Bubble, Bubble, Toil and Trouble

By Jan H. Schut

The latest in foaming technology announced at the Society of Plastics Engineers FOAMS 2010 topical conference this past September in Seattle includes two  new developments in foamed biopolymers. Scion Research (www.biopolymernetwork.com, www.scionresearch.com) in New Zealand described commercial scale production trials of expanded PLA beads (E-PLA) for the first time.  PHBV is a copolymer of polyhydroxybutyrate with 3% hydroxyvalerate, a natural polyester produced by fermentation of corn starch. Dow Chemical Co. (www.dow.com) also described its new  Styrofoam formulation for the first time since the phase-out of the ozone-depleting blowing agent HCFC-142b. Sulzer Chemtech AG in Switzerland (www.sulzerchemtech.com) is also working on a retrofittable system to convert a twin-screw PVC extruder to physical foaming (using CO2).

Commercial PLA pellets contain voids (photo 1), which disappear after impregnation with CO2 (photo 2), indicating that CO2 acts both as a blowing agent and plasticizer. Photos: Biopolymer Network/Scion

COMMERCIAL SCALE TRIALS OF EXPANDED PLA BEADS

Scion/Biopolymer Network Ltd. announced successful commercial-scale production runs of foamed PLA beads (E-PLA) for the first time. Scion’s paper “Expanded Polylactide (E-PLA): A Realistic Alternative to Expanded Polystyrene (EPS)” by Alan Fernyhough was the first look at industry scale trials done over the past two years at several commercial EPS molding manufacturers in New Zealand. The trials “demonstrated the potential of expanded PLA beads as a realistic alternative to EPS,” Scion says. One trial used a commercial EPS steam preformer in batch mode with a mold for an under-floor insulation block. The mold was filled and controlled automatically on commercial equipment and cooled before the part was removed.

The Biopolymer Network, a collaboration among three New Zealand government research groups, including Scion, has developed the technology over the past eight years and presented the process several times before since introducing it in 2007. The patent-applied-for technology (U.S. Pat. Application # 2006-0167122) is noteworthy for using conventional PLA beads as feedstock, where other E-PLA technologies described in patent literature require specialized higher melt strength PLA.

Scion’s process uses moderate temperature and pressure conditions with liquid CO2 as blowing agent, not supercritical CO2. The liquid CO2 acts as a plasticizer, lowering the Tg of the PLA and making it more ductile, Scion says. Interestingly, small voids and cracks observed in the PLA beads before impregnation disappear after impregnation. “It is likely that the polymer chains are moving or realigning as a result of interaction with CO2 under certain conditions,” Scion reports. Impregnation in a liquid also reportedly keeps the beads from sticking together.

The patent describes using several commercial grades of PLA from NatureWorks LLC, Minnetonka, MN (www.natureworksllc.com) including amorphous (PLA 4060D) and blends of 4060D with highly crystalline PLA (PLA 3001). There is 5-8 wt% CO2 in the final pellets. The patent also describes the addition of fillers to the PLA before pelletizing, which includes 10% or 20% calcium carbonate or talc to the PLA pellets, which can act as nucleating agents.  Trials were conducted at Expol Limited and Long Plastics facilities in New Zealand.

A competing process was developed by EPS producer Synbra Technology BV (www.synbra.com, www.biofoam.nl) in the Netherlands, which uses PLA produced by a process developed by Sulzer with Purac Biochem BV (www.purac.com) in the Netherlands. Synbra is in the process of scaling up commercialization of its BioFoam E-PLA now and is already supplying customers in the Netherlands, U.K. and Italy. Synbra’s beads are molded in its own commercial steam chambers and forming equipment. Synbra licenced the Sulzer/Purac process in 2008, but it requires a high melt strength PLA copolymer made with a combination of L and D lactide PLA monomers from Purac.

EXTRUSION FOAMING PHBV

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.com, www.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., and Sharp Interpack Ltd., all in the U.K.

REPLACING HCFC-142B

A paper on “Long Term Performance of Insulating Foams” by Stephane Costeux of Dow was voted best paper by a group of independent reviewers. It describes Dow’s patent-applied-for new formulation efforts (WO/2008/140892 A1) for Styrofoam extrusion-foamed PS boards after the phaseout of the blowing agent HCFC-142b. The Montreal Protocol in 1987 required all manufacturers to stop using HCFC-142b by Jan. 1, 2010.

Dow’s new formulation uses a polystyrene copolymer with acryolinitrile instead of PS homopolymer and a blowing agent based on zero-ozone-depleting HFC-134a with small amounts of CO2 and H2O. (The paper gave test results for samples made with ratios of 8/1.1/1 and 9/0.76/1 as illustrative examples.)  The paper also says that changing from PS homopolymer to a lower molecular weight PS copolymer with acryolinitrile achieved better long-term insulation (R value) performance, over 8.6% better compressive strength, over 5% higher flexural strength, and over 10 degrees C higher HDT, depending on the thickness of the XPS board. Cell size, dimensional stability, water absorption, UV protection and flame retardancy are comparable to the original PS Styrofoam, Dow says.

DEVELOPING A PVC FOAM RETROFIT

Sulzer is adapting its retrofittable Optifoam physical foaming device for PVC. Sulzer’s paper “Recent Developments in Foam Extrusion by the Use of Flexible Retrofit Components” by Christian Schlummer and Nick Ulicney, presented the developmental PVC version for the first time. Sulzer introduced the original Optifoam device in 2005 for high pressure injection molding and in 2008 for lower pressure extrusion. It consists of a spider-type block with a torpedo inside, attached to an extruder. Physical blowing agent (carbon dioxide or nitrogen) is infused into the melt from two sides as the melt passes between the torpedo and cylinder wall. Melt then goes through static mixers to dissolve the blowing agent into the polymer.

For PVC, Sulzer modified the torpedo section with a cooling jacket to lower the melt temperature and designed a special static mixer, called Polyguard, which has round mixing bars instead of flat plates, so there are no dead zones. The bars are also coated to keep polymer from sticking. The device attaches to a twin-screw extruder instead of single-screw. Sulzer’s PVC foaming trials were done on a 52-mm co-rotating twin-screw extruder with throughput of 130 lb/hr, using liquid CO2 as blowing agent. Nucleating agents were also used, as they would be with conventional PVC foaming. The line produced foam boards with density reductions of 25% to 59%, depending on the amount of blowing agent and the PVC formulation. Sulzer plans more trials to study die design and downstream equipment with Optifoam.

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