Optimizing Ozone-Based Pre-Treatment as a Sustainable Alternative to Conventional Bleaching: A Foundation to Achieve Uniform and High-Depth Colour in Textiles

Abstract

Conventional cotton bleaching relies on hydrogen peroxide under high temperatures and alkaline conditions, leading to high water and energy consumption. This study evaluates ozone as a sustainable alternative oxidizing agent to improve process efficiency. An ozone-based process was investigated by analyzing the influence of pH, fabric moisture content, and chemical additives, including stabilizers and surfactants. Performance was assessed using CIELab coordinates together with evaluation of substrate integrity through degree of polymerization to ensure suitability for subsequent dyeing. Results indicate that bath composition is critical, with both acidic and alkaline media outperforming neutral conditions. Fabric moisture was identified as a key parameter, where periodic renewal of the impregnation bath significantly enhanced bleaching efficiency. Under optimal conditions, the process achieved notable bleaching levels within short treatment times and with low energy requirements. These findings demonstrate that ozone bleaching represents a promising, energy-efficient alternative for cotton pre-treatment, capable of providing substrates suitable for high-quality and sustainable textile colouration.

1. Introduction

The global textile industry is currently undergoing a critical transition towards more sustainable production models. Driven by stringent environmental regulations and a growing awareness of ecological footprints, there is an urgent need to align textile processing with sustainable development goals. Wet processing stages, in particular, are notorious for their intensive use of water, energy, and chemical resources [1,2]. Among these wet processes, bleaching represents a fundamental pre-treatment stage. It is essential for removing natural and artificial impurities [3], thereby providing the necessary substrate baseline for uniform and reproducible dyeing. However, it is also one of the most resource-demanding operations. As illustrated in Figure 1, bleaching is the second most resource-intensive stage in textile production, accounting for a substantial fraction of the total water, energy, and chemical demands of the entire manufacturing chain.

Figure 1. Quantification of the impact of the textile processes of bleaching, dyeing, washing, finishing and drying in terms of consumption of chemicals, energy and water [4].

Given this intensive consumption, the bleaching operation presents a clear and necessary opportunity for process optimization. To meet current sustainability metrics, it is imperative to explore alternative methodologies and implement improvements that drastically reduce environmental impact, all while maintaining industrial whiteness standards, typically measured with the whiteness index established by the standard UNE-EN ISO 105-J02 [5]. Dyeing and finishing require a stable and uniform background to obtain high-quality results.

Currently, the most widespread industrial standard for cotton bleaching relies on hydrogen peroxide (H₂O₂) as the primary oxidizing agent, catalyzed by sodium hydroxide (NaOH) at elevated temperatures (typically between 90 and 98 °C) [2,6]. The principal reaction mechanism involves the activation of H₂O₂ in an alkaline medium, where it dissociates to form the perhydroxyl anion (HO₂⁻), a strong nucleophile responsible for delignification (1).

H₂O₂ + OH⁻ ↔ HO₂⁻ + H₂O

(1)

This active species can subsequently recombine with the remaining peroxide, leading to decomposition reactions (2).

H₂O₂ + HO₂⁻ ↔ O₂↑ + H₂O + OH⁻

(2)

While these reactive species effectively remove impurities, their high reactivity lacks strict selectivity. Consequently, the cellulose polymer is also susceptible to oxidative attack, leading to β-elimination reactions that result in chain scission and a physical loss of fabric tenacity. To control this inherent instability and prevent runaway decomposition (often catalyzed by trace metal ions) the conventional process requires a complex and costly mixture of additives, including stabilizers, surfactants, and chelating agents [7].

In the search for greener and more efficient alternatives, ozone (O₃) has emerged as a highly promising oxidizing agent. Ozone possesses a significantly higher redox potential (2.07 V vs. the standard hydrogen electrode) compared to H₂O₂ (1.78 V) and other traditionally used oxidants such as sodium hypochlorite (NaClO) (1.49 V), enabling powerful oxidation at much lower temperatures [8,9,10,11]. Furthermore, ozone naturally decomposes into molecular oxygen (O₂), leaving no hazardous chemical residues in the effluent. The main mechanism that occurs in alkaline media is proposed by Prahabaran and Rao [12]. They claim that bleaching occurs either directly with ozone molecules or with several intermediate species generated by radical decomposition in three main phases. Phase 1 comprises the initiation step, where the ozone reacts directly with hydroxyl ions in the alkaline processing bath, acting as the primary trigger to generate the hydroperoxide (HO₂⁻) precursor (3).

