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Jeans bleaching – advantages and risks of different bleaching processes (3/3)

In the first part of this article series a general list of requirements for jeans bleaching was introduced. In the second part the different processes to bleach denim garments were discussed. In this part all bleaching processes are compared in an overview, and the various chemical, physical and mechanical treatments are assessed against ecological criteria, especially considering the ZDHC program, to identify the processes with lowest impact to the environment. The target is to select processes which have a good price/performance level and good reproducibility, while using environmentally friendly chemicals.

 Characteristics of the different bleaching processes

Table 1 shows some importance aspects of the different bleaching systems, which have been outlined in part 2 of this report. In the upper part of the table the relevant process parameters are shown, and in the lower part of the table the ecologically relevant parameters are shown.

The overview shows that all systems have their advantages and disadvantages. So there is no process, which is per se the optimal process for bleaching of denim jeans. The assessment of the different process shows which possible treatments are necessary to reduce the environmental impact to a minimum, or even better, to reduce it completely. Moreover, there are social aspects which are also relevant for this assessment.

Assessment of the different Bleaching Process in the view of ZDHC

Background

The background of the ZDHC (zero discharge of hazardous chemicals) programme is the global Detox campaign of Greenpeace. In the Detox campaign Greenpeace called upon leading fashion brands to replace in their supply chain the use of  hazardous chemicals by  hazard-free chemicals. Greenpeace published a report  that 70% of Chinese and Mexican waterways are contaminated with hazard chemicals caused by international textile production. The discharge of the textile auxiliaries are blacklisted as a danger to the environment and the public health. Further in the Detox campaign, Greenpeace organized demonstrations in front of international brand stores to oppose the use of hazardous chemicals in the manufacturing process.[1]

About ZDHC

In 2011, in response to the growing pressure to eliminate pollution of the environment, and following the Greenpeace Detox Campaign, a group of major apparel and footwear brands and retailers made a shared commitment to help leading the industry towards zero discharge of hazardous chemicals by 2020.[2] Aa a part of the commitment and first step towards ZDHC, the group of member brands published a Joint Roadmap in November 2011. The document demonstrates the group’s collaborative efforts in leading the apparel and footwear industry towards ZDHC for all products across the pathways by 2020.

The Joint Roadmap is a plan that sets a new standard of environmental performance for the global apparel and footwear industry. It includes specific commitments with timelines to realize this ambitious goal. Ensuring transparency, the group of brands will report regularly and publicity on the progress against the published Joint Roadmap timelines. [3]

Assessment of the different processes for jeans bleaching

Nowadays people have become more environmentally conscious, and eco-friendly processing is becoming more and more popular. In addition, government agencies are putting more restrictions for control on effluent quality. NGO’s are increasing the pressure, most vocally through the Greenpeace Detox campaign. Under these circumstances, textile processors are looking for more eco-friendly alternatives.

All over the world, light washes on blue denim jeans are predominantly bleached with sodium hypochlorite (see table 1). It has certain advantages of being cheap; the reaction takes place at room temperature and gives a pretty blue cast. But it is also a harsh chemical and gives yellowness to the fabric, if not neutralized properly. Being a strong oxidation agent, it also attacks cotton and reduces its strength which is not desirable in light denim qualities. In addition, it cannot be used for elastic garments.

However, there is a major disadvantage: The hypochlorite processing is environmentally very harmful, for two reasons: firstly hypochlorite itself is a harmful chemical and secondly the subsequent neutralization step with sodium bisulphite generates high amounts of salts leading to disposal and pollution problems. The process results in increasing of the biological oxygen demand (BOD) and in the chemical oxygen demand (COD) level in the effluent. Such polluted water increases subsequently the processing costs.

The biggest problem of the hypochlorite bleaching process is the high content of AOX (organic halogen compounds) in the waste water. The AOX value does not cover a single chemical, but a group of substances contained in the effluent. Due to the aggressive nature, hypochlorite may react with a wide range of chemicals applied during textile processing and form additional hazardous chemicals, which differ widely in their ecological and toxicological properties. [4]

In the view of ZDHC, hypochlorite should be considered as a chemical with potentially very high harmful characteristics. With the wrong handling it may form chlorine, an even more aggressive and even more harmful chemical.

Potassium permanganate can be used for local bleaching or complete bleaching in the washing machine in short time at low temperatures. Manganese is not a toxic heavy metal. It is one of the three essential micronutrients. However, with this bleaching process high amounts of manganese products are released in the effluent, and in high concentrations manganese is toxic.

