Abstract
This study focuses on the development of superhydrophobic and alcohol-repellent medical nonwoven fabrics via electrohydrodynamic atomization (electrospraying). It also compares the effectiveness of electrospraying with conventional pad-dry-cure finishing application. A commercial fluorochemical finishing agent was used to prepare fluorochemical solutions at varying concentrations (0.9–9 wt%). Electrospraying characteristics of these solutions were determined with characterizing their solution properties such as viscosity, conductivity and surface tension. After the successful applications of fluorochemical solutions on nonwoven fabrics via padding and electrospraying, wet pick-up ratios and weight gains of these fabrics were calculated. Also, water and alcohol repellencies of the coated fabrics were characterized with water contact angle and alcohol contact angle measurements. According to our findings, electrospraying application yielded less chemical consumption and higher water contact angle and alcohol contact angle results than padding. Increasing solution concentration and application time for electrospraying enhanced water contact angle values, which reached a maximum level (up to 156°) and afterwards remained almost constant depending on these variables. Thus, their limits to achieve superhydrophobic surfaces were able to be determined. Electrosprayed nonwovens were also shown to be alcohol-repellent against alcohol/water mixture of 70/30 (v/v%) whereas that was 30/70 (v/v%) for padded nonwovens. The investigation of the electrosprayed surfaces revealed a very less coating on the uppermost side of surface fibres which mostly led to the enhanced water and alcohol repellencies. One of the other important outcomes of this study is that there was no significant change on the comfort properties of nonwoven fabrics after the electrospraying application.
Keywords
Introduction
Medical textiles serve to conserve human health and welfare as a protector shield against disease, infection and injurious organisms. They also take roles on wound healing and replacing or supporting for unsanitary and injured body parts. For these purposes, gowns, sheets, masks and other medical textiles are in use in hospitals [1]. Pathogens can cause contamination among medical staff and patients via blood borne or other body fluids. Thus, medical textile products which are in direct contact with human skin should not get wet or let liquid pass through fabric and have hospital-related fluid barrier characteristics such as alcohol, water, blood repellencies [2].
Alcohol is typically used in medical industry for its bioactive characteristic. This certain characteristic helps to reduce the undesired microbial contamination [3]. Although alcohol’s bioactivity can reduce the contamination, it may not remove the risk of cross-contamination via alcohol-containing fluids. Therefore, alcohol repellency is required for medical textile products. The alcohol repellency term includes propanol, methanol and ethanol repellencies [4]. Water and alcohol present different behaviour on surface for repellency because of their surface-tension properties (water – 72.8 mN.m–1, isopropyl alcohol – 22 mN.m–1) [3]. Although alcohol repellency is an essential characteristic for medical textiles’ surfaces, it is difficult to obtain this characteristic due to alcohol’s very low surface tension, which facilitates spreading out the alcohol on the textile surfaces.
In order to achieve both alcohol and water repellencies, an advanced hydrophobic characteristic on the surface is needed. Superhydrophobic surface concept is related with wettability, high water contact angle (WCA) and low sliding angle [5]. While hydrophobic surfaces have WCA of 90° or higher, superhydrophobic surfaces have WCA of 150° or higher with sliding angle less than 10° [6]. In nature, superhydrophobicity is in everywhere from polar bears, ducks, butterflies, water walkers to plant leaves [7]. Lotus leaf is a good example from nature as having superhydrophobic surface which was discovered by Wilhelm Barthlott in 1990 [8]. Its advanced hydrophobicity comes from nano-sized waxy cover on micron-sized unsmooth surface. In this way, lotus leaf’s WCA can reach up to 160°, which makes lotus leaf superhydrophobic [5,9].
There are various methods [5,6,8] studied in the literature for achieving superhydrophobicity on textile surfaces. These methods are sol-gel [10], chemical vapour deposition (CVD) [11], electrochemical deposition [12], plasma [13], phase separation [14], organic/inorganic hybrid [15] and pad-dry cure [16,17]. In these methods mainly low surface energy materials such as fluorocarbons [18], silicones [19], organics (polyamide, polycarbonate [PC], alkyl ketene dimer, etc.) [7,20] and inorganics (ZnO, TiO2, etc.) [7,20] are used for either transformation or modification of surfaces.
Among above-mentioned materials fluorocarbons are commonly preferred as textile-repellent agents against other materials due to their excellent ability for minimizing surface energy of fabrics [3,7]. Surface tension is a key characteristic for achieving superhydrophobic surfaces. If fibres’ surface tension in the fabric is greater than or equal to liquid’s surface tension, fabric will be wetted by liquid. On the other hand, if the fibres’ surface tension is less than liquids’ surface tension, liquid will be repelled from the fabric [21]. A large variety of materials were reported in the literature to achieve hydrophobic surfaces. Some of them were fluorocarbons, silicones, PCs, polyamides, alkyl ketene dimers, ZnO, TiO2 etc. [7]. Fluorocarbons stand out amongst all common used materials for hydrophobic performance because they have a lower surface energy of about 10 mN.m–1 [1]. The fluorocarbons can also show extreme oil and soil repellency characteristics on textiles [2]. Besides, they improve surface roughness, which also helps improving hydrophobic performance [5].
