Rice stubble: cotton fly waste composites for acoustic applications

Abstract The comfort level of human being can be increased by effective noise management without damaging the external environmental resources. The burning of rice stubble particularly in rice growing Asian countries makes the situation bad to worst. Rice stubble deserve for an immediate solution of in product development for its sustainable disposal. Rice stubble, cotton fly waste were used to manufacture composite material to be used as acoustic panel. Rice stubble powder 60%, 70%, and 80%, cotton fly waste 20%, 30%, and 40% by weight, and polyurethane (PU) resin were used to manufacture composite as acoustic panels. The composites were tested for pore size, morphology, water absorbability, and noise reduction coefficient (NRC) mainly. Acceptable level of NRC and other features were achieved. This study will provide a new pathway to use the discarded challenging waste materials to manufacture wealth-generating products.


Introduction
Noise is an undesired sound, nevertheless of its severity or period.Sound tainting has been admitted as a vital menace to human health regarding hearing potential and wellness disorders (Thompson, 2017).A noise level higher than 80 decibels (dB) generates physiological impacts, and long-span exposure above 100 dB causes enduring impairment to human hearing.Materials that suppress the acoustic intensity, as the sound wave passes through it by absorption are acoustic materials (Hong et al., 2013).The acoustic mechanism can be divided into two major classes: passive sound absorption and active sound absorption.The external energy is needed to neutralize the sound energy that comes under the active sound absorption mechanism, and the sound energy disseminated and converted into heat is called passive sound suppression (Bingham et al., 2012).
The architecture of acoustic absorbing structures to control the noise in buildings, auditoriums, and vehicles is challenging for material scientists (Talebi et al., 2019).Most of the acoustic materials are porous and can be classified as cellular materials (foam-like structures), granular structures (concrete-like structures), and fibrous composites (Yang et al., 2018).Fiber-based compositions such as knitted, woven, and nonwoven have acquired enough attention in acoustic applications due to their low price and high effectiveness (Kang et al., 2019;Soltani & Zarrebini, 2013;Tang et al., 2018).The nonwovens were found excellent for sound absorption but unable to give a desired esthetic look and thus required a coverall of conventional woven fabric (Shoshani et al., 2019).The knitted structure was also appropriate for sound absorption, but their acoustic potential remains limited due to their low thickness (Liu & Hu, 2010).The natural fibers are explored to engineer acoustic composites to replace conventional synthetic fibers (Yang et al., 2020).The acoustic potential of natural fibers motivated to explore the possibility of some other agricultural waste for sound absorbing purposes.Three-dimensional woven fabrics are still waiting for a systematic study in acoustic applications due to their higher thickness than traditional woven fabrics.A similar thickness is achieved in a composite structure where desired thickness is easily achieved (Echeverria et al., 2019).Micro-perforated panels (MPPs) are also accepted with an average orifice diameter of 0.5-1 mm as an effective acoustic medium.
The sound absorption potential of MPPs was modified significantly by introducing an effective backing in the form of cloth to provide double sound absorption as an effective next generation acoustic solution (Liu et al., 2017).The textile materials have enormous pores naturally in structures that gained ample attention to be used as future acoustic materials.Cavities and pores of acoustic structure that can cause sound absorption (Lee et al., 2017).
The low cost and ease in manufacturing are two additional advantages associated with woven, nonwoven, and knitted textile materials.
3D spacer-knitted fabrics were also influential in indoor acoustic applications (Arumugam et al., 2015).Rice stubble as an agricultural waste is suggested in acoustic panel manufacturing (Fatima et al., 2021).Recycled cotton fibers were used for acoustic panel manufacturing with polyethylene/ polypropylene packaging waste as matrices material (Sezgin et al., 2021).Cotton fabric waste had a higher sound absorption coefficient among the silk, polyester, linen, and viscose fabrics (Jailani & Isa, 2020).Epoxy 951 with hardner 251 and polyester resin with MEK (methylethylketone) hardner and cobalt accelerator were tried to manufacture acoustic panels in which polyurethane (PU) resins were found suitable for underwater applications (Bertolini et al., 2019;Bhingare et al., 2019;Jayakumari et al., 2019).Agriculture wastes were also found useful to manufacture acoustic items.Rice straws were dispersed with cationic starch followed by freeze-drying to form aerogels and coated with methyl trimethoxy silane to get a water repellent acoustic structure (Tran et al., 2020).
The combination of rice stubble, cotton waste, and PU resin has not been opted to manufacture acoustic panels.PU opted as matrices during acoustic panel manufacturing to utilize the rice stubble, the most air polluting material after burning by farmers, and cotton fly waste generated in the cotton industry.Box and Behnken surface response method was used to optimize the acoustic potential of composites.

