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Effect of corona discharge on the stability of the adhesion of thin silicone-organic coating to polyamide fiber surface made by the sol–gel method

Abstract

This paper presents the effect of pretreatment of polyamide (PA6) nonwoven with corona discharge on the stability of the adhesion of thin hydrophobic silicone-organic coating based on vinyltriethoxysilane, made by the sol–gel method. This pretreatment with corona discharge causes a change in the physicochemical properties of the PA6 fiber surface. These changes include, among others, an increase in the fiber surface roughness, wettability, and surface free energy. At the same time, XPS and EDS investigations have shown an increase in the degree of oxidation and the formation of functional polar groups on the fiber surface (C–O–, C–OH, and O=C–O–). As a result of the changes in the surface properties of pretreated PA6 fibers, a higher degree of the sol deposition was obtained compared with that for untreated nonwoven surface. The assessment of the stability of the adhesion of thin hydrophobic coating to the fiber surface was carried out on the basis of changes in the content of silica deposited on fibers and the kinetics of water contact angle after washing and abrasion processes. In the end, the PA6 nonwoven, pretreated with corona discharge, shows a higher stability of the adherence of the thin silicone-organic coating and a higher degree of hydrophobicity than the untreated nonwoven.

Introduction

The formation of thin functional coatings from polymeric organic–inorganic hybrids, mostly based on silica (SiO2) on the surfaces of fibers/fabrics, constitutes a quickly developing area of improving textiles for various applications. Thin coatings made on the fiber surface by the sol–gel method, depending on their structure, can show specified properties, e.g., hydrophobic properties that are imparted to the textile carrier. Such a property is shown by silicone-organic coatings made of a sol synthesized from vinyltriethoxysilane by the sol–gel method. Moreover, such coatings constitute matrices to which one can durably incorporate inorganic or organic functional particles with specified properties and in this way obtain expected functionalization of textile fabrics, such as bioactivity,1,2,3,4,5,6 water resistance,7,8,9 barrier property against UV radiation,10,11 optimized performance durability—resistance to abrasion or pilling formation.12,13,14,15,16 The basic condition of the application of a thin coating based on SiO2, made by the sol–gel method for various so-called nano-coating finishes of textile fabrics, is its stable adhesion to the surface of fiber/fabric. This adhesion stability depends on the type and extent of physicochemical interactions between fiber surface and the thin silicone-organic coating made on it. These interactions mainly depend on the fiber surface morphology, surface energy, and chemical constitution and type of functional groups. The problem of the stability of the adhesion of silicone-organic nano-coating made by the sol–gel method to fiber surface is particularly important in the case of synthetic fibers with a smooth, nondeveloped, and hydrophobic surface. These properties adversely affect the adhesion stability of the coating during use and washing of the fabrics finished in this way, which finally adversely influences their functional value. A change in the surface properties of textiles can occur by their physical and/or chemical modification (e.g., enzymatic hydrolysis, ozone treatment, air plasma).17,18,19,20 The methods of physical modification of fiber surface include the fabric treatment with atmospheric plasma—corona discharge.21,22,23,24,25,26,27,28,29 The advantage of the fiber surface treatment with atmospheric plasma, compared to the conventional processes, mainly chemical treatment in an aqueous medium, is its pro-ecological character. Such a modification does not consume water and chemical finishing products; thus, it does not produce hardy degradable post-production effluents. At the same time, it makes it possible to obtain good results of fabric improvement, often better ones than in the case of conventional processes.21

The commonly used methods of the assessment of the combination of various layers with fiber surface are based on the determination of the reversible work of adhesion or a direct assessment of the delamination power. However, these methods cannot always be used for assessing the stability of the coating–fabric system. The limitations can be due to the state of fabric surface, coating type and its ability to penetrate the fiber/fabric structure as well as the coating thickness. The method of the coating formation is also of importance. In the case of relatively thick coatings (20–120 μm) made of organic film-forming polymers deposited in the form of paste on the textile fabric, the assessment of the coating stability and its adhesion to the textile carrier is carried out by selected functional tests, including resistance to water penetration, bending and abrasion resistance, resistance to multiple crushing/bending, and multiple standardized laundering.30 The wrinkles, cracks, delamination, and wear of the coating resulting from these treatments are assessed by the organoleptic method or by means of microscopic images. The use of such methods is possible owing to the high thickness of the coating made in this way on the fabric surface and not on single fibers. Most of these tests are unsuitable for the assessment of the stability of very thin (100–300 nm) silicone-organic nano-coatings made on the fiber surface by the sol–gel method. In this method, a sol with an appropriate chemical composition is deposited on a fabric by the technique of full-bath padding. Then, as a result of drying and crosslinking processes, a thin silicone-organic layer is formed on the fiber surface (and not on fabric). The whole process is commonly defined as a nano-coating finish.

