Surface modification and performance of wool fibers after combined plasma and enzyme treatments

Abstract

This study explores the impact of various surface modification treatments on wool fibers, emphasizing their morphological, chemical, color, and mechanical properties. The treatments included plasma, enzymes (glutaminase and protease), and their combinations. Morphological analysis demonstrated that enzyme and plasma treatments resulted in considerable scale removal and increased surface roughness, with combined treatments causing the most pronounced structural changes. FTIR analysis revealed distinct changes in the chemical structure of the wool fibers, indicating significant surface modifications and crosslinking effects. Colorimetric analysis showed that lightness and whiteness generally increased with enzyme and plasma-enzyme treatments, whereas the application of mordants significantly enhanced color strength. Color fastness tests indicated that plasma-glutaminase treated samples with mordant exhibited superior washing fastness and staining resistance. The mechanical properties analysis revealed that plasma treatment improved fiber strength, whereas glutaminase treatment enhanced flexibility; combined treatments effectively balanced strength and flexibility. Overall, the synergistic effects of combined plasma and enzyme treatments, especially with glutaminase, led to improved surface modification, color properties, and mechanical performance of wool fibers, providing valuable insights for optimizing wool fiber treatments in textile applications.

Introduction

Wool, a protein fiber derived from sheep’s hair, belongs to the alpha-keratin protein family and is characterized by a high sulfur content due to cysteine amino acids. Wool’s structure comprises three main components: the cuticle, cortex, and medulla. The cuticle, an outer layer rich in sulfur-based keratin, forms protective scales that confer hydrophobic properties. These scales act as barriers, inhibiting the penetration of aqueous solutions, dyes, and finishing agents. Additionally, the cuticle serves as a safeguard during textile processing because it is resistant to oxidation, reduction, alkalinity, acidity, and enzymatic actions¹. Innovations in dyeing technologies and treatments are essential for enhancing the quality of natural dye applications in textiles. In recent decades, the extensive exploration of low-temperature dyeing methods for wool fibers has included approaches such as fiber modification, novel dye structures, and innovative dyeing techniques². Among these methods, physical and chemical modifications of wool fibers have gained prominence for their effectiveness in low-temperature dyeing³.

Traditional methods for surface modification of wool fibers, such as chlorination and resin treatment, are widely used to enhance the properties of wool fibers⁴. Chlorination involves the application of chlorine-based chemicals to remove or modify the surface scales of wool fibers, thereby improving their dye uptake and reducing the felting propensity³. However, these methods pose significant environmental and health concerns due to the harsh chemicals. Additionally, the chemical processes can weaken the wool fibers, reducing their durability and overall quality⁵. These drawbacks highlight the urgent need for sustainable and eco-friendly alternatives that can achieve effective surface modification without compromising the environment or fiber integrity.

In response to the environmental and health concerns associated with traditional methods of wool fiber modification, researchers have increasingly turned to more sustainable and eco-friendly alternatives². Plasma treatment, which involves the use of ionized gases to alter the fiber surface, has emerged as a promising technique⁶. The proposed method operates under relatively mild conditions and does not require the use of harmful chemicals, making it an environmentally benign alternative to conventional methods. Studies have demonstrated that plasma treatment can effectively modify the surface of wool fibers, improving properties such as wettability, dyeability, and adhesion without damaging the fiber structure⁷. Enzyme treatment is another eco-friendly approach that has gained attention^8,9. Enzymes, which are biodegradable and specific in their action, offer a natural and gentle means of fiber modification. Proteases, for example, can selectively break down the proteinaceous scales on wool fibers, enhancing dye uptake and reducing felting¹⁰. Research has shown that enzyme treatments can improve the softness and handle of wool fibers, making them suitable for various textile applications⁸.

The primary objective of this research was to evaluate the effectiveness of protease and glutaminase enzymes, both individually and in combination with plasma surface treatment, in the surface modification of wool fibers. This study aims to systematically assess the impact of these treatments on the surface morphology and chemical composition of wool fibers. We employed Scanning Electron Microscopy (SEM) to observe surface structure changes and Fourier transform infrared (FTIR) spectroscopy to detect chemical changes. Additionally, the study investigated the natural dye absorption properties using Madder dye and assessed the color fastness of the dyed samples. This research is significant because it explores sustainable and environmentally friendly alternatives for wool fiber treatment, addressing the limitations and environmental concerns associated with traditional methods. By utilizing protease and glutaminase enzymes, which are biodegradable and specific in their action, along with plasma treatment, which operates under mild conditions without harmful chemicals, this study aims to develop effective and eco-friendly surface modification techniques. The combined use of plasma and enzymatic treatments is particularly promising, potentially offering synergistic benefits that enhance the overall properties of wool fibers (Scheme 1). The findings of this research can lead to improved wool textiles with better dye uptake, color fastness, and surface characteristics, contributing to more sustainable practices in the textile industry.

