Environmentally Friendly Super-Water-Repellent ... - ACS Publications

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 15346−15351

Environmentally Friendly Super-Water-Repellent Fabrics Prepared from Water-Based Suspensions Ronggang Cai,*,† Karine Glinel,*,† David De Smet,‡ Myriam Vanneste,‡ Nicolas Mannu,§ Benoît Kartheuser,§ Bernard Nysten,*,† and Alain M. Jonas*,† †

Bio & Soft Matter, Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, Croix du Sud 1, 1348 Louvain-la-Neuve, Belgium ‡ Centexbel, Technologiepark 7, 9052 Zwijnaarde, Belgium § Certech, Rue Jules Bordet, 7180 Seneffe, Belgium S Supporting Information *

ABSTRACT: We report on a facile, versatile, and environmentally friendly method to prepare superhydrophobic fabrics by a simple dipcoating method in water-based suspensions and emulsions. All the materials used are fluorine-free and commercially available at a large scale. The method can be easily integrated into standard textile industrial processes and has a strong potential for the mass production of environmentally friendly superwater-repellent fabrics. The produced fabrics show good resistance to machine washing and acidic or alkaline treatments. In addition, it is shown that superhydrophobicity can be quantitatively predicted based on the combination of the roughness of the fabric and of the fiber coating. KEYWORDS: superhydrophobic fabric, PDMS, silica particle, water-based process, roughness

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effective environmentally friendly solutions to fabricate superhydrophobic surfaces still remains an important practical challenge. Generally, the design and fabrication of green coatings should meet several requirements: (i) one should avoid using fluorinated materials: PDMS, fatty acids, and higher alkanes are better choices to provide a low surface tension and to eliminate the issue of bioaccumulation of fluorinated hydrocarbon chains; (ii) the process should involve nontoxic materials and solvents: in this respect, water-based emulsions and suspensions of nontoxic materials are the best candidates for green processes; (iii) the used materials should exist in large quantities and be commercially available and cheap; (iv) the process should allow us to rapidly coat large surfaces of different natures and shapes. For textiles, reasonably low temperature processing is a supplementary requirement, most textile fibers being limited in thermal resistance. Functional textiles showing advanced performances such as water-proofing,14,17,18,23 anti-UV radiation,24,25 liquid purification,26,27 and antimicrobial properties28−30 have recently drawn attention. Among these, superhydrophobic fabrics are especially interesting for outdoor architecture items such as awnings, tents, external shades, or design artifacts, applications for which the resistance to machine washing is not a major concern.

uperhydrophobic surfaces have attracted a large attention because of their importance in basic research and for applications such as self-cleaning1 or anticorrosion2 coatings, and oil−water separation.3 On the basis of Wenzel and Cassie− Baxter theories, superhydrophobic surfaces require sufficiently rough surfaces and low surface tensions.4,5 To achieve the required surface roughness, researchers have reported methods such as templating and molding,6,7 lithographic approaches,8 and plasma treatments.9 However, these approaches can be used only on certain substrates and generally require complex and/or expensive processing equipment and conditions. A more practical way is to deposit nano/micrometer-sized particles to build the roughness. The low surface tension is then provided by low-surface-energy materials such as fluorinated compounds, 1 0 − 1 3 polydimethylsiloxane (PDMS),11,14 fatty acids,15 or higher alkanes.16 The approach of roughening a surface by the direct deposition of nano/ microparticles (e.g., silica particles) followed by a modification of their surface tension has been extensively reported over recent years.11,12,14,17,18 However, the previously reported methods involve either nonenvironmentally friendly materials11,12,18 (e.g., fluorinated hydrocarbon compounds, which can degrade into shorter fluoroalkanes that bioaccumulate in living organisms19), and/or require complex thus expensive processing conditions,14 and/or use environmentally harmful organic solvents such as tetrahydrofuran11 and toluene.12,17 Therefore, although a few reports have proposed possible solutions toward greener processes and materials,15,18,20−22 developing cost© 2018 American Chemical Society

