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Encyclopedia Britannica - Main :: DEM-DIO |
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DEVELOPED CoLouRs.This group. includes certain azo colours which are developed or produced upon the fibre itself (usually cotton
(a) The Insoluble Azo Colours are produced as insoluble coloured precipitates by adding a solution of a 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 a primary aromatic amine. It is usually 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, '3- naphthol being the most useful among the latter. The same coloured precipitates are produced upon the cotton
caustic
chocolate
paranitraniline red, which serves as a substitute for Turkey-red, although it is not so fast to light as the latter. (b) Developed Direct Colours.The primuline colours were the first representatives of this class and are derived from the yellow dyestuff known as primuline, which dyes cotton in the same manner as the direct colours. The primuline yellow thus obtained is fugitive to light and of little practical
bright red colour, resorcin 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 %) and common salt (lo to 20 %). The diazotizing operation consists in passing the dyed and rinsed cotton for 5 to to minutes into a cold solution of nitrous acidi.e. a solution of 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 io minutes in a cold solution of the necessary phenol, and finally washing with water. The only developer of any practical
caustic
If cotton is dyed with other direct colours containing free amido groups, the colour can be diazotized on the dyed fibre exactly in the same manner as in the case of prim uline-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
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. Red.Rosanthiene. Brown.Diazo brown, diamine cutch , diamine brown, cotton brown.Grey and Black.Benzo blue, diazo blue black, diazo black, diamine black, diazo brilliant black. (c) Benzo Nitrol Colours.These are certain Direct Colours, dyed on cotton in the ordinary manner, which are then developed by passing into a diazo solutione.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 )3- 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." Topping " Direct Colours.The direct colours possess the remarkable property of precipitating the basic colours from aqueous solution. Use is frequently made of this property for " topping ' cotton dyed with direct colours either with a view to obtain compound shades or to brighten the colour. Thus by dyeing cotton first yellow in chrysamine and then dyeing it again in a cold bath of methylene blue a brilliant shade of green results. If, on the other hand, a direct blue is topped with' methylene blue, its brilliancy may be enhanced. End of Article: DEVELOPED If you wish, you can link directly to this article.
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