Browsing by Author "Sarver, Joseph Arron"

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    Linking Thermophysical Transitions and Rheological Properties to Polymer Foaming Outcomes with Carbon Dioxide
    Sarver, Joseph Arron (Virginia Tech, 2022-06-01)
    Interest in high-pressure and supercritical fluids as physical blowing agents for polymer foaming is driving a renewed need for the fundamental understanding of polymer thermophysical and rheological properties in the presence of dense fluids. In particular, carbon dioxide is often studied as a physical blowing agent because of its readily accessible critical point (31.1 ℃ and 73.8 MPa) and relatively high solubility levels in polymer materials. The basic principle involved is to dissolve the supercritical fluid in the polymer at high pressures and then impose a pressure reduction to initiate bubble nucleation and growth. The outcomes depend on the thermophysical and rheological properties of the polymer under the prevailing process conditions in the fluid. The present dissertation explores the high-pressure characterization and foaming of thermoplastic elastomers and seeks to link polymer thermophysical and rheological properties to polymer foaming outcomes with carbon dioxide as a physical blowing agent. A major focus of this dissertation has been the development of novel high-pressure characterization techniques to understand polymer behavior at high pressure. These techniques include (1) high-pressure torsional braid analysis (HP-TBA), (2) magnetic suspension balance (MSB), and (3) unique high-pressure batch foaming cells. HP-TBA allows for the assessment of the depression in thermal transitions (Tg and/or Tm/Tc) and the changes in rheological properties like modulus or rigidity of polymer systems exposed to carbon dioxide. MSB provides for the assessment of the amount of carbon dioxide that sorbs into a polymer material at a given temperature and pressure. Unique confined foaming strategies have been developed to translate information learned from batch-scale experimentation to practical industrial applications. The polymer systems of interest are thermoplastic elastomers including poly(ethylene-co-vinyl acetate) (EVA) and poly(ethylene-co-vinyl acetate-co-carbon monoxide) (EVACO). These materials find use in numerous commercial applications including adhesives, compatibilizers, and foams. Their foams are noted to undergo significant degrees of expansion followed by unfavorable post-foaming collapse. In the first part of this study, the foaming of neat EVACO and EVA with carbon dioxide was explored. The blending of these polymers was then explored to regulate foam expansions and control the pore morphology development. The foamability of the polymers and their blends was explored under both isothermal and gradient conditions to assess the temperature effects on foaming outcomes at a given pressure. In the second part of this study foaming of EVACO was explored in relationship to the depressed thermal transitions of the polymer in the presence of carbon dioxide. Accompanying the depressed melting transition is a sharp reduction in the modulus or rigidity of the polymer material. By studying foaming outcomes near the melting transition rational windows for foaming exploration can be evaluated to generate foams that display more favorable bulk foam densities and minimal foam collapse. This part demonstrates that linking foaming conditions to the relative rigidity or melt strength of EVACO in carbon dioxide allows for the determination of the lower pressure where foaming will occur and the upper pressure beyond which further foam density reductions are not significant. The third part of this study explores the foaming of EVACO with carbon dioxide under batch, confined foaming conditions where the foam expansion is restricted in order to again control the foaming outcomes and prevent foam collapse. A practical question is the scale-up of batch foaming processes which likely will be conducted with injection molding or extrusion type processes. Studying batch foaming in confinement allows for a better understanding of the factors that may affect foam development that may be more readily translated to industrial practice. The fourth part of this study examines the role of crystallinity and block copolymer composition in altering the polymer behavior in carbon dioxide. Several EVACO polymers with varying ethylene, vinyl acetate, and carbon monoxide content have been explored to study how block copolymer composition affects the thermophysical and rheological properties along with the sorption of carbon dioxide at high pressure.