Replacing Mineral Base Oils

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The vast majority of lubricants used around the world are made mostly of mineral base oils, but a Swedish academic told an industry conference last month that it is time to look for replacements.

Speaking Jan. 25 at Teknische Akademie Esslingen’s International Colloquium on Tribology, Lulea University Prof. Roland Larsson predicted that the sustainability movement will force lube formulators to switch to a wide variety of base stocks, such as water-diluted glycerol from biofuel refineries.

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Larsson, on the faculty of Lulea’s Division of Machine Elements, acknowledged that such changes could increase costs and require compromises in lubrication performance and equipment design. He added that they should lead to great employment opportunities in lubricant research and development.

Europe is at the forefront of the sustainability movement, and Larsson’s presentation was one of several dealing with the topic at this year’s colloquium, which was held online. He said there are a number of good reasons for mineral oils to have become the dominant base stock for lubricants: They are available in large quantities at reasonable costs; They provide a value-added product for fuels refiners; They have good lubrication properties, even in the boundary regime.

He added that they also have drawbacks – being toxic, flammable and difficult to clean up – but he contended that more importantly they will be ushered out of use by the global movement toward sustainability.

“The biggest problem is that they are made from petroleum,” he said, citing the greenhouse gas emissions generated by supplying them. Industry is beginning to demand that suppliers provide materials with lower carbon footprints, and governments will eventually require that.

Larsson noted that some lubricant marketers have begun taking steps to reduce carbon footprints, at least in part to bolster the popularity of their products. He argued, however, that the industry needs to take a much more aggressive approach where sustainability becomes a fundamental decision-making criteria throughout all aspects of the business.

Until now, he said, decisions have been made based on two criteria: performance and cost. “We must add on a third parameter,” he said. “We cannot just say that sustainability or the green effect is an extra that we can market. It is something we must have – it is an unavoidable requirement.”

Industry will continue to use petroleum-based base oils for some time, Larsson said, but industry should begin now to find replacements. He allowed that alternatives may cost more or in some cases not match performance of mineral oils, but he argued that such sacrifices must be made – and that acceptable solutions can be found.

For example, equipment manufacturers may need to make some components more robust so that they can withstand reduced levels of wear protection. This might require components to be 5% heavier, he said, which could raise costs and increase the amount of fuel needed to make the component. However, such sacrifices could be justified if overall greenhouse gas emissions are reduced. Switching to different base stocks may also require a move away from to-the-limit equipment design in order to achieve milder lubrication conditions.

Replacing mineral base oils will not be easy, Larsson said. The global petroleum industry currently produces around 1.5 cubic meters of base oil per second – or a bit more than half a metric ton – so much material is needed. Larsson suggested a number of potential sources with significantly smaller carbon footprints: recycled waste lubes, water, gas-to-liquids base stocks made from natural gas, bio-based oils from plants such as rapeseed or sunflowers, synthetic hydrocarbons from renewable resources, oils made from biochemical processes.

“Most probably there will be a wide variety of base stocks in the future,” he said. He suggested that the lubricants industry take a cue from the paint industry, which largely replaced oil-based products with latex products that are water soluble. Besides reducing carbon footprints, this would lead to finished lubricants that are easier to clean up, he said.

He offered glycerol as a renewable material that could be used to make water soluble lubricants. It is non-toxic, being present in foods that people regularly consume, and it is produced in large amounts – for example as a by-product of bio-diesel refineries – though less than mineral base oils at 0.08 tons per second.

It has pros and cons in terms of performance properties. Diluted glycerols are superior to mineral oils in cooling capacity, are less flammable and can provide a range of viscosities depending on the amount of water used. They also provide very good lubricity when a lubricating film can be maintained, but maintaining an adequate film is challenging. Film thickness can be improved by raising viscosity and making component surfaces smoother.

“Boundary lubrication is a problem of course, so we need to find a new additive chemistry for glycerol-based lubricants,” Larsson said. “That will be something for all of us to work on in the future.”

During a question session following his presentation, Larsson acknowledged that glycerol-based lubes are also much more limited in the levels of heat that they tolerate. The water content would boil off at 80-90 degrees Celsius, so they clearly could not be used in automotive crankcase oils.

Significant issues still need to solved, but Larsson said this should not stop industry from learning to make functional lubricants from more sustainable raw materials.

“We have spent some thousands and thousands of billions of dollars or euros to develop today’s machines together with today’s oils, and now we are going to do the same thing for completely new base fluids and completely new additives,” he said. “So this is fantastic, in a way, for us doing research into tribology. It is a paradise really. We are really facing a very exciting future because we will have so many fun problems to solve. Even if I do understand that companies are facing a struggle for some period.”