Utilizing Waste Cotton/Pigeon Pea Stalk Fibers Composites for Enhanced Sound Absorption and Insulation in Automotive Interiors

ABSTRACT

This study investigates the synthesis and characterization of composite materials, pigeon pea stem, and cotton fibers blended in different ratios such as 100/0, 70/30, 60/40, 50/50, 30/70, and 0/100. These composite materials were produced using a compression molding technique. According to ASTM standards, the acoustics, thermal conductivity, and physical characteristics of the composite samples were tested to assess their qualities. The impedance tube method detailed in ASTM E1050 was used to determine the sound absorption coefficients (SAC) for acoustics. The SAC values were measured at six frequencies such as 125, 250, 500, 1000, 2000, and 4000 Hz. The results showed that composite samples made from waste cotton and pigeon pea demonstrated sound absorption values of greater than 80%. Superior sound insulation and absorption, moisture absorption, fiber properties have also been demonstrated by waste composites. Especially, the waste cotton/pea stalk waste fiber composites achieved over 75% sound absorption, while the waste 28% composites performed well in terms of sound absorption, moisture absorption, and fiber properties. Even in humid conditions, the composite samples constructed from used cotton and pigeon pea stalks demonstrated good moisture resistance without reducing their insulating qualities. Soundproofing barriers are composite layers of foam or pigeon pea/cotton.

摘要

本研究研究了以 100/0、70/30、60/40、50/50、30/70 和 0/100 等不同比例混合的复合材料、木豆茎和棉纤维的合成和表征。这些复合材料是使用压缩成型技术生产的。根据 ASTM 标准，对复合材料样品的声学、导热性和物理特性进行了测试，以评估其质量。 ASTM E1050 中详述的阻抗管方法用于确定声学吸声系数 (SAC)。 SAC值是在125、250、500、1000、2000和4000Hz等六个频率下测量的。结果表明，由废棉和木豆制成的复合材料样品的吸声值大于80%。废旧复合材料也表现出了优越的隔音吸音、吸湿、纤维特性。特别是废棉/豌豆秆废纤维复合材料的吸声率达到75%以上，而废28%复合材料在吸声、吸湿和纤维性能方面表现良好。即使在潮湿的条件下，由用过的棉花和木豆秆制成的复合材料样品也表现出良好的防潮性，而不会降低其绝缘性能。隔音屏障是泡沫或木豆/棉的复合层。

KEYWORDS:

• Acoustics
• composite
• impedance tube
• Thermal insulation
• pigeon pea stalk fibers
• sound absorption

关键词:

• 声学
• 混合成的
• 阻抗管
• 隔热
• 鸽子豌豆茎纤维
• Sound absorption吸声

Introduction

In recent years, there has been a growing trend among researchers to repurpose production waste and environmentally friendly natural materials for the creation of sound-absorbing materials (Paşayev, Kocatepe, and Maraş 2019). This pursuit of eco-friendly porous sound absorbers has prompted investigations into alternatives to traditional natural and synthetic materials. Life Cycle Assessment studies indicate that natural materials emit significantly less CO₂ than conventional absorbing materials such as minerals and glass wools (Boominathan S. Senthil Kumar et al. 2022). Natural fiber composites exhibit various desirable characteristics, including durability, affordability, biodegradability, lightweight, good mechanical properties, non-abrasiveness, high specific strength, and eco-friendliness (Mohankumar et al. 2022). Waste cotton fibers have been proven to be massive reinforcements in the matrix and additionally pigeon pea is used as natural fiber. The aerospace and automotive industries have shown interest in utilizing natural fibers due to their benefits, like low weight, sound insulation properties, and enhanced crash absorbency. However, their use is systematically limited to internal compositions due to their hydrophilic characteristics, which can be mitigated via chemical treatments. Understanding the hygroscopic performance of natural materials is crucial for their application in various environmental conditions (Meseret et al. 2023). In this discourse, the notion of zero-emission automotive and passive houses has been introduced, emphasizing the pursuit of optimal thermal and sound insulation. Novel materials and insulating solutions with reduced thermal conductivity have been engineered to elevate insulation resistance (Moretti, Belloni, and Agosti 2016).

