Multifunctional Hydrophobic, Oleophobic and Flame-retardant Polyester Fabric Večfunkcionalna vodo- in oljeodbojna ter ognjevarna poliestrska tkanina

Technical textile materials with multifunctional protective properties represent one of the largest and fast growing segments of the textile industry. Multifunctional water- and oil-repellent and ﬂ ame-retardant coating on polyester (PES) fabric was prepared in this research using ﬂ uoroalkyl-functional siloxane (FAS) as the water- and oil-repellent ﬁ nishing agent and organophosphonate (OP) as the ﬂ ame-retardant agent. A ﬁ nishing solution containing FAS and OP of appropriate concentrations was applied to the untreated and oxygen plasma-treated PES fabric samples using the pad-dry-cure method. For comparison, single-compo-nent FAS and OP ﬁ nishing solutions were applied to the fabric samples under the same conditions. The coated PES samples were washed under standard conditions. The morphological, chemical and functional properties of the coated PES samples were determined with scanning electron microscopy, Fourier transform infrared spectroscopy, wet pick up, liquid contact and sliding angles measurements as well as oil repellence and vertical burning tests. The results reveal that oxygen plasma treatment prior to ﬁ nishing sig-niﬁ cantly increased the wettability of the PES ﬁ bres, which directly resulted in increased concentration of the absorbed ﬁ nishing agents. This treatment enabled the creation of PES fabric with simultaneous superhydrophobic, oleophobic and ﬂ ame-retardant properties. Although the superhydrophobic and oil-repellent characteristics of the coating were preserved after washing, the ﬂ ame retardancy was hindered because of the removal of OP in the washing bath.


Introduction
Technical textile materials with multifunctional protective properties represent one of the largest and fast growing segments of the textile industry, and these materials have wide uses in diff erent economic sectors. In technical applications, polyester fi bres are the most frequently used synthetic material because of their low cost, durability, ease of care, good dimensional stability, low moisture absorbency and compatibility with cotton in blends [1]. Th ese extraordinary properties enable polyester to be increasingly applied in the production of textile materials for protective clothing and in the sport and leisure, transportation, construction and agricultural industries. However, in addition to the desired characteristics, polyester fi bres suff er from certain important disadvantages related to their functionality, such as electrostatic charging and fl ammability, which decrease the value and usefulness of the end products. Th e susceptibility of polyester fi bres to electrostatic problems is directly infl uenced by their hydrophobicity, leading to generation and accumulation of electrostatic charges [2−4]. Th e latter attract particulate soils from the air, resulting in fi bre soiling. In contrast, due to the hydrophobic properties of polyester, wetting and swelling of fi bres with detergent solution during laundering is hindered, which importantly decreases the eff ectiveness of removal of the adhered soil [5−7]. To overcome these problems, tailoring of a self-cleaning coating characterised by superhydrophobic, oleophobic and low-adhesive properties is crucial. According to the theory, self-cleaning biomimetic solid surfaces exhibit low-adhesion superhydrophobicity, which is simultaneously characterised by a static water contact angle greater than 150° and a water sliding angle less than 10° as a result of a low contact angle hysteresis [8]. Th ese surfaces include micro-and nanoscale roughness topographies coated with water-repellent polymer fi lms [9−12]. However, in addition to particulate soils, oily stains are important contaminants of textile fi bres, and thus creation of a coating with oleophobicity is of great importance.
A coating that is simultaneously oleophobic and selfcleaning could eff ectively repel diff erent types of soils and prevent their adhesion as well as ensure removal of adherent soils via their collection by water droplets when rolling off the surface. Inherent fl ammability with intensive burning melt/ dripping and release of toxic smoke represents a serious hazardous drawback of polyester, which poses great risk and danger to human lives and material goods [13]. Because highly eff ective fl ame retardants including brominated diphenyl esters, brominated phosphates and tri-aryl-phosphates have been restricted and prohibited by the European Union's Registration, Evaluation and Authorisation of Chemicals (REACH) because of their toxicological problems and environmental unsustainability, diff erent environmentally friendly phosphorous-containing compounds have been synthesised to produce fl ameretardant polyester [14−16]. Th e fl ame-retardant mechanism of phosphorous-containing compounds is directly infl uenced by their chemical structure. In general, depending on the oxidation state of the phosphorous atom, fl ame-retardant substances are active in both the condensed phase and the gas phase [14,17,18]. In the condensed phase, phosphorous compounds promote char formation by infl uencing the fi bre decomposition pathway, and in the gas phase, phosphorous compounds decompose to radical scavengers, which terminate oxidative radical chain reactions in the combustion cycle. In this study, we fi rst prepared multifunctional waterand oil-repellent and fl ame-retardant polyester fabric with the use of two chemical fi nishes, i.e., fl uoroalkylfunctional siloxane as a water-and oil-repellent agent and organophosphonate as a fl ame-retardant agent. To enhance the hydrophilicity of polyester fi bres and consequently increase their absorptivity to the fi nishing solution, fi bre functionalisation with oxygen-rich groups was performed using an oxygen plasma pretreatment. It has been established that oxygen plasma treatment can cause an increase in the surface activity and also an increase in surface roughness [19−23], therefore an important goal of our research was to omogočila pripravo tkanine PES s hkratnimi superhidrofobnimi, oleofobnimi in ognjevarnimi lastnostmi. Medtem ko sta se superhidrofobnost in oleofobnost ohranili tudi po pranju, se je ognjevarnost poslabšala zaradi postopne odstranitve sredstva OP med pranjem. Ključne besede: poliestrsko vlakno, apretura, večfunkcionalne lastnosti, vodo-in oljeodbojnost, ognjevarnost, pralna obstojnost Tekstilec, 2019, 62(1), [12][13][14][15][16][17][18][19][20][21][22] Multifunctional hydrophobic, oleophobic and fl ame-retardant polyester fabric investigate whether the coating exhibits self-cleaning properties. To determine the coating durability, the functional properties of polyester fabric were investigated before and aft er washing.

