Superhydrophobic fabric mixed with polyester and cotton yarns modified by alkaline treatment and thermal aging

Abstract

Superhydrophobic fabric composed of polyester and cotton single yarns was developed by alkali treatment and thermal aging. During the alkali treatment to make the nano-roughness of the polyester fibers, micro-roughness also increased due to differences in the thicknesses of the two yarns arising from the increased polyester surface roughness and swollen cotton. The superhydrophobicity, with a static contact angle of 155.8 ± 3.2° and shedding angle of 11.1 ± 0.8°, was achieved with 90% polyester/10% cotton fabric treated with 20% alkali concentration for 20 min under applied tension, then followed by 24 h thermal aging at 130℃. The tensile strength of the superhydrophobic polyester/cotton fabric (28.7 MPa) was higher than that of 100% polyester fabric (20.1 MPa). The breathability of the superhydrophobic polyester/cotton fabric was improved compared with 100% polyester fabric. In durability assessment, a static contact angle of ≥150° was shown for the tape tests. Five times of repeated adhesion with a clothing tape cleaner were conducted for the five samples each. Although washing and dry-cleaning decreased contact angles to as low as 137.7°, a static contact angle of 150° was achieved by additional thermal aging (130℃, 24 h). We developed a superhydrophobic fabric mixed with polyester and cotton yarns by exploiting differences in the characteristics of the two yarns induced by alkali treatment, which causes fabric surface roughness, and thermal aging without the use of any chemicals. Moreover, this superhydrophobic fabric has improved breathability.

Keywords

superhydrophobic polyester cotton alkaline hydrolysis mercerization thermal aging

In the field of clothing materials, superhydrophobic fabrics with self-cleaning properties are considered eco-friendly materials that can help reduce the amount of used detergent and water by requiring fewer wash cycles than other general materials. Consequently, superhydrophobic fabrics have been researched extensively from both academic and industrial perspectives. Superhydrophobic surfaces can be achieved by controlling the surface roughness and maintaining a low surface energy. For fabrics, this can be realized by creating nano-scale surface roughness via inherent micro-roughness according to the weaving process and lowering the surface energy.1,2

In a study by Shim et al.,3 surface roughness was obtained on polyester fabrics with varying process types and thicknesses of the polyester yarn by introducing nanoparticles via coating with a carbon nanotube–Teflon dispersed solution. The superhydrophobicity and surface roughness depending on the micro-roughness of the fabrics were analyzed by the wetting theory and contact angle. However, the durability of the nanoparticles and issues regarding human health hazards upon detachment of the nanoparticles from the fabric render this approach problematic.4 Thus, studies have focused on plasma treatment methods that can realize nano-roughness by etching the fabric surface. In a study by Kwon et al.,5 lyocell was used to develop a single-sided superhydrophobic fabric, where one side was superhydrophobic with water-repelling properties created by deposition and plasma etching, and the other retained the inherent hydrophilicity of lyocell. However, plasma treatment is limited when applied to mass production.6 In a study by Youn and Park,7 an alkaline hydrolysis method that is commonly used in the polyester textile industry was employed to form nano craters on the surface of polyester fabric according to sodium hydroxide concentration and treatment time, while single-sided blade coating of a fluorinated polymer solution with a low surface energy was applied to develop a superhydrophobic polyester fabric.

Furthermore, the use of fluorinated polymers is regulated and restricted because they are non-biodegradable and thus considered environmentally harmful.8 –12 Accordingly, a mass producible superhydrophobic polyester fabric was developed in a study by Oh et al.13 This alkaline hydrolyzed polyester fabric has dual scale roughness on the surface, and the hydrophobization was achieved using an environmental and eco-friendly thermal aging method, which does not require any chemical compounds. Furthermore, Oh and Park14 developed a colorful fluorine-free superhydrophobic polyester fabric via the modified disperse dye process which is composed of alkaline hydrolysis as a pretreatment, followed by dyeing and thermal aging as the drying process.

Because polyester/cotton blends or mixed fabrics have many merits, they have several practical applications, and the demand for developing self-cleaning polyester/cotton fabric is great. Loss of polyester fiber strength due to alkali treatment is expected to be minimized by mixing with a single cotton yarn. In addition, swelling of cotton yarns and thinner polyester yarns caused by alkali treatment increases surface roughness, thus influencing surface roughness and wettability. Therefore, it will be interesting to study surface phenomena arising from the differences in properties of polyester and cotton depending on the alkali and the mechanical properties and breathability. In the present study, alkali and thermal aging treatments were applied to specimens with polyester single yarns (warp) and polyester and cotton single yarns (weft). To achieve this, polyester and cotton single yarns were mixed in different ratios in the weft, and the optimal conditions for achieving superhydrophobicity by alkali treatments with/without tension and subsequent thermal aging treatments were investigated. Thereafter, morphology and chemical changes of the fiber surface were identified, and the tensile strength and vapor/air permeability of the fabric were measured. Additionally, tape, washing, and dry-cleaning tests were carried out for durability assessment. Furthermore, this study will contribute to industry by employing eco-friendly methods that do not use any chemicals for implementing hydrophobicity, and these developments for very common polyester/cotton fabrics will expand the applicability of superhydrophobic fabrics and facilitate their commercialization.

