Let’s get one thing straight: contrary to what the title might suggest, synthetics are in general superior to mineral based oils. But do you know why? Synthetics might be the preferred choice of lubricant, but more often than not mineral oils are chosen instead. The obvious and most prevailing reason for this is cost. But this is not the only factor that could provide reason to choosing a mineral oil over a synthetic.
It’s a very common question to ask if switching to a different lubricant will remedy a lubricant related issue. In doing so, there is a tendency to choose synthetics because it has been deemed to be the better lubricant. To understand why synthetics may or may not be the right choice for your application, you must avoid searching for the best lubricant and instead begin to calculate the optimal lubricant to meet your conditions. This means recognizing a list of factors that relate to your application before jumping to a conclusion. These factors may include machine criticality, machine susceptibility to contamination (including water), age of machine, history of lubricant related failures, operating temperatures, ease of re-lubricating, etc.
What makes synthetic oils fundamentally different than mineral oils?
Synthetics are often many times more expensive than the equivalent mineral oils, upwards of three times the equivalent cost. Why? Because the way they are created. Mineral oils are extracted and refined from crude oils which pose issues with consistencies in molecular characteristics. Synthetics on the other hand are scientifically built through a process called polymerization which keeps the ideal molecule chains to join together to produce a pure formulation. Fundamentally, both of these base lubricants may function similarly, but depending on specific applications and additive requirements there may be a critical dissimilarity in performance.
When do synthetics provide improved performance characteristics?
In general, synthetics are well known to provide excellent oxidative and thermal stability. This can be a direct correlation to a longer lubricant life. In addition, these types of lubricants can demonstrate a much higher viscosity index (or VI). In circumstances where the lubricant will encounter cold start-ups and high operating temperatures, synthetics naturally remain in the desired viscosity ranges due to the high VI.
Polyalphaolyfins, or PAOs, are one of the more common synthetics. PAOs exhibits the common synthetic benefits explained above as well as a few other improved characteristics including excellent demulsibility (the ability to release water) and hydrolytic stability (the ability to prevent lubricant decomposition due to water). These synthetics are most commonly recommended in engine, gear, and compressor oil applications.
Polyalkaline glycols, or PAGs, are also in line with exhibiting the common synthetic characteristics explained previously. But to their excellence, they exceed performance in areas such as viscosity index and an extremely low tendency to leave deposits on machine surfaces. According to an article in a 2009 issue of Machinery Lubrication, “The low deposit-forming tendency is really due to two properties – the oil’s ability to dissolve deposits and the fact that the oil burns clean. So when they are exposed to a very hot surface or subjected to micro-dieseling by entrained air, PAGs are less likely to leave residue that will form deposits.”
Ester-based synthetic lubricants, such as dibasic acid ester and polyol ester, both contain the typical synthetic characteristics yet provide their own areas of expertise. Dibasic acid esters, or diesters, have excellent solubility. This means that it is a great candidate for additive formulation. They have been commonly known be mixed with PAO synthetic lubricants to assist in accepting substantial additive packages in application such as engine oil. Polyol Esters have one key advantage that contributes it its differentiating efficacy; the ability to resist fire. This makes it a prime candidate for applications with high fire risk environments such as jet engine oils.
When do synthetics provide degraded performance characteristics?
Now that we have brought attention to all the wonderful and glamorous reasons why the world has come to love synthetics, let’s turn sides and highlight its downfalls. Each synthetic has at least one formulary established disadvantage and all synthetics have one common fundamental disadvantage. PAOs are the most popular synthetic for a reason, their one formula-based disadvantage is its poor solubility, and as explained earlier it can be resolved by the addition of diesters. PAGs and esters on the other hand have poor hydrolytic stability. In other words, these synthetics not only having an attraction to water (hydrophilic) they can in many cases create reaction with the water. This causes a common issue with fluid, sealing material and coating compatibility.
Now back to the fundamental disadvantage that is common to all synthetics; higher costs. As it might be understood already, this is due to the complex scientific process of polymerization that produces, or “synthesizes,” these lubricants. Choosing to use a synthetic may increase the life of the lubricant, thus extending the oil drain frequency. But this may not always results in money saved if this life extension is a smaller factor compared to the cost inflation. Yet if other dynamics come into play such as ease of re-lubrication or machine criticality, the cost factor influencing to preferred choice of a mineral oil may be overridden. And furthermore if the operating conditions are such that contamination likelihood is so immense that re-lubrication is a forced to a high rate, then the choice to use a higher cost synthetic may be overridden as well. So as you may realize, solving a lubricant related issue isn’t as easy as choosing a synthetic. Make the decision with reason and thorough considerations depending on your inherent conditions.
This article was previously published in the Reliable Plant 2013 Conference Proceedings.
By Bennett Fitch, Noria Corporation
Don’t miss insightful case studies from these industry leaders…