Surface modification of natural leather using RF plasma discharge

Abstract

Radiofrequency (RF) low-pressure plasma has been applied to improve natural leather's wettability and water absorption. The plasma was generated using RF discharge of the Ar-He gas mixture at 60 and 120 W power using different treatment times (30, 60, 120, 180, and 300s). Before and after treatment, the contact angle, surface free energy, and water absorption ratio were measured. It was observed that a 300s plasma treatment time and plasma power 60 and 120 W is enough to decrease the water contact angle from 83.56 for an untreated sample to 0°. The surface-free energy and the water absorption ratio increase with an increase in the treatment time and plasma power. The findings demonstrate that plasma treatment increases the wettability of the surface of leather samples. In addition, the change in leather properties was analyzed by scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry, Fourier transform infrared spectroscopy.

KEYWORDS:

• RF plasma treatment
• natural leather
• contact angle
• surface free energy
• water absorption ratio

1. Introduction

During leather manufacturing processes (such as preparatory stages, tanning, crusting, and surface coating), a large volume of water and a wide range of chemicals are used to obtain desirable leather characteristics, including durability, flexibility, permeability to air and water, and microbial resistance.

The environmental impact of the tanning industry is generally significant with outputs of waste, i.e. high concentrations of organics, salts, and heavy metals (chromium compounds), both in solid and liquid form. Most of the natural leather dying processes involve toxic chemical treatments that pose health risks to humans. In addition, they produce chemical waste that is detrimental to the environment [1,2].

To bring the tanning industry more in line with present environmental thinking, plasma was used to modify the surface of biomaterials to give certain desirable properties due to its numerous advantages over wet handling strategies since it does not require the use of water and chemicals and has a fast treatment time [3] Surface treatment of leather materials is requisite to improve the adhesion and dyeing properties of the surface [4,5].

Depending on the gas used for plasma generation and experimental conditions, plasma can remove organic contaminants, etch surfaces, cross-link molecules, and form new chemical groups on the surface of materials [6,7].

Heat-sensitive materials can have their surfaces modified using non-thermal plasma. It can be generated inside a vacuum chamber by using a radiofrequency (RF) power supply to induce an electric field between two electrodes [8], the electrical discharge characteristics are also investigated in the presence of a mixture of helium and argon gas [9,10].

Recent studies demonstrated that the plasma treatment of natural material containing collagen and keratin increases hydrophilic groups and improves the dyeing process. Plasma-treated leather materials have attracted the attention of numerous researchers because physical treatment is safer than chemical treatment [11,12].

The hydrophobicity of the plasma-treated leather was determined by measuring the water contact angle of the leather surface. Plasma treatment was found to enhance the hydrophilic and hydrophobic properties of natural leather [1,13,14]. The changes in the properties of the leather surface before and after modification were analyzed with differential scanning calorimetry (DSC),and x-ray diffraction (XRD) [15,16].

The objective of this study is to improve the surface properties of natural leather using RF plasma with Ar-He gas mixture. Changes in the physio-chemical properties and hydrophobicity of surfaces were observed. The effects of different exposure times and RF powers on contact angle and/or wettability, surface free energy, water absorption ratio, and surface morphology were investigated using different diagnostic techniques.

2. Experiment and diagnostic techniques

2.1. Materials

In this study, natural lamb leather that was tanned locally was used as an experimental sample. The leather sample was washed with alcohol and distilled water, dried, and cut into 2 × 3 cm² pieces. All leather samples were treated with Ar-He plasma for different time durations 30, 60, 120, 180, and 300 s, under the working power of 60 and 120 W, with the untreated sample serving as the control.

2.2. Experimental setup

The discharge plasma system consists of two parallel plates of stainless-steel electrodes (cathode and anode) with a diameter of 5 and 2 cm between them. The two electrodes were placed inside dielectric ceramics and connected with an RF power supply and impedance matching network. The lower electrodes were powered by an RF source (type ENI model OEM-6, 13.56 MHz, and 0–200 W power). whereas the upper anode was connected to the ground. A rotary pump brought the chamber to a base pressure of 10⁻² torr [17]. A schematic of the experimental set – up is shown in Figure 1.

Figure 1. The schematic of the experimental setup.

