US2205883A - Tanning - Google Patents

Description

Patented June 25, 1940 PATENT OFFICE TANNING George D. Graves, Wilmington, DeL, assignor to E. I. du Pont de Nemours 85 Company, Wilmington, Del., a corporation 01' Delaware No Drawing. Application June 16, 1938, Serial No. 214,051. Renewed September 9, 1939 14 Claims.

This invention relates to the production of leather and is more specifically directed to processes employing an acidic polymerization product of methacrylic acid for tanning, and to the leather produced. When the term methacrylic" is used herein, the intended meaning is alphamethacrylic." The word acidic, when used in connection with methacrylic'acid polymers, refers of course to carboxyl acidity.

ill The conversion of skins into leather is accompanied by profound changes in their physical characteristics. They become opaque and a distinct fibrous structure becomes apparent. The stiiiness characteristic ofdry rawhides disappears to a great extent and they become comparatively flexible. They assume a characteristic attractive appearance and soft feel. Tanning also renders the product imputrescible. For useful tanning these changes must be relatively permanent, and the leather should be resistant to de-tanning by water, at least at ordinary temperatures.

The ancient and well-developed art of tanning customarily effects the conversion of skins into leather by the use of vegetable preparations that contain tannins. Vegetable tanning materials,

such as quebracho extract, are quite slow in their action and tanning processes employing them require rather excessively long periods of time for completion. The leather produced using vegetable tanning materials is characteristically colored, and it is difficult to obtain good, true colors upon dyeing. It is particularly dimcult to produce satisfactory light-colored leather.

Tanning processes employing metal compounds, such as chromium sulfate, are also known. Such processes are, however, subject to a number of disadvantages in operation, and the leather produced is characteristically colored. White leather, so-called, can be produced by employing bleaching if required, but this leather is not truly white since it has a distinct blue, yellow, green, or gray tint.

Numerous synthetic tanning materials have been proposed, but these are markedly inferior to the vegetable or metal compound tanning agents mentioned. In some instances the leathers produced with known synthetic tanning agents are not sufliciently resistant to de-tanning by water, and in other cases the leather lacks the necessary fullness" and plumpness."

Therefore, these synthetic tanning materials of the prior art are customarily used in conjunction with a vegetable tanning material or a metal compound tanning agent, and they find their principal usefulness because of properties apart 5 from their characteristics as tanning agents.

It is an object of this invention to provide a fundamentally new type of tanning process. It ,is a further object to provide processes which offer a rapid, simple, and easily controlled method w for the tanning of skins and hides. It is a still further object of this invention to provide tanning processes by means of which there may be produced a good quality of leather. It is another object of this invention to produce white leather. 1

Still further objects will become apparent herein- 5 after.

These and other objects of my invention are attained by the use of acidic polymerization products of alpha-methacrylic acid as tanning agents. According to the preferred processes of 20 this invention, an acidic polymerization product of methacrylic acid is introduced into a skin and then the combination of the skin with the polymerization product is accelerated by lowering the pH of the system. The leather so produced is white to the extent of having no noticeable tint, and, when compared with the off-color leather produced by the prior art methods, it is a very true white. The leather is additionally of good character, being opaque, soft, full, and flexible.

Acidic polymerization products of methacrylic acid apparently form some kind of chemical complex, probably thru their free carboxyl groups, 35 with skins or hides, and I believe that the leather produced according to this invention is a reaction product of the methacrylic acid polymer and skin substance.

The acidic polymerization products of meth- 40 acrylic acid used as tanning agents according to my invention may be any of a rather large class of products. The term polymerization" is used herein in its generic sense to include polymerization of methacrylic acid with itself as well as 45 interpolymerization of methacrylic acid with other materials. It is necessary only that the polymer used contain a substantial proportion of free carboxylic acid groups.

The preferred tanning agent of this invention 50 is the .polymer of methacrylic acid. hereinafter called :polymethacryiic acid. However, interpolymeis of methacrylic acid with any of a wide variety of other polymerizable compounds may advantageously be used as tanning agents according to the present invention. There is no particuiar restriction on the type of added polymerizable material except that it be one which does not destroy the free acidity of the final polymer to which I believe the tanning eflect is probably attributable.

