GB2097816A - Impregnating leather - Google Patents

Abstract

A leather sheet is impregnated with an aqueous ionic dispersion of a polyurethane polymer, the polymer ionically coagulated, and the impregnated leather sheet dried to form an impregnated leather composite.

Description

SPECIFICATION Method of impregnating leather and impregnated leather composite This invention relates to methods of impregnating leather and to impregnated leather composite.

The term "low density leather" as used, for example, refers to leather splits, whole non-bovine and reptilian leathers such as goat, pig, sheep, rabbit, raccoon, fox and snake. Typically, these low density leathers have a bulk density in the range of 0.3 to 0.8 g/cc and are distinguished from bovine top grain 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 "leather splits" as used herein refers to the material remaining in a leather hide after removal of the top grain venier.

The top grain is an especially valuable material due to its strength and, when finished in accordance with known finishing procedures, is used in the highest quality footwear. It is recognised that top grain leather can be improved.

Such improvement can be conducted by a method according to the present invention by densification to improve the leather structure. For example, bovine top grain leather can be densified to a density comparable to horse-hide thus providing properties comparable to a "cordovan" leather.

Methods according to the present invention are 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, vegetabletanned or by synthetic tanning agents. Such tanning techniques are conventional.

Leather splits have commercial application in buffed, dyed, "reverse" or suede leather products.

However, the lower strength and relatively coarse texture of the leather splits compared to top grain leather have precluded their use broadly in shoe uppers. Due to the porosity of leather splits, it cannot be finished in the same manner as top grain leather since the solution coatings conventionally used with top grain leather will penetrate leather splits, 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 exterior to the interior. Yet while passage of moisture is undesirable, passage of moisture vapour, that is moisture vapour permeability or breathability, of the shoe upper is recognised to be a necessary characteristic of such leather products. Solution or liquid coating of leather splits, while precluding passage of moisture as liquid water, also precludes passage of moisture vapour, rendering coated products moisture vapour impermeable.

It has long been an objective of workers in the leather industry to use effectively 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. Patent Specifications Nos. 3,827,930 and 4,218,505, which teach laminates of leather splits and polyurethane films.

Low density top grain leathers have been used to make gloves, handbags, clothing etc., but are not useful as shoe uppers because they are 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 fibres.

Thus, low density top grain leathers have reduced physical properties when compared to top grain cowhide. The configuration of fibres is not suitable for finishing for use in shoe uppers.

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.

Thus the present invention seeks to provide a method which upgrades the physical and chemical properties of low density leathers, and which upgrades leather splits to provide a product having all the advantages of top grain cowhide, According to one aspect of the present invention there is provided a method of impregnating leather comprising: impregnating a leather sheet with an aqueous ionic dispersion of a polyurethane polymer; ionically coagulating said polyurethane polymer; and drying the impregnated leather sheet to form an impregnated leather composite.

According to another aspect of the present invention there is provided an impregnated leather composite consisting of a leather sheet having polyurethane polymer impregnated therein, said composite having a bulk density less than its actual density.

The polyurethane polymers useful in methods according to the present invention are those which are, ionically water dispersible. These dispersions, are, in contrast with emulsified isocyanate copolymers such as those disclosed in U.S. Patent Specification No. 2,968,575 prepared and dispersed in water with the aid of detergents under the action of powerful shearing forces.

Emulsified polyurethanes have the disadvantage that a detergent must be used to form the emulsion and such detergent is usually retained 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.

