Top causes of grease thickening in industrial lubrication systems—and how to stop them
Key takeaways
- Cross-contamination can cause grease thickening, restricting flow and accelerating equipment failure.
- Incompatibility between base oils—not chemical reactions—is the primary cause of grease performance issues.
- PFPE/PTFE and silicone greases thicken when exposed to hydrocarbon oils, risking flow loss and downtime.
- Field testing and strict lubrication practices help identify and prevent grease compatibility failures.
Cross-contamination is a fact of life in industrial lubrication, but it brings with it a poorly understood and costly problem: certain greases, especially PFPE/PTFE and silicone-based types, thicken dramatically when exposed to hydraulic oils, gear oils, or incompatible greases. This thickening impairs flowability, clogs lubrication paths, and accelerates failure. Understanding the mechanisms behind this phenomenon helps prevent downtime and protects critical assets.
Understanding grease formulation: Base oils, thickeners, and additive interactions
Greases are engineered systems consisting of three primary components: base oil, thickener, and additives:
- The base oil, be it mineral, synthetic hydrocarbon, ester, PFPE, silicone, or halocarbon, provides lubrication.
- The thickener – lithium complex, calcium sulfonate, polyurea, PTFE, barium complex, or aluminum complex gives structure and consistency.
- Additives enhance performance with EP, anti-wear, oxidation inhibitors, and tackifiers.
When grease thickens after contamination, the cause is rarely chemical reaction. The real culprit is physical incompatibility between the grease’s base oil and the contaminant, whether hydraulic fluid, gear oil, or another grease system. The following are some examples of commonly used greases and their reactions with contaminants.
PFPE/PTFE-based greases and hydrocarbon contamination. PFPE/PTFE greases are widely used in vacuum and high-temperature applications because PTFE is chemically inert and PFPE base oils have excellent thermal stability. However, PFPE oils are immiscible with hydrocarbon-based fluids like hydraulic or gear oils. When contamination occurs, mutual insolubility displaces PFPE oil from the PTFE thickener matrix. The displaced PFPE oil migrates away, leaving a concentrated PTFE structure with less oil, resulting in dramatic thickening. Additionally, hydrocarbon contamination destabilizes PTFE particle dispersion, causing clumping and stiffness. The grease becomes too thick to flow through lubrication lines, leading to friction, heat generation, and early failure.
Some halocarbon-based PFPE/PTFE greases are engineered to overcome typical incompatibility with hydrocarbons. These formulations remain chemically inert and nonflammable while maintaining compatibility with petroleum oils. They retain flowability in contaminated environments and tolerate temperatures up to 288°C and vacuum conditions. Field validation remains essential.
Silicone-based greases and petroleum contamination. Silicone-based greases experience similar problems. Silicone oils are highly immiscible with petroleum oils. When contaminated, the grease stiffens, seals swell, and lubrication films break down, compromising protection and accelerating wear.
How to test grease compatibility: field methods for industrial lubrication reliability
- Identify potential contaminants (hydraulic oils, gear oils, other greases).
- Mix with grease samples at realistic contamination levels (10%, 25%, 50%).
- Heat to operating temperatures for appropriate dwell time.
- Measure penetration (consistency), observe for separation, thickening, or softening.
- Evaluate flowability through typical lubrication paths.
Hydrocarbon-based greases. Better Compatibility. Hydrocarbon-based greases – lithium complex, calcium sulfonate complex, and polyurea – generally tolerate hydraulic and gear oil contamination better:
- Lithium Complex: May soften or thicken slightly, depending on contaminant viscosity and additive interactions.
- Calcium Sulfonate Complex: Excellent stability, maintaining structure and EP performance.
- Polyurea: Generally stable with hydrocarbon fluids, though synthetic base oil polyureas require compatibility verification.
- Barium Complex: Good compatibility but less common due to environmental restrictions.
Gear oil contamination considerations. Gear oils often contain sulfur-phosphorus EP additives, which can soften or thin grease by dissolving or disrupting thickener structures. Alternatively, additive polarity can destabilize the grease matrix, increasing consistency and causing thickening. Both scenarios compromise lubrication performance.
Managing cross-contamination between incompatible greases to prevent lubrication failures
Cross-contamination between different grease types is an underappreciated threat. Incompatible base oils – such as mineral oil greases contaminated with silicone or ester-based greases – may separate, thicken, or soften unpredictably. PFPE or silicone greases contaminated with hydrocarbon greases typically thicken due to mutual insolubility.
Thickener incompatibility also poses risks. Polyurea and lithium complex greases often separate or suffer oil bleed and structural breakdown. Calcium sulfonate complex greases are broadly compatible with lithium greases, but compatibility with polyurea or barium complex greases should never be assumed without testing. Disparate additives can also react to form precipitates or destabilize the matrix, as when sulfur-phosphorus EP systems in gear oils interact with polyurea thickeners, causing structure loss or gelling.
Preventing lubrication failures: Engineering best practices for grease compatibility and reliability
Grease thickening upon contamination is a physical phenomenon rooted in base oil compatibility and thickener interactions, not chemical reactivity. While PTFE itself remains inert, incompatibility with hydrocarbon fluids or dissimilar greases results in flow loss, asset failure, and unplanned downtime. Understanding these mechanisms equips engineers to make informed decisions that protect uptime, reliability, and safety.
When thickening occurs from contamination, lubrication flow is restricted through lines and bearings, starving seals and bushings of lubrication. Friction increases, operating temperatures rise, and accelerated wear or catastrophic failures follow.
Reliability and lubrication engineers should:
- Understand system chemistry and evaluate base oil and thickener compatibility with potential contaminants.
- Perform contamination testing by mixing candidate greases with hydraulic oils, gear oils, and dissimilar greases at realistic ratios (10%, 25%, 50%), heating to operational temperatures, and observing for thickening, softening, or separation.
- Consult grease manufacturers for cross-contamination data and validation support.
- Enforce strict lubrication control programs to prevent unintentional mixing, with dedicated tools, clear labeling, and technician training.
About the Author
Michael Holloway
Michael Holloway
Michael D. Holloway is President of 5th Order Industry which provides training, failure analysis, and designed experiments. He has 40 years' experience in industry starting with research and product development for Olin Chemical and WR Grace, Rohm & Haas, GE Plastics, and reliability engineering and analysis for NCH, ALS, and SGS. He is a subject matter expert in Tribology, oil and failure analysis, reliability engineering, and designed experiments for science and engineering. He holds 16 professional certifications, a patent, a MS Polymer Engineering, BS Chemistry, BA Philosophy, authored 12 books, contributed to several others, cited in over 1000 manuscripts and several hundred master’s theses and doctoral dissertations.