Page 309 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of flax fibers 283

by the specific gravity to lead to the so-called specific characteristics of a composite.
The literature values of flax fiber specific gravity range between 1.4 and 1.6 (Truong
et al., 2009), a range which might be due to various lumen sizes and account for
differences of division of the technical fibers water uptake. For example, the fiber
specific gravity was estimated to be 1.38 in the presence of a lumen representing
10.7% of the fiber volume (Batra, 1998). On the other hand, the cell wall specific
gravity has been estimated between 1.53 and 1.55 (Batra, 1998; Tortora and Collier,
1997; Baley, 2002). The method of specific gravity evaluation might also lead to
differences between laboratory techniques used (Archimede, buoyancy method, gas
pycnometry, liquid pycnometry, etc.) (Batra, 1998; Truong et al., 2009; Amiri et al.,
2017). Inside a composite material, the lumen may be filled (partially or completely)
by the polymer provided that it is accessible (in the case of cut fibers) and a very good
wetting of the fibers by the matrix.

8.3.4 Water absorbed fraction
As seen above, plant fibers consist mainly of polar and hydrophilic polysaccharides.

At the mean value of 65 degrees of relative humidity (temperature, 21 C), the
percentage of water mass sorbed in flax fibers is about 7% (Faruk et al., 2012;
Rowell, 2008). This value can vary with the environment, the fiber composition
and fineness, and the lumen size. Sorbed water acts as a plastifying molecule, which
might impact on the mechanical properties of fibers (Baley et al., 2005)and,as a
consequence, those of unidirectional composites (Baley et al., 2012; Alix et al.,
2011). In the latter case, the transformation cycle (heating or not before processing,
temperature, and time during processing) might impact the water percentage of
sorbed water in the fibers.

8.4 Tensile mechanical properties of elementary
flax fiber

8.4.1 Pertinent information to define a batch of fibers

The development of a cell within a plant is not comparable to the industrial production
of a synthetic fiber. Its characteristics can vary according to natural parameters such as
climatic variations, but not subject to human control. To use this kind of reinforcement
requires the setting up of controls and the management of batches for homogenization.
There are control procedures for textiles, but few for composite materials because they
represent a new industrial outlet. The list of information needed to define a batch of
plant fibers has recently been published (Summerscales et al., 2015):

• The variety
• The dates, areas, and growing conditions
• The cycle imposed on plants and fibers after growth (retting, scutching, hackling, carding, etc.)
• The treatments (surface, for example) undergone by fibers
• The packaging and storage