Ester Base Stocks. J. Szydywar Nyco SA, Pans, France JSL (1)

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1 Ester Base Stocks J. Szydywar Nyco SA, Pans, France Natural esters contained in animal fats such as sperm oil, lard oil or in vegetable oils like rapeseed and castor oil have been the main lubricants from antiquity until the early 19th century. By 1890, petroleum oils had almost completely displaced natural oils. However, fatty products were still used as 'oiliness agents' to reduce friction and wear in bearings and gears lubricated by petroleum products. About 193 7, Dr Herman Zorn of G Farben ndustrie in Germany began an intensive search for lubricants havizg the lubricating properties of natural oils, such as castor oil, while eliminating the tendency of these natural products to gel at low temperatures and to form gums and lacquers when used in engine operation. This paper describes the properties, types and applications of esters, as a main group of synthetic lubricants developed during the last four decades Detailed mechanistic studies on the oxidation of natural fats and oils had led to the conclusion that the instability of these materials was associated with the glycerine portion of the molecule and in particular with the secondary hydroxyl group in glycerine. Hence, triesters were prepared from acids reacted with a trihydric alcohol having only primary hydroxyl groups. These esters performed so well that Dr Zorn thoroughly studied several thousands of esters. The most important commercial esters during wartime were methyladipates. These were the first synthetic products ever used on a large scale in the lubrication area. The rapid development of the gas turbine which took place before the Second World War was accompanied by the concomitant development of lubricants that could withstand the arduous environment of the turbine engine. These early aircraft oils were based on dioctylsebacates which have progressively been replaced by azelates and lately by adipates. n the early 1960s, the US Navy was investigating a class of heavier ester fluids for turbine use. The Navy's primary interest in the fluids JSL (1)

2 154 Szydywar JSL 1-2 was their higher gearload carrying ability for turbo propellor and turbo shaft engines as well as helicopter gearboxes and transmissions. n 1965, the US Navy decided to convert all of their turbine engines to the new specification ML-L based on so-called hindered esters or neopolyol esters (NPE). The same move has been taken up by the engine manufacturers for the civil companies, and is still in force today. Esters found new areas of application in industry and also as automotive lubricants. n 1969, the first semi-synthetic 10W50 with a diester was put out on the market. n 1977 a fully synthetic crankcase oil, low20 containing poly alpha olefins (PAO) associated with esters marked the trend towards low viscosity oils, sometimes referred as fuel saving oils. This glimpse at the historical development of the synthetic ester lubricant shows that esters have been early recognised as potential candidates for lubrication in stringent environments such as gas turbine engines. This paper will examine what is attractive about esters as lubricating basestocks. PHYSCO-CHEMSTRY OF ESTERS As is well known, an ester is relatively simple molecule formed by the reaction of an alcohol and an acid (Figure 1). Figure 1 Ester structure 0 / R-c / + HoH~c-R +R--c + HzO OH \ OH&-R Acid Alcohol Ester Water R and R alkyl and/or aryl chain Schematic representation 1 Linear Ester Branched alkyl chain group alkyl chain of acidlor alcohol of acidlalcohol

3 JSL 1-2 Ester base stocks 155 Physical properties t is quite possible to establish a correlation between the structure of a synthetic basestock and its physical and physico-chemical properties. mportant properties contributed by the basestocks include such physical properties as volatility, liquid range and basic viscosity characteristics and thermal stability. On the other hand, additive packages contributed to oxidation stability, controlled corrosion properties and enhanced lubricity. The physico-chemical properties of esters depend on the structure of the hydrocarbon chain the presence of the ester group. t is instructive to bear in mind that the multiple processing possibilities of esters permit infinite variation of their properties: viscosity pourpoint evaporation miscibility with mineral oils miscibility with water compatibility with additives, V improvers and elastomers. The viscosity of an ester by itself does not necessarily indicate the characteristics of an ester. nfluence of the hydrocarbon chain The hydrocarbon skeleton in the ester plays the same role as in the hydrocarbon chemistry as far as the physical properties are concerned. For example, the viscosity indices (V) of synthetic esters are like mineral oils which likewise depend mostly on the molecular shape: increasing the chain length increases viscosity, improves V and raises freezing points; while the addition of side chains increases viscosity, lowers pour point and impairs V. The position of branched chain influences viscosity and V. These structural guides, and others, have grown out of earlier work on pure hydrocarbons by Dr Zorn, and by Murphy and Zisman from Naval Research Laboratory. Although these rules have been mainly studied on diesters, they are equally applicable to other esters.

