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Файл:Fur Processing Technologies. Тutorial
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31
1.6.3. Dyeing operations
Depending on the type and further application of fur semi-finished products, the purpose of dyeing is to paint an object a specific color, improve
the natural color of valuable types of fur, imitate more valuable species (dyeing a rabbit like a cat, sable, red fox), and eliminate defects of hides.
The dyeing process is divided into four stages:
• diffusion of the dye from the solution to the surface of the material
to be painted;
• sorption of the dye by the outer surface of the material;
• diffusion of the dye into the fiber (internal diffusion);
• binding of the dye.
Internal diffusion is one of the most importance prcesses of fur ski
n
dyeing. When dyeing a guard hair with a core, the dye is to penetrate into it
and accumulate there, filling the cells and capillary space completely.
The penetration of the dye into the hair depends on the structure and properties of the scaly layer, which is characterized by hydrophobicity and chemical
inertness. Therefore, the hair is initially processed in solutions of alkalis, reducing agents and oxidizing agents to increase the hydrophilicity and chemical activity of the surface layer. This operation is carried out before dyeing
and includes neutralization (neutralization with partial splitting of the cuticle – starvation), bleaching, and etching.
Hair dyeing is carried out by various types of dyes: oxidation; acid;
vat; etched; basic; active dyes. Oxidation and acid dyes are widely used for
fur processing. Typically, oxidation dyes are used for painting black and
brown colors, imitating valuable types of furs. Acid dyes are used to paint
the fur special unusual color. Active and acid dyes are used to paint fur skins.
Oxidation dyeing of fur. Oxidation dyes include organic intermediates
of the aromatics: amines, phenols, naphthols and their derivatives. Unlike
ready-made dyes, they are colorless or weakly colored. Dyes from them are
formed as a result of oxidation in the hair structure at a temperature of about
35 ºС and a pH of 8–8.5. In our country, these dyes are produced under
the name "dyes for fur" or "oxidative dyes". Germany companies produce
similar dyes under the names Ursol (BASF), Rodol (Lowenstein), Dorapel
Fast (Pulkra Boehme). In the Czech Republic, they are called Benzofur; in
Poland – futramins; in France – fouramines.
32
The process of fur dyeing based on these intermediate products is possible during their oxidation and formation of dyes in the hair and in the dye
solution. However, the covalent bonds forming between oxidation products
and hair keratin can not provide the conditions for a friction-resistant coloration, especially if oxidation dyeing is used for producing fur of intense dark
tones. Therefore, the process is carried out mainly after pickling skins using
metal salts, the ions of which are ready to form complexes of ionized and
neutral groups. Therefore, strong protein-metal-dye complexes are formed,
which significantly increase the color and light fastness.
The dye solution consists of a number of components. They are intermediates or oxidizing dyes, oxidizing agents, alkali, sodium chloride and
wetting agents. To obtain a good color, several intermediate products are
usually used simultaneously. One of the components is used as the main dye.
Other components are as tint substances.
Aromatic amines (n-phenylenediamine; n-aminophenol; n-aminophenol) are widely used in oxidation dyeing. Along with amines and aminophenols, hydroxy derivatives, phenols and naphthols (catechol, resorcinol, 1.5dihydroxynaphthalene) are also used. When these intermediate products are
oxidized with aromatic amines, intensely colored compounds are formed.
Black dye for fur D (Ursol D) is used as the main component to dye
black. According to the chemical composition, p-phenylenediamine NH
C
–NH2. Pyrocatechol, 1.5-aminonaphthol and 2.7-dioxynaphthalene are
6H4
–
2
used as additional oxidizing agents to obtain black color for chromium peak
mordant. A combination of gray DA (Ursol SLA) and yellow H (Ursol 4G)
are used for chromium-pico-copper mordant.
For brown dyeing, the main component is also black for fur D in combination with resorcinol, pyrogallic acid, m-aminophenol or brown for fur A
(Ursol P-base). The latter is n-aminophenol according to its chemical formula HO–C
6H4
–NH2.
To obtain light brown and beige tones, combinations of dyes are used:
brown A, yellow H or yellow A (Ursol GG). For gray tones, along with basic
black D, pyrocatechol (1.2 dioxybenzene, Rhodol C), gray A (Ursol gray B),
gray D (Ursol AL) are used. When dyeing in brown, gray and yellow colors,
the consumption of dyes is lower.
