FUR DYEING
ANILINE BLACK
Fur seal for a long time has been a fur of distinction and importance in
the fur industry, and consequently the dyeing of seal has constituted an
important, though not very extensive branch of the art of fur dyeing.
In quite recent times the popularity of seal has become so great that
imitations have had to be produced to help supply the demand, and as a
result, French seal, or seal-dyed rabbit, and the so-called Hudson seal,
which is seal-dyed muskrat, have acquired a great vogue. Occasionally
opossum, nutria and other furs are also used for the purpose of producing
seal imitations. While the supply of real seals is relatively small, and
the demand large, the production of seal imitations has assumed large
proportions, and as a result, the dyeing of seal and its imitations or
substitutes has come to be a great branch of the fur dyeing industry.
During the past thirty years, the long and tedious processes of dyeing
seal and seal imitations, involving the use of dyes of vegetable origin,
have largely been superseded by what is known as the Aniline Black dye.
It was the French who first worked out successfully the application of
Aniline Black to furs, and the method has attained much importance and
extensive use in the fur dyeing industry.
Aniline Black is the name given to an insoluble black dyestuff produced
by the oxidation of aniline in an acid medium. As a finished product it
cannot be used in fur dyeing, but if the hair of the furs be impregnated
with a suitable preparation of aniline and then treated with certain
oxidizing agents, the color will be formed on the hair, being firmly
fixed and giving a fast black, resistant to light, washing and rubbing.
The basis of the dye, aniline, is an oily liquid, possessing a peculiar
fishy odor, colorless when pure, but rapidly turning brown when exposed to
the air. It is obtained from benzol, which is distilled from coal-tar, by
treating with nitric acid, forming nitrobenzol, which when subjected to
the action of reducing chemicals is converted into aniline. The process
may be shown schematically as follows:
Coal--coal-tar--benzol--nitrobenzol--aniline oil--Aniline Black. Aniline
Black was by no means a new dye when the French succeeded in producing it
on furs. It had been used for a long time previous on textiles, chiefly
cotton. The history of the development of the Aniline Black process
throws considerable light on its nature and constitution, and so presents
many features of interest. As early as 1834, the chemist Runge observed
the formation of a dark green color when heated aniline nitrate in the
presence of cupric chloride. Fritsche, in 1840, noticed that when chromic
acid was added to solutions of aniline salt, a dark green, and sometimes a
blue-black precipitate was produced, and later the same chemist obtained
a deep blue by the action of potassium chlorate on aniline salt. It is
interesting to note that Perkin, in 1856, conducting similar experiments
on the oxidation of aniline with chromic acid, obtained a blue-black
product from which he extracted the first synthetic coal-tar dye, mauve.
Thus far, all the experiments on the oxidation of aniline proved to be
merely of scientific interest, but in 1862, Lightfoot patented a process
for the practical application of colors formed by the oxidation of aniline
on the fibre, a greenish shade being obtained by that method, to which the
name emeraldine was given, and by subsequent treatment with bichromate
of potash, the green was changed to a deep blue color. Since that time,
the methods for producing and applying Aniline Black have been developed
and improved, although all the processes were based on the principles
incorporated in Lightfoot’s original patent. However, it was not until the
last decade of the nineteenth century that the dyeing of furs by means of
the Aniline Black method was successfully attempted.
A knowledge of the nature and the manner of the chemical changes which
take place in the production of Aniline Black is a valuable aid in
obtaining satisfactory results in practise; and although Aniline Black
was extensively used before the true character of the reaction was
understood, since the successful determination of the constitution of
Aniline Black and the discovery of the real nature of the process by
Green and his collaborators in 1913, the methods have been considerably
improved and simplified, with correspondingly better results in dyeing.
As a consequence, the methods of dyeing furs with Aniline Black have also
become simpler and more efficient.
