Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
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 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 mm2
Figure 1. Permanent dipoles in esters /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.
Figure 4. Anilin point of various base stocks 2.2 Thermo-oxidative stability of esters Figure 2. Volatility of synthetic esters vs other base stocks
• Negatively charged oxygen from ester chemical function will bind to positively charged sites of metal surfaces. In a mixed or boundary lubrication regime, 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 (or “lubricity agents”) capable of reducing friction coefficients and wear in moderately loaded conditions (Figure 3). Long, linear carbon chains improve anti-wear capability and friction modification.
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 b-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].
No.102 page 2
• 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.
Figure 5. b-elimination
Using neopentyl structures (Figure 6), in which no hydrogen is present in b position of oxygen, suppresses any possibility of b-elimination and greatly improves, de facto, the thermal stability of esters. Such structures are called neopolyol esters.
Figure 3. Friction modification of synthetic esters vs other base stocks
• 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.
Figure 6. Neopentyl structure LUBE MAGAZINE NO.131 FEBRUARY 2016 31
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65
