Evans, Tim; (2023) Investigating the use of Ionic Liquids as a novel magnetic lubricant. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

The main body of this work explores ionic liquids (ILs) to be used as a magnetic lubricant and sealant in a vacuum pump. The problem has arisen due to upcoming legislation governed by the Stockholm Convention with respect to persistent organic pollutants; where the current lubricant falls into this category due to it’s fluorinated composition and it’s inability to decompose naturally. Other factors include the current lubricant’s high viscosity and difficulty in removing and cleaning the parts of the pump where the lubricant has come into contact. Further to this, due to a gap in the current market it was hypothesized that a magnetic lubricant could enhance performance by using a magnetic field to hold the lubricant in the relevant parts of the pump, enabling less lubricant to be used, whilst also being used to create a seal and in turn enhance performance of the vacuum pump. In addition to this, a contribution to the IL modelling field with respect to XPS is completed, where novel dicationic ILs are modelled with respect to the C1s region using models derived and developed from existing methods. 12 core samples based on ionic liquids with paramagnetic anions derived from Fe3+ and Dy3+ and imidazolium or pyridinium-based cations are explored as lubricants. Key tests were performed to begin collecting data to allow for informed decisions on what samples may be more suitable for purpose. Viscosity measurements were performed using a Brookfield DV-III digital rheometer equipped with a small sample adapter at room temperature initially, followed by ramped measurements up to 80 °C and then followed by viscosity index (VI) calculations. VI calculations require a known density of the sample, this was completed by weighing 1 mL of the sample at room temperature, this was a fixed density where it is known density will decrease with a temperature increase. Thermal stability was measured using thermogravimetric analysis on a ramped rate moving from room temperature up to 450 °C at a rate of 10 °C per minute. Contact angles were measured on three different substrates namely steel aluminium and a fluoroelastomer comprised of hexafluoropropylene and vinylidene fluoride. The method used a Krüss DSA 25E contact angle goniometer with a standard automated syringe dosing unit, applying the touch on method where an average timed dynamic measurement was recorded. Paramagnetism was measured using an Evans balance and friction behaviour. Specifically, the coefficient of friction was measured using a Bruker UMT Tribolab. This test made used of a ball of specified material applied at a specified force onto a steel plate which reciprocated at a specified Hz with a small amount of sample applied between the surfaces. Further to this, a surface profilometer was employed to analyse the wear scar left on the steel plate after the test had been completed using a Bruker DektakXT stylus profilometer. Friction behaviour also entails analysis of the sample after being subjected to friction testing to determine any chemical composition changes utilising X-Ray Photoelectron Spectroscopy (XPS) and infrared (IR) spectroscopy. At this point, trends relating chemical composition to physical properties have began to be deduced; whereby in it’s simplest form extending the alkyl chain component of the cation within the ionic liquid generally results in an increase in viscosity as well as a decrease in density. With regards to VI, increasing the alkyl chain length results in a decrease in VI. The same effect of alkyl chain length on contact angle can be seen whereby a longer alkyl component results in a decrease in contact angle for all surfaces investigated. The 12 core samples all displayed a paramagnetic response to the Evans balance with the Dysprosium samples showing a higher magnetic moment than the Iron-based samples, an expected result. Work involving coefficient of friction experiments also found that the core samples gave lower COF values than the control lubricants, but XPS discovered the short chain ionic liquid samples were potentially less stable than their longer chain counterparts when compared to their original state based on changes to their spectra for before and after COF studies, IR spectroscopy also deduced subtle changes in composition. The work finishes with one sample being scaled up, based on performances in previous tests and tested in a live pump with mixed results. Despite performing well as a lubricant relative to two control lubricants, compatibility issues arise resulting in problematic changes in chemical and physical composition of the scaled-up sample. The remainder of the work covered troubleshooting the scaled-up sample by utilising the same previous analytical tools such as XPS and IR but also covered Mass Spectrometry (MS). This was used to determine potential impurities present in the used sample and inform on potential degradation pathways. This troubleshooting phase concluded that residual Fomblin (the control lubricant used previously) was causing contamination when analysing with XPS, further to this MS found peaks relating to a brand of antioxidants, known as Irganox which is likely to be from its use as an additive in Fomblin. Furthermore, an unexpected result in this work was the magnetic properties of the samples were deemed not strong enough to fulfil their original objective; this resulted in further work being completed to remedy the issue. Initially, inspired by ferrofluids it was hypothesised that perhaps an ionic liquid could be used as the carrier solvent for magnetic nanoparticles as an option to boost magnetism based on magnetite, Fe3O4 following a co-precipitation method. Confirmation of nanoparticle synthesis was completed using XRD with peaks matching a reference sample taken from ICSD. However, difficulties were experienced with relation to nanoparticle stability within the ionic liquid. Surfactants were briefly explored based on oleic acid and decyl, methylimidazolium bromide with no positive results. Novel in-situ methods using an ionic liquid as reaction media for the preparation of magnetite were explored with preliminary dynamic light scattering (DLS) measurements suggesting differences in size of nanoparticle relating to ionic liquid being used as the medium. Further novelty was beginning to be explored with respect to a new type of poly(Ionic Liquid) based on a dysprosium anion. There were difficulties in characterizing the new material which leaves room for future work, however again the paramagnetic response despite being larger than the core samples explored was still not deemed strong enough for manipulation with a typical magnet. In summary, this work has demonstrated that ionic liquids can be used as a lubricant in a vacuum pump, purely as a lubricant as opposed to a lubricant and sealant which was the aim of this project. The scaled up sample performed well on performance of the pump however clearly it had degraded over the 2000 hours that it had run for. The inability to be manipulated with a standard magnet raised issues quickly with respect to utilising as a sealant within the pump. However, these issues enabled further work to be explored including nanoparticles and polymers and despite only being in the preliminary stages offers optimism that both objectives could perhaps be achieved. This work offers further insight into how ionic liquids may perform in large scale experiments as opposed to bench top experiments in the laboratory. It has been seen that COF measurements may not give the full picture of how ionic liquids perform as lubricants as demonstrated by the unanticipated degradation when in the live test pump. But with respect to the test pump, this is a great contribution to the field where it is believed this type of experiment has not been reported in the literature to date.

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