Dielectric barrier discharge plasma treatment for sustainable leather dyeing

Abstract

Adapting green technologies to the leather industry plays a crucial role in environmental protection by reducing wastewater load, using fewer chemicals, minimizing waste, saving energy and water, reducing costs, and increasing productivity. Plasma treatment, accepted as a clean technology, is mostly used in textiles compared to limited studies in leather. Thus, this study will be the first in this respect. Crust leathers, treated with air and argon atmospheric plasma under the conditions of 130 W, 100 s, were dyed with acid and metal complex dyestuffs for 45 minutes at 20°C in different concentrations (1%–4%), and investigated in terms of dyeing properties, wastewater load, and the characterization of the leathers in terms of surface, mechanical and antimicrobial properties. The K/S results of 2% dyeing obtained after air and argon plasma treatment gave similar values with the untreated leathers dyed with a concentration of 4%. This result revealed that half amount of dyestuff is sufficient for the dyeing process to obtain the same dyeing characteristics with the maximum concentration of dyestuff (4%). In addition to improved dyeing efficacy, a reduction up to 35% in chemical oxygen demand value was achieved in addition to 2 and 3 mm inhibition zones without any negative effect to tensile and tear strength of leathers. Consequently, atmospheric plasma treatment is found to be effective and comprising as environment-friendly, and sustainable technique for the leather industry in terms of reduced dyestuff amount and wastewater load without altering the characteristics of the leathers.

Graphical Abstract

Keywords

Leather dyeing dye exhaustion atmospheric plasma dielectric barrier discharge chemical oxygen demand

Highlights

• Metis-type crust leathers were treated with DBD air/argon atmospheric plasma.

• Wastewater load, antimicrobial, mechanical, surface, dyeing properties was investigated.

• Air/argon plasma treatment enhanced the dyeing characteristics of leathers as well as generated antibacterial activity.

• DBD air/argon atmospheric plasma was determined as non-destructive treatment for leather application by decreased COD values and increased dye uptake and hydrophilicity.

Introduction

Leather has unique characteristics that make it irreplaceable material and is generally used to produce upper, garment, upholstery, saddlery, and double-face leathers. The production technology of these leathers causes pollution and consumes water, energy, and chemical agents. Particularly after the dyeing process, unexhausted dyestuff discharged to wastewater must be significantly considered from an environmental point of view.^1,2

The type of dyes and the surface characteristics of leather has a crucial role in the leather dyeing process and the generated wastewater load. Mostly acid and metal complex dyes are used in the leather dyeing process due to their high fastness values and color hues. After the dyeing process, the wastewater load becomes highly contaminated by residual dyes, auxiliary chemicals, and salts, leading to economic and environmental problems. Regulations and restrictions are becoming stricter over time and force researchers to develop environment-friendly technologies for every process of leather production, including adapting new dyes and technologies to the industry.^1–4

In leather industry, half of the leathers are estimated to be dyed in black and must fulfill the standards of quality and appearance like those dyed in different colors. High fastness values, deep dyeing, brilliant shades, homogeneous and cross-section dyeing are the increased customer demands, forcing manufacturers to redesign the conventional leather dyeing techniques.⁵ Besides, dye consumption is as important as the high-quality standards of leather. The increase of active groups in the leather that can bind dyestuff resulted in enhanced dyestuff consumption in the dye bath, leading to darker dyeing tones and positively impacting the environment.¹

By plasma technology, the formation of reactive groups without altering the basic properties of the materials is achieved due to physicochemical surface modifications.^6,7 Plasma is occurred in a dry state and accepted as a clean technology that provides environmental solutions. The surface modifications following plasma treatment are mainly dependent on the composition of the substrate and the gas employed. Oxygen, air, argon, helium, carbon dioxide, nitrogen, hydrogen, tetrafluoroethane, methane, or ammonia could be used as gases or gas mixtures. Helium, argon, nitrogen, and oxygen plasmas are capable of activating surfaces via ablation or etching and commonly used for the polymer etching. The quantity of etching and surface roughening is determined by the impact energy of the ions and the etching rate is affected by the parameters such as plasma composition, substrate type, and operational conditions (power, gas flow, substrate position). Argon is the most common noble gas used in plasma treatment due to the high ablation efficiency, chemical inertness with surface materials, and relatively low cost.^8–10 Plasma technology brings advantages like easy processing without using chemicals and/or water and surface modification of materials without altering the mechanical properties. Besides, this technology shortens the treatment time, saves energy, and generates no industrial waste.^6,7,11,12

More recently, a great interest has occurred in the plasma treatment of fabrics, fibers, and yarns.¹¹ Mostly, the studies focused on the modification of surface characteristics that affect fire retardancy,^13,14 soil repellency,^15,16 self‐cleaning,¹⁷ micro-roughness,^18,19 antibacterial^20,21 and antistatic properties.²² Additionally, plasma treatment helps to achieve hydrophilic surfaces and improve dyeing and printing properties, including the adhesion strength of the textiles.^23,24 Still, plasma technology applications to leather are not as common as in textiles. There are limited studies on the leather plasma treatments performed under low-pressure plasma conditions using oxygen, argon, and nitrogen gases²⁴ to improve the characteristics of hydrophobicity,^11,25,26 hydrophilicity,^27,28 antibacterial,²⁹ water repellency and flammability resistance.³⁰ Up to now, no study has been found on dielectric barrier discharge plasma treatment of leather, including the effect of plasma on dyeing properties and wastewater parameters like chemical oxygen demand (COD), total dissolved solids (TDS), salinity, and electrical conductivity (EC). Besides, only two studies were found in literature on leather dyeing using plasma technology; one is about the dyeing of leather with natural dyes, and the second is on the use of corona discharge plasma at radio frequency^3,31 that concentrated around a pointed tip within a nonuniform electric field. But dielectric atmospheric plasma generates a more uniform electric field spread over a larger surface area compared to corona discharge.²⁰

In the study, for the first time dielectric barrier discharge atmospheric plasma treatment was applied to metis-type crust leathers to investigate the effects of the plasma on the dyeing properties of leather to reveal the significance of wastewater load of residual dyeing bath. For this purpose, acid and metal complex leather dyes in the proportions of 1, 2, 3, and 4% were used for the dyeing process of plasma-treated leathers, and the effect of the treatment were investigated in terms of dyeing characteristics, wastewater load, antimicrobial, mechanical and surface properties of the leathers. As a result, the dielectric barrier discharge atmospheric plasma treatment is found to be an eco-friendly technique in sustainable leather manufacturing.

