Journal of Reinforced Plastics and Composites

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Some natural fibers used in polymer composites and their extraction processes: A review K Palani Kumar and A Shadrach Jeya Sekaran Journal of Reinforced Plastics and Composites 2014 33: 1879 originally published online 3 September 2014 DOI: 10.1177/0731684414548612

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Downloaded from jrp.sagepub.com by guest on September 26, 2014 Original Article Journal of Reinforced Plastics and Composites 2014, Vol. 33(20) 1879–1892 Some natural fibers used in polymer ! The Author(s) 2014 Reprints and permissions: composites and their extraction sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684414548612 processes: A review jrp.sagepub.com

K Palani Kumar1 and A Shadrach Jeya Sekaran2

Abstract Natural fibers are used as reinforcing materials for more than 2000 years. The need for natural fibers has been emerged due to its weight saving, cost effective, and environmentally superior alternatives to synthetic fibers in composites. The interest in natural fiber reinforced polymer composites has been increased rapidly, due to high performance in mechanical properties, significant processing advantages, and it also provides a solution to environmental pollution. The main objective of this paper is to review and discuss the natural fibers like banana, aloe vera, kenaf, and sisal fibers and their extraction processes.

Keywords Natural fibers, polymer composites, extraction process, review

Introduction natural fiber composites is not a new or recent one. Nature is full of examples where in the idea of compos- Natural fibers have good prospective as reinforcements ite materials is used. The coconut palm leaf, for exam- in polymer (thermoplastics, thermosets, and elasto- ple, is nothing but a cantilever, using the concept of mers) composites. Because of high specific properties fiber reinforcement. Wood is a fibrous composite: cel- and low density of natural fibers, composites based lulose fibers have a lignin matrix. The cellulose fibers on these fibers have very good implications in industry.1 have high tensile strength, but they are very flexible, The use of natural fiber composites has been studied by when the lignin matrix joins the fibers and furnishes many researchers. Saravana Bavan and Mohan the stiffness. Bone is yet another example of a natural Kumar2 have studied the potential use of natural composite that supports the weight of various members fibers in composite materials, their availability, process- of the body. It consists of short and soft collagen fibers ing features, mechanical and physical properties, and embedded in a mineral matrix called apatite.5 Fiber some of their applications in India. Lehtiniemi et al.3 reinforced composites have been more prominent have studied the natural fiber-based reinforcements in than other types of composites because most of the epoxy composites processed by filament winding. materials are stronger and stiffer in the fibrous form Nunna et al.4 have studied the significant aspects of natural fiber-based hybrid composites which are found to be predominantly affected by factors which 1Department of Mechanical Engineering, Sri Sai Ram Institute of include variation in fiber volume/weight fraction, vari- Technology, Chennai, India ation in stacking sequence of fiber layers, fiber treat- 2Department of Mechanical Engineering, St. Peter’s College of ment, and environmental conditions. Engineering and Technology, Anna University, Chennai, India The technological development depends on advances in the field of materials. Whatever the field may be, the Corresponding author: K Palani Kumar, Department of Mechanical Engineering, Sri Sai Ram final limitation on advancement depends on materials. Institute of Technology, Sai Leo Nagar West, Tambaram, Chennai Composite materials, in this regard represent a constant 600 044, India. endeavor of optimization in materials. The idea of Email: [email protected]

Downloaded from jrp.sagepub.com by guest on September 26, 2014 1880 Journal of Reinforced Plastics and Composites 33(20) than in any other form. The ultimate aim of the natural The incorporation of natural fibers such as sisal (in fiber composite is to avoid expensive glass fibers and to the form of fibers), ridge gourd (in the form of natural go for biodegradable materials. The biggest advantages woven mat), and coconut leaf sheath (in the form of of using natural fibers in composites are the cost of natural woven mat) into the epoxy matrix shows the materials, their sustainability and density; mainly they moderate improvement in the tensile properties of the have no health hazard. Natural fiber reinforced com- composites. The hybridization of these natural fibers posite has a plate material, which uses sisal, banana, has provided considerable improvement of tensile and roselle for internal and external fixation on human strength when compared to individual reinforcement; body for fractured bone.6 The benefits of using natural this is due to transfer of loads and sharing of loads fiber reinforced composites include improvement of among the fibers.11 Natural fibers have a good potential plastic shrinkage, settlement cracking, low permeabil- in the future. Many types of natural fibers have been ity, greater impact, abrasion, and shatter resistance of investigated and some are under investigation for repla- the matrix material. However, it is clear that the prob- cing plastics. This paper reviews the processing meth- lems of durability are associated with compatibility of ods of selected natural fibers such as banana, aloe vera, fibers and matrix material and environmental effect on kenaf, and sisal. The natural fibers considered in the stress corrosion resulting from interfacial debonding.7 present review and their woven fabric form is presented Analytical hierarchy process method is used to deter- in Figure 1. mine the most suitable natural fiber composites for automotive dashboard panel. Analysis reveals that Banana fiber kenaf 60% +PP is the most suitable material for automotive dashboard panel, as it has the highest value Banana fiber, a lignocellulosic fiber, obtained from the (5.2%) among other natural fiber materials.8 Abdul pseudostem of banana plant (Musa sapientum), is a bast Khalil et al.9 have indicated that the best alternative fiber with relatively good mechanical properties. Plant for synthetic fiber composites is natural fiber compos- fibers are sclerenchymatous cells with heavily lignified ites. The natural fiber reinforced composites are typic- cell walls having a narrow lumen in cross section. ally filled or reinforced with plant fibers, as well as Natural fibers possess several advantages over synthetic plastics such as polyvinyl chloride or recently, even fibers such as low density, appropriate stiffness, and bioplastics. Sathiskumar et al.10 have reviewed the mechanical properties and also have high disposability mechanical properties such as tensile, flexural, impact, and renewability. Also, they are recyclable and bio- and dynamic properties as well as thermal and machin- degradable. Banana fibers are used for various pur- ability properties of the composites with and without poses such as in textile, paper, or handicrafts chemically treated fibers. The water absorption capabil- industry. Banana paper is versatile, as it is waterproof ity of the composites and its effect on mechanical prop- and stronger than wood-pulp paper, which means it has erties is also reported. been used in packaging and even as a basis for building

