Frequently asked questions about phase change materials

What are phase change materials?

Phase change materials (PCM) are substances that absorb and release thermal energy during the process of melting and freezing. When a PCM freezes, it releases a large amount of energy in the form of latent heat at a relatively constant temperature. Conversely, when such material melts, it absorbs a large amount of heat from the environment. PCMs recharge as ambient temperatures fluctuate, making them ideal for a variety of everyday applications that require temperature control.

The most commonly used PCM is water/ice. Ice is an excellent PCM for maintaining temperatures at 0°C. But water’s freezing point is fixed at 0°C (32°F), which makes it unsuitable for most thermal energy storage applications.

To address that limitation, PCMs have been developed for use across a broad range of temperatures, from -40°C to more than 150°C. They typically store 5 to 14 times more heat per unit volume than materials such as water, masonry or rock. Among various heat storage options, PCMs are particularly attractive because they offer high-density energy storage and store heat within a narrow temperature range.

What is latent heat?

There are two kinds of heat energy: sensible and latent. Most common heat storage systems, such as a conventional water heater, use sensible heat, the energy needed to alter the temperature of a substance with no phase change. Latent heat, which can be 100 times that of sensible heat, is the amount of energy required to change matter from one state to another, liquid to solid or vice versa. Sensible heat and latent heat work together in thermal storage materials like PureTemp. This results in the ability to maintain specific temperatures for extended periods of time.

What are some examples of latent heat?

Ice cubes, using their latent heat capacity, absorb large amounts of heat energy from a drink. When all the ice has melted, the cubes have essentially absorbed all their latent energy capacity and then the drink warms to room temperature. Conversely, when hot wax is dripped on skin, a burning sensation is caused by the wax releasing all its internal latent heat energy.

How are PCMs being used?

The global PCM market is experiencing high growth as a result of increasing demand for eco-friendly, energy-saving materials in industries such as building/construction, refrigeration, HVAC, thermal energy storage, textiles and electronics. PCM applications include:

  • Refrigerators and freezers that use less energy.
  • High-performance textiles that provide relief from hot and cold conditions.
  • Shipping containers that maintain goods at the desired temperature for longer periods.
  • Construction material that keeps living spaces comfortable and reduces heating and cooling costs.
  • Thermal energy storage systems that shift a building’s cooling and heating needs to cheaper, off-peak hours.
What types of PCMs are in use?

PCMs fall into four main categories: water-based, salt hydrates, paraffins and vegetable-based. Here’s a look at each.

Water-based ice and gel packs are extremely popular for keeping materials cold around 0°C. These low-cost devices have several advantages. They perform well and are nontoxic, non-flammable, environmentally friendly and easy to use. The disadvantages: They are useful only in applications requiring a temperature of 0°C and are vulnerable to microbial growth.

Salt hydrates consist of inorganic salts and water. Their melt point temperatures range between 15°C and 80°C. The advantages of salt hydrates are low material costs, high latent heat storage capacity, precise melting point, high thermal conductivity and inflammability. The disadvantages:

  • Poor nucleating properties make them vulnerable to supercooling, the phenomenon in which a substance cools below its freezing point without solidifying. That can be beneficial in some applications, but for most uses nucleating agents must be added to address this problem. 
  • The volume change in the solid/liquid phase change of a salt hydrate is up to 10%. Special packaging must be used to accommodate the changing volume. 
  • Some salt hydrates fail to completely recrystallize following each cycle. Eventually, they lose all latent heat capacity. 
  • Some salt hydrates are toxic and many are corrosive to metals, presenting safety and disposal issues.

Paraffins, typically, are derived from petroleum and have a waxy consistency at room temperature. Their melt point temperatures range between -8°C and 40°C. They have good thermal storage capacity and are proven to freeze without supercooling. They also have the advantage of chemical stability over many heating and freezing cycles. They are non-corrosive and are compatible with most encapsulation materials. But they have a limited range of melting points and their cost is linked to unstable petroleum prices. And, like all fossil fuels, petroleum has significant environmental costs. Many paraffins are hazardous to health and the environment. Some can injure skin, eyes and mucous membranes. Some have narcotic effects if inhaled. Hexadecane, a petroleum alternative to PureTemp 18, is one example.

