Robust fluorine-free and self-healing superhydrophobic coatings by

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Applications of Polymer, Composite, and Coating Materials

Robust fluorine-free and self-healing superhydrophobic coatings by H3BO3 incorporation with SiO2-alkyl-silane@PDMS on cotton fabric Sudip Kumar Lahiri, Pengcheng Zhang, Cheng Zhang, and Lin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20651 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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ACS Applied Materials & Interfaces

Robust fluorine-free and self-healing superhydrophobic coatings by H3BO3 incorporation with SiO2-alkyl-silane@PDMS on cotton fabric Sudip Kumar Lahiri, Pengcheng Zhang, Cheng Zhang, Lin Liu* School of Materials Science and Engineering and State Key Lab for Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan, 430074, China

ABSTRACT: Limited robustness is a serious drawback for superhydrophobic coatings, and degrades the performance of superhydrophobic surfaces in practical applications. Although fluoro-reagents have excellent durability for superhydrophobicity, their use has been restricted due to various health and environmental concerns. In this work, we describe a facile and efficient fabrication strategy for creating robust fluorine-free superhydrophobic composite coatings that are prepared by a simple dip-dry method, in which the H3BO3incorporated SiO2-alkyl-silane coatings are deposited on woven cotton fabric surfaces followed by polydimethylsiloxane (PDMS) modification. The coated surface shows a large water contact angle of 157.95˚±2˚ and a small sliding hysteresis angle of 3.8˚±0.6˚, demonstrating excellent superhydrophobicity. The coated fabric surface also exhibited robustness and durability, withstanding a tape-peeling test (under 48.05 kPa) for around 80 repetitions and sandpaper rubbing (loaded 100 g) for 40 cycles. Furthermore, the coated fabric surface displayed self-healing and oil-water separation capacities. The developed superhydrophobic coatings in this study are robust, environmentally benign, and easy to fabricate, showing promising applications in textile industries.

Corresponding author, Tel.: +86-27-87556894; E-mail: [email protected]

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KEYWORDS: Superhydrophobic coatings; Silica nanoparticles; H3BO3 cross-linking; Robust and selfhealing; Oil-water separation.

1

INTRODUCTION Inspired by the excellent waterproofing and self-cleaning properties of sacred-lotus leaf in nature,

several man-made superhydrophobic surfaces have been fabricated for their wide-ranging potential applications in various industries, with properties such as self-cleaning1–4, friction reduction5, oil-water separation6,7, anti-corrosion8,9, anti-icing10–12, anti-fouling13, and anti-drag14. Specifically, distinct micro/nanoscale-based binary structured superhydrophobic surfaces with a water contact angle (WCA) over 150˚ and a sliding angle (WSA) lower than 10˚ have been studied intensively over the last decades because of their potential applications in industrial and biological areas15–17. To date, surfaces with superhydrophobicity have been prepared by means of patterning proper surface roughness along with stable air pockets, which presents in the micro/nano structures with low surface energy18–20. In recent years, noticeable attempts have been taken to enhance water repellency by constructing unique morphological patterns of various natural superhydrophobic substances. These superhydrophobic materials have great potential for practical application in diverse fields. Textiles are widely used materials in domestic and industrial settings. There are various kinds of synthetic textiles such as acrylic, polyester, and nylon, that are useful for weather and waterproofing, and even those require surface treatments or multilayer modifications21,22. Textiles made from staple fibers, together with cotton fabrics, are low-budget, biodegradable, renewable, flexible, and gentle as compared to synthetic fibers, and they are taken into consideration for appropriate substances to treat commercial oily wastewater and marine oil spillage in oceans23. However, the pristine cotton textiles absorb both water and oil at the same time due to the existence of external hydroxyl groups on their surface, which is particularly unsuitable for achieving dual functional properties such as absorbing oils and removing water 2 ACS Paragon Plus Environment

