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Файл:Fur Processing Technologies. Тutorial
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41
washing. The purpose of this treatment is to restore the substances washed
out from the skin and preserve its softness and plasticity. This process is
called pickling. Sodium chloride is used to preserve the hygroscopic properties of a hide. It absorbs air moisture and creates conditions for more uniform
subsequent drying. The concentration of sodium chloride is usually no more
than 30–40 g/dm3, since its higher content leads to an increase in the weight
of semi-finished fur products. During the dyeing process, glycerin is added
to sodium chloride. It increses both hygroscopicity and gloss of the hair.
Acetic acid with a concentration of 5–10 g/dm3 is used to compensate for
the acid washed out from the fur skin during dyeing. It is not recommended to
use stronger acids, since a significant decrease in pH causes the appearance of
red shades, especially after dyeing in light and medium tones.
To prevent the hair tangling and increase the wear resistance of rabbit
skins and fur sheepskins, antistatic agents (3–5 g/dm3) are added to the salt
solution. Typically, they are polyethylene glycol esters of stearic acid – stearoxes. To increase the density and softness of the fur skin, aluminum and
chrome alums are used for the process of pickling. To compensate for
washed-out fats, salt is often combined with fat-liquoring. This process uses
fat emulsions, mainly of synthetic origin, since they are better retained by
the skin and do not saline out on the surface of fur products.
Acid dyeing. Acid dyes are one of the first independent classes of synthetic dyes. Currently, their production accounts for more than 10 % of
the total world amount. Their numerous brands take the leading positions in
the world.
Acid dyes are widely used to color protein (wool, natural silk) and polyamide fibers, and textile materials based on them. They are used also in
the fur industry, displacing oxidation dyes. If earlier acid dyes were used
mainly for dyeing fur sheepskins and astrakhan fur, now they are widely used
for dyeing various types of fur. Environmentally friendly acid dyes allow us
to produce fur products with strong, bright, rich colors of a wide range of
shades.
A wide range of acid dyes includes compounds with different chemical
structures. A common feature of their structure is that they contain one or
more (usually no more than three) sulfogroups. They are sulfo derivatives of
azo, anthraquinone and other dyes. Chemical industry produces acid dyes in
the form of sodium salts of aromatic sulfonic acids with the general formula
КрSO3Na.
42
Russian companies produce dyes of this group under the name “acid”
dyes; Germany – under the names Lurazol, Bazolan (BASF), Alizarine
(Lowenstein), Dorapel S (Pulkra (Boehme)), Sellazid (TFL).
Acid dyes dissolve in water. Their dissociation leads to the formation of
colored anions [КрSO
]. They refer to anionic dyes. The name "acid" dyes
3
of this group came from their fixation with protein fibers, which takes place
in an acid environment, when the protein (collagen, keratin) has an excess
positive charge. Under these conditions, the dye anions, penetrating into
the fiber, interact with the positively charged amino groups of the protein and
displace the acid anions associated with them (SO
2-
, Cl-, etc.). As a result,
4
an ionic bond is formed between the dye and the fiber:
+
-
N
H3Cl
Б < + [KpSO
]-Na+ → Б< +NaCl
3
N+H
-
[O3SKp]
3
COOH COOH
This exchange of ions and the formation of ionic bonds are possible because the dye anions are more active with the protein in comparison with the
acid anions. It may be explained by the fact that the ionic bond between dye
anions is supported and enhanced by the formation of intermolecular hydrogen
bonds, caused by van der Waals forces, and hydrophobic interaction.
Dye anions of different brands differ in structure and form bonds of different strengths with the protein. In this case, the size of the dye molecule,
the number and position of sulfogroups, the presence of hydroxy and amino
groups, and alkyl residues are of great importance. The lower the ability of
the formed bonds to dissociate and the greater the affinity of the dye anion for
the protein fiber, the stronger the color in liquid solutions. However, the faster
the dye anion binds to the fiber, the more difficult to get an even color. Thus,
there is a close relationship between the structure of the dye and processing of
fur products in liquid solutions.
Although acid dyes are bound by a protein fiber using different intermolecular bonds, the ionic bond is one of the most important. This bond determines the dependence of the sorption of the dye by the fiber on the pH of
the medium. Acid dyes are absorbed only in an acid environment.
