Effect of fibre treatment and moisture content on the vibration and acoustic properties of the basalt/hemp hybrid composites

The hybridization reinforcements of composites allow design engineers to enhance the structural and acoustic properties of synthetic fibre-reinforced materials. Natural fibres are considered to have higher potential for replacing synthetic fibres in the composite industry. The present study aims to develop hybrid composite materials for sound insulation applications in the automotive industry. The hybrid composites were developed by reinforcing Basalt and Hemp fibres with Epoxy using vacuum bagging technique and cured under ambient conditions. Vibration and acoustic experiments were carried out on pristine and aged BHU (Basalt-Hemp-Untreated) and BHT (Basalt-Hemp-Treated) specimens. The fibre treatment and moisture gain influenced the natural frequency and stiffness of the hybrid composites. With the fibre treatment, the natural frequency of the specimens was enhanced by 12.8%. With ageing of both BHU and BHT, specimens showed a reduction in the natural frequency by 16.4% and 23% respectively. Moisture ingression into the composites reduced the stiffness and improved the damping factor of the structure. The aged BHU and BHT showed better acoustic performance compared to the pristine specimens.


Introduction
Composite materials consist of multiple constituents of chemically distinct phases, that exhibit microscopic variation in structural properties.Although heterogeneous at a microscopic scale, they display macroscopic homogeneity.Composite materials comprise of a matrix and a reinforcing phase.Over the time, the matrix experiences degradation that ultimately leads to structural failure.These processes encompass impact-induced damage, delamination, moisture absorption, chemical corrosion, and high-temperature creep.Consequently, the matrix plays a pivotal role in the failure of polymer matrix composites (PMCs).The incorporation of continuous reinforcement into these composites results in enhanced mechanical characteristics, including strength and rigidity.The microstructure of composite materials is tailored based on their constituent elements, with manipulation of volume fraction and arrangement enabling customization of mechanical properties to fulfill specific design requirements [1,2].
Hybrid composites, in comparison to conventional polymer composites, represent a relatively recent development in the composite research field.Hybrids are characterized by the incorporation of more than one type of reinforcement phase along with the thermoset or thermoplastic phases.They offer enhanced flexibility in improving mechanical properties, reducing the weight of the structure, and production costs when compared to other fibre-reinforced composites [3].Typically, hybrid composite consist of a combination of high-modulus and low modulus fibres.The high-modulus fibres contribute to rigidity and load-bearing capacity, while the low-modulus fibres enhance tear resistance and are relatively economical when compared to the high modulus fibres.The mechanical characteristics of hybrid composites can be optimized by altering the volume ratio of fibres and by arranging fibres in different orientations at various layers [4,5].
Fibrous materials play a crucial role as dual insulators, serving as both thermal and acoustic insulators in the construction sector.Traditional fibrous insulators like glass fibre and mineral wool are widely used for sound absorption due to their extensive specific surface area, excellent acoustic properties, and cost-effectiveness [6].In the beginning of 21st century, synthetic fibres such as glass, carbon fibre and mineral wool were the dominant materials in the European insulation market, contributing approximately 60% of the total market share [7].Synthetic fibres are commonly manufactured using high-temperature processes, and their raw materials are frequently derived from petrochemical sources, resulting in a notable carbon footprint.Moreover, synthetic materials require more energy and exhibit greater global warming potential throughout their lifecycle.Therefore, the pursuit of ecologically sustainable and non-harmful materials to replace traditional sound absorbers holds great significance [8][9][10].
The present global scenario mainly focuses on replacing synthetic fibres with environmental friendly alternatives.One such naturally derived material is basalt fibre, formed by solidified magma erupted from volcanoes [1] is obtained through magma cooling processes at temperatures ranging from 1500 °C to 1700 °C [11][12][13].Rapid cooling results in an almost glass-like amorphous structure, while gradual cooling leads to crystallization into a blend of minerals.Predominantly, plagioclase and pyroxene make up around 80% of basalt compositions.The classification of basaltic rocks depends on the proportion of primary mineral contents and contain 42.3% to 47% of Silicon Oxide (SiO 2 ) followed by 11% to 13% of Aluminium Oxide (Al 2 O 3 ) [1,13,14].
Hemp (Cannabis Sativa) is one of the most important natural fibres, producing significantly more fibre per square meter of cultivation than cotton or flax and requires less water for growth.Hemp fibre is antimicrobial in nature and is resistant to ultraviolet light, mould, mildew, and insects, making it suitable for outdoor applications.By treating the fibre with an alkali solution, the density and tensile strength of the fibres can be increased [15].Given the high demand for natural fibre-reinforced composites (NFRC) in sound absorption applications across industries like automotive, aviation, and construction, they present a preferable alternative to synthetic composites.While traditional sound-absorbing materials serve for passive control, natural fibres offer enhanced effectiveness and cost-efficiency [2,16].
The main objective of the present research work is to develop eco-friendly hybrid composites for structural and sound insulation applications in automotive industry.The studies on acoustic behaviour of Basalt/Hemp hybrid composite are yet to be reported.In the current work, the vibration and acoustic performance of the hybrid fibre-reinforced (Hemp and Basalt) composite laminate under the influence of moisture were studied.The experimental studies can help in the design and development of novel Hemp/Basalt hybrid composites in the field of acoustic and vibration applications.Two sets of hybrid composites were prepared by reinforcing Basalt and treated/untreated Hemp fibres with Epoxy matrix by using vacuum bagging technique.To study the influence of moisture on damping, stiffness, natural frequency, and acoustic performance of hybrid composites, the specimens were aged in distilled water for a duration of 90 days.The obtained results were compared with the pristine test results.

