**3. Hydro/hygrothermal aging of natural fibers**

Plant fibers have been shown to be highly sensitive to water molecules, which impacts the functionality of bio-composites [13]. This phenomenon is often related to the morphology of these fibers, their cavity (lumen), and the free hydroxyl groups present on their surface (hydrophilic sites) [13–16]. The components of plant fiber responsible for its hydrophilic character are mainly hemicellulose and pectin [7], although lignin and amorphous cellulose are also hydrophilic: lignin contains fewer -OH groups than the polysaccharides [17], while cellulose is less accessible. Therefore, a change in hygrothermal conditions could affect the degree of crystallinity of natural fibers, their stiffness, dimensional stability (swelling), and tensile strength [13, 17].

**Table 1** shows the results of the saturation rates of the plant fibers after immersion in water. These results depend mainly on the nature of the plant species, the extraction method used, the application of the treatments, and the test protocol applied.

Furthermore, it appears that the hydrophilic character of natural fibers could be reduced by applying treatments, such as chemical and physical treatments [18–20, 23–48]. The reasons for these improvements are multiple and depend on the nature of the treatment used and its effect on the plant fibers. Chemical treatments aim to change the chemical composition of the fiber. For example, NaOH treatment removes some of the hydrophilic non-cellulosic materials such as lignin, hemicellulose, and pectin that cover the natural fiber interface. Also, free OH groups can react with the NaOH molecule forming fiber-cell-O-Na groups, which reduces the hydrophilic hydroxyl groups. The silane molecule, on the other hand, has an end containing alkoxy groups that can react with the hydroxyl groups of the plant fiber, giving it hydrophilic properties on its surface. The same mode of functioning for treatment with acetic acid, acrylic acid, and benzoyl [24, 46, 47]. Physical treatments such as Corona [31–33], Plasma [34–36], and thermal [18, 37, 38] mainly modify the structural and interfacial properties of the plant fibers without significantly changing their chemical composition.

*Hydro/Hygrothermal Behavior of Plant Fibers and Its Influence on Bio-Composite Properties DOI: http://dx.doi.org/10.5772/intechopen.102580*


#### **Table 1.**

*Saturation rates of the plant fibers after immersion in water.*

The hydrophilic character of plant fibers strongly influences the hydro/hygrothermal properties (saturation rates, diffusion kinetics, and dimensional deformation) of bio-composites at the macroscopic scale, an increase in the fiber incorporation rate often leads to a change in these properties, see Section 4.

#### **3.1 Sorption isotherm for plant fibers**

The isotherm of cellulosic fibers generally has a sigmoidal shape with hysteresis loops between the adsorption and desorption curve [16, 17, 20, 25, 49–52], in accordance with the type II isotherm according to the "International Union of Pure and Applied Chemistry (IUPAC)" classification [1]. Since plant fibers are much more hydrophilic than polymer matrices, bio-composites generally exhibit the same type of isothermal sorption curve.

The origin of the hysteresis phenomenon is not yet fully understood and is still a subject of debate in the literature. Considering fiber cell walls as micro-mesoporous materials, some authors have linked this to capillary condensation, which occurs at high RH, and which could also be present at low RH in the micropores [51]. However, it has been shown that capillary condensation could only occur for such material at very high RH [53]. Others have suggested that this may be related to the change of state of the amorphous components, notably hemicellulose and lignin [50, 52, 54]. It has been found that when adsorption occurs above the glass transition temperature of these polymers, also known as the softening point, hysteresis should be absent or minimized [50, 52]. Indeed, at room temperature, amorphous cell wall components of cellulosic fibers such as wood could soften around 65–75% RH [50]. Keating et al. [54] reported a loss of sorption hysteresis in artificial hemicellulose (galactomannan) films when the RH is above 75% at 25°C (**Figure 3**). Furthermore, Salmèn and Larsson [50] found that increasing the temperature and decreasing the crystallinity index of the fibers affects the presence of the hysteresis loop (**Figure 4**). They also noticed that the swelling effect could be involved when RH is below the softening temperature. A priori, when water molecules penetrate the matrix, nanopores could be created in the structure to receive these molecules and under desorption conditions, these nanopores could collapse [54]. In addition, Hill et al. [55] questioned the lignin content, whose magnitude of sorption hysteresis was greater when the content of this component was high.

