Mechanical Interlocking of Polypropylene-Based 3D-Printed Structures with Polyethylene Terephthalate Glycol Inclusions

We report on enhancing the mechanical and structural characteristics of polypropylene (PP) three-dimensional (3D)-printed structures fabricated via fused filament fabrication (FFF) by employing hybrid PP-based filament containing a minute amount of polyethylene terephthalate glycol (PETG) inclusions. The improvement was obtained by increasing adhesion through rapid macromolecular interdiffusion between PETG inclusions, enriching the boundary. The inclusions serve as anchoring elements and enhance layer-to-layer adhesion via the interlocking mechanism. In our work, four different hybrid filaments containing 0.7, 1.4, 2.1%, and 3.5 vol % of PETG were produced via melt extrusion of the PP/PETG blend. The samples with 0.7-2.1% PETG had an average diameter of dispersed phase of about 1.2-1.4 μm. The size of PETG inclusions was noticeably larger for 3.5% PETG samples (∼1.9 μm). It was determined that introducing PETG into the PP matrix has practically no effect on the PP thermal transitions. Via X-ray photoelectron spectroscopy (XPS) measurements, we determined that the concentration of PETG material on the filament surface is 10-20 times higher than that in bulk. As the polyester content increases, the surface area occupied by the inclusions increases from approximately 7 to 55%. Our results show that utilization of the hybrid PP/PETG filaments significantly improves the printed parts' mechanical characteristics and sintering level compared to those made from pure PP filament. Specifically, incorporating 0.7% of PETG into the PP filament increased Young's modulus of the printed structures by 80%. Modulus increase by ∼55% was caused by adding 2.1 and 3.5% of PETG. For the yield stress, the addition of 0.7 and 2.1/3.5% of PETG led to an increase of the stress by about 50 and 30%, respectively. In general, the improvement of mechanical characteristics is higher for the lowest content of the PETG inclusions dispersed in the PP matrix. The result indicated that more efficient interfacial anchoring occurs at reduced PETG concentrations where the layer-to-layer contact is not overpopulated with the polyester domains.