US1848135A - Method of producing an adherent film of rubber on the grain surface of vegetable tanned leather - Google Patents

Description

Patented Mar. 8, 1932 UNITED STATES PATENT OFFICE BRIAN HEAD, OF BELMONT, MASSACHUSETTS, ASSIGNOR TO UNITED SHOE MACHINERY CORPORATION, OF PATERSON, NEW JERSEY, A CORPORATION 01 NEW J METHOD OF PRODUCING AN ADHERENT FILM OI RUEBEB ON THE GRAIN SURFACE OF VEGETABLE TAN NED LEATHER No Drawing.

This invention relates to the production on the grain surface of vegetable tanned leather of an adherent film of rubber to facilitate the cementing to said surface of another piece of leather or similar material.

Cements having as their base rubber latex have been used in various ways in the manufacture of boots and shoes, examples of such uses bein described in United States Letters Patent 0. 1,77 9,368, granted October 21, 1930, upon an application filed in the name of James Cavanagh. For example, latex has been used in the manufacture of leatherboard heels and leatherboard bases for rubber heels to stick together the lifts of which such heels and heel bases are made. When, however, attempts have been made to use latex as a cement in the manufacture of leather heels or heel bases, which are made of vegetable tanned leather, it has been found that a firm joint is not obtained between two lifts if the surface of one of the lifts at the joint is a grain surface.

When the grain surface of a piece of leather is coated with latex, the aqueous dispersing medium (hereinafter referred to as water) apparently enters the leatherwithout carrying into it enough rubber to anchor to it the film of rubber which remains on the surface.

' The film, therefore, does not adhere to the grain surface and consequently any joint between that surface and another surface is very weak. I

The interstices in the grain portion of a hide are much smaller than those in the flesh portion; and when,-in the process of vegetable tanning, the hide is heavily loaded with tannin, the interstices in the grain are filled with tannin to such an extent that only very small openings are left. Consequently, as has been explained above, when latex is aplied to such a surface, the rubber-is strained out of the latex, as it were, and left on the surface in a practically non-adherent film.

I have found that by treating the grain surface of vegetable tanned leather with an alkali of suitable strength and applying latex to the surface either simultaneously with or subsequent to the treatment with the alkali,

I firmly adherent film or coat of rubber can Application filed Octobef 22, 1925. Serial No. 64,282.

be deposited upon the grain surface of vegetable tanned leather, and that consequently a firm joint can be secured between that surface and another coated surface, whether the second surface be grain or flesh. The effect of the alkali is believed to be to dissolve or loosen the tannin and thereby open the pores of the leather to an extent sufiicient to permit the water of the latex to carry with it into the pores of the leather an amount of rubber sufiicient, when the water has been absorbed by the leather, to anchor to the leather the film of rubber.

Commercial rubber latex usually contains ammonia, but this alkali is present in only a small amountcommonly about .4%a concentration quite insufficient to produce the desired effect. By adding more ammonia to the latexfor example, enough to raise the ammonia content to 1.2% or thereaboutsan adherent film of rubber can be produced. There are, however, objections to the use of an alkali such as ammonia because of its volatility, by reason of the quantity of irritating and disagreeable fumes which evaporate when ammonia of the necessary strength is used, and by reason of the lowering of the ammonia. content which results from such evaporation. The disadvantages involved in the use of a liquid which gives off irritating fumes are obvious. With regard to the resulting lowering of the ammonia content, it should be understood that the treated leather piecessuch, for example, as heel liftsmust be allowed to stand for an appreciable interval to give the alkali time to act sufficiently upbn the tannin, and that,.during this interval, the alkali evaporates much more rapidly than the water, its concentration decreases, andits action upon the tannin comes progressively less. Preferably, therefore, a non-volatile alkali is used since no objectionable fumes are then given'ofi and, durin the interval that the cement is permitte to stand while the tannin is being acted upon, the only evaporation which takes place is that of the water; and this evaporation causes the concentration of the nonvolatile alkali-and consequently its action upon the tannin-t0 increase progressively.

Strong alkalis tend to discolor leather by darkening it, and, while such discoloration may not be objectionable in some cases, it may be in others. Preferably, therefore, the alkali used should be of a strength suflicient to dissolve or loosen the tannin in the leather so as to open the pores thereof but insufiicient to cause discoloration of the leather; and at the same time it should be present in suflicient quantity so that the amount used up in combining with the tannin will not materially reduce its effectiveness; A cement which meets these conditions should contain a non-volatile alkali in initial concentration of not less than 1/50 equivalent per liter and of such character that the pH value (which is a measure of its alkalinity) can under no working conditions rise above 11.

