corn husk fiber-polyester composites as sound absorber...

Research ArticleCorn Husk Fiber-Polyester Composites as Sound AbsorberNonacoustical and Acoustical Properties

Nasmi Herlina Sari1 I N G Wardana2 Yudy Surya Irawan2 and Eko Siswanto2

1Department of Mechanical Engineering Faculty of Engineering Mataram University Nusa Tenggara Barat Indonesia2Department of Mechanical Engineering Faculty of Engineering Brawijaya University East Java Indonesia

Correspondence should be addressed to Nasmi Herlina Sari nherlinasariunramacid

Received 13 December 2016 Accepted 29 January 2017 Published 19 February 2017

Academic Editor Marc Thomas

Copyright copy 2017 Nasmi Herlina Sari et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

This study investigates the acoustical and nonacoustical properties of composites using corn husk fiber (CHF) and unsaturatedpolyester as the sound-absorbing materials The influence of the volume fraction of CHF on acoustic performance wasexperimentally investigated In addition the nonacoustical properties such as air-flow resistivity porosity and mechanicalproperties of composites have been analyzedThe results show that the sound absorptions at low frequencies are determined by thenumber of lumens in fiber particularly the absorption coefficient which increases the amount of fiber For high-frequency soundthe absorption coefficient is determined by the arrangement of fibers in the composite An absorption coefficient is close to zerowhen the fibers are arranged in a conventional pattern however when they are arranged in a random pattern a high absorptioncoefficient can be obtainedThe bond interface between the fiber and resin enhances its mechanical properties which increases thelongevity of the composite panel

1 Introduction

Noise pollution and waste management are two problemsthat need to be solved in modern societies The use of newlydeveloped alternative materials to absorb the noise consid-erably minimizes these problems Hence the inexpensiveeasily created thin and lightweight composite materials thatcan absorb soundwaves in broader frequency fields are highlydesirable

The fibrous sound-absorbing materials have been exten-sively investigated [1ndash5] Biot studies [1 2] provide anapproach for the propagation of elastic waves in the fluidmedium-saturated porous material at high and low frequen-cies where factors such as pore geometry fluid and mediumhaving comparable densities are required to be consideredDelany and Bazley [3] developed a simple model for estimat-ing the sound of absorption coefficients and characteristics ofimpedance of different types of fibrous absorbent materialsLee and Chen [4] developed Acoustic Transmission Analysis(ATA) model to estimate the acoustic absorption of a mul-tilayer absorbers Attenborough [5] developed a model for

estimating the acoustical characteristics of fibrous absorbentssoils and sands using flow resistivity formulae

The polymer has been widely utilized as a matrix in fibercomposites because it is easily formed from a material thathas physical and acoustical properties [6ndash13] Veerakumarand Selvakumar [6] studied acoustic properties for compositemade from kapok fiber with polypropylene fiber which werefound to demonstrate good sound absorption behavior inthe frequency range 250ndash2000Hz Jailani et al [7] studiedon panels made from coconut coir fibers which have beenconducted to analyze compression effect of the panel onthe acoustic performance The coir fiber panel is a goodsound absorber at 15ndash5 kHz Zulkifli et al [8] investigated theeffect of the porous layer backing and a perforated panel onthe sound absorption coefficient of coconut coir fiber Theyindicated that increasing the thickness material of the panelwill improve the sound absorption ability especially in thelow-frequency range at 600ndash2400Hz Chen et al [9] studiedthe sound-absorbing properties of ramie fiber-reinforcedpolylactic acidmaterials Putra et al [10] studied the potentialof waste fibers from paddy mixed with polyurethane as

HindawiAdvances in Acoustics and VibrationVolume 2017 Article ID 4319389 7 pageshttpsdoiorg10115520174319389

