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Файл:Innovative Technologies for Manufacturing Leather and Fur Products. A Study Guide
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81
sodium carbonate or hyposulfate. Hyposulfate is preferable because it makes
fur skins more flexible.
To maintain the appearance of the hairs that represent the main value
of fur, tannage is performed at the low concentrations of chrome compounds
having reduced basicity.
Tannage is usually finished as soon as the shrinking temperature
reaches 70–75 °С.
Wool sheepskin tanning. Wool and fur-finished sheepskins are tanned
using one-bath method.
Unlike fur-finished sheepskins, wool sheepskins are valued for
the commercial properties of both skin and fur. In tanning wool sheepskins,
solutions are used at higher concentrations and with higher basicity than in
tanning fur-finished sheepskins.
Unlike fur-finished sheepskins, combined pickling and tanning are
used in producing wool sheepskins. Shrinking temperature must be
at least +78 °C.
Chrome tanning control. The following types of control are used in
tanning processes:
1) Control of mixing the solutions;
2) Control of loading the tanning solution; and
3) Tanning process control:
– Diffusion monitoring by colors of rawhide sections of leather bodies;
– Tanning monitoring by shrinking temperature and by getting tanned.
4 . 2 . N e u t r a l i z i n g
Neutralizing hides already tanned is aimed at their deacidification. Resulting from neutralization, the bonds of fixed chrome complexes get stabilized. At the same time, there is a change observed in the bonded chrome
complexes, namely: Acid residuals are displaced from the internal sphere by
the functional groups of collagen, while anionic residuals of neutralizers and
hydroxyl groups enter the complex fragmentarily.
Sodium bicarbonate NaHCO3, sodium carbonate Na2CO3, sodium oxalate NaOOC–COONa, sodium tetraborate (borax) Na2B4O7, and sodium hydro phosphate Na2HPO4 are used as neutralizers.
82
Thus, for instance, without any neutralizers, the chrome complex contained 35 % of OH- and 65 % of SO
4
2-
in its internal sphere. Upon neutralizing it with sodium bicarbonate (NaHCO3), the following results were obtained: 20 % of HCO3-, 74 % of OH-, and just 6 % of SO
4
2-
the values were
respectively 11 % of HPO
4
2-
, 77 % of OH-, and 12 % of SO
4
2-
after neutral-
izing with sodium hydro phosphate; and 16 % of CO
3
2-
, 81 % of OH-, and
3 % of SO
2
4-
after neutralizing with sodium carbonate (Na2CO3)
Solutions that have the рН value not exceeding 9 are normally used
for neutralizing, since with рН values over 9 chrome complexes get de-
structed and leather becomes detanned.
To ensure the uniform neutralization, it is recommended to use buffers
consisting of ammonia and ammonium salts of pH = 7–8. At this pH value,
collagen-bonded acid gets removed without changing the chrome complex.
During neutralization, acid is removed easier from upper layers than
from middle ones. At the same time, different neutralizers act differently
(Table 4.2). Thus, for instance, the basicity of a chrome salt was 55 % on a fiber without neutralizing, while it increases with introducing a neutralizer.
Table 4.2
Basicity changes in neutralizing
Green hide
characteristic
Without
neutralizing
Neutralizing with
Na
3
PО
4
NaHCO
3
Na
2
B
4
O
7
CH
3
COONa
Na
2
CO
3
Basicity (%)
55.0
86.6
82.7
67.4
62.7
70.6
рН of external layer
4.4
5.5
5.8
6.4
5.4
6.4
рН of middle layer
4.2
4.8
5.8
5.0
5.4
5.2
Neutralizers are usually added as 2 % of the semi-product weight.
Neutralizing process is as follows:
1. Hide shall be rinsed flowing water in a drum at the temperature of
30–40 °C for 30–40 minutes. Rinsing is aimed at removing free acid, neutral
salts, and unbonded chrome compounds as much as possible.
2. Neutralizing proper. For this purpose, a neutralizer shall be poured
into the drum into the 10–20-fold amounts of water in 2–3 stages with an
interval of 10 minutes. FR 2–2.5, temperature 30–40 °C. Neutralization lasts
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45–60 minutes. End of the process is determined by the section рН value that
must be 4.5–5.5.