As noted earlier, the formation of colored compounds in the hair structure occurs under the action of oxidants. For this purpose, hydrogen peroxide
is introduced into the working solution. The amount of oxidizing agent is
proportional to the amount of intermediates. It is assumed that 1 cm
35 % hydrogen peroxide is used for 1 g/dm
3
of the dye. Some oxidation dyes
3
/dm3 of
33
contain salts such as sodium chloride, sodium sulfate. Hydrogen peroxide
creates a mild oxidation regime in a neutral or nearly neutral environment.
Therefore, ammonia is added to the solution, which creates optimal conditions for the oxidative effect of peroxide and regulates the oxidizing ability
of dichromate. Oxidation is the slowest and most uniform at pH 8–8.5. When
dyeing with mordants, the acidity of the dyeing bath, the pickling bath and
the skin is taken into account.
The dyeing temperature is maintained within the range of 30–35 ºС. At
a lower temperature, the dyeing process slows down. A higher temperature
leads to a change in tone towards redness and the formation of oxidation
products not in the fur hair, but in the solution, followed by precipitation.
The float ratio is controlled to create free washing of hides. An increase
in the float rate leads to an increase in the absolute amount of the dye absorbed by the skin. However, it reduces the solution consumption. The float
rate is determined by the type of fur and equipment. It ranges from 25 to 3 at
intense mechanical mixing.
Oxidized intermediates have a low molecular weight. They easily dissolve in hot water. Therefore, they are able to penetrate into the structure of
keratin fiber, which is inaccessible for dyes of other types under these conditions. The common property of these intermediates is their extremely easy
oxidizability. In the process of oxidation, colored compounds of large molecular weight are formed, which do not dissolve in water, alcohol and other
solvents. The color intensity of the guard hair during dyeing is higher than
the color intensity of the down hair, which is very important for valuable
types of fur.
Oxidation dyeing provides colored fur products with shine. An increase
in the intensity of the color leads to an increase in the gloss of fur hair due to
"polishing" effect, which results from the drumming processing of the colored fur skins with sawdust. The insoluble pigments, formed in the hair structure after oxidation, are polished by sawdust, which increases the hair gloss.
Russian typical methods for processing valuable types of furs include
multicomponent systems for dyeing hair in natural colors. For example, compositions consisting of five different intermediates are used to dye mink skins
brown. Special control is required to prepare the mixture, since the smallest
deviation in the dosage of any component can cause a change in a color tone.
In this regard, mixed oxidation dyes are of great interest for practical use.
Their application allows us to achieve good reproducibility of the color tone
from batch to batch, which is one of the most important factors in oxidation
dyeing.
34
Oxidation dyes have the ability to sublimate and oxidize spontaneously
even at ordinary temperatures. Therefore, during long-term storage, they may
lose their original properties, which affects the results of dyeing. To solve
this problem, oxidizable intermediates, in particular diamines, are produced
in the form of sulfuric acid salts. Any diamines can be obtained in the form
of sulfates. Sulfates of aromatic amines do not sublimate during the storage.
They do not have a strong odor compared to the original amines. They are
less dusty, due to which their harmful effect on the fur semi-finished products
is reduced and their shelf life is increased. To convert sulfates to soluble
amines, a certain amount of ammonia is required to reach a pH of at least 7.5.
Oxidation dyeing methods
There are two methods for oxidation dyeing:
• dip dyeing;
• brush dyeing
.
Moreover, each of these methods has its own varieties.
Dip dyeing. Dip dyeing is devided into single-bath (one-stage), two-bat
h
(peroxide-free) and combined (peroxide) dyeing.
Single-bath dyeing is carried out in a solution containing dyes, ammonia
and hydrogen peroxide, without prior etching. Insoluble sediment is formed
intensively in the dye solution resulting in the increased crocking of the fur
hair. Hydrogen peroxide oxidizes faster than intermidiates. This leads to its
high consumption. Weak binding of the dye with keratin leads to a high
crocking of the fur and low lightfastness of the dye. Therefore, the singlebath dyeing method is not widely used.
Two-bath dyeing is carried out in two stages. The first stage is etching.
The second stage is dyeing with oxidation dyes without peroxide. However,
due to an insufficient amount of an oxidizing agent, many non-oxidized intermediates remain in the hair and on its surface. It leads to a high consumption of dichromate and intermediates, which makes the method uneconomical. Therefore, it is used only in combination with discharge dyeing.
At the same time, a protective solution is applied to the ends of the hair before etching. It protects hair from coloring. The method is used for dyeing
white sheepskins, lamb, white hare and rabbit fur gray and brown colors with
white hair tips.
