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Файл:Innovative Technologies for Manufacturing Leather and Fur Products. A Study Guide
.pdf
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Garment-and-fancy leathers are the clothing and head wear leather
and haberdashery leathers, such as those for handbags and gloves. Natural
garment-and-fancy leather is a soft material produced from small-sized raw
skins. To manufacture leather garment, pig- and sheepskins are particularly
used, while soft gloves are usually made of the skins of goats, foals, hogs,
and even dogs.
Skirt leathers are collar backs (harness and link ones); trapping
leathers for people and horses (leathers used for human trappings are
named type L; for horse trappings are type K; for saddle flaps and seats
are types K-S; and stirrup leathers are type P).
Technical leathers are drive belt leathers; machine parts leathers; and
other leather types used for engineering purposes.
Besides, leathers are divided by rawhide types, that is, cattle, pigskin,
etc.; by tanning methods, such as chrome-tanned, synthetically tanned/retanned, vegetable-tanned, or compo-tanned; and by finishing technique and
nature, i.e., varnished, aniline-dyed, etc.
Leathers are also divided by configuration, thickness, and area.
Characteristics of some leather types.
Casual shoe upper leather. These are chrome-tanned leathers, oiltanned leather, and varnished leathers, as well as chamois shoe upper leathers
and lining leathers.
Chrome-tanned leathers must be soft, but compact, resistant against
multiple bends, water action, and increased temperatures, and have a good
vapor and air permeability, as well as sufficient strength and stretch.
Based on the finishing technique, they are divided into smooth and
rifled, with natural or artificial grain surface, suede (with buffed grain or
flesh side), nubuck (with some napped grain surface), or nappa (very soft and
thin).
For chrome-tanned leathers, there are several standards that consider
the weight of the rawhides to be processed, configuration (for instance, made
of ranges), rawhide type (pigskin, etc.), and thickness.
Shoe suede, or chamois, is oil-tanned leather. It is made of deer, elk,
sheep, and goat skins. This type of leathers is distinctive due to no grain layer
and hot (although not above 60 °C) water resistance. Best suede is that made
of deer skins. Suede can be washed with soap; it retains its softness.
Varnished shoe leathers are made of calf, grasser, kip, bull-calf, and
heifer skins, horse-fronts, yearlings, and flesh splits. Best varnished leather
is patent kid, or chevrette (made of goatskins).
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Shoe upper leathers made of flesh splits are leathers manufactured
from reticular dermis (pars reticularis). In terms of its purpose, splits can be
heavy upper, casual, and velour/suede. Such splits are made of cattle hides.
Sandal splits are made of pig skins.
Lining leathers are intended for making shoe lining parts. They are
made of tanned goods unsuitable for manufacturing shoe upper leathers, or
of splits.
Heavy shoe upper leathers. These are 1.5–3 mm thick Russian leathers
for shoes. Requirements for such leathers: Good shape keeping, resistance
against multiple bends and stretches, sufficient vapor and air permeability,
and water resistance. Sandal side, as compared to the shoe leather, must be
more resilient, but not hard, and have a lower fat content. It is made of cattle
hides (cow hides), pigskins, and horsehides.
Bottom leathers. These are harder leathers intended for producing sole
and inner-sole parts of shoes. These leathers are made of cattle hides, pigskins, horse butts, camel hides, and sealskins, using vegetable, synthetic, and
mineral tannins.
In terms of configuration, they are made as hides, croupons, single
butts, skirts, and horse butts. Bottom leathers are divided into six categories,
based on their thickness at standard points:
– Category 1: Over 5 mm;
– Category 2: 4.6–5.0 mm;
– Category 3: 4.1–4.5 mm;
– Category 4: 3.6–4.0 mm;
– Category 5: 3.1–3.5 mm; and
– Category 6: 2.6–3.0 mm.
Categories 5 and 6 are inner soling. Inner soling must be resistant
against sweat, moisture, and heat.
Sole leathers must be resistant against compression and bending deformations, as well as against rubbing in dry and wet conditions.
Bottom leathers for screwed/plugged shoe construction method must
be distinct in considerable density and strength and hold fasteners, such as
screws, plugs, or slugs, in wet or dry state, and have a low absorbability,
since they are intended for being used in harsh climates.
