Industrial Crops and Products 24 (2006) 292–299

Oxidation and low temperature stability of

vegetable oil-based lubricants

Sevim Z. Erhan a,∗, Brajendra K. Sharma a,b, Joseph M. Perez b

a USDA/NCAUR/ARS, Food and Industrial Oil Research, 1815 N. University Street, Peoria, IL 61604, USA

b Department of Chemical Engineering, Pennsylvania State University, University Park, PA 16802, USA

Abstract

Vegetable oils are promising candidates as base fluid for eco-friendly lubricants because of their excellent lubricity, biodegradabi

ity, viscosity–temperature characteristics and low volatility. Their use, however, is restricted due to low thermo-oxidative stabilit

and poor cold flow behavior. This paper presents a systematic approach to improve the oxidation behavior and low temperatur

fluidity of vegetable oil derivatives. Among the various possible avenues available, the combination of chemical additives, diluen

(polyalphaolefin), and high-oleic vegetable oils offer the best option for achieving the ultimate goal. Vegetable oil-based lubr

cants formulated using the above approach exhibit superior oxidative stability, and improved low temperature properties such a

pour points compared to commercially available industrial oils such as bio-based hydraulic fluids. The above vegetable oil-base

formulations compare at par with petroleum-based lubricants for use in high-temperature applications and often outperform th

competition in some of its properties.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Vegetable oils; Oxidative stability; Low temperature stability; Bio-based lubricants; Pressurized differential scanning calorimetry; Rotar

bomb oxidation test

1. Introduction

The industrial and automotive lubricants have a significant lubricant market of 2.4 billion gallons per year

(Lubricants Industry Sourcebook, Lubes ‘n’ Greases,

2004–2005). Vegetable oils currently provide only about

0.1% of this lubricant market. Due to strict environmental regulations and a concern for depletion of world fossil

fuel reserves, there is an increasing demand for environmentally friendly lubricants (Rhee, 1996; Rudnick,

2002; Hwang and Erhan, 2002; Adhvaryu and Erhan,

2002). An opportunity to accelerate the development of

∗ Corresponding author. Tel.: +1 309 681 6532;

fax: +1 309 681 6340.

E-mail address: erhansz@ncaur.usda.gov (S.Z. Erhan).

this market exists in the 2002 Farm Bill (Farm Securit

and Rural Investment Act, published January 11, Fed

eral Register). Section 9002 includes language directin

all Federal Government Agencies to give preference t

“bio-based” products, unless it is unreasonable to do so

based on price, availability or performance. Lubricant i

one category specified in this guideline.

Most current lubricants contain petroleum bas

stocks, which are toxic to environment and difficult t

dispose of after use. Environmental concern continue

to increase of pollution from excessive lubricant use an

disposal, especially total loss lubricants. Over 60% of th

lubricants used in the U.S. are lost to the environmen

Vegetable oils have a capability to contribute toward

the goal of energy independence and security since the

are a renewable resource. Vegetable oils with high olei

content are considered to be potential candidates as sub

0926-6690/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.indcrop.2006.06.008
S.Z. Erhan et al. / Industrial Crops and Products 24 (2006) 292–299 293

stitutes for conventional mineral oil-based lubricating

oils and synthetic esters (Randles and Wright, 1992;

Asadauskas et al., 1996).

Vegetable oils as lubricants are environmentally

preferred to petroleum-based oil because they are

biodegradable and non-toxic (Randles and Wright, 1992;

Battersby et al., 1998). Other advantages include very

low volatility due to the high molecular weight of

the triglyceride molecule and excellent temperature–

viscosity properties. Their polar ester groups are able

to adhere to metal surfaces, and therefore, possess good

lubricity. In addition, vegetable oils have high solubilizing power for polar contaminants and additive

molecules.

