Bellows in rubber expansion joints can be made of different materials, suitable for the medium flowing through. These can be, for example, CSM rubber (Hypalon) or FPM rubber (Viton). Both of them are resistant to a wide range of chemicals and different types of petroleum products. However, rubber expansion joints with additional, PTFE (Teflon) bellow lining have the highest chemical resistance to very aggressive compounds and substances. This lining is 1 to 3 mm thick and covers the entire inner surface of the rubber expansion joint in such a way that the flowing medium does not come into contact with the rubber of the bellow, which has lower chemical resistance. The rubber expansion joints lined with PTFE have excellent resistance to almost all types of chemically aggressive substances. Apart from the lining, the expansion joint can also be equipped with internal vacuum rings made of PTFE, which allow operation in negative pressure conditions.