Superhydrophobic textile: treatment in aqueous solutions of aluminum salts

Textile is currently a promising material. Obtaining hydrophobic surfaces on textiles significantly increases its value when used in various fields. In this work we carried out experiments on textile processing. Treatment of textile materials in solutions containing aluminum allows to obtain a superhydrophobic surface. KAl(SO4)2 and AlCl3 solutions were used. It was found that treatment in AlCl3 solution is more effective and allows to achieve a hydrophobic surface on textile with a contact angle of more than 150º. The hydrophobic surface retained its properties even after 30 days of air storage and after scraping and crumpling of samples. Textile samples were investigated using X-Ray photoelectron spectroscopy (XPS). The X-Ray photoelectron spectroscopy results showed hydrophobicity in the treatment of textile materials is ensured by the formation of aluminum oxide on the surface. The dependence of the coarse calico contact angle on the AlCl3 solution concentration is determined. which demonstrates that when the concentration of AlCl3 solution increases (within the limits of variation considered), the contact angle also increases.


Introduction
Technical textile is widely used in such diverse elds as medical technology, aerospace and automotive industries, modern architecture and building, ltration and transport systems (Holme I. 2007;Çay A. et al. 2020;Karpushko E. N. et al. 2016;Cherston J. and Paradiso J. A. 2019;Thierry V. et al. 2018). To achieve increased hydrophobicity self-cleaning ability and the ability to create barrier coatings on the surface of the bers, a surface modi cation of the materials is required.
Nowadays, there are many known methods for modifying the surface of textile materials, these include: treatment in aqueous solutions of various compositions; sol-gel method (Yang M. et al. 2018;Lin D. et al. 2019;Liang J. et al. 2013); methods of deposition nanoparticle to create roughness on surface , chemical vapor deposition (CVD) (Şimşek B. and Karaman M. 2020) and plasma-chemical treatment (Gaiolas C. et al. 2013;Pandit P. et al. 2020;Belhaj Khalifa I. and Ladhari N. 2019;Xu L. et al. 2019;Samanta K. K. et al. 2016;Molina R. et al. 2017). The essence of the methods consists of the formation of thin hydrophobic layers on the materials surface (Daoud W. A. et al. 2004) or inoculation of various functional groups that change the surface properties cardinally [18].
Fluorocarbons with subsequent thermal stabilization are most often used as hydrophobizing agents (Bahners T. et al. 2008). As an alternative to uorocarbons, hydrophobic silicone-based layers are proposed to be obtained.
The paper (Daoud W. A. et al. 2004) describes a method of producing hydrophobic silicon dioxide lms of nanometer thickness on cotton fabrics, providing a contact angle of 140º. A signi cant disadvantage of the method is the high process time, since the preparation of the necessary solutions may take about a day, which signi cantly affects the total process time.
In Xu L. et al. 2012;Liang J. et al. 2013), the sol-gel method was used to form silicon dioxide coatings from a modi ed hydrosol of SiO 2 , the results of which showed that, compared to untreated samples with a contact angle of 0º, after treatment, the contact angle had reached 152º in the papers (Xu L. et al. 2012;Liang J. et al. 2013), and in ) contact angle was 145-155º. Even higher contact angles (163 ) were achieved ) when using hydrophobic coatings based on TiO 2 being obtained from a TiO 2 sol. The sol-gel method, although it is one of the simplest to obtain thin lms, including hydrophobic ones, but its implementation is di cult to obtain a sol, as well as its stabilization. To stabilize the sols, additional reagents are required, which signi cantly increases the cost of the entire processing process. Another disadvantage of the sol-gel methods is the necessity of repeating certain steps of the production of coatings, which increases the overall process time.
The surface treatment in solutions and emulsions, which include aluminum-containing reagents, is highly effective in achieving the hydrophobicity of various textile materials. One of the main methods of obtaining hydrophobic aluminum-containing coatings is Atomic Layer Deposition (ALD) (Xingfang X. et al. 2015;Hyde G. K., et al. 2010). So, in article (Xingfang X. et al. 2015) aluminum oxide was deposited onto wool fabrics by exposing them to alternating pulses of trimethylaluminum and water at 80°C. The water contact angles of ALD coated wool fabrics increased from 130° to around 160°. The authors of (Hyde G. K., et al. 2010) created aluminum oxide coatings and studied the surface wettability according to the Cassie − Baxter and Wenzel models. They created surfaces with a contact angle about 140º.
Cheaper and simpler than the ALD is the wet chemistry method for obtaining hydrophobic surfaces containing aluminum on textile materials. Despite the simplicity of the method, there is little research that is not aimed at studying the capabilities of the method and various dependencies. Therefore, the researching the possibility of achieving high hydrophobicity of textile materials as a result of their treatment in KAl(SO 4 ) solution and AlCl 3 solution, experimental determination of the effect of treatment parameters on the contact angle, as well as nding out the reasons for the increase surface hydrophobicity.