To counteract the adverse impacts of sound pollution, a diverse array of materials designed for acoustic insulation is readily available in the market. Additionally, ongoing research delves into the acoustic insulation properties of emerging materials (Eyupoglu, Sanver, and Eyupoglu 2017). Various investigations on natural fibers as acoustic materials have centered on stalk fibers. Notably, coir fibers exhibit notable sound absorption capabilities, boasting an average absorption coefficient of 0.8 for frequencies exceeding 1 kHz with a thickness of 20 mm (Murugan, Senthil Kumar, and Sakthivel 2022). The utilization of natural fibers, particularly sourced from renewable reservoirs such as cotton and pigeon peas, is gaining momentum within the polymer industry for crafting bio-composites utilized in textiles and diverse sectors. The allure of natural materials within the polymer industry stems from their renewability, cost efficiency, and diminished abrasiveness (Abedom et al. 2021). As consumer demands for high-performance applications and eco-friendly materials rise, manufacturers are adopting natural fiber-based composite materials to transform their products into greener alternatives.

Canbolat et al. (2015) elucidate that in recent times, especially in compact urban settings, the advancement of modern city life has significantly contributed to human well-being, with a particular emphasis on the imperative of tranquility and the mitigation of noise pollution. To address this concern, the incorporation of acoustic insulation technologies in newly constructed structures has become commonplace. Key to this approach are porous sound absorption materials, typically crafted from fibers such as wool, pea stalk, or polyester. These materials possess an intricate network of interconnected macropores and micropores, establishing a sophisticated channel system within solid frameworks and capillaries (Dessalegn et al. 2021). The Cajanus Cajan (pigeon pea) is a perennial legume, recognized as the sixth notable legume crop globally, with India being a major producer (contributing to 90% of world production). The Cajanus Cajan (pigeon pea) is a perennial legume, recognized as the sixth notable legume crop globally, with India being a major producer (contributing to 90% of world production). Its stalks are considered waste afterward collecting the groceries, making sustainable products. Recent studies have explored the potential of using pigeon pea stalks in the formulation of adhesive-bonded composite panels (S. Senthil Kumar, Sakthivel, and Melese 2021). The ample availability of these stalk materials has prompted researchers to investigate their usage as reinforcements in polymer composites, which is a novel approach as such agricultural waste usage. The researchers are actively seeking natural, environmentally friendly alternatives to traditional sound-absorbing materials. Natural fiber composites, particularly those derived from waste materials like pigeon pea stalks, show promise in various applications and industries, and their potential benefits in terms of sustainability and performance are gaining attention. Morphological analysis reveals that the fibers possess a rugged surface, offering significant interfacial strength when employed as reinforcement in composites (Eyupoglu and Merdan 2022).

Islam et al. (2022) have highlighted that the current acoustic insulation materials, composed of porous synthetic substances such as glass fiber, rock wool, glass wool, polyurethane, or polyester, pose risks to human health and the environment due to their mineral or petrochemical origins. This article is dedicated to exploring the viability of utilizing natural fibers sourced from discarded cotton as components in composite materials. The research focuses on examining various properties like acoustic, thermal, physical, and morphological of Cotton/Pigeon pea composites. Notably, the effect of a compatibilizer on these properties has been closely examined. The results of the morphological study revealed that the incorporation of a compatibilizer led to an improved spreading of stalk materials within the composite. Moreover, the mixing of Pigeon pea stalk fibers with waste cotton resulted in noticeable enhancements in physical attributes, acoustic properties, and thermal stability. The advancements described hold promise for industries necessitating effective acoustic and thermal absorption in automotive interior sectors. This study employs epoxy as the primary matrix material, utilizing polymer composites reinforced with Zanthoxylum acanthopodium bark fibers, both untreated and alkali-treated, ranging from 5 to 25 wt.%. The fabrication process involved hand lay-up techniques for developing the epoxy composites. Subsequent assessment of the physical attributes and sound absorption capabilities of the produced composites was conducted following ASTM protocols (Raghunathan et al. 2023). Natural fibers find extensive utility across various lightweight structural applications, including automobile panels, thermal insulation, aerospace components, windows, doors, and insulating panels. Furthermore, there exist additional promising applications for natural fibers, such as vibration-dampening mechanisms and acoustic insulation in settings like recording studios. Given the imperative for eco-friendly materials, there persists a concerted effort to identify novel resources and elucidate their potential applications (Vinod et al. 2022).

This study investigates the development and characterization of composite materials aimed at enhancing sound absorption and insulation in automobile applications. By blending waste cotton fibers and pigeon pea stem fibers in varying ratios (100/0, 70/30, 60/40, 50/50, 30/70, and 0/100), This explores the potential of these materials to serve as effective soundproofing barriers. The composites were developed using the compression molding technique. Our research spotlights the innovative use of waste cotton and pigeon pea stalks in the development of these composite materials, presenting a sustainable solution to address environmental challenges linked to waste management. Additionally, to offer versatile and easy-to-install solutions for noise reduction, particularly in automotive interiors. Through this research, our goal is to contribute to advancements in noise reduction technologies while also addressing environmental concerns by repurposing waste materials in a meaningful manner.

