Colored PETG Filaments: Effects of Pigment on the Mechanical Properties of Heat-Treated FFF Parts ^†

The good printability, flexibility, and durability of Poly (ethylene terephthalate glycol) (PETG) make this thermoplastic an attractive material for various applications in Fused Filament Fabrication (FFF). However, its natural properties and the printing parameters setting are not always enough to ensure the quality criteria required for FFF products, especially considering mechanical strength. Often, the solution starts with the use of thermal post-processing to reorganize the polymer’s structure and the meso-structure of the manufactured components. Even being an amorphous polymer, the use of heat treatments (HTs) in parts printed on PETG is plausible but still little explored scientifically, especially when printing with pigmented filaments. In this sense, a study was carried out to identify the effects of four colored PETG filaments (white, blue, red, and orange), from the same supplier, on the flexural mechanical properties of FFF models when subjected to two conditions: (i) annealing (1 h at 50 °C plus 1 h at 80 °C); and (ii) reprocessing in a starch bed (20 min at 200 °C). To control the experiments, a group of samples of each color was evaluated right after printing, with no HT. Under all conditions, the samples were manufactured and post-treated with isoparametric configurations. The filaments were further characterized by Fourier Transform Infrared Spectroscopy (FTIR) and density measurement. The presence of pigments did not affect its molecular structure and density.

On the other hand, it significantly influenced the maximum flexural strength (MFS) and flexural modulus (FM) of the parts not exposed to HT. In all colors, the annealing promoted an increase in MFS compared to freshly printed parts. The FM was not affected by annealing in the comparisons among colors and for the same color with its respective value right after printing, with no HT. The thermal aggressiveness of reprocessing in starch bed promoted considerable drops in the mechanical properties of all PETGs due to thermal degradation. The study found that the presence of pigments makes the mechanical behavior of parts unpredictable under printing and post-treatment isoparametric conditions. Additives can change the rheology and transition and degradation temperatures of PETG, requiring specific settings for each color. Annealing was the most promising treatment for PETG, but its efficiency should be tested in future studies with optimized parameters, depending on the color of the filaments.