The processing sector is undoubtedly the most significant stage in the textile value chain. C.N. Sivaramakrishnan discusses the technologies that are contributing to more sustainable processing.
The Indian textile industry is as diverse as the country is and as complex an entity. There is an organised, decentralised sector and down the line, there are weavers, artisans as well as the farmers. The spectrum of technology is widespread, right from handmade to semi-mechanical and highly sophisticated information- and micro-processor based technologies. The processing sector is undoubtedly the most significant stage in the value chain contributing to the end user an array of properties like easy care and wrinkle free finishes besides aesthetic value addition in terms of colours, motifs and designs. The value addition at this stage is often manifold, with a range of other functional finishes like hydrophilic, stretch-back effect, oil and water repellant, and peace effect, to name a few. Specialty chemicals have played a significant role in the production of fibres and textiles. Awareness of chemical reactions, polymer sciences and understanding complex biochemical reactions have resulted in what we see as a dramatic shift in the minds of a processor.
The tools available to a processor today are plenty and technological positioning of ideas from other streams of sciences converts a modern day scientist into a virtual magician, especially when it comes to polymers. Fibre blends bring in a series of substrates available for any type of application including routine clothing to exotic spacewear suits. Filament or staple yarn properties can be enhanced to get synergistic updates with several available techniques.
Textile manufacturing begins with the production or harvest of raw material. Fibre used in textiles can be harvested from natural sources like wool and cotton or manufactured from regenerative cellulosic materials like rayon acetate or they can be entirely synthetic like polyester and nylon. After the raw, natural or manufactured fibres are shipped from the farm or the chemical plant, they pass through four main stages: yarn production, fabric production, finishing and fabrication.
In yarn production, natural fibres, predominantly cotton and wool, are cleaned, carded or combed and then spun into yarn. Fabric production is the second step. It involves either weaving or knitting. Finishing represents the third step. Fabrics undergo a series of operations like singeing, de-sizing, scouring, bleaching, dyeing, printing, finishing and finally, fabric formation.
Textile processing generates many waste streams, including water-based effluents and air emissions, solids and hazardous wastes. The nature of the waste generated depends on the type of textile facility, processes adopted, technologies operated, type of fibres and chemicals used. Most processes performed in the dye houses cause atmospheric emissions. Gaseous emissions have been identified as the second-greatest pollutant after effluents. Unfortunately, there is no clear data available on air emissions. Most of the published data is based on mass balance calculations and not as direct measurements. Air pollution is the most difficult type of pollution to sample, test and quantify in an audit. Measurement techniques such as direct reading tubes and gas chromatography or mass spectrometry have been used to collect more reliable data.
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.
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 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.
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.