Laminate fabrication
Composites Tomorrow, a company situated in Vadodara, India, provided bidirectional basalt fibres with a density of 2.8 g/c.c., an average thickness of 0.5 mm, and a weight of 400 GSM.The untreated unidirectional hemp fibres with a density of 1.4 g/c.c. and Elastic Modulus of 70± 5 GPa were sourced from Go Green Products, Chennai, India.Lapox L-12 epoxy resin, recognized for its extended pot life of 45 min at room temperature was chosen as the matrix system.The curing agent utilized was Lapox K-6.Both these materials were procured from Yuje Enterprises in Bangalore, India.The procured unidirectional Hemp fibres were treated with 5% Potassium Permanganate (KMnO 4 ) solution for a duration of 5 min to improve the wettability and adhesion properties.The treated fibres were washed with running water and completely dried using a hot air oven at 50 °C for 1 h.The laminates were manufactured by interposing three layers of treated/untreated Hemp fibre between two layers of Basalt fibre, as illustrated in figure 1.The laminate containing untreated Hemp fibre and treated Hemp fibre are designated as BHU and BHT respectively.The matrix substance was produced by blending Epoxy and K6 hardener in a ratio of 10:1.To ensure an even dispersion of the epoxy, the laminate layers were manually stacked on a flat plate mould as shown in figure 1. Subsequently, the fibres were uniformly saturated with the matrix material using the vacuum bagging process.The edges of the vacuum bag are sealed with the help of sealant tapes.To maintain uniform surface texture on both sides of the specimen, a peel ply was used during the fabrication process.To improve the wetting of fibres and to remove the excess matrix material, the vacuum was supplied up to the gelling point of the matrix material.The curing procedure was entailed for a duration of 24 h at room temperature, as shown in figure 2. Test specimens for vibration and acoustic evaluations were crafted to meet the required dimensions as per the ASTM E 756-05 and ISO 10534-2 standards respectively, by employing the Abrasive Water Jet Machining technique and the specimens are shown in figure 3. The thickness of the BHU and BHT laminates was measured at different locations.The average thickness of the laminates was found to be 1.66 mm and 1.64 mm, respectively.The experimental density of the fabricated laminate was estimated based on the Archimedes principle for specimens with uniform geometry.The obtained experimental densities of the BHU and BHT were 1.12 and 1.24 g cm −3 , respectively.

