Nonfluorinated Superhydrophobic Chemical Coatings on Polyester

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General Research

Non-fluorinated Superhydrophobic Chemical Coatings on Polyester Fabric Prepared with Kinetically-Controlled Hydrolyzed Methyltrimethoxysilane Haisheng Lin, Cornelia Rosu, Lu Jiang, Vikram A Sundar, Victor Breedveld, and Dennis W. Hess Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02471 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 28, 2019

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Industrial & Engineering Chemistry Research

Non-fluorinated Superhydrophobic Chemical Coatings on Polyester Fabric Prepared with Kinetically-Controlled Hydrolyzed Methyltrimethoxysilane Haisheng Lin,* Cornelia Rosu, Lu Jiang, Vikram A. Sundar, Victor Breedveld, and Dennis W. Hess School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332-0100 KEYWORDS: Methyltrimethoxysilane, Superhydrophobic, Water repellency, Hierarchical, Polyester fabric, Impact contact angle ABSTRACT: Non-fluorinated chemical coatings to generate durable water repellency have become increasingly important as a method of improving materials and product performance. Here we report a kineticallycontrolled fabrication of superhydrophobic polyester fabrics via one-step dip-coating in water-based solutions of fluorine-free hydrolyzed methyltrimethoxysilane (MTMS). The hydrophobicity of coated fabrics was tuned by varying the morphologies of surface-coated silica layers from smooth thin film to hierarchical structures. This was achieved by systematically altering MTMS solution pH and reaction time to control the reaction kinetics of hydrolysis and polymerization. These results offer a new strategy and approach towards fabrication of superhydrophobic coatings with tunable hierarchy. The mechanism of MTMS reaction with NH4OH catalyst was investigated with FTIR and Multi-angle dynamic light scattering (MADLS) for improved coating performance. The stability and durability of MTMS coating were carefully verified after washing and accelerated weathering tests through contact angle, spray test and XPS analysis.

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Impact contact angle (ICA) was invoked to quickly assess liquid droplet adhesion after surface impact; this test proffers a viable substitute for the standard AATCC 22-2005 spray test. The technology presented here can be extended to other organosilanes with one functional group and three hydrolysable methoxy substituents to allow a variety of surface functionalities and open a wide range of applications in the field of green surface chemistry modifications.

INTRODUCTION Inspired by the lotus leaf structure, the fabrication of superhydrophobic surfaces has attracted considerable attention in both scientific research and commercial arenas owing to the diverse applications of such surfaces in various fields including water-repellence, self-cleaning and oil-water separation.1-6 In particular, imparting super-water repellency to fiber-based substrates through chemical coatings has been investigated in the textile industry since the 1940s.7-9 As is well-known, such superhydrophobicity is attributed to a combination of properly sized micro-/nanostructures and low-surface energy materials adsorbed or bound to a substrate. Recently, significant efforts have been made towards fabrication of superhydrophobic textiles using nanoparticles (i.e. SiO2, ZnO and TiO2) in combination with low-surface-energy material coatings such as alkylsilanes.10-14 However, successful silane coatings for superhydrophobic textiles often involve toxic solvents (e.g. toluene),2, 8, 15 complex multi-step processes, and use of fluorinated chemistries,11, 16 which have been shown to persist in the environment with long elimination half-life in wildlife and humans, thereby raising toxicological concerns.17-20 Non-fluorinated chemical coatings for fabric finishes to achieve durable water repellency have risen a broad interest in both academic and industry recently, owing to ever-tightening environmental regulation.1722

Non-fluorinated organosilanes offer an economic and viable alternative to fluorinated products in the

next development of water repellents and surface protection.21-22 For example, methyltrimethoxysilane (MTMS), an organosilane with one methyl group and three hydrolysable methoxy substituents, has successfully been used to impart hydrophobicity and oleophobicity to various substrates such as glass,23


