Optimization of process parameters for the preparation of high-performance sound absorbing boards from sugarcane bagasse

The development of sound-absorbing materials is one of important ways to reduce noise pollution. The use of waste lignocellulose to prepare sound-absorbing boards has the advantages of environmental benefit and low price. Sugarcane bagasse (SCB) is one of lignocellulosic wastes, which can be conveniently collected and transported from the sugar factories. Sugarcane bagasse can be either directly compressed into sound-absorbing boards, combined with other materials such as polyurethane foam or red clay to form composite sound-absorbing boards, or modified and then mixed with materials like bamboo charcoal to produce composite sound-absorbing boards. However, the effect of chemical treatment on the sound absorption performance of SCB has been scarcely explored. This study applied alkaline reagent to treat SCB, and investigated the effects of alkaline treatment conditions on the sound absorption performance of SCB boards, then optimized the preparation of sound-absorbing boards with alkali-treated SCB. Firstly, the Box-Benhnken design was applied to optimize alkaline treatment conditions including liquid-solid ratio (10:1, 15:1, and 20:1), NaOH concentration (0.5%, 2.25%, and 4%), reaction temperature (55 ℃, 70 ℃, and 85 ℃), and reaction time (0.5 h, 1.5 h and 2.5 h) with sound absorption coefficient (SAC) as the function. It found that NaOH treatment could improve the SAC of SCB at medium (1 000～4 000 Hz) and high (4 000～6 300 Hz) frequencies, but reduce the SAC of SCB at low (500～1 000 Hz) frequencies. Quadratic surface response models at 1 000～4 000 Hz and 4 000～6 300 Hz can be established, but their predictive performance is suboptimal, which may be attributed to the influence of board-making conditions during sound absorption coefficient testing. The structural and morphological changes of SCB caused by alkaline treatment were one of factors affecting SAC. Due to the highest SAC at 4 000～6 300 Hz, SCB treated under the condition of liquid-to-solid ratio of 15:1, 4% NaOH, 70 ℃ for 0.5 h was selected for physic-chemical characterization including compositional analysis via two-step acid hydrolysis, measurement of specific surface area and pore size by Brunauer-Emmett-Teller (BET) method, and observation of surface morphology via scanning electron microscope (SEM) and atomic force microscope (AFM). It showed that the SAC was related to the compositional changes and surface roughness of SCB caused by alkali treatment. The NaOH-treated SCB contained higher content of cellulose which can enhance the frictional loss and energy conversion efficiency of medium and high-frequency sound waves via its layered structure of linear macromolecular chain and abundant hydroxyl groups. The rougher surface of NaOH-treated SCB also endows SAC improvement via acting on reflection, scattering, and energy conversion of sound waves. The specific surface area, pore size, and total pore volume of alkali-treated SCB were increased, which had relation to the SAC improvement at 1 000～4 000 Hz and 4 000～6 300 Hz. However, the hot-pressing pressure would result in the compression and collapse of the large-pore-size/high-pore-volume structure formed by alkali treatment. During the gluing process, phenolic resin adhesive would fill the pore channels, thereby offsetting the changes in SAC caused by the modification of specific surface area and pore size. As a result, the differences in the porous characteristics of bagasse samples fail to effectively reflect the sound absorption performance of the boards. Therefore, the above alkali-treated SCB with the best sound absorption performance at 4 000～6 300 Hz was chosen as the material for optimizing board preparation. The preparation conditions including hot pressing temperature (110 ℃, 130 ℃, and 150 ℃), hot pressing pressure (2.5 MPa, 5 MPa, and 7.5 MPa), hot pressing time (5 min, 10 min, and 15 min) and glue amount (12%, 15%, and 18%) were optimized by orthogonal test. It showed that the hot pressing temperature was too low or too high to improve the average SAC. The increase of hot compression pressure showed various influence on the average SAC at different frequencies. Increasing the hot pressing time and glue amount could improve the average SAC. The optimal preparation conditions of SCB boards with better SAC were obtained as follows: hot pressing temperature, pressure, time and glue amount were 130 ℃, 7.5 MPa, 15 min, and 18%, respectively. The average SAC values of the SCB boards with a thickness of 5 mm were 0.376 and 0.909 at the frequencies of 1000～4000 Hz and 4000～6300 Hz, respectively. This study provided a clue for improving SAC of SCB boards. It is necessary to further explore the synergistic impact of the material characteristics and board-making conditions on the sound absorption performance of boards made from lignocelluloses.