This study investigates the acoustical and nonacoustical properties of composites using corn husk fiber (CHF) and unsaturated polyester as the sound-absorbing materials. The influence of the volume fraction of CHF on acoustic performance was experimentally investigated. In addition, the nonacoustical properties, such as air-flow resistivity, porosity, and mechanical properties of composites have been analyzed. The results show that the sound absorptions at low frequencies are determined by the number of lumens in fiber, particularly the absorption coefficient, which increases the amount of fiber. For high-frequency sound, the absorption coefficient is determined by the arrangement of fibers in the composite. An absorption coefficient is close to zero when the fibers are arranged in a conventional pattern; however, when they are arranged in a random pattern, a high absorption coefficient can be obtained. The bond interface between the fiber and resin enhances its mechanical properties, which increases the longevity of the composite panel.
Noise pollution and waste management are two problems that need to be solved in modern societies. The use of newly developed alternative materials to absorb the noise considerably minimizes these problems. Hence, the inexpensive, easily created, thin, and lightweight composite materials that can absorb sound waves in broader frequency fields are highly desirable.
The fibrous sound-absorbing materials have been extensively investigated [
The polymer has been widely utilized as a matrix in fiber composites because it is easily formed from a material that has physical and acoustical properties [
This study primarily investigates the effect of adding corn husk fibers (CHFs) on acoustical and nonacoustics properties of polyester composites. In addition, the effects of fiber content on the tensile properties and microstructures via SEM have been analyzed. The results of this study could contribute to engineering applications, especially as sound absorbers.
CHF is the main raw material used in this study. The fiber contains 46.15% cellulose, 33.79% hemicellulose, and 3.92% lignin. It has been treated with 5% sodium hydroxide (NaOH) for 2 h. The scheme of reaction is given as follows:
The unsaturated polyester resin 2250 BW-EX has a viscosity of 6–8 poise (25°C), the tensile strength of 8.8 Kg/mm2, a tensile modulus of 500 Kg/mm2, the flexural strength of 2.5 Kg/mm2, and elongation of 2.3%.
The weight of polyester resin and CHFs were measured before processing so as to determine the volume fraction of CHFs and polyester in the resulting composite. The composition of different sound absorbers is summarized in Table
The composition of the composite (mean values in volume fraction).
Sample | CHF (%) | Polyester resin (%) |
---|---|---|
PF-E | 20 | 80 |
PF-G | 40 | 60 |
PF-H | 50 | 50 |
PF-I | 60 | 40 |
PF-K | 70 | 30 |
PF-M | 80 | 20 |
The connected porosity of composite sample was nonacoustically measured using the method of water saturation used by Vašina et al. [
There are several empirical and semiempirical equations in the literature that can be used to estimate the flow resistivity of absorber materials based upon fiber radius and material porosity or the bulk density of the materials [
The following empirical formula was used to calculate tortuosity (
The acoustic properties of the composite sample were measured using a two-microphone transfer-function method, according to ASTM E-1050-98/ISO 10534-2 standards. The testing apparatus was part of complete acoustic material testing system Brüel & Kjaer (type 4206, Brüel & Kjær), as it is seen in Figure
Impedance tube kit (type 4206, Brüel & Kjær).
The tensile and Young’s modulus were determined using a Tensilon RTG-1310 universal testing machine with a load cell of 10 kN. All the samples of composites were tested after conditioning for 24 h in a standard testing atmosphere of 70% relative humidity and 28°C. According to the ASTMD 3039 standard, a gauge length of 150 mm and a crosshead speed of 5 mm/min were used for tensile testing. The sample size was 250 mm × 25.4 mm × 6 mm. In total 21 samples were tested for each sample condition and the average and standard deviation values were reported.
The surface morphologies of composites were observed using an Inspect-S50 scanning electron microscope with field emission gun. An accelerating voltage of 10 kV was used to collect SEM images on the surface of the sample. The morphologies of the composites were observed and analyzed via SEM at room temperature. Before testing, the samples were sliced and mounted onto SEM stubs using double-sided adhesive tape. They were gold sputtered for 5 min to a thickness of approximately 10 nm under pressure of 0.1 torr and 18 mA current to make the sample conductive. SEM micrographs were recorded at different magnifications to ensure clear images.
