Results and discussion
The DPPH% values of the samples were measured to assess the antioxidant capacity of the oils, with the results presented in Figure 2.
Both the pharmaceutical and technical grade control samples (with no antioxidant added) exhibited no DPPH radical scavenging capacity. In contrast, all the antioxidants tested significantly enhanced the DPPH scavenging activity of the oils compared to the controls. The difference in performance between the technical and pharmaceutical grades of oil was not statistically significant. Among the antioxidants, Irganox L06 demonstrated the most substantial increase in DPPH scavenging activity across all concentrations, reaching a peak of 83% at a concentration of 0.8%. BHT followed, achieving a 53% increase in DPPH activity at the same concentration. Five other antioxidants (Irganox L101, Irganox L109, Irganox L115, Naugard 445, and Irganox L57) displayed DPPH% values in the range of 30-40%. Irgafos 168 performed the worst, with an average increase of only 17%.
The resulting mixtures of heat tests were maintained at a temperature of 160°C for periods of 8 and 24 hours. To evaluate the effectiveness of these antioxidants, TAN and V/V0 analyses were conducted. The findings from these analyses are summarised in Table 4.
For the control samples, which had no antioxidants added, an increase in TAN was observed as a function of heat exposure in both grades of oil. Specifically, in technical grade oil, TAN increased by 0.016 after 8 hours and by 0.052 after 24 hours. Similarly, in pharmaceutical-grade oil, the increase was 0.021 after 8 hours and 0.052 after 24 hours. Viscosity measurements, however, remained relatively stable across the heat treatment duration for both types of oil.
In the samples with added antioxidants, the results showed that the average ΔTAN and V/V0 values after 8 hours of heat treatment were not significantly different from those obtained after 24 hours, regardless of the oil grade. This suggests that a longer heat exposure (greater than 16 hours) or higher temperatures might be necessary to induce a noticeable difference in oxidation during the treatment period.
When compared to the control samples, the addition of antioxidants effectively inhibited the increase in TAN. After 8 hours of heat treatment, 60 out of 63 samples exhibited lower ΔTAN values, and after 24 hours, all samples with antioxidants showed a reduced ΔTAN compared to the controls. The reduction in TAN due to antioxidant addition was statistically significant. Among the antioxidants, Irganox L101, Irganox L06, and BHT were the most effective, maintaining ΔTAN values below 0.001 post-heat treatment. Naugard 445 and Irgafos 168 showed slightly less protection, with ΔTAN values ranging from 0.004 to 0.007, while Irganox L115 and Irganox L109 were the least effective, with ΔTAN values exceeding 0.010.
In contrast, changes in V/V0 values were negligible and statistically insignificant across all samples with added antioxidants, compared to the control.
Table 4: Effect of additives to oxidation values after heat treatment
Finally, no significant differences were observed between the technical and pharmaceutical-grade oils concerning ΔTAN and V/V0 values. Overall, Irganox L06 emerged as the most effective antioxidant additive even at low concentrations, followed by BHT and Irganox L101.
Continued on page 16 LUBE MAGAZINE NO.184 DECEMBER 2024 15
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