Natural Fiber Reinforced Composites

Polymer Reviews

ISSN: 1558-3724 (Print) 1558-3716 (Online) Journal homepage: http://www.tandfonline.com/loi/lmsc20

Natural Fiber Reinforced Composites Michael A. Fuqua , Shanshan Huo & Chad A. Ulven To cite this article: Michael A. Fuqua , Shanshan Huo & Chad A. Ulven (2012) Natural Fiber Reinforced Composites, Polymer Reviews, 52:3, 259-320, DOI: 10.1080/15583724.2012.705409 To link to this article: http://dx.doi.org/10.1080/15583724.2012.705409

Published online: 04 Sep 2012.

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Date: 15 March 2016, At: 09:48

Polymer Reviews, 52:259–320, 2012 Copyright © Taylor & Francis Group, LLC ISSN: 1558-3724 print / 1558-3716 online DOI: 10.1080/15583724.2012.705409

Natural Fiber Reinforced Composites MICHAEL A. FUQUA, SHANSHAN HUO, AND CHAD A. ULVEN

Downloaded by [North Dakota State University] at 09:48 15 March 2016

Mechanical Engineering Department, North Dakota State University, Fargo, North Dakota, USA In this review, insight into the use of bio-based fibers as composite reinforcement has been addressed. Specifics on the varieties of natural fibers, and the resultant properties from their constituents and hierarchal structures are described. The methods used to enhance the interface of these fibers with a variety of polymer matrices are reviewed. In addition, the influence of textile operations on creating various fiber architectures with resulting reinforcing capabilities, along with the methods in which natural fiber reinforced composites can be processed, are addressed. Finally, discussion of the correlation between structure, processing, and final composite properties are provided. Keywords polymer matrix composites, biocomposites, natural fibers, fiber structure, interface/interphase, mechancial properties

1. Introduction The concept of using naturally derived fibers from plants as reinforcement in a composite structure has been successfully utilized by multiple civilizations throughout world history and therefore is not new. A variety of materials can historically be pointed out as to which plant fiber to use as a means to enhance stiffness, strength, and impact resistance of a given material. However, over the past couple of decades a renewed interest and focus on the advanced utilization of different varieties of naturally occurring fibers as reinforcing agents in polymer matrices has been realized worldwide. Emphasis on improving and stimulating rural economies, reducing the world’s need for petroleum-based materials, and becoming more responsible with materials once their end-of-service-life is reached have been the prime factors involved in revisiting natural fiber reinforced composites. Governments and private interest groups throughout the world have been developing regulatory laws and general societal awareness on pollution, energy, and raw material waste which have stimulated a rapid growth of more novel uses of natural fibers as reinforcements in plastics to replace traditional composite, metallic, and wood structures. While application and implementation of natural fiber reinforced composites is growing, the intent of this article is not to explore their use in different applications, but rather to explore the knowledge base which has been collectively constructed by the many experts in this field of research spanning the last couple decades. Development of composite systems Received April 1, 2012; accepted June 19, 2012. Address correspondence to Chad A. Ulven, Mechanical Engineering Department, North Dakota State University, 111 Dolve Hall, NDSU Dept 2490, Fargo, North Dakota 58108, USA. Tel.: +1 701 231 5641. E-mail: [email protected]

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with natural fibers holds unique challenges not observed with traditional engineered fibers. The variation in constituent content and hierarchal structure of natural fibers across various categories and species, unique fabric construction techniques, and distinctive composite processing traits all yield challenges which do not need to be addressed when working with engineered fibers. Specifically, when comparing and contrasting the effectiveness of natural fibers as structural reinforcing elements in polymeric matrices versus engineered fibers such as carbon and glass, composite designers need to recognize certain stark differences. In terms of geometry, natural fibers are not uniform monofilament cylinders like carbon and glass, but bundles of elementary fibers which consist of voids and defects with irregular cross-sections. In terms of chemical structure, natural fibers have varying surface energy and available bonding sites along their fiber length due to the various natural polymers which create these bundles of elementary fibers. Each of these geometrical and chemical considerations for various natural fiber types will be discussed in great detail to begin this review to demonstrate why composite designers cannot simply treat natural fibers as conventional engineered fibers. In addition, these varying fiber structures and natural polymer constituents are also discussed in order to fully understand how to move natural fiber reinforced composite technology forward into new innovations which have not been considered in the past. Other challenges which continue to stifle the widespread development and use of biocomposite materials include inherent variability in natural fiber properties due to a multitude of different plant varieties and growing conditions, inconsistences in supply resulting from uncertain demand and climate conditions, low thermal degradation of most natural fibers (which limit the types of polymers the fibers can be introduced into), and manipulating the surface characteristics of natural fibers to enhance adhesion to polymers for improved load transfer and protection from moisture and decay. Therefore, the authors of this article will conclude this review with suggestions of the most pressing areas of research and investigation towards developing more robust, structurally performing biocomposites for today and tomorrow’s needs.

2. Biobased Fibers and Fiber Forms 2.1 Lignocellulosic Material Chemical Constituents The physical properties of plant fibers are related to the internal structure and constituents of the plant product being used. Plant fibers are lignocellulosic structures composed of cellulose, hemicelluloses, and lignin, with several minor components such as pectin, wax, protein, tannins, ash, and inorganic salts. These constituents vary depending on the source of the fibers, growing conditions, plant age, and digestion processes.1 These constituent content variances are compiled in Table 1 for a variety of fiber types. Generalized, the content of cellulose plays the largest role in governing the properties of the fibers, and dictates the mechanical performance of the fibers when used as reinforcement within composites. Unlike the cellulose, increases in non-cellulosic components tend to reduce the strength and modulus of the fibers, leading to negative influences on natural fiber reinforced composites. 2.1.1 Constituents. Cellulose Cellulose is a linear hydrophilic glucan polymer consisting of D-glucopyranose units (Fig. 1) which are linked together by β-(1–4)-glucosidic bonds.2–4 The hydrophilic nature stems from a large amount of hydroxyl groups attached to the pyranose rings.

261

Fruit/Seed

Leaf

Bast

Category

Musa textilis Phormium tenax Agave fourcroydes Ananas comosus Musa acuminata, Musa balbisiana Ananas erectifolius Phoenix dactylifera Attalea funifera Gossypium sp. Cocos nucifera Elaeis guineensis Luffa cylindrica Ceiba pentranda

Abaca New Zealand Flax Henequen Pineapple Fiber Banana Fiber

Oil Palm Sponge Gourd Kapok

Curaua Date Palm Piassava Cotton Coir (Coconut)

Linum usitatissimum Cannabis sativa Urtica dioica Corchorus capsularis Hibiscus cannabinus Boehmeria nivea Pueraria thunbergiana Abelmoschus esculentus Hibiscus sabdariffa Agave sisilana

Species

Flax Hemp Nettle Jute Kenaf Ramie Kudzu Okra Roselle Sisal

Fiber Type

48–65 50.2–67.2 64

73.6 33.9 31.6 82.7–92 32–47

60.4 45.1–55.1 77.6 70–82 63–67.6

71–81 70.2–74.4 79–83.6 61–73.2 28–39 68.6–76.2 33 46.6–49.3 70.2 56.5–78

Cellulose (%)

Table 1 Fiber Constituent Content

0–22 15.6–21.2 23

5.7–6 0.3–20

9.9 26.1

20.8 30.1 4–8 0 19

18.6–20.6 17.9–22.4 6.5–12.5 13.6–20.4 21.5–25 13.1–16.7 11.3 17.4–19 7.2 5.6–16.5

Hemicellulose (%)


19–25 11.2–14.4 13

7.5 27.7 48.4 0 31–45

12.4 11.2 13.1 5–12.7 5

2.2–3 3.7–5.7 3.5–4.4 12–16 15–22.7 0.6–1 14 11.3–14 14.9 8–14

Lignin (%)

253 71 48 21, 87 21, 77, 87, 93, 102, 251 93, 251 102, 154, 249, 254 21 (Continued on next page)

21, 77, 87 21, 77, 87 23 11, 21,77, 87, 246 21, 247 21, 77, 87 248 249 250 21, 82, 77, 87, 102, 250, 251 9 142, 252 21 21, 251 21, 251

Source

262

Straw - Wheat Straw - Rice Straw - Rye Alfa (Esparto) Bamboo Wildcane Switchgrass Ailanthas Hardwood (Yellow Birch) (Eucalyptus) (Beech) Softwood (Pine) (Spruce) Rice Husk Rye Husk Wheat Husk Olive Husk Almond Husk Areca Husk Sunflower Hull Soy Hull Palm Kernel Shell Hazlenut Shell Sugarcane Bagasse Sugar beet pulp Stover/Stalk Cob

Grasses

Sugar Processing Corn Processing

Husk/Hull

Wood

Fiber Type

Category

Pinus sp. Picea sp.

Betula alleghaniensis Eucalyptus obliqua Fagus sp.

Stippa tenacissima Bambuseae ge. Arundo donax Panicum virgatum Ailanthas sp.

Species

42–44 45.3–50.8 45 26 36 24–36 14 0 38.1–39.5 14–25 41.1 25.9 36.3–55.2 27.4 38.9–51.2 26.1–52

47 37.6 45.8

28.8–48.8 45 37.9 45.4 48.2–60.8 28.1–36.2 61.2 46.7

Cellulose (%)

22–27 21.2–30.2 19 16 18 22–27 14 35–65 16.5–17.1 14–20 29.2 29.9 16.8–24.7 28.1 20.9–30.7 32–45.9

31 32.9 31.8

35.4–39.1 19.3 36.9 38.5 25.1 20.5–29.8 — 26.6

Hemicellulose (%)

Table 1 Fiber Constituent Content (Continued)


28–31 24.5–27.5 19.5 13 16 26–48.4 13 13–25 23.7–24.7 3–4 20.5 42.5 18.14–25.3 3.1 14.4–21.5 11.3–15

21 19.1 21.9

17.1–18.6 18.9 17.6 14.9 2.1–32.2 15.8–22 9.2 26.2

Lignin (%)

11, 62, 91 255, 260 11, 261 62 62 255, 262 263 251 264 68 265 255 117, 256, 266 267 255, 268 133, 255

11 133 255

11, 134, 255 256 10 257 56, 57 258 259 255

Source

Natural Fiber Reinforced Composites

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Figure 1. The molecular structure of cellulose.

The existence of OH groups in cellulose also leads to a large number of hydrogen bonds. Due to these hydrogen bonds and van der Waals forces, part of the cellulose molecules align together, highly ordered, and form crystalline regions. Meanwhile, molecules with less ordered arrangement constitute an amorphous region. The supramolecular structure of cellulose determines its chemical and physical properties. The degree of polymerization (DP) is the key factor, which varies with the type of natural fiber: ramie has 6500 DP; cotton has 7000 DP; flax has 8000 DP.1 There are two forms cellulose existing in nature (cellulose I): cellulose Iα and cellulose Iβ .1,3,5 While cellulose Iα only has been found to exist in some green algae,5,6 cellulose Iβ exists in all species of plants. Cellulose Iβ crystallizes in monoclinic sphenodic structures with two chains in a parallel fashion.5,7 Cellulose I can be transformed into cellulose II, cellulose III, or cellulose IV by different chemical or thermal treatments. In addition, these different forms of cellulose can be transformed back to cellulose I by treatment. Cellulose II is also a monoclinic structure, but unlike cellulose I, its two chains arrange in an antiparallel fashion. Cellulose II is thus more thermally stable than cellulose I.8 However, cellulose I shows better mechanical properties. Hemicellulose The term hemicellulose designates a group of amorphous polysaccharides, such as xylose, mannose, glucose, galactose, and arabinose, which despite its name is nonrelevant to cellulose. Hemicellulose is a copolymer containing several different sugar units, unlike cellulose which contains only 1,4-β-D-glucopyranose. The degree of polymerization of hemicellulose is between 500–3,000 as opposed to the 7,000–15,000 range found in cellulose. Moreover, hemicellulose is a purely amorphous branched polymer with little strength, compared to semi-crystalline linear cellulose. The structure of hemicellulose varies depending on the type of plant. Hemicellulose is highly hydrophilic, soluble in alkaline solution, and easy to hydrolyze in acid. Hemicellulose usually exists in the interface between cellulose and lignin. There are some hemicellulose enzymes in nature to help hemicellulose hydrolyze.9–12 Lignin Lignin is a disordered, cross-linked polymer which gives rigidity to plants. It is an amorphous phenolic polymer with aromatic and aliphatic constituents formed by phenyl-propane units (Fig. 2.). The polymerization of the lignin monomers is initiated by oxidases or peroxidases. However, the exact mechanism still remains obscure. The molecular masses of isolated lignin are from 1,000 to 20,000 g/mol. However, the DP of lignin is difficult to measure, since it almost always becomes fragmented during extraction and consists of several types of substructures without a regular repeating pattern.13 During biosynthesis of plant cell walls, lignin is developed between the polysaccharide fibers (cellulose and hemicellulose), binding them together. This lignification process causes a stiffening of cell walls. As a plant matures, lignin becomes

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Figure 2. Phenolic monomers: (a) p-coumaryl alcohol (H structure), (b) coniferyl alcohol (G structure), (c) sinapyl alcohol (S structure).

more rigid.14 The structure of lignin differs depending on the original sources, but the basic composition is kept the same. Lignin is highly hydrophobic in nature and is not hydrolyzed by acids. It is soluble in hot alkali, readily oxidized, and easily condensable with phenol.15 The mechanical properties of lignin are lower than those of cellulose. Pectin Pectin is a collective name for heteropolysaccharides, which are a major matrix component of the cell wall in long nonwood fibers, particularly the important bast fibers. Pectin gives flexibility to plants. It is soluble in water only after a partial neutralization with alkali or ammonium hydroxide.1 2.1.2 Fiber Architecture. When referring to plant fibers in the context of their use as reinforcement within polymeric matrices, the term fiber can be in reference to either a single elementary fiber, or else bundles of these elementary units. In cases such as wood or seed hulls, the plant “fiber” is referencing a single elementary division. However, in many plants, such as those derived from basts, leafs, or grasses, the “fiber” is actually a bundle of elementary fibers consisting of several single fiber divisions. A single fiber (elementary fiber) can be considered to be a hollow composite. Cellulose fibrils act as the reinforcements, while hemicelluloses, lignin, pectin and the other amorphous components make up the matrix to hold these fibrils together. As shown in Table 2, the amounts of these chemical constituents vary by species, strain, and other aspects of a particular plant’s genetics. This leads to varied performance and structure development in different plant fibers. There are several distinct layers in a single fiber (Fig. 3.): the center lumen, secondary wall (S3, S2, and S1), and primary wall from inside to outside.16 The primary wall is the first layer deposited, containing hemicelluloses and cellulose during the cell growth encircling the secondary walls.17 The secondary cell wall consists mainly of helically wound cellulose microfibrils. These microfibrils are made up of 30 to 100 cellulose molecules, with a diameter of about 10–30 nm, and provide mechanical strength to the fiber. The S2 layer is thicker than the S1 and S3 layers and contributes approximately 70% of the entire fiber’s Young’s modulus.1 The microfibrillar angle between the fiber axis and the microfibril depends on the species. The spiral angle and S2 play a strong role in determining the mechanical properties of the fiber, as a smaller angle usually dictates a higher fiber strength and modulus.18 The outer layer (middle lamella) is constituted of pectin and lignin to compile and bind fiber bundles together, yielding the final plant fiber structure. The existence of pectin and lignin in

265

Flax Hemp Nettle Jute Kenaf Ramie Kudzu Okra Roselle

Sisal Abaca Henequen Pineapple Fiber Banana Fiber Curaua Date Palm Piassava

Cotton Coir (Coconut) Oil Palm

Leaf

Fruit/Seed

Fiber Type

Bast

Category

1.5–1.6 1.15–1.45 0.7–1.55

1.45 1.5 1.2 1.44 1.35 1.38 0.92 —

1.5–1.54 1.48 — 1.3–1.45 0.749 1.45 — — —

Density (g/cm3)

15.6–21 40–450 150–500

50–200 28 180 20–80 50–280 9–10 100–1000 —

— 53.7 10–63 25–200 43.3–140 34 46–200 40–180 —

Diameter (µm)

287–800 106–175 100–400

80–640 756 500 413–1627 529–914 913 170–275 109–147

450–1500 690–873 1594 393–773 223–624 400–938 130–418 68–282 147–184

Strength (MPa)

Modulus (GPa)

1.1–12.6 1.27–6 1–9

1.46–15.8 31.1 13.2 34.5–82.5 7.7–32 30 5–12 1.1–4.6

27.6–38 9.93 87 2.5–26.5 11–14.5 24.5–128 — 5.74–16.55 2.76

Table 2 Fiber Mechanical Properties

6–9.7 15–59.9 8–18

3–15 2.9 4.8 0.8–1.6 1.8–3.7 3.9 5–10 6.4–21.9

1.5–3.2 1.6–4.7 1–6 1–2 2.7–5.7 1.2–3.8 — 2 11–15

Elongation at Break (%)


21, 271, 279 21, 279, 282 283, 284 (Continued on next page)

25, 276, 277 212 278 276 25, 276, 279 280 281 48

21, 217, 236, 269, 270 21, 271 22, 23 21, 25 21, 271, 272 21, 273, 274 275 29 30

Source

266

Straw - Wheat Straw - Rice Alfa (Esparto) Bamboo Wildcane

Hardwood Softwood E-Glass S-Glass Aramid Carbon

Grasses

Wood

Manmade

Fiber Type

Category

0.3–0.88 0.3–0.59 2.5 2.5 1.4 1.4

1.49 — 0.89 0.6–1.1 0.844

Density (g/cm3)

16 30 9.8–17 9 12 5–17

84–94 — — 88–125 190–560

Diameter (µm)

51–120.7 45.5–111.7 2000–3500 4570 3000–3150 4000

59–140 150–200 350 140–441 159

Strength (MPa)

Table 2 Fiber Mechanical Properties (Continued)

5.2–15.6 3.6–14.3 70 86 63–67 230–240

3.7–4.8 3.3–12.5 22 11–36 11.8

Modulus (GPa)


— — 0.5–4.8 2.8–5.7 3.3–3.7 1.4–1.8

— 3.2–4.6 5.8 1.3–8 1.3

Elongation at Break (%)

288 288 137, 289 137, 289 137 137

217, 285 286 3 3, 287 58

Source



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Figure 3. The structure of a single fiber cell.16

these bundles reduces the mechanical properties of the fiber, also influencing the interfacial properties between fibers and different matrices when used in composites. 2.2 Lignocellulosic Fiber Sources Cellulose based fibers can be classified in a number of different ways. The first distinguishing characteristic is whether the plant fibers are considered true fibers in the composite or textile sense, that they maintain at least a short fiber aspect ratio of 20: 1, or whether they are instead fillers with lower aspect ratios. For true fibers, sources can be classified in two forms: wood fibers, and fiber crops such as bast, fruit, seed, leaf, and grasses. Fillers, on the other hand, can be derived from both wood fibers and fiber crops, as well as a number of non-traditional agricultural sources. These include sources like seed hulls and husks, as well as agricultural crop processing byproducts such as residues from corn or sugar processing. With these variances in fiber type, coupled with variances in fiber geometry and constituent makeup, a wide range of mechanical properties have been documented for different plant fibers, as shown in Table 2. Disparity in fiber diameter is also significant, and it can be observed that there is a general trend in improved mechanical performance with decreased fiber diameter.18 But generally, the lower densities of natural fibers compared to common manmade fibers such as E-glass yields specific mechanical properties that make natural fibers competitively comparable in performance. The tensile properties of natural fibers increase with increasing cellulose content and decrease with increasing content of non-cellulosic chemicals, such as lignin, hemicellulose, pectin, and wax. In addition, the structure, microfibrillar angle, cell dimensions, and defects all affect the overall properties of the fibers.17 However, the physical properties of natural fibers are not only related with their chemical constituents, but also depend on several factors, such as fiber source, age, size, location of the fibers in the plant, maturity, and processing methods adopted for the extraction of the fibers. Most of these factors are difficult to control, and thus focus on how to harvest and extract the fibers becomes the key to improving the productivity and quality of these fibers. 2.2.1 Bast Fiber. Bast fibers are fibers derived from the phloem, or inner bark, of a variety of plants. The phloem, which serves as the strength and stiffness of a plant stem, normally


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surrounds a woody xylem core. Among different types of plant fibers, bast fibers have a long and storied past. The earliest usage of bast fibers can be traced back to 30,000 years during the Upper Paleolithic Age19 when prehistoric hunter-gatherers twisted wild flax fibers to make cords to haft stone tools, weave baskets, and sew garments. Bast fibers have been widely used in textile back to thousands of years ago in Egypt and China. One unique property of bast fibers (although it does occur in some grass fibers such as bamboo), is that they contain fiber nodes at regular intervals along a stem’s length.15 These nodes prove to be points of mechanical weakness, which effectively limits the lengths of fiber that can be harvested despite the potential of rather long plan stems. Bast fibers have received a great deal of renewed attention for composite usage, and as such a great deal of focus has been placed on their harvest and production as it pertains to yielding the most ideal fibers. A typical production cycle for bast fiber is given: planting → weed control → plant growth → desiccation → sowing → swathing, mowing/pulling → drying → threshing → dew retting → baling and storing → breaking and scotching → transportation → further processing.15,20 The primary purpose a bast plant is grown (whether for the plant fibers, or for their seeds) determines the timing of harvest. The optimal time for harvesting for fiber quality is at the beginning of seed maturity for most bast fibers. At this time, about 90% of the plant stalk consists of the secondary cell wall and the lignifications of the stalk is still in processing.15 Moreover, the decortications and separation of the fibers are much easier if the plants are harvested at the optimal time. The fibers obtained from plants which have a primary seed production objective are usually stiffer, coarser, and more brittle compared to those grown primarily for fiber. After harvesting bast fiber crops, the fibers will be separated and extracted from the woody stems. The separation processing is called retting, which can be classified into four types: biological, mechanical, physical, and chemical.15 The retting procedure has a major impact on the quantity and quality of the final fiber. Biological retting can be divided into natural (dew/field retting and cold water retting) and artificial (warm water or canal retting) processes. Dew/field retting is a common method in areas with limited water resources. The plants remain on the field until micro-organisms and moisture have separated the fibers from the stem. The duration of the dew retting process is usually 3 to 6 weeks, the time being affected by the climates, bacteria, and dew produces fermentation in the region it is being performed. Dew retted fibers usually have a darker color and poorer quality than water retted fiber. In addition, the unpredictability of the dew retting process leads to the inconsistent quality of fibers (year by year). A thermal assistant modified field retting procedure15 has been applied to this method to achieve the more consistent fiber quality. However, the cost for modified dew retting increases due to this extra procedure. Cold water retting uses anaerobic bacteria to break down the pectin of the plant straw bundles in water. Water penetrates the central stalk portion, swells the inner cells, and burst the outer layer of the plant to increase the absorption of both moisture and bacteria. It takes between one and two weeks, depending on the water type, temperature of water, and bacterial inoculums. High quality fibers can be obtained by water retting. However, the organic fermentation in wastewater can introduce unwanted levels of pollution to the environment. Warm water retting, as the name suggests, is a variance in water retting in which the bast stems are soaked in warm water (28◦ C to 40◦ C), which can reduce the time of retting to 3 to 5 days. Mechanical retting (green retting) is a simple and cost effective method to separate the bast fibers from the xylem and plant straw. While the plant needs to be slightly retted or technically dried to straw before separation by machines, much of the dependency of



