33 Ecofriendly finishing with enzymes

K Kalaiarasi

epgp books

 

 

Introduction

 

Textile wet processes are shifted towards sustainable processes because of increasing governmental and environmental restrictions and decreasing availability of fresh water. Bio-catalysis has proven to be a reliable and promising alternative for chemical processes. Enzymes are gaining importance in various textile wet processing and finishing processes. Enzymes are very effective tool in providing eco-friendly solution to textile industry. They have replaced conventional textile processes which involve harsh chemicals, alkaline or acidic pH, high temperatures and energy. Enzymes are very specific and they operate at mild temperatures and at neutral pH.Different types of enzymes are used in various stages of textile processing. Cellulase enzyme is used for denim fading and bio-polishing. Amylase enzyme is used for desizing of woven fabrics. For scouring and fibre extraction, pectinase enzyme is used and protease enzyme is used for the treatment of wool and silk. Laccases are used for the decolourisation of dyes.

 

Objectives

  • To understand the importance ofeco-friendly textile processing
  • To enable the students to know about the various applications of enzymes in textile processing

Enzymes are high molecular weight proteins that catalyse or increase the rate of the reaction without itself being changed. Enzymes are called as bio-catalysts or biological catalysts as they are produced by all living cells of plants, animals and microorganisms. They are very specific in their action. The activity of the enzymes is influenced by pH, temperature, time, concentration of enzyme, presence of activators and inhibitors.

 

Advantages of enzymes in textile processing

  • Requires low energy Safe to handle
  • Extremely specific in action and so quality of the fabric is not affected
  • Work under mild conditions Bio-degradable
  • Environment friendly
  • Reduce water consumption
  • Produce high quality textile products

Enzymes are used in various textile processes such as desizing, scouring, bleaching, bio-polishing, silk degumming, fibre extraction, denim fading and in decolorization of textile dye effluent.

 

Enzymatic desizing

 

During the process of weaving, the warp yarn undergoes significant mechanical stress. To prevent the warp yarn from breaking during weaving process, it is coated with gelatinous substances called as size and the process is called as sizing. For cotton or cotton blended fabrics, the warp yarns are mostly coated with starch and its derivatives, polyvinyl alcohol, gelatin and carboxy methyl cellulose. Starch is preferred over other sizing agents because of its film forming capacity, low cost and easy availability. Due to sizing, the fabrics are not able to absorb water or any finishing agent. Hence the sizing agent must be removed after weaving process in order to make the fabric absorbent. The process of removal of starch or any other sizing agent is called as desizing.

 

Conventional chemical desizing method uses harsh oxidizing agents like hypochlorites or mild acids for longer time at high temperatures. It also consumes large amount of water. The conventional process results in degradation of cotton fibre thereby reducing the strength of the fabric. An alternative eco-friendly desizing method is the use of enzymes. The enzyme amylase is used for desizing. Amylase acts on starch and converts it to dextrins,maltose and finally to glucose that can be easily removed by washing. The optimum pH required for enzymatic desizing ranges from 5-7 and optimum temperature between 30℃-50℃.

 

Advantages

 

High efficiency and specificity Completely remove starch without damaging the fabric Eco-friendly process as it does not involve any chemicals Improved working conditions

 

The fabric is treated with enzyme by padding or exhaustion methods. There are two types of amylases α–amylase and β- amylase. α-amylasecleaves starch molecule at random. β-amylase successively removes the maltose units from the reducing ends.α-amylases are produced by bacteria, fungi and yeasts. Bacterial and fungal α-amylases are commonly used in industries. Wetting agents and non-ionic surfactants can be used to enhance enzyme penetration and adsorption, fibre swelling and to promote the removal of waxes and soils. Non-ionic surfactants are suitable for combination with enzymes whereas anionic and cationic surfactant may inactivate the enzyme through denaturation. After the enzymatic treatment, fabrics should be washed off above 80℃, in alkaline liquor followed by a wash in neutral liquor.