Figure 1. Fibers and woven fabrics of different natural fibers. (a) Banana, (b) aloe vera, (c) kenaf, (d) sisal.

Downloaded from jrp.sagepub.com by guest on September 26, 2014 Kumar and Jeya Sekaran 1881 materials.12 Banana is a well-known fruit crop which decrease by increasing the diameter of fibers. The diam- has been grown extensively in Indian peninsula. The eter variability has a normal distribution. At lower banana plantation is presented in Figure 2(a). In fact, strain rate, an increase in strain facilitates the amorph- India is the leading producer of this perennial crop. It is ous to crystalline sharing of load. At higher speed, the estimated that, after the harvest of fruits, huge quantity faults dominate with catastrophic failure at the highest of biomass residues (60–80 t/ha) is left over as waste strain rates. Stress–strain curves show strain harden- that constitutes pseudostem, leaves, sucker, etc. There ing.17 The hybrid composites of equivalent weight exists a vast potential of extracting fibers from the ratio of jute and banana reinforced epoxy hybrid com- banana pseudostem. It is estimated that annually posite posses better thermal property and have less 17,000 tons of fibers are extracted from this waste water absorption capacity comparatively. The addition that has been roughly valued as Rs. 85 crores.13 The of banana fiber in the composites increases the tensile, fiber volume fraction and fibers aspect ratio are the flexural, and impact strength. Moisture absorption most significant factors affecting the hardness strength study of hybrid composite of equivalent mixture of the composites. Although the effect of fiber orienta- shows minimum moisture uptake.18 Tensile properties tion is significantly less for both pseudostem and empty are predicted to be little higher for banana and sisal fruit bunch banana fiber as well, it cannot be ignored, natural fiber reinforced composite by using rule of as it is one of the major load-bearing components in the hybrid mixtures equation.19 Due to low density, high composites.14 The composites of banana empty fruit tensile strength, high tensile modulus, and low elong- bunch fiber are stronger in tension than that of pseu- ation at the break of banana fibers, composites based dostem based on the control factors such as volume on these fibers have very good potential use in various fraction of fibers, aspect ratio of fibers, and fiber orien- sectors like construction, automotive, machinery, etc.20 tation.15 Silane and alkali treatment on plantain empty fruit bunch fiber lead to a higher tensile strength than Extraction process that of untreated fibers. From the mechanical tests, plantain empty fruit bunch has a very promising There are two methods for the extraction of banana future and it is used as a substitute for artificial/glass fiber, namely Bacnis method and Loenit method. fibers. Hence, it has a good superior interface.16 Banana The extraction of banana fiber is presented in Figure fibers have shown high variability along the length and 2(a) to (f). In the Bacnis method, banana fiber is pro- between fibers, which is a characteristic of natural duced from waste stalk of banana plant. The outer fibers. The standard deviation has been found to sheath is tightly covered by layers of fiber. The fiber

Figure 2. Extraction of banana fiber. (a) Banana plantation,21 (b) banana fiber extraction in field,22 (c) hand stripping of banana fiber,23 (d) Raspador,24 (e) banana fiber extraction,23 and (f) banana fiber.25

