The processing sector is undoubtedly the most significantstage in the textile value chain. C.N. Sivaramakrishnan discusses thetechnologies that are contributing to more sustainable processing.
Specialty chemicals have played a significant role in textile production, contributing to the development of innovative fibres and textiles. A deep understanding of chemical reactions, polymer sciences, and complex biochemical processes has led to significant advancements in textile processing.
Modern processors have access to a plethora of tools and technologies, allowing them to enhance yarn properties, create fibre blends for various applications, and produce textiles with diverse characteristics, from routine clothing to specialized spacewear suits.
The textile manufacturing process typically involves several stages, including yarn production, fabric production, finishing, and fabrication. Yarn production starts with the cleaning and processing of raw materials like cotton or wool, followed by spinning into yarn. Fabric production involves weaving or knitting, while finishing includes operations like singeing, de-sizing, scouring, bleaching, dyeing, printing, and final fabric formation.
However, textile processing generates various waste streams, including water-based effluents, air emissions, solids, and hazardous wastes. The type and volume of waste generated depend on factors such as the textile facility's processes, technologies, fibre types, and chemicals used.
Air emissions, resulting from processes carried out in dye houses, represent a significant concern. Gaseous emissions, though challenging to measure and quantify, are considered a significant source of pollution in textile processing. Measurement techniques such as direct reading tubes, gas chromatography, and mass spectrometry have been employed to gather reliable data on air emissions and address this environmental challenge.
The mantra for modern textile wet processing is to stick to aqueous eco-friendly routes, following the principles of green chemistry. Awareness and monitoring the carbon footprint are of paramount importance in the art of present-day textile processing. Further, certain newer tools which are slowly creeping into these textile technologies and which are also being successfully used are manipulation with laser, radio waves drying tools, plasma bonding techniques, ionic liquids for solvent effects, ultrasonic treatments, supercritical fluids and enzymatic treatments. All these technologies are contributing to more sustainable processing.
Environmental concerns
The main environmental concern in the textile industry is about the amount of water discharged and the chemical load that it carries. Air emissions are usually collected at their point of origin because they have long been controlled and there is good historical data on air emissions from specific processes. This is not the case with water contamination. The various streams coming from the different processes are mixed together to produce a final effluent whose characteristics are the result of a complex combination of factors such as the types of fibres processed, the techniques applied and the types of chemicals and auxiliaries used. Chemicals give textiles colour and performance that a consumer demands. Chemicals are not bad per se, but their impact depends on how they are used. A safe chemical used wrongly can be many times more polluting than a classified chemical used correctly.
Emerging technologies in textile processing focus on:
- Minimum use of resources like water and energy by using Best Available Technologies
- Reducing chemical consumption
- No or low pollution load
- Elimination of harmful and toxic chemicals
Emerging technologies
Emerging technologies can be defined as manufacturing processes or product technologies that reduce pollution or waste, energy use, or material use in comparison to the technologies that they replace. Emerging technologies will always use best available technologies keeping energy savings in mind as policy makers and regulators are addressing environmental concerns in industries with the application of abatement strategy. This improves the environmental performance of the industries and consequently limits pollutant discharges and helps the environment.
Man-made cellulose deserves special treatment, as regenerated cellulosics are here to stay. The xanthate process yields viscose rayon, both in the staple and the filament form. This sector is undergoing lot of transformation with fresh capital being injected into it and a totally global approach to all environmental issues. Here, technologies are constantly being upgraded to give desirable features to the various fibres produced. A brand new technology which has made inroads in this area of regenerated cellulosics is the dissolution of cellulose ionic liquids and subsequent regeneration.
Thermal energy in a dye house is generated from boilers and is used in dyeing machines, stenters, dryers, thermic fluid heaters, etc, which normally operate at low efficiencies. This results in proportionately high fuel consumption and high emissions. Most dye houses rely heavily on state electricity boards to meet their electrical energy requirements and regularly face the problem of power shortage. The problem is aggravated by increase in power consumption due to poor electrical equipment like motors. Select dyes with assistants and dyeing equipment have been developed to drastically reduce the dyeing time of polyester resulting in significant savings in time and energy.
EMERGING TECHNOLOGY
Low add-on equipment in dyeing: Such processing equipments operate to uniformly apply the fabric with a minimum amount of liquid necessary in semi-continuous and continuous processing systems to conserve energy. Foam finishing is a novel application technique for treating porous substrates with foamed chemicals at low, wet pick-ups. It involves use of a rapidly-breaking low-density foam or froth as the delivery medium for finishing chemicals, precise metering and flow control for delivery of foam to the substrate, pressure-driven impregnation of the foam into the substrate, and an applicator system designed to allow uniform high-speed application and collapse of the foam in a single step. The semi-stable foam is necessary for spontaneous foam collapse and spreading through the substrate, and is in contrast to stable foams specified in various foam coating processes that normally require a separate step to break and distribute the foam through the textile. Foam finishing leads to energy savings anywhere between 30 to 50 per cent.