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wood,24-26 cotton27, steel28 and paper29-30. Interestingly, as previously reported, MTMS can be incorporated onto paper and steel mesh surfaces to establish tunable hydrophilicity30 and hydrophilicity/underwater superoleophobicity28, respectively, by controlling hydrolysis conditions. Furthermore, the MTMS hydrolysis and coating processes are typically in simple ambient aqueous conditions, which represents a clear advantage and would be readily compatible with existing large-scale textile finish manufacturing processes. However, the above present concentration of MTMS in those original coating solutions was extremely high (25%v/v), which required sonication to prevent gelation of MTMS and an ice bath to remove the large amount of heat generated by the exothermic hydrolysis reaction.30 Also, such coating process always resulted in thick, smooth coatings and high MTMS loadings on porous substrates (>10 wt%), which presents difficulties for super-water-repellence and manufacturing processes. Furthermore, the stability and durability of such MTMS coatings on fabrics have not been well investigated and understood. Challenges therefore remain to develop one-step MTMS coatings for super-water repellency that enable easy and efficient control of nanostructures imparted on a targeted surface to obtain stable, durable coatings at low loadings. In this study, we demonstrate a simple fabrication process for superhydrophobic polyester (PET) fabrics: one-step dip-coating in water-based solutions of fluorine-free hydrolyzed methyltrimethoxysilane (MTMS). The hydrophobicity of coated fabrics was tuned via the morphologies of surface-coated silica, which varied from smooth thin film to hierarchical structures. The morphology of the coated silica layer was systematically studied by kinetically controlling and optimizing water repellency by tuning the synergistic effects of pH and MTMS reaction time. This procedure allows the formation of hierarchical coating structures that exhibit stable, durable superhydrophobicity during abrasion and weathering tests. The combination of easy manufacturing and excellent performance makes functional MTMS-coated fabrics attractive for a number of practical applications. During our studies, a simple method of impact contact angle (ICA) measurement was developed for rapid assessment of liquid droplet adhesion after impact on a surface. This method provides useful insight into contact line hysteresis effects on porous substrates and



was shown to correlate well with results from more complex and time consuming standardized spray tests (e.g. AATCC 22-2005), thereby offering an effective and economical alternative.

EXPERIMENTAL SECTION Materials. Methyltrimethoxysilane (MTMS, purum, ≥98.0%) and hydrochloric acid (HCl, ACS reagent, 37%) were purchased from Sigma-Aldrich. Ammonium hydroxide solution (NH4OH, 28-30% NH3 basis) and pure anhydrous ethanol (200 proof USP, KOPTEC) were purchased from VWR. Pristine fabrics were Anticon 100® Heavyweight Series Cleanroom Wipers, Contec® (VWR). Wipers are made from 100% continuous filament polyester double-knit interlock fabric. All chemicals were used without further purification. Deionized water was used for experiments and tests. Preparation of MTMS-coating solutions. Acid catalyzed-MTMS coating solution: MTMS was mixed with 0.1 M hydrochloric acid (HCl) in a 1:50 v/v ratio at room temperature in a glass vial. The mixture was then magnetically stirred at the rate of ~500 rpm for different lengths of time (between 30 and 240 min) to induce MTMS hydrolysis and condensation. Base catalyzed-MTMS coating solution: In a typical preparation, ammonium hydroxide ((NH4OH,) was mixed with pure anhydrous ethanol (EtOH) in 1:1, 1:2.5 and 1:5 v/v ratios at room temperature in a glass vial and the solution was magnetically stirred for 30 min at the rate of ~500 rpm. MTMS was then added to the above solution (at a final ratio of MTMS:EtOH = 1:25 v/v) and the mixture further stirred for different lengths of time (between 30 and 240 min) to induce MTMS hydrolysis and condensation. Fabrication of superhydrophobic fabrics. In a typical fabrication procedure, a piece of PET fabric (small, 2×2 in., or large, 9 × 9 in.) was fully immersed into the 200 mL coating solution for 5 min. After dipcoating, the excess solution was allowed to drain from the fabric by hanging it and air-drying for at least 2h. Finally, the coated fabric samples were cured in an oven at 120 oC for 2h.


