3D-Printed Bioplastics Analyzed for Material Defects & Degradation

Researchers from Poland and Spain seek more answers in the realm of materials science, releasing their findings in ‘Three-Dimensional Printed PLA and PLA/PHA Dumbbell-Shaped Specimens: Material Defects and Their Impact on Degradation Behavior.’

Exploring how processing conditions impact PLA in medical applications, the authors present new research on how experimental samples from their study reacted to long-term hydrolytic degradation. PLA is one of the most commonly used polymers in 3D printing. While PLA is plant-based, non-toxic, biodegradable, and potentially biocompatible, it also deforms easily at low temperatures. Polyhydroxyalkanoates (PHAs) offer many of the same benefits and are being used in regenerative medicine, fabrication of organs, and tissue.

Both PLA and PLA/PHA filaments were used in the study to create dumbbell samples:

“The PHA component in the PLA/PHA blend mainly contains 3-hydroxybutyrate units and a small number of 3-hydroxyvalerate units,” said the researchers. “However, it was not possible to differentiate/ascertain whether the PHA component is a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) or a poly(3-hydroxybutyrate) (PHB)/PHBV blend.”

A Flashforge Dreamer dual extrusion 3D printer was used to create samples, in triplicate:

“Specimens with unexpected shrinkage phenomena after post-processing heat treatment were prepared according to [4,15] with two different processing build directions, one with a horizontal (flat, XY plane) processing build direction and with raster angle (45°/−45°) and a second with a vertical (upright, ZX plane) processing build direction and with raster angle (90°/0°),” explained the researchers.

“For additional degradation experiments in which an attempt was made to find out the reasons for the shrinkage phenomena after post-processing heat treatment, the dumbbell-shaped specimens (whole and cut specimen) were incubated in the same conditions as before for a period of 7 days only, since the shrinkage had to occur in the initial stage of degradation after applying the elevated temperature.”

Macrographic images of the polylactide (PLA) and PLA/polyhydroxyalkanoate (PHA) dumbbell-shaped specimens obtained by 3D printing in horizontal (H) and vertical (V) directions before (A and C) and after 70 days of hydrolytic degradation test at 70 °C (B—the whole specimens after degradation, D—cut in half specimens after degradation).

Macro- and micro-observations were performed both before and after the degradation process. The researchers noted ‘unexpected shrinkage phenomena’ in the form of shrinkage and warping during post-processing—for both types of material samples.

“Usually, PLA-based material disintegrates already at the initial stage of hydrolytic degradation (at 70 °C after 7 days),” stated the researchers. “However, there was no significant disintegration in this case. In view of the fact that it is a quite significant defect, an attempt was made to find the reasons for this phenomenon and to examine its effect on the degradation process.”

Selected SEM micrographs (200×) of the upper (UH) and underside (BH) layers of PLA and PLA/PHA dumbbell-shaped specimens obtained by 3D printing in horizontal (H) and vertical (V) directions before (A) and after 70 days of hydrolytic degradation test at 70 °C (B) the whole specimens after degradation, (C) cut in half specimens after degradation).

“No shrinkage phenomena were observed for the PLA/PHA dumbbell-shaped specimen obtained by 3D printing in the horizontal direction (cut in half),” said the researchers. “The other specimens underwent contraction. In the case of PLA/PHA dumbbell-shaped specimens obtained by 3D printing in the horizontal direction (whole specimens), additional warping (twisting) of the specimens was observed, while additional stress caused by cutting the specimens in half resulted in quite different effects after degradation.”

Average filament width of the upper (UH) and underside (BH) layers of PLA and PLA/PHA dumbbell-shaped specimens obtained by 3D printing in horizontal (H) and vertical (V) directions before and after 70 days of hydrolytic degradation test at 70 °C.

For the PLA/PHA dumbbell-shaped specimen obtained by 3D printing in the vertical direction, as well as the sample 3D printed horizontally, the researchers noted that loss of filament could only be attributed to the degradation process. Contraction for the specimens 3D printed in the vertical direction was 51 percent, and for those printed horizontally, 40 percent.

Overlay of selected gel permeation chromatography (GPC) chromatograms of PLA and PLA/PHA dumbbell-shaped specimens obtained by 3D printing in horizontal (H) and vertical (V) directions after 70 days of degradation at 70 °C (column with a linear range up to Mw = 25,000 g/mol). A small picture shows the chromatogram of the material before degradation (two columns with a linear range of Mw = 200–2,000,000 g/mol).

“The shrinkage phenomena observed during the degradation experiment as a defect turned out to be significant and repeatable at a temperature of 110 °C. The unexpected shrinkage phenomena after post-processing heat treatment, occurred in both non-conditioned as well as in conditioned specimens, under specific environmental conditions (hydrolytic degradation at 70 °C and conditioning at 110 °C),” concluded the researchers.

“Cutting the specimen in half disrupted its orientation, which resulted in a behavior change during degradation experiments. Furthermore, shrinkage phenomena caused an increased loss of molar mass during the degradation experiment, while on the other hand, cutting the dumbbell-shaped specimens in half led to an increase in molar-mass dispersity. The observation of unexpected shrinkage phenomena after post-processing heat treatment in the case of materials made of PLA or with its addition, which are of great importance in specialized applications, especially with potential for biomedical use, is a quite significant material defect.”

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