Materials and methods

Materials

The study employed agricultural residues sourced from cotton and pigeon pea materials shown in Figure 1. Cotton waste was procured from Tirupur spinning mills, while the pea stalk agricultural residues were obtained from Aruppukottai, Tamil Nadu, India. These raw materials were amalgamated to fabricate composite materials for the investigation. Initially, the agricultural waste underwent a preliminary opening and cleaning process. Following this, the waste was introduced into a waste-opening machine to extract fibers. These fibers were subsequently mechanically processed in a carding machine to form a web structure comprising multiple layers. The composite was bonded together using epoxy resin with hardeners having a molecular mass of 97.08 g/mol. To prepare the binding solution, 15 g of hardeners were dissolved in 90 ml of water, yielding a 10 wt% hardeners solution. The waste composite materials exhibited significant potential in fulfilling sustainability criteria. They demonstrated high suitability for constructing various built environments, contributing to the development of energy-efficient, lightweight, and durable products without compromising performance (Dhanapriya et al. 2021).

Figure 1. A -Cotton fibre and B- Pigeon pea fibers.

Development of composite materials

The experimental design comprises a series of process blueprints utilized to ascertain the relationship between dependent and independent variables. Utilizing Minitab’s simplex latex design, one can determine sample size and composite proportion. Each sample undergoes testing across three runs (specimens) for each test type. In this experiment, a waste fiber preform undergoes a compression molding process. The preform is positioned within a heated mold and combined with epoxy resin, subsequently injected under pressure into the mold. Common epoxy RIM systems typically feature Diglyceryl ether of Bisphenol A (DGEBA) epoxy with amine curing agents. In epoxy/graphite prepreg, the predominant epoxy system involves N, N, N′, iN′-tetra-glycidyl-4,4”-diaminodiphenylmethane (TGDDM) epoxy paired with 4,4”-diaminodiphenyl sulfone (DDS) as the curing agent. Achieving the correct 2:1 mix ratio by volume entails measuring out 2 parts resin to 1 part hardener before component blending. Compression molding is executed at a mold temperature of 350°F and a mold pressure of 100 psi @700 kPa, sustained at 180°C for a curing duration of 3 minutes. Following complete curing, the mold is opened, and the sample package is extracted.

Post-curing of a composite part involves subjecting it to elevated temperatures, expediting the curing process and/or optimizing the composite’s final material properties. Typically, post-cure processes occur within ~12 hours subsequent to the initial cure. Post-curing entails exposing a cured resin object to temperatures at or above the curing temperature for an extended period, anticipating outcomes such as heightened strength, elevated glass transition temperature, diminished residual stress, and decreased outgassing tendency.

Figure 2 shows, samples encompass compound cotton/pea stalk blends in various ratios, including (S1C 100% Cotton), (S2P 100% Pea stalk fibers), and blended compositions like (S3C/P 50/50, Cotton/Pea stalk fiber), (S4C/P 70/30, Cotton/Pea stalk fiber), (S5C/P 60/40, Cotton/Pea stalk fiber), and (S6C/P 30/70 Cotton/Pea stalk fiber). Waste fiber weights are gauged employing an electronic balance. These selected samples, ranging from 4 to 8 mm in width and 180 mm in length, are subjected to sound insulation testing in accordance with ASTM Standards. Additionally, the physical properties of composite samples are scrutinized through experimental testing (Mekdes et al. 2021).

Figure 2. Cotton/Pigeon pea composite materials.

Experimental testing methods

Measurement of sound absorption coefficient

The ASTM1050–10 standard test system serves as the conventional method for computing the sound absorption coefficient (SAC) within a controlled setting. In this capacity, the Automotive Analyzing and Examining Center (ARTC, Taiwan) is adept at managing the evaluation procedure. Illustrated in Figure 3 is the configuration of the impedance tube technique, a method primarily utilized for gauging the sound absorption coefficient of various materials. This technique offers an effective means of assessing material sound absorption performance and has consequently found widespread application in noise absorption research. When exposed to sound flow, materials interact with incoming sound waves through absorption, reflection, or transmission mechanisms. Analysis of this interaction covers a frequency range spanning from 50 to 4000 Hz, delineated into high (2000–4000 Hz), medium (1000–2000 Hz), and low (50–1000 Hz) ranges. Each sample undergoes thorough scrutiny to ascertain its sound-absorbing capacities, enabling comprehensive evaluations across different materials (Temesgen et al. 2019).

