Environmentally Adapted Lubricants, Part II. Hydraulic Fluids

Mahendra Pal and Sudhir Singhal Petroleum Products Application Division, Indian Institute of Petroleum, Dehradun, India

Current technologies relating to hydraulic fluids are mostly either mineral based, synthetic based, or water-based. Mineral oil based hydraulic fluids tend to be used for their excellent lubrication characteristics, economic viability, and so account for almost 90% of hydraulic fluids, while water- based fluids tend to be employed in fire hazard applications. Synthetic hydraulic jluids possess superior properties to mineral oils, but their higher price limits their application to aerospace and special applications under severe conditions. In recent years, biodegradable hydraulic fluids, which are readily decomposed by micro-organisms, have come into focus due to the rise in environmental consciousness in European countries, particularly Sweden and Germany, and with this, vegetable-based fluids.

In the present paper, the main categories of hydraulic jluids, including environmentally friendly fluids, are discussed in an overview of the state of technology in this area. Current governmental regulation, and the properties required of environmentally adapted fluids, and an appraisal of the current practices in various countries are also given.

In an earlier paper", the authors reviewed the key aspects of and require- ments for environmentally adapted lubricants, namely, chemical composition, eco-toxicity, biodegradability, bio-accumulation, eco-labelling schemes, and lije-cycle analysis.

KEYWORDS: environment, hydraulic ,fluids, mineral oil, synthetic, vegetable, bio- degradable

INTRODUCTION

This paper deals with the environment and hydraulic fluids. Hydraulic systems have been widely employed in a variety of industrial applications, including construction and agricultural equipment, metalworking, machining, extrusion of metals and plastics, railways, marine vessels, automotive brakes

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and steering, turbine control systems and aircraft landing gear.'" In hydraulic fluid technology, the main concerns are to improve system performance with increased pressure and speed, greater reliability and contamination control, lower noise, and enhanced efficiency with energy saving, in an environmen- tally friendly operation, that involves the substitution of mineral-based products with renewable and environmentally acceptable l~bricants.4'~

Hydraulic fluids have conventionally been classified as mineral, synthetic, or water-containing. Important characteristics of such fluids include lubricity, chemical stability, anticorrosion properties, compatibility with elas- tomeric seal materials, toxicity, and bacterial growth:

There has been increased awareness of environmental issues through- out the world. Hydraulic fluids, when used in construction machinery, for- estry, and agriculture, can leak into the soil, the sea, rivers, lakes, or marshes, and so can upset the ecological balance. Some countries have switched to eco-friendly hydraulic fluids to protect their environment,' and, in the last few years, India has also promulgated some regulations in this regard.'.'

Excluding metalworking fluids, hydraulic oils make up about 50% of industrial lubricants. This paper presents an overview of some past develop- ments and recent trends in hydraulic fluid technology. There is a wide variety of hydraulic oils according to their composition and function.'" The fluids reviewed are mineral, synthetic, water-containing, and environmentally friendly fluids.

Mineral-oil based hydraulic fluids

The earliest power transmission medium used water as the working fluid. This restricted the operating temperature range, and resulted in poor lubrication, along with rust and corrosion problems. Mineral-oil based hydraulic fluids were introduced because of their better lubrication properties. Rust and oxida- tion inhibiting hydraulic fluids began to be used in the late thirties, but these resulted in wear problems in high-pressure pumps. Several additive systems, such as tricresylphosphate (TCP) and zinc dialkyl dithiophosphate (ZDDP), were incorporated to improve the antiwear, rust inhibition, and antioxidant properties, and this type of oil still remains the main type of antiwear hydraulic fluid.' Although the antiwear performance of the early ZDDP-type oil was comparatively poor, much work has been carried out to improve the antiwear, antirust and antioxidant performance, stability and demulsibility of

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Environmentully adapted lubricants, Part II. Hydraulic.fluids 221

Table 1 Categories of fire-resistant hydraulic fluids according to IS0 6743/4

Category

1. Water-based fluids

HFAE

HFAS

HFB

HFC

2. Water-free fluids

HFDR

HFDS

HFDT

HFDU

Composition

O M emulsions (concentrate c 20%)

Mineral-oil free synthetic solutions (concentrate portion I 20%)

WIO emulsions (mineral-oil portion s SOYO)

Aqueous polymer solutions Water I 35%

Phosphate ester-based

Chlorinated hydrocarbons

H FDRlH FDS blends

Synthetic fluid, other compositions

Operational temperature range,

"C

5t0160

-20 to 2 60

-20 to 70

150 max*

10 to 70 (and 150 max*)

-35 to I 9 0

for hydrodynamic couplings.'6

ZDDP-type oils, to reach the working pressures and speeds required for use in vane pumps.''*'2

High viscosity index hydraulic fluids were developed with the addition of VI improvers specifically for low and for wide temperature range applica- tions. Such oil can also be used in numerically controlled and hydraulic pulse motors and machines. Hydraulic oils possessing anti-stick-slip properties for slideway machine tools also belong to this categ01-y.'~ The mineral-oil based MIL-H-5606 hydraulic fluid was used as the aerospace hydraulic fluid in the early fif t ie~. '~

