## 3 Results and Discussion  
### Morphology of FF Particles  
Figure 2 shows the FESEM images of the flax flour powder, as well as the particle size distribution after each grinding process. Figure 2 includes representative images of each particle size at different magnifications while the particle size distribution plots were obtained by taking at least 50 measurements on different FESEM images corresponding to each particle size, namely fine (F) or coarse (C), using the software analysis included in the FESEM microscope. The following parameters were obtained (area, angle, and length), and the histogram plots included the length.  
As can be seen in the FESEM images at lower magnifications (Figure 2a,d), coarse particles obtained by simple grinding (CFF) as well as fine flax flour particles with smaller dimensions (FFF), tend to form aggregates due to their high hydrophilicity. Quantitatively we can see from the size distribution made by measuring randomly chosen particles (Figure 2c,f), that CFF offers a particle size of 100-220 \(\upmu\)m with an average particle size around 157 \(\upmu\)m and remarkably higher particle content in the 140-160 \(\upmu\)m range. On the other hand, FFF particle distribution shows that their size changes in the 40-140 \(\upmu\)m range with smaller average particle size, of about 91 \(\upmu\)m, and the most abundant content of particles is in the range of 80-100 \(\upmu\)m. Therefore, it is evident that with the grinding of FF by ultra-centrifugation, finer particle sizes were obtained, which resulted in a better filler dispersion in the matrix, as well as improved polymer-filler interactions, with a positive effect on the general properties of the developed composites.  
The grinding process has a relevant effect on both particle aggregate and geometry. Despite this, these differences can be seen in Figure 2 since in general, both CFF and FFF particles had an irregular morphology with a rough surface and the presence of granular fractures (typical morphology of hard lignocellulosic particles after being subjected to crushing processes [54; 55; 56]). Figure 3 shows in a more detailed way the above-mentioned effects. Figure 3 shows the morphology of directly ground flax particles after the cold press process. One can see these particles show very irregular shapes. Moreover, it is possible to find particles with high size and very small particles. This is because no particle size separation has been carried out after this grinding process which led to coarse flax flour (CFF). In addition, aggregate formation is evident (in fact, the adhesive carbon tape cannot be seen).  
Regarding fine flax flour (FFF), as shown in Figure 3, it is evident that the shape is more likely spherical, the particle size range is narrower, and the aggregation phenomenon is less pronounced. The sieving process after ultra-centrifugation gives more homogeneity on the obtained morphologies and less aggregation.  
Figure 2: Field emission scanning electron microscope (FESEM) images corresponding to (**a**) CFF at 100\(\times\) with a scale marker of 100 \(\upmu\)m; (**b**) CFF at 500\(\times\) with a scale marker of 20 \(\upmu\)m; (**c**) particle size histogram of CFF; (**d**) FFF at 100\(\times\) with a scale marker of 100 \(\upmu\)m; (**e**) FFF at 500\(\times\) with a scale marker of 20 \(\upmu\)m; (**f**) particle size histogram of FFF.