The article presents the results of investigating the effect of the treatment of PA6 nonwoven with corona discharge on the stability of adhesion of vinyltriethoxysilane nano-coating to the textile substrate. The assessment of this stability was carried out by means of microscopic methods (SEM/EDS and 3D) and tests of surface properties.

Experimental

Materials

A commercial, spun-bonded polyamide 6 (PA6) nonwoven fabric with a surface weight of 137 ± 8 g/m2 and thickness of 0.73 ± 0.03 mm was used in this study. The nonwoven fabric was rinsed in water at 85–90°C for 15 min and then dried at room temperature, before padding process or corona discharge treatment, in order to minimize contaminations.

Preparation of a hybrid Al2O3/SiO2 sol

The hybrid sol was prepared by combining sols Al2O3 and SiO2.

The Al2O3 sol was prepared by mixing aluminum(III) isopropoxide precursor (Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany) with the mixture of ethanol (EtOH) and water (in a molar ratio of 2.5:1) using a high-speed stirrer for about 2 h at a temperature of about 80°C (boiling point of EtOH).31 The aluminum isopropoxide precursor/water molar ratio was 1:1. After that time, the pH value of the mixture was set at 3 by means of 36% hydrochloric acid followed by the addition of 40 g/l poly(vinyl alcohol) (PVA)—(POCH S.A., Poland). The stirring was continued until a homogeneous transparent colloidal solution was obtained.

The SiO2 sol modified with vinyl groups was prepared by the hydrolysis of 97% vinyltriethoxysilane (Alfa Aesar GmbH & Co. KG, Karlsruhe, Germany) in water with a high-speed stirrer for about 1 h until a homogenous colloidal solution was obtained.32

The Al2O3 and SiO2 sols were mixed in volumetric proportions of 1:1 by intensive stirring for about 2 h at 20–25°C. In this way, colorless and stable hybrid Al2O3/SiO2 sol was obtained.

This hybrid sol was used after dilution with water in proportion of 1:5. The ratio of the sol dilution with water is of great importance for the thickness of the silica coating made, its elasticity, and the degree of binding with the fiber surface.

Preparation of sol–gel coat

Deposition of sols on fabrics

The hybrid Al2O3/SiO2 sol was deposited on the nonwoven fabric by padding method using a laboratory two-roller padding machine type HVF 500 (Werner Mathis AG, Switzerland). The temperature of the padding bath was 25°C. The padding rate was 1.5 m/min with a squeezing rollers pressure amounting to 3.5 bar along the roller contact line. The squeeze out degree was 90%.

The padded fabric samples were dried at a temperature of 60°C and then heated at 160°C for 1 min. Under these conditions, a thin, hard, and elastic hydrophobic silicone-organic coating was constantly formed on the surface of fibers of nonwoven fabric.

Corona discharge treatment

The treatment of polyamide 6 (PA6) nonwoven fabric by corona discharge was carried out with the use of a laboratory device constructed by METALCHEM (Toruń, Poland).22 The treatment was carried out at room temperature at atmospheric pressure, using a system of metal five-tip discharge electrode and counter-roll electrode coated with silicon, and gap distance was fixed at 1.5 mm. The scheme of discharge electrode is presented in Fig. 1. The discharge was produced at high voltage 15 kV and low frequency 30 kHz. The PA6 nonwoven fabric was treated with corona discharge with the activation energy E j amounting to 21 J/cm2. The value of E j was calculated according to the formula:

$$E_{\text{j}} = \frac{P \times B}{L \times A}$$
(1)

where P—maximal rated power of the generator (2100 W), B—discharge power in %, L—the length of discharge electrode (30 cm), and A—fabric passing rate in m/min.

Fig. 1
figure1

Scheme of discharge electrode: 1—five-tip electrode, 2—roll electrode, 3—sample, 4—device for discharge gap setting

The general scheme of sample preparation is presented in Fig. 2.

