The preparation of UV-cured superhydrophobic cotton fabric surfaces by electrospinning method

Abstract

Electrospinning was used for the coating of the polyurethane/silica hybrid solutions on the cotton fabric surface. An ultraviolet curing process was performed on the collector by an ultraviolet lamp positioned inside the electrospinning cabin. In order to investigate the effect of fluorinated, silane-functionalized urethane (inorganic part) content on the water-repellency; five different fluorinated concentrations (10%, 20%, 30%, 40%, and 50% by weight) were used. The disappearance of isocyanate peaks during the silane-functionalized urethane synthesis was observed by Fourier transform infrared spectroscopy. Surface morphology and elemental analysis of the coated cotton fabric surfaces was investigated by scanning electron microscopy and energy dispersive spectroscopy analysis respectively. Water-repellency was evaluated by contact angle measurements. A sample that contained 50% inorganic part showed a contact angle of 154.5°. Samples were thermally characterized by differential scanning calorimetry. Glass transition temperature of the synthesized hybrid polymer increased with increasing inorganic part ratio. Additionally, abrasion resistance and crease recovery angle tests were performed to evaluate the effect of fluorinated part percentage on the mechanical and comfort properties of fabrics.

Keywords

PU/silica hybrids sol-gel electrospinning water-repellency UV curing

The surface of textile materials is very important considering the fiber properties such as abrasion resistance, wetting, adhesion, penetration, antistatic properties, and so on. The sol-gel technique allows for tailoring certain properties in a single coating step on textile surfaces. This technique can be used to enhance some specific properties of textile products such as photochromic effects, water/oil-repellency, water resistance, self cleaning, light absorption, UV protection properties etc.¹ The UV curing process offers energy and time savings, low environmental impact, and cost effectiveness. UV curing can be used in fabric coating and finishing, pigment printing, and nonwoven fabric bonding processes in the textile industry.^2,3

Contact angle is a quantity describing a solid’s wetting by a liquid. Geometrically, it is the angle formed by a liquid drop on the surface at the three phase (solid/liquid/gas) boundaries. If the angle is less than 90°, the liquid is said to wet the surface. If it is greater than 90°, it is accepted as non-wetting. Generation of a superhydrophobic surface can be performed by either roughening the surface due to micro- or nano-structures, or lowering the surface free energy by using a coating material (silane or fluoro-containing polymers).^4,5 In the literature, water-repellent textile surfaces can be obtained by the application of fluorinated polymer coatings, such as blends of fluorochemical urethane and fluorochemical acrylate.^6,7 In a previous study, a fluorine-containing methacrylate copolymer was applied on the cotton fabric surface in order to obtain water-repellency.⁸ A number of studies have been conducted on the hydrophobic textile surfaces through the addition of silica nanoparticles and silane compounds.^9–13 For instance, Nakazumi et al. prepared a fluorinated inorganic/organic hybrid material by using poly(methacrylic acid) (PMAA), tetraethyl orthosilicate (TEOS), and perfluoroalkyltriethoxysilane via sol-gel method and applied it on nylon carpeting for water-repellent behavior.¹¹ It has been reported that a sol-gel based formulation consisting of silane compounds such as alkyl- and amino-functionalized alkoxysilanes were used for the PES/cotton fabric treatment for water-repellency and antistatic behavior.¹⁰

In this study, PU/silica hybrid solutions were prepared by the sol-gel method. Coating of the solution onto the fabric surface as a thin layer was carried out using an electrospinning process combined with a UV curing step. This thin layer cannot be obtained by dip-coating or other textile finishing methods. The innovative method of this study is to choose electrospinning process to obtain a very thin coating layer instead of nanofibers onto just one surface of the fabric with the combination of UV-curing. Thus the other surface of the fabric remains uncoated without any clothing comfort loss. Furthermore, during the electrospinning process, with the help of a continuous UV radiation, a well and deeply crosslinked coating layer can be obtained on the fabric surface. The effect of fluorinated silane-terminated urethane on the water-repellency was investigated by contact angle measurements. Samples were characterized by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and differential scanning calorimetry (DSC). Abrasion resistance and crease recovery angle tests were also performed to find the effect of fluorinated part percentage on the mechanical and comfort properties of fabrics.

