Sound Absorption Behavior Related to Reduce the Melamine-Urea-Formaldehyde (MUF) Adhesives for the Coffee Silver Skin Board Production

ABSTRACT The development of sound-absorbing panels which used coffee-silver-skin (CS) and melamine-urea-formaldehyde (MUF) as a composite board was studied. In this experiment, the objective was to examine the impact on the sound absorption coefficient (SAC) when incorporating wood husk into the composite board, specifically in the cases of CS (100%) and CS (90%) mixed with wood husk (10%), while simultaneously reducing the amount of MUF adhesive and adding corn starch (25%). It was observed that as the MUF content of the board decreased to 75%, the sound absorption coefficient average (SAA) were found to be 0.59 and 0.62 for the middle- and high-frequency ranges, respectively. For the frequency range higher than 3.15 kHz, the maximum SAC value of sample boards was 0.91 and the noise reduction coefficient (NRC) value was 0.47. While, the result showed the maximum SAC and the NRC value of sample boards were 0.93 and 0.48, respectively above the frequency range higher than 1.25 kHz for decreasing the MUF adhesive of board to 50%. Similarly, the addition of wood husk and corn starch mixed to CS could increase the SAA value of sample boards in the middle- and high-frequency range.


Introduction
According to the World Health Organization (Basner and McGuire 2018;Basner et al. 2014;Hahad et al. 2019;Hughes and Jones 2001), noise pollution not only causes nuisances and sleep disturbances but also contributes to heart attacks.Effective and efficient noise control measures are necessary to address such issues.Sound absorbers play a crucial role in industrial noise control systems.Life Cycle Assessment (LCA) is a comprehensive approach that analyzes the potential impacts associated with the entire life cycle of a product, including material extraction, production, transport, construction, operation and management, deconstruction and disposal, as well as recycling and reuse.This assessment allows for the evaluation of environmental pollution parameters, such as nonrenewable energy consumption.Synthetic and mineral materials, such as glass foam, glass wool, and mineral wool, have been found to have the greatest environmental impact, while natural materials, such as coconut fiber, flax fiber, and cellulose, exhibit the least impact (Asdrubali, Schiavoni, and Horoshenkov 2012).The environmental problem is a major concern for governments and people worldwide, as it leads to natural disasters such as wind disasters, floods, and iceberg melting.Consequently, environmental protection has become a paramount policy for organizations globally.Zero waste management is one of the policies that can be studied and applied in both the agricultural and industrial sectors.Agricultural industry waste has been required to be sustainable (Hatfield et al. 2002;Hume, Brink, and Basner 2012;Stansfeld and Clark 2015) as part of circular economy that emphasizes the reuse and regeneration of materials or products.This approach is particularly important for sustaining production in an environmentally friendly and sustainable manner.The sound absorption value depended on the material porosity of fibers, such as foam, glass wool, and rock wool.As is well-known, these materials exhibit good sound absorption performance across the middle-to high-frequency range.However, these sound-absorbing materials have detrimental effects on the environment due to their manufacturing processes, which are also harmful to human health.Therefore, considering their impact is of utmost importance (Berardi and Iannace 2017;Galbrun and Scerri 2017;Pickering, Efendy, and Le 2016).These concerns have captured the attention of researchers and the industry, prompting efforts to develop cost-effective and environmentally friendly sound-absorbing building materials.
