It comes down to molecular shape. Paraffinic base oil has open chains: its viscosity changes little with heat and it resists oxidation best, so most lubricants and greases use it. Naphthenic oil has saturated rings (single bonds only): it dissolves additives and resins better and flows in the cold without forming wax. Aromatic oil has rings with double bonds: it dissolves even more and softens dark rubber. Rule of thumb: heat and long life call for paraffinic; cold and solvency call for naphthenic; dark rubber calls for aromatic.
Base oil is the raw material that makes up most of a lubricant; the rest is additives. Every mineral oil mixes all three structures. The family name tells you which one is present in an unusually large amount: a naphthenic base has far more saturated rings than a paraffinic one, and an aromatic base has much more aromatic ring than the other two. This holds even when the open chain still accounts for most of the carbon in the base. The difference in solvency (the ability to dissolve another substance) is like the difference between two nail polish removers: one clears the nail in a single wipe, the other needs several. Faced with resin, additive and rubber, aromatic oil is the strong remover, naphthenic sits in the middle and paraffinic is the weakest.
When should you use paraffinic, naphthenic or aromatic oil?
Choose by the property the application needs. Three terms appear in the table. A virgin base oil is made from crude oil, not re-refined from used oil. A hydrotreated base has been treated with hydrogen under pressure, which removes sulfur and aromatics. Group II is the American Petroleum Institute (API) class for base oils with 90% or more saturated molecules and no more than 0.03% sulfur, the typical result of hydrotreating. The table shows which family meets each requirement and where Kelpen Oil base oils fit.
| What the application needs | Family | Kelpen Oil line |
|---|---|---|
| Withstand heat and oxidation for many hours (industrial lubricant, hydraulic fluid, grease) | Virgin paraffinic, hydrotreated (Group II) | HBP-I line |
| Viscosity that changes little with temperature, in oil or grease without additives | Virgin paraffinic | HBP line |
| Flow in the cold without a pour point depressant (the pour point is the lowest temperature at which the oil still flows) | Naphthenic | HBN line |
| Dissolve additive, resin or emulsifier (the additive that keeps oil dispersed in water), as in soluble cutting fluid and adhesive | Naphthenic | HBN line |
| Plasticize and extend light-colored rubber | Naphthenic | HBN line |
| Plasticize and extend rubber and tires with a balance between solvency and stability | Paraffinic, or a paraffinic and naphthenic blend | HBP line or HPN line |
| Soften and extend dark rubber, such as polybutadiene (BR) and styrene-butadiene (SBR), or formulate printing ink | Aromatic | HBA line |
| Additive-free mineral oil for reciprocating air compressors with low discharge temperature, when the compressor manufacturer specifies this type of oil (not refrigeration compressors) | Highly refined naphthenic | HBN compressor grades |
In rubber, the choice also depends on the polymer and the color of the part. In those cases, the Kelpen Oil Technical Team confirms the grade before the first batch.
What is paraffinic oil?
Paraffinic oil is a mineral base oil in which paraffins predominate: carbon and hydrogen molecules in an open chain, straight or branched, with single bonds only. This structure gives paraffinic oil the highest viscosity index of the three families and the best oxidation resistance. Oxidation is the reaction between the oil and the oxygen in the air; it darkens the oil, raises its acidity and forms sludge (a dark, sticky deposit that clogs filters and the machine's oil passages).
Viscosity index (VI) is the number that tells you how much the viscosity drops as the oil heats up; it is calculated from the viscosity at 40 °C and at 100 °C (ASTM D2270). The higher the VI, the less the oil thins with heat. The API classification (API 1509) requires a VI of 80 to 120 in Groups I and II, where solvent-refined and hydrotreated paraffinic bases sit; Group III, also paraffinic, has a VI of 120 or more.
The weak point is the cold. Straight chains line up and crystallize as wax when the temperature drops. That is why the refinery dewaxes the oil, removing part of that wax; even so, the pour point of a Group I paraffinic base is usually between -9 °C and -18 °C, depending on viscosity and dewaxing.