O₃ + OH⁻ → HO₂⁻ + O₂

(3)

Then, during phase 2, radical chain propagation starts. The conjugate base hydroperoxide quickly reacts with a second ozone molecule to yield ozonide and hydroperoxyl radical intermediates, initiating a self-sustaining radical cascade (4).

HO₂⁻ + O₃ → O₃⁻+ HO₂^•

(4)

Sequentially, the deprotonation of the hydroperoxyl radical occurs in the alkaline matrix to yield the oxide anion (5).

HO₂^• + OH⁻ → O₂⁻ + H₂O

(5)

The propagation via direct hydroxyl radical interaction with ozone maintains the chain length and cyclic renewal of precursors (6).

^•OH + O₃ → HO₂^• + O₂

(6)

Finally, in phase 3, inter-radical interaction and chain termination occur. The competitive radical–radical interactions dictate the efficiency of the bleaching process. These are bifurcated into active propagation versus complete termination paths, chain evolution (7), and recombination or inhibition (8).

O₃⁻ + ^•OH → O₂⁻ + HO₂^•

(7)

O₃⁻ + ^•OH → O₃ + OH⁻

(8)

Furthermore, several laboratory- and semi-industrial-scale experiments have been carried out demonstrating the effectiveness of ozone as a textile bleaching agent. The promising results achieved by Arooj [13] and Paksoy [14], who obtained high whiteness degrees using mini-jet and modified jigger machines, respectively, demonstrate the potential of this technology for large-scale industrial applications. Nevertheless, the ozone process still requires further optimization to guarantee consistent and reproducible performance [6].

On the other hand, comparative studies on overall performance and cost-effectiveness between the traditional process based on hydrogen peroxide and the method based on ozone have been conducted through life-cycle assessment (LCA) [15]. Results showed that the ozone-based process achieved a decrease of 27% in global warming potential (GWP), a 84.1% decrease in human toxicity, a 27.6% decrease in acidification, a 32.3% decrease in eutrophication potential and a 58.1% decrease in freshwater ecotoxicity. These reduction percentages can be improved by using renewable energy sources such as power cells or aerogenerators for the generation of ozone.

Therefore, this article aims to evaluate and optimize an ozone-based bleaching process for cotton fabrics under controlled experimental conditions, specifically analyzing the influence of pH, moisture content, and the use of selected additives (stabilizers and surfactants). By leveraging ozone’s high oxidation potential, this study seeks to develop a time-efficient, low-energy methodology that secures the requisite substrate whiteness and physical integrity, laying a robust and sustainable foundation for the textile colouration industry.

2. Materials and Methods

2.1. Materials

All experimental assays were conducted using a 100% raw-cotton woven fabric, type percale, with a weight of 140 g/m². The fabric was supplied by Cottonifficio Zambaiti (Cene, Italy) and was used after a standard desizing process using the amylase enzyme preparation ADRAZIM MT (ADRASA, Barcelona, Spain). This process consisted of processing the fabric in a 4 g/L dilution of ADRAZIM MT in deionized water (DI). A liquor ratio of 1:10 was used and the process was set at 60 °C for 30 min. After this step, the fabric was rinsed with DI water and dried at room temperature.

For the preparation of the impregnation baths, various chemical reagents were employed at varying concentrations in accordance with the specific experimental assay under analysis. The primary reagents used included sodium hydroxide (NaOH) in 50% dilution (PANREAC) and acetic acid (CH₃COOH) in 80% dilution (PANREAC, Barcelona, Spain) for neutralization, alongside the stabilizing agent Adrablanc OP (ADRASA), Adrabuffer CAD (ADRASA) as an acid buffer and Adranet FXE (ADRASA) as a surfactant, consisting of a blend of anionic and non-ionic components. This surfactant was added in a dose of 3 g/L as a standard dosage.

2.2. Sample Impregnation Process

Prior to the impregnation process, the fabric samples were dried at 60 °C for 20 min, and their initial dry weight was accurately recorded. Subsequently, each fabric specimen was fully immersed in the corresponding impregnation bath for 5 min to ensure thorough and uniform wetting. As a standard, 100 mL of impregnation bath was prepared in every experiment. After the immersion, the excess solution was removed by passing the fabric through a laboratory foulard with a standard pressure of 3 bar and a speed of 4 m/minute. The samples were immediately re-weighed and the pickup was determined based on the weight difference between the wet and dry fabric as a percentage.

