Synthetic polyisoprene rubbers storage

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. 

Synthetic polyisoprene batches exhibit far less viscosity increase on storage due to the slower crystallisation rate of the polymer. This may be illustrated by storing similar batches based on natural rubber and a synthetic polyisoprene and periodically measuring Mooney viscosities (Fig. 6). 

Natural rubber-based adhesives constitute many nsefnl types of adhesives. They have been made from the latex that is collected from the sap of rubber trees grown in Malaysia and other countries of Sonth East Asia. Rnbber of this type was first collected from the Hevea brasiliensis tree - seeds from the tree were first taken from the Amazon forests in Brazil, propagated in Kew in London, and then supplied, over a century ago, to Malaya and adjacent counuies, where there are now many millions of rubber trees, yielding cis-polyisoprene-based natnral rnbber. This has similar properties to the principal SBR synthetic rubber, derived from styrene and butadiene, both of which are obtained by the cracking process from crnde oil, with subsequent chemical reactions. Natural rubber is obtained from the uee by tapping the bark, when the latex flows out spontaneously as the tree is wounded. This latex is about 33% solids - most natural rubber latex is concentrated to 60% and preserved with ammonia for transport and storage. 

Storage of natural rubber for long periods at ambient temperature, or for short periods at freezing temperatures, causes crystallisation of the rubber rendering it solid and unworkable. This necessitates use of hot-rooms where the rubber has to be thawed for 24-48 h at temperatures of 40-50 °C. In our experience it is extremely rare for synthetic c/5-polyisoprenes to crystallise in the bale form and, when it has occurred, thawing takes place in a few hours at 20 °C. Dilatometric measurements at 23°C have shown that the rate of freezing is ten times slower for Natsyn 2200 compared to natural rubber (RSS 1). 

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