4 156 Szydywar JSL 1-2 Biodegradability is also heavily dependent on the hydrocarbon moiety as can be seen from Figure 2. Biodegradability of lubricants is becoming a growing concern for outboard engine oils. Probably some hydrolysis takes place during this biodegradability process which enhances it. nfluence of the ester group The direct effect of the ester on the physical properties is that of lowering the vapour pressure (or the volatility) and consequently raising the flash point because of the strong dipoles, called the London forces, of the cohesive energy (Figure 3). The presence of the ester group in the molecule affects other properties such as: Thermal stability Hydrolytic stability Solvency The well-known inherent lubricity due to the polarity of the ester and oxidation stability to a lower extent. Figure 2 Biodegradability tests by CEC method nfluence of ester chemical structure on biodegradability: Viscosity % Ester at 100 C biodegraded Type (CSt) after 21 days PE linear acid PE branched acid 10 2 Die (dimerate) Complex A Complex B Complex B + BA additive Mineral oil + BA additive 70 to Figure 3 Volatility: comparison between a monoester (MOE) and a mineral oil (MO) KV -54 C (CSt) Flash point f C) Evaporation at 70 C after 192 hours 384 hours (%) MOE MO

5 JSL 1-2 Ester base stocks 157 PROPERTES Let us review in more detail some of these aspects. Thermal stability Esters prepared with primary alcohols are more stable than those with secondary and tertiary ones (Figure 4). Hence, all esters for lubricants are manufactured with primary alcohols. Figure 4 Thermal stability CH3 TD decomposition temperature CH3--C--CH2ap 310 C < TD < 320 C (343 Primary alcohol CH3 C H 3 - C a CH3 Tertiary alcohol TD < 200 C Esters are more stable with primary > secondary > tertiary alcohols t is well documented that the thermal stability of esters is one of the same order as, or even lower than, that of hydrocarbons (Figures 5 and 6). Neopolyol esters are the most thermally stable molecules among esters. The enhanced thermal stability of NPE is directly related to the absence of hydrogen on P carbon of alcohol moiety. Thermal degradation of NPE results from the intervention of free radicals which necessitates greater energy - and thus higher temperature - compared to low energy path which takes place in the decomposition of diesters. Hydrolytic stability This is the reverse action of esterification and happens in presence of water and catalysts. t should be noted beforehand that the only

6 - 158 S zydyw ar JSL 1-2 Figure 5 Thermal stability TD DOS 284OC TMP heptanoate 3 2OCC Octacosane CZS 350 C Highly refined mineral oil - mostly paraffinic Quater phenyl 338OC 453 C TD Decomposition temperature lsotenoscope measurement serious problem associated with hydrolytic stability has been the storage instability problem of the earlier aircraft oils. t has been found that upon ageing, ester lubricants undergo deterioration and develop a tendency to corrode metals, in particular the lead-indium flashing on the aircraft bearing surfaces. After long studies in an attempt to identify the mechanism responsible for such deterioration, it has been found that a positive method for ensuring long storage life is to package dry synthetic oils (less than 300 ppm of water) in dry cans. Besides this specific problem of storage

7 JSL 1-2 Ester base stocks 159 Figure 6 Thermal stability KV change at 40 C X t : 300 C N, flow: 2L/H DE NPE Acidity change DE PA0 - NPE * Test hours stability, it is unusual to come across serious problems in practice which could be unfailingly attributed to lack of hydrolytic stability of the esters. Nevertheless this question has been studied and it can be said that the hydrolysis rate is apparently directed by two factors: polar substituents of the acid portion of the molecule which favour the rate as in fluoresters

8 160 Szydywar JSL 1-2 steric hindrance of the acid portion of the molecule which slows down the hydrolysis (Figure 7). This is one of the major claims of esters derived from neo-acids, acids in which the a! carbon is substituted with alkyl group (trade mark Versatic acid from Shell Chemicals, Neo-acids from Esso Chemicals). Figure 7 Hydrolytic stability Acid CH3 Alcohol Neo ester Steric hindrance due to alkyl substitution on (Y carbon lowers the rate of hydrolysis. Solvency This concept covers two conflicting performance properties: one beneficial, the aptitude to dissolve oxidation products; the other one, sometimes debatable, the swelling of elastomers. Ester solvency The beneficial effect of this is often considered as an advantage in automotive oils where esters minimise the formation of sludge and varnish (Figures 8 and 9). Attack on elastomers Compared to mineral oils, esters can tend to cause swelling in elastomers. However, it is important to be cautious in not hastily generalising to all esters. A dimerate, for instance, with a KV 100 C of about 12 csi has the same behaviour as 400 N in this respect. Generally speaking, however, esters would be more aggressive against NBR-L and CR than mineral oils having the same viscosity. Molecular size and chain length appear to be the major factors influencing rubber swell. Oxidation stability Like all hydrocarbons, esters go through a degradation process by oxygen with the intermediate formation of peroxides resulting in the

