Category Archives: GREEN BIORENEWABLE. BIOCOMPOSITES

PYRIFORM GLAND

Pyriform glands secrete fibers that are important constituents of attachment discs. Attachment discs function to fasten dragline silk fibers to solid supports, including wood, concrete, glass, and other surfaces. This attachment is central to locomotion and web construction. The exact mechanism of the adhesion to the support has yet to be fully elucidated. Biochemical studies have revealed that the attachment discs fibers are embedded in a liquid matrix that rapidly dries. The major constituent of attachment disc silks from L. hesperus represents Pyriform Spidroin 1 (PySp1).53 Real-time quantitative PCR analysis support high levels of PySp1 transcripts in the pyriform gland relative to other silk-producing glands. From a protein architecture standpoint, PySp1 contains the conserved C-terminal domain characteristic of spi­droin family members, as well as internal block repeats ranging from 238-300 resi­dues that are rich in A, Q and E, along with a 78 amino acid spacer region that is ex­tremely hydrophilic in nature. Traditional GGX, GPGGX, and poly A stretches are absent. Block modules contain submotifs with the sequence AAARAQAQAEARA — KAE and AAARAQAQAE, which have been shown to form beta-sheet structures

when synthetic peptides containing iterations of these sequences were investigated by circular dichroism (unpublished data). Analysis of the amino acid composition profile reveals the protein sequence of PySp1 contains the most hydrophilic residues relative to other spidroin family members, a likely feature that is linked the observa­tion that it is spun into a liquid matrix that readily dries.53 Because these fibers are difficult to collect from spiders, mechanical data have been difficult to obtain from traditional stress-strain analyzes. In orb-weavers, the equivalent spidroin has been reported and dubbed PySp2 or PiSp1.54 MS/MS analysis ofboth the attachment discs and luminal contents from the pyriform gland of orb-weavers, similar to PySp1, has confirmed the presence of PySp2 as one of the major constituents.54^ Comparable to PySp1, PySp2 contains the highest degree of polarity among the spidroin fam­ily members, conservation within its C-terminal domain, but differences in protein sequence within its internal block repeats relative to the cob-weaver PySp1. PySp2 contains block repeats that are approximately 200 amino acids, flanked by spacer regions that are 44 residues; these spacer regions are Pro-rich and have iterations of the submotif PAPRPXPAPX, with X representing a subset of amino acids that mostly contains hydrophobic R groups. The block motifs have repetitive motifs that are Gln-Gln-Ser-Ser-Val-Ala (QQSSVA). Synthetic fibers spun by wet-spinning methodology with purified proteins containing the C-terminal domain, block repeat, and spacer region have been reported.54a The high degree of polarity within the pro­tein sequences of the PySp fibroins, along with their ability to form fibers, suggests strategies to allow for fabrication of fibers in liquid environments.(Fig. 1.4.)

image7

FIGURE 1.4 Attachment discs from a black widow spider, L. hesperus. Left to right: digital photograph of a black widow spider with attachment discs holding down dragline silk, followed by two scanning electron microscope (SEM) images of an attachment disc at 450x and 900x, respectively.53

BIOCOMPOSITES COMPOSED OF BIO-BASED EPOXY RESIN AND TANNIC ACID (TA)

4.4.1 BIO-BASED EPOXY RESINS CURED WITH TA

The GPE, PGPE, SPE, and ESO were cured with TA in order to compare the proper­ties of the cured bio-based epoxy resins.2123 Because TA is soluble in liquid GPE and SPE, the mixing of GPE/TA and SPE/TA is very easy. In case of PGPE/TA and ESO/ TA, it is necessary to add ethanol to get homogenous solutions. The chemical for­mula for the commercial TA is often given as C76H52O46 (Fig. 4.2). However, in fact it contains a mixture of related compounds. Also, it is supposed that all of the three hydroxy groups of PG moiety of TA are hard to react with epoxy groups. Therefore, the curing temperature, curing time and epoxy/hydroxy ratio were optimized for GPE/TA, SPE/TA and ESO/TA. First, curing temperature and time were changed at the fixed epoxy/hydroxy ratio of 1/1. As ESO with alicyclic epoxy groups has a lower reactivity than GPE and SPE with glycidyl groups, ESO/TA was cured at a higher curing temperature range (150-230 °C) than that of GPE/TA and SPE/TA (120-200 °C). The GPE/TA and SPE/TA had the maximal tan 5 peak temperatures (73 and 95 °C), when cured at 160 °C for 3 h and 2 h, respectively. Also, ESO showed the highest tan 5 peak temperature (57 °C), when cured at 210 °C for 2 h (Table 2). These results indicate that the control of curing temperature and time is very important for the bio-based epoxy curing system containing aliphatic and sug­ar-based moieties with relatively low heat resistance. When PGPE/TA with epoxy/ hydroxy 1/1 was cured at the same curing temperature and time (160 °C and 3 h) as GPE/TA, the tan 5 peak temperature was 77 °C. When the tan 5 peak temperatures of the materials cured at epoxy/hydroxy 1/1 are compared, the higher order was SPE/ TA (95 °C) > PGPE/TA (77 °C) > GPE/TA (73 °C) > ESO/TA (57 °C), as is sup­posed from the relationship between epoxy functionality and the shortest distance between the two cross-linked points.

