1 Introduction

The fashion industry is growing faster than ever and the main target of manufacturers and designers is to offer the most up-to-date designs to appease consumer demand by presenting low-cost products. Among textile products, no fabric is as widely accepted as denim [1]. The denim jeans market is expected to reach approximately US$87.4 billion by 2027, up from US$63.5 billion in 2020 [2]. In denim fabrics, only warp threads are dyed using indigo dye. Weft threads are left in their natural color, white. Dyeing methods for denim warp yarn are rope dyeing, beam dyeing and slasher dyeing [3]. Denim garments remain the world’s most polluting textiles, not only because of the indigo dyeing but also because of the amount of chemicals and water it requires to achieve the best washes [4]. Denim washing is an important operation in the ready-made clothing industry to add value to final products. There are many operations available to perform this treatment [5]. Processes such as scouring, bleaching, dyeing, sandblasting and potassium permanganate spray are major causes of environmental concerns. Ozone washing is considered a sustainable chemical process and has many benefits compared to conventional denim washing [6]. Because ozone fades the color of clothes and provides a significant gain from water and chemicals [4]. The application of ozone has several benefits compared to the traditional denim washing. These benefits can be listed as follows;

  1. 1.

    Environmental benefits The ozonation process, especially dry ozonation, is free from the problems of effluent generation and recycling, which reduces environmental pollution. The ozonation process operates at a lower temperature, which saves energy and reduces Green House Gas emissions.

  2. 2.

    Economic benefits The ozonation process saves chemicals, water, and energy, which helps the denim manufacturers to save production cost.

  3. 3.

    Social benefits As the ozonation process reduces the environmental pollution, the society is free from the issues relating to environmental pollution.

  4. 4.

    Other benefits Ozonation process is faster due to rapid color removal by ozone, which saves energy and operational costs [6].

Ozone can be produced artificially by various means, such as corona discharge. Oxygen molecules (O2), which are compressed and collected in a tank that will feed the ozone generator, are broken down by high voltage. “O” atoms form a bond with O2 molecules, causing the formation of a triatomic oxygen molecule (O3) [7]. Molecular ozone is a selective oxidant with a reducing potential of 2.07 V at 25 °C [8]. Therefore, it can attack the glycosidic bond of cotton fiber and cleave the olefinic groups of indigo dye [7]. Ozone gas causes the dyed textile materials to fade by breaking down the chromophores in the dyes as shown in Fig. 1. In fact, ozone decomposition of indigo leads to the formation of isatin. The isatin molecule has a yellowish color. For this reason, during the reaction, the material changes from blue (indigo) to yellow (isatin) [4]. Therefore, post-treatment rinsing is critical to remove any traces of isatin. Ozone is an unstable gas and can react directly or indirectly with the target substance when transferred to water. In the indirect pathway, a promoter catalyzes the breakdown of ozone into radicals. If such promoters are not present, ozone directly oxidizes the target molecules. Under acidic conditions (pH < 4) the direct reaction pathway predominates, while under alkaline conditions (pH > 10) the dominant reaction mechanism is the indirect pathway [8]. In order to obtain the desired efficiency in ozonation, it is very important that ozone dissolves in water [9]. As the temperature increases, the solubility of ozone in water decreases [10], but it cannot be said that ozonation effectiveness decreases as the solubility decreases. Because increasing the temperature increases the reaction rate [11]. On the other hand, temperature has a significant effect on the half-life of ozone. As the temperature increases, the half-life of ozone decreases due to its rapid decomposition and low stability [4].

Fig. 1
figure 1

The chemistry of the ozone interaction with the indigo dyed cotton polymer [4]

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Ozone has various applications in textile. It is used as a process prior to dyeing of cellulosic, protein and synthetic fibers with the aim of bleaching, increasing dye-uptake potential of fiber, etc. Furthermore, it can also be used in washing after reactive dyeing and reductive clearing after disperse dyeing, and in color stripping [12]. The most commonly use of ozone technology is color fading of indigo dyed denim fabrics. There are various studies in the literature on fading the color of indigo dyed denim [1, 4, 7, 8, 13,14,15,16,17].

