Silicone Rubber Composites Reinforced by Carbon Nanofillers and Their Hybrids for Various Applications: A Review

Abstract

Without fillers, rubber types such as silicone rubber exhibit poor mechanical, thermal, and electrical properties. Carbon black (CB) is traditionally used as a filler in the rubber matrix to improve its properties, but a high content (nearly 60 per hundred parts of rubber (phr)) is required. However, this high content of CB often alters the viscoelastic properties of the rubber composite. Thus, nowadays, nanofillers such as graphene (GE) and carbon nanotubes (CNTs) are used, which provide significant improvements to the properties of composites at as low as 2-3 phr. Nanofillers are classified as those fillers consisting of at least one dimension below 100 nanometers (nm). In the present review paper, nanofillers based on carbon nanomaterials such as GE, CNT, and CB are explored in terms of how they improve the properties of rubber composites. These nanofillers can significantly improve the properties of silicone rubber (SR) nanocomposites and have been useful for a wide range of applications, such as strain sensing. Therefore, carbon-nanofiller-reinforced SRs are reviewed here, along with advancements in this research area. The microstructures, defect densities, and crystal structures of different carbon nanofillers for SR nanocomposites are characterized, and their processing and dispersion are described. The dispersion of the rubber composites was reported through atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The effect of these nanofillers on the mechanical (compressive modulus, tensile strength, fracture strain, Young's modulus, glass transition), thermal (thermal conductivity), and electrical properties (electrical conductivity) of SR nanocomposites is also discussed. Finally, the application of the improved SR nanocomposites as strain sensors according to their filler structure and concentration is discussed. This detailed review clearly shows the dependency of SR nanocomposite properties on the characteristics of the carbon nanofillers.

Keywords: carbon black; carbon nanotubes; graphene; nanocomposite properties; silicone rubber; strain sensors.

Scheme 1

Graphene, the mother of all carbon nanomaterials.

Figure 1

(a) SEM micrographs, (b) Raman spectra, and (c) XRD patterns of CB, CNT, and GR [3].

Figure 2

(a) FTIR, (b) XPS, and (c) XRD patterns of graphite, GO, and GE [75].

Scheme 2

Schematic presentation of the preparation of RTV-SR-based nanocomposites and the suggested mechanism for reinforcement of the RTV-SR matrix with carbon nanofillers.

Figure 3

AFM micrographs in RTV-SR matrix: (a,a’,a’’) CB + GR hybrid, (b,b’,b’’) CB + CNT hybrid, (c,c’,c’’) CB, (d,d’,d’’) GR, and (e,e’,e’’) CNT [42].

Figure 4

TEM micrographs of the CNT-GR/SR composites: (a,b) CNT-SR composites; (c,d) CNT-GR/SR composites; (e,f) self-assembled CNT-GR/SR composites; line indicating formation of conductive pathways in SR matrix (blue line for GR and red for CNT) [52].

Figure 5

SEM images of GE/SR composites: (a,b) mechanical mixing, (c,d) solution mixing, and (e,f) ball milling. Black dotted circles denote GE-rich zones, while white dotted circles denote SR-rich zones [87].

Figure 6

Kraus plot of EG/SR composites [95].

Figure 7

Compressive mechanical properties: (a) compressive stress vs. compressive strain for different fillers and (b) elastic modulus for different fillers [97].

Figure 8

Stress–strain curves for (a) CNTs, (b) CNT-GR, and (c) GR. (d) Tensile modulus, (e) reinforcing factor, and (f) fracture strain for the studied fillers [106].

Figure 9

Experimental and theoretical predictions of the modulus in CNT-filled composites [111].

Figure 10

Dynamic mechanical thermal properties of CB-GF/SR composites: (a) storage modulus (E’); (b) storage modulus at −139 °C [86].

Figure 11

(a) Friction coefficients of composites; (b) specific wear rates at different loads; (c) friction coefficients; (d) specific wear rates at different sliding velocities [95].

Figure 12

(a) Thermal conductivity and (b) TGA and (c) DTG curves of composites based on GF and GF-CB hybrid in SR matrix [86].

Figure 13

Electrical conductivity of CNS/SR composites with varying CNS contents. Inset shows logarithmic curve [53].

Figure 14

Cyclic strain and relative resistance with increasing amplitude: (a) stress and relative resistance; (b) multi-hysteresis of stress–strain curve; (c) strain and relative resistance; (d) number of cycles against residual strain and residual relative resistance; (e) contour map of residual strain at different cycle numbers [53].

Figure 15

Change in resistance upon applied cyclic strain: (a) change in resistance at increasing GE content in SR matrix; (b) change in resistance at increasing strain from 5 to 30%; (c) change in resistance at different strain rates increasing from 10 to 50 mm/min; (d) durability cycle for the composites at 30% strain and strain rate of 10 mm/min; (e) application of the strain sensor for monitoring on a rubber seal; (f) cyclic response of the strain sensor under different cycling strains [1].

Figure 16

Mechanism of electromagnetic wave propagation from the specimen investigated: (a) periodic distributed microwave/graphene fibers; (b) randomly distributed microwave/graphene fibers; (c) shielding effectiveness of composites for periodic arrays; (d) randomly dispersed microwave or graphene fibers and their combinations [112].