Preparation Methods and Properties of CNT/CF/G Carbon-Based Nano-Conductive Silicone Rubber

Abstract

Carbon-based nano-conductive silicone rubber is a kind of composite conductive polymer material that has good electrical and thermal conductivities and high magnetic flux. It has good application prospects for replacing most traditional conductive materials, but its mechanical and tensile strengths are poor, which limit its applications. In this study, carbon fiber (CF), graphene (G) and carbon nanotubes (CNT) are used as fillers to prepare carbon-based nano-conductive silicone rubber via solution blending, and the preparation methods and properties are analyzed. The results show that when the carbon fiber content is 7.5 wt%, the volume resistivity of carbon fiber conductive silicone rubber is 9.5 × 10⁴ Ω·cm, the surface resistance is 2.88 × 10⁵ Ω, and the tensile strength reaches 2.12 Mpa. When the graphene content is 5.5 wt%, the volume resistivity of graphene conductive silicone rubber is 8.7 × 10⁴ Ω·cm, and the surface resistance is 2.4 × 10⁶ Ω. When the carbon nanotube content is 1.25 wt%, the volume resistivity of carbon nanotube conductive silicone rubber is 1.34 × 10⁴ Ω·cm, and the surface resistance is 1.0 × 10⁶ Ω. The three conductive nano-fillers in the blended carbon nano-conductive silicone rubber form a stable three-dimensional composite conductive network, which enhances the conductivity and stability. When the tensile rate is 520%, the resistance of the blended rubber increases from 2.69 × 10³ to 9.66 × 10⁴ Ω, and the rubber maintains good resilience and tensile sensitivity under repeated stretching. The results show that the proposed blended carbon nano-conductive silicone rubber has good properties and great application prospects, verifying the employed research method and showing the credibility of the research results.

1. Introduction

In recent years, the development of flexible sensors has been focused on improving the weight, sensitivity, and response as the existing flexible sensors do not meet the actual needs. Thus, scholars worldwide have conducted significant research on the preparation technology and composite fillers of flexible sensors. Zhaoxia Pei et al. [1] prepared nanocomposite hydrogels by doping carbon nanotubes (CNTs) and grafting polyphenols on the surface of CNCs to improve their conductivity. Qingwen Wu et al. [2] used graphene (G) as the reinforcing agent of a porous PVDF film and synthesized a GP-PANI/PVDF nanocomposite film via in-situ polymerization. Their sensor is portable and lightweight. Xiaofeng Yang et al. [3] coated the top and bottom of a flexible porous polyvinyl alcohol ionic liquid membrane with a thin film of ethylene oxide (ITO-PET) to afford a double-layer capacitor between the electron and the film. Renwei Mao et al. [4] used graphene oxide (GO) as the raw material and sprayed GO from a high-speed rotating nozzle into a coagulation bath through a dynamic wet spinning process. GO was dried at ambient pressure, and it transformed into GO aerogel spheres. R Moriche [5] found that graphene nanoplatelets (GNPs) have been widely studied as nano-enhanced materials with enhanced mechanical, electrical, and thermal properties. Since it is a 2D material, the electrical network created through the epoxy matrix is more strain sensitive than 1D and 0D materials. Omid Sam et al. [6] studied the damage mechanism of adhesive layer under shear stress, and studied the influence of defects on the damage process in the adhesive layer through the correlation between fracture surface and impedance change. The results show that f-GNP adhesive is sensitive to the deformation and damage propagation of the adhesive layer. By adding 12 wt% f-GNP into the adhesive joint without embedding defects, the ultimate strength is improved by about 74%. R. Moriche et al. [7] evaluated the electrical and mechanical properties of functionalized graphene nanoplatelet (GNP)-reinforced epoxy resin matrix composites. Studies have shown that the electrical conductivity of 12 wt% GNP-reinforced nanocomposites is about 10⁻⁴ S/m, and the dispersion of GNPs in nanocomposites enhances the mechanical properties; however, due to the weak interface formed between the GNP-filled matrix and fibers in the glass, the performance of fiber composites decreased. Pataniya Pratik M et al. [8] coated MoSe2 on cellulose paper to prepare a pressure sensor with a MoSe2 nanometer that is immune to temperature changes. Although the above methods have greatly improved the sensitivity and mechanical properties of flexible sensors, they are still affected by humidity and their electrical properties need to be improved, which limits their use.

