Natural fiber and hybrid fiber thermoplastic composites

Natural fiber and hybrid fiber thermoplastic composites: advancements in lightweighting applications

4

S. Panthapulakkal1, L. Raghunanan1, M. Sain1, B. KC2 and J. Tjong3 1 University of Toronto, Toronto, ON, Canada, 2PhD Student at the University of Toronto, Toronto, ON, Canada, 3Adjunct Professor at the University of Toronto, Toronto, ON, Canada

4.1

Introduction

Composites are structural materials made up of two or more materials such that each component remains separate on a macroscopic level within the finished material, but for which the composite properties are superior to those of the individual component materials alone (Gibson 2011). In composite materials, one of the materials is typically reinforcing, carrying most of the structural loads and thus providing macroscopic stiffness and strength, while another, the matrix, dominates the composite’s shape, surface appearance, environmental tolerance, and overall durability. Composite materials are primarily employed as structural materials in lightweighting applications due to the high strength weight and modulus weight ratios afforded upon incorporation of less dense reinforcing materials into matrix materials (Mallick, 2007). Glass and carbon fibers have traditionally been the most commonly employed reinforcing materials in lightweight composites on account of their high mechanical strength and stiffness. As the significance of environmental drivers on product commercialization increases, renewably sourced natural fibers are finding increased opportunities in lightweighting applications over traditional fiber reinforcements because of the factors such as low density, low cost, reasonably accepted strength, recyclability, safe handling of fibers, and enhanced energy recovery (Faruk et al., 2014; Ahmad et al., 2015; Pervaiz et al., 2016). A significant amount of research aiming to improve the performance of natural fiber reinforced composites has been reported; however, more ongoing research in this direction is needed in order to extend their capabilities in high-end structural applications where the end-use requirements are higher compared to the esthetic and semistructural applications (Faruk et al., 2014; Ahmad et al., 2015; Pickering et al., 2016). This chapter reviews the recent advances of natural fibers and natural fiber hybrids in lightweighting applications, with a large focus on the work done at the Green Composites. DOI: http://dx.doi.org/10.1016/B978-0-08-100783-9.00003-4 © 2017 Elsevier Ltd. All rights reserved.

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Center for Biomaterials and Biocomposites Processing (CBBP). In general, natural fibers include the fibers originated from natural resources such as plants, animals, and minerals. Note that all references to natural fibers in this chapter refer specifically to plant fibers.

4.2

Natural fibers in composite manufacturing

Natural (plant) fibers are classified into woody and nonwoody fibers and the nonwoody fibers are further classified into different groups based on the feedstock from which they are sourced: bast (stem), leaf, seed, straw, and grass. All of these fibers are lignocellulosic, meaning that they are comprised primarily of cellulose, hemicellulose, and lignin as the three major constituents, along with minor components such as waxes, pectins, and extractives. The representative chemical structures of cellulose, hemicellulose, and lignin are presented in Fig. 4.1. Cellulose is a linear homopolymer of β-(D)-glucose comprising of large areas of highly crystalline segments. Hemicelluloses are a mixture of highly branched low-molecular-weight homo- and heteropolymers comprised of anhydro-β-(1-4)-D-xylopyranose, (A) O HO

O HO

O H

O H

H HO O

O O H O H O

HO O

O O O HO H O

O H O H

H O O HO

O

OH

O H

H HO O

O

O

HO O

OH

O O O HO H O H O O

O

O O O HO H O

O H O H

O HO

O

HO O

O

OH

O

H HO O

O H O H

H O

OH

OH

O H

H HO O

O

O HO

HO O

O O O H

O

H O O O

OH OH

OH

Cellulose OH

(B)

(C) HO2C MeO HO

O HO

O

O OH

HO O HO O

O

OH HOH2C HO

O

O O

O

HO

OH

HO

O

O

O

O

O

O

HO

O

HO

HO

O MeO O

OH OH

OMe

OH O

HO

O

HO

O

OH

OMe

O OMe

O

Lignin

OMe

HO

HO

HO

OH OH

OH O

O

O O

OMe

O

OMe

O

OH

Hemicelluloses

Figure 4.1 Representative structures of (A) hemicellulose, (B) cellulose, and (C) lignin. Adapted from Rahimi et al. (2014) and Seery (2013).

Natural fiber and hybrid fiber thermoplastic composites

41

glucopyranose, mannopyranose, and galactopyranose units (Gurunathan et al., 2015). Lignin is an amorphous polymer whose exact structure is as yet unknown, but is known to comprise of p-coumaryl, coniferyl, and sinapyl alcohols (Chakar and Ragauskas, 2004; Zakzeski et al., 2010).

4.2.1 Properties of natural fibers The widely different chemistries and physical characteristics of the three major components determine the chemical and mechanical properties of the resulting fiber. For example, hemicellulose, being the least crosslinked with the lowest molecular weight and possessing the highest density of polar functional groups, is the least thermally stable (Varhegyi et al., 1989; Orfao et al., 1999) and most hydrophilic (Olsson, 2004). Hence, natural fibers high in hemicellulose content decompose at earlier temperatures and possess high moisture absorption affinity. Similarly, the large degree of inter- and intramolecular hydrogen bonding amongst the linear cellulose molecules contributes to the reinforcing mechanical properties of natural fibers. In fact, the mechanical strength of natural fibers increases with the percentage of cellulose composition, increasing polymerization of the cellulose content, and with decreasing microfibrillar angle of the cellulose chains with respect to the fiber axis (Azwa et al., 2013; Bourmaud et al., 2013; Ahmad et al., 2015). Tensile strength and Young’s modulus values also increase with increasing cellulose content. The relationship between the lignocellulose components and selected material properties are summarized in Fig. 4.2. The lignocellulose composition of common representatives of each class of fiber are presented in Table 4.1. As can be seen, the compositions of the fibers vary significantly both within and across different fiber classifications; for example, both kenaf and hemp are bast fibers, but hemp contains twice as much cellulose as kenaf, while sisal and hemp belong to different classifications (leaf and bast, respectively) but possess similar cellulose contents. This is because of the difference in the fiber composition, which is highly dependent on variables such as geographic location, growth Lignocellulosic components of natural fibers

Cellulose

Strength and stiffness

Hemi-cellulose

Thermal and biological degradation, moisture absorption, and flammability

Lignin

Ultraviolet degradation, and char formation

Figure 4.2 Lignocellulose components responsible for mechanical and physical properties of natural fibers.

Chemical composition and physico-mechanical properties of fibers (Rowell et al., 1997; Biagiotti et al., 2004; Summerscales et al., 2010; Ahmed et al., 2015; Gurunathan et al., 2015; Javad and Abdul Khalil, 2011; Gassan and Bledzki, 1996; Ramamoorthy et al., 2015; Reddy and Yang, 2005) Table 4.1

Fiber name

Chemical composition

Physico-mechanical properties

Cellulose

Hemicellulose

Lignin

Pectin

43 47 40 44

25 35 25 29

16 24 25 31

64.1 71.9 70.2 74.4 61 71.5 45. 57 68.6 91

64.1 71.9 17.9 22.4 17.9 22.4 8 13 5 16.7

2 2.2 3.7 5.7 12 13 21.5 0.4 0.7

1.8 2.3 0.9 0.2 0.6 1.9

56 63 63 64

20 25 10

7 9 5

1.0

Wax

Density (g/cm3)

Tensile strength (MPa)

Tensile modulus (GPa)

1.5

1000

40

1.7 0.8 0.5 0.8 0.3

1.5 1.48 1.46 1.45 1.5

800 550 393 930 220

0.2 3

1.5 1.35

400 529 914

Wood Hard wood Soft wood

Nonwood Bast Flax Hemp Jute Kenaf Ramie

1500 900 800 938

27.6 80 372 608 10 30 53 44 128

Leaf Abacca Banana

12 27 32

Sisal

67 78

10 14

8 11

10

2

1.45

530 640

9.4 22

Curaua Pineapple

73.6 70 83

9.9 15 20

7.5 5 12

2 4

4 7

1.4 1.5

158 729 170 1627

34.5 82.5

82.7 91 36 43

3 5.7 0.15 0.25

0.7 1.6 41 45

0 1 3 4

0.6

1.6 1.2

287 597 175

5.5 12.6 4 6

26 43 57 55.2

20 25 38 25.3

7 9 5 1.8

3

1.1

500

35.91

10 1.25

290

17

39 45 28 36 38 57

15 31 23 28 28 33

13 20 12 14 7 21

8 38

35 45

19 25

20

14 17

Seed/fruit Cotton Coir

Grass Bamboo Sea grass Bagasse

Straw Wheat straw Rice straw Corn stover Other Rice husk

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conditions, plant age, and fiber origin. As a result, the physical and mechanical properties of the different fibers cannot be generalized based on the fiber origin. This is further exacerbated by the fact that fiber quality is also strongly influenced by harvesting and processing conditions. Variations in the chemical composition of the plant fibers affect their physico-mechanical properties as given in Table 4.1. Not surprisingly, the different chemical compositions of fibers and their physicomechanical characteristics influence their reinforcing behavior in composite materials. For example, Fig. 4.3 shows the variation in the tensile properties of polypropylene

90 80

81.9

PLA 74

PP

Tensile strength (MPa)

70

63.5

60 47.9

50 42 40 29.2

30 20 10 0

Neat

Abacca

Jute

12 PLA

Tensile modulus (GPa)

10

PP

9.6 8

8

5.8

6 4.9 4

3.4

2

1.5

0 Neat

Abacca

Jute

Figure 4.3 Tensile strength and modulus of PP and PLA reinforced with abacca and jute fibers. Adapted from Faruk et al. (2012).

