Large-scale fabrication of translucent, stretchable and durable superhydrophobic composite films

Shanlin Wang , Xinquan Yu and Youfa Zhang *
Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, P. R. China. E-mail:

Received 17th September 2017 , Accepted 16th October 2017

First published on 16th October 2017

Nano-scale textures with special wettability impart a broad spectrum of unique properties to the superhydrophobic surfaces that can be applied in various fields. However, there are still many difficulties in large-scale practical applications due to the limitation of the service life of superhydrophobic materials. In this study, we developed a type of novel transplantable superhydrophobic film by two-step spray methods and subsequent demoulding. The comprehensive performances of the film were investigated in translucence, thermal tolerance, stretchability, impact resistance, wear resistance, anti-corrosion, and self-cleaning. The results show that the surface can maintain its superhydrophobicity and color after treatment at 150 °C for 24 h. Further testing revealed that the superhydrophobicity of the film was not lost after toleration of strain up to 100%, impacting by water or weight, man-made destruction by hands or sandpaper, immersing in various corrosive liquids, and pollution by sludge water or dirt. These multiple key properties that have been integrated into our superhydrophobic composite film are expected to provide unique advantages for a wide range of applications in decoration, construction, transport, electronics, and wearable devices.

1. Introduction

Superhydrophobic surfaces have been widely developed due to their potential application in damp-proofing, self-cleaning, anti-condensation, corrosion-resistance, drag reduction, oil–water separation, and heat transfer.1–5 However, the application area of these surfaces is restricted in real-world because of their unsatisfactory mechanical stability.6 To adapt the applied environment, many robust superhydrophobic surfaces, such as hierarchical roughness surfaces fabricated by spray nano-coating on the substrate with a microstructure7,8 and a self-healing coating surface prepared using the self-migration of a particle,9 have been reported by functional and structural design. In recent years, many studies have been reported on the construction of an organic–inorganic composite coating using resin as a binder, such as epoxy resin,10,11 polyurethane,12 and polydimethylsiloxane,13 for immobilization of the hydrophobic nanoparticles on various substrates.14 However, we have to recognize that the service life of the superhydrophobic surface is still limited although its mechanical stability has effectively improved. Therefore, how to expediently rehabilitate or replace the invalidity surface will become a new challenge to maintain the superhydrophobicity in practical applications.15

Currently, functionalized film materials (such as mobile sticker, car film, and wallpaper) can be seen everywhere in our daily life due to their applications in anti-UV radiation, waterproofing, heat preservation, ornaments, corrosion resistance, scratch retardance, and so on. Therefore, we believe that it will become a new breakthrough for large-scale application of self-cleaning surfaces in outdoor if these adhesive films can be designed as superhydrophobic films. Based on previous reports, a transplantable superhydrophobic film could be prepared by the following methods: (1) fabrication of a micro–nano-scale rough structure and the subsequent chemical modification on the adhesive films;13,16 (2) pasting the hydrophobic nanoparticles on the flexible substrate by a binder;17,18 and (3) direct immobilization of the hydrophobic nanoparticle on the film via the curing process.19

However, in the past, there are few studies that have reported a type of transplantable and translucent superhydrophobic film that has remarkable thermostability, stretchability, and durability. Herein, to consider the adaptability for large-scale outdoor installation, a transplantable superhydrophobic composite film was fabricated using mixed fluoroethylene vinyl ether and ethylene vinyl acetate resins as a primer and then covering superhydrophobic nano-particles on the semi-cured resins. The films retained superhydrophobicity after toleration of the stresses from heating, tension, impacting, kneading, friction, corrosive liquids, and dirt.

