Biodegradable

Hydraulic Fluid 20

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Joel Krohn, Kyle Kane,

Matt Puckett

FPT Project Group 23

Abstract Biodegradable Hydraulic fluid is currently being looked at to revolutionize the hydraulic industry. While it is not going to replace current technology, it is useful in many applications within the industry where mineral oil is not practical. This paper is going to explain the chemical makeup of the fluids and their biodegradability. It will then compare the differences and similarities between conventional mineral oil and the three major types of biodegradable oils and point out specific uses for each. Finally it will go into the future innovations in the field and breakthroughs in biodegradable technology.

The emergence of biodegradable hydraulic fluid technology has been born more out of

necessity than innovation. Anywhere you go, practically every day we interact with a hydraulic

application to some extent. The world we live in would not be possible without hydraulic fluids

and the benefits of their use. In order to understand how biodegradable hydraulic fluid can be

applicable, first we have to look at where hydraulic technology came from and the key

innovations that have allowed hydraulic technology to advance to where it is today. The first

people who recognized the potential uses of hydraulic technology were two Englishmen named

Joseph Bramah and Sir William Georges Armstrong, although they were not aware of each

others work at the time. Armstrong found inspiration in observing a water wheel, realizing that

the potential of the fluid to do work was lost. A way he captured this was creating a hydraulic

piston type device using water to operate a crane. These two men were observing and applying

Pascal’s law before it was widely used and adapted into technology.

Modern hydraulic systems came into popularity during the industrial revolution with the

invention of the hydraulic press. It uses the force from two cylinders of different diameter with

the smaller piston being initialized first, which forces the fluid into the larger cylinder to start

the process of compacting the desired object. A modern example of a hydraulic press would be

the car crushers that are used in junkyards to compact full size cars into small cubes of metal.

Perhaps the industry that has benefited the most from the invention of hydraulic technology

would be the construction industry. If you look at any modern piece of construction equipment,

you are likely to find multiple hoses and pistons operating, and more often than not there will

be multiple hydraulic systems on one machine. The capability to lift heavy loads and transport

heavy pieces of construction material safely and efficiently has improved the speed and

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efficiency in which a project can be finished. What used to take months and sometimes even

years to build 50 years ago can often be completed in a matter of weeks with the

implementation of hydraulic technology.

Hydraulic technology can be applied in many fields. For the purpose of this paper, we

are focusing on applications where hydraulic oils can be biodegradable and not impact the

environment. A recent article placed that a conservative estimate is that more than 600 million

gallons of hydraulic fluids are lost each year. In the United States, the companies are held

responsible for the clean up at often very high costs. However, in marine environments or

mobile equipment not easily accessible, many times these spills go unreported and the fluid

contaminates the surrounding environment. This is where biodegradable fluids in hydraulics

can drastically help companies and the environment. There can be catastrophic environmental

results if these spills are not contained or cleaned up quickly. Some examples of applications

where the biodegradable hydraulic fluid would be especially useful are in marine and dredging

equipment, turf and lawn equipment, and mobile lift and construction equipment. A big

technology that would benefit especially from the biodegradable fluid would be submersible

pumps, as any failure directly contaminates the surrounding water source and depending on

the size and location of the spill could potentially contaminate underground freshwater

aquifers therefore endangering human life.

There are many different compositions of hydraulic fluids, but for the purpose of this

paper we will keep it simple. Most mineral oil hydraulic fluids are made from dewaxed paraffin-

based crude oils that are then blended with various additives for the necessary application. The

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variables that may affect what additives can be temperature, pressure, and differing viscosities.

You can control the condition of the fluids for extreme temperature operations by adding

viscosity index improvers (polyalphaolefins, polymethacrylates, and polyalkylstyrenes), which

reduce the dependence of viscosity on the temperature. For high pressure applications, you can

include extreme pressure additives (organic sulfer, phosphorus, and chlorine-containing

compounds), which will help to prevent surface damage under extreme loading. To increase or

decrease the viscosity, you would increase or decrease the carbon number range. Most fluids

are within the range of C15 to C50 with the higher end being the higher viscosities. The choice of

viscosity is a major factor in determining the base stock of the hydraulic fluid, and the more you

refine the base mineral oil, the better the viscosity properties will be.

