US3000689A

US3000689A - Processes for the tanning of hides using zinc compounds

Info

Publication number: US3000689A
Application number: US691799A
Authority: US
Inventors: Harlan A Depew
Original assignee: American Zinc Lead and Smelting Co
Current assignee: American Zinc Lead and Smelting Co
Priority date: 1957-10-23
Filing date: 1957-10-23
Publication date: 1961-09-19
Anticipated expiration: 1978-09-19

Description

3,000,639 PROQESSES FOR THE TANNING F HIDES USING ZINC COMPOS Harlan A. Depew, Kirkwood, Mo, assignor to American Zinc, Lead & Smelting Company, St. Louis, Mo., a

corporation of Maine N0 Drawing. Filed Oct. 23, 1957, Ser. No. 691,799

9 Claims. (Cl. 8-94.25)

This invention relates to processes for the conversion of hides to leather by the use of zinc compounds that precipitate zinc oxide, hydroxide, or basic compounds in the hide and which combine with it and to the resulting products. This application is a continuation-in-part of my application Serial No. 429,672. filed May 13, 1954 now abandoned.

The prior art recognizes that zinc is desirable in leather manufacturing and patents have been issued for the use of zinc compositions as minor additives in various stages of leather manufacture from dehairing to the finished leather. in the prior art the hide invariably has been so acid from pickling during tanning that no significant amount of zinc can have been left in the leather.

In a known process, zinc compounds are deposited in the hide but they are not tanning agents because the hide is fully tanned with chromium before the zinc is added. The reactive groups of the hide are saturated with the chromium compounds. The tanning acid is neutralized with sodium zincate, sodium aluminate, or an ammoniacal copper solution. Zinc compounds, aluminum cornpounds, or copper compounds are deposited in the leather. However, since the leather has been fully tanned prior to the addition of the alkali zinc compound, these compounds are not added as tanning agents and the reason for their use is not clear. The probability is that the zinc hydroxide or basic zinc compound helped fill the leather, an expensive compound for this purpose.

Hides have also been impregnated in the prior art with the salts of the rare earth metals and with zinc sulfate. The hide is then reacted with disodium phosphate whereby the phosphates of the rare earth metals and that of zinc are precipitated within the hide in the form of compounds inert to the leather. In the present invention zinc is deposited in a reactive hydrated form in the hide.

White leathers are known in the industry, and one common method to obtain white leathers is by alum tanning in which basic aluminum compounds are deposited from an acidic solution. These products have very poor dimensional stability at elevated temperatures and the aluminum compound leaches freely. Zirconium has also been used for this purpose but is expensive and is not widely used.

White leathers presently known are so unsatisfactory that the most common commercial method is to tan the hides with chromium; then surface bleach the tanned hides and then white coat the leather which coating is frequently scuffed off in use.

In contrast leather tanned with basic zinc compounds by the method of this invention is white all the way through. When the leather is scuffed off the exposed leather is also white. When this white leather is put in boiling water it loses its white color presumably through reaction of the zinc and the protein; the zinc is not leached. This is not a practical consideration, however, since this white leather would not be used in boiling water. In normal storage there is no lossof whiteness.

I have found that the disadvantages of the prior art are overcome and white leathers are obtained by combining basic zinc compounds with the hide to tan it, giving a white leather with acceptable shrink temperatures.

The present processes for the first time produce a White mates Patented Sept. 19, 1961 leather product by precipitating basic zinc compounds in the hide where some of it combines with the hide and gives the desired properties of tanned leather and part of it is present as a pigment whitening the leather throughout. These white leathers are not only useful in themselves but they serve as a desirable basis for tints and colors.

Zinc hydroxide react readily with zinc chloride and zinc sulfate that are added to form complexes known as basic zinc salts, XZn(OH) -ZnCl and YZn(OH) -ZnSO and ZZn(OH) -Zn(NO for example. These basic zinc compounds include zinc hydroxide. Zinc hydroxide dehydrates to ZnO at relatively low temperatures especially under alkaline conditions and is also included herein. The required chloride and sulfate is obtained not only from the added zinc chloride and zinc sulfate but also from sodium sulfates and chlorides present in the hides.

