Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Fur Processing Technologies. Тutorial
.pdf
21
The extraction and emulsification are the most widespread methods of
degreasing.
Using organic solvents, fatty substances are removed not only from
the hair, but also from the skin. The advantages of this method are:
• a high degree of degreasing;
• the absence of hair flow;
• a short process duration (several minutes).
There are some disadvantages of the method. Almost all solvents are
toxic, flammable and expensive. This method may lead to very deep degreasing of hides, which results in loss of shine, strength and elasticity of the hair.
Solvents do not remove non-fatty contaminants (blood, dirt, etc.). To remove
non-fatty contaminants, additional washing operations using aqueous surfactants are required.
The use of aqueous solutions of surfactants is less expensive, but the fat
is extracted from the hides to a lesser extent. Nevertheless, the quality of
emulsion degreasing is quite good. This method ensures the correct fur skin
processing, since aqueous solutions of surfactants remove effectively solid
contaminants that can not be removed during the washing process. Therefore,
the emulsion method is one of the most widespread.
Various surfactants are used to degrease fur skins. The surfactants are
chosen according to the following requirements. The surfactants must not
damage the hair and change its color. They are to form stable emulsions.
The use of the surfactants is to lead to glazed and soft hair of the hide.
The control of the degreasing process is carried out according to
the Lieberman reaction consisting in the interaction of acetic anhydride and
concentrated sulfuric acid. The reaction is to show a negative qualitative test
for fatty substances. A negative test indicates sufficient removal of fat from
the hair. The qualitative reaction is based on the blue-green coloration of
cholesterol or its esters.
The process of hair degreasing is one of the main operations of fur processing. After degreasing, the hair acquires shine and friability. Degreasing
is carried out until the content of fat is about 1.5–2.0 %. If the content of fat
is lower than 1.5–2.0 %, the physical and mechanical properties of the hair
are weakened that leads to the hair brittleness and fragility, as well as the hair
resistance to abrasion decreases.
22
1.6.2. Fur dressing
Pickling. Pickling is one of the most important processes in the techno-
logical cycle of manufacturing semi-finished fur products. The main change
in the fibrous structure of the dermis occurs mainly during processing with
an acid-salt solution (pickel). Separation of collagen bundles provides high
skin properties. The use of organic acids leads to the good plastic properties
of the semi-finished product, uniform blasting of all layers of the semi-finished product and a higher shrinkage temperature.
The interaction of skin proteins with acid leads to a chemical reaction
with the active groups of the protein. The absorption of acid by the proteins
of the hide causes an increase in its volume. To avoid swelling, sodium chloride is added to the pickle solution.
The process of pickling consists of:
• interaction of active collagen groups with acid;
• separation of fibers into smaller ones;
• dehydration and compaction of fur skin;
• partial preservation of proteins;
• removal of a part of interfiber proteins and carbohydrates that cement
structural elemen
ts.
The more the interfiber substances are removed, the softer and more
viscous the skin is.
The process of pickling reduces the resistance of collagen to high temperatures (the shrinkage temperature decreases). When processing with
pickle solutions, the structure of hair keratin, proteins of the Malpighian layer
of the epidermis and hair follicles does not change significantly. However,
the acidic effect increases the ability of wool to felting and changes the natural color of the hair. The connection between the hair and the skin is weakened during the operations before pickling. The connection becomes stronger
in an acid-salt solution.
Factors affecting the pickling process
1. Skin microstructure
Different topographic areas of the skin, which have different thicknesses
and densities, absorb different amounts of acid and salt. Loose areas absorb
acid faster and in greater quantities than the dense ones. Keratin absorbs
more acid than collagen. Keratin has more developed adsorption surface and
23
its saturation is achieved in a shorter time. Therefore, the process parameters
are chosen according to the state of both hair and skin.
2. Nature and concentration of pickle solution components
When pickling fur skins in solutions of strong mineral acids, dehydration prevails, that leads to an increase in softness and ductility of the skin.
The dispersing effect is observed only in dilute solutions of mineral acids.
