**5. Enzyme used in textile wet processing**

#### **5.1 De-sizing**

The cotton warp yarns are sized to improve the yarn strength. Besides help in the interweaving during the procedure, it protects yarn against abrasion and snagging. Mostly starch-based products are used to apply sizes, synthetic and semi-synthetic sizes are polyvinyl alcohol (PVA) and carboxymethylcellulose (CMC) used [104–106]. The purpose of size is to protect the yarn from the abrasive action of weaving loom.

Desizing is the first step for wet processing in textile finishing technology employed to remove sizing material from the fabric. The size must be removed before bleaching and dyeing, for uniformity of wet processing. Chemically, starch is poly-α-glucopyranose in which amylase and amylopectin are present. However, they are insoluble in water. They can be solubilized by hydrolyzing them to shorter chain compounds. The object of desizing is to convert starch to soluble dextrin. The stages of hydrolyzing are mentioned below:

Starch (insoluble) ! dextrin (insoluble) ! dextrin (soluble) ! maltose (soluble) ! α-glucose (soluble)

Types of desizing methods

Enzyme desizing is the most widely practiced method of desizing starch. *Amylase* can catalyses the breakdown of starch form sugars, dextrin and maltose. The advantage of these enzymes is specific for starch, removing it without damaging to the support fabric. An enzymatic desizing process at low-temperature (30-60°C) and optimum pH is 5.5–6.5 is required for amylase [108]. Rising the temperature desizing facilities starch removal as well reduces process duration therefore the thermophile amylases have gained wider acceptance. Thermophile *Bacillus licheniformis* α-amylase can provide high temperature efficient starch removal efficiency with improved absorbency [109]. The enzymatic desizing process can be divided into three steps:

*Impregnation*: Enzyme solution is absorbed by the fabric. This stage involves through wetting of fabric with enzyme solution at a temperature of 70<sup>o</sup> C or higher with a liquid pick up of 1 liter per kg fabric. During this stage, gelatinization of the size is to the highest possible extent.

*Incubation*: The size is broken down by the enzyme. Long incubation time allows a low enzyme concentration.

*After-wash*: The breakdown products from the size are removed from the fabric. The desizing process is not finished until the size breakdown products have been removed from the fabric. This is best obtained by a subsequent detergent wash at the highest possible temperature.

#### **5.2 Scouring**

In textile terminology "scouring" applies to the impurities removal process. Raw cotton contains about 90% of cellulose and various non-fibrous impurities such as dirt, oils, waxes, gums and seed fragments. Pectins are complex polysaccharides comprised of α-(1, 4) linked D-galacturonic acid backbone. Pectin is non-cellulosic substance in cotton acts as cementing/adhesive material; therefore, removing pectin, will enhance to remove other non-cellulosic substances. In scouring process, the target fabric is usually boiled in the presence of alkali solution using large iron-made vessels called kiers. Classic alkaline scouring using sodium hydroxide removes most of such contaminations, essential to achieve satisfactory wet-ability. In textile processing, the process based on extensive alkali consumption requires heavy rinsing which ultimately leads several by-products in the wastewater effluent and poses severe damage to the cellulose contents of the fabric [110, 111].

The process of bio scouring is based on the concept of decomposition of pectin using enzyme. Bio scouring is ecofriendly, energy conserving alternative based on the idea of specially targeting the non-cellulosic impurities with appropriate enzymes without adversely affecting the substrate. Natural properties of the cotton fiber are preserved; the fabric is softer to the touch than after classic scouring. Pectinases enzyme activate in two medium acidic and alkaline. Acidic pectinases that function in a slightly acidic medium (pH between 4 and 6), as well as alkaline pectinases that function in a slightly alkaline medium (pH between 7 and 9) [112]. Optimum enzyme concentration varies from pectinase to pectinase but in general, pectinases are effective in low concentrations 0.005–2% range. In addition, the optimum temperature of pectinases application is form 40-60o C beyond which enzyme reduces its activity [113]. A high temperature rinsing after bio scouring is required for the removal of waxes (**Figure 12**).

