Quote from _Angel_
Thanks to all who replied! I was just wondering, if a stethoscope has been left unused for a couple years or more, does the plastic tubing harden and crack when you do use it?
It depends on the brand of 'scope, the composition of the 'scope tubing, and the temperature at which it's been stored. PVC tubing is actually a mixture of PVC resin, cheap filler (like ground up limestone or talc), pigments like carbon black, plus plasticizer (an oily stuff that the PVC resin absorbs when heated, making the brittle plastic nice & flexible). Depending on what plasticizer is used in making the tubing, it will either be very durable over time, or will be lousy (subject to cracking).
Cheap PVC tubing will contain dirt cheap plasticizer (which will be more likely to evaporate over time or at higher temperatures), plus loads of cheap filler. Better PVC tubing will contain a proper amount of filler plus a good blend of plasticizers (monomerics for low temperature flex, polymerics or less volatile monomerics to reduce evaporation).
In my past life as an engineer for an automotive supplier, I compounded PVC blends for ~20 years.