Natural fibers have accompanied human society since the start of our life. In early history, humans collected the fiber from the plant for rope and textile. Natural fibers are also used as a paper sheets, fish nets and old rags. According to Mohanty et al.,46 natural fibers are derived from plants and animals as shown in Fig. 9.9. Production of materials from renewable resources has risen, due to the increasing awareness that nonrenewable resources are becoming scarce. This century can be considered as the cellulosic century because numerous renewable plant resources for products are being discovered. The increasing attention to natural fibers is primarily due to their economical production with few requirements for equipment and low specific weight. Such attributes of natural fibers result in higher specific strength and stiff­ness as compared to glass reinforced composites.11 Natural fibers also provide safer handling and working conditions. They are nonabrasive to mixing and molding equipment, which can help in cost reductions.11 The most significant virtue of natu­ral fibers is their positive environmental impact. They are carbon dioxide neutral, meaning they do not emit excess carbon dioxide into the atmosphere as they are composted or combusted. In this way, natural fibers contribute to the mitigation of global warming. Working with natural fibers reduces dermal and respiratory irrita­tion owing to their friendly processing atmosphere with better working conditions.11 The abundant availability of natural fibers provides an added advantage over the use of synthetic fibers.

Among the natural fibers, plant fibers are the main sources of fibers that can be found in a large quantity. Figure 9.10 shows the two main categories of natural fibers from plants. The nonwood natural fibers are classified into bast fibers, leaf fibers, seed fibers, stem fibers, grass and straw fibers.1647 Natural fibers from plant resources can also be categories as primary and secondary depending on their uti­lization. Primary plants are those that are grown solely for their fiber content (i. e., jute, hemp, kenaf, and sisal), whereas secondary plants are the ones in which the fiber are produced as a by-product (i. e., oil palm, pineapple, date palm and coir).

The most widely used natural fibers among the list showed in Table 9.1; flax, jute, hemp, sisal, ramie, and kenaf fibers were extensively researched and used in numerous applications.16 Nowadays, sugar palm and oil palm fibers which are abun­dantly available in tropical regions are gaining more interest and significance in research due to their specific properties.

One of the key parameters of the plant fibers is to understand their chemical composition, as the chemical compositions of natural fibers have strong relationship to its performance for application in composite materials. Plant fiber also referred as lignocellulosics, consist mainly of cellulose, hemicelluloses and lignin. It also consists of minor amounts of free sugars, starch, proteins and other organic com — pounds.48 Cellulose, hemi-cellulose, and lignin are the three main constituents of any plant fibers and the proportion of these components in a fiber depends on the age,

source of the fibers and the extraction conditions used to obtain the fibers.49 Table

9.1

shows the chemical composition of different types of fibers.

Natural fibers are multicellular in nature, consisting of a continuous numbers, mostly in a form of cylindrical honeycomb cells which have different sizes, shapes, and arrangements for different types of fibers.50 Different types of fibers provide dif­ferent properties as shown in Table 9.2.

The rice husk and DDGS were soaked in a 0.5 NaOH solution at room temperature maintaining a ratio of (500 mL alkali solution/50 g reinforcing filler) reinforcement was kept immersed in the alkali solution for 2 h. The fibers were then washed sever­al times with distilled water to remove any NaOH on the fiber surfaces, neutralized with dilute acetic acid, and again washed with distilled water. Below, in Fig. 12.3 are exposed photos of different reinforcements used in our study. Alkali treatment is one of the well-known processes to increase mechanical properties. The process alters the chemical content in crude fiber by removing lignin, pectin, hemi-cellulose, and changing the state of the materials from hydrophilic to hydrophobic. The large amount of hemi-cellulose lost made the fibers lose their cementing capacity and caused them to separate out from each other, making them finer42,43,44.

Although the use of materials from biological sources is very promising as a re­inforcing material in polymer matrices, some challenges must be overcome con­sidering their dispersity and hydrophobicity. Therefore, the suspension stability of nanocrystalline and nanofibrillated celluloses in water and in other organic solvents is an important aspect for the composite preparation considering a large amount of apolar solvent available.