3. Green hides shall be rinsed with water at the temperature of 30–40 °C
for 30–40 minutes. Rinsing is aimed at removing the neutral salts formed
from the green hide. Non-removed neutral salts get crystallized during drying, which leads to forming white bloom on the hide surface.
Upon rinsing, the hide must be transferred immediately to dipping and
greasing. Otherwise, acid remaining in the middle layer will diffuse into
the grain layer surface and impede greasing.
Sheep skins shall be neutralized before dipping. Fur pelts are neutralized by rinsing them first with clean water and then with some sodium bicarbonate added to water.
4 . 3 . R e t a n n a g e
Chrome-tanned hide retannage is aimed at increasing the hide body,
outer layer density, grindability, and embossing capacity. In retannage, vegetable and synthetic tanning agents and amino resins are used, as well
as chrome/aluminum/zirconium compounds. Due to retannage with vegetable and synthetic tanning agents, the properties of chrome-tanned hides approach to those of skins processed by vegetable tannage, which is undesirable for shoe upper leathers.
Due to retannage with amino resins, skin tensile resistance, wear resistance, and water resistance increase, its loose grain reduces, and grindability improves. Amino resins are used for re-tanning nourishing chrometanned leathers with snuffed and natural grain.
For re-tanning natural grain leathers, inorganic tanning agents are often used: Complexes of chrome, aluminum, and zirconium.
Retannage is performed as needed. Retannage with organic tanning
agents is performed upon greasing or nourishing operations.
Defects of chrome-tanned hides. In case of improper tannage, the following defects may occur: Papery leather, wrinkled grain, and brittle grain.
Papery leather means no hide body. This is caused by low tannage
degree, in case of using high-basicity solutions and mellow green hide. This
defect can also occur due to overneutralization that causes detanning of outer
hide layers.
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Wrinkled grain means fixed wrinkles and folds. This is caused by improperly preparing green hides for tanning (insufficient reliming or steep basicity increase in chroming) or high acidity of green hides.
Brittle grain means the brittleness of outer skin layer, whereby it chaps
when bending or laying along leather. Brittle grain results from strongly
binding the tanning agents in the outer skin layer, in case of tanning routine
violation.
Insufficient yield of fur skins means the defect characterized by reducing the skin yield with higher tanning intensity. It should be controlled by
leather cure temperature. Insufficient yield may also be caused by heterogeneously tanned corium.
Properties of chrome-tanned hides. Chrome-tanned leathers are quite
resistant against acids and alkali. Their stretch, softness, and elasticity are
higher than those of leathers tanned with tannins. In humid conditions, such
leathers get wetted quicker and dry slower. Chrome-tanned leathers are better
resistant against high temperatures. Dried chrome-tanned leathers cannot be
completely re-macerated in cold or hot water. When drying, additional bonds
occur that do not disappear upon maceration. Volume yield of chrometanned leathers is 200–300 cm3 per 100 weight fractions of green hide substance. It depends on the density of corium structure and on the number of
bonds among the molecular chains of protein and tanning particles.
Chrome tannage ecology. A distinguishing feature of leather producing enterprises and particularly their tanning yards is high water consumption
and, therefore, large volumes of wastewater.
Suspender tanning liquors contain 2 to 10 g/dm3 of chrome oxide. Their
basicity ranges within 38–55 %. Upon bringing the concentration to the required
one and subsequently filtered, tannage sewage can be re-used for pickling, i.e.,
tanning up to ten times without compromising stock properties.
Another way to use chrome compounds from spent solutions is their reclamation. Its essence consists in using alkali metal hydroxides to precipitate
chrome compounds as chrome hydroxide (+3). The precipitations formed shall
be dissolved in the estimated amount of sulfuric acid, producing solutions containing 80–90 g/dm3 of chrome oxide and having the basicity of 35–38 %.
The solutions obtained are used to tan the next lot of green hides.
Aluminum tannage. Tannage using aluminum compounds has been
known since the earliest times. There are hundred times more aluminum
compounds in within the Earth’s interior than chrome compounds. In their
properties, aluminum salts are like those of chrome. In compounds,
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aluminum, just like chrome, has the oxidation degree of 3 and is a complexing agent with the coordination number of 6.
