Bonding Builds BETTER Vehicles

By on November 15, 2018 in PROCESSES & FABRICATING

Adhesives lower weight and costs.

With a global emphasis on carbon-dioxide reductions, all automotive manufacturers have been working on extensive evaluations of their vehicle fleet to identify the best options for powertrain solutions while satisfying many other factors, including consumer acceptance, cost, safety and global/local government requirements for fuel efficiency and reduction of tailpipe emissions.

Adhesive bonding in automotive vehicle construction is providing many solutions. Following the first commercial application to secure a windshield to a vehicle body in the early 1960s, adhesive-bonded joints have become a widely accepted practice to complement alternative thermal mechanical joining.

Benefits such as reduction of stress concentrations, increased load-carrying capacity, increased body stiffness, improved fatigue-life performance—as well as cost and weight reductions—have meant automotive manufacturers can satisfy customers with increased vehicle content while meeting required regulatory safety requirements.

ADHESIVES FORMULATION

Epoxy-based adhesives applied in the body shop are formulated to be compatible with other joining options such as thermal (spot welding) and mechanical (self-pierce riveting), and can bond to a variety of substrates and coatings without additional surface cleaning or preparation. Once subjected to manufacturers’ normal e-coat/paint-shop oven temperatures, the epoxy adhesive is fully cured.

With Body-in-White of the vehicle structure contributing to 25 to 30 percent of its total curb weight, the benefits of lightweighting accommodations are typically compounded. A lighter vehicle requires less energy, which can, in turn, accommodate a lower mass of other primary and secondary components such as the engine, powertrain and braking systems.

Adhesive solutions combined with other joining techniques can be used to optimize and lower the weight of steel-intensive vehicle content—sometimes by hundreds of pounds per vehicle. When lightweight structural materials such as aluminum, magnesium or structural composites are used, adhesives can be a key joining enabler while providing the maximum weight efficiencies possible with these materials.

CUSTOMIZATION AND OPTIMIZATION

Because adhesives are based on materials science, the formulation can be customized to meet specific requirements for performance and manufacturing. Selection of the proper adhesives depends on understanding the vehicle assembly process, the desired cure window, required strength/stiffness characteristics, substrates and substrates coatings, and functional performance requirements.

Adhesives can be optimized for:

• Manufacturer and tier assembly operations
• Improved processability, shelf life and durability
• Advanced high-strength steel, e.g., hot-stamp and nextgeneration steel
• Accommodation of various rates of thermal expansion
• Reduction or elimination of chemical surface treatment
• Use in the body shop, paint shop, trim shop and repair

While replacing or supporting metal fixtures and welds, adhesives make way for more aerodynamic designs. They also manage thermal expansion differences in dissimilar materials and enable the downgauging of steel.

Adhesives can be further formulated to accommodate manufacturer requirements that range from cycle time to substrate to tooling. Adhesives have steadily grown in popularity as the number of formulations and applications expanded and additional benefits are documented. One study of BETAMATE™ epoxy-toughened structural adhesives showed that 478 grams of adhesive enabled the reduction of sheet-metal thickness, and the final vehicle provided equivalent performance with a mass reduction of 53 pounds.

New developments are emerging quickly. We work with a global team of mechanical engineers who apply their knowledge of material science and applied engineering performance with an in-depth understanding of application performance. The engineering team works very closely with experts in product design, virtual simulations and materials engineering.

Areas of focus include:

• Experimental and computer-aided engineering data generation
• Body stiffness, NVH
• Strength and crashworthiness
• Fatigue and durability
• Dissimilar material bonding

ADHESIVES FOR ELECTRIC VEHICLES

The value of lightweighting is even more pronounced in electric vehicles where manufacturers need to offset the additional battery load—which can easily reach 700 pounds— and extend the vehicle range. Safety is still an important consideration in vehicle design and manufacturing, along with ride and handling, durability and repair. Adding the weight and complexities of a battery to vehicle structures requires significant changes to traditional design, development, manufacturing and testing practices.

Government regulations and purchasing incentives are moving manufacturers and drivers to expand their commitment to hybrid and electric vehicles. Some countries, including France, are even announcing their intention to phase out petroleum- and diesel-powered vehicles.

According to BCG analysis, electric vehicles and hybrid electric vehicles make up about 4 percent of global sales today. By contrast, in 2030, that number is expected to jump to nearly 50 percent. Likewise, sales are around 4 million vehicles today and are expected to grow to more than 18 million vehicles by 2023 (Yole Power Electronics Report for EV/HEV/2018).

