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Lube-Tech No.77 page 1PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE

Oil Soluble SyntheticPolyalkylene Glycols A New Type of Group V Base OilAbstractSince their commercialisation over 50years ago, polyalkylene glycol (PAG)lubricants have continued to solveproblems that mineral oils cannot.Conventional polyalkylene glycols thatare derived from the polymerisation ofethylene oxide and/or propylene oxideprovide many performance benefits suchas excellent friction control, good lowtemperature properties, high viscosityindices and excellent deposit control.One limitation is their poor miscibility inhydrocarbon oils. A new range of oilsoluble PAGs (OSP) has recently beendeveloped and derived from downstreamderivatives of butylene oxide. These newAPI Group V base oils offer many of thetraditional advantages of PAGs.Moreover, they offer formulators anadditional tool for upgradinghydrocarbon oils by using them as a co-base oil or performance enhancingadditive to improve deposit control,friction control and extend fluid life.

IntroductionConventional polyalkylene glycols (PAG)are generally regarded as niche syntheticlubricants and are used across thelubricant industry to solve problems thatpetroleum oils can’t solve. Most PAGs inuse are manufactured from downstreamderivatives of ethylene oxide (EO) andpropylene oxide (PO). They offer manytechnical benefits over mineral oils such asexcellent lubricity, good load bearingcharacteristics, good low temperatureproperties, high viscosity indices and highflash points making them suitable for avariety of applications [1]. Furthermore,synthetic processes used for manufac-turing PAGs are very versatile which allowspolymers to be designed to have manydifferent functional properties [2]. In the

lubricant industry today there are over100 different polymer chemistries that areused and there are many hundreds morefor applications beyond lubricants.

This versatility can be exemplified in thedesign of water soluble PAGs fromcopolymers of ethylene oxide withpropylene oxide. These polymers can besynthesised to have a broad range ofmolecular weights and viscosities (e.g. upto 50,000cSt at 40°C) with differentEO/PO ratios and polymer architecturessuch as random or block structures.These are the only major water solublelubricant base oil available today. Thisunique feature provides functionaladvantages for fire resistant hydraulicfluids, metalworking fluids, quenchant,textile lubricants and many more.

One known disadvantage of conven-tional PAGs is their poor oil miscibility.This has two key implications. Firstlywhen end users wish to convert theirequipment from a hydrocarbon oil to aconventional PAG, extensive flushingprocedures are often required. Secondly,formulators can’t use conventional PAGsas co-base fluids or additives in theirhydrocarbon oils to enhanceperformance. This is in contrast to othersynthetic lubricant chemistries such aspolyalphaolefins (PAO), polyisobutylenes(PIB) and esters which are used to formsemi-synthetic lubricants that can meethigher performance specifications.Therefore it is envisioned that the designof oil soluble PAGs that offer many ofthe inherent benefits of conventionalPAGs can provide opportunities toformulators to upgrade hydrocarbon oilsand solve many of today’s lubricationchallenges by leveraging the inherentfunctional benefits of PAGs.

Synthesis & Physical Properties of OilSoluble PAGsThe design of oil soluble PAGs can beaccomplished by synthesisingdownstream derivatives of butylene oxide(BO) to form polybutoxylate homo-polymers or by synthesising copolymersof PO and BO. Butylene oxide is availableon an industrial scale and is used toproduce polymers for other applicationssuch as fuel additives but has beenlargely unexplored as a building block forlubricant base oils. Figure 1 illustrates atypical polymer architecture of an OSPbase oil.

Figure 1 – Structure of oil soluble PAG

The molecular weight and thereby theviscosity of PAGs can be influencedduring the preparation and can beadjusted within narrow limits. This way, avery broad product design space isavailable and polymers can be tailoredand engineered according to the specificapplication needs. Table 1 shows thephysical properties of the new oil solublePAG base oils. The polymers have beenlabelled OSP-32 to 680 with the numericdigits referring to the ISO viscosity gradeclassification. The PO/BO copolymers areOSP-32 to 220 and the BO homo-polymers are OSP-320 to 680.

Note: for OSP copolymers, PO and BO are randomlydistributed; for BO homo-polymers then m = 0.

