Along with an intense interest in chemistry, Gore was also an avid outdoorsman. He was well aware of the waterproof textile technology, or lack thereof. Once his company had stable sales in the thread joint / sealant department, he turned his attention toward manufacturing a textile that was both waterproof and breathable, and would soon revolutionize the world of technical outerwear.
In 1976, Gore sold his first rain jackets with an ePTFE laminate to the public. He had produced a true waterproof jacket that was also extremely breathable. His customers were ecstatic. Finally, they had a jacket that would keep them dry without soaking them in their own warmth-stealing sweat. Victory!
Well, almost. After multiple uses, customers began to report leaks in the fabric. To understand what was happening, we’re going to need a little science lesson:
Gore’s membrane was an extremely thin layer of ePTFE laminated to a nylon face fabric. The ePTFE was hydrophobic, meaning that liquid water was repelled by the surface of the membrane. It also contained millions of tiny pores, which allowed water vapor to easily escape. Genius.
Chemistry aside #1, Feel free to skip: The fluorine shell of the PTFE polymer is neutrally charged. It experiences minimal Van der Waals forces due to the close proximity of each neighboring fluorine molecule and contains a net neutral dipole due to its symmetry. This means the PTFE polymer has a very low surface energy as opposed to the characteristically high surface energy of water (due to enormous intramolecular forces). In order for the ePTFE membrane to “wet out,” a liquid with a comparable surface energy must be used. Water, having an extremely high surface energy, cannot “wet out” a layer of ePTFE under normal pressures, whereas an alcohol, with low surface energy, could. In short, water will not soak through the membrane because of its high surface energy (commonly called surface tension).
The only way water can get through this membrane is under extremely high pressures. In such situations, water can form small enough droplets to fit through the pores of the membrane. Normal pressures from wind-driven rain don’t come close to the pressures required for the membrane to leak, so what was going on with Gore’s first jackets?
The answer is simple in the membrane world. It’s called “fouling.” The ePTFE membrane was laminated to the nylon face layer without any sort of protection. Over time, the pores in the membrane got clogged with hydrophilic “junk.” This was most likely caused by ions from the wearer’s sweat. Unfortunately, sweat and other similar “junk” particles can have strong intermolecular interactions with the water, greatly raising the surface energy of the ePTFE. Put more simply, this creates a bridge for water to flow through the membrane.
Clearly, this was a big problem. But that badass Bob Gore quickly found a solution.
Chemistry aside #2, Skip this only if you have a plane to catch: Gore solved his fouling problem by laminating his membrane with a protective layer of polyurethane (PU). Wait, what? Isn’t PU that non-breathable coating that was used on the first waterproof nylon fabrics? Yes. But Gore was able to use an extremely thin PU layer because the ePTFE formed a great lattice (structure) for the PU to adhere.
Ok, but who cares how thin it is, doesn’t this defeat the purpose of the ePTFE membrane?
Yes and No.
Chemistry aside #3, Mandatory Reading: PU membranes are monolithic, meaning that they have no holes. So how can water get through a solid layer? The PU membrane is hydrophilic, meaning that water is attracted to its surface. Luckily, it just so happens that if a PU layer is thin enough, water is able to diffuse through the membrane via solid-state diffusion. Individual water vapor molecules adsorb, or chemically bond, to the surface, and pass directly through the membrane to the opposite side. There, they can then desorb, or evaporate, through the ePTFE membrane.
The caveat to this process is the driving force, and how the directionality of this phenomenon is controlled. If water vapor can diffuse out, what is keeping water vapor from diffusing in? The answer is that this process requires a strong driving force. In this case, the driving force is a concentration gradient, meaning that in order for water vapor to diffuse out of the PU membrane, the concentration of water vapor must much be much higher on one side than the other (see illustration below). The greater the concentration gradient across the membrane, the greater the driving force and the greater the mass transfer of water vapor across the membrane.
Now, let’s address the concern of the PU layer defeating the purpose of the ePTFE layer. In one sense, it does: the PU layer serves a bottleneck for water vapor escape rendering the high breathability of the ePTFE membrane useless. But in other ways, it doesn’t. Without the ePTFE membrane present, it would be extremely difficult (almost impossible) to create a thin enough PU layer that could breathe effectively at all.
To this day, GORE-TEX uses this same technology (albeit in an extremely refined form) for their products. Since then, however, many other technologies have sprouted up that achieve similar results. Let’s take a look.