Materials and methods

Materials

Metis-type crust garment leathers tanned with chromium, obtained from Tezcan Deri (Aydın, Izmir), were used in the study with a thickness of 2 mm. They were cut along the backbones into halves, and the left and right sides in 10 cm × 30 cm dimensions were used for the experimental and control processes, respectively. CI Acid Black 210 (acid dyestuff; 938.0 g/mol; C₃₄H₂₅K₂N₁₁O₁₁S₃; 610 nm) and CI Acid Black 172 (metal complex dyestuff; 993.7 g/mol; C₄₀H₂₀CrN₆O₁₄S₂.3Na; 590 nm) were supplied from Stahl company (Netherlands), and the other chemicals (formic acid, and neutral syntan) used in the dyeing process were the same as those which are conventionally used in the leather manufacturing processes. For the analysis, analytical grade chemicals were used.³²

Methods

Plasma treatment

For the plasma treatment of crust garment leathers, a laboratory-scale dielectric barrier discharge (DBD) atmospheric plasma was used (Figure 1). Discharge was produced between four electrode couples with a diameter of 17 mm. One of the electrodes in the couple was covered with dielectric material, and the inter-electrode distance was set as 2 mm. The samples were placed between the electrodes and passed continuously. This device operated at resonance condition, and its value was around 13 KHz for this electrode system. At this condition, the applied voltage was about 20 000 Vpp theoretically.³³

Figure 1.

Dielectric barrier discharge atmospheric plasma device.

In the treatments of crust leathers, air (20.9% Oxygen, 79.1% Nitrogen, and relative humidity <3 × 10⁶ ppt) and argon (purity of >99.99) were used as process gases under the power of 130 W for 100 seconds. The power and the exposure time of the plasma treatment was chosen according to the conditions determined in the preliminary studies.^32,34,35 For each DBD plasma treatment, four leather samples were used and subjected to further processes. Experiments in the study were performed in triplicate and given in mean values.

Dyeing process

After plasma treatment, leather samples were weighed and re-wetted with a non-ionic surfactant (1%) at 20°C for an hour. The re-wetted leathers were subjected to neutralization process with the addition of neutral syntan in the proportion of 1% for 30 min to reach the pH 5. Later, untreated and plasma-treated leather samples were dyed with acid and metal complex dyestuffs at different concentrations of 1, 2, 3, and 4% for 45 minutes at 20°C. The fixation process of the dyestuffs was performed with the addition of formic acid and the dyeing process ended at pH value of 3.8–4.0.³⁶ Untreated leather samples were used as control groups. After the dyeing process, leather samples were dried at room temperature overnight hanging method and conditioned at 23 + 2°C and 50 + 5 humidity before physical tests and chemical analyses according to ISO 2419-2024.³⁷

Surface characterization

Surface morphologies of untreated and plasma-treated leathers were analyzed by Scanning Electron Microscope (FEI Quanta FEG 250 SEM) with 2–3 kV voltages. The leather surfaces were coated with gold and investigated at 100x, and 250x.³⁵ X-ray Photoelectron Spectroscopy analysis of the leathers was performed by a PHI 5000 VersaProbe brand device. Spectra were recorded using a monochromatic Al Kα radiation source at a power of 200 W (10 kV, 10 mA), base pressure of 10−9–10−10 torr in the sample chamber, and EA 200 hemispherical electrostatic energy analyzer.³⁸

The Attenuated Total Reflection-Fourier transform infrared (ATR- FTIR) spectra of the leathers were determined using a Perkin Elmer 100 FTIR spectrometer in the ATR mode using a diamond/zinc selenide crystal in the resolution of 4 cm⁻¹ and wavelengths of 4000-400 cm⁻¹ intervals.³⁹

Contact angle test was performed with the equipment consisting of a camera (PULNIX TM765, UK), a computer, and a monitor to determine the wettability of the leathers. The contact angles of distilled water were measured with a sessile drop method. The captured images were viewed at the monitor Contact angle measurement of leather samples were taken as five repeats, and average values were considered.⁴⁰

Antibacterial tests

After air and argon plasma treatment, the agar diffusion method (DIN EN ISO 20645) was used to evaluate the antimicrobial activity of plasma-treated and untreated leather samples against Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 6538) and Klebsiella pneumonia (ATCC 4352).

Leather samples were cut in a diameter of 1 cm and sterilized with UV light for 15 minutes. The samples were then placed in Petri dishes containing Müller Hinton Agar under aseptic conditions. The inoculated agar plates were incubated at 37°C for 24 hours, and the inhibition zones were measured in millimeters.⁴¹

Performance analyses

Color measurements

Color values were measured at 400–700 nm wavelengths, and averages of 10 measures were used to calculate K/S values (HunterLab ColorQuest II spectrophotometer, USA). The measured reflectance values (R) were used to calculate relative color strength (K/S) values according to the Kubelka–Munk equation (1):

K / S = \frac{{(1 - R)}^{2}}{2 R}

(1)

where K and S are the absorption and scattering coefficients, respectively.⁴²

Dyestuff exhaustion

The quantity of exhausted acid and metal complex dyestuffs was estimated from the absorption of dye solution measured at its λmax in the equipment–UV-Visible spectrophotometer (Shimadzu, Japan) and from a calibration curve.⁴³

Wastewater characterization of dye baths

The chemical oxygen demand (COD) values of the dyeing baths were measured by photometrical methods (Merck SQ300 device).⁴⁴ Conductivity,⁴⁵ pH,⁴⁶ salinity,⁴⁷ and total dissolved solid⁴⁸ ratios of dyeing baths were determined with the versatile measuring instrument of HACH HQ 40D MULTI.⁴⁹

Rubbing fastness

The rubbing fastness properties of the leather samples were determined with a Bally Finish Tester 9029 (USA) according to EN ISO 11640 standard (50 rubs dry and 25 rubs wet) and evaluated using grayscale per the standards ISO 105-A05 and ISO 105-A04.^50–53

Determination of tensile strength and tear load

The tensile (N/mm²) and tear strength (N/mm) values of the leather samples were tested by the use Shimadzu AG-IS Tensile Tester and Trapezium-2 software with a speed of 100 mm/min according to EN ISO 3376 and EN ISO 3377-2 standards, respectively.^53,54 The gauge length is 50 mm and the width is 10 mm for the tensile strength test. The thickness of the leather samples was measured by the Satra STD 483 digital micrometer device (0.01 mm accuracy; Satra, England).

Results and discussion

Scanning electron microscopy analysis

The effect of air and argon plasma treatment on the surface characteristics of the crust leathers was investigated using SEM, and the morphological observations of untreated and plasma-treated leathers were illustrated in Figure 2.

Figure 2.

SEM images of untreated and plasma-treated leather samples (100x, 250x). SEM images of untreated (a, d), air plasma-treated (b, e), and argon plasma-treated leather samples (c, f).

