ISSUE 011 Autumn 2021 Candela C-7 hydrofoil speedboat dossier l In conversation: Robert Hoevers l Battery recycling focus l Vehicle dynamics insight l ZeroAvia hydrogen-electric aircraft digest l Motor materials

and at 3000 bar in race engines. While EVs and IC-engined vehicles alike must work reliably through a Siberian winter or a Death Valley summer, nothing in a battery EV has to handle such pressures. Obviously, the needs of the electric powertrains are common to all, and hybrids have both engines and battery packs to seal, but fuel cell systems present unique challenges connected with hydrogen storage and supply. The fuel cell stack itself, along with the valves, connectors and lines, are very difficult to seal against the leakage of hydrogen gas stored at high pressure – typically 700 bar – and the wide range of temperatures encountered in fuel cell systems. Such pressures put large stresses on seals from the moment the hydrogen filling nozzle is put into a hybrid EV’s filler neck. Because hydrogen is the smallest of all molecules, permeation past seals is difficult to prevent and demands some special materials, designs and coatings. Further, the hydrogen heats up as the pressure in the tank increases during filling. The 700 bar pressure in the tank must therefore be reduced to 7 bar before it can be fed to the fuel cell, and the gas must flow through a pressure regulating valve to achieve that in a controlled manner. This tenfold pressure drop causes a commensurate temperature drop, so the seals must retain the gas over a very wide temperature range. Within the fuel cell itself, the most important components in this context are the stack seals, which are those that separate the individual cells that comprise the stack. These must compensate for manufacturing tolerances to ensure that hydrogen permeation remains ultra-low and that there is no release of components that could poison the catalysts. In fuel cells, sealants play a vital role in preventing leakage of ions, for example. Sealants or adhesives? Sealants and adhesives are often discussed together, as they are often formulated from the same sets of materials and can have similar processing times. The main functional difference is that adhesives have to be stronger, particularly in semi-structural and structural applications, and are usually expected to form permanent bonds, while sealants generally have no structural role and are often intended to be removable to allow for disassembly for maintenance and repairs. The materials used in the automotive industry have evolved significantly in recent years in response to more stringent requirements in a number of areas. In IC-engine applications, including hybrids, the need to withstand high temperatures has grown because of tightly packed engine bays and the widespread use of turbocharging. More robust requirements are also emerging for improved adhesion, and resistance to aggressive lubricants and coolants, as are requirements for fast curing in assembly and for non-flammability. Other key factors include increased vehicle and component durability, faster processing and design freedom, along with weight reduction, safety, sustainability and legislator compliance. There is also an ongoing reduction in the use of hazardous chemicals such as isocyanates and others regarded as substances of very high concern (SHVCs). Industry also wants sealants that cure at lower temperatures and bond better to unprimed or untreated substrates as well as various chemical pre-treatments and finishes such as anodised or painted substrates. Sealants and adhesives now span a broad range of technologies including water-based, hot melt, reactive hot melt (RHM) and solvent-based formulations. Polyurethanes are among the most important compounds used, in one- and two-component form as well as RHM to create, for example, semi-structural and structural adhesives, and elastomeric sealants. Other materials include butyls and block co-polymers, silane- terminated polymers, polyacrylates, two-component methyl methacrylate, epoxies and silicones. Elastomeric applications A typical EV application for an elastomeric sealant would be the joint between the motor and gearbox in an e-axle or similar transmission system. For sealing cooling systems that use a water-glycol mix, an elastomer called ethylene propylene diene monomer is widely used, being favoured over E-axles typically involve the coupling of an oil-lubricated gearbox to an electrical machine that runs with minimal lubrication or even dry at very high rpm (Courtesy of Trelleborg) 66 Autumn 2021 | E-Mobility Engineering Focus | Sealing