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Файл:Innovative Technologies for Manufacturing Leather and Fur Products. A Study Guide
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71
Depending on their charges, complexes can be cationic, neutral, or
anionic:
Cr
H2O
H2O
H2O
H2O
H2O
H2O
3+
Cl
-
S
O
O
O
O
Cr
H
2
O
H
2
O
H
2
O
OH
homogenous cationic
heterogenous neutral
S
O
O
O
O
Cr
H
2
O
H
2
O OH
OH
-
Na
+
homogenous anionic
If there are excessive anions in the solution, cationic complexes may
evolve into anionic ones. Most chrome compounds are hydrolysable in aqueous
solution, group OH penetrating the complex and displacing water molecules.
Chrome complexes get larger dur to condensation or polymerization reactions.
Condensation process accompanied by replacing a water molecule
with the OH-group is called olation. At the same time, polynuclear complexes are being formed, in which two or more atoms are cross-linked
by brdiges and surrounded by ligands:
CrH
2
O (H
2O)4
HO
2+
SO
4
2-
Cr
OH
H2O
(H2O)
4
2+
SO
4
2-
+
Cr(H2O)
4
Cr (H2O)
4
OH
HO
4+
(SO
4
2-
)2 + 2H2O
ol-compound
72
Olated (ol-) compounds continue changing with the time: OH-group
educes a hydrogen ion, and ol-compounds evolve into oxo-compounds. This
process is called ageing:
Cr(H
2
O)
4
Cr (H
2
O)
4
OH
HO
4+
(SO
4
2-
)
2
(SO
4
2-
) + HSO
4
-
3+
O
OH
Cr (H
2
O)
4
Cr(H
2
O)
4
(SO
4
2-
) + H
2
SO
4
2+
O
O
Cr (H
2
O)
4
Cr(H
2
O)
4
oxo-compound
In ageing, the oxo-compound is formed, and the acid is educed, which
results in the reduced pH of the solution.
Tanning is also affected by the molecular weight of chrome complexes. Particles having low molecular weights penetrate corium quite
quickly, while those having higher molecular weights do slowly, but they
still quickly get bound to collagen. Basically, excessive molecular weights
of chrome complexes may impede the tanning process.
In the solutions of basic chrome salts, both ol- and oxo-compounds are
formed and the complex itself gets larger, forming a chrome tetramer and
running under the following scheme:
(SO
4
2-
)4
4+
HO
OH
Cr (H
2
O)
4
Cr2(H
2
O)
4
Cr(H2O)
4
OH
OH Cr (H2O)
4
OH
Cr Cr(H2O)
4
OH (H2O)
4
8+
(SO
4
2-
)4
73
Since only the basic compounds of Cr (+3) have a tanning power, basicity expressed as a percentage is an important characteristic of a tanning
compound. Basicity of a chrome complex is characterized by the number
of OH--groups bound to the chrome atom falling within the oxidation degree
of three. Thus, the simply represented chrome sulfate complex Cr(OH)SO4
has the basicity of 1:3 = 33.3 %, while that of Cr2(OH)4SO4 is 2:3 = 66.6 %.
A mixture of such complexes provides the basicity of 40 % necessary for
tanning. As a maximum, chrome can add three hydroxyl groups to its bonds,
but this forms Cr(OH)3 insoluble in water and having no tanning action.
Chemical basics of chrome tannage. Tanning with basic chrome complexes is a chemical process that results in forming a strong chemical bond
of chrome atoms to collagen. Collagen structure contains various functional
groups: –COOH, –NH2, –NH, –OH, etc. Chrome compounds are bound prevailingly on the –COOH and –NH2-groups of collagen, located in side
chains. At the same time, coordination, ionic, and hydrogen bonds can be
formed.
Coordination bonds are formed due to entering into the internal sphere
of the complex of ionized –COOH-groups and non-ionized –NH2–
or –OH-group:
These strong bonds do not get broken when exposed to water. They determine the irreversible bonding the tanning chrome compounds to collagen.
