What is a phase change material (PCM) in the context of cold packs?

A phase change material (PCM) is a substance that absorbs or releases heat as it changes from one physical state to another, typically from solid to liquid or liquid to solid. In cold packs, water-based gel formulations act as PCMs.

As the cold pack melts from frozen (solid) to liquid, it absorbs heat and helps maintain the temperature of the product being shipped. This controlled phase change is what makes cold packs effective for temperature regulation.