Polyhydroxy acids such as dihydroxystearic acid (DHSA) have been experimented with for some time, one example being in 19552
, although it has never become popular. In this study it has been found that although it will form a grease in non- polar PAO it is very soft and not well suited to grease making. Trihydroxystearic acid synthesised from castor oil was found to be even less well suited.
Conclusions At fi rst glance it appears that 12-HSA is the best solution. However, other fatty acids, in particular stearic acid, have a large cost advantage: 12-HSA costs around 40% more than stearic acid. Yet a signifi cant part of the cost of the soap is the lithium hydroxide, which is a lot more expensive than the fatty acids, and 12-HSA soaps need slightly less lithium hydroxide per kilo of soap. The cost of the 3 raw materials is obviously quite variable, but currently 1 kilo of lithium 12-HSA is about 30% more expensive than 1 kilo of lithium stearate. Since the thickening power of Li-12HSA is >30% better than that of lithium stearate, it can be concluded that Li-12HSA makes more cost-effective soaps.
Although it was found that 12-HSA made better soaps than any of the other (relatively pure) fatty acids tried. It is conceivable that mixtures of fatty acids may give superior thickening ability to that of 12-HSA alone, but that is beyond the scope of this investigation, which was originally focused mainly on high temperature properties. A mixture of fatty acids is unlikely to make a large difference to dropping point, although the addition of acids like azelaic and sebacic acids is well known to make greases (known as lithium complex greases) that have dropping points in excess of 260 ºC. It is also well known that the soap structure can be modifi ed with many different additives such as polymers to confer benefi cial properties.
12-HSA does have some inherent disadvantages, too, such as that the greases it makes are naturally darker than those made with stearic acid, which are naturally very pale. Secondly, 12-HSA soaps are less resistant to hot water, as in the picture (right), where both greases were spread onto a slide and covered in boiling water for an hour. The 12-HSA grease disperses into the water, whilst the stearic acid grease is barely affected. There may be some applications where these benefi ts would be useful. It is also well known that the soap structure can be modifi ed with many different additives such as polymers to improve properties like oil separation and shear stability, which are weaker in greases made with stearic acid. This would require a lot more work to demonstrate, however.
For the time being, at least, it is safe to say that the conventional wisdom that states that 12-HSA is the fatty acid of choice for lithium greases is probably correct!
References 1. Swern et al. J. Amer. Chem soc. 67, 1788, 1945. 2. Mikesca, L. A. and Morway, A. J. USP 2,712,527
For further information please contact Dr. Julian Wilkinson, Lead Chemist - Oil & Gas Technology, of RS Clare & Co Ltd:
Jwilkinson@rsclare.co.uk
LINK
www.rsclare.co.uk
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