Multi feature consequences of morphological defects on the properties of a composite made by Nature, flax fibres .
Topic(s) : Special Sessions
Co-authors :
Loren MORGILLO (FRANCE), Lèna BRIONNE , Marwa ABIDA , Johnny BEAUGRAND (FRANCE), Alain BOURMAUD (FRANCE)Abstract :
Individual flax fibre reveals a multi-components structure that can be described as a complex composite material. They are made of three different cell wall layers (S1, S2-G, S3-Gn), S2 driving the fibre properties due to its large size. This layer consists of a spiral of highly crystalline cellulose fibrils in a matrix of amorphous hemicelluloses and pectins and can be considered a composite material in its own right (Bourmaud et al., 2013). S2 microfibrils are oriented at 3-7°, but can be misoriented in area of defects, where the MFA can reach up to 35-40° (Melelli et al., 2020). These structural defects of the fibres, known as "kink-bands", are mainly generated by the flax fibre extraction processes (scutching and combing). They are often associated with a reduction in the mechanical properties of fibres and fibre-reinforced materials (Richely et al., 2022). In this study, we therefore aim to understand the complex role of kink-bands on the properties of individual flax fibres.
Microtomographic analysis, carried out on fibres of the same variety but which have not undergone the same extraction intensity, revealed that kink-bands are areas of high porosity. Figure 1 shows the differences between kink-bands of uncombed and combed flax fibres. One can notice differences in porosity between the two samples, pores being concentrated in kink-bands regions in the case of combed flax (Morgillo et al., 2023) (Fig 1.F).
Defect zones have therefore a very specific structure which has consequences at several levels. Beyong the MFA deviations, kink-bands have a direct influence on the properties of the cellulose. Indeed, after analysing flax fibres with different kink-bands contents, X-ray diffraction (XRD) analysis show that fibres with a higher kink-bands content have a lower crystallinity than fibres with fewer defects. Moreover, dynamic vapour sorption (DVS) testing demonstrates that for high relative humidity, flax fibres with a high kink-band content are able to absorb more water. According to our results, kink-bands regions are therefore more amorphous and porous areas, which create entry zones for water, but also lead to a reduction in mechanical properties. A multi-scale statistical study was carried out on 96 elementary flax fibres of the Felice (2021) variety. We demonstrated that kink-bands have a direct impact on mechanical properties, in particular, fibres with a low-size kink-bands have a higher tensile strength and modulus than fibres with large kink-bands (Fig. 2). Finally, intensely combed fibres exhibit a type I linear elastic behaviour, demonstrating a significant change in behaviour compared to the uncombed batch, attributed to local reorientation and pre-stretching.
In conclusion, kink-bands are complex defects elements, affecting the properties of the fibre at different scales, which it is important to understand in order to improve the biocomposites of tomorrow.
Microtomographic analysis, carried out on fibres of the same variety but which have not undergone the same extraction intensity, revealed that kink-bands are areas of high porosity. Figure 1 shows the differences between kink-bands of uncombed and combed flax fibres. One can notice differences in porosity between the two samples, pores being concentrated in kink-bands regions in the case of combed flax (Morgillo et al., 2023) (Fig 1.F).
Defect zones have therefore a very specific structure which has consequences at several levels. Beyong the MFA deviations, kink-bands have a direct influence on the properties of the cellulose. Indeed, after analysing flax fibres with different kink-bands contents, X-ray diffraction (XRD) analysis show that fibres with a higher kink-bands content have a lower crystallinity than fibres with fewer defects. Moreover, dynamic vapour sorption (DVS) testing demonstrates that for high relative humidity, flax fibres with a high kink-band content are able to absorb more water. According to our results, kink-bands regions are therefore more amorphous and porous areas, which create entry zones for water, but also lead to a reduction in mechanical properties. A multi-scale statistical study was carried out on 96 elementary flax fibres of the Felice (2021) variety. We demonstrated that kink-bands have a direct impact on mechanical properties, in particular, fibres with a low-size kink-bands have a higher tensile strength and modulus than fibres with large kink-bands (Fig. 2). Finally, intensely combed fibres exhibit a type I linear elastic behaviour, demonstrating a significant change in behaviour compared to the uncombed batch, attributed to local reorientation and pre-stretching.
In conclusion, kink-bands are complex defects elements, affecting the properties of the fibre at different scales, which it is important to understand in order to improve the biocomposites of tomorrow.