The present tanning agents are preferably used in aqueous media. The preferred agents are water-soluble initially, as is polymethacrylic acid, or are capable of beingbrought into solution with aqueous alkali metal hydroxides, amines, or other bases. For best results the solubility in either instance should be such that the solution may be brought to the proper pH for tanning without precipitation of the tanning agent. In some instances, however, agents. that are moderately or even only slightly soluble can be used with some degree of success, due possibly to the fact that, as the small amount in solution is used up, a small additional amount of the polymer in turn goes into solution, and so on.

The polymerization products of methacrylic acid may be used as tanning agents for any type of skin adapted to be converted into leather or fur. It is to be observed that the term skin is used generically to include heavy skins such as cowhide, lighter skins such as goatskin and calfskin, and also skins of fur-bearing animals.

It will naturally be necessary to adapt the tanning processes to the type of skin to be treated. Goatskin may be more rapidly tanned than calfskin, but those skilled in the art are well aware of such factors and may readily adapt the tanning agents of this invention to the specific needs of particular specialized problems.

Hides to be tanned according to the processes of this invention may be prepared in any of the manners customary in the art. They may suitably be unhaired, limed, delimed and bated, and pickled. The skins may be de-pickled. They may also be given a salt liquor treatment to soften them and ,to render them of more uniform condition asis the practice in prior art tanning processes.

While skins prepared in any manner may be treated according to the processes of this invention, the pH of the skins is important. As will be observed below, the tanning is effected at certain conditions of acidity and the skins may tend to change these conditions. The tanning solution may suitably be controlled to compensate for any variance of the pH of the skins from that of the tanning solutions. It may sometimes be found advantageous to adapt the processes for preparing the skins so that a skin prepared for tanning is approximately at the pH of the tan ning solution.

Too rapid reaction of the tanning agent with the skin in the first stages of the process should be avoided in order to prevent surface hardening of the skin. In tanning with the agents of the present invention this result is accomplished by careful adjustment of the pH of the tanning solution. While the exact acidity of a tanning solution according to this invention may be considerably varied depending upon the specific type of skin to be treated and depending upon the specific type of leather to be produced, it is generally desirable to have the tanning solution initially at a pH from about 4.8 to 5.7. Under most conditions of operation it will be found that a pH of about 5 is optimum. At a pH much below 5 the aflinity of skins for the condensation products of methacrylic acid becomes so great that surface hardening takes place, while at a pH much above 5 the afllhity of the hide for the tanning agent is so decreased that the penetration is not sufllciently rapid for the best results.

The acidity of the polymerization products of methacrylic acid may suitably be adjusted by the use of such alkalies as alkali metal hydroxides or carbonates, ammonium hydroxide, and organic bases, such as ethanolamines. As has been observed, the pH of the hide should be considered in adjusting the pH of the tanning solution so that the tanning solution will be employed at an acidity within the range indicated. To assist in maintaining the tanning solution at about the optimum acidity a suitable buffer salt such tanning agent has been effected the rate of tanning may be accelerated by lowering the pH of the solution. The pH to which the solution should be brought to effect complete tanning will vary considerably with the specific type of skin being treated. It will not generally be desirable to go to a pH below' about 3 since, at a higher hydrogen ion concentration, hydrolysis of the skin may take place. As a practical matter it will be found that the pH should be lowered to from about 3.4 to 4.2, while more specifically in many processes a pH of about 4 will be most satisfactory.

The lowering of the pH of the tanning solution may be eifected by the use of any acidic material such as lactic, hydrochloric, sulfuric, acetic, phosphoric, sulfamic, and formic acids.

