Effect of Interfacial Roughness on Mechanical and Thermo-Acoustic Behavior of Corn Husk Fiber

The designing of acoustic material with high efficiency absorption is cutting edge research for acoustician as well as architectures’ in the acoustic indus- tries attracts the material scientist due to its numerous significant character-istic properties. Nondestructive technique such as ultrasonic processing is employed for surface modification of corn husk which changes the interfacial as well as skeletal arrangement in interlocking of fibers with polymer chain. Tensile strength of single corn husk fiber before and after surface treatment was observed to be increasing from 332.57 MPa to 345.16 MPa, which confirms the strong fibrillation due to surface treatment. Further the hardness of the fabricated corn husk composite was found to be 23HV as observed in three different places. Thermal conductivity of the samples increases with temperature supporting the validation of the sample for acoustic application. The high sound absorption performance of the composite classified the material as Class-A type with 0.94 absorption coefficient supported by the different characterization and surface analysis of the composite.


Introduction
The importance on development of quieter technology and acoustic material was highlighted considering the noise pollution as fourth major pollution in the world.Thus recent development in acoustic material has gained its importance in the different domain such as its fabrication, design and selection of materials which can have high acoustic value (Jezdović et al. 2021).The fiber-based acoustic materials are found to be lighter, easier to handle and many interesting properties for which industries and researchers have concentrate the works on it for further development.Although there are several biomaterials accessible in large quantities, none of them have good acoustic properties, such as corn husk (Kambli et al. 2018).Corn husks, which are lignocelluloses, are often discarded or utilized as compost fertilizer since the major component of the corn that is processed is either the kernel alone or the kernel with the cob.After a vigilant literature review on fabrication techniques and manufacture of acoustic building materials from biodegradable materials, it was observed that, a few researchers (Tam and Denvid 2014) have worked on synthesis of composite from corn husk which are compatible with reduction of intensity of noise in an open environment.Berliandika, Yahya and Ubaidillah (2019) have studied the acoustic property of corn husk composite with different mass % and different thickness and found better acoustic absorption in untreated one than in treated one.Lyu et al. (2020) fabricated multilayer composite using corn husk and polylactic acid and investigated the acoustic property which was found to be 1.Sarwati and Mohamed (2021) have investigated the mechanical performance of corn husk fiber bonded with polyethylene matrix with different filler %.Natural fiber reinforced composites have unique characteristics of fiber bridging action that occurs due to formation of multiple cracks during fiber matrix bonding.Due to the presence of polar-OH groups in natural fibers, the moisture absorbing capacity is high which leads to formation of weak interaction between fiber and resin.So, there is a need to increase the binding between the fibers and polymer matrix.To improve interfacial characteristics of the fiber as well as matrix, the surface treatments approach has been used (Guday et al. 2020).The interfacial behavior of a corn husk composite is investigated using a variety of physical and mechanical parameters.High mechanical strength, complex networking structure, renewability, cost efficiency and biodegradable make the corn husk composite a distinguishing bio material for the fabrication of composite compared to other natural fiber.The present study focuses on fabrication of acoustic material based on reinforcement of corn husk with epoxy polymer modified by ethanol blended acetone as surfactants which can be used as a potential sound absorber.The fabrication of corn husk composite involves green method with cost effective and very less energy consumption.

Materials
The biomaterial in consideration is corn residue i.e. husk of corn which was gathered from market.Extracted corn husk was washed with water and dried under sun.Now the dried husk is ready to be used as reinforcement.Figure 1(a) shows corn husks of fresh corn cob.For treatment of corn husks, a mixture of acetone and ethanol was considered.For matrix material blends of epoxy -LY 556 and hardener HY-951 was considered.For fabrication of composite an aluminum mold of 15 × 15 cm was used.C-clamps were used for tightening the mold.

Fiber extraction process
Corn husks were collected and washed in water to enable microbial decomposition and easy fiber extraction.Corn husk was combed to extract the fibers from husk.The recovered fibers were carefully rinsed with regular water and dried in oven at 80°C to eliminate extra moisture.Furthermore, the dried fiber was cut to 15 cm lengths using scissors, as illustrated in Figure 1(b).