269

weather and bacteria is eliminated in this process. Despite this, the obtained fibers are much coarser compared to biological retted fibers, and thus their usefulness is still limited for composite usage. To obtain fine and clean fibers with consistent quality, a wet retting (physical retting) process can be applied. It allows for modification of the fibers for different applications by adjusting the processing parameters.15 There are three methods: ultrasound retting, steam explosion method (STEX), and enzyme retting. Ultrasound retting avoids the unreliable dew retting process in biological retting and mechanical retting. The obtained fibers from ultrasound retting are normally for non-textile applications. STEX can produce very fine fibers, with fineness and properties that are comparable to cotton fibers.15 Enzyme retting is another alternative which yields fine and consistent quality fibers. Enzyme retting takes between 2 to 24 hours, and produces undamaged individual fibers with high intrinsic fiber strength. However, the costs of enzyme retting are much higher than other retting methods. Chemical and surfactant retting is a heated water retting processes with additional chemical modification. There are several chemicals can be used to dissolve pectin, including sulfuric acid, chlorinated lime, sodium or potassium hydroxide, and sodium carbonate.15 Like enzyme retting, this method is also time efficient, but it comes at a higher cost than biological retting. The procedure of fiber separation can significantly affect the fiber quality, the fiber morphology, the fiber surface composition, and the final fiber mechanical properties. Although cost is still prohibitive for some applications, methods such as optimized biological, physical, and chemical retting procedures have shown to result in better and easier separation of the fibers from the woody core while minimizing the mechanical loading history on the fibers. Bast fibers are found to grow in a variety of regions and climates across the world. Bast fibers include flax, hemp, nettle, ramie, jute, kenaf, kudzu, okra, roselle, rattan, and wisteria, among others. A summary of these fiber types are explained in the next several paragraphs. Flax Flax grown for fiber (linen) and grown for seed oil (linseed) has been planted in temperate regions, such as Netherlands, France, Spain, Russia, Belgium, China, India, Argentina, Canada, and the United States. Linen flax is the oldest textile known and an import standard for the modern textile industry. The plant can grow between 80 and 150 cm in about 80 to 110 days, of which 75% of the plant’s height can be used to produce fiber.15 Hemp Hemp is an annual plant native to central Asia. It has been cultivated in central Europe since the Iron Age (400 BC). There are several advantages to plant hemp crops: 1) it does not need herbicides, pesticides, fungicides, and fertilizer; 2) it has an impressive growth rate, and can grow to 4 m in 12 weeks; 3) it restores nutrients to the soil; 4) it is very deep rooting, which provide a good disease break and helps to maintain the soil structure. Hemp fiber has similar chemical contents to flax fiber, but hemp has better moisture resistance than flax. Hemp fiber has high tenacity, which is about 20% higher than flax, but low elongation.21 Hemp fiber has been widely used in paper, textiles, construction, composites, food, medicine, and fuel. Nettle Nettle is another bast fiber which can be used as the reinforcement in composites. The usage of nettle dates back to 3000 BC.15 During World War II nettle was widely used for fiber derivation; however, its use and production has slowed significantly since.

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M. A. Fuqua et al. Nettle is a perennial plant which grows to the height of 2.8 m. The nettle fibers have a remarkable tensile strength and modulus and are finer than other bast fibers.22,23


Jute Jute is one of the most well-known bast fibers and the second most common natural fiber cultivated in the world (next to cotton).20 Jute is native to the Mediterranean, and now grows in India, Bangladesh, China, Nepal, Thailand, Indonesia, and Brazil.24 Jute can grow 2–3.5 m in height and is entirely grown for its fibers. Jute fibers are very brittle, with a low extension to break due to the high lignin content (up to 12–16%). The tensile strength of jute fibers is lower than that of flax and hemp.21.25 Jute fibers have little resistance to moisture, acid and UV light. However, their fine texture as well as their resistance to heat and fire have provided a wide range of applications in industries such as textile, construction, and automotive. Kenaf (Hibiscus cannabinus) Kenaf is a warm season, annual crop native to central Africa, which has been spread to tropical and subtropical Asia. Kenaf grows very fast with a height of 3 m being achievable in 3 months.26 The plant contains two fiber types: long bast fibers (35–40% yield of stem weight) and short core fibers.27 Kenaf fibers are coarse, brittle, and difficult to process. The mechanical properties of bast kenaf fiber is similar to those of jute. There are various industrial applications for kenaf, including paper products, textile, and construction. Ramie Ramie is one of the oldest cultivated fiber crops in East Asia, dating back to 4000 BC. The height of ramie is between 1 to 2.5 m. Ramie is normally harvested two or three times a year but can be harvested up to six times under good growing conditions. Ramie fibers need to be degummed unlike other bast fibers. Ramie fiber is one of the strongest, stiffest, and finest textile fibers. It has a specific tensile strength and modulus similar to that of glass fiber, and higher than that of flax and jute.28 Ramie fibers have excellent resistance to bacteria, mildew, and insect attack. They are also stable in alkaline and mild acids. Kudzu Kudzu is a fiber crop native to southern Japan and southeast China. Kudzu fiber has been used for centuries for basketry and clothing. The crop is also widely used to enhance the soil and control erosion. Kudzu is a conductive plant and can spread by stolons and seeds. Kudzu has been spread all over the world due to its destructive expansion. There are several methods to control the growing of kudzu, including crown removal, mowing, grazing, fire, herbicide, fungi, and helium. Okra Okra is cultivated in tropical, subtropical, and warm temperate regions around the world originating in Africa. It is an annual or perennial plant with the height of 2 m. Okra is one of the most heat and drought tolerant species in the world.29 Roselle (Hibiscus sabdariffa) Roselle is an annual or perennial plant with a height of 2–2.5 m. Roselle is used for the production of bast fiber and as an infusion native to West Africa and India. Nowadays, China and Thailand are the largest producers. Roselle fiber can be used as a substitute for jute.

2.2.2 Leaf. Leaf fibers are another group of long, multiple-celled lignocellulosic fibers extracted from plants, commonly found as foliage. While leaves can be diverse in appearance



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and purpose, leaves used for fiber production tend to be long, linear structures which can be mechanically separated with relative ease. Since there exist a large variety of plants which are used to derive leaf fiber, the harvesting and fiber extraction of these sources also shows a level of diversity. Most leaf fiber is extracted by mechanical decortication.15,30–32 To expedite and assist the fiber separation during decortication, often leaf fibers will be soaked in water from 1–5 days in a form of water retting, although this is not always performed.30,33 During decortication, leaves are crushed using a rotating wheel with blunt blades, which breaks apart the fibers from the other structures and components of the plant leaf. After this, the fibers are normally washed and dried31,32 with proper drying being important to the final mechanical performance of the fibers. After drying, fibers can be carded according to their diameter and length. The leaves of many plants that can be used as reinforcements for polymeric composites, include those of sisal, abaca, phormium tenax, henequen, pineapple fiber, banana fiber, curaua, date palm, piassava, caroa, istle and raphia. A description of these fibers and their attributes is provided in the next several paragraphs. Sisal Sisal is widely grown in tropical countries in Africa, the West Indies, and the Far East. It has a 7–10 year life-span with about 200–250 leaves, each of which can extract around 1000 fiber bundles.34,35 Tanzania and Brazil are the main producing countries. The plant grows up to 2 m tall, and the length of the sisal fiber varies between 0.6 and 1.5 m. The leaf contains three types of fiber: structural, arch, and xylem. The most important commercially are the structural fibers, because they do not tend to split during extraction. However, the arch fibers hold high mechanical strength, and thus are of potential use in short fiber applications. Xylem fiber tends to be lost or destroyed during the extraction process.31 Sisal has been the leading material for agricultural twine because of its strength, durability, ability to stretch, affinity for certain dyestuffs, and resistance to deterioration in saltwater. However, the tensile strength, modulus, and toughness of sisal fiber decreases with increasing temperature.36 Abaca (Manila hemp) Abaca is a species of banana native to the Philippines with a leaf height of 3 to 4 m. Abaca fiber, obtained from the leaf sheaths, has the general designation of “manila hemp,” despite not being a bast fiber. It was originally used for twines and ropes. Abaca is classified as a hard fiber along with coir, henequen, and sisal. Abaca is considered to be one of the strongest of all plant fibers: its tensile strength is three times higher than cotton and twice that of sisal. In addition, abaca has excellent resistance to salt water.37 Phormium tenax Phormium tenax is an evergreen perennial plant, native to New Zealand and Norfolk Island. Phormium tenax is an important fiber plant whose leaves can grow up to 3 m long.38 The traditional use of phormium leaves was making plaiting mats, containers, and ropes. Henequen Henequen is a relative of sisal, but not of as high a quality as sisal. Henequen fibers are commonly used in the textile industry. It grows in the tropical regions of Africa, Central and South America, and Asia. During the leaf defibration, the waste fibers can be processed to obtain cellulose pulps or short henequen fibers as reinforcement to polymeric composites.39 Henequen fibers are smooth, straight, and easy to degrade


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in salt water. Henequen has low elongation at break and a low modulus compared to other leaf fibers. However, henequen also has good toughness and resiliency, which is attractive for some reinforcement applications.15 Pineapple fiber Pineapple fiber (PALF) is obtained from the leaves of the pineapple plant and is found commonly in the Philippines. It is combined with silk or polyester to create a textile fabric, carpets, mops, and curtains. Pineapple spreads in tropical countries for its fruit, and thus PALF can be considered an agricultural byproduct. The crops have very short stems, but the leaves are almost a meter in length. The constituent content in PALF40 is similar to those in flax, and the microfibrillar angle is relatively low (14◦ ).41 The mechanical properties of PALF are comparable with jute fibers, but its structure is without mesh. PALF is very hygroscopic and the fiber bundle strength can decrease by 50% when the fibers are wet, although yarn strength will increase by about 13%.15 Banana Fiber Banana is the largest herbaceous flowering plant, and is often mistaken for trees. The leaves are spirally arranged and can grow up to 2.7 m long. They are often used as ecologically friendly disposable food containers or as plates in Southeast Asia. The banana fiber has been used for textiles dating as far back as the 13th century. The cellulose content in the banana fiber and sisal fiber is almost the same, but the spiral angle of banana (11◦ ) is much lower than sisal (20◦ ).42 Hence the tensile properties of banana fiber are higher than sisal.43 Curaua Curaua is an Amazonian plant growing in semi-arid conditions. The plant has hard leaves which grow to be 1.5–1.7 m in length. Curaua fiber has attracted a lot of attention since the fiber stared being used widely in the Brazilian automotive industry in 1993.44 The content of lignin in curaua is lower than several plant fibers, such as jute, sugarcane bagasse, and coconut.45 In addition, the cellulose content of curaua is nearly 73%, and it has a higher crystallinity (67%) than that of other fibers.45 Date Palm Date Palm is a palm cultivated for its sweet fruit growing in the Middle East, Northern Africa, the Canary Islands, Pakistan, India, and the United States. The leaves are 3–5 m long. The palm tree stem is covered with a mesh made of single fibers.46 These fibers can be used to make ropes and baskets. Piassava Piassava fibers are stiff fibers extracted from the leaves of a native palm tree. They are commonly used in industrial and domestic brooms, industrial brusher, carpets and roofs.47 The tensile properties of piassava fibers are lower than those of jute and sisal.48 However, their mechanical properties are comparable to those of coir fibers. 2.2.3 Fruit and Seed. Fruit fibers are those that are obtained from the fruit shell or bunches such as coir, oil palm fiber, and sponge gourd, while seed fibers, such cotton and kapok, are from the protective boll around the seed. The following paragraphs describe each type in more detail. Cotton Cotton is a fluffy staple fiber grown extensively in tropic and subtropic regions across the world, making it the leading fiber crop currently grown and harvested.20 Despite its prevalence, cotton fiber’s mechanical performance is relatively low compared to other natural fibers, especially those of numerous bast types. This is partly due to the fiber’s



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low degree of orientation.49 Despite its limited application as a reinforcement, it has been proven to be useful as an ancillary fiber in conjunction with other natural fibers during fabric manufacturing.50 Coir Coir is a fiber found between the hard inner shell and the outer coat of coconuts, which grow in tropic and subtropic regions of the world. There are two types of coir, a white fiber derived from harvesting coconuts before they are ripe, and a coarser brown fiber derived from harvesting coconuts after they have ripened. Between the two, coir fiber production worldwide is calculated to be roughly 250,000 tons.51 While the fiber is considered a poor reinforcing fiber because of its low strength and modulus, it has found interest due to its low density, low thermal conductivity and high elongation.52 Oil Palm Oil palm is the highest yielding edible oil crop in the world, native to West Africa and the Congo Basin. Mesocarp and endosperm oil are the major products of oil palms. In addition, there are other products from oil palms, such as palm wine, food, medicine, boiler fuel, and construction materials. The oil palm fibers can be extracted from trunk, frond, fruit mesocarp and empty fruit bunch (EFB).53 In order to develop the applications for oil palm fibers, they need to be extracted from the waste using a retting process. In addition, the fibers need to be cleaned of oily and dirty materials. The fibers are very porous, and their diameters vary greatly. Hence the tensile strength of oil palm fibers is lower than most other plant fibers. 2.2.4 Grasses. Another source of lignocellulosic fiber is from grasses such as alfa, bamboo, canary, wildcane, Indian grass, and switchgrass, as well as various straws, which are dried stalks of cereal plants. Grass fibers biocomposites have been investigated in Europe for use within the automotive industry, and are especially of interest due to their low cost and simple harvesting and processing. Due to their lower mechanical properties compared to other plant fiber types such as bast and leaf fibers, grass fibers are predominantly used as short fiber reinforcements, rather than as long or continuous fibers. Straw Straws are not true grasses by definition, but instead are considered the byproducts of cereal stalks such as wheat, rice, and rye, after they are stripped of grain and chaff and then dried. They hold a variety of agricultural uses, including as bedding, fodder, thatching, and biofuel. While their mechanical performance is not as high as those of most of the bast or leaf fibers, they have found application as short fiber reinforcement. Alfa (Esparto) Alfa, or Esparto grass, is a perennial grass found in the extreme southern Spain and the northwest region of Africa. It is mainly used for paper making from a pulping process, due to its short fiber length but high mechanical properties.3,54 As lignocellulosic fibers have reemerged in interest for material purposes, attention has grown to the potential of alfa to be used as a short fiber reinforcement. Bamboo Bamboo fiber is extracted from the pulp of bamboo plants. Bamboos are the largest member of the grass family and are some of the fastest growing plants in the world. Bamboos are found mainly in Asia, and have been used for a food source, construction, furniture, textile, and paper. Bamboo fiber has upwards of 60% cellulose, about roughly 25% lignin, while the microfibrillar angle is between 2◦ and 10◦ .55–57

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Wildcane Wildcane grass is an abundantly available natural resource. The density of wildcane grass fiber is comparable to other plant fibers, such as sisal, banana, and bamboo. The tensile modulus of wildcane grass fiber is similar to those of sisal and banana fiber.58 2.2.5 Wood. Wood is commonly classified in to two categories, hardwood and softwood. Despite their names, this designation does not indicate the degree of hardness of the woods. Instead, it designate whether the wood is from a gymnosperm tree, and thus a softwood or from an angiosperm tree, and thus a hardwood. The main difference between these two types is that hardwoods have more complex structures consisting of vessels or pores which softwoods lack.59 Both hardwoods and softwood are made up of distinct cells that are connected and interconnected in a continuous fashion. They are oriented in two specific systems, axial cells running parallel to the long axis of a wood plant (the direction of the wood “grain”), and radial cells running perpendicular to the long axis, oriented as radii to a circle. In softwoods, 90% of the structure is made up of axial tracheids, which are cell structures roughly 1–10 mm in length and 100 times longer than they are wide. These tracheids are what give the wood its strength. In hardwoods, the structure is more complex. Axially, the woods are made up of axial parenchyma and axial fibers. The fiber cells are the mechanically supporting structure, and are roughly 0.2–1.2 mm in length and about half the width of tracheids in softwood. It is through the thickness of the fiber cells that density and mechanical strength of hardwoods is determined, with thicker cells yielding stronger but more dense structures.59 For polymer composite use, both hardwoods and softwoods can be used in two forms, as single fibers with higher aspect ratios (wood fiber) and as smaller fiber bundles with low aspect ratios (wood flour).60 Wood flour, due to its low aspect ratio, is used more as a filler than as a reinforcing fiber. Both wood fiber, and more commonly wood flour, can be obtained a number of ways. These wood fractionation methods, include heavy reliance on post-industrial sources such as sawmill and wood working centers, in the forms of sawdust, planar shavings, and ground wood chips.60,61 2.2.6 Husk/Hull. The terms husk and hull are somewhat interchangeable terms that refer to the outer shell or coatings of a variety of seeds. The term chaff is also sometimes used, especially when in reference to the hull of beans or the sheath of a variety of grains. In general, these seed and grain shells are derived as agricultural byproducts from various industry processing. Common sources include soy beans, rice, wheat grain, palm kernels, sunflower seeds, and rye, to name just a few. Though the process is somewhat dependent upon the source and region of a particular agricultural product, husks and hulls are separated from their desired end-goal seed or grain by some means of mechanical dehulling. This process normally involves rolling or crushing the seed or grain so as to split and separate the outer shell from the internal structure. The resulting hulls are then treated as wastestream, often sold as animal bedding or livestock feed due to their relatively high fiber content.62 Due to the large amount of these wastestreams produced, focus is growing into their use as fillers, and in some instances short fibers, in polymeric composite applications. To accomplish this, the hulls and husks are mechanically fractionated by available means. Due to the relatively small nature of the unprocessed hulls from most seeds and grains, their applicability as true fiber reinforcement is limited, and as such their neat mechanical


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performance is not normally studied in serious depth. To date, emphasis has been placed on using hull sources with high cellulose content, due to cellulose’s crystalline configuration.


