The term dye, or dyestuff, refers to a substance used to color another material. Dyes, unlike pigments, are usually water-soluble. However, some dyes become insoluble after application. (McGraw-Hill 2005)
Prior to 1856 dyes were from natural sources.These included the madder plant, mollusks' secretions, cochineal insects, kermes, indigo and woad plants as just a few. In 1856 William Henry Perkins discovered mauveine while trying to synthesize quinine, an anti-malaria drug. While Perkins was not the first to produce a synthetic dye, he did push synthetic dyes into industrial manufacturing in 1859. The synthetic dye industrial grew rapidly after this.
There are many ways in which dyes can be classified. They can be separated by chemical structure, shade, means of application or usage While chemists tend to classify dyes by their chemical structure, dye users and manufacturers are more inclined to use mode of application or usage as a classification system. Many times these two systems are combined.
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Dyes can be classified by their chemical structure. This classification system presents many advantages. First, dyes with similar properties are easily identifiable. For example, azo dyes, which are strong and cost-effective, are easily distinguishable from anthraquinone dyes, which are weaker and more expensive. Also, this classification system is used by both synthetic dye chemists and dye technologists, which leads to easier communication and identification.
Dyes can also be classified by their usage. This method is useful because the intended use of the dye is readily apparent. This method is also the official method used by the Colour Index (C.I.), which is a popular reference database maintained by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. Table 1 tabulates a selection of dyes arranged based on the C.I. application classification. The table also includes the principle substrates of each dye, the methods of application, and the chemical types for each class.
1.3 Naming Systems - industrial dyes
Every dye has a commercial name and a C.I. name. The C.I. includes both of these names to make cross-referencing easier.
Commercial dye names are made up of three parts. The first is a trademark determined by the dye's manufacturer to label the class of the dye. The second is simply the color of the dye. The third element is a combination of coded numbers and letters used to identify the hue and any other noteworthy properties of the dye. These codes can vary from manufacturer to manufacturer and the same color could have many different commercial names attached to it. An example commercial dye name is Solanthrene Green XBN.
The C.I. name for a dye is derived from the application class to which the dye belongs, the color or hue of the dye and a sequential number. Additionally, a five digit constitution number is assigned to a dye when its chemical structure has been disclosed by the manufacturer. Examples of C.I. names include C.I. Acid Yellow 3 and C.I. Basic Blue 41.
Each dye has a commercial name and a Colour Index (C.I.) name. The commercial name is comprised of three parts that gives the manufacturer information about that specific dye.
Size of the Industry - Bamfield
In the early 1990s there were six major European dyestuff manufacturers (BASE Bayer, Ciba-Geigy, Hoechst, ICI and Sandoz), four Japanese (Nippon Kayaku, Sumitomo, Mitsubishi and Mitsui Toatsu) and one in the US (Crompton and Knowles). However, in the early 2000s, many things had changed. There were only four large European companies (Ciba Specialties, Clariant and DyStar, plus the late entrant Yorkshire). The number of Japanese companies was reduced to two (Nippon Kayaku and Sumitomo), and the United States had no major dyestuff manufacturers. The industry has become much more globalized with many operations relocating to different geographical regions and the formation of multiple joint ventures in Asia. At the same time, some localized companies have specialized and become world players in specific areas, such as Everlight of Taiwan's production of reactive dyes. This dye industry restructuring may be better depicted in the tables in Appendix A.
Due to the magnitude of dyes available, only azo and anthraquinone are discussed here.
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Over half of all commercial dyes produced are azo dyes. and having been studied more than any other class. Azo dyes contain at least one azo group (-N=N-) but can contain more azo groups, i.e., disazo, trisazo, polyazo. The azo group is attached to either one or two aromatic groups. The azo goup occurs in the trans form with a bond angle of 120Â° and the nitrogen atoms have a sp2 hybridization.
