With the development of industrial technology, the high speed, high performance, high automation, high efficiency, and long service life required by modern equipment are no longer satisfied by simply using mineral oil lubricating materials in lubrication. Adding a small amount of other substances to lubricating materials can improve their performance and give them new characteristics. These substances are called additives for lubricating oil.
Adding different additives to oil products is the most economical and effective way to improve oil quality. Generally speaking, the variety and quality of lubricating oil often depend on the type and quality of additives. Therefore, developing the production and use of additives has become an important way to make rational and effective use of resources, improve equipment performance, and save energy.
Lubricating oil additives can be divided into detergent dispersant, antioxidant and anti-corrosion agent, extreme pressure and anti-wear agent, oiliness agent and friction modifier, antioxidant and metal deactivator, viscosity index improver, rust inhibitor, pour point depressant, anti foam agent and other groups according to their functions. The action mechanism of common lubricating oil additives is introduced below.
1、 Clean dispersant
Clean dispersants include two types: cleaning agents and dispersants. Mainly used for internal combustion engine oil (gasoline engine oil, diesel engine oil, railway diesel locomotive oil, two-stroke gasoline engine oil, and marine engine oil). Its main function is to keep the interior of the engine clean, so that the insoluble substances generated are suspended in a colloidal state, preventing further formation of carbon deposits, paint films or sludge. Specifically, its functions can be divided into four aspects: acid neutralization, solubilization, dispersion, and washing.
1) Acid neutralization effect: Clean dispersants generally have a certain alkalinity, some even high alkalinity. They can neutralize the organic and inorganic acids generated by the oxidation of lubricating oil, prevent further condensation, thus reducing the paint film, and also prevent the corrosion of engine components by these acidic substances.
2) Solubilization effect: Clean dispersants are surfactants that can solubilize solid or liquid substances that are originally insoluble in oil into the center of micelles composed of 5-20 surfactant molecules. During use, they solubilize oxygen-containing compounds containing hydroxyl, carbonyl, and carboxyl groups, nitro compounds, water, etc. into micelles to form colloids, prevent further oxidation and condensation, and reduce the formation and aggregation of harmful deposits on engine components.
3) Dispersing effect: It can adsorb solid small particles such as carbon deposits and paint films that have already been generated, making them a colloidal solution dispersed in oil, preventing these substances from further condensing into large particles and adhering to machinery, or depositing as sludge.
4) Washing function: It can wash off the paint film and carbon deposits that have been adsorbed on the surface of the components, disperse them in the oil, and keep the engine and metal surfaces clean.
The structure of a detergent dispersant is basically composed of three functional groups: lipophilic, polar, and hydrophilic. Due to the different structures, the performance of the detergent dispersant varies. Generally speaking, the detergent with ash additives has better cleanliness, while the dispersant without ash additives has outstanding dispersibility.
Typical representatives of detergent dispersants include sulfates, alkylphenols, salicylates, succinimides, succinic esters, and polymers. The first three are also known as grey cleaning dispersants, while the latter three are called ash free cleaning dispersants.
2、 Antioxidant
Antioxidants and antioxidants can inhibit oil oxidation and are mainly used in industrial lubricants, internal combustion engines, and process oils. Antioxidants can be divided into two types based on their principle of action: 1) chain reaction terminators; 2) Peroxide decomposition agent. The commonly used antioxidant for blocking phenolic and amine compounds belongs to chain reaction terminators, which can form stable products (ROOH or ROOA) with peroxide groups (ROO.), thereby preventing the oxidation reaction of hydrocarbon compounds in lubricating oil, such as 2,6-phenol, 4,4-methylbisphenol, α - naphthylamine, N, N-di-sec-butyl-p-phenylenediamine, etc.
Peroxide decomposers can decompose peroxides generated in the oxidation reaction of oil products, preventing the chain reaction from continuing and playing an antioxidant role; It can generate inorganic complexes during the thermal decomposition process, form a protective film on the metal surface, and play a role in corrosion resistance; Under extreme pressure conditions, a chemical reaction occurs on the metal surface to form a sulfurized film with load-bearing capacity, which plays an anti-wear role, making it a multifunctional additive. The main varieties of antioxidant and anti-corrosion agents include zinc dialkyldithiophosphate (ZDDP), alkyl zinc thiophosphate, butyl zinc thiophosphate, and their series products.
Phenolic and amine antioxidants are commonly used in transformer oil, industrial lubricants, turbine oil, and hydraulic oil. Zinc salts of dialkyldithiophosphate and other sulfur-containing, phosphorus containing, or organic selenium containing compounds are commonly used in industrial lubricants, internal combustion engine oils, and process oils. However, lubricating oil containing dithiophosphate is not suitable for use on the steel sleeve at the top of the connecting rod of internal combustion locomotives and lubricating engines with silver plated elbow pins. Dialkyldithiocarbamate can meet the requirements for machines with silver plated parts.
