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AbstractThe lubricant industry has a history ofinnovation spanning more than acentury. Recently a new range of APIGroup V base oils, called oil solublepolyalkylene glycols (OSP), has beenintroduced. Unlike conventionalpolyalkylene glycols (PAG), the new OSPscan enhance the properties of existinghydrocarbon oils by providing a range offunctionalities. This article will brieflydescribe OSPs, highlight some of thesefunctionalities and also illustrate theirpractical benefits for use in syntheticgear oils as a selected application toillustrate their multi-functionality.Research in this area is still new butformulators and equipment builders nowhave another tool for solving some oftheir tribology and lubrication challenges.

Introduction Most lubricants in use today are derivedfrom API Group I-III base oils as thedominant formulation component.However for some applications andequipment, semi-synthetic or indeed fullsynthetic lubricants are needed toprovide a desired functionalperformance. These lubricants oftencomprise an API Group IV (polyal-phaolefin) or a Group V base oil in whichthey are used as primary or co-base oilsin formulations. Group V base oilsinclude several classes of products suchas synthetic esters, alkylatednaphthalenes and phosphate esters.These are often polar materials withlower aniline points than classical mineralbase oils and offer a plurality of function-alities some of which can include goodfilm forming behaviour, high solvencypower and cleanliness features, anti-wearand extreme pressure properties and soon. Many of these materials are used asco-base oils in hydrocarbon oils toenhance their performance or asperformance enhancing additives.

Polyalkylene glycols are another class ofGroup V base oil. They were firstinvented in the 1940s and are used inmany niche applications such as wormgear lubricants, rotary screw aircompressors, reciprocating gas

compressors, anhydrous hydraulic fluids[1, 2] and more recently gas turbine oils[3]. In these applications theydemonstrate excellent deposit control,good thermo-oxidative stability and fluidlongevity, and offer excellent frictioncontrol for improving energy efficiency ofequipment. PAGs are also used asadditives in water based lubricants andfunctional fluids such as metal cuttingfluids, fire resistant water glycol hydraulicfluids and quenchants. However unlikeother Group V base oils, their use asadditives to boost the performance ofhydrocarbon oils has not been possible inthe past simply because they have a lowhydrocarbon oil solubility. The majority ofpolyalkylene glycols used today inlubricants are derived from downstreamderivatives of ethylene oxide and/orpropylene oxide. Recent research hasshown that butylene oxide and itscombination with other oxides can leadto oil soluble polyalkylene glycols (OSP)and if carefully designed, these polymerscan offer many of the excellentfunctional properties of traditional PAGsand provide formulators with a newbuilding block for solving problems thatmay not be possible with existingtechnologies.

Research into the functional performanceof OSPs is still in its infancy but in thepast three years some unique featuresand benefits have been discovered. Thisarticle will explain more about OSPs andsome of their key functionalities. It willhave a special focus on gear oils andparticularly their unique air releaseproperties. In recent feature articles in“Lube” magazine [4, 5] the importanceof fast air release for lubricants in sometypes of equipment has been discussedand this article builds on that theme.

Aspects of Oil Soluble PAGsThe oil soluble polyalkylene glycols in usetoday span a wide range of molecularweights and viscosities. Table 1 providesan overview of some of their keyproperties. Since there is a high degreeof flexibility in designing OSPs, it ispossible to extend this range and buildpolymers with even lower or highermolecular weights and functionalities.The polymers range from the classicalISO-32 through to ISO-680 viscositygrades. The lowest viscosity polymer,OSP-18, has a kinematic viscosity of4mm2/sec at 100ºC which is similar to aclassical PAO-4 base oil. Their viscosityindices range from 123 to 196.

Oil Soluble Polyalkylene Glycols

Table 1. Typical physical properties of Oil Soluble Polyalkylene Glycols (OSP).

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The polymers have excellent lowtemperature properties with pour pointsfor the lower viscosity grades less than -50ºC. The lower viscosity grades havehigh levels of biodegradability and thehigher viscosity grades are more bio-resistant.