Nanofibrillated cellulose (NFC) refers to cellulose fibers that have been fibril — lated to achieve agglomerates of cellulose microfibril units. Those materials have nanoscale (less than 100 nm) diameter and typical length of several micrometers.68 The interest in nanofibrillated cellulose (NFC) has increased notably over recent de­cades mainly because its high mechanical reinforcement ability or barrier property in bionanocomposites or in paper applications, respectively. For the first applica­tion, the possibilities to interact with different polymer in matrices can be increased if different functions and/or their contents can be added in its surface. For Missoum and coauthors,68 the two main nanofibrillated cellulose drawbacks that are associ­ated with its mechanical properties are the high number of hydroxyl groups and the high hydrophilicity, which limits its uses for several applications. For both cases, the surface modification is recommended in order to reduce the number of hydroxyl groups and to increase their compatibility with hydrophobic polymer in matrices. Missoum and coauthors made a complete and recently review of nanofibrillated cellulose with focus on surface modification such as physical adsorption, molecular grafting or polymer grafting.

According to a recent review,68 the surface characteristics of the NFC depend on the raw material, the process of pretreating the material, technology of production of NFC and surface modification technique itself. Thus, to obtain NFC features and predefined hydrophobicity and dispersibility, it is essential that the raw materi­als and production steps such as pretreatment, mechanical treatment and surface modification, are previously specified. In summary, wood pulp bleached Kraft or sulfite are most often used as a starting material for the production of NFC but also nonwood fibers have been reported. About the devices, it is common among then the use of high pressure and strong mechanical shearing to fibrillate the fibers

Dhar and coauthors,69 changed nanocrystalline cellulose (NCC) surface from negative to positive by using surfactant, tetradecyl trimethyl ammonium bromide (TTAB). They observed that the addition of electrolyte or high amount of the surfac­tant the degree of phase separation in NCC suspension was reduced and the suspen­sion became more stable. Cationically modified NCC was also studied by Zaman et al.70 NCC, obtained from sulfuric acid hydrolysis of wood cellulose fibers, was ren­dered cationic by grafting with glycidyltrimethyl ammonium chloride (GTMAC). They found that the cationic surface charge density of NCC can be increased by controlling the water content of the reaction system. The optimum water content was found to be 36 wt.% for aqueous based media and 0.5 water to DMSO volume ratio for aqueous-organic solvent reaction media. As Dhar et al., Zaman and coau­thors also found that the cationically modified NCC was well dispersed and stable in aqueous media due to enhanced cationic surface charge density.

Lu, Askeled and Drzal71 studied the effect of surface modification of microfibril — lated cellulose (MCC) in the mechanical properties of composites with epoxy resin matrix. Three different coupling agents were employed to modify a sample of Kraft pulp microfibrillated cellulose from a mix of wood: 3-aminopropyltriethoxy si­lane, 3-glycidoxypropyltrimethoxysilane, and a titanate. The surface modification changed the character of microfibrillated cellulose from hydrophilic to hydropho­bic, maintaining the crystallinity of the material. Among the coupling agents, the titanate showed the most hydrophobic surface. Both treated and untreated materials were easily incorporated into the resin by using acetone as solvent. Better and stron­ger adhesion between the microfibrils and the epoxy polymer matrix was observed for the treated fibers, which resulted in better mechanical properties of the compos­ite materials.

In Zaman et al. study,72 hydrophilic surface finishing agent (glycidyl tri-methyl ammonium chloride) that contains nanocrystalline cellulose (NCC) was used to modify the quality characteristic of the polyethylene terephthalate (PET) fabric, coating durability, moisture regain, and wettability. The results showed that the surface properties of the fabric changed from hydrophobic to hydrophilic after the treatment, and the cationic NCC-containing textile surface finish showed superi­or adhesion onto the cationic dye able (anionic) PET surface over the unmodified NCC. Furthermore, the cationic textile surface finish was capable of withstanding multiple washing cycles.

Non-modified and modified (grafting of n-octadecyl isocyanate) sulfated nano­crystalline cellulose from Luffa cylindrical fibers were used to verify the effect of both NCCs on the glass transition temperature, melting point and degree of crys­tallinity of polycaprolactone (PCL) matrixes.73 The nanoparticles showed an aver­age length and diameter around 242 and 5.2 nm, respectively, with an aspect ratio around 46. The degree of crystallinity was further increased when using modified nanoparticles. Mechanical tests showed an increase of the modulus of the nanocom­posites upon addition of L. cylindrica nanocrystals. This effect was more marked for modified nanoparticles and probably partly due to the increased crystallinity of the PCL matrix. Moreover, chemical grafting promotes the more homogeneous disper­sion of nanocrystals within the PCL as shown by the significant improvement of the elongation at break compared to unmodified nanoparticles.

