Effects of silicon dioxide nanoparticles on the mechanical, acoustic and vibration damping properties of kenaf-fibre polyester composite panels

Abstract

This research aims to systematically analyse the impact of SiO₂ nanoparticles (NPs) in kenaf fibre reinforced polyester resin acoustic panels used for indoor applications. Three variants of the panels were manufactured using Vacuum assisted resin infusion process, each panel consisting of five layers of kenaf fibre with three weight percentages of NPs (0.0, 0.5 and 1.5). Sound Absorption Coefficient (SAC) testing was performed via impedance tube in higher and lower frequency ranges. Tensile testing was carried out using Instron 8801 Universal Testing Machine following ASTM D3039 standard and ASTM D790 standard was used for performing flexural test on a Tinius Olsen H10KL Universal Testing Machine. Free vibration analysis was performed using Atalon-DAQ 042,451. A micro-X-ray computed tomography scan was performed to evaluate the porosity of the panel specimen. Scanning electron microscopy (SEM) images were utilized for studying the dispersion of NPs and other effects. The results showed that the tensile and flexural strength decreased with the addition of NPs with the 1.5% sample showing the lowest tensile strength and flexural strength. The 0.5% sample displayed a SAC of 0.98 at 666 Hz and the 1.5% sample showed the best SAC of 0.99 at 2040 Hz. An increase in void content was observed with an increase in NPs. SEM Imaging revealed the various defects such as agglomerates, debris and cavities. The 0.5% sample showed the highest water absorption capability with an increase of 5.59% in weight. Addition of NPs improved the damping properties of the samples.

Introduction

Composite structural components are increasingly being adopted across various industries due to their advantageous properties, such as low weight and superior performance compared to traditional metal materials. “Noise” is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud or disruptive to mental or hearing faculties. Due to the rapid increase and growth in various industries, noise pollution has become an unavoidable issue. To tackle this issue, a range of materials with unique properties to reduce noise can be used. Comprehensive reviews of natural fiber composites continuously highlight their potential as eco-friendly, cost-effective alternatives to traditional synthetic fibers, though property modifications are often required to meet demanding industrial standards^1,2.

Kenaf is a natural fibre which has emerged as a promising reinforcement due to its fast growth rate³ and good cellulose content, which contributes to favourable mechanical stiffness^4–6. However, natural fiber composites often face challenges regarding interfacial adhesion, and moisture sensitivity when paired with polymer matrices and limitations in absolute mechanical strength compared to glass or carbon fiber composites^7,8, as evidenced by studies demonstrating significant mechanical degradation under accelerated weathering conditions in hemp and Innegra hybrid composites⁹. Studies demonstrate that water absorption significantly impairs flexural properties and induces structural failure^10–12. To resolve these issues, researchers have successfully utilized fiber surface treatments to improve hydrophobicity^13,14. Additionally, pairing kenaf with other elements like glass, flax, natural rubber, or even X-ray films has proven effective in enhancing bending, torsional, and impact strengths^15–18.

Incorporation of nanoparticles as suspended/scattered reinforcements has become a growing research area. Silicon Dioxide (SiO₂) nanoparticles are well-known for their capacity to alter the physical and mechanical characteristics of composites^19,20. These nanoparticles are expected to reinforce the polyester matrix due to their large surface area and high thermal stability²¹. However, the dispersion of nanoparticles heavily affects their effectiveness. Poor dispersion can lead to the formation of agglomerates, which may affect tensile strength. Besides mechanical properties, the potential of natural fibres as sound absorbers is under-explored. The porous structure of plant-based materials like bamboo and bast fibers are good for effective sound energy dissipation^22,23. Introducing silica nanoparticles (NPs) and nanodiamonds into composites changes their vibration damping behavior and high-frequency sound absorption by modifying structural porosity and internal void distribution^24–26. While studies⁸ investigated void content, tensile strength, vibration damping and sound absorption in kenaf/bamboo epoxy composites, their study employed hand-layup rather than VARI, did not incorporate nano-SiO₂ and omitted water absorption and micro-CT characterization. However, we could not find studies where all the aforementioned properties were examined simultaneously.

