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Combining fibers with polymers in biomaterials usually produces composites with totally different characteristics as well as superior desired performance over the utilized constituents. Biobased plastics are dominating the new trends in plastic industry since the petro‐based plastics are nonrenewable (help in depletion of petroleum resources), nondegradable (cause shortage in landfills), and very harmful to the environment. These new trends are focusing more specifically on the renewable plants and on agro waste fiber composites. However, it is not possible to completely replace all the petro‐based productions with biobased ones [25]. In such cases, the concept of combining biomaterials with petro‐based products should be adopted. Natural biocomposites have become well recognized for their low cost and low density. In addition, the ease of shaping and processing due to the low abrasiveness when compared to synthetic fiber‐based composites gives the biobased composites extra advantages. On the other hand, many difficulties arise in using the biobased composites in industry. One of these difficulties is the incompatibility issue between the fiber and the polymer. This is due to both the hydrophilic characteristic of the natural fibers and the hydrophobic characteristic in polymers. Reducing the incompatibility requires physical and chemical treatments for the fibers, as well as using various additives as coupling agents between the fibers and the polymers. Once the composite material is well fabricated, its characteristics are required to be tested and improved. The most critical properties are the mechanical ones, namely the tensile strength, the tensile modulus, the fatigue strength, the creep rate, and the impact strength. Agro waste natural fibers are normally suitable to reinforce polymers due to their relative high mechanical performance and their low densities.

The mechanical properties of the composite materials are the most essential characteristics even if the composites are not used in loaded applications. A certain level of strength is required for the composites to at least maintain their shapes during service. However, for some composites, it is very hard to estimate their mechanical properties as is the case with biocomposites of short natural fibers. This is due to many reasons, such as the fiber dimensions, fiber quality, fiber orientation and distribution, the fiber–matrix interface quality, as well as the matrix characteristics [41, 58]. Table 1.1 demonstrates the mechanical and physical characteristics of some natural fibers.

Improving the composite properties can sometimes be achieved by controlling some key factors, such as the fiber aspect ratio (L/D) and volume fraction of the fibers with respect to the matrix [59]. If the aspect ratio of the fiber is very small, insufficient load will transfer from the matrix to the fiber; in such cases, the fibers will work just as fillers and no considerable improvements will be achieved in the composite's mechanical performance. On the other side, high aspect ratio usually leads to poor fiber dispersion, substantially poor mechanical performance. Regarding the volume fraction, the low percentile causes discontinuities in transferring the load over the fibers; thus, the composite strength will decline. Also, the high percentile can produce the same effect due to fiber clustering.

Table 1.1 Mechanical and physical characteristics of some natural fibers.

Source: AL‐Oqla et al. [16]. © 2015, Elsevier.

Fiber type	Coir	Date palm	Flax	Hemp	Sisal
Density (g/cm3)	1.15–1.46	0.9–1.2	1.4–1.5	1.4–1.5	1.33–1.5
Length (mm)	20–150	20–250	5–900	5–55	900
Diameter (μm)	10–460	100–1000	12–600	25–500	8–200
Tensile strength (MPa)	95–230	97–275	343–2000	270–900	363–700
Tensile modulus (GPa)	2.8–6	2.5–12	27.6–103	23.5–90	9–38
Specific modulus (approx.)	4	7	45	40	17
Elongation to break (%)	15–51.4	2–19	1.2–3.3	1–3.5	2–7

Table 1.2 Informative values on the different properties of the fibers.

Fiber type	Density (g/cm3)	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation to break (%)	Cost per weight (USD/kg)
Coir	1.15–1.46 (1.31)	95–230 (162.5)	2.8–6 (4.4)	15–51.4 (33.2)	0.3
Date palm	0.9–1.2 (1.05)	97–275 (186)	2.5–12 (7.25)	2.0–19 (10.5)	0.02
Jute	1.3–1.49 (1.4)	320–800 (560)	8–78 (43)	1–1.8 (1.4)	0.3
Hemp	1.4–1.5 (1.45)	270–900 (585)	23.5–90 (56.75)	1–3.5 (2.25)	1.3
Kenaf	1.4	223–930 (576.5)	14.5–53 (33.75)	1.5–2.7 (2.1)	0.5
Oil palm	0.7–1.55 (1.13)	80–248 (164.0)	0.5–3.2 (1.85)	17–25 (21)	0.3

On the other hand, an obvious lack of research regarding the evaluation and selection processes of the natural fiber composites (NFCs) is observed. More specifically, evaluating and selecting the proper agro waste natural fibers for the NFCs is not investigated comprehensively regarding the desired features [9]. Hence, more efforts to establish sufficient comparison criteria are required in order to precisely evaluate and select the appropriate fiber type for the biobased products. The overall characteristics and capabilities of the NFCs depend on the physical, mechanical, chemical, and economic features of the composites' constituents. Therefore, in order to exploit the benefits of these materials to the full extent, comprehensive investigations of the previously mentioned features have to be completed as a primary stage in any industrial application. New techniques have been developed by AL‐Oqla and Sapuan [20] for the assessment and selection of the composites. A wide range of valuable criteria has been discussed by AL‐Oqla and Sapuan [20] to demonstrate that natural fibers have a primary role in natural fiber reinforced polymer composites. Another technique to evaluate various raw fibers was presented by AL‐Oqla et al. [10], where six different types of natural fibers were considered in the evaluation process. These were coir, jute, hemp, kenaf, oil palm, as well as date palm. The physical, mechanical, and economic properties of these types were considered simultaneously. Informative values on the different properties of the considered fibers are listed in Table 1.2. They are obtained from literature based on experimental works, where the average values were adopted assuming that the data is uniformly distributed within the data range found in literature.

Table 1.3 Specific properties of the fibers.

Fiber type	Specific tensile strength (MPa)/(g/cm3)	Specific tensile modulus (GPa)/(g/cm3)	Specific elongation (%)/(g/cm3)	Cost ratio
Coir	124.05	3.36	25.34	0.231
Date palm	177.14	6.90	10.00	0.015
Jute	400.00	30.71	1.00	0.231
Hemp	403.45	39.14	1.55	1
Kenaf	411.79	24.11	1.50	0.385
Oil palm	145.13	1.64	18.58	0.231

Table 1.4 Specific properties of the fibers with respect to the cost ratio.

Fiber type	Specific tensile strength (MPa)/(g/cm3)/cost ratio	Specific tensile modulus (GPa)/(g/cm3)/cost ratio	Specific elongation (%)/(g/cm3)/cost ratio
Coir	537.53	14.55	109.82
Date palm	11 514.29	448.81	650.00
Jute	1 733.33	133.10	4.33
Hemp	403.45	39.14	1.55
Kenaf	1 070.64	62.68	3.90
Oil palm	628.91	7.09	80.53

Hence, Table 1.3 lists the specific properties for the fibers (the average values of each property divided by the average values of the density).

The obtained specific properties calculated in Table 1.3 are further calculated with respect to the cost ratio as tabulated in Table 1.4.

It is believed that a comparison of the natural fibers using the combined physical, mechanical, and economic information would result in better evaluation of the available natural fibers and resources. It can be noticed here that the specific tensile strength to cost ratio for the date palm was five times that of jute. Therefore, combined evaluations would lead to better evaluation of fibers as it can be realized that date palm fiber is better than jute once specific properties to cost ratio evaluation criterion is considered.