The table shows that, as would be expected, larger chains have higher melting points because it is easier for longer chains to become entangled with one another. Alternatively, if hydrogen bonding ability and not chain length is more important, would a molecule with 2 or more –OH groups be useful, such as 9,10-dihydroxystearic acid or 9,10,12-trihydroxystearic acid?
An extreme example is the difference between lithium stearate (18 carbon atoms), which has a melting point of 221ºC, and lithium acetate (2 carbon atoms), which has a melting point of 286ºC, more than 60ºC higher, which clearly demonstrates that shorter carbon chains result in higher melting point compounds.
In small molecules, the addition of an –OH group can make a massive difference, as the following comparison of the boiling points (b.p.) of methane and methanol illustrates:
Graph showing dropping point of grease (ºC) as a function of no. of carbon atoms in chain.
Most of the fatty acids were bought from chemical suppliers with the exceptions of the 12-HSA which is of the same grade that RS Clare uses for grease manufacturing. 9,10-dihydroxystearic acid and 9,10,12-trihydroxystearic acid were prepared via reaction of oleic acid and ricinoleic acid, respectively, using an epoxidation/hydrolysis reaction using hydrogen peroxide/formic acid followed by hydrolysis1
.
Greases were made by heating PAO 6 with one of the fatty acids above, then saponifying using a commercially available anhydrous lithium hydroxide dispersion in mineral oil. The formulations were controlled to make greases with 15% soap content. The greases were milled twice using a triple roll mill and further PAO 6 added to make NLGI no. 2 greases with an unworked penetration of 280±5 (i.e. the same consistency). The following tests were then performed:
• Dropping point • Cone penetration (60 and 10,000 double strokes)
• Oil separation (18 hours and 1 week). The results are shown in the table below:
Graph showing amount of soap required to make a grease of 280±5 penetration as a function of number of carbon atoms.
The other commonly held belief is that lithium 12-HSA greases are more shear stable. This has been investigated by comparing the unworked penetration with worked (10,000 strokes) penetration; the results are shown below:
Lithium 12-HSA soap from the above results makes a grease that does not have a particularly good dropping point, being no better than stearic acid and clearly inferior to some of the smaller fatty acids such as capric acid. The lithium 12-HSA soap does have a large advantage in producing a high yield (i.e. less is needed to make a grease of a given consistency), which creates a cost advantage.
This table shows the unworked pentration and change after (a) 60 (b) 10k strokes.
The synthetic fatty acid 9,10-dihydroxystearic acid made only a very soft grease, even with a 20% soap content, whilst 9,10,12-trihydroxystearic acid did not make a grease at all – just a plastic mass that was insoluble in oil.
The dropping points show a clear trend of decreasing with increasing carbon chain length. This is because, as the carbon chain length decreases, the compound becomes more ionic in nature as the ratio of ionic bonds to covalent bonds increases.
Graph showing grease shear stability, measured by difference between unworked penetration and worked (10k), measured in points (0.1mm).
This graph shows clearly that the lithium 12-HSA soap has a definite advantage here, too, producing greases that soften a lot less when sheared.
LUBE MAGAZINE NO.123 OCTOBER 2014
19
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