Despite the reduced sorption of organic acids by collagen in comparison with
mineral acids, the leaching of carbohydrate components is more in the process as well as the dispersing of the acid takes place. This contributes to
a finer fiber separation of the fur skin. Consequently, this leads to an increase
in its elastic properties. Unlike mineral acids, organic acids do not cause
problems during subsequent chromium tanning, since their concentrated solutions (10-15 g/dm
3
) have a higher pH compared to the pH of mineral acid
solutions. The comparison of the effect of mineral acids with organic acids
on rabbit skins and especially furs proved the advantage of pickling with organic acids. It produces softer and more plastic hides with stronger fiber disruption and less hair defects. The disadvantage of organic pickling is long
duration of the process.
3. The amount of neutral salts
Sodium chloride is used to prevent the formation of swelling. An excessive increase in concentration of the salt makes the leather heavier and more
calloused.
4. Float ratio
The float ratio is controlled and maintained at a high value. Otherwise,
the hair of the hide may be felted.
5. Process duration
The duration of the pickling process is not to be stopped at the end o
acid sorption. It is to be set according to the depth and uniformity of acid
penetration into the entire thickness of the skin. The process is performed at
a constant elevated temperature (about 40 ºС) for its intensification. In addition, the process may be intencified by mechanical mixing and setting of optimal pickling parameters required for this type of raw material.
There are no exact indicators to establish the completion of a particular
operation of fur processsing. In practice, we visually determine the readiness
of the product. The completion of the pickling process is determined by
the appearance of a white stripe when the skin is squeezed as well as by skin
ductility, dehydration and roughness.
Ageing. Ageing is performed after the process of pickling. It is an important technological process used for:
f
24
• penetration of acid from the surface layers into the thickness of
the skin and hair, resulting in its uniform distribution in the semifinished product;
• additional binding of reagents with active groups of the protein;
• partial removal of moisture from the skin under the pressure of one
skin to another.
The duration of ageing after pickling is from 12 to 24 hours.
Tanning. Some operations of fur processing lead to the destruction of
the protein structure. They are pickling, softening or souring. The finished
fur product is easily torn in the process of wearing. In addition, the leather of
the finished product may be swelled under the influence of atmospheric
moisture. Skins after pickling have a relatively low shrinkage temperature.
They are unstable to the action of enzymes and various chemical reagents.
In this regard, it is difficult to carry out the subsequent processes of drying
and dyeing using heat and chemicals. To solve these problems the process of
tanning is carried out after pickling. The tanning process involves processing
the hides using various substances which are called tanning agents.
The chemical interaction of the tanning agent with the proteins of the skin
leads to an irreversible decrease in the reactivity of collagen. Thereby,
the process of tanning increases the water and heat resistance of the skin, and
its resistance to subsequent processing operations.
Tanning increases the resistance of collagen to the action of acids, alkalis and enzymes, as well as oxidants and reducing agents. It is very important
for fur skins, which are repeatedly subjected to various treatments (dyeing,
bleaching, enrichment of the hair, etc.). Keratin of the hair also interacts with
tanning agents. However, the amount of tanning compounds bound by
the hair is small compared to the skin.
Many inorganic and organic compounds have tanning properties. Inorganic tanning agents may be compounds of chromium, aluminum, iron, titanium, zirconium, etc. Organic tanning agents are tannides, aldehydes, amino
resins, high-unsaturated fats, etc. However, not all tanning agents can be used
in fur processing. The most suitable are salts of chromium (III) and chromium (VI). Complex salts of chromium (III) are tanning agents. Salts of
chromium (VI) are the initial products used for the preparation of tanning
compounds and pickling of skins before dyeing.
Tanning chromium compounds are multinuclear hydroxo complexes,
which are readily soluble in water and stable over a wide pH range. They
enter slower into substitution reactions with active collagen groups than other
25
mineral tanning agents. This allows us to control the tanning process over
time.
The following factors influence the stage of diffusion of a chrome tanning agent into the dermis:
• degree of separation and destruction of collagen structure;
• size of tanning particles;
• concentration of tanning agents and temperature of solutions;
• mechanical mixing.