It may seem that hides are colored only in strong acids (pH 2–3), since
they provides the maximum degree and rate of sorption of the dye. However,
this is possible only for the dyes with the simplest structure, which are characterized by the presence of at least two sulfogroups in the molecule, low molecular weight and low affinity for the protein. Such dyes are slowly sorbed from
43
the solution and penetrate to the hair and skin forming uniform color. However, they are not stable enough in liquid solutions. Dyes with a higher molecular weight and the highest affinity for a protein in a strong acid (pH 2–3) are
quickly absorbed from the solution and fixed on the fiber. However, they do
not penetrate inside the fiber completely and color only the surface of the fiber,
while unevenly. Moreover, the higher the molecular weight and affinity of
the dye for the protein is, the stronger their effect is.
In practice, the process of dyeing is carried out at a higher pH value of
the system in order to reduce the binding of the dye in the outer layers of
the hair and skin and ensure its deep penetration. Special additional operations
are required to prepare the hides for this process. For example, the process of
neutralization is used. In addition, dyeing solutions with lower acidity are
added to the solution due to a decrease in the consumption of sulfuric acid or
its replacement with organic acid. With a decrease in the active acidity of
the system, the number of positively charged centers in the protein structure
decreases. This leads to a decrease in the rate of sorption of the dye and its
deeper penetration into the hair and skin.
The dyes with a higher molecular weight and a higher affinity for protein
fiber require a higher pH value for dyeing. They are widely used for dyeing
sheepskins, fur velor and other types of semi-finished fur products, since they
provide wear-resistant colors. Typically, they start dyeing even in a weakly
alkaline medium, which creates conditions for a deeper penetration of the dye
into the inner layers of the skin. At the end of the process, an organic acid is
introduced into the system to enhance the fixation of the dye. Generally, acid
dyes are designed specifically for this purpose, as well as certain brands of acid
dyes for leather (for example, anionic dyes) are produced for dyeing hides of
sheepskins and fur velor.
For hair dyeing, acid dyes of simple structure are used. They have a lower
molecular weight and an average affinity for hair and association. Typically,
they refer to monoazo dyes containing one sulfogroup in the molecule.
The fewer hydrophilic groups in the acid dye molecule are, the better they diffuse into the hair. The position of these groups is of great importance for
the process of dyeing.
If the process parameters and components are chosen according to the requirements of dyeing, the dyes penetrate deeply into the hair. The color is even
and resistant to various physical and chemical effects.
The color range of acid dyes is diverse. Acidic dye mixtures are widely
used for dyeing in fancy colors. A correctly selected mixture of dyes behaves
like a new dye with a corresponding set of properties. The final color is
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a result of mixing of diferent components. Thus, to obtain a variety of different colors and shades, a relatively small assortment of dyes is required.
To determine the composition and formula of dye mixtures providing a special color, instrumental colorimetric methods are used. These methods allow
us to create databanks and store them in computers. Such approach is used
to optimize the selection of suitable mixtures of dyes and predict the color
and quality of the dye.
To obtain stable and good results of coloring fur skins using acid dyes,
especially mixed dyes, along with the selection of dyes, it is very important
to choose the appropriate parameters of the dyeing process and ensure their
control. However, in practice, this process may be complicated by the need
to color either skin or hair. Sometimes it is necessary to dye both hair and
skin in the same color or in different colors. These modes require appropriate
conditions of the process. The substrate preparation for dyeing, temperature
regime, acidity of the dye solution, and the use of effective auxiliary substances are important factors of the process. Based on these factors, an engineer can regulate the dyeing process and control it.
The speed of acid dyeing of fur hair, the intensity and uniformity of
the color, its reproducibility, and the result of dyeing depend on the temperature of a dye solution. An increase in temperature leads to an increase in
the dyeing rate due to a decrease in the association degree of the dye and an
increase in the rate of its diffusion into the hair due to an increase in the kinetic energy of molecules (ions) and swelling of the hair, leading to an increase in the pore size. At the same time, an increase in the processing time
leads to an increase in the amount of dye associated with the hair.
In practice, acid dyeing of fur hair is most often carried out at a temperature of 60–65 ºС. A higher temperature accelerates the process and results
in obtaining intense, bright colors of the hair. However, the plastic properties
of the skin deteriorate. The skin becomes harder and its area decreases. At elevated temperatures, the dye may only penetrate into the surface layer of
the skin at the hair-solution interface.
