Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/611

 DYE I N G Grey.—Benzo Grey, Benzo Black, Azo Mauve, Diaminogene, Neutral Grey. (&) Janus Colours.—These comprise a small series of azo colours of a strongly basic character, which possess the unusual property of dyeing cotton in an acid bath. Since they also dye wool and silk under the same conditions, they may be employed for dyeing plain shades on wool, silk, and cotton union fabrics. The material is entered into the dyebath at a low temperature, which is gradually raised to the boiling-point. The necessary additions to the dyebath are 1 to 3 per cent, colouring matter, 2 per cent, sulphuric acid, and 5 to 20 per cent, sodium sulphate. The colours include Janus Red, Janus Yellow, Janus Green, Janus Blue, Janus Brown, Janus Bordeaux, Janus Grey, and Janus Black. (c) Sulphide Colours.—These dyestuffs, which are also called Sulphine Colours and Thiosulphate Colours, are only suitable for dyeing the vegetable fibres. The dyestuff Cachou de Laval, discovered in 1873, was the first member of this group, and was obtained by melting a mixture of sodium sulphide and various organic substances—e.g., bran, sawdust, &c. In recent years several other dyestuffs have been added to the list, namely, grey, blue, green, brown, and especially black colours, by submitting certain definite amido compounds to a similar treatment with sodium sulphide or sodium thiosulphate, and subsequent oxidation. The mode of dyeing with these colours is based on the fact that they are soluble in an alkaline reducing agent, and if the cotton is impregnated with the solution, subsequent oxidation develops the colour and fixes it upon the fibre in an insoluble coqdition. The material is boiled for about one hour in a solution of the colour (10 to 15 per cent.), with the addition of sodium carbonate (1 to 10 per cent.), common salt (10 to 20 per cent.), and sodium sulphide (2 to 20 per cent.); it is then washed quickly in water, and developed for half an hour in a boiling bath containing 2 to 5 per cent, of some metallic salt—e.g., bichromate of potash, copper sulphate, chrome alum, &c. A final washing with water containing a little caustic soda to remove acidity is important. The sulphide colours are remarkable for their fastness to light, alkalis, acids, and washing, but unless proper care is exercised the cotton is apt to be tendered. The following list includes most of the colours of this class :— Green.—Pyrogene Green, Italian Green. Brown.—Pyrogene Brown, Pyrogene Yellow, Cachou de Laval, Thiocatechine, Katigene Black Brown. Grey and Black.—Pyrogene Grey, Yidal Black, Immedial Black, Katigene Black, Anthraquinone Black, St Denis Black, Amidazol Black, Cross Dye Black, Clayton Fast Black, Eclipse Black, Carbide Black, Thiogene Black, Sulphaniline Black, Sulfogene Black, Pyrogene Black. At the present time (1902) this class of colours is continually increasing in number, and for certain purposes in cotton dyeing the group is already becoming important. Developed Colours.—This group includes certain azo colours which are developed or produced upon the fibre itself (usually cotton) by the successive application of their constituent elements. It may be conveniently divided into the following sub-groups :— Insoluble Azo Colours, Primuline Colours, Developed Direct Colours, Benzo Nitrol Colours. (a) The Insoluble Azo Colours are produced as insoluble coloured precipitates by adding a solution of a so-called diazo compound to an alkaline solution of a phenol, or to an acid solution of an amido compound. The necessary diazo compound is prepared by allowing a solution containing nitrous acid to act upon a solution of an amido-base—e.g., aniline. It is desirable to keep the solutions cool with ice, owing to the very unstable nature of the diazo compounds produced. The colour obtained varies according to the particular diazo compound, as well as the amine or phenol employed, beta-naphthol being the most useful among the latter. The same coloured precipitates are produced upon the cotton fibre if the material is first impregnated with an alkaline solution of the phenol, then dried and passed into a cold solution ot the diazo solution. Three operations are therefore necessary when dyeing with these colours: (1) diazotizing of the base ; (2) impregnation of the cotton with the phenol solution; (3) developing. As an example, the method of dyeing an orange-yellow colour derived from aniline and beta-naphthol is here given.—Diazotizing. Aniline (93 parts) is dissolved in hydrochloric acid (sp. gr. 1‘14) (390 parts) and water (200 parts). The solution is cooled with the addition of ice, and a solution of sodium nitrite (75 parts) in water (500 parts) is slowly added with continual stirring. Each base requires a slightly different treatment to ensure the best reavlts.—Impregnation. A stock solution of beta-naphthol is prepared by dissolving 14 parts by weight of this substance in 4 parts caustic soda and 50 parts water. This solution is diluted 5 to 10 times with water, according to the depth of colour required, and 1 part soda-ash (sodium carbonate) is then dissolved in 15 to 30 parts of the dilute solution; in order to give a brighter colour, a small amount of sulphated oil is sometimes added. The cotton is well impregnated with this solution, then wrung out and