KEYWORDS

Bimaterials

Chemical Modification

Lignocellulosics

Polymer

Thermal, and Physical Properties Wood

MURSHID IMAN and TARUN K. MAJI

ABSTRACT

Modern scenario about the conservation of natural resources and recycling has led to the renewed interest concerning biomaterials with the focus on renewable raw materials. The use and removal of traditional composite materials usually made of glass, carbon, aramid fibers being reinforced with unsaturated polyester, epoxy or phenolics are considered crucially due to increasing global awareness and de­mands of legislative authorities. Therefore, the growing awareness of the pressing need for greener and more sustainable technologies has focused attention on use of bio-based polymers instead of conventional petroleum based polymers to fabricate biodegradable materials with high performance. Another aspect, which is receiving more attention, is the use of alternate resource prior to the use of the conventional materials. The important aspect of composite materials is that they can be designed and tailored to meet different desires. Natural fibers such as hemp, ramie, jute, etc. are cheap, biodegradable and most importantly easily available worldwide. The bio composites prepared by using natural fibers and varieties of natural polymers such as soy flour, starch, gluten, poly(lactic acid), etc. have evoked considerable inter­est in recent years due to their ecofriendly nature. These natural polymers have some negative aspects also. Thus modification by cross-linking, grafting, blending and inclusion of nanotechnology provide desired properties and widen the spectrum of applications of bio composites. Biocomposites offer modern world an alterna­tive solution to waste-disposal problems associated with conventional petroleum based plastics. Therefore, the development of commercially viable “green products” based on natural fibers and polymers for a wide range of application is on the rise. Moreover, using nanotechnology for the synthesis of biocomposites provide better mechanical properties and thermal stability. In short, the use of bionanocomposites may provide us a healthier environment owing to its multifaceted advantages over conventional polymers. This chapter discusses on the potential efficacy of natural

polymers and its various derivatives for preparation of biocomposites to be used for varieties of applications.

17.1 INTRODUCTION

With the advancement of science and technology, the people of the current civilized world are becoming more dependent on the advanced materials. In this regard, the chemist from all over the world has contributed a lot for the modernization of our society. One of such major gift-that chemist has ever bestowed to the human society is the “polymer” or “polymeric materials” without which the world have been in a totally different situations. However, as the environmental and health effects of a chemical or chemical process have begun to be considered, therefore, there has been an expanding search for new materials with high performance at affordable costs in recent years. With growing environmental and health awareness, there has been a significant focus within the scientific, industrial, and environmental communities on the use of ecofriendly materials, with terms such as “renewable,” “recyclable,” “sus­tainable,” and “triggered bio-degradable” becoming buzzwords. The development or selection of a material to meet the desired structural and design requirements calls for a compromise between conflicting objectives. This can be overcome by resorting to multiobjective optimization in material design and selection. Composite materials, which are prepared using natural reinforcements and a variety of renew­able matrix, are included in this chapter.

Since, most of the renewable materials are associated with bio-logical and plant based products as a source of raw materials, particularly to plastic industries, and these could generate a non-food source of economic development for farming and rural areas in developing country. The development of such materials has not only been a great motivating factor for materials scientists, but also an important provider of opportunities to improve the living standard of people around the world. This can also provide a potential for economic improvement based on these materials even though major thrust for their use has been driven by the needs in industrialized coun­tries. For example, natural fibers such as jute, sisal, hemp, pineapple, etc., whose ex­traction is an important process that determines the properties of fibers, can generate rural jobs since those fibers have established their potential as reinforcing fillers in many polymers, and products based on these have found increasing use on a com­mercial scale in recent years.131 Another example for the generation ofjobs by agro­based materials is provided by the use of rice husk, which constitutes more than 10% of a world rice production. These examples underline not only the development of new materials, but also the possible generation of additional employment through the collection, transportation and development of new materials. It is reported that increasing use of renewable materials would create or secure employment in rural areas, the distribution of which would be agriculture, forestry, industry, etc.

The use of natural polymers was superseded in the twentieth century as a wide- range of synthetic polymers was developed based on raw materials from low cost petroleum. However, since 1990s, there is a simultaneous and growing interest in developing bio-based products and innovative process technologies that can reduce the dependence of fossil fuel and move to a sustainable materials basis. The main reasons for development of such material are stated below:32

1. Growing interest in reducing the environmental impact of polymers or com­posites due to increased awareness to ecofriendliness;

2. Finite petroleum resources, decreasing pressures for the dependence of pe­troleum products with increasing interest in maximizing the use of renew­able materials; and

3. The availability of improved data on the properties and morphologies of natural materials such as lignocellulosic fibers, through modern instruments at different levels, and hence better understanding of their structure-proper­ty correlations.

These factors have greatly increased the understanding and development of new materials such as biocomposites.