To date, researchers have been exploring the preparation technology of conductive silicone rubbers, mainly studying new preparation technologies and the introduction of new conductive materials or the treatment of conductive fillers to enhance the properties of conductive silicone rubbers. At present, the mainstream is to use carbon-based fillers to conduct research to prepare conductive silicone rubber. The main carbon-based fillers include carbon black, graphene, carbon nanotubes, and carbon fibers. In research on carbon black, Wu Juying et al. [9] prepared carbon black/BR, carbon black/NR, and carbon black/methyl vinyl silicone rubbers using the mechanical blending method. They found that the same conductive filler has different percolation values due to the influence of different substrates. P. Ghosh et al. [10] compounded carbon black with silicone rubber as the conductive filler, and they found that when the conductive filler content was 15~30 wt%, the percolation effect occurred, which significantly enhanced the mechanical and conductive properties of the composite. Zuo Zhewei et al. [11] filled three kinds of carbon fibers (CFs) with high-temperature vulcanized silicone rubber as the matrix and concluded that the percolation value of the conductive silicone rubber filled with CFs is related to the length of the CFs. Ding et al. [12] successfully prepared silicone rubber composites with vertically oriented magnetic carbon fibers (o-MCF/SR) by casting the molding and filler orientation under a uniform magnetic field in a vacuum environment. The results show that the in-plane thermal conductivity of the o-MCF/SR composite exhibits an unusual power law increase with increasing MCF loading. L. Wang et al. [13] assembled conductive silicone rubber layer by layer through graphene oxide and silicone rubber. When the number of layers is 30, the conductivity is 0.82 S/m. Jinliang Zhao et al. [14] compounded silicone rubber with GO meteorological gel beads and modified materials (PCM) to prepare conductive and heat-dissipating silicone rubber. Tiejun Ge et al. [15] prepared FGO/HPDMS composites by mixing room-temperature vulcanized silicone rubber with FGO synthesized by treating dicyandiamide (HMDI) with GO, and the thermal conductivity and mechanical properties of the composites were obviously high. Chun-Yu Chen et al. [16] prepared conductive silicone rubber by mixing graphene nanomaterials (HGNS) with silicone rubber. Its microwave absorption rate is greater than that of other graphene composite materials. In the study of carbon nanotubes, Pan Song et al. [17] grafted carbon black free radicals with modifiers and reacted them with CNTs to form carbon black CNT nano-fillers. P. S. Lin et al. [18] prepared methyl vinyl silicone rubber-based composites filled with alumina (Al₂O₃) powder and carbon nanotubes by a conventional mechanical blending method. The results show that the thermal conductivity, Young’s modulus, and hardness of the composite material are significantly improved by surface modification of Al₂O₃ powder and adding a small amount of carbon nanotubes. Chen et al. [19] introduced a kind of polydimethylsiloxane (PDMS)/multi-walled carbon nanotubes (CNT)/aligned nickel particles (Ni) composite material manufactured under a low magnetic field. The research shows that the compressive modulus of the composite material with 0.23 vol% CNT and 3.93 vol% Ni particles is 4.5 in the direction parallel to (X direction) and perpendicular to (Y direction) Ni particles arrangement, respectively.

The above analysis shows that different conductive fillers and dispersion modes of conductive fillers have great influence on the properties of conductive silicone rubber, and conductive fillers such as carbon fiber, graphene, and carbon nanotubes can obviously improve the properties of conductive silicone rubber. At present, most of the literature has focused on research on the influence of two carbon-based fillers on the properties of conductive silicone rubber, and there is no literature on the preparation of conductive silicone rubber by blending three carbon-based fillers. Therefore, in this paper, carbon fiber, graphene, and carbon nanotubes were blended to prepare conductive silicone rubber, and the effects of blended fillers on the properties of conductive silicone rubber were studied, and the properties of conductive silicone rubber and pure silicone rubber with single filler were compared and analyzed.

2. Experimental Materials and Methods

2.1. Experimental Materials

The following materials were employed: multi-walled CNTs (HQNANO-CNTs-010-0 produced by Suzhou Hengqiu Technology Co., Ltd., Suzhou, China), graphene (multi-layer graphene (within 10 layers) produced by Suzhou Hengqiu Technology Co., Ltd., Suzhou, China), CF (Toray T300 CF powder), organic solvent n-heptane (pure n-heptane produced by Sinopharm Group Chemical Reagents Co., Ltd., Shanghai, China), and flexible base liquid silicone rubber (LSR; LR 3003/10 TRA).

2.2. Main Instruments and Equipment

The test equipment used in this experiment mainly includes an ultrasonic disperser, constant temperature drying oven, electronic leveling instrument, and a scanning electron microscope. The specific models of some equipment are shown in

Table 1. Main experimental equipment.

Name	Manufacturer	Model
Mechanical stirrer	Suzhou Yinganyang Instrument Co., Ltd., Suzhou, China	JJ-1
Ultrasonic disperser	Ningbo Xinzhi Biotechnology Co., Ltd., Ningbo, China	Scientz-750F
Electric blast drying oven	Shanghai Yiheng Technology Co., Ltd., Shanghai, China	BPG-9050BH
Electrometer	Tektronix Technology Co., Ltd., Beaverton, OR, USA	Keithley 6514
Scanning electron microscope	Hitachi, Tokyo, Japan	Hitachi SU8010
X-ray diffractometer	PANalytical Company, Malvern, UK	Empyrean
Raman spectrometer	Renishaw company, Wotton-under-Edge, UK	RM1000
Thermogravimetric analyzer	German NETZSCH Instrument Manufacturing Co., Ltd., Selb, Germany	TG 209 F1
Instron Electronics Universal Materials Testing Machine	Instron Corporation, Norwood, MA, USA	68TM-10

below.

2.3. Preparation Process and Method of Conductive Silicone Rubber

(1)

Preparation of the base solution: LSR is formed by cross-linking and curing of AB glue. AB glue is the silicone rubber (component A) and curing agent (component B). The role of the curing agent is to achieve curing at room temperature. Weigh AB glue at a ratio of 1:1 and mix it with n-heptane at a volume ratio of 1:2. Stir the mixture for 30 min by mechanical stirring and ultrasonically disperse it for 1 h to evenly mix the n-heptane solvent with LSR.

(2)

Preparation of the mix conductive filler solution: weigh the proper amounts of the carbon-based materials and add them into n-heptane. Mechanically stir the mixture for 30 min, and ultrasonically disperse it for 1 h.