What paraffinic oil is used for: it is the base of industrial and automotive lubricants, greases, textile oils, rust preventives, paints, and rubber plasticizers and extenders. At Kelpen Oil, the family includes the HBP line, a virgin base, and the HBP-I line, also virgin and produced by hydrotreating, which is a Group II base oil.
Industrial paraffinic oil is also different from two products with a similar name. Solid paraffin wax, used in candles, is the wax removed during dewaxing. Pharmacy mineral oil is a white mineral oil, refined until it has no color or odor, with its own health requirements.
What is naphthenic oil?
Naphthenic oil is a mineral base oil in which naphthenes predominate: carbon molecules closed in a ring, also with single bonds only. The ring hardly forms wax, so naphthenic oil flows at very low temperatures without dewaxing: low-viscosity naphthenic bases reach pour points below -50 °C.
In exchange, the viscosity of naphthenic oil changes more with temperature, and its VI is usually below 80. That is why the API classification places naphthenic base oil in Group V, the group for bases that do not fit Groups I to IV. At the same viscosity, naphthenic oil is also denser than paraffinic oil.
Its advantage is solvency. Naphthenic oil dissolves additives, resins and emulsifiers better and forms a more stable emulsion, the milky mix of oil in water used in soluble cutting fluid for machining. The Kelpen Oil HBN line serves low-temperature lubricants, soluble cutting fluids, adhesives, oil-based paints, leather softeners and plasticizers for light-colored rubber.
Naphthenic oil is also the most widely used base for transformer insulating oil and the base of mineral oil for refrigeration compressors, because it stays fluid in the cold without a pour point depressant. Refrigeration calls for its own product, formulated to work together with the refrigerant gas; the HBN compressor grades are for air compressors.
What is aromatic oil used for in industry?
In industry, aromatic oil is used mainly as an extender and plasticizer for dark rubber: it goes into the compound to soften the rubber, help mix in the fillers (carbon black, silica) and increase the volume of the compound, hence the name extender. It is a mineral base oil rich in aromatic rings, six-carbon rings with alternating double bonds. It has nothing to do with the aromatic oil used in diffusers and aromatherapy, which is an essential oil from plants.
Aromatic oil is the family with the highest solvency, the lowest VI and the darkest color. That is why it rarely goes into lubricants and shows up where color does not matter: dark rubber parts and printing ink. The Kelpen Oil HBA line is an aromatic mineral oil derived from petroleum residue, indicated for dark rubber, such as polybutadiene (BR) and styrene-butadiene (SBR), and for printing ink components.
Aromatic oils may contain polycyclic aromatic hydrocarbons (molecules made of several fused rings), which are restricted in some markets. In the European Union, extender oil used in tires falls under REACH, the European regulation on chemical substances (Annex XVII, entry 50). If the part is for export, check with the Technical Team what your customer requires before choosing the base.
Why does the shape of the molecule change solvency and viscosity index?
Because each structure reacts differently to heat, to cold and to the other molecules in the product. The sequence is:
Open, flexible chain (paraffinic): the flow resistance of long chains changes little with temperature, which leads to a high VI. The molecule has no double bond and no aromatic ring, the points oxygen attacks first, which leads to longer life in hot service.
Straight chain in the cold (paraffinic): the molecules line up side by side and form wax crystals, which leads to a high pour point when the base is not dewaxed.
Saturated ring (naphthenic): the ring is compact and rigid and does not fit into a crystal, which leads to flow in the cold; the same rigidity makes the viscosity rise sharply in the cold and drop sharply in the heat, which leads to a lower VI. In practice, a naphthenic base carries more aromatic ring and more sulfur than a hydrotreated paraffinic one, and the ring has carbons that oxidize more easily than a straight chain, which leads to intermediate oxidation resistance.
