A Review Of Natural Fibers Used In Biocomposites: Plant, Animal and Regenerated. Cellulose Fibers. Corresponding Author:

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1 A Review Of Natural Fibers Used In Biocomposites: Plant, Animal and Regenerated Cellulose Fibers Sunil Kumar Ramamoorthy 1, Mikael Skrifvars 1, Anders Persson 2 1 School of Engineering, University of Borås, Sweden 2 The Swedish School of Textiles, University of Borås, Sweden Corresponding Author: Mikael.Skrifvars@hb.se Abstract Natural fibers are increasingly finding applications in composite manufacturing. Based on their sustainability benefits plant and animal fibers are substituting synthetic fibers in composites. These fibers are used to manufacture several biocomposites. The chemical composition and properties of each of the fibers change, which demands the detailed comparison of these fibers. The reinforcement potential of natural fibers and their properties have been described in numerous papers. Today, high performance biocomposites are produced from several years of research. Plant fibers, particularly bast and leaf, find applications in automotive industries while most of the other fibers were explored in lab scale but haven t found large scale commercial applications. It is necessary to also consider other fibers such as seed (coir) and animal (chicken feather) fibers since they are secondary or waste product. Few plant fibers such as bast fibers are often reviewed briefly but other plant and animal fibers are not discussed in detail. This review paper discusses all the six types of plant fibers such as bast, leaf, seed, straw, grass and wood, together with animal fibers and regenerated cellulose fibers. Developments dealing with natural fibers and their composites have been presented here. The fiber source, extraction, 1

2 availability, type, composition and mechanical properties are discussed in this study. The advantages and disadvantages of using each biofiber are discussed. Three fabric architectures such as nonwoven, woven and knitted are briefly discussed. Finally, the paper presents the overview of the results from the composites made from each fiber with suitable references for in-depth studies. Keywords: Natural Fiber Composite, Biocomposite, Plant Fiber, Animal Fiber, Bast Fiber, Leaf Fiber 1. Introduction Fiber reinforced composites (FRC) are used in several applications for several years and market is growing continuously. It is known that the addition of fibers to the polymers has several advantages, especially, the mechanical properties of the composites. Synthetic fibers such as carbon/glass are reinforced in polymers to be used in high-performance applications such as automobiles and aircraft industries. 1-3 The performance of these composites was improved continuously through rigorous research, often through mixing of two or more reinforcements/polymers or fillers. 4-7 However, these high-performance composites are difficult to recycle as the separation of the components are quite difficult Therefore, these composites are often disposed in unsatisfactory ways such as landfill or incinerated which causes vast environmental impact It is to be noted that most of these polymers are from petroleum-based non-renewable resources. Ecological problems in recent decades have forced to look for the new alternatives which could replace the traditional FRCs with lower environment impact materials This created renewed interest in natural materials which could be used as reinforcements or fillers in the composites and thus referred as natural fiber reinforced composites or ecocomposites. 15 2

3 It is also termed as biocomposites. 16 Several researchers came up with ideas of reinforcing different natural fibers in polymers to produce eco-friendly composites for several applications which do not require excellent mechanical properties, such as secondary/tertiary building structures, car door panels, packaging etc. 13 The use of natural fibers as reinforcements in composites has been growing since then and replaced several synthetic fiber reinforced composites in many applications such as automotive, marine, aerospace, construction industries etc ,17 This is mainly due to extensive research, environmentally friendly/biodegradability character, low cost of the natural fibers and interesting physical and mechanical properties of these fibers (low density, good strength, processing flexibility, high specific stiffness etc.). 14,16 In chronological order, the first attempts were focused on replacing the synthetic fibers by natural fibers in petroleum-based matrices to form composites. 13 The production and characterization of these composites were studied and the properties were improved substantially These composites gained support due to considerable reduction of nonrenewable resources used in composites; mineral-inorganic materials were replaced by natural-organic ones. 13 The use of natural fibers in composite industry created a new alternative for the farmers. The possibilities of using recycled plastics like polyolefin succeeded as it reduced the consumption of non-biodegradable polymers Furthermore, the biodegradable polymers were derived from renewable resources; polylactic acid thermoplastic polymer from corn starch, soybean based thermoset matrix etc. and the composites were made completely from renewable resources, and thus referred as green composites

4 Natural fiber composites were used in airplane seats as early as The use of natural fibers critically declined and came to near-halt due to the rise of standard plastics and its low cost. Few countries like India sustained the use of natural fibers as reinforcements for composites in applications like pipes, panels etc. 39 Indian government encouraged natural fiber reinforced composites for buildings, eg. Madras-House. 40 Critical discussions about the environmentally friendly materials and preservation of non-renewable resources regained the interest in natural fiber composites. 11 The US market reports that the composite market was 2.7 billion pounds in 2006 and estimated to reach 3.3 billion pounds by 2012 with 3.3% annual growth. 41 The natural fiber market in US experienced 13% growth rate (275 million kilograms) from 1994 to 2004, and the demand for the natural fibers continue to rise. 42 The average global annual market growth for bio-based matrix was 38% from 2003 to 2007; Europe saw the highest annual growth rate of 48%. Bio-based matrix market was 0.36 million metric ton in 2007 and it is expected to reach 3.45 million metric ton in The fiber reinforced market is multibillion dollar business and the natural fiber composites take a fair part of it. 44 Many articles including reviews, conference proceedings and books reflect the growing interest and importance of natural fibers, bio-based matrix and biocomposites Bledzki and Gassan reviewed the cellulose based composites until 1999, and Omar et al reviewed biocomposites from 2000 to ,12 Detailed review about biocomposites, mostly from bast and leaf fibers, and their properties until 2010 have been discussed in the above two papers. There is lack of detailed discussion on other plant and animal fibers. New techniques including prediction of composite properties using different softwares/methods are playing important role in designing the composites. 45,46 The potential of man-made regenerated cellulose fiber is highlighted due to uniformity and superior properties; these fibers are used 4

5 as reinforcements in composites The detailed review of the regenerated cellulose fibers and their composites have not been done. This paper aims to review the natural fibers such as plant and animal fibers, and regenerated fibers. All six types of plant fibers such as bast, leaf, seed, straw, grass and wood are discussed, and animal fibers such as silk, wool and chicken feather are discussed. Regenerated cellulose fibers like Lyocell and viscose are also discussed. Though it covers wide range of fibers for biocomposites, it will certainly be incomplete due to widespread research in this field, but this paper will hopefully provide a sensible overview of the fibers used for biocomposites. 2. Natural fibers and cellulose-based fibers Fibers are broadly classified as natural or man-made fibers. Figure 1 shows the schematic representation of fiber classification. 47 Natural fibers have replaced man-made fibers in many applications in last few decades due to low cost, low density and low tool wear. 11,12 Wide use of natural fibers as reinforcements due to renewability and sustainability has brought several fibers into composite field. Natural fibers are categorized into three types based on their origins; Plant, mineral and animal fibers. Animal fibers such as hair and silk, and mineral fibers have not been widely used as reinforcement fibers. But several plant fibers have been used widely in biocomposites field for applications like automotive, marine and construction. Regenerated cellulose fibers fall between natural and man-made fibers which have been used as reinforcement fibers in recent times. Natural and regenerated cellulose fibers have been discussed in this paper which is key part of biocomposites reinforcements. 5

6 Figure 1. Schematic Representation of Fiber Classification Figure 2. Structure of Biofiber. 14 Reproduced with permission from Elsevier, Copyright Plant fibers 6

7 The fibers from the plants can be in the form of hairs (cotton, kapok), hard fibers (coir, sisal) and fiber sheafs (flax, hemp, jute). The plant fibers are classified depending on their utilization such as primary and secondary. Plants grown for it to be used as fibers belongs to primary such as hemp, jute, kenaf etc., while the by-products of some plants such as coir, pineapple etc. are used as fibers and these plants belong to secondary group. 12 There are six types of plant fibers namely bast fibers (flax, hemp, jute, kenaf and ramie), leaf fibers (abaca, pineapple and sisal), seed fibers (coir, cotton and kapok), straw fibers (corn, rice and wheat), grass fibers (bagasse and bamboo) and wood fibers (softwood and hardwood), Figure 1. Table 1 shows the amount of few fiber produced and their geographical distribution. 13,14,53-59 General structure of biofiber is shown in Figure 2. Table 1. Some Plant Fiber Production and Their Producers 13,14,53-59 Bast Fiber World production (10 3 ton) Largest Producers Flax Hemp Jute Kenaf Ramie Canada, France, Belgium China, France, Philippines India, China, Bangladesh India, Bangladesh, USA China, Brazil, Philippines, India Leaf Abaca Curaua Pineapple 70 >1 74 Philippines, Ecuador, Costa Rica Brazil, Venezuela Philippines, Thailand, Indonesia 7

8 Sisal 378 Tanzania, Brazil Seed Coir Cotton Oil Palm India, Sri Lanka China, India, USA Malaysia, Indonesia Grass Bagasse Bamboo Brazil, India, China India, China, Indonesia Figure 3. Chemical Structure of Cellulose. 57 Reproduced with permission from Elsevier, Copyright The major chemical composition of plant fibers is lignocellulose (cellulose, hemicellulose and lignin) and the amount of these components change from plant to plant. The amount of each component also changes due to age, species and it could also vary in different parts of same plant. These basic components partially determine the physical properties of the fibers. These polymers could be distributed unevenly throughout the plant cell wall which makes it difficult to know the composition and properties of the fibers. Table 2 shows the 8

