The thermal properties of the composite blends containing different reinforcement (rice, wood and DDGS) measured by DSC are shown in Table 12.2.

Figure 12.5 presents the thermograms of the second heating of the PP/reinforce — ment flour blends subjected to the same rate flow. Only one endothermic peak corre­sponding to PP can be observed in these figures. All the composites regardless have invariably exhibited a slightly higher Tm compared to the Tm of neat PP. Figure 12.5 shows DSC curves corresponding to the cooling scan for PP and its composites. All curves show exothermic peaks corresponding to the crystallization of the polymeric matrix. A shift of Tc towards higher temperatures in the presence of MAPP and rein­forcement was observed in the composites. This indicated that the phenomenon of crystallization during the cooling occurred more rapidly in composites containing MAPP than in the pure PP. The effect of rising crystallization rates was clear for all of the composites containing MAPP. The results imply that MAPP acted as a pre­cursor and increased crystallization. The presence of reinforcement (wood, rice or DDGS) decreases the thermal stability, heat in turn causes scissions of chains and all these phenomena generate an early fusion and it is more pronounced whit samples with reinforcement agent.

Figure 12.6 shows DSC curves corresponding to the cooling scan for PP and its composites. All curves show exothermic peaks corresponding to the crystallization of the polymeric matrix. Every bio-filler affects the thermal properties of the com­posite differently4950. The cooling characteristics have shown very interesting be­havior. The temperatures corresponding to onset of crystallization and peak crystal­lization have increased due to presence of filler reinforcement. These temperatures have further increased due to chemical treatment of rice husk and DDGS. Thus it seems that addition of reinforcement is causing early crystallization of PP. This indi­cates that reinforcement is influencing the degree of super cooling of PP. Hattotuwa et al.51 also have reported similar results. The presence of MAPP does not signifi­cantly modify the crystallization temperature but leads to an increase in the degree
of crystallinity. It is recognized that wood and MAPP act as nucleating agents52,16. The presence of these two elements generates the formation of more crystals. Tm, Tc,

ЛHm and xcor are reported in Table 12.2 for composites of PP matrix and differ­ent reinforcements of wood, rice and DDGS. An increase in T was observed when reinforcement was loaded into the polymer matrix. The addition of reinforcement had the effect of shifting Tm to higher values. This increase was accompanied by an increase of the composites’ degree of crystallinityX(%) which was corrected as Xcor (%) by taking into account there enforcement concentration53 54. These results suggest that crystallization occurred earlier with the incorporation of reinforcement, which played the role of a nucleating agent. Reinforcement provided sites for het­erogeneous nucleation; this induced crystallization of the polymeric matrix. This was ascribed to the poor thermal conductivity of reinforcement. In the composite, reinforcement acted as an insulating material, hindering the heat conductivity. As a result, the composites compounds needed more heat to melt. Similar findings were previously reported by Matuana and Kim55 for PVC based wood-plastic composites. They found that the addition of wood flour to the PVC resin caused significant in­creases in the temperature and energy at which fusion between the particles started. The delayed fusion time observed in rigid PVC/wood flour composites was attrib­uted to the poor thermal conductivity of the wood flour; this decreased the transfer of heat and shear throughout the PVC grains. These phenomena were consistent with the results of this study. For a composite, the impact strength depends on the composition and structure as well as the testing method. Adding reinforcement in all cases was shown to increase both the crystallization temperature and extent of crystallization of polymer matrix in WPC systems as compared to controls.

FIGURE 12.6

The thermal stability of the composites was investigated using DSC analysis under nitrogen atmosphere. The results have shown a spectacular improvement of thermal stability of the composites and an increase of the degree of crystallinity. Although the properties of some blends are acceptable for some applications, further improvement will be necessary, mainly by optimizing fiber-polymer.

Low-density polyethylene (LDPE) is most commonly used as plastic and has been extensively used as a backbone for radiation grafting with different monomers (Fig. 15.2). This is essentially due to its excellent chemical resistance and high impact strength. Due to its high chemical inertness against solvents, acids and bases, poly­ethylene (PE) as a matrix material became a very popular membrane after grafting with various hydrophilic monomers.

ccccccccc

HHHHHHHH

FIGURE 15.2 LDPE structure.

Cellulose is one of the most fascinating organic resources, an almost inexhaustible raw material, and a key source of sustainable materials on an industrial scale in the biosphere. Natural cellulose based materials (cotton, wood, linen, hemp, etc.) have been used by our society as engineering materials for millennia and their use contin­ues today as verified by the extent of the world wide industries in building materials, paper, textiles, etc. Generally, cellulose is a fibrous, tough, water-insoluble natural polymer that plays a vital role in maintaining the structure of plant cell walls. It was first discovered and isolated by Anselme Payen58 in 1838, and since then, numerous physical and chemical prospects of cellulose have been extensively studied. As a chemical raw material, cellulose has been used for about 150 years for wide spec­trum of products and materials in daily life. Many polymer researchers are of the opinion that polymer chemistry had its origins with the characterization of cellulose. Cellulose differs in some respects from other polysaccharides produced by plants, the molecular chain being very long and consisting of one repeating unit. Cellu­lose can be characterized as a high molecular weight homopolymer of в -1,4-linked anhydro-D-glucose units in which every unit is corkscrewed 180° with respect to its neighbors, and the repeat segment is frequently taken to be a dimer of glucose, known as cellobiose (Fig. 17.3).

