Lubricant Antioxidants: Key Elements for Ensuring Machinery Operation
Hazards of Lubricant Oxidation
During the daily use and long - term storage of machinery, most lubricants come into contact with air, which inevitably leads to their interaction with oxygen and subsequent oxidation. Once a lubricant oxidizes, its quality deteriorates, and its working efficiency is significantly reduced. The oxidative deterioration of the hydrocarbon components in lubricants is a complex process, influenced by a combination of factors. On one hand, the type of hydrocarbon components and their inherent antioxidant capabilities play a key role. On the other hand, external working conditions such as temperature, the degree of air contact, metals, and moisture cannot be underestimated. Among these, temperature has the most significant impact on lubricant oxidation. Under normal oxidation conditions, for every 10°C increase in temperature, the oxidation rate approximately doubles.
In the field of modern engines, especially in aero - turbine engines, in order to improve the engine's thermal efficiency, increase the thrust - to - weight ratio, and reduce fuel consumption, the gas temperature before the turbine is constantly rising. This makes the working conditions of the lubricants increasingly harsh. In such a high - temperature environment, not only are higher requirements placed on the heat resistance of the mechanical components' materials, but also a huge challenge is presented to the heat resistance of the lubricants.
How Do Antioxidant Additives Combat Oxidation?
Given the hazards posed by lubricant oxidation, mitigating oxidation and reducing its impact have become crucial tasks for ensuring lubricant performance. From the perspective of lubricant performance additives, this can be achieved through three aspects: capturing free radicals, decomposing peroxides, and removing oxidation products.
(I) Antioxidant and Anti - Gum Agents (Free Radical Scavengers)
The main function of antioxidant and anti - gum agents is to react with free radicals, rendering them inactive and thus effectively preventing the occurrence of oxidation reactions. The main types of such additives include phenolic, amine - based, phenol - amine, and borate compounds. They can promptly capture the free radicals generated during the oxidation of lubricants, interrupting the oxidative chain reaction and thereby extending the service life of the lubricants.
(II) Antioxidant and Corrosion Inhibitors (Peroxide Decomposers)
Antioxidant and corrosion inhibitors serve to decompose peroxides, reducing their content in the lubricant and thus delaying the development of oxidative branching. Most of these additives not only have antioxidant properties but also possess the function of preventing metal corrosion. Common antioxidant and corrosion inhibitors mainly include dialkyl dithiophosphates (MDDP) and dialkyl dithiocarbamates (MDDC). They can decompose peroxides into stable products, reducing the promoting effect of peroxides on lubricant oxidation while protecting metal components from corrosion.
(III) Detergent - Dispersants
When internal combustion engine oil is in operation, its oxidation deposits are mainly divided into three types: carbon deposits, gum films, and sludge. Carbon deposits are the oxidation products of lubricants and incompletely burned fuels in the combustion chamber, which can affect spark plug ignition and normal combustion. Gum films are the oxidation products of the lubricant film at the piston ring area, which may cause the piston rings to stick, lose elasticity, and reduce heat dissipation and sealing effects. Sludge is formed by the aggregation of the oxidation products of the lubricant in the crankcase, dust, and moisture, which can block the oil passages and exacerbate engine wear.
Detergent - dispersants can effectively prevent the formation of carbon deposits, gum films, and sludge in engines. Their functions are mainly achieved through four aspects: detergency, dispersion, solubilization, and neutralization. The detergency function is to clean the already - formed carbon deposits and gum films from the metal surface, keeping the mechanical components clean. The dispersion function is to adsorb and disperse solid particles such as gum films and carbon deposits in the oil, enabling them to flow with the oil and be removed by the filter, preventing their deposition on the metal surface and at the bottom of the oil sump. The solubilization function is to increase the solubility of the lubricant in solid oxidation products, enhance the dispersion ability, and simultaneously dissolve intermediate oxides to prevent their further condensation and oxidation, reducing the formation of gum films and carbon deposits. The neutralization function is to neutralize the acidic components in the oxidation products, preventing their further oxidation and reducing the corrosive effect of acidic components on metals. Commonly used detergent - dispersants include sulfonates, alkylphenates, alkylsalicylates, sulfur - phosphorylated polyisobutylene barium, and polyisobutylene succinimides.
Types of Antioxidants
(I) Phenolic Antioxidants
1.Monophenolic Type: Monophenolic antioxidants have good antioxidant properties and can effectively extend the oxidation induction period. However, their applicable temperature is relatively low, and they perform best below 100°C. At high temperatures, they may instead promote oxidation. 2,6 - Di - tert - butyl - p - cresol (BHT) is one of the most widely used monophenolic antioxidants. It is mainly applied in industrial lubricants such as turbine oils, transformer oils, hydraulic oils, and machine tool oils, and is also extensively used in fuel oils.
2.Bisphenolic Type: To meet the application requirements at higher temperatures, bisphenolic antioxidants have emerged. A representative bisphenolic antioxidant is 4,4' - methylenebis(2,6 - di - tert - butylphenol), whose operating temperature can reach 150°C and is suitable for lubricants of piston - type aero - engines, compressor oils, and other oil products.
3.Phenolic Ester Type: Phenolic ester - type antioxidants generally refer to compounds containing ester groups at the para - position of the hydroxyl group, aiming to improve the oil - solubility and thermal stability of the product. Usually, β - (3’,5’ - di - tert - butyl - 4 - hydroxyphenyl) propionic acid methyl ester (abbreviated as 3’,5’ - methyl ester) is used as the parent body, and it is obtained by transesterification with alcohol. Phenolic ester - type antioxidants belong to high - temperature antioxidants and have many advantages. For example, they overcome the volatility defect of traditional phenolic antioxidants, improve high - temperature performance, and can control high - temperature sludge and deposits while having good medium - and low - temperature performance. They have excellent stability in mineral oils and non - traditional base oils, are easy to handle at low temperatures without crystallization, have low volatility, and when combined with alkyl diphenylamines, molybdenum - containing and sulfur - containing antioxidants, they are particularly effective in controlling oxidation and high - temperature deposition. A representative phenolic ester - type antioxidant is β - (3’,5’ - di - tert - butyl - 4 - hydroxyphenyl) propionic acid isooctyl ester.
