# Use of a commingling process for innovative flax fibre reinforced unidirectional composites

Elouan Guillou, Mahadev Bar, Mario Scheel, Thierry Falher, Timm

Weitkamp, Darshil U Shah, Alexandre Beigbeder, Pierre Ouagne, Alain

Bourmaud

► To cite this version:

Elouan Guillou, Mahadev Bar, Mario Scheel, Thierry Falher, Timm Weitkamp, et al.. Use of a commingling process for innovative flax fibre reinforced unidirectional composites. Composites Part B: Engineering, 2024, 270, pp.111150. 10.1016/j.compositesb.2023.111150. hal-05340352

HAL Id: hal-05340352

https://hal.univ-brest.fr/hal-05340352v1

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1 **Use of a commingling process for innovative flax fibre reinforced unidirectional composites**

2 *Elouan Guillou<sup>a,b</sup>, Mahadev Bar<sup>c</sup>, Mario Scheel<sup>d</sup>, Thierry Falher<sup>a</sup>, Timm Weitkamp<sup>d</sup>, Darshil U. Shah<sup>e</sup>, Alexandre

3 Beigbeder<sup>a</sup>, Pierre Ouagne<sup>c</sup>, Alain Bourmaud<sup>b</sup>

4 <sup>a</sup> IPC Laval, Rue Léonard De Vinci, Changé, France

5 <sup>b</sup> Univ. Bretagne Sud, UMR CNRS 6027, IRDL, Lorient, France

6 <sup>c</sup> Laboratoire Génie de Production, LGP, Université de Toulouse, INP-ENIT, Tarbes, France

7 <sup>d</sup> Synchrotron SOLEIL, Saint-Aubin, France

8 <sup>e</sup> Centre for Natural Material Innovation, Department of Architecture, University of Cambridge, Cambridge CB2

9 1PX, UK

10 *Corresponding author: elouan.guillou@univ-ubs.fr

11

12 **Abstract**

13 Optimisation of textile preforms play a crucial role in the development of high-performance biobased
14 composites materials. In this context, the main ambition of this work is to quantify and assess the mechanical
15 properties and behaviour of biocomposite materials made from unidirectional commingled preforms based on
16 flax and poly-(propylene) fibres. To the best of our knowledge, there is no literature examining the effect of the
17 commingling process on the ultrastructure and the mechanical properties of flax fibres. At the scale of the
18 elementary fibre, fibre mechanical properties are observed to be stable after commingling. However, repeated
19 drawing in the commingling process leads to increased cellulose crystallinity and a larger fraction of elementary
20 fibres exhibiting quasi-linear tensile behaviour. The composite materials produced with these commingled
21 flax/poly-(propylene) preforms contain few cortical residues and show a remarkable degree of fibre
22 individualisation. Moreover, they exhibit high Young's modulus and a stress at break of 24 GPa and 194 MPa,
23 respectively, for a fibre volume fraction of 36%. A substantial drop in properties is however noted at high fibre
24 fractions due increased heterogeneity of the materials. Remarkably, the biocomposites achieved
25 unprecedented transverse modulus and stress at break of 2.3 GPa and 16.5 MPa, respectively. Our results
validate the potential and interest in commingling processes for designing a new family of plant fibre

composite materials.

Keywords: A. Fabrics/textiles; B. Microstructure; D. Mechanical testing; E. Thermoplastic resin; F. Commingling process