It has been widely reported that global manufacturers are committed to electric vehicles. For example:

• Toyota plans to electrify its entire lineup by 2025.
• Volvo announced that all models introduced after 2019 will be either hybrids or all-electric.
• Ford is investing $4.5 billion into 13 new EVs.
• PSA will offer an electric option on all models by 2025.
• Renault will offer eight fully electric and 12 electrified vehicle models by 2022, fully electric vehicles will represent 20 percent of the product range.
• FCA announced a series of new all-electric vehicles by 2022: four Jeep SUVs, four Maserati and two Fiat cars.

And of course, suppliers are quickly making commitments of their own. The success of electric and related vehicles depends on innovation throughout the value chain.

As we bring technologies from DuPont and Dow together to serve the transportation market, we have a strong intersection with products that contribute to the advancement of electric vehicles. We have developed AHEAD™—Accelerating Hybrid-Electric Autonomous Driving—to provide a single source for applications in vehicle electrification, autonomy, connectivity and the supporting infrastructure.

AHEAD leverages the portfolios of three DuPont businesses—transportation and advanced polymers, electronics and imaging, and safety and construction—to create a comprehensive offering of technology and materials solutions. This provides integrated materials solutions in lightweighting, battery-pack components and assembly, thermal management/safety, electric motors, powertrain/ chassis and electrical/electronic applications.

THE COMPLEXITY OF THE BATTERY PACK

Initially, battery packs are a major focus for the AHEAD initiative. The goal is to improve battery performance by making them lighter, more durable and able to maintain a longer range—all in a scalable fashion. Cost is an additional concern. Today, the battery is the single most costly item on an electric vehicle.

Development of lightweight materials for the battery cover and assembly is already underway, and adhesives are contributing to the solution by addressing needs for weight savings; safety/thermal management; improvements in noise, vibration and harshness; and durability. In fact, there are many aspects of battery-pack development that are similar to the challenges faced by vehicle designers and engineers.

Battery packs accommodate multiple modules, and each one houses multiple pouch, prismatic or cylindrical cells. These modules are typically contained in aluminum or composite enclosures that are secured in a frame with a protective top and bottom, and an internal crash-reinforcing frame tray. In most cases, the battery and individual modules are designed for easy removal from the vehicle.

Maintaining a proper battery temperature between 25 degrees Celsius and 35 degrees Celsius is critical, especially during operation and charging. Thermal management is also important for safety reasons. New thermal-conductive adhesives bond well to coated, as well as uncoated aluminum. In addition to providing thermal management capabilities, the adhesive can act as a vibration inhibitor and could be used to bond an e-coated aluminum heating/ cooling unit assembly into a battery pack.

Adhesives are already being specified for battery-pack development for the same advantages that they are being specified in other areas of the vehicle. Epoxy adhesives improve crashworthiness and the structural integrity of the battery pack. Polyurethane adhesives are chosen for application areas where they can inhibit vibration and enable bonding of multiple substrates.

Polyurethane-based elastic structural bonding solutions, like those used for glass bonding, are specified when an application requires management of vibrations, gap variations and tensions caused by thermal differences between substrates. These adhesives can enable the assembly of lightweight substrates while keeping process steps and volatile organic compounds (VOCs) to a minimum. This could include bonding battery cells into a battery module tray, frames and cooling units. The substrates we are seeing here include composites, thermoplastics, coated steel and coated or uncoated aluminum.

Today, the battery is the single most costly item on an electric vehicle.

Multi-material bonding solutions like those used on the Body-in-White also enable the use of dissimilar substrates, which helps achieve lightweighting goals. They also manage differences in thermal expansion and support a wide range of robotic and manual application methods.

Adhesives can bond the pack to the vehicle, which will improve structural technology of the vehicle overall.

Looking ahead, further adhesive advancements are expected to address issues like safety and lightweighting. Proven solutions like these will help us identify and implement new technologies quickly and efficiently.

Polyurethane adhesives are chosen for application areas where they can inhibit vibration and enable bonding of multiple substrates.

 

ABOUT THE AUTHORS
Mansour Mirdamadi is chief engineer at Dow Automotive Systems. He has global responsibility for engineering and application development for Dow Automotive Systems products. He brings expertise in design, engineering analysis and methodology development to customers for body structures and lightweight solutions.

Mr. Mirdamadi, who joined Dow in 1988, has a Master of Science degree in engineering mechanics and a Bachelor of Science degree in mechanical engineering from Penn State University. He holds five U.S. patents and has contributed to several international publications in the area of vehicle body structures representing structural adhesives and reinforcement systems.

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