Nowadays, combined dyeing is widely used in Russian companies. It is
used for etched furs with the addition of hydrogen peroxide. Ammonia is
used in this method to create an optimal pH value to oxidate the intermediates. After that, the solution of dyes and skins is loaded. Five-fold water
35
hydrogen peroxide solute is added after 30 minutes of skin loading to provide
the diffusion of the unoxidized intermediate into the etched hair. The combined method of dyeing allows us to regulate the formation of oxidation
products in the hair and combine etching and dyeing. The main advantage of
the method is the reduction of production cycle and water consumption.
Brush dyeing. There are different types of brush dyeing.
Dyeing by punching is used to preserve the natural color of leather or to
level the color of spotted natural fur. The dye solution is applied to the entire
area and depth of the hair by punching with a stiff brush. In this case, the consumption of dyes as 3–4 times higher as for dip dyeing. Typically,
the method is used for dyeing plucked skins of fur seals, as well as an otter,
desman and muskrat in black for imitating seal fur.
There is a method for dyeing only hair of the hide. It is used in combi-
nation with dip dyeing to imitate valuable types of furs. The dyeing solution
is applied with a paint sprayer or a soft brush or goose feathers from the right
wing (in a bundle of 5–12 feathers). It is very complicated operation, because
it requires a gradual transition from the dark color of the upper part of the hair
to the light color of the lower part. The method is used for dyeing brown
color.
Airbrush dyeing is carried out after combined dip dyeing and finely
sprayed dyeing using a paint spray gun. We can create various imitations,
fantasy color drawings and smooth transitions of tones on the surface, as well
as obtain various coloristic effects. The dyeing solution is applied 3–4 times,
after which it is dried. Then, the skin is drummed in a bath with sawdust and
trimmed.
Printing dyeing method consists in applying one or multicolor patterns
to the fur hair using various stencils. The method of photographic film printing is also used. The stencil can be a thin metal sheet with a cut-out pattern
or a wooden cliche with a convex pattern for skins with a short hair, as well
as a stencil grid with a pre-applied photographic emulsion.
Retanning. Retanning is used to prepare a semi-finished product for ox-
idation dyeing. In the process of dyeing, hydrogen peroxide is absorbed by
both the hair and the skin. At the same time, it destructs bonds between
the functional groups of the protein and chromium, and the leather is detanned. This results in a loss of its strength and deterioration of its plastic
properties after oxidation dyeing. To solve this problem, other tanning agents
(formaldehyde, compounds based on polyaldehydes), which are more firmly
fixed in the collagen structure, should be used instead of chromium compounds.
36
Killing. The quality of fur hair color is determined by the degree of its
activity to dyes. The outer layer of the hair, called cuticle, is covered with
a thin protective coat that makes it hydrophobic and chemically inert. Before
dyeing, it is necessary to increase the wettability of the hair. To increase
the wettability, a semi-finished product is processed in solutions of alkaline
reagents in the presence of surfactants. This treatment is called starvation
(neutralization).
The free acid in the hair is neutralized by the action of ammonia or so-
dium carbonate. Surfactants are added to the solution to remove fat and mechanical impurities from the hair surface. The process of killing is very important for fur skins, which are characterized by significant differences in
the structure of guard and down hair (mink skins, rabbit skins, etc.). To
equalize the susceptibility to dyes of different types of hair, the density of the
cuticle of the guard hair should be slightly reduced. This is achieved by a
more intense mode of the starvation process. The simultaneous use of ammonia and sodium carbonate with a concentration of 2 to 5 g/dm
3
is required.
The choice of surfactants plays an important role at this stage of pro-
cessing. Typically, nonionic surfactants with a concentration of 1 g/dm
3
are
used. The starvation parameters are also important because any alkaline
treatment has a destructive effect on keratins, as a result of which the chemical activity of the hair increases and its coloring is facilitated. However,
the elasticity of the hair decreases and it tends to felting. When processing
long-haired species of furs, especially the skins of arctic fox, special substances are added into the solution to prevent felting.
Taking into account the destructive effect of alkaline reagents on the
hair, the temperature and duration control is an essential element of the starvation process. An increase in temperature above 32 ºС may be accompanied
by irreversible changes in the hair structure, which causes a significant loss
of shine, elasticity and strength. The duration of the process is 1–2 hours.
Then a thorough washing of the skins with warm water is carried out. It is
unacceptable to roll the skins after the process of killing, because after
the water runoff, the concentration of residual alkali on the fur increases,
which may damage fur products.
Etching. An etching process is the treatment of fur skins in a solution of
heavy metal salts. It is performed before oxidation dyeing. Typically, sodium
or potassium dichromates are used for this purpose. The etching process aims
at increasing the affinity of the hair to oxidation dyes, increasing the intensity, light-, moisture-, and friction-resistance, and obtaining an in-depth hair
color tone. In this case, the fur skins are retanned during the pickling process,
37
since chromium (VI) is reduced to chromium (III) during the dyeing process.
