Organic Pigments

Organic Pigments Although pigments are widely used in paints, inks, plastics and textile industries, they are also used in many other sectors. However, a better understanding of pigments is beneficial for their more effective use. The purpose of this article is to provide a definition of pigments and a perspective on the history of pigment technology. To broadly define existing pigment families and specify the properties of these pigment families. The word pigment is derived from the Latin word "Pingere," and is initially limited to solid materials added to a medium to impart a coating power and appearance of desired consistency for painting. This definition has long since been extended to include materials used for similar purposes in inks, rubber, plastics and many other media. Adding to the confusion, the same word is also used to describe any coloring substance in animals or vegetables. Here we will address only pigments used in industrial applications. Industrially, a pigment is a finely divided insoluble black, white or colored solid material; an important function of it is to improve or impart color to the appearance of the medium in which it will be used. Pigments and other components are brought together through simple physical mixing, and this characteristic is what distinguishes a pigment from a dye. Pigments and dyes are generally derived from the same basic building blocks; the primary criteria that distinguish them are that dyes are soluble in the media in which they are incorporated and pigments are not. Since there are numerous pigments and dyes, it became necessary to establish a classification system. This system was created by the England-based organization "Society of Dyers and Colourists" and the American "American Association of Textile Chemists and Colorists" as the "Color Index" system. Within the Color Index system, each industrially available product is classified with a "C.I. Generic Name." These are divided into main groups as follows: • Acid dyes, • Azoic components and mixtures, • Basic dyes, • Developers, • Direct dyes, • Disperse dyes, • Fluorescent brighteners, • Food dyes, • Ingrain dyes, • Mordant dyes, • Natural dyes, • Oxidation bases, • Pigments, • Reactive dyes, • Reducing agents, • Solvent dyes, • Sulfur dyes, • Vat dyes. Each product contains a "C.I. Generic Name" and a term that describes its property, for example: Pigment Red 3. In this article, we will focus specifically on colored organic pigments used for industrial purposes. To understand pigment types and how their properties are interrelated, they are fundamentally divided into Classical Pigments and High-Performance Pigments. The definition can be misleading, because some pigments called Classical undoubtedly possess high-performance pigment properties (the most important example being the Phthalocyanine pigment group). Conversely, some pigments called High-Performance can be thought of as complex extensions of Classical pigment structures (Naphthol (Pigment Red 170) pigments). Nevertheless, normally, the performance criteria are what determine whether an Organic Pigment is classified as Classical or High-Performance. It is beneficial to understand some guiding principles! The guiding principles of the chemistry of pigment families are, of course, also related to the availability and cost of basic building blocks and the number of steps involved in the synthesis of the final molecule. As molecular weight (M.W.) and indirectly complexity increases, it is certainly a sound principle that solvent fastness properties also increase. The increase in average molecular weights of Monoazo pigments (340-400) to Diazo pigments (600-830) and from there to Azo Condensation pigments (900-1200) clearly demonstrates this trend. However, this is generally not the most important factor. The most important factor is almost certainly the architecture of the molecule. The most commonly used groups to ensure insolubility are CO and NH groups, which can occur together as an amido (CONH) group or be found in different parts of the molecule, as in Quinacridone. From this perspective, it is interesting to compare the commonly used Classical Azo Pigment, C.I. Pigment Red 3 (C17H13N303; M.W. = 307) and the High-Performance Quinacridone Red (C20H12N20z; M.W.= 312) pigment. The first blooms significantly in most solvents; the second is nearly insoluble in all common solvents due to strategically positioned CO and NH groups. Increasing intermolecular forces such as hydrogen bonding also raise the melting point and thus expand the application range in which a pigment can be used. To achieve these more fundamentally stable molecular forms, unfortunately, the entire production procedure must be much more complex.

Historical Development

This distinction between High-Performance Pigments and Classical Pigments is, as expected, also related to historical development. Most common Classical Pigment forms were developed before World War II; however, since then more complex chromophoric structures have been sold as commercial products. The historical development of organic pigments is summarized below:  

Classical Pigments

The initial development of classical pigments begins in the mid-1800s with the development of synthetic dyes, and the first produced pigments are "Lake Red C" and Toluidine Red. The most common Classical Pigments are AZO compounds (compounds containing N=N chromophore). Major Classical Organic Pigment Groups: I- Neutral AZO Pigments: 1- Neutral AZO Pigments are defined as MONO AZOs. Their distribution: a. Mono Azo Pigments: i. Arylamide Yellows, ii. Naphthol AS Red, iii. Beta Naphthol Reds. b. Disazo Pigments: i. Diarylid Yellows, ii. Pyrazolone Red and Orange. 2- Metal Salt Type Pigments: a. Arylamide Yellow, b. Naphthol AS Red, c. Beta Naphthol Reds. II- Copper Phthalocyanines and Derivatives: a. Phthalocyanine Blue, b. Phthalocyanine Green. III- Basic Dye Complexes: a. Double / Triple Salt Complexes, b. Copper Ferrocyanine Complexes.

High-Performance Pigments

The High-Performance Pigment series currently available can be thought of as attempts at various pigment production to reproduce the excellent properties exhibited by the phthalocyanine Blue and Green pigment group for all other colors. As with Classical Pigments, it is useful to consider each of these structures as a separate pigment family, but due to the general objective of achieving excellent properties, there are not very different property characteristics from family to family. Furthermore, unlike Classical Pigments, which are normally offered by many different suppliers, different High-Performance Pigment types tend to be limited to several per supplier, depending on each company's chemical expertise. The following are the main groups of high-performance pigments currently available: • Complex Naphthols, • Benzimidazolones, • Azo Condensations, • Anthraquinone and Perylenes, • Quinacridones, • Isoindolinones / Isoindolines, • Diaxozines, • Metal Complexes, • Perylenes, • Thioindigos, • Diketo Pyrrolo Pyrrole.

Special Pigment Forms

Besides Classical Organic and High-Performance Pigments, special pigment types are also available and can be defined as follows: • Fluorescent Pigments, • Metallic Pigments: Aluminum pigments, Gold and Bronze pigments. • Pearlescent Pigments.

Conclusion

Among colored pigment groups not covered in this article, inorganic pigments such as chromium, molybdate and cadmium pigments are being phased out due to their adverse environmental properties. Their consumption quantities are decreasing year by year. The use of classical organic pigments and high-performance pigments is increasing steadily. However, when comparing the use of classical organic pigments and high-performance pigments, the use of high-performance pigments remains limited compared to classical organics. The main reasons for this are not only that their unit prices are higher, but also particularly that OEM paints have begun to be sourced largely directly from outside. Although the unit prices of high-performance pigments have dropped significantly, especially in the last 10 or even 20 years, they still create higher costs because their color strength is also weaker compared to classical organics.   Assoc. Prof. Dr. Gürses Öner Regional Sales Manager Sudarshan Chemical Industries Ltd.