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Lubricant Reference - Base Oils
PETROLEUM
Petroleum is a naturally occurring
complex mixture made predominantly of hydrocarbons and other
compounds of carbon and hydrogen frequently containing significant
amounts of nitrogen, sulfur, and oxygen as well as smaller
amounts of metals. It can occur in solid, liquid, or gaseous
form as asphalt, crude oil or natural gas. The economic importance
of crude oil and natural gas has stimulated considerable interest
in their origins, but because of their fluid nature they are
mobile in the subsurface of the earth and may have accumulated
far from the place where they were formed.
The overall process leading to the accumulation of petroleum
in natural reservoirs can be summarized as follows: organic
matter is incorporated into sediments as they are deposited;
there is a possible shallow generation of biologically produced
methane ; organic matter is converted to petroleum-like materials
by the influence of increasing temperature with lower temperatures
being partially offset by longer times; a small percentage
of the lower molecular weight material that is generated subsequently
migrates from the source rock through permeable carrier beds
to the reservoir; and compositional changes may be produced
in the oil in the reservoir by increasing temperature, water
washing, and bacterial degradation.
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FORMATION
OF PETROLEUM BASE OILS
A mineral oil base stock is a
refined fraction of petroleum crude oil. Its characteristics
are a result of the crude source and its refining techniques.
Since petroleum crude oils are complex mixtures of many hydrocarbons
varying from those of low molecular weights and relatively
simple chemical structures on up to those of very high molecular
weights and very complex structures, the first step in oil
refining is fractions of roughly similar molecular weights.
Since the boiling points of separation of the crude into various
petroleum hydrocarbons are approximately proportionate to
their molecular weights, distillation effects a separation
of crude oils into fractions whose molecules are approximately
similar in size or weight. Distillation does not, however
affect any appreciable separation by molecular types, so that
lubricating oil fractions obtained by distillation contain
approximately similar proportions of paraffins, naphthenes
and aromatics as present in the original crude.
Raw lubricating oil fractions accordingly require additional
refining treatment in order to remove the undesirable constituents
and to segregate the more suitable components in the finished
stocks.
Waxy materials present in the base stock fractions may crystallize
and agglomerate or congeal at low temperatures and thereby
impede low temperature flow, These materials may be removed
by solvent dewaxing processes employing solvents such as methylethyl
ketone or propane, or by catalytic dewaxing.
These extraction and modification
processes can be carried out either before or after final
distillation into viscosity fractions. The choice depends
on the processes employed.
The physical and chemical properties
of the finished base stocks (often referred to as 'virgin')
will not be solely a function of crude source, but also will
be dependent on the processes employed and the extent of refining
employed.
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SYNTHETIC
BASE STOCKS
Certain chemical compounds have been
found to be suitable as base stocks for engine oil. These
are referred to as synthetic lubricants and are defined as
having been produced by chemical synthesis. These are manufactured
by organic reactions such as alkylation, condensation, esterification,
polymerization, etc. Starting materials may be one or more
relatively pure organic compounds. Generally of simple composition,
these compounds are obtained by chemically processing fractions
from petroleum, natural gas, vegetable, or animal oil components.
When vegetable or animal oil base lubricants are natural,derived
from non-petroleum sources rather than from synthesis, they
are not considered synthetic lubricants unless the naturally
occurring product has been chemically changed.
Classes of chemical compounds that
might be used as synthetic base stocks after processing are
shown in the following Table along with distinct generic identification
of the resulting fluids. A synthetic lubricant base stock
may consist of any of the fluids shown in the Table or a mixture
of compatible base fluids. This blending is usually practiced
to enhance physical properties.
Some synthetic base stocks are compatible with petroleum base
stocks, and the two types may be blended to obtain desired
physical and chemical properties.
Some synthetic base stocks are not
compatible with other synthetics or with petroleum base stocks.
Therefore, lubricants containing synthetic base stocks should
not be indiscriminately mixed.
The additive agents necessary in petroleum
base stocks, synthetic base stocks, or partial synthetic blends
intended for engine oils are also synthesized materials. However,
even though these materials are synthesized, they should be
referred to as additives and not included in the base stock
description.
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Examples of Synthetic
Base Fluids
Synthetic Hydrocarbons
Alkylated Aromatics
Alkylbenzenes Polyolefins Polyalphaolefins (hydrogenated)
Polybutenes
Organic Esters
Dibasic Acid Esters Adipates, Azelates, Dodecanedioates (diesters)
Polyol Esters Neopentyl or Hindered Esters Polyesters Dimer
Acid Esters
Others
Halogenated Hydrocarbons Chlorofluorocarbon Polymers Fluoroesters,
Fluoroethers Phosphate Esters Phosphate Esters of Isopropyl
Phenol and Cresylic Acids Polyglycols Polyalkylene Glycols
Polyphenyl Ethers Meta bis (m-phenoxyphenyl) benzene Silicate
Esters Disiloxane Derivatives Silicones Silicones
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