Shoes made by sewing/cement construction methods are worn in urban conditions. Therefore, leathers used for them must be elastic (not loose)
and have a sufficient tensile strength. Being too hard, these leathers may
cause rapid thread abrasion.
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Harness leathers. They are made of medium and heavy cattle hides. Pigskins can be used, too. Requirements: High strength and resistance against
sweat, sunlight, multiply moisturizing, low temperatures, and rubbing.
Garment-and-fancy leathers. Garment leathers must be of increased
softness and stretch. They are mostly made of sheepskin (Cape leather) and
pigskins, and cattle hides. In terms of finishing, they are divided into velour
and natural grain leathers with or without topping.
Garment-and-fancy leathers also include suede and kidskin. Kidskin
is a special type of very soft and stretchy leather. It is made of sheepskins,
goatskins, foal skins, and dog skins. In manufacturing kidskins, pickle alum,
cooking salt, egg yolks, and flour are used.
Leathers for garments or bags are made of all raw materials and splits
unsuitable for producing shoe upper leathers, using chrome / chrome-vegetable tannages. In terms of leather finishing, they may be natural grain leathers, rifled leathers, nubuck, and velour. Gloving leathers are especially soft
and stretchy. They are manufactured from sheepskin, goatskin, foal skin, pigskin, dog skin, and horse fronts as natural grain leathers and velour. For instance, kid leather is a soft and stretchy leather made of lamb or baby-goat
skins by chrome or chrome-oil tannage. Its grain is smooth, without wrinkles.
It is nearly exclusively used to manufacture gloves.
Currently, furniture leathers have become widespread, i.e., thin
chrome-tanned cattle leathers.
Technical leathers. They are only made of cattlehide butts. Requirements: High strength, high elasticity, and regularity of substance.
Rawhide leather and bridle leather form a special group of leathers.
Rawhide leather represents dried cattle green hide. It is very hard and
used for protheses and musical instruments.
Bridle leather, like rawhide leather, belongs to the group of untanned
leathers and is made of cattle, camel, pig, and elk hides. It is used in making
harness goods. Bridle leather can be band leather (δ 3.5 mm or higher and
2.0–3.4 mm) and harness leather (δ 3 mm or higher and 2.0–2.9 mm).
Bridle leather must have high plasticity, be well abated and well impregnated with grease throughout the entire area, and it must not be loose.
Classification and characterization of some fur types.
Furs are classified by fur-bearing species. Besides, furs are classified
by their hair condition, color, size, defects, mock/real, etc.
There is no unified standard for all fur types. Fur industry processes
the hides/skins of over 50 animal species, and their properties change based
on their biological properties. There about 100 standards and specifications
for peltries and furs.
Fur sheepskin. It is manufactured both natural and dyed, shearling,
and snuffed/non-snuffed. Dyed wooled sheepskin is usually black, brown,
beige, gray, and some other colors.
Fur sheepskin is manufactured as dyed to simulate the furs of fur seal
or otter, or use frame sprayed coating to mock the haircoat of wild animals,
such as snow leopard, tiger, or leopard.
Long-hair fur sheepskin is also manufactured from semifine-wool
sheepskins having unimodal, but not thick enough fur, mocking Arctic fox,
nutria, or beaver.
Key quality attributes are the haircoat unimodality and thickness, as
well as fur skin thickness. If it is too thick, the goods will be heavier, while
if it is too thin, the goods are not strong enough.
Wooled sheepskin. Wooled sheepskins are coarse wool breed skins.
They are manufactured as natural or straight/surface dyed. Wooled sheepskins are used to manufacture upper garment with the skin outside. Special
requirements are set for fur skin. It must be tanned well. It must have soft
and non-greasy feel and solid color and be spotless.
Fur coney skins are divided into long-hair and sheared, natural and
dyed, such as seal dyed.
Astrakhan/broadtail skins. Manufactured are pure finished dyed black
astrakhan; finished natural or dyed broadtail; and pure finished natural astrakhan, gray or whitened and colored in brown or other hues.
Astrakhan/broadtail skins are used to manufacture men’s and
women’s head wear, neckpieces and collars, women’s coats, fur capes, etc.
Mink skins are characterized by thick and rich hair of different colors,
such as white, blue, silver, platinum, etc. Based on the fur quality, mink skins
are divided into grades, while they can be of 18 colors each. Minks are used
in manufacturing coating, head wear, capes, collars, etc.