Considerations as to the type of apparatus to use, the time to be allowed for tanning, the temperatures of tanning, and the amounts of tanning agent are substantially those known to the art in connection with prior art tanning processes. The time allowed for tanning must be sumcient to permit complete penetration of the tanning agent and the pH should not be lowered too rapidly or there may be puckering and apparent over-tannage of the grain surface. The tanning'can best be effected at room temperature say 20 to 30 C., but temperatures somewhat higher than those used by the art can successfully be employed if desired. The exact amount of the tanning agent to use can best be determined by a few simple tests under the specific conditions of the process selected and with the specific skin to be treated. In general, enough of the tanning agent must be used to effect the tanning desired but unduly large amounts should not be used as they will not. be taken up by the leather in a reasonable period of time.

The leather obtained according to the preferred processes of this invention is white but it may if desired be treated with white pigments or may be colored during or after the tanning process by the inclusion of any of the common leather dyes and colored pigments.

While the polymerization products or methacrylic acid are valuable tanning agents in their own right, they may if desired be used together with any other tanning agent or tanning assistant heretofore known to the art. They may. for instance, be used in conjunction with vegetable tanning materials such as quebracho extract, or with synthetic tanning agents containing sulphonic acid groups. Additional fullnessf' or plumpness may be obtained by treatment with aluminum salts such as aluminum sulfate.

Leather produced according to the processes of this invention may be subjected to any of the various kinds of finishing treatments customarily used.

My invention will be better understood by reference to the following illustrative examples. Parts are given by weight.

Exsurra 1.-POZymethacrylic acid-water polymerized Nine hundred parts of water was placed in a water-jacketed stainless steel kettle equipped with eflicient stirring apparatus, and was heated to 87 C. A solution of 1.5 parts of benzoyl peroxide in one hundred parts of methacrylic acid was then added to the water with vigorous agitation. Heating was continued to about 90 C. when a vigorous exothermic polymerization reaction began. The temperature rose .to about 96 C. without further heating. Stirring was continued for a total of one and three-quarters hours. The product was a fine gelatinous precipitate suspended in the aqueous medium. The reaction mixture was then cooled to 30 C. by running cold water thru the jacket. On cooling, the polymer dissolved in the water to form a colorless, somewhat viscous solution.

A tanning solution was made by dissolving 5.2

parts, dry weight. of the .product produced as above in 44.8 parts of water and adding this solution to 2.5 parts of salt and 5.0 parts of sodium acetate dissolved in 100 parts of water. The pH of this solution was adjusted to 5.0 by the addition of a suitable amount of sodium hydroxide. Fifty parts by weight of drained calfskin, which had previously been pickled in a solution containing 1.5 per cent sulfuric acid and 10 per cent salt (NaCl), was placed in a tanning drum and 157.5 parts of the above-described tanning solution was added. The pH of the tanning solution at this point was 5.0.

The hides were drummed for three hours after which the pH was found to be 4.7. Acid additions were then made, 3 parts of a 3.6 per cent hydrochloric acid solution being added each fifteen minutes with drumming between each addition until six such additions had been made. At the end of this time, the pH of the liquor in the drum was 4.15. The calfskins were then removed from the drum, washed, and dried.

It was found that by the above-described treatment the calfskins had been converted into white leather of excellent quality.

Following the procedure of this example, skins were successfully tanned using successively larger amounts from 4 per cent to 10 per cent of polymethacrylic acid based on the drained pickled weight of the skins. Using 4 per cent of the polymethacrylic acid the tanning liquor was exhausted while with 10 per cent of polymethacrylic acid present complete exhaustion was not obtained. Larger or smaller amounts could be used if desired. Under the conditions of this example and with calfskin it appears that about 5 to 6 of suitable amounts of sodium hydroxide.

per cent of polymethacrylic acid based on the drained pickled weight is most satisfactory.

Exsueu: 2.Polvmethacrylic acid-toluem polymerized A mixture of 0.9 part of benzoyl peroxide, 0.225 part of water, and 100 parts of methacrylic acid was added to 540 parts of toluene. The mixture was heated rapidly to 100 C. in a vessel equipped with reflux condenser with rapid initial stirring to insure solution of benzoyl peroxide. A vigorous exothermic polymerization .began at about 100 C., requiring cooling of the vessel with wa ter to moderate the reaction. The reaction ves sel was held at about 100 C. for five hours. The product was isolated as a fine white powder by centrifuging and air drying.

