**4. Ozone technology**

**Figure 4.** Classification of ultraviolet radiation [35]

110 Eco-Friendly Textile Dyeing and Finishing

range for modifying wool fiber surfaces [38].

**•** more dye to become fixed, producing deeper shades

let technology in dyeing of proteinous fibers is summarized below.

ture) [38]

on untreated fabric [40].

dye sorption characteristics, and photofading of natural or artificial hair color [36]. The chemical changes caused by short-term UV irradiation of wool are confined to fibers at the fabric surface and UV is unable to penetrate beyond the surface to weaken the bulk fibers responsible for the mechanical strength. This has enabled the potential application of UV technology as a surface-specific treatment in several areas of wool processing [37]. For wool the UV-absorbing species are aromatic amino acid and cystine residues in the protein struc‐ ture which absorb strongly below 350 nm. UVC radiation (200-280 nm) is the most effective

The most commercially used process for antifelting and antipilling of wool is based on chlorination. However, recent concern over the release into the environment of adsorbable organohalogens (AOX) in process effluents has prompted the development of alternative, AOX-free processes. Different types of radiation techniques, such as ultraviolet radiation, are utilized as alternatives to chlorination in wool processing [39]. UV treatment can add value in coloration (dyeing and printing), since it is predominantly surface fibers in a fabric that absorb, reflect and scatter light. Photomodification of the surface fibers can allow:

**•** more rapid fixation of dyes dye fixation under less severe conditions (e.g. lower tempera‐

Modification of the dye uptake by exposure of wool fabric to UV radiation before dyeing has been known since the early 1960s. For most dye classes, UV-irradiated fabric takes up significantly more dye than untreated and when fabrics are irradiated through stencils, intri‐ cate tone-ontone effects can be produced [37]. Some literature related to the use of ultravio‐

*Millington (1998)* stated that UV irradiation of wool can significantly increase dyeing color yields. The use of 1:1 metal-complex dyes was found to be particularly effective, and a 3% o.w.f. dyeing on UV-treated fabric could produce a better depth of shade than a 5% dyeing Ozone is a natural occurring gas that can be both beneficial and detrimental to organisms on Earth. It is important that sufficient amount of this pale blue gas is present in the strato‐ sphere, where O3 molecules would shield most of the UV radiation from reaching Earth [43]. Although ozone is a blue colored gas at normal temperatures and pressures; because of its low concentrations in its applications the observation of this blue color of ozone is impossi‐ ble [44]. Ozone gas has a pungent odor readily detectable at concentrations as low as 0.02 to 0.05 ppm (by volume), which is below concentrations of health concern [45].

Ozone was first generated and characterized by a German scientist named Schonbein in 1840 [46]. Ozone is a nonlinear triatomic molecule possessing two interoxygen bonds of equal length (1.278 A) and an average bond angle of 116°49' [47].

**Figure 5.** Ozone molecule [48, 49]

Ozone is formed naturally in the atmosphere by photochemical reaction with solar UV radi‐ ation and by lightening. It can also be generated artificially. Three most common ways of generating ozone artificially are:

**•** Corona discharge: In this method, ozone is generated when free, energetic electrons in the corona dissociate oxygen molecules in oxygen-containing feed gas that passes through the discharge gap of the ozone generator.

in less reactive molecules, such as aliphatic hydrocarbons, carboxylic acids, benzenes or

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The chlorine/ Hercosett process, the most widely used treatment for the wool dyeing proc‐ ess, causes dangerous ecological problems due to the contamination of waste water with ab‐ sorbable organic halides (AOX). Because of legal restrictions and national and international awareness of ecology and pollution control, an AOX-free pretreatment is required to offer environmental advantages [53]. Alternative surface modifications for improving wool dyea‐ bility are therefore being explored. One of them is ozonation process [54]. Ozone treatments of proteinous fibers such as wool, mohair, angora and silk have been investigated by many authors. When literature is examined, it can be well understood that, increase in dyeability of protenious fibers caused by ozonation process depends on the following parameters;