9 average chemical composition of some plant fibers. 12,16,60-72 Cotton fiber has highest amount of cellulose while coir fiber has highest amount of lignin. The unevenness of the chemical composition of the plant is one of the major issues of natural fibers to be used as reinforcements in composites. Cellulose is the strongest and stiffest component of the fiber which is linear 1,4-β-glucan polymer consisting D- anhydroglucose (C 6 H 11 O 5 ) repeating units containing hydroxyl groups, Figure 3. The OH groups form inter and intra-molecular hydrogen bonding making it hydrophilic in nature. The cellulose chemical structure remains same for all the natural fibers while the degree of polymerization changes, which influences the mechanical properties of the fibers. It is found that bast fibers have highest degree of polymerization compared to most of the other plant fibers. 73 Lignin is phenolic compound which is believed to support the structure of the plant, and it is also resistant to microbial degradation until disturbed through physical/chemical treatment. The chemical structure of lignin is not clear until today even though most of the functional groups and units are identified. 56,74 Lignin is believed to be binder which links the celluloses to keep the structure of the plant. Wax content of the fiber plays a crucial role on processing composites as it influences wettability when matrix is introduced, furthermore influencing interfacial fiber-matrix adhesion. Table 2. Chemical Composition of Some Natural Fibers. 12,16,60-72 Fiber Cellulose Lignin Hemicellulose Wax (wt%) (wt%) (wt%) (wt%) Bast Flax Hemp

10 Jute Kenaf Ramie Leaf Abaca Curaua Henequen Pineapple Sisal Seed/Fruit Coir Cotton Oil Palm Grass Bagasse Bamboo Straw Rice Wheat Others 10

11 Rice Husk The mechanical properties of some natural fibers and made-made fibers are listed in Table ,16,60,62,70-71,75-78 Variation in natural fiber properties can be due to several reasons such as geographical location of the plant, maturity, size, chemical composition, part of the plant from which fibers are extracted etc. The variation can also be attributed to different stages: growth, harvesting, fiber extraction and storage, and each stage have several influencing factors. It is demonstrated that the strength of the fiber is also related to fibrillar angle and physical properties of the fiber. 16,80 Small fibrillar angle, small fiber diameter and high aspect ratio gives high mechanical properties. 12,16,80 Owing to low density and relatively good strength and modulus of natural fibers, these fibers are preferred to be used as reinforcements in several applications. The high strength of kenaf and ramie fibers, and low strength of coir fiber could be attributed to high and low cellulose content. The modulus of kenaf and ramie fibers can be compared to that of man-made fibers. The natural fibers are hydrophilic by nature and has substantial amount of moisture content which influences the mechanical properties. Table 4 shows the moisture content of some natural fibers and these could be related to fiber pores, relative humidity, chemical composition and crystallinity of the fibers ,79 In general, high amount of cellulose, high relative humidity, high pore volume and low crystallinity in fibers tend to have high moisture content. 11

12 Figure 4. Ternary Diagram of the Plant Fibers. 17 Reproduced with permission from Elsevier, Copyright The cost of natural fibers is substantially low when compared to synthetic fibers such as carbon, steel and glass. 75 Performance price ratio (modulus/cost) of natural fibers is several times higher than man-made fibers. 16,75 Figure 4 shows the price performance of some plant fibers. As the result of continuous decline in the availability of natural resources to produce synthetic fibers, the cost of these fibers is expected to increase. So, renewable raw material production and contribution to fiber market continues to rise. The data showed the production of jute fibers is higher than that of E-glass; it also shows that the amount of flax and sisal produced together crosses the amount of E-glass produced. 68,81 Table 3. Mechanical properties of some natural and man-made fibers 11-12,16,60,62,70-71,75-78 Fiber Density Tensile E-Modulus Elongation at Strength break (g/cm 3 ) (MPa) (GPa) (%) Bast Flax

13 Hemp Jute Kenaf Ramie Leaf Abaca Curaua Pineapple Sisal Seed/Fruit Coir Cotton Oil Palm Grass Bagasse Bamboo Man-made Aramid Carbon E-glass S-glass

14 Table 4. Moisture Content of Some Natural Fibers ,79 Fiber Moisture Content (wt%) Bast Flax Hemp Jute Ramie Leaf Abaca Pineapple Sisal Seed Coir 8.0 Grass Bagasse Bamboo Bast fibers Flax 14

15 Flax (Linum usitatissimum) is food and fiber crop mainly produced in Canada, France and Belgium. Nowadays, it is predominantly grown for fiber and linseed oil and accounts ton, Table 1. It is one of the oldest fiber crops and its fiber is one of the first to be spun and woven into textiles. Reports claim that the flax fibers were used for many applications well before 5000 BC in Egypt and Georgia. 82,83 High grade long fibers are usually made into yarns for textiles and the low-grade fibers are used as reinforcements/fillers in composites. The structure of flax fibers (Figure 5) is explained in detail by Charlet et al. 84 The average flax plant grows up to 90 cm tall and possesses strong fibers along the stem which is composed of bark, phloem, xylem and centre void. 84 Macro and microscopic analysis s have been performed to see the fiber bundles and hierarchical organization. 84,85 The chemical composition of the flax fibers varied with different authors which is highly dependent on the flax fibers used. 24,86-88 Flax fiber is considered for reinforcement as it has high cellulose content and high degree crystallinity which makes it stronger and stiffer. Figure 5. Multi-Scale Composite Structure of Flax. 84 Reproduced with permission from Elsevier, Copyright

16 The tensile properties of flax fiber are given in Table 3. The tensile strength of the fiber is between 345 and 1100 MPa. The modulus of the fiber is 27.6 GPa and the percentage elongation of the fiber is between 2.7 and 3.2%. Good quality flax fiber has the best tensile properties compared to other bast fibers. Charlet et al have studied the tensile deformation of a flax fiber and the divided the stress-strain curve into three parts: linear strain (0 to 0.3%), non-linear strain (0.3 to 1.5%) and end linear strain (1.5% to fracture). 89 The reason for each segments are loading of fiber, alignment of micro-fibrils along machine direction and response of the micro-fibrils. 89 Similar tensile strain response was obtained by other researchers. 90,91 Variation of tensile properties of flax fibers were studied based on the fiber location in a stem, plant variety and place of cultivation. 84,90,92,93 It shows that that the fibers extracted from middle of the stem had highest strength and modulus; fibers from Hermes variety showed better tensile strength and modulus; and Hivernal cultivated plants fibers showed better tensile strength and modulus (Figure 6). 84,90,92,93 The size distributions of the fiber diameters, lumen diameters and porosity is shown in Figure 7. The effect of moisture absorption and drying of flax fibers were investigated and the results showed that both moisture intake and drying affected the tensile properties. 94 Defects in the cell wall, diameter of the fiber and testing condition (gauge length) are few other reasons for the variation of tensile properties of flax fibers; detailed study was done and all the above factors affected the tensile properties. 95,96 Flax fibers are used as reinforcements in several natural fiber composite applications as it has good tensile properties, availability and studied extensively, apart from environmental and economic benefits. More detailed review of flax fiber and its composites was done by Yan et al

17 Figure 6. Tensile stress-strain curves for the different varieties of oleaginous flax fibres. 54 Reproduced with permission from Elsevier, Copyright Figure 7. Size Distributions of the (a) Fiber Diameters (b) Lumen Diameters (c) Porosity. 93 Reproduced with permission from Elsevier, Copyright Hemp Hemp (Cannabis sativa) is also bast fiber crop cultivated mainly in China and France for fiber, oil and seed. Cannabis family plants are indigenous to central Asia and believed to reach Europe in the Iron Age. 97 Today, it is also widely grown in temperate climate countries such as Chile, North Korea, India, Japan and many European Union (EU) countries. EU considers promoting hemp cultivation in member countries by subsidy and looking forward for further developments. Nowadays, hemp is used in several applications such as textile fiber, paper, composite fiber, seed food, oil, wax, resin, pulp, biofuel etc. and mainly depends on the 17

18 grade/quality of the hemp plant. 12,55 Hemp plant secretes small amounts of tetrahydrocannabinol (THC) which is famous for narcotic substance marijuana; since the amount of THC present in hemp is lower than 0.2%, it cannot be used as drug like other varieties of Cannabis sativa. Reports suggest that an oldest hemp fabric dates back to 8000 BC (The Columbia History of the World). Hemp fiber cultivation was briefly banned between 1971 and 1993 to avoid misuse and on lifting the ban, the hemp cultivation has grown exponentially but still hemp fiber counts less than 0.5% of total natural fiber production: ton, Table 1. The average height of industrial hemp is 10 feet tall and normally very narrow as it is grown close together. Hierarchical organization of hemp fiber and fiber bundle size were analyzed by several authors. 98,99 It is reported that the average fiber bundle diameter is 25 µm and average fiber bundle length is 25 mm. 99 The chemical composition of the hemp fiber varied with different authors; cellulose content varied from 70.2% to 74.4%, Table 2. Even though it has several applications, due to its high strength and stiffness it is also used as reinforcement in biocomposites. 100 The variation in the chemical composition due several factors 101 leads to variability in mechanical properties of the hemp fibers. The plant population affected the hemp fiber morphology and the physical properties of the fiber which was studied by 4 year research by Amaducci et al. 102 Similar research was done in the colder country, Sweden, taking several growing conditions into account. 103 The tensile properties of the hemp fiber are given in Table 3. The tensile strength of the fiber is around 690 MPa and the modulus is between 30 and 60 GPa. The percentage elongation was around 1.6%. The stress-strain curve obtained by tensile test had similar trend to that of jute fibers. The variation of tensile properties is evident and common to all natural fibers, and this will continue as there are several factors which are uncontrollable in large scale ,101 The tensile properties of the hemp fibers were 18

