Time-temperature indicator devices

A method for recording a duration of time over which a temperature has been established above a predetermined temperature, comprising the steps of:

• providing a time-temperature indicator device comprising
• a dye-compatible polymer composition having a softening point at or above said predetermined temperature an said polymer composition being in the form of a film or coating layer having top and bottom surfaces and a predetermine thickness;
• and
• a polymer-compatible dye composition in contact with one of said surfaces of said polymer composition, wherein said dye composition is present in a quantity sufficient for diffusion through said polymer composition whenever said polymer composition is above said predetermined temperature, which quantity is selected so that the extent of said diffusion within said polymer composition corresponds to the duration of exposure above said predetermined temperature relative to the total time required for substantially complete diffusion through said polymer composition, and with the diffusion of the dye within the polymer composition being perpendicular to said predetermined thickness;
• exposing said device above said predetermined temperature so that said dye composition diffuses through said polymer composition;
• and
• evaluating said extent of diffusion of said dye composition within said polymer composition to determine the duration of time said device was exposed above said predetermined temperature.