Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11~79~
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Soaking and liming of hides and skins
The invention relates to a process for soaking
and liming hides and skins under -conv2n-
tional conditions, wherein the surfactanis conventionally
used are replaced by water-soluble salts o~ a carbo~yl-
containing polymer. This prevents foaming during soak-
ing and liming and subsequent processes, and gives better
leather.
Soaking is the first step in the beamhouse,
where all stages preceding actual tanning are carried
out. Soaking is intended to remove adhering dirt,
blood and any preservatives (especially sodium chloride)
from the hide and to restore the hide to the swollen
state which it possessed immediately after slaughtering.
Essentially, therefore, soaking co~prises treat-
ing the hides and skins with water. The water should
` be very soft and (to suppress rotting) should also be
cool. Howe~er, in the case of dried hides and skins,
in particular, soaking with pure water would require a
~ery long time and damage due to rotting would hardly be
avoidable. It is therefore desirable to accelerate
the soaking process and inhibit rotting. In the past
alkalis were employed for this purpose, but nowadays a
great variety of surfactants, some of ~hich have a
bactericidal action, are added for this purpose. Ho~-
ever, the use of surfactants as a rule causes foaming,
which is often a nuisance during soaking and subsequent
processes, ~nd can e;en effec ! tannin6~ not only
because of frothing-over from open vessels, but
~C
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especially because of the problems of levelness during
fatliquoring and dyeing and also during tanning A
further disadvantage of the use of surfactants is that
the liming sludge hardens in the settling tanks.
Finally, it is difficult to wash the surfactants com-
pletely out of the leather, and leather which contains
a surfactant remains water-absorbent,
The next step in the production of leather,
after soaking, is liming It serves primarily to
loosen the hair (hair-saving liming process) or destroy
(pulp) the hair, chemically or enzymatically, and at
the same time also to some extent opens up the skin,
thereby preparing it for tanning. The commonest
liming chemicals are lime and sulfide. More modern
processes employ a mercaptan instead of an inorganic
sulfide, and the lime can also advantageously be re-
placed, namely by sodium hydroxide solution in combina-
tion with sodium sulfate. Surfactants are also used as
auxiliaries for accelerating the liming process and
achieving a moreevenliming effect, but have the dis-
advantage of foaming, as already mentioned above. The
use of sugar or molasses as liming auxiliaries is of
less importance, for economic reasons.
It is an object of the present invention to pro-
vide an economical process for soaking and liming,
having the advantageous effect of the surfactants
hitherto used (namely an accelerating and leveling
effect) without their disadvantages (ie without the
problem of foaming).
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We have found that this object is achieved,
according to the invention, by a soaking and liming
process as set out in the claim. The amount of
water-soluble salt of carboxyl-containing polymer to be
employed is, for hides, from 0.2 to 2, preferably from
0.3 to l, per cent by weight, based on the salted weight
or soaked weight (or twice this percentage, if based on
dry weight), and, for fur skins, from 0.2 to 2, prefer-
ably from 0.5 to 1, g/liter of liquor
The advantages achieved on soaking are that the
problems encountered when adding surfactants, as des-
cribed above, are avoided. Furthermore, the novel
auxiliary not only increases the rate of soaking, as
compared to that achieved without a surfactant, but also
gives cleaner hides, since the polycarboxylic acids have
a fat-dispersing action. Furthermore, the hides
treated by the novel process are more suitable for sub-
sequent liming.
The advantages achieved during liming are that
the problems resulting from added surfactant, as des-
cribed above, areavoided Furthermore, the novel
additive causes the liming process to proceed more
rapidly than when liming without a surfactant; the con-
ventional amount of lime can be reduced by up to 5~/o,
hence producing less pollution of the effluent; the
liming effect is more even, since the solubility of the
lime is increased, so that the lime penetrates the hide
before the latter has become completely plump; creasing
(drawn grain) barely occurs; the grain is
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substantial~y cleaner; pigments, hair roots, sebaceous
glands and residual fat are sukstantially removed.
Because of being cleaner, the leather can also be dyed in
more brilliant hues. A further advantage is that on
subsequent tanning the chrome exhaustion is
improved compared to what is conventionally achieved.