Since the past 10 years, numerous works on acoustic materials from natural materials have been published.Sheng et al. investigated and developed the sound absorption performance of microperforated panels (MPP) made from coconut fiber and polylactic acid composite materials.The impedance tube method was used to determine the sound absorption performance of the specimens.It was found that different MPP containing coconut fiber and polylactic acid (PLA) had different sound absorption performance.This is mainly due to the porosity and winding structure within the specimen (Chin Vui Sheng, Yahya, and Che Din 2022).Liu et al. studied the sound absorption properties of kapok nonwoven fabrics in the frequency band 100-2,500 Hz using the impedance tube method.Comparison between kapok and polypropylene including hollow polyester fibers.It was found that kapok has sound absorption properties at low frequencies (Liu et al. 2015).The maximum sound absorption coefficient (SAC) of natural waste materials is typically observed in the mid-to high-frequency range, whereas this value decreases in the low-frequency range as the thickness of the porous layer increases.The sound absorption performance is influenced by both the thickness of the perforated plate and the porous layer within the composite sound absorber.It was noted that incorporating a perforated sheet in front of a thinly layered porous material significantly enhanced the SAC and absorption bandwidth (Beheshti et al. 2022).
In recent years, significant research has been conducted on natural fiber materials as substitutes for synthetic and mineral materials in sound-absorbing products.This trend is driven by several advantages associated with natural fibers, including relatively low cost, biodegradability, wide availability, environmental friendliness, and favorable sound absorption characteristics (Berardi, Iannace, and Di Gabriele 2017;Taban et al. 2020;Taban, Khavanin, and Ohadi 2019).Notably, studies have demonstrated the efficacy of sound-absorbing panels utilizing coir fiber (Fouladi et al. 2010) and coconut fiber (Mamtaz et al. 2017) as raw materials in the building and construction sectors.However, further investigation is warranted to explore the adhesive compounds used in wall paneling, plywood, particleboard, and sound-absorbing materials.As commonly acknowledged, synthetic adhesives comprise resins, such as urea-formaldehyde (UF) and melamine-urea-formaldehyde (MUF) (Doost-Hoseini, Taghiyari, and Elyasi 2014).It is important to note that these synthetic adhesives are not only expensive but also possess a substantial level of toxicity, posing risks to both human health and the environment throughout their production and application processes, with particular emphasis on urea-formaldehyde.Consequently, there is a pressing need to reduce the utilization of synthetic chemical adhesives and instead promote the use of natural adhesive alternatives (Anisuzzaman et al. 2014;Derkyi, Darkwa, and Yartey 2008;Kamal et al. 2009).
One potential source of waste materials that can be utilized for producing sound-absorbing materials is silver skin coffee (CS).Given the widespread consumption of coffee as a globally popular crop, significant amounts of waste are generated during the production process (Sarasini et al. 2018).Due to its global popularity, coffee has become economically important to several countries with export value in roasted coffee reaching nearly 12 billion USD in 2020 (Food and Agriculture Organization of the United Nations 2020).In 2021, the worldwide production of coffee was expected to be about 5.5 million tons.About 6.4% has been produced in Ethiopia (Sarasini et al. 2018;Woldesenbet, Woldeyes, and Chandravanshi 2014).Thailand produced 18,689 tonnes of coffee in 2022, including 9,135 tonnes of the arabica variety, mainly grown in the North.Some 9,554 tonnes of the robusta variety were produced, largely in the South (Bangkok Post PCL 2023).Thai coffee products were exported to the Lao, Myanmar, and Cambodia.The report of the Office of Agricultural Economics presented that CS; the waste material from the coffee extraction; was forecasted to be 80 tons per year (about 1% of coffee production).A photograph of the CS is shown in Figure 1.
Therefore, the objective of this research was to investigate the sound absorption properties of medium density CS fiber board composites with reduced MUF adhesive content.The samples were prepared using various ingredient combinations, including CS, wood husk, MUF adhesive, and corn starch.The selection of multiple natural fillers was motivated by the goal of utilizing agricultural waste generated from different regions in Thailand to achieve optimal sustainability.Additionally, previous studies have shown that incorporating different materials into composite matrices can lead to improved sound absorption outcomes.The sound absorption coefficient (SAC) was evaluated using an impedance tube, and the experimental data was analyzed with respect to the constant parameters of density and thickness.This study is expected to contribute to the reduction of industrial waste, address environmental concerns, and mitigate potential future health risks.