Figure 3. Impedance tube setup methods.

Measurement of thickness

A thickness gauge instrument serves as a tool for swiftly and accurately measuring material thickness. Its utility spans across various industries, with engineering and manufacturing benefiting the most, ensuring compliance with industrial standards and regulations. This instrument, dedicated to assessing composite material thickness, records multiple readings to obtain the average sample thickness being tested, calculated with a precision of 0.01 m. Each sample undergoes 20 tests, and the resulting average reading is used for measurement. The determination of fabric thickness adheres to the ASTM D5729 standard method, as outlined in the study by Senthil Kumar et al. (2022).

Measurement of density

The predominant unit for measuring density is grams per milliliter (grams/mL). Density assessments serve to evaluate the purity and concentration of a sample, offering valuable insights into its composition. ASTM D4052 relies on the concept of bulk density or mass density (grams per square meter), which considers both the weight of materials and the thickness of the fabric. To ascertain this density, a specimen measuring 50 square centimeters is randomly selected and weighed. The average of 20 readings is then computed to determine the composite sample density using the formula: Density (g/m³) = Areal Density (g/m²)/Thickness (m), as outlined by Mekonnen et al. (2020).

Measurement of specific porosity

Porosity (%) can be calculated using the formula: Porosity (%) = (Total Volume – Volume of the Solid)/Total Volume x 100. The higher the resulting percentage, the greater the porosity, indicating the capacity of a material to hold substances within it. In a study by Ezhil, J. J. et al. conducted in (2014), porosity was defined as the ratio of void volume to total volume within the material. The experiments were conducted 20 times, and the average values were utilized for analysis. This relationship is mathematically expressed as follows: VA represents air voids volume, and Vm denotes the total sample volume. The porosity of six composite samples was determined following the ASTM standard D 3776.

Measurement of airflow resistance

The unit is Pa s/m. Its function is to gauge the specific airflow resistance in accordance with ISO 9053 (1991): Acoustics materials for acoustical applications, determination of airflow resistance, which encompasses related quantities like airflow resistance and airflow resistivity. Airflow resistivity, represented by “r” (Pa. s/m²), denotes the specific airflow resistance per unit thickness and serves to characterize homogeneous materials. It’s important to note that this parameter isn’t applicable to composite materials. The thickness of the porous material obstructing airflow is denoted by “d” (m). The air resistance of tested composites underwent evaluation 20 times, and average values were calculated using ASTM D 737, as documented in experiments conducted by (Boominathan et al., 2022 & Eyupoglu, Sanver, and Eyupoglu 2017).

Measurement of thermal conductivity

The rate at which heat flows through a solid, denoted by Q, with a cross-sectional area A, can be expressed as q = Q/A, defining the heat flux. According to Fourier’s law, the heat flux is directly proportional to the thermal gradient: q = -k dT/dx, where k represents the thermal conductivity. An essential tool for quality control in factories is the heat flow meter (HFM), designed specifically for thermal conduction experiments. Despite requiring larger sample sizes, the HFM boasts unparalleled accuracy in measuring thermal conductivity, achieved through precise control of temperature gradients. In this study, samples of various mixtures, including cotton, pea stalk, and combinations thereof, each 5 cm thick, were placed between labeled discs (A) and (B). The objective was to observe temperature transfer between these discs. Two thermocouples were employed to calculate the thermal conductivity of both discs, facilitating the measurement of heat transfer rates. To determine the total heat (Q) transferred, the heat supplied to the system was equated to the heat given up by the materials within the setup. Preliminary results, based on 20 readings from this experiment, were compared against standards outlined in ASTM D6343. This study draws on prior research conducted by Sakthivel et al. (2021) and Ramachandran and Sakthivel (2012) to contextualize its findings.

Measurement of scanning electron microscopy (SEM)

The world’s most advanced conventional scanning electron microscope (SEM), operating at voltages up to 30 kV, achieves an unprecedented point resolution of 0.4 nm when equipped with a secondary electron detector. Structural analysis adhered to the ASTM standard D 256 and was carried out utilizing the JEOL 3 scanning electron microscope (SEM) setup. This instrument enabled the cryogenic examination of fractured composite samples. After the completion of tensile tests, the waste fiber blends underwent crushing, and their surfaces were meticulously scrutinized using the JEOL JSM-6480LV scanning electron microscope. Accompanied by SEM Images A and B, these micrographs vividly illustrate the fractured surfaces of the waste fiber blends that were subjected to the tensile tests (Eyupoglu et al. 2018; Seblework et al. 2020).