A discussion of the chemical changes which occur in the Aniline Black
process, is out of place here on account of the highly involved and
complicated character of the reactions, to understand which requires a
considerable knowledge of specialized organic chemistry. But the essential
features of practical importance in the production of Aniline Black are
the following: As already noted, one of the characteristic properties of
aniline is its tendency to turn from a colorless to a dark-brown liquid in
the presence of the air. This change is due, together with certain other
causes, to an oxidation brought about by atmospheric oxygen. By employing
oxidizing agents, this oxidation can be accelerated and carried further,
and eventually the Aniline Black is obtained. Among the substances which
may be used to bring about the conversion of aniline to the insoluble
black dye are manganese dioxide, lead peroxide, hydrogen peroxide, chromic
acid, ferric salts, potassium permanganate, chloric acid and chlorates
in the presence of certain metallic salts, particularly those of vanadium
and copper. Chlorates, especially sodium chlorate and potassium chlorate,
are the most commonly employed oxidizing agents, bichromate of soda or
of potash being used, in addition, to complete the oxidation. When using
chlorates it is necessary to have present in the dye mixture a small
quantity of a metallic salt, which, while not entering into the reaction
itself, is nevertheless indispensable as an oxygen carrier. Vanadium
compounds have proved to be the most effective for this purpose, and
according to an authority, one part of vanadium salt is sufficient to
cause the conversion of 270,000 parts of aniline to Aniline Black, the
necessary amount of a chlorate being present of course. Salts of copper,
cerium, and iron are also extensively used, but they are not quite so
efficient as vanadium.
The formation of the Aniline Black in practise takes place in three
well-defined steps, which it is important to be able to recognize and
distinguish in order to obtain the best results. The first stage of the
oxidizing process produces what is called emeraldine, which in the acid
medium of the aniline bath is of a dark green, while in the free state it
is of a blue color. As the oxidation proceeds, the second stage develops,
the emeraldine being converted to a compound called nigraniline. This in
acid solution is blue, and the free base is a dark-blue, almost black.
It was formerly considered that the nigraniline was the Aniline Black
proper, and so when this stage of the oxidation was reached, the process
was often interrupted and not carried to the limit. This can account for
the fact that Aniline Black dyeings usually turned green after a short
time. The reason for this is that nigraniline, when treated with weak
reducing agents, as, for example, sulphurous acid, is at once changed
to emeraldine, with its dark green color. Since there is usually a small
amount of sulphurous acid in the air, especially in places where coal or
gas is burned, an Aniline Black dyeing which has not been carried beyond
the nigraniline stage will be reduced in time to the emeraldine, and cause
the dyeing to become green. The last step in the oxidation changes the
nigraniline into what is properly called the ungreenable Aniline Black.
Weak reducing substances like sulphurous acid do not change this compound
to emeraldine, and stronger reducing agents only convert it to a brownish
compound, which changes back to the black when exposed to the air. It is
quite evident that in order to obtain a black which will not change to
green in time, the oxidation of the aniline must be carried to the last
stage. By making tests during the dyeing of the furs, it can easily be
determined whether the oxidation has proceeded far enough.
In the dyeing of textiles with Aniline Black, it is customary to carry
out the operation at comparatively high temperatures, approaching 100°
centigrade. With furs such temperatures are out of the question, so it is
necessary to repeat the dyeing several times in order to obtain the proper
depth of shade working in the cold. Only the brush method can be used in
applying the Aniline Black dye to furs, on account of the strong acidity
of the dye mixture, which would ruin the leather, if the dyeing were done
in a bath. Indeed, great care must be exercised even by the brush method
to avoid too great penetration of the dye liquid, otherwise the roots
of the hair will be attacked, and the leather may be “burned” from the
hair side. Furs dyed with Aniline Black are frequently after-dyed by the
dip-process with logwood or some other similar dye, in order to add to
the brilliancy of the dyeing. Combined with intensity of color, Aniline
Black on furs is the only dye which will also give fast, lustrous shades,
and leave the hair soft and smooth.
There are several methods of applying Aniline Black on furs, the most
important being
1. One-bath Aniline Black
2. Oxidation Aniline Black
3. Diphenyl Black
4. Aniline Black by Green’s Process
1. _One-bath Aniline Black_
A typical formula for this method is the following given by Beltzer:
Aniline salt 10 kg.
Sodium chlorate 1.5 kg.
Copper sulphate 0.7 kg.
Vanadate of ammonia 10 gr.