Materials And Methods
The textile samples were squares of coarse calico with a density of 142 g/m 2 and chintz with a density of 80 g/m 2 . The warp and weft bers are alternately interconnected in a 1:1 pattern. Sample size was (10x10) mm 2 . The choice of such a sample size is due to preliminary experiments, which showed that the contact angle under the same processing conditions on samples (10x10) mm 2 and (300x400) mm 2 is identical.
Samples were dipped for 15 minutes in a soap solution with a concentration of 35 g/l and a temperature of 75° C in order to clean the surface from unwanted impurities. We used soap with pH = 7 to make the solution. The concentration of KAl(SO 4 ) solution and AlCl 3 solution was 80 g/l and 35 g/l, respectively.
KAl(SO 4 ) solution was prepared from powder of "h" (GOST 4329-77) 2 class of purity and was diluted with distilled water to the desired concentration. AlCl 3 solution was prepared similarly from AlCl 3 powder of the "analytical grade" (GOST 3759-75) purity class.
Resistive heating was used to provide the required temperature of the aqueous solution. Temperature was controlled by a thermocouple. Textile samples were dried using three layers of lter paper, without wringing. After treatment the samples in solutions, they were exposed to the action on an ironing press at a temperature of T = 120 ° C. To maintain constant moisture after treatment, the samples were placed in a desiccator, in which phosphoric anhydride was used as a desiccant.
Wettability control was carried out by measuring the contact angle of wetting using the "sitting" drop method (Vohrer U. et al. 1998;Gao L. et al. 2009;Marmur A. et al. 2017). Error of measuring the contact angle of the surface is 1.5% of the indication. Dosing of the droplet volume was carried out using a Proline® mechanical dispenser with an accuracy of 1.5%. The contact angle was determined using a specialized computer program.
The research of the surface composition of the textile materials after treatment was carried out by X-ray photoelectron spectroscopy on a super-vacuum complex "Nanofab 25" company SPECS. We used Mg Kα radiation with an energy of 1253 eV. The energy linewidth of X-ray radiation without monochromatization was about 0.5 eV. We recorded survey spectra with a step of 1 eV, spectra of individual lines with a step of 0.1 eV.

Results And Discussions
The results of preliminary experiment on the treatment of coarse calico samples in solutions of AlCl 3 testi ed to the achievement of hydrophobization of the treated surfaces (Fig. 1).
It was found that when KAl(SO 4 ) solution was treated in a solution, the contact angle was less than when processed in AlCl 3 solution. Table 1 shows the results of measuring the contact angle. Before the treatment, a drop of water was absorbed instantly on the surfaces of coarse calico and chintz, that means, the surface was completely hydrophilic, and the contact angle can be considered equal to 0 . Due to the fact that the contact angle on the processed samples of coarse calico and chintz was characterized by the same values, the result of treatment did not depend on the sample material, further studies were carried out on only one material, where coarse calico was used.
To determine the surface composition of the samples obtained during processing in AlCl 3 and KAl(SO 4 ) solutions, we used the method of X-ray photoelectron spectroscopy, the results of which are presented in Fig. 2 and Fig. 3.
For a more correct presentation of the data, the spectral regions with low intensity are enlarged and shown in the footnote in the right corner. The spectra of the processed samples were examined in more detail. In Fig. 3 shows the decomposition of the peak of oxygen (O 1s).
Analyzing of the oxygen peak of the treated samples allowed us to conclude that there are oxygen bonds with carbon, as well as with aluminum, on the coarse calico surface. The bonds with carbon correspond to the composition of the textile material, and the bonds of oxygen with aluminum indicate the presence of aluminum oxide on the surface of the treated samples (Moulder J. 1992). According to the obtained XPS spectra, it can be concluded that the hydrophobicity of coarse calico and chintz is ensured by the  Table 2. The results of the experiments showed that the concentration of soap solution does not have a strong effect on the contact angle of the coarse calico surface, so in further experiments this parameter was 35 g/l.

Conclusions
In the second series of experiments, the in uence of the temperature of a solution of AlCl 3 solution on the contact angle of coarse calico was investigated. Temperature (70 С) and the concentration of soap solution (35 g/l), the concentration of AlCl 3 solution (25 g/l), the processing time in the soap solution (25 min) and AlCl 3 solution (25 min), as well as the temperature of thermal exposure (100° C) were unchanged.
The contact angle The contact angle The contact angle Based on the results of the second series of experiments, it was concluded that the temperature of the AlCl 3 solution has no strong effect on the treatment result (contact angle of surface). Therefore, subsequent experiments were carried out at a constant temperature of a solution of AlCl 3 solution.
Then As can be seen from the obtained graph, with an increase in the concentration of the AlCl 3 solution (within the considered variation limits), a slight increase in the contact angle of the coarse calico wetting is observed, nevertheless, this indicates an increase in the hydrophobicity of the surface.
Thus, it turned out that in order to achieve the greatest hydrophobicity of the surface of textile materials, it is necessary to conduct processing in a solution of AlCl 3 solution with a concentration of 25-35 g/l. This treatment allows you to achieve a contact angle of wetting of the surface of more than 150 .
The contact angle on all samples was also measured again after 30 days and its values did not change.
It can be concluded that environmental factors do not harm the coating, and it is durable.
As a result of the study for the production of a hydrophobic surface of textile materials by «wet» chemistry approach, it was found that it is advisable to use a solution of AlCl 3 rather than KAl(SO 4 ) solution. The use of this solution with concentrations of 25-35 g/l allows you to reach a contact angle of wetting of more than 150 . As the XPS analysis showed, the hydrophobicity of the surface is ensured by the formation of alumina on the surface of the samples. The character of the dependence of the contact angle on the concentration of the AlCl 3 solution was experimentally determined. The wetting angle is directly proportional to the increase in the concentration of AlCl 3 solution in the studied range of parameters. When processing coarse calico samples in a solution of aluminum chloride, the hydrophobic coating is durable, even when checking the contact angle after 30 days, it remains. Testing of the coating under physical impact, as well as when washing samples, was not carried out, but similar experiments are planned in further studies.

Declarations
Funding. Funding information is not applicable.
Con ict of interest. The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.
Availability of data and material. The information is not applicable.
Code availability. The information is not applicable.
Ethics approval. The information is not applicable.
Consent to participate. The information is not applicable.
Consent for publication. The information is not applicable.

Figure 1
The contact angle of the sample of textile The plot of dependence of the contact angle on the concentration of AlCl3