PROPERTIES OF VULCANIZED RUBBER Relatively little rubber is used in the unvulcanized state. Rubber cements for adhesive purposes, sealing can ends, surgical adhesive tape, electrical insulating tape (either all rubber or rubberized fabric) and crepe-soled footwear are among the most important uses of unvulcanized rubber. By skilful compounding of rubber with other materials the properties of vulcanized rub ber may be varied to meet a great diversity of conditions of serv ice and the applications of vulcanized rubber compositions in promoting human comfort, health and pleasure or economic proc esses now number many thousands. Probably the outstanding property which has been secured in vulcanized rubber composi tions is resistance to abrasive wear. Their superiority for this pur pose has long been utilized in vehicle tires, heels and soles, and in conveyor belts for carrying ore, sand, gravel, coal, and even broken glass trimmings (cullet) in glass works. Gravel chutes, grinding mills, and mixers for abrasive materials are lined with rubber. Flexibility, combined with one or more of other desirable properties, renders rubber goods superior to metal or other ma terials under certain conditions of service for equipment to con vey gases or liquids, for invalid or operation cushions, and for many other purposes. Pneumatic tires, hose of various kinds, valve discs, paper machine rolls, clothes wringer rolls, rubber stoppers, insulated wire, universal joint parts, nursing nipples and milking machine parts may be mentioned as articles in which flexibility adds to the usefulness of the product.
Extensibility of rubber has been utilized as the dominantly valuable property in stationers' bands and in elastic webbing. Soft rubber compositions vary in extensibility to the breaking point, according to composition, up to i,000% of the original length. Absorption of shock is a valuable property in vehicle tires, in shock-absorber cord for aeroplanes, in engine supports and shackles for automobiles, in heels, in matting and flooring and in steam hammer cushions. Sheets of rubber vulcanized ad herently between metal plates or concentric cylinders are used for eliminating vibration of buildings caused by operating heavy machinery and for eliminating noise from lighter equipment. The
plates on one side of the rubber-metal units are fastened to the floor, those on the other side to the machine, which is thus sus pended on rubber in shear. Floor vibrations from equipment weighing many tons have thus been reduced by as much as 85%. Resistance to diffusion of gases is utilized in air brake hose, bal loons, vacuum hose, football bladders, tennis balls, jar rings for preserving foods, sealing of cans, gas masks for mines or chemi cal plants, and in air mattresses and pillows. Waterproofness is one of the properties of rubber first recognized and adapted to human needs, even before vulcanization was discovered. Coats, capes, hats, footwear, gloves, aprons, diving clothing, bathing caps, hose, dental dam, etc., find their value in this property.
For electrical insulation of conductors used in damp localities or submerged in water specially prepared rubber of unusually low water-absorbing character is used in order to minimize changes in electrical properties of the conductors. These specially pre pared rubbers enable the cable manufacturer to reduce water absorption by so% to 75%. On dry surfaces rubber exhibits an enormously high frictional resistance. Advantage is taken of this property in transmission belting, which transmits power satisfac torily under many conditions where leather belting cannot be used, in pulley lagging for belt drives and for band saws, tires, pencil erasers, flooring and non-slip footings for ladders. On wet surfaces, on the other hand, rubber compositions give rather low frictional resistance. This has led to the development of rubber bearings for propeller shafts and for deep well pumps where oil lubrication is difficult. Sealing rings in hydraulic pumps not only run with low frictional resistance but deliver water with greater operating efficiency. Rubber has replaced metal or lignum vitae for bearings in installations where silt in the water has caused rapid wear, rubber resisting the cutting action of the silt or sand far better than the other materials. Resistance to flow of liquids through rubber-lined pipe is lower than through metal pipe of the same internal diameter.