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374 Lubricant Additives: Chemistry and Applications
involved in orientation process and moreover could restrict orientation of PIB macromolecules and
decrease tackiness.
13.7 CONCLUSIONS
Theoretical model developed in this chapter is similar to that in Ref. 9, where the withdrawn jet is
treated as a slightly cross-linked elastic gel. The model [9] was modi ed in this chapter by includ-
ing two new features. First, we took into account the exudation of the swollen gel under extension,
a common effect for dilute polymer solutions, observed in our experiments. Second, we made a
nonsteady extension of an earlier stationary theory [9]. Fitting the theory with experimental data
allowed us to well interpret and describe the data.
The results of this chapter clearly demonstrate that evaluation of tackiness by the open siphon
technique presents a simple and reliable method useful for lubricant industry applications.
ACKNOWLEDGMENT
We express high gratitude to David DeVore of Functional Products for initiating and supporting
this research.
REFERENCES
1. Litt, F. Tackiness and antimisting additives, in Lubricant Additives: Chemistry and Applications, L.R.
Rudnick Ed., Marcel Dekker, New York, pp. 355–361, 2003.
2. Elias, E.G., Ed. Macromolecules 1 & 2, Plenum Press, New York, 1984.
3. Treloar, L.R.G. Physics of Rubber Elasticity, 3rd ed., Clarendon Press, Oxford, 1975.
4. Larson, R.G. Constitutive Equations for Polymer Melts and Solutions, Butterworth, Boston, MA,
1988.
5. Astarita, G. and L. Nicodemo. Extensional ow behavior of polymer solutions. Chem. Eng. J., 1, 57–61,
1970.
6. Boger, D.V. and K. Walters. Rhelogical Phenomena in Focus. Elsevier, New York, 1993.
7. Leonov, A.I. and A.N. Prokunin. Nonlinear Phenomena in Flows of Viscoelastic Polymer Fluids,
Chapman and Hall, New York, pp. 106–108, 338–342, 1994.
8. Leonov, A.I. and A.N. Prokunin. On spinnability in viscoelastic liquids. Trans. Acad. Sci. USSR, Fluid,
Gas Mech., 4, 24–33, 1973.
9. Prokunin, A.N. A model of elastic deformation for the description of withdrawal of polymer solutions,
Rheol. Acta, 22, 374–379, 1984.
FIGURE 13.11 Tackiness effect versus concentration of ethylene–propylene copolymer added to the 0.025%
PIB solution in lubricant oil.
80
90
100
110
120
130
140
0 0.01 0.02 0.03 0.04 0.05
C (%)
Z (mm)
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Tackifi ers and Antimisting Additives 375
10. Landau, L.D. and V.A. Levich. Upward withdrawal of vertical plate from viscous liquid. Acta Phys.
Chem. USSR (Russian), 17, 42, 1942.
11. Levich, V.G. Physico-Chemical Hydrodynamics (Russian), Fizmatgiz, Moscow, 1959.
12. Vistanex, Properties and Applications, Exxon Corporation, Irving, Texas, 1993.
13. Irvin, R.J., Ed. Environmental Contaminate Encyclopedia, Mineral Oils, General Entry, John Wiley
and Sons, NY, NY, 1989.
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377
14
Seal Swell Additives
Ronald E. Zielinski and Christa M. A. Chilson
CONTENTS
14.1 Introduction ......................................................................................................................... 377
14.2 Fluid/Seal Swell Additives during 1935–1960 ....................................................................378
14.3 Fluid/Seal Swell Additives during 1961–1980 .................................................................... 378
14.4 Fluid/Seal Swell Additives during 1981–2000 .................................................................... 379
14.5 Fluid/Seal Swell Additives during 2001–2007 ....................................................................380
14.6 Trends and the Future ..........................................................................................................380
14.7 Industrial Fluids ..................................................................................................................380
14.8 Seal Swell Dilemma ............................................................................................................ 381
14.9 Cost-Effectiveness ............................................................................................................... 381
14.10 Manufacture ........................................................................................................................382
14.11 Summary ............................................................................................................................. 382
14.1 INTRODUCTION
The use of additives in lubricants, both military and civilian, began around 1930. Before that time,
additives in lubricants were not required as most uid systems operated at low pressure and low
cycling rates. Seal swelling agents were not required because seals were generally made from
leather or similar materials not subject to swelling by the petroleum-based uids in use at that time.
