Natural rubber physical properties

Since most silicone adhesives and sealants are elastomeric in nature, their physical property testing often parallels classical rubber testing approaches. Common tests include durometer, tensile strength, elongation, and modulus. Several methods are available for the measurement of rubber properties, but the most commonly used are the American Society for Testing and Materials (ASTM) D-412, Test Method for Rubber Properties in Tension, and the ASTM C-661, Standard Test for Indentation Hardness of Elastomeric-Type Sealants by Means of a Durometer. These properties vary widely with the product... 

Liquid, vapours and gases can pass through materials by different processes, depending on the nature or physical property of the materials. In the case of porous materials, such as rock or wood, the gas or liquid flows through the holes within the materials. On the other hand, liquids, vapours and gases pass through non-porous materials such as rubber via permeation processes of absorption and diffusion, as shown in Figure 27.1. 

Siace most fabricated elastomer products contain 10—50 vol % of filler, their physical properties and processing characteristics depend to a great extent on the nature and quaUty of the fillers. Rubber technologists manipulate the formula so as to optimize a large number of properties and keep costs down. 

Table 5. Physical Properties of Black-Filled Epoxidized Natural Rubber ...

Oil resistance demands polar (non-hydrocarbon) polymers, particularly in the hard phase. If the soft phase is non-polar but the haid phase polar, then swelling but not dissolution will occur (rather akin to that occurring with vulcanised natural rubber or SBR). If, however, the hard phase is not resistant to a particular solvent or oil, then the useful physical properties of a thermoplastic elastomer will be lost. As with all plastics and rubbers, the chemical resistant will depend on the chemical groups present, as discussed in Section 5.4. 

Natural rubber Solid Good physical properties and resistance to cutting and abrasion. Low heat and ozone resistance. Gaskets. 

Chemical pretreatments with amines, silanes, or addition of dispersants improve physical disaggregation of CNTs and help in better dispersion of the same in rubber matrices. Natural rubber (NR), ethylene-propylene-diene-methylene rubber, butyl rubber, EVA, etc. have been used as the rubber matrices so far. The resultant nanocomposites exhibit superiority in mechanical, thermal, flame retardancy, and processibility. George et al. [26] studied the effect of functionalized and unfunctionalized MWNT on various properties of high vinyl acetate (50 wt%) containing EVA-MWNT composites. Figure 4.5 displays the TEM image of functionalized nanombe-reinforced EVA nanocomposite. 

Akhtar, S. Morphology and Physical Properties of Thin Films of Thermoplastic Elastomers from Blends of Natural Ruhher and Polyethylene, Rubber Chem. Technol. 61, 599-583, 1988. 

Various other chemical agents which by their nature are capable of producing cross-linkages between polymer chains effect the same changes in physical properties that are observed in sulfur vulcanization. One of the best known of these agents is sulfur monochloride, which readily combines with two molecules of an olefin (the mustard gas reaction). Applied to rubber, it induces vulcanization even at moderate temperatures, the probable structure of the cross-linkage being... 

Since the excellent work of Moore and Watson (6, who cross-linked natural rubber with t-butylperoxide, most workers have assumed that physical cross-links contribute to the equilibrium elastic properties of cross-linked elastomers. This idea seems to be fully confirmed in work by Graessley and co-workers who used the Langley method on radiation cross-linked polybutadiene (.7) and ethylene-propylene copolymer (8) to study trapped entanglements. Two-network results on 1,2-polybutadiene (9.10) also indicate that the equilibrium elastic contribution from chain entangling at high degrees of cross-linking is quantitatively equal to the pseudoequilibrium rubber plateau modulus (1 1.) of the uncross-linked polymer. 

Oils of the three types are offered in a range of viscosities and this will influence their processing character to some extent, although there is little evidence that it will have much influence on the ultimate compound physical properties, at least in natural rubber compounds. The small additions of oil to a compound help with filler dispersion by lubricating the polymer molecular chains and thus increasing their mobility. There will also be some wetting out of the filler particles which enables them to achieve earlier compatibility with the rubber and improve their distribution and dispersion speed. 

R.P. Brown and T. Butler, Natural Ageing of Rubber - Changes in Physical Properties Over 40 Years, Rapra Technology Limited, Shawbury, UK, 2000. 

Hermann Staudinger, on developing a new and simple preparation of the monomer, studied the polymerization of isoprene as early as 1910 (42). Stimulated by the differences in physical properties between his synthetic rubber and natural rubber, he turned his full attention to the study of polymers. 

Vulcanisation of rubber Natural rubber becomes soft at high temperature (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity, it Is soluble in non-polar solvents and Is non-resistant to attack by oxidising agents. To improve upon these physical properties, a process of vulcanisation is carried out. This process consists of heating a mixture of raw rubber with sulphur and an appropriate additive at a temperature range between 373 K to 415 K. On vulcanisation, sulphur forms cross links at the reactive sites of double bonds and thus the rubber gets stiffened. 

Most polystyrene products are not homopolystyrene since the latter is relatively brittle with low impact and solvent resistance (Secs. 3-14b, 6-la). Various combinations of copolymerization and blending are used to improve the properties of polystyrene [Moore, 1989]. Copolymerization of styrene with 1,3-butadiene imparts sufficient flexibility to yield elastomeric products [styrene-1,3-butadiene rubbers (SBR)]. Most SBR rubbers (trade names Buna, GR-S, Philprene) are about 25% styrene-75% 1,3-butadiene copolymer produced by emulsion polymerization some are produced by anionic polymerization. About 2 billion pounds per year are produced in the United States. SBR is similar to natural rubber in tensile strength, has somewhat better ozone resistance and weatherability but has poorer resilience and greater heat buildup. SBR can be blended with oil (referred to as oil-extended SBR) to lower raw material costs without excessive loss of physical properties. SBR is also blended with other polymers to combine properties. The major use for SBR is in tires. Other uses include belting, hose, molded and extruded goods, flooring, shoe soles, coated fabrics, and electrical insulation. 

Influence of Interpolymer Properties. As stated earlier, the physical and chemical properties of interpolymers markedly influence the reaction rate after the induction period. If the monomer present yields a polymer comparable in viscosity with the initial mixture the rate of scission will not accelebrate. For example, the polymerization rate of chloroprene on mastication with natural rubber does not increase as markedly with conversion (69), see Fig. 19, as with methyl methacrylate and styrene. The reason is the chloroprene-rubber system remained elastic and softer than the original rubber. 

Types of Latex Compounds. For comparison with dry-rubber compounds, some examples of various latex compounds and the physical properties of their vulcanizates are given in Table 23. Recipes of natural rubber latex compounds, including one without antioxidant, and data on tensile strength and elongation of sheets made from those, both before and after accelerated aging, are also listed. The effects of curing ingredients, accelerator, and antioxidant are also listed. Table 24 also includes similar data for an SBR latex compound. A phenolic antioxidant was used in all cases. 

Obviously, there are many subtle differences in the structure, morphology, or network topology between radiation cured and sulfur cured elastomers, but their physical properties may be nearly equal, provided that precautions are taken to avoid the occurrence of chain scissions. A comparison of radiation cross-linked and sulfur cured natural rubber (gum and carbon-black-reinforced compounds) is in Table 5.4. ... 

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