Special Issue "Functional Colorants"

Guest Editor
Dr. Zhi-Min Hao
Brennerstrasse 60, CH-4123 Allschwil, Switzerland
E-Mail:
Interests: pigments; functional chromophores; latent pigments; pigment modifications; pigment solid state characteristics

Dear Colleagues,

Colorants played an important role in the early development of chemical industry as well as the organic chemistry as a scientific discipline.

Indeed, dyestuffs were central to the first Industrial Revolution, which began in the late 18^th century with the mechanization of the textile industries. The rapid spread of cotton mills, and increased productivity in textile manufacturing, encouraged chemists to investigate the composition of natural dyes. They gave the scientific name alizarin to the natural colorant obtained from madder wood extract, which is the basic compound for Turkey Red.

Dyes also played a prominent role in the second Industrial Revolution, when the quest for synthetic colorants led to the development of science-based industry.

The discovery of Mauveine, the first of the modern synthetic dyes, by William Henry Perkin in 1856 marked the beginning of the synthetic dye industry. It stimulated the colorant research by many other chemists. Of particular interest are the discoveries of diazotization and diazo compounds by P. Griess in 1858, Fuchsine by E. Verguin in 1859, and the production of synthetic alizarin (1869) and indigo (1897). In 1875, Otto N. Witt proposed a theory of color and constitution, with the concept of chromophores and auxochromes, which is still used in modern days.

The work by August von Kekulé on the quadric-valence of carbon in 1858, and on the benzene constitution in 1865, paved the way for the analysis, design and synthesis of organic dyes. Armed with the knowledge, C. Graebe and A. Liebermann were successful in the structural elucidation (1868) and subsequent synthesis of alizarin, the key component in the metal complex dye Turkey Red. They were then followed by the structural elucidation (A. von Baeyer, 1883) and the synthesis (K. Heumann, 1890) of indigo.

The development of the synthetic dye industry led to the emergence of classical organic chemistry, which in turn found rapid application in industry. From the end of the nineteenth century the intermediates employed in the manufacture of synthetic dyes found also use in making pharmaceutical products such as aspirin. Some synthetic dyes exhibited bactericidal properties; they were called medicinal dyes. Sulfonamides, drugs introduced in the 1930s, are actually based on research into dyestuffs and their intermediates. Certain classes of dyes have made color photography possible. A close look at the history of chemical industries would reveal a fact that many chemical companies started their business as dyestuff manufacturers in the early days.

The industries of colorants have reached the maturity phase of the life cycle. Like many other mature sectors of the specialty chemicals industry, colorant industries are facing challenges. The past three decades have seen a steady decline in new introductions of dyestuffs for the textile industry, the principal user of dyes. In addition to the maturity factor, in more recent times, there have been increasing impact of energy and raw material cost, and the introduction of stringent toxicological test requirements for new products due to ecological and environmental concerns. As a result, the colorant industries are currently undergoing restructuring and consolidation, and this is likely to continue in the foreseeable future.

On the other hand, there are also plenty of opportunities, especially in those non-traditional application areas where colorants are needed. As a matter of fact, dyes and pigments are today no longer used only for the coloration of textiles, plastics, paints, inks and lacquers but serve as key components in high-tech applications such as reprographics, optical data storage, display devices, dye sensitized solar cells, energy transfer cascades, light emitting diodes, laser welding processes or heat management systems. Dyes are also of growing importance in the medical and biomedical fields. It is interesting to note that in a number of such non-traditional applications, the color is largely irrelevant. It is the ability of the colorants to absorb visible electromagnetic radiation with high efficiency, or other functional property, that is exploited.

Human beings have the intrinsic desire to improve the quality of life. The insatiable demand for better life is one of the key driving forces behind the technological development. What was hard for the average man to imagine yesterday is in every day use today. To move toward a better future, we need to create and develop new materials with new effects for new applications. New colorants are to be designed for desirable functional properties. New functional colorants for non-traditional applications will be the future of innovative colorant research activities, and we look forward to contributions from those areas.

