Most rubber
lined valve is vulcanized, a process which involves heating with a small quantity of sulphur (or equivalent cross-linking agent so as to stabilise the polymer chains, over a wide range of temperature. This discovery was made by Charles Goodyear in the 1844, but is a process restricted to polymer chains having a double-bond in the backbone. Such materials include natural rubber and polybutadiene. The range of materials available is much wider however, since all polymers become elastomeric above their glass transition temperature. However, the elastomeric state is unstable because chains can slip past one another resulting in creep or stress relaxation under static or dynamic load conditions. Chemical cross links add the stability to the network that is needed for most practical applications.