Aus der Praxis für die Praxis Introduction Synthetic esters for lubrication were first studied and synthesized in the 1940’s. The Second World War actually prompted such studies, in Germany as well as in the USA, as jet aircrafts were replacing piston engine aircrafts. Lubricating oils were then submitted to much higher temperatures, and the first dibasic ester (adipate) based engine oil was developed, in an attempt to improve at the same time low temperature behavior and high temperature deposit formation, and find the best high/ low temperature compromise. Synthetic esters have been used for over 60 years now in various lubricating applications. Their use is well known, for instance, in aviation turbine oils, air compressor oils, high temperature chain oils, 2 stroke and racing engine oils, or even ultra-low temperature hydraulic oils. However, such uses of esters have been limited to specific areas where cleanliness, resistance to high temperatures, good lubricity and added solvency are required. Rapid, recent legislation changes on exhaust gas emissions, and subsequent new technologies and equipment, the quest for higher power density, improved energy efficiency, extended durability, as well as the growing concern over environmental impacts of lubrication are clearly paving the way towards higher performance and better sustainability for lubricants. It appears that synthetic esters have a lot more to offer than what they have been traditionally used for, and the current market drivers will most probably unveil unexpected, sometimes unknown, added benefits of these base fluids, and promote their use in modern, high performance lubricating applications. 1 Traditional uses of synthetic esters The use of synthetic esters is well known in a number of specific applications where their high thermal stability, superior cleanliness, natural lubricity, and polarity have long been exploited. 1.1 Polar agents One of the most traditional uses of esters is probably as a polar additive in non-polar hydrocarbons, such as Poly Alpha Olefins. In such base fluids, diesters in particular have been used as: 52 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 * Siegfried Lucazeau, Dipl.-Ing. Chemical Engineering NYCO, Paris, France New insight into the benefits of synthetic esters in challenging lubricating applications S. Lucazeau* Synthetic esters have been used for over 60 years in various lubricating applications, but they have been limited to specific areas where cleanliness, resistance to temperature and added solvency are required. In this paper, general properties of esters, along with their traditional benefits in lubricating applications, are reviewed. In particular, some structure/ property relationships are discussed and comparisons with traditional base fluids are provided. Specific applications are detailed, explaining why esters have been of interest: air compressor oils, aviation turbine oils, chain oils, 2 stroke and racing engine oils, ultra-low temperature hydraulics. However, recent legislations, new equipment manufacturers’ specifications, the trends towards higher power density, energy efficiency, improved durability and growing concern over the environmental impact of lubrication, are clearly leading the way towards higher performance and better sustainability. Such market drivers are generating new opportunities for synthetic esters and will unveil unexpected, sometimes unknown, added benefits. This paper will show that ester chemistry may be optimized to upgrade performance. Examples of novel uses of neopolyol and complex esters in highly demanding applications like European Ecolabel, and Vessel General Permit compliant hydraulic oils and greases, ultra-high temperature chain oils and greases, and future generation engine oils, will be discussed. Keywords Synthetic esters, neopolyol esters, performance, high temperature, sustainability, environmental impact Abstract T+S_6_16 17.10.16 17: 01 Seite 52 Aus der Praxis für die Praxis • seal swell agents to mitigate seal shrinkage normally observed in such formulations • dispersing agents in fluids that generate sludge • additive solubility auxiliaries in viscous non-polar media 1.2 Aviation gas turbines In aircraft or ground gas turbines, neopolyol esters have been used almost exclusively for decades, for their ability to sustain temperatures of up to 220 °C in bulk, with excellent deposit control on hot metal parts (up to 375 °C), either in liquid or in vapour phase. In addition, excellent low temperature behaviour is also required (pour point is typically lower than -54 °C), which neopolyol esters also deliver. 1.3 Air compressors In a number of volumetric air compressors, compression chambers are lubricated (piston, vane and screw compressors), thus submitting the oil to elevated temperatures of up to 230 °C typically. Esters, particularly diesters (phthalates), have been used for their low volatility, delivering cleaner air, and their low coking propensity, thus protecting the equipment against possible auto-ignition of carbonaceous deposits or valve-sticking issues leading to possible explosions. 1.4 High temperature chains The low volatility features of esters, along with their high resistance to thermo-oxidation and low deposit formation properties, have long been taken advantage of in high temperature chain oils to minimize formation of gummy and carbonaceous residues and preserve lubricity over time. In addition, high flash points delivered by synthetic esters (up to 325 °C) bring additional safety features to such products. Diesters and triesters (trimellitates) in particular have been used in such formulations. 1.5 Two stroke engine oils In such an application, the cleanliness of the oil in operation, its ability to decrease smoke formation, and its lubricity are key features. Diesters, thanks to their clean burning properties, natural detergency, and good lubricity, are excellent base fluid for 2 stroke engine oils. Since a portion of the fuel/ oil/ air mix is released unburnt to the environment (total loss lubrication), the good environmental profile of synthetic esters is also of great value. Compared to mineral oils, synthetic esters will typically reduce wear, deposit and varnish formation, and exhaust smoke. Moreover, a reduction of up to 25 % Polycyclic Aromatic Hydrocarbons content in exhaust gases has been reported [1]. 1.6 Refrigeration compressors This is a major outlet for neopolyol esters. The switch to non-ozone depleting refrigerant fluids (HFC) has generated the need for highly polar lubricants, showing good miscibility and chemical compatibility features with HFC. Neopolyol esters also show good cold flow properties, high thermal stability, and excellent lubricity. These examples underline the main, usual features and applications of synthetic esters. However, they may be used in a much broader scope of lubricating applications, as their technical potential seems greater than what is commonly believed. 