Trends in hydraulic systems evolve, Thomas Ruehle of BASF told the ACI European Base Oils and Lubricants conference held in Krakow, Poland, in the fall. Hence, hydrolytic, thermal and oxidation stability of the fluids, their filterability, corrosion inhibition, longer life and improved wear protection are essential in any modern hydraulic circuit.

BASF has absorbed both Ciba Additives and Cognis Corp., which greatly deepened its capabilities in this area. Fuel and lubricant solutions now account for about 25 percent of sales in its Performance Chemicals division, where 2010 sales totalled 3.1 billion ($3.9 billion). Included in the segment are lubricant additives such as antioxidants, friction modifiers and extreme pressure agents, synthetic lubricants based on polyalkylene glycols and esters, engine coolants, and other useful components such as polyisobutene, Ruehle explained.

In terms of revenue, hydraulic fluids are one of the most important industrial lube markets for additives. Such additives were valued he said at $230.4 million worldwide in 2008, according to a study by the market research firm Kline & Co. By comparison, the additives market for industrial gear oils amounted to $69.3 million worldwide, and for turbine oils it was just $22.5 million.

What are the key trends in hydraulics now? Ruehle, who is technical service manager at the companys lubricants and oil additives division in Ludwigshafen, Germany, noted that todays modern hydraulic equipment has higher power output than seen before, higher system pressures, and hotter operating temperatures. Systems are being sized more compactly, with smaller reservoirs, tighter clearances and finer filter pores – all of which stress the fluid more than ever before.

Every circuit to run smooth would require longer fluid life, with improved wear protection. The fluids have to have hydrolytic, thermal and oxidation stability, filterability and corrosion inhibition, he said.

Hydrolytic stability is one key to maintaining equipment performance, especially in the presence of water contamination. Thermal and oxidation stability could provide long and improved service life at high operating temperatures, Ruehle said. Water contamination can cause many field equipment issues, and hydrolytically stable oil is therefore a key OEM requirement. Hydrolytically stable oil allows the protection of yellow metals in the presence of water.

Filterability and corrosion inhibition are additional concerns. All hydraulic circuits contain filters, and filter blockage can cause unnecessary downtime and maintenance cost to the end user, numerous studies demonstrate. For this reason, OEMs specify filterability performance as an important requirement for top-tier oils, Ruehle said, adding that non-top-tier oils may have poor filtration characteristics due to an inability of the chemistry to deal with the presence of water.

The list of needs does not end there, he said. In all, hydraulic fluids may contain a base oil and an additive package comprising 10 or more components, including antiwear agents to protect pumps from wear; dispersants to control deposits that can impede valves; defoamers to avoid air contamination; phenolic or aminic antioxidants to extend the fluids oxidative life; and demulsifiers to help tolerate water contamination. Youll also find phosphites, viscosity index improvers, pour point depressants and corrosion inhibitors.

Fluids formulated for antiwear perfor-mance, rather than only rust and oxidation inhibition, are a large segment of the hydraulics market, and many anti-wear formulations use zinc dialkyl dithiophosphate (ZDDP), the traditional antiwear agent found in engine oils and other lubricants. However, Ruehle said, ashless additive technology – based on organic compounds, not metals – can provide performance features superior to ZDDP technology.

Ashless technology results in valuable long-term benefits such as longer life-time and less maintenance, lower disposal and effluent treatment costs. It also results in reduced heavy metal contamination for the environment, lower fluid contamination by ash and sludge, improved cleanliness and filterability, and better wear performance, Ruehle said.

The adoption of ashless products is being driven by legislation in specific European countries, and at the European Union level as well. It is also driven by the Ecolabel specification, as well as the urge to use environmentally acceptable products. Furthermore its also driven by worker health and safety issues, and by the market sectors where environmental contamination can be an issue, like forestry.

