A lubricant can be anything, which controls the friction forces between two surfaces. A lubricant, depending upon the necessity and design can, either reduce or increase or control the friction.
A lubricant is made in a blending plant. In a blending plant, the base oils (which may constitute up to 99% of the lubricant, by volume) are mixed with specially selected additives. Before blending, the base oils are mixed etc. Blending is normally done in Kettles, where the oil is heated up to 600 C and heavily agitated.
Either mechanical or pneumatic agitation can be used. Additives are then added and the whole mixture
homogenised by thorough mixing – technically called blending. This is not a process whereby the chemical
characteristics change – there is molecular change. After blending, the product is filtered and then packed. In
special cases – like Transformer Oil – the product is degassed and thoroughly filtered.
The newer form of blending- especially used in large blending plants, is in-line blending, where the different
components mix while flowing inside a pipeline.
The two main constituents of a lubricant are base oils and additives. Base oils can be more than one (depending
upon the final product viscosity needed) and additives can either be a mixture (called an additive package) or
components.
Base Oil is the name given to basic building block of a lubricant. It is sometimes also called base stocks. Base
stocks are mineral (or petroleum) or synthetic origin, although vegetable stocks may be used for specialised
applications. The base stock provides the basic lubricating requirements of a lubricant.
However, unless it is supported with additives, base oil will degrade and deteriorate very rapidly in some
operating conditions. Depending upon the base stock, petroleum, synthetic or others, different additive
chemistries need to be used for making different kinds of lubricant.
Base oils are made and supplied by refineries. A refinery is a plant where the crude oil is distilled into various
fractions. Normally the last but one portion of the heaviest stocks is the lubricant base stock. The main base oil
producers are the big oil companies.
Base oils can be classified according to their basic chemical nature as to paraffinic, naphthenic or aromatic.
According to viscosity, they can be classified into various neutrals – 150 SN, 500 SN. SN stands for Solvent neutral
(see below) and the number stands for the viscosity of the oil is SUS (Saybolt Universal Seconds – see below – ) at
400 C.
Base oils can also be classified by the kind of treatment they get during refining. If they are made by being
selectively treated with different solvents, then the base oil is called solvent neutrals. If they are made by hydro-
finish process they are called hydro-finished base stocks. If they are made by the cracking of heavier molecules,
they are called hydro-cracked base stocks.
Base oils are made in the refinery by a number of processes – basically distillation. Distillation under atmospheric pressure removes the gasoline and distillate fuel components, leaving a “long-residue” containing the lube oil and the asphalt. Further distillation under vacuum yields “neutral distillates” overhead and an asphalt residue. Simple treatment with sulfuric acid, line and clay turns the distillate into acceptable LVI stocks.
For HVI and MVI stocks, some for of solvent extraction is necessary to remove colored, unstable and low VI components. Finally, wax is removed by dissolving the oil into methyl-ethyl ketone (MEK) and chilling and filtering to yield oils with pour points in the -10 to -20 C range. At the refiners option, the oils may be “finished” with hydrogen to remove sulphur, nitrogen and colour bodies.
Napthenic base oils are used when the final lubricant is expected to work at moderately lower temperatures.
Paraffinic base oils are used when the Viscosity Index requirement is high and the temperatures are not that much
lower. These are the largest kind of lubricating base stocks used. Aromatic base oils are seldom used – and find
their main application as processing oil.
Extra Solvent treated paraffinic and naphthenic base oils are used to make turbine oils. XHVI base oils, with very
low volatility, are used to make thinner oil s for the extreme cold whether. Synthetic base oils are used in
applications where the temperature is either too high or too low.
Base oils called mineral base oils because they are obtained from the mineral-deteriorated-crude oil. This is to
differentiate them from the “synthetic” base stocks.
They are called synthetic because their molecules are created rather than the naturally available one of the
petroleum based mineral base oils. For example: poly Alpha Olefin is synthesised in the laboratory by combining
small molecules of a chemical called decene.
The main advantages of the synthetic oils are in their high viscosity indexes, higher flesh points, lower pour points and very low volatility (tendency to evaporate at higher temperatures). This make them valuable blending components when compounding for extreme service at both high and low temperature.
The main disadvantage of synthetic lubricants is that they are inherently more expensive than mineral oils, and
are limited in supply. This limits their use to speciality oils and greases that command premium prices. Esters
suffer further disadvantage of greater seal-swelling tendencies than hydrocarbons: so, caution has to be exercised
in using them in applications where they many contact elastomers designed for use with mineral oils.
