Lignocellulose refers to plant dry matter, so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of bio-fuels, mainly bio-ethanol. It is composed of carbohydrate polymers (cellulose, hemicellulose), and an aromatic polymer (lignin). These carbohydrate polymers contain different sugar monomers (six and five carbon sugars) and they are tightly bound to lignin.

Lignocellulosic materials include a variety of materials such as sawdust, pop­lar trees, sugarcane bagasse, waste paper, brewer’s spent grains, switch grass, and straws, stems, stalks, leaves, husks, shells and peels from cereals like rice, wheat, corn, sorghum and barley, among others.

Lignocellulose wastes are accumulated every year in large quantities, causing environmental problems. The major constituents of lignocellulose are cellulose, hemicellulose, and lignin, polymers that are closely associated with each other con­stituting the cellular complex of the vegetal biomass. Basically, cellulose forms a skeleton, which is surrounded by hemicellulose and lignin. The chemical composi­tion of plants differs considerably and is influenced by genetic and environmental factors (Table 15.1).

On the other hand, amorphous regions within the cellulose crystalline structure have a heterogeneous composition characterized by a variety of different bonds. Ultimately, this asymmetrical arrangement, which characterizes amorphous regions, is crucial to the biodegradation of cellulose. The accessibility of cell wall polysac­charides from the plant to microbial enzymes is dictated by the degree to which they are associated with phenolic polymer.

Lignocellulose is a complex substrate and its biodegradation is not dependent on environmental conditions alone, but also the degradative capacity of the microbial population. The composition of the microbial community charged with lignocellu- lose biodegradation determines the rate and extent thereof.

NCC is the term frequently used for the cellulose nanocrystals or cellulose whiskers prepared from natural cellulose by acid hydrolysis. NCC is the enlightened crystal­line segments of elementary nanofibrils after the amorphous segments have been re­moved via the treatment with strong acids at eminent temperature. The nanocrystals formed from wood pulp are shorter and thinner than the MFC. NCCs have a high as­pect ratio (3-5 nm wide, 50-500 nm in length), are ~100% cellulose, and are highly crystalline (54-88%). Most likely, from the result of acid hydrolysis process, the end of the cellulose nanocrystals are narrowed due to which they are look like whiskers. This hierarchical structure of natural fibers, based on their elementary nanofibrilar components, leads to the unique strength and high-performance properties of differ­ent species of plants. The mechanical properties of cellulose can be characterized by its properties in both the ordered (so-called crystalline) and disordered (so-called amorphous) regions of the molecule. The chain molecules in the disordered regions contribute to the flexibility and the plasticity of the bulk material, while those in the ordered regions contribute to the elasticity of the material. As they are almost defects free, the modulus of cellulosic nanocrystals is close to the theoretical limit for cellulose. It is potentially stronger than steel and similar to Kevlar.6569

1.1.1 DIFFERENT SILK-PRODUCING GLANDS

There are at least seven different fiber types that can be spun from black widow spi­ders. Specialized abdominal silk-producing glands display distinct morphological

features and are responsible for manufacturing the different silk types.1 Each gland has evolved the ability to express different fibroins that can be spun into distinct fibers that perform specific biological tasks, including threads used for locomotion, prey wrapping, web construction, and protection of eggs (females), as well as re­production and adhesion of threads (Table 1.1).1 These glands, which are normally found in pairs in a single individual, include the major and minor ampullate, tubu — liform, flagelliform, aggregate, aciniform and pyriform. Because most laboratories across the world are focusing on elucidating the properties and characteristics of dragline silk, this review, in particular will emphasize the latest information on this fiber type and current progress made with synthetic silk fiber production using re­combinant dragline silk proteins.

