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444 Lubricant Additives: Chemistry and Applications
97. Phillips, W.D., Phosphate ester hydraulic uids, Handbook of Hydraulic Fluid Technology, G.E. Totten,
ed. New York: Marcel Dekker, 2000.
98. Mackerer, C., M.L. Barth, A.J. Krueger, A comparison of neurotoxic effects and potential risks from
oral administration or ingestion of tricresyl phosphate or jet engine oil containing tricresyl phosphate,
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99. DeCordt, F.L.M., N. Svensson, J. Mihelic, M. Blackowski, Corrosion inhibiting composition, U.S.
Patent Appl. 2005/0017220 A1, 2005.
100. Braga, T.G., R.L. Martin, J.A. McMahon, B. Oude Alink, B.T. Outlaw, Combinations of imidazo -
lines and wetting agents as environmentally acceptable corrosion inhibitors, PCT Intl. Pat. Appl.
WO 00/49204, 2000.
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445
18
Additives for Bioderived and
Biodegradable Lubricants
Mark Miller
CONTENTS
18.1 Introduction .........................................................................................................................445
18.2 History .................................................................................................................................447
18.3 Vegetable Base Fluids..........................................................................................................448
18.4 Oil Types and Performance .................................................................................................449
18.5 Additive Packages................................................................................................................449
18.6 Detergents ............................................................................................................................ 450
18.7 Dispersants .......................................................................................................................... 451
18.8 Corrosion Inhibitors/Antirust .............................................................................................. 451
18.9 Antioxidants ........................................................................................................................ 451
18.10 Antiwear .............................................................................................................................. 453
18.11 Antifoam ..............................................................................................................................454
18.12 Viscosity Modi ers/Pour Point Depressants .......................................................................454
References ......................................................................................................................................454
18.1 INTRODUCTION
There is increasing interest in biobased and biodegradable lubricants as alternatives to petroleum-
based products. According to the National Oceanic and Atmospheric Administration (NOAA),
700 million gallons of petroleum is released into the ocean each year. Over half of that, 360 million
gallons, is because of irresponsible maintenance practices as well as routine leaks and spills.
As environmental enforcement agencies increase pressures and costs for petroleum lubricant
spills, many equipment operators are using or considering environmentally safer products. These
types of uids can protect the users against nes, cleanup costs, and downtime, but care must be
given in selecting the right product for each speci c application.
The bene ts of biodegradable hydraulic uids are well known. Their biodegradable properties
allow them to break down in the environment reducing the negative impact from leaks and spills.
They can be nontoxic, meaning they will not hurt operators, animals, or plants that come into
contact with the uid. They are renewable and reduce dependence on foreign petroleum oil.
Conventional knowledge has focused on the limitations of vegetable oils as base stocks for lubri-
cants. The weakness of the oxidative stability, the cold temperature performance, and incompatibility
with elastomers are well documented. Early generation biobased lubricant formulators utilized perfor-
mance chemistry similar to those used in petroleum-based uids creating lubricant products that did
not meet industrial performance requirements. Over the past decade, however, improvements in vege-
table base stocks, performance chemistry, and formulation expertise have allowed for the development
of biodegradable products with performance similar to or better than conventional petroleum uids.
Although there is a wide range of biobased products, most are readily biodegradable, meaning
that they are biodegradable above 60% in 28 days as measured by the ASTM D5864 biodegradability
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446 Lubricant Additives: Chemistry and Applications
test. Petroleum hydraulic uids are either not biodegradable (<30% in 28 days) or inherently
biodegradable (30% < X < 59% in 28 days, where X is biodegradability).
Most of these biobased products are free of toxic performance chemicals such as sulfated ash,
zinc, calcium, and other heavy metals. Biobased products are also usually formulated to be virtually
nontoxic and frequently measure 5–10 times less toxic compared with the ASTM nontoxicity speci-
cation. A conventional petroleum lubricant typically contains a zinc and calcium additive perfor-
mance system and will generally be considered toxic. These uids are toxic and persistent as the
additive system kills the microbes responsible for biodegradation, and the petroleum uid itself is not
readily consumed by the microbes. There are, however, some inherently biodegradable petroleum
hydraulic uids containing ash-free additive systems and as such are less toxic than standard petro-
leum hydraulic uids. However, these products are still petroleum-based, and the microbes cannot
readily degrade (digest) these types of uids. They will persist in the environment for many months
or years and will dramatically reduce water quality, harming local wildlife and ecosystems.
