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Файл:Innovative Technologies for Manufacturing Leather and Fur Products. A Study Guide
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as well as those having weaker hair cover, to process them further. Insufficient and nonuniform hydration may hinder fleshing and cause fur skin
breakages. Semi-finished products may have an insufficient stretch. Strongly
hydrated hides may have a weak fur skin. Large protein losses cause fur skin
looseness. Bacterial damages lead to weaker fur skin, hair slip, and chafemarks that depreciate the hides.
Thorough pre-soaking hide sorting is very important. Green hides with
explicitly pronounced defects shall be processed separately. Green hides infected with bacteria, fungi, or molds shall be soaked in an acidic medium,
with antiseptics added.
Green hides infected with anthrax bacteria are loaded into the soaking solution at the temperature of 20 °С. Upon hydration, up to 100 g/dm3 of sodium
chloride and 20 g/dm3 of hydrochloric acid will be added. Disinfection lasts two
days. After that, the acid is neutralized, and hides are directed for fleshing.
Grease hides can hardly be hydrated. In soaking them, the solution is
used at an increased temperature, and alkali and wetting agents are added.
Sometimes organic solvents are used.
It is reasonable to soak glassy (overdried) hides using surfactants and
basic-type accelerators and alternate soaking with fur skin conditioning.
Soaking control consists in controlling raw materials, i.e., checking
for bacteriality; technical soaking process control (in which FR, temperature,
and process duration are controlled); and hide readiness control, i.e.,
the hides must be soft across the entire area and matt white across the thickness of the hide cut down the back, near its tail. Hydration uniformity across
the hide layers is identified analytically: In the middle layer, the moisture
content must be at least 67 % and sodium chloride content at most 2 %. Sufficient soaking criterion is the increase in the rawhide weight by 8–10 %, as
compared to the fresh hide weight.
3 . 3 . M e c h a n i c a l F u r - P r o c e s s i n g O p e r a t i o n s
a t P r e p a r a t o r y S t a g e s
In fur production, soaking is followed by mechanical operations.
Fleshing. Sheepskins are fleshed using fleshing machines. To flesh
pelts, disk-type machines are used. Valuable peltry types are fleshed manually with special braces or using other tools.
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In sheepskin processing, fleshing is followed by clipping. Hair shall
be shortened from 50–100 down to 16–20 mm. Hair is clipped using choppers or wool-cutting machines.
Hair pressing is made using fleshing machines with blunted blades, warm
(40 °С) water being fed simultaneously. In fact, it is machine washing of hair.
The next mechanical operation is fur skin conditioning. It is aimed
at loosening the skin. As a result, reactant solutions diffuse better into corium
or hair in subsequent treatments. Wool and clothing sheepskins are conditioned using fleshing machines with dull blades. Furs are conditioned/staked
on braces or other tools. Their action principle is fur skin loosening using
blades fixed on a rotating shaft.
Skin shaving. This operation is aimed at aligning the skin thickness
across the entire hide. Mink, coney, and mole skins are shaved using
the disk-type fleshing machine DMZ-30. To reduce the thickness, it is possible to split the skin using belt-knife splitting machines.
Hides are most frequently shaved after pickling or tanning, at which
the skin gets compacted, becomes coarse, and does not slide as well along
the cutting tool blade. Skin shaving through to opening the hair bulb leads to
hair slip and is a nonremovable defect.
In fur production, mechanical operations are follows by the second
soaking, if necessary, and then by degreasing.
3.4 . D e g r e a s i n g
Fur skins and hair coats of sheep, marmots, ground squirrels, seals,
and fur-seals are notable for their considerable fat contents. Fat content in
sheepskin may exceed 25 %. In fur skin, natural fat is in form of enveloped
fat cells distributed across the hide thickness and area. Fat charges the fur
skin, makes it coarse, and causes its oxidation (“burning”) while storing.
Natural greases must be removed not least because they, being in fat
cells, cannot lubricate collagen fibers and, therefore, cannot make them soft
and flexible.