2.3. Ozone-Based Treatment Process

The ozone treatment was carried out using an ozone generator lamp (UV-Consulting Peschl, Geldo, Spain) (Figure 2). To monitor the process, the generated ozone levels were measured using an industrial-grade ozone detector (O3-m, DIGNIFE, Shenzhen, China), recording a concentration that fluctuated between 5 and 12 ppm of ozone. These measurements were recorded at different intervals throughout the experimental assays to ensure consistency.

Figure 2. Ozone generator lamp in the laboratory set-up.

The ozonization methodology consisted of placing the bath-impregnated cotton fabric at rest on a flat, horizontal surface. The ozone generator was positioned at a fixed distance of 4.5 cm above the sample, a parameter that was strictly maintained for the entire duration of the exposure time.

The temperature used for the samples was 40 ± 2 °C as a consequence of the energy generated by the lamp during the generation of ozone.

Upon completion of the ozone exposure, the samples were immediately rinsed with tap water at 80 °C for 2 min. A subsequent neutralization step was performed depending on the pH of the initial impregnation bath:

• Alkaline medium: Samples were neutralized using a 5 g/L acetic acid (PANREAC) solution for 1 min at room temperature.
• Acidic medium: Samples were neutralized using a 2 g/L potassium carbonate (PANREAC) solution for 1 min at room temperature.

Finally, all treated samples were dried at 60 °C until a constant weight was achieved.

To compare the results obtained from the proposed ozone-based bleaching with the conventional hydrogen peroxide-based process, a reference test was established. To do this, a conventional hydrogen peroxide bleaching process on one of the original textile substrates was carried out. Then, the bleached substrate was characterized following the same analyses as those performed with the ozone-based homologues. The conditions of the reference test are shown in Table 1.

Table 1. Conventional hydrogen peroxide-based bleaching process conditions.

Bath Composition	Reaction Time (min)	Temperature (°C)
1.5 mL/L H2O2 (50%)	20	95–98
1 mL/L NaOH (50%)
2 g/L Adrablanc OP
3 g/L Adranet FXE

2.4. Determination of the Yield of the Process

The yield of the process was determined by colorimetric analysis of the samples after the treatment process, using the standard UNE-EN ISO 105-J03 [16], which establishes the parameters for the determination of colour differences. The values used as reference were the coordinates from CIELab space, L*, a*, b* and the colour difference (∆E) of the original sample, calculated according to (9).

∆E = [(∆L*)² + (∆a*)² + (∆b*)²]^1/2

(9)

where ∆L*, ∆a* and ∆b* correspond to the difference between the L*, a* or b* coordinate of the sample and the reference substrate.

Values of L* represent the luminosity of the sample, which is measured in values from 0 to 100, with 100 being the reference value of diffuse white and 0, a complete black reference. The chromatic coordinates a* and b* represent the position in the CIELab space. Positive values of a* indicate red shades, while negative values indicate green hues. For b*, positive values correspond to yellow hues and negative values to blue hues. For both coordinates, a* and b*, values close to 0 are obtained for lighter colours or grey shades.

All the colorimetric measures were determined with a DATACOLOR 500 (DATACOLOR, Barcelona, Spain) spectrometer with D65 illuminator and 10° observer conditions.

On the other hand, the weight loss was determined by calculating the weight difference of each sample before and after the ozone treatment. This value could represent an indirect factor that could determine the yield of the process because it may be related to a decrease in the resistance of the sample. In this study, a low weight loss represents less damage to the fabric and therefore better performance of the process.

Complementarily, the degree of polymerization (DP) of cellulose was determined in order to evaluate the degradation of the cellulose after the bleaching process. DP experiments were conducted in representative samples treated at different pH values.

DP was determined according to the standard ISO 5351:2019 [17], in which a fraction of the cotton sample is dissolved in copper (II) ethylenediamine 0.5 M. Then, the solution is transferred in a Canon–Fenske capillary viscosimeter and the efflux time elapsed for the solution can be related to the viscosity and, subsequently, with the DP

2.5. Influence of Chemicals on Performance

The influence of different impregnation-bath parameters on the yield of the process was analyzed qualitatively, as indicated in Table 2. To better analyze the influence of each parameter, factors were coded.

Table 2. Experimental plan variable levels.

