Thin Film Colorimetric Spectroscopy of Greases

2010 STLE Annual Meeting & Exhibiti

Thin Film Colorimetric Spectroscopy of Greases

2010 STLE Annual Meeting & Exhibition

May 16-20, 2010

Bally’s Las Vegas Hotel and Casino

Las Vegas, Nevada, USA

AUTHORS AND INSTITUTIONS

Dave Wooton, Wooton-Consulting, Beaverdam, VA

Lisa Williams and Eric Straub, MRG Labs, York, PA

INTRODUCTION

New techniques have been developed involving optical spectroscopy and colorimetric determination of new and in-service greases. Utilizing a handheld visible spectrometer, a patent pending spectroscopy box and a thin-film grease substrate, a visible light spectrum (400 nm to 700 nm) can be gathered to measure the transmittance values of new or in-service greases. This method is useful in a variety of applications that involve both new and used greases. Contrary to old lubricant formation, grease manufacturers now formulate grease using a wide variety of vibrant colors. The wide range of colors used leads to several different applications for this technique. The characterization of grease samples to both differentiate between new fluids and follow in-service behavior is an important part of the condition monitoring program. Understand these color differences and behavior and how they are being modified can lead to a better condition monitoring program. This paper is designed to introduce the application of visible spectroscopy to the analyst’s tool-box. These applications currently include but are not limited to batch color confirmation, in-service grease comparison, and chemometric determination of particulate in new grease supplies.

ABSTRACT

A method for analyzing used greases is presented that relies on a thin-film deposition of the grease on a transparent substrate. The transparent substrate is introduced into a visible light pathway, and is analyzed by a spectrometer in the 400-700nm wavelength. New fresh greases are evaluated in this method, and used grease samples are compared for analysis. The method allows for a colorimetric evaluation of changes in the appearance of the grease, and peak analysis is performed to determine grease mixing, oxidation changes, and the presence of specific contaminants. Experimental works with known quantities of contaminants are presented for the development of chemometric methods for quick and low-cost screening of grease samples.

CURRENT CHEMICAL CHARACTERIZATION METHODS FOR IN-SERVICE GREASE

The Current methods for the chemical characterization of in-service greases include visual inspections, FT-IR, RULER, consistency testing, metals spectroscopy, and ferrous debris monitoring techniques. The results of these tests are compared to baseline criteria to determine the condition of the sample. This allows the analysts the ability to evaluate the in-service grease according to its performance in four key areas: wear, consistency, contamination and oxidation.

Visual inspections are often performed by the maintenance personnel in charge of the machinery. It is often first noted when a significant visual change has occurred such as the wrong color lubricant, hardening or softening of the grease, or large amounts of oil exiting the bearing. In these cases the grease is then sampled and sent to a lab for analysis.

The first of the four criteria that is analyzed is consistency. There are three tests that are being used to determine this critical property:

The Cone Penetrometer test is accurate in full scale and ? scale testing the latter of which requires a ? pound of grease [2]. For in-service greases this amount of sample is typically not present in the bearing, making it difficult to properly perform this test. In addition, rheometer testing is a highly accurate instrument that can measure the consistency of grease as well as many of its other visco-elastic properties. This test is often not able to be used due to the large amount of operator experience required, extensive interpretation of the data and the long analysis cycles. The Grease Thief Analyzer delivers a consistency measurement with 1 gram of sample. By creating a thin film substrate, the Grease Thief Analyzer is also used to prepare the sample for thin film colorimetric spectroscopy of greases.

The next method is FT-IR spectroscopy. FTIR yields the chemical information about the fluid. It is used to generate a chemical fingerprint of the fluid that can be used to differentiate fluids as well as define the in-service chemical behavior of the fluid. Fluid oxidation is typically determined by the use of FTIR spectroscopy.

