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“Any structural application”—wherein two pieces of metal are joined—“would be enhanced by the use of adhesives.” Not entirely surprisingly, the person making that statement works for Henkel Technologies (www.automotive.henkel.com). Chris Liddiard, director of Marketing and Technology, Parts & Foams, within Henkel Corp. (www.henkel.com), which owns arguably one of the most famous names in adhesives, Loctite, explains his point quite simply. Consider a body panel that’s welded in place. “Every weld point is a point of strength.” Note that he’s not anti-welding. “But between the welds there is no strength.” He acknowledges that there is the possibility of having a continuous weld, but chances are, the cycle time would be too long, or, if there was, say, a faster process like laser welding deployed, depending on the materials being welded (e.g., he cites some of the new higher-strength steels as being somewhat problematic), there could be heat distortions. “If you adhesively connect two points”—or welds—“then you have a continuum of strength.”
Which leads to the question of whether it might not be possible to simply glue a vehicle together. Liddiard’s colleague, Michael P. Sylvestre, Business Development Manager for Automotive Adhesives & Sealants, says “Where you see welds, mechanical fasteners, or rivets, there are fertile areas for adhesives.” But he goes on to point out that, for example, one couldn’t simply eliminate the spot welds because one of the things that the weld does is to hold the components together as it moves through the process. He notes that regardless of whether it is a structural adhesive that needs to be cured in the e-coat oven or a two-component adhesive that sets up more quickly, there isn’t sufficient clamping to keep an inner and an outer panel from shifting during an assembly operation.
Bus Building. If process time is not an issue, then there can be far more bonding. Rusty Mansel, Henkel Technologies Market Development Manager for Automotive Components, points to the Saf-T-Liner C2—a school bus—manufactured by Thomas Built Buses (www.thomasbus.com) in its plant in High Point, NC. That vehicle is assembled with welding. But there is the extensive use of self-piercing rivets and adhesives. According to Thomas Built figures, the use of the adhesive reduces the number of body rivets and fasteners by more than 65% and the combination provides a much stronger joint: in a pull test with a 8-in. rivet body joint with a 3.5-in. overlap, the joint tears at an average of 8,462 lb.; when the adhesive is used and there is a 2-in. overlap, the metal tears at an average of 13,260 lb.—the joint doesn’t break. Arguably, they’re building better buses.
They do anticipate an increase in the deployment of adhesives in auto applications for at least a couple of reasons. One is that vehicle manufacturers are working to increase the crashworthiness of their vehicles and adhesives can help. That is, with the use of higher strength steels, there can be an issue with yielding under stress. By using structural adhesives (along with spot welds), the stresses are spread more widely than is the case with a series of isolated spots. Another reason is related to NVH: By using adhesives, they say, there is better stiffness and durability, which help minimize the structural squeaks and rattles that can be generated. Companies including Audi, Mercedes, and General Motors are increasing their adhesive use to address these issues.
Potential—& Price. Liddiard suggests that most bodies use about 15% of their potential in adhesives. He sees that going “nearer to 50%” in the next five to six years.
One potential inhibitor to the greater utilization of structural adhesives is that of cost. The chemistries have a broad range of prices. While an epoxy might be available for $35 a gallon, a second-generation high-strength adhesive might cost $150 a gallon. Unless the person who is making the purchasing decision understands the whys and wherefores of the additional expense, he might simply dismiss the latter as being completely unreasonable and not permit the selection of the right material for the applications.