Naturally, it occurs in a crystalline state. From the cell walls, cellulose is isolated in microfibrils by chemical extraction. In all forms, cellulose is a very highly crys­talline, high molecular weight polymer. Because of its infusibility and insolubility, cellulose has driven the step-by-step creation of novel types of materials. Highlights were the development of cellulose esters and cellulose ethers as well as of cellulose regenerates and the discovery of the polymeric state of molecules. The very first thermoplastic polymeric material of cellulose was manufactured by Hyatt Manu — factoring Company in 1870 to make celluloid in which they had reacted cellulose with nitric acid to form cellulose nitrate. The chemical modification of cellulose on an industrial scale led to a broad range of products based on cellulose from wood. The first example was the fabrication of regenerated cellulose filaments by spinning a solution of cellulose in a mixture of copper hydroxide and aqueous ammonia.59

Natural cellulose has earned in the materials society a tremendous level of awareness that does not emerge to be yielding. The cellulose biopolymer imprima­tur such interest not only because of their unsurpassed quintessential physical and chemical properties but also because of their inherent renewability and sustainabil­ity in addition to their abundance. They have been the subject of a wide array of re­search efforts as reinforcing agents in nanocomposites due to their availability, low cost, renewability, light weight, nanoscale dimension, unique morphology and most importantly they have low environmental, animal/human health and safety risks. Currently, the isolation, characterization, and search for applications of novel forms of cellulose, variously termed crystallites, nanocrystals, whiskers, nanofibrils, and nanofibers, is generating much activity. Novel methods for their production range that begins at the highest conceptual level and works down to the details methods in­volving enzymatic/chemical/physical methodologies for their isolation from wood and forest/agricultural residues to the bottom-up production of cellulose nanofibrils from glucose by bacteria.6061 Some fungi can secrete enzymes that catalyze oxi­dation reactions of either cellulose itself or the lower molecular weight oligomers produced from the enzymatic hydrolysis of cellulose. Of these, the peroxidases can provide hydrogen peroxide for free radical attack on the C2-C3 positions of cellulose to form ‘aldehyde’ cellulose, which is very reactive and can hydrolyze to form lower molecular weight fragments while other oxidative enzymes can oxidize glucose and related oligomers to glucuronic acids. Such isolated cellulosic materials with one dimension in the nanometer range are referred to generically as nanocelluloses.46 These nanocelluloses provide important cellulose properties—such as hydrophilic — ity, wide spectrum of chemical-modification capacity, and the formation of versatile semicrystalline fiber with very large aspect ratio which is the specific features of nanoscale materials. On the basis of their dimensions, functions, and preparation methods, which in turn depend mainly on the cellulosic source and on the process­ing conditions, nanocelluloses may be classified in three main subcategories.

During the last few decades, the environmental problems primarily caused by the excessive use of conventional petrochemical based materials such as plastics have become one of the major public concerns all around the globe. Many countries are making enormous efforts to overcome these problems by making and adopting vari­ous policies as well as management programs. With the growing concerns towards the environment, industries and researchers are also looking forward to the use of environmentally friendly materials for product development in a number of appli­cations. Among various environmentally friendly materials, renewable resources — derived polymers (e. g., bio-based polymers) and composites (e. g., natural fibers reinforced composites) are attracting a great deal of attention because of the inherent advantages of these polymers such as conservation of limited petroleum resources, biodegradability, low toxicity, easy availability, economy and the control of carbon dioxide emissions that lead to global warming.

From the view of sustainable development, the new materials associated with biorenewable sources are enormously being explored. Indeed the concerns over new materials from renewable resources, especially in the automotive and biomedical industries, have recently increased because of the economic consequences of de­pleting petroleum resources, the demands from industrialists and customers for high performance low-cost materials and environmental regulations. From biorenewable natural resources, ecofriendly materials can be obtained as native biopolymers, raw materials for monomers and bio-engineered biopolymers to name a few. Biopoly­mer based materials such as cellulose, starch, chitosan/chitin, poly (lactic acid), and poly (hydroxyalkanoate), etc. are among the most abundantly available biopolymers on the planet Earth. They are replacing the materials for many industrial applica­tions where synthetic polymers have been materials of choice, traditionally. One of the important aspects of biobased materials is that they can be designed and tailored to meet different desires. As the native biopolymers are not conventionally process — able, research efforts have been focused on the processing and meeting the require­ments of particular applications. The biobased materials are most frequently being used in the form of biocomposites. These biocomposites materials contain at least one component form the biorenewable resources that may either be the polymer matrix/ reinforcement or may contain both.

