Hydro/Hygrothermal Behavior of Plant Fibers and Its Influence on Bio-Composite Properties

Abstract

Plant fibers have been shown to be highly sensitive to water molecules; this impacts the functionality of composites reinforced with these fibers, commonly known as bio-composites. This review aims to provide a comprehensive description of the behavior of plant fibers in the presence of water molecules in a liquid or gaseous state, as well as the different phenomena and mechanisms involved at the fiber scale and at the bio-composite scale via recent studies in this field. First, we will discuss the physical problem of sorption in polymers in a general way, and then we will focus on the case of plant fibers. Particular attention will be given to the adsorption kinetics of plant fibers and the models used to determine their diffusion parameters. In a second step, the effect of the incorporation of plant fibers in polymer matrices will be examined as well as the different factors influencing the diffusive behavior of bio-composites. In addition, the effect of hydro/hygrothermal aging on the mechanical properties of bio-composites will be discussed.

1. Introduction

The depletion of natural resources, the pollution caused by industry, and the current environmental challenge have led researchers and industrialists to explore new ways of providing green and renewable resources, such as biomass. Among these promising natural resources in the field of composite materials are plant fibers, also known as cellulosic or lignocellulosic fibers. Indeed, the integration of these fibers into the field of composite materials as an alternative to synthetic fibers continues to attract much research and application interest. This is due to their neutrality with regard to CO₂ emissions into the atmosphere, but also to the fact that these fibers have specific mechanical and thermal properties. In addition, these fibers have other advantages, such as their low cost, their biodegradability, and the fact that they are renewable.

However, the hydrophilic nature of these plant fibers and the diversity of their properties represent limitations for their use as reinforcement for composite materials. This is particularly true of hydrophobic matrices, which raises questions about the compatibility between the reinforcement and the matrix on the one hand, and the durability of the bio-composite on the other, especially when the latter is exposed to changing hygrothermal conditions.

This chapter will comprehensively address the hydro/hygrothermal behavior of plant fibers and their influence on the durability of bio-composites. First, we will discuss the physical problem of sorption in polymers in a general way, and then we will focus on the case of plant fibers. Particular attention will be given to the adsorption kinetics of plant fibers and the models used to determine their diffusion parameters. In a second step, the effect of the incorporation of plant fibers in polymer matrices will be examined as well as the different factors influencing the diffusive behavior of bio-composites. In addition, the effect of hydro/hygrothermal aging on the mechanical properties of bio-composites will be discussed.

Before starting this chapter, it is necessary to define some basic knowledge that are essential for understanding the physical phenomena that will be presented.

2. Generality on hydro/hygrothermal transfer

2.1 Terminology

2.1.1 Adsorption, absorption, and sorption

Absorption is a phenomenon of filling the pores of a solid with liquid water; it occurs by capillary condensation. Whereas adsorption is a process of attachment of water molecules, water vapor, to the surface of materials by intermolecular forces. These forces are of the same nature as those responsible for the imperfection of real gases and the condensation of vapors [1]. We, therefore, talk about the absorption of liquid water (hydrothermal aging) and the adsorption of water vapor (hygrothermal aging). As for sorption, it encompasses both phenomena.

2.1.2 Relationship between relative humidity and water activity

Under isothermal conditions, the water activity (a_w) in a material is defined by the ratio of the water vapor pressure at its surface (p_vs) to the pure water vapor pressure (p_ve). When the material is in equilibrium with its environment or there is no mass or heat transfer, p_vs equals the water vapor pressure of air (p_va). Therefore, the water activity of a material in equilibrium with its environment is equal to the relative humidity of the air (RH_a).

$$a_w={\mathit{HR}}_a=\frac{P_{\mathit{vs}}}{P_{\mathit{ve}}\left(T\right)}=\frac{P_{\mathit{va}}}{P_{\mathit{ve}}\left(T\right)}$$E1

2.1.3 Condensation capillaire

This is the phenomenon where water vapor condenses into a liquid phase in a pore at an a_w of less than 1, where a_w = 1 represents the case of pure water.