Downloaded from jrp.sagepub.com by guest on September 26, 2014 1882 Journal of Reinforced Plastics and Composites 33(20) is located primarily adjacent to the outer surface of the is blended easily with cotton fiber or other synthetic sheath and it is peeled off readily in ribbons of strips of fibers to produce blended fabric and textiles. Banana 5–8 cm wide and 2–4 mm thick, from the entire length fiber also finds its use in high quality security/currency of sheath. The banana fiber extraction in the field is paper, packing cloth for agriculture produce, ships shown in Figure 2(b). This stripping process is known towing ropes, wet drilling cables, etc.12 The ‘‘pseu- as tuxying the strips, which is also called as tuxies. Two dostem’’ is a clustered, cylindrical aggregation of leaf methods of tuxying are employed in Philippines. The stalk bases. Banana fiber, at present is a waste product first method called Bacnis method, where the trunks are of banana cultivation and it is not properly utilized. The pulled apart and the sheath is separated according to extraction of fiber from the pseudostem is not a common their position in stalk. They are then flattened and the practice and much of the stem is not used for the pro- fiber is stripped from the stem by cutting the pulpy duction of fibers. The buyers of banana fibers are erratic portion and pulling away the tuxy.12 In Loenit and there is no systematic way to extract the fibers regu- method, the tuxies have pulled off the stalk from one larly. Useful applications of such fibers would regularize sheath at a time. In either of these methods, the tuxies the demand, which would be reflected in a fall of the are tied into bundles of 23–27 kg and they are cleaned prices.12 Bast fibers like banana fibers are complex in by stripping knife. In this process, tuxies are pulled structure. They are generally lignocellulosic, consisting under a knife blade, where they are pressed tightly of helically wound cellulose microfibrils in amorphous against the tuxy in order to scrape away the plant matrix of lignin and hemicellulose. The cellulose content tissue between the fibers. The clean fiber is then dried serves as a deciding factor for mechanical properties in air and it is made as bundles for subsequent grading along with microfibril angle. A high cellulose content and bailing.12 The hand stripping of banana fiber is and low microfibril angle impart desirable mechanical presented in Figure 2(c). In addition to the hand strip- properties for bast fibers. Lignins are composed of nine ping, machines are used as trunks from which the dark carbon units derived from substituted cinnamyl alcohol; outer sheaths have been removed, and they are cut into that is, coumaryl, coniferyl, and syringyl alcohols. sections of 120–180 cm in length. In this process the Lignins are associated with the hemicelluloses and play chopped plant is passed through a machine called an important role in the natural decay resistance of the Raspador which is shown in Figure 2(d). The sections lignocellulosic material.12 are then crushed between rolls and the pulpy tissues are scraped away, one half the length at a time, by two Kenaf fiber large revolving drums, the rim of which are fitted with scrapping blade which scrape the sheath while it Kenaf or its scientific name Hibiscus cannabinus is a is pressed against a bed plate as shown in Figure 2(e). warm season annual fiber crop closely related to The extracted banana fibers are dried by using oven. cotton and jute. Historically, kenaf has been used as a After the drying, the fibers are graded and bundled for cordage crop to produce twine, rope, and sackcloth. further use. The extracted banana fiber is shown in Nowadays, there are various new applications for Figure 2(f). The yield of fiber is almost 10 times more kenaf including paper products, building materials, by mechanical extraction, however, the quality of the absorbents, and animal feeds. Kenaf has a single, fiber is inferior. straight, and branchless stalk. Kenaf stalk is made up Banana fibers are extracted by employing chemical, of an inner woody core and an outer fibrous bark is mechanical, or biological methods. Chemical method surrounded by the core. The fiber derived from the causes environmental pollution, and the mechanical outer fibrous bark is also known as bast fiber. The method fails to remove the gummy material from the kenaf plantation, kenaf leaves, and the kenaf flower fiber bundle surface. Biological procedures yield more are shown in Figure 3(a) to (c). fiber bundles than the other two procedures without any Kenaf bast fiber has superior flexural strength com- harm to the environment. After extracting the fibers, bined with its excellent tensile strength, which makes it degumming is an essential one prior to the utilization as the material of choice for a wide range of extruded, of fibers. The removal of heavily coated, noncellulosic molded, and nonwoven products. Kenaf fiber is utilized gummy material from the cellulosic part of plant fibers is as reinforcement material for polymeric composites, called degumming.12 Banana fiber is a multiple celled which is an alternative to the glass fiber. It also have structure. The lumens are large in relation to the wall some advantages over traditional reinforcement thickness. Cross markings are rare and the fiber tips are materials such as glass fiber in terms of cost, density, pointed and flat; they have ribbon like individual fiber renewability, recyclability, abrasiveness, and bio- diameter range from 14 to 50 mm and the length from degradability. The efficiency of the fiber reinforced 0.25 to 1.3 cm, showing the large oval to round lumen. composites depends on the fiber–matrix interface Banana fiber is a natural fiber with high strength, which which has the ability to transfer stress from the

