C₁₈H₂₉NaO₃S, C₈H₇NaO₃S, and C₁₇H₃₅COONa were used to treat CBp in the experimental process to investigate the best surface activator for CBp, and then the treated CBp was used to prepare rubber composites by wet blending method. Table 1 shows the components, suppliers, and formulation of the rubber composites.

In this paper, the wet mixing of natural latex and CBp was used to improve the homogenous dispersion of CBp in rubber matrix (Tian et al., 2021). This method mainly includes two processes: Preparation of CBp/natural latex masterbatch and preparation of CBp/rubber composite. The specific experimental process is shown in Figure 1.

CBp/rubber composite were made by adding fillers such as zinc oxide to the masterbatch of CBp/natural latex through the internal mixer, which produces a strong shearing effect on the rubber by the rotor of the internal mixer to achieve uniform dispersion of the CBp in the rubber matrix. Then, the CBp/rubber composite was prepared by vulcanization. The specific process was shown in Table 2.

Processability is an essential indicator of the properties of rubber compound (Tian et al., 2020a; Tian et al., 2020b). It is usually characterized by Mooney viscosity and rheological properties. Among them, t10 is the scorch time of the rubber compound; t90 is the positive vulcanization time of the rubber compound; MH and ML represent the highest torque and the lowest torque of the rubber compound, respectively. Table 3 presents the processing performance data of CBp/rubber compound with different surfactants.

The three rubber composite with surfactant-modified CBp have sufficient scorch time, which can ensure the safety of rubber compound processing. The positive vulcanization time of C₁₈H₂₉NaO₃S and C₈H₇NaO₃S is longer than that of C₁₇H₃₅COONa modified rubber composite. C₁₈H₂₉NaO₃S and C₈H₇NaO₃S are sulfonates. The sulfonic acid reacts easily with the amine ion in the amine accelerator, reducing the reactivity of the accelerator and thus increasing the positive vulcanization time of the CBp/rubber composite (Wang et al., 2019). The MH-ML value of the rubber composite prepared by C₈H₇NaO₃S modified CBp is the largest, indicating that the rubber composite has the highest crosslinking degree. Meanwhile, the ML of the rubber composites prepared by C₁₈H₂₉NaO₃S modified CBp is the smallest, indicating that the CBp/rubber compound has better fluidity and low Mooney viscosity.

The CBp/rubber composite prepared by treating CBp with surfactant are tested by carbon black dispersity analyzer. The value of [X] represents the degree of dispersion of carbon black, and the value of [Y] represents the degree of dispersion of large particles of carbon black. The larger the value of [X] and [Y], the better the dispersion of carbon black in the rubber compound. Table 4 presents the value of [X] and [Y] of CBp/rubber composite with different surfactants.

From Table 4, the dispersion of C₁₈H₂₉NaO₃S and C₈H₇NaO₃S modified CBp in the rubber composite matrix is significantly better than that of C₁₇H₃₅COONa. C₁₈H₂₉NaO₃S has been found to be effective in dispersing carbon black, and the lower the degree of sulfonation function, the better the dispersion of carbon black. Lignosulfonate mainly enhances colloidal stability by electrostatic repulsion, and the likely mechanism of stabilization is a combination of electrostatic repulsion and steric hindrance (Subramanian and Øye, 2021). C₈H₇NaO₃S and C₁₈H₂₉NaO₃S have the same functional groups, but the molecular weight of C₁₈H₂₉NaO₃S (348.47 g/mol) is larger than that of C₈H₇NaO₃S (206.19 g/mol) (Kurt et al., 1995). By adding the same mass of surfactant, the number of effective functional groups of C₁₈H₂₉NaO₃S will be less than that of C₈H₇NaO₃S, making the dispersion effect of CBp in rubber matrix slightly lower than that of C₈H₇NaO₃S.

According to Table 5, the △G′ values of C₁₈H₂₉NaO₃S and C₈H₇NaO₃S modified CBp composites are 238.89 kPa and 237.27kPa, respectively. These values are lower than the △G′ value of 264.7 kPa for the C₁₇H₃₅COONa modified CBp rubber composite. A smaller △G′ value indicates a better interaction between CBp and rubber, with reduced CBp-CBp interaction (Tian et al., 2021). This suggests that CBp is more effectively dispersed in the rubber composite. The addition of C₁₈H₂₉NaO₃S and C₈H₇NaO₃S surfactants improves the wettability and uniform dispersion of CBp in natural latex, reducing the CBp-CBp interaction (Zhu et al., 2020). C₁₈H₂₉NaO₃S and C₈H₇NaO₃S are sulfonates and exhibit higher activity compared to C₁₇H₃₅COONa (Subramanian and Øye, 2021). Although C₈H₇NaO₃S and C₁₈H₂₉NaO₃S possess the same functional groups, the molecular weight of C₁₈H₂₉NaO₃S (348.47 g/mol) is greater than that of C₈H₇NaO₃S (206.19 g/mol) (Kurt et al., 1995). When the dosage is the same, C₈H₇NaO₃S has a higher number of hydrophilic functional groups than C₁₈H₂₉NaO₃S, resulting in a stronger surface modification effect on CBp. The enhanced dispersion of CBp in the presence of C₁₈H₂₉NaO₃S and C₈H₇NaO₃S is supported by higher [X] and [Y] values, as reported in Table 4.