Received: February 13, 2018 Accepted: April 24, 2018 Published: April 24, 2018 15346

DOI: 10.1021/acsami.8b02707 ACS Appl. Mater. Interfaces 2018, 10, 15346−15351

Letter

ACS Applied Materials & Interfaces On the basis of these concerns, we report in this work on a facile and versatile dip-coating method to prepare environmentally friendly superwater-repellent polyester (PES) fabrics, which only uses water-emulsified PDMS and silica suspensions in water. In contrast, previous reports of superhydrophobic coatings on fabrics based on aqueous suspensions/emulsions were concerned with fluorinated compounds or sol−gel methodologies.20,31,32 Our method is schematically presented in Figure 1 and described in detail in Supporting Information

Figure 1. Sketch of the preparation of superhydrophobic PES fabrics including the deposition on bare fabrics (step 0), an anchoring layer (step 1), a roughening silica particle-based layer (step 2), and a layer providing low surface tension (step 3). Each layer is deposited by dipcoating from aqueous suspensions, followed by proper annealing.

Figure 2. Superhydrophobic PES fabrics prepared by dip-coating from (a−d) a water-based PDMS emulsion (1.5 wt %) and a silica particle suspension (1 wt %) or (e−f) a water-based PDMS emulsion (1.5 wt %), a silica particle suspension (1 wt %), and a water-based paraffin wax emulsion (4 wt % original emulsion in water): (a) optical profilometry image of the bare PES fabric, used to compute the textural roughness; (b) SEM image of the coated PES fabric, providing a view on the homogeneity of the coating (scale bar: 100 μm); SEM image of a single fiber is shown in the inset, providing a view on the structure deposited on the fiber (scale bar: 1 μm); (c, e) small-scale AFM topography images, used to compute the roughness of the coating deposited on the fibers; (d, f) pictures of a 10 μL water droplet resting on the coated fabrics.

SI2. It involves the deposition of an adhesive anchoring PDMS layer by dipping the PES fabric (Concordia Textiles) in a waterbased emulsion of PDMS (HC 303, Wacker), then the deposition of silica particles (Tixosil 365, Solvay) from an aqueous silica particle suspension to increase surface roughness, and finally the deposition of a supplementary PDMS layer by a second dipping in the water-based PDMS emulsion in order to modify the silica surface and provide low surface tension. According to its material safety data sheet, the selected PDMS consists of α,ω-terminated 3-((2-aminoethyl)amino)propyl)methoxysilyl ether PDMS chains, stabilized by a mixture of nonionic surfactants (diethylene glycol monobutyl ether and ethylene glycol monohexyl ether). The methoxysilyl end groups of the PDMS chains can be used to cross-link the material by condensation upon heating; additionally, they can also react with the silanol groups of the silica particles as any methoxysilane, thereby promoting adhesion. Therefore, an annealing treatment was performed after each dip-coating step to remove the excess of water from the coatings and cross-link/ graft the PDMS chains. The detailed conditions used to process and characterize the coated fabrics are described in Supporting Information SI2 and SI5. The superhydrophobic fabric referred to in Figure 2b−d is a typical sample prepared according to our dipping approach from a 1.5 wt % PDMS emulsion and a 1 wt % silica suspension. The large-scale texture of the bare fabric was obtained by optical profilometry (Figure 2a); SEM images of the modified fabric indicated a good uniformity of the coating and confirmed the construction of the nanoscale structures on the fiber surface (Figure 2b). The water contact angle measured on this sample was larger than 145° as shown in Figure 2d, and the water roll-off angle was smaller than 1.5°. The topography of the surface of the coated fibers in this sample was obtained

