## 4 Conclusions  
The main objective of the present work was to evaluate the influence of FF particle size, i.e., coarse (CFF) and fine (FFF), and the filler content (10, 20, 30 and 40 wt%) on the mechanical, thermal, water uptake and morphological properties of composites of a partially biobased epoxy (BioEP) resin processed by casting. The addition of this lignocellulosic filler into the BioEP matrix resulted in a decrease in flexural strength and impact absorption energy with increasing filler content. This is because of the lack of (or very poor) polymer matrix-particle filler interactions. However, this decrease is lower for composites with finer particles, due to their better dispersion and better interaction with the matrix. In this work, it has been observed that the composite filled with 10 wt% of FFF presents a flexural strength and impact energy absorption 24.2% and 33.3% higher than its counterpart with CFF. Particle size has also a high influence on water absorption. It has been observed that composites with fine particles (FFF) filler offer less water absorption due to the presence of fewer voids in their structure as a result of better dispersion and fewer aggregates. In the case of CFF-filled composites, the larger size increases the lack of adhesion with the matrix generating voids and aggregates that allow the water entering, thus leading to a higher water absorption and diffusion coefficient. Therefore, the present work has revealed that the finer filler particle size results in better mechanical and water absorption properties in these natural fiber reinforced plastics (NFRPs). Besides, a partially biobased, high environmentally friendly material can be obtained. On one hand, the partial biobased epoxy content (31 wt%) can be increased up to almost 70 wt% renewable origin in composites with 40 wt% FF (both CFF or FFF). In addition, this work has revealed an alternative to upgrading wastes from the flaxseed industry. The developed BioEP/FF composites offer an attractive wood-like aesthetic appearance, and therefore, they can be used in sectors such as decoration, furniture, or the automotive industry. Once the particle size and the amount of FF have been optimized to obtain more balanced mechanical, thermal and water absorption properties in the BioEP/FF composite, further studies will focus on the use of highly reactive coupling agents, i.e., silanes such as (3-glycidyloxypropyl) trimethoxy silane and (3-aminopropyl) trimethoxy silane, to provide increased interaction between the epoxy matrix and the embedded FF particles. All these steps, including an industrial scalation and cost study will assess the viability of these materials to contribute to the circular economy in the flax industry. Conceptualization, R.B. and T.B. ; Data curation, D.L., D.G.-G. and T.B. ; Formal analysis, D.L. and L.Q.-C.; Investigation, D.L. and S.R.-L.; Methodology, D.L., D.G.-G. and L.Q.-C.; Project administration, R.B. and T.B. ; Supervision, D.G.-G. and R.B. ; Validation, D.G.-G., R.B. and T.B. ; Writing-original draft, D.L. and S.R.-L.; Writing-review & editing, D.G.-G. and R.B. All authors have read and agreed to the published version of the manuscript. This research was funded by Spanish Ministry of Science, Innovation, and Universities (MICIU), project numbers MAT2017-84909-C2-2-R. This work was supported by the POLISABIO program grant number (2019-A02). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Code** & \(L^{\star}\) & \(a^{\star}\) & \(b^{\star}\) & \(\Delta E\) \\ \hline BioEP & 39.50 & \(-\)0.48 & 1.60 & - \\ BioEP\_10 CFF & 39.51 & 8.97 & 19.02 & 19.82 \\ BioEP\_20 CFF & 40.77 & 9.51 & 19.36 & 20.42 \\ BioEP\_30 CFF & 40.93 & 9.59 & 19.54 & 20.62 \\ BioEP\_40 CFF & 41.48 & 9.68 & 19.83 & 20.97 \\ BioEP\_10 FFF & 35.82 & 9.61 & 15.80 & 17.80 \\ BioEP\_20 FFF & 36.27 & 9.52 & 16.83 & 18.50 \\ BioEP\_30 FFF & 37.77 & 9.44 & 17.11 & 18.49 \\ BioEP\_40 FFF & 39.55 & 9.63 & 18.42 & 19.62 \\ \hline \hline \end{tabular}
\end{table}