Consumers now occasionally see the terms “phase-change materials” or “PCM s” in descriptions of clothing or bedding. Each season, more manufacturers use PCMs in their products, some even making them a selling point. Even a casual reading tells us that phase-change materials have something to do with comfortable temperatures. More exactly, PCMs are used to keep temperatures within a desired range. But what are phase-change materials, and how do they work?
According to Wikipedia, “A phase-change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy.” All materials absorb heat when they melt or vaporize and release heat when they condense or freeze, but not all materials qualify as PCMs. A phase-change material absorbs and/or releases large amount of energy in the process, remaining at the same temperature until the process is completed.
As a solid substance absorbs heat energy, it becomes warmer, eventually becoming hot enough to melt. Wax is a good example. We have all seen light wax becoming soft on a hot day. Only a little warmer and it reaches the melting point. As the wax melts, it absorbs heat from its surroundings. As it solidifies, it releases heat. When the air becomes cooler (or the source of heat is removed), the wax cools by releasing heat. Cool enough, it becomes solid again.
The most well known and longest used phase-change material is ice. A container of water will not fall below 32˚F (0˚C) until all the water is frozen, and that ice will not rise above 32˚F (0˚C) until completely melted. Ice is more efficient for keeping things cold than water is for keeping them from getting too cold; it takes 144 times more energy to melt 16 oz. of ice than to raise the temperature of 16 oz. of water by 1˚F. 
Large bodies of water have a moderating effect on nearby land, and ice is perfectly fine for keeping food and drinks cold, but it is too heavy, bulky and messy for controlling temperatures in mattresses and clothing. Lighter substances which are easier to handle are needed for this, as well as materials that can release enough heat at the lower end of the preferred temperature range.
In the 1970s and 1980s NASA wanted to protect instruments in spacecraft from extreme temperatures without using large amounts of energy to operate heating/cooling systems. The most feasible means was to use materials which would absorb or release large amounts of heat when needed, and phase-change materials were the ideal solution. PCMs were used on the SkyLab and space probes.
After this, NASA sought to apply this technology to spacesuits to protect astronauts on extravehicular excursions. Engineers at Triangle Research & Development in Triangle Research Park, NC, developed means of incorporating PCMs in textile fibers. Using this under license, Outlast Technologies is marketing fibers and fabrics to manufacturers and users of textiles, including bedding producers.
Phase-change materials used in textiles are generally in the form of microencapsulated gels or polymer chain links. Since many of these substances are irritants, the means of application depends on whether the user has body contact with the material with PCMs. With no skin contact, coating–which is the most efficient method–is used. Most of the phase-change materials used are polyolefins (waxes) whose melting points are in the desired ranges. Several PCMs are usually selected and balanced to narrow the effective temperature range to a comfort zone for sleeping.
Phase-change materials in mattresses and pillows are not restricted to the fabrics. For instance, a patent assigned to Sleep Innovations by inventor Walter Mackay is for a method of incorporating certain types of PCMs (“organic compounds of nonmetals other than C, H, O, and N”) into bedding foams.
The use of phase-change materials is one available choice when shopping for a mattress, pillows, or other bedding products. Whether you select a product with this feature depends on several considerations. Availability is no longer a major consideration since so many brands now incorporate PCMs into their mattresses
The first consideration is, “Will this really help me sleep better?” This depends on how temperature affects your sleep, which in turn depends on the ambient room temperature (how you use heating or air conditioning) and the composition of the mattress. Mattresses with foams, especially memory foam, tend to be warmer, sometimes too warm. If your bedroom tends to be too warm or too cool, or if the top layers of your mattress have memory foam, PCMs may be the solution to keeping cool.
The next consideration is cost. Phase-change materials are now so widely used that the cost is not as high. Some mattresses with PCMs, such as the Serenity Gel by Bed In A Box, are in the low-to medium price range.
The most important consideration–not to be forgotten–is, “Does this mattress meet my needs, regardless of whether or not it has PCMs?”
 “Phase-Change Materials,” Wikipedia, http://en.wikipedia.org/wiki/Phase-change_material.
 “Melting to Keep Cool,” PBS Nova, http://www.pbs.org/wgbh/nova/next/tech/melting-to-keep-cool/.
 Mansfield, Richard G., “Phase Change Materials,” Textile World, http://www.textileworld.com/Issues/2004/March/Features/Phase_Change_Materials.
 Patent Docs, http://www.faqs.org/patents/app/20120193572.
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