Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
L 175 310 Fertell et al. -2-
BACKGROUND OF THE INVENTION
1. Field of the Invenl:ion
..
This invention relates to leather treatment and more particularly
to the upgrading of low density leather.
2. Description of the Prior Art
.. _
"Low density leather" as used herein means and refers to leather
splits, whole non-bovine and reptilian leathers such as goat, pig, sheep, rabbit,
raccoon, fox, snake and the like. Typically, these low density leathers have
a bulk density in the range of 0.3 to 0.8 g/cc and are distinguished from
top grain bovine leather which has a high density.
Top grain leather is a venier of the hide which, after removal,
leaves a great deal of leather known as 'leather splits." Thus, the term
'~eather splits" as used herein means and refers to the material remaining
~in a leather hide after removal of the top grain venier. The top grain is
an especiaily valuable material due to its strength and when finished in accord-ance~with known finishing procedures~is used in the highest quality footwear.
It is recognized by those skilled in the art that top grain leather can be
improved. Such improvement can be conducted in accordance with the present
invention by densification to improve the leather structure. For example,
.
bovine top grain leather can be densifled to a density comparable to horse-
hlde thus ~providing properties comparable to a "cordovan" leather.
The present invention is applicable to leather splits from the skins
of reptiles and mammals including cattle and horses and to top grain low
density leather regardless of the method of tanning~, whether the same be
chrome-tanned, zirconium-tanned, vcgetable-tanned or by synthetic tanning
~; agents. Such tanning techniques are well known to those skilled in the art.
Leather splits have commerclal appl~cation in buffed, dyed, "reverse"
or suede leather products. However, the lower strength and relatively coarse
texture of the split products compared to top grain leather have precluded
their use broadly in shoe uppers. Due to the porosity of split leather, it
-2- ~
~;
17531~ Fertell et al -3-
cannot be finished in the same manner as top grain leather since the solution
eoatings coventionally used with top grain leather will penetrate split leather,resulting in a boardy product with no actual top finish.
It is important in leather products used, for example in shoe uppers,
that the interior thereof absorb moisture from the foot while moisture is
desirably precluded from passing from the e2~terior to the interior. Yet while
passage of moisture is undesirable, passag~e of moisture vapor~ that is moisturevapor permeability or breathability, of the shoe upper is recognized to be
a necessary characteristic of such materials. Solution or liquid coating of
split leathers, while precluding passage of moisture as liquid water, also pre-
cludes passage of moisture vapor, rendering coated prodLIcts moisture vapor
impermeable.
It has long been an objective of workers in the leather industry
to effectively use leather splits to form a product useful in a broad range
of products where top grain cowhide is the material of choice. Such efforts
are exemplified by the teachings of U. S. Patents Nos. 3,827,~30 and 4,218,505,
; which teach laminates of leather splits and polyurethane films.
In another aspect of the invention, low density top grain leathers
~; have been used to make gloves, handbags, clothing and the like, but are not
useful as shoe uppers because they a:re low in integrity, memory and flex-fold
characteristics. Even when low density leathers are used in the less demanding
applications they do not have the long term wear characteristics of top grain
cowhide. The low density characteristics of these leathers are attributable
in part to the high concentration of fats in the animal hide which, during
the tanning process, are removed leaving a highly porous network of loosely
bonded fibers. Th~ls, the low density top grain leathers have r educed physical
properties when compared to top grain cowhide. The configuration of fibers
is not suitable for finishing to top grain uses in shoe uppers.
1~531~
In yet another aspect, when low density top grain leathers are
used in clothing, the thickness should be as low as possible to render a
garment which has good hand and drape. However, sufficient thickness must
be maintained to provide a garment with some integrity.
In accordance with the present invention, a process is provided
wherein the physical and chemical properties of low density leathers are
upgraded.
Further, in accordance with the present invention, leather splits
are upgraded to provide a product having all the advantages of top grain
cowhide.
