In porous absorbers, sound propagates through an interconnected pore network re­sulting in sound energy dissipation. Absorbers are only effective at mid to high frequency range, and this is the frequency range, which the ear is most sensitive to.14

Porous sound absorbing materials are used for the control of automotive acous­tics, room acoustics, industrial noise control, and recording studio acoustics. Sound absorbers are generally used to decrease the undesirable sound reflection from hard, rigid and interior surfaces in order to reduce the reverberant noise levels.10 Porous absorbers may be classified as cellular, granular, or fibrous materials.

Cellular materials include foams from polymers such as polyurethane and in­creasingly from metals, like aluminum.10 In order to be an effective absorber, the foam should have an open-pore structure, that is, pores should be interconnected to allow for airflow from one to the other face of the foam. Although there are concerns about fire hazard and release of toxic combustion byproducts and the difficulty in recycling, polyurethane foams are widely used as sound absorbers.217

Wood-chip panels, porous concrete and pervious road surfaces are some ex­amples of granular absorbers. Granular materials can be consolidated with binders or used in the unconsolidated form.17 There is also an interest in producing granular absorbers from recycled materials such as used tires, waste foam,14 and rubber par­ticles.2526 An example of this is seen in Fig. 5.2.

Fibrous materials may be composed of glass, mineral or organic fibers in the form of mats, boards or preformed elements. Whereas granular materials can achieve a broadband absorption of 80% at the most, fibrous absorbers can give higher ab­sorption over a wider range of frequencies approaching 100% dissipation.14

Fibrous and granular absorbers may be produced by bonding the fibers or gran­ules chemically with the use of a binder, mechanically or thermally.10,27 It is a com­mon practice to cover porous absorbers with a thin perforated sheet such as highly perforated panels of metal, wood or gypsum. The reason to do so is to give a more pleasant appearance, to protect them from damage and to prevent the particles harm­ful to humans from polluting the air and the surrounding.10 The next section briefly explains sound absorption mechanisms that take place in porous materials.

Flammability of jute-based biocomposite is quantified by measuring the three pa­rameters of limiting oxygen index, flame propagation and smoke density.

6.5.2.2 LIMITING OXYGEN INDEX TEST

Limiting Oxygen Index (LOI) test was carried out to measure the minimum vol­ume concentration of oxygen that will just support flaming combustion of the jute biocomposite in a flowing mixture of oxygen and nitrogen. Oxygen concentration reported is the volume percent in a mixture of oxygen and nitrogen. Test was per­formed as per ASTM D 2863-97 standard12. The jute composite specimens for the LOI measurements were 152.4 mm ‘ 5 mm ‘ 4 mm in size. The volume percent of oxygen required for combustion is given in Table 6.5.

6.5.2.3 FLAME PROPAGATION

The rate of flame spread was measured as per federal motor vehicle safety system (FMVSS) standard13. Specimen of 152.4 mm ‘ 5 mm ‘ 4 mm in size was exposed

horizontally at its one end to a small flame for 15 seconds. The distance and time duration of burning or the time to burn between two specific marks were measured. The burn rate B, was expressed as the rate of flame spread in mm/min given in Eq. (6).

where B, L and T are burn rate in mm per minute, length of the flame traveled in mm and time in second for the flame to travel L mm, respectively.

From Table 6.6, 2.5% natural rubber latex based jute composite shows a lesser flame propagation rate than 5% natural rubber latex jute composite. Further, with a 1% sodium phosphate (Na3PO4; as a fire retardant) treatment on 5% natural rubber latex composite, the flame propagation rate of the jute composite reduces by almost half.

7.3.3.1 IN-PLANE SHEAR BEHAVIOR

Figure 7.24 shows a typical in plane shear curve for reinforcement 1.

