Base Oil Product

Base Oil Product

Base Oil
Base oils are used to manufacture products including lubricating greases, motor oil and metal processing fluids. Different products require different compositions and properties in the oil. One of the most important factors is the liquid’s viscosity at various temperatures. Whether or not a crude oil is suitable to be made into a base oil is determined by the concentration of base oil molecules as well as how easily these can be extracted.

 

Specification

Method

SN-150

SN-500

SN-650

Appearance

Sight/Melt

Bright, Clear

Bright, Clear

Bright, Clear

Kinematic Viscosity @ 100°C (cSt)

ASTM D445

5.4-7

9.5-11.2

12.1-14

Viscosity Index (Min)

ASTM D2270

95

89

85

Flash Point (°C), COC

ASTM D92

208

240

250

Pour Point (°C)

ASTM D97

-12

-6

-6

Color (Max)

ASTM D1500

1.5

2

3

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Lubricant Base Oils: Analysis and Characterization
1 INTRODUCTION
Base oils are refined from crude oil, re-refined from used oil, or manufactured to give products with specific chemical and physical properties. Mineral oils are base oils produced from the atmospheric residuum of crude oil. The residua are processed to remove aromatics and nitrogen-, sulfur-, and oxygen-containing compounds which degrade the performance of the finished base oil.
Mineral base oils are classified by the molecular type that predominates (paraffinic, naphthenic, or aromatic) and their physical and chemical properties. Paraffinic base oils tend to have high pour points and must be dewaxed to produce base oil with accept-able flow properties at low temperatures. Naphthenic base oils have naturally low wax content and generally have better solvency properties than paraffinic stocks. However, they tend to oxidize more readily than paraffinic oils. Aromatic stocks are valued for their solvency properties rather than their lubricating quality, and are used as process oils in the rubber and printing ink industries. Typical properties of paraffinic, naphthenic, and aromatic oils are shown in Table 1.