Experimental details

Materials and surface modification

The fabric samples consisted of 100% polyester staple single yarns in the warp and different numbers of polyester staple single yarns and cotton staple single yarns were used in the weft (Table 1). Polyester yarn count was 177 denier whereas that of cotton yarn was 30' s (177.15 denier), and the fabric samples were plain weave with a weave density of 70 × 70/in². The numbers of polyester single yarns to cotton single yarns in the weft per inch were 56:14, 45:25, and 35:35, thus the ratios in the number of single yarns of the fabric were 90:10, 82:18, and 75:25, respectively. In the case of 100% cotton fabric, however, single cotton yarns were used in the warp and weft. After weaving the fabric with single yarns, an alkali was used to treat it followed by thermal aging. In accordance with the ratios, each fabric and their sample codes are shown in Table 1. To remove impurities from the woven fabric samples, a 1% scouring solution was prepared by dissolving sodium carbonate anhydrous in distilled water at a liquid ratio of 1:50. Subsequently, fabric samples cut to 5 cm × 5 cm in size were submerged in the solution for treatment at 70℃ for 30 min. The scoured fabric samples were washed with distilled water more than 10 times and dried for 24 h at room temperature. Sodium hydroxide (Junsei Chemical, Japan) was used to apply the alkali treatment to the fabric samples. At a liquid ratio of 1:30, the samples were submerged in an aqueous sodium hydroxide solution at 70℃; the tested sodium hydroxide concentrations were 10%, 20%, and 30% and the submerge times were 10, 20, and 30 min. Moreover, three repeated alkali treatment experiments were conducted for each sample with or without tension. In order to apply tension to the fabric, a stainless steel frame was fabricated to which the fabric was attached. The stainless steel frame had two circular frames for the inner and outer parts that provided tension during alkali treatment. The fabric was placed on the inner circular frame and then the outer circular frame was placed over it and fixed to maintain the original dimension of the fabric. The alkali-treated fabric samples were washed in distilled water until the pH reached 7, after which they were dried for 2 h at room temperature. The dried fabric samples were placed in a 130℃ oven for 24 h as a thermal aging treatment. The experimental processes involved in the alkali and thermal aging treatments for hydrophobization are depicted in Figure 1.

Figure 1.

Schematic of overall process for fabricating superhydrophobic fabric composed of polyester and cotton single yarns.

Table 1.

Ratios in the number of polyester and cotton single yarns for each specimen

The number of single yarns in the fabric			Codes
Warp	Weft
Warp	Polyester	Cotton
Polyester 70	70	0	P100
	56	14	PC90/10
	45	25	PC82/18
	35	35	PC75/25
Cotton 70	0	70	C100

Evaluation of fabric properties

Physical and chemical properties

After alkali treatment, the polyester surface and outer appearance of the cotton fabric were observed by field-emission scanning electron microscopy (SEM) (Supra 55VP, Carl Zeiss, Germany). To prevent the accumulation of electrons, the sample surfaces were sputter coated (EM ACE200, Leica, Austria) for 120 s under vacuum conditions before SEM observations. X-ray photoelectron spectroscopy (XPS, Axis-His, Kratos Inc., USA) was performed at 24.5 W and 15 kV to analyze changes in the chemical compositions of the fabrics.7 X-ray diffraction analysis (2D XRD D8 Discover, USA) was performed to investigate the degree of crystallinity of the fabrics composed of polyester and cotton single yarns, which represents the weight fraction of crystalline regions throughout the entire polymer and was calculated using equation (1). In the equation, X_C represents the degree of crystallinity as the ratio of the area under the peak estimated as the crystalline regions to the total area of the graph. The diffraction patterns were obtained between 5° and 40° in steps of 0.02° (λ = 0.15 nm).¹⁵

X_{c} (%) = \frac{Areaofcrystallinefraction (underpeak)}{{\begin{matrix} Areaofcrystallinefraction \\ + Areaofamorphousfraction \end{matrix}}} \times 100

(1)

To evaluate the micro-roughness of the polyester and cotton fabrics after alkali treatment with or without tension and subsequent thermal aging, an automatic surface tester (Kawabata KES-FB4-Auto A, Kato Tech, Japan) was used to determine the geometrical surface roughness, which was quantified by the mean deviations in the direction of the normal vector of a real surface for measuring the mean deviation surface roughness of samples prepared to a size of 20 cm × 20 cm.16

Surface wettability

To determine the surface wettability, an optical tensiometer (Theta Lite, KSC Instruments, Finland) was employed to measure the static contact angles and shedding angles. The samples were cut to a size of 0.5 cm × 5 cm, placed on a glass slide, and fixed with Scotch tape. Subsequently, 3.5 µL water droplets were dropped onto the sample, and the static contact angle was measured immediately. For the shedding angle, the prepared sample was mounted on a pre-tilted sample holder, and 12.5 µL water droplets were dropped from a height of 1 cm. Subsequently, the minimum tilting angle for the water droplets to roll ≥ 2 cm was determined. Mean values of static contact angles and shedding angles were derived out of 10 different spots on five samples for each condition.17

Mechanical properties

In accordance with ASTM D5035 (strip method), the samples were prepared to a size of 2.5 cm × 15 cm, and a universal testing machine (Instron-5543, USA) was used. Five samples were prepared for each condition to measure the tensile strength (MPa) in the weft direction. The mean value and the standard deviation were derived based on the maximum tensile strength values of the five samples. The space between clamps was set to 7.6 cm, the load was set to 1 kN, and the tensile speed was set to 10 cm/min.18,19

Breathability

Air permeability (FX 3300, TEXTEST, Switzerland) was measured in accordance with ASTM D737-75 (Frasier method). The samples were prepared to a size of 20 cm × 20 cm, and the applied pressure was set to 125 Pa. The mean value was derived out of five samples for each treatment condition.

