Vulcanized rubber, property requirements

Despite of 150-year s history of vulcanization process, it is impossible to consider that fundamental and applied researches in direction of vulcanization systems perfection are completed. For today one of the ways of rubbers properties improvement is the synthesis and application of the new chemicals-additives, including, vulcanization active, that is connected, first of all, with reduction of global stocks of zinc ores as basic raw material for reception of traditional activator - zinc oxide. Besides, modem increase of industrial potential and the accumulation of big quantity wastes derivate the problems of ecological character, which require the emergency decision. Therefore creation of resourcesaving technologies of the new compounds reception from products of secondary raw material processing has paramount importance. 

In summaiy, theie aie a lange of vulcanizing systems which can be used for natural rubber, and the choice is dependent on the combination of properties required. No single one offers ideal, all-around properties combined with good heat resistance. The end user has to be selective, according to the properties required foi the final application. Certain properties such as oil resistance and gas permeability have been omitted from Table 3, because in legaid to these properties natural mbbei is substantially inferior to synthetic mbbers such as acrylonitrile rubber and halobutyl rubber (see Elastomers,... 

Property Requirements of Vulcanized Rubber 2.6.1 Stress-strain Properties... 

Molar mass strongly influences the performance of raw (unvulcanized) rubbers during the preparation of rubber compounds, e.g. addition of fillers and other ingredients. Also the processing characteristics of the compounded rubbers as well as the physical properties of the vulcanized rubbers significantly depend on the molar mass of the unvulcanized rubbers. In order to better meet various requirements, there is not only one BR grade available but... 

A critical requirement for obtaining engineering properties from a rubbery material is its existence in a network structure. Charles Goodyear s discovery of vulcanization changed natural rubber from a material that became sticky when hot and brittle when cold into a material that could be used over a wide range of conditions. Basically, he had found a way to chemically connect the individual polymer chains into a three-dimensional network. Chains that previously could flow past one another under stress now had only limited extensibility, which allowed for the support of considerable stress and retraction upon release of the stress. The terms vulcanization, rubber cure, and cross-linking all refer to the same general phenomenon. 

The most extensively studied block copolymers prepared by anionic polymerization are the styrene-butadiene or styrene-isoprene rubbers. Shell Chemical Company s Kraton thermoplastic elastomers are ABA block copolymers of this type. Their elastomeric properties are excellent, yet they differ from other rubbers in that vulcanization is not required. These elastomers consist of a rubbery polybutadiene matrix with the styrene segments serving as anchors in thermoplastic microdomains. 

Styrenic TPEs have strength properties equal to vulcanized rubber, but they do not require vulcanization. Properties are determined by polymer type and formulation. There is a wide latitude in compounding to meet a wide variety of application properties. According to application-driven formulations, Kratons are compounded with a hardness range from Shore A 28 to 95 (Shore A 95 is approximately equal to Shore D 40), sp gr from 0.90 to 1.18, tensile strengths from 150 to 5000 Ib/in (1.03 to 34.4 MPa), and flexibility down to 112°F (-80 C) (see Table 3.1).2... 

Elastomeric adherends. Vulcanized-rubber parts are often contaminated with mold release and plasticizers or extenders that can migrate to the surface. Solvent washing and abrading are common treatments for most elastomers, but chemical treatment is required for maximum properties. Many synthetic and natural rubbers require cyclizing with concentrated sulfiiric add until hairline fractxires are evident on the surface. 

Cross-link density has only a minor effect on some compound properties such as thermal conductivity, electrical properties, and low temperature brittleness. Another cure system consideration is the compoimd scorch behavior. Prior to vulcanization, rubber is plastic-like and can be processed into desired shapes such as tires, hoses, belts, or other articles. The time available to accomplish this processing depends largely on the cure system and is referred to as the scorch time. If a compound cures prematurely during the processing step, it is usually scraped. Therefore, a key requirement of the vulcanization step is to minimize premature vulcanization or scorch (Fig. 7). 

When compared to synthetic rubbers, NR requires lower curing temperatures, and therefore, longer vulcanization time. It has also poorer abrasion resistance and ageing properties, and cracks easily in tread grooves and tyre side walls. 

The proportion of vulcanized phase which should be used in a formulation is governed by the processing properties required. Often a proportion of 20 parts of vulcanized phase to 80 parts of unvulcanized rubber is adequate. This can be achieved either by complete replacement of the unvulcanized rubber component in the mix with an SP20... 

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. 

TPE s are more economical to produce than traditional thermoset materials because fewer steps are required to manufacture them than to manufacture and vulcanize thermoset rubber. An important property of these polymers is that they are recyclable. 

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