Materials
The raw materials (CS) used in this study were produced by an agricultural waste at a robusta coffee producer community enterprise group in Chumphon Province, Thailand.The original material was dried by hot air until a final moisture content of 14.0% ± 2.0 dry weight basis (d.b.) after it was scaled to obtain flakes for the preparation of the composites, approximately 10 × 15 mm and removed impurities such as coffee beans, dust.The wood husks (W) used in this composite material were sourced from rubber wood husks, measuring 5-8 mm in size, obtained as waste from a furniture assembly plant in Surat Thani, Thailand.The adhesive employed in the experiment was Melamine-Urea-Formaldehyde (MUF) Type E-1 (in accordance with JIS A 5905) from AICA-Hatyai Co., Ltd., Songkhla, Thailand.Preparation of the composite prior to hot pressing involves mixing the weighed materials with MUF using a low-speed blade mixer for 5 minutes (weight ratio of material: adhesive, 2:1).Furthermore, household corn starch obtained from King Milling Co., Ltd., Samut Prakan, Thailand, was mixed with MUF.The process involves mixing corn starch powder with the material, where the corn starch acts as a filler.The objective was to reduce the amount of MUF and investigate the possibility of minimizing the use of chemical adhesives and their impact on sound absorption properties.Subsequently, all the mixtures were placed into molds, and the samples were pressed at 120°C with a pressure of 220 kg/cm 2 for 15 minutes until they reached dimensions of 180 mm in width, 180 mm in length, and 30 mm in thickness, respectively.Each condition was detailed in Table 1.

Sampling methods
Figure 2(a) presents the characteristics of composite sound absorbing panels.For the reduced sound absorption coefficient experiment, Figure 2(b) presents the composite material with a diameter 100 mm and 30 mm was tested by using impedance tube.Subsequently, Figure 2(c) presents the measurements of size, volume, and weight of the sound-absorbing material using the KEYENCE XM Series program (XM-QD1MJN6EU6AK) to calculate the density as follows.
For the density of sound-absorbing materials, where ρ b is bulk densityðkg:m À 3 Þ, mis the mass of sample and Vis the volume of sample.Thus, this was written as follows in Equation ( 1): According to the Archimedes principle (Calis Acikbas et al. 2018), the sampling weight was measured after it was boiled in hot water for 1 hour.Then, it was wiped water before the weight recorded and blown with hot air for 2 h at 110 • C. Thus, the porosity of sample (Φ) was specified by Equation (2): Where W suspend is the sampling weight that suspended in water; W wet is the sampling weight after wiping water on surface; W dry is the sampling weight after drying.The thickness of specimen samples having dimensions of 50 × 50× 30 mm was prepared according to ASTM D570, the thickness swelling (TS) was calculated from Equation (3): Where T o is the thickness of specimens before the test started and T l is the thickness of specimens after immersion in distilled water for 24 hours.Distilled water, renowned for its exceptional purity and absence of impurities, was utilized in lieu of regular tap water.The substitution of regular tap water, which may harbor impurities, has the potential to induce disparities in porosity results due to unforeseen influences, such as chemical reactivity with the specimen.Moreover, the MUF employed in this experiment is widely acknowledged for its insolubility in water subsequent to the completion of the curing or hardening process.Being classified as a thermosetting adhesive, it undergoes a solidification process and does not undergo softening or liquefaction when subjected to heat (Ebnesajjad and Landrock 2014).However, for boards that add filler with corn starch, this method for determining porosity may have inaccuracies.This is because corn starch is water soluble and is extracted in this method.After completing the test, observed residual starch in the water composition.

