Bio-based fluids are ideal because they offer many environmental benefits such as high levels of biodegradability and low eco-toxicity. They can be formulated to provide the same functionality of many standard petroleum-based fluids. These environmental advantages are naturally gaining attention and support from governments and policy makers. In 2002, the United States passed the Farm Security and Rural Investment Act encouraging the use of bio-based products in a variety of industries. Since then, the U.S. Department of Agriculture has written guidelines for a number of biobased lubricants, including hydraulic fluids, penetrating lubricants, lubricating greases and others.

The Europe Union meanwhile developed its Eco-label, and in 2005 extended its use to lubricants. Displaying the Ecolabel differentiates lubricants that are derived from renewable resources andoffers consumers guidance on making a more environmentally friendly choice.These changes seem to indicate that biobased lubricants will be more widely adopted in years to come.

Dow Chemicals UCON Fluids and Lubricants business has been conducting significant research and development in the field of polyalkylene glycol (PAG) lubricants for many years. And since 2006, Dow has expanded that effort even further to include the development of lubricantsmade from renewable resources, like soybean oil or other seed-based raw materials.

Who Wants Bio-Hydraulic?

Agriculture, construction, forestry, marine and even the steel and die-casting industries are all using bio-hydraulic fluids. Not surprisingly, each industry has its own unique and critical fluid needs.

For example, in the construction industry where mobile equipment is used, equipment reliability in both warm and cold climates is crucial. In the forestry and marine industries, high biodegradability and low eco-toxicity is essential, particularly where fluid leakage is a real possibility. The Nordic countries especially have taken the lead with regards to encouraging forestry equipment operators to use only bio-hydraulic fluids. In the food industry, lubricants must be formulated specifically with safety in mind so that if they are accidentally ingested, no harm would come to consumers; for this reason, many ingredients found in conventional lubricants actually are banned from food-grade lubricants, in case of accidental food contact during processing or packaging. Lastly, for the steel and die casting industry, fire-resistant properties of lubricants are critical.

Meanwhile, hydraulic component manufacturers are also demanding more of these fluids. Higher power inputs, greater pressures, smaller fluid reservoirs, faster speeds and new alloys are all having an impact on hydraulic lubricant development.

Since each segment of the marketplace has its own requirements, there has been a proliferation of different hydraulic technologies and chemistries being used. However, Dow has recently discovered that many of these new technologies show significant differences in performancewhen evaluated in hydraulic equipment.

Fluid Development Trends

The demand for environmentally friendly fluids that have low eco-toxicity and biodegradability is growing. In addition to these qualities, users still need fluids that offer performance characteristicslike an extended life, less maintenance and high productivity. The challenge for the fluid manufacturers is to balance product performance and cost.

This balancing act begins with the choice of base oil. Vegetable oils, high oleic vegetable oils and unsaturated oleate polyol esters are some of the more common base fluid choices used incommercially available bio-hydraulic products today. There are also polyalkylene glycol (PAG) and synthetic polyol esters offering biodegradability and low toxicity properties, though they are madefrom petrochemical sources. Sometimes these base oil types are blended to optimize cost versus performance.

Recently, researchers from Dow investigated six commercially available biohydraulic fluids from several suppliers claiming that their products offered good environmental performance. Thesewere fluids with strong, recognized brands. All fluids assessed were ISO-VG 46 viscosity grade, and included compositions derived from the following principle base oils:

A) Canola oil.

B) High oleic canola oil.

C) Oleate-based polyol ester (POE).

D) Oleate-based POE blended with high oleic canola oil.

E) Synthetic POE.

F) Polyalphaolefin (PAO) blended with high oleic sunflower oil.

All of the oils were screened for their performance in hydraulic equipment and laboratory tests. The performance of each then was compared to a synthetic PAG, Fluid G.

Eye on Performance

Each of the six bio-hydraulic fluids was tested for oxidation performance, lowtemperature rheology, hydrolytic stability (resistance to breaking down in the presence of water), elastomer compatibility and traction performance.

The fluids also were assessed in an Eaton (Vickers) Vane Pump Test, according to ASTM test method D7043. This test uses a Vickers V-104C vane pump, operating at 2,000 psi, 1200 rpm and at 65 degrees C bulk fluid temperature for a 100-hour test duration. All fluids displayed excellent, low ring and vane wear (less than 20 mg of weight loss) after the 100- hour test period (Figure 1, page 40). But a closer examination of the condition of each fluid after the tests showed surprisingly severe degradation of the oleatebased POE fluid [Fluid C] and the blend of PAO and sunflower oil [Fluid F].

Oxidation stability was also examined in a blown-air oxidation test method (ASTM D2893) conducted at 120 degrees C. In this test, the viscosity of each lubricant was monitored over a four-month period. The products derived from traditional vegetable oils and high oleic vegetable oil [Fluids A, B, D and F] showed a dramatic increase in viscosity, which would restrict their use in high-temperature hydraulic equipment (Figure 2, page 40). High oxidative stability was observed for the synthetic PAG and POE fluids (E and G). In hydrolytic stability tests as well, these fluids performed much more favorably than the vegetable oil and oleate based versions.

These tests show that todays commercially available bio-hydraulic fluids yield significant differences in performance in vane pump wear, oxidation, low-temperature rheology and hydrolytic stability tests.

Generally, canola based products demonstrate good performance in hydraulic vane pump and traction tests, but are significantly disadvantaged in thermo-oxidative and hydrolytic stability. Unsaturated oleate polyol ester based products demonstrate better thermooxidative stability than canola oils, but hydrolytic instability and shear loss in vane pump tests were significant. And saturated polyol esters display excellent vane pump and oxidative stability, but their higher cost may limit growth in the hydraulic segment.

To overcome the deficiencies in oxidation and hydrolytic stability performance of vegetable oils, Dow is researching the chemical modification of traditional vegetable oils by eliminating unsaturation in the triglyceride structures and sterically Preliminary thermo-oxidative and hydrolytic stability and low-temperature rheology performance of this modified product (Fluid H) is impressive. Furthermore, low wear results and high shear stability in hydraulic equipment has been validated.

In fact, in traction experiments, the new technology shows much lower traction coefficients than saturated polyol esters and very similar performance to the canola-based product (Figure 3).

Based on these tests, Dow is developing a new family of chemically modified vegetable oils that offer improvements over traditional vegetable oil lubricants. These materials offer the combined advantages of both synthetic and naturally derived bio-based fluids. The changes offer improvements in thermo-oxidative stability, low-temperature performance and hydrolytic stability but retain the excellent film-forming properties. As a first generation product, Dow looking to bring a new top-tier biohydraulic fluid to market within 12 months. The company believes that the new chemically modified vegetable oils can be tailored to specific viscosity grades – thereby opening up applications beyond hydraulic fluids.