FUTURE BASE FLUIDS CLASSIFICATION
PROF. M. C. DWIVEDI
CONVENER, LAWPSP* SYMPOSIUM
20A/ 1004 - 10th
Floor, MHADA, POWAI, MUMBAI-400076, INDIA
API classification of base
oils (1990) has been a common criterion for
procurement of base oils by manufacturers and
blenders of lubricant and functional fluid
products. This also severed as a bench mark to the
refiners and trade. However, with the rapid rise in
the crude oil prices, and changing refinery process
configurations, availability of base oils is reduced
and prices increased sharply. Prices of non
petroleum fluids; particularly vegetable oils,
remained relatively stable. This is making many of
these fluids cost competitive and viable
alternatives. Superior native tribological
properties and eco-compatibility makes these fluids
preferable over petroleum based fluids. The future
may therefore see multiple options for base stocks
and performance specific products.
Non-edible vegetable oils
are at lower price than petroleum base stocks and
use of these as base stocks for functional fluid
formulation makes a strong economic sense. Several
other fluids like water or glycerol etc. may workout
economical than petroleum fluids for some
applications. This necessitates a need to
consider functional fluid base stocks as ‘Fluids’
rather than ‘Petroleum Based Fluids” This
pressure will get more intense in future as the
prices of petroleum base oils increase. This is
likely to happen due to following reasons.
1. Increase in crude oil price
2. Decreasing production of base oils due to
changes in refining strategies and technologies.
3. GTL processes may provide substantial
part of base oils in future, but the prices are
likely to be higher than group-I base oils.
4. Increasing cost of
technical services and marketing and low returns
from functional fluid business.
On the other hand, prices
of vegetable oils are likely to remain stable or
even reduce due to following reasons.
Increase in acreage of oil seed cultivation
in Fareast and African countries.
Introduction of high yield varieties of oil
seeds and scientific inputs to agriculture.
Increasing trend of avoiding consumption of
Increasing availability of used frying pan
Standards and literature developed over five decades
to define quality and application practices of
functional fluids are mostly by petroleum industry
with no consideration for other stocks. Many of
these tests are non-scientific and empirical and
have relevance only for petroleum products.
The need for the future is therefore to develop
standards and test protocol,
based on tribological requirements and performance
characteristics of fluid and
eliminate those tests which are biased for petroleum
The issue is to be considered from the view point of
base oil selection, product classification and
(A) BASE FLUIDS
Base fluids are classified as follows.
1. Hydrocarbon fluids
(a) Petroleum based fluids–
(b) Synthetic hydrocarbons, including GTL and STL
2. Vegetable oil based fluids
(a) Native Vegetable Oils (VOs)
(b) Chemically Modified Vegetable Oils (CMVOs or
3. Synthetic ester and ether fluids
(a) Hydrophobic ester fluids
(b) Hydrophilic ester fluids
4. Water based fluids
Fluids containing less than 95 % water
(b) Fluids containing more than 95 % water
Not classified above like, silicon fluids, organo
halogen fluids, etc. and innovative and emerging
1(a) HYDROCARBON FLUIDS
These are common base fluids; products of petroleum
refinery which have dominated after 1930s. These are
constituted of hydrocarbons, generally of paraffin
and naphthene groups. Petroleum based fluids are
constituted of thousands of different hydrocarbon
molecules over the specified boiling range
(molecular size). The mixtures are complex and
random and no two oils are same.
These are characterized by physico-chemical
characteristics. Chemical compositions are averages
and identity of molecule is unimportant. Petroleum
fluids have wide and flexible application range and
meet 85 to 90% of all industrial, defense and
domestic; tribological fluid demand. These are
abundantly available at low cost.
These have wide viscosity range, moderate viscosity
index, moderate low temperature characteristics,
excellent thermal and oxidation stability, good
water repellency and high hydrolytic stability.
These have low to moderate extreme pressure
properties and adherence to metal surface. These
have low biodegradability(20to30percent) and
moderate phytotoxicity and low eco-compatibility.
1(b) SYNTHETIC HYDROCARBONS
Synthetic hydrocarbons are tailor-made hydrocarbon
molecules designed to give
superior performance characteristics. These are
synthesized by a variety of reactions.
Rebuilding the molecules from basic olefins
like, propylenes, butylenes
etc. followed by hydrogenation or alkylation to
2. Olegomerisation, dimerisation, trimerisation of
larger olefins followed by
hydrogenation and /or alkylation.
3. GTL and STL processes designed to synthesize
These fluids have excellent performance
characteristics like high VI, high oxidation and
thermal stability, high water resistance and
hydrolytic stability, low volatility and evaporation
losses, excellent surface properties and EP
characteristics, and excellent low temperature
characteristics. These fluids are designed for
specific service conditions have limited additive
compatibility requiring specific additives packages.
