Production of Superhydrophobic Cotton Fabric by Layer-By-Layer Deposition of SiO2/TiO2-Polydimethylsiloxane

ABSTRACT In this study, the aim was to produce superhydrophobic cotton fabric by TiO2 and SiO2 nanoparticles, and polydimethylsiloxane using layer-by-layer deposition method. It was found that the increase in the bilayer number and PDMS concentration improved superhydrophobicity. In this way, water contact angle values higher than 150° were provided. The scanning electron microscopy and atomic force microscopy analyses demonstrated the formation of surface roughness. Moreover, the physical fabric tests and water and stain repellency tests were also carried out. L-b-L deposited samples were found to be softer and had high water and stain repellency values. The durability of the coatings to washing was also confirmed.


Introduction
Superhydrophobic textile materials are of great interest recently due to many potential applications. Superhydrophobicity firstly observed in nature as many plant leaves and animals have adapted to natural life with their superhydrophobic surfaces, such as lotus leaves (Barthlott and Neinhuis 1997;Zhao et al. 2009), rice leaves (Kwon, Huh, and Lee 2014), water striders (Gao and Jiang 2004;Peng, Meng, and Li 2016;Shi et al. 2007), butterflies (Shi et al. 2011;Zheng, Gao, and Jiang 2007), cicadas (Ohko et al. 2001;Sun et al. 2009), mosquitos ) and sharks (Bixler and Bhushan 2012;Pan, Xiao, and Ye 2019). Superhydrophobic surfaces defined as the surfaces of which the static water contact angle (WCA) is greater than 150° and the contact angle hysteresis is smaller than 10°. It is a complex phenomenon related to the combination of low surface energy and high surface roughness (Feng et al. 2004;Ivanova and Philipchenko 2012). The surface roughness plays an important role in limiting the wetting behavior of a solid surface and developing superhydrophobic surfaces (Zhao et al. 2009). When the surface is rough, the water and dirt particles cannot touch all surface points and cannot adhere and slide away from the surface (Darband et al. 2020). With the creation of the superhydrophobic feature in textile materials, a variety of functional properties such as high water repellency, self-cleaning, and anti-fouling were able to be developed (Zhao et al. 2010).
Cotton fibers are still widely used due to their advantages such as soft handle, breathability, and naturalness. Due to the hydroxyl groups of cellulose, the cotton fibers can easily be wetted and stained by liquids, which limits the use of cotton garments especially as waterproof outer garments. Therefore, it is clear that the development of superhydrophobic property on cotton fabrics can provide an advantage in this respect. To obtain the superhydrophobic effects on cotton fabrics, a variety of chemicals were used in the literature, of which two were TiO 2 and SiO 2 . By applying TiO 2 or SiO 2 particles onto the cotton fabrics, a rough surface was reported to be formed by the deposition of micro/nano scale structures on the surface of the fibers (Chen et al. 2018;Hao et al. 2016;Liu, Xin, and Choi 2012).
