Sunday, December 06, 2009

Heat of sorption in wool

Years ago, I got out my “Handbook of Chemical Property Estimation Methods” and calculated the heat of sorption for wool. Then, I looked at B. V. Holcombe’s mathematical models for moisture and heat in merino fibers and decided that heat of sorption was a largely theoretical issue. That is: a very real effect of very little practical importance.
First, heat of sorption for wool only occurs when the wool is fairly dry. When I was first thinking about this, I was thinking about sailors and fishermen. Their wool never got “fairly dry.” They lived in a damp environment, and their wool always contained a very high percentage of the amount of water that it could absorb.
In a damp environment like Ireland/England, unless there is a determined effort to dry the wool, the wool as it comes off of the sheep will have always contain a very high percentage of the amount of water that it could absorb and cannot absorb more water and thereby it will not release heat.
Certainly, storing the wool in a low humidity, centrally heated environment for weeks will dry the wool so that when remoistened, it will release the heat of sorption, but if you take the garment out and wear it every couple of weeks, the wool will retain enough moisture from wearing to wearing that the evolved heat of sorption on any particular wearing is minimal.
What will dry the wool so you (in Yorkshire) can get enough heat of sorption that you can really feel it, is to put your wool in the clothes dryer and run it for a few cycles on high heat. Take the (now ball of felt) out of the dryer, and for the next few minutes you will be able to feel the heat of sorption as the wool fibers pick up moisture from the atmosphere. Or, you can take your wool with you for a summer in the desert of Saudi Arabia or the Australian Outback. Then, pack the wool in sealed plastic bags, and when you open the plastic bags, for a few minutes, you will be able to feel the heat of sorption in all its glory.
Here in sunny (dry) California, we had bit of rain a couple of weeks ago and cool, dry weather since then that has suddenly turned cold. Thus, this morning my office is unheated (45F), cold, and dry (30%). I brought a synthetic fleece garment with me out to my office. In a short while, my cheeks were cold. I have a good gansey, here in the office that has been packed in a cloth bag all through our long, hot summer and which has not been worn since last spring.
I put the synthetic fleece against one cheek and a sleeve from the gansey against the other. The cheek with the synthetic fleece fabric against it was warmer than the cheek with the wool against it. If there had been perceptible heat of sorption from the wool as moisture from my cheek combined with the wool, I would have felt it. However, the wool still had moisture in it from that damp spell a couple of weeks ago and it could not absorb enough additional moisture to produce warmth that exceeded the ability of the synthetic fleece to reflect warmth to my cheek as a result of better insulation. That gansey was not out in rain. It was not worn. It was merely exposed to a couple of days of 70 F and 70% RH a few days ago. That kind of moisture is common in California or Yorkshire. However, the wool absorbed enough moisture that it impeded the detection of a practical heat of sorption effect in wool a few days later.
Synthetic pile has been so successful because of its ability to feel warm against the skin - even in comparison to well knit wool. If wool’s heat of sorption was of practical significance, synthetic pile would not be as successful as it is. Right now, I am wearing a Patagonia synthetic pile jacket because it feels so warm, despite having some good ganseys on the table behind my desk. If I could get a burst of warmth by putting one or two of them on, I would.
On the other hand, wool fleeces in the part of our house that we keep much warmer (and therefore lower RH) for my elderly and frail aunt, do dry enough in a couple of weeks that if one sits on them, the heat of sorption is very perceptible. Sit on the same fleece every few days and it retains enough moisture for several days that the effect is not felt.
It is a real effect, but one must start with very dry wool to detect the effect. It is not an on-going effect. One cannot expect bursts of warmth from your gansey as a series of rain showers blow over in the course of an afternoon.

Someitme in the near future, I will return to the topic.