TABLE 4.2 Optimization of Curing Temperature and Time for the GPE/TA, SPE/TA and ESO/TA with Epoxy/Hydroxy Ratio 1/1

Sample Curing temperature (°C)

Curing time (h)

Tan 8 peak temperature (°C)

GPE/TA 120

2

68

140

2

70

160

1

63

2

72

3

73

4

66

5

69

180

2

77

200

0

63

SPE/TA 120

2

74

140

2

82

160

1

89

2

95

3

89

4

90

5

90

180

2

84

200

2

65

ESO/TA 150

2

46

170

2

52

190

2

53

210

1

55

2

57

3

55

4

51

5

51

230

2

50

Next, epoxy/hydroxy ratio was changed at the curing temperature/time, which showed the highest tan 5 peak temperature at epoxy/hydroxy 1/1. The tan 5 peak temperatures of GPE/TA and SPE/TA increased with decreasing epoxy/hydroxy ra­tio (Table 4.3). The increase of Tg with TA content should be attributed to an increase of the content of highly hindered aromatic framework rather than an increase of crosslinking density. However, the cured resins at epoxy/hydroxy 1/1 showed the highest tensile strength and modulus. Judging from the trend of tensile properties, it is thought that the incorporation of TA component in the crosslinked structure is in­sufficient for the TA-rich compositions (GPE/TA 1/1.2—1/1.4, SPE/TA 1/1.2—1/1.4). Consequently, the epoxy/hydroxy ratio 1/1 was selected for both GPE/TA and SPE/ TA, considering the balance of T and tensile properties. The epoxy/hydroxy ratio of PGPE/TA was fixed to 1/1, considering the result of GPE/TA, In case of ESO/ TA, both the tan 5 peak temperature and tensile properties increased with decreasing epoxy/hydroxy ratio. When the epoxy/hydroxy ratio was lower than 1/1.4, the ESO/ TA mixture became so viscous that we could not prepare a void-free cured sample. Consequently, the condition of curing temperature 210 °C, curing time 2 h, and epoxy/hydroxy ratio 1/1.4 was selected for the curing of ESO/TA. When petroleum — based PN is used as a hardener of SPE, the tan 5 peak temperatures for SPE/PN resins cured at epoxy/hydroxy ratios 1/0.8-1/1 were 78-81 °C, which were lower than those of SPE/TA resins cured at epoxy hydroxy ratios 1/1-1/1.4 (95-111 °C).

TABLE 4.3 Thermal and Mechanical Properties of the GPE, PGPE and SPE Cured with Various Hardeners

Sample

Epoxy/

hydroxy

ratio

Curing

condition

(°C/h)

Tan 8 peak tem­perature

(°C)

Tensile

strength

(MPa)

Tensile

modulus

(MPa)

5 wt.% loss tempera­ture (°C)

GPE/TA

1/1

160/3

73

36.5

2430

317

1/1.2

79

31.2

2360

312

1/1.4

91

32.1

2260

PGPE/TA

1/1

160/3

77

63.5

2710

316

SPE/TA

1/1

160/2

95

60.6

1710

314

1/1.2

109

25.3

1698

1/1.4

111

30.0

1275

SPE/PN

1/0.8

170/3

81.0

1/0.9

80.6

1/1

78.1

346.3

ESO/TA

1/0.8

210/2

46

6.0

116

1/1

57

12.7

409

1/1.2

58

12.7

450

1/1.4

58

15.1

458

TREATMENT

Different treatments may be applied to biocomposite according to the effect required. Examples of some chemical treatments include alkalization, enzymatic treatments, flame-retardant and antimicrobial agent applications. Physical treatments include compression and thermal treatments.