To analyze the changes in fabric breathability due to the alkali treatment, the water vapor transmission rate was measured by the change in weight according to the amount of vapor permeating into a cup through the sample, which was performed in accordance with ASTM E96-80 (calcium chloride test). All samples were conditioned for 24 h at 20 ± 1℃ and 65 ± 2% relative humidity before treatment. After preparing circular samples with a diameter of 7 cm, 33 g of calcium chloride was added, and the sample was placed concentrically on top of a cup. After leaving it at 40 ± 2℃ and 90 ± 5% relative humidity for 1 h, the sample was weighed immediately. The sample weight was measured again 1 h later, and the values were substituted into equation (2) to calculate the water vapor permeability. Five samples were prepared for each condition, and the measured values were used to derive the mean value.20

P = \frac{a_{2} - a_{1}}{S} \times 24

(2)

where P is the water vapor transmission rate (g/(m²∙h)); a₂ – a₁ is the change in sample mass after 1 h (g/h); and S is the area through which the vapor passes (m²).

Durability

To assess the durability of the surface superhydrophobicity, a clothing tape cleaner (Scotch-Brite, 3 M) was used to perform 5, 10, 15, and 20 repeated trials. Subsequently, the contact angles were measured and the surface was observed through SEM scans to determine the changes in surface roughness. To evaluate the washing durability, 5 cm × 5 cm samples thermal-aged after the alkali treatment with or without tension were prepared. After loading into a laboratory scale washing machine (Terg-O-Tometer, Yasuda Seiki, Japan) at a liquid ratio of 1:300 (w/w), the samples were washed for 20 min at 25℃ under a 40 rpm without detergent. The static contact angles and shedding angles were measured after 5, 10, 15, and 20 repeated cycles. The samples for the dry-cleaning assessment were prepared in the same manner as for the washing durability assessment. After adding a commercial dry-cleaning solvent (1%) containing an anionic surfactant in a beaker at a liquid ratio of 1:300 (w/w), the samples were submerged for 20 min at 25℃ under a 40 rpm spin cycle using a magnetic stirrer. The static contact angles and shedding angles were measured after 5, 10, 15, and 20 repeated cycles.13

Results and discussion

Changes in surface morphology

Changes in the surfaces of the polyester and cotton fabric samples by alkaline hydrolysis were investigated according to alkali concentration and treatment time with or without applied tension. Nano craters were formed on the surface of the polyester samples treated with high alkali concentration and longer treatment time regardless of tension, whereby a dual roughness was formed (Figure 2(a)). This occurred because when the alkali treatment is applied to polyester, the hydroxyl ions of sodium hydroxide attack the carbon with its relatively low electronegativity, in the carbonyl group of the polyester molecule, which is then immediately converted into a carboxylate anion via hydrolysis and breaks off as a short ring.7,13 As a result, polyester molecular chain cleavage occurs, which ultimately forms nano-scale craters on the surface due to the hydrolysis of ethylene glycol and terephthalic acid (Figure 2(b)). On the other hand, no changes occurred on the surface of the cotton samples; round-shaped swelling was observed in the cross-sectional view when the alkaline treatment was applied with tension, in contrast to those treated without tension. Original cotton fiber has a ribbon shape and is folded in the lengthwise direction21 prior to the alkaline treatment, but afterwards the cotton fiber becomes swollen and unfolded with round-shaped cross-section. This is because the Cell-OH of cellulose becomes alkali cellulose (Cell-ONa) as sodium hydroxide forms bonds with the hydroxyl groups of the cellulose molecule. As the hydrogen bonds between the molecules in cellulose are broken, sodium hydroxide and water penetrate between the molecules and cause swelling (Figure 2(c)). In particular, if tension is applied to the fabric, shrink resistance occurs in the warp and weft to improve their orientation and more fiber swells in contrast to fabric treated without tension.22 –24 As a result, a round-shaped cotton fabric was observed more clearly when the alkali treatment was applied with tension.

Figure 2.

(a) Scanning electron microscopy images of polyester and cotton fiber, untreated and treated with alkaline concentration of 20% for 20 min with or without tension; (b) alkaline hydrolysis mechanism occurring on polyester fabric surface;2 (c) mercerization mechanism occurring on cotton fabric surface.3

Superhydrophobicity

To investigate the effects of alkaline treatment on the superhydrophobicity, the treatment time and alkali concentration with or without tension were varied, and thermal aging was applied for 24 h at 130℃. Subsequently, the static contact angles and shedding angles of the P100, PC90/10, PC82/18, PC75/25, and C100 fabric samples (Figure 3) were measured. The results showed that with tension, the static contact angles of P100, PC90/10, PC82/18, PC75/25, and C100 after thermal aging following 20% alkaline treatment for 20 min were 156.6 ± 2.7°, 155.8 ± 3.2°, 151.9 ± 3.8°, 147.8 ± 4.0°, and 137.0 ± 1.9°, respectively (Figure 3). In particular, the static contact angle and shedding angle of the PC90/10 sample were 155.8 ± 3.2° and 11.1 ± 0.8°, respectively, which represented superior superhydrophobicity compared with the fabric subjected to alkali treatment without tension. This tendency occurred similarly for the PC82/18, PC75/25, and C100 samples but not for the P100 sample. This difference was attributed to cotton, which exhibited different morphologies depending on the application of tension during the alkali treatment. Thus, a comparative analysis was performed by measuring the surface roughness mean deviation values of polyester, polyester/cotton and cotton fabrics using an automatic surface tester (Figure 4).

