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Understanding Polyalkylene Glycols (PAG)

PAG lubricants are a family of synthetic base lubricants used across refrigeration, heat pump, and hydrocarbon gas compression applications. For industrial operators, OEMs, distributors, service companies, and technical specialists working in hydrocarbon compression, petrochemical, refrigeration, and CCUS systems, the key point is that PAGs come in several distinct chemistries — each with different solubility, miscibility, dilution behaviour, viscosity response, and film strength in the presence of refrigerants and process gases. Selecting the right type depends on the gas composition, system design, and operating conditions, because the wrong PAG can increase dilution, reduce effective lubrication, and raise carbon or varnish risk in compressors. This guide explains how PAG chemistries behave, how to compare them for specific gases and refrigerants, and which practical selection rules help match lubricant performance to compressor reliability and efficiency.

 

Common PAG Chemistries Used in Compressor Lubricants

A polyalkylene glycol lubricant is an API Group V base stock defined by the oxide monomer used to build its backbone. Propylene oxide (PO) produces a less polar, more hydrophobic polymer. Ethylene oxide (EO) produces a polar one. A PAG lubricant is defined by the oxide monomer used to build its backbone, but it can also be built with different molecular weights and EO:PO ratios to tailor properties. Copolymers sit between the two, with the EO:PO ratio controlling where the final lubricant lands on the polarity spectrum. PAGs can also be produced as random or block copolymers. Polarity is what governs the properties that matter in service: water solubility, refrigerant miscibility, and how much hydrocarbon gas dissolves into the lubricant at pressure. Oil soluble variants are typically derived from butylene oxide.

  • Water-Insoluble PAG (WI-PAG)
    Fully propylene oxide. Relatively apolar, poor water solubility, good solubility with heavy hydrocarbon gases.
  • Water-Soluble PAG (WS-PAG)
    PO/EO copolymer, typically 1:1. Good water solubility, lower hydrocarbon dilution than WI-PAG.
  • Polyethylene Glycol (PEG)
    Fully ethylene oxide. Extremely polar, fully miscible with water, essentially immune to hydrocarbon dilution.

Technical Background

MONOMER CHEMISTRY AND POLARITY
Key solubility, miscibility and dilution behaviour is mainly driven by base-stock chemistry, and in the lubricants industry this is why polyalkylene glycol pag is used as one of several synthetic lubricants, while additives tune performance areas such as oxidation control, wear protection, corrosion and foaming. A PO-based backbone gives a lubricant that dissolves readily into hydrocarbon gases but rejects water. An EO-based backbone does the opposite. Copolymers are built to sit somewhere useful between the two. That tunable solubility is one reason PAG chemistry is used widely, since these are versatile polymers widely applied across demanding services. Because polarity is baked into the base stock, additive packages cannot shift these behaviours meaningfully — the backbone decides what the lubricant will do in service. PAGs also show exceptional thermal stability and resist breakdown effectively at high temperatures. Compared with hydrocarbon fluids and traditional hydrocarbon lubricants, PAG chemistry can deliver lower friction coefficients and higher load-carrying capacity in industrial machinery.

180–250

Typical PAG viscosity index

< 3%

PEG dilution with pentane (R-601)

15–30%

WI-PAG dilution with butane (R-600)

DILUTION BEHAVIOUR WITH HYDROCARBON GASES
At operating pressure, hydrocarbon gas dissolves into the lubricant in the sump. The more it dissolves, the more in-service viscosity drops below nominal grade. In representative butane/heavy-hydrocarbon cases, WI-PAG can show substantial dilution, WS-PAG typically lower dilution, and PEG very low dilution. Actual values depend on pressure, temperature, gas composition and formulation For medium to heavy hydrocarbon streams at high pressure, dilution is the single most important input into nominal grade selection.
MODELLING IN-SERVICE DILUTION

Predicting how much a PAG will dilute in real service requires solving the thermodynamics of gas solubility in the specific lubricant at the actual operating pressure and temperature. NEXT uses a PVT (pressure–volume–temperature) model calibrated against measured solubility data for each base chemistry — WI-PAG, WS-PAG, and PEG, with PAO and mineral baselines for comparison. The model takes the gas composition in mol % (methane, ethane, propane, butane, pentane, CO₂, H₂S, N₂), the suction and discharge pressure, and the expected sump temperature as inputs, and returns the in-service viscosity at those conditions.

That in-service number is what should drive nominal grade selection. For heavy hydrocarbon streams or high-pressure duty, the gap between nominal and in-service viscosity is typically one to two ISO grades — large enough to move a compressor from correctly lubricated to under-lubricated if selection is made from the data sheet alone. The PVT model removes that gap from the decision.

