2, University of Tasmania, Hobart, Tasmania, Australia
An exploration of poly(lactic acid) (PLA), high density polyethyle (HDPE), and low density polyethylene (LDPE) composites containing liquid crystalline graphene oxide (LCGO), edge functionalised graphene (EFG), and laponite in a range of concentrations in both single and dual filler combinations was undertaken. These were extruded as composite filament and tested as feed material for fused deposition modelling (FDM).
Printable polymer nanocomposites have the potential to widen the scope for products made by FDM by improving physical properties. Graphene has been shown to provide improvements in PLA modulus, thermal stability, and tensile modulus [1,2]. LCGO has been developed in-house, and has shown great potential to improve the mechanical, thermal, and electrical properties of polymers . In this study nanocomposites ranging from 0.1wt%-10wt% filler were extruded using a customisable counter-rotating mini twin screw extruder. As graphene has a tendancy to either form bonds with itself that cannot be broken during extrusion and may also restack to form graphite , separation of LCGO and EFG from itself prior to and during the filler addition is key.
Powdered polymer (HDPE/LDPE/PLA) was added to the first open port of the extruder. Once the polymer was molten, a dispersion of fillers (LCGO/EFG/laponite) in an aqueous solution was syringe pumped directly into the melted polymer in a second port along the extrusion screw. As each drop of solution was added the melt, the water evaporated and the filler was taken in to the polymer. This method prevents a lot of the filler-filler contact that occurs in typical powder coating methods, which in turn keeps agglomerate size low. Solution concentrations were up to 10mg/mL, added at at 6mL/min.
LCGO was made on site at UOW , to approximately 9mg/ml (aq. dispersion). EFG was made in house at UOW. Laponite was sourced from BYK Additives and Instruments (Germany). PLA was sourced from Filabot (USA), HDPE from Sigma-Aldrich (USA), LDPE from Visy (Australia). Each additive was diluted to 0.1, 0.5, 1, 2, 5 and 10mg/mL in an aqueous solution for both single and dual filler composites.
Direct solution additon mid extrusion has shown to generate average agglomerate size range from <500nm to 50μm. It is expected that the upper limit will lower to 25μm with a slower addition of filler.
Mechanical strength was seen to improve by 20% in composites of PLA/EFG at 8wt%, and by 10% in dual filler composites PLA/EFG/LCGO at 2wt%.
Dripping water on to the polymer melt did not lower the degradation temperature of any of the polymer, indicating it evaporates off immediately as intended.
The authors thank the Australian National Fabrication Facility - Materials Node for their provision of research facilities, acknowledge the use of facilities within UOW Electron Microscopy Centre and funding provided by the ARC Centre of Excellence for Electromaterials Science.
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