The plasma treatment increased the depth and size of pores according to the observations obtained from the visual displays of SEM. SEM images (Figure 2) reveal changes in the micropores, showing the modification and swelling, as well as a more uniform surface following plasma treatment. The increase in homogeneity and swelling became clearer after argon plasma treatment.³¹ Before plasma treatment, the surface of the leather was not uniform in terms of roughness and fine deep pores. But after the treatment, the average size of pores is increased, which may result from the swelling and additional loosening of the surface along with the overgrowth of the pores.³ The microcracks and groves of the atmospheric plasma-treated leather surfaces are clearly seen in the micrographs due to the etching effect of the treatment.⁴⁰ No pinholes, tears, or defects were detected after either air or argon plasma treatment. Accordingly, DBD treatment can be considered as a non-destructive procedure that affects only a minimal depth of the leather surface layer.²⁷

X-ray photoelectron spectroscopy analysis

XPS (X-ray Photoelectron Spectroscopy) analysis was performed to investigate the changes in the surface chemical composition of plasma-treated and untreated leather samples. Low-resolution scans were made to determine the percentage changes of elements on the surface of leather samples. For all samples, atomic percentages of the oxygen, carbon, and nitrogen components were calculated by using the area sensitivity factor, and percentages were given in Table 1.

Table 1.

Relative ratios of the atoms on the leather samples.

Treatment	C%	O%	N%	O/C
Untreated	81.8	15.9	1.9	0.19
Air plasma	79.5	18.1	1.5	0.22
Argon plasma	81.2	17.0	1.3	0.21

The surface of untreated leather consists mainly of carbon (C1s), oxygen (O1s), nitrogen (N1s), and a low percentage of sulfur (S2p), with peaks located at about 532, 400, 285, and 225 eV. In the survey of air and argon atmospheric plasma-treated leather samples, the peak positions did not change, showing an increase in O and a decrease in C and N. The nitrogen found in leather samples is due to the collagen protein that forms the leather. The decrease in the atomic percentage of C could be due to the removal and oxidation of organic compounds, and the increase in O could result from introducing new oxygenated molecules on the leather surface. The increase in oxygen content indicated that more oxygen-containing groups such as carboxyl, carbonyl and hydroxyl were formed on the leather surface after atmospheric plasma treatment, which increases surface functionality.⁵⁵ The change in oxygen content was found to be higher with air atmospheric plasma treatment than with argon atmospheric plasma treatment.⁵⁶ The argon plasma technique generates free radicals on material surfaces, that readily interact with ambient oxygen and moisture when samples are stored. This technique primarily activates the sample surface through chain fragmentation generating free radicals. In contrast, the air plasma technique mainly forms oxygen-containing groups, and at elevated plasma power levels, such as 300 W, traces of nitrogen could be observed on the surface of the samples.⁵⁸

The O/C ratio of the untreated leather was determined as 0.19, whereas it increased to 0.22 and 0.21 following air and argon plasma treatment, respectively. This increase could be attributed to the formation of new functional groups, including oxygen. The polar nature of oxygen groups led to an increase in hydrophilicity, capillarity, and surface energy of the leather surfaces as a result. The increased number of functional groups has an important role in leather dyeing in terms of providing more sites for dyestuff molecules to bind, which also gives better dyeing, fastness, and wastewater load results.^27,55,56 Besides, XPS results were found to be compatible with decreased contact angle values.⁶²

Fourier transform infrared spectroscopy

ATR-FTIR spectra of the leather surfaces obtained before and after air and argon plasma treatments are given in Figure 3.

Figure 3.

FT-IR spectrums of untreated, air plasma, and argon plasma leather samples.

In the FTIR spectrum of the untreated leather, broadband appeared around 3300 cm⁻¹ due to the stretching vibration of peptide bond -NH groups, which is the conformation of the backbone, very sensitive to the strength of the hydrogen bonds, can be identified as the amide A and O-H stretching vibrations.⁷⁶ The absorption band between 2854 and 2924 cm⁻¹ is related to C–H, CH₂, and CH₃ stretching of aliphatic side chains. The peak at 1743 cm⁻¹ corresponded to the C = O stretching due to the ester fatty acids. A peak that appeared at 1634 cm⁻¹ is associated with the C = O amide in the peptide band identified as amide I. The peak at 1548 cm⁻¹ represents the N-H of amide II. Also, the other amide vibrations assigned to C–N stretching and N–H wagging appear at 1450 cm⁻¹. The characteristic bands of amide III appeared in spectra at 1282, 1236, and 1202 cm⁻¹ assigned to the C–N stretching and N–H bending vibrations from amide linkages, as well as wagging vibrations of CH₂ groups in the glycine backbone and proline side chains. Absorption bands at 1185–1035 cm⁻¹ arise from the stretching vibration of C–O and C–O–C.⁵⁶

The FT-IR spectrums of the untreated and plasma-treated leather samples only showed differences determined by the intensity of the bands that can be the result of plasma-induced reactions. Besides, no new peaks were observed, and the peaks were similar to atmospheric plasma-treated textile surfaces.^33,59 The changes in the absorption ratio may result from minor structural alterations due to plasma-induced chemical modification as the leather thicknesses remain consistent, and the secondary structure does not change significantly. The oxidative action of the air and argon atmospheric plasma treatment is reflected in the spectrum as the development of shoulders around 1742 cm⁻¹ resulting from the development of new carboxylic, keto, and aldehyde groups by the oxidation of some oxidation-prone amino acids by ozone. The alteration in the chemical composition of the surface and the increase in the oxygen content of the leather samples were also confirmed by XPS.³

Contact angle measurements

The effect of air and argon plasma treatment on the wettability of leathers was studied using water contact angle measurements. Contact angle images and average contact angle values of the leather samples against water are given in Table 2.

Table 2.

Contact angle images and average contact angle values of the leather samples.

Sample	Untreated leather	Leather treated with air-plasma	Leather treated with argon-plasma
Average contact angle value^(o)	97.37	81.44	84.52
Contact angle images

Contact angle θ can be defined as the angle between the surface of the liquid and the outline of the solid contact surface. In the case of a liquid and solid interaction, the degree of wetting can be measured by the contact angles. Contact angles below 90° represent hydrophilicity (high wettability), while those above 90° indicate hydrophobicity (low wettability).¹¹

The water contact angle measurement of untreated leather showed hydrophobic properties (97.37°). After the activation of the leather surface by air and argon plasma, the water contact angles decreased to 81.44° and 84.52°, respectively, indicating improved hydrophilicity (Table 2).¹¹ SEM observations revealed an increase in pore width (Figure 2), which enhances surface hydrophilicity as confirmed by FTIR-ATR and XPS in terms of the detection of active hydrophilic groups on the surface and the increase in the oxygen content of the leather samples respectively.^11,60 This is attributed to the dissociated oxygen atoms in the plasma, the increase in oxygen-containing functional groups on the leather surface, and the enlargement in pore diameter and depth. Besides, the increased hydrophilicity in the leather samples is likely due to the synergistic interaction between oxygen ions in the air and argon gas used in atmospheric plasma treatment. New hydrophilic groups resulted from partially decomposing the hydrophobic layer by the atmospheric plasma. These groups can increase the surface energy and decrease the contact angle. Consequently, the average contact angle values reveal that air plasma is more effective than the argon plasma. In the dyeing process, it is crucial to improve the adhesion properties of the leather to ensure better bonding of the dyestuffs to the leather. Therefore, it is suggested that an increase in surface hydrophilicity by the effect of air and argon plasma is attributed to the modified surface, which became so reactive that it can react with dyestuffs and thus provide a better dyeing property. The results of the contact angle measurements were found to be compatible with the K/S results.^{11,28,55,61,62}

Antibacterial characteristics

The antimicrobial properties of the leather samples were evaluated by Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 6538), and Klebsiella pneumonia (ATCC 4352), and the results were shown in Table 3 and Figure 4.