Fig. 2
figure2

General scheme of the samples preparation

Examination of the fiber surface with deposited sol–gel coatings—SEM/EDS analysis

The surface and elemental compositions of the analyzed samples were determined using a scanning electron microscope VEGA 3 (Tescan, Czech Republic) with an EDS INCA Energy Microanalyser (Oxford Instruments Analytical, England). Images were taken after the surface of samples had been coated with a gold layer by vacuum sputtering (Quorum Technologies Ltd., England). EDS examination was carried out in an area 434 µm × 434 µm.

The 3D analysis of fabric surface was performed with the use of Alicona MeX program (Alicona Imaging GmbH, Austria) cooperating with a VEGA3 scanning electron microscope. Based on the 3D topography of fabric surface, the parameters of its roughness were determined:

  • Sa—average height of selected area—average roughness

$${\text{Sa}} = \frac{1}{A}\mathop {\iint }\limits_{A}^{{}} \left| {z\left( {x,y} \right)} \right|{\text{d}}x{\text{d}}y$$
(2)
  • Sdq—root-mean-square gradient (RMS)

$${\text{Sdq}} = \left( {\frac{1}{A}\mathop {\iint }\limits_{A}^{{}} \left( {\frac{\partial z(x,y)}{\partial x}} \right)^{2} + \left( {\frac{\partial z(x,y)}{\partial y}} \right)^{2} {\text{d}}x{\text{d}}y} \right)^{{\frac{1}{2}}}$$
(3)
  • Sdr—developer interfacial area ratio

$${\text{Sdr}} = \frac{1}{A}\left[ {\mathop {\iint }\limits_{A}^{{}} \left( {\left[ {1 + \left( {\frac{\partial z(x,y)}{\partial x}} \right)^{2} + \left( {\frac{\partial z(x,y)}{\partial y}} \right)^{2} } \right]^{{\frac{1}{2}}} - 1} \right){\text{d}}x{\text{d}}y} \right]$$
(4)

where A—defined area, i.e., an area used to define the parameters that characterize the surface; z(x,y)—the value of ordinate—a function representing the height of surface with a limited range in position x, y.

Parameter Sa—amplitude parameter—is the arithmetic mean of the absolute values of ordinates inside the area tested and is determined as mean roughness. Parameters Sdq and Sdq are hybrid parameters: Sdq determines a general measurement of the slopes of particular elevations on the surface tested, while Sdr defines the percentage of an additional surface raised up by the texture compared to the perfectly flat surface with the same size of measurement area. These parameters were determined according to standards ASME B46.1-2009, PN-EN ISO 1302:2004, and PN-EN ISO 25178-2:2012.

XPS analysis

The X-ray photoelectron microscopy (XPS) measurements were taken on ESCALAB-210 (VG Scientific, England) spectrometer with Al Kα (1486.6 eV from an X-ray source operated at 300 W, 14.5 kV, 20 mA). The spectrometer chamber pressure was below 5 × 10−8 mbar. Survey spectra were recorded in the energy range from 0 to 1350 eV with the energy step size of 0.4 eV. High-resolution spectra were recorded with the energy step size of 0.1 eV, 50 ms dwell time, and 25 eV pass energy. Interpretation of XPS spectra was carried out according to the data for polyamide 6.33,34

Measurement of water contact angle

The water contact angles of the tested samples were measured by using a PGX Goniometer (FIBRO Systems, Sweden). Measurements were taken at a temperature of 22 ± 1°C and relative humidity 43 ± 3%. In the measurements, deionized water was used as standard liquid that was deposited on the sample surface tested in the form of 2 μl drops.

Measurement of dynamic contact angle, surface free energy, and work of adhesion evaluation

In order to determine surface free energy, the dynamic contact angle in three standard liquids was measured with the use of a KSV Sigma 701 F tensiometer (KSV Instruments, Finland) and n-hexane (99% pure, POCH S.A., Poland), deionized water, and ethylene glycol (99% pure, POCH S.A., Poland). The values of surface tensions (γL) and polar (γ pL ) and dispersive (γ dL ) components of the liquid used are listed in Table 1.