Textile material and fi nishing agents
Plain-weave 100 % polyester (PES) woven fabric with a weight of 67 g/m 2 was used in the study. Th e fabric was washed with a solution of non-ionic surfactant at a concentration of 2.5 g/l. Aft er washing, the fabric was rinsed in distilled water, squeezed and dried at room temperature. Two commercially available fi nishing agents were chosen, i.e., fl uoroalkyl-functional water-born siloxane (FAS) as a water-and oil-repellent agent under the trade name Dynasylan F 8815 (Degussa, Germany) and organophosphonate (OP) as a fl ame-retardant agent under the trade name Apyrol CEP (Bezema, Switzerland). Both fi nishing agents can be mixed with water to any desired concentration.

Plasma treatment, fi nishing and washing
PES fabric samples with a size of 20 x 20 cm were treated with oxygen plasma (O 2 gas) for 30 seconds under 60 Pa pressure in a low-pressure inductively coupled radiofrequency plasma system. Untreated and plasma-treated PES samples were fi nished with a mixture of 100 g/l FAS and 200 g/l OP using the pad-dry-cure process. Acetic acid was used in pH adjustment of the fi nishing bath to pH 4-5. Th e process included full immersion of samples for one minute at room temperature, squeezing between padded rollers at a constant pressure and roller velocity, followed by drying at 100 °C and curing at 150 °C for 5 minutes. For comparison, singlecomponent FAS and OP fi nishing agents were also applied to the untreated and plasma treated PES samples under the same conditions. Th e PES samples codes and the procedures for the fabric surface modifi cations are listed in Table 1. Th e fi nished PES samples were washed in a Gyrowash 815 (James Heal, UK) testing instrument according to the EN ISO 105C06 standard. Washing was performed in 150 ml of 4 g/l ECE phosphate reference detergent B solution at 40 °C for 45 min in the presence of ten steel balls that supply an accelerated washing treatment that corresponds to 5 domestic washes. Aft er washing, the samples were rinsed in distilled water at 40 °C, rinsed in cold tap water, and dried at room temperature.