Commodity polymer-based composite materials are now well established all over the world. Because of their high specific strength, modulus and long durabil­ity compared to conventional materials such as metals and alloys, these materials have found wide applications. However, the use of large volumes of polymer-based synthetic fiber composites in different sectors has led to disposal problems. There­fore, scientists have been looking for the reduction of such environmentally abusive materials, and triggering greater efforts to find materials based on natural resources in view of the letter’s ecofriendly attributes. Such natural resources are organic in nature and also a source for carbon and a host of other useful materials and chemi­cals, particularly for the production of “green” materials.1’56’22’33

In parallel, researchers have focused their works on the processing of nano­composites (materials with nanosized reinforcement) to enhance mechanical prop­erties. Similar to traditional microcomposites, nanocomposites use a matrix where the nanosized reinforcement elements are dispersed. The reinforcement is currently considered as a nanoparticle when at least one of its dimensions is lower than 100 nm. This particular feature provides nanocomposites unique and outstanding prop­erties never found in conventional composites. Bio-based nanocomposites are the next generation of materials for the future.

2.3.1.1 ELECTROSPINNING

During electrospinning process a high voltage (5-30 kV) is applied between a nee­dle attached to a syringe and a target. The out flowing solution gets charged which takes it to the target. During the flight the solvent evaporates and polymer fibers develop. The fibers on the target are disordered, their diameter can reach 100 nano­meter to few millimeters. The fibers can evolve thicker than the average diameter forming concentrated beads, which significantly decreases the specific surface area. The fiber diameter and the number of beads are the main characteristics of the final samples. In the electrospinning process there are given parameters which cannot be modified: humidity, temperature, pressure, atmosphere, molecular weight of the polymer. Other parameters such as viscosity, surface tension, conductivity, dielec­tric constant may be modified by selecting the appropriate polymer. The others pa­rameters may be modified directly: flow rate, voltage, diameter of the needle and the distance between the needle and the target 46.

Sound absorption coefficient of absorbing materials may be measured according to standard test methods ASTM E 1050-08, ASTM C384-04 (2011) and ASTM C 423-09a.30,32 Normal-incidence sound absorption coefficient (NAC) may be mea­sured according to ASTM E 1050-08, the Standard Test Method for Impedance and Absorption of Acoustical Materials Using A Tube, Two Microphones and A Digital Frequency Analysis, or ASTM C384-04(2011) Standard Test Method for Impedance and Absorption of Acoustical Materials by Impedance Tube Method. Random-inci­dence sound absorption coefficient may be measured according to ASTM C 423-09a Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method.

ASTM C 423-09a includes the size and construction of the sound absorber and a reverberation room. However, the test may be costly and time consuming for an early performance estimation of noise reduction capability of composites. Further­more, it requires the test sample surface to be in massive dimensions as the mini­mum required area of the porous absorber specimen is 5.57 m2.33 While samples in small sizes are sufficient for ASTM C384-04, the testing standard requires sound absorption for each frequency to be measured separately which may take a very long time. This makes the test method ASTM E 1050-08 very feasible, taking into consideration that all data for numerous frequency points are measured simultane­ously, and small dimensions of specimens are used: a specimen diameter of 100 mm is needed for a frequency range 50-1600 Hz, whereas a diameter of 29 mm is used for 500-6400 Hz range, similar to samples shown in Fig. 5.1. Due to its practical­ity, ASTM E 1050-08 test method will be briefly described here. In order to obtain knowledge on ASTM C384-04 and ASTM C423-09a, one may refer to the men­tioned standards’ specifications.

Normal-incidence sound absorption coefficient (NAC), an, can range from 0 (no absorption) to 1 (total absorption). The formulation defines an as follows;

a.=і-И2

where ft is reflection coefficient.3 It is clear from Eq. (1) that the amount of sound absorption decreases when the reflection ratio increases. The reflection coefficient can be given as follows,

ft = . (2)

z1 + Z 0

In the expression above, z1 represents the acoustical surface impedance of the po­rous material and Z0 is the acoustical impedance of free air.3 By merging Eqs. (1) and (2), the following equation can be obtained:

Here, in the case of measurement in the impedance tube, where an is measured with the material backed by a hard wall as shown in Fig 5.2, in accordance with standard ASTM E 1050-08, the acoustical surface impedance of the porous material, z1, takes the following value:

z1 = Z0 coth (kl), (4)

where k is the wave number and l is the thickness of the material. The Eq. (4) is true, provided that the hard wall backing material has a surface impedance of z2=®. As understood from Eq. (2), the more the difference between the impedance of free air and the surface impedance of the porous material, the greater becomes the reflection coefficient.3 The schematic diagram of the impedance tube testing system is shown in Fig. 5.3 and the measurement system is shown in Fig. 5.4.