Initial salts used for preparing tanning aluminum compounds:
Al2(S04)
3
·18Н
2
O, a readily water-soluble aluminum sulfate (aluminum
earth); and Al2(SO4)2Me2SO4·24H2O, “alum”, that dissolve in water harder
than aluminum sulfate. Alum-tanned leather is distinct in softness, smooth
and delicate upper side, and white color.
However, using aluminum salts is limited by tanning leather types
only which are used to produce items that are not exposed to water when
being used, because aluminum complexes can be easily destructed by water.
Hydrolysis of aluminum and chrome compounds runs in three stages
according to a general scheme:
[Al(H2O)6]Cl
3
↔ [Al(H
2O)5
OH]Cl
2
+ HCl;
[Al(H2O)6]Cl3 ↔ [Al(H2O)4(OH)2]Cl+ 2HCl;
[Al(H2O)6]Cl3 ↔ [Al(H2O)3(OH)3] + 3HCl.
However, chrome compounds are hydrolyzed successively, i.e., com-
plex compounds with the basicity of 33.35 % are formed at stage 1, followed
by the second stage and then by the third one; while all three stages are observed simultaneously at the hydrolysis of aluminum compounds. Therefore,
in alkalizing aluminum compound solutions, aluminum hydroxide precipitate is formed at low basicity (20 %) and at pH 4.2 already.
Stability of aluminum complexes can be increased by using the anions
of organic acids, such as acetic, formic, ethanedioic, amber, or citric, as ligands. Aluminum can form stable complexes with vegetable and some synthetic tannins.
Aluminum complexes can be stabilized with water-soluble synthetic
polymers, such as polyacrylic acid, polyvinyl alcohol, and amine resins.
When interacting with collagen, the stabilized aluminum complexes
are bounded to it via both carboxyl and hydroxyl groups, forming a strong
three-dimensional structure stable in aqueous solutions.
“Alum” is used in producing glace and garment leather and taw, as
well as in processing chrome-tanned leathers to increase their heat resistance.
Tanning technique used in producing glace leathers. Tannin compo-
sition as percentage of the green hide weight: “Alum” – 8, dairy salt – 2,
finely dispersed wheat flour – 16, egg yolk – 2–4, and water – 60. The flour
is covered with warm (30 °C) water and stirred until thick and smooth. Then
the water emulsion of yolk is added and stirred again. Then “alum” and dairy
86
salt solutions are added. Being dissolved in water, “alum” forms sulfuric acid
and the basic aluminum oxide salt that has tanning properties. Maximal binding of aluminum with collagen takes place at the “alum” concentration
of 2.5 g/dm3, expressed as aluminum oxide.
Dairy salt prevents green hide from plumping. Flour, as a filler, softens
the leather. Egg yolk is a stuffing material. Drum tannage lasts 3–4 hours.
Unfinished goods stay in the drum for 8–10 hours, then hauled, right sidefaced, dried at the temperature of 40 °C, and sent to finishing.
“Alum” is used for tanning upper and bottom leathers in combination
with a chrome-based tannin. Chrome-aluminum complexes are stable
in aqueous solutions and ensure a strong bond between aluminum and collagen. The strongest bonding of chrome-aluminum complexes to collagen is
achieved by introducing sodium formate in the amount of 0,5 moles per
1 atom of aluminum when tanning.
Aluminum can form stable complexes with vegetable and synthetic
tannins; therefore, combined aluminum and organic tannins are used for tanning in producing the leathers.
Zirconium tannage. Currently, leather industry often uses zirconium
sulfates with the oxidation number of 4 as tanning agents. In terms of the tannage intensity, the main zirconium sulfates are close to chrome compounds,
while they even take precedence over chrome compounds in terms of fullness. Most common zirconium compounds in nature are zirconium sulfate
Zr(S04)
2
·4H
2
0, zirconium dioxide ZrO2, and zirconium hydroxide Zr(OH)4.
Zirconium with oxidation number 4 forms complexes with coordination
numbers 7 and 8.
Zirconium compounds in aqueous solution hydrolize quicker than
those of chrome and aluminum, forming polycyclic ol- and oxo-compounds:
Zr(H
2
O)
n
4+
(SO
4
)
2-
Zr
(H
2
O)
n-2
(OH)
2
2+
SO
4
2-
+ H
2
SO
4
,
where n is a coordination number.
