Ozone Finishing for Denim Reduces Environmental Impact, Processing Costs and Processing Time
Global demand for jeans has been projected to reach $56.2 billion this year.1 Trend forecasts continue to showcase denim as a wardrobe staple. While dark washes are reappearing, consumers worldwide still love bleached, stonewashed jeans, jeans jackets, vests and skirts. Manufacturers achieve these finishes with water, chemicals, enzymes and pumice stones. According to Levi Strauss & Co., the average pair of jeans uses 42 liters of water in the finishing process.2 A recent survey I did in 10 factories overseas supports that figure; factories I visited reported using 20 liters to 60 liters of water to finish each pair. Meanwhile, 900 million people lack access to clean water.3 A 2012 United Nations report says growing demand for water matched against static or shrinking supplies … point(s) unerringly in the direction of greater conflict over water.4 Responsible water use is of health and safety importance.
Forward-thinking brands and manufacturers are seeking alternatives to water, chemicals and stones for achieving the faded look consumers crave. Leading brands are sensitizing consumers to reduced water and chemical use; while conducting market research in Europe and the Middle East last summer, I found brands advertising low water, low chemical and no chemical finishes directly to the consumer.
Two water-saving, bleaching technologies are commercially viable today: laser and ozone.
Low water, low chemical, no chemical hangtags
Laser. Computer-driven laser technology can replicate localized wear, whiskers, and intricate lacelike patterns without water, chemicals or stones. Laser offers precise, repeatable bleaching. However, equipment is costly, each garment must be individually positioned for treatment, and only one side can be treated at a time. Excellent for localized effects, laser is less beneficial for overall bleaching.
Ozone. Ozone technology harnesses the natural bleaching capabilities of ozone gas to give a range of overall and specialty bleach effects with substantially reduced environmental impact. Ozone can be used to clean pocket backstaining from normal washing processes, or to bleach denim to a lighter shade.5 The photograph on the following page shows the range of bleachdown achievable by varying ozone concentration and exposure time. Individual brands are also creating unique specialty looks with the power of ozone.
Ozone does not eliminate water use in jeans finishing. However, it substantially reduces consumption of water as well as energy, chemicals, enzymes and stones. Ozone offers important advantages over traditional wet finishing.
Oxygen (O2) is converted to ozone gas (O3), jeans are dampened, exposed to the ozone, and rinsed; the ozone is reconverted to ordinary oxygen before release into the environment. While chemical bleaching or stonewashing uses six to seven washes and rinses, ozone finishing requires two to three. A production manager I spoke with reported more than 50 percent reductions in water, chemicals and pumice stone consumption when using ozone finishing in sequence with reduced traditional wet finishing methods.
Ozone finishing reduces energy consumption by reducing the amount of water that must be heated for wet finishing, and the temperature required. Furthermore, replacing some traditional finishing with ozone reduces effluent, including the sludge pumice stones create.
Ozone bleaches more quickly than chemicals and stonewashing. Ozone can clean back stains in three seconds. At optimum concentrations, it bleaches denim in 15 minutes to levels commonly desired by fashion today, versus 30 to 45 minutes with traditional methods. Ozone increases production per shift.
Ozone technology reduces environmental impact, processing costs and processing times while achieving desirable fashion looks.
Range of Bleachdown Achievable with Ozone Finishing. Photograph courtesy of Ozone Denim Systems.
Safety, Quality, Performance. Companies considering investment in ozone technology should consider three factors: safety, quality, and performance. Proper ozone bleaching systems are comprised of a filtered/refrigerated air compressor, oxygen concentrator, ozone generator, exposure chamber and ozone destruct unit. Specific equipment features affect the safety and reliability of ozone denim processing equipment.
Safety. The Canadian government reports exposure to ozone at 0.1 parts per million (ppm) causes minor irritation to the eyes, nose and throat and may cause headaches; at 50 ppm, death can be expected.6 For bleaching denim, 5,000 – 7,000 ppm are used. Exposure to ozone can be deadly. The United States Occupational Safety and Health Administration (OSHA) standards set maximum allowable exposure of workers operating ozone technology in the U.S. at 0.1 ppm per 8 hours, time weighted average7 and 0.3 ppm per 15 minutes.