On the other hand, vegetable oils have poor oxidative stability (Gapinski et al., 1994; Becker and Knorr,

1996) primarily due to the presence of bis-allylic protons. These active sites are highly susceptible to radical

attack and subsequently the molecules undergo oxidative degradation and form polar oxy compounds. This

phenomena eventually results in insoluble deposits and

increases in oil acidity and viscosity. Vegetable oils also

show poor corrosion protection (Ohkawa et al., 1995).

The presence of ester functionality renders these oils susceptible to hydrolytic breakdown (Rhodes et al., 1995).

Excessive water contamination must be prevented since

it can result in the formation of emulsions.

Low temperature studies have also shown that most

vegetable oils undergo cloudiness, precipitation, poor

flow, and solidification at −10 ◦C upon long-term exposure to cold temperature (Rhee et al., 1995; Kassfeldt and

Goran, 1997). Not all vegetable oils possess equivalent

chemical and performance properties. Mixing of renewable oils will be required to serve the major market of

lubricants. There is limited data available on blending of

vegetable oils.

This study presents a systematic approach to improve

the oxidation and cold flow behavior of vegetable oil

derivatives using a synergistic combination of additives

in high-oleic vegetable oil blended with synthetic fluid.

The oxidation properties are studied using pressure differential scanning calorimetry (PDSC) and rotary bomb

oxidation test (RBOT), while pour point measurements

are used to study low temperature properties.

2. Experimental procedure

2.1. Materials studied

Five vegetable oils are used in this study: an alkali

refined soybean oil (SB) obtained from ADM Packaged

Oils, Decatur, IL, a high-linoleic soybean oil (HLSB)

and a mid-oleic soybean oil (MOSB) obtained from the

United Soybean Board; a high-oleic soybean oil (HOSB)

obtained from Optimum Quality Grains, LLC, West Des

Moines, IA; a high-oleic safflower oil (HOSF) and a

high-oleic sunflower oil (HOSN) from Spectrum Ingredients, Petaluma, CA. A polyalphaolefin (PAO) from BP,

Naperville, IL was used as a low temperature blending

stock in this study. All these oils were used as received

without any further purification.

A commercially available biodegradable hydraulic

fluid (COM) was also used for comparison in the study.

Two additives used in the study, zinc diamyl dithiocarbamate (ZDDC) and antimony dialkyldithiocarbamate

(ADDC), were obtained from R.T. Vanderbilt, Norwalk,

CT.

2.2. Pressure differential scanning calorimetry

(PDSC) method

The PDSC experiments use a computerized DSC

2910 thermal analyzer from TA Instruments (New Castle, DE). Typically a 2 l sample is placed in a hermetically sealed aluminum pan containing a pinhole

lid for interaction of the sample and the reactant gas

(dry air). The 2l sample results in a film thickness

of less than 1 mm. This ensures proper oil–air interaction and eliminates gas diffusion limitation. The module is temperature calibrated using indium metal (m.p.

156.6 ◦C) and a 10 ◦C/min heating rate. Dry air (Gateway Airgas, St. Louis, MO) is used to pressurize the

module at a constant pressure of 1379 kPa (200 psi). A

scanning rate of 10 ◦C/min was used in the temperature ramping experiments. The onset temperature (OT)

of oxidation is calculated from the exotherm in each

case.

The oxidation stability of vegetable oil-based lubricants was also studied in the PDSC using the oxidation

induction time (OIT) obtained in isothermal experiments

at 200 ◦C in constant pressure mode at 200 psi with air

flow rate of 34 ± 3 ml/min.

2.3. Rotary bomb oxidation test (RBOT)

Rotary bomb oxidation tests (RBOT) of all the oil formulations are carried out in presence of copper catalyst

at 150 ◦C in dry conditions as per ASTM Test Method

D-2272. In the RBOT test, the vessel is sealed, charged

to 90 psi pressure with oxygen, and rotated axially in a

constant temperature oil bath set at 150 ◦C. The pressure

in the bomb is continuously recorded. The RBOT time is

the time at which the pressure of the bomb has dropped

by 25.4 psi.
294 S.Z. Erhan et al. / Industrial Crops and Products 24 (2006) 292–299

2.4. Long-term oven oxidation stability test

In this test, the vegetable oils, additized vegetable oils,

and their PAO blends are placed into glass sample vials

and placed in oven at 85 ◦C for 28 days. The weight

loss/gain were determined and total acid number (TAN)

at 0 and 28 days were obtained.

