2.2.1. Ultrasound

Ultrasound pre-treatment were carried out using a semi-pilot tank (SinapTec, Lezennes, France) at the Université du Littoral (Dunkerque, France). The pre-treatment parameters are an output power of 2500 W and a frequency of 20 kHz. The semi-pilot tank can contain a volume of 50 L of water. Ultrasonic pre-treatment requires immersion in a solvent for the propagation of ultrasonic waves. The pre-treatment was carried out by immersion in water during 30 min. This duration of the pre-treatment was established by preliminary tests, which showed the best individualisation for this duration (data not shown). For ultrasound pre-treatment, 2.5 kg of tows was used. The ultrasound pre-treated tows were named US 30 min. A chiller with an air-cooled refrigerating unit and circulation pump (Unichiller 025 OLE, Huber, Offenburg, Germany) was used to maintain the water temperature at approximately 25 ◦C during the pre-treatment. After the pre-treatment, the flax tows were dried overnight in an oven (BD 56, Binder GmbH, Tuttlingen, Germany) at 60 ◦C.

#### 2.2.2. Gamma Irradiation

Gamma irradiation was carried out industrially by the company Ionisos (Dagneux, France). To preserve the fibre structure and physical properties [28], a low dose of 5–7 kGy was applied to approximately 2.5 kg of flax tows; this sample was named Gamma. Gray is the unit derived from the international system of units for absorbed dose. A Gray represents the energy of ionising radiation from one joule to a homogeneous medium with a mass of one kilogram.