Experimental details

Materials

UV-curable resin bisphenol A glycerolate (1 glycerol/phenol) diacrylate and 1.6 hexanediol diacrylate provided by Sigma Aldrich. 2,2,3,3,4,4,5,5,6,6,7,7-Dodecafluoro-1,8-octanediol; 98%, 3-isocyanatopropyltrimethoxysilane (ICPTMS); 95%, 3-methacryloxypropyltrimethoxysilane (MEMO); 98%, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES); 97%, and tetraethyl orthosilicate (TEOS) ≥ 99.0% were purchased from ABCR Co. Dibutyltin dilaurate (Sigma Aldrich) was used as a catalyst. As photoinitiator, 1-hydroxy-cyclohexyl-phenyl-ketone (Irgacure 184) was supplied by Sigma Aldrich. Para-toluenesulfonic acid (PTSA) ) ≥ 98.5%, ethanol ≥ 99.5%, and N,N-dimethylacetamide were obtained from Sigma Aldrich. Distilled water was obtained from our laboratory. Cotton fabric suitable for shirting and outwear (113 g/m² plain weaved, 30 warp/cm, 22 weft/cm, 0.27 mm thickness) was supplied from Tavsanli Textile Co.

Synthesis

Synthesis of silane-functionalized urethane oligomer

3 g of 2,2,3,3,4,4,5,5,6,6,7,7-Dodecafluoro-1,8-octanediol was charged to a 100 mL three-necked round bottom flask that was fitted with a thermometer pocket, water condenser, and a magnetic stirrer. 3.4 g of ICPTMS was slowly added to the reaction flask. Dibutyltin dilaurate was added to the reaction flask as a catalyst, at a concentration of 0.5% by weight. A schematic representation of this reaction is shown in Figure 1. The temperature was raised to 60℃, and the mixture was stirred for 8 h.

Figure 1.

Synthesis of silane-functionalized urethane oligomer and hydrolysis, polycondensation reactions of silane functional oligomers.

Synthesis of fluorinated methacrylate-functional oligomers

A 100 mL three necked round bottom flask equipped with a magnetic stirrer, a dropping funnel, and a nitrogen inlet was charged with 0.02 mol of PFOTES, 0.03 mol of MEMO, 0.02 mol of TEOS, and 0.01 mol of silane-functionalized urethane oligomer. Ethanol was added to the reaction mixture. The mixture was then homogenized by mixing with a magnetic stirrer. The water/silicone ratio was calculated as r = 3 and the calculated amount of water added to the mixture. 0.1% of PTSA acid as catalyst was added to the above mixture. The mixture was stirred for 4 h at room temperature with a magnetic stirrer. A schematic representation of this reaction is shown in Figure 1. This reaction is called the “inorganic part”.

Electrospinning and UV curing processes

Hybrid resin formulation was prepared by adding inorganic part to organic part in predetermined (10%, 20%, 30%, 40%, and 50% by weight) ratios. Cotton fabric samples were coated with this hybrid polymer solution via an electrospinning process. The organic part of formulation is composed of a UV resin (bisphenol A glycerolate (1 glycerol/phenol) diacrylate) and a reactive diluent (1.6 hexanediol diacrylate). A 3% (by total weight) photoinitiator was added to each of the hybrid formulations. Each formulation was stirred until all the components were miscibilised completely. The efficiency of mixing was judged by the amount of cloudiness visible during the mixing. Then the solutions were loaded into a 5 mL syringe, which was then placed in a syringe pump. The electrospinning process parameters (flow rate, applied voltage) were optimized to find the best operation conditions. Electrospinning process was carried out at 21 kV with a pump rate of 8 mL/h, at room temperature. A drum rotating at 15 rpm, in 6 cm diameter was placed at a distance of 11 cm from the needle and was covered with the cotton fabric sample in 18 cm × 5 cm dimensions. A UV lamp (OSRAM, Ultra-vitalux 300 W) was placed opposite to the drum for the crosslinking process. UV curing was kept on during electrospinning and was measured at 30 min. Figure 2 shows the experimental set-up of the electrospinning cabin.

Figure 2.

Experimental set-up of electrospinning and UV curing processes.