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Characterizations of the prepared MTMS-coated PET fabrics Measurement of static, impact and roll-off water contact angles. All contact angle measurements were performed with a ramé-hart automated goniometer (model 290). Contact angles were determined by standard software (Drop Image, version 2.6.1). For each sample, static contact angles (SCA) were measured by directly placing a 10μL DI-water drop onto the MTMS-coated fabric surface, and an average value (together with the standard deviation) was computed from eight tests performed at eight randomly-selected positions. Inspired by the standardized spray test (AATCC 22-2005) method for textile fabrics in outdoor applications, a new impact water contact angle (ICA) measurement was developed for quick assessment of water droplet adhesion after impact on the surface. Typically, the syringe of the goniometer was positioned with the needle tip 5 cm above the test fabric surface, so that a 10μL drop falling from the needle reaches the surface at an impact velocity of ~1 m/s. The roll-off angle (ROA) after impact was measured as the angle of inclination of the surface (tilt speed: 2 o/s) at which the water drop rolls off the substrate. Spray test. The standardized spray test AATCC 22-2005 was applied to measure the water-repellent efficacy of the MTMS-coated PET fabrics. In this test, the fabric (7×7 in.) is placed on a surface tilted at 45o and 250 ml of DI-water is sprayed from a 6 inch height onto the fabric surface at a prescribed rate (duration ~28 s) through a standardized spray head. Rating the outcome of the spray test (0-100) is accomplished by comparing the wetted pattern with pictures from a standardization chart.31 Tape peeling test: 3M Scotch single-sided tape (giftwrapping; tensile strength of 15 lbs./in. width; synthetic acrylic adhesive with a polyolefin backing; adhesion to steel 15 oz./in. width) was pressed against the MTMS coated PET surfaces firmly by hand to ensure that no visually detectable air gaps existed between the surfaces and kept in contact with the surface for at least 1 min. Then the tape was detached beginning from one end of the PET fabric using tweezers. The contact angle of the sample was systematically measured after a given number of peeling cycles. Falling sand abrasion test: In a typical process, one piece of MTMS coated PET fabric (size: 3×3 in.) was firmly attached on one flat surface that was tilted at 45o. Then the surface was exposed to a jet of ~30 g of



sand particles (~80-120 μm diameter glass beads from McMaster-Carr) that flowed out of a plastic funnel held 40 cm above the substrate for approximately 40 s. Washing test: The washing tests were performed using a whirlpool washing machine with QUICK wash. The washing program includes soaking, washing, rinsing and spinning at room temperature using a minimum water quantity (~50 L) programed by the machine. A 7’x7’ sample of as-prepared MTMS coated PET fabric was used and ~5 ml detergent (Tide Original) was added during the soaking period. Accelerated weathering test: Natural Florida weathering testing was carried out following standard SAE J2527 test protocol in a Black Standard Weatherometer at 500, 1000 and 1000kJ. This SAE Standard (SAE J2527-2004) specifies the operating procedures for controlled irradiance with a Xenon arc lamp apparatus for accelerated exposure of various exterior automotive materials under controlled temperature and humidity cycling. This test method is designed to simulate extreme outdoor environmental conditions such as sunlight, heat, and moisture (in the form of humidity, condensation, or rain) for the purpose of predicting the weathering performance of outdoor textiles. A 7’x7’ sample of MTMS coated PET fabric was used to examine the durability of superhydrophobic MTMS coating under these accelerated weathering conditions. SEM imaging: The surface morphologies of all fabric samples were characterized using Field-Emission Scanning Electron Microscopy (Zeiss Ultra 60, Carl Zeiss SMT, Ltd., Thornwood, NY), at an accelerating potential of 10.0 kV. Before imaging, all samples were coated with a thin layer of gold/palladium (Hummer IV Sputtering System) to prevent sample charging, and then mounted on metal stubs using carbon tape. XPS analysis: X-ray photoelectron spectroscopy (XPS) analyses were conducted using a Thermo Electron Corporation K-Alpha XPS system with a micro-focused monochromatic Al Kα X-ray source. The spot size of the instrument is 400 μm. FTIR analysis: Fourier-Transform Infrared measurements (FTIR) were performed using a Nicolet iS50 FT-IR spectrometer from Thermo Scientific LLC. The instrument is equipped with a DLaTGS Detector that has a KBr window and a KBr beam splitter. The specific accessory was Smart-iTR (ATR) with a diamond window. Fabric samples were placed directly on the diamond window and, for each measurement


