leather(redirected from Pig leather)
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a material made from the skin of an animal. Leather is the processed dermis (with preservation of its basic natural fibrous structure), whose properties are changed by mechanical, physical, and chemical action, depending on the intended use.
In the USSR, leather is divided into four classes according to use: shoe leather, harness and saddle leather, industrial leather, and clothing and haberdashery leather. Shoe leather includes leather intended for the uppers and bottoms of shoes. A distinction is made between bottom leather attached by screws and by thread and glue (welt, sewn, and glued fastening). Such leather is resistant to bending and compression and is made from the hides of cattle, camels, pigs, horses, and marine animals, with the addition of various tannins. Leather for shoe bottoms is classified according to the type of tanning, the character of the finish, the thickness at a standard point (for the fifth to sixth categories, 1.75 to 7 mm), quality (four grades), and configuration (whole hides, half hides, and so on). Combined tanning with chromium compounds, tannides, and syntans and sometimes with compounds of zirconium, aluminum, and other materials is usually used in making leather for shoe soles. Chrome-tanned leathers, in spite of their great wear resistance, are not used widely because of a number of disavantages, including high wettability, inability to hold their shape, low friction coefficient when wet, and difficulty of finishing.
Shoe bottom leather is used to make soles, insoles, and other shoe components. Sole leather must have a good capacity for withstanding the deformations of abrasion, compression, and bending, and it must also maintain its linear dimensions upon wetting and subsequent drying. Leather for welt methods of fastening may be less stiff than leather used for screw fastening, with an elastic modulus under tension of 60–100 meganewtons per sq m (MN/m2), or 600–1,000 kilograms-force per sq cm (kgf/cm2), rather than 100–120 MN/m2 (1,000–1,200 kgf/cm2); however, the leather must be sufficiently dense to retain the thread tacks after part of the stitches have been abraded away on the walking surface of the sole. Leather with greater plasticity is selected for sewn methods of fastening, since excessive stiffness makes the manufacturing process difficult and the threads wear out quickly during use. For the glue fastening method the fibrous structure of the dermis must be compact. The required properties of sole leather are provided by the selection of the raw material.
Leather for shoe uppers is a soft material that is subjected to repeated stretching and compression, bending, and the action of dust, dirt, and chemical substances both during use and in the process of manufacturing shoe parts. The leather should be sufficiently air- and vapor-permeable but impermeable to water. For shoe uppers, a distinction is made between chrome-tanned leather and Russia leather. The earlier method of classifying upper leather according to the method of fastening has become obsolete, since both Russia leather and chrome-tanned leather are widely used to make shoes fastened by various means. Russia leather is used for the uppers of heavy work or army shoes and sandals. It is also sub-divided according to the type of raw material used, the configuration, the method of tanning, the pigmentation, the thickness (from 1.5 to 3 mm), the size, and the class. The necessary properties of Russia leather for shoe (resistance to repeated bending and stretching and high water impermeability) are achieved mainly by strong separation of the structure of the collagen fibers of the dermis (intensive liming) and the introduction of a large quantity of oily substances (absolutely dry leather contains more than 31 percent oil). Russia leather for shoes must also be resistant to perspiration (since the upper of a heavy boot is made without a lining) and vapor-permeable (for evaporation of perspiration). Heat-resistant Russia leather, which is notable for its high content of chromium compounds and several fillers, is used for shoes whose rubber soles are attached by hot vulcanization. Russia leather for sandals must be more elastic, but not stiff, and less lubricated (8–16 percent oil), with a well-finished outer surface.
Chrome-tanned leather for shoe uppers is tanned with chromium or chromium and zirconium, sometimes with additional tanning with synthetic tannins, tannides, or tanning amino resins; it is normally oiled (3.7 to 12 percent oil) and is 0.06 to 2 mm thick.
Varnished leather and lining leather are also made. They must be soft, evenly colored, and without spots and other defects. They are classified according to the type of raw material, configuration, area, thickness, character of the finish (smooth or tooled), method of finishing (with a natural or artificial outer surface, with a matte outer surface, with a glossy outer surface, or with the flesh side out), the type of coating (casein, acrylic, nitrocellulose, and so on), and quality.
Varnished leather is mainly distinguished from ordinary chrome-tanned leather only by its finish (the application to the outer surface of a film of varnish—oil, polyurethane, nitrocellulose, or mixed). Varnished leather must be flexible, with an evenly glossy and noncracking varnish film, a sharply defined grain, and a carefully finished flesh side. It is used predominantly for fashion shoes and haberdashery. Shoe lining leather is made from the discarded semifinished products of the tanning process.
Harness and saddle leather is used to make straps, holsters, bags, and cases, as well as equestrian accessories (parts of the bridle, straps, and saddlebags) and harnesses. Cowhide and pigskin are used for these articles. All types of harness and saddle leather are made using combined tanning methods. Rawhide is used to make the parts of the harness (straps, reins, and joints), because straps made from it are highly resistant to breakage.
Industrial leathers are used to make drive belts and machine parts. Leather for drive belts is made from the back of cowhides (bulls and dry cows). They must have great durability when stretched, high density, great elasticity, and uniform thickness. The high elasticity of such leathers is achieved by introducing significant quantities of oily substances and by intense stretching of the hide backs and the bands cut from them for the belts, as
|Table 1. Properties of some types of artificial and natural sole leather|
|Density (g/cm3)||Average life of shoes (months)||Weight of one pair of soles (g)||Relative consumption of rubber per pair (percent)|
|Natural sole leather||0.9–1.3||4||320||—|
|Nonporous colored rubber||1.1–1.5||3–4||520||100|
|Porous colored rubber||1 .0–1 .2||5–6||360||80|
|Lightened porous rubber||0.40–0.9||7–12||210–320||40–70|
|Extra-light porous rubber||0.15–0.25||7–12||70–130||15–25|
well as by stretching the belts themselves during their manufacture. Industrial leathers are used to make such machine parts as twisting hoses (parts of combing and carding machines), couplings, driving straps for looms, gaskets, packing cups, and dividing straps.
Clothing and haberdashery leather is a soft material made from fine leather raw materials using chrome and combined tanning methods. Clothing kidskin, noted for its great toughness, is made from the skin of sheep. In addition, pigskin is used to make clothing. Haberdashery goods are sometimes made from chrome-tanned leather. Haberdashery leathers must be evenly colored, not fragile, and resistant to friction. In some cases they undergo stamping (cutting of an artificial grain). Leather for gloves is made from the skin of sheep, goats, foals, piglets, dogs, and other animals, with the application of chrome, aluminum-chrome, and oil tanning. Such leathers must be soft, plastic, and tough.
REFERENCEKhimiia i tekhnologiia kozhi i mekha, 2nd ed. Moscow, 1970.
L. P. GAIDAROV