Once the second most important class of dyes, it also includes some of the oldest dyes; they have been found in the wrappings of mummies dating back over 4000 years. In contrast to the azo dyes, which have no natural counterparts, all the important natural red dyes were anthraquinones . However, the importance of anthraquinone dyes has declined due to their low cost-effectiveness. Anthraquinone dyes are based on 9,10-anthraquinone (1), which is essentially colorless. To produce commercially useful dyes, strongly electron donating groups such as amino or hydroxyl are introduced into one or more of the four positions (1, 4, 5, and 8). The most common substitution patterns are 1,4, 1,2,4, and 1,4,5,8-. To optimize the properties, primary and secondary amino groups (not tertiary) and hydroxyl groups are employed. These ensure the maximum degree of Ï€-or-bital orbital overlap, enhanced by intramolecular hydrogen-bonding, with minimum steric hindrance. These features are illustrated in C.I. Disperse Red 60 (2).
The strength of electron-donor groups increases in the order: OH<NH 2<NHR<HNAr. Tetrasubstituted anthraquinones (1,4,5,8-) are more bathochromic than di- (1,4-) or trisubstituted (1,2,4-) anthraquinones. Thus, by an appropriate selection of donor groups and substitution patterns, a wide variety of colors can be achieved.
To ensure a safe work environment, proper use and disposal of chemicals, most nations have ongoing legislation regulating industry. Some of the important United States, European, and Japanese legislation are presented in Table 2.
The two most important pieces of chemical control legislation enacted affecting the dye and pigment industries are the United States' Toxic Substance Control Act (TSCA) and EEC's Classification, Packaging, and Labeling of Dangerous Substances and its amendments. Table 3 is a comparison of TSCA and the 6th Amendment of the EEC classifications.
There are also several additional regulations for the use of dyes in food, food-packaging, or pharmaceuticals.
The reactions for the production of intermediates and dyes are carried out in bomb-shaped reaction vessels made from cast iron, stainless steel, or steel lined with rubber, glass (enamel), brick, or carbon blocks. These vessels have capacities of 2-40 m3 (ca 500-10 000 gallons) and are equipped with mechanical agitators, thermometers, or temperature recorders, condensers, pH probes, etc., depending on the nature of the operation. Jackets or coils are used for heating and cooling by circulation of high-boiling fluids (e.g. hot oil or Dowtherm), steam, or hot water to raise the temperature, and air, cold water, or chilled brine to lower it. Unjacketed vessels are often used for reactions in aqueous solutions in which heating is effected by direct introduction of steam, and cooling by addition of ice or by heat exchangers. The reaction vessels normally span two or more floors in a plant to facilitate ease of operation.
Products are transferred from one piece of equipment to another by gravity flow, pumping, or blowing with air or inert gas. Solid products are separated from liquids in centrifuges, in filter boxes, on continuous belt filters, and, perhaps most frequently, in various designs of plate-and-frame or recessed-plate filter presses. The presses are dressed with cloths of cotton, Dynel, polypropylene, etc. Some provide separate channels for efficient washing, others have membranes for increasing the solids content of the presscake by pneumatic or hydraulic squeezing. The plates and frames are made of wood, cast iron, but more usually hard rubber, polyethylene, or polyester.
The final stage in dye manufacture is grinding or milling. Dry grinding is usually carried out in impact mills (Atritor, KEK, or ST); considerable amounts of dust are generated and well-established methods are available to control this problem.
The principal air pollutants from dye manufacturing are volatile organic compounds (VOCs), nitrogen oxides (NOx), hydrogen chloride (HCl), and sulfur oxides (SOx).
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Liquid effluents resulting from equipment cleaning after batch operation can contain toxic organic residues. Cooling waters are normally recirculated. Wastewater generation rates are of the order of 1-700 liters per kg (l/kg) of product except for vat dyes. The wastewater generation rate for vat dyes can be of the order of 8,000 l/kg of product. Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) levels of reactive and azo dyes can be of the order of 25 kg/kg of product and 80 kg/ kg of product, respectively. Values for other dyes are, for example, BOD5, 6 kg/kg; COD, 25 kg/kg; suspending solids, 6 kg/kg; and oil and grease, 30 kg/kg of product.
Major solid wastes of concern include filtration sludges, process and effluent treatment sludges, and container residues. Examples of wastes considered toxic include wastewater treatment sludges, spent acids, and process residues from the manufacture of chrome yellow and orange pigments, molybdate orange pigments, zinc yellow pigments, chrome and chrome oxide green pigments, iron blue pigments, and azo dyes.