Zhi-Min Hao, Ph.D.
Guest Editor

Related Special Issue: Functional Colorants in the International Journal of Molecular Sciences

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• functional dyes
• functional colorants
• colorants for high-tech applications
• dyes for high-tech applications
• chromophores for displays
• dyes for solar cells
• dyes for optical information storage
• dyes for sensing system

Type of Paper: Review
Title: Color Produced by Awa Natural Indigo: Unique Beauty of Natural Indigo Color Related to Aggregation Size of Indigo Dye Molecules
Authors: Miyoko Kawahito
Affiliations: Tokushima Prefecture Industrial Technology Center,　Life-Style Sciences Division, Japan. E-mail: kawa@itc.pref.tokushima.jp
Abstact: Despite extensive research and general agreement that color produced by natural indigo (*) is superior in some respects to color produced by synthetic indigo (**), there has been limited investigation of that superiority and the reasons for it.
Here, natural indigo and synthetic indigo are compared in terms of three key color characteristics: sae (brightness of color), nijimi (running of color), and iromura (color unevenness).
To examine how natural indigo color can be distinguished from synthetic indigo, natural indigo dyers subjectively evaluated color characteristics. Apparent color difference between cloth dyed with natural indigo and cloth dyed with synthetic indigo was quantitatively measured using modern equipment and techniques. Causes of color difference were studied by extracting coloring materials from dyed cloth and dyestuff, examining indigo dye molecules in dye bath, and examining dye penetration into fiber.
Sae produced by natural indigo is subjectively distinguishable from sae produced by synthetic indigo. Nijimi produced by natural indigo is superior to nijimi produced by synthetic indigo. More iromura is produced by natural indigo than by synthetic indigo.
Regarding sae, which includes the nuances of clarity, softness and brightness, when brightness is used as a scale of sae, cloth dyed with natural indigo is brighter than cloth dyed with synthetic indigo. Nijimi may relate to sophisticated Japanese concepts of beauty in suibokuga (Japanese ink painting using shade and light). Nijimi of cloth tie-dyed with natural indigo is relatively simpler and shorter than nijimi of cloth tie-dyed with synthetic indigo, attracting attention. Iromura, (which may relate to Japanese concepts of geniality and ambiguity) produced by natural indigo is quantitatively more in color produced by natural indigo than in color produced by synthetic indigo.
Quantitative examination supports subjective evaluation in terms of three key characteristics. Natural and synthetic indigo contain coloring materials besides indigo and quality and quantity of those color materials may differ without affecting the color itself. Indigo molecules exist as an aggregation in the dye bath and the aggregation of natural indigo molecules is much higher than that of synthetic indigo molecules, so natural indigo collects on the surface of fiber more slowly and penetration is poorer.
Superiority of color produced by natural indigo in terms of three key characteristics sae, nijimi, and iromura depends on higher aggregation of natural indigo. Sae of natural indigo is brighter due to collection on fiber surfaces that depends on higher aggregation. Because natural indigo has poorer penetration due to higher aggregation, nijimi produced by natural indigo is shorter. More iromura is produced by natural indigo than by synthetic indigo because of lower rate of migration and poorer penetration.
Japanese industrial dyeing is highly evaluated worldwide, but industrial dyeing (e.g. synthetic indigo dyeing) does not seem to be able to represent sophisticated Japanese aesthetic concepts as well as traditional natural indigo dyeing can.
*Natural indigo : One of natural dyestuffs. The main coloring material is indigo. It is said to include from about 1% to 50 % indigo. There are many kinds of indigo plants, in Japan, Polygonum tinctorium Lour. is used.
**Synthetic indigo: Baeyer decided structure of indigo in 1883, later it was synthesized from intermediary substance of oil and coal. Almost all indigo is synthetic and synthetic indigo includes about 100% indigo.

Type of Paper: Article
Title: A Method for Digital Color Analysis of Spalted Wood Using Scion Image
Authors: S. C. Robinson, P. E. Laks and E. J. Turnquist
Affiliations: Michigan Technological University, USA. E-mail: scrobins@mtu.edu
Abstract: Color analysis of spalted wood requires a non-subjective, repeatable method for determining percent of pigmentation on a wood surface. Previously published methods used human visual perception to determine the percent of the pigmentation using a square grid overlay. Our new method uses Scion Image, a graphical software program used for grayscale and color analysis, to separate fungal pigments from the wood background. This human interface processes renders the wood block into HSV (hue, saturation, value, within the RGB color space), allowing subtle and drastic color changes to be visualized, selected and analyzed by the software. Analysis with Scion Image allows for a faster, less subjective, and easily repeatable model that is superior to simple human visual perception.