Article pubs.acs.org/Langmuir
Fabrication of Liquid and Vapor Protective Cotton Fabrics Derek D. Lovingood,*,† W. Bruce Salter,‡ Kara R. Griffith,‡ Katherine M. Simpson,‡ John D. Hearn,§ and Jeffery R. Owens§ †
Oak Ridge Institute for Science and Education, 4692 Millennium Drive, Ste 101, Belcamp, Maryland 21017, United States Universal Technology Corporation, 1270 North Fairfield Rd., Dayton, Ohio 45432, United States § Air Force Research Laboratory, Airbase Technology Division, 139 Barnes Drive, Suite #2, Tyndall AFB, Florida 32403, United States ‡
S Supporting Information *
ABSTRACT: Through microwave-assisted techniques, cotton textiles treated with heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane in the presence of high surface area silica nanoparticles create a material capable of repelling bulk liquid challenges while simultaneously adsorbing organic vapors from bulk liquid droplets. Characterizing the contradictory behavior of adsorption of vapors and repellency of liquids is the primary focus of this article. These procedures reveal a quick and simple method for a one-step deposition of a vapor-sorptive, liquid-repellent, Cassie−Baxter surface onto textiles. Packed column breakthrough and single swatch permeation experiments showed that treated materials possess a high affinity for 3-hepten-2-one vapor, while goniometry revealed contact angles in excess of 120° for surface-deposited, 5 μL droplets of several test liquids. Scanning electron micrograph images confirm a lotus-like, nanorough surface, while ATR-FTIR spectra confirm surface fluorocarbon moieties. The performance of so-treated materials lends itself to the application of chemical protective apparel, while the simplicity of the treatment bodes well for potential commercialization.
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INTRODUCTION The equations that describe and predict both how a liquid will interact with a surface and how a vapor will be adsorbed onto a surface are well established and understood. In the case of bulk liquids, the Young equation1 is used to describe how liquid droplets will behave on a smooth surface, while the Cassie− Baxter2 and the Wenzel3 equations are used to describe how droplets will behave on a roughened surface. Vapor adsorption is typically described in its most simple form using the Langmuir equation4 for monolayer adsorption and Brunauer− Emmett−Teller (BET) theory5 for describing adsorption on solid surfaces and pore filling. Through nanostructuring and control of surface energy, materials scientists have demonstrated the ability to maintain extremely high contact angles even with low surface tension liquids.6−16 The ability to repel a wide range of liquids with vastly different surface tensions has led researchers to dub such materials as “omniphobic”. The lucrative commercial market of protective textiles has propelled omniphobic textiles to cost-competitive commercialization for their use as lightweight, high-comfort protective textiles. Fabrics treated with heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane (FS) demonstrate superhydro/oleophobicity against test liquids; i.e., droplets of water have contact angles greater than 150° and n-hexadecane droplets have contact angles greater than 130° on treated textile compositions of nylon, cotton, 50:50 nylon:cotton, and both meta and para aramids.17−19 However, testing of these materials against droplets of moderately volatile, highly potent poisons such as O-isopropyl methylphosphonofluoridate (sarin) still showed very high vapor breakthrough immediately beneath the droplet. In retrospect, this observation is blatantly apparent had one © 2013 American Chemical Society
considered the effect of a bulk liquid droplet on a nanorough surface with respect to the increased pressure inside the droplet caused by capillary forces immediately under the droplet, as is described in the Young−Laplace equation. Previous studies show that silica nanoparticles (SiO2 NPs) as well as other NPs effectively adsorb carbon dioxide, acetone, dimethyl methylphosphonate (DMMP), and other chemical vapors.20−25 SiO2 NPs possessing a high surface area and porosity are an ideal substrate for vapor/gas mitigation via adsorption onto the abundant hydroxyl groups found on the surface of the particles. Organophosphorus esters, such as DMMP, readily adsorb onto surfaces of SiO2, TiO2, and Al2O3 through hydrogen bonding of the phosphonyl group to the hydroxyl groups.21,26 Recently, microwave synthetic techniques were used to prepare high porosity, high surface area, monodisperse SiO2 NPs in sizes ranging from 40 to 200 nm.27 SiO2 NPs synthesized by these microwave techniques yield monodisperse SiO2 NPs suspended in acetone. These solutions are ideal for preparation of treated textiles because particle aggregation is minimal, the solutions are stable for extended periods of time, and even coatings are produced. To our knowledge, there are no reported methods for creating a breathable single layer textile that repels bulk liquid droplets and mitigates transport of organic vapors in one treatment. While omniphobic treatments for cotton and other textiles are not new, those materials typically repel only liquids and allow gas-phase molecules to freely permeate. The work Received: August 23, 2013 Revised: October 25, 2013 Published: November 12, 2013 15043
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measured before and after the treatment to determine the amount of coating deposited on the fabric (Table 2).