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including the background, 32 scans were acquired at a resolution of 8 cm-1. Omnic software was used to process the data that were baseline and atmospheric suppression corrected. MADLS Analysis: Multi-angle dynamic light scattering experiments (MADLS) were carried out on a custom-built apparatus equipped with an ALV-5000 digital auto correlator and associated software. The scattering volume was illuminated by a focused laser beam (λ0 = 640 nm). The instrument has a classical design with two pinholes and lenses for homodyne detection. Measurements were performed at different scattering angles, θ, from 30º to 120º. The temperature of the toluene bath surrounding the sample cell to suppress reflected light was maintained at 25 °C using a circulating bath. Samples were prepared in precleaned and dust-free vials (Pyrex, 13×100 mm from Fisher) as follows: first, approximately 1.5 mL of anhydrous ethanol was filtered (Whatman PTFE filter, 0.1 μm pore size). Subsequently, a volume of ~ 50 μL of MTMS: NH4OH: EtOH, 1:25:25 dispersion was added and the vials were capped. Aliquots of samples were investigated at 15, 30, 45, 90 and 120 min reaction times. A two-exponential function was used to fit the experimental data. The decay time associated with the correlation function of the fast mode was converted to the decay rate of the electric field, autocorrelation function g(1)(t), Γ. Then, the apparent diffusion coefficient was computed from Dapp= Γ/q2, where q is the scattering vector magnitude, expressed as q= 4π⋅n⋅sin(θ/2)/λ0, where n is the solvent refractive index, θ the scattering angle and λ0 the laser wavelength in vacuo. The Stokes-Einstein equation, Dapp = kBT/(6π⋅ηo⋅Rh,app), was used to calculate the apparent hydrodynamic radius, Rh,app. In this equation, kB is Boltzmann’s constant, T is the absolute temperature at which the experiments were carried out and ηo is the solvent viscosity. Rh,app was then extrapolated to q = 0. The particle uniformity was expressed by the dimensionless ratio of the second cumulant to the square of the average decay rate, μ2/߁ത2.



RESULTS AND DISCUSSION Preparation of superhydrophobic fabrics with thin film MTMS coatings A typical preparation process of methyltrimethoxysilane (MTMS) coating solution involves hydrolysis and condensation before reacting with hydroxyl groups present on the substrate surface. It has been found that the rates of both hydrolysis and condensation are influenced by changing pH levels of the media for both acid- and base-catalyzed reactions.32 However, the optimum pH for either hydrolysis or condensation has not been well understood nor reported for coating applications in acid- and base-catalyzed reactions. Detailed investigation and understanding of the mechanisms of acid- and base-catalyzed hydrolysis of organosilanes is critical for optimizing coating structures for a wide variety of surface modification applications. Therefore, in this work, we studied the correct balance between hydrolysis and condensation processes to prepare the most appropriate MTMS coating solution for the best coating performance (i.e. water repellence, stability and durability) with both HCl (pH ~1.2) and NH4OH-(pH ~8.9-10.8) catalysts in aqueous solution. PET fabric used in this work is made from 100% continuous filament polyester double-knit interlock fabric, which is very light-weight and highly hydroscopic (super-easily absorbing water). The wetting behaviors of as-prepared MTMS-coated PET fabrics were characterized through both static water contact angle (SCA) and impact water contact angle (ICA) to evaluate water repellency and resistance. As shown in Figure 1 and S1, all MTMS-coated PET fabrics exhibited similar SCAs (blue bars) in the range 140-155o for both HCl (pH ~1.2) and NH4OH-(pH ~8.9-9.6) catalysts with reaction times between 30 and 240 min. No obvious difference between the SCAs (152±3o) for all HCl-catalyzed MTMS-coated PET was observed with varied reaction time, while the SCAs of NH4OH catalyzed MTMS coated PET were affected slightly by the MTMS reaction time. For pH ~8.9 and 9.6 coating mediums (Figure S1 and Figure 1b, respectively), the SCAs increased from 130±4o to 148±4o, and from 140±3o to 150±3o, respectively, for MTMS hydrolyzed for 30 and 240 min.


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For the textile industry, SCA is not an effective method to reliably evaluate the efficacy of water repellency and resistance of a chemically finished fabric. Additional testing is needed, but standard wetting tests such as contact angle hysteresis and roll-off angle measurements, that are appropriate for flat substrates, are more difficult to implement and interpret for porous substrates like fabrics. Therefore, specialized methods, such as standardized spray tests (AATCC 22-2005) with water spraying from 6 in height onto the fabric surface, are used to assess performance. However, this spray test not only requires a standard AATCC spray tester, but also large fabric samples (at least 7×7 in.). Thus, inspired by the spray test (AATCC 22-2005) method, an impact contact angle (ICA) test was developed in this work for quick assessment of liquid droplet adhesion after water droplet impact on the surface. The difference between ICAs and SCAs for each surface was obviously shown in Figure 1 and S1, where all ICAs (red bars) were significantly lower than corresponding SCAs, and the ICAs also distinctly expressed the fabric surface wettability with the effect of reaction time of coating solution. Therefore, ICAs is applied as a simple but more accurate parameter to characterize the interaction between liquid drops and a targeted surface. When a surface displayed a poor repellency to impact water drop, its ICA would be much lower than SCA. However, similar values between ICA and SCA (~150o) suggested excellent water-repellency of a tested surface. For HCl-catalyzed MTMS-coated PET, the ICA values were around 125-130o; where the ICAs illustrated the obvious effect of MTMS reaction times on the water-repellent efficacy and resistance of NH4OH-catalyzed MTMS-coated PET. For pH ~8.9 and 9.6 (Figure S1 and 1b), the ICAs increased from 80±3o to 120±4o, and from 90±3o to 130±4o, respectively, for MTMS reaction times between 30 and 240 min. On the other hand, the standard spray test (AATCC 22-2005) showed that the ratings for all MTMScoated PET fabrics were only 70 (ISO 2, partial wetting of coated fabric beyond the spray points), showing very little distinction between the samples. The detailed correlation between ICA and AATCC 22-2005 test was discussed below. To explain the difference in wetting behaviors of MTMS coated PET fabrics and understand the underlying mechanism, the surface morphologies were analyzed by Scanning Electron Microscopy (SEM).