Among physical treatments, compression and thermal treatment play a very im­portant role as they form the basis of molding process, which is a must for most of the noise control composites. Compression of a fibrous mat deteriorates its sound absorption properties according to Castagnede et al.81 For a given homogeneous po­rous layer, compression is followed by a decrease in terms of porosity and thickness, and at the same time by an increase of tortuosity and resistivity. Jayaraman et al.,47 Yilmaz et al.,4 and Yilmaz et al.58 also found a decrease in sound absorption with compression. The finding of Yilmaz et al.20 is presented in Fig. 5.12.

image129

PLA/Hemp/PLA sandwich biocomposites (Yilmaz et al., 2012: DOI: 10.1002/app.34712).20

Yilmaz et al.4,8 investigated the effects of thermal treatment on noise control ca­pability. Yilmaz et al.8 treated three-layered sandwich structures of PLA/Hemp/PLA at temperature points of 125, 145, 165 and 185°C. An increase in air flow resistivity and a decrease in NAC were found. The decrease in NAC reached a substantial ex­tent when the melting point of the constituent thermoplastic fiber, PLA, is reached as seen in Fig. 5.13. Yilmaz et al.4 investigated the effects of thermal treatment on noise reduction performance of three-layered PP/Hemp/PP sandwich structures at 150 and 185°C (see Fig. 5.14). They reported a slight increase in NAC for the lower frequency range for 150°C treated composite and a drastic decrease in NAC for the biocomposite treated at 185°C as shown in Fig. 5.12. Similar to the find­ing of Yilmaz et al.8, temperature exceeding the melting point of the thermoplastic component was reported to be very deteriorating for sound absorption performance. The slight increase in the lower region which is experienced for the 150°C treated sample might be due to increase airflow resistance. Accordingly, by fine-tuning the parameters of thermal treatment, the noise reduction capability of the biocomposite may be preserved or it can even be enhanced.

image179
Подпись: sound DOI:

Among chemical treatments, Jayaraman et al.47 studied the effect of fire retar — dancy treatment on sound absorption. They found that the treatment had a positive impact on the sound absorption of kenaf fibers. Yilmaz et al.9 applied alkalization treatment on PP/hemp/PP to detect the effect of the treatment on sound absorption. As known, the alkalization process partially removes lignin, pectin and hemicel — luloses present in the fibers and leads to a separation between the fibrils of the natural technical fibers. This results in finer fibers and a rougher surface, which might enhance sound attenuation. However, alkalization did not increase the sound absorption coefficient as expected. In contrary, the NAC values decreased with the decrease in the mass of the material due to loss of hemp fiber constituents and pos­sibly the distortion of the pores during the wet treatment as given in Fig. 5.15.

image132

FIGURE 5.15 Effect of alkalization on sound absorption performance of PP/Hemp/PP webs. (a) Treated at room temperature. (b) Treated at boiling temperature. (Yilmaz et al., 2013.9 DOI: 10.1007/s12221-012-0915-0).

Among wet treatments, Huda and Yang55 extracted fibers from cornhusks by al­kalization and further treated a part of them with cellulose and xylanase enzymes to produce finer fibers with the elimination of extracellulosic materials. They blended cornhusk fibers with PP fibers then formed a web by carding and thermally bonded the web to produce a composite structure. They obtained better sound absorption from composites consisting enzyme treated cornhusk fibers compared to those that were untreated as shown in Fig. 5.16.

image133

FIGURE 5.16 Effect of alkalization (trt 1), and subsequent enzymatic (trt 2) treatments on sound absorption of corn husk — PP composites (From Huda, S.; Yang, Y Macromolecular Materials, 2008.55 With permission from Wiley VCH).

5.4 CONCLUDING REMARKS

Fibrous materials act as noise control elements in a wide range of applications as they present a cost-effective, light-weight, and environmentally friendly alterna­tive to conventional materials. Fibrous materials should be designed as composite structures to address the demands in terms of esthetics and performance character­istics such as moldability, durability and enhanced noise reduction capacity. De­signing of an effective composite sound absorbing noise control element requires a good understanding of sound propagation in fibrous materials. This chapter has presented an overview of natural fiber based biocomposites as rigid porous sound absorbers. Sound absorption mechanisms that take place during sound wave propa­gation through fibrous media have been given. An overview of empirical models which may be applicable to explain sound propagation in biocomposites has been explained. Based on the models, factors that affect sound absorption behavior of biocomposites have been investigated referring to previous studies.