30

## 1. Introduction

Over the past decades, there has been tremendous interest and technological development concerning flax fibre reinforced composites due to their high specific mechanical properties and ecological benefits in comparison to synthetic fibres such as glass fibres [1–8]. Nowadays, flax fibres are commercially available in various preform architectures such as fibre mats, non-wovens, 3-D woven, local reinforcing grids, preform rods or bi-axial and tri-axial reinforcements [9–12]. In order to use flax fibre reinforcements in load-bearing/structural applications, these fabrics have to be optimised considering some specific features of flax fibres, especially their anisotropy. Due to their complex cell wall structure, flax fibres display very different longitudinal and transverse modulus around 52 GPa [13] and 8 GPa [14], respectively, but low in-plane shear modulus (GfLT=2.5 GPa), especially compared to glass (GfLT= 29.7 GPa) [15], and a high Poisson's ratio of 0.498 [16]. This anisotropy demonstrates that twisted flax yarns or crimped fabrics are not ideal to take advantage of flax fibre properties as the fibres are not in the direction of the load. To manufacture structural composite parts, long, aligned and continuous fibre reinforcements are used. However, unlike continuous synthetic fibres, flax fibre in planta are discontinuous and grouped in bundles, so they require special attention from extraction to preform manufacturing [17,18]. The quality of the extraction process (retting, scutching and hackling) strongly affects the level of fibre individualisation and subsequent distribution in the matrix. Coroller et al. highlighted the importance of a hackling step for fibre dispersion; this process reduces the number of bundles in the final composite leading to better fibre-matrix stress-transfer [19]. However, like other crop processing steps, hackling induces numerous fibre dislocations, also referred as kink bands [20], which may negatively impact the mechanical properties of the final composite [21],[20]. Therefore, the impact of the process on the mechanical properties of the fibre as well as those of the final composite have to be considered.

In addition to the extraction process, composite manufacturing also requires attention, especially when processing with a thermoplastic matrix. Unlike thermoset resins, conventional thermoplastic resins have
high melting viscosity (500-5000 Pa.s for thermoplastic VS 100 Pa.s for thermoset) which make processing more troublesome in term of resin penetration, poor fibre reinforcement dispersion, mis-alignments and void formation [22]. Therefore, various prior developments of hybrid yarns were considered to overcome these issues [23-25], including flax hybrid yarns [22,26-33]. Various manufacturing methods, including co-wrapping, core spinning and commingling were summarized by Alagirusamy [34]. Among these, wrap spinning appears as a convenient way to produce yarns for composite reinforcements. It consists of twistless commingled staple fibres wrapped by a filament. Studies have already highlighted their benefit on the microstructure homogeneity and the gain of mechanical properties at the composite scale [35],[36]. However, to the best of our knowledge, no study has reported the impact of a commingling process on the ultrastructure and the mechanical properties of the elementary flax fibres.

In the present work, a set of flax / polypropylene (PP) hybrid slivers were manufactured at various flax fibre contents using a lab-scale drawing system. Along the process, elementary flax fibres were collected and tensile tested to measure the impact of the drawing process on their mechanical properties. The hybrids slivers were then consolidated to assess tensile mechanical properties of the unidirectional (UD) composites. A discussion specifically addresses the quality of reinforcement individualisation and composite microstructure resulting from the comingled reinforcement product.

70

## 2. Materials and methods

### 2.1. Materials

Polypropylene (PP) fibres, having a density of 0.905 g/cm³, melting temperature of 165°C and melt flow index 10 g/min (tested at 190°C with 2.16 kg weight) are used as the matrix. PP sliver having a linear density of 10 Ktex, consisting of PP fibres with an average linear density of 6 denier and a staple length of 6 cm was supplied by Peignage Dumortier (Tourcoing, France).

The flax used in this study was cultivated in Normandy, France. Flax seeds were sown in March and pulled 100 days later; then the plants were laid on the soil for dew-retting. Subsequently, the fibres from the flax stems were extracted following classical scutching and hackling processes. Commercial flax reinforcements are usually made of several flax varieties. In this study a flax sliver with a large count (linear mass about 20,000 tex)
81 was supplied by the Natup company (Linière de Saint Martin, France). During sliver manufacturing, flax fibres
82 were also drawn and doubled 3 times.