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As shown in Figure S2b, the fiber surfaces of pristine PET fabrics were smooth with only a small amount of nano-scale debris and fiber diameters of 12±2 μm. After coating with the HCl-catalysed MTMS solutions, the observed fiber diameter of PET fabrics did not visibly change and the surface became smooth without any distinct nanoscale roughness (Figure 2), indicating a thin, smooth film of MTMS on PET fibers. Furthermore, there were no obvious differences between surface morphologies of any HCl-catalysed MTMS-coated PET fabrics with varied reaction times (30-240 min). This observation was consistent with the above results for SCAs and ICAs, suggesting that the reaction time of HCl-catalysed MTMS solution did not significantly influence the coating results. In contrast, the base-catalyzed MTMS solutions were dramatically affected by reaction time. As shown in Figures 3 and S3, with increasing reaction times, the modified PET fiber surfaces became rougher, comprising a hierarchical structure with nano-particles embedded in a thin film. The appearance of hierarchical structures varied with reaction time, which explains the increase of SCAs and ICAs with increased reaction times. To our knowledge, this is the first time that the wetting properties of silane coated fabrics have been carefully controlled and tuned through reaction time in media with different pH to achieve hierarchical structures. A more detailed study on the successful preparation of superhydrophobic MTMS coated PET fabrics and underlying mechanism are presented and discussed in the remainder of this paper.

Preparation of superhydrophobic fabrics with hierarchical MTMS coatings The Stöber method has been widely used for controlled growth of monodisperse silica spheres through the hydrolysis of tetra-alkyl-silicates in ammonium-alcohol solutions.6,

33-35

The silane hydrolysis and

condensation process provides controlled growth of MTMS polymerization products and nanoparticles similar to its tetraorthosilicate counterpart. The composition of the reaction mixture can be tuned by both solution pH and reaction times to achieve the optimum surface morphology desired for specific coating properties. As shown above, in the two ethanol-ammonium solutions (NH4OH : EtOH =1 : 5 and 2

， pH ~8.9;

：5, pH ~9.6) , the MTMS gradually hydrolysed and poly-condensed in a sol-gel system with varied


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nanoparticle sizes and concentrations. Although super-water-repellent MTMS coated PET surfaces were not achieved in these two cases, the trend towards hierarchical MTMS coating structure and resulting beneficial wetting properties was clearly illustrated with increasing solution pH. Generally, adding more ammonium hydroxide to raise the pH of the final solution increases the MTMS condensation rate to form more and larger nanoparticles.32 Subsequently, a third ethanol-ammonium solution was prepared with the ratio of NH4OH : EtOH = 1 : 1 (pH ~10.8) and the pristine PET fabrics were then immersed in the MTMS coating solutions with varied reaction times to obtain superhydrophobic surfaces. As shown in Figures 4 and S4, initially hydrophilic PET fabrics were successfully converted to super-water-repellent after MTMS coating with reaction times between 90 and 180 min, with SCAs and ICAs of ~155o and ~150o, respectively. The MTMS-coated PET fabrics exhibited the same bright white appearance and physical softness as the pristine PET with MTMS loadings ~5 wt% (Table 1). Importantly, water drops impacting these MTMS coated PET surfaces rebounded from the surface and easily rolled off without residual water at tilt angles of ~30o (Figure S4 and video in supporting information). The surface morphologies of MTMS-coated PET fabrics in ethanol-ammonium hydroxide solution (NH4OH : EtOH =1 :1) are shown in Figure 5. At short reaction time (< 1h) of MTMS in the presence of the NH4OH catalyst, the MTMS coating on PET showed a smooth thin film with few nanoparticles (20150 nm) (Figure 5a). This structure was similar to those shown above for HCl-catalyzed and low pH ammonium-catalyzed MTMS coatings on PET; similarly, their ICAs were relatively low (130±4o) and the roll-off angle of all the impact water drops were >85o. Successful hierarchical MTMS coatings on PET surfaces were obtained for reaction times of 90-180 min; under these conditions, the coated PET fiber surfaces displayed a hierarchical pore structure comprising dense nanoparticles (20-100 nm) and crosslinked MTMS-based porous structures (Figure 5b and 5c). These hierarchical pore-containing structures assist surface resistance to water drop penetration into the PET fabric, especially under impact conditions, thus resulting in low adhesion. With even longer reaction times (>3h), the coating solution exhibits partial gelation products and, as shown in Figure 5d, the MTMS polymerization product formed large deposits on