The quality tests and standards as applicable to
petroleum base fluids are applicable to these fluids
as well. These are compatible to petroleum base
fluids. These are predominantly used in IC engine
oils, turbine oils and other applications requiring
wider temperature range. Like petroleum oils these
have low biodegradability, low eco-compatibility and
mild to no phytotoxicity.
2. VEGETABLE OIL BASED FLUIDS
Vegetable oil based stocks for functional fluids and
tribological applications are placed in two
(a) Native Vegetable Oils (VOs.)
(b) Chemically Modified Vegetable Oils (CMVOs or
Native vegetable oils or those with little
processing have several functional characteristics
superior to mineral oils. These include.
1. High flash point and fire point
2. Better oiliness, lubricity and EP characteristics
3. Lower volatility
4. High viscosity index
5. Total biodegradability and eco-compatibility
VOs have some adverse characteristics.
1. High Pour Point –Some
VOs have high concentration of saturated linear
compounds of C16
carbons, with high pour point. This could be
adjusted to product application requirements by
dewaxing / deguming, blending and also by use of
Limited Kinematic Viscosity Range.
- Most of the vegetable oils have viscosities
varying in narrow range 30 to 45 cSt. at 40oC,
because these are constituted mainly by
triglycerides of 7 to 9 fatty acids of C16,
carbon atoms in different structural configurations
and ratios. Some oils like jojoba oil have lower KV
( 23 to 25 cSt.), castor oil has higher KV (256 to
300 cSt). at 40oC. These could be
viscosified by blending small quantities of
vegetable waxes, animal waxes or oxygenated
synthetic polymers like polyglycols. Oxidized VOs
particularly waste oil from food processing have
higher KV and could also be used for blending. Thus
base oil with KVs at 40oC from 25 cSt. to
300, cSt. may be formulated by blending of different
native vegetable oils. Chemically modified VOs like
alkylated esters, dibasic, tribasic and polyol
esters, dimerised, trimerised, polymerised VOs. and
estolides etc. cover higher ranges of viscosity
( up to 4000 cSt. at 40oC). Fatty
acid esters have lower KVs (2 to 10 cSt. at 40oC).
Thus an assortment of native VOs, and MVOs and their
blends can provide
base stocks for the entire range of functional fluid
– Most VOs have substantial unsaturation and are
susceptible to oxidation, particularly in high
temperature applications. Unsaturation can be kept
within limits by blending with saturated stocks.
There is not likely to be any difficulty for lower
temperature applications as these olefins are
thermally stable and resist oxidation. These could
also be protected by use of appropriate antioxidant
The present oxidation tests are designed for
petroleum products and oriented to high temperature
applications. High test temperature accelerates
oxidation and forms different oxidized compounds
than those formed at slow oxidation at lower
temperatures. This may misrepresent performance of
oils meant for lower temperature applications.
There is serious need to revalidate oxidation tests
for deferent application temperature ranges.
measured by Iodine Value (IV) or Bromine Number.
Mineral oil base stocks have very little
unsaturation ( IV< 2). For VOs unsaturation level is
high (IV – 60 to 180). This is equivalent to about
30 percent molecules being monounsaturated.
It is essential to include IV in the
test protocol to have an idea about the
concentration of unsaturated
molecules in the product. This will also indicate
chemical nature of the stock.
– Petroleum base stocks are generally neutral and
acidity limits are low for most products ( less than
0.2 mgKOH/g). Vegetable oils have up to 3 percent
free acids, acidity level may be as high as 1.5mg
KOH /g. Presence of acids in products is detrimental
as it increases rusting and wear of the metal.
Acidity in inorganic form is more corrosive than
that in organic form. Organic acids with short
organic, chains ( less than C6),
are corrosive and reactive to metals.
Larger organic acids like those with C16
and above are non-corrosive and do not react to
metals at lower temperatures (less than 80oC).
Iron salts of fatty
acids may form at a very slow pace and when in
critical concentration these may
act as oxidation catalysts to oxidize the oil,
forming more acids. This chain
reaction may be terminated by use of additives.
Vegetable oils have no inorganic acidity. Acidity is
because of presence of free fatty acids (FFA). These
are non corrosive at low temperatures. However to be
on safer side a limit on FFA for the base stocks
will be desirable.
FFA limit of 0.5 percent is suggested. This
corresponds to about 1 mg
This should work but more rational limits could be
fixed after detailed scientific and statistical
evaluation of data.
5. Saponification Value (SAP) -
SAP value indicates saponifiable material present in
the oil. For mineral oils this is small (less than
1) and indicates presence of additives which may
hydrolyze and form soaps with alkali (KOH). For
synthetic esters SAP value may range from 60 to 120.