(3)

Preparation by the solution blending method: add the LSR solvent into the conductive filler solution, mechanically stir it for 30 min, and ultrasonically vibrate it for 1 min for uniform dispersion.

(4)

Molding preparation: Vacuum filter the mixture in a vacuum drying oven, pour it into a Petri dish, and let it rest for 24 h. After n-heptane completely volatilizes, cure it in a constant-temperature drying oven at 125 °C for 30 min to obtain the final sample.

2.4. Performance Testing and Structural Characterization Methods

(1)

Electrical performance test

The two ends of the conductive silicone rubber sample were wrapped with copper sheets to ensure the accuracy of the resistance value. The vise was clamped to prevent shaking, and the resistance was measured with a resistance meter. When the sample resistance was lower than 10⁶ Ω, the Geely 6514 electrometer was used to measure the sample resistance, and when the sample resistance was higher than 10⁶ Ω, a high resistance meter was used to measure the sample resistance. Thereafter, the volume resistance was calculated using the following formula:

$${\rho }_V=R_v\frac{s}{h}$$

(1)

The unit of ${\rho }_V$ is Ω m (ohm m), h is the thickness of the sample (the distance between the two poles), and S is the area of the electrode. The measurements were repeated three times and the average value was taken. The volume resistance under different concentrations of the fillers was analyzed and the percolation value range of the carbon-based conductive fillers was simultaneously determined.

(2)

Tensile sensitivity test

Both ends of the prepared 30 mm × 10 mm × 1 mm sample were wrapped with copper sheets, and the sample was placed into the fixture of the textile dynamic resistance testing instrument. The maximum effective resistance of the tensile value at the rate of v = 100 mm/min was adopted. Reciprocating stretching was realized. The drawing rate formula is as follows:

$$\mathrm{\epsilon }=\frac{∆\mathrm{L}}{\mathrm{L}}=\frac{\mathrm{v}\mathrm{t}}{\mathrm{L}}$$

(2)

where ε is the drawing rate, v is the drawing rate, and t is the recording time of the universal drawing machine.

(3)

Thermogravimetric analysis and testing

The thermal stability of the conductive silicone rubber was analyzed by weighing about 5 mg of the sample and using the German resistance thermogravimetric analyzer TG209F1. The test conditions were as follows: 50 mL/min nitrogen protection, 20 mL/min purge gas, and heating from room temperature to 800 °C at a heating rate of 10 °C/min.

(4)

Glass transition temperature and melting point test

The effects of conductive fillers on the Tg and melting point of the composites were investigated using an MDSC 2920 DSC analyzer. The test conditions were as follows: first, the sample was heated from room temperature to 220 °C, cooled to −70 °C, and then heated again 220 °C at the rate of 10 °C/min. All the above processes were conducted under the protection of 20 mL/min nitrogen.

(5)

Raman spectrum test

The prepared materials were characterized by Raman spectroscopy using the Renishaw inVia confocal Raman microscope in Lei Nishao, UK. The test conditions were as follows: resolution of 1 cm⁻¹, scanning range of 80–4000 cm⁻¹, and laser wavelength of 532 nm.

(6)

X-ray diffraction test

X-ray diffraction (XRD) analysis of the conductive silicone rubber was conducted using a Panaco Empyrean sharp-edged X-ray diffractometer. The test conditions were as follows: voltage of 40 kV, current of 40 mA, scanning angle of 5–85°, scanning rate of 10°/min, and scanning step size of 0.02.

3. Results and Discussion

3.1. Properties of the PREPARED Single-Filler Conductive Silicone Rubbers

(1)

Properties of the CF conductive silicone rubber

CF conductive silicone rubber (LSR/CF) was prepared by varying the CF content to 1.5, 3.5, 5.5, 7.5, 8.5, 10.5, 11.5, 15, and 25 wt%, and the ratio of silicone rubber to n-heptane was kept unchanged at 1:2. The properties of LSR/CF were analyzed.

(a)

Electrical properties of the CF conductive silicone rubber

The volume resistivity and surface resistance curves of LSR/CF with different CF concentrations are shown in Figure 1 and

Table 2. The electrical properties of different concentrations of carbon fiber fill.

Carbon Fiber Content wt%	Volume Resistivity Ω·cm	Surface Resistance Ω
1.5	2.5 × 1011	7.6 × 1012
3.5	1.8 × 109	5.4 × 1010
5.5	9.5 × 104	2.8 × 105
7.5	7.0 × 104	2.9 × 105
8.5	8.7 × 103	2.6 × 104
10.5	4.8 × 103	8.7 × 103
11.5	2.9 × 103	6.3 × 103
15	1.8 × 103	4.8 × 103

.

As shown in Figure 1, the volume resistivity and surface resistance of LSR/CF sharply decreased when the CF content was increased from 1.5 to 5.5 wt%. When the CF content was 5.5 wt%, the volume resistivity was 1.5 × 10⁵ Ω·cm, the surface resistance was 7.5 × 10⁵ Ω, and the conductive path began to form. When the CF content was 7.5 wt%, the volume resistivity was 9.5 × 10⁴ Ω cm and the surface resistance was 2.88 × 10⁵ Ω. Moreover, the CF filling amount was moderate, and the particles had a great chance of contacting each other, which enabled the formation of a conductive path and greatly improved the conductivity. When the CF content exceeded 7.5 wt%, the volume resistivity and surface resistance rapidly decreased. The results show that CF particles have a considerable influence on the electrical properties of silicone rubber. When the CF concentration was increased from 5.5 to 7.5 wt%, the volume resistivity and surface resistance of LSR/CF slowly decreased and tended to stabilize, and the electrical properties of CF conductive silicone rubber were stable. The volume resistivity of carbon fiber/(boron nitride/silicone rubber) (CF/(BN/SR)) composites studied in reference [20] is 8.6 × 10¹³ Ω cm, which is 9 orders of magnitude higher than the volume resistivity of 7.5 wt% carbon fiber conductive silicone rubber (9.5 × 10⁴ Ω·cm) in this paper, which indicates that the carbon fiber conductive silicone rubber prepared in this paper has good electrical properties.

