Formation of Highly Hydrophobic Surfaces on Cotton and Polyester

Ind. Eng. Chem. Res. 2009, 48, 9797–9803

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Formation of Highly Hydrophobic Surfaces on Cotton and Polyester Fabrics Using Silica Sol Nanoparticles and Nonfluorinated Alkylsilane Qinwen Gao, Quan Zhu,* and Yuliang Guo College of Chemistry, Chemical Engineering and Biotechnology, Donghua UniVersity, Shanghai 201620, China

Charles Q. Yang Department of Textiles, Merchandising and Interiors, The UniVersity of Georgia, Athens, Georgia 30602

Water and soil repellency is one of the most desirable properties for textile fabrics. A surface with a water contact angle higher than 150° is considered to be a practically nonwettable superhydrophobic surface. In this research, we studied the formation of highly hydrophobic surfaces on cotton and polyester fabrics using silica sol formed by hydrolysis and subsequent condensation of tetraethoxysilane under alkaline conditions followed by hydrophobization using hydrolyzed hexadecyltrimethoxysilane (HDTMS). The textile fabrics thus treated showed excellent water repellency with a water contact angle as high as 155° on cotton and 143° on polyester. The high hydrophobicity of the treated fabrics is due to the presence of hydrophobic HDTMS as well as the increase in roughness by silica sol on the surfaces of the treated fabrics. The morphology of the cotton and polyester fabrics were characterized by scanning electron microscopy. We also found that the treated cotton and polyester are resistant to hydrolysis of multiple washing cycles. 1. Introduction Water and soil repellency has been one of the most desirable textile properties for consumers. Textile manufacturers have been consistently improving such properties for their products for many decades. A surface with a water contact angle >150° is considered to be a material having a practically nonwettable superhydrophobic boundary.1-7 Surfaces of some plants and insets, such as lotus leaves and butterfly wings, have naturally formed superhydrophobicity showing extraordinary waterrepellent and self-cleaning functions. Due to potential benefits such superhydrophobicity may bring to a variety of materials, strong interest has immerged to develop technologies of producing superhydrophobic surfaces.8-10 The superhydrophobicity of a material depends on not only its surface chemistry but also on its surface topology as reported in the literature.11 Two distinct theoretical models (Wenzel and Cassie-Baxter)1,2 have been used to guide the generation of superhydrophobic surfaces by either roughening the surface, lowering the surface free energy, or both. Superhydrophobic textile materials have been successfully developed by different technical approaches.11-21 Most of the previous studies have focused on the use of fluorochemicals because of their excellent water repellency.11,12 However, the use of fluoroalkyl compounds also has significant disadvantages, such as high cost and the potential risks that those fluorochemicals create for human health and the environment.22,23 Hence, the development of nonfluorinated alternative modifying agents to produce environmentally friendly hydrophobic textile materials becomes very desirable. Sol-gel methods were used to impart water repellency to a material by producing rough surfaces. Cotton has always been the principal fiber for clothing due to its attractive characteristics such as softness, comfort, high resistance to heat, and high resistance to alkaline. However, its high absorbency as a result of the abundant hydroxyl groups on cotton diminishes the stain-resistance and water repellency * To whom correspondence should be addressed. Tel.: +86-21-67792622. Fax: +86-21-6779-2608. E-mail: [email protected].

