Focus on Improving PLA Mechanical Properties with Addition of Poly(3-Hydroxybutyrate)

In ‘Improving Mechanical Properties for Extrusion-Based Additive Manufacturing of Poly(Lactic Acid) by Annealing and blending with Poly(3-Hydroxybutyrate),’ researchers from Ghent University in Belgium, and Sichuan University in China, explore new ways to refine 3D printing with PLA.

While pointing out the innovations that have been produced via polymers thus far, the authors also explain that for the use of extrusion in additive manufacturing processes, there is a lack of polymers that are commercially available.

(a) Processing scheme for PLA (poly(lactic acid))/PHB (poly(3-hydroxybutyrate)) filament fabrication prior to printing; (b) printing orientation scheme.

Previous researchers have studied the potential for ABS and PLA, as well as many other thermoplastics; however, one of the greatest limitations found in parts produced with such materials lies in ‘unsatisfactory’ mechanical properties. There are three major ways to improve performance, however:

• Reinforced and strengthened fibers and fillers
• Polymer annealing
• Blending with a nucleating agent or polymers like polyhydroxyalkanoates (PHAs)

No matter how performance is optimized though, users must consider both layer and infill issues. In this study overall, the researchers continued evaluating how to improve PLA with a list of techniques, to include:

• Scanning calorimetry (DSC)
• X-ray diffraction (XRD) analysis
• Polarizing microscope (POM)
• Scanning electron microscopy

“Notably, PLA and PHB are bio-degradable; thus, the material can freely enter the eco-cycle. Hence, the present study closed the gap in applying conventional processing optimizations for the field of 3D printing, taking into account sustainability,” explained the researchers.

Printing parameters

Scheme 1. (a) Processing scheme for PLA (poly(lactic acid))/PHB (poly(3-hydroxybutyrate)) filament fabrication prior to printing; (b) printing orientation scheme.

Both dog bones (type 1BA, ISO527) and impact bars (100 × 10 × 4 mm, ISO 179) were created out of both virgin and blended PLA. The researchers fabricated the samples on a Felix 3.0 3D printer, noting a recrystallization peak at high temperatures, as well as opaqueness in the PLA bars after annealing. The team was also optimistic that displayed crystal banks could be ‘beneficial’ to interfacial strength, improving impact strength.

“The annealed PLA samples were characterized by strong reflection peaks of (200/110) and (203) at 17.3° and 19.6°, respectively, that are consistent with the increases seen with DSC (X_c > 50% for annealed samples). The peak values were also consistent with values reported in other studies and were indicative of a typical orthorhombic crystal. Weak reflections at 15.8° and 25.3° belonging to the (010) and (116) plane were also observed,” stated the researchers.

Increased roughness was found in the fracture surfaces of PLA/PHB—in comparison to neat PLA. Post-annealing, it remained coarse. They also explained that creating a composite in combining PHB and PLA resulted in the ability to print at a lower temperature and see refined toughness, as well as a Vicat softening temperature.

“PHB was thus a good supplement to enhance the flowability and thermal properties of PLA. The PLA/PHB blend showed a lower tensile modulus and stress compared to PLA alone due to the intrinsic lower stiffness of PHB and the rather poor miscibility between PLA and PHB,” concluded the researchers.

“Meanwhile, the strain at break increased upon the introduction of a second phase. The modulus and strength of the PLA/PHB blend improved considerably after annealing, due to the strengthened structure of the PLA phase promoted by annealing-mediated crystallization. The presence of PHB did improve PLA’s VST, which is maintained at a high level for the PLA/PHB blend before and after annealing through the facilitation of crystallization in the blend by PHB.”

While the area of materials science continues to expand voluminously, the basics like ABS and PLA are still extremely popular, with researchers and users around the world continuing to explore projects using composites, testing a variety of different patterns and features, creating new filaments, and 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.

(a) Thermal curves, (b) Selected XRD curves, (c) Selected SEM images of PLA/PHB samples before and after annealing (between 0.5 and 2 h; 80 and 100 °C)

(a) Modulus, (b) Tensile stress at maximum load, (c) Tensile strain at break (d) Notched impact strength and VST for PLA/PHB (poly(3-hydroxybutyrate)) blends, shown with the results of neat PLA, black square in (a–c), red points in (d).