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By Deborah Jackman, PhD, PE, LEED AP™ - originally posted on 04/11/2012

The Painting:
‘Child Labor in the Dye Works’ is attributed to the German artist, Heinrich Kley, who lived between 1863 and 1945.  The exact date of the painting is unknown, but based upon the style of dress of the workers and the equipment used, the painting was likely created sometime between the mid 1890s and the start of the World War I (1914).  It depicts an industrial yarn dyeing operation, presumably in Germany.  The portion of the scene that draws the eye is the brilliantly colored yellow yarn being transferred from the dyeing process by the child worker.  The use of child labor is, of course, forbidden in developed countries today, but persists– contrary to international pressures to curtail it– in some developing nations. However, the primary purpose of this article is not to discuss child labor in the developing world. Instead, we will use the image presented in the painting as a springboard to explore the broader environmental implications of textile dyeing operations. Before discussing these environmental impacts and what can be done to minimize them, it is helpful to understand in overview the history of textile dyeing.

A Brief History of Textile Dyeing:
Dyeing of fibers to produce colored cloth is among the oldest of human activities.  Until the first aniline dye, mauve, was synthesized by William Henry Perkins in 1858, all dyes were natural in origin—derived either from plant, animal, or mineral sources.  The initial driving force behind the development of synthetic dyes was the desire to have a wider variety of more vibrant colors available than were obtainable from natural sources.  Then, once synthetic fibers such as nylon began to be introduced in the mid 1940’s, a secondary driver for the continued use of aniline dyes was the fact that synthetic fibers took up synthetic dyes more readily than natural dyes. Following the introduction of synthetic mauve dye, other aniline dye colors were gradually introduced such that by 1900 a full range of colors were available in the form of synthetic aniline dyes [1].  Aniline dyeing of wool had the additional advantage of not requiring a mordanting step, as the use of natural dyes did.  (Mordanting is a chemical pre-treatment of the fiber separate from the dyeing step itself required to ensure that the pigment takes to the fiber and remains color-fast.)  The upshot of all of these factors is that by the mid 20th century, aniline dyes ( and their close chemical derivatives—azo dyes) had supplanted natural dyes in nearly all commercial textile operations world-wide.

The timeline of the technical development of aniline dyes discussed above might suggest that the colored fibers depicted in our painting were the product of aniline yellow, but this is likely not the case.  Rather, the yellow yarn depicted in the painting is likely wool dyed with weld—a natural dye derived from the weld plant (Reseda luteola) [2].  The reasons for this are economic, not technical.  Even though the technology for using aniline dyes was probably known at the time this painting was created (estimated to be the mid 1890s to 1914), the wool textile industry in Germany was depressed until 1904 due to high tariffs imposed by countries to which Germany would have exported its colored fibers.  Thus, textile manufacturers did not invest significant money into updating operations until after at least 1904 because there was little economic incentive for them to do so [3].  To further the irony, during the late 19th century Germany was the center of R&D work on organic chemical synthesis, and German companies held most of the chemical patents for synthetic dyes.  In 1913, 80% of the synthetic dyes exported to the rest of Europe and to America came from Germany and yet, German textile manufacturers lagged behind other nations in adopting the synthetic dyes in their own operations.  During World War I many German chemical plants were destroyed for fear that they were manufacturing mustard gases and other chemical warfare agents, and after the War, many of these German dye patents were seized.  The nexus of synthetic textile dye manufacturing then shifted to the U.S. [4].

Environmental Impacts of Textile Dyeing:
The use of weld to dye wool and other natural fibers, as depicted in the painting, has relatively low environmental impacts compared to synthetic dye operations, but is not totally benign.  The spun fibers or woven, un-dyed cloth is placed in hot (200 F) water to which enough alum (sodium aluminum sulfate, NaAl(SO4)2 ,) has been added to create a 1:4 alum to fiber (by weight) solution.  The fibers are steeped in the hot solution for 12 hours.  This is the mordanting step which prepares the fibers to accept the dye.  Once mordanted, the fibers are transferred to another vat containing a hot (150F to 200F) aqueous weld dye solution.  They are kept in the dye solution until the desired shade of yellow is obtained.  The fibers are then removed, washed with a pH neutral soap and allowed to dry.  The aqueous weld dye solution is prepared by crushing the stems, leaves, and flowers of the weld plant and placing them in a vat covered by water.  The mixture is brought to the boiling point and allowed to steep for 30 minutes.  The colored solution is then strained to remove the solids [5].  Given that both the weld plant and the alum have low toxicity, they do not present a hazard from a material or human health/worker standpoint.  But because of the extreme color intensity of the aqueous dye solution, it does need to be decolorized before being discharged as an industrial wastewater in a manufacturing setting.  Such decolorization can be accomplished by processing the solution through an activated carbon filter or by using ultrafiltration or ion exchange technologies. From a more generalized sustainability standpoint, the weld dyeing process is quite energy intensive (due to the need to heat the solutions and to maintain their temperature for relatively long periods), and is also water intensive (although a given batch of weld dye can be reused to do multiple batches of fibers by re-fortifying the dye bath with more weld).    Although we are focusing on the use of weld dye to produce yellow fibers in reference to our subject painting, other natural dyes used to produce other colors generally exhibit similar environmental impacts to that of weld.