Figure 3.

Static contact and shedding angles of polyester, polyester/cotton, and cotton fabrics treated (a) without tension and (b) with tension; (c) photos of water droplets on the untreated PC90/10 fabric and the treated PC90/10 fabric (with tension; alkali treatment with concentration of 20% and treatment time of 20 min; and thermal aging at 130℃ for 24 h).

Figure 4.

Surface roughness mean deviation of (a) warp and (b) weft, depending on the application of tension during alkali treatment of polyester fabric (P100), fabric composed of polyester and cotton single yarns (PC90/10) and cotton fabric (C100).

In the alkaline treated P100 and PC90/10 fabrics, the surface roughness mean deviation value was higher than untreated fabrics. This occurred because the alkali treatment caused hydrolysis in the polyester fabric, resulting in a decreased fiber and yarn thickness,25,26 and thus a higher surface roughness mean deviation values occurred as the thickness difference increased between the polyester and cotton single yarns.27 On the other hand, in the case of alkaline treated C100 fabric, the surface roughness mean deviation value was lower than untreated cotton fabric. This is due to the swelling of the fibers during mercerization, which produces a smoother surface of the fibers and reduces the gaps among fibers and yarns. However, the roughness values were different in the C100 samples according to the application of tension. When the alkaline treatment was applied with tension, the surface roughness mean deviation values in the warp direction was 11.50 µm, which was higher than that of the fabric treated without tension (8.92 µm). These results could be interpreted as the competitive effect of fabric shrinkage and swelling during the alkaline treatment. It was proven through the fiber size and fabric thickness (Figure 5). The size of polyester fiber was decreased by alkaline hydrolysis whereas that of cotton fiber was increased by alkaline mercerization, compared with untreated. In particular, differences in fiber size of polyester and cotton after alkali treatment with tension were greater than in case of those treated without tension (Figure 5(a)). When the alkali was treated without tension, the fabric shrank freely in the relaxed state and swelling occurred at the same time. As the fabric density increased and because the difference in fiber size was not significant, the geometric deviation decreased.24,27 On the other hand, when tension was applied, the fabric did not shrink; however, the fiber swelled, because of which, the gaps between the fiber sizes were relatively large. Therefore, the geometric variation increased.24,28,29 In addition, the fabric thicknesses of P100, PC90/10 and C100, which were alkali-treated with tension, showed higher values than those without tension (Figure 5(b)). Accordingly, to increase the micro-roughness of polyester and cotton fabrics, applying tension for alkaline treatment was more favorable for achieving superhydrophobicity. In addition, when thermal aging was applied after the alkaline treatment, the surface roughness mean deviation values were reduced compared with those only treated with alkali. This occurred because the molecular chains of amorphous region shows the property of becoming random coil state by heat treatment, which causes shrinkage. In particular, in the case of polyester, above the glass transition temperature, not only the chain of the amorphous region but also the tie chain, which is connecting the crystal to the crystal, becomes shorter in the crystal region.30

Figure 5.

Fabric properties depending on the application of tension during alkali treatment: (a) fiber diameter of polyester and cotton fibers and (b) fabric thickness of P100, PC90/10, and C100.

Add to, as shown in Figure 3(a) and 3(b), the static contact angle decreased and shedding angle increased as the percentage of cotton fibers increased because of the increase of hydrophilic groups. Cotton fiber consists of cellulose which is formed by two β-glucose units forming β-glycosidic bonds with several thousand bonded glucose units. This creates a high number of hydrophilic groups, and thus, the fabric surface becomes more hydrophilic as the percentage of cotton yarns in the fabric increases.28,29

Figure 3(c) shows the images representing the untreated PC90/10 fabric sample and PC90/10 fabric treated alkaline concentration 20% for 20 min with tension and thermal-aged 130℃ 24 h. It is shown that the superhydrophobicity achieved on the surface of the treated PC90/10 fabric sample.

Chemical composition

According to the aforementioned results, the PC90/10 sample treated with 20% alkaline solution for 20 min with tension and thermal aging at 130℃ for 24 h showed the highest superhydrophobicity. Chemical composition analysis by XPS was performed for three different conditions: untreated, alkali treatment alone, alkali treatment followed by thermal aging (Figure 6, Table 2). The analysis of three different peaks (284.50 eV for C-C/C-H bonds, 286.15 eV for C-O bonds, and 288.52 eV for O-C=O bonds) showed that the percentages of C-C/C-H, C-O, and O-C=O bonds in the untreated PC90/10 sample were 70.37%, 16.23%, and 9.88%, respectively. When the sample was subjected to alkali treatment, the percentage of C-C/C-H bonds decreased to 68.12% compared with the untreated sample, whereas the percentages of C-O and O-C=O bonds increased to 20.74% and 11.14%, respectively. These changes occurred because polyester was hydrolyzed by sodium hydroxide, and hydrophilic groups were introduced.25 On the other hand, applying both the alkali treatment and thermal aging increased the percentage of C-C/C-H bonds to 73.89% and decreased the percentages of C-O and O-C=O bonds to 19.16% and 10.47%, respectively. This occurred because when polyester is thermally aged above the glass transition temperature, rearrangement of the polymer occurs, whereby the hydrophilic groups introduced during alkali treatment move into the fabric and the hydrophobic groups are arranged on the surface.13,31

Figure 6.