2.5. Pour point method

Pour points are measured by following the ASTM

D97 method (ASTM method, 2000). A 50 ml sample

is placed in a test jar, stoppered. The stopper contains

thermometer measuring from +20 to −80 ◦C that is

immersed into the sample. These test jars are placed in

a digital pour point bath (Petrolab, NY). Temperature is

measured in 3 ◦C increments until the sample stops pouring. The pour point is defined as the temperature in ◦C

at which the sample still pours when the jar is tilted. Statistically the method has shown quite good consistency

for determining low temperature flow property of fluids.

3. Results and discussion

3.1. Oxidation stability of vegetable base oils

The vegetable base oils used in this study were

selected to provide a wide variation in oleic acid percentage (Table 1). The oleic acid varied from 24.2% in SB t

89.1% in HOSN. The unsaturation number (UN) show

in Table 1 was calculated to indicate the relative amount

of saturated, monounsaturated and polyunsaturated fatt

acids in the vegetable oil. The greater the UN the greate

is the degree of unsaturation in the vegetable oil. The oxi

dation stability of various neat vegetable oils was deter

mined as their onset temperatures (OT) using PDSC an

is shown in Table 2. The OT is the temperature at whic

a rapid increase in the rate of oxidation is observed an

obtained by extrapolating the tangent drawn on the steep

est slope of reaction exotherm to the baseline (Fig. 1). A

high OT would suggest a high oxidation stability of th

oil. The oxidation stability of neat base oils increase

as the degree of unsaturation decreased, which concur

the previous research (Lal and Carrick, 1994; Honary

1996; Asadauskas et al., 1996). The correlation coeffi

cient value (R2) of 0.993 was obtained for correlatio

between unsaturation number and PDSC onset temper

atures of different soybean oils with varying oleic aci

content. The correlation between OT and different fatt

acid percentages did not yield very high R2 values. Th

maximum value (0.93) of R2 was obtained between OT

and oleic acid percentage. This shows that oxidation sta

bility of soybean oil is well predictable using its fatty aci

composition rather than individual fatty acid percent

age. Among the neat high-oleic vegetable oils, HOS

has poorest oxidation stability compared to HOSB an

Table 1

Fatty acid composition of various soybean oils by GC (AACC, 58-18, approved methods of the American Association of Cereal Chemists, 10th ed

vol. II, 2000)

Vegetable oils Palmitic C16:0% Stearic C18:0% Oleic C18:1% Linoleic C18:2% Linolenic C18:3% UN

SB 11.14 4.77 24.20 53.60 6.29 1.5

HLSB 10.61 5.63 27.49 56.27 – 1.4

MOSB 9.13 4.33 60.71 24.18 1.65 1.1

HOSB 7.08 3.30 86.78 1.08 1.76 0.9

HOSF 4.82 2.03 80.27 12.87 – 1.0

HOSN 3.27 2.86 89.11 4.76 – 0.9

Table 2

PDSC onset temperatures (OT) of vegetable oils and their PAO blends with and without additive mixture

Vegetable oils Onset temperature (◦C)

Neat oils 20% PAO8 blend Neat oils + Additive 20% PAO8 blend + Additiv

PAO8 188.0

SB 172.9 173.9 215.0 226.6

HLSB 179.2 184.6 235.8 241.8

MOSB 190.3 191.5 243.1 250.4

HOSB 197.7 199.7 254.4 256.5

HOSF 177.4 187.9 257.1 260.4

HOSN 193.2 198.5 260 264

COM 244.2
S.Z. Erhan et al. / Industrial Crops and Products 24 (2006) 292–299 295

Fig. 1. Typical PDSC exotherm of various soybean oils (SB, HLSB,

MOSB, and HOSB) using temperature ramping method (at 10 ◦C/min

in presence of air at 200 psi pressure).