Most of the research and modeling effort for understanding the sound absorption characteristics of fibrous materials have been devoted to conventional petro-based materials. More study is needed with sound absorbers of biocomposites, which of­fer a safer production and service life with effective utilization of scarce resources.

Natural fibers, engineered environmentally friendly polymers, and recycled ma­terials can be given as examples of “green” materials to be used during production of noise absorber biocomposites based on previous studies. There is especially very limited knowledge pertaining to the acoustical properties of biocomposites consisting of engineered biopolymers or agricultural byproduct materials. Future research ef­forts devoted to the understanding of these materials may enhance our understand­ing of sustainable materials as noise reduction elements.

KEYWORDS

Biocomposites

Fiber reinforced Composites

Noise Control

Noise Pollution

Sound Absorption Materials

Sound Absorption Mechanism

LINED DUCT

HVAC (Heating, ventilation and air-conditioning duct) are usually wrapped in fiber glass for thermal insulation. Fiber glass also have good sound absorbing properties and help in the breakout noise from the sheet metal duct. Jute-based composites have yet another application for noise control in ducts. The jute felt faced with a jute textile is applied to the inner walls of the duct. Since jute felt have good sound absorbing properties they help in the noise reduction. Since they are faced with jute textiles, the fibers do not separated due to the flow of air in the HVAC duct. It is recommended by ASHRAE (American Society of Heating, Refrigeration and Air-conditioning En­gineers) that the maximum flow velocity of air at the exit of a duct from an AHU (Air Handling Unit) should be less than 7.5 m/s64. At these speeds in the duct the fibers do not separate from the felt. Figure 6.24 shows a sheet metal duct lined with 25 mm jute felt faced with jute textile of 400 gsm (grams per square meter). Fiber glass is not rec­ommended to be used inside a duct, since if the fibers get carried by the air flowing inside the duct, they can be harmful to persons breathing the conditioned air, which is not so in the case of jute. Jute being hygroscopic can also withstand and moisture or condensate getting deposited in it in the inner of the duct walls. Jute felt can also be wrapped outside the duct to reduce the breakout noise.

Подпись: FIGURE 6.24 image158

The inside linings of AHU (Air Handling Units) of HVAC systems can also be lined with jute felt with a perforated facing backed with a 5 mm thick composite jute panel, for substantial noise reduction. Particularly in hospitals and classrooms where low noise levels are required, such a treatment can be done.

BIODEGRADABLE POLYMER/BIOPOLYMER MATRICES

Biodegradable polymers can originate from biomass or petroleum-based and can be classified as green polymeric matrices.14 It is essential to differentiate biodegradable polymers from biopolymers because the origin of the raw material can vary. For instance, poly(caprolactone) (PCL) is a petroleum-based polymer but is completely biodegradable and thus can be referred to as green matrix. These polymers can be degraded by anaerobic and aerobic biological processes, which mainly produce carbon dioxide, water, and biomass.13 Biopolymers (bio-based polymers) are con­sidered biodegradable but not all biodegradable polymers are biopolymers. Rein­forcement of the biodegradable materials with natural fibers give improved material properties, desired in various applications without compromising biodegradability. Figure 9.5 shows the life cycle of biodegradable polymers, which includes conser­vation of fossil resources, water and CO2 production. The rate of biodegradation depends on humidity, temperature (50-70°C), amount and type of microbes.

image197

FIGURE 9.5 The life cycle of biodegradable polymers (Siracusa, V; Rocculi, P; Romani, S.; Rosa, M. D., Biodegradable polymers for food packaging: a review. Trend in Food Science and Technology 2008, 19, 634-643. With permission.).