It is, accordingly, an object of the present invention to provide novel processes for tanning hides to leather using basic zinc compounds for this purpose.

Another object is to provide such processes in which zinc oxide, zinc hydroxide and basic zinc compounds are deposited in the hides to tan them.

Another object is to provide such processes in which the zinc compounds deposited in the hides combine With the hides in tanning them.

Another object is to provide such processes for producing white leather.

Another object is to provide novel white leather products tanned by zinc compounds.

Another object is to provide novel white leather products tanned by basic zinc compounds partly combined with the fibers of the hides.

Another object is to provide novel white leather products produced by tanning hides with zinc hydroxide including zinc oxide, and basic zinc compounds deposited in the hides.

Other and further objects of the present invention will appear from the following illustrative descriptions thereof.

A hide treated with basic zinc compounds according to the processes of this invention may act as if it had been bated, and accordingly, for some grades of leather hating can be omitted and the hide limed with conventional dehairing processes can be treated directly With zinc solutions to precipitate zinc hydroxide andb'asic zinc salts therein. if the limed hide is treated with a zinc sulfate solution to precipitate zinc hydroxide, calcium sulfate also will be precipitated in the hide as a desirable filling material. Similarly, barium sulfate is precipitated when barium hydrate is the base.

When zinc sulfate or zinc chloride is used with NaOH in the processes of this invention, Na SO or NaCl is formed. These salts, with added salts as required, control the swelling of the hide.

The scraped and dehaired and limed hide either bated or unbated may go directly to the tanning procedure of this invention if the hide has the necessary a'cid neutralizing capacity.

The processes of the present invention are capable of various procedural modifications and illustrative embodiments thereof are hereinafter described to illustrate the invention and the resulting products vary somewhat with the processes employed.

In general the pH of the liquid exuding from a hide before adding the soluble basic zinc compounds including zinc chloride and zinc sulfate liesbetween 5 and It to give the hide a suitable acid neutralizing capacity. The minimum pH at which, basic zinc compounds precipitate varies with the zinc chloride concentration and other salts present. Ordinarily pl-Is of the liquor surrounding the hide higher than 11 are avoided as injurious o to the hide but there is no upper limit. Actually a high pH is not essential. The COOH and other reactive groups of the hide tend to react with the soluble bases added -by the processes of this invention and give an acceptable acid neutralizing capacity without a high pH.

The bases rather than basic salts that are used in this invention are water-soluble compounds, thenegative ions being (GI-l). Among these bases in addition to lime already mentioned are NaOH, KOH, LiOI-I, and Ba(OH) Ammonium hydroxide is an especially useful base because after tanning any excess can be evaporated. Watersoluble amine base such as dimethyl amine hydroxide and tetramethyl ammonium hydroxide are examples of other bases, of which there are a number than can be used.

In carrying out this invention, the amount of zinc to be added is determined and the hide is then treated when necessary with a water solution of a base in sufficient concentration to provide acid neutralizing capacity so that when a solution of the zinc saltis added, the zinc salt will diffuse into the hide and will precipitated therein.

The zinc salt and base may be added alternately to the hide when a high zinc content is desired. The order of adding is not essential, but the base is usually added first.

In another embodiment of the processes of this invention a hide, if not basic, is given an acid neutralizing capacity using dilute ammonia water of, for example, 1/22% concentration. The hide is then soaked in an ammoniacal solution of a basic or normal zinc salt such as the chloride or sulfate, preferably with agitation and some pounding action as in a tumbling mill. The addition of salt (NaCl) to the solution often improves the zinc penetration in the hide. After the zinc compound has entered the hide, the hide is removed from the solution and much of the free ammonia is evaporated. It is generally accepted that the zinc exists in the ammoniacal solution as Wernerian compounds such as (NH ZnCl and As the ammonia evaporates, the zinc in the hide becomes insoluble and precipitates, presumably largely as a basic zinc chloride but in'part probably as zinc oxide, zinc hydroxide, and zinc diaminochloride (ZnCl -2NH Ammonium salts and other salts may also form in the hide. The treated hide is then washed with water or with dilute ammonia or soda ash. The-hide is then oiled and may be further processed in known manner with other tanning materials, such as syntans, formaldehyde, or chromium compounds, but this last step is unnecessary.

