aging (ageing)

of a polymer
https://doi.org/10.1351/goldbook.AT06849
Processes that occur in a @P04739@ 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.
Source:
PAC, 1996, 68, 2313. (Definitions of terms relating to degradation, aging, and related chemical transformations of polymers (IUPAC Recommendations 1996)) on page 2315 [Terms] [Paper]