US3147138A - Method of coating leather - Google Patents

Description

United States Patent 3,147,138 METHOD OF COATING LEATHER Erwin Muller, Wolfgang Speicher, and Peter Coppe, Leverkusen, and Rudolf Merteu, Cologne-Flittard, Germany, assignors to Farbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, a corporation of Germany No Drawing. Filed Jan. 16, 1962, Ser. No. 166,672 Claims priority, application Germany Jan. 28, 1961 7 Claims. (Cl. 117-142) This invention relates to coated leathers and to proc esses for producing the same. More particularly, it relates to an improved primary or bottoming coating for leathers.

When leather is used as a material for various everyday articles, it is generally provided prior to processing with coatings which are usually dyed or pigmented. These dressings are intended to impart a pleasing appearance to the leather and to protect it as far as possible from moisture and mechanical stresses. The natural character of the leather, for example, grain strength, softness and flexibility, should not be impaired by such dressings. In order to obtain permanently flexible and nevertheless mechanically resistant coatings on leather, it is necessary in most cases to build up the dressing in layers, starting with a bottoming, which is frequently arranged to be soft, and terminating with a finishing layer which is usually hard.

The bottoming layer, which in many cases is decisive as regards the quality of the entire dressing, has various objects according to the type of leather being treated, of which a few are set out by way of examples below.

It should first of all serve as a base for the succeeding dressing layers which are usually harder and prevent the latter from penetrating too deeply into the leather and thus counteracting a hardening of the leather material. Furthermore, it must provide a bond between the leather material and the upper dressing layers. Consequently, it is essential for the adhesion of the entire dressing. By a correctly adjusted degree of softness and a depth of penetration adapted to the leather material, the bottoming can favorably influence the feel, softness, and associated of the grain on the leather. The bottoming is frequently of decisive importance as regards the mechanical properties of the complete dressing, such as resistance to cracking, resistance to cold and fastness to friction glazing.

As the bottoming or primary layer is of great importance with respect to the quality of the finished leather article, numerous methods have been proposed with respect to the bottoming systems for leathers. It has been heretofore known to introduce bottomings containing oil, wax or fat usually in an aqueous emulsion or solution in order to have a particularly favorable influence on the softness and grain association of the leather. Nevertheless,lthe bonding strength of the dressing film can be impaired by such bottoming layers. Furthermore, polymers, for example, esters of acrylic acid or copolymers with butadiene, usually in aqueous emulsion, have frequently been used for the bottoming of leather. Watersoluble or organically soluble natural or synthetic materials, such as casein, albumin, vegetable mucins, shellac, alkyd resins, nitrocellulose and the like have often been recommended as bottoms for leathers. These substances, however, frequently do not conform in their softness to the standards which must be set for leather bottoming layers. Combinations of the said types of bottoming layers, often in combination with pigments or fillers, can on the contrary lead to bottoming layers which are successfully employed in practice and which do not disadvantageously influence the natural properties of the leather, particularly the grain association.

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It has also been known to improve the grain association of hollow-grained leather to use as bottoming layers, solutions of unmodified alkyd resins having free hydroxyl groups in organic solvents, which also contain crosslinking agents, such as isocyanate, see for example, German Patent 889,349. The said urethane based prepolymers which are used in solution in organic solvents, have the property of strengthening the bond between the grain layer of the leather and the recticular layer. This is shown in a better grain association, that is to say, when the leather is bent with the grain layer on the inside, the grain layer is warped into very fine corrugations, even if a coarse creasing, caused by a loose bonding prior to the treatment of the leather With the polyurethanes, using the same test. The consolidation of the grain by the said polyurethanes also leads to an improved resistance to scratching and fastness to friction glazing of the dressing. This is shown when a sharp-edged article is moved under pressure over the dressed leather. The untreated areas become warped in corrugated form, while the leather treated with the polyurethanes remains smooth.

One disadvantage of the use of the previously described polyurethanes is that the organic solvents necessary to dilute them are combustible, can cause physiological injury, can initiate undesired migrations of fatty substances in the leather and in addition substantially increase the cost for dressing or currying. Where the polyurethanes still contain free isocyanate groups, they are in addition restricted as regards their storage capacity and above all are very sensitive to moisture.

It is, therefore, an object of this invention to provide an improved process for bottoming leather. It is another object of this invention to provide an improved bottoming system for leathers which does not impair the bonding strength of the subsequently applied layers to the leathers. It is still another object of this invention to provide leather bottomings which consolidate the grain of the leather to a high degree. It is a further object of this invention to provide a bottoming layer to leather which can be applied from an aqueous solution. It is a still further object of this invention to provide leather having an improved resistance to friction glazing.

