**2.3 Plant fibers**

Plant fibers can be collected from leaves, bast, fruit, seeds or stalks. Common leaf fibers include abaca and sisal. Leaf fibers are characterized by increased strength, most likely due to their high content of lignin, see [4]. The most remarkable type of fruit fiber is the coir of coconuts. Bast fibers are part of the outer cell layers of stems of plants and known examples include Kenaf and hemp. Specific plant fibers used as reinforcement in composite materials:


**Figure 2.** *Chrysotile from Jeffrey mine, Quebec, Canada. Author's collection.*


Plant fibers offer significant advantages over synthetic fibers as reinforcing components. They are renewable, biodegradable, have low densities and lower processing costs. However, they also bring design and fabrication challenges, mostly due to their reduced adhesion to the polymer matrices as a consequence of their hydrophilic character, see [5]. Some images of plants and the related fibers are shown below. The roots of Pandanus – *Pandanus Utilis* – form a pyramidal tract and it holds the trunk of the plant. Pandanus trees grow in tropical and sub-tropical coastlines and islands of all oceans and can withstand salt spray, drought and strong winds, see [6, 7]. The plants growing along seashores have thick aerial roots as anchors in the sand. Such roots keep the trees upright and secure them in the ground - see **Figure 3**.

The Coconut Palmtree is known for its abundance in the equatorial and tropical regions of the world and also for its versatility – see **Figure 4**. The *Cocos Nucifera* or Coconut Palmtree belongs to the family *Arecaceae*, genus *Cocos* [8]. The drupes are used for food, charcoal, oil and cosmetics, and also in the fabrication of biofuel and composite materials. The mechanical strength of the husk fibers is investigated in many research centers around the world [9–12] and the results show widely distributed values.

The coir or coconut fiber is extracted from the outer husk of the fruit. It has a low density, is unsinkable and resistant to saltwater. The individual fiber cells are narrow and hollow, with thick walls made of cellulose – see **Figure 5**. They are pale when immature, but later become hardened and yellowed as a layer of lignin is

#### **Figure 3.**

*Pandanus tree from rainforest habitat, PNG University of Technology campus, Lae, Morobe, Papua New Guinea. Inlet: Pandanus Fibers.*

*Composite Materials with Natural Fibers DOI: http://dx.doi.org/10.5772/intechopen.101818*

#### **Figure 4.**

*Coconut Palmtree from PNG University of Technology campus, Lae, Morobe, Papua New Guinea. Inlet: Coir.*

#### **Figure 5.**

*Cross sections of coir - left - and sisal Fibers – Right – at 200X magnification. Scan electron microscopy on SEMoscope Inovenso IEM 11.*

deposited on their walls. Each cell is about 1 mm (0.04 in) long and 10 to 20 μm in diameter. Fibers are typically 100 to 200 mm long. The two varieties of coir are brown and white. Brown coir harvested from fully ripened coconuts is thick, strong and has high abrasion resistance.

Sisal - botanical name *Agave Sisalana* - is a flowering plant native to Yucatan but widely naturalized in many tropical and subtropical regions of the world. The fibers are extracted from the sword-shaped leaves – see **Figure 6**. During its life of 8 to 10 years, the plant grows hundreds of leaves, each one containing around one thousand fibers [13]. Sisal has been the leading material for binder twine for centuries due to its strength, durability, ability to stretch, and resistance to deterioration

in saltwater. However, its traditional use is limited by the competition from polypropylene and the increased potential as reinforcement in composite materials.

The most important constituents of plant fibers are cellulose, hemicellulose, pectin and lignin. Cellulose is an organic polymer with chemical formula (C6H10O5)n. The high tensile strength of plant stems arises from the arrangement of cellulose fibers and their distribution into the lignin matrix. Cellulose is responsible for the hydrophilic nature of plant fibers. Hemicellulose is a heteropolymer present along cellulose in all terrestrial plant cell walls, see [14]. Hemicellulose has a random, amorphous structure with little strength, is not resistant to hydrolysis and can be easily hydrolyzed by dilute acid or base. Pectin has the function of holding plant fibers together. Lignin is an organic polymer made by cross-linking phenolic precursors, see [15]. It allows the development of structural materials in the cell walls, lending them strength. In adition, lignin is resistant to decay and its contribution to water absorption is negligible. At low magnifications on scan electron microscopes cross sections of coir and sisal fibers reveal similar honeycomb structures constituted of cellulose fibers in lignin matrices, see **Figure 5**.