In hair coat, fat is distributed across the hair shaft surface as a film or
drops. Small amount of fat is inside a hair. Various contaminations, such as
sand, dirt, or food rests, adhere to fat. In rawhide processing, fat hinders running the operations, such as soaking, dyeing, and hair conditioning.
Degreasing the sheepskin hair cover is one of the core processes of its
manufacturing. Upon degreasing, hair cover becomes luster and friable, and
its color becomes uniform and vivid. Degreasing continues until the hair fat
content reaches 1.5–2.0 %. With a lower fat content, the physical and mechanical properties of hair worsen, brittleness and fragility develop, and
abrasion resistance reduces.
In degreasing, wool stearin is hydrolyzed and split into cholesterol and
higher fatty acids that, with acetic anhydride in presence of sulfuric acid, give
typical greening. This is the so-called Liebermann’s reaction. It is used in
the rapid analysis of hair for identifying fat on it.
3.4.1. D e g r e a s i n g T e c h n i q u e s
Adsorption technique is based on using highly dispersed solid adsorbents (special clays). The finest clay particles are enveloped in fat and removed. This technique is not practically used, it is inefficient.
Degreasing with solvents means extracting fat from green hides with
chlorinated hydrocarbons or white spirit in sealed devices. When selecting
a solvent, it is necessary to consider its ability to dissolve fats, volatility, toxicity, fire hazard, and economic efficiency.
Tri- and dichloroethane have the highest degreasing ability.
Petrochemicals are fire-hazardous, the fumes of which provide adverse effects upon human nervous system.
Organic solvents can remove fatty matters from both hair cover and
fur skin. Degreasing runs for several minutes. Solvents cannot remove nonfat contaminations, such as blood, dirt, etc. To eliminate the latter ones, additional washing is required for hides, i.e., washing them with the aqueous
solutions of surfactants.
Degreasing with solvents includes degreasing proper and removing
the solvent and extraction products contained in it. Solvent is removed in
three stages: Hydroextraction, drying, and airing at an increased temperature
to obtain full removal of the solvent.
Thus, the benefits of this technique include a high degreasing degree, no
hair slipping, and a short process duration (several minutes). Its disadvantages:
Practically all solvents are toxic, and most of them are fire hazardous. There is
also a risk of too deep degreasing, which may lead to the worse hair appearance
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and the loss of luster, strength, and flexibility. Solvent cannot remove any nonfat impurities, such as blood, dirt, dung, sand, etc.
Emulsion technique is used the widest. It consists in processing hides
in the aqueous solutions of surfactants.
Degreasing action of surfactants, according to P. A. Rehbinder’s the-
ory, passes through several phases. Contaminated hair has fat droplets and
hydrophobic dirt particles on its surface. Hair surface is hydrophobic, too.
Therefore, contaminations are held quite tightly on it. Fat-to-hair sticking
coefficient is characterized by the water-contact angle.
Surfactant presence on hair decreases the water-contact angle and
leads to completely wetting the contaminated surface with detergents (wetting phase). Detergent solution having a high wetting ability provides
a cleaving effect, i.e., isolates the particles of dirt and fat from each other and
from the hair surface (peptizing phase).
Intensively mixing and shaking lead to forming a stable aqueous emulsion from peptized fat droplets and a suspension of contaminating solids
(emulsification phase). Separated dirt particles are eliminated by floating at
intensive foam formation (foam-forming phase) and replacing the detergent.
Surfactant concentration in foam exceeds that in the solution by 50–60 times.
To degrease fur pelts, various surfactants are used, for which the following requirements are set: They should not damage the hair or change its
color; upon treatment, they should make the hair lustrous, soft, and friable,
and they should produce stable emulsions.
As is known, depending on the structure and dissociation of aqueous
surfactant solutions, they are divided into anion-active, cation-active, and
non-ionogenic. In fur industry, anion-active and non-ionogenic surfactants
are basically used.
Anion-active surfactants include alkyl-sulfates (Novost M, synthapon,
or datalan), alkyl-sulfonates (Volgonat or sulfonate), alkyl-aryl-sulfonates
(sulfanols of various brands), and carboxylate based fatty acid salts.
