Environmental impacts of denim manufacture
W. Schrott1, R. Paul2 1Hof University of Applied Sciences, Münchberg, Germany; 2Hohenstein Institut für Textilinnovation gGmbH, Boennigheim, Germany
20.1 Introduction Denim, especially blue jeans, is the biggest single textile product type sold around the world. This is because of its popularity in all geographic regions, social strata and age groups. The production chain is optimised for bulk production and with the capacity to meet this global demand. This means that the overall environmental impact of denim manufacture is significant. Improvements in each step of denim production offer potentially significant reductions in overall environmental impact as well as cost saving for the industry. In assessing the environmental impact of denim manufacture, it is important to take a holistic approach that accounts for all of the activities involved in the creation of a product, such as raw material extraction, manufacturing, transportation, use and disposal. A standard approach is Life Cycle Assessment (LCA), which is internationally recognised in standards such as ISO 14040 and 14044 (Muthu, 2014). There are four key stages in LCA: • Goal and scope definition. • Inventory analysis. • Impact assessment. • Interpretation.
The first stage is critical in establishing the boundaries of the LCA, for example whether the study includes all production, distribution and use stages for the product from ‘cradle to grave’, including how finished products are used and then disposed of by consumers. Inventory analysis includes categories such as energy requirements, raw material needs, emissions (to air, water and land) and waste related to production of raw material and finished products. Life cycle impact assessment evaluates the potential environmental impacts of a product throughout its life, whilst interpretation identifies the most significant types of impact and makes recommendations for improvement (Escamilla and Paul, 2014). In 2007, Levi Strauss & Co. conducted an LCA study to assess the environmental impact of a pair of Levi’s jeans from cotton seed to landfill. This study has provided insights on the environmental impact caused by jeans outside the bounds of the direct sphere of influence of the company. The environmental impact was assessed in the Denim. http://dx.doi.org/10.1016/B978-0-85709-843-6.00020-2 Copyright © 2015 Elsevier Ltd. All rights reserved.
following categories, which environmental scientists and LCA experts used to calculate overall environmental impact: • Contribution to climate change: quantifies amount of greenhouse gas emissions. • Energy use: quantifies how much energy is used in production. • Renewable energy use: percentage of energy use from renewable sources. • Water consumption: measures water usage in cubic metres. • Land occupation: amount of land needed to produce a product. • Qualified sustainably grown fibre content: a content analysis of fibres grown under a recognised cultivation program to address areas of sustainability. • Waste generation: surveys the primary solid waste content during production and finishing. • Materials efficiency: how much of the primary materials end up in the final product. • Recycled content: assesses the amount of materials used from post consumer recycled sources. • Land transformation: amount of land transformed from its original state by production. • Eutrophication: measures the impact of harmful nutrients discharged to freshwater bodies.
From this work, an idea was born in 2008 to develop a life cycle based product environmental impact assessment method (E-valuate) that relied heavily on primary data, making it both actionable and dynamic. This method was created in the hope that key stakeholders could be informed about how their business decisions influence the environmental attributes of the products they design, produce, merchandise and sell. It was observed that the greatest opportunity to reduce the environmental impact of a new or existing product occurs during the design phase of its life cycle. Therefore, the primary objective of this LCA approach was to provide designers and developers with the information they need to produce more sustainable products. A secondary objective was to provide a scientific method to support any claims of environmental improvement of products. Although not an initial objective of this effort, it was later realised that the methodology also provides a rigorous means to communicate environmental performance to suppliers (Levi Strauss & Co.). Based on these studies, Levi Strauss & Co. has later launched the Water
Environmental impacts of denim manufacture
are manufactured, such as the working conditions of those involved in the industry. This broader concept of sustainability is enshrined in the ISO 26000 standard for social responsibility and sustainability. In the future, the concept of eco-denim will need to account for these wider social and ethical concerns. Whilst there are variants to produce special effects in denim fabrics, the standard manufacturing and distribution process for denim is as follows: • Cotton production (including growing and harvesting). • Yarn production (including spinning and warping). • Warp yarn dyeing (including dyestuff and auxiliaries production and use). • Sizing (including size production and use). • Weaving. • Flat fabric finishing. • Cutting and sewing (garment manufacturing). • Garment washing/finishing (including the production and use of chemicals, auxiliaries, enzymes, etc.). • Retail (marketing, logistics/distribution, sales outlets). • Use by consumers (including laundering). • End of life (disposal or recycling/reuse).
As noted, it is important to look at the entire life cycle of the product including raw material production, distribution and sale, use by consumers and the end of the useful life of the product (recycling or disposal). Current LCA studies identify the washing of jeans, both during the production process and also by consumers during use, as having the greatest environmental impact. Manufacturers also have a responsibility to educate and inform consumers about washing clothing in a more environmentally friendly way (by the use of shorter, lower temperature washes), as well as to manufacture apparel that can be cleaned effectively at lower temperatures and minimises wastewater residue.