described herein overcomes the problem of vapor migration by incorporating high surface area adsorbents (SiO2 NPs) into the treatment, while FS provides repellency against bulk liquids. The incorporation of SiO2 NPs increase surface area, which is expected to promote adsorption of gas-phase molecules, and increase surface roughness, which is also expected to increase liquid repellency. Microwave-assisted techniques are employed to treat cotton textiles with FS and SiO2 NPs, creating encapsulated cotton fibers where the silica matrix serves as the framework necessary for binding SiO2 NPs and FS. The rate of acid-catalyzed condensation of the siloxane network onto the cotton fibrils is mediated by acetone, which stabilizes the siloxane cross-linking agents until microwave energy is applied to the system.28 Irradiation of the reaction solution with microwaves causes cross-linking of the siloxane network prior to evaporation of the solvent, thus ensuring uniform coating of the textile fibers and simultaneous entrainment and crosslinking of the SiO2 NPs. Cotton swatches were tested for vapor adsorption against the physical simulant for sarin, 3-hepten-2one (hereafter referred to as heptenone), by packed column and by single-layer permeation breakthrough measurements. Cotton swatches were also tested by goniometry for repellency against water, ethylene glycol, diiodomethane, n-hexadecane, DMMP, and heptenone.
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Table 2. Summary of Sample Mass, Breakthrough Time, Calculated Mass of Heptenone, and Adsorptive Capacity for Treated Cotton Swatches from Packed Column MS Experiments
EXPERIMENTAL SECTION
Table 1. Description of Prepared Treated Cotton Swatches TMOS
2% TMOS, 0.1 mM HCl dissolved in acetone
NP
150 nm SiO2 NPs, 2% TMOS, 0.1 mM HCl dissolved in acetone 4% FS, 2% TMOS, 10 mM HCl dissolved in acetone
FS NPFS a
description of reaction solutiona
150 nm SiO2 NPs, 4% FS, 2% TMOS, 10 mM HCl dissolved in acetone
treated cotton mass (mg)
heptenone breakthrough time (min)
calculated heptenone mass (μg)
μg of heptenone/ mg of fabric
cotton NP FS NPFS
203 193 205 210
203 212 259 263
0.3 66 42 49
6 647 496 661
0.03 3.05 1.92 2.50
Packed Column. Breakthrough curves of heptenone were measured on packed columns of treated swatches using a modified commercial gas chromatograph−mass spectrometer (GC-MS, Trace GC Ultra and Trace DSQ, Thermo Fisher). Treated swatches were degassed in a vacuum (150° for water) and highly oleophobic (>130° for n-hexadecane) properties. The NPFS swatch was monitored for changes in repellency by comparing the contact angles of the initial droplets and droplets after 1 h of exposure to water and to n-hexadecane. The initial average contact angle for water on the treated swatch was 157 ± 2.0°, and the average contact angle at the 1 h time point was 147 ± 5°. For the n-hexadecane challenge, the initial average contact angle was 137 ± 3° and the 1 h contact angle was 135 ± 5°. Surface Analysis of Treated Fabrics. Figure 1 shows surface analyses of the treated cotton swatches performed by
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Figure 1. Surface analysis of treated cotton fabrics by FTIR shows the presence of FS bound to treated cotton.