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dried quickly.—Developing. The naphthol-prepared cotton should be passed as soon as possible into the diazo or developing solution, in order to prevent the cotton from acquiring a brownish hue through oxidation. The orange-yellow colour desired is developed almost immediately, but it is well to continue working the cotton about in the solution for a few minutes. The dyed cotton is squeezed, washed, soaped slightly, and finally rinsed in water and dried. The colours obtained by this method vary according to the particular diazo compound employed. If the aniline used in the foregoing process is replaced by meta-nitraniline, a yellowish-orange colour is obtained; with para-nitraniline, a bright red; with alphanaphthylamine, a claret-red; with amido-azo-toluene, a brownish red; with benzidine, a dark chocolate; with dianisidine, a dark blue; and so on. The dyed colours are fast to washing and are much used in practice, particularly the Para-nitraniline Red, which serves as a substitute for Turkey-red, although it is not so fast to light as the latter. (5) The Primuline Colours are derived from the yellow dyestuff known as Primuline, which dyes cotton in the same manner as the Direct Colours. The Primuline yellow is fugitive to light and of no practical value, but since the colouring matter is an amido base, the dyed fibre can be diazotized and then developed in solutions of phenols or amines, whereby azo dyes of various hues may be obtained, according to the developer employed ; e.g., beta-naphthol develops a bright red colour (Primuline Red), resorcinol develops an orange, phenol a yellow, naphthylamine a brown, &c. The dyeing of the Primuline yellow is effected by boiling the cotton for one hour in a solution of Primuline (5 per cent.) and common salt (10 to 20 per cent.). The diazotizing operation consists in passing the dyed and rinsed cotton for 5 to 10 minutes into a cold solution of nitrous acid—i.e., a solution of f oz. sodium nitrite per gallon of water, slightly acidified with sulphuric acid. The diazotized material should not be exposed to light, but at once washed in cold water and passed into the developer. The developing process consists in working the diazotized material for 5 to 10 minutes in a cold solution of the necessary phenol or amine, and finally washing with water. If the developer employed is an amido compound, the dyed colour will contain a free amido group, and hence the operations of diazotizing and developing may be repeated again and again with the object of obtaining darker shades. The Primuline colours are best adapted for cotton dyeing, and the colours obtained are fast to washing and to moderate soaping, but they are not very fast to light. (c) Developed Direct Colours.—If cotton is dyed with such Direct Colours as contain free amido groups, the colour can be diazotized on the dyed fibre exactly in the same manner as in the case of Primuline-dyed cotton, and then developed by passing into the solution of an amine or phenol, or by treating it with a warm solution of sodium carbonate. In this manner a new azo dye is produced upon the fibre, which differs from the original one not only in colour, but also by being faster to washing and other influences. A treatment with copper sulphate solution after development is frequently beneficial in rendering the colour faster to light. Some Direct Colours, indeed, are of little value, owing, for example, to their sensibility to acids, until they have been diazotized and developed, the usual developers being beta-naphthol, resorcinol, phenol, and phenylene-diamine. The following Direct Colours, after being applied to cotton, may be submitted to the above treatment, the colours produced being chiefly blue, brown, and black :— Blue.—Diazurine, Diazo Blue, Diamine Blue, Diaminogene. Brown.—Diazo Brown, Diamine Cutch, Diamine Brown, Cotton Brown. Grey and Black.—Benzo Blue, Diazo Blue Black, Diazo Black, Diamine Black, Diazo Brilliant Black. (d) Benzo Nitrol Colours.—These are certain Direct Colours dyed on cotton in the ordinary manner, which are then developed by passing into a diazo solution—e.g., diazotized para-nitraniline, &c. The dyed colour here plays the part of a phenol or amine, and reacts with the diazo compound to produce a new colour. The process is similar to the production of the Insoluble Azo Colours, the beta-naphthol which is there applied to the fibre being here replaced by a Direct Colour. The colour of the latter is rendered much deeper by the process, and also faster to washing and to the action of acids. The dyestuffs recommended for application in the manner described are : Benzo Nitrol Brown, Toluylene Brown, Direct Fast Brown, Pluto Black, Direct Blue Black. Mordant Colours.—The colouring matters of this class include some of the most important dyestuffs employed, since they furnish many colours remarkable for their fastness to light, washing, and other influences. Employed by themselves, Mordant Colours are usually of little or no value as dyestuffs, because, with some exceptions, either they are not attracted by the fibre, particularly in the case of cotton, or they only yield a more or less fugitive stain. Their importance and value as dyestuffs are due to the fact that they act like weak acids and have the property of combining with metallic oxides to form insoluble coloured compounds termed