This article selects commonly used phenolic, amine, benzotriazole, and formate type antioxidants in China, and tests the thermal stability and antioxidant activity of the samples using thermogravimetry, PDSC, and rotary oxygen bomb, providing technical reference for the evaluation and selection of antioxidants. The hydrocarbons in lubricating oil are prone to free radical chain reactions under conditions such as light, heating, and metal catalysis, which oxidize to form acid, aldehyde, and ketone substances, further exacerbating the formation of insoluble impurities such as sludge and carbon deposits inside the oil or on the metal surface, affecting the normal operation of the equipment. Therefore, it is necessary to add antioxidants to the lubricating oil, combine with free radicals to generate stable substances, decompose peroxides generated by chain reactions, reduce metal activity, hinder oxidation reactions, and maintain stable 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 For the commonly used antioxidant brands and types in lubricating oil products, phenolic antioxidants A, B, C, D, amine antioxidants E, F, G, H, benzotriazole type antioxidants I, and formate type antioxidants J were selected. Different suppliers were selected to provide samples for some antioxidant types, totally 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 determine whether there are differences in the same type of antioxidant components provided by different manufacturers. It also provides a reference for the temperature limit values suitable for different antioxidant conditions. The temperature range for thermogravimetric analysis is set to 20~550 ℃, with a heating rate of 20 ℃/min, and protected by nitrogen gas. It shows the thermal decomposition temperatures of different antioxidants at different stages of weight loss. The decomposition temperature of most antioxidants of the same brand is basically similar, among which the amine type E is mainly composed of butyl/octyl substituted diphenylamine. Due to the different synthesis processes of different manufacturers, the proportion of butyl/octyl substituted diphenylamine in the final additive product is different, 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 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 second only to phenolic type A, and they also exhibit 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 types of antioxidants with API II-6+base oil in different mass fractions, and submit them for PDSC (Pressure Differential Scanning Calorimetry) program heating and rotating oxygen bomb (RBOT, SH/T 0193) testing to determine the antioxidant activity of the 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 temperatures of phenolic and amine antioxidants tested by PDSC are relatively close, both around 210~240 ℃. The temperature of benzotriazole type is the lowest, while the temperature of formate type thermal oxidation is around 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 results of the formate ester test 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, phenolic type A1-A3, amine type E1-E3, benzotriazole type I1-I2, and formate type J1-J3 were selected, and dissolved in II-6+at an added dose of 0.3%. 100 mL of sample oil was prepared and placed in a centrifuge tube for a month of storage stability test. By observing the appearance of the sample to be tested, it was found that after mixing and standing for 1 month, the oil became turbid and micro precipitates were generated, indicating that among the four types of antioxidants selected, Perhaps formate type antioxidants have 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.
