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Файл:Innovative Technologies for Manufacturing Leather and Fur Products. A Study Guide
.pdf
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Potential defects while liming. Defects caused by improper liming include cockling and grain crackiness, skin looseness, break and openness,
strength loss, overstretch, or, vice versa, hardness.
Limed cockling can be formed due to the fact that the volume of very
ponderous grain expands, while this is hindered by the middle layers that
have not swollen yet. This results in forming specific grain folds and wrinkles that are difficult to get rid of and cause the lower quality and degrade
leather. Liming-caused skin swelling can be nonuniform due to temperature
abuse, too high alkalinity, and noncompliance of mixing regime.
Superfluous losses of hide/skin albumin during liming lead to breaks
and pipey grain, i.e., the grain divides from subjacent tissues; as well as to
skin looseness.
Processing hides in rotating apparatuses at the smaller values of float
ratio may lead to grain distress.
Lime liquor is normally reused multiply.
Changes in the corium structure that take place in liming, are registered within the subsequent manufacturing cycle; such changes determine
considerably the leather quality and yield.
3 . 6 . M e c h a n i c a l R a w h i d e -
a n d S k i n - P r o c e s s i n g O p e r a t i o n s
Fleshing, or scraping, means mechanically removing the hypodermis
tissue (underskin) and lumps of meat or fat from the skin. This process contributes to quicker and evener running the subsequent liquid-based treatments. Scraping shall be performed upon washing and before soaking.
If rawhides are fleshed at meat factories, then there is no need for this operation at tanneries. To remove hypodermis tissue as completely as possible,
swelling green hides shall undergo the secondary fleshing. In manufacturing
chrome-tanned leathers, green hides are fleshed only once, when swollen.
Hides are fleshed using fleshing machines. A hide is hanged onto
the feed rubbered roll adjacent to transporting fluted rollers. As soon as
the machine is on, the hide is fed towards the knife roller (1,400 rpm),
the helical blades of which cut off the hide cuttings.
Dehairing is performed using dehairing machines. It is aimed at removing weakened hairs from a hide. In terms of design, dehairing machines
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are like fleshing ones. The difference is that the helical blades are blunted,
and the knife roller rotation rate is much lower than that of fleshing machines.
Pigskins shall be dehaired upon soaking and degreasing. Bristles are
removed using a bristle-pulling machine. The skin shall be placed onto
the feeding conveyer with its bristles up and its rump foremost. It is captured
by transporting and pressure rolls from the conveyer and directed into
the opening between transporting and operating rolls that rotate with different speeds towards each other. Their rotation rates differ by 1.6 times. Operating roll represents a steel tube with rubber-fabric rings pivoted on it. Due
to different rotation rates, bristles get shifted and separated from the skin.
Green-hide beading consists in manually removing the remaining
hide trimmings from the hide edges upon machine fleshing. With a proper
raw-material preparation, no beading is needed. Hides are beaded on cutting
tables, simultaneously with cutting off fringes, which prevents green hides
from twisting in the subsequent liquid-based treatments.
Leveling (splitting) means evening a skin/green hide by thickness and
dividing it into several layers. It is performed upon dehairing and fleshing
(green hide splitting) or chrome tanning (leveling semi-finished goods). Upper (grain) layer thickness is standardized, while the resting parts make
splits. Leveling is used in manufacturing chrome-tanned leathers from all
types of raw materials, other than light calf leathers, goat-, and sheepskins.
Green hides for manufacturing shoe oiled and harness leathers are only leveled, if their aitch piece thickness is above 3 mm. Thick ranges are also leveled in manufacturing bottom leathers.
Goods are leveled with splitting machines, the cutting tool of which is
a closed belt steel knife. Fluted and ringed rollers are used to feed and direct
the goods to the moving knife blades. Ringed roller is composed of brass
collars slipped on a copper or steel rod. In case of the uneven thickness of
the green hide being leveled, the rings loosely set on the roller get pressed
into the rubber roll, thus ensuring the even leather thickness.
Leveling performed upon chrome tanning has its special benefits:
Leathers are evener in thickness, and there is no need for separately tanning
upper and lower splits. A disadvantage thereof is the formation of many split
cuttings due to the increased requirements for the quality and substance regularity of the lower split. In manufacturing chrome-tanned leathers, leveling
is most frequently performed at the green-hide stage. This ensures the leather
area-based yield to increase by 6–8 %. All subsequent processes run easier.
This results in obtaining stronger leathers with smoother grain.
