In this paper, phenol, amine, benzotriazole and Formate antioxidants commonly used in China were selected to test the thermal stability and oxidation resistance of samples by thermogravimetry, PDSC, rotating oxygen bomb, etc., providing technical reference for the evaluation and selection of antioxidants. Hydrocarbons in lubricating oil are prone to free radical Chain reaction under the conditions of light, heating, metal catalysis, etc., which oxidizes to produce acid, aldehyde and ketone substances, and further aggravates, forming insoluble impurities such as sludge and carbon deposit in the oil or on the metal surface, affecting the normal operation of equipment. Therefore, it is necessary to add antioxidant in lubricating oil, combine free radicals to generate stable substances, decompose peroxides generated by Chain reaction, reduce metal activity, and hinder the oxidation reaction, so as to maintain the stability of oil performance. There are currently many types of antioxidants available on the market, and different manufacturers have the same brand corresponding products. This article selected multiple commonly used antioxidants in China for performance evaluation, providing technical reference for the evaluation and selection of antioxidants. Test materials Antioxidant selection According to the brands and types of antioxidants commonly used in lubricating oil products, phenolic antioxidants A, B, C, D, amine antioxidants E, F, G, H, benzotriazole type antioxidant I and Formate type antioxidant J were selected, and samples from different suppliers were selected for some antioxidant types, totaling 21 samples. Base Oil Due to the large proportion of automotive internal combustion engine oil in the total lubricating oil, as well as the low viscosity trend of engine oil in the market due to fuel economy and national standard requirements, API Class II base oil is mostly used for blending, and II-6+is the most widely used. Therefore, II-6+base oil is chosen as the dilution oil.
Evaluation of antioxidant performance
Thermogravimetric analysis
Thermogravimetric analysis can test the thermal stability of different antioxidants, and can also roughly judge whether there are differences in the same type of antioxidant components provided by different manufacturers, and also provide a reference for the temperature limits of different antioxidants suitable for working conditions [2]. The temperature range of Thermogravimetric analysis is set as 20~550 ℃, the heating rate is 20 ℃/min, and nitrogen protection is provided.
The decomposition temperature of most antioxidants of the same brand is basically similar. Amine type E is mainly composed of butyl/octyl substituted Diphenylamine. Due to the different synthesis processes of different manufacturers, butyl/octyl substituted Diphenylamine accounts for different proportions in the final additive products, resulting in significant differences in its thermal decomposition temperature; However, there are differences in the alkyl chain length of benzotriazole derivatives, leading to differences in decomposition temperature.
It is generally believed that phenolic antioxidants have a lower temperature range of use, while amine antioxidants have better high-temperature resistance. However, from the data in Table 1, it can be seen that except for phenolic type B, the decomposition temperatures of most phenolic antioxidants with a weight loss of 1%, 50%, and 90% are almost similar to those of amine type antioxidants. Especially when phenolic type A loses 1% weight, the decomposition temperature exceeds 300 ℃, which is significantly better than amine type antioxidants and shows good high-temperature resistance. The decomposition temperature of Formate salts at 1% weight loss is only second to that of phenolic A, and they also show good high temperature resistance. The weight loss of benzotriazole type I1 is 1%, and the decomposition temperature of 50% is lower than that of I2. This may be because there are many light components such as diluted oil, and the 90% decomposition temperature is significantly higher than that of I2, indicating that its main structure has good high-temperature resistance.
In summary, some brands of phenolic and amine type antioxidants have strong high-temperature resistance. In industrial production, the thermal stability of products from different manufacturers of the same brand may differ due to differences in the proportion of diluted oil and synthesis process.
Sensitivity to API Class II base oils
Mix different kinds of antioxidants with API II-6+base oil at different mass fractions, and send them to PDSC (pressure Differential scanning calorimetry) for programmed temperature rise and rotating oxygen bomb (RBOT, SH/T 0193) to test the oxidation resistance of single agent. Among them, the temperature range of PDSC is set at 80-380 ℃, with a heating rate of 10 ℃/min, an oxygen pressure of 1.5 MPa, and an oxygen flow rate of 100 mL/min to obtain the thermal oxidation temperature of the sample under pressure conditions.
From the analysis results of the rotating oxygen bomb and PDSC, it can be seen that the increase in dosage has little effect on the thermal oxidation temperature of benzotriazole type antioxidants, and has a certain impact on the other three types of antioxidants. However, it can significantly increase the rotating oxygen bomb duration of all antioxidants, which increases with the increase in dosage. The thermal oxidation temperature of phenol and amine antioxidants tested by PDSC is relatively close, which is about 210~240 ℃. The temperature of benzotriazole type is the lowest, and that of Formate type is about 200~215 ℃. The results of the rotating oxygen bomb showed that the amine type antioxidant had the longest duration and the benzotriazole type had the shortest duration, reaching the experimental endpoint in only about 1 hour; There are significant differences in the results of phenolic antioxidants; The test results of Formate type are basically consistent.
The antioxidant properties of similar antioxidants provided by different manufacturers are basically the same, with only A1 showing significantly poorer antioxidant properties compared to A2 and A3. When the dosage of D in the phenolic type and E, G, and H in the amine type reaches 0.5%, the duration of the rotating oxygen bomb increases to over 1000 minutes. Among them, when the dosage of phenolic type D is 0.1%, the duration of the rotating oxygen bomb and PDSC is already higher than that of all antioxidants. When the dosage of amine type H is 0.5%, the duration of rotating oxygen bomb and PDSC is higher than that of other antioxidants. Most data indicate that when the dosage of antioxidants is 0.1%~0.3%, the antioxidant performance of phenolic type is better than that of amine type. When the dosage is increased to 0.5%, the antioxidant performance of amine type is significantly improved, indicating that amine type antioxidants may exhibit better antioxidant performance at high dosage.
In addition, phenol type A1-A3, amine type E1-E3, benzotriazole type I1-I2 and Formate type J1-J3 were selected respectively, which were dissolved in II-6+at 0.3% of the added dose. The 100 mL sample oil prepared was placed in a centrifuge tube for one month storage stability test. By observing the appearance of the samples to be tested when they were still, it was found that the oil products of J1-J3 and II-6+mixed and stood for one month became turbid and produced micro precipitation, indicating that among the four types of antioxidants selected, It is possible that Formate antioxidant has the defect of poor storage stability.
Conclusion
The thermal decomposition temperatures of products of the same brand from different manufacturers collected are basically similar, but the structural differences of products of the same brand may lead to significant differences in their decomposition temperatures. Therefore, in actual use and single agent acceptance, it is recommended to judge the single agent structure based on the working conditions and product requirements, in addition to physical and chemical analysis, combined with thermogravimetry;
In II-6+base oil, increasing the dosage of antioxidants can improve its antioxidant performance, but the antioxidant properties of some similar antioxidants provided by different manufacturers show significant differences. To ensure the performance of lubricating oil products, it is recommended to increase single agent antioxidant performance testing when examining products from the same brand and different manufacturers to control product quality;
In the future, due to the increase in the dosage of antioxidants in higher standard passenger car gasoline and heavy-duty diesel engine oil products, the demand for antioxidants is expected to grow the fastest. However, further research is needed to improve the antioxidant resistance, high temperature resistance, and storage stability.