RESULTS Liquid Repellency. To quantitatively determine the hydro/ oleophobicity of treated cotton swatches, 5 μL droplets of water, ethylene glycol, diiodomethane, n-hexadecane, DMMP, and heptenone were deposited on the textiles and each liquid’s contact angle was measured. Contact angle measurements indicated that the incorporation of SiO2 NPs into the silica matrix has no negative impact on bulk liquid repellency. In fact, for some liquids an increase in repellency was observed. The average measured contact angles for the liquid droplet challenges as well as CCD images of the droplets on the substrate are in the Supporting Information (Figure S1 and Table S1). The untreated cotton and TMOS control swatches demonstrated no repellency, immediately absorbing all challenges. NP swatches showed repellency against water, ethylene glycol, and diiodomethane but showed no repellency to n-hexadecane, DMMP, or heptenone. This observed
FTIR. Neat FS was measured as a control and exhibits three distinct stretches at 1090 cm−1 assigned to Si−O−CH3 and 1150 and 1200 cm−1 assigned to −CF3 and C−F. The untreated cotton swatch and NP swatch clearly lack these stretches. The FS and NPFS swatch exhibit stretches at 1150 and 1200 cm−1, indicating the presence of CF moieties. The stretch at 1090 cm−1 is absent in these swatches because the −OCH3 groups are eliminated and the −Si−O−Si− network is formed. In Figure 2, SEM images provide further surface analysis of cotton swatches treated with SiO2 NPs and FS. The untreated (A.i) and treated swatches (B.i) appear similar at 30× magnifications, displaying the weave of the textile with some fraying from broken strands. As the magnification increases (500×), bundles of fiber (A.ii and B.ii) are visible within the weave and distinct differences begin to emerge between the two 15045
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Figure 2. SEM images of (A.i−iii) untreated cotton swatches and (B.i−vi) NPFS treated cotton swatches (∼150 nm SiO2 NPs and FS). Images A.i− iii and B.i−iii are at similar scale to allow comparisons between treated and untreated cotton swatches’ microscale morphology. Images B.iv−vi are at higher magnifications so submicrometer morphology of the NPFS coating is visible.
S2), showing that treatment does not change the physical appearance of the cotton. SEM images of control swatches (FS, NP, and TMOS) can be seen in the Supporting Information (Figures S3−S5). Adsorptive Capacity of Treated Cotton. Packed column analysis was used to measure the total convective adsorptive capacity of the swatches from vapor breakthrough of heptenone. Although packed column breakthrough measurements provide an efficient means to survey the total adsorptive capacity for the textile samples, the experimental setup has a drawback. The packed column geometry requires the challenge vapor to saturate a ∼3 cm plug of fabric before breakthrough, yielding an average adsorption capacity. This method is limited in that it yields the adsorptive capacity under steady-state challenge conditions, which do not correlate to the vapor protection provided by a single layer of fabric. However, this technique does offer an effective means to directly compare the adsorption kinetics and capacities of various textile treatments. In Figure 3, data from packed columns are presented and show the heptenone breakthrough for untreated and treated cotton swatches detected by MS. For the untreated cotton swatch, a breakthrough time of 0.3 min was observed, corresponding to a total heptenone adsorptive capacity of 7 μg. The NP swatch showed the greatest breakthrough time of
swatches. The untreated bundle (A.ii) shows smooth texture on the individual strands whereas the treated bundle (B.ii) shows a coating, which encapsulates the fiber bundles. This coating consists of the silica network formed on the fiber surface by the condensation of TMOS and FS with incorporated SiO2 NPs (although the NPs are not visible at this magnification). Cracks within the coating, visible in the treated swatch (B.ii), possibly formed from the evaporation of solvents (acetone, ethanol, methanol, and water) during the application and microwave processes. At 3.5K× comparable magnification (A.iii and B.iii), major differences in surface roughness of individual fibers are clearly discernible between the two swatches. Image A.iii shows a smooth individual cotton fiber, and image B.iii shows a cotton fiber covered by the silica coating. In the treated swatch, SiO2 NPs are now clearly visible within the silica coating, and a textured roughened surface is apparent with more visible cracks. Images B.iv−vi, taken at magnifications of 6K×, 40K×, and 60K×, respectively show the finer detail of individual covered fibers. Image B.iv is the end of an individual fiber, where the textured surface and the SiO2 NPs are easily visible along the surface of the fiber. Images B.v and B.vi display the roughened silica coating composed of the ∼150 nm SiO2 NPs within the silica network. An additional image comparing untreated cotton and NPFS can be found in the Supporting Information (Figure 15046
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Figure 3. Packed column breakthrough of heptenone for untreated and treated cotton swatches: untreated cotton (black), NP only (red), FS only (blue), NP and FS (teal).
