24th International Colloquium Tribology - January 2024 201 Go Greener by In-situ Characterization of Lubricants for Cold Rolling - Droplet Size Distribution and Physical Separation/ Emulsion stability Arnold Uhl 1* , Stefan Küchler 1 , Sylvain Gressier 2 , Titus Sobisch 1 1 LUM GmbH, Berlin, Germany 2 LUM France, Plaisir, France * Corresponding author: info@lum-gmbh.de 1. Introduction A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. Typically, lubricants contain 90% base oil (most often petroleum fractions, so-called mineral oils) and less than 10% additives. Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters, silicones, fluorocarbons and many others are sometimes used as base oils. Additives deliver reduced friction and wear, increased viscosity, improved viscosity index, resistance to corrosion and oxidation, aging or contamination, etc. [1] There are non-liquid lubricants and dry lubricants available, but they are not subject to this talk. A good lubricant generally possesses the following characteristics: • A high boiling point and low freezing point (in order to stay liquid within a wide range of temperature) • A high viscosity index • Thermal stability • Hydraulic stability • Demulsibility • Corrosion prevention • A high resistance to oxidation. [1] Based on the above, lubricants for cold-rolling are formulated e.g., as oil-in-water (o/ w) emulsions. Instrumental methods for the analytical characterization of emulsion stability/ demulsibility and for particle/ droplet characterization become more and more popular offering a greener approach for the analysis compared to conventional methods based on chemical analysis. This talk presents three case studies, where the patented STEP-Technology ® ( S pace and Time resolved measurement of Extinction Profiles) was applied in combination with accelerated stability/ separation testing. It allows for the quick and reliable characterization of lubricants requiring much less sample volume and avoiding expensive and environmentally unfriendly cleaning of tools and apparatus. 2. STEP-Technology ® STEP-Technology has been developed and patented by LUM GmbH and allows for the in-situ visualization of nanoand microparticle movement using different optical wavelengths, e.g., invisible near infrared (NIR), visible red and blue wavelengths as well as the use of invisible X-radiation. The sample is filled into a closed sample cell of appropriate cell geometry based on sample properties. Then the sample cell is positioned between the light or radiation source and the corresponding detector. Light or X-radiation is sent to the sample simultaneously from top to bottom and the transmitted light or radiation is recorded, again simultaneously from top to bottom at preset time intervals. This can be done either at earth gravity to investigate the real-time separation or at higher gravity for accelerated testing, using relative centrifugal acceleration (RCA) to physically accelerate the particle movement with a predefined multiplicator of earth gravity. The experiments are done either at a constant preset temperature in the range from 4 to 80°C or even in some instruments with a temperature ramp. For further technology details and applications see [2] and the references therein. The three case studies were performed at higher gravity and at a constant temperature, being different for each case. The NIR wavelength 870 nm was used. 3. Case studies 3.1 Accelerated testing vs. conventional The accelerated stability testing of o/ w emulsions within few minutes by using a single analytical instrument, based on ISO/ TR 13097 [3], is compared with a rather complex emulsion stability index (ESI) determination lasting for several hours and having a high negative economic and environmental impact other than the presented method. Three lubricants for cold-rolling - o/ w emulsions - were analysed by accelerated stability testing at higher gravity (11 times physically accelerated). Go Greener by In-situ Characterization of Lubricants for Cold Rolling - Droplet Size Distribution and Physical Separation/ Emulsion stability 202 24th International Colloquium Tribology - January 2024 Clarification speed by LUMiSizer/ LUMiFuge within 17 min, RCA 11, 45°C, high homogenizer speed. ESI values obtained after several hours, provided by customer [4]. Customer ‘s temperature of 45°C for comparative chemical analysis was applied in the analytical instrument, too. During the separation the change of near infrared transmission was recorded and quantified by Integral transmission (area below curve) as function of time. 3.2 Droplet size distributions Two lubricants for cold rolling o/ w emulsions were compared, first the droplet size distribution was determined based on ISO 13318-2 [5]. Second, the combination of different analytical methods (instrumental optical technique and naked eye evaluation) for the reliable separation stability characterization of a challenging sample was applied. Samples in duplicate. For the white lubricant the mean value of the distribution was also determined by another method (impedance based) as x50 = 3.5 μm, which is in good agreement with the LUMiSizer ® result. The much broader droplet size distribution and the large deviations in the duplicates of the brown lubricant are confirmed by having a look at the samples by naked eye. The brown lubricant is a very inhomogeneous sample. 3.3 Instability index of lubricants 6 different lubricants were qualitatively and quantitatively compared. For qualitative comparison the clarification profiles were used, for quantitative comparison the instability index, both as defined in [6]. The instability index after 10 min discriminates all 6 samples. Samples 3, 5, 6 require 40 s only. 4. Conclusions Accelerated stability testing based on STEP-Technology ® is enabled for up to 12 samples simultaneously, requiring only a small sample volume in disposable sample cells and providing analytical results in short time. This is regarded much greener compared to conventional large volume single sample testing, in glassware to be extensively cleaned afterwards, lasting for several hours, based on chemical analysis. Costs and environmental impact are significantly reduced, when applying STEP-Technology. The stability ranking of the lubricants for cold rolling is identical for both methods. The reliable characterization of lubricants including challenging samples consists of two independent results when applying STEP-Technology in combination with physically accelerated testing, the droplet size distribution, and the instability index for the sample in its original concentration. The combination of the instrumental optical approach with the always available visual inspection and possibly further characterization methods is the way of choice. Independent results show the identical ranking. The near infrared clarification profiles recorded under accelerated conditions as well as the derived instability index allow for a quick qualitative and quantitative discrimination of lubricants for cold rolling. To be applied either in formulation or in quality assurance or in application development. References [1] https: / / en.wikipedia.org/ wiki/ Lubricant Available on 12.4.2021 at 13: 51 [2] Comprehensive Characterization of Nanoand Microparticles by In-Situ Visualization of Particle Movement Using Advanced Sedimentation Techniques, Dietmar Lerche, KONA Powder and Particle Journal 2019, 36, 156-186 [3] ISO/ TR 13097: 2013 Guidelines for the characterization of dispersion stability, https: / / www.iso.org/ standard/ 52802.html, 31.10.2023 1945 [4] Private communication from the customer (producer of lubricants, not to be disclosed) to LUM GmbH [5] ISO 13318-2: 2007, Determination of particle size distribution by centrifugal liquid sedimentation methods Part 2: Photocentrifuge method, https: / / www.iso.org/ standard/ 45771.html, 31.10.2023 19: 45 [6] Instability Index, T. Detloff, T. Sobisch, D. Lerche, Dispersion Letters Technical, T4 (2013) 1-4, Update 2014, https: / / www.dispersion-letters.com/ technical-notes/ instability-index, 31.10.2023 19: 45