3、 Oil and extreme pressure anti-wear agents
1) Extreme pressure anti-wear agent refers to an additive that can form a high melting point chemical reaction film with the metal surface in a high-temperature and high-pressure boundary lubrication state, in order to prevent melting, biting, and scratching. Its function is that the products decomposed under high temperature friction can react with metals to generate compounds with lower shear stress and melting point than pure metals, thereby preventing contact surface biting and welding melting, effectively protecting the metal surface. Extreme pressure anti-wear agents are mainly used in lubricating oils with extreme pressure requirements, such as industrial gear oil, hydraulic oil, guide rail oil, cutting oil, etc., to improve the extreme pressure anti-wear performance of the oil.
Extreme pressure anti-wear agents are generally divided into organic sulfides, phosphides, chlorides, organic metal salts, and borate type extreme pressure anti-wear agents. The main varieties of extreme pressure anti-wear agents include chlorinated paraffin, acidic dibutyl phosphate, nitrogen-containing derivatives of thiophosphoric acid, triphenylphosphate, sulfurized isobutene, dibenzyl disulfide, lead cycloalkanoate, borate, etc.
2) Any additive that can increase the oil film strength, reduce the friction coefficient, improve wear resistance, and reduce friction and wear between moving parts in lubricating oil is called an oiling agent.
Oily agents are surfactants with polar groups on one end and oil soluble alkyl groups on the other end. Substances containing such polar groups have strong affinity for metal surfaces and can firmly adsorb onto them, forming a protective film similar to a buffer pad between metals, preventing direct contact between metal surfaces and reducing friction and wear.
Oily agents have high interfacial activity, and they produce physical or chemical adsorption on metal surfaces. Physical adsorption is reversible and works at low temperatures and low loads; Under high temperature and high load, the adsorbent will desorb and lose its effectiveness. Fatty acid based oil-based agents not only have physical adsorption, but also chemical adsorption, which forms metal soaps with metal surfaces at lower temperatures to improve wear resistance.
The commonly used oil-based agents include higher fatty acids (such as stearic acid, palmitic acid, oleic acid, lauric acid, palmitic acid, ricinoleic acid, etc.), esters of fatty acids (such as ethyl stearate, butyl oleate, etc.), fatty acid amines or amide compounds (such as stearylamine, N, N-di (polyethylene glycol) octadecylamine, stearamide, etc.), sulfurized whale oil, sulfurized cottonseed oil, dimer acids, benzotriazole fatty amine salts, and acidic phosphate esters. Oily agents are mainly used in industrial lubricants, hydraulic oils, guide rail oils, gear oils, etc.
4、 Viscosity index improver
Viscosity index improver, also known as thickening agent or viscosity agent, has a yield second only to detergent dispersants. Viscosity index improver is an oil soluble chain polymer with molecular weights ranging from tens of thousands to millions.
Viscosity index improvers dissolve in lubricating oil and exist in the form of coils at low temperatures, with little effect on the viscosity of the lubricating oil. As the temperature of the lubricating oil increases, the coils stretch, the effective volume increases, and the flow resistance of the lubricating oil increases, resulting in a relatively significant increase in the viscosity of the lubricating oil.
Due to the different forms and effects of viscosity index improvers at different temperatures, they can increase viscosity and improve viscosity temperature performance. Therefore, viscosity index improvers are mainly used to increase the viscosity index, improve viscosity temperature performance, and increase viscosity of lubricating oils. Viscosity index improver can be used to prepare thickened engine oil, making the prepared oil have excellent viscosity temperature performance, good low-temperature starting performance, low fuel consumption, and certain anti-wear effect.
Viscosity index improver is widely used in internal combustion engine oil, mainly for producing multi-stage gasoline and diesel engine oil, as well as hydraulic oil and gear oil. Common viscosity index improvers include polyisobutene, polymethacrylate, ethylene/propylene copolymers, styrene diene copolymers, and polyethylene n-butyl ether.
5、 Pour point depressant
After the temperature of the oil drops to a certain degree, it will lose its fluidity and solidify. The main function of a pour point depressant is to lower the freezing point of the oil and ensure that it can flow at low temperatures. Oil contains wax, and at low temperatures, high melting point paraffin hydrocarbons often precipitate as needle shaped or sheet-like crystals, which are interconnected to form a three-dimensional network structure, forming a crystalline skeleton that adsorbs and surrounds low melting point oil, especially like a sponge that absorbs water, causing the entire oil to lose fluidity. Pour point depressants have two functions: adsorption and eutectic. Although pour point depressants cannot prevent the precipitation of wax crystals, they can change the structure of wax.