9 JSL 1-2 Ester base stocks 161 Figure 8 Solvency of esters (from prewar work in Germany) Ring sticking on BM W 132 engine (h) Minimum reg uired Rotring D from vacuum oil 8 8 Polyethylene (PE) 20 to 40 PE: TME ester 50: Showing the clear contribution of the ester. Figure 9 Solvency of esters (Caterpillar 1 G2 test results) PA0 NPE Oil A Mineral base 14% Oil B Mineral base 14% Same additive package Limits 1st deposit groove fill 45% 13% 80 Demerit rating The varnishes of polymeric nature are more soluble in NPE than in PAO. formation of short chain acids, ketones, aldehydes, water, COz, etc. Under practical conditions, this results in an acidity increase, and a viscosity increase, but absence of deposits or gums. The high performance capabilities of esters (DE and NPE) a direct result of their excellent response to anti-oxidants over that of petroleum oils. Unsaturation of the alkyl chain considerably impairs oxidation resistance of esters. Oxidation resistance can also be greatly improved by replacing the hydrogen atom of the backbone by more stable fluorinated groups as in fluoresters (which were thoroughly investigated in the 1950s). MAJOR ESTER BASESTOCKS FOR LUBRCANTS After the review of the general properties of esters, let us now examine the various families actually used as lubricant basestocks.

10 ~ Acetonevia 162 Szydywar JSL 1-2 Figure 10 Synthetic ester basestocks for lubricants Representative S u b-group Code Major Ester Monoester MOE Oleate Heptanoate Pelargonate Diesters of diacids DE Adipate' Dimerate Azelate Neopolyol esters Aromatic esters Complex NPE ARE CXE TMP/PE esters of short acids Phthalate Monoacid/alcohol polyacid + polyol TMPiPE oleate Trimellitate 3 to Many 1. Adipate - an ester used mainly as plasticiser. Classification There are many ways in which they may be presented. ASTM propose a classification of organic esters with two sub-groups: diesters of diacids (DE); neopolyol esters (NPE). Phthalate, an aromatic ester, belongs to the first group but can be hardly compared to, say, an adipate. Figure 11 Synthetic ester basestocks for lubricants (very 7 limited development) Sub-group Feedstock TriMe adipake Dodecanoate Nonadecanoate (and many others...) DE DE DE isophorone Trimer of butadiene 0x0 on oleic acid Deut;ped VEBA Dupont Huls BASF

11 JSL 1-2 Ester base stocks 163 The distinction is justified by the fact that when one refers to a diester it is understood among specialists that it is a product such as adipate or azelate. The physical properties and the thermal stability of a phthalate are quite different from those of an adipate for instance with the same viscosity. Figure 10 gives a general view of the various esters used in lubrication; many others have been studied (see Figure 11). Raw materials for esters The following raw materials are needed for making synthetic esters. They include monoacids, saturated and unsaturated (Figures 13 and Figure 12 Raw materials for esters (acids and alcohols) Monoalcohols Polyols Monoacids saturated MOE NPE Monoacids unsaturated MOE NPE Diacids DE CXE Aromatic acids ARE MOE DE NPE ARE CXE Monoester Diester Neopolyol ester Aromatic ester Complex ester 14); monoalcohols (Figure 15); diacids (Figure 16); polyols (Figure 17). Acids come predominantly from vegetable oils and fats which have the advantage of being renewable sources. Alcohols, on the contrary, are essentially petro-chemicals derivatives. Manufacturing process The fundamental reaction acid + alcohol +ester + water is the basis for the production of esters. The reaction is reversible and removal of water provides the driving force which carries the esterification to completion. The manufacturing process includes three stages (in the case of NPE), as shown in Figure 18. Esterifcation The reaction equipped with a stirrer, a condenser, is loaded with alcohols and an excess of short chain acids which generally

12 164 Szydywar JSL 1-2 Figure 13 Linear monoacids (derived from vegetable oil and fats) Acid Process Producers Caproic Copra Fractional distillation Various Heptanoic Cap ry li c Pelargonic 8 9 Castor oil Copra Oleic acid (tallow) Pyrolysis + oxidation Fractional distillation Ozonolysis Ato Chimie Various Unichema Capric 10 Copra Fractional distillation Various Oleic (unsaturated) 18 Tallow Extraction Various Figure 14 Linear monoacids (derived from petrochemicals) Acid Carbon number Feedstock Process Producers Butyric 4 Propylene Ox0 Various Valeric 5 Butylene Ox0 Union Carbidc Hoechst Heptanoic 7 Hexene (1) Ox0 PCUK Celanese Pelargonic 9 Octene (1 ) Ox0 Celanese cut cut cut C5-C6 c7-c9 C9-C10 Paraffin Paraffin Paraffin Atmospheric oxidation Communist countries: USSR China Poland Renewable. raw materials. occur in liquid state. The reactor is then heated up to OoC for hours. Catalyst and azeotropic solvent are collected together and the solvent, which separates on cooling, is returned to the reaction vessel until the esterification reaches completion.