Fourier transform infrared spectroscopy analysis
The chemical changes induced in the unidirectional Hemp fibres due to the KMnO 4 treatment process were characterized by the Fourier Transform Infrared Spectroscopy (FTIR) technique.The spectra obtained from the untreated Hemp fibre specimen were compared with those obtained from the treated Hemp fibre specimen.The specimens required for the analysis were prepared by chopping untreated and treated Hemp fibres into fine powder form.The chopped treated and untreated Hemp fibres of average length of 1 mm were mixed with Potassium Bromide (KBr).The use of KBr helps in converting treated and untreated Hemp fibre specimens into small pellets of diameter 13 mm.Both the pellets were scanned between a frequency range of 500 cm − 1 to 4000 cm − 1 in a transmittance mode.

Artificial ageing
To investigate the effects of moisture absorption on the vibrational and acoustic behaviour of the hemp and basalt fibre-reinforced hybrid composites, the specimens were aged in distilled water under ambient conditions for three months.This was done to imitate the moisture absorption process in real-life situations.The specimen's moisture gain was monitored at fixed intervals using a digital weighing machine.The moisture absorption study was carried out using gravimetric analysis (equation ( 1)).Where W g represents the percentage of moisture gain, W 2 stands for the wet weight of the specimen, and W 1 signifies the dry weight of the specimen ( ) The moisture diffusion curve representing the moisture absorption characteristics of the specimen was studied through the application of Fick's diffusion equation (equation ( 2)), where, D z represents the bulk diffusion coefficient, M t represents the moisture absorption percentage at the saturation point, and h indicates the thickness of the specimen.T 1 and T 2 were selected from the initial linear segment of the curve, while M 1 and M 2 denoted the absorbed moisture percentages corresponding to times T 1 and T 2 , respectively [17][18][19].

Vibrational test
The natural frequency, damping factor and stiffness of the pristine and aged BHU and BHT were estimated as per the ASTM E756-05 standard.The tests were performed with fixed-free boundary conditions, as illustrated in figure 4. A PCB accelerometer with a sensitivity of 101.6 mV g −1 was affixed to the free end of the sample to record the specimen's response to free vibration.Experimental data acquisition was carried out using NI-9234, a data acquisition (DAQ) interface, along with NI-LabVIEW 2016 software.The Fast Fourier Transformation (FFT) of the time domain data gives the frequency domain graph using LabVIEW graphical programming.The peak value of the frequency domain graph is the natural frequency of the beam.

Acoustic testing
The acoustic tests were carried out in accordance with ISO 10534-2:1998 utilising a BSWA Tech SW422 Impedance Tube configuration as shown in figure 5(a) and (b).Transmission loss values were taken for the frequency range of 63-6300 Hz using two specimens with a diameter of 100 mm and 30 mm.Transmission loss (TL) of a particular material indicates the effectiveness of the material when used as an acoustic barrier.To attenuate sound energy when sound is travelling in air when it passes through a barrier.The portion of transmitted incident energy is called the power Transmission coefficient of sound and is represented by the symbol τ.The transmission loss of any material can be expressed using equation (3) [20].

FTIR analysis
The FTIR analysis of untreated hemp fibre reveals the presence of various functional groups of lignin, hemicellulose, and cellulose, as shown in figure 6. Notably, there is a prominent peak in the range of 3300-3500 cm −1 , attributed to the hydroxyl (-OH) stretching vibration in the hemicellulose arrangement of the fibres.The stretching and vibration at 2920.50 cm −1 show the presence of an alkane group (C-H) (lignin and cellulose components).The small peaks at 2200-2400 cm −1 correspond to the weak alkyne (C ≡ C) and weak nitrile group (C ≡ N) of the lignin.The prominent bending at 1600.00 cm 1 and 1200 cm 1 is associated with the alkene functional group and hydroxyl group of the untreated hemp fibre.
The treatment of hemp fibre with 5% KMnO 4 eliminated the -OH stretching in the range of 3300-3500 cm −1 .Elimination of -OH group from the fibre surface will reduce the affinity of fibre towards the moisture.The bending observed within the range of 3595.01-3505.17cm −1 , shows the presence of an amine group (N-H).Additional bending at 2921.93 cm −1 and 2853.06 cm −1 are a result of C-H stretching, while those within the range of 2427.09-2009.27cm −1 are attributed to C ≡ N groups.The small peak at 1926.26 cm −1 corresponds to carbonyl (C=O) stretching.Notably, a visible bending at 1219.25 cm −1 indicates the presence of O-H in cellulose.