Biodegradable hydraulic fluids use vegetable based oils as a base stock instead of a

mineral oil. The main chains in hydraulic fluid are called estolides, which are a class of long

chain esters. Scientists have experimented and implemented a plant-based estolide from

meadow-foam seed, however the low temperature properties and cost were prohibitive to the

operation. The base stock of the oil began to break down at 80o Fahrenheit, which is not

practical for industrial application. Vegetable oil was chosen as a new base for tests to be

conducted. Over 28 days of continuous use, 30 percent of the petroleum/mineral oil based

fluids degraded versus 80 percent of the vegetable-based estolides. However, A reaction of

combining the vegetable oil with sulfuric acid produced an extremely high yield of estolides was

established. Researchers did this by breaking vegetable oils into their two main components of

glycerin and fatty acids. By doing this the researches found that the sulfuric acid acts as a

catalyst to the forming of estolides, and by having more estolides in the hydraulic fluid you have

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higher performance potential and a practical efficiency for industrial application.

Good environmental habits begin with using biodegradable fluid in useful, practical

applications. One area is in the marine environment, where repair and replacement parts are

often days away, the same goes for clean up equipment. If you use biodegradable hydraulic

fluid in these offshore operations, a spill is not nearly as catastrophic to the marine

environment as a mineral based hydraulic fluid would be. Biodegradable hydraulic fluids have

been treated with additive to reduce biodegradation, bioaccumulation, and toxicity. They are

also formulated so they will not oxidize or hydrolyze any more than the mineral-based hydraulic

fluids they are intended to replace.

Before we get into the differences and similarities between biodegradable and

conventional hydraulic oil, it is important to understand what exactly biodegradation really is.

While there is not a universally accepted definition, it is generally agreed that it is a biochemical

process by which substances have the ability to decompose by natural processes in both soil

and water without leaving any harmful or toxic substances behind. This means that the

biodegradable oil, once decomposed by bacteria and other micro-organisms, will not have

killed any living animals or left a negative alteration on the spill site. Biodegradation breaks

down into two general categories; primary biodegradation and ultimate biodegradation.

Primary biodegradation is simply the conversion of the oil into new chemicals and ultimate

biodegradation is where the oil is converted completely into carbon dioxide and water with a

few simple inorganic substances left over (Biodegradation). Now the three different general

types of biodegradable hydraulic oils are going to be looked at individually and compared to

conventional hydraulic oils made from mineral oil.

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Triglycerides are water-insoluble esters that are usually derived from vegetable oils, and

are composed of saturated and unsaturated fatty acids. This type of biodegradable hydraulic oil

is generally made from rapeseed oil, but other oils such as soybean, sunflower and canola are

emerging. The triglycerides are fortified with additives, that are also biodegradable, to enhance

their performance capabilities. These additives are generally corrosion inhibitors, antioxidants,

and pour point depressants (McManus). The current application of this type of fluid is in

mobile machinery, in particular the forestry industry and farming equipment benefit the most

due to their close proximity to environments that cannot be contaminated with conventional

hydraulic fluid.

Triglycerides are currently available in four different viscosity grades, which are VG 22,

32, 46, and 68. Their viscosity index is usually higher than 200, which means that they work

well across a wide range of temperature changes. Since the primary molecules that compose

triglyceride oils are carbon and hydrogen, the physical properties are very similar to mineral

oils. This entails the specific gravity and specific heat, which makes converting from one to the

other a relatively simple process. It is also acceptable to mix the new triglyceride oil with

conventional hydraulic oils, which also aids in transitioning to the biodegradable oils. The

molecular weight of these oils is also almost twice the amount of mineral oil, which increases its

fire resistance. This is because the vapor pressure and volatility are lower which increases its

flash points. These fluids are very “fatty”, which is a property that gives them excellent lubricity

as well.