The Canadian Ozone Safe Works Practices document cited above also states ozone corrodes most metals, damages most plastics, and hardens rubber (causing it to crack). Ozone bleaching equipment (including seals and gaskets) must be designed and constructed with materials that withstand these effects in order to reduce the potential for accidental exposure.
To prevent harmful or deadly accidental exposure of workers to ozone gas, safety features and safety practices should be of paramount importance in selecting, installing, operating, and maintaining ozone finishing equipment in any facility. While several companies manufacture and sell ozone equipment for the jeans industry, safety features vary in important ways, leaving some workers potentially more at risk than others for accidental ozone exposure.
Three equipment features stand out in best-of-class design for ozone safety: fill mechanisms, door locks and monitor/alarm systems.
Fill Mechanism. Ozone finishing equipment that draws ozone gas into the exposure chamber with negative force reduces potential for accidental worker exposure in the event of a ruptured seal. Should a breach occur, negative fill systems suck air into the exposure chamber rather than spilling ozone into the workplace.
Door Locks. Door locks with a UV light ppb8 ozone sensor that only allow the exposure chamber to open after ozone falls below a 0.2 ppm safety threshold reduce potential for worker exposure to ozone when emptying jeans from the chamber. Time-countdown locks offer no assurance ozone has been properly evacuated before workers open chamber doors.
Monitor and Alarm Systems. Dual alarms (audible and visual) linked to ozone monitors for localized and ambient ozone alert workers, plant management and security personnel in the event of any ozone leaks, allowing personnel to evacuate the immediate area until ozone levels fall.9 Monitors should be sensitive to 0.3 ppm for worker protection.
Quality. Like safety, the quality of finish achieved using ozone varies with machine design. Keys to quality and consistency of ozone bleaching are: 1) specialized oxygen concentrators that enable generation of concentrated ozone without oil or moisture contamination;10 2) real time monitors and controls that ensure consistent ozone levels in the chamber regardless of batch size; 3) manual and automated control capabilities to allow creative flexibility and repeatability, and 4) a closed-loop recirculation mechanism to move ozone throughout the exposure chamber during finishing. Ozone is heavier than oxygen; without continual circulation ozone sinks, causing uneven bleaching. Ozone technology must incorporate these features for uniform finishing results.
Performance. The keys to equipment performance are 1) a filtered/refrigerated air compressor that does not allow contamination with oil, lubricants or moisture; 2) a 95 percent to 99 percent oxygen generator that enables generation of high-concentration ozone; 3) a lint filter to prevent clogging of the ozone destruct unit, and 4) a real time monitor with UV light true-read ppm sensitivity. These features are important for peak performance of ozone denim finishing equipment.
Ozone technology doesn’t fully replace chemical bleaching, stonewashing and enzymes for jeanswear, but used alone or in sequence with traditional processing, it reduces environmental impact, operating costs and processing times. The caveat? When best-of-class equipment is used, ozone technology can be used safely and with consistent, repeatable finishing results.
Margaret Bishop is a global consultant to the textile and apparel industry, and an Adjunct Instructor in the Departments of Textile Development and Marketing, and International Trade and Marketing at the Fashion Institute of Technology in New York. She may be reached at M.L.Bishop.email@example.com.
1 “World Denim Market – A Report on Capacities, Market Size, Forecasts, etc.” Denim and Jeans 13 October 2009. N.p. Web 13 June 2014.
2 “New Jeans. Incredible Finishes. Less Water. | Levi Strauss.” Levi Strauss N.p. 5 November 2010. Web 13 June 2014.
3 “Facts About Water.” The Water Project. N.p. Web 13 June 2014.
4 The Global Water Crisis: Addressing an Urgent Security Issue. Edited by Harriet Bigas. Pp. 12–16.
5 Ozone works well to fade indigo-dyed denim. It also bleaches black denim and some other colors, depending upon the specific dyes used.
6 Workers’ Compensation Board of British Colombia. Ozone Safe Work Practices. 2006. Page 14.
7 Miles, John B. Jr., Director, Occupational Safety & Health Administration, United States Department of Labor, Standard Number 1910.1000; 1910.1200. https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=21606&p_table=INTERPRETATIONS
8 Parts per billion.
9 Ozone breaks down naturally into ordinary oxygen within minutes in water and within hours as a gas.
10 This is not a standard industrial air compressor. Standard industrial air compressors generally contaminate the compressed air with oil or other lubricants that then interferes with the operation of the ozone equipment.