2.2.1. Mechanism of moisture absorption in polymer/natural fiber composites

Polymer/natural fiber composites tend to absorb moisture in humid atmosphere. Therefore, this moisture absorption affects the fiber matrix interface leading to low stress transfer between matrix and fiber. The transport of water in a polymer/fiber composite can be attributed to the imperfections of the matrix (voids, pores, and cracks). The water absorbed in a matrix can be divided into two types, free water and bound water. Free water is the water molecules moving freely through the voids, and bound water is the water molecules bonded to the polar groups of a matrix [30]. Figure 3 shows the two types of absorbed water in polymeric matrix.

When water molecules penetrate polymer/fiber composite, it attaches to the hydrophilic group of the fiber creating intramolecular hydrogen bonds. Thus, this interaction reduces the interfacial adhesion between fiber and matrix which leads to deterioration in the mechanical properties of the composite [30, 31]. Figure 4 explains how moisture absorption by fiber affects the composite.

Polymeric matrices, especially, starch-based reinforced with natural lignocellulosic fibers are sensitive to moisture. Moisture absorption will deteriorate their functionality. Therefore, aspects should be considered when manufacturing such a composite that would be used in a humid atmosphere. The most important aspect is the proper selection of the fiber type according to its moisture resistance.

#### 2.2.2. Effect of fiber type and content on moisture absorption

cellulose content and low fiber diameter. Halpin-Tsai was the best model to predict the composite behavior, and this is due to the randomness of the fibers used to prepare these

Figure 2. Experimental and theoretical tensile modulus of starch-based lignocellulosic fiber composites for different

Owing to its natural abundance and low cost, starch-based biodegradable "green" polymers have attracted great attention. Unfortunately, the use of these plastics in a wide range of applications has been restricted by its low water resistance. Therefore, in order to overcome this disadvantage while preserving the biodegradability and the green property of this polymer, natural lignocellulosic fibers are being used as a biodegradable and eco-friendly reinforcement [22–25]. The incorporation of natural lignocellulosic fibers, which are mainly made up of hydrophilic cellulose, into starch-based matrix is responsible for the reduction of moisture absorption of the resultant composite. This reduction in the moisture absorption of the two hydrophilic materials is attributed to the good interfacial adhesion between starch and cellulose which leads to decreasing the free volume of the starch molecular chains and thus reduce the water absorption; the less hygroscopicity of cellulose when compared with starch; formation of fibrous network around starch thus hinder the moisture penetration; and the high

composites.

2.2. Moisture absorption behavior

fibers: (a) flax, (b) bagasse, (c) DPF, and (d) banana.

50 Composites from Renewable and Sustainable Materials

crystallinity of cellulose when compared with starch [26–29].

According to literature, the moisture absorption of starch/natural fiber composites is highly affected by the fiber type and its content. Each type of fibers has its own cellulose content; thus fibers with high cellulose content highly diminish the moisture absorption of the resultant composite. Mehanny et al. [13] studied the effect of reinforcing thermoplastic starch (TPS) matrix with different contents of NaOH-treated bagasse fiber on the moisture absorption of the resultant composite. Figure 5 shows the moisture absorption of composites reinforced with 0, 20, 40, 60, and 80% bagasse fiber. Their results showed that the moisture absorption after reaching equilibrium of the starch-based matrix with 0% fiber reached more than 53%,

Figure 3. Free water and bound water in polymer matrix. Adopted from Ref. [30].

Figure 4. Effect of water on fiber-matrix interface. Adopted from Ref. [30].

Figure 5. Moisture absorption of TPS/bagasse fiber composites with different fiber wt%. Adopted from Ref. [13].

whereas with the lowest fiber content 20% it reached 48%. By increasing the fiber content up to 80%, the moisture absorption dropped to 36%.

According to Elsayed et al. [14, 15], increasing the NaOH-treated flax fiber content from 0 up to 60% resulted in reducing the moisture absorption of the TPS/flax composite from 48 to 38% as shown in Figure 6. The authors proposed investigating, as well, the effect of changing

Figure 4. Effect of water on fiber-matrix interface. Adopted from Ref. [30].

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Figure 5. Moisture absorption of TPS/bagasse fiber composites with different fiber wt%. Adopted from Ref. [13].

Figure 6. Moisture absorption of (a) TPS/flax fiber and (b) TPS/DPF composites with different fiber wt%. Adopted from Refs. [14, 15].

the reinforcement fiber on the moisture absorption of the resultant composite. NaOH-treated date palm fiber (DPF) with different contents was used. They demonstrated that changing the fiber type caused a slight effect on the moisture absorption property of the resultant composite. The authors attributed this slight difference to the convergence of the cellulosic content of both fibers.

Darwish et al. [16] investigated the effect of increasing the content of the NaOH-treated banana fiber (BF) reinforcement on the moisture absorption property of the starch/BFs composites as shown in Figure 7. The starch matrix was reinforced with 40, 50, and 60% BFs. The moisture absorption dropped from 70 to 41% with increasing the BF content from 0 to 60%.

Figure 8 shows a comparison between the moisture absorption of the TPS composites reinforced with 60 wt% fibers at the equilibrium plateau from the previously stated studies [13–16]. From Figure 8, it can be concluded that the moisture absorption is highly dependent on the fiber type, hence the cellulose content of the fibers. Therefore, flax with the highest cellulose content diminished the moisture absorption of the composite to 35%.

#### 2.2.3. Fick's law and moisture absorption of TPS/natural fiber composites

Fick's second law of diffusion has been widely used to model and characterize the absorption of moisture in many materials. Fick's second law depicts the process of one dimension moisture absorption with respect to exposure time as shown in Eq. (1) [32–35]:

$$\frac{d\mathbb{C}}{dt} = D \frac{d^2 \mathbb{C}}{dX^2} \tag{15}$$

where C is the water concentration, t is the time of diffusion, and D is the diffusion coefficient normal to the surface in the x-direction.