2. Experimental

2.1 Materials

Fluoroethylene vinyl ether (FEVE), ethylene vinyl acetate (EVA), and curing agent were provided by Nanjing WanQing Chemical ClassWare & Instrument Co., Ltd. Silica nanoparticles with a size of 10–20 nm were provided by Evonik Degussa China Co., Ltd. 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES) was purchased from Sikang New Material Co., Ltd. Absolute ethyl alcohol (EtOH), deionized water, ammonium hydroxide (28%), n-butyl acetate, and other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

2.2 Preparation of the FEVE/EVA composite paint

Resin composite paint was acquired by dissolution of FEVE, EVA, and curing agent in n-butyl acetate. The mass ratio of FEVE, EVA, and curing agent (polyisocyanate-type) is 3[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1, and the paint concentration is 100 mg mL−1. In a typical process, FEVE and EVA were first dispersed in n-butyl acetate under an 80 °C dryer for 2–4 h. For application, the biphasic mixtures were prepared at room temperature for 30 min under stirring at a rate of 500 rpm. Subsequently, the curing agent was added to the mixtures, and the mixture was then continuously stirred for 10 min.

2.3 Synthesis of the superhydrophobic paint

The typical silica superhydrophobic nano-particle (SNP) paint was fabricated as follows: 30 g silica nanoparticles were diluted and alkalified in an alkaline solution containing 1.6 L of deionized water and 0.8 L of ammonium hydroxide. After ultrasonic treatment for 10 min, the mixture was slowly added to 16 L of EtOH under stirring at 300 rpm for over 10 min. Subsequently, 60 mL of PFDTES was added to the system. After the addition of PFDTES, the reaction was allowed to proceed at room temperature for 24 h under constant stirring at 300 rpm.

2.4 Fabrication of the superhydrophobic film

A spray painting apparatus equipped with a W-101-152G spray gun (ANEST IWATA Corporation, 1.5 mm of nozzle orifice) and OTS-550 oil-free gas compressor (Taizhou outstanding industry and trade co., ltd., 0.7 MPa of exhaust pressure) was used to first spray the FEVE/EVA composite paint on a cleanly glazed tile for acquisition of primers. After semi-curing for about 10 min, when the environment temperature was about 25–30 °C, the SNP paint was sprayed on the composite primer. Finally, the transplantable superhydrophobic composite film was obtained after complete curing for 12–18 h at room temperature and subsequent depanning. The spray gun was moved (about 0.1 m s−1) according to an S-style spray route from top to bottom, and a distance of about 30–40 cm was maintained. This process could controlling the thickness (about 100 μm) of the coating by spray-steps.

2.5 Characterizations

The microstructure of the sample surface was observed by field-emission scanning electron microscopy (FESEM), which was conducted using a Sirion instrument. Static water contact angles (SCAs) and roll-off angles (RAs) were determined at room temperature using an OCA 15Pro contact angle meter to characterize the hydrophobicity of the specimens. The average CA value was determined by measuring the same sample at 3–5 different positions. The volume of the water droplet was 5 μL for SCAs and 10 μL for RAs unless noted otherwise. Thermogravimetry-differential scanning calorimetry (TG-DSC) measurements were conducted via DSC8000 (PerkinElmer) using a dynamic heating rate of 5 °C min−1 from room temperature to 300 °C. Time-lapse images or videos were obtained using the Photron FASTCAM Mini UX100-type high-speed camera equipped with Navitar 6000 zoom lens. In addition, other optical images or cartoons were obtained using the Nikon D 3400 digital single lens reflex camera.

