Introduction To Synthetic Lubricants & Their Applications

These days, equipment is running faster and hotter than ever to meet productivity demands—and higher performance lubricants are required to meet these demands. Synthetics offer that level of performance.

Synthetics aren’t new; they’ve been around for 70 years. Esters were used in WWII by both Germany and the U.S. to keep equipment running under harsh conditions. Only in the last 20 years, however, have end users throughout industry really recognized the cost benefits of synthetics. Now, more and more uses are being found for these formulations and they are experiencing significantly high growth rates.

Synthetics are formulated by combining low molecular-weight materials in a chemical reaction to produce higher molecular-weight materials. These reactions are controlled to produce products with uniform consistency and targeted performance properties. Mineral-based lubricants don’t exhibit the consistency and uniformity that synthetics exhibit, nor do they have the performance properties.

Synthetic classifications There are many types of synthetic fluids. Within the scope of this particular article, we will deal with the most common.

Synthetics are classified into the following major groups:

• Synthetic Hydrocarbons
  • Polyalphaolefins (PAO)
  • Alkylated Aromatics
  • Polybutenes

• Esters
  • Diesters
  • Polyol Esters
  • Phosphate Esters

• Others
  • Polyglycols
  • Silicones

PAOs are the largest synthetic group, followed by esters and PAGs. Most of the discussion will focus on these three synthetic types.

Advantages/disadvantages
Fig. 1 details the overall advantages of synthetics as a class. Not all synthetics have all these advantages—and some have more than others. Fig. 2 describes some of the disadvantages. Note that this is a composite of all the major synthetic types. The only disadvantage common to all synthetics is cost. For the most common synthetics (PAO, Esters and PAGs) the cost is 3-5 times the cost of a high-quality mineral oil.

The advantages offered by synthetics allow these formulations to be real lubrication problem solvers. The three major categories for synthetic use are:

Temperature extremes—Synthetics are wax-free, so they can be used at very low temperatures. High temperatures (over 200 F) call for the use of synthetics. They should be considered when mineral bulk temperatures reach 180 F. The high- and low-temperature capability of the most common synthetic types is illustrated in Table I. These numbers indicate the highest and lowest levels of operation and are not common operating conditions.

Wear reduction—Synthetics are custom manufactured and the molecular sizes are more uniform than mineralbased products, thereby providing greater film strength and lubricity than mineral oils. Diesters and polyol esters, because of their polarity, have excellent lubricity and film strength, followed by polyalkylene glycols. PAOs, which are non-polar, have the lowest level of lubricity and film strength of this group. The film strength, coupled with additives, helps minimize wear under boundary lubrication conditions.

Energy savings—Many times, the use of synthetics has been justified on energy savings alone. This is particularly true in the case of gearboxes (something that will be discussed in the an upcoming article in this series.) Efficiency is related to lubricity and film strength. Traction coefficient is an important factor with synthetics during elastohydrodynamic lubrication (EHL), which occurs with rolling motion, such as that in bearings and pitch point in gears. EHL lubrication results in a thin film under high pressure that increases the viscosity of the film. The internal resistance of the fluid film to sliding, known as traction coefficient, can affect energy savings in rolling element bearings. PAOs and PAGs have low traction coefficients compared to mineral oils and, as such, can lead to energy savings.

Synthetic types
Polyalphaolefins…
PAOs are produced by the following reaction:

Decene is reacted with itself to produce high molecular weight hydrocarbons that are linked in groups of 10 carbon atoms; thus, we can produce any molecular weight in groups of 10. The initial reaction involves a reaction of a linear alpha olefin to produce molecular weights in groups of 10 carbon atoms. The final reaction saturates the double bond to produce a PAO.

It is also possible to react dodecene (which has 12 carbon atoms) to produce increasing molecular weights in groups of 12 carbon atoms. For the purposes of this article, however, our discussion will focus on carbon atoms in groups of 10.

PAOs are classified in terms of viscosity at 100 C. Table II illustrates some of their viscosity grades.

By blending different viscosities, there is a great deal of flexibility in creating different viscosity grades with PAOs.

Key properties:

• Excellent low-temperature fluidity
• Good high-temperature properties
• High viscosity index
• Low volatility
• Hydrolytic stability
• Highly compatible with mineral oils
• Low biodegradability
• Slight elastomeric seal shrinkage
• Low additive solvency
• Low lubricity

PAOs are formulated with 5-20% ester—which is typically a diester—to overcome the seal shrinkage and non-polarity, resulting in good additive solubility and increased lubricity. PAOs have the widest application of any of the synthetics. This will be discussed in the next article

Esters… Two major groups of esters to be discussed are diesters and polyol esters. Diesters are produced by the following reaction:

This reaction is reversible so, in the presence of heat and water, a diester can decompose back to an acid and an alcohol. The conditions need to be severe to cause this reaction to reverse, but it will occur under high-temperature and high-moisture conditions.

Depending on the alcohol and acid selected, a large number of diester types can be produced and tailored to a particular application.

Key properties:

• Low pour point
• Low volatility
• Good thermal and oxidative stability
• Excellent solvency and cleanliness
• Good metal-wetting properties, resulting in good lubricity
• Good biodegradability
• Poor compatibility with some elastomers, plastics and paints
• Hydrolyze under high-temperature, high-moisture conditions

Polyol esters… 
These synthetics are produced by reacting a highly branched di-functional alcohol with a mono-basic acid as follows:

This ester is highly branched, which results in the following key properties:

Key properties:

• Low pour point
• Low volatility
• Good viscosity index
• Excellent thermal and oxidative stability
• Excellent solvency and cleanliness
• Very good lubricity
• Highly biodegradable
• Slight tendency to hydrolyze under severe conditions
• Nearly 50% more expensive than diesters

Polyalkylene glycol…
PAGs are quite versatile. Many different types can be created, which allows for a wide variance in properties. PAGs are produced as follows:

Either 100% ethylene oxide, 100% propylene oxide or a combination of the two are used to create many different types of PAGs with unique properties— and with many different molecular weights.

PAGs can be made either water soluble or insoluble. Increasing the ethylene oxide (EO) ratio increases the water solubility and decreases the oil solubility. Water soluble PAGs are inversely soluble, meaning that the solubility decreases with increasing temperature.

Table III illustrates the various ratios of ethylene and propylene oxide and their properties.

Key properties:

• Versatile with both water-soluble and water-insoluble grades
• High viscosity indexes
• Hydrolytic stability
• Excellent lubricity
• Low volatility
• High oxidative and thermal stability
• Can be formulated to have limited gas solubility
• Resistant to sludge formation
• Compatible with most elastomeric seals but may cause slight shrinkage
• Incompatible with many paints and polycarbonate and polyurethane
• Incompatible with mineral oil and other non-ester synthetics

Conclusion
Table IV summarizes the strengths and weaknesses associated with each of the major synthetics that have been discussed in this article.

Synthetic fluids are real problem solvers—and very important in improving equipment reliability. Their usage is growing as equipment conditions require higher performing lubricants. In the next installment of this series, selecting the optimal synthetic based on the equipment and conditions will be discussed.

Acknowledgements
The author wishes to thank Dr. Ken Hope, of Chevron Phillips, and Dr. Martin Greaves, of Dow Chemical, for their assistance in the preparation of this article.

Contributing editor Ray Thibault is based in Cypress (Houston), TX. An STLECertified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training in a number of industries. E-mail: rlthibault@msn.com; or telephone: (281) 257-1526.