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Antioxidants can be categorised according to their effects into primary phenolic antioxidants, primary aminic antioxidants, and secondary antioxidants. The selection of antioxidants for this study was based on a thorough review of the literature, with additional choices made based on specific properties. These antioxidants are detailed in Table 2.
indicator of the oxidation rate. Oxidation is a key factor that increases oil viscosity, which can diminish lubricant performance. In this study, the kinematic viscosity of the samples was measured both before (V0) and after (V) the oxidation tests. The ratio of these two values (V/V0) was calculated and reported as the change in viscosity. The viscosity measurements were conducted at 40 °C[8].
Table 2: Properties of the antioxidants used in the study *Suggested by the manufacturer
Experimental set up
For the experiments, 100 ml clear glass bottles were used. Each bottle was filled with 75 g of white oil. A control bottle was maintained for each type of oil, to which no antioxidant was added. The remaining bottles received the selected antioxidants in concentrations of 0.1%, 0.2%, 0.3%, 0.6%, and 0.8% by weight.
Heat testing was conducted in an oven maintained at a constant temperature of 160°C. The bottles were placed in the oven for durations of 8 and 24 hours.
Oxygen and heat testing was conducted using a custom- made apparatus. A piece of copper wire was added to each bottle to serve as a catalyst [7,8]. The bottles were then placed in the apparatus, which was set to either 120°C or 150°C, and dry air was continuously flushed through each bottle at a rate of 3 ml/minute. The treatment lasted for 8 hours, after which the antioxidant activities of each sample were evaluated.
Analytical measurements Total Acid Number(TAN) is a commonly utilised metric for assessing antioxidant activity. During extended oxidation, TAN tends to rise as carboxylic acids are formed. Conversely, when an oxidation inhibitor is employed, a reduction in TAN is typically observed [9,3].
To evaluate the antioxidant activity of the selected antioxidants, TAN values were measured both before and after the oxidation tests. The difference between these two measurements (ΔTAN) was then calculated. Changes in the viscosity of oils can also serve as an
14 LUBE MAGAZINE NO.184 DECEMBER 2024
The antioxidant activity was measured using the 2-diphenyl-1-picrylhydrazyl (DPPH) radical method, a standard approach for evaluating the free radical scavenging abilities of commercial antioxidants in white oils. The procedure involved mixing 5 ml of an isooctane solution containing 0.10 mM DPPH with 56 µL of the oil samples in 30 ml dark-coloured bottles. The prepared samples were then stored in the dark for 30 minutes, after which their absorbance was measured at 517 nm using a UV-VIS spectrophotometer (Optimiser) [10].
The percentage of DPPH radical scavenging activity was then calculated using the following formula(%): (Acontrol - Asample)/(Acontrol) x 100
Figure 2: DPPH% values of the oil samples
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