**4. Conclusions**

PLA was successfully modified via reactive extrusion. By using 0.2 wt% of an organic peroxide, the molar mass and complex viscosity could be increased. To prevent PLA from undergoing hydrolysis, a stabilizer was used. To receive good foaming properties as well as a thermally and hydrolytically stable PLA, the two modifiers were combined. The combination of 0.2 wt% DCUP and 0.5 wt% PCDI leads to the highest molar mass and the best thermal stabilization. By using the acid value determination, it could be shown that the stabilizer reacts with the end groups of PLA and that several processing steps such as extrusion and foaming can lead to a degradation of PLA. As a result, it is indispensable to prevent PLA from hydrolysis to ensure that there is no downgrade of the properties of PLA. In future works, different modifiers and stabilizers could be investigated and also the following processing steps of bead foams, such as welding.

**Author Contributions:** Conceptualization, J.D.; methodology, J.D. and C.B. (Christian Brütting); software, J.D., validation, J.D. and C.B. (Christian Brütting); formal analysis, J.D.; investigation, J.D. and C.B. (Christian Brütting); resources, J.D.; data curation, J.D.; writing—original draft preparation, J.D. and C.B. (Christian Brütting); writing—review and editing, J.D. and C.B. (Christian Brütting); visualization, J.D.; supervision, V.A., H.R. and C.B. (Christian Bonten); funding acquisition, V.A., H.R. and C.B. (Christian Bonten). All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by German Research Foundation (DFG) (392851979).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors kindly like to thank the scientific staff members who contributed with inspiring discussions and all technicians and students for their support during the experiments.

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