Abstract
The properties of polylactic acid (PLA) filaments have not yet been analysed in detail, and they are strongly affected by the extrusion process used in some additive manufacturing systems. Here we present the mechanical, thermal, physical, and fractographical properties of an extruded filament (not the bulk material or scaffolds), the basic building block of any PLA structure printed via material extrusion. This research aims to create a reference point for the modelisation of additively manufactured structures via extrusion processes, as the main building block is characterised in detail for a deep understanding. Furthermore, we investigated the natural ageing (up to one year), the effect of the printing (extruding) temperature (180 and 190 °C), and the effect of the crosshead speed during the tensile tests (10−1 to 102 mm/min) to provide a deeper analysis of the material. The results showed that the material extruded at 190 °C performed better than the material extruded at 180 °C. However, after one hundred days of natural ageing, both materials behaved similarly. This was related to the flow-induced molecular orientation during the extrusion. The crosshead rate produced a logarithmic increase of the mechanical properties, consistent with the Eyring model. Additionally, the ageing produced significant changes in both the elastic modulus and the yield strength: from 2.4 GPa and 40 MPa, in one-day-aged samples, up to 4 GPa and 62 MPa once entirely aged. Finally, it was observed that the glass transition and the enthalpic relaxation increased with ageing, agreeing with the Kohlraushch–William–Watts model.
Highlights
Among synthetic biodegradable plastics, poly(lactic acid) (PLA) is an amorphous and compostable polymer derived from renewable sources [1,2]
Several effects take place during the extrusion of PLA, such as flow-induced molecular orientation [32,33,34] and flow-induced crystallisation [35]; during the ageing, such as hydrolysis, photo-oxidation, thermal decomposition, and natural weathering, which are highly dependent on the ageing conditions [36,37,38,39,40,41,42,43,44,45]; during the thermal tests, such as the crystallisation in different crystalline phases [46,47]; and during the mechanical tests, such as the change of mechanical properties with the strain rate due to the viscoelastic behaviour, which is directly dependant on the amorphous phase of the PLA [48], among many possible examples
This study investigated the influence of the printing temperature, natural ageing, and crosshead speed on the mechanical, thermal, physical, and fractographical properties of PLA printed via material extrusion
Summary
Poly(lactic acid) (PLA) is an amorphous and compostable polymer derived from renewable sources [1,2]. Several effects take place during the extrusion of PLA, such as flow-induced molecular orientation [32,33,34] and flow-induced crystallisation [35]; during the ageing, such as hydrolysis, photo-oxidation, thermal decomposition, and natural weathering, which are highly dependent on the ageing conditions (ambient humidity, temperature, pH, and exposure to ultraviolet light) [36,37,38,39,40,41,42,43,44,45]; during the thermal tests, such as the crystallisation in different crystalline phases [46,47]; and during the mechanical tests, such as the change of mechanical properties with the strain rate due to the viscoelastic behaviour, which is directly dependant on the amorphous phase of the PLA [48], among many possible examples Most of these effects are interrelated and should be carefully analysed [49,50,51]. R squared (R2) was used to measure how good the analytical model was
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