2 Advances in Acoustics and Vibration

acoustic material and found that the absorption coefficientis greater than 05 from 1 kHz and can reach the averagevalue of 08 above 25 kHz Bastos et al [11] developedvegetable fibers coconut palm sisal and acaı as sound-absorbing panels Measurement scale reverberation chamberexposed promising results from acoustic performance forall panels Flammability odor fungal growth and agingtests have been performed on samples to identify theirpractical ability Koizumi et al [12] developed bamboo fiberas sound-absorbing material They reported that the bamboofiber material has equivalent acoustics properties with glasswool Jayamani and Hamdan [13] studied sound absorp-tion coefficient of urea-formaldehyde and polypropylenemixed with kenaf fiber They reported that the kenaf fiberreinforced with polypropylene demonstrates higher soundabsorption coefficients than kenaf fiber reinforced with urea-formaldehyde These previous studies represented that abetter understanding of the microstructure and physicalparameters of a material could help in developing high-performance acoustic materials

This study primarily investigates the effect of addingcorn husk fibers (CHFs) on acoustical and nonacousticsproperties of polyester composites In addition the effects offiber content on the tensile properties and microstructuresvia SEM have been analyzed The results of this study couldcontribute to engineering applications especially as soundabsorbers

2 Materials and Methods

21 Materials and Sample Preparation CHF is the mainraw material used in this study The fiber contains 4615cellulose 3379 hemicellulose and 392 lignin It has beentreated with 5 sodium hydroxide (NaOH) for 2 h Thescheme of reaction is given as follows

CHF minusOH + NaOH 997888rarr CHF minusO-Na+ +H2O (1)

Chemical reactions have been removing impurities on thefiber surface The CHF was rinsed five times with mineralwater in other to remove NaOH sticking from the fibersurfaces They were dried in natural sunlight to remove anyresidual moisture and were then preserved in a dry box witha humidity of 40 The chemical contents of treated CHFare 5437 cellulose 2237 hemicellulose and 564 ligninThe average of diameter of a single CHF is 0133 plusmn 003mmmeasured by a Mitutoyo digital micrometer

Theunsaturated polyester resin 2250 BW-EXhas a viscos-ity of 6ndash8 poise (25∘C) the tensile strength of 88 Kgmm2a tensile modulus of 500Kgmm2 the flexural strength of25 Kgmm2 and elongation of 23

The weight of polyester resin and CHFs were measuredbefore processing so as to determine the volume fractionof CHFs and polyester in the resulting composite Thecomposition of different sound absorbers is summarized inTable 1 The mixtures were hot pressed at 100∘C and 03MPafor 4min into a round mold with a diameter of 32mmfollowed by cooling to room temperature at 5MPa to obtaina round shape to fit in the impedance tube during the sound

Table 1 The composition of the composite (mean values in volumefraction)

Sample CHF () Polyester resin ()PF-E 20 80PF-G 40 60PF-H 50 50PF-I 60 40PF-K 70 30PF-M 80 20

absorption test All the absorbermaterials were obtainedwitha diameter of 29mm and thickness of 20mm Six differentsamples were used for acoustical and porosity tests

22 Porosity The connected porosity of composite samplewas nonacoustically measured using the method of watersaturation used by Vasina et al [14] All the samples weredried at 105∘C for 24 h Subsequently they were weighedbefore being left in a vacuum vessel to saturate under waterthe density of water is 120588

119908= 1000Kgsdotmminus3 After 24 h they

were carefully removed and weighed again The porosity wascomputed using 120576 = 119881

119886119881119904 where 119881

119886is the volume of the

composite occupied by the water and119881119904is the total volume of

the composite The volume of water can be computed using119881119908= (1198982minus1198981)120588119908 where119898

2and119898

1are the wet and the dry

masses of the composite (Kg) respectively

23 Air-Flow Resistivity There are several empirical andsemiempirical equations in the literature that can be used toestimate the flow resistivity of absorber materials based uponfiber radius and material porosity or the bulk density of thematerials [14ndash16] The air-flow resistivity of the samples usedin this study is expressed in [16]

119877 =68120583 (1 minus 120576)1296

11988621205763 (2)

where 120583 is the viscosity of air (184 times 10minus5 Pasdots) 120576 is theporosity and 119886 is the radius of the fiber

