Reactive modification and crystallization behavior of foamed poly(butylene terephthalate)
Otto H. York Department of Chemical Engineering
Doctor of Philosophy
Gogos, Costas G.
Hyun, Kun S.
Todd, David B.
Knox, Dana E.
The objective of this study is to investigate the feasibility of preparing low density foams based on poly(butylene terephthalate) (PBT) through modification of its molecular structure and rheological characteristics by reactive extrusion. The scientific and/or engineering interpretation of the experimental data was based on fundamentals of extrusion, crystallization mechanisms, as well as information on reactivity and tendency toward degradation of PBT.
As a preliminary study, the extrusion foaming behavior of a commercial linear semi-crystalline PBT with a chemical blowing agent (CBA) was investigated. Foam characteristics were functions of extrusion operational variables, such as die set temperature and screw speed. Gas solubility and CBA decomposition kinetics were found to be key elements for controlling and optimizing the quality of the foamed polymer.
Initially, the linear PBT polymer was reactively modified using a tri-functional epoxide in a batch mixer to produce a branched structure with a higher molecular weight (MW) and a broader molecular weight distribution (MWD) as evidenced by rheological analysis. These experiments provided information on the PBT/modifier reaction kinetics. Chain branching was then carried out by single screw extrusion, where the competing degradative chain scission reaction needed to be taken into account.
Applications of fundamentals of extrusion theory to the reactive extrusion process enabled the optimization of the extrusion operational conditions. The optimization led to the production of a branched polymer with viscoelastic characteristics suitable for low density extrusion foaming by injection of a physical blowing agent (PBA).
The branched product made under the optimized conditions could be foamed to densities as low as 0.33 g/cc and showed high expansion ratio of 4.0 and cell structure resistant to coalescence. The possibility of overlap of the polymer crystal formation with the nucleation/growth of gaseous cells was investigated based on theoretical predictions. Slow pressure drop rate was shown to decrease cell nucleation rate. Fast crystallization rate stabilized cell morphology.
The occurrence of flow induced crystallization during bubble formation was investigated, to our knowledge, for the first time. Smaller size spherulitic structures at the bubble walls of the branched PBT compared with the linear polymer, suggested longer crystal nuclei lifetime, which may indicate that the formation of the crystalline structures was affected by the bi-axial deformation occurring during bubble growth.
njit-etd2007-020 (141 pages ~ 9,448 KB pdf)
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Created September 16, 2008