Dynamic Mechanical Analysis of 3D printed (fused deposition modelling) PLA

Three-dimensional printing (3DP) is a relatively new prototyping technique based on computer-aided design models that are manipulated by a terminal computer. The applications of 3DP systems are quickly spreading in many manufacturing areas thank to their unprecedented versatility; in fact, several different materials, such as ceramics, metals, polymers, and composites can be processed to create appropriate geometries. 3DP systems have recently attracted interest as emerging technologies in the bio-medical and pharmaceutical field for the manufacturing of scaffolds and drug delivery systems (DDS). However, these applications require materials having specific properties such as biocompatibility and biodegradability. Actually, among the several materials available for the 3DP technologies only few of them can be used in pharma applications. One of the most interesting material is represented by the poly-lactic acid (PLA), since it displays both biocompatibility and biodegradability together with excellent 3DP processability. This work deals with the characterization of PLA in order to evaluate the effect of 3DP processing parameters on its thermo-mechanical properties. PLA blocks were prepared varying the processing temperature and the filling space using the fused deposition modeling (FDM) technology with a 3D printer (Delta Wasp, CSP s.r.l., IT) and analysed using a Dynamic Mechanical Analyser (DMA 8000, Perkin Elmer, USA) equipped with a closed furnace. All the tests were performed in bending mode applying a constant deformation amplitude at four different frequencies during a heating ramp between 20° and 200°C at 3°C/min. Results were analysed in order to highlight thermal transitions and mechanical properties of PLA blocks. Time temperature superposition (TTS), together with William-Landel and Ferry (WLF) fitting, were performed to study the glass transition in term of free fractional volume, thermal expansion coefficient, apparent activation energy and dynamic fragility. Moreover, the materials mechanical relaxation time spectra (H(t)) were calculated using the 2nd approximation of the William and Ferry methods. All the 3D printed PLA showed similar mechanical behaviour, with dynamic moduli over the range from the rubbery plateau to the glassy region. However, specific differences were found as a function of the 3D manufacturing conditions. Specifically, samples processed at higher temperature displayed more elastic behaviour while the sample filling ratio influences only samples prepared at intermediate temperature.