Anion-active surfactants are strong foam-makers. They usually contain a sulfonic group and are resistant against calcareous water. Their concentration is 0.5–2 g/dm3 during soaking and 3–6 g/dm3 in degreasing.
Non-ionogenic surfactants include synthanols (DT-7, DS-10, VN-7,
and DT-308). They can be oxidized in sewage and are recommended for fur
degreasing. OP-7 and OP-10 are also included in non-ionogenic surfactants,
but they are not oxidized in sewage.
Non-ionogenic surfactants are resistant against the actions of acids
and alkalis, which allows using them in a weak acid medium. Concentration
of non-ionogenic surfactants (0.5–2 g/dm3) is lower than that of anion-active
ones, since they are adsorbed by hair to a lesser extent.
Degreasing action of surfactants can be increased by using the mixture
of anion-active and non-ionogenic surfactants. For instance, taking Novost
and synthanol DS-10 on a 10:1 ratio causes the reduction of detergent consumption by 30–40 % at the good degreasing quality.
Fats contained in hair cover have the melting temperature of 38–40 °C.
Therefore, the degreasing solution temperature shall be maintained within
the range of 40–42 °C. In isoelectric point (pH 4.9), fur is damaged least of
all, does not swell strongly, and is the least reactive. In an alkaline medium,
hair-to-skin bonding becomes less strong, and its mechanical properties
worsen. It is not reasonable to degrease in a weak acid medium, since fatty
soaps are hydrolyzed in acidic media, forming fatty acids, and their degreasing ability falls. Therefore, degreasing is performed in a neutral or weak alkaline medium, in presence of sodium carbonate.
Mechanical mixing intensifies the degreasing process.
To achieve complete corium degreasing, combined compounds are
used, based on surfactants and solvents. Solvents transform fat into a solution, while a water-medium stable emulsion is formed in presence of surfactants. In recent years, enzymes are introduced in degreasing solutions
to eliminate protein contaminations more efficiently. Thus, for instance, adding alkaline protease G10X to the surfactant accelerates the hydration of coney skins and does not require any additional washing of hair cover.
3.4.2. D e g r e a s i n g P r a c t i c e s
Fur-finished sheepskin is degreased in several stages:
1. First degreasing with the Novost compound or similar ones, such
as Atezan-Ultra, Gamma, Gelon RK, De-Sol-A, etc. In case of Novost,
the process is performed in presence of calcine soda and formaldehyde
at the temperature of 40 °C and FR = 10 for 45 minutes.
2. Mechanical operations, if necessary.
3. Second degreasing in the same solution, but at FR = 7 and the temperature of 42 °C for 1 hour.
Upon soaking, rawhide is directed for dehairing and liming.
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3.5 . D e h a i r i n g a n d L i m i n g
Dehairing and liming mean raw material processing in a strongly alkaline medium by calcium hydroxide suspension, sodium sulfide being
added. Dehairing is aimed at removing hairs, while liming should change
the corium structure.
Hairs and epidermis are removed by the following techniques:
– Non-hair-retaining liming;
– Hair-retaining liming;
– Lime-paste technique; and
– Ara-liming.
Non-hair-retaining liming means the chemical degradation of hairs by
sodium sulfide in an alkaline medium or using oxidizers until the hair is completely dissolved and transformed into the tail lime liquor. This technique is
used in manufacturing chrome-tanned shoe-upper leathers.
Hair-retaining liming means loosening the hair-corium bond under
the action of sodium sulfide having concentration lower than that in the preceding technique. Upon such treatment, the hair together with epidermis can
be easily separated from corium on unhairing machines. Then the hairs are
used in felt manufacture. This technique is used in making bottom leathers.
With lime-paste technique, chemical agents that loose the hair-corium
bond are applied as a paste onto the flesh side of the hide and held until they
penetrate to the hair-root. Then hairs are removed mechanically, using an
unhairing machine. This technique is most frequently used in making leathers of goat- and sheepskins.