Material
Rice stubble was collected from local field and found bulk density 90-150kg/m 3 .The rice stubble composition was analyzed in National Institute of Natural Fiber Engineering and Technology (NINFET), Kolkata, India Laboratory and found, cellulose, hemi-cellulose, lignin and silica as major component.The rice stubble consist silica 11%, lignin 19.9%, cellulose 39.8%, pectin 2.3% fat and waxes 11.5%, and hemicellulose 20.4%.The presence of amorphous silica determines the pozzolanic effect of Rice stubble.Pozzolanic effect exhibits cementitious properties that increase the rate at which the material gains strength.
Cotton fly waste having 12-15 mm length and fineness of 5.92 micronair value, was collected from Shatabdi Knitware Industry, Kanpur.Cotton is a natural absorber due its porous structure, and provides a better propagation toward sound waves because of the small diameter, spiral shape and the presence of convolutions.
Polyurethane is a pre polymer of isocyanate with freeflowing consistency and density of 1.125 g/cm 3 .Tensile modulus and strength were 36 and 48 MPa, respectively.Melting temperature was 210 C and hardness was 55D.Polyurethane (PU) resin was procured from Kumar Rotoflex Pvt Ltd Kanpur (Figure 1 (a-d)).
To exclude extra trials and create higher order response surface, the Box and Behnken design of experiment is opted in this study as shown in Table 1.

Method
The rice stubble were dried at 80 C for 4 h and then chipping into small pieces .To develop composite samples, a mold of 20 Â 20 Â 1 cm was prepared.Firstly, PU resin was uniformly distributed within the boundaries of the mold, followed by distribution of rice stubble powder as shown in Figure 1e.The cotton fly waste was spread layer by layer to minimize the mass variation and to get 10 mm thickness (Figure 1).The molds were kept for 12 h to get it fixed at room temperature.The Box and Behnken surface response methodology with rice stubble content 60%, 70%, and 80%, cotton fly waste 20, 30 and 40%, PU 5, 7.5, and 10 volumetric ratio of raw material including rice stubble and cotton waste was used as shown in Table 1.The samples were chopped to get the standard size for various characterization techniques as shown in Figure 1f.

Acoustic performance test
A simple 4 0 Â2 0 Â2 0 wooden box is fabricated to measure sound transmission loss through the composite material.The outer wall of the box is covered by a ceramic sheet and the inner wall is covered by acoustic foam to absorb the reverberation sound.
The JBL GO Bluetooth speakers were mounted on an inner side of vertical wall of the instrument, and two decibel metering devices were fixed in the exterior of top wall.First speaker was fixed and kept stationary, while the second was dynamic to do fine tuning of sound intensity between source and detecting decibel meter as shown in Figure 1h.These decibel meters (Mextech SL 36) were purchased from global medical shop.A 20 Â 20 cm composite material piece was mounted vertically in sliding condition between these two-decibel meters.The least count of decibel meter was 0.1 Db with tolerance limit of ±1.5 Db.
The sound absorption coefficient of samples was obtained by using indigenous sound absorption tester (inventor has applied for Indian patent) as per ISO 10534-2:1998.The noise reduction coefficient (NRC) and sound transmission loss (R) were determined using the following equations: where d is the difference in sound power between transmitting room and receiving room in decibel.
where L 1 is the sound pressure in transmitting compartment, L 2 is the sound pressure in receiving compartment

Scanning electron microscopy (SEM)
Joel, JCM 7000 NeoScope, scanning electron microscope was used to study the morphology and fine structure of rice stubble, and composite material at 10 kV.The sample surfaces were made conductive after depositing a very layer of gold by Cressington Q108 sputter coating machine.The captured images were analyzed by using ImageJ 1.47 image analysis software.
The air space content of composite material was measured and analyzed as per ASTM D 273-416.
where q ct is the calculated, and q e is the measured density of the manufactured composite sheet.The following equation was used to estimate the ideal density of the composite sheet.

Water absorption test
The ASTM D570 test standard was used to estimate the water absorption potential of the samples after conditioning at 65 C for 48h.The water adhering was permitted at 25 C for 216 h till achieved the perfect absorption level as shown in Figure 1g.This test will help to understand the water retention capacity of fibers and the composite panel.This test helps to evaluate the acoustic performance of the composite panel if it were to be used under wet conditions.

