US2945737A - Method of dehairing hides - Google Patents

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This invention is concerned with the preparation of leather and is directed specifically to the problem of unhairing cattle hides, or dewooled sheep or goatskins, preparatory to their conversion to leather. It aims to provide a process which gives much faster hair removal tharhconventional methods, without injury to the hide or to the hair which is removed, and with less disturbance of the hide substance than in the conventional processes. According to my invention, .I treat the soaked hides or skins first with an aqueous alkaline solution of a strong reducing agent at a relatively low pH (not above 11.0 Within a few minutes after the addition of the hides), maintain contactfor a few hours, draw off the reducing agent and preferably wash the hides or skins, then treat the hides or skins for a few hours with an alkaline bath which is characterized by the presence of ammonium a to 100 F.) are used throughout the process.

In the conventional tannery, hair removal. is accomplished in the beam-house. Cattle hides, or sheep or goat skins from which the wool has been removed leaving the shorter hairs, are dehaired for further processing using techniques which save the hair. In the standard processes, the hides are soaked for four to six days in a depilatory solution, consisting of an alkaline reducing agent and lime, which softens and loosens the hair, which is then removed in machines which scrape it from the skins and hides. The hair is washed and dried, and has an appreciable market value.' However, some injury may be done to the skin in the process; and the long time required has been the source of considerable additional cost of operation.

Many attempts have been made to shorten the time required for the dehairing operation, and a number of processes have been suggested. But none of the methods have succeeded in getting the combination of shortened time, satisfactory hair removal, satisfactory hair recovery and satisfactory skin condition for leather manufacture.

According to my invention, I obtain all these desired objects by treating soaked hides or skins at ordinary ambient temperatures with an aqueous alkaline solution of a strong reducing agent, at a pH of below 11 for a period of a few hours, then preferably washing the hides or skins with Water, then treating the skins for a few hours with an aqueous alkaline bath containing ammonium ions at a pH of or more, but under 12, and then mechanically removing the loose and loosened hair from the hides or skins. Ordinary ambient temperatures are used throughout the process.

In the first bath, it is essential to. use alkaline reducing agents which are strong enough to affect the 8-8 link'in' the cystine present in the older hair, and to reduce at least a portion of the cystine to cysteine, whereby the hydrolysis of the keratinous proteins of the hair follicles and epidermal layer can be accomplished by Patent 2,945,737 Patented July 19, 1 960 ment, are just as effective but slightly less desirable economically; and the sulfites, bisulfites and hydrosulfites are all satisfactory, although a trifle slower in action. But sodium thiocyanate, for example, is not a powerful enough reducing agent to affect the SS linkage in the cystine under the conditionsnecessary for preserving the 'hair and skin.

Generally, the reducing agent is present in the bath at a concentration of between 0.5 and 2.0% (about 2 to 10% based on hide weight). More reducing agent than 2% is generally wasted, while concentrations under 0.5% act so slowly that the contact time must be prolonged to such an extent that quality suffers.

The pH of this solution is held below 11.0 during the period of treatment since higher pH will cause deterioration in the quality of the recovered hair and substantial reduction in yield of saleable hair. In addition, there is some danger of attacking the skin itself. With some reducing agents, such as the cyanides, which contribute to raising the pH and which are rapidly used up with marked reduction, of pH, the initial pH may be slightly over 11.0, since the pH drops so rapidly below 11.0 that no damage is done.

Enough bath is used to effectively cover the hides, and permit agitation of the hides in the solution. Inmost plant equipment, the solution will run 3 to 5 times the hide Weight, although this can vary widely;

The first treatment should continue for at least an hour, and a two hour treatment under agitation is most elfective. Where convenient, the skinsmay be left in the bath for longer periods, up to 12 hours or more, without real dam-age, due to the low pH.

The skins are preferably washed after the first 'treatment to minimize carryover of solution. The washing is not essential; but since carryover varies from batch to batch, better control of the operation is obtained if the skins are washed to reduce the actual carryover of chemi-' 'cals.

economically unsound. The necessary pH may be obtained by bufiering with.

a phosphate, or in any other desired manner. It is essential that the pH in this bath be at least 10, since hair loosening is too slow at lower pHs. At pH 12 or above,

the tendency toward hair damage becomes marked and this is often accompanied by an actual decrease in the ease of hair removal. Hence, a pH of 12.0. or higher should be avoided.

Preferably, the alkali. treatment should be limited to about 2 hours, with good agitation. Poor agitation results in excessive hair loss locally; prolonged contact results in increasing hair damage with time, often accompanied by a decrease in the ease of hair removal.

The invention is exemplified by the following specific examples:

, Example 1.-Dehairing of cow hiq'es Four hides, weighing 200 pounds in the green salted state, were washed in a wheel, trimmed, fleshed, and 'cut I into two. They were then placed in a paddle agitator containing gramsofwater. at 803 F.:containing --1 concentrated.