X-ray studies have indicated that basic zinc chloride such as is precipitated in the hides by the processes of this invention crystallizes in flat hexagonal plates with the zinc atoms in one plane and with the Cl atoms and OH groups oriented in an adjoining plane. In each location the presence of an OH radical or a Cl atom is a random arrangement. The percentages of Zn, OH, and C1 in the compound may vary widely. A well recognized ratio is expressed by the empirical formula-- 4Zn(OH) -ZnCl The ratio of four Zn(OH) to one ZnCl is the basicity. As the basicity increases, the maximum amount of zinc in solution decreases and the stability of the solution becomes poorer. For this reason when high concentrations of zinc chloride are desired in the hide it is preferable to use solutions of lower basicity as represented by 2Zn(OI-I) -ZnCl Zn(OH) -ZnCl or even lower ratios.

The one extreme of basicity is essentially pure zinc hydroxide dissolved in ammonia, and the other extreme is zinc chloride dissolved in ammonia and NH Cl.

When zinc chloride is dissolved in aqua ammonia and the solution is evaporated, a large number of compounds are precipitated ranging from zinc hydroxide through a range of basic zinc chlorides to ammonium chloride and the mixture will analyze approximately ZnCl -ZNH .zinc diaminochloride. In the case of Zn(OH) dissolved i in aqua ammonia and precipitated, the deposit is largely Zn(OH) Although the deposit on evaporation of ammoniacal basic zinc chloride solution analyze approximately xZn(OH) -ZnCl 2NH X-ray studies and stability tests indicate that ammonia with water is absorbed on the basic zinc chloride, rather than as an essential part of the crystal.

In any case, there is available approximately two NH for each ZnCl in both the zinc diaminochloride and in the basic zinc chloride.

When hides containing zine diarninochloride in the residue from ammoniacal basic zinc chloride solution are washed, the compounds hydrolyze and the end product tends toward Zn(OH) The basic zinc compound before hydrolysis in and around the fiber in part reacts with the fibers of the hide and converts the hide to leather andthe remainder of the basic zinc compounds deposits around the fibers whitening, separating and lubricating them.

When a hide treated with an ammoniacal zinc solution is partially dried in the air, the free ammonia concentration is greater on the interior of the hide than on the surface. The hide may then be treated with an acidic zinc salt solution to deposit less basic zinc compounds in the hide. The hide is then leached and NH Cl is removed leaving basic zinc compounds in the hide.

In general, increasing the amount of precipitated zinc in the hide increases the leather qualities of the hide and approximately %-1% of precipitated zinc are needed to make the treated hide show pronounced leathery qualities. However much larger quantities of precipitated zinc maybe used.

Zinc oxide and zinc hydroxide are very insoluble and Lhgir basic compounds produce a permanently tanned The processes described above refer especially to basic zinc compounds containing chloride, but other zinc compounds such as the sulfates, nitrates, etc. may be used.

White zinc tanned leather produced by these processes may be dyed to desired colors or it may be reacted with chemicals to develop uniform colors through the leather.

Leather tanned with zinc according to these processes has unusually good tensile strength and tear resistance attributable to the avoidance of the highly acid solutions that are used in conventional tanning.

These processes and the resulting products are illustrated by the following examples in which a moist basic zinc chloride was dissolved in 2628% aqua ammonia to form the solution:

Percent Zn -s 9.0

Cl 3.1 N-H approx -s 21 Evap. residue approx 17 Example I The solution above was diluted with three parts of water and a oalfbelly prepared as inExample I was immersed in it for 18 days. During this period the treatment was carried on out of doors and the liquid froze and thawed and some ammonia was lost. The treated hide was then dried for ten hours-and leached and oiled. The product is designated hereinafter as B.