Electrohydrodynamic atomization or electrospraying is a novel and promising method for adding functionality to textile surfaces. This method peculiarly does not need any extra mechanical energy; instead electrical field is used for the application [22]. It can provide obtaining nano-sized coating which can bring several advantages for many textile surface treatment applications. These advantages are high surface area, thinner coating layer and low coating weight [23]. In addition to these advantages, fabric breathing characteristic and fluid barrier characteristic can be provided [5,24,25]. From environmental and industrial aspects, nano-sized finishing is environmentally friendly and becomes feasible to industrial applications [25]. Monodisperse distribution [26,27] and controllable size of the finishing agents [28] are another important advantages of the application.
The main differences between electrospraying and electrospinning are the properties of polymeric solutions used and the form of materials produced. Solution properties for electrospraying differ in a way that concentration, viscosity and in most cases molecular weight are typically chosen lower than electrospinning counterparts. Also, while the final product of electrospraying can be very small-sized particles, that of electrospinning can be micron- or nano-sized fibres. On the other hand, electrospraying and electrospinning can show a major similarity in the processing of polymer solution, which is the evaporation of substantial amount of solvent during the traveling of polymer solution jet from needle to collector. Thus, the formed materials are mostly dried after their journey. In both electrospraying and electrospinning, the formation and size of the final products are similarly affected from some process and solution parameters. For instance, conductivity which determines the charge capacity of droplet usually shows similar effect on the properties of nanoparticles produced. That is to say, increasing the conductivity leads to size reduction on both nanoparticles and nanofibers [29,30].
There are very limited studies for achieving superhydrophobic surfaces via electrospraying method. Burkater et al. [31] electrosprayed polytetrafluoroethylene (PTFE) on glass substrates for achieving high WCA. They reached WCA of 150° and above on fluorine doped tin oxide (FTO)-coated glass slides surfaces. Gunesoglu [32] conducted a statistical study on the influence of process parameters and solution viscosity on deposition time for electrospraying. It was found that the shorter deposition time could be achieved with higher solution viscosity, flow rate and voltage and lower tip-to-collector distance. Although in this study the electrospraying application was performed on a textile substrate (cotton fabric), the hydrophobic characteristic of the fabric and the degree of the hydrophobicity were not investigated. Papadopoulou et al. [33] studied on the formation of superhydrophobic porous film surfaces for the application of corrosion and oxidation protection in building materials. Electrospraying and electrospinning methods were used to form porous films of poly (ethyl methacrylate) (PEMA) and PC. Hydrophobic characteristics of these films were compared with the one produced with phase separation method. Although WCAs around 150° were achieved from all of these methods, film surfaces obtained from electrospinning /electrospraying methods had better uniformity and more controllable thickness compared with phase separation method. Also, phase separation method had considerably lesser effectivity because of its hysteresis contact angles and random film surface.
In the literature, there is no study that investigates the hydrophobic performance of electrosprayed fluorochemical-applied textile surfaces. Also, the potential of electrospraying application for the achievement of superhydrophobic and alcohol-repellent textile surfaces has not been explicitly discovered yet. In this study, it was aimed to obtain superhydrophobic and alcohol-repellent medical nonwoven textile surfaces by using electrospraying application. A commercial fluorocarbon textile-finishing chemical with varying concentrations (0.9–9 wt%) were used. In order to determine the suitability of these solutions for electrospraying viscosity, surface tension and conductivity measurements were conducted. The concentration of the solution and application time (10–240 min) were optimized for achieving high repellency on the nonwoven textile surface. Also, electrospraying application was compared with conventional padding application at same concentrations and application time. For hydrophobic performance assessment WCA and alcohol contact angle (ACA) measurements were obtained from all samples. In our study, the alcohol repellencies of medical nonwoven fabrics were measured against isopropyl alcohol and its mixture with water. This is because isopropyl alcohol known as rubbing alcohol is widely used in medical industry. It is also popular in pharmaceutical sector due to its low toxicity [34]. In order to characterize the structure of surface coatings, scanning electron microscope (SEM) images were taken. Pick-up ratios and weight gains of coated fabrics were determined to find the chemical material consumption. Also, the changes on comfort properties before and after applications were monitored using water vapour permeability and air permeability tests.