2 Structure/ performance relationships 2.1 Polarity The ester chemical function displays a permanent dipole, due to oxygen electronegativity (Figure 1). This intrinsic polarity has a number of consequences and imparts specific properties of interest from a lubrication standpoint: • Permanent dipoles attract each other through electrostatic forces. Such intermolecular forces, called Keesom forces (a specific component of Van der Waals forces), are roughly 100 times weaker than covalent bonds and 5 times weaker than hydrogen bonds [2]. They impart greater internal cohesion than in pure hydrocarbons, resulting in lower volatility and evaporation rates (4 mm 2 / s @ 100 °C neopolyol ester is able to show a NOACK volatility - 1h @ 250 °C of roughly 7 %) , and consequently higher flash points. Synthetic esters may show flash points of up to 310 °C at ISO VG 46 (Figure 2). Additionally, long, linear carbon chains in ester structures further reduce volatility. • Negatively charged oxygen from ester chemical function will bind to positively charged sites of metal surfaces. Whilst non-polar hydrocarbons will tend to be squeezed away from metal surfaces in a mixed or boundary lubrication regime, where surface asperities start coming into contact, esters will stick to the surface and provide protection against friction and wear, to some extent, thanks to their natural affinity with metal surfaces. As a result esters may be viewed as friction modifiers capable of reducing friction coefficients and wear Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 53 · seal swell agents to mitigate seal shrinkage normally observed in such formulations · dispersing agents in fluids that generate sludge · additive solubility auxiliaries in viscous nonpolar media 1.2 Aviation gas turbines In aircraft or ground gas turbines, neopolyol esters have been used almost exclusively for decades, for their ability to sustain temperatures of up to 220°C in bulk, with excellent deposit control on hot metal parts (up to 375°C), either in liquid or in vapour phase. In addition, excellent low temperature behaviour is also required (pour point is typically lower than -54°C), which neopolyol esters also deliver. 1.3 Air compressors In a number of volumetric air compressors, compression chambers are lubricated (piston, vane and screw compressors), thus submitting the oil to elevated temperatures of up to 230°C typically. Esters, particularly diesters (phthalates), have been used for their low volatility, delivering cleaner air, and their low coking propensity, thus protecting the equipment against possible auto-ignition of carbonaceous deposits or valve-sticking issues leading to possible explosions. 1.4 High temperature chains The low volatility features of esters, along with their high resistance to thermo-oxidation and low deposit formation properties, have long been taken advantage of in high temperature chain oils to minimize formation of gummy and carbonaceous residues and preserve lubricity over time. In addition, high flash points delivered by synthetic esters (up to 325°C) bring additional safety features to such products. Diesters and triesters (trimellitates) in particular have been used in such formulations. 1.5 Two stroke engine oils In such an application, the cleanliness of the oil in operation, its ability to decrease smoke formation, and its lubricity are key features. Diesters, thanks to their clean burning properties, natural detergency, and good lubricity, are excellent base fluid for 2 stroke engine oils. Since a portion of the fuel/ oil/ air mix is released unburnt to the environment (total loss lubrication), the good environmental profile of synthetic esters is also of great value. Compared to mineral oils, synthetic esters will typically reduce wear, deposit and varnish formation, and exhaust smoke. Moreover, a reduction of up to 25% Polycyclic Aromatic Hydrocarbons content in exhaust gases has been reported [1]. 1.6 Refrigeration compressors This is a major outlet for neopolyol esters. The switch to non-ozone depleting refrigerant fluids (HFC) has generated the need for highly polar lubricants, showing good miscibility and chemical compatibility features with HFC. Neopolyol esters also show good cold flow properties, high thermal stability, and excellent lubricity. These examples underline the main, usual features and applications of synthetic esters. However, they may be used in a much broader scope of lubricating applications, as their technical potential seems greater than what is commonly believed. 2 Structure/ performance relationships 2.1 Polarity Figure 1 : Permanaent dipole in ester chemical function The ester chemical function displays a permanent dipole, due to oxygen electronegativity (Figure 1). This intrinsic polarity has a number of consequences and imparts specific properties of interest from a lubrication standpoint: · Permanent dipoles attract each other through electrostatic forces. Such intermolecular forces, called Keesom forces (a specific component of Van der Waals forces), are roughly 100 times weaker than covalent bonds and 5 times weaker than hydrogen bonds [2]. They impart greater internal cohesion than in pure hydrocarbons, resulting in lower volatility and evaporation rates (4 mm 2 / s @ 100°C neopolyol ester is able to show a NOACK volatility - 1h @ 250°C of roughly 7%) , and consequently higher flash points. Synthetic esters may show flash points of up Figure 1: Permanent dipole in ester chemical function T+S_6_16 17.10.16 17: 01 Seite 53 Aus der Praxis für die Praxis in moderately loaded conditions. Esters may also be qualified of good “lubricity” agents (Figure 3). Long, linear carbon chains improve anti-wear capability and friction modification. • The co-existence of polar sites with non-polar hydrocarbon chains gives esters amphiphilic properties: they show dispersancy and detergency features. As a result, esters do contribute to minimizing the formation of deposits and varnishes on surfaces and help keep oxidation products in suspension. In addition, esters will also help dissolve poorly soluble additives in non-polar base fluids, through similar mechanisms. As an illustration, non-polar Poly Alpha Olefins usually show weak solvency with regards to particulate matter, oxidation products, and a number of polar additives, leading to possible sludge, deposit and varnish formation even though such base fluids normally exhibit high resistance to oxidation. The use of ester will compensate for the poor solvency of the medium, will help keep surfaces clean and dissolve additives. • Esters (diesters in particular), are very good plasticizers. Their polarity makes them interact with a number of polymers. The measurement of the Anilin Point (ASTM D611), initially used as an indicator of the aromaticity of oils, may also supply an estimate of the potential effect of esters on polymers. Esters typically show Anilin Points revolving around 10 °C, which is much lower than any hydrocarbon, including naphthenics or aromatics. As a consequence, esters may be used as seal swelling agents, especially in non-polar media, where elastomeric seals may shrink (Figure 4). Using long, linear chains in esters mitigate the impact on elastomers and improve seal compatibility if needed. 