pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabricating Superhydrophilic Wool Fabrics Dong Chen,† Longfei Tan,‡,§ Huiyu Liu,‡ Junyan Hu,† Yi Li,*,†,‡ and Fangqiong Tang*,‡ †
Institute of Textiles and Clothing, Hong Kong Polytechnic University, Hung Hom, Hong Kong, People’s Republic of China, ‡Laboratory of Controllable Preparation and Application of Nanomaterials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and §Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China Received September 21, 2009. Revised Manuscript Received October 21, 2009 A simple method for fabricating environmentally stable superhydrophilic wool fabrics is reported here. An ultrathin silica layer coated on the wool altered both the surface roughness and the surface energy of the fiber and endowed the wool fabrics with excellent water absorption. The process of coating silica sols was dependent on an acid solution of low pH, which influenced the electrostatic interactions between nanoparticles and wool fibers. The morphology and composition of silica-sol-coated wool fabrics were characterized by a combination of SEM, TEM, EDX, FTIR, and XPS measurements. The possible mechanism and size effect of silica nanoparticles on the hydrophilic property of wool fabric were discussed. The washing fastness of the superhydrophilic wool fabrics in perchlorethylene and water was also evaluated. This study shows that wool fabrics modified by optical transparence, chemical stability, and nontoxic silica sols are promising in constructing smart textiles.
Introduction Wool fiber, know for its lightness, softness, warmness, and smoothness, is known as a natural clothing material. Because wool is mainly composed of keratin, the outermost part of the fiber is the cuticle cell, of which the surface is a fatty layer of 18-methyl eicosanoic acid covalently bound to the protein layer of the wool cuticle via a thioester linkage.1,2 Because of the presence of this fatty layer, the surface of wool is hydrophobic. Therefore, the water absorption and sweat venting properties of wool fiber are not very good, which affects the wearing comfort of wool textiles.3 The wool hydrophobic surface layer is also a barrier to anticrease finishing, dyeing, and grafting of hydrophilic agents, which is not favorable to adding smart functionalities to wool fabrics.4-6 A number of investigations, such as nonthermal plasma7,8 and enzyme treatment9,10 of wool fabric, have been explored to improve wool fabrics’ hydrophilic property. However, besides the demand for an effective wettability enhancement, another issue that has increased in importance and has attracted attention over the years is the stability of the wool hydrophilic property.8 The hydrophilicity of fabric treated with nonthermal plasma decreases progressively in air after plasma treatment, which can *To whom correspondence should be addressed. (Y.L.) Fax: 852-27661432. E-mail:
[email protected]. (F.T.) Fax: 8610-82543521. E-mail: tangfq@ mail.ipc.ac.cn.
(1) Negri, A. P.; Cornell, H. J.; Rivett, D. E. Text. Res. J. 1993, 63, 109–115. (2) Huson, M.; Evans, D.; Church, J.; Hutchinson, S.; Maxwell, J.; Corino, G. J. Struct. Biol. 2008, 163, 127–136. (3) Li, Y. Text. Prog. 2001, 31, 1–135. (4) Molina, R.; Jovancic, P.; Comelles, F.; Bertran, E.; Erra, P. J. Adhes. Sci. Technol. 2002, 16, 1469–1485. (5) Canal, C.; Erra, P.; Molina, R.; Bertran, E. Text. Res. J. 2007, 77, 559–564. (6) Jocic, D.; Vlchez, S.; Topalovic, T.; Molina, R.; Navarro, A.; Jovancic, P.; Julia, M. R.; Erra, P. J. Appl. Polym. Sci. 2005, 97, 2204–2214. (7) Kan, C. W.; Yuen, C. W. M. Text. Prog. 2007, 39, 121–187. (8) Morent, R.; de Geyter, N.; Verschuren, J.; de Clerck, K.; Kiekens, P.; Leys, C. Surf. Coat. Technol. 2008, 202, 3427–3449. (9) Jovancic, P.; Jocic, D.; Dumic, J. J. Text. Inst. 1998, 89, 390–400. (10) Parvinzadeh, M. Enzyme Microb. Technol. 2007, 40, 1719–1722. (11) Canal, C.; Molina, R.; Bertran, E.; Navarro, A.; Erra, P. Fiber. Polym. 2008, 9, 293–300.