High-quality re-refined base oils are processed to be indistinguishable from virgin mineral oils derived from crude oil. They are derived from contaminated used oils from a variety of sources. Analyses of re-refined base oils may include tests for contaminants in used oil that are not found in virgin base oils. (Refer to the appropriate American Society for Testing and Materials (ASTM) standard.
Unless stated otherwise, all references to ASTM methods are from the Annual Book of ASTM Standards, Vol. 05.)
The most commonly used synthetic base oils fall in to one of five classes: olefin oligomers, especially polyalphaolefins (PAOs), esters (dibasic and polyol), alkylated aromatics, polyalkylene glycols, and metalloorganic oligomers (phosphate or silicate esters, siloxanes).
Chlorine and/or fluorine-substituted hydrocarbons or polyethers are specialty lubricants used where extreme operating conditions are encountered.
Several general references on base oil technology, applications, and characterization have been published and are widely available. Neale has edited a tribology handbook with an excellent review of base oil types and their properties.
Klaus and Tewksbury provide a comprehensive review of base oil and finished lubricant properties.
The ASTM has published a monograph on testing petroleum products following ASTM methods
And an educational guide on base oil characterization tests which covers lubricant performance and safe handling, composition, performance, and consistency.
The journal Analytical Chemistry issues a comprehensive, biennial review on the analysis of petroleum and coal which includes lubricants and base oils.
For a broader perspective on lubricants, applications, lubricant characterization, and performance, Caines and Haycock have written a reference book on automotive lubricants, and the Association of Iron and Steel Engineers (AISE) has published a manual on industrial lubricants.
2- BASE OIL REFINING PROCESSES
Base oils are traditionally made by solvent refining to physically extract undesirable aromatics and heteroatomic compounds from the feed, solvent-assisted wax crystallization (paraffinic crudes), and clay treatment to decolorize and stabilize the finished product. In more modern processes, some refiners have replaced solvent refining with hydrocracking or severe hydro treating, solvent dewaxing with catalytic dewaxing or wax isomerization, and clay treating with hydro finishing. Mineral base oils with properties approaching those of PAOs have been manufactured by isomerizing wax derived from slack wax or Fischer – Tropsch synthesis.
Base oils produced from vacuum distillates are called neutral oils; bright stocks are base oils refined from vacuum.
3- BASE OIL COMPOSITIONS
Base oil characterization is important for understanding the relationship between composition and performance of finished products in end-use applications. Rig or engine tests, developed to predict field performance, are often expensive or take a long time to complete. Consequently, a need exists for relatively simple, low-cost tests to predict the performance of a finished lubricant in the customer’s equipment.
3.1 Hydrocarbon Type Analysis
Base Oil ProductBase oils derived from petroleum are complex mixtures of thousands of individual compounds. To better under-stand how base oil molecular structure, composition, and properties relate to product performance, investigators have examined individual model compounds and characterized the bulk molecular hydrocarbon types in base oils. Over a 26-year period, American Petroleum Institute (API) sponsored a program to measure the properties of 321 high-molecular-weight hydrocarbons.
Briant has provided a thorough analysis of these and other data on model compounds.
ASTM has published several test methods to obtain information on the composition of base oils. ASTM D 2140 and D 3238 give calculated values for the carbon number distribution among aromatic, naphthenic, and paraffinic compounds based on viscosity (ASTM D 445), density/specific gravity (ASTM D 1481), and refractive index (ASTM D 1218). ASTM D 3238, also known as the refractive index – density – molecular weight (n–d–M) method, gives similar, though not identical, results to ASTM D 2140.
More detailed base oil compositional information can be obtained by first separating base oil into aromatic and non-aromatic fractions on a chromatographic column and analyzing each fraction separately by mass spectrometry (MS). ASTM D 2549 describes a chromatographic procedure for separating hydrocarbon mixtures boiling between 232°C and 538°C on a packed column. The non-aromatics fraction is subjected to analysis by high ionizing voltage MS (ASTM D 2786) to give composition by seven saturated hydrocarbon types and one aromatic type. The aromatics fraction is analyzed by high ionizing voltage, low-resolution MS.
ASTM D 2007 (Clay-gel Analysis) and a technique developed by Hirsch et al. are column chromatographic procedures used to separate high-boiling petroleum fractions into saturated, aromatic, and polar compounds. Barman showed that ASTM D 2007 is subject to cross-contamination of these hydrocarbon classes. Thin-layer chromatography (TLC) with flame ionization detection (FID) is more rapid and accurate.
Mohindroo and Preston.20/ used supercritical-fluid chromatography (SFC) with flame ionization and ultraviolet (UV) detectors to separate base oils into saturates, polars, monoaromatics, and polynuclear aromatics. Hsu et al. developed a rapid, preparative scale, high-pressure liquid chromatography (LC) procedure to separate base oils into compound groups using two columns and solvents of different polarity.