Low liquor ratio dyeing machine: Reduction in water use will contribute to significant energy savings in the dyeing process including various wet treatment and drying unit operations. Water consumption needs to be reduced because it is linked to the overall water supply cost including that of drainage.
To reduce processing bath ratio, it is necessary to investigate some measures. In general, dyeing and finishing methods are classified into the batch and continuous processing methods and it is recommended to use the latter method where a low bath ratio is desired. However, depending on the details of processing requirements, there are often instances in which the batch method has to be employed. In such cases, batch processing machines which allow lower bath ratios such as the jigger, wince, beam, pad roll and jet flow types should be selected.
Bath ratio has a direct influence on production cost. Recently, low bath ratio processing machines which are built-in with the above mechanisms have been developed and put on the market.
Automated chemical dosing and colour kitchen: Special mention has to be made of the dye bath monitoring system which enables dyers to monitor dye concentration in the dye bath while measuring temperature, pH and conductivity of the dye bath simultaneously. A good, automated colour kitchen considerably reduces the number of dyes added and the levels of reprocessing. The right first time ratio shows a good percentage anywhere from 40 to 80 per cent. This effectively translates into average savings of 5 to 10 per cent in energy and water use, and a reduction on the consumption of dyestuffs and chemicals to around 10 to 20 per cent.
Equipment modification: Modifying existing production equipment and utilities by adding measuring and controlling devices runs the processes at higher efficiency and lower waste and emission generation rates.
Technology change: Replacing technology, processing sequence and/or synthesis pathways minimises waste and emission during production.
Modernisation in dyeing and printing technologies
The quality of dyeing can be improved by the use of computer product design, measured by computer colour matching and other computer graphic arts technologies and methods. High purity dyes with short processing sequences are used in waterless technologies, ink jet printing and low temperature plasma processing. Digital printing is a growing segment which is replacing flat screen printing machines due to similar costs and production speeds.
Process modifications: Cationic and anionic dyeable fibres
Almost all types of polymers including cellulosics are made with cationic dye by injecting anionic species in the dope. This is similar to the acrylic fibre technology where some anionically charged co-monomer forms an integral part of the fibre's backbone. Anionic dye dyeable fibres can be obtained by binding certain cationic molecules on the fibre's polymeric. The processing parameters and workability dictates the selection of chemical molecules, some typical type for regenerated cellulosics whereas some other types for hydrophobic synthetic fibres. Nitrogen and quaternary ammonium compounds chemistry play a pivotal role here and success has dawned on quaternary polymeric molecules that do not affect the spinnability characteristics of these polymers. Naturally occurring Chitosan has emerged a forerunner. Its dosages and stability are being studied and perfected.
Salt-free, high fixation dyeing of reactive dyes
This technology will catch up in the foreseeable future as pollution norms will only get stricter. Techniques have evolved where pigment dyeable substrates are made available for routine processing by these techniques. The charging type cationiser creates positive charge sites along the polymer body. This helps to suck in the reactive dyes to bring them in close proximity with the fibre without the aid of electrolytes like salt, only to get reacted by the alkaline treatment and subsequent fixation.
The cationiser treated textile, essentially for the cellulosics, does not require salt to push the dye molecules on to the fibres, helping in offering a system that is salt-free or low in salt. The products that are acceptable for this type of treatment are expected to give such effects without any heat treatment or curing before dyeing. Controlled molecular weights quaternaries following the cationic molecule chemistry are synthesised for this activity.
Role of specialty chemicals
Specialty chemicals play a very important role in textile processing. New chemical technologies have played a pivotal role in maintaining the growth of textile chemicals in accordance with legislation on health, safety and environment. Textile wet processing chemicals can lower the cost of textile chemical production. An approach to lower chemical costs is to provide the chemicals in bulk or semi-bulk containers doing away with the cost of drums and drum disposal. Rising quality specifications accompanied by increasing cost pressure makes for a very challenging situation for textile processors and the textile industry as a whole.
There is tremendous pressure to deliver textile auxiliaries at low cost. Continuing the decade-old trend of having the strongest growth in the textile industry, textile chemical manufacturers have come up with unique ideas for making the textile chemicals available in concentrated form by reducing cost.
Low temperature bleaching: Pre-treatment of fabric in any form whether yarn, woven fabric or knitted hosiery is the basic requirement for further textile processing for whites or dyeing or printing. This pre-treatment makes the fabric uniformly absorbent and white and is the basic requirement for successful dyeing and finishing. Special bleach activators have been developed to bleach at low temperatures. Other emerging trends in the areas of processing include reusing the dye bath, recovery of synthetic sizes and counter-current washing.
Textile producers can sustain their competitiveness in a liberalised and competition-driven market only when they are able to develop new markets and boost productivity by raising their real net output and investing in emerging technology. Added value can be obtained only by shifting away from labour-intensive mass products and concentrating on new, high-quality specialised products.
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