**4. Conclusions**

A wide variety of commercial PLA grades was investigated in this work. The grades can firstly be distinguished by their molecular weight, which leads secondly to different rheological behavior (i.e., melt strength and zero shear viscosity) and varied thermal properties (i.e., crystallization kinetics). Additionally, different D-contents were considered, which also determine the thermal properties (i.e., crystallinity). Besides these material-specific properties, different process parameters were changed during the pressure induced batch foaming (i.e., saturation temperature, -pressure, -time, and pressure drop rate). Consequently, a wide variety of factors that could influence the expansion behavior of PLA were conclusively studied.

Low density foams (volume expansion ratio VER ≥ 10) could be achieved for most of the PLA grades. Exceptions were the injection molding grades, whose low molecular weights lead to lower zero shear viscosities, vanishing low melt strengths, and high crystallization kinetics, which, in total, limit the expansion. It also could be shown that the rheological properties (melt strength and zero shear viscosity) are not the only factors governing the expandability as the crystallization behavior, which is known to be very complex for PLA can tremendously hinder the expansion, as it is the case for the injection molding grades. The D-content governs the foaming temperature and the melt temperatures of the crystallized foams. PLA with lower D-content needs to be foamed at higher temperatures, as it was also observed that a D-content of approx. 4% leads to melting temperature of around 150 ◦C, while at 2% and lower it is around 170 ◦C. A D-content of 12% commonly leads to no crystallization and consequently to much lower foaming temperatures that were way below 100 ◦C. The saturation pressure strongly governs the foaming temperature on which the lowest density can be achieved, as well as the nucleation, and thereby the fineness of the cells. Surprisingly, a shorter saturation time leads to lower densities, because the CO2-induced crystallization intensifies with ongoing saturation, allowing for less expansion, as we hypothesized. Even for PLA with a high D-content of 12%, which can usually be considered as totally amorphous, a divergent foaming behavior (tsat > 4 h) was shown, which is presumably due to an additional phase transition, induced by the long saturation. Furthermore, a high pressure drop rate is favorable for achieving low density foams with fine morphology, as the nucleation rate is increased independently from the D-content.

**Author Contributions:** T.S.: conceptualization, writing—review and editing sections "1. Introduction", "2. Materials and Methods" and Sections 3.2 and 3.3, abstract and conclusions, review, editing and preparation of original draft. H.L.: investigation and data curation of foam experiments, writing Sections 3.2 and 3.3, S.M.C.: investigation and data curation of rheological experiments and writing Section 3.1. C.B. (Christian Brütting) writing—review and editing section "1. Introduction" and "3 Results and Discussion". C.B. (Christian Bonten) and V.A. supervision, reviewing and funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by German Research Foundation (DFG), grant number AL474/33-1, and BO1600/40-1. Open access charges were funded by the German Research Foundation (DFG) and the University of Bayreuth in the funding program Open Access Publishing.

**Acknowledgments:** The authors would like to acknowledge the Bavarian Polymer Institute (BPI) for providing access to different analysis methods. Furthermore, we thank mechanical workshop at the University of Bayreuth for construction and installation of the high pressure autoclave used in this work. Also, we would like to acknowledge Rika Schneider (MC II, University of Bayreuth) for additional GPC measurements.

**Conflicts of Interest:** The authors declare no conflict of interest.

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