Natural rubber characteristics

Cure Characteristics. Methods of natural rubber production and raw material properties vary from factory to factory and area to area. Consequentiy, the cure characteristics of natural mbber can vary, even within a particular grade. Factors such as maturation, method and pH of coagulation, preservatives, dry mbber content and viscosity-stabilizing agents, eg, hydroxylamine-neutral sulfate, influence the cure characteristics of natural mbber. Therefore the consistency of cure for different grades of mbber is determined from compounds mixed to the ACSl formulation (27). The ACSl formulation is as follows natural mbber, 100 stearic acid, 0.5 zinc oxide, 6.0 sulfur, 3.5 and 2-mercaptobenzothiazole (MBT), 0.5. 

The ability to bond natural rubber to itself and to steel makes it ideal for lining tanks. Many of the synthetic elastomers, while more chemically resistant than natural rubber, have veiy poor bonding characteristics and hence are not well suited for hning tanks. 

Current production of NR is about 5.2 X 10 tonnes. For some years it has enjoyed a premium price over SBR because of its desirable characteristics described above and, compared with other large tonnage polymers, a somewhat restricted supply. Clearly it is difficult to substantially increase the production of such a material in a short period of time and indeed the attractions of other crops such as palm oil as well as the desire to move away from a monoculture economy mitigate against this. The indications are that, unless there is undue intervention of political factors, the future of natural rubber as a major elastomer remains secure. 

Polychloroprene rubber possesses the characteristics of natural rubber and has the advantage of higher polarity. 

One effect of this strain-induced crystallisation is that there is a characteristic upswing in the plot of stress against strain for natural rubbers, as illustrated in Figure 7.11. 

Figure 7.11 Stress relaxation curve for vulcanized natural rubber showing characteristic upswing at higher stresses...

Vulcanised butyl rubber is very similar to vulcanised natural rubber in various physical characteristics but has better resistance to oxidation and has low permeability to gases. Hence, it is widely used in tubes for cycles, scooters, motor cars, etc. It is also used as rubber in many other applications. 

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. 

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent. Solution polymerization, used at approximately 13 plants, is a newer, less conventional process than emulsion polymerization for the commercial production of crumb mbber. Polymerization generally proceeds by ionic mechanisms. This system permits the use of stereospecific catalysts of the Ziegler-Natta or alkyl lithium types which make it possible to polymerize monomers into a cis structure characteristic that is very similar to that of natural rubber. This cis structure yields a rubbery product, as opposed to a trans stmcture which produces a rigid product similar to plastics. 

ISO 1658 Natural Rubber - Test Recipes and Evaluation of Vulcanisation Characteristics, International Oiganization for Standardization, Geneva,... 

This property depends on filler particle size and filler loading. Higher filler loading gives lower resilience. Rubber type plays a great role here in that no synthetic rubber can match the characteristic of high resilience of natural rubber. 

If solid polymer objects are fluorinated or polymer particles much larger than 100 mesh are used, only surface conversion to fluorocarbon results. Penetration of fluorine and conversion of the hydrocarbon to fluorocarbon to depths of at least 0.1 mm is a result routinely obtained and this assures nearly complete conversion of finely powdered polymers. These fluorocarbon coatings appear to have a number of potentially useful applications ranging from increasing the thermal stability of the surface and increasing the resistance of polymer surfaces to solvents and corrosive chemicals, to improving friction and wear properties of polymer surfaces. It is also possible to fluorinate polymers and polymer surfaces partially to produce a number of unusual surface effects. The fluorination process can be used for the fluorination of natural rubber and other elastomeric surfaces to improve frictional characteristics and increase resistance to chemical attack. 

B 4. Bateman, L., R. W. Glazebrook, C. G. Moore, R. W. Saville Structural characteristics of the sulfur linkage in natural-rubber vulcanizates. Proc. 3rd Rubber Technol. Conf. London 298 (1954, pnbl. 1956). 

Many workers have studied and published correlations between various types of plastimeter, often to show that they do not agree and to illustrate the superiority of the instrument which supposedly agrees best with processing behaviour. Several comparisons are included in the literature already noted and other examples are shown in Figure 6.6. Figure 6.6a shows the relatively close correlation obtained between two compression instruments, the Wallace rapid and Williams plastimeters, for materials of similar flow characteristics (plasticised natural rubber). When such rubbers are compared on two basically different instruments (compression and extrusion) the... 

ISO 1658, 1994. Natural Rubber. Test recipes and evaluation of vulcanisation characteristics. 

In addition, some rubbers, for example natural rubber and polychloroprene, stiffen at low temperatures by partial crystallisation. This is a gradual process continuing over many days or weeks and is most rapid at a particular temperature characteristic of each polymer, for example -25°C for natural rubber. Hence, tests intended to measure the effect of crystallisation must detect changes in stiffness or recovery after periods of ageing at a low temperature. 

Butyl Rubber. A synthetic rubber produced by copolymerization of isobutene(98%) with a small proportion(ca 2%) of isoprene or butadiene. Polymerization is conducted at-50 to 100° in a liquid hydrocarbon, with A1C13 as catalyst. Its outstanding property compared with other rubbers is impermeability to gases. The uncured rubber is tacky, but it may be compounded like natural rubber and vulcanized. Butyl rubber has good resistance to chemical attack and to aging even at high temps. It has superior vibration insulation characteristics and abrasion resistance, but relatively low tensile strength and poor flame resistance... 

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