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08.12.10

 

Extrusion Foaming PHBV


Extract from the November blog of Plastics Engineering Magazine. Imerys is part of the consortium involved in this research.


 

Brunel's paper "Rheology and Extrusion Foaming of PHBV" by Damian Szegda reported on two years of research on extrusion foaming of PHBV. It is believed to be the first processing data on foamed PHBV not coming from a supplier of the resin. Metabolix Inc. in Cambridge, Mass. (www.metabolix.comwww.mirelplastics.com), dba Telles, which produces Mirel PHBV, published data on foaming it at the Biofoams 2009 conference in Toronto, sponsored by the University of Toronto.


PHBV has moisture barrier of 5-26 WVTR, HDT of up to 140 C, and is home compostable. Negative attributes are its narrow processing window, high crystallinity (58%), brittleness (1-1.30 Charpy Notched Impact, K/Jm3), slow crystallization, and high cost ($7.50-$10.50/lb depending on quantity). It also degrades readily above its melt temperature (168°C). Other quirks include low viscosity, sharp transition from solid to liquid phase, and room temperature (22°C) Tg.


Brunel used PHBV (ENMAT Y1000P) from Tianan Biologic Material Co. in Ningbo, China (www.tianen-enmat.com) and foamed it with an endothermic blowing agent (BA.F4.E MG) made by Adeka Palmarole SAS in Saint Louis, France (www.adeka-palmarole.com). The blowing agent is based on sodium bicarbonate and citric acid in an LLDPE carrier, which decomposes into CO2 and H2O. Brunel tested 1.25%, 2%, 2.5%, 5% and 7.5% blowing agent, achieving density reduction of 58% with 5% blowing agent. The optimum amount of blowing agent, however, was 2% because water released by the blowing agent causes degradation of the polymer.


Brunel also tested compounds of PHBV with 5%, 12% and 20% calcium carbonate as a nucleating agent, which reduced cell size and increased the population of cells. This improved foam quality, but didn't reduce density because of the high specific gravity of calcium carbonate.


Brunel used a 30-mm-diameter, 30:1 L/D Betol co-rotating twin screw extruder with five heating/cooling zones, melting the polymer in the first zones, then super cooling it below equilibrium melting temperature towards the die to avoid degrading the heat sensitive polymer and to increase melt strength. Tests were done with both a sheet and strand die.


The major difficulty was buildup of PHBV in the die because of the super cooling and because of stress induced crystallization in narrow parts of the die, Brunel reports. As PHBV built up, the accumulation altered processing conditions, increased die pressure and caused foam quality to deteriorate. "It was possible to extrude quality foams for only a limited amount of time," Brunel's Szegda says. "It's a lot harder to foam PHBV than PLA."  Brunel's research was supported by a research consortium that includes Sainsburys Supermarkets Ltd., recycling consultant Nextek Pty. Ltd., Wells Plastics Ltd., Imerys Minerals Ltd. and Sharp Interpack Ltd., all in the U.K.

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