Polyalkylene Glycol (PAG) lubricants have been gaining attention in the industry for their many benefits, including better lubricity and friction control. However, the mechanisms that enable these properties are often overlooked, leading to PAGs being labelled as a “mystery fluid.” My experience is that PAGs are almost “feared” on-site due to compatibility concerns. Still, whenever my customers have made the switch, I rarely see them change back to standard hydrocarbon alternatives. It’s time to explore the reasons behind the enhanced lubricity of PAG lubricants and how they can offer significant advantages in various applications.
How the PAG Molecule Shape Influences Performance
The key to understanding PAG lubricants’ superior performance is their surface activity and polymer flexibility. One way to analyse lubricants’ traction values (a measure of their friction control) is by using a friction testing apparatus called the mini traction machine (MTM). This machine consists of a steel ball rotating on a steel disc, simulating the high contact pressures, sliding, and temperatures in gear systems. Researchers have tested various base oils, from group one to group five, using the MTM, and the data is readily available.
Among PAGs, it is found that ethylene oxide (EO) and propylene oxide (PO) copolymers exhibit remarkably low traction values, significantly lower than those of hydrocarbon oils (group one to group four). This means that if you’re designing a fluid requiring energy efficiency, PAGs can be an excellent starting point. However, only some PAGs have high traction values. Water-insoluble PO homo polymers, for instance, have traction values similar to hydrocarbon oils. Their polymer structure is less flexible than EO/PO copolymers. The oxygen atoms in the EO/PO copolymers bring flexibility to the system, contributing to their impressive traction behaviour.
In contrast, oil-soluble PAGs (OSPs) fall between EO/PO copolymers and PO homo polymers on the traction curve. These OSPs were designed to upgrade hydrocarbons and, like other PAGs, are oxygen-rich polymers. Their surface-active nature enables them to be included in hydrocarbon oils to improve friction control under boundary or elastohydrodynamic (EHD) conditions. While OSPs can’t be considered additives, they exhibit anti-wear characteristics, thanks to the oxygen-rich nature of their polymers. The classical four-ball wear test can measure these anti-wear properties.
Fixing Weaknesses of Mineral Oils – Pour Point and Oxidation Stability
In the world of hydrocarbon-based lubricants, straight-chain paraffins which possess fantastic viscosity index and flexibility, do tend to suffer from relatively high pour points due to their propensity to form wax crystals. PAGs exhibit different pour point behaviour. Short branches, typically methyl or ethyl, emerge from the polymer backbone during polymerisation. These branches contribute to improved low-temperature properties, resulting in lower pour points for PAGs. Classical ethylene oxide (EO) and propylene oxide (PO) polymers have pour points around -30 degrees Celsius, while oil-soluble PAGs can reach even lower pour points due to their increased branching.
Another characteristic of PAGs that raises questions is their oxidation stability. The presence of oxygen within the molecular structure of PAGs might lead some to assume that they cannot oxidise further – “you can’t attach more oxygen if it’s already there”. Counterintuitively, while PAGs possess fairly good oxidation stability (certainly superior to most mineral oils), other synthetic base fluids may trump them. PAGs can be used continuously in gear systems or compressors at temperatures up to 120 degrees Celsius when protected by a suitable antioxidant package, such as alkylated diphenyl amines. However, when operating temperatures exceed 150 degrees Celsius, the stability of PAG lubricants may be compromised.
However, an essential advantage of PAG’s is the nature of the oxidation byproducts. When PAGs degrade, they break down into smaller oligomers, which further degrade into small organic molecules like aldehydes, ketones, and acids. These oxidation byproducts are polar and soluble in the parent base oil, which is why PAGs tend to run clean in equipment, relatively free from deposits and varnish. This characteristic is in stark contrast to hydrocarbon oils, which, when oxidised, produce polar byproducts that can precipitate out of the non-polar base oil, leading to potential challenges, especially with groups two and three hydrocarbon base oils.
Are PAGs Actually “Varnish-Free”?
Polyalkylene Glycols (PAGs) are often marketed as “varnish-free” lubricants because they keep clean and prevent deposit formation when they degrade. When PAGs oxidise, they form polar molecules like other base oils. Oil-soluble PAGs can keep these molecules in solution rather than allowing them to plate out and become sludge.