Natural fiber and hybrid fiber thermoplastic composites

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and polylactic acid (PLA) composites reinforced with two different types of natural fibers (at constant loading; 30% by weight) (Faruk et al., 2012). Other characteristics of the fibers that influence their reinforcing abilities are (1) the microfibrillar angle of the cellulose microfibrils in the secondary layer of the fiber cell walls and (2) the moisture content of the fibers. In fibers, the cellulose fibrils were arranged in multiple construction in the primary and secondary layers of the fiber cell wall. Each fibril in the fiber bundle has a complex layered structure comprised of a thin primary wall (the first layer deposited during cell growth) encircling a secondary wall. The secondary wall itself is made up of three layers and the mechanical properties of the fibers are determined by these layers, especially the thick middle layer. In the middle layer, cellulose micofibrils formed from long cellulose chain molecules, are wound in helical patterns. The angle between the fiber axis and the microfibrils is called the microfibrillar angle and the characteristic value of this microfibrillar angle varies from one fiber to another (Azwa et al., 2013). The typical microfibrillar angle of different fibers is given in Table 4.2. The microfibrilllar angle has a pronounced influence on the mechanical properties of the fiber, as it is the stiffness-determining parameter of the fibers. Gassan et al. (2001) reported that the elastic modulus decreases with increasing spiral angle, whereas John and Thomas (2008) reported that if the microfibrils have a spiral orientation to the fiber axis, the resultant fibers will be more ductile, and if the microfibrils are oriented parallel to the fiber axis, the fibers will be rigid, inflexible, and have high tensile strength. The moisture content of the plant fibers is another factor that affects the mechanical properties of the fibers. The amount of moisture associated with the fibers is dependent on the content of noncrystalline parts and void content of the fibers. Table 4.2 shows the equilibrium moisture content of some natural fibers affecting the reinforcing ability of the fibers (Rowell, 2008). Processing of the composites

Microfibrillar angle and moisture content of the natural fibers (Faruk et al., 2012; Reddy and Yang, 2005; Rowell, 2008)

Table 4.2

Fiber Sisal Hemp Jute Flax Kenaf Abacca Ramie Pinapple Coir Bagasse Bamboo

Microfibrillar angle (degree) 2 6.2 8 5 10 2 6.2 20 25 69 83 30.45

Equilibrium moisture content (%) 11 6.2 1.2 12.5 13.7 8 12 6.2 12 15 9 10 13 8.8 11.36 8.8 8.9 9.16

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using fibers with high moisture content might lead to higher void content of the composites and can affect the ultimate strength of the composites. Chemical, physical, and mechanical properties of natural fibers can be improved by modifying the chemical composition. Cellulose is known for its high strength and stiffness, and plant fibers with high cellulose content but low hemicellulose and lignin contents are typically desired for the best mechanical properties. In general, in the hierarchy structure of plant cell walls, cellulose molecules pack into microfibrils (,5 nm), microfibril bundles form single fibers (5 50 μm), single fibers form elementary fibers (10 20 μm), and elementary fibers form technical bundles (50 100 μm), which are embedded in the matrices of hemicelluloses and lignin in the different macro- and microstructural components of the cell wall (Reddy and Yang, 2005; Azwa et al., 2013). Hence, depending on the methods of removal of hemicelluloses and lignin, natural fibers of different scales can be obtained. The macroscale fibers, fiber bundles, possess large numbers of defects and flaws which prevent the crystal modulus of cellulose, and therefore the maximum mechanical fiber strength, from being realized. On the other hand, micro- or nanoscale fibrils possess less defects on account of their smaller sizes and can therefore reach modulus values close to those of perfect cellulose, giving the maximum fiber strength possible of natural fibers. The isolation and separation of high-quality microfibrils/nanofibrils from natural fibers have been achieved by means of mechanical as well as chemico-mehanical means (Chakraborty et al., 2005; Wang et al., 2007; Alemdar and Sain, 2008). Mechanical defibrillation of the fiber bundle requires the shearing action to separate the fiber bundles and can be achieved by means of mechanical refiners (Chakraborty et al., 2005), high shear mixers (Sain et al., 2014), or high-pressure homogenizers (Bhatnagar and Sain, 2005). Chemical treatments include successive treatments using alkali or acids to remove the hemicelluloses, followed by delignification to generate high strength cellulosic fiber bundles (Wang et al., 2007; Alemdar and Sain, 2008; Panthapulakkal and Sain, 2013). A combination of chemical and mechanical treatments can defibrillate the fiber bundles into high-strength cellulosic fibrils (Wang et al., 2007; Alemdar and Sain, 2008; Panthapulakkal and Sain, 2013). Alternatively, fiber separation using microbes or enzymatic means in combination with mechanical defibrillation can also result in high strength cellulosic fibrils (Janardhnan and Sain, 2006, 2011).

4.3

Natural fiber reinforced thermoplastics composites

Generally, the matrix protects the fibers during processing, keeps the fibers in place, and distributes load throughout the composite. In lightweighting applications, polymers are the preferred matrices due to their lower densities and comparable specific tensile properties compared to high-performance engineering materials such as metals. In fact, natural fiber reinforced plastic composites already find significant commercial applications as lightweighting materials in the automobile, construction, aerospace, sports, textiles, and furniture industries (Mohammed et al.,

Natural fiber and hybrid fiber thermoplastic composites

47

2015; Pervaiz et al., 2016). Polymers used in lightweight composite materials can be either thermoplastics or thermosets. Thermoplastics are heat-reprocessable polymers, whereas thermosets are not. Thus, thermoplastics present greater opportunities for the manufacture of reprocessable and/or recyclable lightweighting materials and are discussed in this chapter.

4.3.1 Types of thermoplastic composites Based on the material resource, natural fiber thermoplastic composites can be classified into two types: composites with nondegradable but recyclable synthetic thermoplastics, and green composites comprising biodegradable thermoplastics. Of these, polypropylene, a synthetic commodity thermoplastic, is the most employed polymer owing to its low cost, low density, excellent processability, good mechanical properties, high temperature resistance, excellent electrical properties, good dimensional stability, and good impact strength. Commonly used biodegradable thermoplastics include PLA, polyglycolic acid, poly-β-hydroxyalkanoates, and poly(ε-caprolactone) (PCL). The physical, mechanical, and thermal properties of the more commonly employed synthetic and biodegradable thermoplastics are presented in Table 4.3. Note that all of these polymer matrices possess melting temperatures below 200 C and can be processed with natural fibers without degrading the fibers and hence can provide their full potential of reinforcing ability, provided good interfacial interaction between the fibers and matrix.

Table 4.3 Physical and mechanical properties of polyolefins and degradable polymers (Lechner, 2005; Ashori, 2008; Gurunathan et al., 2015) Polymer

Tg ( C)

Tm ( C)

PP PE (HDPE) PS PLA PCL P (3HB) P (4HB) Starch

210 to 223 2110

160 175 0.90 0.92 26 41.4 126 135 0.95 28

0.95 1.78 15 700 1.04 30

100 55 65 260 to 265 5 15 248

110 135 120 175 58 65 168 182 53 110 115

4 5 0.35 3.5 0.21 0.44 3.5 149 0.125

Density (g/cm3)

1.04 1.21 1.11 1.18

1.09 1.25 1.15 1.26

1 1.39

Tensile strength (MPa)

30 60 21 60 20.7 42 25 40 104 35 80

Tensile modulus (GPa)

Strain at break (%)

1 2.5 2.5 6 300 1000 5 8 1000 31

PP, polypropylne; PE, polyethylene; HDPE, high-density polyethylene; PLA, polylactic acid; PCL, poly- ε caprolactam; PHB, polyhydroxyl butyrate.

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Mechanical properties of the composite with and without interface modifiers (CBBP lab results)

Table 4.4

Composite material

Tensile strength (MPa)

Flexural strength (MPa)

Flexural modulus (MPa)

Un-notched impact strength (J/m)

Rice husk Rice husk with 2.5 wt% interface modifier

14.4 6 0.9 19.5 6 0.8

30.4 6 2.1 41.3 6 3.8

2.84 6 0.08 3.14 6 0.33

40 6 8 71 6 9

4.3.2 Factors influencing natural fiber reinforced composites Besides the nature of the natural fiber and the matrix materials themselves, the performance of natural fiber reinforced composites is highly influenced by fiber loading and dispersion, fiber length and distribution, fiber orientation, and interactions between the matrix and the fiber. The first three are typically addressed by the judicious selection of processing conditions, whilst improvements in the fiber matrix adhesion generally require fiber surface treatment. Fiber matrix interaction is one of the most important parameters that determine the reinforcing capability of the lignocellulosic fibers in hydrophobic polymeric matrix. For example, mechanical properties of rice husk reinforced composites (Table 4.4) show that the presence of interface modifier enhances the properties of the composites. If the fiber matrix interaction is poor, even a high-performance fiber will give poor mechanical properties due to the inability of the matrix to effectively transfer the load onto the reinforcing fiber.

4.3.2.1 Fiber loading and dispersion Despite general acceptance that the mechanical properties of natural fiber composites increase with increasing fiber loading (Ku et al., 2011), many works report conflicting influences of fiber loading on mechanical properties. This is because of the cumulative influence of processing conditions on the fiber matrix interactions (Ku et al., 2011). For example, for hemp fiber reinforced polypropylene composites prepared by different methods, Pervaiz and Sain (2003) reported improvements in tensile strength, flexural strength, and tensile modulus with increasing fiber loading, while Hajnalka et al. (2008) presented decreasing tensile strength and polynomial maxima singularities in the Young’s modulus with increasing fiber loadings. At higher fiber loading, the fibers cannot be completely wetted by the polymer matrix leading to a poor interface between the fiber and matrix and hence poor load transfer through the interface. Further, fibers cannot be dispersed in the matrix leading to agglomerates in the polymer matrices leading to poor performance. For polymer matrices like PP and PE, composites with a fiber loading of up to 40 50% can be processed without the above-mentioned inadequate wetting and poor dispersion.