Furthermore, standard petroleum products contain aromatic, cyclic (ring structure) hydrocarbons.
These aromatics cause the familiar rainbow sheen on a water surface. Biobased products do not con-
tain aromatics and as such do not produce a rainbow sheen on a water surface when spilled. Severely
hydrotreating petroleum oil during the re ning process will remove most aromatics. These uids might
not produce a sheen but will persist on the water surface harming the aquatic wildlife and ecosystem.
There are no universal de nitions of biobased, environmental, or biodegradable for lubricants.
The focus of this chapter is on vegetable or biobased lubricants with the vegetable oil content maxi-
mized at >60%. The Farm Security and Rural Investment Act (FSRIA or Farm Bill) was signed
into law in 2002. A goal of that legislation is to increase the government’s purchases and use of
biobased products. Under this legislation, the United States Department of Agriculture (USDA)
has selected and prioritized items for designation as preferred biobased products and set minimum
biobased content levels. Each level of prioritization is issued in a round of designated products. In
the rst round, lubricant-related products were speci ed (Table 18.1) [1].
Despite the levels required by federal minimum biobased content, high-performance vegetable-
based products can sometimes contain >90% biobased content. As a result, they will be readily
(rapidly) biodegradable [2].
Also, this chapter looks primarily at environmentally compatible performance chemistry,
which is de ned as no heavy metals, no ash, low treatment volume, low toxicity, and low environ-
mental persistence. Use of these types of chemistries will not adversely affect the environmental
performance of biobased products and are very less toxic. One standard of measuring toxicity of a
substance is the LC 50, that is, the concentration, in parts per million (ppm), of a substance that is
lethal to 50% of the laboratory animals exposed to it in a 96 h test [3]. Therefore, the higher the LC
50, the lesser the toxicity. As most commercially available biobased lubricants can be formulated
at LC 50s ranging from 5,000 to 10,000 ppm, only formulations less toxic than 5,000 ppm will be
considered. This chapter, however, touches lightly on chemistry that seems to have a positive effect
TABLE 18.1
Government-Specifi ed Minimum Biobased Content
Product Minimum Biobased Content (%)
Diesel fuel additives 90
Hydraulic uids (mobile equipment) 44
Penetrating lubricants 68
Source: Extracted from United States Department of Agriculture Biopreferred
Web site, http://www.biopreferred.gov.
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Additives for Bioderived and Biodegradable Lubricants 447
on vegetable-based products yet might be < 5000 ppm LC 50 level. The additives will include both
traditional petroleum oil-type additives and other novel additives.
Signi cantly, more work has been done in hydraulic and tractor transmission uids as well as
total loss lubricants, that is, lubricants that are consumed through their use. Engine oils, although a
large market, typically do not present an environmental threat due to leaks or spills. They require
extremely high operating temperatures that are not suitable for vegetable oils. Although some
work has been done with engine oils to date, the Chemical Manufacturers Association (CMA) or
International Lubricant Standardization and Approval Committee (ILSAC) has certi ed no veg-
etable-based engine oil. Some of the development work on engine oil performance has shown the
performance of several additive types and will be addressed.
The use of re ned, bleached, and deodorized (RBD) food-grade vegetable oils in industrial
lubricant applications has been limited due to their inherent performance de ciencies regarding
oxidative and thermal stabilities and their limited cold temperature ow properties. The use of
genetically modi ed vegetable base stocks such as high oleic, reaction modi cation such as esteri-
cation, and chemical modi cation combined with improved vegetable oil-speci c performance
additives can fully address the concerns surrounding the use of vegetable oils in hydraulic applica-
tions. Commercial applications and previous research have demonstrated that mixtures of modi ed
vegetable oils provide performance levels required for hydraulic original equipment manufacturers
(OEM). Much work has been done evaluating the performance bene ts of genetically modi ed
vegetable oils and mixtures of base uids including ester and polyalphaole ns, but it is outside the
scope of this chapter. For the most part, the bene ts that an additive provides to a conventional veg-
etable oil will also apply to genetically modi ed oils and mixtures.
Before modern additives are explored, the history of vegetable oils will be reviewed.
18.2 HISTORY
Early generation biobased lubricant formulators utilized performance chemistry similar to those
used in petroleum-based uids creating lubricant products that frequently did not meet industrial
performance requirements.