Results and discussions

The physical characteristics of a blend comprising 100% recycled cotton and pigeon pea stalk fibers were meticulously evaluated, with the ensuing average values meticulously recorded. Table 1 elucidates the array of samples subjected to scrutiny, encompassing pure cotton (S1C 100%), pure pigeon pea stalk (S2P 100%), an equitable blend of cotton and pea stalk (S3C/P 50/50), a blend favoring 70% cotton and 30% pea stalk (S4C/P 70/30), a blend comprising 60% cotton and 40% pea stalk (S5C/P 60/40), and a blend weighted toward 30% cotton and 70% pea stalk (S6C/P 30/70). The testing adhered rigorously to ASTM standard procedures.

Table 1. Properties of composite materials.

Sample	Thickness (mm)	Density (g/cm^3)	Porosity %	Airflow resistance (cm^3/s/cm^2)	Thermal Conductivity (W/mK)	Sound Absorption Coefficient % (SAC)
S1C	12.03	0.1323	0.769	34.8	0.137	0.15
S2P	12.85	0.1497	0.919	35.6	0.166	0.31
S3C/P	13.05	0.1366	0.924	37.6	0.128	0.18
S4C/P	12.78	0.1212	0.891	38.5	0.147	0.33
S5C/P	13.04	0.1244	0.884	36.5	0.127	0.232
S6C/P	13.02	0.1539	0.894	39.3	0.138	0.361

Scanning electron microscopy analysis

The SEM images in Figure 4 (A) and (B) illustrate the fracture surfaces of waste composite samples following the tensile test. In these images, it can be observed that the fibers have become separated from the resin face due to interfacial enablement. Especially, the composite with 6% weight fiber content and 4 mm length exhibits visible pulled-out fibers. On the other hand, the composite containing 20%wt. fiber and 14 mm length demonstrate the highest quality matrix/fiber adhesion. These findings align with previous studies conducted by Moges, K. A. et al. in 2022 and Fasika Abedom et al. in 2021, who also obtained similar results.

Figure 4. SEM images of composite (A) cotton fibers composites (B) Pigeon pea fibers composites.

Sound absorbing properties of developed composite materials

Composite materials designed for sound absorption play a pivotal role in various noise control scenarios. Traditionally, their acoustic properties are assessed using impedance tubes with small-sized samples. However, real-world noise control situations involve materials of diverse sizes and shapes, with diffuse acoustic fields rather than planar waves. Figure 5 illustrates the sound absorption coefficient (SAC) values for colorful waste cotton/pea stalk-blended samples. Notably, as frequency increases, SAC also rises consistently across samples (S1C, S2P, S3C/P, S4C/P, S5C/P, and S6C/P), indicating improved sound-absorbing capabilities. For example, at an outside frequency of 4000 Hz, SAC values for the samples were: S1C–0.15%, S2P–0.31%, S3C/P − 0.18%, S4C/P − 0.33%, S5C/P − 0.232%, and S6C/P − 0.361%. Furthermore, it’s noteworthy that the sound absorption efficiency of porous materials correlates with their mass density. Higher mass density enhances absorption efficiency for low-frequency sounds but diminishes it for high-frequency ones. Remarkably, composite materials excel in absorbing high-frequency sounds, particularly above 4000 Hz. This finding is consistent with prior research by Hongisto et al. (2022) and Vadivel Santhanam et al. (2012), who also observed similar trends regarding the interplay between frequencies, sound direction, and absorption coefficient. In conclusion, sound absorption composite materials, especially those incorporating waste cotton/pea stalk blends, show significant promise for noise control applications, especially in effectively managing high-frequency sounds. A study by Sair et al. (2019) highlights the favorable sound absorption values of various broom varieties across different sample thicknesses, particularly noteworthy above 500 Hz, endorsing the use of this eco-friendly material for sound absorption treatments. Additionally, natural fibers integrated into composite materials exhibit commendable insulating properties (Sair et al. 2019). Acoustic insulation remains crucial in automotive and building construction to ensure indoor comfort conditions and meet energy-efficiency standards (Aly et al. 2021). ANOVA results underscore the significant impact of composite sound insulation on the overall sound insulation value of developed cotton/pigeon pea stalk fiber composites (p < .0001, R2 = 0.902234), indicating a substantial difference between groups rather than within them, thus affirming the substantial effect of sound insulation on composite performance.