Natural fiber and hybrid fiber thermoplastic composites

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Table 4.5 Mechanical properties of wheat straw fibers prepared by different processes and their composites (Panthapulakkal et al., 2006) Process for preparing wheat straw fibers

Wheat straw fibers Tensile strength (MPa)

Mechanical processing Chemical processing

58.7 6 51 146.3 6 53

Tensile modulus (GPa) 3.7 6 2.6 7.9 6 3.7

Wheat straw PP composite Tensile strength (MPa)

Tensile modulus (GPa)

40.8 6 0.5 35.5 6 0.5

3.0 6 0.04 2.4 6 0.03

Fiber dispersion is important in the development of high-performing composites. Since lignocellulosic fibers are hydrophilic, the fibers tend to agglomerate in the composites and the poorly dispersed fibers/agglomerates can act as stress concentrators that can ultimately lead to premature failure of the composites. Table 4.5 shows the mechanical properties of wheat straw fibers prepared by two different process and their polypropylene composites (30 wt% fiber loading) prepared by injection molding. It is clear that the straw fibers prepared by chemical means have higher mechanical strength, and are expected to provide high-strength composites compared to the mechanically produced fibers. However, the strength of the composites is contrary and it is reported that this is because of the poor dispersion of the highly hydrophilic wheat straw fibers prepared by chemical process (Panthapulakkal et al., 2006).

4.3.2.2 Fiber length Fiber length is one of the other important factors influencing the mechanical properties of composites. In short fiber composites, tensile load is transferred from the matrix to the fiber through shear at the fiber matrix interface. Tensile stress experienced at the fiber ends are zero and the stress increases along the fiber length and hence the fibers need to have a length greater than a minimum, which is the critical fiber length, in order for the fiber to be able to carry the load (Matthews and Rawlings, 1999). Ideally, for the fibers to be efficient reinforcements, fiber length needs to be much greater than the critical fiber length. In such cases, during the tensile tests, the majority of the fibers can be loaded as if they are continuous fibers. Critical fiber length, Lc, can be defined in terms of the aspect ratio for a fiber (length/diameter) and can be expressed as, Lc σf 5 d 2τ i where d is the fiber diameter, σf is the tensile strength of fiber, and τ i is the interfacial shear strength. Critical fiber length varies depending on the fiber type, matrix, fiber content in the matrix, and fiber treatment.

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Green Composites

Thermoplastic composites are classified according to the length of the reinforcing fiber; they can either be short fiber composites or long fiber composites. Long fiber composites are typically stronger on account of increased matrix fiber interface interactions (Pervaiz and Sain, 2003; Bos et al., 2006), but are difficult to process using the existing thermoplastics processing technology because of entanglement and breakage (Bos et al., 2006). On the other hand, short fiber composites can be prepared using existing thermoplastics technology, and are thus more ubiquitous despite their lower mechanical strength. Too-short fibers, however, result in nonreinforcing behavior, as the aspect ratio of these fibers is much less than the critical fiber length.

4.3.2.3 Fiber orientation Closely associated with fiber length is fiber orientation. Generally, the composite is stiffer and stronger in the direction of greatest orientation, and weaker and more compliant in the direction of least orientation (Advani and Tucker, 1987). In other words, when the fibers are aligned parallel to the direction of the applied load, the mechanical properties of the composites will be high, whereas when the fibers are aligned perpendicular to the applied load, the composite mechanical properties will be the least. For example, Hajnalka et al. (2008) demonstrated that the tensile strength of hemp reinforced polypropylene composites in which the fibers were perpendicular to the applied load was 20 40% lower than that of the same composites when the fibers were parallel to the load. In another study on hemp/PLA composites, it was found that composites with the fibers aligned at 45 and 90 exhibited 48% and 30% of the strength and 53% and 42% of the Young’s moduli compared to the composites where the fibers were aligned in the flow direction (Baghaei et al., 2014). It is very difficult to obtain the fiber alignment in the case of natural fiber reinforcements, even though some alignments are possible during injection molding depending on the viscosity of the polymer melt and mold design (Pickering et al., 2016). The random orientation of short fiber reinforced composites, therefore, provides more of an advantage in that all directions are likely to be equally as strong, compared to fibers aligned in one direction.

4.3.2.4 Fiber matrix adhesion Natural fibers are primarily hydrophilic on account of their polar cellulose and hemicellulose content, whilst thermoplastic polymers are generally hydrophobic on account of their long aliphatic chain segments. This difference in electronic affinities results in poor interactions between the two materials, thus giving poor fiber matrix interaction and hence reinforcement effect. This is considered as one of the main disadvantages of natural fibers in the reinforcement of polymer matrices. To alleviate this problem, the surfaces of the fibers can be modified such that their miscibility with the hydrophobic polymer matrices improves. A weak interaction between the fiber and the matrix leads to inefficient stress transfer leading to poor performance properties whereas a strong interface can lead to efficient stress transfer between

Natural fiber and hybrid fiber thermoplastic composites

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the fiber and matrix and demonstrates a brittle nature, with easy crack propagation through the matrix and fiber. A very strong interface leads to higher tensile and flexural properties and low-impact strength properties. The interface needs to be engineered to have a good balance of impact and tensile properties. A significant amount of research in this direction has been reported to-date which includes both physical and chemical treatments for the fiber surface modification. Physical methods such as stretching, calendaring, thermotreatment, and the production of hybrid yarns have been reported for the modification of natural fibers (Faruk et al., 2012). Physical modifications of the fibers bring changes in the structural and surface properties of the fibers and can enhance the interface between the fiber and matrix and hence physical or mechanical interaction between the fibers and the polymer. In general, chemical modification of the fibers has been achieved by the introduction of a third material that has compatibility of properties between the fibers and the polymer matrix. Several compatibilizing or coupling mechanisms such as weak boundary layers, deformable layers, restrained layers, wettability, chemical bonding, and acid base effect have been suggested for the chemical modification of the interface between the fibers and the matrix (Faruk et al., 2012). Table 4.6 shows the different types of fiber surface modification to improve the interfacial interaction between different types of fibers and matrices and Fig. 4.4 shows the possible interaction of cellulose fiber with different types of modifiers. Because of the extra processing steps required during the chemical modification of natural fibers, alternate methods such as the incorporation of additives during processing are employed. The widely used technique is the use of maleated coupling agents as an additive during the processing of the composites. The fundamental difference in the use of maleated coupling agents with other chemical treatments is that maleic anhydride is not only used to modify the fiber surface but also to modify the polymeric matrix to achieve better interfacial bonding between fiber and matrix. A significant amount of published literatures are available to-date on the effect of different types of maleated coupling agents on the mechanical properties of natural fiber reinforced composites.

4.4

Developments in the processing of natural fiber reinforced composites

Generally, natural fiber reinforced composites are manufactured using the traditional techniques already developed and optimized for thermoplastics. The processing of typical natural fiber reinforced composites can be classified into two stages: compounding and molding. These are presented in further detail below. The first stage in all of these processing methods involves the melt blending of the fiber and matrix components, along with other components, such as additives including coupling agents, processing aids, pigments, heat stabilizers, etc. The result of this stage is a pelletized feesdstock or a melt blended composite that can be further processed similar to thermoplastics to give the final molded composite

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Green Composites

Different methods of surface treatments reported in the literature (Mohanty et al., 2004; Yuan et al., 2004; Huda et al., 2008; Pickering et al., 2011; Ragoubi et al., 2012; George et al., 2014)

Table 4.6

Fiber matrix composites

Treatment

Measurements

Inference

Miscanthus fiber-PLA

Corona discharge treatment

Mechanical, DMA, SEM, XPS

Sisal-PP

Mercerization, cyanoethylation, MAPP treatment

Thermal and mechanical properties

Flax and hemp fiber

Enzymatic treatment

Mechanical, SEM, and XPS

Flax-PLA

10% organosilane, 5% NaOH

Physicochemical and mechanical

Hemp-PLA

5% NaOH, 0.5% silane

Mechanical and POM

Wood-PP

Argon and air plasma

Mechanical, SEM, and XPS

Significantly enhanced interfacial adhesion compared to untreated fibers (Ragoubi et al., 2012) Mechanical properties improved significantly by the addition of MAPP compared to other properties (Mohanty et al., 2004) Surface topography were free of contaminants and individual fiber bundles were exposed (George et al., 2014) Increase in the hydrophobicity of fibers and moderate improvement in the mechanical properties of the composites (Huda et al., 2008) 5% NaOH increased adhesion by 100% and 0.5% increased adhesion by 45% (Pickering et al., 2011) Some extent of increase in the tensile strength and modulus of the composites (Yuan et al., 2004)

products. Compounding can be done either as a continuous process such as extrusion or as a batch process such as high shear mixing. Fiber attrition occurs during most extrusion and compounding and the degree of fiber attrition depends on the initial fiber length, fiber loading, and process variables such as screw design, shear rate, and viscosity of the melt compound. The selection of the compounding method is influenced by the feeding difficulty due to the difference in the bulk density of the natural fiber and the thermoplastic matrix, and the poor dispersion of the natural fiber in the thermoplastic matrix due to poor fiber matrix adhesion properties. Compression molding (CM), injection molding, and extrusion are the common manufacturing methods for short fiber reinforced composites.