In our previous works, TiC based nanocomposites have been deposited between 25 °C and 800 °C. As it has been showed, the nanocomposite film deposited at 200 °C ex­hibited the best mechanical properties (nanohardness 18 GPa and elastic modulus 205 GPa) 53,54 The film consisted of TiC crystallites separated by thin carbon matrix (Fig. 2.16a) as showed by TEM investigations. The crystallites have columnar structure with average width around 10-15 nm (Fig. 2.4.2b). The mechanical properties of films, namely hardness and elastic modulus may be compare with mechanical proper­ties of bulk Ti implants (Table 2.6). In the case of TiC film, the 18GPahardness and 205 GPa elastic modulus value was measured53,54. TiC phase was the reinforcing phase to enhance the hardness of films. Highest H/E ratio ~ 0.094 and elastic recovery ~ 0.634 indicate that the deformation of the TiC nanocomposites arises mainly from elastic deformation of the C matrix which determines the elasticity of the asperities in a tribological contact.

Selected area electron diffraction (SAED) confirmed the stable cubic TiC phase (Fig. 2.17. JCPSFWIN 32-1383). According to Balden et al.,55 the optimal content from the structure (crystallites separated by thin carbon matrix) of the doped C films is one with a low metal concentration within 1-20 at.%. EDS elemental analysis of the film composition resulted in ~62-72 at.% C and 23-26 at.% Ti composition of the films (Table 2.7).

FIGURE 2.17 SAED of TiC/A:C thin film.

The film prepared at argon-contained ~30 at% Ti and ~70 at% C according to the EDS. The Auger Electron Spectroscopy (AES) depth profiling was applied to deter­mine the in-depth compositions. This surface sensitive analysis obviously gave the concentration values in the surface close region as well, which might play a role in the interaction with the biological substances. To minimize the ion bombardment induced roughening Ar+ ions of 1 keV with angle of incidence (with respect to the surface normal) of 78° were applied, and the sample was rotated during sputtering. The mor­phology of films was studied by atomic force microscopy (AFM AIST-NT, Smart SPM 1010), in semicontact mode. AES analysis gave somewhat higher C and lower Ti concentrations, which can be accepted considering the difficulties of the C analysis56. The concentration of oxygen according to the EDS was below than the detectability limit, while AES detected oxygen in all of the 300 nm thickness of TiC film as shown in Fig. 2.18. The oxygen concentration was constant, around 2%, inside the layer, while it strongly increased at the outer surface as well as at the TiC/SiO2 interface.

The carbon concentration followed similar trend; it was constant inside the layer, while it changed at the outer and inside interface. The carbon concentration, however, decreased at the surface (Fig. 2.18b). Thus the surface of the film consists Fig. 2.18b. AES spectra of carbon in TiC film of amorphous carbon, titanium carbide and tita­nium oxide.

Different fiber shapes result in different surface areas; and different surface areas, in turn, lead to different viscous and thermal effects.14 Irregular cross section of fibers also increases the sound absorption due to increased surface area.61 Greater
fiber surface area allows greater sound absorption friction between fibers and air.64 Watanabe et al.64 and Narang et al.65 report a direct correlation between sound ab­sorption and fiber surface area. In the frequency range 1125-5000 Hz, fibers with serrated cross sections absorb more sound compared to ones with a round cross sectional area. Hur et al.66 explain that sound absorption increases with specific sur­face area of fiber with an increase of relative density and friction of the pore wall. Accordingly, Ta§can and Vaughn67 report greater sound transmission loss in fibrous structures of polyester fibers with deep grooves compared to that made from round polyester fibers.

In this regard, biocomposites are advantageous. Plant fiber component of bio­composites generally has increased surface areas caused by their inherent irregular shapes as shown in Fig 5.6 in contrast to smooth surfaces of man-made fibers. This might be the reason for the general acceptance that bio-fibers act as good noise reduction elements.