A practical tool to express noise reduction capacities of sound absorbers is the noise reduction coefficient (NRC). NRC of a material is the average of the sound absorption coefficient values at 250, 500, 1000 and 2000 Hz frequencies.32

As shown in the aforementioned expressions, acoustic impedance, z, is a very important material parameter in terms of the noise reduction performance of porous materials. Acoustical impedance is the ratio between the sound pressure, p, and the particle vibration velocity, ux, as presented by Eq. (5),10

Acoustical impedance determines a porous material’s sound absorption capabil­ity as shown in Eqs. (2) and (3). The acoustical impedance includes two compo­nents, resistance and reactance as shown in Eq. (6),

z = r + X,

where z stands for the impedance, r represents the resistance, which is a real quan­tity, and x denominates the reactance, which is an imaginary quantity.35

Among the physical parameters, flow resistance is the most critical factor de­termining the sound absorptive properties of porous materials. A good number of researchers have used air flow resistivity to model sound absorption.18,20,28,36 Even though different authors may use different denomination, terms as used in ASTM C522-03 Standard Test Method for Airflow Resistance of Acoustical Materials are adopted here and defined below accordingly.37

Flow resistance, R, in mks acoustic ohms (Pa-s-m-3), is the pressure drop across a specimen divided by the volume velocity of airflow through the specimen. Specific flow resistance, r, in mks rayls (Pa-s-m1), is the product of the flow resistance of a specimen and its area. It is equivalent to the pressure difference across the specimen divided by the linear velocity of flow measured outside the specimen. Flow resistiv­ity, r0, in mks rayl/m (Pa-s-m~2), of a homogeneous material, is the quotient of its specific flow resistance divided by its thickness. The flow resistance, R, the specific flow resistance, r, and the flow resistivity, r0 of porous materials can be given as Eqs. (7)-(9),3’37

ity (denoted by a in Delany and Bazley28, S in Mechel18) S is the area, in m2, l is the thickness, in m, of the porous material, and u is the volumetric velocity of the fluid in m3/s. Even though all these terms are concerned with steady flow and are not val­id for sound with frequencies above a few hundred Hz for some sound absorbers,13 they have been adopted by the majority of the researchers for the sake of simplicity.

The specific flow resistance is linearly related to the material thickness provided that the material is uniform. Thus, if the specific flow resistance is divided by the thickness, it will give the flow resistivity, in mks rayl/m, which is characteristic of the material independent of the thickness.29

Some part of the sound energy incident on a noise control material gets reflected, some get absorbed and the rest get transmitted, as shown in Fig. 6.4. This phenom­enon of acoustical interaction of the sound energy with the material is very strongly dependent on the physical structure and physical properties of the material. The sound absorption coefficient is defined as the amount of energy absorbed by the noise control material to the energy incident on the material. The materials which have high sound absorbing coefficients are usually used as sound absorbers. A good barrier material “blocks” the sound and reflects it back to the incident medium. A good sound absorbing material has a poor reflecting capability. The sound transmis­sion coefficient of an acoustical panel is the fraction of sound power in the incident airborne that appear in the transmitted airborne wave on the opposite or rear side of the acoustical panel. These coefficients are a function of the frequency of the sound wave, and thus need to be known for the noise control materials in the frequency or a frequency band at which noise has to be controlled15,16. Traditional sound absorbers are open cell porous polyurethane foam, fiber glass and naturally occurring materi­als like coir, cotton, hemp and jute. Traditionally used sound barrier material for industrial noise control are heavy concrete, steel, lead and so forth.

The mechanism of sound absorption in a material is due to the viscous loss be­tween the sound pressure waves while interacting with the walls of the pores in the material. The sound energy is thus dissipated as heat at the pores. At low frequen­cies this energy exchange is isothermal and at high frequencies it is adiabatic17. The sound absorption coefficient of a material depends upon its acoustical impedance at the surface. Several experimental techniques exist for the measurement of the same18,19. The fiber size, porosity, tortuosity, flow resistivity affect the impedance of the material and some of these values for the jute derivatives are measured by experiments and some of them can be estimated from analytical formulations. Many a times during numerical simulations of computer aided engineering models using techniques like the finite element method and the boundary element method the knowledge of impedance of these biocomposite materials are required to estimate the noise reduction obtained20. The sound absorption coefficients of recycled fibrous materials and effects of thickness, surface facing and compression on the sound absorption coefficients of some fibrous material are available in the literature.21,22

