## 1 Introduction  
Natural rubber (NR) with a cis-1,4-polyisoprene structure has high tensile strength, elasticity, and dynamics, and it is widely used for various purposes, including the manufacture of solid tires [1, 2]. Solid tires in the manufacturing process are inseparable from natural rubber, synthetic rubber, activators, coactivators, fillers, softeners, antioxidants, accelerators, vulcanization agents, and other additives. Zinc oxide (ZnO) and stearic acid (SA) are reactive substances that easily diffuse into the rubber matrix. These two materials are widely used in the rubber industry and have an important role in the rubber vulcanization process [3]. The annual global production of ZnO is more than 25 million tons, and more than 50% of it is used for activators in the rubber industry [4].  
In addition, accelerators in rubber vulcanization are widely used from elements of thiazole, sulfenamide, thiuram, dithiocarbamate, and guanidine [4]. This material plays an important role in the vulcanization process as it forms cross-links in the rubber matrix. Cross-links are formed in the rubber matrix due to the presence of elemental sulfur as a vulcanizing agent [5, 6]. Vulcanization is a chemical reaction process that forms rubber products [7]. The vulcanizing agent serves to link polymer chains through cross-linking reactions.  
ZnO and SA in rubber compounds are used in various ratios, according to the rubber product produced [8]. According to [PERSON] et al. [9], using ZnO with a ratio of 2 phr can increase the tensile strength and hardness of vulcanized rubber before and after aging. [PERSON] et al. [10] reported that using ZnO 3 phr can improve the mechanical properties of a rubber vulcanizate. [PERSON] et al. [11] used ZnO 3 phr and stearin 2 phr to study NR rubber-based magnetic composites. [PERSON] and [PERSON]. [12] used ZnO 5 and stearic acid 2 in an abrasion resistance study of two levels of epoxidized NR (ENR 25 and ENR 50). [PERSON] and [PERSON] [13] used ZnO 5 phr and SA 3 phr in their study on making rubber compounds using filler from rice husk ash. Both ZnO and SA at various ratios have been widely used to manufacture solid tires. [PERSON] and [PERSON] [14] used ZnO 4.85 phr and stearic acid 2.15 phr to manufacture solid tires for forklifts. [PERSON] and [PERSON] [15] have studied the effect of ZnO on tire rubber compounds' physical and mechanical properties.  
The use of zinc stearate (ZS) as an activator and coactivator in various rubber industries and by researchers has not been widely studied. This study used zinc stearate [Zn(C\({}_{18}\)H\({}_{35}\)O)\({}_{2}\)] for the development of rubber composites into solid tires. ZnO as an activator and SA as a coactivator in the vulcanization process the accelerator's performance to form cross-links [16,17]. The advantages of ZS in the manufacture of rubber compounds are accelerating curing time and scorch time, increasing the density of vulcanizate cross-links, and improving the mechanical properties of the vulcanizate. In addition, the use of ZS for the manufacture of rubber compounds can minimize the excess ratio of ZnO and stearic acid [18]. Cross-links density was defined and proposed by [PERSON] in the 1950s based on measurements of swelling equilibrium [19]. The use of ZS in this study aims to accelerate the curing time (t\({}_{90}\)), score time (ts\({}_{2}\)), and delta torque (\(\Delta\)s) in the manufacture of a rubber vulcanizate. In addition, using ZS improves the additives' distribution into the rubber vulcanization matrix.