New technique: Analyzing nanoparticles in lubricants

Dr. Neil Canter, Contributing Editor | TLT Tech Beat April 2009

Originally developed to evaluate viruses in water-based systems, Nanoparticle Tracking Analysis measures and quantifies nanoparticles in oil. 

KEY CONCEPTS
• Nanoparticle Tracking Analysis has been developed to determine the type and quantity of nanoparticles in lubricant oil.
• Brownian Motion of the nanoparticles makes their analysis more challenging. Parameters such as viscosity, refractive index and temperature must be known to obtain accurate NTA.
• The accuracy of the NTA test, considered to be an important supplement to the QSA test, is better than +/- 5%.

Varnish buildup remains a constant threat to optimum lubricant performance over a long period of time. This material is formed through the polymerization and agglomeration of insoluble, oil degradation byproducts. These species are generated due to thermal, mechanical and electrostatic spark discharge stresses placed on the lubricant.

Varnish is amorphous and can be found in a range of colors and consistencies. It also consists of particles that are less than 1 micron in diameter. This makes analysis of varnish very difficult and problematic. In a previous TLT article, a colorimetric technique called Quantitative Spectrophotometric Analysis (QSA) was described (1). This method predicts the level of varnish in a lubricant system to an accuracy of +/- 10% (1).

STLE-member Gerald Munson, CLS, managing partner of Fluid Assets, LLC, in Madison, Conn., says, “There is need for a better technique to quantify the number of and size of nanoparticles in an industrial lubricant such as turbine oil. Analytical procedures such as PCS (Photon Correlation Spectroscopy of DLS, Dynamic Light Scattering) have been available to detect and evaluate nanoparticles, but they have limitations.”

PCS provides useful data on nanoparticles, but it has a significant weakness when particles of different sizes are present. Munson explains, “Large particles interfere with the ability of PCS to measure the size and amount of smaller nanoparticles.” In the real world of lubrication systems, as we all know, particles come in many different shapes and sizes.

Development of a technique that can more accurately measure smaller nanoparticles that are frequently contributors to lubricant and machinery failure is needed. Such a technique has not been available until now.

NANOPARTICLE TRACKING ANALYSIS
A technique just developed over the past few years by Fluid Assets measures and quantifies nanoparticles of many different sizes in oil. It is known as Nanoparticle Tracking Analysis (NTA) Model OM-30.

Munson indicates that NTA was developed for use in biological applications in water-based systems. He says, “The initial application of NTA is to evaluate viruses, which typically range in size from 10 to 100 nanometers.”

NTA measures the size and quantity of submicron particles through the use of video microscopy by tracking the Brownian Motion of particles in all types of liquids. The imaging is done by focusing a fine laser beam of light with a wavelength of 638 nanometers through a sample containing the nanoparticles. These particles are seen by an optical microscope and then analyzed with proprietary software.

Water has proven to be a fairly easy medium to use in NTA analysis. Munson says, “Water is very transparent, and a high contrast can be seen between clean and relatively contaminated water. The viscosity of water is also low, which helps with the analysis.”

In contrast, using NTA to do particle analysis in oils is much more challenging. Munson adds, “Viscosity and refractive index vary depending upon the oil. Temperature is also a critical factor to ensure that the results are accurate. The reason is that these parameters all affect the Brownian Motion of the nanoparticles. We have developed a technique to overcome these difficulties.”

On the higher particle size end, NTA is able to detect and quickly evaluate particles up to a size of 0.8 microns. Munson adds, “At particle sizes above 1-2 microns, the time required to track particles is too long to be practical.” This capability dovetails nicely with existing and new ASTM standards such as the F-312 patch test down to 0.5 micron and those being developed.

The accuracy of the NTA test is better than +/- 5%, according to Munson. From a precisions standpoint, additional testing is needed to evaluate a larger number of samples.

The ability of NTA to determine the type and quantity of nanoparticles in a specific turbine oil system is shown in Figure 2. The red curve shows the number of nanoparticles by quantity (count on the y axis) and size (on the x axis) for the system prior to being cleaned up. Afterward, the blue curve displays the reduction in the number of nanoparticles in the lubricant system over a broad range of particle sizes in one pass through a unique electronic kidney loop filtration system.


Figure 2. Nanoparticle Tracking Analysis (NTA) determines the type and quantity of nanoparticles in a specific turbine oil system, as shown in the curves. The red curve shows the nanoparticles prior to system cleanup, while the blue curve displays the nanoparticles left after filtration. (Courtesy of Fluid Assets, LLC)

Munson sees NTA as an important supplement for the QSA test. He adds, “We hope that the data generated by NTA will prove to be as important as results obtained from existing varnish tests.”

Munson is continuing to collect NTA data from mainly turbine oil systems to better understand the capabilities of the method. Future work will involve evaluation of lubricants formulated with other synthetic basestocks such as esters and polyalkylene glycols.

Further information can be obtained by contacting Munson at gmunson@fluid-assets.com. He will also be presenting a paper on NTA at the upcoming 2009 STLE Annual Meeting & Exhibition in May.

REFERENCE
1. Canter, N. (2006), “Strategies for Monitoring and Improving Lubricant Quality,” TLT, 62 (9), pp. 36–44.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.