Foot warmer patches are a tool for heating. The reaction principle is to use the galvanic battery to accelerate the oxidation reaction speed and convert chemical energy into heat energy. In order to make the temperature last longer, the mineral material vermiculite is used for heat preservation.

Because it cannot react before use, the material of the bag must be very special, consisting of a raw material layer, a gelatin layer and a non-woven bag. The non-woven bag is made of a microporous breathable film, and it has to have a conventional airtight outer bag. When in use, remove the outer bag and let the inner bag (non-woven bag) be exposed to the air, and the oxygen in the air enters the inside through the breathable membrane. The time and temperature of heat release are controlled by the oxygen permeability rate of the breathable membrane. If the oxygen permeation is too fast, the heat will be released at once and the skin may be burned. If the oxygen permeation is too slow, there will be no temperature. After use, it is a dark brown solid, which contains carbon powder, NaCl solid, Fe2O3 solid and magnesium-aluminum-containing salts.

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