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Figure 3. (a) Kenaf plant. Kenaf plantation,26 (b) kenaf leaves,27 (c) kenaf flower.28 matrix to the fiber. The main obstacles in the use of manufacturing process, which has never been asso- natural fibers in plastics have been the poor compati- ciated with other natural fibers before, such as pultru- bility between the fibers and the matrix, since it inher- sion and potentially filament winding.36 The use of ent high moisture absorption results in dimensional kenaf fiber reinforced composite helps to generate changes of the fibers. It also leads to microcracking of jobs in both rural and urban areas, in addition by help- the composite and degradation of mechanical proper- ing to reduce waste and thus contributes to a healthier ties. Various chemical treatments have been used to environment.37 improve the mechanical performance of the natural Kenaf is primarily grown for its fiber. It has an amaz- fiber, which include jute and hemp by many researchers ing ability to grow up to 14 ft in one growing season, by in the past.29 Exposure of the natural fiber (kenaf) com- yielding around 6–10 tons of fiber per acre. Fibers in posite material to environmental conditions such as dis- kenaf are usually found in the bark, which constitute tilled water, sea water, and rain water results has 40% of the plant, and at the core which makes 60% of decreased the fracture toughness. The decrement of the plant. For more than thousands of years Kenaf the fracture toughness is due to the water absorption fibers are used for making textiles. The Egyptians have characteristic, which depends on the content of the used these fibers to make boat for sails and other textiles. fiber, fiber orientation, area of exposed surface, and Fibers made from the outer layer of the plant are used permeability of fiber, void content, and the hydrophil- for making cords, ropes, and storage bags. Bast fibers icity of the individual component.30 A cross sectional are simple to process. Blended with cotton, kenaf fibers area evaluation for natural fiber (kenaf) is treated with are made into yarn and woven into fabrics. These tex- alkali solution by data-based approximation method, tiles are esthetically pleasing, lightweight, and have a which increases the tensile strength.31 Twisted kenaf soft feel. Apparels have the appearance of linen. They hybrid material, fabricated by hot impregnation are the most sustainable fabrics, due to its growth rate method presents good mechanical properties. It can and ability to replenish. It does not require much water be utilized in bumper beams of automotive compo- for its cultivation, almost no fertilizer or pesticides, and nents; moreover, impact properties could be improved grows to its full length, approximately in 150 days. by optimizing the structural parameter like thick- Kenaf is completely biodegradable, as it does not ness, beam curvature, and strengthening ribs.32 require many chemicals for its degradation. Deterioration in tensile modulus of hybrid composites uses kenaf and fiber glass with polyester due to longer Separation of kenaf fibers immersion time in distilled water, sea water, and acidic solutions at room temperature. The main reason is the The process of separating the long and short fibers formation of hydrogen bonding between the water mol- depends on the method of harvesting. In frost free ecules and cellulose fiber.33 Kenaf composites cannot regions, the kenaf stalk is cut with special equipment. compete with glass composites. The dry specific proper- In cooler regions, the plant is typically frost killed and a ties are slightly lower for the kenaf composites, where natural drying of the stalk occurs. The separation is the wet samples are drastically lower.34 The factors done in either whole stalk or chopped.29 which affect the mechanical properties and some chemical modification in woven natural fiber, especially Mechanical separation method banana and kenaf increase the scope for better future.35 Kenaf fiber reinforced composite has utilized There are many methods to separate the kenaf its availability and readiness with various fibers. Mechanical separation of fiber is a quite

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Figure 4. Kenaf fiber extractor.38,39 economical way. The kenaf fiber extractor is shown in depends on the availability of water and the cost of Figure 4. The bast kenaf fibers that separate most of the retting process. To extract fine fibers from kenaf core materials are fed to the Rando Cleaner, a roller plant, a small stalk is harvested for preretting. type cleaner equipped with fine saw tooth wire. There Usually, this small stalk is brought before 2 weeks of are two kinds of Rando cleaner, they are stick machine harvesting time. The fiber is easily removed from the and trash master. Standard stick machine is equipped kenaf core and tied into bundles and submerged in soft with three 355.6 mm diameter saws with and without running water. The stalk stays submerged in water for Pelxiglas. The trash master is a six-cylinder incline clea- 20 days. However, the retting process requires less time, ner with 12.7 mm space between grid bars and the if the quality of the fiber is better. In most cases, the degree of incline, which is different, for example, 30 fiber extraction process of bast fibers in water retting is or 45. In order to get the cleanest fiber, it is necessary done by the farmers while standing under water. The to process the bast fibers through both stick machine Kenaf water retting process is shown in Figure 5(a). and trash master which is equipped with different saws The popular method and process in extracting fine at inclination. In the procedure of separating kenaf into bast fibers is mechanical retting. The fiber is pulled its two fractions, moisture content is a significant pre- out from the hurd or core and then it is hit with a dictor of final fiber content; in this case, moisture con- wooden hammer and washed with water and at last tent of whole stalk kenaf and humidity of ambient air the water is squeezed out and stocked.29 The Kenaf need to be tested carefully. The separation efficiency stem showing the fibers is shown in Figure 5(b) and and processing rate are the two important parameters the produced bundle of fiber is shown in Figure 5(c). to be considered. By using drying or separation during periods of low ambient humidity, separation efficiencies Sisal fiber (SF) are drastically improved to 99%. Whole stalk kenaf fiber above the moisture content of about 18–20% is SF is a hard fiber that has been extracted from the difficult to separate. Separation efficiency is low when leaves of the sisal plant (Agave sisalana). Though it is the ambient humidity is above 60% and it is accom- the native to tropical and subtropical North and South plished with the aid of drying. The stick machine is America, sisal plant is now widely grown in tropical more efficient at fiber and core separation of kenaf countries of Africa, West Indies, and Far East. SFs stalk than the six-cylinder cleaner. The slower process are extracted from its leaves. SF is one of the most rate had increased the separation efficiency.29 widely used natural fibers and it is very easily culti- vated. It has short renewal times and grows wild in Water retting method the hedges of fields. Tanzania and Brazil are the two main producing countries. A sisal plant produces about It is a wet process by which the bundles of cells in the 200–250 leaves and each leaf contains 1000–1200 fiber outer layers of the stalk are separated from nonfibrous bundles, which is composed of 4% fiber, 0.75% cuticle, matter by the removal of pectins and other gummy 8% dry matter, and 87.25% water. So, normally a leaf substances. The available retting processes are called weighing about 600 g yields about 3% by weight of mechanical retting which means by hammering, chem- fiber with each leaf containing about 1000 fibers.42 ical retting (boiling and applying chemicals), steam or The sisal plant, fiber extraction, and the extracted vapor retting, and water or microbial retting. Among fiber are shown in Figure 6(a) to (c). them, the water or microbial retting is a century old but The incorporation of natural fibers such as sisal (in it is the most popular process in extracting fine bast the form of fibers), ridge gourd (in the form of natural fibers. However, selection of these retting processes woven mat, and coconut leaf sheath (in the form of

