Foaming thermoplastics: Heterogeneous nucleation and extensional rheology

The continued growth of new applications for thermoplastic foams depends on the ability to enhance control over the cellular structure. The goal of this research is to improve the understanding of cell nucleation and growth, which together determine the cellular structure and physical properties of the foamed material. In this study, two processing methods are used to accomplish this goal: batch and continuous foaming.; The batch process is used with an emphasis on cell nucleation, aiming to determine the effectiveness of traditional and novel nucleants. To nucleate the 1 billion or more cells/cm3 needed to produce microcellular foam, an overview of nucleation theory points to the need for an even dispersion of very small nucleants. Uniformly distributed spherical sites in the form of block copolymer micelles are explored as nucleants in the batch foaming of polystyrene. Polystyrene-based block copolymers with either a poly(ethylene propylene) or poly(methyl methacrylate) block were not able to increase the cell concentration. But polystyrene-b-poly(dimethylsiloxane) diblocks showed a doubling of the cell concentration. The increased solubility of carbon dioxide in poly(dimethylsiloxane), and the reduced surface tension of PDMS are key factors in improving the cell concentration.; The continuous foam extrusion process was developed and used with an emphasis on understanding cell growth and coalescence. Rheological properties, particularly strain hardening during extension, can have a significant impact on cell growth. Foams with lower bulk density and higher cell concentrations are obtained by using blends of linear and branched polypropylene as compared to either the linear or branched homopolymer. This synergistic effect is attributed to both strain hardening in the branched polypropylenes, as well as a lower temperature for the onset of crystallization in the linear polypropylene. Strain hardening stabilizes growing cells, whereas a delayed onset of crystallization allows for increased nucleation. Cell nucleation and growth together determine the final foam structure. This work has also been extended to the foaming of thermoplastic elastomers containing these polypropylenes as their thermoplastic phase. Thermoplastic elastomers containing 25% branched polypropylene in their thermoplastic phase foamed better than formulations containing only linear polypropylene.