2.2.7 Other Agricultural Byproducts. Like seed and grain hulls and husks, a variety of other agricultural byproduct streams have come to emerge as natural filler sources. With agricultural product use focused not only on food for consumption and feedstock purposes, but also industrial application, new processing methods are yielding interesting byproduct streams. These include byproducts from sugar plants such as sugarcane bagasse and subarbeet pulp, as well as various corn byproducts such as corn stover and cob from the large amount of corn processing seen in North America. Sugarcane Bagasse In the processing of sugarcane, bagasse is yielded as a byproduct of crushing the cane for juice extraction. Uses for bagasse have been readily sought, and it has found applicability in a wide variety of applications, including as a biofuel source, and a source for paper pulp.63,64 However, when processed it also has a 1.2 mm length and 15 µm width (aspect ratio 80: 1),64 and allows for similar modulus performance as wood flour when incorporated in composites,192 thus making it a prime candidate as a short fiber reinforcement. 2.3 Protein Fibers Beyond lignocellulosic sources, natural fibers can also be sourced from various protein based systems. For these fibers and fillers, the structural backbone is the amino acid protein structure. Proteins consist of four levels of structure. The primary structure is the amino acid sequence, which is the order that various amino acids connected by peptide bonds, lie within a polypeptide. These amino acid sequences when regularly repeated and stabilized by hydrogen bonds make up the secondary structure. There are three major types of secondary structure: α-helix, a right-handed coiled or spiral conformation which is the most prevalent, regular, and predictable of sequencing; β-sheet, a slightly less prevalent structure of connected laterally by at minimum of two to three backbone hydrogen bonds, forming a generally twisted, pleated sheet; and turns, a large collection of structure types in which the polypeptide chain reverses its direction. From these secondary structures, the overall shape of a single protein molecule is defined by the tertiary structure, or protein fold. The fold will also determine the spatial relationships of the secondary structures, and will be responsible for the basic function of the protein. They are stabilized by nonlocal interactions, such as through the formation of a hydrophobic core, or by hydrogen or disulfide bonds, of salt bridges. The final quaternary structure of a protein is the arrangement of multiple folded proteins into a single protein complex.293 For fiber reinforcement purposes, only a subset of all proteins are considered. These are scleroproteins, which are fibrous proteins in the form of long protein filaments which make up connective tissue, muscle fiber, bones matrices, and tendons. Fibrous proteins include keratin, collagen, elastin, and fibroin.294 It is from these fibrous proteins that animal derived natural fibers have been used. Of most importance are keratin and fibroin. Collagen, being the most abundant protein within vertebrates, is predominantly found in bones, tendons, and skin, while elastin is primarily found in ligaments and arterial blood vessels. As such, neither is particularly a good source for harvestable protein fiber for composite use. 2.3.1 Keratin. Keratin proteins are the key structural proteins in animal hair, nails and a major fraction of animal skin. There are two major forms of keratin, α-keratin which is

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made up of predominantly α-helix amino-acid sequencing, and β-keratin which contains more β pleated sheet secondary structures. The α-keratins are the primary protein of hair, horns, hooves, nails, and claws of mammals, while β-keratins are the primary protein structure of the scales, shell, and claws of reptiles, as well as the feathers, beaks, and claws of birds.295 Wool Wool, the most commonly harvested from sheep, but also from goats, alpaca, and camels, has been explored for composite usage.296,297 However, its use has been stymied by the relatively high cost in comparison to it being a relatively weak textile fiber, having a tensile strength of approximately 120–180 MPa.298,299 Due to a large amount of cysteine, a number of disulfide bonds that crosslink with adjacent polypeptide chains occur within the α-keratin of the wool, causing it to act characteristically similar to a cross-linked rubber. The tensile modulus of a dry fiber is approximately 2.3–3.4 GPa, and the fiber has an approximately 25–40% strain to failure.298,299 On a macro level, wool is made up of both para-cortex cells, which align parallel to the length of the fibers, and ortho-cortex cells, which is a fibril-matrix composite which forms a helical array in macrofibrils. These structures affect the crimp of the fiber, which is what causes wool to have bulk. When wool dries after being wetted, the ortho-cortex reduces its helix angle, causing the macrofibrils to want to increase in length. However, the para-cortex does not see a change in length. This leads to a discrepancy in the two structures, causing the fiber to bend and results in a helical crimp.298 Feathers Feathers, an abundantly available product stream yielded from the harvesting of poultry, are made up predominantly of β-keratin. Industrial derived feathers can yield fiber material that ranges from 5 to 1700 µm in length, and 10 to 40 µm in diameter, for fiber aspect ratios which range from 1:1 to 170:1.300 Feather hold good acoustic and thermal insulation properties due to air pockets that develop between the keratin microfibrils.164,301 For their density, which ranges from 0.8 to 0.89 g/cm3, and size, feather fibers have relatively impressive mechanical properties, with tensile strengths around 160 MPa and elastic modulus of around 4.0 GPa.300,302,303 2.3.2 Fibroin. Fibroin is a form of protein created in the production of silk (along with sericin, which is a non-structural protein which acts as a sticky coating along a silk fiber’s fibroin center structure). Fibroin protein consists of layers of antiparallel β sheets, and it is considered to be a type of β-keratin, although an established phylogenetic connection with vertebrate β-keratin has yet to be established. Its primary structure consists mainly of the recurrent amino acid sequence (Gly-Ser-Gly-Ala-Gly-Ala)n . Due to the amino acid’s high glycine content, β sheet packing is tight, leading fibroin to have excellent tensile strength and modulus performance.294,295 Silk Silks are made by a variety of arthropods; however, the most abundantly harvested silk comes from the Bombyz mori silkworm larvae, which spins a cocoon of silk fiber that can be harvested in a controlled environment. Silkworm silk has a tensile strength of approximately 125–420 MPa and a modulus of 8.9–17.4 GPa, on a fiber which has a density of approximately 1.3 g/cm3 and ranges in diameter from 7 to 12 µm.298,303–306 Compared to other natural fibers, silk is rather uniform and can be obtained in a continuous form. But it is expensive compared to other natural fibers, making its


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practical use within composites comparatively limited.305 Another source of silk which has been explored has been spider silk, as produced by a variety of arachnoids. Spider silk holds great performance potential, however farming and harvesting of spider silk is currently infeasible, and exploration into spider silk use is limited to exploratory synthesis as of current.298


3. Fiber Treatment There are several factors that can affect the mechanical performance of composites, including the properties of the reinforcements and matrices, the orientation of fibers, and the fiber-matrix interaction. With natural fiber composites, the fiber-matrix interface is an especially important point of focus, as the assumption of perfect adhesion is not a viable one. Since this interface determines the ability of the composite to effectively transfer stress to the fibers through the matrix, it plays a significant role on the mechanical performance of the final composite. The chemical constituents that make up lignocellulosic fibers play varied roles in affecting the interfacial interaction between the fibers and the polymer matrices used in their composite production. The non-cellulosic chemicals, while binding and providing protection to the cellulose fibrils, can interfere with potential bonding between the cellulose and polymer matrices. This can limit the direct interaction between a composite’s matrix and the actual reinforcing cellulose component of the fibers. Furthermore, the hydrophilic nature of these components, along with cellulose itself, can lead to severe polar mismatching between the plant fibers and hydrophobic polymers used as matrices. Finally, the intermolecular hydrogen bonding of the various chemical constituents can hinder the dispersion of plant fibers, especial for short fibers and particulate dispersion in thermoplastics, yielding inconsistency in the final composites. To improve the physical and mechanical properties of natural fibers reinforced composites, a number of surface treatments have been explored. These range in focus and application. Some approaches focus on the removal of non-cellulosic constituents from the surface of the fiber to provide a cleaner surface. Others focus on separating the large fiber bundles into smaller bundles of elemental fibers so as to provide greater surface area for matrix interaction. And yet other focus on introducing certain chemicals or free radicals to form covalent bonds between plant fibers and polymer matrices, to help improve the interfacial adhesion. Some treatments work with the intent of meeting multiple of these improvements. In pursuit of these different directions, three major categories of surface modification have come under development: physical methods, physico-chemical treatments, and chemical modifications. 3.1 Physical Treatments The intent of all the physical treatments that are applied to natural fibers is to partially change the structural and surface properties of the lignocellulosic fibers. In doing so, improvements in the fiber-matrix adhesion can be yielded. Physical treatments include fiber stretching, calendaring, rolling or swaging, solvent extraction, electric discharge, laser, γ -ray and thermotreatment. 3.1.1 Simple Mechanical. Stretching,65 calendaring,66 and rolling or swaging67 are traditional mechanical techniques used for long plant fibers, which are used to yield fiber bundle separation and improve the surface area available for matrix interaction. However, these

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techniques bring risk, including the potential of damage to the fibers during the processing. This can subsequently lead to decreased fiber performance, negating some benefits of improved matrix interface.65 However, overall these types of processes tend to improve composite performance. 3.1.2 Solvent Extraction. The simplest way of increasing the surface area for short fibers and fillers is through mechanical fractionation.68,69 However, this method can cause significant degradation of the fibers’ reinforcement capability due to reductions in fiber aspect ratios. One manner to avoid this and still increase fiber yield from short fibers and particulates is though solvent extraction (liquid extraction). Using this method, lignocellulosic fibers can be separated from plant sources by selective solvent action, obtaining fibers with high content of cellulose.70–72 However the use of chemical solvents brings with it negative aspects, especially when trying to maintain a “green” friendly approach to material development. During solvent extraction there are certain amounts of hazardous substances and vapors produced which can pollute the environment. 3.1.3 Electric Discharge. A fairly low environmental impact method of physically separating fibers is by electric discharge. Electric discharge activates and modifies the surface of the hydrophobic lignocellulosic fibers to achieve good compatibilization between them and non-polar matrices. This can entail corona treatment, plasma treatments, or ionized air treatments.45,73,74 Corona treatment is a surface oxidation activation method which uses a luminous, audible discharge that takes place at or near atmospheric pressure.75 There are several corona forms that can be released to cause activation, such as pulse corona, pulseless corona, streamer corona, glow corona and spark. The type depends on the polarity of the field and the electrode geometrical configuration. Corona treatments can be used as pretreatments to activate the cellulose for further chemical treatments, such as grafting76, or they can be used as standalone surface modifications. Corona treatment has been shown to increase fiber surface polarity, which is good for interacting with hydrophilic polymer matrices. On the downside, a corona discharge can decrease the fiber tenacity due to the surface ablation and etching.75 Plasma treatment can also be used as a modifier to increase or decrease fiber surface energy, cause surface crosslinking, and introduce reactive free groups.77 Plasma treatment applied to lignocellulosic fiber uses a cold plasma, in which the electrons have a very high temperature, which can provide a sputtering effect on the surface of fiber, increasing the possibility of chemical modification73,75. The cold plasma treatment can be processed under vacuum, or more commonly at atmosphere since air pressure cold plasma does not have the same volume limitation as vacuum plasma. The effects of plasma treatment are similar to corona treatment: surface cleaning, ablation or etching, modification the surface chemical structure, and introduction of free radicals.78 In addition, surface crosslinking can be introduced which may strengthen the surface layer of the fiber.77 However, the modification can be very heterogeneous depending on the treatment conditions.61 Another electric discharge method is ionized air treatment. This method can minimize the aggregation of fiber bundles through charging neutralization using corona-like electron discharges. The aggregation of the fiber bundles is related to both electrostatic buildup on the fiber surfaces as well as the retaining of charges in the inner part of the fiber.74 These charges can be neutralized by subjecting the fibers to the charged species present in ionized air. Like corona treatment, ionized air treatment can be combined with other


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chemical treatments.79 Although ionized air treatment increases the wettability of a fiber,80 the improvement to the interfacial properties are lower than many chemical treatments.45


3.1.4 Thermal Treatment. Thermal treatments involve autoclaving fibers around or below the glass transition temperature of lignin (around 200◦ C) for different durations of time times to dry and separate the fiber bundles to single fibers.75,78,81 During this process, other non-cellulose constituent and chemicals, which have similar or lower glass transition temperatures than lignin, depolymerize or release from the fiber bundles. Thermal treatments can increase the crystallinity and dimensional stability of lignocellulosic fibers. In addition, the hydrophobicity of the surface of the fibers can be increased due to non-cellulose chemicals melting and flowing to the surface.7,82,83 3.1.5 Steam Explosion. Steam explosion is a method used separate the lignocellulosic fiber bundles into their elementary fibers and main components.75 This process is widely used in extracting wood fibers, but can also be applied to high cellulose long fibers to further isolate the cellulose.81 Steam explosion reduces the amount of minerals, water-extractables, and pectin within the fibers, however it often also has the adverse effect of reducing the length of longer fibers. Just like its use on wood, steam explosion can be applied to various biomass feedstocks, using saturated steam and various reaction times. During the treatment of biomass (or wood pulp), the biomass is pressurized with steam for a short time and then exposed to atmospheric pressure, which leads the biomass defibrillation. As such, steam explosion can significantly reduce the size of fibers or particles, while potentially increasing the crystallinity of the fibers, resulting in a higher modulus of the fibers.84 3.2 Chemical Treatments 3.2.1 Alkaline Treatment. Alkaline treatment is a common fiber surface treatment that can also be used as a pre-treatment when combined with other chemical modifications. Originally, these treatments were carried out to improve a fiber’s dye affinity and luster.85 Researchers subsequently discovered that alkalization also has a positive effect on the mechanical properties of fiber-reinforced composites, yielding significant improvements in the interfacial performance.39.86 The structure of cellulose in plant fibers exhibits a monoclinic crystalline lattice of Cellulose-I that can be changed into different polymorphous forms through alkaline treatment.85,87 The effect of alkali on a cellulose fiber is a swelling reaction, during which the natural crystalline structure of the cellulose relaxes. The type of alkali and its concentration will influence the degree of swelling and the degree of lattice transformation into Cellulose-II (Fig. 4.). Usually, sodium hydroxide (NaOH) is used because Na+ is able to widen the smallest pores in between the lattice planes and penetrate into them. After removing the excess NaOH, the new Na-Cellulose-I lattice is formed. Meanwhile, the OH groups of the cellulose are converted into ONa- groups. Rinsing with water removes the linked Na ions and converts the cellulose to a new crystalline structure: Cellulose-II, whose lattice is more stable than Cellulose-I. The total reaction is shown in Fig. 5.88 After alkali treatment the surface of the lignocellulosic fibers reveal a smoothing effect. This is partially due to the removal of the outer surface of the fiber layer through dissolution in chemical solutions during the treatment. Alkaline treatments promote the interfacial adhesion between fibers and matrices, particularly improving the mechanical


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Figure 4. A schematic which represents the crystalline lattices of Cellulose-I, Na-Cellulose-I, and Cellulose-II.85

interlocking. The tensile and flexural properties of NaOH treated flax/epoxy (Vf = 40%) showed significantly improvement compared to untreated flax/epoxy:85 approximate 30% increase in both longitudinal tensile strength and flexural strength; about 27% increase in both longitudinal tensile modulus and flexural modulus. Usually, alkali treated long plant fibers shrink if no tension is applied to the fiber during the treatment. However, when sodium hydroxide/ethanol solutions are used instead of sodium hydroxide/water solutions, it is found that the shrinkage is negligible and the fibers remain easy to process.89 3.2.2 Coupling Agents. Coupling is a common method of chemical modification. A coupling agent contains chemical groups which react with the fiber surface and the polymer matrix. Covalent and hydrogen bonds are formed in the reaction, which improves the interfacial adhesion. Many coupling agents have been used in the surface modification of natural fibers. These coupling agents tend to vary with each polymer matrix system. Silanes Silane treatment is a widely used surface modification method used for fiber/polymermatrix interfaces. Various silanes have been effective in improving the interface properties of natural fiber/polymer composites. The type of silane chosen is decided by the matrix system, with formulations designed for both thermoset and thermoplastic polymers. The results of tensile tests, pull-out tests, and single fiber fragmentation tests suggests that silane treatments yield a greater property improvement over other chemical treatments like alkalines.28,90,91 Usually, the first step of a silane treatment is the mercerization process, which activates the OH groups of the cellulose and cleans the surface of lignocellulosic fibers using an alkaline sodium hydroxide bath. Silanes are first hydrolyzed in water, as shown in Fig. 6., for a triethoxy vinyl silane. The silane is then reacted with the lignocellulosic fiber, forming polysiloxane structures

Figure 5. Sodium hydroxide fiber reaction.


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Figure 6. Silane hydrolyzing in water.

through reaction with the hydroxyl groups of the fibers (Fig. 7). This process starts with the silanes interacting with the fiber’s hydroxyl groups, undergoing hydrolysis, condensing, and finally yielding bond formation.28,86 The surface topology of a given fiber will play a role in the interfacial adhesion between fibers and matrices. SEM analysis of the surface topology of both untreated and treated fibers shows evidence that physical microstructure changes occur at the fiber surface during silane treatment (Fig. 8). Many micropores were observed on the surface of flax fiber, while the silane coupling penetrated these micropores and formed a mechanically-interlocked coating. Silane modification effectively results in improved surface tension, wettability, swelling, adhesion, and compatibility with polymeric materials. Acylation Acylation is another method of modifying the surface of lignocellulosic fibers with acyl groups to improve a fiber’s hydrophobicity. There are two ways to add the acyl groups to the surface of lignocellulosic fiber: acetylation using acetate92–94, and valerylation using valerate. The principle of the method is to react the hydroxyl groups (OH) of the lignocellulosic fiber constituents with acyl groups (RCO). In acetylation, many acetates can be used, including acetyl chloride, acetic acid, and acetic anhydride. Figure 9 shows a acetylation treatment of fibers that starts with an acetic acid treatments followed by a second treatment with acetic anhydride containing concentrated H2 SO4 .88 Beyond helping to change the polarity to make the fibers more hydrophobic, partial hydrolysis of cell walls during the processing can help inhibit fermentative microorganisms, which is beneficial to composite environmental durability.95 Acetylation is one of the most studied reactions of lignocellulosic materials.96 But despite being so well examined, evidence suggests that acylation may not be as effective as alkaline treatments are for improving the interfacial interaction with matrices.97 Graft Copolymerization Graft copolymerizations are another common method used to chemically compatibilize natural fibers and polymer matrices. In this method, functional groups which can interact with cellulose or other constituents of the natural fibers are grafted to

Figure 7. Fiber reaction in silane.


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Figure 8. SEM micrographs showing the fiber surfaces of (a) untreated flax fiber, and (b) silane treated flax fiber89 (Color figure available online).

polymers that are either the same as, or have a polarity similarity to a given matrix. In this way, the grafted systems are able to act as bridges to alleviate the mismatches in polarity between hydrophilic fibers and hydrophobic matrices. The functional groups which are employed include methyl groups, isocyanates,1,98 triazine,1 benzoylation,55,82 maleic anhydride,55 and organosilanes.54 Overall, due to performance, cost, and commercial availability, the most prominent functional group for compatibilization by graft polymerization is maleic anhydride (MA). Due to the high hydrophobicity of polyolefin matrices, grafting of MA to polyolefins, such as with MA grafted polypropylene (MAPP),69,99–102 and MA grafted polyethylene (MAPE),103–106 has become a common approach to improve composite mechanical performance through aided interfacial interaction. However, MA has also been explored as a functional group grafted to other thermoplastic matrices, including styrene-(ethylene-cobutylene)-styrene (SEBS),101,107–109 polystyrene,110 poly(lactic acid) (PLA),111,112 and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV).113 Figure 10 shows the basic reaction mechanism for the grafting and subsequent interaction with lignocellulosic fibers as it applies to MAPP.114 The MA group of the MAPP is able to react with the hydroxyl groups in the fiber’s cellulose, forming a bond between the MA’s carbon and the oxygen from the cellulose. At the same time, the PP portion of MAPP is able to co-crystallize with the unmodified PP, which allows for the MAPP to act as a bridge between the fiber and matrix.110,115 MA grafted polymers are also able to compensate for insufficient breakup forces during processing when dealing with the melt compounding of short fiber thermoplastic composites, by reducing the interfacial tension between the polymer matrix and the fibers themselves. This can lead to finer dispersion of the fiber throughout the system, improving mechanical performance.116 Another functional group widely used is acrylic acid. The graft polymerization of acrylic acid, initiated by free radicals on the surface of cellulosic fiber, has been investigated in much the same manner that maleic anhydride has been.117,118 Acrylic

Figure 9. Fiber acetylation treatment.8



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Figure 10. Chemical reaction mechanism of MAPP and lignocellulosic fiber.114

acid is used as a hydrophilic monomer for the advantages of its ionic character and its gelling capacity as a polymer. Acrylic acid undergoes the typical reactions of a carboxylic acid. When acrylic acid reacts with the hydroxyl groups (OH) of a natural fiber, it forms corresponding esters. The acrylic acid and its esters then readily combine with themselves or other monomers by reacting at their double bond, forming homopolymers or copolymers. Likewise, other vinyl monomers can be used in a similar manner, as has been explored with acrylonitrile119,120 and acrylamide.121 Different coupling methods have different chemical effects on both fiber and polymer matrices. However, all the coupling treatments improve the mechanical performance of natural fibers/fillers composites. The coupling agents give rise to a degree of chemical types of interfacial adhesion. The interfacial shear strength of acetylation flax/polyester composites showed an approximately 12% increase compared to untreated flax/polyester composites.89 Better interfacial bonding usually indicates a better load transfer between fibers/fillers and polymer matrices. The HDPE-g-MAH enhanced the tensile properties and impact strength of teak wood flour/HDPE-g-MAH/HDPE significantly.104 3.2.3 Bleaching. Bleaching treatments are typically used on cotton fiber and synthetic fibers to deduce the color and increase fiber whiteness.122 The underlying principle of bleaching is the reaction of a bleaching agent with the functional groups of a fiber, which show different colors, changing the structure of the groups to cause whitening. The color imposed by the inclusion of lignin in a fiber is due to the conjugation between aromatic rings and carbon double bonds. Since lignocellulosic fibers will contain certain amounts of lignin and pectin, which are difficult to clean by other chemical treatments, bleaching can be used as an effective way to degrade lignin on the surface of flax fiber, as well as hydrolize pectins. The two major types of bleaching agents that exist are classified according to their mechanisms of chemical reaction: oxidation bleaching and reduction bleaching.

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Figure 11. Oxidation bleaching with sodium hypochlorite.

Reduction Bleaching Reduction bleaching is performed using either sulfur dioxide (SO2 ) or sodium dithionite (Na2 S2 O4 ). It is usually only performed on protein fiber textiles, as higher cellulose fibers see significant losses in durability. Thus it is rarely explored for modification of lignocellulosic fibers for composite purposes. Oxidation Bleaching Oxidation bleaching is the most commonly used method, including bleaching using sodium hypochlorite (NAClO), hydrogen peroxide (H2 O2 ), or sodium chlorite (NAClO2 ). Sodium hypochlorite can attack the hydroxyl groups in the lignin to form aldehyde groups (CH O), degrading the lignin as shown in Fig. 11. (where R and R’ represent the structures of lignin). However, the existing aldehyde groups by this approach can cause the cellulose to degrade, leading to detrimental performance of the fibers. To alleviate this, hydrogen peroxide can be used to react with the hydroxyl groups of the lignin, forming carbonyl group (C O) which deduce the lignin as shown in Fig. 12 (where R and R represent the structures of lignin). During this bleaching process, the degradation of lignin is the highest among all oxidation methods yet the cellulose fibers will not break down. However, the cost of hydrogen peroxide is much higher than many other bleaching agents with similar capabilities. Sodium chlorite bleaching’s mechanisms are still controversial, and there exist several assumptions to explain the reaction. Despite this, sodium chlorite bleaching can reduce lignin and pectin with minimal amounts of cellulose degradation, at a reasonable cost compared to sodium hypochlorite and hydrogen peroxide. 3.2.4 Enzyme. The use of enzymes, chelators, and enzyme/chelator systems has also been explored as an environmentally friendly means of improving the quality of lignocellulosic fibers for composite applications. Pectinases applied on lignocellulosic fibers are commercial mixtures of several types of enzymes, which can include Flaxzyme, Viscozyme, and Pectinex AR.92,123 Pectinolytic enzyme preparation and ethylene diamine tetraacetic acid (EDTA) can be used separately or in combination to treat lignocellulosic fibers.124 Through the use of enzymatic systems, improvement in fiber cleanliness and separation of the fiber bundles has been observed.125 However, enzymatic treatment can promote the growth of molds on fibers (300–900 fold increase in microbial counts) due to the removal of pectin which binds the fiber bundles.81 Depending on the mold type and propensity, this can somewhat reduce the effectiveness of their use as interface modifiers. It was observed in EDTA modified fiber reinforced composites that tensile strength improvements in excess of 50% could be achieved.126 However, the strength of fibers can also be decreased due to the hydrolysis of fibers in the acid solutions, since the optimal

Figure 12. Oxidation bleaching with hydrogen peroxide.


