Synthetic base oils
There are 5 types of base oils used for the formulation of lubricants. The 2 last groups are called Synthetic Oils.
Synthetic base oils are made up of molecules of identical structure and they are scientifically designed. The properties of these base oils are consistent and highly predictable unlike those of mineral oils.
If we compare synthetic base oils with mineral base oils, we can notice the following differences.
| Characteristic | Mineral Base Oil | Synthetic Base Oil |
|---|---|---|
| Molecular structure | Million combinations | Repeating molecules |
| Molecule size (weight) | Varies a lot | Constant |
| Purity | Cotains aromatics, wax, sulfur and nitrogen | Very pure |
The main characteristics of the most common synthetic base oils are shown below:
GROUP IV (PAO - POLYALFAOLEFINE)
Production process: Pure branched hydrocarbon, made from ethylene gas (C2H4) and decene (C10H20).
Advantages: Low pour point, high visocity index, excellent thermal stability, low volatility, very good oxidation stability, compatible with mineral oils.
Disadvantages: May shrink seals (leaks), poor solubility with additives, susceptible to settling of varnish-like insolubles.
Applications: Engine oils, gears, turbines, bearings, hydraulic, compressors and high temperature greases.
GROUP V (PAG - POLYALKYLENE GLYCOL)
Production process: Made by polymerization of alkylene oxides: ethylene oxide (EO) - soluble in water or propylene oxide (OP) - insoluble in water.
Advantages: Natural detergency, high viscosity index (160-280), does not form deposits, good lubricity and good thermal and oxidative stability.
Disadvantages: Incompatible with polycarbonate sight glasses, not compatible with other fluids, may soften some paints and coatings.
Applications: Refrigeration compressors, water soluble brake fluids, water soluble fire resistant fluids, gas compressors (low solubility with gas), high temperature and worm gears, chain lubrication, metal working and tempering, food grade H1.
GROUP V (DIESTER)
Production process: Made by reacting an oxygen-containing acid with an alcohol.
Advantages: Viscosity index above 140, pour point below -50 ° C, natural detergency, good oxidation stability.
Disadvantages: Poor hydrolytic stability, can swell seals and remove some paints, requires special additives, poor demulsibility.
Applications: Oil for compressors, bearings, gears, high temperature greases, co-base with PAO.
GROUP V (POLYOL ESTER)
Production process: Made by a reaction of an acid with an alcohol containing two or more OH groups.
Advantages: Viscosity index between 130-190, thermal stability, better hydrolytic stability than diesters, oxidation stability, pour point from -30 to -70ºC.
Disadvantages: It can swell seals and remove some paints, risk of hydrolyzing, not compatible with other fluids.
Applications: Oils for aviation turbines, compressors, gears, internal combustion engines, high temperature applications in general.
GROUP V (PHOSPHATED ESTER)
Production process: Made by reaction between alcohols and phosphoric acid. The triaril version does not support its own combustion (fire resistant). The trialkyl and butyl versions do support combustion (not fire resistant).
Advantages: Excellent fire resistance, good thermal stability, pour point ranges from -25 to -5ºC, excellent boundary film lubrication property.
Disadvantages: Very low viscosity index (from 60 to -30), degradation products are phosphate soaps (consistency like black mud), can be hydrolyzed.
Applications: Fire Resistant Hydraulic Fluids, Aviation Hydraulic Fluids, Electrohydraulic Steam Turbine Control (EHC) Fluid.
GROUP V (SILICONES)
Production process: Polymeric composition with replacement of the carbons in the central structure by silicon.
Advantages: Very high viscosity index (300 or more), chemically inert, water repellent, low volatility, excellent thermal and oxidative stability.
Disadvantages: High compressibility, oxidation products are abrasive (silicon oxide), low surface tension, poor wear protection, poor solubility with additives.
Applications: High temperature greases and oils, lubricants exposed to radiation or oxygen, brake fluid.