AMS Spring 2023

Recycled PLA Shows Highly Variable Strength

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3D printing, as well as 4D printing, have opened up an ever-expanding realm of hardware, software, and unique methods for constructing complex geometries. Along with that though also comes a vast array of materials which is continually growing—but many researchers and engineers still use the old standbys like ABS and PLA.

Efforts to recycle 3D printed items are a constant source of study too, as researchers worry about the amount of plastic that could be left sitting in landfills, even if it is eventually biodegradable, as is the case with PLA. In ‘A comparison between mechanical properties of specimens 3D printed with virgin and recycled PLA,’ Italian researchers Antonio Lanzottia, Massimo Martorelli, Saverio Maietta, Salvatore Gerbino, Francesco Penta, and Antonio Gloria further explore the realities of using recycled PLA for functional parts.

An image of the filament extrusion

PLA is popular in comparison to ABS because it is a bio based polymer. The authors point out that composting the material is probably not a very realistic solution due to the amount of time it takes parts to degrade. But, what if we could recycle PLA? How feasible is this and how does this affect the material? What happens to mechanical strength of material that has been recycled—especially if it has been recycled repeatedly:

“Specifically, it has been proved that the use of a filament recycled twenty times through an extrusion-based process minimally affected the tensile strength and modulus of PLA,” state the researchers.  “In  addition, a study on recycled polypropylene blends in injection moulding procedure was performed and  an  appropriate  blending  ratio  of  virgin  and  recycled  polymer  was  assessed,  showing  that  the  decrease  in  the mechanical properties of devices fabricated from recycled polymers may be improved optimizing the process parameters during  the  injection moulding.”

Further studies also showed that weakening in mechanical properties was minimal in recycled PLA, motivating the authors to form an intense study comparing both virgin PLA and recycled PLA, testing both interlaminar properties and short-beam strength. PLA samples were printed at 200°C using a Prusa I3, with a .4 mm nozzle. The first set was tested, then ground up and recycled with a homemade extruder into 1.75 mm material. It was then used to make new samples for mechanical property testing.

Schematic representation of the experimental setup – horizontal shear load diagram (adapted from ASTM D2344)

The researchers included three different recycling phases, with testing for short-beam strength on both virgin and recycled material. They noted that the PLA recycled once and even twice over was not ‘significantly’ affected in short-beam strength, but after that it did experience substantial degradation. In the samples that had been recycled three times, there was ‘great variability.’

In conclusion, the researchers offered more specific data:

“The  one-time and twice recycled  specimens  showed  a  short-beam  strength  (106.8  ±  9.0  MPa  and  108.5  ±  9.9  MPa,  respectively)  which  was  similar  to  that  of  the  virgin  specimens  (119.1  ±  6.6  MPa).  However,  a  third  recycling  process  negatively  affected  the  values  of  the  short-beam  strength  also  producing  a  great  variability in the results (75.0 ± 16.2 MPa).”

3D printing, in existence since the 80s, has only just begun to really hit its stride—and the study of materials science has become of substantial interest to many, whether they are interested in using PLA or recycled materials to create items like energy storage devices, prosthetics, sustainable thermoplastics, and so much more. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

[Source / Images: A comparison between mechanical properties of specimens 3D printed with virgin and recycled PLA]

Results from mechanical tests: typical failure modes

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