Example Ill The solution above was diluted with 7 parts of water to make a solution having 1.1% Zn. The reduced ammonia content makes the solution less stable and with further dilution zinc hydroxide would precipitate. However, at this concentration, there is no precipitate. A third calfskin belly was immersed in this solution simultaneously with the calf skin in Example 11. It was removed in the same length of time and dried, washed, oiled, and dried as in Example II. This product is designated hereinafter as C.

Within less than 24 hours after the hides were inimersed in the treating solution, an increase in stillness of the hide was noticeable in A and B.

X-ray examination of C showed no ZnO, Zn(O l-l) ZnCO NH Cl, zinc diaminochloride, nor basic zinc chloride, although small amounts may be present, but showed a diffuse unknown pattern that could only be a zinc-protein compound. A and B contained, in addition, zinc hydroxide lines.

The three leathers were white and flexible and withstood boiling water with less shrinkage and hardening than a piece of commercial alum tanned leather.

Zinc analyses of these leathers were:

, Percent Zn A 1.4

27 parts of ZNSO -H O were dissolved in 200 parts of water. 100 parts of 26-28% NH (aq.) were stirred in to obtain a colorless solution that was diluted to 500 parts with water. The calf-belly hide was ball milled in this solution for 6 hours and was allowed to stand in the milling solution for another 10 hours. The hide was equeezed by hand to remove excess liquid and was hung to air dry for 5-5 /2 hours.

The hide was milled for 6-6 /2 hours in a solution consisting of 13.7 parts ZnSO -H O and 237 parts water. It was next rinsed and washed well with water and finally milled for 3 hours in 200 parts of 1:1 sulfonated oil water emulsion. The oiled hide was rinsed with water and allowed to air dry until the required dryness was reached to obtain a moderately stifi white leather.

Example V Bated calf-belly hides were soaked in 2% NH water to preserve them from bacterial decomposition and to allow uniform penetration of the NH A 6" x 12" piece of this hide was soaked with occasional stirring for 24 hours in a solution consisting of 41.4 parts ZnSO -6H O in 460 parts of water. The hide was removed, allowed to drain to remove excess solution, and was hung to dry in air for 15-20 hours. While still moist, the hide was further dried for 3-4 hours in a circulating air oven at 40-41. The hide was next soaked in water overnight then was milled for 8 hours in a ball mill containing equal parts of water and Nopco alum oil (anionic surfactant). After further soaking overnight in the oil emulsion, the hide was removed, rinsed lightly with water, and air dried at 25-30 C. The final dry product was a well tanned, white leather having excellent pliability and strength.

Example VI A 5" x 8" piece of NI'Ig'SOELkCd, bated calf-belly hide was placed in a ball mill with a solutionconsisting of 25 parts ZnCl and 475 parts water and was milled with porcelain balls for 7-8 hours. The hide after standing in the milling solution was removed and squeezed to re-' move excess ZnCl The hide was returned to the mill and was milled for 77% hours in a slurry comprising 50 parts slaked lime and 450 parts water. The hide was next washed with water, and was allowed to soak overnight in a solution of 25 parts ZnCl and 475 parts water. The hide was removed, rinsed in running water for 15 minutes, and was milled for 4 hours in an emulsion consisting of 250 parts Water and 250 parts of Nopco alum oil. After rinsing with water, the hide was air dried to obtain a white leather having good pliability and no visible rawhide on cutting. Analysis of the air-dried leather shows 1.4% Zn.

Example VII A 5" x 12" piece of limed but unbated calf-shoulder hide was stored overnight in a solution consisting of 165 parts ZnSO -H O, 75 parts of salt, and 1260 parts water. The hide then was milled in the ZnS O solution for 2 hours and was allowed to stand in the milling solution for another 35-40 hours. It was next rinsed in water and squeezed to remove excess ZnSO After milling for 4 hours in an emulsion consisting of 100 parts Water and 100 parts Nopco alum oil, the hide was rinsed lightly with water and was hung to air-dry. The final dry leather had a good opaque white color and analyzed 3.8% Zn.