Thermal diffusion allows separation of hydrocarbons by molecular size under the influence of a thermal gradient. Cyclic and linear molecules diffuse by convection towards opposite ends of the apparatus. The process is time-consuming, but it provides detailed information not available by bulk chromatographic separation.
Chromatography separates compounds into distinct molecular types but provides no quantitative information on the number of aromatic and paraffinic carbon or hydrogen atoms in each fraction. Nuclear magnetic resonance (NMR) and infrared (IR) analyses provide this information directly. ASTM D 5292 describes an NMR method to measure the mole or mass percent of aromatic carbon or aromatic hydrogen atoms in a sample. A. van de Ven et al. developed an improved correlation between C NMR and IR analysis using the aromatic bands at 1610 cm and 815 cmto quantify themole percent aromatic carbon in a base oil.
UV absorption (ASTM D 2008) is used to detect totalaromatics in a sample, especially in severely hydropro-cessed base oils, waxes, and white oils where the aromatics content is low. In ASTM D 2269, aromatics are concentrated before UV analysis by dimethyl sulfox-ide (DMSO) extraction. Similar procedures have been developed to assess the potential carcinogenicity of base oils and aromatic extract distillates.
3.2 Elemental Analysis
Compounds containing nitrogen, oxygen, and sulfur in base oils can have profound effects on the properties and performance of finished lubricants. Elemental analyses are conducted to detect base oil contamination or impurities which survive the process of re-refining used oils. ASTM D 6074 provides a comprehensive list of tests to characterize hydrocarbon base oils (mineral oils) including those for elemental analysis.
Sieber and Salmon give an excellent summary of the instrumentation and methodology for spectrometric methods of lube oil analysis.
4- PHYSICAL AND CHEMICAL PROPERTIES
Physical and chemical properties of base oils can be related, ultimately, to their bulk or molecular properties.
Physical property tests are used for manufacturing process control, QC, QA, consistency of finished products, and to set specifications between buyer and seller.
4.1 Viscosity
Viscosity is the critical factor in establishing hydro-dynamic or full-film lubrication which reduces friction and wear between moving parts. Viscosity is defined as the ratio of shear stress to shear rate and measures resistance to flow. Newtonian fluids, including base oils above the cloud point, obey this equation. Klaus and Tewksbury,Briant et al. and Alexander provide detailed descriptions of rheology theory and applications to lubricants.
Dynamic, or absolute, viscosities are most often measured in coaxial-cylinder viscometers.
Kinematic viscosities are determined by ASTM D 445 which measures the time for a liquid to move through a narrow capillary segment. Absolute viscosity in millipascal sec. or centipoise (cP) equals kinematic viscosity in mm2 or centistokes (cSt) times density.
 Viscosities are typically reported at two temperatures, 40°Cand 100°C, and a property defined as the viscosity index (VI) is calculated from the values (ASTM D 2270). High VI oils exhibit a relatively small change in viscosity with temperature. Aliphatic paraffin predominates in high VI oils (Table 1). ASTM D 341 describes a procedure and conditions to calculate the viscosity at one temperature from viscosities measured at two other temperatures.
Viscosity also changes with respect to pressure, but the effect is much smaller than the temperature effect. Viscosity – pressure relationships are important in highly loaded contacts in gears or rolling element bearings, where the pressures under elastohydrodynamic lubrication can exceed 345 MPa (50 000 psi). Viscosity as a function of pressure can be measured in falling-weight viscometers, capillary viscometers, vibrating-crystal viscometers, and optical viscometers.
The effect of pressure on viscosity decreases in the order naphthenics>paraffinics>esters.
Apparent viscosities of non-Newtonian fluids vary withshear rate and shear stress. These include base oils belowthe cloud point and fully formulated lubricants containing high-molecular-weight components (VI improvers).
For engine oils, high-temperature – high-shear viscosityat 150°Cand 106 shear rate represents lubrication conditions in high-speed bearings. Methods tomeasure viscosity under these conditions include the
Tapered-bearing Simulator (ASTM D 4683), Tapered-plug Viscometer (ASTM D 4741), and  MulticellCapillaryViscometer (ASTM D 5481).
The Cold-cranking Simulator (CCS) (ASTM D5293) measures low-temperature base oil viscosities at shear rates. Wax-related viscosity effects areminimized, and measured viscosities are similar to extrapolated viscosities from data obtained above the cloudpoint. CCS viscosities of fully formulated engine oilscorrelate with low-temperature engine startup.
4.2 Wax-related Properties
As paraffinic base oils are cooled, wax crystals grow and form interlocking networks throughout the fluid which eventually stop oil flow at low shear rates. The shear stress required to break down the wax structure and resume lubricant flow is called the yield stress. Several tests have been developed to measure low-temperature, engine oil viscosities at low shear and low shear stress after long cooling cycles. Test conditions mimic, but don’t necessarily duplicate, conditions that can cause low-temperature engine oil pump ability failures in the field. Except for the cloud- and pour-point tests, methods listed in Table 2 were developed specifically for fully formulated engine oils.
 
