Finer fibers lead to more effective sound absorption due to the greater number of fi­bers per volume, more contact area, and more tortuous channels.61 Furthermore, the presence of finer fibers decreases the chances of pore connectivity62 and increases the flow resistivity. Additionally, finer fibers can be vibrated easily compared to coarser ones; and cause acoustic energy dissipation.61 Jayaraman et al.,61 Lee and Joo,62 and Koizumi et al.63 reported higher sound absorption with finer fibers. Koi­zumi et al.63 found substantial increase in absorption properties with incorporation of micro denier fibers. Technical fibers generally have Poisson distribution of fiber diameter which may be taken into account during modeling.18 One disadvantage with plant fibers is that they generally are coarser than petroleum-based fibers due to the fact that a technical plant fiber is comprised of a great number of elementary fibers as seen in Fig 5.6.

Usually in certain applications when air flows over the jute fibers at grazing inci­dence, at times the fibers may come out of the jute felt with the airflow. In order to prevent the fibers from separating natural rubber is sprayed on to the jute felt sur­face. However, this coating of natural rubber may close some of the pores of the jute felt and thus decreases its sound absorbing capability. Thus rubber spraying has to be done with caution. Table 6.10 shows that untreated jute felt (density 117 kg/m3) has higher NRC value (0.85) as compared to 1% natural rubber latex jute composite (density 219 kg/m3).

The screw barrel temperature is an important process parameter as it directly influ­ences the properties of the end product. The melting of the polymer matrix takes place in the screw barrel. It is divided into various heating zones from feeding zone to the nozzle. The temperature of these zones depends upon the processing tem­perature of polymer matrix used. The temperature of the heaters has to be adjusted carefully as heating of the mixture in the barrel is not only done by the heaters alone. The shearing action of the screw also provides some heat during plasticizing. The viscosity of the fiber matrix mixture reduces as the temperature is increased but at the same time higher temperatures can lead to degradation of matrix as well as natural fibers used.

Recently, development of biodegradable, ecofriendly polymer composite materi­als has been focused towards monocomponent, all-cellulose composites32. As it was previously mentioned, cellulose is one of the most abundant renewable and biodegradable biopolymer resources with high mechanical performance. In their cell walls, the spirally oriented cellulose plays the role of reinforcements in a soft hemi-cellulose and lignin matrix. All-cellulose composites were produced based on the original concept of self-reinforced composites, a composite with a matrix and reinforcement from the same polymer, which has been primary developed for ther­moplastic high-density polyethylene33. In this new type of so-called self-reinforced composites, the interfacial bonding problems are circumvented by the use of cel­lulose for both the reinforcement and the matrix. These composites have exhibited significant prospects as bio-based and biodegradable materials that have excellent mechanical properties. Because cellulose does not exhibit a melting point, it must be dissolved in order to aid in processing. Fiber surface selective dissolution of aligned cellulose fibers has been employed in the solvent by controlling the immersion time. Since the cell wall of the natural fibers is build of several layers, the surface layer of the fibers can be partially dissolved and transformed into the composite matrix phase. The remaining fiber cell cores maintain their original structure and impact a reinforcing effect to the composite. Due to the fact that both the fiber and matrix phases of this cellulose composite are from the same origin, and they are chemically identical, a strong interfacial adhesion could be expected between them33.

In this procedure, activated fibers are immersed in lithium chloride/N, N-dimeth- ylacetamide (LiCl/DMAc) for a specified immersion times. The fibers are then re­moved from the solvent and the partially dissolved fibers start to gel. Finally, this fiber-incorporated gel are coagulated in a nonsolvent system to extract the DMAc and LiCl, and then dried under vacuum. The cell wall of a cellulose fibers is consti­tuted by a number of layers; therefore, the surface layer of the fibers can be partially dissolved and transformed into the matrix phase of the composites, whereas the
undissolved part of the fibers, preserve the original structure, thus imparting the reinforcing effect to the composite.

Currently, several kinds of solvent systems have been used to dissolve cellulose, such as lithium chloride/N, N-dimethylacetamide (LiCl/DMAc), dimethyl sulfox­ide (DMSO)/tetrabutylammonium fluoride, NH3/NH4SCN, NaOH/urea, ionic liq­uids, PEG/NaOH, etc.34 All-cellulose composites have been prepared by dissolving pretreated cellulose pulp and then impregnation of the cellulose solution into the aligned fibers followed by coagulation in methanol and drying. Examples of starting materials are pulp,35 filter paper36- and long fibers37. Nishino et al. have prepared all­cellulose composites from pure cellulose and ramie fibers in LiCl/DMAc32. Duch — emin et al. have studied the effect of dissolution time and cellulose concentration on the crystallography of precipitated cellulose, using microcrystalline cellulose (MCC) as a model material38. The results of their work have contributed to a further understanding of the phase transformations that occur during the formation of all­cellulose composites by partial dissolution.

