**5. Conclusion**

Plant fibers have been shown to be highly sensitive to water molecules, which impacts the functionality of bio-composites. This phenomenon is often related to the morphology of these fibers, their cavity (lumen), and the free hydroxyl groups present on their surface. It appears that the hydrophilic character of natural fibers could be reduced by applying treatments, such as chemical and physical treatments.

The isotherm of cellulosic fibers generally has a sigmoidal shape with hysteresis loops between the adsorption and desorption curve in accordance with the type II isotherm. The Guggenheim, Anderson, and de Boer (GAB), Hailwood Horrobin (H-H), and Generalized D'Arcy and Watt (GDW) models are the most widely used in the literature to describe the isotherm of plant fibers.

The diffusive behavior of plant fibers are often based on the Fick model. However, it is sometimes described by non-Fick diffusion models. The same conclusions have been reported for bio-composites. On the other hand, the process of water diffusion through bio-composites is influenced, mainly, by two types of factors, namely internal factors (related to the bio-composite structure and the nature of its phases) and external factors (relative humidity and temperature).

Hydro/hygrothermal aging can cause irreversible damage to the bio-composite due to the different nature of the matrix and the plant fibers. This is caused by a differential swelling between these components.
