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.

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In contemporary civilization, the electromagnetic radiation from electronic devices and communication systems has become a substantial pollutant. High-performance electromagnetic absorbers have become a solution for absorbing unwanted electromagnetic waves. This research proposed a lightweight and flexible electromagnetic absorber produced from natural rubber filled with conductive carbon black (CCB) and Fe3O4. The effect of CCB, Fe3O4, and a combination of CCB and Fe3O4 as a hybrid filler on foam morpholog, electromagnetic reflectivity, tensile strength, and compression set properties were investigated. In addition, the effect of the alternating layered structure of CCB and Fe3O4 on electromagnetic absorption was investigated. The results indicated that the composite foam exhibited an interconnected network structure that enhanced the electromagnetic attenuation in the absorber. CCB increased the electromagnetic absorption of the foam, whereas Fe3O4 had less of an effect. The foam filled with the hybrid filler at the CCB/Fe3O4 ratio of 8/2 exhibited excellent electromagnetic absorption. The composite foam had a higher tensile modulus and higher strength compared to neat foam. The addition of CCB decreased the compression set; however, the compression set was improved by the incorporation of Fe3O4. Composite foams filled with hybrid filler can serve as highly efficient electromagnetic absorbing materials.

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