Synthetic lubricants offer a number of operational advantages over mineral-based products across a wide array of applications, including bearings, gearboxes, compressors, vacuum and diaphragm pumps, rotary shaft seals, valves, hydraulic systems, and instrumentation. Synthetics are the logical choice if:
- Performance demands are beyond petroleum product capabilities
- Equipment failure of excessive downtime is traceable to ineffective lubrication products of practices
- High or low temperatures are encountered (-75 to about 500 F)
- Situations exist in which conventional products are ineffective.
Some man-made lubricants cause less metal wear because they create a lower coefficient of friction in the load zone of non-conforming surfaces. This condition cuts maintenance, parts replacement, and energy costs. In addition, synthetics provide long life because of enhanced thermal and oxidative stability; and minimal sludge, corrosive, and deposit formations because of high temperature oxidation stability.
The major drawback to the synthetic approach in an appropriate application is a significantly higher initial cost three times or more. However, this cost is usually recovered over the extended life of the product six times or more. Also, systems subject to leakage or contamination are not usually good candidates for synthetic products.
The chart on the following pages serves as a guide to selecting and applying synthetic oils and greases. The listings are based on information supplied by the 67 companies and are categorized according to viscosity. However other important variables should be considered when selecting and applying the products. These factors include pour and flash points, demulsibility, lubricity, rust and corrosion protection, thermal and oxidation stability, antiwear properties, compatibility with seals and paints, and compliance with various testing and standard requirements.
The products presented in each category are not necessarily interchangeable or compatible. These two features depend on an assortment of interrelated factors, and each situation requires an individual analysis.
A properly selected synthetic does the same job as any mineral oil, if not better, but no single product exceeds petroleum in every characteristic. Nevertheless, it is possible to formulate synthetic lubricants containing those features that are most important for a specific application. The result is an optimum compromise that petroleum products cannot usually match.
Performance characteristics of synthetic lubricants come from the physical and chemical properties of the base fluids, and chemical properties of the base fluids, and the effects of additives introduced into the final product. Physical and chemical qualities include viscosity-temperature behavior, low temperature fluidity, volatility, compatibility with paints and elastomers, ability to dissolve chemical additives, compatibility with petroleum, and hydrolytic stability. Additives are included to influence to a greater or lesser degree, oxidation stability, load bearing ability, and corrosion protection.
Table I shows the relative performance characteristics of seven general types of synthetic lubricants and a paraffinic mineral oil.
There are several classes of synthetic lubricants. The most common are synthesized hydrocarbons, such as polyalphaolefins (PAO) and dialkylated benezenes. These products are the closest to the performance characteristics of mineral oils and are compatible with them. Other commonly used synthetic lubricants are organic esters, including dibasic acid esters (diesters) and polyol esters; phosphate esters; polyalkylene glycols (polyglycols); and silicones.
SYNTHETIC HYDROCARBON FLUIDS (SHF) equal or exceed the best lubricating properties of mineral oils, but without the drawbacks. These synthetic base fluids are manufactured from specific chemical compounds that are often petroleum derived. The SHF base fluids are made by chemically combining (synthesizing) various low-molecular-weight compounds to obtain base stocks with predictable desired properties.
These man-made fluids are available in several viscosity grades and operate over a wide temperature range. The products serve as engine and turbine oils, hydraulic fluids, gear and bearing circulating oils, and compressor lubricants. SHF’s are wax free, offer excellent hydrolytic and chemical stability, and provide low volatility.
ORGANIC ESTERS have shear-stable viscosities over a wide temperature range (-l00 to 400 F), high film strength, good metal wetting, and low vapor pressure at elevated temperatures. Organic esters easily accept additives, enhancing their applicability for finished product formulations such as crankcase oils and compressor lubricants.
POLYOL ESTERS feature many of the advantages associated with dibasic acid esters, but perform at even higher temperatures. These esters are applied as high temperature chain lubricants and in industrial turbines.
PHOSPHATE ESTERS are organic lubricants well suited for fire resistance applications.
POLYGLYCOLS offer excellent viscosity and temperature properties, and resist sludge buildup. Formulated products are used in applications from -40 to 400 F, and are low in toxicity. Because of their solubility characteristics, polyglycols are well suited for lubricating gears and bearings and compressors handling hydrocarbon gases.
SILICONES are chemically inert, nontoxic, fire resistant, and water repellant. The products have low pour points and volatility, good low-temperature fluidity, and good oxidation and thermal stability up to very high temperatures.
A disadvantage of silicones is their low surface tension, which permits excessive spreading on metal surfaces. In addition, this product offers poor response to wear and friction-reducing additives.
It is important to remember that synthetics are as different from each other as they are from petroleum lubricants. The performance of a synthetic and its applicability to any individual situation depends on the quality of the synthetic base stock and the additive package.
Table II outlines several major applications for synthetics.
Suitability of a specific synthetic for a given application often depends on the chemical compatibility with system components. In general, synthetic hydrocarbons, like PAOs, match well with seals, elastomers, paints, and plastics, while esters may not.
Compatibility should be checked with the lubricant previously used in the equipment. Some synthetics are compatible with mineral oils; others are not. Polyglycols are not generally miscible or compatible with mineral oils. Phosphate ester hydraulic fluids are not miscible with water and waterglycol hydraulic fluids. The compatibility of the thickeners must be considered when choosing synthetic greases.
It is not usually advisable to mix various types of synthetics. The system should be drained and flushed when changing to a different lubricant.
It is advisable to consult a manufacturers’s representative before introducing a synthetic to guarantee that the right product is selected, and to obtain the maximum benefits from its use.