Artificial Leather

a man-made polymeric material used instead of natural leather for making shoes, clothing, headgear, haberdashery, and some industrial goods. Certain types of artificial leather, made with natural rubber and nitrocellulose, were used as early as the late 19th century.

In the USSR the industrial manufacture of artificial leather dates to 1930. At that time, fabrics with a nitrocellulose coating, plastic leather, and shoe pasteboard were manufactured. The technology of production of those materials was borrowed from already developed branches of industry, including textiles, rubber and paper. These types of artificial leather were inferior to natural leather in their properties and appearance and were called leather substitutes. They gradually lost their importance. With the development of polymer chemistry, the appearance of new types of raw material, and the higher technical level of processing polymers, the assortment of manufactured goods increased and their quality improved. Goods made of artificial leather began to compete successfully with goods of natural leather and even surpassed them in some respects.

Artificial leather is classified according to its use and according to its structure and method of manufacture, as shoe rubber, shoe and haberdashery pasteboard, and soft artificial leather.

Shoe rubber is one of the most widely used substitutes for natural leather, and it was one of the first used in industry. It is used mainly to make components of the bottoms of shoes (soles, heels, and heel taps). This type of artificial leather is a highly filled rubber based mainly on synthetic rubbers, usually butadiene-styrene rubber. Small quantities of thermoplastics or thermosetting resins may be added to it to increase its hardness and wear resistance.

The steps in the manufacture of shoe rubber are the mixing of the rubber with the ingredients, molding of the mixture (by means of calendering, injection, or other methods), and the production and vulcanization of the raw uppers. The artificial leather may be porous or solid, black or colored. The porous resins are produced by introducing pore-forming substances. Such rubbers absorb virtually no water. The use of soles made from porous rubbers significantly lightens the shoe and improves its shock-absorbing and thermal insulation properties (see Table 1). For this reason, such synthetic leather may be considered superior to natural leather. The presence of a group of valuable qualities in shoe rubber has led it its substitution for natural leather in more than 70 percent of shoe soles and in various industrial articles (gaskets and shock absorbers).

Shoe and haberdashery pasteboard are made from various fibrous materials according to a technology borrowed from the manufacture of paper and cardboard. The raw materials are cellulose fiber, cotton, and leather fiber—the ground waste products of natural leather (predominantly vegetable and chrome-vegetable tanned). The fibrous materials are glued together with rosin glues, bituminous rosin emulsions, and latices. The fibrous mass is molded on mesh machines, forming single-layer or multilayer sheets or ribbons. They are condensed by pressure and then dried and calendered. Leather pasteboard for insoles and counters is made in this way. Shoe pasteboards with low apparent density and good usage characteristics (for example, Texon) are made by molding unglued fibrous strips from refined pulp, with impregnation by latex, heat treatment, and finishing. The main advantages of such artificial leathers are their ability to retain their mechanical properties when wet, together with sufficient hygroscopicity and moisture capacity (from the point of view of hygiene requirements). In foreign industry, pasteboard-type artificial leathers have almost entirely replaced natural leather in the manufacture of inner shoe components, and in the USSR more than 60 percent of such components are made from them. The main properties of pasteboard artificial and natural leather are shown in Table 2.

Soft artificial leathers are made by treating the foundation (fabric, knitted material, nonwoven material, or paper) with film-forming rubber cement or latices (shoe kersey or elastic artificial leather), polyvinyl chloride pastes and plasticized rubbers (vinyl artificial leather), polyamides (amide nitrocellulose bases (nitro artificial leathers), and polyester urethanes. The foundation is treated with film-forming substances of various consistencies on machines with application assemblies, heating chambers for drying or gelling (when using polyvinyl chloride pastes), vulcanization chambers, and finishing apparatus. Calenders, extruders, and cashiering and other machines are used

to process highly viscous mixtures. Soft artificial leathers are made dense or porous. In the case of porous artificial leathers, vapor-forming agents are introduced into the original compounds, or porosity is achieved by other means, such as frothing with latex mixtures. The finishing operations (embossing, coloring, and application of a colored imprint) are done on special equipment. There is an extremely wide assortment of manufactured soft artificial leathers.

A special group of artificial leathers consists of those based on nonwoven materials. The use of nonwoven fibrous foundations makes possible the production of homogeneous and isotropic artificial leathers with relatively high moisture capacity and ability to elongate under the stress to which shoes are subjected during their manufacture. In additon, they are a good imitation of leather.

Materials whose qualities approach those of natural soft leather are made in the USSR and abroad; they are called synthetic leathers. Various kinds of fibers are used to make their foundation; polymeric hydrophilic substances (frequently polyester urethanes) are used to impregnate them and form the outside coating. Materials having the required durability and hygienic characteristics are manufactured with fibrous-porous, porous, and reinforced interlayer structures.