Ben Hmida et al. [1] found that the main oxidation by-product was isatin (1H-indole-2,3-dione), which formed in excess amounts on wet ozonated denim. Ben Hmida and Ladhari [4] identified the residual products of the indigo oxidation by ozone, the cause of the yellowing observed on the surface of the fabric through spectrophotometric measurements, pH variation and FTIR analysis. Ozone effect on mechanical properties of the fabric was studied through the measurement of ultimate tensile strength. Fraj and Jaouachi [7] applied ozone treatment to denim fabric samples to improve the effect of the washing conditions on the appearance and physical characteristics of the fabrics. Based on the obtained results, ozone treatment increased the whiteness value of treated fabrics. Moreover, it induced certain changes to the physical, mechanical and bagging properties of denim fabrics. Kamppuri et al. [8] observed that ozone was able to bleach the denim jeans and decreased the backstaining without the significant loss of the strength of the fabrics. Sancar Beşen and Balcı [14] investigated the possibility of obtaining of fading effect to indigo dyed denim yarns via ozonation process. Therefore, a novel approach was attempted for fading the denim materials in the form of yarn before weaving or garment processes. He et al. [15] investigated yarns of cotton, lyocell and PET treated by denim color fading ozonation in terms of the effects of time, pH and water content on their physical properties. It is found that, the cellulose in cotton and lyocell yarns has a loss of strength and elongation, while only slight impacts on their crystalline structures are observed. Hu et al. [16] studied three process factors used in the industrial scale color fading process, i.e. (i) ozone feed rate; (ii) moisture content in the fabric; and (iii) treatment time. Additionally, the color fading and tactility of fabric through ozonation were analyzed. Results showed that the three investigated factors have a significant impact on the fading effect, in which the treatment time was the most significant.

In this study, it was aimed to determine the effects of water content, treatment time and pH on both color fading and strength of the fabric during ozone treatment of denim textiles. Thus, optimum conditions of color fading with ozone in industrial applications have been revealed. Although studies conducted on small-scale samples provide insight into the subject, studies conducted under industrial-scale conditions are thought to be very important. Therefore, it is believed that this article, which was conducted directly under industrial-scale production conditions, is original and will contribute to the literature.

2 Materials and Methods

2.1 Materials

In this study, 14.5 oz 3/1 Z twill 100% organic cotton denim fabric, which is frequently used in the production of denim trousers was used. All the experiments were carried out by using soft mill water (0–1°F).

2.2 Ozone Treatment

Ozone treatments were carried out in a Jeanologia G2 ATMOS (Valencia, Spain) machine. The capacity of the ozone generator was 1200 g/h. In this study, water content, time and pH parameters affecting fading degree with ozone treatment were examined.

The treatments were carried out at room temperature due to the reasons explained in introduction part. After treatment the samples were rinsed with warm and cold water. The water content of the fabric was adjusted by spraying water whose pH was adjusted to 4, 7 or 11 on it with E-Flow Lab (Nanobubble micronization: 105–106 bubbles/cm3).

To optimize the ozone treatment, fabrics were treated at different water content values, ozone treatment time and pH of the impregnation liquor. Water content value optimization was tested at 10, 23, 30, 38 and 54% while the pH of the impregnation liquor and ozone treatment time were kept constant at pH 7 and 10 min respectively. The ozonation time was tested at 5, 10 and 15 min. while the pH of the impregnation liquor and water content value were kept constant at pH 7 and 30% respectively. The effect of pH was tested at pH 4 (adjusted with HCOOH), pH 7 and pH 11 (adjusted with NaOH) while the ozonation time and water content value were kept constant at 10 min and 30% respectively.

L* [lightness–darkness value (0: ideal black, 100: ideal white)] values of the fabric samples were measured in the spectral region of 400–700 nm at the maximum absorption wavelength by using X-RITE NIST TRACEABLE (COLOR i 5DV) spectrophotometer (D 65/10°). Then, the color fading values of samples treated with ozone in different conditions were calculated by using the below formula;

$${\text{Color}}\;{\text{fading}}\;{\text{value}}\;\left( \% \right) = \left( {L_{2} - L_{1} } \right)*100/L_{1} ,$$
(1)

L1: L* value of untreated sample; L2: L* value of ozonated sample.

Statistical evaluation of the color fading values (%) was made using Minitab 19 program through analysis of variance (ANOVA).

Fabric samples were also tested for tear strength according to BS EN ISO 13937-1 standard [18] using Elmatear Digital Tear Tester and breaking strength tests according to BS EN ISO ASTM D-5034 standard [19] using James H. Heal Model 510 titan device.

In order to determine the differences in chemical structures of the untreated and ozonated fibers, fabric samples were subjected to Fourier-transform infrared (FTIR) analysis. FTIR spectrophotometer, model Vertex 70 ATR, made by Buriker was used over the range 500–4000 cm−1. The obtained data was used to draw FT-IR spectra by using origin2019b program.