For most native vegetable oils it is around 80 to
190. SAP value does not indicate any adverse
tribological property. It indicates possibility of
hydrolysis in presence of moisture particularly at
elevated temperatures (more than 120oC).
In a situation where multiplicity of lubricants is
assumed, it may help in guessing the type of fluid.
6. Environmental and Toxicological Issues -
VOs are fully biodegradable and eco-compatible.
Some additives or blending components may have
limited biodegradability, this may reduce the
biodegradability of product. As these constituents
are in small concentrations, more than 90 %
biodegradability of product is assured.
Toxicity may sometimes be a problem if VOs used are
non-edible or used VOs. Oxidized used oils have no
toxicity. Non edible VOs are toxic, because of
presence of some minor toxic constituents. Toxicity
is mild and some of these are being used for soap
making and other applications. Toxicity of VOs could
be removed or reduced by detoxification processes.
However, it would be essential to specify some
limits for human and animal
2(a) NATIVE VEGETABLE OIL BASE STOCKS
Chemical structure of the constituent molecules of
VOs has some basic limitations.
1 Presence of unsaturation in some
2. Presence of slightly polar triglyceride
3. High pour point due to presence of linear
saturated chain compounds.
4. Presence of hydroxyl ‘OH’ group in some
VOs like castor oil.
5. Presence of toxic compounds in low
These limitations could be controlled by a variety
Therefore in spite of structural limitation native
VOs could be used
as base stocks for formulation of all types of
tribofluids for low and moderate
( less than 80oC)
This constitutes 60 to 70 percent of total
functional fluids demand.
2(b) CHEMICAL MODIFICATION OF VEGETABLE OILS
For extreme pressure, high temperature and cryogenic
applications, vegetable oils can be chemically
modified to create molecules of desired size and
configuration. Constituent molecules of VOs serve as
basic blocks to build these molecules. Size or
carbon number and structural configuration can be
changed. These fluids may be designed to give.
1. Low pour point and low temperature properties.
2. Excellent EP characteristics
3. Wide viscosity range and high VI
4. High thermal and oxidation stability
Chemical reactions generally used to modify VO
molecules are -
by appropriate organic alcohol to built desired
carbon number linear chain of esters. This also
reduces polarity of the molecule.
of unsaturated sites by an appropriate organic group
to eliminate double bond and to create side chain to
give lower pour point and higher thermal, oxidation
and hydrolytic stability. The carbon number can also
be changed by proper selection of alkylation groups
to get proper KV.
or self alkylation and condensation of olefin bonds
to form large isoalkyl molecules followed by
hydrogenation to saturate the residual unsaturation.
Such molecules give excellent EP, high oxidation
stability, high VI, and excellent low temperature
performance characteristics. Fluids of wide
viscosity range (KV-30 to 4000cSt. at 40oC)
could be created.
of double bond gives molecules with high thermal,
oxidation and hydrolytic stability. These will also
have excellent corrosion protection and rust
prevention. However epoxy molecules are known to be
incompatible to other oil constituents and may not
give physically stable formulations. Special
additives system will be required for such fluids.
In addition, there are other chemical reactions
which may be used to suitably alter structure of VOs
constituents. Such functionally oriented molecules
derived from VOs can give functional fluids to meet
all extreme conditions of application.
Thus VOs, MVOs and their blends can give functional
fluids for all types of tribological applications.
3. SYNTHETIC FLUIDS
Synthetic fluids were developed during the war in
1940s to meet extreme military requirements. Later
these found acceptance in aerospace, power
generation heavy construction and earth moving
machinery. These perform under extreme conditions of
temperature and load where petroleum based fluids
fail. However, over the last fifty years; their
market share has not exceeded ten percent because of
high costs. These fluids are synthesized from basic
hydrocarbons, petrochemicals, or natural raw
materials. These are classified based on chemical
structure. The general nature of these molecules is
3.1 SYNTHETIC HYDROCARBONS
These are hydrocarbons with appropriate chemical
structure and linearity to give
desired performance characteristics. These
constitute major part (60-70 percent)
of synthetic lubricants in automotive sector. These
discussed in (1b).
3.2 ESTER AND ETHER FLUIDS
Esters, ether and such derivatives constitute about
20-30 percent of synthetic fluid demand. These are
of mono, di, or tribasic esters. Esters of diols,
triols and polyols also form molecules of similar
structure. The esterification groups are selected
based on requirements of KV and VI of the fluid and
desired performance properties. Number of carbon
atoms and polarity could be changed by appropriate
selection of esterification group. Corresponding
ethers also have similar tribological
characteristics. These are high performance fluids
recommended for high load, and extreme conditions of
temperature. These have high temperature oxidation
resistance and stability, good low temperature
viscosity. In most cases hydrolytic stability is low
particularly as the acidity increases during
Biodegradability of these is from 40 to 70
percent. Eco-compatibility and toxicity vary widely.