Scheme 1.

Synergistic surface modification of wool fibers via plasma and enzymatic treatments.

Materials and methods

Materials

The wool fibers used in this study were obtained from Merinos Company and were characterized by an average diameter of approximately 34 μm, with a standard deviation of 6 μm. For surface modification, protease and glutaminase enzymes were sourced from Arnova. Acetic acid, sodium carbonate, and ammonia were purchased from Sigma-Aldrich and used to adjust the pH of the enzyme solutions. Hydrated double-sulfate salt of aluminum (alum) was also obtained from Sigma-Aldrich and used as a mordant in the dyeing process. The natural Madder dye (Rubia tinctorum) was obtained from a local shop and used without further purification.

Plasma treatment

Plasma treatment of wool fibers was conducted using an atmospheric pressure glow discharge device (Basafan Engineering Company, Iran). Wool yarn samples were placed in a plasma machine containing a mixture of argon and oxygen. The treatment was performed under optimized conditions^2,11,12 by adjusting the device to a voltage of 85 V and a frequency of 15 MHz. Each wool sample was treated for 5 min. This plasma treatment aims to modify the surface properties of wool fibers, enhancing their receptivity to subsequent enzyme treatment and dyeing processes. The treated samples are listed in Table 1.

Table 1.

Comprehensive list of abbreviations with descriptions for key terms used in the study.

U	Untraded sample
Pl	Plasma treated sample
Pl-G	Plasma-glutaminase treated sample
Pl-P	Plasma-protease treated sample
P	Protease treated sample
G	Glutaminase treated sample
Pl	Dyed plasma treated sample without mordant
UWM	Dyed untreated sample without mordant
G	Dyed glutaminase treated sample without mordant
Pl-P	Dyed plasma-protease treated sample without mordant
P	Dyed protease treated sample without mordant
Pl-G	Dyed plasma-glutaminase treated sample without mordant
Pl-G-Al	Dyed plasma-glutaminase treated sample with mordant
P-Al	Dyed protease treated sample with mordant
G-Al	Dyed glutaminase treated sample with mordant
Pl-P-Al	Dyed plasma-protease treated sample with mordant
U-Al	Dyed untreated sample with mordant
Pl-Al	Dyed plasma treated sample with mordant

Enzymatic treatment of wool fibers

For the enzyme treatments, both protease and glutaminase enzymes were used. A sodium carbonate solution was prepared and its pH was adjusted to 8.5. The solution was then heated to 37 °C. A 0.5% enzyme (either protease or glutaminase) was added to the alkaline solution, and wool fibers were immersed in it at a liquor ratio of 25:1. The fibers were maintained in the enzyme solution for 1 h to ensure effective surface modification¹³. Following enzyme treatment, the samples were neutralized with an acidic solution adjusted to pH 5 at room temperature for 15 min to ensure complete removal of any residual enzyme activity. The fibers were then allowed to cool and then washed thoroughly with distilled water. This procedure was applied to both untreated and plasma-treated wool fibers to compare the effects of enzyme treatment on different fiber preparations.

Enzymes treatment of wool fibers

For the enzyme treatments, both protease and glutaminase enzymes were used. A sodium carbonate solution was prepared and its pH adjusted to 8.5. This solution was then heated to 37 °C. A concentration of 0.5% enzyme (either protease or glutaminase) was added to the alkaline solution, and wool fibers were immersed in it at a liquor ratio of 25:1. The fibers were maintained in the enzyme solution for 1 h to ensure effective surface modification¹³. Following the enzyme treatment, the samples were neutralized with an acidic solution adjusted to pH 5 at room temperature for 15 min to ensure complete removal of any residual enzyme activity. After that, the fibers were allowed to cool and then washed thoroughly with distilled water. This procedure was applied to both untreated wool fibers and plasma-treated wool fibers to compare the effects of the enzyme treatments on different fiber preparations.

FTIR assay

FTIR spectroscopy was used to analyze the chemical changes in wool fibers before and after various treatments, including enzyme-treated, plasma-treated, and plasma-enzyme-treated samples. Samples of untreated and treated wool were examined using an FTIR spectrometer (Bruker Tensor 27). The spectra were collected over a range of 4000 to 400 cm⁻¹ with a resolution of 4 cm⁻¹. For the analysis, the wool samples were ground into a fine powder and combined with potassium bromide (KBr) to create pellets suitable for FTIR spectroscopy. This method allowed for a detailed examination of the chemical alterations induced by the treatments, providing insights into the modifications at the molecular level.