Langmuir 2010, 26(7), 4675–4679
be explained by the rotation of chemical hydrophilic groups toward the bulk of the fiber and depends on the storage conditions.11,12 Moreover, plasma treatment requires expensive equipment. Enzyme treatments eliminate the fatty layer of the wool fiber and destroy the scale layer of the wool fiber, which lead to irreversible damage to the surface structure of wool fiber. Therefore, obtaining environmentally stable superhydrophilic wool fabrics without damaging the fiber is still a problem. Recently, extensive research has been conducted to realize superhydrophobic13-17 and superhydrophilic18,19 solid surfaces by regulating both surface roughness and surface energy. Considering the significant potential of such surfaces for both scientific and industrial applications, carbon nanotubes,20 gold nanoparticles,21 hydrophobic silica nanoparticles,22-26 and nanofilaments27,28 have been successfully applied to hydrophilic fabrics to realize superhydrophobic textiles with self-cleaning (12) Canal, C.; Molina, R.; Bertran, E.; Erra, P. Fiber. Polym. 2008, 9, 444–449. (13) Gao, X.; Jiang, L. Nature 2004, 432, 36–36. (14) Li, X. M.; Reinhoudt, D.; Calama, M. C. Chem. Soc. Rev. 2007, 36, 1350– 1368. (15) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Adv. Mater. 2002, 14, 1857–1860. (16) Ming, W.; Wu, D.; vanBenthem, R.; de With, G. Nano Lett. 2005, 5, 2298– 2301. (17) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y. G.; Wang, Z. Q. J. Mater. Chem. 2008, 18, 621–633. (18) Liu, X. M.; He, J. H. J. Colloid Interface Sci. 2007, 314, 341–345. (19) Namavar, F.; Cheung, C. L.; Sabirianov, R. F.; Mei, W. N.; Zeng, X. C.; Wang, G.; Haider, H.; Garvin, K. L. Nano Lett. 2008, 8, 988–996. (20) Liu, Y. Y.; Tang, J.; Wang, R. H.; Lu, H. F.; Li, L.; Kong, Y. Y.; Qi, K. H.; Xin, J. H. J. Mater. Chem. 2007, 17, 1071–1078. (21) Wang, T.; Hu, X. G.; Dong, S. G. Chem. Commun. 2007, 1849–1851. (22) Li, S. G.; Xie, H. B.; Zhang, S. B.; Wang, X. H. Chem. Commun. 2007, 4857–4859. (23) Hoefnagels, H. F.; Wu, D.; de With, G.; Ming, W. Langmuir 2007, 23, 13158–13163. (24) Ramaratnam, K.; Tsyalkovsky, V.; Klep, V.; Luzinov, I. Chem. Commun. 2007, 4510–4512. (25) Wang, H. X.; Fang, J.; Cheng, T.; Ding, J.; Qu, L. T.; Dai, L. M.; Wang, X. G.; Lin, T. Chem. Commun. 2008, 877–879. (26) Li, S. G.; Zhang, S. B.; Wang, X. H. Langmuir 2008, 24, 5585–5590. (27) Artus, G. R. J.; Jung, S.; Zimmermann, J.; Gautschi, H.-P.; Marquardt, K. Adv. Mater. 2006, 18, 2758–2762. (28) Zimmermann, J.; Reifler, F. A.; Fortunato, G.; Gerhardt, L.-C.; Seeger, S. Adv. Funct. Mater. 2008, 18, 3662–3669.
Published on Web 11/12/2009
DOI: 10.1021/la903562h
4675
Article
Chen et al. Scheme 1. Possible Formation Mechanism of a Superhydrophilic Ultrathin Silica Layer on a Wool Fiber
properties by many approaches. To the best of our knowledge, however, this strategy has rarely been applied to the fabrication of superhydrophilic fabrics, which is of great importance for applications influencing our daily lives. In the current work, we present a simple method for fabricating environmentally stable superhydrophilic wool fabrics. Silica sols are applied to pristine wool fibers to form an ultrathin layer on the surface of wool fibers, increasing both the surface roughness and surface energy of the wool fabrics; then funcionalized fabrics can be obtained.
