Synthetic polyisoprene rubbers comparison with natural rubber

Among the variety of synthetic rubbers, EPM and EPDM are particularly known for their excellent ozone resistance in comparison with natural rubber (cfs-l,4-polyisoprene) and its synthetic counterparts IR (isoprene rubber), SBR (styrene-butadiene rubber), and BR (butadiene rubber). Secondly, EPDM rubber can be extended with fillers and plasticizers to a very high level in comparison with the other elastomers mentioned, and still give good processability and properties in end articles. This leads to an attractive price/performance ratio for these poljmiers. 

Thermodynamic Analysis. As reported previously, the storage modulus G of PDMS networks with tetrafunctional crosslinks is independent of frequency between 10 3 and 1 Hz (21). This behaviour which is entirely different from that of vulcanized natural rubber or synthetic polyisoprene networks, was attributed to the lack of entanglements, both trapped and untrapped, in these PDMS networks. Figure 4 shows that G of a network with comb-like crosslinks is also frequency independent within an error of 0.5%. For comparison, two curves for PDMS having tetrafunctional crosslinks are also shown. The flat curves imply that slower relaxations are highly unlikely. Hence a thermodynamic analysis of the G data below 1 Hz can be made as they equal equilibrium moduli. 

Rheological properties and mixing behaviour of natural rubber In order to study the effect of rheological properties of elastomers on their behaviour in the internal mixer, some experiments were performed using various natural rubber kindly supplied by MRPRA (x). Natural rubber rheology has not been deeply studied despite the commercial importance of this material, and only a few recent papers deal with the mixing of natural rubber (19, 20, 21) and rheological comparison between natural and synthetic cis-1,4 polyisoprenes (22). 

Partially isomerized natural rubber has been of some interest as a non-crystallizing material (Cunneen and Higgins, 1%3) with better rubberiness at low temperatures than usually experienced with natural rubber (comparisons being made in the vulcanized state). Commercially, however, the material has not been able to compete with oil-extended or plasticized rubber blends, in particular those containing polybutadiene or with synthetic polyisoprenes. 

In contrast, the physical properties of synthetic high cw-polyisoprene compare very favourably with those of high quality grades of natural rubber. Such a comparison of SMR CV and four commercially available polyisoprenes is shown in Table 8. In tests carried out at ambient... 

However, the excellent cold properties of the lithium polymer can be explained on the basis of microstructure in Table II. It seems reasonable to assume that of the three possible microstructures the 1,2 structure is the least desirable for low temperature flexibility followed by the frans-1,4 structure, with the cis-1,4 structure the most desirable. A comparison of the low temperature flexibility of balata (or gutta-percha) vs. Hevea rubber would indicate a preference for the cis-1,4 structure over the trans-1,4 structure, although these natural products are polyisoprenes rather than polybutadienes. In the case of the 1,2 structure, it is generally assumed that the prevalence of this structure in sodium-catalyzed polybutadiene, or butadiene copolymers, accounts for its poor cold properties however, the occurrence of a natural or synthetic product with an entirely 1,2 structure would help to confirm this more definitely. The relative predominance of any single structure is another important consideration in the performance of a rubber at low temperatures because a polymer with a large percentage of one structure would be more likely to crystallize at a low temperature. 

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