Nanoparticle agglomeration can cause porosity or voids, which might weaken mechanical integrity. However, this effect can unexpectedly improve acoustic properties because these voids allow sound energy to dissipate. A significant research gap exists in understanding the compromise between mechanical structural integrity and acoustic performance when using nanoparticles-reinforced natural composites. This study aims to bridge this gap. We fabricated Kenaf fiber-reinforced polyester composites using the Vacuum Assisted Resin Infusion (VARI) process, incorporating varying weight percentages of SiO₂ nanoparticles (0.0%, 0.5%, and 1.5%) as a nanofiller^17,27.

Materials and manufacturing methods

Materials

To fabricate the specimens, we used polyester (GP-0051) resin for the matrix obtained from Covai Seenu & Co located in Coimbatore, Tamil Nadu. The nanoparticles were obtained from Adnano Technologies Pvt Ltd. situated in Karnataka, India and the kenaf fibres for reinforcement were sourced from M/S Vruksha Composites located in Tenali, Andhra Pradesh.

Kenaf fibre

Kenaf (Hibiscus cannabinus) (Fig. 1) is a natural fibre which belongs to the Malvaceae family. It has a fast growth rate³ with excellent cellulose content. Its high porosity and hollow fibre structure is suspected to be a good sound absorber to help with acoustic indoor applications. Due to it being widely researched, several insights can be found to better understand the results of the research and understand its viability for the intended use.

Fig. 1.

Kenaf fibre fabric used.

Polyester resin

Polyester resin is a thermosetting polymer resin formed by the polycondensation of dibasic organic acids and polyhydric alcohols. It has versatile applications in the composites industry, often paired with fibreglass, provides very good dimensional strength and dimensional stability due to low shrinkage, and good weathering resistance. It is up to five times cheaper than other performative alternatives like epoxy which makes it favourable for mass production. Its ease of storing and processing at room temperatures are other factors towards its selection.

SiO₂ Nanoparticles

Nano-silica (Fig. 2) was chosen as the modifier due to its high specific surface area and reactivity which are suspected to significantly influence the material’s properties. They help to alter the porosity of the fibres affecting their capillary action. Hence, the choice was made to use silica nanoparticles as filler. As per the supplier’s technical data sheet, the nanoparticles had a purity of 99%, average particle size (APS) of less than 100 nm, bulk density of 0.3 g/cm³, specific surface area (SSA) of 220 m²/g, near-spherical morphology, and white powder physical form (CAS No. 7631-86-9). No surface functionalization was applied prior to use. The high specific surface area promotes interfacial interaction with the polyester matrix, though it also increases the tendency for agglomeration at higher loading levels.

Fig. 2.

SiO₂ Nanoparticles.

Manufacturing

Vacuum resin infusion process was used to fabricate the specimens in the form of panels. It is a commonly utilized method where the resin is pulled through the reinforcement using vacuum. A work moldboard is coated with 10 layers of wax which acts as a release agent. Kenaf fibres are then arranged in a stack on the board in a mat form. Five layers of kenaf fibres are used. After arranging the kenaf fibres, a layer of peel ply was placed on top of the stack. To infuse the resin evenly in the fibres, an infusion mesh was placed on top of the peel ply. This setup is then sealed using a vacuum bagging sheet.

