Advances in adhesive technology have dramatically expanded the scope of potential metal bonding applications. Until recently, most structural acrylic adhesives would lose strength over time on galvanized steel. However, the latest adhesive formulas provide long-term durability on galvanized substrates. Structural acrylic adhesives are also excellent alternatives for bonding metal to plastic, metal to composites and metal to metal, as has been shown in the light-gauge-steel construction industry, where structural adhesives for metal bonding were first introduced 2 years ago.

In this sector, adhesives have been used in combination with metal pins to replace expensive sheet metal screws. Structural adhesives have also been formulated to bond trusses, joists, walls, headers, studs and cantilevered beams, where their use can reduce overall component costs by 60 percent, thanks to savings found in assembly time. Obviously, because they are being used in both residential and nonresidential structures, these adhesives absolutely must maintain their strength through varying temperatures and weather conditions without corroding or failing for the life of a building.

In manufacturing, structural adhesives are now being used to bond steel, galvanized steel and aluminum in the HVAC and appliance industries, where corrosion resistance is critical. Manufacturers are also using structural acrylic adhesives to assemble metal, plastic and composite frames, panels, booms and cabs on specialty vehicles such as trailers, truck bodies, buses and construction equipment; to galvanize steel frames to fiberglass and ABS tubs and spas; to bond plain, painted or powder-coated metals and plastics in chair, desk and cabinet applications; and to assemble metal, plastic and composite signs and outdoor displays. And, these are just a few examples of the many applications where structural adhesives are being used.

Three Ways to Get Attached

For example, thermal methods work very well when joining homogenous materials with similar melting points. They also create a very strong bond and can fill gaps. Mechanical fastening will not seal, but can create strong, durable joints between dissimilar substrates. Chemical assembly works well when securing dissimilar substrates, and can fill large gaps and seal the bond joint. As an added benefit, structural adhesives resist moisture and can stand up to all kinds of weather in harsh outdoor applications.

Among the most important limitations with regard to thermal joining and mechanical fastening are cost and appearance. For example, thermal joining requires specialized labor, and welded joints are often non-uniform and lack the clean aesthetics desired for high-end applications. Welded, brazed or soldered parts are also very difficult to disassemble.

Mechanical fastening is also an expensive process, requiring time, and the correct inventory and equipment to drill holes and insert fasteners. Depending on the application, mechanical fastening can require multiple operators. Furthermore, the requisite holes can cause quality issues down the line as they create leak paths, a starting point for corrosion, and may detract from the visual aesthetics of the end product.

Finally, because fasteners and thermal joining concentrate stress at a single point, they may cause premature joint failure and have difficulty withstanding stresses caused by flex or vibration.

In contrast, two-part structural acrylic adhesives provide a relatively quick and easy means of filling large gaps, sealing joints and joining dissimilar materials, all the while imparting a neat appearance to the finished product. In addition, these materials deliver thermal and chemical resistance, and distribute stresses evenly across the entire bond line, making for a stronger assembly. In some vehicle applications, steel has actually been known to fracture and split before adhesive joints fail. By eliminating the holes made with mechanical fasteners, structural acrylics help reduce corrosion and deliver an aesthetically appealing, long-lasting joint. By eliminating the stress concentrations produced by fasteners, some manufacturers have even been able to use thinner gauge metals than had previously been possible.

Structural acrylic adhesives cure by mixing two separate parts-a resin and an activator. Once the two components are mixed, a room-temperature chemical reaction occurs, delivering a very strong bond to metals, plastics and composites. Structural acrylic adhesives require little surface preparation and can be formulated to deliver application-specific open times from minutes to hours.

The mix ratio for the adhesives is forgiving and allows some margin for error, although assemblers should be aware that, once mixed, the adhesives generate heat during the exothermic curing process. Luckily, manufacturers can minimize the effects of this heat by controlling the amount of adhesive dispensed, the size of the assembly and the substrates used. For example, a large metal assembly will dissipate heat faster than a small metal, plastic or composite part.

Note that acrylic adhesives will not bond well to wood and rubber. Also, because most structural acrylics cannot resist temperatures above 250 F, processing that involves elevated temperatures, such as a paint bake cycle, may present problems. However, a few structural acrylics can withstand temperatures up to 400 F for short periods of time, allowing for use in paint bake cycles without a significant loss in bond strength.

