How Much Do You Know About the Mechanisms of Action of Common Lubricating Oil Additives? (Part 1)

With the development of industrial technology, the high speed, high performance, high automation, high efficiency, and long lifespan required by modern equipment can no longer be met by simply using mineral oil lubricants. Adding small amounts of other substances to lubricants can improve their performance and give them new properties. These substances are called lubricating oil additives.

Adding different additives to oils is an economical and effective way to improve oil quality. Generally speaking, the variety and quality of lubricating oils often depend on the variety and quality of additives. Therefore, developing the production and use of additives has become an important way to rationally and effectively utilize resources, improve equipment performance, and save energy.

Lubricating oil additives can be classified according to their function into detergents and dispersants, antioxidants and corrosion inhibitors, extreme pressure anti-wear agents, oiliness agents and friction modifiers, antioxidants and metal deactivators, viscosity index improvers, rust inhibitors, pour point depressants, and antifoaming agents. The following introduces the mechanisms of action of commonly used lubricating oil additives.

1. Detergents and Dispersants

Detergents and dispersants include detergents and dispersants. Primarily used in internal combustion engine oils (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 engine interior clean, keeping insoluble substances in a colloidal suspension to prevent further formation of carbon deposits, varnish, or sludge. Specifically, its function can be divided into four aspects: acid neutralization, solubilization, dispersion, and cleaning.

1) Acid Neutralization: Detergents and dispersants generally have a certain degree of alkalinity, some even highly alkaline. They can neutralize the organic and inorganic acids generated by the oxidation of lubricating oil, preventing further condensation and thus reducing varnish. They also prevent these acidic substances from corroding engine components.

2) Solubilizing effect: Detergents and dispersants are surfactants that solubilize solids or liquids that are normally insoluble in oil within micelle centers composed of 5-20 surfactant molecules. During use, they solubilize oxygen-containing compounds (hydroxyl, carbonyl, carboxyl groups), nitro compounds, and water into the micelles, forming colloids that prevent further oxidation and condensation, reducing the formation and accumulation of harmful deposits on engine parts.

3) Dispersing effect: They adsorb existing carbon deposits and varnish, dispersing them in the oil as a colloidal solution. This prevents these substances from further agglomerating into larger particles that adhere to engine parts or deposit as sludge.

4) Scrubbing effect: They wash away varnish and carbon deposits adsorbed on component surfaces, dispersing them in the oil and keeping the engine and metal surfaces clean.

The structure of detergent-dispersants is basically composed of three groups: lipophilic, polar, and hydrophilic. Due to differences in structure, the properties of detergent-dispersants vary. Generally speaking, ash-containing additives have better detergency, while ashless additives have superior dispersibility.

Typical examples of detergent-dispersants include sulfonates, alkylphenol salts, salicylates, succinimides, succinates, and polymers. The first three are also called ash-containing detergent-dispersants, and the latter three are called ashless detergent-dispersants.

2. Antioxidants

Antioxidants and antioxidant corrosion inhibitors can inhibit oil oxidation and are mainly used in industrial lubricants, internal combustion engines, and process oils.

Antioxidants can be divided into two types according to their mechanism of action: 1) chain reaction terminators; 2) peroxide decomposers. Commonly used phenolic and amine-type antioxidants are chain reaction terminators. They can react with peroxide groups (ROO.) to form stable products (ROOH or ROO), thus preventing the oxidation of hydrocarbon compounds in lubricating oils, such as 2,6-phenol, 4,4-methylenebisphenol, α-naphthylamine, and N,N-di-sec-butyl-p-phenylenediamine.

Peroxide decomposers can decompose peroxides generated during oil oxidation, preventing the chain reaction from continuing and thus providing antioxidant effects. They can also produce inorganic complexes during thermal decomposition, forming a protective film on metal surfaces for corrosion resistance. Under extreme pressure conditions, they react chemically on metal surfaces to form a load-bearing sulfide film, providing anti-wear effects. Therefore, they are multi-functional additives. The main types of antioxidants and corrosion inhibitors include zinc dialkyl dithiophosphate (ZDDP), zinc thiophosphoric acid alkyl salts, zinc thiophosphoric acid butyl octyl salts, and their related products.

Phenolic and amine-type antioxidants are widely used in transformer oils, industrial lubricating oils, turbine oils, and hydraulic oils. Dialkyl dithiophosphate zinc salts and other sulfur-, phosphorus-, or selenium-containing compounds are commonly used in handicraft lubricants, internal combustion engine oils, and process oils. However, lubricants containing dithiophosphates are not suitable for use on internal combustion locomotives with silver-plated elbow pins or on the connecting rod top steel bushings of engines. Dialkyl dithiocarbamates meet the requirements for use on machines with silver-plated parts.

3. Oiliness and Extreme Pressure Anti-wear Agents

1) Extreme pressure anti-wear agents are additives that, under high temperature and high pressure boundary lubrication conditions, can form a high-melting-point chemical reaction film with the metal surface to prevent fusion, seizing, and scratching. Their function is that the products of decomposition under high frictional temperatures react with the metal to generate compounds with lower shear stress and melting points than pure metals, thereby preventing contact surface seizing and welding, effectively protecting the metal surface. Extreme pressure anti-wear agents are mainly used in industrial gear oils, hydraulic oils, guide rail oils, cutting oils, and other lubricants with extreme pressure requirements to improve the extreme pressure anti-wear performance of the oil.

Extreme pressure anti-wear agents are generally classified into organosulfur compounds, phosphides, chlorides, organometallic salts, and borate-type extreme pressure anti-wear agents. The main types of extreme pressure anti-wear agents include: chlorinated paraffin, dibutyl phosphite, nitrogen-containing derivatives of thiophosphate, tricresyl phosphate, isobutylene sulfide, dibenzyl disulfide, lead naphthenate, and borates.

2) Any additive that can increase the oil film strength of lubricating oil, reduce the coefficient of friction, improve wear resistance, and reduce friction and wear between moving parts is called an oiliness agent.

An oiliness agent is a surfactant, with a polar group at one end of its molecule and an oil-soluble hydrocarbon group at the other. Substances containing this polar group have a strong affinity for metal surfaces; they can firmly and directionally adsorb onto the metal surface, forming a protective film similar to a buffer between the metals, preventing direct contact between the metal surfaces and reducing friction and wear.

Oiliness agents have high interfacial activity; they undergo physical or chemical adsorption on metal surfaces. Physical adsorption is reversible; it works at low temperatures and under low loads, but desorbents lose their effectiveness under high temperatures and loads. Fatty acid-based oiliness agents, in addition to physical adsorption, also exhibit chemical adsorption, forming metallic soaps with metal surfaces at lower temperatures, thus improving wear resistance.

Commonly used oiliness agents include higher fatty acids (such as stearic acid, palmitic acid, oleic acid, lauric acid, palmitic acid, ricinoleic acid, etc.), fatty acid esters (such as ethyl stearate, butyl oleate, etc.), fatty acid amines or amide compounds (such as stearamide, N,N-di(polyethylene glycol)octadecylamine, stearamide, etc.), sulfurized whale oil, sulfurized cottonseed oil, dimer acids, benzotriazole fatty amine salts, and acidic phosphate esters. Oiliness agents are mainly used in industrial lubricants, hydraulic oils, slideway oils, gear oils, etc.