Natural fiber and hybrid fiber thermoplastic composites

HO HO {cell} O HO

53

Si HC

CH2

O C O {cell} OH

RHN

Silanization

Cyano methylation HO

{cell}

O–Na+

Alkali treatment

O C R

O {cell} OH

Maleic anhydride treatment

Figure 4.4 Mechanism of fiber surface modification using different methods. Adapted from Gurunathan et al. (2015).

In CM process, loose chopped fibers or mats of short or long fibers either randomly oriented or aligned, are compressed with thermoplastic usually in sheet form under pressure and temperature. Viscosity of the polymer matrix needs to be carefully controlled in order to ensure that the fibers are completely impregnated by the polymer. The processing variables such as viscosity of the polymer, pressure, temperature, and holding time need to be taken care of for the production of good-quality composites, to provide good interfacial interaction between the polymer and matrix (Ho et al., 2012). Injection molding is commonly used for the composites with thermoplastic matrices, though thermoset composites can also be achieved by this process. During this process, the molten composite is injected through the nozzle to the mold, and solidifies in the mold. Fiber orientation in the composite varies because of the variation in the shear flow along the walls and the center. The fibers align along the mold wall, while at the center of the mold the fibers take up a transverse orientation with respect to the mold flow, leading to a skin core structure (Kim et al., 2001). The factors affecting the injection molded composite strength are: residual stress in thermoplastic matrix composites due to pressure gradients, nonuniform temperature profiles, polymer chain alignment, and differences in fiber and matrix thermal expansion coefficients (Ho et al., 2012). The selection of the compounding and processing method significantly influences the fiber dispersion, length and length distribution, and fibrillation of the natural fibers. For example, Gatenholm and Mathiasson (1994) reported fiber size reduction during the processing of PHB with cellulose, and was related to the reduction in molecular weight of PHB. These authors reported that during processing, chain scission of PHB led to the formation of crotonic acid that hydrolyzed the cellulose fiber leading to significant fiber attrition, but at the same time increased dispersion of the fibers in the matrix. Fiber attrition during high shear mixing, such as thermo-kinetic process used by extrusion and injection molding, was reported in the study of Karmaker and Youngquist (1996).

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Their study also found that these fibers can have orientations and the ability to overlap and interlock, which can result in better mechanical properties. One other process of manufacturing lightweight composites that is gaining attraction in the composite industry is the direct long fiber injection process (D-LFT), in which the fibers are spooled and fed in to a heating zone of the twinscrew extruder, where the thermoplastic is integrated with the fiber bundles. These bundles can be used either for a direct CM in a cold tool, or cut at a desired length and fed continuously into an injection molding machine to mold parts continuously. D-LFT is widely used to manufacture long glass fiber reinforced thermoplastics in high throughput (Schut, 2002). Natural fiber enhanced direct long fiber thermoplastic technology (NF-DLFT) is a modified form of the existing D-LFT approach for producing large semistructural and structural natural fiber reinforced thermoplastic composites. Several companies involved in the development of natural fiber composites, such as Composite Products Inc., Winona, Magna International (in cooperation with NRC), Rieter Automotive Systems (Winterthur, Switzerland), Fraunhofer Gesellschaft (Munich, Germany) are exploring NF-DLFT as the key processing method in their commercializing efforts to maximize the production (Bureau et al., 2011; Ruch et al., 2002; Knights, 2003; Schut, 2002, 2004). Different methods were reported for the processing of natural fibers using the D-LFT process. In one process, called the express method, the basic thought of the combination of extrusion and compression method is utilized to combine the natural fiber mats with the polymer melt directly in the pressing tool (Faruk et al., 2012). In this process, a film of molten polymer and a layer of natural fiber fleece is pressed together using the pressing tool. The molten polymer layer is placed on the pressing tool with the help of an adjustable extruder. In another NF-D-LFT process, the long natural fibers were directly fed off fiber yarns or slivers into either a twin-screw extruder or directly into an injection molding machine. NF-DLFT pulls continuous fiber rovings into a twin-screw extruder in which the screws cut the roving and blend them gently with a premelted polymer. The advantages of NF-DLFT include reduced screw wear, single heat history, custom formulation, in-line recycling, just in time compounding, and overall cost reduction (Schut, 2002, 2004; Markarian, 2007; Knights, 2003; Malnatti, 2007; Schemme, 2008a,b). Research in this direction has shown that it is possible to retain the long fiber structure through an optimal configuration of the screws, producing better performance; however, handling of the slivers is rather problematic for industrial application because it leads to inconsistent fiber distribution. Table 4.7 shows the length of fiber in composites prepared by different processing methods. A vast number of publications are available related to the processing of natural fiber reinforced composites, where the authors focus on the composite properties prepared by various processing methods. For example, Table 4.8 shows the different molding methods for manufacturing wheat straw, one of the vastly available agricultural residues, used by different authors.

Natural fiber and hybrid fiber thermoplastic composites

55

Length of fibers in composites prepared by different processing methods (Fakirov and Bhattacharyya, 2001; Faruk et al., 2012)

Table 4.7

Manufacturing process

Fiber loading (%)

Fiber length (mm)

Mixing (cascade mixing) Extruder compounding Pultrusion Injection molding Precompounded long fiber granules by extrusion DLFT extrusion compression molding Compression molding

NA NA NA 40% 40%

.3 .3 10 30 0.1 1 1 25

40% 40%

1 25 .10

4.4.1 Recent developments in short fiber composites processing The primary challenges for the development of a manufacturing process for highperformance structural materials from short lignocellulosic filled thermoplastic materials include: (1) retention of the fiber length required for the effective stress transfer from the matrix to the fibers, (2) well dispersion of the hydrophilic fibers in the matrix to avoid stress concentrating agglomerates, and (3) a good fiber/matrix interfacial adhesion which enhances the stress transfer to the fibers. One of the critical factors in the processing of the natural fiber-based composites is the agglomeration of the hydrophilic fibers, either due to moisture absorption or the improper selection of process variables, leading to poor dispersion of the fibers in the matrix. These unevenly distributed agglomerates act as stress concentrators in the polymer matrix resulting in the premature failure of the composites and, hence, low performance. Several factors, such as the shearing forces generated in the compounding equipment, residence time, temperature, and viscosity of blends, have to be considered to achieve the proper dispersion of the fibers without fiber length reduction. Center for Biocomposites and Biomaterials Processing at the University of Toronto (U of T) has developed a microfiber technology to alleviate this critical issue of agglomeration and produce high-performance microfiber composites. Using this technology, the short biofibers are defibrillated to microfibers, which also disperses the fibers well in the polymer matrix, and leads to the manufacture of high-performing composites within very short cycle times (Sain et al., 2014). This compounding process is capable of mixing short fibers, such as wood cellulose, into thermoplastics creating a naturally reinforced polymer matrix, thus forming a new microfiber containing composite with greater structural properties. X-ray tomographic pictures of microfiber composites clearly show that fiber dispersion is better when composites are prepared by microfiber technology developed by the U of T (MF technology) compared to conventional methods (Fig. 4.5).

Table 4.8

Different processing methods reported in the literature for wheat straw reinforced composites

Polymer

Wheat straw fiber size

Fiber Processing method content (%)

Properties studied

Reference

Polyethylene (MFI 5 11 g/10 min) and polypropylene (MFI 5 8 g/ 10 min)

50 mesh

35

Thermal degradation properties and coefficient of thermal expansion

Zabihzadeh (2010)

Polypropylene (MFI 5 12 g/10 min)

Average length 0.3 mm

20

Pan et al. (2010)

Polypropylene (MFI—not reported)

Wheat fiber—type 1: 40 length 20 40 mm; type 2: length 0.5 1.5 mm

Polypropylene, (MFI 5 12 g/10 min)

Wheat flour; wheat fiber 1.87 mm; fungal modified wheat flour

Tensile, dynamic mechanical, and thermal properties, melt rheological properties, and morphological analysis Mechanical properties, melt flow, heat deflection temperature, coefficient of thermal expansion, and water absorption Mechanical properties

30, 40

Melt blending using Haake Buchler system (180 C for 10 min at 40 pm) followed by granulation and injection molding (temp: 185 C, P— 3 MPa) Melt blending using Thermo Haake Rheomix (170 C for 7 min at 55 rpm) followed by granulation and compression molding (180 C for 4 min) Melt blending uisng a highintensity K-mixer at 175 C followed by granulation and injection molding

Wheat flour: melt blending using high-intensity K-mixer at a preset temp. of 185 C Wheat fiber: melt blending using a

Mishra and Sain (2009)

Panthapulakkal and Sain (2006)

Brabender mixer at 180 C, for 5 min at 60 rpm, followed by granulation and injection molding

Polypropylene powder (MFI 5 12 g/min)

Mechanically refined 30 wheat fiber (1.87 mm) and chemically pulped wheat fiber (2.70 mm) Wheat flour 35 mesh 30 50 size and 2 5% clay

PLA

Wheat straw fibers

Polypropylene (MFI 5 12 g/10 min)

Melt blending using Brabender mixing at 180 C, 60 rpm for 5 min

Mechanical properties

Panthapulakkal et al. (2006)

Melt blending using a Haake Minilab microcompounder (corotating conical twin-screw extruder) at 190 C and 40 rpm followed by pelletization and injection molding (190 C at 100 psi) Melt blending using microextruder under temperature conditions between 195 C and 205 C, screw speed 100 rpm and cycle time 3 min, followed by granulation and injection molding

Flexural properties and water absorption properties

Reddy et al. (2010)

Biodegradability

Pradhan et al. (2010)

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Figure 4.5 X-ray tomographic images of microfiber composites. Well-dispersed microfibers using MF technology (left) and agglomerations of microfibers using conventional technology (right). Source: Unpublished lab results at CBBP.

