Monitoring early warnings signs of additive depletion should be a fundamental goal of any well-organized, proactive oil analysis program. The reasons for this are clear. Because oil additives play such a fundamental role in the health of the base oil and in providing basic lubricant functions, measuring additive depletion rates and mechanisms allows for early corrective action, before depletion compromises the base oil condition and/or equipment.
Many of the test methodologies considered routine in oil analysis measure additive depletion, to some degree. These tests can loosely be separated into two basic classes: 1) physical (viscosity, filterability, etc.) and chemical tests (acid number, base number, etc.) and 2) spectroscopic tests (Table 1). Generally, physical and chemical tests do not actually measure additives themselves, but rather, the observable effect additive depletion may have on a bulk oil property.
|ICP/RDE||ASTM D5185/6595||Spectroscopic||Metal analyses|
Table 1. Spectroscopic Methods
Take acid number (AN) for example. Though there are several ASTM standards for AN, they all basically measure the same thing. In each case, a sample of oil is titrated against a known standard titrant (potassium hydroxide, KOH) with the volume of reagent used indicating the acid content of the oil. The main problem with AN testing, and indeed most physical and chemical tests of this type, is that they are not specific to measuring additive condition, but rather a bulk oil property that is in some way related to the presence or absence of certain additives.
Figure 1. Factors affecting the acid number
of antiwear and extreme pressure oils
This is illustrated in Figure 1. For oils that contain appreciable concentrations of zinc-based antiwear additives such as ZDDP, or sulfur-phosphorus extreme pressure (EP) additives, new oil ANs generally range from 0.3 to 1.5.
This is simply related to the fact that phosphorous additives of this type are mildly acidic and react with the KOH, yielding a small positive AN reading. It may be believed that a downward trending AN might be a good indicator of additive depletion. While this may be true in a controlled lab environment, in the real world there are various factors that could cause a change (up or down) in AN which are unrelated to additive depletion.
For example, additives such as ZDDP and sulfur-phosphorus EP additives are designed to plate out on metal surfaces under operating conditions. This causes the AN to drop as the concentration of these additives in the bulk oil drops.
However, as the oil ages, base oil oxidation which causes the AN to rise, may mask the effects of additive depletion by causing an equivalent rise. In effect, the casual observer may see no change in AN and incorrectly conclude that the additive is in good shape, when in fact, depletion is being masked by base oil oxidation.
Now consider the environment in which the oil is being used. In an environment with excessive acidic ingressed contaminants (such as a sulfuric acid plant, phosphate mine, etc.), the AN may increase, while it may decrease in other environments, say for example a plant where caustic washdown using sodium hydroxide is commonplace.
The take-home message concludes that while chemical and physical tests do play a role, without additional corroborative evidence they cannot solely provide conclusive evidence of additive health.
For organometallic additives - those containing elements such as calcium, magnesium, boron, zinc, phosphorus, etc. – elemental spectroscopy, using either inductively coupled plasma (ICP) or rotating disc electrode (RDE), spectrometers can be used. However, when it comes to monitoring additive depletion, elemental spectroscopy suffers one fatal flaw.
While elemental spectroscopy can determine sudden and massive additive loss, due to effects such as water washout or the addition of wrong oil, the technique is generally not sensitive enough to provide an early enough warning of additive depletion, and in fact may miss the problem all together.
Again, the reasons are simple. Elemental spectroscopy measures elements. While the elements in question (zinc and phosphorus from ZDDP) may be bound up as a healthy, fully functional additive, additive depletion resulting in destruction of the additive molecules and hence loss of additive functionality would not necessarily be identified because the elements themselves will still be present in some other (nonfunctional) form. The elements are not destroyed, just changed in form and still capable of being measured identically.
For some organic additives – those that contain only carbon, hydrogen, and nitrogen or oxygen - ICP or RDE provides no benefit. For these additives, which include phenolic and phenylamine antioxidants, dispersants and VI improvers, fourier transform infrared spectroscopy (FTIR) can be a useful tool. However, in many instances, an inability to obtain a correct new oil reference, or competing and overlapping absorbance make it difficult to accurately determine additive concentrations using FTIR.
One method that can be used to fill in the gaps left by ICP, RDE or FTIR is nuclear magnetic resonance spectroscopy (NMR). NMR is used in a host of different research and practical applications, from drug development to medical diagnostics and is one of the mainstays of organic chemistry laboratories. NMR is closely related to magnetic resonance imaging (MRI) most commonly used to capture images of soft tissue damage in human medicine.
Though the fundamentals of NMR are embedded in the laws of quantum mechanics and are far beyond the level covered in this article, the basic principles can be understood by comparing NMR with the other more commonly used spectroscopic techniques in oil analysis, specifically ICP (or RDE) and FTIR.
