Structural adhesives are gaining in usage popularity as engineers employ the bonding agent with lightweight materials to help achieve a continuous bond for improved vehicle rigidity and other benefits.

“We’re seeing the application of structural adhesives totaling several meters in certain passenger vehicles in comparison to minimal usage just a couple of years ago,” noted David Anderson, Senior Director of the Automotive Market, Steel Market Development Institute (SMDI.)

Anderson and other materials-technology experts spoke about adhesives’ growing role with Automotive Engineering during SMDI’s 2017 Great Designs in Steel seminar in Livonia, MI.

Structural adhesives commonly are used in tandem with a traditional joining technique. “As automakers choose to go with thinner and thinner steels on vehicle bodies, the methodology of just using spot welds isn’t necessarily the best option,” Anderson explained.

A combination of spot welds and adhesives, known as weld bonding, was used on the 2018 Honda Odyssey’s body-in-white. The BIW is comprised of low strength steels (41%), press hardened steels (7%), high strength steels (6%), advanced high strength steels (37%), ultra high strength steels (8%) and aluminum (1%).

The fifth-generation Odyssey doubled the amount of structural adhesives from the prior model. And that 44 meters (144 ft) of bonding material, which enabled the body engineers to avoid 5 kg (11 lb) of additional stampings and reinforcements, helped increase the vehicle’s torsional rigidity 10%, according to Nicholas Goldsberry, 2018 Odyssey Body Development Leader at Honda R&D.

After body and closure panels arrive at a body assembly plant, spray treatment systems are used on the metal to remove cutting oils and other debris. But that cleaning process poses problems for the uncured adhesives on those structures, according to Tyler Auvil, Ph.D, Senior Chemist, Structural Adhesives R&D at Dow Chemical.

If adhesives are washed out of the bond line, the joint’s strength is reduced, Auvil noted. The cleansing process also could re-deposit the uncured adhesive back onto the body surface before the panel and adhesive go through a bake oven. In that scenario, the cured adhesive would require sanding off and repair work prior to paint application.

“The wash-off problem usually was addressed by increasing the adhesive’s viscosity, but then the adhesive could no longer by pumped through a robotic applicator,” he said.

Dow’s next-generation structural adhesive, called Toughened Betamate, could have its first vehicle production application in early 2018. This epoxy-based structural adhesive demonstrated better wash-off resistance during tests at the company’s R&D facility in Auburn Hills, MI, Auvil claimed. He added that the toughened adhesive maintains the ability to be pumped through a robotic applicator.

Toughened Betamate's low temperature cure provides consistent lap-shear strength at a broad range of curing temperatures, he said. The adhesive’s increased tensile modulus makes it well-suited for advanced high strength and downgauged, ultra-high-strength steel alloys.