Relaxation of PET Orientation at Temperatures Below the Glass Transition

Download or read book Relaxation of PET Orientation at Temperatures Below the Glass Transition written by Brian M. Johnson and published by . This book was released on 2013 with total page 71 pages. Available in PDF, EPUB and Kindle. Book excerpt: Polyethylene terephthalate (PET) is a semi-crystalline polyester widely used in industry as a material for food and beverage containers. Although the properties of amorphous PET make it a poor material to use for this application, its properties can be improved through molecular orientation, which is accomplished by deforming the polymer while it is heated above its glass transition temperature (Tg). Orientation is frozen-in by cooling PET below Tg, and can be unfrozen and rapidly relaxed by raising the temperature above this point. Although orientation often results in some degree of strain-induced crystallization (SIC), completely crystalline PET is not possible, and some orientation will exist in the amorphous phase. Additionally, PET molecules retain a small degree of mobility below Tg. A concern for oriented PET, then, is the potential of orientation relaxation when the material is exposed to sub-Tg temperatures of 30-40 °C for extended periods of time. The goal of this study was to determine the extent of relaxation that occurs when oriented PET is exposed to sub-Tg temperatures, as well as how its physical and mechanical properties change correspondingly. Samples of extruded PET sheet were oriented above Tg by stretching them biaxially to two different stretch ratios and rapidly quenched to room temperature. As-quenched birefringence, film dimensions, and mechanical properties of the oriented films were measured. The films were then stored in an air circulating oven at 40, 50, or 60 °C for up to a week. Every 24 hours a film was removed from the oven, and post-exposure birefringence, dimensions and density were measured. Mechanical properties were measured after 7 days of exposure. Measurements of film dimensions provided noteworthy results, with all films experiencing a small degree of shrinkage. Films exposed at 60 °C exhibited the most shrinkage, with a 0.5% decrease in dimensions after a week. Shrinkage followed a trend of exponential decay, and its full extent lessened as temperature was decreased. By fitting a model to the data, it was predicted that the amount of shrinkage after a week of exposure at 30 °C would be negligibly small. The results of birefringence did not follow any clear trends and require further investigation. Density of films following exposure did not show expected trends with time. An explanation proposed for this is the presence of physical aging, which is expected to increase density in unoriented amorphous regions and compete with the effects of relaxation on density. This possibility is supported by post-exposure mechanical properties, which exhibited changes consistent with physical aging.