' Although various non-volatile alkalis, which willnot discolor leather, may be used, such, for example, as sodium arsenite or a mixture of sodium bicarbonate and sodium carbonate, use will preferably be made of borax, a satisfactory cement resulting when there is added to rubber latex 20 grams (about 1/10 equivalent) of borax per liter.

The great advantage from a practical standpoint in using borax is that, because borax is a weak alkali, a considerable amount of it can be added to the latex so as to make sure that there shall be enough present to combine with a considerable amount of the tannin without raising the pH value to an extent sufiicient to cause discoloration of the leather, since even a saturated solution of borax, which is a substance freely soluble in water, has a low pH value. It should be noted, however, that a stronger alkali which is sparingly enough soluble may be used since, owing to the sparing solubility of such an alkali, its solution will not become concentrated enough to raise the pH value above the limit set. It should, of course, be understood that the handling of latex takes place at room temperature and that the solubility referred to above is the solubility at 25 C. or thereabouts.

In carrying out the method of the present invention, the grain surface of a piece of vegetable tanned leather is coated with the cement and allowed to stand for a short time durin' which the alkali opens the pores of the leat er so that the water, as it goes into the ores, carries with it a certain amount 0 rubber; and when the water has entered the leather, there is left on the grain surface an adherent film of rubber. A second piece of stock, which may or may not e leather, having had one surface provided with a like adherent rubber film may be joined firmly to the leather piece by pressing the two pieces firmly togetlier with their films in contact.

Throughout the specification the term rubber latex has been used in order to promote brevity. Rubber latex is essentially an aqueous dispersion of rubber in which the rubber content is commonly from 33 to 38% by weight. Rubber latex may be used undiluted with water in case a particularly strong bond is required between two articles; but for most purposes the latex may be diluted to a considerable extent. Just what is the limit tex, whether diluted or not, ingredients which tendto prevent coagulation or to vary the viscosity or affect the adhesive quality. The term rubber latex, then, as used in the ape pended claims, is intended .to cover any cement, the basis of which is water-dispersed rubber.

Having described my invention, what I claim as new and desire to secure by Letters Patent of the United States is 1. The method of roducing an adherent film on the grain sur ace of vegetable tanned leather which comprises treating the natural grain surface of the leather with a weak nonvolatile alkali in a concentration suflicient to open the pores of the grain surface of the leather, and applying rubber latex to said grain surface.

2. The method of producing an adherent film on the grain surface of vegetable tanned leather which comprises treating said surface with a weak non-volatile alkali in a concentration sufiicient to open the pores of the leather, and simultaneously applying rubber latex to said surface.

3. The method of producing an adherent film on the grain surface of vegetable tanned leather which comprises treating said surface with rubber latex containing a non-volatile alkali in sufficient concentration to open thepores of the leather and of such character that the pH value does not exceed 11.

4. The method of causing the grain sur- ,derstood that there may be added to the lay face of a piece of vegetable tanned leather 0 to adhere to another piece of material which comprises, coating said grain surface with rubber latex containing a non-volatile alkali of such a character that the pH value does not exceed 11, and in sufficient concentration so that an adherent film is formed on said grain surface, forming on a surface of the other piece an adherent film, and subsequently pressing together the two films.

5. The method "of producing an adherent film on. the grain surface of vegetable tanned leather which comprises treating said sur face withrubber latex containing borax in.

a concentration suflicient to open the pores of the leather.

6. The method of producing an adherent film on the surface of leather which com prises coating said surface with rubber latex containing borax in a concentration sufiicient to open the pores of the leather.

. 7. The method of producing an adherent film on the grain surface of vegetable tanned leather which comprises coating the surface with rubber latex containing a non-volatile alkali in initial concentration of not less than 1/50 equivalent per liter and of such character that the pH value can under no conditions rise above 11.

8. The method of causing the grain surface of a piece of vegetable tanned leather to adhere to another iece of material which comprises coating sa1d grain surface with rubber 2o latex containing an alkali in suflicient concentration to open the pores of the leather and of such character that the pH value can under no conditions rise above 11, allowing the leather piece to stand until a dry adherent film of rubber has formed, forming upon the other piece a dry adherent film of rubber,- and pressing together the two films.

- In testimony whereof I have signed my name to this specification.

so BRIAN MEAD.