PAOs vs PAGs

Q: What’s the main difference between PAOs & PAGs?

Let’s start off with definitions!

PAO: Polyalphaolefin

PAG: Polyalklene Glycol

While both are synthetic oils they are classified under different Groups of Base oils. PAOs have their own Base oil Group IV while PAGs fall into the Group V (catch all).

PAOs

PAOs are actually hydrogenated oligomers of an α-olefin and there are different methods of oligomerisation. Due to this process, PAOs have very good low temperature properties and the products are wax free! Additionally, their lower volatilities also allow them to operate over a wide temperature range. Usually, they can be used in a lot of versatile applications such as gearboxes, screw compressors, fans, motors and even automotive!

However, PAOs have a low polarity which gives rise to poor solvency of polar compounds and issues with seal performance.¹

PAGs

On the other hand, PAGs can differ depending on their structure. For instance, Ethylene is water soluble while Propylene is not, however, neither are oil soluble. Both experience significant chemical reactions producing sludge like deposits when mixed with mineral oil.

Usually, their properties include a wide viscosity range, low pour points, good lubricity, low toxicity and non-flammable in aqueous solutions. PAGs are typically always found in fire-resistant hydraulic fluids as well as industrial gear oils, compressor lubricants, heat transfer liquids and metalworking fluids.¹

Compatibility

Both products need to be tested for compatibility with mineral oils before any mixing occurs. Additionally, most lubricant suppliers deem PAOs & PAGS as “filled for life” solutions which last for a longer time compared to mineral oils. Typically, the purchase of these products are more expensive than mineral oils, however if one looks at the cost of waste disposal and reduced downtime (due to decreased shutdowns for oil changes) the overall cost of the lubricant is by far less than that of mineral oil.

References:

Chemistry and Technology of Lubricants 3^rd Edition, Chapter 2, R.M. Mortier, M.F. Fox, S.T. Orszulik

Industrial / Strategic Tips

Mixing viscosities

Can I mix different viscosities of oils to get the viscosity that I want?

It can be done but this is not an ideal situation.

There are times when the only available viscosity is an ISO 46 (on a rig) but the equipment requires an ISO 68 and the new stock will not be delivered in time to avoid shutdown. Can the ISO 46 be used instead?

An ISO 46 oil is lighter in viscosity than an ISO 68 however, for most oils, there is a chart that depicts the viscosity of the oil at operating temperature. In these cases, one can consult this chart and determine if the viscosity at operating temperature will still fall within operating limits.

If we mix an ISO 46 with an ISO 68 oil we cannot be certain of where the new viscosity will fall especially if we do not know the ratios that are being used. There is a viscosity calculator that can help guide this decision available at: https://www.widman.biz/English/Calculators/Mixtures.html

This can be used as a guide and the actual values of the oil should be verified via oil analysis.

While this situation is not ideal, we need to remember that compatibility is also key.

As such, we should stick with the same line of lubricants that we being used. Typically, lubricant suppliers have the same formulation but change the viscosities for lubricants of the same line.

Grease

Grease compatibility

Are all greases compatible?

We recently touched on greases being available in a wide variety based on application but the real question is, “Are all greases compatible?”

The short answer is, “No”.

All greases contain a thickener (which helps with its physical state). Thickeners vary depending on application (such as temperature, water resistance etc). As such, to verify whether two greases are compatible or not, Machinery Lubrication developed a Compatibility Chart based on thickener type.

You can determine the thickener type by looking at your Data Sheet or talking to your OEM.

Not all greases are compatible, so be careful when mixing greases!

Tagged: compatibility

Are all greases compatible?