FUELS & LUBRICATION
GLYCEROL IS A PROMISING BASE FOR SUSTAINABLE LUBRICANTS Speaking at the 23rd International Colloquium Tribology in January 2022, Dr Roland Larsson of Luleå University of Technology, Sweden, highlighted the potential of glycerol as a new, sustainable lubricant base
8 The high performance of mineral oil-based lubricants’ lubricant film properties will be challenging
Currently over 1.5m3 of lubricant is manufactured per second around the world. In most cases, crude oil is used as the lubricant base. It provides appropriate viscosity at reasonable cost, and it is available in large quantities. However, there are some drawbacks with oil. It is flammable, difficult to clean, slow to degrade, and toxic to some plants and animals. It has been estimated that approximately 50% of all lubricants sold worldwide end up in the environment via total loss applications, volatility, spills or accidents. A rethink of petroleum-based lubricants is required, because they are not sustainable. Base stocks will slowly disappear, and their cost may become prohibitive. Customers may no longer wish to buy fossil products, and legislation may stop them from using them. Therefore, the need for fossil-free lubricants will increase in the future. The need for sustainability will make it necessary to find new products, and so a new optimal lubricant formulation will be required. When considering a new lubricant base, tribologists need to consider the tribo-optimisation of the whole system – not just the lubricant itself. Performance must be balanced between performance and cost, and sustainability or circularity is now an unavoidable goal. This may mean that, initially at least, sustainability becomes more important to what is optimal than price or performance.
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Finding a new optimum for lubricant performance may therefore initially result in lower performance or higher costs as design strategies mature. For example, a more robust component may have a slightly higher weight, so energy consumption during manufacturing and operation will increase. This might be acceptable if the component doesn’t need replacement or if it can be reused. Both suppliers and customers aim for “eternal service life” but an optimal design may not be “design to the limit” and this creates milder triboconditions for a new lubrication to endure. What are the options for renewable lubricant bases? We could re-condition oils, or find bio-based oils (rapeseed, sunflower), or other fossil-free alternatives, including water. We could use coatings and surface treatments instead of lubrication. But, why not do what has been done in the paint industry for many years? That is, use water-based or watersoluble lubricants. One advantage of water-based lubricants is that there are many biomass base stock candidates to choose from. Water solubility also makes them easier to disperse in ground and water and makes it easier to keep surfaces and machines clean. For myself and my colleagues in Division of Machine Elements at Luleå, Dr Marcus Björling, Dr Yijun Shi, and Dr Anders Pettersson, glycerol (C3H8O3) is a promising choice. It is non-toxic, water soluble, odourless, colourless, and
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