20.2 Cotton production for denim The most important raw material in denim production by volume and value is cotton, and denim accounts for nearly 20% of global cotton production. As a natural product, the quality of cotton varies depending on its geographical origin as a result of different soil and climate conditions, and from season to season. As a result, the skilled mixing or blending of different fibre varieties is necessary to ensure consistent raw material quality as the basis for good product quality at the end of the production chain. This need for quality can conflict with environmental needs. As an example, good quality cotton fibre may depend on extensive irrigation, the use of fertilisers and various kinds of pest control. Globally, cotton production can be segmented into: • Conventional cotton farming systems. • Integrated pest management (IPM) systems. • Organic cotton production.
The environmental pros and cons of each method are compared and discussed in detail in the literature. It is important to be aware that organic production may not
always be the best option, if conventional and IPM systems make the best use of scare land resources and produce the highest yields. On the other hand, all support processes and products used to optimise cotton yields must be harmless to the environment as assessed through LCA, and should not have an intensive energy consumption or carbon footprint. It is important to promote the use of chemicals that are not harmful to the environment and that biodegrade to leave harmless residues. In addition, it is essential to optimise dosages of fertilisers to avoid excessive consumption of chemicals that may be environmentally damaging to produce and use. One important aspect of cotton production is recyclability, and at the end of life of a garment, there are many possibilities for recycling it. High quality cotton fibres may be converted to superabsorbent polymers by chemical modification, and can be used for the production of medical textiles such as diapers, incontinence products, etc. This may not apply to cotton from denim that was previously dyed and finished, but in that case the denim may be triturated and used for the production of nonwoven felts to be used as thermal and acoustic insulation materials in automobile and construction sectors. It has also the potential to be converted into art and drawing paper, by proper dissolution and further deposition of the pulp. It may also be possible to produce cellulose in powder form, which can be used as fillers or for blending with other polymers to develop composite materials (Schmidt and Paul, 2014). Denim cotton can also be used as a raw material for developing new types of regenerated cellulosic fibres. Cotton can be recycled via the lyocell process (Firgo et al., 1996) by dissolving used cellulose material and spinning a fresh cellulosic fibre from N-methylmorpholine N-oxide as a highly recyclable solvent for the spinning process. The lyocell fibres can be modified by the nozzle structure and other spinning conditions, and by adding products to the fibre production process, including crosslinking agents, softeners and other modulators. In the lyocell process, recycled cotton, other cellulosic textiles, newspaper or wood material can be mixed with fresh cotton to make cellulose yarn of appropriate quality for production of denim or any other cellulose based textiles. The benefits for environmental and sustainability parameters can be calculated for pure lyocell fibre denim and several blends such as 50:50 cotton/lyocell, compared with those of 100% fresh cotton using standard cotton (from Cotton Inc., United States) and lyocell (from Lenzing AG, Austria) production process parameters. In the case of a 50:50 blend of cotton and lyocell, the saving would be about 50% of current energy use in conventional cotton growing and harvesting, as well as 50% of the chemicals such as fertilisers that are needed for cotton cultivation. Transportation costs and environmental impacts are comparable to those of traditional cotton production. Yarn production (spinning) from cotton/lyocell blends is well established and cost effective, with only the addition of extra spinning oil required to ensure an efficient process.
20.3 Dyes for denim dyeing Denim is mostly associated with blue jeans and characterised by the typical blue colour of indigo. As a result, indigo accounts for about 70%–80% of all dyestuffs used in denim production. The second most important colour is black, followed by
Environmental impacts of denim manufacture
minor colours used to produce various fashion effects. To achieve the wash down effect required for most denim jeans, ring dyeing of the substrate fibre is used. The core of the fibre stays undyed (white), and only the surface of the yarn is dyed. Abrasion of the outer coloured covering of the yarn, later in the production process, exposes the white core of the fibre to produce the typical wash down or vintage effect. In the future, environmental requirements may increase the demand for denim that is fully dyed with optimised fastness of the dyed fabric. These products would potentially be simpler to manufacture and easier for consumers to wash at low temperatures. The most important dyestuff for denim is indigo (C.I. Vat Blue 1), followed by sulphur black (C.I. Sulphur Black 1). Direct and reactive dyes are used in garment dyeing. Better fastness and a preference for a clean (rather than vintage) denim look, as well as for colours other than blue, may increase future demand for reactive dyes. This might also reduce the process gap between denim and the continuous dyed fabric products.
