Rheology of Foaming Polymers and Its Influence on Microcellular Processing

Links

URL: http://hdl.handle.net/1807/19107

Abstract

The rheological properties of polymer melts and polymer/blowing agent (BA) solutions are determined experimentally and the influences of material rheological properties and crystallization on low-density foaming behaviour of polylactic acid (PLA) are investigated. Understanding the rheological properties of foaming polymers allows the optimization of polymer chemical structure and the development of technologies that produce desired cell morphologies.

Although the technology for producing CO₂-blown polystyrene (PS) foams is well established, the rheological properties of a PS/CO₂ solution, especially its extensional property, are not well understood. In this study, these properties are determined with an in-house developed, online technique, and the measured data are compared with those from commercial rheometers. The online measurement system consists of a tandem foam extrusion system and a die for measuring pressure drops. Shear viscosity is determined from the pressure drop over a straight rectangular channel, while planar extensional viscosity from the pressure drop over a thin hyperbolic channel, taking into account the pressure drop due to shearing. Measured viscosities of the polystyrene without CO₂ compare well with those from commercial rheometers. With the presence of dissolved CO₂, both the shear and extensional viscosities of the polystyrene are significantly reduced. The influence of CO₂ on the two viscosities is found to be similar to an increase of temperature.

Polylactic acid is the first mass-produced biodegradable polymer, and has potential to replace petroleum-based polymers in foaming applications. In this study, the influences of material rheological properties and crystallization on the low-density, microcellular extrusion foaming behaviour of polylactic acids (PLAs) are investigated. Comparisons are made between linear and branched PLAs and between amorphous and crystalline PLAs. The branched PLAs are found to produce foams with higher expansion ratios and reduced open-cell content compared to the linear PLA. The foaming behaviour of the linear PLA, then, is significantly improved by adding a small amount of long-chain-branched PLA. The improved cell structure with branched PLAs is attributed to their relatively high melt strength and strain to break. For the first time, it is shown that crystallization, induced by cooling and macroscopic flow during processing, increases melt strength, which aids the production of low-density foams.

Cited Works (7)

Year	Entry
1996	Park CB, Suh NP. Filamentary extrusion of microcellular polymers using a rapid decompressive element. Polym Eng Sci. January 1996;36(1):34-48.
1998	Park CB, Behravesh AH, Venter RD. Low density microcellular foam processing in extrusion using CO₂. Polym Eng Sci. November 1998;38(11):1812-1823.
2008	Li YGG. Development of a Novel Visualization and Measurement Apparatus for the PVT Behaviours of Polymer/gas Solutions [PhD thesis]. University of Toronto; 2008.
2002	Naguib HE, Park CB, Panzer U, Reichelt N. Strategies for achieving ultra low-density polypropylene foams. Polym Eng Sci. July 2002;42(7):1481-1492.
1980	Ferry JD. Viscoelastic Properties of Polymers. 3rd ed. New York, NY: John Wiley & Sons; 1980.
2003	Xu X, Park CB, Xu D, Pop-Iliev R. Effects of die geometry on cell nucleation of PS foams blown with CO₂. Polym Eng Sci. July 2003;43(7):1378-1390.
1993	Langer R, Vacanti JP. Tissue engineering. Science. May 14, 1993;260(5110):920-926.

Cited By (3)

Year	Entry
2012	Chen N. The Effects of Crosslinking on Foaming of EVA [PhD thesis]. University of Toronto; 2012.
2016	Ding W. Development of Polylactic Acid/cellulose Nanofiber Biocomposite Foams [PhD thesis]. University of Toronto; 2016.
2018	Romero Diez S. Crystallization Study of Polymers Under High Pressure Gas / Supercritical Fluid [Master's thesis]. University of Vermont; October 2018.