Performance of Natural Multi-layer Coir and Kenaf Fibers as Sustainable Acoustic Absorber

Natural fiber materials are often favored as a replacement to synthetically made sound absorbers due to its comparable sound absorption properties as well as in the effort to reduce the environmental issue. However, studies are weighted heavily on evaluating the performance of a single or composite fiber and only few studies focused on the multi-layer fiber. In this paper, the sound absorption performance of multi-layer coir and kapok fibers is presented. The kapok fiber is used as the assisting layer element for the coir fiber. A combination of coir fibers with different layer thicknesses and with additional 2 mm thick kapok layer shows significant improvement of the absorption bandwidth. It is also found that the kapok fiber layer placed in between the coir fiber layers show the best improvement of the absorption frequency bandwidth.

Sound absorbers play a significant role in keeping a quiet and noise-free environment. Both synthetic and natural materials can use to control noise. Currently, natural materials are a valid alternative to conventional synthetic materials. This paper aims to study the physical parameters of Grewia Optiva fibers. The theoretical models’ assessment has been carried out to calculate the sound absorption coefficient with the help of Scilab software for synthetic (melamine foam, polyurethane foam, and mineral wool) and natural (kenaf fiber, coir fiber, and Grewia Optiva fiber) materials. The results from the simulation conclude that the sound absorption coefficient of Grewia Optiva fiber is almost similar to or much better than other synthetic and natural sound absorbers. Hence, Grewia Optiva fibers can be a valid substitute for traditional sound absorbers for sound absorption purposes.

This research was carried out to study the acoustic properties of natural organic fibres; kenaf and coir fibres using impedance tube method. Kenaf fibre was used as noise absorber filler in an insulation panel while the coir fibre as reinforcement in the perforated composite panel. The perforated panel was made from coir fibre/polyester composites with coir fibre volume fraction of 10%, 20% and 30%. The perforation area of the perforated panel was also varied at 10%, 20% and 30%. During the processing stage, the kenaf fibre sheet has been treated with PVA and cut into 100 mm and 30 mm diameter sample for low and high frequency test. The density of the coir fibre is determined to be 32.2 g/cm3 while the density of the kenaf fibre is 42.6 g/cm3. The tests were carried out using impedance tube at acoustic lab, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia using ISO 10534-2 standard for noise absorption coefficients to determine their noise absorption coefficient. The results obtained show that the optimum noise absorption coefficient index for kenaf fibre is 0.8 with 10% fibre volume fraction of coir fibre/polyester perforated composites panel at 10% perforation areas.

Cement concrete is used in small structure to big multi-storied buildings. It is noteworthy to concentrate on concrete elements and mix ratio to achieve desirable strength with feasible cost of raw material. In this investigation, fibers reinforced concrete specimens (M50 grade) were prepared. The strength and durability were tested and compared with the properties of plain concrete (PC). The fibers such as coir, kenaf, and Polypropylene (PP) were used as reinforcement and the compressive, split tensile, flexural, and durability analysis were performed. The compressive test resulted that the natural fiber-based PC/Coir & PC/PP-Coir specimens displayed the maximum compressive strength of 64.5 and 66 MPa owing to the small density and slighter water up taking properties of coir and PP fiber. The split tensile test showed that the PC/kenaf specimen exhibited the higher split tensile strength of 7.3 MPa due to the presence of kenaf fiber with high young’s modulus. Flexural test reported that the PC/PP-Coir/Kenaf specimen exhibited the higher flexural strength of 7.6 MPa because of reinforcing effect offered by coir, kenaf, and PP fibers. Durability study showed that the PC/PP and PC/PP-coir specimens displayed the minimum weight loss of 1.05 and 2.1% and the minimum loss in compressive strength of 3.4 and 4.6%, respectively, in alkaline medium. It was concluded from this study that the coir fiber could be a suitable candidate to improve the strength and durability of concrete.

An analytical study based on rigid frame model is demonstrated to evaluate the acoustic absorption of coir fiber. Effects of different conditions such as combination of air gap and perforated plate (PP) are studied in this work. Materials used here are treated as rigid rather than elastic, since the flow resistivity of coir fiber is very low. The well-known rigid frame Johnson-Allard equivalent-fluid model is applied to obtain the acoustic impedance of single layer coir fiber. Atalla and Sgard model is employed to estimate the surface impedance of PP. Acoustic transmission approach (ATA) is utilized for adding various consecutive layers in multilayer structure. Models are examined in different conditions such as single layer coir fiber, coir fiber backed with air gap, single layer PP in combination with coir fiber and air gap. Experiments are conducted in impedance tube on normal incidence sound absorption to validate the results. Results from the measurement are found to be in well agreement with the theoretical absorption coefficients. The performance of the rigid frame modeling method is checked more specifically in all conditions, by the mean prediction error rate of normal incidence sound absorption coefficients. Comparison between the measured absorption coefficients and predicted by rigid frame method shows discrepancy lower than 20 and 15% for most of the conditions in the frequency range of 0.2–1.5 and 1.5–5 kHz, respectively. Moreover, acoustic absorption of various single and multilayer structures is compared with the simpler empirical methods such as Delany-Bazley and Miki model; and complicated method such as Biot-Allard Model and Allard Transfer Function (TF) method. Comparisons show that the presented method offers a better accuracy of the results than the empirical models. Subsequently, it can provide almost same absorption plot with Biot-Allard model (single layer combination) and TF method (multilayer combination) proving it to be a comprehensively easy and general analytical tool. Therefore, the rigid frame model can be implemented relatively easier than other similar models to analyze the acoustic absorption of coir fiber in most of the conditions.

