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<\/figure><\/div>\n\n\nThe results showed that the modification of fibers with stearic acid (STA) can improve the contact angle and hydrophobicity of bamboo fibers, so that for modified fibers with a concentration of 0.3 M, the contact angle was 143.8 degrees, placing the fibers in the superhydrophobic range. Additionally, the thermogravimetric results showed that aluminum hydroxide (ATH) loading increased the coal residue at 600\u00b0C by 12%, indicating an improvement in hydrophobic properties and thermal stability. <\/p>\n\n\n\n
The average sound absorption for composites based on raw fiber and modified with STA and ATH in the range of 630\u20136300 Hz was 0.51 and 0.472, respectively. This shows that chemical modification has reduced the sound absorption of the composite to a small extent. On the other hand, the average sound absorption coefficient for frequencies above 630 Hz was 0.84 and 0.79, respectively, indicating that the sound absorption composite has good absorption. Overall, these observations provide strong evidence that chemical modification of natural fibers can lead to the development of more stable and safer porous acoustic absorbers.<\/p>\n\n\n\n
Introduction<\/h3>\n\n\n\n
Noise has become a major environmental issue with negative effects on human health as urbanization and industrialization develop1<\/a><\/sup>. According to the World Health Organization (WHO), over 1.6 million healthy life years are lost annually in Western Europe due to traffic noise2<\/a><\/sup>. Noise not only significantly affects work quality and living standards but can also cause a series of health issues, including hearing loss, sleep disorders, fatigue, cardiovascular problems, and psycho-physiological disorders. That makes noise control an important and necessary action3<\/a><\/sup>. Employing porous sound-absorbing materials is a practical solution for noise reduction. <\/p>\n\n\n\n Meanwhile, porous fibrous materials have been extensively used as sound-absorbing materials in the field of sound-control engineering4<\/a><\/sup>. Synthetic or natural fibers can be used to make fiber composites5<\/a><\/sup>. Compared to synthetic fibers, the use of natural alternatives, either on their own or as part of composite materials, offers significant advantages, especially in terms of environmental impact. Natural fibers are relatively inexpensive, readily available, abundant, and generally have low densities. They are also a renewable resource and contribute to enhanced CO2<\/sub> sequestration. Additionally, they result in lower emissions of toxic fumes and gases during manufacturing or incineration, as well as reduced dermal and respiratory irritation6<\/a>,7<\/a><\/sup>. Such properties have made these materials innovative sources for developing thermal and acoustical insulation materials. This is especially true in the building sector of developing countries, where the lack of proper recycling policies is a significant concern8<\/a><\/sup>. <\/p>\n\n\n\n So far, numerous studies have investigated the use of various natural fibers as sound absorbers9<\/a>,10<\/a>,11<\/a><\/sup>. Their results indicate that with the selection of an appropriate chemical modification method and proper structural parameter adjustment, natural fibers can become an efficient and effective replacement for synthetic fibers12<\/a><\/sup>. But still, some issues remain, such as the low thermal stability and hydrophilicity of natural fibers13<\/a><\/sup>, which limit the practical use of bio-composites based on natural fibers due to their low thermal stability and rapid flammability, posing safety concerns. A composite becomes highly unsafe if it cannot withstand high temperatures. <\/p>\n\n\n\n Various methods can be employed to enhance the durability of natural fiber composites against thermal degradation. Flame retardants (FRs) are one of the most widely used methods to improve the thermal properties of natural fiber composites14<\/a><\/sup>. Among flame retardant compounds, aluminum hydroxide (ATH) is considered one of the most classic inorganic flame retardants due to its good stability, low or non-toxicity, lack of corrosive gases, and long-term flame-retardant effect15<\/a>,16<\/a><\/sup>. However, to ensure acceptable flame resistance, the amount of mineral retardant added generally needs to be high. This is a significant drawback, as adding a large amount of mineral retardants can considerably diminish the mechanical properties of polymeric materials due to the incompatibility between organic and inorganic materials15<\/a><\/sup>. Recently, nano-scale flame retardant compounds have been used to overcome this problem17<\/a><\/sup>.<\/p>\n\n\n\n On the other hand, natural fibers swell and shrink due to changes in moisture in the cell wall. These dimensional changes can lead to easier decay of composites that are based on natural fibers, reducing their resistance to biological attacks such as fungi and bacteria, as well as chemicals and radiation18<\/a><\/sup>. Consequently, enhancing the compatibility of natural fibers with organic matrices and their application as reinforcement in composite parts can be improved by making them hydrophobic. <\/p>\n\n\n\n To accomplish this, researchers are modifying natural fibers with various chemicals and nanomaterials to improve the properties of the fibers for the development of engineering materials19<\/a><\/sup>. Among these chemicals, stearic acid (STA) has recently been used to improve the hydrophobic properties of natural fibers20<\/a><\/sup>. STA is a saturated fatty acid with an 18-carbon chain found in many animal and vegetable fats and oils21<\/a><\/sup>. However, the process of making natural fibers hydrophobic and improving their thermal properties often involves the use of toxic chemicals, which reduces the ecological advantage of using natural fibers22<\/a><\/sup>. <\/p>\n\n\n\n Moreover, despite bamboo being one of the most attractive natural sources for producing natural fibers and the third-largest source of wood and cellulose products after wood and cotton, there have been limited studies on bamboo fibers in this regard. Only a few studies have explored its use as a sound absorber23<\/a><\/sup>. Bamboo fibers, also known as natural glass fibers, are more desirable than other fibers due to their rapid growth rate, high global production, low cost, resistance to fungi, and insect attacks24<\/a>,25<\/a>,26<\/a><\/sup>.<\/p>\n\n\n\n According to the provided content, the aim of this study is to improve the hydrophobic and thermal properties of bamboo fibers by using STA and ATH coatings and to investigate the acoustic behavior of the raw and modified composite. To achieve this, the bamboo fibers were first treated with varying concentrations of STA. The hydrophobic properties were assessed using the water droplet contact angle test to determine the optimal STA concentration. Next, to enhance the thermal properties of the bamboo biocomposite, ATH nanoparticles were added to both the modified fibers from the previous step and the polyvinyl alcohol (PVA) binder. The thermal properties of the prepared biocomposites were then evaluated using a thermogravimetric test. Finally, the acoustic properties of both the raw and modified biocomposite were analyzed using an impedance tube.<\/p>\n\n\n\n <\/p>\n\n\n\nMaterials and methods<\/strong><\/h3>\n\n\n\n