The rubber cement used for patches is extremely volatile and the
solvent will slowly leak out of the crimp of the metal tube container,
hardening the cement. I don't think this varies too much between
brands.
I choose to keep those metal tubes unopened and used only on the
roadside for emergency repairs, in case my spare tube also gets a flat.
When I get home, I patch the tube using rubber cement contained in a
screw top lid and pack the patched tube back into my saddlebag.
IME, once you open those small metal tubes, the life span decreases
substantially. I squeeze the container in my patch kit about once a
week to be sure it hasn't hardened.
John Forester recommends carrying rubber cement in a small, screw top
jar for patches (to minimize volatile loss). It's in the archives.
Cheers,
John