Wettability of PES fabric samples
Th e wettability of PES fabric samples was determined based on the amount of the fi nishing solution applied to the samples in the "wet on dry" process. To this end, the pressure and the velocity of the padded rollers were set to 300 kPa and 1.5 m/min, respectively, and held constant during the squeezing process. Th e amount of the applied fi nishing solution was referred to as the wet pickup (WPU), which was calculated by the following equation [2]: Five measurements were performed for each sample, and the corresponding WPU value was reported in terms of the mean value and the standard error.

Scanning electron microscopy (SEM)
SEM images of the untreated and treated PES fi bres were obtained using a JSM 6060 LV scanning electron microscope (JEOL, Japan) operated with a primary electron beam accelerated at 10 kV. All samples were coated with a thin layer of gold prior to observation to supply conductivity and enhance the quality of the images. Fourier transform infrared (FT-IR) spectroscopy Fourier transform infrared (FT-IR) spectra were obtained on a Spectrum GX I spectrophotometer (Perkin Elmer, Great Britain) equipped with an attenuated total refl ection (ATR) cell and a diamond crystal (n = 2.0). Th e spectra were recorded over a range of 4000 cm -1 to 600 cm -1 using 32 scans at a resolution of 4 cm -1 .

Contact angle measurements
Th e static contact angles θ of water and n-hexadecane on the PES samples were measured using a DSA 100 contact angle goniometer (Krüss, Germany). Liquid droplets of 5 μl were placed on diff erent points of each fabric sample, and the values of θ were determined aft er 30 seconds of droplet deposition using the Young-Laplace fi tting method. Ten measurements were collected for each fabric sample, and the corresponding θ value was reported as the mean value and the standard error.

Sliding angle measurements
Th e water-sliding (or roll-off ) angles α were measured in the warp direction of the fabric samples and determined as the critical angle at which the droplet of 50 μl began to slide or roll off the gradually inclined fabric surface. Five measurements were collected for each fabric sample, and the corresponding α value was reported as the mean value of the standard error.

Oil-repellent properties
Th e oil repellence of the PES samples was determined under static conditions using AATCC test method 118-1978 with eight hydrocarbon liquids in a series of decreasing surface tension. Paraffi n oil was denoted with the rating number 1 and n-heptane was given the rating number 8. Drops of the standard test liquids were placed on the fabric surface and observed for wetting. Th e repellence rating was the highest numbered test liquid that did not wet the fabric in 30 seconds.

Vertical test of fl ammability
Th e combustion behaviour was determined via the vertical test of fl ammability according to DIN 53906. A fabric sample of size 15 × 7.5 cm, arranged vertically, was exposed to a propane fl ame for 6 s at the bottom of the sample. Aft er removal of the fl ame source, the aft er-fl ame time and aft er-glow time were determined. Seven measurements were collected for each sample in the warp direction, and the measured quantities were reported as the mean values and the standard deviations.
3 Results and discussion