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Figure 5. Kenaf fiber and its processing. (a) Kenaf water retting process,39 (b) kenaf stem showing fibers,40 (c) kenaf fiber.41

Figure 6. (a) Sisal fiber extraction. Sisal plantation,43 (b) sisal fiber extraction,44 and (c) sisal fiber.45 natural woven mat) into the epoxy matrix shows mod- having a lustrous color, while the retting process erate improvement in the tensile properties of the com- yields a large quantity of poor quality fibers. After posites, when compared to individual reinforcement; extraction, the fibers are washed thoroughly in plenty this is due to transfer of loads and sharing of loads of clean water to remove the surplus wastes such as among the fibers.11 chlorophyll, leaf juices, and adhesive solids.43 The sisal leaf contains three types of fibers: (1) mech- The length of SF is between 1.0 and 1.5 m and the anical, (2) ribbon, and (3) xylem.43 diameter is about 100–300 mm. The fiber is actually a bundle of hollow subfibers. Their cell walls are rein- 1. The mechanical fibers are mostly extracted from the forced with spirally oriented cellulose in a hemicellulose periphery of the leaf. They have a roughly thickened- and lignin matrix. So, the cell wall is a composite struc- horseshoe shape and seldom divide during the ture of lignocellulosic material reinforced by helical extraction processes. microfibrillar bands of cellulose. The composition of 2. Ribbon fibers occur in association with the conduct- the external surface of the cell wall is a layer of lignac- ing tissues in the median line of the leaf. They are the eous material and waxy substances, which bonds the longest fibers when compared with mechanical fibers cell to its adjacent neighbors. Hence, this surface does and they are easily split longitudinally during not form a strong bond with a polymer matrix. Also, processing. cellulose is a hydrophilic glucan polymer consisting of a 3. Xylem fibers have an irregular shape and occur linear chain of 1, 4-b-bonded anhydroglucose units and opposite to the ribbon fibers through the connection this large amount of hydroxyl groups gives SF the of vascular bundles. They are composed of thin- hydrophilic properties. This leads to a very poor inter- walled cells and therefore they are easily broken up face between SF and the hydrophobic matrix and has and lost during the extraction process. very poor moisture absorption resistance.43 The incorporation of SF with Glass fiber reinforced plastics The processing methods for extracting SFs include (GFRP) has exhibited superior properties than the (1) retting followed by scraping and (2) mechanical by jute fiber reinforced GFRP composites in tensile prop- means of using decorticators. Mechanical process yields erties and jute fiber reinforced GFRP composites have about 2–4% fiber (15 kg per 8 h) with good quality performed better in flexural properties.46