Ionic bonds are formed between opposite-charged groups:
1) Between the negatively charged (anionic) complexes and the posi-
tively charged protein; and
2) Between the positively charged (cationic) complexes and the nega-
tively charged protein:
NH
3
+
(CH2)
4
-
Cr(H2O)
2
S
O
O
O O
HO OH
-
COO
-
Cr OH
OH
(H
2
O)
4
+
Ionic bonds get broken when exposed to water and are intermediate in
evolving into stronger coordination bonds. It is considered that, in the first
stage of tanning, chrome is being bound to protein involving considerably
ionic bonds that evolve into strong coordination bonds with the time, especially during neutralizing and drying.
Hydrogen bonds are formed between the hydrogen atom
of the chrome complex OH-groups and the nitrogen or oxygen atoms
of the functional groups of collagen:
These bonds are weaker as compared to the other ones. They do not
ensure an irreversible chrome-collagen bond.
Chrome complexes can bond to protein via one or more bonds. If there
are at least two bonds, a bridge is formed that cross-links the adjacent chains of
collagen. It is the cross-linking of the molecular chains of protein that account
for the following properties of tanned leather: Increased shrinking temperature,
chemical resistance, hygrothermical stability, and enzymic stability.
Factors affecting the process of chrome tannage. Chrome tannage is
74
performed by drumming or paddling with an aqueous solution of chrome
75
compounds and suggests two main stages: Diffusion of the tannin into corium and binding it with the protein macromolecule.
Initially, tannin diffuses via capillaries, from which chrome compounds enter the reaction center of protein. Some factors may affect diffusion. Let us consider the most important ones.
Corium structure separation and loosening degree in preparatory
process and operations. The looser the corium structure is, the higher
the chrome compound diffusion rate in corium. Tannins penetrate the looser
types of green hides or looser areas, such as belly or shoulder, than the denser
ones. Other conditions being equal, long liming, abating, and pickling accelerate diffusion process. Rawhide swelling (plumping) hinders it.
Tanning particle sizes. The larger the tannin molecules are, the lower
the diffusion rate is. Their sizes are, in turn, related to the tanning solution
basicity. The higher the basicity is, the higher the probability is that
the olation processes may run, oxo-compounds may form, and polymerization may run, and the larger the tanning particles are. Thus, the molecular
weight of chrome-sulfate compounds is 796 at the basicity of 33 % and 947
at the basicity of 50 %. Therefore, the diffusion rate of the 33-% basicity solution is considerably higher than that of the 50-% one.
Green-hide acidity. Partial saturation of collagen with acid during
pickling ensures a higher diffusion rate of the tanning chrome compounds
into the corium structure. As acidity increases, basicity decreases, and therefore, so does the molecular weight. Moreover, acid blocks the nitrogen-containing groups of protein, thus preventing the premature tannin binding
to collagen. To ensure a normal diffusion process for tanning chrome compounds, raw skins must have a pH of 2.8–3.2 upon pickling. Optimal value
of pH for hair skins is 2.2–2.8.
Concentration of tannins and solution temperature. Increasing both
tannin concentration and solution temperature leads to faster diffusion.
Mechanical impact. Tannins diffuse faster into the half-finished product when drummed. Mechanical impact accelerates diffusion due to balancing the concentrations in the layers adjacent to the half-finished product
through destructing them when mixing the solution.
Diffusion process is monitored based on the rawhide section color
changing from white to light blue or green.
Process of binding the chrome tannins to protein is affected by some
factors. Here are some of them.
Tanning solution basicity. As basicity increases, binding increases,
too, and vice versa. Excessive basification may lead to binding the tannin
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with preliminarily surface layers of corium, which may hinder diffusion and
result in nonuniform tanning. Thus, with the basicity of under 40 %, the middle layer of corium contains the highest amounts of chrome in terms
of Cr2O3, while basicity of over 40 % results in the highest amounts of it in
the outer layer. For tanning the pickled raw hides, a tannin having the basicity of 36–42 % is normally used.