Mixed enzymatic treatments of unscoured cotton fabric conducted by German scientists involved pectinase, cellulase, protease, and lipase. Beside the temperature, the pH of the environment is crucial for the activity and stability of the enzyme. An assistance of lipases removes natural fats & lubricants for better absorbency and levelness in dyeing. Bio-scouring for mutations containing lipase are more effective in attaining good hydrophilicity for cellulosic textiles [114]. In recent years, pectinases have been immobilized by ion exchange resins, aminated silica gel and macroporuos polyacrylamide for cotton scouring.

#### **Figure 12.**

*SEM images of a Bio-scoured cotton fabric (a) Bio-scoured with pectinase enzyme, (b) Bio-scoured with cellulase and pectinase mixed enzyme.*

**Figure 13.** *Dissociation of hydrogen peroxide.*

#### **5.3 Bleaching**

Bleaching is a process for improving the whiteness of textile materials with or without removing the natural coloring matter or extraneous substances. Bleaching produces permanent and basic white effect on fabric, which is required for level dyeing and sharp printing. Among the different oxidizing and reducing bleaching agent, H2O2 is mostly used as a universal bleaching agent from last two decades. The dissociation of hydrogen peroxide increased with rising temperature and form perhydroxyl anion shown in **Figure 2**. Perhydroxyl ions (HO2 ) demobilize the mobile electrons of conjugated double bonds in chromophores and caused decolorization. However, hydrogen peroxide bleaching process required high temperature and long processing time, which leads to higher energy consumption and increased fiber damage, which would cause problems in dyeing [115] (**Figure 13**).

Many researchers explored the alternative eco-friendly bleaching method for cotton processing, such as laccase/mediator or glucose-oxidase/peroxidase and bleaching with enzymatically in situ generated per acids. Lacasses with copper containing oxidoreductases enzymes used for bio bleaching to bleach textiles, modify fabric surfaces and coloration of cotton [116]. Another important bio-bleaching method for producing H2O2 is glucose oxidase. Generation of peroxidase with glucose oxidase requires slightly acidic to neutral conditions at low temperatures, however these conditions is insignificant. In addition, at the temperature 80-90<sup>o</sup> C and alkaline pH 11, glucose oxidase provides efficient results for improving the whiteness of the cotton fabric [117]. On the other hand, in addition of bleach activators such as Tetraacetylethylenediamine (TAED), nanoyloxybenzene sulphonate (NOBS), N-[4-(triethyl ammoniomethyl) benzoyl] caprolactum chloride (TBCC) enhances the bleaching performance. Combined laccase and glucose oxidase can perform better bleaching effect on linen fabric (**Table 11**).

#### **5.4 Bleach clean up**

In textile industry, bleaching is carried out by H2O2 after scouring and before dyeing. However, 10–15% of H2O2 retains on fabric, which can degrade the cellulose and formed pinhole on the fabric surface, it can reduce the strength of fiber.


**Table 11.**

*Whiteness of different bleaching process.*

Different reducing agent is used to destroy the hydrogen peroxide, or water to rinse out the hydrogen peroxide bleach. However, catalase enzyme can now be used to decompose excess H2O2 [118]. This eliminates the use of strong reducing agent and minimizes the water consumption. The cost of enzyme for degradation of hydrogen peroxide in bleaching effluents could be reduced by the immobilized catalase enzymes [119]. The process of bleach clean-up is very straightforward. A summary of the methodology is- 1) Drain the bleach liquor after bleaching 2) Fresh cold water filled 3) Maintain the pH is in the range 6.5–7 and the temperature 45<sup>o</sup> C 4) Add catalase (*Terminox Ultra*) enzyme 5) After 10–20 minutes checking the H2O2 removed by using Merck peroxide test strips 6) Start the dyeing process. Dyeing without and with bleach clean-up has shown in **Table 12**.