Plasma was generated from a mixture of Ar-He gas (60% Ar and 40% He) and the system pressure was fixed at 0.5 torr and maintained at this value during all measurements. The leather samples were placed on the sample stage and treated with the RF plasma power 60 and 120 W for 30, 60, 120,180, and 300 s treatment time.

2.3. Diagnostic techniques

2.3.1. Contact angle technique

The contact angle technique is used to describe the wettability of surfaces [18]. Water contact angle of the leather samples was determined using the drop method. High wettability (hydrophilic) is related to small contact angles (<90°), whereas low wettability is related to large contact angles (>90°) (hydrophobic) [13].

Distilled water (20 μl) was dropped multiple times to the leather surface using a micropipette at a temperature (25 ± 1 °C). The height was fixed at 2 cm above the sample, five water drops were measured for each parameter (expouser time, plasma power) and the standard deviation calculate. Digital cameras were used to record the droplet's photos. For each photograph of a droplet, the contact angle (θ) that was created between the drop and the surface was measured using the Image J programme [19,20]. Figure 2 illustrates of contact angles measurement.

Figure 2. Contact angle measurement. Images on water drops on the leather surface.

Figure 2 the contact angles measurement for the untreated sample and treated at plasma power 60 and 120 W, and different treatment times.

2.3.2. Surface free energy (γ_S)

The surface free energy (γ_S) measures the change in the surface hydrophobicity for plasma-treated materials [17]. The γ_S of a solid surface indicates the relationship between the contact angle (θ) and the surface tension of water (γ_L= 72.8 mN/m). The γ_S influences the amount of liquid retained on the surface and hence affects the efficiency of wet processes such as dyeing. The relationship between γ_S, γ_L and θ is given by [21]. (1) $\mathrm{cos\theta }+1=\frac{2({\gamma }_S{\gamma }_L{)}^{1/2}}{{\gamma }_L}$(1)

2.3.3. Water absorption ratio

The water absorption ability and water quantity penetrated through plasma-treated leather samples at RF powers of 60 and 120 W for treated times 30, 60, 120, and 150 s were analyzed. All samples were tested after 2 and 24 h of immersion in 5 ml of water. All samples were weighted using an electronic balance before and after water absorption. Using the following equation, the percentage change in sample weight was calculated [22]: (2) $\mathrm{Weight}\mathrm{change}\mathrm{\%}=\frac{w-w_o}{w_o}$(2) where w_o and w are the sample weight before and after water absorption, respectively.

3. Results and discussion

3.1. Contact angle measurement

The contact angle values on the leather surfaces at plasma power 60 and 120 W and treated time 30, 60, 120, 180 s are shown in Figure 3. The plasma-treated leather samples exhibited improved water absorption relative to the untreated sample. The highest contact angle for the untreated sample is 83.56^o± 1.6. As the treatment time and plasma power increase, the contact angle decreased from 80.56 ± 1.63 and 79.66 ± 13 for sample treatment at time 30 s and plasma power 60,120W, respectively, to reaching 0° at treatment time of 300 s. For a comparison, I note that Štěpánová et al. [23] presented similar results on natural caw leather plasma treated at different exposure time. Table 1 shows the decrease in contact angle with increasing plasma power and exposure time. This indicates the improved hydrophilic property of the plasma-treated leather samples. The contact angles of polar liquids decrease on Ar-He plasma-treated surfaces. These liquids have polarized covalent bonds and have more ability to create hydrogen bonds. When the contact angles decreased, the wettability of the leather’s surface increased. This result is consistent with the findings of previous studies [4,24,25].

Figure 3. The contact angle of untreated and plasma-treated leather samples at plasma power 60 and 120 W and different treatment time, error bar is the standard deviation.

Table 1. The contact angle and surface free energy of untreated and plasma-treated leather samples.