Materials and methods
Materials
Polypropylene-based spunbond-meltblown-spunbond (SMS) nonwoven fabrics used in medical drapes were kindly provided by RITAS A.S. (Gaziantep/Turkey). These fabrics have 35 g.m–2 basis weight in which each spunbond layer has 14 g.m–2 and meltblown layer has 7 g.m–2. They had fibres with denier values in the range of 1.6 to 2 and were calendered with a speed of 90 m.min–1 and a temperature of 145℃.
Fluorochemical-based commercial textile surface finishing agent was kindly supplied by Eksoy Chemicals Inc. (Istanbul/Turkey). This fluorochemical consists of fluoroalkyl acrylate copolymer, emulsifiers (25–35%), tripropylene glycol (5–10%) and water (55–65%). The fluoroalkyl acrylate copolymer inside this chemical has comparatively low molecular weight [35]. Therefore, when this copolymer-based solutions are subjected to high electric field, it is likely to form droplets instead of fibres in which an appropriate molecular weight and concentration are required to induce desired molecular entanglement [7].
Aqueous solution of fluorochemicals with 0.9, 1.8, 3, 4.5, 6 and 9 wt% concentrations were prepared at room temperature and were used for both electrospraying and padding treatment.
Characterization of fluorocarbon solutions
The conductivity of fluorocarbon-containing solutions was measured on Thermo Scientific Orion 4 Star and four measurements for each concentration were taken. Characterization of surface tension was carried out by Theta Optical Tensiometer. Six surface tension measurements were taken for each concentration. Viscosities of the solutions were determined by using Brookfield DV III Ultra Programmable Rheometer at 100 r/min with three repeats for each solution.
Human keratinocyte (HaCaT cell line) cell viability analysis
Cytotoxicity of fluorochemical textile finishing agent on fabric was determined with HaCaT cell viability assay. In this test, HaCaT cells were cultured in Dulbecco’s modified Eagle’s medium (DMEM) containing 10% fetal bovine serum, 1% L-glutamine at 37℃ in air containing 5% CO2 [36]. The cells (5 × 103 cell.mL–1) were seeded to 96-well plate and were incubated overnight. Fluorochemical was diluted with cell culture medium and 100 µl of the medium was pipetted on the cells. Cells were incubated for 4 h at 37℃. After the incubation fluorochemical-containing mediums were removed and 10% MTT solution (2.5 mg.mL–1 MTT dissolved in PBS) containing cell culture mediums were pipetted to each well, then plate was incubated at 37℃ for 4 h. The mediums were removed and 100 µl of 0.08 M HCl in isopropanol solution was added to each well to dissolve formazan crystals. Formazan absorbance was measured at 570 nm. Cell culture medium was used as a control in the measurement.
Electrospraying and pad-dry-cure applications
Electrospraying application has been carried out via vertical electrospraying set-up (Figure 1). This set-up contains a syringe pump (New Era NE-1000X), a high-voltage power supply (Gamma High Voltage Series ES100P Models 20 W) and a grounded collector. During the electrospraying application, while supplied voltage, flow rate of solution and needle tip to collector distance were fixed at 10 kV, 10 µL.min–1 and 10 cm, processing time was varied at 10, 60 and 240 min.
Electrospraying set-up.
Pad-dry-cure, a conventional textile finishing method, was also used to coat nonwoven fabrics with fluorocarbon finishing agents. In padding application nonwoven fabrics were immersed in aqueous fluorochemical solution bath from 10 s to 240 min and then they were squeezed between two pressure rollers at 300 kPa pressure and 2 m.min–1 velocity. Dimensions of the nonwoven fabric sample were 20 cm width and 20 cm length for padding and electrospraying applications. After both electrospraying and pad-dry applications, treated nonwoven fabrics were dried for chemical fixation purpose in a vacuum oven (Nuve EV 018) at 110℃ for 5 min.
Characterization of electrosprayed and padded nonwoven fabrics
Contact angle measurements were conducted on raw and treated nonwoven fabrics by using Theta Optical Tensiometer according to ASTM D7334−08 [37] standard. In these measurements, 100% water, 100% isopropyl alcohol and varying alcohol-water mixtures (10/90, 30/70, 50/50, 70/30, 90/10 v/v%) were used as the liquids for the sessile droplet formation.
Raw and treated nonwoven fabrics were examined with taken SEM (JEOL JSM-6390) pictures for the assessment of coating structure. Before SEM investigation, samples were coated with Au/Pd in SC 7620 Sputter Coater. SEM pictures were taken at 5 kV accelerating voltage and 12- to 15-mm working distance with magnifications between 100 and 5000.
Water vapour transmission rate (WVTR) of raw and treated nonwoven fabrics were measured in SDL ATLAS M261 according to the BS EN 7209 [38] standard. Also, air permeability test was performed for these fabrics according to ASTM D737-04 [39] standard in SDL ATLAS M021A air permeability tester.
