Base oils are one half a lubricant formulation. The other being additives. I have been planning on an article to cover different base oils for a while but it ended up so long I have had to split it into three sections. Here is the first in the base oil series covering mineral oils. As the most popular type of lubricant world wide I thought they deserved their own section. Enjoy!
1. Introduction
Mineral oils dominate the industrial and motor oil lubricant markets, offering various types of refining and performance characteristics that cater to different applications.
Mineral Oils vs. Other Base Oils
As mentioned mineral oils play such an important role in lubrication I have given them their own section. Mineral base oils are derived from crude oil. Crude comes from little creatures that were in the oceans millions of years ago. They are refined to achieve specific performance characteristics, which are then enhanced with additives. They are the most commonly used lubricants and are classified based on their refining methods and molecular composition. In contrast, synthetic base oils are artificially engineered to provide superior performance in areas such as oxidation stability, low-temperature fluidity, and thermal stability. This leads to the ongoing debate between mineral and synthetic oils.
The mineral vs. synthetic debate often centres around the trade-offs between cost and performance. Mineral oils, particularly those from Group I, are typically less expensive to produce and are suitable for general-purpose applications. However, they may lack the high performance required for modern engines and industrial machinery. Synthetic oils, such as PAOs (Polyalphaolefins group IV), offer better viscosity index, oxidative stability, and lower volatility. They are ideal for high-performance applications but are generally more expensive. Group III Plus base oils, such as those derived from Gas-to-Liquid (GTL) technology, attempt to bridge this gap, offering near-synthetic performance at a lower cost compared to fully synthetic PAOs. Ultimately, the choice between mineral and synthetic oils depends on application-specific requirements, environmental factors, and budget considerations. This article discusses base oils, focusing on the different API groups, their properties, refining processes, and distinctions between mineral and synthetic lubricants. We explore Groups I, II, III, and Group III Plus oils, outlining their production methods, chemical properties, and applications to help understand their contributions to the lubricants industry.
2. Base Oil Molecules
Base oils originate from crude oil, which is extracted from deep beneath the Earth’s surface. Crude oil is often found in sandstone reservoirs, made up of a mixture of various sizes and types of molecules—comparable to a mixed box of LEGO bricks. These molecules fall into three main families:
- Paraffins: Linear hydrocarbons that exhibit excellent viscosity index (VI) and oxidation stability. Group I, II, and III base oils are primarily composed of paraffinic molecules. These oils offer good high-temperature performance and stability. However, paraffins suffer from poor low-temperature performance, leading to high pour points. Their linear structure allows for easy molecular stacking, which results in solidification at lower temperatures, making them less effective in cold environments.
- Naphthenes: Naphthenes are cyclic hydrocarbons that generally perform moderately across most properties. They have better pour points compared to paraffins due to their ring structure, which reduces the ability of molecules to stack effectively. This characteristic allows naphthenes to maintain better flow at lower temperatures, making them useful in applications where cold-temperature performance is needed. Naphthenes in contrast to paraffins exhibit poor viscosity indices sometimes having negative VIs. This makes them excellent for heat transfer or transformer oils where you want them to go really thin with temperature increase to allow for convection of heat, but not for eg an engine oil where you want consistent viscosity.
- Aromatics: Aromatics are molecules that contain one or more benzene rings. They generally exhibit poor oxidation stability and viscosity index properties due to the presence of unsaturated bonds in the ring structure. These double bonds can easily react with oxygen, leading to increased degradation of the oil. Furthermore, aromatics can be toxic and are therefore undesirable in finished lubricants. However, their solubility properties are beneficial in certain additive formulations, and they can help dissolve other components effectively when present in small quantities. As a laboratory owner they also make excellent lab solvents for extracting various compounds or cleaning glassware etc.
These molecular families contribute to the various properties of finished lubricants, influencing viscosity index, oxidative stability, pour point, and solubility. Different refining processes are employed to manipulate these properties and create oils tailored for specific applications.
