HETG fluids are triglycerides derived from vegetable or animal oils —with soybean, sunflower, and rapeseed (Canola) being the most common sources. They frequently contain soluble thickeners to increase their natural viscosity, which is approximately 35 mm2/sec at 40°C.

Triglycerides are long-chain fatty acids combined with alcohol in the form of glycerin. Natural triglycerides have excellent lubricity but poor thermal and hydrolytic stability. They also oxidize rapidly. Hydrolytic stability and oxidation resistance can be improved with additives, chemical modification, and even genetic modification of the seeds used to produce the base stock.

HETG fluids offer many advantages. For one, they are highly biodegradable and non-toxic. They offer excellent lubricity and anti-corrosion properties. And because they are made from natural, renewable resources, they are available in plentiful supply. In addition, they have a high viscosity index and high flash point.

Of course, HETG fluids have some drawbacks. They are adversely impacted by high temperature operation, which causes quick aging, rapid oxidation, and extreme thickening and gumming. In addition, they are susceptible to water contamination, which causes hydrolysis and TAN increases. They tend to thicken and gel because they do not perform well at low temperature. Finally, they are double the cost of mineral oils; they require special system designs; and because they are miscible with mineral oil, this can have a negative effect on biodegradability.

HEES fluid characteristics
HEES fluids are synthetic esters that are derived from either petroleum or vegetable — typically rapeseed— oil feedstocks. Petroleum sourced HEES fluids are a combination of an organic acid and alcohol, whereas the vegetable sourced fluids are a combination of a fatty acid and alcohol.

HEES fluids are available as unsaturated, partially saturated, and fully saturated products. Of these, the fully saturated products generally offer the best performance and command the highest price.

HEES fluids offer longer service life due to their higher thermal and oxidative stability and better fluidity at low temperatures. They are also available in a broad viscosity range (ISO VG 32/46/68). However, they have more disadvantages than advantages. For example, their cost is greater, and like HETG fluids, they require special system design requirements. They also hydrolyze in the presence of water. And like HETG, because they are miscible with mineral oil, this can have a negative effect on biodegradability.

HETG and HEES fluids are often considered for the same applications, so the chart on page 38 is intended to provide a quick comparison of their major characteristics.

HEPG fluid characteristics
HEPG fluids are polymers made from the reaction of such alkylene oxide monomers as ethylene oxide, propylene glycol, or propylene oxide with glycol. Those with 50 to 100% ethylene oxide are water soluble, while those with 100% propylene oxide are water insoluble. Both types have inherent fire-resistant characteristics.

The biodegradability of HEPG fluids depends on the ratio of propylene to ethylene oxides. The higher the molecular weight, the lower the biodegradability of the fluid.

HEPG fluids are offered in a very broad viscosity range. They have many fire-resistant characteristics and have an operating temperature range limited to –20° to 80°C. In addition, water soluble polyglycols can be used as anhydrous lubricants.

However, they, too, require special system designs, and they are incompatible with polyurethane seals. Finally, pumps and motors may need to be de-rated when used with HEPG fluids.