Exploring The Processing Of HDPE-Exfoliated Graphene Nanocomposites For Fuel Tanks
21. August 2014
There were three best paper award winners just announced by the organizing committee of the SPE Automotive Composites Conference & Exhibition (ACCE) for its
fourteenth-annual show coming up Sept. 9-11, in Novi, Mich. I will be attending this event and, judging from the roster of papers to be presented, expect to further report on emerging developments, particularly in the thermoplastic composites arena.
One of the three winning papers, “Processing Methods of High Density Polyethylene-Exfoliated Graphene Nanoplatelet Nanocomposites for Automotive Fuel Tank Applications, is based on a study jointly conducted by the Composite Materials and Structures Center at Michigan State University and the Hyundai-Kia America Technical Center. Lead author Keith Honaker, a graduate student at Michigan State University and a 2013-2014 SPE ACCEE graduate scholarship award winner, will discuss how HDPE-exfoliated graphene (GnP) nanoplatelet composites were synthesized and tested.
In order to further enhance the mechanical and barrier properties of the composite, different processing techniques were explored including microlayer coextrusion and solution mixing with sonication followed by extrusion. The researchers investigated multiple modifications to the nanocomposite constituents including cryo-milling of the HDPE pellets and coating the platelets with a wax or polyolefin elastomer before extrusion.
Among the key results: simple melt mixing of HDPE and GnP resulted in an increase in stiffness, a decrease in Izod impact resistance, and a 50% decrease to both oxygen and fuel permeation with 5% wt. GnP. Meanwhile, microlayer coextrusion yielded a high alignement of the nanoplatelets in the direction of the flow and resulted in improved permeation resistance at low GnP concentrations, but did not result in improvement of barrier properties at concentrations above 5% wt.
In contrast, cryo-milling the HDPE pellets into a powder resulted in a minor decrease in mechanical properties and a 35% decrease in oxygen permeation. A wax coating on the platelets before melt extrusion resulted in an increase in both Izod impact resistance and barrier properties, but a decrease in flexural modulus. A polyolefin elastomeric coating on the GnP resulted in retaining the flexural modulus properties with only a slight improvement to Izod impact resistance and barrier properties.
Overall, the researchers concluded that obtaining outstanding barrier properties without affecting the mechanical properties of HDPE-GnP nanocomposites using melt mixing, extrusion and injection molding processes is very challenging. For this reason, the focus of their current research is on combining a few of these methods to create synergistic effects.
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