1,000 pounds of water at 90 'F., in which was dissolved 12;5'-*pounds"ofsodium-'sulfhydrater The pH ofth'e'solu tion was 9.5. The hides were paddled for a total of 2 hours, and then held 6 hours.

The stock wasremoved*andwashed%5 minutes in'water at 801""FI torfliish' oif thefirstsolutionr The' hatcfiwas" thenplacedina'se'cond batli'consistingfof P'ounds' Water-M80 F. 1;0'00 Tchammoniumthydroxide (28%? NH 10 Tfisodiixmmhosphate f 400' pounds -water'= 2 pounds anhydrous-sodium sulfite pH 8.5;

Thebatch .wasmilledlfor an.hour,zand. the skins were pulled and fleshed; The fieshed'hides .were returnedto the. mill, from. which the. soak liquor. had: been removed,

and'lthey; werezmilledIfon-anhoui; at 75 F. in a bath. comprising 200 pounds water 3 -p'ounds sodium sulfhydrate This. solution was dr'ainedi'well andlreplaced. by a vsolution consisting of 200#' water at 75 F:

2011- urea: (toi 'improvet dispersing actiom) The batch .was milled- 15 minutes,.all'owed to rest' 2 hours, milled'fo'r a few minutes,.and"allowed to rest 1V2 hours. Atithis point (just short. of"4'hours tota1) vhair removal wasiexcellenf, th'e hides were of fine quality, andthe .hair. was recoverable;

A few of'the hides were left in thebathfonobservaf tion,.withoutstirringfiir. another,4 hours. At this. point, the hides were definitelyswollen, andiunhai'ring,.was. no. longer as satisfactory. This indicatedclearly that .the hydrolysis bath must not be allowed 'to actlmorethan-a few hours.

Exampla-le-Sodiitmr sulflte: unh'airing:

2000 grams ofgreen'ca'ttle'hides were soaked overnight in water at*80?" Ff, andthentumbled, with mild"agitation, for two hours in a 1% solution'of anhydrous .sodium sulfite in 8000 grams of' water at 80." F.;, the batch was drained, rinsed,=.and. tumbled: for. 2 hoursin 8000 ammonium'.hydroxide-.(28% NI-I 1%. sodium carbonate and 0,1 caustic soda (pH 12.2). Unhairingwas difiicult becauseof the-hightpH. "I-herecovered' hair LWaS of=in-- ferior quality. V

Example 4..S0dium sulfitmunhairing 7 Example 3 was repeated except the -second.-.bath containedll %;.ammonium hydroxide andl2% trisodium.phosphate (pH 11.6 to 11.8) This gave far superior unhairing.;to-. that obtained.. in. Example: 3.,= .andnillus trates,

4 with that example, the necessity for controlling the pH of thesecondbath:

Example 5.P0tassium cyanide unhairing 6 cowhide sides, weight 100# in the green salted state, and which had been soaked overnight in 400 pounds of a 1% aqueous solutionvofi ureacontaining 0.2 pound 4 sodium polysulfide, were washed in a paddle mill for 15 minutes witha=-1% -aqueous -solution of ammonium hydroxide... The mill was drained, the ,stock trimmed. and-- fleshed, and the stock -was. milled for 2 hours atJi F. in a solution consisting of Pounds I Water"- ...1 1 00 29 B. ammonium hydroxide 1 Potassium cyanide 2 /2 The water clinging to the hides immediately reduced the cyanide.concentrationsto,1.5%; .originalpHwas 10.31,

droppingjo .933 during the .run, despite, the addition of.:

2# ammonium hydroxideatthe end'of 15 minutes..

Theucyanidessoliition'was1drained ofiiinto. a .lime pit,

the. stock was washed and' again mill'ed' for 2* hours at,

FI after adding; a.sol'ution of Originalv ,pH 1 1.67, final pH 110.

Example 61-Potmssium cyanide unh'airing 6 cowhide sides, weighing 101 pounds, in the green salt'ed state, were soakedlin conventionalfashion torev move salt. etc., and"placed'in a mill; and clinging water.

largely removed. A solution containing ;pounds water -at"75f F5 1- pound ammoniumhydioxide; 29 B.

V 2V2:pounds'zpotassium cyanide;

was added, and milling ,comm'enced. This solution was immediatelyreduced to 2.25% potassium cyanide bythe watericlingingto' the hides; its initial? pH of 11.7"wa's reducedinaf'ew'minutes to 11.0; was l0l9 at' 15' minutes,"- and'10.7at the end of the twohour milling cycle.

The solution 'waswithdrawn after 2 hours, and" replaced by a solution containing 200 'pounds=water:at:-.75* F. 4 pounds? ammoniunr hydroxide 29- B. 5"pound;trisodiumxphosphate. 1 pounds: :dimethylamine': sulfate The mill was: rurrfor five minutes, and" then turned over lminute every'fifteenminutes. At the end of two'hours; the solution was. drained. The pH ranged between 1110 and 111' during; the run.