24 Tortuosity The following empirical formula was used tocalculate tortuosity (120590) in terms of porosity The tortuosity isexpressed in [5]

120590 = 1 +(1 minus 120576)

(2120576) (3)

25 Sound Absorption Measurement The acoustic proper-ties of the composite sample were measured using a two-microphone transfer-function method according to ASTME-1050-98ISO 10534-2 standards The testing apparatus waspart of complete acoustic material testing system Bruel ampKjaer (type 4206 Bruel amp Kjaeligr) as it is seen in Figure 1A small tube setup was employed to measure differentacoustical parameters in the frequency range of 100Hzndash64 kHz At one end of the tube a loudspeaker was situated

Advances in Acoustics and Vibration 3

Power amplifierAcoustic material test

Computer

Loudspeaker

Microphones

Sample

Incident signal

Reflected signal

(100Hzndash64kHz)Impedance tube kit

Figure 1 Impedance tube kit (type 4206 Bruel amp Kjaeligr)

to act as a sound source and the test material was placedat the other end to measure sound absorption propertiesTwo acoustic microphones (type 4187 Bruel amp Kjaeligr) werelocated in front of the sample to record the incident soundfrom the loudspeaker and the reflected sound from thematerial The recorded signals in the analyzer in terms of thetransfer function between the microphones were processedusing Bruel amp Kjaeligr material testing software to obtain theabsorption coefficient of the sample under test Each set of theexperiment was repeated three times in order to have averagemeasurements

26 Mechanical Properties The tensile and Youngrsquos modu-lus were determined using a Tensilon RTG-1310 universaltesting machine with a load cell of 10 kN All the samplesof composites were tested after conditioning for 24 h in astandard testing atmosphere of 70 relative humidity and28∘CAccording to theASTMD3039 standard a gauge lengthof 150mm and a crosshead speed of 5mmmin were usedfor tensile testing The sample size was 250mm times 254mmtimes 6mm In total 21 samples were tested for each samplecondition and the average and standard deviation values werereported

27 Scanning Electron Microscope The surface morphologiesof composites were observed using an Inspect-S50 scanningelectronmicroscope with field emission gun An acceleratingvoltage of 10 kV was used to collect SEM images on thesurface of the sample The morphologies of the compositeswere observed and analyzed via SEM at room temperatureBefore testing the samples were sliced and mounted ontoSEM stubs using double-sided adhesive tape They weregold sputtered for 5min to a thickness of approximately

10 nm under pressure of 01 torr and 18mA current to makethe sample conductive SEM micrographs were recorded atdifferent magnifications to ensure clear images

3 Results and Discussion

31 Nonacoustic Composites Properties Large differenceswere observed in nonacoustical properties of the compositesamples because of their different microstructures as aresult of the addition of the CHF in the polyester Thisdiversity is very interesting because it provides differentporous microstructures and consequently different acousticproperties Porosity tortuosity and flow resistivity values arelisted in Table 2

Increasing the amount of fiber volume fraction in thepolyester resin increases the porosity and decreases bothtortuosity and air-flow resistivity in the absorbent material(seen Table 2) The porosity value of the sample PF-M was08267 whichwas higher than those of the other samples usedin this study The presence of lumen in the fiber indicatesthat the porosity of the sample increases when the number offibers increases (Figure 2) In other words the lower value ofporosity and higher value of the flow resistivity of the samplecan be attributed to the higher volume fraction of polyesterresin

All the composite samples demonstrate an open porestructure wherein the pores are interconnected This is oneof the most important factors for noise absorption becausesuch a structure decreases air-flow resistivity and thus thedissipation of the wave energy in the pores In these samplesthe multiscale fiber structure with the lumen inside fiberbundle has a pore shape and the pore size can differ by severalorders of magnitude (Figures 2(a) and 2(b))