Historically, vegetable-based lubricants have not exhibited suf cient performance for industrial
applications. There were several reasons for this inability. The rst reason is that vegetable-based
lubricants were misformulated. Traditionally, a lubricant is compounded from base oil and various
performance chemistries. Early formulators in the vegetable-based lubricant market used the same
chemistry that was used for petroleum lubricants for vegetable base oils. This approach was not
effective as the characteristics of vegetable oils are vastly different than those of petroleum oils.
Typical characteristics of vegetable oils as compared with petroleum are summarized in Table 18.2.
TABLE 18.2
Typical Characteristics of Various Base Fluids
Characteristic Petroleum Oil Vegetable Oil Saturated Ester PAO
Lubricity Low High High Low
Oxidative stability RPVOT 300 50 180 300
Viscosity Index (VI) 100 200 165 150
Hydrolytic stability High High Low High
Polarity Low polar Highly polar Polar Low polar
Saturation Saturated Unsaturated Saturated Saturated
Flash point (°F) 200 450 400 350
Pour point (°F) –35 –35 –40 –50
Source: Derived from Terresolve Technologies In-house Knowledge Base.
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448 Lubricant Additives: Chemistry and Applications
Vegetable oils have to be formulated for their individual characteristics. Some modern vegetable-
based products currently in the market offer good performance and a fair price. Figure 18.1 shows
relative costs for a wide range of lubricants.
Although all triglyceride vegetable-based lubricants have temperature limitations, there are some
that are better than others. Although most vegetable-based lubricants have a maximum operating tem-
perature of 140°F, there are some that offer protection as high as 220°F. Similarly, most vegetable-
based lubricants offer good performance down to 30°F, yet there are some that ow below –30°F.
These temperature limitations led to another major reason for early technical failures and that
was the wrong uid choice. Even the highest-performing biobased uids have operating limitations
in terms of temperature and life expectancy. Using a biobased uid in an application over 220°F
(and as low as 160°F for some uids) will cause premature and possibly catastrophic equipment
failure. This, combined with sensitivity to moisture, which is another characteristic of vegetable-
based lubricants, has caused numerous cases in which using a vegetable-based uid was a major
contributor to the failure. In extreme high-temperature, environmentally sensitive applications,
readily biodegradable synthetic uids should be utilized.
Finally, biobased uids require special care to maximize its useful life. Water in any lubricant
system is bad for many reasons, which include causing many additives to fall out, increasing the onset
of acid formation, deteriorating seals, creating rust, and accelerating wear. Most biobased uids are
more susceptible to hydrolytic breakdown, the result of which can be acid formation, more suscep-
tible to additive precipitation, and as previously mentioned are prone to oxidative instability.
Over the past decade, however, improvements in vegetable base stocks including low–euricic
acid rapeseed oils, high oleic soy and rapeseed oils, and improved low-temperature castor oil;
improvements in additive chemistry; and improvements in formulation expertise have allowed the
development of biodegradable products with performance similar to or better than conventional
petroleum uids.
18.3 VEGETABLE BASE FLUIDS
Vegetable oils and other fats are triglycerides that are essentially triesters of fatty acids and glycerol.
They are soluble in most organic solvents and are insoluble in polar substrates such as water. They can
have a broad range of fatty acid pro les, but most commonly used vegetable oils will be C18—oleic, lin-
oleic, or lineleic acids. The proportion of each of these acids depends on the vegetable type, the growing
season, and the geography. These factors can dramatically affect the performance of the vegetable oil
in terms of oxidative stability, cold ow, hydrolytic stability, and other features of the nal product. For
example, the higher the oleic acid content, the better the oxidative stability but the higher (worse) the
pour point. Further discussion of acid content and oil performance is reviewed in other sections.
0
1
2
3
4
5
6
Group I Group II Group III Vegetable Poly alpha
olefin
Polyalkyl-
ene
g
lycol
High visco-
sity PAO
Ester
FIGURE 18.1 Relative cost of various base uids. (Derived from Terresolve Technologies In-house Knowl-
edge Base.)
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Additives for Bioderived and Biodegradable Lubricants 449
18.4 OIL TYPES AND PERFORMANCE
The following chart shows the effect of oleic acid level on oxidative stability as measured by the
pressure differential scanning calorimeter (PDSC). The PDSC is a test frequently used to evaluate
thin- lm oxidation of lubricants and base uids. It is also known as the CEC L-85-T-99. The test
measures the minutes to induction time, and the longer the time, the better the oxidative stability.