Using the recently developed techniques and technologies, biocomposites with better mechanical properties and thermal stability can be efficiently developed de­pending upon the applications. In fact the use of biocomposites may provide us a healthier environment owing to their multifaceted advantages over conventional polymers.

Keeping in mind the advantages of bio-based materials, this book focuses on the potential efficacy of different biocomposites procured from diverse natural re­sources and preparation and processing of the biocomposites to be used for a variety of applications. The book consists of 17 chapters, and each chapter gives an over­view of a particular biocomposite material, its processing, and successful utilization for selected applications. The chapters summarize recently developed research con­cerning spider silk biocomposites; biogenic hydroxyapatite based implant biocom­posites; liquid crystals and cellulose derivatives biocomposites; bio-based epoxy resins, bio-based polyphenols and lignocellulosic fibers; wood based biocompos­ites; flame retardant biocomposites; biocomposites for industrial noise control, cel­lulose based bionanocomposites, etc. Each individual chapter also focuses on the knowledge and understanding of the interfaces manifested in these biocomposites systems and optimization of different parameters for novel properties. In addition to this, the book also summarizes the recent developments made in the area of injection molding of biocomposites; chemical functionalization of natural fibers, processing of biocomposites and their applications in automotive and biomedical. A number of critical issues and suggestions for future work are discussed in a number of chapters, underscoring the roles of researchers for the efficient development of biocomposite materials through value addition to enhance their use.

As the editors of Green Biorenewable Biocomposites: From Knowledge to In­dustrial Applications, we have attempted to compile, unify, and present the emerg­ing research trends in biopolymers based biocomposites. We hope that this book will contribute to the advancement of both science and technology in this exciting area.

—Vijay Kumar Thakur, PhD

Washington State University — U. S.A.

—Michael R. Kessler, PhD, P. E.

Washington State University — U. S.A.

Titanium (Ti) is most commonly used as orthopedic implant materials or bone substi­tute materials. Ti possesses the good biocompatibility and the sufficient mechanical properties for medical applications (Table 2.6) 48. One negative property of Ti is a low abrasion resistance and minute Ti abrasion powders may cause inflammatory reac­tions 49. Scaling treatment is only method for removal of plaque and dental calculus adheres, and it is a necessary treatment process to obtain good prognosis throughout the long-term maintenance of implant 50. Therefore, for abutment division of implant, it is important to possess high abrasion resistance to keep the smoothness of the im­plant surface after scaling treatment 51. Biomaterials must be nontoxic, noncarcinogen­ic, chemically inert, stable and mechanically strong enough to withstand the repeated forces of a lifetime. From this point of view, TiC is a very stable phase in comparison to pure Ti or Ti alloys. Titanium carbide (TiC) is an useful material for biomedical instruments because it possesses a range of desirable properties. The combination of very high hardness, high melting temperature, and excellent thermal and chemical stabilities makes TiC suited to a number of commercial applications. TiC is often used in abrasives, cutting tools, grinding wheels, and coated cutting tips 52.

Ti based implants are classified as bioinert. Bioinert refers to a material that retains its structure in the body after implantation and does not include any immunologic host reaction. Bioactive materials should be used for modification of the surface that occurs upon implantation. Bioactive refers to materials that direct chemical bonds with bone or even with soft tissue of a living organism. One of most used bioactive materials is a hydroxyapatite. The major inorganic constituent of bones and teeth is calcium phosphate, whose composition is similar to that of synthetic hydroxyapatite (HAp; Ca10(PO4)6OH)2. This similarity provides HAp based materials excellent bioactivities like bone bonding capability, osteoconductivity, and biocompatibility. The major dis­advantage of HAp is its poor adhesion, poor mechanical integrity, high brittleness, degradation in acidic/basic conditions and incomplete bone growth, which restricts its application only in non load-bearing areas of the human body. The creation of nanocomposites of ceramic materials with particle size few ten nanometers can sig­nificantly improve the bioactivity of the implant and enhance the osteoblast adhesion.

5.3.1.1 FIBER TYPE

A wide variety of fibers are used in biocomposite sound absorbers, including con­ventional synthetic fibers, conventional plant fibers, exotic plant fibers, animal fi­bers, reclaimed fibers of chemical and natural origin and engineered compostable fibers like poly (lactic acid), as given in Table 5.2. The characteristics of the con­stituent fibers also have an important effect on sound absorption.