In fur processing, the tanning solutions of even lower basicity are
used. Thus, in tanning the Astrakhans, the basicity is 15–20 %. For sheepskins and coneys, it is 35–40 %, while peltries are tanned at the basicity
of 5–10 %.
Solution pH. A pH decreases, binding decreases, too, and vice versa.
Increasing pH from 3 to 5 leads to increasing the protein –COOH-group ion-
ization degree from 75 to 100 % and promotes the emergency of non-ionized
NH2-groups. This results in increased binding.
Chrome complex nature and composition. Skins tanned with cation
complexes have the highest shrinking temperature, while those tanned with
anion complexes have a much lower one.
Concentration of chrome compounds. Most chrome is bound to protein at the chrome concentrations of 10–30 g/dm3 in terms of chrome oxide.
As the concentration continues increasing, the binding decreases, which is
explained by changes in chrome complexes in concentrated solutions, i.e.,
less active neutral and anion complexes are formed.
Tanning solution temperature. Heating makes hydrolysis of chrome
complexes stronger, basicity higher, and therefore, binding increases, too.
Increased binding of chrome to collagen at higher temperatures is especially noticeable in dilute solutions. For this reason, in the end of tanning,
the solution shall be heated up to 40 °C by adding water heated up to 60 °C.
This process is called heat treatment of leather. It promotes a more uniform
distribution of tannin over the thickness of skin and an increase in the content
of Cr2O3.
Availability of neutral salts. Neutral salts present in the solution
change chrome complexes, sodium sulfate affecting more than sodium chloride, since ions SO
4
2-
are bound stronger and enter more actively into
the complex. At the same time, neutral and anion complexes are formed with
lower tanning properties. Sodium chloride reduces swelling (raw skin
plumping) and promotes the tanning process, to accelerate which aluminum
sulfate is added. Polynuclear complexes are formed, which simultaneously
contain chrome and aluminum as central atoms. Such complexes allow
achieving the required chrome content at lower chrome tannin consumption
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and at the practically complete depletion of the tanning solution. This results
in reducing the contents of toxic chrome salts in wastewater.
Impact of organic acid salts / masking substances. Introducing them
into the chrome tannin improves the leather qualities: It becomes fuller and
smoother; chrome compounds get distributed over its thickness more uniformly; and the total chrome content increases in leather and decreases in
the spent solutions. However, this effect manifests when adding at most two
moles of organic acids per 1 mole of chrome; therefore, large anions enter
the complex, it grows and binds to the collagen better. In case of more organic acids, chrome binding reduces, since they are not easily substituted by
the functional groups of collagens at a too high content of organic ligands in
the internal sphere of the complex.
Tanning solution ageing. Maximum binding is achieved at the tanning
solution age of 48 hours. Further ageing reduces the chrome binding to collagen due to changes in chrome complexes.
Tannage duration. The maximum binding is achieved during the first
2–4 hours of tanning, then the process gets slower and then stops practically
completely. This is explained by changing chrome complexes due to the migration of a part of the acid from the pickled raw skins into the solution.
At the same time, basicity reduces strongly.
Aging of the tanned semi-finished leather. Noticeable increase in binding chrome with collagen takes place after 24 hours of ageing the semi-finished goods after tanning. Therefore, ageing is included into the processing
methods of producing chrome-tanned leather.
Chrome tanning technologies. One- and two-bath methods are
the most popular technologies. One-bath tanning means tanning the skins in
a single bath with the solution of tanning chrome compounds. Pickling reduces the basicity of the chrome tannin early in the process, which leads to
a slow binding of chrome compounds to protein and promotes diffusion and
the uniform distribution of the tannin in the corium thickness. To strengthen
the binding, a high-basicity tanning solution is added, while sodium carbonate, Na2CO3, is added in the end of the process. Depending on the float
ratio, tanning can be normal (FR 1–1.5), half-dry (FR 0.6–0.9), and dry
(FR 0.3–0.5). One-bath tanning can be single-phase or double-phase.