The foregoing objects and others which will become apparent from the following description are accomplished in accordance with this invention, generally speaking, by applying to the leather an aqueous solution of a Water soluble reaction product of a polyhydric alcohol and an aldehyde or an organic polyisocyanate or an aldehyde and a polyisocyanate. The water soluble condensation products according to the invention are suitable for bottoming layers and also for barrier layers on leathers which are subsequently to be dressed or curried with reaction lacquers comprising polyfunctional isocyanates and polyfunctional soft resins containing active hydrogen atoms.

The bottoming layers in accordance with this invention can also be prepared by reacting a polyhydric alcohol with an aldehyde or by first reacting the polyhydric alcohol with the aldehyde and then reacting the product with an organic polyisocyanate. In either event the polyisocyanate is used in an amount insuflicient to react with all of the hydroxyl groups present to thereby insure that hydroXyl groups will terminate the polymer forming the bottoming layer. The resulting polymer is then dissolved in water or an aqueous solution. The use of the bottoming or priming layers in accordance with this invention in an aqueous solution greatly simplifies the practice of coating leathers and concurrently reduces the cost of the process over the alkyd type coatings which require organic solvents, while greatly increasing the bonding strength of subsequent layers to the leather.

Any suitable divalent or polyvalent hydroxyl compound free of ester groups may be used in reaction with an aldehyde, an isocyanate, or an aldehyde and an isocyanate in accordance with this invention such as, for example, ethylene glycol, propylene glycol, butanediols, hexanediols, glycerine, trimethylol alkanes, pentaerythritols, tetramethylolcyclohexanol, sorbitol, glucose, saccharose, invert sugar, acidly degraded starches, thiodiglycol and the like; adducts of alkylene oxides such as, for example, ethylene oxide, propylene oxide, butylene oxide, amylene oxide and the like with any of the aforesaid polyalcohols, polyfunctional amines or amino alcohols such as, for example, ethylene diamine, propylene diamine, hexamethylene diamine, 3,3'-dichlorobenzidine, 2,5-dichloro-p-phenylene diamine, hydrazine, phenylene diamine, tolylene diamine, 4,4 diamino diphenyl methane, aminoethyl alcohol, aminopropyl alcohol, aminobutyl alcohol and the like; polyalkylene ethers such as, for example, diethylene glycol, triethylene glycol, octa-ethylene glycol, polyalkylene glycols such as polyethylene glycol, polypropylene glycol with molecular weights in particular between 500 and 2000, oxalkylated glycerine, trimethylol propane, sorbitol, carbohydrates, hydroquinone, ethylene diamine, mixed ethers of thiodiglycol and ethylene glycols and the like. Monofunctional alcohols can also be concurrently used in minor proportions.

Further, an additional modification consists in the concurrent use of phenols, more especially phenol, cresols, ethylphenols or tertiary butylphenols, during further or initial lengthening with aldehydes. The condensation products which are claimed are produced by lengthening the aforesaid polyhydroxyl compounds with aldehydes and/or polyisocyanates and it is also possible for several of the aforesaid lengthening agents to be used simultaneously or in a timed sequence for the lengthening operation.

Any suitable aldehyde may be used such as, for example, formaldehyde in its various forms, acetaldehyde, butyraldehyde, acrolein, crotonaldehyde, furfurol, glyoxal and the like. Other compounds which split off formaldehyde may also be used. For successfully conducting the condensation acid catalysts such as sulphuric acid, sulphonic acids, phosphoric acid, fluoboric acid or perchloric acid should be added in catalytic quantities and preferably in the amount of 0.01 to by weight.

Any suitable organic polyisocyanate can be used for the production of the water soluble polycondensation products such as, for example, hexamethylene diisocyanate, decamethylene diisocyanate, 1,7-diisocyanato-4-oxa-heptane, o-, mand p-phenylene diisocyanate, toluylene diisocyanates and their isomeric mixtures, diphenyl methane diisocyanate, naphthalene diisocyanates, cyclohexylene diisocyanate, furfurylidene diisocyanate, 4,4,4"-triisocyanato-triphenyl phosphate and other compounds as well as reaction products containing NCO groups and obtained from the above polyisocyanates with polyalcohols or partial polymers of the above polyisocyanates and the like. When using the compounds containing isocyanate groups, care must be taken that the reaction of the polyisocyanate is carried out as far as possible in the absence of water.

