# Evaluation of Adhesion and Hysteresis Friction of Rubber-Pavement System
## 7 Test Results and Discussion  
### Adhesion Friction  
The average contact angle between rubber samples and the test probe liquids was recorded using the Sessile drop device. Each measurement represents the average contact angle measured from right and left. The results for the measured contact angles in degrees are presented in Table 4. Based on the contact angle measurements, the surface energy components were calculated for each rubber material using Equation (7), which is discussed in detail by [PERSON] and [PERSON] [7]. \[\mathrm{W_{LS}}\ =\ \gamma_{\mathrm{L}}\ (1\ +\ \mathrm{cos}\ \theta)\ =\ 2\sqrt{\gamma_{\mathrm{s}}^{\mathrm{LW}}\gamma_{\mathrm{V}}^{ \mathrm{LW}}}+2\sqrt{\gamma_{\mathrm{s}}^{+}\gamma_{\mathrm{V}}^{-}}+2\sqrt{ \gamma_{\mathrm{s}}^{-}\gamma_{\mathrm{V}}^{+}} \tag{7}\]  
The surface free energy components are summarized in Table 5. The surface free energy components of the aggregates were measured using a Universal Sorption Device (USD) from a previous study [20]. The surface free energy components for the gabbro and limestone aggregates are shown in Table 6. It was found that gabbro rock has the higher total surface free energy compared to limestone. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Probe Liquid** & \multicolumn{2}{c}{**Water**} & \multicolumn{2}{c}{**Glycerol**} & \multicolumn{2}{c}{**Ethylene Glycol**} & \multicolumn{2}{c}{**Formamide**} & \multicolumn{2}{c}{**Diidomethane**} \\ \hline \multirow{2}{*}{**Rubber Type**} & **Contact** & **Std.** & **Contact** & **Std.** & **Contact** & **Std.** & **Contact** & **Std.** & **Contact** & **Std.** \\  & **Angle** & **Dev** & **Angle** & **Dev** & **Angle** & **Dev** & **Angle** & **Dev** & **Angle** & **Dev** \\ \hline
**Pure gum** & 89.47 & 0.69 & 120.18 & 0.86 & 87.14 & 0.74 & 83.03 & 1.21 & 67.43 & 0.76 \\
**SBR** & 100.98 & 0.86 & 101.28 & 1.47 & 87.16 & 1.05 & 84.19 & 0.66 & 71.74 & 0.55 \\
**Nitrile** & 89.04 & 0.74 & 94.77 & 0.82 & 84.32 & 0.89 & 90.35 & 0.99 & 63.03 & 1.04 \\
**EPDM** & 108.36 & 0.74 & 112.45 & 0.67 & 89.61 & 0.75 & 93.29 & 0.87 & 72.22 & 13.12 \\
**Neoprene** & 124.58 & 0.89 & 112.02 & 0.74 & 90.23 & 0.95 & 87.54 & 1.75 & 69.63 & 1.64 \\
**Butyl** & 111.32 & 0.96 & 117.02 & 1.21 & 92.49 & 0.71 & 107.16 & 0.93 & 64.43 & 0.98 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average contact angle between rubbers and probe liquids using Sessile drop device (degrees). Figure 7: Testing rubber specimen in the dynamic mechanical analyzer (DMA) device inside a temperature-control chamber. The adhesive bond energy between different rubber and aggregate samples was calculated from their surface free energy components according to Equation (3). Figure 8 shows the calculated adhesive bond energies between the rubber and aggregates. The SBR and Pure Gum rubber materials were found to provide the highest adhesion with test substrates made with gabbro and limestone. The adhesive bond energy between limestone and SBR rubber was 79.91 ergs/cm\({}^{2}\). The Neoprene was found to provide the lowest adhesion with HMA substrates compared to other rubber materials. Also, the limestone was found to provide higher adhesion with rubber materials compared to gabbro in most cases. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Rubber**} & \multicolumn{3}{c}{**Surface Energy Components (ergs/cm\({}^{2}\))**} & \multicolumn{2}{c}{**Standard Deviation (ergs/cm\({}^{2}\))**} \\ \cline{2-7}  & \(\gamma^{\text{LW}}\) & \(\gamma^{+}\) & \(\gamma^{-}\) & \(\gamma^{\text{Total}}\) & \(\gamma^{\text{LW}}\) & \(\gamma^{+}\) & \(\gamma^{-}\) \\ \hline
**Pure gum** & 21.21 & 0.00 & 2.26 & 21.21 & 0.44 & 0.00 & 0.42 \\
**SBR** & 22.11 & 0.00 & 2.24 & 22.11 & 0.46 & 0.00 & 0.40 \\
**Nitrile** & 17.08 & 0.00 & 10.33 & 17.08 & 0.54 & 0.00 & 0.67 \\
**EPDM** & 16.77 & 0.00 & 0.35 & 16.77 & 1.53 & 0.00 & 0.17 \\
**Neoprene** & 15.30 & 0.00 & 0.00 & 15.30 & 0.69 & 0.00 & 0.00 \\
**Butyl** & 18.79 & 0.00 & 0.00 & 18.79 & 0.51 & 0.00 & 0.00 \\ \hline \hline \end{tabular}
\end{table}