The development of superhydrophobic textiles have been obtained by a number of different methods such as sol-gel (Guo et al. 2017;Liu et al. 2015;Shi et al. 2012), vapor deposition , plasma (Zhang et al. 2003), electrospinning (Sheng et al. 2017), dip-coating (Pan, Xiao, and Ye 2019), spray coating (Lei et al. 2017;Li et al. 2014;Zhang, Ge, and Yang 2014), and layer-by-layer deposition (Nanda et al. 2017;Zhang et al. 2008Zhang et al. , 2012Zhou et al. 2013), etc. The principle of action of all methods to produce a superhydrophobic surface on fabric is based on coating the surface of microscale fibers with nanoscale particles and then, modifying the surface with low surface energy . Within these methods, layer-by-layer deposition method stands out due to the possible modification of textile materials without altering comfort and handle properties. The principle of the layer-by-layer deposition method is the coating of the surface of the material by positively and negatively charged polyelectrolytes up to desired number of bilayers. In this way, extremely thin films can be formed on the surface of the material. L-b-L deposition can be ensured by dip coating, spray coating, and spin coating methods. As a result, L-b-L deposition is an easy-to-apply, low-cost, environmentally friendly, and suitable method for the production of special fabrics in the field of technical textiles (Michel et al. 2012).
Previous research on the production of superhydrophobic fabrics with L-b-L deposition method were appeared in the literature. Accordingly, Lin et al. (Lin et al. 2018) produced superhydrophobic and flame retardant cotton fabrics by L-b-L deposition by using branched polyethyleneimine, ammonium polyphosphate, and fluorinated silica/polydimethylsiloxane solutions. They reported that self-cleaning and antifouling properties were also provided. In the study of Lee et al. (Lee, Rubner, and Cohen 2006), anti-reflection, anti-fogging, and self-cleaning properties were investigated by L-b-L deposition of TiO 2 and SiO 2 nanoparticles. Chen et al. (Chen et al. 2015) applied branched polyethyleneimine, ammonium polyphosphate, and fluorinated-decyl polyhedral oligomeric silsesquioxane to cotton fabrics by L-b-L method to enhance flame retardant and superhydrophobic fabrics. Durability of the coating to abrasion was also indicated. In another study, Zhang et al. (Zhang et al. 2012) applied cationic poly (dimethyldialylammonium chloride) and silica particles to obtain superhydrophobic cotton fabric by L-b-L deposition. Zhao et al. (Zhao et al. 2013) prepared superhydrophobic and UV protective cotton fabric by using L-b-L deposition method. Wang et al. (Wang, Tian, and Zhang 2020) fabricated superhydrophobic coatings by spraying the suspension of SiO 2 /polydimethylsiloxane on various substrates including textile materials. Resultant superhydrophobic SiO 2 /PDMS coatings were shown to have good mechanical durability, chemical stability, and self-cleaning performances. Uğur et al. (Uğur et al. 2010a) investigated the L-b-L deposition of TiO 2 nanoparticles onto cationic cotton fabric and photocatalytic activity and UV blocking properties were investigated as well as the durability of the coatings to washing. Within the present study, L-b-L deposition of SiO 2 , TiO 2 , and polydimethylsiloxane (PDMS) onto cotton fabric was investigated to obtain superhydrophobicity. When the previous studies were evaluated in general, it was shown that the number of studies on the production of superhydrophobic cotton fabric by L-b-L deposition and especially the use of TiO 2 , SiO 2 , and PDMS compounds together was very limited. In addition, the durability of the superhydrophobic effect obtained in these studies to washing was not investigated. Therefore, it is thought that our research will contribute to the solution of existing problems and scientific studies in this field. For this purpose, the effects of PDMS concentration and the number of bilayers on superhydrophobicity and washing durability were investigated. Both characterization and fabric tests were applied as well as water contact angle measurements.