An interesting property of OSPs is theirlow aniline points which are typically lessthan -20ºC. The aniline pointmeasurement is considered to be ameasure of the degree of solvency poweror polarity of a base oil. Thus base oilsthat are rich in ester, ether or aromaticfunctionalities tend to have lower anilinepoints than those simply derived fromparaffinic or iso-paraffinic feed stocks.Figure 1 illustrates the typical anilinepoints for a wide range of API Group I-Vbase oils. API Group I base oils havetypical values of about 100ºC. Thisincreases for modern Group II and III oilssince these base oils have much lowerlevels of aromaticity and polarcompounds in their compositions.Polyalphaolefins (PAO) usually have thehighest values which are typically about130ºC for commodity lower viscosityPAOs or significantly higher for theheavier viscosity grades. Formulators ofmodern hydrocarbon lubricants based onGroup II-IV oils sometimes find conven-tional additives are less soluble in baseoils with high aniline points and toimprove their solubility they often includea more polar base oil such as an ester oran alkylated naphthalene at low treat

levels. Esters have typical values in therange 5-40ºC and alkylated naphthaleneshave values which are slightly higher.Since OSPs have very low aniline pointsthey provide formulators another option.Unlike esters which are sometimeshydrolytically unstable in environmentswhere water or moisture ingress occursin a lubricant, OSPs are more hydrolyt-ically stable and may provide greaterfluid longevity and peace of mind.

Research and application testing hasshown that OSPs offer a number of keyfeatures [6, 7]. For example fieldexperience has shown that they have theability to help control deposits when

used as co-base oils or additives inhydrocarbon oils such as hydraulic fluidsand compressor oils. They are alsosurface active polymers and at low treatlevels can reduce friction under mixedelasto-hydrodynamic and boundaryconditions. Furthermore OSPs haveunique air release properties. Theseperformance attributes will be explainedin more detail.

OSPs and Deposit ControlIt is well known that the thermal andthermo-oxidative degradation ofhydrocarbon lubricants and ester basedproducts can lead to deposits, gums andvarnish formation in equipment. Thepresence of these degradation productscan cause serious equipment damage orpoor equipment reliability. This isespecially true in turbine, hydraulic andcompressor equipment. Equipmentbuilders continue to add new specifi-cation requirements or tighten existingones to improve deposit control oflubricants. For example specifications forpassenger car engine oils continue toevolve such that better piston cleanlinessand engine sludge control are attained.Oil soluble PAGs can play a role inhelping our industry solve some of theseproblems. To illustrate this, twoformulations were assessed in a modifiedASTM D2893B test in which the fluidswere aged for two months at 121ºC inwhich air was blown through the fluid ata fixed rate. Both compressor fluidscontained an API Group II base oil andone contained 5% of an OSP. Figure 2Figure 1. Typical aniline points of common API Group I-V base oils.

Figure 2. Visual assessment of the deposit control characteristics of two compressor oils that have been agedin a modified ASTM D2893B test.

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shows the visual effect of including anOSP as a co-base oil. After two months ofageing visual evidence of deposits wereobserved in the compressor fluid that didnot contain the OSP. However theinclusion of the OSP showed a fluid withsignificantly improved cleanliness. Ashydrocarbon oils age, they usually formmore polar oxidation by-products that aresometimes not soluble in the parent baseoil. When this occurs, deposition ontosurfaces can occur that can ultimatelylead to varnish formation. Since OSPs areinherently polar materials, they helpsolubilize the degradation products ofhydrocarbon oils and therefore minimizethe effect of deposit formation.

Improved deposit control and systemcleanliness has been observed in manytypes of equipment that have operatedin the field using API Group I-IVhydrocarbon oils including hydraulic,gear, compressor and metalworkingfluids. But in addition this effect has alsobeen observed when OSPs have beenincluded into natural esters (vegetableoils). When vegetable oils oxidize theycan form highly viscous gums as theolefinic moieties of the acid fractions ofthe triglycerides oxidize and then cross-link. The inclusion of an OSP tovegetable oils at typical treat levels of10% by weight can help solubilize thedegradation products and keepequipment cleaner for longer periods.Furthermore, hydrolytic stability testsshow that OSPs can help improve thestability of vegetable oils to hydrolysis byacting as polymeric sponges to wateringress into a lubricant.

OSPs and Friction ControlOil soluble polyalkylene glycols also actas friction control additives. Since everythird atom along the alkoxide backboneis an oxygen atom, the materials offer ahigh degree of polarity and surfaceactivity. It is well known that conven-tional PAGs, that are polymers derivedfrom ethylene oxide and propylene oxidederivatives, offer excellent film formingproperties. In some applications forexample in worm gears, significantlybetter energy efficiency can be achievedversus using hydrocarbon oils. Howeversince conventional PAGs are not solublein hydrocarbon oils, this inherent featurehas not been exploited. OSPs are nowoffering this functionality.