Comparison of the mechanical performance of composites manufactured using MF technology and conventional technology using wood fibers (CBBP lab results)

Table 4.9

Performance property

MF technology composites with well-dispersed microfibers

Composites with undispersed microfibers

Fiber content (wt%) Tensile strength (MPa) Flexural strength (MPa) Flexural modulus (GPa) Notched Izod impact strength (J/m)

50 72 116 5 35

50 56 91 4.5 23

To understand how critical the microfibre dispersion is in the composites to achieve higher mechanical properties, a comparison of the mechanical performance of the composites is presented in Table 4.9. The mechanical performance of the resultant composites can meet the OEM requirements for structural applications such as door modules. Three of the products manufactured using MF technology are shown in Fig. 4.6. Although microfiber technology leads to high-performance composites, relatively poor impact performance of the composites still restricts their use in high-end applications. However, we found that the high-impact performance can be achieved by the addition of small amount of synthetic fibers.

4.5

Thermoplastic hybrid composites

Hybrid composites are, in general, composites containing more than one reinforcement in the same polymer matrix. Note that the hybrid composites mentioned in

Natural fiber and hybrid fiber thermoplastic composites

59

Figure 4.6 Products using microfiber technology. Source: CBBP products.

this chapter are hybrid fiber composites where two different types of fiber are used for reinforcing the same matrix. Hybrid fiber composites are widely used in the composite industry and have gained significant attention for their capability of providing new freedom of tailoring the composites with the required properties that cannot be achieved in single fiber reinforced composites. By careful selection of the fibers and processing techniques, it is possible to design engineered hybrid composites to various applications with economic benefits. The long fiber hybrid composites can be prepared in many different configurations. The most important configurations are the interlayer configuration where the layers of two different fiber types are stacked onto each other, and intralayer configuration, where the two fibers are mixed within the layers (Swolfs et al., 2014). In the case of short fiber composites, the two different types of fibers are concurrently distributed in the polymer matrix, and this type of composite is the type of interest in this chapter. The properties of the hybrid fiber composites are influenced mainly by the relative amount of the fibers, elastic properties of the fibers, fiber strength distribution, degree of the dispersion of fibers in the matrix, type of matrix used, and interfacial interaction between the fibers and the matrix. Hybridization of natural fibers with a small amount of moisture-resistant and corrosion-resistant synthetic fibers is one way of improving the toughness or impact strength and resistance to moisture absorption of the natural fiber composites. Several studies has been reported in the literature that shows the incorporation of glass fibers with natural fibers like sisal (Kalaprasad et al., 2004; Nayak and Mohanty, 2010; Nayak et al., 2010; KC et al., 2015), oil palm fruit bunch fibers

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Table 4.10 Properties of composites prepared using U of T microfiber technology (Sain et al., 2015) Composite

Composition (fibers, in wt%)

PP hemp/glass 35/15 PP flax/hemp 35/15 PP woodfiber/glass 35/15

Tensile strength (MPa)

Tensile modulus (GPa)

Un-notched impact strength (J/m)

77 78 83

5.5 5.5 4.4

287 315 293

(Rozman et al., 2001), jute fibers (Esfandiari, 2007), banana fibers (Samal et al., 2009a; Nayak et al., 2010), flax (Arbelaiz et al., 2005), hemp (Panthapulakkal and Sain, 2007), wood flour and wood fibers (Jiang et al., 2003; Sain and Li, 2003), bamboo fibers (Nayak et al., 2009, 2010; Thwe and Liao, 2002, 2003) in thermoplastic matrices such as PP, PE, and PVC resulted in improved performance. Our studies (Panthapulakkal and Sain, 2007) on the injection molded hybrid hemp and glass fiber composites demonstrated that hybridization of 10 wt% of glass fiber with 30 wt% of short hemp fibers enhanced the strength and stiffness considerably. Tensile strength and stiffness increased from 52.5 MPa and 3.7 GPa (40 wt% hemp fiber composites) to 59.5 MPa and 4.4 GPa, respectively. The microfiber technology that we have developed at the CBBP for short fiber composite processing, as discussed in the processing section of the composites, leads to high-performance composites; however the poor impact performance of the composites restricts their use in high-end applications. In order to improve the impact performance of the composites, U of T microfiber technology has been modified to integrate a small amount of synthetic fiber, such as glass fiber. Table 4.10 shows some of the results that have been obtained in this research. The results indicate that these hybrid composites can be used in many of the structural applications in the automotive industry.

4.6

Advanced natural fiber/hybrid fiber composites in lightweighting applications

The lightweighting concept has significantly gained attention in various industries, especially the automotive and aerospace industries, because of the fuel efficiency and environmentally friendly design concept. During the last 10 20 years, different types of materials and processing methods have been invented and this has given a resurgence of different types of hybrid composites for lightweighting applications. This section deals with our research advancement in this direction using cellulosic microfiber and synthetic fiber composites for the manufacture of high-performance lightweight composites.

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61

The major challenge for the development of high-performing bio-based structural composites with natural microfibers or natural microfiber glass fiber hybrids is the lack of impact strength required to comply with the structural OEM requirements. The use of NF-DLFT, which is the current method for manufacturing long fiber composites economically, is one method of improving the impact strength of the composites because of the increased length of the fibers in the composites. However, unlike flax and sisal, cellulose microfibers are not found in continuous form in nature and must be processed from cellulosic or lignocellulosic entities to produce continuous yarns to be incorporated into the NF-DLFT process. In order to obtain high-performance microfiber reinforced hybrid composites, the microfiber technology has been integrated with the NF-DLFT process to provide the required dispersion of the fibers, and at the same time, keep the length of the fibers long enough to provide high-impact strength. Using this technology we have designed and developed many composite formulations using cellulose microfiber and long glass and long carbon fiber for lightweighting applications, especially in the automotive industry. Carbon fiber, being twice as strong and 30% lighter compared to glass fiber (Brookband et al., 2015; Fua et al., 2000) has been used in automotive applications for some time. However, due to their very high cost, these materials are usually employed in high-end products like sports vehicles or luxury cars. Hybrid design of carbon and glass fiber reinforced composites have been recently introduced with encouraging results. Hybrid composite structures have been developed using varying ratios of glass and carbon woven fabric in epoxy matrices and it is shown that, when employed at the exterior, composite laminates having 50% ratio of carbon fiber reinforcement exhibit optimum flexural properties and alternating carbon/glass lay-up arrangement ensures best compressive strength (Zhang et al., 2012). The manufacturing cost of carbon fiber is the main bottleneck for its ultimate use in mass scale lightweight applications. A number of research efforts are underway to develop low cost manufacturing techniques or conversion techniques to reduce the overall cost of carbon fiber from the traditional precursor, PAN. Oak Ridge National Laboratory (ORNL), USA, has been working to develop a higherspeed, lower cost oxidative stabilization process and, very recently, in collaboration with RMX Technologies (RMX), they have scaled up a plasma-based oxidation process to the capacity of 1 t/y. ORNL has further reported net savings of 30% in energy consumption per kg of carbon fiber compared to conventional methods (DOE Report, 2013; US Drive, 2015). The use of recycled carbon fiber instead of using virgin carbon fiber is another alternative to reduce the cost of the carbon fiber composites. We have demonstrated microfiber direct long fiber thermoplastic technology (MF-DLFT), in association with the industrial partners, in order to manufacture high-performing hybrid composites—cellulose microfiber and carbon fiber reinforced hybrid composites for automotive under-the-hood applications. The hybridization of cellulosic fiber with long glass and carbon fiber increased the performance of the composites, including the toughness of the composites. The OEM requirements of the two under-the-hood parts are given in Table 4.11.

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Table 4.11 Comparison of the properties of the cellulose microfiber hybrid composites developed at the CBBP with the OEM specifications Property specification

Engine cover

Cam cover

CBBP MF-DLFT hybrid composite

Tensile strength (MPa) Flexural modulus (GPa) Impact strength (at 23 C, kJ/m2 ) HDT (at 1.82 MPa,  C) Flammability (mm/min) Density (g/cm3 ) Current materials

110 7.1 3.1

85 7.2 4

5, 1 5, 1 B

187 ,100 1.32 1.42 30% glassfilled PA6

170 ,100 1.47 40% glassfilled PA66

B, 5



11 Cellulose microfiber 1 carbon fiber reinforced PP

5, equal; 1, exceeds; B, about the same; 11, exceeds well above.

Microfiber-enabled composites have several unique advantages compared to conventional glass-filled thermoplastic structures; the major ones being 15 30% of weight reduction and the associated savings in the fuel consumption (14%). CBBP has developed various cellulose microfiber carbon fiber hybrid composites intended for use in various applications, and the details are given in Table 4.12. Various under-the-hood parts were successfully prototyped in association with one of our industrial partners, FORD Canada, and some of the parts are shown in Fig. 4.7. Life cycle analysis of these prototypes to evaluate their environmental impact is currently undergoing and will be reported soon.

