Difference between revisions of "Bleaching"
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| − | ! style="background-color:#66cdaa;width: | + | ! style="background-color:#66cdaa;width:250px;color:#ffffff" | OPPORTUNITY |
| − | ! style="background-color:#66cdaa;width: | + | ! style="background-color:#66cdaa;width:250px;color:#ffffff" | CONSIDERATIONS |
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| − | | '''Look for design opportunities to avoid bleaching. Darker, duller shades may not require bleaching prior to | + | | '''Look for design opportunities to avoid bleaching. Darker, duller shades may not require bleaching prior to dyeing/finishing.''' || |
| − | dyeing/finishing.''' || Lighter, brighter shades do need to be chlorine bleached before dyeing. | + | Lighter, brighter shades do need to be chlorine bleached before dyeing. |
|- | |- | ||
| − | | '''Promote suppliers who use ozone bleaching processes.<br/><br/> | + | | '''Promote suppliers who use ozone bleaching processes. <br/><br/> Promote the aesthetic of ozone bleach effects.''' |
| − | Promote the aesthetic of ozone bleach effects.''' || • Ozone has limited availability, and is relatively expensive since it requires investment in an ozone generator.<br/> • Ozone produces a different aesthetic than chlorine derivative or permanganate bleaching. | + | ||• Ozone has limited availability, and is relatively expensive since it requires investment in an ozone generator.<br/> • Ozone produces a different aesthetic than chlorine derivative or permanganate bleaching. |
|- | |- | ||
| − | | '''Promote suppliers who use enzyme process bleaches.<br/><br/> | + | | '''Promote suppliers who use enzyme process bleaches.<br/><br/> Promote the aesthetic of enzyme bleach effects.''' |
| − | Promote the aesthetic of enzyme bleach effects.''' || Enzyme produces a different aesthetic than chlorine derivative or permanganate bleaching. | + | || Enzyme produces a different aesthetic than chlorine derivative or permanganate bleaching. |
|- | |- | ||
| − | | '''Promote suppliers who use low-temperature peroxide bleach processes, such as pad-batch systems.''' || Pad-batch bleaching is much slower than high-temperature peroxide bleaching, and requires significant floor space to store batches for long periods of time. | + | | '''Promote suppliers who use low-temperature peroxide bleach processes, such as pad-batch systems.''' |
| + | || Pad-batch bleaching is much slower than high-temperature peroxide bleaching, and requires significant floor space to store batches for long periods of time. | ||
|- | |- | ||
| − | | '''Avoid chlorine bleaching if possible.<br/><br/> | + | | '''Avoid chlorine bleaching if possible.<br/><br/> For fabric bleaching prior to dyeing/finishing, do not accept chlorine-derivative bleaches when peroxide will suffice.''' |
| − | For fabric bleaching prior to dyeing/finishing, do not accept chlorine-derivative bleaches when peroxide will suffice.''' || Peroxide might be slightly more expensive than chlorine-derivative bleaches, but they are less toxic. | + | || Peroxide might be slightly more expensive than chlorine-derivative bleaches, but they are less toxic. |
|- | |- | ||
| − | | '''Promote proper wastewater treatment.''' || Develop standards for laundries and/or fabric dye-houses. | + | | '''Promote proper wastewater treatment.''' |
| + | || Develop standards for laundries and/or fabric dye-houses. | ||
|} | |} | ||
| + | |||
==Availability== | ==Availability== | ||
Hydrogen peroxide is widely available globally. | Hydrogen peroxide is widely available globally. | ||
Latest revision as of 15:42, 24 April 2015
Bleaching is the process of removing colour and is typically used to whiten cloth made from natural fibres where naturally occurring pigments are present.Bleaching is often used in conjunction with other cleaning processes such as desizing or scouring as preparatory steps for subsequent dyeing, printing, and/or finishing processes, since most of these processes work best on clean, white fabric. Fabric or garments can also be bleached, finished and sold white.
Bleach is also effective at removing colour from previously dyed/printed products, and is commonly applied in industrial laundries to achieve "vintage" or "faded" looks (e.g., bleaching of indigo-dyed denim garments).