## 2.2. Preparation of the commingled slivers

84 The flax fibres were blended with PP fibres manually in three different blend proportions (w/w) (30/70, 50/50,
85 70/30). Then, the fibre blends were fed to the laboratory-based drawing machine (Trytex, India) which consists
86 of one gill-drawing unit used for bast fibre drawing. Following this preliminary mix, the large count sliver is
87 submitted to six-drawing stages where the linear mass of the sliver decreased to 150 tex using a lab scale
88 drawing system (Linimpianti company, Villa d'Alme, Italy). During the different stages of this process, six
89 parallel "Gill type" systems perform the different drawing operations. During this stage, the sliver mass is
90 reduced but it is also homogenised as between each drawing stage, six drawn slivers are each time grouped
91 together before the following drawing. Thus, the technical fibre diameter may also be reduced when the
92 technical fibres are pulled from the Gill system pin.

93

## 2.3. Composite manufacturing

95 Unidirectional composites were made from the flax hybrid slivers obtained after the six drawing operations
described previously. For longitudinal specimens, micro-unidirectional PP-flax composites were manufactured
96 using a 100-mm-long aluminium mould with a 6 x 2 mm² section. Three sections of 100 mm length were cut in
97 the flax hybrid sliver and stacked in the mould before thermal compression using a Labtech 50T moulding
98 press. The press was preheated to 190 °C and the composite assembly remained for 2 min at this temperature.
99
100 Then, first compression at 20 bars was applied for two minutes. The last compression at 50 bars was performed
101 for three minutes before the assembly was cooled for four minutes at 25 °C under 100 bars.
102 For transverses specimens, only the 50/50 flax-PP sliver was transformed into a veil by Ecotechnilin (Yvetot,
103 France) following a confidential process. The flax hybrid veil obtained has an areal density of (100 ± 10) g/m². A
104 lay-up of twelve 100-by-100-mm plies of the flax hybrid veil were assembled and put in a mould of dimensions
105 100 mm x 100 mm x 2 mm. Composites were consolidated following the manufacturing process described
106 above. Then, samples measuring 100 mm x 6 mm were cut using a milling machine. Table 1 describes the
107 sample nomenclature of the composites. The fibre volume fraction of composites was evaluated through
108 analysis of composite's cross-sections. 5 surfaces images were collected by SEM and analysed with FIJI
109 software.
<table><caption>Table 1- Sample nomenclature of the composites.</caption><thead><tr><th></th><th>Sample ID</th><th>Fibre weight content targeted (%)</th><th>Fibre volume fraction (%)</th></tr></thead><tbody><tr><td>FHSC 25</td><td>Flax hybrid sliver composite</td><td>30</td><td>25 ± 0.8</td></tr><tr><td>FHSC 36</td><td>Flax hybrid sliver composite</td><td>50</td><td>36 ± 1.6</td></tr><tr><td>FHSC 52</td><td>Flax hybrid sliver composite</td><td>70</td><td>52 ± 2.3</td></tr><tr><td>FHVC 37</td><td>Flax hybrid veil composite</td><td>50</td><td>37 ± 1.2</td></tr></tbody></table>

## 2.4. Tensile testing

To study the mechanical properties of elementary flax fibers, the FDAS (Fibre Dimensional Analysis System) device and the extensometer LEX820, developed by Dia-stron (Diastron Ltd, Hampshire, UK), were used following the XPT25-501-2 standard [37]. The FDAS measures, by ombroscopy, the apparent diameter of the fibres in an elliptical pattern using a laser photodetector (LSM 500S, Mitutoyo, Japan). Five sections were analysed on each elementary fibre. According to Garat et al. [38] for fibre bundles and [39] for elementary fibres, the elliptical model is the most appropriate for measuring the cross-section area of natural fibres such as flax. The LEX820 device allows tests to be carried out on elementary fibres with low-stress breaks. The displacement rate was set at 1 mm/min with a gauge force of 0.05 N and a gauge length of 10 mm. For each batch, more than 50 elementary flax fibres were manually collected at different locations in the sliver. Then, each collected elementary fibre was positioned in a plastic tab before being fixed quickly (approximately 5 s) with a photo-curing glue (DYMAX, Wiesbaden, Germany). Before testing, all the specimens were stabilised during at least 48 hours at 50%RH and 23°C before testing in a room with controlled environmental conditions (50% RH and 23°C).