MISCIBILITY WITH REFRIGERANTS
Certain PAG formulations are highly miscible with ammonia (R717), making them suitable for ammonia DX systems where oil return depends on refrigerant/lubricant circulation. With CO₂ (R-744), miscibility is partial and formulation-specific — dedicated CO₂ PAGs are built to manage this. PAG oil is also widely used in automotive air conditioning systems with modern refrigerants, even though PAGs are not standard with most HFC or HFO stationary systems, where POE remains the dominant chemistry.
LUBRICITY AND FILM STRENGTH
PAGs are naturally high-lubricity. The polar backbone adsorbs onto metal surfaces, reducing friction at metal contacts and supporting higher load-carrying capacity under boundary conditions for more reliable extreme pressure protection than synthetic hydrocarbons can match without additive support. Combined with a high viscosity index of 180 to 250, a PAG typically outperforms a mineral or PAO of the same nominal grade across a wide temperature range.
CARBON AND VARNISH BEHAVIOUR
PAGs are known for low deposit tendency and clean burn-off behaviour compared with many hydrocarbon-based oils, and they also show exceptional thermal stability, resisting breakdown at elevated temperatures better than many hydrocarbon oils, which helps reduce carbon and varnish risk in suitable applications. For high-ratio gas compressors and heat pumps running at elevated discharge temperatures, especially in high temperature applications, this behaviour is often the deciding factor over base-stock cost.

Key Factors

Four variables determine whether a given PAG will perform in a given system. They interact — a change in one usually shifts the requirements on the others.

Base Lubricant Chemistry

The monomer type (PO, EO, BO, or copolymer) determines polarity, water solubility, and solubility with both refrigerants and hydrocarbon gases.

Gas Composition

Light hydrocarbons (methane, ethane) cause limited dilution; medium fractions (propane, butane) cause moderate dilution; heavy fractions (pentane+) cause significant viscosity reduction, particularly in WI-PAG.

Operating Pressure

Higher discharge pressures increase gas solubility in the lubricant. This effect is most pronounced with medium and heavy hydrocarbon gases and must be factored into viscosity grade selection.

Operating Viscosity

In-situ viscosity can drop significantly under gas dilution. Nominal grade selection must account for expected dilution across the full range of operating pressures and gas compositions.

Applications

PAG Selection by Application

PAG selection starts with what the compressor is moving, because choosing within a long-established synthetic base-oil family means matching the job to one of several high performance lubricants. Gas composition and refrigerant chemistry determine which backbone works.

Natural Gas Processing & NGL

Lubricants for feed and residue gas compression, NGL recovery, fractionation and gas-processing facilities.
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Natural Gas Gathering

Lubricants for compressors handling raw, wet and variable-composition gas from wells and field gathering systems.

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Vapor Recovery Units

Lubricants for low-pressure hydrocarbon vapour recovery from tanks, production facilities and oil-and-gas operations.

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Sour Gas Compression

Lubricants for gas streams containing significant H₂S, CO₂ and other acid-gas components.
Learn More >

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Frequently Asked Questions

Common questions about gas dilution and its effects on compressor lubrication systems.

It depends on what the compressor is moving. For ammonia refrigeration and heat pumps, WI-PAG is standard because of its high miscibility with ammonia. For medium-to-heavy hydrocarbon gas compression, WS-PAG is the usual choice as it dilutes less than WI-PAG at pressure. For immiscible heat pump systems running on hydrocarbon refrigerants, PEG is the default because dilution is negligible. For CO₂ systems, dedicated CO₂ PAGs are formulated specifically for that refrigerant and should not be substituted with general WI-PAG or WS-PAG.

No. PAG is chemically incompatible with mineral, PAO, and POE lubricants and forms sludge on contact. A proper conversion requires draining the system, flushing with a compatible fluid, and confirming that residual contamination is below 1% before charging the new lubricant. Skipping the flush is the most common cause of premature PAG failure after a base-stock change.

The model is a thermodynamic tool that calculates how much of each gas component dissolves into the lubricant at the system’s actual operating pressure and temperature. It is calibrated against measured solubility data for WI-PAG, WS-PAG, and PEG, with PAO and mineral baselines included for comparison. Inputs are gas composition in mol %, suction and discharge pressure, and expected sump temperature. Outputs are total dilution percentage and the resulting in-service viscosity. For compressors running on hydrocarbon streams or at elevated pressures, this in-service viscosity is the number that matters for grade selection — and it can differ from the nominal data-sheet value by one or more ISO grades. To run your case through the model, use the Request Lubricant Recommendation form or contact the NEXT technical team directly.

They are the same material. PPG is the chemical name for a polymer built entirely from propylene oxide; WI-PAG is the industry designation used in the lubricant market. The terms are used interchangeably in technical literature and in NEXT product documentation.

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