Surface treatment process
Surface modification of corn husks was accomplished using appropriate optimal surfactant solutions.The main advantage of ethyl alcohol and acetone (E-A) treatment is that it decreases the rate of moisture absorption in natural fibers as well as decreases the fiber diameter.It also increases the interlocking between interface of fiber and epoxy more efficiently as compared to the other treatments.The suitability of (E-A) was assessed by computing the ultrasonic compressibility in various mole fractions based on ultrasonic velocity.From the data, it was found that 0.4 molar concentration of E-A is suitable for treatment of fibers (Ibrahim et al. 2020).The mixture of E-A was kept in sonicator for 1 hour for proper blending of the surfactants.Density of E-A was determined with pycnometer with accuracy ±0.0001 kg/m 3 .Corn husks of size 15 cm were soaked in E-A solution for 12 hours for Then the corn husks are dried in room temperature for one day post treatment.Equations 1 and 2 depict the reaction of acetone and ethanol on fiber surface.

Main reaction:
Side reaction:

Process of fabrication
For the curing process, the treated fibers were kept inside hot air oven at 60°C.To prepare matrix, Epoxy polymer of LY 556 grade and hardener HY-991 grades were taken in 10:1 ratio were stirred with mechanical stirrer till smooth colorless mixture with no bubbles is formed.Corn fibers were placed longitudinally inside the mold.The matrix was poured on the fibers and covered with an aluminum sheet.Four C-clamps were put in each side of the mold to tighten up the mold to make it air proof.The mixture was left to dry for 24 hours.The dried composite was detached from the mold and cut with different shapes as required for different testing.Figure 2 illustrates the overall process of fabrication of corn composite in a single picture.

Measurement of thermal conductivity
Thermal analyzer from METER Group, inc., USA was considered for measurement of thermal conductivity of the fabricated corn fiber composite.A rectangular corn husk composite was taken and three regions were located to make drilled holes for insertion of sensor tip as shown in Figure 3. Thermal grease was used inside the hole to improve contact with testing sample.Single needle TR-3 sensor of 24 mm in diameter and 100 mm in length was inserted into the material for readings.Infinite line heat pulse (ILHP) technique was used for determination of thermal conductivity.Figure 3 presents portable thermal conductivity testing arrangement of corn husk composite.

Characterization techniques
A HITACHI SU 3500 scanning electron microscope (SEM) set to a voltage of 5kV was used to study the anatomical changes occurring during the various stages of processing corn husk composite.The samples of the composite were scratched from its surface and placed over an aluminum specimen holder attached with gold-coated adhesive tape.The composite's elementary makeup was investigated with Energy Dispersive X-Ray Spectroscopy (EDS).Further, (Bruker Alpha-II) Fourier Transform Infrared Spectrometer (FTIR) was used in the transmittance mode, to get the information about the presence of functional groups in the treated and untreated fiber varying between 4000 cm −1 -500 cm −1 .Analysis of surface roughness was performed using Mitutoyo Surface Tester (Model-SJ-210).Average roughness (Ra) and (Rz) were measured according to ISO 1997 standard.A stylus of size 5 µm was used to trace the surface of the sample in three different places and the average of the surface roughness was taken into account.

Laboratory arrangement for sound absorption
Sound absorption coefficients (SAC) of fabricated composite are measured using an advance impedance tube from HOLMARCas per ISO-105342 standardization.Transfer-function-based measurement is done using a 50 mm inner diameter anodized tube made up of aluminum with a sound source at one end and a backing screw at another end.Two microphones separated by a specific distance were connected to a digital signal analyzer and data gathering system with aid of signal conditioner.The measurement is done within 1/3 octaves frequency range of 500-3150 Hz using HOLMARC Wave analyzer 4C software as shown in Figure 4.

