Lubricant additives

An additive is a chemical substance added to a lubricant base stock that imparts a new or desirable property not naturally occurring in the base stock. An additive may also reinforce a desirable property that the base stock already possesses. A formulated fluid consists of a base stock and a performance package.
By Joseph L. Foszcz, Senior Editor, Plant Engineering Magazine May 13, 2002
Key concepts
Additives improve the lubricating ability of base oil stocks.
Additives must be compatible with each other.
Additive packages should be put together by experts.
Sections:
Types of additives
Don’t do it yourself
Sidebars:
Chemically active additives


An additive is a chemical substance added to a lubricant base stock that imparts a new or desirable property not naturally occurring in the base stock. An additive may also reinforce a desirable property that the base stock already possesses.

A formulated fluid consists of a base stock and a performance package. The performance package can contain a number of additives. The quality and quantity of the additives depends on the quality of the base stock and the use proposed for the finished lubricant.

Lubricant additives are categorized as chemically active and chemically inert. Active additives have the capacity to interact with metals to form protective films and with polar oxidation and degradation products to make them harmless.

Chemically inert additives improve the physical properties that are critical to the effective performance of a lubricant.

Base oils may be petroleum, synthetic, or biological in origin. Petroleum-derived base oils currently account for about 97% of total lubricant production.

The base stock and additives work together and must be carefully selected and balanced to allow the finished product to do its intended job, which includes protecting moving parts from wear, removing heat and dirt, preventing corrosion and rust, and improving energy efficiency.

Types of additives

Additives are a major tool in adjusting lubricants for specific applications, enabling lubrication systems to be fine-tuned for maximum operating performance. Additives ideally should be oil soluble, water insoluble, and compatible with equipment materials.

  • Dispersants and detergents

    • Dispersants are used to suspend oil-insoluble, resinous oxidation products and particulate contaminants. They minimize sludge formation, particulate-related wear, viscosity increase, and oxygen-related deposit formation.

      Detergents perform functions similar to those of dispersants. Additionally, they neutralize acidic and oxidation products, controlling rust and corrosion. Detergents chemically interact with the chemicals that eventually form deposits, rendering them harmless.

    • Antiwear and extreme-pressure agents

      • Fig. 1. Surfaces separated by a lubricant film don’t wear.

        Under normal conditions of speed and load, a lubricant film separates two metal surfaces (Fig. 1). An increase in load or a decrease in speed reduces the oil film, allowing metal-to-metal contact and raising the temperature of the contact zone. The lubricant then loses viscosity, which decreases its film-forming ability.

        Under these conditions, the lubricant film changes from hydrodynamic to boundary or thin-film. Antiwear additives and extreme-pressure (EP) agents help minimize surface damage on parts in contact.

        Both antiwear and EP additives function by thermal decomposition to form products that react with a metal surface to form a solid, protective layer. This solid, metal alloy film fills surface asperities, reducing friction and preventing welding and surface wear (Fig. 2). EP additives require higher activation temperatures and loading conditions than antiwear additives.

        Fig. 2. EP additives provide an alloy film to prevent surface wear.

        Since most antiwear and EP agents contain sulfur, chlorine, phosphorus, boron, or combinations of these chemicals, many are corrosive to metals. Lubricants are formulated to optimize the balance between EP protection, antiwear protection, and corrosiveness.

      • Oxidation inhibitors

        • Equipment operating at high temperatures may require an oxidation-inhibiting additive. Lubricants undergo a complex series of oxidation reactions resulting in increased viscosity, formation of acidic contaminants that attack metal surfaces, and deposition of sludge and varnish.

          The severity of the oxidation process increases dramatically with a rise in temperature. For every 18-deg F rise, the oxidation rate approximately doubles.

        • Rust and corrosion inhibitors

          • Corrosion is largely due to the attack of acids created by oil oxidation. To combat this effect, inhibitors are used that react chemically with nonferrous components to form a corrosion-resistant protective film. There are also corrosion-resistant inhibitors that chemically neutralize acid contaminants as they appear in lubricants.

            Another aspect of corrosion is the rusting of metal (Fig. 3). Rusting is the chemical attack of water on iron surfaces. Two types of inhibitors are used to combat this effect. One type forms a protective barrier film on the metal to keep water out. The other type emulsifies the water into the oil, removing it from metal surfaces.