4.Phenol - Amine Type: Phenol - amine - type antioxidants are obtained by introducing amino groups into phenolic molecules to improve their antioxidant properties. They are anti - aging agents for natural synthetic latex and antioxidants for polyolefins, polyesters, lubricants, mineral oils, and cellulose plastics. This product has the characteristics of non - pollution, non - discoloration, and good dispersibility. A representative phenol - amine - type antioxidant is 2,6 - di - tert - butyl - α - dimethylamino - p - cresol.
(II) Amine - Based Antioxidants
Amine - based antioxidants have a higher operating temperature and better antioxidant durability than phenolic antioxidants, and are remarkable in extending the induction period and inhibiting the late - stage oxidation of oils. In terms of controlling the viscosity increase of oils, amine - based antioxidants perform better than phenolic antioxidants. When phenolic antioxidants are used in combination with amine - based antioxidants, they are more effective in controlling piston deposits than using only amine - based antioxidants. However, amine - based antioxidants also have some drawbacks. For example, they are relatively expensive, lose their effectiveness in the presence of acidic substances, have relatively high toxicity, easily discolor the oil, and have a potential risk of forming precipitates. These factors have limited their application to a certain extent. Nevertheless, due to their excellent high - temperature performance, their application in some specific fields exceeds that of phenolic antioxidants. Research has found that diphenylamine and α - naphthylamine remain stable at temperatures above 200°C and are expected to become future high - temperature antioxidants.
1.p - Di - isooctyldiphenylamine: This is a multi - purpose liquid amine - based antioxidant and a general - purpose antioxidant for turbine oils, hydraulic oils, guideway oils, compressor oils, and other industrial lubricants. It is also an excellent antioxidant for greases that does not cause product color darkening and can be used to formulate ash - free crankcase oils for automobiles and aero - engines. It has a synergistic effect when combined with phenolic antioxidants.
2. Phenyl-α- naphthylamine: It has outstanding high - temperature antioxidant properties and is often used in combination with phenolic antioxidants in turbine oils and industrial gear oils.
(III) Peroxide Decomposers
Peroxide decomposers mainly include sulfur - containing compounds, sulfur - phosphorus compounds, and sulfur - nitrogen compounds. Among them, sulfur - phosphorus compounds are more widely used, such as ZDDP. A representative sulfur - nitrogen compound peroxide decomposer is ash - free dialkyl dithiocarbamate. At low concentrations, it functions as a hydroperoxide - decomposing antioxidant, and at high concentrations, it acts as an extreme - pressure agent.
Synergistic Enhancement of Antioxidants
When two or more additives are used in combination, they may produce synergistic (enhancing) or antagonistic effects. When using antioxidants, people usually hope to obtain a synergistic effect through combination.
(I) Synergistic Effect between Phenolic and Amine - Based Antioxidants
Research shows that there is a synergistic effect when hindered phenols (HP) and alkyl diphenylamines (ADPA) are combined. This is because in the initial stage of the reaction, the reaction rate of aromatic amines with hydrocarbon peroxyl radicals is higher than that of hindered phenols, and hindered phenols can provide hydrogen atoms for the regeneration of amines from amine radicals, thus achieving a synergistic antioxidant effect.
(II) Synergistic Effect between Free Radical Scavenging Antioxidants and Peroxide Decomposers
When free radical scavenging antioxidants and peroxide decomposers are used in combination, they can also exert a synergistic effect. Free radical scavenging antioxidants can promptly capture free radicals and interrupt the oxidative chain reaction, while peroxide decomposers can decompose peroxides, reducing their promoting effect on the oxidation reaction. The two cooperate with each other to jointly improve the antioxidant performance of the lubricant.
In conclusion, the oxidation problem of lubricants seriously affects their performance and the normal operation of equipment. Antioxidant additives, as a key means of combating oxidation, can effectively improve the antioxidant performance of lubricants, extend their service life, and ensure the efficient and stable operation of mechanical equipment through different types of additives and their rational combination. In future research and applications, further in - depth exploration of the performance optimization and synergistic mechanism of antioxidants will contribute to the development of higher - quality and more efficient lubricant products to meet the ever - evolving industrial demands.
The Price of Antioxidants
The price of Antioxidants depending on factors such as brand, specification, composition, and sales channels. If you are interested in Antioxidants, please feel free to contact us.
Supplier of Antioxidants
UNPChemicals is a professional supplier of high-quality and effective Antioxidants. We offer several remarkable products, namely 2,6-Di-tert-butyl-p-cresol (T501) OSAIL T501,Liquid phenyl-α-naphthylamine OSAIL APAN,Octyl phenyl-a-naphthylamine OSAIL L060,etc.
Professional Lubricant Additive Manufacturer
UNPChemicals, aka Luoyang Pacific United Petrochemical Co., Ltd., focuses on the application and development of special lubricating grease additives such as MODTC, MODTP, molybdenum amide, thiadiazole metal deactivators, and phosphate esters. With nearly 30 products in seven series, including extreme pressure anti-wear additives and special grease additives, it is a global manufacturer of special lubricating grease additives and a national high-tech enterprise with great influence and leading role in the industry. If you are looking for Lubricant Additive or technical information, feel free to contact UNPChemicals.