19 studied based on the age of the plant and testing parameters. 104 It was found that fully matured plant has better tensile strength than the partly matured plant; it was also found that testing parameter, gauge length, affected the tensile strength of the hemp fiber. 104 Renowned automotive company, Bayerische Motoren Werke AG (BMW), makes car parts out of hemp fiber reinforced composites. 100 Detailed review of hemp fiber and its composites was done by Shahzad Jute Jute (Corchorus capsularis/ Corchorus olitorius) is mainly grown for its fiber and it is one of most important natural fibers after cotton. 105 It belongs to bast fiber and it is one of the cheapest fibers grown in tropical regions. History of jute dates back to 206 BC 221 AD; jute paper was discovered in Dunhuang, Gansu Province, China which was believed to be produced during Western Han Dynasty (Jute Paper from Western Han Dynasty). 106,107 Historical documents show that the jute fibers were used predominantly in India during the era of Mughal Emperor Akbar ( ). 108,109 The British East India Company traded jute from India. Since then, jute has been one of the chief fibers in terms of usage, consumption, production and availability. The jute fiber industry grew big during 1800 s in Dundee, Scotland. 108,109 The global production of jute fibers is around ton and major portion of it is contributed by India, China and Bangladesh, Table 1. The height range of the jute plant is between 2 4 m and the fibers are drawn by retting process. The jute fibers were studied to understand the fiber structure and the properties. 105 The chemical composition of jute fibers was studied by various researchers and the results show the range of cellulose content; 61% 71.5%, Table 2. Availability of the jute fiber in large quantities makes it popular reinforcement among German automotive manufacturers

20 Figure 8a. Weight Gain and Swelling in Thickness of Jute Composites at Different Humidity Levels and Immersed Water Conditions. 114 Reproduced with permission from Elsevier, Copyright Figure 8b. Effect of Humid Conditions on the Tensile and Flexural Strength of Jute Composites. 114 Reproduced with permission from Elsevier, Copyright

21 The tensile strength of the jute fibers varies from 393 MPa to 773 MPa according to different authors, Table 3. The modulus of the fiber is MPa and the elongation is %. Jute fibers absorb most water in bast fibers, Table 4. It is one of the most explored fibers for reinforcements; epoxy composites 111, PLA composites 112, polyester amide composites 113, phenolic composites 114,115 etc. Water uptake and morphological analysis of the jute fibers were performed. 115 Fiber length dependence upon impact strength in jute-based composites was studied by Razera et al. 115 Figure 8a and 8b show the effect of humidity on jute fibers. There was more than 500% growth in use of jute fibers for car door panels from the year 1996 to 1999 (from 4000 to tonnes. 110 Detailed review of jute fibers and its composites was done by Mohanty et al Kenaf Kenaf (Hisbiscus cannabinus) is cultivated in tropical regions mainly for its fiber and seed oil. It is primarily considered as central Africa and southern Asia crop, and today it is grown primarily in India and Bangladesh, Table 1. It is a new crop in USA and has shown good potential in biocomposite applications. It is believed that kenaf is cultivated since around 4000 BC. In ambient conditions the plant may grow up to 10 cm/day and matures in 3 months. 101,116 As it is fast growing crop it can annually yield up to kg/ha and latest varieties may yield kg/hg annually. The average plant grows 3 m tall with woody base diameter 3 5 cm. The bast (bark) contributes about 40% of the plant where the fibers are extracted and the remaining part is core wood. 117 The bark has high crystalline fiber orientation while the core wood has amorphous pattern. The stem diameter is often 1 2 cm. It has environmental and economic advantages; energy consumption to produce 1kg of kenaf is 15MJ while it takes 54 MJ for glass fibers and the cost of the fibers is considerably 21

22 low. 101 The properties of the kenaf fiber is similar to that of jute fibers and the structure of kenaf fiber is same as other natural fiber. The chemical composition of the kenaf fibers is given in Table 2; it has lowest cellulose content (31 39%) and highest lignin content (15 19%) among bast fibers. Several authors suggest using kenaf fiber as reinforcement in composites due to their superior toughness. 57, Table 3 shows the tensile properties of the kenaf fibers: tensile strength and modulus of about 930 MPa and 53 GPa repectively. The percentage elongation is around 1.6%. Several composites were made be reinforcing kenaf fibers in thermoset and thermoplastic matrix Chin et al. studied the potential of kenaf fiber composites in tribological applications 125 and Serizawa et al. considered these composites for electronic applications 126. Few other applications of kenaf fibers are discussed by Ramaswamy et al. 127 Chemical analysis of the fibers were performed to see the chemical composition of the kenaf seed oil and maturing of core/bast. 128,129 Properties of the fiber based on growing conditions were studied by Ogbonnaya et al. 130 Kenaf fiber and its composites are reviewed in detail by Akil et al Ramie Ramie (Boehmeria nivea) is the bast fiber that is native to Asia and today it is mainly produced in China and Brazil, Table 1. It has been grown in China for many centuries and commonly called as China Grass. 131 It is herbaceous perennial plant belonging to Nettle family and can be harvested 3 6 times a year. The plant last for around 7 20 years and grows 1 2.5m tall. 132 The presence of gum, pectin and other substances in the bark makes the chemical treatment mandatory before the fibers could be used. The fibers are coarse and have thick walls. 133 The global production of these fibers is around ton and it is lowest among bast fibers, Table 1. The ramie fiber has least amount of lignin content 22

23 amount bast fibers ( %) and has cellulose content of about %, Table 2. The low production and impure nature of the plant makes it difficult to consider as reinforcement for composite in large scale. Table 3 shows the tensile properties of the fiber: tensile strength and modulus is MPa and GPa respectively. The elongation is about %. Ramie fiber is stiffest among bast fibers, Table 3. The fiber yield of the perennial plant each year was studied by Hearle et al. 134 Physical properties of the ramie fibers were studied by Nam et al. 135 and the composites were prepared by same authors 136. Ramie fibers were reinforced in many thermoset and thermoplastic resins to produce biocomposites Ramie fibers are comparatively less explored than any other mentioned bast fibers due to above stated issues (availability and impure) and most of the ramie fibers are consumed domestically. Summerscales et al. and other researchers reviewed about bast fibers and use of these fibers as reinforcements in composites. 132, Few more bast fibers were also used as reinforcements for biocomposites, Table 5. The properties of these fibers, their reinforcement potential and the composite properties were not studied in detail. Table 5. Some more important fibers from plants. 14 Fiber Type Fiber Species 23

24 Bast China jute Isora Kudzu Nettle Roselle Sun hemp Cadillo/urena Wood Abutilon theophrasti Helicteres isora Pueraria thunbergiana Urtica dioica Hibiscus sabdariffa Crorolaria juncea Urena lobata (>10,000 species) Leaf Banana Cantala Caroa Date Palm Henequen Istle Mauritius Hemp Piassava Phormium Sansevieria Musa indica Agava cantala Neoglaziovia variegata Phoenix Dactylifera Agave fourcroydes Samuela carnerosana Furcraea gigantea Attalea funifera Phormium tenas Sansevieria Leaf fibers Seed/Fruit/Root Kapok Sponge Gourd Broom Root Ceiba pentranda Luffa cylinderica Muhlenbergia macroura 24

25 Abaca Abaca (Musa textilis) is predominantly grown in the Philippines, Ecuador and Costa Rica, Table 1. It belongs to banana family (Musaceae) and mainly cultivated for its fiber. It is native to Philippines and first large scale cultivation was done in Sumatra in 1925, later it was cultivated in America by funding of US Department of Agriculture. 142 The lifetime of the plant is around 10 years and the fibers are harvested periodically (two or three times a year) after initial growth. Cultivation, extraction and processing of abaca fibers are discussed in detail by Goltenboth and Muhlbauer. 142 The average height of the plant is roughly 4 meters with leaves from the trunk and the fibers are extracted from leaves. The chemical composition of the abaca fibers vary like other natural fibers: cellulose (56-63%), hemicellulose (20-25%) and lignin (7-9%), Table 2. Several authors have used these fibers are used as reinforcements in composites as it has good mechanical properties Other application include marine rope as it is resistant to salt water. The tensile strength and modulus of the abaca fiber is around 400 MPa and 12 GPa, Table 3. The percentage elongation is between 3 and 10%. Abaca fiber reinforced thermoset composites were produced and good tensile properties were achieved. 150,151 Similarly, thermoplastic composites reinforced with abaca fibers possessed good tensile properties Bledzki at al. compared the properties of abaca/pp composites with jute/pp and flax/pp composites. 148 Abaca/PP composites had comparable tensile strength to jute/pp and flax/pp composites. 148 Abaca/PP composites had higher flexural and impact strength than jute/pp and flax/pp composites. 148 The odour emission test done by Bledzki at al. showed that odour emission concentration is more than 2 times in abaca/pp composites when compared to jute/pp and flax/pp fibers which makes it difficult to be used in several indoor 25

26 applications. 148 Abaca fibers are vulnerable to pathogens and due to this the cultivation was affected severely Banana fiber which is very close to abaca fiber is reviewed by Jarman Curaua Curaua (Ananas erectifolius) is cultivated in Amazon regions (Brazil and Venezuela), mainly semi-arid areas. Brazil is the one country to produce curaua fibers for commercial purpose; it produced around 150 ton in the year The curaua plant leaves are hard and erect with flat surfaces and required minimum of 2000 mm of precipitation annually. The plant produces about leaves annually and the leaves grow about 1.5 m in eight months period while the dry fiber content is only about 5 8%. 157 The plant lifetime is about five years and each plant produce up to seven scions a year. 158 This fiber is often replaced by sisal fiber due to low production/cultivation of curaua and higher demand of natural fibers. Curaua fibers along with other fibers are used in interior and exterior automotive applications. 159,160 But the commercial use is quite limited considering the global production of only 150 tons and 5 8 wt% dry content. Marcia et al. studied the curaua fiber regarding density, diameter, tensile strength, moisture absorption, thermal behaviour, chemical composition and morphology. 161 Table 2 shows the chemical composition of the curaua fiber: cellulose (73.6%), hemicellulose (9.9%) and lignin (7.5%). The high cellulose content fiber of curaua is one of the reason for its high strength and this fiber is used as reinforcement in composites

