aging (ageing)

of a polymer
Processes that occur in a polymeric material during a specified period of time, and that usually result in changes in physical and/or chemical structure and the values of the properties of the material.
PAC, 1996, 68, 2313 (Definitions of terms relating to degradation, aging, and related chemical transformations of polymers (IUPAC Recommendations 1996)) on page 2315