**6. Conclusions**

In this study the possibility to process successfully, at the semi-industrial scale, PLAbased composites containing hazelnut shell powder (HSP) was investigated. A first labscale production was carried out in order to individuate the best HSPs amount for the subsequent scaling-up step. Two different HSPs typologies of different sizes were added

from 20 up to 40 wt.%. The thermal, mechanical, and melt fluidity analysis showed poor stress transfer, which led to a decrement in tensile strength. The fillers seem to act as nucleating sites for PLA that increased its crystallinity; however, a marked decrement of the melt viscosity was recorded, especially for fillers small in size due to their major water uptake. The composition including 30 wt.% of HSP was selected for the successive scale-up in a semi-industrial extruder. Interesting results were obtained considering the scaled-up composites, as their melt fluidity was decreased thanks to the presence of the venting system in the extruder that efficiently removed the residual humidity. The scaled-up composites showed improved mechanical properties, respect to the lab-scaled composites, demonstrating that these composites are effectively processable and can be easily scaled-up The prepared biocomposites showed the possibility of achieving an optimized balance between improvement of mechanical properties and the valorization of a significantly high HSP content.

In future work, a further step towards more efficient exploitation of HSPs should concern their functionalization. The HSPs' superficial modification, coupled with the optimization of the extrusion process parameters, would allow to obtain biocomposites with further improved mechanical properties.

**Author Contributions:** Conceptualization, M.-B.C. and L.A.; experimental work, A.V. and L.A.; theoretical analysis, L.A. and A.L.; data curation and elaboration, A.V., L.A. and M.-B.C.; writing original draft preparation, L.A. and A.V.; writing—review and editing M.-B.C.; supervision, D.B. and A.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** Arianna Fiber s.r.l. received partial funding by Tuscany Region on POR FESR 2018–2020 thanks to the Finap Bio project and by Fondazione Cassa di Risparmio di Pistoia (CRPT), thanks to project GREEN SPEAR.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

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