The following factors influence the stage of binding of tanning chromium compounds with protein:
• basicity of the tanning solution;
• pH of solution;
• nature and composition of chromium complex;
• concentration of chromium compounds;
• temperature of tanning solution;
• presence of neutral salts;
• salts of organic acids (masking substances);
• ageing of tanning solution;
• duration of tanning;
• ageing of tanned semi-finished product.
All operations of chrome tanning are controlled (preparation of solutions, loading of tanning solutions, and tanning process).
With an increase in the temperature of the tanning solution, the rate of
interaction of chromium compounds and their binding with collagen increases, as well as the uniformity of chromium distribution over the layers.
However, a significant increase in temperature leads to a decrease in plastic
properties of a fur semi-finished product. Therefore, an increase in a temperature above 45 ºС is not recommended.
Nowadays, chrome complexes are most widely used for tanning in combination with other types of tanning agents. Chrome tanning allows us to
produce semi-finished products of high quality. Moreover, chrome tanning
has fewer disadvantages compared to other types of tanning.
However, there are some disadvantages of chrome tanning. One of them
is high water consumption associated with the concentration limitations of
working solutions and the toxicity of chromium compounds. Environmental
requirements for leather and fur products, as well as for the treatment or disposal of industrial wastewater, make this problem one of the most important.
26
According to sanitary and environmental standards of some countries,
chromium compounds must not be presented in products or industrial effluents, or their content must be minimized. Therefore, research projects aimed
at developing chromium-free or chromium-saving technologies are of great
importance.
One of the tanning methods is tanning in organic solvents. The method
is used to intensify the process, reduce water consumption and almost eliminate the formation of waste water, while increasing the quality of hides.
The use of organic solvents in the tanning process is being developed mainly
in two directions. The most common direction is based on the use of polar
and non-polar solvents (for example, water and perchlorethylene). A nonpolar solvent acts as a medium and is introduced into the solution to provide
the required float ratio. Tanning is carried out in the presence of highly concentrated aqueous solution of a tanning agent. The system is used in the form
of an emulsion. A nonionic surfactant is used to form this emulsion.
The emulsion diffuses more intensively into the thickness of the skin, promotes its dehydration, creates porosity and increases the chemical activity of
various collagen zones. The penetration of an organic solvent into the structure of the dermis prevents adhesion of its individual elements and reduces
shrinkage during the drying process.
The second direction is the use of organic solvents as a medium in which
a tanning agent is completely dissolved. In this method, polar and non-polar
solvents or mixtures of a polar solvent and water can be used. The most
promising solvents are chlorinated and fluorinated hydrocarbons, since they
are non-combustible, non-explosive, have low heat capacity and heat of vaporization, which allows them to be regenerated at a low heat consumption.
Several methods of tanning based on organic solvents are widely used
abroad. The most promising methods are Coratill and Sekotan methods.
Coratill method. It is based on the use of non-polar hydrophobic solvents. The skins, depending on the type and method of their conservation,
are subjected to soaking, washing and fleshing, then squeezed to a moisture
content of 30–60 % and processed in the Bovet apparatus in an organic solvent. Chemicals required for pickling and tanning are applied in the form of
concentrated solutions or without prior dissolution in water. Thereafter,
the solvent is pumped into an intermediate recovery tank. The skins remaining in the apparatus are wrung out and dried. By the time of unloading,
the hair of the hide is dry.
Sekotan method. It is carried out both in an organic solvent and in an
aqueous medium. The process of tanning is carried out in an aqueous
27
medium, and the solvent is used to dry the semi-finished product. Acetone
or alcohol is used as a solvent. The disadvantages of this method are the high
consumption of solvents, their flammability and explosion hazard.
Nowadays, there are many research projects aimed at developing
the synthesis of new tanning materials capable of dissolving in an organic
medium and creating high-performance equipment for this method.
According to the environmental requirements, a great attention is paid
to the partial or complete elimination of chromium from tanning processes.
For this reason, many international companies are trying to improve the tanning with the use of aluminum salts and various organic tanning agents.
