Research#
Market research#
Current products in the market utilizing sodium acetate trihydrate crystals#
Top: Hot Snapz brand hand warmers; Bottom: HeatWave brand handwarmers, by Bent Grass Concepts. Shown here is how one would make the handwarmer bag reusable. This list is not exhaustive.
How does crystal formation happen?#
Schematic of how current products work#
Above: Example of salt acetate-based hand warmer encased in vinyl bag. The flex disc has to be manually ‘clicked’ to trigger formation of crystals. The handwarmer can be reused by boiling the bag in water to melt all the crystlas. (Reference: thinkgeek.com, but annotations by me)
Above: Video of heating pad in action. (Reference: Wikipedia)
Design rules for triggering crystal formation#
- Nucleation usually happens from a seed salt crystal that is added to an aqueous salt solution (salt dissolved in water)
- The nucleation happens in an unstable concentration of salt dissolved in water (i.e, in a ratio greater than 1.25:1 salt:water)
- The temperature at which nucleation occurs can vary with pressure, and this window is very small (Ref)
- In traditional hand warmers, there is a metal disk that can be flexed to start nucleation. It's believed that seed crystals reside in the submicron cracks of this metal disk, and when its flexed it releases these salt crystals (Ref)
Helpful tutorials and links#
- How crystal formation changes with different salt solution concentrations
- Berg, J. et al. "Thermal Conductivity Enhancement of Sodium Acetate Trihydrate by Adding Graphite Powder and the Effect on Stability of Supercooling." 2015
- Dröscher, H. "The chemical system of sodium acetate / water as phase change material (PCM) for the use in seasonal energy storage." 2015