Characterisation of PES fabric samples
Th e results of WPU are presented in Figure 1. It can be observed that the value of WPU is directly infl uenced by the sample pretreatment as well as the characteristics of the fi nishing solutions. Plasma treatment of fabric samples prior to the fi nishing process signifi cantly increased the WPU of all fi nishing solutions regardless of their properties, which was attributed to the increased wettability of the plasma-treated PES fi bres. Th is result confi rms that the oxygen plasma treatment caused the formation of new polar functional groups on the surface of PES fi bres, which signifi cantly increases their hydrophilicity and thus their wettability. Th e enhanced fi bre wettability directly resulted in an increased concentration of the absorbed fi nishing agents. Furthermore, in the case of the untreated fabric samples, the surface tension of the fi nishing solution importantly infl uenced the WPU. Accordingly, the WPU of the FAS solution with low surface tension was 2 times lower than the WPU of the high-surface-tension PO solution. Because this phenomenon was insignifi cant in the case of the plasma-treated samples, this diff erence represents an important advantage of oxygen plasma treatment of hydrophobic textile fi bres. SEM images of the PES fi bres before and aft er different procedures for the fabric surface modifi cations are presented in Figure 2. It can be observed from the images that the oxygen plasma treatment did not signifi cantly change the surface morphology of the fi bres, suggesting that the bulk properties of the fi bres remained undamaged. It is also evident that the applied fi nishing agents coated the fi bres, which caused light thickening and gluing of the fibres in certain places on the surface. Th e latter was the most pronounced when the mixture of FAS and OP was applied to the plasma-treated fabric sample. Figure 3 shows the ATR FT-IR spectra of representative plasma-treated and fi nished PES fabric samples as well as the untreated sample for comparison. In all spectra, the following bands that are characteristic for PES fi bres can be observed: the absorption band of low intensity at 3340 cm -1 due to intermolecular O-H bonds; the absorption bands in the 3000−2850 cm -1 spectral region attributed to stretching of νCH 2 , νCH 3 and C-H; the absorption band at 1710 cm -1 due to strong C=O stretching vibrations of the carbonyl group of the ester bond; the band at 1577 cm -1 due to asymmetric stretching of the C-O bond of the carboxylate anions; the absorption bands at 1372, 1338, 1240 and 1095 cm -1 caused by the δ(C-O) and ν as (C-O-C) vibrations of the polyester fi bres; and the absorption bands at 848, 793 and 721 cm -1 due to the C-H and C-C vibrations of the benzene ring [24,25]. Th e oxygen plasma treatment did not change the spectrum of the PES fi bres, which suggests that the concentration of new functional groups incorporated onto the fi bre surface was too low to be detected by FT-IR spectroscopy. Furthermore, in the case of the PES-P/FAS sample, the bands belonging to the FAS fi nishing agent at 1238 cm -1 due to ν a (CF 2 ) mixed with rocking (CF 2 ), at 1144 cm -1 due to ν s (CF 2 ) modes, and at 1208 cm -1 due to ν a (CF 2 ) and ν a (CF 3 ) vibrations [24,26,27] were blurred by the polyester fi ngerprint. Th e same applies to the band at 1227 cm -1 in the spectrum of the PES-P/OP sample, which corresponds to the P=O bonds of phosphonate [24,28] and is characteristic of the OP fi nishing agent. However, a detailed insight into the spectrum of the PES-P/OP sample reveals an appearance of a broad band of low intensity at 930 cm -1 due to P-O stretching vibrations of phosphonate [24].  plasma did not improve their water repellence, despite the fact that the WPU and the concentration of the applied FAS were increased on the plasma-treated sample. Th is result suggests that the FAS coating can create a superhydrophobic fabric surface at notably low concentration. Th e higher WPU of the FAS and OP mixture for the PES-P/FAS+OP sample compared with the PES/FAS+OP sample resulted in a slight reduction in water repellence. Th e reason for this result was attributed to a higher uptake of the hydrophilic OP fi nishing agent in the mixture, which hindered the superhydrophobic performance of FAS but still resulted in notably high hydrophobicity with a water contact angle equal to 148°. Th e FAS coating exhibited excellent washing fastness with an insignifi cant decrease of the water contact angles in the case of all washed samples. However, the concentration of FAS uptake by the untreated PES/FAS and PES/FAS+OP samples was too low to supply suffi cient oleophobicity of the PES fi bres. On these samples, n-hexadecane did not form stable drops of constant shapes on the fabric surface but slowly spread and penetrated into its porous structure, which resulted in a decrease of the contact angles and therefore prevented the static contact angle measurements. In contrast, the increase of the WPU of the oxygen plasma-treated PES fi bres (PES-P/FAS and PES-P/FAS+OP samples) infl uenced the creation of the uniform oleophobic FAS coating with n-hexadecane contact angles in the range of 120 to 124°, which exceeded 119° even aft er sample washing. Accordingly, oxygen plasma treatment prior to the fi nishing process is crucial to supply simultaneous water repellence and oil repellence properties to PES fi bres.