PET surfaces from a non-uniform dispersion, although there is still an MTMS thin film present on the fibers. Accordingly, the ICAs were 135±3o, which are lower than the SCAs (152±3o) as well as those evaluated at intermediate reaction times (90-180 min). In addition, for textiles, reasonable stability of coating solutions is a supplementary requirement for storage, transportation and coating process in the design and fabrication of green coatings. As described above, successful hierarchical MTMS coatings on PET surfaces were obtained for reaction times of 90-180 min, which indicated the best time window of coating was only in 1-2 hour (Figure 4 and 4S). In order to prolong the best time window and further storage of coating solution, it is found that the coating solutions must be stored at below 2 oC refrigerated conditions after 90 min MTMS reaction time in above ethanolammonium hydroxide solution (NH4OH : EtOH =1 :1), where the MTMS reaction may be very slow or stopped. The very similar coating performance was observed for the same coating solution stored up to 2 days at below 2 oC refrigerated conditions. The chemical composition of the PET surfaces analyzed via XPS is shown in Figure 6 and atomic compositions are listed in Table 1. As expected, the surface of pristine PET fabric showed only C 1s and O 1s signals, while the surfaces of MTMS-coated PET were dominated by new signals associated with Si 2s (154.1 eV) and Si 2p (102.8 eV). This demonstrates that MTMS was successfully incorporated onto the surface of PET fibers. Figure 6b shows C 1s spectra of pristine PET fibers with three main peak components of binding energies at ~284.5, 286.3 and 288.8 eV, attributable to the C-H/C=C, C-O-C, and O-C=O bonds, respectively. The MTMS coating thickness and homogeneity can be estimated based on the XPS signal intensities of the C 1s peaks. For PET fibers coated after MTMS reacting times 180 min), the MTMS coating on PET fibers became heterogeneous with a strong O-C=O signal at ~288.8 eV binding


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energy, suggesting that the coating thickness on at least part of the fiber surface was less than 5 nm. This is consistent with the morphologies shown in Figure 5. The hypothesis of heterogeneous coatings for reaction times >180 min was also supported by a slight decrease of Si atomic composition with longer reaction times. However, it is interesting that the overall loading of each MTMS-coated PET fabric was essentially the same at ~5wt% (Table 1), suggesting that similar amounts of MTMS hydrolysis and condensation products coated the PET fabric surfaces at varied reaction times. On the other hand, the chemistry and structures of these MTMS hydrolysis and condensation products were kinetically controlled by the reaction conditions (i.e. pH and time), which could be readily used to tune the wetting properties and coating performance.

Correlation of custom impact angle and standard AATCC 22-2005 spray testing As demonstrated above, the standard AATCC spray test method (22-2005) is commonly applied to assess the water-repellent efficacy of coatings for textile fabrics with a water-repellent finish. Evaluation of this