HOSN, which can be explained on the basis of higher

linoleic acid content present in HOSF. PAO is an oxidatively more stable base fluid with an onset temperature

of 188 ◦C compared to 173 ◦C of SB (Table 2). The PAO

is used as a diluent in various vegetable oils in 20–40%

range. There is some increase in oxidation stability as

seen from increased OT (Table 2) and improvement in

low temperature stability when adding up to 20% PAO.

Higher amounts of PAO did not show further improvement. Also higher amounts of PAO8 will decrease the

biodegradability of these oils, as PAO8 itself has 30%

biodegradability by OECD 301B method compared to

>70% in case of vegetable oils (Lawate, 2002). The

choice and concentration of diluent in vegetable oils is

thus critical for environmentally friendly lubricants since

it dictates the biodegradability and ecotoxicity of finished lubricants. Therefore, all these vegetable oils were

tested for their oxidation stability and low temperature

stability using 20% PAO. To offset the poor oxidation

stability of the vegetable oils in these PAO–vegetable oil

blends, it is necessary to optimize the antioxidants and

pour depressants used in the blends.

3.2. Effect of antioxidant–antiwear additive mixture

ZDDC is selected because of its outstanding performance as antioxidant in vegetable oils as reported

by Becker and Knorr (1996). In a study on the optimization of additives by Sharma et al. (2006), it was

established that a mixture of additives is required to substantially improve the oxidation stability of soybean oil.

The combination of 2% ZDDC and 2% ADDC provided

a synergistic effect and it was concluded that in soybean

oil, the combination of ZDDC as the antioxidant and

ADDC as the antiwear additive provide the best results.

As a result, these additives were selected for use in the

vegetable oils in this study.

ZDDC has been reported to function both as radical

scavenger and hydroperoxide decomposer (Becker and

Knorr, 1996). It converts the hydroperoxides formed during the oxidation process to non-radical products, thus

preventing chain propagation. The mechanism of oxidation inhibition by ZDDC involves complex interactions

between alkylperoxy radicals, organic hydroperoxides

and intermediate decomposition products of initial compounds. The mechanism of oxidation inhibition appears

to be very effective. The possible reason for the synergism between antioxidant ZDDC and antiwear ADDC is

the multifunctional behavior of ADDC. Although it is an

antiwear and extreme pressure additive, it also exhibits

antioxidant properties in the RBOT and PDSC. In evaluations of ADDC with soybean oil a time of 30 min

compared to 14 min of soybean oil and onset temperature of 185 ◦C compared to 172 ◦C were obtained.

The effectiveness of this additive combination was

tested in various vegetable oils and their 20% PAO

blends. The PDSC temperature ramping method was

used to study the effect of the additives on base oils

oxidation stability. This method has an advantage that

it can be used for oxidation stability of neat base oils

as well as additized oils, while the PDSC isothermal

method is good only for additized oils. The PDSC onset

temperatures of neat vegetable oils, their PAO blends,

and additized oils are shown in Table 2. The ZDDC and

ADDC additive combination increased the oxidation stability of all the vegetable oils by 50 ◦C. The differences

between different high-oleic oils are also narrowed on

use of PAO and the additive combination. Also the difference between vegetable oil and their PAO blends are

more visible in additized samples. The additized higholeic oils have higher OT compared to the commercially

available biodegradable hydraulic fluid (COM).

A modified PDSC isothermal method was also used

to assess the oxidative stability of formulated products.

