Analysis of Common Lubricant Additives
Common functional additives for internal combustion engine oil include detergent dispersants, antioxidants, extreme pressure anti-wear agents, metal deactivators, and rust inhibitors. The following is a brief introduction to various additives.
1. Metal detergents Detergents are important additives for internal combustion engine oils and are the most widely used additives. Their main function is to neutralize the acidic substances produced by the combustion and decay of oil products and prevent the production of paint film carbon deposition on the metal surface. They mainly have solubilization, peptization, acid neutralization, and cleaning. According to different organic functional groups, they can be divided into alkyl benzene sulfonates, alkyl salicylates, sulfurized alkylphenol salts, cyclopentaneates, sulfurized phosphates, and other carboxylates. According to different alkaline components, they can be divided into barium salts, calcium salts, magnesium salts, sodium salts, etc., which are made into low alkalinity, medium alkalinity, and high alkalinity, respectively. Among them, high-alkalinity calcium salts are the most widely used.
2. Ashless dispersants: The main functions of dispersants in lubricating oils are dispersion and solubilization. The dispersing effect means that the oil-soluble groups provided by the dispersant can effectively inhibit the aggregation of soot and oxides so that these particles are effectively dispersed in the oil; the solubilizing effect means that the dispersant can directly react with the carbonyl and hydroxyl groups The that The generate the sludge, thereby achieving the dissolution effect. The main dispersants are polyisobutylene succinimide boronized ashless succinate ashless phosphate benzylamine polyisobutyl succinimide is the most used one.
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3. Antioxidants The oxidation of lubricating oil is an important reason for the deterioration of oil quality. Antioxidants can slow down the process of oxidation and deterioration and extend the service life of lubricating oil.
Antioxidants are divided into primary antioxidants and secondary antioxidants. Primary antioxidants are also called chain reaction stoppers. They mainly use amines and hindered phenol compounds to react with peroxy free radicals (ROO·) quickly and then react with alkyl free radicals (RO·) to prevent (cut off) oxidation chain reactions.
Secondary antioxidants are also called peroxide decomposers. They mainly use thioethers, phosphates, sulfur phosphates and other compounds to react with peroxides (ROOH) to produce non-free radicals and non-active substances to prevent oxidation. The most widely used phenolic antioxidant is 2,6-di-tert-butyl-p-cresol (T501). Representative compounds of amine antioxidants include N, N-di-sec-butyl-p-phenylenediamine, N-phenyl-N-sec-butyl-p-phenylenediamine and other metal alkylthiophosphoric acid compounds (such as Cu, Zn, Mo, Sb, etc.) have certain antioxidant, anti-wear, anti-corrosion and extreme pressure effects. In particular, zinc dialkyl dithiophosphate (ZDDP) has excellent antioxidant, anti-wear and anti-corrosion properties and can be widely used in lubricants, especially engine oils.
4. Extreme pressure anti-wear agent Extreme pressure anti-wear agents can reduce the friction coefficient and reduce wear. First, explain the concepts of "anti-wear agent" and "extreme pressure agent." Anti-wear agent: Under medium load and speed conditions, the friction surface will increase in temperature due to a large amount of heat release, which will cause the oily agent adsorbed on the friction surface to desorb and lose its anti-friction and anti-wear effect. Under such conditions, it is necessary to use surface-active substances that can react with the new metal surface at a higher temperature to form a chemical adsorption film to prevent the friction surface from bonding. Such substances are anti-wear agents. Extreme pressure agents: Under low-speed and high-load or high-speed impact friction conditions, the friction surface is prone to sintering, and anti-wear agents are powerless, but extreme pressure agents can prevent sintering.