Figure 2 illustrates that the C₁₈H₂₉NaO₃S modified CBp rubber composite exhibits the highest modulus. C₁₇H₃₅COONa, C₁₈H₂₉NaO₃S, and C₈H₇NaO₃S belong to metal soap salts derived from fatty acids. These compounds consist of hydrophilic functional groups and hydrophobic carbon chains (Kerenkan et al., 2016). The hydrophobic carbon chains align to form micelles within the rubber, providing internal lubrication. The hydrophilic functional groups react with the surface groups of pyrolytic carbon black, serving as a means of surface modification (Wang et al., 2017). C₁₈H₂₉NaO₃S and C₈H₇NaO₃S, which are sulfonic acid metal soap salts, exhibit higher activity than C₁₇H₃₅COONa (Subramanian and Øye, 2021). The modification of CBp using C₁₈H₂₉NaO₃S and C₈H₇NaO₃S improves CBp dispersion and enhances CBp-rubber interaction. Additionally, the presence of a benzene ring in C₁₈H₂₉NaO₃S reduces the flexibility of its molecular chain, resulting in a more effective internal lubrication effect compared to C₈H₇NaO₃S (Shahabuddin et al., 2021). The combined effects of these factors contribute to the highest modulus observed in rubber composites containing C₁₈H₂₉NaO₃S -modified CBp.

Table 6 shows the physical and mechanical properties of CBp/rubber composites modified by different surfactants.

The C₁₈H₂₉NaO₃S surfactant-modified CBp/rubber composite has the highest tensile strength of 25.63 MPa, followed by C₈H₇NaO₃S and C₁₇H₃₅COONa, with 24.74 MPa and 22.17 MPa, respectively. The C₁₈H₂₉NaO₃S surfactant-modified CBp/rubber composite has 15.6% increment in tensile strength compared to C₁₇H₃₅COONa surfactant-modified CBp/rubber composite. There are three surfactants used in this study, the functional group of C₁₇H₃₅COONa is carboxyl group, while the functional group of The C₁₈H₂₉NaO₃S and C₈H₇NaO₃S is sulfo group. From Table 6, surfactants with sulfo functional group have higher tensile strength compare to carboxyl functional group surfactant. The presence of sulfonate group increases the water solubility of the C₁₈H₂₉NaO₃S and C₈H₇NaO₃S, and its surface charged functional groups contribute to the uniform dispersion of CBp in natural latex (Zhu et al., 2020), which enhances the compatibility of CBp with rubber and thus increases the tensile strength of rubber composites. In general, the more uniform the dispersion of CBp and the stronger the CBp-rubber interaction, the greater the tensile strength of the composite (Alex et al., 2013). This is also the main reason for the high tear strength and resilience value of the C₁₈H₂₉NaO₃S and C₈H₇NaO₃S surfactant-modified CBp/rubber composites. The C₁₈H₂₉NaO₃S surfactant-modified CBp/rubber composites has the lowest hardness of 59°, followed by C₈H₇NaO₃S and C₁₇H₃₅COONa, with 65° and 67°, respectively. The hardness of rubber composites generally related to the dispersion of the modified CBp and the degree of crosslinking of CBp/rubber composite. Typically, the higher MH-ML value and the more uniform the dispersion of CBp, the higher the crosslinking degree of the rubber composite (Shan et al., 2023). The higher degree of crosslinking enhanced the mechanical properties of rubber composite (Sun et al., 2022). This could be the primary contribution of the higher hardness values of the C₈H₇NaO₃S and C₁₇H₃₅COONa surfactant-modified CBp/rubber composites. These findings show that the CBp can be uniformly dispersed in natural latex after being treated with C₁₈H₂₉NaO₃S, resulting in better comprehensive mechanical properties of the composite.

The dynamic mechanical properties of the composites refer to the mechanical response of composites under the action of alternating force. Usually, the dynamic mechanical properties of composites are studied through the changes in storage modulus and loss modulus under the influence of temperature and vibration frequency. Loss factor (tanδ) refers to the ratio of loss modulus G″ to storage modulus G′, which is an essential parameter of rubber composites and directly reflects the internal friction of rubber compounds. To a certain extent, it can characterize the dispersion of fillers in the rubber compound, the network degree of fillers, and the rolling resistance and wet skid resistance of the rubber compound. Typically, the higher the peak value of the loss factor (tanδ), the better the dispersion of the filler and the lower the degree of networking of the filler (Tian et al., 2021).

It can be seen from Figure 3 that the rubber composite prepared by C₈H₇NaO₃S modified CBp has the highest tanδ peak, followed by C₁₈H₂₉NaO₃S modified CBp composite, and C₁₇H₃₅COONa modified CBp composite has the lowest tanδ peak. This shows that C₈H₇NaO₃S modified CBp has the best dispersibility in rubber composite and the weakest filler network effect, this is also in agreement with the payne effect and the dispersion properties of the composite. Besides, the C₈H₇NaO₃S modified CBp/rubber composite has the lowest tanδ value at 60°C, and the C₁₈H₂₉NaO₃S modified CBp/rubber composite has the highest tanδ value at 0°C. In the industry, tanδ at 0°C is commonly used to characterize the wet skid resistance of rubber composites. The larger the value, the better the wet skid resistance of the rubber composite. The rolling resistance of rubber composites is characterized by tanδ at 60°C. The smaller the value, the better the rolling resistance of the rubber composite (Tian et al., 2021). CBp modified by C₈H₇NaO₃S improves the surface activity of the CBp and makes it evenly dispersed in the rubber composite, thereby improving the dynamic mechanical properties of the rubber composite (Dong et al., 2017).