**2.2. Rheological properties**

The rheological properties were determined through kinematic viscosity (ASTM D445) and viscosity index, VI (ASTMD2270). The kinematic viscosity was measured using a viscometer at 40 and 100°C. It was calculated from the ratio of dynamic viscosity over density at the same temperature. The correlation between viscosity and temperature was further associated with VI. The testing was repeated for three times and the average value was recorded.