Figure 18.2 [4] shows that the there is a direct correlation between the oleic acid content of a vegeta-
ble oil and its oxidative stability and that the higher the oleic content the higher the oxidative stability.
18.5 ADDITIVE PACKAGES
As previously mentioned, early vegetable oil formulators used performance additives similar to
the performance additives used for petroleum base uids. Petroleum oils are, for the most part,
nonpolar, whereas triglycerides are highly polar. As such, conventional petroleum additives some-
times have solubility problems with vegetable oils. Frequently, to utilize typical additive packages,
a solubilizing agent must be used. Additive packages without metals, such as ashless additives, are
more soluble in vegetable oils.
Even today, several additive manufacturers try to promote additive packages targeted to vegetable
oils. Although major additive companies are highly skilled at performance chemistry for petroleum
base oils, the expertise needed for formulating vegetable oils is highly specialized. Adding to it is
the complication of maintaining environmental integrity, which makes it a daunting task.
Most additive packages supplied by the major additive companies are speci cally designed for
petroleum base oils. Even those targeted toward vegetable oils utilize core chemistry tailored for
petroleum. Additionally, the carrier uid from the various components is usually petroleum, and
when a multicomponent package is created, several percent of petroleum is included. This will
reduce biodegradability and increase toxicity of the nal uid, that is, counter the objective of the
product.
To make high-performance uids from vegetable oils, a clean sheet approach must be utilized,
and the uid must be designed with the speci c characteristics of the base uids and the speci c
requirements of the end products. Some typical petroleum additives can be utilized and are effec-
tive in vegetable oils. Frequently food-grade additives are superior and have lower toxicity. When
formulating vegetable-based lubricants, it is useful to expand the search into food additive and other
nonlubricant industries. Table 18.3 [5] summarizes the performance of some conventional additive
packages designed for both petroleum and vegetable base oils as compared with a custom-designed
0
10
20
30
40
50
60
70
80
Minutes to induction
60 75 85
Oleic acid content
FIGURE 18.2 PDSC oxidation stability. (Based on Bergstra, R., Emerging Opportunities for Natural Oil
Based Chemicals, MTN Consulting Associates, Plant Bio-Industrial Workshop, Saskatoon, Saskatchewan,
Canada, February 27, 2007.)
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450 Lubricant Additives: Chemistry and Applications
performance formulation using vegetable oil and speci c additives. The rotary pressure vessel oxi-
dation test (RPVOT) known as the ASTM D-2272 is a common test procedure that compared the
oxidative stability of lubricants and base uids. The ASTM D-665B rust test is also a typical indica-
tor of the rust protecting characteristics of a nished uid.
For the past several years, Valvoline has been working toward the development of a bio-containing
engine oil. In this work, they compared results of a mid-oleic soy oil and petroleum oil mixture with
a conventional additive package with the results from a clean sheet design. Taking the characteristic
of bio uid under consideration, they were able to achieve vastly superior results in the Sequence VII
bearing corrosion test (Table 18.4) [6].
18.6 DETERGENTS
Vegetable oils have a high level of solvency and by themselves act as a detergent for many
applications. Vegetable oils and methyl esters of vegetable oils have been successfully used as
solvents and cleaners for many years.
When attempting to add detergent performance, phenates and sulfonates should be avoided, as
they are frequently toxic for environmentally safe uids. The phenate and sulfonate functional groups
are not the problem, but conventional petroleum detergent additives that are typically attached to
heavy metals, such as calcium, sodium, and barium, are highly toxic. Specialty metal-free phenates
and sulfonates have been used as a component to protect seal materials. These products have shown
collateral bene ts. For example, some classes of phenates have shown improvements in thermal
stability to natural oils, and some sulfonates show an additional side bene t of corrosion inhibition.
TABLE 18.3
Performance of Various Additive Packages in Vegetable Oil
Custom
Formulation
Vegetable
Additive
Package 1
Vegetable
Additive
Package 2
Petroleum
Additive
Package 1
Petroleum
Additive
Package 2
Petroleum additive package 1 X
Petroleum additive package 2 X
Vegetable additive package 1 X
Vegetable additive package 2 X
RPVOT D2272 125 76 16 41 16
Rust D665B Pass Pass Fail Pass Fail
Source: Derived from In-house Terresolve Technologies Proprietary Development Research, October 11, 2005 through
January 8, 2007.
