Acoustical and Thermal Characterization of Insulating Materials Made from Wool and Sugarcane Bagasse

ABSTRACT Utilizing agricultural waste and natural fibers minimizes environmental impact and can improve the acoustic and thermal conditions of buildings. Natural fibers can be an alternative to non-biodegradable synthetic sound-absorbing materials. This study aimed to investigate the acoustic and thermal properties of insulating materials made from wool and sugarcane bagasse. Thermal conductivity, thermal resistance, acoustic and moisture absorption, and fire properties of five insulating materials made from sheep wool, goat fiber, camel wool as well as pith and fiber bundles of sugarcane bagasse were determined. The measurement of the sound absorption coefficient was performed in an impedance tube. The thermal resistance and thermal conductivity coefficient were measured according to the ASTM D5334–08 Standard. The findings show that camel wool has the highest sound-absorbing performance, thermal insulation, and fire-resistant properties. The lowest value of the noise reduction coefficient (NRC) was 0.52 for goat fiber, and the highest was 0.74 for camel wool. The maximum sound absorption coefficient of camel wool was 0.95 at a frequency above 1000 Hz. Thermal conductivity varies between 0.038–0.046W/(M.K). Hence, all materials tested can be considered thermally insulating. The results showed insulating materials made from wool, especially camel wool, had better performance than fiber and pith of sugarcane bagasse.


Introduction
Today, using thermal and acoustic insulation in buildings is a significant challenge.Currently, various methods and materials such as mineral wools, synthetic fibers, and polymeric foams have been developed and used in the building and construction sectors.Over the last decades, synthetic fibrous sound-absorbing materials such as slag wool, fiberglass, polyurethane, or polyester, with high production costs and generally based on petroleum polymers, have been extensively used for constructing acoustic chambers (Balaji et al. 2019).Over the last decade, natural fibers have been introduced as an appropriate material for the production of low-cost and eco-friendly panels (S.M. Cascone, Cascone, and Vitale 2020).Several studies have reported the acoustical properties of hemp fibers (Liao, Zhang, and Tang 2022), typha (Moghaddam et al. 2016), and sugarcane waste (Othmani et al. 2016).
Wool has many unique features, including excellent insulation performance and low flammability (Tămaş-Gavrea and Dénes 2020).Wool is self-extinguishing and will not ignite easily.In addition, wool does not release toxic particulates or gases during combustion (Berardi and Iannace 2015).Animal wool fibers are commonly found in most countries, including Australia, New Zealand, China, the United Kingdom, South Africa, and other parts of Europe and Asia (Asis et al. 2015).Several studies investigated the acoustic and thermal properties of sheep wool (Zach et al. 2012;Berardi, Iannace, and Di Gabriele 2016;Del Rey et al. 2017).However, large quantities of waste wool fibers are owned by camels and goats, which probably have different acoustic and physical properties such as the thermal characteristics of sheep wool (Asis et al. 2015).
Sugarcane bagasse, which is a cellulosic agro-industrial byproduct released after crushing and extracting juice from the canes (Hajiha and Sain 2015), is found in many tropical countries including Thailand, Indonesia, Pakistan, Malaysia, India, and Brazil.Bagasse contains fiber bundles and other structural elements such as vessels, parenchyma, and epithelial cells, The latter can be summarized under the technical term"pith" (Sanjuán et al. 2001).Pith causes severe problems in pulp and papermaking, so it is separated from the bagasse as much as possible and discarded.It increases the consumption of chemicals in the pulping and bleaching process.Accordingly, the present study examined pith and bagasse fiber separately.Flammability and moisture absorption are the limitations of natural fibers which considerably reduce their use (Kim, Dutta, and Bhattacharyya 2018).Overall, there is little research on the use of wool waste in the development of double insulation (sound and thermal insulation).Mainly these studies focus on sheep wool and other animal wool such as camel wool and goat fiber have not been studied.In this paper, we focus on sugarcane bagasse and waste wool fibers, and their acoustic and thermal behavior.In addition to the testing of acoustic and thermal properties, other experiments, including morphological analysis, fire properties, and moisture absorption, were conducted.Hopefully, these insulating materials will contribute to the development of a green building.

