# Optimizing Viscoelastic Properties of Rubber Compounds for Ballistic Applications
## 3 Results
### Impact Behavior of Rubber-Coated Plates  
In Section 3.2, it is shown that induced frequencies during ballistic impact are located in the area of maximized damping for the butyl rubber compound used, as the material undergoes the glass transition in this frequency range. To provide proof to the claim that this phenomenon positively effects ballistic protection, ballistic experiments were conducted with three different rubber formulations with different glass transition temperatures to cover three cases: an ideal glass transition temperature (in agreement with the results from Section 3.2), a glass transition temperature that is higher than the ideal one, and one that is lower than the ideal one. If an optimized glass transition temperature and thus the maximized damping in this area has a positive effect on ballistic protection, then the rubber compound with the ideal \(T_{g}\) should show the best results.  
As the influence of fillers and crosslinking system on the glass transition temperature is limited, it has to be varied by choice of different polymers. Butyl rubber (IIR), styrene-butadiene rubber (SBR), and a butadiene rubber (BR)/natural rubber (NR) blend (ratio of 9:1) were used. The elastomers and their respective glass transition temperatures are listed in Table 4. In attempt to reduce other possible influence factors on the results, e.g., hardness, all three formulations were prepared with different carbon black contents to ensure that all three rubber compounds possess the same hardness (~50 \({}^{\circ}\)Sha)  
In the first experimental series, the impact behavior of the rubber-coated aluminum plates was observed with a high-speed camera. The images of the impact experiments are shown in Figure 8.  
The impact responses of the materials with different glass transition temperatures vary strongly. BR/NR (Figure 8a) shows a ductile behavior whereas the rubber bulges away from the projectile. SBR (Figure 8c) responds in a brittle manner and the rubber splinters away. This behavior is expected  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Elastomer** & \(T_{g}\) I\({}^{\circ}\)Cl \\ \hline SBR & \(-19\) \({}^{\circ}\)C \\ IIR & \(-42\) \({}^{\circ}\)C \\ BR/NR (9:1) & \(-100\) \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 4: Rubber compounds used for the interlayer in the armored laminates tested.  
Figure 8: High-speed photographs of the impact behavior of (**a**) BR/NR (9:1), (**b**) IIR, (**c**) SBR. Photographs were taken after 0, 32, 64, 128 and 404 \(\upmu\)s. A schematic colorized overlay in the first photograph (**a**) highlights the rubber front-layer and aluminum back-layer.  
since the glass transition temperature of SBR is too high, meaning the impact frequency is higher than the glass transition frequency, and thus the material undergoes the transition while the glass transition temperature of BR/NR is too low, leading to the impact not triggering the glass transition.  
For the IIR specimen (Figure 8b), the material shows a mix of both responses. The rubber splintered away like in the case of SBR, but bulging can also be observed. This can be interpreted as the glass transition being in the correct magnitude. As the mean molecular mass of elastomers and polymers, in general, has a broad distribution, its properties also show a larger variety. Some of the polymer chains already underwent the phase transition from ductile to brittle, others still behave in a ductile manner.  
The results of the DMA experiments in Section 3.2, stating that impact frequencies are in the range of the glass transition of IIR, can be validated by these impact behavior experiments. By using an ultra-high-speed camera, it is clearly possible to differentiate between IIR and other rubbers with non-optimized glass transition temperatures.