BRIEF DESCRIPTION OF THE I~ENTION
According to one aspect of the present invention there is pro-
vlded a method of impregnating leather comprising:
impregnating a leather sheet, preferably a low density leather
sheet, by completely saturating the leather sheet to eliminate all voids
therefrom with an aqueous ionic dispersion of a polyurethane polymer;
ionically coagulating said polyurethane polymer Erom said disper-
sion impregnated in said leather sheet; and
drying the impregnant to form an impregnated leather having im-
proved tear strength.
According to another aspect of the present invention there is
provided an impregnated leather composite comprised of a leather sheet
having an ionically coagulated aquaously ionically solubilized polyurethane
polymer impregnated throughout said leather sheet at a level sufficient to
improve the tear strength of said leather, said composite having a bulk
density less than its actual density.
When the leather sheet is a low density leather split, subsequent
to drying the impregnated leather composition, a grain layer is imparted to
at least one surface of the composition by the application of heat and
pressure thereto.
~ ~7~3:~0
BRIEF DESCRIPTION OF T~E INVENTION
The polyurethanes useful in the practice of the present invention
are those recognized in the art as ionically water dispersible. These
dispersions are in contrast with emulsified isocyanate copolymers such as
those disclosed in U.S. Patent No. 2,968,575, and prepared and dispersed in
water with the aid of detergents under the action of powerful shearing
forces. The emulsified polyurethanes have the disadvantage that a deter-
gent must be used to form the emulsion and such detergent is usually re-
tained in the dried emulsion coating, thus seriously detracting from the
overall physical and chemical properties of the final product. Further,
insufficient shearing force results in unstable products, and the material
cannot usually be produced in conventional reaction kettles because of the
need for a high shearing force.
- 4a -
1 ~753~ ~
The preferred system for preparing an ionic aqueolls polyurethane
dispersion is to prepare polymers that have free acid groups, preferably
carboxylic acid groups covalently bonded to the polymer backbone. Neutral-
ization of these carboxyl groups with an amine, preferably a water eoluble
mono-amine, affords water dilutability. Careful selection of the compound
bearing the carboxylic group must be made because isocyanates, necessary
components in any polyurethane system, are generally reactive with carbox-
ylic groups. However, as disclosed in U.S. Patent No. 3,412,054, 2,2-
hydroxymethyl-substituted carboxylic acids can be reacted with organic
polyisocyanate~ without significant reaction between the acid and isocyan-
ate groups due to the stearic hinderance of the carboxyl by the adjacent
alkyl groups. This approach provides the desired carboxyl containing poly-
mer with the carboxylic groups being neutralized with the tertiary mono-
amine to provide an integral quaternary ammonium salt and hence, water
dilutability.
Suitable carboxylic acids and preferably the stearically hindered
carboxylic acids are well known and readily available. For example, they
may be prepared from an aldehyde that contains at least two hydrogens in
the alpha position which are reacted in the presence of a base with two
equivalents of formaldehyde to form a 2,2-hydroxymethyl aldehyde. The
aldehyde is then oxidized to the acid by procedures known to those skilled
in the art. Such acids are represented by the structural formula,
fH2H
R f COOH
CE~20H
wherein R represents hydrogen or alkyl of up to 20 carbon atoms and prefer-
ably up to eight carbon atoms. A preferred acid is 2/2-di-(hydroxymethyl)
propionic acid. The polymers with the pendent carboxyl groups are charac-
terized as anionic polyurethane polymers.
1 175310
Further, in accordance with the present invention, an alternate
route to confer water dilutability is to use a cationic polyurethane having
pendent amino groups. Such cationic polyurethanes are disclosed in U.S.
Patent No. 4,066,591, and particularly in Example XVIII. In the context of
the present inven~ion it is preferred that the anionic polyurethane be
used.