SEM analysis

The SEM analysis was conducted using an electron microscope (KyKy, EM-8000 F). Small sections of wool fiber were attached on carbon adhesive tabs and coated with a thin gold layer to enhance the conductivity and imaging quality. The SEM images were captured at an accelerating voltage of 5 kV. For a comprehensive analysis, both untreated wool samples and those treated with enzyme, plasma, and combined plasma-enzyme treatments were imaged at magnifications ranging from 100x to 5000x. This detailed imaging enabled the observation of surface morphological changes resulting from different treatments.

Dyeing wool yarn using madder

The dyeing process of the wool yarn was conducted using a pre-mordanting method. Initially, the wool samples were mordanted with 5% alum on the weight of the fiber (owf) and 3% acetic acid. The wool was immersed in a mordanting bath with a liquor ratio (L: G) of 40:1. The bath temperature was initially 40 °C and was gradually raised to boiling over 25 min. The mordanting process was continued for an additional 45 min (2 °C/min). After mordanting, the wool samples were thoroughly washed with distilled water. Following mordanting, the wool samples were dyed with 100% of madder and 3% acetic acid. The pre-mordanted wool samples were immersed in a dye bath with a liquor ratio (L: G) of 40:1. The bath temperature was initially set to 50 °C and then gradually raised to boiling for 30 min (1.5 °C/min). The dyeing process was continued for another 60 min. After dyeing, the wool samples were thoroughly washed with distilled water to remove any unbound dye.

Color assessment and fastness evaluation

The dye absorption and color strength of the dyed wool samples were assessed using the K/S value, which was calculated using the Kubelka-Munk equation:

1

In this equation, K represents the absorption coefficient, S is the scattering coefficient, and R is the minimum reflectance value, which corresponds to the highest absorption. Both S and K are approximately proportional to the concentration of each dye in the sample. To evaluate the color characteristics, the dyed yarn samples were evenly wrapped on cardboard and analyzed for L*, a*, b∗, ℎ, and C^∗ values using a reflective spectrophotometer under a D₆₅ standard light source. Measurements were conducted with a CIE standard observer at a 10-degree viewing angle, and the color components and reflection spectra were recorded across the 400–700 nm range.

The color fastness properties of the dyed wool yarns were assessed using ISO standard testing methods. Specifically, ISO 105-C02:1989 was used to evaluate the color fastness to washing, while ISO 105-B02:1994 was used to assess the color fastness to light. In addition, to ensure a thorough comparison, the color characteristics of the dyed wool yarns were measured before and after the fastness tests in triplicate, ensuring the reliability and reproducibility of the data. Color differences were quantified using the following equation:

2

In the context of color fastness evaluation, the values ΔL*, Δa*, and Δb* quantify the differences in color coordinates before and after the fastness tests. ΔL* indicates the difference in lightness (L*), where positive values signify an increase in lightness (fading) and negative values indicate darkening. Δa* reflects the change in the red-green chromaticity coordinate (a*), with positive values indicating a shift toward red and negative values toward green. Similarly, Δb* measures the difference in the yellow-blue chromaticity coordinate (b*), where positive and negative values denote a shift toward yellow and blue, respectively. These values were used to calculate the overall color difference (ΔE) and to assess the extent of color change during fastness tests. Lower ΔE values generally indicate better color fastness and minimal color change, whereas higher values indicate greater color variation.

The tensile strength of wool yarn

The mechanical properties and breaking resistance of the dyed wool yarn samples were examined by evaluating their tensile strength. This evaluation was carried out using a specific tensile testing machine (Kardotex Co., Iran) following the ASTM D 2256 standard. Each yarn sample was clamped between the grips of the machine with a gauge length of 10 cm. A consistent crosshead speed of 25 cm/min was maintained during the test until the sample reached its breaking point. The maximum force needed to break the yarn was measured in centinewton (cN). To determine the tensile strength, the maximum force recorded was divided by the linear density (Tex) of the unstressed yarn. The ultimate tensile strength was calculated by averaging the outcomes of at least 10 repeated trials.