**• pH:** Typically, at pH<4 direct ozonation dominates and above pH>9 the indirect pathway dominates. In the range of pH 4-9, both of them are important. The pH influences the gen‐

Generally at neutral medium reaction rate of ozone gas is slow due to its low solubility. While molecular ozone reacts at low pH values, at high pH values radicals react. Since the oxidation potential of hydroxyl radicals exceeds that of ozone molecules, oxidation is faster in indirect reactions. Additionally HO• is not the only radical that is formed. Even though




**Figure 6.** Scheme of reactions of ozone added to an aqueous solution [50]

eration of hydroxyl radicals [55].

chlorobenzenes [50].


Ozone is a very powerful oxidizing agent, which is able to participate in a great number of reactions with organic and inorganic compounds. Among the most common oxidizing agents, it is only surpassed in oxidant power by fluorine and hydroxyl radicals (see Table 1) [50]. Ozone has strong tendency to react with almost any organic substance as well as with water. The reaction proceeds via several intermediates such as peroxides, epoxides and per‐ hydroxyl and hydroxyl radicals [51].


**Table 1.** Oxidation power of selected oxidizing species [50]

In an ozonation process two possible pathways have to be considered [50]: direct oxidation with ozone molecules or the generation of free-radical intermediates, such as the OH radi‐ cal, which is a powerful, effective, and non-selective oxidizing agent [52].

$$\rm{H}\_{3}\rm{O}\_{3} + \rm{H}\_{2}\rm{O} \rightarrow 2\rm{HO}\_{2}\tag{1}$$

O + HO • HO• + 2O 3 2 ® <sup>2</sup> (2)

Molecular ozone can directly react with dissolved pollutants by electrophilic attack of the major electronic density positions of the molecule. This mechanism will take place with pol‐ lutants such as phenols, phenolates or tiocompounds. The radical mechanism predominates in less reactive molecules, such as aliphatic hydrocarbons, carboxylic acids, benzenes or chlorobenzenes [50].

**Figure 6.** Scheme of reactions of ozone added to an aqueous solution [50]

**•** Corona discharge: In this method, ozone is generated when free, energetic electrons in the corona dissociate oxygen molecules in oxygen-containing feed gas that passes through

**•** UV light: Ozone can also be generated by UV light. The high energy UV light ruptures the oxygen molecules into oxygen atoms, and the subsequent combination of an oxygen atom

**•** Electrolysis: A third method for generating ozone is electrolysis, which uses an electrolyt‐ ic cell. Specifically, electrolysis involves converting oxygen in the water to ozone by pass‐

Ozone is a very powerful oxidizing agent, which is able to participate in a great number of reactions with organic and inorganic compounds. Among the most common oxidizing agents, it is only surpassed in oxidant power by fluorine and hydroxyl radicals (see Table 1) [50]. Ozone has strong tendency to react with almost any organic substance as well as with water. The reaction proceeds via several intermediates such as peroxides, epoxides and per‐

**Oxidation species Oxidation power (V) Oxidation species Oxidation power (V)** Fluorine 3.03 Chlorine dioxide 1.50 Hydroxyl radical 2.80 Hypochlorous acid 1.49 Atomic oxygen 2.42 Hypoiodous acid 1.45 Ozone 2.07 Chlorine 1.36 Hydrogen peroxide 1.77 Bromide 1.09 Permanganate 1.67 Iodine 0.54

In an ozonation process two possible pathways have to be considered [50]: direct oxidation with ozone molecules or the generation of free-radical intermediates, such as the OH radi‐

Molecular ozone can directly react with dissolved pollutants by electrophilic attack of the major electronic density positions of the molecule. This mechanism will take place with pol‐ lutants such as phenols, phenolates or tiocompounds. The radical mechanism predominates

O + H O 2HO 32 2 ® (1)

O + HO • HO• + 2O 3 2 ® <sup>2</sup> (2)

cal, which is a powerful, effective, and non-selective oxidizing agent [52].

ing the water through positively and negatively charged surfaces [46].

the discharge gap of the ozone generator.