Code	Factor	Concentration (g/L)	Dosing Level	Coded Factor
X	Alkaline (NaOH 50% w/w)	10	High	X (1)
		4	Low	X (−1)
		0	No dosing	0
Y	Bleaching Stabilizer (Adrablanc OP)	6	High	Y (1)
		2	Low	Y (−1)
		0	No dosing	0
Z	Acidic Buffer (Adrabuffer CAD)	3	High	Z (1)
		1	Low	Z (−1)
		0	No dosing	0

The variations in the different parameters of the impregnation-bath composition were organized with the aim of screening a wide range of conditions and finding approximations for the best performance of the process, in terms of chemical additives and pH conditions.

The experimental design begins with alkaline media and neutral conditions, to check if the pH and the bleaching stabilizer had any effect on the performance (Table 3). After this first approach to the process, the second series was designed in acidic conditions, using the buffer Adrabuffer CAD. With these experiments, a wide range of pH conditions are covered.

Table 3. Experiments according to the bath composition.

Experiment	pH of Bath	X	Y	Z
1	Alkaline	−1	−1	-
2		−1	1	-
3		1	−1	-
4		1	1	-
5		−1	-	-
6		1	-	-
7	Neutral	-	−1	-
8		-	1	-
9		-	-	-
10	Acidic	-	−1	−1
11		-	−1	1
12		-	1	−1
13		-	1	1

2.6. Water Content Influence on Performance

Due to the temperature that the textile sample achieves during the ozonization process (40 °C), the water content of the sample may vary during the treatment. To some extent, the pickup value of each sample would be related to the liquor ratio of the experiment, which is a key parameter in textile wet processing. This parameter represents the relationship between the volume of the bath and the weight of the treated textile and directly affects the results. This is why the influence of water content (or quantity of aqueous bath) in the process was tracked in order to fully understand its influence on the yield of the ozonization treatment.

To determine the evaporation rate of water during the ozonation, the fabric was immersed in a solution of DI water with 3 g/L of surfactant (Adranet FXE, ADRASA, Barcelona, Spain) to assure complete impregnation of the fabric. Then, the sample was weighed and placed in the ozone generator. The sample was repeatedly weighed at fixed intervals of time.

In order to determine whether the water content may influence the overall performance, different scenarios were planned to be investigated (Table 4). Parameters such as the content of the impregnation bath, the time of ozonation, and the re-impregnation of the fabric in the bath were analyzed.

Table 4. Experimental plan determining the effect of water content on performance.

Experiment	Bath Composition	Ozonization Time (min)	Re-Impregnation
14	None	60	No
15	Deionized water (DI)	5	No
16		15	No
17		30	No
18		15	Yes, every 5 min
19		30	Yes, every 5 min
20	3 g/L surfactant (Adranet FXE)	15	Yes, every 5 min
21		30	Yes, every 5 min
22		30	No
23	Bath X (1)Y (1)	15	Yes, every 5 min
24		30	Yes, every 5 min
25		30	No

For all experiments, the yield and weight were recorded at intervals of 5 min to determine the evolution during the process.

3. Results

3.1. Influence of Chemicals on Performance

The bleaching process performance, in terms of L*, a*, b*, ΔE, weight loss and DP (of the representative samples), was evaluated based on the combination of the reagents used to prepare the bath, according to the experimental design (Table 3). The results can be seen in Table 5. This data gives relevant information about the effect of the process on the fabric and should be evaluated altogether.

Table 5. Results for experimental plan in alkaline and acidic conditions.

Experiment	L*	a*	b*	∆E	Weight Loss (%)	DP
Raw cotton	87.89	1.47	10.39	-	-	2000
Conventional bleaching	95.20	−0.14	2.85	12.21	4.55	1560
1	90.12	1.62	9.53	3.63	0.3	1290
2	90.27	1.25	11.46	2.49	1.5
3	91.00	1.02	9.56	3.14	0.8
4	90.25	1.16	9.53	3.25	0.8
5	90.61	0.98	10.33	2.42	1.0
6	89.79	1.06	9.73	2.84	2.5
7	90.27	1.10	10.37	1.95	0.4	1270
8	89.70	0.91	10.33	2.46	0.9
9	90.44	1.33	10.99	1.92	1.2
10	90.91	1.11	9.50	3.20	2.6	1075
11	90.56	0.95	9.51	3.20	1.1
12	91.02	1.12	10.20	2.71	2.0
13	91.40	0.92	10.20	3.19	0.9

A uniform increase in the lightness L* was observed across all experimental conditions in experiments 1 to 13 compared to the raw-cotton lightness. This confirmed the bleaching efficacy of the ozone-based treatments. The most substantial whitening effects were recorded in experiment 3 (L* = 91.00), experiment 12 (L* = 91.02) and experiment 13 (L* = 91.40), which had different bath compositions (alkaline, acidic and acidic media, respectively). However, these variations were not very significant and no conclusive information could be obtained with the L* parameter alone.