Preparation of wool
Camel wool, sheep wool, and goat fiber were obtained from livestock farms in Khorasan Razavi province, Iran.It should be noted that animal wool was obtained from animals of the same breed and the feeding, climate, and maintenance conditions were the same.Wool samples were taken as a representative sample from the whole flock.First, the wool is washed in hot soapy water to remove dirt, grease, and dry plant matter from the fleece, then the thorns, debris, and other undesired substances are charred by sulfuric acid and hydrochloric acid.Following the charring process, the wool was sundried for 3 days and again heated in the standard oven at 70-90°C for 15 minutes to let the excess water in the fiber evaporate.Fibers were then stored in a desiccator to avoid moisture absorption before the sample preparation.

Preparation of pith and fiber bundles of sugarcane bagasse
Fresh sugarcane bagasse used in this study was purchased from the Sugarcane Cultivation and Development Research Center located in Khuzestan province, Iran.A manual shredder was used to shred the bagasse into small pieces.Then, the shredded fibers were sieved to mesh size 2 mm and the sugarcane bagasse components were separated manually into 2 fractions, including the fiber bundles and the pith.Fiber bundles were extracted by soaking sugarcane in water for two days at room temperature followed by heating at 80•C for 3 h in our study.The dried pith was transformed into fine particles in two milling steps.Fiber bundles and the pith were then placed in desiccators to avoid moisture absorption before the sample preparation.The parameters of the fibers are shown in Table 1.The thickness and length of fibers were determined by ImageJ software and a field emission scanning electron microscope.

Preparation of samples
A manual shredder was used to shred the raw material into small pieces.Subsequently, the shredded fibers were soaked into the molds with 6 and 3-cm diameters (depending on the size of the impedance tube) in uniform layers with a bulk density of 150 kg/m3 and thickness of 5 cm.Moreover, cylindrical samples with a diameter of 10 cm and a thickness of 7 cm were made to measure thermal conductivity and thermal resistance.The composites were pressed to a nominal thickness of 4 h under 200 bars using a designed hydraulic cold press machine.The adhesive used was polyvinyl alcohol (PVA) made by Sigma Company.The adhesive applied to all samples was set at 6% weight.Three samples were made for each absorber.The flow of the process is shown in Figure 1.
Sound absorption, thermal conductivity, thermal resistance, moisture absorption, morphological structure, and fire properties were measured in these prepared samples (insulating materials made from wool and sugarcane bagasse).

Acoustic property
The measurement of the sound absorption coefficient (SAC) was performed using an impedance tube according to ISO 10,534-2.The impedance tube is equipped with 2 tubes, a 6 cm diameter tube is used to measure the SAC at low and medium frequencies (63-1600), and a 3 cm diameter tube is used to measure SAC at high frequencies (1600-6300).The different parts of the impedance tube device and the position of the specimen within it are shown in Figure 2 (Taban et al. 2019).
Measurements for each specimen in the low, medium, and high-frequency bands were repeated four times separately and the average (SAC) was determined for each frequency range based on the graph obtained from the impedance tube device.All measurements were performed at 22 •C and an atmospheric pressure of 101.7 kPa.The noise reduction coefficient (NRC) was calculated using the following formula:

Thermal conductivity and thermal resistance
Thermal properties measurements are carried out under steady-state conditions.The thermal resistance and thermal conductivity coefficient were measured by ASTM D5334-08 (Huang and Shu Liu 2009) using a KD2 Pro thermal properties analyzer (Decagon KD2 Pro Instrument, Paretavous Research Institute, Mashhad, Iran).Decagon kd2 pro thermal analyzer is a portable device used to measure thermal properties.It consists of a single-needle transducer inserted in the samples to measure thermal conductivity and resistivity (Figure 3).Thermal resistance and thermal conductivity were measured at a temperature of 28-±2 °C and measurements for each specimen were repeated 3 times.

Moisture absorption and humidity testing
Moisture absorption measurement was performed in a climatic chamber by SANS 1381-1: 2013.Five samples were prepared from each insulating material (sheep wool, camel wool, goat fiber, pith, and fibers of sugarcane bagasse) and experiments were conducted on these materials.The prepared samples were dried in an oven at 135°C for 30 min to ensure the removal of excess moisture in the material.Then, the dried fibers were weighed with high-precision scales, re-dried for 30 minutes and re-weighed, and re-dried for 15 minutes and re-weighed to achieve a constant weight of specimens (W d ).Following this, the samples were placed in a climate chamber for 72 hours and weighed (W w ).
The relative humidity within the chamber was 90%±5 with a temperature of 29 ± 2°C.The moisture absorbance in the samples is calculated by Equation (2):

Fire properties
The fire tests of the samples were performed at the environmental health laboratory according to SANS 10,177.The purpose of this test was to compare the fire behavior of insulating material made from sheep wool, camel wool, goat fiber, pith, and fibers of sugarcane bagasse in fire conditions.The prepared samples were placed in a climatic chamber with a standard test environment (humidity 65 ± 5% and temperature 20 ± 2) for 3 minutes.Then it was put in a full-size furnace by SANS 10,177.Initially, the furnace temperature was set to 400 ± 5°C for 5 minutes, then it was raised to 750°C.The burning behavior of the sample, including the decomposing of samples, smoke generation, ignition, and continuous burning time were observed.

