Evaluating Thermal Ageing of Recycled Bio-based Filament for 3D Printing as Function of Extruder Temperature

In this work, the effect of thermal ageing on printed parts, produced using Fused Deposition Modelling (FDM) technology, was investigated. The constituting material was made from a common bio-based thermoplastic (polylactide acid, PLA) derived from waste recycling of separate collection bio-bags. The filament was extruded in a 3D printing machine using optimized processing conditions and different extruder temperatures (190°C and 210°C). Specimens were aged for various days (up to 175) in a vacuum oven at 70°C and characterized using dynamic-mechanical analysis (DMA) and infrared spectroscopy in attenuated total reflection (ATR) modality. The findings were presented in terms of storage modulus (E') at 30°C and glass transition temperature (Tg) evaluated at various heat treatment periods (50, 70, 130, 175 days). Although samples printed at 190 and 210 °C had a different initial average storage modulus (about 20%), they came to be almost the same value after 175 days of heat treatment. However, in both cases, the storage modulus (E’) at 30°C showed a non-monotonous trend as a function of ageing period, reaching a maximum in correspondence of around 80-90 days. On the contrary, an increasing trend of glass transition temperature from 65 to 70°C was confirmed in two sample series by increasing the ageing time. These outcomes were attributed to changes in physical and chemical arrangements of polymer macromolecules that occurred during the thermal treatment. The remarkable differences in absorbance peaks in ATR spectra of aged specimens were interpreted as confirmation of chemical changes induced in the polymer chain structure by heat at different treatment times.