Single-phase tanning is performed in a spent pickling solution, into
which dry chrome tannin or a concentrated solution of chrome tannins
(150–170 g/dm3 of chrome oxide) with the basicity of 36–42 % is added in
two stages with an interval of 30 minutes. In case of using a tanning solution,
pickle is poured off in the amounts equaling to the chrome tannin solution
78
(FR 0.7–0.8). In 3–4 hours of the tannage start with full penetration
(the section must be bluish-green), 5–10-% solution of Na2CO3 in
the amount of 0.1–0.3 % of raw hide wight is poured into the drum in several
stages. Sodium carbonate solution is poured slowly to avoid depositing of
chrome hydroxide on the grain face of the half-ready hide, thus preventing
stains and grain cracking. In 6–8 hours from the tannage beginning, the tannage extent shall be tested. If the test is negative, sodium bicarbonate
Na2CO3 is added, while chrome tannin is added in case of a low (2–3 g/dm3)
concentration of chrome in terms of Cr2O3.
Tannage duration is 10–12 hours for raw cowhides, 8–10 hours for
goatskins, and 6–8 hours for sheepskins. Upon finishing the tannage, raw
hides are unloaded, folded face-to-face, and put onto a tray for laying away
for 24 hours. If the semi-finished hides are laid away in an undiligent manner, the non-flattened grain gets overtanned, which results in forming irreversible folds.
Double-phase one-bath tannage is used to process cowhides of medium and large sizes and performed with the solutions of tanning chrome
compounds of two basicity levels (24–27 and 48–50 %) at FR 0.6–0.7.
The process lasts 8–10 hours, starting temperature is 18–22 °C and
it reaches 40 °C by the end of the process. Upon pickling, a solution with
the basicity of 24–27 % is poured into the drum in the amount of 1.3–1.5 %
of the raw material, in terms of chrome oxide. In 1.5–2 hours, a solution with
the basicity of 48–50 % is introduced in the amount of 0.7–0.8 % of the raw
material weight, in terms of chrome oxide, while the sodium sulfite solution
is added in the amount of 0.8 % of the raw hide weight in 3–4 hours of
the tannage beginning.
Varieties of one-bath tannage are masked chrome compound tannage
and dry tannage. Chrome complexes with the anions of organic acids as ligands are called masked compounds.
Tanning with the masked chrome compounds helps produce leather
having a smoother and denser surface grain and well filled-in peripheral areas. Masked chrome compounds are absorbed by corium faster and in greater
amounts. This happens, because masked anions weaken the chrome bonds to
other ligands. At the same time, multiple labile groups are formed
in the chrome complex, which can be easily replaced by the functional
groups of collagen. Sodium formates and phthalates are mostly used as
masking agents.
Using the masking chrome compounds does not require increasing
the tannin basicity in the end of the process to increase binding. Masked
79
tannin is distributed uniformly in the corium thickness and is bounded well.
Masked chrome compounds have buffer properties, i.e., maintain pH within
certain range. Therefore, they diffuse easily and are distributed in the corium
uniformly. Masked chrome compounds are used to tan unpickled raw hides.
Unpickled tannage is used in producing elk uppers and lining leather.
In producing leather from pig skins, upon abating, the raw skins are rinsed
by water at the temperature 35–37 °C for 1.5–2 hours and then processed by
dicyandiamide-formaldehyde (DDAF) resin. DDAF increases the corium
porosity and permeability, promoting the diffusion of chrome compounds.
Chrome-emulsion tannage is also unpickled. In this case, raw skins
are processed with fat liquor before tanning. Fat components of emulsions
protect collagen fibers against premature binding to tanning chrome compounds.