Material morphology
FESEM) Field emission scanning electron microscope) (LMU TESCAN BRNO-Mira 3 ) in the Comprehensive Research Laboratory of Mashhad University of Medical Sciences, Iran was utilized to assess the surface morphology of the waste fibers.Before measurement, the fibers were sputtered with gold for scanning electron microscopy under a vacuum to ensure good conductivity.Because of its high electrical conductivity and small grain size, gold produces high-quality microscopic images.
The morphology of all prepared samples was examined and their results were reported.

Data analysis
Acoustical thermal morphological, fire properties and moisture absorption of the insulation samples produced in the current study were statistically analyzed for comparison of different fibers.ANOVA tests the null hypothesis that all group means are the same.Significant differences between groups would suggest that the experimental manipulation had some effects on the dependent variables.

Acoustic property
In this study, the acoustic and thermal behavior of various insulating materials made from goat fiber, camel wool, sheep wool as well as pith and fibers of sugarcane bagasse were reported.Figure 4 compares the sound absorption coefficient of insulating materials made from wool fiber, pith, and fiber of sugarcane bagasse with a thickness of 5 cm.A good performance of SAC of more than 0.7 is shown for frequencies above 1000 Hz.The absorption coefficient increases until it reaches the value of 6000 Hz at 0.9.This applies to all types of studied samples.Figure 4 shows that the maximum sound absorption coefficient of insulating materials made from sheep, camel, and goat fiber was 1, 0.97, and 0.94 at 2000, 1000, and 2000 Hz respectively.
The results of the one-way analysis of variance (ANOVA) show that there is a significant difference between the average sound absorption coefficients in different samples.The results show that with the same thickness, insulating materials made from wool have a higher sound absorption than fiber and pith of sugarcane bagasse.Insulating materials made from camel wool have a higher sound absorption compared to sheep and goat fiber.The test results are shown in Table 2.
Like other wool absorbers, the obtained materials possess low sound absorption capacity at low sound wave frequency and excellent capacity at medium and higher frequencies (Figure 4).This has been confirmed in studies (Beheshti et al. 2022).Insulating materials made from camel and sheep wool are excellent sound absorbers because of the micro-cavities that are present in the wool.The absorption obtained in this study was generally higher than that reported in Ref (Oldham, Egan, and Cookson 2011), especially at high frequency.At 2000 Hz, an absorption of 0.95 was obtained in this study versus a value of 0.7 reported in Ref.This study found values more similar to those in Refs (Ballagh 1996), which reported an absorption coefficient of 0.9 from 800 Hz to 2 kHz (18) and 0.95 at 2 kHz.(Berardi, Iannace, and Di Gabriele 2016) in their study reported that wool has good acoustic absorption and can be a valid alternative to traditional mineral acoustics.Also, the study by del Rey et al. 2017 shows that sheep wool has an acoustic absorption performance comparable to that of mineral wool or recycled polyurethane foam.
Table 2 shows the comparison of the NRC of the wools with that of the pith and bagasse fiber.The lowest NRC score was 0.52 for goat fiber and the highest was 0.74 for camel wool and was more similar to the results of other studies, which reported the noise reduction coefficients (NRCs) of 0.55 and 0.7 for the 4 and 6 cm thick (Berardi and Iannace 2015).In the current study, the NRC values were generally higher than some studies which had reported the NRC value was between 0.4 and 0.43 (Kobiela-Mendrek et al. 2022), consequently allowing us to recognize insulating material made from wool fibers as a highly promising sound-absorbing material.This has been confirmed in other studies (Ilangovan et al. 2022).
The maximum sound absorption of insulating material made from pith and bagasse fiber was 0.95 and 0.98.At 3100 and 510 Hz, respectively.In a study by (Malawade and Jadhav 2020), the maximum absorption coefficient of insulating material made from bagasse fibers with a thickness of 30 mm was 0.7 and 0.78 for the sound frequency of 3000 and 2250 Hz and the results indicate the high sound absorption performance of the insulation sample of sugarcane in the present study.