Polymers from renewable resources have attracted tremendous amount of atten­tion over two decades. The increasing appreciation for biopolymers is predominant­ly due to: environmental concerns, and the rapid petroleum resources depletion.14 Figure 9.6 presents the classification of polymers in four different groups.1115 All the three groups of polymer are derived from renewable resources except the last group, which is from petroleum products.11 Generally, biopolymers can be classified into three groups: (1) natural polymers, like starch, protein and cellulose; (2) synthetic polymers from natural monomers, such as polylactic acid (PLA); and (3) polymers from microbial fermentation, such as polyhydroxybutyrate (PHB).14

image198

FIGURE 9.6 A classification of biodegradable polymers based on their origin (Adapted from John, M. J., Thomas, S., Biofibers and biocomposites. Carbohydrate Polymers 2008, 71, 343-364. With permission.).

Numerous new polymer materials were developed from renewable resources, such as starch, which is a natural polymer. Other biopolymers are poly lactic acid from fermentable sugar and polyhydroxyalkanoate (PHAs) from vegetable oils next to other bio-based feedstocks.16

TENSILE PROPERTIES OF FIBER TOWS

It is very difficult to test the tensile properties of individual fibers thus the test was performed on the fiber tow removed from the fabric tested instead in accordance with the procedure described above. Table 11.6 lists the fiber tows come from dif­ferent treated and untreated fibers that were tested as well as their tensile properties. The tows in the longitudinal direction in the fabric are denoted as parallel, whereas the ones in the orthogonal direction are denoted as perpendicular.

It is evident from Table 11.6 that the tensile properties of the fiber tow did not change much for most of the systems indicating that the treatment did not generally have a detrimental effect on tensile properties except for fibers treated with alkali metal hydroxide alone (e. g., KOH and NaOH). It is clear, therefore, that cellulose materials treated with both alkaline earth metal salt and alkali metal hydroxide are advantageously fire retardant, often self-extinguishing, while retaining good tensile

properties,

in contrast to fibers treated only with alkali metal hydroxide or treated

with another metal salt.

TABLE 11.6

Tensile Force of Tows of Treated C2 Fibers

Max load pounds force (N)

Fiber

Description

Parallel Perpendicular

C2

Untreated C2

4.6 (20.4) 5.4 (23.8)

C2-1/P1

Ba(OH)2

4.9 (21.7) 5.4 (24.1)

C2-2/P1

BaCl2

4.7 (21.1) 5.6 (25.1)

C2-4/P1

Mg(NO3)2

5.4 (23.8) 5.7 (25.5)

C2-7/P1

Mg((OH)2

4.3 (19.2) 5.3 (23.6)

C2-10/P1

KOH

3.6 (15.8) 4.5 (20.2)

TABLE 11.6 (Continued)

Max load pounds force (N)

Fiber

Description

Parallel

Perpendicular

C2-11/P1

NaOH

3.6 (15.8)

4.3 (19.0)

C2-13/P1

NaOH then washed

3.0 (13.2)

4.1 (18.4)

C2-15/P1

Al(OH)3

5.1 (22.8)

5.3 (23.6)

C2-17/P1

MgCl2 + NaOH

4.8 (21.3)

5.3 (23.7)

C2-18/P1

MgSO4 + NaOH

5.2 (23.0)

5.8 (25.8)

C2-19/P1

CaCl2 + NaOH

4.4 (19.5)

5.6 (24.9)

C2-20/P1

Ca(NO3)2 + NaOH

5.5 (24.4)

5.6 (24.8)

MANNANS-BASED COMPOSITES

Few studies on mannan-based composites are emphasized on the use of glucoman — nan as a film-forming component. Films produced from pure glucomannan have excellent mechanical properties and the improvement of those properties has been sought by blending glucomannans with other polysaccharides, proteins (chitosan, soy protein, sodium alginate, carboxymethyl cellulose, cellulose, gelatin, starch, etc.) or synthetic polymers.33

14.5.6 FACTORS INFLUENCING THE CHEMICAL, THERMAL, AND PHYSICAL PROPERTIES OF BIOCOMPOSITES

In recent decades, the interest in wood-based materials has increased especially for their biodegradability and abundance in nature. The main bio-based materials as al­ready have been targeted within this chapter are natural fibers (NF), microfibrillated cellulose (MFC), nanofibrillated cellulose (NFC), nanocrystalline cellulose (NCC), Xylan and lignin. The majority of these materials can be used alone or especially in combination with other polymeric materials. Except lignin, which has a hydro­phobic nature, the combination of those materials with other polymers usually faces the issue of the hydrophilic-hydrophobic incompatibility, an issue that has generally been evident with most synthetic polymers from base oil. Therefore, the surface modification of those materials is an excellent strategy to achieve the desired prop­erties of the final composite.