Characterization

PU/silica hybrid-coated fabric samples were examined on a JOEL JSM-5910 LV SEM. Elemental concentration of silicone, fluorine, carbon, and oxygen atoms was determined with an Oxford Instruments-INCA energy dispersive spectrum (EDS) system. The FT-IR spectrum of the silane-functionalized urethane oligomer was recorded on a Shimadzu 8300 FT-IR spectrophotometer in the range of 380–4000 cm⁻¹ at a resolution of 4 cm⁻¹. DSC analysis of the hybrid films was performed using a Perkin Elmer Jade model DSC. The tests were run from 20℃ to 220℃ with a heating rate of 10℃/min under N₂ atmosphere. Polymer uptake of the coated fabric samples was characterized by coating thickness¹⁴ and weight increment measurements. In order to investigate the handle properties of the PU/silica hybrid polymer coated fabrics, crease recovery angle and abrasion tests have been chosen for mechanical characterization. The crease recovery angle test was carried out according to the TS 390 EN 22313 standard in a crease recovery angle tester.¹⁵ For crease recovery angle measurement, fabric samples were cut in 40 mm × 15 mm dimensions in both weft and warp directions. Then samples were horizontally folded and kept under 10 N forces for 5 min. After this time, the load was removed and the angle between folded parts was measured by using a crease recovery angle tester. Abrasion resistance test was performed according to the TS EN ISO 12947-2 standard¹⁶ in a Martindale pilling and abrasion instrument. Water contact angles were measured by using a Gardco PGX+ goniometer. The volume of droplets was controlled to be about 3 µL. Water contact angle test was repeated after a domestic laundering process, which was performed at 40℃ for 30 min using 5 g/l of ECE reference detergent in a Gyrowash machine according to the ISO 105 C06.¹⁷

Results and discussion

FTIR spectroscopy

Completion of the urethane reaction was confirmed by the disappearance of the characteristic –NCO peak at 2275 cm⁻¹ in the FT-IR spectrum as shown in Figure 3. The carbonyl stretching vibration of ester bonds of the polymer (C = O) appears at 1742 cm⁻¹. The characteristic absorbance peaks of C–F can be observed at 1192–1080 cm⁻¹ and 815–765 cm⁻¹ respectively.

Figure 3.

FT-IR spectrum of silane-functionalized urethane structure.

Surface morphology (SEM-EDS analysis)

SEM images of the untreated fabric and the fabric coated by 40% fluorinated methacrylate-functional oligomers at various magnifications are given in Figures 4(a–d). From the images, it can be seen that polymer solution was settled homogeneously on the fiber surfaces. Neither agglomeration nor any irregular shape can be observed.

Figure 4.

SEM images of untreated fabric (a) and coated with 40% inorganic part added polymer at a magnification of ×100 (b), ×10,000 (c), and ×50,000 (d).

There are few advantages of coating the fabric surface by the electrospinning method. According to the SEM images, the electrospinning process enables the coating of only one surface of the fabric in a very thin layer, thus keeping the other surface uncoated. Fabric texture and porosity remain similar to its original form because polymer deposition was performed only onto the fiber surface. The synthesized fluorinated methacrylate-functional oligomer coating material is a UV curable system. During the electrospinning process, with the help of UV light, each coating layer was exposed to UV light; thus a well and deeply crosslinked coating layer was obtained on the fabric surface. Additionally, thickness of the coating layer can be regulated via the electrospinning process. These factors are important considering clothing comfort issues. Other coating techniques do not posses the mentioned advantages.

Elemental analysis of 40% inorganic part-containing sample is presented in Figure 5. The EDS spectrum reveals the presence of carbon, oxygen, fluorine, and silicon that was deposited on the sample surface. The percentage of elements is used for the determination of semi-quantitative composition of the sample. The presence of silicon confirms the effective incorporation of the inorganic precursor to the system.

Figure 5.

SEM–EDS spectrum of 40% inorganic part-containing sample.

Measurement of contact angle and surface energy

Table 1 shows the contact angle and surface energy values of the hybrid polymer coated fabric surfaces with various percentages of fluorinated methacrylate-functional oligomer before and after domestic laundering step. The contact angle measurement cannot be performed on the un-treated cotton fabric surface, because of the hydroxyl groups in cotton structure; the water droplet is absorbed immediately by the fabric. According to the table, the water-repellency of the sol-gel finished cotton fabrics was improved by increasing the percentage of fluorinated methacrylate-functional oligomer in the coating solution both before and after washing steps. Before the washing step, the best contact angle, thus the lowest surface energy, was obtained as 154.5° and 9.1 mJ/m² respectively on the 50% inorganic part containing sample. This surface is regarded as superhydrophobic since the contact angle is greater than 150°.¹⁸

Table 1.