Example VIII A piece of hide was cut from the shoulder area of bated cow hide, approximately 4" x 6" with a wet drained weight of 73 g. The hide was washed in running tap water and then placed in a S-qt. ball mill about half full of V2? porcelain balls in a solution of 1381 cc. of water, 107 cc. of 28% NH (2% water soln.), 112.5 g. NaCl (7.5% soln.) and 30 g. of NH Cl as a butter to maintain a pH of 10.5. It was ball milled for 5% hours. The pH of the hide surface at this time was 10.5.

The hide was now placed in a l-qt. ball mill about half full of /2 balls. Solutions of 5.1 g. (7% of wet hide weight) of NaCl, 7.3 g. ZnSO -H O (10% of wet hide Wt.), enough 5 N. H 50 to bring the pH to 2, and then 2.2 g. (3% of wet hide weight) of sodilun citrate were added and enough water in the solution to cover the hide in the ball mill. The final pH of solution was 3. The hide was ball milled 1% hours and the pH increased to 5.5. The hide was left standing in the mill for 36 hours. It was ball milled an additional 1% hours and the pH reach 6.2.

The milling solution was poured into a beaker to which oils were added-8.8 g. of Tanoyl 1881 (12% of wet hide wt.) and 2 g. of Nopcolene K.Q.W. The solution was heated to C. and stirred. When the oil was well dispersed, the solution was poured back into the mill with the leather and the leather was ball milled for 4 /2 hours. Then the leather was removed and lightly washed in running water, and hung to air dry at room temperature.

The wet tanned hide dried into a slightly stiff very good white leather with a shrink temperature of 79 C. compared with about 52 C. for the raw hide.

The oils were furnished by Nopco Chemical Co. and are described by the producers as follows: Tan Oil 1881 is anionic with a pH of 6 /2-7. It is a slightly bleached sulfated sperm and COFA oil intended for white chrome. Nopcolene K.Q.W. is non-ionic cationic. It is a mixture of blown vegetable oil, sulfonated neat and sperm oil, wax, and tallow. It is a fulling type of oil. It makes the leather feel thicker and spongy.

Example IX A 14" x 14" square piece of bated rawhide cut from the flank portion of a cow hide was washed and rinsed thoroughly in running tap water. It was wet drained and the weight was 1105 g. The hide was placed in a S-qt. ball mill about half full of pebbles. A solution of 1 liter of water, 75 g. common salt, and 5 cc. of cone. NH (28%), was added. It was milled 5 /2 hours, then stopped, and .left standing :in the mill-overnight. .It was milled an additional hour the following morning. The solution pH at this time was 95-10. This solution was discarded. The hide Was placed back in the ball mill in a fresh solution of 1. I. water, 110.5 g. (10% of Wet hide wt.) ZnS -H O, 77.4 g. (7% wet hide wt.) NaCl. The pH was adjusted to 3 with addition of N. H 80 and 4-5 g. of sodium citrate were added as a buffer. The hide was ball milled 3 hours, stopped and 111 cc. of 40% HCHO (4% HCHO of wet hide wt.'), and 5-6 g.'ZnO to buffer the solution at pH 5 were added. It was ball milled 3 hours, left in the mill overnight and ball milled an additional hour the following morning.

The milling solution was poured into a beaker and 166 g. of Nopco Tanoyl 1881 of wet hide wt.) were added with 40 g. of Nopcolene K.Q.W. The solution was warmed to 80 C. and stirred until the oil was well dispersed. The oiling solution was poured-back into the ball mill on the hide and the ball milling continued for 5 hours.

The hide was removed, rinsed in running tap water and hung to air dry at room temperature overnight. The following morning the hide was placed in a circulating oven and finished drying at 60 C.

