Survey of azo-colorants in Denmark

Technical Aspects of Azo Colorants

General chemistry

Azo colorants

Azo colorants encompass substances, which have one or more chromophoric groups in their chemical structure and therefore are capable of colouring diverse substances by selective reflection or by transmission of daylight. Azo colorants include both azo dyes and azo pigments.

Azo colorants range in shade from greenish yellow to orange, red, violet and brown. The colours depend largely on the chemical constitution, whereas different shades rather depend on the physical properties. However, the important disadvantage limiting their commercial application is that most of them are red and none are green.

Azo group

The part of an azo colorant molecule which produces colour, the chromophore group, is a double bonded azo linkage. The chromophoric group of azo colorants alters colour of a substrate, either by selective absorption or by scattering of visible light, i.e. light with wavelengths of approximately 400-750 nm.

The azo linkage consists of two nitrogen atoms, which are also linked to carbon atoms. At least one of these carbon atoms belongs to an aromatic carbocycle, an aryl moiety, usually benzene or naphthalene derivatives or a heterocycle, e.g. pyrazolone, thiazole. The second carbon adjoining the azo group may also be part of an aliphatic derivative, e.g. acetoacetic acid.

In general, an azo colorant molecule can be summarised as follows:

aryl - N = N - R,

where R can be an aryl, heteroaryl or -CH = C(OH) - alkyl derivatives.

Stability of azo linkage

The azo linkage is considered the most labile portion of an azo dye. The linkage easily undergoes enzymatic breakdown, but thermal or photochemical breakdown may also take place. The breakdown results in cleavage of the molecule and in release of the component amines. However, the azo linkage of azo pigments is, due to very low solubility in water not available for intracellular enzymatic breakdown.

The component amines which may be released from azo dyes are mostly aromatic amines (compounds where an amine group or amine-generating group(s) are connected to an aryl moiety). In general, aromatic amines known as carcinogenic may be grouped into five groups (Clayson & Garner, 1976).

	Anilines, e.g. o-toluidine.
	Extended anilines, e.g. benzidine.
	Fused ring amines, e.g. 2-naphthylamine.
	Aminoazo and other azo compounds, e.g. 4-(phenylazo)aniline.
	Heterocyclic amines.

The aromatic amines containing moieties of anilines, extended anilines and fused ring amines are components of the majority of the industrially important azo dyes.

Azo dyes

Azo dyes are, due to their relative simple synthesis and almost unlimited numbers of substituents, the most numerous group of synthetic dyes. Azo dyes do not occur naturally.

Azo dyes may have one or more azo groups. Azo dyes with one azo group are called mono azo dyes, with two azo groups, diazo dyes, followed by triazo and polyazo dyes. Azo dyes with more than three azo linkages are designated polyazo dyes. The most commercial important are mono- and diazo dyes, triazo dyes, whereas polyazo are much less important.

Nomenclature

Due to the complexity of the chemical names, azo colorants are only rarely referred to using the IUPAC or CAS nomenclatures. Technical literature has adopted the classification of azo colorants either by the chemical constitution or by the colour.

All commercial important azo colorants are identified by the Colour Index system. Each colorant is given a generic name, e.g. Direct Brown, which briefly gives information on application and colour. In addition to the generic name, a five-digit number is allocated which unambiguously identifies the chemical structure of the colorant.

In the Colour Index system, the azo colorants are provided with numbers ranging from 11,000 to 39,999 in correspondence with the Chemical Class shown in Table 3.1:

Klassificering af azofarvestoffer i henhold til Colour Index systemet.

Azo pigments

Azo pigments constitute the largest group of organic pigments due to the relatively easy synthesis and the good technical performance.

In principle, the chemical structure of azo pigments is identical to the chemical structure of azo dyes where the azo linkage is the chromophore group. The necessary low solubility is achieved by avoiding solubilising groups or by incorporating groups reducing solubility, e.g. amide groups, or by forming insoluble salts (lake formation) of carboxylic or sulfonic acids.

Azo pigments are particulate solids, which are almost insoluble in water or other media in which they may be dispersed for application. They colour other substances by being physically attached to or incorporated into it. Furthermore, they are physically and chemically unaffected by the substrates, which they are intended to colour.

Technical properties of azo dyes

Azo dyes represent the largest, in number, group of synthetic dyes and the most widely, in tonnage, manufactured. These dyes are, compared to natural dyes, better capable of meeting requirements regarding technical properties, e.g. fastness to light.

