Structural Adhesives Unable to Replace Spot Welds in Light Vehicles & How that has Helped Them Grow

Posted on 2/19/2015 3:30:30 PM By Marc Benevento

To start, I have to say how excited I am to have the opportunity to be contributor to the ASC blog.  This is the first of what I hope to be many posts on the subject of adhesives in transportation (automotive, heavy truck, bus, and marine) and “wood” products, such as doors and windows, which can be constructed of wood, plastics, or composites. 

With so much change taking place in automotive materials, the use of structural adhesives in light vehicles is great subject on which to start.

Much has been written about the growth of structural adhesive usage related to the use of multi-material structures to reduce vehicle weight, and rightly so.  Increasingly stringent regulations on emissions and fuel economy are forcing automakers to explore every option to improve vehicle efficiency.  This includes the use of a broadening array of body materials such as high strength steel, aluminum, magnesium, plastics, and composites.  Because spot welding cannot be used to join dissimilar metals or plastics, new joining techniques are required—structural adhesives included among them.

The ability to join dissimilar materials, however, is not the only driver of increased structural adhesive usage in automobiles.  In fact, it isn’t even the primary reason.  This is curious because the value proposition of structural adhesives is normally associated with the elimination of spot welds and mechanical fasteners.  However, automotive engineers have redefined the value proposition of structural adhesives by finding additional benefits the products provide.

At the same time automakers are trying to reduce weight, they strive to improve vehicle safety, handling, comfort and perceived quality.  It is the combination of driving forces, many of which oppose each other that creates opportunities for structural adhesives.

Vehicle safety, reduced to its simplest form, is an exercise in energy conservation.  The kinetic energy of a moving vehicle (or two) must be dissipated until it reaches zero and the vehicle is at rest.  The rate and manner in which this takes place dictates how much energy is transferred to the occupants and how well they are protected.  Toughened structural adhesives used in conjunction with spot welding, known as weld-bonding, can increase the energy absorption of automotive frame rails and serve as a tool to improve vehicle crash safety.  In this case, the primary function of the adhesive is not to join but to reinforce the rails.

In addition to helping improve vehicle safety, weld-bonding automotive frame rails offer other performance enhancements.  A bead of adhesive effectively reduces spot weld pitch to zero, which increases stiffness of the section.  Used in the appropriate locations, structural adhesives can increase torsional rigidity of the white body, which is critical to delivering a responsive driving experience.   Again, the primary function of the adhesive is to supplement the performance of existing welds, rather than replace them.

In short, structural adhesives enable OEMs to create superior vehicles without changing the way they are manufactured.  As a result, the use of structural adhesives is increasing in all vehicle segments, but particularly in premium coupes, sedans, and crossover vehicles where dynamic performance is a priority.  In fact, Audi and Cadillac claim that some current vehicles contain over 100 linear meters of structural adhesive in each white body, and they expect usage to increase in the future.  This consumption is based on what a structural adhesive supplier might consider a secondary benefit, reinforcement and stiffening, rather than the primary benefit their product offers, which is the ability to join two different materials without additional fastening methods.

Given the many benefits structural adhesives offer automotive OEMs, I imagine that creative new applications will be found for them as the material mix evolves and the performance bar continues to escalate.  These new applications may not be limited to the primary benefit a product offers. Maintaining an open dialogue between automotive engineers and the supplier community that is focused on functional objectives, rather than products, will lead to discovery of unmet needs.  This, in turn, will lead to faster and more efficient development of new products and applications.

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