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3 . P R I M A R Y P R O C E S S E S A N D O P E R A T I O N S
I N L E A T H E R A N D F U R P R O D U C T I O N
Fur and leather manufacturing is a complex of sequential processes
and operations, in which raw materials are transformed into ready leather or
fur with a given consumer performance.
By the nature of their effect on raw and green hides, processing
treatments can be conditionally divided into two groups: The first one,
processes that are based on chemical or physical regularities and performed in liquid media; and the second one, operations based on mechanical effects.
Processes are normally used for a batch of raw or green hides; therefore, they are called batch treatments. They are usually performed by dipping
a batch of raw or green hides into the treatment liquid and in special devices,
such as paddles, drums, or screw apparatuses.
Operations, i.e., the second-group activities, are normally used to treat
individual peltries or hides. As compared to the batch treatments, the duration of which is defined by hours, piece work is short-termed.
In fur production, the process treatment of hides/skins can be conditionally divided into the following standard groups:
– Preparing raw materials, i.e., completing the batches;
– Preparatory processes and operations, such as soaking, washing,
fleshing, and degreasing;
– Dressing, i.e., pickling, tanning, and stuffing/degreasing;
– Semifinished goods processing, i.e., drying, sawdust cleaning, and
fur/skin finishing;
– Dying, i.e., neutralizing, etching, decoloring, bleaching, and dying
proper; and
– Dyed semifinished goods finishing, i.e., drying, cleaning, fur correcting, and skin/fur finishing.
Leather industry process treatment consists of the following processes
and operations: Preparatory, resulting in obtaining green hides; tanning, resulting in obtaining tanned semifinished goods; and finishing, resulting
in obtaining leather.
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3 . 1 . B a s i c P r o c e s s F l o w P a r a m e t e r s
Main parameters of liquid treatments include float ratio (FR), temperature, reactant concentration, process duration, and the degree of mechanical
impact.
FR is the volume (m3 or dm3) of treatment liquid, such as water or
a solution, per rawhide weight unit (t or kg).
In leather industry, FR is considered to belong to green skin mass or
to pelt masses at different process stages (normally, upon liming and upon
shaving). In fur production, FR is considered to belong to air-dried mass,
while in sheep-skin production – to green skin mass or pelt mass, such as
while dyeing.
In leather manufacturing processes, FR is taken as equal to 0.6–2.0,
while it is 7–15 in fur production and must be 20–25 in processing fox and
Arctic fox skins. When using fixed apparatuses, FRs are taken higher than in
processing skins in mixing machines. FRs can be lower when processing
skins having smooth and straight hair than when processing those having soft
and curled hair. Apparatus performance, water consumption, and sewage
volume and contamination depend on the FR value.
Process duration. Duration of rawhide or green-hide processing is determined by the time of absorbing the agent by corium/hair and by its uniform
distribution across the layers of the object to be processed. Process duration
depends on the raw material type and curing method. Thus, the duration of
soaking the green-salted ship skins is 6–12 hours, while it is 20–24 hours for
air-dried ones.
Agent conservation. Increasing the agent concentration accelerates its
diffusion into the semi-finished product. However, the excessive increase in
the agent concentration may cause its hyperfunctioning in upper layers and
blocking its diffusion paths. This may result in defects appearing in semifinished products, skin, and fur.
Temperature. Increasing temperature accelerates the process, on
the one hand, but it activates protein breakdown, on the other hand. The process cannot be performed at a temperature close to the leather cure temperature. As a rule, the processes are performed at temperatures that are about
20–25 °C lower than the semi-finished product cure temperature.
Mechanical effects (mixing). It accelerates the process, while the diffusion of agents into the corium and hair speeds very quickly. However, it
should be kept in mind that intensively and continuously mixing semi-
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finished fur products or rawhides in paddles can cause hair matting. Long
mixing of semi-finished products or rawhides in drums, especially at a high
rotation speed, may lead to obtaining loose and weak grain leather.
3 . 2 . S o a k i n g o f R a w H i d e s
This is the process of hydrating the cured raw materials, characterized
by a low moisture content. Soaking means processing rawhides with water,
frequently with electrolytes added.