The use of additives to improve uid performance began with the military uids. Industrial uid
development paralleled or followed the military uid development.
With the enhanced development of military aircraft and reciprocating engines spurred by World
War II starting approximately in 1935, advanced ghters and bombers required new, high-perfor-
mance lubricating uids and seals. The new military uids were based largely on petroleum base
stocks or mineral oils for hydraulic systems. These consisted of paraf nic, aromatic, and alicyclic
(naphthenic) components. Various additives including rust inhibitors, oxidation inhibitors, deter-
gents/dispersants, viscosity index (VI) improves, pour point depressants, antiwear components, and
antifoam materials were used to develop additive packages for these hydraulic uids depending
on the aircraft type and intended missions. These development efforts resulted in the creation of
a military speci cation—MIL-H-5606—for these uids. The speci cation allowed for some ex-
ibility in formulation for competitive purposes and guaranteed a performance band for the uid and
its compatibility with elastomer seals.
Similarly, new reciprocating engine lubricants based primarily on diesters of both aromatic and ali-
phatic acids and additive packages for uids intended for each aircraft engine type, that is, high- ying
bombers (B-17s) and ghters (P-47s), were developed. These were the early versions of MIL-L-7808.
High-acrylonitrile-content butadiene–acrylonitrile (NBR—Acrylonitrile-Butadiene Rubber) seals
were used for these aircraft hydraulics and they worked ne except when strategic aircraft ew for
long time periods at very high altitudes (i.e., >30,000 ft): the seals leaked, thus the base polymer had
to be modi ed with a lower acrylonitrile content to improve low-temperature performance. Of course,
seals made from NBR with reduced acrylonitrile content have higher butadiene content and thus
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378 Lubricant Additives: Chemistry and Applications
would have increased volume swell in petroleum-based uids. This caused many problems. Programs
driven by the military had material and chemical engineers working together and adjusting both uid
and seal formulations to solve the problems. The results of these coordinated efforts were the develop-
ment of uids and seals that were complementary.
It is very dif cult to speak in speci c terms with regard to seal swell additives, as commercial
uids have additive packages, which are considered very proprietary and highly con dential. To
address seal swell additives, we will follow the development of military uids and seals in three
basic time frames: 1935–1960, 1961–1980, and 1981–2007. In general, speci c information of com-
positions of most aircraft uids, including seal swell additives, is extremely limited and is usually,
again, company con dential. However, because these developments were, at least, in part supported
by the military, more information is available. We describe what is known about swelling agents, yet
recognize that generalities will have to suf ce due to the con dential/proprietary nature of formu-
lated uids. Industrial uid development, including seal swell additives, followed the military uid
development, and the additives used in military uids were used in the industrial uids, therefore,
in essence, by following the military uids, we are also following the industrial uids as both used
essentially the same seal swell additives.
14.2 FLUID/SEAL SWELL ADDITIVES DURING 1935–1960
During this period, petroleum-based uids predominated as the uid of choice for hydraulic uids.
The earliest versions of the military uid designated MIL-H-5606 (mineral oil) had a higher aromatic
content and thus provided suf cient volume swell for the high-acrylonitrile (ACN) NBR (nitrile) seals
used in the low-pressure (∼1500 psi) hydraulic systems in piston engine aircraft. As aircraft develop-
ment proceeded, more powerful piston engines were developed for the very long-range bomber mis-
sions, primarily the B-17, to be used by the Army and Air Force for attacks on enemies in Europe and
the Far East theaters. The long-range bombers ying at very high altitudes, 35,000–45,000 ft, caused
oil thickening and seal leakage, which resulted in sluggish performance. The seals used were nitrile
seals designated by the military as MIL-R-25732. Fighter aircraft, both land-based and aircraft car-
rier-based, did not experience problems because they ew at lower altitudes and for shorter missions.