Under alkaline conditions, experiments 1 through 6 exhibited distinct numerical trends; however, these variations only gained significance upon comparative evaluation. Specifically, experiments 3, 4 and 6 (characterized by high alkaline dosages) demonstrated a reduction in the b* coordinate and a concomitant increase in total colour difference (ΔE). These shifts indicate a chromatic transition toward a lighter yellow hue, with experiment 4 recording among the lowest b* values. Regarding the weight loss results, experiment 6 exhibited the highest loss, which correlates with a bath composition devoid of bleaching stabilizers. Moreover, experiment 1 yielded one of the lowest b* values alongside a high ΔE, despite containing neither a high alkaline dosage nor a stabilizer. Conversely, experiments 2 and 5 (low alkaline dosage) resulted in increased b* values and reduced ΔE. In all of these scenarios, the a* values fluctuate around the original value of the sample. These variations do not represent a great change, as they lie very close to the original value, which at the same time is relatively close to 0. The change in the b* parameter was selected for analysis, as it provides a clearer response to the bleaching process.

On the other hand, in neutral conditions, no significant trends were observed for the b* or a* coordinates. However, gravimetric analysis indicated that the presence of a stabilizer facilitates process control, as evidenced by the lower weight loss recorded in experiment 7. This low weight loss may be indicative of less damage to the fabric.

Under acidic conditions, the b* values demonstrated a direct correlation with the stabilizer dosage. Furthermore, these conditions resulted in higher weight loss compared to other media, suggesting that acidic environments are significantly more aggressive, resulting in a higher degradation of the sample.

In conclusion, the treatments generally induced a decrease in the a* coordinate (red–green axis), dropping from 1.47 in the raw sample to values predominantly around 0.90 to 1.30. The b* coordinate (yellow–blue axis) showed some fluctuation but generally decreased in the most effectively bleached samples (e.g., dropping from 10.39 to 9.56 in experiment 3), indicating a reduction in the fabric’s inherent yellowness.

Regarding the effect on the degree of polymerization (DP), a significant difference is observed among alkaline, neutral, and acidic media. Experiment 10, conducted in an acidic medium, resulted in an approximate 50% reduction in DP, indicating that the acidic process is highly aggressive towards the fibre. Furthermore, the stabilization of ozone in water may lead to poor selectivity due to its intrinsic reactivity. Conversely, while the DP reduction in alkaline and neutral media is almost identical, comparison with the standard bleaching process suggests that the ozonation process degrades cotton fibres slightly more rapidly than the conventional one.

3.2. Influence of Water Content on Performance

As previously mentioned, the water content of the sample could decrease due to evaporation during the ozone treatment since the temperature reaches 40 °C. To control this parameter, the evaporation rate was first determined by immersing the sample into an aqueous bath. This bath composition was not altered by any additive, as they may interfere with the evaporation rates and impregnation of the fabric. After the impregnation, the sample was placed in the ozone generator and the weight evolution of the fabric was measured at certain time intervals until completely dry (Figure 3). The weight when completely dry was obtained by placing the sample in the oven at 60 °C for 20 min.

Figure 3. Evolution of water content over time under experimental conditions.

The evaporation rate was very high initially, achieving 33.3% of the total evaporation after 5 min. Consequently, after 10 min, over 60% of the total content of water had evaporated, reaching a plateau region after 20 min, where the fabric was almost dry. At the same time that the water content was tracked during ozonation, the L* values were analyzed in order to see if there was a correlation between the water content and the luminosity of the sample (Figure 4).

Figure 4. Evolution of the L* of the sample during the ozonization process.