Biobased PCMs are organic compounds derived from animal fat and plant oils. Their melt point temperatures range between -40°C and 151°C. The most common biobased PCMs are derived from fatty acids and have higher efficiency than salt hydrates and petroleum-based phase change material. The other advantages of biobased phase change material:

  • They are nontoxic.
  • They experience minimal volume change between phases.
  • They are stable.
  • They can last for decades.
  • They have high latent heat.
  • They are fire-resistant.
  • They are cheaper than petroleum-based PCMs.
What factors determine the quality of a PCM?

Latent heat – Latent heat is measured in joules per gram. Preferred latent heats are greater than 150 J/g and preferably greater than 180 J/g. A high latent heat of fusion means 
that a lesser amount of material is needed to store a given amount of energy.

Sharpness of latent heat release and absorption – To a first approximation, the release and absorption of latent heat over a narrow temperature range is an important performance criterion. However, for some applications, broad heat release rates of up to 10C or even 30C will not adversely affect performance in a properly engineered consumer device.

Stability to cycling and aging – Over the course of hundreds or thousands of 
freezes and melts, the phase change transition temperature and latent heat energies should remain consistent. Changes in the hydration sphere of some PCM molecules over time can affect the melt and freeze points and their respective latent heats. This is the primary reason that one of the most used stability tests for PCMs is the thermal cycle test that involves multiple melt and freeze cycles.

Non-corrosive to encapsulation – Encapsulation of the PCMs involves a PCM core and an outer shell to prevent leakage, degradation and contamination. Careful consideration must be given to the compatibility of the individual PCM with the plastic or metal chosen for the application. Plastic tends to be the least expensive container for PCM chemicals. However, containers with thick plastic walls suffer from low thermal conductivity. Various metals can be used for PCM encapsulation to increase thermal conductivity. However, metals tend to be more costly and can corrode with some PCMs.

Cost effective – The useful heat in joules per gram of phase change material is a primary performance criterion for PCM chemicals. On a cost basis, a minimum in dollars per joule of useful heat is desired.

Safe to use – The PCM should be nontoxic, nonflammable and environmentally friendly.

Temperature – Applications are typically temperature-specific. Often the temperature desired is within 1 or 2 degrees. Therefore, 200 different PCMs would be needed to target each degree in a temperature range of -50°C to 150°C.

End of life cycle - At the end of the life of a PCM application the PCM should be able to be land-filled and degrade naturally within 6-12 months.

What is a thermal cycle test?

In a thermal cycle test, a PCM is frozen and melted multiple times. The test is an important measure of a PCM’s durability. RAL, an independent quality assurance association based in Germany, has set standards for PCM quality. The RAL's Quality and Testing Regulations for Phase Change Materials specifies these increments for thermal cycling testing: 50, 100, 500, 1,000, 5,000 and 10,000. 

What is special about PureTemp® PCMs?

Most PCMs today are chemical formulations derived from petroleum products, salts or water. Such PCMs are limited in temperature range options, containment methods and thermal cycles. PureTemp PCMs with comparable latent heats have been proven to be superior by those measures and more, including stability, flammability, toxicity and sustainable performance. Each production lot undergoes more than 10 separate quantitative and qualitative tests, including DSC, to guarantee quality.

Are PureTemp® PCMs certified as biobased?

USDA biobased product decalYes. PureTemp phase change materials have earned the U.S. Department of Agriculture’s Certified Biobased Product Label. The label verifies that the amount of renewable biobased material in the product meets or exceeds levels set by the USDA. Nineteen PureTemp products, with temperature ranges from 4°C to 68°C, have been designated as 100 percent biobased. All claims of a product’s biobased amounts are verified by independent labs and monitored by the USDA. PureTemp is listed in the USDA BioPreferred Catalog in two categories: phase change materials and heat transfer fluids. PureTemp has also been awarded the Federal Procurement Preference, meaning that the PureTemp product line is now preferred by federal agencies and their contractors when making purchasing decisions.

What is DSC?