These are expensive fluids used for critical
applications like, aviation, high temperature gas
turbine lubrication for power generation, heavy
metallurgical machines etc. Some of the derivative
esters like; organic phosphate esters are used as
fire retardant power transmission fluids in aircraft
and metallurgical operations.
Synthetics differ from CMVOs as these are pure
compounds with narrow range of properties, while
CMVOs are complex mixtures of several such esters
over a wider carbon number range.
Organo-esters from glycols, diols, triols, polyols,
ethylene oxide and propylene oxide condensed
alkylene glycols, particularly those with short
organic chains are hydrophilic. These are used in
tribological applications such as fire resistant
fluids, metal working fluids etc. In some power
transmission fluids applications, these are used in
pure form. These fluids can be formulated in desired
viscosity range, and have high density, fire
resistance, high heat capacity, high thermal and
oxidation resistance. Some of these have high
linearity and high VI. Biodegradability is medium 30
to 60 percent) and eco-compatibility may vary
4. WATER BASED FLUIDS
Water based fluids are used in several functional
applications over moderate range of temperatures (4
These are, low cost, in desired viscosity range,
high VI, highest heat capacity and eco-friendly
fluids. A group of these are not true solutions and
are emulsions, suspensions / dispersions of
hydrophilic polymers in water. These are shear
sensitive and breakdown at high shear rates. These
are classified in two groups.
4.1 ORGANIC AQUEOUS SOLUTIONS
These are true solution of polyhydric organic
alcohols, like glycols, glycerols etc. in water at
various concentrations covering the whole range of
viscosity from 1 to 16000 cSt. at 40oC.
There are two types of these fluids.
1. More Than 95 % Organic Constituent
These are aqueous organic solutions of polyhydric
alcohols, amides and amino derivatives. The cost is
high because of substantial concentration of organic
component. Application temperatures are extended
below + 4oC
and above 60oC
depending on concentration of organic component.
Glycols, glycerol; PVA solutions have freezing
temperature well below ─15oC
and boiling point well above 130oC.
Water sugar solutions also give high viscosities and
may be used for some applications. These have high
eco-compatibility, high fire resistance, high heat
capacity, high oxidation and thermal stability, high
VI. These are widely used as fire retardant fluids,
cutting and metal working fluids etc.
2. Less Than 5 Percent Organic Constituents
These are less expensive, moderate viscosity, high
heat capacity and highly eco-compatible
formulations. The application temperature range is
narrow (4 to 60oC).
This covers more than 70 percent industrial
applications at atmospheric conditions, like metal
cutting, metal working, quenching, grinding etc.
These can also be used as fire retardant hydraulic
Water mineral oil emulsions are used in applications
like metal cutting, grinding, rolling, stone cutting
and grinding. Some of these have more than 5 %
mineral oil ( 10-30 %) while others have less
than 5% mineral oil ( 3 to 0.5%).
4.2 WATER POLYMER SUSPENSIONS AND GELS
A small amount of water soluble polymer can give a
solution with viscosity as high as 16,000 cSt. For
example 0.1 percent polyacrylamide can viscosify
water to about 200 cSt. and give a gel with KV
16,000 cSt. at 40oC,
at 2 percent concentration. These are not true
solutions and are shear sensitive and breakdown at
high shear rate. These are also temperature
sensitive and cream out at temperature above 80oC.
These are eco-compatible and have highest heat
capacity. Many of these have been used for
atmospheric applications as cutting fluids,
hydraulic fluids, gear oils, etc.
(B) BASE OIL CLASSIFICATION
B.1 API CLASSIFICATION OF BASE OILS
Performance of a functional fluid depends on base
oil composition as the base oil constitutes more
than 95 % of the product. All rheological, and
physico-chemical characteristics of fluid are
attributes of the chemical structure of the
constituent molecules of base oil. It is the base
oil that performs and carries out the intended
mechanical function. Additives provide marginal
enhancement to specific characteristics. The
molecules constituting the base oil are subjected to
physical stresses and chemical severity. Their
chemical or morphological breakdown means breakdown
of the fluid. Therefore base oil quality is primary
requirement of oil formulations. Base oils are bulk
products and a general quality index is required to
assess suitability for a particular application. The
common criteria for petroleum and hydrocarbon base
oil quality are “API Classification”. This grades
base oils in five groups based on three
1. Sulfur content,
2. Saturates content,
3. Viscosity Index
1. SULFUR CONTENT
is a minor constituent and does not influence bulk
rheological or physicochemical properties of the
fluid. Sulfur compounds are also incorporated as
additives. Some sulfur compounds present in base oil
may be corrosive; these could be taken care by use
of appropriate additives, while others impart
lubricity and EP characteristics. In all the
functions where the fluid remains in liquid form;
little sulfur (0.3%) present in the oil cannot be a
potential environmental or health hazard. In IC
engine oils, a part of lubricating oil may burn or
evaporate with fuel, but the quantity as compared to
fuel is so small that it cannot contribute any
significant sulfur concentration in the exhaust
gases. Special significance attached to sulfur, in
API classification appears to be out of place.