Thermal insulation property
The results of the thermal insulation properties of the samples are given in Table 3.
The thermal resistance value was calculated from Equation (3).
Where: d: thickness of the material (m) λ : Thermal conductivity of the material (W/m.K) Thermal conductivity values of all insulation samples, including sheep wool, goat fiber, camel wool as well as pith and fiber bundles of sugarcane bagasse with a density of 150 kg/m3, range from 0.038-0.046W/mK.It can be noted that the thermal resistance value meets the requirement imposed by the national standard.According to Romanian Norm C107/2-2005 (Tămaş-Gavrea et al. 2020), a material with a thermal conductivity of less than 0.065 and a thermal resistance of more than 0.50 can be considered as thermal insulating.(Asdrubali, D'Alessandro, and Schiavoni 2015) suggested that the thermal conductivity of insulating materials should be less than 0.07.Thus, all materials considered in this article may be considered thermal insulators.Zach et al. 2012 carried out several measurements to evaluate the thermal insulation of sheep's wool.The results showed that the thermal insulation of sheep wool was comparable to that of mineral wool.In the study of (Bousshine et al. 2022).It was also found that sheep wool has the main properties required for sound and thermal insulation applications and has a good potential to replace synthetic materials.Measured absorption coefficient and obtained thermal conductivity (0.95, 0.044 W/m.K) for sheep wool which is consistent with the results of the study by (Dénes, Florea, and Lucia Manea 2019), in which the thermal conductivity of wool varied between 0.038 and 0.054 W/mK.Similarly, in the study of (Ramlee, Naveen, and Jawaid 2021), it was stated that OPEFB fibers and bagasse based on agricultural wastes can act as efficient thermal insulation, thereby significantly reducing additional energy consumption and costs.This has been confirmed in studies (Mehrzad et al. 2022;Ramlee et al. 2021).Results from other studies also indicate that wools are very good thermal insulators and confirm the results of this study (Asis et al. 2015;Tămaş-Gavrea et al. 2020;Zach et al. 2012).:

Moisture absorption property
Overall, all-natural fibers absorb moisture (Morton and Hearle 2008).The moisture absorption results from the various insulating materials made from wool waste fiber as well as the pith and fibers of sugarcane bagasse are presented in Table 4.The results indicated that all studied samples absorbed more than 3.8% of the moisture, which is higher than the specific requirement of 2% (Asis et al. 2015).
The average value of moisture content must be in the range of 4-13% according to the TIS 876-2547 standard.It can be seen that the moisture content of insulation samples of animal wool fibers was 6.66%, 9.71%, and 8.85%, which all of them agree well with the TIS 876-2547 standard.The results were consistent with the study that reported values of 10.12% for insulation samples of sheep wool and 10.85% for goat wool (Ahmed and Qayoum 2021).
Moreover, the ANOVA test showed that insulation samples of wool absorb more moisture than the pith and fiber of sugarcane bagasse (P-value <.05).However, the moisture absorption coefficient of insulation samples of pith and bagasse fiber was also high.
The structure of the pith cell fraction has an impact on the water absorption characteristics of corresponding tissues or their water retention values.The Pith fraction has a large water-absorbing capacity.The pith can absorb 1300% of dry weight, while the water absorption capacity of the bagasse fiber fraction is only 500% (Sanjuán et al. 2001).Also, the results of the study of (Dénes, Florea, and Lucia Manea 2019).It showed that the amount of water absorption of sheep wool fibers is up to 35%, and therefore wool can help control indoor air humidity.An important point is that moisture absorption affects the sound and heat insulation of materials.Patnaik et al (Asis et al. 2015) showed that the spraying of silicon on the waste wool components acts as a barrier to moisture penetration and it prevents significant changes in the acoustic and thermal behavior of the sample under high humidity conditions.