Aromatic ring: the double bonds form an electron cloud shared by the whole ring, easy to deform, that attracts additives, resins and rubber chains, which leads to the highest solvency. The bonds next to the ring oxidize more easily, which leads to darkening and sludge when the oil runs hot.
| Property | Paraffinic | Naphthenic | Aromatic |
|---|---|---|---|
| Structure behind the name | Open chain | Saturated ring | Ring with double bonds |
| Viscosity index | High (80 to 120 in Groups I and II) | Usually below 80 | The lowest |
| Cold behavior (pour point) | Forms wax; usually -9 °C to -18 °C after dewaxing | Flows without wax; below -50 °C in light bases | High pour point in rubber grades: they thicken in the cold because of viscosity, not wax |
| Solvency (aniline point) | The lowest (90 to 130 °C or more) | Intermediate (70 to 100 °C) | The highest (20 to 40 °C in highly aromatic oils) |
| Oxidation resistance | The highest | Intermediate | The lowest |
| Density at the same viscosity | The lowest | Higher | The highest |
| Typical use | Lubricant, grease, textile oil, rubber plasticizer and extender | Soluble cutting fluid, cold service, light rubber, insulating oil | Dark rubber extender, printing ink |
How this shows up in your plant: a paraffinic base where cold rules shows up as cloudy or thickened oil in the tank on a cold morning and wax on the filter. A base with low solvency shows up as additive that will not dissolve and leaves a deposit at the bottom of the blending tank, or as soluble cutting fluid with cream separating on the surface of the emulsion. A naphthenic or aromatic base in equipment that runs hot for long periods may darken and form sludge ahead of schedule. In rubber, oil that is incompatible with the polymer migrates to the surface of the part, which turns oily.
Figure 1. A representative section of the molecule of each family and the same resin in three bases: in the paraffinic oil it stays clumped at the bottom, in the naphthenic oil it partly dissolves and in the aromatic oil it spreads through the whole oil. Below, the viscosity index of each family and the published aniline point range for each: 90 to 130 °C or more for paraffinic, 70 to 100 °C for naphthenic and 20 to 40 °C for highly aromatic oils.
What is aniline point?
Aniline point is the lowest temperature at which equal volumes of oil and aniline (an aromatic laboratory solvent) mix completely and form a single phase (ASTM D611). The lower the aniline point, the more the oil resembles aniline and the stronger a solvent it is.
The published ranges for each family are: highly aromatic oils, 20 °C to 40 °C; naphthenic bases, 70 °C to 100 °C, depending on refining and viscosity; Group I paraffinic bases, 90 °C to 110 °C; and hydrotreated Group II and III paraffinic bases, 100 °C to 130 °C or more. Taken together, paraffinic bases start at about 90 °C and go up to 130 °C or more. Within the same family the number rises with viscosity, so always compare bases of similar viscosity. Aniline point also predicts the oil's effect on rubber: the lower it is, the more the oil tends to swell seals and hoses made of rubber (elastomer).
How can you tell if a base oil is paraffinic or naphthenic?
By its carbon type composition, measured in the laboratory. The test calculates how much of the carbon in the base sits in paraffinic chains (CP), in naphthenic rings (CN) and in aromatic rings (CA), from the refractive index (how much light bends as it passes through the oil), the density and the viscosity (ASTM D2140 and ASTM D3238). No mineral oil is pure: even in a naphthenic or aromatic base, the paraffinic chain may be the largest share. So the reading compares contents against those of a paraffinic base. A much higher CN points to a naphthenic base; a much higher CA points to an aromatic base. The viscosity-gravity constant (ASTM D2501), a number calculated from density and viscosity that rises from paraffinic to aromatic, confirms the reading.
Our in-house laboratory in Arujá (SP) runs this calculation with the refractive index measured on the Abbemat 350 refractometer and the density and viscosity measured on the SVM 4001 viscometer, and it measures the aniline point on an automatic Tanaka instrument. The Technical Team uses these tests from the lubricants laboratory to characterize each base. Kelpen Oil, a Brazilian lubricant manufacturer since 1999, supplies paraffinic bases (HBP and HBP-I), naphthenic bases (HBN), aromatic bases (HBA) and a paraffinic and naphthenic blend (HPN).
