It becomes more and more common: rain or sportswear that is waterproof and breathable at the same time. Porous materials that allow water vapour (perspiration) to pass while rejecting liquid (rain) water form the basis. The secret lies in the combination of the open structure and the water repellence of tissues. In fact, any tissue can ‘breathe’ via the open structures in between the yarns or the fibres of which the yarn is composed. These pores and the rate at which yarns absorb water determine the degree of protection against rain. By treating the yarns with a water repellent compound – for example based on fluorocarbons like Teflon – a water drop can not penetrate the tissue. The surface tension of water keeps the drop together, preventing penetration. Water vapour – without surface tension – will still be able to pass.
In this application Teflon is best known under the trademark GoreTex®. In 1969, Bob Gore discovered a way to process polytetrafluoroethylene (PTFE) into a thin, porous layer. He did this by stretching the material in such a way that pores were created, about 1400 million tiny holes per square centimeter. Generally, this waterproof, but vapour-permeable layer known as ‘expanded PTFE’ is applied between the lining and the outer fabric. Sometimes the Teflon layer is provided with a dirt-repelling layer to prevent oil and other fatty substances from blocking the pores.