The chemical diversity of azo dyes permits a wide spectrum of shades, mainly within the scale of red. A disadvantage limiting their application is, however, that none of the azo dyes are green.

The great majority of azo dyes are water soluble and they colour different substrates by becoming physically attached. The attachment may be due to adsorption, absorption or mechanical adherence.

Azo dyes have a broad industrial application field. They are used for colouring of synthetic and natural textile fibres, plastics, leather, paper, mineral oils and waxes. Their abilities of keeping an intense colour and fastness to light are quite good in most cellulose fabrics but are relatively poor in colouring of cotton and wool.

A number of azo dyes are used as food colorants in cosmetics and as drugs for treatment of bacterial infections.

Most of the commercial available azo dyes are in fact formulations of several components in order to improve the technical properties of the dyeing process. The content of a specific dye lies in the range of 10 to 98%.

The grouping of dyes, including azo dyes, often reflects a strict defined concept of application. The majority of industrial important azo dyes belongs to the following groups:

	Acid dyes
	Basic dyes
	Direct dyes
	Disperse dyes
	Mordant dyes
	Reactive dyes
	Solvent dyes

The acid, basic, direct and reactive azo dyes are ionic, whereas disperse, mordant and solvent azo dyes are non-ionic dyes.

Acid dyes

Acid dyes are the most widely used azo dye in Europe. The dyes are manufactured and employed as water-soluble sodium salts of the sulfonic or carboxylic acid groups.

Acid dyes, which are anionic, are used in the textile industry for dyeing of all natural fibres, e.g. wool, cotton, silk and synthetics, e.g. polyesters, acrylic and rayon. To a less extent they are used in a variety of application fields such as in paints, inks, plastics and leather.

Basic dyes

Basic dyes include water-soluble cationic azo dyes, characterised by positive charge(s) introduced to the molecule.

Basic azo dyes belong to the oldest known class of synthetic dyes. Their first application was in colouring of natural fibres, e.g. cotton, silk and wool. Later, they were applied for the colouring of synthetics, like e.g. polyesters, acrylics and rayon. Azo dyes with several cationic charges are important dyes for polyacrylonitril fibres.

Some of the basic azo dyes are used in medicine for treatment of bacterial infections.

Direct dyes

Direct dyes include water-soluble anionic azo dyes, which require the presence of electrolytes for the dyeing process. Most of the direct dyes are benzidine-based. They are classified as direct dyes, because they may be applied directly to celluloid fibres. Furthermore, they are used for co- louring of rayon, paper, leather and to a less extent nylon.

Disperse dyes

Disperse dyes encompass azo dyes, which are sparingly soluble in water and mainly used for dyeing of synthetic (hydrophobic) fibres. The disperse dyes are clearly the dominating group within azo dyes used world-wide. The fibres shall be in an organic medium, in which the dye is more soluble than in water. The disperse dyes have been used for cellulose acetate fibres, but now they are used in large quantities for dyeing of polyester, polyamide and acrylic fibres.

Mordant dyes

Mordant dyes include azo dyes, which are converted into their final, insoluble form on the fibres. A mordant is a metal, most commonly chromium, aluminium, copper or iron. The dye forms together with a mordant, an insoluble metal-dye complex and precipitates on the natural fibre. The application area is limited to the colouring of wool, leather, furs and anodised aluminium.

Reactive dyes

Reactive dyes encompass azo dyes, which form covalent bonds with the fibres they colour, e.g. cotton, rayon, cotton, wool silk and nylon. The dye molecule contains specific functional groups, which can undergo addition or substitution reactions with the -OH, -SH and -NH₂ groups present in the fibres. Due to very good fastness of the substrate, the reactive dyes are one of the most important group of dyes for colouring of textiles.

Solvent dyes are used on a large scale in many industrial sectors. They are dissoluted in the substrate they colour. The small fastness to light of these dyes depends heavily on the substrates being coloured. They are used for coloration of inks, plastics (mainly for polystyrene and resins of polymethacrylate), wax and fat products and mineral oil products (gasoline, fuels lubricants and greases).

