Page 308 - Handbook of Properties of Textile and Technical Fibres

282 Handbook of Properties of Textile and Technical Fibres

the growth environment of the fiber, consists of the size of the so-called lumen
(originally the living cell compartment), which usually represents 6%e8% of the fiber
section (Charlet et al., 2010).

8.3.2 Fiber composition
Flax fibers have been described as multilayer composite structures composed of
three main layers: S1 (w0.5e2 mm), S2 (w5e10 mm), and S3 (w0.5e1 mm) (Hearle,
1963; Roland et al., 1995). The main S2 structural layer is responsible for most of the
mechanical performance and the physical properties of the fibers. The main constituent
of elementary fibers is cellulose, the percentage of which is generally greater than 80%
(Baley et al., 2014; Bourmaud et al., 2013b) and with a high percentage of crystallinity
(Nishiyama et al., 1999). The presence of other constituents, summarized in several
reviews (McDougall, 1993; Morvan et al., 2003; Gorshkova et al., 2010), reveals
the main difference from cotton, which is composed mainly of cellulose (van Dam
and Gorshkova, 2003). Among the main noncellulosic polysaccharides (ncps)
are the so-called “pectic” polysaccharides, such as homogalacturonan (HGA) and
rhamnogalacturonan (RGI), containing galacturonic acid (GalA). They account for
about half of the ncps (Alix et al., 2009; Lefeuvre et al., 2014a; Rihouey et al.,
2017). A significant part of these ncps is easily extractable and constitutes the gel-
like matrix in which the coated-cellulose microfibrils are embedded, whereas some
HGA and long galactans side chains of RGI (RGI-GalL) are difficult to extract
when compared to gel-matrix polymers because they are extractable in boiling alkali,
being considered as structuring ncps (Rihouey et al., 2017). “Neutral” polysaccharides,
such as glucomannan (GM), xylan (Xn), and xyloglucan (XG), which were originally
named hemicelluloses and extracted with alkali, are the main structuring ncps.
They are tightly hydrogen-associated with cellulose microfibrils, coating the cellulose
surface or being trapped between two surfaces, interacting between themselves
and possibly crossing the gap between adjacent cellulose microfibrils, as described
in Alix et al. (2009). Together, the structuring ncps constitute the interface/interphase
between cellulose microfibrils and the gel-matrix ncps (Baley et al., 2014).
Despite their relatively low content, ncps have been shown to play an important role
in flax fiber structure and properties. For example, the ratio of structuring and gel-
matrix HGA and RGI was correlated with the fiber tensile properties (Bourmaud
et al., 2013b). Rihouey et al. (2017) have proposed a structural model in which
the ncps are distributed differently in the cell wall layers and interact differently
with cellulose and between themselves. Especially, GM and RGI-GalL were found
to be the most tightly bound and are thought to be located in the S2 layer. Xylan
and XG, which were easily extracted with alkali at moderate temperature, were local-
ized in the most external layers of the fibers.

8.3.3 Specific gravity

The fiber specific gravity is an important parameter when the fibers are used as
reinforcing elements in a polymer because the mechanical parameters are often divided