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Figure 13. Peroxide reaction with cellulose.


pH of pectin lyase and pectate lyase solutions is between 3–6. To alleviate these concerns, exploration in to the use of alkaline pectate lyase has been examined. This resulted in alkaline pectate lyase treated fiber strengths that were higher than those treated with an acidic polygalacturonase solutions.127 3.2.5 Peroxide. Peroxide treatment is a process similar to the initiation step of the free radical polymerization, which induces adhesion in cellulose fiber-reinforced thermoplastic composites, has drawn the attention of a number of researchers.94,128 Chemicals which can be used as initiators of the polymerization include dicumyl peroxide (DCP)120 and benzoyl peroxide (BPO).129 An alkali pre-treatment is required before the peroxide treatment to separate wax, hemicelluloses, and lignin. Figure 13 shows the decomposition of the peroxide and the subsequent reaction at the interface, in which the final step should occur at the time of curing of composites. Since peroxide treatment is the first step of free radical polymerization, it works best as an interface modifier when the curing mechanism of the matrix polymer is free radical polymerization. The peroxide initiated cellulose free radicals may react with the radicals of the matrix. Using polyethylene as an example matrix, the following reactions can occur during the processing of the composites: increases in molecular weight and crosslinking of the polymer matrix by the combination of macro-radicals of polyethylene (Fig. 14), or else grafting of the matrix onto the cellulose fibers by combing cellulose and polyethylene radicals (Fig. 15). In either case, the hydrophilicity of the fibers decreases after the peroxide treatment. Permaganate While technically not a peroxide treatment, another similar treatment method is with permanganate. Permaganate treatment uses the oxidation property of KMnO4 to reduce the hydrophilicity of cellulosic fibers. The degree of reduction in hydrophilicity is found to grow with increases in KMnO4 concentration.61,98 Peroxide treatments can significantly reduce the water absorption of natural fibers/fillers, which in turn can improve the interfacial adhesion between fibers/fillers and hydrophobic polymer matrices.291 Meanwhile, the tensile properties of peroxide treated natural fiber reinforced thermoplastic composites showed clear improvement.292 On the other hand, the rate of peroxide decomposition can also negatively affect the mechanical properties of treated composites. From the fracture surface of DCP treated and BPO treated composites, it can be understood that the interfacial bonding is stronger in the DCP treatment.292

Figure 14. Peroxide reaction by combination of macro-radicals of polyethylene.

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Figure 15. Peroxide reaction by combining cellulose and polyethylene radicals.

3.3 Physico-Chemical Treatments


Physico-chemical treatments are those which combine physical treatments, such as thermal treatment or steam explosion, with chemical treatments to create assistance to the chemical reactions and better separation of fiber bundles. These types of treatments provide clean and fine natural fibers or fibrils which have very high cellulose content. The mechanical properties of these fine fibers are close to the pure cellulose fiber, which can significantly improve the mechanical properties of their composites. 3.3.1 Hydrothermal Treatment. Hydrothermal treatment of fibers is a combination of chemical treatment and autoclave processing, making it a combination of both thermal treatment and chemical modifications.92,124 At its base, hydrothermal treatments can provide benefits over base chemical treatments due to the high pressures and temperatures of autoclaving, assisting the chemical penetration and increasing the chemical reaction rate. While the chemical modifications play the largest role on the finial interfacial properties of the fibers, the thermal treatment assists in the separation of the fiber bundle, improving surface area exposure for both treatment and matrix interaction. Alkaline solutions are the most common chemicals used in the hydrothermal treatment. However, other chemicals can be used in the autoclave reactor, including silicone oil emulsion,130 phosphoric acid,131 and Na2 CO3 with alkaline conditions.132 Temperatures ranging from 150◦ C to 500◦ C have been reported used.131,133 3.3.2 Steam Explosion. Steam explosion techniques using chemical solution are another approach to physico-chemical treatment of fibers. Like the purely physical process, fibers are pressurized then exposed to atmospheric pressure, causing them to defibrillate into smaller fibers with increased crystallinity. However, the process is done in conjunction with chemical treatments such as Na OH solution92,134 or Na2 CO3 solution.132 In doing so, further removal of lignin and other impurities is obtained, yielding higher cellulose fibers for filler and short fiber applications.

4. Fiber Weave and Mat Preparation The length of fibers yielded from plants is dictated by a number of factors, ranging from the fiber plant source itself, to variances within particular plants based on growing conditions and harvest times. But inherently, unlike the case for manmade fibers, lignocellulosic fibers will always have finite lengths for individual fibers. This makes their use in mat form and in continuous tow applications uniquely challenging compared to traditional manmade fibers. 4.1 Random Oriented The simplest manner of incorporating natural fibers into a mat for reinforcement purposes is through a random, non-woven orientation. Non-woven mats are beneficial because they can be produced using a wide variety of plant sources, including those that yield shorter fiber lengths. Likewise, fibers of longer length run less risk of dependency on maintained fiber integrity during extraction. This makes mat production possible without the need



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for significant additional processing, an ideal factor in developing low cost composite reinforcement. One of the major problems with random oriented mats is that they inherently lack preferential stress directions and are incapable of producing composites with high stress/strain levels. In composite production, this mechanical defect is compounded by the relatively high porosity that exists within the structures of natural fiber. This not only can lead to discontinuity in the fiber, but also raises the propensity for composite voids. Random oriented fiber mats tend to have higher porosity than woven or aligned mats to start with. This severely limits the overall fiber volume fraction achievable for non-woven natural fiber mats. It has been estimate that random oriented natural fiber mats are limited to achieving composite fiber volume fractions of about 30% due to their high porosity, compared to 65% for natural fiber woven mats, and 75% for unidirectional bio-fibers.20 Non-woven natural fiber reinforcement can be incorporated into composites by a number of means. This simplest, although least optimal method, is to randomly disperse chopped fiber as layers of preform for impregnation with liquid resins.43 It is also used with thermoplastics, where sheets of plastic are heated and pressed to impregnate the layers of dispersed fiber.135 However, in both instances, the production yields low quality composites due to fiber distribution issues and high levels of porosity invoked. Randomly dispersed chopped fibers can also be more effectively incorporated as preforms when using binders to form sheets by means of sheet molding compound (SMC).136,137 In doing so, random nonwoven fibers are able to be handled and processed by secondary means such as compression molding, without the risk of large fiber movement and porosity changes. The most prevalent, though still being heavily explored, method of non-woven fiber mat creation is by needle-punch. Needle punching imparts mechanical binding between fibers by orienting and interlocking the fibers of a spunbonded or carded web by passing a barbed needle in and out of the web. This allows fibers to be connected, though unwoven, and leads to better dispersion and random orientation compared to just dispersing chopped fibers. It also imparts a degree of through-thickness fiber orientation that is less likely to be achieved through chopped fiber dispersion. However, due to the short nature of many natural fibers, often even needle punching will not be enough to yield a handleable mat, and secondary chemical binders are needed. The use of these binders then holds the potential to be either beneficial or harmful to subsequent composite productions, depending on formulation of the binder and matrix resin.138,139 4.2 Tow/Yarn Creation To use plant fibers in applications that require long, continuous strands of fiber, additional manipulation beyond fiber extraction is necessary. Natural fibers rarely are much longer than a meter in length at longest. This limits their ability to be used as continuous individual fibers. To be used in applications that require fiber tow weaving or continuous tow placement, multiple fibers are spun together into yarns. In doing so, individual fiber ends get encompassed and twisted with middle regions of other individual fibers, creating larger continuous filaments. Fiber yarns can be spun by hand, or more commonly for industrial applications, by winding machines which can precisely control fiber alignment and yarn thickness.140 Filament density and diameter can be controlled by the number of individual fibers spun together, the degree of twist, and the method of spinning. Two major spinning methods are used: wet spinning, which allows the creation of tight, finer yarns which are saturated in water; and dry spinning, which creates coarser, larger diameter filaments.15,20


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The amount of twist imparted during the spinning of a yarn has a large bearing on the filament’s mechanical performance. A large degree of twist is detrimental to both the strength and the modulus of the resultant yarns compared to the theoretical fiber performance.141 This is because natural fibers have an inherent anisotropy that is biased in favor of the fiber’s longitudinal direction, and a higher degree of twist directs fibers in a manner that lessens the longitudinal direction of individual fibers along the length of a yarn.140,141 Often, a high degree of twist is necessary to maintain the integrity of a yarn, as is the case with shorter starting fibers.142 Insufficient twist can also be detrimental to yarn performance despite allowing better orientation of the individual fibers, due to lack of frictional binding. Longer starting fibers do not require as high a degree of twist to maintain integrity, and thus can benefit greatly from twist control. The degree of twist can also play a role on the tightness and permeability of a yarn, which can dictate the wettability of the filament during composite processing. As such, both processing and performance of a natural fiber composite using spun yarn is partially dictated by the degree of twist. Beyond the ability to create weaves or use continuous fiber toes, creation of natural fiber yarns can also be beneficial in composite processing due to the surface characteristics yarn exhibits. Due to their makeup of multiple discontinuous fibers, yarns have fiber ends protruding out along their surfaces, known as yarn hairiness.143 This hairiness can promote better fiber matrix mechanical interface interaction and interlock, especially compared to the surface of unspun individual fibers. 4.3 Weaves Woven fabrics allow for fibers to be incorporated into mat form while deriving benefits from continuity and anisotropy. But as detailed in previously, natural fibers are not continuous unless spun into yarns, and thus are not as simple to weave. Natural fiber yarns tend to be coarser and thicker than synthetic fibers. While the coarseness (or hairiness) can be an asset for composite mechanics, it makes the fibers more difficult to process using traditional looms, which are designed for finer, smoother fibers.144 Despite this, a number of knit and weave techniques have been found viable for natural fiber mat production. 4.3.1 Uniaxial. The simplest way to create uniaxial natural fiber composites, like that which is done with random dispersed short fibers is to place individual fibers downs with preferential directionality. This method can be done at low cost, while removing the necessity to spin yarns before creating the preform. But in laying fibers by hand, issues of uniform directionality, as well as even fiber dispersion, porosity, and final part thickness all arise. As such, weaving unidirectional mats is an important processing consideration. Natural fiber uniaxial mats are created by warp knitting a yarn in interlacing loops, while interlacing a second yarn in the weft direction to maintain a binding of the yarn tows in the warp direction (Fig. 16). This yields a woven, uniaxial fabric which does not have any fiber crimping, a characteristic that is especially important in uniaxial applications.141 The yarn in the weft direction can be used to a greater or lesser extent depending on the necessity of binding needed. Thinner yarns of different fiber type are commonly used to reduce the extraneous fiber’s footprint, with cotton (a relatively fine, smooth natural fiber) often being used in the weft with bast fibers for reinforcement in the warp direction. 4.3.2 Biaxial. Biaxial weaving of natural fibers involves interlacing sets of yarn in structured, aligned patterns that form a fabric. The main advantage of biaxial weaves is the ability



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Figure 16. Uniaxial fabric made from flax fiber yarn141 (Color figure available online).

to align “continuous” fiber tows into pre-oriented, handleable preforms. Beyond their preferential stress directionality, they also are able to be used in higher fiber volume fractions than random mats. Weaves can also be crossed into three dimensional architectures; however, to date there are no commercially available three dimensional natural fiber weaves. As for two dimensional, there are three main types of biaxial weaves or architectures which can be used to create natural fiber mats: plain, twill, and satin. Plain Plain weaves are the simplest weave to construct. As shown in Fig. 17, warp and weft fibers are simply aligned in a criss-cross pattern. However, this method introduces a high degree of crimp and poor drapability on the fibers, especially for thicker yarns. As such, it can cause detrimental mechanical performance compared to other biaxial weave techniques.17,138 In cases of heavier yarns, a modification of the plain weave is often used, called a basket weave (Fig. 18). The basket weave is a modification of the plain weave in which two or more yarns per warp and weft are woven in criss-cross.

Figure 17. Plain weave roving scheme.17


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Figure 18. Basket weave roving scheme.17

In doing so, the weave imparts less crimp upon the fibers and yields a flatter, stronger fabric. However, it suffers in that it is less drapable, and less stable than a plain weave fabric. As such, it is used mainly when fiber yarn thickness poses too much risk of crimp by plain weaving.17 Twill In a twill weave, one or more warp fibers is woven over and under two of more weft fibers in a repeated manner, with offsets between rows as shown in Fig. 19. This results in a distinct diagonal rib to appear on the fabric, known as a wale. Because of wale, twill-weave result in fabrics with high drapability compared to plain weaves, with only a small loss in stability. The fewer the interlacings between twills, the more free flowing

Figure 19. Twill weave roving scheme.17



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Figure 20. Satin weave roving scheme.17

the fabric becomes. The twill architectures also result in better fiber wetout and less fiber crimp, resulting in better mechanical performance in composite applications.17,138 Satin A satin weave is essentially a modified twill weave in which a larger number of warp fibers (normally 4, 5, or 8) pass under a single weft fiber, and then vice versa in a repeating pattern (Fig. 20). Like the twill weave, satin weaves are much flatter, impart less crimp on yarn, and yield stronger fiber performance compared to plain weaves. But it is also less stable of a fabric than either the plain or twill weaves. The satin weave also has a degree of asymmetry through the fabric’s thickness, with the faces having alternate weft or warp directionality to them.17

5. Composites As is the case for more traditional fiber and filled composites, composites yielded from natural fibers range greatly in their processing methodologies, obtainable fiber volume fraction, and void contents, and resultant material properties. Especially in relation to final composite material performance, the number of variables that exist make it difficult to create unconditional performance declarations. As is explained in Sections 2, 3, and 4, attention must be placed on natural fiber selection, treatment, and reinforcement architecture. Natural fiber composites can be produced using a wide variety of polymer matrices and processing methods. Broadly, these reinforced systems can be placed into three categories: thermoset composites, continuous short/long fiber thermoplastic composites, and filled thermoplastic blends. 5.1 Thermoset Composites Most thermosetting resins used for traditional composite production can be used as matrices for natural fiber composites. Common thermosetting resins used as matrices include phenolic resin,80,145–147 epoxy,16,49,141,148,149 unsaturated polyester,120,141,150,151 vinyl ester,141,152–154 and polyurethane.80,155–157 The degree of hydrophilicity of these matrices

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provides for acceptable interfacial bonding with the lignocellulosic fibers, even before surface modification, through the formation of covalent bonds. Furthermore, most thermoset resins can be processed and cured at moderate temperatures, normally below the degradation point of lignocellulosic material, providing distinct advantageous for processing versus thermoplastic materials. Different sizes and types of lignocellulosic fibers can be used for thermoset composite production, include loose long and short fibers, woven and non-woven fiber mats, and filler particles. With the use of physical and chemical surface modifications, their composites have yielded promising performance, demonstrating the potential to act as replacements for man-made fibers.

5.1.1 Processing. The processing methods used for lignocellulosic fibers reinforced thermosetting composites are comparable to the methods used for synthetic or mineral fiber composites, including hand layups, liquid infusion methods such as resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM), prepreg development, compression molding, and pultrusion. Despite being able to be processed by a number of comparable methods, the production of composites with natural fibers does have limitations. Due to the necessity of yarn production, which yields twisted fibers with relatively poor yarn tensile strength, methods such as filament winding are not regularly practiced. Furthermore, due to the loftiness and high porosity of natural fibers, production of high fiber volume fraction parts can be difficult. Natural fibers have permeability roughly a magnitude lower than comparable glass fiber mats, meaning that higher volume fractions are preventative to fiber impregnation by traditional means.15,159

VARTM/RTM Liquid infusion molding techniques such as RTM and VARTM allow for the closed mold infusion of fiber preforms, producing high quality parts with reduced atmospheric volatile release. RTM is commonly used for both continuous-strand mat and woven rovings as well as discontinuous dispersed lignocellulosic fibers.82,141,160–162 RTM processing provides good control over the orientation of the fibers. However, it is sensitive to the concentration of fibers, as fibers displacement may be shown when the fiber concentration is low.162 Limitations in permeability limit fiber volume fraction to a maximum of roughly 35–40%, considerably lower than what is obtainable for glass fiber preforms.82,141,159,163 VARTM, an offshoot of RTM, has grown in general industrial use because of its reduction in tooling costs compared to RTM, coupled with the ability to produce larger parts. VARTM has been applied in natural fiber composite production;163–165 however, due to the lower applied pressure and inherent loftiness of most lignocellulosic fibers, fiber volume fractions tend to be low and VARTM is not used as commonly as RTM for natural fiber composite development. Prepreg Development Prepregs can be created using natural fibers in a number of ways. The most common method is by sheet molding compound (SMC).137,167 SMC can be performed just as would be in normal composite production, although care must be taken to prevent excessive moisture uptake after processing. To prevent this, prepregs are often stored in air tight bags.167 Unidirectional prepregs can also be achieved through drum winding of



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fibers and resin, although winding difficulties due to fiber tension and yarn impregnation can make this process challenging.85 Compression Molding Compression molding is a common processing technique used for natural fiber composite production due to its simplicity and versatility. Approaches include pre-mixing short fibers into resins then using heated or non-heated compression to achieve full impregnation and shape,33,71,166 hand-impregnating woven or non-woven fibers mats with liquid resin then pressing with or without heat to achieve resin cure and part formation,43,55,120,148,149 and pressing of natural fibers impregnated in resin by a sheet molding compound (SMC) process.137,167 With different pressure applied, the fiber volume fraction of the composite can be controlled, and yields of 60–65% are possible.33,55,168 In addition, composite parts with a number of complex geometries and fiber orientations are possible, although the size of the composites yielded is limited by the size of presses. Furthermore, the processing method is both labor intensive and brings processors into contact with the liquid resins and their volatile emissions. Pultrusion Pultrusion is an automated process to produce continuous, constant-cross-section composites. It has been used with some success in producing unidirectional composites with natural fiber reinforcement with flax, hemp, kenaf, and jute fibers in yarn forms.141,152,169,170 Because the methods relies on the pulling of continuous rovings through a resin bath/impregnator, care must be taken in the production using natural fibers due to their yarns low pull strength. Pull speeds have been limited to 100–200 mm/min,169,170 which is low compared to the pull speeds used during glass fiber processing.171 Resultant fiber volume fractions achievable are roughly 35–40%.141,169 5.1.2 Performance. Like traditional composite performance, properties yielded by lignocellulosic fiber reinforcement of thermosetting resins is extremely dependent on the fiber source, orientation, fiber volume fraction, and interfacial interaction due to degree of surface modification. Table 3 lists the ranges in strength and modulus performance that can be obtained using natural fiber reinforcements in a number of matrices. Overall, mechanical performance is dictated by traditional composite theory, with a heavy emphasis on the

Table 3 Natural fiber reinforced thermoset mechanical performance Tensile

Flexural

Natural Fiber Composite

Strength Modulus Strength Modulus (MPa) (GPa) (MPa) (GPa)

Unidirectional - Vinyl Ester Unidirectional - Polyester

82–248 45–143

11–24 4–17

95–146 63–280

13 4–41

Unidirectional - Epoxy Random Mat - Polyester

65–535 18–92

6–16 2–7

84–325 50–155

5–24 4–5

Random Mat - Epoxy

49–59

60–11

77–180

2–8

Source 141, 152 58, 149, 161, 168 16, 141, 149 82, 141, 145, 172, 290 33, 72, 141, 145, 148, 166


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Figure 21. Flexural strength of epoxy reinforced with random oriented New Zealand flax (◦) and glass fibers ().72

effects of fiber modification with the intent of achieving good interfacial interaction with a given matrix. Strength Thermoset composites developed using natural fibers can yield a wide variety of strength performance, as shown in Table 3. Surface modification has been shown to yield significant improvements in performance, with researchers reporting 40–80% improvements in composite strength due to surface modification of natural fibers.82,117,120 But despite these improvements, lignocellulosic fibers are on average weaker than man-made fibers like E-glass, and as such, they yield composites which are not as strong as traditional reinforced systems, as shown in Fig. 21. However, the lower density of natural fibers compared to fibers such as E-glass means that on a specific basis, lignocellulosic sources are a viable consideration for low-weight, high strength material development. Modulus Just as was the case for composite strength performance, thermoset composites developed using natural fibers will yield a wide variety of modulus performance depending on fiber type and degree of loading, as shown in Table 3. Due to the high stiffness of many lignocellulosic fibers, natural fiber composites can have relatively high moduli, and are able to directly compare to glass filled composites in many cases on an absolute basis, as shown in Fig. 22. Thus, on a specific basis, thermoset composites reinforced with natural fibers can be extremely good performers. While composite modulus is not as dependent upon the interfacial interaction between fiber and matric, surface modification has been shown to yield small improvements in performance in some cases.82,117,120. Impact Versus neat resins, thermoset composites reinforced with natural fibers will see improvements in impact performance.79,120,151,172,173 Impact performance is based on both



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Figure 22. Flexural modulus of epoxy reinforced with random oriented New Zealand flax (◦) and glass fibers ().72

the properties of the respective matrix and that of the fibers, in addition to the interfacial properties of the two. While the inclusion of natural fibers provides a means of absorbing and redirecting impact energy as is common for most reinforcement types, the effects of surface modification are found to be varied. In some cases, surface modification is found to lead to improved impact performance versus untreated fibers.79,120 However, impact strength is sensitive to this adhesion, and it is more commonly found that improved interfacial bonding results in detrimental effects to composite impact performance.82,151,173 Water Absorption In thermoset composites, traditional water absorption occurs by water diffusion through defects in the interface fiber via the microvoids and microcracks, as well as along the crosslinking points. Due to the hydrophilicity of natural fibers, resultant composites are extremely susceptible to moisture uptake along this interface. Versus glass fibers, untreated lignocellulosic fiber composites are observed to yield significant saturated weight gains (13.5% versus 1% for glass), leading to significant detrimental mechanical properties.174 However, fiber surface modification can significantly reduce moisture absorption by closing off open -OH groups and reduce the void content at the fiber/matrix interface.82,94,117,146 Despite this, natural fiber reinforcement will still lead to higher degrees of water absorption compared to man-made reinforcement, due to the capillary porous cross sectional geometry of most plant fibers.