The above analysis shows that the conductive properties of LSR/CF become stronger with increasing CF concentration. Furthermore, for the mass ratio of 5.5–7.5 wt%, the electrical properties of CF conductive silicone rubber were good.

(b)

Mechanical properties of the CF conductive silicone rubber

The tensile strength, elongation at break, and elastic modulus of LSR/CF with different CF concentrations are shown in Figure 2 and

Table 3. The resynamic properties of different concentrations of carbon fiber fill.

Carbon Fiber Content wt%	1.5	3.5	5.5	7.5	8.5	10.5	11.5	15	25
Tensile-Strength /Mpa	2.81	2.26	2.20	2.12	2.02	1.99	1.99	1.91	1.81
Elongation at Break /%	111.5	103.4	100	90.6	88.2	88.3	88.9	86.8	85.9
Elastic Modulus /Mpa	0.67	0.46	0.37	0.38	0.40	0.31	0.35	0.21	0.36

.

The figure shows that, as the CF concentration increases, the mechanical properties generally rapidly decline first and then slowly decline. Miiela et al. [21] found that adding too much carbon fiber would reduce the mechanical properties of composites. In this paper, when the carbon fiber content is 1.5 wt%, the tensile properties of conductive silicone rubber are the best, with tensile strength of 2.81 Mpa, elongation at break of 111.3%, and elastic modulus of 0.67 MPa. When the CF concentration was increased from 1.5 to 5.5 wt%, the mechanical properties of the conductive silicone rubber rapidly decreased, because part of the CF filler mixed with LSR to yield the conductive silicone rubber and another part precipitated to the lower layer of the mixed solution in the molding preparation stage to form a CF-like conductive film. The mechanical properties of LSR/CF improved when the CF content was increased from 5.5 to 7.5 wt%; the tensile strength exceeded 2 Mpa and the elastic modulus exceeded 0.4 MPa. X. Zhao et al. [22] found that in a certain range, with the increase of CF load, CF is not always surrounded by SR matrix, which makes the interaction between filler and matrix weak. Therefore, both tensile strength and elongation at break decrease. When the carbon fiber content exceeds 7.5 wt%, the mechanical properties tend to decrease rapidly. When the mass fraction of carbon fiber reaches 25.5 wt%, the tensile strength is 1.81 Mpa, the elongation at break is 85.9%, and the elastic modulus is 0.36 MPa. Compared with 7.5 wt% carbon fiber, the tensile strength, elastic modulus, and elongation at break of carbon fiber conductive silicone rubber decreased by 0.41 MPa and 0.02 Mpa, respectively, from 90.6% to 85.9%. Therefore, under the experimental conditions of a single carbon fiber-filled silicone rubber, the carbon fiber conductive silicone rubber with a mass ratio of 5.5–7.5 wt% has better mechanical properties. The tensile strength of spherical boron nitride 10/pitch-based carbon fiber 20/silicone rubber (s-BN10/PCF20/SR) prepared by reference [23] is 1.08 MPa, which is obviously lower than the tensile strength of 7.5 wt% carbon fiber conductive silicone rubber prepared in this paper (2.12 MPa), which indicates that the carbon fiber conductive silicone rubber prepared in this paper has good mechanical properties.

(c)

Dispersion and heat resistance analysis of LSR/CF.

According to the analyses of the electrical and mechanical properties, the ideal CF concentration is 5.5–7.5 wt%. Thus, LSR/CF with 7.5 wt% CF was tested, and its properties were comprehensively analyzed.

• (1)

  Dispersion analysis

Figure 3 displays the Raman spectra of LSR/CF. In the figure, for pure LSR, the peak at 1340 cm⁻¹ is ascribed to –CH2 bending and that at 2899 cm⁻¹ is the characteristic peak of –CH2 stretching vibration. For LSR/CF, the bending and stretching vibration characteristic peaks of –CH2 were also observed at 1340 and 2899 cm⁻¹, which proves that CF has good dispersion performance in conductive silicone rubber.

• (2)

  Heat resistance

The thermogravimetric analysis diagram of LSR/CF with 7.5 wt% CF is shown in Figure 4.

The corresponding temperature when losing 5%, 10%, and 50% of mass due to high temperature is T_d.5%, T_d.10%, and T_d.50%, respectively, and Tmax occurs when reaching the highest experimental temperature of 700 °C. As shown in Figure 4, the mass of LSR/CF rapidly decreased with increasing temperature, and the mass loss stabilized when the combustible substances were completely burned. For a CF concentration of 7.5 wt%, the T_d.5%, T_d.10%, and T_d.50% of the conductive silicone rubber were 457 °C, 523 °C, and 708 °C, respectively, and the corresponding temperature and weight loss at Tmax were 779 °C and 48.05%, respectively. For pure LSR, T_d.5%, T_d.10%, and T_d.50% were 441 °C, 520.1 °C, and 789.7 °C, respectively, and the corresponding temperature and weight loss at Tmax were 789.7 °C and 50.75%, respectively. When LSR/CF initially decomposed under the influence of temperature, its tensile sensitivity gradually decreased. The initial decomposition temperature of LSR/CF was 457 °C, which is 16 °C higher than that of pure LSR, indicating that CF improves the heat resistance of the silicone rubber.