Moisture absorption behaviour
Figure 7 illustrates the moisture absorption characteristics of the Basalt/Hemp fibre-reinforced Hybrid composites.In the initial ageing period (7-10 days), all the specimens are seen to exhibit swift moisture absorption.The BHU specimens exhibited the most substantial moisture gain of 13.23%, while the BHT specimens demonstrated a comparatively lower moisture gain of 10.08%.At the onset of the second phase, all specimens experienced steady growth in moisture gain and reached a saturation point (between 75 and 80th day).The moisture absorption curve for all the specimens aligned with the general Fickinian diffusion curve.From the moisture absorption curve, the diffusion coefficient of the specimens is estimated as per equation (2).The rate of moisture ingression into the specimen is shown in table 1.By the end of the ageing period, the BHU specimens absorbed 14.03% moisture, while the BHT specimens absorbed 10.65% moisture.The interaction between the fibres and the matrix significantly influences the moisture absorption behaviour of the Fibre-Reinforced Polymer (FRP).The diffusion coefficient of the specimen reveals the dependence of moisture gain on the treatment of hemp with KMnO 4 .The BHU specimens absorbed a higher percentage of moisture compared to the treated specimens.The breakdown of -OH group in the fibre due to the treatment enhanced resistance to moisture infiltration, indicating a reduced impact on the overall structural properties of the material.

Vibrational analysis
The effect of treated and untreated hemp fibre with ageing on the dynamic properties of the hybrid composite was examined using free vibration experiments.The damping coefficient of the hybrid composite was determined by analysing the time-domain graph acquired from the free vibration tests and applying equation (4) [21].
Where d is the logarithmic decay, X n-1 and X n are amplitudes of oscillation at times t n-1 and t n, respectively, and z is the damping ratio.The reinforcement of the untreated Hemp fibre increased the mass of the specimen.This in turn decreases the specimen's stiffness and natural frequency, as given by equation (5) [22].
Where w n is the natural frequency of the test specimen, k is the stiffness, and m is the modal mass in kg. Figure 8(a) and (b) show the effect of moisture gain on the natural frequency and stiffness of the BHU and BHT specimens.The pristine BHT specimens have shown the highest natural frequency and stiffness values.After further investigations of the specimens and the post-ageing process, it was revealed that the introduction of moisture resulted in a decline in both stiffness and natural frequency values at the conclusion of the ageing period.The BHT specimens exhibited a significant reduction in stiffness value following the ageing process when compared with the BHU specimens.As a result of the ageing process, all the specimens exhibited a reduction in the natural frequency.BHU specimens have shown very low changes compared with BHT specimens.Furthermore, the value of the damping ratio for a material can be calculated using equation (6) [22], Where c is the damping coefficient (Ns m −1 ), and c .c. is the critical damping coefficient of the material.The damping ratio of the specimen is a result of the combined influence of the matrix and fibre's inherent damping properties, along with the relative displacement between them under dynamic conditions.From equation (6) it can be deuced that with ageing, the reduction in stiffness due to moisture absorption is resulting in a slight  enhancement in the damping ratio in the tested specimen.Figure 9 shows the effect of ageing on the damping factor of the BHT and BHU specimens.The experimental results indicated that KMnO 4 treated Hemp fibrereinforced specimens exhibited a higher damping ratio at the end of the ageing period (from 0.02 at the beginning of the ageing to 0.04 at the end), whereas untreated fibre showed a relatively lower damping ratio when compared with treated fibre-reinforced specimens, but a similar trend was observed.The untreated specimen damping ratio increased from 0.015 to 0.0225.It was observed that the stiffness of both specimens showed a reduction with ageing process due to moisture absorption.However, the rate of reduction of stiffness was greater than the rate at which the mass addition occurred, resulting in an overall enhancement of the damping ratio, with a corresponding reduction in natural frequency.These trends are clearly observed in figures 8 and 9 respectively.It can be concluded that stiffness reduction and mass gain are the factors resulting in a reduction of natural frequency and an enhancement of the damping ratio.This demonstrates unequivocally how weight gain and stiffness loss lead to an overall improvement in damping ratio for all the specimens exposed to a moist environment.Similar kind of observations reported in [18,[22][23][24] 3.4.Acoustic characterisation Sound absorption tests were executed using an Impedance tube, encompassing frequencies from 63 Hz to 6300 Hz. Figure 10(a) and (b) portray the transmission loss patterns of both pristine and aged hybrid (hemp and basalt) composites.All specimens exhibited a consistent transmission loss pattern across all the frequencies.Among the tested specimens, the maximum transmission loss was observed at 2650 Hz with a value of 21.74 dB in BHU aged, as illustrated in figure 10(a).A BHU pristine specimen showed a maximum peak of 14.38 dB at 1900 Hz, as shown in figure 10(a).The velocity of wave propagation through solids is given by the equation (7) [25], Where, E represents the modulus of elasticity (N m −2 ), and ρ denotes the specimen's density (kg m −3 ).The denser the material, the higher the velocity of wave propagation.It is clear that materials with a higher density will have lower transmission losses.The density of the BHU sample is lower than of the BHT and consequently, the overall transmission loss of the untreated specimen is relatively better, as demonstrated in the experimental results displayed in figure 10(a).The pristine BHT specimen has a maximum peak of 11.36 dB at 2240 Hz as shown in figure 10(b), and then there is a subsequent drop in transmission loss value.BHT-pristine and aged specimens showed the most severe variation in peaks throughout the analysis.The transmission loss only increases in the range of 2000 Hz-3000 Hz, and after 3000 Hz, the transmission loss showed an increasing trend with the frequency.But in the case of the BHT-aged specimen, the maximum peak of 12.38 dB was observed at 2650 Hz as shown in figure 10(b).
The transmission loss analysis of both pristine and aged BHU and BHT specimens showed random peaks at different frequencies throughout the analysis.The transmission loss is directly affected by the stiffness and density of the medium.As the moisture gain increased, the stiffness of the specimens reduced, resulting in an overall improvement in transmission loss.The moisture absorption rate is lower in the treated specimen, as shown in figure 7.With age, even though the stiffness of both specimens reduced, the rate of reduction in the treated specimen was higher.Due to this, the transmission loss characteristics of the treated specimen were lower than those of the untreated hybrid specimen.