The RULER test utilizes Linear Sweep Voltammetry to determine the amount of antioxidant remaining in a sample to control the fluid’s oxidation [3]. The methodology for running grease samples using RULER are currently under review with ASTM. Trending the amount of anti-oxidant remaining in the grease not only allows for trending of the remaining useful life of the anti-oxidant to protect the fluid, but also allows for determination of the proper re-lubrication cycle for the equipment.

Metal spectroscopy and ferrous debris monitoring techniques are used to evaluate the wear and contamination ingress. There are many ways to determine the elemental content of in-service greases – including Rotating Disc Electrode spectroscopy, Inductively Coupled Plasma spectroscopy, Atomic Absorption spectroscopy, and X-Ray Fluorescence Spectroscopy. These metal spectroscopy techniques are used to detect mixing of different thickeners, wear metals present and anti-oxidant elements remaining by determining the elemental make up of the samples. Ferro-magnetic material is detected using ferrous debris monitoring instrumentation. A Hall Effect type sensor is used to measure disturbances in the electromagnetic field caused by magnetic particles – allowing a quick and non-destructive way to analyze the total ferro-magnetic content in a sample [4].

COLOR CHARACTERIZATION METHODS

Color Characterization Methods are an extension of the spectroscopic analyses like FT-IR. Color of a sample is actually the spectroscopic response within the visible range (400-700 nm) which is seen by the human eye. When one expands the color measurement to that measured by a spectrophotometer, one now has another important fingerprint characterization method.

One can study the color spectrum using the tristimulus values of the CIE LAB measurements to summarize the spectrum into three variables representing what the human eye can see. This color representation contains three variables (L*, a*, b*), which are opponent-color scales, in which a* is positive in the red direction and negative in the green direction; b* is positive in the yellow direction and negative in the blue direction; and L* is the gray scale being more positive in the lightness direction and closer to zero in the darkness direction. One can develop an assignment to these tristimulus colors and the new fluid’s color properties. Using this, the new fluid can be color characterized.

Since the full color spectrum is also available, studying the changes in this spectrum will allow even more characterization. The progression from new to in-service life can be followed with spectral changes. For example, the ingress of metals has the tendency to promote gray and black colors that yield a recognizable color spectral behavior.

Grease Color Examples

Greases are known to come in an array of colors, depending upon application and manufacturer. Proper characterization of this relationship allows another parameter to add to characterization. Cross-contamination, improper fluids and contaminations can often be identified through the fluid’s spectral information. Below are a couple of clear examples to show this fact.

Correlation between Sample and Color Measurements and Potential Uses

In-Service In service fluid has been shown to progress toward gray/black coloration, partially due to the ingress of wear metals and fluid oxidation/degradation. The color spectrum can easily distinguish these changes.

Freshness of the samples can therefore be assessed as to the extent the color has progressed to its gray/black coloration – or end of fluid life.

Chemometric techniques are used to evaluate the particulate contamination of new and in-service greases. Facilities can use this technique to evaluate the presence of a common contaminant, such as coal dust in a coal-fired power plant. By enacting contaminant target levels, and utilizing the colorimetric method to quantify stored new greases or in-service greases, the same life-extension benefits seen in oil cleanliness efforts can be pursued.

Colorimetric Spectroscopy is versatile test that can aid in the improvement of quality control programs for grease manufacturers and provide additional information regarding contamination of new and in-service greases. This 3

test can quantify and confirm deviations from baseline values and can bring validity and peace of mind to visual inspectors who see color variations from batch to batch. The non-destructive nature of the test also permits a cost effective analysis solution that laboratories can easily incorporate into their grease analysis package

REFERENCES

[1] Annual Book of ASTM Standards, "D1403-02" (American Society for Testing and Materials, West Conshohocken, PA, 2000).

[2] Annual Book of ASTM Standards, "E308-08" (American Society for Testing and Materials, West Conshohocken, PA, 2000).

[3] Fluitec International. RULER Users Guide. 2003. Print.

[4] Kittiwake. AnalexfdMplus User Manual. Print.

KEYWORDS

Grease, In-Service, Color, Visible Spectroscopy

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