Chrome-emulsion tanning techniques consist in the following. Delimed and softened raw skins is rinsed in a drum, then fresh water
(up to FR 0.6) and fat emulsion in the amounts of 2.3–2.6 % of the raw skin
weight are poured into the drum. Fat liquor composition: Spindle lubricant –
47.5 %, water – 47.5 %, and surfactants – 5.0 %. Processing by emulsion
lasts 20 minutes, then the chrome tannin with the basicity of 32–34 %
is introduced into the drum. Temperature is 25–28 °C. Tanning duration
is 6–8 hours.
Chrome-tanned skins are fuller than those tanned upon pickling.
This method is used to produce suede leather from pigskins.
Dry tannage is conducted at low FR (0.3) and at the high chrome oxide
concentration (40–50 g/dm3). High concentration facilitates diffusing tanning compounds into corium since there are fewer high-basicity and highmolecular tannin particles in highly concentrated solutions than in diluted
ones. Defects are prevented, such as wrinkled grain. When using dry tannage
for unpickled raw hides, they must be delimed completely. Otherwise, casehardening bands may occur at the boundary between the basic and the neutral layers. Despite some advantages, dry tannage has not been used widely
due to the lack of appropriate equipment.
Two-bath tannage is performed in two solutions (baths): A chroming
one and a reducing one. In the chroming solution, raw skins are processed
with the sodium bichromate (Na2Cr2O7) solution in presence of mineral acid,
dichromatic acid being produced: Na2Cr2O7 + H2SO4 → H2Cr2O7 + Na2SO4.
Raw hides absorb and bind dichromatic acid, becoming yellow. Collagen
bound to acid –Р1NH
2
.
H2Cr2O
7
.
H2NР2– cannot be decomposed by water or
80
destructed by mechanical impacts. However, dichromatic acid (Cr+6) does
not provide any tanning action.
Chroming usually lasts 3–4 hours. Shrinking temperature increases by
1–2 °C.
In the reduction solution, chromed pelt is processed by sodium thiosulfate in acidic medium. With this processing, dichromatic acid reduces on
the collagen fibers, forming chrome complexes that interact with the amino
groups of collagen. Reaction of reducing dichromatic acid to the basic
chrome compounds:
H
2
Cr
2 O 7
+ 2 H
2
S O4 + 3 Na
2 S2 O3
2 Cr (OH) SO4 + 3 S + 3 Na 2 S O4 + 2 H2 O.
The produced sulfur deposits on corium fibers, fills the skin out, and
provides softness and delicacy to it. Reduction process is considered finished, if the raw skin section is bluish green in its densest area.
Two-bath tanned leather has a greater yield in terms of area, it has
smoother grain, delicate feel, and filled well. It also stretches less than onebath tannage skins. This may be due the fact that anion chrome complexes
interact with collagen in two-bath tannage, while in one-bath tannage these
are cation chrome complexes.
Tanning by the Original method represents a variety of the two-bath tanning method and is used to produce goat- and sheepskins. The summary of
the method is the use of bichromate and chrome potash alum in the first bath and
only thiosulfate in the second one. In dissolving bichromate in the pickling liquid, dichromatic acid is formed that diffuses into raw hides and binds to the basic
groups of collagen. Then Cr(+6) is reduced by thiosulfate, anion-type chrome
complexes being formed, characteristic of two-bath tannage. Chrome potash
alum is hydrolyzed simultaneously, forming a mixture of neutral and basic
chrome sulfates that also diffuse in the raw skins well and form bonds to the carboxyl groups of collagen, like in one-bath tannage.
Thus, in tanning by the Original method, the tanning chrome compounds are bound according to both the two- and one-bath tannage types.
Shrinking temperature of the leather samples after tannage by
the Original method must be 90–95 °C.
Tannage in fur production. Tanning semi-finished furs has its distinctions. It is known that fur skins must be very flexible and soft. To maintain
the plasticity of the pelt, it undergoes a high acid saturation at the increased
temperature in pickling. Considering that acid saturation hinders the binding
of chrome compounds. During tannage, the pickled pelts are neutralized by
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