Natural Fibers: The Sustainable Alternatives for Textile and Non-Textile Applications

Abstract

The increasing environmental concerns and depletion of petroleum resources have increased the importance of natural fibers and have stimulated researchers and industries to use sustainable fibers instead of conventional synthetic fibers. Besides exceptionally brilliant mechanical and physical properties are also attractive aspects of natural fibers enabling the utilization of natural fibers in myriad of textile and non-textile applications such as clothing, and reinforced composite products in various industries such as automotive, building, and furniture. Natural fiber composites are composite materials comprising of reinforcing fibers derived from renewable and carbon dioxide neutral resources such as wood or plants. NFCs find application in molded articles that demand moderate strength for acceptable performance for various indoor and outdoor applications. A rapid drift from oil-derived polymers and mineral-reinforced materials to sustainable alternatives has fostered automotive and packaging industries to start utilizing natural fiber composites in their designs. Accordingly, natural fiber composites are serving as energy efficient and sustainable alternatives replacing traditional materials such as metals, polymeric resins, and reinforcement fibers. A worldwide clamor for green products and thus upsurge in sustainable alternatives have been witnessed as a result of diminishing petroleum reserves worldwide, exorbitant prices of petroleum, and high disposal costs of petroleum-based composites along with inability of decomposition of some petroleum-based composites. Contrastingly, natural materials outshine the petroleum-based products in being renewable, inexpensive, biodegradable, and eco-friendly.

1. Introduction

The significance of natural fibers to fulfill the basic human needs of clothing and shelter has been well established since time immemorial. However, with the advent of synthetic fibers in 1900s, the popularity and usage of natural fibers became archaic. Nevertheless, the world wide clamor for sustainable and eco-friendly approaches in textile supply chain and depletion of petroleum resources have stimulated the usage of natural fibers thereby replacing synthetic fibers with sustainable natural fibers.

Natural fibers are grown naturally and do not pose any detrimental environmental impact except for situations where fertilizers, pesticide and other toxic chemicals are extensively utilized to improve their yield. The adverse impacts of synthetic fibers on environment and their fossil fuel based origin fosters industrial establishments, researchers and technologists to explore latest and innovative methods of growth, development, cultivation and usage of natural fibers via eco-friendly mode. Cotton holds lion’s share of market as far as textile applications are concerned but the fiber cannot be considered sustainable owing to massive utilization of water, pesticides, fertilizers and toxic chemicals thereby leading to environmental and economic distress. Apart from cotton, some resource efficient fibers that are replacing cotton in varied textile applications include linen, hemp, flax, jute and bamboo.

Linen is gaining interest among textile designers for designing clothing, footwear and handbags. Hemp and jute are rope like fibers and exhibit coarseness compared to linen and generally preferred for apparels and accessories that demand rough texture and durability. Jute is a plant based multicellular fiber characterized by nodes and cross markings in longitudinal view and polygon shapes in cross-section. Flax is cellulosic fiber in crystalline form featuring a length of 90 cm and diameter of 12–16 μm. Flax is mainly cultivated in Canada, Netherlands, Belgium and France. The stem of plant Linum usitatissimum is the fiber source. The 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 and straw are another plant based fibers gaining popularity among fashion designers. Bamboo is wood like tropical grass while the sources for straw are wheat stalks, grasses, sisal hemp and rice paper.

The extensive utilization of natural fibers for a range of textile applications is attributed to their exceptionally brilliant mechanical and physical properties like good specific modulus, low density, toughness properties, low cost, recyclability and nontoxicity. Apart from textile and fashion industry, the natural fibers find application in various industries such as automotive, building, furniture and fiber reinforced composite products [1]. A myriad of natural fibers like jute, hemp, kenaf, ramie, flax, sisal, bamboo, coir, oil palm, etc. are used for development of polymer composites on account of their biodegradability, high performance profile, sustainable attributes, lightweight and economic viability.

Furthermore, low weight, better crash absorbance and sound insulation properties of natural fibers make them ideal choice for automotive and aerospace sectors. However, the applications of natural fibers are restricted to interior structures owing to their hydrophilic nature thereby demanding chemical treatment for improving their moisture related properties. The inherent property of moisture absorption on exposure to different temperatures and humidity conditions by natural fibers presents a key challenge for their usage in different environmental conditions [2, 3, 4].

