SealSkinz - Waterproof and Breathable: Why Both?
Elemental protective outerwear needs to be both waterproof AND breathable. A totally waterpoof but non-breathable garment would be very uncomfortable to wear, as perspiration (as water vapour) would have no means of escaping, thus condensing on the inside of the garment, leaving the wearer 'wet from within' and feeling cold.
Conversely, a very breathable but non-waterproof garment would be so open, allowing air and perspiration (as water vapour) to pass freely through, that it would offer little protection from the elements (e.g. canvas tents let water through if you lean against them!)
Types of Waterproofing ...
There are two commercial ways to make a fabric resist water:
Coatings
Some garments are made water-resistant by coating the outer surface with a layer of hydrophobic ('water hating') chemical or wax. When water droplets fall onto the fabric they 'bead' due to the surface tension of the water and roll off. However, coatings are not always durable and can be damaged during repeated washing and flexing (wear).
Laminates
A more durable method is to use a waterproof breathable film or membrane, which is laminated ('glued') to a layer of fabric inside the garment. SealSkinz socks and gloves incorporate a Porelle membrane which is laminated between the inner and outer layers. The Porelle membrane is both waterproof AND breathable and will never wash away.
Waterproof Membranes
Introduction
There are two types of polyurethane film used to make waterproof, breathable products:
1. Microporous
Microporous films are full of microscopic holes or pores. These pores are approximately 5 microns in diameter whereas water droplets are typically 500-5000 microns and 'drizzle' droplets can be as small as 200 microns in diameter and are therefore too large to penetrate the pores in the film. It is impossible for water droplets to pass through and therefore the film is 100% waterproof.
Molecule size* |
Pore size |
Droplet size |
|
Size (microns) |
0.0003 |
5 |
200+ |
Ratio |
1 |
10,000 |
1,000,000 |
Water vapour molecules generated by the body perspiring are much smaller (typically less than 0.0003 microns) and pass easily through the holes, carrying moisture away.
*A WV molecule is a million times smaller than a typical water droplet.
2. Hydrophilic
SealSkinz socks and gloves use a different type of membrane called hydrophilic ('water loving'). Hydrophilic membranes are totally solid, without pores. It is therefore physically impossible for droplets of liquid water to pass through the membrane, however it is breathable because water vapour molecules are transmitted from molecule to molecule through the membrane on the polymer chain and expelled on the other side. The driving forces are the differences in humidity and temperature between the atmospheres on each side of the film.
The internal climate, next to the skin, is hot and humid because the body is generating heat and moisture as it exercises. The external climate is much drier and cooler. Consequently, water vapour is driven from the inside of the garment to the outside. As the foot gets hotter during an activity, so the film becomes more efficient at expelling moisture.
SealSkinz 100% Waterproof Testing
Every single SealSkinz product is tested before it leaves the factory. The sock or glove is mounted onto a machine which subjects it to water pressure. If any product leaks, the item is rejected and diced. Every SealSkinz product is therefore guaranteed to be 100% waterproof.
The waterproof film can withstand 10 psi of hydrostatic head pressure, the equivalent of a column of water 700 cm high standing on the fabric surface. This is 7 times the level required to meet European Standards for waterproof clothing.
Breathability Index
Breathability is measured by a water vapour permeability index in accordance with BS 3424: Part 34: 1992 Method 37. In this test, pieces of laminated fabric cover a small cup holding a measured mass of water.
The cup is allowed to stand in a controlled climate for several hours. At the end of the test, the cup is re-weighed to determine how much moisture from within the cup has evaporated and been expelled through the sample fabric.
A water vapour permeability (WVP) rating is then determined by comparison to a standard fabric given the same treatment. The SealSkinz Mid light, Long light, Mid thermal Socks and the SealSkinz Glove all have a water vapour permeability index of 50-60% when measured in accordance with BS 3424: Part 34: 1992 Method 37.