The research on natural fibers shows that Egyptian cotton can be used as an acoustical material in different forms23. It is an effective, cheap product and pos­sesses a high sound reducing capability. Also, it is easy to use and creates no health risks compared to common commercial acoustical materials. Work has been done on natural fibers such as jute, coir and sisal where the structure-property relation­ship of these fibers including fracture modes has been determined. Attempts to in­corporate them in polymers and characterization of these new composites, with and without subjecting them to environmental conditions, have also been reported24. Relationship between the sound absorption coefficients of a cover made of woven cotton fabric with its intrinsic parameters has been determined25. The effect of air space, behind and/or between the layers of new sample made of local textile ma­terial (100% cotton) which is produced from a specially woven structure on the absorption coefficient has been studied for use as a sound absorbing curtain26. Stud­ies show that natural fiber composites are likely to be environmentally superior to glass fiber composites in most cases for the following reasons: (1) natural fiber production has a lower environmental impact compared to glass fiber production; (2) natural fiber composites have higher fiber content for equivalent performance, reducing the more polluting base polymer content; (3) the light-weight natural fiber composites improve fuel efficiency and reduce emissions, especially in automobile applications; and (4) end of life incineration of natural fibers results in recovered energy and carbon credits27.

Sound proofing properties such as absorption coefficient and transmission loss index of natural organic multilayer coir fiber has been studied28. The effect of perfo­rated size and air gap thickness on acoustical properties of coir fiber has also been studied. Comparison of acoustical properties between coir fiber and oil palm fiber has also been reported. The results obtained show that the coconut coir fiber gives an average noise absorption coefficient of 0.50. It shows a good noise absorption coefficient for higher frequencies but less for the lower frequencies. The oil palm fiber gives an average noise absorption coefficient of 0.64. The oil palm fiber shows a good noise absorption coefficient for higher frequency region compared to lower frequency region. Both fibers have a high potential to be used as sound absorber ma­terials. The potential of using coconut coir fiber as sound absorber is also there. The effects of porous layer backing and perforated plate on sound absorption coefficient of sound absorber using coconut coir fiber were studied and implemented using woven cotton cloth as a layer type porous material in car boot liners in automobile industry29. Another type of natural sound absorbing material such as industrial tea — leaf-fiber waste material for its sound absorption properties has been investigated30. The experimental data indicate that a 1 cm thick tea-leaf-fiber waste material with backing provides sound absorption which is almost equivalent to that provided by six layers of woven textile cloth.

The technology based on the synthetic fiber composites made up of glass, Kev­lar or carbon has played a vital role in noise reduction applications in the aerospace industry since 1950. The advancement in the composites design after reaching the aerospace requirements is targeted for the general industrial and domestic sectors. In contrast, the increased usage of electrical and mechanical appliances at home and industries has created a concern for noise pollution. Even though synthetic compos­ites possess specific properties like light-weight, high strength-to-weight ratio and stiffness, they are not much applicable for the industrial and domestic sectors due to the high cost of the raw materials. Further these materials are harmful when kept exposed in the open environment. Though recently rice hull has been added to open cell polyurethane foam and its acoustical properties evaluated31. On the basis of these aspects, the new direction in industrial application on sound proofing materi­als based on natural biocomposites reinforced with fibers of sisal, coir, jute, etc. is steadily developing in the past few years.

There are many theoretical models available for predicting the sound absorption coefficients of sound absorbing materials32. Though the sound absorption coefficient can be measured, the knowledge of certain acoustical parameters help the material developer to estimate the sound absorption coefficients well before the material is made. Some such parameters are density, fiber size, porosity, tortuosity, flow resis­tivity and characteristic lengths.

A Solution consisting in increasing the blank holder pressure has been already men­tioned in Section 7.3.3.1 to get rid of wrinkles. However, low blank holder pressure

should be preferred to avoid to homogeneity defects caused by too high strained tows. The use of high resistance tows could also be a solution to prevent this defect. Using low blank holder pressure would also delay the appearance of tow sliding de­scribed in Section 7.3.2.2 as this defect only appears when the blank holder pressure is increased. This therefore means that compromises need to be found already at this level to prevent the appearance of these defects.

To prevent the appearance of buckles different solutions may be developed. A first solution consists in designing specific fabric architecture as it was showed in Section 7.3.2.1 that the architecture of the fabric was a critical parameter. Indeed, for the two orientations the buckles appear on the warp tows (on the face C and its opposed edge for 0°orientation and on faces A and B for the 90°orientation).