Chemical compositions from x-ray photoelectron spectroscopy analysis of PC90/10 fabrics: (a) untreated, (b) alkali-treated, and (c) alkali-treated and thermally aged.

Table 2.

Percentages of different carbon bonds in PC90/10 fabrics

PC90/10	C-C/C-H at 284.50 eV	C-O at 286.15 eV	O-C=O at 288.52 eV
Untreated	70.37	16.23	9.88
Alkali-treated	68.12	20.74	11.14
Alkali-treated and thermally aged	73.89	19.16	10.47

In the case of C100 sample, with a static contact angle of 137.0 ± 4.6°, the percentages of C-C/C-H, C-O, and C=O bonds were 55.99%, 33.90%, and 10.10%, respectively (Figure S1, Table S1, supplemental material). When C100 sample was subjected to alkali treatment (20% concentration, 20 min treatment time), the percentage of C-C/C-H bonds decreased to 54.94% compared with the untreated sample, whereas the percentages of C-O and C=O bonds increased to 35.16% and 11.79%, respectively. This occurred because when the alkali treatment was applied, the bonds of the cellulose molecules were broken by the introduced sodium hydroxide, and as cellulose became Cell-ONa, the C-C bonds were reduced.22 On the other hand, when both alkali treatment and thermal aging were applied, the percentage of C-C/C-H bonds increased to 57.58%, and the percentages of C-O and C=O bonds decreased to 30.63% and 9.90%, respectively. Because mercerized cotton has more amorphous regions than the untreated sample, polymer chains in the amorphous regions gain a greater mobility from heat.30 Because the higher surface energy is the more unstable state, so the reaction occurs in the direction to lower the surface energy.31,32 As a result, thermal aging caused more hydrophobic groups to migrate to the surface for the surface to become stable.

Figure 10.

Static contact angles and shedding angles of superhydrophobic PC90/10 fabric after (a) tape tests, (b) washing tests, (c) washing and thermal aging at 130℃ for 24 h, (d) dry-cleaning tests, and (e) dry-cleaning and thermal aging at 130℃ for 24 h.

Mechanical properties

Tensile strength for P100, PC90/10, PC82/18, PC75/25, and C100 samples subjected to alkali treatment with tension were compared with the same samples treated without tension (Figure 7). The PC90/10 sample subjected to alkali treatment with tension showed a tensile strength of 28.7 MPa, whereas that of the same sample treated without tension was 24.7 MPa, indicating that strength loss by alkaline treatment could be reduced by applying the tension. The C100 sample treated without tension showed a tensile strength of 26.6 MPa, which was lower than that of the untreated sample (35.6 MPa), whereas the same sample treated with tension showed an increased tensile strength of 39.2 MPa, indicating that the tensile strength of the fabric composed of polyester and cotton was increased by cotton. This is possible because changes due to crystalline structure transitions could have a major impact on the mechanical properties. X-ray diffraction analysis was performed to analyze the change in crystallinity of polyester/cotton fabrics treated with/without tension. The X-ray diffraction patterns of the untreated PC90/10 fabric showed four main peaks at 2θ = 14.4°, 16.3°, 23.2°, and 26.6° (Figure 8). In the untreated fabric, the peaks at 14.4°, 16.3°, and 23.2° correspond to the cotton fiber,33 whereas the peaks at 16.3°, 23.2°, and 26.6° correspond to the polyester fiber.34

Figure 7.

Tensile strength of fabrics composed of polyester and cotton single yarns alkali-treated with and without tension.

Figure 8.

X-ray diffraction spectra of fabrics (PC90/10) before and after alkali-treated with or without tension.

The diffraction pattern of the alkali-treated fabric with and without tension showed peaks at 14.4°, 16.3°, 21.9° and 26.6°. The peaks at 14.4° and 16.3° are attributed to cellulose I, but the peaks at 21.9° were newly detected.35 This is because cotton had deformed cellulose due to the O6H-O3 intermolecular hydrogen bonds of cellulose I and the O6H-O2 intermolecular hydrogen bonds of cellulose II after alkaline hydrolysis. Consequently, the direction of the chain in the molecule and the angle of the crystal changed from 84° to 62°, resulting in a different crystal structure.36 In addition, the polyester peak appeared at 23.2° but after alkaline hydrolysis the peak position was changed to 21.9° and the peak width broadened. This is because the crystallite size decreased and the amorphous region increased, due to breakage of the broken polyester molecular chain caused by OH anion attacks during alkaline hydrolysis.37 The untreated PC90/10 fabric showed the crystallinity of 49.0%, whereas the samples alkali-treated with and without tension showed crystallinities of 44.2% and 38.9%, respectively (Table 3). This could be attributed to modification of the cellulose crystalline phase in the cotton fabric depending on the applied external conditions. When the tension was applied, crystalline formed as a result of molecular orientation due to the shear stress generated by the flow.38 In addition, alkaline treatment also affects the morphology of the crystalline phase, and cellulose I is transformed into cellulose II by the alkaline treatment. Cellulose I has a high crystallinity due to parallel alignment of the molecular chains, whereas cellulose II has a lower crystallinity due to anti-parallel chain alignment.39,40 Moreover, when the fabric sample was submerged in the alkali solution without tension, the degree of freedom of the fiber increased in the direction of buoyancy in the solution. As a result, the alignment of the yarns in the fabric became poorer without tension, whereas the alignment improved with tension, which resulted in a higher strength.41 –43 Therefore, applying thermal aging and alkali treatment with tension could be suitable for developing superhydrophobic fabric that can overcome the strength loss.

