Keywords

1 Introduction

Over the past few years, there has been an increase in demand for sustainable products. Innovative techniques to recycle, social impact and sustainable marketing methods have prompted society to use sustainable and degradable products [1]. Composite materials consist of natural fibers, and their arrangement influences the properties of materials. For reinforced composites, different fibers such as carbon fiber, glass fiber and natural fibers are used and as polymer matrices, namely plastic, metal, resin or rubber [2]. Natural fiber consists of plants fiber which includes leaf fiber (sisal, pineapple and abaca), main components (hemp, jute and kenaf), reed and grass fibers (corn, wheat and rice), seed fibers (coir, kapok and cotton), bast fibers (kenaf, jute, ramie, hemp and flax) and each and every other sort (roots and wood) [3]. In contrast to synthetic fibers, after its purpose, natural fibers can be burned without any CO2 emission and improved recovery of energy which results in positive carbon credits and reduction in global warming [4]. For this research, we have considered kenaf, sisal, pineapple, bamboo and banana fibers. The sources and mechanical properties of all these fibers have been discussed. These fibers are used as reinforcement for different composites which is studied here. Also, the applications of all these fibers have been concentrated. Figure 1 shows the basic constituents of natural fiber-reinforced composites.

Fig. 1
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Basic constituents of natural fiber-reinforced composites [1]

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2 Source and Properties of Fibers

2.1 Kenaf

The main source of kenaf fiber is hibiscus [5]. The scientific name is Hibiscus cannabinus L. The fiber length in mm is 6. The fiber diameter is about 20–30 μm. The density of kenaf fiber is 1.2 g/cm3. The main properties of kenaf are its excellent durability [6]. The main kenaf production countries of the world are India (45%), China, Malaysia and USA [7, 8]. For the production of rope and paper, the primary fiber used was kenaf, and it is considered as one of the substantial fiber in bast type [6]. Kenaf plants are rugged. They are really tough and stiff. They are highly immune to the effects of bug sprays. The kenaf plant has its origins from Asia, Africa, America and a few regions in Europe. They are present here on Earth for over a period of 4000 years. The inner and outer core and flowers are used to extract the fiber. The majority of the dry weight of the stalk is constituted by the inner core (60%), and the rest of the dry weight is from the bast (40%), also known as external fiber [9]. After harvest the plants are sent to a fiber separator, the tail section is entirely used in the pulping process. To isolate the extracted fibers from the influence of external substances like pectin or wax, they are treated using bacterial or synthetic method. These fiber strands are converted to finely woven textures. These fibers are completely biodegradable and hence are nature friendly. In ancient times, the Egyptians used these fibers to make vessels, Nowadays, they are used for manufacturing sacks, strings and ropes [6].

2.2 Sisal

Agave (Agave sisalana) is the source of sisal. This fiber has length up to 1000 mm, and fiber diameter lies between 200 and 400 μm. The density of fiber is 1.2 g/cm3. The characteristic features of the sisal fibers are its hardness, high strength, stretchability, coarse nature, durability, its resistance toward moisture absorption, saltwater deterioration and its surface, which readily accepts variety of dyes. Due to these abilities, the sisal fiber is considered as one of the top reinforcement fibers. Sisal fiber, which is mainly produced in Brazil, is one of among the most used natural fiber. Native to Mexico, the mother plant is a rosette and can grow up to 2 m height. In its 6–7 years of life, it gives about 210–260 leaves for industrial use [5].

2.3 Pineapple

The source of pineapple leaf is ananasmagdalenae. The fiber length of pineapple fiber is up to 200 mm, and fiber diameter is 106 μm [9]. The density of fiber is 1.5 g/cm3. The main properties of pineapple fiber are its saltwater resistance and wear resistance. Philippines is one of the main pineapple producer of the world [10]. The source plant of pineapple leaf fiber is an easily accessible one and is also one of the most developed. The pineapple fiber is a waste by-product of pineapple processing. The plant which can reach up to 2 m height grows mainly in the tropical region and has a group of 20–30 leaves each about 6 cm wide [9]. Almost 90–100 tons pineapple leaves can be produced from a hectare. Comparing all natural fibers, very good mechanical properties are shown by pineapple leaf fibers [10]. These fibers are multicellular, and they are extracted from the pineapple leaf strands through scrapers.