20.3.1 Indigo dye Indigo is available as both a natural and synthetic product. Natural indigo is extracted from various plants growing in tropical and subtropical areas of the world. Optimised indigo plant growing and extraction into a powder dyestuff product, compared with optimised chemical dyestuff production, is a clear example that natural production of dyestuffs is not necessarily more environmentally friendly than synthetic production (Schmidt, 1997). The use of natural indigo dye for denim requires significant land area for the cultivation of plants such as Indigofera tinctoria, mostly grown in India and other subtropical regions, that contain the indigo dyestuff in 3%–4% of the plant by weight. A precursor of the chromophoric system, named indican, is fixed in the plant via a sugar type molecule bridge. This has to be cleaved by fermentation to release the indican that forms indigo molecules in the presence of air. The indigo molecules form the water insoluble pigment used as vat dyestuffs. Only 70%–80% of the dyestuff content can be extracted from the plant material, by the use of hot and strong alkali in a reductive extraction medium. The crude indigo dyestuff from plant extraction also has to be further purified from small particles out of organic plant material, which might otherwise cause problems in continuous indigo dyeing machines. This process means the energy balance of growing, harvesting and extraction of natural indigo to achieve pure dyestuff material is negative in comparison with a fully automated indigo dyestuff synthesis. For economic reasons, the alkali used for the extraction procedure is not a highly purified chemical substance, but mostly a crude material that is slightly purified following its use in other industrial processes. This alkali normally contains a significant amount of heavy metals that mostly stay with the dyestuff during concentration. This heavy metal contaminated natural dyestuff is put into the indigo dyeing machine. Under the reductive and alkaline dyeing conditions, heavy metal ions exhaust onto the cotton fibre together with the dye, and remain
after washing due to their high affinity for cellulosic fibres. The presence of heavy metal traces compromises product quality and may be regarded as unacceptable by some consumers. There are significant quality variations between synthetic indigo dyes, with variations of 5%–20% in byproduct content that cause problems in subsequent wastewater treatment. The cheapest formulation is often (dedusted) powder, but this can pose an inhalation safety hazard for dye house workers. Dust free alternatives are Indigo Granular and Indigo Paste, a slurry of 20%–30% indigo by weight in water, which are both free of small dust particles that might otherwise be inhaled by dye house workers. The latest development in the indigo market is a pre-reduced form of indigo developed by BASF as 20% solution that was launched to the global market by DyStar of Germany as 40% solution. This product is manufactured by catalytic hydrogenation from crude indigo (Blackburn et al., 2009). It is available as a product containing 20%–40% indigo by weight. This concentration process requires additional energy and cost, but eliminates volatile components from the dyestuff solution. The amount of aniline, a starting material in indigo synthesis, can be reduced to significantly below 1% of the dyestuff liquor. The aniline is then below the concentration that can be detected in the final product, as required by several eco-labels. Indigo as vat dye has to be reduced by a reducing agent so that it can be converted into the water soluble leuco form that exhausts onto the cotton fibre. After the exhaustion phase in the dye bath liquor, the yarn needs to be aired to oxidise the leuco form of indigo and fix it as an insoluble pigment on the fibre. After the airing, the yarn passes through a second dye bath box with an alkaline and reductive medium. This process will be repeated five to eight times to ensure a strong ring dyeing process with indigo fully fixed on the surface of the yarn. This means repeated use of sodium hydrosulphite as a reducing agent to reduce the oxidised indigo after the airing phase. The final product derived from hydrosulphite use is sulphate, which is not poisonous but is corrosive against concrete. This creates problems in the treatment and reuse of wastewater. Reusing of indigo dye baths for shade development on denim fabrics, instead of using fresh dye baths, could enhance the economic and environmental viability of indigo dyeing. It was found that there were no significant differences in colour yield between reused and fresh dye baths up to a certain level of reuse (Deo and Paul, 2004a). Additionally, repeated use of an indigo vat of very high concentration provided a wide range of lighter shades on denim fabrics, with a considerable cost saving, in the meantime offering the possibility of reusing the fully exhausted bath by replenishing (Deo and Paul, 2003, 2004b).
20.3.2 Sulphur dyes Sulphur black, the most important non blue dyestuff in denim, also must be reduced by a reducing agent into water soluble leuco form. Sulphur dyes are more easily reduced (redox potential) and more soluble than indigo dyes. Therefore, weaker reducing agents can be used and dyeing with sulphur black can be achieved with only one reduction/airing production cycle (Bechtold et al., 2000b).
