*3.1. Curing Characteristics*

The curing characteristics of the mixed compounds are given in Table 2. It is evident that the minimum torque *ML* significantly decreased with an increase in the primary carbon black particle size. This is caused by the decreasing amount of the immobilized rubber chains on the decreasing carbon black surface, as well as the structure. The low aggregate structure and surface area of the N 990 type leads to weak interaction with rubber chains. Moreover, the thermal production process leads to a higher purity of the carbon black surface, with low content of active groups. This fact can result in a lower stiffening effect. However, in the case of the torque *ML*, interactions between rubber chains and carbon black are mainly caused by physical forces, because the property is in an uncured state [25–28].


**Table 2.** Curing characteristics of the studied compounds.

The difference between the maximum (*MH*) and the minimum (*ML*) torques is marked as Δ*M*, which is a parameter demonstrating the degree of chemical crosslinking. The same amount and type of curing system was observed in the compounds; thus, the degree of Δ*M* is supposed to be comparable for all compounds. In reality, the Δ*M* is significantly different. Evidently, the reaction between the rubber and the curing system is one of the factors affecting the crosslinking process [28,29].

The other factor affecting the crosslinking process is the chemical reaction of the rubber with functional groups on the carbon black surface, which differs depending on the carbon black grade. It was found that these functional groups could have either positive or negative effects on the curing characteristics, depending on carbon black type [29]. Although the N 330 type has a higher surface area compared to the N 550 type, its curing level is lower due to the presence of cure-retarding groups [30]. This phenomenon causes various *t*90 values for the compounds in this study.
