Phase change materials (PCMs) store and release heat as they change between solid and liquid states. PCMs were researched by NASA in the 1970s-1980s for thermal regulation applications. Some PCMs like lithium chloride are used to manage electronics temperatures, while paraffinic hydrocarbons and plastic crystals have been incorporated into textile fibers and fabrics. When microencapsulated PCMs in fabrics melt upon heating or freeze upon cooling, they absorb or release latent heat to buffer the wearer's temperature changes.

1. Phase Changing
Material

2. PCM - Definition
Substances that undergo the process of
phase change are also known as phase
change materials (PCMs).
These materials store, release or absorb
heat as they oscillate between solid and
liquid form.
Giving off heat as they change to a solid
state and absorbing it as they return to a
liquid state.

3. PCM - Development
IN 1970-80 NASA’s Space Research included considerable research and
development on new materials that could protect delicate instruments in
space from the temperature extremes they encountered.
NASA used PCMs on the Lunar Rover and Skylab projects in the
A NASA study identified more than 100 PCMs. Some - including pure
lithium chloride - are being used for heat management of
electronics, telecommunications and microprocessor equipment.
Lithium chloride, however, is not suitable for normal textile applications.

4. PCM - Development
In 1987, (Triangle R&D), demonstrated the feasibility of incorporating PCMs
within textile fibers, and were issued a patent in 1994.
Materials used in this work included
Paraffinic hydrocarbons - such as eicosane; and
Plastic crystals - such as 2,2-dimethyl-1,3-propanediol (DMP) and
2-hydroxymethyl-2-methyl-1,3-propanediol (DMP).
When plastic crystals absorb thermal energy, the molecular structure is
temporarily modified without changing the phase of the material.
The thermal properties can be optimized by the choice of the paraffinic
hydrocarbon.
Compounds with a higher number of carbon atoms have higher temperature
stability.
The patent issued to Triangle R&D on the inclusion of PCM microcapsules
within a fiber covers a broad range of fibers including polyester, nylon,
acrylics and modacrylics.

5. PCM - Development
The incorporation of PCMs within a fiber requires first that the PCM be
microencapsulated.
In order to produce a microencapsulated PCM, some of the following criteria
had to be met
• particle size;
• uniformity of particle size;
• stability to mechanical action and chemicals; and
• core-to-shell ratio, with PCM content as high as possible.
In the mid- to late 1980s, work on temperature-adaptable fabrics was
undertaken at the US Department of Agriculture's Southern Regional
Research Center (SRRC) in New Orleans.
The work was based on the durable binding of polyethylene glycol to
cotton and cotton/polyester blends using poly-functional crosslinking
agents and acid catalysts.
The resultant products were designated "Polytherm" fabrics.
The action of the polyethylene glycol polymer complex as similar to a helix
that coils and uncoils and acts like a thermal spring.

6. PCM - Development
Frisby Technologies Inc., The company's business was based on sales of
its thermal management products Thermasorb® and ComforTemp®.
Frisby developed and patented gel-coated microcapsule technology for
the preparation and application of thermal management materials.
SteeleVest® cooling vests.
These vests, worn by workers facing extreme heating conditions in
industrial and military environments, can provide up to four hours of
cooling.
Thermocules® is the name for its PCM material that is applied as a matrix
coating on fabric. This type of coating allows the fabric to maintain air
permeability.
• lifestyle apparel - smart fleece vests, men's and women's hats, gloves
and rainwear;
• outdoor sports apparel - jackets and jacket linings, boots, golf shoes,
running shoes, socks, and ski and snowboard gloves;
• bedding - mattresses, mattress pads, pillows and comforters; and
• automotive seating.

7. PCM - Working Concept
When the encapsulated PCM is heated to the melting point, it
absorbs heat energy as it moves from a solid state to a liquid state.
This phase change produces a temporary cooling effect in the
clothing layers (Figure 1).
The heat energy may come from the body (e.g. when the wearer first
dons the garment) or from a warm environment.
Once the PCM has completely melted, the storage of heat stops.

8. PCM - Working Concept
If the PCM garment is worn in a cold environment where the
temperature is below the PCM's freezing point and the fabric
temperature drops below the transition temperature, the
microencapsulated liquid PCM will change back to a solid state.
This generating heat energy and a temporary warming effect (Figure 2).
The developers claim that this heat exchange produces a buffering
effect in clothing, minimizing changes in skin temperature and
prolonging the thermal comfort of the wearer.

9. PCM - Working Concept
The clothing layer(s) containing PCMs must go through the
transition temperature range before the PCMs will change phase
and either generate or absorb heat.
Consequently, the wearer has to do something to cause the
temperature of the PCM fabric to change.
PCMs are a transient phenomenon.
They have no effect under steady-state thermal conditions.