The effect of fiber type on sound absorption is hard to detect as it is often ac­companied by differences in fiber size and shape as seen in Figs. 5.5 and 5.6. Fiber type determines the relationship between the fiber size and flow resistivity.14 The following relationships have been reported:

where r0,g, r0,p, and r0,w represent flow resistivity values of glassfiber, polyester14 and wool46 webs in mks rayl/m, respectively; ц stands for viscosity of air which is 1.84×10-5 kg m1 s1, h denominates porosity, a is fiber radius in m, and p represents density of the fiber mat in kgm-3.

Fiber/material Production method Investigated parameters Measured parameters Frequency range (Hz) Thickness (mm) Fiber diam­ eter (ціп) Max. NAC Publication PP, PLA, glass — fiber, hemp Air laying, needle- punching, thermal treatment Heat treatment NAC, air flow permeability 500-5000 3.90-13.1 9-42 0.99 Yilmaz et al.8 PES, PP, cotton, wool, jute, rice straw, sawdust, jute, Needle-punching Thickness, cover plate, air gap, composition NAC 100-6300 2.53-22.6 N. S. 0.99 Seddeq et al.56 PP, PLA, glass — fiber, hemp Air laying, needle — punching Compression NAC, air flow permeability 500-5000 7.91-13.1 9-42 0.99 Yilmaz et al.9 PP, hemp Air laying, needle- punching, alkaliza­tion Alkalization NAC, air flow permeability 500-6400 10.61-12.53 32M2 0.99 Yilmaz et al.20 PP, Bamboo strips Laying stacking com­pression molding Blend ratio, thick­ness, fiber type, fiber orientation NAC, Noise Reduction Coefficient (NR) 0-3000 1.16-10.12 75 0.80 Huda et al.48 Coir fiber, wood particle debris, phenolic resin Needle-punching, resin bonding Blend ratio, needle-punching, fiber placement NAC, Noise Reduction Coefficient (NR) 125M000 N. S N. S 0.99 Yao et al.57 PU binder, pine sawdust, recycled rubber Resin bonding Blending ratio, thickness, mate­rial type, NAC 50-10,000 20-40 1-4* mm 0.92 Borlea et al.26 PP, PLA, glass — fiber, hemp Air laying, needle — punching Porosity, fiber type and size, layer sequence NAC, air flow permeability 500-6400 11.45-12.68 9-42 0.99 Yilmaz et al.58

Fiber/material Production method Investigated parameters Measured parameters Frequency range (Hz) Thickness (mm) Fiber diam­ eter (Mm) Max. NAC Publication PES, formalde­hyde, recycled PS, woodchip, furnace slag, municipal waste, power plant ash Granulation Resin bonding Material type 10-3,150 2.5-10* mm 1-2* mm 0.91 Bratu et al.51 Flax tow Grinding, washing, microwave, molding Grinding, micro — wave treating, molding, thick­ness NAC 100^1000 2-10 N. S 0.82 El Hajj et al.54 Recycled pulp, luffa fibers, yam waste Wet laying, cold pressing Blend ratio, mate­rial type NAC 500^1800 N. S N. S 0.13 Karademir et al.52 PP, Jute, PES Carding, needle — punching Material density, number of layers Sound insula­tion N/S 2.6-51 8.7 N/A Sengupta59 Jute, bamboo, banana, jute Carding, needle — punching Fiber type NAC 100-1600 49-6.4 N. S 0.20 Thilagivath21 PP, hemp, rapeseed straw, beech and flax Extrusion granulat­ing, compression molding Fiber type NAC 1000 -6500 N/S N. S 0.32 Markiewicz et al.49 PP, mechani­cally split corn husks, jute Spunbonding, mold­ing Fiber type, blend ratio, NAC 300-3000 3.2 1.3 0.42 Huda and Yang60

N/A: Not applicable, N. S: Not stated, *: granule diameter, PES: polyester, PP: polypropylene, PU: polyurethane, PS: polystyrene, PLA: poly lactic acid.

Studies related to sound absorption properties of biocomposites include struc­tures made up of different fibers. Surface properties of fibers and their cross-sections also play an important role. Accordingly, Nick et al.45 found greater absorption for cotton-polypropylene (PP) blend fibrous material for automotive applications com­pared to the flax-polypropylene and hemp-polypropylene blends. This was probably due to the inherent superior fineness of cotton fibers as compared to flax and hemp.

Jayaraman et al.47 examined the effect of kenaf fiber inclusion, which is a natural bast fiber, on the absorption of sound in fibrous absorbers. The addition of kenaf had a negative effect on the noise reduction performance compared to polyester and re­claimed polyester fibers, however, this effect is less pronounced in high frequencies. This negative effect may also be due to natural coarseness of kenaf fiber compared to synthetic fibers.

Parikh et al.22 developed composites in various weight ratios of natural and syn­thetic fibers including kenaf, jute, waste cotton, and flax with recycled polyester and off-quality polypropylene and compared to absorbers of conventional fibers, that is, 70% polyester and 30% polypropylene. They reported that each of the natural fibers contributed to noise reduction because of their absorptive properties in comparison with the conventional material. Furthermore, adding a soft cotton underpad was found to greatly enhance the sound absorption properties of the nonwoven floor coverings.