To understand any spectroscopic technique, one must first understand that atoms or elements (Zn or P) and molecules (ZDDP), exist in certain energy states. All molecules strive to exist in their lowest energy state, referred to by physicists as the ground state. In ICP or RDE, the oil sample is subjected to a high-energy source – either an argon plasma or electric discharge.
The effect of this high-energy source causes the material contained in the sample to instantaneously vaporize and atomize. However, in the process of vaporizing and atomizing, a significant number of these atoms are forced into a higher energy state than the ground state (referred to as an excited state).
Unhappy in the excited state, the atoms almost instantaneously lose energy and return to the ground state. In doing so, energy is emitted in the form of light. Because the energy of the ground and excited states for each element are defined by the laws of quantum mechanics, the emitted energy (the difference between the energy levels of the ground and excited states), and hence the wavelength of light are specific to each atom or element. By recording the intensity of light at each wavelength, one can obtain concentrations for each element in parts-per-million.
FTIR involves a fairly similar process. In this case, one talks about molecules (ZDDP) rather than individual atoms. In FTIR, the molecules contained within the sample are at rest in their ground energy state. Once again, the molecule has a series of excited states where energy is clearly defined by quantum mechanics, and unique to specific molecules.
By scanning the infrared light across a range of wavelengths, light is absorbed by specific molecules when the infrared energy exactly matches – or is in resonance with – the energy separation of the ground and excited states of the molecules.
In this case, measuring the amount of light absorbed as a function of wavelength allows the FTIR spectrum to be recorded. The FTIR spectrum becomes an X,Y-graph. The x-axis can be thought of as the chemistry axis, the y-axis as the concentration axis. A peak position location along the x-axis (wavelength) can tell the analyst what type of chemistry is present in the sample.
Every atom is comprised of a nucleus containing protons and neutrons, and a series of surrounding electrons. In atomic spectroscopy (ICP or RDE) and molecular spectroscopy (FTIR), the electrons are responsible for the energy levels described previously. However, not only do electrons exist in discrete energy states, protons do as well.
Under normal circumstances, these energy states are said to be degenerate – having the same energy level. However, if a strong magnetic field is applied, these states can be split, with one state increasing in energy while the other decreases. The degree to which the states increase or decrease in energy is proportional to the strength of the applied magnetic field.
For certain nuclei (atoms), this effect can be used to record a nuclear spectrum similar to the electronic spectrum recorded in FTIR. This NMR spectrum also becomes an X,Y-graph, having the x-axis thought of as the chemistry axis. For oil analysis, the most commonly used atoms for which NMR spectra can be recorded are hydrogen (1H), carbon-13 (13C) and phosphorus (31P).
Figure 2. 1H NMR spectrum for toluene 3
NMR is a valuable tool because it detects not only the presence of these atoms, but more importantly that it is also sensitive to the chemical environment around the atom. Take for example the simple organic molecule toluene (Figure 2). Toluene contains two different types of hydrogen atoms. Those chemically bonded to the benzene ring (aromatic hydrogens) and those attached to the methyl group (aliphatic hydrogens).
These differences are reflected in the NMR spectrum (Figure 2). The spectrum contains two distinct peaks along the x-axis, one due to the aromatic hydrogen’s (7.38 to 7.00 ppm) and one due to the methyl hydrogen’s (2.34 ppm); their peak heights (or more correctly areas) are directly proportional to the relative concentration of the two different types of hydrogen atoms present.
The most commonly used element for which NMR is most useful for lubricant analysis is phosphorus (31P) NMR. Various lubricants include phosphorus-containing additives, the most common being ZDDP (zinc dialkyldithiophosphate), TCP (tricresyl phosphate) and sulfur phosphorus-type EP additives. The chemistry and fate of these phosphorous compounds is important to the lubricant’s life cycle.
In each case, 31P NMR can be used to track both the depletion rates and pathway. This may be particularly useful because these additives decompose in numerous ways - including, but not limited to, hydrolysis, oxidation and thermal degradation. By understanding the mechanism of degradation, one can determine the underlying root cause and allow corrective action to address the fundamental reason for premature additive depletion.
Figure 3. 31P NMR spectrum of new crankcase oil
showing the different type of P species present as part
of the ZDDP antiwear additive package of a new engine oil.
Figure 3 shows the 31P NMR spectrum of a new engine oil containing ZDDP. The presence of multiple peaks along the x-axis of the 31P NMR spectrum indicates that phosphorus is present in several different chemical environments. These include:
Overbased ZDDP (seen at 100 to 104 ppm). There are several types of potential overbased ZDDP compounds.
Neutral ZDDP (seen at 94 to 98 ppm). This is a broader peak representing the tetrahedron structure of (RO)2P(S)S Zn S(S)P(OR)2.
Starting dialkyldithiophosphate acid: (RO)2P(S)SH (seen at 86 to 87 ppm).
