The Dyeing of Cotton Fabrics Part 24

C{6}H{5}NH{2} + HCl + HNO{2} 2H{2}O + C{6}H{5}N:NCl Hydrochloric acid Nitrous Water, Diazo-benzene aniline, acid, chloride.

These diazo compounds are distinguished by their active properties, especially in combining with amines in acid solutions, or with phenols in alkaline solution to form the azo dyes, thus diazobenzene chloride will combine with naphthol to form naphthol-azo-benzene, thus:--

C{6}H{5}N:NCl + C{10}H{7}OH + NaOH = Diazo-benzene chloride, Naphthol, Caustic soda.

C{10}H{6}OHN:NC{6}H{5} + NaCl + H{2}O Naphthol-azo-benzene, Salt, Water.

These azo compounds are coloured, but are perfectly insoluble in water, alkalies, or acids; on the other hand the sulphonates of these bodies are easily soluble and form the numerous azo dyes now so largely made and used in wool and silk dyeing, but which on account of their being sulphonates cannot be used for cotton dyeing.

Methods have been devised for producing the insoluble azo colours direct upon the fibres. They are also called naphthol colours from the use of beta-and alpha-naphthol in their production. Although these azo dyes, when produced on the fibre, do not possess the fastness of the alizarine dyes, yet, on account of their cheapness and relative great fastness to soap and the action of sunlight, they are better than many of the newer cotton dyes.

By this method (first introduced in England by Holliday) colours of exceptional brightness and fastness can be obtained which were not obtainable with the dyes then known. Those which are obtained from phenols are of the first importance.

_The Diazotisation of the Amido Bases._

With most bases this must be accomplished as cold as possible below 65 F. At a higher temperature, and when allowed to stand, most diazo compounds decompose quickly with evolution of nitrogen, which decomposition results in the mixture losing its power of producing colour, or at the most gives unsatisfactory results. For this reason it is therefore always necessary to work as cold and as quickly as possible.

The amido-azo bodies, whose compounds with the phenols are also distinguished by their great fastness, are in this respect an exception.

They can be diazotised at the ordinary temperature, and their diazo compounds are much stabler than those, for example, of alpha-and beta-naphthylamine or of aniline, which must always be used as quickly as possible.

From anisidine, phenetidine and amido-diphenylamine, still more stable diazo compounds can be obtained, but the prices of these bases are rather high, and the colours produced with them are not fast to light.

The cheapest and most convenient method of obtaining nitrous acid for diazotising is by the action of a mineral acid, preferably hydrochloric acid, upon nitrite of soda.

For diazotising one molecule of base requires one molecule of hydrochloric acid to form a salt of the base, a molecule of nitrite of soda, and another molecule of hydrochloric acid to decompose the nitrite. The diazotisation is better carried out and the diazo solution rendered more stable if another molecule of hydrochloric acid and an excess of nitrite of soda are used. The presence of an excess of nitrite can be determined by testing the diazo solution with pota.s.sium iodide starch paper, which in the presence of excess of nitrite gives the blue iodine starch reaction.

In carrying out the diazotisation, the base is first dissolved in the whole amount of hydrochloric acid which has to be used, and the solution is filtered. The diazotisation takes place in the manner shown in the equation:--

C{6}H{5}NH{2} + HCl + HCl + NaNO{2} = Aniline hydrochloride, Hydrochloric acid, Sodium nitrite,

NaCl + C{6}H{5}N:NCl + H{2}0 Salt, Diazo-benzene chloride, Water.

The bases which form salts soluble with difficulty, such as nitroaniline and the amido-azo bodies, offer special difficulties in diazotising.

It has been found that the operation with these is best carried out if the chemically pure bases in paste form are mixed with the requisite amount of nitrite, and the diluted paste then poured into the hydrochloric acid.