Ara-liming is used to process green pigskins or to dehair sheepskins.
To loosen the hair-corium bond, it is necessary to decompose epidermal tissues lining the follicles and holding the hair within the corium. Keratin is
the core protein of both hairs and epidermal tissues. Its strength is determined
by the presence of cross-linking disulfide bonds -S-S-. Dehairing reduces
to damaging these bonds by alkalis and reducing or oxidizing agents.
In an alkaline medium, disulfide bond is hydrolyzed as shown below:
CH
CNO
H
CH2S S CH2CH + H2O
NCH
O
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C
CH
N
CH
2
SH +
O
H
N
CH CH
2
SOH
H
C O
Sulfenic acids get decomposed in an alkaline medium, generating hydrogen sulfide, H2S and sulfur, S, while forming alcohols, R-CH2OH, and
aldehydes. Alcohols can react with keratin or its decomposition products,
forming strong lanthionine bonds, -CH2-S-CH2, that strengthen the hair-corium bond. This phenomenon has the name of hair immunization and is undesired:
HC
C
NH
CH
2
SH + HOCH
2
...
O
...CH
2
SCH
2
...
Hair immunization is prevented in the presence of sodium sulfide that
is a sharpening agent. Sodium sulfide blocks the active groups of matters
formed in the disulfide bond hydrolysis in the presence of an alkali:
HC CH
2
SOH + Na
2
S
NH
CO
HC CH
2
S ONa + NaHS
NH
CO
Sodium hydroxide diffuses into the deep hide layers from the flesh
side. Complete impregnation can be achieved within about 12 hours. In presence of sodium sulfide, sodium hydroxide diffuses into the hide from both
sides with the same rate, due to the sodium sulfide effect upon the keratinized epidermis layer. Complete impregnation is achieved withing 2–3 hours.
Amines can block the active groups of sulfenic acids:
HC CH
2
NH
CO
SOH + CH3NH2 HC CH2S NHCH
3
NH
CO
-H2O
Dimethylamine, (CH3)2NH, shows the most intensive dehairing action.
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Disulfide bond can be damaged by oxidizing agents, such as chloramine B, chlorourea, or chlorates. The bond is damaged due to the interaction
between chlorine oxide generated in splitting chlorine-containing compounds in a weak acid medium and keratin:
2 HC CH
2
NH
CO
S S CH2CH + 6 ClO
2
NH
CO
4 HC CH2SO3H
NH
CO
-Cl
2
sulfo-acid
Dehairing is aimed at loosening the hair-corium bond, followed by removing hairs using dehairing machines or chemically, by dissolving them,
while liming is aimed at separating or loosening the corium structure.
Interfibrillar proteins may be of different nature. Thus, albumins are
water-soluble and can be basically removed by soaking. Globulins, mucins,
and mucoids are not soluble in water and should be removed by liming.
In the liming process, corium swells strongly (it is called plumpness).
Plumpness is accompanied by increasing the skin weight, thickness, and tensile property.
It is known that collagen fibers are in the corium as bundles of various
thickness and shapes. Collagen protein consists of polypeptide chains interrelated by hydrogen and electrostatic bonds.
In liming, due to the interaction with calcium hydroxide or sodium
sulfide, the positively charged NH
3
+
groups transform into the uncharged
NH2 ones according to the following scheme:
Thus, only similar charges are kept, while repulsion occurs among polypeptide chains, and water penetrates easier the internal corium structures.
Osmotic pressure develops in this case. Structural elements of corium are
reduced lengthwise and enhanced edgewise dramatically.
At the same time, the charged centers of protein attract intensively water dipoles, which leads to ion-dipole interaction.
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Thus, plumpness is a cumulative effect of three factors: Osmotic pressure, ion-dipole interaction of water dipole with the charged protein groups,
and the electrostatic repulsion of similarly charged groups.
Corium plumpness extent depends on the nature of alkali used. Liming
with calcium hydroxide only causes a smaller plumpness than that with its
mixture with sodium sulfide. The reason for this is that calcium ion is bonded
with the protein very strongly and makes the СООˉ groups not exactly available to water molecules.