The proportions of the polyhydroxyl compounds and lengthening agents, that is the aldehyde and/or polyisocyanate, should be such that the reaction products obtained still contain free hydroxyl groups and have hydroxyl numbers preferably between 1 and 800. It is essential that the polycondensates obtained should be soluble in water, and this can be achieved either by a corresponding accumulation of hydroxyl groups or by the concurrent use of hydrophilic components, such as di-, tri-, or polyethylene glycols in the condensation. The preparation of some of the previously described condensation products is described by way of example in US. Patent 2,961,428 and in US. application Serial No. 71,175.

The bottoming or priming agents according ot the invention are usually applied to the grain side of the undressed, buffed or unbufr'ed leather in an aqueous solution either once or several times by the conventional methods, by hand, by spraying, brushing, or by using a machine. The quantity and the concentration of the coating applied depends on the degree of hollow graininess and the absorption capacity of the leather to be treated. After drying the bottoming, the leather can be ground, polished or ironed. The dressing of currying is thereafter effected with normal commercial surface colors and binding agents or with reaction lacquers consisting of polyfunctional soft resins containing hydroxyl groups in the normal manner.

Wetting agents, solvents and emulsifiers can be added to the bottoming or priming layers to be used according to the invention for regulating their penetration power. Dyestuffs or pigment preparations can be concurrently used for coloring the priming or bottoming agents. It is also possible simultaneously to use other auxiliaries for the dressing of leather, such as polymer emulsions, casein solutions and others, which intensify the barrier layer effect of the priming solutions according to the invention.

While any of the compositions set forth above are suitable in accordance with this invention as bottoming layers applied to leather in an aqueous solution, it is preferred that the bottoming composition be prepared by reacting a triethylene glycol with formaldehyde and then reacting this composition with a quantity of toluylene diisocyanate insufiicient to react with all of the hydroxyl groups present.

The invention is further illustrated, but not limited by the following examples, in which parts and percentages are by weight unless otherwise specified.

Example 1 A polyhydroxyl compound used in the bottoming or priming is prepared as follows:

About 1.5 kg. of triethylene glycol, about 340 g. of paraformaldehyde and about 3 g. of concentrated H are heated While stirring over carbon dioxide to 80-90 C. in a flask provided with a reflux condenser, until a clear melt is formed, this taking about 1 hour. The water split off is then extracted, initially under light vacuum and finally at a pressure of 12 mm./Hg and condensation is carried out at 80 to C. until no more water distills over. The quantity of distillate is 214 g. The condensation product which has formed and which is a yellowish water-soluble oil, has an hydroxyl number of 88. The 1.6 kg. of polyacetyl which are obtained are adjusted with alkali agents to pH 7. About 200 g. of toluylene diisocyanate are then added with stirring. After careful homogenization, about 1.6 cc. of tri-nbutylamine are added dropwise. The mixture is heated for 4 hours to 80 C. After cooling, the product obtained is dissolved in water.

A 10% aqueous solution of this polyhydroxyl compound is applied by means of a plush wheel or brush or by spraying to a chrome-tanned cowhide which is retanned with vegetable tanning agents and the grain side of which is roughened with finegrain emery paper. After drying the bottom priming coat, the leather is pressed by hand or by the use of a hydraulic press. Thereafter, a conventional dressing or currying, consisting for example of casein-pigment dispersion and polymer emulsions, is carried out by the methods usual in practice. The resultant dressed or curried leather is distinguished from a leather dressed in the same manner but not with the bottoming agents according to the invention by an improved grain effect and an improved resistance to friction glazing.

Example 2 A polyhydroxyl compound is prepared from about 10,000 parts of cane sugar, about 23,000 pars of 40% formalin (81 g.=1 mol CH O), about 19,100 parts of diethylene glycol, about 13,950 parts of trioxethylated trimethylol propane and about 300 parts of 30% fluoboric acid.

The pH value of the experimental batch is 2.7 in the form of its 50% aqueous solution. This mixture is heated for 4 hours to 90 C. and the water which is added and which forms is then removed as far as possible quantitatively at 90 C./12-18 mm. Hg. Dry ammonia is then introduced for another 4 hours at 40 C. and the substances left for 1 hour at 40 C. at a pressure of 12 mm. Hg. C.P. ==4700; 10.5% OH; 0.4 acid number; 0.2% CH O; 0.58% H O, pH-value of 50% aqueous solution=8.6.

An aqueous solution for the bottoming composition is prepared from about 100 parts of a normal commercial aqueous leather surface color with a casein base, about 250 parts of the above described polyhydroxyl compound, about 250 parts of water and about 200 parts of a normal commercial aqueous polymer emulsion.