According to different mechanisms of action, extreme pressure anti-wear agents can be roughly divided into two categories: active and inactive additives. The molecular structure of active additives contains active elements such as sulfur, phosphorus, and nitrogen, which can react chemically with the metal surface to form a protective film. Inactive additives are additives that form a protective film on the friction surface by themselves or their decomposition products, such as boron compounds, silicon compounds, aluminum compounds, etc. According to the molecular structure of additives, they can be divided into chlorine series, sulfur series, phosphorus series, nitrogen series, borates, molybdenum series, rare earth compounds, nanomaterials, etc.
Chlorine-based chlorine-containing anti-wear agents mainly include chlorinated paraffin and chlorinated biphenyl. This type of additive reacts with metal on the friction surface to form low-melting-point metal salt films such as FeCl3 and FeCl2, which are easy to shear and thus play a lubricating role. Due to the easy hydrolysis of chlorides to generate HCl to corrode metal surfaces and the harmfulness of chlorine to the environment, the use of chlorine-based additives is increasingly restricted.
Sulfur-based sulfur-containing additives mainly include sulfurized animal and vegetable oils, sulfided hydrocarbons, thioesters, polysulfides, etc. Sulfur-containing compounds will decompose at a certain temperature, and the generated active sulfur will react with iron to form a protective film. Sulfurized isobutylene is the most important product of sulfur-based extreme-pressure anti-wear agents.
Phosphorus-based phosphorus-containing additives are mainly used as anti-wear agents. The extreme pressure of phosphorus-containing agents is less than that of sulfur-containing additives, but phosphorus has a good anti-wear effect on steel friction pairs. Nitrogen-based nitrogen-containing compounds were originally used as corrosion inhibitors, dispersants and antioxidants. Subsequent studies have found that many nitrogen-containing compounds have anti-wear and load-bearing capacity. Because they do not contain phosphorus, sulfur and metals, they have less impact on the environment and have become one of the hot spots in the current additive development work. Borates Borate additives are characterized by excellent load-bearing capacity and good anti-wear properties. Inorganic borates are highly efficient multifunctional lubricant additives with excellent extreme pressure anti-wear properties, thermal oxidation stability, anti-rust and anti-corrosion properties, and sealing adaptability. They have been used in industrial gear oils, two-stroke engine oils, and vehicle gear oils. However, inorganic borates are insoluble in oil and easily soluble in water, which greatly limits their application. In addition to having good anti-wear effects, organic borate extreme pressure anti-wear agents also have good anti-corrosion, anti-oxidation, and oil solubility and have good development prospects.
Molybdenum-based molybdenum compound lubricating materials occupy an important position among many lubricating materials due to their excellent tribological properties. Molybdenum disulfide (MoS2), oil-soluble dialkyl dithiocarbamate molybdenum sulfide (MoDTC), and dialkyl dithiophosphate molybdenum sulfide (MoDTP) have been widely used in aviation, aerospace, navigation, machinery and other fields. The working principle of molybdenum-based lubricating additives is that sulfur-containing organic metal complexes react with a metal matrix with the help of sulfur atoms to form extreme pressure chemical lubricating film and partially decompose into MoS2 solid lubricating film. MoS2 solid lubricating film fills the concave and convex friction surface and relieves friction resistance.
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Rare earth compounds have been a hot topic in tribology research in recent years, and rare earth halides, rare earth oxides and rare earth metals have been studied more. Most researchers believe that the reason why rare earth compounds have excellent anti-wear properties is that rare earth metals will form chemical reaction films on the friction surface in the form of simple substances or oxides, thereby playing an anti-wear and friction-reducing role; on the other hand, rare earth metals will "diffusion" into the friction pair subsurface and improve the grain boundary structure of the friction pair surface, thereby greatly improving the wear resistance and corrosion resistance of the material.
Nanomaterials have also been a hot topic in recent years. Nano anti-wear additives have excellent anti-compression and anti-wear properties and can significantly improve the lubrication performance of lubricating oil under harsh working conditions. However, nanoparticles are easy to agglomerate and difficult to disperse evenly in lubricating oil, which is also a problem that nano-lubricating materials must solve.
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