Table 1 Physicochemical properties of standard liquids

The surface free energy of the sample tested was determined on the basis of the standard liquid contact angle according to method Wu (harmonic mean) according to the following equations35,36:

$$\gamma_{\text{S}} = \gamma_{\text{SL}} + \gamma_{\text{L}} { \cos }\varTheta_{\text{L}}$$
(5)
$$\gamma_{\text{S}} = \gamma_{\text{S}}^{\text{d}} + \gamma_{\text{S}}^{\text{p}}$$
(6)
$${{\gamma_{\text{SL}} = \gamma_{\text{S}} + \gamma_{\text{L}} - \frac{{4 \gamma_{\text{S}}^{\text{d}} \gamma_{\text{L}}^{\text{d}} }}{{\gamma_{\text{S}}^{\text{d}} + \gamma_{\text{L}}^{\text{d}} }} {-} \frac{{4 \gamma_{\text{S}}^{\text{p}} \gamma_{\text{L}}^{\text{p}} }}{{\gamma_{\text{S}}^{\text{p}} + \gamma_{\text{L}}^{\text{p}} }} }}$$
(7)

where γ S—surface free energy of the material tested, γ SL—interfacial free energy of the material tested—standard liquid system, γ L—surface free energy of the standard liquid, γ dS , γ pS , γ dL , γ pL —components of the surface free energy: dispersive component (d) and polar component (p) on the material tested or standard liquid, and ΘL—standard liquid contact angle of the material tested. On the basis of the determined value of γ S, γ L and ΘL, the interfacial free energy and the reversible work of adhesion (W adh) of the material tested—standard liquid system were estimated according to the following equations:

$$\gamma_{\text{SL}} = \gamma_{\text{S}} {-}\gamma_{\text{L}} { \cos }\Theta_{\text{L}}$$
(8)
$$W_{\text{adh}} = \gamma_{\text{S}} + \gamma_{\text{L}} - \gamma_{\text{SL}}$$
(9)

Testing the resistance to abrasion of nonwoven fabric finished with nano-coatings

The tests were carried out according to PN-EN ISO 12947-1:2000 + AC:2006 Textiles. Determination of the abrasion resistance of fabrics by the Martindale method. Determination of specimen breakdown with the use of Nu-Martindale 864 apparatus (James H. Heal & Co. Ltd., England). The pressure of abrasive heads was 12 kPa. A standard wool woven fabric was used as the abrasive element.

Testing the resistance to washing of nonwoven fabric finished nano-coatings

The washing was carried out according to PN-EN ISO 105-C06:2010, procedure A1S (40°C) “Textiles—Tests for colour fastness—Part C06: Colour fastness to domestic and commercial laundering” in a Rotawash M228 (SDL Atlas, USA).

Results and discussion

Nonwoven surface modification

The functional use of a textile fabric containing a thin silicone-organic coating on the fiber surface depends on the stability of its adhesion to a textile substrate. In turn, the stability of this adhesion depends  considerably on the surface properties of both fibers and fabric. In the case of forming thin silicone-organic coatings on the fiber surface by the sol–gel method, when an appropriately synthesized sol is deposited on the fabric by padding, it is the hydrophilic character of fibers that is of great importance for the quantity and uniformity of the sol deposition and consequently for the properties of the nano-coating obtained. The PA6 nonwoven used in the investigations is characterized by hydrophobic properties. These properties were changed into hydrophilic properties by the treatment of the nonwoven with corona discharge. Figure 3 presents SEM images of the surface of PA6 nonwoven before and after the treatment with corona discharge. Comparing the SEM images recorded in the macroscale, the morphological/topographical changes in the treated PA6 fibers are almost invisible. The effect of corona discharge on the fiber surface topography is visible in the 3D images (Fig. 4). As a result of the occurring fiber surface nano-structurization, the fiber roughness is increased and consequently the fiber specific surface is developed. The value of the average roughness, Sa, of PA6 fibers, determined by means of stereometric measurements, amounts to 19.81 nm, whereas after treatment, it increases to 49.13 nm. A considerable increase in the amplitude parameters, Sdq and Sdr, especially in the coefficient of surface area development Sdr, was also obtained. These parameters characterize the oscillations of the topographical elements (Sdq and Sdr) as well as their distribution and number. For the untreated PA6 nonwoven, the values of Sdq and Sdr amounted to 0.44 and 9.98%, respectively, whereas after treatment with corona discharge—1.68 and 115.75%, respectively. This indicates the formation of a developed structure of fiber surface and consequently an increased surface of the PA6 fiber contact with the silicone-organic nano-coating. Apart from an increase in roughness, the treated PA6 fibers/nonwoven gained hydrophilic properties. As a result of the treatment with corona discharge under air/oxygen, there occurs the addition of oxygen present in air and the formation of functional groups on the fiber surface. These groups, in turn, cause a change in the surface free energy (γ S) of PA6 fibers. According to equation (6), the surface free energy is a sum of two components: dispersion (γ d) and polar (γ p) components. It is assumed that the dispersion component mainly refers to van der Waals forces, whereas the polar contribution is mainly due to intermolecular forces (e.g., dipole/dipole interactions, hydrogen bonding, and π-cloud/π-cloud interactions). Determined on the basis of hexane, water, and ethylene glycol contact angles, the γ S value of untreated PA6 nonwoven amounted to 37.92 mJ/m2 and its polar component 24.53 mJ/m2. As a result of the treatment of PA6 nonwoven with corona discharge, the value of γ S is increased and is due to the increase in the polar component (Table 2). The value of γ d after treatment did not change and was similar to the value determined for untreated PA6 nonwoven. The changes observed indicate a high contribution of polar groups on the surface of PA6 fiber that result from chemical processes proceeding on the fiber surface, especially the reaction of oxygen with alkyl radicals in the polymer chain.37,38,39