by AFM by scanning the tip on a region of size much smaller than the fiber diameter of the fabric (Figure 2c). The average values of root-mean-square (rms) roughness σ and Wenzel roughness r (r is the ratio between developed and projected areas; the process used to extract these roughness parameters from AFM images is described in SI5) measured for coated fibers were ca. 305 nm and 1.74, respectively (Table 1). This large roughness of the fiber surface, together with the low surface tension provided by PDMS, contributed to achieve superwater-repellency as evidenced by the large water contact angle and the small roll-off angle compared to the bare fabric or PDMS-only coated fabrics (Table 1). Movie S1 shows the easy water roll-off on such a modified PES fabric sample. To better understand how the surface structure and roughness influence the water-repellent behavior, we prepared a series of fabric samples from the PDMS emulsion and the silica suspension using different combinations of concentrations. The measured water contact angles and roll-off angles on these samples are reported in Table 1, together with the rms and Wenzel roughness measured by AFM on the fibers. As expected, a higher fiber roughness generally corresponds to larger contact angles and smaller roll-off angles. The results measured for reference samples consisting of a 100 nm-thick 15347


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ACS Applied Materials & Interfaces

Table 1. Roughnesses, Water Contact, and Roll-off Angles Measured on PES Fabrics Prepared from Water-Based PDMS Emulsions and Silica Particle Suspensions of Different Concentrationsa PDMS concentration (wt %)

silica particle concentration (wt %)

1 1.5 2 1 1.5 2 1 1.5 2 1 1.5 2 1b 1.5b 2b 1.5c 17d 0e

0.5

0.75

1

1.25

0

1 0 0

fiber rms roughness, σ (nm) 135 140 130 186 197 165 267 305 135 172 146 135 14 16 6 299 5 16

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

57 25 27 51 55 52 55 83 27 86 50 55 4 4 3 65 2 4

fiber Wenzel roughness, r 1.46 1.40 1.31 1.61 1.52 1.42 1.69 1.74 1.34 1.61 1.40 1.42 1.01 1.02 1.01 1.75 1.00 1.04

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.23 0.13 0.03 0.23 0.20 0.12 0.16 0.19 0.11 0.17 0.04 0.16 0.01 0.01 0.01 0.14 0.00 0.02

total Wenzel roughness, rt = 1.44 r

water contact angle (deg)

water roll-off angle (deg)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

144 ± 2.1 143 ± 2.4 130 ± 3.7 144 ± 1.3 144 ± 1.4 141 ± 2.8 145 ± 1.7 145 ± 0.5 140 ± 0.2 140 ± 0.6 142 ± 0.5 140 ± 0.6 129 ± 3.2 130 ± 3.3 129 ± 5.4 106 ± 9.0 117 ± 0.7 N/A

7−15 7−20 no 5−10 10−20 20−60 0−2 0−1.5 no 10−15 15−20 40−60 no no no no no N/A

2.10 2.01 1.88 2.31 2.18 2.04 2.43 2.50 1.92 2.31 2.01 2.04 1.45 1.46 1.45 2.52 1.00 1.50

0.32 0.19 0.04 0.33 0.28 0.17 0.23 0.27 0.16 0.24 0.06 0.23 0.01 0.01 0.01 0.21 0.00 0.03

Standard deviations of all the measured values are also indicated. bPDMS film without silica particle prepared by dip-coating on a PES fabric. Fabric modified with PDMS anchoring layer and silica particles deposited by dip-coating, but without the top PDMS layer. On this sample, the water droplet wetted the surface slowly, and therefore, the contact angle values decreased with contact time. The contact angle data shown in the table was taken ∼5 s after putting the water droplet on the sample surface. dFlat PDMS film without silica particle prepared by spin-coating on a silicon wafer; the thickness of this film measured by ellipsometry was 100 nm. eBare PES fabric. Water wetted and penetrated the bare fabric immediately after being put on the surface. Contact and roll-off angles could not be measured. a c

