Dimensional stability is the change in length of an unrestrained film sample subjected to a specific elevated temperature. ExxonMobil typically measures this property at 275°F (135°C). Machine direction (MD) and transverse direction (TD) values are evaluated and reported separately. Units are reported as % change from the original dimension.
Relevance to performance
In general, materials expand when subjected to elevated temperatures. Oriented films, on the other hand, are likely to shrink because the polymer has "memory" and tries to return to its unoriented dimensions. Dimensional stability values for different films (measured at the same temperature) provide a relative comparison of how much film distortion will occur in heated processes, like oven drying or package sealing. Acceptable temperatures for processing a film vary and are dependent on the film properties, the type of process (contact with heated air or heated metal), the dwell time, and whether the film is restrained or not.
OPP films are typically designed for minimal shrinkage. Most ExxonMobil OPP films can be used in very high-speed, low-dwell heat seal applications with actual crimp jaw temperatures of up to approximately 355°F (180°C) without causing unsightly seal distortion. However, longer dwell exposures (> ½ second) require that temperatures do not exceed about 300°F (149°C) to prevent severe shrinkage. OPP is commonly used at temperatures between 220°F and 300°F (104.5°C to 149°C), where it can have a slight dimensional change. Test values at 275°F (135°C) for tenter-oriented OPP are typically -2% to -8% in both the machine and transverse directions.
Some films are designed to shrink in a predictable way, like the new, developmental ExxonMobil film Bicor TYTE. TYTE provides a crisp, tight overwrap for products like CD jewel cases, video tapes, and food or pharmaceutical boxes. Its dimensional stability values are -9% in the MD and -14% in the TD.
What affects dimensional stability
Dimensional stability is mostly determined by the OPP film's residence time and temperature in the annealing section of the orientation process. The annealing section involves the last zones of the transverse direction orienter and is where the oriented film continues to be held at the edges with tenter clips while surrounded by temperature-controlled oven air. This relieves residual stresses and creates "heat set" OPP. Without proper annealing, OPP is prone to greater shrinkage in heated environments.
ASTM D 1204 and ExxonMobil procedure #438 follow the same principles, but differ in some specific protocols. Both tests involve placing a film sample of known original dimensions into a temperature-controlled convection oven for a certain period of time and measuring the length of the sample after conditioning. Results are reported as % change. Negative numbers indicate shrinkage, while positive numbers indicate expansion. Pertinent details of the two procedures are summarized in Table 6.
|Specimen Size||Oven Temperature||Time in Oven||Pre/Post Conditioning||Precision||Reporting|
|ExxonMobil #438||1" x 7" cut in MD, and one in TD||Convection oven controlled to target, typically 275°F (135°C)||7 min||None||Nearest .02 inches||% change|
|ASTM D 1204||10" x 10"||Convection oven controlled to target ± 1°C||As appropriate depending on film testing||Yes, as standard laboratory temperature and humidity||Nearest .01 inches||% change|
|Table 6: Comparison of dimensional stability test conditions between ExxonMobil and ASTM procedures|