2.6 Test methods

The heat resistance tests were carried out on the samples in an oven at 25–200 °C for 2–24 h. The mechanical stabilities of the superhydrophobic composite films were expressed by tensile, impacting, and friction. After this, the tensile tests were performed by the following methods: (1) vertical free hanging test was executed by suspending a weight of 200 g under the bottom of the film and then free motion from initial position; (2) horizontal tensile tests were operated by a miniature electric motor with a radial velocity of 20 cm min−1; (3) periodic horizontal tensile tests were carried out by a telescopic arm with a velocity of 20 m min−1 and the strain was about 20%. In addition, the impacting tests use running water or weight to impact the surfaces of the horizontal film from a height of 50 mm. The friction tests have been listed as follows: (1) finger-wipe, single-hand-grasp, both-hand-rub, and both-hand-kneading tests have been conducted by finger or hand with a glove to periodically wipe, grasp, rub or knead the film and (2) according to the literature,14 sandpaper abrasion test of one cycle has been carried out by positioning the samples face-down to 240# sandpaper and moving for 10 cm along the ruler with a loaded 100 g weight in vertical and horizontal direction. In this test, the glazed tile has been replaced by a glass slide to prevent the shedding of the film. Moreover, to avoid the interference of the residue powder from the samples rubbed by sandpaper, the new sandpapers were used after abrasion per 10 cycles. Long-term chemical durability was designed by a static immersion test to place the film into a corrosive liquid, such as water with pH = 1–13, 1 M NaCl and urine. Self-cleaning properties were demonstrated using sludge water to pollute the film or employing water droplets for removal of dirt on the surfaces.

3. Results and discussion

3.1 Surface performance

Fig. 1a shows a universal preparation process of the transplantable superhydrophobic composite film by two-step spray methods on a glazed tile using a suitable resin as a primer to immobilize the functionalized nanoparticle (herein, it is SNPs). Thus, how to select the suitable resin in this work should arouse sufficient attention. The selection mechanisms of the transplantable superhydrophobic composite film could be summarized as following key technologies: (1) high bonding strength between in-resin and functionalized nano-particle; (2) the cured resin easily pulled off from the glazed tile or other substrates (Video S1, ESI); and (3) the specific function (e.g. stretchability (Fig. S1), wearability, transparency, and collapsibility) of the resin in some application situations. As shown in Fig. 1b, we employed FEVE/EVA composite resins as the primer for investigation of the comprehensive performance of the superhydrophobic composite film under the stress from heating, tension, impacting, kneading, friction, corrosive liquids, and dirt. For demonstration of the flexibility, we tailored the film, just as a paper, into a specified size by a paper knife (Video S2, ESI). Although the transmittance of the film is very poor, Fig. 1c shows that it has enough transparency to distinguish graphics and characters on a backlight device; thus, this film is a good candidate for self-cleaning of electronic visual display in the future.
image file: c7ta08203g-f1.tif
Fig. 1 (a) Schematic for the preparation of the superhydrophobic composite film by two-step spray methods and subsequent demoulding. (b) and (c) Optical image of the superhydrophobic composite film and its superhydrophobicity. (inset: SCA image of the film. The SCA and RA are about 162.1° and 3.1°, respectively.) FESEM images at different magnifications from (d): (e) cross-section, and (f); (g) surface of the superhydrophobic composite film.

Microscopic morphology of the superhydrophobic composite film was characterized by FESEM from the cross-section and surface. Fig. 1d indicates that the film contains 25 μm of SNP coating and 75 μm resin in the natural state. To demonstrate the thickness uniformity, we also observed the cross-section of the film using a magnification at 100×, as shown in Fig. S2a (ESI). In addition, a hybrid layer with a thickness of about 10 μm was also found under the contact surface between the nano-coating and resin (Fig. S2b). As shown in Fig. 1e, the nano-clusters composed of nano-particles with a size of 10–20 nm (Fig. S2c) were immersed in the resin. On the surface of the film with superhydrophobicity, as shown in Fig. 1f and g, the SNP coating was evenly distributed on the composite resin with a ubiquitous microcrack. This microcrack could divide the rigid inorganic nano-coating into countless nano-clusters with a size of 10–20 μm (Fig. S3, ESI), which was favorable to maintain the uniform distribution of SNPs under stretching and bending conditions.

