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The Science of Wool

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Heat of Adsorption

Wool hates water, but loves mist. The outermost layer (epicuticle) of a woolen fiber is made of scales that do not absorb water at all, and the overlapping arrangement of the scales cause them to shed water like the shingles of a roof. However, water vapor can slip through the extremely small spaces between the scales.
Wool adsorbs water.  Once inside the woolen fiber there is a temporary chemical bond attaching water molecules to the inner structures of the fiber.  All adsorbtion is exothermic, meaning that is generates heat. When wool releases the water molecule, desorption, it uses up heat.  That is how wool can be cooling as well as heating!

With WeatherWool, we have done everything we can (tried to stay out of the way!) to maximize this phenomenon, using fine (thin) but strong fiber in a Jacquard weave, to increase surface area of the fibers, thereby enabling more adsorption. And we avoid the use of liner fabrics in our garments, which are pure wool. We also do not use superwash fiber (except, currently, for our Neck Gaiter), because superwashing detracts from wool's ability to adsorb water. 

Regarding how wool handles water in its various forms, I haven't done lab tests, and I don't claim to understand the research I've read. I've just been out in the weather and know something is going on! Here are some of the published explanations for why wool handles water so well:

  • Typical wool can adsorb 30% of its weight in water without feeling wet because the water is adsorbed (trapped inside the fibers).
  • And again (sorry) the outside of a wool fiber sheds water ... so even when wool is literally soaking wet, it will quickly dry off on the outside, and feel dry to the touch, because all the water is held inside the fiber.
  • Wool is also a hygroscopic material.  That  is, when wool is completely dry it will adsorb water from the air until it comes to an equilibrium with the surrounding air.  This process of ‘equilibration’ with the surrounding air is called ‘conditioning’.
  • Inside the wool fiber, water vapor binds to the cortex, producing the heat of adsorption.  Again, all adsorption is exothermic ...heat generating.
  • The body burns a lot of heat to turn liquid water into vapor ... using up excess heat is the reason for perspiration. When perspiration vapor is adsorbed within a wool fiber, and then condenses, it slowly releases the heat of condensation ... the heat the body expended to create the vapor.
  • Water molecules are polar, having both a positive and negative side. The poles of the water molecules react with the bonds that exist within the wool fiber. These reactions create new bonds and release heat in the process.
These are some reasons why wool is famously warm even when wet. Probably there are others. This behavior is radically different from synthetics, and from cotton, which absorbs water like a sponge. 

In 1858, French Scientist Coulier was the first to observe that when dry wool was moved to a moist room – when it adsorbed water -- it produced heat. Water vapor binds to the chemical structure of wool fiber and small amounts of energy are released in the form of heat. This is known as the "heat of sorption". Experiments have shown that human subjects can perceive the heat of sorption of water vapor by wool garments in typical winter conditions, particularly if the woolens are thoroughly dried before use. Some claim a kilogram of merino wool can release as much heat over 8 hours as an electric blanket. This is just one of the reasons wool keeps you warm even when it is wet. So woolens are the rare exception to the clothing rule - they generate heat.

The process of internal adsorption is very gradual, and is relatively impervious to liquid water. This is important in cold weather, where heat and energy must be conserved. Physical exertion, even on a cold day, causes the body to produce warm water vapor.  Wool will adsorb the warm vapor, trapping the moisture and the heat inside the fibers. Wool does this even on a cold, rainy day!   It's remarkable to be working in the rain and just feel warm and comfortable inside your wool. Work hard in rain gear and you just get soaked from the inside, even in the clothing that claims to breathe.
This is all separate from the insulation properties of wool created by trapping air in the crimp of the fibers.

Water-Resistant Properties
The water-resistant properties of wool can be life-saving. If you fall in a freezing river and reach dry land within minutes, your wool will be wet on the outside, like your hair, but will not have taken on any water internally. Because the wool cuticle is hydrophobic, your wool will dry quickly and you will be relatively unaffected, assuming your base layers and socks are also wool. The behavior of cotton is the opposite. Typical cotton garments will immediately soak up as much water as they can hold, and wet cotton fiber is wet on the inside and the outside, so it pulls immense amounts of heat from your body. Cotton Kills. There is no cotton whatsoever in WeatherWool garments.

Click here to see me jump in a river in winter, and then simply carry on, comfortably, and enjoy the day ... no problemo. 

Pure Wool
One important factor separating WeatherWool from inferior woolen garments is that WeatherWool Fabric is pure wool. Most woolen garments contain significant amounts of cotton. A garment label stating “100% Virgin Wool” means that the wool in the garment has not been recycled, but it does not mean that the fabric is pure wool. WeatherWool Fabric is always pure wool because any other material will degrade performance. We use no fillers or lower grade fibers in our yarn or weaving.

Heat Conduction
When you touch wool on a cold day, it feels warm because wool is a poor conductor of heat. That is, wool does not absorb heat from your body. There are two reasons for this. First, when you touch wool, you don't touch very much of it. The structure of wool is not flat … it is very curly and kinky and on a microscopic level, its surface is very rough. So when skin comes in contact with woolen fabric, it simply does not touch as much of the wool as other fabrics. Secondly, the cellular, chemical and biophysical structures of woolen fiber are all fine insulators.


Why We Get Cold!
Our skin radiates heat into the air. That warming air, which is lighter than cold air, rises up and away from the body. This convection increases with wind. Even in mild temperatures hypothermia can set in.

Heat loss is also exacerbated by metals and water, which conduct heat quickly. Moisture greatly quickens our loss of heat. Wet skin loses heat up to 25X faster than dry skin. In the cold, avoid getting sweaty or wearing jewelry. Sit on a fallen tree rather than a stone or cold ground. And rest your feet on a large branch or thick moss if possible.

Heat is also lost when cold air is inhaled. Your nasal passages heat the air going into your lungs. Breathe through your nose, not your mouth.

The body prioritizes where to maintain essential heat. Blood flow to arms and legs is reduced in order to maintain heat around vital organs and the head. The head receives about 25% of human blood flow. A head without a hat funnels body heat away. If your feet are cold -- put on a hat!

Dress in Layers
Start with a base layer next to the skin. Base layers are designed to wick moisture away from the skin. Cool and cold-weather base layers -- we recommend only wool -- will also insulate and capture the heat contained in moisture released from the body.

Mid-layers add extra insulation of air. In cold weather with low activity level, a thick layer is needed.  Mid-layers should also act in concert with moisture (sweat) transpiration. If your mid-layer creates a barrier between your wool base layer and wool outer layer, the moisture is trapped,  This is why we use no liners. Wearers of WeatherWool have observed how moist air is released to the outside, where it then freezes!

Top layers help insulate, regulate heat and moisture loss, and provide overall protection from elements. Outer layers also should handle abrasion, resist flame, odors, stains, dirt and electric-arc, and be absolutely silent. The ideal outer layer will also feel luxurious, provide concealment in nature and be admired in social settings. WeatherWool can handle a huge variety of weather conditions, but we do recommend a storm shield for extreme wind and rain conditions.

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27 March 2020