Compounding, rubber

Natural rubber, and with a few exceptions the many synthetic rubber polymers commercially available, are unsaturated that is, they contain double bonds which provide sites for the vulcanization (crosslinking) reaction. The polymers are mostly linear or branched, but some also contain (either intentionally or unintentionally) minor amounts of gel (crosslinked particles) prior to vulcanization. This can have a profound effect on processing properties. The following substances are also included in rubber compounds. 

1 Reinforcing Fillers. Most rubber compounds that are intended to develop a reasonable tensile strength, abrasion, and tear resistance will contain up to 50 phr (parts per hundred parts rubber by weight) or more of a reinforcing filler, nearly always a carbon black. The use of carbon black in rubbers is quite different than in plastics, where it is strictly a pigment and limited to much lower loadings. 

Carbon blacks are not simply carbon. Basic blacks have hydroxyl groups at the particle surfaces and acid blacks have carboxylic add functionality. It has been shown that the rubber polymer forms strong secondary and primary covalent bonds with the carbon black, which accounts for its reinforcing ability. A wide variety of carbon blacks is available. In addition to chemical functionality, they differ in such factors as particle size, degree of aggregation, and surface area, and different types of rubber polymers require particular kinds of black for optimum reinforcement. Silicone rubbers are sometimes reinforced with finely divided silica (S1O2). 

Fillers. As with plastics, the function offiUers in rubber is mainly to reduce the cost of the compound. Most rubber fillers are finely divided inorganics such 

4- Vulcanizing or Curing Systems. The function of these is to crosslink the polymer. The most common curing systems are based on sulfur. While sulfur 

2) drying by evaporation. During these processes the rubber globules in the latex give up their separate existence and fuse into one homogeneous mass of crude rubber. 

D is the handle for opening/locking the lid C, and F is the masticated rubber. A is the hollow cylinder. B is the clamp and E is the studded core.  

In later stages, the technology developed and rubbers were compounded, mixed in mills, calendered in calenders, extruded in extruders, molded in hydraulic presses, cured in autoclaves and many hand operations of forming and curing became possible. State-of the art techniques of compounding were developed to produce rubber products of any shape and dimensions for many requirements. 

Economics and price of the final article often dictate a specific type of mbber that can be used. The expected usable life for the product is controlled by many factors including end customer awareness, competitive situation in the marketplace, safety, reUabiUty, and other factors. Rubber is almost always used as a functional part of another system. For example tires, hoses, belts, O-rings, and numerous mbber components are used in manufacturing automobiles and tmcks. The overall life of the vehicle as well as its performance level often control or direcdy relate to the service life or quaUty level of the mbber parts. 

The principal component of a mbber compound is the elastomer or blend of elastomers chosen for a specific component appHcation. There are 25—30 different chemical classifications of elastomers six of these classes represent over 90% of all elastomers used (see Elastomers, synthetic). 

According to the International Institute of Synthetic Producers (IISRP) there were 3.8 x 10 t of elastomer consumed in the United States in 1994, 27% natural mbber, and 73% synthetic mbber. Elastomer growth in the United States is 1.5% and closely follows GDP growth (1). 

Natural mbber comes generally from southeast Asia. Synthetic mbbers are produced from monomers obtained from the cracking and refining of petroleum (qv). The most common monomers are styrene, butadiene, isobutylene, isoprene, ethylene, propylene, and acrylonitrile. There are numerous others for specialty elastomers which include acryUcs, chlorosulfonated polyethylene, chlorinated polyethylene, epichlorohydrin, ethylene—acryUc, ethylene octene mbber, ethylene—propylene mbber, fluoroelastomers, polynorbomene, polysulftdes, siUcone, thermoplastic elastomers, urethanes, and ethylene—vinyl acetate. 

Elastomer Specific gravity Hardness, Shore A Tensile strength, MPa Elongation, % Resihence Compression e set Impermeability e to gases 

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Ghoreishy, M. H. R. and Nassehi, V., 1997. Modelling the transient flow of rubber compounds in the dispersive section of an internal mixer with slip-stick boundary conditions. Adv. Poly. Tech. 16, 45-68. 

Clarke,. 1. and Freakley, P.K., 1995. Modes of dispersive mixing and filler agglomerate size distributions in rubber compounds, Plast. Rubber Compos. Process. Appl. 24, 261-266. 

Rubber compounds Rubber content Rubber latex... 

Cyclohexylamine condensed with mercaptobenzothiazole produces the large volume moderated mbber accelerator JV-cyclohexyl-2-henzothiazolesulfenamide (41) [95-33-0] (see Rubber compounding). DCHA similady is used in piepaiing... 

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