Huda et al.48 produced unconsolidated light-weight (0.312 g/cm3) composites by laying fine bamboo strips on a PP web and by a subsequent compression molding process. They reported better mechanical and noise reduction capabilities for the mentioned composites compared to jute-based composites.

Markiewicz et al.49 produced composites including PP and lignocellulosic fillers and measured their sound absorption performance in the 1000-6500 Hz frequency range. They reported the hemp filler addition allowed for significant increase in noise reduction over 3000 Hz, whereas rapeseed straw, beech and flax filler added to PP suppressed sound in the 3000-4000 Hz range.

Brencis et al.50 presented a research study with an aim to develop a sound ab­sorber from gypsum foam reinforced by fibrous hemp. They claimed that the gyp­sum, Gypsies rock, a local resource in Latvia, can have performance characteristics comparable to other state-of-the-art thermal and sound insulation materials. Addi­tionally, gypsum poses an important fire-resistance characteristic. Fragility, which is the disadvantage of the gypsum material, claimed to be avoided with the use of plant fibers, such as hemp, as a reinforcement element.

Bratu et al.51 studied composite materials including pellets from plastic bottles, sawdust, and ash from plant and sterile municipal wastes in a polymer type organic matrix in different blend ratios. The effects of the blend ratio and the type of the waste material on the sound absorption performance were investigated. They re­ported that use of sawdust and woodchips were advantageous in terms of noise reduction compared to the other recycled materials.

Karademir et al.52 prepared biocomposites through a wet laying process from recycled corrugated boards with addition of 30% yarn waste and 15% luffa fibers. They found that the addition of luffa fibers and yarn waste led to an increase in sound absorption together with an increase in air permeability at the expense of tensile strength.

Among the very few examples of biocomposites containing materials of ani­mal origin, Huda and Yang examined the sound absorption performance of ground chicken quill based PP composite and compared it with jute-based PP composites.53 They reported that the chicken quill based composites resulted in better sound ab­sorption performance in 500-2200 Hz frequency range as shown in Fig. 5.7.

ё 0.30 —

tL 0.20

0.10

0. 00

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2

Frequency (kHz)

FIGURE 5.7 Sound absorption of PP-chicken quill composites compared to PP — jute mats at different thicknesses and blend ratios (From Huda, S.; Yang, Y Composites Science and Technology, 2008.53 With Permission from Elsevier.).

The sound absorption coefficient improves with frequency since at high frequencies the wavelengths reduce and for a given sample the velocity is a maximum at the in­terface. Better sound absorption occurs when the incident velocity is at a maximum at the interface. Thus to improve the low frequency sound absorption either the thickness of the sound absorbing material can be increased or an air gap between the material and the rigid backing can be provided. Usually in many applications where low frequency absorption is to be improved an air gap is usually provided as shown in Fig. 6.8. Figure 6.9 shows the effect of air gap on the sound absorption coefficient of a 50.8 mm jute fiber with a density of 348.2 kg/m3.

The typical injection-molding machine is designed for the fabrication of thermo­plastics and the same is used for fabrication of NFCs without any major change in the machine tool. As the properties of the natural fibers used as reinforcement are different from that of synthetic fibers used in traditional PMCs, an understanding of the various process parameters of injection molding machine and their effects is necessary to achieve the desired flawless composites. In case of natural fiber reinforced composites the polymer pellets and the fibers are first dried in the hot air circulation dryer, hopper dryer, or dehumidification dryer depending upon the material predrying recommendations to remove excessive moisture. The fibers in the chopped form and matrix in pellet form are either precompounded or directly fed into the injection-molding hopper. The common precompounding device used prior to injection molding is twin-screw extruder or a melt mixer, which uniformly blends the matrix and fibers prior to injection molding resulting in uniform distribution of natural fibers in the composite. There are various process parameters to be taken care of while injection molding of NFCs. Optimization of these process parameters leads to reduction in the cycle time and as a result a reduction in the operating cost and increased productivity. The important process parameters are identified as screw barrel temperature, screw speed, injection speed, injection time, injection pressure, mold temperature and back pressure.

The interest in using natural flax fibers as reinforcement in biocomposites has in­creased dramatically, and in the same time, it also represents one of the most impor­tant uses. However, flax fibers are hygroscopic in nature; moisture absorption can result in swelling of the fibers, which may lead to microcracking of the composite and degradation of mechanical properties. This problem can be resolved by treating these fibers with suitable chemicals to decrease the hydroxyl groups; these groups may be involved in the hydrogen bonding within the cellulose molecules or by dif­ferent type of pretreatments.232425 Chemical treatments may activate these groups or can introduce new moieties that can effectively interlock with the matrix.