It has been found by experience that the colour is developed much brighter upon the fibre when the diazo solution contains acetic acid and no free mineral acid. However, the diazotisation is better carried out with hydrochloric acid, and the presence of the latter is necessary to give stability to the solution. If before the diazo solution is used a quant.i.ty of acetate of soda be added to it, the free hydrochloric acid liberates acetic acid from the acetate, and the chloride of the diazo body changes into its acetate. It is better to add an excess above the two molecules of acetate of soda which are required.

The combination when aniline and beta-naphthol are used, as the amine and phenol respectively, is shown in the following equations:--

C{6}H{5}N:NCl + C{10}H{7}OH + Diazo-benzene chloride, B. naphthol,

NaOH = 2NaCl + C{6}H{5}N:NC{10}H{6}OH + H{2}O Caustic soda, Benzene- azo-naphthol, Water.

Or, with naphthylamine and naphthol, thus:--

C{10}H{7}N:NCl + C{10}H{7}OH + NaOH =

NaCl + C{10}H{7}N:NC{10}H{6}OH + H{2}O Naphthalene azo-naphthol.

By the action of nitrous acid upon amido-azo bodies a group of bodies called diazo-azo compounds are obtained which contain the group N:N twice over, thus:--

C{6}H{5}N:NC{6}H{4}NH{2}HCl + NaNO{2} + 2HCl = Benzene-azo-aniline-hydrochloride,

NaCl + C{6}H{5}N:NC{6}H{4}N:NCl + 2H{2}O.

Diazo-azo-benzene-chloride.

When this compound is combined with naphthol diazo-azo dyes are produced.

C{6}H{5}N:NC{6}H{4}N:NC{10}H{6}OH.

Benzene-azo-benzene-azo-naphthol.

The molecular weights of the bases, phenols and chemicals employed are the following:--

1. Hydrochloric acid, HCl--36.5. 2. Caustic soda, NaOH--40. 3. Nitrite of soda, NaNO{2}--69. 4. Acetate of soda, NaC{2}H{3}O{2}3H{2}O--136.

1. Commercial hydrochloric acid at 32 Tw. contains about 365 grams of HCl in a litre, or 3-1/2 lb. in a gallon.

2. The commercial 77 per cent. soda must always be used, and for practical purposes it may be taken as pure. It is best to make a solution which contains 160 grams NaOH in a litre of water.

3. The nitrite supplied is almost chemically pure, and is easily soluble in water. In order to make a solution 140 or 290 grams are dissolved per litre.

4. Crystallised acetate of soda contains 3 molecules of water of crystallisation, and is usually somewhat moist. Instead of 136 grams 140 are taken to allow for moisture. The amount is dissolved in about 500 cubic centimetres of water.

_Bases._

1. Aniline, C{6}H{5}NH{2}--93.

2. Toluidine, C{7}H{7}NH{2}--107.

3. Alpha-and beta-naphthylamine, C{10}H{7}NH{2}--143.

4. Para-or meta-nitroaniline, C{6}H{4}NO{2}NH{2}--138.

5. Nitro-para-toluidine, C{7}H{6}NO{2}NH{2}--152.

6. Amidoazobenzene (base), C{6}H{5}N:NC{6}H{4}NH{2}--197.

7. Orthoamidoazotoluol (base), C{7}H{7}N:NC{7}H{6}NH{2}--225.

8. Alpha-or beta-naphthol C{10}H{7}OH--144.

_Example of Quant.i.ties Taken._

Molecular Weight.

1. Molecule nitrite 69 grams.

2. Molecule aniline 93 "

3. Molecule hydrochloric acid 365 "

4. Molecule acetate of soda 136 "

5. Molecule of naphthol 144 "

6. Molecule caustic soda 40 "

Applying the principles which have just been described to the dyeing of cotton, it is found that the cotton may be dyed by taking the base and preparing the diazo body, impregnating the cotton with this, and developing the colour by pa.s.sing into a bath of the phenol. On the other hand, the cotton can be prepared with the phenol and the colour developed by pa.s.sing into a bath of the diazotised base, and practice has shown that this latter proceeding is the best. Practically the only phenol that is used is the beta-naphthol; alpha-naphthol is occasionally used, but not often.