PAG/ PEG End Capping ServicesAbout Banner

End-capping service

Lubricants reduce friction and wear on moving parts and while natural mineral oil-based fluids represent the majority of the market, technological advances in equipment and machinery would not be possible without the benefits offered by synthetic products. One of the fastest growing synthetic lubricants is polyalkylene glycol (PAG).

PAGs are Polyalkylene Glycols – homopolymers of ethylene oxide or propylene oxide, or co-polymers of ethylene oxide or propylene oxide . In addition, PEGs (also known as PEO and POE)  – polyethylene glycol or oligomers or polymer of ethylene oxide are used as lubricant coatings for various surfaces in aqueous and non-aqueous environments. Both product ranges can benefit from end capping technology showing improved stability and functionality.

Chemoxy offer end-capping as a toll processing service.

Methyl capping converts a reactive hydroxyl (-OH) group into an unreactive ether group (-OMe). For example in the capping of polyethylene glycol monomethyl ether.



methyl capping



Chemoxy offer end-capping as a toll processing service. We have over 20 years’ experience and are set up to handle bulk methyl chloride at our facilities in Teesside, UK.

So, why end cap?

End-capping can improve HYGROSCOPICITY  STABILITY, and increase the VISCOSITY INDEX, allowing you to offer a lubricant which reduces the risk of rust, improving the integrity of the system and which will operate over a wider temperature range.

What is Hygroscopicity?

Hygroscopicity is the measurement of a material’s ability to absorb or release water as a function of humidity.

The effect of the main chain and end group relative to the hygroscopicity of the PAG structure is well researched. The hygroscopicity is known to increase as the number of hydroxyl groups is increased. This is due to the hydrogen bond of the hydroxyl group (end group) and ethylene oxide, EO. It is also known that EO is more active than propylene oxide, PO. The PO has steric hindrance.

The hydrogen bonding strength of the hydroxide group of EO is ten times stronger than that of PO. In addition it is known that the hygroscopicity of the diester type and dicarbonate type are same as the diether type. However the water which is absorbed causes hydrolysis. As a result, the diether type PEG/PAG is most suitable to use in automotive A/C systems since permeation of moisture at seals or through barrier hoses is likely. Thus the end capping process reduces the hygroscopicity of the lubricant by converting the hydroxyl to an ether, thereby removing the hydroxyl and the hydrogen bonding ability, hence water attraction is reduced.

This is important, because the more water that the lubricant absorbs, the more the risk of rust increases, and thus the degradation of the integrity of the system and the system itself.  Chemoxy are able to modify your PAG and PEG products, using methyl capping chemistry to reduce the hygroscopicity  and improve the lubricants performance in service.


It is also known that the thermal stability of PAG’s that have un-capped end groups, i.e. free hydroxyl groups, are also less thermally stable than those that are capped – in chemical terms, it is relatively easy to dehydrate an alcohol to make an alkene, i.e. producing water, and thereby making hygroscopicity worse, as well as generating polymerisable alkenes. Consider also, that one way to make symmetrical ethers is to heat an alcohol with acid:

               RCH2CH2OH  →   RCH2CH=CH2 + H2

                RCH2CH2OH  → RCH2CH2OCH2CH2R


The holy grail of lubricants is oil where the viscosity doesn’t change when heated.  Such an oil would be thin enough to give better fuel economy when the engine is cold but would retain its lubricant properties when hot.  The change in viscosity with temperature is known as the “viscosity index”.  High viscosity index = viscosity doesn’t change much.  Low viscosity index = viscosity changes significantly so ideally lubricants should have a high viscosity index. End-capping increases the viscosity index (VI) which maintains optimum lubricity over a wide temperature range.

Thus to improve properties of PAG’S OR PEG’s, or any other lubricant or oil with a terminal hydroxyl function as regards water absorption, rust, degradation under temperature/pressure of operating conditions – end-capping is required. Chemoxy offer a custom processing service to end cap PAGs and PEGs, Contact us to  find out more.