Table 3.

Antibacterial activity of leather samples.

	Escherichia coli	Staphylococcus aureus	Klebsiella pneumonia
Untreated	None	None	None
Air plasma	2 mm	3 mm	2 mm
Argon plasma	2 mm	3 mm	2 mm

Figure 4.

Antibacterial activity of untreated, air plasma, and argon plasma treated leather samples.

No inhibition zone was observed for the untreated leather samples (Table 3; Figure 4). On the contrary, the diameter of inhibition zones obtained from air and argon plasma-treated leather samples was determined 3 mm for Staphylococcus aureus (ATCC 6538), 2 mm for Escherichia coli (ATCC 25922), and Klebsiella pneumonia (ATCC 4352). Atmospheric plasma treatment of the leather samples was found effective in terms of antibacterial characteristics against Gram-positive and Gram-negative microorganisms. This is due to the synergistic sterilization effect of plasma through UV radiation that damages the DNA of microorganisms. Then the chemical compounds in the microorganism break down into chemically unstable volatile molecules (CO, CHx), which later form stable compounds under the influence of plasma. The -OH ions decomposed by ozone induce an oxidative stress effect called lipid peroxidation, leading to the breakdown of the microorganism’s cell membranes.⁶³ This characteristic could result from the atmospheric plasma treatment that gives a sanitation effect to crust leather samples together with improved hydrophilicity and a more porous surface which leads to an increase in the level of humidity and breathing on the leather.^64,65 Atmospheric plasma modification, provides sterilization below 50°C without any chemical usage, can be a good alternative for obtaining antibacterial leather characteristics.^63,66

Color measurements

In this study, the effect of different dyestuff concentrations on atmospheric plasma-treated leather samples, including the wastewater characteristics, was investigated. The color measurement results (L*, a*, b*, and dE*) and K/S values of the leather samples dyed in the proportions of 1, 2, 3, and 4% are given in Table 4.

Table 4.

Color measurement and dyestuff consumption (DC) results of the leather samples.

Dyestuff	Dyestuff concentration (%)	Treatment	K/S	L*	a*	b*	dE*	DC concentration (ppt)* *×10⁶
Acid (black 210)	1	Untreated	3.84	41.95	−2.83	0.25		15 ± 0.4
		Air plasma	5.86	35.06	−0.76	−0.94	7.29	7 ± 0.6
		Argon plasma	5.82	35.50	−0.90	−1.09	6.86	4 ± 0.1
	2	Untreated	5.38	36.77	−2.83	0.16		17 ± 0.7
		Air plasma	7.82	30.39	−0.54	0.66	6.80	9 ± 0.3
		Argon plasma	7.00	31.59	0.01	0.71	5.94	5 ± 0.1
	3	Untreated	5.75	36.51	−4.14	−0.09		40 ± 0.3
		Air plasma	8.16	29.92	−0.17	0.42	7.71	16 ± 0.4
		Argon plasma	8.01	29.75	−0.16	0.29	7.85	10 ± 1.1
	4	Untreated	6.90	33.65	−3.70	−0.35		105 ± 2.7
		Air plasma	8.76	28.80	−0.15	0.20	6.04	90 ± 1.6
		Argon plasma	8.80	28.07	0.64	1.08	7.21	31 ± 2.5
Metal complex (black 172)	1	Untreated	2.95	46.01	−0.81	−5.67		2 ± 0.07
		Air plasma	4.40	40.17	−0.31	−6.22	5.89	1 ± 0.07
		Argon plasma	4.35	40.09	0.02	−5.84	5.98	1 ± 0.0
	2	Untreated	4.88	42.53	−0.07	−5.87		5 ± 0.5
		Air plasma	6.86	35.77	0.61	−5.03	6.83	2 ± 0.2
		Argon plasma	6.90	35.37	0.07	−6.67	7.20	2 ± 0.7
	3	Untreated	5.14	37.49	−0.13	−5.29		5 ± 0.06
		Air plasma	8.88	29.41	0.61	−4.09	8.20	4 ± 0.1
		Argon plasma	8.01	30.92	0.70	−4.88	6.64	2 ± 0.1
	4	Untreated	6.24	34.76	0.38	−5.10		12 ± 0.3
		Air plasma	8.02	31.04	0.76	−4.61	3.77	10 ± 0.5
		Argon plasma	8.33	30.50	0.73	−4.63	4.30	7 ± 0.3

The inclusion of polar groups and the expansion of pores on the leather surface after plasma treatment facilitated the rapid diffusion of water molecules. The uptake of the dyeing bath is improved in plasma-treated samples, as seen by the noticeable percentage increase in color depth.⁶⁰ Additionally, the plasma treatment of leather has been demonstrated to activate the leather surface through the incorporation of functional groups, as confirmed by FTIR-ATR and XPS results (Table 1 and Figure 3), which significantly influence the rate of dyeability. Since electrostatic force plays a vital role in the acid dyeing of leather, the increase in functional groups provides binding sites for dye molecules on the substrate. The results clearly indicate that plasma treatment enhanced the dye in-take capacity of leather, as demonstrated by spectroscopic measurements (Table 4).³

The K/S values of untreated, air, and argon plasma-treated leather samples showed that plasma treatment increases the color yield of the samples dyed with acid dyestuffs (Table 4). Air and argon plasma treatments enhanced the color yields by approximately 26%–52% compared to untreated samples. In addition, after air and argon plasma treatments, the K/S values of leathers dyed at a 2% concentration were similar to those of untreated leathers dyed at a 4% concentration for both acid and metal complex dyes. The color strength of plasma-treated leather samples dyed with 2% dyestuff is comparable to that of untreated samples dyed with 4% dyestuff. This indicates a 50% reduction in dyestuff usage, which provides significant environmental benefits by reducing wastewater pollution. Thus, atmospheric plasma led to less wastewater and environmental burden for the same color intensity during the dyeing process. The high dE* values clearly highlighted the effectiveness of plasma treatment in enhancing the color yield of the leathers, likely resulting from surface activation and increased hydrophilicity.³ It could be stated that atmospheric plasma treatments had positive effects on color values in terms of efficiency and density of the dyeing, indicating an increase in absorption of the dyestuff on the leather.^31,61,63 The images of 2% acid and metal complex dyed leather samples are given in Figure 5.