The three characteristic zones usually observed during in plane analysis of wo­ven fabrics are well defined in Fig. 7.24. In zone 1, the stiffness of the fabric is week, and the tows can easily rotate. The low stiffness is due to tow-tow frictions. In zone 2, lateral contact and compression between neighboring yarns takes place. A stiffness that can be nonlinear is observed up to defined angle called the locking angle. From that angle, the stiffness rises again, and out of plane bending of the fabric takes place due to its week bending stiffness. As a consequence, wrinkles may appear in the faces of the fabric. For the flax woven fabric studied in this work, Fig. 7.24 shows that the value of the locking angle can be estimated to be around 30°. This value confirms that the values of shear angle observed on the faces of the tetrahedron are lower than the locking angle. It is therefore normal that no wrinkles due to high shear angles appeared on the Faces of the preform. However, the locking angle is much lower than the shear angles measured in the corner of the tetrahedron. It is therefore normal that wrinkles were observed for low blank holder pressure (1 bar). When the blank holder pressure was raised the membrane became tighter and the wrinkles disappeared.

SPF/epoxy composite tensile properties in different fiber orientations (long random, chopped random, and woven roving) and contents (10, 15, and 20%) was investi — gated.70 The results of different orientations proved that the 10 wt.% woven roving of SPF had the highest tensile strength and tensile modulus values, that is, 51.725 MPa and 1255.825 MPa. The high tensile properties of the woven roving SPF ori­entation were ascribed to the longitude and transversal orientation of woven roving fiber formation. Consequently, the bonding between their fibers are stronger com­pared to the long random SPF and chopped random SPF. Meanwhile, increasing the fiber loading from 10 to 15 wt.% for both long random and chopped random SPF orientations improved the tensile strength and tensile modulus of their respective composites. Further addition of long random and chopped random SPFs up to 20 wt.% in their composites decreased their tensile strength and tensile modulus. This might be due to the inadequate wetting of the fiber by the matrix resin as well as the easy crack initiation and propagation at the fiber ends.

Lignin displays high structural complexity characterized succinctly by a seemingly random (highly polydisperse), highly cross-linked polymeric network composed of monomeric phenylpropane units cross-linked through ether linkages and carbon — carbon bonds. It is an amorphous component of plants able to confer strength and rigidity to the cell wall. In addition to the multiplicity of lignin forms potentially available based on their origin, lignin structures and content vary widely depend­ing on the plant. In the case of wood, the amount of lignin ranges from ca. 12% to 39%.27

The monomer structures in lignin consist of phenylpropane units, but differ in the degree of oxygen substitution on the phenyl ring. Basically, three structures of lignin; H-subunit (4-hydroxy phenyl), G-subunit (guaiacyl) and the S-subunit (sy — ringyl) are conjugated to produce a three-dimensional lignin polymer that has been proposed by the majority of scientists to arise by radical (combinatorial) biosynthe­sis (Fig. 14.6). Thus, because enzymatic pathways for lignification have not been unequivocally identified, lignin is not expected to have a putative regular structure such as cellulose or other major macromolecules and therefore is a physically and chemically heterogeneous material with an unknown chemical structure.27

Historically, lignin has been considered as an unwelcome by-product from the pulping process, because the main objective of pulping is to extract cellulose from wood. However, based on the biorefinery portfolio, an effective and cost-saving utilization of industrial lignin it is essential, because lignin has the possibility to even partly replace petroleum-based products because of its chemical similarity and potential cost-performance benefits.

The extraction of lignin from lignocellulosic materials always leads to frag­ments of low molecular weight, due to the method of extraction, thus changing its physic-chemical properties. Besides, the method of isolation, the source from which lignin is obtained also has an influence on its properties. Lignin itself possesses very attractive chemical motifs that allow it to be modified. For example, its phenolic hydroxyl is a reactive site for cross-coupling, networking, modification, and polym­erization. A variety of polymers such as polyurethanes can be effectively produced from it. Indeed, by judicious control of decomposition (chemical and thermal) a panoply of chemicals can be derived for use as monomers to construct popular and valuable polymers such as polyesters, polyetheres, and polystyrenes.27