Natural fiber composites are composite materials comprising reinforcing fibers derived from renewable and carbon dioxide neutral resources like wood or plants. NFCs find application in molded articles that demand moderate strength for acceptable performance like large diameter piping, equipment housings, roofing for economical or low-budgeted housing. A rapid drift from oil derived polymers and mineral reinforced materials to sustainable alternatives has fostered automotive and packaging industries to start utilizing natural fiber composites in their designs. Accordingly, natural fiber composites are serving as an energy efficient and sustainable alternatives replacing traditional materials such as metals, polymeric resins and reinforcement fibers. A worldwide clamor for green products and thus upsurge in sustainable alternatives has been witnessed as a result of diminishing petroleum reserves worldwide, exorbitant prices of petroleum and high disposal costs of petroleum-based composites along with inability of decomposition of some petroleum based composites. Contrastingly, natural materials outshine the petroleum based products in being renewable, inexpensive, biodegradable and eco- friendly.

The demerits associated with glass fibers have prompted the emergence and wide spread acceptance of natural fibers for making composites to be suitable in automotive, furniture, packaging and building industries. Although natural fiber composites offer innumerous advantages such as lower cost, lightweight composites, biodegradability and renewable sources, however disadvantages associated with natural fibers like variations in fiber geometry and physical properties, lower mechanical properties, poor interfacial adhesion and incompatibility with hydrophobic matrix resin systems cannot be undermined.

The challenges ahead in design and manufacturing of natural fiber reinforced composites for varied applications lie in overcoming the aforesaid demerits associated with natural fiber composites.

The next section of the chapter shall discuss the fiber properties suitable for textile applications and the broad fiber classification.

2. Fiber properties for textile applications

The properties which qualify the textile fibers to be used for textile applications are being discussed in detail in the following section:

2.1 Length and appearance

Length of fiber influence their selection for fabric production. Short staple natural fibers like cotton, jute, wool impart rough look to fabrics made from these fibers. However, fabrics made of nylon, polyester filament yarns exhibit smooth and shiny look. Moreover, short wavy fibers like cotton and wool have affinity for dirt particles while long, continuous filament fibers like silk or synthetic fibers are easy to clean and do not get as dirty as their natural, staple counterparts. Accordingly, wool, jute fibers are used for apparels and fashion accessories where designer aims to explore rough, crimpy fabric appearance as the design feature while accessories like scarves, stoles which demand sheen and smooth, soft look and feel, utilizes silk or filament fiber yarns and fabrics in their designing.

2.2 Moisture absorption

The fibers vary in their affinity for water molecules with natural fibers like cotton inherently hydrophilic while synthetic fibers like polyester, nylon exhibit hydrophobicity or low moisture absorption. The moisture absorption property of fibers determine their selection for apparels and accessories suitable for a particular season. Summer wear apparels and accessories should provide rapid sweat absorption from wearer’s skin and thus utilizes hydrophilic fibers like cotton. Fabrics made of synthetic fibers, being hydrophobic do not soak sweat and are uncomfortable in summers. A special consideration of fiber type is important while designing textile end products like gloves, socks which are in intimate contact with skin where sweat accumulation may lead to dampness and discomfort to the wearer.

2.3 Thermal conductivity

The ability of the fiber to conduct heat away from the wearer’s skin is yet another crucial factor influencing the selection of fiber for a particular end use. Cotton and rayon exhibit high thermal conductivity and have the ability to conduct heat from body, lower the body temperature and providing cool feel next to skin, Accordingly, fibers with higher thermal conductivity are preferred for accessories intended for summers like driving gloves, socks, stockings. However, the requirements in winter are quite contrasting with such material selection that can provide thermal insulation and prevent the body heat to escape out. Therefore, fabrics made of wool, acrylic and synthetic fibers, being poor heat conductors of heat aid in keeping the wearer warm in winters.

2.4 Strength

The strength of fibers determine the ease of washing of fabric made from the chosen fiber. Fibers have property of strength variation in dry and wet states. Fibers like wool, silk, rayon loose strength in wet state while cotton and synthetics exhibit high strength even when wet enabling easy laundering of fabrics made from such fibers. Therefore, cotton and other synthetic fibers should be preferred for daily wear apparels and accessories which require frequent cleaning and laundering. However, occasional wear accessories like silk scarves, stoles are made of delicate fabrics prone to strength loss on repeated washings and need dry cleaning and less frequent washing cycles.