All-cellulose composites have been obtained, as an example, from aligned ra­mie fibers32. Due to the high fiber volume fraction in these composites (up to 80%), the tensile strength of these uniaxially reinforced all-cellulose composites has been found as high as 480-540 MPa. A similar approach has been also used to prepare random all-cellulose composites from filter paper36. As concerning the preparation methodology, by increasing the immersion time of cellulose fibers, larger fractions of the fiber skin are dissolved to form a matrix phase. Therefore, an improvement of interfacial adhesion has been observed by increasing the dissolution time. In the case of aligned ramie fibers, longitudinal tensile tests have shown that an immersion time of 2 h is the optimum processing condition to produce all-cellulose composites. In these conditions, it has been found that the amount of fiber surface selectively dissolved to form the matrix phase. This is adequate to provide sufficient interfacial adhesion to the composite, whereas the undissolved fiber cores retain their original structure and strength.

Lu et al.39 have published the results of their work on all-cellulose composites prepared by molding slightly benzylated sisal fibers. In contrast to plant fiber/syn — thetic polymer composites, water resistance of the current composites was greatly increased as characterized by the insignificant variation in the mechanical properties of the composites before and after being aged in water. They have found that sisal/ cyanoethylated wood sawdust and sisal/benzylated wood sawdust all-composites exhibit mechanical properties similar to those of glass fiber reinforced composites39. Physical heterogeneity in these all-plant fiber composites was favorable for the in­terfacial interaction. Biodegradability of the self-reinforced sisal composites was also followed. They found that in the case of enzymolysis aided by cellulose, the degradation rate of the composites gradually slowed down due to the hindrance of the lignin, which cannot be hydrolyzed by cellulose39.

A. Grozdanov et al. have worked on all-cellulose composites based on cotton textile fabrics, prepared by partial fiber surface dissolution in lithium chloride dis­solved in A, A-dimethylacetamide (LiCl/DMAc)40. Two different parameters have been studied: (i) the influence on type of scouring (alkaline or enzymatic) and (ii) the cotton textile preforms (knits, woven). In order to improve the interface and pro­tect against fiber degradation for the all-cellulose composites, alkaline scouring was performed by using 4% NaOH for 60 min treatment at 100°C. Enzymatic scouring was done with two conditions: alkaline pectinase-BioPrep3000 L at 55°C for 30 min and acid pectinase-NS 29048 at 45°C for 30 min2829. All-cellulose composites with ~90-95% fiber volume fraction were successfully prepared by using solutions of 3 (wt./v) cellulose concentrations in 8% (wt./v) LiCl/DMAc for impregnation of cotton textile preforms. Characterization protocols of the obtained all-cellulose composites have included FTIR, SEM, TGA/DTA, 13C-NMR, mechanical tests and Dp-determination.

It was found that a dissolution time of 24 h lead to bio-based materials with the best overall mechanical performance, since this time allowed dissolution of a suf­ficient amount of the fiber surface to obtain good interfacial bonding between fibers, while keeping a considerable amount of remaining fiber cores that provide a strong reinforcement to the composite. Characteristic mechanical curves of the studied all­cellulose composites based on various treated cotton woven fabrics are presented in Fig. 10.4. The measured data for the maximum load and deformations are presented in Table 10.7. Comparison of the mechanical performances of all-cellulose com­posites based on alkali — and enzyme-treated cotton-woven preforms has shown that the treatments can effectively improve the mechanical strength of the composites. The higher values for the mechanical strength were obtained for the all-cellulose composites based on enzyme-treated cotton-woven preforms. Han et al.34 have con­firmed the same effect. Moreover, they found that with an increase of the immersion time from 1 to 3h, the values of the mechanical strength sharply increased.