To solve the bomber uid problem, the mineral oil base uid was further re ned to replace some aro-
matic content with aliphatic content to improve low-temperature viscosity at −65°F. This resulted in
a revised version of MIL-H-5606 hydraulic uid. Low-temperature sealing performance of the high-
ACN NBR-content MIL-R-25732 seals was improved by replacing some of the high-ACN NBR with
medium- or low-ACN NBR as the base polymer or replacing part of di-2-ethylhexylphthalae (DOP)
processing oil used in seal compounding formulations with di-2-ethylhexylsebacate (DOS) oil. Fluid
formulators added suf cient diester uid, such as DOS, to the MIL-H-5606 to maintain 18–30%
seal volume swell. Although very early piston engines used petroleum-based oils as lubricants, this
gave way to pentaerythritol ester-based uids to improve low-temperature performance at −65°F
and maintain high-temperature performance at 275°F. MIL-R-25732 seals were used for engine lube
sealing. For naval aircraft, antirust additives were added to uid formulations for obvious reasons.
Commercial aircraft versions of military aircraft were developed during this era, that is, Douglas
DC-3 (C-47 military transport), Douglas DC-6, and Lockheed Constellation. These aircraft initially
used military aircraft hydraulic oils/seals and engine lubrication oils/seals. However, because of the
short ight duration and many takeoffs and landings of these aircraft, a new, more re-resistant uid
was introduced. This new uid had a phosphate ester base stock, which created the need for seals
made with a polymer compatible with this new base stock.
14.3 FLUID/SEAL SWELL ADDITIVES DURING 1961–1980
As advanced military aircraft with enhanced performance ( jet aircraft) and missions were
developed, both hydraulic system and engine operating temperatures increased. Oil companies
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Seal Swell Additives 379
and seal manufacturers responded by enhanced synthetic oil development and correspond-
ing seal development to meet aircraft and gas turbine engine manufacturer’s needs for −65 to
350°F performance. Oil companies synthesized thermally stable polyalphaole n (PAO) u-
ids to meet the desired 350°F high-temperature uid requirement; this was formulated with
various additives such as antioxidants, lubricity additives, antiforms, metal deactivators, rust
inhibitors, VIIs, and seal swell materials to meet 18–30% volume swell. The rst synthetic
military hydraulic uid developed was less ammable than MIL-H-5606 and was designated
as MIL-H-83282. MIL-H-83282 had an operational range of −40 to 350°F. Seals based on
sulfur-donor or peroxide-cured NBR were developed for this uid. The new seals were designated
as MIL-R-83461 and were developed to meet the −65 to 350°F operational requirements.
The MIL-H-83282 uid contained a signi cant amount of diesters to achieve desired seal vol-
ume swell. This uid found almost immediate use in most military aircraft, except for the Air
Force strategic aircraft; for example, the B-52, B-1B, KC-135, and KC-10, which required −65°F
viscosity, did not use this uid, which became very viscous below −40°F. This shortcoming was
later resolved by the development of a PAO-based uid designated MIL-H-87257, which is cur-
rently being implemented. This low-temperature PAO uid was achieved by the synthesis of lower-
molecular-weight PAOs and replacing part of the higher-molecular-weight fractions present in
MIL-H-83282. This approach caused a small loss in non ammability capability but met the desired
capability of a universal less- ammable hydraulic uid for all military aircraft and with a −65 to
350°F range capability.
During this time frame, engine lubrication uid requirements were also increased to −65 to
350°F, which led to the development of diesters of both adipic and sebacic acids, especially diocty-
ladipate (DOA) and DOS, with the latter predominating. These were formulated to later versions of
MIL-L-7808 for both military and commercial aircrafts. During this time, seals were also formu-
lated with uorocarbon polymers to meet a −40 to 350°F operational requirement for these uids.
These seals were designated by the military speci cation MIL-R-83485. Seal swelling additives
were not needed for the MIL-L-7808 lubrication uids, as both NBR and uorocarbon (FKM) seals
exhibit swelling in these uids.
During this 1961–1980 time frame, commercial aircraft were using the non ammable phos-
phate ester uids. Ethylene-propylene-(EPM) or ethylene-propylene diene-(EPDM)-based elasto-
meric seals were developed for these uids. As the phosphate ester base stock acts as a plasticizer,
there was no need for seal swell additives in these uids.