112 Eco-Friendly Textile Dyeing and Finishing

hydroxyl and hydroxyl radicals [51].

Hypobromous acid 1.59

**Table 1.** Oxidation power of selected oxidizing species [50]

with an oxygen molecule produces ozone (O3).

The chlorine/ Hercosett process, the most widely used treatment for the wool dyeing proc‐ ess, causes dangerous ecological problems due to the contamination of waste water with ab‐ sorbable organic halides (AOX). Because of legal restrictions and national and international awareness of ecology and pollution control, an AOX-free pretreatment is required to offer environmental advantages [53]. Alternative surface modifications for improving wool dyea‐ bility are therefore being explored. One of them is ozonation process [54]. Ozone treatments of proteinous fibers such as wool, mohair, angora and silk have been investigated by many authors. When literature is examined, it can be well understood that, increase in dyeability of protenious fibers caused by ozonation process depends on the following parameters;

**• pH:** Typically, at pH<4 direct ozonation dominates and above pH>9 the indirect pathway dominates. In the range of pH 4-9, both of them are important. The pH influences the gen‐ eration of hydroxyl radicals [55].

$$\bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \tag{3}$$

$$\bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \tag{4}$$

$$\bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \tag{5}$$

Generally at neutral medium reaction rate of ozone gas is slow due to its low solubility. While molecular ozone reacts at low pH values, at high pH values radicals react. Since the oxidation potential of hydroxyl radicals exceeds that of ozone molecules, oxidation is faster in indirect reactions. Additionally HO• is not the only radical that is formed. Even though the HO• radical is the most powerful radical with a 2.8 V oxidation potential, HO2•, HO3• and HO4• radicals are also formed as shown in Eq. 6-11 [56].

$$\bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \bullet \tag{6}$$

**• Ozone dose:** Because oxidation reactions are caused by molecular ozone or radical spe‐ cies formed by the reactions of ozone, with the increase in ozone dose ozonation efficien‐

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**• Water content of the fiber:** In literature it was stated that the rate of oxidation is accelerat‐ ed by the hydration of hydrophilic groups in the fiber and above a critical level of mois‐ ture 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 during ozonation process was reported [51, 57, 58]. Prabaharan and Rao have suggested a model in order to explain this phenomenon. According to this model it can be said that for ozonation to take place, water should be present inside of the fiber. However

Fig. 7 gives the schematic representation of ozone path from gas phase to reaction site in the fiber. When sufficient water is not present, d1 and d2 are absent and hence O is transported by convection across the distances d1 and d2 and then by diffusion across d3 and d4. Since sufficient water is not supplied, entire hydrophilic group in the fiber is not hydrated and hence the extent of attack at R is low. When sufficient water is supplied, d1 is absent and d2 is either fully or partially absent depending on the quantity of water available and hence O is transported by convection across d1 and convection or diffusion across d2 followed by dif‐ fusion across d3 and d4. Since sufficient water is supplied, entire hydrophilic group in the fiber is hydrated and hence the extent of attack at R is maximum. When excess water is present, d1 and d2 are present and hence O is transported by diffusion across d1, d2, d3, and d4. Since excess water is present at d1 and d2, dilution of ozone takes place and hence ozone

**Figure 7.** Schematic representation of ozone path from gas phase to reaction site [58]; (d1: the distance occupied by surface water, d2: the distance occupied by mobile water phase, d3: immobile water phase, d4: the distance between

Some literature related to the effect of ozonation on dyeing properties of various proteinous

*Micheal and El-Zaher (2003)* has evaluated the effect of ultraviolet/ozone treatments for differ‐ ent times on the characteristics of wool fabrics with respect to wettability, permeability, yel‐

the quantity of water present has a definite effect on the rate of reaction [51].

attack at R is lower in spite of complete hydration at d3 [58].