The determination of the tensile properties of composites, in accordance with the ISO-527-5 standard [40], was carried out on micro-UD composites using a MTS Synergie RT/1000 universal testing machine at a controlled temperature of 23°C, a relative humidity of 50% and a crosshead speed of 1 mm/min, considering the loading frame compliance. The load was measured with a 5-kN sensor and an extensometer with a nominal length of 25 mm used for measuring the sample strain. A first tangent modulus was calculated between a strain of 0.02% and 0.1%. A second tangent modulus was recorded for the longitudinal specimens at a strain of 0.6%-0.8%, due to the bi-linear behaviour of UD flax composites [41]. For each formulation, seven specimens were tested.

## 2.5. Microstructural analysis
135 Flax fibres and flax-PP slivers were investigated by scanning electron microscopy (SEM) to assess the
136 individualization rate, surface quality and the presence of cortical parenchyma residues. Prior to investigations,
137 samples were gold sputter coated (Edwards Sputter Coater) and then analysed with a JEOL JSM-IT500HR SEM.
138 Transverse sections of UD composites were also investigated through SEM observations to particularly
139 determine the porosity content of composites. Images were then analysed by ImageJ software using a
140 morphological analysis tool to get the area of each fibre in the picture. For each composite, five random cross
141 sections were used. Threshold was automatically adjusted on the same min and max grey value for each image.
142 Further investigations were performed by X-ray phase contrast microtomography measurement at Synchrotron
143 SOLEIL on the ANATOMIX beamline [42]. For this tomography measurement a monochromatic X-ray beam of a
144 photon energy of 10 keV was collected from a detector (LuAG:Ce scintillator, 20x Mitutoyo objective (Na 0.48),
145 camera Orca Flash 4.0 V2) resulting in an effective pixel size of 0.325 µm. The sample detector distance was 5
146 mm, and 2000 projections were collected over an angle of 180°. The exposure time was 0.25 s resulting in a
147 scan duration of about 9 min. Volumes were reconstructed with PyHST2 [43] (ESRF, Grenoble, France). A
148 paganin filter was applied during the data reconstruction with a kernel length of 8 µm. The analysis, including
149 segmentation, was then performed with the Avizo 2021.1 software (Thermo Fisher Scientific). Volume were
150 segmented into the material phase of fibres by defining a threshold intensity value.

151

152 2.6. Measurement of cellulose crystallinity

153 A Siemens D500 diffractometer CuKα radiation was used to estimate the cellulose crystallinity by wide-angle X-
154 ray diffraction (WAXRD). Samples were loaded on a silicon wafer and scans were collected from 2θ = 5 to 40°
155 with step size of 0.03° at 2 s/step, at 30 kV and 20 mA; measurements performed under ambient conditions
156 and two repeat tests were performed. Crystallinity was calculated using Eq. 1,

$$
157 \qquad C = \frac{l_{tot} - l_{am}}{l_{tot}} * 100 \qquad (\text{Eq. 1})
$$

158 Where $l_{tot}$ is the intensity at the primary peak for cellulose I (at $2\theta \approx 22.5^{\circ}$) and $l_{am}$ the intensity from the
159 amorphous phase evaluated as the minimum intensity (at $2\theta \approx 18.5^{\circ}$). Standard deviation of the crystallinity was
160 based on 3 samples.

161 2.7. Statistical analysis
Statistical tests, named two sample t-test [44], were performed in order to check whether the tensile properties were statistically different between the two batches in terms of their mean values. A confidence interval of 95% was considered. When the P-value is below 0.05, the mechanical properties being tested are assumed to be significantly different.

