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Adhesive and Sealants Use in Automotive Drive Train: How do Internal Combustion Engines differ from Electric Motors?

As a generality, the similarities of adhesive and sealant use in internal combustion engines (IC) and electric vehicle motors (EV) are actually greater than the differences.  Both systems have mechanical assemblies like bearings and gears that are retained with adhesives, threaded fasteners that are subject to vibration and thermal cycling necessitating adhesive augmentation, and systems that require sealing against fluid migration, dust, and air flow. Since I am still painting with a broad brush, the drivetrain of EV’s isn’t really all that different from other electric motors that have been built for a century.  The primary differences of the EV motor lie in scale, environment, and source of electricity.

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Figure 1:  All-electric vehicle
Source: The U.S. Department of Energy

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Figure 2:  IC engine vehicle
Source: The U.S. Department of Energy

Of course, the devil is in the details with respect to each use and environment: IC applications can range from the relatively benign temperature requirements of ambient conditions for the vehicle (if the application is toward the cooling or non-combustion side) to extreme environmental conditions on the combustion or exhaust side.  Exhaust components including the catalytic converter run too hot during operation for organic materials like epoxy, acrylic, or urethane.  As temperatures rise, engineers turn to silicones, ceramic adhesives, and non-adhesive assembly methods such as welds, brazed joints, crimps, and fasteners. 

Conversely, on the cooler side, fluid exposure (not temperature) is often the Achilles’ heel for  adhesive or sealant use.  Exposure to ethylene glycol and water (plus a myriad of buffers, stabilizers and modifiers) requires testing for adhesive and sealant materials in contact with engine coolants. Demonstrating ethanol/gasoline blend resistance for fuel delivery applications with the higher alcohol content tends to be more problematic.  From an adhesives and sealants standpoint, diesel and biodiesel blends present fewer challenges than alcohol blended fuels. 

However, as the application approaches the combustion chamber, the influence of temperature is largely a factor: not only must the adhesive or sealant survive the extreme cold of certain ambient environments, but it also must survive the higher temperature created by combustion and the thermal cycling caused by those intermittent conditions. The same temperature cycling effect makes the interface with substrates more challenging since CTE’s (coefficient of thermal expansion) are rarely the same or similar.  The application is also now more likely to be exposed to coolant chemistry and/or lubrication chemistry (petrochemical, synthetic, and blends).  This convergence of stresses – thermal, mechanical, and chemical – is extreme.  Understandably, materials approved for these uses are substituted only amid dramatic condition changes.

I characterize IC and EV differences as sealing in substances versus sealing them out.  That is a statement full of holes requiring clarification, but it is not too far off as a summary: besides holding together parts in the harsh automotive environment, IC drivetrain applications focus on sealing in coolants, fuel, lubricants and air/gases while simultaneously keeping contaminants out.  With the exception of coolant applications that are similar to IC, EV blocks moisture, water, air, dust, and other contaminants out of the system. 

Purposely, I am avoiding discussion of control systems and related electronic and electrical systems since these are similar in IC and EV applications.Those control systems and the learnings from those applications are influential in EV drivetrain.  Sealing connectors against intrusion while simultaneously acting as electrically isolating or conducting is an obvious example.  Battery sealing for EV is another.  

Interestingly, comparing a standard lead-acid cell battery used in IC to the most common Lithium Ion batteries used in EV one will note that sealing in the acid-based electrolyte is the primary function of materials used for lead-acid batteries while sealing out moisture is critical for Lithium Ion batteries.An EV motor’s temperature rating may be higher than other motors such as windshield-wiper motors and air movement fans from IC, yet it is still well within range of most reactive chemistries, thus not a particularly high barrier. 

Taking a page from smart phones and other devices, forecasting future EV applications will involve managing heat to maximize operational efficiency of motors and their batteries due to densification. Densification will be driven by weight reduction and the need to pack more energy into batteries or other electrical delivery systems.  Achieving purity of raw materials to avoid ionic contamination in tandem with enhancing adhesive properties and requirements beyond bonding or sealing,as well asdiscreet material application,will be the focus of engineers and suppliers collaborating on solutions in the not-so-distant future.  Thermal management will become as important to EV as it is in smaller portable electronics.  Future vehicle manufacturing will look more like smart phone manufacturing and less like Henry Ford’s original production line.

I’ve excluded hybrid drive trains (HEV) –  the reader can accurately assume the intersection of IC and EV is worst-case, combining all of the aforementioned requirements. This level of complexity portends pure EV – at least in theory, disruptive innovation may be just a battery breakthrough or hydrogen fuel cell development away.

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Figure 3:  Plug-in hybrid electric car
Source: The U.S. Department of Energy

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Figure 4:  Hydrogen Fuel Cell Vehicle
Source: The U.S. Department of Energy

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Figure 5:  Flexible Fuel vehicle
Source: The U.S. Department of Energy

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This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° [605658].

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