Manufacturing products from lignin depends on its physicochemical properties. Because of its wide varying functionality, parameters such as functional groups dis­tribution are relevant to end-use properties of lignin. The reactivities of these lignins will impact on the attributes of the end products. Table 14.4 shows molecular weight and functional groups of lignins.28

The Tg is a critical parameter to evaluate for any polymer because it is an in­direct measure of crystallinity degree and crosslinking (higher crosslinking would lead to a higher temperature for phase transition to liquid state) that relate to the rubbery region of the material.29 For example, comparing hardwood and softwood milled wood lignin, hardwoods presents a Tg lower (110-130°C) than softwoods (138-160°C). The value of the Tg will to a great degree depend on the moisture content and chemical functionalization and will be lower if the lignin has greater mobility. In addition, a greater molecular weight of the polymer typically translates to a higher Tg, but for lignin the impact of structural variation will need to be calculated into the overall rheological characteristics because it influences degree of polymerization.

The ability to introduce lignin into composite blends is greatly enhanced if its miscibility is increased; the miscibility of a hydrophobic polymer like lignin into a more hydrophilic matrix can be increased by appropriate modification of the phe­nolic group (hydroxypropyl, butyrate, etc.)30 or it can be introduced into hydropho­bic matrices by the formation of lignin copolymers.28 At a value of 4 GPa, the me­chanical properties of lignin are significantly lower compared to cellulose pulp (40 GPa).31 Therefore, lignin itself is not appropriate to be used as reinforcing material as has been demonstrated for cellulose. On the other hand, many commercial ap­plications of low-value lignin already require it to be tailored for specific end goals with high value (extrusion moldings, composites, etc.).

CH is currently sold in the USA as a dietary supplement to aid weight loss and lower cholesterol and is approved as food additive in Japan, Italy, and Finland. However, although pure CH has very attractive properties, it lacks bioactivity and is mechani­cally weak.20 These drawbacks limit its biomedical applications. For these reasons, it is highly desirable to develop a hybrid material made of CH and appropriate filler, hoping that it can combine the favorable properties of the materials, and further en­hance tissue regenerative efficacy. Of particular relevance is that CH is ideally suit­ed to complex with anionic form of cellulose (carboxymethyl cellulose), to enable a number of applications and can be conjugated with functional molecules, antibod­ies, biotin, and heparin.2125 The advantage of blending CH is not only to improve its biodegradability and its antibacterial activity, but also the hydrophilicity, which is introduced by addition of the polar groups able to form secondary interactions (-OH and — NH2 groups involved in H bonds with other polymers). The most promising developments at present are in pharmaceutical and tissue engineering areas.

Clay based biocomposites using biopolymers as matrices are continuously gaining widespread attention from scientific and industrial world. Montmorillon — ite (MMT) clay and chitosan nanocomposites have been reported to possess excel­lent mechanical, thermal and bioactive properties.1112 Cellulose nanocrystals, have attracted a great deal of interest in the development of nanocomposites owing to

their appealing intrinsic properties such as nanoscale dimensions, high surface area, unique morphology, low density, and mechanical strength. Furthermore, they are easily modified, readily available, renewable, and biodegradable and are known to improve the limited mechanical and barrier properties of biopolymers.26’27

