By: Ajay Joshi, Tanveer Malik and Sushanta Naik
Assistant Professor, Shri Vaishnav Institute of Textile Technology
Shri Vaishnav Vidyapeeth Vishwavidyalaya, Indore
ABSTRACT
Textile pre-treatment is a cleaning process wherein all the primary impurities, which are present naturally in fibre and secondary impurities which get in due of action during spinning, weaving and further process. There are many processes in this pre-treatment including desizing, bleaching and scouring etc. wherein many different types of chemicals are used which may sometimes cause drastic effects on the fabric and may hinder further process
badly. Here in concerned paper, we have discussed these pre-treatment process in terms of biological manner that is with application of enzyme, what are the benefits in this conventional process with our main focus on pre-treatment of cotton. We have also focused on ways of reduction of wastage of water which is also the biggest problem in case of
chemical pre-treatment.
INTRODUCTION
Natural fibres and Synthetic fibres contain primary impurities that are contained naturally, and secondary impurities that are added during spinning, knitting and weaving process. Textile pre-treatment is a series of cleaning operation. All impurities, which cause adverse effect during dyeing and printing is removed in the pre-treatment process. Pre-treatment process includes desizing, scouring and bleaching which make subsequent dyeing and softening process easy. Uneven pre-treatment might cause drastic deterioration in qualities of processed products.
Biotechnology can be defined as the “application of scientific and engineering principles to the processing of material by biological agents to provide goods and services”.
The earliest evidence of biotechnology includes baking of bread using yeast by ancient Egyptians. Early methods of producing coloured pigments from natural viable resources may also be cited as primitive technology. Today enzymes can be used in large scale in medicine, food analysis, transgenic animals and plants and also in domestic detergents fields. In textile applications, the knowledge of specific action of enzymes-amylases for starch splitting began around 1857, when malt extract was used to remove size from fabrics before printing. The use of enzyme in pre-treatment process has found much broader acceptance. At present priority areas are scouring and bleaching of cellulosic fibres and carbonising, bleaching and shrink-resistance treatment of wool. Enzymes were traditionally used for stone washing and bio-polishing of cotton fabrics and garments. Also, enzymes have been incorporated with
detergents to remove fibre fuzz and brighten the colour of fabric.
Pre-treatment processes, process steps and techniques which are made in order to improve the hydrophilic characteristics and affinity for dyestuffs and finishing treatments of the fibres depend on the kind of fibre to be treated. However, these processes can be both harmful to the environment and to the fibre. Therefore, studies about the usage of enzymes in the pre-treatment of protein based textile materials has increased. The enzymes are natural
proteins so they can be easily and quickly biologically broken into pieces. By means of enzyme usage, the pre-treatment processes become ecological and the natural properties of the product are kept.
ENZYMES
Enzymes are biological catalysts that accelerate the rate of chemical reactions. The reaction happens with lower activation energy which is reached by forming an intermediate enzyme – substrate. In the reaction itself the enzymes are not used up, they do not become a part of the final product of the reaction, but only change the chemical bonds of other compounds. After the reaction is complete, the enzyme is released again, ready to start another reaction.
Usually, most enzymes are used only once and discarded after their catalytic action. All known enzymes are proteins.
They therefore consist of one or more polypeptide chains and display properties that are typical of proteins. Some enzymes require small non-protein molecules, known as cofactors, in order to function as catalysts. Generally, they are active at mild temperatures. Above certain temperature the enzyme is denaturated. Enzymes have a characteristic pH at which their activity is maximal. Extreme pH values influence on the electrostatic interactions within the enzyme, leading to inactivation of enzyme.
Other important factors that influence the effect of enzymatic processes are the concentration of enzyme, the time of treatment, additives like surfactants and chelators and mechanical stress. Enzymes can work at atmospheric pressure and in mild conditions with respect to temperature and acidity (pH). Most enzymes function optimally at a temperature of 30ºC- 70ºC and at pH values, which are near the neutral point (pH 7). Enzyme processes are potentially energy saving and save investing in special equipment resistant to heat, pressure or corrosion. Due to their efficiency, specific action, the mild conditions in which they work and their high biodegradability, enzymes are very well suited for a wide range of industrial applications.
Enzymes work only on renewable raw materials. Fruit, cereals, milk, fats, cotton, leather and wood are some typical candidates for enzymatic conversion in industry. Enzymes are used in the textile industry because they accelerate reactions, act only on specific substrates, operate under mild conditions, are safe and easy to control, can replace harsh chemicals and enzymes are biologically degradable i.e. biodegradable.
PROPERTIES OF ENZYMES USED IN TEXTILES:
1. Enzyme accelerates the reaction
An enzyme accelerates the rate of particular reaction by lowering the activation energy of reaction.
