Rapid advances in adhesives technology will allow the design of significantly lighter cars and also enable use of thinner steels. Some new platforms are already employing the new approach with much broader gains expected soon.
Ford reduced the weight of the 2008 Ford Focus 3.7 lb through increased use of crash-durable structural adhesives in the upper body, says Shawn Morgans, Ford's body structure technical leader. “We have reduced spot welds,” Morgans told Design News. “The focus from here on will be to reduce the gauge of the steel.”
General Motors' engineers also have their eyes on the increased use of the brand-new super-bonders.
“It would be very nice to use crash-toughened adhesives extensively,” says Mark Verbrugge, the director for GM's Materials and Processes Lab., “and not have riv bonding, thermal joining, friction stir welding or anything else. That would help reduce weight. The fixturing in the plant is also problematic if you end up having to put in rivets or something else to make sure the structure stays put. What you would really like to have is an adhesive bond that fixtures and sets up immediately.”
New GM vehicles developed under the Epsilon and Delta banners, in Europe and Asia, respectively, will utilize newly developed crash-resistant adhesives. New Opels are also expected to use the advanced systems.
Use of structural adhesives for car bodies is most advanced in Europe where manufacturers are committed to reducing carbon dioxide emissions to a maximum of 120 g/km for all new passenger cars by 2012. Crash-stable adhesives are being used in the structures of new Mercedes and BMW models, for example.
Adhesive bonding in cars has a long history including the bonding of glass into the body flange. Bodies represent 25 percent of a car's weight and are key targets for weight savings. Until recent years, high-strength adhesives used in aircraft and other applications were not suitable for car bodies because they were brittle in crash situations, particularly at low temperatures. The technical breakthrough came with the development of a two-stage system that modifies impact resistance. A chemical reaction creates toughened particles and a flexibilizer in a process called synergistic rubber toughening. The toughened particles reduce fracturing and absorb energy. Soft particles, only a few nanometers in size, are uniformly distributed throughout the matrix before the curing of the adhesive.
Another significant technical advance makes crash-stable adhesives suited to conventional car production systems. They typically cure at temperatures around 356F, allowing the pre-treatment electrocoat process to cure the structure without a dedicated curing oven. The newest of the materials can be used in temperatures ranges from -40 to 176F.
The techniques advancing fastest today employ hybrid bonding. As Verbrugge points out, spot welding or some other conventional fastening system is required to provide initial strength. Another factor — simulations are limited in predicting the behavior of the very new crash-stable adhesives as part of a full vehicle test.
“Another area where we are seeing a lot of uptake is in high-strength steels,” says Chris Liddiard, a director of structural solutions for Henkel. “The steel doesn't yield very much so you tend to push that energy to the weld very quickly if you aren't careful.” Use of the new adhesives improves seam integrity by providing a dual-energy management system.
The big next-generation breakthroughs for structural adhesives will come when reliable predictions can be made of the bond's behavior on a car-size scale and faster-setting-up adhesives are developed. But rapid progress is under way, with implications for many types of mechanical design.
The Lotus Take
“Globally, the challenge for the automotive industry is to increase the fuel efficiency of vehicles, while improving safety, performance and maintaining affordability,” says Jason Rowe, chief mechanical engineer for Lotus. In a recent research report, Lowe writes: “One of the biggest recent advances in joining technologies is in the performance of structural adhesives.”
Lotus is using structural adhesives with mechanical fastening techniques on its VVA (versatile vehicle architecture) structure, introduced in 2005 that is made mostly of aluminum shapes. Self-piercing rivets are used in place of spot welding with flow-drill screws used for single-sided access on closed sections. Both hold the structure together during the bonding cure cycle and prevent adhesive joint peeling in a crash. Heat-cured structural adhesive is the main joining medium. The system is said to provide exceptional torsional stiffness.
“The combination of self-piercing rivets (SPR) and adhesives provides a joint with a far greater peel resistance than bonding alone and at least three times the strength of riveting alone,” says Rowe. “In addition, the SPRs allow de-jigging of the primary structure before the adhesive is cured, holding the geometry and dimensions without specialist fixturing through electrocoat.” The new approach was required because spot and MIG welding can degrade aluminum.
Lotus is using the VVA in its upcoming 2+2 sports car. Once part of GM, Lotus makes sports cars and operates as an engineering consultancy under the ownership of a Malaysian manufacturing company.
The first American vehicle to make extensive use of the new crash-durable adhesives was the Ford F-150 pickup truck, which received the highest crash-resistant rating in its class by the U.S. Insurance Institute for Highway Safety. It was also the first crash-resistant adhesive that didn't require heated pumps.