In this method, the oil is heated in an air atmosphere at

200 ◦C and the time at which oxidation starts was determined as oxidation induction time (OIT). This method

is more sensitive to differences in oxidation stability of

additized oils (Sharma and Stipanovic, 2003). The OIT

of additized vegetable oils and additized PAO blends

are shown in Fig. 2. There is a significant difference

in OIT of SB and HLSB. The OIT for MOSB doubled

to 26 min from 13.4 min for HLSB, which is due to twofold increase in oleic acid content of MOSB compared to

HLSB. High polyunsaturation (linoleic + linolenic acid

content) in triacylglycerol molecule of vegetable oils

decreases the oxidation induction time, while high sat-
296 S.Z. Erhan et al. / Industrial Crops and Products 24 (2006) 292–299

Fig. 2. Oxidation induction time (OIT) of vegetable oils and their PAO

blend based lubricant formulations containing additive mixture using

PDSC isothermal experiment.

uration and monounsaturation (palmitic + stearic + oleic

acid content) increases the OIT of vegetable oil based

lubricants. Among the additized high-oleic oils, HOSN

has higher OIT compared to HOSB and HOSF. This is

attributed to the high oleic fatty acid content and the lowest polyunsaturation content compared to the other two.

All three exhibit a higher OIT than the COM. The addition of 20% PAO8 to additized vegetable oils improved

the effectiveness of the additives by increasing the OIT

through 35%.

The oxidation stability of the vegetable oil based

lubricants was also evaluated using the RBOT method.

The fluids were evaluated with and without additives.

The additives concentrations were similar in all tests.

The RBOT time usually coincides with an inductiontype period. A rapid pressure drop due to an increase in

the rate of consumption of oxygen occurs after a period

of time. This signals the depletion of the antioxidant and

rapid oxidation of the lubricant. With this test, in the

absence of added antioxidants, soybean oil demonstrates

RBOT time of only 14 min and an order of magnitude

increase with the PAO and additives.

The RBOT results of vegetable oil and vegetable

oil–PAO blend based lubricants along with the COM for

comparison are found in Fig. 3. With the exception of

SB, all of the oils achieved oxidation stability comparaFig. 3. RBOT times of vegetable oils and their PAO blend based lubricant formulations containing additive mixture.

ble to the COM. The RBOT time increased more from

SB (109 min) to HLSB (233 min), than from HLSB t

MOSB (284 min) although oleic acid content increase

more in the latter case than former. This is attribute

to the negligible amount of linolenic acid present i

HLSB compared to SB. The importance of reducin

the polyunsaturation in the vegetable oils is also show

in the high-oleic oils. HOSN has a much higher (40%

RBOT time compared to HOSB and HOSF. This can

not be solely explained on the basis of higher oleic aci

content of HOSN compared to HOSB and HOSF. It als

shows that the type of polyunsaturation (linoleic ver

sus linolenic) contributing to the UN is also a factor. A

similar trend was obtained in vegetable oil–PAO blen

based lubricants. Addition of 20% PAO8 in vegetable oil

increased the oxidation stability by as much as 35%. Thi

PAO response on various vegetable oils using RBOT i

similar to that obtained in PDSC isothermal experiments

This data suggests that an increase in oxidation stabilit

of normal soybean oil (SB) can be achieved by modifi

cation of the composition of the fatty acids and use o

suitable additive combinations.

The oven oxidation test performed on neat vegetabl

oils, additized vegetable oils and their blends is some

what similar to Baader oxidation test (Lawate, 2002), i

which the test is conducted at 95 ◦C for 72 h. The poo

oxidation stability of vegetable oils prevent their use i

high temperature applications like engine oils. There

fore, the test temperature at 85 ◦C is selected, which i

suitable for other applications such as drip, hydraulic

elevator, compressor fluids for agricultural and heav

equipments. In this study, the oil samples were heate

at 85 ◦C in oven for 28 days. The weight loss/gain an

total acid number increase after 28 days were measure

for all these vegetable oils and their formulations. I

case of neat oils, there is some weight gain, which i

due to oxidation of neat oils in the absence of antioxi

dants resulting in formation of some oxopolymerizatio

products (Adhvaryu et al., 2000; Erhan and Adhvaryu

2002). The weight gain is less in high-oleic oils, whil

it is slightly higher in SB and HLSB, due to the pres

ence of more polyunsaturation in these oils (Table 3)