66 min and a calculated total adsorptive capacity of 647 μg. The FS swatch resulted in a breakthrough time of 42 min with a calculated total adsorptive capacity of 496 μg, while the NPFS swatch showed a breakthrough time of 49 min and a calculated total adsorptive capacity of 661 μg. The mass imparted by the coating is an important factor in the development of textile treatments. The different coatings result in different mass gains of the swatch where 10, 26, and 25% mass increases are observed for the NP, FS, and NPFS swatches, respectively. To properly compare the effectiveness of the different coatings and total adsorptive capacity, the results are normalized based on mass of the tested swatch. Comparing the fabrics in this manner, the adsorptive capacity of the different coatings is unchanged with respect to rankings, where the NP swatch adsorbs 3.05 μg of heptenone/mg of swatch while the NPFS adsorbs at 2.50 μg/mg and then the FS at 1.92 μg/mg. The total adsorptive capacities and treatment masses are summarized in Table 2. Breathability. As previously mentioned, the NP swatch is the most effective material for total vapor adsorption but fails to repel liquid droplets of n-hexadecane, DMMP, and heptenone. The incorporation of the coating on the swatch as well as the mass increases from the treatments may negatively affect fabric breathabilityan important parameter in protective coating formulations for textile applications. To investigate this effect, pressure drop measurements were conducted on the treated cotton samples, and in the Supporting Information (Figure S6) the comparative results of the pressure drop breathability measurements are reported for the untreated and treated swatches. It should be noted that greater measured pressure across the swatch translates into a more restrictive (less breathable) material. The pressure drop across the untreated cotton swatch measured 0.66 ± 0.01 in. of H2O, while the NP, FS, and NPFS swatches measured 0.50 ± 0.01, 0.70 ± 0.01, and 0.78 ± 0.01 in. of H2O, respectively. These results suggest that the NP coating creates a more breathable swatch while the other coatings are slightly more restrictive. The FS swatch is 6% more restrictive than the untreated cotton swatch while the NPFS swatch is 18% more restrictive than the untreated cotton. Permeation. In Figure 4 and the Supporting Information, permeation data for heptenone are presented for the untreated and treated cotton swatches. Unlike the packed column analysis
Figure 4. (a) Headspace permeation of heptenone on swatches. (b) Heptenone breakthrough of treated swatches relative to untreated cotton.
where the entire swatch must be saturated before breakthrough is measured, permeation measures migration of the challenge through a single layer of the treated swatches. Vapor challenges of heptenone were created in a modified permeation cell, where the liquid was placed in an aluminum weigh boat (roughly 12 × 3 × 3 mm) so that vapor permeation could be measured independently of any liquid−substrate interactions. In the Supporting Information (Figure S7), the permeation results are presented for a 150 μg/cm2 challenge, which resulted in 314 ± 39 μg of heptenone permeating the cotton swatch after 1 h. The NP swatch reduced cumulative breakthrough to 191 ± 25 μg. The FS swatch allowed 104 ± 56 μg to permeate. The NPFS demonstrated the lowest permeability with 62 ± 27 μg of heptenone as detected by GC-MS. Because the simulant is contained in an enclosed headspace, the only available space for the vapor to migrate is through the opposite side of the fabric; thus, a diffusion gradient exists across the textile, which drives the vapor across the material to the effluent side. This creates a more difficult scenario than challenges using static headspacesuch as the HSSPME method reported in the next sectionbecause the partial pressures of heptenone on both sides of the material can never reach equilibrium. Permeation is driven by vapor pressure and diffusion; however, since the challenge vapor is constantly swept from the lower chamber, diffusion continues until the challenge is depleted. Turbulent air flow in the lower chamber also produces eddy currents that can further enhance vapor flux across the sample. Additionally, differences in the weave and in the homogeneity of the coating will impart some variability in 15047
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the NP treatments, RH showed no statistically significant effect on the permeation performance of the FS treated swatches (2 ± 2% for increasing RH). Substantiating the results determined in the permeation and goniometry experiments, the best material for heptenone vapor mitigation with broad-spectrum liquid repellency was the NPFS treated swatch, where breakthrough is marginally affected at RH > 0. The incorporation of FS into the NPFS treatments shows that increasing RH minimally reduces the effect on breakthrough time compared to the NP only treatment. As the RH increases, an average reduction of 12 ± 0.1% is observed for the NPFS under humid conditions. The rationale for these results is that the interpenetrating network of the FS prevents water molecules from interacting with surface hydroxyl sites of the SiO2 NPs. This assertion is supported by the measured adsorption of water on the NP and FS treated fabrics. At RH = 0.75 and T = 25 °C, the NP treated fabric adsorbs 1.75× more water than the FS treated fabric.