Fig. 3
figure3

SEM images of the PA6 nonwoven fiber surface: (a) untreated, (b) pretreated with corona discharge. Magnification ×10,000

Fig. 4
figure4

3D SEM images of the topography of the PA6 nonwoven fiber surface: (a) untreated, (b) pretreated with corona discharge; defined area 4 μm × 4 μm

Table 2 Values of surface free energy (γ S) and its components (γ dS , γ pS ), work of adhesion (W adh), and average values of contact angles for PA6 nonwoven fabrics

The formation of new polar groups on the surface of treated PA6 fibers is also confirmed by the increase in the adhesion work value (W adh). The value of W adh determined for the solid–liquid system increased by 12% for the system with water and by about 9% for the system with ethylene glycol (Table 2). The determined value of the surface tension of hybrid silicone-organic sol amounts to 54.42 mJ/m2; thus, it remains within the range between the values of surface tension of two standard liquids. This also suggests that the adhesion of the coating made to the surface of PA6 fiber treated with corona discharge will be more stable than that with the untreated surface.

Because of the presence of atmospheric oxygen during the treatment with corona discharge, the new functional polar groups formed on the PA6 fiber surface contain oxygen, contributing to a change in the oxidation degree of the PA6 fiber surface and consequently to a change in the fiber chemical properties. The determination of the element content by the EDS technique has shown that the degree of fiber surface oxidation measured with the ratio O/C increased by 6% in relation to that of the nonwoven untreated with corona discharge, whereas the increase in the N/C ratio was 1.8%. To identify the groups formed and gain information about the oxidation degree of the fiber surface, nonwoven samples were subjected to XPS examinations. Spectrum analysis was performed with reference to the peaks of carbon (C1s), oxygen (O1s), and nitrogen (N1s) with binding energy at 285, 531.35, and 399.77 eV, respectively. For untreated PA6 nonwoven fabric, the C1s core-level spectrum was deconvoluted with four individual carbon components (Fig. 5). The peak C1 at 285 eV is attributed to aliphatic carbon atoms in the C–C chain CH2 groups. The peak C2 at 285.31 eV can be associated with the amido-carbonyls C–(C=O)–. The peak C3 at 286 eV represents the carbon atoms neighboring the amide nitrogen –C–NH–(C=O), and that of at 288 eV is assigned to the amide carbonyl group –(C=O)–N. After the corona discharge treatment of the PA6 nonwoven fabric, a fifth peak C5 at 286.39 eV was observed. This C5 peak can be assigned to the newly formed C–O or C–OH functional groups. The deconvoluted O1s spectrum of the untreated and corona-discharge-treated PA6 nonwoven fabric consisted of two peaks, at 531.7 and 533.3 eV. The spectrum with higher intensity is attributed to the oxygen atom at 531.7 eV characteristic of the carboxyl acid group, whereas the spectrum with lower intensity is related to oxygen atoms absorbed from air (for untreated PA6 nonwoven fabric) or to the oxygen atom in the newly formed C–O or C–OH bond (for treated PA6 nonwoven fabric).40,41,42

Fig. 5
figure5

The C1s, O1s, and N1s XPS spectra of PA6 nonwoven fabric: (a, c) untreated and (b, d, e) pretreatment with corona discharge