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Rounding means splitting semifinished goods in manufacturing bottom and harness leathers into topographic sections, i.e., into range, sides, and
croupon. Rounding allows using hides more efficiently according
to the thickness and properties of each section.
Green hides can be rounded, but chromed semifinished goods are
rounded more frequently, in which it is easier to define the topographic section
borders. In rounding, fore legs are separated together with belly or range.
There are standards set for the semifinished goods component yields. Thus,
when manufacturing bottom leathers from light and heavy steer hides and from
bull-calves, the yields are 21.5, 47, and 31.5 % for range, croupon, and belly,
respectively.
In some tanneries, when manufacturing chrome-tanned leathers,
the tanned semifinished goods are cut into two parts along the backbone
upon leveling and then processed as side leathers.
3.7 . Re- L i m i n g a n d Aba t i n g
Obtained upon completion of soaking and liming processes and mechanical operations, green hides contain considerable amounts of calcium
compounds (~ 4 %) both as sorbates and as chemically bonded matters
(~ 1.7 %). Limed green hides are in a swollen state. Papillary layer follicles
contain the hide protein and fat decay products, scuds, that have not been
removed in the preceding processes and operations.
Re-liming is aimed at eliminating the swelling and removing calcium and
sodium sulfur from green hides. Re-liming follows rinsing the green hides with
current water; dirt, scud, and adsorbed calcium hydroxide being removed.
To remove calcium bonded chemically to collagen, acids and salts are
used, which form soluble compounds with calcium. Acid-based re-liming
scheme is as follows:
,
where P is protein, A is acid anion.
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In re-liming, inorganic (sulfuric, hydrochloric, or boracic) and organic
(formic or other) acids are used. Using strong acids may cause acidic swelling. Calcium is predominantly removed from outer layers. At pH below 5,
scud may be deposited in follicles, while it is easily washable at pH above 5.
Using boracic acid excludes grain swelling. Organic acids do not
cause any acidic swelling either, while they remove calcium well.
When re-liming with salts, such as ammonium sulfate, the process
runs according the scheme below:
(NH
4
)
2
SO
4
+ H
2
O
NH
4
OH + H
2
SO
4
NH
4
OH NH
3
+ H
2
O
Benefit of ammonium salts is that the acid is formed as far as it is
used, i.e., the process is regulated automatically. Ammonium hydroxide being
evolved remains in green hides, maintaining the alkaline medium and preventing from forming the acidic swelling. In re-liming, excessive ammonium sulfate
is used, since, in this case, binary water-soluble salt, (NH4)2Ca(SO4)2, is formed.
Calcium sulfate (CaSO4) is hardly soluble in water.
Ammonium chloride is used for re-liming, too. Calcium chloride
formed in this process is well soluble in water. However, green hides relimed with ammonium chloride has a too loose corium structure.
Ammonium chloride is used for re-liming pigskins in manufacturing
flexible leathers, as well as for re-liming green hides that have been stored in
the open air for a long time, in order to eliminate the emerged lime stains due
to transforming the insoluble calcium carbonate into calcium chloride.
Acid-salt re-liming technique is known, using a mixture of ammonium
sulfate and phthalic anhydride that transforms into phthalic acid.
Re-liming process end is identified by the off-color of green hide section at the densest site when testing it for phenolphthalein. Non-re-limed
green hides becomes deep crimson. Upon completion of re-liming, green
hides are cleansed thoroughly.
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Insufficient re-liming in manufacturing bottom leather, skirt leather,
and Russian leather may cause grain cockling and crackiness due to tannins
being deposited with calcium compounds.
Abating. Green hides intended for producing shoe upper leathers shall
be abated immediately upon re-liming. Upon abating, the green hides lose
their plumpness definitively and become air permeable and soft, while their
grain sides become smooth and silky.
Abating means short-term green hide treatment with ferments that
cause removing scud and epidermal remnants from green hide pores. In domestic leather industry, enzymic products are used, such as pancreas, pancreatin, and protosubtilin.
The core characteristic of the intensity of effects provided by enzymic
products upon green hides is their activity that is defined by the casein digestion degree of the product in standard conditions. Quantitatively, activity is
measured in conditional units, i.e., milliliters 0.1n of the sodium hydroxide
solution per a gram of the product. Casein is deposited with 0.1n of hydrochloric acid, the residuals of which are titrated with 0.1n of the sodium hydroxide solution. Then a work experiment and a blank test are performed.
Activity is computed as the difference between the amount of 0.1n of the sodium hydroxide solution used in titrating the work experiment (a1) and
the control test (a2): A = a1-a2. Along with conditional units relevant to
the number of milliliters 0.1n of NaOH solution, activity is also evaluated by
the number of enzyme units per 1 g of the product.
