*2.3. Characterisation*

#### 2.3.1. Particle Size Analysis

The morphology of the particles was studied with an automed dynamic morphological analyser (SympaTec GmbH, Clausthal, Germany) called QICPIC and further use in the text as its model name by measuring the particle diameter (DIFI). DIFI was defined as the quotient of the total projection area and the sum of all the lengths of the branches of the fibre backbone:

$$\text{Diameter} = \frac{\text{total projection area}}{\text{sum of the lengths of all the tenths}} \tag{1}$$

Flax tows were studied in a water solution using a dispersion unit, LIXELL (SympaTec GmbH, Clausthal, Germany), to prevent particle aggregation. Each sample was weighed to approximately 50 mg and then pre-dispersed in 5 mL of ethanol and then 45 mL of distilled water before undergoing further dispersion using an ultrasound probe (Vibra-CellTM 75022, Bioblock Scientific, Hampton, NH, USA). The solution was finally dispersed in 950 mL of distilled water under magnetic stirring. All samples were analysed using a M7 lens, which allows precise measurement of particles with dimensions ranging from 4.2 to 8665 μm (ISO). The number of particles analysed varied between hundreds of thousands to a million depending on the samples. For each sample, measurements were carried out in triplicate to ensure the reproducibility of the data. PAQXOS software (SympaTec GmbH, Clausthal, Germany) was used to calculate the particle diameter (DIFI).

#### 2.3.2. SEM Observations

SEM images of fibres and composites were recorded by using a Jeol JSM IT-500 HR (JEOL Ltd., Tokyo, Japan). All samples were spin coated with gold for 180 s using an Edwards Scancoat Six device before observations. Secondary emission electrons were used, with an accelerating voltage of 3.0 kV.

For the composite cross-section observations, a section of composite was inserted into a mixture of 34 g of catalyser and 100 g of epoxy resin. The sample was placed overnight in an oven at 50 ◦C to promote crosslinking. Then, the surface of the sample, which was subsequently characterised using a SEM was polished using an automatic polisher (TegraForce-5, Struers, Copenhagen, Denmark). Several polishes were carried out with different polishing times and grains: 60 s/500, 90 s/800, 120 s/1000, 150/1200, 180 s/2000 and 210 s/4000.

#### 2.3.3. Monosaccharide Composition

The quantification and identification of the neutral monosaccharides constituting flax tows was carried out by gas chromatography after acid hydrolysis and conversion of the monomers into alditol acetates [30]. A DB 225 capillary column (J&W Scientific, Folsorn, CA, USA; temperature 205 ◦C, carrier gas H2) was used to perform the chromatography. For calibration, a range of glucose solutions and inositol as an internal standard were used. Approximately 12 g of fibres in total were tested, and the measurements were performed in duplicate.

#### 2.3.4. Water Sorption/Desorption

The sorption and desorption isotherms for water were established with a dynamic vapour sorption device (IGAsorpt-HT, Hiden Isochema, Warrington, UK). The flax tows were cut to approximately 3–4 mm, and 25 mg was placed in a microbalance located in the hermetic reactor. Prior to adsorption, the samples were dried at 105 ◦C for 1 h. Inside the reactor, the temperature and relative humidity (RH) were controlled. The sorption/desorption sequence was programmed as follows: Increase from 0% to 90% RH, and then decrease to 0% RH at 20% RH steps. For each RH step, the sample mass was continuously measured until equilibrium was reached (i.e., when the mass variation became less than 0.1 μg/min over a 1 h window).

#### 2.3.5. Mechanical Properties of the Composite Plates

The specimens were cut from the manufactured composite plates with a milling machine. For each plate, five tensile (20 × 8 mm2), four bending (80 × 10 mm2) and two SEM (20 × 20 mm2) test specimens were cut. The tensile properties of the flax-PLA non-woven composites were determined in accordance with the ISO 527 standard. The uniaxial tensile tests are performed in the static state during loading. An MTS Criterion Model 42 machine (MTS, Eden Prairie, MN, USA) with an MTS extensometer (nominal length 25 mm) was used with a 5 kN load cell (MTS, Eden Prairie, MN, USA) and a constant displacement rate of 1 mm/min. Load cell indicated the maximum force that the (traction) cell can provide. Five specimens were tested for each composite.

The bending properties of flax-PLA non-woven composites were determined according to the ISO 178 standard using an MTS Synergie 1000 R/T apparatus (MTS, Eden Prairie, MN, USA). The three-point bending tests were carried out at a constant displacement rate of 1 mm/min with a 250 N load cell. Four specimens were tested for each composite.

#### *2.4. Environmental Analysis*

The purpose of this study was to determine the environmental impacts of two physical treatments applied to flax tows: ultrasound (see Section 2.2.1) and gamma irradiation (see Section 2.2.2). Here, a simplified environmental analysis was conducted and not a full LCA (ISO 14044). The model was realised with the software Simapro. The Ecoinvent V3.5 database was used. The system boundaries are based on the cradle-to-gate approach and are represented in Figures 1 and 2. The locations of the production sites were included. The flax tows data were obtained from the company Depestele, located in Normandy, France as part of the program FARBioTY. The model works by mass allocation, which means that if a process has several products, the impact of each of the products is allocated in proportion to its mass. The studied impacts are abiotic depletion, global warming, ozone layer depletion, human toxicity, acidification and eutrophication based on the CML 2 baseline 2000 calculation method. The energy consumption, fossil and nuclear, was calculated with the Cumulative Energy Demand V1.11 method. Evaluations of indicators are available in the literature [31]. The results are presented as diagrams with a relative scale. Due to the uncertainties, for the same environmental indicator, if the difference between two studied pre-treatments is less than 5%, they are considered equal. Interventionary studies involving animals or humans, and other studies that require ethical approval, must list the authority that provided approval and the corresponding ethical approval code.

**Figure 1.** Flow chart of flax tows production.

**Figure 2.** Flow chart for ultrasound and gamma irradiation pre-treatments.