Plasma power 120 W			Plasma power 60 W
Surface free energy mN/m^2	Contact angle(^o)	Treatment time (s)	Surface free energy mN/m^2	Contact angle(^o)	Treatment time (s)
22.51	83.56 ± 1.63	0	22.51	83.56 ± 1.63	0
25.32	79.66 ± 1.3	30	24.69	80.51 ± 1.4	30
29.29	74.41 ± 1.8	60	27.72	76.45 ± 1.6	60
51.46	47.04 ± 2.6	120	42.72	57.85 ± 2.7	120
56.27	40.68 ± 2.03	180	49.48	49.55 ± 2.5	180
60.17	35.09 ± 2.8	240	56.64	40.17 ± 2.3	240

3.2. Surface free energy measurement

The contact angle results indicated that plasma treatment enhanced the hydrophilicity of the natural leather as their values were less than 90°. Surface free energy γ_S was calculated using Eq. (1). Table 1 shows the surface free energy values of treated and untreated leather samples. The increase in plasma power and treatment time led to an increase in surface hydrophilicity based on the surface free energy results. During surface plasma treatment, there is an increase of polar chemical functional groups, surface energy, and surface roughness, which results in a substantial difference between the surface free energies.

3.3. Absorption ratio measurement

Due to the abundance of hydrophilic groups in collagen, including the amino group (-NH2), carboxyl group (-COOH), and hydroxyl group (-OH), leather is a naturally hydrophilic substance. Therefore, measurements were made to determine the water absorption ratio for plasma-treated and untreated samples. Figure 4 shows the improvement of hydrophilic properties after plasma treatment with Ar-He plasma at 60 W for treatment times of 30, 60, 120, 180, and 300 s. The water absorption ratio increased to 37.3% and 31.01% for the sample treated at 300 s compared to the untreated sample after 2 and 24 h of immersion in water, respectively. Figure 5 depicts the increase in water absorption ratio with the increase in plasma power due to increased surface free energy. Therefore, Ar-He plasma treatment improved the wettability and increase water absorption of leather samples [12, 24].

Figure 4. Water absorption after 2 and 24 h at different treatment time and at plasma power of 60 W.

Figure 5. Water absorption at different treatment times and at plasma power of 120 W.

3.4. SEM measurement

The surface morphology of the leather samples treated by Ar-He plasma were examined using scanning electron microscopy (SEM Model S50, FEI Qunta Inspect, Netherlands) operated at 15 kV.

Figure 6 shows the SEM images of plasma-treated leather surface for various treatment times and plasma power of 60 W. These images revealed that pores are clearly visible on the leather surface. This effect may be caused by surface physical etching and chemical modification introduced by plasma treatment, which increases water permeabilities as plasma treatment time increases. This improvement was observed in the cross-sectional view of the plasma-treated sample and made the leather surface more uniform. These results were supported by similar experiments in previous studies [24,26,27].

Figure 6. SEM image untreated and plasma-treated leather surface at various treatment times and plasma power of 60 W.

3.5. XRD measurement

XRD is a non-destructive method used to analyze the structure of crystalline materials. XRD for the leather surface of the untreated and plasma-treated samples were obtained using Lab XRD-6100 (Shimadzu, Japan; 2 kW, NF, Cu tube, APD 2000 PRO). The 2θ angle was scanned 10–60°, with 0.0200^o and 0.6 s per step.

Figure 7 shows the distinct diffraction peaks at 2θ values of 21.9°, 43.72°, and 50.84° for the untreated sample and peaks at 21.4°, 44.16°, and 51.08° for the plasma-treated sample at 300 s and 60 W. In addition, new peaks were generated due to the functional groups found in plasma, which are caused by the scattering of amorphous collagen. The intensity values of the peaks shown for the untreated samples are greater than that of the treated sample for 300 s by 184.05%. This indicates the effect of plasma on the surface composition of the leather samples [28,29].

Figure 7. X-ray diffraction (XRD) patterns of untreated sample, plasma-treated samples at 30,120 and 300 s and power 60 W.

3.6. DSC measurement

DSC is a thermal analysis method that measures the heat flows into or out of a sample as a function of temperature or time while the sample is exposed to a controlled temperature.

Calorimetric measurements were conducted using the Universal Analysis 2000 of DSC-Q2000 (TA Instruments Co. Germany). The DSC temperature and heat flow were calibrated with In and Pb using the previously described method [30] Prior to DSC measurements, the leather samples were treated with Ar-He plasma at plasma power 60 W for 30 and 300 s. Together with the untreated sample, the leather samples (7 mg) were placed in aluminium pans. In an inert environment of nitrogen gas (30 mL/min), all measurements were made at a heating ratio of 10 °C/min, and all transitions were noted during the second heating scan.

