Antraquinonoid Pigments - Color Fundamentals

Gerhard Pfaff, Gerhard Pfaff

• Publisher: De Gruyter
• Year: 2022
• ISBN:
  9783110586633
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Volume 1

From Antraquinonoid Pigments to Color Fundamentals

With contributions on various industrially important dyes, inorganic and organic pigments, on color fundamentals and colorants in various application systems (building materials, coatings, cosmetics, plastics, printing inks)

• A comprehensive overview on all important dyes, inorganic and organic pigments supplemented by information on all relevant applications and color fundamentals
• The chapters are clearly structured and arranged in alphabetical order in the 3-volumes.

1 Anthraquinonoid pigments

Robert Christie

School of Textiles & Design, Heriot-Watt University, Galashiels, United Kingdom

Adrian Abel

DCC Europe, Rossendale, United Kingdom

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: R. Christie, A. Abel, Anthraquinonoid Pigments Physical Sciences Reviews [Online] 2021, 6. DOI: 10.1515/psr-2020-0146

Abstract

Colorants based on the anthraquinone structure are categorized as a subclass of carbonyl colorants. Anthraquinone textile dyes rank second in importance to azo dyes, especially within the vat dye application class. Vat dyes became of interest to the pigment industry because of their insolubility. This insolubility and generally excellent fastness properties inspired investigations into the selection of suitable established anthraquinonoid vat dyes for use as pigments after conversion to a physical form that is appropriate for their applications. Originally this proved difficult, but was eventually achieved following the development of appropriate conditioning after treatment processes. The structural chemistry of the various types of anthraquinonoid pigments in relation to their technical and coloristic performance is discussed. The chapter concludes with an illustrated description of the main synthetic routes and finally with a description of the principal applications of the individual commercial products. Anthraquinonoid pigments are generally regarded as high-performance products, suitable for highly demanding applications, although they tend to be expensive.

Keywords: anthraquinonoid, madder, alizarin, purpurin, quinizarin, vat dyes, vat pigments, aminoanthraquinones, heterocyclic, polycarbocyclic, anthrapyrimidine, indanthrone, flavanthrone, pyranthrone, anthanthrone, isoviolanthrone,

1.1 Fundamentals

Colorants based on the anthraquinone structure, categorized as a sub-class of carbonyl colorants, hold a special place in the field of color chemistry. The parent unsubstituted anthraquinone molecule (1a) is illustrated in Figure 1.1, with reference to Table 1.1. Anthraquinone textile dyes rank second in importance to azo dyes. Many textile dyes are anthraquinones carrying a range of substituents. However, the important commercial anthraquinonoid pigments are invariably large polycyclic molecular structures, a feature that is primarily responsible for their high levels of technical performance, including fastness to light, weather, solvents, chemicals, and heat [1,2,3].

Figure 1.1: Structures of anthraquinones (1a-1d).

Table 1.1:Substituent pattern in some simple anthraquinone molecules.

Compound	R^1	R^2	R^3
1a	H	H	H
1b	OH	OH	H
1c	OH	OH	OH
1d	OH	H	OH

1.2 History

Madder, one of the earliest natural dyes, is obtained from the root of Rubia tinctorum, also known as dyer’s madder. It is an herbaceous perennial plant related to the Rubiaceae family, which includes coffee. The roots are harvested after two years, and an inner layer provides the best quality dye. The dye is applied to the cloth with alum (a hydrated sulfate salt of aluminum), which acts as a mordant that fixes the dye to the cloth. It is claimed that the dye was being used in the Indian sub-continent as early as 2300 BC, based on the discovery of a piece of cotton colored with madder in Mohenjo-daro, an archaeological site in the Indus Valley, Pakistan [4]. This dye was later traded around the world and was of such importance that it was the cause of trade wars between various European countries and the colonized lands in the Americas. It was one of Sir Isaac Newton’s “colour spectrum”, that he postulated in 1672. The main active components of this dye are alizarin (1b) and purpurin (1c), hydroxy derivatives of anthraquinone, as illustrated in Figure 1.1 with reference to Table 1.1. The dye could be converted to an insoluble pigment, madder lake, by precipitating (laking) with alum. This lake pigment has low lightfastness and is remembered mainly as a component of mixtures used to paint miniatures, popular in the fifteenth and sixteenth centuries, especially in the Flemish region of Belgium. German chemists, Graebe and Liebermann discovered a synthetic route to alizarin in 1868 [5], around the same time as Perkin, who had discovered Mauveine, devised an alternative process that he patented in the UK. A year later, Perkin developed a more practical route, which he adopted at his Greenford manufacturing plant, London and quickly brought it into production. Soon after these events, the cultivation of madder root virtually ceased. A pigment that was originally made from natural madder, marketed as CI Pigment Red 83, is still made but using synthetic alizarin. This calcium lake, strictly a metal salt pigment, was discovered in 1826 by Robiquet and Colin. Older versions of the Colour Index refer to it as a metal complex. The main current use of this pigment, still referred to as madder lake, is in the coloration of soap, cosmetics, and artists’ colors. It has very poor fastness to solvents and is not fast to light. Confusingly, this product is often referred to as CI Pigment Red 83:1 (especially in artists’ colors), but there is no such product listed in the Colour Index. Another metal salt pigment, made from quinizarin (1d), was discovered by Bayer and introduced as Helio Fast Rubine 4BL, either as a bright violet sodium salt (CI Pigment Violet 5) or, more often, as an aluminum salt (CI Pigment Violet 5:1), which has a bright reddish violet hue. Despite its poor tinctorial strength, low solvent resistance and low lightfastness in reductions, the aluminum salt was still being used for general industrial paints until relatively recently, because it offered an inexpensive way to obtain violet colors.

In terms of volume, anthraquinones were second only to azo colorants until the discovery of copper phthalocyanine. Many anthraquinone textile dyes provided the highest quality dyeings with respect to lightfastness and washfastness. Anthraquinones have been developed as acid, disperse, mordant, reactive, and vat dyes. Vat dyes were the application class of most interest to the pigment industry because of their insolubility. They constitute a group of insoluble colorants that are applied to cellulosic fibers (e. g., cotton) via a water-soluble (leuco) form that is obtained by reduction with sodium dithionite in alkaline solution. They are then oxidized to regenerate the pigment as insoluble particles trapped within the fibers. Vat dyes set new standards for washfastness in textiles, to such an extent that the German dye industry allowed vat dyed articles to carry a special label, the Indanthren label, assuring purchasers of goods that had been dyed in this way that they would remain fast during washing. Application of the commercial forms of the early vat dyes as pigments was not straightforward, because of difficulties in wetting out the particle surfaces and in producing fine particle dispersions, and consequently they lacked tinctorial strength and brightness. However, the insolubility and very good fastness properties of vat dyes inspired investigations into the selection of suitable established products for use as pigments after conversion to a physical form that is appropriate for their applications. Lessons learned, especially from experience of the conditioning aftertreatment processes developed for copper phthalocyanine pigments, were applied to many of these vat dyes, and a few became important as pigments, generally referred to as vat pigments. These products offered the technical and coloristic advantages, notably high levels of fastness performance and bright colo...