The conventional soaking conditions are a liquor
length of from 70 to 400%, based on salted weight (if
based on dry weight, the values are twice as great);
the chemicals used are alkalis to bring the pH to 7-ll,
and/or surfactants in amounts of up to 2%, with or with-
out bactericides, the amount of which depends on their
effectiveness and on the desired effect; the temperature
should be from 12 to 32C, preferably from 20 to 25C,
and soaking requires from 5 to 48 hours.
The conventional chemical liming conditions are:
a liquor length of from 50 to 40~/0, based on salted
weight or soaked weight (these two roughly correspond);
conventional liming chemicals, as already mentioned, the
lime being used in excess (Z - 6% of the salted weight),
ie. in larger amount than dissolves, so that there is
always a saturated solution present, the inorganic sul-
fide being used in an amount of the order of 0.5 - 4%,
based on salted weight, and the mercaptan in an amount
of from 0.5 to 5%, depending onthe desired degree of
pulping.
As regards the use of sodium hydroxide and sodium
sulfate, with or without from 2 to 20 g/liter of sub-
stances which have a hydrotropic effect on albumen (for
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a definitio~, cf. H. Rompp, Chemielexikon, 6th edition,
Franck'sche Verlagshandlung Stuttgart, 1966, page 2,838),
ie., in the main, water-soluble salts of organic acids,
especially of sulfonic acids, carboxylic acids, hydroxy-
sulfonic acids or sulfocarboxylic acids, and more parti-
cularly of 4-sulfophthalic acid, instead of lime and
sulfide, reference may be made to German Patent 2,714,814.
In enzymatic liming, surfactants, with or without bac-
tericides~ are employed in addition to the enzyme; the
temperature should be about 37C and the liquor length
from 50 to 400/0, based on salted weight.
The carboxyl-containing polymer or salt thereof
may, according to the invention, be employed either only
during soaking or only during liming or, preferably,
during both. The K value of the fully neutralized
salt (ie. the salt obtained with 1 mole of NaOH or NH40H
per mole of carboxyl groups,the pH ofa ~%strength solution of
such a salt being from 8 to 9), determined by the method
of H. Fikentscher, Cellulosechemie 13 (1932), 58 - 64
and 71 - 74, should, in 5% strength sodium chloride solu-
tion at 20C, be from 10 to 150, preferably from 30 to
100.
For the purposes of the invention, carboxyl-
containing polymers are especially the homopolymers of acrylicacid or methacrylic acid, and the copolymers of acrylic
acid, methacrylic acid, maleic acid or maleic anhydride
-
-
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with other ethylenically unsaturated compounds, pro-
vided that their sodium salts and ammonium salts are
soluble at the stated use concentrations and under the
use conditions (ie , for soaking of salted hides and
skins, at relatively high concentrations of salt in the
soaking liquor; for liming, in a saturated -
calcium hydroxide solution). The copolymers may
contain up to 80, preferably up to 60, per cent by
weight of ethylenically unsaturated compounds which do
not contain a carboxyl group and are copolymerizable
with acrylic acid, methacrylic acid or maleic anhydride,
eg. acrylamide, methacrylamide, acrylonitriie, meth-
acrylonitrile, acrylic acid esters, methacrylic acid
esters, ethylene, isobutene, styrene and vinyl isobutyl
ether. Of course, with a view to the neutralized
polymers being water-soluble, higher proportions of
hydrophilic comonomersthan of hydrophobic comonomers can
be tsed. The particularly preferred polymers con-
tain from 0 to 50% by weight of the carboxyl-free
comonomers; they may also contain a plurality of
comonomers as copolymerized units - for example, copoly-
mers of acrylic acid, acrylonitrile and acrylamide may
be used. The appropriate copolymers are known and
are obtained by polymerizing acrylic acid or meth-
acrylic acid or copolymerizing these, or maleic anhy-
dride, with suitable comonomers. It is important that
the polymers should be water-soluble at a neutral pH
(when they are partially
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neutralized~ or at least at a slightly alkaline pH (when
they are fully neutralized3, ie. at a pH of from 7 to 9
In principle, not only the sodium salts and
ammonium salts, but other water-soluble sa~ts of the
stated polymers could be used, but these are ruled out
for industrial purposes, on economic grounds
The salts are obtained by polymerizing the sodium
salts or ammonium salts of acrylic acid or methacrylic
acid, or copolymerizing them with suitable comonomers,
or by neutralizing the polymers of the stated acids
with ammonia or preferably with sodium hydroxide solu-
tion, or by copolymerizing maleic anhydride with co-
monomers, eg. styrene, ethylene, l-olefins, such as
isobutene, or vinyl isobutyl ether, preferably in a
molar ratio of l : 1, and then hydrolyzing the anhydride
groups with an e~uivalent amount of sodium hydroxide
solution.