Table 2 Nonacoustical properties of samples

Sample Thickness (mm) Density (Kgsdotm3) Porosity120576

Air-flow resistivity R (Pasdotssdotmminus2) Tortuosity120590

PF-E 20 6405 06474 44980 1272PF-G 20 3834 07053 29353 1208PF-H 20 3041 07247 25424 1190PF-I 20 2441 07457 21576 1171PF-K 20 1980 07582 19568 1160PF-M 20 1583 07954 14435 1128

(a) (b)

Figure 2 SEM photomicrographs of corn husk fibers 5 NaOH treated (a) surface and (b) cross-sectional features

PF-M

PF-H PF-I

PF-K

Polyester

PF-E

PF-G

1000 2000 3000 4000 5000 60000Frequency (Hz)

00

02

04

06

08

10

Soun

d ab

sorp

tion

coeffi

cien

t

Figure 3 The sound absorption coefficients of composite samples

32 Acoustical Properties The normal sound absorptionproperties for all samples of CHF-polyester composites aregraphically illustrated in Figure 3 The zero value in the 119910-axis indicates perfect sound reflection and the value of oneimplies complete sound absorption These results show thatall composite samples demonstrated an increase in soundabsorption coefficient in the range of 1 kHzndash25 kHz This isbecause lumen inside the fiber bundle increases the amount

of fiber which results in high absorption coefficient Theadditional thermal energy is dissipated more rapidly due tothe increased frictional surface area The sound absorptioncoefficient of the PF-M sample is therefore correspondinglyhigher than those of the other samples The sound wavespropagate vibration energy through the air spaces in theindividual lumina inside the fiber A part of this sound energyis converted into heat in the lumina which is then absorbedby the surrounding walls The larger the air cavities andlumina inside the fiber the longer the wavelength of thesound that is absorbed somore dominant at low frequenciesSEM micrograph analysis (Figures 6(a) 6(c) 6(e) 6(g) 6(i)and 6(k)) illustrates that there are many lumens inside thefiber and continuous channels in the porous structure ofpolyester composites

At frequencies above 2 kHz the sound absorption capa-bility of PF-E PF-G PF-I and PF-K samples decreasesThe decrease caused by the interface of the fiberresin andorderly fiber arrangement that cause the higher value ofthe flow resistivity of the sample makes movements of thesound difficult to pass through the samples An absorptioncoefficient is close to zero when the fibers are arranged in aconventional pattern SEM micrographs (Figure 6) illustratethat there is a distinct interface between fibers and resin inall samples Interface surfaces between fibers and resin of PF-E PF-G PF-I and PF-K samples (Figures 6(b) 6(d) 6(h)


PF-GPolyester

PF-E

PF-I

PF-K

PF-HPF-M

1000 2000 3000 4000 5000 60000Frequency (Hz)

05

101520253035404550

Real

par

t of i

mpe

danc

e rat

io

Figure 4 The real part of the impedance ratio of different samples

and 6(j) resp) are quite dense and contain orderly fiberbundles arrangement Interface strength not only influencescomposite mechanical property but also influences soundabsorption quality When sufficient amount of resin is usedthe interface area between the fibers and the resin is smoothand strong (Figures 6(a) and 6(b)) When the incidentsound waves are continuously transmitted onto a compositeinterface the sound waves will be reflected and refractedand the acoustic damping phenomena will consume a smallamount of energy reducing heat losses and thus obtaining alower absorption coefficient at high frequencies This wouldalso explain why the sound absorption of PF-E is lower thanthose of sample patterns of composites which are similar

Sample pure polyester resin (PE) had the absorption coef-ficient under 02 Althoughpolyestermay be a valuable optionin noise absorption applications these results discourage itsuse as a sound-absorbing material

Figure 3 also shows that the PF-H and PF-M samplesdemonstrated the ability to absorb sound at high frequenciesabove 4 kHz This is due to the random distribution of thefiber The random distribution of the fibers in the fibrousabsorbent materials allows the sound waves to hit the lumenof the fiber bundle and strengthen the sound absorptioneffect a high absorption coefficient can be obtained SEMmicrographs (Figures 6(e) 6(f) 6(k) and 6(l)) illustrate therandom distribution of the fibers in PF-H and PF-M samples

