**4. Conclusion**

There are evolving global challenges on the utilization of non-renewable resources in the manufacturing industry and increasingly stringent environmental legislation. Both consumers and regulatory agencies are thriving for products that reduce dependency on fossil fuels and thus, are more environmentally friendly. As such, this paves for an opportunity to embrace the use of natural fibers in products and composites leading to significant growth of biobased economy, which the present chapter intends to stimulate.

The field of study of plant fibers that can be industrially exploited remains open. In this chapter, a particular emphasis has been put on their production, in particular on the methods that are generally used to separate them from their originating plants. To date, the question of improving the quality of the extracted fiber has been satisfactorily answered, particularly as regards the possibility of combining several methods when necessary. Some other questions still require research. These include, among others, growing conditions for seed multiplication and fiber production, harvesting methods, optimisation of fiber separation, the molecular basis for improving fiber decortication and performance. The knowledge gained from this work could be used to design new varieties of fibers, tailored for specific industrial applications. Similarly, the recourse to proteomics [68, 69], to isolate genes involved in the biosynthesis of cell wall lignin and hemicellulose in tobacco. Variations in these constituents can affect the fiber quality and cellulose availability. This could then lead to a new orientation on molecular selection research as well as genetic modifications studies to improve the quality of plant fibers.

Morphology and surface behavior of plant fibers are studied using various techniques such as XRD, FTIR, SEM, AFM, TEM and thermogravimetric analysis that helps in understanding the nature of natural fibers.

In terms of the mechanical behavior of plant fibers, important milestones have been achieved to highlight the influence of the chemical composition and structural parameters of the plant wall on their tensile properties. The microstructure of plant fibers is very complex, precisely when it comes to defining generalizable geometric and analytical models that describe it. As mentioned above, improving the mechanical properties of fibers may require the introduction of new types of fibers. And we could mention in this regard the ongoing research on spinning with solvents [70, 71], to obtain fibers of greater strength and low scattered properties. Understanding how fiber morphology affects the properties of composite materials is essential. More precisely, it is important for the selection of new fibers and for the cultivation of fibrous plants genetically selected. This would help to predict their potential for reinforcement in other materials to achieve desired properties.

Investigation of the viscoelastic properties of plant fibers has also been outlined. A variety of dynamic modulus measurement methods exists including ultrasonic wave propagation and the flexural resonance method presented here, for which normal modes of vibration are monitored. Stress relaxation tests are to be carried out to retrieve stress over time as well as the elastic modulus of the fiber material. A mathematical method for extracting the relaxation modulus from relaxation experimental data has to be proposed to this end. Proper selection of the testing vibrational mode and machine cross-head speed (during relaxation) appear important in the suggested methods in order to avoid dispersive results. The Young's modulus that is obtained from the dynamic behavior of the specimen should, therefore, reflects the frequency dependence of the material.