As the oil goes into service, the ZDDP additives molecules start to degrade. The question is, how quickly are they degrading and what is causing degradation?
Under normal circumstances, ZDDP in an engine oil decomposes due to oxidation. As this occurs, oxidation converts the phosphorous-sulfur species into phosphorous-oxygen compounds. This migrates the chemical shift of the by-products along the x-axis to the right of the spectrum. There are several products that are produced during operation:
O,O,S-trialkyldithiophosphate, (RO)2P(S)SR' (seen at 86 to 93 ppm) is a thermal rearrangement product that forms when the ZDDP (typically the neutral ZDDP) is heated above its decomposition temperature. This product is thermally more stable than the ZDDP itself. Large quantities of this by-product indicate thermal stress of the lubricant.
O,O,O-trialkyldithiophosphate, (RO)3P(S) (seen at 68 to 70 ppm) is another thermal rearrangement product of the ZDDP. It is not a major product, but typically forms after (RO)2P(S)SR' starts to decompose.
Monothiophosphate, (RO)2P(S)O-Salt (seen at 40 to 58 ppm), is produced from a further oxidative mechanism.
Acid phosphate, (RO)2P(O)O-Salt and (RO)3P(O) (seen at 10 to -8 ppm), are the final oxidation products before phosphate production, and are potentially precipitated out of the oil as zinc or calcium phosphate.
By plotting the concentration of each species as a function of sampling time (miles on the engine oil), both the rate and mechanism of degradation can be assessed (Figure 4).
Figure 4. Decomposition chemistry of a ZDDP. By tracking
the concentration of the different chemical species present
(using NMR) as a function of operating hours, the depletion
mechanism of ZDDP in in-service engine oil can be tracked.
In this example, the ZDDP additives deplete rapidly, in about 2,000 miles. However, equally instructive are the by-products that are formed. As seen in Figure 4, the initial degradation by-products are thermal degradation by-products.
Only after the by-products are themselves oxidized can the formation of oxidation by-products be seen, and ultimately phosphates, the final end product of ZDDP degradation, are observed. This type of plot shows that the severe stressing conditions this oil has been subjected to have caused the additives to exceed their ability to resist thermal stress.
Figure 5. Comparison of new (left) and used (right) sulfur phosphorus EP gear oils. By comparing the two gear oil samples (new and used), there are phosphorous additives that have depleted (components E, D and C), while there are components that have changed or been added to (components B and F). Following these chemistries will lead to a better understanding of the lubricants operation and its remaining useable life.
Like in the ZDDP example, a wealth of information can be obtained about sulfur-phosphorous gear lubricants using 31P NMR.2 These types of gear oils are typically formulated with several different types of phosphorous additives. The 31P NMR of new gear oil shows five major types of phosphorus species (Figure 5):
As the lubricant degrades in-service, the chemical composition of the additive changes, even though the P concentration in the ICP or RDE elemental analysis may remain unchanged (Figure 5). This is the value of NMR spectroscopy: not only does it indicate the occurrence of degradation, but by looking at the relative changes of the different chemical species formed, a clear understanding of the degradation mechanism can be determined.
Tracking additive depletion can be one of the most challenging aspects of used oil analysis. However for certain additives, and in particular those containing phosphorus, such as ZDDP, TCP and sulfur phosphorus NMR offer perhaps an unparalleled insight into both depletion rates and mechanism.
NMR is a fairly specialized technique requiring highly specialized equipment, usually beyond the capabilities of mainstay commercial oil analysis labs. However, several specialized labs offer NMR services on petroleum products at moderate costs ($100 to $300 per sample).
Understanding NMR Spectra
NMR spectra, such as those shown in Figures 2 and 3, plot concentration (on the y-axis) as a function of parts per million (ppm). However, unlike ppm used to determine concentrations of wear metals, or water in conventional oil analysis, ppm is used to indicate the shift in the NMR resonance frequency from a standard run with each sample. For proton (1H) NMR, the standard is usually tetramethylsilane (TMS). While for 31P NMR, it is usual to use phosphoric acid. The chemical shift can be calculated using the following formula:
Chemical shift (in ppm) = (RFS-RFTMS)/RFTMS x 106
Where RFS is the resonance frequency of the sample and RFTMS is the resonance frequency of the TMS reference. Some typical chemical shifts for 1H in hydrocarbon molecules are shown in Table 2.
The key to using NMR spectrometry in the analysis of hydrocarbon molecules is to recognize the chemical shifts observed in the NMR spectra and relate the relative concentrations of nuclei in these different environments. Because chemical shifts relate to the surrounding environment of the nucleus in question (for example, aromatic versus aliphatic hydrogen nuclei), recognizing the relative chemical shifts in ppm allows for the nature of chemical species present to be determined.