-

spray test rating (0 100) is accomplished by comparing the wetted pattern with pictures from a standardization chart.31 Here, we first introduce the impact contact angle (ICA) method for simple, accurate and quick assessment of liquid droplet adhesion after impact on the surface. The syringe from a standard contact angle goniometer was positioned with the needle tip 5 cm above the test fabric surface, which means that a 10 μL drop falling from the needle reaches the surface at an impact velocity of ~1 m/s, which has previously been used as an efficient impact velocity for testing the droplet interaction with a superhydrophobic surface.36 The correlation between contact angle values acquired during custom impact conditions and ratings of standard AATCC 22-2005 spray testing appears in Table 2. The results of AATCC 22-2005 spray test for pristine PET and MTMS coated PET fabrics are shown in Figure S5, as fully wetting (0 rating) and no wetting (100 rating), respectively. For a ‘hydrophilic’ surface (i.e. pristine PET, unstable SCA = 110 ± 3o, water drops were fully absorbed within a few seconds), the ICA was 0 and spray test rating 0, indicating



full absorption of water and completely surface wetting. Hydrophobic surfaces with an ICA around 90-130o always showed wetting across the entire fabric surface after spray test, but not complete wet-out, and spray test ratings were 50. When the ICA increased to around 130-150o and ROA was higher than 30o, the fabric surface only displayed wetting at the spray points and partial wetting beyond the spray points, which is described by rating 70-80. The superhydrophobic surface of a textile fabric with 90-100 spray test rating required an ICA higher than 150o and ROA lower than 30o. These results indicated that the simple and quick ICA measurement correlates well with and can be used as substitute for the more complicated and time-consuming standardized AATCC 22-2005 spray test. Furthermore, the ICA measurement are not only limited to water drops, but can easily be adopted to other testing liquids, such as oils, while the AATCC 22-2005 spray test can be only applied for water repellency measurement.

Mechanism of MTMS hydrolysis and condensation for improved coating performance In order to understand the mechanism of MTMS hydrolysis and condensation process and resulting coating performance, Fourier-transform infrared spectroscopy (FTIR) was used to decipher the mechanisms behind the MTMS-based coating formation. Figure 7 and Figure S6 (Supporting Information) display the characteristic vibration signals associated with species resulting from hydrolysis and condensation of MTMS and identified in dry 1:25:25 formulation at different reaction times. As seen in Figure 7a, the FTIR traces were marked by the presence of a broad absorption band with peaks centered at 1095 cm-1 and 1020 cm-1. These absorptions arise from asymmetric Si-O-Si vibrations, typically assigned to insoluble silica derivatives.37 The asymmetric Si-OH vibration was detected at 903 cm-1. The C-H vibration band was positioned at 1270 cm-1, identical to pure MTMS. The MTMS characteristic Si-C (1189 cm-1) and Si-O (1077 cm-1) absorption peaks disappeared while Si-O-C (836 cm-1) stretching band significantly decreased in intensity and shifted upfield to 856 cm-1 in the 1:25:25 formulation. The presence of small traces of the latter signal in the condensation products suggests that the resulting silica species may have a few active


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sites that can further undergo condensation under favorable reaction conditions (e.g. presence of NH4OH catalyst).38 The gelation tendency of the mixture, especially at very early and after long reaction times (15 min, 90 min, 120 min) and upon resting (see Figure S7 Supporting Information) supports this assumption. This behavior is not characteristic of silica produced from the pure tetraethyl orthosilicate homologue. The absorption bands associated with free hydroxyl (OH) groups and H-bonding were apparent in the 4000 – 3000 cm-1 range, as discerned in Figure 7b. At early reaction times (15 min) the stretching of OH in Hbonding was more pronounced (~3400 cm-1) than that of free OH (~3600 cm-1) which became dominant at intermediate times (45 min). Between 60 – 120 min, the absorption band associated with H-bonding (3600 cm-1) was the main spectral feature. These observations suggest that the reacting species have terminal silanol groups (Si-OH) as a result of MTMS hydrolysis and condensation and, more importantly, are highly interactive. Drying effects are not ruled out: they may accentuate these interactions. Multi-angle dynamic light scattering (MADLS) was used to investigate the size and polydispersity of the reaction mixture species. Autocorrelation functions extracted from the scattering light intensity showed a bimodal (fast and slow decay) behavior and were fitted to a two-exponential function.39 Figure 8a shows the apparent hydrodynamic radius, Rh, app extrapolated to zero scattering vector magnitude, q, and plotted as a function of reaction time. These values are associated with the fast decay mode. The slow mode behavior of the correlation function did not match accurately to the fitting, likely due to the low number of large particles/aggregates in the scattering volume that cause severe fluctuations in the scattered intensity and masks those arising from Brownian motion.40 Contrary to what was expected, the Rh, app values decreased with an increase in reaction time. At the same time, the polydispersity index, taken as the ratio between the second-order cumulant and squared decay rate, µ 2∙Γത-2, did not change significantly from 0.35 ± 0.02 (30 min) to 0.37 ± 0.04 (45 min) and remained constant at longer reaction times. These values indicate that the system is highly polydisperse; a polydispersity index up to 0.08 is typically assigned to nearly monodisperse scattering objects. The trends in size and polydispersity were