Materials
The scoured and bleached cotton woven fabric (plain-woven fabric with a weight per unit area of 246 g/m 2 ) supplied from Batı Basma Company (Turkey) was used. Tetraethyl orthosilicate (TEOS), titanium(IV) isopropoxide (TTIP) and polydimethylsiloxane (PDMS) were purchased from Sigma-Aldrich. Anhydrous ethanol was obtained from Tekkim Chemical Company (Turkey).

Method
It is known that the active sites should be introduced onto the substrates to fabricate multilayer films by L-b-L technique. For this purpose, the surface of the cotton fabric was charged by cationization with a reactive quaternary ammonium-based commercial chemical (Hydrocol KNR, Rudolf Duraner Company) by pad-batch method. The fabric samples were impregnated with aqueous solution of 250 g/l Hydrocol KNR and 85 g/l NaOH with a pick up ratio of 80%, and kept at room temperature for 24 h. After that, the samples were rinsed, washed at room temperature with 1 g/l washing agent, rinsed at 60°C for 10 min and rinsed at room temperature for 5 min, followed by drying at 120°C for 2 min.
SiO 2 and TiO 2 nanoparticles and PDMS were used for L-b-L deposition by dip coating method. SiO 2 nanoparticles were prepared according to Li and He (2013) by mixing of 120 ml of TEOS, 180 ml of ethanol, and 2 ml of NaOH, followed by stirring at 60°C for 2 h, and then kept stable for two weeks. In order to prepare TiO 2 nanoparticles (according to Li and He 2013), 20 ml of TTIP, 100 ml of ethanol, 300 ml of deionized water, and 6 ml of nitric acid (65%) were mixed, followed by stirring at 60°C for 2 h, and then kept stable for two weeks.
For the preparation of L-b-L solutions 1 g of PDMS was added into 50 ml of each solution (SiO 2 and TiO 2 solutions) and stirred at 60°C for 15 minutes. The pH of the resultant solutions was adjusted to be 10.5 and 2.5 for SiO 2 -PDMS and TiO 2 -PDMS, respectively. Samples were first immersed into SiO 2 -PDMS solution for 5 min followed by rinsing in deionized water for 2.5 min. Then, the samples were immersed in TiO 2 -PDMS solution for 5 min followed by rinsing in deionized water again for 2.5 min. In this way, one bilayer of SiO 2 -PDMS/TiO 2 -PDMS was deposited on the surface of the cotton fabrics. After the immersion processes, the samples were dried at 80°C for 10 min. The same procedure was repeated until the desired bilayer number was obtained. In this study, the deposited bilayer number was selected to be 1, 4, and 8. In the final stage, L-b-L deposited samples were immersed into a PDMS solution in 50 ml ethanol including 10% cross-linker (modified glyoxal resin, Reacel ZF CAT, Bozzetto Chemical Company) and the samples were cured at 150°C for 30 min. Three different PDMS amounts (1, 2, and 3 g in 50 ml solution; that is 20, 40, and 60 g/l) was used in this stage. The resultant samples were encoded as "C-PDMS amount in final PDMS solution-number of bilayers deposited" sequence (e.g. C24 represented that 2 g of PDMS was used in the final bath and 4 numbers of bilayers were obtained by the L-b-L deposition process). "C" alone denotes the untreated fabric. Moreover, only PDMS treated sample was also produced by the application of the aforementioned final step to untreated cotton fabric, which was encoded as C-P.

Testing and characterization
The water contact angles (WCA) of all fabrics were measured using the water drop method by CSV Cam 101 model device at room temperature. A drop of pure water is dripped onto the fabric with the help of a syringe. The image of this drop when it falls on the fabric surface is captured by the camera system. The contact angle between the pure water drop and the fabric surface is calculated by the formulation according to the Young model. After the applications, the standard washings were carried out to check the washing durability of the L-b-L deposited samples with high contact angles. The samples were washed in 5 times at the Wascator machine according to ISO 6330 (2012) standard (40ºC for 58 minutes) and dried at room temperature.
Considering the WCA results, the morphological and surface properties of the selected samples were characterized. Accordingly, the surface morphology of the samples was visualized by scanning electron microscopy (SEM, Thermo Scientific Apreo S). The surface topography of the selected sample (C38) was displayed by atomic force microscopy (AFM, BRUCER Dimension Edge with ScanAsys). X-ray photoelectron spectrometer (×PS, Thermo Scientific C-Alpha) was employed to characterize the surface chemistry of the samples.
The water repellency test was carried out in the Spray Tester according to the AATCC 22 (2017) standard. The stain repellency test was carried out according to the AATCC 130 (2018) standard. In this test, tea, coffee, and cherry juice stains were dripped onto the fabric. After washings, the samples were evaluated with the special scale of the standard, where 1 is worst and 5 is the best stain repellency value. As well as instrumental analyses, physical fabric tests were also applied to selected samples. Accordingly, fabric thickness was measured by SDL ATLAS F×44 device according to the TS 7128 EN ISO 5084 (1998) standard. The tensile forces of the fabrics were determined in accordance with the TS EN ISO 13,934-1 (2013) standard, using the Strip method (ZwickZ010). Circular flexural strength test was carried out by SDL Atlas circular flexural strength device according to ASTM D4032-08 (2016) standard. The air permeability test was carried out in a 20 cm 2 measurement area and a pressure difference of 100 Pa by F×3300 Textest device according to TS 391 EN ISO 9237 (1999) standard. The surface roughness test of fabrics was carried out in Mitutuyo SJ 310 brand surface roughness measuring device according to JIS B 0601 (2013) standard. At the end of the test, the Rv value is obtained as the maximum depth at one sampling length. All physical tests were done in 3 repetitions.