Environmental impacts of denim manufacture
Most of the cheap sulphur based reducing agents such as sodium sulphide have been banned due to toxicity. Glucose is now often used as the most environmentally friendly reducing agent for sulphur dyes. The reducing agent causes high chemical oxygen demand (COD) content in the wastewater, but the sugar products ending in the wastewater can be easily mineralised by bacteria in the biological segment of a wastewater treatment plant, ending up as carbon dioxide in the atmosphere (Teli et al., 2001). The dyeing procedure with other sulphur dyes is similar to that of sulphur black. All sulphur dyes are oligomeric in character, with different sulphur containing structures. The chemical structures of the dyes are based on the raw material used as well as on synthesis conditions. The use of baking to produce sulphur dyes generates more byproducts than in other dyestuff classes, which then have to be treated in wastewater. A groundbreaking eco-efficient technology from Archroma of Spain involving pre-reduced sulphur dyes is Advanced Denim which combines leading edge technologies with environmental and health benefits. It significantly reduces water use, power consumption and cotton waste, as well as totally eliminating the wastewater problem. More importantly, the visual effects and finishes made possible by Advanced Denim are often beyond the capabilities of conventional dyes.
20.3.3 Natural dyes There has been research into supplementing indigo with the use of natural dyes. One example is the dyeing of denim fabric with onion extract using eco-friendly natural mordants such as harda, tartaric acid and tannic acid, instead of using metallic mordants. It was also found that denim fabric dyed with turmeric and Indian madder using natural mordants or without any mordants exhibited wash, light and crocking fastness properties comparable to denim dyed with metallic mordants such as copper sulphate and stannous chloride. Techniques such as these have the potential to promote totally eco-friendly natural denim wear that is dyed using natural dyes without any metallic mordants. Even though it is highly improbable that natural dyes can ever replace synthetic ones, natural dyed denim wear can find some niche markets, especially for children and women (Deo and Paul, 2000, 2004c).
20.4 Dyeing auxiliaries 20.4.1 Reducing agents All dyeing processes using indigo, vat and sulphur dyes need a reducing agent in the dyeing process to form the water soluble leuco form that exhausts on the substrate and is then oxidised to the water insoluble pigment that gives the appropriate level of fastness. Significant amounts of reducing agent are needed to ensure full reduction of the dyestuff and appropriate solubility and homogeneity for optimum dyeing. Several reducing agents are used in industrial scale production. Besides cost and reductive strength (redox potential), an important consideration is environmental impact, especially potentially harmful residues in wastewater after dyeing.
Sodium hydrosulphite is still the most common reducing agent for vat dyes, but can pose a potential fire risk as well as requiring treatment as a wastewater residue. Organic sulphites have been used as alternative reducing agents, but have not been successful in denim due to problems such as higher cost and limited application (redox potential). BASF and DyStar have developed a catalytic hydrogenation process that reduces the need for hydrosulphite as well as showing improvements in quality and new application effects (deeper colour). To minimise reducing agent consumption in the future, reduced forms of vat dyes are also being used, and dyeing processes are being optimised. Reduced forms include pre-reduced indigo (20%–40% solution or paste) and reduced forms of sulphur dyes. Pre-reduced vat dyes are currently less common due to problems in application. Reduced sulphur dyes offered by Archroma show saving comparable to pre-reduced indigo. The environmental benefits from reduced sulphur content are closely combined with high quality sulphur dye production to minimise sulphur containing byproducts. This means avoiding the use of cheap sulphur based reducing agents such as sodium hydrogen sulphide and other sulphides that are harmful to the environment. The use of organic reducing agents is restricted by limits in their range of application, since their redox potential is not suitable for all vat dyes. Hydroxyacetone and other α-hydroxy ketones (Federer-Lerch, 1995; Jermini, 1997) emit a strong smell that makes them difficult to work with. Other inorganic agents, including thiourea dioxide (Olip, 1995), have been tested against chemical (redox potential), technical (dyeing result), ecological (LCA) and commercial (total process cost analysis) criteria, but have not been found to be viable. The combination of boron hydride with hydrosulphite can significantly reduce the amount of hydrosulphite required.
20.4.2 Process chemicals In general, an optimum dyeing process should achieve a homogeneous colouration of the substrate in a short period of time. In vat and sulphur dyeing, auxiliaries can help to solubilise pigments effectively and keep leuco forms of dye in their soluble, reduced state. Alkali, mostly caustic soda, is necessary in the reduction process, to solubilise leuco forms of dye and to open (swell) the fibre substrates. Modulators are also added to the alkaline indigo dyestuff liquor to avoid full penetration of the yarn by the dye, as it should only dye the surface. Quick de-airing of the liquor and substrate is also important after the substrate enters the dyeing liquor, to ensure a homogeneous dyeing result. Typical process chemicals used in the dye house include: • Wetting agents: alkyl sulphates, alkyl sulphonates, phosphoric acid esters, fatty alcohol ethoxylates, alkylphenol ethoylates and fatty amine ethoxylates. • Washing agents: detergents, tensides. • Defoaming and de-airing agents: from many different chemical groups. • Dispersing agents: lingo sulphonate, condensation products of naphthalene sulphonic acid, fatty acid esters. • Sequestering agents: phosphonates, polyphosphates, amino carboxylic acids, polyacrylates, gluconate.
