Ind. Eng. Chem. Res. 2010, 49, 6075–6079
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Preparation and Comparison of Hydrophobic Cotton Fabric Obtained by Direct Fluorination and Admicellar Polymerization of Fluoromonomers Jayanta Maity,† Pratik Kothary,† Edgar A. O’Rear,*,† and Chacko Jacob‡ School of Chemical, Biological and Materials Engineering, UniVersity of Oklahoma, 100 East Boyd SEC T335, Norman, Oklahoma 73019, and Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India
Two different methods of surface modification of cotton fabrics are compared: direct fluorination using elemental fluorine and admicellar polymerization, which uses a surfactant and fluoromonomer system. Sample characterization includes infrared spectroscopy, analysis of mechanical properties, analysis of wetting times, contact angle measurements, scanning electron microscopy, elemental analysis, and atomic force microscopy analysis. Fourier transform infrared spectroscopy and elemental analysis showed the presence of fluorine moieties. For direct fluorinated cotton fabric, the static contact angle was found to be 117°, and for cotton fabric modified by admicellar polymerization, the contact angle was 125°. In all cases, the presence of a very thin fluorocarbon layer on the surface was confirmed, but differences in terms of the amount of fluorochemical formed and in surface structure were also found. Introduction Surface modification of a substrate is a significant and multipurpose method for creating materials with enhanced performance in some specific areas. Materials modified so possess surface properties that are different from their bulk properties. For example, application of a low surface energy fluorochemical film can reduce friction, inhibit adhesion, and impart water repellency. Many techniques have been used to modify substrate surfaces and alter the mechanical, chemical, or optical properties of a material. The initiated chemical vapor deposition technique has recently been used to polymerize perfluoroalkylethyl methacrylate (CH2dC(CH3)COOCH2CH2(CF2)nCF3 where n ) 5-13) and create low surface energy copolymer films1 and superhydrophobic mats.2 A solvent-free initiated chemical vapor deposition process was studied by Gupta et al. to create low surface energy poly-(1H,1H,2H,2Hperfluorodecyl acrylate) thin films on silicon wafers.3 Surfactants adsorbed on a surface have been utilized as a guide for polymerization to alter surface properties of materials. This method has been termed admicellar polymerization. Admicellar polymerization is an in situ polymerization reaction in surfactants adsorbed onto the substrate surface, forming a thin polymeric film on the substrate surface.4,5 The process can be described in four main steps: admicelle formation, monomer adsolubilization, polymeric film formation, and surfactant removal. Admicellar polymerization has been successfully used to form various types of polymeric thin films on different substrates, such as polystyrene on silica,6 alumina7,8 and cotton9,10 styrene-isoprene copolymer on glass fiber,11 poly(methylmethacrylate) on aluminum metal,12 and polypyrrole on mica.13 Le applied a fluoropolymer to aluminum plates by admicellar polymerization to block corrosion in crevices.14 This method is simple, with low energy consumption, and when used on textile fabrics, it proceeds without blockage of the interstices between fibers and yarns, thus maintaining air permeability of the fabric. Since the thickness of the applied film is typically * To whom correspondence should be addressed. Tel: 1-405-3254379. Fax: 1-405-325-5813. E-mail:
[email protected]. † University of Oklahoma. ‡ Indian Institute of Technology.