To obtain a well reproducible and even color, the temperature regime is
to be controlled. The temperature control is the most important for the dyeing
process based on mixtures of dyes. Even small deviations from the optimum
temperature may cause color changes and variations. Therefore, dyeing
equipment is to be provided with automatic temperature control sensors. According to the experience of industrial companies, the dyeing temperature
may be reduced by increasing the duration of the process and the dosage of
the corresponding auxiliary substances. When the dyeing temperature is
45
10 ºС lower than the optimum value, the duration of the process is doubled.
If you do not follow this rule, the color of the hair will be faint, insufficiently
light-resistant, and uneven along the length of the hair.
Color uniformity is one of the most important indicators of dyeing quality. Fur raw materials differ in their structure and properties of fur hair. Guard
and intermediate hairs are less susceptible to dyes than down hair.
The greater the differences in hair structure, the more difficult it is to achieve
uniform color. At the same time, the color uniformity also depends on the nature of the dyes, as well as on the degree of their affinity for the dye.
Poorly levelling dyes have a high affinity for the hair due to their hydrophobicity and their resulting colors are surface and uneven. However,
they are distinguished by high strength. To increase the color uniformity,
auxiliary substances are used to regulate the affinity of the dye for the fiber
at the initial stage of the process. The simplest way to reduce the affinity of
the dye is to add neutral electrolytes (sodium sulfate or sodium chloride) to
the solution. The disadvantage of acid dyes is poor levelling of color. In practice, leveling agents are added to the dyeing solution. There are different substances such as wetting agents and emulsifiers, which affect the properties of
hair keratin and dye solutions.
Surfactants are the most effective leveling agents. The choice of surfactants depends on the dye nature. For poorly levelling dyes, surfactants which
have a dispersing effect on dyes and reduce the affinity of dyes to the fiber,
are used. Nonionic surfactants form a weak hydrogen bond with acid dyes in
the solution. Due to the formation of associates of the dye and surfactant,
the affinity of the dye for the hair decreases. The more hydrophobic the dye
is, the more intensive the process of its solubilization by the surfactant is and
it is less sorbed by the hair. The associates are decomposed gradually, which
leads to the uniform sorption of the dye.
During the dyeing process, the smallest disaggregated dye particles penetrate into the hair due to their thermal movement. As they are removed from
the dyebath, the larger aggregates disintegrate and replenish the concentration of smaller particles. It was found that the introduction of a nonionic surfactant into the dyebath leads to a 3.2-fold increase in the sorption of the dye
compared to dyeing in a pure dye solution. The introduction of a nonionic
surfactant reduces the aggregation of dye particles.
The introduction of cationic surfactants into the dye solution leads to
their interaction with both the dye and the hair. As a result of preliminary
alkaline washing, the hair acquires a negative charge. This leads to the interaction of cationic surfactants with acidic groups of keratin, which is
46
accompanied by the formation of positively charged groups on the surface of
the hair and increases the sorption of acidic dyes, especially those that have
a greater negative charge and a lower affinity for the material to be painted.
Anionic surfactants and anionic dyes are competitors in relation to
the main protein groups. The addition of anionic surfactants complicates
the sorption of acidic dyes, especially those with low affinity for the hair.
This is due to the electrostatic repulsion of like-charged groups.
Thus, the addition of nonionic or cationic surfactants in the process of
hair dyeing allows us to regulate the degree of affinity of dyes to the dyed
material and their selectivity from the solution. The use of surfactants as leveling agents is the most important for dyeing with dye mixtures. In this case,
they are used to equalize the degree of affinity for a fiber of individual dyes
of the mixture, which provides the most efficient use of all dye components
and stable reproducibility of the processed materials.
If the dyeing process is based on the mixtures of dyes, the color tone
may change depending on the type of leveler, because of the different degree
of change in the affinity of individual dyes to the fiber. According to the
nature of the leveler and colorant, this difference may be more or less observed.
The concentration of leveling agents in the solution has a significant effect
on the quality of the resulting color. Typically, their consumption is 0.5–
1.5 g/dm3. The amount of leveling agents is to be controlled to obtain the required color uniformity. The excessive concentration of these subsances may
reduce the color intensity and lead to material waste.
Special levelers have been developed for acid dyeing of fur hair:
• Alizarine dyes – Level A or Level P;
• Dorapel S dyes – Doregal P or Doregal PEW;
• Sellazid dyes – Inwaderm AL.