7 Excellent unhairing. was obtained;

Itwill-be noted that in this example, theoriginahpH in the firsttreating h'ath is somewhatabove 11.0; but is rapidly reduced to below this figure, so that the act'ual treatment ofFthe hides is almost all at" a pH' below 1110;

I have shown ,zbuta few examples of the .process, and it is1obviousthat-these examples can be widely'. modified without departingfrorn thescope of the invention which. is defined. in. theclaiins ILclaim:

- 1. The. process of. dehairing cattle, sheep and goat hides in. such fashiongas to produce recoverable. hair without. damage-to the hides, which comprises'treate ing wet hides with a first, lime-flee,-bathrcomprising an. aqueous. alkaline. solution of. 2 to 10%, based on. hide weight ofl an: alkalilmetal salt. which has areducing; action strongzenough to reduce the S--S linkage in! cystine; for r a period of: atleast an. hour, the .pH of.the bath beingmaintained. at a point' not above -11.0-fo11-suba stantiallw but. not necessarily the: entire. treatment period;

removing the first hath, treating the hides for a few hours at ordinary ambient temperatures with a second bath comprising a lime-free aqueous alkaline solution containing ammonium ions representing at least 0.15% NH; and sodium ions, and maintained during the treatment at a pH of at least and under 12.0, and then mechanically separating the loosened recoverable hair from the hides.

2. The process of claim 1, in which the reducing agent is selected from the group consisting of alkali metal sulfhydrates and, cyanides.

3. The process of claim 1, in which the reducing agent is present in the first bath in an amount between 2 and 10% of hide weight.

4. The process of claim 1, in which the NH concentration in the second bath is 0.3%.

5. The process of dehairing cattle, sheep and goat hides in such fashion as to produce recoverable hair without damage to the hides, which comprises treating wet hides with a first, lime-free bath comprising an aqueous alkaline solution of 2 to 10%, based on hide weight, of an alkali metal salt which has a reducing action strong enough to reduce the S-S linkage in cystine, for a period of at least an hour, the pH of the bath being maintained at a point not above 11.0 for substantially but not necessarily the entire treatment period, removing the first bath, wash- References Cited in the file of this patent UNITED STATES PATENTS 2,487,558 Kamlet Nov. 8, 1948 2,775,504 Zalcrnan Dec. 25, 1956 FOREIGN PATENTS 186,608 Great Britain Ian. 21, 1924 OTHER REFERENCES OFlaherty: Chem. and Tech. of Leather, Reinhold Publ. C0,, N.Y., 1956, pp. 283, 282, 286 and 287.

Progress in Leather Science, Br. Leather Manuf. Assoc., London, 1948 pp. 159, 161, 162, 164, 168, 169 and -177.

Alderton: The Leather World, 1944, vol. 36, pp. 316-317, 396, 397, 462-464.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,945fl37 July 19, 1960 Thomas C, Thorstensen It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected'below.

Column 4, after line 28, insert the following sentence as line 29:

Excellent unhairing was obtained.

column 5, line 11 after "and" strike out the comma.

Signed and sealed this 31st day of January 1961.

(SEAL) Attest: KARL H" AXLINE ROBERT c. WATSON Commissioner of Patents Attesting Ofi'lcer

Claims (1)

1. THE PROCESS OF DEHAIRING CATTLE, SHEEP AND GOAT HIDES IN SUCH FASHION AS TO PRODUCE RECOVERABLE HAIR WITHOUT DAMAGE TO THE HIDES, WHICH COMPRISES TREATING WET HIDES WITH A FIRST, LIME-FREE, BATH COMPRISING AN AQUEOUS ALKALINE SOLUTION OF 2 TO 10%, BASED ON HIDE WEIGHT, OF AN ALKALI METAL SALT WHICH HAS A REDUCING ACTION STRONG ENOUGH TO REDUCE THE S-S LINKAGE IN CYSTINE, FOR A PERIOD OF AT LEAST AN HOUR THE PH OF THE BATH BEING MAINTAINED AT A POINT NOT ABOVE 11.0 FOR SUBSTANTIALLY BUT NOT NECESSARILY THE ENTIRE TREATMENT PERIOD, REMOVING THE FIRST BATH, TREATING THE HIDES FOR A FEW HOURS AT ORDINARY AMBIENT TEMPERATURES WITH A SECOND BATH COMPRISING A LIME-FREE AQUEOUS ALKALINE SOLUTION CONTAINING AMMONIUM IONS REPRESENTING AT LEAST 0.15% NH3 AND SODIUM IONS, AND MAINTAINED DURING THE TREATMENT AT A PH OF AT LEAST 10 AND UNDER 12.0 AND THEN MECHANICALLY SEPARATING THE LOOSENED RECOVERABLE HAIR FROM THE HIDES.