 

Enzymatic Scouring

 

Greige fabric (untreated) contains natural impurities like waxes, fats, pectins, proteins, organic acids, mineral salts and hemicelluloses in the primary cell wall and cuticle of the fibre. These impurities decrease the absorbency property of the fibre and prevent evenness in dyeing and printing. Hence these impurities have to be removed. The process of removal of natural impurities is called scouring. Conventional scouring process uses strong alkalies like sodium hydroxide and the reaction takes place at high temperatures (90-100℃). These chemicals, damages the fibre and results in loss of strength and weight. Due to high sodium hydroxide concentration extensive washing and rinsing is required, causing increased water consumption. The waste water discharged ishighly alkaline in nature and has high COD(ChemicalOxygen Demand) and TDS(Total Dissolved Solids).The waste water should be neutralised before discharge in to the environment. The scouring process need to be improved to meet today’s energy and environmental demands.

 

Enzymatic scouring or bio-scouring is an alternative ecofriendly method to alkaline scouring. This method uses enzymes which hydrolyse the non-cellulosic substances responsible for hydrophobicity of the fibre. A combination of enzymes including pectinase,protease, cellulaseand lipase are used for bio-scouring. Pectinases decompose pectinin the outer layers of the fibre. The weakened outer layer is removed by subsequent wash process. Protease degrades protein and lipase acts on natural waxes and fats. Wetting agents and non-ionic surfactants can be used to enhance enzyme penetration and adsorption, and removal of waxes and soils.

  Advantages

  • Fabric strength and weight is not affected Performance at neutral pH
  • Lower consumption of water Increases softness of fibre
  • Ecofriendly process.

   BLEACHING

 

Bleaching is the process of removal of natural colourants from the fabric and make the fabric white in appearance. Bleaching is done after desizing and scouring,but before dyeing. Hydrogen peroxide is commonly used for bleaching. It is called as the universal bleaching agent. Bleaching with hydrogen peroxide is carried out at 90-100◦C.The hydrogen peroxide left after bleaching has to be completely removed or the oxidation effect of hydrogen peroxide makes the dyeing uneven. Hydrogen peroxide is decomposed by reducing agents such as sodium thio sulphate. High temperature and alkaline pH maintained during bleachingprocess weakens the fibres.Hence, Nowadays catalase enzyme is used which eliminates the need for reducing agents and also the usage of water is minimized. This results in discharge of less waste water.

 

Catalase enzymes belong to hydrolase class of enzymes. They are also called as hydroperoxidases. They are produced by several bacteria and fungi. They act at neutral pH and have optimum temperature at 20- 50◦C. Catalase enzyme is also obtained from animal source (Bovine, liver). They decompose hydrogen peroxide to water and oxygen. One molecule of catalase hydrolyse 700 time its own weight of peroxide. Dyeing can be carried out in the same bath resulting in lower water consumption and waste water generation.

 

Another enzyme used for bleaching is laccase. Laccase bring about whiteness of the fabric by oxidation of flavonoids. Laccases are isolated from Trameteshirsuta. Laccases are group of multi copper enzymes which oxidize both phenolic and non-phenolic compounds. The fabric to be bleached is treated with laccase as a pre-treatment step followed by peroxide bleaching to attain high levels of whiteness. The combination of chemical bleaching with enzyme clean up action leads to less fibre damage and lower water consumption.

 

The enzyme glucose-oxidase releases hydrogen peroxide during breakdown of glucose. Hence the process of desizing and bleaching can be carried out in a single bath which saves water and energy.

 

DENIM FADING

 

Denim the universal fashion fabric is made of twill weave with indigo dyed warps. The indigo dye adheres to the surface of the yarns. In conventional stonewash process, the faded effect of denim is brought about by washing the blue jeans with pumice stones. The stone wash effect is due to the removal of surface bound indigo dyewhichexposes the white interior of the yarn. The pumice stones damage the washer drum and results in the loss of fabric strength due to abrasion.

 

Disadvantages of conventional method of denim fading

  • Machine damage
  • Blocking of drainage system
  • Difficult in removing pumice stone residues Excessive damage to garments.

The use ofenzymes for denim fading is an environment friendly process that results in soft handle of the fabrics. The enzyme used for denim washing is cellulase. Cellulases are hydrolytic enzymes that cleave the cellulose molecule to small oligosaccharides. Cellulase partially hydrolyse the cellulose present in the surface leaving the interior part of cotton fiber intact.

