# Physical and Chemical Methods to Assess Performance of TPO-Modified Asphalt Binder
## 4 Discussion
### FTIR Testing Comparison  
After standardized DSR testing was performed, the FTIR results were gathered and compared. The FTIR spectra were evaluated based on the absorbance in different regions, as shown in Figure 2. The hardest to analyze was the fingerprint region because of the numerous overlapping peaks that were difficult to resolve. In this study, the fingerprint region did not change significantly with an increase in TPO percentage, resulting in little to no changes in the functional groups present. This could be due to a few reasons: (1) The TPO already contains similar functional groups to the asphalt binder. While there are some differences, there may not be sufficient TPO present to significantly alter the chemical compounds in the binder, (2) research has shown that TPO is sensitive to heat and that components can decompose even under mild conditions [10]. The reduced chemical changes as detected by FTIR analysis could be due to the TPO converting or decomposing during blending and aging, and (3) the TPO is only interacting with one of the colloidal phases, and the effects are too small to significantly be detected. Other research has shown that TPO could contain alkene groups at 3000-3100 cm\({}^{-1}\) or 1575-1675 cm\({}^{-1}\) in moderate fractions [6]. However, there does not seem to be any evidence of a noticeable fraction of alkene groups in the TPO used in this experiment. This could be a result of the loss of double bonds through chemical reactions or specific production conditions. There are no production quality standards for TPO, resulting in highly variable TPO products. Because of the complexity of the asphalt binder and TPO's structures, unknown interactions or reaction mechanisms could be taking place which could be addressed in future research projects. While the fingerprint region is chemically dense, other peaks are more distinguishable. Water vapor was detected in the 2% blend which could be the reason for the enhanced stiffness detected by DSR testing. The alkane peaks are the strongest peaks of each sample and were similar for every blend. TPO itself shows a strong peak around 1700 cm\({}^{-1}\). Therefore, it may be obvious that the TPO-modified blends show a slightly larger peak around that region compared to the original asphalt binder. This impact on the C=O peak complicates the comparison of the widely used carbonyl index. Because aging does not seem to affect the functional groups of the binder blends as seen in Figures 5, 9 and 12, a further analysis into the relative ratio of the carbonyl groups was a more useful way to analyze the impacts of TPO on short- and long-term aging. The carbonyl information in Figure 14 indicated that other than the 2% sample, long-term aging had the largest ratio of carbonyl groups compared with the other aging methods. This was expected for long-term aged samples since PAV aging occurs under increased air pressure which increases the exposure to oxygen and the formation of carbonyl groups. The unaged and short-term aged blends had varying results. The 0% TPO blend had about equal carbonyl indices for the unaged and short-term aged samples. However, as the percentage of TPO increased in the unaged samples, the carbonyl index also increased due to the amount already present in the TPO. The 2% blend had a much higher unaged carbonyl index, most likely due to water contamination from the TPO. The short-term aged carbonyl indices were more similar across the TPO blends. The 8% TPO blend had \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Unaged Grade** & **RTFOT Grade** & **High PG** & **Intermediate PG** \\  & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** \\ \hline
0\% TPO & 64 & 64 & 64 & 22 \\
2\% TPO & 64 & 64 & 64 & 22 \\
4\% TPO & 64 & 64 & 64 & 22 \\
8\% TPO & 64 & 58 & 58 & 22 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Performance grades of TPO-modified binder blends. the lowest short-term aged carbonyl index compared to its unaged value.