Table 3.

Crystallinity of fabrics (PC90/10) before and after alkali-treated with or without tension

PC90/10	Crystallinity (X_c, %)
Untreated	49.0
Alkali-treated with tension	44.2
Alkali-treated without tension	38.9

Breathability

To assess the breathability of the fabric samples, air permeability and water vapor transmission rate were measured for the P100, PC90/10, PC82/18, PC75/25, and C100 samples that were untreated, subjected to alkali treatments with or without tension and then thermal aging treatments with tension. As shown in Figure 9(a), when the samples were subjected to both alkali and thermal aging treatments without tension, the air permeability tended to decrease as cotton content increased. This occurred because when subjected to the alkali treatment without tension, the cotton fabric swelled within the same area40 which decreased the air permeability due to the higher weave density as the fabric shrank. On the other hand, when subjected to both alkali and thermal aging treatments with tension, the air permeability was higher than that of the untreated samples or the samples alkali/heat-treated without tension. This is attributed to the increase in spaces between yarns arising from shrinkage resistance caused by tension and reduced yarn thickness caused by alkali hydrolysis. As a result, the fiber diameter decreased44 causing the space between the yarns of fabric to widen.

Figure 9.

Plots of (a) air permeability and (b) water vapor transmission rate for untreated, treated without tension, and treated with tension fabrics composed of polyester and cotton single yarns.

The water vapor transmission rate results (Figure 9(b)) showed that the samples subjected to both alkali and thermal aging treatments had a higher breathability than that of the untreated samples. This is potentially attributable to the fabric becoming thinner due to hydrolysis on the polyester fabric surface during the alkali treatment, which formed pores between the yarns that facilitated the penetration of moisture.45 In addition, a comparison of breathability between fabric samples subjected to alkali and thermal aging treatments with or without tension showed that samples treated with tension had a higher breathability than that of the samples treated without tension. The reason for this was the same as that of the air permeability results: a high porosity occurred when the alkali treatment was applied with tension. In a study by Han et al., the superhydrophobic polyester fabric was developed by alkaline treatment and dip-coating in a fluoropolymer solution. However, the water vapor transmission rate was decreased by the hydrophobic dip-coating because of the decrease in air permeability and the reduction of surface energy by fluoro-chemical dip-coating.18 However, the superhydrophobic fabric developed in this study exhibited improved vapor permeability, partially due to the increased air permeability by thermal aging with tension and due to the mixing of hydrophilic cotton yarns.

Durability

For the PC90/10 fabric sample, which represented the optimal conditions (with tension; alkali treatment with concentration of 20% and treatment time of 20 min; and thermal aging at 130℃ for 24 h) tape tests were repeated 5, 10, 15, and 20 times, after which the surface wettability was measured (Figure 10(a)). After 20 repeated trials, the static contact angle decreased to 150.2 ± 5.5° and shedding angle increased to 14.1 ±0.8°. This occurred because the nano craters formed on the polyester fabric by the alkaline hydrolysis,46 which caused the micro-nano hierarchical roughness that played a role in creating the superhydrophobicity, were partly collapsed (Figure S2, supplemental material). However, it was confirmed that the superhydrophobicity of the sample was almost maintained, even after 20 repeated tape tests.

With respect to the washing durability of the PC90/10 sample, as shown in Figure 10(b), the static contact angle decreased to 148.8 ± 2.9° and shedding angle increased to 16.0 ± 1.2° after a single wash cycle. These results could be interpreted as hydrophobic groups on the fiber surface aligned by thermal aging are submerged in water, they move towards the bulk and hydrophilic groups are rearranged directing to the fiber surface.44,47 However, when thermal aging (130℃, 24 h) was re-applied after washing (Figure 10(c)), the surface molecular chains were rearranged, and the static contact angle increased to 153.5 ± 4.0° and the shedding angle was 14.4 ± 1.1°. In addition, the superhydrophobic PC90/10 sample was used to assess the changes in surface wettability according to repeated dry-cleaning (Figure 10(d)). After one cycle of dry-cleaning, the static contact angles and shedding angles were 150.9 ± 3.5° and 14.8 ± 1.4°, respectively. The commercial dry-cleaning solvent used in the experiment is a hydrocarbon-based solvent containing the anionic surfactant. The hydrophilic groups of the surfactant are adsorbed and arranged while facing the fabric surface,48,49 then the hydrophilic groups in the fibers are migrated onto the fiber surface. Thus, as the number of dry-cleaning cycles increases, the static contact angles were decreased and shedding angles were increased. However, when thermal aging was applied after dry-cleaning, the static contact angles and shedding angles were 153.2 ± 4.2° and 13.4 ± 1.1°, respectively, which indicated recovery of the superhydrophobicity (Figure 10(e)).

Conclusions

In the present study, fabric samples with polyester in the warp and composed of polyester and cotton single yarns in the weft were developed to expand the application of superhydrophobic clothing materials. Moreover, alkali treatments with or without tension and subsequent thermal aging treatments were applied to the samples. The results showed that at the condition of alkali treatment (concentration 20%, 20 min) with tension and subsequent thermal aging (130℃, 24 h), 90% polyester and 10% cotton specimen showed the highest superhydrophobicity with static contact angles and shedding angles of 155.8 ± 3.2° and 11.1 ± 0.8°, respectively. These properties occurred due to the formation of nano craters on the polyester fabric surface upon alkali treatment, due to the micro-roughness increase attributed to the fabric thickness difference, due to the thickness reduction in the polyester fabric by hydrolysis and due to the thickness increase in the cotton fabric caused by mercerization and swelling. In the subsequent thermal aging, superhydrophobic groups were aligned on the polyester surface as polymer rearrangement occurred at a temperature above the glass transition temperature, whereas superhydrophobic groups were aligned on the cotton surface due to the increased mobility from the greater amount of amorphous regions in the alkali-treated fabric compared with the untreated cotton fabric.