Fire properties
The fire test results of the investigated materials are presented in Table 5. Insulation samples of wools showed very good flame retardancy and durability performance better than pith and sugarcane bagasse fiber (P-value <.05).Insulation samples of Camel wool and sheep wool took a bit longer to ignite in comparison to that of pith and bagasse fiber.This was due to the inherent fire-retardant properties of waste wool fibers, which delayed the burning.Unlike pith and bagasse fiber, wools shrink first and then decompose at higher temperatures.Wool is naturally flame resistant.This is due to the inherent fire resistance properties of waste wool fibers that delayed the burning.All samples were not combustible at the start of the fire (the temperature was about 400°C), but initially decomposed, generated smoke, and burned at higher temperatures (about 750°C).The results of a study by (Dénes, Florea, and Lucia Manea 2019) showed that sheep's wool does not help the spread of flame and does not support combustion, which is consistent with the results of the current study.

Material structure
SEM (scanning electron microscopy) was used to characterize the pith and fibers of bagasse and waste wool fibers (Figure 5-7).In the pith fraction, shown in Figure 5, one can see a normal structure due to the presence of chemical compounds that provide mechanical and thermal resistance to the biocomposite (Balaji et al. 2019).
The microscopic structure of the pith shows that the tissue of the pith is mainly formed by vascular bundles, sclerenchymatous cells, and parenchyma (Tsuchida et al. 2014).Parenchyma is a tissue in the plant body with an evenly thin cell wall.Sclerenchyma is characterized by a thick secondary cell wall inside its primary cell wall.Sclerenchyma cells provide the elasticity of the plant body (Zhu et al. 2018).The fire-retardant properties of sheep wool have been studied several times [ (Zhu et al. 2018)] but and fibers of sugarcane bagasse are not flame retardant, Bagasse and fiber are not fire-resistant materials, hence they are easily combustible, greatly limiting the safe use of the material.The chemical structure and morphology of the pith are different from fiber bundles.Fiber bundles have a high cell wall thickness, which contributes to tissue stability.Nevertheless, parenchyma cells are very small and have a very thin wall, which affects their acoustic and thermal properties.This result is also proven by a study conducted by Sanjuán et al. (2001).
Microscopic images show that the outer surface of sheep and camel wool contains keratin scales, which are not found in goat fiber.The microscopic structure of the fibers plays an important role in the acoustic, thermal insulation, moisture absorption, and fire properties of materials.In addition, chemical or physical treatment of the surface of the fibers may enhance their properties.

Conclusion
In this study, five different insulating materials made from sheep wool, camel wool, goat fiber, pith, and fibers of sugarcane bagasse were prepared and tested based on acoustic absorption, thermal conductivity and resistivity, humidity absorption, and fire properties.The insulation sample of Camel wool showed the best acoustic absorption, thermal insulation, and fire-resistant properties.It absorbed more than 70% of the incident noise (63-6300 Hz).However, high moisture absorption is a major drawback that can reduce its performance under high humidity conditions.The experimental results obtained in this study showed that the thermal conductivity of the materials studied varies from 0.038-0.046W/mK Therefore, all materials reported in this article could be used effectively as thermal insulation materials.Insulating material made from wool has better fire properties than pith and bagasse fiber.Wool is a flame-retardant fiber.However, wool has a protein structure and is not likely to be attacked by mold, but chemical modification is necessary to prevent attacks by insects and parasites.
Consequently, the current insulation used in construction is a synthetic material that can affect human health and well-being, by emitting toxic substances such as benzene, formaldehyde, and other hazardous volatile organic compounds (VOCs).Natural fibers may be a good alternative to synthetic sound absorbents in buildings.

Highlights
• Camel wool showed the best acoustic absorption, thermal insulation and good fire properties.
• Natural fibers may be a good alternative to synthetic sound absorbents in buildings.
• Wools, especially camel wool, have better performance than fiber and pith of sugarcane bagasse.However, their performance in wet conditions may be severely reduced.

Figure 1 .
Figure 1.Process of preparation of samples.

Figure 4 .
Figure 4. Sound absorption coefficient of the wool waste fibers, pith and fibers of sugarcane bagasse, 5 cm thick.

Figure 5 .
Figure 5. SEM micrographs of surface of pith fractions of sugarcane bagasse at different scales.

Figure 6 .
Figure 6.SEM micrographs of the surface of sugarcane bagasse fiber at different scales.

Table 1 .
The parameters of the fibers.

Table 2 .
One-way analysis of variance to compare the average sound absorption coefficient in all types of investigated natural fibers.

Table 3 .
Thermal conductivity and thermal resistance of the wool waste fibers, pith, and fibers of sugarcane bagasse.

Table 4 .
The results of moisture absorption measurement of the studied samples.

Table 5 .
The fire properties of the studied samples.