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The triglycerides also possess drawbacks to them. The first is that their operating

temperatures are lower than that of mineral oils. This increases the necessity of keeping the

oils at lower temperatures because they degrade very quickly in extreme heats. Anything over

eighty degrees is too high for this type of oil. Also the life of the oil is shorter than the life of

mineral oils. One of the largest drawbacks to this type of oil is that it is very susceptible to

water. The water content must be kept lower than 0.1 percent, or the water will begin the

biodegradation process. If water gets into the system, it will begin breaking down the fluid into

alcohol and acids rendering the oil useless as a hydraulic fluid. The storage of the oil must also

be controlled, with the chemical properties dramatically changing as temperatures vary

(Hydraulic Fluid Textbook).

The maintenance associated with triglycerides is pretty similar to that of conventional

mineral oil. The average life of the oil is lower, so more frequent changes are necessary. Also

since water is so damaging to the oil, water traps must be installed into the system to prevent it

from doing damage. This should also be closely monitored throughout the life of the oil.

Calcium should also be monitored in this type of oil as it is very damaging when it comes to the

ability of the filter to perform as it is expected.

Synthetic esters are the next category of biodegradable hydraulic oils. They are very

similar to triglycerides, with the difference being that they are chemically produced instead of

naturally derived. This chemical process is what allows such a wide range of performance

characteristics being achieved with this type of oil. Currently this type of biodegradable oil is

used in applications such as excavators and mobile machinery that are used in cold conditions.

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These oils currently come in viscosity grades VG 22, 32, 46, and 68 with their viscosity index in

the 140-190 range. The need for additives in this type of oil is minimal, which increases the

biodegradability of it. The thermal stability of these oils is better than that of triglycerides and

oxidation is also better. The anti-wear performance is on par with the current mineral oils, and

they are actually able to mix with mineral oils as well with caution exercised in the filtering

process. These oils work very well in higher temperatures and are the choice when it comes to

heavy load bearing applications (Hydraulic Fluids Textbook). The life of these oils is much

longer than that of the triglycerides and is even better than that of mineral oils.

The biggest drawback of synthetic esters is the price. Currently the price of this type of

oil is very high, which is offset a little by the length of time that the oil lasts. The synthetic

esters also perform poorly when water gets into the system. They are not as bad as the

triglycerides but extreme care should be taken to keep water out of the system. Another

problem seen with synthetic esters is that they are not compatible with all of the current seals

used in traditional mineral oil systems. So if plans are to incorporate the oil into an existing

system, care should be taken and testing should be run. This problem is being addressed

though in the form of an additive that will make the oil more compatible.

Maintenance on synthetic esters is very similar to the triglycerides. The life of the fluid

is longer than that of mineral oils so changes should occur less frequently. With water being a

very harmful element to this oil as well, water traps should be installed into the system and

monitored frequently. Calcium buildup can also affect synthetic esters as well, but it is more

tolerant than triglycerides.

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The third type of biodegradable oil is polyglycols. Specifically polyethylene and

polyalkylene are the primary two different glycols used in the polyglycol formation. Currently

these oils are used when applications are going to be near water. This includes bridge lifts and

swimming pool uses. They are available in the viscosity grades of VG 22, 32, 46, and 68 as well,

but only come in lower viscosity indexes around the 150 range. These oils are commonly used

in the food industries already as lubrication. On many levels polyglycols are comparable with

mineral oils. They both have almost equal use life and similar anti-wear performances, which

makes the in between changes very similar. The oil makes for great lubrication and is much

more fire resistant than mineral oil. They also possess the ability to operate in much higher

temperatures than the previous two types of biodegradable hydraulic oil.

