Thermal Transition
A change in the temperature at which a polymer moves from a hard, glassy state to a soft, rubbery state indicates an alteration in the molecular mobility of the material. This glass transition shift often occurs when a coating on paperboard absorbs moisture or is exposed to chemical plasticizers. In the food packaging industry, such a change can compromise the stiffness and barrier properties of a clear window or a moisture-resistant layer.
Material Degradation
Exposure to ultraviolet light or extreme heat during storage can drive the transition temperature higher by causing cross-linking in the polymer chains. This glass transition shift results in a more brittle coating that is prone to cracking when the box is folded. Converters must monitor these changes to ensure that the finished product remains functional through the entire lifecycle from the warehouse to the consumer.
Measurement Technique
Differential scanning calorimetry provides the data needed to identify the exact point where the heat capacity of the sample changes. By comparing a fresh sample to one that has undergone aging, technicians can quantify the glass transition shift precisely. This measurement helps in predicting the shelf life of specialty laminates used for frozen foods where the temperature must stay well below the transition point to prevent the film from becoming tacky.