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The polymers span many of the classicalISO viscosity grades but it is possible todesign even lower and higher viscositygrades using the same synthesisapproach. These OSP base oils havehigher viscosity indices than mineral oils,excellent low temperature propertieswith pour points as low as -57°C andvery low aniline points. Details of theirperformance benefits of such polymerswill be discussed.

Oil SolubilityThe oil solubility of OSP-32 to 680 wasassessed in API Group I to Group IIImineral oils (ISO-46) and three API Group

IV polyalphaolefins. These PAO’s havebeen labelled in Tables 2 and 3 as GroupIV-H (PAO-40), Group IV-M (PAO-8) andGroup IV-L (PAO-4). Blends containing10 and 50% by weight of OSP base oilin each of the hydrocarbon oils wereprepared and assessed for solubility atambient temperature for 1 week. Thevisual appearance of the fluids wasassessed before and after the test.Blends which appeared homogeneousthroughout the test were characterisedas being soluble (green) and those whichwere not homogeneous were charac-terised as non soluble (red). Tables 2 and3 show the results.

Generally excellent solubility is seen forthe lower viscosity OSP grades (OSP-32,46 and 68) and the higher viscositygrades (OSP-320, 460 and 680) in all theGroup I-IV base oils except the highviscosity PAO. OSP-150 and 220 are lessmiscible in Group II and III oils.

One macro trend in the base oilindustry is the transition of manylubricant formulations from Group I toGroup II and III base oils. Newerhydrocarbon base oils are much moreiso-paraffinic with significantly loweraromatic content and lower sulphurlevels. In Group I base oils thesecomponents often provide a degree ofsolvency which helps to impartsolubility for performance enhancingadditives. The absence of theseunwanted polar compounds in Group II,III and also Group IV base oils has madeit more challenging for formulators todevelop compositions using manyconventional types of performanceenhancing additives. The excellent lowaniline points of the polar OSP base oilsmeans their addition to hydrocarbonoils can overcome this barrier and alsoprovide additional benefits describedbelow.

Deposit ControlThe excellent deposit control charac-teristic of conventional PAG lubricantsis well known [3]. In some applicationscontrolling deposits and varnishformation is a critical concern [4]. PAGshave been used extensively in someapplications such as rotary screw aircompressor lubricants to essentiallyeliminate this problem. The depositcontrol behaviour of blends of OSP-46in a Group I solvent neutral 150(S.N.150) mineral oil was evaluatedusing a modified ASTM D-2893B testmethod. To S.N.150 base oil wasadded 10, 5 and 1% by weight ofOSP-46 and a phenolic antioxidant(0.5% w/w). The fluids were heated to121°C with dry air blown through thefluid at a flow rate of 10L/hour for anextended period of 50 days. The visualappearance of each product wasassessed and any deposits observedand reported. Results are visuallyshown in Figure 2.

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Table 1 – Physical properties of OSP Polymers

Table 2 – Typical oil solubility for 10% OSP base oils in API Group I-IV hydrocarbon oils

Table 3 – Typical oil solubility for 50% OSP base oils in API Group I-IV hydrocarbon oils

Green = fully soluble at ambient, red = not soluble.

Green = fully soluble at ambient, red = not soluble.

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Lube-TechThe inclusion of an oil soluble PAG signif-icantly improves oil cleanliness anddeposit control. Heavy deposits wereformed in the mineral oil fluid that didnot contain any OSP base oil and yet allfluids which contained the OSP base oilwere clear translucent and deposit freeat 10 and 5%. Even at 1% evidence ofimproved cleanliness was observed. Oneplausible explanation for the absence ofdeposits is that the oxidation by-productsof mineral oils are more polarcompounds. These are not soluble inhydrocarbon base oils and lead todeposit formation. However they aresoluble in the more polar OSP polymers.Conceptually this is a significantobservation since it is well known thathydrocarbon lubricants are prone todeposit formation in engine oils, turbinefluids, compressor oils and gearlubricants. The inclusion of an oil solublePAG to hydrocarbon oils, could upgradethem, providing longer fluid life andgreater equipment reliability. In theautomotive industry specificationspertaining to deposit control continue totighten and solutions are desired thatminimise this impact.