Figure 5.

Images of 2% acid and metal complex dyed leather samples. Images of 2% acid and metal complex dyed untreated (a, d), air plasma-treated (b, e), and argon plasma-treated leather samples (c, f).

Plasma treatment of the fiber surface causes an oxidation effect, which includes the production of polar groups by oxidation, which contributes to the increased color yield for plasma-treated fibers dyed with metal complex dyes. In addition, the plasma treatment improves the absorption of anionic dyes by introducing additional amino groups into the surface fiber of the leather.^67,68

The dyestuff consumption values decreased dramatically for the air and argon plasma-treated leather dyeing floats. The increase in dye consumption after air plasma was found higher than the argon plasma treatment. Also, the increase in dye consumption was more significant after plasma treatment when dyeing at high concentrations.³¹ As previously mentioned, the efficiency and density of the dyeing were increased after air and argon plasma treatments. Similarly, the amount of remaining dye in the effluent was significantly reduced. An increase in the consumption of metal complex and acid dyes will enable a reduction in the discharge of wastewater to the environment and cause an increase in the color quality of the leather.^31,63

Wastewater characterization of the effluents

After assessing the consumption values of acid and metal complex dyestuffs, various pollution parameters, including chemical oxygen demand (COD), total dissolved solids (TDS), salinity, and electrical conductivity (EC), were used to evaluate the wastewater quality with the results provided in Table 5.

Table 5.

Chemical oxygen demand, total dissolved solids, salinity, and electrical conductivity values of the residual dyeing baths.

Dyestuff	Dyestuff concentration (%)	Treatment	COD (mg/L)	EC (µS)	Salinity (ppt)	TDS (%)
Acid (black 210)	1	Untreated	1080 ± 0.0	3.91	1.9	0.22
		Air plasma	980 ± 14.1	3.69	1.8	0.19
		Argon plasma	965 ± 7.1	3.56	1.8	0.19
	2	Untreated	1720 ± 14.1	5.49	2.9	0.31
		Air plasma	1585 ± 7.1	5.21	2.8	0.24
		Argon plasma	1565 ± 7.1	5.26	2.8	0.24
	3	Untreated	2925 ± 7.1	6.63	3.6	0.45
		Air plasma	2410 ± 56.6	6.21	3.3	0.39
		Argon plasma	1950 ± 28.3	5.95	3.2	0.38
	4	Untreated	4345 ± 7.1	7.91	4.6	0.45
		Air plasma	3925 ± 35.4	7.45	4.1	0.43
		Argon plasma	2895 ± 7.1	7.50	4.1	0.43
Metal complex (black 172)	1	Untreated	1120 ± 0.0	3.41	1.9	0.22
		Air plasma	1095 ± 7.1	3.40	1.8	0.13
		Argon plasma	1095 ± 21.2	3.39	1.8	0.17
	2	Untreated	1455 ± 7.1	4.76	2.7	0.30
		Air plasma	1445 ± 7.1	4.50	2.4	0.30
		Argon plasma	1410 ± 28.3	4.40	2.3	0.27
	3	Untreated	1795 ± 7.1	5.54	3.0	0.41
		Air plasma	1740 ± 28.3	5.44	2.9	0.39
		Argon plasma	1725 ± 7.1	5.28	2.9	0.38
	4	Untreated	1975 ± 7.1	6.17	3.4	0.50
		Air plasma	1910 ± 14.1	6.11	3.3	0.46
		Argon plasma	1900 ± 14.1	6.12	3.3	0.41

The environmental pollution loads of the residual dyeing baths decreased after atmospheric plasma treatments. The chemical oxygen demand (COD) of residual dyeing baths, representing the oxygen consumed during the complete chemical oxidation of organic compounds to inorganic by-products, can be regarded as an approximate measure of the theoretical oxygen demand. The chemical oxygen demand (COD) of residual dyeing baths, representing the oxygen consumed during the complete chemical oxidation of organic compounds to inorganic by-products, can be considered as an approximate measure of the theoretical oxygen demand, and COD is a good indicator of the amount of chemically oxidizable organic matter in water.⁶⁹ A reduction of up to 35% in the COD value was observed in acid dyeing with a 4% acid dye concentration, compared to untreated leather samples. However, the reduction (%) in COD values decreased as the dyeing concentration decreased. Also, a significant reduction was observed in dyeing processes performed at 1, 2, and 3 % dyestuff concentrations.

Similarly, while a decrease in COD values was observed for metal complex dyes, the percentage reduction was not as significant as that seen with acid dyes. The reason for the high dye consumption of metal complex dyestuffs is the binding capacity of the dyestuff in its nature. Besides, the reduction of COD values could be due to the formation of active groups (O, OH, and O₃) on the leather surfaces.^{44,69–71,74}

TDS refers to the overall concentration of dissolved substances in water, including salts, minerals, metals, ions etc. Measuring TDS helps to evaluate water quality, with elevated TDS levels serving as a sign of water pollution. The plasma-treated leathers dyed with 1% acid dyestuff resulted in a minimum TDS% value of 0.19, while those dyed with 4% acid dyestuff gave a maximum TDS% value of 0.43. Similarly, minimum values of 0.13 and 0.17 were obtained after dyeing with 1% metal complex dyestuffs, while values of 0.41 and 0.46 were achieved with 4% metal complex dyestuff application. In addition, TDS values increased with the increased dyestuff concentration, and a slight decrease in TDS values was obtained compared to untreated leather samples. Lastly, TDS values of the metal complex and acid dyestuffs were found similar (Table 5).⁷²

The total components ionized in wastewater were ascertained using electrical conductivity (EC)..^72,73 The EC values of effluents containing metal complex dyestuffs varied between 3.39 μS/cm and 6.17 μS/cm, whereas those of effluents containing acid dye varied between 3.56 μS/cm and 7.5 0 μS/cm (Table 5). When the EC values of the effluents were compared with the control leathers after the plasma treatment, an increase was determined in parallel with the increasing dye concentration.

Salinity test is used to detect dissolved salts in the structure of water.⁷² As indicated in Table 5, the wastewater salinity values for the samples contaminated with acid and metal complex dyes ranged from a minimum of 1.8 ppt to a maximum of 4.6 ppt for acid dyes, and from 1.8 ppt to 3.4 ppt for metal complex dyes. Salinity values also increased in parallel with increasing dye concentrations.

Color fastness properties

The dry and wet rubbing fastness values of leather samples dyed with acid and metal complex dyestuffs are given in Table 6.

Table 6.

Rubbing fastness values of leather samples.