The polyurethanes useful in the practice of the invention more
particularly involve the xeaction of di- or polyisocyanate and compounds
with multiple reactive hydroyens suitable for the preparation of poly-
urethanas. Such diisocyanates and reactive hydrogen compounds are more
fully disclosed in U.S. Patents Nos. 3,412,054 and 4,046,729. Further, the
process to prepare such polyurethanes is well recognized as exemplified by
the aforementioned patents. In accordance with the present invention, aro-
matic, aliphatic and cycloaliphatic diisocyanates or mixtures thereof can
be used in forming the polymer. Such diisocyanates, for example, are toly-
lene-2,4-diisocyanate; tolylene-2,6-diisocyanate; meta-phenylene diiso-
cyanate; biphenylene-4,4'-diisocyanate; methylene bis~4-phenyl isocyanate);
4-chloro-1,3-phenylene aiisocyanate; naphthylene-1,5-diisocyanate; tetra-
methylene-1,4-diisocyanate; hexamethylene-1,6-diisocyanate; decamethylene-
1,10-diisocyanate; cyclohexylene-1,4-diisocyanate methylene bis (4-cyclo-
hexyl isocyanate); tetrahydronaphthylene diisocyanate; isophorone diiso-
cyanate and the like. Preferably~ the arylene and cycloaliphatic diiso-
cyanates are used most advantageously in the practice of the invention.
Characteristically, the arylene diisocyanates encompass those in
which the isocyanate group is attached to the aromatic ring. The most
preferred isocyanates are the 2,4 and 2,6 isomers of tolylene diisocyanate
and mixtures thereof, due to their ready availability and their reactivity.
Further, the cycloaliphatic diisocyanates used most advantageously in the
practice of the present invention are 4,4-methylene bis(cyclohexyl isocy-
anate) and isophorone diisocyanate.
ll7~3la
Fertell et al. -7-
The isocyanate is reacted with the multiple reactive hydrogen com-
pounds such as diols, diarnines, or triols. In the case of diols or triols, theyare typically either polyalkylene ether or polyester polyols. A polyalkylene
ether polyol is the preferred active hydrogen containing polymeric material for
formulation of the polyurethane. The most useful polyglycols have a molecular
weight of 50 to 10,000, and in the context of the present invention, the most
preferred is from about 4no to 7,000. Further, the polyester polyols improve
flexibility proportionally with the increase in their molecular weight.
Examples of the polyether polyols are, but not limited to, polyethylene
ether glycol, polypropylene ether glycol, polytetramethylene ether glycol, poly-hexamethylene ether glycol, polyoctamethylene ether glycol, polydecametllylene
ether glycol, polydodecamethylene ether glycol and mi2~tures thereof. Polyglycols
containlng several different radicals in the molecular chain, such as, for example,
the compound HO(CH2OC2H4O)nH, wherein n is an integer greater than one,
can also be used.
; ~ The polyol may also be a hydroxy terminated or a hydroxy pendent
polyester which can be used instead of or in combination with the polyalkylene
ether glycols. Exemp]ary of such polyesters are those formed by reacting acids,
esters or acid halides with glycols. Suitable glycols are polymethylene glycols
~20 such as ethylene, propylene, tetramethylene or decamethylene glycol; substituted
methylene glycols such as 2,2-dimethyl-1,3-propane diol, cyclic glycols such as
:
cyclohexanediol and aromatic glycols. Aliphatic glycols are generally preferred
to impart flexibility. These glycols are reacted with aliphatic, cycloaliphatic
or aromatic dicarboxylic acids or lower alkyl esters or ester forming derivatives
2S to produce relatively low molecular weight polymers, preferably having a melting
point of less than about 70C. and a molecular weight like those indicated for
the polyalkylene ether glycols. Acids for preparing such polyesters are, for
example, phthalic, rnaleicj succinic, adipic, suberic, sebacic, terephthalic and
t ~7~31~
hexahydrophthalic acids and the alkyl and halogen substituted derivatives
of these acids. In addition, polycaprolactone terminated with hydroxyl
groups may also be used.
one particularly useful polyurethane system is the crosslinked
polyurethane system which is more fully disclosed in Canadian Patent No.
