Fabrication of High-Performance Natural Rubber Composites with Enhanced Filler-Rubber Interactions by Stearic Acid-Modified Diatomaceous Earth and Carbon Nanotubes for Mechanical and Energy Harvesting Applications

Abstract

Mechanical robustness and high energy efficiency of composite materials are immensely important in modern stretchable, self-powered electronic devices. However, the availability of these materials and their toxicities are challenging factors. This paper presents the mechanical and energy-harvesting performances of low-cost natural rubber composites made of stearic acid-modified diatomaceous earth (mDE) and carbon nanotubes (CNTs). The obtained mechanical properties were significantly better than those of unfilled rubber. Compared to pristine diatomaceous earth, mDE has higher reinforcing efficiencies in terms of mechanical properties because of the effective chemical surface modification by stearic acid and enhanced filler-rubber interactions. The addition of a small amount of CNT as a component in the hybrid filler systems not only improves the mechanical properties but also improves the electrical properties of the rubber composites and has electromechanical sensitivity. For example, the fracture toughness of unfilled rubber (9.74 MJ/m³) can be enhanced by approximately 484% in a composite (56.86 MJ/m³) with 40 phr (per hundred grams of rubber) hybrid filler, whereas the composite showed electrical conductivity. At a similar mechanical load, the energy-harvesting efficiency of the composite containing 57 phr mDE and 3 phr CNT hybrid filler was nearly double that of the only 3 phr CNT-containing composite. The higher energy-harvesting efficiency of the mDE-filled conductive composites may be due to their increased dielectric behaviour. Because of their bio-based materials, rubber composites made by mDE can be considered eco-friendly composites for mechanical and energy harvesting applications and suitable electronic health monitoring devices.

Keywords: carbon nanotubes; diatomaceous earth; electrical properties; energy harvesting; mechanical properties; rubber nanocomposites.

Figure 1

SEM images of (a) multi-walled carbon nanotube, (b) pristine diatomaceous earth, and (c) stearic acid-modified diatomaceous earth; (d) FT-IR spectroscopies of pristine DE (black line), stearic acid (red line), and modified DE (mDE) (blue line).

Figure 2

Schematics for the modification of diatomaceous earth by stearic acid.

Figure 3

Compressive mechanical properties, (a,b) compressive stress–strain curves, and (c,d) Young’s modulus as a function of filler amounts.

Figure 4

Effect on compressive load at 20% deformation with loading–unloading cycles in different rubber composites: (a) NR/20-mDE, (b) NR/17-mDE/3-CNT, (c) NR/40-mDE, (d) NR/37-mDE/3-CNT, (e) NR/60-mDE, and (f) NR/57-mDE/3-CNT.

Figure 5

Tensile mechanical properties: (a,b) stress–strain curves, (c) modulus at 10% deformation, (d) tensile strength, (e) elongation at break, and (f) fracture toughness.

Figure 6

Swelling properties: (a) variation in swelling indexes with filler amounts, (b) variation in cross-link densities with filler amounts, and (c) filler efficiencies on cross-link densities to the hybrid filler systems.

Figure 7

SEM images of rubber composites; (a) NR/20-DE, (b) NR/20-mDE, (c) NR/40-mDE, (d) NR/60-mDE, (e) NR/17-mDE/3-CNT, (f) NR/37-mDE/3-CNT, (g) NR/57-mDE/3-CNT, and (h) NR/57-mDE/3-CNT at higher resolution.

Figure 8

Possible reinforcing mechanism of rubber by stearic acid-modified diatomaceous earth.

Figure 9

Electrical properties of mDE/CNT–filled rubber composites: (a) resistivity and (b) conductivity.

Figure 10

(a) Energy harvesting specimens, (b–f) voltage output with dynamic loading–unloading cycles for different rubber composites, and (g,h) areas in one cyclic loading–unloading.