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Retting water flow can be exercised over water conditions and thus the quality of fiber obtained is better and more A process called retting is employed to extract fiber consistent. Tanks constructed for the purpose are from plants. This process involves the action of bacteria used for submerging plant stalks. Water is changed and moisture on plants to dissolve and rot away cellular after the initial 8 h of submerging. This aids the retting tissues and gummy substances that surrounds the fiber process, because a lot of waste and toxins are removed bundles in the plant. Once the surrounding tissue and along with this water. The waste water that is removed other substances are dissolved, they fall away, and the has been treated and used as liquid fertilizer, because it fiber is then easily separated from the stem. Retting is is rich in chemicals.47 done either with the help of water or with the help of dew.47 Scuthing. Straw is the name used to refer to the stalks that have completed the retting process. This straw is Dew retting. Dew retting process is used in areas where then put through the process of drying in open air and water resources are scarce. To make the process effect- then stored for a while to allow curing to occur. Curing ive, the night-time dew has to be quite heavy and the aids the process of fiber removal. The next stage is the daytime temperature has to be warm. In the dew retting breaking process in which the woody portion of the process, the harvested plant stalks are spread out evenly straw is broken. This process has been done either by on grassy surfaces. Here, the sun, air, dew, and the hand or through machines. Machines with rollers are natural decaying process involving bacteria produce used for this process when done on a large scale. fermentation and as a result the cellular fleshy matter Scuthing is the process in which the broken pieces are surrounding the fiber in the stalks falls away. removed by either beating or scraping the straw. Some Depending upon the existing climatic conditions, this machines perform both the beating and the scuthing process takes 2–3 weeks. Dew retted fiber can easily be process. The waste material from the scuthing process distinguished from water retted fiber due to its darker is used for paper manufacture. Fibers extracted from it color. When compared to the water retted fiber, dew are allowed to dry and twin into ropes and also they are retted fiber is poorer in quality.47 woven into mats, etc. Another by-product of the scuthing process is the woody pieces referred to as shives that Water retting. The preferred method of retting is water are used as fuel for heating the water used for retting.47 retting, as it yields superior quality fiber. In this The SF extractor, drying of SF, and the fabricated sisal method, bundles of the plant stalks are submerged in rope are shown in Figure 7(a) to (c). water. The time duration for the plants to remain submerged in water is carefully monitored. If the submer- Aloe vera ging time allowed is not enough, the separation process becomes very difficult and so the yield is affected. On The aloe vera plant has been known and used for cen- the other hand, if the submerging time allowed is too turies for its health, beauty, medicinal, and skin care much, the quality is affected and the extracted fiber is properties. The name aloe vera derives from the Arabic weak. word ‘‘Alloeh’’ meaning ‘‘shining bitter substance,’’ Trial and error methods have resulted in a process while ‘‘vera’’ in Latin means ‘‘true.’’ Two thousand known as double retting. In this process, plant stalks years ago, the Greek scientists have regarded aloe are retted in water for a lesser time than optimum, vera as the universal panacea. The Egyptians call taken out, and dried for a long time and then they Aloe ‘‘the plant of immortality.’’ The botanical name are retted again. The fiber extracted after this process of aloe vera is Aloe barbadensis miller. It belongs to generally has a very superior quality.47 Asphodelaceae (Liliaceae) family and is a shrubby or arborescent, perennial, xerophytic, succulent, pea-green Stagnant water retting. Another method employed is nat- color plant. It grows mainly in the dry regions of ural water retting. Stagnant or slow moving water like Africa, Asia, Europe, and America. In India, it is ponds and bogs are used for this. The stalk bundles are found in Rajasthan, Andhra Pradesh, Gujarat, dropped into the water and are weighted down with Maharashtra, and Tamil Nadu. The plant has triangu- stones or logs. The submerging time is decided depend- lar, fleshy leaves with serrated edges, yellow tubular ing upon the temperature of the water and the mineral flowers and fruits that contain numerous seeds. Each content of the water. The submerging time allowed leaf is composed of three layers: (1) An inner clear gel varies between 10 days and 2 weeks.47 that contains 99% water and rest is made of glucoman- nans, amino acids, lipids, sterols, and vitamins. (2) The Tank water retting. Tank retting is yet another method middle layer of latex which is the bitter yellow sap and used for retting fiber yielding plant stalks. Control of contains anthraquinones and glycosides. (3) The outer

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Figure 7. Sisal fiber processing and fabricated sisal ropes. (a) Sisal fiber extractor,48 (b) drying of sisal fiber,49 and (c) sisal fiber rope.50

thick layer of 15–20 cells called as rind has protective researchers have carried out several studies on natural function and synthesizes carbohydrates and proteins. fiber composites and their mechanical properties. Table Inside the rind are vascular bundles responsible for 1 presents the summary of the natural fibers used in this transportation of substances such as water (xylem) paper such as banana, kenaf, sisal, and aloe vera; the and starch (phloem). Aloe vera plant has good medic- resin used; the manufacturing process employed for the inal values and is extensively used in cosmetics and manufacturing of the composites; and its mechanical medicines. Since yarn prices started soaring up, the properties. jute weavers started looking for natural fibers to manu- The results from the table indicate that the property facture saris. After using banana fiber, the (tensile strength and flexural strength) of the natural Anakaputhur weavers are now using fiber from aloe fiber composites varied depending on the type of vera plant.51 In the western countries like Germany fibers, type of resin, and the manufacturing process. and Spain, aloe vera enriched clothing is in the initial The mechanical properties of banana fiber/phenol- stages of development. It has been claimed that this formaldehyde resin composites fabricated by resin herb in garments prevents aging of the skin, rejuvenates transfer molding (RTM) and compression molding skin cells, and keeps skin free from the microbial infec- (CM) techniques are analyzed as a function of fiber tions. Dermatology tests related to the textile applica- length and fiber loading. Tensile and flexural properties tions of these fibers are conducted by a research are found to be higher in RTM composites as com- university in Germany. Presently, these garments are pared to CM composites at all values of fiber loading. manufactured in Barcelona, Spain. It is mainly used Thus, the mechanical properties of the composites are for manufacturing inner garments, stockings, etc. due intimately dependent on the fabrication techniques.56 to the claimed additional functions like absorbing bad An alkali-treated banana-coir epoxy hybrid composite smell and providing antibacterial features. By looking has greater tensile strength and impact strength than an at the initial stages of development of garments from untreated banana-coir epoxy hybrid composite. aloe vera, the achievement of Tamil Nadu weavers is However, the alkali-treated banana-coir epoxy hybrid really commendable, as they are making full fledged composite has less flexural strength than the untreated embellished sarees with it.52 The aloe vera plant, the banana-coir epoxy hybrid composite. The properties aloe vera fiber extractor, and the aloe vera fiber are are improved by the alkali treatment process.57 The shown in Figure 8(a) to (c). tensile strength of abaca composite is relatively more than abaca and banana composite and much higher Comparisons of different composites when compared with banana composite. The single consisting of different fibers with composite withstands more strain before failure in ten- different resins sile testing than the hybrid fiber composite. The flexural strength of the composite is in decreasing order from Natural fiber composites are finding increased applica- banana, abaca and banana hybrid, and abaca compos- tion in many areas due to their environment friendly ite. Abaca has the highest flexural strength since its nature. These composites are manufactured either by strength increases with increase in interfacial adhesion. using synthetic resin or biodegradable resin. Banana composite can be used in applications where Thermoplastics or thermosetting plastic can be used high impact strength is necessary.58 Chemical treatment as a matrix material for these composites. The like NaOH increases the flexural strength of the fiber up