Results and discussion

Morphological analysis of treated wool

Morphological analysis of the treated wool fibers was conducted using Scanning Electron Microscopy (SEM). Figure 1 illustrates the effects of different treatments, including enzyme-treated samples, plasma-treated samples, and plasma-enzyme-treated samples. The SEM images of the raw wool sample reveal fiber strands with a smooth, polished surface. The presence of intact scales, which play a protective role and influence the dyeing properties, is clearly visible. These scales are critical for determining the hydrophobic nature of wool and its resistance to environmental factors^2,3,14. In contrast, all treated samples exhibited significant structural changes. The treatment results in the disruption of the edges of the fibers, scale breaking, and formation of grooves on the fiber surface^15–18. Among the various treatments, protease and plasma-protease treatment resulted in the highest scale removal, indicating substantial surface modification^19,20. Plasma treatment, on the other hand, resulted in the lowest disruption on the surface of wool fibers, with a milder effect compared to protease treatment. The combination of plasma and protease treatment resulted in the highest surface disruption and scale removal, indicating a synergistic effect of both treatments. The synergistic effect observed in the combination of plasma and protease treatment is attributable to the complementary actions of both processes, each enhancing the efficacy of the other. Plasma treatment primarily involves introducing reactive species, such as radicals and ions, to the fiber surface²¹. The argon-oxygen plasma generates high-energy particles that bombard the wool fibers, leading to surface etching and microscale roughness¹². Additionally, plasma treatment increases surface hydrophilicity by introducing oxygen-containing functional groups, such as hydroxyl, carbonyl, and carboxyl groups²². This change in surface chemistry enhances the wettability of fibers, rendering them more sensitive to aqueous solutions and subsequent treatments. Protease treatment, however, involves the selective hydrolysis of peptide bonds (Scheme 2) in the keratin structure of wool fibers²³. The protease specifically targets these bonds, leading to the removal of scales and exposing the underlying cortex¹⁰. Due to the increased hydrophilicity and surface roughness created by plasma treatment²⁴, protease can penetrate deeper and act more efficiently. The initial etching and introduction of functional groups by plasma treatment²¹ allow enzymes to access more of the fiber surface area and internal structures.

Fig. 1.

SEM images of wool fibers illustrating the morphological changes induced by enzyme treatments, plasma treatments, and combined plasma-enzyme treatments.

Scheme 2.

Mechanism of protease-mediated peptide bond cleavage in wool fiber keratin, illustrating the enzymatic hydrolysis process that targets specific peptide bonds in the keratin structure.

In the combined plasma-protease treatment, plasma treatment disrupts the tightly bound scales on the wool surface, creating pathways for the enzyme to penetrate more effectively. This pre-treatment effectively “opens up” the fiber, making it more susceptible to enzymatic action. The roughened surface and increased hydrophilicity enhance the binding and activity of proteases, allowing them to work more efficiently in breaking down keratin. This leads to more extensive removal and surface modification than either treatment alone. Physical etching by plasma treatment and biochemical hydrolysis by protease act in a complementary manner, creating a more reactive and accessible surface for the enzymes to perform precise and selective modifications. Together, they achieve a level of surface disruption and modification that neither can accomplish independently. The observations in our study support this proposed mechanism, as plasma-protease-treated samples exhibited the highest levels of surface disruption and scale removal. This combined approach influences the strengths of both treatments, leading to superior fiber modification and enhanced performance for dye absorption and other applications.

Glutaminase treatment also resulted in significant morphological changes in the wool fibers as well. The enzyme primarily catalyzes the acyl transfer between peptide-bound glutamine residues and various primary amines, leading to protein crosslinking (Scheme 3). This crosslinking increases the structural integrity of wool fibers, leading to a smoother surface appearance compared to untreated samples and introducing slight surface roughness. Unlike protease treatments, which focus on breaking down fiber cuticles, glutaminase treatment reinforces fiber structures, and minimizes scale disruption. In agreement with literature findings, our results show that glutaminase-treated samples exhibit improved surface crosslinking and minimal scale disruption.

Scheme 3.

Suggested reaction mechanism for glutaminase treatment on wool fiber, demonstrating the enzymatic conversion of glutamine residues into glutamic acid.

FTIR results of treated wool Fiber

FTIR spectroscopy was used to analyze the chemical changes in untreated, enzyme-treated, plasma-treated, and plasma-enzyme-treated wool fibers (Fig. 2). For untreated wool fibers, characteristic absorption bands were observed at 3550 cm⁻¹ (N-H stretching), 1640 cm⁻¹ (C = O stretching), 1500 cm⁻¹ (N-H bending), and 1250 cm⁻¹ (C-N stretching). Peaks around 1015 and 1065 cm⁻¹ were also observed, which are associated with C-O stretching in carbohydrates and glycoproteins²⁵. The FTIR spectra were normalized on the basis of the 1255 cm⁻¹ peak intensity to ensure an accurate comparison between the untreated and treated wool fibers. The peaks corresponding to C-H stretching vibrations (2900 cm⁻¹) and O-H stretching vibrations (3550 cm⁻¹) showed distinct variations across the different treatments. Glutaminase-treated wool fibers exhibited higher peak intensities at both 2900 cm⁻¹ and 3550 cm⁻¹ compared to the untreated sample. This increase (Fig. 2a) can be attributed to the exposure of additional aliphatic C-H bonds and the formation of new hydroxyl groups as a result of enzymatic activity. Conversely, protease and plasma treatments resulted in lower peak intensities at 2900 cm⁻¹, demonstrating as shoulders, and a shift with reduced intensity for the peak at 3620 cm⁻¹, indicating partial removal or modification of surface lipids and proteins.