film of the pure PDMS deposited onto a flat silicon wafer, of the fabrics with silica-free PDMS coatings, and of the fabric with the anchoring PDMS layer and silica particles layer (without the top PDMS layer), are also given in Table 1. The poor water repellency (i.e., smaller water contact angles and no water roll-off on the surface) observed on these reference samples confirmed the necessity to deposit PDMS/silica particles/PDMS layers on the fabrics by the three-step dipping method proposed in this work in order to achieve the superhydrophobic properties. A more quantitative account of the water-repellency can be obtained based on Wenzel’s and Cassie−Baxter’s theories.33 For a moderate roughness, conditions for which the Wenzel (impregnation) regime applies, cos(θ) = rt cos(θflat), where θflat and θ are the contact angles on the flat reference sample and on a rough fabric, respectively; in this equation, rt is the total Wenzel roughness of the coated fabric, i.e., the ratio between developed and projected areas. When the roughness increases above a critical value rt* corresponding to a contact angle θ*, the surface enters the Cassie−Baxter “fakir” regime wherein the droplet only feels a fraction of the surface, air being trapped in cavities; at this stage, the contact angle remains essentially constant at the critical value θ* (although it varies slightly depending on the proportion of air-filled cavities felt by the droplet). For our coated fabrics, the Wenzel roughness originates from two different phenomena at markedly different spatial scales.34 First, the fabric texture generates a textural Wenzel roughness rf at the 10−500 μm lateral scale, which was measured by optical profilometry to be 1.44 (Figure 2a and Supporting Information SI6). Second, the coating adds a fiber roughness component r at the 50−1000 nm lateral scale, which can be measured by AFM as shown in Figure 2c, and varies according to the concentration of the PDMS and silica particle suspensions

(Table 1). If the coating is of negligible thickness compared to the amplitude of the fabric roughness, and if the lateral scale of the fiber coating roughness is well below the one of the fabric textural roughness, two conditions which are met by our samples, the total Wenzel roughness is simply rt = rf r, which expresses that the two types of roughness are independent stochastic phenomena. The so-computed total Wenzel roughness rt is reported in Table 1, and the contact angle is plotted versus rt in Figure 3a. A fit of the Wenzel equation cos(θ) = rt cos(θflat) to the data was performed in the range 1 ≤ rt ≤ rt*, taking as fit parameters the contact angle of the reference flat surface θflat and the critical Wenzel roughness rt*, with the data weighted according

Figure 3. (a) Variation in the water contact angle with the total Wenzel roughness (rt), resulting from the texture of the fabric and from the rough fiber coating. The continuous line is a fit to the Wenzel equation. (b) Roll-off angle of water on the coated fabrics. Stars indicate the absence of roll-off; vertical bars provide the range of angles over which the roll-off occurs. The dotted curves are drawn to support the eye. 15348