Dyestuff	Dyestuff concentration (%)	Treatment	Dry rubbing		Wet rubbing
			Leather	Felt	Leather	Felt
Acid (black 210)	1	Untreated	5	4/5	3/4	4
		Air plasma	5	4/5	3/4	4
		Argon plasma	5	4/5	3/4	4
	2	Untreated	4/5	4/5	3/4	4
		Air plasma	5	4/5	4	4
		Argon plasma	4/5	4/5	3/4	4
	3	Untreated	5	4/5	3	4
		Air plasma	5	4/5	3/4	4
		Argon plasma	5	4/5	3/4	4
	4	Untreated	4/5	4/5	3/4	4
		Air plasma	5	4/5	3/4	4
		Argon plasma	5	4/5	3/4	4
Metal complex (black 172)	1	Untreated	5	4/5	4	4
		Air plasma	5	4/5	4	4
		Argon plasma	5	4/5	4	4
	2	Untreated	5	4/5	3/4	4
		Air plasma	5	4/5	3/4	4
		Argon plasma	5	4/5	3/4	4
	3	Untreated	5	4/5	3/4	4
		Air plasma	5	4/5	3/4	4
		Argon plasma	5	4/5	3/4	4
	4	Untreated	5	4/5	3/4	4
		Air plasma	5	4/5	3/4	4
		Argon plasma	5	4/5	3/4	4

The results in Table 6 demonstrated that air and argon plasma treatments did not adversely affect the rubbing fastness properties of the leathers dyed with acid and metal complex dyestuffs. This finding is consistent with our previous studies.^32,35 At the same time, the plasma treated samples were dyed darker at the same concentration and the fastness values were found similar or better due to its high binding capacity. Although some chemical modifications can be created on leather by plasma application, the air and argon atmospheric plasma device had a gentle plasma effect, and this can be associated with the impact of plasma, which contributes positively to the binding of the dyestuff to the leather surface.^3,31,63

Tensile and tear strength values

Since the physical properties of the leathers are important parameters for leather-goods production, tensile and tear strength properties of the leather samples were measured to determine the effects of plasma treatment. The results are given in Table 7.

Table 7.

Tensile and tear strength values of leather samples.

Treatment	Tensile strength (N/mm²)	Tear load (N/mm)
Untreated	12.04 ± 4.4	44.04 ± 0.89
Air plasma	13.43 ± 3.7	42.17 ± 0.22
Argon plasma	13.33 ± 2.2	42.12 ± 4.03

The tensile and tear strength values of the leather samples were found as 12.04, 13.43, and 13.33 N/mm² and 44.04, 42.17, and 42.12 N/mm for untreated, air, and argon plasma treated samples, respectively. No negative effect has been occurred on tensile and tear strength values of the leather samples after plasma treatments. The tensile and tear strength values of the samples were found higher than those indicated by UNIDO and BASF (10 N/mm² and 12 N/mm² respectively for tensile strength: 15 N/mm and 20 N/mm, respectively, for tear load) for garment leathers.

The internal structure of the leather could be affected by the ozone and UV radiation produced by the plasma. However, these internal structural changes cannot be observed like changes in surface morphology, which can be studied by scanning electron microscopy. Therefore, we can also hypothesize, like other research studies.^27,31 that the increase in strength and decrease in tear load after plasma treatment could be due to the modification of the fibrous structure of the collagen network by ozone and UV radiation. As plasma species can only penetrate to a depth of approximately 1000 A°, leading to the removal of materials, the atmospheric plasma treatment affects the surface properties of the leather without changing the interior part.⁷⁵ According to the tensile strength and tear load results, it can be concluded that air and argon atmospheric plasma modification is not a damaging method for leather goods.^27,31

Conclusion

In this study, the effect of dielectric barrier discharge atmospheric plasma treatment on leather dyeability, wastewater quality, surface morphology, and physical characteristics of leathers were investigated. The results revealed that air and argon plasma treatment considerably improved the dyeing efficiency of the leather samples by using the half of the maximum dye without negatively affecting their physical properties. The consumption of acid and metal complex dyestuffs has found to be effective. Furthermore, the 4% acid dye concentration reduced the COD load of the acid dyeing effluent by about 35% compared to the control group. Due to the high affinity and better binding capacity to leather, the same effect was not obtained from the metal complex dyeing effluent. The leather samples gained hydrophilicity properties and antibacterial effects against Gram-positive and Gram-negative microorganisms without using any chemical agent. The presented results proved that it was possible to obtain the desired properties on plasma-treated leather samples successfully and the argon plasma treatment is considered as a better alternative for improving the leather processing and quality in terms of cross section dyeing and wastewater quality.

Plasma technology has various advantages for modifying leather surfaces over conventional wet finishing treatments, including the absence of harmful chemicals and effluents. In addition, the results showed that plasma technology could be easily used and integrated into leather production processes. Plasma treatment seems to be promising in several fields of leather surface modifications. Therefore, it is expected to be incorporated into large-scale production for commercialization as a fast, effective, low-cost, and environmentally friendly method despite the high investment cost and the necessity to control the process conditions.

Footnotes

Author contributions

Conceptualization, E.B, T.G. and A.C.A.Z.; methodology, E.B, T.G., A.A., A.D., E.Ö., G.G. and A.C.A.Z.; formal analysis, E.B., T.G. and A.C.A.Z.; investigation, E.B., T.G., A.A., A.D., E.Ö., G.G. and A.C.A.Z.; resources, E.B., T.G., A.A., A.D., E.Ö., G.G. and A.C.A.Z.; writing—original draft preparation, E.B., T.G. and A.C.A.Z.; writing—review and editing, A.A., A.D., E.Ö., and G.G. supervision, T.G., A.A., A.D., E.Ö., and G.G. All authors have read and agreed to the published version of the manuscript.

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, of this article: The authors are grateful for the funding from the Ege University Rectorate Scientific Research Project Number 13-TKUAM-004.

ORCID iD

Ebru Bozaci

References

Kanagaraj

Senthilvelan

Panda

, et al. Eco-friendly waste management strategies for greener environment towards sustainable development in leather industry: a comprehensive review. J Clean Prod 2015; 89: 1–17.

George

Chauhan

Kumar

, et al. Approach to ecofriendly leather: characterization and application of an alkaline protease for chemical free dehairing of skins and hides at pilot scale. J Clean Prod 2017; 79: 249–257.

Dave

Ledwani

Nema

. Surface modification by atmospheric pressure air plasma treatment to improve dyeing with natural dyes: an environment friendly approach for leather processing. Plasma Chem Plasma Process 2016; 36(2): 599–613.

Sivakumar

Rao

. Studies on the use of power ultrasound in leather dyeing. Ultrason Sonochem 2003; 10(2): 85–94.

Haroun

. Evaluation of modified leather dyeing technique using black dyestuffs from the economical view. Dyes Pigments 2005; 67(3): 215–221.