Although the biocomposites based on enzyme — and bleach-treated preforms have shown the best mechanical properties, alkali-treated cotton-woven preforms have shown higher lateral crystalline indices in the obtained composites compared to enzyme treated ones. Crystalline indices for the studied all-cellulose composites, obtained as a ratio of the FTIR bands at 1430 cm-1 (CH2 symmetric band) and 898 cm-1 (Group Clfrequency: — CH2=C-R) are presented in Table 10.8. The results obtained for the crystalline indices confirmed there were not significant changes in the crystalline structure of the cotton based composites.

The crystalline structure was studied also by 13C-NMR spectroscopy. The ob­tained results are shown in Table 10.9. The crystalline fraction Xc was calculated by deconvolution of the spectra in the 80-90 ppm region, according to the following equation:

I88 5 and I83 5 are the intensity of the peaks assigned to the crystalline and amorphous fraction, respectively.

No significant changes in cellulose structure were evidenced, as all the spectra are very similar (almost identical) (Fig. 10.5). For all of the studied samples, it is evident that the fiber core was not damaged and the fibers kept their structural and strength performance.

Characteristic SEM microphotographs of the morphology in the obtained all­cellulose composites are shown in Figs. 10.6 and 10.7. Figure 10.6 show morphol­ogy of alkali-treated cotton all-cellulose composites, while in Fig. 10.7, morphology of enzyme (alkali pectinase) treated cotton performs are shown. SEM images pro­vide direct information regarding the interfacial bonding of the studied all-cellulose composites based on alkali and enzyme treated cotton performs confirming where

good fiber-fiber adhesion was registered in both types of cotton performs. For alkali scoured performs, progressive build-up of covering thermoplastic films around the fibers were found, while for enzymatic scoured performs bonding bridges were reg­istered between two fibers.

Therefore, it can be affirmed that all-cellulose composites based on various treated cotton fiber performs show very interesting mechanical properties and rep­resent a new class of bio-derived and biodegradable composite materials with inter­esting possibilities of future development.

TERESA CRISTINA FONSECA SILVA, DEUSANILDE SILVA, and LUCIAN A. LUCIA

ABSTRACT

This century has been witnessing the increasing development of ecofriendly materi­als derived from natural fibers to reinforce composites. In this chapter, wood-based polymers (i. e., cellulose, hemicellulose and lignin) have been chosen as the chief biopolymeric templates for review because together they comprise the most abun­dant resource on the planet, viz., lignocellulosics. Moreover, although wood has long been used as a raw material for building, fuel, papermaking, and various other ap­plications, the potential of wood-based polymers to reinforce composites has shown significant progress. One of the greatest challenges to advancing this area had been the lack of economic and abundant alternatives for natural fiber-reinforced compos­ites. This issue is currently being addressed by the application of lignocellulosics. The chemical, thermal, and physical properties of the biopolymers and resultant composites are influenced by the molecular weight distribution and composition of the biopolymers with the miscibility of the individual components being of great significance and often being the limiting aspect to the optimization of the physi­cal properties of the final blends. To overcome miscibility issues between many naturally occurring polymers and associated composites, chemical modification and graft polymerization of the surfaces of such biopolymers are common approaches.

This chapter will review the overall characteristics as and mechanical properties of the reinforcing biopolymers that come from wood and are used in the final bio­composites. In addition, the methods employed for polymer modification, mainly the chemical methods, will be discussed.

14.1 INTRODUCTION

Broadly defined, biocomposites tend to be composite materials formed from natu — ral/bio-based fibers as the reinforcing elements and petroleum-derived nonbiode­gradable polymers (e. g., PP, PE) or biodegradable polymers (e. g., PLA, PHA) as the matrix material. Biocomposites from plants and related biomaterials are more ecofriendly because the traditional composite structures (reinforced with epoxy, un­saturated polyester resins, polyurethanes, or phenolics) tend to have negative im­pacts on the environment because of their nonrenewable (nonbiodegradable) nature. Biocomposites can be classified as partially or completely biodegradable as shown in Fig. 14.1.

In general, an appropriate selection of biopolymers is usually determined by the stiffness and tensile strength of the resultant composite. Other deciding factors are thermal stability, self-adhesion of fibers, and matrix, dynamic and long-term behav­ior, economics and processing costs.

Although bio-fibers confer some attractive properties to manufacturers such as flexibility during processing, high specific stiffness, and low cost, the formed composite still lacks the necessary thermal and mechanical properties desirable for engineering plastics compared to synthetic polymers. Also, an enhanced miscibility between natural fibers and matrix should provide better biocomposites. Intensive research on these new compositions and processes has triggered an increased level of development and application. For instance, the worldwide capacity of bio-based plastics is expected to increase from 0.36 million metric ton (2007) to 2.33 million metric ton by 2013 and to 3.45 million metric ton by 2020.