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Figure 8. Aloe vera fiber extraction. (a) Aloe vera plant,53 (b) aloe vera fiber extractor,54 and (c) aloe vera fiber.55

Table 1. Natural fibers, resin used, and its properties.

Manufacturing process Tensile strength Flexural strength Type of fiber Resin used employed (MPa) (MPa) References

Banana Phenol formaldehyde Compression molding 24 44 56 Resin transfer molding 28 48 Epoxy Hand lay up 16.43 36 57 Epoxy Hand lay up 37.33 37.25 58 Bisphenol-A Epoxy Hand lay up 54.15 163.1 59 Epoxy Resin transfer molding 22.58 32.618 60 Thermoplastic polyurethane Compression molding 16.34–23.74 6.64–11.21 61 Kenaf Epoxy Resin transfer molding 45 77.4 62 Polyester Vacuum infusion 90.8 93.4 63 Isopthalic polyester Hand lay up 38.18 166.2 64 Polyester Hand lay up 10.58 82.63 65 Polyester Hand lay up 74.46 – 66 Epoxy Compression molding 100.56 – 67 Epoxy Hand lay up 78.92 – 68 Polyester 76.67 – Vinyl ester 78.34 – Sisal Urea Compression molding – 58.58 69 Formaldehyde Epoxy Hand lay up – 17.52 70 Epoxy Hand lay up 41.80 – 71 Polypropylene 44.40 – Polystyrene 45.06 – Polyethylene 31.12 – Polyurethane Compression molding – 3.7 72 Phenol – 11.2 Isophthalic polyester Hand lay up 97.1 138.87 73 Epoxy Hand lay up 56 66 74 Polyester Compression molding 35 64 75 Aloe vera Novolac Condensation – – 76 Formaldehyde Phenol Epoxy Hand lay up – – 77

Downloaded from jrp.sagepub.com by guest on September 26, 2014 Kumar and Jeya Sekaran 1889 to 20–30% and removes the moisture content of the characterization of the composites reveals that the fiber. The tensile strength and flexural strength increase hybridization is having significant effect on the mech- with increasing fiber volume fraction. Among all the anical properties of composites. Among all the compos- hybrid fiber composites tested, banana reinforced ites, the composite having outer layer of kenaf and core epoxy hybrid composites registered the highest mech- of banana had the highest modulus, tensile, and flexural anical properties. The mechanical properties of the nat- strength and composite having skin of banana and core ural fiber and synthetic fiber plates composites tested of kenaf shows lowest mechanical properties.65 The are found to compare favorably with the corresponding results from the research66 indicated that 40% tensile and flexural properties.59 Tensile tests are car- reinforcement of kenaf gives better tensile strength. ried out at the strain rates of 0.1 mm/s. The ultimate The results indicated that the fatigue life is affected by tensile strength is highest for the banana–silica compos- the amount of fiber volume ratio but it may not have ite though it is still less than that of the epoxy resin. It is shown any significant improvement at very high seen that addition of silica and fibers significantly number of cycles.67 The kenaf/epoxy composites have affects the modulus of elasticity. About 12% increase the highest ultimate tensile strength compared to kenaf/ in modulus of elasticity has been seen due to addition of polyester composites and kenaf/vinyl ester composites. 10 wt% of banana fibers.60 Sharma and Kumar61 have The composites strength gradually increased when the fabricated banana fiber reinforced thermoplastic poly- fiber volume fraction increased.68 urethane (Grade: LPR5725EG) composites. They have The mechanical properties of SF reinforced urea– used up to 25% banana fibers in variation of 5%. The formaldehyde resin composites with 50 wt% SF have results from their studies indicated that 15 wt% banana the optimal Charpy impact strength and it reaches fibers shows an increase of modulus by 359% (max- 9.42 kJ/m2. Whereas the flexural, wear resistance and imum) and then it decreases as the banana fiber wt% water absorption properties are proved to be excellent increases. Tensile strength increases continuously up to in the composite with 30 wt% SF.69 A polymer matrix 15%, and the trend is continued in 20% reinforcement composite contains coir, banana, and sisal as the also, but 25% shows a decrease in tensile strength. The reinforcement phase has been successfully fabricated tensile strength of kenaf bast fiber is between 400 and by Sakthivel and Ramesh.70 The material properties 550 MPa which is higher than some natural fiber, of fabricated natural fiber reinforced composites are namely sisal and jute. Therefore, kenaf fiber has the observed. It is found that banana reinforced natural potential to be used as reinforcement in the polymer composites are the best natural composites among the composite. For tensile and flexural strength properties various combination. It can be used for manufacturing of the Kenaf fiber reinforced polymer (KFRP) compos- of automotive seat shells among the other natural fiber ite, the higher overlapped length resulted in higher ten- combinations.70 SFs have good potential as reinforce- sile and bending strength. From the results of both ments in polymer (thermoplastics, thermosets, and rub- tests, the KFRP composites with 40 mm overlapped bers) composites. Due to the low density and high length have the highest value of tensile and flexural specific properties of SFs, composites based on these strength.62 The mechanical properties of kenaf fiber fibers may have very good implications in the automo- reinforced polyester composites made by vacuum infu- tive and transportation industry. Moreover, reduced sion method are investigated. It has been found that the equipment abrasion and subsequent reduction of alkalization treatment used has improved the mechan- retooling costs make these composites more attractive. ical properties of the composites. Kenaf fiber can be a The use of SFs as a source of raw material in plastic good reinforcement candidate for high performance industry not only provides a renewable resource, but polymer composites. The kenaf–polyester composite could also generate a nonfood source of economic manufactured by vacuum infusion process provides development for farming and rural areas. SF polymer an opportunity of replacing existing materials with a composites with and without hybridization should be higher strength, low-cost alternative that is environ- developed and characterized so as to arrive at a series of mentally friendly.63 Short natural fiber composite composites which may find use in several areas such as laminate is prepared using the Kenaf fiber and polyes- marine, structural, consumer articles, and industrials ter isopthalic resin. Fiber matrix weight ratio of 1:20 applications.71 Thermal treatment of sisal fabric has been employed. The fiber treatment has improved reinforcement applied before molding the polyurethane the tensile properties of composite lamina. But it is composite is suitable when considering its flexural observed that the flexural strength is decreased by behavior, once the treatment increases notable flexural 4.4% for Kenaf UT-8 mm laminate when compared strength. The best performance in flexural mode to to resin lamina. Surface treatment of fiber had a signifi- reinforce structures, when considering the flexural cant effect on fiber/matrix adhesion and due to this strength and its deflection, is the thermally treated there is an increase in flexural strength.64 The sisal/phenolic followed by untreated sisal/phenolic,