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ACS Applied Materials & Interfaces to the standard deviation. Above the critical roughness, in the Cassie−Baxter regime, the contact angle data was approximated as a constant equal to the Wenzel value at rt*. The experimental data are well-represented with θflat = 113.4 ± 0.1°, close to our own experimentally measured value of 117° and the ca. 113.5° reported for PDMS;35 the fitted critical Wenzel roughness rt* is 2.05 ± 0.01, corresponding to a critical contact angle θ* = 144.5°. The critical roughness and the contact angle on the flat surface are normally related by rt* = 1 + 0.25tan2θflat;33 using the rt* value obtained by our fitting procedure, a contact angle θflat = 116.0° is obtained, again in good agreement with the 113−117° expected from literature and our own experiments (Table 1), thereby fully confirming the consistency of the model. Above the critical roughness, the sample enters the Cassie− Baxter regime; the initial fraction of the sample in contact with the droplet is then33,36 ϕ* = (1 + cos θ*)/(1 + cos θflat) ≈ 0.3; hence, at the critical roughness, about 70% of the surface consists of entrapped air pockets. For higher total roughnesses, the fraction of entrapped air does not increase significantly, as testified by the limited variation of the contact angle in this regime (Figure 3a). Nevertheless, the superhydrophobicity is improved when the roughness increases further, as shown by the decrease of the water roll-off angle (Figure 3b), which reaches virtually zero for a total roughness of 2.5 (corresponding in the case of our fabric to a fiber roughness r of ca. 1.7). For the sake of comparison, a hydrocarbon material (i.e., a paraffin wax) was also tested as top layer instead of PDMS to provide the low surface tension. Therefore, a water-based paraffin wax emulsion (CONTRAQUA WE from Thor; 4 wt %) was deposited by dipping atop a PDMS (1.5 wt %)/silica particle (1 wt %) coating. The detailed preparation steps of this fabric sample are described in the Supporting Information SI3. The water contact angle measured on this sample was larger than 145° (Figure 2f) and the water roll-off angle was smaller than 5°. Figure 2e shows the typical AFM topography image of this sample with a rms fiber surface roughness of ca. 240 nm and a fiber Wenzel roughness of ca. 1.72. Movie S2 shows the water roll-off on this coated fabric. The results are comparable to those obtained for the PDMS/silica particle/PDMS coated sample of Figure 2b−d, indicating that the method developed in this work can be used with other water-based coating formulations of low surface tension. The resistance to machine washing of the superhydrophobic fabrics prepared from a 1.5 wt % PDMS emulsion and a 1 wt % silica suspension was also checked (see conditions in the Supporting Information SI5). The variation of the contact and roll-off angles with the number of washing cycles is shown in Figure 4. After four cycles of machine washing (i.e., washing with soap, rinsing, and spinning) programmed at 30 °C, followed by drying at room temperature, the coated PES fabric described in Figure 2b−d remained superhydrophobic with only a slight decrease of its water repellent properties (water contact angle of ca. 141° and water roll-off angles of 7−8.5°), as shown in Figure 4a, b. The reason for the good machine washing resistance of the original, simply annealed coating results from two factors. First, a strong electrostatic interaction exists between the terminal protonated amino groups of the PDMS chains and the negatively charged silica particles. This strong interaction was evidenced by the immediate precipitation which occurred when mixing the PDMS emulsion and the silica particle suspension. This is also the reason why a three-step dipping procedure was needed in this work instead

Figure 4. Variation in the (a) water contact angle and (b) roll-off angle versus the number of machine washing cycles, measured for simply annealed PDMS(1.5 wt %)/silica particle(1 wt %)/PDMS(1.5 wt %) coatings.

of a one-step dipping based on a suspension containing a mixture of PDMS and silica particles. As a result, silica particles act as physical cross-linkers between PDMS chains. The second factor are the possible reactions of the methoxysilyl end groups of the PDMS chains upon annealing, which lead to chain grafting onto the silica particles and chemical cross-linking. The existence of a cross-linked network after annealing was confirmed by the fact that tetrahydrofuran, although swelling the layer, did not dissolve it. In an attempt to improve further the durability of the superhydrophobic coating toward washing, the amine end groups of the PDMS chains in the coating were further crosslinked with an oxime-blocked bis-isocyanate (Phobol XAN, Huntsman; see cross-linking conditions in SI2). The supplementary cross-linking was visually confirmed by the fact that swelling by tetrahydrofuran decreased after Phobol XAN addition. Contact angle measurements performed on the sample with such a supplementary cross-linked coating showed very similar variations of the water repellent properties as on the original coating (Supporting Information SI8). For example, after four washing cycles, the cross-linked coating showed a water contact angle of ca. 140° and a water roll-off angle of 10−15°. The supplementary cross-linking of PDMS thus has no significant effect on the mechanical durability of the coated fabrics toward machine washing. The mechanical resistance of the simply annealed superhydrophobic PDMS(1.5 wt %)/silica particle(1 wt %)/PDMS(1.5 wt %) coating was also checked by performing peeling tests with an adhesive tape as previously reported in the literature (Supporting Information SI5).37 The variation of the contact and roll-off angles measured on the sample versus the number of tape peeling cycles is shown in Figure S2. After five cycles of tape peeling, the surface remained superhydrophobic with a slight decrease of the contact angle and a slight increase of roll-off angle, indicating the good mechanical resistance of the superhydrophobic coatings. In addition, the chemical stability of the same superhydrophobic coating was also tested in strongly acidic (pH 2, solution prepared by adding aqueous hydrochloric acid in water) and alkaline (pH 12, solution prepared by adding sodium hydroxide in water) conditions. After immersing the superhydrophobic fabrics into these solutions for 1 h, it was observed that the water contact angle and the roll-off angle were very similar to the values measured on the original samples before immersion. This again indicates a good chemical stability even in harsh aqueous 15349