The dried hide was slightly stiff. Upon working, the leather softened. The shrink temperature was 88 'C. A sample of the leather analyzed 2.51% Zn.

area of a cow hide that had been limed, dehaired, and

bated in the regular factory procedure used in preparing hides for chrome tanning. The exuded'liquor had a pH of approximately 7 but it had an appreciable amount of acid neutralizing capacity that was capable of reacting with soluble zinc salts. and placed in a S-qt. ball mill about half full of pebbles in a solution of 1 l. or" water, 92 g. 10% of .wet hide wt.), ZnSO -H O, 64 g. (5% of Wet hide wti), salt. The pH of the solution was adjusted to 3 with 5 N. H SO and buffered with 5.6 g. of sodium citrate. The hide was ball milled 6 hours, when there was added 92 cc. of 40% HCHO (10% of wet hide wt.) and 5-6 g. ZnO to increase the pH to 5. The hide was ball milled 1 hour then left in the mill overnight. It was ballmilled an additional hour the following morning, then removed and discarded the solution. At this point the hide had a shrink temperature of 90 C.

The hide was returned to the ball mill in a solution of 1 1. water and 30 g. of Neil- S0 and ball milled for 2% hours to removeexcess HCHO. [The solution was discarded at the end of the ball milling period.

A fresh solution of 750 cc. H2O,92 g. ZIISOgH O,

64 g. salt and 183 g. of Tanoyl 1881 of wet hide wt.) was prepared. The solution was heated to 70 C., stirred until the oil was dispersed well, and then poured into the ball mill on the hide. It was ball milled'2 hours, removed and rinsed in tap water andhungto air dry at room temperature. The dried leather became softer and whiter upon working. It had a shrink temperature of 84 C. and analyzed 3.29%.Zn.

The NaHSO treatment reduced the added tan given by the formaldehyde.

Example XI A 14" x 14" piece of hide out from the shoulder area of a bated hide, wet wt. 780 g., was rinsed thoroughly in tap water and placed in a S-qt. ball mill, half full of pebbles, in a solution of 1 l. of water, 5 cc. of 28% NH and 55 g. NaCl (7% of wet hide wt.). The pH of the solution was 10-10.5. The hide was ball milled 4% hours and left standing in the ball mill overnight.

The hide was removed from the ball mill the following morning and thesolution was discarded. A fresh solution of 1 l. of water, 78 g. (10% wet hide wt.)

ZnSO 'H O, and 55 g. (7% hide wt.) .NaCl was pre- The hide was rinsed thoroughly,

pared in the ball mill. The hide was placed in the ball mill and the pH was adjusted to 3-4 with 5 N. H SO and sodium citrate buffer. After ball milling for 2% hours, the solution pH was 4-5, and 5-6 g. of ZnO were added to give a pH of 5.5-6. The hide was ball milled 3 hours and left standing overnight.

The following morning 177 g..of Tanoyl 1881 (15% of hide wt.) were added to the milling solution which was removed from the ball mill and warmed to 60 C. with stirring until the oil was dispersed. The solution was poured back into the mill and the ball milling continued for 3 hours. The hide was removed, washed in lukewarm water .to remove excess oil and hung to air dry at room temperature.

The leather dried to an off-white which improved upon working. It had a shrink temperature while still wet of 73 C., and upon drying the shrink temperature was 68 C. The Zn analysis was 7.03% Zn. After 24-hour leaching in distilled water a sample analyzed 5.3% Zn.

Example XII A 4 x 6" (77 g. wet wt.) piece of bated rawhide cut from the shoulder area of a large cowhide, was rinsed in running tap water and then placed in a 'l-qt. ball mill with about 2 doz. pebbles. Enough water was added to cover the hide (approx. 100 cc.), together with 7.7 g. (10% wet hide wt.) ZnSO -H O, and 5.4 g. NaCl (7% hide wt.) 5 N. H 80 was added to a pH of 2, and 2.3 g. (3% of hide wt.) of sodium citrate then added to buffer at a pH of 3. The hide was'ball milled 4 /2 hou-rs, allowed to stand overnight in the milling solution and then ball milled 5 additional hours. At this point the pH of the solution was raised to 5 with the addition of NaOH. The ball milling] was then continued for one additional hour.

