2.1. Tensile properties

#### 2.1.1. Strength

Role of mixture (ROM) and inverse role of mixture (IROM) are the most common theories to predict composite mechanical behavior. ROM assumes that there is a perfect bond between fibers and matrix, also that the strain (and stress for IROM) in the fibers is equal to the strain in the matrix. This model deals with aligned continuous fiber composites [19, 20].

ROM equation predicts tensile strength in longitudinal direction:

1. Introduction

46 Composites from Renewable and Sustainable Materials

In the past two decades, environmentally friendly biodegradable plastics have been receiving attention because of the need to diminish the worldwide pollution caused by petroleum-based synthetic plastics [1, 2]. Biodegradability by definition corresponds to the capacity of the material to be completely assimilated by indigenous micro-organisms in the ecosystem. Every year more than 300 million tons of plastics are produced for different applications [3]. In addition to nonrenewability of the synthetic plastics resource, disposing of those types of plastics in landfill will inevitably release toxins in the soil and underground water. Moreover, contaminants can be

Bioplastics from natural resources provide reliable and sustainable alternative to synthetic polymer. Starch is the most commonly studied materials as an eco-friendly polymer that is based on renewable plant material, fully biodegradable, and low cost [6, 7]. There are some challenges that oppose the development of starch-based plastics. Those challenges are mainly poor long-term stability, low water resistance, deterioration of mechanical properties due to moisture uptake, and the relatively fast biodegradability [8]. To overcome those challenges, natural lignocellulosic fibers have been widely utilized to improve the properties of the starchbased plastics [9]. Comparable to synthetic fibers, natural fibers are less dense in addition to being fully degradable. Furthermore, the reinforcement of lignocellulosic fibers significantly improves the mechanical properties of starch-based matrix [10]. Sisal, jute, kenaf, coir, wood, pulp, cellulose, bagasse, banana, orange, and flax fibers are lignocellulosic fibers that have been all studied and found to substantiate enhancement for the starch-based matrix's versatile properties [10–14]. The objective of this article is to review and conduct further analysis to understand the effect of high-fiber content (flax, palm, banana, bagasse, bamboo, and hemp) on mechanical and physical properties. Moreover, experimental results reported before will be

For flax, bagasse, banana, and date palm fiber (DPF) preparation took place by preheating the composite, pressing at 5 MPa and 150°C for 30 min [13–16]. Bamboo-reinforced starch composite was prepared by pressing at 20 MPa and 130 C for 5 min [17]. Hemp-reinforced starch composite was prepared by pressing at 10 MPa and 130 C for 5 min [18]. All fibers except bamboo and hemp were subjected to alkaline (NaOH) treatment before composite preparation. In this chapter, we will deal with tensile, moisture, and thermal results, and further

Role of mixture (ROM) and inverse role of mixture (IROM) are the most common theories to predict composite mechanical behavior. ROM assumes that there is a perfect bond between

absorbed in food resources and eventually accumulated in the human body [4, 5].

compared with theoretical models, selecting the most appropriate model.

details concerning experimental procedures can be found in the cited articles.

1.1. Preparation and characterization

2. Results and properties

2.1. Tensile properties

2.1.1. Strength

$$
\sigma\_L = \sigma\_F V\_F + \sigma\_M V\_M \tag{1}
$$

IROM equation predicts tensile strength in transversal direction:

$$
\sigma\_T = \frac{\sigma\_F \sigma\_M}{V\_M \sigma\_F + V\_F \sigma\_M} \tag{2}
$$

where σ<sup>F</sup> is the fiber tensile strength, σ<sup>M</sup> is the matrix tensile strength, VF is the fibers volume fraction, and VM is the matrix volume fraction. In order to transform fiber weight fraction (WF) to volume fraction (VF), we will be in need to the following equation:

$$V\_F = \frac{\rho\_M W\_F}{\rho\_M W\_F + \rho\_F W\_M} \tag{3}$$

where ρ<sup>F</sup> is the fiber density, ρ<sup>M</sup> is the matrix density, and WM is the matrix weight fraction.

