Grease is made by combining three ingredients: a base oil, a thickener, and performance additives. The base oil makes up 70 to 95 percent of the finished product, the thickener accounts for 3 to 30 percent, and additives fill in the remaining portion (up to about 10 percent). The process involves heating, mixing, cooling, and milling these components into a smooth, consistent paste that clings to metal surfaces and reduces friction.
The Three Core Ingredients
Every grease starts with a base oil. This is the actual lubricant doing the work. Mineral oils refined from petroleum are the most common choice, but synthetic oils handle extreme temperatures better. For environmentally sensitive applications, vegetable-based oils like rapeseed, sunflower, soybean, or even used cooking oil can serve as the base. Rapeseed oil dominates the European bio-lubricant market, while sunflower and soybean oils are more common in the United States.
The thickener is what turns liquid oil into a semi-solid paste. Think of it like a sponge that holds and slowly releases oil where it’s needed. Without a thickener, you’d just have oil running off every surface. The type of thickener determines how the grease performs under heat, pressure, and moisture.
Additives are the final piece. They give grease specific capabilities: resisting extreme pressure, preventing rust, fighting oxidation, or reducing wear on metal parts. The exact additive package depends entirely on what the grease needs to do.
Choosing the Right Thickener
Lithium complex is the most widely used thickener in the world. It handles high temperatures well, offers solid mechanical stability, and is relatively inexpensive to formulate. If you need a general-purpose grease that works across many applications, lithium complex is the standard starting point.
Polyurea thickeners excel in a different way. They resist oil separation (called “bleed”), hold up exceptionally well against oxidation, and run quietly. That makes polyurea grease the go-to for electric motor bearings and sealed-for-life applications where the grease has to last without ever being reapplied.
Calcium sulfonate thickeners naturally resist water and corrosion, making them popular in marine and food-processing environments. The choice of thickener isn’t interchangeable. Mixing greases with different thickener types can soften or harden the mixture unpredictably, so manufacturers pick one thickener chemistry and build the entire formulation around it.
How Additives Shape Performance
Extreme pressure (EP) additives prevent metal surfaces from welding together under heavy loads. They work by reacting with the metal at contact points to form a protective film right where the pressure is greatest. Most EP additives are sulfur-based compounds, including sulfurized fatty oils and sulfurized olefins. Some formulations also use chlorine-based chemistry, though environmental concerns have reduced that practice.
Anti-wear additives serve a related but different purpose. Where EP additives prevent catastrophic seizure, anti-wear additives reduce the gradual loss of metal that happens during normal operation. These are typically phosphorus-based compounds. Many formulations combine both sulfur and phosphorus chemistry to cover the full range of contact conditions a bearing or gear might experience.
Beyond these two categories, grease can contain rust inhibitors, oxidation stabilizers (which extend shelf life and service life), and tackifiers that help the grease cling to surfaces instead of being flung off by spinning parts.
The Manufacturing Process Step by Step
Grease production begins with heating the base oil in a large kettle reactor. The thickener components are added at specific temperatures, and the mixture is heated further to trigger the chemical reactions that form the thickener structure. For lithium-based greases, this typically involves reacting a lithium compound with fatty acids in the presence of the hot base oil. The temperature, heating rate, and mixing speed during this stage all affect the final product’s texture and consistency.
Once the thickener has fully formed, the mixture is cooled gradually while being stirred. Additives are blended in during the cooling phase, usually at temperatures low enough to avoid breaking down their active chemistry. The cooling rate matters: cool too fast and the thickener crystals form unevenly, producing a lumpy or unstable grease. Cool too slowly and production time becomes impractical.
After cooling, the grease passes through a colloid mill, which is essentially a high-precision grinding machine. The mill forces the grease through a tiny gap, breaking down any clumps and distributing every ingredient evenly throughout the batch. Modern manufacturing demands uniformity down to about one-thousandth of a millimeter. Without milling, the grease would have inconsistent pockets of thickener and oil, leading to unpredictable performance.
The final step before packaging is deaeration, where air bubbles trapped during mixing are removed under vacuum. Air pockets degrade the grease over time by accelerating oxidation and reducing the density of the product. Deaerated grease lasts longer on the shelf and performs more consistently in service. After deaeration, the grease passes through a filter to catch any remaining contaminants or oversized particles, then gets pumped into tubes, cartridges, drums, or storage tanks.
How Grease Consistency Is Measured
The thickness or firmness of grease is rated on the NLGI scale, which runs from 000 (nearly liquid, like heavy cooking oil) up to 6 (a hard block, almost like a bar of soap). The rating is determined by a penetration test: a standard cone is dropped into the grease, and how far it sinks in tenths of a millimeter determines the grade.
- NLGI 000 to 0: Semi-fluid greases used in centralized lubrication systems and enclosed gearboxes. Penetration values range from 355 to 475 tenths of a millimeter.
- NLGI 1: Soft grease for cold-weather applications or systems needing easy flow. Penetration of 310 to 340.
- NLGI 2: The most common grade, used in the majority of automotive and industrial bearings. Penetration of 265 to 295. If a spec sheet doesn’t specify a grade, NLGI 2 is usually the default.
- NLGI 3 to 6: Progressively stiffer greases for high-temperature, vertical, or vibration-heavy applications where softer grease would drip or be thrown off. Penetration drops from 250 all the way down to 85.
Manufacturers control the NLGI grade by adjusting the ratio of thickener to base oil. More thickener produces a firmer grease with a higher NLGI number. The milling step also affects final consistency, since more aggressive milling can soften the product slightly by breaking down thickener fibers.
Vegetable-Based and Biodegradable Greases
For applications near waterways, forests, or food-contact surfaces, biodegradable greases replace petroleum base oils with plant-derived alternatives. Rapeseed, sunflower, palm, castor, and even olive pomace oil have all been tested as base stocks. These vegetable oils naturally contain long carbon chains (C18 to C22) that give them good lubricity, meaning they’re inherently slippery at a molecular level.
The challenge with vegetable oils is that they oxidize faster than mineral oils and don’t perform as well at extreme temperatures. Sulfurized vegetable oils can double as both base stock and extreme pressure additive, which simplifies formulation. Used cooking oil is also being explored as a low-cost, recycled alternative, though it requires more processing to remove food residues and water before it can serve as a grease base.
The thickener and additive chemistry for bio-greases follows the same principles as conventional grease. The difference is in selecting components that are also biodegradable, so the entire product breaks down safely if it enters the environment.