Contact angle, standard deviation (SD), and surface energy values of fabrics treated with various percentages of fluorinated methacrylate-functional oligomer

Fluorinated methacrylate- functional oligomer ratio (%)	Before washing			After washing
Fluorinated methacrylate- functional oligomer ratio (%)	Contact angle (°)	SD	Surface energy (mJ/m²)	Contact angle (°)	SD	Surface energy (mJ/m²)
0	65.9	1.08	41.2	51.5	0.47	46.4
10	128.9	1.45	18.4	109.5	0.68	25.4
20	139.1	1.00	14.7	111.6	0.75	24.7
30	142.6	1.24	13.5	117.6	0.56	22.5
40	144.1	1.18	12.9	125.3	1.34	19.7
50	154.5	1.67	9.1	137	0.52	15.5

The covalent bonding between the silane groups on hybrid polymer and hydroxyl group on cotton structure enables a coating layer durable to the washing step. According to Table 1, after the washing step, the highest contact angle and the lowest surface energy were obtained as 137° and 15.5 mJ/m² respectively on the 50% fluorinated methacrylate-functional oligomer containing sample. The decrease in contact angle value after washing can be derived from the wetting agent residue of detergent remaining on fabric surface.

Figures 6 and 7 show the images of the water droplets on the coated fabric surfaces before and after washing respectively. Accordingly, after washing, neither fiber shedding nor any unevenness on coating layer was observed. The shape of the droplet became circular with increasing amount of fluorinated methacrylate-functional oligomer.

Figure 6.

Contact angles of water droplets on fabrics before washing, treated with various percentages of fluorinated methacrylate-functional oligomers: (a) 0%, (b) 10%, (c) 20%, (d) 30%, (e) 40%, (f) 50%.

Figure 7.

Contact angles of water droplets on fabrics after washing, treated with various percentages of fluorinated methacrylate-functional oligomers: (a) 0%, (b) 10%, (c) 20%, (d) 30%, (e) 40%, (f) 50%.

Contact angle change with time before and after the washing process can be seen in Figure 8. It is important to illustrate that there was no remarkable change in contact angle values during 100 s. Thus the water droplet cannot be absorbed over time.

Figure 8.

Contact angle change with time before and after washing with various percentages of fluorinated methacrylate-functional oligomers: 0%, 10%, 20%, 30%, 40%, 50%.

Coating thickness and weight increment

Table 2 shows the thickness and weight increment values of the hybrid polymer coated fabric samples. It is possible to obtain different coating layers in intended thickness values via the electrospinning process. With the increasing fluorinated methacrylate-functional oligomer ratio, thickness and weight increment values of the samples were decreased. Coating thickness values were not change remarkably for all samples. A small decline in micro-scale was observed in thicknesses of the samples. This can be related to the decrease of the bulkiness of the fabric with increasing inorganic part ratio. Weight of the hybrid polymer coated fabric samples was increased on average by 20% compared to the uncoated fabric weight. The weight increment handicap may be solved by using a lower molecular weight polymer solution with a thinner coating layer.

Table 2.

Thickness and weight increment of the samples after coating by fluorinated methacrylate-functional oligomers

Ratio of fluorinated methacrylate-functional oligomers (%)	Thickness of the samples (µm)	Weight increment (%)
Uncoated fabric	380	0
0	460	32.1
10	450	21.1
20	445	20.2
30	440	18.2
40	430	17.7
50	420	17.3

DSC analysis

DSC thermograms of the hybrid-free films are given in Figure 9. Glass transition temperature increases with the increasing ratio of the inorganic part. Polymer without the inorganic part has a glass transition temperature of 60.8℃, whereas the 50% inorganic part-containing hybrid polymer exhibited a glass transition temperature of 78.4℃. Table 3 shows the change in glass transition temperature with the increasing ratio of the inorganic part. According to the table, it can be seen that inorganic polymers have better thermal stability than other polymers. Inorganic components hinder the movements of polymeric chains, thus increasing the inorganic part ratio increases the glass transition temperature, meanwhile increasing thermal stability. In hybrid polymeric systems, DSC analysis illustrates the thermal stability and shows the bonding of the organic/inorganic components of the system.^19,20

Figure 9.

