Machine Design – May 2019

STEPHEN MRAZ| Senior Editor

ePTFE

Keeps Athletic Shoes

Dry and Comfortable

Pores in ePTFE membrane can be small enough to keep water molecules out but

large enough to let air in and out, letting the fabric “breathe.”

I

n 1958, former DuPont research-

er Bill Gore started up a company

that would develop new ways to

use polytetrafluoroethylene, or

PTFE. He came up with hundreds, may-

be thousands.

In 1960, the son, Bob Gore discov-

ered how to transform PTFE into a

strong, porous, adjustable membrane

by expanding it into ePTFE. PTFE and

ePTFE are chemically identical, but the

expanded version can be made with

pores of various sizes, and it is much less

dense, so its mechanical properties are

different.

ePTFE really became popular after

W.L. Gore & Assoc. took advantage of

the pores in the expanded polymer. Engi-

neers could make them too small to let

water in but large enough to let air and

water vapor out, making it the first fabric

that is truly breathable. But in one indus-

try sector the breathable polymer made a

big splash: athletic gear, especially shoes.

EXPANDING PTFE

ePTFE is among the more innovative

variations in processing and using PTFE.

Turing PTFE into ePTFE doesn’t require

soluble fillers, foaming agents, or chemi-

cal additives—just a little mechanical

manipulation and some heat treatments.

The expansion process begins with

an extrusion of a paste consisting of fine

PTFE powder and typical lubricants or

mineral spirits. Lubricant are removed

by a quick heat treatment. The lubri-

cant-free extrusion, which can be in the

shape of a rod, tube, or tape, next gets

expansion.

In this process, unsintered PTFE

extrusions are first heated to anywhere

from 35° to 320°C while held and

restrained in a device that can stretch

extrusions at rates varying from 10% per

second to 40,000% per second.

The stretched part is then heated to

above 330°C, usually to between 350°C

and 370°C, while held in a device that

prevents the stretched part from shrink-

ing for anywhere from a few seconds

to a few hours. After this heat treat-

ment, called “amorphous locking,” the

expanded part cools and is removed.

Higher stretching rate yields ePTFE

with more uniform pores, and opera-

tors can control pore size and number,

or density. Pores can range in size from

1 to 6 μm. Porosity can range from 90%

to 95%. Sintered PTFE parts have den-

sities of about 2.15 g/cm^3 ; unsintered,

unexpanded parts have densities of 1.5

g/cm^3 , but they can be as low as less than

Materials/Recreation

32 MAY 2019 MACHINE DESIGN