5.2 Continuous Short/Long Fiber Thermoplastic Composites Continuous fiber thermoplastic composites are materials produced using a preform of fiber (either non-woven or woven) that is bound by a thermoplastic matrix. Due to the difficulty in processing, this class of materials is still relatively uncommon for natural fiber production. However, the low costs of both lignocellulosic fibers and many commodity thermoplastic

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matrices provide the potential of economical, high performance structural components, and as such holds room for a great amount of growth and development. 5.2.1 Processing. The production of thermoplastic natural fiber composites is strongly limited by the temperature and processing times that the fibers can be exposed to without undergoing significant deterioration.115,137,175 Lignocellulosic fibers will begin seeing significant thermal degradation around 180–210◦ C.176 Therefore, the processing of thermoplastic composites must be limited to temperatures at the lower range of this degradation range and for limited processing times. Otherwise, potential exists for significant fiber damage and reduced composite performance.175 This limits the thermoplastics which can be used for composite production significantly. The most prominently used matrix system has been polypropylene (PP), due to the polymer’s low processing temperature range, advantageous economical standing, and widespread use in traditional glass reinforced thermoplastic composites.49,177–181 Other low melt thermoplastics, such as polyethylene (PE),179,180,182 poly(ethylene terephthalate) (PET),179 bio-polyesters such as poly(hydroxyalkanoate)s (PHA),183,184 and poly(lactic acid) (PLA)185,186 have also been used as matrices for bio-based composites. Continuous fiber thermoplastic composite production is also hindered by challenges in achieving proper matrix flow in order to yield good fiber-matrix interfacing. Thermoplastics melt at much higher viscosities then thermosets, and as such are difficult to yield good impregnation without high pressures due to limited flow. When coupled with the relatively high porosity of natural fibers, continuous natural fiber thermoplastic composites have been found to have relatively high void contents, especially as fiber loading is increased as shown in Fig. 23.187

Figure 23. Void contents of long natural fiber reinforced PP composites according to nominal fiber fraction by weight.187


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However, natural fibers are not as strong as man-made fibers such as E-glass, and thus longer fiber composites run the risk of fiber damage occurring during processing at high pressures, hindering composite mechanical performance. While yarns can help achieve more fiber continuity, they also become more difficult to fully impregnate. Due to these concerns, production of thermoplastic composites using continuous short and long natural fibers is still limited in exploration. Currently, methods of production such as film stacking, concurrent weaving, and suspension impregnation have been implemented which rely on high compression during processing to yield impregnation. Film Stacking In film stacking, pre-dried mats of natural fiber (random oriented177,178,180,184 or unidirectional49,140,186) are stacked in combination with film blown or film extruded sheets of a given thermoplastic matrix. By alternating polymer film layers and fiber layers, a simple lay-up laminate can be built. The fiber mats are then impregnated by hot pressing the laminate, causing matrix flow by melt and compression, yielding composites with fiber volumes of upwards of 70%.177,178 Concurrent Polymer Fiber/Natural Fiber Weaving In concurrent polymer fiber/natural fiber weaving, natural fiber and fibers made up of the desired matrix polymer or woven together to yield preforms which can subsequently be compression molded to yield final composites. The commingling of fibers can be done in a number of ways, ranging from the use of carding machines to create webs of individual natural fiber bundles and polymer fibrils,185 to creating woven175 or unidirectional179 preforms by drum co-winding natural fiber yarns and tows of polymer fibers on a drum. The benefit of concurrent weaving of fiber and matrix fibers is the ability to better distribute the polymer and potentially yield better final fiber/matrix interaction. Suspension Impregnation In suspension impregnation, the thermoplastic matrix polymer is dissolved in an applicable chemical solvent. Dried natural fibers can then be immersed in the mixture, coating them with the matrix-solvent solution. During this process, mechanical agitation can assist in separating the fiber bundles in the solution. The fibers are then removed and dried in a vacuum oven to evaporate out the solvent, yielding matrixcoated fibers. The matrix coated fibers are then placed in a mold and compression molded at elevated temperature, yielding a random or unidirectional composite.183 5.2.2 Performance. Strength Random oriented natural fiber mats used as reinforcement in thermoplastics tend to have lower absolute strength performance in relation to comparable E-glass fiber composites due to the relatively low absolute strength of natural fibers compared to the man-made ones,177,178,184 as demonstrated in Fig. 24. For shorter fiber random mat thermoplastic composites, the use of surface interface modification has been found to not be greatly important to improving the strength of the composites versus untreated fibers. In these composites, the upper adhesion limits for the matrix dominated shear failure are reached, as interfacial bonding is greatly influenced by the impregnation pressure during molding.177,178 This can be attributed to the high degree of air inclusion that is introduced when attempting to impregnate the fibers with the high viscosity melts.185


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Figure 24. Tensile strength of PP/flax composites (◦) and MAPP modified flax composites (•) as a function of fiber volume fraction. The solid line represents the data for commercially available glass mat thermoplastic (GMT).177

Unlike in short fiber random mat thermoplastics, the use of surface modification is found to greatly improve the strength performance of yarn derived woven and unidirectional natural fiber thermoplastic composites.49,175 For example, reports of 90% increases in flexural strength and 50% increase in tensile strength have been documented for flax/PP composites when a MAPP surface compatibilizer was used.49 Modulus Random oriented natural fiber mats used as reinforcement in thermoplastics are capable of yielding composites with comparable absolute modulus properties to those produced using traditional man-made fibers such as E-glass,177,184 as shown in Fig. 25. When accounting for the lower density of most lignocellulosic fibers compared to E-glass, this demonstrates that the specific modulus of the natural fiber composites is able to outperform traditional man-made fibers in certain cases. The fiber length to achieve maximum stiffness from a natural fiber (approximately 1mm) is shorter than that required for maximum strength (approximately 8mm), allowing for greater improvements in modulus with additional fiber loading compared to strength with random oriented fibers.184 Reports vary as to the effectiveness of surface modifiers on the final modulus performance of these composites. Some reports suggest that modifiers will play little effect, since the elastic domain is not strongly dictated by surface interaction177. However, other researchers have seen significant improvements in modulus with the use of compatibilization, suggesting that the improvements that can be yielded in strength may also extend to material stiffness115,175. Impact Due to the comparably low fiber strength of natural fibers compared to E-glass, randomly oriented natural fiber composites are significantly more brittle under impact than traditional glass composites177,178, as shown in Fig. 26. Due to the lower fiber



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Figure 25. Tensile modulus of PP/flax composites (◦) and MAPP modified flax composites (•) as a function of fiber volume fraction. The solid line represents the data for commercially available glass mat thermoplastic (GMT).177

strength, the failure mechanism under impact see less contribution from energy dissipation by fiber pullout and debonding, and instead fail by brittle fiber fracture, causing significantly lower impact performance than man-made fiber composites. 5.3 Short Fiber/Filler Thermoplastic Composites Due to the challenges of achieving melt flow when processing thermoplastic composites with continuous fiber preforms, alternative methods to introducing natural fiber reinforcement have been approached. These methods use dry or melt blending to disperse short fibers or particles throughout thermoplastic matrices, which can then be processed to final shape

Figure 26. Notched Charpy impact strength of PP/flax composites (◦) as a function of fiber volume fraction. The solid line represents data for commercially available glass mat thermoplastic (GMT).177

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using various molding and shaping techniques. This normally results in serious reductions in fiber length, yielding composites that are either short fiber reinforced, or particulate filled in the case of non-fiber lignocellulosic inclusions. Using industry standard final shape processing methods for thermoplastics, polymer composites reinforced with short natural fibers derived from high cellulose sources76,186,188–191 can be created. However, thermoplastic molding also allows for the incorporation of lower aspect ratio lignocellulosic systems as reinforcing fillers. Fillers from wood fiber and flour,99,107,192–194 seed husks and hulls,62,99,195–198 and other agricultural byproducts64,69 have all been examined with different degrees of success. With large amounts of these materials being produced with little commercial value, addressing their potential as functional fillers serves a strong market potential. 5.3.1 Processing. The production of thermoplastic composites with lignocellulosic fillers or short fibers, like for continuous fibers, is limited by the temperatures and processing times that the fillers/fibers can be exposed to before significant degradation. This limits the polymers to lower melt temperature polymers such as polypropylene,102,199–201 polyethylene,202–205 poly(vinyl chloride) (PVC),198,206,207 PLA,28,112,208,209 and bio-polyesters such as poly(butylene adipate-co-terephtha-late) (PBAT),210,211 and PHAs.113,212,213 Compounding In order to achieve fiber dispersion and low void contents, short fibers and fillers must be compounded into thermoplastic matrices before they are processed to final shape. This can be done in a number of manners. The most prominent method for melt compounding is by twin screw extrusion, which allows continuous processing and pelletizing of filler and short fiber composite blends.186,188,189,214,215 While the process yields homogeneous mixtures with limited thermal dwell, it can be very damaging to short fibers lengths, as a high amounts of shear are induced in the process.112,216,217 Fiber loadings of upwards of 70 wt% have been achieved218, although at these high loadings processability and surface quality deteriorate, and as such loadings of 20–50 wt% are more common. Fibers and fillers can also extrusion compounded by melt blending with a single screw extruder.76,214,219,220 While this can be useful for short fibers in an attempt to maintain fiber length, the reduction in shear and dispersion of fiber and fillers can yield lower composite mechanical properties compared to twin-screw extrusion.214 Batch mixers mixer have also been used, however these are more limited due to their scalability, and are thus more common for laboratory sized productions.28,192,200,209,221 Final Shape Parts can be produced to final shape by traditional molding methods, such as injection molding,90,188,189,220,222,223 and compression molding.76,224,225 Molding natural fiber/filler reinforced thermoplastics can done with little variation from traditional molding techniques. However, due to the hydrophilic nature of the fillers, and the negative effects moisture can have upon molding (such as part swelling, poor surface quality, and voids), attention must be given to ensuring that the material is thoroughly dried before processing.60,226 Continuous part production can also be achieved by methods such as profile extrusion.204,226–228 Profile extrusion is the most common final shape processing method for wood-thermoplastic composites, and processors have explored a variety of extruder types and processing strategies. These include using traditional single screw extruders to process compounded composite blends, as well


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Figure 27. PP composite strength as a function of fiber loading for composites of (a) treated and untreated jute fiber, and (b) treated and untreated jute fiber (NF) and E-glass (GF).235

as using compounding twin screw extruders with final shape dies to yield final parts directly during compounding.229,230 5.3.2 Performance. Strength In general, without surface modification, the tensile strength of thermoplastics will be decreased with the addition of low aspect ratio lignocellulosic derived fillers. As filler loading is increased, losses in material strength will grow.133,219,231–233 This is due to the low aspect ratio of the fillers and their poor interfacial interaction causing them to act as flaws in the continuity of the matrix. As filler loading is increased, surface void formation also increases, causing detrimental strength performance. However, modification of the matrix-filler interface through filler surface treatments can mitigate this issue. Application of treated fillers has been shown to result in composites with strength performances that are nearly equivalent to that of virgin polymers, even at high filler loading levels.196,201,218,234 As the aspect ratio of the fillers is increased, and the lignocellulosic materials can be defined as short fibers which allow for stress transfer, they become less detrimental to the strength of thermoplastics. However, the strength of the fiber still has little influence on final composite strength when the fiber/matrix interface is poor.186,190,235 But with the use of surface modification, enhancements to composite strength can be achieved,191,232,235,236 as shown in Fig. 27a. Due to the comparatively low strengths of lignocellulosic fibers versus the traditional short fiber such as E-glass, the improvements in composite strength are still significantly less than those gained using established reinforcements (Fig. 27b).235 This provides one of the major hindrances to natural fiber replacement of glass fiber in thermoplastic systems. Modulus For a given fiber or filler, inclusion and subsequent increases in reinforcement loading will yield near linear increases in the modulus performance of a thermoplastic system.133,191,231,237,238 Modulus improvements are the result of the lignocellulosic fibers and fillers, which have higher moduli than the corresponding thermoplastic matrices, exerting a resistance against the deformation of the matrix, in turn restricting polymer chain elongation. As the aspect ratio of the lignocellulosic materials increases, more of the fiber is fully stressed, since stress transfer through the ends of the fibers becomes less significant as length increases. As such, filler made up of chopped short fiber


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Figure 28. Elastic modulus values of different fibers/filler reinforced high density polyethylene composites with at various fiber loadings.237

like hemp, flax, and sisal will yield higher composite modulus values at comparable loadings to smaller aspect ratio fillers derived from ground plant sources, as shown in Fig. 28.237 Figure 28 also demonstrates the general performance of short fiber and filler reinforced thermoplastics in comparison to short fiber E-glass reinforcement. E-glass composites tend to outperform natural fiber composites of similar fiber aspect ratio and loading.191,237 However, when accounting for the significant differences in fiber densities, on a specific basis, short natural fiber thermoplastics can provide proportionate modulus performance. Impact In general, the loading of lignocellulosic fillers will result in significant decreases in overall impact strength of thermoplastics.104,213–215,239,240 Natural fillers have relatively high moduli compared to must respective thermoplastic matrices. They tend to provide points of stress concentration within the matrix since the fillers are not able to absorb enough energy to cause toughening. Since particulates exhibit low aspect ratios, they fail to provide a large surface area for crack redirection, and tend to act as crack initiators rather than diversions for the absorption of imparted energy. Impact toughness in filled systems can be improved using compatiblization, however due to low aspect ratios, this will still yield poor, or at best equivalent, impact performance in relation to most matrices.108 As the filler fiber’s length (and thus aspect ratio) is increased, the potential of lignocellulosic material to not be detrimental, but rather beneficial to impact performance, are observed.190,241 Since the interface between natural fillers and most thermoplastics is relatively weak, crack redirection along the giber length becomes the method of shear yielding in the matrix. Longer fibers redirect crack propagation and increase overall toughness.



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Figure 29. Water absorption of PLA/kenaf bio-composites at 50 wt% fiber loading for different concentrations of glycidoxypropyl trimethoxy silane (GPS) as compatibilizer244 (Color figure available online).

Thermal Short fiber and filler reinforcement of thermoplastics without modification shows minimal, if any, effect upon the melt or glass transition temperature of the resultant thermoplastic composites99,190,213,242–244. For most thermoplastics, the degree of crystallinity observed in resultant systems is also found not to change due to lignocellulosic reinforcement addition.69,99,213,242 This is due to the fibers and fillers being too large to normally restrict the mobilization of the macromolecular polymer chains. However, in certain polymer systems, such as PLA190,244 and PHAs,213 polymer crystallinity was found to be increased due to fiber inclusion. In these, crystalline cellulose exposed at the surface of the fibers is able to act as a nucleating agent.189,244 Water Absorption Lignocellulosic fibers, having open OH groups, are susceptible to significant water absorption, making absorption and swell a concern in their resultant composites. This hydrophilic natures causes swelling and water absorption of the thermoplastic systems to increase with increasing fiber content.213,242,244,245 This is further hindered in cases of poor bonding, which causes increased micro voids thus increased water absorption in the composites. However, through the use of compatibilizers which interact with the -OH groups of the cellulose, significant lowering of composite water absorption can be achieved,102,244 as shown in Fig. 29.

6. Concluding Remarks The importance of fiber type, length, architecture, and surface characteristics as they pertain specifically to natural fibers as reinforcements in a variety of plastics were reviewed in this article. In addition, a thorough description of the natural fibers themselves as naturally occurring composites composed of lignin and hemicellulose polymers bonding reinforcing cellulose polymer chains together was provided.


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For innovations in natural fiber based composite materials moving forward, a deeper understanding of the natural polymers involved in the formation of natural fibers and their ability to be manipulated through chemical, physical, or biological routes is imperative. Some specific thoughts which emerged from this review include how the spiral angle of the cellulose structures in the primary and secondary walls vary from one plant species to the next and how they move elastically during deformation within the fiber structure, ultimately influencing composite performance. In addition, are there ways in the future to manipulate or convert the structure of these naturally occurring binding polymers such as lignin, hemicellulose, pectin, etc., into more structurally robust polymer matrices? Such systems could provide superior bonding to the cellulose and repair these naturally defect ridden structures and therefore improved load transfer for their composites than those produced today. From an agricultural standpoint, areas of research moving forward include the genetic manipulation or modification of agronomic practices of different fiber varieties for improved fiber yield, ease of retting (i.e., separation of the fiber from the stalk and one another), and better uniformity or reduced imperfections as a result of growing conditions. There are obvious degrees of performance variety which arise not only across species and type, but within a single fiber classification. While this can be attributed to an inherent variance within grown product, reducing this variation through careful agricultural study could open a new platform of reliable reinforcement beyond manufactured fibers. Related, another emerging need is in developing analytical fiber grading techniques which can easily and consistently be employed to grade the quality of fiber as they are processed, shipped, stored, traded, etc. prior to being processed into composites for various applications. Perhaps one grade of fiber which is not of high enough quality for a structural, load bearing composite application is still suitable for a non-structural housing or covering composite? Finally, composite designers need to further explore and continue to develop fiber hybridization strategies for reducing the variations in properties observed with 100% natural fiber based composites and to strategically improve certain weaknesses of natural fiber based composites such as tensile strength. From the survey of work which has been accomplished in this area of research most recently, several issues with the current state of technology can be identified which require deeper investigation and understanding before bio-fiber use can become a class of engineering fibers in a wide variety of structural and semi-structural applications. These can range in focus, from continued growth of recognized areas of research, to the development of new platforms of discovery and innovation. Areas where further growth and progress are needed include the continued tailoring of chemical, physical, and biological surface treatments for the wide variety of fiber types that can be introduced into an even larger number of polymer matrix candidates for improved composite performance. However, other avenues exist which show potential to grow bio-composite use. These can range from exploring the ability to stabilize natural fiber structures to allow for introduction in higher melt temperature thermoplastics, as well as developing additives, secondary coatings, etc. which can improve the environmental stability of natural fiber based composites to withstand harsher environments of application. There also exists a large realm of potential in developing both improvements to existing composite manufacturing methods, and new processing routes entirely to improve natural fiber based composite performance. What is evident is that there is a large amount of fiber to be sourced and worked with, and a strongly established platform of development and understanding which is helping to push bio-fibers into a realm of utilization that makes them potential engineering materials. If continued research thrusts are taken, it should not be a matter of if, but a matter of how


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long before natural fiber composite materials are able to become an accepted and reliable structural element for a wide variety of applications.


References 1. Bledzki, A. K.; Gassan, J. “Composites reinforced with cellulose based fibres,” Progress in Polymer Science. 1999, 24, 221–274. 2. Ludueña, L.; Vázquez, A.; Alvarez, V. “Effect of lignocellulosic filler type and content on the behavior of polycaprolactone based eco-composites for packaging applications,” Carbohydrate Polymers. 2012, 87, 411–421. 3. John, M. J.; Anandjiwala, R. D. “Recent developments in chemical modification and characterization of natural fiber-reinforced composites,” Polymer Composites. 2008, 29, 187– 207. 4. Topalovic, T.; Nierstrasz, V.; Bautista, L.; Jocic, D.; Navarro, A.; Warmoeskerken, M. “XPS and contact angle study of cotton surface oxidation by catalytic bleaching,” Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2007, 296, 76–85. 5. Hori, R.; Wada, M. “The thermal expansion of wood cellulose crystals,” Cellulose. 2005, 12, 479–484. 6. Yamamoto, H.; Horii, F.; Odani, H. “Structural changes of native cellulose crystals induced by annealing in aqueous alkaline and acidic solutions at high temperatures,” Macromolecules. 1989, 22, 4130–4132. 7. Zhang, Q.; Bulone, V.; Å gren, H.; Tu, Y. “A molecular dynamics study of the thermal response of crystalline cellulose Iβ,” Cellulose. 2011, 18, 207–221. 8. Dányádi, L. “Interfacial interactions in fiber reinforced thermoplastic composites,” Phd dissertation, Budapest University of Technology and Economics, 2009. 9. Sun, R.; Fang, J.; Goodwin, A.; Lawther, J.; Bolton, A. “Fractionation and characterization of polysaccharides from abaca fibre,” Carbohydrate Polymers. 1998, 37, 351–359. 10. Fang, J.; Sun, R.; Tomkinson, J. “Isolation and characterization of hemicelluloses and cellulose from rye straw by alkaline peroxide extraction,” Cellulose. 2000, 7, 87–107. 11. Mart´ı-Ferrer, F.; Vilaplana, F.; Ribes-Greus, A.; Benedito-Borrás, A.; Sanz-Box, C. “Flour rice husk as filler in block copolymer polypropylene: Effect of different coupling agents,” Journal of Applied Polymer Science. 2006, 99, 1823–1831. 12. Liu, W.; Mohanty, A. K.; Askeland, P.; Drzal, L. T.; Misra, M. “Influence of fiber surface treatment on properties of Indian grass fiber reinforced soy protein based biocomposites,” Polymer. 2004, 45, 7589–7596. 13. Doherty, W. O.; Mousavioun, P.; Fellows, C. M. “Value-adding to cellulosic ethanol: Lignin polymers,” Industrial Crops and Products. 2011, 33, 259–276. 14. Boyce, C. K.; Zwieniecki, M. A.; Cody, G. D.; Jacobsen, C.; Wirick, S.; Knoll, A. H.; Holbrook, N. M. “Evolution of xylem lignification and hydrogel transport regulation,” Proceedings of the National Academy of Sciences of the United States of America. 2004, 101, 17555–17558. 15. Bismarck, A.; Mishra, S.; Lampke, T. “plant fibers as reinforcement for green composites”, In Natural Fibers, Biopolymers, and Biocomposites; Mohnaty, A.K., Misra, M., Drzal, L.T., Eds. CRC Press, Boca Raton, FL, 2005; Ch. 2. 16. Rong, M. Z.; Zhang, M. Q.; Liu, Y.; Yang, G. C.; Zeng, H. M. “The effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites,” Composites Science and Technology. 2001, 61, 1437–1447. 17. John, M.; Thomas, S. “Biofibres and biocomposites,” Carbohydrate Polymers. 2008, 71, 343–364. 18. Mohanty, A.; Misra, M.; Hinrichsen, G. “Biofibres, biodegradable polymers and biocomposites: An overview,” Macromolecular Materials and Engineering. 2000, 276–277, 1–24. 19. Kvavadze, E.; Bar-Yosef, O.; Belfer-Cohen, A.; Boaretto, E.; Jakeli, N.; Matskevich, Z.; Meshvelian, T. “30,000-year-old wild flax fibers,” Science. 2009, 325, 1359–1359.