2.1.4 Sorption isotherms

Under isothermal conditions, the curve representing the equilibrium moisture content (EMC) of a material as a function of a_w is called the sorption isotherm if it was determined from the dry state of the material, and the desorption isotherm if the material was saturated at the initial state [2].

Isotherms are classified according to the new IUPAC classification [1] into six types (Figure 1(1)). The shape of the isotherm provides information about the pore size (macropores, mesopores, or micropores) and the types of adsorption (monolayer or multilayer). When the desorption process is different from the sorption process, a hysteresis loop is observed. These loops are also classified by IUPAC [1] into five categories (Figure 1(2)).

Figure 1.

IUPAC classification of physisorption isotherms (1) and hysteresis loops (2) [1].

2.1.5 Pore size

According to the IUPAC pore classification [1], macropores, mesopores, and micropores are, respectively, those pores whose width exceeds 50 nm, between 2 and 50 nm, and do not exceed 2 nm, respectively.

2.1.6 Aging

Aging is defined according to Berges [3] as an evolution of one or more properties of the material through a modification of the structure, the composition, or the morphology of the constituents. These changes can be temporary and dependent on the presence of the aging source (reversible aging), or permanent (irreversible aging) [4]. Furthermore, the ability of a material to resist irreversible aging is defined by its durability.

2.2 Sorption and diffusion of water molecules in polymers

Generally, polymers are penetrable by water molecules. The latter propagate progressively through the polymer macromolecular network when it is in direct contact with water molecules (gaseous or liquid) [5]. Over time, this leads to a weight gain that continues until the material reaches a saturation plateau [5]. This adsorbed amount is related to the total amount of hydrophilic sites, polymeric areas sensitive to receive water molecules, available in the polymer chains [6]. This also means that the chemical potentials of water in the polymer and in the ambient environment are equal [7]. The kinetics of water sorption and the amount of water adsorbed/absorbed depends on the polymer nature (hydrophobic or hydrophilic), the characteristics of the water (pH, deionized or salted water), and other thermodynamic parameters [4].

The most common method for evaluating water sorption processes in polymers is the recording of mass gain versus time data. The gravimetric curve represents the plot of this data; the mass of water absorbed/adsorbed (M_t) versus time (LF curve in Figure 2). This curve contains important information: the linear part of this curve informs about the penetration rate of water (the diffusion), while the saturation level presents the mass adsorbed/absorbed at infinity (M_∞) [7].

Figure 2.

Typical gravimetric absorption curves of a fluid: (LF) Fick behavior, (A) pseudo-Fick behavior, (B) 2-step diffusion, (C) diffusion with mechanical damage, and (D) diffusion with chemical damage [8].

The diffusion behavior of polymers is often based on the Fick model. Nevertheless, some materials may exhibit non-Fick behavior in the presence of anomalies (curves A B, C, and D in Figure 2). The diffusion behavior can be identified mathematically using Eq. (2) [9]. If the parameter n is close to 0.5, this indicates that the diffusion of water, in this case, follows Fick’s law.

$$\frac{M_t}{M_{\infty }}=k{t}^n$$E2

Where k, n represents the diffusion kinetic parameters.

In the case of bio-composites, we can see in the literature that the gravimetric curve of this kind of material shows a Fickian behavior at the macroscopic scale, see Section 4. However, at the microscopic scale, the problem is more complicated, water molecules can penetrate bio-composites by three different mechanisms: between polymer chains (matrix and fiber), by capillary action in micro-voids, and at the interface level (between fibers and the matrix) when chemical adhesion is absent.

