Is the ICE Decline an Upturn for Silicones?
Unfamiliar to many in the industry, silicone base stocks have limited application in lubricants. The rise of electric vehicles may herald new opportunities for these high-value synthetic fluids.
Amongst the dominant synthetic base stocks—polyalphaolefin, esters and polyalkylene glycols—the use of silicones has been devoted to niche applications. Despite their advantages over other synthetic base stocks, including chemical inertness, oxidation stability and very high viscosity indices, the limiting factor of silicones is their widely perceived poor lubricity.
The raw materials for the production of silicones are mainly methanol and quartz sand, with the latter readily available. However, methanol demand has increased rapidly over the past decades, as it is also needed for the re-esterfication of vegetable oils to make bio-diesel.
Manufacturers are scattered throughout the United States, Europe and Asia, and include Dow Silicones, Elkem Silicones, Evonik, Momentive Performance Materials, Shin-Etsu Chemical and Wacker Chemie. Like many other segments of the lubricants industry, the market has been shaped through a series of mergers and aquisitions. Dow Chemical took over Dow Corning in 2016 and formed Dow Silicones in 2018; Elkem Silicones was renamed in 2017 from Bluestar Silicones, which had taken over Rhone Poulenc’s silicones division; and Momentive Performance Materials resulted from GE and Bayer silicone divisions spin-offs, which were then bought by Apollo Management in 2006.
Given their unique chemical and physical properties, silicone products can be used to manage foam, add gloss and shine, release, soften and condition, emulsify, waterproof substrates and lubricate. The majority of silicones are found in a long list of industrial applications, including food production and processing, household cleaning products, paints and coatings, construction industry sealants, pressure-sensitive adhesives, mold-making elastomers, personal care, electronics and high-voltage electrical applications, as well as the plastics, textile and paper industries.
For lubricant formulations, there are historically three types of silicone fluids used: dimethyl siloxane polymers, known as PDMS or dimethyl silicone; phenylmethyl dimethyl siloxane copolymers with phenyl substitution from 10% to 90%, known as phenyl silicone; and trifluoropropylmethyl dimethylsiloxane copolymers, known as fluorosilicone.
Fluorosilicones show reasonable lubricating properties with very good chemical intertness, while phenylsilicones are used for high-temperature bearing greases. More recently, formulations have been developed that use copolymers of fluorosilicones and phenylsilicones that combine inertness, high-temperature resistance and reasonable lubricity.
However, only PDMS materials are economically viable in volumes needed for use as synthetic base fluids; the others are five to 20 times more expensive. PDMS fluids are mainly used in lubricant applications where contact with plastic or rubber make compatibility with these materials crucial. In metal-to-metal applications, they do build lubricating films comparable to hydrocarbon fluids at medium- to high-speed operation. But in low-speed, highly loaded contacts, they show much higher wear than hydrocarbon materials of the same viscosity.
For combustion engine applications, PDMS fluids would show fuel economy savings due to their high viscosity index of over 350—more than twice that of polyalphaolefins. But even if an engine could use coatings or another non-lubricant method to overcome wear in heavily loaded contacts, PDMS would still not be a feasible lubricant, as blow-by gases in the piston rings would create silica residues on spark plugs or valves.
Electric vehicle drive trains have different requirements for a lubricant. An EV fluid needs to balance tribological properties such as wear protection, elasto-hydrodynamic lubricating film formation, low fluid friction, shear stability, air release and low foaming tendency with thermal and electrical properties such as thermal stability, heat transfer properties, low viscosity, sufficient electrical properties such as specific conductivity, specific resistance and breakdown voltage, and materials compatibility.
Introducing hydrocarbon chains and rings to the silicone backbone can change the silicone’s properties to create better wear protection without losing too much of the higher viscosity index relative to polyalphaolefins. Alkyl-modified silicones show higher values than hydrocarbons in specific heat capacity and thermal conductivity, which is a result of silicones‘ higher density.
The development of such new, modified silicone base stocks on laboratory and pilot scale is possible, but the interest of the big silicone suppliers in lubricant base fluid development is dictated by competing demand coming from previously mentioned industries. Interest from the lubricant industry certainly exists, but so far it is mainly in specialty applications, as the transfer of former Dow Corning’s Molykote lubricant product line from Dow’s Materials Science division to Dupont’s Specialty Products division a few years ago shows.
The use of silicones in electric vehicles could lead to larger-volume applications. The aquisition of Sanford, North Carolina-based high-performance silicone lubricant manufacturer Polysi Technologies by Fuchs late last year documents interest in silicone base stock technology from a big player in the lubricant industry.
Manfred Jungk , Ph.D., is a chemist and director of MJ Tribology consulting, based in Geisenheim, Germany. He has more than 30 years of experience in the lubricants industry. Contact him at email@example.com.