27 Figure 9. SEM Image of Curaua Fiber with leaf residue adhered to surface. 162 Reproduced with permission from SAGE, Copyright The tensile properties of the curaua fiber are given in Table 3: tensile strength ( MPa), modulus (11.8 GPa) and elongation ( %). Two articles conclude that curaua is the best natural fiber for composite applications due to its performance. 157,163 Thermoset and thermoplastic composites reinforced with curaua fibers were prepared and the properties of the composites are discussed by several authors Curaua fiber production for industrial applications with respect to characterization and micropropagation was discussed by Leao et al. 172 Figure 9 shows the SEM image of the fiber with leaf residue adhered to surface. Curaua fibers were reinforced in recycled polypropylene and polystyrene and good composite properties were obtained. 169,173 Detailed viscoelastic analysis of the curaua fibers reinforced composites was done by Ornaghi et al. where cole-cole plots were used as identify the homogeneity of the system. 174 Furthermore, curaua fibers are reinforced in cement for construction applications by d Almeida et al. 175 Curaua fibers has good potential to be used as reinforcement in biocomposites but the low amount of production hinders the use of fibers in industrial scale and often replaced by other plant fibers. Brief review of curaua fiber and its composites was done by authors. 161,162,176 27

28 Pineapple Pineapple (Ananas comosus) plant is indigenous to South America but today it is primarily grown in Southeast Asian countries such as Philippines, Thailand and Indonesia. It is also grown widely in Costa Rica, Chile, Brazil and India. The botany, cultivation and utilization of the pineapple were discussed by Collins. 177 Pineapple fiber is obtained from the leaves of the plant, Pineapple leaf fiber (PALF). The plant is about m tall and has leaves which are cm long. Each plant may have 30 or more leaves which are dark green in colour, sword shaped and bear spines. 178 The fibers are extracted by mechanical or retting process and it has been spun to fine yarn. 178 The pineapple fibers are inexpensive and it is not the primary product; fibers are extracted from leaves which were considered to be waste and now it can be used for industrial applications. PALF has cellulose content of 70 82% and lignin content of 5 12%, Table 2. PALF are limited due to low production which makes it difficult to be used in industrial applications. The high cellulose content and low microfibrillar spiral angle (14 deg) are associated with good tensile properties. 178 Tensile properties of the pineapple fiber is given in Table 3: tensile strength ( MPa), modulus ( GPa) and elongation (1.6%). PALF has highest modulus among leaf fibers and good quality fiber modulus can be compared to manmade fibers such as Aramid and glass, Table 3. The tensile strength of good quality PALF is highest among leaf fibers, Table 3. On the other hand, PALF has lowest elongation at break among leaf fibers (1.6%), Table 3. Like other plant fibers, PALF also reinforced in thermosets and thermoplastics Matrix from natural sources were reinforced with PALF to produce green composites. 179,184 Thermal properties such as thermal conductivity and thermal diffusivity of pineapple fiber reinforced phenolformaldehyde composites were studied using 28

29 transient plane source (TPS) technique by Mangal et al. 185 Inclusion of fiber decreased the thermal conductivity and thermal diffusivity of the composite, and this is applicable to all plant fiber composites. 185 The X-ray analysis and di-electric properties were done to see the crystallinity of the fibers, orientation of spiral angle and dielectric behaviour. 186,187 It was found that the PALF had high degree of crystallinity (0.63 to 0.68), low microfibrillar spiral angle (14 deg), dielectric properties was comparable to that of jute fibers The properties and structure of PALF were studied in detail by researchers. 67 The pineapple leaf fibers and its composites are reviewed by Mishra et al Sisal Sisal (Agave sisilana) is commercially crop produced mainly in Tanzania and Brazil, Table 1. It is also widely cultivated in China and Kenya. It is believed that sisal is native to Central America and its fiber was used already at pre-columbian times. 189 The sisal plant was explained in terms of botany, cultivation and utilization by Lock. 190 The plant grows about 1 m tall and 28 mm wide with leaves. The leaves are dark green in colour, rigid, fleshy and lance-shaped and grown in a rosette from the stalk. 189 The lifetime of the plant is about 7 10 years and after mature period the fibers are extracted from leaves; each leaf has about 1000 fiber bundles of which only 4% is fiber. 191 The fibers are traditionally used for rope, twine and as textile fiber. Decortication is common process by which the fibers are extracted. These fibers were classified into three types based on the place of extraction, namely mechanical, ribbon and xylem. 75 The mechanical fibers have highest strength among three while xylem fibers has the lowest strength, and this is because mechanical fibers are extracted from periphery of the leaf and has defined shape whereas xylem fibers are irregular in shape and has thin walled cells. 75 Ribbon fibers are intermediate fibers which are 29

30 extracted from conducting tissues in the median line of the leaf; have considerable mechanical strength. 75 The sisal fiber dimensions and their mechanical properties were studied by Bisanda et al. 192 The fiber extraction was studied by several researchers. 191,193 Table 2 shows the chemical composition of sisal fibers: cellulose (67 78%), hemicellulose (10 14%) and lignin (8 11%). These fibers are widely used as reinforcement in composites. 58,75,178,192 Several authors came up with different tensile properties of sisal fibers and this variation was expected in plant fibers. Table 3 shows the tensile properties of the sisal fiber: tensile strength ( MPa), modulus ( GPa) and elongation (3 7%). The sisal fibers are reinforced in thermoset and thermoplastic matrix like other plant fibers; and mechanical, thermal, viscoelastic, dielectric, water absorption and morphological properties of the composites were published Morphological analysis is very common among natural fiber composites research to study interfacial adhesion, pores, fiber pull-out etc. Evaluation of fiber pull-out studied in detail by Sydenstricker et al. 203 Oksman et al. studied scanning electron microscopy images of sisal-epoxy composites. 194 The sisal fiber and its composites are reviewed by many researchers. 58,178,204 Other leaf fibers (Table 5) such as banana, henequen etc. were also used as reinforcement in composites. The characterization of the fibers, properties of the fiber, reinforcement potential and composite properties were studied briefly Seed/Fruit fibers Coir 30

31 Coir (Cocos nucifera) is natural fiber extracted from coconut husk; mainly produced in India and Sri Lanka (Table 1). Coir is the by-product of coconut and it is inexpensive material in local markets. 12,16 The coconut refers to the entire fruit which has hard inner shell and outer fibrous material. The fibrous layers are separated either by manual dehusking or through crushing machine. 212 The fibers are white in colour when the coconut is unripe (white fiber) and the colour changes to brown when coconut ripens (brown fiber). 212 These fibers are mainly used to make ropes, twines, doormats, mattresses, brushes etc. The coir fibers have cellulose (36 43%), hemicellulose ( %) and lignin (41 45%), Table 2, but the chemical composition changes with maturity. These fibers have highest lignin content among natural fibers, Table 2. A coir fiber cell is narrow, hollow, stiff and coarse with thick walls made up of cellulose. 212 The coir fiber is the only plant fiber which is relatively waterproof and absorbs low amount to salt water Low water absorption can be related to lower cellulose content and high amount of lignin. Table 3 shows that the coir fiber has low tensile strength and a modulus of about MPa and 4 6 GPa respectively, but it has high elongation (15 40%). These fibers can be stretched beyond its elastic limit without rupture due to microfibrillar spiral angle (45 ). Water absorption of coir fiber was explored by several researchers as it is the lowest moisture absorption among plant fibers The coir dust was studied by Prasad with respect to physical, chemical and biological properties. 218 Satyanarayana et al. studied the coconut fibers with respect to density and electrical resistivity. 219 Various matrix were reinforced with coir fibers and the composite properties were published: viscoelastic 220, physic-mechanical 212, , thermal 69,223, electric 224, morphological 225,226 and sound absorption 227. Varma et al. studied coir fibers and its composites in detail. 66,228 31

32 Cotton Cotton (Gossypium) is the most famous natural fiber which is native to tropical and subtropical regions around the world. Today, China, India and United States are the top three cotton producers in the world. The cotton fibers have been used since 5000 BC and woven cotton fabrics are known from 900 to 200 BC. 229,230 The cotton fibers are grown in a protective capsule (boll) in the cotton plant and harvested on maturity. The cotton crops and fibers were widespread between 2000 and 1000 BC. 231 It is predominantly used as textile fiber; the growth and the demand for the cotton have been increasing. The development of cotton fibers, their growth kinetics, cytology of early fiber development, chemical changes during fiber development and cell wall were studied by Basra et al. 232 During the fiber development, it undergoes striking amount of elongation; 1000 to 3000 times longer than their diameter. 232 Genetically modified (GM) cotton crops were developed to reduce the dependence on pesticides; its effect on farmers health 233 and economic impact 234 were studied. The chemical composition of the cotton fiber is given in Table 2: cellulose (82.7%) and hemicellulose (5.7%). The cotton plant produces one of the purest forms of cellulose known to man. 232 The hydrophilicity of the cotton fibers are high due to large amount of cellulose (OH groups). The compatibility between hydrophilic fibers and hydrophobic matrix is poor; which makes fiber surface modification necessary for technical applications that involve hydrophobic matrixes. The tensile properties are given in Table 3; cotton fibers have tensile strength between MPa, modulus between GPa and elongation between 7 8%. Moisture absorption and release performance of knitted fabrics made from composite yarns were studied by water absorption capacity, diffusion rate and drying rate. 235 The performance of 32