2.3 Fick’s mechanism

Fick’s laws were established by the analogy between conductive heat transfer and mass transfer [10]. In an isotropic case, Fick’s second law is given by the equation below:

$$\frac{\partial C}{\partial t}=D\left(\frac{{\partial }^{2}C}{\partial x^2}+\frac{{\partial }^{2}C}{\partial y^2}+\frac{{\partial }^{2}C}{\partial z^2}\right)$$E3

Where: D is the diffusivity coefficient mm²/s, it is a scalar that defines the diffusion kinetics. C is the equilibrium water concentration.

The above equation could also be written in a case of radial diffusion in a solid cylinder of radius r as follows:

$$\frac{\partial C}{\partial t}=D\left(\frac{1}{r}\frac{\partial C}{\partial r}+\frac{{\partial }^{2}C}{\partial r^2}\right)$$E4

In the case where a uniform concentration is imposed on the borders, the simplified solution for long times ($0<M_t/M_{\infty }<0.5$) and short times ($0.5<M_t/M_{\infty }<1$) of the above relationship becomes, respectively, as follows [11]:

$$\frac{M_t}{M_{\infty }}=\frac{4}{\sqrt{\pi }}{\tau }^{1/2}\left[1-\frac{\sqrt{\pi }}{4}{\tau }^{\frac{1}{2}}-\frac{\tau }{12}\dots .\right]$$E5

$$\frac{M_t}{M_{\infty }}=1-\sum _{n=1}^{\infty }\frac{4}{{\propto }_n^2}\mathit{exp}\left(-{\alpha }_n^2\tau \right)$$E6

With $\tau =\mathit{Dt}/r^2$ and ${\alpha }_n$ the solutions of the Bessel equation of order n.

Equations (5) and (6) can be decomposed even more precisely according to the M_t/M_∞ intervals as follows [12]:

$0<M_t/M_{\infty }<0.2:$

$$\frac{M_t}{M_{\infty }}=\frac{4}{\sqrt{\pi }}{\tau }^{1/2}$$E7

$0.2<M_t/M_{\infty }<0.5:$

$$\frac{M_t}{M_{\infty }}=\frac{4}{\sqrt{\pi }}{\tau }^{1/2}\left[1-\frac{\sqrt{\pi }}{4}{\tau }^{\frac{1}{2}}-\frac{\tau }{12}\dots .\right]$$E8

$0.5<M_t/M_{\infty }<0.7:$

$$\frac{M_t}{M_{\infty }}=1-4\left(\frac{\mathit{\text{expexp}}\left(-{\propto }_1^2\tau \right)}{{\propto }_1^2}+\frac{\mathit{\text{expexp}}\left(-{\propto }_2^2\tau \right)}{{\propto }_2^2}\right)$$E9

$0.7<M_t/M_{\infty }<1:$

$$\frac{M_t}{M_{\infty }}=1-4\left(\frac{\mathit{\text{expexp}}\left(-{\propto }_1^2\tau \right)}{{\propto }_1^2}\right)$$E10

With ${\alpha }_1=2.40483\mathit{et}{\alpha }_n=5.52008$.

For the case of a plane plate of thickness h with a uniform initial concentration on both surfaces, the approximate analytical solution of Fick’s law (Eq. (3)) for unidirectional diffusion along the thickness is expressed as follows [12]:

For the first half-absorption ($M_t/M_{\infty }<0.5$):

$$\frac{M_t}{M_{\infty }}=\frac{4}{h}\sqrt{\frac{\mathit{Dt}}{\pi }}$$E11

For the second half-absorption ($M_t/M_{\infty }\ge 0.5$):

$$\mathit{ln}\left(1-\frac{M_t}{M_{\infty }}\right)=\mathit{ln}\frac{8}{{\pi }^2}-\frac{{\pi }^2\mathit{Dt}}{h^2}$$E12

This is often used to illustrate the transfer of water vapor in continuous media and has been adopted by several authors to simulate the transfer of liquid water in materials assumed to be continuous at the macroscopic scale such as bio-composites and plant fibers, see Sections 3 and 4.

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, 14, 15, 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, 19, 20, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 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, 32, 33], Plasma [34, 35, 36], and thermal [18, 37, 38] mainly modify the structural and interfacial properties of the plant fibers without significantly changing their chemical composition.

