How Much Do You Know About the Mechanisms of Action of Common Lubricating Oil Additives? (Part 2)

4. Viscosity Index Improvers

Viscosity index improvers, also known as thickeners or viscosity modifiers, are second only to detergents and dispersants in production volume. They are oil-soluble, chain-like polymers with molecular weights ranging from tens of thousands to millions.

When dissolved in lubricating oil, viscosity index improvers exist in filamentous form at low temperatures, having little effect on the oil's viscosity. As the lubricating oil temperature rises, the filaments stretch, increasing their effective volume and thus increasing the flow resistance, leading to a relatively significant increase in viscosity.

Because viscosity index improvers exhibit different forms and have different effects on viscosity at different temperatures, they can increase viscosity and improve viscosity-temperature properties. Therefore, viscosity index improvers are mainly used to increase the viscosity index of lubricating oil, improve viscosity-temperature properties, and increase viscosity. They can be used to formulate thickened engine oils, giving the formulated oil excellent viscosity-temperature properties, good low-temperature starting performance, low oil consumption, and a certain degree of anti-wear effect.

Viscosity index improvers are widely used in internal combustion engine oils, primarily in the production of multi-grade gasoline and diesel engine oils. They are also used in hydraulic oils and gear oils. Commonly used viscosity index improvers include: polyisobutylene, polymethyl methacrylate, ethylene/propylene copolymers, styrene-diene copolymers, and polyethylene n-butyl ether.

5. Pour Point Depressants

When the temperature of an oil drops to a certain level, it loses its fluidity and solidifies. The main function of pour point depressants is to lower the pour point of the oil and ensure its fluidity at low temperatures. Oils contain waxes. At low temperatures, high-melting-point paraffinic hydrocarbons often precipitate as needle-like or plate-like crystals, which interconnect to form a three-dimensional network structure, creating a crystalline skeleton that adsorbs and surrounds the low-melting-point oil, much like a water-absorbing sponge, causing the entire oil to lose its fluidity. Pour point depressants have both adsorption and eutectic effects. While they cannot prevent the precipitation of wax crystals, they can alter the structure of the wax.

Pour point depressants adsorb onto the surface of wax crystals or form eutectics with them, altering the shape and size of the wax crystals and preventing them from agglomerating into a three-dimensional network structure, thus maintaining the fluidity of the oil at low temperatures. Pour point depressants are widely used in various lubricating oils; typical examples are alkyl naphthalenes, polymethyl methacrylates, and polyalphaolefins.

6. Rust Inhibitors

The function of rust inhibitors is to form a strong adsorption film on the metal surface to inhibit the contact of oxygen and water, especially water, with the metal surface, preventing corrosion. As petroleum additives, rust inhibitors must have sufficient adsorption to metals and solubility in oil; therefore, rust inhibitors are composed of strong polar groups and appropriate lipophilic groups. Currently, the most widely used and effective rust inhibitors include: sulfonates (calcium sulfonate, sodium sulfonate, and barium sulfonate), carboxylic acids and their salts (dodecenylsuccinic acid, zinc naphthenate, N-oleoylsarcosine octadecylamine salt), organophosphates, imidazoline salts, ester-type rust inhibitors (lanolin and lanolin soap, styrene-60 or 80, oxidized petroleum grease), heterocyclic compounds (benzotriazole), and organic amines.

Water-soluble rust inhibitors mainly include: sodium nitrite, potassium dichromate, trisodium phosphate, diammonium hydrogen phosphate, sodium benzoate, and triethanolamine. Rust inhibitors are mainly used in industrial lubricating oils, metalworking cooling fluids, and metal protective oils.

7. Demulsifiers

Oil products can be contaminated by water during use, such as from leaking machinery or the need to spray large amounts of cooling water to cool processed parts. These situations introduce water into the oil, requiring the oil to have a certain degree of water separation and to prevent emulsification into a W/O (water/oil) emulsion. Because lubricating oils, after emulsification or with poor demulsibility, lose fluidity (W/O emulsions can multiply oil viscosity) and lubricity, they can also cause metal corrosion and wear. Industrial gear oils, turbine oils, and hydraulic oils (such as those containing zinc salts) are all susceptible to water contamination, therefore these oils have high requirements for anti-emulsification performance.

The causes of poor water separation or emulsification in lubricating oils are multifaceted.

1) High-viscosity oils contain some polar components;

2) Industrial lubricating oils contain various additives, especially detergents, dispersants, rust inhibitors, and extreme pressure anti-wear agents. Most of these additives are surfactants, which reduce the oil's demulsibility after addition;

3) During use, the oil is oxidized to form easily emulsifiable compounds such as carboxylic acids, further worsening its demulsibility.

While increasing the refining depth of the base oil and selecting suitable additives are primary considerations, adding demulsifiers is the main way to improve the demulsibility of lubricating oils. Adding demulsifiers to oils alters the oil/water interfacial tension, thereby improving the oil's demulsibility. This is because demulsifiers eliminate barriers to droplet bonding (i.e., remove the protective film on the outside of the droplets), making them easier to combine. Furthermore, demulsifiers can induce phase inversion in emulsions, changing the w/o (whole/oil) type to an o/w (oil/water) type, thus separating water. Commonly used demulsifiers include polyoxypropylene derivatives.

8. Antifoaming Agents

Even after refining, lubricating oil base oils may retain small amounts of polar substances. With the use of various additives to meet the high-performance requirements of various mechanical equipment, foaming can occur in circulating lubrication systems. This not only affects the pumping speed of the lubricating oil but also damages the oil film strength and stability, causing unnecessary wear or preventing normal machine operation. Phenomena such as oil starvation, vapor lock, and sintering will then occur.

Antifoaming agents inhibit foam formation to prevent the formation of stable foam. They adsorb onto the foam film, forming an unstable film and thus destroying the foam. A commonly used antifoaming agent is methyl silicone oil antifoaming agent. It is insoluble in oil and is highly dispersed in the oil using methods such as colloid milling. Its dosage is generally 1-100 ppm. There is also a non-silicone antifoaming agent, which is a polyacrylate-type polymer ester. Compared with silicone oil, it effectively improves the air release properties of the oil.

9. Composite Additives

As the quality grade of oils increases, functional additives are gradually shifting from single-agent additives to composite additives. The performance of composite additives depends not only on improving the quality of individual additives but also on determining the synergistic effect of additives through the study of additive compounding principles to obtain composite additives with excellent overall performance. Using composite additives can simplify formula screening, reduce the cost of lubricant production, and stabilize oil production quality. Currently, composite additives play an increasingly important role in lubricants.