Accordingly, despite the hydrophilic nature of the cotton fabric, a superhydrophobic surface was achieved on the fabric composed with polyester and cotton single yarns. In this study, the limitation of loss of strength due to alkaline hydrolysis in 100% polyester fabric was overcome by approximately 8.6 MPa by mixing with cotton; moreover, the air permeability and water vapor transmission rate were also found to be superior to those of the untreated fabric samples. Therefore, by mixing polyester yarns with cotton yarns, it was possible to develop the superhydrophobic fabric with increased breathability while maintaining mechanical strength; this is widely applicable to apparel products. Additionally, the durability assessment results confirmed that a static contact angle of ≥150° was maintained even after repeated tape tests. Furthermore, the washing and dry-cleaning assessment results confirmed that the superhydrophobicity could be recovered by additional thermal aging. Although the study results show low hydrophobicity compared with previous studies, we have developed a practical fabric with increased tensile strength and breathability. Accordingly, the superhydrophobic fabric composed of polyester and cotton single yarns developed in the present study using the common alkali treatment method and eco-friendly thermal aging hydrophobization is expected to have a high utilization value in the apparel industry and is believed to have high potential for commercialization. Further study is needed to determine conditions that can reduce the thermal aging time by adjusting the roughness of the fabric or finding a more facile thermal treatment.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Foundation of Korea funded by the Korean government (Grant No. NRF-2018R1A2B6003526).

ORCID iD

Chung Hee Park

Supplemental material

Supplemental material for this article is available online.

References

Park

Kim

Park

. Superhydrophobic textiles: review of theoretical definitions, fabrication and functional evaluation. J Eng Fibers Fabr 2015; 10: 1–18.

Jiang

Zhao

Zhai

. A lotus-leaf-like superhydrophobic surface: a porous microsphere/nanofiber composite film prepared by electrohydrodynamics. Angew Chem 2004; 116: 4438–4441.

Shim

Kim

Park

. The effects of surface energy and roughness on the hydrophobicity of woven fabrics. Text Res J 2014; 84: 1268–1278.

Park

Kim

Park

. Influence of micro and nano-scale roughness on hydrophobicity of a plasma-treated woven fabric. Text Res J 2017; 87: 193–207.

Kwon

, et al. Nanostructured self-cleaning lyocell fabrics with asymmetric wettability and moisture absorbency. RSC Adv 2014; 4: 45442–45448.

Park

Kim

. A quantitative analysis on the surface roughness and the level of hydrophobicity for superhydrophobic ZnO nanorods grown textiles. Text Res J 2014; 84: 1776–1788.

Youn

Park

. Development of breathable Janus superhydrophobic polyester fabrics using alkaline hydrolysis and blade coating. Text Res J 2019; 89: 959–974.

Park

. Robust fluorine-free superhydrophobic PET fabric using alkaline hydrolysis and thermal hydrophobic aging process. Macromol Mater Eng 2018; 303: 1700673–1700673.

Yun

, et al. Development of a superhydrophobic electrospun poly(vinylidene fluoride) web via plasma etching and water immersion for energy harvesting applications. RSC Adv 2018; 8: 28825–28835.

10.

Wang

Fang

Cheng

, et al. One-step coating of fluoro-containing silica nanoparticles for universal generation of surface superhydrophobicity. Chem Commun 2008, pp. 877–879.

11.

Hao

, et al. Synthesis of fluoro-containing superhydrophobic cotton fabric with washing resistant property using nano-SiO₂ sol-gel method. Adv Mater Res 2010; 121–122: 23–26.

12.

Zhang

, et al. Facile preparation of durable and robust superhydrophobic textiles by dip coating in nanocomposite solution of organosilanes. Chem Commun 2013; 49: 11509–11511.

13.

Moon

, et al. Nanostructured superhydrophobic silk fabric fabricated using the ion beam method. RSC Adv 2014; 4: 38966–38973.

14.

Park

. Colorful fluorine-free superhydrophobic polyester fabric prepared via disperse dyeing process. Adv Mater Interfaces 2020; 7: 2000127–2000127.

15.

Zeronian

Collins

. Surface modification of polyester by alkaline treatments. Text Progr 1989; 20: 1–26.

16.

Kolpak

Weih

Blackwell

. Mercerization of cellulose: 1. Determination of the structure of mercerized cotton. Polymer 1978; 19: 123–131.

17.

Vassiliadis

Provatidis

. Structural characterization of textile fabrics using surface roughness data. Int J Cloth Sci Tech 2004; 16: 445–457.

18.

Han

Park

. Development of superhydrophobic polyester fabrics using alkaline hydrolysis and coating with fluorinated polymers. Fibers Polym 2016; 17: 241–247.

19.

Gholamzad

Karimi

Masoomi

. Effective conversion of waste polyester–cotton textile to ethanol and recovery of polyester by alkaline pretreatment. Chem Eng J 2014; 253: 40–45.

20.

Ramgulam

Amirbayat

Porat

. Measurement of fabric roughness by a noncontact method. J Text Inst 1993; 84: 99–106.

21.