2.2 Thermo-oxidative stability of esters 2.2.1 Thermal stability At elevated temperatures, esters undergo thermal degradation phenomena, in which oxygen do not play any role. From that standpoint, the ester chemical function may be viewed as a weak point, as it may undergo β-elimination, leading to alkenes and acids (Figure 5). Such a reaction is probably becoming significant at temperatures of 275 °C to 315 °C. However, metals like iron or copper will have a strong catalytic effect and will dramatically lower the temperatures at which this reaction takes place to about 200 °C [3]. 54 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 to 310°C at ISO VG 46 (Figure 2). Additionally, long, linear carbon chains in ester structures further reduce volatility. · Negatively charged oxygen from ester chemical function will bind to positively charged sites of metal surfaces. Whilst nonpolar hydrocarbons will tend to be squeezed away from metal surfaces in a mixed or boundary lubrication regime, where surface asperities start coming into contact, esters will stick to the surface and provide protection against friction and wear, to some extent, thanks to their natural affinity with metal surfaces. As a result esters may be viewed as friction modifiers capable of reducing friction coefficients and wear in moderately loaded conditions. Esters may also be qualified of good “lubricity” agents (Figure 3). Long, linear carbon chains improve anti-wear capability and friction modification. · The co-existence of polar sites with non-polar hydrocarbon chains gives esters amphiphilic properties: they show dispersancy and detergency features. As a result, esters do contribute to minimizing the formation of deposits and varnishes on surfaces and help keep oxidation products in suspension. In addition, esters will also help dissolve poorly soluble additives in non-polar base fluids, through similar mechanisms. As an illustration, non-polar Poly Alpha Olefins usually show weak solvency with regards to particulate matter, oxidation products, and a number of polar additives, leading to possible sludge, deposit and varnish formation even though such base fluids normally exhibit high resistance to oxidation. The use of ester will compensate for the poor solvency of the medium, will help keep surfaces clean and dissolve additives. · Esters (diesters in particular), are very good plasticizers. Their polarity makes them interact with a number of polymers. The measurement of the Anilin Point (ASTM D611), initially used as an indicator of the aromaticity of oils, may also supply an estimate of the potential effect of esters on polymers. Esters typically show Anilin Points revolving around 10°C, which is much lower than any hydrocarbon, including naphthenics or aromatics. As a consequence, esters may be used as seal swelling agents, especially in non-polar media, where elastomeric seals may shrink (Figure 4). Using long, linear chains in esters mitigate the impact on elastomers and improve seal compatibility if needed. Property Unit Naphthenic Gr I Gr III PAO Alkyl Naphthalene Diester Neopolyol ester Complex ester Viscosity at 100°C mm²/ s 4.6 5.0 5.1 5.1 4.8 5.3 4.9 5.7 Viscosity at 40°C mm²/ s 31.3 29.8 26 24.1 28.7 26.6 22.6 28 Flash point COC °C 194 195 226 245 232 234 267 265 Evaporation by TGA 250°C - 1 h* % 43.4 38.3 19.3 5.6 12.0 4.7 2.3 2.6 Figure 2 : Volatility of synthetic esters vs other basestocks Property Diester mm 0.91 0.85 0.64 -- Figure 3 : Friction modification of synthetic esters vs other basestocks Figure 2: Volatility of synthetic esters vs other basestocks to 310°C at ISO VG 46 (Figure 2). Additionally, long, linear carbon chains in ester structures further reduce volatility. · Negatively charged oxygen from ester chemical function will bind to positively charged sites of metal surfaces. Whilst nonpolar hydrocarbons will tend to be squeezed away from metal surfaces in a mixed or boundary lubrication regime, where surface asperities start coming into contact, esters will stick to the surface and provide protection against friction and wear, to some extent, thanks to their natural affinity with metal surfaces. As a result esters may be viewed as friction modifiers capable of reducing friction coefficients and wear in moderately loaded conditions. Esters may also be qualified of good “lubricity” agents (Figure 3). Long, linear carbon chains improve anti-wear capability and friction modification. · The co-existence of polar sites with non-polar hydrocarbon chains gives esters amphiphilic properties: they show dispersancy and detergency features. As a result, esters do contribute to minimizing the formation of deposits and varnishes on surfaces and help keep oxidation products in suspension. In addition, esters will also help dissolve poorly soluble additives in non-polar base fluids, through similar mechanisms. As an illustration, non-polar Poly Alpha Olefins usually show weak solvency with regards to particulate matter, oxidation products, and a number of polar additives, leading to possible sludge, deposit and varnish formation even though such base fluids normally exhibit high resistance to oxidation. The use of ester will compensate for the poor solvency of the medium, will help keep surfaces clean and dissolve additives. · Esters (diesters in particular), are very good plasticizers. Their polarity makes them interact with a number of polymers. The measurement of the Anilin Point (ASTM D611), initially used as an indicator of the aromaticity of oils, may also supply an estimate of the potential effect of esters on polymers. Esters typically show Anilin Points revolving around 10°C, which is much lower than any hydrocarbon, including naphthenics or aromatics. As a consequence, esters may be used as seal swelling agents, especially in non-polar media, where elastomeric seals may shrink (Figure 4). Using long, linear chains in esters mitigate the impact on elastomers and improve seal compatibility if needed. Property Alkyl ester % 43.4 12.0 2.3 Figure 2 : Volatility of synthetic esters vs other basestocks Property Unit Gr I Gr III Alkyl Naphthalene PAO Diester Neopolyol ester Viscosity at 100°C mm²/ s 4.18 4.25 4.82 3.95 3.56 4.40 Viscosity at 40°C mm²/ s 21.3 19.7 28.7 17.3 13.7 19.7 4 ball Wear Scar 40 kg, 1 h mm 1.10 0.76 0.91 0.73 0.85 0.64 60 kg, 30 min mm - - - 1.78 - 0.71 HFRR 5N - 60°C - 75 mn Friction coefficient -- 0.492 0.311 0.296 0.225 0.230 0.220 Figure 3 : Friction modification of synthetic esters vs other basestocks Figure 3: Friction modification of synthetic esters vs other