Kelly-Tyson formulates an approach to deal with discontinuous (random and unidirectional) fiber, overcoming the problem of unequal strain in the fibers and the matrix by assuming perfect bonding between fibers and matrix. If the fiber is shorter than the critical length (Lc), it cannot be loaded to its failure stress, and the strength of the composite is then determined by the strength of the fiber/matrix bond, in addition to fiber strength. So for fibers' length greater than Lc, the composite tensile strength can be calculated as follows [19]:

$$L\_{\mathbb{C}} = \frac{\sigma\_{\mathbb{P}} d}{2\pi} \tag{4}$$

$$
\sigma\_{\mathbb{C}} = \varepsilon \left( 1 - \frac{L\_{\mathbb{C}}}{2L} \right) \sigma\_{\mathbb{F}} V\_F + \sigma\_M V\_M \tag{5}
$$

where Lc is the critical length, d is the fibers' average diameter, τ is the shear strength of fiber/ matrix bond (about 0.5 of matrix strength), ε equals 1 in case of unidirectional distribution and three-eighth in random distribution. L is the fibers' average length, σ<sup>F</sup> is the fibers' tensile strength, σ<sup>M</sup> is the matrix tensile strength, VF is the fibers' volume fraction, and VM is the matrix volume fraction. Different properties have been investigated from low to high fiber weight content (50–80%; Figure 1). For all types of fibers, mechanical properties improved as the fiber content increased from 0 to 50–70%. Strength of 0% (starch matrix) was around 4 MPa for the flax, bagasse, and DPF. Starch matrix recorded lower strength in the case of banana composite (2 MPa), whereas starch-based resin strength used in bamboo and hemp composites was 12 MPa. Among the first four types of composites, 50–60% fibers' composite was 8–15 times higher than 0%, depending on the fiber type. Flax composite exhibited the highest mechanical properties with tensile strength surpassing 60 MPa, banana composite was the lowest in strength (27 MPa). About 80% composite was flimsy with

Figure 1. Experimental and theoretical tensile strength of starch-based lignocellulosic fiber composites for different fibers: (a) flax, (b) bagasse, (c) DPF, (d) banana, (e) bamboo, and (f) hemp.

severely deteriorated tensile properties due to lack of matrix material; hence, the load cannot be transferred from fiber to another. For the last two types, the trials did not continue to recognize the maximum possible fiber content for maximal strength (no reduction in strength after maximum strength point). Hemp fiber composite recorded 365 MPa for 70% of the fibers, the high strength of hemp fiber (above 700 MPa), and availability of hemp in long bundles are reasons for this high strength. Kelly-Tyson's (random 2d) was the closest model to experimental results for flax, bagasse, and DPF composites. In the case of banana and bamboo fibers, the experimental results were between Kelly-Tyson (random 2d) and IROM, with more inclined trend to IROM, and this can be due to higher levels of randomness (3d), especially for shorter fibers (banana). Hemp fibers were continuous fibers; hence, the results were close to ROM model.

#### 2.1.2. Elasticity

Similar to ROM and IROM strength models, the two models predict longitudinal and transversal elasticity values for unidirectional continuous fiber composites in which the actual composite elasticity value must lie between. ROM equation predicts elastic modulus in longitudinal direction (EL):

$$E\_L = E\_F V\_F + E\_M V\_M \tag{6}$$

And IROM equation predicts elastic modulus in transversal direction (ET):

High-Content Lignocellulosic Fibers Reinforcing Starch-Based Biodegradable Composites: Properties and Applications http://dx.doi.org/10.5772/65262 49

$$E\_T = \frac{E\_F E\_M}{V\_M E\_F + V\_F E\_M} \tag{7}$$

where EF is the fibers modulus of elasticity, EM is the matrix modulus of elasticity, VF is the fibers volume fraction, and VM is the matrix volume fraction.