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droplets simultaneously. Nevertheless, building up a super-anti-wetting surface24–26 on natural fiber-based textiles (cotton textile) along with stable superhydrophobicity and superoleophilicity has recently received much attention27,28. Due to their selective wetting property, these super-anti-wetting surfaces have potential applications in oil-water separation. There are several techniques by which to fabricate superhydrophobic coatings on cotton textile surfaces, such as sol-gel methods29, chemical etching30,31, dip-dry coating32, layer-by-layer technique33, and spray-coating method34. Among them, dip-dry deposition, a flexible wet chemical method, has received much attention35. Through this method, superhydrophobic coatings can be easily fabricated through covering the fiber surface with micro/nanoparticles such as SiO236–38, CuO39, TiO240, ZnO41 or composites, followed by surface modification with low surface energy materials. Among those nanoparticles, SiO2 nanoparticles are widely used because of their lower toxicity, chemical immobility, optical transparency, and low environmental impact42. Perylene-functionalized silica particles with selfcleaning properties have been fabricated43 and have shown desirable characteristics including excellent controllable size and thermal, and environmental stability with a large surface area-to-volume ratio. Wang et al. fabricated superhydrophobic wood surfaces by introducing polydimethylsiloxane and silica nanoparticles in order to achieve mechanical durability44. Lin et al. demonstrated a straightforward, environment-friendly pleasant way to design superhydrophobic coatings utilizing aggregated silica nanoparticles and decanoic acid-modified TiO245. Chen et al. acquainted another economical technique to achieve non-fluorinated superhydrophobicity by hydrophobic silica nanoparticles with low surface energy and optimum surface roughness at the micro- and nanoscales46. However, achieving durability and robustness is still a great challenge for the practical application of these hydrophobic surfaces. To overcome this limitation, further development of cost-effective, simple, and broad-scale strategies for manufacturing superhydrophobic coating is required. The broad use of superhydrophobic coatings is limited by the poor mechanical durability of nonfluorine containing materials. One of the major problems is that the micro/nanoscale roughness of the 3 ACS Paragon Plus Environment


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surface becomes damaged by distinct actions, such as abrasion and erosion, causing the loss of superhydrophobicity of the surface47,48. Hence, designing a mechanical durable superhydrophobic surface is a vital subject in this field. Recently, a straight-forward approach of enhancing the bonding strength of a polymeric coating was developed just simply by increasing the cross-linking between functional groups, which makes the coating robust. For example, Yang et al. mixed two silanes, tetraethyl orthosilicate (TEOS) and heptadecafluoro-1,1,2,2-tetrahydrodecyltriethoxysilane (HDFTES), with strings of silica nanoparticles in an ammonia/ethanol solution to prepare a bridged nanoparticle/sol solution for spray coating49. Although the robustness of the coating was enhanced, the fabrication process was too complicated, and the coatings contained fluoro-reagents which are expensive and toxic to humans and the environment. On the other hand, through supramolecular complexation, Zhang et al. chose chitosan as a cross-linker with transition metal ions to construct multi-stimuli-responsive hydrogels. However, the biopolymer cross-linker lacked adhesion strength50. Another simple one-step method of making a crosslinked assembly coating was described by Ejima et al. who employed the natural polyphenol tannic acid and FeIII inorganic cross-linking method to fabricate several coatings and particles through coordination complex chemistry51. However, surface durability performance was not taken into consideration. While the key methodology was to create more cross-linking points to enhance the network of the whole coating, there were limitations in durability along with environmental issues. Therefore, it is still essentially required to develop new materials and strategies to increase the consistency of the nanostructured coating while maintaining scalable feasibility. In this study, H3BO3 was deposited on woven cotton fabric surfaces to create strong cross-linking bonds between silica nanoparticles and alkyl-silanes to form micro-nano hierarchical structures and to achieve superhydrophobicity. H3BO3 has a central boron atom that is connected to three hydroxyl (-OH) groups, which are capable of strong hydrogen bonding, thus providing a very high bond dissociation energy (̴ 806 kJ.mol-1) with most of the hydroxyl group containing particles, such as silica, formed through condensation52. Motivated by this cross linking approach, we believed that it is simple to create 4 ACS Paragon Plus Environment