Figures 4 and 5 show that the real part is the resistanceassociated with energy losses and the imaginary part is thereactance associated with phase changes respectively In thiscase we can observe a better performance of PF-H and PF-Msamples than other materials studied Increasing the amountof fiber reduces the number of impedance values andmaterialstiffnessThe reduced impedance values increase the fractionof wave energy that can be transmitted through the sample

Furthermore sound absorption at lower frequencies(over 10ndash2 kHz) is desirable for automotive applicationsbecause of this frequency range according to noise from thewind engine running tires road and conversation therebymaking CHF-polyester composites a promising candidate forautomotive interior sound absorption

PF-G

Polyester

PF-E

PF-H

PF-M

PF-I

PF-K

1000 2000 3000 4000 5000 60000Frequency (Hz)

minus20

minus10

0

10

20

30

Imag

inar

y pa

rt o

f im

peda

nce r

atio

Figure 5 The imaginary part of the impedance ratio of samples

33 Mechanical Properties Theoretically there should be aninteraction between hydrophobic polyester and hydrophiliccellulose The disappearance of the noncellulose material onthe surface of the fiber enables surface interaction with thepolyester matrix The void fraction is mainly formed becausethe composites have not been consolidated (not sufficientlypressed to form a contiguous solid structure) in order tomanufacture composites

Figures 7 and 8 show that the increase in the fibervolume fraction leads to increase in the tensile strength valueand Youngrsquos modulus of the composite from 1881 plusmn 85to 2573 plusmn 319MPa The increase in mechanical strengthcan be attributed to the bond interface between the fibersand resin The mechanical properties of the PF-M sample(or composite sample with 20 resin and 80 CHF) aretherefore correspondingly higher than those of the othersamples

For PF-H sample there was a 1253 decrease in thetensile strength values with a strength value of 2040 plusmn11MPa The probability of the overlapping of multiple CHFin composite samples thereby causes the weaker transferenceof load between fiber and matrix occurring due to thepoor interfacial adhesion causing lowering in the mechanicalproperties However the value of the modulus of elasticityof the sample PF-H is higher than that of the material usedin this study contributing to the sound absorption of thesample

The tensile strength value of the PF-E sample is the lowercompared to other samples This is due to the fiber volumefraction less than the other samples The tensile strength ofthe fiber of 23743MPa is higher than the tensile strength ofthe resin

4 Conclusions

In this study a CHF-polyester sound absorber was proposedand the sound absorption capability of the material wassignificantly enhanced through the simple method Thepresence of a number of lumen structures in the fiber bundlefacilitates sound absorption at low frequencies in the range


(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)

Figure 6 Scanning electron microscope (SEM) images of surfaces of the samples and cross-sectional features of composite samples (a b)PF-E (c d) PF-G (e f) PF-H (g h) PF-I (i j) PF-K and (k l) PF-M


PF-IPF-E PF-G PF-H PF-K PF-M000

500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573

Figure 7 Tensile strength of each sample

PF-IPF-E PF-G PF-H PF-K PF-MSamples

0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299

Figure 8 Modulus of elasticity of each sample

of 1 kHzndash2 kHz The interface between the surface of thefiberresin and orderly arrangement of fibers within the resinof PF-E PF-G PF-I and PF-K samples caused a decrease inthe sound absorption properties at frequencies above 2 kHzHigh frequencies above 4 kHz (PF-H and P F-M samples) areobtained due to the random distribution of the fiber

Increased resin lowers friction between the fibers reduc-ing heat losses and subsequently its sound absorption coeffi-cient

All samples used in this study have the potential to beused as sound-absorbing materials These results indicatethat alternative high-performance sound-absorbing materi-als could be obtained using CHF which can solve environ-mental problems and reduce noise pollution

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] M A Biot ldquoTheory of propagation of elastic waves in a fluid-saturated porous solid I Low-frequency rangerdquoThe Journal oftheAcoustical Society of America vol 28 no 2 pp 168ndash178 1956

