Flax (Linum usitatissimum L.) Fibers for Composite Reinforcement: Exploring the Link Between Plant Growth, Cell Walls Development, and Fiber Properties

Abstract

Due to the combination of high mechanical performances and plant-based origin, flax fibers are interesting reinforcement for environmentally friendly composite materials. An increasing amount of research articles and reviews focuses on the processing and properties of flax-based products, without taking into account the original key role of flax fibers, namely, reinforcement elements of the flax stem (Linum usitatissimum L.). The ontogeny of the plant, scattering of fiber properties along the plant, or the plant growth conditions are rarely considered. Conversely, exploring the development of flax fibers and parameters influencing the plant mechanical properties (at the whole plant or fiber scale) could be an interesting way to control and/or optimize fiber performances, and to a greater extent, flax fiber-based products. The first part of the present review synthesized the general knowledge about the growth stages of flax plants and the internal organization of the stem biological tissues. Additionally, key findings regarding the development of its fibers, from elongation to thickening, are reviewed to offer a piece of explanation of the uncommon morphological properties of flax fibers. Then, the slenderness of flax is illustrated by comparison of data given in scientific research on herbaceous plants and woody ones. In the second section, a state of the art of the varietal selection of several main industrial crops is given. This section includes the different selection criteria as well as an overview of their impact on plant characteristics. A particular interest is given to the lodging resistance and the understanding of this undesired phenomenon. The third section reviews the influence of the cultural conditions, including seedling rate and its relation with the wind in a plant canopy, as well as the impact of main tropisms (namely, thigmotropism, seismotropism, and gravitropism) on the stem and fiber characteristics. This section illustrates the mechanisms of plant adaptation, and how the environment can modify the plant biomechanical properties. Finally, this review asks botanists, breeders, and farmers' knowledge toward the selection of potential flax varieties dedicated to composite applications, through optimized fiber performances. All along the paper, both fibers morphology and mechanical properties are discussed, in constant link with their use for composite materials reinforcement.

Keywords: biomechanics; breeding; cell wall; fiber crops; flax fiber; growth conditions; plant development.

FIGURE 1

Scheme illustrating the growth of flax starting from the sowing day, according to the time after sowing and corresponding cumulative growing degree-day (GDD). G: germination; VS: vegetative stage; F: flowering; SF: seed formation; FM: fiber maturity; SM: seed maturity; and S: senescence.

FIGURE 2

Description of identifiable tissues and cells on the transverse cross-section in the bottom part of a growing flax stained with toluidine blue.

FIGURE 3

Scheme illustrating the fiber elongation, with fibers represented in green. (A) First phase of the coordinated growth; (B) more advanced step of the coordinated growth; (C) beginning of the intrusive growth with fibers having “knees” on both ends; (D) more advanced phase of the intrusive growth, with fibers becoming a much longer structure than the neighboring cells and showing tapered ends.

FIGURE 4

(A) Pictures of two flax stems of different heights having a different snap point location. (B) Scheme illustrating the different stages of the fiber formation along a flax stem according to the localization of the snap point.

FIGURE 5

(A) Scheme illustrating the different stages of the fiber thickening, starting from a cell having only a primary cell wall (PW) and ending with a fiber having a thick G-layer, a small lumen and a possibly remaining thin Gn-layer. (B) AFM image of an intermediate stage of cell wall thickening showing the G-layer and the innermost newest Gn-layer. (C) AFM image of a final stage of cell wall thickening showing the thick G-layer and the innermost residual Gn-layer.

FIGURE 6

Scheme illustrating the cell wall thickening in relation to the remodeling of the Gn-layer. Cellulose microfibrils are originally separated by long galactan chains present in nascent RG I [5]. The long galactan chains are then trimmed off which leads to the G-layer with compact cellulose microfibrils. Scheme inspired from Goudenhooft et al. (2018).

FIGURE 7

Interspecific scaling of plant height (H) as a function of the plant basal diameter (D) of flax in comparison with scaling formulas and maximal dispersion of non-woody plants (190 species) and woody plants (420 species) from data proposed by Niklas (1993b).

FIGURE 8

Scheme illustrating the main steps of the varietal selection in the case of flax, from the parents P₁ and P₂ to the numerous filial generations F_i to achieve the obtainment of a new selected variety. Scheme inspired from Bert (2013) and Evert and Eichorn (2013).

FIGURE 9

Lodged flax plants in a field at the end of the F stage.

FIGURE 10

Main notions addressed in this section, with corresponding examples of illustration for flax. (A) Sowing density. (B) Crop canopy. (C) Response to a mechanical stimulus (plants in a field are visible on the left picture whereas they are grown in a greenhouse on the right picture). (D) Gravitropic response. The written informed consent was obtained from the two individuals for the publication of this image.

FIGURE 11

Flax plants moving in the wind in a wave-like movement. The wave is visible as it bends over the plants as it passes.