One of the key components that enhances the performance of metalworking fluids is extreme pressure additives. They are typically formulated into medium- to heavy-duty machining and forming fluids where a high degree of lubricity is required and are used in both oil-based and water-based fluids. The choice of extreme pressure additive technology matters for metalworking fluid formulators. The amount and type of extreme pressure additives are typically determined by the metalworking operation, the metallurgy and a wide range of other considerations.

In this article, we will explore the function of extreme pressure additives in metalworking fluids and discuss some of the machining operations in which they provide benefits. We will then discuss the different types of extreme pressure additives along with the benefits and drawbacks of each class of chemistries. Finally, we will provide considerations regarding selection of additives.

Extreme Pressure 101

Extreme pressure additives are a key technology in the metalworking fluid formulator’s toolbox. Whereas traditional boundary lubricants function by adsorbing to metal surfaces and providing a physical barrier between contacting metal surfaces, extreme pressure additives chemically react with metal surfaces to form a film. 

In metal cutting and metal forming operations, deformation due to metal-on-metal contact between the tooling and the workpiece generates very high localized temperatures. At these temperatures, extreme pressure additives decompose to form reactive species that chemically react with the metal surface. For example, sulfurized additives form metal sulfide layers, while chlorinated additives produce metal chlorides. The film created by the reaction acts as a protective barrier to minimize direct metal-to-metal contact. This helps reduce the coefficient of friction, which allows for easier removal or easier deformation.

Some of the key metalworking applications where extreme pressure additives are required include:

• Cutting. Processes such as turning, milling and drilling can see severe conditions that involve significant friction and heat. In these applications, extreme pressure additives help in reducing tool wear, improving surface finish and preventing unnecessary downtime due to premature tool failure.
• Drawing and stamping. In operations where metals are deformed, such as stamping and forging, extreme pressure additives are essential to prevent galling and scoring on both the metal workpiece and the dies.
• Grinding. The high-speed nature of grinding operations generates substantial heat. Extreme pressure additives can help in reducing this heat by reducing the coefficient of friction, thereby helping to reduce wheel wear and improve longevity.
• Extrusion. These processes involve the reduction of metal cross-sections under high pressure. Extreme pressure additives ensure smooth metal flow and reduce die wear.

By minimizing wear in these applications, extreme pressure additives significantly extend the life of cutting tools, saving operators on repair and replacement costs and minimizing downtime. The lubricity provided by extreme pressure additives further ensures a smooth finish on the machined parts, enabling the manufacturer to deliver high-quality products to their customers. Finally, lower wear rates mean that machines can operate at faster speeds, improving overall productivity and efficiency in operations. 

Evaluating Different Types of Extreme Pressure Additives

A variety of different chemistries are often deployed to deliver extreme pressure performance in metalworking fluids. The choice typically depends on the needs of the end-use application, so for formulators, it is worth knowing the details of each type. 

Chlorine. Chlorinated paraffins have historically been the most common class of extreme pressure additives in heavy-duty metalworking fluids and are therefore often the initial choice for formulators. Chlorinated paraffins are relatively inexpensive and provide exceptional extreme pressure performance on ferrous metals like steel and cast iron. These additives activate at relatively low temperatures and over a wide temperature range. Furthermore, because chlorinated paraffins tend to be viscous, they also provide good boundary lubrication performance—a characteristic that can be challenging to replicate using other extreme pressure chemistries. They are oil soluble and can typically be emulsified. 

Despite their reliability for extreme pressure performance, chlorinated paraffins do have some drawbacks. A potential byproduct of their use is hydrochloric acid generation, which can be corrosive to several ferrous alloys, especially in hot and humid environments. Care should be taken when formulating chlorinated fluids to reduce this. They have also come under regulatory scrutiny in the past several decades due to their tendency to bioaccumulate in waterways. Because of this, short-chain chlorinated paraffins (C10-C13) have been eliminated from use globally.  Medium-chain (C14-C17), long-chain (C18-20) and very long chain (C20+) chlorinated paraffins are still available; however, the longer chain lengths typically contain less chlorine and can be more difficult to emulsify.

While regulatory pressure has pushed formulators to look to other chemistries, end-user preference has also contributed to the decline in chlorinated paraffins. General sustainability mindfulness as well as worker health and safety has led some companies to eliminate their use on production floors. Disposal of fluids containing chlorinated paraffin can also come at an increased cost for end users, as it is classified as hazardous waste and must be managed accordingly. 