Functional properties of PES fabric samples
To determine whether the superhydrophobic PES fabric samples are self-cleaning, the sliding angles of water were determined and are presented in Figure 6. As shown in Figure 6, the lowest water sliding angles of 15° and 13° were obtained for the PES/ FAS and PES-P/FAS samples, respectively, indicating the nearly full self-cleaning properties of these samples. However, to decrease the water sliding angle, the low surface free energy micro-to nanostructured roughness of the fi bres surface should be created in the chemical modifi cation process, which could allow air to become trapped in the fi bre topography, thus creating a composite surface that minimises the solid/water interface and maximises the water/air surface area. However, according to the SEM images, the oxygen plasma treatment and the fi nishing process did not signifi cantly aff ect the topography of the PES fi bres, which remained nearly unchanged. Th e results also show that the presence of OP in the coating increased the water sliding angles of the PES/FAS+OP and PES-P/FAS+OP samples to a great extent due to the sticking of the water droplet to the fi bre surfaces. Th is phenomenon indicates that the hydrophilic character of OP strongly increased the adhesion between water and the coating. It is clear that OP does not contribute to creation of the self-cleaning properties of the coating. Although the water sliding angles for the PES/FAS and PES-P/FAS samples only slightly increased aft er washing, the water sliding angles dramatically decreased for the PES/FAS+OP and PES-P/ FAS+OP samples. Th is result suggests that the coating structure was changed during washing and that the OP fi nishing agent released from the fi bre surface. Th e results of the oil repellence rating summarised in Table 2 gave additional information to the results presented in Figure 5. Although n-hexadecane penetrated into the PES fabric porous structure of the PES/FAS and PES/FAS+OP samples, the mixture of paraffi n oil and n-hexadecane with a 1.6 mN/m higher surface tension than n-hexadecane did not wet these samples in 30 minutes. Because the same applies for paraffi n oil, this result indicates that the PES/FAS and PES/FAS+OP samples were still repellent for diff erent oils. Th e results in Table 2 also show the high oleophobicity of the PES-P/FAS and PES-P/FAS+OP samples, which repelled even n-decane with much a lower surface tension than n-hexadecane. Th e sample repellence only slightly deteriorated aft er washing. Th e results of the burning behaviour of the PES fabric samples determined by the vertical test of fl ammability are summarised in Table 3.

Conclusion
In this research, we successfully tailored the multifunctional superhydrophobic, oleophobic, and fl ame-retardant coating on PES fabric using a twostep chemical modifi cation procedure consisting of oxygen plasma treatment followed by pad-dry-cure application of an FAS and OP mixture. A comparison of the functional properties of PES fabric treated by the FAS and OP mixture with those treated by single-component FAS or OP fi nishing solutions reveals the following: Application of the FAS fi nishing agent supplied • washing-resistant superhydrophobic properties to the PES fi bres, which was insignifi cantly aff ected by the presence of OP in the mixture; Oxygen plasma treatment of the surface of PES • fi bres dramatically increased the wet pick up of the fi bres and therefore preserved the conditions for the creation of the uniform oleophobic coating with n-hexadecane contact angles in the range of 120 to 124°, which exceeded 119° even aft er sample washing; Th e presence of OP in the coating notably increa-• sed the water sliding angles because of the enhanced adhesion between water and the PES fi bres surface, resulting in complete deterioration of the self-cleaning properties; Application of the OP fi nishing agent created • excellent fl ame-retardant behaviour of the PES fi bres, which did not burn aft er fl ame withdrawal and showed superior self-extinguishing behaviour; Th e fl ame retardancy of the PES fi bres was not • wash-resistant because OP was only physically incorporated into the coating and was removed during the washing process.