Reinforcement 1 considered in this study is not balanced since an important space is observed between the weft tows whereas it is almost nonexistent between the warp tows as shown schematically in Fig. 7.26.a. This space controls the appear­ance of buckles. Its presence between the weft tows allows the warp tows to bend out of plane. Between the warp tows the lack of space prevents the movement of the weft tows. As a consequence, a balanced fabric with no space between two consecu­tive warp and weft tows could prevent tow buckling (see Fig. 7.26.b).

This new fabric (reinforcement 3) manufactured with the same un-twisted tows was manufactured by GroupeDepestele and tested under the same process condi­tions as the previous studied fabric. The results presented in Fig. 7.5 confirm the absence of buckles.

A second type of solution was investigated to prevent buckling on the final pre­form; it focuses on the optimization of the forming process parameters so that the local tensions in the preform can be changed in the defect zones. Nevertheless, the change of the local tensions with no modification of stresses in the rest of the fabric is not easy with the geometry of the bank holders used in this study (Fig. 7.1.b).

To reach this goal, new specially designed blank holders have been elaborated to apply minimum pressure to the tows passing by the triple point. New tests are conducted on reinforcement 1 for the 0°orientation. The final preform presented in Fig. 7.28 is obtained for a blank holders’ pressure of 3bar, applied on the warp tows

on which the buckles appeared previously. No buckle is observed on the Face C and its opposed edge, unlike with the previous blank holder system.

7.5 CONCLUSIONS

The possibility of manufacturing complex shape composite parts with a good pro­duction rate is crucial for the automotive industry. The sheet forming of woven reinforcements is particularly interesting as complex shapes with double or triple curvatures with low curvature radiuses can be obtained. To limit the impact of the part on the environment, the use of flax fiber based reinforcements may be consid­ered for structural or semistructural parts. This study examines the possibility to develop composite parts with complex geometries such as a tetrahedron without defect by using flax based fabrics. An experimental approach is used to identify and quantify the defects that may take place during the sheet forming process of woven natural fiber reinforcements. Wrinkling, tow sliding, tow homogeneity defects and tow buckling are discussed. The origins of the defects are discussed, and solutions to prevent their appearance are proposed. Particularly, solutions to avoid tow buckling caused by the bending of tows during forming are developed. Specially designed flax based reinforcement architecture has been developed. However, if this fabric design has been successful for the tetrahedron shape, it may not be sufficient for other types of shapes and that is why the optimization of the process parameters to prevent occurrence of buckles from a wide range of commercial fabrics was also investigated with success.

The effect of polystyrene-block-poly(ethylene-ran-butylene)-block-poly(strene- graft-maleic-anhydride) as compatibilizing agent (2 and 4%) and alkali treatment (4 and 6%) of short SPF on the flexural strength and flexural modulus of SPF/ HIPS composites were studied by Bachtiar et al.116 using 40 wt.% of fiber content. SPF alkali treatment using 6% NaOH solution improved the flexural strength, flex­ural modulus and impact strength of the composites as compared to the untreated composites by 12%, 19% and 34%, respectively. On the contrary, the SPF/HIPS composites treated with compatibilizing agent indicated no improvement in flex­ural strength and flexural modulus. However, significant improvements of impact strength of the alkali and compatibilizing agent treated composites were obtained. The impact strength of the 4% alkali and 3% compatibilizing agent treated compos­ites were about 16% higher than the untreated SPF/HIPS composites. The enhance­ment of the impact strength of alkali treated SPF/HIPS composites were due to: (1) development of rough surface fibers which offers good fiber-matrix adhesion; and (2) removal of hemicellulose and lignin parts of the SPF fibers, whereas the strong cellulose components on the fibers remained. The compatibilizing agent also en­hanced the impact strength of the composites due to chemical reaction of hydroxyl groups of SPF fibers with the anhydride groups of the copolymers which resulted into good interface adhesion between SPF fiber and HIPS matrix.116

9.4 CONCLUSIONS

The rapid advancement in the development of greener materials based on natural fibers and biopolymers is gaining more attention due to increase environmental awareness coupled with depletion of petroleum resources. Utilization of fibers and polymers that are biodegradable and obtained from renewable resources will help to preserve our environment. Hence, sugar palm tree is a potential ‘green resource’ for natural fibers and biocomposites.

