The following search on ASK.COM - "exothermic wool" provides this link,
www.izwool.com/qualities-of-wool-2.htm
With the following information. (there are diagrams also but I'm afraid they do not copy/paste well).
Wool's Hygroscopic Ability
Endothermic and exothermic reactions of wool.
The wool fibre is a complex fibre. The outer structure is made up of several cuticle layers, these layers form scales that are impermeable to liquids, permeable for vapour.
A wool fibre is always in a dynamic equilibrium within the surrounding environment, constantly absorbing and desorbing moisture vapour as a process to maintain a set equilibrium. The equilibrium is set to create a "least energy state". This is a physical molecular make up that controls a chemical equilibrium between polar sites with in the wool fibre and moisture molecules or vapour in the surrounding environment. The "Least energy state" is set by nature/ the sheep to assist maintaining homeostasis.
Polar sites are distributed throughout the wool fibre and are responsible for chemically attracting moisture molecules and expelling them according to the surrounding air environment and do so to maintain the chemical equilibrium. Energy in the form of heat is released and taken up again as this process occurs. The process of absorbing and desorbing moisture vapour is known as "hygroscopic".
PLUS.....
When a water molecule is attracted to a polar site the molecule is slowed down and releases energy in the form of heat, an exothermic reaction and known as "Heat of sorption". This process is not linear, fig 1, the drier the wool fibre the more moisture it can attract and the more heat can be given off. As the internal polar regions become filled, the less heat is given off.. This feature is useful for the build up of heat during warm up exercise and the affect rapidly decreases as the fibre fills with moisture vapour
If the surrounding environment is becoming drier the polar regions will release the water molecules, they will take up energy and be released back out into the environment, an endothermic reaction. It will only release enough vapour to try and maintain the equilibrium. Important for warm down as the fibre cools with the body and not any more than necessary which can cause chills.
The wool fibre can absorb up to 35% of its weight in moisture vapour internally. The wool fibre usually maintains approximately 14 - 18% moisture depending on the surrounding environment.
In most cases our surrounding environment is constantly changing, hence so is the wool fibre in trying to maintain its chemical equilibrium. Essentially the wool fibre is never stable unless in a controlled atmosphere and so is in an "dynamic equilibrium", a constant state of change.
Wool has a natural ability to hold moisture vapour within and between the fibres whereas synthetics will only hold liquid either within OR between the fibres. A synthetic product that is designed to hold moisture within the fibre is acting in the same manner as a sponge and as such has no ability to release the liquid unless it is physically forced to do so.
END