A prime example of this benefit can be observed in the 1980s when rotary screw air compressors faced significant varnish and deposit formation challenges while using hydrocarbon oils. The transition to PAGs resolved these issues, providing longer life and improved performance. As a result, PAGs are now the preferred synthetic lubricant for rotary screw air compressors over many other technologies.
Around 2005, gas turbines began experiencing varnish formation due to the degradation of group two and group three hydrocarbon oils used in these turbines. As a result, some OEMs decided to transition from hydrocarbon-based gas turbine oils to PAG-based oils, drawing on the successful experience with rotary screw air compressors. This shift led to many gas turbines converting to PAG technology and the emergence of claims about varnish-free PAG-based lubricants for turbines.
I have seen several instances of black, carbon-like deposits observed due to PAG breakdown in gas turbines. The root cause analysis suggests that these occurrences are not due to oxidative degradation but thermal degradation, likely caused by a significant thermal event within the turbine. In these cases, a hotspot within the turbine may be starved of oxygen, preventing oxidative degradation and resulting in the development of carbon-rich deposits – analogous to the coking experienced in the di- and polyol-esters.
Condition Monitoring Basics for PAG-Based Lubricants
Another crucial factor that end-users should be aware of is the importance of condition-monitoring PAGs by tracking their acid value. Over time, the acid value of PAGs will gradually increase. Therefore, users should monitor the fluid closely and consider changing it when it rises by about one-milligram KOH per gram over its initial starting value.
In addition to the rising acid value, PAGs may exhibit a dramatic viscosity drop near the end of their lifespan. Unlike hydrocarbon oils, PAGs appear stable for long periods when monitoring viscosity, but a sudden decrease in viscosity indicates that they have reached the end of their useful life.
The Affinity of PAG-Lubricants for Water
Another notable feature of PAGs is their ability to absorb water, which can be observed in condition monitoring programs. PAGs typically show higher water values than other lubricants due to their hygroscopic nature, meaning they can absorb atmospheric moisture. For some operators this might cause concern, especially when they have experience with the hydrolytic sensitivity of esters.
Fortunately, PAGs are relatively inert to the presence of water, with some types capable of absorbing up to 5,000 or even 10,000 PPM water with no negative effects. In addition, the water absorbed by PAGs is typically held within the polymer matrix, rendering it inert. This allows PAG-based lubricants to operate at much higher water levels than their counterparts. For example, it is typical for air compressor lubricants based on PAGs to run at levels of 2,000 to 4,000 PPM water, as they are exposed to varying humidity levels and seasonal fluctuations.
PAG Use Cases
PAGs have proven valuable in various applications, such as compressors, turbines, and worm drive gearboxes. In these applications, PAGs address issues that standard hydrocarbon oils cannot effectively handle, like varnish formation, incompatibility with yellow metals, or lower solubility with methane and propane.
Another feature of some PAGs is their biodegradability, which is particularly relevant for environmentally sensitive applications. Lower molecular weight PAGs, with molecular weights less than 1500 grams per mol (roughly ISO 150 viscosity grade), often exhibit some level of biodegradability. Many ISO 32 and ISO 46 PAGs are readily biodegradable. This makes PAGs an excellent choice for developing environmentally acceptable lubricants, and low toxicity can also make them suitable for use in NSF H1 certified food-grade lubricants.
The Future of PAGs
As we look toward the future, PAGs have the advantage of energy efficiency benefits which should be considered when selecting base fluids for new formulations. PAGs play a significant role in this regard. Additionally, as equipment becomes smaller and more compact with higher power densities and increased thermal stresses, hydrocarbon oils may need help to keep up with the demands of this new generation of machinery.
In such cases, PAGs could solve the challenges posed by equipment with higher thermal stress and degradation products that form deposits and varnish. Ultimately, PAGs provide a versatile and environmentally friendly option for lubrication applications. Their biodegradability, ability to address issues common to hydrocarbon oils and potential for use in new applications make them an attractive choice for formulators and equipment builders. As the industry continues to evolve and prioritise energy efficiency and sustainability, PAGs are well-positioned to play an increasingly important role in lubrication technology.