The preferred system for preparing an ionic aqueous polurethane polymer dispersion for use in a method according to the present invention is to prepare polymers that have free acid groups, preferably carboxylic acid groups covalently bonded to the polymer backbone. Neutralization of these carboxyl groups with an amine, preferably a water soluble mono-amine, affords water dilutability. Careful seiection of the compound bearing the carboxylic group must be made because isocyanates, necessary components in any polyurethane system, are generally reactive with carboxylic groups. However, as disclosed in U.S. Patent Specification No. 3,412,054, 2,2hydroxymethyl-substituted carboxylic acids can be reacted with organic polyisocyanates without significant reaction between the acid and isocyanate groups due to the stearic hinderance of the carboxyl by the adjacent alkyl groups.This approach provides the desired carboxyl containing polymer 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 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 oxidised to the acid by known procedures. Such acids are represented by the structural formula,

wherein R represents hydrogen or an alkyl group of up to 20 carbon atoms and preferably up to eight carbon atoms. A preferred acid is 2,2-di (hydroxymethyl) propionic acid. The polymers with the pendent carboxyl groups are characterized as anionic polyurethane polymers.

Further, an alternate route to confer water dilutability is to use a cationic polurethane having pendent amino groups. Such cationic polyurethanes are disclosed in U.S. Patent Specification No. 4,066,591, and particularly in Example XVIII. In the context of the present invention it is preferred that the anionic polyurethane be used.

The polyurethanes useful in methods according to the present invention more particularly involve the reaction of di- or polyisocyanate compounds and multiple reactive hydrogen compounds suitable for the preparation of polyurethanes. Such diisocyanate compounds and reactive hydrogen compounds are more fully disclosed in U.S. Patent Specification Nos. 3,412,054 and 4,046,729.

Further, the process for preparing such polyurethanes is well known as exemplified by the aforementioned specifications. In accordance with the present invention, aromatic, aliphatic and cycloaliphatic diisocyanates or mixtures thereof can be used in forming polymers. Such diisocyanates, for example, are tolylene-2,4diisocyanate; tolylene-2,6-diisocyanate; metaphenylene diisocyanate; biphenylene-4,4'diisocyanate; methylene bis(4-phenyl isocyanate); 4-choro-1 ,3-phenylene diisocyanate; naphthylene-1 ,5-diisocyanate; tetramethylene1 ,4-diisocyanate; hexamethylene-1,6diisocyanate; decamethylene-1, 1 O-diisocyanate; cyclohexylene-1 ,4-diisocyanate; methylene bis (4-cyclohexyl isocyanate); tetrahydronaphthylene diisocyanate; and isophorone diisocyanate.

Preferably, arylene and cycloaliphatic diisocyanates are used most advantageously in the practice of the present 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 diisocyanates 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 isocynate) and isophorone diisocyanate.

The isocyanate is reacted with the multiple reactive hydrogen compounds such as diols, diamines, or triols. In the case of dials or triols, they are typically either polyalkylene ether of polyester polyols. A polyalkylene ether polyol is the preferred reactive 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 400 to 7,000. Further the polyester polyols improve flexibility proportionally with the increase in their molecular weight.

Examples of polyether polyols are, but not limited to, polyethylene ether glycol, polypropylene ether glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, polyoctamethylene ether glycol, polydecamethylene ether glycol, polydodecamethylene ether glycol and mixtures thereof. Polyglycols containing several different radicals in the molecular chain, such as, for example, the compound HO(CH20C2H40), 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. Exemplary of such polyesters are those formed by reacting acids, esters or acid halides with glycols. Suitable glycols are polymethylene glycols such as ethylene, propylene, tetramethylene or decamethylene glycol; substituted methylene glycols such as 2,2dimethyl-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 to produce relatively low molecular weight polymers, preferably having a melting point of less than about 700C and a molecular weight like those indicated for the polyalkylene ether glycols. Acids for preparing such polyesters are, for example, phthalic, maleic, succinic, adipic, suberic, sebacic, terephthalic and 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 British Specification No. A 2031920, along with the crosslinked and non-crosslinked polyurethane compositions recited in U.S. Patent Specification No.4,171,391.