Over the last few years, a number of research efforts have focused on investigat­ing the future development of natural fibers as load bearing constituents in compos­ite materials. The use of such materials in composites has increased mainly due to their abundance and relative low cost, their ability to recycle, and fact that they can compete well in terms of strength per weight of material.

Table 15.8 shows the FTIR bands of composite (LDPE, wheat husk with benzophe — none) where the characteristic peaks associated different components are observed. The NH stretching was assigned to 3969 and 3500 cm1 45 and the asymmetric and symmetric stretching vibrations of NH2 grouping are observed at 5964, 3472 and 3361 cm1, respectively.67 Highly intense and well defined peaks observed at 2743 and 1750 cm1 are due to the C=O stretching vibration of carbonyl group correspond to strong absorption benzophenone group.45 The NH and CH stretching modes aris­ing from amino groups appear around 5611 and 3462 cm-1. The CH2 asymmetric stretching (wheat gluten husk) corresponds to 5411, 4876 and 4319 cm-1 and scis­soring mode at 3920 cm-1.

The bands at 4564 and 784 cm-1 are due to aromatic CH bends. The aromatic CH stretching vibrations appear weak just above 4012 cm-1. The C=C and C=O vibra­tions of aromatic ring are confirmed at 3715, 1677 and 3234 cm-1. At 3394 cm-1 was assigned at OH group of materials. The C=O stretching was attributed at 3258 and 2853 cm-1 (wheat gluten husk).5 The absorbance at 1455 and 909 cm-1were observed due to methylene groups which are proportional to the relative total concentration of carbonyl groups.8 The peaks of LDPE were observed at 2914 cm1 correspond at CH2 asymmetric stretching, the CH2 deformation were assigned at 1561 and 1439 cm1 and finally the CH3 symmetric stretching at 1341 cm-1.9

Aciniform silks are manufactured by the aciniform gland, a structure that can de­scribed in the cob-weaver as somewhat “finger-like” shaped (Fig. 1.1). These silks are used for swathing prey, building sperm webs, web decorations, and egg sacs (Fig. 1.3).1 Partial cDNA sequences coding the major protein constituent, Acini­form Spidroin 1 (AcSp1), were initially reported, and more recently, the entire ge­netic blueprint for AcSp1 was completed.51 Aciniform silks have also been shown to be constituents of egg sacs and MS/MS analysis has demonstrated the presence of AcSp1 in egg sacs and prey wrapping silk.51b Inspection of the genomic DNA reveals the AcSp1 gene consists of a single exon that exceeds the largest exons reported from humans, chimpanzees, mouse, and zebrafish, predicting a protein size of 630 kDa. Analyzes of the 16 block repeats of L. hesperus AcSpl, which each com­prise 375 amino acids, show extreme conservation relative to block repeats reported from other spidroin family members.51c This represents approximately twice the size of the 200 amino acid block repeats reported for Argiope trifasciata AcSp1.51a Al­though the mechanical data available for aciniform silks is relatively scarce, stress — strain curves collected from a handful of orb-weaver spiders support that this fiber represents one of the toughest spider silk threads.51a Purified recombinant AcSp1 proteins from A. trifasciata carrying 2, 3, or 4 block repeats (denoted W2, W3, and W4) can be induced to form fiber-like threads by shear forces in physiological buf- fer.52 Mechanical analysis of hand-pulled, synthetic fibers produced from purified recombinant protein W4 have reported breaking stress and strain values of 116 ± 24 MPa and 0.37 ± 0.11, respectively.

In the past studies, various natural fibers such as flax, jute, hemp, kenaf, sisal, abaca, pineapple leaf fiber, cotton, coir, bamboo, and wood flour have been investigated as reinforcing fibers for biocomposites. Their properties and the application to biocom­posites are described in detail in some review articles.50,52 We used wood flour (WF) made from Sanbu cedar crushed into powders through 3 mm screen mesh and mi — crofibrillated cellulose (MFC). The reason for the use of WF is as follows: Among the natural fibers, the use of waste wood generated from forest-thinning and wreck­ing of wooden building, etc. is very important for solving the severe environmental problem.53,54 Also, a massive outbreak of damaged cedar trees infected by Cerco — spora sequoiae Ellis et Everhart (“Sugi-Mizogusare” disease) which cannot be used as log and lumber is becoming a serious problem in Chiba, Japan. Fig. 4.4 shows the FE-SEM micrographs of the WF supplied from Kowa Technos, Co. Ltd. (Sammu, Chiba, Japan). The photograph at a low magnification shows that the WF particles are mainly composed of fibrous substance of ca. 0.2-1.0 mm in length. The average length and aspect ratio of the WF fibers were ca. 0.7 mm and 4.2, respectively. The photograph at a high magnification revealed that each particle is composed of fiber bunch with a rough surface.