306

M. A. Fuqua et al.

20. Summerscales, J.; Dissanayake, N. P.; Virk, A. S.; Hall, W. “A review of bast fibres and their composites. Part 1: Fibres as reinforcements,” Composites Part A: Applied Science and Manufacturing. 2010, 41, 1329–1335. 21. Malkapuram, R.; Kumar, V. “Recent development in natural fiber reinforced polypropylene composites,” Journal of Reinforced Plastics and Composites. 2008, 28, 1169–1189. 22. Bodros, E.; Baley, C. “Study of the tensile properties of stinging nettle fibres (Urtica dioica),” Materials Letters. 2008, 62, 2143–2145. 23. Bacci, L.; Baronti, S.; Predieri, S.; di Virgilio, N. “Fiber yield and quality of fiber nettle (Urtica dioica L.) cultivated in Italy,” Industrial Crops and Products. 2009, 29, 480–484. 24. Alves, C.; Ferrão, P.; a.J., Silva; Reis, L.; Freitas, M.; Rodrigues, L.; Alves, D. “Ecodesign of automotive components making use of natural jute fiber composites,” Journal of Cleaner Production. 2010, 18, 313–327. 25. Satyanarayana, K.; Sukumaran, K.; Kulkarni, A.; Pillai, S.; Rohatgi, P. “Performance of banana fabric-polyester composites”. In Marshall, I. (ed.) Proceedings of Second International Conference on Composite Structures, 535–538 (Applied Science Publishers, London, 1984). 26. Akil, H. M.; Omar, M. F.; Mazuki, A. A. M.; Safiee, S.; Ishak, Z. A. M.; Bakar, A. A.; Abu Bakar, A. “Kenaf fiber reinforced composites: A review,” Materials & Design. 2011, 32, 4107– 4121. 27. Nishimura, A.; Katayama, H.; Kawahara, Y.; Sugimura, Y. “Characterization of kenaf phloem fibers in relation to stem growth,” Industrial Crops and Products. 2012, 37, 547–552. 28. Yu, T.; Ren, J.; Li, S.; Yuan, H.; Li, Y. “Effect of fiber surface-treatments on the properties of poly(lactic acid)/ramie composites,” Composites Part A: Applied Science and Manufacturing. 2010, 41, 499–505. 29. De Rosa, I. M.; Kenny, J. M.; Puglia, D.; Santulli, C.; Sarasini, F. “Morphological, thermal and mechanical characterization of okra (Abelmoschus esculentus) fibres as potential reinforcement in polymer composites,” Composites Science and Technology. 2010, 70, 116–122. 30. Athijayamani, A.; Thiruchitrambalam, M.; Natarajan, U.; Pazhanivel, B. “Effect of moisture absorption on the mechanical properties of randomly oriented natural fibers/polyester hybrid composite,” Materials Science and Engineering: A. 2009, 517, 344–353. 31. De Almeida, J.; Aquino, R.; Monteiro, S. “Tensile behavior of high performance natural (sisal) fibers,” Composites Science and Technology. 2008, 68, 3438–3443. 32. Satyanarayana, K.; Guimarães, J.; Wypych, F. “Studies on lignocellulosic fibers of Brazil. Part I: Source, production, morphology, properties and applications,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1694–1709. 33. Srinivasa, C.; Arifulla, A.; Goutham, N.; Santhosh, T.; Jaeethendra, H.; Ravikumar, R.; Anil, S.; Santhosh Kumar, D.; Ashish, J. “Static bending and impact behaviour of areca fibers composites,” Materials & Design. 2011, 32, 2469–2475. 34. Li, Y.; Mai, Y.-w.; Ye, L. “Sisal fibre and its composites: A review of recent developments,” Composites Science and Technology. 2000, 60, 2037–2055. 35. Bisanda, E.; Ansell, M. “Properties of sisal-CNSL composites,” Journal of Materials Science. 1992, 27, 1690–1700. 36. Chand, N.; Hashmi, S. “Mechanical properties of sisal fibre at elevated temperatures,” Journal of Materials Science. 1993, 28, 6724–6728. 37. Shibata, M.; Ozawa, K.; Teramoto, N.; Yosomiya, R.; Takeishi, H. “Biocomposites made from short abaca fiber and biodegradable polyesters,” Macromolecular Materials and Engineering. 2003, 288, 35–43. 38. De Rosa, I. M.; Santulli, C.; Sarasini, F. “Mechanical and thermal characterization of epoxy composites reinforced with random and quasi-unidirectional untreated Phormium tenax leaf fibers,” Materials & Design. 2010, 31, 2397–2405. 39. Valadez-Gonzalez, A. “Chemical modification of henequén fibers with an organosilane coupling agent,” Composites Part B: Engineering. 1999, 30, 321–331. 40. Devi, L. U.; Bhagawan, S.; Thomas, S. “Mechanical properties of pineapple leaf fiber-reinforced polyester composites,” Journal of Applied Polymer Science. 1997, 64, 1739–1748.



307

41. Mishra, S.; Misra, M.; Tripathy, S.; Nayak, S.; Mohanty, A. “Potentiality of pineapple leaf fibre as reinforcement in PALF-polyester composite: Surface modification and mechanical performance,” Journal of Reinforced Plastics and Composites. 2001, 20, 321–334. 42. Idicula, M.; Joseph, K.; Thomas, S. “Mechanical performance of short banana/sisal hybrid fiber reinforced polyester composites,” Journal of Reinforced Plastics and Composites. 2009, 29, 12–29. 43. Idicula, M.; Neelakantan, N.; Oommen, Z.; Joseph, K.; Thomas, S. “A study of the mechanical properties of randomly oriented short banana and sisal hybrid fiber reinforced polyester composites,” Journal of Applied Polymer Science. 2005, 96, 1699–1709. 44. Tomczak, F.; Satyanarayana, K. G.; Sydenstricker, T. H. D. “Studies on lignocellulosic fibers of Brazil: Part III: Morphology and properties of Brazilian curauá fibers,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 2227–2236. 45. Trindade, W. G.; de Paiva, J. M. F.; Leão, A. L.; Frollini, E. “Ionized-air-treated curaua fibers as reinforcement for phenolic matrices,” Macromolecular Materials and Engineering. 2008, 293, 521–528. 46. Alawar, A.; Hamed, A. M.; Al-Kaabi, K. “Characterization of treated date palm tree fiber as composite reinforcement,” Composites Part B: Engineering. 2009, 40, 601–606. 47. de Deus, J.; Monteiro, S.; D’Almeida, J. “Effect of drying, molding pressure, and strain rate on the flexural mechanical behavior of piassava (Attalea funifera Mart) fiber-polyester composites,” Polymer Testing. 2005, 24, 750–755. 48. D’Almeida, J.; Aquino, R.; Monteiro, S. “Tensile mechanical properties, morphological aspects and chemical characterization of piassava (Attalea funifera) fibers,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 1473–1479. 49. Bledzki, A.; Fink, H.-P.; Specht, K. “Unidirectional hemp and flax EP- and PP-composites: Influence of defined fiber treatments,” Journal of Applied Polymer Science. 2004, 93, 2150– 2156. 50. Jawaid, M.; Abdul Khalil, H. “Cellulosic/synthetic fibre reinforced polymer hybrid composites: A review” 2011 51. Harish, S.; Michael, D.; Bensely, A.; Lal, D.; Rajadurai, A. “Mechanical property evaluation of natural fiber coir composite,” Materials Characterization. 2009, 60, 44–49. 52. Biswas, S.; Kindo, S.; Patnaik, A. “Effect of fiber length on mechanical behavior of coir fiber reinforced epoxy composites,” Fibers and Polymers. 2011, 12, 73–78. 53. Shinoj, S.; Visvanathan, R.; Panigrahi, S.; Kochubabu, M. “Oil palm fiber (OPF) and its composites: A review,” Industrial Crops and Products. 2011, 33, 7–22. 54. Arous, M.; Amor, I. B.; Boufi, S.; Kallel, A. “Experimental study on dielectric relaxation in alfa fiber reinforced epoxy composites,” Journal of Applied Polymer Science. 2007, 106, 3631–3640. 55. Kushwaha, P.; Kumar, R. “Influence of chemical treatments on the mechanical and water absorption properties of bamboo fiber composites,” Journal of Reinforced Plastics and Composites. 2010, 30, 73–85. 56. Ismail, H.; Edham, M.; Wirjosentono, B. “Bamboo fibre filled natural rubber composites: The effects of filler loading and bonding agent,” Polymer Testing. 2002, 21, 139–144. 57. Jain, S.; Kumar, R.; Jindal, U. “Mechanical behaviour of bamboo and bamboo composite,” Journal of Materials Science. 1992, 27, 4598–4604. 58. Prasad, A. R.; Rao, K. M.; Gupta, A.; Reddy, B. “A Study on flexural properties of wildcane grass fiber-reinforced polyester composites,” Journal of Materials Science. 2010, 46, 2627– 2634. 59. Wiedenhoeft, A. “Structure and function of wood”, In Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-190, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 2010; Chapters 3, 3.1–3.18. 60. Stark, N. M.; Cai, Z.; Carll, C. “Wood-based composite materials panel products , gluedlaminated timber , structural materials”, In Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-190, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 2010; chap. 11, 11.1–11.28


308

M. A. Fuqua et al.

61. La Mantia, F.; Morreale, M. “Green composites: A brief review,” Composites Part A: Applied Science and Manufacturing. 2011, 42, 579–588. 62. Bledzki, A. K.; Mamun, A. A.; Volk, J. “Physical, chemical and surface properties of wheat husk, rye husk and soft wood and their polypropylene composites,” Composites Part A: Applied Science and Manufacturing. 2010, 41, 480–488. 63. de Sousa, M.; Monteiro, S.; D’Almeida, J. “Evaluation of pre-treatment, size and molding pressure on flexural mechanical behavior of chopped bagasse/polyester composites,” Polymer Testing. 2004, 23, 253–258. 64. Luz, S.; Goncalves, A.; Delarcojr, A. “Mechanical behavior and microstructural analysis of sugarcane bagasse fibers reinforced polypropylene composites,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1455–1461. 65. Haig Zeronian, S.; Kawabata, H.; Alger, K. “Factors affecting the tensile properties of nonmercerized and mercerized cotton fibers,” Textile Research Journal. 1990, 60, 179–183. 66. Semsarzadeh, M. A. “Fiber matrix interactions in jute reinforced polyester resin,” Polymer Composites. 1986, 7, 23–25. 67. Satyanarayana, K. G.; Arizaga, G. G.; Wypych, F. “Biodegradable composites based on lignocellulosic fibers: An overview,” Progress in Polymer Science. 2009, 34, 982–1021. 68. Mullin, W. J.; Xu, W. “Study of soybean seed coat components and their relationship to water absorption,” Journal of Agricultural and Food Chemistry. 2001, 49, 5331–5335. 69. Fuqua, M. A.; Ulven, C. A. “Characterization of polypropylene/corn fiber composites with polypropylene grafted maleic anhydride,” Journal of Biobased Materials and Bioenergy. 2008, 2, 258–263. 70. Le Digabel, F.; Avérous, L. “Effects of lignin content on the properties of lignocellulose-based biocomposites,” Carbohydrate Polymers. 2006, 66, 537–545. 71. Kaddami, H.; Dufresne, A.; Khelifi, B.; Bendahou, A.; Taourirte, M.; Raihane, M.; Issartel, N.; Sautereau, H.; Gérard, J.-F.; Sami, N. “Short palm tree fibers thermoset matrices composites,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 1413–1422. 72. Le Guen, M. J.; Newman, R. H. “Pulped Phormium tenax leaf fibres as reinforcement for epoxy composites,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 2109– 2115. 73. George, J.; Sreekala, M.; Thomas, S. “A review on interface modification and characterization of natural fiber reinforced plastic composites,” Polymer Engineering & Science. 2001, 41, 1471–1485. 74. Faulstich de Paiva, J. M.; Frollini, E. “Unmodified and modified surface sisal fibers as reinforcement of phenolic and lignophenolic matrices composites: Thermal analyses of fibers and composites,” Macromolecular Materials and Engineering. 2006, 291, 405–417. 75. Mukhopadhyay, S.; Fangueiro, R. “Physical modification of natural fibers and thermoplastic films for composites – A review,” Journal of Thermoplastic Composite Materials. 2009, 22, 135–162. 76. Ragoubi, M.; Bienaimé, D.; Molina, S.; George, B.; Merlin, A. “Impact of corona treated hemp fibres onto mechanical properties of polypropylene composites made thereof,” Industrial Crops and Products. 2010, 31, 344–349. 77. Bledzki, A. K.; Reihmane, S.; Gassan, J. “Properties and modification methods for vegetable fibers for natural fiber composites,” Journal of Applied Polymer Science. 1996, 59, 1329–1336. 78. Marais, S.; Gouanvé, F.; Bonnesoeur, a.; Grenet, J.; Poncin-Epaillard, F.; Morvan, C.; Métayer, M. “Unsaturated polyester composites reinforced with flax fibers: effect of cold plasma and autoclave treatments on mechanical and permeation properties,” Composites Part A: Applied Science and Manufacturing. 2005, 36, 975–986. 79. Razera, I.; Frollini, E. “Composites based on jute fibers and phenolic matrices: Properties of fibers and composites,” Journal of Applied Polymer Science. 2004, 91, 1077–1085. 80. Milanese, A. C.; Cioffi, M. O. H.; Voorwald, H. J. C. “Mechanical behavior of natural fiber composites,” Procedia Engineering. 2011, 10, 2022–2027.



309

81. Nykter, M.; Kymäläinen, H.-R.; Thomsen, A. B.; Lilholt, H.; Koponen, H.; Sjöberg, A.-M.; Thygesen, A. “Effects of thermal and enzymatic treatments and harvesting time on the microbial quality and chemical composition of fibre hemp (Cannabis sativa L.),” Biomass and Bioenergy. 2008, 32, 392–399. 82. Sreekumar, P.; Thomas, S. P.; marc Saiter, J.; Joseph, K.; Unnikrishnan, G.; Thomas, S. “Effect of fiber surface modification on the mechanical and water absorption characteristics of sisal/polyester composites fabricated by resin transfer molding,” Composites Part A: Applied Science and Manufacturing. 2009, 40, 1777–1784. 83. Ayrilmis, N.; Jarusombuti, S.; Fueangvivat, V.; Bauchongkol, P. “Effect of thermal-treatment of wood fibres on properties of flat-pressed wood plastic composites,” Polymer Degradation and Stability. 2011, 96, 818–822. 84. Ibrahim, M. M.; Dufresne, A.; El-Zawawy, W. K.; Agblevor, F. A. “Banana fibers and microfibrils as lignocellulosic reinforcements in polymer composites,” Carbohydrate Polymers. 2010, 81, 811–819. 85. Vandeweyenberg, I.; Chitruong, T.; Vangrimde, B.; Verpoest, I. “Improving the properties of UD flax fibre reinforced composites by applying an alkaline fibre treatment,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 1368–1376. 86. Valadez-Gonzalez, A. “Effect of fiber surface treatment on the fiber matrix bond strength of natural fiber reinforced composites,” Composites Part B: Engineering. 1999, 30, 309– 320. 87. Mwaikambo, L. Y.; Ansell, M. P. “Chemical modification of hemp, sisal, jute, and kapok fibers by alkalization,” Journal of Applied Polymer Science. 2002, 84, 2222–2234. 88. Agrawal, R.; Saxena, N.; Sreekala, M.; Thomas, S. “Effect of treatment on the thermal conductivity and thermal diffusivity of oil-palm-fiber-reinforced phenolformaldehyde composites,” Journal of Polymer Science Part B: Polymer Physics. 2000, 38, 916–921. 89. Baley, C.; Busnel, F.; Grohens, Y.; Sire, O. “Influence of chemical treatments on surface properties and adhesion of flax fibre polyester resin,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 1626–1637. 90. Gwon, J.; Lee, S.; Chun, S.; Doh, G.; Kim, J. “Effect of chemical treatments of wood fibers on the physical strength of polypropylene based composites,” Korean Journal of Chemical Engineering. 2010, 27, 651–657. 91. Gwon, J. G.; Lee, S. Y.; Doh, G. H.; Kim, J. H. “Characterization of chemically modified wood fibers using FTIR spectroscopy for biocomposites,” Journal of Applied Polymer Science. 2010, 116, 3212–3219. 92. Pietak, A.; Korte, S.; Tan, E.; Downard, A.; Staiger, M. P. “Atomic force microscopy characterization of the surface wettability of natural fibres,” Applied Surface Science. 2007, 253, 3627–3635. 93. Hill, C.; Abdul Khalil, H. “Effect of fiber treatments on mechanical properties of coir or oil palm fiber reinforced polyester composites,” Journal of Applied Polymer Science. 2000, 78, 1685–1697. 94. Pothan, L. A.; Thomas, S. “Effect of hybridization and chemical modification on the waterabsorption behavior of banana fiber-reinforced polyester composites,” Journal of Applied Polymer Science. 2004, 91, 3856–3865. 95. Weng, J.-K.; Li, X.; Bonawitz, N. D.; Chapple, C. “Emerging strategies of lignin engineering and degradation for cellulosic biofuel production,” Current Opinion in Biotechnology. 2008, 19, 166–172. 96. Khalil, H.; Ismail, H.; Rozman, H.; Ahmad, M. “The effect of acetylation on interfacial shear strength between plant fibres and various matrices,” European Polymer Journal. 2001, 37, 1037–1045. 97. Alvarez, V. a.; Vázquez, A. “Influence of fiber chemical modification procedure on the mechanical properties and water absorption of MaterBi-Y/sisal fiber composites,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 1672–1680.


310

M. A. Fuqua et al.

98. Kalia, S.; Kaith, B.; Kaur, I. “Pretreatments of natural fibers and their application as reinforcing material in polymer composites-A review,” Polymer Engineering & Science. 2009, 49, 1253–1272. 99. Kim, H.; Lee, B.; Choi, S.; Kim, S. “The effect of types of maleic anhydride-grafted polypropylene (MAPP) on the interfacial adhesion properties of bio-flour-filled polypropylene composites” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1473–1482. 100. Mohanty, S.; Nayak, S.; Verma, S.; Tripathy, S. “Effect of MAPP as a Coupling Agent on the Performance of Jute/PP Composites,” Journal of Reinforced Plastics and Composites. 2004, 23, 625–637. 101. Michaud, F.; Riedl, B.; Castéra, P. “Improving wood/polypropylene fiberboards properties with an original MAPP coating process,” Holzals Rohund Werkstoff. 2005, 63, 380–387. 102. Espert, A.; Vilaplana, F.; Karlsson, S. “Comparison of water absorption in natural cellulosic fibres from wood and one-year crops in polypropylene composites and its influence on their mechanical properties,” Composites Part A: Applied Science and Manufacturing. 2004, 35, 1267–1276. 103. Liu, H.; Wu, Q.; Zhang, Q. “Preparation and properties of banana fiber-reinforced composites based on high density polyethylene (HDPE)/Nylon-6 blends,” Bioresource Technology. 2009, 100, 6088–6097. 104. Sewda, K.; Maiti, S. “Mechanical properties of teak wood flour-reinforced HDPE composites,” Journal of Applied Polymer Science. 2009, 112, 1826–1834. 105. Rahmat, A.; Maradzi, M. “Mechanical properties of rotational moulded empty fruit bunch fiber reinforced polyethylene composites,” Journal of Chemical & Natural Resources Engineering. 2008, 2, 41–52. 106. Tajvidi, M.; Takemura, A. “Effect of fiber content and type, compatibilizer, and heating rate on thermogravimetric properties of natural fiber high density polyethylene composites,” Polymer Composites. 2009, 30, 1226–1233. 107. Song, Y.-m.; Wang, Q.-w.; Han, G.-p.; Wang, H.-g.; Gao, H. “Effects of two modification methods on the mechanical properties of wood flour/recycled plastic blends composites: Addition of thermoplastic elastomer SEBS-g-MAH and in-situ grafting MAH,” Journal of Forestry Research. 2010, 21, 373–378. 108. Xie, X.; Fung, K.; Li, R.; Tjong, S.; Mai, Y.-W. “Structural and mechanical behavior of polypropylene/ maleated styrene-(ethylene-co-butylene)-styrene/sisal fiber composites prepared by injection molding,” Journal of Polymer Science Part B: Polymer Physics. 2002, 40, 1214–1222. 109. Fu, S.; Feng, X.; Lauke, B.; Mai, Y. “Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate/polymer composites,” Composites Part B. 2008, 39, 933–961. 110. Trejo-O’reilly, J.; Cavaillé, J.; Paillet, M.; Gandini, A.; Herrera-Franco, P.; Cauich, J. “Interfacial properties of regenerated cellulose fiber/polystyrene composite materials. Effect of the coupling agent’s structure on the micromechanical behavior,” Polymer Composites. 2000, 21, 65–71. 111. Wu, C.-S. “Renewable resource-based composites of recycled natural fibers and maleated polylactide bioplastic: Characterization and biodegradability,” Polymer Degradation and Stability. 2009, 94, 1076–1084. 112. Avella, M.; Bogoeva-Gaceva, G.; Buˇzarovska, A.; Errico, M. E.; Gentile, G.; Grozdanov, A. “Poly(lactic acid)-based biocomposites reinforced with kenaf fibers,” Journal of Applied Polymer Science. 2008, 108, 3542–3551. 113. Jiang, L.; Huang, J.; Qian, J.; Chen, F.; Zhang, J.; Wolcott, M. P.; Zhu, Y. “Study of poly(3hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/bamboo pulp fiber composites: Effects of nucleation agent and compatibilizer,” Journal of Polymers and the Environment. 2008, 16, 83–93. 114. Park, J.-M.; Quang, S. T.; Hwang, B.-S.; DeVries, K. L. “Interfacial evaluation of modified Jute and Hemp fibers/polypropylene (PP)-maleic anhydride polypropylene copolymers (PPMAPP) composites using micromechanical technique and nondestructive acoustic emission,” Composites Science and Technology. 2006, 66, 2686–2699.