A buffed chrome calf leather is treated by hand or spraying with the priming or bottoming mixture above described. After drying, the leather is pressed by hand or by using a hydraulic press. This bottoming layer has applied thereto, a reaction lacquer consisting of solutions of polyfunctional soft resins containing hydroxyl groups and the quantity of polyfunctional isocyanates required for the cross-linking, the application being made by spraying. After drying the reaction lacquer, a lacquered leather is obtained, which is distinguished by a very fine grain formation, softness, good bonding strength of the lacquer and excellent fastness properties.

Example 3 A 50% aqueous solution of a polyhydroxyl compound is prepared as follows:

Abous 500 g. of a polyethylene oxide having a molecular weight of about 1,000 and an hydroxyl number of about 1 12 are neutralized with gaseous hydrochloric acid to remove the alkali still present from the production of the product and, after dehydration at 130 C./ 12 mm. Hg, about 80 g. of toluylene diisocyanate are incorporated by stirring at about 90 C. The temperature is raised to from about 130 to about 140 C. After about 2 hours, the viscous melt is poured onto metal sheets and heated for about 12 hours at about 100 C. The thermoplastic material thus obtained and having a rubber-like consistency is dissolved in about 586 g. of water.

Example 4 To an unbufied dyed chromium calf leather there is applied by means of a plush a aqueous emulsion of a copolymerisate from butadiene-acrylonitrile-styrene which may be dyed with water-soluble dyestuffs or caseinpigment dispersions. After drying of this coating, the base mixture described below is applied again by means of plush. The leather is then dried by airing over night and subsequently pressed by hand or by means of a hydraulic press. To these bottom layers a reaction lacquer is applied by spraying which is prepared from solutions of soft resins containing polyfunctional hydroxyl groups and of the quantity of polyfunctional isocyanates required for cross-linking. The enamelled leather obtained after drying of the reaction lacquer is excellently suitable for upper leather and bag leather on account of its good fastness properties, fine grain and supple feel.

The base mixture mentioned above is composed as follows:

'15 parts of a 20% aqueous solution of the polyhydroxyl compound described in Example 1,

parts of a 20% commercial aqueous emulsion of a polyamide from adipic acid-hexamethylene-diamine and/ or caprolactam,

200 parts of water.

It is, of course, to be understood that the invention is not to be limited by the working examples, but that any of the hydroxyl compounds prepared using the compounds set forth above such as any of the hydroxyl terminated compounds, aldehydes and isocyanates for those specifically used in the working examples.

Although the invention has been described in considerable detail for the purpose of illustration, it is to be understood that variations can be made by those skilled in the art without departing from the spirit of the invention and scope of the claims.

What is claimed is:

1. A process for bottoming leather which comprises applying to the leather in an aqueous solution an bydroxyl terminated reaction product of a polyhydric alcohol and an organic polyisocyanate.

2. A process for bottoming leather which comprises applying to the leather in an aqueous solution an hydroxyl terminated reaction product prepared by reacting a polyhydric alcohol with an aldehyde in a first step and reacting the reaction product of the first step with an organic polyisocyanate.

3. A method for bottoming leather which comprises applying to the leather an aqueous solution of the reaction product of triethylene glycol, formaldehyde and toluylene diisocyanate.

4. A process for bottoming leather which comprises applying to the leather in an aqueous solution an hydroxyl terminated reaction product of a polyhydric alcohol and an organic polyisocyanate, said hydroxyl terminated reaction product being substantially free of ester groups.

5. A process for bottoming leather which comprises applying to the leather in an aqueous solution an hydroxyl terminated reaction product prepared by reacting a polyhydric alcohol with an aldehyde in a first step and reacting the reaction product of said first step with an organic polyisocyanate, said hydroxyl terminated reaction product being substantially free of ester groups.

6. The process of claim 2 wherein the aldehyde is a lower aliphatic aldehyde.

7. The process of claim 2 wherein the aldehyde is formaldehyde.

References Cited in the file of this patent UNITED STATES PATENTS 2,786,081 Kress Mar. 19, 1957 2,946,767 Gassmann July 26, 1960 2,956,963 Baird Oct. 18, 1960 2,961,428 Muller et al. Nov. 22, 1960 3,066,997 Neher et al Dec. 4, 1962

Claims (1)

1. A PROCESS FOR BOTTOMING LEATER WHICH COMPRISES APPLYING TO THE LEATHER IN AN AQUEOUS SOLUTION AN HYDROXYL TERMINATED REACTION PRODUCT OF A POLYHYDRIC ALCOHOL AND AN ORGANIC POLYISOCYANATE.