3. Classification of textile fibers and textile based applications

The various types of fibers used in the textile industry exhibit their own individual, unique properties. The properties fibers display, depend largely on their source of origin. The fiber properties desirous for textile applications have been discussed so far. This section is devoted to elaborate classification of fibers based on their nature and origin, botanical, zoological or chemical name, moisture absorption properties, thermo plastic behavior and utility. The chapter will mainly focus on origin based fiber classification.

3.1 Classification based on origin

The primary classification of fibers is based on their origin and thus can classified as natural and synthetic fibers.

3.1.1 Natural fibers

Natural fibers, as the name suggests are obtained from nature. The natural sources for these fibers can be plants, animals or minerals (Figure 1). Figure 2 shows the classification of vegetable based cellulosic fibers. The fibers obtained from plants include cotton, kapok (from seeds), sisal, banana, pineapple (obtained from leaves) and bast fibers like jute, flax, hemp, kenaf, ramie, etc. (obtained from plant’s stems). Cellulose fiber extensively used in textile industry include cotton, linen, flax, hemp and jute. A variety of fashion ensembles ranging from cotton canvas satchels, tote bags, tapestry luggage to sports socks, sneakers, bandanas, handkerchiefs, scarfs, stoles, hats and caps uses cotton as the major raw material.

Figure 1.

Classification of fibers.

Figure 2.

Classification of vegetable based cellulosic fibers.

The salient features of cotton fiber that makes it suitable for a range of apparels and accessories include its softness, breathability, moisture absorbency and temperature regulation property. The natural textured surface of fiber in addition to its moisture absorption provides comfortable feel and breathability next to skin. Accordingly, the fiber is preferred for applications where rapid water absorption is the prime concern like terry-cloth apparel such as beach coats. Moreover, cotton is versatile, easy to care and handle fiber that is not just static free, hypoallergenic, and pill free but also has the ability to retain its original feel and color. The above stated attributes make cotton year round fiber suitable in both warm as well as cold weather.

Kapok belongs to the Bombacaceae family and is generally cultivated in tropical regions. Kapok seeds are enclosed within the fiber. The cellulosic fiber features yellowish or light brown color, light weight and hydrophobicity. Kapok fiber is preferably utilized as adsorption, oil absorbing, buoyancy material, apart from being used as biofuel and reinforcement material.

Flax is cellulosic fiber in crystalline form featuring a length of 90 cm and diameter of 12–16 μm. Flax is mainly cultivated in Canada, Netherlands, Belgium and France. The stem of plant L. usitatissimum is the fiber source. The 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.

Ramie fiber is herbaceous perennial plants extracted from the plant steam, is extensively grown in China, Japan and Malaysia. Ramie is considered to be one of the fast growing, strongest and longest among natural bast fibers featuring a height of 1–2 m. Ramie fiber is processed in a similar manner as linen from flax. The fiber finds wide range of applications such as apparels like sweaters and cardigans, upholstery, marine packing, gas mantle, fishing nets, automotive, furniture, construction, pulp, paper, agrochemicals and composites etc.

Jute is primarily grown in Asian countries like India, Bangladesh, China, and Myanmar and it takes about 4 months to grows up to height of 15–20 cm. The extraction of fibers is accomplished about 4 months after its cultivation. The fibers are subjected to chemical or biological retting process which is essential for pectin removal between bast and wood core. The chemical retting process involves the application of chemicals like N₂H₈C₂O₄, Na₂SO₃, etc. while biological retting involves soaking the bundled stalks in water for about 20 days enabling the removal of pectin between the bast and the wood core thereby assisting in easy fiber separation. Subsequently, the fibers are allowed to dry.

Kenaf belongs to bast fibers and are extracted from flowers, outer fiber (bast comprising 40% of the stalks dry weight) and inner core (comprising 60% of stalks dry weight). The processing of kenaf plants upon harvesting is accomplished with a mechanical fiber separator, consuming whole stalk in pulping followed by chemical or bacterial treatment of extracted fibers for their separation from the non-fibrous content such as wax, pectin, and other substances. The fiber exhibits unique properties like biodegradability and eco-friendly attributes, stiffness, strength, toughness and high resistance to insecticides. Kenaf fiber primarily find application in production of paper, rope, cords, storage bags along with textile applications. More recently, kenaf fiber is utilized for composites apart from application in furniture, construction, packaging, automotive sector.

Coir Fiber, one of the thickest natural fiber is mainly cultivated in tropical regions including India, Sri Lanka, Indonesia, Philippines, and Malaysia and is obtained from the husk of the coconut fruit.