The biggest drawback of polyglycols is that they are not able to mix with other oils, so

converting a system over to this type of oil is not really practical. There are also issues with all

of the materials commonly used in systems today that make changing a system to this type of

oil impractical. Of course a new machine can be built with this type of oil in mind and still be

very practical (Schneider). Another drawback is that this oil mixes almost too well with water,

which can pose problems where it is used, which is by water. It is also the least biodegradable

out of the three, which makes it less desirable under most conditions.

The maintenance of polyglycols is very similar to the maintenance of mineral oil. They

both have the same usage life so the intervals between changes will be very similar. Water is

not such a detrimental aspect so it is not as necessary to keep it water free. Care should still be

taken as with any hydraulic fluid because too much will change all of its properties and

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decrease performance. Also it is possible that the oil can become corrosive which will damage

the entire system.

The variety of biodegradable hydraulic fluid contributes to its success in the industry

today. It comes in three primary different types with properties very similar to that of mineral

oils. The physical properties are very diverse and most are even adaptable to specific

applications with the addition of additives. The fluid is sustainable, which can become very

important in the next fifty years when cheap mineral oil is thought to possibly run out. It also

does not do damage to the environment when it is spilled, which happens very frequently. The

next section will delve into the upcoming biodegradable hydraulic oil technology and future

applications and innovations in the different oils.

Vegetable oil based biodegradable hydraulic fluids have, and will continue to have, a

niche in agricultural applications. Vegetable based biofluids perform well against their

synthetic counterparts in these applications except for low temperature flow, oxidation, and

thermal decomposition. Hydraulic biofluid vegetable oil is made up of triglycerides.

Triglycerides consist of one glycerol molecule and three fatty acids. The degree to which the

fatty acids are unsaturated influences its oxidation ability. To better the oxidation quality of the

vegetable based biofluid more oleic acid is needed. Increasing oleic acid is done by plant

hybridization and various oil chemical processes to change the acyl chains’ unsaturated acids to

have a higher concentration of oleic acid. This slightly improves the oxidative stability of the

vegetable oil. These areas in which most biodegradable hydraulic fluids underperform in will be

studied and researched in depth.

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The majority of vegetable biofluids are mono-unsaturated. This gives the biofluid poor

oxidation resistance and poor lubrication properties within the hydraulic system. These subpar

properties, as compared with synthetic oils, still allow the hydraulic system to operate

sufficiently. For example, Sandia National Laboratories now use a soybean based hydraulic fluid

in their equipment after a twenty five gallon hydraulic oil spill. Johnson & Johnson uses a

canola based hydraulic biofluid in their elevators (Alias, Yunus, and Idris 89). Although these

biofluids mentioned above work adequately in their respective hydraulic systems, recent

research regarding palm based trimethylolpropane ester (TMPE) has been done to improve on

the poor lubrication properties of current vegetable oil in hydraulics. The tested biodegradable

hydraulic fluid was created by using TMPE as the base oil and utilizing trimethylolpropane

(TMP) fatty acid methyl ester. The TMP fatty acid methyl ester was prepared through use of

sodium methoxide and palm oil as a catalyst. This hydraulic biofluid was found to be

comparable to current commercial hydraulic biofluid in many aspects except for its high pour

point. To increase the lubrication performance and pour point of the TMPE biofluid; Iragox

L135 was added. Iragox L135 is a phenolic antioxidant of high molecular weight normally used

in lubrication, and makes up 1% by volume of the TMPE biofluid. This formulated TMPE was

tested on the performance of its pour point, wear and friction, and filterability. The test

gathered data from the biofluids properties before and after 800 hours of use in the

laboratory’s hydraulic machinery. The results were then evaluated and compared with

commercial grade biodegradable hydraulic fluid under the same conditions. The commercial

grade biodegradable hydraulic fluid used for comparison was Bio-HVO2-42HYD. This fluid is

classified as ISO 46 and is ultimately biodegradable.

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This research was completed in a controlled atmosphere laboratory hydraulic system.