Thus, the dye diffusion into the hair thickness is intensified by varying
both the pH of a dye solution and the use of various auxiliary substances.
Dyeing operations
Retanning. The quality of hair dyeing is determined by the preliminary
processing of the semi-finished product. One of the most important preliminary operations is retanning. To preserve the plastic properties of the skin
and prevent shrinkage of the semi-finished product, all processes occuring in
liquid solutions should be carried out at a temperature of 25–30 ºС which is
lower than the shrinkage temperature of the skin. As mentioned earlier, hair
47
dyeing is performed at a temperature of 65 ºС. Therefore, a shrinkage temperature of the skin is to be at least 90 ºС. The increased thermal stability of
the skin is achieved by adding chromium compounds during the process of
retanning, since only these tanning agents are able to increase significantly
the thermal resistance of the skin without deteriorating its softness and plasticity. Chrome tanning agents can be used individually or in combination
with tanning agents.
The main goal of the retanning process is to create favorable conditions
for the deep penetration of the dye into the hair, its uniform localization and
strong binding of the dye to the hair. At the same time, the process aims at
increasing intensity, uniformity, high lightfastness, color fastness to wet and
dry friction, to the action of aqueous solutions of acids, and color reducing
agents, as well as increasing wear-resistance of fur products.
Neutralization. The neutralization process plays an important role.
The purpose of neutralization is to remove dirt and grease from the surface
of the hair and skin, neutralize excess acidity, increase wettability, and
change the structure of the hair.
Dyeing. The process of dyeing is performed for 2–3 hours. The initial
conditions of the process are created to provide the diffusion of dyes into
the hair, which is very important not only to improve the evenness of
the color, but also to increase its lightfastness and resistance to various effects. Dyeing begins at a pH of about 4.5.
For the first 15 minutes, the process is carried out in a solution of levelers, the concentration of which is set depending on the float ratio. The higher
the float ratio is, the less amount of levelling agents is required. Then a solution of dyes is added. The consumption of dyes depends on the type of raw
materials, the required color saturation, and the concentration of the active
substance in the dye. As a rule, to obtain intense coloring of sheepskins and
astrakhan fur, the concentration of dyes in the solution 3–4 times as less as
for dyeing furs. For getting dark colors, it is recommended to introduce dyes
into the solution in two stages with intermediate acidification.
Firm fixation of the dye on the fiber is ensured by adding formic acid at
the end of the process to decrease gradually the pH of the solution. Formic acid
is added in two doses with an interval of 30–60 minutes. The total consumption
of acid is 2–3 cm
3
/dm3. If all technological parameters are controlled, the de-
gree of dyebath depletion may be achieved at the level of 97–99 %.
After dyeing, the hides are washed in a solution containing a non-ionic
surfactant. Then, fat-liquoring is performed with pure water. This is followed
by finishing processing of the dyed semi-finished product.
48
1.6.4. Dyeing of fur velour
Until recently, only the skin of fur velor was dyed and curried. Nowadays,
the process of dyeing is used both for hair and skin of fur velour. There is onecolor hair and skin coloring or contrasting (bicolor) coloring. There are many
dyeing techniques that can be used to obtain a variety of colouristic effects on
fur velor. They are based on the general rules of dyeing of fur hides.
Skin collagen and hair keratin interact with dyes in different ways. This
allows us to create required finishing effects. As you know, hair has a more
dense structure in comparison with skin, so the diffusion of dyes into the hair
is more difficult. The larger the dye molecule is, the better the skin is colored
and the worse the hair is colored. The hair is more hydrophobic. Therefore,
it is better colored with dyes which are soluble in hydrophobic media. An increase in the content of hydrophilic groups leads to an increase in the solubility of the dye and the ability to color the skin.
Thus, the differences between keratin and collagen characterize the affinity to dyes and create special conditions for dyeing both hair and skin, and
either hair or skin.
Dyeing skin of fur velour. This method is used to color only skin without
shading or with weak shading of the fur hair.
The process of dyeing begins with neutralization. It is recommended to
neutralize velour with a mixture of formate and sodium bicarbonate in an
equal ratio of 2 g/dm
3
for two hours. Neutralization is followed by washing
with the addition of surfactants to prevent the absorption of dyes by the hair.
Next, the semi-finished product is washed with clean water and sent to
the stage of dyeing.