The term, "ionic dispersing agent" used herein means an ionizable acid or base capable of forming a salt with the solubilizing agent. These "ionic dispersing agents" are amines and preferably water soluble amines such as, for example, triethylamine, tripropylamine, and Nethyl piperidine; also, acids and preferably water soluble acids such as, for example, acetic, propionic, and lactic. Naturally, an acid or amine will be selected contingent on the solubilizing group pendent on the polymer chain.

It is preferred that an impregnated leather composite according to the present invention is highly flexible and approximate the flexibility of top grain cowhide when leather splits are the low density leather starting material. When low density top grain leathers are used as the low density leather starting material, the final impregnated leather composition should be about as flexible as the top grain leather starting material. Therefore, the polyurethan polymer must behave in an elastomeric manner. The desired elastomeric behaviour would generally require about 25 to 80 percent by weight of a long chain polyol (i.e., 700 to 2,000 eq.wt.) in the polymer.

The degree of elongation and elasticity may vary widely depending upon the desired properties of the final product.

In forming the polyurethane polymers useful in the practice of the present invention, the polyol and a molar excess of diisocyanate are reacted to form an isocyanate terminated polymer. It will be appreciated that reaction conditions and reaction times and temperatures are variable within the context of the particular isocyanate and polyol utilized. It will be recognized that reactivity of the ingredients involved requires the balance of reaction rate with undesirable secondary reactions leading to colour and molecular weight degradation. Typically, the reaction is carried out with stirring at 500C to 1200C 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 500C to 1200C to form isocyanate terminated prepolymer.The acid is desirably added as a solution, for example, in N-methyl-1,2pyrrolidone or N-N-dimethylformamide. The solvent for the acid will typically be no more than about 5% of the total charge in order to minimize the organic solvent concentration in the polyurethane composition. After the dihydroxy acid is reacted into the polymer chain, the pendent carboxyl groups are neutralized with an amine at about 580C to 75OC for about twenty minutes and chain extension and dispersion are accomplished by addition to water with stirring. A water soluble diamine may be added to the water as an additional chain extender.The chain extension involves the reaction of the remaining isocyanate groups with water to form urea groups and further polymerize the polymeric material with the result that all the isocyanate groups are reacted by virtue of the addition to a large stoichiometric excess of water. It is to be noted that the polyurethane polymers are thermoplastic in nature, i.e., not capable of extensive further curing after formation except by the addition of an externai curing agent. Preferably, no such curing agent is added to form an impregnated leather composite according to the present invention.

Sufficient water is used to disperse the polyurethane polymer at a concentration of 10 to 40% 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 low density leather sheet and by the particular dispersion composition which are all dictated by the final characteristics of the impregnated leather composite. It should be noted that no emulsifiers or thickeners are required for the stability of the dispersions.

Ways to modify the primary polyurethane dispersions according to the use of the end product will be appreciated, for example, by the addition of coloring agents, compatible vinyl polymer dispersions, ultraviolet filtering compounds, stabilizers against oxidation, etc.

The characterization of the dispersions used in the present invention is done by measurements of non-volatile content, particle size, viscosity measurements and by stress strain properties on strips of cast fiim.

The concentration range of polyurethane polymer in the dispersion is governed by the desirable percent add on of the polyurethane polymer into the low density leather. The dispersion viscosity is generally in the range from 10 to 1 ,000 centipoise. The low viscosity, relative to that of identical polymers at the same solids level in organic solvent polymer solutions, assists rapid and complete penetration of the aqueous dispersion into the low density leather, Useful solutions of polyurethane polymers will, in contrast, generally have viscosities of several thousand centipoise ranging as high as 50,000 centipoise at concentrations of 20 to 30 percent by weight. Particle size, as a useful measure of stability, may be measured by light scattering.

Useful dispersions have non-settling characteristics will have particles of a diameter of less than 1 micron.

In the method according to the present invention, low density leather sheet is impregnated with polyurethane up to about 40% by weight of the total weight of the impregnated leather composite. As low as 5% polyurethane in the impregnated leather composite improves properties, and desirably 10 to 20% is the most advantageous level. Properties such as tensile strength, tear strength and bias elongation characterise the impregnated leather composite as pertinent to leather products in the shoe, upholstery and garment industries.