Materials and Methods Chemicals and Reagents. Acetic acid (99.5%), ammonium hydroxide (25%), perchlorethylene, and acetone were obtained from the Beijing Chemical Reagent Company. Sodium silicate (silicate module M = 3.3, M = SiO2/Na2O molar ratio) was obtained from the Beijing Redstar Chemical Construction Materials Company. The woven wool fabrics used in this work were purchased from a common shop. Deionized water was used for all of the experimental processes. Preparation of Silica Sols. Silica sols with diameters about 27 nm were prepared by a modified Bechtold-Snyders method.29 The preparation process consisted of two steps: seed formation and colloidal particle growth. Preparation of the silicic acid monomer was realized by passing sodium silicate solution through a self-made Hþ-form ion-exchange column, after which the pH value of solution was adjusted to 8.0 by ammonium hydroxide. To prepare seeds, a portion of this silicic acid monomer was heated to boiling over a period of 20 min. Then, freshly prepared silicic acid monomer was added to the seed solution under intensive stirring, maintaining pH 10.0 and volume constant, which means that the rate of adding silicic acid monomer must be equal to the water evaporation rate. As a result, 27 nm silica sols containing 10% SiO2 were obtained. Preparation of Superhydrophilic Fabrics. The silica sols were coated onto wool fibers by the immersion process, a wet chemistry method, which appears to be more robust and able to modify the surface of natural wool fibers than common physical methods.30 A piece of woven wool fabric (20 cm 20 cm) was purified by washing in acetone for 20 min using an ultrasonic bath to remove surface impurities. Then, this fabric was taken out and placed into a vacuum drier at 40 °C for 2 h so that a clean, dry fabric could be obtained. Acetic acid was used to adjust the pH of silica sols solution to pH 3 to 4. The weight ratio of wool fabric to water is 1:30, and that of SiO2 to water is 1:200. After the wool fabric was immersed in the pH-adjusted silica sol solution, the temperature was raised from ca. 25 to 80 °C over 30 min and held at 80 °C for up to 20 min. Meanwhile, this system was oscillating in a constant-temperature (29) Bechtold, M. F.; Pa, K. S.; Snyder, O. E. U.S. Patent 2,574,902, 1951. (30) Hinestroza, J. P. Mater. Today 2007, 10, 9.
4676 DOI: 10.1021/la903562h
oscillator during the whole process. Then, the treated wool fabric was washed with water and dried in an oven at 80 °C. Process of Washing and Wetting Time Testing. The weight ratio of superhydrophilic wool fabric to perchlorethylene or water was 1:200, and testing was carried out under vigorous magnetic stirring for 5 min at 25 °C. After being dried in a vacuum drier at 60 °C, the test samples were conditioned for at least 24 h in the atmosphere (relative humidity of 65 ( 2% and temperature of 20 ( 2 °C). The wetting time for a drop of distilled water to sink into the sample after the washing process was taken with a stopwatch. According to British Standard 4554:1970 (Method of Test for Wettability of Textile Fabrics), fabrics giving times greater than 200 s with water are considered to be unwettable. Each sample was measured three times, and the average value was recorded. Characterization. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopic measurements were performed with a Hitachi S-4300 scanning electron field-emission microscope. Transmission electron microscopy (TEM) images were obtained using a JEM-2100 electron microscope operating at 200 kV. Fourier transform infrared (FTIR) spectra were characterized by a Fourier transform infrared spectrum (FTIR-1730) in attenuated total reflection mode. X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI Quantera SXM X-ray photoelectron spectrometer using an Al KR monochromator source. Water contact angles were measured using a contact angle meter (Phoenix-300 contact angle and surface tension analyzer).