In recent years, MFC with the diameters in the range of 10-100 nm, which is obtained through a simple mechanical process which includes refining and high pressure homogenizing, has received significant research attention as a reinforcing fibers of polymers.52,55,63 Yano et al. reported that the poly(lactic acid)/MFC bio­composites prepared by the method using acetone as a mixing solvent exhibit high strength and modulus.59 Drzal et al. reported that the mechanical properties of the composites of MFC and the bisphenol F-type epoxy resin cured with a polyether amine are improved by the silane coupling treatment of the MFC.60 It is important for the preparation of superior polymer/MFC composites to devise the dispersion method of MFC in the hydrophobic polymers and the surface treatment method of MFC. The merit of the use of bio-based epoxy resin system instead of petroleum — based epoxy resin system is that MFC can be easily dispersed in aqueous solution of GPE/TA or SPE/TA without a special surface modification of MFC. We used the MFC, trade name Celish KY-100G was supplied by Daicel Chemical Industries, Ltd. (Tokyo, Japan). This product is a 10 wt.% solid content in water suspension.

Process parameters during absorbent material formation have an important impact on sound absorption due to their effects on the characteristics of the absorbent mate­rial. Process parameters have been classified into production and treatment param­eter categories here. Different production and treatment processes used in biocom­posite sound absorber formation are given in Table 5.2.

5.3.3.1 PRODUCTION

The fiber component of biocomposites is mainly manufactured by web formation and bonding methods before combining with other components. Web formation can be classified into three categories:

• Drylaid system (carding or airlaying)

• Wetlaid system (similar to paper production from pulp)

• Polymer-laid system (spunbonding, meltblowing, etc.).79

• There are also three classes of web bonding processes:

• Chemical bonding (use of binders)

• Thermal bonding (calendaring, through-air blowing, or ultra-sonic impact)

• Mechanical bonding (needling, stitching, water-jet entangling).27

Among web forming types, Jayaraman et al.47 report higher sound absorption for fibrous structures formed by airlaying compared to carded ones irrespective of the fiber content. This finding was agreed by the findings of Parikh et al.22. This might be due to relatively random placement of fibers, and thus, higher tortuosity, higher number of pores with smaller sizes, higher number of fiber-to-fiber contact points, and gradient in porosity due to gravity.

Among web bonding methods, for needle-punching, the factors which affect noise reduction properties are given as the number of needling passes,136 and punch­ing density80.

Genis et al.80 found that the absorption coefficient reaches its maximum at mate­rial density rw = 100 kg-m-3 for thermo-bonded polypropylene, and punching den­sity P = 28 cm2 for needle-punched polypropylene and polyamide materials in the sound frequency range of 63 to 8000 Hz, for fiber diameters between 10 to 40 pm, and material thicknesses of 3 to 20 mm. They found the absorption coefficient of their needle-punched samples to have more dependency on the frequency range compared to thermo-bonded ones; thus they have a narrower absorption efficiency frequency range. The absorption in needle-punched materials was also more depen­dent on the diameter of fibers compared to thermally bonded webs. Jayaraman et al.47 did not report a significant difference between needled and needled plus ther­mally bonded fibrous structures.

Jute can withstand high temperatures and are thus suitable for use in machinery enclosures placed around engines and compressors for noise control. In machinery enclosures they can be used in the inside lining of the walls. Usually such materi­als are faced with perforated sheets, so that most of the material is exposed to the
incident sound field. They can also be used has lagging material around hot exhaust pipes and in HVAC ducts.

In noisy shop floors particularly in a punch press or a forging shop to reduce the reverberant field, usually sound absorber type panels are hung from the ceilings. Such sound absorbing panels can be made of the biocomposites like jute, coir, and cotton. The walls can be padded with biocomposite materials, curtains on factory windows and doors can be made up of such biocomposite textiles.