In the denim factory where this study was carried out, an economic comparison of the two methods was also made in case it was desired to obtain similar washing effects with ozone and conventional washing. The required process flows in ozone and conventional washing to obtain similar effects are as follows:

  • Ozone Rinse washing → Rinse washing → Stone washing → Rinse washing → Rinse washing → Centrifuge → Drying → Spraying (Spray 30%) → Ozone → Enzyme → Neutralization → Rinse washing → Centrifuge → Drying.

  • Conventional washing Rinse washing → Rinse washing → Stone washing → Rinse washing → Rinse washing → Bleaching → Neutralization → Antiperoxide enzyme → Rinse washing → Centrifuge → Drying → Rinse washing → Centrifuge → Random → Neutralization → Antiperoxide enzyme → Rinse washing → Centrifuge → Drying.

3 Results and Discussion

3.1 Effect of Water Content, Time and pH on Color Fading Obtained During Ozonation

The results of analysis of variance (ANOVA) regarding the color fading (%) values of denim trousers that were undergone ozonation under different conditions are given in Table 1.

Table 1 Analysis of variance results regarding the effect of various factors on the degree of color fading in ozonation process (df degrees of freedom, Adj SS adjusted sum of squares, Adj MS adjusted mean square, difference is meaningful at the level of \(\alpha =0.05\))
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When Table 1 is examined, it is seen that the effects of water content, time and pH parameters on the degree of color fading during the ozonation process are statistically significant (p < 0.05). Tukey analyzes were also done to determine the source of the difference, and the results are given in Tables 2, 3, and 4.

Table 2 Tukey test results on the effect of water content in ozonation process
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Table 3 Tukey test results on the effect of time in ozonation process
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Table 4 Tukey test results on the effect of pH in ozonation process
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When Table 2 is examined, it is understood that there are 3 groups. It can be said that the best color fading value is obtained at 30% water content. At lower or higher water content values, ozonation efficiency decreases. These results can be explained by assuming that the rate of oxidation is accelerated by the hydration of hydrophilic groups in the fiber and above a critical level of water content (at which fiber is completely hydrated) it is retarded by the water which enters the intermicellar and interfibrillar space. In many previous studies, the importance of water content during ozonation process was reported [20,21,22,23]. If excess water is present, ozone also reacts with this excess water and hence all ozone molecules will not be available for reaction with the reaction site in the fiber [20]. From these results, it can be concluded that the water content of fabric during the ozonation processes has an important effect on color fading. It is recommended to make ozonation of indigo dyed denim fabric at 30% water content value.

When the effect of the time is investigated, it can be said that the ozonation of 5, 10 and 15 min are all in separate groups, so the differences between them are important, and the intensity of color fading increases as the time increases. This is also normal since more dye molecule could be destroyed in longer period of time. Ozonation efficiency is dependent on time as previously stated [23].

When Table 4 is examined, it can be said that there is no significant difference between pH 4 and pH 7, but pH 11 is significantly different from them and the best color fading is obtained at pH 4. This shows that ozonation efficiency decreases in the alkali medium when compared to acidic and neutral mediums. Because in an alkali solution, more hydroxide ions are present which act as an initiator for the decay of ozone [22]. Moreover, it is reported that the dissolved ozone concentration in water decreases from 4.3 × 10−4 mol/L at pH 4 to 1.5 × 10−4 mol/L at pH 10 [21]. Since there is no significant difference between pH 4 and pH 7, it is thought that the pH 7 condition, which is simpler in practice and does not require the use of chemicals, would be optimal. These results are parallel with previous studies [21,22,23].

All these results can be seen from the main interaction plot for color fading given in Fig. 2.

Fig. 2
figure 2

Main effects plot for color fading

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When Fig. 2 is examined, it can be understood that it is most appropriate to work at 30% water content and pH 7, but the time should be changed according to the desired degree of color fading.

3.2 Loss of Tensile and Tearing Strength

The results of the analysis of variance regarding the tensile strength and tearing strength loss in denim textiles that were undergone ozonation under different conditions are given in Tables 5 and 6, respectively.

Table 5 Analysis of variance regarding the effect of various factors on loss of tensile strength in ozonation process
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Table 6 Analysis of variance regarding the effect of various factors on loss of tearing strength in ozonation process
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When Tables 5 and 6 are examined, it is seen that the effects of water content, time and pH parameters on the loss of tensile and tearing strength during the ozonation process are statistically insignificant (p > 0.05). All these results can be seen from the main effects plot for tensile and tearing strength loss given in Figs. 3 and 4, respectively.