Thorsen

. A corona discharge method of producing shrink-resistant wool and mohair: Part II: effects of temperature, chlorine gas, and moisture. Textil Res J 1968; 38(6): 644–650.

Karahan

Yaman

Demir

, et al.

Tekstilde plazma teknolojisinin kullanım olanakları: plazma nedir?

Tekstil ve Konfeksiyon 2006; 16(1): 302–309.

Finson

Kaplan

Wood

. Plasma treatment of webs and films. 38th annual technical conference proceedings, Society of Vacuum Coaters. Chicago, 1995, pp. 2–7.

Shahidi

Wiener

Ghoranneviss

. Surface modification methods for improving the dyeability of textiles fabrics. In: Gunay

(ed). Eco-friendly textile dyeing and finishing. Rijeka: InTech, 2013, pp. 33–52.

10.

Matthews

Hwang

McCord

, et al. Investigation into etching mechanism of polyethylene terephthalate (PET) films treated in helium and oxygenated-helium atmospheric plasmas. J Appl Polym Sci 2014; 94: 2383–2389.

11.

Kaygusuz

Meyer

Junghans

, et al. Modification of leather surface with atmospheric pressure plasma and nanofinishing. Polym-Plast Technol Eng 2018; 57(4): 260–268.

12.

Özdoğan

. The effect of plasma treatment over the adhesion property of polyamide 6 fabrics. Tekstil ve Konfeksiyon 2006; 16(2): 128–133.

13.

Tsafack

Levalois-Grützmacher

. Flame retardancy of cotton textiles by plasma-induced graft-polymerization (PIGP). Surf Coating Technol 2006; 201(6): 2599–2610.

14.

Errifai

Jama

Le Bras

, et al. Elaboration of a fire-retardant coating for polyamide-6 using cold plasma polymerization of a fluorinated acrylate. Surf Coating Technol 2004; 180-181: 297–301.

15.

Öktem

Ayhan

Seventekin

, et al. Modification of polyester fabrics by in situ plasma or post‐plasma polymerisation of acrylic acid. Color Technol 1999; 115(9): 274–279.

16.

Ceria

Hauser

. Atmospheric plasma treatment to improve durability of a water and oil repellent finishing for acrylic fabrics. Surf Coating Technol 2010; 204(9-10): 1535–1541.

17.

Shahidi

Ahmadi

Rashidi

, et al. Effect of plasma treatment on self-cleaning of textile fabric using titanium dioxide. Micro & Nano Lett 2015; 10(8): 408–413.

18.

Haji

Naebe

. Cleaner dyeing of textiles using plasma treatment and natural dyes: a review. J Clean Prod 2020; 265: 121866.

19.

Gasi

Petraconi

Bittencourt

, et al. Plasma treatment of polyamide fabric surface by hybrid corona-dielectric barrier discharge: material characterization and dyeing/washing processes. Mat Res 2019; 23(1): 1–5.

20.

Schutze

Jeong

Babayan

, et al. The atmospheric-pressure plasma jet: a review and comparison to other plasma sources. IEEE Trans Plasma Sci 1998; 26(6): 1685–1694.

21.

Morais

Guedes

Lopes

. Antimicrobial approaches for textiles: from research to market. Materials 2016; 9(6): 498.

22.

Shishoo

. Plasma treatment of fibres and textiles. In Plasma technologies for textiles. Elsevier, 2007, pp. 282–298.

23.

Gotmare

Samanta

Patil

, et al. Surface modification of cotton textile using low-temperature plasma. International Journal of Bioresource Science 2015; 2(1): 37–45.

24.

Tudoran

Roşu

Coroş

. A concise overview on plasma treatment for application on textile and leather materials. Plasma Process Polym 2020; 17(8): 2000046.

25.

Kaygusuz

Meyer

Junghans

, et al. Surface activation and coating on leather by dielectric barrier discharge (DBD) plasma at atmospheric pressure. J Soc Leather Technol Chem 2017; 101(2): 86–93.

26.

Feng

Liao

Wang

. Improvement in leather surface hydrophobicity through low-pressure cold plasma polymerization. Journal of American Leather and Chemistry Association 2014; 109: 89–93.

27.

Štěpánová

Kelar

Slavíček

, et al. Surface modification of natural leather using diffuse ambient air plasma. Int J Adhesion Adhes 2017; 77: 198–203.

28.

You

Gou

Tong

. Improvement in surface hydrophilicity and resistance to deformation of natural leather through O₂/H₂O low-temperature plasma treatment. Appl Surf Sci 2015; 360: 398–402.

29.

Idris

Majidnia

Valipour

, et al. Antibacterial improvement of leather by surface modification using corona discharge and silver nanoparticles application. J Sci Technol 2013; 5(2): 1–15.

30.

Gaidau

Niculescu

Surdu

, et al. Research on cold plasma treatment of leather and Fur-based materials as ecological alternative. Ind Textil 2017; 68(5): 350–356.

31.

Acikel

Aslan

Oksuz

, et al. Effects of atmospheric pressure plasma treatments on various properties of leathers. J Am Leather Chem Assoc 2013; 108(07): 266–276.

32.

Gülümser

Adıgüzel Zengin

Bozaci

, et al. Developing of leather dyeing properties by plasma technology, 3rd international leather engineering congress. Izmir, Turkey, 2015, pp. 45–49.

33.

Yaman

Özdogan

Kocum

, et al. Improvement surface properties of polypropylene and polyester fabrics by glow discharge plasma system under atmospheric condition. Textile and Apparel 2009; 19(1): 45–51.

34.

Demir

Bozaci

Gülümser

, et al. An ecological approach for the surface modification of aramid fibers. Textile and Apparel 2016; 26(3): 256–261.

35.

Gülümser

Aslan

Adıgüzel Zengin

, et al. Effects of atmospheric plasma treatment over leather dyeing properties. 7th international conference of textiles. Tirane, Albenia, 2016, pp. 163–168.

36.

Zengin

Özgünay

Ayan

, et al. Determination of dyestuffs remaining in dyeing processes of vegetable-tanned leathers and their removal using shavings. Pol J Environ Stud 2012; 21(2): 479–506.

37.

ISO 2419 :2024 Specifies the preparation of leather for physical and mechanical testing together with standard atmospheres for conditioning and testing.

38.

Özgüney

Bozacι

Demir

, et al. Inkjet printing of linen fabrics pretreated with atmospheric plasma and various print pastes. Aatcc Journal of Research 2017; 4(1): 22–27.

39.

Bozaci

Sever

Sarikanat

, et al. Effects of the atmospheric plasma treatments on surface and mechanical properties of flax fiber and adhesion between fiber–matrix for composite materials. Compos B Eng 2013; 45(1): 565–572.

40.

