Azocolorants in Textiles and Toys

2 Azo-colorants in Textile Dyeing and Printing

2.1 Azo-colorants
2.2 Azo-dyestuffs and azo-pigments on the market
2.3 Conclusion

Definition

Azo-dyestuffs and azo-pigments are referred to collectively as azo-colorants.

Azo-colorants are a group of chemical compounds, containing one ore more azo-group(s). The azo-group contains a double bond between two nitrogen atoms.

Figure 2.1
Azo-group (Verband der chemischen Industrie).

-N = N-

Azo-colorants are subdivided in azo-dyestuffs, which are soluble in the application medium, and azo-pigments, which are insoluble in the application medium. Azo-dyestuffs can also subdivided in water-soluble (hydrophilic) and fat-soluble (lipophilic) (Verband der chemischen Industrie).

The characteristic property of pigments is their extremely low solubility in organic solvents and in the substrate. They generally exhibit very low solubility in organic solvents and for this reason they remain almost totally in the solid form during time dyeing process and when applied to the substrate (Anliker, 1986).

Azo-dyestuffs are used for both dyeing and printing, whereas azo-pigments are used for printing only.

Manufacture of azo-colorants

Azo-colorants are manufactured by a so-called diazotization process. First the aromatic amine (in Figure 2.2 also called diazo component) is transformed into a diazonium compound. The reaction proceeds at low temperature and in the presence of sodium nitrite and for example hydrochloric acid.

Figure 2.2
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The reaction in the manufacture of azo-colorants (Verband der chemischen Industrie).

The diazonium component then reacts with the coupling component (which may be a phenol, a naphtol or an amine) to form the actual dyestuff (Peter, 1989).

Because of the free choice between a large number of both diazo and coupling components, the range of variation in the manufacture of azo-colorants is very wide. The number of combinations further increases because a dyestuff may contain more than one azo-compound. This results in a wide range of shades, applications and fastness properties.

The azo-colorants all share the -N=N- group. To this functional group a large variety of aromatic constituents can be linked so as to give colorants of appropriate characteristics.

The behaviour of the azo-colorant is determined by thie character of substituent groups rather than by the presence of the azo group.

In the case of certain azo-colorants it is also known that the aromatic

amine can be split off again by reductive cleavage of the N-N bond.

According to time German textile industry the finishing and dyeing of textiles involves some 600 different finishing and auxiliary agents, in addition to about 800 colorants (Platzek, 1996).

The Colour Index, which is the recognized international register of commercial dyestuffs and pigments, assigns names to dyestuffs, not based upon the chemical class of the molecule, but based upon the method of application or the end-use. Azo-dyestuffs are named as Acid, Basic, Mordant, Reactive, Disperse, Direct, Solvent or Food dyestuffs, but not all Acid-dyestuffs are azo-dyestuffs, etc.

The Dutch Regulations contain a list (Enclosure 1) of azo-dyestuffs, which can split off the aromatic amines forbidden in Germany and the Netherlands. A similar list is not included in the German Regulations, and the German authorities do not intend to prepare such a list.

The German Regulations (Enclosure 2) do not specify which dyestuffs or pigments are forbidden, but instead specifies which aromatic amines are forbidden.

The reason is that countless colorants (several hundred) are used, and it would be neither possible nor justifiable to prove the suspicion in each case by the required experiments on animals. (Moll, 1995).

According to the Bundesgesundheitsministerium, other reasons for not intending to prepare a list are:
- the list would be difficult to keep up to date, because new developments are frequently announced
- some dyestuff manufacturers will not publish detailed information about the chemical composition of their products because they wish to keep them confidential.

ETAD (Ecological and Toxicological Association of the Dyes and Organic Pigments Manufacturers) is of the same opinion.

Experience also show that such a list would be difficult to maintain, because dyestuffs and pigments are withdrawn and new dyestuffs and pigments are constantly put on the market because of changes in fashion.

According to (AQUIRE DATABASE, 1993) about 100 azo-dyestuffs, which are affected by the prohibition in the German Regulations, are on the market. Table 2.1 shows the distribution of these azo-dyestuffs grouped according to their content of the 20 aromatic amines, which are forbidden in the German Regulations, and a further two aromatic amines (o-anisidine, p-aminoazobenzol), both categorised as carcinogenic in the EU.

Table 2.1
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Commonly used azo-dyestuffs (generic types) on the market, which by reductive cleavage processes can split off carcinogenic aromatic amines considered carcinogenic by the German authorities.

According to Table 2.1 azo-dyestuffs, which can split off the aromatic amines 2,4,5-trimethylaniline, 2,4-diaminoanisole, 2-naphthylamine, 3,3’dimethyl-4,4’-diaminodiphenyl-methmane, 4,4’-methylene-bis-(2-chloraniline), 4,4’-oxydiarmiline, 4-aminodiphenyl and p-cresidine, should not be present on the market.

DTI knows by experience, that particularly in the Far East some local dyestuff manufacturers exist. There is no control of the produced colorants, and it is not known what types of colorants the manufacturers are producing. Production statistics etc. from such manufacturers do not exist. This means that in this case there would actually be a risk of presence of azo-colorants based on one or more of the 20 aromatic amines prohibited in Germany.

Figure 2.3
Azo-dyestuffs classified by dye class (AQUIRE DATABASE, 1993).

Marketable azodyes which under reducing conditions can cleave off carcinogenic aromatic amines.

Number of individuals (generic names) which are still manufactured today. The inquiries are made on Colour Indeks and internal databases.

Figure 2.3 shows that direct dyestuffs with 61 % make up the greatest part of the azo-dyestuffs which can split off carcinogenic aromatic amines, followed by acid and disperse dyes. Additional literature of more recent date with similar information does not exist. As was the case for Table 2.1, Figure 2.3 is based on the 20 aromatic amines prohibited in Germany and the Netherlands and the 2 aromatic amines (o-anisidine, p-aminoazobenzol) both categorised as carcinogenic in the EU (AQUIRE DATABASE, 1993).

Figure 2.4 lists the dye classes with respect to their application to the different textile materials.

According to Figure 2.4, direct dyestuffs are mainly used for dyeing of cotton and regenerated cellulose, acid dyestuffs are mainly used for dyeing of wool, natural silk and polyamide. Disperse-dyes are mainly used for dyeing of acetate, polyamide and polyester.

The relationship between azo-colorants and the textile material for which they may be used (Figure 2.3 and 2.4) is one of the points considered in the selection of samples (chapter 5).

Figure 2.4
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The dye classes and their application to different fibre (Sørensen, 1996).

According to (Klaschka, 1994) azo-dyestuffs based on benzidine have not been manufactured in Germany nor in other EU-countries for decades. However, they may still be present in goods imported from other countries. This confirms the experience of DTI. According to Table 2.1 (AQUIRE DATABASE, 1993) the most frequently occurring azo-dyestuffs on the world market are in fact based on benzidine.

The reasons that azo-colorants make up the majority of colorants used for textiles are the vast nurnber of shades and applications available and the fastness properties.

Due to the vast number of azo-colorants used it is not possible to work out a list of banned azo-colorants. In addition it is often difficult to get any information from suppliers regarding the chemical constitution of some of their products because such information is kept confidential. The use of both azo-dyestuffs and azo-pigments also depends on changes in fashion, in that new colorants appear and old disappear constantly. This means that a list of forbidden azo-colorants would be difficult to maintain.

The relationship between azo-colorants and the textile materials for which they may be used is taken into consideration in the selection of samples for analysis.