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, 50, 51, 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].

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, 58, 59, 60, 61, 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 (M_d) and monolayer water (M_h). 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.

Fibers	Saturation rate (%)	References
Flax	42.58	Stamboulis et al. [18]
Alfa	140.60	Bessadok et al. [19]
Agave	138	Bessadok et al. [20]
Hemp	62	Saikia [21]
Hemp	63	Célino et al. [22]
Jute	67.8
Lin	62.5
Sisal	60.6
Diss	40	Nouri et al. [23]

Table 1.

Saturation rates of the plant fibers after immersion in water.

Modèles	Equation	Paramètres
Guggenheim, Anderson and de Boer (GAB).	${\mathit{EMC}}_{\mathit{GAP}}\left(a_w\right)=\frac{{\mathit{km}}_{0}c{a}_w}{\left(1-{\mathit{ka}}_w\right)\left(1-k{a}_w+k{\mathit{ca}}_w\right)}$	m0 the adsorption capacity in the first layer in contact with the solid, c, and k are parameters related to the sorption enthalpy.
Generalized D’Arcy and Watt (GDW).	${\mathit{EMC}}_{\mathit{GDW}}\left(a_w\right)=\frac{{\mathit{km}}_0{a}_w}{1+{\mathit{ka}}_w}\frac{1-k\left(1-\omega \right)a_w}{1-k{a}_w}$	$\omega$ represents the molecules adsorbed rate on primary sites which transform into secondary adsorption sites.
Hailwood and Horrobin (HH) model.	${\mathit{EMC}}_{H-H}\left(a_w\right)=\frac{18}{W}\left(\frac{K_1{K}_2{a}_w}{1+K_1{K}_2{a}_w}\right)+\frac{18}{W}\left(\frac{K_2{a}_w}{1+K_2{a}_w}\right)$	W is the molecular the dry fiber weight per mole of sorption sites, K1 and K2 are constants.

Table 2.

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

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)).

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].

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 [12, 25, 57]: D₁ for the first half-sorption and D₂ for the second half-sorption. Gouanvé et al. [12] found during their work that the D₁ and D₂ values of flax fibers are similar throughout the a_w range studied. However, Alix et al. [25] observed a distinct behavior on the same type of fiber, the value of D₂ was found to be significantly larger than D₁ over the whole range of a_w studied. According to the authors, this dissimilarity is caused by the heterogeneity of the fibers where D₂ should be more representative of water diffusion in the core of the fiber while D₁ 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.

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 10⁻² 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⁻⁰⁶ mm²/s and 2.00 e⁻⁰⁴ mm²/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 same order of magnitude for the different case studies. As an example, fibers treated with 5% NaOH showed a D₁ when immersed in water and conditioned under a RH = 68% of 4.30 × 10⁻⁸ mm²/s and 1.6 × 10⁻⁸ mm²/s, respectively.

Fibers	Condition	Diffusion model	D (mm^2/s)	References
Jute	51% RH	Fick	3.38 e^−07	Mannan and Talukder [63]
Flax	Water immersion	Fick	1.93 e^−04	Stamboulis et al. [18]
Flax	Under a wide range of RH	Fick	6.50 e^−07	Gouavné et al. [12]
Agave	57%	Fick	D1 = 7.27 e^−03; D2 = 3.29 e^−02	Bessadok et al. [57]
	75%	Fick	D1 = 4.55 e^−03; D2 = 2.24 e^−03
	87%	Fick	D1 = 7.64 e^−03; D2 = 2.51 e^−03
Hemp	Water immersion at 300 K	Fick	5.20 e^−04	Saikia [21]
Flax	33% RH	Fick	2.06 e^−04	Roudier [10]
	50% RH	Fick	2.79 e^−04
	75% RH	Fick	3.69 e^−04
Hemp	Water immersion	Langmuir	5.6 e^−06	Célino et al. [22]
Jute		Langmuir	5.9 e^−06
Flax		Langmuir	6.8 e^−06
Sisal		Langmuir	9.1 e^−06
Hemp	80% RH	Fick	2.27 e^−04
Jute		Fick	4.02 e^−04
Flax		Fick	2.00 e^−04
Sisal		Fick	1.17 e^−04
Diss	Water immersion	Fick	D1 = 7.90 e^−08; D2 = 5.6 e^−07	Nouri et al. [23]

Table 3.