The acoustical transmission loss of an isotropic material can be estimated by Eq. (13), which characterizes the sound blocking capabilities of sound barrier materi­als. This equation states that for every doubling of the frequency the transmission loss increases by 6 dB. For a uniform density of the material, the transmission loss also increases by 6 dB. This is true below the first coincident frequency of the panel made out of such materials. However for composite materials Eq. (13) is not ap­plicable since the mass density is not uniform. It is recommended to measure the transmission loss in such case. The transmission loss (TL) of materials can be de­termined experimentally and standards exist for the same37,38. A view of the setup used to measure the TL of the biocomposite panels is shown in Fig. 6.10. The two microphone sound intensity probe is used to measure the sound intensity with and without the sample whose transmission loss is to be determined. A random noise source is placed at the bottom of the box39.

FIGURE 6.10 Transmission loss being measured.

TL = 20log10(m/) -42 [dB]

where, m is surface density in kg/m2 and f is frequency in Hz.

The coincident frequency, / for a panel can be estimated by Eq. (14). At the coincident frequency there is drop in the value of the transmission loss of the material.

where E is the modulus of elasticity of the material, h is the thickness of the panel, c is the speed of the longitudinal sound wave in the material, rs is the density of the material and v is the poison’s ratio.

The natural rubber based jute composites were prepared as per the following procedure39. Jute felt of 400 gsm specimens were dried in an oven for lhour to re­move the water content in the specimen. The jute felt were treated with 1% NaOH (alkali) solution for 1 h. This alkali treatment was used to remove the impurities in the specimens. These alkali treated jute felts were again washed by water till they became alkali free. The washed jute felts were dried in an oven at 80°C for an hour. The dried felt was then dipped in 1% (by volume) natural rubber solution for 1 h. Excess rubber latex was drained off and the rubber treated jute felts were dried in a dry room for 1 h. Jute-based natural rubber latex composite was prepared by press­ing ten pieces of natural rubber treated jute felts in a hydraulic press at 140°C with a load of 8 ton for 15 min. Similarly 2.5% natural rubber, 5% natural rubber and 10% natural rubber jute composites were prepared keeping all other parameters same. In all the sample preparations, natural rubber was used as bonding agent between the interfaces of the fibers.

The measured TL of all the samples are given in Table 6.1136. Usually a single number is preferred to represent the transmission characteristic of a barrier material known as the sound transmission class (STC)37. The STC ratings of the measured natural rubber latex treated jute composite of 5 mm thickness is shown in the last line of Table 6.9. These jute base composite panels of 5 mm thickness have compa­rable STC ratings to that of 3 mm Aluminum plate. The mass density of the panels have a strong influence on the TL. Higher the surface density higher will be the TL.

The screw speed during plasticizing of the mixture is responsible for the homogene­ity of the NFCs. Higher screw speed leads to the high shear forces leading to heat generation resulting in the reduction in viscosity of the mixture. At the same time high screw speed leads to fiber attrition (fiber breakage) and in extreme cases causes the polymer properties to degrade (due to high heat generation). A proper balance between the screw speed and the backpressure should be maintained to allow mix­ing of the fibers and matrix during the entire cooling cycle.

8.3.1.1 INJECTION SPEED

Injection speed is an important process parameter during injection molding of ther­moplastics. Injection speed is dependent upon the melt viscosity of the polymer and the fiber load. The injection speed has to be increased with the rising viscosity of the melt as it becomes difficult for the viscous mixture to flow into thin cavities. High injection speed causes high shear rates while passing through the mold, which reduces the viscosity of the melt. The same has been reported in a study where, when the fiber content of natural fiber was increased above 30% the injection speed was increased in order to overcome the increased viscosity and fill the mold cavi­ties completely.6Injection speed should be faster in case of precision parts having thin cross-section or in case of multi cavity mold. Injection speed is kept compara­tively slower for thick parts. Injection molding machines now come equipped with programmable injection speed control, which can vary the injection speed during injection process.

Today, natural fiber reinforced biocomposites are mainly applied in the automotive industry. From 1996 to 2000 year, the use of natural fibers in the European automo­tive sector has increased from 5000 to 28,000 tons, respectively. It is evident that flax, hemp and kenaf fibers are among the most applied types of natural fibers. According to automotive industry reports, about 5 to 10 kg of natural fibers are incorporated into every European car5. This figure includes flax fibers, hemp, jute, sisal and kenaf, which all are used in composite production41. The use of flax was reported by the suppliers to be circa 1.6 k ton in 1999, and is expected to rise to 15 to 20 k ton in the near future. The German and Austrian car industry alone employed

8.5 to 9 k ton of flax fibers in 200142. The introduction of every new car model in­creases the demand, depending on the model, by 0.5 to 3 k ton per year.

