Page 314 - Handbook of Properties of Textile and Technical Fibres

288 Handbook of Properties of Textile and Technical Fibres

properties and a specific gravity of 1.5, flax fibers have specific tensile properties
similar to those of glass fibers (Baley and Bourmaud, 2014).

8.4.7 Analysis of the reproducibility of mechanical properties
from 1 year to another
Reproducibility of flax performance, from one harvesting year to another, is often
considered an issue for natural fiber composite development (Lefeuvre et al.,
2013; Haag et al., 2017). During four harvesting years (2009e12), this parameter
has been studied for one variety (Marylin) and one crop area (plateau du Neubourg,
France) by Lefeuvre et al. (2014). Crop rotation is necessary for flax harvesting:
generally flax is harvested every 7 years from any one field. A careful field selec-
tion was therefore made whilst seeds, seeding rate, harvesting rules and the
retting process used were carefully maintained similar. Weather conditions during
growing steps were seen to differ (2 standard years, one rainy year, and the last
one with drought), which drastically influenced stem yield (Lefeuvre et al.,
2014b). Thirteen fiber batches were selected within these 4 years and then character-
ized in tension. Observations showed that the mean mechanical properties evolved
slightly with an increase of stiffness from53.3to 58.9 GPa,strengthfrom970
to 1109 MPa, and strain at break from 2% to 2.2% (Lefeuvre et al., 2014b).
Thus, considering the large differences of weather condition, the mechanical
response of flax fiber was hardly modified. In addition, the weather is not the only
parameter to be taken into account. Retting conditions on the field cannot be
controlled, which bring supplementary heterogeneities between fibers (Martin
et al., 2013). Mechanical extraction of fibers (scutching and hackling) could also
affect fiber properties (Baley, 2004). In an industrial context, fibers are used depend-
ing on the production date (not on their production location), and mixtures of
different batches are made.

8.4.8 Impact of growing conditions
2
2
The number of plants harvested per m (from 1200 to 2500 seeds/m )isanother
parameter that needs to be taken into account (Bourmaud et al., 2016a). As harvesters
are looking for an optimal yield, a close analysis reveals that this parameter influ-
ences not only the stem morphology (height and diameter), the lodging stability,
2
but also the fiber mechanical properties. Beyond 1700 plant/m , not all the seeds
germinate.
Generally speaking, numerous parameters need to be taken into account at the plant
scale (variety, genetic patrimony, seeding quality, etc.), growing conditions (physico-
chemical and biological soil characteristics, soil structuration, previous crops, water
feeding, temperature, sunshine, wind, day duration, etc.), harvesting rules (plowing,
seeding date, seeding rate, fertilization, treatments, etc.), plant dehydration, retting,
plant storage, fiber extraction (scotching, hackling, carding, etc.).