New models based on General Motors Delta and Epsilon platforms will make extensive use of new crash-resistant structural adhesives.mOne example is expected to be the 2009 Daewoo/Chevrolet compact sedan, although General Motors will not make comments about future designs until vehicles are released. The new adhesives improve the crash worthiness of cars and also allow use of thinner gauges of steel. The new generation of structural adhesives has a crash resistance that is more than ten times higher than conventional reinforcing adhesives.
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, however, 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.
New technical breakthroughs 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 curing of the adhesive.
Another important development came when adhesives producers developed grades that are compatible with existing automotive production lines. The new grades typically cure at temperatures around 356°F, allowing the electro coat process to cure the structure without a dedicated curing oven. The newest of the materials can be used in temperatures ranges from minus 40 to 176°F.
Two leaders in the field are Henkel and Dow Automotive. Another factor is that the adhesives enhance new high-strength steels. The new adhesives improve seam integrity by providing a dual energy management system. High-strength steels don’t yield very much so energy often is pushed to point of the spot weld, unless the adhesives are used.
“So far, the new adhesives have been used mostly as an enhancement to spot welding,” comments Chris Liddiard, a director of structural solutions for Henkel North America. “The trend we see going forward is the gauge reduction. A lot of these applications are in development now.”