Evaluation of contact angle results
The degree of hydrophobicity obtained by L-b-L deposition was investigated by WCA tests. The number of WCA measurements was 3. The WCA of the untreated fabric was measured to be 21.4°, which points out a hydrophilic structure. WCA values of only PDMS applied cotton fabric were also tested and it was found to be 130.4° which categorically showed that superhydrophobic property could not be achieved. It was also observed that although a relatively high contact angle was provided at the beginning, the contact angle gradually decreased as the drop on the surface was slowly absorbed as time progressed. As shown in Figure 1, L-b-L deposited samples became highly hydrophobic. Considering 150° as the limit value for the superhydrophobicity, it was observed that when one bilayer was formed, high amount of PDMS was required to achieve superhydrophobicity. When the number of bilayers increased, it was observed that need for PDMS was decreased and superhydrophobic properties could be achieved for all samples. This categorically indicated that the L-b-L deposition of SiO 2 -PDMS/TiO 2 -PDMS created adequate roughness on the surface of the cotton fibers. In addition, since PDMS has a high hydrophobic effect and low surface energy, a superhydrophobic effect was provided. Results showed that the effect of PDMS amount was more pronounced for 8 bilayers deposited samples. On the other hand, the increase in the number of bilayers obtained by L-b-L deposition process led to an increase in WCA. For this reason, 3 g PDMS impregnated samples containing different number of bilayers were used for the following parts of the study.
The standard washings were carried out to check the washing durability of the samples with high contact angles (C31, C34, and C38). The measured WCA of the samples after 5 consecutive washing are given in Figure 1. The PDMS-only sample (C-P) showed a significant decrease in WCA after washing. On the other hand, there was a very little loss in WCA values of L-b-L deposited samples after the washing processes. The loss in WCA due to washing increased with the increase in the number of deposited bilayers. It was indicated that the deposited bilayers were durable to washing to a great extent and the loss of superhydrophobicity after washing was negligible.

Surface characteristics
In order to investigate the effects of the number of bilayers on surface morphology of L-b-L deposited samples, SEM analyses were applied to C31, C34, and C38, as shown in Figure 2. It was observed that very small nano-sized particles were formed on the surface of the fibers. Also, homogeneous distribution of TiO 2 and SiO 2 nanoparticles was detected. In general, in terms of the nanoparticle density (from the densest to the lowest) and the surface roughness (from the roughest to the least roughness),  an order can be made as follows: C38˃C34˃C31. It is clear that the surface roughness was improved with the increase in bilayer number, which is in line with WCA test results. Furthermore, the change in the surface morphology after L-b-L deposition for C38 sample (which had the highest WCA) was scanned by AFM analysis. In the study of Tian et al. (Tian et al. 2016), it was reported that the surface of untreated cotton fabric is smooth and the micro-fibrillary structure of cotton fiber has been investigated to be visible as parallel narrow stripes. Zhu et al. (Zhu et al. 2017) reported that the root mean square (RMS) value of untreated cotton fabric was 15.4 nm in AFM analysis. In this study, AFM result of C38 sample showed a rough topographical structure with a RMS value of 57.6 nm rather than a parallel striped surface structure. As a result, it was proven that L-b-L deposition was roughened the surface. This result also supported contact angle and SEM images.