Experimental setup
The SAC was measured and conducted by the tested impedance tube according to ISO 10,534-2 (de Carvalho, Dalla Nora, and da Rosa 2020; Jung et al. 2001;Putra et al. 2018).The experimental system diagram is shown in Figure 3.For each experimental sample, three were performed in which the specimen was pulled out and placed back into the holder.This helps ensure the repeatability of the test and unwanted errors from installing fibers inside the pipe can be ignored.All results of the measured sound absorption coefficients presented in this paper have the same general variance (Or, Putra, and Selamat 2017).The physical environment such as temperature and humidity in the chamber was controlled at constant.Table 2 presents the SAC of the composite board at 63 Hz − 6.3 kHz frequency ranges.For the samples which had the ingredient ratio and MUF adhesive different, the SAC of these samples were recorded and considered.The experimental data; the Sound absorption coefficient average (SAA); was divided into three groups: low frequency (63 Hz−1 kHz), Medium frequency (1 kHz−3 kHz) and High frequency (3 kHz−6.3kHz).The noise reduction coefficient (NRC); the arithmetic means of the absorption coefficients for a specific material and installation conditions; was determined by the center octave frequencies of 250, 500, 1,000, and 2,000 Hz.Thus, it could be calculated from Equation (4):

Scanning electron microscopy (SEM) techniques
SEM is a useful technique for the investigation of surface structure of biological samples.In this study, the structures of sample boards include CS, wood husk, corn-starch, and MUF adhesives was imaged with a low-magnification camera Canon super-ED Lens10X model SEM (Canon, Tokyo, Japan).

Results and discussion
The results of the experiments showed the characteristic of the sound-absorbing composite materials that were produced from CS with addition of wood husk mixed to CS, and addition corn-starch for decreasing the volume of MUF adhesives.

The characterization of the boards
Table 1 presents the observed increase in porosity and swelling of the samples following the reduction of the MUF glue content.The sample plates after hot pressing with thickness 30 mm, had an average density of 35.66 ± 1.06 kg/m 3 .The addition of wood husk to CS resulted in a higher porosity of the sample board compared to the CS100-G100.This finding suggests that the different ingredient combinations had a significant impact on augmenting the gaps within the composite board across all samples.Moreover, in the 7-8 condition, a noticeable increase in porosity was observed.A comparison of the porosity properties of the materials under similar conditions (CS100-G100 and CS100-G75-CP25, respectively) revealed that the addition of corn starch increased the porosity of the materials.However, the evaluation of porosity by water on materials filled with corn starch fillers may affect the material, causing the corn starch to be extracted and water absorbed.Therefore, this corn starch has a great impact on humidity, water contact, and swelling.It is imperative to take this into account when considering the utilization of corn starch fillers, such as in the development of sound-absorbing materials for outdoor architectural use, among others.
The TS value is a significant parameter that profoundly influences the installation of the composite board and serves as a standard for assessing the swelling resulting from liquid absorption.The measured TS value provides an indication of the sample board's resistance to liquid penetration.Furthermore, the impact of incorporating additives, such as wood husk and corn starch, on the swelling characteristics of CS composite panels remains to be explored.This study is expected to offer valuable insights for the selection of appropriate materials.For the sample board in the group containing ingredients from CS, the board with a 50% reduction in MUF adhesive content (CS100-G50) exhibited a higher TS value compared to the CS100-G75 and CS100-G100 boards.This finding suggests that the quantity of MUF adhesive has a significant impact on reducing the TS, as the MUF adhesive plays a crucial role in protecting the boards from moisture and preventing swelling caused by liquids.Similarly, in the group of CS mixed with wood husk, reducing the amount of MUF glue resulted in higher TS values.Furthermore, the addition of cornstarch as a replacement for MUF glue did not lead to a decrease in the TS.These findings highlight the importance of selecting the suitable composite material and appropriately reducing the amount of adhesive to achieve a low TS value.