basestocks Figure 4: Anilin points of various base fluids Figure 4 : Anilin points of various base fluids 2.2 Thermo-oxidative stability of esters 2.2.1 Thermal stability At elevated temperatures, esters undergo thermal degradation phenomena, in which oxygen do not play any role. From that standpoint, the ester chemical function may be viewed as a weak point, as it may undergo βelimination, leading to alkenes and acids (Figure 5). Such a reaction is probably becoming significant at temperatures of 275°C to 315°C. However, metals like iron or copper will have a strong catalytic effect and will dramatically lower the temperatures at which this reaction takes place to about 200°C [3]. Using neopentyl structures (Figure 6), in which no hydrogen is present in β position of oxygen, suppresses any possibility of β-elimination and greatly improves, de facto, the thermal stability of esters. Such structures are called neopolyol esters. Figure 5 : β-elimination reaction Figure 6 : General structure of neopolyol esters 2.2.2 Resistance to oxidation The first step of the commonly accepted oxidation mechanisms is the abduction by oxygen of a hydrogen atom, thus producing a free radical. -CH 2 groups from alcohol chains are expected to be highly reactive, however they are protected by steric hindrance from acid chains on neopolyol esters. As a result, -CH 2 groups from acid chains are the main sites of oxygen attack [4]. On the acid chain, hydrogen atoms exhibit different reactivities towards oxygen, depending on their positions. Hydrogen atoms bonded to tertiary and secondary carbons are the most likely to be oxidized, i.e. -CHand CH 2 groups. This is directly related to the thermodynamic stability of the resulting free radical. Therefore, if the number of -CH 3 hydrogens increases with respect to the number of -CH 2 and - CHhydrogens, the kinetics of the oxidation reactions will slow and oxidation stability will increase [3]. The direct consequence of this is: · the shorter the acid chains, the more stable the structure against oxidation (less -CH 2 - ) · the more branched the acid chains, the more stable the structure against oxidation (more - CH 3 -, added protection from steric hindrance) (Figure 7) Oxidation reactions, like thermal degradation reactions, are strongly catalyzed by transition metals (iron in particular). Test tube C8- Iso-C8, iso-C9 Figure 7: Oxidation and corrosion test 2.2.3 Coking propensity Whatever their structure, esters (like any other compound) will eventually start to degrade at elevated temperatures. For ultra-high temperature applications (oven chains oils for instance), the question of what T+S_6_16 17.10.16 17: 01 Seite 54 Aus der Praxis für die Praxis Using neopentyl structures (Figure 6), in which no hydrogen is present in β position of oxygen, suppresses any possibility of β-elimination and greatly improves, de facto, the thermal stability of esters. Such structures are called neopolyol esters. 2.2.2 Resistance to oxidation The first step of the commonly accepted oxidation mechanisms is the abduction by oxygen of a hydrogen atom, thus producing a free radical. -CH 2 groups from alcohol chains are expected to be highly reactive, however they are protected by steric hindrance from acid chains on neopolyol esters. As a result, -CH 2 groups from acid chains are the main sites of oxygen attack [4]. On the acid chain, hydrogen atoms exhibit different reactivities towards oxygen, depending on their positions. Hydrogen atoms bonded to tertiary and secondary carbons are the most likely to be oxidized, i. e. -CHand CH 2 groups. This is directly related to the thermodynamic stability of the resulting free radical. Therefore, if the number of -CH 3 hydrogens increases with respect to the number of -CH 2 and -CHhydrogens, the kinetics of the oxidation reactions will slow and oxidation stability will increase [3]. The direct consequence of this is: • the shorter the acid chains, the more stable the structure against oxidation (less -CH 2 - ) • the more branched the acid chains, the more stable the structure against oxidation (more -CH 3 -, added protection from steric hindrance) (Figure 7) Oxidation reactions, like thermal degradation reactions, are strongly catalyzed by transition metals (iron in particular). 2.2.3 Coking propensity Whatever their structure, esters (like any other compound) will eventually start to degrade at elevated temperatures. For ultra-high temperature applications (oven chains oils for instance), the question of what happens when the oil practically “burns” must be raised: • it may polymerize, get viscous, and generate sludge and insoluble particulate matter • it may generate coke (hard, carbonaceous deposits on surfaces) • it may also decompose and break down into light, volatile fractions, in a kind of pyrolytic mechanism Esters were reported to show gas evolution during oxidation process (CO 2 , H 2 , and CO) [5], and some structures will favour such decomposition pathways that preserve cleanliness in operation over polymerization and coking mechanisms. It is believed that highly branched structures in particular will favour such chemical decomposition reactions (Figure 8). Cleanliness is not only a matter of resistance to oxidation: it is also linked to the ability of an oil to decompose cleanly. Highly branched neopolyol esters will strongly resist elevated temperatures and will eventually decompose cleanly, leaving little or no residue. A fully branched neopolyol ester, for instance, will start showing initial signs of degradation at roughly 210 °C. Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 55 Figure 4 : Anilin points of various base fluids 2.2 Thermo-oxidative stability of esters 2.2.1 Thermal stability At elevated temperatures, esters undergo thermal degradation phenomena, in which oxygen do not play any role. From that standpoint, the ester chemical function may be viewed as a weak point, as it may undergo βelimination, leading to alkenes and acids (Figure 5). Such a reaction is probably becoming significant at temperatures of 275°C to 315°C. However, metals like iron or copper will have a strong catalytic effect and will dramatically lower the temperatures at which this reaction takes place to about 200°C [3]. Using neopentyl structures (Figure 6), in which no hydrogen is present in β position of oxygen, suppresses any possibility of β-elimination and greatly improves, de facto, the thermal stability of esters. Such structures are called neopolyol esters. Figure 5 : β-elimination reaction Figure 6 : General structure of neopolyol esters 2.2.2 Resistance to oxidation The first step of the commonly accepted oxidation mechanisms is the abduction by oxygen of a hydrogen atom, thus producing a free radical. -CH 2 groups from alcohol chains are expected to be highly reactive, however they are protected by steric hindrance from acid chains on neopolyol esters. As a result, -CH 2 groups from acid chains