In the following sections, the effect of the hydrophilic nature of wood base ma­terials, the type of material and its concentration, the degree of dispersibility of such materials in nonpolar solvents and their characteristics according to obtaining the targeted mechanical, thermal, and surface energy properties of the composites will be considered.

IMPROVEMENT IN BARRIER PROPERTIES

The incorporation of cellulose nanocrystals in the chitosan matrix was observed to decrease water vapor permeability values, for example, from 3.31 to 2.23 gm/m2 day kPa, with 10 wt.% loading of CNC.36 The protection against oxidation of food or meat products can be reduced by packaging films with reduced oxygen perme­ability. The significant reduction in oxygen permeability was demonstrated with nanocomposite films prepared via layer by layer assembly by alternatively dipping in chitosan solution and CNC dispersion.43 The reduction in oxygen permeability of 180 mm thick A-PET substrate was about 94%, after 30 numbers of bilayers coating of chitosan and CNC via LBL assembly (Fig. 16.5).

image272

FIGURE 16.5 Oxygen permeability of A-PET substrate coated with chitosan/CNC nanocomposites via layer-by-layer assembly and inset: reciprocal value of the oxygen permeability coefficient (KPO2). Reprinted with permission from Elsevier Ltd., (Carbohydrate Polymers, 2013, 92, 2128-2134).

PREPARATION OF HYDROXYAPATITE

Eggshells were collected and washed with detergent, then calcined in air at 900 °C for 10 h. During the first 30 min most of the organic materials were burnt out, then the eggshells were converted to calcium oxide. Calcined shells were crushed and milled in a ball mill or an attritor mill. The ball mill (Fritsch GmBH, Fig. 2.2a) was equipped with alumina balls and bowls, the attritor mill (Union Process, Fig. 2.2b) was fitted with zirconia tanks and zirconia balls (02 mm). The crushed eggshells were reacted with phosphoric acid (H3PO4) in an exothermic reaction. The mixtures were milled for 5 h at 4000 rpm (attritor milling) or for 10 h at 350 rpm (ball milling), to achieve homogenous mixtures and to prevent agglomeration. In all cases, the used shell: H3PO4 ratio was 50 : 50 wt.%16,17.

After milling, a small amount (approximately 0.5 g) of each type of HAp pow­der was sintered at 900 °C for 2 h in air. HAp powders usually degrade at high temperatures, the most common problem being CaO formation.

image11

FIGURE 2.2 Mills used for hydroxyapatite preparation. (a) ball mill, (b) attritor mill.

BIOCOMPOSITE STRUCTURES AS SOUND ABSORBER MATERIALS

NAZIRE DENIZ YILMAZ and NANCY B. POWELL

ABSTRACT

Biocomposites, provided that they are produced in porous form, that is, unconsoli­dated structure, act as noise control elements in a wide range of applications as they present a cost-effective, light-weight, and environmentally friendly alternative to conventional sound absorbers. This chapter presents an overview of biocompos­ites as rigid, porous sound absorbers. Sound absorption mechanisms that take place in porous biocomposites are explained. Methods of measuring sound absorption performance are presented. Some models to predict sound absorption capacity are described. Based on these models, factors that affect sound absorption behavior of biocomposites are given. An overview of biocomposite sound absorbers developed by researchers is reviewed. Suggestions for future research are listed.

5.1 INTRODUCTION

Advances in new technologies are often accompanied by noise pollution, besides air, soil, and water pollution.1 To give an example, transportation is a major source of noise pollution with the ever-increasing number of more powerful and larger vehicles on the road. Vehicle passengers are affected by the noise generated by vehi­cles as much as the people outside the car. In addition to affecting the comfort of the passengers, it has negative effects also on the driver such as fatigue and distraction; hence, reduces the safety of the occupants. Not only the comfort and safety of pas­sengers, but also the quality perception of the vehicle is deteriorated by unwanted sound.2

While progress in technologies has resulted in higher standards of living, devel­opment of advanced materials should be carried out with responsible environmental practices. In this respect, ever-tightening regulations, together with growing public awareness, force the manufacturing industry to select environmentally friendly ma­terials and processes.3 Biocomposites, which contain bio-based or biodegradable

components, offer a viable alternative to their conventional counterparts: glassfi — ber based synthetic polymer composites. Automotives is a promising market seg­ment for fiber-reinforced biocomposites with increasing product quantity, quality and variety. More than 40 automobile components including trunk and hood liners, floor mats, carpets, padding and door panels are conventionally made of fibrous structures and composites. This fact presents the significant potential for the use of bio-fibers as substitution for conventional petro-based fibers.4 Bio-fiber based composites, that is, biocomposites have already found commercial uses by major vehicle producers since the 90s, in automobile components including door linings and panels, package shelves, and seatback linings,5 for all of which, noise control is a requirement.