Coir fiber in contrast to other natural fibers has higher lignin and lower cellulose and hemicellulose content and features a high microfibrillar angle thereby rendering it several valuable properties, such as strength, resilience, resistance to weathering, and high elongation at break. The salient properties of coir fiber make it a suitable candidate for a range of applications such as upholstery, ropes, mats, mattresses, agriculture, construction, brushes etc. [5, 6, 7].

Another natural fiber that is gaining interest among textile designers for designing clothing, footwear and handbags is linen. Hemp and jute are rope like fibers and exhibit coarseness compared to linen and generally preferred for apparels and accessories that demand rough texture and durability. Hemp, jute is finding applications in accessories like fashion jewelry, handbags, belts, hair accessories, footwear, bag-pack, tote, gunny-bags and mobile covers.

Jute is a plant based multicellular fiber characterized by nodes and cross markings in longitudinal view and polygon shapes in cross-section. Jute is hygroscopic in nature with moisture regain of 12–14% and its color varies from yellow to brown to gray and is good insulator of heat and electricity. Jute fiber is used for making burlap, hessian, gunny cloth which serve as raw materials for accessories like handbags, jewelry etc.

Bamboo and straw are another plant based fibers gaining popularity among fashion designers. Bamboo is wood like tropical grass while the sources for straw are wheat stalks, grasses, sisal hemp and rice paper. Summer wear hats, hair accessories, different handbags are designed from straw, bamboo or strips of bamboo woven like straw. The pliability and sturdy hand of bamboo canes make them suitable for gripping elements and handles for handbags. However, straw is generally preferred for delicate fad accessories trending in a single season which is accounted to its inferior durability and flex abrasion resistance. Repeated bending in one position may cause fiber breakage hampering the esthetic appeal of accessory and rendering it useless.

The fibers such as wool (obtained from sheep) and silk (obtained from silk worm cocoons) are protein based fibers and finding applications in a variety of accessories. Pashmina (cashmere or cashmere/silk blend), wool and silk are used for designing exclusive and high priced stoles, scaves and other winter wear accessories. Pashmina is obtained from the underbelly of the Capras goat found in India’s Himalayan mountains. Wool is easily distinguishable from hair or fur fibers showcasing crimpy appearance and elasticity. Fabrics made from wool fiber exhibit durability owing to tear and snag resistance and anti-pill properties. Moreover, wool fabrics provide easy care and handling properties, drapes stunningly, and maintains resilience in wet condition. The most crucial property of wool fiber of providing insulation by holding air layer next to the skin makes the fiber a preferred choice for winter wear apparels and accessories. Henceforth, winter wardrobe is incomplete without the inclusion of wool based hats and scarfs.

Another natural protein fiber namely silk is composed of fibroin and is the product of insect larvae that forms cocoon. The most exclusive variety of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori. Silk is considered to be the most luxurious of all fibers characterized by unique natural luster. Triangular prism like structure of the silk fiber is responsible for its shimmering appearance allowing silk fabric to refract incident light at different angles, thereby producing varying color effects. The silk fiber has an additional advantage of being one of the strongest natural fibers and offers remarkable abrasion resistance with many years of service to wearer.

Furthermore, the fiber provides extremely soft texture, is elastic and displays the ability to retain its original shape with minimal shrinkage.

The unique properties of this protenecious fiber available in filament form make it the most anticipated option for apparel and accessory designers to render a loyal, pleasing look to end products. The fabrics produced from this luxurious fiber however, need special care as far as dry cleaning, washing and pressing is concerned. Manual washing of silk apparels and accessories is usually recommended unless the end products have been processed for machine laundering. Moreover, the exorbitant prices of silk fiber prompt designers to look for cheaper substitutes that resemble silk, such as polyester and nylon microfibers.

Vegetable fibers like ramie, jute and hemp in contrast to hair fibers are economical and are characterized by rigid feel, coarseness and brittleness and thus find application in designing textile products where strength, abrasion resistance and rough texture are the requisites. The fibers are thus generally preferred for designing lower priced accessories like fashion jewelry, belts, handbags. Asbestos is the natural mineral fiber known for its fireproofing and insulating properties. Accordingly, the fiber was utilized for flame retardant protective clothing. However, the fiber has limited application in textile arena in recent times owing to its carcinogenic nature and hazards to lungs by continued inhalation of asbestos fiber.

