*2.2. Hazelnut Shell Powders (HSPs) Characterization*

In order to quantify the humidity present in the HSPs, about 0.5 g of HSP for each sample were put in a Petri dish (previously weighed) and they were weighed before and after the drying process in a ventilated oven at 60 ◦C for 16 h. For each fiber typology, at least 3 measurements were carried out.

To investigate the possible degradation of the fillers during the extrusion compounding and to evaluate differences in chemical compositions between H0210 and HM200 HSPs, thermogravimetric (TGA) and FT-IR analysis were carried out.

TGA was performed on a TA Q-500 instrument (TA Instruments, Waters LLC, New Castle, DE, USA). Few milligrams were heated at 10 ◦C/min from room temperature up to 700 ◦C at 10 ◦C/min in nitrogen atmosphere.

FT-IR analysis was carried out on a Nicolet T380 FT-IR (Thermo Scientific, Madison, WI, USA) spectrometer equipped with an ATR Smart iTX accessory. Infrared spectrum of HSP was recorded in the 550–4000 cm−<sup>1</sup> range, collecting 256 scans at 4 cm−<sup>1</sup> resolutions.

The powders morphology was investigated by scanning electron microscopy (SEM) analysis using a FEI Quanta 450 FEG (Thermo Fisher Scientific, Waltham, MA, USA). The samples were prior sputtered with platinum to enhance their conductivity and generate the images, thanks to the secondary electrons. For each fiber typology, different images were acquired in order to obtain the filler distributions. The HSPs distributions were obtained, according to literature [33,34], measuring the dimensions of at least 200 filler particles by using Image-J software.
