US 3,953,566 teaches a method to make a water proof breathable membrane based expandable, amorphous locked Poly tetrafluoroethylene (ePTFE).
ePTFE has the following microstructure (crystalline structures interconnected by amorphous fibrils. Due to the high porosity and hydrophobic properties, it allows vapor to pass through and blocks liquid water until the pressure is larger than the blocking surface tension (Mechanism behind the” water proof and breathable”). The capability is related to the porous sizes.
Schematic manufacturing process.