Figure 5. Sound absorption performances of S1C, S2P, S3C/P, S4C/P, and S5C/P & S6C/P.

Impact of thickness on sound absorption

Increasing the thickness of a material can significantly boost its capacity to absorb low-frequency sound waves while exerting minimal influence on the absorption of higher frequencies. This phenomenon stems from the intricate interplay between the size and abundance of pores within the material. Generally, an increase in the number of pores, especially in smaller sizes, enhances the material’s sound absorption capabilities. Conversely, materials with larger pores tend to exhibit weaker sound absorption properties. In Figure 6, we observe a composite sample comprising a blend of 50% cotton and 50% pea stalk fiber, with a uniform thickness of 13.05 mm. This composite, denoted as S1C, S2P, S3C/P, S4C/P, S5C/P, and S6C/P, showcases notably superior sound absorption qualities. Conversely, waste composites with thicknesses ranging from 12.03 mm to 13.02 mm exhibit only marginal sound absorption, particularly when the uniformity falls below 3.5 mm. The optimal sound absorption is achieved with a uniformity exceeding 13.05 mm, boasting an absorption rate of 27%. Materials with higher densities, such as concrete or solid plywood, tend to reflect more sound than they absorb. Furthermore, denser materials have the capability to absorb a wider spectrum of sound frequencies compared to thinner counterparts. These findings are consistent with the experiments conducted by Archana et al. (2012), which demonstrated that increasing both thickness and pore size concurrently elevates the peak value of the sound absorption coefficient (SAC), shifting the peak toward higher frequencies and narrowing the bandwidth. Statistical analysis, employing one-way ANOVA with a 95% confidence level, underscores the significant impact of composite material thickness on both sound and thermal insulation of the developed samples (p < .0001, R2 = 0.900277). This analysis reveals a noteworthy difference (p < .05) among the samples, further validating the importance of material thickness in sound absorption and insulation properties.

Figure 6. Influence of thickness on sound absorption.

Impact of density on sound absorption

The effectiveness of sound absorption depends on various factors, with the mass density of the material being a significant determinant. In the case of porous materials, higher mass density generally results in better absorption of low-frequency sounds but may reduce absorption of high-frequency sounds. As depicted in Figure 7, increasing the thickness of a sample enhances its sound absorption properties. Research conducted by Kathiresan (2012) demonstrated that as sample thickness increased, sound immersion in the center improved, and there was a broader spread of frequencies. Thicker structures exhibited superior performance for frequencies above 4000 Hz but showed less effective absorption at lower frequencies, typically around 500 Hz.

Figure 7. Influence of density on sound absorption.

Interestingly, cotton composites boasting a density variance of 0.1539 g/cm3, paired in a 50/50 ratio of cotton to pea (C/P), demonstrated a noteworthy 24% surge in their sound absorption coefficient (SAC). Likewise, blends of cotton and pea stalks (C/P) at a 70/30 ratio, coupled with a density fluctuation of 0.1497 g/cm3, showcased an amplified mean SAC. Moreover, SAC exhibited an incremental boost of 0.361% for C/P blends at a 60/40 ratio, characterized by density variations of 0.1244 g/cm3, 0.1323 g/cm3, 0.1366 g/cm3, and 0.1212 g/cm3, respectively, with an augmented fiber content per unit thickness. These findings align with a study by Hasina et al. (2016), corroborating similar outcomes. The density of a material emerges as a pivotal determinant governing its sound absorption capacity. Notably, sound absorption values tend to escalate at mid and higher frequencies with an increase in sample density. These principles find validation through statistical scrutiny utilizing a one-way ANOVA, which rendered significant outcomes at a 95% confidence level. The ANOVA analysis underscores the substantial impact of composite material weight on the sound and thermal insulation performance of natural cotton/pea stalk fiber composites (p = .0001, R2 = 0.89719).

