**1. Introduction**

The intensification of global warming coupled with a gradual depletion of petrosourced resources is pushing our societies and industries to use an increasing number of biomass resources in an ecologically responsible approach. The composite materials sector is not an exception to this trend; for several decades, plant fibres, especially flax, have offered a solution to replace predominant synthetic reinforcements, such as glass fibres. Indeed, flax fibres have some advantages: a lower density [1], specific mechanical

**Citation:** Gautreau, M.; Kervoelen, A.; Barteau, G.; Delattre, F.; Colinart, T.; Pierre, F.; Hauguel, M.; Le Moigne, N.; Guillon, F.; Bourmaud, A.; et al. Fibre Individualisation and Mechanical Properties of a Flax-PLA Non-Woven Composite Following Physical Pre-Treatments. *Coatings* **2021**, *11*, 846. https://doi.org/ 10.3390/coatings11070846

Received: 23 June 2021 Accepted: 11 July 2021 Published: 14 July 2021

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properties equivalent to glass fibres [2,3] and a natural and renewable origin. Generally, the use of flax fibres makes it possible to reduce the carbon footprint of performance composite materials [4], thus enables one to obtain a less impactful life cycle.

Flax processing provides a range of products of different qualities. At the scutching step, good-quality scutched fibres are produced but also low-cost and qualitative scutching tows, which comprise a mixture of elementary fibres, fibre bundles and shives [5]. In industry, they are mainly used for the production of non-woven preforms. These products, manufactured at low cost [6] and in high volume, are particularly popular in the automotive sector to design and produce non-structural interior composite parts [7]. Currently, such preforms are made using a mix of plant fibres and thermoplastic fibres, such as poly-(propylene) (PP), which has good mechanical properties as well as a low melting temperature, which is an important parameter to prevent plant fibre damage during the processing stage [8,9]. To improve the end of life of these materials [10], it is interesting to study the coupling of flax fibres with a biobased and biodegradable polymer matrix such as poly-(lactid) (PLA) or poly-(β-hydroxybutyrate) (PHB).

Cellulose is the main component of flax fibres, which varies in content between 50% and 80% depending on the variety considered [11]. Other non-cellulosic components are hemicelluloses (between 11% and 20%), pectins (2% and 8%) and lignins (approximately 1%–3%). The structure of an elementary flax fibre is characterised by a primary wall, a secondary wall divided into several sub-layers, generally 3, denoted S1, S2 and S3, and a cavity in its centre called the lumen [12]. The thickest S2 sub-layer (5–10 μm) largely influences the mechanical properties of the fibre. The cohesion of elementary fibres within fibre bundles is ensured by the pectin-rich middle lamella [13]. However, hemicelluloses and lignins can also be found in this latter part. Non-wovens are made using tows, which contain both elementary fibres and bundles but also some woody sticks to a certain extent; these materials are named shives and are considered contaminants by materials engineers. The shives composition and mechanical properties strongly differ from those of technical fibres [14,15], and contain a reduced fraction of cellulose and more lignin.

For industrial materials, mechanical properties are among the predominant characteristics determining future functionalities and uses. Non-woven composites have interesting mechanical properties; according to Ashby material charts, they are superior to injectionmoulded composites thanks to longer fibres but inferior to unidirectional composites (UDs) [16]. In general, in plant fibre-reinforced composites, individualisation of fibre bundles into elementary fibres is one of the key parameters to obtain good mechanical properties. Indeed, the middle lamella, which ensures fibre cohesion within fibre bundles, is an area of weakness due to its poor mechanical properties [17]. In addition, an aspect ratio (length/diameter) for the fibres of greater than 10 is considered the minimum value for efficient stress transfer during mechanical stress loading [18]. As an example, a positive impact of individualisation was observed on the mechanical properties of flax/epoxy UD composites [19]. These different elements show that by damaging or solubilising the middle lamella, an increase in elementary fibre content, and, at the same time, a decrease in fibre bundle content are promoted. At the composite scale, this modification of the architecture of the fibres allows for a larger specific fibre/matrix interfacial area as well as an optimised aspect ratio, and; therefore, better mechanical properties.

To induce this individualisation for fibre bundles, several types of pre-treatments exist, potentially used as alternatives or post-assisting to retting [20]. First, chemical pre-treatments, such as the use of chelating agents such as ethylenediaminetetraacetic acid (EDTA), are frequently used for the defibrization of plant fibres. Chelation removes calcium ions from the pectins of the middle lamella [21]. Alkaline treatments of the NaOH or KOH type are commonly used to eliminate non-cellulosic components of the fibre, such as hemicelluloses and pectins [22]. However, using chemical agents used to modify plant fibres penalise their eco-friendly aspect and can possibly reduce fibre and composite performance [23]. In this regard, physical and chemical-free pre-treatments are needed to increase the individualisation for fibre bundles. Ultrasonic treatment of hemp fibres has been shown to reduce the diameter of fibre bundles by removing pectic and hemicellulosic structures [24]. This elimination of cell-wall polymers is possibly due to the phenomenon of cavitation; the asymmetric implosion of bubbles near the material results in turbulence, which can damage certain binding constituents, such as those in the middle lamella in particular. Ultrasound treatment allows further changes to the fibre. In fact, ultrasonically treated fibres show a significant reduction in water sorption [25] as well as a delignification process [26]. Other physical pre-treatments that do not involve immersion in a solvent such as gamma irradiation can be used to promote individualisation for fibre bundles. The use of gamma irradiation for lignocellulosic fibres shows interesting effects. Scanning Electron Microscope (SEM) observations showed that the primary wall and the middle lamella of henequen fibres are strongly impacted by gamma irradiation [27]. In addition, non-cellulosic polymers such as low DP hemicelluloses, impurities and waxes undergo degradation at low irradiation doses (<10 kGy) [28]. Therefore, it appears to be possible to preferentially degrade the pectins in the middle lamella while preserving the cellulose structure and thermal and mechanical properties of plant fibres, providing that low irradiation doses are used (<30 kGy) [28]. Gamma irradiation can decrease water uptake for fibres treated at low doses thanks to the removal of hydrophilic components; jute fibres irradiated and then immersed in water for 30 days indicate a 14% drop in water absorption [29]. Gamma irradiation can also promote fibre/matrix interfacial adhesion by modifying the surface topography of plant fibres and enhancing their roughness [27], thus allowing better mechanical interlocking and a potential increase in interphase properties with a polymer matrix.

The aim of this work is to apply free-of-chemicals pre-treatments to maximise the environmental benefit of flax technical fibres. Two physical pre-treatments: ultrasound and gamma irradiation were applied to flax tows with the aim of increasing the specific surface area and aspect ratio of the flax reinforcements. The efficiency of these physical pre-treatments was evaluated by monitoring the individualisation into elementary fibres and the reduction in the size of the fibre bundles by means of a dynamic morphological analysis. In addition, the chemical composition and water uptake of (un)treated tows was evaluated. Then, treated flax tows were used to produce non-woven flax-PLA composites, and the impact of pre-treatments on their microstructure and mechanical properties was studied. Finally, an environmental analysis of the various treatments was undertaken to assess whether or not the environmental impact of the pre-treatments is justified by the potential gain in the mechanical properties of the related composites.

#### **2. Materials and Methods**
