**4. Natural fibers as reinforcement for composites materials**

There has been an upsurge toward attainment of superior mechanical and tribological properties for varied applications by replacing existing materials with more advanced, innovative and sustainable materials. Accordingly, monolithic materials are fast being replaced by materials like glass, carbon, aramid fibers for sporting, aerospace, automotive and construction sectors. Nonetheless, the non-biodegradability, non-renewability and energy intensive production processes render the materials unsustainable and thus doomed to be hazardous to environment. Consequently, natural fiber reinforced composites with potential to replace the synthetic fibers are gaining momentum to avert the deleterious impact of conventional, unsustainable materials. Accordingly, a gamut of natural fibers obtained from fruits, seeds, leaves, stem, animals, etc. are being explored for their viability in composites. The composites are tailor-made materials possessing unique qualities which can be altered by variation

in reinforcement and matrix phase. The salient features of natural fibers particularly their low density (1.2–1.6 g/cm3 compared to glass fiber with density of 2.4 g/cm3 ) make them preferred choice for light weight composites in contrast to synthetic fibers. Consequently, there is an upsurge in the demand of natural fibers based composites commercially in gamut of industrial establishments such as automotive interior linings (roof, rear wall, side panel lining), furniture, construction, packaging etc. Lightweight composites primarily utilize natural fibers like hemp, jute, sisal, banana, coir, and kenaf for their production process. Animal hair fibers owing to good mechanical properties such as ductility of 20% elongation on the average and strength to failure of 250–300 MPa on the average are other suitable candidates for myriad of textile and non-textile applications. Likewise, coir fibers exhibit high impact strength/resistance.

The modification of properties of the natural fibers by different chemical treatments and blending them with polymers and other synthetic materials is accomplished to enhance the properties of the natural fibers and in turn the properties of hybrid composites. Hybridization involves combination of fillers and natural fiber resulting in enhanced mechanical properties of the composites. A number of factors such as volume or weight fraction of the reinforcement, fiber alignment, distribution, orientation and aspect ratio, fiber-matrix adhesion, usage of additives, and chemical treatment of fibers affect the mechanical performance of fiber-reinforced composites. Natural fibers are potential sustainable candidates replacing synthetic fibers in architecture, cladding, walling, and flooring. Flax, hemp, sisal, and wool are used in car interiors and exteriors like in Mercedes-Benz components. The coir/polyesterreinforced composites find application in the mirror casing, paperweights, voltage stabilizer cover, projector cover, helmet etc. Likewise, rice husk fiber, cotton, ramie, jute fiber, kenaf are increasingly being used in clothing, fishing nets, packing materials, ropes, sewing threads [5, 6].

The following section will discuss salient features of natural fibers suitable for composites and varied application areas:

The source of natural fibers include a variety of plants and animals (hair, chicken feather). The plant based fibers constitute of cellulose, lignin, hemicellulose, pectin, waxes, and water-soluble substances. **Figures 3** and **4** shows the various natural fibers suitable as composite materials and other associated sectors.

*Cotton (Gossypium)* is considered to one of the most crucial agricultural crop holding lion's share of market as far as textile industry is concerned. The fiber belongs to the sub-tribe Hibisceae and family of Malvaceae. The cotton cultivation is generally practiced in tropical and subtropical regions, with China being the largest cotton producer followed by India and the United States. Upland cotton (*Gossypium hirsutum*) and pima cotton (*Gossypium barbadense*) are the most popular and extensively utilized cotton species. The comfort, moisture absorbing and hydrophilic attributes make it a preferred choice for summer wear clothing and accessories. More recently, the potential of cotton for development of composites for industrial applications is being explored.

*Silk (Bombyx mori)* is an animal based natural fiber extracted from cocoons of silkworms. The major producers of silk include China, South Asia, and Europe. The exceptionally brilliant properties of fiber such as good mechanical strength, extensibility and compressibility makes it a preferred choice as luxury material for exorbitant, high end range of apparels and fashion items.

*Hemp* belongs to plants species and is generally cultivated in European and Asian regions. The fiber is approximately 2 cm in diameter and grows to a height of 1.2–4.5 m. Hemp comprises of inner layer surrounded by outer core of bast fiber *Natural Fibers: The Sustainable Alternatives for Textile and Non-Textile Applications DOI: http://dx.doi.org/10.5772/intechopen.106393*

**Figure 3.** *Natural fibers.*

**Figure 4.** *Logitudinal & cross-sectional view of natural fibers.*

attaching to the former by glue-like substance or pectin. The harvesting of hemp fiber is followed by separation of woody core from bast fibers through mechanical process. Thereafter, the separated woody core is cleaned for obtaining the required core content and may be cut to the desired size. The subsequent processing of separated bast fibers result in yarn or bundle formation. Hemp fiber finds application in range of textile and non-textile applications such as sustainable apparels, bags, ropes, garden mulch, fabrication of composites, building material and animal beddings.

The flax fiber extraction is accomplished by two processes namely retting and scorching to alter the fiber properties. The enzymatic application during retting process causes pectin degradation thereby resulting in separation of fibers. Flax fiber is extensively used for production of linen besides being used in furniture, home textiles and interior decor items.

*Bamboo* is also referred to as natural glass fiber owing to fiber alignment in the longitudinal direction. Bamboo is available in the dense forests of China and features as many as 400 species. The par excellence properties of fiber such as light-weight, low cost, high strength, and stiffness renders it suitable as reinforcement in polymeric materials, building and bridge construction, bridges, traditional boats, etc.

*Pineapple Leaf fiber* is one of the abundantly cultivated leaf fiber obtained from crop waste after cultivation of pineapple. The short tropical plant features a height of 1–2 m and 20–30 clustered leaves. Pineapple leaf fibers are multicellular and lingocellulosic. The fiber exhibits good mechanical properties thereby making is a preferred choice for varied applications such as automobiles, textile, mats, construction, conveyor belt cord, air-bag and advanced composites.

*Sisal* is mainly grown in Brazil and South Mexico. The fiber comprises of the rosette of leaves and grows up to a height of 1.5–2 m. Sisal fiber on account of its good mechanical properties is extensively utilized for a range of textile and non-textile applications such as fiber core of the steel wire cables deployed in elevators, automotive sector, shipping industry, civil constructions etc.