1,154,191 of Andrea Russiello entitled "Crosslinked Polyurethane Disper-
sions", along with the crosslinked and non-crosslinked polyurethane com-
positions recited in U.S. Patent No- 4,171,391.
When used herein, "ionic dispersing agent" means an ionizable
acid or base capable of forming a salt with the solubilizing agent. These
"ionia dispersing agents" are amines and preferably water soluble amines
such as triethylamine, tripropylamine, N-ethyl piperidine, and the like î
also, acid and preferably water soluble acids such as acetic, propionic,
lactic, and the like. Naturally, an acid or amine will be selected contin-
gent on the solubilizing group pendent on the polymer chain.
It is preferred that the impregnated leather composition in
accordance with the invention be highly flexible and approximate the flexi-
bility of top grain cowhide when low density leather splits are the leather
starting material. When top grain low density leathers are used as the
leather starting material, the final impregnated leather composition should
be about as flexible as the top grain leather starting material. There-
fore, the polyurethane polymer must behave in an elastomeric manner. The
desired elastomeric behavior would generally re~uire about 25 to 80 percent
by weight of a long chain polyol (i.~., 700 to 2,000 eq. wt.) in the poly-
mer. The degree of elongation and elasticity may vary widely from product
to product depending upon the desired properties of the final product.
~17~310
~ertell et al. -9-
In forming the polyurethanes use~ul in the practice of the invention,
the polyol and a molar excess of diisocyanate are reacted to form an isocyanate
terminated polymer. Although suitable reaction conditions and reaction times
and temperatures are variable within the context of tlle particular isocyanate
and polyol utilized, those skilled in the art well recognize these variations.
Such skilled artisans recognize that reactivity of the ingredients involved requires
the balance of reaction rate with undesirable secondary reactions leading to
color and molecular weight degradation. Typically, the reaction is carried out
with stirring at about 50C. to about 120C. for about one to four hours. To
provide pendent carboxyl groups, the isocyanate terminated polymer is reacted
with a molar deficiency of dihydroxy acid for one to four hours at 50C. to
120C. to form isocyanate terminated prepolymer. The acid is desirably added
as a solution, for example, in N-methyl-1,2-pyrrolidone or N-N-dimethylformamide.
The solvent for the acid will typically be no more than about 5 percent of the
total charge in order to minimize the organic solvent concentration in the poly-urethane composition. After the dihydroxy acid is reacted into the polymer
chain, the pendent carboxyl groups are neutralized with an amine at about 58C.
to ~5C. for about twenty minutes and chain extension and dispersion are accom-
~:
plished by addition to water with stirring. A water soluble diamine may be
added to the water as an additional chain extender. The chain extension involvesthe reaction of the remaining isocyanate groups with water to form urea groups
; ~ ~ and further polymerize the polymeric material with the result that all the iso-
cyanate groups are reacted by Virtlle of the addition to a large stoichiometric
excess of water. It is to be noted that the polyurethanes of the invention are
thermoplastic in nature, i.e., not capable of extensive further curing after for-
mation except by the addition of an external curing agent. Preferably, no such
curing agent is added to form the impregnated leather composition.
~. ~753~
Sufficient water is used to disperse the polyurethane at a con
centration of about 10 to 40 percent by weight solids and a dispersion
viscosity in the range of 10 to 1,000 centipoise. Viscosity may be
adjusted in accordance with the particular impregnation properties desired
and adapted to the density of the leather sheet and by the particular dis-
persion composition which are all dictated by the final characteristics of
the impregnated leather sheet. It should be noted that no emulsifiers or
thickeners are required for the stability of the dispersions.
Those of ordinary skill in the art recognize ways to modiy the
primary polyurethane dispersions accordiny to end product uses, for
example, by the addition of coloring agents, compatible vinyl polymer dis-
persions, ultraviolet filtering compounds, stabilizers against oxidation,
and the like.
The characterization of the dispersions prepared in accordance
with the invention is done by measurements of non-volatile content, part-
icle size, viscosity measurements and by stress strain properties on strips
of cast film.