Mechanical property
Single-fiber tensile test of untreated and treated corn husk fiber has been done using an INSTRON 3382 UTM machine with a 1Newton load cell at a controlled atmosphere of 24°C temperature and 55% relative humidity.60 mm gauge length samples were prepared and placed within the sample holder and test is performed at a 1 mm per minute cross head speed as per ASTM D -337975 standard.For both untreated and treated fibers, twelve samples are tested for each category and average value were estimated for analysis.Figure 5 shows the experimental setup for single fiber tensile test.The micro hardness of corn husk composite was measured using Viker's hardness testing machine (VH3300) according to ASTME 384 standard.Each test was conducted at three different positions and the average value was considered as described in Figure 5.

Result and discussion
The SEM image of both untreated and treated fiber with composite has been shown in Figure 6(a-e).
The figures shows the transformations occurring in raw fiber due to action of E-A which is the cause for interfacial bonding between fiber and matrix for fabrication of composite.Figure 6(a,b) shows the longitudinal as well as cross sectional view of the untreated fiber where presence of impurities and lignocellulosic masses like lignin, hemicellulose and cellulose are clearly observed.Figure 6(c,d) clearly shows the SEM of E-A treated corn fiber.Here, it is clearly seen that with the action of E-A on fiber surface most of the lignocellulosic mass are removed so as the fibrils are clearly visible.Pores and shallow grooves appear on the surface due to the action of surfactants.These void places on the fiber

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enhance the interfacial bonding of epoxy polymer with fiber surface thus forming a strong composite.
Figure 6(e) shows the SEM of composite where pore are clearly observed which influence the absorptive property of the composite.
For EDS of corn fiber composite, its dust form was considered.Figure 7 shows the elemental composition of composite with their atomic weight and peaks.According to the EDS graph, the major components of the composite constitutes carbon and oxygen.While compared to other natural fiber composites, the fabricated corn composite has 60.60% of carbon which is quite unusual.Cellulosic parts makes the composite brittle so the presence of carbon is might be due to the chemical treatment which leaves behind various non-cellulosic components (Yilmaz 2013).It is also seen from the EDS profile that with increase in carbon % the weight % of oxygen decreases.This may be due to the removal of lingo cellulosic biomass from the fiber surface due to the action of ethyl alcohol.Trace levels of silica has been found in the EDS profile indicating the improved stiffness of the composite due to the enhanced epoxy and fiber adhesion.The presence of silica is also responsible for the enhanced tensile property of the composite (Mohanta and Acharya 2015).
FTIR spectroscopy was used to detect the presence of functional groups in the powder sample of corn fiber composite, as shown in Figure 8(a,b).The O-H stretching is confirmed by a broad band of transmittance peaks from 2925 cm −1 to 1640.49cm −1 .The absence of the spectra from 1140.7 cm −1 to 1640.5 cm −1 in the treated composite clearly indicates that lignin has been removed completely from the fiber surface (Garside and Wyeth 2003).Existence of absorption spectra 1424.7 cm −1 indicates the presence of carboxyl groups as salts for which presence of Na was detected in the EDS.A change in band position can be seen in the spectra of both treated and untreated composite materials.This change in spectrum is caused by an increase in intermolecular interactions between the hydroxyl ions, which is responsible for the enhanced compatibility and stronger binding of fiber to matrix (Tang et al. 2018).FTIR confirms the loss of OH ions from the fiber surface which is clearly shown in Figure 8. Removal of hydroxyl ions increases the hydrophobicity of the fiber creating many active sites on the fiber bed.Therefore, formations of rough surface due to the chemical deformations are confirmed from the FTIR graph.The development of elastic structures and porosity, which were validated by the SEM image, contribute to the composite's acoustic performance.
Surface roughness was measured using a contact based stylus instrument.Figure 9 shows the surface profilometer used for measurement of surface roughness.The tip of stylus is 5 µm in radius.Λc

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(wavelength between upper cutoff) and λs (wavelength between lower cutoff) values were 2.5 mm and 8 mm, respectively.The tip speed of stylus was set to 0.25 mm/sec.The stylus tip was made to move vertically over the surface of the composite which generated electrical signals and undergoes amplification to measure the roughness parameters like R a , R z , R q .The formula for measurement of surface roughness include