Comparison of the protein architectures of the spidroin family members reveals common structural themes. Spidroins contain iterations of internal block segments flanked by nonrepetitive N — and C-termini. The N-terminal domain consists of approximately 155 amino acids that include a secretion signal. X-ray diffraction studies of the N-terminal domain of MaSp1 reveal a dimeric structure with each subunit forming an antiparallel five-helix bundle.11 In-vitro studies using purified recombinant proteins demonstrate that alterations in pH can affect spidroin aggrega­tion, suggesting that this domain functions as a pH-sensitive relay to control protein aggregation and solubilization.12 This experimental observation is consistent with reports that have demonstrated that the acidity increases moving down the spin­ning duct from a pH 6.9 to 6.3, a process apparently regulated by proton pumps positioned right before exiting via the spigots.13 Specifically, as the pH approaches 6.0, the N-terminal domain has been proposed to change conformations, accelerat­ing the self-assembly process of the fibroins. Consistent with the fact that hydrogen ion concentrations serve an important trigger for fiber formation is the observa­tion that homodimer stability and assembly is favored under acidic pH.12a Although the N-terminal domain appears to be more conserved relative to the nonrepetitive C-terminus, the C-terminal domain is also one of the most highly conserved re­gions within the spidroin family members. From a length or size perspective, the C-terminal domain is approximately 100 amino acids and its sequence shares no similarity relative to the N-terminal domain. Much attention has also been focused on elucidating the function of the C-terminal domain of MaSp1. The C-terminal do­main has been implicated in the control of solubility and fiber assembly.14 Analysis of the amino acid residues within the C-terminal domain of dragline silk fibroins has revealed the presence of a conserved Cys residue. This Cys residue appears to direct fiber formation, an observation supported by the expression of recombinant fibroins lacking the C-terminal domain that assemble improperly when manufactured inside insect cells.15 Additionally, experimental evidence supports its direct participation in disulfide bonds and association in large molecular weight complexes in the glan­dular contents of the MA gland.16 Interestingly, mutation of the conserved Cys to

Ser leads to inefficient dimerization of recombinant proteins purified from bacteria, suggesting this residue controls the aggregation process (unpublished data).

The internal block segments of the MA fibroins, as previously stated, are 40-50 amino acids in length and are repeated approximately 60 times. Each block module can be further broken into submotifs that are classified into four categories: 1) poly A (An) blocks or runs of Gly Ala couplets [GA]; 2) Gly X repeats [GGX]; 3) Gly Pro-Gly X-X motifs [GPGXX]; and 4) spacer regions. Different combinations of these submodules control the secondary structure of the protein and ultimately are responsible for a large part of the mechanical properties of the fiber. Solid-state NMR, Raman spectroscopy and X-ray diffraction studies support that the poly A stretches and GA couplets form the crystalline regions of the fibers, contributing to the high tensile strength.17 The GGX motifs have been hypothesized to be present within the amorphous region of the fibers and biophysical studies support these sub­modules form alpha-helical structures.18 It also has been proposed that the GPGXX modules, which are present only within MaSp2 and not MaSp1, form a helical shape that contributes extensibility to the fibers. In part, the mechanical properties of the fibers can be controlled by the ratios of MaSp1 and MaSp2; these ratios can vary from spider species to species and can be influenced by diet.19

Recently, the literature has pointed out intense interest in investigating new solvents, in particular for cellulose compounds.47,48 The mechanism of cellulose dissolution is important in description of the strategies employed for the synthesis of cellu­lose derivatives, and in the future perspectives to produce some complex structures, including nano-composites, “smart” polymers that respond reversibly to external stimuli, and bio-compatible materials. In this respect, in relation with the molecular structure of cellulose (Schemes 1 and 2), both intra and intermolecular hydrogen bonding interactions, influencing various properties must be mentioned.39,49 The in­tramolecular hydrogen bond interactions between O-2-H and O-5’ of the adjacent glucopyranose unit and O-2-H and O-6’ contribute to single-chain conformation and stiffness.

SCHEME 1 Molecular structure of cellulose.

The intermolecular hydrogen bonding interactions in cellulose, responsible for the sheet-like nature of the native polymer, are localized between the hydroxyl groups, — OH group at the C-6’ and C-2’ positions of cellulose molecules adjacently present in the same lattice plane.

SCHEME 2 Hydrogen bonds in cellulose.

Literature has demonstrated the versatility of multiple hydrogen bonding inter­actions in the formation of derivative cellulosic structures.

