It is known that the chemical composition of the natural fibers is of paramount in determining their suitability for different industrial applications particularly for NFRPC. That is, several characteristics of these composites like degradability and

Disorderly arranged

crystalline cellulose

microfibrils

Fig. 1.6 Structure of bio-fiber. Adapted from Azwa et al. (2013) recyclability, weather resistance, fungi attack, etc., strongly depend on the chemical composition of filler (fiber) (Al-Oqla and Sapuan 2014; Azwa et al. 2013). Actually, a variation in the fiber quality can be achieved for the same fiber type due to several factors. Some of these factors are: soil quality, fiber location on the plant, weather conditions, crop variety, fertilization, climate, and harvest timing (Dittenber and GangaRao 2011; Kalia et al. 2011b). In addition, extraction processing methods, fibers cross-sectional area variation, and the differences in drying processes will also affect the quality of the natural fibers (Dittenber and GangaRao 2011). Consequently, differences of natural fiber chemical and physical properties can be found in literature. Plant fibers consist mainly of cellulose fibrils embedded in lignin matrix. The bio-fiber structure is shown in Fig. 1.6 ; A primary cell wall and other

three secondary walls form the fiber’s complex layered structure whereas secondary thick middle layer of the cell walls consists of a series of helically wound cellular micro-fibrils formed from long chain cellulose molecules can determine the mechanical properties of fiber. Each cell wall is formed from three main compo­nents which are cellulose, hemicelluloses, and lignin. The lignin-hemicelluloses have a matrix-like role while the micro-fibrils which are made up of cellulose mol­ecules act as fibers (Dittenber and GangaRao 2011; John and Thomas 2008). Pectin, oil, and waxes can be found as other components (John and Thomas 2008; Wong et al. 2010). Due to the existence of Lumen, the natural fiber has a hollow structure unlike synthetic ones (Liu et al. 2012).

Cellulose and lignin are the most important structural components in many natu­ral fibers. In plants, cellulose is usually found as a slender rod like crystalline micro-fibrils and aligned along the fiber’s length (Azwa et al. 2013). Although Cellulose is resistant to hydrolysis, strong alkali, and oxidizing agents, it is degrad­able to some extent when exposed to chemical treatments (Azwa et al. 2013). Lignin is a complex hydrocarbon polymer. It usually gives rigidity to plant and assists in water transportation. It is hydrophobic, resists most of microorganisms attacks as well as acid hydrolysis, it is usually soluble in hot alkali, readily oxi­dized, and easily condensable with phenol. The nature of cellulose and its crystal­linity can determine the reinforcing efficiency of natural fibers (John and Anandjiwala 2007). Filaments are bonded into a bundle by lignin and are attached to stem by pectin. Lignin and pectin are weaker polymers than cellulose. They have to be removed by retting and scotching for effective composite reinforcements (Dittenber and GangaRao 2011). The average weight percentage of chemical com­position of the date palm tree frond and their fiber properties (Mirmehdi et al. 2014; Sbiai et al. 2010) are shown in Table 1.2.

It can be noticed that there are some variation in the measured values of the date palm fiber’s chemical composition due to inherent parameters mentioned previ­ously. A comparison between average values of both cellulose and lignin for the date palm fiber with other natural fibers can demonstrate the appropriateness and competitiveness of the date palm fibers for being potential type of fillers for natural fiber composites. Such comparison is demonstrated in Fig. 1.7.

It can be seen from the comparison that the date palm fiber has an added value over both hemp and sisal, because it has less cellulose content than they do which reduces the ability of the date palm fiber to absorb water comparing with hemp and sisal (Al-Oqla and Sapuan 2014). On the other hand, this can give the date palm fiber more desired mechanical properties over the coir one. Moreover, the cellulose content in date palm fiber is greater than that of lignin, which allows it to be competitive for automotive applications (Al-Oqla and Sapuan 2014).