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robust micro and nanostructures via the development of a H3BO3-incorporated silica coordination network with pre-hydrolyzed alkyl-silane, followed by polydimethylsiloxane modification. Here the trifunctional H3BO3 performs dual properties in the coating synthesis process, in one side it reacts with SiO2 nanoparticles to form nanoporous structures, and other side it creates strong bonds between nanoporous SiO2 and pre-hydrolyzed HDTMS to make the coating robust due to high bonding energy between silanol groups and H3BO3. Although various types of researches have been conducted in recent years to achieve the superhydrophobic coatings with self-healing properties, none of them reported that in synthesis one material could carry out the dual functional properties at the same time. Moreover, the optimum usage of alkyl-silane and PDMS shows good self-healing property, which is also vital for practical applications. The usages of H3BO3 do not generate any harmful effect on the cotton fabrics or add extra manufacturing expenses. Furthermore, the methods for the preparation of the superhydrophobic coatings are unique and absolutely simple, such as no demand of applying further expensive chemicals or physical modifications to achieve nanoporous structures which is essential to gain optimum superhydrophobicity and large scale production in industries. In this work, we will show that the coated fabrics with H3BO3-incorporated SiO2-alkyl-silane@PDMS composite coatings are environmental friendly, inexpensive, sustainable, and easy to scale-up, and they can be extended to other substrates, such as various kinds of cellulose sponge, wood, and paper, via simple dip-dry process. This prepared superhydrophobic cotton fabric exhibits multifunctional properties, including self-cleaning, self-healing, and oil-water separation as well as excellent mechanical durability, thus showing promising application in the textile industry.

2 2.1

EXPERIMENTAL WORK Materials and reagents Tetraethyl orthosilicate (TEOS), ethanol, ammonia solution, boric acid (H3BO3), sodium hydroxide,

tetrahydrofuran (THF), chloroform, and methylene blue were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. HDTMS (Hexadecyltrimethoxysilane) was purchased from Aladdin Industrial Corporation, Shanghai, China. Polydimethylsiloxane (PDMS) prepolymer (Sylgard 184A) and 5 ACS Paragon Plus Environment


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the curing agent (Sylgard 184B) were purchased from Dow Corning Corporation (Shanghai, China). All the reagents are of analytical grade. The pristine woven cotton fabrics were purchased from a nearby fabric store and were ultrasonically rinsed with ethanol and deionized water sequentially to eliminate viable contaminants and then dried in an oven at 60 C for further processing.

2.2

Preparation of SiO2 sol The silica nanoparticles were synthesized through a prominent sol-gel technique established by

Stöber et al.53, wherein 40 mL of ethanol and 2 mL of aqueous ammonia were uniformly combined at ambient temperature. Then, a solution of 46 mL of TEOS and 40 mL of deionized water was added and mechanically stirred for 4 hrs in a water bath at room temperature to form silica-sol. 2.3

Nanolinking and functionalization of silica nanoparticles The prepared SiO2 sols (35 mL) and 100 mL of ethanol were placed into a round-bottomed flask

and mixed by stirring for 2 hrs to form a uniform SiO2-sol solution. Then, H3BO3 (1 mol) was added dropwise into the SiO2-sol solution while adjusting the pH to 9-10. The reaction was kept for another 2 hrs at 85 C to ensure the poly-condensation and in-situ growth of nanoparticles. This process led to nano linking between the SiO2 nanoparticles and H3BO3. Different amounts of prehydrolyzed HDTMS were added to the H3BO3@SiO2 composite to achieve the condensation (end capping) properly. The mixture was cooled to room temperature and the polymer composite was dried at 80 C prior to being ground into fine powder using a mortar and pestle. For comparisons, a certain amount of bare SiO2 and H3BO3-SiO2 as well as SiO2 and H3BO3-SiO2 modified with HDTMS and/or PDMS samples were also prepared and dried in an oven at 80 C for further processing.