Aluminum compounds allow us to produce hides with white skin, dense
thin pile and high quality of final products. However, the hydrothermal stability of products is low. The combined using of aluminum tanning and oxymethylated aldehyde tanning or tanning based on BASF polymers increases
the shrinkage temperature of the skin. After this operation, the retanning of
the skin with chromium compounds is not required. After tanning with aluminum compounds using citric acid and glutaraldehyde, the shrinkage temperature of the skin reaches 86 ºС. If subsequent dyeing of the semi-finished
product at high temperatures is required, it is retanned with a small amount
of chromium compounds.
The Department of Plasma Technology and Nanotechnology of High
Molecular Weight Materials at KNRTU conducts research in the field of partial or complete replacement of toxic chromium compounds with environmentally friendly tanning materials. Thus, chromium-saving technologies
have been developed for obtaining a fur semi-finished product using nonisocyanate urethanes. The research is based on low-temperature and lowpressure plasma treatment of materials using ammonium sulfate titanylate.
Lying. The process of lying is carried out after the tanning process for
additional diffusion of chromium salts into the inner layers of the skin and
their stronger binding to collagen. The shrinkage temperature of the skin in
the process of lying, especially after drying, rises. According to the recommendations of the Standard, the duration of lying is 60–72 hours. After
the use of modern chemical reagents increasing the intensity of liquid processes, the recommended value of lying after tanning has decreased to 12–
14 hours, after brushing the duration reduces to 2–3 hours.
Greasing. Greasing is the processing of fur skins using fatty materials
after pickling and tanning. It is used to produce hides with high softness and
plasticity. Fatty materials are adsorbed on the surface of the structural elements of the skin, separate them and create fatty shells around the fibers.
28
It results in the increased water resistance of the skin. Fatty materials prevent
the fibers from sticking together during drying and facilitate their mutual
sliding under the influence of deforming forces. However, fatty materials
may chemically bond with tanning agents and collagen.
One of the most widely used greasing methods is dipping of hides in
diluted emulsions. Emulsions are stable over time. They penetrate into
the skin and do not delaminate in it. Therefore, the skin fibers will not be
greased. Unstable emulsions delaminate very quickly that causes the deposition of fatty substances on the skin surface. In this case, the fatty substances
can not petetrate inside the fibers. The emulsion with optimal stability penetrates and evenly distributes in the skin and after that it loses the stability.
Therefore, the fatty substances are completely absorbed by the fibers. For
a more complete decomposition of the emulsion at the end of greasing, sodium chloride is used (50 g/dm
3
).
Recently, a method of greasing skins, combined with tanning and pickling, has been used to reach deep penetration of the emulsion into the skin
and its delamination directly on the surface of the fibers. This method reduces significantly the duration of the processing cycle. The absorption of
fats is 60–80 %. Therefore, the solutions may be reused in this process.
Sammimg. A semi-finished fur product absorbs a large amount of moisture (in some cases 80 %). For high-quality subsequent operations, moisture
is to be removed by samming the semi-finished product in a centrifuge. Centrifuge is a cylindrical vessel with a perforated surface, rotating around its
axis at high speed. During the rotation, the semi-finished product is tightly
pressed against the walls of the drum due to centrifugal force. The moisture
is removed to the outside through the cylinder walls. Thus, the moisture content can be reduced to 50 %.
Shaving. Shaving is used to process skins with a thick and dense skin or
uneven thickness over topographic areas.
Drying. The purpose of this operation is to remove excess moisture from
the semi-finished product and prepare it for finishing operations. A significant amount of the moisture is removed from the skins before spin drying in
a centrifuge. The moisture fills large capillaries and pores of the semi-finished product (wetting moisture). The pressed semi-finished product contains 45–60 % moisture, and after the main drying the moisture content is
12–15%. The process of drying produces semi-finished products of high
quality with minimal shrinkage. The drying parameters are to provide lowest
labor, heat and electricity costs.
29
There are several methods of drying:
• contact drying;
• convective drying;
• radiative drying;
• dielectric or high-frequency drying
.
The contact and convective methods are the most widespread because
they are inexpensive compared to the radiative and dielectric ones.