are the main sites of oxygen attack [4]. On the acid chain, hydrogen atoms exhibit different reactivities towards oxygen, depending on their positions. Hydrogen atoms bonded to tertiary and secondary carbons are the most likely to be oxidized, i.e. -CHand CH 2 groups. This is directly related to the thermodynamic stability of the resulting free radical. Therefore, if the number of -CH 3 hydrogens increases with respect to the number of -CH 2 and - CHhydrogens, the kinetics of the oxidation reactions will slow and oxidation stability will increase [3]. The direct consequence of this is: · the shorter the acid chains, the more stable the structure against oxidation (less -CH 2 - ) · the more branched the acid chains, the more stable the structure against oxidation (more - CH 3 -, added protection from steric hindrance) (Figure 7) Oxidation reactions, like thermal degradation reactions, are strongly catalyzed by transition metals (iron in particular). Test tube C8- Iso-C8, iso-C9 Figure 7: Oxidation and corrosion test 2.2.3 Coking propensity Whatever their structure, esters (like any other compound) will eventually start to degrade at elevated temperatures. For ultra-high temperature applications (oven chains oils for instance), the question of what Figure 5: β-elimination reaction Figure 4 : Anilin points of various base fluids 2.2 Thermo-oxidative stability of esters 2.2.1 Thermal stability At elevated temperatures, esters undergo thermal degradation phenomena, in which oxygen do not play any role. From that standpoint, the ester chemical function may be viewed as a weak point, as it may undergo βelimination, leading to alkenes and acids (Figure 5). Such a reaction is probably becoming significant at temperatures of 275°C to 315°C. However, metals like iron or copper will have a strong catalytic effect and will dramatically lower the temperatures at which this reaction takes place to about 200°C [3]. Using neopentyl structures (Figure 6), in which no hydrogen is present in β position of oxygen, suppresses any possibility of β-elimination and greatly improves, de facto, the thermal stability of esters. Such structures are called neopolyol esters. Figure 5 : β-elimination reaction Figure 6 : General structure of neopolyol esters 2.2.2 Resistance to oxidation The first step of the commonly accepted oxidation mechanisms is the abduction by oxygen of a hydrogen atom, thus producing a free radical. -CH 2 groups from alcohol chains are expected to be highly reactive, however they are protected by steric hindrance from acid chains on neopolyol esters. As a result, -CH 2 groups from acid chains are the main sites of oxygen attack [4]. On the acid chain, hydrogen atoms exhibit different reactivities towards oxygen, depending on their positions. Hydrogen atoms bonded to tertiary and secondary carbons are the most likely to be oxidized, i.e. -CHand CH 2 groups. This is directly related to the thermodynamic stability of the resulting free radical. Therefore, if the number of -CH 3 hydrogens increases with respect to the number of -CH 2 and - CHhydrogens, the kinetics of the oxidation reactions will slow and oxidation stability will increase [3]. The direct consequence of this is: · the shorter the acid chains, the more stable the structure against oxidation (less -CH 2 - ) · the more branched the acid chains, the more stable the structure against oxidation (more - CH 3 -, added protection from steric hindrance) (Figure 7) Oxidation reactions, like thermal degradation reactions, are strongly catalyzed by transition metals (iron in particular). Test tube C8- Iso-C8, iso-C9 Figure 7: Oxidation and corrosion test 2.2.3 Coking propensity Whatever their structure, esters (like any other compound) will eventually start to degrade at elevated temperatures. For ultra-high temperature applications (oven chains oils for instance), the question of what Figure 6: General structure of neopolyol esters Figure 4 : Anilin points of various base fluids 2.2 Thermo-oxidative stability of esters 2.2.1 Thermal stability At elevated temperatures, esters undergo thermal degradation phenomena, in which oxygen do not play any role. From that standpoint, the ester chemical function may be viewed as a weak point, as it may undergo βelimination, leading to alkenes and acids (Figure 5). Such a reaction is probably becoming significant at temperatures of 275°C to 315°C. However, metals like iron or copper will have a strong catalytic effect and will dramatically lower the temperatures at which this reaction takes place to about 200°C [3]. Using neopentyl structures (Figure 6), in which no hydrogen is present in β position of oxygen, suppresses any possibility of β-elimination and greatly improves, de facto, the thermal stability of esters. Such structures are called neopolyol esters. Figure 5 : β-elimination reaction Figure 6 : General structure of neopolyol esters 2.2.2 Resistance to oxidation The first step of the commonly accepted oxidation mechanisms is the abduction by oxygen of a hydrogen atom, thus producing a free radical. -CH 2 groups from alcohol chains are expected to be highly reactive, however they are protected by steric hindrance from acid chains on neopolyol esters. As a result, -CH 2 groups from acid chains are the main sites of oxygen attack [4]. On the acid chain, hydrogen atoms exhibit different reactivities towards oxygen, depending on their positions. Hydrogen atoms bonded to tertiary and secondary carbons are the most likely to be oxidized, i.e. -CHand CH 2 groups. This is directly related to the thermodynamic stability of the resulting free radical. Therefore, if the number of -CH 3 hydrogens increases with respect to the number of -CH 2 and - CHhydrogens, the kinetics of the oxidation reactions will slow and oxidation stability will increase [3]. The direct consequence of this is: · the shorter the acid chains, the more stable the structure against oxidation (less -CH 2 - ) · the more branched the acid chains, the more stable the structure against oxidation (more - CH 3 -, added protection from steric hindrance) (Figure 7) Oxidation reactions, like thermal degradation reactions, are strongly catalyzed by transition metals (iron in particular). ASTM D4636 204°C Test tube Deposits C8-C10 ester Iso-C8, iso-C9 ester Figure 7: Oxidation and corrosion test 2.2.3 Coking propensity Whatever their structure, esters (like any other compound) will eventually start to degrade at elevated temperatures. For ultra-high temperature applications (oven chains oils for instance), the question of what Figure 7: Oxidation and corrosion test happens when the oil practically “burns” must be raised: · it may polymerize, get viscous, and generate sludge and insoluble particulate matter · it may generate coke (hard, carbonaceous deposits on surfaces) · it may also decompose and break down into light, volatile fractions, in a kind of pyrolytic mechanism Esters were reported to show gas evolution