Due to the use of NPs, preparation of resin is to be given importance to. Resin to hardener mixing ratio of 100 parts of resin for 1 part of hardener was used. The NPs were incorporated at wt% of 0.5% and 1.5% respectively. The NPs and resin were measured carefully and mixed using a magnetic stirrer for 45 min where the speed was gradually increased to 1000 rpm and gradually heated up to 60 °C as can be seen in Fig. 3a, and then cooled down to room temperature. Post resin preparation, the infusion spirals were placed at opposite edges of the setup within the vacuum bagging sheet and extended into tubes/pathways, respectively, for the resin. The inlet tube was fed using a reservoir of the prepared resin. The outlet tube was attached to a vacuum suction pump. The pump was turned on to induce a vacuum within the setup and once a zero-pressure region was created, the reservoir was opened to create a resin flow in the fibres. Once the resin is evenly distributed within the fibres, the vacuum is sealed and the tubes are disconnected.

Fig. 3.

(a) Polyester resin being mixed with NPs. (b) Manufactured Laminate marked for trimming.

This workpiece is left to cure over a period of 24 h. Once cured, the laminates are trimmed into uniform specimens as per the testing standard’s requirement, with a thickness of 5 mm. (Fig. 3b).

Experimental studies

Mechanical testing

The tensile and flexural tests were conducted using an Instron 8801 and Tinius Olsen H10KL universal testing apparatus respectively (Fig. 4). The device was outfitted with a loading cell with a 50 KN capacity. For the tensile test, five specimen samples from each fraction of NPs were cut in compliance with ASTM D3039. Each specimen had dimensions of 250 mm × 25 mm × 5 mm and was evaluated at a crosshead speed of 2 mm/min. In accordance with ASTM D790, a three-point bending test was used to assess the laminates’ flexural strength. The laminates measured 127 mm × 12.7 mm × 5 mm and were tested at a crosshead speed of 2 mm/min. The span length was 80 mm in accordance with the 16:1 span to depth ratio followed by ASTM D790 standard. Five specimens of each test had their average values collected for analysis.

Fig. 4.

(a) Tensile test (b) Flexural test.

Free vibration and damping analysis

The samples went through an impulse test based on ASTM E756. The samples were 25 mm wide, 100 mm long, and 5 mm thick. Each sample was hit at eight equally spaced spots. Free vibration analysis was used to determine the natural frequency and damping factor. A diagram of the modal analysis setup is shown in Fig. 5. The samples were held in place to act like a cantilever beam. A small impact hammer (Model 5800SL from Dytran, S/N-8224, Sensitivity: 104 Mv/Lb) and a small accelerometer sensor (352C22/NC from LW282645) were connected to a DAQ. The DAQ card was connected to a computer and worked with DewesoftX software (DewesoftX_2023.3_230616_x64). The DAQ changed the signal from time to frequency using a fast Fourier transform. The frequency response curve was carefully looked at, and the natural frequency and damping factor were recorded from DEWE tools. The damping values were found using the circle fit method with this formula²⁵:

where ω₀ is the angular frequency at resonance, α₁ and α₂ are angles between angular frequencies, and ω₁ and ω₂ are the angular frequencies.

Fig. 5.

Impulse test setup.

CT scan for void content detection

A Micro-CT scan of the manufactured panels is performed to find any voids in the specimen. A typical term for voids in laminates is “void content” or “vacant volume fraction.” The Bruker Skyscan1273 (Fig. 6), a 3D X-ray microscopy system as seen in the figure, is the device used in this instance. This kind of testing is non-destructive.

Fig. 6.

Micro-CT scan.

FE-SEM imaging

To study the effects of the dispersion of NPs, the samples were plated with gold to help produce clear images. Hitachi S-3400N SEM instrument was used at an acceleration of 10 kV.

Sound/Sabin absorption coefficient (SAC)

ASTM E1050-08 standard was used for the transfer functions of the impedance tube, the setup can be seen Fig. 7. The samples were cut in circular shape of diameter 30 mm and 100 mm respectively with a thickness of 5 mm. The sample and loudspeaker are placed precisely 320 mm apart for the testing setup. To ensure consistent sound intensity per predetermined bandwidth/Hz inside the tube, sound was injected after placing the sample inside the tube. To capture both the incident sound from the sound source and the reflected sound from the sample, two acoustic microphones were placed in front of the sample. Following transmission, the sound in the form of signal were picked up by an analyser. The auto and cross-spectra required to generate the transfer function were obtained by computer analysis of the data. The measurement transfer function was used to determine the absorption coefficient. The SAC was calculated using measurements in the frequency range of 500 Hz to 6300 Hz.