## 3. Results and discussion

### 3.1 Impact of the commingling process on mechanical properties of elementary flax fibres

Table 2 gathers the values of the average tensile properties of the flax fibre before and after the commingling process. The mechanical properties of elementary flax fibres extracted from the flax sliver shows high mechanical properties in comparison to the literature data on elementary French flax fibre [13] represented in Figure 1.b. However, such high mechanical properties have already been measured by Charlet et al. on hackled fibres [45] who reported an average Young's modulus value of 76.7±40.8 GPa, an average tensile strength at break of 1795±1127 MPa and a strain at break of 2.4±0.7 %. It is worth noting that flax fibre cross-sectional area (CSA) has been for long assessed by 2D measurement considering their CSA as circular. Recent studies highlighted the importance of accurate morphometric characterisation to perform reliable assessment of the mechanical properties of elementary flax fibre [38], [39]. In this work the elliptical CSA of flax fibre are measured by ombroscopy through FDAS.

**Table 2.** Tensile properties of elementary flax fibres. The letters "a" and "b" refer to the two-sample t-test. Statistically different values belong to the group "b".

<table><thead><tr><th>Sample</th><th>Diameter (µm)</th><th>E (GPa)</th><th>σ (MPa)</th><th>ε (%)</th><th>Crystallinity (%)</th></tr></thead><tbody><tr><td>Flax sliver</td><td>13.74 ± 3.95<br>a</td><td>74.1 ± 17.8<br>a</td><td>1698 ± 465<br>a</td><td>2.44 ± 0.49<br>a</td><td>81.8 ± 0.9</td></tr><tr><td>Flax hybrid sliver</td><td>13.23 ± 3.43<br>a</td><td>67.4 ± 16.6<br>b</td><td>1632 ± 468<br>a</td><td>2.70 ± 0.60<br>a</td><td>84.6 ± 2.0</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** XRD spectra of the flax sliver and the hybrid flax sliver (a). Comparison of tensile modulus versus tensile strength of elementary flax fibre with literature data (b). The blue triangle refers to the flax hybrid sliver and the orange circle refers to the flax sliver.</figcaption></figure>

Nonetheless, the high standard deviation values must be considered in that case. In flax textile industry, the significant scattering of mechanical properties can be tempered by blending fibre batches, which is observed in this study. Regarding the impact of the commingling process on the mechanical properties, the strength at break and the strain at break were not significantly different before and after combing (p-value of 0.35 and 0.08 respectively) whereas a slight but statistically significant decrease of 9% of the Young's modulus is noticed after the commingling stage.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** a: Schematic diagram illustrating stress-strain deformation curves of T1 (Linear), T11 (two distinct linear sections), T111 (one non-linear section followed by an increase in the tangent modulus); b: tensile behaviour distribution of elementary flax fibres extracted from the sliver and from the hybrid sliver.</figcaption></figure>
Elementary flax fibres have different tensile behaviour than glass fibres; Figure 2.a shows the three most common behaviour types (TI, TII and TIII), due to the specific ultrastructure and bio-chemical composition of flax fibres as described by Richely et al [46]. Indeed, tensile behaviour is mainly impacted by the cellulose content, microfibril orientation, stick-slip mechanisms during stretching and the presence of defects such as kink bands [47], [48]. In literature, a TIII predominant behaviour is mainly observed for non-hackled flax fibres [49], [50]. This non-linear behaviour has been well documented and explained by complementary phenomena[46]: (I) The elastic deformation of the constitutive polymer in addition to partial re-arrangement of cellulose microfibrils and subsequent change in microfibril angle (MFA) even at low loading levels [49], [47]; (II) The stick-slip mechanism presented by Placet et al. [51], [47]; (III) The strain-induced crystallisation of amorphous cellulose upon loading, evidenced in Figure 1a/Table 2, leading to irreversible stiffening of the fibre in the axial direction [52]. In addition to the aforementioned mechanisms, significant reversible re-alignment of the MFA is observed in the dislocation zones [47], [53], [54].