The automotive industry gives a long list of presumed benefits of natural fiber composites, which includes the general reasons for the application of natural fibers as discussed briefly in the previous sections:

• Low density, which may lead to a weight reduction of 10 to 30%.

• Acceptable mechanical properties, good acoustic properties.

• Favorable processing properties, for instance low wear on tools.

• Options for new production technologies and materials.

• Favorable accident performance, high stability, less splintering.

• Favorable ecobalance for part production.

• Favorable ecobalance during vehicle operation due to weight savings.

• Occupational health benefits compared to glass fibers during production.

• No off-gassing of toxic compounds (in contrast to phenol resin bonded wood and recycled cotton fiber parts).

• Relatively easy recycling (it is not clear whether they mean thermal recycling here).

• Price advantages both for the fibers and the applied technologies.

Obviously the production and application of natural fiber reinforced parts also

brings along some difficulties:

• For the production of nonwovens: presence of shives, dust, very short fibers.

• Uneven length distribution and uneven decortications of the fibers (especially for nonwovens).

• Irreproducible fiber quality combined with availability.

• Variations in nonwoven quality and uniformity due to fiber quality variation.

• Moisture sensitivity, both during processing and during application.

• Limited heat resistance of the fibers.

• Specific smell of the parts.

• Limited fire retardancy.

• Variations in quality and uniformity of produced parts.

• Possible molding and rotting.

TABLE 10.10 Application of Natural Fibers in Automotive Parts

Manufacturer Model Application (dependent on model)

Audi A3, A4, A4 Avant, A6, A8, Roadster, Coupe

Seat back, side and back door panel, boot lining, hat rack, spare tire lining

BMW 3, 5 and 7 Series and others

Door panels, headliner panel, boot lining, seat back

Daimler/ A-Series, C-Series, E-Series, S-Series

Chrysler Door panels, windshields/dashboard, business table, pillar cover panel

Fiat Punto, Brava, Marea, Alfa Romeo 146, 156

Ford Mondeo CD 162, Focus

Door panels, B-pillar, boot liner

Opel Astra, Vectra, Zafira

Headliner panel, door panels, pillar cover panel, instrument panel

Peugeot New model 406

Renault Clio

Rover 2000 and others

Insulation, rear storage shelf/panel

Saab Door panels

SEAT Door panels, seat back

Volkswagen Golf A4, Passat Variant, Bora

Door panel, seat back, boot lid finish panel, boot liner

Volvo C70, V70

The higher volume fraction of lower density natural fibers in natural fiber com­posites also reduces the weight of the final component. Joshi et al. have reported that natural fiber composite components based on hemp fibers applied in Audi-A3 car resulted in 20-30% reduction in weight43. In fact, natural fiber composites are be­coming popular in automotive applications because of this weight reduction. Lower weight components improve fuel efficiency and in turn significantly lower emis­sions during the use phase of the component life cycle. It was estimated that the coefficient for reduction in fuel consumption on gasoline powered vehicles ranges from 0.34 to 0.48 L/(100 kg x 100 km) in the New European Driving Cycle, while the saving on diesel vehicles ranges from 0.29 to 0.33 L/(100 kg x 100 km). In other words, over the lifetime travel of 175,000 km an automobile, a kilogram of weight reduction can result in fuel savings of 5.95-8.4 L of gasoline or 5.1-5.8 L of diesel, and corresponding avoided emissions from production and burning of these fuels.