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ACS Applied Materials & Interfaces conditions. Additional tests also evidenced that the coated fabrics (simply annealed) resist UV irradiation, even after 20 days of exposure (data not shown, conditions described in the Supporting Information SI5). The method that we developed can also be used to modify other substrates, provided they have a sufficiently large textural contribution to the roughness. As illustrative proof, we tested the application of the coating on a cellulose sponge, a very good water absorber in the absence of superhydrophobic coating. After the water-repellent treatment by water-based PDMS and silica suspensions (preparation steps are described in SI4), water droplets could stand on the sponge surface as shown in Figure 5. The inset picture in this figure shows a water droplet

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Detailed information on the preparation and characterization of superhydrophobic samples and extraction of the textural roughness (PDF) Movie S1, water roll-off on a superwater-repellent fabric prepared from water-based PDMS and silica particle suspensions (AVI) Movie S2, water roll-off performed on a superwaterrepellent fabric prepared from water-based PDMS, silica particle and paraffin wax suspensions (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Ronggang Cai: 0000-0001-9095-7823 Alain M. Jonas: 0000-0002-4083-0688 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Solvay and Wacker Chemie for kindly providing Tixosil 365 and Wacker HC 303, respectively. CONTRAQUA WE emulsion and Phobol XAN cross-linker were kindly provided by Thor and Huntsman, respectively. This work was financed by the Interreg V program FranceWallonia-Flanders (http://www.interreg-fwvl.eu/nl), a crossborder collaboration program with financial support of the European Fund for Regional Development, and cofinanced by the province West Flanders and the Walloon Region through the project Duratex. K.G. is Research Associate of the F.R.S.− FNRS.

Figure 5. Images of water droplets deposited on a scrubbing sponge samples (∼2 × 2 × 2 cm3) treated by a PDMS(1.5 wt %)/silica particle(0.75 wt %)/PDMS(1.5 wt %) coating. The inset on the top right shows a zoomed-in image of a water droplet (10 μL) deposited on the treated sponge surface.

on the treated sponge with a contact angle of ca. 140°. The true value was probably larger than 140° but could not be measured with more accuracy because the very rough surface of the sponge covered the bottom of the water droplet. In conclusion, we have developed a facile and versatile method to prepare superwater-repellent fabrics. The superwater-repellency is achieved by combining the fabric roughness with the one provided by silica particles, added to the low surface tension provided by a low surface energy material such as PDMS or paraffin wax. The role of each component of the roughness (textural roughness from the fabric, and fiber roughness from the silica particle coating) was deconvoluted, and a simple Wenzel/Cassie−Baxter model was able to quantitatively reproduce the experimental measurements. The method uses a simple dip-coating technique and commercially available water-based suspensions, and is fully environmentally friendly and cost-effective. The superhydrophobic coatings could resist at least four cycles of machine washing and were very stable even in harsh aqueous conditions. In addition, the good versatility of the method offers possibilities to produce superhydrophobic surfaces on different substrates. Therefore, it can be easily integrated into industrial processes and has a strong potential for the mass production of environmentally friendly and cost-effective superwater-repellent surfaces.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02707. 15350


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