Solvent dyes

Technical properties of azo pigments

Pigments are widely used. The most important area of use is in the graphic printing inks, where approximately 50% of all pigments are used. 25% of the pigments are used in paints and coatings and less than 20% in plastics and fibres. The remaining application fields are e.g. textile printing, office articles, wood, paper, cosmetics and food and feed colouring.

The industrial production and use of pigments, including azo pigments, are expanding world-wide. Most probably 50% of the organic colorants applied within industrial processes are today organic pigments (Ullmann, 5^th Edition).

Physical properties like size and shape of pigment particles, crystal geometry and presence of impurities are responsible for the efficacy of the colouring process. The maximum particle size of most of the commercial pigments is less than 1 µm and often even smaller than 0.3 to 0.5 µm. The smallest particles may be one to more than two orders of magnitude smaller. The small particles tend to agglomerate and form crystallites, and this tendency increases with decreasing particle size. Organic pigments, as powders, will therefore comprise of a mixture of such crystallites and single crystals.

Pigment particles may assume a variety of shapes, such as cubes, platelets and needles as well as a number of irregular shapes in combination.

Commercial pigments are available as powdered crystalline solids or already dispersed forms. Dispersion is performed by the manufacturer and may contain carrier material and dispersing agent. The efficiency of dispersion is very important for the process of colouring. After dispersion of the pigment, particles may be stabilised in order to avoid flocculation. This is particularly important for the application of pigments in thermoplastic materials, e.g. polyvinylchloride.

Technical properties of azo pigments always refer to the complete pigment system, which beside a pigment constitutes of e.g. solvents and binders etc. Of particular interest are migration, thermal stability, fastness to light and weather resistance. In solvent-based printing inks, pigments must be extremely resistant to the solvent used in the ink.

The rough grouping of azo pigments may be based on the numbers of azo groups and/or the type of coupling component. Azo pigments may be allocated to the following groups (Ullmann, ^5th Edition):

	Benzimidazolone pigments
	Diazo pigments
	Disazo condensation pigments
	Monoazo Yellow and Orange pigments
	Naphthol AS pigments
	b -Naphthol pigments
	Azo pigment lakes

Benzimidazolone pigments

Benzimidazolone pigments provide a range of colours ranging from greenish yellow to orange, medium red to carmine, bordeaux and brown shades. The technical performance is excellent. Benzimidazolone pigments are used for exterior-use paints of a high quality, e.g. car finishes. Furthermore they are used for colour plastics and for high grade printing inks.

Diazo pigments

Disazo pigments may be characterised by a double azo and/or by double coupling components. Diazo pigments provide colours in the range from very greenish yellow to reddish yellow and orange and red. In comparison with the yellow and orange pigments of monoazo, the diazo pigments provide better solvent and migration fastness, but poorer fastness to light and weather resistance. These pigments are economically very important, particularly in the production of printing inks. The main use encompasses printing inks and plastics.

Disazo condensation pigments

Condensation of two monoazo pigments provides a pigment "double in size". The final colours range from greenish yellow to orange, red and brown. Due to their large molecular size, they are of very good technical properties, particularly very good migration fastness and thermal stability. These properties make disazo condensation pigments suitable for colouring of plastics and paints.

Orange pigments

Monoazo pigments provide a range of colours from yellow to orange. The yellow pigments were introduced 80 years ago and they are relatively cheap and very light fast. Therefore they are still very widely used, mainly in coating materials and especially in air-drying and emulsion paints. They are also used in the printing industry.

Naphthol AS pigments

Naphthol AS pigments, so-called naphthol reds, are all red, providing a range of colours from yellowish and medium deeply red to brown and violet. The technical properties vary. In general Naphthol AS pigments have a good fastness to light and are weather resistant, but they tend to migrate. The main area of use is in printing inks and interior paints.

b -Naphthol pigments

b -Naphthol pigments belong to the oldest known synthetic colorants. They are characterised by good fastness to light and weather resistance. On the other hand they have a poor migration fastness. Today only a few b -naphthol pigments are in use, mainly for colouring of inexpensive coating materials.

Azo pigment lakes

Azo pigment lakes are synthesised from monoazo dyes, which are converted to an insoluble form by formation of salt with metals. Azo pigment lakes provide colours from yellow to red. The red pigments have a brilliant shade and are of great industrial importance. The technical properties of azo pigment lakes vary, but they have a good fastness to light, weather resistance and a high thermal stability, whereas some tend to migrate. They are used in almost all printing sectors and for colouring of plastics.