Coir fibres and jute fibres are natural materials that are abundantly available in tropical regions of our country. Waste generated by the industrial and agricultural processes has created disposal and management problems, which causes serious challenges for environmental conservation. A considerable amount of coir fibres and jute fibres remain in the environment as waste, so the utilization of these materials for construction is an important step to improve sustainability and eco-friendly construction. The current study deals with the addition of coir and jute fibres to concrete such as attain sustainable construction material without sacrificing the strength of concrete. To check its suitability, test for compressive strength, split tensile strength, flexure strength and water absorption test were performed. To ensure the suitability of coir fibres and jute fibres as ingredients of concrete the results obtained from above were compared with conventional concrete. Also, a comparative study of fibre reinforced concrete and self-compacting fibre reinforced concrete was carried out. The work was carried out by conducting tests on the raw materials to determine their properties and suitability as engineering material. Concrete mix designs were prepared using the IS method for an M30 grade concrete. The specimens were cast with percentage addition of 0.7 jute fibre and 0.3 coir fibre, 0.3 Jute fibre and 0.7 coir fibre and 0.5 Jute fibre and 0.5 coir fibre.

Tensile and flexural properties of polylactic acid-based hybrid green composites reinforced by kenaf, bamboo and coir fibers

In the current decade, a number of industries have moved their attention towards emerging sustainable technologies in order to better support socio-economic and environmental considerations. The present research investigates a unique hybrid composite developed by the amalgamation of natural kenaf-coir fibers, with resin of epoxy, incorporated with titanium carbide (TiC) nanoparticles. This study also presents the development process involved in manufacturing the composites, along with mechanical testing and optimization of these composite samples. The nanofillers of TiC are utilized in wt. percentages of 0%, 3%, 4%, and 5%, while coir and kenaf fibers are incorporated at 0%, 3%, 4%, and 5% by weight, and the thickness of the samples is varied at 2, 3, 4, and 5mm. The mechanical attributes of composites are evaluated using a vacuum bag molding process, with subsequent testing and optimization performed through Taguchi and ANOVA analysis to discover the optimal sample combination. The findings indicate that the most effective composite formulation includes 4% TiC, 5% kenaf, 5% coir, and a thickness of 3 mm, which provides the highest tensile modulus and strength among all tested samples. The integration of kenaf fibers with coir fibers and TiCs as fillers significantly improves the tensile and flexural attributes of the hybrid composite in contrast to composites made with coir or kenaf fibers alone.

In the last few years, concerns about the adverse effects of petroleum-based materials on the environment and public health have gradually increased. It is worthy and meaningful to find natural materials to substitute the conventional synthetic sound absorbers. The sound absorption properties of Norway maple leaves were investigated in this paper. The leaves laminated in 5 to 50 layers were measured in the low- to mid-frequency and mid- to high-frequency ranges in the impedance tubes. Later, the sound absorption performance of laminated leaves was compared with polyester, kapok, coir, and broom fibers. The results show that the 15 layers of leaves exhibit good sound absorption properties in the two frequency ranges. Moreover, the 15 layers leaves have comparable sound absorption performance to polyester and coir fibers. The results indicate that the maple leaves are the potential alternative material to synthetic material in a low-frequency range with a thin structure.

Chapter 7 - Manufacturing and design of coir fiber composites

Use of natural material for development of sound absorbers received considerable significance since in past few years. Natural materials are preferred due to their ease of availability, low environmental impact, distinctive internal structure and reusability. One such natural material Peanut shell is made up of natural cellulose fibers and has good internal pores which can be utilized for sound absorption application. In the current study samples of Peanut shell specimens were made of 100mm diameter having different material to binder weight ratios. The test specimens were made with thickness 10mm, 15mm, 20mm, 25mm, 30mm, 35mm and 40mm respectively. Normal incidence sound absorption coefficient for the specimens is measured using ASTM E1050-98 (Standard Test Method for Impedance and Absorption of Acoustical Materials Using a Tube, Two Microphones, and a Digital Frequency Analysis System) and values of the normal incidence sound absorption are compared with values obtained using Delany and Bazley Model. It was observed that Peanut shell test specimen with material to binder weight ratio 70:30 gives optimum average sound absorption for frequency range between 250Hz to 4500Hz. The current paper concludes that Peanut shell can prove to be good alternative natural material over existing conventional sound absorbing materials for application in field of sound absorption.