3.1.2 Man-made fibers

Man-made fibers also referred to as synthetic or artificial fibers are the fibers that are developed by mankind to meet the ever increasing demands of fibers in textile and fashion industry since the sources of natural fibers are on the verge of depletion. Moreover, the stringent animal rights discourage slaughtering of animals for their skin, fur and hair fibers. Therefore, the man-made fiber industry is bound to grow tremendously. The man-made fibers are manufactured with the aim of cutting down costs associated with natural fibers and at the same time achieving desirable properties like high strength, abrasion resistance, soft feel, drape ability and varied textures. Manmade fibers are classified into regenerated, synthetic and miscellaneous inorganic fibers on the basis of raw materials employed for their manufacture. Regenerated fibers like viscose rayon, cuprammonium rayon, acetate, triacetate, casein, rubber are the man-made fibers that belong to cellulosic group and are produced using natural polymers i.e. cellulosic base materials thus requiring minimum chemical steps [1, 2, 3, 4].

Acetate it has trade name of Ariloft, Chromspun had a luxurious feel and appearance, is available in a wide color range, drapes extremely well, is relatively fast drying and is resistant to shrinkage, mildew and moth. It is used for blouses, dresses, foundation garments, lingerie, linings, shirts, slacks and sportswear.

The non-cellulosic group include fibers like nylon, polyester, acrylic, polyolefin, spandex, glass and teflon which are protein based fibers. These fibers constitute of molecules of carbon, hydrogen, nitrogen and oxygen derived from petroleum and natural gas. The fibers manufactured by synthesizing various chemicals like the petroleum products are classified as synthetic manmade fibers. Natural raw materials are not required as a base material for their manufacture unlike the regenerated ones.

Nylon fiber belonging to this group is manufactured using hexamethylenediamine and adipic acid while polyester uses dimethyl terephthalate and ethylene glycol in its production. The process basically involves conversion of chemicals into fiber forming substances which can be drawn into filaments of required coarseness or fineness.

Nylon is known for its exceptional strength, abrasion resistance and easy care properties making it desirable fabric for hosiery, blouses, dresses, lingerie, underwear, raincoats, ski wear and suits.

Acrylic is also referred to as Acrilan, Orlon and Zefran. The fiber is characterized by its soft feel, warmness, wool like texture and light weight. Moreover, the acrylic fiber fabrics display considerable strength, resilience, drying ability, and are resistant to moths, sunlight, oil, and chemicals. Therefore, fabrics made of acrylic fiber namely fleece, fake furs, jerseys find application in winter wear and knitted accessories along with apparels like dresses, infant wear, knitted garments, skirts and sweaters. Modacrylic, another synthetic fiber is used for fabrication of fleece and knit pile fabrics. The fabrics owing to their softness, resilience, abrasion and flame resistance, drying ability, and shape retention, are preferred for deep-pile coats, linings, simulated fur accessories and hair accessories.

Olefin is another organic man-made fiber that exhibits excellent colorfastness, extreme strength, resistance to mildew and perspiration and is thus, is garnering designer’s attention for designing of knitted accessories, winter wear and sportswear apparels and accessories.

Another fiber that has taken the apparel and accessory industry by storm is rayon fiber which offers multitude advantages of being soft, comfortable and easily dyeable. Consequently, the fiber is preferred choice for a variety of apparels and accessories like sportswear, blouses, jackets, lingerie, rainwear, shirts, scarves, stoles and ties.

Spandex Produced extensively by Dupont as Lycra, its major advantage is its ability to be stretched 500 percent over and over without breaking, always returning to its original length. It is lightweight, more durable than rubber, and resistant to body oils, it is used wherever stretch is required such as in athletic apparel, bathing suits, foundation garments, ski pants and sportswear.

Miscellaneous inorganic group comprises of fibers which use substances like metal and glass in their manufacture. The malleability and ductility of the inorganic fibers render them usable for textile applications. However, high cost and technical difficulties offers hindrance to wide spread acceptance of the fibers [5, 8].

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/polyester-reinforced 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.

Figure 3.

Natural fibers.

Figure 4.

Logitudinal & cross-sectional view of natural fibers.

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 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 lingo-cellulosic. 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.

5. Natural fibers composites—Some salient features and associated challenges

Fiber reinforced polymeric composites consist of reinforcement fibers held by surrounding resin matrix. Continuous filaments or short fibers comprise the reinforcement fibers. Reinforcement preforms like nonwoven mats, yarns, fabrics and 3D fabrics are obtained from fibers.

