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| Issue date:01/12/2008 |
| ATA Journal for Asia on Textile & Apparel - Dec 2008 Issue |
| Source:Journal for Asia on Textile & Apparel |
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The use of enzymes in textile wet processing is increasing every year because of their undoubted advantages over traditional chemical processing methods, writes Ian Holme
Although enzymes have been used in textile wet processing since around 1857, their main utilisation has been for desizing starch-based sizes. The more widespread application in pretreatment, dyeing, printing and finishing has intensified over the last decade, largely stimulated by the success of biopolishing of cellulosic fabrics, such as cotton and lyocell fabrics.
Enzymes are biocatalysts that essentially function by facilitating and increasing the rates of chemical reactions without themselves suffering any overall change, provided that the application conditions are optimised for the particular treatment.
They are now being actively researched for textile wet processing in order to promote innovative fibre, yarn or fabric treatments, which offer novel effects as well as cost savings in terms of energy, water and effluent treatment costs.
Enzymes are high molecular weight proteins composed if amino-acid residues linked together in a specific chemical sequence or repeat unit.
Many enzyme structures require a cofactor to be present in order to provide the requisite catalytic activity. The cofactor is known as a coenzyme and may be a metal ion or an inorganic molecule. For example, laccase enzymes are copper-containing oxidase enzymes that catalyse oxidative reactions. Other types of enzymes include hydrolases that catalyse hydrolysis, e.g. of the glucosidic linkages in cellulose, of ester bonds in polyester, and of amino-acids in wool and silk.
Enzymes are substrate-specific because their chemical structure is such that the enzyme is bound to the substrate through specific sites to form an enzyme-substrate complex. In all enzymatic treatments, the groups taking part in the reaction are held in close proximity to one another and to catalytic sites (termed the active sites) in the three-dimensional enzyme structure. Active sites consist usually of only a few amino-acid residues and will also include any coenzyme required to facilitate the particular reaction.
New ways of pre-treatment being explored
There are now a wide variety of enzymatic treatments used for pretreatment, dyeing and finishing processes. Of particular importance are the novel enzyme treatments that can be applied in fibre or fabric pretreatment. At present, these enzyme pretreatments are largely confined to cotton but the possibility of using innovative enzymes to separate the fibres from the stems of the flax plant and other lignocellulosic fibres like ramie, hemp and etc, could open up new possibilities for such fibres.
Major types of enzymes used in textile wet processing include:
Pretreatment
Desizing: use of -amylases for removal of starch-based sizes from woven cotton fabrics
Bioscouring: removal of cotton waxes and other impurities from cotton using pectate lyase enzymes
Bleach clean up: use of catalase enzymes to rapidly destroy any traces of residual hydrogen peroxide remaining in the fabric after bleaching
Dyeing
Excess dye removal: achieved by using peroxidase enzymes on cotton denim fabric
Removal of unfixed reactive dyes: removal of coloured components from the unfixed reactive dye hydrolysates in dye liquor
Finishing
Biopolishing: removal of unsightly surface fibres (surface fuzz) from cellulosic fabrics using acid cellulases e.g. on cotton to produce a smooth surfaced fabric that exhibits decreased pill formation during garment wear. Also used as a pretreatment on lyocell fabrics in order to obtain uniform peachskin finishes by subsequent mechanical fibrillation treatments
Stone washing: production of stonewash effects using neutral cellulases
Denim bleaching: bleaching of dyed denim using laccase / mediator systems to provide a variety of shades.
Gentle enzyme desizing protect natural fibers
Many fabrics based upon cotton contain starch-based size and polyester / cotton and polyester / cellulosic fabrics contain starch-based size together with other sizes. Starch consists of two components, namely amylose (straight chain) and amylopectin (branched chain). Starches with a higher amylose content, e.g. wheat and corn (maize) starch are considered easier to remove than sizes based on rice and tapioca (with lower amylose contents).
Enzymatic desizing is considered to be more gentle upon the cellulosic fibres because amylase enzymes only degrade starch. However, desizing methods involving mineral acid or oxidation desizing can result in overall or localised fabric damage if the process control is not optimised. The starch size molecules are removed by degradation (enzymatic hydrolysis), so the degraded products cannot be recycled as a size as the average molecular weight (or degree of polymerisation) of the chain molecules is too low. Nor can the amylase enzyme be recycled, because it cannot be separated yet from the mass of degraded material removed from the fabric.