Various natural sound-absorbing materials such as rice by-products, coir fiber, date palm fiber, peanut husks, hardwood cross-sections, and forest by-products have been introduced to replace petroleum-based sound-absorbing materials in previous studies, and their sound-absorbing performance was significant. This study investigated the sound-absorbing performance of pure coffee grounds as an eco-friendly sound-absorbing material. After inserting coffee grounds into cylindrical holders with lengths of 20, 30, and 40 mm, the density of the coffee grounds was adjusted from 0.2 to 0.5 g/cm3. Then, the sound absorption coefficients were measured by an impedance tube. As the thickness and density increased, the sound absorption coefficient at low frequencies improved. However, the sound absorption coefficient at high frequencies decreased. The optimal noise reduction coefficient (NRC) of coffee grounds investigated in this study was 0.61 at a density of 0.3 g/cm3 and thickness of 50 mm. This result shows a sound-absorbing performance that is comparable to other natural sound-absorbing materials. This study concludes that coffee grounds have high use-value as an eco-friendly sound-absorbing material.

The normal incidence sound absorption coefficient of single-layered porous materials predicted using some prediction models is well known. The published acoustic behaviors prediction models, such as Biot model, Zwikker and Kosten model, Delany and Bazley model, and Champoux and Allard model, can give acceptable prediction results by only taking specific flow resistivity and material thickness as independent variables to estimate the normal incidence sound absorption coefficient. However, the existing literature fails to provide proper knowledge regarding the acoustic characteristics of dual-layered porous nonwoven absorbers. So, the aim of this paper was to propose a theoretical acoustic model for dual-layered porous nonwoven absorber and to verify the proposed model experimentally. In theory aspect, the study focused on the extension algorithm of the Zwikker and Kosten model for dual-layered nonwoven absorber. The theoretical analysis of the impact of thickness and porosity of outer and inner layer on sound absorption coefficient was detailed using numerical simulation method. In experiment aspect, we particularly designed 20 dual-layered nonwoven absorbers with four types of meltblown polypropylene nonwoven materials and five types of hydroentangled E-glass fiber nonwoven materials firstly. Secondly, the calculated sound absorption coefficients using the proposed model were compared with the measured ones of the 20 dual-layered nonwoven absorbers at 250, 500, 1000, and 2000 Hz. Experimental results indicate that the measured and the calculated data have very similar trend with the change of thickness, porosity, and the sound frequency, apart from the obvious difference between them at low frequency.

Wheat straw, which is a by-product of wheat production and has a tubular structure, is typically used for animal feed and compost. This study estimated the sound absorption coefficient of wheat straw based on cross-sectional computed tomography (CT) images. After image processing, the surface area of the wheat straw skeletal outline and the volume of the void area were determined. The propagation constant and characteristic impedance of the void area were obtained by approximating the clearance between two parallel planes representing the void area walls. Each CT image was represented as a transfer matrix to calculate the sound pressure and particle velocity, and the transfer matrix method was used to derive the normal incidence sound absorption coefficients. The measured tortuosity was considered when calculating the normal incidence sound absorption coefficient. The CT images were corrected to reflect the lack of sound absorption by the porous part of the thick-walled portion by considering it as a solid structure. The theoretical sound absorption coefficients calculated from the corrected images were in good agreement with the measured sound absorption coefficients.

Natural fibre has been conventionally and widely utilised as a sound absorber in order to replace the traditional synthetic absorber materials. In this study, coir fibre (CF) was prepared as an acoustic absorber and subjected to an additional surface treatment by using sodium hydroxide (NaOH) at various concentrations ranging from 1% to 8%. This was geared towards analysing the effect of alkalisation on the fibre morphology, diameter, and changes occurring in the CF functional groups, thus resulting in enhanced sound absorption properties. To this end, the fibre surface was analysed using a scanning electron microscopy (SEM) to study the surface morphology of treated and untreated CF materials, whereas the implementation of Fourier-transform infrared (FTIR) allowed an analysis of CF characterisation. The absorber sample was fabricated at a constant thickness of 45mm and a density of 0.4g/cm3 density prior to testing for the sound absorption coefficient (SAC) by using an impedance tube. The morphology of CF revealed the treated fibres to be free of impurities including lignin and hemicellulose layer, which were removed from their surface. This finding was supported by the peak changes observed on the FTIR spectra. Furthermore, the fibre diameter was reduced as the concentrations of NaOH increased. The results conclusively indicated that treated CF at the concentrations of 7% and 8% NaOH gained the highest SAC values across the low and high-frequency ranges, yielding an α coefficient average of 0.9 and above.