Photoactivatable leuco base time-temperature indicator

• US 4,917,503 A
• Filed: 09/30/1987
• Issued: 04/17/1990
• Est. Priority Date: 12/02/1985
• Status: Expired due to Term

• Current Assignee: Temptime Corporation (Zebra Technologies Corporation)
• Original Assignee: Lifelines Technology Incorporated (Zebra Technologies Corporation)
• Inventors: Bhattacharjee, Himangshu R.
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): WORTH, WILLIE M
• Application Number:
  US07/103,418
• Time in Patent Office: 930 Days
• Field of Search: 116/206, 116/207, 116/216, 252/408.1, 252/586, 252/962, 252/963, 374/102, 374/106, 374/161, 374/162, 422/56-58, 422/86, 426/88, 428/913, 430/338, 430/344, 430/346, 436/2, 436/902, 436/904, 436/905
• US Class Current: 374/102
• CPC Class Codes: G01K 3/04 in respect of time
• CPC Class Codes: G01N 31/229 for investigating time/temperature history
• CPC Class Codes: Y10S 252/962 Temperature or thermal history
• CPC Class Codes: Y10S 436/905 Photochemical activation of reactions

Abstract

A photoactivated time-temperature indicator is based on a leuco base system. A thermally insensitive, white ("inactive") leuco base (or a mixture of such leuco bases) is mixed, preferably in a polymeric matrix, with a material that generates acid upon exposure to light. Photoexcitation, preferably by UV or near UV light, causes the formation of a thermally sensitive, color-forming ("active") product. Following this activation step, a progressive color development occurs at a rate that increases with temperature. The indicator is useful for monitoring the freshness of perishable products, particularly those stored at subambient temperature.

Claims

1. A photoactivatable time-temperature indicator, comprising a mixture of

(a) a thermally inactive compound that comprises a leuco base and

(b) a photosensitive compound that, on exposure to actinic radiation, forms an acid that converts the leuco base to a thermally active product.

2. A photoactivatable time-temperature indicator, comprising a mixture of

(a) a thermally inactive compound that comprises a leuco base and

(b) a photosensitive compound that, on exposure to actinic radiation, forms an acid that converts the leuco base to a thermally active product, wherein said mixture is dispersed in a medium and coated onto a substrate.

3. The indicator of claim 2 in which the thermally inactive compound comprises one or more leuco bases of triarylmethane dyes.

4. The indicator of claim 3 in which the leuco bases include at least one selected from the group consisting of malachite green leuco, brilliant green leuco, and crystal violet leuco.

5. The indicator of claim 4 in which the leuco bases include malachite green leuco.

6. The indicator of claim 3 in which the photosensitive compound is o-nitrobenzaldehyde or 2,2,2-tribromoethanol.

7. The indicator of claim 2 in which the medium comprises a polymer.

8. The indicator of claim 7 in which the medium comprises polyvinyl alcohol.

9. The indicator of claim 7 in which the medium comprises polyvinyl acetate.

10. The indicator of claim 2 in which the substrate comprises paper.

11. The indicator of claim 2 further comprising a cover sheet.

12. The indicator of claim 11 in which the cover sheet comprises polyester.

13. A method of measuring an incremental time-temperature exposure, which comprises the steps:

(a) exposing a photoactivatable indicator comprising a mixture of a thermally inactive compound which includes essentially a leuco base and a photosensitive compound which on exposure to actinic radiation forms an acid that converts the leuco base to a thermally active product, to actinic radiation to render said indicator thermally active,

(b) measuring the reflectivity of the indicator at a specified wave length,

(c) measuring the reflectively of the indicator at the specified wave length after the incremental time-temperature environmental exposure, and

(d) calculating the incremental time-temperature exposure by using a pre-established relationship between a change in reflectivity of the indicator and time-temperature exposure.

14. The method of claim 13 in which the photoacti vatable indicator is a mixture of o-nitrobenzaldehyde, malachite green leuco, and polyvinyl acetate.

15. The method of claim 13 in which the actinic radiation is electromagnetic radiation having a wavelength in the range between about 200 nm and about 400 nm.

16. The method of claim 13 in which the temperature exposure is in the range between about -20°

and 60°

C.

17. A method of measuring an incremental time-temperature exposure of a perishable article by

(a) applying to the article an indicator comprising a mixture of a thermally inactive compound that includes essentially a leuco base and a photosensitive compound which on exposure to actinic radiation forms an acid that converts the leuco base to a thermally active product,

(b) exposing the indicator to actinic radiation to render it thermally active,

(c) measuring the reflectivity of the indicator at a specified wavelength,

(d) measuring the reflectivity of the indicator at the specified wavelength after incremental time-temperature exposure, and

(e) calculating the incremental time-temperature exposure by using a pre-established relationship between a change in reflectivity of the indicator and time-temperature exposure.