2.4

Fabrication of superhydrophobic fabric One and a half grams of polymer composite powder described above was dissolved in THF (28 g)

at 35 C (solution A). Then, PDMS (0.5 g) and curing agent (0.05 g) were dissolved into 28.9 g of THF to 6 ACS Paragon Plus Environment

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prepare a PDMS solution. The solution was ultrasonicated for 30 min (solution B). Before coating treatment, solutions A and B were mixed together to form a uniform coating solution (solution C). Finally, the woven cotton fabric (5 cm × 5 cm) was dipped in solution C for 15 min with ultrasonication and then cured for 4 hrs to obtain the coated fabric which showed superhydrophobicity. 2.5

Abrasion and tape peeling testing To carry out the abrasion test, the resultant coated fabric was placed on a glass slide secured with

commercial adhesive tape, and then a silicon carbide sandpaper moved on the fabric surface in horizontal and vertical directions for 15 cm each under a load of 100 g. The sandpapers with Cw-800, Cw-1000, and Cw-2000 meshes were used in this experiment. For the tape peeling test, the coated fabric surface (4 cm x 2 cm) was pressed by a load of 48.05 kPa with industrial grade scotch tape, and then the tape was removed. Eighty repeats were performed to test the durability of the coated fabric. 2.6

Assessment of the self-healing property The self-healing capability of the superhydrophobic cotton fabrics was evaluated by an air-plasma

treatment by a laboratory setup micro-plasma-jet generator at atmospheric pressure for several cycles, in each cycle, the cotton fabrics were exposed under air-plasma for 3 min followed by storage at room temperature for 12 hrs. The generation of air-plasma was described in details in our previous work54. After plasma treatments, the WCA of the cotton fabrics was measured again to evaluate the self-healing property. 2.7

Structure and wetting characterizations The surface morphologies were determined by a field emission scanning electron microscope

(FESEM, Gemini-SEM 300) with an acceleration voltage of 5 kV. In general, before the FESEM examination, a thin layer of Pt was coated on the specimens by sputter-coating to enhance conductivity. The infrared spectrum was recorded to observe the chemical structure of the composite coating by using a VERTEX-70 Fourier transform infrared (FTIR) spectrometer. XPS observations were accomplished by a 7 ACS Paragon Plus Environment


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Thermo Electron Corporation VG Multilab 2000 XPS spectrometer. The water contact angles (WCAs) and sliding hysteresis angles (SHAs) on the fabric surface were evaluated with a deionized water droplet of 5 μL at ambient temperature on a professional contact angle measurement instrument (Kino SL200KB). The recorded values of WCAs and sliding hysteresis angles (SHAs) were averaged at five different positions on each sample surface. The surface roughness measurements were carried out using a LSCM (KEYENCE Laser Microscope VK-X200). The specific surface area and average pore diameter of the powders were measured by the conventional Brunauer–Emmett–Teller (BET, ASAP 2020) technique.

3 3.1

RESULTS AND DISCUSSIONS Composition analysis The chemical composition of the formed polymer coating covering SiO2 particles and alkyl-silane

