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We're all aware of various advertising mediums flogging super-excellent lubricants with outrageous claims, that buying lots of them will solve all your lubrication woes. Just take a look around your local automotive or hardware store. Does anybody buy that stuff? Sure they do! Check out the costly floor space allocated to displaying these magic lubricant potions. Check out the infomercials. If nobody bought them, they wouldn't be there, and advertising is not cheap. Do the products work? That's another story, and the answers can be highly subjective.
This article addresses the introduction and methodology of spectral analysis.
In the field of lubricant analysis, condition monitoring is a general term used to describe analysis of in-service lubricants. Fourier transform infrared (FTIR) spectroscopy is an analytical tool that may be used to indicate the quality of a particular fluid before, during and after its designated functional service life.
Historically, lubricant testing was accomplished by color, taste and smell. Modern lubricant data is collected and processed ensuring accurate, reproducible data in accordance to relevant industry recognized standards, such as one prescribed by the American Society of Testing Materials International (ASTM), The American Petroleum Institute (API), The Society of Automotive Engineers (SAE), and others. Only the strict adherence to respected peer-reviewed test procedures should be used to verify the validity of these so-called "super excellent lubricants/lubricant additives". Anecdotal claims without scientifically proven facts are just a marketer's way of fooling the public to make a profit.
Of the many different technologies available to the modern tribologist, spectroscopy has always played an important role in both lubricant formulations as well as in-service lube analysis. The detailed information on the chemical composition it provides is reliable, repeatable and nonsubjective. Although there are many types of infrared (IR) spectrometers employed in the lubricant industry, FTIR spectroscopy instruments are some of the most widely used for formulation and in-service lube analysis.
Measuring the chemical composition of a lubricant, FTIR spectroscopy is used as a condition monitoring tool with its inherent advantages over conventional IR instrumentation including simpler instrumentation, faster spectral acquisition, and spectral data manipulation capabilities.
Molecular analysis of lubricants and hydraulic fluids by FTIR spectro- scopy produces direct information on molecular species of interest, including additives, fluid breakdown products and external contamination.2
Table 1 presents the parameters measured using FTIR spectroscopy.3
ASTM International has standardized analytical procedures designed to determine condition monitoring parameters associated with more common conventional lubricant quality. The ASTM sub committee D02.96 is currently writing new analytical standards for in-service lubricants using FTIR spectroscopy. Although generally applicable to petroleum hydrocarbons,4 only the FTIR condition monitoring of petroleum hydrocarbons is relatively well-defined. The methodology becomes more tenuous as synthetic oils, esters and glycols come into play.
Figure 1 shows a spectral snapshot of the "fingerprint region" for most common in-service mineral oil lubricants. The defined spectral regions (on the horizontal axis) for performance additives hardly change from one lubricant to the next. The amount present, however, will affect the amplitude of the peaks (on the vertical axis). In lubricant analysis, it is normal for additives to deplete over time, while new bands will be created relative to the oxidative state of the lubricant.
One of the most versatile advantages of FTIR spectroscopy is the fact that spectra themselves can be added or subtracted from each other. For example, if we use the sample spectra in Figure 1 as a (a) "benchmark" oil and then subtract (b) the "candidate" oil (which, in this case, is the same as (a)), we will generate (c) a "flat-line" Figure 2. The resultant flat-line (c) proves (b) to be a perfect clone of (a).
In reality, this would be difficult to accomplish, as any "blip" in (c) will indicate a differential of a component succinctly identified by its horizontal axis position and its relative quantity depicted by the vertical axis amplitude - on the positive side or negative side of the horizontal "flat-line". Lubricant manufacturers may use this methodology as part of their quality control system where (a) will be stored in a data library as the targeted lubricant, (b) will be the production lot manufactured and (c) will indicate any components deviating from the original formulation.