Animal pancreas activity is 200 U/g, or 0.4 cm3 0.1n of NaOH solution, that of pancreatin is 500 U/g, or 1 cm3 0.1n of NaOH solution, while
that of protosubtilin (a bacterial product) with and without filler is 16 and
70 U/g, respectively.
Ferments are very sensitive to the medium pH changes. For pancreatic
ferments, the optimal pH value is 7.8–8.7, while 7.4–7.6 is optimal for protosubtilin G3x. With temperature growing up to 40 °C, ferment activity increases gradually reaching its maximum at 50 °C, and then falls rapidly due
to their destruction. Abating duration depends on the rawhide type and purpose. Longer abating is required to produce soft dressing and gloving leathers. Abating is a very demanding process. With insufficiently abated rawhides, the resulting leather has a rather hard and coarse grain face. With excessive abating, leather becomes loose, and grain breaks may be formed.
Abating is checked organoleptically by the hide plasticity and air permeability and by the silky surface (hand slides easily). Fingerprints are visible on well-abated skins.
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3.8 . P i c k l i n g
Upon finishing the abating process, green hides will be pickled, that is,
processed by a solution of a neutral salt and an acid, which is referred to as
pickle solution, or just pickle. Sulfuric, formic, or acetic acid and sodium
chloride or ammonia sulfate are used conventionally.
Pickling is aimed at acidifying the green hides without swelling them
before chrome tannage and at curing the green hides before tanning. Pickles
consisting of sulfuric acid and sodium chloride are used most often, as well
as those made of sulfuric acid, an organic acid, such as formic, acetic, or
phthalic acid and sodium chloride.
Salt is absorbed by green hides, preventing them from acidic plumping
(swelling). Acid ensures reaching the required pH value of the green hides.
No swelling ensures the sodium chloride concentration of 50–80 g/dm3.
In pickling, green hides are dehydrated, with the volume of fiber bundles being reduced and the corium permeability being increased for tannin particles,
both mineral and organic.
Pickling is performed up to pH = 2.8–3.2 of the green hide section
throughout its thickness. At this pH value, appearance of defects is excluded,
such as casehardening and cockling of the hide.
Acid absorbed interacts with collagen as per the scheme:
Chemical interaction between the acid and the collagen is accompanied by destructing hydrogen bonds between the neighboring collagen chains
and therefore, by teasing the corium structure, which ensures the softness
and stretch of leather and increases its area yield.
Pickling fluids form the corium volume. Its structural elements do not
adhere when being dried, retaining the porosity of corium.
Hides are pickled at 18–23 °C, FR 0.6–1. Duration depends on the raw
hide type: 5–7 hours in producing chrome-tanned leather from larger cattle
hides and 2.5–3 hours for green pigskins; that is, the thicker and denser
the green hide is, the longer it is pickled. Unsplit green hides shall be pickled
2 times longer than the split ones. Upon pickling, green hides must be ragged
handle and white.
In practical fur production, sulfuric, acetic, formic, and other organic
acids are used in pickling. Acid saturation shall reach pH = 2.2–2.8.
Upon preparation processes and operations in leather manufacture,
hides are passed to tanning. Upon preparation operations in fur production,
fur dressing shall start.
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4 . T A N N A G E
Producing leather is a total of chemical, physicochemical, and mechanical processes and operations, among which tanning is one of the key
importance.
Humans have been mastering the art of producing leathers of animal
hides or skins since antiquity. The very first way of tanning was that
the hunter greased an animal skin and then crumpled it manually, thus performing fat tannage. Later, people started using the leaves, seeds, and barks
of various trees. Today, a wide range of various chemicals is used for tanning.
Tannage means the skin protein texturization with a tannin.
Skin texturization, or structurization, means fixing the increased fiber
separability achieved in preparatory processes, which results in considerably
changing the physicochemical properties of corium.
Scleroproteins, such as collagen (up to 80 %) and elastin, are known
to be the core component of the skin corium. Besides, globular proteins, such
as albumins and globulins, are found in hides and skins.
A protein macromolecule consists of amino-acid residues bonded via
a peptide bond. In tanning, tannins penetrate into the corium structure and
interact with the functional groups of protein peptide chains, forming stable
cross-linkages among them. Protein macromolecules get matched. Schematically, this can be shown as follows:
Here, Т is a fragment of the tannin molecule.
