Enhancing sound absorption properties of open-cell natural rubber foams with treated bagasse and oil palm fibers

The aim of this work was to study the effects of natural fiber type and loading content on the sound absorption efficiency of natural rubber (NR)/treated natural fiber composite foams. This was investigated by measuring the cell characteristics of composite foam, sound absorption coefficient (SAC), and viscoelastic behavior. Bagasse (BF) and Oil Palm (OPF) fibers were treated with sodium hydroxide (NaOH) solution and the optimal treatment conditions for BF and OPF were determined by varying treatment times and NaOH concentrations. Potassium oleate (K-oleate) was used as a blowing agent to create open-cell NR foam. The results indicated that the most suitable NaOH concentration for both BF and OPF was 10%wt. and optimal treatment times were 30 and 10 min, respectively. At low fiber loadings, the addition of treated BF and OPF resulted in a decrease in the average cell size and an increase in the number of foam cells. As loading increased above 5%wt., cell size and cell number exhibited the opposite trends. Both treated BF and OPF enhanced the sound absorption efficiency of NR foams, especially at medium and high frequencies. NaOH treatment improved the interfacial bonding between the matrix and natural fibers, and increased the roughness on the surface of BF and OPF, leading to an enhanced ability for BF and OPF to absorb sound waves. The results indicated that treated BF was more effective than treated OPF for increasing SAC values. Type and dispersion of fiber and viscoelastic behavior were important factors on SAC of composite foams more than cell characteristic.