Soaking is aimed at bringing the hide into a condition that is as close
to the fresh one as possible, in terms of both the hydration degree and microstructure. During soaking, dirt, blood, curing agents, and soluble proteins,
i.e., albumins and globulins, are removed from the hide. At the same time,
the hide absorbs a considerable amount of water. Water is bonded with
the active groups of collagen peptide chains, i.e., CO-, -NH-, NH
3
+
, COO-,
and OH-, due to hydrogen bonds and electrostatic attraction forces. This is
the so-called hydration water. It does not get removed by mechanical samming and cannot dissolve any other substances. Hydration water content is
40–50 % оof the dry collagen weight.
The resting moisture contained in a hide upon soaking is named swell-
ing moisture. Especially strong corium swelling, called plumpness, manifests
in the solutions of alkali and acids.
Corium plumpness degree in preparatory processes affects the readyproduct quality essentially. Visually, plumpness consists in increasing
the corium thickness by 30–100 %, depending on the topographic area. With
a strong plumpness, the corium structures become homogenous and vitreous.
Water forms a single system with protein. Resulting from plumpness, the corium becomes flexible. This flexibility degree is used as plumpness criterion.
Uniformity and, therefore, the soaking schedule depend on the hide
weight, its curing method, and the equipment utilized.
Heavy and thicker hides need longer soaking to achieve the required
hydration degree. Wet-salted hides are hydrated am easiest, while it is harder
with the sun-dried ones. Increased temperature accelerates hide hydration
considerable. At the same time, hydration degree decreases. In standard procedures, it is recommended to soak at the temperature of 18–22 °C.
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It should be kept in mind that, due its biologic origin, leather and fur
raw materials are the carriers of bacteria that get activated in an aqueous medium. At the first soaking stage (within 2 hours at the temperature of 37 °C),
bacteria adapt to the environment – this is the so-called latent period, and
then their explosive development starts. Bacteria act very strongly upon elastin that forms the basis of the outer layer of corium. With their explosive
growth, there is a risk of breaking the grain integrity. To prevent this,
the hides must be completely hydrated within the latent period. Therefore,
short soaking or pre-soaking is preferable. To prevent bacteria development,
antiseptics are introduced into the soaking liquid. In classical technology,
this was sodium hexfluorosilicate, Na2SiF6. Recently, sodium hexafluorosilicate is more frequently replaced by bactericides, such as FKh that represents
a mixture of organic acid salts and special additives, or Verinol based on
non-ionogenic and anion-active surfactants, antiseptic agents, and special additives. Bacteria also develop slower at pH above 9 or lower than 4.
To accelerate soaking, sharpening agents are used, which cause
the ionization of active collagen groups and break down ionic and hydrogen
bonds. All these changes enhance the corium permeability and contribute to
its quicker hydration. The following may be used as sharpening agents: Sodium carbonate, Na2CO3, and sodium sulfide, Na2S, or sodium sulfite,
Na2SO3, as well as surfactants that reduce water surface tension and improve
the wetting ability of hides.
In soaking, sodium sulfide gets hydrolyzed and forms sodium hydroxide that accelerates rawhide hydration, as well as sodium hydrosulfide that
promotes rawhide dehairing:
Na2S NaOH + NaHS
H2O
.
Mechanical action induced by the equipment rotation or by mixing accelerates soaking considerably. However, too long drum rotation or too intensive mixing may lead to hide looseness.
The highest hydration of wet-salted hides can be achieved within
6–8 hours of soaking. However, the uniform water distribution across
the hide thickness is only observed at the temperature of 27–30 °C after
10–12 hours or 22–25 °C after 16–18 hours.
Resulting from hydration and washing out soluble proteins while
soaking, there is an increase in the internal surface of the structural elements of corium and changes in its physical and mechanical properties.
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A hydrated hide becomes softer, more flexible, and able to interact with
chemical substances.
Insufficient and nonuniform hydration of a hide across its thickness
and area during soaking may cause grain cockling and crackiness in the subsequent processes or its hardness manifestation. Large losses of proteinic
matters in soaking lead to leather looseness and loose grain. Bacterial contamination of rawhides causes damaged grain areas on the leather.
Core principles and purpose of rawhide soaking are also maintained
for soaking raw pelts. However, there are some differences. While the hairto-corium bonding is weakened in soaking the rawhides, maintaining
the hair cover is one of the most important tasks when soaking raw pelts.
A right choice of surfactants is very important. They must be soluble
and active at the temperature of 20–25 °C, and resistant in salt solutions.