TABLE 18.4
Sequence VII Performance
Engine Test Result Weight Loss (mg) Stripped Viscosity (cSt) at 10 h
GF 4 Spec limit 26 Stay in grade
Traditional package 108.6 11.51
Unique package 4.0 11.09
Source: Based on McCoy, S., United Soybean Council Technical Advisory Panel, The Valv-
oline Company, September 20, 2005.
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Additives for Bioderived and Biodegradable Lubricants 451
Low amounts of phosphate and phosphate esters have been used successfully in vegetable
products showing good performance without adversely affecting the environmental performance.
Sulferized phenates and salicylate chemistries are typically not used in bio oils as they adversely
affect the environmental bene ts of a vegetable oil. There has been some work, although very sparse,
on the detergent and antiwear bene ts of thiophosphates, phosphonates, and thiophosphonates.
These additives and their bene ts, however, are of limited value in formulating vegetable-based
lubricants because vegetable oils on their own have suf cient detergent and antiwear capabilities
and do not require supplementation.
18.7 DISPERSANTS
Almost all dispersants found are designed for petroleum oils. They typically contain a long tail
that will be soluble in the oil and polar head to keep certain contaminants in the oil. These
contaminants can be ltered out later or are swept away during an oil change. The vegetable oil
molecular size is too small to act as a dispersant, and while the polar head is highly polar, the
molecular chain length is too small to accommodate dirt and sludge.
Typical vegetable-based lubricant applications such as hydraulic uids and total loss lubricants
(bar-chain oil and 2-cycle oil) have little need for dispersants. There has, however, been some work
done evaluating dispersants for vegetable oil-containing engine oils. This work has shown that the
most effective way to add dispersancy without signi cantly reducing other performance character-
istics is through a dispersant polymer. As it is one additive that can be used for multiple purposes,
a side bene t to a dispersant polymer is that it keeps the amount of nonbiodegradable, nonbiobased
products to a minimum. Conventional high-molecular-weight dispersants such as polyisobutylene
(PIB), PIB succinimide, and PIB succinate ester are not used as they adversely affect the environ-
mental performance.
18.8 CORROSION INHIBITORS/ANTIRUST
Rust is a chemical reaction between water and ferrous metals, whereas corrosion is a chemical reac-
tion between chemicals (usually acids) and metals. As previously mentioned, water in any lubricant
system is bad. It can cause rust on metal surfaces and reacts with chemicals in the system to produce
acids that cause corrosion. Vegetable oils can utilize more novel (expensive) acidic rust inhibitors
helping to protect against corrosion and rust. Typically, acid inhibitors in conventional additive
packages form precipitates that cause lter plugging. Sulfonates and acid rust inhibitors are more
surface active and perform better in vegetable-based oils.
18.9 ANTIOXIDANTS
Vegetable oil’s primary weakness for use in industrial applications is its oxidative instability.
Oxidation occurs with the interaction of oil and oxygen. Vegetable oils (triglycerides) have a number
of double bonds depending on the type of the oil. Through use, these uids develop free radical ions.
The double bonds in the oil are searching for ions (free elections). The double bond reacts with the
free electrons and will change the characteristics of the vegetable oil. This will open a double bond
and allows for degradation of by-products. There is a very good correlation between the degree
of unsaturation of a vegetable oil and its oxidative stability. Table 18.5 [7] summarizes several oil
types and their oxidation characteristics as measured by the PDSC (see Section 18.4 for a detailed
description). The onset oxidation temperature and the oxidative stability can be predicted using
fatty acid composition rather than individual fatty acid percentage.
Vegetable oils have signi cantly lower oxidative stability than petroleum uids. The stability can
be improved through the proper choice of vegetable type, chemical modi cation, and
antioxidant
additives.
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452 Lubricant Additives: Chemistry and Applications
The study on oxidative stability (Section 18.5, Table 18.3) mentioned earlier was continued to
include antioxidant additives to both the vegetable additive package and the petroleum additive pack-
age. One can see from Table 18.6 that although both additive packs can have oxidative stability improve-
ments, neither was able to reach the performance level of a custom formulation hydraulic uid.
Some unique antioxidants such as zinc diamyl dithiocarbamate (ZDDC), alkylated diphenyl
amines (ADPA), and butylated hydroxyl toluene (BHT) improve in both PDSC and RPVOT in soy
oils. These also have shown a synergistic effect with some antiwear agents such as antimony dialkyl
dithiocarbamate (ADDC) as shown in Figure 18.3.
