*2.1. Materials*

#### 2.1.1. Flax Tows

Flax tows (*Linum usitatissimum* L.) from the Bolchoï variety were supplied by Depestele (Bourguebus, France). Flax plants were cultivated in 2017 (Normandy, France) and dewretted in the field. These plants had a length of approximately 80 mm and a shive rate less than 5%. This sample was named Native, no-treated flax tows.

#### 2.1.2. Flax-PLA Non-Woven Preforms

The non-woven preforms were composed of 50:50 wt% of flax tows and PLA fibres and manufactured in October 2020 by the EcoTechnilin Company (Yvetot, France). PLA fibres had a length of 60 mm and a diameter of approximately 30 μm. Three non-woven preforms were produced with the different batches of flax: untreated, gamma irradiation and ultrasound pre-treated. All non-woven preforms had a basis weight of approximately 100 g/m2.

#### 2.1.3. Composite Manufacturing

The flax-PLA non-woven composites were produced by hot pressing using a Darragon 35T hydraulic press. The dimensions of the composite plates were 160 × 160 × 1.5 mm3. The time-temperature-pressure cycle used is described as follows: Heat up to 190 ◦C, 3 min of preheating on contact, followed by the application of 50 bar pressure for 3 min at 190 ◦C, and, finally, a 15 min cooling under pressure to reach 10 ◦C.

#### *2.2. Physical Pre-Treatments*