Moreover, solvents contribute essentially to the modification of solution behav­ior (association, complex, micelle, and core-shell structure of the polymer chains), and also to the processes, which determine the morphology of cellulose derivatives. Most of the parameters that play an important role in the formation of cellulose films have been presented in detail in this chapter. Among these variables, one should mention the nature of cellulose and the substitution degree, the nature of the casting solvent mixtures, their composition and temperature.5053 Conformational properties of cellulose acetate phthalate (obtained from cellulose acetate with a 1.93 substitu­tion degree according to Scheme 3)54,55 in solutions of 2-methoxyethanol (2-Me)/ water (W) (solvent/nonsolvent mixtures), are evaluated from the modification of coil density, (p — Eq. (1) or (2)), and gyration radius, (Rg — Eq. (3)), with concentra­tion over both a dilute and a large domain of concentrations (Fig. 3.1).56,58

The thermodynamic parameters represented by intrinsic viscosities, ([p]), ra­dii of gyration in perturbed, Rg, c=0 , and unperturbed, Rg, c=e, states, the critical concentrations delimiting the dilute — semidilute domain (c*), semidilute unentan­gled — semidilute entangled domain (c**), and semidilute — concentrated domain (c *** = c * (Rg, =0 / Rg, c=e)8), presented in Table 3.1, show the influence of sol­vent mixtures.

The perturbed and unperturbed dimensions of cellulose acetate phthalate de­crease with increasing the water content, which also reflects modification of the solvation power of solvents. 2-methoxyethanol is a good solvent for CAP, and the addition of water reduces the quality of the solvent mixtures in 0-0.30 volume fractions of water. At the same time, 2-methoxyethanol is preferentially adsorbed (preferential adsorption coefficient, shows positive values of 0.460, 0.685 and 0.526 for 80, 75, and 70% 2-Me contents, respectively) over the mentioned do­mains, and both solvents tend to minimize preferential adsorption at the extreme values of 2-methoxyethanol volume fractions (Фі). Consequently, from preferential adsorption data, two distinct ranges of water composition can be distinguished, over which the solvent mixture evidences different interactive properties. Therefore, the polymer chains have the tendency to surround themselves with the thermodynami­cally most efficient solvent, at a given composition of the solvent mixture.

Generally, for nonassociating polymers appears that:59,60

nsp x c **125 in semidilute unentangled regime;

nsp x c **48 in semidilute entangled regime — if the polymer is in a theta sol­vent; and

nsp x c **37 in semidilute entangled regime if the polymer is in a good solvent.

Entanglement concentration, c**, defined as the transition from the semidilute unentangled regime to the semidilute entangled one, can be measured using the change in slope at the onset of the entangled regime. Figure 3.2 illustrates the depen­dence of specific viscosity for CAP solutions in 2-Me/W solvent mixtures at 25 °C as a function of a dimensionless parameter, c ■ [n], defined as the coil overlap param­eter, which provides an index of the total volume occupied by the polymer. A slope modification in these dependencies occurring at critical concentrations, c * *, which separates the semidilute unentangled/semidilute entangled domain. As a function of solvent mixture composition, these concentrations take the following values: 8.69, 9.55 and 10.00 g/dL for 100/0, 80/20 and 70/30% v/v 2-Me/W, respectively. In ad­dition, the dependencies between specific viscosity and concentration are expressed by the following equations:

log tfsp = 1.603 +1.215′(c'[П) + 5.207′(c [П)2 +1.093′(c [П)3 for 100/0 v/v 2-Me/W

2 3 (7)

log np = 1.673 +1.205′ (c’ ft]) + 4.781′ (c’ ft])2 +1.124′ (c’ ft])3 for 80/20 v/v 2-Me/W

2 3 (8)

log np = 2.759 +1.379′ (c’ ft]) + 4.045′ (c’ ft])2 +1.190′ (c’ ft])3 for 70/30 v/v 2-Me/W

A lower quality of the solvent mixture leads to higher values for % and also for c** concentrations; increase of the water content leads to a decrease in the qual­ity of the solvent mixtures, and increase of polarity by water addition leads to an increase in the number of intermolecular polymer associations. The %> power law exponent in semidilute entanglement regime increases, from 4.5 to 5.5 for 100/0 — 70/30 v/v solvent mixture compositions. In the dilute and semidilute unentangle­ment domain, %> increases with the increase of the 2-Me content, suggesting that 2-Me is a better solvent for CAP than 80/20 v/v 2-Me/W or 70/30 v/v 2-Me/W (Table 3.1 and Fig. 3.2).

