Environmentally safe hydraulic oils - Part 1

Spills of standard, petroleum-based hydraulic fluids are known to kill marine life and contaminate soil. Environmentally safe hydraulic fluids are formulated to avoid these undesirable results. In Part 1 this month, we look at the features which are desirable in a non-petroleum-based hydraulic fluid.

To be classified as environmentally safe, a fluid must be readily biodegradable, which means that more than 80% of the fluid must break down into innocuous products when exposed to the atmosphere over a 28-day period. They must also be virtually non-toxic. These fluids reduce the cost of spill clean-ups, and the fluid is unlikely to harm plant life, fish, animals or humans that come into contact with it.

Renewable, biodegradable hydraulic oils based on vegetable oil base stock easily meet the environmental requirements, but they must also be a useful hydraulic fluid and meet or outperform other forms of hydraulic fluid if they are to gain widespread adoption.

Selecting the right hydraulic fluid for an application means balancing various fluid characteristics to achieve the desired overall properties. An ideal hydraulic fluid would have the following characteristics:

• Constant viscosity, regardless of temperature
• High anti-wear characteristics
• Thermal stability
• Hydrolytic stability
• Low chemical corrosiveness
• Low tendency to cavitate
• Long life
• Fire resistance
• Readily biodegradable
• Low toxicity
• Low cost

No single fluid has all these ideal properties, so selection of the best compromise involves understanding the basic characteristics of the system in which the fluid will be used. The main influential factors other than toxicity and biodegradability are:

• Viscosity - Maximum and minimum operating temperatures, along with the load, determine the system's viscosity requirements. A minimum viscosity must be maintained at higher temperatures, and the fluid must not be so viscous at low temperatures that it cannot be poured. Vegetable oil hydraulic fluids have viscosity indexes comparable with petroleum-based fluids.
• Wear - Wear caused by metal-to-metal contact can be prevented by using a fluid with additives that create a thin protective film between moving metal parts. The most common anti-wear additive used to date in petrochemical hydraulic oils has been zinc dithiophosphate (ZDP), but this heavy metal is water soluble and highly toxic to marine environments, and these types of fluids are being replaced by 'ashless' anti-wear fluids. Vegetable oils provide natural anti-wear characteristics to a certain extent, but are still supported by anti-wear additives, especially for higher temperature operation.
• Corrosion and Oxidation - System rusting is caused by water carried within the fluid attacking ferrous metal parts, so rust inhibitors are normally added. Ideally, the oil should exhibit good stability in the presence of water (hydrolytic stability). Other additives are often also used to prevent chemical corrosion as fluids age, creating acids sludge and varnish as they oxidise. These problems can be mitigated by changing the fluid regularly, and by not allowing it to be used at high temperatures, which accelerates decomposition.
• Flammability - Even though petroleum-based fluids have a relatively high flash point (>150 C), they can be a significant hazard if hydraulic machinery is operating in an environment with potential ignition sources - open flames, sparks or hot metal. In these environments, a leak spraying from a high-pressure hydraulic system could cause a serious fire and result in major property damage, personnel injury or death. Vegetable oils are inherently more resistant to fire than petroleum-based fluids.

The types of non-petroleum base oils

Three different types of base oils have been tried for use as 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.

Next month

Synthetic and vegetable oils have their various advantages and disadvantages when compared with petroleum-based oils. We will discuss them in more detail in Part 2 of our feature story next month.