Large differences were observed in nonacoustical properties of the composite samples, because of their different microstructures as a result of the addition of the CHF in the polyester. This diversity is very interesting because it provides different porous microstructures and consequently different acoustic properties. Porosity, tortuosity, and flow resistivity values are listed in Table
Nonacoustical properties of samples.
Sample | Thickness (mm) | Density (Kg⋅m3) | Porosity | Air-flow resistivity, | Tortuosity, |
---|---|---|---|---|---|
PF-E | 20 | 640.5 | 0.6474 | 44,980 | 1.272 |
PF-G | 20 | 383.4 | 0.7053 | 29,353 | 1.208 |
PF-H | 20 | 304.1 | 0.7247 | 25,424 | 1.190 |
PF-I | 20 | 244.1 | 0.7457 | 21,576 | 1.171 |
PF-K | 20 | 198.0 | 0.7582 | 19,568 | 1.160 |
PF-M | 20 | 158.3 | 0.7954 | 14,435 | 1.128 |
Increasing the amount of fiber volume fraction in the polyester resin increases the porosity and decreases both tortuosity and air-flow resistivity in the absorbent material (seen Table
SEM photomicrographs of corn husk fibers 5% NaOH treated: (a) surface and (b) cross-sectional features.
All the composite samples demonstrate an open pore structure wherein the pores are interconnected. This is one of the most important factors for noise absorption because such a structure decreases air-flow resistivity and thus the dissipation of the wave energy in the pores. In these samples, the multiscale fiber structure with the
The normal sound absorption properties for all samples of CHF-polyester composites are graphically illustrated in Figure
The sound absorption coefficients of composite samples.
At frequencies above 2 kHz, the sound absorption capability of PF-E, PF-G, PF-I, and PF-K samples decreases. The decrease caused by the interface of the fiber/resin and orderly fiber arrangement that cause the higher value of the flow resistivity of the sample makes movements of the sound difficult to pass through the samples. An absorption coefficient is close to zero when the fibers are arranged in a conventional pattern. SEM micrographs (Figure
Sample pure polyester resin (PE) had the absorption coefficient under 0.2. Although polyester may be a valuable option in noise absorption applications, these results discourage its use as a sound-absorbing material.
Figure
Figures
The real part of the impedance ratio of different samples.
The imaginary part of the impedance ratio of samples.
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.
Furthermore, sound absorption at lower frequencies (over 1.0–2 kHz) is desirable for automotive applications because of this frequency range according to noise from the wind, engine running, tires, road, and conversation, thereby making CHF-polyester composites a promising candidate for automotive interior sound absorption.
Theoretically, there should be an interaction between hydrophobic polyester and hydrophilic cellulose. The disappearance of the noncellulose material on the surface of the fiber enables surface interaction with the polyester matrix. The void fraction is mainly formed because the composites have not been consolidated (not sufficiently pressed to form a contiguous solid structure) in order to manufacture composites.
Figures
Tensile strength of each sample.
Modulus of elasticity of each sample.
For PF-H sample, there was a 12.53% decrease in the tensile strength values, with a strength value of
The tensile strength value of the PF-E sample is the lower compared to other samples. This is due to the fiber volume fraction less than the other samples. The tensile strength of the fiber of 237.43 MPa is higher than the tensile strength of the resin.
In this study, a CHF-polyester sound absorber was proposed and the sound absorption capability of the material was significantly enhanced through the simple method. The presence of a number of lumen structures in the fiber bundle facilitates sound absorption at low frequencies in the range of 1 kHz–2 kHz. The interface between the surface of the fiber/resin and orderly arrangement of fibers within the resin of PF-E, PF-G, PF-I, and PF-K samples caused a decrease in the sound absorption properties at frequencies above 2 kHz. High frequencies above 4 kHz (PF-H and P F-M samples) are obtained due to the random distribution of the fiber.
Increased resin lowers friction between the fibers, reducing heat losses and subsequently its sound absorption coefficient.
All samples used in this study have the potential to be used as sound-absorbing materials. These results indicate that alternative high-performance sound-absorbing materials could be obtained using CHF, which can solve environmental problems and reduce noise pollution.
The authors declare that there is no conflict of interests regarding the publication of this paper.