Lignocellulosic fibers have been used as reinforcing composites for over 3000 years,1 in combination with polymeric materials. Due to their moderately high spe­cific strength and stiffness, they serve as an excellent reinforcing agent for plastics (thermosets as well as thermoplastics) besides their advantageous ecological charac­ter. The study of fibers to reinforce plastics began in 1908 with the advent of cellu­lose material in phenolics, extending to urea and melamine and reaching commodity status with glass fiber-reinforced plastics.2 However, in the last several decades, much better biocomposite materials have been developed. They have higher fiber contents, better interfacial properties, improved processing technologies, and more effective additives.1 Their biodegradability, lightness in weight, abundance and wide variety of fiber types are very important factors for their inclusion in large volume markets such as the automotive and construction industry.3

The use of lignocellulosic fibers derived from renewable resources as a reinforc­ing phase in polymeric matrix composites provides positive environmental benefits. The main advantages of lignocellulosics are: biodegradability, low costs, nonabra­sive, and nonhazardous nature, low density, abundance, wide variety of fiber types, high specific strength and modulus, relatively reactive surface, which can be used for grafting specific groups. However, there are drawbacks for using lignocellulosic fibers as reinforcing materials. One of the major disadvantages is the poor compat­ibility exhibited between the fibers (hydrophilic nature) and the polymeric matrices (hydrophobic nature), forming flocs or aggregates during processing and thus result­ing in a heterogeneous dispersion of fibers within the matrix. A low thermal stability is also a problem because the composites-based lignocellulosic fibers undergo deg­radation at temperatures higher than 200 °C. Another drawback is low resistance to moisture that leads to swelling and creation of pockets at the air-substrate interface leading to a compromise of mechanical properties and reduction in dimensional stability.4In addition, the low microbial resistance and the nonuniformity of the fiber dimensions pose further problems.

What is evident in all of these studies is that a “bandage” approach via chemical treatments is applied to solving most of the composite shortcomings. Yet, lignocel — lulosic fibers have distinct advantages over synthetics because they tend to deform rather than break during the manufacturing process. Also, cellulose fibers in particu­lar have a flattened oval cross section that enhances stress transfer by a high aspect ratio.5

A summary of the advantages and disadvantages of lignocellulosic fibers to re­inforce composites are shown in Table 14.1.6

TABLE 14.1 Summary of the Advantages and Disadvantages of Lignocellulosic Fibers to Reinforce Composites

Advantages Disadvantages

In general, reinforcing fibers can be classified as: straw fibers, nonwood fibers and wood fibers shown in Fig. 14.2.

BAST	LEAF	SEED /		Softwood
Ex.:	Ex.: Sisal	FRUIT		Hardwood
Kenaf,		Ex.: Cotton
hemp

Corn/Wheat/ Rice Straws

FIGURE 14.2 Classification of reinforcing fibers.

It is generally accepted that wood fibers are the most abundant biomass resource on earth. They are a class of natural composites that are principally found in trees and other vascular tissue and are composed of tubes made up of cellulose microfi­brils embedded in a matrix of lignin and hemicellulose. Cellulose is a polydisperse linear polymer composed of в-D-glucopyranose monomers in which the monomers are linked together by the chemical process of dehydration condensation to form glycosidic oxygen bridges between the saccharides. In natural fibers, cellulose chains have a degree of polymerization of between 500-10,000 glucopyranose units in wood cellulose, dependent upon the type of wood examined. The respective cel­lulose polymer chains of saccharide units are ordered hierarchically to form nano­fibrils, which are aligned along the major axis of the chain whose structural integrity is maintained by lateral hydrogen bonding forces among the hydroxyl and oxygen functionalities between chains providing wood its inherent high mechanical strength properties and high strength-to-weight ratio, in addition to rigidity. The nano-fibrils aggregate to form microfibrils that are responsible for the cell wall composition that displays a very pronounced crystalline phase interdispersed with noncrystalline (amorphous) regions. The diameters can range from 2-20 nm and possess lengths up to tens of microns thus offering very high aspect ratios as a function of biomolecular origin (e. g., valonia makes a form of cellulose that has one of the highest aspect ratios among the celluloses)7 (Fig. 14.3).

The reinforcing effect imparted by cellulosic fibers is based on the nature of cel­lulose and its crystallinity.

