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.

Different fragments of martensitic Cu-14.86 Zn-5.81 Al-0.5 Fe (mass.%) shape memory alloy were subjected to heating, up to 453 K, with different rates ranging from 1.66 ×10⁻² K s⁻¹ to 54.6 × 1.66 × 10⁻² K s⁻¹, performed by means of a differential scanning calorimeter. In all cases, during heating, an endothermic peak was observed which was associated with the martensite reversion to parent phase. By means of the differential scanning calorimeter charts the critical transformation temperatures of martensite reversion were determined using the tangent method. The effects of heating rate were evaluated from the point of view: (i) of variation tendencies of critical transformation temperatures; (ii) of deviations of experimental values from linear fit and extrapolation to zero heating rate; and (iii) of corroborating morphological changes of martensite (sub)plates with heat flow variation particularities. The results prove that there is an obvious tendency of critical transformation temperatures, of reverse martensitic transformation, to linearly increase with heating rate. The effectiveness of the linear relationships was checked for two heating rate values located inside and outside the above mentioned range, respectively and the difference between the experimental and calculated values of critical transformation temperatures fell within the range (−3 … +4) ‰.

The reproducibility of thermally induced reversible martensitic transformation, occurring in a Cu–15Zn–6Al (mass-) shape memory alloy during heating, was studied on three different fragments of martensitic alloy subjected to complex thermal cycling comprising four series of five heating–cooling cycles, applied on a differential scanning calorimetry (DSC) device, between room temperature (RT) and three maximum temperatures: 453, 463 and 473 K respectively. After each series of heating–cooling cycles the fragments were naturally aged at RT for 37 days (∼3·2 Ms). Thermograms of DSC revealed a gradual diminution in the magnitude of martensite reversion to parent phase, accompanied by decreasing tendencies of both critical temperatures and transformation rates, which reflect a gradual loss of thermal memory, associated with ‘amnesia’ occurrence. Scanning electron microscopy observations coupled with energy dispersive X-ray spectroscopy performed at the end of complex thermal cycling revealed that ‘amnesia’ occurrence was associated with changes in the morphology of martensite plates, from parallel plate-like to interblocking needle-like, and with an increase of chemical fluctuations, as an effect of the intensification of atomic migration.

From commercial pellets of recycled polyethylene terephthalate glycol (R-PETG), 1.75 mm diameter filaments for 3D printing were produced. By varying the filament’s deposition direction between 10° and 40° to the transversal axis, parallelepiped specimens were fabricated by additive manufacturing. When bent at room temperature (RT), both the filaments and the 3D-printed specimens recovered their shape during heating, either without any constraint or while lifting a load over a certain distance. In this way, free-recovery and work-generating shape memory effects (SMEs) were developed. The former could be repeated without any visible fatigue marks for as much as 20 heating (to 90 °C)-RT cooling–bending cycles, while the latter enabled the lifting of loads over 50 times heavier than the active specimens. Tensile static failure tests revealed the superiority of the specimens printed at larger angles over those printed at 10°, since the specimens printed at 40° had tensile failure stresses and strains over 35 MPa and 8.5%, respectively. Scanning electron microscopy (SEM) fractographs displayed the structure of the successively deposited layers and a shredding tendency enhanced by the increase in the deposition angle. Differential scanning calorimetry (DSC) analysis enabled the identification of the glass transition between 67.5 and 77.3 °C, which might explain the occurrence of SMEs in both the filament and 3D-printed specimens. Dynamic mechanical analysis (DMA) emphasized a local increase in storage modulus of 0.87–1.66 GPa that occurred during heating, which might explain the development of work-generating SME in both filament and 3D-printed specimens. These properties recommend 3D-printed parts made of R-PETG as active elements in low-price lightweight actuators operating between RT and 63 °C.