**1. Introduction**

76 Textile Dyeing

Wang, P. Y. & Wu, C.J (1997) Photochromic Behaviour of Some Phenoxyanthraquinone Dyes

288

in Solution and on Polyester Substrate; *Dyes and Pigments*, Vol. 35, No. 3, pp. 297-

Raw cotton fibres have to go through several chemical processes to obtain properties suitable for further dyeing and use. With scouring, non-cellulose substances (wax, pectin, proteins, hemicelluloses…) that surround the fibre cellulose core are removed, and as a result, fibres become hydrophilic. Conventional scouring processes of cotton are conducted at temperatures up to 130 °C in a very alkaline medium (pH 10–12) with sodium hydroxide. Since a non-specific reagent is used in the treatment, it attacks impurities but it also causes damages to the cellulose portion of the fibres. Several auxiliary agents, such as wetting agents, emulsifiers and sequestering agents, which improve the efficiency of scouring and reduce the damage of fibres, are also added to the scouring bath.

Scouring is regularly followed by a bleaching process, which removes the natural pigments of cotton fibres. Cellulose fibres are most frequently bleached with hydrogen peroxide resulting in high and uniform degrees of whiteness. The water absorbency also increases, however, during the decomposition of hydrogen peroxide, radicals that can damage the fibres are formed. For this reason, organic and inorganic stabilizers and sequestering agents are added to the treatment bath.

Hydrogen peroxide is not ecologically disputable. The large amount of water used to rinse and neutralize the alkaline scoured and peroxide bleached textiles is ecologically disputable. Namely, the bleaching process is conducted in an alkaline bath at pH 10 to 12 and at temperatures up to 120 °C. Due to high working temperature, a large amount of energy is consumed. Auxiliary chemicals added into the bath increase the TOC and COD values of effluents. Upon neutralization of highly alkaline waste baths, large amounts of salts are produced. Consequently, the textile industry is considered one of the biggest water, energy and chemical consumers (Alaton et al., 2006; Warke & Chandrate, 2003).

To comply with more and more rigorous environmental regulations and to save water and energy, biotechnology and several types of enzymes have entered the textile sector. Many review (Jayani et al., 2005; Galante et al. 2003) and scientific (Gummandi & Panda, 2003; Buchert et al., 2000) papers describe the use of different enzymes for textile finishing. Pectinases are an efficient alternative to sodium hydroxide in the removal of non-cellulose substances from the cotton fibre surface (Preša & Tavčer, 2008a; 2008b). This process occurs at moderate temperatures in a slightly acidic (Calafell & Garriga, 2004; Li & Hardin, 1998;

Dyeing of Environmentaly Friendly Pretreated Cotton Fabric 79

Dyeability of differently pretreated fabrics is very important for industry and was studied from many aspects before. The dyeing behaviour of cotton fabrics treated with different enzymes by using reactive, cationic and acid dyes showed that cationic and acid dyes were more sensitive to the enzymatic treatment used as reactive dyes (Canal et al., 2004). When dyed with bireactive dyes the dyeing showed excellent evenness and their capacity to cover differences in whiteness arising from different pretreatment processes was significant. On the contrary, the dyeings with acid and cationic dyes revealed, with large differences in dye exhaustions, that each enzymatic system produces different ionisable residues on the primary wall of the cotton fibre (Canal et al., 2004). The importance of charges and functional availability on the fibre surface after different bioscouring treatments for their dyeing behaviour was also exposed by Calafell and co-workers

The ability of reactive dyes to cover the differences in whiteness from different bioscouring and bleaching processes when dyed in dark shades was confirmed by several researchers

Our research will upgrade the above results with reactive dyeing of differently scoured and differently bleached cotton fibres in medium and light shades and estimate the processes

Desized cotton fabric, 100 g/m2, was obtained from Tekstina, Slovenia. Acid pectinase Forylase KL (AP) was supplied from Cognis, Germany, and alkaline pectinase Bioprep 3000L (BP) from Novozymes, Denmark. Cotoblanc HTD-N (anionic wetting and dispersing agent, alkansulphonate with chelator) was supplied from CHT, Germany. H2O2 35% (HP) and peracetic acid (PAA) as a 15% equilibrium solution in the commercial bleaching agent Persan S15 were obtained from Belinka, Slovenia. Foryl JA (nonionic wetting agent) and Locanit S (ionic-nonionic dispersing agent) were obtained from Cognis, Germany and Lawotan RWS (nonionic wetting agent) was obtained from CHT, Germany. Sodium hydroxide was supplied from Šampionka, Slovenia, and acetic acid and sodium carbonate

The cotton fabric was scoured according to three different procedures using sodium hydroxide, acid pectinase or alkaline pectinase. The scoured fabrics were bleached with two bleaching agents: hydrogen peroxide and Persan S15. The abbreviation of processes and treatment conditions are displayed in Table 1. Enzymatic scouring and one-step treatments were performed 60 minutes at 55 °C, than the temperature of the bath was increased to 80 °C for 10 minutes to deactivate the enzymes. To activate peracetic acid in AP/PAA treatment, the pH was adjusted to 8 after 30 minutes. Demineralised water was used in all processes. The treatments were performed on the Jet JFL apparatus manufactured by Werner Mathis AG loaded with 50 g of fabric at a liquor ratio of 1:20. After all treatments, the bath was discharged and the jet was filled sequentially with fresh water heated to 80 °C, 60 °C and 25 °C to rinse the fabric. After alkaline scouring and peroxide bleaching, the fabrics were additionally neutralized with a neutralizing bath containing acetic acid and rinsed with cold

(Calafell et al., 2005).