When you switch base oil suppliers, compare carbon type composition and aniline point, not just viscosity. Two bases sold as paraffinic, in the same grade, can dissolve the same additive differently and change the stability of the formulation.
| What we measure | What it reveals | Standard |
|---|---|---|
| Carbon type composition (CA, CN and CP, in %) | The family of the base: how much is aromatic, naphthenic and paraffinic | ASTM D2140 and ASTM D3238 |
| Aniline point (°C) | Solvency and tendency to swell seal rubber (elastomer) | ASTM D611 |
| Viscosity index | How much the viscosity drops with heat | NBR 14358, with viscosity at 40 °C and 100 °C by ASTM D7042 |
| Pour point (°C) | Presence of wax and cold performance | NBR 11349 |
| Density (g/mL) | Naphthenic and aromatic oils are denser at the same viscosity | Measured on the SVM 4001 viscometer, together with viscosity |
| ASTM color (scale from 0.5 to 8, lightest to darkest) | Degree of refining and presence of aromatics | NBR 14483 |
| Sulfur (%) | Degree of refining: more sulfur points to a less refined base | ASTM D6481 (X-ray fluorescence) |
Frequently asked questions
Is mineral oil polar or nonpolar? Nonpolar. All three families are hydrocarbons, which is why they do not mix with water. Aromatic oil is the most polarizable: the electron cloud of the ring deforms easily, and that is what lets it dissolve resins and additives that paraffinic oil cannot.
Is mineral oil an aromatic hydrocarbon? Partly. Every mineral oil mixes paraffinic, naphthenic and aromatic hydrocarbons, and refining decides how much aromatic content remains. Hydrotreating reduces aromatics, so a Group II base has far less aromatic content than a Group I base, and an aromatic base keeps the most.
What is the difference between paraffinic and naphthenic transformer oil? Naphthenic insulating oil stays fluid at lower temperatures without additives, which is why it is the most widely used base in transformers and switchgear (the breakers and switches that open and close the circuit). Paraffinic insulating oil is also used. The type to use comes from the equipment specification; in Brazil, the National Petroleum Agency (ANP) calls them type A (naphthenic) and type B (paraffinic) in Resolution No. 900/2022.
Is the paraffinic oil used on crops the same as base oil? No. Paraffinic oil sold as an insecticide or as an agricultural adjuvant (a product mixed into the spray solution to improve how the crop protection product works) is a formulated product, registered with the agricultural authority for that use. Paraffinic base oil is a raw material: HBP-B, a light-colored paraffinic oil from Kelpen Oil, is indicated as a raw material for crop protection and agricultural product manufacturers. Base oil does not go straight onto crops.
Glossary
Base oil: the raw material that makes up most of a lubricant, before additives.
CA, CN and CP: percentage of the carbon in the base that sits in aromatic rings, in naphthenic rings and in paraffinic chains.
Viscosity index (VI): number that tells how much the viscosity drops with heat; the higher it is, the less the oil thins.
Pour point: lowest temperature at which the oil still flows.
Aniline point: temperature at which the oil mixes completely with aniline; the lower it is, the stronger a solvent the oil is.
Solvency: the oil's ability to dissolve additives, resins and polymers.
Dewaxing: refining step that removes wax from a paraffinic base so it flows in the cold.
Hydrotreating: treatment with hydrogen under pressure that removes sulfur, nitrogen and aromatics from the base.
Talk to the Technical Team
The Kelpen Oil Technical Team reviews the application, the viscosity, the operating temperature, the additive or polymer to be dissolved and your customer's requirements, and confirms the right base oil. If you already use a base and want to check what you receive, the Arujá (SP) laboratory runs an analysis of the oil you already use as a paid service. Save this article or send it to whoever buys base oil or formulates lubricants at your company.