Fig. 7.

Impedance tube setup.

Water absorption test

The samples were cut in 40 mm x 40 mm sizes and weighed. They were then immersed in water for 24 h, as can be seen in Fig. 8, and weighed again. The readings were noted down and the process was repeated over 5 days. The overall increase in weight percentage was calculated using the following formula¹¹:

where w_n is the change in weight percent, wt₀ is the initial weight on day 0, and wt₁ is the weight on day 5.

Fig. 8.

Sample immersed in water.

Results and discussions

Mechanical properties

Tensile strength

The results show that the introduction of NPs has decreased the ultimate tensile strength compared to the sample with 0% NPs even though the overall load-bearing capacity has increased as shown in Fig. 9. While the sample with 0.5% NPs was able to bear the highest load of 4.23 kN, there was a decrease in tensile strength of 2.13% and 48.5% (1.5% NPs) when compared to the sample containing 0% NPs. Thus, it can be inferred that the tensile strength decreases with the addition of NPs. This is due to the agglomeration of the NPs and increase in the voids. The overall performance of composites is determined by the quality of interfacial bonding and the proper filler-to-polyester resin ratio. As the NPs clump together, higher weight percentages of NPs in the specimens cause interfacial bonding failure. Strong covalent and non-covalent connections between the fibre and polyester matrix are necessary to transfer the maximum amount of load from the polyester resin to the composite reinforcement. Shear and tensile failure were the two types of failure found in the samples during testing. As a result, the polyester matrix began to crack, the fibres separated from the matrix, and eventually the fibres broke.

Fig. 9.

Results on tensile test (a) Load vs displacement (b) Tensile stress vs Tensile strain (c) Comparison of UT across samples.

The interaction between the fiber and matrix, along with the bond between the filler and matrix, is vital in facilitating the transfer of stresses within the composite. Addition of NPs resulted in the reduction of adhesion between the reinforcement and matrix. This resulted in the formation of agglomerated areas and voids. Hence, incorporating NPs into the polyester matrix reduces the tensile property.

Similar studies were performed using an epoxy-matrix for kenaf fibre composite^28,29. It was determined that 2–3% is the percolation threshold for SiO₂ composites before severe agglomeration leading to mechanical failure. Since a polyester matrix was used in this research, the mechanical failure can be observed at lower wt.%.

Flexural strength

Figure 10 shows that increasing NPs only reduces the flexural strength of the samples. While the 0.0% sample showed the highest flexural strength at 59.93 MPa, the 0.5% and 1.5% samples showed flexural strengths of 37.58 and 38.95 MPa respectively. Adding NPs reduced the flexural strength by 37.3% and 35% for 0.5% and 1.5% samples respectively. This can be explained by the failure of interfacial bonding which increased due to increase in NPs and hindering effective load transfer between reinforcement fibre and matrix.

Fig. 10.

Flexural stress vs. Flexural strain.

Nanoparticle aggregation and inadequate dispersion within the matrix are responsible for the observed decrease in flexural strength with increasing nano-SiO₂ content. Higher loadings cause nano-SiO₂ to cluster, weakening the composite structure and generating stress concentration spots. Research has demonstrated that mechanical qualities only get better up to an ideal nano-SiO₂ level, after which agglomeration causes flexural strength to decline. Furthermore, too much nano-SiO₂ might weaken the interfacial connection between the fibre and matrix, decreasing efficient load transfer and causing early failure. Mechanical degradation is further exacerbated by poor interfacial adhesion and increased void formation. According to reports, excessive inclusion of nano-SiO₂ may have a negative impact on strength because of microstructural flaws and particle clustering, even if it can increase characteristics and refine pore structure at lower contents. Moreover, improved acoustic absorption qualities can be achieved by increasing porosity and void content, but mechanical performance is sacrificed in the process. Because of agglomeration, weak interfacial bonding, and increased void content, larger concentrations of nano-SiO₂ result in decreased flexural strength even though they improve several functional qualities^30,31.

