Temperature sensitive films

1. A temperature-sensitive film comprising:

(a) a colored backing layer; and

(b) a temperature-sensitive layer disposed over the backing layer, the temperature-sensitive layer comprising:

(i) a binder;

(ii) a set of lower-melting light-scattering particles dispersed in the binder, the lower-melting light-scattering particles having a melting point at or near a threshold temperature, such that they melt and transparentize at the threshold temperature; and

(iii) a set of higher-melting light-scattering particles dispersed in the binder, the higher-melting light-scattering particles having a melting point above the threshold temperature;

wherein the transparency of the temperature-sensitive layer and the visibility of the colored backing layer are irreversibly increased at the threshold temperature.

2. The temperature-sensitive film of claim 1 , wherein the temperature-sensitive layer is initially opaque below the threshold temperature and transparent after the threshold temperature has been reached.

3. The temperature-sensitive film of claim 1 , having an initial Y-value of at least 40 prior to the melting of the lower-melting light-scattering particles and a Y-value that differs by less than 2 from the Y-value of the colored backing layer in the absence of the temperature-sensitive layer after the lower-melting light-scattering particles have melted.

4. The temperature-sensitive film of claim 1 , wherein the lower-melting light-scattering particles comprise a wax.

5. The temperature-sensitive film of claim 1 , wherein the higher-melting light-scattering particles comprise silica.

6. The temperature-sensitive film of claim 5 , wherein the lower-melting light-scattering particles comprise a wax.

7. The temperature-sensitive film of claim 6 , wherein the binder comprises a thermoplastic elastomer.

8. The temperature-sensitive film of claim 6 , wherein the temperature-sensitive layer comprises about 20 to 45 wt. % binder, about 40 to 65 wt. % lower-melting light-scattering particles and about 5 to 20 wt. % higher-melting light-scattering particles, based on the solids content of the temperature-sensitive layer.

9. The temperature-sensitive film of claim 1 , wherein the binder comprises a thermoplastic elastomer.

10. The temperature-sensitive film of claim 9 , wherein the colored backing layer comprises a polymeric film.

11. The temperature-sensitive film of claim 9 , wherein the threshold temperature is 150° C. or lower.

12. The temperature-sensitive film of claim 1 , wherein the temperature-sensitive layer comprises about 20 to 45 wt. % binder, about 40 to 65 wt. % lower-melting light-scattering particles and about 5 to 20 wt. % higher-melting light-scattering particles, based on the solids content of the temperature-sensitive layer.

13. The temperature-sensitive film of claim 1 , further comprising an adhesive coating on the colored backing layer.

14. A method of indicating that an object has reached a threshold temperature using a temperature-sensitive film, the film comprising:

(a) a colored backing layer; and

(b) a temperature-sensitive layer disposed over the backing layer, the temperature-sensitive layer comprising:

(i) a binder;

(ii) a set of lower-melting light-scattering particles dispersed in the binder, the lower-melting light-scattering particles having a melting point at or near the threshold temperature, such that they melt and transparentize at the threshold temperature; and

(iii) a set of higher-melting light-scattering particles dispersed in the binder, the higher-melting light-scattering particles having a melting point above the threshold temperature;

the method comprising placing the temperature-sensitive film in thermal contact with the object and observing an increase in the visibility of the colored backing layer after the object reaches the threshold temperature.