Manganese gets via the waste water into surface water and groundwater. Plants extract manganese from the ground and transport it to the leaf. Extreme high concentration results in withering of the leaf and on this way to high amounts of manganese come into the food chain.

Bleaching with hydrogen peroxide is predominately used for black denim to obtain a nice clear grey colour. The peroxide bleaching process is carried out at temperatures as high as  95°C. At these temperatures a stabilizer is needed, so that the oxygen cannot escape too fast, but is active on the garment. The mostly used stabilizer is sodium silicate (Na₂SiO₃), because it is the cheapest product. On the other hand sodium silicate must be completely removed after the bleaching process, to avoid a harsh handle of the fabric.

Another problem at the peroxide bleach is the catalytic damage due to  presence of iron on the fabric. Iron speeds up the catalysis of the peroxide what results in fabric damage. A high pH (9-12) can reduce the risk, but not completely exclude.

Organic peroxide has the same reaction mechanism as hydrogen peroxide; except that the fibre damage is much lower. The product can also be used for elastic garments.

The products have a very high reactivity and are potentially explosive. For that they are normally used in a diluted form with an inert material. Mostly used is calcium chloride as inert material, which has the disadvantage that the products decline to dust formation. If the dust falls on a wet garment the fabric becomes white spots.

The dust formation has also the problem, that it can be used in bigger amounts only in ex-protected areas, because also the dust of the diluted product is explosive.

Laccase completes the whole wet garment processing with enzyme products. In the denim garment processing enzymes are used for scouring (pectinase), desizing (amylase) and stonewashing (cellulase). Being biodegradable, enzymes are eco-friendly. Thus, laccases as bleaching agents for blue denim are a sound  ecological alternative. But in jeans bleaching laccases work exclusively on the indigo. That has the advantage of minimal damage of the fibre, but it cannot lighten toppings or bottoming of other dyestuff classes (e.g. sulphur dyes).

The laccase-mediator process has following advantages over the existing commercial processes: denim fibres are not attacked during the colouration, avoiding the use of stones, cleaning expenses due to stone recovery will be eliminated and the environmental impact will be reduced because this system produces no halogenated by-products, and the indigo oxidation by-products are easily biodegraded in either aerobic or anaerobic waste water treatment. So the process has a considerable reduction of chemicals, water, time and energy.

The disadvantage of the enzyme process is the product cost of the laccase compared with the cost of hypochlorite. Also the storage of the product is a problem, especially in Asian countries, because the laccase must be stored under cold conditions to avoid activity loss of the enzyme. Another problem is the cost and toxicity of the redox mediators, or their reaction products. The availability of natural and less polluting mediators could be increase the industrial biotechnological process.[5]

With the ozone bleaching concept significant saving of water in the denim finish can be achieved. On the Bred & Butter fair (July 2011, Berlin) the US based denim giant Levis presented a “waterless” concept. One element of the concept is the use of ozone as bleaching agent.[6]

First trails were made on incompletely sealed machines, so that the ozone is released in the environment in an uncontrolled way. At the end of the process the complete ozone get out of the machine when the door will be opened.

An environmentally improved version was presented by Joker. The joker production has used ozone since 20 year for their waste water decolourisation and has long experience with ozone. They carry out the complete bleaching process without water and chemicals. The machines of joker are hermetically sealed, so that ozone cannot pass off the machine. The bleaching process takes 40 to 50 minutes, depending on the raw material and the degree of bleaching effect. After the bleaching the ozone reconverts to oxygen, aided by a catalyst.[7]

The bleaching process with has been considered for a long time as the most environmentally friendly method to achieve a strong bleaching effect of garments. But the process requires much energy due to high temperature, and the waste water has an high pH and contains a very high COD, which requires an additional chemical/biological waste water treatment.

Another problem of this process is the high risk of backstaining. Also, a very strong dispersing agent is needed to prevent that the leuko form falls back on the fabric and stain the weft yarn and the pocket linen. The dispersing agent regards another point of the ZDHC project. Unfortunately, most dispersing agents are on the chemical base of APEO or Alkylphenole which is a strict no-go in terms of the Detox/ZDHC program.

Sandblasting is dispelled in many countries and stand on many restricted lists of the main brand. On the other side Levi Strauss still allows sandblasting, with certain restrictions, wherein workers have to use aluminium oxide, aluminium silicate, copper slag or any other material containing less than 1% crystalline silica. These products are recommended by the United States Occupational Safety and Health Administration (OSHA) to help eliminate the health risks associated by silica. They also require proper ventilation for workers and call for all workers to wear masks as added protection against dust inhalation.[8]

But in the reality the biggest problems of the sandblasting are the working conditions. If the process is made on the one hand in an extractor hood with a high exhaust of the sand and quartz particles, and on the other hand with good breathing equipment, the process has no health problems anymore. Moreover, the grounded air must be filtered before it goes in the environment.