Environmental impacts of denim manufacture
• Rebeaming agents (for rope dyeing machines): cationic softeners, silicone emulsions, polyethylene (PE) emulsions, waxes, polyacrylates.
All of these auxiliaries potentially end up in wastewater with a high COD value, which poses a potential environmental hazard unless further treated. It is important to metre quantities precisely to minimise unnecessary use. And wherever possible, auxiliary containing liquors should be recycled.
20.4.3 Electrochemical reduction The environmental problems caused by the use of reducing agents, which typically end up in wastewater that needs further treatment, can be solved completely by replacing the use of a chemical reducing agent with an alternative physical process. The best current alternative is electrochemical dyeing. Electrochemistry involves the use of electrical energy to initiate chemical reactions, replacing traditional chemical agents. Current technology involves indirect electrochemical reduction using another strong oxidising/reducing agent as a medium to make the technology applicable to different types of dyes. An example is a multicathode electrolyser using iron complexes as cathodically regenerable reducing agents. Although the initial capital cost of the equipment is high, running costs are low. Over time, the process has been found to be cheaper as well as more environmentally friendly, because it reduces the use of chemical energy to produce reducing agents, and lessens the problem of hazardous wastewater with a high COD value and its associated costs of treatment and disposal. The process also allows better quality control and can be automated, as well as being adaptable to all types of vat dyestuffs and many types of dyeing equipment. The current cost of investment may come down in the future as production is scaled up. The return of investment (payback period) will become shorter with increasing costs for reducing agents and chemicals, water, wastewater cleaning and labour in the dye house. In the future, if electrochemical dyeing gets established as an example of BAT in the dyeing process, it will be increasingly utilised by consumers and regulators (Roessler et al., 2002; Bechtold and Turcanu, 2009).
20.5 Combined dyeing processes Due to the low affinity of indigo with cellulose, the denim dyeing process is a stepwise process. It achieves the required colour depth by adding successive layers on the yarn surface with each dyeing cycle. This layering process can be made more efficient by processes such as bottoming or topping.
20.5.1 Bottoming To achieve darker blue shades using indigo, a black layer can be put onto the cellulose fibre first, before continuing with the regular indigo dyeing and building up of the blue
dyestuff layers. In the case of later washing, the colour turns darker because firstly the blue colour disappears and the black bottom can be seen. If more colour is removed and the white core of the yarn is liberated, a stronger contrast is noted, compared with that of pure indigo dyeing. Bottoming is carried out by passing the warp yarn through a first dyeing box containing sulphur black, then passing through at least one wash box before passing regularly through several indigo boxes.
20.5.2 Topping An even darker denim shade is produced by topping, where the warp yarn passes through several indigo boxes, then two wash boxes and finally one or two sulphur black boxes. This process produces the regular indigo layers on the yarn surface and on top a black layer. The colour of a topping is therefore darker, compared with a bottoming. Washing shows more intensive changes towards lighter shades, but the contrast is not as big as in the case of bottoming.
20.5.3 Overdyeing A similar effect is achieved when standard indigo dyed denim (warp yarn dyed) is overdyed in flat fabric form. This can be sulphur black, but other colours and dyestuff classes used in continuous dyeing can also be used. The cost for an additional and separate dyeing process is higher than for bottoming or topping, and therefore is only used when the process on an indigo dyeing machine is not possible, like with colours other than vat dyes, or for special fashion effects.
20.5.4 Sandwich dyeing The combination of bottoming and topping is known as sandwich dyeing, because the resulting dyeing structure shows the indigo layers between the bottom and top layer. This effect is used by a few denim producers to achieve special effects that can be seen after selective washing procedures. This process is not used often because of cost and problems in controlling the quality. The colour strength of individual layers may differ between the beginning and end of the dyeing lot. This causes different wash down effects between early and late dyeing positions.
20.6 Minimal application technologies Minimal application (MINAP) technologies minimise the use of dyestuffs, textile auxiliaries (and energy). All MINAP technologies are related to ring dyeing of denim, because the colouration remains only on the surface of the substrate and not in the core of the fibres. These technologies are mostly well established for special applications, but will need further development to be fully deployed in denim production (Meyer et al., 2011).
Environmental impacts of denim manufacture
20.6.1 Inkjet printing Inkjet printers need to achieve competitive production speeds and running costs, compared with continuous dyeing processes, before this technology can be widely used in denim manufacture (Pai and Paul, 2005). However, inkjet technology has a role in developing new effects like mixing indigo with other dyes and printing several layers. The technology will enable new designs that are not available in the traditional denim dyeing process and should become very attractive for high end fashion garments.