Differential scanning calorimetry (DSC) is a thermoanalytical technique used to determine the peak melting points and latent heat values of all PureTemp PCMs. The instrument used is called a differential scanning calorimeter. When a DSC is conducted for a PureTemp PCM, a temperature range that is 25 degrees below and 30 degrees above the peak melt of the PCM is scanned at a rate of 1°C/minute. This allows for accurate melting point and latent heat values to be obtained.

What are PureTemp® PCMs made of?

PureTemp’s patented phase change materials are made from natural sources such as palm oil, palm kernel oil, rapeseed oil, coconut oil and soybean oil. They are nontoxic and biodegradable. Properly contained, these fully hydrogenated compounds will not oxidize or become rancid. Fully hydrogenated fats and oils can be stable for decades because they do not have chemical sites for oxidation to occur.

How many PureTemp® formulations are available?

More than 200 PureTemp formulations ranging from -40°C to 151°C have been produced and tested by expert chemists at the Entropy Solutions lab in Tuscaloosa, Ala. About 30 formulations, ranging from PureTemp -37 to PureTemp 68, are in production mode. Here are some commonly ordered formulations and their attributes:

                                        Peak melting point     Heat storage capacity   Liquid density      
PureTemp -37 -37°C / -34.6°F 147 J/g 0.88 g/ml
PureTemp 4 5°C / 41°F 195 J/g 0.88 g/ml
PureTemp 8 8° / 46.4°F 180 J/g 0.86 g/ml
PureTemp 18 18°C / 64.4°F 189 J/g 0.86 g/ml
PureTemp 23 23°C / 73.4°F 203 J/g 0.83 g/ml
PureTemp 28 29°C / 84.2°F 205 J/g 0.86 g/ml
PureTemp 29 29°C / 84.2°F 189 J/g 0.85 g/ml
PureTemp 37 38°C / 100.4°F 222 J/g 0.84 g/ml
PureTemp 60 61°C / 141.8°F 230 J/g 0.87 g/ml
PureTemp 68 68°C / 154.4°F 198 J/g 0.87 g/ml
What do PureTemp® PCMs look like?

Like a vegetable oil when liquid, like a wax when solid:

What is the average latent heat of PureTemp® PCMs?

The average latent heat ranges from 170 to 270 J/g.

How do PureTemp® PCMs measure up against paraffin?

PureTemp’s biobased PCMs offer thermal performance that meets or exceeds that of paraffin-based PCMs and with greater versatility. Here’s a comparison:

PureTemp Petroleum/paraffin
Latent heat range 170-270 J/g 170-240 J/g
Renewable Yes No
Bio-friendly Yes No
Cost per unit Low Medium-High
Flammability Low High
Degradability 6 months Non-biodegradable
Rancidity None None
Thermal life cycle Limitless Limitless
Toxicity Non-toxic Potentially
Microencapsulation Yes Yes
Are PureTemp® PCMs safe?

Unlike petroleum-based phase change materials, PureTemp PCMs are nontoxic and have a low flammability rating. Many PureTemp® PCMs are composed of materials certified by the U.S. Food and Drug Administration as food-grade chemicals.

Are PureTemp® PCMs biodegradable?

Yes. At the end of an application’s useful life, PureTemp PCMs can be buried in landfills and will naturally degrade within about six months. For more information, see "Biodegradability of phase change material."

Are PureTemp® PCMs corrosive?

PureTemp PCMs are compatible with many polymers and resins. PureTemp PCMs are compatible with crystalline polymers (HDPE and fluorinated polymers, for example) and will leach through amorphous polymers (PES, PET and PS, for example).

A few PCMs may interact with some metals and should be tested on a case-by-case basis. Some are not compatible with copper, aluminum or carbon steel but are compatible with 316 stainless steel and perhaps 304 stainless steel.

But note that compatibility is not an issue for encapsulated PCMs, which do not come into contact with the environment.

How long will PureTemp® PCMs last?

In more than five years of research supported by the U.S. Department of Agriculture, the National Science Foundation and the Department of Defense, PureTemp phase change materials experienced zero thermal degradation while exposed to over 65,000 thermal cycles.

How does a PureTemp® product get its number?