Synthetic base oils are used to make specialised lubricants for use in extreme conditions. For example: To make
Gulf Formula G which is 5W40 viscosity grade lubricant, it is a necessity to use synthetic base stocks, because
normal mineral based lubricants cannot work at the lower temperatures at which Formula G is expected to work.
Another mandatory application of synthetic fluids is the Fire Resistant Hydraulic Fluid.
Hydrofinishing is a process by which the base oil is treated with hydrogen, usually following the conventional
solvent refining to saturate olefins. Results in improved colour, demulsibility and foam characteristics. This is
different from the hydrocracking process.
Viscosity is a measure of a fluid’s resistance to flow. For lubricating oil in general, viscosity is the most important
controlling property. In a bearing that is operating properly with a fluid film between its surfaces, the viscosity of
the oil at the operating temperature is the property which determines the bearing friction, heat generation and
the rate of oil flow through the bearing under given conditions of load, speed and bearing design.
The oil should have a viscosity at the operating temperature that is correct for maintaining a fluid film between
the bearing surfaces, despite the pressure tending to squeeze it out. While a reasonable factor of safety is usually
desirable, excessive viscosity should be avoided because of unnecessary friction and heat generation.
Viscosity is also useful for identification of grades of oil and for following the performance of oils in service. An
increase in viscosity usually indicates that the oil has deteriorated to some extent, a decrease ordinarily indicates
dilution. The permissible extent of viscosity increase before corrective measures are taken is largely a matter of
experience and judgement of the operator.
The Flash point of a petroleum product is the temperature at which enough vapour is produced so that air-vapour mixture will flash in the presence of small flame. From the view point of safety,, flash points are of most significance at or slightly above the maximum temperature that may be encountered in storage, transportation, and use of liquid petroleum products, either enclosed or open containers.
In this temperature range, the relative fire and explosion hazard can be estimated from the flash point. For products with flashpoints below 100 F, special precautions are necessary for safe handling. Flash point is also used by manufacturers and marketers of petroleum products to detect contamination.
A substantially lower flash point than expected for the product is a reliable indicator that a product has become contaminated with a more volatile product such as gasoline. The flash point is also an aid in establishing the identity of a particular petroleum product.
The pour point of petroleum product is the minimum temperature at which the fluid will pour or flow under test
conditions. This is an indicator of the ability of an oil or distillate fuel to flow at cold operating temperatures. The
pour point of a product is often mistakenly taken as coldest ambient temperature at which equipment using the
product as fuel or lubricant can be operated.
The product in question may have a Cloud Point 10 to 200F above the pour Point and this may become the
limiting temperature. ( At cloud Point, the wax crystals in the oil start forming in the product, the wax crystals
tend to clog filter screens in fuel and lubricant circulation systems, preventing proper equipment operation)
This problem can occur in situations where a product has been artificially lowered by means of an additive (Pour
Point Depressant) the additive does not lower the Cloud Point. There are other reasons for setting a minimum
operating temperature well above a product’s Pour Point especially with lube oils.
The viscosity may become so high at the pour point that large amounts of power are wasted overcoming the high
fluid friction. Insufficient lubricant flow to location where needed is another consideration.
Additives are used in lubricating oil to change or alter or enhance its properties. Base oil as such cannot be used in
most of the present-day lubricating applications. Their properties – like resistance to heat, oxygen, wear etc – have
to be increased. This increment is done with the use of these additives. To increase the resistance to oxidation, we
add ‘antioxidants’, to increase resistance to wear, we add ‘antiwear additives’
Oxidation Inhibitor Prevents varnish and sludge formation on bearings or in circulating systems. Retards aging of
the oil. Lengthens service and storage life of oil.
Protects oil itself directly (indirect protects metal parts – varnish and acids) Reacts more readily with oxygen (from
air).than does the oil itself, thereby retarding oxidation of the oil. Rust Inhibitor Prevents rusting of ferrous (iron or
steel) machine parts Forms a film on ferrous metallic parts thus protecting them from attack by water or other
destructive material.
Corrosion Inhibitor Prevents corrosive attack on non-ferrous metallic surfaces Forms a film on non-ferrous
metallic parts thus protecting these parts from attack by contaminants in the oil. Detergent Prevents oxidation
products (sludge) which have formed in oil from sticking to metal components.
May also remove deposits already formed on metallic components. Usually combined with dispersant additive By
chemical reaction, oxidation products (sludge) remain soluble in the oil and do not stick to the metal surfaces.