Understanding Technical fabrics

So, what are we really talking about when we use the phrase ‘technical fabrics’? Basically, we are looking at fabric technologies with a very specific end use in mind that balance the three key areas of waterproofness (read general weatherproof-ness), breathability and durability.

I’ll use the well-known GORETEX products as an example here, as they really are a market leader and Gore are continually developing new, ground breaking fabric technologies to meet the needs of their Brand partners.

Breaking it down W L Gore and Associates use two main forms of construction in their garments, Two and Three Layer (although there are other specific constructions in their massive portfolio). Two Layer construction essentially means that their ePTFE (expanded Polytetrafluoroethylene) membrane is attached directly to an outer face fabric with a loose lining; Three Layer construction means that the face fabric, membrane and inner lining fabric are all bonded together. Many prefer the Three Layer construction, as it’s easier to get on and off, as a loose mesh liner can sometimes snag on intermediate layers or other gear.

Different seam tapes (to ensure seams are 100% waterproof and do not let water in through stitch holes) are used in the different constructions; Two Layer seam sealing tape is applied to the GORE-TEX membrane directly, whilst in Three Layer, the sealing takes place over the inner lining fabric. Seam sealing is an art form in itself!

Keeping the weather at bay

Let’s take a look first at waterproofness and how technical fabrics achieve this. The GORE-TEX membrane is the heart of all their excellent products. It contains over 9 billion microscopic pores per square inch. These pores are 20,000 times smaller than a water droplet, but 700 times larger than a water vapor molecule, which makes the membrane durably waterproof, while allowing perspiration (moisture as a vapour) to escape from the inside. An oleophobic, or oil-hating, substance is integrated into the membrane, preventing the penetration of body oils, insect repellents and the like.

Simply put, imagine an elephant, a mouse hole and an ant! The mouse hole is the microscopic pore in the membrane, the ant is a water vapour molecule and the elephant a water droplet; it’s pretty easy to see that although the ant can escape through the mouse hole with ease, there is no way that you could push the elephant through it without breaking the wall down! In garment terms, that would mean tearing a ruddy great hole in your jacket!

Testing times

So, water cannot penetrate the membrane, or can it? Logic dictates that eventually any fabric technology can be pushed to a point of failure and this can be tested; in this case the applicable British Standard is ‘BS EN 343:2003 Protective Clothing. Protection against Rain’. The Standard describes the tests that should be carried out to ascertain a garment’s level of waterproofness and sets out the performance standards that items carrying this description should meet. A Test House would carry out these tests in order to ascertain the garment’s ability to keep out water. This is measured in PSI (pounds per square inch) or Hydrostatic Head. Hydrostatic Head is measured in mm and pressure is measured in pounds per square inch. The Hydrostatic Head is determined by a test whereby a fabric is held taut underneath a sealed tube of water one-inch -in diameter. Over 24-hours it is observed to see how many millimetres of water the fabric can withstand before it leaks through. When the fabric begins to seep water, the ‘height’ in millimetres is noted. This is known as the fabric’s ‘hydrostatic head’, which literally means: Hydro - Water, Static - Not moving, Head - Height.

The PSI relates to the amount of water pressure that a garment can withstand, both from the pressure within the garment (the wearer), and the external conditions (the weather).

To be 100% waterproof, the British Standards of 3 PSI has to be met, which means that a fabric can withhold 1500mm of water pressure. Pressure is not just as simple as the pressure of a rain storm; simply moving a garment when it is worn will exert pressure on the fabric.

Let’s put this in perspective though; if you were to sit on a wet, flat surface, the mean pressure you generate is around 10,000mm!  If you were kneeling in a firing position, then the point of the knee generates   15-17,000mm   of pressure (smaller surface area, greater pressure) so let’s face it, a minimum legal rating of 1500mm just does not do the job. It does begin to explain though how a garment can be legally classed as ‘waterproof’, but in reality it really isn’t.

Back to GORE-TEX; what’s the hydrostatic head I hear you ask? Simple one this, as all GORE-TEX products have a MINIMUM testing level of 28,000mm…  that’s right, 28-metres! Put another way I’ve seen top end GORE-TEX fabrics tested at over 70 PSI and not leak until they quite literally explode! Belt and braces time here folks.
Keeping you comfortable

Comfort is a relative thing,  but if you’re too hot, that can lead to heat exhaustion; too cold, hypothermia. Controlling the micro-climate inside your clothing system helps to prevent either. The body regulates itself effectively; as activity levels increase you sweat and the evaporation of this moisture helps to cool the skin. If moisture is not allowed to move away, you will become ‘uncomfortable’ inside your clothing system. We’ll get properly into the layering system in a future article.

Letting moisture escape from the system is a tricky balancing act. Behind it all is a law of nature, the Law of Partial Pressures. This law states that moisture as a vapour will naturally move from a warm, high pressure environment to a cooler low pressure one. If we are keeping a well-balanced temperature inside our clothing system, then on a foul weather day moisture will move away from our skin through the pores in our clothing and escape to the outside. Sometimes we call this the ‘Temperature Gradient’ as it sounds like moisture vapour is moving ‘down’ from hot to cold.

So, does a breathable fabric technology work in a rain forest? Yes, is the answer, but due to the temperature inside the garment and outside the garment being more in equilibrium, the transfer rate will obviously slow down. We all know we can work with Ma Nature, but never beat her!

This, of course, is also subject to testing; many methods can be used here to establish a Moisture Vapour Transmission Rate (MVTR) or a Resistance to Evaporative Transmission (RET) figure. Of the two, I prefer an RET figure, as it just plain makes more sense to me.

Skin is 100% breathable and has an RET of Zero. As soon as you cover it, though, you build up a resistance that slows down the rate at which moisture can be transmitted away; the thicker the barrier the more resistance and the slower the transmission. In essence, this means that as you cover the skin with layers your system becomes less breathable, so we will always be looking for technical fabrics with as low an RET figure as possible.

In some cases, the clever folks at Gore have got the RET figure well down into single figures, especially on their bespoke technologies and the GORE-TEX 3-Layer garments. They’ve developed a completely new and innovative lining fabric too called a Micro Grid Backer, which is thinner, but more durable, than the industry standard linings available elsewhere. Thinner, less resistance, right?

Getting the balance right!

The third and final part of the precarious balancing act is putting this membrane technology together in a garment with a lining fabric and an outer face fabric that will ensure long term durability, whilst not degrading performance. These fabrics are chosen with extreme care and tested for all kinds of heat, cold, flex and abrasion. The tests are carried out both by Gore themselves and also independently.

So, with our GORE-TEX fabrics in place we have exceptionally durable garments, which have at least a 28+ metre hydrostatic head and which can sit right at the very top of the of the ‘Extremely Breathable’ category.

The fact of the matter is that many people find GORETEX garments expensive but at the risk of being blunt they must have no real conception of what is going on behind the scenes! When you’re paying for a GORE-TEX garment you’re getting ‘Formula 1’ levels of performance and that doesn’t come cheap.

Many other companies use very similar membranes these days, so Gore don’t have the monopoly any longer and several examples are included here.