From the obtained mechanical properties results, it can be said that due to an increase in void content, although the acoustic properties might increase, the mechanical properties would degrade.

Sound/Sabin absorption coefficient

Low frequency

This study analysed the Sabin absorption coefficient (SAC) of polyester composites reinforced with NPs, specifically focusing on low-frequency performance. As shown in Fig. 11a, adding NPs up to 0.5% did enhance the sound absorption compared to pure polyester samples in the range up to 1600 Hz. While all specimens initially exhibited low absorption below 200 Hz, the samples showed a significant increase above 200 Hz, reaching a peak SAC of 0.98 at 666 Hz for the 0.5% sample while the 0% and 1.5% samples showed lower values with the SAC falling even more with the addition of NPs beyond 0.5%.

Fig. 11.

Sound absorption coefficient vs. frequency (a) Low frequency (b) High frequency.

Here, due to the longer wavelengths that interact primarily with the macroscopic architecture of the samples, the waves get trapped in the voids and reverberate repeatedly within the voids. Hence, the 0.5% sample shows the best sound absorption in the lower frequency range as it has the highest void content in the medium to high diameter range (Discussed further) as revealed by the CT-Scan results.

High frequency

The Sabin absorption coefficient (SAC) was evaluated in the higher-frequency range of 1600 Hz–6300 Hz with a sample size of 30 mm in diameter. In Fig. 11b, the 1.5% sample showed an increase in SAC reaching 0.99 at 2040 Hz. The 0.5% sample was reduced to 0.96 SAC and the 0% sample showed a SAC of 0.85. Interestingly, at higher frequency, increasing the concentration increases the SAC whereas in the lower frequency range, the SAC increases up to 0.5% then falls. While shorter wavelengths at high frequencies may hinder energy dissipation in the laminate, the inclusion of internal voids successfully promotes higher sound absorption in this frequency band.

Due to the shorter wavelengths possessed by the waves in the higher frequency range, they instead interact with the microscopic obstacles such as agglomerates instead of getting trapped in the voids. Due to the higher agglomerates and obstacles such as fibres present in the 1.5% sample, it provides the best SAC in this case. Similar trends were found in studies where the void content was directly proportional to the sound absorption capabilities of composite materials^32–35.

Void content-CT scan

Figure 12 shows the CT Scan Images of the samples with their respective transparent images displaying the porosity. Figure 12a1, a2 shows the 0.0% sample with several pores in all the diameter ranges indicating decreased sound absorption qualities when compared to 0.5% sample, Fig. 12b1, b2, which shows porosity in the lower diameter range (Red and green). But, Fig. 12c1, c2 shows the porosity of the 1.5% sample, displaying majorly smaller pores (red). In summary, the 0.0% sample displays the lowest void volume spread across all diameter ranges, while the 0.5% and 1.5% samples show an increased void content which helps absorb sound more effectively, which can be seen in the results of the Sabin absorption coefficient test. This is the reason the 1.5% sample has the highest SAC in the higher frequency range.

Fig. 12.

CT scan images of (a1, a2) 0.0% sample (b1, b2) 0.5% sample (c1, c2) 1.5% sample.

Voids can be present in the laminates due to several reasons such as agglomeration of NPs, heat generated while curing or poor interfacial bonding between the fibres and matrix. In the end, the presence of pores improves the SAC of the sample as the longitudinal waves enter the voids and reverberate within instead of escaping through, releasing heat energy which is negligible.

Due to the addition of NPs, the dynamic viscosity of the resin increases, impeding the resin flow during the manufacturing process. This results in air getting trapped between the matrix which results in the increase of void content with the addition of NPs^36,37.