Figure 3 illustrates the film formingbehaviour of OSP-46 in a Group III baseoil (ISOVG-46). The test was conductedusing a Mini-Traction Machine in which asteel ball rotates on a steel disc such thatthe slide-to-roll ratio is 10%, speed is 15mm/sec, contact pressure is 0.75GPa anda fluid temperature is 80ºC. The rubbingcycle was about 30 minutes. In theabsence of OSP, friction graduallyincreases over time and the data pointsshow increased scatter suggesting thesurfaces are becoming rougher. Howeverwith the inclusion of an OSP, the frictioncoefficient stabilizes after less than 5minutes. Simplistically this provides someevidence of friction reducing behaviour.This effect has also been observed inother types of Group I-III base oils andpolyalphaolefins. Figure 4 shows afurther example in which friction profileshave been recorded for a Group III baseoil (ISOVG46) with 1% of an OSP andcompared to the base oil alone.Boundary friction is reduced when theOSP is included and again supports thegood film forming properties of thesepolymers.

OSPs as components of Gear OilsToday OSPs are being adopted in a broadrange of applications and as moreresearch is conducted into theirfunctionality and performance theirpractical use into solving tribologyproblems is expected to grow. One areawhere OSPs could provide multi-functionality is in industrial gear oilswhere they can be used as primary baseoils in fully synthetic gear oils or asperformance additives in for example,PAO gear oils. Some of the reasons whyan OSP solution should be consideredwill be explained.

Research has shown that OSPs haveunique and very fast air releaseproperties. The air release properties ofgear oils and other lubricants are oftenmeasured using ASTM D3427. Theentrainment of air into lubricants inoperation is a common occurrence andup to 10% of air by volume can beentrained. The rapid release of the air inhydraulic, turbine and gear equipment isa critical requirement for modernequipment. Fluids with poor air releaseproperties can cause erosion, cavitation,spongy controls, noise and faster ageingof the fluid through oxidation. The airrelease properties of most classes of fluidgenerally become worse (i.e. highervalues) as its viscosity increases. Forexample an ISO-320 fluid will usuallyhave a much higher air release valuethan an ISO-46 fluid. Some of the trendsin gear box design are shown in Figure 5.

Figure 3. Friction profile of a Group III base oil (blue)and a Group III base oil with 10% OSP-46 (red).Measurements made using a Mini-Traction Machine,(steel ball on steel disc) at 80oC, a contact pressure of0.75GPa, speed 15 mm/sec and 10% slide-roll-ratio.

Figure 4. Friction performance of a Group III base oilwith and without OSP-46 using a ball on disc Mini-Traction Machine operating at 80ºC, contact pressure1GPa and slide roll ratio of 50%.

Figure 5. Trends in gear equipment design andfluid development.

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Modern gear boxes are being designed tohave lower weights and size, higherpower and sometimes use newermetallurgies. There is also a trend todesign equipment with smaller reservoirs.These trends will have an impact onfuture lubricant requirements and it isexpected they will need to have improvedthermo-physical properties such as higherthermal conductivities and faster heattransfer, improved thermo-oxidativestability, and preferentially highervolumetric heat capacity. But in additionfaster air release times may be needed.The requirements for air release times oflubricants have been trending down inequipment builder specifications.

Figure 6 shows the air release times oftwo types of commercially availablesynthetic gear oils that are based onPAOs and conventional PAGs for ISO-150to 460 viscosity grades. These times arecompared to a range of formulated OSPgear lubricants in which the OSPrepresents more than 95% of theircompositions. For example one cancompare the data for the ISO-320products which is a common class ofviscosity grade for industrial gear andwind turbine lubricants. ConventionalPAG and PAO products give typical airrelease times which are 15 minutes orhigher. However the OSP product showsa significantly lower value of about 2minutes. This differentiation is observedacross all viscosity grades. Furthermore,very low air release times of less than 1minute have been recorded for othertypes of fully synthetic OSP productssuch as hydraulic oils in the viscosityrange ISO-32-68. For equipment buildersthat are seeking to redesign their systemsto include much smaller oil reservoirs andperhaps reduce capital costs, the newOSP technology may provide a logicalsolution.

