# _molecules_
## 5 Curing Behavior of High-Vinyl SSBR Versus Low-Vinyl SSBR  
To answer the question if high-vinyl SSBR follows a direct radical coupling mechanism and does not need any activators for this, two different SSBR types were investigated: the SSBR Sprintan SLR 3402 with a low and the SSBR Sprintan SLR 4601 with a high-vinyl content (Table 1).  
Each of these two types were tested in a very basic unfilled compound with 2.0 phr \(N\)-tert-butyl-2-benzothiazyl sulphenamide (TBBS) and 1.4 phr sulfur and compared to the same compounds, where additional 2.5 phr ZnO and 2.5 phr stearic acid (called in the following \"activators\") were added. Figure 9 shows their different curing behaviors. Additional information about the materials and methods used in this chapter are displayed in Appendix A.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **SSBR Sprintan SLR 3402** & **SSBR Sprintan SLR 4601** \\ \hline \hline Styrene content in \% & 15 & 21 \\ \hline \hline Vinyl content in \% & 30 & 63 \\ \hline \hline Functionalization & yes & yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Selected SSBR types.  
Figure 8: Two different mechanisms of sulfur coupling.  
Figure 9: Curing behaviors of a low-vinyl (**left**) and a high-vinyl (**right**) unfilled SSBR compound with and without activators.  
There are several remarkable observations, starting from the graph on the right-hand side: the high-vinyl SSBR shows the same final torque with and without activators. This already delivers the first hint that the above proposed direct radical mechanism from an opened S8 ring to a vinyl bond indeed takes place. [PERSON] and [PERSON] already considered this direct coupling as well; they proposed that \"Zinc-accelerator complexes are either not needed or are acting catalytically\" in an SSBR [14].  
Furthermore, the curing of high-vinyl SSBR starts earlier without activators and the scorch time is significantly lower. This behavior can also be explained by the assumed radical process: a direct sulfur coupling to vinyl via sulfur radical starts earlier than the \"traditional\" ionic process but has a lower reaction rate. Normally, radical processes can be assumed to be very fast and can even cause explosions [15]. This assumes a chain reaction. In the current case of the direct coupling between a sulfur radical and the vinyl group, many different sulfur radicals need to be formed all over the matrix. The radical reaction can be terminated in many different ways; no chain reaction is assumed to take place all over the compound. This also means that the supporting substance for this formation cannot be a catalyst but a substance which is well distributed in the compound, similar to sulfur, to initiate the radical formation spread all over the compound. The other option for such a trigger is the temperature.  
The presence of activators in the high-vinyl SSBR compound increases the scorch time. This leads to the conclusion that ZnO and stearic acid do not act as activators in the system but as retarders. They seem to interact with sulfur as well as with accelerators, capturing the sulfur radicals or avoiding even the formation of the radicals.  
On the left-hand side, the low-vinyl-containing SSBR cures were much slower in the absence of activators, reaching only 60% of the final torque of the compound after 60 min in the presence of activators. Here, the amount of vinyl groups is too low to enable an efficient direct coupling.  
The absolute value of the final torque of the low-vinyl SSBR with activators is higher in comparison to the high-vinyl SSBR. This is connected to the fact that the low-vinyl SSBR also contains a significant lower amount of styrene, leading to a higher degree of butadiene units in the SSBR and ultimately meaning a higher degree of crosslinkable sites. This higher degree of double bonds in the main chain seems to lead to a higher crosslink density indicated by the higher observed end torque.