Items such as food and pharmaceuticals exposed over time to temperatures outside the specified range can pose a serious health risk. Determining the heat exposure history of these items is a key step in addressing these issues. In particular, there is a need for determining when an item has been exposed to specific temperatures, how long such exposures occurred, in what order they occurred, and the expected remaining shelf life of an item.

One method manufacturers typically use to help avoid unsatisfactory degradation is date stamping — labeling a product with a date, such as the date of manufacture or expiration. Often, thermal and related stressors, such as direct sunlight, account for the most serious effects of product degradation. Date stamping cannot accurately assess the degree or rate of degradation.

Efforts have been made to account for the effect of high temperatures and to assess whether such temperatures have yet been reached. These techniques include a sensor that acts like a fuse, indicating if it has been exposed to a specific temperature. Time Temperature Indicators are sensors that are sensitive to both time and temperature. The sensor typically may be visually inspected to determine whether a product has yet degraded past a certain point. While such techniques may be useful for indicating that an item was exposed to a specific temperature, they generally do not reveal by how much the specific temperature was exceeded, or for how long the item was exposed to any given temperature.

A polymeric thermal history sensor was developed that determines the thermal history of an item (such as a consumer product, for example) subject to thermal exposure. The sensor contains a set of different polymeric films or substrates. The infrared (IR) absorption spectra and, in some cases, the visual absorption or transmittance of these materials, correlates with the degree of crystallization of the polymer, which in turn depends on the thermal conditioning of the materials. The sensor is positioned adjacent to the item of interest, and because the sensor is exposed to essentially the same thermal conditioning as the item itself, the sensor can reveal important information about the thermal history of the item.

The sensor initially contains substrates in the amorphous state. Thereafter, each polymeric substrate will undergo crystallization in response to thermal exposure. By exposing the sensor to a thermal stressor, each substrate — having a unique composition — responds with a different degree of crystallization. This difference results in different IR absorption spectra. By measuring the different IR absorption spectra for each polymeric substrate, a fingerprint is obtained corresponding to the thermal history of the item. The sensor is most effective for use with temperature exposures varying from approximately 20 °C to approximately 250 °C. The sensor may be re-used after quenching the substrates by exposing it to sufficiently high temperatures.

A reflective surface, for infrared wavelengths, may be positioned under the polymeric substrates to allow infrared light, directed through the substrates, to be reflected back through the same substrates in order to facilitate the measurement. Alternatively, an IR transparent surface may be used to facilitate measurement in some cases, such as where the sensor is removed from the item and passed through a measurement device. A variety of materials may be applied to the sensor, such as an adhesive layer, to attach to the item of interest. A handheld device similar to a standard barcode reader may be used to simultaneously measure the IR absorption spectra of each polymeric substrate.

For more information, contact Dr. Brian Metzger at This email address is being protected from spambots. You need JavaScript enabled to view it.; 406-994-7782.

Tech Briefs Magazine

This article first appeared in the October, 2017 issue of Tech Briefs Magazine.

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