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In air and heat experiment, The mixtures were subjected to aeration at a rate of 3 ml/min, with one set maintained at 120°C and another at 150°C for 8 hours. The results are summarised in Table 5.
of oxidation rather than heat alone under the tested conditions.
Compared to control samples, the addition of antioxidants effectively inhibited the increase in TAN. After 8 hours of treatment, all antioxidant-treated samples showed lower ΔTAN values at both 120°C and 150°C. This reduction in TAN was statistically significant compared to the controls. All antioxidants performed well (ΔTAN < 0.003), with Irgafos 168 and Irganox L06 showing slightly superior performance.
Similarly, the addition of antioxidants mitigated the increase in viscosity (V/V0) compared to control samples. The difference between controls and antioxidant-treated samples was statistically significant at both 120°C and 150°C for both oil grades. Among the antioxidants, Irganox L101 and Irganox L06 performed slightly better (V/V0 < 1.0055) than Irganox L109, Irganox L57, Irgafos 168 (1.0066 < V/ V0 < 1.0070), and BHT (V/V0 = 1.0083).
Table 5: Effect of additives to oxidation values after heat&oxygen treatment
For control samples (without antioxidants), TAN values increased with exposure to air and heat in both oil grades. Specifically, technical-grade oil exhibited a TAN increase of 0.058 at 120°C and 0.063 at 150°C, while pharmaceutical-grade oil showed increases of 0.048 at 120°C and 0.055 at 150°C.
Additionally, viscosity increased slightly (2.2%) in the technical-grade oil after treatment, whereas the increase in pharmaceutical-grade oil (0.5%) was not significant. These findings suggest that the combined effect of oxygen exposure (via aeration) and heat resulted in more pronounced oxidation compared to other conditions (such as light and heat), leading to observable degradation in the oil properties.
In the antioxidant-treated samples, the ΔTAN values did not significantly differ between the two temperature conditions (120°C vs. 150°C) for either grade of oil. However, significant differences were observed in V/V0 values. These findings, along with the results from the heat-only treatment (160°C), suggest that oxygen exposure was the primary driver
16 LUBE MAGAZINE NO.184 DECEMBER 2024
Lastly, while the ΔTAN values showed no statistically significant difference between technical and pharmaceutical-grade oils, the V/V0 values did. The technical-grade oils exhibited a more significant increase in viscosity compared to pharmaceutical- grade oils after treatment, both in control and antioxidant-treated samples.
Evaluation of the results To evaluate the effectiveness of the antioxidants tested, a comparative table was created using the results obtained from the study. Each antioxidant was then assigned a relative score based on its performance (refer to Table 6).
Table 6. Effect of additives to oxidation values after heat&oxygen treatment
Table 6: Effect of additives to oxidation values after heat&oxygen treatment (Scoring from best to worst: ++++, +++, ++, +, +/-, -)
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