Aniline yellow and other aniline and azoic synthetic dyes exhibit more severe environmental impacts than natural dyes.  Most synthetic dyes are delivered to textiles by creating an aqueous (i.e. water based) dye solution, immersing the fibers in the dye solution at elevated temperatures  (100 to 130 C) for a specified period, and then removing the fibers or woven cloth for further finishing, including drying.  Most synthetic dyes, including aniline and azoic dyes, do not require a mordanting step, which is one of the distinct advantages of synthetic dyes.  Various pieces of specialized machinery have been developed by the modern textile industry to optimize and mechanize such dying operations.    Reference [6] provides a detailed technical review of various modern textile dyeing processes and related equipment.  However, regardless of the specifics of a particular technique, the general process involves the use of considerable amounts of energy and water, just as the natural dyeing process does.  In this regard, there is little difference in the environmental impacts or sustainability of the use of natural versus synthetic dyes.

The differential environmental impacts between natural dyes and synthetic aniline/azo dyes occur at two other points in the life cycle of the dye–  its manufacture and the disposal of resulting dye wastewaters.  At both of these points in the life cycle, aniline/azo-based dyes have much more negative and severe impacts than do natural dyes.

The manufacture of azoic dyes involves a complex, multi-step process of organic chemical synthesis. The root source of the chemicals used to synthesize azoic dyes is petroleum (i.e. crude oil). The precursor chemical to Yellow Azo dye (and other shades of azo dyes) is aniline which is converted into the dye via an oxidation process.  Aniline, itself, is an aromatic (i.e. based upon the benzene ring structure) amine.  It is acutely toxic to humans and also likely carcinogenic although evidence of its carcinogenicity is somewhat contradictory. Various azo dyes have been banned by the European Union due to concerns about their carcinogenicity [7],[8],[9].  Each step in the chemical synthesis of azo dyes from the extraction of the crude oil to the final synthesis from aniline has significant negative environmental impacts.

Large scale agricultural production of weld and other dye plants would also use petroleum to fuel farm machinery and to transport the harvested plants, but since there are not as many steps and chemical reaction processes between the planting of the weld and its final use, and because the intermediaries in the processes do not involve the use of aromatic chemicals such as aniline and benzene, the production of natural dyes is arguably far more sustainable than the production of synthetic dyes.  The weld plant itself is a hardy perennial, native to temperate regions of Eurasia.  It is drought tolerant, thrives in poor soils, and is not subject to insect infestations.  Therefore, cultivation of weld for use in dyes is relatively low impact, not requiring significant amounts of fertilizers or pesticides.

Once synthetic dyes are spent, the steps involved in treating the resulting dye-laden wastewaters are expensive and complex. Singh and Arora [10] provide a critical review of present treatment technologies being employed to treat wastewaters containing azo dyes.  A variety of physical, chemical, and biological treatment strategies are being employed singly and in combination to remove azo dyes prior to discharge of these industrial wastewaters.  Because azo dyes can be degraded biologically in the natural environment to produce by-products such as aniline and related toxic aromatic chemicals, it is necessary to remove these dyes prior to discharge.    Treatment strategies such as carbon adsorption; coagulation, flocculation and settling; filtration; membrane processes; ion exchange; direct chemical oxidation; UV irradiation; and aerobic and anaerobic biological treatments are being used to capture or destroy the azo dye chemicals before they reach the environment.  In those cases where the azo chemical is merely transferred to another medium rather than destroyed (e.g. coagulation/flocculation/filtration), the environmental hazard is not eliminated but must still be dealt with through yet another series of steps such as incineration or land filling.  In contrast, wastewaters containing natural dyestuffs or alum mordanting solutions do not require as extreme of treatment steps—simple decolorization using carbon adsorption or via UV irradiation is typically sufficient because the dyes are not toxic.  Treatment is done simply to meet water standards related to color and turbidity.  Again, in terms of what is required to treat the wastes generated in textile dyeing operations, natural dyes exhibit far more sustainability.  Potential pollution is prevented rather than having to be remediated.

Future Developments to Promote Sustainability:
As we have discussed, both natural and synthetic textile dyeing operations have adverse environmental impacts; although arguably, those involving natural dyestuffs have fewer impacts. Nevertheless, because even natural dyeing operations use large amounts of energy and water, the textile industry continues to seek even more sustainable methods to color fibers.

