The microstructure composition of natural fibers is complex due to the hierarchical organization of the different compounds present at various amounts. Natural fibers are composed of a plurality of walls. These walls are formed of crystalline microfi­brils based on cellulose connected by lignin and hemicelluloses. These walls vary in their composition (ratio cellulose, lignin, pectin, and hemicelluloses ratios) and the orientation of microcellulose fibril.

Chemical composition varies with the type of fibers. These constituents contribute to the overall properties of the fibers. The concept of variability of fiber is important and must be considered in the case of the natural fibers. The origin and method of extraction of fibers lead to variation in dimensional and structural properties of fibers (density, diameter, length, cellulose percentage, microfibrillar angle). These different structural and dimensional characteristics will influence mechanical and thermal properties of fibers. The technique used to determine the structural proper­ties of natural fibers is the Fourier transform infrared spectroscopy (FTIR) (Fig. 14.1).

Figure 14.1 shows the FTIR curves of some raw natural fibers performed in the range of 4,000-600 cm-1. It was observed the main peaks of fiber constituent as lignocellulosic compounds: cellulose, hemicelluloses, and lignin (Yang et al. 2007) (1,730, 1,630, and 1,030 cm-1).

Generally, in a composite material the content and orientation of reinforcement fibers determine the elastic properties. Similarly, natural fibers, the characteristic properties of fibers are mainly determined by chemical and physical composition such as structure, cellulose percentage, microfibrillar angle, and polymerization degree (Rowell et al. 2000; Bledzki and Gassan 1999; Arrakhiz et al. 2013b). Similarly, the microfibrillar angle is inversely proportional to the resistance and hardness of fiber (Yang et al. 2007).

The scanning electron microscopy is used for determination of morphologic properties of the fibers. The samples were coated with gold prior measurement. The Fig. 14.2 shows the Alfa, Doum, Pine cone, Hemp, Coir, and Bagasse fibers of

cross-section and along the fiber. It’s clearly observed that the microfibrillar shape is tubular and the fiber section is circular; at high magnification a single fiber is composed of several microfibers. It was also shown that the fibers are twisted.

The natural fibers area subjected to degradation during composite processing. The thermal degradation of fibers (a critical feature for their application as filler or reinforcement) is reduced to a low level of hemicelluloses. The temperature degra­dation of fibers limits the choice of the matrix. The thermal decomposition of natu­ral fibers was investigated by thermogravimetric analysis (TGA). Natural fibers were heated under air to 600 °C at a rate of 10 °C/min to provide the mass loss, decomposition temperature, and maximum decomposition peak.

The TGA curves of fibers (Hemp and Coir) generally show two major degrada­tion temperature peaks, which are known as the two main-stage degradation of natu­ral fibers, the first at range of 220-260 °C and the second at range of 430-460 °C (Fig. 14.3). The first shoulder peak is corresponding to the hemicelluloses degrada­tion and the second shoulder peak is due to the cellulose and lignin degradation.