The enzyme remains intact at the end of reaction by acting as catalyst.
2. Enzymes operate under milder condition
Each enzyme have optimum temperature and optimum pH i.e. activity of enzyme at that pH and temperature is on the peak.
For most of the enzyme activity degrades on the both sides of optimum condition.
3.Alternative for polluting chemicals
Enzymes can be used as best alternative to toxic, hazardous, pollution making chemicals.
Also some pollutant chemicals are even carcinogenic. When we use enzymes there is no pollution.
4. Enzyme acts only on specific substrate
Most enzymes have high degree of specificity and will catalyse the reaction with one or few substrates.
One particular enzyme will only catalyse a specific type of reaction. Enzymes used in desizing do not affect cellulose hence there is no loss of strength of cotton.
5. Enzymes are biodegradable
At the end of reaction in which enzymes used we can simly drain the remaining solution because enzymes are biodegradable and do not produce toxic waste on degradation hence there is no pollution.
PRETREATMENT OF COTTON
Cellulosic fibers, whether they are natural or regenerated need some form of treatment to make them suitable for dyeing or finishing. This treatment, which removes natural or added impurities, is called preparation pre-treatment. It can be carried out on loose fibers, yarns or fabrics.Unless the fibre is uniform in whiteness, absorbency and chemical composition and has law levels of impurities it is unlikely that it will take up dye or finish in a uniform way or to the maximum extent possible. The prepared fabrics must have ‘fitness for purpose’. For example, a fabric to be given a water-repellent finish must be free from residual surfactant. Fibers used for medicinal end uses must have a low ash content.
All such requirements must be met against economic constraints relating to the costs of water, labour, plant, chemicals and energy. The first, water, is particularly important. Not only can the purity of the water affect many of the requirements, but the volume and the environmental impact of effluent can, and is, adding increasingly to the cost per kilogram of textile produced. The effluent with the highest biological/chemical oxygen demand stems
from pre-treatment.
Successful preparation depends on four factors:
The level and type of impurities present.
The chemicals used in the various stages of preparation.
The water supplies.
The type of machinery used.
ENZYME APPLICATION IN PRETREATMENT PROCESS
The fabric should be free from natural and added impurities before it goes colouration. Some of the chemicals like caustic soda, soda ash, hydrogen peroxide, hydrochloric acid, detergent and auxiliaries that are used at different stages preparatory process to remove such impurities are found to be harmful to the environment. Modern wet
processing industries are followed the enzymes in the preparatory process instead of using harmful chemical because enzyme are more convenient, effective and environment friendly.
The application of enzymes has many advantages compared to conventional, non-enzymatic processes. Enzymes can be used in catalytic concentrations at low temperatures and at pH-values near to neutral.
VARIOUS APPLICATIONS OF ENZYMES DESIZING
In the textile industry, one of the first areas which enzyme research opened up was the field of desizing of textiles. A size is a substance that coats and strengthens the fibers to prevent damage during the weaving process. Size is usually applied to the warp yarn, since this is particularly prone to mechanical strain during weaving. The size must be removed before a fabric can be bleached and dyed, since it affects the uniformity of wet processing.
Previously, in order to remove the size, textiles were treated with acid, alkali or oxidising agents, or soaked in water for several days so that naturally occurring microorganisms could break down the starch. However, both of these methods were difficult to control and sometimes damaged or discoloured the material. But by using enzymes, which are specific for starch, the size can be removed without damaging the fibers.
During the weaving process the warp (chain) threads are exposed to considerable mechanical strain. In order to prevent breaking, they are usually reinforced by coating (sizing) with a gelatinous substance (size). Cotton fibres and cotton/synthetic fibre blends are sized, i.e. they are coated with a strengthening, adhesive like material (usually starch in native or modified form or a starch based material) to prevent damage during the weaving process.
Small amounts of fats or oils may be also added to the size, with the aim of lubricating the warp coat surface. Sizing is the process where size is applied to warp yarns for weaving. The purpose of size is to protect the yarn from the abrasive action of the loom. The size must be removed (desizing) before a fabric can be bleached and dyed, since it affects the uniformity of wet processing.
Desizing is the process of removing the size material from the warp yarns in woven fabrics. Sizing agents are selected on the basis of type of fabric, environmental friendliness, ease of removal, cost considerations, effluent treatment, etc. Desizing, irrespective of what the desizing agent is, involves impregnation of the fabric with the desizing agent, allowing the desizing agent to degrade or solubilise the size material, and finally to wash out the degradation products. Various types of desizing methods are available. If the size is water soluble, an alkali wash with detergents may be used. Oxidative chemicals such as persulphate and alkali or bromide and alkali may also be used at high pH and temperature. Alternative eco-friendly desizing agents are available in the market in the form of enzymes. Amylases are used to remove starch.
