Recently, an engine oil sample was tested to determine the root cause of a failure that resulted in an engine seizing. Initial observation of the oil sample showed it was thick and almost gel-like.
Common causes of engine oil thickening include highly oxidized oil, glycol contamination, thermal degradation and severe soot contamination. Initial analysis of the oil sample revealed that the oil was relatively new (additive elements were similar to the reference oil), it did not appear to be oxidized (reasonable acid number), and there was no sign of glycol contamination or wrong oil added (no unusual additive elements were present in the used oil that did not show up in the reference oil).
To sample for soot load, a pentane and toluene coagulated insolubles test (ASTM D893) was run on the sample and results indicated less than 1 percent solids in the sample. However, when a thermogravimetric analysis (TGA) test (ASTM D5967) was run, it reported about 15 percent soot, a huge discrepancy. Generally, results of these tests should correlate, but in this case they seemed to disagree with each other. To investigate further, two additional pentane insolubles tests by membrane filtration (ASTM D4055) were run.
The first test was conducted using a 0.45 micron filter and the second with a 0.30 micron filter. The 0.45 micron filter yielded 2 percent soot while the 0.30 micron filter found about 22 percent soot. It appeared that a high soot load caused the samples to become thick, but it was strange that the pentane insolubles test utilizing centrifugal solids separation did not report similar results. After more investigation, the reason why one test was more successful than the other was discovered.
To understand why the results of the pentane and toluene coagulated insolubles test did not coincide with TGA and membrane filtration tests, it is important to know how each test is performed and understand the underlying theories.
Understanding Each Test
Pentane and Toluene Coagulate Insolubles Test - ASTM D893
Basically, this test separates insolubles from the oil after it has been mixed with solvents. First, a pentane solvent is mixed in with the oil to separate solids and oxidation products from the oil by lowering the viscosity. The mixture is then centrifuged to separate the insoluble material and the insoluble material is measured to determine the percentage of insolubles present. The test is then repeated using toluene instead of pentane. The toluene dissolves organic oxides, but not soot. The mixture is centrifuged again and the insoluble material is weighed. The difference between the two weights (pentane and toluene) is the estimated soot content in the sample.
Soot Percent by Thermal Gravimetric Analysis (TGA) - ASTM D5967
The TGA method involves placing the sample in an oven, where it is weighed continuously as the temperature is increased at specific increments. An inert nitrogen purge gas is used over the sample to enable the oil to evaporate without other reactions (oxidation) taking place.
When the weight stabilizes and remains stable for a certain duration, this signifies that all the oil has evaporated and only insolubles remain. At this point, oxygen, rather than nitrogen, is introduced, which allows all the carbonaceous material (mainly soot) to combust and leaves the other metal insolubles in the sample. The soot percentage is calculated by measuring the difference between the weight of the sample before oxygen was introduced to the weight of the sample after it stabilized and all the soot is removed.
Pentane Insolubles by Membrane Filtration - ASTM D4055
This method is similar to ASTM D893 in that it also uses a pentane solvent. However, instead of centrifuging to separate the solid particles from the oil, the oil is passed through a submicron membrane. As the sample passes through the fine mesh patch, the insoluble material remains on the upstream surface of the patch while the remaining sample passes through. The insolubles are then dried and weighed, and the insolubles percentage is calculated.
Upon speaking to the laboratory personnel, it was discovered that when the pentane insolubles test was run, the additives (dispersants) in the oil appeared to have kept the soot suspended in the sample. When the sample was centrifuged, the soot was moved along the side of the test tube instead of being forced to the bottom with the other insolubles. Therefore, test results indicated a low insolubles reading. If the oil’s additives had been depleted, the soot might not have remained suspended in the oil. This is further supported by the fact that the soot passed through a 0.45 micron membrane, but failed to pass through a 0.30 micron membrane. Dispersant additives keep soot from agglomerating.
Two lessons were learned from this discovery. First, had the TGA test or the ASTM D4055 pentane insolubles test using a membrane not also been conducted, the oil analysis results would have shown that soot was not above critical levels (usually about 2 percent or 3 percent). Although more expensive in certain cases, these tests are more appropriate for measuring soot than pentane and toluene insolubles by centrifuge.
The second lesson is that the customer should communicate with his/her laboratory and ask specific questions about the oil. The customer can oftentimes help when decisions are being made about what tests are most appropriate in a particular application. In addition, if questionable test results are obtained, the laboratory can serve as a good second opinion and troubleshooter.