Kozlowski et al. modified flax fiber with plasma, boiling and bleaching for PLA/ Flax composite production. Modified flax gives composites with better mechanical properties that are more resistant to flame than unmodified ones25.

As mentioned, the type of pretreatment has a strong influence on the natural composite properties. Susheel Kalia et al.26, have reported numerous treatments of natural fibers. Mercerization, bleaching, grafting, coupling, treatment with silane, benzoylation as well as plasma treatment to fibers improve most of the usable prop­erties of natural fibers composites.

B. Wang et al. presented results of the modification of short flax fibers, which were derived from Saskat-chewan-grown flax straws use in fiber-reinforced com­posites, performed by mercerization, silane treatment, benzoylation and peroxide treatment27. SEM analysis has shown that physical microstructure changes occurred at the fiber surface. Silane treatment provided surface coating to the fibers. Benzo — ylation treatment produced a smooth fiber surface, while after dicumyl peroxide treatment the fibrillar structure of the individual ultimate fibers was observed. This may be due to the leaching out of waxes and pectic substances. Micropores, par­ticles adhering to the surface, groove like portions and protruding structures made the fiber surface very rough. Application of the coupling agents has been shown to be effective improving the surface properties of flax fibers, forming a mechanically interlocked coating on its surface. The results have shown that silane and peroxide treatment on flax fiber bundles lead to a higher tensile strength than that of the untreated fiber bundles. Comparatively lower tensile strength was observed in ben — zoylation treated fibers27.

Recently, introduction of enzymes in numerous wet processing of natural fibers, such as desizing, bio-scouring, bio-polishing, etc., are powerful methods for surface modification. Enzymes alone, or with combination with mercerization gives cottons that have less hydrophilic surface relative to traditionally treated fibers, but with increased numbers and types of functional groups.2829

A. Grozdanov et al. have worked on comparison of Flax/PLA biocomposites versus Kenaf/PLA biocomposites.3031 Natural fibers as a nonwoven preforms, flax and kenaf were kindly supplied by KEFI-Italy. Surface modification was carried out as follows: (a) dewaxing: the fibers were treated with 1:2 mixture of ethanol/ benzene for 72 h at 50 °C, followed by washing with distilled water and air drying to get defatted fibers; (b) vinyl monomer grafting: acrylonitrile (ACN) graft copoly­merization onto dewatted fibers was carried out using 0.01 M Ce4+/0.1 M HNO3 as initiator at temperature of 50°C; (c) alkali treatment: the defatted fibers were treated with 2 and 10% NaOH solution for 1 h at 30 °C; (d) acetylation; dewatted fibers were placed in 100 mL flask and covered with the appropriate amount of acetic an­hydride for 0.5 h at 20 °C, followed by Soxhlet extraction and drying. Biocompos­ites based on PLA matrix reinforced with flax and kenaf nonwoven preforms (20% wt. fiber content) have been prepared by compression molding at 170°C under the pressure of 50 bars. The flexural strength and the flexural modulus were measured in three-points bending mode using an Instron machine (model 5564), at a cross-head speed of 1 mm/min and at room temperature. The test span was 48.0 mm. For each sample 10 specimens were tested and the average values of the flexural strength and modulus were calculated. Thermogravimetrtic analysis (TGA) was performed in the range of 25 to 800°C, which had heating rate of 20 K/min (under nitrogen), using the Perkin Elmer DIAMOND system. The morphology of chemically treated and dried (12 h in vacuum) flax and kenaf fibers, as well as of their composites was examined by SEM (JEOL, model JSM-T20 (Uw = 20 kV).

Characteristic TGA data for different treated flax and kenaf nonwoven preforms are presented in Table 10.4. Comparison of the decomposition temperatures of flax and kenaf nonwoven preforms has shown that higher thermal stability (about 70°C) exhibited flax nonwoven forms probably as a result of higher crystallinity as well as higher cellulose and hemicellulose content.

Due to the chemical treatments, overall morphology of the flax and kenaf fibers has been changed. Characteristic SEM photos of alkali treated flax and kenaf fibers are shown in Fig. 10.1 (kenaf fib.) and 10.2 (flax fib.). The untreated fibers represent the bundles with relatively smooth surface (Figs. 10.1a and 10.2a), although small particles attached to the surface are also seen. The alkali treated fibers have a rough surface topography with significant defibrillation of individual fibers (Figs. 10.1b and 10.2b).

The flexural modules and flexural strength for the obtained PLA/Flax biocom­posites with various treated Flax preforms were shown in Table 10.5. It is evident that flax fibers improved the flexural modulus and flexural strength of the polymer matrix compared to neat PLA.

Mechanical properties of the obtained biocomposites reinforced with flax and kenaf nonwoven preforms have been shown in Table 10.6. Evidently, flax and kenaf nonwoven preforms increased the mechanical properties of the PLA neat polymer. Higher flexural modulus was obtained for the composites reinforced with kenaf fibers, while higher flexural strength was measured for PLA/Flax biocomposites.