Another model used to predict discontinuous fiber composite behavior was called Halpin-Tsai. The composite elastic modulus in longitudinal direction (EL) can be obtained as follows [15, 19]:

$$E\_L = E\_M \frac{\left(1 + \xi\_L \eta\_L V\_F\right)}{\left(1 - \eta\_L V\_F\right)}\tag{8}$$

where (ξ) is a factor dependant on the shape and distribution of the reinforcement. The parameter (η) is a function of the ratio of the relevant fiber and matrix moduli with respect to the reinforcement factor (ξ). In this model, they can be calculated by the following formulas:

$$
\eta\_L = \frac{(E\_F - E\_M)}{(E\_F + \xi\_L E\_M)} \tag{9}
$$

$$\mathcal{E}\_L = 2\left(\frac{L}{d}\right) + 40V\_F^{10} \tag{10}$$

Also, the composite elastic modulus in transversal direction (ET) can be obtained as follows:

$$E\_T = E\_M \frac{\left(1 + \xi\_T \eta\_T V\_F\right)}{\left(1 - \eta\_T V\_F\right)}\tag{11}$$

where

severely deteriorated tensile properties due to lack of matrix material; hence, the load cannot be transferred from fiber to another. For the last two types, the trials did not continue to recognize the maximum possible fiber content for maximal strength (no reduction in strength after maximum strength point). Hemp fiber composite recorded 365 MPa for 70% of the fibers, the high strength of hemp fiber (above 700 MPa), and availability of hemp in long bundles are reasons for this high strength. Kelly-Tyson's (random 2d) was the closest model to experimental results for flax, bagasse, and DPF composites. In the case of banana and bamboo fibers, the experimental results were between Kelly-Tyson (random 2d) and IROM, with more inclined trend to IROM, and this can be due to higher levels of randomness (3d), especially for shorter fibers (banana). Hemp fibers were continuous fibers; hence,

Figure 1. Experimental and theoretical tensile strength of starch-based lignocellulosic fiber composites for different fibers:

Similar to ROM and IROM strength models, the two models predict longitudinal and transversal elasticity values for unidirectional continuous fiber composites in which the actual composite elasticity value must lie between. ROM equation predicts elastic modulus in longi-

And IROM equation predicts elastic modulus in transversal direction (ET):

EL ¼ EF VF þ EMVM (6)

the results were close to ROM model.

(a) flax, (b) bagasse, (c) DPF, (d) banana, (e) bamboo, and (f) hemp.

48 Composites from Renewable and Sustainable Materials

2.1.2. Elasticity

tudinal direction (EL):

$$
\eta\_T = \frac{(E\_F - E\_M)}{(E\_F + \xi\_T E\_M)} \tag{12}
$$

$$
\xi\_T = 2 + 40V\_F^{10} \tag{13}
$$

The two previous moduli are for oriented fibers, the following equation is a result of an averaging process made by Tsai and Pagano [21] to predict the modulus of an isotropic composite (Ec) based on random fibers reinforcement.

$$E\_{\mathbb{C}} = \frac{3}{8}E\_L + \frac{5}{8}E\_T \tag{14}$$

Figure 2 shows the modulus of elasticity for the first four composite types discussed in the strength section. Young's modulus followed the same pattern as tensile strength. Modulus increased with fiber content till an optimum point (50–60%), then property deterioration took place at higher content (80%) due to the scarcity of resin and inability to transfer load between fibers. Flax composite exhibited the highest modulus (4.7 GPa) in comparison with other types. Superior mechanical properties for flax were realized elsewhere and attributed to high

Figure 2. Experimental and theoretical tensile modulus of starch-based lignocellulosic fiber composites for different fibers: (a) flax, (b) bagasse, (c) DPF, and (d) banana.

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 composites.