**Figure 3.** *Sorption isotherm of a nanocomposite film and a guar film as a function of RH, showing hysteresis [54].*

#### **Figure 4.**

*Moisture adsorption and desorption curves as a function of RH for cellulose samples with different degrees of modification. Measurements were carried out at 25 and 65°C. The higher the degree of oxidation (ox), the lower the crystallinity [50].*

## *Hydro/Hygrothermal Behavior of Plant Fibers and Its Influence on Bio-Composite Properties DOI: http://dx.doi.org/10.5772/intechopen.102580*

In addition, several mathematical models exist in the literature to describe the isotherm of this kind of material, including theoretical, semi-theoretical, and empirical models [56]. 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 for plant fibers [20, 25, 55, 57–62] (**Table 2**). These models also make physical sense for the attachment of water molecules at the pore scale and can be explained by an extension of Langmuir's theory for multilayer adsorption. Indeed, each adsorption site can only adsorb one water molecule. These adsorbed molecules could subsequently be secondary adsorption sites for subsequent molecules. The GDW model assumes that some of these sites have the potential to become secondary adsorption sites. The H-H model, on the other hand, considers that water adsorbed by the cell wall can exist in two forms: multilayer water (*Md*) and monolayer water (*Mh*). However, while the monolayer in the GAB and DGW model is invariant, the monolayer described in the H-H model can change over the entire RH range.

#### **3.2 Water adsorption kinetics of plant fibers**

The diffusive behavior of plant fibers are often based on the Fick model [10, 12, 18, 21, 22, 25, 57, 63]. However, it is sometimes described by non-Fick diffusion models. Célino et al. [22] used different classical diffusion models to define this phenomenon for plant fibers: the Fick model, the two-stage Fick model developed by Loh et al. [64], and the Langmuir model developed by Kibler et al. [65]. They concluded that the Fick model better represents the kinetics of water vapor adsorption in the studied fibers. However, the Langmuir model seems to fit better in the case of liquid water absorption during the immersion process (**Figure 5**(1)). In addition, Saikia [21] observed during his work on hemp, okra, and betel nut fibers that absorption behaved in two stages. The first stage took place very quickly and obeyed Fick's law of diffusion. The second absorption step represents a non-Fickian diffusion (**Figure 5**(2)).

On the other hand, the analytical solution of Fick's law in the case of plant fibers are approximate, the sorption is generally subdivided into two or even four zones, each of which is defined by its own law of behavior (see Eqs. (7)–(10)). Therefore, some authors propose a single diffusion coefficient to describe the diffusive behavior within plant fibers [18, 63], but others suggest two different diffusion coefficients


#### **Table 2.**

*Mathematical description of the GAB, GDW, and HH models.*

#### **Figure 5.**

*(1) Immersion absorption curve for different plant fibers [22]. (2) Water content absorbed by: (a) hemp, (b) okra, and (c) betel nut as a function of the square root of time at different temperatures [21].*

[12, 25, 57]: *D*<sup>1</sup> for the first half-sorption and *D*<sup>2</sup> for the second half-sorption. Gouanvé et al. [12] found during their work that the *D*<sup>1</sup> and *D*<sup>2</sup> values of flax fibers are similar throughout the *aw* range studied. However, Alix et al. [25] observed a distinct behavior on the same type of fiber, the value of *D*<sup>2</sup> was found to be significantly larger than *D*<sup>1</sup> over the whole range of *aw* studied. According to the authors, this dissimilarity is caused by the heterogeneity of the fibers where *D*<sup>2</sup> should be more representative of water diffusion in the core of the fiber while *D*<sup>1</sup> characterizes diffusion through the surface. The same findings were raised by Bessadok et al. [57] on Agave fibers and Nouri et al. [23] on Diss fibers.