**2 Experimental** 

**2.2 Treatment methods** 

water.

**2.1 Materials** 

(Fakin et al., 2008, Loszonci et al., 2004).

were supplied from Riedel-de Haen, Germany.

from the ecological point of view.

Sawada et al., 1998, Buschle-Diller et al., 1998) or alkaline (Etters, 1999; Durden et al., 2001; Lenting & Warmoeskerken, 2004) medium and is dependent on the type of pectinase. Pectin acts as a sort of cement or matrix that stabilizes the primary cell wall of cotton fibres. Pectinases decompose insoluble pectin into smaller particles thereby destabilizing the structure in the outer layers. The weakened outer layers can be removed in a subsequent wash process to such extent that following finishing processes and dyeing can be easily preformed (Etters, 1999; Losconzi et al., 2004; Cavaco-Paulo & Gubitz, 2003). It has been established that agitation of the treatment bath is very important for pectinase to function efficiently (Anis & Eren, 2002; Li & Hardin, 1998) and that selected sequestering agents can improve their effectiveness (Losonczi, 2005). Since enzymes act selectively, no damage to fibres occur during treatment. Also, it has been observed that after enzymatic treatment, baths are less polluted than baths after scouring with sodium hydroxide (Losonczi et al., 2005; Choe et al., 2004).

Bleaching with peracetic acid (PAA) is an alternative to bleaching with hydrogen peroxide (Tavčer, 2010). It is a powerful oxidizing agent (redox potential: 1.81 eV) (Preša & Tavčer, 2009) with excellent antimicrobial and bleaching properties. It is efficient at low concentrations, temperatures and in neutral to slightly alkaline medium. Its products of decomposition are biologically degradable. In the past, it was prepared in situ from acetic acid anhydride and hydrogen peroxide (Rucker, 1989; Wurster, 1992). However, the risk of explosion during the synthesis reaction prevented affirmation of peracetic acid as a bleaching agent in industry. In recent years, peracetic acid has become interesting (Hickman, 2002; Križman et al., 2005; Križman Lavrič et al., 2007). Several commercial products are available as balanced mixtures of peracetic acid, acetic acid and hydrogen peroxide (Equation 1). They are stabilized with a minimum amount of sequestering agent. Today, peracetic acid products available in the market are safe, simple to use, and priceeffective. Equation 2 shows the reaction that occurs when peracetic acid is used for bleaching.

$$\text{C}^{\text{H}}\_{\text{CH}^{3-}} \overset{\text{O}^{\text{O}}}{\overset{\text{O}^{\text{O}}}{\text{}}} + \text{H}^{-\text{O}-\text{O}-\text{H}} \xrightarrow[\text{stability}]{\text{catalyst}} \text{CH}\_{\text{2}} \overset{\text{catalyst}}{\overset{\text{catalyst}}{}} \text{C}^{\text{H}}\_{\text{2}} \overset{\text{O}^{\text{O}}}{\overset{\text{O}^{\text{O}}}{}} + \text{H}\_{2}\text{O} \quad \text{(1)}$$

CH3COOOH + impurities CH3COOH + oxidised impurities (2)

Both processes, scouring with pectinases and bleaching with peracetic acid, are conducted at temperatures of 50–60 °C for 40–60 minutes and pH 5–8. If both processes could be combined into one process, huge amounts of water, energy, time, and auxiliary agents can be saved. In a recent study (Preša & Tavčer, 2008b), it was confirmed using a viscosimetric method that pectinases retain their activity in the presence of peracetic acid and that combined processes are feasible.

The objective of our work was to compare the properties of enzymatically-scoured and peracetic acid-bleached cotton fabrics treated by two-bath and one-bath scouring/bleaching methods, with respect to conventionally-treated fabrics (alkaline scoured and bleached with hydrogen peroxide) with emphasis on their degree of whiteness and dyeability.

Dyeability of differently pretreated fabrics is very important for industry and was studied from many aspects before. The dyeing behaviour of cotton fabrics treated with different enzymes by using reactive, cationic and acid dyes showed that cationic and acid dyes were more sensitive to the enzymatic treatment used as reactive dyes (Canal et al., 2004). When dyed with bireactive dyes the dyeing showed excellent evenness and their capacity to cover differences in whiteness arising from different pretreatment processes was significant. On the contrary, the dyeings with acid and cationic dyes revealed, with large differences in dye exhaustions, that each enzymatic system produces different ionisable residues on the primary wall of the cotton fibre (Canal et al., 2004). The importance of charges and functional availability on the fibre surface after different bioscouring treatments for their dyeing behaviour was also exposed by Calafell and co-workers (Calafell et al., 2005).

The ability of reactive dyes to cover the differences in whiteness from different bioscouring and bleaching processes when dyed in dark shades was confirmed by several researchers (Fakin et al., 2008, Loszonci et al., 2004).

Our research will upgrade the above results with reactive dyeing of differently scoured and differently bleached cotton fibres in medium and light shades and estimate the processes from the ecological point of view.
