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Synthetic Esters in High Temperature Grease

December 01, 2012
Tyler Housel, Inolex
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High viscosity synthetic esters have been used as basestocks in high temperature lubricants for many years, and are now available with the NSF HX1 designation for food processing applications.  Synthetic ester based lubricants are effective for continuous operation at 260°C (500°F) and show little evaporation  or oxidative degradation compared to other common basestocks.  These benefits make synthetic esters uniquely suited for high temperature greases.

Although grease is defined as thickened oil, the demands of the oil within a grease can be quite different than a fluid lubricant.  The thickener holds the oil in place so the oil must remain liquid to maintain the consistency and stability of the grease.  If the oil evaporates, a hard layer of thickener is left behind.  Similarly if the oil thermally polymerizes, the grease will become a solid mass.  Excessive evaporation or oxidation of the oil destroys the grease structure. 

When the grease does not flow properly it cannot lubricate moving parts and may even become abrasive or adhesive.  Even worse, most grease fittings cannot effectively flush thermally degraded grease from the bearing.  Once the grease becomes skinned, it is usually just a matter of time before the bearing will fail because the solid residues block fresh grease from getting in. 

For this reason, high temperature greases must be made from oils that resist evaporation and are thermally stable under the expected use conditions.  Synthetic esters are known for outstanding oxidative stability and also have very low evaporation relative to other lubricant base oils.  Mineral oils and PAO perform well under most common machinery operating conditions.  However, when temperatures go above 204°C (400°F), the hydrocarbon oils (API groups 1-4) degrade too quickly to be effective.  Synthetic esters are preferred in industrial and food processing lubricants when temperatures go above 225°C (440°F). 

To demonstrate the benefits of synthetic esters as basestocks for high temperature grease, we prepared an NLGI grade 2 food grade grease from a high viscosity synthetic polyol ester (Lexolube FG-350 HX1).  For comparison, we purchased 17 high temperature, food grade (NSF-H1 certified) greases.  The commercial greases were all NLGI grades 1 or 2 and were made from petroleum, PAO and vegetable basestocks.  The thickeners were aluminum complex, PTFE, silica and calcium sulfonates.  Several claimed to be useful at 500°F and above.

To show how high temperatures affect both the thickener and base oil, we weighed two grams of each grease into a 50 mm aluminum pan and then topped each one with a 70 mm cover.  A group of nine specimens were placed in an oven for 20 hours at the specified temperature.  After cooling, the grease structure was noted and the grease and cover pan are weighed again to determine evaporation loss and weigh the varnish deposited on the cover.  Failure occurs when either the grease forms a hard skin or the grease loses its structure.  We started testing 400°F (204°C).  Samples that remained viable were retested at a higher temperature. 

Testing 20 hours at 400°F
At 400°F we were surprised to find that 9 out of 17 commercial samples lost their grease structure even though many of these were rated for use at 500°F.  Two other samples developed a thin skin on the surface.  Evaporation loss ranged from 4% to 44% and some stained the cover with more than 1 milligram of varnish. The synthetic ester grease structure was perfect.  Weight loss was less than 1%, and there was no residue on the cover.

Testing 20 hours at 450°F
Of the 8 remaining commercial products, 5 had skin layers due to oxidative polymerization or base oil evaporation and two others lost their grease structure at 450°F.  Evaporation loss ranged from 5% to 55%, and the varnish ranged from 0.3 to 2.4 mg.  The synthetic ester grease structure was still excellent.  Evaporation was only 2% and there were 0.1 mg of deposits.

Testing 20 hours at 500°F
The 7 remaining samples all showed a heavy skin layer and significant oil bleed.  Evaporation loss ranged from 11% to 35%, and deposits weighed from 4 to 27 mg. The synthetic ester grease still maintained a good structure with some oil bleeding.  Although the grease had thickened somewhat, there was no skin on the surface.  The weight loss was 10% and vapor deposits were less than 4 mg. 

Testing 4 hours at 550°F
The test duration was 4 hours at 550°F, yet all of the commercial samples had a heavy skin.  Weight loss ranged from 7% to 29% and deposits were 8 to 53 mg.  The synthetic ester grease still had good structure with some bleeding and no skin.  The weight loss was 7% and deposits were only 2.2 mg. 

Many NSF-H1 food grade greases give outstanding performance at temperatures up to 400°F (204° C), but most of these cannot withstand continuous operation (20+ hours) above 400°F.  Many high temperature failures are attributed to evaporation or thermal degradation of the basestock. 

Please note that the Lexolube(R) FG products are covered under pending patents. Tyler Housel is currently Business Director at Inolex. You can find his contact information in the membership database.

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