Demir

Bozaci

Gülümser

, et al. An ecological approach for the surface modification of aramid fibers. Textile and Apparel 2016; 26(3): 256–261.

41.

ISO 20645 . Determination of antibacterial activity-agar diffusion plate test. 2004.

42.

Karahan

Özdoğan

Demir

, et al. Effects of atmospheric plasma treatment on the dyeability of cotton fabrics by acid dyes. Color Technol 2008; 124(2): 106–110.

43.

Ujhelyiova

Bolhova

Oravkinova

, et al. Kinetics of dyeing process of blend polypropylene/polyester fibres with disperse dye. Dyes Pigments 2007; 72(2): 212–216.

44.

Kilic

. Evaluation of degreasing process with plant derived biosurfactant for leather making: an ecological approach. Textile and Apparel 2013; 23(2): 181–187.

45.

SM 2510 B . 2510 Conductivity Standard Methods For the Examination of Water and Wastewater 2018; 23rd. DOI: 10.2105/SMWW.2882.027.

46.

H+B SM4500 . 4500-H+ pH standard methods for the examination of water and wastewater. DOI: 10.2105/SMWW.2882.082 (2018).23rd.

47.

SM 2520

. 2520 SALINITY Standard Methods For the Examination of Water and Wastewater 2018; 23rd. DOI: 10.2105/SMWW.2882.028.

48.

SM 2540

. 2540 SOLIDS Standard Methods For the Examination of Water and Wastewater 2018; 23rd. DOI: 10.2105/SMWW.2882.030.

49.

Zengin

Çolak

Özgünay

. Usage of hydrogen peroxide in leather manufacturing processes and its effects on leather characteristics. Textile and Apparel 2010; 20(3): 236–240.

50.

Aslan

. Improved dyeing properties of chrome-tanned leathers and parameters of the wastewater using chitosan formate. Ekoloji Dergisi. 2014; 23(91).

51.

EN ISO 11640 . Leather- Tests for colour fastness- Colour fastness to cycles of to-and-fro rubbing. 2018.

52.

EN ISO 105-A05 . Textiles-Tests for colour fastness-Part A05: instrumental assessment of change in colour for determination of grey scale rating. 1997.

53.

ISO 105-A04 . Textiles-Tests for colour fastness-Part A04: method for the instrumental assessment of the degree of staining of adjacent fabrics. 2013.

54.

Kılıç

Zengin

. Effect of viscosity on the characteristic properties of solvent free patent finished leathers. Textile and Apparel 2021; 31(2): 137–145.

55.

EN ISO 3376 . Leather-Physical and mechanical tests Determination of tensile strength and percentage extension. 2020.

56.

Jiang

Liu

, et al. The effects of surface modification using O₂ low temperature plasma on chrome tanning properties of natural leather. J Ind Textil 2019; 49(4): 534–547.

57.

Silvestre

Blasco

MPC

López

, et al. Hydrophobic leather coating for footwear applications by a low-pressure plasma polymerisation process. Polymers 2021; 13(20): 3549.

58.

Bozaci

Sever

Sarikanat

59.

Inbakumar

Morent

De Geyter

, et al. Chemical and physical analysis of cotton fabrics plasma-treated with a low-pressure DC glow discharge. Cellulose 2010; 17(2): 417–426.

60.

Xuewei

Xingye

. Improvement in surface hydrophilicity and resistance to deformation of natural leather through O₂/H₂O low-temperature plasma treatment. Appl Surf Sci 2016; 360: 398–402.

61.

Arik

Demir

Özdoğan

, et al. Effects of novel antibacterial chemicals on low temperature plasma functionalized cotton surface. Journal of Textile & Apparel/Tekstil ve Konfeksiyon 2011; 21(4): 356–363.

62.

Açikel

Kaygusuz

Gültek

, et al. Investigation of the effect of argon atmospheric pressure plasma (APP) on the finishing process of garment leathers. J Soc Leather Technol Chem 2020; 104(6): 300–306.

63.

Jiang

Liu

, et al. The effects of surface modification using O2 low temperature plasma on chrome tanning properties of natural leather. J Ind Textil 2019; 49(4): 534–547.

64.

Ikel

Kaygusuz

. Leather surface functionalization by plasma application. . Academic Researches in Architecture, Engineering Planning and Design 2018: 143–162.

65.

Lkhagvajav

Koizhaiganova

Yasa

, et al. Characterization and antimicrobial performance of nano silver coatings on leather materials. Braz J Microbiol 2015; 46: 41–48.

66.

Ražić

Akalović

Ivanković

, et al. Plasma pre-treatments improve antimicrobial properties of bovine splitted leather. 13th international scientific-professional symposium textile science and economy 18th september 2020. Zagreb, Croatia, pp. 1–6.

67.

Kan

Chan

Yuen

CWM

. Surface characterization of low temperature plasma treated wool fiber. Fibers Polym 2004; 5: 52–58.

68.

Atav

Yurdakul

. Low temperature dyeing of plasma treated luxury fibres. Part I: results for mohair (angora goat). Fibres Text East Eur 2011; 19(2): 84–89.

69.

Moisan

Barbeau

Moreau

, et al. Low-temperature sterilization using gas plasmas: a review of the experiments and an analysis of the inactivation mechanisms. Int J Pharm 2001; 226(1-2): 1–21.

70.

Syakur

Zaman

Affif

, et al. Application of dielectric barrier discharge plasma for reducing Chemical Oxygen Demand (COD) on industrial rubber wastewater. 2016 3rd International Conference on Information Technology, Computer, and Electrical Engineering (ICITACEE), 2016, pp. 1–5.

71.

Kanagaraj

Panda

Senthilvelan

, et al. Cleaner approach in leather dyeing using graft copolymer as high-performance auxiliary: related kinetics and mechanism. J Clean Prod 2016; 112: 4863–4878.

72.

Zengin

ACA

. Potential application of quillaja saponaria saponins as an antimicrobial soaking agent in leather industry. Journal of Textile & Apparel/Tekstil ve Konfeksiyon 2013; 23(1): 55–61.

73.

Clesceri

Greenberg

Eaton

. Standard methods for the examination of water and wastewater. American water work association, water environment federation. 20th ed. Washington, DC: American Public Health Association, 1999. APHA-AWWA-WEF.

74.

Camp Dresser&McKee, Inc . Guidelines for water reuse. Washington, DC: U.S. Environmental Protection Agency, 2004. EPA/625/R-04/108 (NTIS PB2005 106542).

75.

Karahan

Özdogğan

Demir

, et al. Effects of atmospheric pressure plasma treatments on some physical properties of wool fibers. Textil Res J 2009; 79(14): 1260–1265.

76.

Cosma

Tudoran

Coroș

, et al. Modification of cotton and leather surfaces using cold atmospheric pressure plasma and TiO₂-SiO₂-reduced graphene oxide nanopowders. Materials 2023; 16(4): 1397.