of cotton textiles. Therefore, additional finishes are required to impart superhydrophobicity and self-cleaning properties to cotton fabrics.13-15 Composite sols were also applied to the fabrics of synthetic fibers, such as polyamide and polyester, to achieve high hydrophobic properties.11,16 The purpose of this paper was to use silica sol and hexadecyltrimethoxysilane (HDTMS) to impart extremely high hydrophobicity to both cotton and polyester fabrics. The representative data are presented as follows. 2. Experimental Procedures 2.1. Materials. The cotton fabric was a plain weave fabric (548 ends/284 picks) with both warp and filling yarns having 15 tex. The polyester is a plain woven fabric (646 ends/478 picks) with both warp and filling yarns as a staple fiber yarn having 6 tex. The cotton and polyester fabrics were first washed using a nonionic detergent to remove wax, grease, and other possible finishing agents before chemical modification. Tetraethoxysilane (TEOS), ethanol, NH4OH, and acetic acid were provided by Shanghai Chemical Reagent Corporation, Shanghai, China. HDTMS was supplied by Fluka, Germany. 2.2. Sample Preparation. 2.2.1. Silica sol. Silica sol was prepared using alkaline hydrolysis of TEOS in a NH4OHethanol solution. A typical preparation procedure is described as follows. NH4OH (5 mL) is gradually added into ethanol (100 mL) with stirring at 60 °C, and the stirring is continued for 30 min. Then, TEOS (6 mL) was added dropwise to the solution with stirring, and the stirring is continued for 90 min to form a silica sol. 2.2.2. Hydration of HDTMS. HDTMS (3%, w/w) was gradually added to ethanol to form a solution, and its pH was adjusted to 5.0 using acetic acid. The solution thus prepared was stirred for 60 min to form an alkylsilanol solution. 2.2.3. Fabric Treatment. A fabric specimen was first immersed in a silica sol for 20 min, then padded using a laboratory padder with a wet pickup ranging from 70 to 80%, and dried at 80 °C for 3 min. The samples thus treated were immersed in the

10.1021/ie9005518 CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009

Scheme 1. Formation of a Superhydrophobic Surface on Cotton Based on Silica Sol and HDTMS

ethanol solution of hydrolyzed HDTMS for 1 h, dried at room temperature, and finally cured at 120 °C for 1 h. 2.3. Material Characterization. 2.3.1. Particle Size Measurement. The number-average particle sizes of the silica sol samples were measured using a particle size and zeta potential analyzer (Model Zetasizer Nano ZS, Malvern, Britain) 1 h after each sol sample was prepared. 2.3.2. Water Contact Angle (CA) Measurement. The CA was measured using an optical video contact angle instrument (Model OCA 40, Dataphysics, Germany) at room temperature. The water CA was determined 60 s after a water droplet of 6 µL was placed on the fabric. Each CA presented was the average of those measured at five different locations of each fabric specimen. 2.3.3. Scanning Electron Microscopy (SEM). The surface morphology of the treated fabrics were measured using SEM (Model JSM-5600LV, Jeol, Japan). 2.3.4. Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectra were collected using a Nicolet Nexus670 Fourier transform spectrophotometer with an attenuated total reflection (ATR) accessory. The resolution was 4 cm-1, and the number of scans was 100 for each spectrum. 2.3.5. Measurement of Laundering Durability of the Treated Fabrics. Evaluation of laundering durability of the chemically modified fabrics was carried out according to AATCC (American Association of Textile Chemist and Colorist) Test Method 61-2003. The study was performed using a standard accelerated laundering machine (Model Washtec P, Roaches, Great Britain) equipped with 500 mL stainless-steel lever-lock canisters. The water temperature was 40 °C. The fabric was laundered in a canister containing 200 mL aqueous solution of an AATCC standard reference detergent (0.37%, w/w).

2.3.6. Measurement of Physical Properties of the Treated Fabrics. The physical properties of the cotton and polyester fabrics, including tensile strengths, whiteness, and air permeability, were measured before and after the coating procedures. The tensile strength was measured using an universal material testing machine (Model H10K-S, Tinius Olsen, America) according to Chinese National Standard Test Method GB/T39321997. The whiteness was measured using a color measuring and matching system (Model Datacolor 650, Datacolor, America) according to Chinese National Standard Test Method GB/ T8424.2-2001. The air permeability was measured using gas permeability tester (Model YG461E, Ningbo Textile Instrument Co., Ltd., China) according to Chinese National Standard Test Method GB/T5453-1997. 3. Results and Discussion The procedure for forming a superhydrophobic surface on cotton fabric using silica sol and HDTMS described in the Experimental Procedures section is summarized in Scheme 1. TEOS is hydrolyzed in an ethanol/H2O solution to form silica sol, whereas HDTMS is hydrolyzed in an aqueous ethanol solution to form alkylsilanol. A cotton fabric specimen is first treated with the silica sol using a pad-dry method, then immersed in the hydrolyzed HDTMS solution, and finally dried and cured. The self-assembly of HDTMS is formed by the reaction between the alkylsilanol and the surface hydroxyl groups of silica nanoparticals on the cotton fiber surface (Scheme 1). A series of silica sol samples (sol 1-5) with different particle sizes are prepared by changing the NH4OH concentration during the formation of the silica sol as discussed previously (Table