Result and discussion
The acoustic panels were subjected to different characterization techniques to verify its performance.Table 3 concludes the multivariate data of composite porous structure and ANOVA analysis.The P-value and F-value of multivariate analysis are the ratio between mean square results to the mean square error, respectively.The null hypothesis can be rejected in case of P-value lesser than 0.1.The P-value 0.4429 indicates the insignificant "Lack of Fit" and satisfactory fit of data to the proposed mathematical model.The high value of correlation coefficients R ð Þ 2 0.80 established the fitness of models and preciseness of estimated constants.The number of predictors for porosity was mentioned by adjusted R 2 was 0.5451 in the given model and the anticipated R 2 values were found was 0.5680.The coefficient of correlation, adjusted R 2 , and anticipated R 2 , presented strong correlation between predicted and experimental results (Tables 2 and 3).
The quadratic polynomial model (Eq.3) is proposed here to predict the porosity of composite by using cotton fiber content, rice stubble content, and PU content as input parameters.
Porosity ¼ 110:05þ8:87 Â cottonfly wasteÀ2:89 Ârice stubbleÀ0:23ÂPUÀ13:70Âcottonfly waste 2 þ23:72Ârice stubble 2 À37:45ÂPU 2 þ15:05 Âcottonfly wasteÂrice stubble À26:17 Âcottonfly wasteÂPUÀ8:2Ârice stubbleÂPU (5) Figure 2a indicates the interaction of rice stubble and cotton fly waste on porosity of composite sheet keeping the PU contains kept 7.5 times of rice stubble and cotton fly waste.The free space or porosity of composite panel was decreased from 141.3 lm pore size to 93.4 lm by increasing in rice stubble content from 60 to 80%.It may be due to the fine particle size of rice stubble with low density, which is blocking the available pores.
The interconnectivity between rice stubble and PU resin on composite sheet porosity was plotted in Figure 2b, when cotton fly waste add-on was kept constant at 30%.This figure indicates optimum pore size 104.4 lm was obtained at 8 volumetric ratio of PU.Effect of rice stubble follow same trends as previous one Figure 2a The effect of cotton fly waste and PU on porosity is shown in Figure 2c, which indicates that porosity was increased from 24.1 to 90.2 lm by increasing both parameters cotton fly waste and PU.
The effect of cotton fly waste and PU is clearly visible on porosity of acoustic panel.

Optimization of porosity
The effect of these parameters: cotton waste %, PU and rice stubble % on porosity of acoustic panel was analyzed for optimization purposes.The response optimizer component of design expert tool was employed for porosity optimization.For this piece of work the utmost values for porosity from optimization via software was obtained as 136.1 lm and the corresponding values for rice stubble fiber loading, cotton fly waste and PU are 80, 40, and 5.98 g volumetric ratio demonstrated in Figure 3.

Water absorbency test
The water absorbency of acoustic panel is given in Table 4.The response surface methodology was used to predict the interconnectivity, which describes the dependency of water absorption on rice stubble, cotton fly waste and PU content in acoustic panel.Figure 4a indicates the interaction of rice stubble and cotton fly waste on water absorbency of composite sheet keeping the PU contains kept 7.5 times of rice stubble and cotton fly waste.The water uptake was increased from 54.0% to 66.8% and then slowly starts to decrease up to 62% by increasing the rice stubble content from 60% to 80%.The cotton fly waste loading increases the water absorbency of acoustic panel from 54.0% to 62.4% when cotton fly waste content was increased from 20% to 40%.
Interaction of rice stubble and cotton fly waste on water absorbency of composite sheet shown in Figure 4b.PU contains kept constant at 7.5 times of rice stubble and cotton fly waste.The water absorbency was decreased from 74.8% to 54.8% by increasing the PU content from 5 to 10 times.It may be based on fact that PU is a hydroscopic material and decreased the possibility of water absorption.
Water absorbency increased from 53.5% to 72% by increasing PU content from 5 to 10 times of rice stubble and cotton fly waste.Water absorbency also increased by increasing cotton fly waste.It follows the same trend as PU clearly seen from Figure 4c.
From the following trends, it can be seen that cotton fly waste play very crucial role in water absorbency of acoustic panel.

Scanning electron microscopy
The scanning electron microscope photographs are shown in Figure 5.It is clearly shown in SEM photographs that   pores are available throughout the surface which ensures the free space for optimum sound absorption.Effect of rice stubble and cotton fly waste is different but combined effect of these parameters maintains sufficient porosity throughout the surface.As the rice stubble content increases, the pore size is decreasing in the ratio.The role of cotton fly content is also clearly illustrated in SEM photograph.The pore shape irregularity was increased by increasing the cotton fly waste.The pore shape is irregular and shape can be not standardized.However in PU foam, the pore size distribution is quite uniform and small.The fiber content in acoustic panel composite reflect its effect on pore size and pore size distribution as shown in Figure 5.