The leather crystallinity was measured by using DSC to quantify the heat associated with its melting (fusion). By comparing the observed heat of fusion to values from the literature, this heat is expressed as a percentage of crystallinity. The equation for calculation of the crystallinity degree of leather is given by: [28]. (3) $\text{Crystallinity \%}=\frac{\mathrm{\Delta }{H}_m-\mathrm{\Delta }{H}_c}{\mathrm{\Delta }{H}_{\mathrm{lit}}}\mathrm{\%},$(3) where $\mathrm{\Delta }{H}_m$ is the melting enthalpy, $\mathrm{\Delta }{H}_c$ is the crystallization enthalpy, and $\mathrm{\Delta }{H}_{\mathrm{lit}}$ the literature value for the melting enthalpy [31,32].

The DSC curves of untreated and plasma-treated samples (60 s and 300 s) are shown in Figure 8. The heat flow decreased as the treatment time increases. The crystallinity of plasma-treated leather samples at 60 and 300 s is higher than that of the untreated sample. This may be due to the elevated temperature of the plasma-treated samples. It was observed that an endothermic peak around 322.01 was related to the degradation of collagen chains that constitute the leather. Table 2 shows the crystallization enthalpy and melting peak temperature of untreated and plasma-treated leather samples. The melting enthalpy for percent crystallinity of plasma-treated samples increased compared to that of the untreated sample. This agrees with the polymer crystallization results of a previous study [7].

Figure 8. DSC curves of untreated and plasma-treated samples at 60 and 300 s and plasma power of 60 W.

Table 2. The melting peak temperature and crystallization enthalpy of untreated and plasma-treated samples 60 and 300 s and plasma power of 60 W.

Sample	Melt peak temperature (°C)	Enthalpy (J/g)	Crystallinity %
S1 untreated	325.76	134.4.0 J/g	46.3
S2 60 s	327.57	136.5 J/g	67.5
S4 300 s	329.56	118.5 J/g	80.7

3.7. FTIR measurement

Fourier transform infrared spectroscopy FTIR is an important technique for studying the spectral characteristics of the treated leather surfaces by classifying the molecular function groups that contribute to the hydrophilic layer. This technique was used to determine the characteristics of natural leather during plasma treatment and their effect on This technique was used to determine the characteristics of the reactive species produced during plasma treatment and their effect on the leather surface properties [33].

The FTIR spectroscopy analysis was conducted using an IRAffinity-1S FTIR spectroscopy produced by Shimadzu Co., USA. A small amount of substance was scraped from the samples to obtain the spectral transmittance in the range of 400–4000 cm⁻¹, with a maximum resolution of 1 cm⁻¹. FTIR spectra were obtained at a steady room temperature of (25 ± 1°C).

The leather powder underwent plasma treatment at two different power levels 60 and 120 W for 300 s. The results were then compared with the FTIR spectra of an untreated sample. As shown in Figure 9, all analyzed leather samples had nearly identical spectra. The peak at wavenumber 2335 cm^-1 shows that the peak that was obtained from untreated leather was fainted, and it is not sharp compared to the sharp peak in the spectra obtained from samples of leather that were treated with Ar-He plasma..

Figure 9. FTIR spectra of untreated and plasma-treated samples at 300 s and plasma power of 60 and 120 W.

It was found that all the spectra of the leather samples were almost the same. This is because leather is made up of the protein collagen, which is the major protein from which leather is formed [34]. The main characteristic peaks of collagen that have the most intense peak in spectra are the collagen characteristic bands centred at 1634 cm⁻¹,1545 cm⁻¹, and 1241 cm⁻¹, which correspond to amide I, amide II, and amide III. The other two main peaks of the collagen structure at 3315 cm⁻¹ and 3073 cm⁻¹ are related to amide A and amide B [34,35].

4. Conclusions

In this paper, low-pressure RF plasma using Ar-He mixture has been employed to improve the surface properties of natural leather. Plasma treatment of the leather surface increased the surface energy and hydrophilic properties of natural leather. Furthermore, the contact angle values on the leather surface decreased with increased plasma power and treatment time. This indicates that the wettability of the leather surface was increased. SEM images showed a more uniform leather surface with increased plasma treatment time. Therefore, it can be concluded that plasma treatment can be used to modify the surface properties of leather. Future work will include more analytical studies on the positive effects of plasma treatment on different types of natural leather and under different physical conditions.

Disclosure statement

No potential conflict of interest was reported by the author.