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Table 2. Water CAs of the Cotton Fabric Sample Treated First with Silica Sols (Sol 1-5) Then with Hydrolyzed HDTMS sample code sample C1 sample C2 sample C3 sample C4 sample C5 sample C6 a

Figure 1. Number percentage of the silica particles of sol 1-5 as a function of particle diameter. Table 1. Preparation of Silica Sols with Different Particle Sizes

sample code

TEOS (mL)

EtOH (mL)

NH4OH (mL)

silica particle average size (nm)

sol 1 sol 2 sol 3 sol 4 sol 5

6.0 6.0 6.0 6.0 6.0

100 100 100 100 100

3.0 4.0 4.5 5.0 6.0

30.37 33.80 46.06 52.33 71.48

polydispersity index (PDI) 0.226 0.241 0.356 0.135 0.159

1). The number-average particle size and polydispersity of the silica sol samples (sol 1-5) thus prepared are presented in Table 1. The number percentage of the silica sol samples sol 1-5 is shown as a function of particle diameter in Figure 1. The data presented here show that the number-averaged particle size of the silica sol increases as the NH4OH concentration used is increased, because increasing the NH4OH concentration accelerates the condensation and aggregation of the silica sol particles. The silica sol thus prepared can easily be coated on the surface of a material using methods including dipping, spraying, or spin coating. Water CA is used in this research for evaluating the hydrophobicity of the chemically modified cotton and polyester

first treatment

contact angle (deg)a


143.7 ( 1.4 151.2 ( 2.1 151.8 ( 1.8 155.1 ( 2.5 144.8 ( 2.2 140.0 ( 1.9

The water CAs expressed here are the means ( standard deviations.

fabrics. The limitation of this method, however, should be noted. Due to the fibers sticking out from the surface of a fabric sample, as seen in Figure 2, the determination of the baseline of the water droplet is more difficult, which may in turn lead to possible underestimation of the CA data. Additionally, because the protruding fibers may exhibit forces on the water droplet, it is also difficult to yield accurate values for advancing and receding water CAs. In this research, we only measure and report static CAs for 6 µL water droplets. It is well-known that water can completely wet pure cotton. A water drop placed on the surface of a cotton fabric sinks completely into the fabric after a few seconds as shown in Figure 2A. A polyester fabric surface cannot support the formation of any spherical water droplets as shown in Figure 2D. Therefore, one cannot measure water CA on the surfaces of cotton and polyester surfaces before chemical modification. The cotton fabric is treated first with silica sol then with hydrolyzed HDTMS as described previously. The water CA values of the cotton fabric samples thus treated (sample C1-C5) and that treated with hydrolyzed HDTMS only (sample C6) are presented Table 2. The cotton fabric samples treated with hydrolyzed in HDTMS only has a water CA of 140.0°, and it becomes 143.7° when the cotton is first treated with silica sol (sol 1) then with HDTMS, indicating that treatment of cotton by silica sol before the treatment using HDTMS increases the water CA of the fabric. The fabric

Figure 2. Images of the 6 µL water droplet on (a) untreated cotton; (b) the cotton treated with hydrolyzed HDTMS only; (c) the cotton treated with sol 4 silica sol and hydrolyzed HDTMS; (d) untreated polyester; (e) the polyester treated with hydrolyzed HDTMS only; (f) the polyester treated with sol 4 silica sol and hydrolyzed HDTMS.

