Natural rubber belts

A thin layer of a mix of natural rubber, sulfur, precipitated silica, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180 °C). A modi-... 

Most polystyrene products are not homopolystyrene since the latter is relatively brittle with low impact and solvent resistance (Secs. 3-14b, 6-la). Various combinations of copolymerization and blending are used to improve the properties of polystyrene [Moore, 1989]. Copolymerization of styrene with 1,3-butadiene imparts sufficient flexibility to yield elastomeric products [styrene-1,3-butadiene rubbers (SBR)]. Most SBR rubbers (trade names Buna, GR-S, Philprene) are about 25% styrene-75% 1,3-butadiene copolymer produced by emulsion polymerization some are produced by anionic polymerization. About 2 billion pounds per year are produced in the United States. SBR is similar to natural rubber in tensile strength, has somewhat better ozone resistance and weatherability but has poorer resilience and greater heat buildup. SBR can be blended with oil (referred to as oil-extended SBR) to lower raw material costs without excessive loss of physical properties. SBR is also blended with other polymers to combine properties. The major use for SBR is in tires. Other uses include belting, hose, molded and extruded goods, flooring, shoe soles, coated fabrics, and electrical insulation. 

Insoluble Sulfur. In natural rubber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of rubber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

The best known polymeric materials are natural rubber and synthetic rubber. These materials have low modulus of elasticity. The flexibility of these materials enables their application in tubing, belting and automotive tires as encountered in everyday usage. Resistance to chemicals, abrasive attack and insulating property can be advantageous in corrosion control applications. 

The largest use of natural rubber is in the manufacture of tires. Over 70 percent of its consumption is in this area. The next largest use is as latex in dipped goods, adhesives, rubber thread, and foam. These uses account for approximately another 10 percent. The remainder is used in a variety of applications such as conveyor belts, hoses, gaskets, footwear, and antivibration devices such as engine mounts. 

Despite the introduction of synthetic elastomers, ceramics and other abrasion resisting metals such as manganese, natural rubber holds a dominant position in this field of application and is the primary choice for abrasion resistance. Synthetic rubbers (particularly styrene-butadiene rubber which is dominant in the tyre industry sector) are used in dry abrasion application such as general purpose abrasion resistant sheets and conveyor belt covers, since the rubber can be reinforced with fine particles of carbon blacks to achieve dry abrasion resistance close to that of natural rubber. It should be noted that styrene-butadiene rubber is inferior to natural rubber in cutting and chipping resistance. 

Natural rubber Excellent properties of vulcanizates under conditions not demanding high levels of heat, oil, and chemical resistance Tires, bushing, couplings, seals, footwear and belting second place in global tonnage... 

Polyisoprene Similar to natural rubber, but excellent flow characteristics during molding Tires, belting, footwear, flooring... 

Rubber consumption is dominated by tyre production. In these, conveyor belts, and pressure hoses, thin layers of either steel wire or polymeric fibre reinforcement take the main mechanical loads. These layers, with rubber interlayers, allow flexibility in bending, whereas the reinforcement limits the in-plane stretching of the product. The applications are dominated by natural rubber and styrene butadiene copolymer rubber (SBR). Other rubbers have specialised properties butyl rubbers have low air permeability, nitrile rubbers have good oil resistance, while silicone rubbers have high and low temperature resistance. Rubbers play a relatively small role in this book, but the rubbery behaviour of the amorphous phase in semi-crystalline thermoplastics is important. 

The adhesion between rubber and brass-plated steel (e.g., steel tire cords for belted radial tires) has been the subject of much study and speculation. Brass plating is the major method of obtaining adhesion between natural rubber and the steel of tire cords. Over the years there has been much speculation about its mechanism, but there is agreement on one aspect of the adhesion of natural rubber to brass-plated steel the actual adhesion between the natural rubber and the brass-plated cord, formed in situ during the vulcanization process, is an interfacial layer of sulfides and oxides of copper (Buchan, 1959 van Ooij, 1979, 1984). 

The increase in natural rubber usage translates into approximately 21 kg per tire for a radial construction compared with approximately 9 kg found in a bias buck tire. Natural rubber compounds also tend to And use in covers of high-performance conveyor belts where a similar set of performance parameters such as those of a buck tire bead compound are found. Low hysterebc properties, high tensile sbength, and good abrasion resistance are required for both products. 

Depending on the grade of polymer, polychloroprene can be vulcanized by zinc oxide or magnesium oxide. Tetramethylthiuram disulfide can serve as a retarder. Polychloroprene is inferior to NBR for oil resistance but is still significantly better than natural rubber, SBR, or BR. Like NBR it also finds extensive use in such products as oil seals, gaskets, hose linings, and automotive engine transmission belts where resistance to oil absorption is important. Butyl rubbers are a copolymer of isobutylene and isoprene ... 

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