By KIMBERLEY GILLES, associate editor

Preparing aluminum parts for adhesive bonding at Mickey Truck bodies.


While adhesives may join materials better than welding or mechanical fasteners in some applications, a revolutionary breakthrough in adhesive technology will be needed before they eliminate the need for welding or metal fasteners.

While fabricators and builders have found that adhesives are a valid joining technology in some applications that improves product quality, reduces costs and speeds up production, welds and fasteners are simply better for creating such things as high-pressure tanks, and critical structural components for vehicles, buildings and civil engineering structures.

However, adhesive technology continues to evolve, and many companies are continuing to evaluate and implement adhesives to improve products.

Transportation equipment
Aerospace and automotive were among the earliest industries to adopt adhesives, mainly because adhesives can create seals that keep water out of metal fabrications while providing corrosion protection. However, the poor fracturing characteristics that early adhesives had limited their use.

Now, adhesive makers are formulating fracture-toughened compounds that provide distribution of stress and load transference while being relatively low in weight. Also, the new adhesives help to cut manufacturing costs because they reduce the number of mechanical fasteners, spot welds and filler metal that are required to create a joint or to attach metal to metal.

It was a combination corrosion resistance, stress relief and cost reduction that led Groupe Hesse (www.grouphesse.com) and Mickey Truck Bodies (www.mickeybody.com) to use structural adhesives in producing the components of the truck bodies and trucks that they produce.

Group Hesse Inc. built the industry's first all-aluminum beverage truck body. Until it changed its manufacturing processes, it was welding together the frames, attaching side walls with rivets and covering the rivets with plastic to create a clean space on the outside of the truck for painted-on advertising. But the rivets slowed the company's assembly process and, after the trucks were put into service, the rivets caused the paint to peel. Meanwhile, plastic rails the company used to cover the rivets were prone to damage and could not be customized to match the paint job.

The company, with the aid of Henkel Corp.'s Loctite unit (www.loctite.com), launched an 18-month study of in-service trucks that were manufactured with a Loctite adhesive. As a result, Group Hesse eliminated the rivets and plastic rails it previously used while reducing manufacturing time for the a rear exterior walls of the truck bodies it builds by about 25 percent. It now takes the Kansas City, Mo., company four hours to build those components, instead of 5.25 hours.

Separately, Mickey Truck Bodies Inc., a manufacturer of aluminum beverage delivery trucks and dry freight vans in High Point, N.C., also cut manufacturing costs and improved the aesthetics of its vans by eliminating the self-sealing through-bolts that were inserted every two feet along the length of the trucks bodies it built. Now it uses a structural acrylic from Loctite. Formerly, the process of attaching the bolts required two operators, but the adhesive process takes only one operator to apply the adhesive, position the bar on the wall and tack it with non-penetrating metal screws that provide stability until the adhesive cures, all in 20 percent less installation time.

Mickey Truck Bodies also used structural adhesives to replace metal-to-metal welding on the front bulkhead and back roll-up door units of its aluminum beverage trucks, says John Hargett, manager of engineering for the beverage truck division. It replaced welding because using adhesive reduces vehicle weight and joins the thin outer shells with the thicker structural extrusions without heat distortion, Hargett said.

Degussa AG (www.degussa.com), a specialty chemical company that includes adhesives in its products, says there are about 5,000 weld points in a mid-sized car, and that adhesives can replace about half of those welds.

Eliminating that many welds could cut the weight of a vehicle by about 55 lbs., while producing an auto body that is more rigid because adhesives create joints that cover wider areas of sheet metal than spot-welds, Degussa says.

Additionally, Degussa says the increased rigidity provided by adhesives could allow car-makers to use thinner gauge steel for car bodies, further reducing the weight of cars and increasing fuel efficiency.

However, reducing the gauge of the sheet metal and cutting the number of spot welds on car bodies requires adhesives to be more resistant to fracture under loads and for widely ranging temperatures that would include the heat of dry summers in Arizona and the cold of a moist Minnesota winter.

Dow Automotive (http://automotive.dow.com/), a business unit of Dow Chemical Inc., has started to address these requirements through a collaborative effort that includes with auto makers and suppliers. Dow Automotive says it has developed adhesive systems that bond to a variety of substrates, including oily sheets, without surface preparation — a major step forward in adhesive technology.

The ability of these and other adhesives to increase the stiffness of joints is an important feature as more automotive manufacturers consider the use of high-strength steels that have better energy absorption than lighter gauges of steel, but also have the same modulus as mild steel.