Diffusion parameters of plant fibers under different aging conditions.

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, 67, 68, 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 (experimental-numerical) 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 D₁ and D₂ of flax fibers when a_w 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 well as the braking effect of swelling at high a_w. 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 a_w (low mobility of the aggregates). Alix et al. [25] observed similar behavior for flax fibers with a decrease in diffusion coefficients when a_w is less than 0.1. Above this value, D₁, D₂ 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 (D₁, D₂) with increasing a_w (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.

Figure 6.

Evolution of the water diffusion coefficients (D₁ and D₂) as a function of a_w for: (a) flax fibers [12], (b) agave fibers [57], (c) flax fibers [25], and (d) Diss fibers [23].

Figure 7.

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

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 bio-composites 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.

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 bio-composites.

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 × 180 × 3 mm³) the edge effect is no longer observable and diffusion occurs along with the thickness similar to the case of the samples (20 × 20 × 3 mm³) 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 (D_c) in the case of finite sample dimensions for a homogeneous material:

$$D_c=\frac{D}{{\left[1+\frac{h}{l}+\frac{h}{w}\right]}^2}$$E13

This correction is often applied in the literature on bio-composites to avoid the edge effect [72, 83, 84, 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, 87, 88], others that represent it in a more realistic way by taking into account the fibers [80, 89, 90, 91, 92]. These models can be made in 3D or 2D. In addition, the use of Fick’s law is often observed [80, 86, 87, 88, 89, 90, 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 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 bio-composite microstructure. This was achieved based on microscopic pictures of the bio-composite microstructure. The difficulty in this kind of modeling is the non-adaptation 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.

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].

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 pine sawdust at the same loading rate and the diffusivity was about two times higher (Table 4).

Biocomposites	Conditions	D (mm^2/s)	References
Kraft fiber (40%)/PP	Water immersion at 30°C	2.90 e^−7	Beg and Pickering [98]
	Water immersion at 50°C	5.70 e^−7
Kraft fiber (40%)/PP + 4% MAPP	Water immersion at 30°C	2.50 e^−7	Beg and Pickering [98, 101]
Agave fiber (30%)/PP	Water immersion	7.10 e^−7	Pérez-Fonseca et al. [100]
Pine sawdust (30%)/PP		4.10 e^−7
Wood flour (30%)/PP	Water immersion at 27°C	5.70 e^−4	Mishra and Verma [96]
Kenaf fibers (30%)/PP	Water immersion	1.30 e^−6	Law et al. [76, 99]
Kenaf fibers (30%)/PP + 5% MAPP		8.70 e^−7
Kenaf fibers (25%)/PP + 1% MAPP	Water immersion	9.40 e^−6	Tajvidi et al. [102]
Kenaf fibers (50%)/PP + 2% MAPP		1.30 e^−4
Wood flour (25%)/PP + 1% MAPP		1.60 e^−5
Wood flour (50%)/PP + 2% MAPP		1.30 e^−5
Sisal fibers (30%)/PP	Water immersion	3.62 e^−8	Joseph et al. [97]
Diss fibers (20%)/PP	Water immersion	4.30 e^−7	Nouri and Tahlaiti [94]

Table 4.

Diffusion coefficients of different biocomposites reported in the literature.

Furthermore, as the ambient temperature increases, the activity of the polymer molecules increases, which accelerates the diffusion of water molecules within the bio-composites. In the same line, Beg and Pickering [98] observed that increasing temperature increases the water uptake kinetics of bio-composites (Figure 8).