These chemistries can have a positive effect on the oxidative performance of vegetable oils.
However, they can be toxic and persistent and reduce the environmental bene ts of the uids.
There are two other widely used methods for oxidation control. The rst is free radical
scavenging and the other is hydroperoxide decomposition. Radical scavenging will lock up free
radicals and prevent oxidation. Commonly used free radical scavengers for vegetable oils are aryl-
amines and hindered phenol. Zinc dialkyldithiophosphate (ZDDP) is a widely used free radical
scavenger and an excellent antiwear agent but is not often used in vegetable oil formulations due
to its high level of toxicity.
TABLE 18.5
Degree of Saturation and PDSC Onset Temperature in Soybean Oils
Base Fluid Name Unsaturation
PDSC Onset
Temperature (°C)
Soybean oil SO 1.5 173
High linoleic SO HLSO 1.4 179
Mid oleic SO MOSO 1.14 190
High oleic SO HOSO 0.94 198
Source: Extracted from Erhan, S. Z., Oxidative Stability of Mid-Oleic Soybean Oil: Syner-
gistic Effect of Antioxidant-Antiwear Additives, National Center for Agricultural
Utilization Research, USDA/ARS, Peoria, IL, 2006.
TABLE 18.6
Effect of Various Antioxidants on Vegetable Oil Formulations
12 3 5 6
Custom
Formulation
Vegetable
Additive
Package
Vegetable Additive
Package +
Supplement 1
Petroleum Additive
Package +
Supplement 3
Petroleum Additive
Package +
Supplement 2
Petroleum additive
package 1
XX
Vegetable additive
package 1
XX
AO supplement 1 X
AO supplement 2 X
AO supplement 3 X X
RPVOT D2272 125 76 100 41 52
Rust D665B Pass Pass Pass Pass Not tested
Source: Derived from In-house Terresolve Technologies Proprietary Development Research, October 11, 2005 through
January 8, 2007.
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Additives for Bioderived and Biodegradable Lubricants 453
The other widely used oxidation inhibitor mechanism is hydroperoxide decomposition.
Hydroperoxide is another precursor to oxidation, and breaking it down can signi cantly improve
oxidation stability. The main additive components for hydroperoxide decomposition are phosphorus-
based additives as they are mostly environmentally benign and very effective. Other types of
hydroperoxide decomposers such as sul des, phosphates, and ole ns are not used due to environ-
mental incompatibility.
18.10 ANTIWEAR
Vegetable oils can have excellent lubricity, far superior than that of mineral oil. The polarity of veg-
etable oil improves antiwear characteristics as they have an af nity to metal and protect the surface.
Some RBD vegetable oils have passed hydraulic pump wear tests, such as ASTM D2882 and ASTM
D2271, in their natural state with no additives. These unadditized oils thermally break down due to
their oxidative instability but perform well in the wear tests.
Amine phosphate compounds (APCs) are used in readily and inherently biodegradable prod-
ucts. They do not work well in re ned petroleum as they do in vegetable oil.
Some commonly used antiwear additives have been utilized in vegetable oils and found to
reduce oxidative stability when combined with certain antioxidants. These include, for example,
APC and molybdenum dialkyl phosophordithioate (MDPDT) with ZDDC (Figure 18.4).
0
20
40
60
80
100
120
140
160
SO 1% ZDDC 2% ZDDC 3% ZDDC 2% ZDDC +
1% ADDC
FIGURE 18.3 Effects of antioxidant and antiwear on soy oil RPVOT. (Extracted from Erhan, S. Z., Oxidative
Stability of Mid-Oleic Soybean Oil: Synergistic Effect of Antioxidant-Antiwear Additives, National Center for
Agricultural Utilization Research, USDA/ARS, Peoria, IL, 2006.)
FIGURE 18.4 Oxidation effects of various antiwear additives in vegetable oil formulations. (Extracted
from Erhan, S. Z., Oxidative Stability of Mid-Oleic Soybean Oil: Synergistic Effect of Antioxidant-Antiwear
Additives, National Center for Agricultural Utilization Research, USDA/ARS, Peoria, IL, 2006.)
0
50
100
150
200
250
2% ZDDC 2% ZDDC
+ 2% APC
2% ZDDC
+ 2% APC
MDPDT
2% ADDC
PDSC
RPVOT
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