 

Partial hydrolysis of the fiber surface removes indigo dye and results in light areas or faded effect and the process is known as bio-stone washing. Cellulases are active at a temperature range of 30 -60◦C and pH 6-7. A small amount of enzyme can replace several kilograms of pumice stones. Denim washing with cellulases is an eco-friendly process that results in good appearance and soft handle of fabrics. Fungal Cellulase isolated from Trichoderma reesei is effective in producing a stone washed effect.Cellulase enzymes need to be inactivated after the desired stonewash effect is obtained. Insufficient inactivation will result in extended degradation of cellulose that leads to undesirable strength loss and weight reduction.

   

Advantages of bio-stone washing

  • Shorter treatment time
  • Less damage to fabric and machine Less pumice dust in the environment less solid waste

   BIO-POLISHING

 

A ball or fuzz (microfibers) emerging from the fabric is called a “pill”. Pills arise due to heavy processing of the fabric or garment. These pills reduce fabric quality as they result in an unattractive, knotty fabric appearance.

 

Cellulase enzyme acts on the microfibers protruding from the surface and weaken it by degradation. On application of sufficient shear forces the fibre will break from the surface and give the cotton fabrics a renewed smooth appearance. The process is called as bio-polishing or depilling. The fabric after treatment with cellulase enzyme exhibit much lower pilling tendency. Enzymatic depilling is carried out after bleaching and desizing. Enzymatic treatment after dyeing may result in partial dye removal.

 

Benefits of bio-polishing

  • Smoother and softer feel of the fabric Improved pilling resistance
  • Cleaner surface
  • Superior color brightness. Increased water retention

   SILK DEGUMMING

 

Silk fibres are composed of double filament of fibroin surrounded by a layer called sericin. Both sericin and fibroin are proteins. The process of removal of sericin, a protein that covers the silk filament is called as degumming. In conventional method, degumming is carried out in an alkaline solution containing soap, which damages the protein fibroin. The use of specific proteolytic enzyme is preferred because they act only on sericin without attacking fibroin.

 

Enzymatic degumming is performed using papain, a cysteine protease. Papain can be isolated from papaya fruit. The optimum pH for papain ranges from 5.8 to 7.0 and temperature 50 -57°C.

 

Advantages

  • Improve handle, smoothness and lustre No fiber damage
  • Retains the strength of silk filaments

  Enzymatic treatment of wool

 

Untreated wool is hydrophobic in nature because of the presence of wax, grease and fatty acids in the epicuticle surface. These hydrophobic impurities are removed by alkaline scouring which uses harsh chemicals such as sodium carbonate. The conventional chemical method has certain drawbacks, Yellowing of fibres Reduced durability Poor handling quality Adverse impact on environment Difficulties in dyeing To protect the environment and the fibre, proteases are used as an alternative to harsh chemical treatment of wool. Enzymatic treatment of wool fibres with proteases improves shrink resistance, tensile strength retention, handle, softness, wettability and dye uptake. It also reduces the felting property of wool. Enzymatic process is preferred because it does not involve any harsh chemicals and it brings changes in the surface properties without affecting the interior part of the fibre. Transglutaminase, a thiol enzyme is also used for the treatment of wool fabric.

 

Facing up of the surface of woollen fabrics occurs during dyeing by abrasive action. Enzymatic treatment with protease reduces “facing up” and enhances the pilling performance of the fabrics thereby increasing the smoothness of the fabrics.

 

Shrink proofing of wool

 

Wool fibres have the tendency to felt during the wet processing steps. Felting of wool is due to the presence of scales on the wool surface. Removal or modifications of these scales prevent felting. To prevent shrinkage, wool fibres are treated with oxidisers like KMnO4 or NaClO. Chemicals such as halogen compounds used for the modification or removal of scales are environmentally harmful. The use of protein disulphide isomerase improves the shrinkage property of wool fabrics.

 

Enzymes in detergent industry

 

Enzymes are used in detergent formulation. Due to increased environmental awareness, enzymes are used in detergent making since 1980s. The enzymes present in the detergents, enhance the removal of soil particles by degrading them into simpler water soluble particles which can be easily washed off. Proteases that are stable at alkaline pH and having low temperature optimum are suitable for detergent applications. These proteases hydrolyse proteinaceous stains. Amylases are used to remove starch based stains that stick on textile fibres.

