These rings are similar in design to the piston rings in automobile engines. These gaskets are composed of interlocked plies of preformed corrugated metal and asbestos strips, called a filler. The filler may or may not be encased in a solid metal outer ring. The term rubber and synthetic rubbers, covers many natural each of which can be compounded into numerous varieties. The characteristics of these varieties have a wide range, as shown in table 7-1.
Natural rubbers have many of the charac-teristics required in an effective seal.
However, their very poor resistance to petroleum fluids and rapid aging when exposed to oxygen or ozone limit their use. Today their use has almost ceased. There are two general classes of synthetic rubber seals. One class is made entirely of a certain synthetic rubber. The term homogeneous, which means having uniform structure or composition throughout, is frequently used to describe this class of seal.