the performance of the materials. Tighter control of these variables should yield more repeatable and consistent results. In Figure 4a, results from headspace permeation using the HSSPME method are presented for the untreated and treated cotton swatches. This method also measures vapor permeation through a single layer of material, the effective variables being the vapor pressure of the challenge, quality of the coating, and adsorption affinity of the challenge vapor to the test material. Headspace permeation measurements eliminate effects such as eddy currents at the air−material interface, which could influence the results reported in Figure S7. This method provides for accurate comparison of the samples’ permeation characteristics with fewer interfering factors; however, HSSPME measurements do not provide the same level of quantitative data as reported in the permeation cell experiment. Figure 4a shows the effectiveness of the textile treatments and reveals gross differences in the heptenone breakthrough times between the treated and untreated swatches. The permeation data shown in Figure 4b are normalized to the measured heptenone breakthrough for untreated cotton at each exposure time point. The NP treated swatch shows remarkable improvements over untreated cotton, where a 91% reduction (averaged over the three time points) in heptenone breakthrough is measured. The FS swatch shows an 82% reduction in breakthrough while the NPFS shows a 93% reduction for the time points. The results from the two permeation studies (Figure 4 and Figure S7) show that the NPFS treatment is much more effective at attenuating heptenone vapor permeation than the FS treatment alone. While the NP treatment shows comparable vapor adsorption results to the NPFS, it does not repel low surface tension liquids. (Figure S1 and Table S1). Effect of RH on Treated Fabric. Packed column breakthrough measurements were performed under different RH conditions to determine the impact of humidity on the performance of the treatments. In Figure 5, normalized
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CONCLUSION A method was established by the use of microwave-assisted synthetic techniques as part of a one-step treatment for cotton textiles to provide both broad-spectrum liquid repellency and attenuation of 3-hepten-2-one vapor, a physical simulant for sarin. SiO2 NPs with mean diameters of ∼150 nm were prepared by microwave synthesis, and cotton swatches were treated with these SiO2 NPs to increase the textile’s adsorptive capacity for vapors while heptadecafluoro-1,1,2,2-tetrahydrodecyltrimethoxysilane was added to impart liquid repellency. The data demonstrate that materials containing SiO2 NPs with FS deliver the best overall performance in repellency against the selected liquid challenges and against heptenone vapor. Goniometry measurements show materials treated with SiO2 NPs alone fail to repel nonpolar and low-surface tension liquids, while the NPFS materials are omniphobic with contact angles that are indicative of a superhydrophobic and highly oleophobic substrate. Materials containing the SiO2 NP treatment are overall more effective for heptenone vapor capture. Packed column analysis, which examines the total adsorptive capacity as the amount of time required for a vapor to saturate a plug of the treated textile, shows the SiO2 NP treatment clearly performs best, followed by the NPFS and FS treated cottons. For single layer breakthrough experiments conducted in a permeation cell and by GC-MS headspace analysis, the data demonstrate that the NPFS treated cotton possesses the highest capacity for heptenone vapor for a single layer of swatch. The use of FS was shown to be critical for conditions of 0.25 RH and above as the NPFS material experienced only a slight reduction in performance (12 ± 0.1%) as RH increased. Virtually no effect from RH was observed for FS treated cotton, while NP treated cotton yielded a 53 ± 14% average reduction in heptenone breakthrough time. All treatments were accomplished with only minimal sacrifices in mass burden and breathability to the cotton. These tests suggest that textile products developed using these formulations and methods would prove effective in personal protective equipment, chemical protective apparel, medical, and household applications. The complexity of the treated material makes theoretical modeling difficult; however, future work will include modeling of equivalently treated flat surfaces. These experiments would eliminate the complexity of the fiber shapes and the loss of vapor through the weave of the fiber. Successful modification of cotton indicates that these
Figure 5. Effects of relative humidity on heptenone breakthrough for treated cotton swatches. RH = 0, 0.25, 0.50, and 0.90 at 25 °C.
heptenone breakthrough for the treated swatches is plotted against the different RH values (0, 0.25, 0.50, and 0.90) at 25 °C. The data in Figure 5 show that the SiO2 NPs treatments are adversely affected when RH > 0 compared to the FS treatments, which are minimally affected over the RH test range. For the NP treatment, a 53 ± 14% average reduction in the heptenone permeation time is observed when RH > 0. This is attributed to water vapor occupying binding sites in or on the SiO2 NPs, resulting in faster heptenone breakthrough. Unlike 15048
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treatments have potential benefit with other substrates such as nylon, nylon−cotton blends, wool, and aramids. The ability to produce these materials on an industrial scale has yet to be demonstrated but will be investigated in the future.
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ASSOCIATED CONTENT
S Supporting Information *
Images of repellent liquid droplets and table of average contact angles, image showing visual comparison of untreated vs NPFS treated cotton swatches, SEM images for the control swatches (FS only, NP only, and TMOS only), plot of fabric breathability, and plot of single layer vapor permeation. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (D.D.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported in part by an appointment to the Postgraduate Research Participation Program at the Air Force Research Laboratory administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the U.S. Department of Energy and the Air Force Research Laboratory, Materials and Manufacturing Directorate, Airbase Technologies Division (AFRL/RXQ).
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