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Three Ways Phosphate Esters Degrade*

June 01, 2013
Jean Van Rensselar
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Phosphate esters degrade in three ways. Each avenue becomes significant in certain environmental conditions that aircraft hydraulic systems are exposed to. All three paths produce acid phosphate (phosphoric acid derivatives) as the destructive degradation product.

1. Pyrolysis. Pyrolysis or thermal degradation of phosphate esters only becomes significant at very high temperatures above 300 F (150 C), which would be unusual in aviation hydraulic systems. It would probably only happen in the case of equipment malfunction or a few special situations such as brake system cylinders where the fluid is not sufficiently insulated to withstand the high temperature of carbon brakes. During pyrolysis, alkyl phosphate esters form unsaturated hydrocarbons and leave a highly acidic residue. Aryl groups are more resistant to pyrolysis.

2. Oxidation. Because phosphate esters are highly resistant to oxidation and aircraft hydraulic systems are closed systems with limited availability of ambient oxygen, oxidation is not a significant degradation path for hydraulic aircraft fluids.

3. Hydrolysis (reaction with water). The most common way that hydraulic aircraft fluids degrade is through exposure to water. This is due to the fact that hydrolysis occurs even at moderate temperatures (and faster at higher temperatures). Phosphate esters absorb water from the atmosphere very rapidly. Because of this, the hydraulic fluid normally contains 1,000s of parts per million of water capable of decomposing the fluid. Aryl phosphates are more prone to hydrolytic degradation than alkyl phosphates, as are alky and aryl-mixed phosphates.

All three types of degradation increase as the temperature increases. Oxidation and hydrolysis are also exacerbated by metals such as iron and copper. The harmful byproduct is always a phosphoric acid derivative—an acid so strong that it can damage hydraulic system components, attack and degrade elastic polymers and etch metal parts so badly that they fail.

*From: http://www.exxonmobil.com/lubes/exxonmobil/emal/files/HyJetV_Technical_Bulletin.pdf  
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