A tanning solution was made by dissolving 92 parts by weight of the above toluene-polymerized polymethacrylic acid in 818 parts of water, adding 17.25 parts of sodium hydroxide dissolved in parts of water, and diluting with 330 parts 215 water. The pH of this tanning solution was Eleven hundred and fifty parts of drained calfskins which had previously been pickled in a solution containing 1.5 per cent 'sulfuric acid and 10 per .cent salt were placed in a tanning drum, and a solution'containlng 46 parts of salt in 575 parts 01 water was added. The skins were drummed for ten minutes. The 1412.25 parts of tanning solution prepared as above described was then added to the tanning drum and the hides were drummed for one hour and allowed to stand in the tanning'solution for sixteen hours without drumming.

Combination of the tanning agent with the hide was then effected by adding 30 parts by weight of 100 per cent lactic acid as a 10 per cent solution at intervals over a period of about six hours of drumming. These lactic acid additions efiected a lowering of the pH to 3.78. The hides were then washed and dried.

It was found that the calfskins had been converted into white leather of excellent quality.

1 ExAuPLn 3.- -Polz' methacrylic acid-ultraviolet polymerized Methacrylic acid was placed in a closed glass vessel and exposed to radiation from a mercury vapor are for five days, at which time the reaction product was a hard, glassy, semi-opaque solid. It was soluble in dilute ammonia solution.

solution was placed in a tanning drum, and there was added twenty-five parts of drained goatskin which had previously been pickled with a solution containing 1.5 per cent sulfuric acid and 10 per cent salt and then de-pickled with sodium acetate and washed.

The skins were drummed for four hours and allowed to stand in the tanning solution for an additional sixteen hours without agitation. Small amounts of a 10 per cent solution of lactic acid were then added with drumming between each addition until the pH had been lowered to 3.6. The skins were then washed, fatliquored with a commercial fatliquor, and dried.

It was found that the goatskins had been converted into white leather of excellent quality.

This

and the solution filtered over kieselguhr.

EXAMPLE 4.Partially hydrolyzed methyl methacrylate Twelve parts of methyl methacrylate polymer was dissolved in ninety parts of benzene. The solution was heated to the boiling point in a vessel equipped with a reflux condenser. A solution of eighteen parts of potassium hydroxide in seventy-two parts of absolute ethyl alcohol was slowly added. The solution turned dark but remained clear. After forty-eight hours further efiuxing, a gelatinous precipitate was formed. The alcohol and benzene were decanted and a solution of forty-eight parts of potassium hydroxide in one hundred ninety-two parts of water was added to the residue in the reaction vessel. The precipitate slowly dissolved on heating. The resulting solution was refluxed for seventy-two hours and then acidified to Congo red with concentrated hydrochloric acid. The resulting precipitate was redissolved in a small amount of 5 per cent aqueous potassium hydroxide solution The product was reprecipitated by dropping the solution slowly into a large volume of boiling concentrated hydrochloric acid, filtered oil rapidly, and air dried overnight. Analytical data indicated that hydrolysis of the carbomethoxyl groups to free carboxyl groups was 58 per cent complete.

A tanning solution was made by dissolving three parts by weight of this product in one hundred parts of water containing a few drops of concentrated ammonium hydroxide. This solution was then adjusted to pH 6.4 by addition of hydrochloric acid.

To this solution contained in a tanning drum was added twenty-five parts by weight of drained goatsklns, which had previously been subjected to pickling and de-pickling, as described in Example 3, together with fifty parts of 5 per cent sodium chloride solution. The skins were drummed for four hours, and lactic acid additions were then made at intervals of one-half hour'with drumming between each addition until the pH had been lowered to 3.7. The skins were then removed, washed, fatliquored with a commercial fatliquor and dried. They were found to have been converted into white leather of good quality.