Another major component of natural fibers are hemicelluloses, lignin, pectins, and waxes. Lignin is a highly cross-linked, rather amorphous polymer with a very high polydispersity consisting of substituted phenyl propane units that plays the role of the matrix. Hemicelluloses are also part of the wood biopolymer matrix and may be characterized as branched polymers of galactose, glucose, mannose, and xylose. Cellulose acts as a reinforcing material, that is, in wood fibers, cellulose fibers, microfibrils and microcrystalline cellulose, bulk materials that have varying elastic moduli as shown in Fig. 14.4. The modulus of variegated biomaterial such as wood can be up to 10GPa, from which the isolation of the cellulose component can be as high as 40GPa (upon separation by appropriate pulping/mechanical treatments). Indeed, further separation into the microfibrils allows moduli of up to 70GPa to be accessed.8

Young’s Modulus

10 GPa

40 GPa 70 GPa 250 GPa

FIGURE 14.4 Correlation between structure, process, component, and modulus (adapted from Michell9).

During the last several decades, a number of useful materials that use the rein­forcing properties of wood cellulose have found a number of major markets. For example, wood plastic composites (WPCs) are one of the most attractive. WPCs are a type of composite that contain lignocellulosics combined with thermoset or thermoplastic polymers. Thermosets do not reversibly cure and can be represented as epoxies and phenolics, whereas thermoplastics can be repeatedly melted to allow other materials, such as wood biopolymers, to be blended with them. Polypropylene (PP), polyethylene (PE), and polyvinylchloride (PVC) are among the most widely thermoplastics applied in WPCs for building, marine, electronic, furniture, aero­space, construction, and automotive.10 Figure 14.5 shows the uses of wood plastic
composites in 2002.3 They are typically produced by blending a lignocellulosic — based polymer or composite (e. g., wood fibers) with the epoxies/phenolic resins (for example) to form a filler/polymer matrix and then pressing or molding it under high pressures and temperatures. A preeminent prerequisite for success in reinforcement of plastics is the availability of large quantities of the lignocellulosic-based fibers.1

Consumer

Electronic products Appliances

components

Automotives

Construction

FIGURE 14.5 A breakdown of the total wood plastic composites used in 2002.3

The future growth of WPCs, cellulose-based plastics, “plastic” lumber, and analogous natural fiber composites was approximately 2.4 MM tons in 2011 with an expectation of reaching 4.6 MM tons in 2016.11

PE, PVC, and PP are the predominant matrices used in WPCs although several types of WPCs with lignocellulosic matrices and conventional polymers have al­ready received attention and subsequent development.12 For example, composites from maple wood fibers and a bacterial polyester (poly(P-hydroxybutyrate-co-P — valerate)) have been manufactured by an extrusion-injection molding process. When the composite was reinforced with 40 wt.% of maple wood fiber, the tensile and flexural moduli of the resultant biocomposites improved by approximately 170% relative to neat bacterial polyester. Such behavior was observed to linearly depend on the regulated enrichment of the biocomposite with the wood fibers.13

The physical properties of lignocellulosic fibers are critically important to the successful design of biocomposites because their characteristics are highly depen­dent on fiber chemical and physical properties, such as the structure of fibers, cel­lulose content, angle of fibrils, cross-section, and the degree of polymerization. Additionally, well-defined mechanical properties are a general prerequisite for the successful use of composites and the fibers have to be specially prepared or modi­fied with respect to the following: 1) homogenization of the fiber’s properties; 2) degrees of separation and degumming; 3) degrees of polymerization and crystal­lization; 4) good adhesion between fiber and matrix; 5) moisture repellency; and 6) flame retardancy properties.8

The wood fibers, as any lignocellulosic fiber, can be processed in different ways to yield reinforcing elements having different mechanical properties. The fibrilla­tion of pulp fiber to obtain microfibrillated cellulose is obtained through a mechani­cal treatment of pulp fibers consisting of refining and high pressure homogenizing processes. Also, cellulose whiskers (also known as cellulose nanocrystals) can yield individual reinforcing elements of excellent physical properties. Cellulose nano­crystals have been investigated as fillers in a number of matrix systems, including siloxanes, poly(caprolactone), glycerol-plasticized starch, styrene-butyl acrylate la­tex, cellulose acetate butyrate, and epoxy resins.14

The reinforcing ability of the cellulose whiskers lies in their high surface area and good mechanical properties. However, to obtain a significant increase in mate­rial properties, the whiskers should be well separated and evenly distributed in the matrix material. 15Because amorphous regions are structural defects, short mono­crystals can be obtained under acid hydrolysis from various sources including wood, sisal, tunicin, ramie, cotton stalks, wheat straw, bacterial cellulose, etc.