[2] M A Biot ldquoTheory of propagation of elastic waves in a fluid-saturated porous solid II Higher frequency rangerdquoThe Journalof the Acoustical Society of America vol 28 no 2 pp 179ndash1911956

[3] M E Delany and E N Bazley ldquoAcoustical properties of fibrousabsorbentmaterialsrdquoApplied Acoustics vol 3 no 2 pp 105ndash1161969

[4] F-C Lee and W-H Chen ldquoAcoustic transmission analysis ofmulti-layer absorbersrdquo Journal of Sound and Vibration vol 248no 4 pp 621ndash634 2001

[5] K Attenborough ldquoAcoustical characteristics of rigid fibrousabsorbents and granularmaterialsrdquoThe Journal of the AcousticalSociety of America vol 73 no 3 pp 785ndash799 1983

[6] A Veerakumar and N Selvakumar ldquoA preliminary investiga-tion on kapokpolypropylene nonwoven composite for soundabsorptionrdquo Indian Journal of Fibre and Textile Research vol 37no 4 pp 385ndash388 2012

[7] M Jailani M Nor and R Zulkifli ldquoEffect of compression onthe acoustic absorption of coir fiberrdquoAmerica Journal of AppliedSciences vol 7 no 9 pp 1285ndash1290 2010

[8] R Zulkifli Zulkarnain and M J M Nor ldquoNoise control usingcoconut coir fiber sound absorber with porous layer backingand perforated panelrdquoAmerican Journal of Applied Sciences vol7 no 2 pp 260ndash264 2010

[9] D Chen J Li and J Ren ldquoStudy on sound absorption propertyof ramie fiber reinforced poly(L-lactic acid) composites mor-phology and propertiesrdquoComposites Part A Applied Science andManufacturing vol 41 no 8 pp 1012ndash1018 2010

[10] A Putra Y Abdullah H Efendy W M F W Mohamad andN L Salleh ldquoBiomass from paddy waste fibers as sustainableacoustic materialrdquo Advances in Acoustics and Vibration vol2013 Article ID 605932 7 pages 2013

[11] L P Bastos G D S V De Melo and N S Soeiro ldquoPanelsmanufactured from vegetable fibers an alternative approachfor controlling noises in indoor environmentsrdquo Advances inAcoustics and Vibration vol 2012 Article ID 698737 9 pages2012

[12] T Koizumi N Tsujiuchi and A Adachi ldquoThe development ofsound absorbing materials using natural bamboo fibersrdquo HighPerformance Structures and Materials vol 4 pp 157ndash166 2002

[13] E Jayamani and SHamdan ldquoSound absorption coefficients nat-ural fibre reinforced compositesrdquo Advanced Materials Researchvol 701 pp 53ndash58 2013

[14] M Vasina D C Hughes K V Horoshenkov and L LapcıkJr ldquoThe acoustical properties of consolidated expanded claygranulatesrdquo Applied Acoustics vol 67 no 8 pp 787ndash796 2006

[15] F P Mechel Formulas of Acoustics Springer Berlin Germany2nd edition 2008

[16] R Maderuelo-Sanz A V Nadal-Gisbert J E Crespo-Amorosand F Parres-Garcıa ldquoA novel sound absorber with recycledfibers coming from end of life tires (ELTs)rdquo Applied Acousticsvol 73 no 4 pp 402ndash408 2012

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Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpswwwhindawicom

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



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Journal of

Advances inOptoElectronics


Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

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Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks



acoustic material and found that the absorption coefficientis greater than 05 from 1 kHz and can reach the averagevalue of 08 above 25 kHz Bastos et al [11] developedvegetable fibers coconut palm sisal and acaı as sound-absorbing panels Measurement scale reverberation chamberexposed promising results from acoustic performance forall panels Flammability odor fungal growth and agingtests have been performed on samples to identify theirpractical ability Koizumi et al [12] developed bamboo fiberas sound-absorbing material They reported that the bamboofiber material has equivalent acoustics properties with glasswool Jayamani and Hamdan [13] studied sound absorp-tion coefficient of urea-formaldehyde and polypropylenemixed with kenaf fiber They reported that the kenaf fiberreinforced with polypropylene demonstrates higher soundabsorption coefficients than kenaf fiber reinforced with urea-formaldehyde These previous studies represented that abetter understanding of the microstructure and physicalparameters of a material could help in developing high-performance acoustic materials