EXAMPLE 5.-Methacrylic acid-styrene interpolymer Five parts of styrene, thirty-five parts of methacrylic acid, and 0.5 part of benzoyl peroxide were added to two hundred twenty-six parts of toluene and placed in a vessel equipped with a reflux condenser. The reaction mixture was rapidly heated to 90 C. A vigorous exothermic polymerization reaction took place. The reaction mixture was heated at 90-100 C. forsix hours. It was then cooled and filtered. The product, isolated as a fine white powder, was dried under vacuum. I

A tanning solution was made by dissolving ten parts by weight of this interpolymer in two hundred parts of water containing a few drops of concentrated ammonium hydroxide. This solution was then adjusted to pH 5 by addition of lactic acid.

To this solution contained in a tanning drum was added twenty-five parts by weight of drained calfskins which had previously been subjected to pickling and depickling as described in Example 3. The skins were drummed for one and onehalf hours, and lactic acid additions were then made at intervals of one-half hour with drumming between each addition until the pH had been lowered to 3.6. The skins were then removed, washed and dried, and were found to have been converted into white leather of good quality.

EXAMPLE 6.Methacrylic acid-methyl methacrylate interpolymer One part of methylated starch was dissolved in five hundred parts of water at 80 C. in a water jacketed vessel equipped with reflux condenser and an efficient stirring apparatus. A solution of one part of benzoyl peroxide in 87.5 parts of methacrylic acid and 12.5 parts of methyl methacrylate was then added to the water solution with efiicient agitation. A vigorous polymerization reaction began. The reaction mixture was heated to 92.5 C. with vigorous stirring for one hour and then cooled. The product was a granular powder suspended in the aqueous medium. The reaction mixture was cooled below 70 C. and 40.7 g. of NaOI-I dissolved in 359.3 parts of water was added. Stirring was continued until a smooth viscous solution was obtained.

A tanning solution was made by dissolving in one hundred thirty-four parts by weight of water ten parts, dry weight, of the above interpolymer. The pH of the solution was adjusted to 6.0 by adding sodium hydroxide. A batch of skins was prepared for tanning by drumming for ten min utes in one hundred parts by weight of drained pickled calfskins with a solution containing four parts of sodium. chloride in 50 parts of water. The above-described methacrylic acid-metha-- crylate solution was then added to the tanning drum, and the hides were drummed for three hours. Lactic acid was then added in small portions with drumming until the pH had been lowered to 3.85. The skins were then removed from the drum, washed, and dried. It was found that the calfskins had been converted to white leather of good quality.

acidic methacrylic acid polymers suitable for use in the present invention may be made in a number of ways. The polymerization may be effected by any of several well-known procedures, such asat elevated temperatures with benzoyl peroxmers prepared by a variety of methods, as shown above, but it is to be observed that in the case of polymers of very high molecular weight, as judged by solution viscosity, the rate and degree of tanning maybe decreased somewhat, probably because penetration of the agent into the skin is impaired. Very low polymers also should be avoided.

In addition to the polymers of the examples, the following illustrative interpolymers are suitable for use according to the present invention. All of these polymers, with the exception of the interpolymer of methacrylic acid with methacrylamide, which is a yellowish friable resin, are obtained as white powders of varying degrees of water and alkali solubility. All of them have been tested in accordance with the methods given With difierent As is apparent from the foregoing examples the 6. Leather'which comprises a reaction product of polymethacrylic acid and skin substance.

in the examples. and in each instance white leather of good quality was produced.

Methacrz llic acid interpolumers 5. Leather which comprises a reaction product of an acidic methacrylic acid polymer and skin substance.

ing in water forming a tanning solution with a pH of about 4.8 to 5.7.

GEORGE D. GRAVES.

' Proportions 5 a gggggg gg Method of preparing interpolymer Solubilities Original By analysis Diethyl iumarate-- 15% toluene solution 01 monomers heated at 50:50 83:17 Readily soluble in water.

- 100:1 C. for 16 hrs. with 1.33% benzoyl per- Butadiene 25%solutionoimonomersinmixtureoi'l4parts 80:20 Part1 soluble in water.

of toluene and 1 part carbon tetrachloride Read y soluble in dilute sodium heateddior 24 hrs. at 65 C. with 1% benzoyl hydroxide.