3. Group I Base Oils: Solvent Refined
Group I base oils are produced using a process called solvent refining, which involves several steps to transform crude oil into a refined lubricant:
- Fractional Distillation: The first stage of processing crude oil is fractional distillation. Crude oil is heated in an atmospheric distillation tower, where it is separated by boiling point, allowing different molecular fractions to be collected. Lighter fractions such as methane, propane, and butane exit the top of the distillation tower, while heavier fractions such as kerosene and light gas oils are collected lower down the column. The heaviest fractions remain at the bottom and require further processing.
- Vacuum Distillation: The heavy residue left from atmospheric distillation is further processed using vacuum distillation. The reduced pressure in the vacuum distillation tower allows separation at lower temperatures, which is important for preventing thermal cracking of the heavier molecules. The outcome is multiple distillate cuts of varying viscosities, which are used for lubricant production.
- Solvent Extraction: To improve the quality of the base oil, aromatics are removed using solvent extraction. Aromatics are undesirable due to their poor viscosity index and oxidation stability, as well as potential toxicity. A highly polar solvent, such as furfural, is used to selectively dissolve aromatic molecules while leaving paraffinic and naphthenic molecules intact. This extraction process enhances the overall oxidative stability and viscosity index of the base oil.
- Solvent Dewaxing: To ensure good low-temperature properties, long-chain paraffins that contribute to a high pour point are removed using solvent dewaxing. A solvent like methyl ethyl ketone (MEK) or toluene is used to dissolve the wax components, which are then crystallized out by chilling the mixture. The wax is filtered out, resulting in a base oil with improved flow properties at low temperatures.
- Hydrofinishing: The final step in Group I base oil production is hydrofinishing, where the dewaxed oil is treated with hydrogen under moderate pressure in the presence of a catalyst. This process helps to saturate any remaining double bonds, improving the colour, stability, and performance characteristics of the base oil. Hydrofinishing also reduces the reactivity of the oil, enhancing its resistance to oxidation.
The final Group I base oil may include solvent neutral oils (e.g., SN150, SN500) or bright stock, depending on the specific refining steps used. Solvent neutral oils are light to medium-viscosity base oils, while bright stock is a high-viscosity product obtained from the heaviest fractions.
4. Group II Base Oils: Hydrocracked Oils
Group II base oils differ from Group I oils primarily due to the use of hydrocracking instead of solvent refining. Hydrocracking is a more advanced refining process that involves subjecting the crude fractions to hydrogen at high pressure in the presence of a catalyst:
- Hydrocracking: In hydrocracking, the goal is to saturate carbon-carbon bonds and remove impurities such as sulphur and nitrogen. Hydrogen is used under high pressure (typically 1500-3000 psi) and high temperature (typically 350-400°C) in the presence of a metal-based catalyst, such as nickel-molybdenum or cobalt-molybdenum. This process breaks down larger, complex molecules into more stable, saturated hydrocarbons, resulting in base oils with improved properties. Aromatic rings are opened, and sulphur and nitrogen are removed as hydrogen sulphide and ammonia, respectively. Hydrocracking significantly enhances the viscosity index, oxidation stability, and purity of the oil compared to Group I.
- Catalytic Dewaxing: In Group II base oil production, catalytic dewaxing is used instead of solvent dewaxing. In catalytic dewaxing, straight-chain paraffins are chemically altered to form isoparaffins, which have lower pour points. A catalyst, often containing zeolite, is used to selectively reshape paraffinic molecules, which not only reduces the pour point but also maintains or improves the viscosity index. The conversion of straight-chain paraffins to branched isoparaffins ensures that the base oil has excellent low-temperature properties.
Group II base oils are moderately hydrocracked and have very low aromatic content, making them suitable for automotive and industrial lubricants. They offer improved stability, reduced impurities, and a higher viscosity index compared to Group I oils.
5. Group III Base Oils: Severely Hydrocracked Oils
Group III base oils also utilise hydrocracking but undergo a more severe version of the process compared to Group II oils:
- Severe Hydrocracking: Group III base oils are produced with more intense hydrocracking conditions, including higher pressures and temperatures. This results in oils with even higher viscosity indices, lower volatility, and a near-complete removal of aromatic compounds. The resulting hydrocarbons are highly saturated and exhibit properties that are often comparable to synthetic oils.
