Challenging
Conventional Wisdom: Is 12-hydroxystearic acid the best fatty acid for making high dropping point lithium greases?
Greases are principally composed of 3 parts: an oil, a thickener and an additive package. For decades, 12-hydroxystearic acid (12-HSA) has been the fatty acid of choice to make the thickener part of lithium soap greases, which in 2013 made up 77% of the world grease market. Most grease makers would say that its main desirable qualities are:
• It has the highest melting point • It has the highest thickening efficiency, meaning that less is needed hence reducing cost
• The greases it produces have superior shear stability compared to greases made from other fatty acids or fats such as tallow • It produces greases with low oil separation.
These beliefs have become well established amongst grease makers and are based largely on research from the middle of the last century, which is sometimes contradictory and often uses poor quality materials. This study aims to establish whether with the large improvements in base oil quality, purity of lithium hydroxide (LiOH) and the fatty acids available, the above statements are still true. In this study 12-HSA has been tested against a variety of other natural and synthetic fatty acids to discover whether or not it is still the best choice for making lithium greases.
Only saturated fatty acids have been chosen (not containing carbon-carbon double bonds) because unsaturated fatty acids are known to make softer, inferior greases.
This paper was originally presented at the 2014 ELGI Conference, the theme of which was ‘High Temperature – Extreme Conditions’ so an emphasis was placed on formulating for and testing high temperature performance. For this reason, fully synthetic PAO 6 was chosen as the base oil because of its superior high temperature performance.
To make a lithium grease it is usual to melt the fatty acid (12-HSA in this case) into a portion of the oil to be used. LiOH, usually with some water, is then added at about 90ºC. As the temperature rises, a reaction, known as a saponification reaction, takes place to form a lithium soap. A diagram is shown below:
Greases can be made from many different fatty acids, both natural and synthetic. The fatty acids used in this study have a variety of chain lengths, from the short-chain capric acid with 10 carbon atoms to the long-chain behenic acid with 22 carbon atoms. 12-HSA has 18 carbon atoms, but there is no particular reason to believe that 18 carbon atoms is the perfect number, hence the effect of chain length has been investigated. Although chain length has a clear effect on the melting point of the fatty acid (table below), will the effect be the same in a grease? The table below shows the naturally occurring fatty acids used in this study:
The lithium 12-hydroxystearate then forms a 3D network within the oil that partially immobilises the oil and causes the mixture to become semi-solid. Various conditions cause the 3D structure to break down: at high temperatures the soap, which in its pure form is a solid at room temperature melts; this causes the soap and hence the grease to liquefy completely. The reason it is widely believed that 12-HSA forms superior soaps is due to hydrogen bonding, where the –OH group forms intermolecular bonds between different molecules (dashed lines), as shown opposite:
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LUBE MAGAZINE NO.123 OCTOBER 2014
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