Characteristic morphology in the obtained PLA/Flax biocomposites have shown that flax fibers were well covered with the polymer matrix, which resulted in good stress transfer between the flax fibers and PLA polymer matrix. SEM photographs of the studied PLA/Flax biocomposites with various treated flax performs are shown in Fig. 10.3. Surface treatments have resulted in defibrillation of the flax fibers and increased the possibility for mechanical interlocking of flax fibers and polymer ma­trix, which consequently increased the fiber/matrix interfacial strength.

Each composite has its own unique chemical properties which is attributable to the filler type and method of preparation which in turn affects the composite’s thermal properties.26,49,80,81 For example, DDGS contains a higher concentration of protein («26%) than found in most wood and lignocellulosic fiber particles («1.5-7%). The DSC thermal properties of the DDGS, PW and PINEW composites are shown in Table 13.7. Only single endothermic (melting) temperature and exothermic (crystal­lization) peaks were observed in the DSC curves for all formulations conducted. Little difference was found between the melting points (Tm) of neat HDPE (i. e., 130.4°C) and the original DDGS formulations (HDPE-25DDGS and HDPE — 25DDGS-MAPE). Chemical modification (A or AM) treatments caused DDGS for­mulations to exhibit slightly higher Tms than HDPE or the nonchemically treated — DDGS composite controls. This trend was duplicated with the PW formulations given the same chemical modification treatments. Generally, biocomposites will exhibit a slightly higher Tm compared to that the neat thermoplastic resin.49,80 The increase in Tm in the composite is probably due to the disruption of the HDPE crystal lattice network by the presence of the filler particles. This is demonstrated by the HDPE-40PINEW formulation, which contains 40% Pine wood filler and subsequently exhibits the highest Tm value (133.5°C) recorded in this study (Table 13.7). Interest­ingly, the HDPE-10STDDGS/30PINEW, which also contains 40% filler had a lower Tm value (131.8°C). Although both of these formulations contained 40% filler, de­cidedly different mechanical and physical properties were exhibited by them (Tables 13.2-13.5). Considerably more research needs to be conducted to determine how the mixing fillers of different chemical composites interact and affect their resulting thermal properties.

The addition of fillers to the HDPE results in composites with lower crystalliza­tion enthalpy (DHc) and melting enthalpy (DHm) values compared to neat HDPE (Table 13.7). It has been suggested that that wood fillers absorb more heat energy in the melting of composites which results in their lower DHm values when compared to neat thermoplastic resins.43

The degree of crystallinity (xc) varied considerably depending on the filler type employed in the composite. Other investigators have also observed a variation in the degree of crystallinity values associated with various LPC.3049 The degree of crystal­linity values for DDGS, PW and PINEW biocomposites were higher when MAPE was included in the formulation (Table 13.7). This situation may also be related to the HDPE resin employed. For example, PW blended with a different HDPE source showed distinctly lower x values than neat HDPE employed in this study.30 When the concentration of PINEW is increased to 40%, x values decreased markedly be­low that of neat HDPE. One explanation for this phenomenon for the reduction in X values is due to the amount of free volume occurring between the polymer chains capable of allowing filler to be intermixed.81 As the volume of the filler increases less resin polymer intermolecular free volume is available for dissipating the filler material.81