Impact of porosity on sound absorption

The porosity in this region corresponds to the ascending segment of the porosity-sound absorption curve, where sound absorption enhances with increasing porosity. However, beyond this peak, heightened porosity results in diminished sound absorption performance. Figure 8 illustrates the influence of porosity pressure on the sound absorption of a composite made from waste cotton and pea stalks, with the following porosity values for the respective lead samples: S1C 0.769%, S2P 0.919%, S3C/P 0.924%, S4C/P 0.891%, S5C/P 0.884%, and S6C/P 0.894%. It was noted that samples exhibiting lower air permeability demonstrated reduced sound absorption at lower frequencies but amplified absorption at higher frequencies. The distinction in sound absorption among the finer pores arises from the interaction between fibers and cohesion, leading to superior sound energy absorption in certain samples. Similar conclusions have been drawn by previous researchers (Abedom et al. 2021; Nandanwar, Kiran, and Ch Varadarajulu 2017). Porous materials for sound absorption consist of channels, cracks, or voids that allow sound waves to penetrate. Sound energy dissipates through thermal loss induced by air molecules’ friction against pore walls, while viscous loss occurs due to airflow viscosity within the materials.

Figure 8. Influence of porosity on sound absorption.

Impact of airflow resistance on sound absorption

Increasing the airflow resistivity of a material leads to decreased air permeability. Consequently, it reduces the likelihood of sound waves passing through, resulting in diminished sound absorption. Conversely, lower airflow resistivity reduces the efficiency of converting sound energy into thermal energy. Figure 9 depicts the correlation between sound absorption coefficient and specific airflow resistance. According to this data, composite materials with higher airflow resistance typically demonstrate superior sound absorption characteristics.

Figure 9. Influence of airflow resistance on sound absorption.

Enhancing the acoustic performance of materials often hinges on optimizing airflow dynamics. Elevated airflow levels notably augment the sound absorption capability. Moreover, heightened airflow resistance and augmented fabric density wield significant influence over sound absorbency. Within this study, the composite materials demonstrate airflow resistances ranging from 34.8 to 39 cm3/S/cm2, correlating with sound absorption coefficients (SAC) spanning 0.15 to 0.31%. Notably, increased fabric density leads to diminished airflow resistance due to enhanced consolidation of air voids, thereby impeding airflow. Additionally, higher short fiber content aids in air void filling, further impacting airflow resistance. Of the tested samples, the waste cotton/pigeon pea composite exhibits the most substantial airflow resistance, registering a SAC of 0.361%. This value surpasses those of S1C, S2P, S3C/P, S4C/P, S5C/P, and S6C/P, as corroborated by Alyousef (2022) and Chagas Rodrigues et al. (2022). As airflow resistance rises, sound absorption curves tend to shift toward lower frequencies. Materials with lower airflow resistance typically exhibit subdued absorption coefficients at lower frequencies, with a pronounced increase observed at medium and high frequencies. The efficacy of sound absorption improves with heightened resistivity of fibrous materials, albeit it diminishes beyond a certain threshold. Insufficient airflow resistance results in minor acoustic energy attenuation and poor absorption, while excessive resistance causes substantial wave reflection, weakening absorption. The airflow resistivity of fibrous porous materials depends on factors such as fiber morphology, size, density, porosity, tortuosity, and arrangement. ANOVA results underscore the significant impact of composite airflow resistance on both sound and thermal insulation values (p < .0001, R2 = 0.87045), revealing an inverse relationship between airflow resistance and the insulation properties of the developed composites.

Impact of thermal conductivity on sound absorption

Samples measuring 300 × 300 × 15 cm³ (width × length × thickness) were utilized for experimentation. Placed between two plates, these samples facilitated the establishment of a temperature gradient across their thickness. The measurements were conducted at intervals of 10, 20, 30, and 40°C. Figure 10 illustrates the thermal conductivity of composite materials, highlighting their superior ability to conduct thermal energy compared to their individual components. Lower thermal conductivity values signify heightened resistance to heat transfer within the composite samples, resulting in better temperature retention. These composite samples comprised 50% waste pea stalk and two-sub cast waste cotton fibers, yielding an aesthetically pleasing blend. The findings underscore potential applications for manufacturing items with similar thermal conductivity properties. Notably, the composite material containing waste pea stalk fibers exhibited a thermal conductivity of 0.147 W/mK, surpassing values observed in samples S1C, S3C/P, S4C/P, S5C/P, and S6C/P. These composite materials prove suitable for ceiling applications, corroborated by similar conclusions in prior studies (Dhanapriya et al. 2021; Rodríguez Juan et al. 2022). Thermal insulation refers to materials or combinations thereof that impede the rate of heat flow via conduction, convection, and radiation when appropriately applied. At a mass fraction of 0.75% of carbon nanotubes (CNTs), the thermal conductivity of foamed hybrid composites reaches its peak. Sound absorption performance of foamed composites exhibits a non-linear relationship with test frequency, with a peak around 1000 Hz. Key factors influencing this performance include temperature, moisture content, and density, alongside secondary factors such as thickness, air velocity, pressing, and aging time. ANOVA results confirm the significant impact of composite thermal conductivity on both sound and thermal insulation properties of the developed natural fiber cotton/pigeon pea stalk fiber composites (p < .0001, R² = 0.902234).