Results and Discussion Possible Mechanism for Coating Ultrathin Silica Layers on Wool Fibers. As shown in Scheme 1, coating an ultrathin silica layer onto wool fibers could be dependent on electrostatic interactions between wool fibers and silica sols.31 Silica sols behave in a similar manner to dye molecules during wet chemical processing. Wool fiber has been known to have free amino and carboxyl groups in the wool backbone as important reactive sites. At pH 3 to 4, a considerable number of internal amino groups are protonated but the carboxyl groups are not substantially protonated until the pH approaches 2, leading to the wool fiber having a positive surface charge. Therefore, acid solution swells wool and will benefit silica sols coating the wool surface. Morphology and Composition Characterization. Silica sols of 27 nm were prepared by a modified Bechtold-Snyders method.29 Monodisperse and nearly spherical silica sols can be seen from the TEM image, shown in Figure S1 (Supporting Information). The morphologies of pristine wool fibers and silicasol-treated fibers are shown in Figure 1. Figure 1a,b shows the SEM images of the pristine wool fibers, indicating that the surface (31) King, D. G.; Pierlot, A .P. Color. Technol. 2009, 125, 111–116.
Langmuir 2010, 26(7), 4675–4679
Chen et al.
Article
Figure 1. SEM images of wool fibers (a and b, pristine wool fibers; c and d, ultrathin silica-layer-coated wool fibers). The inset in d is a high-magnification SEM image of silica-layer-coated wool fibers.
Figure 3. XPS spectrum of wool fabrics: (a) pristine wool fibers and (b) ultrathin silica-layer-coated wool fibers.
Figure 2. FTIR spectrum of wool fibers: (a) pristine wool fibers and (b) ultrathin silica-layer-coated wool fibers.
of the fibers is smooth with scales. Figure 1c,d reveals the SEM images of wool fibers coated with silica sols of 27 nm, indicating that after being treated with silica sols the wool fibers have been effectively coated with a thin layer of silica sols. In addition, the ultrathin silica layer appeared not to lead to deleterious effects on the morphology of the native wool fiber, shown in Figure 1c. The cross-sectional dimension of the wool fiber is in the micrometer range, so the presence of an ultrathin layer of nanoparticles is not perceptible to the touch.30 To determine the composition of the ultrathin silica-layercoated wool fibers, an EDX measurement was performed. The characteristic peaks for different elements in the superhydrophilic layer are presented in Figure S2 (Supporting Information). It shows the existence of the silica element on the surface of the fiber, indicating that the silica layer was coated onto the wool fiber surface. Fourier transform infrared spectra (in attenuated total reflection mode) analysis was performed here to study the chemical nature of the wool fabric before and after the silica coating was applied. Upon comparing spectra a and b of Figure 2, it is seen that the band at 1100-900 cm-1 is quite different in the two spectra. The peak at 1110 cm-1 belongs to the Si-O-Si stretching vibration band. The peak at 1071 cm-1 in the IR spectrum can also be Langmuir 2010, 26(7), 4675–4679
assigned to Si-O-Si vibrations in an amorphous silica/silicate. This shows that the groups of Si-O-Si were grafted onto the surface of the wool fiber.32 XPS was also used to characterize the wool fabric before and after the ultrathin silica layer coating was applied, as shown in Figure 3. XPS is a popular and powerful technique for the investigation of surface composition; it does provide qualitative information on the chemical changes. For the pristine wool textile, peaks corresponding to C, O, N, and S were observed (Figure 3a). After surface modification with a silica layer, new peaks appeared at 154.6 and 103.7 eV that are attributed to Si 2s and Si 2p signals,26 respectively (Figure 3b), whereas the S 2s and S 2p peaks disappeared completely. This suggests that a layer of silica has covered the surface of the wool fibers. From a combination of EDX, FTIR, and XPS measurements, we conclude that an ultrathin silica layer was coated onto the wool fiber surface successfully. Superhydrophilicity of Ultrathin Silica-Layer-Coated Wool Fabric. The wettability of the fiber was measured in terms of the water contact angle and the rate of water absorption. The contact angle of the pristine wool fabric was about 112° because of the hydrophobic nature of the keratins (Figure 4a). After the silica coating was applied, the water contact angle on the wool fabric became 0° in less than 1 s. (Figure 4b). A video clip shows (32) Xu, B. S.; Niu, M.; Wei, L. Q; Hou, W. S.; Liu, X. G. J. Photochem. Photobiol., A 2007, 188, 98–105.
