Natural rubber also properties

Thermoplastic Elastomers Thermoplastic Elastomers are a very flexible type of thermoplastic that has the properties of natural rubber. Also called TPE. 

Non-reinforcing fillers, for a given increase in vulcanizate stiffness, generally give better permanent set, creep and dynamic properties than reinforcing types. The basic properties of natural rubber also show up to advantage in the fabrication of components. 

Based on infrared evidence, ester groups in natural rubber were initially reported to be due to the presence of lactone functional groups. " Subsequently, C-NMR studies on deproteinized natural rubber showed that the ester groups were due to the presence of fatty acids bonded to the rubber molecule, of which 80% was saturated fatty acids and 20% unsaturated fatty acids. The fatty acids were later identified to be the acyl component of phosphoHpids hnked to the a-terminal end of the rubber molecule. The phospholipids were also postulated to be the branching point of the natural rubber molecule. The effects of these groups on natural rubber s properties have been discussed above. 

Natural rubber also undergoes reaction with atmospheric oxygen. This interaction is a major cause of deterioration in physical properties of the rubber on aging and has been extensively investigated. In this book it is possible to give only a brief account of these investigations into an extremely complex process more detailed reviews may be found elsewhere [2,4,6]. 

Rubber Natural rubber, also called elastomer, provides the industry worldwide with certain thermoset material properties that to date are not equaled by synthetic elastomers. Examples include tires (with their relative heat buildup resistance), certain type vibrators, etc. However both synthetic TSE and TPE have made major inroads to product markets previously held by natural rubber and also expanded into new markets. The three basic processing types are conventional (vulcanizable) elastomer, reactive type, and thermoplastic elastomer. More synthetic types are used than the natural worldwide. 

The coordinate catalysts enable the s5mthesis on an industrial scale of isoprene rubbers, which in terms of composition, structure and physicochemical properties are similar to natural rubber. Also with these catalysts one can obtain the cw-butadiene rubbers of a regular structure, known for valuable utilization performances. 

This lower has a number of ramifications on the properties of polybutadiene. For example, at room temperature polybutadiene compounds generally have a higher resilience than similar natural rubber compounds. In turn this means that the polybutadiene rubbers have a lower heat build-up and this is important in tyre applications. On the other hand, these rubbers have poor tear resistance, poor tack and poor tensile strength. For this reason, the polybutadiene rubbers are seldom used on their own but more commonly in conjunction with other materials. For example, they are blended with natural rubber in the manufacture of truck tyres and, widely, with SBR in the manufacture of passenger car tyres. The rubbers are also widely used in the manufacture of high-impact polystyrene. 

Compared with the natural material, raw SBR is more uniform in a variety of ways. Not only is it more uniform in quality so that compounds are more consistent in both processing and product properties but it is also more uniform in the sense that it usually contains fewer undesired contaminants. In addition, over a period of years it has been generally less subject to large price variations. These differences in uniformity have, however, tended to lessen with the advent of improved grades of natural rubber such as Standard Malaysian Rubber which have appeared in recent years. 

Acrylonitrile-butadiene rubber (also called nitrile or nitrile butadiene rubber) was commercially available in 1936 under the name Buna-N. It was obtained by emulsion polymerization of acrylonitrile and butadiene. During World War II, NBR was used to replace natural rubber. After World War II, NBR was still used due to its excellent properties, such as high oil and plasticizer resistance, excellent heat resistance, good adhesion to metallic substrates, and good compatibility with several compounding ingredients. 

Standard-grade PSAs are usually made from styrene-butadiene rubber (SBR), natural rubber, or blends thereof in solution. In addition to rubbers, polyacrylates, polymethylacrylates, polyfvinyl ethers), polychloroprene, and polyisobutenes are often components of the system ([198], pp. 25-39). These are often modified with phenolic resins, or resins based on rosin esters, coumarones, or hydrocarbons. Phenolic resins improve temperature resistance, solvent resistance, and cohesive strength of PSA ([196], pp. 276-278). Antioxidants and tackifiers are also essential components. Sometimes the tackifier will be a lower molecular weight component of the high polymer system. The phenolic resins may be standard resoles, alkyl phenolics, or terpene-phenolic systems ([198], pp. 25-39 and 80-81). Pressure-sensitive dispersions are normally comprised of special acrylic ester copolymers with resin modifiers. The high polymer base used determines adhesive and cohesive properties of the PSA. 

Anorin-38 has also shown an interesting effect as a multifunctional additive (a single additive to replace many of the conventional additives) for natural rubber (NR). It showed excellent blending behavior and compatibility with NR. Aorin-38 enhances the tensile properties and percent elongation, decreases fatigue, acts as an antioxidant and antiozonant, and positively affects many of the other properties, apart from acting as a process aid and a cure enhancer [183-186]. 

TPEs from blends of rubber and plastics constitute an important category of TPEs. These can be prepared either by the melt mixing of plastics and rubbers in an internal mixer or by solvent casting from a suitable solvent. The commonly used plastics and rubbers include polypropylene (PP), polyethylene (PE), polystyrene (PS), nylon, ethylene propylene diene monomer rubber (EPDM), natural rubber (NR), butyl rubber, nitrile rubber, etc. TPEs from blends of rubbers and plastics have certain typical advantages over the other TPEs. In this case, the required properties can easily be achieved by the proper selection of rubbers and plastics and by the proper change in their ratios. The overall performance of the resultant TPEs can be improved by changing the phase structure and crystallinity of plastics and also by the proper incorporation of suitable fillers, crosslinkers, and interfacial agents. 

Elastomers, of which vulcanized natural rubber is the most important example, also undergo dramatic changes in mechanical properties when filled with particulate solids. In part, knowledge of this particular type of system has been developed empirically as the technology of car-tyre manufacture has advanced. 

They have studied the properties of NR-epoxidized natural rubber (ENR) blend nanocomposites also. Vulcanization kinetics of natural mbber-based nanocomposite was also smdied. The effect of different nanoclays on the properties of NR-based nanocomposite was studied. The tensile properties of different nanocomposites are shown in Figure 2.7 [33]. 

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. 

A convenient term for any material possessing the properties of a rubber but produced from other than natural sources. A synthetic version of natural rubber has been available for many years with the same chemical formula, i.e., cis-1,4-polyisoprene, but it has not displaced the natural form. See also Butyl Rubber, Chloroprene Rubber, Ethylene-Propylene Rubber, Nitrile Rubber, Silicone Rubber and Styrene-Butadiene Rubber. 

When the USA and Germany were cut off from the supplies of natural rubber during the Second World War both countries sought to produce a synthetic alternative SBR was the result, and at one stage it was the most commonly used synthetic rubber. It can be produced by both emulsion and solution polymerisation techniques, with the emulsion grades being the most widely used. Emulsion polymerisation yields a random copolymer, but the temperature of the polymerisation reaction also controls the resultant properties obtained. Cold polymerisation yields polymers with superior properties to the hot polymerised types. 

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. 

Chain flexibility also effects the ability of a polymer to crystallize. Excessive flexibility in a polymer chain as in polysiloxanes and natural rubber leads to an inability of the chains to pack. The chain conformations required for packing cannot be maintained because of the high flexibility of the chains. The flexibility in the cases of the polysiloxanes and natural rubber is due to the bulky Si—O and rxv-olelin groups, respectively. Such polymers remain as almost completely amorphous materials, which, however, show the important property of elastic behavior. 

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. 

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. 

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