The thermal properties of biocomposites need to be determined because the pro­cessing temperatures (extrusion and injection molding) may often exceed 200°C. Commercial HDPE products are often made with high melt temperature resins. The thermogravimetric curves for the various composites are plotted in Fig. 13.7 and these results are summarized in Table 13.8. The degradation of neat HDPE em­ployed in this study, occurs in a single stage, beginning at 461.7°C, with a maximum decomposition peak occurring at 478.3°C. HDPE degradation was 99.7% complete at end of this stage. Similarly, the HDPE-MAPE blend mimicked these parameters, although exhibiting somewhat lower degradation and peak maximum temperatures. In contrast, there are several earlier degradation peaks for the DDGS composites. Examining the HDPE-25DDGS formulation reveals a major degradation tempera­ture (Td) for the DDGS flour occurring at ~ 242.2°C which subsequently results in maximum peak temperature at 322°C. Minor degradation peaks also occur and are the decomposition of low molecular weight components such as hemicellulose which degrades between 225 to 325°C.782 Next a larger second higher degradation peak occurs with a maximum at 321°C, which is corresponds to the decomposi­tion of cellulose which degrades in the 300 to 400°C.82 Further, a third degradation peak corresponds to lignin decomposition which is reported occurring near 420°C; however it is not readily seen in this study.82 This peak was obscured by the de­composition of the HDPE. The DDGS composite has a residual weight of 5.9% ash residue from the heterogeneous ingredients associated with the filler. Differences among the DDGS composite Td’s are due to the association of the filler material and the plastic resin. Higher Td’s and maximum peak temperatures occurred for STD — DGS composites compared to the DDGS composites; this can be attributed to the occurrence of higher levels of low-molecular-weight organic compounds in DDGS composites compared to that found in the STDDGS composites. Similarly, other investigators report that addition of extractables (clay) caused a decrease in Td val­ues to occur.12 The results presented here confirm a previous study using DDGS formulations blended with a different HDPE resin.30 The addition of the coupling agent MAPE had a somewhat complex influence on the decomposing behavior the DDGS composites. In some cases, inclusion of MAPE in the DDGS and STDDGS formulations (HDPE-25DDGS-MAPE and HDPE-25STDDGS-MAPE) resulted in occurrence of lower degradation temperatures (1st Td) compared to formulations without MAPE (HDPE-25DDGS and HDPE-25STDDGS) (Table 13.8). Chemi­cally modified DDGS formulations (HDPE-25 STDDG/A) showed considerably higher 1st peak degradation initiation temperatures (Td) and 1st degradation maxi­mum peaks compared to the untreated DDGS formulations (HDPE-25DDGS). The chemical modification treatments (A and AM) improves the thermal stability of for­mulations. This phenomenon has been reported by other investigators where the 1st decomposition temperatures of HDPE-acetylated-WF were reported to higher when compared to HDPE-WF formulations.51 Likewise, chemically treated PW formula­tions (HDPE-25STPW/A and HDPE-25STPW/AM) exhibited higher 1st peak deg­radation initiation temperatures (Td) and 1st degradation maximum peaks compared to the untreated PW formulations (HDPE-25PW).

“Mixed” filler formulations (HDPE-12.5 STDDGS/12.5PINEW) and HDPE- 12.5DDGS/12.5PINEW-MAPE) exhibited much higher 1st degradation initiation temperatures (Td) and 1st degradation maximum peaks than the nonmixed DDGS filler formulations (HDPE-25STDDGS and HDPE-25STDDGS-MAPE). This is at­tributed to the presence of PINEW in the mixture, which probably masks the pres­ence of DDGS particles in these formulations. Perhaps, mixed filler composites containing PINEW flour and DDGS could be considered more “thermally stable” than the DDGS formulations tested (Table 13.8). More study is necessary to prove this contention. However, these “mixed” fillers composites were not as stable as em­ployment of formulations containing PINEW alone. Based on the TGA analysis and since the processing temperatures did not exceed 210°C the DDGS, PW and DDGS — PINEW composites were thermally stable for the temperatures in which they were subjected to in this study.

13.4 CONCLUSIONS

The mechanical properties of two potential lignocellulosic material reinforcements, DDGS and PW, for use in commercial LPC was conducted in this study. Further, the benefit of “mixing” chemically dissimilar fillers, DDGS with PINEW, was assessed. The tensile, flexural, impact strength, environmental durability (soaking responses), and thermal properties of injection molded test specimens were measured. Compari­son of the composites to neat HDPE that was processed with the same conditions was conducted to determine the relative merits of using a filler against a control. Using DDGS subjected to solvent extraction (STDDGS) produces a composite with superior mechanical properties (HDPE-25 STDDGS) compared to the composite made with the original DDGS material (HDPE-25DDGS). Further, formulations of STDDGS with MAPE (HDPE-25STDDGS-MAPE) exhibited slightly lower tensile and flexural moduli but slightly higher ultimate stresses than similar formulations made without MAPE. The flexural properties and the tensile modulus of a solvent extracted DDGS with MAPE exceeded those of neat HDPE. The PW composites in general exhibited greater tensile and flexural properties than the DDGS com­posites made with similar formulations. Chemical modification by acetylation and malation of DDGS and PW fillers prior to compounding had mixed effects on im­proving the mechanical properties of the composites studied when compared to the untreated controls. In fact, the tensile and flexural moduli of composites containing chemically modified fillers were slightly lower than the baseline solvent treated filler with MAPE. The mixing of PINEW and STDDGS resulted in formulations (HDPE-12.5DDGS/12.5PINEW and HDPE-12.5DDGS/12.5PINEW-MAPE) with
improved mechanical properties compared to the STDDGS formulations (HDPE- 25STDDG or HDPE-25STDDGS-MAPE). All DDGS and PW composites soaked in water for 872 h exhibited weight gain, color changes, and some alteration in their mechanical properties, especially El%. The thermal stability of LPC formulations can be improved by mixing with wood filler and employing chemical modification.

13.5 ACKNOWLEDGEMENTS

The authors acknowledge Kimberly Pelphrey for technical assistance and Dr. N. Joshee for Paulownia wood material. Mention of a trade names or commercial prod­ucts in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Ag­riculture. USDA is an equal opportunity provider and employer.