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There has been much said in recent years about the increased propensity of some modern oil formulations to produce sludge and varnish. Ironically, many of the oils accused of contributing to varnish are the same oils endowed with exceptionally high oxidation stability due to advanced refining technology and formulation science. To better understand this paradox, let’s begin by defining a new term.
Lubricants have an intrinsic Impurity- holding Capacity (IHC) relating to their ability to retain very small (submicron) impurities in solution. These impurities are largely oil-degradation products, but include other oil suspensions as well.
This holding capacity is like an imaginary silo or cup within the oil. If the cup is half empty, it has unutilized or reserve capacity. However, conditions can occur that can cause an overfill, leading to insoluble suspensions in the oil and varnish (sometimes called the “dumping point”).
For instance, if a half-full, eight-ounce cup of water was suddenly shrunk to a cup two ounces in size, then two ounces of water would spill onto the table. Or, if you poured 12 ounces of water into an eight-ounce cup, then four ounces would spill onto the table.
The overflow in the water/cup examples is analogous to what occurs in lubricating oils when impurities are supersaturated and the IHC is breached (cold temperatures alone can cause this). In other words, our cup “runneth over.”
As in the example, this occurs due to low IHC, an exceedingly high impurity concentration, or both. Either way, what remains in the cup are impurity “solubles” (oxide or carbon insolubles) and what overflows becomes our impurity “insolubles,” sometimes referred to as varnish potential.
Now, imagine a teeter-totter like the one shown in Figure 1. On one side is oxidation stability, and on the other side is IHC. When you increase an oil’s oxidation stability, you often decrease its IHC.
Conversely, if you introduce polar constituents into the oil to increase its IHC, you risk impairing its oxidation stability – or, perhaps, its demulsibility and air handling ability. Hence, IHC can be said to be inversely proportional to oxidation stability, although there are some exceptions.
Unfortunately, there’s more bad news. IHC shows a positive correlation to electrostatic discharge (ESD) risk. This is another common root cause of varnish, resulting in periodic lightning strikes within the oil.
Many modern turbine oil formulations have not only low IHC but also related properties (dryness, cleanliness and purity) that are direct contributors to static discharge. So while the high quality of these oils translates to better oxidation stability, they are figuratively punished for having low IHC and being more prone to static discharge problems.
As mentioned, an oil’s total impurities include those in solution and those not in solution, including impurities in transition between these two states. Another good analogy is the states of coexistence of water in oil. Dissolved water corresponds to soluble impurities, and emulsified water corresponds to insoluble impurities.
Dissolved impurities are anything in the oil that consumes fractional space within the IHC imaginary silo. These would primarily include oil- degradation products from oxidation, microdieseling and electrostatic discharge, but they may also include some additives. While additives aren’t normally regarded as impurities, they do alter the oil’s state of being a pure, saturated hydrocarbon.
Relative humidity (RH) relates to moisture in air (100 percent RH corresponds to the saturation point). But in the same sense, oil has a relative humidity (100 percent RH corresponds to the saturation point of dissolved water in oil).
Taking this further, we can also refer to an oil’s relative IHC (R-IHC) because the actual impurity capacity varies with temperature just as the moisture-holding capacity of water in air also varies with temperature (the higher the temperature, the higher the moisture-holding capacity).
In fact, IHC should correlate well to the soluble moisture-holding capacity for oil due to the fact that they are both largely influenced by temperature and the density of polar constituents in the base oil. Aniline point, interfacial tension, dielectric constant and percent polars are similar but not identical properties.
IHC is best quantified gravimetrically in parts per million (ppm), milligrams per kilogram (mg/kg) or milligrams per liter (mg/L) using a suitable reference material serving as the surrogate impurity.
A number of common additives susceptible to “additive dropout” might work well as the reference material. Perhaps two materials with dissimilar solubility should both be reported to best characterize IHC range. During measurement, the reference material (titrant) would be added in gravimetric increments (titrated) to an anhydrous sample of the test oil until a terminal point of flocculation is reached.
Like viscosity, IHC would have to be reported at a standardized test temperature, say 20 degrees and 40 degrees Celsius. One could perhaps develop an IHC index (similar to viscosity index) representing the relative change in IHC with respect to change in temperature. One could also assess an oil’s impurity “dew point” temperature, which would define the reserve impurity-holding capacity.
I propose the term “impurity floc point” be used; however, it should be noted that this type of measure will be challenged by the fact that some oils would also have dew and cloud points in the same temperature range. Because all three cause optical turbidity, it would be difficult to differentiate between them.
IHC values for additized new or in-service lubricants would have to be reported as fractional capacity because part of the IHC for neat oils is occupied by the additive system itself. Additionally, many additives, like dispersants, may actually increase IHC, while others may consume it.
When additives drop out by normal depletion (some additives), a portion of the original base oil IHC may be restored. Both the additive treat rate (dosage) and additive types influence the residual IHC after they are blended in a finished oil. Table 1 shows some examples of how base oil properties influence IHC.
Despite the fact that there is not currently a standardized test protocol to provide a quantifiable measure of IHC, it still can be referred to in relative terms. To my knowledge, there’s also no standardized test method to quantify the soluble fraction of water in oil.
However, the concept of an oil’s dew point, saturation point or relatively humidity is abundantly clear to those performing oil analysis and formulating lubricants. The same applies to an oil’s relative IHC (instead of relative humidity), IHC (instead of saturation point) and impurity floc point (instead of dew point).
On the surface, IHC may seem a little abstract to be of practical value to oil analysis and maintenance professionals. However, with sludge and varnish issues becoming more widespread and pronounced, and the consequences more acute, there is a need in many cases for oils to possess reserve impurity-holding capacity. This serves as an impurity safety net in case of cold temperature excursions (or zones) or the buildup of oil-degradation products.
Lubricants facing static discharge or microdieseling distress may need to be periodically tested for reserve IHC. After all, we can rightly refer to impurity “solubles” as incipient varnish and impurity “insolubles” as impending varnish. Lubricants with IHCs loaded to full capacity are likely to be throwing sludge and producing varnish, and are prime candidates for oil reclamation and, possibly, additive reconstruction.