Precision lubrication is a simple concept - at least at face value. Simply put, it requires the right lubricant, be it oil or grease, to be put in the right place, at the right time; that the lubricant and equipment it is lubricating be kept clean, dry and cool; and that rotating equipment be kept appropriately aligned and balanced.
While sound lubrication engineering principles can be used to ensure that lubrication selection is appropriate, used oil analysis’ role, to a large extent, is to ensure that the properties for which the lubricant was selected are still intact and appropriate for the stated application.
Many of the important properties of a lubricant are dependent on the base oil. Its ability to support a dynamic load, its ability to control and dissipate heat and its ability to provide motive power in hydraulic systems are just a few of the important properties. However, in many instances, it is the additives in the lubricant that are just as important - in some instances, almost more important - than the base oil in providing appropriate lubricant functionality. Despite this, many oil analysis programs focus simply on measuring base oil degradation, using tests such as viscosity and acid number, and fail to address the condition of the additives.
Trending Additive Depletion through Used Oil Analysis
Perhaps the simplest used oil analysis tool for monitoring additives is elemental analysis. Using either rotating disk electrode (RDE) or inductively coupled plasma (ICP) spectroscopy, elements such as zinc, phosphorus, calcium, barium and magnesium can be measured and trended to provide an indication of additive concentration. However, elemental spectroscopy has two major limitations with respect to tracking additives. First, the technique does not actually measure additives, but rather the individual elements or atoms contained within the additive molecule. While this comment may seem obvious, it has serious implications when talking about trending additive depletion.
To understand the potential problem, consider the fate of one of the most common additives, zinc dialkyl dithiophosphate (ZDDP), an antiwear and antioxidant additive. Depending on formulation, a common AW hydraulic fluid may contain anywhere from 100 ppm to 500 ppm of ZDDP, as measured by the elemental concentrations of zinc and phosphorus. Subjecting an oil containing ZDDP to high temperatures and high levels of moisture will likely result in significant additive depletion due to hydrolysis - a chemical reaction between the ZDDP molecule and water. Under such circumstances, the ultimate by-products of the hydrolysis reaction will likely be zinc salts and phosphates, which although no longer chemically ZDDP, may remain in solution in the oil. The result is that by considering only zinc and phosphorus concentrations, the difference between “good” zinc and phosphorus in the form of ZDDP and “bad” zinc and phosphorus from reaction by-products will be next to impossible to determine.
The second limitation of using elemental spectroscopy for tracking additive depletion is perhaps even more fundamental. Many common additives such as antioxidants, dispersants, VI improvers and some antifoam additives are organic molecules. Simply stated, an organic additive molecule contains carbon, hydrogen and perhaps oxygen, nitrogen or sulfur. Because none of these elements are routinely detected using elemental spectroscopy, ICP or RDE offers little or no help in monitoring organic additive health.
Molecular Spectroscopy vs. Atomic Spectroscopy
Elemental spectroscopy is sometimes termed atomic spectroscopy. The reason is obvious - a spectroscopic tool is used to determine the concentration of atoms in the oil sample. Molecular spectroscopy by contrast uses similar principles of chemical physics to atomic spectroscopy, but determines molecule concentrations not elemental concentrations. This has some distinct benefits for tracking additives. First, because molecular concentrations are being measured directly, additive decomposition, like the hydrolysis of ZDDP cited above, become immediately apparent. Secondly, many molecular spectroscopic tools are well-suited for determining concentrations of organic molecules.
The term molecular spectroscopy covers a multitude of analytical techniques. Some tools such as Fourier transform infrared (FTIR) are common; others such as nuclear magnetic resonance (NMR), gas chromatography in conjunction with mass spectrometry (GC-MS) and liquid chromatography in conjunction with FTIR (LC-FTIR) are perhaps not so common. A detailed discussion of some of these molecular spectroscopic tools can be found in a recent ASTM publication.1
The following discussion provides an overview of some of the analytical tools and issues related to using molecular spectroscopy to monitor additive condition of in-service oils, helping to illustrate the value of molecular spectroscopy in trending additive depletion.