To be considered truly ashless, additive technologies should comprise purely organic materials, without zinc dithiophosphates, and no metal, according to Ruehle. They should be zinc-free and use instead calcium and magnesium, he said, adding that many current hydraulic fluid formulations that use ZDDP can be divided into low-zinc grades, typically 300-500 ppm, and high-zinc grades, typically 500-1,000 ppm.

In a similar vein, biohydraulics are gaining more attention, but bio needs to be defined, Ruehle commented. BASF produces three types of additives for biohydraulic fluids: antioxidants, antiwear additives and corrosion inhibitors.

The general causes of a pump failure could be dirt, improper assembly or alignment, insufficient fluid, overload and corrosion, Ruehle said. Dirt is implicated in 47 percent of failures, while insufficient fluid can be blamed in about 11 percent of cases. Corrosion was the culprit 4 percent of the time, and improper assembly and alignment caused 28 percent of pump failures. Pump failure could be avoided with well-balanced hydraulic formulations.

He added that formulators should consider the risk of antagonism between the antiwear agent and corrosion inhibitor. Otherwise, the components will be competing with each other for the surface of the metal they are meant to protect.

Demand is also growing for hydraulic fluids that can reduce the fuel consumption of equipment, typically by eliminating friction losses. By adjusting the optimum viscosity index of the hydraulic fluids, the energy efficiency can be maximized. High efficiency fluids pay off, Ruehle said.

Given the above needs, its not surprising that the landscape of hydraulic specifications has become crowded with testing hurdles for fluids to pass, in order to win approval from original equipment manufacturers. In 1950, lubricant suppliers faced a simple Vickers vane pump test for wear; today there are requirements from Denison, Rexroth, Eaton and others. Many of these tests, such as the tough Denison T6H20C pump test, put the fluid through both dry and wet test regimes.

The principal OEM standards now for hydraulic pumps and systems are Denisons HFO, HF1 and HF2; Bosch Rexroths RE 220, RE 221, RE 223; and Eaton I-286-S and M-2950-S, according to Ruehle.

OEMs also require that hydraulic fluids show good air release – hence the need for defoamers – and compatibility with system components. Lubricants and additives have to be chemically inert to the system components, like paint and seals, Ruehle pointed out. For example, rubber is a common material for hoses and seals in many hydraulic systems. Unfortunately, these elastomers are made of compounds similar to those found in lubricants, and when used together both materials can interact, Ruehle cautioned.

Interaction of elastomers with oils has shown a diffusion of oil into the elastomer and adsorption of the oil on the polymer, he observed. As a result it may alter the oils volume and weight, or that of the rubber material. With diffusion and extraction of rubber ingredients, additives from the elastomers could be enriched in the media (oil). The reverse is also possible, with diffusion and extraction of media and rubber ingredients as well as addi-tives into and from elastomer.

It helps to remember that cross-linking agents for rubber are zinc dibenzyldithiocarbamate, zinc dibutyldithiocarbamate and zinc dibutyldithiophosphate. All these are similar compounds that are used in lubricants, Ruehle said. In such cases, the hydraulic fluids additive package should be reformulated, otherwise interaction is unavoidable.

With all the above performance needs, user expectations and OEM demands, Group I base oils will become less important for hydraulic oils in the future, Ruehle declared. The share of Group II and III base oils reached almost 30 percent of the total base stocks distribution for hydraulics in 2009, he estimated, and the demand for Group I base stocks for hydraulic fluids is expected to decline further. From 2006 to 2009, naphthenic base oils were slightly less then 10 percent of the total base stock distribution for hydraulic fluids.

The shifting base oil mix also will require reformulated additive packages, Ruehle said in closing. Group IV base oil – PAO – showed the best performance in stability, temperature regime, lubrication and viscosity index characteristics, followed by Group III base oils.

Overall though, esters and PAGs have the best polarity and solvency characteristics compared to Group I, Group II, Group III base stocks, and polyalphaolefins. Those two characteristics are essential for an additive-friendly fluid, so the future should see more esters and PAGs in the hydraulic fluid barrel.