Sound absorption behavior
Table 2 shows the SAC value, which was measured and recorded by the impedance tube experiment.The result showed that the reference board (CS100-G100) had the SAA values at 0.69 and 0.73 for the middle-and the high-frequency range, respectively.For the frequency range higher than 4 kHz, the maximum SAC value of sample boards was 0.90 and the NRC value was 0.44.When the MUF adhesive of board decreased to 75% (CS100-G75), the SAA values found to be 0.59 and 0.62 for the middle and the high frequency range, respectively.For the frequency range higher than 3.15 kHz, the maximum SAC value of sample boards was 0.91 and the NRC value was 0.47.While, the result showed the maximum SAC and the NRC value of sample boards were 0.93 and 0.48, respectively above the frequency range higher than 1.25 kHz for decreasing the MUF adhesive of board to 50% (CS100-G50).For this reason, this indicated that the decreased value of MUF adhesive had a strong influence on increasing the SAC value of sample boards in the middle-and the high-frequency range.Similarly, the addition of wood husk mixed to CS could increase the SAA values of sample boards in the middle and the high frequency range.For the decreased MUF adhesive of board having wood husk mixture in CS, the result indicated the maximum SAC and the NRC value of sample boards were increased.
While the addition of CP substitutes for the decreased MUF adhesive of board, the result indicated the maximum SAC and the NRC value was increased.For this reason, this indicated that the addition of wood husks and CP could have increased the porous in the sound absorbing material.These findings align with previous research, indicating that the incorporation of different natural fibers in the appropriate proportions, along with the inclusion of other binder additives, can enhance porosity and ultimately result in a higher sound absorption coefficient.This can be attributed to the increased complexity of the porous structure (Samaei et al. 2021;Sujon, Islam, and Nadimpalli 2021).
Figure 4 shows the sound absorption behavior between CS and CS mixed with wood husk and the decreased MUF adhesive at 63-6.3 kHz frequency ranges.Figure 4a shows that the board of CS100-G100 had the maximum SAC values at 0.88 above 1.5 kHz frequency and this value was decreased to 0.30 above 2.5 kHz frequency.The SAC values increased to a maximum peak of 0.90 above 4 kHz frequency.This experimental result was similar to the sound absorption behavior of the CS100-G75 and CS100-G50 boards.the SAC value of the sample board increases as the content of MUF adhesive decreases.Figure 4b presents the SAC value of sample board which was produced by addition of wood husk to CS (CS90-W10-G100).The results showed that the absorption behavior was similar to that of the CS100-G100 board at low frequencies.For the higher frequency, the sound-absorbing behavior of this board was constant.For the sample with reduced MUF adhesive, the SAC value of the composite board increases similarly to what was observed for the reference board of CS100-G100.This result indicated that the addition of wood husk mixed to CS could increase the sound absorption performance for all samples.Even so, from the aforementioned sound absorption behavior, it can be observed that reducing the MUF glue content affects the increase in SAC, but there is not a consistent trend since the increase in SAC depends on the frequency.This variability may be caused by various structural characteristics, such as gap size, the adhesion characteristics of CS, and the addition of wood husk.
Figure 5 presents a comparison of sound absorption coefficients between CS and CS mixed with wood husk.In Figure 5a, it is evident that the CS90-W10-G100 board exhibited superior sound absorption performance to CS100-G100 in the frequency range of 2 kHz to 3 kHz.Meanwhile, the board which decreased 25% quantity of MUF adhesives (CS90-W10-G75) had a good sound absorption range results at low frequency and then constantly increased at high frequency (Figure 5b).For the board which decreased 50% (Figure 5c) quantity of MUF adhesives (CS90-W10-G50), this sound absorption results are similar to Figure 5a, b.Thus, the addition of wood husk mixed to CS could increase the sound absorption performance.This result would be helpful to the CS board development by addition quantity of wood husk and decreased volume of MUF adhesives in the future.This result indicated that the suitable MUF adhesive decreased had the influence of increasing the SAC value of the composite board.Similarly, the addition of wood husks could have increased the porous in the sound absorbing material.It caused an increase the SAC value.
In comparing the SAC between the board which addition of CS, wood husk and corn-starch mixed with MUF adhesive and the referent board (CS100-G100) results (Figure 6), it was shown that the CS100-G75-CP25 board were not significantly different from the referent board at 63 Hz−3 kHz frequency ranges (Figure 6a).However, the SAC value of the board with the addition of wood husk mixed with CS (Figure 6b) was significantly different from the referent board at 3-6.3 kHz frequency ranges.It was observed that the cornstarch-added board resulted in an increase in the SAC value in the sample (CS90-W10-G75-CP25) to 0.95 at 1 kHz and 0.94 at 3 kHz.
Figure 7 presents the structure of sound-absorbing composite boards by using SEM images.Figure 7a shows the MUF adhesive which spread and coated on some CS surface.Similarly, Figure 7b presents the structure of surface boards when the wood husk and the MUF adhesive were mixed to CS.This image shows the gap and alignment of CS surface board were complicated.This result indicated that the complicated surface board could increase the SAC value.For this reason, this suggested that the different ingredient combinations had the influence of increasing the gaps and porosity in the composite board for all samples.Figure 7c shows the cornstarch acts as a filler of the MUF.The contrasting structures of MUF glue and cornstarch are evident upon observation.Cornstarch appears as round granules (Ito and Aguiar 2009;Ziegler-Borowska et al. 2018), distinct from the cohesive nature of MUF glue, which forms a bonded mass (Ding, Yan, and Zhao 2022;Nuryawan and Park 2017).As previously demonstrated in samples prepared with lower MUF content, reducing the MUF content leads to an increase in SAC.
The pore complex structures of the composite board are shown in Figure 8.For the image by a low magnification camera, Figure 8a shows the structure of CS composite board.This board, which had the circular porous spaces and the compacted small gaps, were alternated.When comparing with Figure 8b, the addition of wood husk mixed to CS had the influence of increasing the gaps and porosity in the composite board.This result caused to the increased SAC value.

