understanding-lubricant-properties-from-basic-physicochemical-characteristics-to-specialized-performance
Lubricants are technology-intensive products composed of complex mixtures of hydrocarbons. Their real-world performance is the result of combined physical and chemical processes occurring during use.
To evaluate lubricant quality and suitability for specific applications, a wide range of properties are tested and analyzed.
Lubricant performance characteristics are generally divided into three categories:
- Basic physicochemical properties
- Special physicochemical properties
- Simulated bench and rig tests
This article focuses on the physicochemical properties that define lubricant quality and performance.
1. Basic Physicochemical Properties of Lubricants
All types of lubricating oils and greases share a set of fundamental physicochemical properties that reflect their intrinsic quality.
(1) Density
Density is one of the simplest and most commonly used physical indicators of lubricant quality.
It increases with higher contents of carbon, oxygen, and sulfur in the oil composition.
For lubricants with similar viscosity or molecular weight:
- Oils rich in aromatics, resins, and asphaltenes have the highest density
- Naphthenic oils are intermediate
- Paraffinic oils have the lowest density
(2) Appearance (Color)
Color often reflects the refining depth and stability of base oils.
Generally, higher refining severity removes more oxidation and sulfur compounds, resulting in a lighter color.
However, even under identical refining conditions, base oils derived from different crude sources may show different color and clarity.
For finished lubricants containing additives, color is no longer a reliable indicator of base oil quality.
(3) Viscosity Index (VI)
The viscosity index indicates how much an oil’s viscosity changes with temperature.
- High VI: less viscosity change, better viscosity–temperature behavior
- Low VI: greater sensitivity to temperature changes
(4) Viscosity
Viscosity represents the internal resistance to flow and is a key indicator of lubricating film strength and fluidity.
Under the condition of no functional additives:
- Higher viscosity → stronger oil film but poorer flow
- Lower viscosity → better flow but weaker film strength
(5) Flash Point
The flash point indicates volatility and fire safety.
- Lighter fractions → higher volatility → lower flash point
- Heavier fractions → lower volatility → higher flash point
From a safety perspective:
- Flash point < 45 °C: flammable liquid
- Flash point ≥ 45 °C: combustible liquid
As a general guideline, the flash point should be 20–30 °C higher than the operating temperature.
(6) Acid Value, Base Value, and Neutralization Number
- Total Acid Number (TAN): measures acidic substances (mg KOH/g)
- Total Base Number (TBN): measures alkaline substances (mg KOH/g)
Although the term neutralization number may include both acid and base values, in practice it usually refers to TAN, unless otherwise specified.
(7) Pour Point and Cloud Point
The pour point is the highest temperature at which the oil ceases to flow under specified cooling conditions.
Lubricant solidification differs from pure compounds:
- Not all components crystallize simultaneously
- Loss of flow is gradual
Key considerations:
- Pour point should be 5–7 °C below the lowest ambient temperature
- Lower pour point generally means higher production cost
- Low-temperature viscosity and viscosity–temperature behavior must also be considered
The cloud point and pour point both indicate low-temperature flow behavior.
Typically, the cloud point is 2–3 °C higher than the pour point.
(8) Mechanical Impurities
Mechanical impurities are insoluble sediments or colloidal materials, such as:
- Sand
- Metal particles
- Insoluble additive residues
High-quality base oils usually contain less than 0.005% mechanical impurities.
(9) Water Content
Water content is expressed as a weight percentage.
Water in lubricants:
- Breaks down the oil film
- Accelerates corrosion
- Promotes sludge formation
Therefore, lower water content is always preferred.
(10) Ash and Sulfated Ash
Ash is the non-combustible residue remaining after combustion and typically consists of metallic elements and their salts.
- For base oils, ash reflects refining depth
- For additive-containing oils, ash indicates additive concentration
Sulfated ash is measured after treating the residue with sulfuric acid to convert metals into sulfates and is commonly used in international standards.
(11) Carbon Residue
Carbon residue refers to the coke-like residue formed after heating under specified conditions.
It reflects:
- Chemical composition
- Refining depth
Primary contributors include resins, asphaltenes, and polycyclic aromatics.
For additive-free base oils, lower carbon residue is better.
However, in additive-containing oils, this parameter loses its original significance due to high-carbon additives.
2. Special Physicochemical Properties
Beyond general properties, lubricants must exhibit application-specific performance characteristics, especially for high-quality or specialty products.
(1) Thermal Stability
Thermal stability indicates resistance to thermal decomposition at high temperatures.
It mainly depends on base oil composition, while many additives may negatively affect it.
(2) Oxidation Stability
Oxidation stability reflects resistance to aging and degradation.
Testing usually involves:
- Oxidation in the presence of air or oxygen
- Metal catalysts
- Measurement of acid value increase, viscosity change, and sludge formation
(3) Corrosion and Rust Protection
- Corrosion tests evaluate metal discoloration (e.g., copper strip test at 100 °C)
- Rust tests assess steel protection in the presence of water or saltwater
Both are mandatory in most industrial lubricant standards.
(4) Oiliness and Extreme Pressure (EP) Performance
- Oiliness: polar molecules form adsorbed films that reduce friction and wear
- EP performance: chemical reactions under high load and temperature form soft protective films
(5) Foam Resistance
Foaming disrupts oil films, accelerates oxidation, and impairs system operation.
Anti-foaming performance is therefore a critical quality indicator.
(6) Demulsibility
Demulsibility reflects the ability to separate from water.
Poor demulsibility leads to:
- Emulsification
- Inadequate lubrication
- Water retention in systems
(7) Hydrolytic Stability
Hydrolytic stability measures resistance to degradation in the presence of water and metals (typically copper).
It is evaluated by monitoring water acidity and metal weight loss.
(8) Air Release Properties
In hydraulic systems, trapped air reduces response accuracy.
This property measures how quickly dissolved air escapes from the oil.
(9) Elastomer Compatibility
Lubricants must not cause rubber seals to swell, shrink, harden, or crack.
Seal compatibility is typically assessed by dimensional changes after oil immersion.
(10) Shear Stability
Viscosity index improvers may shear under mechanical stress, causing viscosity loss.
Shear stability is evaluated by measuring viscosity reduction after standardized shear tests.
(11) Volatility
Base oil volatility affects:
- Oil consumption
- Viscosity stability
- Oxidation resistance
This property is particularly important for multi-grade and energy-saving oils.
(12) Solvency
Solvency is commonly expressed by the aniline point and indicates the oil’s ability to dissolve additives.
(13) Electrical Properties
Exclusive to insulating oils, including:
- Dielectric loss factor
- Dielectric constant
- Breakdown voltage
- Impulse voltage resistance
(14) Rust Prevention Performance
Rust-preventive oils are evaluated through:
- Humidity tests
- Salt spray tests
- Storage and long-term exposure tests
(15) Special Properties of Greases
Depending on application, grease testing may include:
- Water washout resistance
- Low-temperature torque
- EP and anti-wear performance
- Bearing life tests
(16) Other Application-Specific Properties
Different lubricants require specialized tests, such as:
- Cooling rate for quenching oils
- Emulsion stability for soluble oils
- Stick–slip behavior for slideway oils
- Oil mist dispersion
- Floc point for refrigeration oils
These properties depend on specific base oil chemistry and tailored additive systems.
