DMA Investigation of the Factors Influencing the Glass Transition in 3D Printed Specimens of Shape Memory Recycled PET

Abstract

Polyethylene terephthalate (PET) is used worldwide for packing, and for this reason, it is the main material in plastic waste. The paper uses granules of recycled PET (R-PET) as raw material for producing filaments for 3D printing, subsequently used for printing the test specimens in different ways: longitudinally and at angles between 10° and 40° in this direction. Both the filaments and the printed specimens experience thermally driven shape memory effect (SME) since they have been able to recover their straight shape during heating, after being bent to a certain angle, at room temperature (RT). SME could be reproduced three times, in the case of printed specimens, and was investigated by cinematographic analysis. Then, differential scanning calorimetry (DSC) was used, in R-PET granules, filaments and 3D printed specimens, to emphasize the existence of glass transition, which represents the governing mechanism of SME occurrence in thermoplastic polymers, as well as a recrystallization reaction. Subsequently, the paper investigated the 3D printed specimens by dynamic mechanical analysis (DMA) using a dual cantilever specimen holder. Temperature (DMA-TS) and isothermal scans (DMA-Izo) were performed, with the aim to discuss the variations of storage modulus and loss modulus with temperature and time, respectively.

Characterisation of the shape memory effect of PET polymer by FFF 3D printing

“…The sample deformation was made at room temperature in two testing cycles compared to the standard testing procedure. Shape memory specimens' cold deformation is documented in the literature [10,16,17]. The results show PET shape recovery is possible even after two cold deformation cycles.…”

“…Polyethene terephthalate (PET) is a semicrystalline thermoplastic material and is characterised by both amorphous (i.e., transparent) and crystalline (i.e., opaque) phases [11,12]. It is widely used in the food industry for food and liquid containers [13,14] and as feedstock for additive manufacturing technologies such as FFF as a new or recycled material [15,16].…”

On the Thermomechanical Behavior of 3D-Printed Specimens of Shape Memory R-PETG

“…The grains were processed by heating, extrusion into filament, deposition, and cooling. The entire technology, including the experimental line for 3D filament production, was previously described in detail [ 27 ]. Table S1 in the Supplementary Material provides the printer’s specifications.…”

“…Thermal analysis was performed using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) with a NETZSCH DSC 200 F3 Maia device (Netzsch, Selb, Germany) calibrated with Bi, In, Sn, Zn, and Hg standards and a NETZSCH DMA 242 Artemis device (Netzsch, Selb, Germany) equipped with a dual-cantilever specimen holder, respectively, using the previously detailed procedure [ 27 ]. The accuracy of the DSC device is given by the following parameters: digital resolution: 0.2 µW/digit; signal noise: 0.7 µW RMS at 130 °C (equivalent to 3.6 µW peak-to-peak); signal time constant: >2.5 s (indium melting peak); reproducibility onset temperature: +/−0.1 K (indium melting peak); and reproducibility of peak areas: +/−1% (indium melting peak).…”

“…To the best of the present authors’ knowledge, there are no reports on the study of SME in recycled PETG. Taking into account the huge recycling effort of PETG waste [ 26 ] and the previous results reported by some of the present authors on the shape memory properties of recycled PET (R-PET) [ 27 ], this article aims to investigate the performance of 3D-printed parts from filaments fabricated from this material.…”