Available Mechanical and Chemical Properties of Natural Fibers: Critical Review

Available Mechanical and Chemical Properties of Natural Fibers: Critical Review

DOI: 10.4018/978-1-5225-4837-9.ch004
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Natural fibers are so attracting in comparison to synthetic fibers since they exhibit good properties like the low cost, good specific mechanical properties and their requirements of low energy during production. However, natural fibers hold some drawbacks which must be consider in comparison to the synthetic fibers like their high moisture absorption, low mechanical properties, heat resistance and durability and the variation in their prices and quality. In order to choose the suitable natural fiber for the selected application, their mechanical and chemical properties can be helpful for taking the right decision. In this chapter, a wide research is done in order to provide as much as possible the available mechanical and chemical properties of natural fibers of bast, leaf, seed, stalk, and wood categories from the most trusted publications. The specific mechanical properties of natural fibers are then calculated for a better comparison at the level of composites.
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Structural Constitution

Plant fiber’s membrane is heterogeneous and its structure consists of: cellulose fibers, lignin, hemi-cellulose, cell wall (primary and secondary wall), middle layer and microfibrils as shown in Figure 1. Cellulose fibers consist of helically wound cellulose microfibrils where an amorphous lignin matrix acts to bound them together. Lignin helps in protecting the fiber against biological attack and keeps water inside the fiber. It adds the stiffness to the stem and makes it able to resist the gravitational force and wind. Hemi-cellulose is a compatibilizer between cellulose and lignin of natural fibers (Kalia, Kaith, & Kaur, 2009).

Each fiber has a layer of complex structure consisting of a thin primary wall which is the first layer formed during the growth of the cell and surrounding a secondary wall. Its secondary wall has three layers, where the middle one is thick and determines the fiber’s mechanical properties. The middle layer of the secondary wall is composed of a series of helically wound cellular microfibrils which are formed from long-chain cellulose molecules. These microfibrils offer the mechanical strength to the fiber and have approximately 10–30 nm diameter containing 30–100 cellulose molecules in extended chain conformation. Waxy substances which bond the cell to its adjacent neighbors prevent the fiber’s surface from forming a strong bond with a polymer matrix (Li, Mai, & Ye, 2000).

Figure 1.

Structural constitution of natural fiber cell (Rong, Zhang, Liu, Yang, & Zeng, 2001)


Cellulosic fibers exhibit different properties which can vary due to the weather conditions, level of maturity, harvest, retting degree, decortications, fiber modification and technical processes (spinning and carding) (Van de Velde & Kiekens, 2001). Dividing natural fibers into elementary cellulosic constituents, such as microfibrils are gaining much interest since they have high strength and stiffness (Bledzki, Reihmane, & Gassan, 1996), high reinforcing potential (Hornsby, Hinrichsen, & Tarverdi, 1997), and due to their biodegradability and renewability. Hydrophobic layers like lignin are not preferred for the interaction between natural fibers and most hydrophilic resins. It leads to their poor adhesion and results in having bio-composites with low mechanical properties. Such phenomena can be overcome by applying chemical or physical surface treatments for natural fibers.


Mechanical Behavior

The ultimate tensile strength, Young’s modulus and elongation at failure, in addition to the density, are described.

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