(2)

Properties of graphene conductive silicone rubber

Graphene conductive silicone rubber (LSR/G) with a graphene content of 1.5, 2.5, 3.5, 5.5, 7.5, 8.5, 9.5, and 11.5 wt% was prepared and its properties were analyzed. The ratio of silicone rubber to n-heptane was kept unchanged at 1:2.

(a)

Electrical properties of graphene conductive silicone rubber

The volume resistivity and surface resistance of LSR/G with different graphene concentrations are shown in Figure 5 and

Table 4. The electrical properties of different concentrations of graphene fill.

Graphene Content /wt%	1.5	2.5	3.5	4.5	5.5	7.5	8.5	9.5
volume Resistivity /Ω·cm	4.6 × 108	2.3 × 108	5.7 × 107	2.2 × 107	8.7 × 104	1.6 × 104	8.7 × 103	2.0 × 103
surface Resistance /Ω	1.3 × 1010	7.5 × 109	1.7 × 109	6.0 × 108	2.4 × 106	5.2 × 105	2.6 × 105	6.1 × 104

.

As shown in Figure 5, when the graphene filling amount was 1.5–4.5 wt%, under the condition of less graphene filling, no conductive path formed in the matrix, showing good insulation. When the graphene filling amount reaches 5.5 wt%, the volume resistivity is 8.72 × 10⁴ Ω·cm, and the surface resistance is 2.48 × 10⁶ Ω. The filling amount of graphene was moderate and the conductive particles were in contact with each other, which enabled the formation of conductive paths and greatly improved the conductivity. When the graphene content exceeded 5.5 wt%, the volume resistivity and surface resistance sharply decreased and the conductivity was unstable. The volume resistivity of boron nitride-multilayer graphene/silicone rubber (BN-MG/SR) prepared by the literature [24] is as high as 4 × 10¹¹ Ω·cm, which is higher than that of the 5.5 wt% graphene conductive silicone rubber prepared in this paper. The ratio (8.7 × 10⁴ Ω·cm) is 7 orders of magnitude higher, which shows that the graphene conductive silicone rubber prepared in this paper has better electrical properties.

In the concentration range of 4.5–5.5 wt%, graphene particles considerably influence the electrical properties of silicone rubber, and the best electrical properties of LSR/G are observed in this range.

(b)

Mechanical properties of LSR/G

The tensile strength, elongation at break, and elastic modulus of LSR/G are shown in Figure 6 and

Table 5. The resynamic properties of different concentrations of graphene fill.

Graphene Content wt%	1.5	2.5	3.5	5.5	7.5	8.5	9.5	11.5
Tensile Strength /Mpa	1.26387	1.1018	1.08860	0.07479	0.03127	0.24404	0.00917	0.00759
Elongation at Break /%	103.2555	88.6389	85.4772	67.3561	70.4911	71.3661	58.9406	53.29772
Elastic Modulus /Mpa	0.21906	0.18840	0.18864	0.18089	0.10840	0.11815	0.11145	0.10566

.

As can be seen from Figure 6, when the mass fraction of graphene is 1.5 wt%, the tensile strength of the prepared conductive silicone rubber is 1.26387 Mpa. When the mass fraction of graphene is 1.5~4.5 wt%, the tensile strength, elongation at break, and elastic modulus of the prepared conductive silicone rubber decrease rapidly. When the mass fraction of graphene is in the range of 4.5~5.5 wt%, the changes of tensile strength, elongation at break, and elastic modulus of the prepared conductive silicone rubber are relatively stable. When the mass fraction of graphene is 5.5 wt%, the tensile strength, elongation at break, and elastic modulus of conductive silicone rubber are 0.07479 Mpa, 67.3561%, and 0.18089 Mpa, respectively. As the graphene content was further increased, the tensile strength obviously decreased and the elongation at break and elastic modulus slowly decreased.

In the LSR/G experiments, the mechanical properties of graphene rapidly decreased due to the self-lubrication between molecules. To ensure the mechanical properties of LSR/G, the graphene concentration of 4.5–5.5 wt% was determined to be ideal, and within this range, the conductive silicone rubber formed a conductive network. The graphene-filled silicone rubber prepared by the literature [25] has a maximum tensile strength of 1 Mpa, which is lower than that of the graphene conductive silicone rubber prepared in this paper (1.26387 Mpa), which proves that the graphene conductive silicone rubber prepared in this paper has good mechanical properties.

(a)

Analysis of dispersibility and heat resistance of LSR/G.

The electrical conductivity of LSR/G increased with the graphene content, and the resistance value considerably changed in the graphene concentration range of 4.5–5.5 wt%. The composite properties of LSR/G with 5.5 wt% graphene were tested and analyzed.

• (1)

  Dispersion analysis

Figure 7 displays the Raman spectra of LSR/G. The bending peak at 1340 cm⁻¹ and the –CH2 stretching vibration characteristic peak at 2899 cm⁻¹ were not detected for LSR/G. This proves that graphene can be well combined with silicone rubber substrate.

• (2)

  Heat resistance

Figure 8 displays the thermogravimetric analysis of LSR/G with 5.5 wt% graphene.

