## 3 Results and Discussion
### Mechanical Properties  
Table 1 shows the mechanical behavior of unfilled BioEP resin and BioEP/FF composites reinforced with different contents of CFF and FFF. Regarding the flexural properties, it can be seen that FF addition to the BioEP matrix results in a significant decrease in flexural strength compared to the unfilled BioEP resin. This decrease is much more pronounced as the FF content (both CFF and FFF) increases. As expected, the lowest flexural strength is obtained in composites reinforced with 40 wt% of CFF and FFF, obtaining a flexural strength of 22.5 and 22.9 MPa, respectively, which represents a decrease of 76.4% and 75.9% in comparison to the flexural strength of the unfilled BioEP resin (95.2 MPa). This decrease in flexural strength is due to the lack of adhesion between the lignocellulosic filler and the surrounding matrix, which causes stress concentration phenomena that promote breakage [20]. Another issue with a high influence on the mechanical properties of composites is the filler aspect ratio. High filler aspect ratios (above 6) result in a better stress transfer from the matrix to the filler/reinforcement, thus improving their mechanical properties. However, this stresses transfer is poorer in the case of low aspect ratio filler, such as the FF used, with an aspect ratio comprised between 1-2 [57]. If the two types  
Figure 3: FESEM images corresponding to (**a**) CFF at 100\(\times\) with a scale marker of 100 \(\upmu\)m; (**b**) FFF at 100\(\times\) with a scale marker of 100 \(\upmu\)m.  
FF, i.e., coarse and fine FF (CFF and FFF, respectively) are compared, it can be seen that the flexural strength is always lower for all composites containing CFF. This is because the bigger particle size causes a larger weak interfacial area between the BioEP resin and the reinforcement, i.e., it decreases the adhesion between the hydrophobic matrix and the hydrophilic reinforcement which generates an increase in the stress concentration phenomena that negatively affect the mechanical properties of composites [58]. The biggest difference in flexural strength between composites with CFF and FFF is obtained for samples with 10 wt% of FF. For this composition, the flexural strength was 24.2% higher for FFF-reinforced composite compared to CFF-reinforced composite. Regarding the flexural modulus, different trends can be observed for each of the fillers. The flexural modulus of CFF-reinforced composites is hardly affected by 10, 20, and 30 wt% filler and the modulus values are similar to those obtained for BioEP resin (2840-2985 MPa). However, the flexural modulus of the composite reinforced with 40 wt% CFF decreases significantly to 2290 MPa, representing a decrease of 23.3% compared to the unfilled BioEP resin. This is directly related to an embrittlement process which is much more evident at higher filler loadings. It is important to bear in mind that the flexural modulus is directly related to the supported stress and inversely related to the flexural deformation. BioEP is intrinsically brittle, therefore, the change in the flexural deflection before fracture is very low. This is even reduced in BioEP/FF composites, but the most important parameter is a clear decrease of the flexural strength from 95.2 MPa down to 22.9 MPa. For this reason, at 40 wt% CFF, the modulus decreases in a remarkable way since the flexural stress is remarkably reduced while the elongation is almost identical to neat BioEP, which is an intrinsically brittle material. In the case of FFF-reinforced composites, the addition of 10 wt% FFF to the BioEP matrix significantly increases the flexural modulus from 2985 MPa (unreinforced BioEP) resin up to 3579 MPa, which is a % increase of nearly 20%. This increase in the flexural modulus compared to the 10 wt% CFF reinforced composite may be due to the better dispersion of fine particles into the matrix and improved polymer-particle interaction due to its smaller size, which results in comparatively higher flexural strength values, and consequently higher modulus and stiffness.