Downloaded from jrp.sagepub.com by guest on September 26, 2014 1890 Journal of Reinforced Plastics and Composites 33(20) thermally treated sisal/Polyurethane (PU), and treatment. When carefully treated and developed, nat- untreated sisal/PU composites, respectively.72 The com- ural fiber composites can have comparable properties posite with 50% sisal–glass fiber and 50% resin com- to the existing synthetic fiber composites. bination has maximum tensile strength.73 The effect of moisture absorption on mechanical properties of coco- Conclusion nut coir and SF reinforced hybrid epoxy composite is compared with the composites containing the dried Research on natural fibers composites has created enthu- fibers. Increasing the fiber content at dry condition, siastic attention due to its marvelous properties and eco- the tensile and the flexural strength increased. At wet logical considerations. A brief discussion on some of the condition, the tensile and flexural strength have a high- natural fibers and its extraction is given along with a level reduction. Exposure to moisture caused a signifi- review. Also the comparison between the different resin cant drop in the mechanical properties due to the deg- used, manufacturing process employed for the manufac- radation of the fiber–matrix interface.74 The impact turing of the composites, its mechanical properties are strength and flexural strength of the composites discussed in brief. The main aim to focus on the attention increase for randomly taken fiber length and fiber of the researchers is to look at the potential of the natural weight percentage by the addition of red mud, but ten- fibers as an alternative medium to replace the conven- sile strength value decreases due to the distribution of tional materials or synthetic fibers as reinforcement in the particulates along with matrix which results in poor composites. In general, the use of natural fiber reinforced stress interface between matrix and filler. Impact and composite helps to generate jobs in both rural and urban flexural for red mud filled sisal polyester composites are areas, and supports for green environment. more when compared with that of banana fiber composites. The addition of red mud along with sisal and Acknowledgements banana fiber increases impact and flexural strength. The review is prepared based on the references and web Due to high impact and flexural strength, red mud resources, for which the authors expresses their sincere filled polymer matrix composites are suitable for high thanks to them. load withstanding and load-bearing capacity applica- 75 tions. The resin composites with three different bio- Conflict of interest polymers (cane chaff, aloe vera, and rice husk) and None declared. extensive study of its thermal stability, flame retard- ancy, and thermal degradation mechanism with differ- Funding ent natural fibers can be successfully incorporated into thermosetting resin matrix like phenolics and substi- This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. tuted phenolics by condensation method using acid catalyst as is evident from spectral studies. References Biopolymeric composites with phenolics are having higher thermal stability than the biopolymers them- 1. Malhotra N, Sheikh K and Rani S. A review on mechan- selves. It opens a new opportunity for production of ical characterization of natural fiber reinforced polymer J Eng Res Stud cost-effective (as it uses wastes) heat-resistant advanced composites. 2012; III: 75–80. 2. 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