• What Are the Chemical & Physical Properties of Lubricants?

Fuel for Thought

What Are the Chemical & Physical Properties of Lubricants?

Mar 07 2022

From maximising the efficiency of machinery in heavy-duty oil and gas refineries to repelling rust, dirt and other contaminants from bicycle chains, lubricants are used for a wide range of applications. Choosing the right lubricant for the job is largely dependent on chemical and physical properties. These determine performance characteristics and help operators predict how a lubricant will behave during use. Ultimately, this information determines the ability of a lubricant to meet certain goals and benchmarks.

Read on to find out more about why the physical and chemical properties of lubricants are so important and how these variables are used to help determine factors like quality, suitability and price.


Viscosity describes how resistant a lubricant is to flow. Also known as internal friction, viscosity is one of the most important properties considered when assessing lubricants and determining characteristics like grade and suitability. Lubricants with high viscosity are thicker and offer more internal resistance to flow. For example, steam cylinder oils used to lubricate heavy-duty engines must be highly viscous to withstand the high-pressure operating conditions. Gear oils used to lubricate machinery in flour mills, sugar refineries and food manufacturing plants are another example of high viscosity lubricants.

Low viscosity lubricants are thinner and fast flowing, similar to water. Kerosene used to lubricate rolling metals is an example of a low viscosity lubricant. Over the past few years many trucking fleets have been making the switch to low viscosity engine lubricants, which are marketed as a more sustainable and cost-efficient option.

Temperature can affect viscosity, which is why motorists often switch to lighter, more viscous engine oils in colder weather. Guidelines published by the International Standards Organisation (ISO) use temperatures of 40ËšC to measure the viscosity of lubricants. The grading system rates oils and lubricants on a scale of ISO VG 2 to ISO VG 1500, with a lower number indicating a lower viscosity index.  

Sometimes, viscosity can decrease when a lubricant is exposed to high temperatures. If this is undesirable, chemical index improvers such as methacrylate molecules can be used to prevent the thinning process and maintain the desired viscosity index.

Chemical stability

Lubricating oils with good chemical stability shouldn’t be at risk of oxidising, carbonising or undergoing other undesirable reactions when exposed to oxygen. A variety of factors determine the chemical stability of a lubricant, including concentrations of catalysts such as copper.   

Sulphur content

The sulphur content of lubricants is an important property to consider as the chemical element can fast-track corrosion. That said, sulphur can also be added to certain oils to improve lubricating capabilities.

American Petroleum Institute (API) gravity

API gravity measures the density of a lubricant, with a hydrometer used to calculate a rating based on how light or heavy the product is compared to water. Lubricants with API gravity of 10 or more are classed as light while lubricants with API gravity of less than 10 are heavy.

Neutralisation number

Even after being refined, a lubricant may contain impurities, including acids and alkaline products. A neutralisation number is assigned based on how many milligrams of potassium hydroxide is needed to defuse acid content in one gram of oil.


Demulsibility describes the ability of a lubricant to separate from H2O. Oils with high demulsibility are generally preferred as water can compromise the lubricating properties of a product. This can lead to issues such as increased energy consumption due to resistance, surface corrosion and contamination caused by the release of abrasive wear particles. Additives can be a good way to improve the demulsibility of a lubricant and prevent it from mixing with water.

Oxidation stability

Oxidation is a chemical reaction that occurs when oxygen interacts with the lubricant. Variables such as water, acids, catalysts and temperature can influence oxidation and have a negative impact on the lifespan and performance characteristics of a lubricant. For applications where lubricant is in circulation for long periods without being changed, such as large mechanical systems, good oxidation stability is essential.

Corrosion resistance

Corrosion occurs when electrochemical action takes place on a refined metal. This reaction gradually deteriorates the surface and converts the refined metal back to a more stable form such as oxide, sulphide, carbonate or hydroxide.

Pour Point

Pour point describes the lowest temperature at which a lubricant will continue to flow in liquid form. This property is important as it determines the suitability of lubricants for cold climates and operating conditions, or when pressure will not be applied. Pour point depressants are often used to reduce the solidification temperatures of lubricants and improve performance when undergoing ASTM tests.

