automotive structural adhesives
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Glue power: Acura’s 2019 RDX added 121 ft. of adhesives, substantially enhancing its body rigidity—yet also helping to cut total vehicle weight. (Acura)

Stuck on structural adhesives

A funny thing happened on the way to the total-vehicle lightweighting that’s now a common engineering dictate for nearly every new vehicle: structural adhesives—which entered low-volume automotive use in the 1980s—got pulled into the mainstream.

The widening reliance on high- and ultrahigh-strength steels and aluminum for body structures, particularly unitized bodies-in-white (BIW), to cut weight without sacrificing crash-mitigating strength or handling-degrading rigidity, is the prime factor driving the expanding use of structural adhesives. Particularly where dissimilar materials need to be joined, engineered adhesives are the way to go—alone or in tandem with more-conventional joining technologies such as riveting and spot-welding.

In the mid-1990s, several automakers, including Ford and Jaguar, were experimenting with aluminum for BIW and were using adhesives as part of the materials-joining strategy. But it was more recently that adhesives began to literally and figuratively “take hold” in CAE design software and assembly-plant body shops all over the world.

“I think there’s been a lot of technological breakthroughs in the past ten years on structural adhesives,” offered Ben Meaige, senior materials research engineer for Honda R&D Americas, noting the explosion of adhesives applications for body and chassis. Meaige believes about a decade ago, adhesives suppliers including Henkel, DuPont, Lord, 3M and Sika, among others, made significant breakthroughs in the epoxies tyically used in most adhesives’ base chemistry. It’s probably no coincidence, then, that some industry estimates indicate adhesives use has nearly doubled since around 2005.

“Previously, epoxy had an inherent problem of being brittle. The adhesive suppliers have been working on toughening—and “toughening” is adding rubber groups into the epoxy formulation to give it more flexibility,” Meaige explained. “About ten years ago, they made some major breakthroughs in the quality of that toughening.”

“We’ve used adhesives forever,” said Sandy Munro, CEO at Munro & Associates, the benchmarking and competitive analysis engineering firm renowned for its revealing competitive-analysis “teardowns” of popular and innovative vehicles. “It’s just that the amount of confidence in them has drastically changed [in recent years].”

Ramp up to big programs, big gains
Lightweighting for electric vehicles (EVs) with their frightfully heavy battery packs, as well as coming automated vehicles (AVs), also is a gathering force behind the expanding use of structural adhesives, suggests Eric Aldstadt, Henkel’s head of engineering, automotive acoustics and structurals NA/LAN. Conductive adhesives are particularly useful for strengthening battery packs, he said, and total EV weight can be reduced by downgauging body materials without impacting crash performance.

But on the way to the EV and AV revolution, today’s conventional vehicles are proving the point.

The BMW i3, launched in 2013, still might be considered the poster vehicle for structural adhesives’ coming of age. The unique car’s BIW is made of carbon-composite panels and assemblies that are adhesively bonded, while this entire “Life Module” is affixed to the chassis with structural adhesives. Both DuPont and Sika have applications on the car.

“Anything you can possibly imagine, they glued together. The polyurethanes they are using are unreal,” observed Munro. The adhesives’ advantages are many on the i3: Apart from the obvious joining of material that is difficult to mechanically join, the adhesives ably provide for varying material expansion rates, enhance stiffness with a more continuous and consistent bond line and help counter any areas of potential galvanic corrosion around fasteners and locators.

For the current Ford F-150, now famous for its aluminum-intensive bodyshell, Ford naturally turned to structural adhesives for wedding various areas of the body structure. DuPont’s Betamate epoxy-based material was the choice to combine with specially-developed mechanical fasteners. Ford said this monument to adhesives-as-lightweighting-enabler uses some 350 ft (107 m) of structural adhesives.

General Motors also has been on pitched strategy to lightweight vehicles with an appropriately applied mix of materials. “[Cadillac’s] CT6 is still our production vehicle that represents the leading edge in structural adhesive usage,” said Jeff McGarry, executive general manager - BIW Dispense Technologies and executive general manager - Body Execution at General Motors’ Bowling Green, Ky. assembly plant “It’s got the most material. We’re bonding aluminum to aluminum and aluminum to steel, with a primary joiner of either a rivet or a flow-drill screw.