Statistical analysis
Analysis of variance, ANOVA, was used to determine the statistical importance of variables on comfort properties. The level of significance was considered at
Results and discussion
Determination of electrospraying characteristics of fluorocarbon solutions
Properties of fluorochemical solutions (standard errors are given in parentheses).
The viscosities of our fluorocarbon solutions were found to be in the range of 16.6–22.3 mPa.s (Table 1). When these results were evaluated, it was concluded that there was no significant change in viscosity at low concentration values (0.9–3 wt%). This is probably due to the fact that fluoroalkyl acrylate copolymer chains in fluorochemical solution were not sufficiently close to each other at dilute solutions for inducing any viscosity change. On the other hand, when concentration values exceeded 3 wt%, fluoroalkyl acrylate copolymer chains came close enough to interact with each other. Thus, solution viscosity increased steadily as concentration increased. As a result of the increasing concentration, the interaction between chains triggers intermolecular entanglement and restricts the freedom of motion of individual chains [40]. Solution viscosity determines whether the process follows the electrospin or electrospray routes [8,41]. It also has effect on fibre/particle dimension [8]. If liquid viscosity reaches high values, electrospinning process takes place because of unbroken jet travels to collector as fibre [8]. Jaworek et al. [27] studied the effect of viscosity on electrospraying and found that the viscosity of electrosprayed mixtures were in between 1 mPa.s and 22 mPa.s. Acatay et al. [5] electrospun blend of fluorolink-D with poly(AN-co-TMI) and found that electrospinning occurred only at viscosity values higher than 31 mPa.s. Considering all of these studies, viscosity values obtained in our study were convenient for electrospraying.
When conductivity values of fluorocarbon solutions were evaluated, it was seen that the conductivity of solutions increased sharply from 182 (±2) µS.cm–1 to 1456 (±1) µS.cm–1 with increasing concentration (Table 1). Results indicated that high fluorocarbon concentration enhanced the conductivity. Water itself has very small degree of conductivity (in our study it was found to be 4.1 µS.cm–1). On the other hand, fluorocarbons’ dielectric strength and electronegativity are very high [31]. The electronegativity of fluorine brings partially ionic character via partial charges on fluorine and carbon atoms. So the addition of the fluorocarbon improves the conductivity of water-originated solution [42]. Also, the acidic nature of the fluorochemical can improve the concentration of ions in the solution. Takahashi et al. [43] measured the conductivity of fluorocarbon-containing hybrid surfactant. They also found that as the concentration of this hybrid surfactant increased, conductivity value increased linearly. Conductivity is crucial characteristic for electrospraying and electrospinning. It determines droplet charge capacity [8]. Solution with low conductivity values (<10–11 µS.cm–1) cannot be electrosprayed. But there is not any upper limit of conductivity value for electrospraying [44]. Since the conductivity values of our solutions were found to be higher than 182 (±2) µS.cm–1, it can be said that our solutions had enough conductivity values for electrospraying.
Surface tension of the solution has also an important role on electrospraying process like viscosity and conductivity. Therefore, the surface tension values of our solutions were measured and were found to be in between 37.6 (±0.2) mN.m–1 and 42.9 (±0.5) mN.m–1. Water has very high surface tension value (in our study it was found to be 66.2 mN.m–1). This is due to the fact that water molecules form hydrogen bonds with each other. These strong hydrogen bonds result in inward force at water surface and keep water molecules together. This causes water to have a high surface tension. For this reason, as water concentration in the solution is increased, surface tension is expected to increase. Therefore, diluted solutions showed higher surface tension values than less-diluted solutions. In the literature, it was reported that electrospraying liquids’ surface tension should not be higher than 50 mN.m–1 [32]. In this regard, the surface tensions of our fluorocarbon solutions were convenient for electrospraying. It was also found that the surface tension slightly decreased with increasing fluorocarbon solution concentration. The reason for this can be explained with solution components’ characteristics. Reported surface tension value of distilled water was 72.7 mN.m–1 [45] while that of fluorocarbon was lower than 10 mN.m–1 [1]. Since the diluted solutions had higher water content than concentrated solutions, it can be expected to have higher surface tension values for more diluted solutions.