Of the three sources of amylase enzymes available, namely bacterial, malt and pancreatic amylases, modern pretreatment systems rely upon bacterial -amylases which can be engineered to operate under different processing conditions and degrade starch components by random chain scission, producing dextrins that can be rapidly washed out of the fabric.
Bactosol PHC liquid hc of Clariant is a highly flexible bacterial -amylase product designed for rapid desizing of starch-based sizes, and starch size mixtures. The product has now been engineered for application to woven fabrics containing sizes based upon starch; starch / PVA (polyvinyl alcohol); starch / PVA / CMC (carboxymethyl cellulose); starch / acrylate; starch / PVA / acrylate; and modified starch, e.g. carboxymethyl starch.
In response to the demands placed upon modern fabric pretreatment systems for flexibility of operating conditions and versatility in terms of application methods, Clariant Bactosol PHC liquid hc can be applied under a wide range of conditions / processes, for instance:
Cold desizing (20-30°C by the cold pad batch method)
Warm desizing (50-70°C by the warm pad batch method)
Hot desizing (80-90°C using J-box, U-box, winch or jig-based procedures)
HT (high temperature) desizing (90-100°C using the pad-steam method)
Enzymatic shock desizing (80-95°C applied on a washing range)
Injecta (Benninger) module desizing
The product is compatible with organic sequestrants and it can be used with anionic wetting agents, although with some anionic products the enzyme effectiveness is decreased.
Combined desizing and bioscouring for eco-friendlier processing
Textile industrial interest with enzymes has aroused green awareness (Photo source: Novozymes) |
Novozymes from Denmark introduced a commercial pectase lyase enzyme for the bioscouring of cotton in 1999. It has been established that the pectins in native cotton fibres are present in the fibre primary wall and function as water-insoluble substances that bind the waxes and proteins together in the primary wall matrix.
Pectate lyase enzymes act by hydrolysing the pectins, thereby degrading the pectin-wax interface in-between the wax and the cotton primary wall. The cotton waxes are further broken down by the enzyme enabling a bioscouring action to be accomplished. The reduction in the residual pectin levels in the cotton fibres was sufficient to ensure that the enzyme treated fibres exhibited good wetting ability and dyeing properties.
More recently, Novozymes introduced an alkaline amylase with a broad activity spectrum, capable of application over the pH range 5-10 and from 20-85°C. This has enabled combined desizing and bioscouring to achieve commercial acceptance in a number of textile mills. This combination of a pectate lyase with an alkaline amylase from Novozymes can be applied not only to cotton fabrics, but has also been applied to linen and linen blends in China with the advantages of reducing the number of processing steps and the chemical auxiliaries necessary for treatment.
For linen and linen blend combined desizing and bioscouring pretreatment, the method was found to offer the following benefits:
Greater flexibility in the combined desizing / bioscouring stage
A single pretreatment recipe for all production
Softer fabric handle combined with a smoother surface appearance and less hairiness
Lower cost and less effluent (caustic soda from mercerising)
Chemical cost saving due to decreased auxiliary use
More consistent dyeing
A major advantage was the increase in production capacity (around 30%) because of the decrease in the number of pretreatment steps.
The application of Novozymes’ combined enzyme desizing and bioscouring system to tightly woven light weight cotton fabrics in an Indian mill resulted in better final quality fabric, and ease of operation, combined with lower operating costs.
It is clear that enzyme treatments for textile wet processing are now yielding substantial benefits to textile wet processors in terms of improved and safer fabric pretreatment, reduction in the number of processing steps, improved fabric appearance and performance, improved production economics, and lower effluent loads. In an industry sector in which cost-competitiveness is global, and reduced lead times and improved service to customers are paramount, enzyme treatments also now offer the potential for improved fabric appearance, handle and performance.
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Copyright © Adsale Publishing Limited. Credit goes to Adsale Industry Portal when used.
We reserve the right to take legal action against any party who reprint any part of this article without acknowledgement. For enquiries, please contact the Editorial Department. |
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