Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Figure 2. Noack volatility properties of Group lll, IV and esters vs kinematic viscosity at 100O
C
Low Temperature Performance With regards to the hydrocarbon base stocks, PAOs have the advantage in that they have pour points (< - 60 O
C) which are
very much lower than equivalent ISO grade Group I, Group II or Group III base oils (-10 to -20 O
on the other hand can be synthesised to have either high pour points / melting points (> 100O
OC), depending upon what is required in the end application.
For synthetic oleochemical and petrochemical esters, pour point characteristics can be influenced by the choice of raw materials, both alcohols and acids. Consider three commonly used C8/ C10 esters based on glycerol, trimethylolpropane (TMP) and pentaerythritol (PE) and three TMP esters based on different C18 acids.
Low pour point esters are characterised by irregular structures which makes molecular packing within a fluid difficult or chaotic. Small, controllable changes in structure through branching, saturation / unsaturation and mixtures of fatty acids can be used to tune pour point characteristics, as can be seen in table 3.
C). Synthetic oleochemical esters C) or very low pour points (< -60
Table 4. CCS at -35O C for a selection of base stocks
Now let’s consider esters where the chemical structure is once again very important when fine tuning the low temperature flow properties of ester base oils. Two esters with similar viscosity at 100°C (approximately 4.4 and 4.5 mm2
/s) display contrasting
CCS viscosity at -35°C: - the 4.4 mm2
- the 4.5 mm2 /s polyol ester has a high CCS viscosity (2343
mPa.s), somewhat lower than Group II and Group III base stocks but much higher than PAO 4.
/s diester of similar KV100 has a much lower CCS viscosity (990 mPa.s) than the polyol ester and indeed has the lowest CCS at -35°C of all the 4 – 4.5 mm2 stocks, including PAO 4.
/s base The effect is not limited to 4 mm2 /s polyol- and di-esters. Consider also two esters with a KV 100 of 3.3 mm2 /s. Once
again the influence of chemical structure can be seen on the low viscosity flow properties with the mono-ester having lower CCS viscosity (550mPa.s) than the di-ester (715 mPa.s).
Table 3. Pour point characteristics of C8/C10 esters and TMP C18 esters
Whilst we generally consider low pour points to be a good thing, there are applications where a higher pour point is highly desirable, for example, some esters are used to provide lubrication and then temporary corrosion protection through the formation of a thin wax like film in the manufacture of sheet metal.
As with all base stock selection, it is a combination of properties that are required and a balancing act usually needs to be performed. In the case of base fluid selection for an engine oil a combination of excellent low temperature flow properties and low Noack volatility are required and therefore both properties need to be considered when designing ester base fluids for engine oils.
Figure 3 shows the relationship between CCS at -35°C and Noack volatility and clearly demonstrates that esters have the best combination of CCS and Noack volatility properties when
LUBE MAGAZINE NO.129 OCTOBER 2015 35
No.100 page 4
Low temperature performance doesn’t just mean pour point properties but it also includes kinematic and dynamic viscosity properties at a range of low temperatures.
Low temperature viscosity properties are also a function of chemistry. As an example, Cold Cranking Simulator (CCS) dynamic viscosity measured at -35°C for Group II, Group III, Group IV and a selection of chemically different esters are shown in table 4.
If we consider hydrocarbon base oils with the same kinematic viscosity at 100°C then the Group II base fluid has the highest CCS viscosity, followed by Group III (1). Group III (2) is a more highly refined Group III base stock and as such has a lower viscosity at -35°C than Group III (1). Of the hydrocarbon base fluids PAO 4 has the lowest viscosity at -35°C (1374mPa.s)
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