Ultimately, a structural acrylic adhesive will require a cure time of up to 24 hours to achieve full strength. However, many formulations allow handling of assemblies in just minutes (fixture time). To keep production lines moving, mechanical fasteners are sometimes used to temporarily hold the assembly in place while the adhesive cures. These small fasteners are used only sparingly and do not require through-holes.

High-performance adhesive formulations are available for bonding specific substrates such as aluminum, stainless steel, carbon steel or galvanized materials. Adhesives are also being formulated with "toughening" agents to improve impact and peel resistance. For example, rubber-toughened structural acrylic formulations can be used in those applications requiring excellent cold impact resistance, long-term fatigue resistance and durability.

Real-World Applications

For example, Group Hesse (Quebec), the oldest beverage trailer and truck body manufacturer in North America, is now using adhesives in place of rivets to attach exterior walls to aluminum trailer frames.

Previously, Group Hesse had to use an expensive, time-consuming, three-step process that began with drilling dozens of holes into the trailer walls in preparation for the rivets. Once that was done, operators would rivet the wall panels in place and install a white, plastic rail around the edges of the front and back walls to cover the rivets. In theory, at least, this plastic rail served to improve the appearance of the front and back walls, which are used for advertising space.

Unfortunately, the plastic rails did not match many of Group Hesse's customers' custom paint jobs, and they were easily damaged. The rivets also caused the paint to peel, resulting in rust and corrosion in Canada's extreme weather conditions. It was pretty obvious that Group Hesse needed an assembly process that was simpler, less expensive, more attractive and offered better long-term performance.

Ultimately, the company decided to attach the walls using Loctite H8000 Speedbonder adhesive from Henkel Corp., a fast-fixturing structural acrylic that offers excellent impact and peel resistance on aluminum. The adhesive completely replaced the rivets on both the front and rear exterior walls, eliminating the plastic rails. Application and assembly are fast and simple. The adhesive is manually applied to the frame using a pneumatic cartridge dispenser. Afterward, operators clamp the panel in position, remove any excess adhesive, and allow the assembly to cure for 2 hours. Total process time is now 4 hours-compared to 5.25 hours in the past. No additional reinforcements are required.

After incorporating the adhesive, Group Hesse monitored a fleet of the new beverage trucks for 18 months as they went about their business on Canadian roads. Tests found that the longevity and aesthetics of the painted graphics improved dramatically. By eliminating the rivets and the plastic rail, Group Hesse's customers were also able to extend their painted graphics all the way across the panel. The corrosion that historically started at the rivets disappeared.

Group Hesse is now evaluating whether to use adhesives to assemble the truck body's aluminum frame. In doing so, the company hopes to reduce its reliance on difficult and expensive welds. In addition, because adhesives will more evenly distribute joint stress, Group Hesse hopes to increase the overall structural integrity of the assembly.

"It is obvious to me that welds will be used less and less in our design, replaced by stronger, more efficient technologies like Speedbonder H8000," says Group Hesse chief engineer, Martin Barrette.

Life Without Through-bolts

In the case of Mickey Truck Bodies' product, the challenge lay in a series of galvanized steel e-track bars that are attached to the fiberglass-reinforced plastic interior walls of its truck bodies. Truckers secure ratchet straps to the bars to hold cargo in place during transportation. If the e-track bars fail, cargo can move throughout the truck body, potentially causing dangerous weight shifts.

Historically, the company would use self-sealing through-bolts inserted every 2 feet along the entire length of the truck. This required a pair of operators working in tandem and resulted in an unappealing bolt-strewn exterior.

To speed up the process and create a better-looking product, Mickey Truck Bodies began using Loctite H8600 Speedbonder, a structural acrylic formulated for long-term strength on galvanized steel. This product has replaced the through-bolts along the e-track and has made the installation process a one-man job.

In practice, an operator applies the adhesive along the e-track bar and then positions the bar on a wall using sheet metal screws to tack the track in place. These screws do not go all the way through the wall, because they are only there to provide stability while the adhesive cures.

In the end, Mickey Truck Bodies found it was able to reduce e-track installation time by 20 percent, relying on just one operator. The company further reduced costs by eliminating the expensive through-bolts, and the look of the truck bodies is much improved. Finally, the e-track bar attachment is actually stronger than in the past, because the stress is spread over the entire surface area of the bar, instead of being focused around the individual fasteners.