Phase Change Materials

Thermal energy storage is the major application of phase change materials (PCMs). As a PCM undergoes a liquid/solid phase transition at a constant temperature, they store/release heat as the latent heat of fusion. Parrafin wax and polyethylene glycol (PEG) are PCMs with this capability. They can be incorporated into clothing fabric to maintain a comfortable temperature in cold weather. For example, while a person is physically active in the cold outdoors, the body is giving off enough heat to melt the PCM and thermal energy is stored in the PCM. When the person is at rest, the body is not expelling as much heat. Thus, the PCM freezes and releases the stored thermal energy, providing warmth to the person.

Parrafin and PEG are polymer materials. A narrow temperature range of phase transition is controlled by their molecular weight. These PCMs operate in a temperature range suitable for the described application, 25-40°C. A way to incorporate parrifin wax into fabric is to first encapsulate it into capusules of diameter less then 100 microns. Then these microcapsules are placed into fibers that can be woven into fabric. Advantages to parrafin are high heat of fusion and commercial availability at reasonable cost. PEG with a molecular weight of 1000 gives the desired temperature range. An advantage to PEG is that a permanent press finishing technique utilized in the fabric industry can be employed to bind PEG to fabric. Clothing with PEG can endure numerous washing cycles, and even though PEG is in direct contact with the skin, it does not cause irriation.

Evaluation of using Phase Change Materials
Thermasorb is the tradename of a phase change material (PCM) created and distributed by Frisby Technologies. The PCM is made of parrafin wax. Different types of Thermasorb are classified by the temperature at which they undergo phase transformation, which is determined by the molecular weight of the paraffin polymer.
Properties of Thermasorb 95:
Completely transforms from solid to liquid at 35°C (95°F)
Latent heat capacity = 180 J/g
Contained within microcapsules of diameter range 15-40 micrometers
density = 0.90 g/cc

Suppose we add three grams of Thermasorb 95 into a glove:
This must add 3g/(0.90g/cc) = 3.3cc volume to the glove.
When the PCM completely changes phase from solid to liquid at 35 C, it releases 3g*180J/g = 540J of latent heat. The supply of this amount of latent heat is distributed over time. For comparision, this same amount of heat would be supplied by our 1 Watt battery and heating element design in 9 minutes.

The properties of other types of Thermasorb can be found at http://www.thermasorb.com/technical.html.

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