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Degree

Doctor of Philosophy in Engineering, University of Toledo, Chemical Engineering, 2004.

Abstract

Poly (Ethylene Terephthalate), PET, has become the packaging material of choice for many packaging applications. PET containers have virtually replaced glass bottles in many market segments. PET markets that require cold or hot products (up to 85°C) to be filled within have essentially reached commodity status. There are many applications that could be served by PET, if PET bottles with a higher degree of thermal stability could be commercially produced. Such non-commodity applications would provide added value to the marketplace, and enable more favorable economic return. Much work has been published regarding the kinetics of crystallization of PET, in both its oriented and unoriented states. Within the published volume of literature, there are key elements to the methods of processing PET that are disclosed and can be used to construct a new crystallization (heat set) process for the blow molding of PET bottles. For this work commercial stretch blow molding machines were adapted to accept a circulation of hot air on the interior of the bottle during the blow molding cycle. A combination of hot molds at the bottle exterior and heated compressed air at the interior bottle surface provide energy to the oriented bottle sidewall, with the energy sufficient to induce an increase in the crystalline structure of the PET. Additionally, the energy allows rearrangements of the morphology of the non-crystalline region from a stressed to a more relaxed structure, resulting in an improvement of bottle thermal stability up to 121°C. Once a process was established and performance of the bottles from the process was validated, the differential equation describing the energy transfer was written. Through numerical methods, this equation was solved for the temperature gradient, and PET kinetics were added to provide a final model that predicts both temperature and crystallinity gradients through the bottle sidewall as a function of the process parameters. The model predictions were tested against measured values, and reasonable agreement was found. The final model provides a means of describing the crystallization of PET that occurs immediately after formation of the bottle (i.e. as the bottle reaches the blow mold). Due to assumptions made during the modeling of the physical characteristics of PET (e.g. thermal conductivity, heat capacity), the model would only be valid for 100°C < T < 250°C. Once the process was designed, and the model describing the process was validated, the ultimate thermal properties of the resulting container were measured. Containers capable of withstanding a retorting process (e.g. filled package heated for 45 minutes at 121°C) were demonstrated. An assessment of level of crystallinity versus relaxation potential was performed. It has been clearly demonstrated that crystallinity is not an absolute predictor of thermal performance, as the relaxation potential is the key property to consider. Finally, the impact of increasing the amount of crystallinity on gas barrier is delineated.

Keywords

Poly (ethylene terephthalate); PET; heat-set; crystallization; bottle; packaging

Advisor

Saleh Jabarin

Document number: toledo1091111913
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=toledo1091111913

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