Microstructural properties

The SEM images of all the samples are shown in Fig. 13. While the 0.0% sample shows some amount of debris, presumably from the wax releasing agent and shallow cavities, Fig. 13a1, a2, a significant difference can be seen when increasing the amount of NPs in the samples. The 0.5% sample shows minimal agglomeration with increased amount of shallow cavities, Fig. 13b1, b2. As the amount of NPs are increased, an increase in agglomerates can be noticed. In the 1.5% sample, interfacial bonding failure was observed along with a drastic increase in cavities/voids which can be seen in Fig. 13c1, c2.

Fig. 13.

SEM image of (a1, a2) 0.0% sample (b1, b2) 0.5% sample (c1, c2) 1.5% sample.

This increase in voids, cavities and agglomerates explains the better sound absorption capabilities at higher frequencies in higher wt% samples. By incorporating NPs in the matrix, the bonding between the fibre reinforcement and matrix fails. This interfacial bonding failure is directly proportional to the wt% of NPs and the same can be observed in images above. This explains why an increase in NPs reduces the tensile and flexural properties of the samples by a significant margin.

Due to the high van der Waals forces possessed by the NPs, they overcome the low shear forces during mixing and form agglomerates³⁸. These agglomerates serve as critical stress concentrators. Under tensile or flexural loading, they mimic the behavior of structural flaws, initiating premature interfacial bonding failure as observed.

Water absorption

From observing Table 1, it can be noted that the 0.0% sample had an increase of 4.82% in its weight and the 0.5% and 1.5% samples had an increase of 5.59% and 4.34% respectively. The void content in the respective samples could be the reason behind the 0.5% sample having the highest water absorption capabilities. Due to the high amount of voids present in the 0.5% sample in the higher diameter region, the amount of water that seeps into the sample increases. As the 1.5% sample has voids in the smaller diameter region, it has the worst water absorption capability of the samples.

Table 1.

Influence of NPs on water absorption properties of the samples.

Sample (wt %)	Day 0	Day 1	Day 2	Day 3	Day 4	Day 5
0	6.9148	7.0797	7.146	7.1994	7.2276	7.2479
0.5	8.803	9.0433	9.1258	9.2247	9.2793	9.2953
1.5	7.8408	8.0174	8.0846	8.1456	8.1611	8.1814

Silica NPs serve as a hydro-barrier. Studies show that silicon dioxide nano-interphases physically block the hydrophilic cell wall receptors of natural fibers. Additionally, the nanoparticles physically seal the capillary micro-pores located at the fiber-matrix boundary, successfully dropping saturated water absorption rates in kenaf composites from 29.6% down to 12.9%^39,40. The 0.5% sample has the highest water absorption capacity, which can be credited to the higher void content present as revealed by the CT scan. But the 1.5% sample has the lowest capacity due to the presence of higher concentration of NPs and lower voids than the 0.5% sample. Although void content increases with NP concentration, the water absorption behaviour is governed not merely by the total void volume but by the size distribution of the voids. The CT scan results reveal that the 0.5 wt.% sample contains voids predominantly in the medium-to-large diameter range, which form interconnected capillary pathways that facilitate greater water ingress. In contrast, the 1.5 wt.% sample, despite having a higher total void content, contains voids concentrated in the smaller diameter range. Smaller, isolated voids offer less capillary action and restrict water penetration, resulting in lower overall water absorption despite the higher void fraction. This explains why the 0.5 wt.% sample exhibits the highest water absorption at 5.59%, even though the 1.5 wt.% sample has a greater total void content.

Free vibration properties

Table 2 shows the influence of nanoparticle (NP) loading on the free vibration characteristics of kenaf fibre reinforced polyester composites under cantilever boundary conditions. The results present that both natural frequency and damping ratio are affected by NPs content.

Table 2.

Influence of NPs on free vibration characteristics of kenaf-fibre reinforced polyester composites under cantilever conditions.