2. SATURATES CONTENT
content in group I base oils is below 90 percent and
for other groups it is more than 90 percent.
Saturates mean n-paraffins, i-paraffins, and
cycloparaffins or naphthenes.
have high pour point, high viscosity index, and also
high emulsibility. I-paraffins have moderate
to low, VI, low pour point, high oxidation stability
and low emulsifiability.
have low VI, low pour point, high oxidation and
hydrolytic stability. Some characteristics oppose
each other, therefore an appropriate combination of
saturates will result in an optimum performance and
quality of base oil.
have to be less than 10 percent in all the base oils
except for those of group-I. Aromatics are
characterized by low VI, low pour point, low
oxidation stability and low emulsifiability. These
also have higher carbon formation tendency.
Therefore while aromatics are undesirable for IC
engine lubrication where oil is exposed to high
temperatures and is likely to decompose, these are
desirable in gear oils and hydraulic oils where
oiliness is important. Even though, aromatics are
undesirable their presences in the range 8 to 12
percent enhances overall oxidation stability.
Appropriate level of aromatics can enhance the oil
is another group of hydrocarbons found in distillate
stocks in small concentration (up to 2%). These are
more reactive and have poor oxidation stability and
high carbon forming tendency. At low concentration
(up to 2 %) these may offer better EP
characteristics in gear oils, particularly for low
temperature high load applications.
The API base oil classification is not comprehensive
and does not identify these differences within
‘Saturates’ or functions of ‘Aromatic’ constituents.
This may lead to inaccuracy in selection of proper
quality of base oil.
3. VISCOSITY INDEX
is another parameter of API base oil classification.
VI is high (90 to100) for normal paraffins and
low (80-95) for i-paraffins and naphthenics.
Aromatics have lower VI ( 70-80). Not all-i-parrafins
have low VIs; it dependent on the linearity of the
molecule. If a free linear chain is more than 8
carbon atoms VI will be 90 or higher.
Lube oil base stocks have carbon number from C26
Therefore most iso-paraffins with one side chain
will have free linear chains of more than 8 carbon
atoms and VI higher than 90. Iso- paraffin with two
or more short iso-chains will have rigid molecules
with lower VIs. Thus VI is not an independent
property; it is derivative of ‘Saturates’
concentration and chemical structure of saturate
constituents. Range of VI indicated in API
classification (80-120) is too wide to be index of
quality. Range of VI for mineral base oils is from
80-95, except for some synthetic oils of group III.
range of 80-120 appears to be absurd for group I and
II oils. Such a range makes it impossible to make a
distinction between good or bad quality base oils.
Selection of base oil depends on viscosity and these
are designated accordingly (150N, 300N, 500N,
etc.). Viscosity grade oils are prepared by
blending high and low viscosity oils. Viscosity can
also be modified by using small quantity (1-2 %) of
polymers as viscosifier. Such blended stocks have
poor shear stability and do not give as good
performance as distillates of the same viscosity
Viscosity of the stock is
required to be specified with API group to correctly
specify the base oil.
API grouping does not address to
performance and composition factors and cannot
provide performance or quality assurance. This can
be made comprehensive and more accurate by
incorporating additional parameters and information
to give better quality reflection.
B.2 Standard Guide For Characteristics of
Hydrocarbon Lubricants Base Oil
This has larger spectrum of properties, but does not
quantify (Table-2). It is unnecessarily loaded with
extra concern for presence of minor constituents;
metals etc. Specifications are preoccupied with the
base oils from re-refining. This is a
futile exercise as, for a user it does not matter
whether the base oil is virgin or re-refined.
There is no way how two types of oils could be
In absence of any quantitative limits for the base
oils of different viscosity ranges, ASTM D- 6074 is
of no use. Specification limits of contaminants are
also meaningless as base oil is not the end product.
The end product is formulated from base oil by
blending appropriate additives package. Pollution
potential of end product for a particular
application should be considered for environmental
impact. How and in what form the product is used and
what is rejected in the environment, is important;
which should have rational limits. There is no
logic in applying contamination limits on base oil
or intermediate products.