cy increases [56]

the immobile water phase and R)

fibers is summarized below.

$$\bullet \text{ HO}\_2\bullet \leftrightarrow \bullet \text{O}\_2\bullet + \text{H}^\* \tag{7}$$

$$\bullet \bullet\_{3} + \bullet\_{2} \bullet \bullet \bullet\_{3} + \bullet\_{2} \tag{8}$$

$$\text{HO}\_3^- + \text{H}^+ \rightarrow \text{HO}\_3^- \tag{9}$$

$$\rm{HO}\_3\bullet \rightarrow \rm{HO}\bullet + \rm{O}\_2 \tag{10}$$

$$\bullet \text{ HO} \\ \bullet \text{+O}\_3 \rightarrow \text{HO}\_4 \tag{11}$$

At pH < 9, the highly selective ozone molecules react rapidly at sites of high electron density (such as aliphatic and aromatic double bonds) and slowly at less reactive sites (such as C-H bonds of saturated hydrocarbons). The presence of the •OH radicals above pH 9 have less selectivity and high oxidation potential (2.8 V) [51]. Ozonation efficiency decreases in the ba‐ sic pH values when compared to acidic and neutral pH. Because in a basic solution, more hydroxide ions are present and these hydroxide ions act as an initiator for the decay of ozone [57]. Moreover, it is reported that the dissolved ozone concentration in water decreas‐ es from 4.3X10-4 mol/L at pH 4 to 1.5X10-4 mol/L at pH 10 [58].

**• Temperature:** As temperature increases, ozone becomes less soluble (see Table 2) [45]; however, it can not be said that ozonation efficiency reduces with the decrease in the sol‐ ubility of ozone, because temperature rise also increases the reaction rate [56].


**Table 2.** Water solubility of ozone [59]

**• Ozone dose:** Because oxidation reactions are caused by molecular ozone or radical spe‐ cies formed by the reactions of ozone, with the increase in ozone dose ozonation efficien‐ cy increases [56]

the HO• radical is the most powerful radical with a 2.8 V oxidation potential, HO2•, HO3•

At pH < 9, the highly selective ozone molecules react rapidly at sites of high electron density (such as aliphatic and aromatic double bonds) and slowly at less reactive sites (such as C-H bonds of saturated hydrocarbons). The presence of the •OH radicals above pH 9 have less selectivity and high oxidation potential (2.8 V) [51]. Ozonation efficiency decreases in the ba‐ sic pH values when compared to acidic and neutral pH. Because in a basic solution, more hydroxide ions are present and these hydroxide ions act as an initiator for the decay of ozone [57]. Moreover, it is reported that the dissolved ozone concentration in water decreas‐

**• Temperature:** As temperature increases, ozone becomes less soluble (see Table 2) [45]; however, it can not be said that ozonation efficiency reduces with the decrease in the sol‐

> **Temperature (°C) Solubility (kg.m-3)** 0 1,09 10 0,78 20 0,57 30 0,40 40 0,27 50 0,19 60 0,14

ubility of ozone, because temperature rise also increases the reaction rate [56].





HO • HO• + O 3 2 ® (10)

HO• + O HO 3 4 ® (11)

and HO4• radicals are also formed as shown in Eq. 6-11 [56].

114 Eco-Friendly Textile Dyeing and Finishing

es from 4.3X10-4 mol/L at pH 4 to 1.5X10-4 mol/L at pH 10 [58].

**Table 2.** Water solubility of ozone [59]

**• Water content of the fiber:** In literature it was stated that the rate of oxidation is accelerat‐ ed by the hydration of hydrophilic groups in the fiber and above a critical level of mois‐ ture 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 during ozonation process was reported [51, 57, 58]. Prabaharan and Rao have suggested a model in order to explain this phenomenon. According to this model it can be said that for ozonation to take place, water should be present inside of the fiber. However the quantity of water present has a definite effect on the rate of reaction [51].