33 cotton based composites were studied by Mwaikambo et al. 236,237 The comparison of the mechanical properties of wood and cotton fiber composites were studied, and the properties of cotton based composites were improved by increase in fiber % in the composite while the trend in wood based composites were contrary. 238 This was due to the better interface between fiber and matrix in cotton based composites than wood based composites. Nonwoven cotton-based composites were produced for automotive applications by Kamath et al. 239 Cotton fibers are used in many value added products and the demand for the fibers have increased which makes the fibers expensive. Recycled cotton blend fabrics were used as reinforcement to reduce the cost. 240,241 Cotton fiber composites were studied by Mueller et al Oil Palm Oil Palm (Elaeis guineensis) is the West African tree that is the main source of palm oil. It is closely related to American oil palm, Elaeis oleifera, which is also used for oil production. African oil palm is native to west and southwest Africa, mainly between Angola and Gambia whereas the American oil palm is native to Central and South America. The use of oil palm dates back to 3000 BC in Africa. 243 Today, Elaeis guineensis is the highest yielding edible oil crop and mostly cultivated outside African continent; Malaysia and Indonesia produce most of the world supply. 244 Oil palm fiber (OPF) is secondary product which is extracted from empty fruit bunches which were used as reinforcement in biocomposites; it is also possible to extract fibers from other parts of the tree but the yield is very low compared to that of fruit bunch. 245,246 Empty fruit bunches create waste disposal problems, while fibers can be extracted from this waste. The retting process is used to extract the fibers and water retting is most common. 247 The more sophisticated machine extraction was developed by Jayashree 33

34 et al. 248 The chemical composition, anatomy, lignin distribution and cell wall structure was studied in detail by Abdul Khalil et al. 244,249,250 These fibers are hard and tough and properties similar to coir fibers. 250 The OPF consists around 65% cellulose and 29% hemicellulose, Table 2. OPF has tensile strength about 248 MPa, modulus 3.2 GPa and elongation 25%, Table 3. The environmental exposure of plant fiber composites has big impact on mechanical properties of the composites and this was studied by Hill et al. 251 The effect on the mass loss, tensile and flexural properties were evaluated and it was found that mechanical properties fell drastically. Water sorption of OPF reinforced composites was studied by Sreekala et al. 252 Viscoelastic analysis of OPF composites was studied by Jacob et al. and storage modulus was improved. 253 Figure 10a and 10b show the SEM images of OPF. Unlike cotton fibers, OPF has only few applications such as ethanol production 254, gasification 255 and biocomposites. Many researchers have prepared composites reinforcing OPF in various thermoset and thermoplastics; a detailed review was done on OPF and its composites by Shinoj et al. 256 Figure 10a. SEM Images of Transverse Sections of OPF(4x) F: fiber, L: lacuna. 256 Reproduced with permission from Elsevier, Copyright

35 Figure 10b. SEM Images of Longitudinal Section of OPF (750x). 256 Reproduced with permission from Elsevier, Copyright Other leaf/fruit fibers such as kapok 236,237,257,258 and sponge gourd were used as reinforcement in biocomposites Grass Bagasse Bagasse is the fibrous residue that remains after sugarcane stalks are crushed to extract juice. It was principally a waste material that causes disposal cost for sugar mills. Wet bagasse consists 30% of sugarcane which means 3 ton of bagasse is produced for each 10 ton of sugarcane. The bagasse is comprised of 49% moisture, 48.7% fiber and the rest soluble solids. 264 Currently, bagasse is used as raw material in some applications such as bioethanol , combustion , cellulose source for dissolving pulp, value-added products 274 and biocomposites. The low calorific value and low level of sucrose left in the bagasse makes energy and bioethanol productions as low efficiency process. The world bagasse production is high ( ton) and mainly produced in Brazil, India, South Africa and China, Table 1. The cellulose content in bagasse fiber is around 55.2% while hemicellulose and lignin content is about 16.8% and 25.3% respectively, Table 2. 35

36 The bagasse fiber has inferior tensile properties when compared to bast and leaf fibers; strength (290 MPa) and modulus (17 GPa), Table 3. Different composite processing like extrusion, compression molding etc. were adopted to produced bagasse reinforced thermoset and thermoplastic composites The comparison between compression and injection molding with respective to mechanical behaviour and microstructural analysis of bagasse composites was done by Luz et al. 276 Preparation and properties of recycled PE/bagasse composites were studied by Lei et al. 279, impact properties of bagasse composites was studied by Stael et al. 278 and dimensional stability of esterified bagasse composites was analyzed by Hassan et al. 280 Bagasse reinforced cement composites were prepared and analyzed by Bilba et al. 281 The potential of the bagasse fiber and its composites were studied in detail. 264, Bamboo Bamboo (Bambusa Shreb) is a perennial plant mainly grown in Asian countries such as India, China and Indonesia, Table 1. It has more than 1250 species and the high quality bamboo is stronger than steel; which is used as building material. 285 It belongs to grass family and the fibers are extracted in two kinds; natural original bamboo fiber and bamboo pulp fiber (regenerated bamboo viscose fiber). The world production has reached ton and used in textiles and paper industry Microstructural characterization of bamboo was studied by Ray et al. where fracture of fiber and cross-section of the fibrils was shown. 289 The chemical composition of bamboo fiber is given in Table 2; cellulose (26 43%), hemicellulose (30%) and lignin (21 31%). The tensile properties of the bamboo fiber are given in Table 3; strength ( MPa) and modulus (11 17 GPa). Green composites were prepared by reinforcing bamboo fibers in 36

37 polylactic acid (PLA) and their properties were evaluated. 290 Thermal conductivity of bamboo/pla were evaluated by Takagi et al. and it was found out that these composites have lower thermal conductivity compared to that of woods. 291 Xiao et al. studied the design and construction of modern bamboo bridges. 292 Computer simulations of the field test were done and it showed the trend of midspan deflection. 292 Modern bamboo structures were discussed by several authors in conference and a book was published based on several papers. 293 The durability of bamboo-based fibers were studied by cyclic loading and results showed that the composites were produced with good fatigue resistance. 294 Mechanical properties, crystallization and interfacial morphology of bamboo/pp composites were studied by Chen at al. 295,296 A detailed review of bamboo fiber reinforced structural concrete elements was done by Ghavami. 297 Bamboo fiber and its composites were reviewed in detail by Khalil et al. 298 The review based on structure and properties of bamboo fiber and its composites was also done Straw Straw is an agro-residue or by-product obtained from cereal plants when grain and chaff have been removed. The straw is obtained from several cereal crops such as rice, wheat, barley, oats and rye. It is normally gathered and stored in a bale (straw bale) and it is used in various applications: Animal feed 300, biofuels 301, biomass 302, construction material 303, paper 304, packaging 304 etc. The straw is also gaining importance in biocomposite research Rice Straw Rice is the most widely consumed staple food in the world, especially in Asia. The genetic evidence show that the rice originated between 6200 and BC. 305 It is usually annual plant which is native to Asia; and today it is grown worldwide. Straw makes up about half of 37

38 the yield of rice. Rice production, morphology, growth, development and harvesting was clearly discussed by Datta. 306 After harvesting, the rice is separated from the straw either manually or by machine process. The rice straw consists of 41 57% cellulose, 33% hemicellulose and 8 19% lignin, Table 2. This lignocellulosic fiber could be used as reinforcement in biocomposite production. Rice straw was mainly reinforced in thermoplastic polymers such as polyvinyl chloride, polyhydroxybutyrate, polyethylene, polypropylene Tensile, impact, bending and viscoelastic properties of rice straw based composites were evaluated The tensile strength and modulus of rice straw fibers was 74.6 MPa and 3.3 GPa respectively. 311 Although, it is very low compared to other plant fibers, it could be used in low stress applications. It was concluded that straw-based composites have potential to be used as core reinforcement material for structural board products. 311 Rice straw based composites were also tested for acoustic properties and found that these composites could be suitable for sound absorbing insulation material in wooden construction. 312 Rice straw can partially replace the wood particles in the composites without reducing the bending strength Wheat straw Wheat is third most produced cereal after corn and rice with annual production of about 607 million ton. 313 Wheat is grown worldwide while USA, China and India being the major producers. 313 It is food crop which could also be used to produce alcoholic beverages and biofuel. The wheat production, utilization, development and harvesting was discussed in detail by Gooding and Davies. 314 Grains are separated from straw on harvest and the straw is considered as by-product or residue. Traditionally, wheat straw is used as cattle feed and construction material. 303 Table 2 shows the chemical composition of wheat straw: cellulose 38

39 (39 45%), hemicellulose (15 31%) and lignin (13 20%). The researchers have used this lignocellulosic material to produce biocomposites. These fibers were reinforced in polypropylene and their dispersion was studied with increase in the fiber loading. 315 Mechanical properties, flowability and fracture surface morphology of the composites were evaluated. 315 Furthermore, wheat straw/pp and jute/pp composites were compared; superior properties were obtained by lightweight wheat straw/pp composites. 316 Morphology (Figure 11) and thermal properties of wheat straw based biocomposites were studied and it showed that the composites could be used in packaging and medical science. 317 A detailed study of wheat straw/phbv composites was done by Avella et al. where mechanical, thermal and biodegradation properties of the composites were explored. 318 The rate of PHBV crystallization increased on addition of wheat straw fibers due to nucleating effect and these fibers did not reduce the biodegradability rate. 318 Wheat straw was used as reinforcement material by other researchers and the composite properties are discussed High-performance composite panels were produced by Han 320 and mechanism of bondability between fiber and resin was discussed Barley straw Barley is fourth most produced cereal crop in the world and the world s biggest producers are Russia, Ukraine, France and Germany. Barley production, harvesting, processing and utilization were discussed in detail by Shands et al. 321 Ullrich discussed about the improvement and uses of barley. 322 It is food crop which is also used as animal fodder, beer production, distilled beverages and health foods. Large quantities of barley straw were not used until recently but the potential of the barley straw has been explored for energy production and composite material preparation. Researchers used barley straw as 39