Fig. 2.

Shows the complete FTIR spectra analysis of enzyme-treated wool fiber, plasma-treated wool fiber, and wool fiber treated with both enzyme and plasma. The spectra are displayed within the wavenumber range of 4000 to 500 cm⁻¹, and specific wavenumber ranges are zoomed in for comparison: (a) 2500–3750 cm⁻¹, (b) 1350–1800 cm⁻¹, and (c) 900–1100 cm⁻¹.

The amide I band at 1640 cm⁻¹, which corresponds to the C = O stretching vibration in proteins, exhibited a higher peak intensity in glutaminase-treated wool fibers compared with the untreated sample (Fig. 2b). This increase reflects the crosslinking activity of glutaminase, which enhances the structural integrity of wool fibers by forming additional peptide bonds. This observation is consistent with findings in the literature^26,27, where transglutaminase enzymes facilitate protein crosslinking, thereby increasing the stability and strength of wool fibers. The intensity of the carbonyl peak at 1750 cm⁻¹ decreased in the plasma- and protease-treated samples compared with the untreated wool fibers (Fig. 2b). This reduction indicates the partial degradation or removal of surface lipids and fatty acids, which is commonly observed in treatments aimed at increasing fiber hydrophilicity and dye absorption. Previous studies^10,26–30 have reported similar findings, in which enzyme treatments lead to the removal of fatty acids and enhance the dyeing properties of wool fibers.

The characteristic sulfoxide peaks at 1015 cm⁻¹ and 1065 cm⁻¹ showed a notable increase in absorbance intensity for the plasma, protease, and plasma-protease-treated samples compared with the untreated wool fibers (Fig. 2c). This heightened absorbance indicates the successful oxidation of sulfur-containing groups within the wool fibers. Studies^7,12,21,31 have shown that treatments that disrupt disulfide bonds in wool fibers often lead to the formation of sulfoxides, indicating an increase in fiber reactivity and dye uptake properties. These observations are in line with studies that highlighted the role of enzymes in modifying the surface characteristics and chemical composition of wool fibers.

Lightness value and whiteness index of treated samples

The lightness (L^*) and chromaticity (a^* and b^*) values for the treated wool samples are presented in Fig. 3a and b. Plasma treatment alone slightly decreases the lightness, suggesting a possible surface modification effect that reduces reflectance. In contrast, glutaminase treatment alone increased the lightness value to 85.33, indicating the effective removal of surface impurities and fatty acids, which enhanced the fiber brightness. The combination treatment yielded notable results. The glutaminase plasma-treated sample had a lightness value of 83.53, which was slightly higher than that of the untreated sample, indicating an additive effect of both treatments. Protease treatments demonstrated the highest lightness values, with the protease-treated sample reaching 86.81 and the protease-plasma-treated sample achieving 85.49. These results confirm that protease treatments are particularly effective at cleaning the wool surface and removing scales, resulting in higher reflectance and brightness.

Fig. 3.

Impact of different treatment methods on the colorimetric properties of wool samples: (a) changes in lightness (L*) of untreated, plasma-treated, enzyme-treated, and plasma-enzyme-treated samples, (b) comparative analysis of color characteristics (H*, and C*) for treated and dyed wool samples, and (c) color strength (K/S) values of dyed wool samples, highlighting the influence of surface modification on dye uptake and color properties. Error bars represent standard deviations based on triplicate measurements.

The Whiteness Index (WI) for each sample was calculated using the CIELAB coordinates, with the untreated sample having a WI of 74.94, the plasma-treated sample at 73.72, the glutaminase-plasma-treated sample at 75.17, the protease-plasma-treated sample at 76.87, the protease-treated sample at 78.24, and the glutaminase-treated sample at 76.62. The reduction in the lightness and whiteness index for the plasma-treated sample is consistent with the findings of Zhang et al.³², where plasma treatments caused slight surface etching and increased roughness, leading to reduced reflectance. Although the exact cause of the reduction in whiteness remains unclear, it is highly likely that the decrease in the whiteness index of the fibers is due to plasma treatment inducing oxidation on the fiber surface. The slight increase in lightness and whiteness index for glutaminase-plasma-treated samples (WI = 75.17) compared with untreated samples (WI = 74.94) indicates an additive effect of both treatments. This is consistent with the findings of previous studies^30,33, which indicated that enzymatic treatments enhance fiber brightness by eliminating impurities and improving reflectance. The significant increase in lightness and whiteness index for both protease-treated (WI = 78.24) and protease-plasma-treated samples (WI = 76.87) is in agreement with findings from previous research²⁹. Protease treatments effectively break down protein chains and remove surface scales, resulting in enhanced brightness. The higher lightness and whiteness index observed in the glutaminase-treated sample (WI = 76.62) is consistent with studies that emphasize the role of glutaminase in eliminating impurities and improving fiber brightness, thus reinforcing the conclusions of prior research^34,35.

