A phase change material absorbs and releases thermal energy in order to maintain a regulated temperature.
When a PCM is in its' solid phase it will absorb heat as the external temperature rises. The temperature of the PCM will mirror the external temperature until the PCM's melt point is reached. When the external temperature reaches the melt point of the PCM, the PCM will begin to melt, i.e. "change phase". During the phase change process the PCM will absorb large amounts of heat with almost no change in temperature. During this time period, the PCM is providing a cooling effect. The amount of time the PCM will provide a cooling effect is determined by the PCM's enthalpy of melting, also called the latent heat of fusion of melting. The enthalpy varies depending on the PCM material itself. In the case of PCMs, the enthalpy is typically measured in Joules/gram. The higher the number of Joules per gram, the longer the PCM will provide a cooling effect.
The reverse cycle occurs as the external temperature cools. The PCM, now in its' liquid phase, can release the heat it absorbed as the external temperature decreases. During this time period, the PCM solidifies and provides a warming effect.