Amount of PDMS (g)
To prove that TiO 2 and SiO 2 were deposited onto the cotton fiber, XPS analyses were carried out. XPS analysis of C38 sample which had the highest WCA is shown in Figure 3 through characterizing the chemical compositions of the surface in the range of 0-1200 eV. In this spectra, the characteristic peaks at 284.97 and 532.39 were attributed to carbon and oxygen atoms, respectively (Uğur et al. 2010b). The peaks at 455.88 eV and 102.43 eV indicated the presence of Titanium (Ti) and Silicon (Si) atoms, respectively (Patrocinio et al. 2014;Zhao et al. 2010). XPS elemental analysis results showed that Si and Ti content was generated after L-b-L deposition. The Si content of L-b-L deposited sample was found to be quite high compared to Ti content, due to the use of PDMS besides SiO 2 . XPS results categorically showed that the TiO 2 nanoparticles and, SiO 2 nanoparticles and PDMS were successfully deposited onto cotton fabric by the L-b-L deposition.

Evaluation of physical test
The water repellency, air permeability, and physical test results of selected fabrics are given in Table 1 for untreated sample and L-b-L deposited sample which exhibited the highest WCA (C38). It was observed that the fabric thickness increased by L-b-L deposition possibly due to the formation of bilayers on the surface of the treated fabrics. The tensile force of untreated cotton fabric was higher than those of other samples. In L-b-L process, HCl and NaOH were used to impart anionic and cationic character to nanoparticle dispersions. Also, in the final PDMS application, a cross-linking agent was used to impart durability of the coating. Thus, the decrease in tensile force could be as a result of both the degradation of cellulose due to the use of highly acidic TiO 2 solutions and the restriction of the mobility of molecular segments due to intermolecular crosslinking.
The circular bending strength values of C-P and C38 samples were found to be lower compared to untreated sample. The lower the circular bending strength, the softer the handle of the fabric. Although the increase in fabric thickness after the processes created an expectation of hardening in the fabric handle, the increase in softness revealed that the positive effect of PDMS use was more pronounced in terms of handle. The reason why the touch is softer after the treatment is that the PDMS material containing siloxane was transferred onto the fabric. The air permeability, on the other hand, decreased after L-b-L deposition, which is due to the closure of the pores as can be clearly seen in SEM images in Figure 2. The surface roughness of L-b-L deposited sample was found to be higher compared to untreated and only PDMS applied samples, as expected. This result is in agreement with the SEM images and AFM analysis result. As expected, L-b-L deposition of TiO 2 /SiO 2 -PDMS led to a high degree of the water repellency due to the formation of superhydrophobic coating on the fiber surfaces. Figure 4 illustrates the stained untreated, C-P and C38 samples with tea, coffee, and cherry juice. Due to the imparted superhydrophobicity, it was shown that the stain solutions could not spread on the fabric surface and remained on the surface as droplets. On the contrary, in the untreated fabric and C-P, it was seen that the stains spread on the fabric surface and penetrated into the fabric. The stain repellency grade of the samples is given in Figure 5. The number of measurements was 3. While the stain repellency values of untreated sample stained with tea and cherry juice and washed 5 times were 1.5, the stain repellency values were increased by L-b-L deposition. It was observed that C38 and C-P samples had the highest stain repellency values in all fabrics. The increase in the bilayer number led to an increase in stain repellency for tea and cherry juice stains. Since coffee stain is a heavier stain, the stain repellency was found to be lower compared to other stains and samples with 4 and 8 bilayers showed similar stain repellency.

Conclusion
L-b-L deposition of TiO 2 and SiO 2 nanoparticles, and PDMS was proven to be effective for the production of superhydrophobic cotton fabrics with WCA values higher than 150°. The formation of a rough surface on cotton fabric was confirmed by SEM and AFM analyzes. In this context, high surface roughness was obtained by the self-assembly of TiO 2 and SiO 2 and low surface energy is provided with the final PDMS coating. The increase in the number of bilayers led to an increase in WCA. For 8 bilayers deposited samples, the amount of final PDMS application was found to be significant, where the WCA value was increased from 151.9° to 158.0° for 1 and 3 g PDMS concentration, respectively. The resultant fabrics were reported to be softer in handle and had higher water repellency. Moreover, it was investigated that the deposited bilayers were durable to washing without significant loss of WCA. Further studies on textile materials made of other natural fibers or synthetic fibers are planned to determine the scope of applicability of the methodology.