Due to the nature of dyes small quantities present in wastewater are obvious. This can cause more of a visual disaster than a hazardous situation. Dyes, because they are intensely colored, present special problems in wastewater; even a very small amount is noticeable. However, the effect is more aestetically displeasing rather than hazardous, e.g., red dyes discharged into rivers and oceans. Of more concern is the discharge of toxic heavy metals such as mercury and chromium.
Wastewaters from both dye manufacturing plants and dyehouses are treated before leaving the plant, e.g., by neutralization of acidic and alkaline liquors and removal of heavy metals, and in municipal sewage works. Various treatments are used .
Biological treatment is the most common and most widespread technique used in effluent treatment, having been employed for over 150 years. There are two types of treatment, aerobic and anaerobic. The aerobic system needs air (oxygen) for the bacteria to perform the degradation process on the activated sludge, whereas anaerobic bacteria operate in the absence of air. Activated sludge usually removes only a moderate amount (10-20 %) of the color.
Removal of color by adsorption on activated carbon is also employed. Activated carbon is very effective in removing low concentrations of soluble chemicals, including dyes. Its main drawback is its limited capacity. Consequently, activated carbon is best for removing color from dilute effluent.
Chemical treatment of the effluent with a flocculating agent is generally the most efficient and most robust way to remove color. The process involves adding a flocculating agent, such as ferric (Fe3+) or aluminum (Al3+) ions, to the effluent. This induces flocculation. A coagulant may also be added to assist the process. The final product is a concentrated sludge that is easy to dispose of.
Health and Safety Issues - (Hunger 2005)
Like all other chemical production industries, dye manufacturers must consider the health and safety aspects. One way to look at these issues is through the analysis of toxicological testing.
Acute toxicity comprises the three exposure routes (oral, dermal and by inhalation) as well as the irritation of the skin and mucous membranes (eye). The acute toxicity test first provides information about the toxic potential of a chemical. The test results are also helpful for the determination of the dosage for chronic studies and can give indications of target organs.
"A comprehensive review â€¦ including skin and eye irritation of numerous commercial dyes, derived from Safety Data Sheets, showed that the potential for acute toxicity effects ('harmful' or 'toxic') was very low (Hunger 2005)."
Symptoms of an allergic reaction do not generally show up at the first time of contact with an allergising substance but after repeated contact. The state following the development of hypersensitivity against a certain chemical substance (or biological material) is called sensitisation. There are two ways of possible exposure, resulting either in skin or respiratory sensitisation. Contrary to an irritation effect, the sensitisation reaction is increased with repeated exposure.
"There is evidence that some reactive dyes cause contact dermatitis, allergic conjunctivitis or rhinitis, occupational asthma or other allergic reactions to textile workers. [Also] disperse dyes with a sensitising potential may cause an allergic skin reaction if dyed on polyamide or semi-acetate, where due to a low wet fastness, the dyes could migrate onto the skin (Hunger 2005)." A list of reactive dyes classified as sensitisers and a list of disperse dyes harmful to consumers are available in Appendix B.
"Some dyes exhibit a mutagenic potential that has an effect on the genetic material. The Ames test [or the Prival test for azo dyes] is commonly used as a first screening for the prediction of mutagenicity of a substance (Hunger 2005)." For the mutagenicity testing of azo dyes, the Prival test (a modification of the Ames test) was found to be superior.
"In a comprehensive study with more than 200 dyes of various structures, more than two thirds were reported to be non-mutagenic. Reevaluations have shown that in some cases impurities could be responsible for positive test results (Hunger 2005)."
Some side effects of exposure will not appear until after many years of contact. "Strict regulations concerning the handling of known carcinogens have been imposed in most industrial nations (Hunger 2005)."
"The carcinogenicity of dyes is ascertained by animal testing, and some dyes have been listed as proven carcinogens to animals, and probable carcinogens to humans (Hunger 2005)." A list of dyes classified as potential human carcinogens can be found in Appendix B.
Conclusions - industrial dyes
The dye industry is intimately connected to the textile industry. Dye manufacturers tend to concentrate their resources on textile dye production more specifically for cotton and polyester. However, current consumer demand in areas such as ink-jet printing is becoming increasingly important. These high-tech dyes are considered specialty dyes that require low volume with high price value.