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Where, R a is the roughness average of the profile obtained using profilometer and Y i is the difference between the highest and lowest peak of the average line.Roughness average of the composite was found to be 5.155 µm.
Here, Δi j j is the deviation in the slope.The root mean square average from the surface of corn husk composite was found to be 39.358 µm.R Z is the difference between the highest and lowest peak in the profile.The R z value for corn husk composite was found to be 7.962 µm.The increased rate of stylus generates heat between the tip and the surface hence the wear value increases and in turn the roughness value also increases.Increase in tip rate often causes chatter, resulting in unfinished processing at a greater traverse speed, resulting in increased surface roughness.Furthermore, deep fiber pull-outs, is also a cause for greater roughness value of the composite.As a result, the interfacial region expands and interlocking between the fiber and the polymer matrix improves (Shaohua and Chen 2013), resulting in a high bond strength.Interface roughness is significant in the strengthening of interfacial adhesions.
Surface modification brings out significant rise in tensile stress and elastic modulus of single corn fiber (Fidelis et al. 2013).The chemical structure of corn husk fiber and epoxy resin are dissimilar, so strong interfacial adhesion is needed for stronger tensile strength of composite and to keep the epoxy resin and fibers intact in place at all points.Figure 10(a,b) presents the results of single corn husk fiber tensile test.Increment in both the elastic moduli and the strength of corn fiber are seen when they are treated with the 0.4 mole concentration of acetone and ethanol.It was noticed that the tensile strength of treated corn husk fiber rises to 345.16 MPa from 332.57MPa strength of untreated fibers.The elastic modulus of untreated corn fiber was 17.02235 GPa, but after treatment it raised to 17.958 GPa.The elastic modulus of all samples that were treated with alcohol got better because amorphous parts were stripped away.The major constituent of epoxy polymer is the secondary hydroxyl group which reacts with the amine group of hardener to form three dimensional rigid networking structures to provide a good binding with the fiber surface (Garadimani, Raju and Kodancha 2015).The average diameter of untreated corn husk fiber reduces from 0.2 mm to 0.12 mm after treatment with alcohol is shown in the Figure 10(c).The reduction of diameter is due to the alcoholic surface modification causing removal of cellulose, lignin, and hemicellulose from fiber surface, which enhances the stiffness of the corn husk fibers (Herlina et al. 2017).Surface treatment decreases the cross sectional area of the fiber and the surface roughness increases, which might also improve the composite's tensile strength.
For hardness test, the corn husk composite was cut into a square shape of size 5 cm with thickness of 3 cm according to ASTM E384 standard.The indentor was provided a constant load of 0.8 Kgf and dwelling time as 10 sec. Figure 11(a) shows the image of the indentor impression.The micro harness was measured with the formula Here, F is the applied force on the indentor, L is the diagonal of the indentor impression in cm, X is the horizontal length in cm and Y is the vertical length incm (Dhal and Mishra 2013).Hardness testing is done at three different positions to get accuracy in hardness number as shown in Figure 11(b).Table 1 shows the parameters related to hardness value.The average hardness of corn husk composite was found to be 23HV.This value of hardness can be considered to explain the porous nature of the composite as porosity is inversely proportional to hardness of any material.This occurs primarily due to the collapsing of pores under the load.From Figure 11(a), it can be clearly observed that the composite surface is filled with large numbers of pores which confirms that hardness has a good involvement in increasing the acoustic behavior of the material.Figure 11(c) shows the regression of hardness test which reveals the R 2 value as 0.8929.As the R 2 value is quite nearer to 1, it can be said that the hardness value of the material is quite nearer to the theoretical value.
Impedance tube method has been implemented to measure the sound absorption coefficient (SAC) of the corn fiber composite.The graph in Figure 12 shows that the range of frequency has been considered from 500 Hz to 3150 Hz.From the figure, it is clearly seen that with increase in frequency of sound the composite's SAC also increases.At lower frequency (500 Hz), absorption is much more less in comparison to higher frequency range (Singh and Nath 2021a).The sound reflection of the treated composite is reduced as lignocellulosic content is removed and carbon and oxygen content is increased, resulting in greater sound absorption (Chen, Li, and Ren 2010).Yet, the SAC varies slightly due to the effect of physical property like density, tortuosity, flow resistivity and porosity.When incident high frequency sound strikes the pores in the composite, air molecules in vacant areas vibrate spontaneously.The radiated heat energy caused by the loss of vibrational energy inside the composite decreases the material's tortuosity (Singh and Nath 2021b).The corn fiber blended with E-A decreases the thickness of the fibers, increasing the airflow resistance within the void spaces.The density of fibrils rises as the diameter of the fibers decreases, thus increasing the porosity of the composite.The porosity of the fiber was improved after E-A treatment, as a result of which the tortuosity is reduced and the flow resistivity is increased.Decrease in tortuosity results in formation of shorter air channels, thus increases the airflow resistance and therefore increases sound absorption (Sarangi et al. 2014).When compared to untreated fiber, the treated composite exhibits improved sound absorption coefficient, increasing from 0.86 to 0.94.,With increase in frequency, the sound wavelength drops to 1/3 octave wave resulting in superimpose of incident sound and reflected sound inside the voids.This enhances a large sound absorption through the fabricated corn composite (Mishra and Nath 2019).According to international SREN ISO 1165-2002 standard, the fabricated corn husk composite can be accepted as Class -A sound absorber (Singh and Nath 2021b).
Figure 13 presents temperature dependence thermal conductivity value of both treated and untreated corn husk fiber reinforced epoxy composites.The value of thermal conductivity rises from 0.0625W/mK to 0.0689 W/mK in treated composites.Figure 13 inferred that at 46°C maximum heat flux transferred through the composite as after the maximum temperature thermal conductivity starts decreasing.As the temperature increases, vibration of solid molecules increases which balances the scattering and propagation of phonons.Thermal conductivity reduces as the temperature inside the pores increases progressively (Rojas et al. 2019).In current study, epoxy polymer blended with corn As a response, the heat energy passes through the composite poorly which result in bad conduction of heat (Mendes et al. 2015).So the fabricated composite works as a good heat insulator after 46°C.