Thiry, [11], discusses several new and more sustainable strategies for coloring fibers and fabrics that are likely to replace, at least in part, traditional solution-based dyeing operations in the near future.  Among the new strategies being developed are 1) ultra low liquor ratio dyeing; 2) selective plant breeding to produce naturally colored cotton fibers; 3) digital printing of fabrics; 4) cationic cotton printing; and 5) waterless dyeing involving the use of supercritical CO2.  Ultra low liquor ratio dyeing reduces both water and energy usage by reducing the weight ratio of the dye solution to the fabric to levels as low as 3:1 for some fabrics.  The smaller the amount of water used, the less energy is required to heat the solution and the smaller the amount of wastewater ultimately generated.  Cotton is the most commonly used natural fiber in the world, largely because of its use in denim fabrics.  Prior to relatively recent attempts to breed color out of cotton in order to produce higher agricultural yields, cotton grew with various naturally occurring pigmentations such as red, green, and brown.  Recently, growers are working to breed these natural color variations back into cotton, producing fibers requiring no dyeing whatsoever.  Digital printing of fabrics using technologies similar to the ink jet printing used with paper use no water whatsoever.  However, the production of the ink jet cartridges and the chemicals contained in them involve certain environmental impacts. (As with any discussion of sustainability, one must consider not just the impacts of the immediate process, but of the entire life cycle associated with that process.)  Cationic cotton printing involves treating cotton fabric so that it has a positive (cationic) charge.  The charged cotton is then immersed in a dye bath containing a reactive dye that attaches itself to the charged sites on the fabric.  Using the correct chemical ratio of dye to fabric results in water in the dye bath which is free from all chemicals and color at the end of the batch process.  That water can then be reused in dyeing subsequent batches of fabric, thereby conserving water.  Cationic dyeing is conducted at room temperature, meaning it is less energy intensive than traditional chemical dyeing operations.  Details on the production of these cationic dyes were unavailable and so we cannot draw a conclusion of their broader sustainability over the entire life cycle of the process.  Finally, some textile manufacturers are attempting to develop a dye process using supercritical CO2 .The carbon dioxide is exposed to extremely high pressures at relatively low temperatures (room temperature and below).  As any student of basic thermodynamics knows, this will cause the CO2 to exist in the liquid state.  The pigments are suspended in the liquid CO2 and the fabric is introduced.  Then the pressure of the system is dropped and the CO2 evaporates, leaving a completely dry dyed fabric.  While technically feasible, it has not been scaled up for full scale production because it is not economically competitive with conventional dyeing processes at this time.

In addition to the various innovative strategies discussed above, the textile industry is also experiencing a resurgence in interest in the use of natural dyes and associated processes, as a way to mitigate environmental impacts.  Allegro Natural Dyes, a company located in Longmont, Colorado, was an industry start-up in 1995, created with the intent to re-introduce natural dyes into the textile industry, [12].  The company has subsequently obtained a number of patents involving various proprietary processes centered around the production of natural dyes.  Allegro is an interesting case because it supports the premise that the use of natural dyes can be economically competitive to synthetic dye-based practices in today’s modern textile industry.  Hence, in some ways, the textile industry has come full circle.

Today there clearly exists an opportunity to employ the best parts of both traditional and modern dyeing practices to produce a more sustainable textile industry overall.  The use of natural dyes combined with modern water treatment technologies such as ion exchange and membrane technologies, and combined with various energy recovery techniques, could produce industry processes that would capitalize on the low environmental impacts of natural dye production and yet could mitigate the water and energy intensiveness of the dyeing process.  This would result in a sustainable process over the life cycle of the dyeing operation.  It represents an exciting modern sharing of new technology with age-old traditional practices.

References:

  1. The Cambridge History of Western Textiles, Jenkins, David, editor, Cambridge University Press, 2003, p.764.
  2. New International Encyclopedia, “Weld”, Gilman, Daniel Coit; Peck, Harry Thurston; Colby, Frank Moore; Dodd, Mead & Company, New York, 1905.  [Wikisource].
  3. Jenkins, op. cit., p. 782.
  4. Jenkins, op. cit., p.1082-1084.
  5. The Art and Craft of Natural Dyeing: Traditional Recipes for Modern Use, Liles, J.N., Knoxville: University of Tennessee Press, 1990.
  6. “ A Review of Textile Dyeing Processes”, Perkins, Warren, S., Textile Chemist and Colorist, Vol.23, No.9, August, 1991.
  7. “Aniline”,  http://en.wikipedia.org/wiki/Aniline , January 21, 2012.
  8. “Azo compound”, http://en.wikipedia.org/wiki/Azo_dye , February 16, 2012.
  9. Environmental Impact of Textiles; Slater, Keith; Woodhead Publishing, LTD.; 2003, p. 81-82.
  10. “Removal of Synthetic Textile Dyes From Wastewaters:  A Critical Review on Present Treatment Technologies”; Singh, K., and Arora, S.; Critical Reviews in Environmental Science and Technology, 41: 807-878, 2011.
  11. “Color it Greener”; Thiry, Maria, C.; AATCC Review, Vol. 10, Issue 3, p.32-39,      2010.
  12. “Natural Dye Startup”; Chemical Week; Vol.157, Issue 7, 0009272X. August 23, 1995.