Consequently, %> increases with solvent quality for dilute or semidilute unen­tangled chains, due to a stronger apparent repulsive force among chain segments, whereas %> decreases with increasing solvent quality of the chains in entangled re­gime. This phenomenon has been described by a two-parameter scaling relationship for polymers in entangled regime,60 and the increase in %> with decreasing solvent quality in the entangled regime has been reported to be caused by a higher number of entanglement couplings in the poor solvent, which result from the intense attrac­tive forces between the chain segments.5961 Increase of the water content over the

0.70 volume fraction displays significant hydrogen bonding with cellulose acetate phthalate, generating the entanglement process among other types of interactions generated by carboxyl groups. CAP, which contains carboxyl and hydroxyl groups, is able to develop hydrogen bonding, easily dissociating into a relatively low polar­ity solvent, such as 2-Me, and leading to strong intermolecular interactions with the addition of higher polarity water, which shows the influence of hydrogen bonding on the thermodynamic properties. Therefore, transition from the semidilute unen­tangled regime to the semidilute entangled one is determined by solvent polarity. The concentration corresponding to this transition increases from 8.69 g/dL in 100/0 v/v 2-Me/W to 9.98 g/dL in 70/30 v/v 2-Me/W. In addition, literature62 establishes an optimal concentration of around 30 g/dL for obtaining nano-fibers. According to Table 3.1, this concentration is located at the supper limit of the semidilute — con­centrated domain. In this context, it is stated that the solvent plays an important role in the electrospinning process and that the optimal concentration of CAP for obtain­ing nano-fibers is: 35%, 25% and 12.5% in 2-Me, 50/42.5/7.5 v/v/v 2-Me/acetone (Ac)/W, in 85/15 v/v Ac/water, respectively. These concentration values correspond to the entangled domain and decrease with decreasing solvent mixtures’ quality.

Knowledge on conformational properties of cellulose derivatives in solution is important in various applications, such as those involving the production of films.4363 For the above illustrated system, the water content in the casting solutions favored the occurrence of roughness (according to the atomic force microscopy (AFM) im­ages from Figs. 3.3-3.5), more visible for intermediary compositions of the mixed solvents, corresponding to a better preferential adsorption of 2-methoxyethanol from the solvent mixtures.

For extreme water compositions, lower average roughness values and root — mean-square roughness values appear, the phenomenon being more intense at high­er water contents (Fig. 3.6, Table 3.2).

This changing trend in morphology is due to the modification of polymer chain conformation in solution that can be speculated according to the applied area. In this context, modification of the rheological properties and some morphological aspects of cellulose acetate phthalate in 2-methoxyethanol/acetone/water, at different com­positions of solvent mixtures, allowed the establishment of optimal composition of solvent mixtures for obtaining fibers with controlled diameters.64 Figure 3.7 plots the modification of dynamic viscosity, Л, versus shear rate, Y, and water content.

Increasing water content leads to decrease in dynamic viscosity, concomitantly with increasing the Newtonian plateau and flexibility. At the same time, at constant val­ues of shear rate, mentioned literature shows that dynamic viscosity varies insignifi­cantly until an approx. 25% vol. water content, while, for a 27.5% vol. composition, dynamic viscosity increases, attaining approx. the same values for different shear rates. At higher water composition, decreasing of dynamic viscosity signifies rear­rangement of macromolecules in solution.

The values of transition frequency from viscous to elastic domain and the values at which storage, G’, and loss, G», moduli are equal, slightly decrease with increas­ing the water content (Fig. 3.8). A sharp increase, up to a water content exceeding 25% vol., is also evidenced.

