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

In addition, a multitude of results on the diffusion coefficients of plant fibers have been found in the literature (**Table 3**), this is due mainly to the adopted test protocol, especially for the case of immersion in water, and to the assumed hypothesis (the diffusion occurs from the cross-section and the latter is assumed to be circular). Célino et al. [22] studied the sorption mechanism of different plant fibers (flax, hemp, sisal, and jute) under different conditions: immersion in water and conditioned at 80% RH. The results showed, for each type of fiber, a significant difference of the order of 10<sup>2</sup> between the diffusion coefficients of each case studied. For example, flax fibers showed a diffusion coefficient in the case of water immersion of 5.9 e<sup>06</sup> mm<sup>2</sup> /s and 2.00 e<sup>04</sup> mm<sup>2</sup> /s under 80% RH. In the case of relative humidity conditions, similar results were also obtained by Rouidier [10] on flax fibers. However, Gouavné et al. [12] noted a difference of more than three decades under the same conditions. Furthermore, in the case of water immersion, Stamboulis et al. [18] noted a *D* value of the same order of magnitude as those found by Célino et al. [22] and Rouidier [10] under relative humidity for flax fibers. On the other hand, Nouri et al. [23] studied the effect of different treatments on the diffusive behavior of Diss fibers under different conditions. The results showed diffusion coefficients of the


**Table 3.**

*Diffusion parameters of plant fibers under different aging conditions.*

same order of magnitude for the different case studies. As an example, fibers treated with 5% NaOH showed a *D*<sup>1</sup> when immersed in water and conditioned under a *RH* = 68% of 4.30 <sup>10</sup><sup>8</sup> mm<sup>2</sup> /s and 1.6 <sup>10</sup><sup>8</sup> mm<sup>2</sup> /s, respectively.

In addition, most analytical approaches consider the cross-sectional shape of vegetal fiber bundles as circular. However, this hypothesis is not necessarily applicable to all vegetal fibers as shown by the observations made on the different vegetal fibers [66–69]. Assuming that the diffusion occurs from the cross-section, the morphology of the cross-section can influence the results obtained when calculating the diffusion coefficient. Recently, Nouri et al. [70] have proposed a coupled (experimentalnumerical) approach to improve the modeling of water diffusion through Diss considering two fibers geometries, one often suggested in the literature (circular) and the other one revealed by microscopy (ellipsoidal). The modeling was applied for untreated and treated Diss fibers to determine the diffusion coefficient. Moreover, a single diffusion coefficient was enough to describe diffusion behavior in the case of Diss fibers, contrary to what is practiced with analytical approaches. It was demonstrated that for the same fiber cross-section area, faster diffusion will occur on high perimeter fibers. This means that when the fibers are represented by an ellipsoidal section instead of a circular one for the same cross-sectional area, the diffusion coefficient is less important. Therefore, a significant decrease of 1.5–2.4 in the diffusion coefficient was observed for the fibers studied.

Concerning the evolution of diffusion coefficients as a function of the evolution of relative humidity, Gouanvé et al. [12] observed an increase in *D*<sup>1</sup> and *D*<sup>2</sup> of flax fibers when *aw* is lower than 0.50; above this value, a decrease was observed for these coefficients (**Figure 6a**). These findings were explained, according to the authors, by the dual-mode of sorption, Langmuir, and Henry, in the first part of the isotherm as

#### **Figure 6.**

*Evolution of the water diffusion coefficients (*D*<sup>1</sup> and* D*2) as a function of* aw *for: (a) flax fibers [12], (b) agave fibers [57], (c) flax fibers [25], and (d) Diss fibers [23].*

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

well as the braking effect of swelling at high *aw*. The same findings were raised by Bessadok et al. [57] (**Figure 6b**). According to them, this indicates a water sorption mechanism according to Park's model: part of the water is adsorbed on specific sites (low mobility of the fixed water molecules) and the rest is dissolved according to Henry's process (higher mobility of the dissolved molecules) and then the subsequent formation of aggregates at high *aw* (low mobility of the aggregates). Alix et al. [25] observed similar behavior for flax fibers with a decrease in diffusion coefficients when aw is less than 0.1. Above this value, *D*1, *D*<sup>2</sup> follow the behavior observed by Gouanvé et al. [12] (**Figure 6c**). This early decrease was explained by the fact that water molecules interact with polar fiber groups leading to hydrogen bonds that increase the cohesion between cellulose chains, thus reducing the mobility of water. In contrast to the previous authors, Nouri et al. [23] found a decrease in *D* (*D*1, *D*2) with increasing *aw* (**Figure 6d**). On the other hand, Roudier [10] found that as the relative humidity increases, the diffusion coefficient for flax fibers increase. According to him, this relationship is described by a linear law (**Figure 7**). It should be noted that the fibers studied here were dried before each conditioning, contrary to the other works which are based on the results of isotherm adsorption.

Furthermore, treatments also have an important impact on the hygrothermal properties of plant fibers. Mannan et al. [63] found that after various treatments on jute fibers, the diffusion coefficients decreased during delignification, bleaching, and soap washing, suggesting that moisture was absorbed in the amorphous region of the fibers. Stamboulis et al. [18] studied the effect of the heat treatment, Durbalin, on the hygrothermal properties of flax fibers. The raw fibers always showed a higher diffusion coefficient than the treated fibers, independent of the *RH* studied. Bessadok et al. [57] observed a decrease in the diffusion coefficient of Agave fibers after their chemical treatment (acrylic acid, styrene, maleic anhydride, and acetylation). On the same line, Nouri et al. [23] noticed a decrease in the diffusion coefficient of Diss fibers for the water absorption case after the different treatments carried out (thermal, acetic acid, NaOH, and silane treatments).

The relationship between the diffusion properties of plant fibers and biocomposites is little studied in the literature due to the lack of reliable experimental protocols for plant fibers (for the case of immersion in water) on the one hand, and the complexity of the problem on the other hand. In the next section, we give more explanation about the effect of integrating plant fibers as reinforcement to the polymer matrix on the diffusive behavior of bio-composites.

#### **Figure 7.**

*Linear evolution of the water diffusion coefficient for flax fibers as a function of relative humidity [10].*