DSC thermograms of the hybrid-free films with various percentages of fluorinated methacrylate-functional oligomers: 0%, 10%, 20%, 30%, 40%, 50%.

Table 3.

Change of glass transition temperature with the variation of the ratio of fluorinated methacrylate-functional oligomers

Ratio of fluorinated methacrylate-functional oligomers (%)	Glass transition temperature (Tg ℃)
0	60.8
10	62.6
20	70.8
30	72
40	76.2
50	78.4

Measurement of mechanical properties

Abrasion resistance test

Wear resistance of the untreated and treated fabric samples were evaluated by an abrasion resistance tester. Table 4 shows the abrasion resistance properties of the untreated and sol-gel finished fabric samples in different inorganic part ratios. Untreated cotton fabric endures 21,000 cycles in abrasion resistance tests, while after coating by only acrylate (organic part), wear resistance was improved and increased up to 208,000 cycles. Results showed that increasing the fluorinated methacrylate-functional oligomers ratio causes a reduction in the abrasion resistance. Increasing the inorganic part ratio of the solution causes a hard handle on the fabric and gives stiffness to the surface, thus affecting abrasion properties negatively compared to the only organic part coated samples.

Table 4.

Abrasion resistance values and crease recovery angles of the untreated and hybrid polymer coated fabric samples in both weft and warp directions

Ratio of fluorinated methacrylate-functional oligomers (%)	Abrasion resistance (cycle) (×1000)	Crease recovery angle
Ratio of fluorinated methacrylate-functional oligomers (%)	Abrasion resistance (cycle) (×1000)	Weft direction (°)	Warp direction (°)
Untreated fabric	21	80	115
0	208	50	64
10	140	57	64
20	127	60	65
30	121	60	70
40	108	65	72
50	97	65	72

Crease recovery angle test

Table 4 shows the crease recovery angle testing results of the untreated and PU/silica hybrid-coated samples in both weft and warp directions. Untreated fabric has shown the highest crease recovery angle values because of the resilience and softness of the fabric. After the force was removed, untreated cotton fabrics tend to turn back their initial position. After coating with only acrylate (organic part), fabrics gained a rigid handle and exhibited less crease recovery angles. After the addition of the inorganic part into the hybrid polymer, crease recovery angle values increased with the increasing amount of the inorganic part. As in the abrasion resistance test, increasing the inorganic part on the textile surface increases the stiffness of the fabric and affects the fabric drapeability negatively. Therefore crease recovery angle values were increased as a result of the stiffness increment.

Conclusion

Water-repellent cotton fabric surfaces were successfully obtained by the sol-gel processed fluorinated methacrylate-functional oligomers coatings containing an organic part. Electrospinning was chosen as the coating method and UV light was used as a crosslinking medium. The synthesized silane-functionalized urethane structure was characterized by FT-IR spectroscopy. PU/silica hybrid films were characterized by DSC analysis. Hybrid polymer coated fabric samples were morphologically characterized by SEM and elementally characterized by EDS analysis. Water repellency of the coated fabric samples was measured by contact angle measurement. Coated samples were mechanically tested by abrasion resistance and crease recovery angle tests respectively.

FT-IR spectrum showed characteristic ester bonds (C = O) and C–F absorbance peaks at 1742 and 1192–1080/815–765 cm⁻¹, respectively. SEM images proved the well-dispersed hybrid polymer deposition on the fabric surface. EDS spectrum showed the presence of C, O, F, and Si atoms on the fabric surface. The best contact angle of 154.5° was obtained with the 50% inorganic part added fabric sample. After the domestic laundering step, this contact angle value decreased to 137° because of the surfactant residue of detergent. Coating thickness and weight increment values decreased with increasing inorganic part ratio depending on the decline of the bulkiness of the fabric surface. DSC thermograms showed that the thermal stability of the sample and glass transition temperature increase with increasing inorganic part ratio in the hybrid polymer solution. Increasing the amount of inorganic part on the textile surface increases the stiffness of the fabric; thus abrasion resistance was affected negatively and crease recovery angle increased in both weft and warp directions. It was concluded that superhydrophobic, low surface energy textile substrates can be obtained by hybrid polymeric coatings.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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