**5. Discussion**

In attempt to better correlate the obtained results with the mechanical properties, it was noticed that in general the tensile strength of the prepared biocomposites decreased by increasing the melt fluidity, as shown in Figure 10, where the data related to composites containing 20, 30, and 40% of HSP are reported. Interestingly, for the 30% HSP biocomposites, the data obtained for the scaled-up samples follow a similar trend, but as yet observed, the tensile strength is higher and the MFR is lower. Moreover, finer HSP (HM200) results in the highest value of tensile strength and lowest value of MFR. Hence, by avoiding the chain scission of PLA thanks to the optimized processing conditions, the fluidity is greatly decreased.

**Figure 10.** Tensile strength as a function of MFR for biocomposites containing 20, 30, and 40% of HSP.

The production of the biocomposites including PLA and HSP resulted in strong interactions or reactions (Figure 11, reaction 1) between the polymer matrix and the functional groups on the HSP surface. Hydroxyl groups, belonging to cellulose and hemicellulose, that represent the major component of HSP, were mainly considered.

**Figure 11.** Main reactions occurring during the preparation of PLA/HSP biocomposites; reaction 1 involves an HSP solid particle.

Reaction 1's occurrence depends on the surficial area of HSP and can induce an increase in tensile strength, thanks to the improved matrix-filler adhesion. On the other hand, reaction 2 (Figure 11, reaction 2) is PLA hydrolysis due to humidity, occurring more in the composites containing the finer HSP. In a lab-scale extruder configuration, reaction 2 affects properties more than reaction 1 because of the higher residence time and absence of devolatilization. Thus, as demonstrated by the study of B parameter obtained as the slope of the Pukanszky's plot (Figure 6), the dispersion in the matrix of the HSP with the lowest dimension was less efficient. On the contrary, when the preparation is scaled-up, reaction 1 as well as the fibre-matrix interaction are more significant. In good agreement, the finer HSP, with the highest surface area, resulted in the highest tensile strength and highest melt viscosity.