Sulfur. Sulfur is another historically common chemistry deployed for extreme pressure performance. These types of additives include sulfurized olefins, sulfurized vegetable oils and sulfurized fats. Sulfur additives can be light or dark in color depending on the raw materials used and the specific sulfurization process. Sulfur additives can be classified as either “active” or “inactive” depending on whether they will stain yellow metals (active) or not (inactive). Often, fluids containing active sulfur will also use a yellow metal corrosion inhibitor to avoid staining any brass or other yellow metal components in the machine. Regardless of yellow metal activity, both types are reactive toward ferrous metals at typical cutting temperatures. Sulfur additives tend to require higher temperatures to activate relative to chlorine and phosphorous additives. Sulfurized fats and vegetable oils can also provide some boundary lubrication due to their higher viscosity and polar functionality. Most sulfur additives are oil soluble, and many can be emulsified.

Because many sulfur-based additives are derived from animal fats and vegetable oils, they are considered less hazardous and more environmentally friendly compared to chlorinated paraffins. As such, they do not face the same types of regulatory scrutiny that chlorinated paraffins do. Although sulfur additives are generally not classified as hazardous, some sulfurized additives can have a stronger odor than other classes of extreme pressure additives. Thus, operators’ concerns regarding odor may come into play here as well, especially if machines are not enclosed or properly vented.

Phosphorous. Phosphorous additives comprise a range of organic phosphates (PO₄) and phosphites (PO₃). Typically considered anti-wear additives in other lubrication industries, phosphorous additives activate at slightly lower temperatures than the other extreme pressure additives and can provide protection to both ferrous metals and nonferrous metals, such as aluminum. Phosphorous additives are available in oil-soluble, water-dispersible and water-soluble variations. These attributes make phosphorous extreme pressure additives versatile and easier to incorporate into formulas. 

While phosphorus-based additives can deliver exceptional performance, especially on nonferrous alloys, they do have some drawbacks. Phosphates and phosphites are both very easily broken down by microbes and are therefore sometimes referred to colloquially as “bug food.” Thus, although they are generally regarded as nonhazardous, there can be some concerns about groundwater contamination when utilizing phosphorus-based extreme pressure additives.  This has led to various regulatory reviews concerning their use, and while no action has been taken as of now, that may change in the future.

Overbased metal sulfonates. Overbased metal sulfonates are another option to provide extreme pressure performance, but they do not function in the same way as the more typical chemistries discussed above. Instead, these additives provide what is known as “passive extreme pressure” lubrication and are used in conjunction with other additives—most typically sulfur—to provide synergistic extreme pressure performance. 

A significant limitation of overbased metal sulfonates is that they tend to have compatibility issues with water-based fluids, making them useful primarily in oil-based fluids.

Selecting an Optimal Extreme Pressure Additive

For metalworking fluid formulators, the choice of which extreme pressure additive or additives to use is not straightforward. The primary functions of the metalworking fluid are to extend tool life, improve surface quality, and provide overall throughput and efficiency gains in manufacturing by removing heat and lubricating the interface between tool and workpiece. The ability of a fluid to accomplish these goals varies greatly depending on the additives that are used. 

Unlike many other lubricant industries, metalworking fluids are not governed by industry-wide standards or specifications, so the right combination of additives must be chosen based on a range of performance attributes that depend on the operation, metallurgy and other factors. Some light-duty operations may require only boundary lubrication, whereas other more extreme operations may require high levels of boundary and multiple extreme pressure additives. For example, some of the most severe operations like deep drawing and cold heading may necessitate the use of chlorinated paraffins. Sulfur extreme pressure additives strike a balance of performance and ease-of-use for a variety of mild to severe operations on ferrous metals. If a single fluid is to be used to machine both ferrous and nonferrous metals such as aluminum, then a phosphorous-based additive may be most appropriate. 

The development of more effective, environmentally friendly extreme pressure additives is an active area of research. New synergistic blends of different chemistries are entering the market as effective alternatives with enhanced performance and fewer drawbacks relative to current offerings. Consulting with additive suppliers may help you to identify the right extreme pressure additive or combination of additives for your application, as they can assist you in testing and evaluating your formulations and provide guidance as to whether one additive or fluid may perform better than another. For formulators, staying educated on new developments, product offerings and the applications of these additives is crucial for optimizing metalworking fluids and achieving superior performance.  

Ryan Weber joined Lubrizol as a technology development manager in the Metalworking Additives group in 2015. Since then, he has been active in the fundamental study and development of sulfur-based extreme pressure additives, ferrous corrosion inhibitors and emulsifiers for the industrial lubricants market.

Andrew Yoder joined Lubrizol in 2021 as a technology development manager in the Metalworking Additives group. With over 20 years of formulating experience, his work centers on developing lubricant additives for the metalworking industry.