OSPs provide some interesting benefits asperformance additives in synthetic gearoils and especially those based on PAOs.Formulations of PAO gear oils oftencontain a synthetic ester as a co-base oil.The ester provides a range of function-alities. One of these is to provide a sealswelling effect on certain elastomers tonegate the seal shrinkage effect of higherviscosity PAOs. In addition the esterprovides a degree of friction control andhelps to maintain good fluid cleanlinessduring its service life. One knowndisadvantage of esters is that they areprone to hydrolysis especially under mildlyacidic conditions. Therefore as a fluidages to form acidic by-products,hydrolytic degradation can accelerate.Formulators of future gear oils may wishto consider using OSPs as an alternativeto esters. For example it is known thatthe ISO-VG32 grade of OSP has similarseal swelling effects to an adipate diesterthat is commonly recommended as a co-base oil in formulating PAO gear oils.

Table 2 compares their simple viscometricproperties and suggests a direct replacementis possible on these properties alone.

Figure 7 compares the effect of the twoco-base oils as seal swell additives onnitrile butadiene rubber (NBR) under ashort duration elastomer test. In theabsence of an ester or OSP co-base oil,the PAO shrinks NBR. However when theco-base oils are included, it is possible tonegate the shrinkage to achieve a mildseal swelling effect.

Figure 8 compares the frictionperformance of both compositions in aMini-traction Machine test in which a

steel ball rotates on a steel disc at acontact pressure of 0.75GPa, the fluidtemperature is 80ºC, speed 15 mm/secand a 10% slide roll ratio. Esters areknown to be surface active and act asfriction modifiers or friction reducingagents. There is a greater degree offriction reduction for the OSP containingproduct versus the ester under these testconditions. This provides supportingevidence that OSPs may help improvefriction control and improve energyefficiency of gear oils when they are usedas co-base oils or additives informulations. Therefore in synthetic PAOgear oils the inclusion an OSP canprovide multi-functionality and improvedeposit control as fluids age, improvefriction control, act as seal swell additivesand also provide enhanced additivesolubility. OSPs may be a preferred choiceover esters especially if there areconcerns about hydrolysis when using anester and if longer fluid life is desired.

Article written by Martin R. GreavesResearch LeaderThe Dow Chemical CompanyHorgen, SwitzerlandEmail: mrgreaves@dow.com

Figure 7. NBR elastomer volume swell of formulatedPAO blends for 168 hours at 100ºC.

Figure 8. Friction performance of a PAO gear oilcontaining 20% diester (red) and a PAO gear oilcontaining 20% OSP-32 (blue).

Figure 6. Air release values measured at 75ºC usingASTM D3427 of commercially available PAO and PAGsynthetic gear oils (ISO VG150-460) and new OSPs. Table 2. Physical properties of adipate ester and

OSP-32 co-base oils.

References[1] Matlock, P.L. and Clinton, N.A. (1993), “Polyalkylene

glycols”, Shubkin R. (eds), Synthetic Lubricants and HighPerformance Functional Fluids, Marcel Dekker, pp. 101-123.

[2] Greaves, M.R. (2013) “Polyalkylene glycols”, Rudnick,L.R. (eds), Synthetics, Mineral Oils and Bio-basedLubricants, Chapter 6, CRC Press.

[3] Khemchandani, G.V., (2012) Non-varnishing andtribological characteristics of polyalkylene glycol-basedsynthetic turbine fluid, Lub., Sci., 24. No. 1 pp. 11-21.

[4] T. Dopichay, J. Unth, T. Lang, R. Rahmfeld, Comparisonof oil types regarding air release and foaming in drycase operation, Lube magazine, No. 116, August 2013.

[5] T. Dopichay, J. Unth, T. Lang, R. Rahmfeld, Comparisonof oil types regarding air release and foaming in drycase operation, Lube magazine, No. 117, October 2013.

[6] Greaves, M.R., Van Voorst, R., Meertens, M., Khelidj N.,Zaugg-Hoozemans E. (2012), “Performance propertiesof oil-soluble synthetic polyalkylene glycols”, Lubr. Sci.,Vol. 24 No.6, pp. 251-262.

[7] Greaves, M.R., Oil Soluble Polyalkylene Glycols – A NewType of API Group V Base Oil, Mineraloeltechnik, UNITIconference, Stuttgart, Germany, April 2013.

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