14.4 FLUID/SEAL SWELL ADDITIVES DURING 1981–2000
As mentioned previously, MIL-H-83282, a PAO-based hydraulic uid, was developed during the late
1960s and during 1970s, with implementations in most nonstrategic military (i.e., F-16, F-14, and
F-18 ghters) aircraft beginning in the late 1970s after extensive ight-testing. The Air Force’s stra-
tegic aircraft—long-range/high-altitude bombers, tankers, and reconnaissance aircraft—continued
to use MIL-H-5606 to accommodate its need for a hydraulic uid with a −65 to 275°F capability.
The desire for a less- ammable uid with better low-temperature viscosity led to the development of
MIL-H-87257. This uid balanced the PAO content with the proper molecular weight range of PAO
components. With the proper additive package, including suf cient quantities of diesters (i.e., DOS)
to achieve seal swell of 18–30%, dynamic cycling testing indicated that this new uid had the desired
capability of −65 to 350°F as well as compatibility with both MIL-R-82485 ( uorocarbon) and MIL-
R-83461 (NBR) seals. Flight-testing was conducted on all military aircraft, including strategic air-
craft, and it appeared likely that MIL-H-87257 would eventually be the universal hydraulic uid (−65
to 350°F) for military aircraft. During this time, commercial aircraft continued to use phosphate ester
uids for hydraulics with the previously mentioned EPM/EPDM-based seals. Engine lubrication uids
were still primarily the DOS-type, MIL-L-7808, uids for both military and commercial jet aircraft.
Seal swelling materials are not required as the diester uids accomplish this purpose.
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380 Lubricant Additives: Chemistry and Applications
14.5 FLUID/SEAL SWELL ADDITIVES DURING 2001–2007
MIL-H-83282, a PAO-based hydraulic uid, has replaced MIL-H-5606 as the principal hydraulic
uid used in military aircraft. The advanced military ghter aircraft, namely, the F-22, F-35, and
the latest model F-18, require MIL-H-83282. Currently, seals for these systems are MIL-P-83461
NBR and the higher-temperature capability MIL-R-83485 uorocarbon materials. MIL-H-83282 is
currently required in the main systems of all eet aircraft previously using MIL-H-5606; however,
MIL-H-5606 is still used in some areas, such as viscous dampers, due to its better low-temperature
(–65°F) characteristics. Older aircraft that had been using MIL-H-5606 are gradually changing over
to MIL-H-83282 by attrition.
Even with the trend of newer commercial aircraft, namely, Airbus’ A380 and Boeing’s 787, to
have higher hydraulic system pressures up to 5000 psi, commercial aircraft continue to use phosphate
ester uids with the previously mentioned EPM/EPDM-based seals. Improvements in re properties
and other areas of performance continue to be made with these phosphate ester uids; however, seal
swelling materials are still not required because the diester uids accomplish this purpose. No new
types of uids or seal swell additives have been introduced for engine lubrication uids. The DOS-
type, MIL-L-7808, uids are still primarily used for both military and commercial jet aircrafts.
14.6 TRENDS AND THE FUTURE
It is very likely that advanced military ghter aircraft will continue to have the requirement of
hydraulic uid with a −40 to −350°F capability with a pressure of 4000 psi. As MIL-H-83282
meets these requirements, it should continue to be the primary hydraulic uid for these aircraft.
MIL-H-5606 will still be used in areas requiring better low-temperature (–65°F) performance until
progress is made on improving the low-temperature properties of PAO-based hydraulic uid or
new base uids are developed. Newer polymers are also investigated as potential seal materials for
these systems as nitrile elastomers oxidize when exposed to temperatures above 250°F for extended
periods of time, and uorocarbon elastomers cannot achieve the low-temperature requirement
of −65°F.
Strategic aircraft needs over the next 20 years will likely be met by the updated C-17 transport,
the new A300, KC-10, or a tanker version of the C-17 (a KC-17), whereas strategic bomber needs
are likely to be met by the B-1B, B-2, and an advanced version of the B-2 stealth aircraft. Although
there could be some changing needs in both hydraulic and engine lubrication uid needs, it is
currently not possible to determine speci c uid/seal needs or seal swelling agent needs.