In the meantime there are many sandblasting facilities offered in the markets, in which the workers have no direct contact with the silica dust. The dust is exhausted by ventilation out of the sandblasting chamber.

But all this equipment make only sense, if there is no permanent control that these are used from the workers and the filters of the masks and the extractor hood are changed continuous. Only factories in which this is guaranteed should be allowed to operate this process, but unfortunately mostly this treatment is carried out in low cost countries such as Pakistan, China, Bangladesh and Egypt.[9] [10]

According to a report in July 2013, potentially hazardous sandblasting techniques are still used in China’s garment industry (Guangdong), with machinery hidden from safety auditors, in some factories which were linked to brand such as Levis Strauss and H&M, which have previously announced supply chain bans on this very  controversial technique.[11]

The reductive electrochemical bleaching of garments in drum washers was proven in lab and pilot plant, but not yet established in the market place. In this process the chemicals are completely replaced by “electrons”. The problems of the ECB process are the high costs compared with the cost-efficient and global available hypochlorite.

For the reductive bleaching system a high sealed washing machine needed as for the ozone bleaching. Compared with the costs of an ozone generator the costs of the electrochemical cell are much higher.

Laser application investments are increasing in recent last years. Due to the laser beam the machines are completely encapsulated. The laser beam “burn” the style, which was designed on the computer, on the garment. It is a much more precise process with a high reproducibility as manual work. It can also handle digital data such as photographs, illustrations and logos and programs can be used to edit and change the details like gradation and form. This is an unique feature, which can only made with the laser treatment.

Most of the machines exhaust the fume and fibre dust out of the encapsulated treatment cabin, so that the users do not need protection equipment. The process is discontinuous, because one garment after and other from both sides must be treated. Every garment must be laid on a transportation band (e.g. Golden Laser) or put on a “Mannequin” (e.g. Tonello) in the right position to have the effects on the right place.

The strength of the laser beam must be adjusted very accurately on the fabric and the design. If the final effect should be a complete white, more of the surface must be burned. But too much abrasion has the risk of fibre damage by losing tear strength. That is also the reason why light weight garment cannot be treated with a laser.

 Conclusion

 In terms of an eco-efficiency analysis a sustainable denim production surely has to yield an optimum balance for the environment and economics- it must be commercially feasible.

In principle, innovative and eco-friendly products and processes should be economically attractive if the costs of waste water cleaning and other environmental protections are calculated realistically. The philosophy of ZDHC is to bring no harmful chemicals in the production process, which must be removed out of the water and air cycle. If there is only a look on the bleaching process, it must be considered which products are due to processing steps that occurred prior to the bleaching process. In some cases there are cross-media effects which depend on previous processing steps.

The comparison of the bleaching system (table 1) shows that every bleaching system has limitations of either their effectiveness or their environmental impact. So it is not possible to say, which is per se the best available technology (BAT) for bleaching denim. The priority and selection of the most appropriate technology could be different depending on local conditions.[12]

Some general site-related factors, which are crucial for the BAT selection, are subsequently listed:

• Size and structure of the factory (complete denim production or only wet garment processing)
• Infrastructure of the factory (e.g. fresh water supply)
• Existing machinery outfit (new machines, special equipment)
• Producing the effects by handwork or with special machines
• Working conditions (use of proper workwear, safety precautions, dust mask, etc.)
• Local conditions related to the waste water cleaning situation
• Identification of the key environmental issues of the complete factory
• Storage of the chemicals according to the instruction in the MSDS
• Legal issues, i.e. with regard to REACh in the EU

In most cases it is easier to change a chemical rather than a complete machinery outfit. But in a longer process the identification, development and introduction of customized technique must be implemented, which fulfills the requirements of high efficiency and integrated prevention and degradation of environmental pollution based on the BAT. This means a selection of the used chemical, process technology and end-of-pipe techniques. That applies following sequence of priority: 1. Prevention, 2. Minimizing, 3. Reduction (aftertreatment)[13].

The most sustainable denim production in future will be always a process considering  all single steps of the entire denim production chain. Hereby, the optimum comes from harmonizing the used products, machinery equipment and process parameters to an optimum. In this process of development it would be very helpful, if the retailers (global jeans brands) could not only look for low cost production but also orientate the future eco-labelling on BAT parameters. The ZDHC is the first step to minimize the environmental impact and the use of minimal resources, chemicals, energy and water as well as minimal CO2 emission.