Polychloroprene crystallisation

The Cluster Model Application for the Description of the Process and Properties of Polychloroprene Crystallisation... 

Since it is not possible to commercially produce a polymer that is based on the cis 1,4 form, commercial polymers are based on the Irons 1,4 form which has a crystalline melting point, Tm, of +75 °C and a Tg of -45 °C. Pure 1,4 trans polychloroprene thus crystallises readily and would normally be considered to be of limited use for a rubber. Such a polymer, however, does not crystallise when dissolved in a solvent, but will do so when the solvent evaporates. This feature is used to good effect in the production of contact adhesives. 

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. 

The mercaptan modified polychloroprenes are vastly superior to the sulphur modified (G) types in set resistance although Neoprene GW has partly closed the gap. For good set resistance at low temperatures, the crystallisation resistant copolymers must be used, with compounding appropriate to low temperature resistance. 

---