Upon tanning, the goods become porous, and leather fibers do not adhere to each other when being dried. This increases its tensile strength, rot
proofing, and the effects of increased temperature and ferments.
Т
COO
COO
NH2
Т
NH
2
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However, all types of tanning lead to shrinkage of the goods area.
Shrinkage gets partially restored in subsequent mechanical operations. Thermal stability of leather is characterized by the curing temperature Tc (Table 4.1), at which the test sample resizes (shrinks).
Table 4.1
Changes in the thermal stability of corium upon tannage
Corium
Tc in °C (GOST 938.25-73)
Before tannage
60–65
After tannage with:
– Tannins
70–90
– Chrome salts
90–140
– Synthetic tannins
65–75
– Formaldehydes
85–90
Thus, upon tannage and subsequent finishing, corium becomes suitable for producing various goods, such as shoes, clothes, fancy goods, as well
as saddlery and harness. Occasionally, leather is manufactured without tanning. Parchment is one of such leathers.
Tanning is an irreversible process. Tannin cannot be washed out
or transfer into a solution from the tanned semifinished goods. According
to the tannin used, there are tannage with inorganic and organic matters,
combined tannage, and tanning with polymers.
4.1. Inorganic Tanning Matters
Inorganic tanning matters include the basic complex compounds
of chrome, aluminum, iron, titanium, and zirconium.
Tanning action of inorganic compounds depends on the ability
of the metal ion to form stable complexes with the functional groups of collagen. Chrome compounds have the best tanning properties.
Tanning chrome compounds. As tanning materials, the complex
chrome compounds are used that have the oxidation number of +3 and
the coordination number of 6 and contain a hydroxyl group (OH). Chrome
compounds with the oxidation number of +6 have no tanning aptitude.
Natural source of chrome compounds is a mineral named chromite,
FeO.Cr2O3. As the source compounds for obtaining chrome tanning agents,
leather industry uses potassium and sodium bichromates, K2Cr2O7 and
Na2Cr2O
7
.
2H2O, obtained from chromite.
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Tanning chrome complexes are obtained from bichromates by reducing the chrome (+6) compounds with sodium thiosulphate, glucose, or syrup
according the following reaction:
4K2Cr2O7 + 12 H2SO4 + C6H12O
6
4K2SO4 + 8Cr (OH)SO
4
+ 6CO2 + 14H2O.
Solution color gradually changes from orange to green during reduction. Reduction completeness was estimated from the color. It must be purely
green, without any yellow or olive tints. A ready chrome tannin represents
the basic chrome sulfate, Сr2(SO4)n(OH)
6-2n
, in form of green powder or
granules. Dry chrome tannin can enter directly to the processing equipment,
or it can be used to prepare an aqueous solution at a chemical plant. Typically, they obtain solutions with the concentration of 150–170 g/dm3. Ready
chrome tannin may contain the residues of non-reduced chrome (+6) that
renders toxicity to the tannin.
Chrome (+3) has a strongly pronounced affinity of forming complexes.
Various groups and ligands named ligands are included into the internal sphere
of the complex. Coordination number of the complex is determined by
the number of ligands bound by the central atom. Chrome complexes have
the coordination number of six. Hence, ligands can be molecules, such as H2O
and NH3, or groups, such as OH-, SO
4
2-
, SO
3
2-
, HCOO-, CO
3
2-
, etc. Some ligands can be displaced by the other ones. This depends on the central atom
binding affinity, as well as on their concentration in the solution. By their affinity, ligands can be arranged as follows: Cl < SO
4
2-
< SO
3
2-
< HCOO-<
< CH3COO- < C6H5COO-< (COO)2-< HOC(CH2COOH)2COO-<
< (HOCHCOO)2-< OH-. Acidic residues characterized by the low dissociation constant, i.e., weak acids, are bound stronger to the chrome atom. For
instance, when heating the complex chrome salt solution, sulfate ion is replaced by sulfite ion, formate, acetate, phthalate, oxalate, citrate, and tartrate,
which are the residues of weakly dissociating organic acids.
When entering the internal sphere of a tanning compound, organic acids increase its particle sizes and, in certain conditions, such as two moles of
organic acid per one mole of the chrome complex and improve the tanning
process. However, at a higher concentration of the organic acid, ligands are
bound to chrome so strongly that the functional groups of collagen cannot
displace them, which hampers binding chrome to collagen.
Chrome complex compounds can be homo- or heterogenous. Complex
compounds that have identical ions or molecules in their internal sphere are
called homogenous, while those having different groups in their internal
sphere are called heterogenous.
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