Effect of solution concentration, application type and time on chemical consumption
Fluorocarbon solutions were electrosprayed successfully on SMS nonwoven substrates. Application time (10–240 min) and fluorocarbon solution concentration (0.9–9 wt%) were varied for electrospraying. During all electrospraying applications, cone-jet and multi-jet formations were observed. In addition to electrospraying, padding (pad-dry-cure) application was performed at different times (10 s to 240 min) to obtain coated nonwoven fabrics for comparison purpose. After electrospraying and padding applications, the consumption rate of chemicals and weight change on the fabric were reported. Thus, wet pick-up ratios and weight gains of fabrics could be determined. Figures 2 and 3 show results for wet pick-up ratios and weight gains after both coating applications. ES-10, ES-60 and ES-240 represent for the electrospraying applied at 10 min, 60 min and 240 min, respectively. Also, Padding, Pad-10, Pad-60, Pad-240 represent for the padding applied at 10 sec, 10 min, 60 min and 240 min, respectively. It was found that wet pick-up ratios for padding applications ranged in between 72% and 125% and did not point out any significant trend (Figure 2a). The reason can be explained by the intrinsic characteristic of the PP-based nonwoven fabric, which did not have hydrophilic characteristic. Therefore, only surface wetting can be accomplished and the fabric surface can be saturated with fluorochemical solution even at short application time and low concentration. This resulted in no strong dependency of wet- pick-up on time and concentration during padding application. On the other hand, wet pick-up ratios for electrospraying started from very low ratio (0.1%) and reached up to 53% (Figure 2b). The application concept of electrospraying was not based on surface wetting treatment unlike padding. During electrospraying process, a great deal of solvent evaporates and most of the functional agents reach on to the fabric surface. Therefore, wet pick-up ratios increased with increasing either concentration or application time (Figure 2b). Also, when we compare wet pick-up ratios for electrospraying and padding, it could be said that there was a great deal of difference in between results obtained for these applications. For instance, wet pick-up ratio for the electrosprayed nonwoven fabric (ES-10) with solution having 0.9 wt% fluorocarbon concentration was 0.1%, while this ratio was 87.9% for padded fabric (Pad-10) at same concentration and application time. Even at the longest application time (240 min) and the highest concentration (9 wt%), wet pick-up ratio of electrosprayed fabric (53%) was much lower than that of padded fabric (119%). This trend was observed the same for all concentrations and time scales used in this study. This is mainly due to the fact that during electrospraying only the face of the nonwoven fabric could be coated with fluorocarbon chemical and water could be substantially evaporated. But for padding application, the entire fabric was wetted with aqueous fluorocarbon chemicals. High pick-up ratios were also reported in the literature for padding [1,46]. Shyr et al. [46] used padding method to apply water-repellent finishing agents on polyester fabric and obtained approximately 40% pick-up ratio. Thilagavathi and Kannaian [1] also processed cotton fabric via padding method to achieve blood repellency with fluoropolymer and measured the pick-up ratio as about 75%.
Pick-up ratios of padded (a) and electrosprayed (b) nonwoven fabrics. Weight gains of padded (a) and electrosprayed (b) nonwoven fabrics.
After the applications of electrospraying and padding, nonwoven fabrics were dried in oven to remove water and to fix the chemicals on the surface of the fabric. Our results revealed that weight gains of fabrics showed rising tendency when fluorochemical concentrations were increased for both applications (Figure 3). This is expected since more fluorochemicals were accumulated on to fibers with the increase of fluorochemical concentrations. Although such a relationship was observed for the application time variable on electrospraying, there is no such situation existing for padding. The reason for not observing a gradual increase on weight gain with increasing the padding time can be explained by the saturation of the fabric during wetting. It means that the amount of chemicals adsorbed on to the fabric did not significantly different from each other at investigated time range. This is also supported by our findings obtained for wet pick-up ratios in padding application (Figure 2a).
When we compare the weight gain results for electrospraying and padding applications, we can say that padded fabrics had considerably higher weight gain than electrosprayed fabrics under 240 min application time at all concentration values. For instance, weight gains for the fabrics were recorded as 0.1% and 2.3% for electrosprayed (ES-10) and padded (Pad-10) fabrics at 0.9 wt% fluorocarbon chemical concentration, respectively. This is because finishing agent was applied to padded fabric via wetting and therefore the fabric was entirely coated with finishing agent. But for electrosprayed fabric, finishing agents were only collected on to the face of the fabric. However, this trend was not observed for comparatively long electrospraying application time of 240 min. This is because fabrics were already saturated with fluorochemicals during padding, but this is not a valid case for electrosprayed fabrics in which high amount of finishing agent can be collected on the surface of the fabric when the duration of application is increased (Figure 2).
When we compare pick-up ratio and weight gain results, there were significant differences between them for padded nonwovens, which contained finishing agent and water together. For electrosprayed nonwovens this difference was suppressed due to the evaporation of substantial amount of water during electrospraying application. For padded nonwovens high difference between weight gain and pick-up ratio could cause more energy consumption for drying of excessive water from fabric especially for industrial applications. Souma [47] reported the drawbacks of the padding method as high chemical and water usage that means high expenditure. Leroux et al. [48] also supported this phenomenon and pointed out that padding as a traditional method is not environmentally friendly since a huge amount of water and chemical products are used, and high energy is required to evaporate this water. On the contrary, for electrospraying application, less amount of finishing agent can be required to obtain the same performance levels. Therefore, water usage and energy requirement can be minimized.
