Лако-красочные материалы — производство

(a) Aminoazo Dyes. Dyes of this group which contain several amino groups, such as combinations with m-phenylenediamine, have, in general, pronounced basic characteristics (chrysoidine, Bismarck brown). They form water-stable salts with mineral acids and are gen­erally used in this form. Such salts can be isolated by acidifying the coupling reaction mixture with the acid concerned (e. g., hydrochloric acid) and salting out the dye with a salt of this acid (e. g., sodium chloride). Alternatively, the dye base can be precipitated with alkali, filtered off, and redissolved in the desired acid. The salt is then re­covered by crystallization.

Aminoazo dyes with only one amino group (aminoazobenzene type) are weak bases giving, in general, difficultly soluble salts which are readily hydrolyzed. If the dye is to be isolated as one of its salts, for example, the hydrochloride, the salt is precipitated from the reaction mixture by means of a considerable excess of hydrochloric acid. The precipitate is filtered off and washed with dilute hydrochloric acid, not with water. On the other hand, if the dye is to be used in coloring oils, fats, etc., it must be isolated as the free base and must contain no salts. For this purpose, the coupling reaction mixture is made alkaline and the precipitated dye is filtered off and washed thoroughly with water. The dye can be purified further by. recrystallization from an organic solvent.

(b) Hydroxyazo Dyes. In this group, the dyes usually concerned are, with few exceptions, the o hydroxyazo dyes which are insoluble in water and soda solution and which, therefore, are precipitated as they are formed. These dyes are used as colors for spirit varnishes, oils, and fats, and also as pigments in the preparation of opaque colors. For the former applications, the physical form of the dye is immaterial and the precipitated dye can simply be filtered off, washed thoroughly with water, and dried. If necessary, the dye can be purified by recrystalliza­tion from an organic solvent. Pigment colors, on the other hand, are only filtered off, washed, and then made up in the form of a paste. Sub­sequent purification or other treatment is excluded. Steps must be taken, therefore, to ensure that the dye separates from the coupling reaction mixture in a pure condition and in a finely divided state. The latter is achieved, in cases where it is necessary, by adding Turkey red oil to the coupler solution before coupling.

2, Dye» with Sulfo or Carboxyl Group»

These dyes are usually isolated as their water soluble, alkali metal salts by a salting out process. The isolation must be carried out in such a way that the dye is precipitated as completely as possible while ac­companying materials remain in solution. Also, the finished dye should not contain an unnecssarily large amount of inorganic salts. In some cases, the salts formed in the dye forming reaction suffice to effect the precipitation, but usually additional salt is required. A salt solution is used whenever possible in order to be certain that the precipitate does not include undissolved salt, and also to avoid contamination of the product by the small amount of insoluble material which is always present in salt. Solid salt is added only in the case of dyes which are so soluble that the solution must be saturated with salt to bring about precipitation. In certain instances, potassium chloride or ammonium sulfate gives more complete precipitation than sodium chloride. If the neutral alkali metal salt of the dye is appreciably soluble even in satur­ated salt solution, it is frequently possible to isolate the dye as the free acid or as an acid salt by adding acid, perhaps in combination with salt. In the event that such a dye acid is too insoluble for use in dyeing, it can be converted subsequently to the alkali metal salt by dissolving it in the required amount of alkali in a minimum quantity of water and evaporating the solution to dryness. Alternatively, the dye acid, after being dried, can be mixed with the required quantity of dry soda; the dry mixture can easily be dissolved by stirring in water.

The greatest difficulty is often encountered in getting the dye pre­cipitated in a form which is easy to filter and wash. Usually, but not always, it is advantageous to carry out the precipitation at about 70 to 80°C., less often at the boiling point, and to stir the mixture for a time at this temperature. This treatment often converts an originally slimy pre­cipitate into a crystalline or granular form. In other cases, however, the reverse situation prevails, and a dye which is precipitated well in the cold becomes slimy on heating, so no general rules can be laid down. Test tube experiments must be carried out to establish the optimum conditions for each individual dye. The alkalinity of the solution is sometimes an important factor in determining the character of the pre­cipitate, along with the temperature and salt concentration. It may be, for example, that an easily filterable precipitate can be obtained only from a solution which is exactly neutral.

If the dye has been precipitated from a hot solution, it is usually desirable to filter and wash at higher temperatures also; filtration is more rapid under these conditions, and the impurities are held in solu­tion more completely. It is necessary to cool before filtration only if precipitation is incomplete in the hot solution. Washing is done with a salt solution corresponding in salt concentration and temperature to the mixture filtered. More complete removal of by-product dyes may be achieved if some soda solution is added to the wash liquid. If the product is still insufficiently freed from by-products, it must be purified by reprecipitation. For this purpose, the dye is dissolved in hot water, the solution is filtered if necessary and then salted out using about the same temperature and salt concentration as in the first precipitation.

The washed product is pressed out strongly to remove adhering salt solution as completely as possible, and then dried and powdered. Diffi­cultly soluble dyes are preferably prepared finally in paste form and the dry weight determined with a test sample. Drying of dyes containing several nitro groups should be done with care at moderate temperatures, if they are dried at all, because of the danger of explosions.