Conclusion
This research presents a study on the behavior of sound absorption in natural fiber composites derived from CS.The study investigates the impact of reducing the amount of MUF and utilizing a mixture of wood bark and cornstarch.The following were the conclusions from this research: (1) This sound-absorbing composite material uses natural fibers, which are waste from the agricultural production process, with CS and wood husks as the main ingredients.MUF, a synthetic chemical, has been reduced in quantity to increase environmental friendliness, and corn starch filler has been added.(2) The suitable MUF adhesive decreased had the influence of increasing the SAC value of the composite board.
(3) The addition of wood husk mixed with CS resulted in an improved sound absorption performance.This finding highlights the influence of different ingredient combinations in increasing the gaps and porosity within the composite board across all samples.When comparing this composite board to those made from rubber wood husk or Medium Density Fiber Board (MDF), which are commonly utilized in the furniture industry, it was determined that the SAC and NRC values of the tested material met the acceptable criteria for sound absorption.However, it is important to note that these results are dependent on the specific production process employed (Nandanwar and Varadarajulu, 2017;Ribeiro et al. 2019).The findings of this study provide evidence that reducing adhesive usage and incorporating agricultural waste can effectively enhance sound absorption properties and contribute to the overall sustainability of the product.(4) Incorporating corn starch as a filler in sound-absorbing materials may enhance soundabsorbing properties.However, caution must be exercised as corn starch has limitations in resisting moisture.Therefore, further experimentation or application techniques are required.

Highlights
The study investigated the development of sound-absorbing panels using coffee silver-skin (CS) and Melamine-Urea-Formaldehyde (MUF) as composite materials.The objective of this experiment was to assess the impact on the Sound Absorption Coefficient when incorporating wood husk into the composite board, specifically in the cases of CS and CS mixed with wood husk, while simultaneously reducing the amount of MUF adhesive and introducing corn starch as a substitute.
(1) Sound absorbing materials were produced from environmentally friendly coffee silver-skin.(2) Reducing the amount of MUF glue has the effect of increasing the SAC value of the composite panel.
(3) The addition of corn starch may result in improved sound absorption performance.

Figure 1 .
Figure 1.A photograph of coffee silver skin.

Figure 3 .
Figure 3.The experimental setup of the tested impedance tube according to ISO 10,534-2.

Table 1 .
The physical parameters of the composite board samples.

Table 2 .
The sound absorption coefficient values of samples.