When you imagine the assembly line at an auto plant, you probably picture welding machines, rivets, bolts and screws fastening all the parts of a car together. You might be surprised to learn that many automakers are using glue to bond some parts together on modern vehicles. This isn't the kind of glue you'll find in a school kid's backpack, or the kind of car glue that you might use to assemble a plastic model -- automotive structural adhesives are advanced materials called epoxies that can be customized to bond to almost any surface and endure a wide range of temperature extremes.
Using glue in auto manufacturing isn't a way to cut corners or make an inferior car. Modern adhesives offer a lot of engineering advantages over traditional methods of fastening two parts together. In fact, adhesives usually form a bond stronger than the materials they're bonding together. Adhesives could be the key to building lighter, more efficient cars, and when new materials such as carbon composites are used, adhesives might be the only way to bond them as carbon fiber panels cannot be welded together.
Of course, auto adhesives aren't perfect -- there are a few environmental concerns and some applications for which they just don't work very well. Which adhesives work best for the many different parts of a car? And how will the use of adhesives change the auto industry? As a result, will we see less expensive cars? If so, will they be safe? Keep reading to find out.
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.
Pros and Cons of Adhesives
Using adhesives has some major advantages over other fastening methods. An adhesive distributes loads better than a spot weld or a rivet. Imagine two body panels that are attached with a few spot welds. Every time the car hits a bump, the force of the bump travels through the car. When it passes across those two body panels, the spot welds carry all the force. This leads to stress fractures in the area near the welds. An adhesive would allow the panels to be attached across a wider surface area, distributing the force in a more efficient manner. A good seam weld would accomplish the same thing, except welds are so rigid they don't transfer force very well. Adhesives, especially polyurethane epoxies, have the ability to flex and absorb some of the force, and transmit the rest of it evenly across the surface area. This is one of the reasons airplane manufacturers have been using adhesives for years.
Adhesives also allow for the use of lighter structural materials. Welds and bolts require thick, heavy steel pieces to anchor them. With an adhesive, thinner-grade steel can be used, or the manufacturer can switch to alternate materials such as aluminum or carbon composites. These materials can't be welded the way steel can, so adhesives are often the best option. Indian automaker Tata reduced the weight of their ultra compact Nano in part by using adhesives to attach certain body panels.
Temperature does affect adhesives, but the epoxies used to assemble cars don't melt until they are heated to 400 degrees Fahrenheit (204.4 degrees Celsius) or more, and they don't crack until they are cooled to minus 40 degrees Fahrenheit (minus 40 degrees Celsius). Obviously, these are extreme temperature variances that most people would never encounter in their lifetime. For the average driver, temperature is no worry.
That's not to say that adhesives are without their flaws. The auto industry doesn't have a lot of long-term data on the use of adhesives, and some question their ability to stand up to 10 or 20 years of use. Structural adhesives have been used by European manufacturers for several years, so confidence is growing in adhesives' longevity.
There is one potential negative side effect of adhesive use -- more expensive repairs. If an adhesive-bonded structural part breaks, most repair shops will not have the facilities necessary to rebond the adhesive. Instead, they would have to use a pre-bonded component sent by the manufacturer, possibly forcing the car's owner to pay for more than just the broken part. A good body shop can handle a cosmetic repair by cutting away the damaged section and patching in a new part with adhesives specifically designed for this purpose.
While adhesives are gaining wider use today, their use in auto assembly is not really new. Car windshields have been held in place by polyurethane epoxies for decades, and in the last ten years or so, manufacturers such as Saturn and even BMW have attached plastic cladding to their cars with adhesives instead of bolts or welds. As auto designers explore new technologies and new materials, we may see the use of adhesives skyrocket in the next few years. Even so, there will probably always be a place on the assembly line for a welder.
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More Great Links
- Adhesives and Sealants Industry. "Structural Adhesives for Advanced Automotive Manufacturing." Sept. 1, 2007. (Aug. 7, 2008) http://www.adhesivesmag.com/Articles/Cover_Story/BNP_GUID_ 9-5-2006_A_10000000000000161632
- Gilles, Kimberley. "Making joints with structural adhesives." Welding Magazine. (Aug. 7, 2008) http://weldingmag.com/processes/news/wdf_38771/
- Sabatini, Jeff. "Can You Glue A Car Together?" Automotive Design and Production. August 1999. (Aug. 8, 2008) http://www.autofieldguide.com/articles/089903.html
- Smock, Doug. "GM Will Use New Structural Adhesives Extensively." Design News. March 24, 2008. (Aug. 8, 2008) http://www.designnews.com/blog/Engineering_Materials/858-GM_Will_Use_ New_Structural_Adhesives_Extensively.php