Staking. After drying, the semi-finished fur product is staked. The operation is performed in a staker. The fibers of the skin are separated, the skin is
loosened, as a result of which it becomes softer and more pliable. Typically,
the staking is carried out after drumming. The operation of skin loosening is
carried out with cleaning. In the process of staking, the area of the skins increases. The most effective staking process is performed in the RM-2 machine.
If the surface of the shaft between the knives is filled with emery powder,
the machine simultaneously cleans up the skin along with the staking.
The mashine operation is safe, although the quality of the staking is not as high
as for staking in a special continuous staker MRP.
Drumming. Final drumming is an essential operation of fur processing.
The operation is performed many times. Depending on the mode, it is divided into hair drumming and skin drumming. During the process of drumming, the skin of a semi-finished fur product is under the influence of mechanical forces, which increase its softness. The hair is grinded by sawdust.
It becomes clean, friable, and shining. The quality of drumming depends
on the type, shape and size of the sawdust. Sawdust is prepared from hardwood. The use of oak sawdust is not allowed. Sawdust has a moisture content of 25–30 % (wet sawdust) and 10–12 % (dry sawdust). It is preferable
to use square cross-cut sawdust with a side length of about 3 mm. Sawdust
of smaller size and needle-shaped are not used for drumming. It causes hair
felting and soiling, does not contribute to additional loosening of the skin.
To obtain a good shine of the hair, the sawdust moistened with turpentine is used for the process of drumming. Drumming is an important process for dyed skins, since it removes loosely fixed dye and produces an
easy-care semi-finished fur product. The process of drumming is also used
to clean the hair and skin from excess fat.
During the process of drumming, wood chips and auxiliary materials
are placed in the drum. When the drum rotates, the skins are kneaded, and
the impurities from them are adsorbed by sawdust. Various surfactants, fat
solvents, technical flour, ammonia are used to enhance the drumming effeciency. The choice of auxiliary materials depends on the purpose of
30
drumming and the type of skins being processed. The drumming effeciency
also depends on the ratio between the weights of the semi-finished product
and sawdust, the degree of drum filling, the frequency of its rotation and
the processing duration. The consumption of auxiliary materials depends
on the size and type of the processed skins. A low load of the drum leads
to the low mechanical action created by the mass of the loaded materials.
A large load of the drum does not provide a good mixing of the skins. A low
drum rotation speed does not provide the required parameters of drumming.
At a high rotation speed, the skins are pressed against the drum walls under
the action of centrifugal force, which also reduces the mechanical impact.
One of the promising methods of drumming is hair cleaning. It is used
to replace sawdust.
Steam or water-air mixtures are used to moisten fur raw materials with
long, soft and thin hair. In this process, humidified air (temperature 40–
45 ºС, relative humidity 98–100 %) is fed into the drum with hides for 20–
25 minutes. After that, the hides are perched with rubber rings or balls.
During the first drumming, the required cleanliness, shine, softness
and friability of the hair are not achieved. Therefore, drumming is carried
out again with dry sawdust (moisture content 10–12 %). In addition to sawdust, fine river sand without sharp edges that may scratch the hair is also
used. In some cases, flour is used. It is used to clean the hair and produce
the skin with a white color and some roughness, which is desirable for subsequent cleaning. To obtain a good shine of the hair, the second drumming
is recommended to be carried out with sawdust moistened with turpentine.
The use of acetic acid and glycerin also increases the gloss of the hair.
The second drumming is very important for dyed skins since it removes
the loosely fixed dye and produces easy-care fur products.
The sawdust is removed from the skins by suction using combined
drums or shaking in mesh drums.
The drumming process may lead to the following defects of hides:
• hair sealing due to uneven drying and increased fat content;
• hair without shine due to insufficient drumming;
• increased hair soiling as a result of poor cleaning from loosely fixed
dyes;
• skin stiffness caused by the insufficient moisture and poor drumming or high moisture and subsequent drying;
• skin delamination due to the increased time of drumming
.
The defects may also be caused by the use of poor quality sawdust.
Соседние файлы в предмете [НЕСОРТИРОВАННОЕ]