Development of Injection-Molded Polylactide Pieces with High Toughness by the Addition of Lactic Acid Oligomer and Characterization of Their Shape Memory Behavior

Abstract

This work reports the effect of the addition of an oligomer of lactic acid (OLA), in the 5-20 wt% range, on the processing and properties of polylactide (PLA) pieces prepared by injection molding. The obtained results suggested that the here-tested OLA mainly performs as an impact modifier for PLA, showing a percentage increase in the impact strength of approximately 171% for the injection-molded pieces containing 15 wt% OLA. A slight plasticization was observed by the decrease of the glass transition temperature (T g ) of PLA of up to 12.5 • C. The OLA addition also promoted a reduction of the cold crystallization temperature (T cc ) of more than 10 • C due to an increased motion of the biopolymer chains and the potential nucleating effect of the short oligomer chains. Moreover, the shape memory behavior of the PLA samples was characterized by flexural tests with different deformation angles, that is, 15 • , 30 • , 60 • , and 90 • . The obtained results confirmed the extraordinary effect of OLA on the shape memory recovery (R r ) of PLA, which increased linearly as the OLA loading increased. In particular, the OLA-containing PLA samples were able to successfully recover over 95% of their original shape for low deformation angles, while they still reached nearly 70% of recovery for the highest angles. Therefore, the present OLA can be successfully used as a novel additive to improve the toughness and shape memory behavior of compostable packaging articles based on PLA in the new frame of the Circular Economy.

Tailoring the Properties of Thermo-Compressed Polylactide Films for Food Packaging Applications by Individual and Combined Additions of Lactic Acid Oligomer and Halloysite Nanotubes

“…In particular, the neat PLA film showed respective T 5% and T deg values of 342.2 and 383 • C. The inset image within Figure 7a shows that the addition of OLA gradually reduced the thermal stability of PLA, being more intensely for the onset of degradation since the T 5% values gradually reduced to 335.3, 313.3, and 292.5 • C for additions of 10, 20, and 30 wt%, respectively. This is in agreement with previous studies reporting about the thermal stability of PLA/OLA blends [12,36,44], which related the lower stability of the blends to the lower T g and the inherently poor thermal stability of OLA. Finally, all the PLA/OLA blends showed residual mass values below 2 wt%.…”

“…However, contents of 10 wt% and 20 wt% of OLA, respectively shown in Figures 3c,d, yielded to a rough surface with multiple macrocracks and thin plastic filaments. As also observed in our previous study [36], the presence of OLA successfully inhibited microcrack formation and the cracks grew to a greater extent, thus suggesting that the film absorbed more energy and deformed more intensively during fracture. It should be noted that no sign of phase separation could be detected since PLA and OLA have the same chemical structure.…”

“…However, it should be noticed that the T m value that corresponds to the higher melting peak was significantly not different than that attained in the neat PLA sample and the other OLA-containing PLA films, suggesting that similar crystallites were formed in all the materials. Furthermore, the χ c max values increased with the OLA content, which has been previously related to the inherent plasticizing effect of OLA [12,36]. Therefore, the presence of OLA in the PLA matrix hindered crystallization to some extent by impairing proper chain packing.…”

“…In this regard, Martin and Avérous [13] found that 20 wt% of OLA reduced E t by 63% whereas ε b increased up to 200%. However, our recent study reported that the use of impact-modifier type of OLA unexpectedly reduced the ductility of injection-molded pieces of PLA due to it rather acted as rubber-like filler in the PLA matrix [36]. Therefore, the different values attained can be mainly related to the OLA type since the here-used grade is mainly aimed to plasticize biopolyesters and it will be discussed later.…”

“…Furthermore, the presence of a single T g , located between that of PLA and OLA, being the latter reported in the 10-30 • C range [39], suggests that the biopolymer and its oligomer were fully miscible at all the given composition studied here. The fact that PLA cold crystallized at lower temperature can also be related to the plasticization phenomenon of PLA provided by OLA since it is known to promote the motion of the PLA chains [36]. Other authors have ascribed this effect to a heterogeneous nucleation of the short length OLA molecules by which the cold crystallization process of PLA is favored [40].…”