13 JSL 1-2 Ester base stocks 165 Figure 15 Monoalcohols Carbon Alcohol number Feedstock Process Producers 2-Ethyl hexanol C8 Butyraldehyde Aldol Various s0 octyl C8 Heptene (C3-t C4) Ox0 Various s0 nonyl c9 Diiso butylene Ox0 Various s0 decyl c10 Triprop ylene Ox0 Various s0 tridecyl C13 Tetrapropylene Ox0 Various Octanol (linear) C8 Ethylene Alfol Condea Decanol (linear) c10 Ethylene Alfol Condea From petrochemistry only. Figure 16 Di- and aromatic acids Diacid Carbon number Feedstock Process Producers Adipic Azelaic Sebacic Dodecanoic Dimer Phthalic Trimellitic C6 c9 c10 c12 C3 6 C8 c9 Benzene Oleic acid Castor oil Butadiene trimer Unsaturated C18 Xylene Tri Me - benzene Oxidation Ozonolysis Pyrolysis Oxidation Dimerisation Oxidation Oxidation Various Unichema Union Cam] Du Pont Hiils Various Various Amoco Alcohol Alcohol group Feedstock Process Producers NPG TMP PE MPE DPE 2 sobutyraldehyde Aldol BASF 3 Butyraldehyde Aldol Various Acetaldehyde Aldol Various 4 Acetaldehyde Aldol 6 Acetaldehyde Aldoi

14 166 Szydywar JSL 1-2 Figure 18 Manufacture diagram Acid Alcohol 4 ESTERFCATON Elimination of water 1 Recycling Filtration Recycling T c --t Tapping point possible for esters. The final procedure depends on: - the type of ester - the desired quality 1 Physical andlor chemical DistiZZation The crude ester is then distilled under vacuum to top up excess reactants. Refining The distilled ester can be physically orland chemically refined in order to improve various properties.

15 JSL 1-2 Ester base stocks 167 MAN APPLCATONS OF SYNTHETC ESTERS The main outlet for synthetic esters is aviation oils, followed by automotive oils. Figure 19 summarises the main applications. NPE will remain the main basestock for aviation oils in the foreseeable future. Even the most advanced US Air Force specification is built around NPE. Automotive oils contain up to 15% of esters of adipate or NPE type, a little more if associated with PAO. n many instances esters are complementary to polyalphaolefins in crankcase oils. The percentage limitation comes from the swelling tendency and sometimes from wear problems. Esters bring: very low volatility, especially with NPE; very good thermal stability; good oxidation resistance; Figure 19 Synthetic ester basestocks for lubricants Subgroup Designation Major Application Minor Yearly estimated tonnage in Europe MOE DE NPE ARE CXE Oleates Others Adipates Dimerate Azelates TMP/PE Short acids TMPPE oleates Phthalate Trimellitate Divers MWA Automotive Automotive Aviation Aviation Hydraulics ACO Dielectric Automotive Automotive thickener Aviation ACO Automotive Automotive Textile MWA Automotive < 500 < 100 1,500-2,500 $000 < 500 5,000 5,000 < 200 < 200 < 100 MWA: Metal working additive ACO: Air compressed oil

16 168 Szydywar JSL 1-2 no deposit formation; 0 and solvent ability. Some industrial oils take advantage of these properties in air compressor oils. Oleates have found applications in hydraulic fluids where some fire hazards exist; these cannot be compared to chlorine- or phosphorus- Figure 20 Ester as an excellent chemical structure for compromise Polyalkyleneglycol (PAG) --o-clu-- OH ether group Polyalphaolefin (PAO) / CH \ PAG and PA0 are not miscible as such but, by addition of NPE, the mixture is homogeneous +* Affinity

17 JSL 1-2 Ester base stocks 169 containing compounds from the fire resistance point of view. They fill a gap between mineral oils and phosphates, with the advantage of not requiring special seal systems and not releasing toxic fumes. Esters have an excellent chemical structure for compromise. Their properties can be balanced through control of the starting raw materials (see Figure 20). Polyglycols and PAOs are not miscible as such, but by addition of esters the mixture becomes homogeneous. CONCLUSON The scope of applications of these versatile molecules could probably be widened, were not the price, as for other synthetics, a major deterrent.