Conclusion
The low cost, green processing and high efficient acoustic material was synthesized from the carbon rich corn husk bio materials.Ultrasonic technique for compatibility and surface treatment was employed to increase the durability, surface roughness and bond strength of corn husk fiber.The FTIR spectral analysis confirms the different mechanical and shielding performance for sound and heat of the composite.Ductility and yield strength of the fiber has been studied using destructive tensile strength test indicate the deformation in cellulose as well as anti cellulose.From the analysis of thermal properties of the composite, it is concluded that the bio composite has ability to absorb heat due to random distribution of fibers and interfacial adhesion of the fiber-polymer matrix, which makes the composite an insulator.The optimum sound absorption coefficient was found to be 0.94 at a frequency of 3000 Hz, which is classified as Class-A-type acoustic shielding material as per the international SREN ISO 1165-2002 standard.The synthesized acoustic material may be proposed for its commercial and industrial application particularly as a noise absorber in in different home appliances, machineries, automobile parts.

Figure 2 .
Figure 2. Preparation process of corn husk composite.

Figure 4 .
Figure 4. Experimental setup of sound absorption test using impedance tube.

Figure 5 .Figure 6 .
Figure 5. (a) Set up for tensile strength test (b) Single fibre specimen for tensile strength test.R E T R A CT E D

Figure 9 .Figure 10 .
Figure 9. Surface roughness measurement by Surface profilometer.R E T R A CT E D

Figure 12 .
Figure 12.Variation of sound absorption coefficient with frequency.

Figure 13 .
Figure 13.Thermal conductivity of corn husk composite at different temperature.

Table 1 .
Parameters related to hardness value.