To simulate actual hydraulic circuit conditions, the TMPE hydraulic biofluid was run in a

hydraulic circuit consisting of a hydraulic motor, load bearing double acting cylinder, spring

retracted solenoid activated 4/3 DCV, flow controlling relief valve, a Program Logic Controller,

and a 24 V direct current supply. 7.54 liters of TMPE formulated oil was used in this system;

7.54 liters was the capacity of the tank in the circuit described above. The fluid was exposed to

pressures from 240 to 910 psi with a maximum shear of 3120 rpm. The circuit was computer

run and the temperature never exceeded 60° C. The experiment ran 8 hours every day for 100

days. To safeguard against contaminants in the system, unformulated TMPE was used to flush

out the hydraulic circuit before the tests were run. Pour point, wear and friction, and

filterability measurements were taken of the formulated TMPE oil just before the system was

turned on at operation hour 0 and then again at operation hour 800.

To measure the pour point properties of the fluid the methods set forth in the ASTM

D97 were followed. The wear and friction data was compiled by performing the test described

according to ASTM D2266. The biodegradable hydraulic oil was put under loads of 15 kg and 40

kg for one hour at a constant temperature of 75° C with the motor at 1200 rpm. Using values

obtained from this experiment, the coefficient of friction was calculated. Filterability data was

collected using a filtration unit that was connected to a vacuum pump. The filter used was

Grade 120H and 1F.

The pour point of the formulated TMPE biofluid was high compared with the

commercial grade hydraulic oil; these results are to be expected from biodegradable hydraulic

fluid. The formulated TMPE pour point was recorded at 10° C while the commercial grade

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biofluid’s pour point is -15° C. TMPE has a high concentration of palmitic material which greatly

decreases the rate of flow at higher temperatures than that of other hydraulic fluids. This gives

the formulated TMPE a high pour point. Although the TMPE biofluid had the same pour point

of 10° C before and after the 800 hours of operation. The chemical makeup of TMPE makes for

a slower degradation process which in turn slows the polymerization rate of the molecules

(Alias, Yunus, and Idris 91). The slower polymerization rate increases the stability of the

biofluids pour point. The additive in the formulated oil acts in a way that suggests it

manipulates the molecular double bond structure which also increases the stability of the oil’s

pour point. The unformulated oil had a higher pour point, at 12°C, than the formulated which

reinforces the benefit, albeit not a much greater benefit, of including the additive in the

hydraulic fluid.

To quantify wear and friction the Wear Scar Diameter (WSD) and coefficient of friction

were measured and calculated for the formulated and unformulated TMPE. The WSD were

both average for the 15 kg and 40 kg loads at hour 0. After 800 hours of operation the WSD for

the 15 kg and 40 0 kg loads were 1049 mm and 3364 mm, respectively. These results are due

to particles formed from the oxidation of the biofluid and particles of the system material which

cause scars in the components. Acidic material along the walls of the circuit generates some of

the particles that cause these wear scars. Aldehyde and carboxylic acid form from the oxidation

of ketone from alcohol in the hydraulic fluid; this process creates the buildup of the particle

producing acidic material along the walls of the circuit. This wear process occurs in all hydraulic

systems and therefore the outcome was expected. Although the commercial grade hydraulic

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biofluid outperformed both the formulated and unformulated TMPE oils, the unformulated

TMPE performed better in the WSD testing than the formulated mixture.

After 800 hours of operation the filterability trial resulted with the filter gaining .65 g of

particles from the system. To combat this outcome for future applications using a filter paper

with finer pores is recommended. Using a finer filter paper after the circulation pump will

decrease the amount of particles in the system which prolongs the life of the equipment.

TMPE as a biodegradable base oil for hydraulic circuits is a viable option but steps must

be taken to increase the performance in certain areas. The high pour point must be decreased

with pour point depressants if TMPE is to be used as a common biodegradable base fluid for

hydraulic systems. The main area of concern is with the wear induced with the formulated

TMPE fluid. Under conditions of prolonged mechanical work and relatively high temperatures

the WSD increased at an unsatisfactory rate. Mitigating the effects of the acidic compounds

could lessen the amount of particles that are causing the wear scars.

