Why do reusable heat packs have a limited time frame before the
reaction becomes irreversible?
This reaction should never become irreversible until and unless there is bacterial growth or loss of water for some reasons. For example, one manufacturer explains that the pads can re-used for 18 years! http://www.thermo-pad.com/frequently-asked-questions/
We normally think that plastics are impermeable to gas exchange or other small molecules. They are not just like our skin. Do remember that a balloon eventually gets deflated with time? Moisture is no exception.
Search Moisture permeability of polymers and you will find 100s of articles.
How many times can they be used?
There is no limit as to the number of times a Thermo-Pad can be used.
There are pads still in use after eighteen years. The pad will not
operate if there is a hole in the vinyl. Thermo-Pad is manufactured
from the same vinyl as is used in water beds and comes with a one year
What are the white crystals that form in the liquid pad?
If the Thermo-Pad has been stored in its crystallized state for an
extended period of time, a small amount of water can evaporate from
the pad and cause the solution to become more concentrated. This
higher concentration is indicated by “snowflake like” crystals in the
liquid pad. The pad will still operate but at a slightly higher
temperature. This water loss can be substantially reduced by storing
the pad in its liquid state and may be reversed by extending the
boiling times and by leaving the pad in water to cool.