**4. Hydro/hygrothermal aging of bio-composites**

In contrast to plant fibers, water sensitivity is not as noticeable in most polymer matrices, especially petroleum-based matrices. In this section, we investigate the effect of incorporating plant fibers on the hydro/hygrothermal behavior of biocomposites.

#### **4.1 Diffusion behavior of bio-composites**

We can see in the literature that most of the plant fiber/PP bio-composites show a Fickian diffusion: wood powder/polypropylene (PP) [71], jute fiber/PP [72, 73], date palm fiber/PP [74], hemp fiber/P and bagasse fiber/PP [75], Kenaf fiber/PP [76], wood fiber/PP [77], wood fiber/high density polyethylene (HDPE) [78, 79], flax fiber/ Elium and flax fiber/epoxy [7], flax/epoxy fiber [3], short jute fiber/PLA [80], Areca fine fibers, and *Calotropis gigantea* fiber/phenol formaldehyde [81].

For the case of a flat plate of thickness *h* with infinite dimensions, the approximate analytical solution of Fick's law for unidirectional diffusion through the thickness is expressed by Eqs. (11) and (12). This is of course to determine the homogenized properties of the bio-composite at the macroscopic scale (effective properties). Furthermore, in the case of finite dimensions, the direction of diffusion can be perturbed by so-called edge effects; the diffusion is not only unidirectional along with the thickness but also occurs in other directions. Chilali [7] studied the influence of the width (*w*)/thickness (*h*) ratio on the water diffusion coefficient of Lin fiber/epoxy and Lin fiber/Elium composites. The results showed that when this ratio is equal to 60 (180 � <sup>180</sup> � 3 mm<sup>3</sup> ) the edge effect is no longer observable and diffusion occurs along with the thickness similar to the case of the samples (20 � <sup>20</sup> � 3 mm<sup>3</sup> ) sealed on the edges. Therefore, and in addition to what was explained in Section 2.3 for the plane plate case, Shen and Springer [82] proposed a correction to the diffusion coefficient (*Dc*) in the case of finite sample dimensions for a homogeneous material:

$$D\_c = \frac{D}{\left[\mathbf{1} + \frac{h}{l} + \frac{h}{w}\right]^2} \tag{13}$$

This correction is often applied in the literature on bio-composites to avoid the edge effect [72, 83–85].

On the other hand, the interpretation of the diffusion process of composites by analytical approaches often does not reveal all the secrets of this phenomenon, especially when such a complex material is studied.

For a better understanding, the use of numerical approaches using the finite element method is frequently reported in the literature. We can distinguish two types of modeling of the diffusive behavior of conventional composites in the literature: those that consider the composite as a homogeneous material defined by its effective properties [86–88], others that represent it in a more realistic way by taking into account the fibers [80, 89–92]. These models can be made in 3D or 2D. In addition, the use of Fick's law is often observed [80, 86–91], although we can sometimes see the use of non-Fick's laws such as Langmuir's [89, 92, 93]. However, there is little work on bio-composites. Chilali [7] modeled the diffusive behavior of bidirectional flax fiber/ epoxy and flax fiber/Elium bio-composites by a 2D biphasic model using the 2D Fick model. Berges [3] reproduced the same methodology for unidirectional flax/epoxy

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

fiber composites. Jiang et al. [80] modeled the 3D diffusive behavior of short jute/PLA composites based on microstructure identification by X-ray tomography. Nouri [94] proposed a 2D numerical model for Diss/PP bio-composites considering three phases: fibers, matrix, and interface. This was done based on microscopic pictures of the biocomposite microstructure. This was achieved based on microscopic pictures of the bio-composite microstructure. The difficulty in this kind of modeling is the nonadaptation of the values determined at the fiber scale, notably the diffusion coefficient as explained in Section 3, to the value that should be implemented in the model. Generally, the authors need to recalculate the fiber diffusion coefficient by an inverse method.

#### **4.2 Factors influencing the hydro/hygrothermal behavior of bio-composites**

In general, water molecules penetrate bio-composites by three different mechanisms: between polymer chains, by capillary action in micro-voids, and in the interfaces between fibers and the matrix [81]. Therefore, 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).

The literature has shown that the fiber content influences the hydro/hygrothermal properties of bio-composites, Aloa et al. [28] studied the water absorption behavior of hemp fiber/PLA bio-composites and found that water absorption and swelling increased with the addition of hemp fibers to the PLA matrix. The same results were observed by Reza and Krishna [95], Sanjeevi et al. [81], and Mishra and Verma [96] for water absorption. The latter authors explained this, in their study of wood flour/PP composites, by the increase in free OH groups with increasing wood flour; this makes the bio-composite more hydrophilic. However, the diffusion coefficient does not seem to be impacted by increasing the loading rate, and the bio-composites show a lower diffusion coefficient than PP (a slowing wood flour effect). In contrast, results obtained by Law and Ishak [33] show that the diffusion coefficient of Kenaf/PP fiber composites increases with increasing loading rate (an accelerating Kenaf fiber effect). The authors explained this by the ability of the matrix to surround most of the fibers at low loading rates acting as a barrier to water diffusion. This barrier effect of the matrix decreases with increasing loading rate, leading to accelerated diffusivity. Similar results were reported by Joseph et al. [97] on Sisal/PP fiber composites.

On the other hand, a better fiber/matrix adhesion can lead to a reduction in the number of hydrophilic sites in the bio-composite and consequently influence its diffusive behavior. Beg and Pickering [98] found a decrease in the diffusion coefficient of kraft fiber/PP composites with a loading rate of 40% after the addition of 4% by weight MA-g-PP (coupling agent), see **Figure 8**. This decrease has also been observed by many researchers [76, 99]. However, the work of Mishra and Verma [96] has shown the opposite.

On the other hand, Sanjevvi et al. [81] found that fiber size had a significant influence on the adsorption rate of Areca fine fibers (AFFs) and *C. gigantea* fiber/ phenol formaldehyde bio-composites whose long-fiber composite showed the highest water uptake of about 20% when the volume loading rate was 25%. In addition, Pérez-Fonseca et al. [100] found that the size of the fibers (short: 150–212 mm and long: 300–425 mm) did not have a significant effect on the water absorption kinetics of Agave/PP and Pine Sawdust/PP composites. Nevertheless, the composites containing Agave fibers had about three times higher absorption than the composites loaded with

#### **Figure 8.**

*Evolution of moisture content of composites (kraft fiber/PP) as a function of water immersion time during hygrothermal aging at temperatures of 30, 50, and 70°C (dotted line: with 4 wt.% MAPP; solid line: without MAPP) [98].*


#### **Table 4.**

*Diffusion coefficients of different biocomposites reported in the literature.*