            Fig. 3. Corrosion and rust create particles that can cause wear and blocked passages.

          • Emulsifiers and demulsifiers

            • Emulsifiers are chemical compounds that reduce the surface tension of water, facilitating the thorough mixing of oil and water. This enables the two immiscible fluids to form an intimate mixture known as an emulsion.

              Emulsions have to possess a number of desirable properties: they should be stable over long periods of time, possess good lubricating abilities, resist attacking seals and metals, and be easy to demulsify for disposal.

              In the presence of water, certain lubricants have an increased tendency to form emulsions due to the presence of chemical additives, such as demulsifiers, that act as surfactants. Demulsifiers are added to enhance water separation and suppress foam formation.

              Demulsifiers concentrate at the water-oil interface and create low-viscosity zones, promoting droplet coalescence and gravity-driven phase separation.

            • Pour point depressants

              • Pour point depressants enable mineral oils to function efficiently at low temperatures. The depressant can lower the pour point by as much as 100 deg F. These additives are commonly used in applications that require the use of mineral oil below 32 F.

              • Foam inhibitors

                • Almost every lubricant application involves some kind of agitation, which encourages foam formation through air entrainment (Fig. 4). Excessive foaming results in ineffective lubrication and promotes oxidative degradation of the lubricant.

                  Fig. 4. Foam reduces lubricity and increases fluid degradation.

                  The viscosity and surface tension of the lubricant determine the stability of the foam. The presence of surface-active materials such as dispersants and detergents increases the foaming tendency of lubricants.

                  Foam inhibitors retard foam formation by altering the surface tension of the oil and by facilitating the separation of air bubbles from the oil phase.

                • Viscosity index improvers

                  • As with most fluids, viscosity can be expected to change with temperature changes. Generally, viscosity is high at low temperatures and becomes lower as the temperature increases. Viscosity index improvers are added to lubricants when the system to be lubricated operates over a large temperature range. The additive lessens the viscosity decrease at higher temperatures.

                    Don’t do it yourself

                    The success of the lubricating fluid’s performance is related to a critical balance of additives and additive types used. Some of the chemistries of additives can react or interact with each other. Some additives are not soluble in certain base oils and come out of solution, rendering them ineffective. Additive combinations in lubricants are the result of years of research and development by qualified experts. Altering or reformulating a lubricant is strongly discouraged.

                    Additive properties

                    Additive type Purpose Functions
                    Antiwear and EP agent Reduce friction and wear and prevent scoring and seizure Chemical reaction with metal surfaces to form a film with lower shear strength than the metal, preventing metal-to-metal contact
                    Corrosion & rust inhibitor Prevent corrosion and rusting of metal parts in contact with the lubricant Preferential adsorption of the polar constituent on metal surfaces to provide a protective film or neutralize corrosive acids
                    Detergent Keep surfaces free of deposits Chemical reaction with sludge and varnish precursors to neutralize them and keep them soluble
                    Dispersant Keep insoluble contaminants dispersed in the lubricant Contaminants are bonded by polar attraction to the dispersant molecules, preventing them from agglomerating and keeping them in suspension
                    Pour point depressant Enable lubricant to flow at low temperatures Modify wax crystal formation to reduce interlocking
                    Viscosity modifier Reduce the rate of viscosity change with temperature Polymers expand with increasing temperature to counteract oil thinning
                    Antifoamant Prevent lubricant from forming a persistent foam Reduces surface tension to speed collapse of foam
                    Antioxidant free Retard oxidative decomposition Decompose peroxides and terminate radical reactions
                    Metal deactivator Reduce catalytic effect of metals on oxidation rate Form inactive film on metal surfaces by joining with metallic ions

                    Acknowledgements
                    Plant Engineering extends its appreciation to Dow Corning Corp. and Tower Oil & Technology Co. for their assistance in the preparation of this article.
                    Author Information
                    Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@cahners.com

                    Chemically active additives

                    Dispersants

                    Detergents

                    Antiwear agents

                    Extreme-pressure agents

                    Oxidation inhibitors

                    Rust inhibitors

                    Corrosion inhibitors

                    Chemically inert additives

                    Emulsifiers

                    Demulsifiers

                    Pour point depressants

                    Foam inhibitors

                    Viscosity index improvers