Limits on concentration of each contaminant in a
product should be decided based on detailed
evaluation of environmental impact.
MODIFIED BASE OIL CLASSIFICATION
In an earlier paper, a more comprehensive
classification of hydrocarbon base oils (Table-3)
in line with the API has been suggested. This
includes two additional parameters (1) Kinematic
viscosity at 40oC
(KV) and (2) Linearity Index (LI) that take care of
chemical structure of molecules constituting the
oil. API designation requires KV at 40oC
to be specified with any group, like “150N Group I”.
In the suggested classification each group is
divided into five sub-groups of common KV values as
Sub group Kinematic viscosity at 40oC.
(a) 13 cSt. (70SUS) and lower
(b) 32 cSt. (150 SUS)
(c) 65 cSt. (300 SUS)
(d) 110 cSt. (500 SUS)
(e) 150 cSt. (750 SUS) and above
Group I, II and III are essentially those in API
classification, except that there is special mention
of GTL and STL products and group II+. Group IV also
includes all other synthetic hydrocarbons besides
PAOs. Even though better than API, for hydrocarbon
stocks, this classification does not take care of
other base stocks. A more comprehensive product
identity and quality is desirable to meet the
challenges of the future.
PERFORMANCE CHARACTERISTICS AND MOLECULAR STRUCTURE
OFBASE OIL CONSTITUENTS.
Physical, physico-chemical and performance
characteristics of base oil are attributes ofthe
chemical structure and nature of the constituent
molecules. Influence of these molecular parameters
are discussed as follows.
1. MOLECULAR SIZE
Blending of close viscosity oils gives better
products rather than blending of dissimilar oils.
Viscosity should be natural molecular viscosity and
not the made up blend viscosity. The oil should have
least variation of molecular size or carbon number.
This is controlled by “Boling Range”. Oils of narrow
boiling range have less variation in molecular
weight and size and are better base oils. These also
have lower volatility losses. Viscosity, density and
other bulk properties depend on molecular weight.
2. MOLECULAR STRUCTURE
Intensive characteristics like, VI, pour point etc.
depend on the type of molecule i.e. paraffins,
i-paraffins, naphthenes, aromatics, diaromatics etc.
Chemical characteristics like thermal decomposition,
oxidation stability, EP characteristics etc. also
depend on molecular structure. No single group of
hydrocarbons has all the desirable characteristics
of a functional fluid. Therefore an optimum
combination is to be formulated for a particular
A large number of studies have been reported on
influence of oil constituents on performance. Low
temperature studies are confined to effect of oil
constituents on VI, and EP characteristics. EP
effect is often not clear because of presence of
sulfur or additives. High temperature studies mainly
cover oxidation stability or carbon and varnish
formation in IC engines. Interfacial properties like
foaming, air release, demulsification, etc. are
sensitive to contaminations. It is not possible to
make generalizations as contaminants are not
In general, influence of molecular size and
structure on fluids characteristics is summarized as
1. Functional fluids have carbon numbers from C24
2. Each ring compound group has 6 carbon atoms,
(sometimes five). Higher and
lower carbon rings are not present. Fused ring
components and diaromatics are
less than 2 percent.
3. VI is high for hydrocarbons with more than 8
carbon free chain length.
4. Oxidation stability is low, for free linear
chain larger than 16 carbons.
5. Aromatics give better EP characteristics,
oxidation protection and additives compatibility.
Based on the above factors a model lubricant
molecule could be conceptualized as; An
isolinear compound with C8
free chains with alkane linkage with 8 to 11 carbon
If the side linkage is constituted by an aromatic or
naphthenic ring, the molecule will become thermally
stronger but will have 8 to 11 carbons in non-liner
configuration that will reduce flexibility of
molecule and lead to lower VI. Table -4 gives some
of these assumed liner molecules of API group III
oils. Same number of carbon atoms could be arranged
to give low linearity and low VIs. Petroleum oils
are mixtures of a variety of hydrocarbons some with
high and other with low VIs.
Constituent molecules of base oil could be compared
with the ‘Ideal Base Fluids’ structure in terms of
Linearity Index (LI). This is defined as the
percent of carbon atoms associated with ‘Flexible’
linear chains. That means free liner chains of 8
to 12 carbon atoms, giving high VI and high
oxidation stability. Compact chemical structure of
ring compounds and those of short linear chains
(less than 8 carbon atoms) are considered ‘Rigid’.
Long linear chains (more than 12 carbon atoms) are
considered ‘Weak’ as these have low thermal and
oxidation stability. Carbon atoms linked to a double
bond or a polar group are considered ‘Rigid’.
