2.1 Materials

Natural rubber (NR) 100 Phr was used as the matrix material; antioxidant (4020) 2 Phr was used to resist the effect of aging on the composite properties; zinc oxide (ZnO) 2 Phr, stearic acid (SAD) 2 Phr, and accelerator (CZ) 1 Phr were used as vulcanization activators to promote the vulcanization reaction; sulfur (S) 2 Phr was used to carry out the rubber molecular vulcanization crosslinking reaction. CNFs and thermally cracked CB (CBp) were used as reinforcing fillers, mainly to enhance the properties of the composites. The CBp/CNFs ratios and sample numbers are shown in Table 1.

The specifications of CNFs and CBp are as follows: CNFs, JCGCF (OD: 20–30 nm, Length: 3–12 um, SSA: 180–230 m²/g), Jia Cai Technology; CBp, N990 (DBP: 44 ± 10 ml/100 g, PH: 8.0–10, Ash: ≤0.5), Tianjin Yiborui Chemical Co.

2.2 Sample preparation

Cut the NR into small pieces to facilitate feeding. Set the speed of the internal mixer (XSM-500 type, Shanghai Kechuang Rubber & Plastic Machinery Equipment Co., Ltd.) to 80 r/min and the initial temperature to 40°C. Put the cutting NR into the mixer, add 1/2CBp for mixing at 1:20, add other fillers except for CZ and S for mixing at 2:00, and discharge the rubber at 5:00. Continue to mix and add CZ and S on the open mixer (BL-6157 type, Baolun Precision Testing Instruments Co., Ltd.), and then drop the sheet after sufficient mixing, and leave the mixed rubber for 8 h. Vulcanize the compound using a plate vulcanizing machine (QLB-400X400X2 type, Shanghai No.1 Rubber Machinery Factory), place the quantitative mixed rubber in the vulcanizing mold, the vulcanizing temperature is 150°C, and the vulcanizing time is T₉₀ × 1.3 (T₉₀ is obtained by vulcanizing characteristics).

2.3 Performance test

Vulcanized rubber samples were impregnated in liquid nitrogen, removed and subjected to brittle fracture to obtain cross-sections. A field emission scanning electron microscope (SEM, SU8000, Hitachi Group) was used to characterize the cross-sections and to analyze the dispersion of the filler in the rubber. 3D laser measuring microscope (LEXT OLS5000, Olympus, Japan) was used to watch the 3D morphology of the brittle fracture surface of samples. The analysis of the fracture surface can reflect the size of the filler agglomerates within the rubber collective to a certain extent, and the greater the roughness of the fracture surface indicates the more serious filler agglomeration.

No rotor vulcanizing instrument (M-2000-AN, Taiwan High-Tech Testing Instruments Co., Ltd.) was set to 150°C, pressure 0.6 MPa, and test time 60 min. A total of 5.8 g of mixed rubber was weighed and tested on no rotor vulcanizing instrument to obtain the vulcanization characteristics data.

The vulcanized rubber was cut into standard samples for physical and mechanical property testing, and the thickness of each sample was measured before testing. The tensile samples are shown in Figure 1a and the tear samples are shown in Figure 1b. Set the tensile rate of Tensile testing machine (TS 2005b type, Taiwan Ucan Technology Co., Ltd.) to 500 mm/min for the physical and mechanical property test of vulcanized samples, and measure three samples in each group and take the average value. The hardness of the vulcanized samples was measured by using Shore A hardness tester (LX-A type, Shanghai Liuling Instrument Factory), and three standard specimens were superimposed for hardness measurement.

A conductivity instrument (JSM-2100, Japan Electronics Corporation) and a high resistance meter (ST2254) were used to build a test platform for sample volume resistivity measurements, and the sample surface was cleaned and the sample thickness was measured before testing. The conductive samples are shown in Figure 1c.

4.1 Cross-sectional SEM characterization

The SEM characterization of the sample sections is shown in Figure 4. From Figures 4a,b, it can be seen that there will be an obvious filler agglomeration phenomenon inside the composites when CNFs are added alone, which is not conducive to the dispersion of fillers and the improvement of composite properties. The agglomeration phenomenon also exists when CBp is added, which has an impact on the reinforcement of the rubber material. As can be seen from Figures 4c–e, the filler agglomeration problem is obviously improved when CBp and CNFs are added to the rubber matrix as fillers, and the CNFs are interspersed between the CBp particles to prevent the agglomeration of CBp particles. CBp and CNFs are isolated from each other and connected to each other, which solves the filler agglomeration and establishes a continuous filler network inside the compound, so that the filler reinforcement can be maximized. With the increase of CNFs content, CNFs aggregation will start to appear inside the composites again, indicating the saturation phenomenon of filler synergy. Only when the ratio of CBp and CNF is optimal, the synergistic effect will show the best effect.

Figure 4f can characterize the specific states of CBp and CNFs inside the composite. The CNFs are present between the CBp particles and interconnected at the larger gaps, forming a continuous network of CNFs. This continuous network isolates the CBp particles from each other and prevents the agglomeration of CBp. At the same time, this network has better reinforcement and conductivity, which is helpful for the enhancement of the basic characteristics and conductivity of the composites. SEM characterization of the composite sections is consistent with the analysis of the filler particle distribution state above.