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cos θC-B ) fls cos θs - flv ) rf cos θs + f - 1 ) f(r cos θs + 1) - 1

Table 3. Water CAs of the Polyester Fabric Treated First with Silica Sols (Sol 1-5) Then with Hydrolyzed HDTMS sample code sample P1 sample P2 sample P3 sample P4 sample P5 sample P6 a

first treatment

contact angle (deg)a


139.8 ( 1.5 139.7 ( 2.4 140.4 ( 2.1 143.8 ( 1.7 139.8 ( 2.6 132.9 ( 1.8

The water CAs expressed here are the means ( standard deviations.

treated with sol 4 and HDTMS has the highest water CA of 155.1° as shown in Figure 2C. The water CA values of the polyester fabrics treated first with sol 1-5 then with hydrolyzed HDTMS are presented in Table 3. One observes phenomenon similar to those of the treated cotton fabric discussed above. The polyester fabric treated with the silica sol and hydrolyzed HDTMS is modestly higher than that treated with HDTMS only, and the polyester fabric treated with sol 4 and HDTMS has the highest water CA of 143.8° (Table 3 and Figure 2F). Therefore, we can conclude that treating the cotton and polyester fabrics using silica sol and hydrolyzed HDTMS is able to drastically increase their hydrophobicity. The increase in water repellency of a textile fabric as a result of the presence of the silica nanoparticles on the fabric surface can be attributed to the increased roughness on the surface. The wettability of surfaces with liquids is controlled not only by the chemical composition but also by the geometry of the surface. The effect of surface roughness on wettability has been explained by the Wenzel and Cassie-Baxter models. The basic assumption in Wenzel’s theory is that the water droplet penetrates the asperities while the water droplet suspends above the asperities for the Cassie-Baxter model. These models form the basic guidelines for the study of superhydrophobic surfaces. For the Wenzel model, there is a linear relationship between the apparent contact angle of the surface and the roughness factor of the given surface.4,24 cos θw ) r cos θY

(1)

where θw is the apparent water CA on a rough surface, θY is the ideal contact angle on a smooth surface, and r is the roughness factor, which is defined as the ratio of actual area of liquid-solid contact to the projected area. This equation shows that when the surface is hydrophobic (θY > 90°), the CA increases as the roughness increases. The wetting behavior can also be described by the following equation in the Cassie-Baxter model.2,13 cos θC-B ) fls cos θs + flv cos θv

(2)

Where θC-B is the apparent water CA, fls is the liquid/solid contact area divided by the projected area, and flv is the liquid/ vapor contact area divided by the projected area. The terms θs and θv are the corresponding water CAs on the smooth solid surface and vapor surface. The liquid-air contact angle θv is 180°, and cos θv gives the overall minus sign in the equation. Given f is the fraction of the projected area of the tops of the solids in contact with the liquid (flv ) 1 - f) and r is the roughness of the wetted solid area (r g 1), eq 2 can be modified as follows.

(3)

