Closed Cell Rubbers
The technically correct term for closed cell sponge rubber is expanded rubber.
The cells are like individual balloons, holding gas and not
allowing moisture to pass through as long as the cell wall has not
burst.
The raw compound is mixed on the same type of equipment used to mix
solid rubber compounds, but a chemical blowing agent is also
added.
Under the heat and pressure of a pre-cure or pre-forming process,
this additive decomposes and mixes with the rubber compound and
generates nitrogen gas.
As this reaction is occurring, the mixed batch is placed into an
oven and allowed to expand into a bun from 2" to 4" thick. The bun
of closed cell sponge rubber is then aged and split into
sheets.