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Date of Award

6-1999

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Supervisor

Professor J. Vlachopoulos

Abstract

The creation of a homogeneous polymer melt from powder particles, which involves particle coalescence followed by the formation and dissolution of bubbles, is a situation encountered in processes such as rotational molding and powder coating. The present work focuses on the study of the transformation of a loosely packed, low density powder compact, to a fully densified polymer part, when processed at temperatures above the melting (or glass transition) point of the polymer. The purpose of this work is to study the appearance of air pockets, or bubbles, which are trapped during the melting-sintering of the polymer particles, to elucidate the mechanisms involved in their formation and subsequent dissolution and to propose models which are suitable for the description of the overall densification of the powder compact. A comparative study of the processing characteristics and properties offered by various polymers has been undertaken. During this work, the problem of the presence of excessive bubbles has been encountered mainly in polymers with high amorphous contents and low crystallinities. Rheological characterization suggested that these types of polymers typically exhibit weak viscosity dependence on temperature and higher melt elasticities. The formation of bubbles and their subsequent dissolution have been studied experimentally. Various types of polymers have been examined, in an effort to identify the important material parameters affecting bubble formation and to elucidate the mechanisms involved. The evolution of density as a function of time during sinter-melting was measured experimentally, using a heating oven. The results revealed that the overall process consists of two stages: Particle coalescence, which depends on viscosity, surface tension and powder properties, occurs during the first stage, during which air pockets, which eventually become bubbles, are entrapped inside the melt. The second stage involves the shrinkage and eventual disappearance of the bubbles. The experimental results were compared to models commonly used in the ceramics, glass and metals processing literature for the densification of particulate compacts. Application of models based solely on viscosity and surface tension phenomena, can describe satisfactorily the process until the point where closed pores (bubbles) form. The latter stage of bubble dissolution has been addressed by modeling the dissolution of a single spherical bubble in an infinite polymer melt under isothermal conditions. The bubble dissolution model has been successfully applied to provide predictions of density as a function of time for the late stages of densification.

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