311

115. Bullions, T.; Gillespie, R.; Price-O’Brien, J.; Loos, A. “The effect of maleic anhydride modified polypropylene on the mechanical properties of feather fiber, kraft pulp, polypropylene composites,” Journal of Applied Polymer Science. 2004, 92, 3771–3783. 116. Kim, K.-N.; Kimm, H.; Lee, J.-W. “Effect of interlayer structure, matrix viscosity and composition of a functionaiized polymer on the phase structure of polypropylene-montmorillonite nanocomposites,” Polymer Engineering & Science. 2001, 41, 1963–1969. 117. Vilay, V.; Mariatti, M.; Mat Taib, R.; Todo, M. “Effect of fiber surface treatment and fiber loading on the properties of bagasse fiber-reinforced unsaturated polyester composites,” Composites Science and Technology. 2008, 68, 631–638. 118. Khan, M.; Khan, R.; Zaman, H.; Noor-A Alam, M.; Hoque, M. “Effect of Surface Modification of Jute with Acrylic Monomers on the Performance of Polypropylene Composites,” Journal of Reinforced Plastics and Composites. 2009, 29, 1195–1205. 119. Singha, A.; Rana, R. “Chemically induced graft copolymerization of acrylonitrile onto lignocellulosic fibers,” Journal of Applied Polymer Science. 2012, 124, 1891–1898. 120. Mehta, G.; Mohanty, A.; Misra, M.; Drzal, L. “Effect of novel sizing on the mechanical and morphological characteristics of natural fiber reinforced unsaturated polyester resin based biocomposites,” Journal of Materials Science. 2004, 39, 2961–2964. 121. Kaith, B.; Jindal, R.; Jana, A.; Maiti, M. “Development of corn starch based green composites reinforced with Saccharum spontaneum L fiber and graft copolymers–evaluation of thermal, physico-chemical and mechanical properties,” Bioresource Technology. 2010, 101, 6843–6851. 122. Rosa, M. F.; Chiou, B.-s.; Medeiros, E. S.; Wood, D. F.; Williams, T. G.; Mattoso, L. H. C.; Orts, W. J.; Imam, S. H. “Effect of fiber treatments on tensile and thermal properties of starch/ethylene vinyl alcohol copolymers/coir biocomposites,” Bioresource Technology. 2009, 100, 5196–202. 123. Alix, S.; Lebrun, L.; Marais, S.; Philippe, E.; Bourmaud, A.; Baley, C.; Morvan, C. “Pectinase treatments on technical fibres of flax: Effects on water sorption and mechanical properties,” Carbohydrate Polymers. 2012, 87, 177–185. 124. Liu, Q.; Stuart, T.; Hughes, M.; Sharma, H.; Lyons, G. “Structural biocomposites from flax-Part II: The use of PEG and PVA as interfacial compatibilising agents,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1403–1413. 125. Stuart, T.; Liu, Q.; Hughes, M.; Mccall, R.; Sharma, H.; Norton, A. “Structural biocomposites from flax Part I: Effect of bio-technical fibre modification on composite properties,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 393–404. 126. Akin, D. E.; Dodd, R. B.; Foulk, J. A. “Pilot plant for processing flax fiber,” Industrial Crops and Products. 2005, 21, 369–378. 127. Akin, D. E.; Condon, B.; Sohn, M.; Foulk, J. A.; Dodd, R. B.; Rigsby, L. L. “Optimization for enzyme-retting of flax with pectate lyase,” Industrial Crops and Products. 2007, 25, 136–146. 128. Devi, L. U.; Bhagawan, S.; Thomas, S. “Dynamic mechanical analysis of pineapple leaf/glass hybrid fiber reinforced polyester composites,” Polymer Composites. 2010, 31, 956–965. 129. Alix, S.; Philippe, E.; Bessadok, A.; Lebrun, L.; Morvan, C.; Marais, S. “Effect of chemical treatments on water sorption and mechanical properties of flax fibres,” Bioresource Technology. 2009, 100, 4742–4749. 130. Li, X.; He, L.; Zhou, H.; Li, W.; Zha, W. “Influence of silicone oil modification on properties of ramie fiber reinforced polypropylene composites,” Carbohydrate Polymers. 2012, 87, 2000–2004. 131. Romero-Anaya, A.; Lillo-Ródenas, M.; Salinas-Mart´ınez de Lecea, C.; Linares-Solano, A. “Hydrothermal and conventional H3PO4 activation of two natural bio-fibers,” Carbon, 2012, 50, 3158–3169. 132. Thomsen, A.; Thygesen, A.; Bohn, V.; Nielsen, K.; Pallesen, B.; Jorgensen, M. “Effects of chemical/physical pre-treatment processes on hemp fibres for reinforcement of composites and for textiles,” Industrial Crops and Products. 2006, 24, 113–118. 133. Georgopoulos, S.; Tarantili, P.; Avgerinos, E.; Andreopoulos, A.; Koukios, E. “Thermoplastic polymers reinforced with fibrous agricultural residues,” Polymer Degradation and Stability. 2005, 90, 303–312.


312

M. A. Fuqua et al.

134. Kaushik, A.; Singh, M.; Verma, G. “Green nanocomposites based on thermoplastic starch and steam exploded cellulose nanofibrils from wheat straw,” Carbohydrate Polymers. 2010, 82, 337–345. 135. Wambua, P.; Ivens, J.; Verpoest, I. “Natural fibres: can they replace glass in fibre reinforced plastics?” Composites Science and Technology. 2003, 63, 1259–1264. 136. Lee, S.; Shi, S. Q.; Groom, L. H.; Xue, Y. “Properties of unidirectional kenaf fiber-polyolefin laminates,” Polymer Composites. 2010, 31, 1067–1074. 137. Holbery, J.; Houston, D. “Natural-fiber-reinforced polymer composites in automotive applications,” JOM. 2006, 58, 80–86. 138. Cristaldi, G.; Latteri, A.; Recca, G.; Cicala, G. “Composites based on natural fibre fabrics”, In Woven Fabric Engineering; Dubrovski, P.D., Ed.; InTech, 2010; Chap. 17, 317–342. 139. Pervaiz, M.; Sain, M. M. “Carbon storage potential in natural fiber composites,” Resources, Conservation and Recycling. 2003, 39, 325–340. 140. Madsen, B.; Lilholt, H. “Physical and mechanical properties of unidirectional plant fibre composites: An evaluation of the influence of porosity,” Composites Science and Technology. 2003, 63, 1265–1272. 141. Goutianos, S.; Peijs, T.; Nystrom, B.; Skrifvars, M. “Development of flax fibre based textile reinforcements for composite applications,” Applied Composite Materials. 2006, 13, 199– 215. 142. Carr, D.; Cruthers, N.; Laing, R.; Niven, B. “Fibers from three cultivars of New zealand flax (phormium tenax),” Textile Research Journal. 2005, 75, 93–98. 143. Chabba, S.; Matthews, G.; Netravali, A. “Green composites using cross-linked soy flour and flax yarns,” Green Chemistry. 2005, 7, 576–581. 144. De Carvalho, L.; Moraes, G.; D’Almeida, J. “Influence of water absorption and pre-drying conditions on the tensile mechanical properties of hybrid lignocellulosic fiber/polyester composites,” Journal of Reinforced Plastics and Composites. 2008, 28, 1921–1932. 145. Joseph, K.; Varghese, S.; Kalaprasad, G.; Thomas, S.; Prasannakumari, L.; Koshy, P.; Pavithran, C. “Influence of interfacial adhesion on the mechanical properties and fracture behaviour of short sisal fibre reinforced polymer composites,” European Polymer Journal. 1996, 32, 1243– 1250. 146. Megiatto, J. D.; Ramires, E. C.; Frollini, E. “Phenolic matrices and sisal fibers modified with hydroxy terminated polybutadiene rubber: Impact strength, water absorption, and morphological aspects of thermosets and composites,” Industrial Crops and Products. 2010, 31, 178–184. 147. Saha, P.; Manna, S.; Sen, R.; Roy, D.; Adhikari, B. “Durability of lignocellulosic fibers treated with vegetable oil/phenolic resin,” Carbohydrate Polymers. 2011, 87, 1628–1636. 148. Islam, M. S.; Pickering, K. L.; Foreman, N. J. “Influence of alkali fiber treatment and fiber processing on the mechanical properties of hemp/epoxy composites,” Journal of Applied Polymer Science. 2011, 119, 3696–3707. 149. Seki, Y. “Innovative multifunctional siloxane treatment of jute fiber surface and its effect on the mechanical properties of jute/thermoset composites,” Materials Science and Engineering: A. 2009, 508, 247–252. 150. Burgueño, R.; Quagliata, M. J.; Mohanty, A. K.; Mehta, G.; Drzal, L. T.; Misra, M. “Loadbearing natural fiber composite cellular beams and panels,” Composites Part A: Applied Science and Manufacturing. 2004, 35, 645–656. 151. de Albuquerque, A.; Joseph, K.; Hecker de Carvalho, L.; D’Almeida, J. R. M. “Effect of wettability and ageing conditions on the physical and mechanical properties of uniaxially oriented jute-roving-reinforced polyester composites,” Composites Science and Technology. 2000, 60, 833–844. 152. Peng, X.; Fan, M.; Hartley, J.; Al-Zubaidy, M. “Properties of natural fibre composites made by pultrusion process,” Journal of Composite Materials. 2012, 46, 237–246. 153. Chen, H.; Miao, M.; Ding, X. “Influence of moisture absorption on the interfacial strength of bamboo/vinyl ester composites,” Composites Part A: Applied Science and Manufacturing. 2009, 40, 2013–2019.



313

154. Botaro, V. R.; Novack, K. M.; Siqueira, E. J. “Dynamic mechanical behavior of vinylester matrix composites reinforced by Luffa cylindrica modified fibers,” Journal of Applied Polymer Science. 2012, 124, 1967–1975. 155. Merlini, C.; Soldi, V.; Barra, G. M. “Influence of fiber surface treatment and length on physicochemical properties of short random banana fiber-reinforced castor oil polyurethane composites,” Polymer Testing. 2011, 30, 833–840. 156. Rozman, H. “The mechanical and physical properties of polyurethane composites based on rice husk and polyethylene glycol,” Polymer Testing. 2003, 22, 617–623. 157. Rials, T. G.; Wolcott, M. P.; Nassar, J. M. “Interfacial Contributions in Lignocellulosic FiberReinforced Polyurethane Composites,” Journal of Applied Polymer Science. 2001, 80, 546–555. 158. Rodriguez, E.; Giacomelli, F.; Vazquez, A. “Permeability-porosity relationship in RTM for different fiberglass and natural reinforcements,” Journal of Composite Materials. 2004, 38, 259–268. 159. Rouison, D.; Sain, M.; Couturier, M. “Resin transfer molding of hemp fiber composites: optimization of the process and mechanical properties of the materials,” Composites Science and Technology. 2006, 66, 895–906. 160. Williams, G. I.; Wool, R. P. “Composites from natural fibers and soy oil resins,” Applied Composite Materials. 2000, 7, 421–432. 161. Dansiri, N.; Yanumet, N.; Ellis, J. W.; Ishida, H. “Resin transfer molding of natural fiber reinforced polybenzoxazine composities,” Polymer Composites. 2002, 23, 352–360. 162. Rouison, D.; Sain, M.; Couturier, M. “Resin transfer molding of natural fiber reinforced composites: cure simulation,” Composites Science and Technology. 2004, 64, 629–644. 163. O’Donnell, A.; Dweib, M.; Wool, R. “Natural fiber composites with plant oil-based resin,” Composites Science and Technology. 2004, 64, 1135–1145. 164. Dweib, M.; Hu, B.; O’Donnell, A.; Shenton, H.; Wool, R. “All natural composite sandwich beams for structural applications,” Composite Structures. 2004, 63, 147–157. 165. Thielemans, W.; Wool, R. P. “Butyrated kraft lignin as compatibilizing agent for natural fiber reinforced thermoset composites,” Composites Part A: Applied Science and Manufacturing. 2004, 35, 327–338. 166. Sbiai, A.; Kaddami, H.; Fleury, E.; Maazouz, A.; Erchiqui, F.; Koubaa, A.; Soucy, J.; Dufresne, A. “Effect of the fiber size on the physicochemical and mechanical properties of composites of epoxy and date palm tree fibers,” Macromolecular Materials and Engineering. 2008, 293, 684–691. 167. van Voorn, B.; Smit, H.; Sinke, R.; de Klerk, B. “Natural fibre reinforced sheet moulding compound,” Science. 2001, 32, 1271–1279. 168. Pal, S.; Mukhopadhyay, D.; Sanyal, S.; Mukherjea, R. “Studies on process variables for natural fiber composites’ effect of polyesteramide polyol as interfacial agent,” Journal of Applied Polymer Science. 1988, 35, 973–985. 169. Akil, H. M.; De Rosa, I. M.; Santulli, C.; Sarasini, F. “Flexural behaviour of pultruded jute/glass and kenaf/glass hybrid composites monitored using acoustic emission,” Materials Science and Engineering: A. 2010, 527, 2942–2950. 170. Nosbi, N.; Akil, H. M.; Ishak, Z. M.; Abu Bakar, A. “Degradation of compressive properties of pultruded kenaf fiber reinforced composites after immersion in various solutions,” Materials & Design. 2010, 31, 4960–4964. 171. Liu, X.; W., H. “Heat transfer and cure analysis for the pultrusion of a fiberglass-vinyl ester I beam,” Composite Structures. 1999, 47, 581–588. 172. Mishra, S.; Tripathy, S.; Misra, M.; Mohanty, A.; Nayak, S. “Novel eco-friendly biocomposites: Biofiber reinforced biodegradable polyester amide composites–fabrication and properties evaluation,” Journal of Reinforced Plastics and Composites. 2002, 21, 55–70. 173. Megiatto, J. D.; Oliveira, F. B.; Rosa, D. S.; Gardrat, C.; Castellan, A.; Frollini, E. “Renewable resources as reinforcement of polymeric matrices: composites based on phenolic thermosets and chemically modified sisal fibers,” Macromolecular Bioscience. 2007, 7, 1121– 1131.


314

M. A. Fuqua et al.

174. Assarar, M.; Scida, D.; El Mahi, A.; Poilâne, C.; Ayad, R. “Influence of water ageing on mechanical properties and damage events of two reinforced composite materials: Flax fibres and glass fibres,” Materials & Design. 2011, 32, 788–795. 175. George, G.; Tomlal Jose, E.; Jayanarayanan, K.; Nagarajan, E.; Skrifvars, M.; Joseph, K. “Novel bio-commingled composites based on jute/polypropylene yarns: - Effect of chemical treatments on the mechanical properties,” Composites Part A: Applied Science and Manufacturing. 2012, 43, 219–230. 176. Schniewind, A. (ed.) Concise Encyclopedia of Wood & Wood Based Materials; Pergamon Press: Elmsford, NY, 1989. 177. Garkhail, S.; Heijenrath, R.; Peijs, T. “Mechanical properties of natural-fibre-mat- reinforced thermoplastics based on flax fibres and polypropylene,” Applied Composite Materials. 2000, 7, 351–372. 178. Pervaiz, M.; Sain, M. M. “Sheet-molded polyolefin natural fiber composites for automotive applications,” Macromolecular Materials and Engineering. 2003, 288, 553–557. 179. Madsen, B.; Hoffmeyer, P.; Lilholt, H. “Hemp yarn reinforced composites II. Tensile properties,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 2204–2215. 180. Zaman, H. U.; Khan, A.; Hossain, M.; Khan, M. A.; Khan, R. A. “Mechanical and electrical properties of jute fabrics reinforced polyethylene/polypropylene composites: role of gamma radiation,” Polymer-Plastics Technology and Engineering. 2009, 48, 760–766. 181. Khan, R.; Khan, M.; Zaman, H.; Pervin, S.; Khan, N.; Sultana, S.; Saha, M.; Mustafa, A. “Comparative studies of mechanical and interfacial properties between jute and e-glass fiberreinforced polypropylene composites,” Journal of Reinforced Plastics and Composites. 2009, 29, 1078–1088. 182. Khan, R. A.; Zaman, H. U.; Khan, M. A.; Nigar, F.; Islam, T.; Islam, R.; Saha, S.; Rahman, M. M.; Mustafa, A.; Gafur, M. “Effect of the incorporation of PVC on the mechanical properties of the jute-reinforced LLDPE composite,” Polymer-Plastics Technology and Engineering. 2010, 49, 707–712. 183. Shanks, R.; Hodzic, A.; Wong, S. “Thermoplastic biopolyester natural fiber composites,” Journal of Applied Polymer Science. 2004, 91, 2114–2121. 184. Barkoula, N.; Garkhail, S.; Peijs, T. “Biodegradable composites based on flax/ polyhydroxybutyrate and its copolymer with hydroxyvalerate,” Industrial Crops and Products. 2010, 31, 34–42. 185. Graupner, N.; Müssig, J. “A comparison of the mechanical characteristics of kenaf and lyocell fibre reinforced poly(lactic acid) (PLA) and poly(3-hydroxybutyrate) (PHB) composites,” Composites Part A: Applied Science and Manufacturing. 2011, 42, 2010–2019. 186. Sawpan, M. a.; Pickering, K. L.; Fernyhough, A. “Improvement of mechanical performance of industrial hemp fibre reinforced polylactide biocomposites,” Composites Part A: Applied Science and Manufacturing. 2011, 42, 310–319. 187. Lee, B.-H.; Kim, H.-J.; Yu, W.-R. “Fabrication of long and discontinuous natural fiber reinforced polypropylene biocomposites and their mechanical properties,” Fibers and Polymers. 2009, 10, 83–90. 188. Niu, Pengfei; Liu, Baoying; Wie, Xiaoming; Wang, Xiaojun; Yang, Jie. “Study on mechanical properties and thermal stability of polypropylene/hemp fiber composites,” Journal of Reinforced Plastics and Composites. 2010, 30, 36–44. 189. Pickering, K. L.; Sawpan, M. A.; Jayaraman, J.; Fernyhough, A. “Influence of loading rate, alkali fibre treatment and crystallinity on fracture toughness of random short hemp fibre reinforced polylactide bio-composites,” Composites Part A: Applied Science and Manufacturing. 2011, 42, 1148–1156. 190. Gonzalez, D.; Santos, V.; Parajo, J. “Silane-treated lignocellulosic fibers as reinforcement material in polylactic acid biocomposites,” Journal of Thermoplastic Composite Materials. 2011, 1–18. 191. Nystrom, B.; Joffe, R.; Langstrom, R. “Microstructure and strength of injection molded natural fiber composites,” Journal of Reinforced Plastics and Composites. 2007, 26, 579–599.



315

192. Lei, Y.; Wu, Q.; Yao, F.; Xu, Y. “Preparation and properties of recycled HDPE/natural fiber composites,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1664–1674. 193. Kim, S.; Moon, J.; Kim, G.; Ha, C. “Mechanical properties of polypropylene/natural fiber composites: Comparison of wood fiber and cotton fiber,” Polymer Testing. 2008, 27, 801–x806. 194. Migneault, S.; Koubaa, A.; Erchiqui, F.; Chaala, A.; Englund, K.; Wolcott, M. P. “Effects of processing method and fiber size on the structure and properties of wood-plastic composites,” Composites Part A: Applied Science and Manufacturing. 2009, 40, 80–85. 195. Sui, G.; Fuqua, M.; Ulven, C.; Zhong, W. “A plant fiber reinforced polymer composite prepared by a twin-screw extruder,” Bioresource Technology. 2009, 100, 1246–1251. 196. Yang, H.; Wolcott, M.; Kim, H.; Kim, S. “Effect of different compatibilizing agents on the mechanical properties of lignocellulosic material filled polyethylene bio-composites,” Composite Structures. 2007, 79, 369–375. 197. Chevali, V. S.; Huo, S.; Fuqua, M. A.; Ulven, C. A. “Utilization of agricultural by-products as fillers and reinforcements in ABS,” SAE International Journal of Materials and Manufacturing. 2010, 3, 221–229. 198. Crespo, J.; Sanchez, L.; Garcia, D.; Lopez, J. “Study of the mechanical and morphological properties of plasticized PVC composites containing rice husk fillers,” Journal of Reinforced Plastics and Composites. 2007, 27, 229–243. 199. Lee, S.; Yang, H.; Kim, H.; Jeong, C.; Lim, B.; Lee, J. “Creep behavior and manufacturing parameters of wood flour filled polypropylene composites,” Composite Structures. 2004, 65, 459–469. 200. Arbelaiz, A.; Fernandez, B.; Ramos, J.; Retegi, A.; Llanoponte, R.; Mondragon, I. “Mechanical properties of short flax fibre bundle/polypropylene composites: Influence of matrix/fibre modification, fibre content, water uptake and recycling,” Composites Science and Technology. 2005, 65, 1582–1592. 201. Anglès, M. N.; Salvadó, J.; Dufresne, A. “Steam-exploded residual softwood-filled polypropylene composites,” Journal of Applied Polymer Science. 1999, 74, 1962–1977. 202. Mirzaei, B.; Tajvidi, M.; Falk, R.; Felton, C. “Stress-relaxation behavior of lignocellulosic high-density polyethylene composites,” Journal of Reinforced Plastics and Composites. 2011, 30, 875–881. 203. Najafi, S. K.; Tajvidi, M.; Chaharmahli, M. “Long-term water uptake behavior of lignocellulosichigh density polyethylene composites,” Journal of Applied Polymer Science. 2006, 102, 3907–3911. 204. Stark, N. M.; Matuana, L. M. “Characterization of weathered wood-plastic composite surfaces using FTIR spectroscopy, contact angle, and XPS,” Polymer Degradation and Stability. 2007, 92, 1883–1890. 205. Cui, Y.; Tao, J.; Noruziaan, B.; Cheung, M.; Lee, S. “DSC Analysis and Mechanical Properties of Wood–Plastic Composites,” Journal of Reinforced Plastics and Composites. 2008, 29, 278– 289. 206. Kiani, H.; Ashori, A.; Mozaffari, S. A. “Water resistance and thermal stability of hybrid lignocellulosic filler-PVC composites,” Polymer Bulletin. 2010, 66, 797–802. 207. Huang, Z.; Wang, N.; Zhang, Y.; Hu, H.; Luo, Y. “Effect of mechanical activation pretreatment on the properties of sugarcane bagasse/poly(vinyl chloride) composites,” Composites Part A: Applied Science and Manufacturing. 2012, 43, 114–120. 208. Huda, M.; Drzal, L.; Misra, M.; Mohanty, A. “Wood-fiber-reinforced poly(lactic acid) composites: Evaluation of the physicomechanical and morphological properties,” Journal of Applied Polymer Science. 2006, 102, 4856–4869. 209. Tao, Y.; Yan, L.; Jie, R. “Preparation and properties of short natural fiber reinforced poly (lactic acid) composites,” Transactions of Nonferrous Metals Society of China. 2009, 19, 651– 655. 210. Le Digabel, F.; Boquillon, N.; Dole, P.; Monties, B.; Avérous, L. “Properties of thermoplastic composites based on wheat-straw lignocellulosic fillers,” Journal of Applied Polymer Science. 2004, 93, 428–436.