In hydrolysis, ion Н+ gets removed from the complex compound mol-
ecule, while the solution acidity increases.
With increasing the temperature and pH and decreasing the solution
concentration, the hydrolysis degree increases. Hydrolysis stops where acids
87
are added. In neutralizing the free acid in the zirconium sulfate solution,
highly basic compounds can be obtained. Zirconium sulfate solutions are stable until the basicity is 50 %. With higher basicity, they become muddy and
settle out. Hydrolysis product is zirconium hydroxide Zr(OH)4 that settles
out when basified. Therefore, tanning with the main zirconium compounds
shall be performed in an acidic medium.
Stability of zirconium complexes can be increased by adding the salts
of organic acids, such as lactic acid СНз-СН-(ОН)-СООН, or by obtaining
mixed complexes containing chrome, aluminum, and titanium.
When increasing the concentration of solutions in an alkaline medium,
zirconium complexes grow forming ol- and oxo-compounds according to
the scheme below:
...
Zr H + OH Zr
...
Zr
OH
OH
Zr
+ H
2
O
Zr O Zr
... ...
Freshly made zirconium sulfate solutions contain cationic (38 %), anionic
(11 %), and neutral (51 %) complexes. When heating zirconium sulfates, white
fine-crystalline precipitate settles, zirconium tetrahydrate-disulfate:
Zr(H
2
O)
4
O
O Zr (H
2
O)
4
Zr(H
2
O)
4
O Zr (H
2
O)
4
O
(SO
3
H)
2
(SO
3
H)
2
(SO
3
H)
2
(SO
3
H)
2
Sizes of complexes are of great importance regarding their tanning
properties. The larger the complexes are, the quicker they get fixed by green
hides, but their diffusion is hampered. Molecular weight of the zirconium
compounds used for tanning is 600–2,100, while the tanning particle contains at least four zirconium atoms.
88
Shrinking temperature of leather tanned with zirconium compounds is
89–91 °C.
Interaction of zirconium compounds with collagen is generally of adsorption nature. At the same time, an 8–15-nm-thick “shield” of zirconium
compounds is formed on the structural elements of collagen. Basically, zirconium compounds are bonded to collagen via nitrogen-containing groups
(–ΝΗ2, –NΗ–) according to the scheme below:
Hydrogen- or weaker bonds can also be formed. In this case, corium
is just slightly structured, but its volume is formed well. Tanning compounds
hamper corium shrinkage when being dried. Tanning with zirconium slats is
used to produce shoe upper and bottom leathers.
In our country, the tanning zirconium compounds are used, which represent the sodium sulfozirconate, i.e., ZrО2·1.2SО4·1.1Na2SO4·nH2O. It represents a white or slightly yellowish crystalline mass containing 27–30 %
of ZrО2 and having the basicity of 36–42 %.
Zirconium-tanned leathers have high resistance to light, increased
density and abrasive resistance, pure white color, good grindability, and perspiration resistance.
Zirconium compounds are used to tan both pickled and non-pickled
green hides. When tanning pickled green hides to produce shoe upper leathers, the hide products shall be pre-treated with sodium formate or synthetic
tannins. This is necessary for deeper and more uniform distribution of zirconium compounds in corium.
Zirconium-tanned leather is more compact and firmer than chrometanned one; therefore, the stuffing yield increases. Zirconium compounds are
most frequently used combined with the tanning chrome or titanium compounds.
Titan tannage. Titanium is one of the common elements in the earth
crust. Its most important compounds include titanium chloride TiCl4, titanium oxychloride TiOCl2, titanium oxysulfate TiОSO4, titanium hydroxide
Ti(OH)4, and titanium dioxide ΤiО2. Basic titanium compounds have slightly
lower tanning properties than those of chrome and zirconium.
89
For tanning, titanium and ammonium sulfate is used, i.e.,
(NH4)2TiO(S04)·nН2О. This salt is highly water-soluble and more hydrolysis-resistant than titanyl sulfate TiO(SO4)·nH2O. Basicity of titanium and
ammonium sulfate is 42–47 %, while the content of titanium oxide ТiO2 is
at least 19 %. By its appearance, titanium and ammonium sulfate is a white
crystalline powder. Structure, composition, and properties of titanium- and
zirconium salts with the oxidation number of 4 are not dissimilar. Titanium
salts hydrolyze in water, forming the basic titanium salt and sulfuric acid,
which is why the solution is acidic.
