Лако-красочные материалы — производство

(a) Fundamental Considerations

The following points are of great importance: (1) The starting materials should be pure; (2) The quantities of materials should be measured accurately; (3) Vigorous and continuous stirring, or shaking, should be used, both in the diazotization and in the coupling; and (4) Diazo compounds should be protected from heat and light. They should be used immediately after being prepared.

(1) It is especially important that the starting materials should be free from isomers or closely related compounds (e. g., monosulfonic acid impurities in disul — fonic acids, or vice versa) which can also enter into the dye-forming reaction. On the other hand, the slight brown or gray color, exhibited by many technical products, is usually unimportant. Extremely dark coloration or resinification indicates oxidation or decomposition of the material, and purification by distillation or crystallization may be necessary. The presence of sodium chloride or other alkali metal salts usually has no deleterious effect, but must be taken into account in calculating the quantities to be used.

(2) The amount of nitrite used in a diazotization must correspond exactly to the quantity required theoretically if the coupling reaction is carried out in acid solution, or if the coupling is done in alkaline solution and the mixture must be acidified subsequently in isolating the dye. Any excess of nitrite under these condi­tions would diazotize or nitrosate the coupler, or, in the latter case, would alter the finished dye. When both the coupling reaction and the isolation of the dye are carried out in alkaline solution, a small excess of nitrite does no harm. The use of too little nitrite is bad because it leads to the formation of diazoamino compounds or to coupling of the diazotized base with itself.

It is customary, to use a small excess (3 to 5 per cent) of an end component so that the coupling reaction goes rapidly to completion. Middle components in dis­and polyazo dyes, however, must be used in exactly the theoretical amount in order to prevent the formation of by-products.

In order to know the exact quantities of materials to use, it is essential to know their purity. Commercial products, which are isolated by distillation and which are taken from fresh stocks, are usually quite pure and can be considered 100 per cent materials. Examples of these are: aniline and its homologs, chloroaniline, ani — sidine, phenetidine, naphthylamine, phenol, cresol, resorcinol, naphthol, diphenyl — amine, etc. If the compounds have become dark colored or tarry on long standing, they should be purified before use by distillation, if necessary in vacuum. Undis — tillable materials, on the other hand, are rarely 100 per cent pure. This applies particularly to sulfonic acids and their salts which usually contain inorganic salts, and often water of crystallization. The purity of such materials must be determined if it is not given by the supplier. The determination, in the case of diazotizable amines, is done by titration with nitrite solution, and in the case of coupling com­ponents, with diazo compounds (see Analytical Section). The purity of such a material is expressed most simply in terms of its apparent molecular weight (M), i. e., as the number of grams of the technical product which contains 1 mole of the pure material. If, for example, 1 gram of technical H acid requires 26.8 cc. 0.1 N diazo solution, M is 10,000/26.8, or 373. Then, to prepare 0.1 mole of a dye, 37.3 grams of the technical H acid would be used.

Technical sodium nitrite is never quite 100 per cent pure, and should be titrated before use. When diazotizations are being carried out frequently, it is con­venient to have nitrite available in three forms: (a) l N nitrite solution (1 mole of ЫаЫОг in a liter); for preparation, see page 385; (h) 5 N nitrite solution (1 mole of NaNOj in 200 cc.); ana (c) solid nitrite.—The technical product is dried in a steam heated oven, then powdered finely and stored in an air-tight container (since it takes up moisture). The NaNOj content is determined by titration.

(4) Decomposition of diazo compounds is accelerated by heat and frequently by light. This decomposition takes place more rapidly the more alkaline the solution, insofar as the alkali does not cause rearrangement to the isodiazotates. The latter compounds are quite stable and either do not couple at all or couple very slowly. Excess acid improves the stability of diazo compounds. The first sign of decomposi­tion is generation of nitrogen. In all work with diazo compounds, decomposition is held in check by strong cooling, protection from bright light, and, insofar as possible, maintaining high acidity. In general, the more basic an amine, the more unstable is the diazo compound derived from it; conversely, negative (acidic) substituents, such as halogen, nitro, sulfo, etc., increase the stability of diazo compounds. The o-hydroxydiazo compounds have unique characteristics, due to their particular structure, and exhibit extraordinarily high stability, at least in the absence of alkali.

Many diazo compounds undergo still another type of decomposition reaction in addition to the usual conversion to the phenol with the liberation of nitrogen. This second decomposition reaction involves a replacement, by hydroxyl, of a substituent located ortho to the diazo group. The substituents which are subject to this replace­ment are halogen, alkoxyl, nitro, and sulfo groups. The reaction is favored by higher temperature and alkalinity, and by the presence of negative substituents (e. g., halogen, nitro, sulfo, carbonyl, etc.). It takes place especially easily in compounds having two diazo groups located meta to each other. This reaction is actually used technically for the preparation of o-hydroxydiazo compounds. In cases where the replacement reaction is undesired, it must be minimized by working at low tempera* tures and in as strongly acidic medium as possible.

The work should always be organized so that the coupling reaction can be carried out immediately after the diazo compound is made, and in no case should the diazo compound be held for a prolonged period of time. If the diazo compound is isolated in the solid form, precautions must be taken to keep it moist since even relatively stable diazo compounds may explode violently when prepared in the dry state; o-hydroxydiazo compounds of the naphthalene series are exceptions.