Rubber typical

The process of anionic polymerisation was first used some 60 or more years ago in the sodium-catalysed production of polybutadiene (Buna Rubbers). Typical catalysts include alkali metals, alkali metal alkyls and sodium naphthalene, and these may be used for opening either a double bond or a ring structure to bring about polymerisation. Although the process is not of major importance with the production of plastics materials, it is very important in the production of synthetic rubbers. In addition the method has certain special features that make it of particular interest. 

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

Stress-Strain Data. Tensile tests were made with an Instron tester at some seven crosshead speeds from 0.02 to 20 inches per minute at five or six temperatures from 30° to —46°C. The tests were made on rings cut with a special rotary cutter from the circular sheets of the elastomers. The dimensions of each ring were determined from the weights of the ring and the disc from its center, the thickness of the ring, accurately measured, and the density of the rubber. Typically, the outside and inside diameters were 1.45 and 1.25 inches, respectively, and the thickness was about 0.085 inch. The test procedure used is described elsewhere (11), and the cubic equation, eq 4 in ref. j 2, was used to compute the average strain in a ring from the crosshead displacement. 

Commercial grades of HR (butyl rubber) are prepared by copolymerising small amounts of isoprene with polyisobutylene. The isoprene content of the copolymer is normally quoted as the mole percent unsaturation , and it influences the rate of cure with sulphur, and the resistance of the copolymer to attack by oxygen, ozone and UV light. The polyisobutylene, being saturated, however, naturally confers on the polymer an increased level of resistance to these agencies when compared to natural rubber. Commercial butyl rubbers typically contain 0.5-3.0% mole unsaturation. 

The majority of plasticiser consumption is in CR and NBR. Plasticisers are also technically important in chlorosulphonated polyethylene, hydrogenated nitrile, ethyl acrylate copolymer, epichlorohydrin copolymer and ethylene-acrylic terpolymer. At around 10 kt/annum (Europe), total consumption of plasticisers is on a much smaller scale than the process oils used in hydrocarbon rubbers. Typical addition levels are below 20 phr. 

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

For peroxide cross-linking, organic peroxides, such as dicumyl, di-t-butyl, and benzoyl peroxides, are used in amounts 1 to 3 phr (parts per hundred parts of rubber). Typical cure cycles are 5 to 10 min at temperatures 115 to 170°C (239 to 338°F), depending on the type of peroxide used. Each peroxide has a specific use. A postcure is recommended to complete the cross-linking reaction and to remove the residues from the decomposition of peroxide. This improves the long-term heat aging properties.62... 

Impact-modified polystyrene is mainly produced by mass polymerization, either in tower cascades or tank/tower cascades. In the latter case, particle size and morphology can be defined by variation of the viscosity ratio between the continuous and the discontinuous phases, the stirrer velocity, the molecular weight of the poly butadiene rubber and the amount of rubber. Typical particles sizes are 2-20 xm, this being the optimum for effectively dissipating impact energy. 

In the case of mass ABS, the variety of rubber particle morphology is less diverse. Typical examples of morphology are shown in Figure 14.6. If polybutadiene rubber is used (linear or star), cellular particles are obtained with SAN occlusions. In the case of styrene-butadiene block rubber (typically 30% styrene) also cellular particles are obtained but besides the SAN occlusions, polystyrene domains are clearly visible in the particles. To be able to make the other morphologies that are possible in HIPS, the interfacial tension has to be manipulated. Controlling the grafting reaction is a way to achieve this but the possibilities are limited with the tools (mainly initiator) that are currently available. 

Fillers are materials that modify rubber characteristics (e.g., hardness) and improve its physical characteristics (e.g., tensile strength), in addition to reducing costs. Rubber is sometimes compounded without the use of fillers the resultant product is called gum rubber. Typical fillers are calcined and hydrated clays, magnesium silicate (talc), magnesium oxide, and silicas. Carbon black, a common filler used to increase the heat resistance in industrial components such as tires, is not used as a filler in pharmaceutical components but it is used in smaller amounts as a black pigment. Polynuclear aromatic (PNA) hydrocarbons are a concern with carbon blacks but the grades used by manufacturers of pharmaceutical components contain very low concentrations. 

Vulcanization is an industrial process applied to various polymers from the class of unsaturated polyhydrocarbons. The major practical use of vulcanized elastomers is the tire industry. Tires are made from various polymer blends, including natural rubber, typically between 20 and 50%. The other polymers used in various blends that can be vulcanized include copolymers such as poly(styrene-co-1,3-butadiene) or SBR, poly(acrylonitrile-co-1,3-butadiene-co-styrene) or ABS, poly(isobutylene-co-isoprene), poly(ethylene-co-propylene-co-1,4-hexadiene, etc. 

Elastomers include natural rubber (polyisoprene), synthetic polyisoprene, styrene-butadiene rubbers, butyl rubber (isobutylene-isoprene), polybutadiene, ethylene-propylene-diene (EPDM), neoprene (polychloroprene), acrylonitrile-butadiene rubbers, polysulfide rubbers, polyurethane rubbers, crosslinked polyethylene rubber and polynorbomene rubbers. Typically in elastomer mixing the elastomer is mixed with other additives such as carbon black, fillers, oils/plasticizers and accelerators/antioxidants. 

Thus, as an elastomer is compressed in, say, the Z-direction (as in an isolator on a rubber grommet, engine mount, or transmission mount), the mount will deform in the X and Y directions. This value is nearly 0.5 for natural rubbers (typically used for mounts in automotive systems). For steel, Poisson ratios are around 0.3. The Poisson ratio has no units. 

Figure 2.75 shows the constructions of a standard bias (diagonal) ply tire and a radial ply tire. The major components of a tire are bead, carcass, sidewall, and tread. In terms of material composition, a tire on an average contains nearly 50% of its weight in actual rubber for oil extended rubbers (typically containing 25 parts of aromatic or cycloparafiBnic oils to 75 parts of rubber), it is less. The remainder included carbon black, textile cord, and other compounding ingredients plus the beads. 

The EVA copolymers are slightly less flexible than normal rubber compounds but have the advantage of simpler processing since no vulcanization is necessary. The materials have thus been largely used in injection molding in place of plasticized PVC or vulcanized rubber. Typical applications include turntable mats, based pads for small items of office equipment, buttons, car door protection strips, and for other parts where a soft product of good appearance is required. 

Silicone elastomers are either room-temperature vulcanization (RTV) or heat-cured silicone rubbers, depending on whether cross-linking is accomplished at ambient or elevated temperature. [The term vulcanization (see Chapter 1 and Chapter 2) is a synonym for cross-linking. While curing is also a synonym for cross-linking, it often refers to a combination of additional polymerization plus cross-linking.] RTV and heat-cured silicone rubbers typically involve polysiloxanes with degrees of polymerizations of 200-1500 and 2500-11,000, respectively. 

The content of the fiber component is typically 3-15%. The elastomeric binder comprises about 3-15%. The rest are fillers. Aramids are used in fiber reinforced gaskets instead of asbestos fibers. Binders typically are synthetic rubbers. Typical components of a gasket formulation are shown in Table 13.5. 

REL. See Recommended Exposure Limit, release agent. A substance that aids in the separation from a mold, used in baking, plastics, and rubber. Typical release agents for plastics and rubber are paraffin and tallow for baking, vegetable oils. 

---