“Yeah, we’ve tried some ‘green’ fluids and we have had some bad experiences.” This is a statement I have heard more than once from clients who have jumped on the biofluid bandwagon before looking closely at what they were buying.

Over the past several years, global regulations have been passed requiring companies working in or near waterways to respect wildlife. In terms of hydraulic systems, manufacturing has traditionally used mineral oil and viewed it as a commodity. Yet, with growing requirements and restrictions, these manufacturers are being forced to use biofluids that are significantly more expensive without understanding the differences in fluid types, which can mean everything when it comes to performance.

Biofluid classifications

Biofluids can be broken down into four major classifications: HEPG, HETG, HEPR and HEES. HEPG fluids, or polyglycols, may be water-based but are not miscible with other hydraulic fluids. They are often incompatible with some seal materials. HETG fluids are plant or animal based and highly biodegradable. However, they generally cannot tolerate temperatures above 160°F for more than a few hundred hours. HEPR fluids, or polyalphaolefins, have good hydrolytic stability and biodegradability but may lose their viscosity as they run through a hydraulic system over time, see Figure 1.

HEES fluids, or synthetic esters, have good biodegradability. However, the wide variety of HEES products can perform drastically differently, depending on what type of HEES it is and its base composition. Unfortunately, HEES fluids are often generalized into one major category despite major differences in performance and longevity within the HEES class of products.

Saturated vs. unsaturated synthetic ester

Esters are formed by condensing, or combining, an acid with an alcohol, a process called esterification. Just as not all fluids are created equal, neither are all synthetic fluids. All synthetic ester products can be broken into two categories—saturated and unsaturated. The saturation of a fluid is based on the chemical bonding within the fluid itself.

Oxidation, or aging of a fluid, is caused when a fluid reacts with oxygen. The result is extreme thickening and gumming of the fluid, along with deposits and shellac, which lead to major catastrophic system failures. Chemically speaking, an unsaturated ester has many open bonds that react with oxygen and cause the fluid to age more rapidly. Saturated esters, on the other hand, have significantly fewer open bonds, so they do not oxidize rapidly and will last much longer when subjected to  high temperatures, Figure 1.

So how can you tell the difference? The easiest way to tell the difference is to ask the fluid manufacturer for the iodine number. This number identifies the number of open bonds available in a fluid. The higher the Iodine number, the greater the number of bonds that can interact and oxidize. Generally speaking, saturated synthetic ester products have an iodine number of less than 15.

Hygroscopic properties of fluids

Water content in an oil is largely influenced by temperature, relative humidity and chemical composition of the oil itself. Typically, the concentration of water varies from 300 to 800 ppm, where 0.1% is equal to about 1000 ppm. Any oil can take in and dissolve water to an extent. When fluid is mixed with a small amount of water — about 100 ppm (0.01%) — the fluid’s appearance will not change. It will appear to be transparent because water is dissolved at a molecular level, and even over time the oil and water will not separate.

However, all fluids also have a saturation limit, the maximum possible concentration of dissolved water in an oil. Once the molecules in the oil can take in no more water, the fluid is considered saturated. Any water beyond the saturation limit does not bind to the molecules in the oil and it becomes free water. You will often see this water sink to the bottom of a container while the oil floats to the top. It is extremely important to remove free water from any hydraulic system because it will hinder lubrication of internal parts (pump pistons, etc) and reduce life span of the oil. 

Hydrolytic stability

Hydrolytic stability, a fluid’s resistance to decomposition in the presence of water, is important to consider with biofluids. For equipment operating near waterways or hydraulic systems prone to condensation in the reservoir, hydrolytic stability is extremely important. Users need a fluid that is stable and will not break down or alter its composition when a small amount of water is present in the system.

A good example of a substance with good hydrolytic stability is table salt. Salt can be dissolved in water and recaptured later by evaporating the water. In this example, salt does not change its chemical properties when in contact with water. Edible oils do not behave the same way. When these oils are cooked with water for 500 hr at about 200°F, they become cleaved (bonded), forming new chemicals. This process is called hydrolytic fat cleavage, and it alters the oil’s chemical composition. Unlike the salt example, this change cannot be undone without a complex chemical process.