3.2 Thermostability

Thermostability of resins is an important indicator to estimate their environmental adaptability under extreme conditions. According to the TG-DSC curve shown in Fig. 2a, the superhydrophobic composite film could withstand 300 °C temperature in air environment. However, further investigation from heat resistance test displays that the surface begins to turn yellow when it is treated at 175 °C for 2 h although its superhydrophobicity can be sustained (Fig. S4a, ESI). After treatment at 200 °C for 2 h, the yellow film has been firmly adhered on the glass slide by a softening mechanism (Fig. S4b, ESI). As shown in Fig. 2b, we have listed the SCAs and RAs on the films after treatment at different temperatures for 2 h; the values suggest that their superhydrophobicity are steady before heating at 200 °C. In addition, the superhydrophobicity (Fig. 2c) and color (Fig. S4c and d, ESI) of the films were investigates at 150 °C for 2–24 h to explore that the heat treatment time had hardly any influence. Under cryogenic environment, the harden effect and brittle fracture are very common phenomena in most of the resins. Herein, we displayed the harden effect of the film by immersing it in liquid nitrogen (Fig. S5, ESI). Interestingly, the low-temperature brittle fracture has not happened on the film. When placed at ambient temperature, the hardened film gradually softens, and its superhydrophobicity and flexibility are also maintained (Video S3, ESI).
image file: c7ta08203g-f2.tif
Fig. 2 (a) TG-DSC curve of the superhydrophobic composite film from room temperature to 300 °C at a heating rate of 5 °C min−1. SCAs and RAs on the films after treatment under different conditions: (b) 25–200 °C for 2 h and (c) 150 °C for 0.5–24 h.

3.3 Tensile strength

For consideration of the robustness and flexibility, we designed a series of experimental studies to investigate the endurance of the superhydrophobic composite films (size of 10 × 2 × 0.1 cm3) under the action of an external force. In Fig. 3a, the evaluation of the elasticity of the film by a tensile stress–strain measurement is shown, which suggests that the maximal tensile strength and strain are about 1.53 MPa and 239.8%, respectively. As shown in Fig. S6 (ESI), the film has sufficient resilience to support the weight with a mass of 200 g. When the weight was released from the initial position, the maximum and minimum strains (ε) were about 32.2% and 17.2%, respectively. After the weight relaxed into gravitational equilibrium, the strain was about 26.1%, and then, the Young's modulus was calculated to be 3.75 MPa (for more details, see Video S4, ESI). In addition, we bond a transparent tape under the superhydrophobic composite film to improve the tensile strength. Fig. S7 (ESI) shows the tensile stress–strain curve of the enhanced film. The maximal tensile strength and strain are about 32.59 MPa and 142.9%, respectively.
image file: c7ta08203g-f3.tif
Fig. 3 (a) Stress–strain curves of the superhydrophobic composite film with a size of 10 × 2 × 0.1 cm3. (b) Optical image of the film and its superhydrophobicity under natural and stretch state. All the SCAs and RAs are over 150° and below 10° on the films with (c) different strains and after (d) different cycles with a strain of about 20%.

For quantization of the tensile performance, the superhydrophobicity of the surfaces was demonstrated under different strains. As shown in Fig. 3b, the superhydrophobicity was maintained when the strains of the film increased to 100% from 0%. Their SCAs and RAs are drawn in Fig. 3c, indicating that the film has excellent adaptability to deal with the distortion during application. Moreover, as shown in Video S5 (ESI), the superhydrophobic composite film could return to normalcy without any deformation and damage after stretching with a strain of about 20%. Further tensile test suggested that the superhydrophobicity of the film still emerged even when it was stretched (ε = 20%) for 500 cycles (Fig. 3d).

