Synergistic effect of plasticizer and nucleating agent on crystallization behavior of polylactide during fused filament fabrication

Polylactide (PLA) materials manufactured by fused filament fabrication (FFF) technique are usually in amorphous form and thereby exhibit poor mechanical properties and thermal resistance. Overcoming this issue requires a good knowledge of the crystallization behavior of PLA materials during the FFF process. In this work, a plasticizer (polyethylene glycol, PEG) and a nucleating agent (tetramethylene-dicarboxylic dibenzoyl-hydrazide, TMC-306), were added in PLA matrix, separately and synergistically, to tailor crystallization behavior of FFF-printed PLA parts. At a bed temperature of 60 degrees C, PEG played a stronger effect on crystallization behavior than the nucleating agent TMC and even the combination of TMC and PEG. This resulted in an increase of the crystallinity from 8% for both neat PLA and PLA/TMC samples to 18% for the samples containing PEG. With the increase of the bed temperature to 90 degrees C, TMC and PEG separately or synergistically had prominent effects on enhancing the crystallization ability of PLA during the FFF process, leading to the highly crystalline PLA parts with the crystallinities in the range of 30-40%. By means of wide angle X-ray diffraction and scanning electron microscopy measurements, shear-induced crystal structures were indentified at the filament surface as well as the weld interface. Nevertheless, the shear-induced effect had negligible influence on the final crystallinity of PLA parts. Instead, PLA parts with a high crystallinity can be attained, only when the material characteristics and the printing conditions are in favor of cold crystallization of PLA. The unique crystallization behavior of FFF-printed PLA materials offers guidelines for the fabrication of products with controlled crystallinity and hierarchical structures for specific applications.