2.4 Bamboo

Bambusoideae is the scientific name of grass pulp, which is the source of bamboo. Fiber length is up to 90 mm, and fiber diameter is about 10–20 μm. The density of the fiber is 0.85 g/cm3. The countries that vastly produce bamboo include Japan, Ecuador, China, India and Chile. Bamboo possesses some excellent properties like high durability and stability, excellent flexibility. They show great resistance to UV radiation. Bamboo fiber is otherwise called common glass fiber because of the arrangement of strands in the longitudinal ways. Bamboo is abundantly found in China; around 400 species are discovered [7]. Because of its better quality, hardness, low weightiness and easy availability, bamboo fibers are used as a major reinforcement for different polymeric materials. The extracted bamboo fibers are used for manufacturing various composites in a wide variety of businesses [11]. Bamboo has been customarily utilized for making houses, spans, customary vessels and so on.

2.5 Banana

Plantain or Musa acuminata is the source of banana fiber. The fiber Length of banana is 5000 mm and diameter is 10 μm. The density of the fiber is 1.35 g/cm3. The main properties are better fineness and spinnability, very strong, less elastic, shiny in appearance, light and less absorption. The main producer of banana are Thailand, South East Asia, India and Bangladesh [12,13,14]. Banana fiber is a waste by-product of banana cultivation at the present time. For industrial usage, banana fibers can be extracted without any extra cost. The fiber length strongly influences the properties of the fiber. As banana fibers have good potential, it can be developed into strong biocomposite materials [7, 8, 11]. Due to its durability, ability to stretch, affinity for certain dyestuff and resistance against saltwater, banana has been the forerunning material traditionally for making twines. Banana is also used as an eco-friendly strengthening agent that has properties which can replace fiber glass and asbestos in composites in many industrial uses.

3 Mechanical Characteristics of Natural Fibers

While choosing the composite, whether it should be used for structural applications or else for mechanical properties play an important role. Natural fibers are generally used as reinforcement in composites because they are lighter. It has better Young’s modulus and tensile strength compared to the polymer matrix, so there is no need for additional treatment of reinforced composites. Table 1 shows the mechanical properties of natural plant fibers. Graphical analysis of mechanical properties is shown in Fig. 2a–c. Mechanical properties including tensile strength, Young’s modulus and elongation at break are considered here.

Table 1 Mechanical properties of natural fibers [15,16,17,18]
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Fig. 2
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a Tensile strength of natural plant fibers [15,16,17,18] b Young’s modulus of natural plant fibers [15,16,17,18]. c Elongation at break of natural plant fibers [15,16,17,18]

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4 Natural Fiber as Reinforcement for Composites Materials

4.1 Kenaf Fiber Composite

Kenaf fiber could be used as reinforcement in a variety of composites, due its superior properties. Epoxy-based FRP (fiber strengthened polymer) composites have generally high thermal steadiness. Be that as it may, it shows helpless split development obstruction that limits its mechanical applications. Organoclay is an organically modified phyllosilicate, derived from a naturally occurring clay mineral. As organoclay is incorporated in the composite, it displays improved flexible properties by allowing a higher limit for fiber stacking [19]. Delamination, formation of voids and air bubbles and fiber pull out are the major failure mechanisms in reinforced composites. Highly stable polymeric structure and a lower damping factor are shown by kenaf reinforced epoxy composites [20].

4.2 Sisal Fiber Composite

The various properties of polystyrene composites reinforced using sisal fibers are observed on basis of the fibers length and content. It has been represented that benzoylation on the fiber improves the fiber arrange connection and thusly extended the quality amazingly [5, 9]. The properties were seen to be essentially self-ruling of fiber length regardless of the way that the extraordinary unbending nature exhibits fringe change at 10 mm fiber length [20, 21].