As shown in Figure 8, for LSR/G with 5.5 wt% graphene, T_d.5%, T_d.10%, and T_d.50% were 470 °C, 509 °C, and 569 °C, respectively, and the corresponding temperature and weight loss at Tmax were 798 °C and 32.124%, respectively. Furthermore, according to the thermogravimetric analysis, the heat resistance of LSR/G was higher than that of pure LSR. The initial decomposition temperature of LSR/G was 29 °C higher than that of pure LSR (441 °C), and the heat resistance enhancement effect of the LSR/G was higher than that of LSR/CF.

LSR/G with 5.5 wt% graphene has good heat resistance, but its heat resistance is lower than that of LSR/CF, and the bonding performance between graphene and the silicone rubber matrix is better than that between CF and the silicone rubber matrix.

(3)

Properties of carbon nanotube conductive silicone rubber

Carbon nanotube conductive silicone rubber (LSR/CNT) was prepared with 0.2, 0.5, 0.75, 1, 1.25, 1.5, and 2.5 wt% CNTs. The ratio of silicone rubber to n-heptane was kept unchanged at 1:2. The properties of the samples were analyzed.

(a)

Electrical properties of LSR/CNT.

The volume resistivity and surface resistance of the LSR/CNT with different CNT concentrations are shown in Figure 9 and

Table 6. The electrical properties of different concentrations of carbon nanotubes fill.

Carbon Nanotube Content wt%	0.2	0.5	0.75	1	1.25	1.5	2.5
Volume Resistivity Ω·cm	0.83 × 1010	1.61 × 107	1.55 × 107	4.32 × 104	1.34 × 104	0.50 × 104	5.1 × 103
Surface Resistance Ω	1.33 × 1011	1.71 × 1010	8.01 × 108	5.16 × 107	1.00 × 106	9.87 × 104	7.00 × 104

.

As shown in Figure 9, for CNT content of less than 1 wt%, the proportion of CNTs in the conductive silicone rubber was low, the spacing between CNTs particles was large, and the conductive path did not form. When the CNT content was increased from 1 to 1.25 wt%, the resistivity stabilized, and the conductive path easily formed. When the mass ratio of CNTs is 1.25%, the volume resistivity of conductive silicone rubber is 1.34 × 10⁴ Ω·cm, and the surface resistance is 1.00 × 10⁶ Ω. When the CNT content was further increased, the volume resistivity and surface resistance remained basically unchanged. The volume resistivity of polydimethylsiloxane (PDMS)/multi-walled carbon nanotubes (CNT) prepared in literature [20] is 1 × 10⁶ Ω·cm, which is higher than that of the 1.25 wt% carbon nanotube conductive silicone rubber prepared in this paper. The volume resistivity (1.34 × 10⁴ Ω·cm) is 2 orders of magnitude higher, which indicates that the carbon nanotube conductive silicone rubber prepared in this paper has better electrical properties.

The above experiments show that CNTs have excellent conductivity and can form conductive paths in the matrix when filled at low concentrations. The CNT content of 1–1.5 wt% was selected as the best technological ratio, and the electrical properties are good in this range.

(b)

Mechanical properties of LSR/CNT.

The tensile strength, elongation at break, and elastic modulus of LSR/CNT with different CNT concentrations are shown in Figure 10 and

Table 7. The resynamic properties of different concentrations of carbon nanotubes fill.

Carbon Nanotube Content wt%	0.2	0.5	0.75	1	1.25	1.5	2.5	3.5
Tensile Strength /Mpa	3.10	2.02	1.80	1.69	1.60	1.58	1.30	0.004
Elongation at Break /%	113.0	90.4	89.0	85.0	75.9	65.9	64.5	63.2
Elastic Modulus /Mpa	0.738	0.43	0.42	0.40	0.35	0.29	0.26	0.20

.

Figure 10 shows that the tensile strength, elongation at break, and elastic modulus of LSR/CNT decreased when the CNT concentration was increased from 0.5 to 1 wt%, because the mass fraction of the silicone rubber decreased while the content of CNTs increased. When the filling amount of CNTs was small, the CNT particles had little influence on the conductive silicone rubber composite system. When the concentration of CNTs was increased from 1 to 1.25 wt%, its tensile strength, elongation at break, and elastic modulus decreased relatively slowly; the tensile strength decreased from 1.69 Mpa to 1.60 Mpa, the elongation at break decreased from 850% to 759%, and the elastic modulus decreased from 0.40 Mpa to 0.35 Mpa, forming an infiltrated filler network at a lower CNT content [26]. In addition, LSR/CNT exhibited good mechanical properties. When the CNT concentration exceeded 1.25 wt%, the tensile strength, elongation at break, and elastic modulus rapidly decreased and were relatively unstable. The maximum tensile strength of 0.5 wt% carbon nanotube silicone rubber (SiR) prepared in reference [27] is only 0.51 MPa, which is lower than the tensile strength of 1.25 wt% carbon nanotube conductive silicone rubber (1.60 MPa) prepared in this paper, which indicates that the mechanical properties of carbon nanotube conductive silicone rubber prepared in this paper are good.

(a)

Spectrum and heat resistance analysis of LSR/CNT.

According to the results of electrical and mechanical properties, the ideal CNT concentration is 1–1.25 wt%, and the composite properties of LSR/CNT with 1–1.25 wt% CNTs were explored.