40 reinforcement in biocomposites. 323 The chemical composition of the barley straw was studied; cellulose (48.6%) and lignin (16.4%). 324 Bouhicha et al. investigated the barley reinforced composite material for construction in Algeria; the effect of fiber length and fiber fraction on shrinkage was studied along with mechanical properties. 323 Thermal conductivity of the composite was studied and the results showed that straw lowers the thermal conductivity. 325 The comparative study of mechanical properties between wheat and barley straw fiber reinforced composites revealed that barley fiber composites had lower compressive strength than wheat straw fiber composites. 325 Figure 11. SEM Images of the Wheat Straw (a) Cross Section (b) Microfibers (c) TEM Image of Wheat Straw Nanofibers. 317 Reproduced with permission from Elsevier, Copyright

41 White et al. studied straw-reinforced polyester composites. 324 Structural and thermal characterization of various straws such as rice, wheat, rye and barley were studied by Sun et al Husk Rice husk Rice husk (or rice hull) is the hard protecting cover of the rice grain. After rice harvesting, the grain together with husk goes further processing to remove the hull. The husk separation was manual process until late 1800 s, after which modern rice hulling machine was invented. The rice husks could be used as filler in construction, insulation material, fertilizer or fuel. The husk is formed from hard materials including silica and lignin. Amorphous reactive silica is obtained on burning rice husk (ash) which could be used in various applications in material science. 327 The chemical composition of rice husk is given in Table 2: cellulose (35 45%), hemicellulose (19 25%) and lignin (20%). Rice husk based composites were prepared and their properties were evaluated with respective to hygrothermal aging, mechanical, morphological and thermal properties The study showed that the potential of rice husk as filler was lower than that of talc as the processing of talc composites were easier than rice husk composites. 329 The talc composites showed higher tensile strength, E-modulus, flexural modulus and impact strength than rice husk composites. 329 Similar inferior properties of rice husk composites was obtained by Yang et al. 330 The enhancement of processability of rice husk/pe composites was investigated by Panthapulakkal et al. 332 ; water absorption, swelling, density and extrusion rate were studied. The limitation in mechanical properties makes the rice husk not to be used as fillers in several applications. 41

42 Figure 12. TGA Analysis of Barley Husk, Coconut Shell and Softwood. 333 Reproduced with permission from Elsevier, Copyright There was limited research on composites based on husk from other grains; Barley Husk 333, Corn Husk 334,335, Wheat Husk 336 and Rye Husk 336. The moisture absorption of the barley husk was lower than the coconut shell which makes barley husk as possible reinforcement for moisture sensitive applications. 333 The tensile strength and impact properties of barley husk composites were better than coconut shell and softwood composites. 333 Figure 12 shows that the thermal stability of barley husk (235 C) was comparable to softwood fiber (245 C) and it was higher than coconut shell (195 C) which makes husk as possible alternative filler for softwood in composite applications. 333 Figure 13 shows the comparison of particle size distribution of barley husk with coconut shell and softwood. Elemental and FTIR analysis of barley husk are shown in Figure 14a and 14b. Huda et al. used corn husk as fillers in biocomposites where the tensile, flexural and impact properties were investigated and compared to jute composites. 334 Bledzki et al. studied the thermal, physical, chemical and surface properties of wheat and rye husk composites. 336 It was found that the rye husk (180 C) has lower thermal stability than wheat husk (205 C). 336 Surface morphology of these 42

43 two husks were studied by SEM which showed that rye husk had smoother surface than that of wheat. The chemical composition of these husks was investigated and the results showed that wheat husk had higher cellulose (36%) and lignin content (16%) than that of rye (cellulose-26%, lignin-13%). The tensile and impact strength of wheat husk composites were better than the composites with rye husk fillers. 336 Figure 13. Particle Size Distribution of Barley Husk, Coconut Shell and Soft Wood. 333 Reproduced with permission from Elsevier, Copyright Figure 14a. Elemental Analysis of Barley Husk. 333 Reproduced with permission from Elsevier, Copyright

44 Figure 14b. FTIR Analysis of Barley Husk and Coconut Shell. 333 Reproduced with permission from Elsevier, Copyright Wood fiber There are two types of wood reinforcements, namely, softwood (gymnosperm) and hardwood (angiosperm). Harwood is from deciduous tree which loses its leaves annually while the softwood is from conifer which keeps their leaves throughout the year. Hardwoods are denser than softwoods with exceptions like balsa wood. Growth rate of softwood is faster than that of hardwood. Both of these are used for several applications but hardwood is less frequently used than softwood Softwood Softwoods such as Pinus (Pines), Picea (spruces) and Larix (larches) are relatively less complex than hardwood in terms of cells present. All softwood species have two main cell types, longitudinal tracheids and ray parenchyma. It might contain resin canals, ray tracheids and longitudinal parenchyma depending on the species. The softwood anatomy is straight 44

45 forward in most cases as 90% or more of softwood volume is composed of longitudinal tracheids which transports water and gives mechanical strength to the wood Hardwood Hardwoods (aspen, birch) are more complex when compared to softwood. Unlike softwood, the roles of conduction and support are carried out by different cells in hardwood. The presence of specialized water-conducting cells, vessel elements, is a primarily feature that distinguishes hardwood from softwood. These vessel elements are narrow tubes and open at the ends, and called as pores; therefore, hardwood is called as porous wood. Hardwood has ray parenchyma which varies in the structure and dimensions in different species. It may contain longitudinal parenchyma and other cells depending upon the species. The lignin content varies between these two types of wood; spruce (softwood) 28% and aspen (hardwood) 18%. Softwood is preferred for composite applications as it has higher aspect ratio than hardwood. The properties and cell types of both softwood and hardwood was discussed in detail. 337 Morphological analysis was done on hardwood and softwood. 338 The composite processing with wood reinforcement and the mechanical properties of the composites were discussed. 337,338 The wood reinforcement were mostly reinforced in thermoplastic composites, especially PP Softwood composites had better stiffness than hardwood composites and this could be due to higher lignin content in softwood compared to hardwood. 342,344 But hardwood composites showed better tensile strength, impact strength and elongation which could be attributed to higher cellulose content. 342 A comparative study of wood fiber reinforced PLA and PP composites showed that fibers dispersed better in PLA than PP. 343 Fiber morphology and composite properties were also investigated. 343 The wood fibers were 45

46 reinforced in HDPE and the composite properties were studied. 341 Detailed review of wood composites was done by review articles and books. 338, There are other plant fibers which were used in composite production and some of them are listed in Table 5. The most commonly used fibers are described in detail and out of these bast and leaf fibers are widely preferred in industrial applications. These plant fibers were well explored in terms of chemical composition, mechanical and thermal properties, reinforcement potential. There are many books which describes the potential of the plant fibers in composites. 337, The industrial applications of natural fibers with respect to structure, properties and technical applications were discussed. 351 Several fibers, especially bast and leaf fibers, and its composites were reviewed previously ,66,156,161,162,176,178,204,228,242,256,264, ,299,345 General review on plant fiber and its composites was done by several researchers. 13,16-19,22-26,139,140 Bledzki and Gassan have reviewed the plant fiber reinforced composites until Faruk et al. have reviewed the plant fiber reinforced composites from 2000 to Animal fibers Animal fibers such as silk, wool and chicken feathers is the second most important source of natural fiber after plant fibers for reinforcement in composites. Researchers have analyzed these fibers potential as reinforcement in composites. 351 Silk and wool are used in textile industries for many purposes and it could be expensive to be used as reinforcement in composites but chicken feathers are waste which are obtained from slaughter house. The chemical composition, mechanical and thermal properties of these fibers are crucial to be used as reinforcement. Like the plant fibers, there is a variation in the composition and the properties of animal fibers which could be an issue. There are several journal articles about 46

47 each of these fibers as reinforcement but there is no review paper on animal fibers as reinforcement till date Silk The potential of silk is outstanding due to their structure and properties. It consists of highly structured proteins and wide range of properties; high tensile strength, high elongation and resistant to chemicals. 351 Silk is obtained from several sources and their properties vary; several insects and most of the larvae of butterfly species (approx Lepidoptera) produce silk during metamorphosis. 352 The silk is also obtained from spiders (approx species) and they can have seven different types of silk with unique properties in their lifespan. 351 Out the these fibers, Mulberry silk (Bombyx mori) from silkworm and dragline silk (Nephila) from spider were widely discussed. Table 6. Properties of two types of silk. 351, Fiber Degree of crystallinity Maximum use temperature Thermal (%) ( C) degradation ( C) Mulberry Silk Dragline Silk Table 7. Tensile strength and extensibility of two types of silk. 357 Fiber Tensile Strength (GPa) Extensibility (%) Mulberry Silk

48 Dragline Silk Figure 15. (a)raw Cocoon silks (b) Side view of silk fiber. 21 Reproduced with permission from Elsevier, Copyright The mulberry silk (Bombyx mori), Figure 15, is most commonly used silk and it is the only silk that has commercial applications. The two main components of mulberry silk are fibroin (fiber) and sericin (gum). Secricin accounts about one-third of mulberry silk which are washed off and the fibroin is used as fiber in several applications. The fibroin consists of two types of amino acids: light chain and heavy chain. 351 Mulberry silk fiber is semi-crystalline and the degree of crystallinity is between 38 and 66%, Table 6. The fibers could be used below 170 C without degradation of fibers and thermal degradation occurs at 250 C, Table 6. This fiber is stable against many chemicals; insoluble in most alcohols and acetone, resistant against mild acids and absorbs less water. The structure, crystallinity and chemical properties of mulberry silk was discussed in detail. 351 The production, extraction, processing and applications of mulberry silk were explained. 351 The mulberry silk has tensile strength of 0.6 GPa which is higher than most of the plant fibers, Table 7. Mulberry silk was reinforced in different blends of HDPE, LDPE and natural rubber. 358 The tensile, tear and morphological properties of the composites were analyzed. 48