Mueller and Krobjilowski have studied application of nonwoven composite fabrics in automotive interior components. They have compared carded and nee­dle-punched nonwoven fabrics of 750 g/m2 produced from 50/50 flax/PP, hemp/PP and kenaf/PP5. They concluded that fine fibers improve the mechanical properties on natural composites. For that purpose cotton fiber composites were prepared as acoustic materials and compared with synthetic based composites. The type of res­ins slightly influence on the acoustic properties, while composite thickness depends from the type of synthetic composite should be replaced.

Besides the automobile industry, a growing number of the nonautomotive ap­plications and products are being presented for natural fiber biocomposites. Some of these applications are in the field of energy and impact absorption, such as floor cov­erings that use excellent acoustic properties of the natural fibers, bicycle helmets, security helmets for the construction area, and monitor housings for the computers5.

The construction sector and infrastructure are the second large sector with huge potential for increased applications. This application field is particularly interesting for rural and agri-cultural societies as well as for the countries where natural fiber production is very high.

Other uses of natural fiber based composites are for various furniture elements such as, deck surface boards, and picnic tables42.

10.4 CONCLUSIONS

It is very clear and evident that natural fiber reinforced biocomposites offer a huge potential for future applications not just in the automotive industry, but also in other sectors such as construction, infrastructure and furniture production. The main chal­lenges related to the lower moisture absorption, higher fire resistance, better me­chanical properties, durability, variability, and manufacturing/processing of natural fiber reinforced biocomposites are being addressed by many recent research efforts. Moisture absorption can be reduced through surface modifications of fibers and/or by special coatings. Fire resistance can be improved by the use of in tumescent coat­ings, which eventually may also be made from renewable resources. Mechanical properties and durability are the main areas of research into natural fiber reinforced biocomposites, and many proposed solutions have been found to improve the fiber/ matrix interface. Fiber variability is itself largely uncontrollable, but the develop­ment of quality assurance protocols and diversification of fiber growing sources can address the issue before the fibers reach composite manufacturers. Natural fi­ber reinforced biocomposites have been successfully adapted to nearly every major manufacturing process currently used with synthetic composites, usually with few or no modifications to the processes themselves.

New types of all-cellulose composites were successfully prepared by a surface selective or partial dissolution method of cotton woven textile fabrics. Two different media were used for the fiber surface treatment: i) alkaline scouring with bleaching and ii) enzymatic scouring with acid and alkali pectinases combined with bleaching.

Therefore, future research in the field of natural fiber reinforced biocomposites for infrastructure applications would be most beneficial if directed at one of the highlighted challenging areas, particularly focused on continuing to improve me­chanical properties, moisture resistance, and durability.

KEYWORDS

Biocomposites

Cellulose

Mechanical Properties Natural Fibers PLA

Polymer Matrix Renewable Resources

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.

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

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

PRATHEEP K. ANNAMALAI and DILIP DEPAN

ABSTRACT

Biobased nanocomposites have gained a huge attention from industrialists and aca­demic researchers. Chitosan is Ж-deacetylated derivative of the most abundant nitro­gen-rich polysaccharide in nature chitin. Chitosan is nontoxic, biodegradable, biocom­patible and antibacterial and biologically renewable. In recent decades, by combining the benefits of chitosan and reinforcement with various nanoparticles, a wide range of materials is developed for packaging, agricultural, medical and automotive applica­tions. In this chapter, we discuss the recent developments on chitosan-based nano­composites using the biologically renewable nanofillers called ‘nano-cellulose.’ Na­no-cellulose is obtained either as shorter nanocrystals or longer nanofibrils from the most abundant natural polymer cellulose, via acid hydrolysis, enzymatic treatment or mechanical shearing methods. Their reinforcement potential in the different matrices is attributed to the mechanical properties of individual nanofibers and the formation of a percolation network that connects the well dispersed cellulose nanocrystals by hy­drogen bonds and provides superior performance like mechanical, barrier, controlled drug release, and antibiotic properties. Here we review the recent trends and develop­ments on chitosan-cellulose nanocomposites including processing, properties required for packaging and biomedical engineering applications.

16.1 INTRODUCTION