Effect of intrinsic viscosity on solid-state microcellular foaming of polyethylene terephthalate
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Krishna Nadella MicroGREEN Polymers, Inc., Arlington, Washington 98223
Vipin Kumara),b) Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195 (Received 12 February 2013; accepted 14 June 2013)
Microcellular foams, with cells of the order 10 lm, have been studied for over two decades. But little research has been done to study the effect of molecular weight on solid-state microcellular foaming. In this study, polyethylene terephthalate (PET) with a range of intrinsic viscosity (IV) 0.68–0.81 dL/g was used to investigate the effect of molecular weight on microcellular foaming process and resulting foam microstructures and properties. In the saturation step, IV showed negligible effect on sorption and desorption of CO2 in PET for all the conditions explored. In the foaming step, we found relative density increased with increasing IV. Also, as IV increased, cell size decreased and cell nucleation density increased. We hypothesize that lower chain mobility in higher IV samples led to more localized cell nucleation, resulting in a higher nucleation density, and also more constraints for cell growth, resulting in a smaller cell size. In addition, higher IV foams were found to have smaller skin thicknesses.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2013.191
the gas, driving the gas out of the polymer matrix and into nucleated bubbles. Molecular weight is known to influence mechanical properties of polymers.8,9 But little research has been done to examine its effect on the solid-state microcellular foaming process. Lee10 studied the effect of molecular weight on foam melt extrusion process and found that the cell size and degree of foaming decreased with increasing molecular weight. Stafford11 studied the effect of molecular weight on supercritical CO2 foaming of polystyrene and found that molecular weight did not significantly affect cell size and cell densities. However, the effects of molecular weight on each step of microcellular foaming and resulting foam properties are still largely unexplored. PET has good strength, ductility, gas barrier properties, and food compatibility. As the most recycled plastic, it has been widely used for various packaging applications. The sponsor of this research, which makes microcellular products from recycled PET, was concerned with the variability of intrinsic viscosity (IV) in incoming recycled sheets. The PET recycling stream is mainly from water bottles and soft drink bottles, in which IV typically ranges from 0.7 to 0.85 dL/g.12 IV range investigated in this study (0.68–0.81 dL/g) reflected the typical IV variations in recycled PET stream. In this paper, we investigated the effect of IV (an indirect measure of molecular weight) on sorption, desorption, and foaming o
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