This study primarily investigates the effect of addingcorn husk fibers (CHFs) on acoustical and nonacousticsproperties of polyester composites In addition the effects offiber content on the tensile properties and microstructuresvia SEM have been analyzed The results of this study couldcontribute to engineering applications especially as soundabsorbers

2 Materials and Methods

21 Materials and Sample Preparation CHF is the mainraw material used in this study The fiber contains 4615cellulose 3379 hemicellulose and 392 lignin It has beentreated with 5 sodium hydroxide (NaOH) for 2 h Thescheme of reaction is given as follows

CHF minusOH + NaOH 997888rarr CHF minusO-Na+ +H2O (1)

Chemical reactions have been removing impurities on thefiber surface The CHF was rinsed five times with mineralwater in other to remove NaOH sticking from the fibersurfaces They were dried in natural sunlight to remove anyresidual moisture and were then preserved in a dry box witha humidity of 40 The chemical contents of treated CHFare 5437 cellulose 2237 hemicellulose and 564 ligninThe average of diameter of a single CHF is 0133 plusmn 003mmmeasured by a Mitutoyo digital micrometer

Theunsaturated polyester resin 2250 BW-EXhas a viscos-ity of 6ndash8 poise (25∘C) the tensile strength of 88 Kgmm2a tensile modulus of 500Kgmm2 the flexural strength of25 Kgmm2 and elongation of 23

The weight of polyester resin and CHFs were measuredbefore processing so as to determine the volume fractionof CHFs and polyester in the resulting composite Thecomposition of different sound absorbers is summarized inTable 1 The mixtures were hot pressed at 100∘C and 03MPafor 4min into a round mold with a diameter of 32mmfollowed by cooling to room temperature at 5MPa to obtaina round shape to fit in the impedance tube during the sound

Table 1 The composition of the composite (mean values in volumefraction)

Sample CHF () Polyester resin ()PF-E 20 80PF-G 40 60PF-H 50 50PF-I 60 40PF-K 70 30PF-M 80 20

absorption test All the absorbermaterials were obtainedwitha diameter of 29mm and thickness of 20mm Six differentsamples were used for acoustical and porosity tests

22 Porosity The connected porosity of composite samplewas nonacoustically measured using the method of watersaturation used by Vasina et al [14] All the samples weredried at 105∘C for 24 h Subsequently they were weighedbefore being left in a vacuum vessel to saturate under waterthe density of water is 120588

119908= 1000Kgsdotmminus3 After 24 h they

were carefully removed and weighed again The porosity wascomputed using 120576 = 119881

119886119881119904 where 119881

119886is the volume of the

composite occupied by the water and119881119904is the total volume of

the composite The volume of water can be computed using119881119908= (1198982minus1198981)120588119908 where119898

2and119898

1are the wet and the dry

masses of the composite (Kg) respectively

23 Air-Flow Resistivity There are several empirical andsemiempirical equations in the literature that can be used toestimate the flow resistivity of absorber materials based uponfiber radius and material porosity or the bulk density of thematerials [14ndash16] The air-flow resistivity of the samples usedin this study is expressed in [16]

119877 =68120583 (1 minus 120576)1296

11988621205763 (2)

where 120583 is the viscosity of air (184 times 10minus5 Pasdots) 120576 is theporosity and 119886 is the radius of the fiber

24 Tortuosity The following empirical formula was used tocalculate tortuosity (120590) in terms of porosity The tortuosity isexpressed in [5]

120590 = 1 +(1 minus 120576)

(2120576) (3)