4.3 Pineapple Fiber Composite

Pineapple leaf fiber (PALF)/polypropylene (PP) composite, PALF/PP fiber shows unrivaled elasticity than other normal fiber strengthened composites. Thusly, PALF/PP fiber can be a potential regular fiber fortified composite for different applications. PALF might be utilized in manufacture of eco-friendly composite items for diversified applications furthermore, and henceforth, manufactured strands can undoubtedly be supplanted with PALF [22]. The usage of pineapple leaf fiber in composite material is another wellspring of materials which can be monetary, eco-friendly and furthermore recyclable. Treatment using alkali can be done to improve various properties. The mechanical properties of the composite can be further improved by incrementing the fiber stacking [20].

4.4 Bamboo Fiber Composite

These composites have a potential for building applications [23]. When contrasted with slick epoxy, the flexural modulus for bamboo fiber composites was consistently higher and monotonically increments with fiber length and substance. Nearness of bamboo filaments could give critical protection from break development, subsequently changing the general disappointment mode from absolutely fragile to malleable. Bamboo strands demonstrated solid holding with the epoxy lattice in the composite [20]. Fiber breakage, network splitting, debonding and fiber pull out are significant deformation types.

4.5 Banana Fiber Composite

Banana fiber is discovered to be a decent support in polyester resin. Excellent flexural strength and flexural modulus are exhibited by banana fiber-reinforced polyester composites. Composites have most extreme mechanical quality, and it tends to be utilized as a substitute material for customary fiber strengthened polymer composites. Banana fiber can be utilized as strengthened specialist effectively in the composite business as an economical structure material. Crossover epoxy composites displayed predominant mechanical properties [20, 24].

5 Applications

Natural fibers find application in almost every day-to-day activities. It finds uses from household activities to even automobile, aerospace and marine industries [25]. Kenaf fibers are utilized in automotive development, bundling, furniture, materials, mats, paper mash, material for packing, carry baggage and cases for mobiles [5, 26]. Sisal fibers can be used in various industrial sectors like transportation, construction and automobile. It also finds many uses in our day-to-day life like reinforcement in steel wire links, used for making ropes [27]. It is used to manufacture paper. It also finds application in the manufacturing of roofing sheet, panels, doors and shutting plates [9, 26, 28]. Pineapple fibers of treated and surface-adjusted strands are utilized for making transport line rope, air pack and progressed composites [7, 29]. Other application includes textile, sports item, baggage, automobiles, cabinets, mats and furniture. Fibers of bamboo are used in the production of seats, other furniture and panels. Banana fibers are utilized in low-cost production of paper [30]. They are also used in carpets and mattresses, cloth, ropes, twines, panel and handicraft production. They are also be very useful in sound proofing and insulation [13, 31, 32]. Table 2 and Fig. 3 show the usage of natural fibers and wood for composites in automotive industries in Europe 2012 (total fiber volume of 80,000 tons).

Table 2 Usage of natural fibers for composites in Europe 2012 (total fiber volume of 80,000 tons) [27]
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Fig. 3
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Natural fiber composite usage in Europe 2012 [27]

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6 Conclusion

The increased concern and awareness of the society toward environmental conservation has turned out to be one of the main reasons for the selection of natural fibers as the reinforcement in polymer composites. When these fibers are physically and chemically treated, they can be effectively used as a supplement for the existing materials. The results shows that the mechanical properties are increased by various chemical treatments. The adhesion between the fiber and the matrix are also improved by the same. Usage of natural fibers results in cheap composite manufacturing costs. The impact of chemical processes on natural fiber properties is also considered. The kenaf fiber shows low-density and high-specific mechanical properties. The fibers of sisal have high tensile intensity, resistance toward saltwater, alkali and acid deteriorations and toward abrasion. It also shows high tenacity. Pineapple fiber shows very good mechanical, physical and thermal properties. It is known that bamboo fiber is light, stiffer and stronger when compared to glass fiber thus making it a very good substitute. Studies showed that banana fiber has very good durability, ability to stretch, affinity for certain dyestuff and resistance against saltwater; banana has been largely used in manufacturing even from early times and is used even now. It was found that among the mentioned fibers, pineapple showed highest tensile strength and Young’s modulus. Also, it was observed that banana fiber had the highest elongation at break. With these results, it is understood that in the coming years, natural fibers will be extensively used for the making of composites as they are the non-polluting, harmless, renewable as well as sustainable source of fiber.