Effects of different treatment methods on color strength

The color strength (K/S value), hue, and chromaticity (a and b) of the dyed wool samples, both with and without mordant, were analyzed to determine the effect of various treatment methods. The untreated wool sample without mordant exhibited a K/S value of 11.1 and a hue of 41.23 (Fig. 3c). Plasma treatment increased the K/S value to 11.7 with a hue of 42.56, indicating improved dye uptake, likely due to the increased surface area after etching. Plasma treatment enhances the surface morphology of wool fibers by increasing roughness and specific surface area¹², which improves dye absorption and penetration. This treatment also enhances hydrophilicity^21,24, allowing for better moisture retention and more uniform dye uptake. As a result, plasma-treated wool exhibits greater color strength while reducing the dye concentration needed in the dye bath, contributing to a more sustainable dyeing process. The glutaminase-treated sample without mordant had a K/S value of 11.1, similar to the untreated sample, with a hue of 40.52, indicating minimal impact on dye uptake. This is consistent with previous studies^26,27, which indicated that glutaminase treatment can improve dye uptake, particularly in the presence of mordants. Protease treatment elevated the K/S value to 12.01 and resulted in a hue of 41.45, indicating increased uptake of dye by exposing additional dye sites, which is in line with the findings of²⁸. Protease treatment enhances dye uptake in wool fibers through two main mechanisms. First, keratin hydrolysis occurs as proteases, degrade the keratin structure of wool, allowing for better penetration of dye molecules into the fiber¹⁰. Second, surface modification happens as protease treatment removes the cuticle scales³⁶, resulting in a smoother surface that improves dye bonding to the fiber. Together, these processes significantly increase the dye absorption and color strength in treated wool. The protease-plasma-treated sample had a K/S value of 10.3, and a hue of 41.45, indicating a decrease in color strength compared with protease treatment alone. The glutaminase-plasma-treated sample had the lowest K/S value of 8.6, with a hue of 41.00, indicating excessive surface modification reduced dye affinity.

The only mordant-treated sample had a K/S value of 15.1 and a hue of 46.82, indicating significant enhancement in dye uptake. The mechanism of dye-mordant interaction involves several key processes. First, aluminum ions from Alum form coordination complexes with anthraquinone compounds in Madder, which enhances dye uptake on wool fibers³⁷. Additionally, the presence of aluminum ions modifies the wool’s surface, increasing its affinity for dye molecules and leading to improved color saturation³⁸. Plasma treatment with mordant further increased the K/S value to 17.7 with a hue of 47.46. Glutaminase treatment with mordant significantly increased the K/S value to 18.2, with a hue of 38.21, demonstrating improved dye binding, consistent with the literature. Protease treatment with mordant resulted in a K/S value of 18.6, a hue of 46.49, and high color strength due to effective dye site exposure. The protease-plasma-treated mordant-treated sample had a K/S value of 19.8, a hue of 43.85, demonstrating synergistic effects of combined treatments. The glutaminase-plasma-treated mordant-treated sample exhibited the highest K/S value of 20.3, with a hue of 33.21, indicating superior dye uptake. The use of mordants significantly improved dye uptake across all treatments, with the combined glutaminase-plasma treatment showing the most substantial effect, demonstrating the potential for superior dyeing performance on wool fibers.

Effect of surface modification method on color fastness

The evaluation of color fastness properties, including washing fastness, staining, and light fastness, of dyed wool samples treated with various surface modification methods revealed distinct trends and significant differences in performance (Table 2). Washing fastness results showed that enzyme treatments generally improved this property.

Table 2.

Color fastness evaluation of dyed wool samples subjected to various surface modification methods, including plasma, enzyme, and combined plasma-enzyme treatments.