The adsorption or eutectic formation of pour point depressants on the surface of wax crystals changes the shape and size of wax crystals, preventing the formation of a three-dimensional network structure by the bonding of wax grains, thereby maintaining the fluidity of oil products at low temperatures. Pour point depressants are widely used in various lubricating oils, with typical representatives being alkyl naphthalene, poly (methyl methacrylate), and poly (alpha olefin).
6、 Rust inhibitor
The function of rust inhibitor is to form a strong adsorption film on the metal surface to inhibit the contact of oxygen and water, especially water, on the metal surface, so that the metal does not rust. As a rust inhibitor for petroleum additives, it must have sufficient adsorption properties for metals and solubility for oil. Therefore, rust inhibitors are composed of strong polar groups and appropriate lipophilic groups. At present, the following types are widely used and have good effects: sulfonates (calcium sulfonate, sodium sulfonate, and barium sulfonate), carboxylic acids and their salts (dodecenyl succinic acid, zinc cycloalkanoate, N-oleoyl sarcosinate octadecylamine salt), organic phosphates, imidazoline salts, ester rust inhibitors (lanolin and lanolin soap, benzene-60 or 80, oxidized stone oil), heterocyclic compounds (benzotriazole), organic amines, etc.
Water soluble rust inhibitors mainly include sodium nitrite, potassium dichromate, trisodium phosphate, diammonium hydrogen phosphate, sodium benzoate, triethanolamine, etc. Rust inhibitors are mainly used in industrial lubricants, metal processing cooling lubricants, metal protective oils, etc.
7、 Anti emulsifier
Oil products can be contaminated by water during use, such as mechanical equipment leakage or the need to spray a large amount of cooling water to cool processed parts, which can introduce a certain amount of moisture into the oil. This requires the oil products to have a certain degree of water separation and not be emulsified into W/O (water/oil) emulsions by water. Because lubricating oil loses its fluidity (W/O emulsions can double the viscosity of the oil) and lubricity after emulsification or its poor anti emulsification properties, it can also cause metal corrosion and wear. Industrial gear oil, turbine oil, and hydraulic oil (such as oil containing zinc salts) are all susceptible to water pollution, so these oils have high requirements for anti emulsification performance.
The reasons for poor water separation or emulsification of lubricating oil are multifaceted.
1) High viscosity oils may contain some polar components;
2) Various additives are added to industrial lubricants, especially dispersants, rust inhibitors, and extreme pressure anti-wear agents. Most of these additives are surfactants and need to be added to reduce the oil's emulsification resistance;
3) During use, oil is oxidized to form easily emulsifiable compounds such as carboxylic acids, which reduces the oil's resistance to emulsification.
Deepening the refining depth of base oil and selecting suitable additives are certainly the first considerations, but adding emulsifiers is the main way to improve the emulsification resistance of lubricating oil. Adding emulsifiers to oil can change the interfacial tension between oil and water, in order to improve the emulsification resistance of the oil. Because the addition of anti emulsion can eliminate the obstacles to the binding of dispersed phase droplets (i.e. remove the protective film outside the droplets), making it easier for the droplets to bind together. In addition, emulsifiers can cause phase transition of emulsions, transforming W/O type into O/W type, achieving the purpose of water separation. Commonly used emulsifiers include derivatives of polyoxypropane type.
8、 Anti foam agent
After refining, there will still be a small amount of polar substances remaining in the base oil of lubricating oil. With the use of various additives to meet the high-performance requirements of various mechanical equipment, foaming will occur in the circulating lubrication system, which not only affects the pumping of lubricating oil, but also breaks the strength and stability of the oil film, causing unnecessary wear accidents or making the machine unable to operate normally. Phenomena such as oil cut-off, air resistance, sintering, etc. will continue to occur.
Antifoaming agent is used to inhibit the production of foam, so as to avoid the formation of stable foam. It can be adsorbed on the foam film to form unstable film, so as to achieve the purpose of destroying foam. The most commonly used anti foaming agent is methyl silicone oil anti foaming agent. It is insoluble in oil and is highly dispersed and distributed in oil through methods such as colloid mills. Its dosage is generally 1-100ppm. There is also a non silicone anti foaming agent, which belongs to the high molecular weight ester of polyacrylate type. Compared with silicone oil, it can effectively improve the air release of oil products.
9、 Composite additive
With the improvement of oil quality grades, functional additives are gradually shifting from single agents to composite agents. The performance of composite additives not only depends on the improvement of the quality of individual additives, but also needs to determine the essence of the synergistic effect of additives through the study of additive composite rules, in order to obtain the composite agent with the best comprehensive performance. The use of composite additives can reduce the difficulty of formula screening, lower the cost of lubricant production, and stabilize the quality of oil production. Nowadays, the position of composite additives in lubricating oil is becoming increasingly important.
