Rubber is used as a part of the shaft seals that prevent the pumped media from running out of the pump.
To ensure that the pumped media does not run out of the pumps, shaft seals in rubber (among other materials) are used as sealing materials. Different types of rubber are used, depending upon the media and pressure conditions.
The term rubber includes both natural rubber and synthetic rubber. Rubbers are flexible long-chain polymers that can be stretched easily to several times their unstretched length and which rapidly return to their original dimensions when the applied stress is released. Rubbers are cross-linked (vulcanized), but have a low crosslink density. The cross-link is the key to the elastic, or rubbery, properties of these materials. The elasticity provides resiliency in sealing applications at temperatures up to about 100 °C.