To understand that the superhydrophobicity could be held on the stretched films, we designed a simple model for acquisition of theoretical analyses. According to Fig. 4a, we speculated that the square columns from SNPs were uniformly distributed on the resin. Under the tensile state, the square columns were separated to form a patterned composite surface, and their relative geometrical parameters were labeled as side length (D), height (H), and side-to-side distance (L). The strain (ε) of the film was defined as ε = L/D. Then, the wettability on the patterned surface was predicted using a hybrid mode, as shown in Fig. S8. The water drops (with a diameter of D0L) turned to the Wenzel state from the apparent Cassie state on the stretched film when LmD. Therefore, contact angles (θ) on the hybrid state film (0 < ε < m) could be calculated by the following formula:20,21

image file: c7ta08203g-t1.tif(1)
where θ0 = 162.1° is the apparent contact angle on the SNP surface. For more details, see modeling calculation and analysis in the ESI.Fig. 4b and c exhibit that a number of islet-like prominences with a size of 10–20 μm were evenly dispersed on the film after stretching. As shown in Fig. 4d, the prominence maintains the original nano-porous structure by accumulation of nano-particles with a size of 10–20 nm, which is significative to retain the superhydrophobicity of the film after stretching.

image file: c7ta08203g-f4.tif
Fig. 4 (a) Schematic of the geometric parameters of the square columns from SNPs coating after stretching. (b–d) FESEM images for the stretched film (ε ≈ 50%).

3.4 Impact resistance

For use as an elastomer, the vertical impact resistance was also examined by an impacting test on the horizontal film with a size of 10 × 2 × 0.1 cm3 to reveal that the superhydrophobic composite films have adequate toughness for toleration of impact force from soft and hard foreign intruders. Fig. 5a shows that a water droplet can also completely leave the surface without wetting, penetrating or damaging due to the Cassie state of superhydrophobicity. In consideration of the elastic deformation, we have compared the bouncing process of water droplet with a size of 2.1 mm on the stretchy film (Fig. 5a) and rigid glass slide treated by the SNP coating (Fig. 5b). The results suggest that the droplet was sputtered and dispersed as a lot of little drops after contact with the rigid superhydrophobic surface, and then, the sectional water in heartland was re-collected and bounced off the substrate with a contacting time of 24.8 ms. The difference is that all the water was completely rebounded on the stretchy surface with a contacting time of 31.6 ms, which was advantageous to reduce damage from tangential stress (for more details, see Video S6, ESI). Moreover, Fig. 5c indicates that a weight with a mass of 100 g can be bounced off the film without penetration or damage because of the remarkable elasticity. The contacting time between the weight and the film from encounter to separation is about 82.2 ms. The maximum strain was obtained when the weight drove to the bottom at 30.0 ms with a strain of 4.04% and Young's modulus of 0.17 MPa. In addition, we surveyed smaller droplet or weight to impact the film for decrease of the contacting time. Fig. S9 shows that the contacting time between the droplet with a size of 1.3 mm or weight with a mass of 10 g and the film from encounter to separation is reduced to 13.0 ms or 27.2 ms, respectively.
image file: c7ta08203g-f5.tif
Fig. 5 Typical time-lapse images of the impacting test. Water droplets (with volume of 40 μL) perpendicularly impacting the (a) cantilevered elastic films and (b) rigid surfaces with SNPs coating by free fall from about 10 cm height. (c) A 100 g weight replacing the water droplet to bounce the cantilevered elastic films with a kinetic energy of 0.1 J.

3.5 Wear resistance

As is known, wear resistance is an important factor to affect the service life of superhydrophobic materials because superhydrophobicity will be lost once the rough structure is damaged. Herein, we used different methods to demonstrate the anti-wear property of the superhydrophobic composite film. As shown in Fig. 6a–d, four types of man-made destructions were carried out by finger-wipe, single-hand-grasp, both-hand-rub, and both-hand-kneading tests to exhibit the prominent mechanical endurance of the film. Video S7 (ESI) shows various man-made destructions on the superhydrophobic composite films. The results showed that the water droplet that was dyed by blue could completely deviate from the films without wetting or contaminating the surfaces; this revealed that the films still retained favorable superhydrophobicity after sufferance of man-made destruction. Moreover, to quantize the wear resistance of the superhydrophobic composite film, we orderly sprayed the composite resin and SNP paint on a glass substrate to prevent decrustation. After curing, the sandpaper abrasion test was implemented (Fig. S10, ESI), and the SCAs and RAs of water were obtained, as shown in Fig. 6e. The results indicated that the SCAs were maintained over 150° after abrasion of 70 cycles, indicating that superhydrophobicity was not lost by mechanical abrasion. However, the RAs of water droplet with a volume of 10 μL rapidly increased from below 15° to over 80° during 30–70 cycles. Interestingly, the RAs could also be retained below 15° after 70 cycles while using the water droplet with a volume of 25 μL.
image file: c7ta08203g-f6.tif
Fig. 6 Images of the processes and results of man-made destructions by (a) finger-wipe, (b) single-hand-grasp, (c) both-hand-rub, and (d) both-hand-kneading. (e) SCAs and RAs on the superhydrophobic composite film after abrasion of 0–70 cycles on the 240# sandpaper.