Commercial aircraft requirements over the next 20–30 years are not likely to involve major
changes. Airframe designs and engine requirements will not change substantially but will likely
feature incremental improvements in performance and, as always, be dependent on customer
requirements. Therefore, a dramatic change is not expected in hydraulic or lubrication uid/seal
needs for commercial aircraft.
14.7 INDUSTRIAL FLUIDS
As stated earlier, industrial uid development has closely followed the development of military
uids. Natural base oils including mineral oils have been used in industrial uids. These uids are
not as controlled as military uids, which have developed speci cations to meet and essentially
perform the same within a narrow band of additive variability, allowed by the speci cation for com-
petitive bid requirements. Industrial uids vary widely as a result of their crude source, for example,
whether paraf nic, naphthenic, mixed, paraf nic–naphthenic, and the like, as well as their forma-
tion, for example, distillation range, straight run or cracked, hydrore ned, solvent extracted, and the
like. Because these uids vary widely, the additive packages are very proprietary, but again the seal
swell additives parallel those used in military uids.
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Seal Swell Additives 381
DOS and DOP are used in industrial uids. Other seal swell additives include mineral oils with
aliphatic alcohols such as tridecyl alcohol. Oil-soluble, saturated, aliphatic, or aromatic hydrocar-
bon esters such as dihexylphthalate are very popular. Trisphosphite ester in combination with a
hydrocarbonyl-substituted phenol is also used as a seal swell additive. As more highly re ned min-
eral oils and synthetics based on PAO enter the market, it is not likely that there will be signi cant
differences in seal swell additives.
14.8 SEAL SWELL DILEMMA
When a uid is formulated, the usual concentration of seal swell additive is 0.6–1.0% by volume.
During development, the uid manufacturer uses standard test elastomers to determine uid seal
swell, which is normally targeted at 18–30%. These tests are normally 70–168 h in duration and
do not allow for a full measure of uid/seal compatibility evaluation. The uid is then marketed
as compatible with the elastomer seal. The dilemma is that the seal used in the actual system is
much different than the elastomer used for seal swell determination, and swell beyond 20% is
normally not allowed by elastomer speci cation or desired from a performance standpoint. Thus,
we have uid, developed to produce high seal swell, and an elastomer seal that is not allowed to
swell beyond 20%. This dilemma occurs because uid and seal development are no longer coordi-
nated. In the past with the military driving parallel development programs, uids and seals were
simultaneously developed; this is not true in today’s commercial arena. The uid manufacturer has
been taught that high seal swell is good. It allows for more seal force and less compression set. The
problem is that seals that swell excessively soften and completely ll the seal gland. These condi-
tions for dynamic seals lead to nibbling of the seal as it over ows the gland and to excessive seal
wear. This leads to premature seal failure and leakage, which results in costly downtime while
seals are replaced.
To put uid developers on the same track as seal developers, ASTM International Committee
D 02 has developed ASTM D6546. This speci cation details a uid–elastomer seal test program,
which involves the testing of commercially available elastomers in the new uid at actual service
temperatures for long periods of time (1000 h). These test conditions with recommended limits
allow for a more complete evaluation of uid–seal compatibility and should allow for optimized
uid formulations with a minimum amount of swell additive.
Changes to increase test times are also gradually made in new uid speci cations as more
importance is placed on reliability and long-term performance. The commercial aerospace industry
is becoming the driving force behind newer and updated speci cations, such as SAE AS5780, with
increased test times using commercially available uids tested with commercially available seal
materials. The military is slowly following.
14.9 COST-EFFECTIVENESS
It is important to minimize the amount of seal swell additive in uid formulations. Seal swell addi-
tives increase the cost of mineral- and PAO-based hydraulic uids. This is because the rather large
amounts of DOS is expensive when compared to the cost of the base stocks, but it accomplishes the
purpose of seal swell. It must be remembered that quali cation testing of uids and seals, not to
mention quality control testing of both, is inherently expensive. This is why the implementation of
ASTM D 6546 should be a cost-effective, integral part of the uid development process. The uid
developer can also use the results of the tests to instill con dence in the uid users that they are
provided with a uid and recommendations for seals substantiated by long-term testing.