In the present conditions, the weight loss in additize

oils and their PAO blends is <0.5%, which is lowe

than COM sample (1%). Therefore, in vegetable oil for

mulations, volatility is not a problem, which is due t

higher molecular weight associated with triacylglycero

structure of vegetable oils. The volatility relates to th

potential loss of oil and to the environmental impact o

the volatile components of the fluid. Both European an

U.S. OEM and industry organizations include volatilit

in their specifications (Rudnick, 2002).
S.Z. Erhan et al. / Industrial Crops and Products 24 (2006) 292–299 297

Table 3

Weight loss or gain of neat vegetable oils, additized oils and their PAO

blends in oven oxidation stability test at 85 ◦C for 28 days

Vegetable oils % Weight loss (+) or gain (−)

Neat oils Neat

oils + additive

20% PAO8

blend + additive

SB −1.56 0.3 0.4

HLSB −1.7 0.5 0.4

MOSB – 0.4 0.5

HOSF −1.1 0.5 0.5

HOSN −1.0 0.4 0.4

COM 1.0

The TAN values of the vegetable oils are in the

range of 0.18–0.37 mg KOH/g. The TAN increased to

2.0–3.0 mg KOH/g after 28 days in the absence of antioxidants. There was no change in TAN of additized vegetable oils and their PAO blends. Under the test conditions of the oven oxidation test, the additive combination

used in this study does an excellent job by preventing

oxidation of vegetable oils and their PAO blends.

3.3. Low temperature improvement using PAO and

PPD

The low temperature flow property of vegetable oils

is extremely poor and this limits their use at low operating temperatures especially as automotive and industrial

fluids. Vegetable oils have a tendency to form macrocrystalline structures at low temperature through uniform stacking of the ‘bend’ triglyceride backbone. Such

macro-crystals restrict the easy flow of the system due

to loss of kinetic energy of individual molecules during

self-stacking (Erhan and Adhvaryu, 2002). SB, HLSB,

and MOSB have a pour point of −9 ◦C, whereas, higholeic oils have pour-points in the range of −18 to −24 ◦C

(Fig. 4). The excellent thermal and oxidative stability of

high-oleic oils due to removal of polyunsaturation (low

Fig. 4. Pour points of vegetable oils and their PAO blends with and

without pour point depressant (additive).

unsaturation numbers) also results in improved cold flow

property due to reduction in saturated fatty acids. However, the high oleic oils are still limited in their use in

low temperature applications.

In order to improve cold storage stability, polyalphaolefin (PAO) having pour point of −57 ◦C was added

as a diluent with various soybean oils. Twenty, 30

and 40% PAO blends were prepared through blending

and pour point was determined in each case. Addition

of 20% PAO to various soybean oils SB, HLSB, and

MOSB improved their pour point slightly by −3 to

−6 ◦C, but no difference was observed in high-oleic

vegetable oils. Further addition of PAO to 40% made

no significant improvement in the pour point, therefore, only 20% PAO-blends were used in final formulations. The use of higher amounts of PAO also

results in decreased biodegradability of final formulations. Further improvement in the low temperature fluidity of soybean oil and soybean oil–PAO blends can be

achieved by using suitable pour point depressant (PPD)

additives.

The PPD concentration was optimized by determining the pour points of several blends. The purpose of

PPD is to hinder crystallization of triglyceride molecules

at low temperature by further disrupting the stacking

mechanism. An optimum PPD concentration of 1% in

final formulations of various soybean oils and soybean

oil–PAO blends significantly improved the cold flow

behavior (Fig. 4). The response of PPD is more visible

in HLSB, MOSB, and high-oleic oils, where pour point

decreased by −12, −9, and −9 ◦C, respectively, while in

SB, pour point decreased by −6 ◦C. The PPD response

is slightly strong in PAO blends, where in pour point

decreased by −9 to −15 ◦C in vegetable oil–PAO blends.