Sample	Washing fastness		Staining				Light fastness
			Wool		Cotton
	ΔE	Gray scale	ΔE	Gray scale	ΔE	Gray scale	ΔE	Blue scale
Pl	13.73	4	18.75	3–4	3.60	4–5	1.26	6–7
U	14.84	3–4	19.29	3	5.56	4–5	1.56	6–7
G	12.61	4–5	31.55	4	6.26	4–5	2.20	6–7
Pl-P	14.31	4–5	19.22	3–4	4.00	4–5	1.98	6–7
P	13.40	4–5	33.62	3–4	6.79	4–5	2.26	6–7
Pl-G	12.11	4–5	30.49	4	7.58	4–5	1.79	6–7
Pl-G-Al	4.25	4–5	15.00	3–4	1.81	4–5	1.98	6–7
P-Al	10.70	3–4	18.79	3	2.77	4–5	0.96	6–7
G-Al	2.77	4–5	16.41	3–4	2.32	4–5	0.81	6–7
Pl-P-Al	12.69	3–4	16.84	3–4	2.72	4–5	1.13	6–7
U-Al	11.81	3	17.32	3–4	2.41	4–5	0.79	6–7
Pl-Al	12.35	3	19.53	3	2.68	4–5	4.96	6–7

The mechanism of enzyme treatments in improving washing fastness involves the enzymatic breakdown of complex molecules, which enhances dye fixation and reduces color loss during washing. Enzymes such as proteases hydrolyze dye molecules, making them more responsive to fixation on fibers. For instance, protease treatment in wool dyed with natural alizarin significantly improves washing fastness³⁹. Without mordant, glutaminase-treated samples exhibited the best washing fastness, with lower ΔE values and higher gray scale ratings, compared to untreated and plasma-treated samples. Treatment with protease and plasma protease also showed improvement, but to a lesser extent. When mordant was applied, all treatments significantly enhanced washing fastness, with the glutaminase-treated samples exhibiting the best performance. These findings are consistent with literature^40–42, which indicates that enzyme treatments, especially with glutaminase, can improve the washing fastness by modifying the fiber surface, making it more resistant to washing processes. Staining of wool and cotton revealed that the untreated samples had the highest ΔE values, indicating the most staining. Plasma and enzyme treatments, particularly glutaminase, improved staining resistance in both wool and cotton. The application of mordant further enhanced staining resistance across all treatments, with glutaminase- and plasma-glutaminase-treated samples showing the best performance. The mechanism of alum mordant in improving washing fastness primarily involves enhancing the bonding between dye molecules and fibers, which results in increased color retention during washing processes. Alum acts as a mordant by forming coordination complexes with dye molecules, effectively anchoring the dye to the fabric fibers³⁷. Also, the alum-dye-fiber complex proves to be more resistant to washing, thereby reducing the possibility of dye leaching during laundering⁴³.

Light fastness, as evaluated using blue scale ratings, remained stable across all treatments, both with and without mordant. All samples maintained high blue-scale ratings, indicating good light fastness. Overall, enzyme treatments, particularly glutaminase, showed the most significant improvements in washing fastness and staining resistance. The addition of mordant further enhances these properties, supporting findings from various studies that highlight the benefits of combined surface modification and mordanting techniques in improving the color fastness of dyed wool fibers.

Mechanical properties of dyed wool yarn

The mechanical properties of the wool samples, specifically maximum stress (cN/Tex) and strain at break (%), were evaluated after applying various surface modification treatments. Tukey’s HSD analysis was conducted to assess the statistical significance of the differences among the treatment methods. The results revealed distinct treatment groupings into homogeneous subsets, providing further insights into observed trends.

Plasma-treated wool exhibited the highest maximum stress value, as shown in Table 3, and was confirmed by Tukey’s analysis (Table 4), which placed it in the second subset al.ong with the untreated sample. The mean stress value for plasma-treated wool was 8.1 cN/Tex, which was slightly higher than that of the untreated sample (8.0 cN/Tex), and the difference between these two treatments was statistically insignificant (Sig. = 0.998). Plasma treatment induces surface etching, increasing fiber surface roughness, which enhances inter-fiber friction and improves mechanical interlocking between fibers, contributing to higher tensile strength and stiffness^2,11,12.

Table 3.

Mechanical properties of dyed wool samples subjected to various surface modification methods, including plasma, enzyme, and combined plasma-enzyme treatments.

Sample	Tenacity (cN/tex)	Breaking extension (%)
U	8.0 ± 0.4	15.7 ± 1.6
Pl	8.1 ± 0.3	15.6 ± 1.0
P	7.1 ± 0.3	15.9 ± 1.5
G	7.2 ± 0.4	17.2 ± 1.2
Pl- P	7.0 ± 0.48	16.1 ± 1.4
Pl- G	7.3 ± 0.53	16.7 ± 1.8

Table 4.

Tukey HSD analysis of tensile properties in dyed wool samples subjected to various surface modification methods, including plasma, enzyme, and combined plasma-enzyme treatments.