Figure 7. Moisture absorption of TPS/banana fiber composites with different fiber wt%. Adopted from Ref. [16].

High-Content Lignocellulosic Fibers Reinforcing Starch-Based Biodegradable Composites: Properties and Applications http://dx.doi.org/10.5772/65262 55

Figure 8. Moisture absorption of TPS composites reinforced with 60% fibers at the equilibrium plateau.

Diffusion behavior, Fickian or non-Fickian, can be recognized theoretically by the shape of the absorption curve represented by

$$\frac{M\_t}{M\_m} = K. \, t^n \tag{16}$$

where Mt and Mm are moisture absorption at time t and at equilibrium state, respectively. K and n are the diffusion kinetic parameters. Mt can be calculated from

$$M\_t = \frac{W\_t - W\_0}{W\_0} 100(\%) \tag{17}$$

where W<sup>0</sup> is the weight of dry sample and Wt is the weight of wet sample at time t [32–35].

The diffusion parameter n indicates whether the diffusion is Fickian or non-Fickian. When n = 0.5, the diffusion is Fickian. When n ≥ 1, the diffusion is non-Fickian. Moisture absorption in lignocellulosic fiber-reinforced polymer composites always follows Fickian behavior [32–36].

One-dimensional approach of Fick's law shows that the moisture absorption is directly proportional to the square root of time, then slows down until an equilibrium plateau is reached. For values of Mt/Mm < 0.6, the initial part of the curve can be deduced by [36]:

$$\frac{M\_t}{M\_m} = \frac{4}{h} \sqrt{\frac{D.t}{\pi}}\tag{18}$$

where h is the thickness of the sample.

the reinforcement fiber on the moisture absorption of the resultant composite. NaOH-treated date palm fiber (DPF) with different contents was used. They demonstrated that changing the fiber type caused a slight effect on the moisture absorption property of the resultant composite. The authors attributed this slight difference to the convergence of the cellulosic content of both fibers. Darwish et al. [16] investigated the effect of increasing the content of the NaOH-treated banana fiber (BF) reinforcement on the moisture absorption property of the starch/BFs composites as shown in Figure 7. The starch matrix was reinforced with 40, 50, and 60% BFs. The moisture

Figure 8 shows a comparison between the moisture absorption of the TPS composites reinforced with 60 wt% fibers at the equilibrium plateau from the previously stated studies [13–16]. From Figure 8, it can be concluded that the moisture absorption is highly dependent on the fiber type, hence the cellulose content of the fibers. Therefore, flax with the highest

Fick's second law of diffusion has been widely used to model and characterize the absorption of moisture in many materials. Fick's second law depicts the process of one dimension mois-

where C is the water concentration, t is the time of diffusion, and D is the diffusion coefficient

C

dX<sup>2</sup> (15)

dC dt <sup>¼</sup> <sup>D</sup> <sup>d</sup><sup>2</sup>

Figure 7. Moisture absorption of TPS/banana fiber composites with different fiber wt%. Adopted from Ref. [16].

absorption dropped from 70 to 41% with increasing the BF content from 0 to 60%.

cellulose content diminished the moisture absorption of the composite to 35%.

2.2.3. Fick's law and moisture absorption of TPS/natural fiber composites

normal to the surface in the x-direction.

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ture absorption with respect to exposure time as shown in Eq. (1) [32–35]:

For the second half of the absorption curve, where Mt/Mm > 0.6, Springer [37] proposed the following approximation:

$$\frac{M\_t}{M\_m} = 1 - \exp\left[-7.3\left(\frac{D.t}{h^2}\right)^{0.75}\right] \tag{19}$$

Figure 9 shows a comparison between the Fick's law predicted and the experimental moisture absorption test results of TPS composites reinforced with 60 wt% of different fibers (banana, bagasse, DPF, and flax). The four curves show that the experimental data are in a good agreement with the predicted values. Thus, TPS/lignocellulosic fiber composites follow a Fickian behavior.

Figure 9. Experimental and predicted moisture absorption of TPS composites reinforced with 60% (a) BFs, (b) bagasse fibers, (c) DPFs, and (d) flax fibers.

#### 2.2.4. Effect of moisture absorption on mechanical properties of composites

Mechanical properties deterioration is one of the drawbacks of moisture absorption on polymer/natural fiber composites. This deterioration is attributed to the swelling of the cellulose fibers. Due to this swelling, development of shear stress at the fiber/matrix interface occurs. Therefore, this leads to debonding of the fibers, delamination, and loss of structural integrity [30, 38]. Fiber surface chemical treatments are proven to promote the moisture resistance by reducing fiber hydrophilicity. Moreover, these treatments improve the interfacial adhesion between matrix and fiber thus, tightens the water penetration pathways and consequently, lowers the moisture absorption [30, 39–41].

In Ref. [25], the authors studied the effect of moisture absorption on the mechanical properties of starch-based composites reinforced with different contents of NaOH-treated flax and DP fibers. Tensile strength test results showed that the strength of both flax and DPFs composites decreased to the half of its original value as shown in Figure 10.

In sum, from the previously illustrated studies, moisture absorption of TPS/lignocellulosic fiber composites is affected by the type and the content of the reinforcement fiber. Moreover, different fiber surface treatment techniques highly modify its water resistance. Therefore, proper selection of fiber type, content, and surface treatment technique is of crucial importance while manufacturing a composite exposed to humid atmosphere in order to preserve its strength and structural integrity.

Figure 10. Effect of moisture absorption on the tensile strength of (a) flax composites and (b) DPFs composite. Adopted from Ref. [42].