In this context, the FTIR spectrometer has become increasingly prominent in lubricant identification and analysis. Its power is based on the fact that specific molecular functional groups absorb in unique regions of the mid-infrared spectrum, allowing identification of base oils, additives, contaminants and breakdown products.
Consider an aftermarket lubricant additive (ALA) marketed as a metal conditioner (even though this product is added to a lubricant), which retails for approximately $85.00 for an 11-ounce bottle and claims the following performance benefits:
Provides a lubricity between moving components, which improves tolerances to their correct degree and increases efficiency with less wear.
Penetrates metal to form a tough, slippery, residual coating to all friction points.
Eliminates the rapid oxidation of motor oils.
Prevents buildup of engine contaminants and actually prepares deposits to be entrapped by the motor oil filter.
Gas mileage may be increased by as much as eight to ten percent with long- term use.
Circulates throughout the oil system to seal costly and dangerous leaks. Reaches all gaskets and increases their seal protection and service life.
An initial hypothesis was formed based entirely on the performance benefits of the metal conditioner listed above:
Definitely contains oil, of which adequate amounts are already present in the motor oil.
Possibly contains an ester (a polar molecule that adheres to and wets out metallic surfaces) and also possibly polytetrafluoroethylene (PTFE), also known as Teflon® because:
Definitely contains a detergent/dispersant. Adequate amounts are already in the motor oil. It's the job of the filter to eliminate the dispersed contaminants. Nothing unusual about this.
The esters will wet out and penetrate seals. The product will circulate with the oil and reach wherever the oil is supposed to reach. Esters will inherently perform the following:
An FTIR spectrum was taken of the metal conditioner and compared to the spectral library of lubricant additives and lubricants in Figure 3. Using UMPIRE® software, the program was able to identify components of the metal conditioner on the basis of their spectral fingerprints.
In Figure 3, specific regions in the spectrum of the metal conditioner are indicated in #D. Note the peaks labeled phenol antioxidant, ester and halocarbon peaks. These conform to the hypothesis above.
Spectrum #A depicts a commercially available antioxidant which closely matches the peak of #D.
Spectrum #B depicts a commercially available colloidal suspension of PTFE in mineral oil and ester. Notice how the spectrum looks like #D.
Spectra #C is an added spectrum of one percent #A and #B.
Dispersants/detergents, rust inhibitors, defoamers, etc. were also found using the same methodology and were added to the spectra.
It can now be observed that the fingerprint of #D may be successfully reproduced. A rough cost estimate indicates the material blended in the 11-ounce container is approximately $1.25. However, most of the ingredients in the bottle are already present in premium motor oil and are functioning in their designated jobs. If we eliminate the duplication, then this 11-ounce container actually contains approximately $0.50 worth of product, which in essence is useless and perhaps detrimental to the engine itself.
Given the power of FTIR spectroscopy, one has to question why it is not used more often and more effectively. IR spectroscopy is a powerful tool, simply because it can provide substantial information about oil condition using a single instrument. Indications about the state of oxidation, nitration and sulfation and levels of soot, moisture, glycol and various additives, among others, are available.
Remember, beauty is only skin deep. You have to look behind the label and inside the bottle to understand the true meaning of a lubricant.
1. J. Coates and L. Setti. "Infrared Spectroscopy as a Tool for Monitoring Oil Degradation." Aspects of Lubricant Oxidation. ASTM STP 916, W.Stadtmiller and A. Smith, Editors. American Society for Testing and Materials. Philadelphia, 1986.
2. U.S. Department of Energy Efficiency and Renewable Energy. O&M Best Practices Guide, Release 2.0 (Chapter 6).
3. ASTM Standard Practice for Condition Monitoring of Used Lubricants by Trend Analysis Using Fourier Transform Infrared (FTIR) Spectrometry.
4. R. Shubkin. Synthetic Lubricants and High-performance Functional Fluids. Marcel Dekker, Inc. New York, 1992.
5. UMPIRE® proprietary software is a registered trade mark of Thermal-Lube Inc.