The concentration range of polyurethane in the dispersion useful
in the practice of the invention is governed by the desirable percent add
on of polymer into the leather sheet.
The dispersion viscosity is generally in the range from 10 to
l,000 centipoise. The low viscosity, relative to that of identical poly-
mers at the same solids level in organic solvent polymer solutions, assists
rapid and complete penetration of the aqueous dispersion into the leather.
Useful solutions of polyurethanes will, in contrast, generally have visco-
sities of several thousand centipoise ranging as high as 50,000 centipoise
at concentrations of 20 to 30 percent.
Particle size, as a useful measure of stability, may be measured
by light scattering. Useful dispersions having non-settling characteris-
tics will have particles of a diameter of less than l micron.
-- 10 --
1 1753~
In the process of the invention, the leather sheet material isimpregnated with polyure-thane up to about 40 percent by weight of the total
impregnated leather composition weight. As low as 5 percent polyurethane
in the impregnated leather composition improves properties, and desirably
10 to 20 parcent is the most advankageous level. Properties such as ten-
sile strength, -tear strength and bias elongation characterize the impreg-
nated composition as pertinent to leather products in the shoe, upholstery
and garment industries.
The leather sheet can be impregnated with the polyurethane dis-
persion by standard impregnating techniques. However, the most preferred
- method of impregnation is by "full impregnation" wherein the leather sheet
is completely saturated with polyurethane dispersion thereby eliminating
all voids within the leather sheet. This method of full impregnation
allows for controlled final polyurethane add on by the adjustment of the
solids concentration of the polyurethane dispersion.
Coagulation is accomplished by contacting the impregnated leather
sheet with an aqueous solution of an ionic media designed to ionically
replace the solubilizing ion. In theory, although not intended to be bound
by such theory, in the case of an anionically solubilized pol~urethane, the
amine which neutralizes the carboxyl containing polyurethane is replaced
with a hydrogen ion whlch reverts the anionic pendent carboxyl ion, thus
reverting the polyurethane polymer to its original, "non-dilutable"
condition. This causes coagulation of the polymer within the leather
structure.
In the case of the anionic poloymer, aqueous acetic acid solu-
tions at concentrations of 0.5 to about 10 percent are suitable ionic coag-
ulants for the anionic dispersions and are preferred over stronger acids
because of the relative ease of handling, low corrosion potential and
disposability. Other acids substantially soluble in water at equivalent
concentrations may be used. The coagulation is quite rapid, so rapid, in
fact, that polymer is substantially entirely retained within the leather
structure, with no polymer loss by migration into the ionic solution.
- 1 1-
~ ~53~
"Salting-out" to coagulate the dispersion by the addition of the
neutral salt is feasible, but is not favored because of the large amounts
of salt needed, about ten times the concentration of aGid, and attendant
problems of product contamination.
Another, and most preferred method, of coagulation an anionic
polyurethane dispersion is by thermal coagulation. In this method a salt
of hydrofluorosilicic acid is added to the dispersion prior to impregnation
and subsequent to impregnation, the impregnate is heated thereby generating
acid which causes coagulation of the dispersion. This method is more fully
disclosed in Canadian Patent Application Serial No. 395,481, filed
February 3, 1982 by John McCartney entitled "Thermal Coagulation of Poly-
urethane Dispersions".
Retained aqueous phase after the coagulation step is removed by
conventional means. For example, the impregnated leather sheet may be
passed through squeeze rolls, rinsed in water, and dried by heated air or
infrared radiation.
In a typical process, the leather sheet is fully saturated with
polyurethane dispersion in a suitable vessel. The surface of the impreg-
nated leather sheet is wiped to remove excess aqueous dispersion. The
polyurethane is then coagulated either thermally or with a solution of
counterion. The impregnate is then squeezed to remove excess water and
dried in an oven.