Friction PropertiesTraction measurements were performedusing a Mini-Traction Machine (MTM).The MTM is a computer controlledprecision traction measurement system.The test contact is formed between apolished 3/4 inch steel ball and a 46-mmdiameter steel disc, each independentlydriven to produce a sliding/rollingcontact. Traction measurements onblends of a polyalphaolefin base oil (PAO-8) and OSP-46 at concentrations of 5 and8% were recorded at a contact pressureof 0.75GPa, a temperature of 80°C and aslide roll ratio of 10% over a 30 minuteperiod. Traction data is shown in Figure 3.

Simplistically the polar nature of the OSPpolymer appears to have good surfaceaffinity and provides a film that lowersand stabilises friction. Under these testconditions, excellent stability is observedat a level of 8% OSP-46. Friction beginsto increase when it is used at 5%. Insome applications such as automotiveengine oils, synthetic esters are used asfriction modifiers or indeed boundary

friction additives. However it is widelyknown that many esters are susceptibleto hydrolysis [6]. Since PAGs are hydrolyt-ically stable, oil soluble PAGs may provideanother tool to formulators that wish toimprove friction control.

Noack VolatilityVolatility measurements were made forOSP-32 to 68 using method CEC-L-40.Measurements were made on the virginbase oil (no anti-oxidant). Measurementswere also made for OSP-32, 46 and 68when 1 and 2% w/w of an alkylateddiphenylamine antioxidant was added.Results are shown in Figure 4.

The NOACK volatility test is conducted at250°C. At this temperature lowermolecular weight oligomers arevolatilised. Since the composition of thelower molecular weight oligomersdecreases as the average polymermolecular weight and viscosity increases,volatility decreases accordingly as isshown. Furthermore the addition of anti-oxidant to the base oil suppresses thevolatility since it reduces the rate ofoxidation of the polymers. PAGs formlower molecular weight oligomers onoxidation which are more volatile thantheir parent polymer.

Rotary Pressure Vessel Oxidation TestThe Rotary Pressure Vessel Oxidation Test(ASTM D2272) was used to measure theoxidation stability of one model polymer– OSP-46 – and it’s response to aminicand phenolic anti-oxidants. The anti-oxidants were screened at levels of 2%w/w. Included in the assessment are adialkylated diphenylamine (DADP),alkylated phenylnaphthylamine (APNA)and an alkylated phenolic (AP) antiox-idants. Results are shown in Table 4.

The virgin OSP-46 base oil showed a lowvalue of 15 minutes. However, additionof aminic anti-oxidants and acombination of aminic and phenolic anti-oxidants yield significantly higher values.

A proprietary anti-oxidant packageshows values above 1500 minutes arepossible through simple experimentation.

Practical Uses of OSPsThe new oil soluble polyalkylene glycolsprovide additional functionality overtraditional PAGs. Simplistically they can beused as a primary base oil, co-base oiland as a performance enhancing additive.As a primary base oil, it is envisioned thatnew industrial lubricant products can beformulated such as gear lubricants,greases and compressor fluids. Onepractical advantage is that equipmentconversions from a hydrocarbon oil to anoil soluble PAG lubricant will be muchsimpler. Furthermore the new OSPpolymers are kinder to elastomers andseals and are also less hygroscopic thanconventional PAGs.

Their use as a co-base oil in mineral oil toform semi-synthetic lubricants or as aperformance enhancing additive inhydrocarbon oils will open up theapplication envelope for PAGs. For thefirst time hydrocarbon oils and PAGs arecompatible. As deposit control additives,OSPs can help solve many of thechallenges of varnish formation in thegas turbine industry or eliminatechallenges with servo-valve sticking orfiltration blockages in hydraulic systems.In recent years there has been a prolif-eration of specifications for automotiveengine oils around deposit control. Oilsoluble PAGs provide an additional toolto formulators to meet these specifi-cations and also provide benefits infriction control as film forming additives.