The N1s core-level spectra of untreated and corona-discharge-treated PA6 nonwoven fabric show a single peak at 399.9 eV related to nitrogen atom in O=C–NH– bond.43

Stability of adhesion of the silicone-organic coating

In the process of making thin silicone-organic coating on the fiber surface by the sol–gel method with the use of full-bath padding, the hydrophilic properties of textile fabric are of great importance. These determine the degree of the deposition of the padding bath. In the case of corona-discharge-pretreated PA6 nonwoven, we obtained a higher degree of the deposition of hybrid Al2O3/SiO2 sol than that for the untreated nonwoven. The quantities of deposition amounted to 1.00 ± 0.02 and 0.40 ± 0.06 g/m2, respectively. The hybrid Al2O3/SiO2 sol synthesized on the basis of vinyltriethoxysilane and deposited on the PA6 nonwoven after drying and thermal crosslinking forms, on the fiber surface, a thin hydrophobic silicone-organic coating. For both untreated and treated with corona discharge PA6 nonwovens with such coatings, we obtained water contact angle within the range of 130°–140°.

The change in the original hydrophobic properties of PA6 fiber/fabric into hydrophilic ones obtained by the corona discharge pretreatment, and then another change to hydrophobic properties due to the formation of a thin silicon-organic coating on the fiber surface, caused a change in the values of surface free energy of the nonwoven with the nano-coating finish (Table 2). For the fiber/fabric samples with silicone-organic coating, pretreated with corona discharge, differences in the values of the water and glycol contact angles and in the surface free energy were higher than those for the untreated samples. This results from the higher deposition of hybrid Al2O3/SiO2 sol on the surface of treated PA6 fibers that have better hydrophilic properties than those of untreated fibers.

To assess the stability of the adherence of the silicone-organic coating made to PA6 fibers, the samples were subjected to washing and abrasion processes. Because the silicone-organic coating was made on the fiber surface, the stability of its adherence was determined on the basis of changes in the percentage content of silicon on the PA6 fiber surface using the EDS technique. The choice of silicon for the assessment of this stability was justified by the fact that silicon is an element that, apart from carbon, aluminum, and chlorine, is in the composition of the hybrid silica sol and whose percentage content (among the elements absent in the PA6 nonwoven) is the highest amounting to about 11%.

The EDS analysis has shown that after washing, the decrease in the silicon content was observed for both the pretreated and untreated samples with corona discharge. This indicates that the silicone-organic coating made on the PA6 surface was partly washed out. However, the decrease in the silicon content for the untreated PA6 nonwoven was higher, amounting to about 37%, than that for the pretreated nonwoven, amounting to 27% (Fig. 6). It should be noticed that despite a partial washout of the silicone-organic coating, the silicon content on the surface of PA6 was higher than that in the case of untreated nonwoven. For this sample, the water contact angle was 130°, similar to the value obtained before washing (Fig. 7b). Differences in hydrophobic properties were observed for the sample untreated with corona discharge. After washing, the hydrophobic effect was instable. The time of water drop penetration amounted to 160 s, and it could be divided into two stages. The first one, lasting about 120 s, proceeded very slowly, and the contact angle obtained was at the level of 120°. After this time, an instantaneous spreading of water drop was observed (Fig. 7a). This deterioration in the barrier properties for water penetration may result from a partial washout of the silicone-organic coating, and consequently from a decrease in its thickness. In this case, water easily penetrates the thinner silicone-organic coating. At the same time, as in the considerably thicker silicone-organic coatings, also in thin coatings, some sort of “ducts” can be formed to facilitate the water penetration and adsorption on the fiber surface.44

Fig. 6
figure6

Weight percentage of silica on the PA6 nonwoven fiber surface with silica coating: 1—before washing and abrasion, 2—after washing, 3—after abrasion

Fig. 7
figure7

Water drop penetration time on the untreated (a) and corona-discharge-pretreated (b) PA6 nonwoven fabric with silica coating: 1—before washing and abrasion, 2—after washing, 3—after abrasion

Differences in the wettability of untreated samples also resulted in a change in the value of their surface free energy after washing. An increase in the value of γ S from 30.90 to 32.62 mJ/m2 was observed, as a result of the decrease in the water contact angle (Table 2). Contrary to untreated samples, the value of γ S for the PA6 nonwoven with the silicone-organic coating, pretreated with corona discharge, was at a similar level as that for the sample that was not subjected to washing. This indicates a stable hydrophobic effect and a better adhesion of the silicone-organic coating to the surface of PA6 fibers pretreated with corona discharge.