316

M. A. Fuqua et al.

211. Avérous, L.; Le Digabel, F. “Properties of biocomposites based on lignocellulosic fillers,” Carbohydrate Polymers. 2006, 66, 480–493. 212. Shibata, M.; Takachiyo, K.-I.; Ozawa, K.; Yosomiya, R.; Takeishi, H. “Biodegradable polyester composites reinforced with short abaca fiber,” Journal of Applied Polymer Science. 2002, 85, 129–138. 213. Nanda, M. R.; Misra, M.; Mohanty, A. K. “Mechanical performance of soy-hull-reinforced bioplastic green composites: A comparison with polypropylene composites,” Macromolecular Materials and Engineering. 2012, 297, 184–194. 214. Yang, H.-S.; Wolcott, M. P.; Kim, H.-S.; Kim, S.; Kim, H.-J. “Properties of lignocellulosic material filled polypropylene bio-composites made with different manufacturing processes,” Polymer Testing. 2006, 25, 668–676. 215. Tasdemr, M.; Biltekin, H.; Caneba, G. T. “Preparation and characterization of LDPE and PPWood fiber composites,” Journal of Applied Polymer Science. 2009, 112, 3095–3102. 216. Joseph, P. “Effect of processing variables on the mechanical properties of sisal-fiber-reinforced polypropylene composites,” Composites Science and Technology. 1999, 59, 1625–1640. 217. Hornsby, P.; Hinrichsen, E.; Tarverdi, K. “Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres. Part II: Analysis of composite microstructure and mechanical properties,” Journal of Materials Science. 1997, 32, 1009–1015. 218. Dányádi, L.; Jankecska, T.; Szabó, Z.; Nagy, G.; Móczó, J.; Pukánszky, B. “Wood flour filled PP composites: Compatibilization and adhesion,” Composites Science and Technology. 2007, 67, 2838–2846. 219. Rahman, M. R.; Huque, M. M.; Islam, M. N.; Hasan, M. “Mechanical properties of polypropylene composites reinforced with chemically treated abaca,” Composites Part A: Applied Science and Manufacturing. 2009, 40, 511–517. 220. Islam, M. N.; Rahman, M. R.; Haque, M. M.; Huque, M. M. “Physico-mechanical properties of chemically treated coir reinforced polypropylene composites,” Composites Part A: Applied Science and Manufacturing. 2010, 41, 192–198. 221. Lu, J. Z.; Wu, Q.; Negulescu, I. I.; Chen, Y. “The influences of fiber feature and polymer melt index on mechanical properties of sugarcane fiber/polymer composites,” Journal of Applied Polymer Science. 2006, 102, 5607–5619. 222. Mohanty, A.; Drzal, L.; Misra, M. “Novel hybrid coupling agent as an adhesion promoter in natural fiber reinforced powder polypropylene composites,” Journal of Materials Science Letters. 2002, 21, 1885–1888. 223. Albano, C.; Ichazo, M.; González, J.; Delgado, M.; Poleo, R. “Effects of filler treatments on the mechanical and morphological behavior of PP+wood flour and PP+sisal fiber,” Materials Research Innovations. 2001, 4, 284–293. 224. Selke, S. E.; Wichman, I. “Wood fiber/polyolefin composites,” Composites Part A: Applied Science and Manufacturing. 2004, 35, 321–326. 225. Mulinari, D.; Voorwald, H.; Cioffi, M.; da Silva, M.; da Cruz, T.; Saron, C. “Sugarcane bagasse cellulose/HDPE composites obtained by extrusion,” Composites Science and Technology. 2009, 69, 214–219. ˇ 226. Sercer, M.; Raos, P.; Rujnić-Sokele, M. “Processing of wood-thermoplastic composites,” International Journal of Material Forming. 2009, 2, 721–724. 227. Panthapulakkal, S. “Enhancement of processability of rice husk filled high-density polyethylene composite profiles,” Journal of Thermoplastic Composite Materials. 2005, 18, 445–458. 228. Pramanick, A. “Temperature-stress equivalency in nonlinear viscoelastic creep characterization of thermoplastic/agro-fiber composites,” Journal of Thermoplastic Composite Materials. 2006, 19, 35–60. 229. Kaboorani, A. “Effects of formulation design on thermal properties of wood/thermoplastic composites,” Journal of Composite Materials. 2010, 44, 2205–2215. 230. Jiang, H.; Kamdem, D. P. “Development of poly(vinyl chloride)/wood composites. A literature review,” Journal of Vinyl and Additive Technology. 2004, 10, 59–69.



317

231. Donmez Cavdar, A.; Kalaycioglu, H.; Mengeloglu, F. “Tea mill waste fibers filled thermoplastic composites: the effects of plastic type and fiber loading,” Journal of Reinforced Plastics and Composites. 2011, 30, 833–844. 232. Poletto, M.; Zeni, M.; Zattera, A. “Effects of wood flour addition and coupling agent content on mechanical properties of recycled polystyrene/ wood flour composites,” Journal of Thermoplastic Composite Materials. 2011, (In Press). 233. Singh, S.; Mohanty, A.; Sugie, T.; Takai, Y.; Hamada, H. “Renewable resource based biocomposites from natural fiber and polyhydroxybutyrate-co-valerate (PHBV) bioplastic,” Composites Part A: Applied Science and Manufacturing. 2008, 39, 875–886. 234. Dányádi, L.; Renner, K.; Szabó, Z.; Nagy, G.; Móczó, J.; Pukánszky, B. “Wood flour filled PP composites: adhesion, deformation, failure,” Polymers for Advanced Technologies. 2006, 17, 967–974. 235. Thomason, J. “Dependence of interfacial strength on the anisotropic fiber properties of jute reinforced composites,” Polymer Composites. 2010, 31, 1525–1534. 236. Barkoula, N.; Garkhail, S.; Peijs, T. “Effect of compounding and injection molding on the mechanical properties of flax fiber polypropylene composites,” Journal of Reinforced Plastics and Composites. 2009, 29, 1366–1385. 237. Facca, A. G.; Kortschot, M. T.; Yan, N. “Predicting the elastic modulus of natural fibre reinforced thermoplastics,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 1660– 1671. 238. El-Sabbagh, A.; Steuernagel, L.; Ziegmann, G. “Processing and modeling of the mechanical behavior of natural fiber thermoplastic composite: Flax/polypropylene,” Polymer Composites. 2009, 30, 510–519. 239. Morreale, M.; Scaffaro, R.; Maio, A.; La Mantia, F. “Effect of adding wood flour to the physical properties of a biodegradable polymer,” Composites Part A: Applied Science and Manufacturing. 2008, 39, 503–513. 240. Ashori, A. “Hybrid composites from waste materials,” Journal of Polymers and the Environment. 2009, 18, 65–70. 241. Paul, S. A.; Joseph, K.; Mathew, G.; Pothen, L. A.; Thomas, S. “Preparation of polypropylene fiber/banana fiber composites by novel commingling method,” Polymer Composites. 2009, 31, 816–824. 242. Abdelmouleh, M.; Boufi, S.; Belgacem, M.; Dufresne, A. “Short natural-fibre reinforced polyethylene and natural rubber composites: Effect of silane coupling agents and fibres loading,” Composites Science and Technology. 2007, 67, 1627–1639. 243. Awal, A.; Ghosh, S.; Sain, M. “Thermal properties and spectral characterization of wood pulp reinforced bio-composite fibers,” Journal of Thermal Analysis and Calorimetry. 2009, 99, 695–701. 244. Lee, B.-H.; Kim, H.-S.; Lee, S.; Kim, H.-J.; Dorgan, J. R. “Bio-composites of kenaf fibers in polylactide: Role of improved interfacial adhesion in the carding process,” Composites Science and Technology. 2009, 69, 2573–2579. 245. Yang, H.; Kim, H.; Park, H.; Lee, B.; Hwang, T. “Water absorption behavior and mechanical properties of lignocellulosic filler/polyolefin bio-composites,” Composite Structures. 2006, 72, 429–437. 246. Saha, P.; Manna, S.; Chowdhury, S. R.; Sen, R.; Roy, D.; Adhikari, B. “Enhancement of tensile strength of lignocellulosic jute fibers by alkali-steam treatment,” Bioresource Technology. 2010, 101, 3182–3187. 247. Ramakrishna, G.; Sundararajan, T. “Studies on the durability of natural fibres and the effect of corroded fibres on the strength of mortar,” Cement and Concrete Composites. 2005, 27, 575–582. 248. Tanner, R. D.; Prokop, A.; Bajpai, R. K. “Removal of fiber from vines by solid state fermentation/enzymatic degradation: A comparison of flax and kudzu retting,” Biotechnology Advances. 1993, 11, 635–643.


318

M. A. Fuqua et al.

249. Camciuc, M.; Deplagne, M.; Vilarem, G.; Gaset, A. “Okra Abelmoschus esculentus L. (Moench.) a crop with economic potential for set aside acreage in France,” Industrial Crops and Products. 1998, 7, 257–264. 250. Methacanon, P.; Weerawatsophon, U.; Sumransin, N.; Prahsarn, C.; Bergado, D. “Properties and potential application of the selected natural fibers as limited life geotextiles,” Carbohydrate Polymers. 2010, 82, 1090–1096. 251. Swamy, R.; Kumar, G. M.; Vrushabhendrappa, Y.; Joseph, V. “Study of areca-reinforced phenol formaldehyde composites,” Journal of Reinforced Plastics and Composites. 2004, 23, 1373–1382. 252. Daniels, V. “Factors affecting the deterioration of the cellulosic fibres in black-dyed New Zealand flax (Phormium tenax),” Studies in Conservation. 1999, 44, 73–85. 253. Hoareau, W.; Trindade, W.; Siegmund, B.; Castellan, A.; Frollini, E. “Sugar cane bagasse and curaua lignins oxidized by chlorine dioxide and reacted with furfuryl alcohol: characterization and stability,” Polymer Degradation and Stability. 2004, 86, 567–576. 254. Tanobe, V.; Sydenstricker, T.; Munaro, M.; Amico, S. “A comprehensive characterization of chemically treated Brazilian sponge-gourds (Luffa cylindrica),” Polymer Testing. 2005, 24, 474–482. 255. Demirbas, A. “Calculation of higher heating values of biomass fuels,” Fuel. 1997, 76, 431–434. 256. Hassan, M. L.; Nada, A.-A. M. “Utilization of lignocellulosic fibers in molded polyester composites,” Journal of Applied Polymer Science. 2003, 87, 653–660. 257. Hattalli, S.; Benaboura, A.; Castellan, A. “Adding value to Alfa grass (Stipa tenacissima L.) soda lignin as phenolic resins,” Polymer Degradation and Stability. 2002, 75, 259– 264. 258. Pascoal Neto, C.; Seca, A.; Nunes, A.; Coimbra, M.; Domingues, F.; Evtuguin, D.; Silvestre, A.; Cavaleiro, J. “Variations in chemical composition and structure of macromolecular components in different morphological regions and maturity stages of Arundo donax,” Industrial Crops and Products. 1997, 6, 51–58. 259. Reddy, N.; Yang, Y. “Natural cellulose fibers from switchgrass with tensile properties similar to cotton and linen,” Biotechnology and Bioengineering. 2007, 97, 1021–1027. 260. Marklund, E.; Varna, J. “Modeling the effect of helical fiber structure on wood fiber composite elastic properties,” Applied Composite Materials. 2009, 16, 245–262. 261. Lauricio, F. M. Technology Manual on Rice Husk Ash Cements (UNDP/UNIDO (RENASBMTCS); Makati, Philippines, 1987. 262. Heredia-Moreno, A. “Olive stones as a source of fermentable sugars,” Biomass. 1987, 14, 143–148. 263. Yalchi, T. “Determination of digestibility of almond hull in sheep,” Journal of Biotechnology. 2011, 10, 3022–3026. 264. Conghos, M.; Aguirre, M.; Santamaria, R. “Biodegradation of sunflower hulls with different nitrogen sources under mesophilic and thermophilic incubations,” Biology and Fertility of Soils. 2003, 38, 282–287. 265. O’Mara, F. “The nutritive value of palm kernel meal measured in vivo and using rumen fluid and enzymatic techniques,” Livestock Production Science. 1999, 60, 305–316. 266. Trindade, W. G.; Hoareau, W.; Razera, I. A.; Ruggiero, R.; Frollini, E.; Castellan, A. “Phenolic thermoset matrix reinforced with sugar cane bagasse fibers: Attempt to develop a new fiber surface chemical modification involving formation of quinones followed by reaction with furfuryl alcohol,” Macromolecular Materials and Engineering. 2004, 289, 728–736. 267. Cheilas, T.; Stoupis, T.; Christakopoulos, P.; Katapodis, P.; Mamma, D.; Hatzinikolaou, D.; Kekos, D.; Macris, B. “Hemicellulolytic activity of Fusarium oxysporum grown on sugar beet pulp. Production of extracellular arabinanase,” Process Biochemistry. 2000, 35, 557– 561. 268. Zhang, M.-L.; Fan, Y.-T.; Xing, Y.; Pan, C.-M.; Zhang, G.-S.; Lay, J.-J. “Enhanced biohydrogen production from cornstalk wastes with acidification pretreatment by mixed anaerobic cultures,” Biomass and Bioenergy. 2007, 31, 250–254.



319

269. Kessler, R.; Becker, U.; Kohler, R.; Goth, B. “Steam explosion of flax-a superior technique for upgrading fibre value,” Biomass and Bioenergy. 1998, 14, 237–249. 270. Stamboulis, A.; Baillie, C.; Peijs, T. “Effects of environmental conditions on mechanical and physical properties of flax fibers,” Composites Part A: Applied Science and Manufacturing. 2001, 32, 1105–1115. 271. Graupner, N.; Herrmann, A. S.; Müssig, J. “Natural and man-made cellulose fibre-reinforced poly(lactic acid) (PLA) composites: An overview about mechanical characteristics and application areas,” Composites Part A: Applied Science and Manufacturing. 2009, 40, 810– 821. 272. Shibata, S.; Cao, Y.; Fukumoto, I. “Press forming of short natural fiber-reinforced biodegradable resin: Effects of fiber volume and length on flexural properties,” Polymer Testing. 2005, 24, 1005–1011. 273. Goda, K.; Sreekala, M.; Gomes, A.; Kaji, T.; Ohgi, J. “Improvement of plant based natural fibers for toughening green composites-Effect of load application during mercerization of ramie fibers,” Composites Part A: Applied Science and Manufacturing. 2006, 37, 2213– 2220. 274. Angelini, L.; Lazzeri, A.; Levita, G.; Fontanelli, D.; Bozzi, C. “Ramie (Boehmeria nivea (L.) Gaud.) and Spanish Broom (Spartium junceum L.) fibres for composite materials: Agronomical aspects, morphology and mechanical properties,” Industrial Crops and Products. 2000, 11, 145–161. 275. Luo, X.; Benson, R. S.; Kit, K. M.; Dever, M. “Kudzu fiber-reinforced polypropylene composite,” Journal of Applied Polymer Science. 2002, 85, 1961–1969. 276. Satyanarayana, K.; Sukumaran, K.; Mukherjee, P.; Pillai, S. “Materials Science of Some Lignocellulosic Fibers,” Metallography. 1986, 19, 389–400. 277. Jayaraman, K. “Manufacturing sisal/polypropylene composites with minimum fibre degradation,” Composites Science and Technology. 2003, 63, 367–374. 278. Herrera-Franco, P.; Valadez-Gonzalez, A. “A study of the mechanical properties of short naturalfiber reinforced composites,” Composites Part B: Engineering. 2005, 36, 608–597. 279. Satyanarayana, K.; Sukumaran, K.; Kulkarni, A.; Pillai, S.; Rohatgi, P. “Fabrication and properties of natural fibre-reinforced polyester composites,” Composites. 1986, 17, 329– 333. 280. Gomes, A.; Matsuo, T.; Goda, K.; Ohgi, J. “Development and effect of alkali treatment on tensile properties of curaua fiber green composites,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1811–1820. 281. Al-Khanbashi, A.; Al-Kaabi, K.; Hammami, A. “Date palm fibers as polymeric matrix reinforcement: Fiber characterization,” Polymer Composites. 2005, 26, 486–497. 282. Tomczak, F.; Sydenstricker, T. H. D.; Satyanarayana, K. G. “Studies on lignocellulosic fibers of Brazil. Part II: Morphology and properties of Brazilian coconut fibers,” Composites Part A: Applied Science and Manufacturing. 2007, 38, 1710–1721. 283. Hariharan, A. “Lignocellulose-based hybrid bilayer laminate composite: part i - studies on tensile and impact behavior of oil palm fiber-glass fiber-reinforced epoxy resin,” Journal of Composite Materials. 2005, 39, 663–684. 284. Sreekala, M.; Kumaran, M.; Thomas, S. “Oil palm fibers: Morphology, chemical composition, surface modification, and mechanical properties,” Journal of Applied Polymer Science. 1997, 66, 821–835. 285. Sain, M.; Panthapulakkal, S. “Bioprocess preparation of wheat straw fibers and their characterization,” Industrial Crops and Products. 2006, 23, 1–8. 286. Zhao, Y.; Qiu, J.; Feng, H.; Zhang, M.; Lei, L.; Wu, X. “Improvement of tensile and thermal properties of poly(lactic acid) composites with admicellar-treated rice straw fiber,” Chemical Engineering Journal. 2011, 173, 659–666. 287. Okubo, K.; Fujii, T.; Yamamoto, Y. “Development of bamboo-based polymer composites and their mechanical properties,” Composites Part A: Applied Science and Manufacturing. 2004, 35, 377–383.


320

M. A. Fuqua et al.

288. Kretschmann, D. E. “Mechanical properties of wood”, In Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-190, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI, 2010; Chap. 5, 5.1–5.46. 289. Thomason, J. “The influence of fibre properties of the performance of glass- fibre-reinforced polyamide 6,6,” Composites Science and Technology. 1999, 59, 2315–2328. 290. Rout, J.; Tripathy, S.; Misra, M.; Mohanty, A.; Nayak, S. “The influence of fiber surface modification on the mechanical properties of coir-polyester composites,” Polymer Composites. 2001, 22, 468–476. 291. Wang, B.; Panigrabi, S.; Tabil, L.; Crerar, W. Effects of Chemical Treatments on Mechanical and Physical Properties of Flax Fiber-Reinforced Rotationally Molded Composites. 2004 ASAE/CSAE Annual International Meeting, 2004. 292. Joseph, K.; Thomas, S.; Pavithran, C. “Effects of chemical treatment on the tensile properties of short sisal fibre-reinforced polyethylene composites,” Polymer. 1996, 37, 5139–5149. 293. Murray, R. F.; Harper, H. W.; Granner, D. K.; Mayes, P. A.; Rodwell, V. W. Harper’s Illustrated Biochemistry; Lange Medical Books/McGraw-Hill: New York, 2006. 294. Mathews, C. K.; van Holde, K. E.; Ahern, K. G. Biochemistry, 3rd Edition. Prentice Hall: New York, 1999. 295. Voet, D.; Voet, J.; Pratt, C. Fundamentals of Biochemistry: Life at the Molecular Level, 3rd Edition. Wiley: New Jersey, 2008. 296. Peng, X.; Fan, M.; Hartley, J.; Al-Zubaidy, M. “Properties of natural fibre composites made by pultrusion process,” Journal of Composite Materials. 2010, 42, 237–246. 297. Tsukada, M.; Shiozaki, H.; Freddi, G.; Crighton, J. S. “Graft copolymerization of benzyl methacrylate onto wool fibers,” Journal of Applied Polymer Science. 1997, 64, 343–350. 298. Hearle, J. W. S. “Protein fibers: Structural mechanics and future opportunities,” Journal of Materials Science. 2007, 42, 8010–8019. 299. Cheung, H.; Ho, M.; Lau, K.; Cardona, F.; Hui, D. “Natural fibre-reinforced composites for bioengineering and environmental engineering applications,” Composites Part B. 2009, 40, 655–663. 300. Huda, S.; Yang, Y. “Composites from ground chicken quill and polypropylene,” Composites Science and Technology. 2008, 68, 790–798. 301. Suyama, K.; Fukazawa, Y.; Suzumura, H. “Biosorption of precious metal ions by chicken feather,” Applied Biochemistry and Biotechnology. 1996, 57/58, 67–74. 302. Bullions, T.A.; Hoffman, D.; Gillespie, R.A.; Price-O’Brien, J.; Loos, A.C. “Contributions of feather fibers and various cellulose fibers to the mechanical properties of polypropylene matrix composites,” Composites Science and Technology. 2006, 66, 102–114. 303. Poole, A.J.; Church, J.S.; Huson, M.G. “Environmentally sustainable fibers from regenerated protein,” Biomacromolecules. 2009, 10, 1–8. 304. Perez-Rigueiro, J.; Viney, C.; Llorca, J.; Elices, M. “Silkworm silk as an engineering material,” Journal of Applied Polymer Science. 1998, 70, 2439–2447. 305. Setua, D.K.; Dutta, B. “Short silk fiber-reinforced polychloroprene rubber composites,” Journal of Applied Polymer Science. 1984, 29, 3097–3114. 306. Ok Han, S.; Muk Lee, S.; Ho Park, W.; Cho, D. “Mechanical and thermal properties of waste silk fiber-reinforced poly(butylene succinate) biocomposites,” Journal of Applied Polymer Science. 2006, 100, 4972–4980. 307. Ashby, M.F.; Gibson, L.J.; Wegst, U.; Olive, R. “The mechanical properties of natural materials. I. Material property charts,” Proceedings: Mathematical and Physical Sciences. 1995, 450, 123–140.

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Carbon Fiber-Reinforced Polyurethane Composites ... - Springer Link

Bending Properties of Fiber-Reinforced Composites Retainers ...

Short carbon fiber reinforced polycarbonate composites

STEADY-STATE CREEP OF FIBER-REINFORCED COMPOSITES ...

Free Download Fiber-Reinforced Composites - Google Sites

Audiobook Download Fiber-Reinforced Composites ... - Google Sites

Abaca Fiber Reinforced Epoxy Composites

Oil palm fiber reinforced polypropylene composites