Sound absorption properties: NRC
The NRC and sound transmission loss (R) were determined using the following equations: where d is the difference in sound power between transmitting room and receiving room in decibel where L 1 is the sound pressure in transmitting compartment, L 2 is the sound pressure in receiving compartment.
The ANOVA analysis and multivariate parameters for the NRC are presented in Tables 6.
The ANOVA analysis represents the total of mean squares of every variable.
The model adequately fitted the data, as evidenced by the insignificant "Lack of Fit" with a p-value of 0.70 for sound absorption potential of acoustic panel.
The accuracy and appropriateness of statistical model were confirmed by the higher correlation coefficients, 0.99.The high correlation coefficients confirm a adequately fit of the model to the experimental data.The adjusted R 2 (R 2 ¼0.9997) confirms the significance of number of predictors in the model.The anticipated R 2 values were found was 0.9994.
The adjusted R 2 and anticipated R 2 proved the high association between perceived and anticipated results.The second-order polynomial model is given in Equation ( 8) to predict the NRC by feeding the rice stubble, fly waste and PU content in the acoustic panel manufacturing.
The stereographic behavior of the acoustic panel is plotted in Figure 6.The NRC was increased by increasing rice stubble and cotton waste content in acoustic panel manufacturing.The PU content was kept constant at 7.5 times of cotton fly waste and rice stubble content to study the effect of rice stubble and cotton fly waste on NRC, as shown in Figure 6a.The NRC increased from 0.29 to 0.32 as rice stubble powder increases from 60 to 80%.The cotton fly waste content was also found as noise absorbing material.The NRC was increased from 0.29 to 0.35 by increasing the cotton fly waste loading from 20% to 40%, higher than a commercial acoustic panel of the same thickness (0.33).
The cotton fly waste was kept constant at 30% to study the effect of rice stubble and PU on NRC, as shown in Figure 6b.PU content does not have a remarkable impact on NRC, but the rice stubble powder was again found decisive to enhance NRC from 0.30 to 0.33.The cotton waste content in composite acoustic panel also increased the NRC.The NRC increased from 0.29 to 0.33 by increase in cotton waste content from 20% to 40% but the effect of PU resin content on NRC is not linear in nature as explained in Figure 6c.
From the following trends, it can be seen that rice stubble and cotton fly waste have significant effect on NRC of acoustic panel.

Optimization of sound absorption properties
The effect of these parameters cotton waste %, PU and rice stubble % on NRC of acoustic panel was analyzed for optimization purposes.The NRC optimization was performed by design expert software through response optimization feature and found the utmost values for NRC as 0.35.The NRC of 0.35 was achieved for rice stubble, cotton waste, and PU loading of 80%, 40% and 7.08 times of rice stubble and cotton fly waste (volumetric ratio) respectively as shown in Figure 7.

Conclusion
Rice stubble a challenging agriculture waste is successfully used to convert it into wealth in the form of acoustic composite panel with another industrial cotton fly waste.The composite acoustic panels were found comparable with acoustic panel available in the market.Rice stubble in powder form is useful to enhance NRC.Cotton fly waste enhances the binding potential of rice stubble with PU resin, which jointly increases the NRC values of the panels.Effective porosity, an essential feature for acoustic application has been achieved up to 141 lm maximum pore diameter.Maximum 0.4 NRC was achieved by using rice stubble 70%, cotton fly waste 30% by weight and PU resin 7.5 times of rice stubble and cotton fly waste by optimization of process parameters by surface response method.Some more waste materials like Rye waste, Pigeon pea waste can be added for the manufacturing of acoustic panel for sustainable development.

Disclosure statement
This is original research work and did not submitted to any other journal for publication.Authors declare that this research work does not have any conflict of interest.We both authors declare the interest statement.

Figure 2 .
Figure 2. Effect of (a) rice stubble % and cotton waste %, (b) PU and rice stubble %, and (c) PU and cotton waste % on porosity.

Figure 4 .
Figure 4. Effect of (a) rice stubble % and cotton waste % (b) rice stubble % and PU content, and (c) cotton waste % and PU content on water absorbency.

Figure 6 .
Figure 6.Effect of (a) rice stubble % and cotton waste %, (b) rice stubble % and PU content, and (c) cotton waste % and PU content on NRC.

Table 1 .
Box and Behnken surface response design.

Table 3 .
Multivariate specifications and ANOVA for pore size.

Table 2 .
Pore size distribution, NRC, water absorbency of acoustic panel.

Table 4 .
ANOVA Analysis for NRC.