Page 305 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of flax fibers 279

3. Structuring of the cell wall is a thickening step that proceeds for around 2 months, almost
to plant maturity (van Dam andand Gorshkova, 2003). Structuring also involves the
consolidation of the fiber junctions. In a stem, the shared junctions between fibers ensure
the load transfer between fibers (within a bundle). The junctions containing two main types
of pectins, homogalacturonan and rhamnogalacturonan (And eme-Onzighi et al., 2000),
consist of two morphological domains: (1) the middle lamellae between the primary cell
walls of two neighboring fibers and (2) the tricellular junctions of three fibers in the corners
(Jauneau et al., 1994). The pectic compositions of fiber junctions vary based on space and
time during stem growth and adapt to the differentiation stages of the fibers’ development
(elongation, expansion, and cell wall thickening). Especially, junction remodeling may be
catalyzed by enzymes (e.g., de-esterification of pectins and pectin cross-linking) either via
calcium ions (leading to the well-known calcium pectate) or via some phenolic compounds
(e.g., ferulic and coumaric acids). Junction chemical variation might also occur by apposition
of newly synthesized pectic polysaccharides. The strength of the junctions generally
increases during the expansion and thickening of the fibers, but the detailed molecular
mechanisms depend on the culture environment (Baley et al., 2014).

8.2.3 Control of the maturity of the plants before uprooting
In France, the plants are pulled out at seed maturity, when the accumulated temperature
received by the plants reaches between 950 and 1100 C(Bert, 2013). The accumulated

temperature has been used for years by farmers to estimate the development and the
harvesting time of flax. The formula used to calculate this parameter is

n
T max þ T min
X
ST ¼ 5
2
i¼1

where ST is the accumulated temperature, T max ( C) is the maximal temperature, and

T min ( C) is the minimum temperature all reached on day i. Moreover, i ¼ 1 on the day
of sowing.
There is no growth under 5 C and there are specific values of ST for every stage

of plant development, i.e., 50 C for the germination, 550 C for the flowering,

650 C for the seed formation, 950e1100 Cat fiber maturity. After 950 C ST, there

is no increase of fiber mass whatever the environment and the variety (Bert, 2013).

The seed maturity occurs at ST ¼ 1150 C, and lignification of fibers might happen,
which shifts down the fiber quality: the retting process is slowed down by the fiber
lignification, which increases the risk of overretting (Section 8.2.5), and impact on
the scutching strength to be applied on the stems to extract and divide the fiber bundle.
The plant-maturity control is hence important to avoid the fiber lignification.
8.2.4 The retting step
To extract the fibers from the stem, they must be freed from the external cortical
tissues and the internal woody part, which is performed via the partial degradation
of cell junctions during the retting process (Baley et al., 2014).