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  (1)

  Dispersion analysis

Figure 11 depicts the Raman spectra of LSR/CNT. Two distinct characteristic peaks were observed in the spectra at 1189 and 1237 cm⁻¹. The former is called the G mode and the latter is called the D mode. They stem from the impurities in CNTs and their own tubular defects. The strength ratio of the D and G modes is used to express the density of the tubular structure defects in multiwall CNTs. That is, R = Id/Ig (where I denotes the integral of the characteristic peak area), which signifies that the greater the ratio, the greater the defects of CNTs, the more uniform the dispersion, and the lower the degree of order.

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  (2)

  Heat resistance

Figure 12 shows the thermogravimetric analysis of LSR/CNT with 1.25 wt% CNTs.

As shown in Figure 12, for CNT content of 1.25 wt%, T_d.5% was 464.3 °C, T_d.10% was 531.716 °C, and the corresponding temperature and weight loss at Tmax were 799 °C and 56.325%, respectively. Among the three studied carbon-based nano-conductive silicone rubber samples, LSR/CNT exhibited the best electrical properties, but its heat resistance reinforcement effect on silicone rubber was less than that of LSR/G and LSR/CF. A stable conductive network can be constructed when carbon nanotubes are filled with 1.25 wt%, and tensile sensitivity can be achieved even after bearing 500% tensile strain.

Based on the data of the single-filler conductive silicone rubbers, the optimal proportion of the nano-fillers was obtained: 5.5 wt% for graphene, 1.25 wt% for CNTs, and 7.5 wt% for CFs.

3.2. Properties of Blended Carbon Nano-Conductive Silicone Rubber

Blended carbon nano-conductive silicone rubber (LSR/CNT/CF/G) was prepared with the optimal proportions of the nano-fillers. The properties of LSR/CNT/CF/G were compared with those of LSR/CF, LSR/CNT, and LSR/G via Raman spectroscopy, thermogravimetry analysis, DSC, XRD, scanning electron microscopy, and tensile resistance analysis.

(1)

Analysis of the tensile sensitivity of LSR/CNT/CF/G.

As shown in Figure 13, LSR/CNT/CF/G obviously has good tensile resistance characteristics. Its initial resistance was 2.69 × 10³ Ω, and when the elongation was 100%, the resistance was 3.86 × 10³ Ω. The resistance of LSR/CF with 7.5 wt% CF was 5.34 × 10⁷ Ω, As shown in Figure 14,The resistance value of 5.5 wt% graphene conductive silicone rubber is 8.33 × 10⁸ Ω. The stretching sensitivity of blended carbon nano conductive silicone rubber is better than that of carbon fiber conductive silicone rubber and graphene conductive silicone rubber. When the stretch rate reaches 250%, the resistance value 9.71 × 10³Ω, As shown in Figure 15, 1.25 wt% carbon nanotube conductive silicone rubber resistance value is 7.09 × 10⁶ Ω. As shown in Figure 16,The tensile sensitivity of LSR/CNT/CF/G was better than that of LSR/CNT, and the resistance value increased from 1.01 × 10⁴ to 9.66 × 10⁴ Ω when the tensile strength was increased from 250% to 520%. Blended carbon nano-conductive silicone rubber is in the range of 520–816%. The maximum resistance was 1.04 × 10⁶ Ω when the elongation was 827%.

(2)

Dispersion analysis of LSR/CNT/CF/G

Next, the dispersion of LSR/CNT/CF/G was analyzed. Figure 17 displays the Raman spectra of LSR/CNT/CF/G in comparison to the spectra of the single-filler conductive silicone rubbers. For pure LSR, the peak of the bending of –CH2 was at 1340 cm⁻¹, and the characteristic peak of the stretching vibration of –CH2 was at 2899 cm⁻¹. The LSR/CNT/CF/G sample curve comprised two peaks: D (1280 cm⁻¹) and G (2430 cm⁻¹). The D peak stemmed from the defect of the sp3 orbital in the graphene sheet, while the G peak stemmed from the phonon mode of the sp2 carbon atom in the plane. As shown in Figure 18, for LSR/CF, the bending and stretching vibration characteristic peaks of –CH2 were still observed at 1340 and 2899 cm⁻¹, respectively. The characteristic peaks in the curve of the pure LSR sample gradually weakened or disappeared, proving that good interfacial adhesion exists between the multi-filler blend and the matrix and that the dispersion is good.

(3)

Thermal stability analysis of the carbon-based nano-conductive silicone rubbers

When comparing the thermal stability of the carbon-based nano-conductive silicone rubbers prepared with different conductive fillers, the data need to be recorded under different weightlessness conditions. Then, the thermal stabilities of the different carbon-based nano-conductive silicone rubbers were compared. Figure 19 displays the thermogravimetric analysis diagram of LSR/CNT/CF/G. Figure 20 and Figure 21 compare the thermogravimetric analysis and derivative thermogravimetric results of the different carbon-based nano-conductive silicone rubbers and LSR, respectively.

A shown in Figure 19, for LSR/CNT/CF/G, T_d.5% was 476.8 °C, T_d.10% was 526.02 °C, and T_d.50% was 645.6 °C, and the corresponding temperature and weight loss at Tmax were 719.0 °C and 43.99% respectively.