The biological origin of at least one of the components of natural fiber composite make them potential sustainable candidates with long textile fibers (e.g., flax, hemp, kenaf, jute, ramie and sisal), short fibers (wood fibers, by-products from long fiber processing, and recycled fibers) and fiber fibrils being used as reinforcement. Biomaterials like various epoxidized plant oils and soy protein serve as matrix materials. The fabrication of natural fiber composites is accomplished in a similar manner as manufacturing methods namely resin transfer molding (RTM), vacuum infusion, compression molding, direct extrusion and compounding, and injection molding deployed for conventional composites comprising thermoset matrix and thermoplastic composites. The pre-treated process of fiber and manufacturing techniques of composites primarily govern the properties of natural fiber reinforced polymer composites. Composites with diverse properties can be obtained by varying the manufacturing techniques and the constituents of composite materials. Thus it can be inferred that precise selection of fibers, matrix, additives and manufacturing method enables tailoring the properties of natural fiber composites for varied applications.

Natural fiber composites offer several environmental benefits in contrast to their synthetic counterparts as the former result in less pollution during fabrication, lower fuel consumption and CO₂ emissions during transport to the constructions sites and lastly, considerable reduction of the disposal and energy-consuming disposal efforts.

Natural fiber composites owing to replacement of synthetic materials by bio-base and renewable sources are classic examples of sustainable resources for industrial applications. The materials exhibit the potential of being cost effective for identical structural characteristics, can be grown in controlled facilities or farms and can substantially bring down the carbon footprints. The problems posed by synthetic fibers and resins in their disposal accounts for approximately 20% of the total landfill space thereby fostering ardent replacement of synthetic composites with natural fiber composites.

Nevertheless, the applications of natural fibers are restricted to interior structures owing to their hydrophilic nature thereby demanding chemical treatment for improving their moisture related properties. The variability in fiber properties namely apparent variability and actual property variability presents a challenge in their usage as reinforcement in composite materials. Apparent variability arises due to experimental methods, measurement and testing techniques while the actual property variability is the inherent variability present in fiber. Moreover, the structural defects in the fibers leads to fiber deformation which also restrict the usage of natural fibers.

The inherent property of moisture absorption on exposure to different temperatures and humidity conditions by natural fibers presents a key challenge for their usage in different environmental conditions [2, 3]. The presence of hydrophilic group affects the interfacial bonding between polymer matrix and the fiber in composite structures owing to hydrophobic characteristics of matrix. The interaction between fibers and polymer matrix can be optimized by chemical treatment of natural fibers by reduction in hydroxyl functional groups on the fiber surface thereby increasing surface roughness and the interfacial interaction between the matrix and the fibers [6, 7].

6. Conclusions

The significance of natural fibers to fulfill the basic human needs of clothing and shelter has been well established since time immemorial. However, with the advent and upsurge in the popularity and demand of synthetic fibers, the global consumption of natural fibers became archaic. Nevertheless, the world wide clamor for sustainable and eco-friendly approaches in textile supply chain and depletion of petroleum resources have stimulated the usage of natural fibers thereby replacing synthetic fibers with sustainable natural fibers.

Cotton holds lion’s share of market as far as textile applications are concerned but the fiber cannot be considered sustainable owing to massive utilization of water, pesticides, fertilizers and toxic chemicals thereby leading to environmental and economic distress. Apart from cotton, some resource efficient fibers namely hemp, flax, organic cotton, bamboo, jute, kenaf, ramie, sisal, coir are replacing cotton in varied textile applications and for development of polymer composites on account of their biodegradability, high performance profile, sustainable attributes, lightweight and economic viability.

Natural fiber composites are composite materials comprising reinforcing fibers derived from renewable and carbon dioxide neutral resources like wood or plants. NFCs find application in molded articles that demand moderate strength for acceptable performance like large diameter piping, equipment housings, roofing for economical or low-budgeted housing. Natural fiber composites owing to replacement of synthetic materials by bio-base and renewable sources are classic examples of sustainable resources for industrial applications. The materials exhibit the potential of being cost effective for identical structural characteristics, can be grown in controlled facilities or farms and can substantially bring down the carbon footprints. Natural fiber composites offer several environmental benefits in contrast to their synthetic counterparts.

However, the applications of natural fibers are restricted to interior structures owing to their hydrophilic nature thereby demanding chemical treatment for improving their moisture related properties. Undoubtedly, natural fibers are potential sustainable candidates to replace synthetic fibers for myriad of textile and non-textile applications.