Tsinghua University Investigates if Blends of PLA and PBS are Suitable for FDM 3D Printing

A trio of researchers from Tsinghua University in Beijing recently published a paper, titled “Preparation and Characterization of Poly(butylene succinate)/Polylactide Blends for Fused Deposition Modeling 3D Printing,” about preparing material blends of PLA and PBS with various compositions, then validating if they are suitable for use as filaments for FDM 3D printing.

The abstract reads, “To obtain a new type of biodegradable material with high toughness and strength used for fused deposition modeling (FDM) printing, a series of poly(butylene succinate) (PBS)-based polymer materials was prepared via blending with polylactide (PLA). The rheological, thermal, and mechanical properties as well as FDM printing performances of the blends, such as distortion, cross section, and the interlayer bond strength, were characterized. The results show that with increasing PLA content, the blends possess higher melt viscosity, larger tensile strength, and modulus, which are more suitable for FDM printing. Especially, when the content of PLA is more than 40%, distortion due to residual stress caused by volume shrinkage disappears during the printing process and thus products with good dimensional accuracy and pearl-like gloss are obtained. The results demonstrate that the blend compositions with moderate viscosity, low degree of crystallinity, and high modulus are more suitable for FDM printing. Compared with the low elongation upon breaking of commercially FDM-printed material, the PBS/PLA blend materials exhibit a typical ductile behavior with elongation of 90−300%. Therefore, besides biodegradability, the PBS/PLA blends present excellent mechanical properties and suitability as materials for FDM printing. In addition, our study is expected to provide methods for valuating the suitability of whether a thermoplastic polymer material is suitable for FDM printing or not.”

When it comes to prototyping, FDM is one of the most widely adopted technologies, and plenty of materials research has been conducted for the technology. Researchers have been working hard to develop new polymer materials for FDM 3D printing with both high dimensional accuracy and good mechanical properties. PLA, which theoretically can be degraded into just carbon dioxide and water under natural conditions, is often used, but it’s unfortunately a brittle material, which limits its applications.

PBS, with great thermal stability, has a decently low melting point and excellent ductility, which would make it good for FDM 3D printing. But, there haven’t been a lot of studies published on the use of the material as a 3D printing filament.

“One reason is that its low melt strength makes it difficult to continually form monofilament when extruded, which makes printing fail halfway,” the researchers explained. “Moreover, the distortion caused by the relatively large volume shrinkage during cooling probably happens after crystallization, thus resulting in defective products. Therefore, modification of PBS is quite necessary to solve the drawbacks mentioned above and make the material suitable for FDM printing.”

By blending materials, the advantages of these two components can be combined – that’s why this modification method is used so often for polymer materials. There is little research about the use of PBS blends in FDM 3D printing, so the Tsinghua research team stepped up.

“The rheological, thermal, and mechanical properties of the blends were investigated, and different specimens were printed with these filaments to evaluate their suitability for FDM system,” the researchers wrote. “Interlayer bond strength in the printed products was also measured. Furthermore, we expect to find a relationship between the properties of materials and the performance of FDM printing so as to give a reference for judging whether a thermoplastic polymer material, not limited to polymer blends, is suitable for FDM printing or not.”

The team first dried PBS and PLA pellets at 65°C for 12 hours in a vacuum oven before processing them and extruding the blended pellets into filaments for FDM 3D printing.  In addition to a few other shapes, like a rabbit, a cuboid model was printed to show distortion, which can be an obstacle to overcome in FDM.

The shear viscosity of the polymer blend melt was measured, along with the thermal properties, such as glass transition temperatures. The researchers also injection-molded the polymer blend pellets to make dumbbell-shaped and cuboid bars for tensile and impact tests, in addition to performing a thermal analysis on these bars to “investigate the effect of FDM printing process on the crystallization behavior of the PBS/PLA blends.”

“All blends exhibit excellent processing properties and can be extruded as monofilaments with 1.75 mm diameter via a single-screw extruder. With increasing PBS content, the elongation at break and impact strength of the blends arise,” the researchers explained. “However, distortion of the printed bars increases due to larger volume shrinkage resulting from the higher degree of crystallinity in the blends. In addition, the interlayer bond strength improves due to the decreased melt viscosity. When PLA content in the blends is not less than 40 wt %, FDM printing can proceed smoothly with neither observable distortion nor detachment from the platform at room temperature.”

The paper also states that PBS60/PLA40 and PBS40/PLA60, in terms of interlayer bond strength, material toughness, and distortion, are the “optimal blend compositions” for use in FDM 3D printing.

“Therefore, with pearl-like gloss and good mechanical properties as well as dimensional accuracy, the bio-based PBS/PLA blends are new promising materials for producing FDM filaments for applications in many fields, especially for architectural design,” the researchers concluded. “Furthermore, our study is expected to provide methods for evaluating whether a thermoplastic polymer material is suitable for FDM printing or not.”

Co-authors of the paper are Qing Ou-Yang, Baohua Guo, and Jun Xu.

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