Acoustic properties of composites materials based on bentonite and coconut or cactus (nopal) fibers

Abstract

Materials made from natural fibers or waste have been studied to acquire a composite with good acoustical isolation properties, commercial materials used in this concern are polyurethane foam, fiberglass, Rockwool, and drywall. This work proposes using waste coconut and cactus ( Opuntia ficus-indica) fibers to synthesize bentonite composites and study their physicochemical and acoustic properties. Physicochemical properties of precursor and composites (75% wt. fiber) were determined by XRD, FTIR, N2 adsorption, optical, and SEM microscopies. The transmission, reflection, and absorption coefficients, 50 to 10,000 Hz, were obtained in an impedance tube and were compared with commercial materials. The composites were subjected to surface humidity and thermal (30, 50, and 70°C) tests to evaluate the stability of acoustic properties. The mechanical properties of the composites are improved with the addition of bentonite, hardness is increased by 6.7% and 0.9%, and the density is decreased by 73.6% and 45.5%, for coconut and nopal, respectively, porosity also changes. We corroborated the significant presence of montmorillonite in bentonite (XRD), while the nopal or coconut fibers are composed of hemicellulose, cellulose, and lignin (FTIR). Also, mucilage was identified in the nopal samples as adherent biopolymer that generates stability either thermally or in wetting conditions. Coconut fiber has a high absorption coefficient (similar to polyurethane), which is reduced with humidity. However, by adding bentonite, the reflection is more elevated, and transmission is lower than commercial materials. The bentonite and nopal (fiber and composite) presented stability in the acoustic coefficients as a function of humidity and temperature.