25 Sound Absorption Measurement The acoustic proper-ties of the composite sample were measured using a two-microphone transfer-function method according to ASTME-1050-98ISO 10534-2 standards The testing apparatus waspart of complete acoustic material testing system Bruel ampKjaer (type 4206 Bruel amp Kjaeligr) as it is seen in Figure 1A small tube setup was employed to measure differentacoustical parameters in the frequency range of 100Hzndash64 kHz At one end of the tube a loudspeaker was situated



Computer

Loudspeaker

Microphones

Sample

Incident signal

Reflected signal
















(a) (b)


PF-M

PF-H PF-I

PF-K

Polyester

PF-E

PF-G

1000 2000 3000 4000 5000 60000Frequency (Hz)

00

02

04

06

08

10

Soun

d ab

sorp

tion

coeffi

cien

t






PF-GPolyester

PF-E

PF-I

PF-K

PF-HPF-M

1000 2000 3000 4000 5000 60000Frequency (Hz)

05

101520253035404550

Real

par

t of i

mpe

danc

e rat

io







PF-G

Polyester

PF-E

PF-H

PF-M

PF-I

PF-K

1000 2000 3000 4000 5000 60000Frequency (Hz)

minus20

minus10

0

10

20

30

Imag

inar

y pa

rt o

f im

peda

nce r

atio






4 Conclusions



(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)




500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573



0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299





Competing Interests


References



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











Computer

Loudspeaker

Microphones

Sample

Incident signal

Reflected signal
















(a) (b)


PF-M

PF-H PF-I

PF-K

Polyester

PF-E

PF-G

1000 2000 3000 4000 5000 60000Frequency (Hz)

00

02

04

06

08

10

Soun

d ab

sorp

tion

coeffi

cien

t






PF-GPolyester

PF-E

PF-I

PF-K

PF-HPF-M

1000 2000 3000 4000 5000 60000Frequency (Hz)

05

101520253035404550

Real

par

t of i

mpe

danc

e rat

io







PF-G

Polyester

PF-E

PF-H

PF-M

PF-I

PF-K

1000 2000 3000 4000 5000 60000Frequency (Hz)

minus20

minus10

0

10

20

30

Imag

inar

y pa

rt o

f im

peda

nce r

atio






4 Conclusions



(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)




500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573



0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299





Competing Interests


References



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation














(a) (b)


PF-M

PF-H PF-I

PF-K

Polyester

PF-E

PF-G

1000 2000 3000 4000 5000 60000Frequency (Hz)

00

02

04

06

08

10

Soun

d ab

sorp

tion

coeffi

cien

t






PF-GPolyester

PF-E

PF-I

PF-K

PF-HPF-M

1000 2000 3000 4000 5000 60000Frequency (Hz)

05

101520253035404550

Real

par

t of i

mpe

danc

e rat

io







PF-G

Polyester

PF-E

PF-H

PF-M

PF-I

PF-K

1000 2000 3000 4000 5000 60000Frequency (Hz)

minus20

minus10

0

10

20

30

Imag

inar

y pa

rt o

f im

peda

nce r

atio






4 Conclusions



(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)




500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573



0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299





Competing Interests


References



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










PF-GPolyester

PF-E

PF-I

PF-K

PF-HPF-M

1000 2000 3000 4000 5000 60000Frequency (Hz)

05

101520253035404550

Real

par

t of i

mpe

danc

e rat

io







PF-G

Polyester

PF-E

PF-H

PF-M

PF-I

PF-K

1000 2000 3000 4000 5000 60000Frequency (Hz)

minus20

minus10

0

10

20

30

Imag

inar

y pa

rt o

f im

peda

nce r

atio






4 Conclusions



(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)




500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573



0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299





Competing Interests


References



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)




500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573



0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299





Competing Interests


References



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











500

1000

1500

2000

2500

3000

3500

Tens

ile st

reng

th (M

Pa)

Samples

1881

22952040

2368 2323 2573



0

500

1000

1500

2000

2500

Mod

ulus

of E

lasti

city

(MPa

)

123149969

16957

1142913775 13299





Competing Interests


References



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









corn husk fiber-polyester composites as sound absorber...

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