The low density leather sheet can be impregnated with the polyurethane dispersion by standard impregnating techniques. However, the most preferred method of impregnation is by "full impregnation" wherein the low density 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 low density leather sheet with the 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 or an anionically solubilized polyurethane, the amine which neutralizes the carboxyl containing polyurethane is replaced with a hydrogen ion which 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 structure of the leather sheet.

In the case of the anionic polymer, aqueous acetic acid solutions at concentrations of 0.5 to 10% are suitable ionic coagulants 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 the polymer is substantially entirely retained within the structure of the leather sheet with no polymer loss by migration into the ionic solution.

"Salting-out" to coagulate the dispersion by the addition of a neutral salt is feasible, but it is not favoured because of the large amounts of salt needed, about ten times the concentration of acid, and attendant problems of product contamination.

Another and most preferred method, of coagulation of 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 impregnated leather sheet is heated thereby generating acid which causes coagulation of the dispersion. This method is more fully disclosed in British Patent Application No. 82/03613 (U.S. Patent Application Serial No. 234,464, filed February 17, 1981 by John McCartney entitled "Thermal Coagulation of Polyurethane 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 low density leather sheet is fully saturated with polyurethane dispersion in a suitable vessel. The surface of the impregnated leather sheet is wiped to remove excess aqueous dispersion. The polyurethane is then coagulated either thermally or with a solution of counterion. The impregnated leather sheet is then squeezed to remove excess water and dried in an oven.

In the case where the low density leather sheet is leather splits, the dried impregnated leather composite is placed in a heated press and pressure is applied to at least one side of the impregnated leather composite. The heat and pressure fuse the polymer to itself within the impregnated leather composite 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 developed. Other physical techniques for developing a grain layer are more fully disclosed in British Patent Specification No. A 2085043.

Subsequent to forming the impregnated leather composite, it may be further split and treated by finishing by standard leather processing techniques.

The following examples are provided to illustrate the present invention further.

EXAMPLE I A cowhide leather split which was dyed and ready for sale had the following characteristics: Instron tongue tear (kgs) 3.63 Basis weight (g/m2) 1080.00 Density (g/cc) 0.63 Thickness (cm) 0.17 The leather sheet was immersed in a polyurethane polymer dispersion composed of 2 parts by weight of a crosslinked polyurethane dispersion prepared in accordance with Example Ill of British Specification No. A 2031 920, and one part by weight of a crosslinked polyurethane dispersion disclosed in U.S. Patent Specification No. 4,1 71,391. The dispersion blend was adjusted to 25% solids. After the leather sheet was fully impregnated with polyurethane polymer dispersion, it was removed from the bath and excess dispersion was removed from the sheet by wiping. The impregnated leather sheet 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 570C (1350F) for 2 minutes in a heated press with no application of pressure except to ensure heat transfer from the plates of the press to the impregnated leather composite.

The impregnated leather composite had the following characteristics: Instron tongue tear (kgs) 9.07 Basis weight (g/m2) 1490.00 Density (g/cc) 0.85 Thickness (cm) 0.17 As is shown, the impregnated leather composite had a greater than threefold increase in tear strength over the leather split starting material.

Further, the impregnated leather composite exhibited good hand and drape.

EXAMPLE II Two sheets of the impregnated leather composite of Example I were placed back to back and pressed at 1 500C for 2 minutes at 1.34 x 106 kg/m2 (1,900 psi) in a heated press. The surfaces of the leather composite contacting the press plates developed a high density grain layer while the interior contacting surfaces of the leather composite remained as leather splits. The two sheets were separated and each sheet had the following characteristics: Instron tongue tear (kgs) 10.43 Basis weight (g/m2) 1450.00 Density (g/cc) 0.80 Thickness (cm) 0.1 8 The impregnated leather composite of this example 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 similar properties to the leather split of Examples I and II, except that it was pressed at 2.46 x 106 kg/m2 (3,500 psi).