In most engine oil formulations, the additive present in the highest concentrations are dispersants, typically present in the 3 percent to 6 percent by weight range. Dispersant additives are designed to hold sludge and other contaminants such as soot in suspension, until they can be filtered out, or otherwise removed through an oil change. The chemistry of a dispersant consists of a polar head group with a long hydrocarbon tail. This acts something like laundry soap, trapping the dirt into what is commonly called micelle (Figure 1).
Figure 1. Micelle Action of a Crankcase Dispersant Additive
The basic functionality of the dispersant additive is simple. As the dirt, soot or sludge enters the oil, it is trapped in the core of the dispersant’s micelle. This trapping action prevents contaminants from depositing on engine parts like rings and valves and causing premature equipment failures.
The most common chemistry of a dispersant is a polyisobutylene succinimide. The polyisobutylene’s molecular weight is greater than 1,000 and forms the hydrocarbon tail. The succinimide portion of the molecule contains a polyamine and forms the polar head. In the new oil, this additive can be observed in the FTIR spectrum (Figure 2).
Figure 2. FTIR Spectrum of New Oil
This FTIR spectrum allows the analyst to study both the succinimide’s carbonyl bands of the polar head-group, as well as the polyisobutylene’s hydrocarbon bands of the tail group. The strong acid-base number (BN) method (ASTM D2896) allows the amine portion of the polar head group to be analyzed.1
In an engine oil formulation, the next most common additive is the detergent. This additive is typically present in the oil at about 2 percent to 3 percent. Detergents are used primarily to control acids formed by the combustion of impurities found in the fuel. However, it does have some ability to “wash” the metal surfaces of organic deposits. There are several types of detergents that are used in these formulations. The most important of these are sulfonates. The chemistry of the sulfonate is somewhat similar to the dispersant in that it has a hydrocarbon tail and a polar head. The hydrocarbon tail is much shorter, being only a C-16 to C-30 alkyl benzene. The polar head is the salt of a sulfonic acid, typically calcium, magnesium or sodium. In addition to this neutral sulfonate salt, the detergent typically contains an excessive amount of metal carbonate, which is incorporated in the center of the micelle. This carbonate contributes much of the oil’s BN and is used to neutralize acids that enter the oil during use. This additive’s functionality can be observed in the new oil by measuring the oil’s BN. There are two types of BN that are defined by ASTM: strong acid-BN (ASTM D2896), which uses perchloric acid as the titrants, and the weak acid-BN (ASTM D4739), which uses hydrochloric acid.2 The strong acid-BN method measures both the dispersant and the detergent, while the weak acid-BN method measures primarily the sulfonate’s carbonate. These BN methods are effective for trending detergent health in used oil analysis. ICP and FTIR can also be used to follow the additive by monitoring calcium, magnesium or sodium concentrations, by ICP and the sulfonate-detergent’s S=O bands and its carbonate’s CO3 bands using FTIR (Figure 2).3
ZDDP is perhaps the most common additive in many different classes of lubricants, from engine oils to antiwear hydraulic fluids. The initial concentration of ZDDP in any oil can be determined by measuring zinc and phosphorus levels using ICP. However, for the reasons stated earlier, ICP should not be used to monitor in-service ZDDP additive depletion. In-service ZDDP depletion is best measured by FTIR. In the FTIR fingerprint region, there are several infrared stretching vibrations that are clearly observed in the oil (Figure 2). These are the P-O-C bond (seen between 950 cm-1 and 1020 cm-1) and P=S (seen between 640 cm-1 and 665 cm-1).
Figure 3. 31P NMR Spectrum of New Crankcase
Oil Showing the ZDDP Chemistry.
Perhaps the best analytical tool to study ZDDP and its decomposition is NMR spectroscopy.4 This method uses a complex procedure (too detailed to outline here) to measures the chemical environment and chemical structure of atoms and, more specifically, the nucleus of the atom contained within a molecule.1 There are a number of nuclei which can be studied by NMR, the most common being hydrogen (1H) and carbon-13 (13C). For ZDDP, phosphorus, specifically 31P, can be used to help evaluate the chemical environment of the phosphorus atoms present in the additive molecule. Because NMR is sensitive to the chemical environment surrounding the nucleus in question, 31P-NMR can be used to monitor changes in the additive molecule during the in-service life of a ZDDP additized oil.
