Page 318 - Handbook of Properties of Textile and Technical Fibres

292 Handbook of Properties of Textile and Technical Fibres

family of polymers, its melting is necessary during the process and the thermal
degradation of the plant fibers is often considered as a limitation for this process.
The nature of the constituents of cell walls, the temperature, and water content all exert
strong influences on the mechanical properties of plant fibers. During processing, the
reinforcing fibers are subjected to high temperatures (melting of a thermoplastic matrix
for example) that affects their properties. This thermal cycle can modify the structure
of the cell walls and the water absorption capacities (Gourier et al., 2014). A significant

decrease of the mechanical performances at 250 C has thus been established. In real-
ity, two parameters are important: the temperature and the time (Batra, 1998; Corteen
and Illingworth, 1953). To illustrate a noncatastrophic behavior following a thermal
cycle, two examples are presented immediately below.
Polypropylene/flax composites were successfully made by AFP (Baley et al.,
2016a). This process used unidirectional tapes and three successive imposed thermal

cycles at 190 C: the production of tapes by calendaring, the drop-off by a robot of the
heated tapes with a laser, and the postconsolidation in press. A polyamide matrix
(PA11) has a process temperature of 210 C; it is usable with flax fibers and allows

a composite material to be made with good mechanical properties (Bourmaud et al.,
2016b) and which is recyclable (Gourier et al., 2017). During recycling, the material

undergoes several cycles at 210 C and remains usable.
Biocomposites are becoming a major asset to the automotive industry, especially
in the interior parts of cars (Merotte et al., 2016). In these applications, vehicle
manufacturers do not want any odor inside of the car. This makes it necessary not
to have degradation of some elements (at the fibers’ surfaces in the most cases)
during the process by the thermal cycle. Today, many industrial applications exist,
which meet these odor specifications established by the car manufacturers (Koronis
et al., 2013).

8.5 Conclusion

Flax fibers are mainly used in the textile industry, but the composite materials indus-
try is a new and growing outlet. Flax is grown in Western Europe, Eastern Europe,
China, and Egypt. The main production area is Western Europe, and particularly
in France. In this country, cultivated area has increased since 2013, because demand
exists and prices are attractive to the farmer. Apart from the economic aspects, the
interest in this plant is explained by the available fiber volumes, the existence of a
complete innovation chain (for example, new varieties arrive each year on the
market), length of the elementary fibers, and their good mechanical properties.
This latter characteristic can be explained by their structural function and the
long slender shape of the plant (ratio of the height of the stem to its diameter). Fiber
bundles are distributed outside the stem, which ensure the resistance to loading and
bending. Current knowledge allows a better understanding of the influence of
numerous parameters (from the influence of growth conditions, retting, and fiber
extraction) on the fiber properties and better control of their production.