In the Examples, liquor lengths and
percentages are based either on salted weight or soaked
weight, or on dry weight, of the hides and skins, which
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ever condition of the hides and skins is referred to in
the introductory sentence of the particular Example,
EXAMPLE 1
Soaking of salted raw hides.
200 % of water at 28C
0.5% of sodium polyacrylate, K value 40
0.3% of calcined sodium carbonate
6 hours
The soaking time is about 30~0 shorter than when
using the same formulation without the additive accord-
ing to the invention.
EXAMPLE 2
Soaking of air-dried raw hides:
400 % of water at 28C
1 % of the sodium salt of a copolymer of 60
parts by weight of acrylonitrile and 40
parts by weight of acrylic acid; K ~alue
0.5% of calcined sodium carbonate
lO hours~ drain off the liquor.
Res~oaking~
300 % of water at 28C
0.5~o of the above copolymer salt
0,2% of calcined sodium carbonate
8 hours
The soaking is 30-40% faster than when using
the same formulation without the additive according to
the invention; the water pick-up of the soaked hide is
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substantially greater; fleshing after soaking is
facilitated
EXAMPLE 3
Soaking of salted raw fur skins:
500 % of water at 28C
0.5 g/l of the ammonium salt of a copolymer of
35 parts by weight of acrylonitrile and
65 parts by weight o~ acrylic acid, K
value 125
8 hours in a paddle vat, running lO minutes
- per hour.
Soaking is about 30% faster; fleshing after
soaking is facilitated; the flesh side and, in the case
of light-colored furs, ~lso the hair, i5 substantially
lighter than after soaking without the additive accord-
ing to the invention
EXA~LE 4
Soaking of air-dried raw furs:
l,000 % of water at 28C
l g/l of the sodium salt of a polyacrylic
acid of K value 40
15 hours in a paddle vat, running for 0 min-
utes per hour
- Soaking is 50% faster than without the additive
according to the invention; fleshing after soaking is
substantially facilitated, due to the greater water
pick-up; the flesh side and, where relevant, the hair
are once again substantially lighter.
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. EXAMPLE 5
Liming of salted raw hides: -
150 % of water at 28C
0,3% of the ammonium salt of a polyacrylic
acid of K value 40
0.5% of sodium hydrosulfide (NaHS)
2 % of 600/o strength sodium sulfide
2.5% of slaked lime
18 hours
The liming chemicals penetrate the pelts sub-
stantially more rapidly; the chemicals are more uni-
formly distributed through the cross-section of the
hide, giving smoother pelts (less drawn grain); the
color of the pelts is lighter, due to the easier removal
of the scud (hair pigments); mechanical processing on
the fleshing machine is easier; due to the improved
solubility of the lime, lime blast after rinsing is less
pronounced.
EXAMPLE 6
Liming of dried goatskins:
100 % of water at 28C
0,5% of the sodium salt of a copolymer of
styrene and maleic anhydride (molar ratio
l : 1), of K value 40
3 % of 60% strength sodium sulfide
3 % of slaked lime
18 hours
Resoaking.
200 % of water at 28C
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2 % of slaked lime
0.2% of the above copolymer salt
12 hours
The opening-up action of the soak is faster. the
pelts are smoother and cleaner, the color is lighter,
the mechanical working of the peits (fleshing and scud-
ding) is easier and on rinsing with hard water, lime
blast is avoided.