All these substances are dissolved hot in 50 liters of water, and allowed
to cool, forming solution A. Aniline salt is aniline oil which has been
neutralized with the exact quantity of hydrochloric acid to form the
hydrochloride. It forms white or greyish crystalline lumps very easily
soluble in water. The sodium chlorate is the oxidizing agent, and the
copper sulphate and the vanadate of ammonia are the oxygen carriers.
15 kg. of sodium bichromate are also dissolved in 50 liters of water,
forming solution B. The bichromate is also an oxidizing agent and serves
to complete the oxidation of the aniline to the black.
Immediately before using, solutions A and B are mixed together, both being
cool. In general practise it is customary to mix only small quantities
at a time, as a considerable precipitate forms when the whole batch
is mixed at once, the precipitate being so much waste dye substance.
Usually a liter of A and a liter of B are mixed at a time, and the furs
brushed with the mixture. The brushing must be varied according as the
hair is hard and stiff, or soft and tender. The hair must be thoroughly
impregnated in all directions, and the penetration must not be too deep
to affect the leather. With experience and dexterity satisfactory results
can easily be achieved. After the skins have been properly treated, they
are dried at a temperature of about 35 degrees centigrade. When dry, they
are returned to the dye bench, where they receive another application
of the dye mixture, and are again dried. This operation may be repeated
as often as six or seven times before a sufficiently intense black is
obtained. Another way of producing the desired depth of shade with fewer
applications is by using more concentrated dye mixtures. Each method
has its disadvantages, the greater number of brushings requiring the
expenditure of more time and labor, and the greater concentration of
the bath resulting in a considerable loss of dye substance due to the
formation of a large precipitate when the two solutions are mixed, and
moreover, not all furs can be treated with concentrated mixtures. The best
results with this method usually require the application of six coats of
a mixture of moderate concentration.
2. _Oxidation Aniline Black_
In order to overcome the difficulty of employing very concentrated dye
mixtures, or of making many applications of the dye, a method was devised
whereby the two solutions of the previous process, instead of being mixed
together, are applied successively to the hair of the furs, the following
formula, also by Beltzer, being an example:
Aniline oil 10 liters
Nitric acid 36° Beaumé, or
Hydrochloric acid 22° Beaumé 20 liters
Cold water 20 liters
This is solution A, and is merely a solution of aniline hydrochloride,
or nitrate, depending on which acid has been used. Nitric acid, although
more costly than the hydrochloric acid, is to be preferred, because it
is an oxidizing acid, and so assists in the oxidation of the aniline, and
besides, has a more beneficial effect on the hair than the hydrochloric,
in the matter of softness and luster.
Sodium chlorate 4 kg.
Copper sulphate 1 kg.
Vanadate of ammonia 10 gr.
Water 50 liters
This is solution B, containing the oxidizing agent, and the oxygen
carriers. Just before using, equal quantities of A and B are mixed, and
the skins brushed with the mixture. The skins are then dried at 35–45°
centigrade, at which temperature the color begins to develop. When almost,
but not entirely dried, the skins are subjected to the action of warm
vapor, which is allowed to enter the drying chamber, so as to keep the
temperature about 40° centigrade, the color developing better in this
way. This operation may be repeated, or the skins are directly treated
with a solution of 25 kg. of sodium bichromate in 100 liters of water,
to complete the oxidation. The moist skins are exposed to the air for a
time, and then dried at 35° C.
This method of dyeing has several advantages over the One-bath Aniline
Black. It requires fewer brushings, and enables the complete utilization
of the dye solutions without loss. With three applications of the dye
mixture by the Oxidation process, as deep and intense a black can be
obtained as with six brushings by the One-bath method. The dyeings, too,
are nearly, but not fully as brilliant and even as in the latter case.
The greater the number of coats of dye that are applied the more regular
will the dyeing be.
3. _Diphenyl Black_
In 1902, the Farbwerke Hoechst, a large German producer of coal tar
intermediates and dyes, invented an Aniline Black process to which they
gave the name Diphenyl Black. The chief departure from the previous
Aniline Black methods was the replacing of part of the aniline oil of the
dye mixture by Diphenyl Black Base I, which is para-aminodiphenylamine.