during oxidation process (CO 2 , H 2 , and CO) [5], and some structures will favour such decomposition pathways that preserve cleanliness in operation over polymerization and coking mechanisms. It is believed that highly branched structures in particular will favour such chemical decomposition reactions (Figure 8). C8-10 ester Iso-C8, iso-C9 ester Figure 8 : Micro-Coking Test, 230-280°C GFC-Lu-27-A-13 Cleanliness is not only a matter of resistance to oxidation: it is also linked to the ability of an oil to decompose cleanly. Highly branched neopolyol esters will strongly resist elevated temperatures and will eventually decompose cleanly, leaving little or no residue. A fully branched neopolyol ester, for instance, will start showing initial signs of degradation at roughly 210°C. 2.2.4 Additives Anti-oxidant response is excellent in esters in general. Preferred antioxidants are generally alkylated diphenylamines. Also, taking in consideration that metals (iron and copper in particular) do catalyze oxidation reactions, any additive capable of deactivating such a catalytic effect will have a positive impact on oxidation resistance. Phosphorus additives are useful for inhibiting catalytic effect of iron, whilst metal deactivators like heterocyclic compounds are used to passivate yellow metals. Additives play a major role in the high temperature performance of synthetic esters. 2.3 Biodegradability The ester chemical function can be degraded by bacteria. It is believed that the initial step of this process is hydrolysis of ester. As a consequence, esters that are highly hydrolytically stable tend to show low biodegradability features, even though this is not verified for all esters. The majority of esters do show high levels of biodegradability as measured according to OECD 301B (typically 70-80%, up to 100%). They generally show higher biodegradability levels than any other base stock. In addition, whilst oil soluble PAGs or PAOs for instance only demonstrate significant biodegradability for the lower viscosity grades, synthetic ester technology is not limited by viscosity : 79% biodegradability (OECD 301B) can be achieved with an ISO VG 1000 synthetic ester. 2.4 Esters are designed Synthetic esters are produced from defined, mostly pure raw materials. These alcohols and acids may be chosen from a variety of available compounds. This ultimately means that: · Synthetic esters are mostly pure materials. The chemical structures are usually well defined and they are composed of a very limited number of compounds, if not one single molecule. There is not any undesirable material in the base fluid, and properties and the behaviour of synthetic esters remain very consistent in operation, thus ensuring optimum performance. This represents a great difference with mineral base stocks which may be viewed as a continuum of chemical species, including compounds that are not well identified and undesirable materials. · Synthetic esters are clean products, which means they do not contain impurities or other compounds not introduced deliberately that may be detrimental to quality - provided the manufacturing process is optimized that way. Mineral or organic impurities may have a significant impact on the performance of the Figure 8: Micro-Coking Test, 230-280 °C GFC-Lu-27-A-13 T+S_6_16 17.10.16 17: 01 Seite 55 Aus der Praxis für die Praxis 2.2.4 Additives Anti-oxidant response is excellent in esters in general. Preferred antioxidants are generally alkylated diphenylamines. Also, taking in consideration that metals (iron and copper in particular) do catalyze oxidation reactions, any additive capable of deactivating such a catalytic effect will have a positive impact on oxidation resistance. Phosphorus additives are useful for inhibiting catalytic effect of iron, whilst metal deactivators like heterocyclic compounds are used to passivate yellow metals. Additives play a major role in the high temperature performance of synthetic esters. 2.3 Biodegradability The ester chemical function can be degraded by bacteria. It is believed that the initial step of this process is hydrolysis of ester. As a consequence, esters that are highly hydrolytically stable tend to show low biodegradability features, even though this is not verified for all esters. The majority of esters do show high levels of biodegradability as measured according to OECD 301B (typically 70 - 80 %, up to 100 %). They generally show higher biodegradability levels than any other base stock. In addition, whilst oil soluble PAGs or PAOs for instance only demonstrate significant biodegradability for the lower viscosity grades, synthetic ester technology is not limited by viscosity : 79 % biodegradability (OECD 301B) can be achieved with an ISO VG 1000 synthetic ester. 2.4 Esters are designed Synthetic esters are produced from defined, mostly pure raw materials. These alcohols and acids may be chosen from a variety of available compounds. This ultimately means that: • Synthetic esters are mostly pure materials. The chemical structures are usually well defined and they are composed of a very limited number of compounds, if not one single molecule. There is not any undesirable material in the base fluid, and properties and the behaviour of synthetic esters remain very consistent in operation, thus ensuring optimum performance. This represents a great difference with mineral base stocks which may be viewed as a continuum of chemical species, including compounds that are not well identified and undesirable materials. • Synthetic esters are clean products, which means they do not contain impurities or other compounds not introduced deliberately that may be detrimental to quality - provided the manufacturing process is optimized that way. Mineral or organic impurities may have a significant impact on the performance of the base fluid: even though refining processes do tend to remove them in mineral base stocks, they will still show a sizeable amount of a variety of impurities. • The chemical structure of synthetic esters can be chosen. Trade-offs do exist for synthetic esters, and one may choose to maximize some properties over others, and minimize undesirable features. This can be achieved through careful design of the structure to match, as closely as possible, lubricating needs. This gives some precious flexibility to formulators. • Whilst most neopolyols derive from petroleum industry (even though biosourced alcohols are coming up), fatty acids may be chosen from vegetable sources, thus allowing the production of esters showing high contents of renewable carbon. This content may be as high as 100 % but typically revolves around 70 to 80 %. Synthetic esters constitute a big family of compounds, which may be very different in performance from one another. However, most of them will offer a unique combination of high performance features, including low volatility, high viscosity indices, low pour points, excellent thermo-oxidative stability and low propensity to deposit formation, excellent lubricity, whilst showing a very good environmental profile. 