Treatment method	N	Subset for alpha = 0.05
		1	2
Plasma-protease	10	7.0
Protease	10	7.1
Glutaminase	10	7.2
Plasma-glutaminase	10	7.3
Plasma	10		8.1
Untreated	10		8.0
Sig.		0.964	0.998

Means for groups in homogeneous subsets are displayed.

Uses Harmonic Mean Sample Size = 10.

Protease-treated wool, both alone and in combination with plasma, demonstrated the lowest maximum stress values (7.1 cN/Tex and 7.0 cN/Tex, respectively), belonging to the first subset of the Tukey analysis. This indicates a significant reduction in fiber strength compared to untreated and plasma-treated samples. These results are consistent with the literature, as protease treatments disrupt the structural integrity of wool fibers more extensively because of their aggressive enzymatic action.

Glutaminase-treated wool had a mean maximum stress of 7.2 cN/Tex, which was slightly higher than that of protease-treated samples but significantly lower than that of plasma-treated and untreated wool. However, when combined with plasma treatment, the stress level increased to 7.3 cN/Tex, indicating a synergistic effect between plasma and glutaminase treatment. This result aligns with previous findings^44–46, which showed that plasma can diminish the loss of strength caused by enzyme treatment by enhancing the fiber surface’s mechanical integrity.

Regarding strain at break (%), glutaminase-treated wool had the highest value, indicating improved fiber flexibility compared with the other treatments. This result is consistent with literature findings that enzyme treatments enhance fiber flexibility by partially hydrolyzing surface proteins, allowing for greater elongation⁴⁷. The protease-treated samples exhibited slightly improved flexibility despite their lower tensile strength.

The untreated and plasma-treated wool samples had similar strain at break values similar to one another, with no significant differences observed between the two groups. This indicates that plasma treatment primarily modifies surface properties without extensively altering bulk extensibility, thereby maintaining fiber flexibility.

In summary, plasma treatment enhances fiber strength without significantly affecting flexibility, whereas enzyme treatments, particularly with glutaminase, improve fiber flexibility at the expense of tensile strength. Tukey’s HSD analysis confirms these observations, showing that plasma and untreated wool belong to a statistically distinct group with higher maximum stress values, whereas enzyme-treated samples fall into a lower strength subset. Combined plasma and enzyme treatment offers a balanced approach, slightly improving both strength and flexibility. These findings emphasize the trade-offs and synergies between surface modification methods and their influence on wool fiber mechanical properties.

Conclusion

This study investigated the effects of various surface modification treatments on wool fibers, focusing on their morphological, chemical, color, and mechanical properties. Treatments included plasma, enzyme (glutaminase and protease), and combined plasma-enzyme applications. FTIR analysis revealed that plasma treatment increased the peak intensity at 1630 cm⁻¹, indicating the introduction of new functional groups. Glutaminase treatment specifically demonstrated a crosslinking effect, as evidenced by changes in the FTIR spectra, which indicated the formation of new bonds and interactions between amino acid residues in the wool fibers. Combined plasma-enzyme treatment exhibited unique spectral changes, reflecting synergistic effects on the wool fiber surface. Morphological analysis showed that the untreated wool fibers had a smooth surface with intact scales. Enzyme treatments, especially protease, resulted in significant surface roughness and removal of the scaffold. Plasma and plasma-enzyme treatments caused the most pronounced structural changes, indicating enhanced surface etching and disruption. Color properties varied with treatment, with enzyme and plasma-enzyme treatments generally increasing lightness and whiteness compared with untreated samples. Mordant application significantly improved color strength, particularly in plasma-glutaminase-treated samples. In terms of color fastness, mordant-treated plasma-glutaminase-treated samples exhibited the best washing fastness and staining resistance, indicating enhanced dye retention and stability. Enzyme and plasma treatments without mordant showed moderate improvements in color fastness compared with untreated samples.

The mechanical properties analysis indicated that plasma treatment enhanced fiber strength without significantly affecting flexibility (strain at break). Glutaminase treatment improved fiber flexibility but slightly reduced fiber strength. Combined plasma-enzyme treatment offered balanced improvements in strength and flexibility.

Overall, combined plasma and enzyme treatments, particularly with glutaminase, demonstrated synergistic effects, enhancing the surface modification, color properties, and mechanical performance of wool fibers. These findings provide valuable insights into optimizing wool-fiber treatment for improved performance in textile applications.

Author contributions

In this study, the research team collaborated effectively to achieve their objectives. Rahele Ghasemian conducted experiments that ensured high-quality data collection, while project lead Hossein Barani designed the project, developed the methodology, and guided the manuscript writing. Faezeh Khazaei contributed essential expertise in color assessments. All authors played significant roles in the research and collectively approved the final manuscript.

Funding

This work was supported by University of Birjand.

Data availability

Availability of data and materials: The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.