20.6.2 Coating Using pre-reduced or regular indigo in combination with eco-friendly binder systems on modern coating equipment will also make denim effects available without producing significant effluent problems. Coating is more attractive than other processes such as padding because it generates no wastewater and shows lower water and energy consumption. There are choices between solvent based coatings and water based coatings, with various quality, cost and ecological issues involved in determining an appropriate choice.
20.6.3 Spraying Spraying onto fabric or warp yarn from a dyeing beam is a possible alternative technique that has already been tested on a lab and industrial scale. Unlike in the exhaustion method, in spraying it is possible to apply dyes only to the surface and to desired parts of the denim garment.
20.6.4 Foam application Pre-reduced indigo is suitable for foam application on fabric and yarn, if an inert gas such as nitrogen is used as a foam forming medium. Lab and industrial trials have shown significant saving compared with conventional dyeing, as well as the ability to develop new effects by combining foam application with other products. The process can be automated and allows individual, handmade designs appropriate for high end fashion products (Aurich et al., 2011; Sütsch and Schrott, 2012).
20.7 Auxiliaries and finishing chemicals Standard denim is produced from raw cotton that is not pretreated. In general, prewashing, bleaching and other pretreatment processes are not necessary, apart from applying some alkali on the fibres in the first wetting before entering the first dyeing box. However, a number of chemicals such as size or softeners are used during production, either to facilitate further processing (size) or to provide an appropriate finish (softeners). Manufacturers need to assess whether they can achieve the effects they want during existing processes such as dyeing, rather than adding a further process
step. In using auxiliary chemicals, manufacturers also need to assess environmental impact by accounting for characteristics such as biodegradability and contribution to the COD value of wastewater.
20.7.1 Size After warp dyeing, the ropes of yarn are opened and put onto a beam before weaving. Size is a natural or synthetic polymeric material that is put onto the dyed yarn before weaving. It strengthens (lubricate and protect) the cotton yarn so it can survive the high stress of the high speed weaving process without breaking. Sizes can be categorised as natural or synthetic. Natural sizes are starch (natural starch and starch derivatives) and derivatives of cellulose. Starch can be obtained from potatoes, corn, cassava (in South America) and other natural products. Synthetic sizes are acid based and ester based polyacrylates and polyvinyl alcohol. Manufacturers often use mixes of natural and synthetic products at ratios between 3:1 and 5:1 because these have the best technical profiles in terms of bonding power, elasticity and film formation. The dyeing beam of a slasher indigo dyeing range can be used directly for sizing after dyeing. After weaving, the size is typically washed off before any fabric finishing is done, because the size interferes with some finishing procedures and gives denim a harsh hand that most customers would not prefer. Typically, 5%–15% of size is applied to the cotton yarn. When it is washed off, it ends up in the wastewater and can represent over 70% of the total COD value of wastewater from a denim mill. In many cases, desizing agents are used to achieve quick and complete removal of the size. Besides special washing agents, enzymatic desizing of starch based sizes is carried out using 1–2 g/L amylases. If the size can be separated from the washing liquor, the wastewater needs less further treatment, though there is still the problem of disposing of the separated size residue. Biodegradability is the most important environmental requirement for size, since recycling is currently not feasible on the grounds of both cost and the quality of recycled size.
20.7.2 Softeners Softeners may be applied in several steps of the production chain, but are most often used in the last rinse of the garment finish. They provide a smooth or soft hand to the garment, and only small quantities enter the wastewater. The following types of softeners are commonly in use: • Cationic softeners: ester based cationic and quaternary softeners are readily biodegradable but can be toxic to aquatic life. • PE emulsions: show COD input as well as biodegradability. • Silicone emulsions: non biodegradable and potentially toxic to fish. • Wax: biodegradable and partly removable by adsorption to sludge. • Polyacrylates: non biodegradable, but residual monomers may be vapourised.
Environmental impacts of denim manufacture
20.7.3 Fabric finishing chemicals Fabric finishing or overdyeing is an exception in the denim production process. The major processes involve coating and foam applications with various finishing chemicals to give a soft hand, crease resistance or a shiny appearance. Only expensive denim articles requiring a particular appearance and hand that cannot be achieved in the garment finish are given a special fabric finish.