4.7

Emerging trend: utilization of waste or recycled fibers in composites

In the recent years, the concept of “green” or “eco-friendly” or “eco-materials” has gained significant importance because of the need to protect our ecosystem. Ecofriendly materials can be assessed in terms of their life span, life time requirements, complexity of product shape, volume of items to be produced, cost-effectiveness, and lightweighting. Natural fiber composites are considered as eco-materials because of their environmentally friendly properties such as their low density, costeffectiveness, low energy-intensive processing, and carbon neutrality compared to their synthetic counterparts. As discussed in the fiber section, there is wide availability of natural fibers in different forms. Millions of tons of solid waste materials are being generated world-wide annually as a byproduct during agricultural, mining, municipal, and other processes. The use of the waste materials in composites or

Details of the hybrid composites prepared at the CBBP center and the percentage weight reduction compared to the currently used composite materials in the automotive applications

Table 4.12

Composites

Renewable content (wt%)

Intended application

Prototype build to-date

Weight reduction (%)

Cellulose microfiber 1 carbon fiber in PP matrix Cellulose microfiber 1 carbon fiber in PP matrix Cellulose microfiber 1 carbon fiber in PP matrix Cellulose microfiber 1 carbon fiber in PP matrix

20 30

Engine cover, extension panel dash, battery tray, door carrier plates, air inlet box Oil pan, cam cover, windage tray, engine front cover, intake manifold Engine cover, extension panel dash, battery tray, door carrier plates, air inlet box Interior parts with glass/mineral filled PP

Engine cover

30

Oil pan, cam cover Battery tray

20 25

Door cladding

15

20 35 20 25

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Figure 4.7 Prototypes of engine cover, beauty shield, cam cover, and oil pan developed by CBBP with the support of FORD Canada.

Classification of major waste streams available (Kumar et al., 2015)

Table 4.13

Types of solid wastes

Sources

Recycling and utilization

Agro waste

Rice and wheat straw and husk, cotton stalk, saw mill waste (wood flour), nut shell, bagasse, banana stalk, sisal and vegetable residues Coal combustion residues, steel slag, bauxite red mud, construction debris Coal water waste, mining overburden waste, tailing from iron, copper, zinc, gold, and aluminum industries

Fiber reinforced composites, particle boards, printing papers and corrugating mediums, bricks, roofing sheets, etc. Cement, bricks, tiles, paint, concrete, ceramic products, and wood substitute products Brick, tiles, lightweight aggregate fuel

Industrial waste Mining mineral waste

recycling materials can open up a stream of low cost materials for industrial applications such as building, packaging, and automotive industries. The advantages of using waste or recycled materials in composites include (1) reduction of cost, (2) alleviate waste disposal, (3) reduce pollution by reducing the burning of the waste, and (4) socioeconomic benefits, such as creation of jobs and improving the economy of rural areas. The major classification of waste materials is given in Table 4.13. Agricultural residues are the major inexpensive resources for lignocellulosic fiber production. Usually after harvesting, large amount of residues are discarded in the field and may pose environmental problems. These residues are annually

Natural fiber and hybrid fiber thermoplastic composites

65

Table 4.14 Availability of agricultural lignocellulosics (Reddy and Yang, 2005) Agricultural residues

Availability (103 t)

Corn stover Coir Bagasse Wheat straw Rice straw Barley straw Sorghum stalks

727 100 100 568 579 196 252

renewable, available in abundance, and of limited value at present. All parts of these residues can be used for plastic reinforcement, enabling maximum utilization of these inexpensive resources. Examples of primary lignocellulosic agricultural byproducts that are available in substantial quantities and at low cost include wheat, rice, and barley straw, corn stover, sorghum stalks, coconut husks (coir), sugarcane bagasse, and pineapple and banana leaves. The world-wide availability of these fiber residues is given in Table 4.14. Significant amounts of research have been reported on the exploitation and utilization of various agro-residues for manufacturing different type of composites. For example, the use of wheat straw and corn stover as such in plastic reinforcement had been reported by Panthapulakkal et al. (2006) and the results showed that it is possible to tailor make the properties according to the requirement. A similar study on the utilization of all parts of pineapple leaf waste in the reinforcement of PP has been reported by Kengkhetkit and Amornsakchai (2014). Use of such waste or unattended residues not only reduces the material cost but also bring in opportunities to adjust the price-performance ratio and make it viable for industrial applications and could be an additional source of revenue for farmers, without adversely affecting soil fertility (www.fao.org/ DOCREP/004/Y1873E/y1873eob.htm). Growing fiber crops and the use of processed fibers and crop residues will be the most promising alternatives to reduce the dependence on the utilization of the fossil fuel-based resources. However, the major limitations of using these agricultural byproducts and/or residues include the lack of an established collection, storage, and handling systems to prevent the degradation of these residues when stored for long period. The above-mentioned needs to be reconsidered for the effective utilization of agro-residues or waste fibers. It has been reported that, in most cases, the use of natural fibers in composites is likely to be environmentally superior to the use of fossil fuel-based glass fiber composites; the reasons being (1) production of natural fiber has lower environmental impacts compared to the production of glass fiber, (2) natural fiber composites have higher fiber content for equivalent performance than that of glass fiber composites, reducing the fossil fuel-based polymer content, (3) the lightweight natural fiber composites improve fuel efficiency and reduce emissions in the use phase of the component compared to their synthetic

66

Green Composites

counterparts, especially in auto applications, and (4) carbon neutrality of the natural fibers (Joshi et al., 2004). Even though fossil fuel-based fibers are environmentally inferior compared to natural fibers, this can be mitigated partly by the use of waste or recycled fibers in the composites and these composites can be considered as “green composite” even though they do not use natural fibers. For example, waste streams of carbon fibers from the aerospace or aviation industry or other manufacturing industries can be used as a cost-effective reinforcement of polymer matrices for the production of high-performing composites.

4.8

Environmental benefits of using lightweight composites and future trends

The demand for sustainable products based on renewable resources is escalating as a result of the growing awareness of the public about the depletion of resources as well as global climate changes. The industrial world is looking for replacement of petroleum-based materials with bio-based renewable materials in order to reduce the burden on the environment. Many of the manufacturing sectors set their priority in the design of eco-friendly materials. Plant fibers are renewable and abundant; the primary production of these fibers per year is estimated to be 2 3 1011 t, whereas production of synthetic polymers is estimated to be 1.5 3 108 t (Dreyer et al., 2002). The estimated fibrous raw materials from agricultural crops is about 2.5 3 109 t (Hon, 1988). The projected growth for the natural fiber market in North America from the year 2000 (155 million) to 2025 (1.38 billion) is tremendous (Satyanarayana et al., 2009). Biocomposites and hybrid biocomposites have been gaining momentum in the last decade, especially in the automotive sector, because of the lightweighting advantages of these composites (Gurunathan et al., 2015). Replacement of glass fibers with natural fibers allows lighter components as the density of natural fibers (1.5 g/ml) is lower compared to glass fibers (2.5 g/ml), while simultaneously increasing the proportion of renewable resource content within the vehicle. Many manufacturers are using these green fiber composites for non and semistructural applications in their vehicles and examples are given in Table 4.15. Lightweight design of automotives has become of paramount importance to not only reduce the carbon footprint of their final products but also to conserve valuable and depleting resources. The use of lightweight materials in cars or automobiles helps to reduce the cost in terms of price as well as fuel consumption, which in turn reduces the carbon-dioxide emission, which is one of the strict environmental legislations in various jurisdictions such as Europe, North America, and Asia. Europe has set the CO2 emission target for 2020 as 95 g CO2/km, and in order to meet this legislative requirement a 200 300 kg weight reduction of vehicles is required (Reinforced Plastics, 2014). Lan Mair (2000) reported that a 25% reduction in the weight of vehicles is equivalent to 250 million barrels of crude oil and a reduction of 220 billion pounds of CO2 emission per annum. Automakers such as Volkswagen, Ford, Honda, and General Motors already use natural fiber-based

Natural fiber and hybrid fiber thermoplastic composites

67

Table 4.15 Natural fiber-based composite parts used in vehicles by different manufacturers (Pervaiz et al., 2016) Manufacturer

Vehicle model

Natural fiber-based composite parts

Ford

Mondeo CD 162, Focus A2, A3, A4, Avant, A6 Brevis, Harrier, Celsior, Raum Trucks

Door panels, B-pillar, and boot liner

Audi Toyota Mercedes-Benz

BMW

3, 5, and 7 series and others

Volkswagen

Golf, Passat, Variant, Bora, Fox, Polo

Seat backs, side and back door panel, boot lining, hat track, and spare tire lining Door panels, seat backs, and spare tire cover Internal engine cover, engine insulation, sun visor, interior insulation, bumper, wheel box, and roof cover Door panels, headliner panel, noise insulation panels, seat backs, molded foot, and well linings Door panels, seat backs, boot liner, and boot lid finish panel

products such as car seats, dashboard coverings, roofs, and trunk lids (Pegoretti et al., 2014). Life cycle analysis of plant fiber-based composites in automotives, in comparison with the currently used materials was reported by researchers to demonstrate their environmental benefits (Luz et al., 2010; Witik et al., 2011; Pegoretti et al., 2014). Microfiber-enabled composites developed at our center have several unique advantages compared to conventional glass-filled thermoplastic structures as explained in the above sections. In summary, these lightweight materials can be the 21st century green materials for transportation, providing improved functional stability during use and storage, and environmental degradation on disposal.

