If you’re reading this you’ve probably already read my very brief history of natural dyes. The rest of the following discussion is not quite so brief and does get quite technical in a chemical sense but I’ve tried to omit as much of that as possible to make the information accessible to normal readers like you and me.
Within the space of 50 years mankind had changed almost totally from natural dyes to synthetic ones. Phase 1 of this change was the addition of Chromium. The subsequent, often parallel, phases are discussed below with a bias towards how they apply to fabrics.
The 8 subsequent developments of synthetic dyes from Chromium onwards are:
2. Triphenylmethane dyes
3. Anthraquinone dyes
4. Xanthene and related dyes
5. Azo dyes
6. Reactive dyes
7. Phthalocyanine compounds
8. Quinacridone compounds
9. Fluorescent brighteners
Phase 2: Triphenylmethane dyes
In 1858 Verguin (France) discovers ‘fuchsine’ a rose coloured dye made from aniline and tin chloride. This was the first of a series of dyes later called Triphenylmethanes and this marked the second phase of the growth of the synthetic dye industry. Adding excess aniline made aniline blue. Soon other variations were discovered and the chemistry understood; and before 1900 several hundred colours had been documented.
One problem with these dyes was solubility. That was overcome by the addition of sulphuric acid.
Phase 3: Anthraquinone dyes
In the 1850s and 1860s the understanding of how carbon is structured (tetravalency) led to scientists being able to plan how chemicals might react before doing the experiments. Alizarin and its derivatives (the anthraquinones) gave a huge number of dyes which constitute the second biggest grouping of dyes.
The addition of sulphur created a group of bright, fast dyes for wool. Indrathone blue, a brilliant blue vat dye, was discovered at the turn of the century and related compounds are still today used as pigments spanning colours from blue to yellow.
Phase 4: Xanthene related dyes
Fluorescein was discovered in 1871 but related discoveries were seldom used with fabrics until the late 1880s when some were used for silk. However the dyes had poor lightfastness and usage was stopped – only to be re-used 70 years later when they were found to be particularly good on acrylic fibres. Better products have been found since and now only one chemical dye in this class is used commercially (Safranine T).
Phase 5: Azo dyes
These form more than half of the commercial dyes used today.
The key is the reaction of nitrous acid with arylamine and then with phenols and aryl amines. This chemical reaction forms part of the production process of 50% of dyes in use today having been used since 1875, firstly for wool.
Methyl- related azo dyes were used extensively up until the 1970s but this has now stopped in many countries as they were carcinogenic.
The discovery of the azo dyes led to the development of method called ingrain dyeing. Here the dye is ‘made’ within the fabric. Since the process was carried out at around freezing point, some dyes were called ice colours. In 1912 Naphtol was found to form a water-soluble compound with an affinity for cotton, a major step in the development of the ingrain dyes. Naphtol is able to form a great number of possible end colours although many of these are not adequately colourfast.
Other Azo dyes became the most important commercial colorants because of their wide colour range, good fastness properties, and tinctorial strength (colour density), which is twice that of the anthraquinones, the second most important group of dyes. Azo dyes are easily prepared from many readily available, inexpensive compounds and meet the demands of a wide range of end uses. Cost advantages tend to offset the fact that these are less brilliant and less lightfast than the anthraquinones.
Phase 6: Reactive dyes
Reactive dyes are very adaptable and can create a huge number of colours.
The first reactive dyes utilized monoazo for bright yellow and red shades. Adding aniline gave the azo dye used in the first Procion Red.
Dichlorotriazinyl dyes are now produced by more than 30 dye manufacturers, since the early patents on these dyes have expired.
With the introduction of reactive dyes, cotton could finally be dyed in bright shades with azo dyes for yellows to reds, with anthraquinones for blues, and with copper phthalocyanines for bright turquoise colours.
Phase 7: Phthalocyanine compounds
Phthalocyanines, the most important chromium derivatives developed in the 20th century being introduced in 1934 and marketed as Monastral Fast Blue B and Monastral Fast Blue G.
Copper phthalocyanine is the most important and can be formed directly on cotton. Although not useful for PET and acrylics, some complexes are utilized with nylon. Chemical bleaching alters the shade to bluish-green and green.
Water-soluble versions were developed later by the introduction of sulphur based chemicals also producing a direct dye for cotton (Chlorantine Fast Turquoise Blue Gll), the first commercial phthalocyanine dye.
Such colourants all display strong, bright blue to green shades with remarkable chemical stability. These compounds exhibit excellent lightfastness, and their properties are in striking contrast to those of natural pigments that are destroyed by intense light or heat and mild chemical reagents. The high stability, strength, and brightness of the phthalocyanines render them cost-effective, illustrated by the wide use of blue and green labels on many products.
Phase 8: Quinacridone compounds
A second group of pigments developed in the 20th century were the quinacridone compounds. Quinacridone itself was introduced in 1958. Its seven crystalline forms range in colour from yellowish-red to violet.
Phase 9: Fluorescent brighteners
Raw natural fibres, paper, and plastics tend to appear yellowish because of weak light absorption. Bleaching can reduce this but the bleach must be mild to avoid damaging the material. Alternatively a bluing agent can mask the yellowish tint to make the material ‘appear’ whiter (hence the phrase a ‘bluey whiteness’), or the material can be treated with a fluorescent compound that weakly emits blue visible light. These compounds, also called “optical brighteners,” they are not dyes in the usual sense. The major industrial applications are as textile finishers, pulp and paper brighteners.
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