160 to about 150 , the SiO 2 /epoxy coatings retained their superhydrophobicity.
To further quantify the abrasion-resistance of these materials, water contact angle, sliding angle and retention ratio were plotted as a function of sandpaper abrasion distance on the SiO 2 /epoxy resin coating and Never Wet, respectively, as shown in Fig. 7. It is clear that aer a 4000 cm abrasion distance, the retention ratio was about 78% for the obtained SiO 2 /epoxy resin coatings, whilst, the retention ratio was about 58% for the Never Wet coating, which indicates that the SiO 2 /epoxy resin coating has better abrasion resistance than the commercial coating (Never Wet). Their surface morphologies did not signicantly change before and aer abrasion for 4000 cm of distance as shown in Fig. 8, and the surface always maintained its rough structures, which is necessary for superhydrophobicity. The elemental mapping of C, N, O and Si before and aer the abrasion test are shown in Fig. S6 in the ESI. † It is clear that all the elements are distributed on the coating surface uniformly aer abrasion, suggesting the excellent abrasion resistance.

Oil and water separation
The coating can be fabricated on a porous window screening mesh or cotton via a dip-coating method and the superhydrophobicity as well as superoleophilicity of the coating make      it a good candidate for oil/water separation. As shown in Fig. 9(a-c) and Video S1 in the ESI, † if stirred with an oil/water mixture solution at a high speed for a few seconds, the cotton could totally separate the water and oil. The obtained coating can also be used to separate heavy oil, and the oil could be absorbed immediately into the cotton on contact ( Fig. 9(d-f) and Video S2 in the ESI †). In addition, the robust coating can be fabricated on a window screening mesh and be used to separate oil/water mixture solutions, as shown in Fig. 9(g and h) and Video S3 in the ESI. † Note that the coating demonstrates stable superhydrophobicity no matter how the window screening mesh was abraded, as shown in Fig. S7. † Fig. 9(i) shows the different separating efficiencies of the superhydrophobic window screening mesh to various solvents and oils. For the oils and solvents with different densities and viscosities, such as paraffin oil or n-hexane, the separating efficiency was above 95%. It is clear that the separating efficiency increases with lower viscosities, as shown in Fig. 9(j). It is believed that the oils or organic solvents with high viscosity (e.g., paraffin oil) tended to block the pores of the coating, resulting in a lower separation efficiency. Aer being rinsed thoroughly with alcohol and dried, the as-prepared window screening mesh could be reused for oil/ water separation aer many cycles and maintained its separating efficiency above 90% ( Fig. 9(k)).

Conclusion
In conclusion, we have demonstrated a facile and low-cost method to fabricate superhydrophobic coatings by anchoring functionalized SiO 2 nanoparticles to epoxy resin coatings based on co-curing and thus avoiding phase separation between inorganic nanoparticles and the organic epoxy resin. The uorine-free coatings have strong adhesion to the substrates and also demonstrate excellent mechanical abrasion resistance and anti-corrosion properties. The coatings can also be fabricated on porous substrates and be used to separate oil and water mixture solutions efficiently. It is expected that the superrobust and strong coatings with durable superhydrophobicity will have a wide range of applications for outdoor use.

Conflicts of interest
There are no conicts of interest to declare.