Machine Design – May 2019

ePTFE and Athletic Shoes

0.1 g/cm^3 , with a porosity of 96%. Den-
sity and porosity are linearly related. If
pore size is quite small, less than 1 μm,
parts can have porosities of up to 90%
porosity; larger size pores (1-6 μm) con-
tribute to driving porosity above 95%.
The heat-treated ePTFE web has a
higher permeability than PTFE to gases

and liquids due to its many pores. It can

also be a semi-permeable membrane by

letting the wetting liquids through and

turning away non-wetting fluids. For

example, a gas-saturated membrane in

contact with gas and water will let gas

through and keep water out as long as

the water pressure is below the water

entry pressure. When porosity climbs

above 9%, air permeability soars while

water-entry pressure decreases to a fair-

ly small value. There is a range where

porosity can be selected to balance the

air permeability and water imperme-

ability, which is useful to applications

such as clothing and shoes.

KEEPING FEET DRY

Bob Gore used ePTFE to make fabrics

and fabric assemblies for companies that

make athletic shoes and clothing. For

shoes, ePTFE, which they trademarked

as Gore-Tex, was packaged as booties

that shoe companies would put between

the exterior fabric and an interior liner.

The exterior fabric is usually not water-

proof, but the Gore-Tex membrane was

and would keep the wearer’s foot and lin-

er dry. By staying relatively dry, the liner

lasts longer and is more comfortable.

Gore-Tex could make socks out of

ePTFE, and they would keep feet dry.

Bob Gore discovered how to expand PTFE and create an adjustable membrane that was
waterproof but would let air go through it, the first truly breathable fabric.

34

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MAY 2019 MACHINE DESIGN