“Our new trucks are more representative of current design in lightweighting—we’re using six times more adhesive on the current trucks than on our outgoing trucks,” McGarry continued. “If you compare a new truck BIW to one of our latest SUVs, they look very similar in terms of panel break, material gauge, when and where we use HSS. The adhesives play a much larger role in getting to our vehicle-performance goals of durability, ride and handling and lightweighting.”

GM’s next showcase is the 2020 Corvette, a multi-material matrix that utilizes aluminum castings, sheet and extrusions, composites and steel.

With six times more adhesives on a high-volume pickup, it’s little wonder that DuPont forecasts growth at 3% to 5% annually for the NA transportation market.

“The adoption rate is accelerating, for sure,” Frank Billotto, strategic market manager for the Americas, DuPont Transportation and Industrial, told Automotive Engineering. “[Automakers] have validated the ability of adhesives in conjunction with welding and riveting,” to enhance structures and improve load-transferring ability. He said 18 global automakers are using some form of his company’s adhesives.

And it doesn’t stop with adhesives. Henkel is excited about the potential for epoxy-based 3D structural-foam that adds strength in vital areas with markedly less weight than metal. These “hybrid reinforcements” can be tactically applied in places such as door panels, fenders, bumper beams and even hinges to deliver required strength without excess weight.

The structural foam only now is coming into production use. “We’re trying to get the word out,” said Henkel’s Aldstadt.

Japan comes on strong
Equally intriguing, Japanese automakers have in the past few years visibly accelerated their use of structural adhesives. Some sources believe the Japanese companies were at first cautious in some part because of their reluctance to devote precious assembly-plant floorspace to adhesive-application “cells” that even the most ardent adhesives advocates admit can be square-footage hogs.

But that thinking clearly has changed.

Honda’s begun employing vastly more body adhesive in its newer vehicle programs. The fifth-generation 2018 Odyssey minivan doubled adhesives use to 144 ft (44 m), which Honda reckoned saved 11 lb (5 kg) of metal stampings and reinforcement. Engineers added 115 ft (35 m) of structural adhesives to help the new tenth-generation 2018 Accord sedan increase bending and torsional rigidity by a respective 24% and 32%—while simultaneously helping cut the car’s overall weight by 110 to 176 lb (50 to 80 kg), depending on trim level.

“If we can add in, let’s say 50 m (164 ft) of adhesive and take out .2 mm thickness of our steel, the reduction in our weight from the steel reduction is significantly greater than the addition from the weight from the adhesive,” said Honda’s Meaige.

For the all-new 2019 Acura RDX, for example, 37 m (121 ft) of added adhesives to augment the structural welds netted a weight savings of 9 kg (20 lb), while body rigidity increased by a not-insignificant 38%.

When Toyota’s current 4th-generation Prius migrated to the company’s wide-ranging TNGA platform, its body-in-white moved from 3% high-tensile steel to 19%. Adhesives paired with other advanced joining techniques helped to increase the car’s torsional rigidity by a stomping 60%.

What’s the hangup?
Although structural adhesives’ benefits are undeniable, there are impediments to unbridled acceptance.

GM’s McGarry said adding adhesives late in a program can be difficult to incorporate at the assembly plant.

“While we do have templated cell layouts with adhesive equipment in it, once you have a cell layout ‘footprint’ in a plant and product engineering comes back and says, ‘we need adhesive here,’ it can become cumbersome sometimes to execute a ‘wet’ joint versus a ‘dry’ joint.”

DuPont’s Billotto agrees that coveted body-shop floorspace can inhibit the enthusiasm for increasing the use of structural adhesive. “Some OEMs have been reluctant” to add adhesives, he explained, because of the space requirements. Properly preparing the substrates to be adhered and curbing “washout” of the adhesive also can be challenging, adding to the stigma of lengthy cycle times for adhesives, Billotto noted.

Sandy Munro adds that despite proven advantages, use of adhesives still can be inhibited by lack of education for BIW engineers and assembly-plant veterans alike.

“Nobody really wants to take a risk upstream,” he said. And at the assembly plant, the ‘old hands’ can be reluctant to incorporate new joining methods, he noted.

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