The spinning process of CAP in acetone/water solvent mixtures leads to short fibers with small diameter. When 2-methoxyethanol was added to the systems, the diameter of the fibers becomes larger and the irregularities disappear. Fiber diam­eters were found to vary insignificantly with the water content, yet, at an approx. 25% water content, the fibers evidence a smaller diameter, when the viscosity of solutions and the boiling point of the solvents mixtures increase.

2

On the other hand, the AFM images (with 20 x 20 mm scanning area) from Fig. 3.9 show the influence of casting solutions on films morphology. Table 3.3 lists the average values of pore characteristics and of the surface roughness parameters.

Increasing of the water content in the solvent mixtures determines modification of pores number and of their characteristics, so that, at approx. 25% vol., the pores number is maximum, while the area, perimeter and diameter are minimum.

Modification of morphology is due to the modification in the chain conforma­tion of the polymer, which is influenced by the quality of the mixed solvents.4563 Also, it may be assumed that the association phenomena of 2-methoxyethanol, ac­etone or water over different composition domains of their mixtures may influence the preferential adsorption of one of the solvents by the macromolecular chain.65

Therefore, conformational modifications generated by the interaction from the system change the solubility of cellulose derivatives, and affect the rheological and morphological properties.

The resistance to air flow is achieved through the depth of the material. The thicker the material, the higher is the sound absorption. Generally, when the thickness of the material matches one tenth of the wavelengths of the incidence sound, effective sound absorption is achieved. At a resonance frequency of one-quarter wavelength of the incidence sound, peak sound absorption occurs. The necessity for the sig­nificant thickness to wavelength ratio renders porous materials inefficient sound absorbers at low frequency due to their greater wavelengths. This ratio is extremely small at low frequencies as the wavelength may reach values that are in the order of 10 meters.14 To give some examples, the wavelength of 10-Hz sound is 34.3 m and 100-Hz sound is 3.43 m, whereas the wavelength of 1000-Hz sound is 34.3 cm.

Parikh et al.22 tested velour-surface fibrous materials made of recycled fibers and PP/PES bicomponent fibers to see the effects of thickness, mass per area, and the production method on the sound absorption. The results showed that increase in thickness and mass per area leads to an increase in NAC as expected. A decrease in sound absorption accompanying the decrease in thickness was also reported by Jayaraman et al.,47 el Hajj et al.54 and Yilmaz et al.20

NAC (normal incidence sound absorption coefficient) increases as the thick­ness increases. However, for every frequency there is an upper limit of the thick­ness, a quarter of a wavelength, beyond which NAC decreases slightly.17 This fact is contrary to the belief that NAC should improve continuously as the thickness increases.3 As shown in Fig. 5.9, with increasing thickness, the frequency where the peak absorption takes place decreases, which is often desirable.70

The jute derivatives in the form of fiber, felt, woven textile, composite panels have wide engineering applications for noise control. In architecture and building acous­tics, noise control materials are used for treatment in the walls and ceiling for im­proving the reverberation time and speech intelligibility52. Due to consumer aware­
ness and global competition among manufacturers of home appliances like vacuum cleaners, refrigerator, washing machines, room air-conditioners and the like there is a constant endeavor to improve the product quality by implementing many noise and vibration reduction technologies in their products. In order to control noise of a product the noise sources has to be ranked. Nowadays many experimental tech­niques exist for such noise source identification like the sound pressure level map­ping, sound intensity method and the acoustical holography method17. There is a strict vehicle pass-by noise regulation in each country, which has to be adhered to the automobile manufacturers. Every automobile manufacturer is putting in efforts to bring about noise, vibration and harshness (NVH) reduction in the vehicles they manufacture. Reports of many automobile companies using biocomposite materials for NVH reduction are available6-53’54. The authors have helped few manufacturers in reducing the noise of their products by using such jute-based green technology, few such cases have been reported55,56.