Effect of solution concentration, application type and time on hydrophobicity
After the application of finishing agent on nonwovens via electrospraying and padding methods, hydrophobic characteristics of fabrics were firstly evaluated with WCA measurements. WCA results for raw, padded and electrosprayed fabrics are shown in Figure 4. Applied fluorocarbon solution concentrations ranged between 0.9 wt% and 9 wt% for padded and electrosprayed fabrics while application time varied from 10 to 240 min for electrosprayed fabrics. The application time for padding was not varied at this part of the study since it was shown that it did not affect the amount of fluorochemicals adsorbed by fabric (Figures 2 and 3).
Water repellencies of raw, padded and electrosprayed nonwoven fabrics.
WCA of raw fabric was found to be 128.4° (±2.5°). After the application of fluorocarbon chemical, WCA of padded nonwovens changed between 134.9° (±2.3°) and 145.2° (±3.8°). As the concentration of fluorocarbon increased, WCA increased until it reached a plateau at about 4.5 wt%. For electrosprayed fabrics, WCA significantly increased up to 153.9° (±2.5°) compared to raw and padded fabrics and superhydrophobic characteristic was achieved. Similarly, WCA increased with increasing the concentration of fluorocarbon solution and then it reached a plateau for electrosprayed fabrics at all application time. This was expected since more fluorochemicals were collected on to fabric with increasing the fluorocarbon concentration. Our results also indicate that the concentration values in which WCA values reached to a plateau varied depending on the application type and time (4.5 wt% for Padding; 6 wt% for ES-10 and ES-60; 3 wt% for ES-240). This is due to the fact that the different amounts of fluorochemicals were collected on the surface of fibres with changing time and application type (Figure 3). Thus, fabrics were saturated with fluorochemicals at different concentrations and therefore WCA values reached their maximum value and followed a plateau afterwards.
When we compared WCA results obtained from padded and electrosprayed nonwovens at the same concentration values, we found that electrosprayed nonwovens gave superior results. Even at the shortest application time of 10 min, electrosprayed nonwovens showed better hydrophobic characteristics than padded nonwovens. This was due to the fact that for electrosprayed fabrics fluorochemicals were accumulated on to the surface of the top layer, whereas for padded fabrics fluorochemicals were embedded through the fabric. Also, the charged liquid droplets in electrospraying application can be more efficiently deposited on to the surface than uncharged liquid. These charged droplets repulse each other and widely spread on fabric in the small particle form. By this way more surface area can be coated. This is an advantage for effective surface coating with electrospraying method that produces nano- and micron-sized particles [26]. These particles have larger surface area-to-volume ratio and better affinity for textile surfaces [8]. By taking into consideration all these characteristics, electrosprayed fabric is expected to show higher WCA compared to padded fabrics.
Increasing the application time for electrospraying resulted in an incremental increase for the hydrophobic performance difference between electrosprayed and padded nonwovens. This was expected since more chemicals were deposited on the surface which can reduce surface tension on fibres more and further improve hydrophobicity. It was observed that there is a significant increase in measured weight gain after electrospraying applications with increased process time for all fluorocarbon solutions (Figure 3). To give an example, weight gain for 10-min electrosprayed fabric with 4.5 wt% fluorocarbon concentration contained solution was 0.4%, while this value was about 12.6% for 240 min electrosprayed fabric at the same concentration. However, the difference in between 10 and 60 min electrospraying applications was not so significant. Thus, our results indicated that even 10 min electrospraying application was sufficient for obtaining better WCA than padding application. If pick-up ratios and weight gains of padding and electrospraying methods were taken into consideration, for the same concentration value much lower amount of solutions were adsorbed on to fabric and less chemicals were consumed for electrospraying that could yield higher WCA than padded nonwovens.
Papadopoulou et al. [33] used electrospinning and electrospraying methods to produce superhydrophobic films with PEMA and PC polymers. Polymer concentrations were used between 5 w/v% and 25 w/v% for both PEMA and PC. They reached WCAs of 130° and 150° for PEMA and PC hydrophobic films, respectively. In our study, we achieved larger WCA with lower finishing agent usage. Burkater et al. [31] also used electrospraying method to coat glass slides with fluoropolymer suspension which consisted of 60 wt% PTFE. They obtained WCA of 167° for PTFE-coated surface. When we compare this study with ours, it can be said that the achievement of high WCA results in this study could be attributed to much higher PTFE concentration (60 wt%). Also, unlike our study electrospraying application was performed on non-textile surface.