on the order of nanometers13 to tens of nanometers, the fabric also retains its softness and original feel. Direct modification of polymer surfaces with fluorine gas15,16 is also an attractive option, since it is a simple and fast method that allows the simultaneous treatment of the outer and inner surfaces of complex-shaped products. It yields a stable coating with fluorine atoms covalently linked to a substrate. The highly exothermal process can be controlled by lowering pressure (typically on the order of millibars) and adjusting fluorine concentration (several volume percent of F2 in N2 or He). Direct fluorination is a well-known method for the surface modification of polymers.17,18 The thickness of the modified layer of polymer is controlled over a 0.01-10 µm range. This technology is the so-called dry one with only gases being used, and articles of any shape can be modified. The process proceeds at room temperature or below and does not need initiation or catalyst. One of the main advantages of direct fluorination is that only a thin surface layer of the material is modified, and hence, the bulk properties of the material are practically unchanged. There are several different ways to apply direct fluorination to modify materials. In this paper, we describe and compare two distinct methods to form a fluorine coating on cotton. One of these, admicellar polymerization, is an aqueous-based process, and the other, direct fluorination, is a dry technique in an inert atmosphere. Materials A pique´ cotton fabric was purchased from Alamac American Knits. The fabric was desized, scoured, and bleached. Prior to use, the fabric was washed several times in a washing machine until it was free from any remaining surfactant. The monomers used were 2,2,3,3,4,4,5,5-octafluoropentylmethacrylate (OFPM) and 2,2,2-trifluoroethyl methacrylate (TFEM) purchased from Synquest Laboratories Inc. (Alachua, FL). The surfactants used were the fluoroaliphatic amine oxide nonionic surfactant Masurf FS230 and a fluoroaliphatic quaternary ammonium cationic surfactant Masurf FS1620 obtained from Mason Chemicals (Arlington Heights, IL). Potassium persulfate (Fisher, Pittsburgh, PA) was the initiator for admicellar polymerization. All chemicals were used without further purification. Fluorine gas
10.1021/ie100564y 2010 American Chemical Society Published on Web 06/08/2010
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mixture, supplied by Air Products and Chemicals Ltd. (Allentown, PA), contained 20% F2 and 80% He. Diluting He gas was also supplied by Air Products and Chemicals. The fluorine source gas was >97% pure; the helium was 99.99% pure. Experimental Work Preparation of Samples. Admicellar Polymerization. The admicellar polymerization experiments were conducted at The University of Oklahoma. A 20-mL solution containing 30 ppm FS1620, 162 ppm FS230, 4 mM TFEM, 4 mM OFPM and the corresponding concentration of potassium sulfate in DI water at pH 4 was placed in a 24-mL glass vial reactor. A swatch of washed cotton fabric weighing 2.0 g was added to the vial. Polymerization was carried out in two steps. Surfactant adsorption and monomer adsolubilization were carried out for 2 h at 60 °C in a shaker bath at 80 rpm. Potassium persulfate at an initiator/monomer ratio of 1:1 was then added, the vial was resealed, and the polymerization was allowed to proceed for an additional 2 h at 80 °C. Excess surfactant was rinsed away with several volumes of water, and the sample was dried in an oven at 80 °C for an additional 2 h. Direct Fluorination of Cotton Fabric. Fluorination of the cotton fabric of the same source and size was carried out at the Materials Science Centre, IIT, Kharagpur. This involved treating cotton fabric with a fluorine-helium mixture (2% F2 + 98% He with O2 impurity no more than 0.05%) in a closed vessel at a total mixture pressure of 0.56 bar and an initial temperature of 23 °C. The F2-He mixture was prepared beforehand in a separate vessel and then introduced into the reactor for 1.5 h. Fabric samples were degassed in the reactor during pumpdown and prior to introduction of F2-He mixture. The reactor was made of stainless steel and was cylindrical in shape. Characterization of Treated Cotton Fabric. The aim of the current study was to compare the chemical and physical surface changes of cotton fabrics treated by the direct fluorination process and admicellar polymerization and to identify appropriate methods for evaluating the effects of fluorination on cotton fabrics. We therefore investigated the surface characteristics of cotton fabric treated with elemental fluorine and cotton fabric subjected to admicellar polymerization using fluoromonomers and compared them using wetting time analysis, contact angle analysis, FTIR-ATR spectroscopy, SEM, elemental analysis, and AFM. Contact Angle Measurement. Contact angle measurement of the samples at 24 °C was done by using an I. T. Concept Tracker contact angle analyzer. A 20-µL drop of deionized water of surface tension 72.75 mN/m was deposited on the fabric by a syringe. The drop was formed at the tip of a capillary fitted to a syringe, whose plunger was driven by a motor. Measurement was carried out for 10 min at three sites on each sample with the average value reported. FTIR-ATR Study. Fourier-transform infrared (FT-IR) attenuated total reflectance (ATR) spectroscopy can highlight changes in the functionalities present in different untreated and treated cotton fabrics. For structural analysis, IR study of the different fabrics was performed using a Thermo Nicolet, NEXUS 870 FTIR spectrophotometer. The IR-spectrum was taken over the wavenumber range 4000-500 cm-1. SEM Study. Scanning electron microscopy images of samples were taken at the Noble Microscopy Laboratory at the University of Oklahoma. All samples, modified and unmodified, were gold-coated and observed in a scanning electron microscope (SEM) JEOL JSM 880. Elemental analysis of the coated and noncoated cotton samples was carried out using Zeiss 960A
Figure 1. FTIR spectrum of untreated pique´ fabric (UPF), fluorinated pique´ fabric (PF-DF), and fluoromonomer treated pique´ fabric (PF-AP).
