Hydrolytic Stability (Hydrolysis) of Synthetic Ester-Based Lubricants
by David Doyle, CLS, OMA I & II
General Manager
, Tribology

Esters are an assorted family of synthetic lubricant base stocks which can be engineered to meet particular performance properties. These products are primarily known for hydraulic system applications. Synthetic ester-based oils exhibit excellent biodegradation, good lubricating characteristics, fire resistance, and low-toxicity characteristics. Synthetic esters used for lubricant formulations are organic compounds formed through a reaction with different acid and alcohol feed stocks, which produces an ester and water. Diesters and polyolesters are the most widely used ester families for synthetic lubricant formulation.

Product formulations are generally engineered for specific applications. The manufacturing of these fluids is mainly related to the choice and blending of selected acids and alcohols for the specific properties favorable to an application.

While synthetic ester-based lubricants provide superior performance in select applications, they have one common drawback; they have an attraction to water (hydrophilic) and can exhibit poor hydrolytic stability. Because esters are prone to hydrolysis, excess water consumed can break down the ester, which produces acids and alcohol, reversing the chemical process that originally created the ester compound. This results in the synthetic ester readily decomposing in the presence of excess water to form carboxylic acid and other acids, leading to fluid degradation. Like all chemical reactions, the process depends on temperature.

Hydrolytic breakdown of these fluids contributes to several issues:

  • Fluid stability and degradation
  • Machinery and seal damage
  • Coating compatibility
  • Deposit formation
  • Additive degradation

Although all esters can be hydrolyzed, it does not mean all esters have poor hydrolytic stability. The difference is in the acid/alcohol chemical formula used to create the ester compound. The chemical structure of the products used dictates the properties of the ester. Saturated esters, known as polyolesters have improved hydrolytic stability over diesters. Additives can also improve hydrolytic stability. Some additives work to attract the water molecules so that they are not available to react with the ester molecules. Hydrolytic stability can also be improved by blending with another synthetic based product, while still providing some of the performance advantages of a synthetic ester.

The speed of hydrolysis reactions is influenced by several factors:

  • The chemistry of the base stock
  • Amount of water present
  • Temperature
  • Formation of reaction products that catalyze further fluid degradation
  • Use of additives that support or inhibit hydrolysis
  • Presence of copper, which can further act as a catalyst for hydrolysis

Hydrolysis does have some advantage in certain applications. Since synthetic esters do have the favorable advantage of rapid biodegradability they provide an alternative for use in marine applications or environmentally sensitive areas. The hydrolysis process is a key factor in the biodegradability advantage in using an ester based lubricant.

In order to take full advantage of synthetic ester based lubricants, systems need to be well maintained and inspected. Sources of excess water contamination need to be kept under control. Remember, if a lubricant is able to leak product while in operation, it is most likely pulling atmospheric moisture in once the system is shut down. If an operator is using a synthetic ester based lubricant due to concerns about chronic leakage in the working environment, then the inherent situation will likely create a situation for accelerated hydrolysis in the same environment.

For more information on testing services for your in-service lubricants, coolants and industrial fuels contact one of our regional testing laboratories or view us at www.alstribology.com.