W. L. Gore stretched PTFE fast to make a membrane both porous and stronger

PTFE was prized for chemical inertness and water-repellency, but the patent's own background concedes that in porous applications — one very large area of demand — the polymer had little presence. Making it porous the obvious way wrecked it: holes mean weakness. In 1973 W. L. Gore & Associates filed for a patent on exactly that gap.

The invention is startlingly simple to state. Take paste-formed, dried, unsintered PTFE and stretch it under preferential temperature and rate conditions. The claims run from about 30% per second up to about 40,000% per second. Stretched this fast, the polymer forms a microstructure of nodes interconnected by very small fibrils, with porosity from about 40% to 97% — and, as the patent puts it, a tremendous increase in strength, often greater than tenfold, despite the added voids. Heating above roughly 345–350°C raises the amorphous content and locks the structure in place.

The result is a membrane whose tiny connected channels pass vapor easily — far more permeable than conventional PTFE film — while staying waterproof. By 1979 the Gore-Tex laminate was reviewed as a waterproof-breathable fabric in its own right in Sen'i Gakkaishi, written from the Gore-Tex division of Junkosha, Gore's Japanese affiliate. Because the patent claimed shaped articles generally — films, tubes, rods, continuous filaments — one mechanism could seed many product lines instead of one.

Stretch it fast: pores and strength arrive together

Rate was the missing parameter. The strength gain appears only within preferential temperature and stretch-rate conditions, so a claim range from 30% to 40,000% per second is the invention itself.

The node-and-fibril structure creates tiny connected channels: permeability to gases is far higher than conventional PTFE film, while the material still repels water.

Heat-treating above about 345°C raises amorphous content and locks the structure, so the strength gain survives being turned into products.

Porosity and strength stopped trading off: up to about 97% porosity alongside a tenfold strength gain collapses the usual porous-material dilemma.

When a material resists a process, question the process's tempo. Rate can be the decisive parameter: push the same operation hard enough and two outcomes that normally trade off can arrive together.

The patent, granted in April 1976, anchored the ePTFE materials family and the Gore-Tex waterproof-breathable laminate. Japanese technical literature reviewed the laminate as an established fabric by 1979. The fast-stretch principle kept defining the field: a 2006 Journal of Materials Science study used co-stretching to shrink ePTFE pores for fine filtration and still cited Gore's 1970s patents as the founding references. One process — films, tubes, rods, filaments, laminations — carried products from waterproof clothing to sealing material.