Examples of other new models making increased use of structural adhesives include:
New models using the Delta and Epsilon platforms. One example is expected to be the 2009 Daewoo/Chevrolet compact sedan, although specific confirmation could not be made.
Betamate 1496 is used on Audi A5 structural elements such as pillars, bottom construction and front walls. Dow Automotive officials say the new adhesives provide up to 25 percent improved body stiffness and up to 15 percent more energy uptake in a crash. Their durability improves the strength of bonded joints up to 1,000 times the break time versus spot welds and gap protection is also provided by the sealing characteristics of the adhesive.
Dow developed a new grade for the BMW X5 front rails, front of the dash, shock towers and underbody. A plant in Michigan was modified to produce the material which is formulated to stay secured to the body structure during body wash, phosphate and e-coat baths. Other features are low viscosity, advanced processing characteristics and excellent corrosion resistance.
The external panels of all six luggage doors of the new Domino from Italian coach builder IRISBUS are bonded to the aluminum body with Betamate 2096.
Use of the new materials isn't limited to high-end cars. Adhesives developers say they see high interest from Asian and Indian OEMs developing a new, inexpensive world car. Adhesives allow more process flexibility compared to welding systems.
A Few Caveats
Some challenges still face increased use of crash-resistant structural adhesives. One is the basic conservatism of design engineers, who are familiar with, and trust, conventional joining methods, particularly spot welding. Another is the lack of a program that can simulate the mechanical properties of the new materials on a large scale. The thin bond lines in the new adhesives require an extra-fine mesh that increases the number of finite elements beyond the capabilities of even high-performance computers. Experts are working with the Fraunhofer Institute for Mfg. and Applied Materials research to solve the problem.
There is also a need for adhesives with higher heat resistance that will allow use of the new materials close to the engine block. There will also be a push for adhesives that cure at lower temperatures, allowing OEMs to save on e-coat oven operations. Also look for grades with shorter cure times.
Use of adhesives is not interfering with auto recycling in Europe, according to Marc Van Den Biggelaar, global market manager, Body Structure Solutions for Dow Automotive. Shredded body structures are bundled and sold to remelters, such as the electric arc furnaces used in the steel industry. “There's no special treatment for the adhesives. They burn off at higher temperatures,” says Van Den Biggelaar.
Recycling could become problematic, however, when steel is bonded to aluminum or recycling, one of the advantages of the adhesives systems.
Adhesives Play Critical Role in Composite Aircraft
The use of advanced adhesives to lighten weight has its roots in the aircraft industry, where adhesives are rising to new heights with all-composite aircraft, such as the Boeing Dreamliner and others under development.
3M recently developed two new adhesives that significantly increase the strength and rigidity of composite aircraft structures. “Bonding aircraft with composites versus metals bring some significant challenges,” says Tim Dietz, 3M Aerospace senior technical manager. For example, composites can absorb up to 3 percent of their weight in moisture from the environment. “We've developed a formulation to tolerate those levels of moisture during cure,” Dietz says in an interview with Design News.
The newly developed Scotch-Weld Structural Adhesive Film AF 555 is a thermosetting, modified epoxy adhesive film that is used with a honeycomb or part of a monolithic composite structure. It can be cured from 300 to 355F. Another version, AF 500, is designed for use in 250F curing applications.
The materials are applied at room temperature via either manual lay-up or automated systems. Typically, adhesive systems are kept frozen to avoid any premature reactions. The amount of time the adhesives are out of the freezer (“out time”) is significantly longer for composite aircraft because of the size of the sections. The AF 555 has an out time of one year, a step change for adhesives. Typical out times are in the five- to 45-day range.
At the request of a customer, 3M also developed a version with lightweight conductive screens to dissipate lightning strikes on composite aircraft surfaces.
The new systems were developed with several partners, according to Troy Ferrero, 3M's marketing manager for aerospace structural adhesives. Interestingly, Ferrero and Dietz both say each OEM has its own proprietary specifications for the strength required from the adhesive system. No OEMs would comment to Design News on specifics in their systems. Boeing announced recently though that the Dreamliner fuselage has passed ultimate load testing with flying colors.
While the Dreamliner 787 is the most famous of the new composite aircraft, several more are in development. In a major shift, Airbus announced late last year it would build the frame of the A350 from composites not metal. One expert says demand for carbon fiber in the aerospace industry will rise from 5,210 metric tons in 2006 to 10,700 metric tons in 2010.
The redesigned 2008 Ford Focus cuts curb weight with extensive use of new crash-resistant adhesives.
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