SEM equipped with Oxford Link energy dispersive spectroscopy (EDS) with a thin window and using IXRF EDS 2008 software at a beam energy of 5 keV. AFM Study. The AFM studies were carried out using a Digital Instruments (Veeco) Nanoscope III system. Scanning was carried out in tapping mode using a back-side Al-coated silicon probe (NSC15/AlBS) of radius 600 PF-AP >600
0 117 ( 3 125 ( 3
0 1.9 2.8
AFM roughness (nm)a Rq
Ra
Rmax
31.6 ( 5.7 24.6 ( 5.2 218.1 ( 15.1 77.1 ( 5.9 65.8 ( 5.8 609.8 ( 15.6 157.5 ( 7.0 136.4 ( 6.3 679.7 ( 15.8
a Rq: the root mean square roughness; Ra: the average roughness; Rmax: the difference in height between the highest and lowest point on the surface relative to the mean plane.
treated cotton and the untreated cotton. The relative order in roughness is not surprising, since direct fluorination proceeds by atomic substitution and rearrangement and admicellar polymerization results in deposition of polymer on the surface of the fiber. The contact angle measurement and SEM and AFM images combined with the FTIR- ATR analysis supported the formation of a hydrophobic fluoropolymer layer on the cotton surface by both methods. The Young modulus of the fluoromonomer-treated cotton fabric and the untreated fabric does not differ greatly, but the PF-AP does become somewhat stiffer than the UPF. As for PFDF, the Young modulus decreases, which may be due to some
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Figure 4. Stress-strain plot of untreated pique´ fabric (UPF), fluorinated pique´ fabric (PF-DF), and fluoromonomer-treated pique´ fabric (PF-AP).
degradation of cellulose by reaction with fluorine or with byproduct such as HF. The greater stiffness of the PF-AP textile gives some direct indication of the adhesion between the fibers and might be attributed to the formation of polymeric bridges between the adjacent fibers of the fabric. The stress-strain plot of the three samples is shown in Figure 4. There are some advantages and disadvantages to both direct fluorination and admicellar polymerization. The advantages for using elemental fluorine are that modification proceeds through the formation of stable covalent bonds with no requirement of initiator or catalyst. The reaction can be carried out at room temperature as a completely dry process. Its disadvantage is that control of the exothermic reaction is effected by varying the concentration of fluorine gas. Degradation of cellulose may also occur. In the case of admicellar polymerization, a polymeric layer of fluorocarbon is formed in an aqueous-based system that does not require equipment for handling gases. The approach may be more compatible with existing textile processing methods, although scale-up poses challenges for admicellar polymerization. Conclusions Direct fluorination and admicellar polymerization have been used to modify cotton fabric and impart hydrophobicity to the coated surfaces. The former proceeds by direct reaction with the cellulose, whereas the latter overlays a very thin polymer coating. It was found that the cotton fabric obtained after admicellar polymerization shows a higher contact angle and better mechanical properties than the cotton fabric obtained after direct fluorination. Admicellar polymerization yields greater roughness on length scales on the order of hundreds of nanometers. Acknowledgment This project was supported in part by funds provided by the Oklahoma Center for the Advancement of Science and Technology (OCAST AR081-050), which is gratefully acknowledged. Literature Cited (1) Mao, Y.; Gleason, K. K. Vapor-deposited fluorinated glycidal copolymer thin films with low surface energy and improved mechanical properties. Macromolecules 2006, 39, 3895–3900. (2) Ma, M.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Superhydrophobic fabrics produced by electrospinning and chemical vapor deposition. Macromolecules 2005, 38, 9742–9748.
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ReceiVed for reView March 9, 2010 ReVised manuscript receiVed May 13, 2010 Accepted May 24, 2010 IE100564Y