Fig. 7 gives the schematic representation of ozone path from gas phase to reaction site in the fiber. When sufficient water is not present, d1 and d2 are absent and hence O is transported by convection across the distances d1 and d2 and then by diffusion across d3 and d4. Since sufficient water is not supplied, entire hydrophilic group in the fiber is not hydrated and hence the extent of attack at R is low. When sufficient water is supplied, d1 is absent and d2 is either fully or partially absent depending on the quantity of water available and hence O is transported by convection across d1 and convection or diffusion across d2 followed by dif‐ fusion across d3 and d4. Since sufficient water is supplied, entire hydrophilic group in the fiber is hydrated and hence the extent of attack at R is maximum. When excess water is present, d1 and d2 are present and hence O is transported by diffusion across d1, d2, d3, and d4. Since excess water is present at d1 and d2, dilution of ozone takes place and hence ozone attack at R is lower in spite of complete hydration at d3 [58].

**Figure 7.** Schematic representation of ozone path from gas phase to reaction site [58]; (d1: the distance occupied by surface water, d2: the distance occupied by mobile water phase, d3: immobile water phase, d4: the distance between the immobile water phase and R)

Some literature related to the effect of ozonation on dyeing properties of various proteinous fibers is summarized below.

*Micheal and El-Zaher (2003)* has evaluated the effect of ultraviolet/ozone treatments for differ‐ ent times on the characteristics of wool fabrics with respect to wettability, permeability, yel‐ lowness index, and weight loss. The beneficial effects of this treatment on dyeability, color parameters, light fastness characteristics, and the change in color difference after exposure of the treated dyed samples to artificial daylight for about 150 hours were investigated. The results indicated that the improvement in wetting processes may have been due to surface modifications; this meant that an increase in the amorphousity of the treated samples, the oxidation of the cystine linkage on the surface of the fabrics, and the formation of free-radi‐ cal species encouraged dye uptake [60].

*Sargunamani and Selvakumar (2007)* investigated the effects of process parameters (pick-up value, pH and time) in the ozone treatment of raw and degummed tassar silk fabrics on their properties such as yellowness index, breaking strength, breaking elongation, weight, amino group content. Decrease in yellowness index, breaking strength, breaking elongation, and weight as well an increase in amino group content was observed [58].

**Figure 8.** States of matter [64]

ent excited particles in dependence of the used gas [66].

**Figure 9.** The principle of the plasma treatment [66]

The plasma is an ionized gas with equal density of positive and negative charges which ex‐ ist over an extremely wide range of temperature and pressure [65]. As shown in Fig. 9, the plasma atmosphere consists of free electrons, radicals, ions, UV-radiation and a lot of differ‐

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*Perincek et.al (2008)* investigated a novel bleaching technique for Angora rabbit fiber. For this purpose, a detailed investigation on the role of the fiber moisture, pH, and treatment time during ozonation was carried out. Also, the effect of ozonation on the dyeing properties of Angora rabbit fibers was researched. Consequently, it was found that ozonation improved the degree of whiteness and dyeability of Angora rabbit fiber [57].

*Atav and Yurdakul (2010)* investigated the use of ozonation to achieve dyeability of the angora fibers at lower temperatures without causing any decrease in dye uptake by modifying the fiber surfaces. The study was carried out with known concentration of ozone, involving process parameters such as wet pick-up (WP), pH, and treatment time. The effect of fiber ozonation was assessed in terms of color, and test samples were also evaluated using scanning electron microscopy (SEM). The optimum conditions of ozona‐ tion process were determined as WP 60%, pH 7 and 40 min. According to the experi‐ mental results it can be concluded that, ozonated angora fibers can be dyed at 90°C with acid and reactive dye classes without causing any decrease in color yield [61]. In an oth‐ er study on ozonation process carried out by *Atav and Yurdakul (2011)* the optimum con‐ ditions for mohair fiber were determined as WP 60%, pH 7 and 30 min. Dyeing kinetics also studied and it was demonstrated that the rate constant and the standard affinity of ozonated sample increased [54].