Figure 10. Influence of thermal conductivity on sound absorption.

Impact of water absorption

The composite material’s water absorption was examined across various compositions including S1C, S2P, S3C/P, S4C/P, S5C/P, and S6C/P. Results indicated that S1C exhibited 1.53% absorption, S2P 1.33%, and S3C/P 1.19%, while S4C/P showed 1.10%, S5C/P 1.13%, and S6C/P 1.08%. Notably, SC1 demonstrated the highest absorption, whereas S3C/P showcased the lowest among the samples. When analyzing water absorption inversely, SC1 had the highest while S3C/P had the lowest values. It’s noteworthy that as the proportion of pigeon pea stalk decreased and that of cotton fiber increased, the water absorption of the composite material saw a corresponding rise. Generally, the material strengths decreased post-moisture uptake due to the influence of water molecules altering the structure and properties of the fiber, matrix, and their interface. Upon moisture infiltration, the fibers tended to swell, corroborating findings by Fasika Abedom et al. (2021).

Conclusion

A collection of six distinct waste fiber composite materials, denoted as S1C, S2P, S3C/P, S4C/P, S5C/P, and S6C/P, underwent both acoustics and thermal absorption assessments. Notably, among these samples, S1C and S2P, which comprise waste cotton and pea stalks, showcased remarkable sound absorption and insulation characteristics. Additionally, S3C/P, S4C/P, and other composite variants exhibited exceptional noise absorption, surpassing 75% in the frequency range of 50 – 5000 Hz, even enduring high moisture conditions without compromising their absorption capabilities.

Scanning Electron Microscopy (SEM) uncovered some degradation in the scales of waste cotton and pea stalk fibers within S1C, S2P, and S6C/P composites. However, this degradation did not fundamentally alter the overall morphology of the waste pea stalk fiber blends. These vital composite materials not only offer cost-effective benefits but also align with green structural initiatives by harnessing natural waste resources. Comparative evaluations with other natural fiber-based composites revealed that cotton and pigeon pea stalk-reinforced epoxy composites exhibited similar advantages in physical, acoustic, and thermal properties. This suggests a promising potential for this new category of composites as viable substitutes for certain natural fiber-reinforced alternatives. Such materials hold significant promise for various lightweight operational components, including indoor civil constructions, furniture manufacturing, packaging containers, and automobile interior parts, thereby promoting eco-friendly practices and sustainable development.

The identification of waste fiber composites as a high-impact alternative to synthetic counterparts is noteworthy. The trend toward replacing synthetic fibers with waste fibers in epoxy composites represents a contemporary phenomenon with extensive potential in construction materials, potentially supplanting traditional structural materials across diverse applications. Simultaneously, there exists a growing interest in discovering novel, high-performance materials at affordable costs, with a strong emphasis on eco-friendliness and a shift away from nonrenewable resources.

Highlight of the paper

The study investigates waste cotton and pigeon pea stalk composite materials for sound absorption, thermal conductivity, and physical characteristics. The waste composites demonstrated over 80% sound absorption and excellent insulation, moisture resistance, and fiber properties. Notably, the waste cotton/pea stalk composites achieved over 75% sound absorption and maintained their qualities even in humid conditions. These materials offer cost-benefit advantages and contribute to green structures, making them promising substitutes for natural fiber composites in various applications, promoting eco-friendly practices and sustainable development.

Key findings and contributions of the study:

• The study successfully synthesized composite materials using waste cotton and pigeon pea stem fibers in different ratios. These waste composites offer potential solutions for sustainable development, as they utilize agricultural waste materials that would otherwise be discarded

• The waste composite materials demonstrated excellent sound absorption capabilities, with some samples achieving sound absorption values of over 80%. The composites showed consistent enhancement in sound-absorbing performance as the frequency increased, making them suitable for noise control applications, especially in handling high-frequency sounds effectively

• The utilization of waste cotton and pigeon pea materials in composite production contributes to green building initiatives and sustainable practices, promoting eco-friendly products and reducing environmental waste.

Novelty of the paper

The paper explores the synthesis and characterization of waste composite materials using pigeon pea stem and cotton fibers. The use of agricultural waste materials for composite production contributes to sustainable practices and environmental benefits.

Disclosure statement

No potential conflict of interest was reported by the author(s).