The presence of more oleic acid and less saturated fatty

acids (palmitic acid and stearic acid) in triacylglycerols

of high-oleic vegetable oils does not allow individual

molecules to come close for easy stacking due to break in

symmetry of molecules. This results in the formation of

microcrystalline structures rather than macro-structures.

At lower temperatures, such microcrystalline structures

can easily tumble and glide over one another resulting

in better fluidity of the total matrix. The optimized formulation containing HOSB, PPD, and PAO diluent had

a pour point of −36 ◦C.

4. Conclusions

The study describes a systematic approach to improve

the oxidation stability and low temperature flow property

of soybean oil. It is shown that thermo-oxidative stability

and cold flow property can be best improved using a
298 S.Z. Erhan et al. / Industrial Crops and Products 24 (2006) 292–299

Table 4

Commercial biodegradable hydraulic fluid (COM) compared with formulated lubricant using HOSN–20% PAO8, antioxidants and pour poin

depressant

ASTM test method HOSN–20% PAO blend + additives COM

General performance

Viscosity at 40 ◦C D445 42.33 53.45

Viscosity at 100 ◦C D445 8.93 –

Pour point (◦C) D97 −36 −52

Oxidation performance

RBOT, min to fail D2272 768 278

PDSC OIT (min) As described 98.1 42.6

PDSC OT (◦C) As described 264.7 244.1

Oven test, % weight loss 0.4 1.0

Friction and wear

Four-ball wear; scar diameter (mm) 30 min, RT, 1200 rpm, 40 kg (D4172) 0.316 0.30

combination of proper blending of chemical additives,

diluent (PAO) and high-oleic vegetable oils.

Comparisons were made with a commercially available biodegradable hydraulic fluid. The oxidation behavior of the COM and the fluids in this study are compared

in Tables 2 and 3, and Figs. 2 and 3. HLSB–PAO blend,

HLSB and MOSB show essentially the same onset temperature (Table 1) as COM, while MOSB–PAO blend and

all high-oleic oils and their PAO blends are oxidatively

more stable than COM (244 ◦C) as shown by PDSC

ramping experiment.

Similarly in PDSC isothermal experiments, all additized high-oleic oils and their PAO blends have higher

OIT compared to COM (Fig. 2). RBOT results show

that HLSB–PAO blend, mid-oleic, high oleic and their

PAO blends have excellent oxidation stability compared

to COM (Fig. 3).

The other properties of COM fluid are compared

with the best lubricant formulated in this study using

HOSN–PAO8 blend and are listed in Table 4. The additive combination used in this study improves the oxidation stability of the SB (109 min) and SB–PAO blend

(145 min) formulations. The oxidation stability exceeds

that of another commercially available soy based lubricants (83 min).

Cold flow properties have also been improved using a

combination of PPD and PAO diluent. Among vegetable

oils, SB is not very useful due to their poor oxidation stability. The formulations with mid-oleic soybean oil and

high-oleic oils shows excellent thermo-oxidative stability and low temperature flow property when blended with

PAO and additive combinations, and is acceptable for

most industrial applications. It is possible that with better bean initiative of United Soybean Board, MOSB will

be widely available with price much less than high-oleic

oils. High-oleic oils cost about twice as much as norma

vegetable oils but only about half compared to syntheti

fluids such as PAO8, trimethylol propane (TMP) tri

oleate. It is expected that as high-oleic oils become mor

readily available, the price difference between high-olei

oils and normal soybean oil will also decrease.

Acknowledgements

Names are necessary to report factually on availabl

data; however, the USDA neither guarantees nor war

rants the standard of the product, and the use of th

name by USDA implies no approval of the product t

the exclusion of others that may also be suitable.

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