(HDTMS) with H3BO3 was analyzed by FTIR. Fig. 1 shows the FTIR spectra of unmodified SiO2 nanoparticles, H3BO3 incorporated nano-SiO2 and cross-linked polymer coatings (i.e., H3BO3-SiO2HDTMS), respectively. The unmodified silica particles have two characteristic peaks (see curve (a)) at 1051 cm-1 and 791 cm-1, corresponding to the stretching vibrations of Si-O-Si55. A small peak at 951 cm-1 is associated with the stretching vibration of Si-OH. After incorporating H3BO3 to SiO2 particles, a new peak of B-O-B appeared at 1438 cm-1, indicating that H3BO3 had been introduced onto the surface of the SiO2 particles (see curve (b)). Moreover, the adsorption bands at 3225 cm-1 and 1634 cm-1 are referred to the stretching vibration of O-H and bending vibration of H-O-H (see curve (b)), which verifies the existence of crystal water and absorbed water in H3BO3 incorporated nano-SiO2. After HDTMS addition, two additional peaks at 2919 cm-1 and 2851 cm-1 appeared (see curve (c)), which are attributed to the asymmetric and symmetric stretching of the CH2 group (see ref. 43), respectively. The above results suggest that the long-chain alkyl groups (–C16H33) of HDTMS were successfully grafted onto the surface of the H3BO3-SiO2 particles. Besides, the H3BO3-SiO2-HDTMS cross-linked polymer coating showed stronger absorption peaks of Si-OH and Si-O-B at 3380 cm-1 and 1343 cm-1, 8 ACS Paragon Plus Environment

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respectively than the H3BO3 incorporated nano-SiO2, which is due to the possible absorption of the Si-OB group, implying that the modified nano-SiO2 was partially covered due to the introduction of H3BO3 (see curve (c)). Moreover, the absorption peak of B-O-B positioned at 1465 cm-1 changed narrow and sharp (see curve (c)) compared with the peak of B-O-B located at 1438 cm-1 (see curve (b)), which indicated that the H3BO3 was incorporated with both nanosilica and HDTMS to form a cross-linked polymer network. 3.2

Morphological analysis of the coated fabric surface The morphologies of the coated fabric surfaces with 21 wt.% SiO2, 9 wt.% and 18 wt.% H3BO3-

SiO2 under various magnifications are shown in Fig. 2. As seen in Fig. 2(a1, a2), the fabric surface coated with 21 wt.% SiO2 has a relatively less porous network due to an inadequate aggregation of SiO2 nanoparticles on the surface. However, when H3BO3 was doped at 9 wt.% of H3BO3-SiO2, a noticeable nanoporous structure with hierarchical roughness was developed on the surface of the fabric as shown in Fig. 2(b1, b2). By further increasing the amount of H3BO3-SiO2 to 18 wt.%, both micro/nanopores and randomly distributed nanoparticles were observed to be aggregated on the fabric surface (see Fig. 2(c1, c2)), which is quite similar to the morphology of pumice-stone. The high-magnification SEM image (Fig. 2(c2)) shows that some H3BO3-SiO2 nanoparticles modified with HDTMS/PDMS aggregated into nanoclusters (20 nm), giving hierarchical roughness with micro/nano-structures on the surface. Two reasons may account for this aggregation: One is the grafting of long-chain alkyl groups onto the H3BO3SiO2 nanoparticles, and the other is the in-situ growth of nano-SiO2, which leads to the formation of the hierarchical micro/nano porous structure on the coated surface, and finally obtainment of the superhydrophobicity56. In addition, the porous structure has a strong ability to adsorb various solvents57,58. After further heat treatment at 60 °C for 4 hrs, the solvent adsorbed by H3BO3-SiO2 could perform as pore-forming media to create a porous structure inside the coating when vaporization occurred (see Fig. 2(c2)). It can thus be concluded that the addition of nano linked H3BO3-SiO2 has a high impact on the surface morphology of the coated fabric surface for achieving superhydrophobicity. To clarify the binding 9 ACS Paragon Plus Environment


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status of the coatings on the fabric, the cross-section images were obtained by SEM (see Supplementary Fig. S1). It shows that the coatings in all samples are not only on the surface but also diffuse into the fabrics to make the even coatings. 3.3

Surface wettability The wettability of coated fabrics with various contents of the SiO2-HDTMS and H3BO3-SiO2-