The properties of the composite materials depend on the properties of their in­dividual components, but also on their morphology and interfacial characteristics. One of the drawbacks of cellulose whiskers with polar surfaces is poor dispers- ibility/compatibility with nonpolar solvents or resins. Thus, their incorporation as reinforcing materials for nanocomposites has so far been largely limited to aqueous or polar systems. To overcome this problem and broaden the type of possible poly­mer matrices, surface modification efforts have been made as will be discussed later.

Because the physical properties of lignocellulosic fibers are mainly determined by their composition such as structure of fibers, cellulose content, angle of fibrils, cross-section, degree of polymerization, it is necessary to give a little background on the wood types to clarify structural differences between the formed wood-based bio­composites. It is common to classify wood as softwood (gymnosperm) or hardwood (angiosperm), with basic differences in their anatomical features.

Angiosperm (hardwoods) trees present more complex and heterogeneous struc­tures than gymnosperms (softwoods). The dominant feature separating angiosperms from gymnosperms is the presence of vessel elements for transport functions and shorter fiber cells. The vessels may show considerable variation in size, shape of perforation plates (simple, scalariform, reticulate, foraminate), and structure of cell wall, such as spiral thickenings. According to past work, the spirally layered outer secondary wall (S1 layer) restricts the flexibility of hardwood mechanical pulp fi­bers and thus prevents access to the subjacent inner secondary wall (S2 layer).16

One of the critical parameters influencing the strength properties of wood plastic composites (WPCs) is the size of the fibers. Short and tiny fibers (average particle size 0.24-0.35 mm), typically found in hardwoods, should be preferred. They pro­vide a higher specific surface area and the fibers are distributed more homogeneous­ly compared to composites with long fibers and so the compatibility of fiber and matrix is improved. Given this, swelling decreases and breaks during processing are reduced.3 To support these arguments, wood polypropylene composites of different compositions (30, 40, and 50%) have been prepared using maleic anhydride-poly­propylene copolymer of different percentage and from the results, it was observed that the hardwood fiber-polypropylene composites, by using maleate polypropylene (MAH-PP), show comparatively better performance to softwood fiber-polypropyl­ene composites.17In another study, dissolving wood fiber pulps (Eucalyptus hard­wood and conifer softwood) were used to produce composites. Surprisingly, soft­wood fiber biocomposites showed a tensile strength (76 MPa) significantly higher than that of hardwood.18

The ability of cellulose microfibrils from BSKP (bleached softwood Kraft pulp) to act as a reinforcing agent in a matrix in PVA (polyvinyl alcohol) was demonstrat­ed by the two-fold increase in tensile and two-and-a-half-fold increase in stiffness (at 5 microfibril loading). It was further demonstrated that having a minimal aspect ratio (L/W ratio) is far more important than crystallinity in determining composite reinforcement gains when the composite was compared to MCC (microcrystalline cellulose).19

The overall nanometric effect exhibited by cellulose (i. e., amplification of spe­cific macro properties by enhanced surface area or related nanoscopic parameters) is found in its nanocrystalline form. Table 14.2 illustrates several key attributes with respect to reinforcement efficiency of nanocrystals and macrofibers.14 The dramatic enhancement in surface area, close spacing, very high stiffness and strength, and high aspect ratio allow cellulose nanocrystals to behave a high-performance rein­forcement for advanced materials.19

TABLE 14.2 Typical Properties of Cellulose Nanofibril and Softwood Kraft Pulp Fibers Property Cellulose Nanofibril Softwood Kraft Pulp Length, nm 500 1 500 000 Diameter, nm 5 30 000 Specific surface, 1/nm 0.048 Vf* 0.000008 Vf* Fiber spacing, nm 5 Vf-05 30 000 Vf-05 Aspect Ratio 100 50 Tensile strength, MPa 10 000 700 Elastic Modulus, GPa 150 20 * Vf: fiber volume fraction.

High-strength composites from softwood fibers and nanofibrillated cellulose (NFC) demonstrated increases in the tensile strength from 98 MPa to 160 MPa and the work needed to attain fracture was more than doubled with the addition of 10% NFC to wood fibers. A hierarchical structure was obtained in the composites in the form of a microscale wood fiber network and an additional NFC nanofiber network linking wood fibers and also occupying some of the microscale porosity.20