DOI: 10.1021/la903562h
4677
Article
Chen et al.
Figure 4. Contact angle of a water droplet on wool fabrics: (a) pristine wool fabric and (b) an ultrathin silica-layer-coated wool fabric.
the absorption of water droplets on the superhydrophilic wool fabrics. In the video, sample 1 is a pristine wool fabric and sample 2 is a superhydropilic wool fabric. We can see that the water absorption rate of the superhydrophilic wool fabric is very fast (Supporting Information). Possible Mechanism for the Superhydrophilicity of Ultrathin Silica-Layer-Coated Wool Fabrics. The possible mechanism of the superhydrophilicity will be discussed in the following text. When liquid water is placed on the surface of the hydrophilic fabric, it transfers through the fabric. The process is caused by capillary penetration. The original force drive the penetration is the wettability of the fiber, which strongly depends on the surface energy and the surface roughness. In our work, the surface roughness was increased by coating silica layer onto the surface of the wool fiber, as shown in Figure 1c,d. A thin layer of nanoscale spherical protuberances is observed on the microscale wool scale layer. Furthermore, the contribution of woven structures of the fabric to the surface roughness cannot be excluded.33,34 As for the surface energy, the outer layer of wool fibers is rich in covalently bound fatty acids, with 18-methyleicosanoic acid predominating.1,2 The surface of wool fibers is hydrophobic according to water contact angles, as shown in Figure 4a. Hence, the surface energy of the pristine wool fiber is low. However, by using an ultrathin silica layer coating, the possible effect of the fiber nature on the surface wettability can be minimized. We need to take into consideration only the silica layer wettability, which is hydrophilic because of the presence of a large number of Si-OH groups on the surface of silica clusters. The water contact angle on a planar SiO2 surface was previously reported to be ca. 20°.35 Therefore, we obtained superhydropilic wool fabrics by coating an ultrathin silica layer onto the surface of the wool fiber, which increases both the surface roughness and surface energy. As for these two contributions;surface energy and surface roughness;we still do not know which effect carries more weight with respect to superhydrophilicity. An investigation of this effect is ongoing in our group. Size Effect of the Silica Nanoparticles on the Hydrophilic Property of Wool Fabrics. To investigate the size effect of silica nanoparticles on the hydrophilic property of wool fabric, silica nanoparticles of 50, 150, and 300 nm diameter were used. Because of the limitation of the Bechtold-Snyders method in preparing silica nanoparticles with large diameters,29 the silica nanoparticles used here were prepared by a modified St€ober method (Supporting Information). The morphologies of silicananoparticle-coated fibers are shown in Figure 5. Figure 5a,c,e shows the SEM images of wool fibers coated with 50, 150, and 300 nm silica nanoparticles, and Figure 5b,d,f shows the highmagnification SEM image of the left image. From the SEM (33) Michielsen, S.; Lee, H. J. Langmuir 2007, 23, 6004–6010. (34) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 5998–6000. (35) Cebeci, F. C.; Wu, Z. Z.; Zhai, L.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 2856–2862.
4678 DOI: 10.1021/la903562h
Figure 5. SEM images of wool fibers coated with silica nanoparticles of different diameters: (a) 50, (c) 150, (e) 300 nm. b, d, and f are the high-magnification SEM images of a, c, and e, respectively.