Marine adhesives
The boat and shipbuilding industries and naval forces around the world have been slower to adopt structural adhesives as a method for fastening metal to metal than the aerospace, automotive and truck body industries.

Shipbuilders have used adhesives in non-load bearing applications, and the U.S. Navy and Navy Joining Center at The Edison Welding Institute (www.ewi.org) are evaluating bonding carbon fiber-based composites to metals on primary load-bearing paths for the U.S Navy's new DD(X) destroyer program. However, the Navy is evaluating adhesives that are currently available on the market; not working with adhesive manufacturers to develop new adhesive technologies for the DD(X), says Larry Brown, project manager of the government programs office at the Navy Joining Center.

On the other side of the Atlantic Ocean, the European Union is evaluating the use of adhesives in ships through its BONDSHIP research program.

Documents published on the evaluation say that shipbuilders of large passenger ships could cut costs by 20 percent by using adhesive bonding in place of fastening supports, stiffeners and other attachments in outfitting passenger ships. Additionally, however, the reports say that shipbuilders are concerned about the performance of adhesives during a fire: Intense fire can damage the integrity of adhesive joints and compromise them. That concern also has the attention of the building construction industry, which has created an adhesive-mechanical fastener that is being considered as a replacement for traditional metal joining methods.

Construction
Structural adhesives have tended to lose their strength over time when used on galvanized materials. However, new adhesive technology has increased their durability on these treated metals. That's an important feature in the light-gauge steel construction industry, which also is known as cold-formed steel framing or steel-stud framing. Structural adhesives now can be used in combination with metal pins to replace spot welds and sheet metal screws in both residential and commercial construction to build trusses, joists, shear walls, studs and other structural elements, and they are said to reduce construction costs and time when they are used in combination metal fasteners

One of the arguments against using steel on steel or steel versus wood in residential construction is that using steel is labor intensive and costly. For example, constructing a 4 ft. by 4 ft. shear wall that meets the Seismic Zone 4 requirements typically required in California takes about 300 collated screws and 45 min. of assembly time. However, building that same shear wall with adhesive and pneumatic pins takes 5 min. and 75 pins, for an 85 percent reduction in time. The number of fasteners can be reduced because adhesives distribute the load better. Fasteners continue to be used to hold the metal in place while the adhesive cures, and to insure the integrity of the building during a fire.

Adhesives seldom are discussed for use in metal-to-metal bonding in civil engineering projects, especially when projects may have a design life of 100 years because durability and the tendency for adhesives — even the most advanced adhesives — to lose their strength over that period of time remain issues that discourage their use. Welding, screws, bolts and rivets are well known and well established joining technologies, while synthetic adhesives have been around for about 50 years. However, their use and effectiveness during that time has been noted: Two-component, heat-curing epoxy adhesives were used to bond exterior metal gas distribution pipes for the housing market. These pipes are still leak-proof after 30 years to 50 years, under 5 bars pressure.

Joint Design

The successful use of any structural adhesive depends on a number of factors, including the selection of the right product for the application and joint design. Adhesive joints should be designed so that basic stress is shear or tensile, and cleavage and peel should be minimized. All the bonded areas should share the load equally. The accompanying diagrams explain which joints are good and which to avoid, and alternative designs.


The basics

The advantages of adhesives and adhesive bonding include:

  • Their ability to spread or transfer loads over the bonded area, making a more uniform distribution of stress compared to spot welds and mechanical fasteners.
  • Their tendency to reduce the weight of the structures they are used in because less filler metals and fewer mechanical fasteners are used.
  • Their ability to increase stiffness of structures and fatigue resistance while providing reduced noise and the transference of vibration because of their visco-elastic characteristics.
  • Their ability to join dissimilar materials and materials with different thicknesses.
  • Their ability to maintain the integrity of the bonded material because they require less heat input.

The disadvantages of adhesives and adhesive bonding include:

  • Their lower strength when compared with welding or fastener technologies.
  • Their low peel strength for loads that are out of the plane of their design.
  • Their limited resistance to heat.
  • The requirement for surface pre-treatment prior to application.
  • The need to ensure total wetting of the joined surfaces, often in places that cannot be seen.


The following people provided their time and expertise to this article:

Steve Ouellette, platform manager for structural adhesive technology for Henkel Loctite.
Joe Dequinque, market development manager for agricultural and construction machinery and specialty vehicles for Henkel Loctite.
Greg Morgan, director of product management for Sika Corp., industry adhesives and sealants.
Manny Dias, manager of research & development for Permabond Engineering Adhesives.
Adam Conklin, account manager, Lord Corp.