4.3 Volatility
Base oil volatility is an important factor in crankcase oil consumption, especially at high engine temperatures.
Volatility decreases as VI increases for a given viscosity base oil and distillation cut width.
Volatility is measured by an evaporative-loss test (Noack) or gas chromatography (GC) (simulated dis-tillation). In the Noack test, the weight of oil evaporated at 250°C in one hour in flowing air is measured. Varia-tions of the Noack test exist as Coordinating European
Council (CEC) L-40-T-87,Japanese Petroleum Institute (JPI)-5S-41-93, Method B, and ASTM D 5800.
An alternative apparatus has been proposed which eliminates use of toxic Woods metal as the heat-transfer medium and allows nearly quantitative collection of the volatiles for further analysis.
GC simulates distillation, and is more rapid and precise than Noack methods.
ASTM D 2887 measures boiling-point distribution for hydrocarbons boiling between 55°C and 538°C. ASTM D 5480 uses an internal standard and was developed specifically to measure volatility of engine oils. Noack volatility is typically 5 – 15% higher than ASTM D 2887. Choi and Deane, showed that two base oils with the same Noack volatility could have different volatilities by ASTM D 2887, and vice versa. Engine oil volatility at high engine operating temperatures appears to correlate better with Noack volatility.
4.4 Other Properties
ASTM D 6074 summarizes the tests used most often to characterize base oils and finished oils. ASTM D 6158 lists the specific tests and specifications for mineral hydraulic oils, including noncompounded base oils. Klaus and Tewksbury.and Godfrey and Herguth describe base oil properties from a more fundamental viewpoint and provide typical properties for different chemical classes of lubricants. The Annual Book of ASTM Standards, Volume 05, describes over 150 test procedures useful fortesting or characterizing base oils.
Base oil characterization involves testing and assessment of appearance, viscometrics, safety, interfacial properties, solvency, oxidation and thermal stability, seal compatibility, and volatility. Appearance is determined by color (ASTM D 1500), cleanliness (International Organization for Standardization (ISO) 4406), and clarity (ASTM D 4860). Water above 80 – 100 ppm, for example, will impart a haze to base oils at room temperature.
Color stability can be estimated by light irradiation, either by UV source or sunlight. Mills and Melchior showed that color instability is due to nitrogen and sulfur heterocyclic compounds in base oils. Safety aspects include flash point (ASTM D 92/93) and toxicity (sections 6 and 7). Flash point is a rough measure of volatility and is used to detect low-boiling contaminants in base oils.
Interfacial properties between water and lube oil determine the demulsibility or the speed and efficiency of separating a mixture of the two (ASTM D 2711). Gas release (ASTM D 3827) and foaming tendency (ASTM D 892/6082) relate to the interfacial properties between base oils and gases. Copper corrosion (ASTM D 130) indicates presence of reactive sulfur compounds in base oils.
Solvency for additives and seal compatibility are governed by the chemical composition of base oils.
Highly paraffinic base oils (hydrocracked mineral base oils and PAO) tend to shrink rubber seal materials and have lower solubility for additives than mineral oils containing aromatics and naphthenes, or synthetic base oils containing esters. Aniline point (ASTM D 611) is a useful guide to mineral base oil aromaticity and solvency properties. The effect of base oils and formulated lubricants on seal materials is determined by methods ASTM D 412/471/2240.and 5662.
5- EFFECT OF BASE OIL COMPOSITION ON LUBRICANT PERFORMANCE
Bench and rig test studies of base oils and formulated lubricants are used as guides to performance of fully formulated oils in field applications. The goal is to reduce the testing cost and maintain a high confidence level of performance when one base oil is substituted for another in a fully formulated lubricant. Hsu conducted a thorough review of bench tests used to correlate performance of crankcase oils.
5.1 Oxidation Tests
Base Oil ProductBase oil oxidation tests can be classified as oxygen diffusion-limited (bulk), or reaction rate-controlled (thin film). Tests are run with or without catalyst activator.
Catalysts simulate surfaces or contaminants that might accelerate oxidation. Oxidation-test results on uninhibited oils can be misleading. Highly paraffinic oils with low sulfur and nitrogen content oxidize more readily than base oils containing sulfur and aromatics, but are more stable with added oxidation inhibitor.
Bulk oil oxidation tests include the oxidation test for turbine oils (The Institute of Petroleum (IP) 114), turbine oil stability test (ASTM D 943/4310/IP 157), rotary bomb oxidation test (ASTM D 2272), corrosiveness and oxidation stability of highly refined oils (ASTM D 4636/FTM 791-5308), and the universal oxidation/thermal stability test (ASTM D 4871/5846). Except for the rotary bomb test, which is conducted in a pressurized vessel, oxidation stability is evaluated at atmospheric pressure in flowing air.
Hsu and Cummings developed a thin-film, thermo-gravimetric analysis (TGA) test to determine oxidation, volatility and extent of polymerization of base oils.