After 15 min of ozonization, L* reached a maximum value of 89.92 and subsequently decreased. This behaviour was attributed to the coupled effects of ozone mass transfer and water evaporation. Maximum whitening (L*) was achieved at an intermediate moisture level, where sufficient water was present to enable ozone dissolution and diffusion into the fibre, while partial evaporation enhanced the ozone concentration in the system, typically corresponding to a water content of approximately 60–75% (w/w). Further moisture depletion limited ozone solubility and transport, thereby reducing bleaching efficiency. With this approximation, it was concluded that the water content (or pickup) of the fabric had to be maintained during the bleaching reaction to ensure the best conditions. To further investigate the influence of moisture dynamics, fabrics were “re-wetted” or re-impregnated in the bath every 5 min under different conditions (Table 4) and the corresponding L*, a*, b*, weight loss and DP (of representative samples) values were determined. Samples were analyzed in different batches, according to the parameter under study (Table 6).

Table 6. Results for experimental plan with water content control.

Experiment	L*	a*	b*	∆E	Weight Loss (%)	DP
Raw cotton	87.89	1.47	10.39	-	-	2000
Conventional bleaching	95.20	−0.14	2.85	12.21	4.55	1560
14	86.50	1.84	20.37	10.49	1.0
15	89.24	1.22	10.67	1.41	1.7
16	89.21	1.17	11.35	1.67	0.9
17	89.68	0.87	11.72	2.32	4.0
18	90.12	1.19	9.81	2.33	1.2
19	91.67	0.61	9.36	4.02	3.4
20	90.43	1.06	9.88	2.63	1.6
21	93.46	−0.37	8.61	6.14	0.2
22	89.62	1.02	13.46	3.56	2.7
23	91.04	0.65	8.80	4.15	4.0
24	92.15	0.22	6.42	6.67	3.4	1275
25	92.02	0.21	6.98	6.20	3.5

The experimental results confirmed that water content is a critical factor that must be strictly controlled to optimize process performance. Generally, a trend towards increased L* and a reduction in a* was observed compared to the raw-cotton sample. However, the overall ozone-bleaching efficacy was strongly conditioned by the evolution of b*. The comparative analysis between experiments 17 and 19 clearly illustrated this colorimetric relationship. Although both treatments successfully increased lightness relative to the original sample (reaching L* values of 89.68 and 91.67, respectively), their chromatic impact diverged significantly. Experiment 17 induced an increase in the b* coordinate to 11.72, a value higher than that of the untreated cotton (10.39), which was translated to an undesirable yellowing of the fabric. In contrast, experiment 19 not only achieved greater lightness but also reduced the b* value to 9.36, indicating an effective removal of the fibre’s intrinsic yellowish hue. This irregular behaviour was even more drastically evident under suboptimal conditions, such as those in experiment 14. Here, a decrease in L* was accompanied by a severe increase in yellowing (b* = 20.37). This phenomenon generated the highest colour variation in the series (ΔE = 10.49), which, in this instance, was indicative not of effective bleaching, but rather of the optical degradation of the sample. Regarding the structural integrity of the fabric, evaluated through weight loss, the results exhibited significant dispersion, ranging from 0.2% to 4.0%. Severely oxidative treatments, such as experiments 17 and 23, caused maximum fibre degradation (a 4.0% weight loss).

Synthesizing the results, experiment 21 stands out as the optimal condition within this series. This treatment achieved the highest degree of whiteness, characterized by the maximum lightness value (L* = 93.46) and the most profound reduction in the chromatic components (a* = −0.37, b* = 8.61). Notably, this high bleaching efficacy (ΔE = 6.14) was achieved while causing minimal structural damage to the fabric, with a practically negligible weight loss of 0.2%. This demonstrated the exceptional selectivity and efficiency of the treatment under these specific operational conditions.

Another experiment with high performance was experiment 24, which achieved the lowest b* values of the series (b* = 6.42), meaning that the yellow hue was reduced and thus the bleaching effect was more predominant. On the contrary, this sample exhibited a high weight loss, meaning that the process was still very aggressive and produced damage to the fabric. This effect is confirmed by the evident decrease in the DP value obtained for experiment 24 when compared with the value obtained for the conventional bleaching.