In today’s conditions, environmentally friendly industrial practices cannot be carried out at the expense of quality performance.6 Within this context, acoustic biocomposites should be able to compete with conventional sound absorbers such as glassfiber composites.3 Glassfiber presents some critical disadvantages in terms of human and environment ecology, including being unsafe to handle and posing health risks when inhaled, in addition to being nonrecyclable.2-7’8 Due to these afore mentioned drawbacks, bio-fibers are gaining increased attention in a variety of en­gineering fields to replace glassfibers.9 Natural plant fibers offer some advantages compared to glassfibers. The specific gravity of plant fibers (~1.5 g/cc) is lower than that of glassfibers (~2.5 g/cc). If used in transportation, this, in turn, leads to lower gas consumption, that is, higher mileage per gallon and lower greenhouse gas emis­sions. Other advantages can be listed as lower cost, lower weight, and better heat insulation and noise reduction characteristics.1

There are three major methods to reduce unwanted noise. Primary methods consider modifications at noise and vibration sources. Secondary methods include alterations along the sound propagation path, and tertiary methods engage in sound receivers. Primary methods are restrained by economical and technical parameters to a great extent; while tertiary methods have to deal with each receiving person separately. This situation renders the secondary methods relatively advantageous in a number of applications.310 The secondary methods concerning the control of air­borne noise include the use of sound barrier and absorbers.11 This chapter is focused primarily on sound absorbers.

Sound absorbers are porous materials. In this context, sound absorber biocom­posites should be allowed to have pores as shown in Fig. 5.1. Noise is attenuated in tortuous channels of pores present in the porous materials due to viscosity and heat conductivity of the medium.1213 Porous sound absorbers can be classified into three groups: cellular, granular and fibrous materials.10 Among sound absorbers, fibrous materials are promising materials for noise reduction applications. Fibrous materi­als are advantageous in that they absorb more sound over a broader frequency range compared to other materials.14 Fibrous materials may also be more environmentally friendly in terms of production and after-service life practices.115

image116

FIGURE 5.1 SEM images of unconsolidated sound absorber biocomposites from mechanically split corn husk (MSH) and PP at different MSH concentrations at (a) 35 wt.%, (b) 55 wt.%, and (c) 75 wt.%, respectively (From Huda, S.; Yang, Y. Industrial Crops and Products, 2009.16 With permission from Elsevier).

Among parameters of porous materials, air flow resistivity, porosity, and tortu­osity are the main factors that affect sound absorption.13,17 In fibrous materials, flow resistivity increases with decreasing pore dimensions and fiber diameter. In addition to fiber size, fiber orientation,18 web density, porosity, tortuosity,19 mean pore size, pore size distribution, and absorber surface characteristics also affect flow resistiv- ity.20 Fiber reinforced composites offer some advantages compared to conventional sound absorber materials, including the economical price of the raw materials, effient thermo-processing, and lower specific weight.21

A thorough knowledge of sound propagation through fibrous materials is of prime importance for evaluating the noise absorption capacities of biocomposites, which are designed to serve as noise control elements in a wide range of applica­tions. Sound absorber biocomposites are mostly produced by natural fiber nonwo — vens bonded by some means to produce three-dimensional rigid materials.

In this work, the term “biocomposite” refers to a material made up of distinct parts such as fibers or resin either of which is of biological origin. As the topic is related to noise control in terms of sound absorption, most of the biocomposite ex­amples given in the open literature are in their unconsolidated form as they include pores to allow for sound wave dissipation.

This chapter presents an overview of biocomposites as rigid, porous sound ab­sorbers. Sound absorption mechanisms that take place in porous biocomposites are explained. Methods of measuring sound absorption performance are presented. Some models to predict sound absorption capacity are described. Based on these models, factors that affect sound absorption behavior of biocomposites are given. An overview of biocomposite sound absorbers developed by researchers is re­viewed. Suggestions for future research are listed.