Some hydraulic applications cannot use regular petroleum based oil as hydraulic fluid

because of the hazards they pose. Coal mining and metal working applications are unfit for

synthetic petroleum based hydraulic fluid because of the fire hazards from the exposure to high

temperatures. When leakage occurs in these systems they are not only a hazard to the

environment but to the machine operators as well. Research has provided insight into the use

of water based biodegradable hydraulic fluid for these and other applications. Recently, water-

glycol has become a major area of study as an alternative to petroleum based hydraulic fluid.

Water glycol is incompressible, incombustible, highly non-flammable, biodegradable, and

inexpensive. This fluid also maintains good stability and has a low pour point. Unfortunately

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this water based biofluid does not have good lubrication, antirust, or anticorrosion properties.

According to Wang et al. using common additives and solvents does not improve upon these

qualities (236).

Boron containing thiophosphite derivative (BTP) is a water soluble additive whose

molecular structure could increase the antirust, tribological, and anticorrosion properties of a

water-glycol mixture. To test this theory a hydraulic fluid composed of (by volume) 45%

deionized water, 36% glycol, and 19% water soluble polyether was examined. Two samples of

this liquid were then tested, one with BTP and one with TP, which is thiophosphite. A sample of

TP was taken as the control to understand the full extent of borons role in improving the water-

glycol hydraulic fluid. To measure the tribological properties the two samples were obtained by

lever type four ball friction and wear testing machines. The test used GB/T 3142-82 standard

which is very similar to the ASTM D 2783 method. To gather data on the corrosive properties of

the biofluids GB/T 5095-85 method was used; this method is similar to the ASTM D 130 copper

strip corrosion standards. Rust performance tests and wear scar tests were also done to gather

data on the BTP and TP water-glycol hydraulic fluid.

The load carrying capacity of the water-glycol fluid greatly increased with the addition of

BTP and TP. Small amounts of the additives BTP and TP were added as measurements were

taken. All readings from the BTP fluid were larger than the TP readings for the maximum non-

seizure loads. When BTP reached 2% by volume of the fluid the non-shear loads were triple

that of the base water glycol. Although going over 2% BTP resulted in the same non seizure

load capacity. The welding load data is similar for both of the fluids but BTP increased and

plateaued quickly while TP increased slowly but eventually met BTP.

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The antiwear performance evaluation resulted in BTP outperforming TP. Both fluids

decreased the WSD but the system with BTP had smaller wear scars. The sulfur and

phosphorus found in TP assists in the corrosive wear of the components. BTP also contains the

sulfur and phosphorus but the addition of boron impedes their chemical reactions that induce

the corrosive wear. Therefore the boron in BTP also improves upon the anticorrosive

properties of the water glycol hydraulic fluid. According to Wang et al. water glycol with 2%

BTP by volume can greatly reduce the friction coefficient on loads up to 500 N (240). The base

water glycol hydraulic fluids coefficient of friction at the same loads fluctuate greatly and are at

least double that of the BTP coefficient of friction.

In the antirust performance test, boron was found to inhibit the formation of rust. No

rust occurred for the BTP sample while the TP sample had severe rust even with 3% TP. It is

believed that the alkanolamine boron, that is present in BTP but not TP, is the factor which

prevents rust from forming. Utilizing BTP in water-glycol hydraulic fluid greatly increases its

performance in almost every area. For applications where there are environmental concerns or

fire safety hazards water-glycol with BTP is helpful. The comparatively poor friction and

anticorrosion property hinder this biodegradable hydraulic fluid from becoming a threat to the

common petroleum based synthetic hydraulic fluids.

Biodegradable hydraulic fluids are an emerging technology that still needs much work

and refining. The main issues faced in developing these new fluids are the limitations on

performance capabilities that keep the biodegradable fluids from performing at the same

standards of the petroleum based fluids. It is important that this technology be researched and

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developed as we are making an investment in our environment and our future, taking care of

the world while creating incredible innovations to make it a better place for all.

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