Bleaching can be vital to reducing the ecological impact profile of a garment—it can ensure proper first time dyeing and avoid re-dyeing, which is resource-intensive and pollutive.[1]
Contents
Types of bleaching processes
Hydrogen peroxide
About 90% of fabric bleaching carried out prior to dyeing/printing is performed using hydrogen peroxide, a relatively safe chemical that decomposes into water and oxygen. Hydrogen peroxide is economical and readily available.
Potential Impacts:
Hydrogen peroxide bleaching requires high temperatures (above 60°C), which makes it an energy-intensive process. In addition, hydrogen peroxide is relatively unstable and requires chemical stabilizers and additional additives to optimize the bleaching process. These can be pollutive if left untreated and discharged into wastewater.[1]
Chlorine derivatives
About 90% of garment bleaching, used mainly for fading pre-dyed and pre-printed items, is performed using chlorine derivatives (usually sodium or calcium hypochlorite) or potassium permanganate (also known as PP). Hypochlorite is reactive enough to work well at cooler temperatures and breaks down into table salt, oxygen, and water. Like hypochlorite, potassium permanganate is a strong oxidizing agent. Potassium permanganate is particularly suitable for selective, or precision, applications (e.g., spraying or sponging) to achieve patterned effects, since its deep purple colour makes it easy for the operator to see in real time and to visually track its application to the garment.
Potential Impacts:
If hypochlorite is exposed to organic material before it breaks down, it can react with that material to form halogenated organic compounds (organochlorines). Halogenated organic compounds are persistent, toxic compounds, may bioaccumulate in the food chain, are known teratogens/mutagens and suspected human carcinogens, and may cause reproductive harm. Hypochlorite can also react with acids, ammonia and even dirt particles to form toxic gases. Due to its highly toxic nature, personal protective equipment is required for workers handling PP. Besides stripping colour from the garments to which they are applied, chlorine derivatives and other oxidizing agents also attack and weaken the fibre and are particularly destructive to elastic fibre (spandex) in stretch fabrics, rendering the finished garment more fragile and susceptible to being discarded by the consumer more quickly.
Alternative technologies
Ozone
Ozone gas is comprised of three oxygen atoms (03) and is a much more powerful oxidizing agent than chlorine. It is very effective at fading pre-dyed/pre-printed fabrics and garments, can be performed at lower temperatures than hydrogen peroxide, and uses no water at all. In addition, ozone is completely biodegradable, as it reverts rapidly back to oxygen (02), leaving no chemical residue. However, the ozone bleaching process still requires an alkali addition to swell the fibre and remove motes from cotton, and scouring and sequestering agents still need to be used.[2] Additionally, ozone is highly unstable and cannot be stored. It must therefore be generated on-site as needed, meaning that any manufacturing facility intending to use ozone as a bleaching agent must invest in ozone generating equipment.
Laccase
Laccase is an enzyme that has proven effective at decolourizing or fading pre-dyed/pre-printed apparel products. Generally speaking, enzymes are considered low impact because they are biodegradable (no waste products) and typically work well at low temperatures, thereby minimizing energy consumption. A small amount of enzyme often saves significant amounts of water, energy and chemicals used in bleaching.[3] For example, Gentle Power Bleach™, a bleaching technology developed by Huntsman Textile Effects and DuPont, is an enzyme-based peroxide process that enables bleaching of textiles at 65°C using a neutral pH. The company claims that using Gentle Power Bleach™ lowers treatment and rinsing temperatures and requires fewer rinse baths, resulting in significant water and energy consumption savings and eliminating the need for caustic soda altogether.4 Laccase's effectiveness varies depending on the specific dye-stuff(s) used to dye/print the fabric or garment, but it works quite well on indigo-dyed denim products. Moreover, bleaching with laccase affects only the dyestuff, and will not weaken the fibre or fabric. However, enzymes are often more expensive than chemical options for bleaching, and some of the more popular denim shades may be difficult to achieve through enzyme technologies. For example, heavily faded denim effects cannot be replicated using laccase, and laccase often imparts a grey cast to indigo-dyed fabrics.