4.2 Rubber processing properties

Figure 5 shows the change graphs of energy storage modulus $G^{\prime }$ and loss modulus $G^{\prime \prime }$. It can be seen that the energy storage modulus $G^{\prime }$ and loss modulus $G^{\prime \prime }$ of the composite when CBp and CNFs are blended are significantly higher than that of the single filler. The possible reasons are in two aspects, firstly, the CNFs have strong activity on the surface and can physically/chemically bond with the CBp, and the synergistic effect of the two promotes the dispersion of the fillers in the matrix and increases the bonding point inside the composite. Second, the number of filler particles in the rubber increases after the blending of the two, and the number of filler binding points inside the composite is naturally added. With the increase of CNFs content, the filler binding points in the matrix keep increasing, and the energy storage modulus $G^{\prime }$ and loss modulus $G^{\prime \prime }$ show an increasing trend.

Compared to large strains, at small strains, the energy storage modulus $G^{\prime }$ and loss modulus $G^{\prime \prime }$ vary more with the filler. This is mainly because at low strains the external force first destroys the filler network in the rubber matrix, while causing some degree of damage to the bond between the matrix and the filler. At higher strains, the energy storage modulus $G^{\prime }$ and loss modulus $G^{\prime \prime }$ change less because the filler network inside the composite has been destroyed, and mainly the rubber macromolecular chains inside the composite play a dominant role.

Figure 6 shows Payne effect. Compare and Contrast C1, C2, and C3, it was shown that Payne effect increased significantly when 2 Phr CNFs and 40 Phr CBp were blended together, increasing by 230% compared to when 2 Phr CNFs have added alone and by 59% compared to when 40 Phr CBp has added alone. It indicates that the synergistic effect of CBp/CNFs increased the Payne effect of the composites.

The Payne effect changed less when the CNFS content was increased from 2 to 4 Phr while the CBp was kept constant. The rubber matrix contains a large amount of CBp particles, which is sufficient to segregate the 4 Phr CNFs, and the CNFs are better dispersed inside the rubber matrix to prevent agglomeration among CNFs, when the synergistic effect reaches its best. Continuing to increase the content of CNFs, the Payne effect started to appear more substantially. The main reason is that the CBp content inside the rubber matrix is limited, and the agglomeration phenomenon occurs when the CNFs exceed a certain amount, which causes a significant increase in the bonding point inside the composite.

4.3 Vulcanization properties

Figure 7 shows the M_L, M_H, and ΔM variation curves of the mixed rubber, and the T₁₀ and T₉₀ variations of the mixed rubber are shown in Figure 8, and the corresponding values are shown in Table 2. It is obvious from the figure that the torque of CBp/CNFs/NR blends is significantly higher than that of CNFs/NR and CBp/NR blends, indicating a significant increase in the crosslink density within the rubber compound. The vulcanization time was reduced. The first reason, the increase in the number of filler particles inside the compound during filler blending, contributed to the increase in crosslink density and faster vulcanization rate. The second reason is that the physical and chemical combination of CBp/CNFs by the synergistic effect creates a complete filler network inside the rubber composite, which multiplies the number of filler-rubber bonding points and increases the crosslink density due to the combination between the fillers and the combination between the filler network and the rubber molecules. The complete filler network provides more pathways for heat transfer during the vulcanization process and increases the thermal conductivity of the compound, thus promoting vulcanization. The second reason is more important and plays a major role in the change of vulcanization properties.

With the increase of CNFs content, the M_L, M_H, and ΔM curves of the compound tend to be smooth and the variation of the vulcanization time decreases, indicating that the influence of CNFs on the vulcanization properties of the compound decreases. The possible reason is that the filler network inside the compound is gradually improved, and the continuous increase of CNFs has little effect on the filler network, so the variation of the vulcanization properties is reduced.

4.4 Physical and mechanical characteristics

The physical and mechanical characteristics of the vulcanized rubber were tested experimentally as shown in Table 3, and Figure 9 shows the variation curves of the data. The CBp/CNFs/NR composites were compared with CBp/NR and CNFs/NR. CBp/CNFs/NR showed a significant increase in tensile strength, tear strength and hardness, and a certain degree of decrease in elongation at break. At lower strains, there was little difference in the stress required to produce the same strain. However, as the strain increases, the stress required to produce the same deformation gradually increases. The possible reason for this is that at low strains the damage to the filler network and rubber macromolecular chains inside the composite is small and does not require much stress. In the case of large tensile strains, the stresses required to break the filler network and the rubber molecular chains inside the composite increase.

And with the increase of CNFs content, the physical and mechanical properties of CBp/CNFs/NR composites gradually stabilized. It is mainly because the filler network is gradually improved, the crosslink density inside the material changes less, and the resistance of the composite to damage is basically unchanged.

4.5 Resistance characteristics

The resistivities of the vulcanized rubber are shown in Table 4. The analysis shows that the volume resistivity of the composites was 4.33E13 and 4.03E14 when CNFs and CBp were added alone, and 1.25E12 when CBp/CNFs have blended, which respectively decreased by 1 and 2 orders of magnitude. The volume resistivity stabilized with increasing CNFs content. The surface resistivity of the composites reflected the same trend. The CBp/CNFs blending promoted the dispersion of the filler inside the composites and established a more complete filler network. The improved filler network has better electrical conductivity, which has a significant promotion on raising the electrical conductivity of the composites. The experimental results are consistent with the analysis of filler dispersion effect, which indicates that the above analysis has some accuracy.