Unsaturated hydrocarbon elastomers

Under some circumstances it is possible to observe very low 5 values for polymer solids, namely when the energy to fracture chemical bonds is provided by chemical reaction. An excellent example of this is the ozone cracking of unsaturated hydrocarbon elastomers where crack propagation occurs at threshold values of as low as 0.05 Jm Under such low stresses the toss function of Eq. (5) is effectively... 

Peroxide Vulcanization of Unsaturated Hydrocarbon Elastomers. The initiation step in peroxide-induced vulcanization is the decomposition of the peroxide to give free radicals. If the elastomer is derived from butadiene or isoprene, the next step is either the abstraction of a hydrogen atom from an allyUc position on the polymer molecule or the addition of the peroxide-derived radical to a double bond of the polymer molecule. In either case, polymeric free radicals are the result (Scheme 17). 

Static deflection AFM can be used to measure local mechanical properties of polymer surfaces, but only after consideration of the relative stiffness of the cantilever and the surface under study. Cantilevers with stiffness in excess of 50 N/m are necessary to indent materials with a bulk modulus in excess of 1 GPa (10 N/m ). Soft levers with a spring constant less than 1 N/m are sufficient to indent elastomers. Conventional staining techniques used in electron microscopy provide a viable way to harden unsaturated, hydrocarbon elastomers for imaging with soft cantilevers. Alternatively, low bulk modulus polymers (E < 1 MPa or 10 N/m ) require resonant imaging techniques such as Tapping Mode for direct imaging. 

The majority of elastomer blends are phase separated, but of interest, as crosslinking can achieve mechanical compatibilization due to crosslinking between the phases as noted in Chapter 3. This compatibilization method can lead to unique and useful blends with a compromise in properties, offering useful commercial products as well illustrated by the applications in tire construction. The unsaturated hydrocarbon elastomers without polar functional groups are rarely miscible with each other, because no specific interactions are present to achieve the necessary thermodynamic driving force for miscibility. The few miscible examples noted generally exhibit matched solubility parameters. 

Polymeric hydrocarbon elastomers, such as natural rubber, are cross-linked or vulcanized by the use of sulfur, which reacts with the carbon of the unsaturated bonds in polymer molecules to form a bridge between two molecules so that one polymer molecule is covalently bonded to a second polymer molecule (6). 

Any polymer with unsaturated hydrocarbon groups, either in the main chain or as side groups, can be hydrogenated. Early research on the hydrogenation of elastomers focused on destructive hydrogenation with consequent... 

A weakness of many polymers is their tendency to fail at fairly low stress levels due to the impact of some hostile environments. This phenomenon is known as environmental stress cracking. Cracking occurs when the polymer is stressed for a long time under loads that are relatively small compared to the yield point of the material. A well-known example includes the failure of vulcanized natural rubber in the presence of ozone. It reacts with unsaturated hydrocarbons at the surface and, even when the elastomers are subjected to low stresses, cracks can... 

Because of the unusual reactivity of the DCPD molecule, there are a number of wide and varying end use areas. The primary uses in the U.S. are DCPD-based unsaturated polyester resins (36%) hydrocarbon type resins, based on DCPD alone or with other reactive olefins (39%) EPDM elastomers via a third monomer ethylidenenorhornene or DCPD (16%) and miscellaneous uses (9%), including polychlorinated pesticides, polyhalogenated flame retardants, and polydicyclopentadiene for reaction injection mol ding (39). 

The compositions consist of a heat-plastified mixture of an ethylene homopolymer or copolymer, about 3 to 30 pbw of an elastomer, a stability control agent, which is a partial ester of a long chain fatty acid with a polyol, higher allyl amine, fatty acid amide or olefinically unsaturated carboxylic acid copolymer, and a hydrocarbon blowing agent having from 1 to 6 carbon atoms and a boiling point between -175 and 50C. 

The properties of these elastomers are widely different. All require vulcanization. In general, sulfur is used only for unsaturated polymers, peroxides, quinones, metallic oxides, or cliisocyauates effect vulcanization with saturated types. Many are special-puipose rubbers, some can be used in tires when loaded with carbon black, others have high resistance to attack by heat and hydrocarbon oils and thus are superior to natural rubber for steam hose, gasoline and oil-loading hose. Most are available in latex form. See also Elastomers. 

Proof of the high reactivity of the unsaturated units present in the elastomers we used was obtained by polymerizing vinyl chloride in the presence of different amounts of two hydrocarbons, n-hexane and 2-methyl-2-butene these may be taken as models of the saturated polyhydrocarbon and of the allyl system present in the EPDM terpolymer. AIBN (0.13%) was used as initiator. 

Styrene-based thermoplastic elastomers (see Chapter 4) are sensitive to oxidation since they contain unsaturated soft segments. These elastomers are manufactured by solution polymerization process in aliphatic hydrocarbons. In order to prevent autoxidation during the finishing steps (stripping, drying), which manifests itself by a rise in melt flow index and discoloration of the raw polymer, antioxidant is added to the polymer solution before finishing. Hence the antioxidant has to be soluble in the polymerization solvent. 

Saturated hydrocarbon polymers are also crosslinked by the action of organic peroxides, though branching reduces the efficiency. Polyethylene is crosslinked by dicumyl peroxide at an efficiency of about 1.0, saturated EPR gives an efficiency of about 0.4, while butyl rubber cannot be cured at all. For polyethylene, the reaction scheme is similar to that of the unsaturated elastomers. 

Ethylene-propylene rubber is a synthetic hydrocarbon-based rubber made either from ethylene-propylene diene monomer or ethylene-propylene terpolymer. These monomers are combined in such a manner as to produce an elastomer with a completely saturated backbone and pendant unsaturation for sulfur vulcanization. As a result of this configuration, vulcanizates of EPDM elastomers are extremely resistant to attack by ozone, oxygen, and weather. 

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