The material is called ammonium biacetate, and it is useful as a heat storage material because of a phase transition. As the material is heated, it changes from a solid to a liquid, which then allows it to give off heat as it return to a solid state. I am actually planning on building this material into gloves, hats, socks etc and then those would be microwaved for activation. Temperature would be around 65 degrees celsius, so very little material is needed. Obviously these would be more expensive than the air activated warmers, but would be infinitely reusable. Let me know if this helps.

Thanks for all the responses as well, it is greatly appreciated.