Non-ionogenic surfactants obtained based on ethylene oxide and on higher
fatty acids have such properties to the largest extent. Non-ionogenic surfactants do not lose their activity when interacting with fats. At the same time,
anion-active surfactants (aSAS) are the most active in the alkaline medium
for they can easier emulsify the fat (soap) derivates than the initial non-split
fats. As a rule, aSASs are used combined with alkali agents, such as sodium
carbonate or sulfite. In soaking raw pelts, it is undesirable to use alkali
agents, since they reduce the hair cover luster, enhance its tendency to felting, and cause hair slipping. Therefore, it is preferable to use non-ionogenic
surfactants at the soaking stage in fur production. For instance, Lowenstein
proposes to use WetterHAC for this purpose, a non-ionogenic wetting agent
that also has bactericide properties.
Wide range and special features of raw pelts do not allow setting a unified soaking mode.
Fur-finished shearling soaking. Air-dried and dry-salted fur-finished shearling shall be soaked in presence of bactericide, SNPKh 1.0 g/dm3, at pH 4.5–5.5.
Soaking duration is 16 –24 hours, temperature 25–30 °C, and FR 10.
For acid-salt-cured sheepskins, it is sufficient to hydrate them for
2–4 hours in plain water.
Wet-salted wool sheepskins can be soaked in presence of WetterHAC
(1 g/dm3) at the temperature of 25 °C and FR 10 for 3–5 hours, while the second soaking is performed in presence of 20 g/dm3 of sodium chloride and
1g/dm3 of WetterHAC at the temperature of 25 °С and FR 10 for 6–9 hours.
Wet-salted wool sheepskins can also be soaked in presence of biocide
Don in two stages. Duration of the first soaking is 3–5 hours. The second
soaking lasts 12 hours. Soaking is performed at 35 °С and FR 10.
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Coney skin soaking. Coney skins are soaked in two stages. The first
one is performed at FR 9 and 40 °С in a surfactant-containing solution for
4 hours. The second soaking is longer. For coney skins that are 0.3 mm thick
or thinner, soaking is performed in a solution containing antiseptic, sodium
chloride, surfactant, and sodium hydrosulfite. The second soaking is performed at FR 9 and 40 °С. Using enzymes when soaking coney skins with
the skin thickness of 0.5–0.7 mm allows reducing the total duration of soaking down to 6 hours and obtaining a soft fur skin.
Soaking of merlushka lamb skins and fur moleskin. Soaking is performed at 30–35 °C and FR 10 for 16–18 hours. It is recommended to soak
strongly polluted rawhides, adding alkali sharpening agents and surfactants.
White fox soaking. In soaking white fox skins, sodium chloride
(20 g/dm3) is used. To enhance the skin wettability and accelerate
the process, various surfactants are added to the solution. Duration
of soaking depends on curing method and ranges within 15–24 hours.
Temperature 18 –20 °С, FR 25.
How polymer additives affect soaking. There are known works on using oligomer-polymer matters in soaking solutions. Adding acryl-series polymer dispersions to the soaking solution contributes to a more intensive hydration of cured hides. Using water-soluble polymer dispersions in penetrating compositions at the soaking stage promotes both skin tissue hydration
and a more intensive separation of corium structure, which allows intensifying the subsequent tanning process.
There are known works on using polyacrylamide at the soaking stage
in presence of dialdehyde to maximally reduce the grain cracking of wool
sheepskins having the defect called “grain cracking”. There is also
a strengthening of fur skins. Thus, for instance, grain cracking strength limit
increases by 2.5 times.
One of oligomer-polymer matters is urea formaldehyde resin modified with aliphatic alcohols. There are known works on using synthesized
amino resins at the soaking stage in leather and fur production. The methods proposed allow processing fur pelts with weak fur skins and obtaining
semi-finished fur products meeting the requirements set by standards
GOST 10596-77 and GOST 2974-75. When using amino resins at the soaking stage, the weak fur skin is strengthened, heat resistance increases, and
the fur skin properties get balanced across topographic areas, the hygienic
properties of the processed pelts being preserved.
Soaking control. Basic parameters are controlled: Composition and
concentration of solutions, FR, temperature, duration, pH of solutions, and
fur skin hydration degree. Upon soaking, underhydrated hides are sorted out,
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