In the present study, for each fibre, the type of behaviour was determined and the distributions were addressed in Figure 2.b. The stress-strain response TI was predominant, accounting for 69% and 79% of the flax sliver and the flax hybrid sliver respectively. Also, a transformation of the TII behaviour into TI behaviour was noticed after the commingling process. To the best of our knowledge, no similar results were reported on hackled elementary flax fibre. Only Barbulé has estimated the tensile properties of stretched flax bundles [55]. No significant impact of the drawing process on the Young's modulus and the tensile strength of the bundle was mentioned. Fibre extraction is known to promote the formation of dislocation zones [20], [56], [57] but up to now, no straightforward relationship between the tensile behaviour of elementary flax fibre and the number of defects has been clearly established [58], [59], [60].

To explain this TI predominant behaviour, we assumed that the drawing step acts in the form of progressive tensile cycles. As mentioned in the methods section, the hybrid sliver was stretched 6 times. Placet et al. have already demonstrated the stiffening of type III hemp fibre with increasing strain and the deployment of the cellulose microfibrils in the dislocation zones during repeated and progressive loadings [47]. They have shown that the level of the yield point, characteristic of a TIII behaviour increased after each additional progressive loading phase. Correlated with the evolution of the MFA observed during unidirectional loading by the authors [53], [54] we assume that the cellulose microfibrils are prone to realign and the dislocation zones induced during the drawing process are submitted to recrystallization. In our case, a significant increase of the fibre
227 crystallization degree is demonstrated by XRD analysis after drawing, confirming this literature A (Table2,
228 Fig.1.a).

229 Beyond the linear behaviour of the stretched elementary flax fibre, it is worth noting that tensile properties are
230 not significantly lower after the commingling process, which appears as a promising technique in the
231 development of plant-fibre thermoplastic composites.

232

233 **3.2. Impact of the process on the fibre individualisation and composite microstructure**

234 To reach high mechanical performance, optimising composite microstructure to translate the properties of the
235 reinforcement fibres is crucial. The interest of the commingling process has already been established but a
236 focus is addressed regarding the benefit on reinforcement individualisation, especially flax bundles.
237 contain single fibres linked together by a middle lamella which induces a modification of the damage
238 mechanism and act as weaknesses inside the composite [14],[61]. Figure 3 shows the composite's cross
239 section, as obtained by SEM analysis. All composites are highly individualised and have a relatively
240 homogeneous distribution of fibre despite the presence of few bundles. The reinforcement area distribution
241 obtained through the unidirectional cross section analysis (fig4a) is well correlated with the SEM images (Fig.3).
242 All composites show a content of individualised fibres above 80%. A slight increase of the individualisation has
243 been noticed after processing the hybrid flax sliver into a veil due to an additional drawing step.

244

<figure><img src="image_4.png" /></figure>

245

<caption>246 **Figure 3** Cross section SEM image of FHSC 25 (a), FHSC 36 (b), FHSC 52 (c) and FHVC 37 (d).</caption>
<figure><img src="image_5.png" /><figcaption>b</figcaption></figure>

<table><thead><tr><td>Sample</td><td>Elementary fiber (&lt;25um) %</td><td>SD</td></tr></thead><tbody><tr><td>FHSC 25</td><td>86.9</td><td>1.69</td></tr><tr><td>FHSC 36</td><td>79.8</td><td>1.98</td></tr><tr><td>FHSC 52</td><td>79.1</td><td>9.86</td></tr><tr><td>FHVC 37</td><td>83.0</td><td>1.55</td></tr></tbody></table>

**Figure 4** Reinforcement area distribution in the four composites (a) and their corresponding content of elementary flax fibre (b).