20.8 Garment washing and finishing As noted, denim washing has a significant environmental impact. Originally this was a pure washing process supported by detergents to clean the denim by removing the indigo that had not fixed to the substrate surface. However, in response to the desire for a worn out appearance amongst a fashion conscious younger generation, new processes have been developed. Some processes, like the sandblasting of denim fabrics, have subsequently been banned to protect the health and safety of workers. The most widely used process is stonewashing, developed in the 1970s. Using naturally occurring stones (pumice stone) and detergents in laundry machines, not only excess chemicals and unfixed dyestuff are removed from the garment, but dye is rubbed off the most exposed parts of the garment to give it a more used appearance with contrasting light and dark coloured areas. In addition, the stonewash process gives the fabric a softer hand (Paul and Naik, 1997a). The stonewash process has a number of environmental drawbacks. Removal of pumice stone debris after the process can be labour intensive. The process requires higher energy and water input than conventional washing and leads to increased machine wear and breakdown. Developments to improve the process include a perlite wash using a naturally occurring silicon rock that swells at high temperature (above 87 °C) and is softer to the denim garment and the washing machines. Synthetic stones made from plastic material also can be used, and also reused since they are not destroyed by the process. Nowadays, the stonewash process can be supplemented or even substituted with an enzyme washing process called biostoning. The enzymes used, mostly cellulases, attack the most exposed garment surfaces through a biochemical reaction and achieve effects similar to those of the mechanical garment surface treatment using stones, but cause less damage to the fibre surface. Additionally, the quantity of water used for washing off is less, compared with the quantity used for pumice stones. In general, enzyme washing has a smaller environmental impact than that of stonewashing, because there is less energy use, water consumption, wastewater and damage to machines (Paul and Naik, 1997b). But it is important to be aware of other factors, such as potential allergic reactions to some enzymes amongst operators. Enzymes can be used at various stages of denim production, including: • Desizing with α-amylases and aminoglucosidases. • Biostoning of cellulose.
• Biopolishing with cellulases. • Enzymatic washing with lipases, proteases and hemicellulases. • Enzymatic bleaching with laccases and glucose oxidases.
The ecological impact of enzymes is in general lower than it is with other chemicals. There is some COD input, but with lower energy consumption in production since enzymes are active at lower temperatures. It is important to note that solid enzyme forms are finished with dispersing agents, mainly ethoxylate based surfactants, which can be toxic. The general use of enzymatic techniques is discussed in Paul and Genesca (2013). One disadvantage of cellulases is back staining, which is caused by unwanted recolouration of threads and the lighter inner side of the denim fabric. This can be minimised by detergents and polymeric auxiliaries, which keep the dye waste in emulsion before washing off. However, it is possible that consumers will increasingly accept back staining as a byproduct of more environmentally friendly denim production. In contrast, they will be less inclined to accept more environmentally damaging processes and chemicals like moon washing with potassium permanganate, yellowing by hypochlorite and other chemicals and bleaching with chlorine containing chemicals or oxidative bleaching agents such as persulphate (which is toxic and liberates sulphate) and peroxide. Several developments have focussed on minimising water consumption in industrial washing and the substitution of detergents with oxidising chemicals such as oxygen, ozone, hydrogen peroxide and other products. Biodegradable oxidised waste can be dealt with by conventional wastewater treatment plants. However, from an LCA perspective, it is important that water saved in industrial washing does not result in significant waste remaining on the denim garment that must then be removed in household washing by consumers. This creates a much more serious waste management problem than dealing with the problem during production. One non chemical high end finishing technique that offers all kinds of structural effects is laser technology, developed by companies such as Jeanologia (Spain). Directing a laser beam onto the surface of the denim article allows local sublimations of indigo that show up as lighter (white) areas. In 2011, the main inventor of the stonewash, François Girbaud, launched Wattwash jeans with laser based effects, saving 97.5% of fresh water when compared with the stonewash treatment. If predyeing (before indigo dyeing) with non sublimatable dyes is carried out, laser treatment can be used to highlight the predyed material to produce additional complex colour effects. Another area where significant environmental saving is possible is in the industrial laundry sector. Globally, this is a small scale industry, with many production units having 10 laundry machines or fewer. This sector was developed with low capital investment, production units with varied machinery and equipment, limited production flexibility and no potential for recycling. This sector will benefit from consolidation and the associated investment in new production units in strategic locations (regional distribution centres) with a production size based on 50 or more laundry machines. This enables maximum automation, and recycling of used water by wastewater segmentation, separation and optimised cleaning. This could result in a 50% saving of current energy consumption and water use in this sector.
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Modern laundry machines not only are suitable for washing but also can be used for final colouration and other garment finishing processes. Small tonal changes in colour effected through the use of sulphur, vat, reactive and direct dyes, or by using pigments, are called tinting. If a strong colouration is done in the laundry machine with a predyed (indigo or sulphur black) or white denim article, the process is called garment dyeing. Softeners and a variety of functional finishes can also be applied to the garments in laundry machines (Paul, 2014). The finishing chemicals applied in laundry machines at the end of the production cycle are often not as permanent as those applied in a yarn or fabric process step.