For the HDTMS-modified cotton fiber in the absence of silica particles, the curvature of the cotton fiber renders r > 1, which, in comparison with a smooth wetted area, can enhance the surface hydrophobicity. The morphology of the cotton and polyester fabrics treated with and without silica sol is investigated using SEM. The SEM micrographs of the untreated cotton fabric, that treated with sol 4, and that treated with sol 4 and HDTMS are shown in Figure 3. The surface of the untreated cotton fabric shows grooves and cracks (Figure 3A). In contrast, such characteristics completely disappear on the surface of the cotton fabric treated with sol 4, on which a compact coating with monosized spherical silica nanoparticles appears (Figure 3B). It is the nanoparticles which make the surface rough, thus enhancing the fabric’s water repellency. After the cotton fabric is treated with sol 4 and then with hydrolyzed HDTMS, the silica nanoparticals still remain visible on the surface of the fabric as shown in Figure 3C. For the untreated polyester fabric, its surface is smooth (Figure 4A). In contrast, many nubs or burls appear on the surface of the polyester fabric treated with sol 4 (Figure 4B). Those nubs or burls significantly increase the surface roughness, thus enhancing the fabric’s water repellency. The surface of the polyester fabric treated with both sol 4 and hydrolyzed HDTMS shows no significant further changes (Figure 4C). The superhydrophobic cotton fabric discussed above is also studied by FTIR spectroscopy (Figure 5). The typical absorption peaks of the Si-O-Si bonds of the silica sol and HDTMS in the 1100-1000 cm-1 region appear to be overlapped by the cellulose bands due to C-O bending modes. However, a new band appears at 791 cm-1 corresponding to Si-C stretching in the spectrum of the cotton fabric treated with sol 4 and HDTMS (Figure 5B). For practical use by consumers, the water repellency of textile fabrics needs to be resistant to multiple laundering cycles. The laundering durability of the hydrophobicity imparted to the cotton and polyester fabrics by the treatment using silica sol and hydrolyzed HDTMS is investigated in this research. The cotton and polyester fabrics are first treated with sol 4 and hydrolyzed HDTMS, and the fabrics thus treated are subjected to 30 home laundering cycles. Presented in Figure 6 are the changes in water CA as a function of the number of laundering cycles. The water CA of the treated cotton fabric decreases from 155° to 95° after 30 wash cycles whereas that for the treated polyester fabric decreases from 143° to 110°. Nevertheless, the water CA of both the treated cotton and the polyester fabrics remains above the 90° threshold for hydrophobic surfaces. Figure 6 also shows that most of the decrease in the hydrophobic properties occurs during the first 15 laundering cycles and the water CA decreases only slightly after 15 laundering cycles for both fabrics. The laundering durability of the hydrophobicity of the treated fabrics has been further improved most recently in our research and will be reported in a future paper. Further SEM analysis is applied to investigate the surface morphology of the cotton and polyester fabric after 30 home laundering cycles (Figure 7). The scattered particles together with particulate agglomerate are visible on cotton and polyester fiber surfaces after 30 laundering cycles. For cotton fabric, the silica gel film on the fiber surface is cracked


Figure 3. SEM micrographs of (a) the untreated cotton, (b) the cotton treated with sol 4, and (c) the cotton treated with sol 4 and hydrolyzed HDTMS.

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Figure 4. SEM micrographs of (a) the untreated polyester, (b) the polyester treated with sol 4, and (c) the polyester treated with sol 4 and hydrolyzed HDTMS.

probably due to swelling of cotton. The retention of the hydrophobic properties of the treated cotton and polyester fabrics after repeated laundering can probably be attributed to the formation of interfacial chemical bonds.25 For the cotton fabric, the surface cellulosic hydroxyl groups undergo condensation with the hydrolyzed TEOS (via Si-OH groups) to form interfacial Si-O-C bonds. Under alkaline conditions, polyester possibly forms surface carboxylic groups, which subsequently cocondenses with the Si-OH groups, leading to the formation of small quantity of interfacial ester bonds.26 It was important that the physical properties of cotton and polyester are not adversely affected by the coating procedures. Presented in Table 4 is the comparison of the physical properties of cotton and polyester substrates before treatment and those of the fabrics after the coating. For the cotton fabric, the treatment using silica sol and hydrolyzed HDTMS causes approximately 9% decrease of tensile strength in the warp direction and approximately 3% decrease in the filling

Figure 5. FTIR spectra of the surfaces of (a) the untreated cotton fabric and (b) the cotton fabric treated with sol 4 and hydrolyzed HDTMS.

direction. For polyester fabric, the decrease in fabric tensile strength is even lower (2-3%). One also observes that the changes in whiteness and air permeability of the cotton and polyester fabrics are negligible.

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treatment of the cotton fabric causes limited reduction in the fabric tensile strength (

Formation of Highly Hydrophobic Surfaces on Cotton and Polyester

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