Figure 20 and Figure 21 show that the thermal stability of the carbon-based nano-conductive silicone rubbers is obviously higher than that of pure LSR. In T_d.5% decomposition, the decomposition temperature of pure LSR, LSR/CNT/CF/G, LSR/CNT, LSR/CF, and LSR/G was 441 °C, 464 °C, 457 °C, and 470 °C, respectively. Compared to the other carbon-based nano-filler conductive silicone rubber, the thermal stability of LSR/CNT was the best, and the thermal stability of LSR/CNT/CF/G was similar to that of LSR/CNT.

As shown in Figure 21, the initial decomposition temperature of pure LSR was 360 °C, while that of LSR/CNT/CF/G was about 400 °C. This is because the conductive fillers graphene, CNTs, and CFs all exhibit good thermal stability and thermal conductivity; thus, the carbon-based nano-conductive silicone rubber compounded by them has a higher initial decomposition temperature than pure LSR. When the mass loss was 10%, the temperature of pure LSR was 520 °C and of LSR/CNT/CF/G was 526 °C. This shows that the heat resistance of LSR improved due to the combination of the various fillers. LSR/CNT/CF/G has good heat resistance and can be used as a varistor sensor.

(4)

Analysis of the glass transition temperature of LSR/CNT/CF/G.

The Tg value is an important parameter for rubber materials; it affects the performance and service life of the materials. The internal motion of polymer materials can be divided into glassy, adhesive, high elastic, and viscous states. In general, the transition of polymer materials from glassy state to high elastic state is called as the glass transition, and its temperature is called the glass transition temperature. When the material temperature is lower than the Tg value, the material is brittle, and when the material temperature is higher than the Tg value [28], the material is highly elastic. Figure 22 displays the Tg of LSR/CNT/CF/G obtained using DSC, and Figure 23 compares it with that of the other four silicone rubbers.

As shown in Figure 22, the Tg of LSR/CNT/CF/G with 14.7 wt% conductive filler was −51.126 °C.

As shown in Figure 23, the Tg of pure LSR, LSR/CNT/CF/G, LSR/CNT, LSR/G, and LSR/CF were −49 °C, −51.126 °C, −51.479 °C, −49.944 °C, −50.97 °C, and −50.72 °C, respectively. The Tg values were similar; all were around −50 °C. This is because silicone rubber accounts for a large mass fraction, and its cold resistance determines the Tg value. The working temperature of −50 °C meets the working environment of most varistors.

(5)

Crystal structure analysis of the LSR/CNT/CF/G.

Figure 24 presents the XRD analysis of LSR/CNT/CF/G, and Figure 25 presents the XRD comparison analysis chart of the five kinds of silicone rubber. As shown in Figure 24, pure LSR exhibited two broad peaks in the center range of 2θ = 10° and 22.5°, which proves that the LSR polymer has an amorphous structure, and two broad peaks were also observed for LSR/CNT/CF/G in the center range of 2θ = 10° and 22.5°. Figure 25 shows that for LSR/CNT/CF/G, other than the above two characteristic peaks, the characteristic peak at 2θ = 24° was absent and that at 2θ = 28.2° was obviously diminished. This proves that due to the interaction of the conductive fillers, the fillers are uniformly dispersed during the ultrasonic dispersion and have strong interfacial force and the fillers are dispersed when the LSR solution is cured.

The heat resistance, cold resistance, Raman spectra, and tensile sensitivity of the conductive silicone rubbers blended with multiple fillers and single filler were assessed. The results showed that the conductive silicone rubber blended with multiple fillers had the advantages of maximum initial decomposition temperature, good adhesion with matrix, maximum strain range of 520%, low resistance, and high sensitivity compared to the other materials, which indicated that the properties of the conductive silicone rubber blended with multiple fillers were greatly improved compared to those of pure LSR.

4. Conclusions

In this study, the preparation method of conductive silicone rubber with carbon-based nanomaterials as fillers was studied and the properties of the conductive silicone rubbers were tested and characterized.

(1)

Raman spectrum analysis showed that, compared to single-filler nano-conductive silicone rubbers, the interfacial adhesion and dispersibility of blended carbon-based nano-conductive silicone rubber were better.

(2)

Thermogravimetric analysis showed that the heat resistance of LSR/CNT/CF/G was better than that of LSR/CF, LSR/G, LSR/CNT, and pure LSR, and the initial decomposition temperature reached 476.8 °C.

(3)

DSC analysis showed that the Tg of LSR/CNT/CF/G was similar to that of the single-filler nano-conductive silicone rubbers (around −50 °C), which can meet the working environment of most varistors.

(4)

XRD analysis showed that all the fillers were uniformly dispersed during ultrasonic dispersion and had strong interfacial forces.

(5)

The tensile conductivity test showed that the effective tensile rates of LSR/CF, LSR/G, and LSR/CNT were 100%, 140%, and 300%, respectively, and the maximum effective tensile rate of LSR/CNT/CF/G was 520%. In addition, the resistance fluctuation range of LSR/CNT/CF/G was smaller than that of the single-filler nano-conductive silicone rubbers, which has good resistance and pressure sensitivity characteristics.

In this paper, the preparation method of conductive silicone rubber by blending carbon fiber, graphene, and carbon nanotubes was preliminarily studied, but the research on the types and proportions of conductive silicone rubber fillers still has some limitations, and the follow-up research work can be carried out in the following directions:

(1)

Enrich the types of conductive fillers, carry out surface treatment or modification treatment on the fillers, and improve the process according to the performance of different fillers.

(2)

The fatigue strength and durability of conductive silicone rubber with mixed fillers were further investigated, which laid a foundation for the application of conductive silicone rubber in flexible sensors and other fields.