Environmentally safe hydraulic oils - Part 2

In Part 1 of our story last month, we looked at the desirable features of a non-petroleum-based hydraulic fluid. This month we continue by looking at the advantages and disadvantages of hydraulic fluids based on vegetable oils.

The types of non-petroleum base oils

To re-cap from last month, three different types of base oils have been used in attempts to achieve environmentally safe hydraulic fluids — synthetic esters, polyglycols and vegetable oils.

• Synthetic esters have superior lubrication performance, but at a high cost, which has limited their usage.
• Polyglycols are less costly than synthetic esters, but they lack the required level of biodegradability and are potentially toxic when mixed with lubricating additives.
• Vegetable oils have excellent natural biodegradability and are in plentiful supply.

The base fluids used for biodegradable hydraulic fluids are usually vegetable oils, selected synthetic esters or a blend of the two. They usually contain ashless (no zinc dithiophosphate or ZDP) corrosion and wear inhibitors and performance-enhancing additives, and can provide wear resistance similar to anti-wear petroleum fluids.

Vegetable oils contain unsaturated hydrocarbons and are naturally occurring esters. The downside of them being biodegradable in this way, however, is that the unsaturation means that the fluid has poor low-temperature flow properties and is more susceptible to oxidation, especially at higher operating temperatures. They also have less water resistance. The use of a blend of vegetable oil with a synthetic ester is therefore often used to improve these characteristics, and low toxicity additives can help with some performance aspects, but oxidation stability has remained an issue.

Since vegetable oils generally have the same range of operating temperatures as petroleum-based oils, they are still not suitable for extreme cold or heat — they are best used between temperatures between -15 and 80°C. Synthetic ester-based hydraulic fluids are best used outside this temperature range.

On the positive side, vegetable oils offer excellent lubricity and have a high intrinsic viscosity and extreme-pressure properties. Well-formulated vegetable oil-based hydraulic fluids can pass the demanding Vickers 35VQ25 or Denison T5D-42 vane pump wear tests. They can perform satisfactorily for years under mild climate and operating conditions, provided they are kept free of water contamination.

And unlike petroleum-based hydraulic oils, they come from renewable feedstock.

Vegetable oil feedstock

Vegetable oil hydraulic fluids are mainly made from rapeseed (or canola), sunflower or soybean oil. Each have their respective differences in properties, but with different processing and additive formulations, there is little significant difference between them. Historically, rapeseed has been the most commonly used, particularly in Europe. Soybean oil has been researched and tested in more depth in the United States where it is a more prevalent crop.

Because of the inherent poor oxidation behaviour, as mentioned previously, makers of vegetable oil-based hydraulic fluids have had to use chemical additives or combine them with synthetic ester base stock to improve their life and high temperature characteristics.

But now we live in the age of genetically modified crops and hybrid breeding. Recent advances in hybrid breeding technology have made it possible to alter the physical properties of vegetable oils by changing their fatty acid profiles. This has allowed some researchers to improve oxidation stability by increasing the oleic content of the oil. The resulting high oleic base stock (HOBS) oils with additional antioxidants have been shown to be as good as or better than petroleum oils in oxidation stability trials. For example, US-based Renewable Lubricants Inc has produced a hydraulic fluid that outperforms petroleum oils, synthetics, synthetic blends and commercial vegetable oil formulations in independent third-party tests.

Cost

Cost varies with specific application, but in general plant-based oils are 2 to 2.5 times more expensive than mineral oils, and half as much as synthetic formulations. The price differential between plant-based and mineral oil lubricants is attributable both to the higher raw material cost of vegetable base oils and to more costly additives.

Purchase price, however, is not an accurate measure of overall cost. Because vegetable oil-based lubricants evaporate less quickly and adhere better to metal surfaces, end users often use less product per application. Other cost benefits associated with vegetable oils may include reductions in environmental and safety penalties in the case of spills, parts wear and maintenance costs and disposal fees. As is the case with many plant-based products, once factors such as these are considered, plant-based lubricants are competitive in cost with petroleum oils.