The long-term testing can also provide useful information to the uid user, which can help
plan preventative maintenance programs, which would avoid or minimize costly downtime. Fluid
stability tests are already performed for 1000 h; these additional tests would be extremely cost-
effective in optimizing the uid formulation and addressing total system needs— uid and seals.
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382 Lubricant Additives: Chemistry and Applications
14.10 MANUFACTURE
Manufacturing of both uids and seals is an extremely competitive and very proprietary business.
This has not changed over the years and is not likely to change in the foreseeable future. In general,
the following are true: (1) the competitive nature of the two business types has controlled product
costs and (2) the uids with the reputation for long life with a minimum of problems are the most
successful. Fluid formulation plans that include testing to ASTM D 6546 or similar speci cations
can be very cost-effective over the long run as uids would be developed with optimum concentra-
tions of expensive additives, and long service life with compatible seals would be demonstrated.
14.11 SUMMARY
Seal swell additives are necessary in many uid formulations; seal swell can be bene cial in slow-
ing compression set. However, excessive seal swell can be deleterious to seal life and system per-
formance. Seal swell additives add cost to the nal uid formulation, therefore, minimizing the
concentration of these additives is important from a nancial standpoint. For both the uid and the
seal developers, ASTM D 6546 represents a method to ensure that the seal and the uid with its
additive package are fully compatible. Whatever the proprietary formulation is, the uid developers
can demonstrate seal compatibility with con dence over a long period of time. They can also tailor
the amount of seal swell additive for maximum seal performance and minimum cost.
SAE AS5780 Rev. A October 2005 Speci cation for Aero and Aero-Derived Gas Turbine
Engine Lubricants.
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383
15
Antimicrobial Additives for
Metalworking Lubricants
William R. Schwingel and Alan C. Eachus
CONTENTS
15.1 Introduction ........................................................................................................................ 383
15.2 Microbiological Growth .....................................................................................................384
15.2.1 Bacteria ..................................................................................................................384
15.2.2 Fungi ......................................................................................................................384
15.2.3 Bio lms .................................................................................................................384
15.3 A Microbe-Friendly Environment ......................................................................................385
15.4 Design, Maintain, and Monitor .......................................................................................... 385
15.5 Complications and Challenges ........................................................................................... 386
15.6 Testing for Contamination ..................................................................................................387
15.7 Treatment Options ..............................................................................................................388
15.7.1 Chemical Methods ................................................................................................388
15.7.2 Popular Chemistries ..............................................................................................389
15.7.3 Biocide Combinations ...........................................................................................392
15.7.4 Methods of Biocide Application ........................................................................... 392
15.7.5 Biocide Selection ................................................................................................... 393
15.7.6 Biocide Handling ..................................................................................................394
15.7.7 Physical and Nonchemical Control Methods ........................................................ 394
15.7.8 Enhancement of Fluid Protection ......................................................................... 395
15.8 The Future of Microbiological Control ..............................................................................396
References......................................................................................................................................396
15.1 INTRODUCTION
Metalworking uids, the class of lubricants (along with water-containing hydraulic uids) most
susceptible to microbial attack, provide a number of vital functions in metal removal and form-
ing operations. Designed as a coolant (to remove heat) and a lubricant (to reduce heat generation)
and to provide corrosion resistance, electrochemical resistance, extended resistance to microbial
degradation, and eventual biodegradability in the environment, a metalworking uid must also be
safe for human use and exposure. The failure of the uid to perform any one of these has the poten-
tial to result in operational problems, process shutdowns, decreased tool life, and product-quality
issues, all of which will result in increased costs. Perhaps one of the most common and control-
lable complications is microbial degradation. Metalworking uids contain the nutrients that can
permit unchecked microbial growth, including mineral oil base stocks, glycols, fatty acid soaps and
esters, amines, sulfonates, and other organic components. Use-diluted emulsions, synthetic chemi-
cal solution uids, and aqueous uid concentrates are all vulnerable because they contain water, an
important microbial growth requirement and the major source of microbiological contamination.
Conversely, straight oils, although not immune to contamination problems, show greater overall
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