Effect of application type on alcohol repellency
For the evaluation and comparison of alcohol repellencies, contact angle measurements of raw, padded and electrosprayed nonwoven fabrics were taken with 10/90, 30/70, 50/50, 70/30, 90/10, 100/0 isopropyl alcohol/water (v/v%) mixtures. In all, 3 wt% fluorocarbon solution were used in padding and electrospraying applications (ES-240). Figure 5 shows the measured ACAs for raw and coated nonwoven fabrics. ACA of the raw nonwoven fabric at 10/90 alcohol/water (v/v%) mixture was found to be 120° (±1.6°), while that of padded and electrosprayed fabrics were 130.3° (±3.4°) and 142.5 (±2.1°), respectively. Raw nonwoven fabric did not show any alcohol repellency for alcohol/water mixtures of above 30/70 (v/v%). But padded and electrosprayed nonwovens still showed alcohol repellency up to 100% alcohol. Degrees of alcohol repellencies for electrosprayed fabrics were higher than these for padded fabrics at all alcohol/water mixtures. Also electrosprayed fabric maintained hydrophobic characteristic (ACA > 90°) against alcohol/water mixture even at 70% alcohol concentration with ACA of 96.5° (±5.7°). This was expected since we obtained high WCA of 153.4° (±5.1°) for the same electrosprayed fabric. For alcohol/water mixtures, increasing the alcohol concentration reduces the surface tension of the fluids that makes spreading easy and reduces the contact angle. This is because isopropyl alcohol has lower surface tension value (γisopropyl alcohol = 21.7 mN.m–1) than water (γwater = 66.2 mN.m–1). On the other hand, the surface tension value of the PP fabric was reported to be in between 29 and 31 mN.m––1 [49]. When the surface tension of liquid is lower than that of fabric surface, the liquid can easily wet the fabric [21]. For this reason, increase of isopropyl alcohol content in the mixture made fabric wetting easy and led to a decrease in the surface tension and contact angle. Nevertheless, we were able to achieve high degree of alcohol repellency for the electrosprayed fabrics. Ho et al. [4] also sought for a new approach to increase hydrophobicity on nonwovens and produced them from synthetic polymer mixed with surface modifier, which was responsible for characteristics of hydrophobicity and alcohol repellency. By this way, they could obtain ACA of 45° at 60/40 alcohol/water (v/v%) mixture. But in our study, we reached higher ACAs for electrosprayed fabrics even against 100% alcohol (70.9° ± 6.3°).
Alcohol repellencies of raw, padded and electrosprayed nonwoven fabrics.
Investigation of coated surfaces
In order to investigate the morphology of the coated surfaces, SEM images were taken for raw nonwovens and electrosprayed nonwovens in which 3 wt% fluorochemical was applied for 10 and 240 min (Figure 6). It was observed that electrosprayed fluorochemicals resided on only surface fibres of nonwovens. Also, it seems that there was more surface coatings for 240-min application than 10-min application as expected. Coatings on the fibre surfaces were very less, especially for ES-10. Also, they were mostly observed on the uppermost side of the surface fibres. Thus, it was concluded that these properties of the coating obtained by electrospraying led to the achievement of high degree of water and alcohol repellencies in this study. Shyr et al. [46] used padding method to impart water repellency on to polyester fabric with fluorochemical and nanosilver particles. They observed that fibre surfaces were covered with self-aggregated particles after padding application. It can be speculated that electrospraying application not only provides a better coated surface in the manner of fineness but may also prevent the aggregation of functional finishing agents. SEM images in Figure 6 also show that there is a small rough surface formation for electrosprayed samples. Although, the achieved water repellencies for these samples mostly arised from chemicals presented mostly on the uppermost side of surface fibres and fluorocarbon’s lowering surface tension effect, there must be some contribution of small roughness effect achieved on the fibre surfaces (Figure 6). Hence, it can be speculated that WCA values for these samples can be slightly increased with the participation of roughness effect seen on the coated fibre surfaces.
SEM and water contact angle images of raw, 10-min electrosprayed, 240-min electrosprayed nonwoven fabrics.
Cytotoxicity of fluorochemicals applied to nonwoven fabrics
Cytotoxicity of fluorochemicals that were applied on nonwoven fabrics was determined from the HaCaT cell viability assay. HaCaT cells (human keratinocyte line) were chosen because they have been widely used to assess cytotoxicity of chemicals on human skin [50]. In our assessment, the fluorochemical amount on the treated nonwoven fabrics was estimated as 1 wt% for the comparison of different concentration values of fluorochemical finishing agents on these fabrics. For this purpose, the concentration values given in Figure 7 were normalized to be the 1/100th of their original concentration. When we evaluate the cytotoxicity results presented in Figure 7, we can say that the cytotoxicity of samples increased in relation to increasing concentrations as expected. Our results also showed that the concentration value of 6 wt% can be considered as the critical point in terms of cytotoxicity. In other words, this concentration can be stated as the effective concentration EC50 that caused 50% of cells to die. Above this concentration value more than 50% of the cells died. According to Freire et al. [51], the cytotoxicity of the fluorinated compounds mainly originated from perfluorooctanic acid (PFOA), which can disturb the cell cycle and can create genotoxic effect in the human cells. Based upon our findings, it should be noted that fluorochemical concentration above 6 wt% applied to the fabric with 1 wt% chemical uptake would cause adverse effect.