49 The dragline silk (Nephila) is one of the silk that is produced by spider in its lifespan. This fiber consists of two highly repetitive proteins, namely, spidroin I and spidroin II. It is the toughest and biggest fiber in the Nephila spider s silk set. 351 It is semi-crystalline and exhibits lower degree of crystallinity than mulberry silk, Table 6, and the fibers are stable up to 150 C above which degradation starts. The tensile strength of dragline silk fiber is higher than that of polyamide fibers and comparable strength to several synthetic fibers, Table 7. It has tensile strength of 1.1 GPa which is comparable to high-tensile steel (1.5 GPa) while the density of the dragline silk fiber is very less. The extensibility of the dragline silk fiber is about 30% while steel has only 0.8%. High tensile strength and extensibility makes this fiber unique and there are attempts to make fibers with these qualities. Energy dissipation and mechanical hysteresis are important properties of dragline silk fiber and the results show 65-68% of stretching energy is dissipated. 351 Detailed study was done on structure, properties, influence of water and temperature, production, extraction and processing. 351 Mechanical, thermal and morphological properties of dragline silk reinforced composites studied by Cunniff et al. 354 High strength and elongation makes it potential reinforcement in composite applications. The biggest drawback of these fibers is that it cannot be produced in large quantities due to practical reasons. 351 The silk fibers, their properties and their applications are discussed by several authors The silk reinforced composites have not been in the interest of many due to high cost and low availability. Low availability of the fibers makes it difficult to produce composites in industrial scale Wool 49

50 Wool fiber is commonly a textile fiber which is obtained from sheep, goat, camel, rabbit and certain other mammals; while sheep wool is the most commonly used in commercial scale. The annual production of sheep wool is approximately 1.2 million metric ton and 90% is consumed by textile industry. 363,364 The major producer is Australia, New Zealand and China. The wool is cleaned to remove the wool grease before it is used as textile fiber. It is mainly composed of keratin, animal protein. 365 Keratin is more susceptible to chemical damage and unfavourable environmental conditions than the cellulose in the plant fibers. 365 Wool s scaling and crimp make it easier to spin the fleece which is used in textile fabrics. It is mainly used for insulation purpose as it holds the air and retains the heat. Wool fibers are hydrophilic like plant fibers and absorb water one-third of its weight. 366 The nature, fineness and colour of the wool fiber changes with breed types but the most common wool is creamy white in colour. The thermal properties of the wool fibers are different from cotton and other synthetic fibers; rate of flame spread is low, rate of heat release is low and heat of combustion is low. A char which is self-extinguishing is formed during combustion. Wool fiber is coarser than other textile fibers such as cotton and silk; and the length of the fiber is dependent on coarse behaviour. 365 The breathability, heat generation, structure, felting, shrinking and thermal insulation of wool are discussed in detail Arunkumar et al. 366 It was suggested that the waste wool could be used as reinforcement instead of virgin wool due to high cost of virgin material. 366 Wool industries produce lot of waste which could be used as reinforcement/filler in manufacturing composite. Conzatti et al. produced composites by reinforcing short wool fibers in polypropylene fibers. 367 Morphological, thermal and mechanical properties of wood fiber composites were analyzed. Similar research was done by reinforcing raw wool fibers in polyester resin and their properties were studied and compared to theoretical models. 368,369 Homogeneous distribution of wool 50

51 fibers in composites was achieved by melt blending. 369 The wool fibers could be potential reinforcement for composites for certain applications Chicken feather Chicken feather, Figure 16, is waste from poultry industry and it constitutes 4 billion pounds annually in the Unites States. 370 These feathers are dumped in landfills at a cost of $30/ton or sold at low price ($250/ton) as livestock feed. 370 The poultry feather cannot not be used as feed in European Union as it banned the practice since 2001 due to health concern. The high volume of feathers could be consumed by composite industries which cannot be consumed by many other industries. It is reported that chicken feathers exhibit unique properties such as low density, good thermal and acoustic insulation. 370 The chicken feathers consists 91% keratin (protein), 1% lipids and 8% water. 366 The chemical composition, elemental analysis, morphological structure, aspect ratio, apparent specific gravity, chemical durability and thermal insulation of these feathers were studied Arunkumar et al. 366 Mechanical properties and the structure of the feathers were reported previously. 371 The study shows the good potential of these fibers as reinforcements. The chicken feathers were used as whole fibers or processed to separate keratin fibers which were used as reinforcements. 372 These feathers were used as reinforcement by several researchers The whole chicken feather reinforced polypropylene composites showed higher tensile strength, E-modulus and flexural strength than processed feather composites. 372 They also have higher sound absorption than plant fiber composites. 372 Mechanical and thermal properties of PLA composites reinforced with chicken feathers were studied by Cheng et al. 373 The acoustic and electrical properties of chicken feather reinforced composites were studied in detail. 374,375 The study revealed that the composites can be used 51

52 for printed circuit boards (PCB). 375 Figure 17 shows the microscopic image of chicken feather. Detailed study of chicken fibers and their composites was done by Kock. 370 Figure 16. Typical chicken feather fiber. 21 Reproduced with permission from Elsevier, Copyright

53 Figure 17. (a) Flight chicken feather fiber (b) Down chicken feather fiber (c) chicken feather fiber (d) Cross section of chicken feather fiber. 21 Reproduced with permission from Elsevier, Copyright The fibers are obtained from hair, fur and feather of various animals and birds which could be used as reinforcement in composites. The potential of human hair as reinforcement has been explored and their composite properties were presented Regenerated fibers Regenerated fibers are produced from natural source with human interference. Several regenerated cellulose fibers were produced from wood pulp such as Lyocell, viscose, rayon and modal which has been used as reinforcement in composites Detailed discussion on history, processes, properties, applications and market of regenerated cellulose fibers was done by Woodings. 47 The advancement in biotechnology led to the production of regenerated silk fibers. 377,378 The wood based regenerated fibers have been used to produce fiber reinforced composites while regenerated silk fibers were used for biomedical applications such as scaffolds Lyocell Lyocell fiber is produced by direct dissolution of wood pulp in N-methylmorpholine N-oxide (NMMO). 380 It cannot be utterly called as synthetic fiber as these fibers are produced from biomass origin. The latest techniques allow the recycling of the solvent which could make the process more environmentally friendly in the near future. 381 Initially, it was developed as Tencel by Courtaulds Fibers and sold the fibers to public as type of rayon as it is produced by similar process. Currently, Tencel is owned by Lenzing AG and it is only producer of these 53

54 fibers. 381 Lyocell exhibits some unique characteristics such as soft, absorbent, wrinkle resistant, strong when dry or wet and can be easily washed. 381 It possesses good textile properties; it drapes well, different color dyes can be used, texture can be altered to get suede, leather and silk finishing. 381 It is distinctive fiber as it exhibits beneficial characteristics of both synthetic and natural fibers; uniform mechanical, morphological and physical properties of synthetic fibers while biodegradability, non-abrasiveness to equipment and low density of natural fibers. 382 Lyocell fibers overcome the issue of plant and animal fibers as it is highly pure, uniform and the properties are reproducible. 383 This is one of the main reason that the Lyocell could be used as reinforcement in composites which demands even and reproducible properties. The chemical composition of Lyocell change largely with the wood pulp used. Pulp containing high α-cellulose (over 90%) or high hemicellulose (over 20%) could be used to produce the fibers. 384 The cost of pulp from high hemicellulose is cheaper when compared to the high α-cellulose pulp. 384 The crystallinity changes due to above reason and several authors have reported different crystallinity. 49,50 Generally, high α-cellulose pulp has been used to produce Lyocell which in-turn gave high crystallinity (80%). 48,50 The Lyocell fibers exhibits high tensile properties; strength 1400 MPa, modulus 36 GPa and elongation 6%. 385 Lyocell fibers have been used as reinforcement to produce biocomposites and their properties were studied. 48,51,52,382, Mechanical, morphological and water absorption properties of the Lyocell composites were studied and compared to flax composites. 48 Lyocell hybrid composites were prepared to improve the properties of the composites. 51,52,382 The biodegradability of the composites reinforced with Lyocell were studied by Shibata et al. and it was found that Lyocell/PLA composites have high biodegradability. 386 Main drawback of Lyocell fibers is that it is very expensive when 54

55 compared to natural fiber such as cotton or regenerated cellulose such as viscose. It is used mainly in textile industries to make every day fabrics. The properties of these composites are promising which makes Lyocell as potential reinforcement for composites. 3.2 Viscose Viscose is another important regenerated cellulose fiber in terms of production volume which is prepared by treatment of cellulose with sodium hydroxide and carbon disulfide. These fibers are produced from biomass origin similar to that of Lyocell. Viscose was produced and patented by Cross et al. in It was also widely called as artificial silk. Global viscose production increased from 14,000 in 1920 to 225,000 in 1931 ton/year mainly due to the expiration of viscose patents and more than 100 companies started producing this fiber. 389 These fibers are used in industrial applications such as tyre cords and textiles. The use of viscose increased even after emergence of Lyocell and reached 3.5 million ton in It was weaker than cotton and tends to shrink more easily. It lacked bulk, resilience and abrasion resistance when compared to wool fiber. 389 Similar to Lyocell, it exhibits beneficial characteristics of both synthetic and natural fibers. It has good properties such as silk-like aesthetic, different color dyes can be used and drapes well. The viscose fiber could be used as reinforcement due to it good properties if few production environmental issues are addressed. The pulp used to produce the fibers determines the chemical composition of the fibers. Generally, the pulp used to make viscose has high α-cellulose by removal of hemicellulose from the pulp. 390,391 The crystallinity of the viscose fibers (41%) are lower than that of Lyocell fibers (80%). 50 This is one of the reason that viscose fibers possess lower tensile strength, lower modulus and higher elongation than Lyocell fibers. It was also noted that the thermal 55