Sample (wt%)	Mode 1		Mode 2		Mode 3
	Natural frequency	Damping ratio	Natural frequency	Damping ratio	Natural frequency	Damping ratio
0	30.649	0.0008	147.51	0.0008	409.3	0.0008
0.5	31.409	0.04	213.06	0.001	601.53	0.0001
1.5	32.022	0.018	204.68	0.004	560.7	0.0004

Natural frequency increases steadily with nano-SiO₂ content across all three modes. Mode 1 rises from 30.649 Hz for the neat composite to 32.022 Hz at 1.5 wt.%. Rigid nanoparticles raise the stiffness-to-mass ratio, which drives frequency up consistent with classical vibration theory. Similar studies reported similar improvements in elastic modulus and dynamic response with nano- SiO₂ addition^30,41,42

The damping ratio tells a different story. At 0.5 wt%, it reaches 0.04 in Mode 1, against 0.0008 for the neat composite. Well-dispersed nanoparticles at this loading increase internal friction and interfacial sliding, dissipating energy effectively. At 1.5 wt%, damping drops agglomeration reduces the effective interfacial area and restricts polymer chain mobility. Voids and agglomerate clusters add localised stiffness variation, further limiting damping efficiency. Similar studies showed that optimal dispersion enhances damping through interfacial energy dissipation, while excess filler loading leads to clustering and a weaker viscoelastic response^31,43. Other similar studies⁴⁴ observed analogous behaviour in MWCNT-modified kapok/epoxy composites, where an optimal nanoparticle loading of 0.5 wt.% augmented natural frequencies by as much as 36% under cantilever boundary conditions, attributed to enhanced matrix stiffness via nanoparticle reinforcement.

The damping behaviour is mode-dependent and non-linear throughout. This is largely attributed to the viscoelastic nature of the kenaf fibres and frictional interactions at the fibre-polyester matrix interface. At 0.5 wt.%, damping peaks beyond that, agglomeration and void formation take over.

Conclusions

This article discussed the effect of NPs in kenaf fiber reinforced polyester composites for indoor sound absorption applications. The VARI method is used to fabricate the panels. The tensile strength, flexural strength, SAC, water absorption properties of panels are evaluated and compared. The following findings are reached in light of the analysis, visual observations, and experimental data:

1. The tensile strength is inversely proportional to the concentration of NPs with 0.0% sample showing the highest tensile strength at 43.23 MPa.

2. The flexural strength is inversely proportional to the concentration of NPs with the 0.0% sample showing the highest flexural strength at 59.93 MPa.

3. As void content is an important factor affecting the SAC, in the low frequency range, the 0.5% sample showed the highest SAC at 0.98. While in the high frequency range, the 1.5% sample showed the highest SAC at 0.99.

4. Micro-CT Scanning shows an increase in void content directly proportional to the wt% of NPs. The 0.5% sample has voids in the medium diameter range while the 1.5% sample has the void content in the lower diameter range.

5. Microstructural analysis shows the agglomerates and other features of the samples.

6. The 0.5% sample has the highest water absorption capability with a weight increase of 5.59%, the 1.5% sample has the least at 4.34% weight increase.

7. Adding NPs at optimal loading (0.5 wt%) improves the damping properties though the response is mode dependent and non-linear at higher concentration.

The results obtained strongly support the use of the samples for acoustic material for indoor applications.

Author contributions

Conceptualization, S.K.S. and S.K.; methodology, S.S.S. and E.S.S.V.R.; formal analysis, A.A.R. and R.R.; investigation, S.S.S. and E.S.S.V.R.; writing—original draft preparation, S.S.S. and E.S.S.V.R.; writing—review and editing, S.K. and S.K.S.; visualization, S.K.; supervision, S.K.S.; project administrations, S.K.S.

Funding

Open access funding provided by Vellore Institute of Technology.

Data availability

All data is available within the manuscript.

Declarations

Competing Interests

The authors declare no competing interests.