Figure 5 shows tomography representations of the flax bundles extracted from the sliver, from the hybrid sliver and from the hybrid veil. Residual cortical parenchyma and middle lamellae residue are still present around and within the bundles of the flax sliver (Fig.5.b and d), even after hackling. After the commingling process, this amount is drastically reduced (Fig.5.f) and almost no residues are observed on the flax fibres extracted from the veil. Hackling and drawing processes are known to promote fibre individualisation as well as improving the percentage of non-polluted fibre interfaces [55], leading to better load transfer. However, their benefits on tensile properties at the composite scale may be balanced by the formation of defects, such as crushed fibres (Fig.5.g) and kink-bands (Fig.5.k). Although their impacts at the fibre scale remain unclear, their negative impacts within composite were reported by many authors. During loading, stress concentration occurs in these areas, resulting in fibre-matrix debonding, matrix micro-cracking and fibre failure [61], [62], [63]. Images of damaged fibres are highlighted in Fig.5.j and k. Even though no kink-band quantification was performed, no obvious apparent increase in defects was noticed after the commingling process on these tomography observations, which is correlated with the hypothesis of dislocation recrystallization during drawing, as discussed in the previous section 3.1.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** SEM images and 3D views from micro-CT of flax bundles from the raw hackled sliver (a,b,c,d; yellow boxes); flax bundles from the hybrid sliver (e,f,g; green boxes) and flax bundles from the hybrid veil (h,i,j,k; blue boxes). Cortical parenchyma are coloured in brown.</figcaption></figure>

This point highlights the important role of hackling in obtaining highly individualised fibres in the composite without decreasing the tensile properties of the elementary flax fibres.

**3.4. Mechanical properties of UD composites**

First, supported by SEM images (fig3), all composites exhibit a very low porosity content, below 1.5% (Table.3), even if a greater porosity content was noticed for FHSC 25 specimens. Up to 36%-Vf of flax fibre reinforcement, specimens are in the upper range of longitudinal tensile properties measured on UD flax reinforced PP composites, whereas the benefit of increasing the fibre volume fraction up to 52% is less pronounced, especially for the Young's modulus (Fig.6a,b). Regarding FHSC25, the longitudinal modulus and the longitudinal strength are, respectively, 25% and 52% higher than the average trend line obtained from literature values.

Similar observations were made for FHSC36 samples with 16% and 18% increase regarding, respectively, the longitudinal modulus and the longitudinal strength. In order to estimate the impact of the consolidation step, it is possible to back-calculate the longitudinal fibre modulus using the 'simple' rule of mixtures [64]. Results are presented in table 3. Until a flax fibre volume fraction of 36%, composite benefits of the best of the tensile
properties of the reinforcement (Table3). However, a significant decrease of 38% of the estimated back-calculated fibre tensile modulus Ef1 is noticed for FHSC 52 compared to the measured Young's modulus of elementary flax fibres from the hybrid yarn. Such a drop may be explained by an artificial reconstruction of flax bundles through agglomeration of individualised elementary fibres during the consolidation step, introducing a notion of 'maximum' fibre volume, penalizing the impregnation and the homogeneity of the material. This assumption is supported by the high standard deviation measured when estimating the reinforcement individualisation rate within FHSC 52 (Fig4.b).

292

<table><caption>Table 3. Unidirectional tensile properties of the composites and predicted Young modulus.</caption><thead><tr><th>Sample</th><th>Fibre volume fraction (%)</th><th>Porosity (%)</th><th>Stress at break (MPa)</th><th>E1 (Gpa)</th><th>E2 (Gpa)</th><th>Strain at break (%)</th><th>Ef1 (GPa)</th></tr></thead><tbody><tr><td>FHSC 25</td><td>25 ± 0.8</td><td>1.32 ± 1.1</td><td>155.7 ± 4.2</td><td>17.9 ± 0.15</td><td>11.6 ± 0.9</td><td>1.20 ± 0.12</td><td>67.73</td></tr><tr><td>FHSC 36</td><td>36 ± 1.6</td><td>0.6 ± 0.3</td><td>194.0 ± 29.2</td><td>24.2 ± 4.90</td><td>16.6 ± 1.9</td><td>1.07 ± 0.20</td><td>64.27</td></tr><tr><td>FHSC 52</td><td>52 ± 2.3</td><td>0.7 ± 0.1</td><td>264.9 ± 28.2</td><td>25.7 ± 2.31</td><td>20.9 ± 0.9</td><td>1.30 ± 0.16</td><td>41.43</td></tr><tr><td>FHVC 37</td><td>37 ± 1.2</td><td>0.4 ± 0.2</td><td>195.9 ± 19.4</td><td>23.8 ± 3.02</td><td>15.9 ± 1.4</td><td>1.13 ± 0.08</td><td>63.11</td></tr></tbody></table>