Dyeing begins with the treatment of sheepskins in a 25 % ammonia so-
lution (1.5 g/dm
3
) and an anionic leveler. After 30 minutes, the dyes are
added. For this process, Leather and Lowanil dyes (Lowenstein), Sel-laext,
Sellaflor (TFL), Doranil Fast P (Pulkra (Boehme)) may be used. These dyes
color the hair slightly. The recommended concentration of them in the solution when dyeing in colored tones is 2 g/dm
3
, when dyeing in black – 3 g/dm3.
Typically, the process of dyeing is combined with fatliquoring to produce soft leather and provide a good “writing” effect. At the final stage of
the process, an acid is introduced into the working solution for binding acid
dyes with the skin collagen.
After dyeing, washing is carried out again in order to remove the dye
from the hair. In addition, optical brighteners are used to make the hair whiter
and to obtain a pure tone of the hair.
49
Dyeing hair and skin of fur velour. This method is used to color the hair
and skin of fur velour in the same color. Dyeing may be performed in one
bath or in two baths. In the second case, hair and skin dyeing is carried out
in different baths. First, the hair is dyed according to the usual scheme based
on acid dyes and levelers. At the end of dyeing, hair is washed with clean
water, and then the process of skin dyeing is carried out.
To provide the purity of the color tone, it is important to create conditions under which only hair would be dyed in the first bath, and only skin in
the second one. Therefore, in the second bath, the process is carried out at
a low temperature: 32–37 ºС. In addition to the temperature factor, the affinity of the dye, as in the first bath, is regulated by the introduction of auxiliary
substances. Any dyes may be used for this method of dyeing (for example,
Leather, Low-anil, Lowapel, Sellaext, Doranil Fast P).
The dyeing process is combined with neutralization, which is carried
out in a sodium thiosulfate solution with a concentration of 3 g/dm
hour after, 1.5 g/dm
After 30 minutes, a solution of dyes is added, the concentration of which is
usually 2 g/dm
3
of ammonia and an equalizer are added to this solution.
3
. After 30 minutes, a fatliquoring material is added. After 4–
3
. One
5 hours from the beginning of the process, formic acid is dosed to lower the
pH and ensure the fixation of the dyes.
The two-bath method of dyeing is long, labor-consuming process. It requires a lot of water and chemicals. However, it allows us to get any color
tone and any color saturation, especially if you use a set of dyes.
The one-bath method is simpler, more economical, but its coloristic effects are poorer in comparison with the two-bath method. Dyeing of skin and
hair in one bath is carried out with special dyes (for example, Solow from
Lowenstein). The process includes neutralization, washing and dyeing.
To achieve an intense and uniform color of hair and skin, the temperature
and pH regulation and control are required. Dyeing is carried out at a temperature of 65 ºС. Lower temperatures will lead to a decrease in the saturation
of the hair color. The processing of a semi-finished product in a solution of
auxiliary substances is carried out for 30 minutes. Then dyes with a concentration of 2.0–2.5 g/dm
3
are added. After another 30 minutes, a fatty material
is added. One hour before the process ends, the pH of the solution decreases
as a result of the addition of formic acid.
Thus, the technology of fur processing requires the use of a great number of chemical materials. Each technological operation affects significantly
the quality of manufactured products.
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2 . CHEMICALS FOR FUR AND LEATHER
Chemical product
Application
Wetter НАС
It is a 100 % active, non-ionic wetting and soaking
agent. It is produced with the addition of special bactericides and fungicides. It is used to manufacture
soft leather, protect fur hair from bacteria, and prevent hair slipping.
Its concentration is 1 g/dm
3
during the 1st and 2nd
soaking operations.
De-Sol-А
It is a detergent for washing and degreasing furs and
fur sheepskins.
Its concentration is 1.5 g/dm
3
for the operations of
washing and degreasing.
Elbro 100-С
It is an enzyme, used for the operation of soaking. It
is used to produce soft leather.
Its concentration is 0.5 g/dm3.
PROCESSING
Chemical companies have been developing a huge number of different
chemicals for the processing of fur raw materials. Here we describe the main
chemical companies and their products developed for different operations of
fur and leather processing.
2 .1. Joseph H Lowenstein & Sons Inc Products
Joseph H. Lowenstein and Sons, Inc. is an Americal company, which
manufactures chemical products for fur and leather industries. The company
focuses on the production of organic dyes and pigments.
Table 2. Chemical products for soaking, washing and degreasing
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