- Catalytic Dewaxing: Similar to Group II production, catalytic dewaxing is used in Group III oils, but the focus is on achieving even greater consistency and performance. The isomerisation of paraffins results in oils with exceptional low-temperature properties, making Group III oils particularly useful in high-performance lubricant formulations.
Group III base oils are often marketed as synthetic oils in certain countries, given their high level of purity, improved stability, and high viscosity index. They are frequently used in high-performance lubricants where stability and efficiency are critical.
Group III Oils and Synthetic Labelling
In some regions, Group III mineral base oils are considered synthetic due to their advanced hydrocracking process and the resulting high purity and performance characteristics. This classification was influenced by a notable court case in the United States between Mobil and Castrol in the late 1990s. Mobil challenged Castrol’s marketing of Group III base oils as synthetic, arguing that they did not meet the traditional definition of synthetic oils, which were typically PAOs (Polyalphaolefins). However, the ruling allowed Castrol to continue labelling Group III oils as synthetic, setting a precedent that persists today in several markets. This decision reflects the high quality and performance of Group III oils, despite their origin from mineral oil.
6. Group III Plus Base Oils: Gas-to-Liquid (GTL) Oils
Group III Plus base oils are produced using Gas-to-Liquid (GTL) technology, a unique process that transforms natural gas into high-quality base oils. GTL base oils are often considered synthetic due to their high level of purity and uniform molecular structure, despite being derived from natural gas:
- Fischer-Tropsch Process: The production of GTL base oils begins with the Fischer-Tropsch synthesis, which converts methane (the primary component of natural gas) into longer hydrocarbon chains. Methane is reacted with oxygen and steam to produce a mixture of carbon monoxide and hydrogen, known as synthesis gas or syngas. This syngas is then passed over a cobalt-based catalyst under high temperature and pressure to form liquid hydrocarbons. The Fischer-Tropsch process produces a waxy intermediate product, which can be further processed to create high-quality base oils.
- Hydroisomerisation: The waxy hydrocarbons produced by the Fischer-Tropsch process are subjected to hydroisomerisation to convert them into isoparaffins. This process involves the use of a catalyst and hydrogen to rearrange the molecular structure, improving the cold flow properties of the final product. Hydroisomerisation helps achieve the desired balance between high viscosity index, low pour point, and excellent oxidation stability.
- Performance Characteristics: GTL base oils are known for their excellent performance characteristics, including high viscosity index, outstanding oxidation stability, and low volatility. They are also free of sulphur, nitrogen, and other impurities commonly found in crude oil-derived base oils. GTL base oils have properties similar to Polyalphaolefins (PAOs), such as excellent cleanliness, low volatility, and good low-temperature performance. They are often used in premium automotive and industrial lubricants, where performance requirements are stringent but a fully synthetic PAO may be cost-prohibitive.
7. Oil Analysis Tests for Base Oils Classification
In order to classify base oils into different groups and evaluate their suitability for various applications, a number of oil analysis tests are performed. These tests help assess the physical and chemical properties of the base oils, providing insights into their quality and performance characteristics:
- Viscosity Measurement: Viscosity is one of the most important parameters for classifying base oils. It is typically measured at both 40°C and 100°C to determine the oil’s resistance to flow at different temperatures. The Viscosity Index (VI) is then calculated to indicate the oil’s stability across a range of temperatures. A higher VI suggests that the oil will maintain its performance better when subjected to temperature changes. In addition to this a lab may do for base oil manufacturers more extensive temperature testing ranging from -20 to +100’C in 1,5 or 10 degree increments to allow calculations for applications at all conceivable operating temperatures. My lab has assisted a few manufactures specifically with this research.
- Pour Point Test: The pour point is the lowest temperature at which the oil remains pourable. It is a key factor in determining the suitability of a base oil for low-temperature environments. The Pour Point Test helps identify the presence of waxes and long-chain paraffins that might solidify under cold conditions. Group II, III, and GTL base oils typically have lower pour points compared to Group I due to better refining processes and catalytic dewaxing. In addition to pour point a cloud point may also be useful and give a better indicator of when wax crystals begin to form, although with pour point depressant additives the gap between cloud and pour point can increase significantly.