4.9

Future trends

Recent environmental concerns related to global climate change and greenhouse gas emissions have prompted automotive manufacturers to focus on the development of lightweight and fuel-efficient vehicles. The greenhouse gas emissions (GHG) associated with road transport vehicles account for 27% of all combined emissions in the USA, which translates into 1800 million metric tons of CO2 equivalent (EPA, 2013). Environmental and consumer awareness have forced governments and environmental protection agencies to implement strict regulations in curbing the emissions. “Lightweighting” in the transport industry has become a major theme of research in recent years; the main motives are the anticipated fuel savings, reduced emisions, and the need to meet stricter environmental legislation in various jurisdictions such as Europe, North America, and Asia. It is observed that the demand for lightweight materials, polymers, and composites, in the North

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Green Composites

American automotive market is increasing significantly and is projected to rise to 8020 million pounds in 2019, which is 53% higher than in 2008 (Holmes, 2014). Since a 10% reduction in vehicle weight has the potential to save 6 8% of fuel consumption (Cheah, 2010; DOE, VTO, 2015), and since lighter objects need less energy for acceleration compared to heavy ones, lightweight materials provide a better opportunity to enhance a vehicle’s fuel economy and mitigate GHG emissions. North American automotive OEMs are striving hard to reduce their overall fleet weight in significant numbers for both luxury and standard cars and some companies even have set an ambitious target of reducing up to 350 kg (about 20%) of car weight by 2020 (Shankar, 2013). Natural fiber composites and hybrid fiber composites with natural fiber and a small portion of synthetic fibers are gaining momentum as lightweight materials in various industry sectors, especially the automotive and aerospace industries. The popularity of these composites has resulted mainly from their lightweight, design friendliness, and awareness of the sustainable product design. Though hybrid composites with natural fiber and glass fiber combination have opened up new applications, the authors see the future trend as the composites based on natural fiber and carbon fiber composites, as these composites are very efficient in lightweighting compared to their glass counterparts. There are many researches going in the direction of reducing the cost of carbon fiber production. Alternatively, initiatives using the recycled carbon fiber and recycled polymers for the composite manufacture have already been started (BMW Press Club Global, 2015; Caliendo, 2015; Gardiner, 2014; Reinforced Plastics, 2014; Yang et al., 2012). Another trend that the authors have foreseen is the use of the nanofiber-based composites. Any waste material containing cellulose can be a resource for the production of nanocellulose, and these nanofibrils have very high strength compared to cellulosic fiber bundles and microfibers. Isolation of nanofibrils from resources such as wheat straw, soy hull, and pine has been reported by our research group (Wang and Sain, 2007; Alemdar and Sain, 2008; Nedunuri et al., 2016). Current production of nanofibrils from lignocellulosic resources is energy-intensive. Further, dispersion of these nanofibrils in the polymer matrices is found to be very difficult. In the future, technologies similar to the one used for the in situ production of microfibrils can also be modified to produce nanofibrils in situ during the compounding process. The production of nanofibrils and melt blending of these fibers with the plastics without exposing the fibrils to the environment can lead to better dispersion of these fibrils in the polymer matrices and can produce highperforming lightweight composites for various applications.

Acknowledgments The authors acknowledge financial support from the NSERC Automotive Partnership Canada, NSERC Green Fiber Network, and Ford Motors Canada for their in-kind support. Technical help from Shiang Law is also acknowledged.

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References Advani, S.G., Tucker III, C.L., 1987. J. Rheol. 31 (8), 751 784. Ahmad, F., Choi, H.S., Park, M.K., 2015. Macromol. Mater. Eng. 300 (1), 10 24. Alemdar, A., Sain, M., 2008. Bioresour. Technol. 99 (6), 1664 1671. Arbelaiz, A., Fernandez, B., Cantero, G., Llano-Ponte, R., Valea, A., Mondragon, I., 2005. Comp. A: Appl. Sci. Manuf. 36 (12), 1637 1644. Ashori, A., 2008. Bioresour. Technol. 99, 4661 4667. Azwa, Z.N., Yousif, B.F., Manalo, A.C., Karunasena, W., 2013. Mater. Des. 47, 424 442. Baghaei, B., Skrifvars, M., Salehi, M., Bashir, T., Rissanen, M., Nousiainen, P., 2014. Comp. A: Appl. Sci. Manuf. 61, 1 12. Bhatnagar, A., Sain, M., 2005. J. Reinf. Plast. Compos. 24 (12), 1259 1268. BMW Press Club Global (2015) https://www.press.bmwgroup.com/global/startpage.html. Biagiotti, J., Puglia, D., Kenny, J.M., 2004. A review on natural fiber based composites part I: structure, processing and properties of vegetable fibres. J. Nat. Fibr. 1 (2), 37 68. Bos, L.H., Mussig, J., van den Oever, M.J.A., 2006. Comp. A: Appl. Sci. Manuf. 37, 1591 1604. Bourmaud, A., et al., 2013. Ind. Crops Prod. 44, 343 351. Brookbank, P., Savage, L., Evans, K.E., 2015. J. Reinf. Plast. Compos. 2015; http://dx.org/ 10.1177/0731684415572437. Bureau, M.N., Bravo, V., Mihai, M., McLeod, M., Baril, E., He´tu, J.-F., 23rd International Polyolefins Conference 2011: Evolving Technology for the Global Economy, 27 February 2 March 2011, Houston, TX. Abstract available from: ,http://nparc.cistiicist.nrc-cnrc.gc.ca/eng/view/.. Caliendo, H. New Bill Requests Study on Carbon Fiber Recycling. Industry News, Composites World, 22 June 2015; http://www.compositesworld.com/news/new-billrequests-study-on-carbon-fiber-recycling. Chakar, F.S., Ragauskas, A.J., 2004. Ind. Crops Prod. 20 (2), 131 141. Chakraborty, A., Sain, M., Kortschot, M., 2005. Holzforschung. 59 (1), 102 107. Cheah, L.W., 2010. Cars on a Diet: The Material and Energy Impacts of Passenger Vehicle Weight Reduction in the U.S. PhD thesis. The Engineering Systems Division, Massachusetts Institute of Technology. DOE, VTO. Vehicle Technologies Office: Lightweight Materials R&D Annual Progress Report. 2013; http://energy.gov/eere/vehicles/downloads/vehicle-technologies-office-2013lightweight-materials-rd-annual-progress. DOE, VTO. Vehicle Technologies Office: Materials Technologies. Office of Energy Efficiency & Renewable Energy, 2015; http://energy.gov/eere/vehicles/vehicle-technologies-officematerials-technologies. Dreyer, J., Mu¨ssig, J., Koschke, N., Ibenthal, W.D., Harig, H., 2002. J. Ind. Hemp. 7, 43 59. Esfandiari, A., 2007. J. Appl. Sci. 7 (24), 3943 3950. EPA, 2013. Transportation Sector Emissions; Emissions and Trends. 2013; http://www.epa. gov/climatechange/ghgemissions/sources/transportation.html. Fakirov S., Bhattacharyya D. (2001) Handbook of engineering biopolymers: homopolymers, blends and composites. In: Processing of Natural and Manmade Cellulose FiberReinforced Composites; pp. 267 284. Faruk, O., et al., 2012. Prog. Polym. Sci. 37 (11), 1552 1596. Faruk, O., et al., 2014. Macromol. Mater. Eng. 299 (1), 9 26. Fua, S.Y., Laukeb, B., Ma€derb, E., 2000. Comp. A: Appl. Sci. Manuf. 31, 1117 1125.

70

Green Composites

Gardiner, G. Recycled Carbon Fiber Update: Closing the CFRP Lifecycle Loop. Composites World, 30 November 2014; http://www.compositesworld.com/articles/recycled-carbonfiber-update-closing-the-cfrp-lifecycle-loop. Gassan, J., Bledzki, A.K., 1996. J. Eng. Appl. Sci. 2, 2552 2557. Gassan, J., Chate, A., Bledzki, A.K., 2001. Calculation of elastic properties of natural fibers. J. Mater. Sci. 36, 3715 3720. Gatenholm, P., Mathiasson, A., 1994. J. Appl. Polym. Sci. 51 (7), 1231 1237. George, M., Mussone, P.G., Bressler, D.C., 2014. Ind. Crops Prod. 53, 365 373. Gibson, R.F., 2011. Principles of composite material mechanics. CRC Press, Boca Raton. Gurunathan, T., Mohanty, S., Nayak, S.K., 2015. Comp. A: Appl. Sci. Manuf. 77, 1 25. Hajnalka, H., Racz, I., Anandjiwala, R.D., 2008. J. Thermoplast. Compos. Mater. 21, 165 174. Ho, M.-P., Wang, H., Lee, J.-H., Ho, C.-K., Lau, K.-T., Leng, J., et al., 2012. Compos. B. 43 (8), 3549 3562. Holmes M. June 2014. Demand for lightweight automotive materials in North America to continue to rise. Reinforced Plastics News. http://www.materialstoday.com/compositeapplications/news/demand-for-lightweight-automotive-materials-in/. Hon, D.N.S., 1988. Polym. News. 13, 34 140. Huda, M.S., Drzal, L.T., Mohanty, A.K., Misra, M., 2008. Compos. Sci. Technol. 68 (2), 424 432. Janardhnan, S., Sain, M., 2006. BioResources. 1 (1), 1 5. Janardhnan, S., Sain, M., 2011. J. Polym. Environ. 19, 615 621. Javad, M., Abdul Khalil, H.P.S., 2011. Carbohydr. Polym. 86, 1 18. Jiang, H., Kamdem, D.P., Bezubic, B., Ruede, P., 2003. J. Vinyl Addit. Technol. 9 (3), 138 145. John, M.J., Thomas, S., 2008. Carbohydr. Polym. 71, 343 364. Joshi, S.V., Drzal, L.T., Mohanty, A.K., Arora, S., 2004. Comp. A: Appl. Sci. Manuf. 35, 371 376. Karmaker, A.C., Youngquist, J.A., 1996. J. Appl. Polym. Sci. 62, 1147 1151. KC, B., Panthapulakkal, S., Kronka, A., Agnelli, J.A.M., Tjong, J., Sain, M., 2015. J. Appl. Polym. Sci. 132 (39). http://dx.doi.org/10.1002/app.421452. Kalaprasad, G., Francis, B., Thomas, S., Kumar, C.R., Pavithran, C., Groeninckx, G., 2004. Polym. Int. 53 (11), 1624 1638. Kengkhetkit, N., Amornsakchai, T., 2014. Mater. Des. 55, 292 299. Kim, E.G., Park, J.K., Jo, S.H., 2001. J. Mater. Process. Technol. 111 (1 3), 225 232. Knights, M., April 2003. Plastic Technology. ,http://www.ptonline.com/articles/long-fiberthermoplastics-extend-their-reach.. Ku, H., Wang, H., Pattarachaiyakoop, N., 2011. Compos. B: Eng. 42, 856 871. Kumar, R., Singh, T., Singh, H., 2015. J. Reinf. Plast. Compos. 34 (23), 1979 1985. Lan Mair, R., Jul Aug 2000. Tomorrow’s plastic cars. ATSE Focus, 113. Lechner, M.D., 2005. Polymers. In: Martienssen, W., Warlimont, H. (Eds.), Handbook of Condensed Matter and Materials. Springer, pp. 477 522. Luz, S., Pires, A.C., Ferro, P.M., 2010. Resour. Conserv. Recycl. 54, 1135 1144. Mallick, P.K., 2007. Fiber-Reinforced Composites: Materials, Manufacturing, and Design. CRC Press. Malnatti, P., December 2007. Composites technology. ,http://www.compositesworld.com/ articles/reinforced-thermoplastics-lfrt-vs-gmt.. Markarian, J., 2007. Long fibre reinforced thermoplastics continue growth in automotive. Plast. Addit. Compd. 9 (2), 20 22.