3 Esters in modern, challenging lubricating applications 3.1 Tougher requirements in modern lubrication Commercial competition remains an important driver for better performance and durability, and reduced operation costs in lubricants. However, other factors such as increasing public awareness over environmental issues and depletion of fossil resources are leading to some demand for more energy efficiency, lower environmental impact, and the inclusion of the notion of sustainability in the use of lubricants. Improved safety is also an increasing expectation. Eventually, this translates into new legislation prompting OEMs to issue specifications that reflect this in simple terms: added power density, energy efficiency, and durability, whilst environmental impact should be minimized. From the lubricant manufacturer’s point of view this translates into: • stronger oxidative stress on lubricants • reducing friction, possibly by reducing viscosity • keeping volatility at a low level • harmless, biodegradable and renewable lubricants The following is a number of examples of how synthetic esters may contribute to developing such modern - or future - lubricants. 56 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 T+S_6_16 17.10.16 17: 01 Seite 56 Aus der Praxis für die Praxis 3.2 Latest generation Passenger Car Motor Oil The future 4 stroke engine oils will have to be: • thinner, for fuel economy improvement • submitted to higher temperatures, for longer times, for better energy efficiency and durability • submitted to the increasing presence of biofuels Therefore 4 stroke engine oils will have to show lower volatilities, higher thermo-oxidative stabilities, and improved cleanliness features. This is typically what high performance esters should be able to deliver. A laboratory experiment was conducted, in which 5W- 30 oils, one being ILSAC GF-5 compliant, and the other ACEA C1-2012 compliant, were modified by replacing 20 % of PAO 4 by a neopolyol ester of equivalent viscosity. Looking at the neopolyol ester properties (Figure 9), Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 57 base fluid: even though refining processes do tend to remove them in mineral base stocks, they will still show a sizeable amount of a variety of impurities. · The chemical structure of synthetic esters can be chosen. Trade-offs do exist for synthetic esters, and one may choose to maximize some properties over others, and minimize undesirable features. This can be achieved through careful design of the structure to match, as closely as possible, lubricating needs. This gives some precious flexibility to formulators. · Whilst most neopolyols derive from petroleum industry (even though biosourced alcohols are coming up), fatty acids may be chosen from vegetable sources, thus allowing the production of esters showing high contents of renewable carbon. This content may be as high as 100% but typically revolves around 70 to 80%. Synthetic esters constitute a big family of compounds, which may be very different in performance from one another. However, most of them will offer a unique combination of high performance features, including low volatility, high viscosity indices, low pour points, excellent thermo-oxidative stability and low propensity to deposit formation, excellent lubricity, whilst showing a very good environmental profile. 3 Esters in modern, challenging lubricating applications 3.1 Tougher requirements in modern lubrication Commercial competition remains an important driver for better performance and durability, and reduced operation costs in lubricants. However, other factors such as increasing public awareness over environmental issues and depletion of fossil resources are leading to some demand for more energy efficiency, lower environmental impact, and the inclusion of the notion of sustainability in the use of lubricants. Improved safety is also an increasing expectation. Eventually, this translates into new legislation prompting OEMs to issue specifications that reflect this in simple terms: added power density, energy efficiency, and durability, whilst environmental impact should be minimized. From the lubricant manufacturer’s point of view this translates into: · stronger oxidative stress on lubricants · reducing friction, possibly by reducing viscosity · keeping volatility at a low level · harmless, biodegradable and renewable lubricants The following is a number of examples of how synthetic esters may contribute to developing such modern - or future - lubricants. 3.2 Latest generation Passenger Car Motor Oil The future 4 stroke engine oils will have to be: · thinner, for fuel economy improvement · submitted to higher temperatures, for longer times, for better energy efficiency and durability · submitted to the increasing presence of biofuels Therefore 4 stroke engine oils will have to show lower volatilities, higher thermo-oxidative stabilities, and improved cleanliness features. This is typically what high performance esters should be able to deliver. A laboratory experiment was conducted, in which 5W- 30 oils, one being ILSAC GF-5 compliant, and the other ACEA C1-2012 compliant, were modified by replacing 20% of PAO 4 by a neopolyol ester of equivalent viscosity. Looking at the neopolyol ester properties (Figure 9), improved cleanliness, weaker deposit formation, and lower volatility (as measured by NOACK, ASTM D6375) are expected with this change. PAO 4 POLYOL ESTER KV40 3.9 4 KV100 17.4 17.4 VI 124 134 Pour Point -68 -63 CCS-35 1424 1766 NOACK 13.2 6.4 Figure 9 : comparison of PAO 4 and polyol ester The Micro-Coking Test clearly demonstrates improved deposit formation features with the oils that contain the ester (Figure 10). This was confirmed on the GF-5 oils Figure 9: comparison of PAO 4 and polyol ester by running a TEOST 33C test that shows a 30% reduction on the total mass of deposit (Figure 11). With regards to volatility, isothermal thermogravimetric analyses display obvious reduction of the overall volatility of the modified oils (Figure 13), which is confirmed by NOACK test measurements that exhibit up to 1.6% mass loss reduction (Figure 12). Figure 10 : Micro-Coking Test Figure 11 : TEOST 33C (ASTM D6335) Figure 12 : NOACK test Figure 13 - TGA, 200C, O 2 MCT 230-280 GF-5 GF-5 + ester C1-2012 C1-2012 ester TDD 232,00 243,00 248,00 248,00 Merit A 7,68 8,59 5,26 7,68 Merit B 8,39 9,02 9,64 9,75 Moyenne 8,04 8,81 7,45 8,72 Figure 10 : Micro-Coking Test Figure 13: TGA, 200C, O 2 by running a TEOST 33C test that shows a 30% reduction on the total mass of deposit (Figure 11). With regards to volatility, isothermal thermogravimetric analyses display obvious reduction of the overall volatility of the modified oils (Figure 13), which is confirmed by NOACK test measurements that exhibit up to 1.6% mass loss reduction (Figure 12). Figure 10 : Micro-Coking Test Figure 11 : TEOST 33C (ASTM D6335) Figure 12 : NOACK test Figure 13 - TGA, 200C, O 2 TEOST 33C GF-5 GF-5 + ester Rod Deposit mg 18,4 13,1 Filter Deposit mg 5 3,2 Total Deposit mg 23,4 16,3 Figure 11: TEOST 33C (ASTM D6335) by running a TEOST 33C test that shows a 30% reduction on the total mass of deposit (Figure 11). With regards to volatility, isothermal thermogravimetric analyses display obvious reduction of the overall volatility of the modified oils (Figure 13), which is confirmed by NOACK test measurements that exhibit up to 1.6% mass loss reduction (Figure 12). Figure 10 : Micro-Coking Test Figure 11 : TEOST 33C (ASTM D6335) Figure 12 : NOACK test Figure 13 - TGA, 200C, O 2 Figure 12 : NOACK test improved cleanliness, weaker deposit formation, and lower volatility (as measured by NOACK, ASTM D6375) are expected with this change. The Micro-Coking Test clearly demonstrates improved deposit formation features with the oils that contain the ester (Figure 10). This was confirmed on the GF-5 oils by running a TEOST 33C test that shows a 30 % reduction on the total mass of deposit (Figure 11). With regards to volatility, isothermal thermogravimetric analyses display obvious reduction of the overall volatility of the modified oils (Figure 13), which is confirmed by NOACK test measurements that exhibit up to 1.6 % mass loss reduction (Figure 12). T+S_6_16 17.10.16 17: 01 Seite 57 Aus der Praxis für die Praxis 3.3 Taking high temperature chain oils a step further Specifically designed neopolyol esters (with a high amount of branched acids) do show an ideal profile in use in high temperature chain oils: • they exhibit low volatility and resist high temperatures for a long time • when they start degrading, they actually decompose quickly into light, volatile fractions • they leave very little or no residue Such a behavior is significantly different from synthetic hydrocarbons, or even diesters or triesters like trimellitate, that tend to evaporate/ get oxidized very quickly and leave a high amount of residue (Figure 14 and 15). Optimized ester chemistry, suitably additized with high performance anti-oxidants specifically developed to optimize resistance to oxidation and clean degradation, achieves outstanding high temperature performance. Temperatures of up to 300 °C may be sustained on high temperature oils using such technology. With regards to safety, such products are able to show Flash Points of more than 300 °C easily. 3.4 Pushing temperature limits on greases A high temperature chain oil as described above was thickened with both clay and silica, in order to explore the possibility of producing a high temperature grease based on neopolyol ester technology. Whilst clay yielded a mechanically stable grease, silica, as expected, did not. However, silica proved to be more 58 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 Figure 14: TGA, 250 °C, oxygen Figure 15: Micro-coking tests on high temperature chain oils T+S_6_16 17.10.16 17: 01 Seite 58 Aus der Praxis für die Praxis neutral towards oxidation mechanisms, whilst organically modified clay did interfere with oxidation stability. It was however possible to obtain a clay grease resisting temperatures of up to 230 °C, maybe more (Figure 16 and 17). Such technologies may be able to compete with some silicone based greases in a temperature area where PAO based products start being strongly unstable. 3.5 Combination of high performance and low environmental impact Hydraulics is a specific area where future fluids will have to be: • more resistant to rising temperatures, as a result of increased power density • robust anti-wear fluids, to protect pumps and deliver energy efficiency • cleaner products, to facilitate filtration on finer filters • more consistent in viscosity over temperature for improved energy efficiency • fire resistant fluids, as liability in case of fire is a growing concern • environmentally friendly fluids Suitably selected neopolyol esters do provide high performance as base fluids for hydraulic oils. They show high Viscosity Indices, without using any polymer, thus ensuring excellent shear stability and hydraulic efficiency. They are resistant to oxidation and provide extended lifetime as well as superior cleanliness. They are excellent lubricity fluids, as demonstrated by wear test results on vane pumps. They also show high flash points, for improved fire safety. Seal compatibility and water or air separation are of a very good level. Finally, they show high levels of biodegradability and renewability, thus complying with European Ecolabel or Vessel General Permit requirements (Figure 18). Neopolyol ester based hydraulic fluids are high performance lubricants that also demonstrate excellent environmental profile. In a different area, specific high viscosity synthetic esters do exhibit high biodegradability and renewability levels, which make them excellent fluids for the formulation of greases meeting environmental standards like the European Ecolabel or the Vessel General Permit. In particular, they may be used to formulate marine greases used for bearings, wire ropes, and open gears thanks to their high degree of tackiness and excellent overall performance level. Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 59 Figure 16: Grease 1 (bentonite/ ester/ AO 1) Grease 2 (bentonite/ ester/ AO 2) Figure 17: TGA, 230°C, O 2 T+S_6_16 17.10.16 17: 01 Seite 59 Aus der Praxis für die Praxis Conclusion Market drivers are clearly changing the world of lubrication by promoting: • Higher power density • Higher energy efficiency • Increased durability • Lower environmental impact • More sustainability • Improved safety Synthetic esters do provide a unique combination of high performance features and may be specifically designed to match challenging requirements. They can also show excellent environmental profile without compromising performance. New market trends should therefore give more room to synthetic ester technology. References [1] Cosmachi, E., Cottia, D., Pozzoli, L., and Leoni, R., PAH emissions of synthetic organic esters used as lubricants in two-stroke engines, J. Synth. Lubr., 3, 251 (1998) [2] Organic Chemistry: Structure and Reactivity by Seyhan Ege, pp.30-33, 67 [3] Synthetics, Mineral Oils, And Bio-Based Lubricants: Chemistry and Technology, edited by Leslie R. Rudnick, Taylor & Francis [4] Sniegoski, P.J., Selectivity of the oxidative attack on a model ester lubricant, ASLE Trans. 20, 4, 282-6 (1977) [5] Martem’yanov, V.S., and Kukovitskii, M.M., Mechanism of oxidation and antioxidative stabilization of polyol esters as base fluids for high temperature lubricants, Neftekhimiya, 18, 4, 539-45 (1978) 60 Tribologie + Schmierungstechnik 63. Jahrgang 6/ 2016 Figure 18: European Ecolabel compliant hydraulic fluid Umzug oder Adressenänderung? Bitte T+S nicht vergessen! 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