images, we can observe that silica layers were coated onto the wool fibers successfully. The hydrophilic properties of wool fabrics were measured in terms of wetting time. After wool fabrics were coated with 50, 150, and 300 nm silica nanoparticles, the wetting times were 2, 30, and 180 s, respectively, indicating that wool fabrics with smaller nanoparticles will have better wettability. Because fabrics giving times greater than 200 s with water are considered to be unwettable, the size effects of silica nanoparticles with diameter larger than 300 nm were not further investigated. The mechanism of this trend may be mainly attributed to the porosity of the wool fabric surface.36,37 By coating silica nanoparticles with different diameters, the porosity of the wool fabric surface will be different. When the diameters of silica nanoparticles increase from 27 to 300 nm, the intervals between the nanoparticles of the silica layer will increase, which weaken the capillary penetration of water droplets. Therefore, wool fabric coated with 300 nm silica nanoparticles has the slowest water absorption rate. Moreover, with the increase in nanoparticle size, the specific surface area decreases, the coalesce force between the fiber and the nanoparticles becomes weak, and the coverage of silica-coated wool fibers decreases appreciably. Considering the above reasons, smaller-diameter silica nanoparticles will be more suitable for the fabrication of superhydrophilic wool fabrics. Washing Fastness of Ultrathin Silica-Layer-Coated Wool Fabrics. Robust washing fastness is critical in the use of wool. In our daily life, wool clothes are often washed by dry cleaning process. Therefore, we evaluated the washing fastness of the superhydrophilic wool fabrics by simulating dry cleaning process. Perchlorethylene was chosen as a dry cleaning solvent, which is very effective in cleaning clothing and is used by a (36) Bico, J.; Thiele, U.; Quere, D. Colloids Surf., A 2002, 206, 41–46. (37) Liu, X. M.; Du, X.; He, J. H. ChemPhysChem 2008, 9, 305–309.
Langmuir 2010, 26(7), 4675–4679
Chen et al.
Article
the surface of the wool fiber after water scrubbing. Therefore, from the wetting time and SEM images, we can see that there is a coalesce force between the fiber and the nanoparticles and that the durability can meet the needs of dry cleaning. It is also found that the durability of the superhydrophilic wool fabrics in water needs to be enhanced. Relevant work is ongoing in our group.
Conclusions A simple method was adopted to achieve environmentally stable superhydrophilic wool fabrics by coating an ultrathin silica layer onto natural wool fabrics. Thanks to the silica layer’s optical transparence, chemical stability, and nontoxicity, fabric with silica sols does not lead to deleterious effects on the color and morphology of wool fabrics. Besides this, this silica layer would be of great importance for further modification with bioactive agents or stimuli-responsive molecules on the wool fiber surface, which have great potential to be constructed into smart textiles.38,39 Figure 6. Washing fastness of superhydrophilic wool fabrics in terms of wetting time after different washing times: (a) dry cleaning and (b) water scrubbing.
majority of dry cleaners, yet the byproducts released in the cleaning process are not very good for the environment or for humans. Machine-washable wool clothes are a growing trend that helps people to do their part to reduce toxins that pollute the environment. Therefore, the durability of superhydrophilic wool fabrics with respect to water scrubbing was also evaluated. As shown in Figures 6 and S3, after the superhydrophilic wool fabrics were washed in perchlorethylene 20 times, their wetting time was less than 3 s and the morphology of the silica-coated wool fiber scarcely changed (Figures 6a and S3a). After water scrubbing for 20 times, the wetting time of the specimen was longer than that of ones washed for 3 time, but still around 10s (Figure 6b). The SEM images of the wool fibers after water scrubbing 10 and 20 times are also shown in Figure S3b,c, indicating that some silica nanoparticles were washed away from
Langmuir 2010, 26(7), 4675–4679
Acknowledgment. We acknowledge financial support from the Hong Kong Innovation Technology Commission/Hong Kong Research Institute of Textiles and Apparel Project ITP/001/07TP and ITP/031/08TP, the National Hi-Tech Research and Development Program (863 program) of China (2007AA021803), and the National Science Foundation of China (60736001). Supporting Information Available: Video clip of the absorption of water droplets on superhydrophilic wool fabrics. Preparation of 50, 150, 300 nm silica nanoparticles and coating silica nanoparticles onto wool fibers. TEM image of 27 nm silica sols. EDX spectrum of ultrathin silica-layercoated wool fibers. SEM images of superhydrophilic wool fibers after washing. This material is available free of charge via the Internet at http://pubs.acs.org. (38) Mahltig, B.; Haufe, H.; Bottcher, H. J. Mater. Chem. 2005, 15, 4385–4398. (39) Xia, F.; Jiang, L. Adv. Mater. 2008, 20, 2842–2858.
DOI: 10.1021/la903562h
4679