At the same time, the skin density, heat and water resistance increase.
Many oxidation dyes are used to color fur products without preetching.
However, the light-resistance of the color in this case may be reduced.
The operation is carried out using the dip method of dyeing. Copper (II) sulfate and iron (II) sulfate are used as etching agents in addition to bichromate.
These catalysts are used to decompose hydrogen peroxide, which is one of
the components of the dyeing solution. Copper sulfate is used for pickling
rabbit skins. In the process of oxidation dyeing of fur after copper etching,
the oxidation products are bound with a larger number of chemical groups of
keratin than in the case of etching with chromium salts. This allows us to
obtain a light-resistant color. It should be noted that the color of the fur hair
changes depending on the type of mordant. This is especially noticeable for
copper and chrome mordants, when the color is more saturated.
The absorption of copper and the strength of its bond with fur proteins
depend on the pH of the mordant solution. The chemical bond of copper ions
with hair in an alkaline environment is higher than in an acidic one. To create
an alokiline environment, ammonia is added to the solution. An increase in
the concentration of copper sulfate in the mordant solution and liquid crystal
leads to its absorption by both hair and skin, which causes an increase in the
color intensity. To improve the wettability of the fur semi-finished product,
a surfactant is added to the mordant solution.
The addition of dichromates into the acid layer in the process of etching
leads to the formation of dichromic acid. It interacts with hair keratin according to the mechanism of salt formation. This process is more intensive at a
temperature of 30 ºС and pH 4.5–5.0. Therefore, some formic acid is added
to the pickling bath. It is recommended to use additives to maintain the required pH value.
During the process of dyeing, dichromate is not the only oxidizing
agent. Hydrogen peroxide plays the main role in the oxidation of intermediate products. It is added directly to the dyeing solution. In this regard, and
also taking into account the toxicity of Cr6+ compounds, their consumption
during the etching process should be optimal and correspond to the required
color intensity. For dyeing black, it is 3.0 g/dm
2.5 g/dm
3
. It may be less than 2.0 g/dm3 when we need to obtain light tones.
3
. For dyeing brown, it is 2.0–
When processing valuable types of fur, the duration of the pickling is at
least 8 hours. It is recommended to leave the skins in the solution overnight.
Before dyeing, a thorough wringing of the semi-finished product is required
to remove excess dichromate from its surface. Otherwise, in the initial stage
38
of dyeing, intermediate products may be oxidized intensively in the outer
layers of the hair. The resulting sediment of oxidation products will prevent
the diffusion of dyes into deeper layers of the fiber, and the color will be
superficial, uneven and less durable.
There are many regularities for the process of interaction of chromium
salts with fur skins:
1. Hair absorbs bichromate about twice as much as skin.
2. Skins after the killing process absorb less bichromate than skins
which are not treated in an alkaline solution.
3. The process of absorption of dichromic acid occurs during the first
hour of etching. After that, the concentrations of acid in the hair
and
the solution are in equilibrium.
4. An increase in the concentration of dichromate in the solution leads
to an increase in its absorption by the fur. However, the relative use
of dichromate decreases.
5. The process does not depend on the temperature. A slight increase in
binding at temperatures above 30 ºС is explained by the regenerati
ve
effect of hair keratin. Chromium (VI) is reduced to chromium (III).
In this case, compounds of mixed valence are formed in the hair.
They are orange-brown. This results in coloring of ha
ir.
6. An increase in the float ratio leads to an increase in the absorption of
dichromate. However, the percentage of its absorption from the initial content decreases, which leads to incomplete use of etchi
ng
baths.
7. An increase in the color intensity is observed with an increase in the
dichromate content in the hair to 1% of the hair weight. A furthe
increase in the concentration of dichromate leads to a decrease in
the color intensity, and then to the appearance of a violet and red tint.
This phenomenon is called overchroming. During this process,
the oxidant (bichromic acid) in the hair interacts with the intermediate reducing agent in the solution surrounding the hair. This leads t
o
the formation of an insoluble dye precipitate, which dyes hair. At
a high concentration of dichromate, a dense sedimentary layer is
formed in the peripheral layers of the hair due to the rapid and vigorous oxidation process. This process prevents further diffusion of
the dye into the deep layers of the hair.
The process of overchroming may be avoided by reducing the concentration of dichromate and acid. The addition of ammonia to the etching solution increases the pH of the bath and delays the rapid absorption of
r
39
dichromate at the beginning of the treatment. Thereby, it provides smooth
etching process and uniform color during dyeing. The introduction of a reducing agent (sodium hyposulfite) into the solution before dyeing eliminates
the excess of dichromate.