Cytotoxicity of fluorochemicals applied on nonwoven fabrics.
Effect of application type and time on comfort properties
Transmission of water vapour through fabric which is an indicator for perspiration transfer is important especially for wearer comfort. In this regard, WVTRs of nonwoven fabrics before and after applications were examined (Figure 8). WVTR of raw fabric was found to be 888 g.m–2.24 h–1 (±6 g.m–2.24 h–1) while that of padded and electrosprayed (ES-10) fabrics were 875 g.m–2.24 h–1 (±4 g.m–2.24 h–1) and 888 g.m–2.24 h–1 (±26 g.m–2.24 h–1), respectively. Thus, after the application of fluorochemical (3 wt%) with padding and electrospraying, WVTR of the fabrics did not significantly change compared to raw fabric. When we compared WVTR results obtained from ES-10 (888 g.m–2.24 h–1) and ES-240 (884 g.m–2.24 h–1), it can be said that electrospraying application time did not affect the WVTR of fabrics. Statistically, an insignificant difference in the WVTR results was observed between raw, padded, ES-10 and ES-240 fabrics ( Water vapour permeability of raw, padded and electrosprayed nonwoven fabrics.
Air permeability is another important comfort characteristic for medical textiles that are directly in contact with human skin [2]. Therefore, air breathability of raw, electrosprayed and padded nonwoven fabrics were examined and results are shown in Figure 9.
Air permeability of raw, padded and electrosprayed nonwoven fabrics.
One-way ANOVA analysis was performed to reveal any difference in between the air permeability of samples. It was found that there was no statistically significant difference between the air permeability values of raw and treated nonwoven fabrics at all fluorochemical concentration used in this study (
Conclusion
In this study, superhydrophobic and alcohol-repellent medical nonwoven fabrics were obtained by means of electrospraying application. The nonwoven fabrics were treated with fluorochemical-containing aqueous solutions with fluorochemical concentrations ranged from 0.9 wt% to 9 wt%. After analysing viscosity, surface tension and conductivity properties of these solutions, it was shown that they were in the suitable range for electrospraying. A conventional pad-dry-cure and electrospraying processes were used at the same time to compare fluorochemical consumptions and final properties of the treated nonwoven fabrics. In addition to solution concentration, application time was also taken as a variable for both processes.
According to our findings, after electrospraying application weight gains and wet pick-ups of fabrics increased with increasing solution concentration and application time. However, they did not significantly change after padding when the duration of application was altered. This is because only surface wetting can be accomplished during padding and the fabric surface was already saturated with fluorochemical solution at even short application time of 10 s. Our findings also indicate that electrospraying application yielded less pick-up ratios and chemical consumption than padding. This is due to the fact that the whole fabric was coated with fluorochemical solution during padding, whereas only the fabric surface was coated during electrospraying. Furthermore, we found that electrosprayed nonwoven fabrics yielded higher water and alcohol contact angle (WCA and ACA) results than raw and padded nonwoven fabrics. It is because electrospraying application not only provides thinner coating on the uppermost side of surface fibres but may also prevent the aggregation of functional finishing agents. Also, it was speculated that there must be some additional contribution of small roughness effect achieved on the fibre surfaces. We showed that increasing solution concentration and application time in electrospraying application yielded higher WCA values which reached a maximum level (up to 156°) and afterwards remained almost constant depending on these variables. This allowed us to determine the actual solution concentration and application time limits to achieve superhydrophobic surfaces on the electrosprayed nonwovens. These nonwovens were also shown to be alcohol-repellent against alcohol/water mixture of 70/30 (v/v%), whereas that was 30/70 (v/v%) for padded nonwovens. In addition to these, it was revealed that comfort properties of fabrics did not change after the coating, which is an advantage for the wearer.
Cytotoxicity test results revealed that 6 wt% solution concentration was the critical point (EC50) when fabric had 1 wt% fluorochemical on it. Therefore, it should be avoided using higher concentrations and applying more fluorochemicals on to the fabric. At this point electrospraying application emerges as a promising method to impart less chemicals for obtaining comparatively better properties. It also allows us to coat only the desired surface of fabric (e.g. outer surface) and therefore decreases direct contact with the skin unlike padding in which the entire fabric is coated. Thus, we can conclude that electrospraying application offers great potentials for textile industry because it can provide opportunity to manufacture cost-effective functional textiles without using excessive finishing chemicals and water which can also lead to energy conservation for post treatments (e.g. drying) and cost reduction for chemicals. For academia, this study can be provided as a useful reference for future studies on the investigation of electrospraying application of wide variety of materials on several textile and film surfaces.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was fully supported by the Scientific and Technological Research Council of Turkey (TUBITAK-MAG-113M517).