Page 306 - Handbook of Properties of Textile and Technical Fibres

280 Handbook of Properties of Textile and Technical Fibres

Rettingdknown as dew retting in Northern Europedconsists of enzyme-catalyzed
degradation of cell junctions and cell walls caused by the combined actions of moisture
and heat (Brown et al., 1986). Retting may be divided into two spatiotemporal steps.
First, the epidermis and cortical tissues of stems layered on soil are colonized by fungi
that release several enzymes that degrade pectins and polysaccharides (Meijer et al.,
1995). As a result, the epidermis and cortical parenchyma are partially degraded and
the fiber bundles split (Akin et al., 1996). Calcium pectates are considered resistant
to retting and are thus limiting factors (Sharma, 1988).
In a second step, the fungi colonize the fiber bundles where the complex
pectins present in the fiber junctions may be partially degraded (And eme-Onzighi
et al., 2000; Jauneau et al., 1992). The two-step retting process facilitates fiber
decortication (Kessler et al., 1998; Sharma et al., 1999). On the other hand, if
continuous rains at the latter retting stage prevent harvest, the fungi continue their
development within the elementary fibers and initiate rotting. Indeed, the fungi
induce the degradation of the cellulose microfibrils in the successive cell wall layers,
due to the secretion of glucanase and cellulose enzymes (Brown and Sharma, 1986).
This step is called overretting. For practical and economic reasons, it is hardly
possible to harvest and store wet flax straw (Pallesen, 1996) because of mold
development during storage. Therefore, retting is a critical agricultural process that
determines the divisions of the bundles and the surface quality of the so-called
technical fibers.
There are alternative methods to field dew retting. Water retting is no longer an
option due to pollution of the water by substances created by the anaerobic degradation
of the plants. Various enzyme retting processes have been developed (Akin, 2013b;
Alix et al., 2012; Jauneau et al., 1997). They are very efficient in avoiding fiber
rot and maintain the mechanical properties of the elementary fibers. However, these
alternative methods are not used because they are costly.

8.2.5 Mechanical extraction of fibers (scutching, hackling)
To be used in textile or composite applications, fibers have to be extracted from the
stem. Industrial production of flax fibers from plants is performed by a decortication
process also called scutching. The aim of this step is to extract the fibers from the
stem by removing the woody core (Akin et al., 2005; Salmon-Minotte, 2005; Sultana,
1992). This process yields scutched fibers that consist of the main product and a series
of by-products mainly flax shives, tows, and seeds. Scutched fibers are made of aligned
long flax fibers containing minor amount of shives, depending on the efficiency of the
scutching process and the degree of retting. Shives are formed by the fragmentation of
the woody core. During scutching, some parts of the plants are withdrawn from the
material flow and fall into a secondary circuit. The fibrous parts are called tows.
They are made of misaligned and entangled flax fibers, and contain a large quantity
of shives (Martin et al., 2014).
Scutched fibers have the highest value of all flax products. Scutched flax can
be directly hackled to be used for yarn spinning. Tows have less value as they must
undergo further opening and carding operations to be cleaned of shives and aligned