Finally, the high polarity and low anilinepoints of OSPs will help formulators tosolubilise critical additives in newerGroup II and III base oils which are moreiso-paraffinic and less polar in nature.But also macro-changes in the dynamicsof the base oil industry and the reducedavailability of bright stock has created ashortage of high viscosity base fluids.The inclusion of an OSP as a bright stockreplacement may offer formulators analternative approach to boosting alubricants viscosity and also provide someof the additional functionality describedabove.

PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE

No.77 page 3

Table 4

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Lube-TechPUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE

No.77 page 4

Rotary Pressure Vessel Oxidation TestThe Rotary Pressure Vessel Oxidation Test(ASTM D2272) was used to measure theoxidation stability of one model polymer– OSP-46 – and it’s response to aminicand phenolic anti-oxidants. The anti-oxidants were screened at levels of 2%w/w. Included in the assessment are adialkylated diphenylamine (DADP),alkylated phenylnaphthylamine (APNA)and an alkylated phenolic (AP) antiox-idants. Results are shown in Table 4.

The virgin OSP-46 base oil showed a lowvalue of 15 minutes. However, additionof aminic anti-oxidants and acombination of aminic and phenolic anti-oxidants yield significantly higher values.A proprietary anti-oxidant packageshows values above 1500 minutes arepossible through simple experimentation.

Practical Uses of OSPsThe new oil soluble polyalkylene glycolsprovide additional functionality overtraditional PAGs. Simplistically they canbe used as a primary base oil, co-base oiland as a performance enhancingadditive. As a primary base oil, it isenvisioned that new industrial lubricantproducts can be formulated such as gearlubricants, greases and compressor fluids.One practical advantage is thatequipment conversions from ahydrocarbon oil to an oil soluble PAG

lubricant will be much

simpler. Furthermore the new OSPpolymers are kinder to elastomers andseals and are also less hygroscopic thanconventional PAGs.

Their use as a co-base oil in mineral oilto form semi-synthetic lubricants or as aperformance enhancing additive inhydrocarbon oils will open up theapplication envelope for PAGs. For thefirst time hydrocarbon oils and PAGs arecompatible. As deposit control additives,OSPs can help solve many of thechallenges of varnish formation in thegas turbine industry or eliminatechallenges with servo-valve sticking orfiltration blockages in hydraulic systems.In recent years there has been a prolif-eration of specifications for automotiveengine oils around deposit control. Oilsoluble PAGs provide an additional toolto formulators to meet these specifi-cations and also provide benefits infriction control as film formingadditives.

Finally, the high polarity and low anilinepoints of OSPs will help formulators tosolubilise critical additives in newerGroup II and III base oils which are moreiso-paraffinic and less polar in nature.But also macro-changes in the dynamicsof the base oil industry and the reducedavailability of bright stock has created ashortage of high viscosity base fluids.The inclusion of an OSP as a bright stockreplacement may offer formulators analternative approach to boosting alubricants viscosity and also provide someof the additional functionality describedabove.

References[1] Lawford, S., ‘Polyalkylene glycols’,

Synthetics, Mineral Oils and Bio-based Lubricants, Ed. L. Rudnick,2006; 119-138, CRC Press

[2] Matlock, P.L. and Clinton, N.A.,Polyalkylene glycols, ed. R.L.Shubkin, 1993, Chapter 4, 101-123,Marcel Dekker Inc.

[3] Greaves, M.R. CompoundingsMagazine, April 2009, 59, 4, 25-26.

[4] Livingstone, G. Varnish: LookingAhead, Turbo-machineryInternational. Sept/Oct 2008.

[5] Profliet, R., Keep your hydraulicsystem clean of varnish or pay theconsequences, Lube Magazine. April2008.

[6] Greaves, M.R. Knoell, J., Acomparison of the performance ofenvironmentally friendly anhydrousfire resistant hydraulic fluids, J.ASTMInt. 2009, Vol. 6, No.10, 1-10.

Contact Details

Dr Martin Greaves, Dow Europe GmbHBachtobelstrasse 3HorgenSwitzerland, CH-8810e-mail: mrgreaves@dow.com

Figure 4 – NOACK volatility by CEC-L-40

Table 4

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