Liming hides

Trimming and Fleshing. The limed hides not fleshed before curing are usually trimmed and fleshed at this point in the production. The trimming is done by hand to remove any portions of the hide that could interfere with the subsequent machine processes, eg, the shanks, ears, and snout. 

Deliming and Bating. The limed hides have a pH around 12. Because chrome tanning is done at pH 2—4, the lime must be removed for pH adjustment. In addition, the undesirable materials in the hide, ie, both natural and the degradation products from the unhairing, must be removed (7,9). 

Limed hides Cattle hides Non gelatin base proteins, carbohydrates Aqueous CaOH 40.000-130,000... 

Protein-Based Adhesives. Proteia-based adhesives are aormaHy used as stmctural adhesives they are all polyamino acids that are derived from blood, fish skin, caseia [9000-71 -9] soybeans, or animal hides, bones, and connective tissue (coUagen). Setting or cross-linking methods typically used are iasolubilization by means of hydrated lime and denaturation. Denaturation methods require energy which can come from heat, pressure, or radiation, as well as chemical denaturants such as carbon disulfide [75-15-0] or thiourea [62-56-6]. Complexiag salts such as those based upon cobalt, copper, or chromium have also been used. Formaldehyde and formaldehyde donors such as h exam ethyl en etetra am in e can be used to form cross-links. Removal of water from a proteia will also often denature the material. 

Unha.iring, Unhairing can be done either by a hair save or a hair pulp system. The hair pulp system is preferred by most tanners for its speed and labor efficiency. In the hair pulp system the hides are treated with sodium sulfide (sulfhydrate) and lime (calcium hydroxide). The hair is quickly destroyed by the strong alkaline reducing conditions. 

The differences in the amino acid chemistry of the hide coUagen and the hair keratin are the basis of the lime-sulfide unhairing system. Hair contains the amino acid cystine. This sulfur-containing amino acid cross-links the polypeptide chains of mature hair proteins. In modem production of bovine leathers the quantity of sulfide, as Na2S or NaSH, is normally 2—4% based on the weight of the hides. The lime is essentially an unhmited supply of alkah buffered to pH 12—12.5. The sulfide breaks the polypeptide S—S cross-links by reduction. Unhairing without sulfide may take several days or weeks. The keratin can be easily hydrolyzed once there is a breakdown in the hair fiber stmcture and the hair can be removed mechanically. The coUagen hydrolysis is not affected by the presence of the sulfides (1—4,7). 

After about 4 to 12 hours the hides are drained, then floated with lime without additional sulfides. The dmm or hide processor is mn at slow speeds intermittently during the unhairing process. The continued action of the lime solution on the hides brings the pH to 12—13. At this pH the hides swell to about twice the original thickness. This swelling has a beneficial opening of the fibers to permit better tannage (Fig. 3a). 

Splitting. In most modem large tanneries that make upholstery leather, and in some that make shoe uppers leather, the hides are spHt in the lime condition. In splitting the hides are cut to the desired thickness with a horizontal belt knife. The hides are fed into the machine grain up. The clearance between the grain and the blade is maintained by a series of narrow rollers supported by a mbber roUer underneath the spacing roUers. The grain layer is then cut to the thickness desired to an accuracy of about 0.1 mm. 

For deliming, ammonium salts and acids are used. The proportion of ammonium salts to acids and the type of acids employed is a matter of the tanner s choice. The acid neutralizes the lime, Ca(OH)2, thereby adjusting the pH. The ammonium salts have two functions to buffer the solution to a pH required for bating, and to form calcium ammonium complexes. The acidity and the complex formation solubilize the calcium and serve to bring the hide to the desired pH. 

The usual procedure in the manufacture of chrome-tanned leathers is the use of a continuous bate, pickle, and tan method. The hides or skins remain in the dmm from the lime washing through the chrome tanning stages. The entire process usually takes about 22—24 hours. The bate, pickle, and tan can be done faster, but most tanners find that a one-day cycle fits well into production scheduling and results in a quaUty leather. 

Anschwemmfilter, n, settling filter, Anschwddebreit m. Leather) liming paste, aoschwdden, v.t. paint (the flesh side of hides) with lime,... 

Schwbde, /. place where hides are limed state of being limed, -blei, m. (Leather) lime... 

The usual alkali employed is lime. The raw material for gelatine is tropocollagen, which is present in the original hides or bones. This protein consists of three polypeptide chains arranged in a triple helix. In contrast, gelatine consists of several free or interassociated chains, ranging in molecular weight from around ten thousand to several hundred thousand. On extraction, monomers (a-chains MW 100 000), dimers (P-chains) and trimers ( -chains) and some lower order peptides are released. 

The highest level of NDMA (0.5 yg/ml) was found in a sample from a 36.5% aqueous solution of dimethylamine sulfate (DMAS). According to plant personnel, 117 liters of DMAS are used each day in the hide unhairing process. This would amount to less than 60 mg NDMA/day potential exposure from the contaminated DMAS. NDMA (0.0015 yg/ml) was also found in the re-lime pit (unhairing vat) process water. This process water contained 5.7 liters of DMAS in 8000 L of lime-saturated water. The other samples which contained NDMA (0.004 yg/ml and 0.006 yg/ml) were waste-water from the tannery outlet pipe collected at the local municipal waste-... 

In many parts of France and Belgium, they employ a weals solution of lime to assist in cleansing and freshening the hides, with the view, at the same time, of rendering the succeeding operations of depilation and tanning more easy and efficacious. 

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