Yoshida, Stipanovic et al, used high-pressure differential scanning calorimetry (DSC) to measure base oil oxidative stability and correlated results with ASTM D 943. Klaus et al. developed the Penn State Micro-oxidation Test to evaluate oxidative and thermal stability of base oils, and correlate lubricant degradation processes with those observed in automotive and diesel engine tests.
Lubricant is oxidized in a thin film. Residues are analyzed by gel permeation chromatography (GPC). Selby recommended the thin-film oxygen uptake test (TFOUT), ASTM D 4742, to determine base oil consistency and quality.
5.2 Correlation of Base oil Composition and Lubricant Performance
As a general rule, base oil oxidation stability increases with increasing saturates content, and sulfur compounds are beneficial at high temperatures in uninhibited oils or when inhibitors have been depleted. Murray et al. concluded that the relationship between VI and oxidation performance of solvent-refined base oils held only for base oils from the same crude source and refining process.
Robson found that oxidation of lubricants formulated with hydrocracked base oils correlated inversely with aromatics content. Sulfur did not correlate at the low levels found in the hydroprocessed lubes. Stipanovic et al. used base oil compositional analysis, physical property measurements, and engine test results to develop predictive models based on partial least squares (PLS) and neural networks analyses. Multi-ring naphthene and polyaromatic compounds, thioaromatics, basic nitrogen compounds, and ‘‘resins’’ increase oxidation, thermal decomposition, and deposit formation. Supp et al. also found that base oil sulfur increased sludge formation in the Sequence VE test, but noted that aromatics, especially highly polar base oil components, were beneficial, possibly because they solubilize sludge precursors.
6- TOXICOLOGY
Crude oil atmospheric residua contain polynuclear aromatic, nitrogen-, and sulfur-heterocyclic compounds,some of which may be carcinogenic. While base oilrefining removes some or all of these materials, themanufacturer is still responsible for assuring that thefinal products are non-carcinogenic. Blackburnhasreviewed the subject, and background information ontest procedures and legal definitions of carcinogenicityare discussed in ASTM D 6074.
Mouse-skin bioassay is the most recognized, directmethod for predicting the carcinogenicity of base oils.
The time of tumor development and the percentageof animals developing tumors relative to the controlsare used to assess carcinogenicity. A bioassay test andDMSO extraction tests have been developed to predictthe results of mouse-painting tests to reduce the timeand cost to assess carcinogenicity. The MutagenicityIndex (Modified Ames Test), ASTM E1687,isa short-term, microbiological, salmonella mutagenesisassay developed to detect mutagens in base oils. Thecorrelation of mutagenicity and carcinogenicity with PNAcontent is defined for oils with viscosities>18 cSt at 40°C,distilling between 250 – 550°C. A new test to predictbase oil carcinogenicity, DNA adduct formation, is nowavailable.
Method IP 346 specifies DMSO extraction of poly-cyclic aromatics and condensed-ring polar compounds invirgin base oil and measurement of the polycyclic aromatics in the extract.
The scope and limitations aredescribed in the CONCAWE review and the European Union Dangerous Substances Directive.
Haaset al.developed another DMSO extraction procedurewhich is similar to ASTM D 2269. The index they developed includes both PNA content and viscosity to predictresults from mouse-painting studies
7- ENVIRONMENTAL IMPACTS OF BASE OILS
Environmental impact is assessed by estimating the eco-toxicity and biodegradation of lubricants. Biodegradation is the oxidative decomposition of carbon, hydrogen, and oxygen compounds by microorganisms. Eco toxicity is the tendency of a material to negatively impact nonhuman organisms.
The general order of biodegradation of base oils is vegetable oils>synthetic esters>mineral oilsalkylbenze-nes>polyalkyleneglycols>PAOs.
The relative order depends on the test conditions and method of reporting. Within a family group, lower-molecular-weight compounds degrade faster and more completely than higher-molecular-weight materials.
Biodegradation processes follow two paths. ‘‘Ultimate’’ biodegradation leads to complete biochemical oxidation to carbon dioxide, water, and inorganic salts. ‘‘Bioaccumulation’’ or ‘‘primary biodegradation’’ is a process in which the original molecule is partially degraded and incorporated into a microorganism as biomass. ‘‘Primary biodegradation’’ is followed by monitoring the disappearance of the original substance, while ultimate biodegradation is measured by carbon dioxide production or oxygen consumption.
ASTM D 6046 is a standard classification of hydraulic fluids for their environmental impact. While the document pertains specifically to the biodegradability and acute aquatic toxicity of hydraulic fluids, the methodology and test procedures identified also apply to base oils. ASTM published a companion document, ASTM D 6081, which covers standard practices for aquatic toxicity testing of lubricants and their components. Voltz et al. and Kiovsky et al. reviewed lubricant biodegradation test methods, and provided comparative test data on base oils and fully formulated oils.