Flash Point

Flash point describes the lowest temperature needed to ignite a lubricant when it comes into contact with a small flame. This ‘flash’ occurs when light particles in the oil are volatised. Lubricants with high flash points are generally preferred as they’re less likely to ignite under normal operating temperatures and conditions.  

Fire Point

Fire point builds on flash point and exposes the lubricant to even more heat. The fire point is determined by the lowest temperature at which the lubricant will continuously burn. Similar to flash point, higher fire points are desirable as they reduce the risk of the lubricant burning while in circulation.

Cloud Point

At low temperatures, lubricants can start to change from liquid to solid state. This can solidify oil particles and create a cloudy appearance. The highest temperature at which this process occurs is called the cloud point.

Freezing point

Lubricants with low freezing points are generally more useful as they continue to function across a range of temperatures and operating conditions. For example, a lubricant with a low freezing point wouldn’t lose its functionality if operational temperatures unexpectedly dropped.

Why predicting lubricant performance characteristics is important

The underlying goal of assessing the chemical and physical properties of a lubricant is to determine its performance characteristics. Lubricants are used for a myriad of different reasons, though all share the same goal of maximising efficiency.

Below, we spotlight some of the main benefits of lubricants and explore how a deep understanding of chemical and physical properties helps to achieve these goals.

  • Reducing mechanical wear and friction

Metal-to-metal contact of moving parts can compromise mechanical efficiency, accelerate wear and release debris into the operating system. Mechanical lubricants form a protective layer between moving parts to prevent friction and eliminate contact between metal surfaces.

  • Minimising asset maintenance costs

Whether it’s repairing a hydraulic pump system at an oil drilling site or replacing a rusted bicycle chain, operating mechanical equipment comes with inevitable maintenance costs. Lubricants can drastically reduce the costs associated with maintaining assets. Ideally, a good lubrication strategy should actively prevent issues such as corrosion, wear and contamination from particulate matter and as a result, reduce asset maintenance costs.

  • Extending the lifespan of assets

By preventing mechanical issues such as corrosion and overheating, lubricants can significantly extend the lifespan of assets. For example, a well-lubricated Hyundai Tucson engine will drastically outperform an engine that isn’t properly lubricated. In fact, if the engine isn’t properly lubricated it could result in seizure, or catastrophic failure as it’s often called in the oil and gas industry. Similarly, the lifespans of high-value clean energy assets such as Haliade-X offshore wind turbines can be significantly increased with a good lubrication strategy.

  • Reducing unplanned downtime

Mechanical breakdowns can spell disaster for industrial operators, where uptime has a direct correlation with profits. For major Fortune 500 manufacturers, the costs of unplanned downtime sit at a huge US$1 trillion, according to a recent study from Senseye. A large portion of this unplanned downtime is caused by machine failure, often due to lubrication issues. 

Even for everyday citizens riding bicycles or driving cars, ‘unplanned downtime’ can be extremely inconvenient. Whatever the context, lubricants help to reduce the risk of mechanical failure and maximise uptime.

  • Avoiding catastrophic failure

While unplanned downtime can take a big toll on profits, catastrophic failure is worse. A recent Safety Notice issued by the Health and Safety Executive (HSE) underscored the importance of proper lubrication of circuit breakers across a range of industries, including agriculture, oil and gas, retail, transport, forestry and more.

“Investigation into an explosion of a HV circuit breaker indicated that recently carried out maintenance may have been a causal factor. The incident resulted in catastrophic failure of the HV circuit breaker leading to fire / explosion and could have resulted in fatal injuries,” reads the Safety Notice.

“Maintenance of HV and LV circuit breakers typically involves both the cleaning and lubricating of the operating mechanism. The HSE investigation found that the same physical product was used for both maintenance activities, but evidence shows the product was in fact only suitable for cleaning and NOT lubrication. This situation may have arisen because of the availability of different products, (for different purposes), within the same product range, or changes to the products composition over time whilst retaining the original name. It is essential that the correct product is used for each task.”

The future of petroleomics

Petroleum chemists, the experts that explore how to transform crude oil and natural gas into useful products such as lubricants, play a critical role in developing new products. Every year, the brightest minds, scientists and industry leaders come together for the annual PEFTEC conference. Find out more about the most recent petroleomics advancements and trends, including exciting leaps in gas chromatography for quantitative analysis of gas oils, in ‘Latest Breakthroughs in Petrochemistry’.

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