*3.1.2 Analysis of oxidation stability*

High oxidation stability of TBHQ is because of the longer alkyl chains present in it [23]. Oxidation stability of the oil samples are estimated by measuring the change in viscosity at 40°C after hot oil oxidation test (HOOT) from the fresh sample without subjected to HOOT [24]. Change in dynamic viscosity of neat PO, PO oil samples blended with different weight percentages of TBHQ and SAE20W40 at 40°C are calculated and plotted as shown in **Figure 5**. From the analysis of different oil samples blended with different weight percentages of TBHQ , PO + 2.5% TBHQ showed the least change in viscosity. It is observed that for the weight percentage of TBHQ beyond 2.5%, it is difficult to dissolve in PO. The change in viscosity at 40°C of PO + 2.5% TBHQ is found to be very lesser than neat PO comparable with that of SAE20W40. Thus PO + 2.5% TBHQ have excellent oxidation stability.

**Figure 5.** *Change in dynamic viscosities of oil samples at 40°C after HOOT.*