Natural fiber and hybrid fiber thermoplastic composites

71

Matthews, F.L., Rawlings, R.D., 1999. Composite Materials: Engineering and Science. Woodhead Publishing, Cambridge, UK. Mishra, S., Sain, M., 2009. J. Nat. Fibres. 6, 83 97. Mohammed, L., et al., 2015. Int. J. Polym. Sci. 2015. 2015. Mohanty, S., Verma, S.K., Nayak, S.K., Tripathy, S.S., 2004. J. Appl. Polym. Sci. 94, 1336 1345. Nayak, S.K., Mohanty, S., 2010. J. Reinf. Plast. Compos. 29 (10), 1551 1568. Nayak, S.K., Mohanty, S., Samal, S.K., 2009. Mater. Sci. Eng. A. 523 (1 2), 32 38. Nayak, S.K., Mohanty, S., Samal, S.K., 2010. Polym. Compos. 31 (7), 1247 1257. Nedunuri, R., Panthapulakkal, S., Sain, M., Dalai, A., 2016. Ind. Crops Prod. 83, 746 754. Olsson, A.-M., Salme´n, L., 2004. Carbohydr. Res. 339 (4), 813 818. Orfao, J., Antunes, F., Figueiredo, J., 1999. Fuel. 78 (3), 349 358. Pan, M., Zhang, S.Y., Zhou, D., 2010. J. Compos. Mater. 44 (9), 1061 1072. Panthapulakkal, S., Sain, M., 2006. J. Polym. Environ. 14 (3), 265 272. Panthapulakkal, S., Sain, M., 2007. J. Appl. Polym. Sci. 103, 2432 2441. Panthapulakkal, S., Sain, M., 2013. J. Plastic Polym. Technol. 2 (1), 9 16. Panthapulakkal, S., Zereshkian, A., Sain, M., 2006. Bioresour. Technol. 97 (2), 265 272. Pegoretti, T.S., Mathieux, F., Evrard, D., Brissaud, D., Arruda, J.R.F., 2014. Resours. Conserv. Recycl. 84, 1 14. Pervaiz, M., Sain, M., 2003. Macromol. Mater. Eng. 288 (7), 553 557. Pervaiz, M., Panthapulakkal, S., KC, B., Sain, M., Tjong, J., 2016. Mater. Sci. Appl. 7 (01), 26. Pickering, K.L., Sawpan, M.A., Jayaraman, J., Fernyhough, A., 2011. Comp. A: Appl. Sci. Manuf. 42, 1148 1156. Pickering, K.L., Efendy, M.G.A., Le, T.M., 2016. Comp. A: Appl. Sci. Manuf. 83, 98 112. Pradhan, R., Misra, M., Erickson, L., Mohanty, A., 2010. Bioresour. Technol. 101, 8489 8491. Ragoubi, M., George, B., Molina, S., Bienaime´, D., Merlin, A., Hiver, J.M., et al., 2012. Comp. A: Appl. Sci. Manuf. 43, 675 685. Rahimi, A., Ulbrich, A., Coon, J.J., Stahl, S.S., 2014. Nature. 515, 249 252. Reinforced Plastics, 2014. Composite developments drive auto industry forward. www. reinforcedplastics.com. Ramamoorthy, S.K., Skrifvars, M., Persson, A., 2015. Polym. Rev. 55 (1), 107 162. Reddy, N., Yang, Y., 2005. TRENDS Biotechnol. 23 (1), 22 27. Reddy, R., Sardshti, A.P., Simon, L.C., 2010. Compos. Sci. Technol. 70, 1674 1680. Rowell, R.M., Young, R.A., Rowell, J.K., 1997. Processing of agro-based resources into pulp and paper. In: Rowell, R.M. (Ed.), Paper and Composites from Agro-Based Resources. Lewis Publishers/CRC Press, Boca Raton, FL. Rowell, R.M., 2008. Natural fibres: types and properties. In: Pickering, K.L. (Ed.), Properties and Performance of Natural-Fibre Composites. Woodhead Publishing Limited, Cambridge, UK, pp. 3 66. Rozman, H.D., Tay, G.S., Kumar, R.N., Abusamah, A., Ismail, H., Ishak, Z.A.M., 2001. Eur. Polym. J. 37 (6), 1283 1291. Ruch, J., Fritz, H.G., et al., 2002. Kunststoffe. 92 (2), 28 34. Sain, M., Li, H., 2003. Polym. Plastics Technol. Eng. 42 (5), 853 862. Sain, M., Panthapulakkal, S., Law, S., Manufacturing process for high performance short lingo-cellulosic fiber thermoplastic composite materials. Canadian Patent, Patent Number: CA 2141485. October 7 2014. Sain, M., Panthapulakkal, S., Law, S.F., Manufacturing process for hybrid organic and inorganic fibre-filled composite materials. US Patent, Patent Number: US8940132 B2, 2015.

72

Green Composites

Samal, S.K., Mohanty, S., Nayak, S.K., 2009a. Polym.-Plastics Technol. Eng. 48 (4), 397 414. Satyanarayana, K.G., Arizaga, G.G.C., Wypych, F., 2009. Progr. Polym. Sci. 34, 982 1021. Schemme, M., 2008a. Reinforced Plastics, 32 39. Schemme, M., 2008b. Plastics Additives & Compounding. 10 (2), 38 43. Schut, J.H., 2002. Plastic technology on-line48, ,http://www.ptonline.com/articles/longglass-leader-how-faurecia-helped-put-tp-composites-in-the-driver’s-seat.. Schut, J.H., 2004. Plastics technology on-line37, ,http://www.ptonline.com/articles/d-lftcomposites-aim-for-auto-body-panels.. Seery, M., 2013. Education in Chemistry, www.rsc.org/eic. Shankar, V., 2013. Global Automotive OEMs Embrace Lightweighting to Attain Fuel Economy and Emission Goals, Frost & Sullivan Market Report. http://www.frost.com/ sublib/display-market-insight.do?id5279328612&ctxixpLink5FcmCtx1&ctxixpLabel 5FcmCtx2. Summerscales, J., Dissanayake, N.P.J., Virk, A.S., Hall, W., 2010. A review of bast fibres and their composites. Part 1 Fibres as reinforcements. Composites, Part A. 41 (10), 1329 1335. Swolfs, Y., Gorbatikh, Verpoest, I., 2014. Comp. A: Appl. Sci. Manuf. 67, 181 200. Thwe, M.M., Liao, K., 2002. Comp. A: Appl. Sci. Manuf. 33, 43. Thwe, M.M., Liao, K., 2003. J. Mater. Sci. 38 (2), 363 376. US Drive. Highlights of Technical Accomplishments 2014; Overview. Published March, 2015; http://energy.gov/sites/prod/files/2015/04/f21/2014%20U.S.%20DRIVE%20Accomplishments %20Report.pdf. Varhegyi, G., et al., 1989. Energy Fuels. 3 (3), 329 335. Wang, B., Sain, M., Oksman, K., 2007. Appl. Compos. Mater. 14 (2), 89 103. Witik, R.A., Payet, J., Michaud, V., Ludwig, C., Manson, J., 2011. Compos. A: Appl. Sci. Manuf. 42, 1694 1709. Yang, Y., Boom, R., Irion, B., Heerden, D.J.V., Kuiper, P., Wit, H.D., 2012. Chem. Eng. Process. 51, 53 68. Yuan, X.W., Jayaraman, K., Bhattacharyya, D., 2004. Compos. A: Appl. Sci. Manuf. 35, 1363 1374. Zabihzadeh, S.M., 2010. J. Thermoplas. Compos. Mater. 23, 817 825. Zakzeski, J., et al., 2010. Chem. Rev. 110 (6), 3552 3599. Zhang, J., Chaisombat, K., He, S., 2012. Mater. Des. 36, 75 80.