Natural fibers have accompanied human society since the start of our life. In early history, humans collected the fiber from the plant for rope and textile. Natural fibers are also used as a paper sheets, fish nets and old rags. According to Mohanty et al.,46 natural fibers are derived from plants and animals as shown in Fig. 9.9. Production of materials from renewable resources has risen, due to the increasing awareness that nonrenewable resources are becoming scarce. This century can be considered as the cellulosic century because numerous renewable plant resources for products are being discovered. The increasing attention to natural fibers is primarily due to their economical production with few requirements for equipment and low specific weight. Such attributes of natural fibers result in higher specific strength and stiff­ness as compared to glass reinforced composites.11 Natural fibers also provide safer handling and working conditions. They are nonabrasive to mixing and molding equipment, which can help in cost reductions.11 The most significant virtue of natu­ral fibers is their positive environmental impact. They are carbon dioxide neutral, meaning they do not emit excess carbon dioxide into the atmosphere as they are composted or combusted. In this way, natural fibers contribute to the mitigation of global warming. Working with natural fibers reduces dermal and respiratory irrita­tion owing to their friendly processing atmosphere with better working conditions.11 The abundant availability of natural fibers provides an added advantage over the use of synthetic fibers.

Among the natural fibers, plant fibers are the main sources of fibers that can be found in a large quantity. Figure 9.10 shows the two main categories of natural fibers from plants. The nonwood natural fibers are classified into bast fibers, leaf fibers, seed fibers, stem fibers, grass and straw fibers.1647 Natural fibers from plant resources can also be categories as primary and secondary depending on their uti­lization. Primary plants are those that are grown solely for their fiber content (i. e., jute, hemp, kenaf, and sisal), whereas secondary plants are the ones in which the fiber are produced as a by-product (i. e., oil palm, pineapple, date palm and coir).

The most widely used natural fibers among the list showed in Table 9.1; flax, jute, hemp, sisal, ramie, and kenaf fibers were extensively researched and used in numerous applications.16 Nowadays, sugar palm and oil palm fibers which are abun­dantly available in tropical regions are gaining more interest and significance in research due to their specific properties.

One of the key parameters of the plant fibers is to understand their chemical composition, as the chemical compositions of natural fibers have strong relationship to its performance for application in composite materials. Plant fiber also referred as lignocellulosics, consist mainly of cellulose, hemicelluloses and lignin. It also consists of minor amounts of free sugars, starch, proteins and other organic com — pounds.48 Cellulose, hemi-cellulose, and lignin are the three main constituents of any plant fibers and the proportion of these components in a fiber depends on the age,

source of the fibers and the extraction conditions used to obtain the fibers.49 Table

9.1

shows the chemical composition of different types of fibers.

Natural fibers are multicellular in nature, consisting of a continuous numbers, mostly in a form of cylindrical honeycomb cells which have different sizes, shapes, and arrangements for different types of fibers.50 Different types of fibers provide dif­ferent properties as shown in Table 9.2.