3.6 Chemical durability and self-cleaning properties

In large-scale practical applications, long-term chemical durability of superhydrophobicity is one of the most important issues that should be considered. Herein, for the convenience of observation, we used a double-sided tape to adhere the film to a glass slide and then immersed it in different corrosive liquids. After placing it in a closed plastic wrap for 2 weeks, the SCAs and RAs of water were measured, as displayed in Fig. 7a. All the SCAs remained larger than 150°, and RAs were still below 10°; this revealed that the superhydrophobicity of the film was not lost by chemical immersion. In addition, to demonstrate the self-cleaning property, we used sludge water as a representative for pollution of the superhydrophobic composite film. As shown in Video S8 (ESI), the complete process of sludge water deviation from the film without any residuum is depicted, indicating that the self-cleaning property is retained even after sludge contamination. Correspondingly, in the rim of film, large stains from residual sludge were examined where the sludge water could arrive. Based on the principle of superlubric surfaces,22 the water droplets could be easily slipped off from when it was contaminated by hexadecane (Fig. 7b and c); this suggested that the self-cleaning property was also maintained even after oil-contamination. Moreover, Video S9 (ESI) shows that a dirt-removal test was carried out on the superhydrophobic composite film to remove soil and dust by passing water over the surface.
image file: c7ta08203g-f7.tif
Fig. 7 a) SCAs and RAs on the film were measured after immersing the film in different corrosive liquids for two weeks. (b) Water droplet was repelled by the superhydrophobic composite film and the untreated beaker was dyed by methylene blue water when immersed in hexadecane. (inset: the SCA is about 167.1°.) (c) The film retained its superhydrophobicity after contamination by hexadecane. (inset: the SCA is about 67.3°).

4. Conclusion

In conclusion, we have developed a type of novel transplantable superhydrophobic composite film by two-step spray methods using FEVE/EVA composite resin as a primer to immobilize SNPs. The translucent film with a thickness of about 100 μm demonstrates significant comprehensive performance in a series of thermal, mechanical, and chemical tests. The heat resistance tests indicate that the superhydrophobicity and color of the film were still retained after treatment at 150 °C for 24 h or immersed in liquid nitrogen. To demonstrate the excellent stretchability, impact resistance, and wear resistance, we also systematically evaluated the superhydrophobicity of the film after toleration of various loads such as extensive and cyclic stretching, impacting by liquid or solid, friction by hands or sandpaper. In addition, the film could maintain its water repellency upon immersion in acid (pH = 1), alkali (pH = 13), salt (1 M NaCl), and even urine solution for two weeks, or being polluted by sludge water and dirt. In large-scale practical applications, the function and performance designability, production flexibility, and replaceability endowed the superhydrophobic composite film with more significant adaptation and selectivity for application in decoration, construction, transport, electronics, and wearable devices.

Conflicts of interest

There are no conflicts to declare.


We are grateful for the supports received from the National Natural Science Foundation of China (Grants 51671055, 51676033), the China National Key R&D Program (2016YFC0700304), the National Natural Science Foundation of Jiangsu Province (BK20151135), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1675).


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta08203g

This journal is © The Royal Society of Chemistry 2017