The major desizing processes are:
1. Enzymatic desizing of starches on cotton fabrics
2. Oxidative desizing
3. Acid desizing
4. Removal of water-soluble sizes
ENZYMATIC DESIZING
Enzyme desizing is the most widely practiced method of desizing starch. In the textile industry amylases are used to remove starch-based size for improved and uniform wet processing. Amylase is a hydrolytic enzyme which catalyses the breakdown of dietary starch to short chain sugars, dextrin and maltose. The advantage of these enzymes is that they are specific for starch, removing it without damaging to the support fabric. An amylase enzyme can be used for desizing processes at low-temperature (30-60ºC) and optimum pH is 5,5-6,5.
The enzymatic desizing process can be divided into three stages:
1. Impregnation: Enzyme solution is absorbed by the fabric. This stage involves thorough wetting of fabric with enzyme solution at a temperature of 70ºC or higher with a liquid pick up of 1 litre per kg fabric. Under these conditions there is sufficient enzyme stability (temperature, pH, calcium ion level govern the stability). During this stage gelatinization of the size (starch) is to the highest possible extent.
2.Incubation: The size is broken down by the enzyme. Long incubation time allows a low enzyme concentration.
3.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 (with NaOH) at the highest possible temperature. The use of the enzyme depends on the kind of size. If there are chemicals used in the size to which the enzyme is not resistant, then it is impossible to use the
enzyme.
Environmental and Economic Benefits:
Avoidance of chemical fiber damage
Increased biodegradability of effluent
Less handling of aggressive chemicals
SCOURING
Scouring of cotton textiles is an essential treatment in textile wet processing in order to obtain a sufficiently hydrophilic fabric. During scouring, waxes and other hydrophobic materials are removed from the cotton fibers. In nature these non-cellulosic materials create a physical hydrophobic barrier to protect the fiber from the environment throughout its development. In aqueous textile processing the waxes and pectins impede wetting and wicking, subsequently obstructing aqueous treatments. Conventionally, scouring is done in a hot aqueous solution ofNaOH to remove hydrophobic components from the primary wall (e.g. pectin, protein and organic acids) and the cuticle (waxes and fats).
However, alkaline scouring is a nonspecific process. The use of high concentrations of NaOH also requires neutralization of wastewater. Even though alkaline scouring is effective and the costs of NaOH are low, the scouring process is rather inefficient because it consumes large quantities of water and energy. It is clear that this process needs to be improved considerably to meet today's energy and environmental demands. In the last couple
of years, a lot of research has been directed to replace this process with an enzymatic one.
BLEACHING
Scouring is regularly followed by a bleaching process, which removes the natural pigments of cotton fibres. Mainly flavonoids are responsible for the colour of cotton. The most common industrial bleaching agent is hydrogen peroxide. Cellulose fibres are most frequently bleached with hydrogen peroxide resulting in high and uniform degrees of whiteness. Namely, thebleaching 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.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 chelators are added to the treatment bath.
Hydrogen peroxide (redox potential is 1.78 eV) (1) 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.
Drawback of the conventional bleaching process
1. Destabilization of peroxide by selection of a higher bleaching temperature.
2. Addition of external stabiliser in order to stabilise the peroxide.
3. This makes us think as to why not lower the temperature of bleaching to a level that without decreasing the scouring efficiency we can achieve same bleaching efficiency at that obtained in conventional high temperature process.
4. On this idea, Rossari, which has always believed in innovating ways to make the process more eco- friendly and cost effective, has brought a new molecule to revolutionize the bleaching process used in textile industry. We are launching this molecule in the name of ‘Koolwhite 2020’ in the commercial market.
ENZYMATIC BLEACHING
Conventional preparation of cotton requires high amounts of alkaline chemicals and consequently, huge quantities of rinse water are generated. However, radical reactions of bleaching agents with the fibre can lead to a decrease in the degree of polymerisation and, thus, to severe damage. Therefore, replacement of hydrogen peroxide by an enzymatic bleaching system would not only lead to better product quality due to less fibre damage but also to substantial savings on washing water needed for the removal of hydrogen peroxide.
An alternative to this process is to use a combination of suitable enzyme systems. Amyloglucosidases, pectinases, and glucose oxidases are selected that are compatible concerning their active pH and temperature range.
Compared with the traditional methods of removing bleach, the enzymatic process results in less polluted wastewater and/or reduced energy and water consumption, depending on the process that enzymes replace. Most of the chemical agents used for the neutralisation of residual bleach are hazardous to handle and problematic to the environment. These reducing agents also need to be adequately removed prior to dyeing. Residual reducing agents
can have a detrimental effect on shade reproducibility from batch to batch. In contrast, enzymes can be handled safely, have no effect on dye and can even be used in the same bath as dyeing. Furthermore, they are completely degradable in nature.