HDTMS particles with 0.5 g PDMS was defined by the water contact angle (WCA) and sliding hysteresis angle (SHA), as evidenced in Fig. 3((a) and (b)). It can be seen from Fig. 3(a) that the hydrophobic capability of the surface of the fabric was increased with increasing content of the SiO2-HDTMS particles and reached the best hydrophobicity at 21 wt.%, which exhibited a WCA of 142˚±1.7˚ and SHA of 22˚±2.1˚. However, further increasing the SiO2-HDTMS content could lead to a reduction in the hydrophobicity, for example, the WCA reduced to 139˚±2˚ and SHA increased to 25˚±1.9˚ when SiO2HDTMS content was increased to 24 wt.%, indicating that excessive SiO2-HDTMS content could not keep the balanced hydrophobicity. To enhance the wettability of the fabric surface, the modified H3BO3-SiO2-HDTMS cross-linked particles were coated with 0.5 g PDMS on the fabric surface (see Fig. 3(b)). In this case, the adhesion functionality of the coated fabrics decreased and the superhydrophobicity was achieved when the added H3BO3-SiO2-HDTMS particles reached 18 wt.%. In such a case, the WCA and SHA have the values of 155.79˚±1˚ and 3.7˚±0.8˚, respectively. However, a further increase in the H3BO3-SiO2-HDTMS content up to 24 wt.% just slightly increased the superhydrophobicity of the surface with a WCA of 157.95˚±2˚ and SHA of 3.8˚±0.6˚. In addition, the relationship between PDMS content with the content of H3BO3-SiO2-HDTMS particles and wettability of the coated fabric surface was also examined and the results are shown in Fig. 3(c). The wettability of the fabric surface coated by the mixed particles of H3BO3-SiO2/HDTMS/PDMS could not be increased by further increasing the PDMS content when the amount is higher than 0.5 g. 10 ACS Paragon Plus Environment

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Moreover, a higher concentration of PDMS might fill in all of the holes and extensively reduce the roughness of the surface, which is not conductive to trapping air among water and cotton to form a balanced Cassie state. The optimal superhydrophobic surface was obtained by adding 24 wt.% H3BO3SiO2-HDTMS particles and modifying with 0.5 g of PDMS. The variation of the wettability is due to in-situ growth and nano-linking of H3BO3 with nano-SiO2, which produces a large specific surface area, lowers the solid-liquid contact area, and captures much more air than bare SiO2 alone. Fig. 3(d) shows the water contact angle and SEM images of the fabric coated by SiO2-HDTMS particles and modified H3BO3-SiO2-HDTMS particles. It is clearly observed that the water contact angle of the coated fabric is enhanced from 142˚ to 157.95˚ with the addition of H3BO3 to the composite. Additionally, the corresponding SEM images (Fig. 3(d)) showed that once the modified H3BO3-SiO2-HDTMS cross-linked filling particles are applied on the fabric surface, the specific surface area is distinctly increased compared with the surface coated by SiO2-HDTMS particles alone. Furthermore, to discover the mechanism regarding superhydrophobicity of the coated fabrics, the laser scanning confocal microscopy (LSCM) was used to measure the surface roughness of the coated fabrics, and the results of the 4 coated fabrics (coated with 19.50 wt% SiO2-HDTMS, 21 wt% SiO2HDTMS, 16.50 wt% H3BO3-SiO2-HDTMS, and 18 wt% H3BO3-SiO2-HDTMS@PDMS, respectively) and the corresponding WCAs are presented in Table 1. It can be seen that the roughness and WCAs of the coated fabrics significantly rely on the content and nature of the coating materials. The fabrics coated with a higher content SiO2-HDTMS mixed particles (i.e., fabric 1 and fabric 2 in Table 1) exhibit a small roughness (Ra

Robust fluorine-free and self-healing superhydrophobic coatings by

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