Date of Award

Fall 1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering - (Ph.D.)

Department

Chemical Engineering, Chemistry and Environmental Science

First Advisor

Marino Xanthos

Second Advisor

Dana E. Knox

Third Advisor

Costas G. Gogos

Fourth Advisor

Gordon Lewandowski

Fifth Advisor

Donald H. Sebastian

Sixth Advisor

David B. Todd

Abstract

A novel in-line method for measuring physical blowing agent solubility in thermoplastic melts in extrusion foaming equipment has been developed in this study. This method uses video microscopy in connection with a foaming extruder (single or twin-screw) to generate solubility data by observing the onset of gas bubble formation/dissolution. This method offers a number of advantages over the existing conventional methods. It can be easily used to obtain solubility data for many foamable thermoplastic-physical blowing agent systems at high pressure and temperature near actual foaming conditions. Data generated using this method could be directly used to characterize and control a foaming process and also provide guidelines for blowing agent selection, foam process design and optimization.

Solubility data of three different inert gases, CO2, Ar and N2, in polystyrene under different temperatures have been obtained. Overall, the results compare favorably with literature equilibrium data obtained with off-line methods. Solubility data of the same gases in polyethylene terephthalate have also been generated. To the author's knowledge, solubility data for this system at melt temperatures are not currently available in the literature. Data obtained from the twin-screw extrusion experiments show much better consistency compared with those obtained from single screw experiments; this indicates the importance of enhanced mixing in affecting the solubility values obtained from the in-line method.

A simple surface renewal model is used to study the gas dissolution behavior over the gas injection section during the foaming process. The model was found insufficient to explain the very complex phenomena occurring inside the extruder, suggesting the existence of other mechanisms that contribute to gas dissolution over the entire extruder length.

The effects of processing conditions on solubility and gas dissolution characteristics are studied using the in-line window system with a co-rotating twin-screw extruder. The results show that the measured CO2 take-up values in polystyrene are affected by polymer throughput, suggesting that an "apparent solubility.' is measured.

Gas solubility and dissolution results are applied to analyze the extrusion foaming process in which a commercially available polyethylene terephthalate is foamed in a single screw extruder by injection of atmospheric gases (CO2 Ar and N2) that were shown to exhibit different degrees of solubility. The results and their general implication on the extrusion foaming process of thermoplastics are discussed.

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