TYPES OF BASIC OILS
Mineral base oils (Min)
Mineral oils are extracted from crude oil and, depending on their structure, may be paraffinic, naphthenic or mixed base oils. Thanks to vacuum distillation, these oils are free of undesirable components which could have a negative impact on, for example, their aging stability or corrosion resistance. The viscosity-temperature behavior is average, ranging from 70 to 100, however it can be increased with viscosity index (VI) improvers. These are oils used to formulate most multipurpose greases for industrial purposes, but also for mining, civil engineering and construction. Their use is widespread because of their low cost and compatibility with a wide variety of materials and paints that may be present in the components to be lubricated.
Polyalphaolephine base oils (PAO)
Synthetic hydrocarbons (HC), such as polyalphaolephines, are oils whose low temperature fluidity and resistance to oxidation are far superior to those of mineral oils. Their viscosity-temperature behavior is very good, between 130 and 140, and it can even reach values of 180 for polyalphaolephines last generation. Thanks to its compatibility with a wide variety of metal, plastic and elastomer components, it is the most widely used synthetic base oil in the automotive industry. With its many applications, both inside and outside the cabin, where, in addition to the respect of the materials that surround it, excellent performance at low temperatures is required. Thanks to the high viscosity PAO oil, it is also suitable for the formulation of greases for high temperature applications. They are perfectly miscible with mineral oils. It is therefore common to find fats with a mixture of the two base oils. Polyglycol base oils (PAG)
In terms of resistance to oxidation, freezing point and polarity, polyglycols or polyalkylene glycols exceed mineral oils and polyalphaolefins. The natural viscosity index can vary from 160 to 200. Because of their low coefficient of friction, they are also used on helical gears because they improve sliding under heavy load. Depending on the type of polyglycol oil used in the base and the temperature, it will be necessary to ensure the miscibility of the greases. In general, polyglycol oils should not mix with mineral oils or PAOs. They have excellent compatibility with plastics and elastomers, especially with those used to support extreme temperatures or exposed to aggressive media (eg EPDM).
Ester base oils (E)
Its natural viscosity-temperature behavior is excellent and the losses by evaporation are lower than those of mineral oils and polyglycols. They can have natural viscosity index values up to 170, when these are high quality synthetic esters. Its fluidity at low temperature and its anti-wear behavior are better than those of mineral oils. Most ester oils are perfectly miscible with mineral oils. One of the main advantages of ester base oils is rapid biodegradability: they become indispensable. Silicone base oils (Si) The silicone base oils have a viscosity-temperature behavior much higher than that of the above-mentioned oils, by their viscosity index of up to 300. They are thermally stable up to 300 ºC, and have a low volatility. These base oils have a low load capacity and protection against corrosion and very low wear. Being chemically inert, they are the ideal basis for the formulation of greases respectful of plastics and elastomers, as well as for those intended for the lubrication or welding of elements exposed to aggressive environments. They can be used in the food industry because they are physiologically safe. They are not miscible with mineral oils or synthetic oils.
Polyether oils (PEP / PFPE)
Phenolic polyether (PPE) and perfluorinated (PFPE) oils have high thermal stability, good load capacity and extremely low volatility. It is for this reason that they are the basis of fat formulation at high temperature. They are inert to chemical reactions and have excellent resistance to aggressive agents such as acids, alkalis, fuels, gases and solvents. The perfluorinated polyether oils are not flammable and have a very high compatibility with plastics.
The behavior of synthetic and mineral oils, in contact with metals, plastics, paints and elastomers, must be systematically controlled in its actual conditions of use, especially when it comes to lubricants applied in the production of series components, including the start-up can be done some time after the application of the lubricant.
CHOICE OF OIL
The choice of the base oil for the formulation of a lubricating grease depends on several factors. In general, mineral oils are used for the lubrication of bearings and other mechanical elements that work under standard conditions. However, it is necessary to choose synthetic oils when one meets more severe conditions: resistance to extreme temperatures, high loads, vibrations, aggressive chemical environments or other physicochemical phenomena.
CHOICE OF VISCOSITY
The lubricating film, directly related to the viscosity of the selected base oil, separates two contact surfaces and prolongs the life of the mechanical components, thanks to a reduction in friction, wear and fatigue . The machines are subject to a combination of requirements that require tradeoffs in choosing the viscosity of the base oil.
Classification of viscosity ranges generally used for specific applications and requirements:
- Heavy load, low speed or high temperature: 2000-320 mm2 / s
- General industrial applications (moderate load and speed): 320-150 mm2 / s
- Medium / high speed Light load bearings. Motors and other machines: 150-46 mm2 / s
- High or very high speed (machine tool) Low temperatures: 46-10 mm2 / s
In some very special cases, the operative viscosity can not be reached because: The choice of oil is limited by the requirement of a machine element that requires a lower viscosity grade. The application requires the use of a grease with a more fluid base oil, which helps dissipate heat and remove pollutants from the bearing. The work is performed at high temperature or very low speed, which requires a theoretical operating viscosity that is impossible to obtain. In this case, it is usual to reinforce the formulation of the grease by adding extreme pressure additives (EP) or solid additives, depending on the application, which minimize the effects of wear, fatigue, abrasion, etc