- Flash Point Test: The flash point is the temperature at which the oil vapours ignite in the presence of an ignition source. It is crucial for understanding the safety and volatility of the oil. Higher flash points indicate better resistance to evaporation at elevated temperatures, which is especially important in automotive and industrial lubricants to prevent losses. They also help in selecting lubricants for their thermal safety properties. Commonly performed tests in this category include PMCC closed flash point, the lowest and most conservative value, the open flash point the value most manufacturers quote, the fire point and even higher autoignition points to aid for very high risk situations where operating temperatures exceed the flash point and to identify at what point it catches fire with ignition or spontaneously.
- Sulphur Content Analysis: Sulphur compounds in base oils can lead to corrosive wear in bearings, gears and machinery. By measuring sulphur content, particularly for Group II and III oils, manufacturers can ensure compliance with environmental regulations and compatibility with modern catalytic converters. Lower sulphur content is generally achieved through hydrocracking.
- Fourier-Transform Infrared Spectroscopy (FTIR): FTIR is used to identify the molecular structure and composition of the base oil. This analysis can determine the presence of aromatic, paraffinic, and naphthenic compounds. The balance between these molecular families directly influences the oil’s properties such as oxidation stability and solubility. Speaking of solubility tests like aniline point can also help with this too.
- Oxidation Stability Test: Oxidation stability is crucial for predicting the oil’s lifespan and its ability to resist degradation in high-temperature conditions. Tests such as the Rotating Pressure Vessel Oxidation Test (RPVOT) evaluate how well the oil resists reacting with oxygen, which helps classify oils into Groups I to III. Hydrocracked oils (Groups II and III) generally exhibit higher oxidation stability due to the removal of aromatics and other reactive components.
- Noack Volatility Test: The Noack Volatility Test measures the evaporation loss of an oil at high temperatures. This property is important for engine oils, as high volatility can lead to oil consumption and the formation of deposits. Group III and GTL base oils tend to have lower volatility compared to Group I, making them better suited for modern high-performance engines.
- Colour and Appearance: The visual appearance of base oil, including its colour, can provide information on its level of refinement and purity. Group I base oils are often darker due to the presence of aromatic and sulphur compounds, whereas Group II, III, and GTL oils are typically lighter in colour, indicating higher levels of purity.
These analysis tests are essential for classifying base oils into Groups I, II, III, and beyond. The results of these tests allow manufacturers and users to select the appropriate base oil for specific applications, ensuring optimal performance and reliability.
Base Oil Classification Table
| Base Oil Group | Viscosity Index (VI) | Sulphur Content | Saturates (%) | Aromatics (%) | Key Properties |
|---|---|---|---|---|---|
| Group I | 80-120 | High (>0.03%) | Low (<90%) | High | Cost-effective, general purpose |
| Group II | 100-120 | Low (<0.03%) | High (>90%) | Low | Improved oxidation stability, low sulphur |
| Group III | >120 | Very Low (<0.03%) | Very High (>90%) | Very Low | High VI, often considered synthetic in some regions |
| Group III Plus | >120 | Very Low (<0.03%) | Very High (>90%) | Very Low | Near synthetic performance, produced via GTL technology |
This table summarizes the classification of base oils, highlighting key differences in their viscosity index, sulphur content, saturates, aromatics, and specific properties that determine their applications.
8. Conclusion
The journey from crude oil to finished lubricant involves a complex series of refining processes tailored to achieve the desired performance characteristics. Group I base oils rely on solvent refining to produce versatile but less refined products suitable for general use, including automotive and industrial applications where cost-effectiveness is important. Group II and III oils use hydrocracking and catalytic dewaxing to produce higher-quality base oils with better stability, higher viscosity indices, and lower impurity levels, making them suitable for modern engines and high-performance applications. Group III Plus oils, produced via GTL technology, bridge the gap between mineral and synthetic oils, offering premium performance at a reasonable cost.
Understanding the molecular families and refining processes involved provides insight into how these base oils meet specific requirements for applications ranging from automotive lubricants to industrial machinery. By optimising refining processes, producers can control the properties of base oils, ensuring they deliver the necessary performance for increasingly demanding applications.
Join us for part 2 as we cover synthetics.