Antioxidants, such as di-t-butyl phenols and diaryl amines, are found in most lubricants. They are used to control the oxidation of the base oil and other additives in the formulation during use. Antioxidants are typically present in fairly low concentrations (typically 1 percent or less) making them more difficult to follow by conventional techniques. Both the phenolic and aromatic amine can be observed in the new oil by FTIR (Figure 4).
Figure 4. FTIR Spectrum of New Oil with
Phenolic and Amine Antioxidants.
However, as their concentration decreases due to use, antioxidants become increasingly difficult to detect, making it necessary to utilize more sophisticated methods like gas chromatography (GC), high-performance liquid chromatography (HPLC) or gas chromatography-mass spectroscopy (GC-MS).1
The following examples illustrate the value of using molecular spectroscopy in combination with more conventional wet chemistry techniques such as BN to monitor additive health.
Used oil analysis methods today often include characterizing several infrared spectroscopic frequencies. One of these is the P-O-C absorption for the ZDDP or phosphorous antiwear. It is observed in the 1025 cm-1 to 960 cm-1 region. In a crankcase lubricant, whether the oil is from a passenger car or heavy-duty diesel engine, this additive component tells a great deal about the lubricant’s life expectancy. It is depleting from both antioxidant and antiwear pathways. Thus, its concentration reflects both of the lubricant’s functions.
Figure 5. Crankcase ZDDP Depletion.
Figure 5 shows an example of this additive depletion rate. The oil in this example has been used beyond its additive’s useful lifetime. As a result, significant increases in the wear metals were observed during the last few miles of the application.
The chemistry of the phosphorous additives is complex. Some of this complexity cannot be observed in its infrared spectrum and requires more detailed study using 31P-NMR.
Figure 6. 31P NMR Data for ZDDP's Depletion.
Figure 6 shows several of the by-products associated with the depletion of a ZDDP in a crankcase application.
Figure 6 illustrates how ZDDP depletes reasonably early in this application; however, there are still several intermediate components that are formed which last well into the oil’s lifecycle. Some of the thermal and oxidative decomposition by-products still exhibit the antiwear or antioxidant properties of the original additive. The final decomposition product, phosphate, starts to appear in this used oil about one-third into the lifecycle. It was shown to grow to a steady state concentration through the remainder of the oil’s life.
As with the ZDDP chemistry, the phosphorous chemistry of gear oil is also best analyzed by 31P NMR (Figure 7).
Figure 7. 31NMR Spectra of New and Used Gear Oils.
This figure shows two spectra, one of the new oil and one of the used oil. Between these two samples, there are resonance peaks due to phosphorous additives that have decreased (components E, D and C), indicating additive depletion, while there are other features in this spectrum that have either changed or appeared (component B and F). Quantitation of these changes in additive concentration can be related to the performance of the oil.
Overbase Detergent Depletion
BN of used engine oils can provide information about the detergent and the dispersant. As discussed earlier, the detergent contains reserve-carbonate to react with the acids being introduced into the oil. The BN from this carbonate is a direct measure of the concentration of this carbonate - the base reserve of the oil. However, the carbonate can also be measured from the FTIR spectrum as shown in Figure 2.
Sulfur-Phosphorus EP Gear Oil Degradation
Figure 9 shows the infrared spectra of new and in-service sulfur-phosphorus additized gear oil. From the two spectra, it is obvious that there are significant observable changes in absorbance peak at 1112 cm-1, 893 cm-1 and 814 cm-1. These peaks are associated with the extreme-pressure/antiwear additive - a sulfurized isobutylene. In this case, measuring changes in one or all of these regions can be used to quantify the condition of the oil’s EP additive, allowing for a condition based oil change, before the EP performance of the oil is compromised.
Figure 8. The Correlation Between the FTIR Carbonate
Absorbance, and the Two Commonly Used BN Wet Chemistry
Methods, ASTM D2896 and ASTM D4739.
Figure 9. FTIR Spectra of New and Used Gear Oil Samples
Tribochemistry - the reaction of molecules during in-service lubricant usage - is a complex subject. Depending on circumstance and application, many reactions can occur that produce new components as well as destroy the formulation additives.
However, by applying advanced analytical spectroscopic tools like FTIR, NMR and chromatographic techniques, science offers an unprecedented window into the fate of additive molecules throughout their in-service life.