Tanning titanium salts in the solution are as complex compounds, predominantly of anionic nature, while ol- and oxo-compounds are formed, as
well. Ol-compounds are bi- or multinuclear compounds and transfer to oxocompounds at increased temperature and basicity.
Titanium complexes are less stable than chrome compounds. They are
normally stabilized using organic oxy-acids, dibasic acids, and polyatomic
alcohols. Mixed complexes of titanium and zirconium are quite stable.
Solutions used for tanning have a basicity of 40–60 %.
Cure temperature of a green hide tanned with titanium salts 80–85 °C,
while it may reach 100 °C after neutralization. Materials are neutralized with
the mixture of sodium sulfite and hexamethylenetetramine. Titanium tanning
agent consumption is 10 % of untanned hide weight expressed as TiO2.
Titanium-tanned leather is white. Titanium complexes are used in currying bottom leather. They are also used in combination with the compounds,
such as Cr(+3), Al(+3), and Zr(+4).
Tanning with iron compounds. Basic salts of Fe(+3) have tanning
properties, while Fe(+2) does not.
Ferric salts with oxidation degree 3 in tanning solutions, as well as
chrome and aluminum compounds, form complexes. An example of a firm
complex iron compound is potassium ferricyanide K3Fe(CN)6. Other complex iron compounds with mineral acids anions are instable and, because of
hydrolysis, form ferric hydroxide residues.
Hydrolysis resistance of tanning iron compounds is increased by
introducing organic acid salts and other organic substances containing
OH-groups into solutions.
Where iron compounds interact with collagen, coordination bonds are
formed between them. Functional groups –NH2 and –COOH penetrate
the internal sphere of complex Р1NH2---Fe-OCO-Р2. To neutralize the irontanned skins, ammonium chloride (NH4Cl) is used in combination with ammonium hydroxide (NH4OН).
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Sodium carbonate (Na2CO3) and sodium hydrogen carbonate
(NaНСO3) break the iron-collagen bonds. Therefore, they are not used as
neutralizers.
Tanning using iron compounds has not been spread widely in practice,
since Fe(+3) compounds are catalysts for collagen oxidation and, therefore,
for leather decomposition during storage, and for heating in a wet state.
Silicate tannage. Natural silicon reserves are large. Sodium silicate,
Na2SiO3, is used as a source substance for making a tanning compound, its
aqueous solution is called waterglass.
Tanning action is also a property of silica sol formed by inorganic acid
acting on sodium silicate:
Na2SiO3 + 2HCl
H2SiO3 + 2NaCl
.
Upon some time, silicic acid transfers to a colloid state, the composition of which can be represented as follows:
[x(SiO2 + nH2O)ySiO3H]- + Н+.
To obtain a stable sol that would not transfer into gel when souring, it
is necessary to pour the diluted sodium silicate solution into a weak solution
of an acid, such as hydrochloric, sulfuric, etc., rather than the reverse. The sol
is only stable in a base or strongly acidic medium.
Silicate tannage is performed by freshly prepared sols. The sols used
contain SiO2 of at least 30 g/dm3 and have рН of 3–3.5. Silicic acid consump-
tion shall be 15 % of green-hide weight, calculated for silicon oxide.
Pickled pelts are impregnated with the silicic acid sol. The green hide
becomes whit, soft, and light-fast, AIT being 60–64 °C. However, when be-
ing stored, it becomes brittle and fragile due to the further polymerization
and dehydration of silicic acid on collagen fibers. Tannage with silicic acid
only is not practically used. Techniques are developed for the two-phase tannage for bottom leather. The hide is first tanned with chrome- or aluminumbased syntanes and then with chrome silicates.
Tanning using hetero-polynuclear complexes. In terms of its in-place
reserves, chrome takes the sixth place, while aluminum, titanium, and aluminum do the first, the third, and the fifth ones, respectively. Therefore,
looking for ways of using the latter ones as tanning agents is quite reasonable, even given the fact that they are exceeded by chrome-based tanning
agents in relevant properties. One of such ways is the combined use of them
as hetero-polynuclear complexes, in which the central ions are Cr(+3),
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