 

Cellulases and lipases are also incorporated in detergents. Cellulases remove pills or fuzzy fibres on the surface of fabrics and liberate entrapped soil at disrupted fibre surfaces. The cleaning of fibre surfaces from soils and loss of micro-fibrils will give a brighter effect to fabrics and garments making it look renewed. It restores smoothness and lustre. Lipases remove oily stains. It hydrolyses ester bonds of fats, thereby producing glycerol and fatty acids. The enzymes used in detergent manufacturing are isolated from microbes.

 

Cutinases are extracellular esterases that hydrolyse ester bonds in cutin, the structural polyester of plant cuticles. Cutinases also hydrolyse triacyl glycerols as efficiently as lipases. Therefore they are used in laundry industry and dish washing detergentsfor removal of fats.

 

Surface modification of synthetic fibres

 

About 50% of worldwide textile market is represented by synthetic fibres. Synthetic fibres such as poly ethylene terephthalate (PET), polyamide(PA) and polyacrylonitrile(PAN) exhibit good tensile strength and resistance to chemicals, abrasion and shrinkage. Main drawback of synthetic fibres is its hydrophobicity which makes the fibre uncomfortable to wear. Also application of finishing and coloring agents is difficult, as most of them are water-dependent. Hence processing of the fibre requires absorbency property or hydrophilicity.

 

In conventional method, sodium hydroxide is used to improve hydrophilic and flexural properties of the fibres. But these conventional chemical treatments have certain disadvantages. They arepoor process control loss of weight and strength of the fibre yellowing of fibres require large amounts of chemicals energy requirement is more not an eco-friendly process

 

Due to the drawbacks of chemical treatment, enzymes are used for the surface modification of synthetic fibres. Lipases and esterases are used for the bio-modification of poly ethylene terephthalate. The enzymatic hydrolysis resulted in increased hydrophilicity, affinity to dyes and depilling. Cutinase isolated from F.solani acts on poly ethylene terephthalate (PET) and polyamide (PA) fibres and increased its hydrophilic property and affinity towards dye.

 

Extraction of bast fibres

 

Bast fibres such as flax, hemp, jute etc., consist of about 50% of cellulose, hemicellulose, pectin, fats, waxes and lignin. The bast fibres are extracted from the stem by a process called “retting”. Retting partially degrades the gummy substances that binds the fibres and helps in the extraction.

 

In conventional process, the plants are spread on the grass field so as to enable the bacteria to act upon it, in presence of moisture or the stem is immersed in slow rivers. Enzymes such as hemi-cellulases and pectinases are used for the extraction of bast fibres.

 

Advantages of enzymatic extraction

  • Controlled degradation of fibres
  • Reduction in generation of effluent Fibre strength is not affected

Decolourization of textile dye effluent

 

Laccases are extracellular, multi-copper enzymes that oxidise phenols, aromatic and non-aromatic compounds.Laccases are widely distributed in higher plants, fungi and bacteria. Laccases are used for the degradation of synthetic dyes as they can react with diverse structures. Decolourization by laccase is an environment friendly process.

 

Conclusion

 

The use of enzymes in textile industry is one of the most rapidly growing fields in industrial biotechnology. Enzyme technology can be used to develop a more environmental friendly and economically competitive process. Among the various textile processes, textile wet processing consumes high energy, chemicals and water. Enzymes can be used for almost all textile wet processes as an alternative to harsh chemical treatment. Enzymes are used for various textile wet processes such as amylase for desizing, cellulase for bio-polishing and denim fading, protease for wool and silk degumming. Further research is required for the bio-modification of synthetic and natural fibres. Utilization of enzymatic processes not only decreases the pollution load but also improves the quality of the products and theenvironment. Enzymatic process requires lessenergy, time and water.Hence, use of enzymes in textile wet processes is a solution to keep better environment for next generation. Thermostable enzymes and enzymes obtained from genetically modified microorganisms found application in textile field in recent years.

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Web links

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