The two sheets, when separated, had the following characteristics: Instron tongue tear (kgs) 14.51 Basis weight (g/m2) 1800.00 Density (g/cc) 1.30 Thickness (cm) 0.13 The leather of this example had properties similar to top grain cordovan leather.

In addition to leather splits which are dyed and processed in accordance with standard leather processing techniques, wet splits may be used as the starting material.

Further, a cowhide and horsehide grain layer may be improved by impregnation by a method according to the present invention.

Claims (26)

1. A method of impregnating leather comprising: impregnating a leather sheet with an aqueous ionic dispersion of a polyurethane polymer; ionically coagulating said polyurethane polymer; and drying the impregnated leather sheet to form an impregnated leather composite.

2. A method as claimed in claim 1 in which said polyurethane polymer has solubilizing ionizable groups covalently bonded to the polymer chain which are reacted with an ionic dispersing agent.

3. A method as claimed in claim 1 or 2 in which said polyurethane polymer has substantially no unreacted N=C=O groups.

4. A method as claimed in any preceding claim in which said leather sheet is a low density leather sheet.

5. A method as claimed in claim 4 including heating the impregnated low density leather sheet under heat and pressure, said heat and pressure being applied to at least one surface thereof to develop a grain layer.

6. A method as claimed in claim 4 or 5 in which said low density leather sheet is fully impregnated with the polyurethane dispersion.

7. A method as claimed in any preceding claim including removing substantially all of the dispersing agent of the aqueous ionic dispersions from the impregnated leather sheet prior to said drying.

8. A method as claimed in any preceding claim in which said polyurethane dispersion is coagulated by thermal coagulation.

9. A method as claimed in any preceding claim in which said aqueous ionic dispersion has a solids content of 5 to 50 percent by weight.

1 0. A method as claimed in any preceding claim in which said aqueous ionic dispersion has a viscosity of 10 to 5,000 centipoise.

11. A method as claimed in any preceding claim in which said leather composite is composed of up to 40% by weight of polyurethane polymer.

12. A method as claimed in any preceding claim in which said leather sheet has a bulk density of 0.3 to 0.8 g/cc.

13. A method as claimed in any preceding claim in which said aqueous ionic dispersion is a crosslinked polyurethane dispersion.

14. A method as claimed in any preceding claim in which said leather sheet is a leather split.

1 5. A method as claimed in any of claims 1 to 1 3 in which said leather sheet is a pig skin sheet or a sheep skin sheet.

1 6. A method as claimed in any preceding claim in which said impregnated leather composite has a bulk density of 0.5 to 1.3 g/cc.

17. An impregnated leather composite consisting of a leather sheet having polyurethane polymer impregnated therein, said composite having a bulk density less than its actual density.

1 8. An impregnated leather composite as claimed in claim 1 7 in which said polyurethane polymer is a crosslinked polyurethane polymers

1 9. An impregnated leather composite as claimed in claim 1 7 or 1 8 having at least one grain layer.

20. An impregnated leather composite as claimed in any of claims 17 to 19 having up to 40% by weight of polyurethane polymer impregnated therein.

21. An impregnated leather composite as claimed in any of claims 17 to 20 in which said leather sheet is low density leather.

22. An impregnated leather composite as claimed in claim 21 in which the leather sheet is a leather split.

23. An impregnated leather composite as claimed in claim 21 or 22 in which the leather sheet has a bulk density of 0.3 to 0.8 g/cc.

24. An impregnated leather composite as claimed in any of claims 21 to 23 in which the leather sheet is a pig skin sheet or a sheep skin sheet.

25. A method of impregnating leather as herein described with reference to the Examples.

26. An impregnated leather composite as claimed in claim 17 and substantially as herein described.