The tanning solution was poured into a beaker and 9.2 g. Tanoyl 1881 (12% of wet hide wt.) and 10 g. of Nopcolene K.Q.W. were added. The mixture was warmed to 60 C. with stirring until the oil was well dispersed, and then poured back intothe ball mill on the leather. The leather was milled for 2 hours, left standing overnight and then milled an additional 1 /2 hours. The hide was then removed, rinsed, and-hung to air dry at room temperature.

The leather dried soft, white, and very pliable. It analyzed 3.55% Zn.

In general, zinc treated leathers tanned at the lower pI-Is show a slightly lower shrink temperature and a brighter white with superior flexibility and less fullness. I have found that I can immerse the tanned hide in a solution mildly alkaline with NaOH or the other bases described in paragraph 2 of page 5 for example without loss in desirable properties. I have used solutions having pHs of 10.5 to 8.0 for this end pointing but I prefer the latter although the exact pH is not critical. The time of treatment can vary from a short period to several days. Some oil in the end pointing solution is desirable to prevent leaching of oil from the tanned leather.

Alkaline endpointing increases the shrink temperature, especially for low shrink temperature leathers.

Although the zinc in basic zinc compounds is extremely insoluble in pure water, in the presence of salts such as NaCl and Na SO complexes form that may cause some loss of zinc. Where this loss occurs the described alkaline end-pointing reduces this zinc loss.

Unlike ordinary leather, the zinc tanned leather of this invention can be fastened with metal fasteners and wet with water and the leatheris resistant to discoloration.

Further, the leather of the present invention upon repeated experiment shows no mildew attack even in warm Weather when commercial leathers used as the control are heavily covered by a slimy growth. By mildew is included all fungi and lower forms of life that form these disagreeable coatings. This high mildew resistance is due to the basic zinc compounds with a range of low basicity to high basicity.

The leather of this invention has been examined by many experts in leather technology and they agree that the zinc tanned leather of this invention has unusual and superior strength as shown by hand tearing. The fibers of the leather can be separated as strong, individual fibers. The Zinc compounds have not only tanned the leather by combining in part with the protein thereof but they have coated and whitened the fibers and kept the fibers separate and lubricated. I attribute the strength of the leather in part to the fact that the hide has not had to undergo the severe acid attack characteristic of tanning where pickling is used and of chrome tanning in particular.

It should now be apparent that the present invention provides novel processes for the tanning of hides using zinc compounds with resulting novel products which in every way satisfy the objects described above.

Changes in or modifications to the above-described illustrative embodiments of this invention may now be suggested -to those skilled in the art without departing from the present inventive concept and reference should be had to the appended claims to determine the scope thereof.

What is claimed is:

1. In a process for tanning hides the steps of soaking the hides prepared for tanning in an aqueous solution of a base selected from the group consisting of Ca(OI-I) NaOH, KOH, LiOH, Ba(OH) NH OH, dimethylamine hydroxide and tetramethyl amonium hydroxide having an alkalinity not above an approximate pH of 11 and then soaking the hides in an aqueous solution of a zinc salt selected from the group consisting of zinc chloride, Zinc sulphate, zinc nitrate, a solution of zinc chloride in ammonia, a solution of basic zinc chloride in ammonia, a solution of Zinc sulphate in ammonia, and a solution of basic zinc sulphate in ammonia and precipitating basic zinc compound in the hides by reacting in the hides the base with the zinc salt forming zinc hydroxide which reacts with the zinc salt to form basic zinc compounds whereby the hides are tanned.

2. A process as described in claim 1 the additional step of soaking the tanned hide in an aqueous solution of a base selected from the group consisting of Ca(OH) NaOH, KOH, LiOH, Ba(OH) NH OH, dimethylamine hydroxide, and tetramethyl ammonium hydroxide.

3. In a process for tanning hides the steps of soaking basic hides having an alkalinity not above an approximate pH of 11 in an aqueous solution of a zinc chloride and precipitating zinc compounds in the hides by reacting the base in the hides with the zinc chloride forming zinc hydroxide which reacts with the zinc chloride to form basic zinc chlorides whereby the hides are tanned.

