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Sunday, March 1, 2009

Lab Tests Provide Insights Into Degradation of Phosphate Ester Hydraulic Fluids

By Shlomo Antika, PHD, ExxonMobil Aviation, Lubricants Global Technical Advisor

In conjunction with ExxonMobil’s September 2008 introduction of a new hydraulic fluid called HyJet V for commercial aircraft applications, engineers at the company have produced a technical paper that discusses the stability characteristics of phosphate esters.

The information should be useful to aircraft engineers and maintenance personnel.

In particular, the document explores why hydraulic fluids can degrade and compares the stability of different types of commercially available hydraulic fluids.

Phosphate Esters in Aviation Hydraulic Fluids

Commercial aircraft hydraulic system fluid may require replacement because of contamination from solids or fluids such as water, engine oil, strut fluid or cleaning solvent. The hydraulic fluid may also need to be replaced because it has degraded to a level that could be harmful to hydraulic system materials and components. Degradation is the consequence of the limited stability of phosphate ester base oils used in commercial hydraulic fluids.

Understanding the chemical properties of phosphate esters is basic to understanding why aviation hydraulic fluids degrade and require periodic replacement.

Aviation hydraulic fluids use a mixture of alkyl and aryl phosphate ester base oils formed by the reaction of an acid and an alcohol or a phenol. The acid portion of the molecule imparts fire-resistant properties. The alcohol/phenol portion imparts desirable flow properties.

Pathways to Decomposition

Three mechanisms contribute to the degradation of phosphate esters: thermal degradation, oxidation and hydrolysis (See chart on page 32). Thermal degradation of phosphate esters, known as pyrolysis, becomes significant only at very high temperatures (above 300ºF).

Such temperatures are not normally encountered in aviation hydraulic systems and only become significant in cases of equipment malfunction or in some special situations, such as brake system cylinders where the fluid is not sufficiently insulated from high temperatures reached by carbon brakes. Oxidation can also promote degradation, although phosphate esters are quite resistant to oxidation. In the case of aircraft hydraulic systems, this is not a significant degradation pathway because normal system temperatures are moderate and aircraft hydraulic systems are closed, with limited availability of oxygen from the air.

The most significant path to fluid degradation is through hydrolysis (reaction with water). Phosphate esters absorb water from the atmosphere rapidly.

Hydraulic fluids normally contain several thousand parts per million of water that can decompose the esters. Degradation can occur even at moderate temperatures but will accelerate at higher temperatures.

The harmful byproduct in all degradation pathways is a phosphoric acid derivative. This "strong" acid can damage hydraulic system components, attack and degrade elastomers, and etch metal parts and tubing.

Controlling Acid Formation

In the early days, the utilization of phosphate ester aviation hydraulic fluids involved high maintenance. Acid formed by hydrolysis was prevalent, and acidified fluids had to be replaced frequently. However, in the late 1960s, acid control additives, called epoxides, were developed to address the problem. These additives react with phosphoric acid derivatives formed by degradation of the phosphate esters, rendering them harmless.

Today, an aircraft’s aviation hydraulic system remains free of strong acids until the acid control additives are essentially depleted.

Now, all commercial Type IV and Type V hydraulic fluids contain similar amounts of acid control additives. However, not all of these fluids are equally stable.

The resistance of the fluid base oil constituents to hydrolysis can explain differences. In addition, supplementary additives can be used to slow down the rate of hydrolysis.

Resistance to Hydrolysis

Strong acids may ultimately form in a hydraulic system if the hydraulic fluid make-up rate is not sufficient to balance the extent of water absorption. Acid number measures acidity, and a fluid that exceeds an acid number of 1.5 should be replaced. The time it takes a fluid to reach an acid number of 1.5 can be considered the in-service life of the fluid. (See chart above for a comparison of ampoule stability with various Type IV and Type V fluids.)

Fluid containing a high concentration of strong acids would react with iron and aluminum to damage parts and form deposits and gels. Fresh hydraulic fluid typically has the additive capacity to neutralize the hydrolysis products of about 0.4 percent water. Thus, there exists quite a bit of acid control capacity.

However, hydraulic fluid continuously absorbs water from the environment, and water levels that exceed the capacity of the acid control additive are not uncommon.

For example, most aircraft manufacturers allow operation up to 0.8 percent water in the hydraulic system. This is based on the expectation that high levels of strong acidity will not occur before the next sampling of the fluid due to normal make-up with new fluid.

It is also based on the typically low hydrolysis rate at the moderate temperatures encountered in hydraulic systems.

Testing Hydraulic Fluids

ExxonMobil researchers used the Airbus NSA 307110 Ampoule stability test to evaluate the comparative degradation resistance of various fluids. The test is designed to measure the hydrolytic and thermal stability of phosphate ester aviation hydraulic fluids.

In this test, the fluids are contaminated with precise water concentrations and sealed in glass tubes in which small strips of steel and copper wire are used to simulate the metals in hydraulic systems that would tend to catalyze the hydrolysis reactions. The tubes are kept in a controlled-temperature oven for long periods of time.

At regular intervals the fluid is tested to determine whether it has exceeded an acid number of 1.5, at which point the end of fluid life has been reached.

Establishing Fluid Life Curves

The Airbus NSA 307110 Ampoule stability test can be run in a range of temperatures and water concentrations to produce what are called "Fluid Life" curves. These can be charted to examine the results of competitive fluids in side-by-side testing of samples.

Hydrolysis is the main degradation mechanism in tests run with water contents of 0.5 percent and higher. (0.5 percent water results are shown in the above chart.)

Other tests determine fluid life at high temperatures but lower water content (for example a water content of 0.2 percent), when thermal stability of the base oils becomes a factor in fluid degradation.

By combining results obtained at a constant water content but at different temperatures, it is possible to build a Fluid Life curve as a function of hydraulic system temperature.

Comparing Competing Fluids

Again the company used the Airbus NSA 307110 Ampoule stability test to compare its new HyJet V product with HyJet IV-Aplus and other competitive fluids, based on side-by-side tested samples. The tests demonstrate HyJet V’s performance at 0.5 percent water and a range of temperatures chosen to replicate aircraft in service conditions. The same conclusions were reached from testing conducted at other water concentrations. According to the technical paper, the following conclusion applies: "The fluid life for HyJet V as a function of temperature in test conditions is about twice that of HyJet IV-A plus and several times that of other commercial Type IV fluids."

Technical Advantages of HyJet V

HyJet V was developed as a new fire-resistant Type V phosphate ester hydraulic fluid in response to the aviation industry’s request for a product with increased thermal and hydrolytic stability and longer service life than Type IV hydraulic fluids. It is the first Type V phosphate ester hydraulic fluid with highest-grade approvals from Airbus and Boeing.

According to ExxonMobil, HyJet V offers: improved stability, corrosion protection and wear protection than competitive Type IV and Type V hydraulic fluids, as well as longer service life. To learn more about HyJet V, maintenance professionals can contact their local ExxonMobil Aviation Lubricants representative or visit www.exxonmobil.com/lubes/aviation.

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