Hydrogen bonds are weakened in liming, too. Protein gets broken
down to some extent, and the corium structure is split. This is evidenced by
the shrinking temperature reduction from 65 down to 55 °C. Corium permeability increases in the liming process.
On long liming in the calcium hydroxide solution at pH = 13, the amides of amino acids are hydrolyzed by the following scheme:
RCH2CONH2 + H
2
O → RCH
2
COOH + NH3 ↑
This accounts for the ammonia smell in lime yards. Sodium sulfide
accelerates the hydrolysis. Sodium sulfide and other alkaline additives are
called the soaking sharpening agents.
Sharpened lime liquors are used to dehair/lime hides, i.e., those with
additives to the suspension of calcium hydroxide, sodium sulfide, or other
alkaline matters.
In manufacturing flexible chrome leathers, the so-called “white
liming” is used. White liming is performed in the pure calcium hydroxide,
10–12 or 18–22 g/dm3, in case of especially soft flexible leathers.
Factors affecting the liming process.
Temperature. With rising temperature, the liming process accelerates.
However, the excessive increase in temperature may lead to the serious loss
of albuminous substances. Optimal temperature: 23–26 °C.
Lime liquor ageing. Lime liquors are normally used several times.
They accumulate amines, ammonia, dairy salt, and sodium sulfide. Such lime
liquors are called old ones. Old liquors, unlike fresh ones, accelerate dehairing, break down the corium structure well, but provide less plumpness.
Old lime liquors are purified, supported, made up to the necessary concentration, and reused. Sometimes old liquors are added to fresh ones (up
to 50 % of the total volume), obtaining lime liquors with optimal properties.
Mechanical action. It accelerates liming. Where machines rotate,
the concentrations of reactants become even at the skin surface, and
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therefore, their diffusion increases. However, long continuous rotation may
cause the increase plumpness of outer skin layer, as compared to the corium
sublayers, and lead to cockling and weakening the corium structure. Therefore, there is a stepwise turnaround of machines.
Liming duration. It is determined by the green-hide purpose. Thus,
chrome leathers made of pig skins are limed according to the following
methods: Liming – 18–20 hours, stripping – 2 hours, and white liming –
24 hours. For especially soft and flexible leathers, liming lasts 48–60 hours.
Shoe and harness Russian leather is limed for 72 hours, 12 of them
being white liming.
Upon their bristles having been removed, pig skins are limed in a drum
at FR 1.5 in two stages. First, with the 8–10 g/dm3 sodium sulfide solution
for 12 hours, then with 11–13 g/dm3 calcium hydroxide for 24–36 hours.
At the second stage, surfactants are added.
With the increasing liming duration, corium is changed deeper, and its
porosity grows, while its strength falls.
How liming mode affects leather properties. Liming affects the leather
properties, such as tensile strength, expansion, porosity, air permeability, and
softness, as well as the leather area yield. With the increase in liming duration, tensile strength decreases slightly, while total and plastic extension increases. Other things being equal, lime-based liquors ensure a softer leather
than sulfide-based ones.
Liming promotes better running of the subsequent processes, such as
tanning. The longer liming is, the more intensive the corium absorbs chrome
compounds.
Dehairing and liming control. Raw materials readiness for mechanical
dehairing is assessed by organoleptic techniques: When pushing with a finger, hairs must be easily detached and shifted. Green-hide liming degree is
determined by its elasticity level: No fingermarks should remain upon pushing with a finger.
While liming, lime liquor concentration and process temperature must
be controlled.
An objective control method is the definition of the resistance
to enzymes and temperatures. The better green hides are limed,
the sooner its sample dissolves when exposed to an enzyme. Very thin
slices of green hides are placed into a cup containing the preliminarily
prepared enzyme extract. Then they are heated at 57 °C. Dissolving duration characterizes the green-hide liming degree: 25–35 minutes for
milk-fed calf leather and 50–60 minutes for heifer, cow hide, and steerhide.
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