Vinyl acetate 153315116119 solution of monomers heated at 75:25 83516.5 Soluble in water.

90-103:1 BC. [or 30 min. with 1.25% benzoyl Unsym dichloretliylene l5 g toluene solution of monomers heated at 75:25 86:14 Partly soluble in water.

90-100 C. for 15-16 hrs. with 1.25% benzoyl Readily soluble in dilute sodium peroxide. hydroxide.

Indene 15% toluene solution of monomers heated at 87.5:12.5 9413 Partly soluble in water.

I 9011127" 0. for 1 hr. with 1.25% benzoyl perg Rilaladgixisioguble in dilute sodium Vinyl chloride 25% toluene solution of monomers heated in 80:20 Diflqcultiy soluble in water.

autoclave at65 C.for 24 hrs. with 1.0% ben- Readily soluble in dilute sodium zoyl peroxide. hydroxide.

Vinyl chloroacetate benzene solution of monomers heated 26.5 :50 72:28 Insoluble in water. Soluble in hrs. at C. with 0.5% benzoyl peroxide. alkali.

Methecrylamide 15% toluene solution of monomers heated 16 :25 74:26 Diflicultly soluble in water.

hrs. at -i00 0. with 1.25% benzoyl per- Readily soluble in dilute sodium oxide. hydroxide. 25

Methyl methacry1ate As in Examplefl 75:25 Soluble in dilute alkali.

Methyl methacrylate-. AsinExampleG 62.5:37.5 Soluble in dilutealkali.

' In both columns, the percentage of methacrylic acid is given first.

Also suitable for use in the present invention '7. In a process for the tanning of skins, the 30 are the acidic polymers obtained by partial or step comprising subjecting a skin to the action complete hydrolysis of polymers or'interpolymers of a methacrylic acid-methyl methacrylate inof methacrylic acid derivatives, such as esters terpolymer. and amides, and of methacrylic nitrile. The use 8. Leather which comprises a reaction product of one such polymer is illustrated in Example 4.v of a methacrylic acid-methyl methacrylate in- 35 I While I have shown certain specific illustrative terpolymer and skin substance.

processes and compositions, it will be understood 9. In a process for the tanning of skins, the that without departing from the spirit of my instep comprising subjecting a. skin to the action vention one skilled in the art may readily devise of a methacrylic acid-styrene interpolymer. numerous tanning solutions and processes eml0. Leather which comprises a reaction prod- 40 ploying polymerization products 01 methacrylic uct of a methacrylic acid-styrene interpolymer acid. and skin substance.

I claim: 11. A tanning composition comprising a par- 1. In a process for the tanning of skins, the tially neutralized acidic polymerization product step comprising subjecting a skin to the action of methacrylic acid, the tanning composition upof an acidic polymerization product of methaon dissolving in water forming a tanning solucrylic acid. tion with a pH of about 4.8 to 5.7.

2. In a process for the tanning of skins, the 12. A taming composition comprising a parstep comprising subjecting a skin to the action tially neutralized polymethacrylic acid, the tanor polymethacrylic acid. v ning composition upon dissolving in water form- 0 3. In a process for the tanning of skins, the ing a tanning solution with a pH of about 4.8 to step comprising subjecting a skin to the action 5.7. 1 of an aqueous solution of an acidic polymeriza- 13. A tanning composition comprising a partion product of methacrylic acid at a pH of about tially neutralized methacrylic acid-methyl metha- 4.8 to 5.7, and then increasing the acidity. cryiate interpolymer, the taming composition 4. In a process for the tanning of skins, the upon dissolving in water forming a tanning sostep comprising subjecting a skin to the action of lution with a pH of about 4.8 to 5.7. an aqueous solution of polymethacrylic acid at 14. A tanning composition comprising a para pH of about 4.8 to 5.7, and then increasing tially neutralized methacrylic acid-styrene interthe acidity. polymer, the tanning composition upon dissolv-