56 stability of viscose fiber is lower than Lyocell fiber. 50 These fibers have been used as reinforcement to produce composites. 51,52,392,393 The composites produced from Lyocell and viscose were compared and it was found that superior properties of Lyocell fiber made their composites perform better than viscose composites. 51,52 But the properties of viscose fiber reinforced composites were comparable to the plant and animal fiber reinforced composites which certainly indicates the reinforcement potential of viscose fibers. 52 Other types of regenerated cellulose such as rayon and modal could be also as reinforcements to produce composites. The crystallinity analysis showed that modal fibers crystallinity was lower than Lyocell fibers. 49,50 The moisture absorption of modal fibers was higher than Lyocell and viscose fibers, and thermal stability of modal fibers was lower than the other regenerated cellulose fiber. 49,50 4. Fiber separation and extraction 4.1 Plant fibers The fibers should be separated and extracted from woody tissue of the fiber crops after the plant harvesting. The plant fibers used in commercial applications, mostly bast fibers, are separated from the fiber crops by retting process. Retting is a process which separates the fiber bundles from central stem which loosens the fibers from woody tissue of the fiber crops. The fiber extraction has impact on fiber yield, fiber quality, chemical composition, structure and properties of the fiber. 337 There are several retting processes as shown in Figure 18a. It can be divided into four major separation processes such as biological, mechanical, physical and chemical. Biological activity of microorganisms such as bacteria and fungi in the environment plays a major role to 56

57 degrade the pectic polysaccharides from non-fiber tissue and separated fiber bundles. Sometimes, the retting process could be challenging due to under-retting which results in contaminated fibers. 337 Figure 18a. Classification of retting processes. 337 Dew retting Dew retting is the oldest and most commonly used retting process to separate fibers. It is also called as field retting. This process needs appropriate moisture and temperature ranges, and therefore could not be used globally. The plant remains in the field (spread in uniform and thin non-overlapping swaths) after harvesting for the microorganisms to separate fibers from cortex and xylem. The plants are turned over on regular basis to ensure homogeneous retting. The retting process should be monitored and stopped at right time to prevent degradation of cellulose by microorganisms (fungi); and this is called as over-retting. The over-retting reduces the mechanical performance of the fibers while under-retting makes further fiber processing difficult. Dew retting depends on weather conditions and normally takes 3 to 6 weeks. There are several disadvantages of this process such as poor quality of fibers compared to other processes, dirty dark fibers due to contact with soil, limited to 57

58 geographical location, uncontrollable weather conditions and occupation of land until the retting is complete. 337 Dew retting remains the choice of the farmers of various plant fiber crops, due to low labor cost and sustainable. Stand retting A modified field retting was attempted to overcome the limitations of dew retting in Northern Ireland. A pre-harvest desiccant, glyphosate (N-phosphonomethyl glycine), was used to facilitate retting. It was found that fungal spread and retting was slower than dew retting but the fibers had better strength when stand retting method was used. 351 Another stand retting method is the thermally induced stand retting. The plant growth is terminated by open gas flames and the plant bases are heated approximately 100 C. The plants dry in couple of days which allows the replacement of weather vulnerable dew retting. The risk of weather and crop damage is reduced by using this method but the cost of retting increases. 337 Cold water retting Traditional water retting involves steeping the fiber section of the plant in running water such as rivers or streams. Later, this was replaced by water tanks which are sealed or opened depending upon the weather and the water change takes place every 2 days. This process utilizes anaerobic bacteria that break down the pectin and it is mainly followed in Eastern Europe. Cold water retting normally takes between 7 and 14 days but largely depend on the temperature of the water, the water type and the bacterial function. 58

59 This retting process produces high quality fibers than field retting. There are some drawbacks of this process such as high amount of water consumption, high cost of artificial drying after retting, environmental pollution from organic wastewater, malodor of fermentation gases. This process is terminated in most parts of Europe due to environmental concerns. 337 Warm water retting This is an accelerated water retting process which produces clean and high quality of fibers in 3 to 4 days. The water tanks are heated between 28 and 40 C. The process and limitations are similar to that of cold water retting, and abolished in most parts of Europe. 337 The wastewater could be used as liquid fertilizer if it is treated and removed toxic elements. Mechanical retting Mechanical retting, also known as green retting, is simple and cost effective process to separate the fibers. Mechanical retting is performed after field or technical drying. Field drying normally takes about 2 to 3 days. The fibers are separated from woody tissue by mechanically. Fibers produced by mechanical retting are less fine than field or water retting process. 337 Ultrasound retting The crushed and washed fiber section of the plant is fed into hot water bath to separate the fiber. Hot water bath is kept at 70 C with small amounts of alkali and surfactants; and exposed to high intensity ultrasound (40 khz). The fibers are separated from the hurds in this continuous process. The advantage of this process is that it does not need field drying as raw fiber section of the plant is fed directly

60 Steam explosion The steam explosion is good alternative to the traditional retting process. The steam and the additives penetrate the fiber interspaces of the fiber bundles under pressure and high temperature; that removes the center lamella at optimum conditions. The resulting relaxation leads to breaking up of fiber section and results in decomposition into fine fibers. This fibers produced by this method are fine and has good properties. 337 Enzyme retting The pectin degrading enzymes are used to separate the fibers from the woody tissue. This gives controlled retting of the fiber crops through selective biodegradation of the pectinaceous substances. The enzyme activity could increase with increasing temperature up to an optimum temperature above which the enzyme starts to denature. The fibers produced are of high and consistent quality. 337 Chemical and surfactant retting This refers to all retting process in which fiber section of the plant is submerged in heated tanks with water solutions of sulphuric acid, chlorinated lime, sodium or potassium hydroxide and soda ash to dissolve pectin component. The surface active agents could be used in retting to remove the unwanted non-cellulosic components adhering to the fibers (dispersion and emulsion forming process). Chemical and surfactant retting produces high quality fibers but the cost is higher than traditional methods. 337 There are other processes such as duralin and tuxying to separate the fibers. The fiber extraction and cleaning takes place after the retting process where the non-fibrous materials are removed entirely. The extraction could take place with one step or several steps such as 60

61 breaking, milling, scotching and decortication. The fibers are cleaned after extraction to be used in several applications. 4.2 Animal fibers Silk fibers from silkworms and spiders are extracted in different ways. Cocoons from mulberry silkworm are boiled gently in a mild soap solution which dissolves the sericin gum binding the fibers and unravel the silk fibers. Then, the fibers can be reeled onto wheels. The silk fiber is washed to remove remaining sericin and other contaminations. The extraction of silk from spider is labour intensive, space and time consuming. The spider is sedated at cool temperature which is then fixed upside down to expose spinnerets. A small brush is used to provoke fiber production and microscope is used to separate fibers. It has to be done for each fiber which is uneconomic for industrial scale production. 351 Wool fibers are extracted by manually shearing and collecting greasy wool fibers which have various impurities. So, skirting and classing are done manually. The greasy wool is scoured to extract the clean wool. 351 Various fibers are extracted in different ways which are then cleaned to remove the impurities before the fibers are used. 5. Reinforcement architecture Fibers are reinforced in matrix in different ways, either continuous or discontinuous fibers. Fibers (short or long) could be directly reinforced in matrix or fabrics made out of fibers could be used as reinforcement. The fabric architecture is divided into three types such as nonwoven, woven and knitted, Figure 18b. The engineered architecture has major impact on 61

62 mechanical properties of fabric and resulting composites. It influences on the method to be used for composite manufacturing. Figure 18b. Schematic representation of different fiber and fabric structure reinforced in matrix Nonwoven fabrics are made from short or long staple fibers which are used as reinforcement. The nonwoven fabrics are made by entangling fibers mechanically, thermally or chemically. Several products such as filters, tissue paper, surgical drapes and many other consumer products are nonwoven fabrics. Carding and needling are well known mechanical technique to produce nonwoven fabric by entangling the fibers. Nonwoven fabric has good tensile strength in fiber direction but poor strength in direction perpendicular to the fiber. Due to this reason, this structure is used in disposable tissue paper where it has to disperse quickly on flushing. Nonwoven fabric is also used as reinforcement in composites where the stress in applied in one direction (fiber direction). 51,52 62

63 Woven fabrics are produced from yarns by weaving (yarn interlacing) on loom. Unlike nonwovens, woven fabric has good strength in both the directions as fibers are oriented in 0 and 90 as warp and weft respectively. There are several patterns in weaving such as plain, twill, satin, basket, leno and mock leno. Figure 19 shows the some weave patterns. Detailed discussion was made about the composites and their properties made out of different weave structures. 14 Figure 19. Weave patterns (a) Satin (b) Basket (c) Leno. 14 Reproduced with permission from Elsevier, Copyright Knitted fabrics are produced from yarns by knitting (yarn interloping). It is third major class of fabric after woven and nonwoven fabrics. The longitudinal loops are called wales and the horizontal loops are called courses. There are two types of knitting such as warp and weft knitting. The knitted fabrics are mostly used in textile industry. Knitted fabric has higher elastic property but lower dimensional stability than woven fabric. These fabric properties have significant effect on resulting composites. 6. Summary 63