Table 4 summarizes the diffusion coefficient values of different bio-composites reported in the literature.

4.3 Hydro/hygrothermal aging

Hydro/hygrothermal aging can be qualified as a physicochemical degradation. Indeed, in the short term, the composite undergoes plasticization due to the infiltration of water molecules between the polymer chains. This type of aging is mostly reversible when the material returns to its initial state. In addition, plant fibers are known to have an affinity for water, unlike most matrices. This causes differential swelling which can also lead to micro-cracks at the fiber/matrix interphase. These cracks can propagate further through a succession of adsorption/desorption cycles (Figure 9). This damage is irreversible and affects the performance of bio-composites. In addition, chemical degradation of the matrix and even of the fibers can occur; this is called hydrolysis. The presence of temperature can aggravate these phenomena [103].

Figure 9.

Degradation mechanism during hygrothermal aging [13]: (a) fibre swells and matrix microcrack at the interface (b) water infiltration into microcracks and the interface (c) dissolution of some fibre component (d) fibres-matrix debonding.

Law and Ishak [99] found a plasticization effect on Kenaf/PP composites after saturation, with a decrease of between 14 and 35% in all tensile and flexural mechanical properties for the composites at 40% loading. After drying the samples again, a partial recovery of their initial mechanical properties (in flexion and in traction) was noted.

Beg and Pickering [98] observed degradation in the tensile mechanical properties of kraft fiber (40%)/PP composites, with and without 4% MAPP, aged for 238 days at different temperatures of 30, 50, and 70°C. The property retention capacity was lower with increasing temperature. The authors associate these results with the degradation of the fibers and/or the fiber/matrix interface (Figure 10).

Figure 10.

Effects of hygrothermal aging on the fracture surface SEMs of composites (kraft fiber (40%)/PP) of: (a) unaged composites and (b) composites aged at 70C for 238 days [98].

Freund [104] carried out a study of cyclic hygrothermal aging at 80°C on Lin/Elium composites during his thesis. Each cycle contains two phases: a saturation phase at 80% relative humidity and a drying phase at 10% relative humidity. It was found that after each cycle the composites lost mass and mechanical properties with a decrease in stiffness of about 50% after five cycles and a decrease in tensile stress from 110 to 30 MPa (Figure 11). These degradations also had an effect on the adsorption kinetics: the more the material undergoes aging cycles, the faster its adsorption becomes.

Figure 11.

Evolution of Young’s modulus and maximum stress of composites flax/Elium as a function of humidity [104].

The author summarized this change in behavior into two reasons: swelling of the composite, especially the fibers, and degradation of the fibers. These findings were justified by a change in the failure mechanism of the composite due to a degradation of the fiber/matrix interface (Figure 12), and the potential degradation of the flax fibers were justified by a decrease in their crystallinity index from 77.2 to 38.4%.

Figure 12.

Fracture facies of flax/Elium composite before and after aging [104].

On the other hand, Berges [3] studied the cyclic aging of composites at 70°C with two humidity conditions 90% RH and 15% RH for adsorption and desorption, respectively. For each cycle, a total duration of 4 days was chosen, that is, 2 days for each half cycle. Tensile tests carried out after four and nine cycles revealed a reproducible behavior for tensile modulus with a decrease in elongation at break and stress at the break with the number of cycles undergone. This reduction was related to fiber/matrix decohesion and resin damage.

Wang and Petru [105] compared the results of the mechanical properties of unidirectional (30% by volume)/epoxy flax fiber composites undergoing natural aging outdoors and artificial aging by immersion in water at 60°C. It was concluded that 60, 120, and 180 days of natural aging correspond to 1.1, 37.2, and 167.9 h of artificial aging, respectively. Comparisons were made regarding flexural strength and flexural modulus.

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.

Acknowledgments

This research work has been conducted with the financial support of FEDER—Region Pays de la Loire in the framework of the CIPTAP R&D project.