Wang

, et al. Designing breathable superhydrophobic cotton fabrics. RSC Adv 2015; 5: 27752–27752.

22.

Solbrig

Obendorf

. Alkaline hydrolysis of titanium dioxide delustered poly(ethylene terephthalate) yarns. Text Res J 1991; 61: 177–181.

23.

Yan

Yao

. One-step extraction and functionalization of cellulose nanospheres from lyocell fibers with cellulose II crystal structure. Cellulose 2015; 22: 3773–3788.

24.

Mooneghi

Saharkhiz

Varkiani

SMH

. Surface roughness evaluation of textile fabrics: a literature review. J Eng Fibers Fabr 2014; 9: 1–18.

25.

Yue

Han

. Transitional properties of cotton fibers from cellulose I to cellulose II structure. Bioresources 2013; 8: 6460–6471.

26.

Nguyen-Tri

El Aidani

Leborgne

, et al. Chemical ageing of a polyester nonwoven membrane used in aerosol and drainage filter. Polym Degrad Stabil 2014; 101: 71–80.

27.

Mazrouei-Sebdani

Khoddami

. Alkaline hydrolysis: a facile method to manufacture superhydrophobic polyester fabric by fluorocarbon coating. Progr Organic Coatings 2011; 72: 638–646.

28.

Somashekar

Kulshreshtha

Narasimham

, et al. Effect of slack mercerization and tension mercerization on the breaking load distribution of cotton fibers. J Appl Polym Sci 1977; 21: 3417–3426.

29.

Boryo

DEA

Bello

Ibrahim

, et al. Effect of alternative scouring agents on dyeing properties of cotton/polyester blend fabric. IOSR-JAC 2013; 5: 11–21.

30.

Moon

Park

. Effect of crystallinity on the recovery rate of superhydrophobicity in plasma-nanostructured polymers. RSC Adv 2020; 10: 10939–10948.

31.

Moon

, et al. Nanostructured fabric with robust superhydrophobicity induced by a thermal hydrophobic ageing process. RSC Adv 2017; 7: 25597–25604.

32.

Datye

Palan

. Effect of alkali on filaments of poly(ethylene terephthalate) and its copolyesters. J Appl Polym Sci 1989; 38: 1447–1468.

33.

Jia

Zhang

, et al. Facile synthesis of an eco-friendly nitrogen–phosphorus ammonium salt to enhance the durability and flame retardancy of cotton. J Mater Chem A 2017; 5: 9970–9981.

34.

Zhou

Fei

, et al. Integrating nano-Cu(2)O@ZrP into in situ polymerized polyethylene terephthalate (PET) fibers with enhanced mechanical properties and antibacterial activities. Polymers 2019; 11: 113–113.

35.

Gupta

Uniyal

Naithani

. Polymorphic transformation of cellulose I to cellulose II by alkali pretreatment and urea as an additive. Carbohydr Polym 2013; 94: 843–849.

36.

Credou

Berthelot

. Cellulose: from biocompatible to bioactive material. J Mater Chem B 2014; 2: 4767–4788.

37.

Bal

Behera

. Analysis of structural parameters of acid and alkali treated polyester fibers using SAXS and other techniques. Indian J Eng Mater Sci 2007; 14: 240–252.

38.

Heuvel

Huisman

. Effect of winding speed on the physical structure of as-spun poly(ethylene terephthalate) fibers, including orientation-induced crystallization. J Appl Polym Sci 1978; 22: 2229–2243.

39.

Toth

Borsa

Takács

. Effect of preswelling on radiation degradation of cotton cellulose. Radiat Phys Chem 2003; 67: 513–515.

40.

Montazer

Sadighi

. Optimization of the hot alkali treatment of polyester/cotton fabric with sodium hydrosulfite. J Appl Polym Sci 2006; 100: 5049–5055.

41.

Rana

Chen

Yang

, et al. Biomimetic superoleophobicity of cotton fabrics for efficient oil–water separation. Adv Mater Interfaces 2016; 3: 1600128–1600128.

42.

Sameii

Mortazavi

Rashidi

, et al. An investigation on the effect of hot mercerization on cotton fabrics made up of open-end yarns. J Appl Sci 2008; 8: 4204–4209.

43.

Achukwu

Dauda

Ishiaku

. Mechanical properties of plied cotton fabric-coated unsaturated polyester composites: effects of alkali treatments. Int J Compos Mater 2015; 5: 71–78.

44.

Mao

Jiang

Miao

, et al. Post-treatment and evaluation of polyester filament cotton-like fabric. Fibers Polym 2016; 17: 1735–1740.

45.

Macedo

JRN

Carmona

dos Santos Rosa

. Differences of alkali and thermal treatment on cotton residues applicable to green composites. Mater Sci Eng Adv Res 2016; 1: 13–18.

46.

Qiang

Chen

Yin

, et al. Robust UV-cured superhydrophobic cotton fabric surfaces with self-healing ability. Mater Design 2017; 116: 395–402.

47.

Ray

Sarkar

Basak

, et al. Study of the thermal behavior of alkali-treated jute fibers. J Appl Polym Sci 2002; 85: 2594–2599.

48.

Valk

Jellinek

Schroder

. The noncrystalline state within PET fiber—meaning and characterization by mechanical relaxation measurement. Text Res J 1980; 50: 46–54.

49.

Smith

Wentz

Martin

. Comparison of soil-deposition and redeposition tests in evaluating drycleaning detergents. J Am Oil Chem Soc 1968; 45: 83–86.