Thermally-activated time-temperature indicator

Claim 1

An article for irreversibly displaying visual evidence of exposure within a predetermined temperature range for at least a predetermined length of time, comprising in combination: a. an opaque open cell microporous diffusely light-reflective layer having first and second surfaces and being formed of components which will not melt at the upper end of said predetermined temperature range, b. a colored stratum firmly joined to the first surface of said microporous layer, and c. overlying, but not appreciably penetrating, at least a portion of the second surface of said microporous layer, a substantially transparent coating of a composition comprising a solid solution of (1) an amorphous rubbery polymer having a glass transition temperature below the lower end of said predetermined temperature range, dissolved in (2) a crystallizable solvent for said rubbery polymer, said solvent having a melting point below the lower end of said predetermined temperature range, the solvent:polymer ratio being selected to control viscosity and assure wicking penetration of the entire depth of said microporous layer by said composition at the expiration of said predetermined length of time in said temperature range to thereby make the colored stratum visible.