Various advantages of using the enzyme are that there is no:
Adverse effect on dyestuffs
Need for heating
Need for rinsing prior to dyeing
Risk of harmful overdosing
Environmental assessments of enzymatic scouring and bleach clean-up The use of enzymes in scouring and bleach clean-up as alternatives to chemical treatment and rinsing with hot water, respectively, led to considerable environmental improvements at the two production lines at textile mills in China. The explanation is that a small amount of enzyme saves considerable amounts of energy and water in both cases and also chemicals in the case of scouring. Sensitivity analyses indicate that the general conclusion of the assessment holds up under different energy supply scenarios although the sizes of the reductions in environmental impacts are subject to much variation and
uncertainty. The impact of the transport of enzymes from the manufacturer to the final user is insignificant even though the transportation distance is long. The main findings of the study are therefore applicable to other textile mills with similar production systems elsewhere in the world.
The magnitude of the environmental improvements obtained by replacing the existing production methods with the enzymatic technologies are highly dependent on the type of fuel used and the actual production conditions. An estimation of environmental improvements at other factories must therefore rely on specific information on production processes and energy supply systems. The study has not addressed the removal of bleach with a reducing
agent and further environmental assessments are required before any conclusions can be made about this method.
COMPARISON OF CONVENTIONAL & ENZYMATIC PROCESSING:
1. ABSORBENCY: In case of cotton, best absorbency was obtained in with conventional process. Bioprocess processing shows good absorbency as such but somewhat uneven. But on viscose both same actions.
2. WEIGHT LOSS ON PROCESSING: Weight loss is higher in case of conventional process compared to bioprocess. This becomes our assumption that more impurities have been removed from fabric treated by conventional rather than in bioprocessing.
3. STRENGTH LOSS: Compared to conventional process, loss of strength is lower in bioprocessing. This means that very little damage occurs when fibre are treated with enzymes.
4. DYE UPTAKE %: Cotton treated by bioprocess absorbed more dyes than those treated conventionally, irrespective of shade%. While for viscose the reverse occurred. The result for this might be possibly that while treating cotton, enzyme might have reduced crystallinity and increased the amorphous region as a result the fabric absorbed more dyes. But structure of viscose is already more amorphous than cotton and therefore the enzyme had no effect on viscose.
CONCLUSION
Biotechnology offers a wide range of alternative environmentally friendly processes for the textile industry to complement or improve the conventional technologies. The use of various enzyme is in the early stages of development, but their innovative applications are increasing and spreading rapidly into all areas of textile processing. The textile industry was identified as a key sector where opportunities available from adapting biotechnology are high but current awareness of biotechnology is low. In textile processing the enzyme can be successfully used for preparatory process like desizing, scouring and bleaching. These enzymatic processes are giving the similar results as that of conventional methods. Though these enzymatic processes we can reduce the water consumption, power energy, pollution, time, and increasing quality. These are just a few applications of Biotechnology; however, many such potentials are yet to be explored.
Pollution free processes are gaining ground all over the world. In this scenario, enzymes emerging as the best alternative to the polluting textile processing methods. Enzymes are not only beneficial from ecological point of view, but they are also saving lot of money by reducing water and energy consumption which ultimately reduce the cost of
production. It seems that in the future it will be possible to do every process using enzymes.Enzyme producing companies constantly improve their products for more flexible application conditions and a more wide-spread use. The textile industry can greatly benefit from the expanded use of these enzymes as highly specific and efficient, non-toxic, environmentally friendly compounds, work under mild conditions (pH, temperature) with low water consumption that results in reduced the use of harsh chemicals in the textile industry, process times, energy and water savings and improved product quality. The main hindrance in using enzymes is their high cost. Enzymes are emerging in a big way in the field of textile wet processing. If their cost can be managed, enzymes can be put to use in a much bigger way for textile processing applications.
There is a huge potential to reduce energy costs in the pre-treatment processes in a textile mill. For example, the process of bleaching needs to be addressed in future in order to reduce energy costs and enhance the quality of bleached fabric. Enzymes could provide the answer. It should be mentioned that the vast majority of enzyme applications today are used for the treatment of cotton. There is also a need to find alternative ways of dealing with an array of different sizing agents other than starch. Enzymes could even be used in future to break down dyestuffs in the effluents from dyehouses and denim finishing laundries. Dyestuffs remain one of the most difficult substances to remove at an effluent treatment plant. With all these opportunities and the need to move towards sustainable development, industrial enzymes could provide some of thesolutions the textile industry is looking for in the future.
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