294

295

Regarding transverse tensile loading (Fig.6.c,d), FHVC 37 stands out from the mechanical properties reported from literature. Samples display a transverse Young's modulus of (2.36 ± 0.08) GPa and a transverse strength of 16 ± 0.9 MPa. Transverse mechanical properties are known to be a major weakness of unidirectional composites materials. Therefore, many previous studies have examined the parameters which influence the transverse strength of plant fibre composites and have been summarized by Baley et al [14]. Among others, reinforcement heterogeneity appears as a main cause of failure due to high strain concentration between fibre-rich and resin-rich areas, leading to micro cracks initiation at the interfaces. Additionally, interfaces have an essential role in the stress transfer, regarding either the adhesion of the polymer on the fibre or the surface quality of the fibre itself [65]. As mentioned in the section 3.2, the commingling process appears as a promising way to promote fibre reinforcement individualisation and to remove residual cortical parenchyma on elementary and flax bundles. At the composite scale, this benefit is expressed as an increase of the transverse mechanical properties, especially the strength at break (Fig6.d). It is worth noting that in case of transverse loading it is better to work at low fibre volume fraction to prevent the formation of fibre/fibre interfaces, which drastically reduce the stress transfer within the composite.

310
<figure><img src="image_7.png" /><figcaption>**Figure 7.** Comparison of longitudinal tensile modulus (a) and tensile strength (b) as a function of fibre volume fraction for unidirectional PP flax fibre reinforced composites [36],[65],[66],[67],[68],[69]. Comparison of transverse tensile modulus (c) and tensile strength (d) as a function of fibre volume fraction for unidirectional PP flax fibre reinforced composites.</figcaption></figure>

## 4. Conclusions

This work has shown that the commingling process offers opportunities to optimise the reinforcement effect of flax fibre in thermoplastic composites. Transverse tensile properties are remarkably higher than in literature, and longitudinal tensile properties are in the upper range of conventional PP-flax composites; for example, the longitudinal Young's modulus and strength are, respectively, 25% and 52% higher than the average trend line obtained from literature values. This is probably due to improved fibre reinforcement individualisation and distribution, with elementary fibre rate reaching 86.3% for our best batch, but also to the high flax fibre surface quality, demonstrated through SEM and micro-CT analysis and to the crystallisation increase of flax fibre during sliver manufacturing. This is particularly interesting for the development of semi-consolidated tape manufacturing for automated fibre placement (AFP). A forthcoming study is underway to assess the potential of such a hybrid sliver manufactured by AFP process in load bearing applications; the added value of poly-(lactide) or polyamide matrix will be also investigated, in order to reach higher mechanical performances.
329 Acknowledgements

330 Elouan Guillou would like to gratefully acknowledge Laval agglomération (https://www.agglo-laval.fr) and the
331 Région Pays de Loire (https://www.paysdelaloire.fr/) for PhD funding. The authors thank Synchrotron SOLEIL
332 for the ANATOMIX beamline time (proposal no. 20211421). ANATOMIX is an Equipment of Excellence
333 (EQUIPEX) funded by the *Investments for the Future* program of the French National Research Agency (ANR),
334 project *NanoimagesX*, grant no. ANR-11-EQPX-0031.

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