Bleaching. Bleaching of furs aims at:
• destructing yellow shades of white fur hair (ermine, polar bear, polar
fox);
• eliminating the existing natural color of fur in order to obtain various
color imitations.
Thus, during bleaching, the natural color of the hair is partially or completely discolored to prepare it for dyeing in various colors or to imitate valuable furs. In addition, the whiteness of unpigmented hair also increases.
Bleaching is achieved by the destruction of hair pigments (melanins), which
are products of tyrosine oxidation.
The process is performed with the use of either oxidizing agents or reducing agents. Typically, hydrogen peroxide is used as an oxidant. Potassium
permanganate, sodium peroxide, chromic peak, persulfates may also be used.
Reducing agents are sodium sulfite, bisulfite, oxalic acid, sulfur dioxide, and
sodium hydrosulfite. The process is carried out in oxidizing solutions, mainly
in a solution of hydrogen peroxide. In the process of catalytic bleaching, as
a result of pretreatment with iron (II) salts, hydrogen peroxide decomposes
with the release of atomic oxygen, which degrades melanins. Collagen and
keratin are also affected. Therefore, stabilizers and protective agents are
added to the oxidizing solutions to form a hydrophobic film on the hair. They
slow down the decomposition of hydrogen peroxide. They are formaldehyde,
lignosulfonic acid and its derivatives, such as Protectol W, etc. Before
bleaching, fur is treated in an alkaline solution of formaldehyde in the presence of sodium carbonate, and then in a solution of urea.
Dyeing. Aromatic oxidizable intermediates, having a low molecular
weight, are able to diffuse into the hair structure even at low temperatures.
Typically, oxidation dyeing is carried out at a temperature of 34–35 ºС,
which is an important advantage of the method. The required color tone is
achieved by selecting the ratio of various intermediate products or by using
mixed oxidation dyes. The process of oxidation of intermediates into insoluble colored compounds occurs under the influence of two oxidants: potassium or sodium dichromate and hydrogen peroxide. Potassium or sodium dichromate is introduced into the hair structure at the etching stage, and hydrogen peroxide is added to the dyeing solution. Oxidation is carried out in
40
a slightly alkaline medium at pH 8.0–8.5. The required pH value is created
by introducing a small amount of ammonia.
The best dyeing results are achieved if hydrogen peroxide is dosed into
the dyeing solution gradually, since the intensity of oxidation of intermediate
products in the solution and on the surface depends on the concentration of
hydrogen peroxide. At a high concentration of Н
, the formation of insol-
2О2
uble colored compounds will prevail in the solution and on the surface of
the fiber, but not in its depth. As a result, dyed hair will have increased soiling. The gradual introduction of hydrogen peroxide into the solution creates
favorable conditions for the diffusion of intermediates deeper into the fiber
and their gradual oxidation in the internal structure of the hair. This allows
us to use fully all components of the dyeing system, including both oxidants.
Insoluble colored products are retained on the hair mainly by hydrogen
bonds or by adsorption forces. The higher the molecular weight of the oxidation products is, the greater the adsorption forses are. Therefore, the complete oxidation of the starting products results in a strong and rich color. This
factor determines the process duration, which is usually 5–8 hours. When
dyeing black, the duration of the process is maximum.
Washing. A semi-finished product after the dyeing process has residues
of unfixed dyes on the surface, which reduce its quality as well as some finishing operations of hair processing become difficult. Some of the non-oxidized and intermediate oxidation products may cause eczematic diseases.
Therefore, the processes of washing of hides are required. Their parameters
depend on the type of raw materials and previous operations. Semi-finished
products of black color are washed twice in the presence of detergents (usually non-ionic surfactants) and once with clean water. Semi-finished products
of brown color are washed first with surfactants, then with pure water.
The indamine reaction is used to define the end of the washing process.
The indamine reaction is a characteristic reaction which is very sensitive to
even small amounts of p-phenylenediamine. When a few drops of hydrochloric acid aniline and ferric chloride are poured into the water extract of the hair
after dyeing, the appearance of a blue-green or green color indicates the presence of an unoxidized black dye for fur products. In this case, additional oxidative treatment is carried out in a solution containing hydrogen peroxide
and ammonia (approximately 1 g/dm
3
), followed by washing with water.
Pickling. In the process of dyeing, various compounds, introduced into
it during the previous stages, are washed out of the fur skin to get the semifinished product with required properties. This leads mainly to the loss of
the plastic properties of fur skins. Therefore, the fur is to be treated after
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