Linearity of molecule is quantified as linearity
Index (LI) defined as the percent of carbon atoms in
flexible linear chains (C8
out of total carbon atoms. LI of a normal paraffin
of 8 to 12 carbon atoms will be 100 and that of
aromatics like benzene, toluene, xylene etc. and
naphthenics like cyclohexane etc. zero. This is
– Total number of carbon atoms in the molecule.
of ‘Rigid’ and ‘Weak’ carbon atoms in the molecule.
Additional ‘rigid’ carbon atoms should also be
counted in CR
1. Carbon atoms in alkyl chains of aromatic and
naphthenic rings, if these chains are
smaller than C7.
For larger chains one carbon atoms attached to ring
should be counted as ‘rigid’.
2. A double bond should be considered as hindrance
and two carbon atoms attached
to it should be taken as ‘rigid’, provided the chain
associated to it has between C7
3. Carbon atoms attached to polar groups like
oxygen, phosphorous. Sulfur, hydroxyl, amine, amide
etc. should be considered ‘rigid’.
In Table -4, linear hydrocarbon with 24 to 34
carbon, ideal lubricant structure, with optimal side
chain placement for linear and with one aromatic or
naphthenic ring are listed. LI is calculated and
corresponding VI ranges are reported. LI can
incorporate both saturation and VI characteristics
as well as oxidation stability and may be taken as a
parameter for deciding the quality of base oils.
This makes reporting ‘Saturates’ as in API
Values of LI over different ranges could be
specified as quality index. This eliminates the risk
of mixing of wide viscosity difference oils. Table
-3 gives base oil classification based on this
concept. It is applicable to only hydrocarbon stocks
of API group I, II, III and IV, and is likely to
give more accurate quality assessment of the base
LI should be 65 - 85 to for high VI and high
oxidation stability of base stocks.
CARBON DISTRIBUTION IN MOLECULES
Common methods to determine carbon atoms in various
structural configurations in hydrocarbon and
petroleum base oils are as follows. These could be
used for experimental determination of CT
in petroleum and hydrocarbon stocks.
1. ASTM D – 3238
Carbon distribution and structural group analysis of
petroleum oils by the n-d-m method.
2. ASTM D -6352
Boiling range distribution of petroleum distillates-
170 to 700oC by gas chromatography
3. ASTM D - 2549
Separation of representative aromatic and
monoaromatic, fraction of high boiling oils by
4. ASTM D - 1319
Hydrocarbon types in liquid petroleum products by
5. G.C. M.S. based analysis.
6. Urea adducible.
Linearity index for non-hydrocarbon fluids may also
be calculated on the basis of these
guidelines, based on the chemical structure of the
compound. For mixtures like VOs and
CMVOs, LI for each constituent may be calculated and
LI of the product may be proportionally averaged. In
Table-5 estimated linearity index of some fatty
acids, vegetable oils and CMVOs are reported. Values
for fatty acids are generally low, those for VOs
(triglycerides ) are moderate ( 50-80), and for
CMVOs high ( 70-95).
Linearity Index for synthetic fluids may also be
similarly calculated from the chemicaltructure. LI
for some synthetic fluids; diester, pthalates,
trimallitates, polyol esters andphosphate esters are
reported in Table-6. Phosphate esters, particularly
those with shortalkyl chains or aryl group
substitutions have lower LIs. These are generally
not preferredas functional fluids and are chosen for
some specialized applications for fire resistanceand
superior EP properties.
BLENDED BASE STOCKS
Refineries produce a limited
number of base stocks (mostly three to five) in
common viscosity grades; because of processing
limitations. Manufacturers of synthetic or VOs
stocks also have such limitations. Viscosity grade
requirements of blenders are much larger. ASTM D
2422 has 20 viscosity grades; from KV (at 40oC);
2 cSt. to 3200 cSt. (ISO VG 2 to ISO VG 3200).
Intermediate grades are prepared by blending two or
more oils of adjustment lower and higher viscosity
grades. Available base stocks are four to five and
all the intermediate grades are to be prepared by
blending these. This causes wider distribution of
molecules in the oil with widely differing boiling
points and volatilities in the product.
During application lighter
constituents vaporize more as a result the oil
‘thickens’ as KV increases. In practice, for
petroleum and hydrocarbon stock normally 15 % change
in KV is considered detrimental to performance
requiring oil change. This corresponds to 5 to 10%
loss of lighter low KV component in blend.
Therefore a maximum evaporation loss or volatility
value of 5 % could be fixed for a good quality
stock. A better assessment of the blend could be
done by GLC based tests like ASTM D 6417 or ASTM D
5480. These tests may not be easily available.
Volatility loss limit of 5 percent is fixed for
petroleum and hydrocarbon base stocks. This is
about 2 percent for synthetics and VO stocks as
these have narrow distribution of molecular weight.