This base has the property of being oxidized to Aniline Black, just like
aniline oil, and the advantage claimed for the Diphenyl Black is that it
produces an absolutely ungreenable black. The method of application is
practically the same as for the other Aniline Black processes, chlorates
being used as the oxidizing agents, in the presence of oxygen carriers
such as salts of copper and vanadium. The use of bichromates is dispensed
with. On account of the comparatively high cost of the Diphenyl Black
Base I, this method has not found very extensive application, especially
as highly satisfactory ungreenable blacks can now be produced by other
methods.
4. _Aniline Black by Green’s Process_
In 1907, Green, who has done much work in the direction of elucidating the
character of the Aniline Black process, obtained a patent for a method
of applying Aniline Black in a manner which was different from all the
previously known formulas. The invention created great interest, and
although in its original form it did not find a wide application, many of
the methods used at the present time are in one way or another derived
from the idea of Green. A resumé of the patent will therefore be given
here: “The invention relates to the production of an Aniline Black, the
new process differing from all other known processes by the fact that
the oxidation of aniline is effected solely or mainly by the oxygen of
air. The possibility of dispensing with an oxidizing agent depends on the
discovery that the addition of a small quantity of a para-diamine, or of
a para-amido-phenol to a mixture containing aniline and a suitable oxygen
carrier, such as a salt of copper, greatly accelerates the oxidation of
the aniline by the atmospheric oxygen. Further, whereas in the ordinary
processes of Aniline Black, the quantity of mineral acid employed cannot
be materially reduced below the proportion of one equivalent to one
equivalent of the base, under the new conditions the mineral acid may
be wholly or partially replaced by an organic acid such as formic acid,
without the quality of the black being materially affected. As suitable
oxygen carriers the chlorides of copper have been found to give the best
results, it being preferrable to use the copper in the form of a cuprous
salt. This is effected by adding to the dye mixture cupric chloride,
together with a sulphite or bisulphite in sufficient quantity to reduce
the cupric salt to the cuprous state, and a sufficient quantity of a
soluble chloride to keep the cuprous chloride in solution. Among the
compounds suitable for the production of this black in conjunction with
aniline are, para-phenylene-diamine, dimethyl-para-phenylene-diamine,
para-amido-diphenylamine, para-amido-phenol, etc.”
This method may be used alone as the other Aniline Blacks, or the dyed
skins may be after-dyed in a bath containing a logwood dye, or it may be
used in conjunction with mineral dyes, or with the Oxidation Colors (see
next chapter). A typical formula for the black by Green’s process is the
following:
Para-amido-phenol 0.5 kg.
Aniline oil 10 liters
Hydrochloric acid 22° Bé. 10 liters
Acetic acid 40% 5 liters
Cold water 25 liters
This is solution A. Solution B is prepared by dissolving
Copper sulphate 2 kg.
Salammoniac 10 kg.
Cold water 50 liters
A and B are mixed, and the mixture applied to the hair of the furs several
times, drying each time at 35°–40° C. After three coats of dye have
been applied, a pretty and fairly intense black shade is obtained, which
is developed further by treating with a solution of 25 grams of sodium
bichromate per liter of water. The skins are then allowed to dry in air,
and then if desired, an after-dyeing is made with some other dye.
On account of its extreme fastness, Aniline Black, produced by any of
the methods outlined above, has attained a justifiable popularity for
the dyeing of furs, in spite of the necessity of using the more or less
cumbersome brush method of applying the dye. Very recently there was
issued to a German company a patent in which is described a method whereby
furs can be dyed with Aniline Black by the dip process. An abstract of
the patent (D. R. P. 33402) is as follows: “As is known, aniline salt,
and similar salts, together with oxidizing agents like bichromates,
chlorates, etc., cannot be used for dyeing furs by the dip process,
because the strongly dissociated mineral acid is injurious to the leather.
The dissociation of the acid can be reduced by adding neutral salts, like
common salt, or Glauber’s salt, so that good results can be obtained by
dyeing in a bath of the dye mixture, the leather retaining its softness.”
Thus far there have been no reports of the successful practical
application of this patent, so its value cannot be discussed. It is
extremely doubtful, however, that furs will ever be dyed in the dyebath
with the present type of Aniline Black formulas, no matter what substances
are added to prevent the leather from being affected.