Recycling Plastic Bags into Filament: Consideration of Emissions during Melting, Extrusion, and 3-D Printing

Billions of plastic bags (PBs) are consumed per day and are discarded after use. One way to reduce PB waste is distributed recycling and conversion into filament for 3-D printing as part of a circular economy, though little is understood about emissions during these processes. Herein, a "green" method was used to mix high-density polyethylene (HDPE) PBs from South Africa or the United States with virgin HDPE and extrude into filaments that were used to 3-D print tensile test specimens. Particle- and gas-phase emissions were measured throughout processing. On a particle number basis, during filament production, emissions mostly had sizes 1.2–5.2 nm, whereas during 3-D printing, emissions were mostly 5.6–560 nm. Particle yields (no./g plastic processed) were significantly (2–3 orders of magnitude) higher during 3-D printing compared with filament making. Acetone (range: 1.4–39.4 μg/g printed) and formaldehyde (range: 9.9–16.1 μg/g printed), the latter a potential occupational carcinogen, were released during 3-D printing. The Young's modulus of test specimens was comparable to literature values for 3-D-printed virgin and recycled HDPE. Recycling waste PBs into filament for 3-D printing has myriad sustainability benefits, though the potential for human exposures to particles and gases is an important consideration for future life cycle analyses.