Rapid Quality Control for Lubricating Oils: Fast, Chemical-Free Analysis of Acid Number, Viscosity, Color Number and Moisture with Vis-NIRS

Lubricating oils are vital for the efficient operation of machine components and ensuring smooth workflow. Because these oils have limited service life, regular inspection and renewal at set intervals is an unavoidable requirement in industrial applications. Changing such oils before the required time can result in major costs in industry, and sometimes the quality control analyses that need to be performed become far more costly than the oil change itself. Accepted quality control analyses for lubricating oils typically require multiple analytical methods, specific instrumentation, sample preparations, trained personnel and time-consuming analytical procedures. Routine analyses that can be performed quickly and easily on lubricating oils using Vis-NIR spectroscopy can help extend oil change intervals significantly and reduce the high cost burden resulting from conventional analyses. Quality control analyses performed with Vis-NIR systems also increase laboratory efficiency while allowing minimization of the investment required in multiple analytical instruments. Within this review, we aim to examine closely the analyses performed in quality control of lubricating oils and present detailed information about the advantages that Near-Infrared Spectroscopy (NIRS) techniques can provide in these analyses.

Near-Infrared Spectroscopy (NIRS)

Near-infrared spectroscopy (NIRS) is one of the molecular spectroscopy methods and enables monitoring of measurement parameters in various quality control analyses in a more practical and faster manner. NIRS is a non-destructive technique that requires no sample pre-treatment or chemicals, making it more environmentally friendly and economical compared to wet chemistry methods. Compared to conventional wet chemistry methods, near-infrared spectroscopy (NIRS), when proper method development is provided, can measure multiple parameters precisely and accurately with analysis times under 1 minute. [caption id="attachment_124268" align="aligncenter"] Table 1. Result acquisition times by parameters with conventional techniques[/caption]   Calibration development procedures using near-infrared spectroscopy are clearly specified in different standards. Examples of these standards include ASTM E1655[2] and ASTM D6122[3], and the methods can be used directly in applications as long as reference values are collected from primary method reference analyses recognized in the standards. For example, when acid number reference values obtained through potentiometric titration, which is a standard test method for petroleum products, are used in NIRS calibration development, NIRS can also be accepted as a reference method like titration or viscometry. Typical quality control parameters of importance in lubricating oils and the reasons for monitoring them can be briefly summarized as follows: Acid Number (AN): Acid number appears as a measure of both weak organic and strong inorganic acid contamination in the oil. Acidic contaminants, incorrect oil type, depletion of alkaline reserve and oxidation byproducts can cause lubricant degradation. Acid number is typically determined by performing potentiometric or coulometric titration under ASTM D664. Viscosity: Lubrication capacity and oil condition depend essentially on viscosity. Inadequate lubrication leads to increased friction, wear and failures important due to heat rise. Any significant change in viscosity requires rapid action and in this context must be monitored in routine oil analyses. ASTM D445 describes the conventional analytical method applied for kinematic viscosity. Moisture: Moisture in lubricating oil eliminates oil film strength and causes erosive wear. Even relatively low moisture concentrations (<500 ppm) can cause significant damage. Moisture content is conventionally determined according to ASTM D6304. Color Number: In addition to aging, oxidation and contamination, excessive heating and filter problems usually manifest themselves through changes in color. Color number analysis is conventionally performed with a colorimeter according to ASTM D1500.

Configuration

In our review, we evaluate acid number, viscosity, moisture and color number analyses performed using a Metrohm NIRS XDS RLA system on 260 different lubricating oil samples collected from different manufacturers and having different service lives. [caption id="attachment_124269" align="aligncenter"] Figure 1. Metrohm NIRS XDS Rapid Liquid Analyzer[/caption]   With Metrohm's XDS Rapid Liquid Analyzer (XDS RLA) system, which combines visible (Vis) and near-infrared (NIR) techniques, it is possible to perform non-destructive, chemical-free, fast and multiple quality control parameter analyses without any sample pre-treatment.

Experimental Content and Workflow

All samples were placed in 8 mm single-use vials without any additional sample preparation as shown in Figure 2, and measurements were taken with the Metrohm NIRS XDS RLA system in transmission mode, covering the entire Vis-NIR wavelength range (400-2500 nm). [caption id="attachment_124270" align="aligncenter"] Table 2. Metrohm NIRS XDS RLA equipment and software content[/caption]   Analysis temperature was set at 40°C to ensure kinematic viscosity measurements were taken at similar temperatures, and a 30-second analysis delay condition was added to the measurement procedure to allow the sample to reach thermal equilibrium. The Vision Air 2.0 Complete software package was used for data collection, model building and quantification method development processes. [caption id="attachment_124271" align="aligncenter"] Figure 2. Image of lubricating oil samples measured in transmission mode with Metrohm NIRS XDS RLA.[/caption]  

Method Development

Quantitative method development for the four selected parameters was performed using Vision 4.1 (Metrohm chemometrics software) contained within the Vision Air Complete package software. Data was pre-processed using first or second derivative based on where the best reliability data (Figures of merit/FoM) were obtained, and models were developed using the Partial Least Square Regression (PLSR) algorithm with different wavelength ranges. Although NIRS XDS RLA allows spectral data collection from 400 nm to 2500 nm, PLSR work was performed using specific spectral ranges. For example, water bands (900-950 nm, 1350-1450 nm and 1850-1950 nm regions) were used for determining moisture content, while the 400-600 nm region in the visible range was used for determining color number. For acid number and viscosity, the NIR wavelength range of 1120-2480 nm was used. The data set was divided into one calibration set and a validation set to verify the performance of the created quantitative models. Because fewer samples were available for color number and moisture content quantification models, validation was performed using the cross-validation function of Vision software.

Analysis Results

Typical Vis-NIR spectra obtained from lubricating oils (see Figure 3) were used in developing the predictive models required for quantitative determination of acid number, viscosity, moisture content and color number parameters. [caption id="attachment_124272" align="aligncenter"] Figure 3. Typical raw Vis-NIR spectral images of selected lubricating oils (Offset has been applied to the spectra due to image adjustment.)[/caption]   The quality of the predictive models was evaluated through correlation graphs showing the relationship between values obtained from primary methods and Vis-NIR data, and is clearly shown in Figures 4, 5, 6 and 7. This review demonstrates that key quality control parameter analyses in lubricating oils can be performed with Near-Infrared Spectroscopy (NIRS) technique as precisely as reference analytical methods while being faster and more economical. With NIRS technique, which can perform analyses of multiple analytical instruments simultaneously, quickly and in a more environmentally friendly manner, you can increase workflow efficiency in your quality control laboratory and achieve significant savings in analysis costs. As Metrohm, in addition to being the global sector leader in reference methods (titration & Karl Fischer titration) that form the foundation of NIRS analytical techniques in the petrochemical sector, we also provide solutions with our Vis-NIRS systems compatible with international standards to deliver fast, precise and reliable results.  

References 1. Metrohm Application Note AN-NIR-041 Quality Control of Lubricants 2. ASTM E1655-05(2012), Standard Practices for Infrared Multivariate Quantitative Analysis, ASTM International, West Conshohocken, PA, 2012, www.astm.org 3. ASTM D6122-15, Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems, ASTM International, West Conshohocken, PA, 2015, www.astm.org