4. A process as described in claim 3 the additional step of soaking the tanned hides in an aqueous solution of a base selected from the group consisting of (Ba(OH) NaOH, KOH, LiOH, Ba(OH) NH OH, dimethylamine hydroxide and tetramethyl ammonium hydroxide.

5. In a process for tanning hides, the steps of soaking the hides prepared for tanning in an aqueous solution of a normal zinc salt dissolved in aqua ammonia having an alkalinity not above an approximate pH of 11, and precipitating in the hides basic zinc compounds by evaporation of ammonia. F

6. In a process as described in claim 5, the additional step of soaking the tanned hides in an aqueous solution of zinc chloride.

7. In a process for tanning hides the step of soaking basic hides having an alkalinity not above an approximate pH of 11 in an aqueous solution of zinc sulphate thus precipitating zinc compounds in the hides by reacting the base in the hides with the zinc salt forming zinc hydroxide which reacts with the zinc salt to form basic compounds whereby the hides are tanned.

8. In a process for tanning hides and the step of soaking basic hides having an alkalinity not above an approximate pH of 11 in an aqueous solution of zinc nitrate thus precipitating the zinc compound in the hides by reacting the base in the hides with the zinc salt forming zinc hydroxide which reacts with the zinc salt to form basic zinc compounds whereby the hides are tanned.

9. In a process for tanning hides, the steps of soaking the hides prepared for tanning in an aqueous solution of a basic zinc salt dissolved in aqua ammonia having an alkalinity not above an approximate pH of 11 and precipitating in the hides basic zinc compounds by evaporation of ammonia.

References Cited in the file of this patent UNITED STATES PATENTS 281,411 Smallridge July 17, 1883 915,638 Trenckmann Mar. 16, 1909 FOREIGN PATENTS 508,960 France Oct. 28, 1920 OTHER REFERENCES Cohn et al.: Separation of Protein Components of Human Plasma, J.A.C.S. 72 (Jan. 1950) pp. 465-474.

Progress in Leather Science: 1920-1945 British Leather Manufacturers Assoc. London, 1948, pp. 527-529.

UNITED STATES PATENT OFFICE GERTIFICATE OF CORRECTION Patent No.. 3,000,689 September 19, 1961 Harlan A Depew It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 1, after no" insert sharp same column 3, lines 1 and 2, strike out "Actually a high pH is not essential."; line 74, for "ZnCl -ZNH read ZNCL -2NH column 4, line 89, for "chloride" read chlorine column 5, line 35 for "ZNSO H O" read -ZnS.O -H O column 7 line 5, for "l. 1." read 1 16 Signed and sealed this 17th day of April 1962.

(SEAL) Attest:

ESTON G JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

Claims

1. IN A PROCESS FOR TANNING HIDES THE STEPS OF SOAKING THE HIDES PREPARED FOR TANNING IN AN AQUEOUS SOLUTION OF A BASE SELECTED FROM THE GROUP CONSISTING OF CA(OH)2, NAOH, KOH, LIOH, BA(OH)2, NH4OH, DIMETHYLAMINE HYDROXIDE AND TETRAMETHYL AMONIUM HYDROXIDE HAVING AN ALKALINITY NOT ABOVE AN APPROXIMATE PH OF 11 AND THEN SOAKING THE HIDES IN AN AQUEOUS SOLUTION OF A ZINC SALT SELECTED FROM THE GROUP CONSISTING OF ZINC CHLORIDE, ZINC SULPHATE, ZINC NITRATE, A SOLUTION OF ZINC CHLORIDE IN AMMONIA, A SOLUTION OF BASIC ZINC CHLORIDE IN AMMONIA, A SOLUTION OF ZINC SULPHATE IN AMMONIA, AND A SOLUTION OF BASIC ZINC SULPHATE IN AMMONIA AND PRECIPITATING BASIC ZINC COMPOUND IN THE HIDES BY REACTING IN THE HIDES THE BASE WITH ZINC SALT FORMING ZINC HYDROXIDE WHICH REACTS WITH THE ZINC SALT TO FORM BASIC ZINC COMPOUNDS WHEREBY THE HIDES ARE TANNED.