confirmed by SEM analysis. At the reaction time of 30 min, the dispersion of 1:25:25 formulation consisted of clearly shaped spherical particles ranging from a few hundred nanometers to approximately three microns (Figure 8b). Significantly smaller and non-spherically shaped particulates were visible at long reaction times (120 min, Figure 8c). Large aggregates and coalesced particles were also identified. Very early stages of the reaction (90 28 ± 3 30 ± 4 40 ± 4 >90

Spray Rating 0 50 100 100 70 50

Table 3. The SCA, ICA, ROA and standardized spray test rating of NH4OH-catalyzed MTMS coated PET fabrics at 120 min reaction times (MTMS : NH4OH : EtOH = 1 : 25 :25 ) before and after mechanical tests. Samples

SCA (o)

ICA (o)

ROA (o)

Spray Rating

Control sample

154 ± 3

152 ± 3

30 ± 2

100

After tape peeling (5 cycles) After falling sand test (5 cycles) After laundry test (5 cycle) After accelerated weathering test

148 ± 3

144 ± 2

39 ± 4

90

150 ± 4

149 ± 4

33 ± 2

100

149 ± 3

147 ± 2

35 ± 3

90

148 ± 4

130 ± 4

>90

50


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Figure 1. Contact angles of MTMS coated PET fabrics at varied reaction times: (a) acid catalyzed, MTMS: HCl (0.1 M) = 1:50, pH = 1.2 ± 0.3; (b) base catalyzed, MTMS : NH4OH : EtOH = 1 : 10 :25, pH = 9.6 ± 0.3. SCA = static contact angle, ICA = impact contact angle.

Figure 2. SEM images of HCl-catalyzed MTMS coated PET fabrics at varied reaction times: (a) 30 min, (b) 120 min, (c) 180 min, and (d) 240 min. The white scale bar = 10 μm.



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Figure 3. SEM images of NH4OH-catalyzed MTMS coated PET at varied reaction times (MTMS : NH4OH : EtOH = 1 : 10 :25): (a) 120 min, (c) 180 min, and (d) 240 min. The white scale bar = 20 μm.

Figure 4. Static and impact contact angles (SCA and ICA), and roll-off angles (ROA) for NH4OHcatalyzed MTMS coated PET fabrics (MTMS: NH4OH : EtOH = 1 : 25 :25, pH = 10.8 ± 0.4) at varied reaction times. 26


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Figure 5. SEM images of NH4OH-catalyzed MTMS coated PET fabrics at varied reaction times (MTMS : NH4OH : EtOH = 1 : 25 :25 ): (a) 30 min, (b) 120 min, (c) 180 min, and (d) 240 min. The white scale bar = 10 μm. The inset images are at high magnification with the black scale bar = 4 μm.

Figure 6. XPS survey spectra (a) and high-resolution C 1s spectra (b) for NH4OH-catalyzed MTMS coated PET fabrics at varied reaction times (MTMS: NH4OH: EtOH = 1 : 25 :25)

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Figure 7. FTIR traces of dried 1:25:25 formulation at different reaction times and MTMS control plotted in the 1400 – 800 cm-1 (a) and 4000 – 3000 cm-1 (b) wavenumber ranges.

Figure 8. Apparent hydrodynamic radius, Rh, app and polydispersity index, µ 2 ∙ Γ-2, as a function of reaction time (a), SEM images of dried 1:25:25 formulation at 30 min (b) and 120 min (c).

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Figure 9. Variation of contact angles (SCA and ICA) and roll-off angles (ROA) as a function of the number of adhesive tape peeling cycles (a) and the number of machine washing cycles (b) performed on a superhydrophobic PET fabric coated with NH4OH-catalyzed hydrolyzed MTMS (MTMS: NH4OH : EtOH = 1 : 25 :25, reaction time 120 min). 29



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Graphical Abstract: Stable and durable superhydrophobic polyester fabrics with hierarchical silica coating were fabricated via one-step dip-coating in water-based solutions of fluorine-free hydrolyzed methyltrimethoxysilane (MTMS) with a facile kinetically-controlled method.

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Nonfluorinated Superhydrophobic Chemical Coatings on Polyester

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