Optimize sustainability benefits
| OPPORTUNITY | CONSIDERATIONS |
|---|---|
| Look for design opportunities to avoid bleaching. Darker, duller shades may not require bleaching prior to dyeing/finishing. |
Lighter, brighter shades do need to be chlorine bleached before dyeing. |
| Promote suppliers who use ozone bleaching processes. Promote the aesthetic of ozone bleach effects. |
• Ozone has limited availability, and is relatively expensive since it requires investment in an ozone generator. • Ozone produces a different aesthetic than chlorine derivative or permanganate bleaching. |
| Promote suppliers who use enzyme process bleaches. Promote the aesthetic of enzyme bleach effects. |
Enzyme produces a different aesthetic than chlorine derivative or permanganate bleaching. |
| Promote suppliers who use low-temperature peroxide bleach processes, such as pad-batch systems. | Pad-batch bleaching is much slower than high-temperature peroxide bleaching, and requires significant floor space to store batches for long periods of time. |
| Avoid chlorine bleaching if possible. For fabric bleaching prior to dyeing/finishing, do not accept chlorine-derivative bleaches when peroxide will suffice. |
Peroxide might be slightly more expensive than chlorine-derivative bleaches, but they are less toxic. |
| Promote proper wastewater treatment. | Develop standards for laundries and/or fabric dye-houses. |
Availability
Hydrogen peroxide is widely available globally. Chlorine derivatives and potassium permanganate are used by the majority of garment laundries to fade pre-dyed/pre-printed garments. Ozone is relatively expensive and the equipment is not yet widely available.1 Enzyme bleaching technologies are readily available globally.
Applications
Not all fabrics require bleaching before they are dyed and finished. As a rule, only fabrics which contain naturally-occurring pigments (such as hemp and linen) require bleaching with chlorine. Cotton fibre is naturally cream coloured and hydrogen peroxide bleaches will suffice in preparing cotton fabrics for dye. Wool and silk fibres yellow with the use of chlorine bleaches. Intended shade is another factor that determines the necessity of bleaching. Many darker, duller shades (dark browns, dark navys, black, etc.) are relatively easy to achieve on unbleached substrates, whereas lighter and brighter shades require a cleaner base. Hydrogen peroxide, ozone and laccase bleaches can be effective substitutes for chlorine derivates and potassium permanganate depending on the desired degree of fading, the desired shade/cast, the specific colourants involved, the equipment available, and the application technique(s) employed.
Marketing opportunities
• Gallons of water per jean saved when ozone bleaching is used. • non-chlorine bleached If alternative bleach is used. • Providing educational information on the corporate/brand website (perhaps in the same place as the CSR reports) could detail information about alternatives used for garment bleaching.
Innovation opportunities
• Promote suppliers who use ozone and laccase (or other enzyme) bleaches. Promote the process on POS items. Look for opportunities to promote the different aesthetic and shades/casts common to ozone/laccase fading and build a collection around the eco-story.
• Creative exploration of the effects of different types of bleaching on different fabrics could yield a white-on-white story, typically a great retail seller for summer. The garments would become a vehicle for communicating the different bleaching options and their different ecological impact/benefit.
• Showcase your company’s alternative bleaching methods on a plain T-shirt.
• Develop graphic symbols for hydrogen peroxide, ozone, laccase/enzyme, etc., to communicate with the customer.
Sources
- Grose, Lynda and Kate Fletcher.Fashion & Sustainablity: Design for Change. London: Laurence King Publishing Ltd, 2012.
- Cotton Incorporated (2013) e-mail conversation March, 2013.
- Nielsen, P H, Kuilderd, H, Zhou W, Lu X, (2009) ‘Enzyme Biotechnology for Sustainable Textiles (hydroxyalkanoates) (PHA) and other fibres, in Sustainable Textiles: Lifecycle and environmental impacts by R.S. Blackburn (Ed.) Woodhead Publishing, Cambridge, p113-138.