20.9 Distribution and retail Since its origins as a type of workwear in the United States during the nineteenth century, denim has become a global fashion brand sold all over the world to many different types of consumers. This has created a complex and sometimes inefficient distribution and retail network. Selling is optimised to offer the widest choice to consumers. However, having a wide range of types and sizes of denim articles available can waste resources, because significant quantities of the final article produced are not bought by consumers and then need to be dealt with by returning the articles to a distribution centre, recycling, etc. Transporting articles to the point of sale and potentially back up the supply chain if they are not sold involves significant transport, storage and other logistic costs and environmental impacts. In the past, the most important consumption areas were the United States, Europe and Japan. Production was carried out mostly in other regions, especially in China, India and Southeast Asian countries having lower raw material and labour costs. This has resulted in high distribution costs. Air cargo transport is known to be a factor in global warming, but shipping also contributes to greenhouse gas emissions. This situation will improve in part because of shifts in global consumption from more mature economies to emerging economies including China, India, Southeast Asian countries and Central and South America. Other factors that may reduce the environmental impact of distribution and retail operations include a move to a more regional structure for denim production, with production located in the same region as consumers in self sufficient regional value chains. Other factors include the growth of e-business and the development of production on demand systems. The use of body measurement data gathered using 3D body scanners in the shop for a specific consumer, together with online ordering linked directly to a production facility, will enable garment production tailored precisely to demand. Another benefit from the increased interest in sustainability concepts will be increased use of recycled material in denim wear as well as reuse of apparel. Recycling will impose new constraints on production in that it works best with standardised materials that can be easily repurposed. In practice, this means minimising variation in raw material, processing and chemical use.
20.10 Future trends In the future, there may be two different segments in denim: standard articles and premium fashion articles. For standard articles, colours and designs will be limited, with individual variations achieved primarily by physical and surface modification processes that have less environmental impact. All dyestuffs and chemicals should, wherever possible, be easily removed and recycled so that the articles themselves can be recycled, by reusing the base material in another garment. Small individual variations in denim articles, like logos, may become more important in differentiating garments. These can be placed onto the final garment by processes such as laser technology, stamping or other individual manufacturing processes. Features such as stitching (with cotton yarn) may also allow differentiation in different garments. Fundamentally, consumers will use garments for longer periods, adding variation through the greater use of accessories. This trend requires taking greater care of textiles. Therefore, in city and urban living areas, a new logistic care system could be established with more laundering done by commercial services, saving time and effort for consumers and enabling optimal water recycling compared with that provided by typical household laundering. Variation in premium fashion articles will be due to colour, effects, cut and changes in fashion cycles, much as it is today. More sophisticated production methods will be possible due to a higher price level for premium articles, and mixes with other chemicals and products will be allowed as long as all selected products fit specifications for final recycling or reuse. The integration of substrates other than cellulosic material (polyester, other synthetic fibre substrates, leather, metal foil and yarn) will be allowed only if a simple separation of the different substrates before recycling is possible. Environmentally preferable MINAP technologies will be used to bring new structures and designs (by inkjet or laser technology) or effects (by coating or foam application) onto the surface. Finally, the premium fashion article segment will be characterised in the future by a high level of tailor made products, and wherever possible, specialist fashion effects will be added as late as possible in the production chain.
20.11 Conclusion Denim is one of the most widely used textile fabrics. Its manufacture has a significant environmental impact, especially in such areas as cotton growing, dyeing and finishing, and a number of ways for reducing this environmental impact are reviewed in this chapter. Following the principles of LCA, emphasis has been put on taking a whole life cycle approach, from raw material production to consumer use and disposal. It has explored ways of reducing chemical use by allowing more recycling or by producing wastewater that is easier to treat. It has discussed replacing chemicals, either with more environmentally friendly alternatives, or with alternative production techniques such as electrochemical dyeing and MINAP technologies. Among all the processes, denim washing in production and household laundering seems to have the highest potential for
Environmental impacts of denim manufacture
environmental improvement. It is very clear that in the future, consumers will insist on denim that has been manufactured to the highest environmental standards, and will focus on the sustainability of the entire life cycle including production, use and recycling.
Sources of further information http://www.advanceddenim.archroma.com http://www.iso.org/iso/home/standards/iso26000.htm http://www.unctad.org/en/Docs/osgdp20112_en.pdf http://www.levistrauss.com/sustainability/products/#intro http://www.emsc.ch/cost628/assets/009_ToblerSchaerer.pdf http://www.dystar.com/econfidence.cfm https://www.basf.com/en/company/sustainability/management-and-instruments/ quantifying-sustainability/eco-efficiency-analysis.html http://www.ecotextile.com/2013071612110/materials-production-news/study-outlinesdenims-water-footprint.html
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