Exploring thermal annealing and graphene-carbon nanotube additives to enhance crystallinity, thermal, electrical and tensile properties of aged poly(lactic) acid-based filament for 3D printing

The shelf life performance of polylactic acid (PLA) filaments for 3D printing is limited by aging mechanisms in terms of durability. In this work, the aged PLA and composite filaments filled with 12 wt% of graphene nanoplatelets (GNP), multiwall carbon nanotubes (MWCNT), and their hybrid mixture (1:1) were studied, after 24 months storage in a laboratory environment. Solid annealing at 80 °C for 4 h and pre-melt annealing at 120 °C for 3 h were applied in order to improve the performances of the aged filaments. It was found that the graphene-carbon nanotube fillers enhance the crystallinity, thermal stability, electrical conductivity and tensile Young's modulus, along with reduced tensile strength, elongation and toughness, compared to the neat PLA. The annealing was found efficient to substantially improve mechanical, thermal and electrical properties of the aged PLA-based composite filaments however, the annealing temperature have to be tuned according to the type of carbon nanofiller and the target properties.