The number refers to the product’s peak melting point in degrees Celsius as measured by a differential scanning calorimeter (DSC). The DSC plots energy released or absorbed versus temperature. This gives the melt and freeze points and the latent heat associated with the compound. Accurate DSC calibration is needed for temperature and latent heat determination. To get accurate response factors from a temperature range of -50°C through 200°C, a temperature calibration curve is created based on indium, gallium and mercury samples heated through their melting transition temperatures. Samples of ~2-8 mg provided an optimal combination of mass accuracy and response to the instrument.

Where is PureTemp® made?

PureTemp is manufactured at various toll chemical manufacturers in accordance with our exacting standards and ISO quality policies.

What containment options are available?

PureTemp PCMs are available in three forms: unencapsulated, microencapsulated and macroencapsulated. Each form is suited to particular applications. The proper containment option depends on many factors. Entropy Solutions experts and encapsulation partners work with each customer to determine the optimal containment choice.

Unencapsulated: Phase change material sold in bulk is known as unencapsulated PCM. The material is solid in ambient temperatures below its melting point and liquid in ambient temperatures above its melting point. Unencapsulated PCMs must be contained in some form – micro- or macroencapsulation – before they can be incorporated in a product.

► To order unencapsulated PureTemp in bulk or request a sample, visit

Microencapsulated: The process of coating small amounts of phase change material, from 10 to 1000 microns, is called microencapsulation. The three basic forms:

Powder: Particles of PCM are encased within a shell and immersed in a water solution. The water is then removed, leaving a powder of encapsulated PCM.

Cake: Particles of PCM are encased within a shell and then immersed in a water solution. Most of the water is removed, leaving a cake that is 30% water and 70% solids.

Slurry: Particles of PCM are encased within a shell and then immersed in a water solution. Some water is removed, leaving a slurry that is about 60% water and 40% solids.

Microencapsulated PCMs are sold to manufacturers for use in high-performance textiles, adhesives, bedding, flooring, paint, roofing and electronics.

► Contact one of our containment partners, Encapsys or Microtek Laboratories, for information on microencapsulated PureTemp.

Macroencapsulated: Phase change material contained in flexible films or plastic vessels of various size, shapes or formats is referred to as macroencapsulated PCM. Also known as bulk containment. Macroencapsulated PCMs are sold to manufacturers for use in cooling vests, car seats, warming blankets, electronics and thermal energy storage.

► Vesl devices containing PureTemp are temporarily unavailable. To learn more about Vesl solutions, visit

Where can I find details on each PureTemp® product?

Technical and safety data sheets are available for dozens of the most commonly used formulations of PureTemp. The technical sheets provide information on melting point, heat storage capacity, appearance, density, DSC analysis and more. The safety sheets provide information on possible health effects and flammability rating. Biodegradability reports conducted by Situ Biosciences of Skokie, Ill., are available for nine PureTemp formulations. 

How much do PureTemp® PCMs cost?

Unencapsulated PureTemp PCMs cost $20 per pound for samples. Commercial amounts of more than 1,000 pounds cost less than $5 a pound, depending on grade and volume. Contact one of our containment partners, Encapsys or Microtek Laboratories, for pricing information for microencapsulated PureTemp.

How are PureTemp® PCMs shipped?

Most PureTemp PCMs can be packaged in their liquid state, even if they eventually become solid at room temperature. They are available in 2-pound (1 liter) and 7-pound (1 gallon) samples. PureTemp formulations of 58°C and above are available in solid formats – granulated, flaked or pelletized – in 1-pound and 4-pound samples. Commercial amounts are shipped in 55-gallon drums and 275-gallon totes. 

How can I order?

To order unencapsulated PureTemp in bulk or request a sample, visit Samples generally ship within one week; bulk PCM shipments are dependent upon inventory level.

Vesl devices containing PureTemp are temporarily unavailable. To learn more about Vesl solutions, visit

Contact one of our containment partners, Encapsys or Microtek Laboratories, for information on microencapsulated PureTemp.

I have a question not answered here.

Two Entropy Solutions advisors, Dr. Mohammed Farid of the University of Auckland and Lucas B. Hyman of Goss Engineering, are pleased to take your questions about PCMs and thermal energy storage.

Send questions to Each week we select the best and post responses on the Phase Change Matters blog,

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