1. Introduction
Vulcanization is the process of forming a three-dimensional network structure using a linear polymer via chemical reactions involving sulfur [
1]. The formation of a network structure is crucial for obtaining rubber vulcanizates with elastomeric properties. The various reagents associated with the sulfur vulcanization of polydienes, such as styrene-butadiene rubber (SBR), include vulcanization agents, accelerators, activators, retarders, and prevulcanization inhibitors. Vulcanization agents include elemental sulfur and/or an organic sulfur donor, such as tetramethylthiuram disulfide (TMTD). The most important classes of accelerators are those based on sulfenamides, benzothiazoles, guanadines, or dithiocarbamic acid (
Table 1).
Rubber vulcanization using sulfur without accelerators takes several hours and is not commercially viable. The application of accelerators makes the process significantly faster, enabling curing within a period of several minutes. Sometimes known as secondary accelerators, such activators include metal oxides (usually zinc oxide (ZnO)), fatty acids (usually stearic acid), and nitrogen-containing bases. Retarders and prevulcanization inhibitors may also be added so as to prolong the processing times and avoid premature vulcanization (scorch). A typical basic recipe for accelerated sulfur vulcanization includes sulfur and/or a sulfur donor (0.5–4 phr), an accelerator or a mixture of accelerators, ZnO (2–10 phr), and stearic acid (1–4 phr). The type and the amount of the sulfur donor and the accelerator are the major variables. Vulcanization systems are classified as conventional (CV), semi-efficient (semi-EV), or efficient (EV), depending on the level of sulfur and the ratio of accelerator to sulfur (
A/
S), as shown in
Table 2.
Rubber vulcanizates show various levels of crosslinking efficiency and develop different crosslink structures, depending on the curing system used (
Table 3).
The energy, length, and stiffness of the crosslinks together determine the mechanical and thermal properties of rubber vulcanizates [
3]. ZnO acts as an activator of sulfur vulcanization, forming zinc-based complexes during the first steps of the reaction. These zinc-based complexes have a major influence on both the kinetics of the reaction and the nature of the crosslink product [
4]. However, its low affinity towards rubber requires the use of large amounts (3–5 phr) of ZnO in order to achieve a good distribution in the matrix. Various possible ways of reducing the amount of ZnO needed in the process of rubber compounding have been proposed [
5]. Different zinc complexes have been tested as alternatives to the conventional ZnO and fatty acid activator system [
6]. Of particular interest is nano zinc oxide (n-ZnO). Its nanoscale particle distribution could potentially increase the accessibility of zinc ions to curing agents. Cure characteristics suggest that the use of n-ZnO can reduce the consumption of zinc by a factor of 10 [
7].
Many studies have focused on the synthesis of n-ZnO. However, there has been little research into how the morphology of n-ZnO particles might influence vulcanization. Przybyszewska and Zaborski [
8] studied the effect of the size (in terms of surface area) and shape (spheres, whiskers, or snowflakes) of ZnO particles on the crosslink density and mechanical behavior of carboxylated nitrile rubber. The rubber vulcanizates prepared with n-ZnO showed a higher crosslink density and improved mechanical properties compared with the vulcanizates obtained using conventional ZnO. However, no specific trends were found correlating the particle size and specific surface area of the n-ZnO particles with the crosslink efficiency. Panampilly et al. [
9] studied the effect of nanosized ZnO on NR vulcanization. Even with a very small amount of n-ZnO (0.5 phr), they observed a reduction in the optimum cure time (t90) and a higher cure rate index (CRI). These findings were attributed to an increase in rubber–filler interactions, as confirmed by the higher bound rubber (BdR) content [
10]. Likewise, Roy et al. [
11] found that the addition of 0.5 phr of n-ZnO led to a decrease in t90 and a higher cure rate index (CRI), compared with the addition of 5 phr of conventional ZnO. A thermo gravimetric analysis (TGA) revealed that the rubber vulcanizates containing n-ZnO had a better thermal stability because of a decrease in the thermal motion of the polymer chains within the network structure. Cui et al. [
12] analyzed the efficiency of using n-ZnO for the vulcanization of SBR in two different structural forms, but with approximately the same particle size and specific surface area—“table-like” (T-ZnO) and “rod-like” nanosized ZnO (R-ZnO). The curing rate was faster in the vulcanization system with R-ZnO and the vulcanizates also showed both higher crosslink densities and better mechanical properties compared with those prepared with T-ZnO. The differences in the behavior of the n-ZnO structures were attributed to the larger amounts of free Zn
2+ ions that formed in the system with R-ZnO.
In summary, according to the literature, reducing the size of ZnO particles can be an effective strategy for enhancing the vulcanization efficiency. Nanometric dimensions favor a better dispersion and distribution of the activator in the polymer matrix, because of the higher surface area of n-ZnO. This promotes the interaction of the zinc centers with the curing agents and rubber chains. The aim of the present research was to increase our understanding of the effect of the shape of n-ZnO particles on the curing activity. We also examined the influence of the particle shape on the crosslink structures of SBR vulcanizates cured with sulfur systems of varying efficiencies.
4. Conclusions
Nano-powder particles of zinc oxide (n-ZnO) behave differently in sulfur crosslinking systems, depending on their morphology and SSA. Their performance also depends on the activity of the curing system used. The course and kinetics of vulcanization, as well as the crosslink density, structure, and mechanical properties of the vulcanizates, seem to depend more on the morphology of the n-ZnO particles than on their SSA. In this study, nanoparticles with a cauliflower morphology exhibited a higher squalene wettability than rod-like nanoparticles, irrespective of their SSA. However, the curing systems (especially EV) containing cauliflower-shaped n-ZnO required a higher activation energy. Both the activation energy and the speed of vulcanization (dα/dt) were found to depend on the morphology of the n-ZnO particles. In the CV system, vulcanization was faster when the n-ZnO particles with a rod-like morphology were used, compared with particles with a cauliflower morphology. The situation was the opposite in the case of the EV systems, in which the cauliflower-shaped particles sped up the vulcanization process. The reduction in activation energy was most visible in the case of EV systems, but again only when n-ZnO particles with a rod-like morphology were applied. In the EV system, the application of cauliflower structures (E and G), despite resulting in a higher activation energy, accelerated the process compared with the reference micro-ZnO powder. The crosslink density of the vulcanizates did not correlate with either the morphology or the SSA of the applied n-ZnO particles. However, a slight increase in the crosslink density was associated with cauliflower-shaped n-ZnO in the EV system. The morphology of the n-ZnO particles had practically no influence on the crosslink structure and related mechanical properties of the CV vulcanizates. However, rod-like nanoactivators used in the EV system resulted in a greater reduction in the content of polysulfidic crosslinks in comparison to the rubber vulcanizates containing cauliflower-shaped n-ZnO. This effect was the most pronounced in vulcanizates containing n-ZnO with a hybrid morphology (D). These vulcanizates showed both the highest crosslink density and the greatest reduction in polysulfidic crosslinks.
Based on the results of this study, the morphology of n-ZnO activator particles appears to influence the parameters and activation energy for vulcanization, as well as the crosslink density, structure, and mechanical properties of the rubber vulcanizates. This influence is independent of the SSA of the n-ZnO particles, but depends on the efficiency of the curing system.