Volatility test has no meaning for water based
fluids as the test (ASTM D 972) is conducted at 100oC.
Also WBFs are recommended at atmospheric
conditions.Sometimes KV of a light fraction is made
up by blending polymer ‘viscosifiers’ to meet the KV
specifications of required base oil. Such oils are
sensitive to shear stress and break down at high
shear applications. These may also cream out at
higher temperature (more then 90oC).
API classification does not
provide any safe guard for such blended stocks.
Inclusion of volatility or evaporation loss limits
(ASTM D 972) appears to be essential in base oil
classification for quality assurance.
COMPREHENSIVE BASE OIL CLASSIFICATION
future appears to be for multiplicity of base oils.
Formulators will select a variety of
base stocks depending on performance requirements
and economics. A comprehensive
base oil classification therefore should take into
consideration all these options. It should
also provide information about the chemical nature
of the stock. This will help in selection of
The comprehensive base oil classification is given
in table-7. This includes all the features and
groups of API classification and further includes
native vegetable oils, chemically modified vegetable
oils, synthetic esters and ether fluids,
polyalkylene glycols and water based fluids.
The properties listed are, Viscosity Index (VI),
Iodine Value (IV), Saponification value (SAP), Water
content, and biodegradability. This also has KV at
Each group is divided into five sub groups according
to KV at 40oC.
Therefore separate mention of viscosity along with
the group as in API is not required. Group I, II,
II+, III and IV are same as those for API, except
that re-refined stocks are included. ‘Sulfur
Content’ and ‘Saturates’ as in API classification
are not included as ‘Sulfur’ is considered
‘Superfluous’ and ‘Saturates’ are adequately
represented in VI, and IV.
Group V includes native vegetable oils as these are
complex mixtures of triglycerides.
These are identified by high IV (more than 80), and
high SAP value.
Group VI, includes esters and non-hydrocarbons as in
group V of API classification. These are saturated
esters and would be identified by low IV (less than
5) and high SAP
(more than 60).
Chemically modified vegetable oils (CMVOs) are
included in group VII. These are characterized by
high VI (>140), lower IV (less than 20) and high SAP
(more than 80). Group VIII has hydrophobic esters of
polyhydric alcohol glycols and polyols. These are
characterized by high VI, high SAP and low IV and
also low to moderate water content
(less than 1 %).
Hydrophilic glycol esters and polyakylene esters and
their water solutions are placed in
group IX. These are characterized by low IV, high
SAP and low water content (2 – 30
Water based fluids; emulsions of different fluids in
water or viscosified water with polymer dispersions,
are included in group X. These are characterized by
physical appearance, low SAP, low IV and moderate to
high (30 to 99 percent) water content.
An additional group XI is also included to take care
of other fluids, like silicons, halo-organics,
organosilicons, and innovative fluids. These are not
well defined and will be identified by individual
Linearity index has not been included in the final
format, even though it gives important information
about the structure of molecules, which may be
useful in selection of additive package. This is
because of following reasons.
Methods available for determination of
distribution and form of association of carbon atoms
are not so accurate, particularly for mixtures.
These are not commonly available in laboratories and
need experienced operators and expensive equipment.
LI varies from a low to high value for all
types of base fluids. Therefore fluid type cannot
be identified by LI. For a fluid to be a good stock
for formulation of functional fluids, LI has to be
higher than 70.
Parameters as included in Table -7 can give
all the required information about stock. Also,
these could be determined in a common laboratory.
If the viscosity grade (a) to (e) is not fixed, and
KV of stock at 40oC is to be specified as
in API classification, the table -7 can be presented
in compact form (Table-8). LI should be greater
than 70 if the carbon distribution data is
The final classification is given in Table -8.
Properties required to be reported for
characterization of a base stock are given in table
– 9. with typical methods.
The classification provides information about
viscosity grade, chemical nature and characteristics
of base fluids for an intended application. This
provides guidance for selection of additive system.
It also has information about biodegradability and
1. Shubkin R. L.,
Synthetic Lubricants And High-Performance Functional
Marcel Dekkar (1993)
2. Gunderson R.C. and Hart A.W.;
Reinhold Publishing Co.
Bailey’s Industrial oil & Fat. Vol. 1 to Vol. 6, 5th
Wiley Interscience (1996)
4. Dwivedi M. C.,
Hydrocarbon Composition Based, Base Oil
LAWPSP Publication Vol. 14, BS.03, (2004)
5. Dwivedi M.C. ,
Standardization of Vegetable Oil Based Functional
LAWPSP Publication Vol. 15, SL.02, (2006. )