Vulcanization plastics/crosslinked rubbers

The characteristic property of elastomers is their rubber-elastic behavior. Their softening temperature lies below room temperature. In the unvulcanized state, i.e. without crosslinking of the molecular chains, elastomers are plastic and thermo-formable, but in the vulcanized state—within a certain temperature range — they deform elastically. Vulcanization converts natural rubber into the elastic state. A large number of synthetic rubber types and elastomers are known and available on the market. They have a number of specially improved properties over crude rubber, some of them having substantially improved elasticity, heat, low-temperature, weathering and oxidation resistance, wear resistance, resistance to different chemicals, oils etc. 

When the crosslinked rubber level in the blend is high, the blends are rubbery with good elastic recovery properties, have low compression set values at elevated temperatures, and are processable as plastics. At low crosslinked rubber levels (<25%), these blends are tough and have very good impact resistance. Dynamic vulcanization was first practiced by Gessler [23] around 1958, when he described the preparation ofhalogenated butyl mbber and polypropylene on a heated roll mill. Most of the blends in the examples had only a small amount of rubber (20 wt%) and the curative used was a metal oxide. The mixing time was about 20 min at 320 ° F. It was shown that the dynamically vulcanized blends had better tensile properties than identical blends vulcanized at 320 °F under static conditions. Fischer [24] prepared... 

Blends of Plastics and Crosslinked Rubbers by Dynamic Vulcanization... 

The length of the sulfide crosslinks formed during vulcanization will affect rubber properties. Mono- and disulfide crosslinks are more stable (less prone to scission) than polysulfide links and so promote better thermal and aging characteristics. Polysulfide links, on the other hand, provide somewhat better molecular flexibility. This can result in better dynamic fatigue resistance. Physical properties aside, the more stable crosslinks are often preferred to provide reversion resistance. Reversion is the cleavage of sulfide crosslinks during vulcanization which results from extending the cure beyond the time required to obtain the desired optimized balance of vulcanizate properties. When overcured in this way, certain elastomers, particularly natural rubber, will revert to the soft, more plastic, less elastic condition characteristic of the uncured compound. 

In another case where the twin-screw extruder was used, the rubber and plastic were melt mixed with all ingredients in a similar manner as described in blend compositions for static vulcanizations. The product was then dumped, cooled, and granulated. The premixed granules were then fed into a twin-screw extruder where a very narrow temperature profile was maintained with a relative high compression (2 1), and the screw speed was adjusted depending on the final torque and the flow behavior of the extruded stock. The stock was cured by shear force and temperature enforced by the twin-screw extruder. The dynamically crosslinked blend was taken out in the form of a strip or solid rod to determine the... 

Filler-free formulations of two-component silicone rubbers crosslinkable by condensation or addition are utilized, with or without added solvent, for the coating of papers and plastic foil, optionally as an aqueous emulsion, e.g. for strippable paper for self-adhesive labels or packaging foil for bitumen. Vulcanization is carried out at high temperature (100 to 180°C), to attain as short hardening times as possible. 

A growing volume of waste materials, especially vulcanized rubbers and crosslinked polymers are proving difficult to recycle. As an alternative to their disposal in landfills, there have been many attempts to grind these materials and use the products as a substitute for fillers in composite materials. Other non-plastic materials such as glass, paper, natural fibrous materials, and fly ash are also used for filler replacement. There is extensive literature on the use of ground tires as filler replacements. This is a specialized topic with only a minor relationship to fillers. 

Dynamic vulcanization (DV) is also considered. DV is the crosslinking of one polymer in a blend of polymers during its mixing, all polymers of the blend being in the molten state. The process is used in the preparation of thermoplastic elastomers from rubber-plastic blends. 

Rubbers and resins normally vulcanize/crosslink in heated molds at 150-220 °C. The cycle times are normally much longer than for uncrosslinked plastics [7, 63]. 

Polymers of lower n-alkyl acrylates are used in plastics to only a limited extent. Ethyl and butyl acrylates are, however, major components of acrylic elastomers. The polymers are usually formed by free-radical emulsion polymerization. Because acrylate esters are sensitive to hydrolysis under basic conditions, the polymerizations are usually conducted at neutral or acidic pH. The acrylic rubbers, like other elastomers, must be crosslinked or vulcanized to obtain optimum properties. Crosslinking can be accomplished by reactions with peroxides through abstractions of tertiary hydrogens by free radicals ... 

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