In the instance where the leather sheet is low density leather
splits, the dried impregnated leather composition is placed in a heated
press and pressure is applied to at least one side of the leather composi-
tion. The heat and pressure fuse the polymer to itself within the impreg-
nate at the surfaces of the material; but yet insufficient to completely
fuse the polymer at the interior of the material. Thus, a density gradient
from the interior of the material to the exterior of the material is devel-
oped. Other physical techniques for developing a grain layer are more
~r - 12 -
~ I 7S3~ ~
fully disclosed in Canadian Patent Application Serial No. 385,081, filed
September 2, 1981 entitled "Simulated Leather Sheet Material" by
John McCartney.
Subsequent to forming the impregnated leather composition, it may
be further split and treated by finishing and the like, by standard leather
processing techniques.
The following Examples are illustrative of the invention.
EXAMPLE 1
A cowhide leather split which was dyed and ready for sale had the
following characteristics:
Instron tongue tear (lbs) 8.00
Basis weight (g/m2)1080.00
Density (g/cc) 0.63
Thickness Icm) 0.17
This split was immersed in a polyurethane dispersion composed of
2 parts by weight of a crosslinked polyurethane dispersion prepared in
accordance with Example III of Canadian Patent No. 1,154,191, previously
cited herein, and one part by weight of a crosslinked polyurethane disper-
sion recited in U.S. Patent No. 4,171,391. The dispersion blend was ad-
justed to 25 percent solids. After the leather sheet was fully impregnatedwith polyurethane dispersion, it was removed from the bath and excess dis-
persion was removed from the sheet by wiping. The impregnate was placed in
a 10 percent aqueous acetic acid bath for ten minutes to coagulate the
polyurethane dispersion. The impregnated sheet was washed with water,
squeezed, and dried at 135F. for 2 minutes in a heated press with no
application of pressure except to insure heat transfer from the plates of
the press to the impregnated leather composition.
.
1 17 5 31 0 Eertell et al. -14-
! The impregnated leather composition had the following ch~racteristics:
- Instron tongue tear (Ibs.) 20.00
Basis weight tg/m2) 1490.00
Density (g/cc) 0.85
Thicl~ness (cm) 0.17
As is shown in Example 1, the impregnated leather composite in accord-
ance with the invention had a greater than threefold increase in tear strength
over the leather split starting material. Further, the impregnated leather com-
posite exhibited good hand and drape.
E~AMPLE Il
Two sheets of the impregnated leather composition of Example I
were placed back to back and pressed at 150C. for 2 minutes at 1,900 psi
in a heated press. The surfaces of the ieather composition contacting the
press plates developed a high density grain layer while the interior contacting
15 ~ surfaces of the leather composite remained as splits. The two sheets were
separated and each sheet had the followlng characteristics:
Instron tongue tear (lbs.) 23.00
Basis weight (g/m2) -1450.00
:
~ Density (g/cc) 0.80
20` ~ ~ ~ ~ ~ ~ Thickness (cm) 0.18
`: : :
~ ; T he impregnated leather composition of this ~xample had good hand
and drape and characteristics similar to top grain cowhide.
~` EXAMPLE III
Example II was repeated with a second cowhide leather split having
; 2S simllar properties to the split of Examples I and IL, except that it was pressed
~ at 3,500 psi.
::
~ '
~ ~ -14-
~l~ " ~
~ 3 1 0
Fertell et al. -lS-
. , '.
The two sheets, when separated, had the follow1ing chnractcristics:
Instron tongue tear (lbs.) 32.00
Basis weight (g/m2) 1800.00
Density (g/cc) 1.30
Thickness (cm) 0.13
The leather composite of this Example had properties similar to top
grain cordovan leather.
In addition to splits which are dyed and processed in accordance
with standard leather manufacturing techniques, wet splits may be used as the
lO starting material in accordance with the invention.
Further, a cowhide and horsehide grain layer may be improved by
impregnation in accordance with the invention.
Although the lnvention has been described with reference to specific
materials and specific processes, it is only to be limited so far as is set forth
lS in the accompanying claims.
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