Although the whiteness index could not be determined for most samples because the tint values fell outside the ±3 range required by the ISO 105-J02 [5] standard, three experiments (experiments 24, 25 and 26) fulfilled this condition and therefore allowed a reliable whiteness evaluation. According to ISO 105-J02, whiteness is an indicator of the perceived degree of whiteness by an average observer, while tint reflects deviations toward reddish or greenish hues from the neutral bluish reference (dominant wavelength of 466 nm). These assays were performed under identical bath conditions, differing only in the maintenance of fabric moisture: in experiments 24 and 25, the solution was continuously replenished to prevent fabric drying for 15 and 30 min, respectively, whereas in sample 26 the bath was not replenished, allowing partial drying of the fabric during treatment. The corresponding tint values were −2.6, −1.4 and −1.8 for samples 24, 25 and 26, respectively, all within the acceptable range for whiteness determination. The calculated whiteness indices were 40.8, 54.9 and 51.7, respectively. These results indicate that increasing treatment time under controlled moisture conditions enhances the whitening effect, while the absence of bath replenishment leads to slightly lower performance. Furthermore, when applicable, the whiteness index provides a more sensitive and discriminative parameter than the CIELab coordinates, which exhibited only minor variations across the study. In this sense, these three samples—showing the highest whitening performance and enabling reliable whiteness evaluation—constitute the most representative conditions of the process and are therefore selected as the basis for further optimization and continuation of the study.

Finally, in order to confirm the influence of the re-impregnation on the bleaching process, experiments 17 and 19 were compared (Figure 5). These two experiments were chosen as a representative example of how the re-impregnation process directly affects the yield, as experiment 17 is set as a baseline representing the process without re-impregnation, while experiment 19 showed the behaviour of the process under constant-water-content conditions.

Figure 5. Evolution of L* values over time for experiments 17 and 19.

The results demonstrated that beyond the 15 min threshold, in the absence of re-impregnation, the L* value plateaus and ceases to increase. Conversely, if the liquor content remained constant (as observed in sample 19) the system continued to evolve, resulting in a sustained increase in L*. As previously noted, the L* parameter correlates with the luminous reflectance of the sample. Analysis of the remaining colour coordinates revealed that the b* values in experiment 17 were higher, even surpassing those of the raw cotton. This behaviour revealed that working in dry conditions only results in yellowing, as previously discussed.

4. Conclusions

The results of this study demonstrate that ozone-based bleaching performance is strongly governed by both chemical conditions and fabric moisture content, which act as key parameters controlling process efficiency and selectivity.

Regarding the influence of chemical composition, all treatments led to an increase in fabric lightness (L*), confirming the bleaching capability of the process. However, L* alone proved insufficient to fully describe bleaching performance. The evolution of the b* coordinate emerged as a more reliable indicator, as effective bleaching was consistently associated with a reduction in yellowness. Both alkaline and acidic media enhanced bleaching efficiency compared with neutral conditions, although acidic environments exhibited more aggressive behaviour, leading to higher weight loss and thus greater fibre degradation. The presence of stabilizers was shown to play a critical role in controlling the process, reducing excessive degradation and improving overall selectivity.

Fabric moisture content was identified as a decisive factor in the ozone bleaching mechanism. Due to rapid evaporation under the operating conditions, the process efficiency decreases significantly as the fabric dries. Optimal performance was achieved at water contents above 50%, specifically in the range of 60–75% (w/w). Maintaining this moisture level through periodic re-impregnation allowed sustained reaction progress and significantly improved bleaching outcomes.

The combined analysis of colorimetric parameters and weight loss revealed that high bleaching efficiency can be achieved without compromising substrate integrity. Under optimal conditions, it was possible to reach high levels of whiteness while maintaining minimal fibre degradation, demonstrating the potential of the process.

Furthermore, DP analysis provided insights into the direct degradation inflicted on the fabric according to the applied bleaching treatment. The reduction in DP from 2000 (raw cotton) to 1075 in experiment 10 (acidic medium) suggested that acidic conditions are suboptimal, despite a potentially enhanced bleaching effect. Conversely, the results under alkaline conditions (experiments 1 and 24) still indicated significant fabric damage, reducing the DP to values around 1270. In both media, fabric degradation was more pronounced than in the conventional process. These findings provided critical data for process optimization in future experiments, which should focus on controlling the high reactivity of ozone.

Overall, the results confirm that ozone bleaching is a highly sensitive process that requires precise control of both chemical environment and moisture content. In contrast with traditional methods, which operate at high temperatures around 90–95 °C, work in 45 to 60 min and use lots of water and chemicals, ozonation provides a much cheaper option in terms of energy and water consumption, which nowadays is not being explored. These experiments, although they are only a qualitative exploration of the process, open the path for further investigations and optimization, leading to an effective and selective alternative for cotton bleaching.