A similar effect was obtained after the abrasion process. The pretreated PA6 nonwoven with the hydrophobic silicone-organic coating made on its surface, after 100,000 abrasion cycles, retained its hydrophobic properties. Despite a considerable decrease in the silicon content, amounting to about 57%, compared to the initial value, the water contact angle did not change and amounted to about 130°. In the case of untreated samples, after abrasion, no stable hydrophobic effect was obtained for them. A water drop completely spilled out after 110 s (Fig. 7). Despite the fact that the decrease in the silicon content on the surface of untreated fibers after abrasion was the same as in the case of treated nonwoven (decrease by about 57%), the final silicon content was considerably lower and amounted to 0.07% by wt. (Fig. 6). This results from the lower deposition of hybrid Al2O3/SiO2 sol and the formation of a considerably thinner hydrophobic silicone-organic coating during the thermal treatment process (drying and crosslinking) than that on the corona-discharge-pretreated surface. Consequently, the silicone-organic coating easily undergoes abrasion/damage and does not maintain its protective properties against water penetration to the surface of PA6 fibers.

The analysis of test results has shown that the pretreatment of the surface of polyamide fibers with corona discharge changes their surface properties and hydrophobic character into hydrophilic one. The increase in the fiber wettability, surface free energy, development of surface topography, and surface structure provides a higher degree of the deposition of hybrid Al2O3/SiO2 sol and a greater surface of the contact of the silicone-organic coating formed with the fiber surface. At the same time, it contributes to the increase in the adhesion stability of silicone-organic coating adhesion to the surface of PA6 fibers. It is of particular importance in the finishing processes of textiles, especially in making thin coatings with an increased functional stability (e.g., resistance to washing processes and external abrasive forces).

Conclusion

  1. 1.

    Corona discharge causes a change in the physicochemical properties of the polyamide fiber surface, especially an improvement in wettability, an increase in surface free energy, formation of functional groups, an increase in the oxidation degree of fiber surface, an increase in the adhesion work, and an increase in the roughness of the fiber surface.

  2. 2.

    In the case of the PA6 nonwoven pretreated with corona discharge, on account of its hydrophilic character, one can obtain a higher degree of the deposition of hybrid Al2O3/SiO2 sol than that for untreated nonwoven.

  3. 3.

    The formation, on the fiber surface, of thin silicone-organic coatings obtained from the hybrid Al2O3/SiO2 sol based on vinyltriethoxysilane, imparts hydrophobic properties to the fiber/nonwoven.

  4. 4.

    After washing of the PA6 nonwoven pretreated with corona discharge, a lower decrease in its silicon content was obtained as a result of a partial washout of the silicone-organic coating.

  5. 5.

    After washing and abrasion, the nonwoven pretreated with corona discharge, containing the hybrid silicone-organic coating, shows better hydrophobic properties than those of untreated nonwoven.

  6. 6.

    As a result of the treatment with corona discharge, it was possible to obtain a textile fabric characterized by an increased stability of the adhesion of the silicone-organic coating, made by the sol–gel method, to the surface of polyamide fibers.

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Acknowledgment

The study has been carried out within the Project WND-RPLD.03.01.00-00-001/09 and the statutory research work of the Textile Research Institute (BZT 01 30/2013) financed by the Polish Ministry of Science and Higher Education. The authors also wish to express their thanks to Ms Beata Borak, PhD of the Institute of Materials Science and Applied Mechanics, Wroclaw University of Technology, for preparing Al2O3/SiO2 sol, and Mr Janusz W. Sobczak, PhD of the Mazovia Center for Surface Analysis Department of Soft Condensed Matter and Fluids, Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, for XPS measurements.

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Correspondence to Dorota Kowalczyk.

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Kowalczyk, D., Kamińska, I. Effect of corona discharge on the stability of the adhesion of thin silicone-organic coating to polyamide fiber surface made by the sol–gel method. J Coat Technol Res 14, 1115–1125 (2017). https://doi.org/10.1007/s11998-016-9899-9

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Keywords

  • Polyamide 6 nonwoven fabric
  • Corona discharge
  • Sol–gel
  • Adhesion
  • Surface modification
  • Hydrophobic properties