**5. Conclusions**

The impact of ultrasound and gamma irradiation pre-treatment was investigated at the scale of tows and non-woven composite materials. Morphological analysis showed better individualisation for pre-treated tows, with a 20% increase in elementary fibres for gamma irradiation and a more than 15% increase for ultrasound pre-treatment. Both pre-treatments partially removed the middle lamella responsible for the cohesion of the fibre bundles, as targeted. Consequently, hemicellulosic and pectic components known to belong to the middle lamella were also removed, which also impacted the sorptiondesorption behaviour of flax tow. Both pre-treatments showed a decrease in water uptake behaviour. At the non-woven composite scale for the tensile test, a 10% increase in stress at break for the US 30 min composite in the machine direction was observed. However, no improvement in Young's modulus or elongation at break was observed for Gamma and US 30 min composites compared to the Native composite. Fine quantification of the porosity in the non-woven composites showed that the ultrasound pre-treatment displays a lower porosity content, arguably leading to the observation of a slight mechanical improvement for non-woven flax-PLA composites. Finally, environmental analysis showed that if any additional pre-treatment weighs down the environmental balance compared to native tows, gamma irradiation is much more suitable than US processing.

**Author Contributions:** Conceptualization, M.G., F.G., A.B. and J.B.; Investigation, M.G., G.B. and T.C.; Supervision, F.G., A.B. and J.B.; Validation, F.G., A.B. and J.B.; Writing—original draft, M.G.; Writing—review & editing, M.G., A.K., G.B., F.D., T.C., F.P., M.H., N.L.M., F.G., A.B. and J.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by by Interreg V.A Cross-Channel Programme through, FLOWER: Flax composites, LOW weight, End of life and Recycling (http://flower-project.eu), Grant No. 23) and Pays de la Loire region.

**Institutional Review Board Statement:** Not Applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not Applicable.

**Acknowledgments:** We thank Sylviane Daniel (INRAe Nantes) for the monosaccharide results and analysis. We also thank Antohny Magueresse (Univ Bretagne Sud) for the different SEM observations and Bastien Watbled (Littoral Côte d'Opale University) for its availability during ultrasound pre-treatment.

**Conflicts of Interest:** The authors declare no conflict of interest.
