Epoxies: Chemicals That Stick Together
Applying adhesive in auto manufacturing is a slightly more complicated process than just slathering on some glue and holding the two parts together. For one thing, automotive structural adhesives are made from chemicals called epoxies. An epoxy comes in two parts, a resin and a catalyst. When the two components are combined, the catalyst starts a chemical reaction in the resin, and the resin develops its bonding properties as the mixture cures. Curing times for the epoxy can range from just a few minutes to a day or even longer. In some cases, curing may be aided by baking or some other form of applied heat. Some epoxies even cure with the application of ultraviolet light.
There are two types of epoxies commonly used: polyurethane epoxies and glassy matrix epoxies. Glassy matrix epoxies are extremely strong and rigid, and they resist shearing -- lateral separation of the bonded parts -- at very high force levels. Polyurethane epoxies are more flexible, but they break under shearing forces at much lower force levels than glassy matrix epoxies. A newer two-stage application process called synergistic rubber toughening gives high-strength epoxies greater flexibility so they are less likely to fracture under stress [source: Smock].
Further complicating the adhesive application process are the unique needs of an automotive production line. In some cases, adhesives must be applied to an entire section of the car in a single batch. After some additional parts are attached and bonded, the section may need to go through a cleaning and painting process before the final pieces are attached. The adhesive needs the ability to stay in place when subjected to liquid sprays, electrodeposition and other assembly processes. To avoid the use of expensive pre-curing ovens, this curing ability can be built-in to the adhesive, but it requires a lot of testing and some impressive chemistry.
The best place to use a structural adhesive is in a location where the primary forces are either compression, where the joined pieces are pushed together, or shear, where the force tries to slide the joined surfaces against one another, like pressing your hands together and trying to slide them apart.
Structural adhesives are typically not good to use if the force acting on the joint would pull the two pieces apart as most adhesives have poor peel strength. Another poor location for a structural adhesive would be one where there are forces that would bend the joint, which could allow the joint to cleave and then peel [source: Gilles].
In addition to the epoxies, urethane foams are also used in certain areas of a car. While they do have adhesive properties, these foams are used more for their properties as sealants than as structural fasteners.