Sugar palm tree is one of the multipurpose trees grown in tropical regions. Of recent, sugar palm fiber with its desirable properties has manifested high potential to be used as reinforcement in polymer composites. Sugar palm fibers can be used as reinforcement for bio-based polymers to produce 100% biodegradable compos­ites. The use of sugar palm fiber and bio-based polymers or even petroleum-based polymers to develop greener composites helps in: (1) reducing the negative environ­mental impact of synthetic polymers and fibers; (2) decreasing the pressure for the dependence on petroleum products; and (3) developing sugar palm trees as new crop in the future for tropical countries most especially in Malaysia. When biocomposite materials from sugar palm tree are increasingly used for industrial applications, such would boost Malaysia’s status as global promoter, developer and manufacturer of green composites. This would lead to increasing revenues and create more jobs. The successful development of green composites from sugar palm tree would provide opportunities to improve the standard of living of the sugar palm tree farmers in Ma­laysia. These would generate nonfood source of economic development for farming and rural areas in Malaysia.

9.5 ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support from Ministry of Education Malaysia with Exploratory Research Grant Scheme (ERGS) project vote number 5527190. Thanks also due to Universiti Putra Malaysia for granting sabbatical leave to S. M. Sapuan in 2013-2014 and to Ministry of Education Malaysia for scholar­ship award (MyPhD) to J. Sahari. The assistance of Dr. Nukman Yusoff of Universiti Malaya is highly appreciated.

Experimental data obtained was analyzed statistically by analysis of variance for statistical significance and multiple comparisons of means were accomplished with Duncan’s Multiple Range Test (p £ 0.05).

13.3 RESULTS AND DISCUSSION

13.3.1 FT-IR SPECTRAL INTERPRETATION

FT-IR is a common means to evaluate the chemical modification of lignocellulosic/ wood fibers.848’5563 Because the effect of solvent extraction and malation are a criti­cal aspect of this study, the FT-IR spectra are shown first, refer to Fig. 13.1.

The FT-IR of the untreated DDGS (Original DDGS) as shown in Fig. 1 indicates the following absorbance modes intensities cm1: 3377 (very strong), 3012 (weak — medium), 2926 (medium-strong), 2857 (medium), 1734 (medium), 1653 (strong), 1521(medium), 1451 (medium), 1374 (medium), 1238 (medium), and 1041 (very strong). The 3377 cm-1 band is a composite of the protein N-H and the O-H of the carbohydrate moieties present; whereas the small 3012 cm-1 band is that of the ole — finic — C=C-H stretching mode of the oil content. The 2926 and 2857 cm-1 peaks are, respectively the alkyl groups (-CH2-, -CH3), whereas the 1734 cm-1 and 1238 cm-1 absorption bands are mainly those of the oil component although both the carbohy­drate and protein components have a small contribution to the 2926 and 2857 cm-1 bands as well. The 3377 and 1041 cm-1 bands are mainly protein and carbohydrate contributions to the spectrum.

In the solvent (hexane/acetone) extracted-DDGS material (STDDGS) the in­tensity of the carbonyl band of the oil (1736 cm-1) decreased from 0.25 to 0.15 absorbance units as most of the oil was removed (Fig. 13.1). Following acetyla­tion (STDDGS/A) two bands 1743 and 1235 cm-1 were boosted in intensity. The ester — C=O (1743 cm-1) and the ester — C-C=O stretch (1235 cm-1) are observed. A competition between the malation-acetylation reaction (STDDGS/AM) resulted in an isolated product whose infrared spectrum gave a broadened 1730 cm-1 band ac­counting for the ester carbonyl.

The IR spectrum of the unmodified PW shown in Fig. 13.1 gives a characteristi­cally strong OH band around 3419 cm-1 and the — CH2O — band at 1047 cm-1, respectively. Also present is an unresolved band around 2928 cm-1 for the alkyls (-CH3, — CH2-) modes. There are also two reasonably sharp medium intensity bands at 1739 and at 1239 cm-1 that indicate carbonyl absorption bands betraying the presence of some oil in the wood. In addition, a pair of bands at 1622 and 1505 cm-1 occur that could represent the presence of some protein component in the wood.

The solvent (hexane/acetone) extracted PW (STPW) did not seem to have re­leased all its oil components since the its spectrum displayed the same intensity in the 1739 and 1238 cm-1 bands as the PW spectrum shown in Fig. 13.1. A noticeable change occurred in the amide I band of the protein which was attenuated. Upon acetylation (STPW/A), two noticeable changes were evident: the 3440 cm-1 OH band was truncated from about 0.85 absorbance units to 0.6 absorbance units. The major evidence following acetylation was the increase in intensity of the carbonyl band at 1744 cm-1 from 0.24 absorbance units in the unmodified to 0.55 units in the acetylated product. Following this was the corresponding increased intensity of the 1238 cm-1 band (-C-C=O) from 0.4 to 0.62 absorbance units, whereas the 1047 cm-1 band remained unchanged. The acetylated/maleate spectrum (STPW/AM) closely mimics the STPW/A spectrum.