Conclusion
This study has yielded significant insights through the experimental analysis into the damping and acoustic performance of an aged hybrid composite comprising hemp and basalt fibres.The study revealed that ageing significantly impacts the acoustic behaviour of the composite material.Moisture ingression during ageing process led to an increase in mass, resulting in a decrease in the natural frequency and stiffness of the hybrid specimens.The density of the medium proved to be a crucial factor affecting transmission loss.The treatment of fibre with KMnO 4 resulted in an increase in specimen density and a reduction in moisture gain with ageing.Specimens with high density (Pristine and aged BHT) showed lower transmission loss.The density of both aged specimens was reduced, and the damping ratio enhanced.The transmission loss of the specimens improved with ageing process.The pristine and aged BHU specimen showed the best performance among the tested specimens.It may be concluded that the BHU specimen is a potential choice of material for vibration damping and as a sound-absorbing barrier in the interior panels of automobiles as an economical replacement for synthetic materials.

Acknowledgments
Authors were greatly thankful to Manipal Institute of Technology, Manipal Academy of Higher Education for providing composite manufacturing and testing facility.

Figure 1 .
Figure 1.Illustrative depiction of the arrangement Basalt and Hemp fabrics in the laminate.

Figure 4 .
Figure 4. Schematic representation of free-vibration test set up.

Figure 5 .
Figure 5. (a) Schematic representation of the impedance tube setup; (b) Experimental set up.

Figure 6 .
Figure 6.FTIR spectroscopy of the untreated and treated hemp fibre.

Figure 8
Figure 8 Effects of moisture gain on vibrational characteristics of BHU and BHT (a) Natural frequency, (b) Stiffness.

Figure 9 .
Figure 9. Effects of moisture gain on the damping factor of BHT and BHU specimens.

Figure 10 .
Figure 10.Effects of moisture gain on transmission loss a) BHU, b) BHT.

Table 1 .
Moisture gain at equilibrium and diffusion coefficient hybrid composites.