Fig. 3
figure 3

Main effects plot for tensile strength loss

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Fig. 4
figure 4

Main effects plot for tearing strength loss

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When Fig. 3 and 4 are examined, it is seen that the strength loss is normally the highest at water content levels (30–38%) where ozone can show its effect best. However, since the differences between the strength losses at different water content levels are insignificant, working at 30% humidity will be advantageous in terms of the best fading performance to be obtained. Similarly, if the time is changed between 5 and 15 min, it can be said that the strength loss gradually increases, but the differences are not significant. In terms of pH, the least strength loss occurred in the process performed at pH 4. However, since the differences between the strength losses occurring in the processes performed at different pHs are statistically insignificant, it can be said that working at pH 7 would be the most appropriate in terms of eliminating the need for chemical use as previously suggested by other authors [22, 23]. As is known, in strongly alkali environments, ozone is decomposed into species such as OH• radical by hydroxyl ion. However, superoxide radicals have a long enough half-life to move in solution and diffuse into the fiber. If these reach the fiber, they will quickly form hydroxyl radicals, which will cause damage to the fibers. If superoxide radicals are eliminated, very small amounts of hydroxyl radicals will be formed. In order to limit superoxide radicals, acid can be added to the medium and the conversion of superoxides into hydrogen peroxide can be supported by the addition of protons [9]. All of these explain why the strength loss in fibers is minimal when the pH is acidic. However, at this point, it should not be forgotten that fibers may also undergo hydrolysis at very low pH (very strongly acidic environment). Prabaharan and Rao [20] also stated that if the pH is acidic, 80% whiteness can be achieved in the shortest time and therefore the strength loss in the fibers will be minimal. The change in the strength of the fibers as a result of the ozonation process depends on the process pH as well as the process time. If the treatment time with ozone gas is extended, the strength loss that will occur increases in proportion to the time.

3.3 FTIR Analysis

FTIR spectra of denim fabrics before and after ozone treatment are illustrated in Fig. 5.

As can be seen from Fig. 5, there is no significant differences between untreated and ozonated samples in the β-glucosidic bond in the cellulose macromolecules which is represented by the peaks at 895 cm−1 as previously stated by Fraj and Jaouachi. Moreover, it was observed that after ozone treatment, the peaks at 3330 and 3280 cm−1, corresponding to hydrogen-bonded OH stretching, underwent a slight modification to 3325 and 3272 cm−1, respectively. This slight modification causes weakening of the hydrogen bonds between the cellulose macromolecules and this causes decrease in the mechanical properties. Also, the slight peak at 1628 cm−1 shifted to around 1618 cm−1 for ozonated sample [7].

Fig. 5
figure 5

FTIR spectra of denim fabrics before and after ozone treatment

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3.4 Economic Analysis

Economic comparison of the ozone and conventional washing is given in Table 7.

Table 7 Economic comparison of the ozone and conventional washing per washing of a single jean
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From Table 7, it is clearly seen that ozone process has a cost advantage over conventional washings since same washing effect can be obtained with less process steps. Also, the water usage is less which is very important in terms of sustainable production.

4 Conclusions

As a clean technology, ozone offers significant opportunities to reduce the environmental burden of the denim industry. An important process step in denim washing is bleaching for the purpose of color fading. Ozone is thought to be an ecologically good alternative to chemicals known to cause adsorbable organic halogens (AOX) problems, such as hypochlorite. The advantages of ozonation can be listed as: minimum loss of strength, being a simple method, water- and chemical-free that is environment friendly, processing low energy costs and short treatment time. Furthermore, the ozonized water after laundering can easily be debosonized by ultraviolet radiation [5]. Nayak et al. [6] represented the results related to the comparison between the traditional and ozonation process in denim manufacturing. Biochemical oxygen demand (mg/L) was 42 for traditional process, while it was 24 for ozonation. Similarly chemical oxygen demand (mg/L) was 122 for traditional process, while it was 78 for ozonation. Furthermore, water and energy consumption and carbon footprint were high for traditional process, while they were low for ozonation.

It is worth noting that the correct setting of the process conditions is very important for an efficient process in ozone bleaching alone. In the light of the findings obtained within the scope of this study, it can be said that the water content of the fabric should be 30% and the process pH should be 7 in order to obtain the best color fading effect with minimum loss in fabric strength, but the processing time should be adjusted according to the desired color fading level.