Water-based hydraulic fluids

Mineral-oil based hydraulic fluid possesses most of the desired properties required by a hydraulic system, with the exception of fire resistance. The in- herent flammability of the mineral fluid was a source of serious fire risk in un- derground mining, military, industrial and commercial aircraft applications.15

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222 Pal and Singhal

This problem led to the reconsideration of the use of water-based fluids in fire hazard applications. Water-based hydraulic fluids in the category of fire-resis- tant hydraulic fluids according to I S 0 6743/4 and German Standard DIN 5 I502 can be classified into four categories, shown in Table 1.16

In the field of fire-resistant hydraulic fluids, products constituting serious hazards to natural water, such as halogenated hydrocarbons belonging to the categories of HFDS and HFDT, are banned in European countrie~. '~ HFDR and HFB fluids tend not to be used in European coal mining, HFAS and HFC categories are used instead. These fluids meet the specifications of environ- mental compatibility, and pose little or no danger to watercourses.I6

Among water-based hydraulic fluids, the category of water-glycol type hydraulic fluids is the largest and constitutes about 7 0 4 0 % of the total pro- duction of FRHFs. Water-glycol hydraulic fluids contain approximately 40% water, 40% glycol, 15% thickeners, and 5% additives, such as wear inhibitors, alkali conditioners, and corrosion inhibitors. W/O emulsion hydraulic fluids are the next most used water-based fluids, particularly in the mining industry and in steel plants. These are composed of about 50-55% mineral oil, 4045% water, and 5-10% additives, such as emulsifying agents and wear inhibitors. O N microemulsions are now becoming quite popular as a result of their per- formance in terms of lubricity, stability, and techno-economic viability.'"

Synthetic hydraulic fluids

Synthetic hydraulic fluids are usually based on either phosphate esters, fatty- acid esters or synthetic hydrocarbons.' Phosphate-ester based hydraulic fluids are employed in aviation, continuous casting equipment in steel plants, in power stations, and in the mining industry, due to their relatively lower ignit- ability compared with mineral oil." Fatty-acid ester based hydraulic fluids tend to be used in high-pressure applications, such as fire-resistant hydraulic fluids in steel plants.' Synthetic hydrocarbon based hydraulic fluids are used in aerospace; MIL-H-83282 is an example of this class of polyalphaolefin based hydraulic fluid, which was substituted for MIL-H-5606 (which was mineral-oil based) in US military applications.I9

ENVIRONMENTALLY ADAPTED HYDRAULIC FLUIDS

There has been increasing pressure to develop biodegradable, environment- ally friendly hydraulic fluids in recent years. Many local and central

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Environmentally adapted lubricants, Part II. Hydraulic fluids 223

Table 2 Environmentally adapted hydraulic fluids

Category Composition WGK* Application Operating temperature

HEPG Polyalkylene glycol soluble in 0 Hydrostatic -30 to < 90°C water drives

HEES Synthetic esters (polyol esters) 0-1 Hydrostatic -35 to c 90°C

HETG Vegetable oils (triglycerides) 0 Hydrostatic -20 to < 80°C

WGK water endangering number.

drive

drive

government regulations have been introduced, and initiatives have been taken by industry and the private sector in Europe to reduce pollution and help protect the environment. Lubricant formulators have introduced three types of environ- mentally friendly hydraulic fluids for use in industrial applications?" polyethyl- ene glycol based (PAG), synthetic ester based, and vegetable oil based.

Polyethylene glycol (PAG) based hydraulic fluids

Polyethylene glycol based hydraulic fluids have wide application in the food industry, agriculture, and construction. PAG has temperature stability with a range from -45 to 250°C and a biodegradability of up to 99%. The oil change intervals are similar to those for mineral oil, being 2000 h or once a year.

Synthetic ester based hydraulic fluids

Synthetic ester based hydraulic fluids possess excellent flow properties at low and high temperatures and ageing stability. Although they have excellent tribological characteristics, they are more expensive than mineral-oil based hydraulic fluids.

Vegetable oil based hydraulic fluids

Vegetable oil based hydraulic fluids contain products such as rapeseed oi1;O or ahyssinica crambe oil," and have good lubrication characteristics. Rape- seed oil does not produce carbon dioxide, offers good corrosion protection, and does not attack sealing materials, varnish, or paint. This type of vegetable oil, however, has some operating temperature limitations depending on ex- treme operating and climatic conditions.

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224 Pal and Singhal

These three hydraulic fluids are good substitutes for mineral-oil based hydraulic fluids, and are water-free and environmentally acceptable as measured by their WGK numbers.’6 Table 2 gives some details.

SUMMARY

In the present paper, the main categories of hydraulic fluids, including envir- onmentally friendly fluids, have been described. In Part I*, the authors re- viewed the key aspects of and requirements for environmentally adapted lubricants.

Acknowledgement

The authors wish to thank the Director, Indian Institute of Petroleum, Dehra- dun, for permission to publish this paper.

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