Adhesives grasp new tasks

What do a hearing aid, golf club, and race car have in common? They all benefit from the bonding versatility of adhesives.

In many cases, engineers look to adhesives to do more than just replace mechanical fasteners. For instance, epoxies, acrylics, urethanes, and polyester adhesives can reduce costs, bear loads, resist hostile environments, and add cushioning and structural integrity. They can also enhance aesthetics, trim weight and assembly time, or assemble components where mechanical fasteners can't go. More than ever, adhesives are called on to join difficult-to-bond substrates such as dissimilar materials, components with different coefficients of thermal expansion, or coated surfaces that resist adhesion. New formulas cure quickly, adapt to most manufacturing techniques, and deliver durable, flexible bonds.

For bonding metals, glass, or plastic, adhesives often do double-duty sealing joints and dampening vibration. The following examples illustrate some of the benefits of these versatile systems.

Acrylics bring a custom fit to hearing aids

At Starkey Laboratories offices in the UK, Germany, and Eden Prarie, MN, a new approach to hearing-aid design called for a change in adhesive. The challenge: Speed production cycles and improve tolerances-without sacrificing quality or performance.

For their custom-fit devices, Starkey engineers use compounds in Loctite's 190 series to bond electrical components in place and to act as the shell of the hearing aid. "People's ears are like fingerprints," says Loctite Senior Application Engineer Patrick Courtney. "The hearing aids have to be custom-fit to be comfortable."

With the help of engineers from Loctite, Rocky Hill, CT, the Starkey team developed a formula and curing process now used at the company's laboratories worldwide. After rigorous testing, they switched from a two-part, room-temperature-curing adhesive to a one-part, low-viscosity, UV-cure acrylic.

"The room-temperature-cure adhesive had created some problems in processing time and control of tolerances," explains Courtney. To custom-fit the aid's tiny electronics, shell-wall thickness had to be held to close tolerances.

By molding the shell with a self-leveling, UV-curing formulation, engineers shortened process time and improved tolerance control. But the design change wasn't a simple swap. "The difficult aspect was that the hearing aids are flesh-tones; users can choose from four colors," explains Courtney. Because UV-cure adhesives are typically transparent, Courtney and his colleagues had to work closely with pigment suppliers to select color-matching equipment and blend the colors correctly.

Once they had the right colors, engineers had to grapple with two other performance factors: cure time and viscosity. After testing several chemistries, Loctite and Starkey engineers developed a mix of UV-sensitive photoinitiators and visible-light photoinitiators to fine-tune the shell's fast cure cycle. They also achieved a viscosity that suited the existing process equipment, without changing durometer, modulus, or other performance parameters.

"We've taken a specialized process and given it rapid turn-around," says Courtney. "Now, it's 24 hours from when the mold comes in to delivery."

Polyester adds speed on the raceway

A race track may be one of the most punishing environments for testing how well cars hold together. It might surprise some enthusiasts to learn that Sports Car Club of America (SCCA) race cars, like the Chevy Camaro driven by Dick Valentine, rely on adhesives in some of the most demanding joining applications.

When chassis-design team Riley & Scott, Inc., Indianapolis, looked for a fastening system to bond titanium and aluminum body panels and suspension and drivetrain components to their specialized tube-frame chassis, they chose adhesives.

The chassis, which provides the core rigidity and strength of the Camaro, is exposed to solvents, temperatures exceeding 177 degrees C (350 degrees F), and severe vibrations. Wildly varying dynamic conditions mean the adhesives must join substrates with different coefficients of thermal expansion. What's more, in an application where every ounce counts, there's no room for extra weight on board.

Design Engineer Bill Riley chose formula 1155 high-bond polyester film from Venture Tape Corp., Rockland, MA, for the job. The double-coated film conforms well to the very flat panels and uneven surfaces of the chassis frame, he says. Weight is another critical factor. "The film saves weight because you can put it right where you want it. You don't have to worry about run-off or having the panel caked with extra adhesive," adds Riley.

Perhaps more importantly, the film's high shear strength adds to the structural integrity of the car, says Principal Mark Scott. "And a big cost savings comes from the time you save applying it."

To test the cars, engineers mount one end of the car at the suspension mounts and twist the other end with a lever arm. "We do a lot of static testing for torsional rigidity of the chassis, and we could tell from past experience that the film was going to do its job right away," Riley explains.

Other Venture Tape adhesives are used worldwide in Ford and Chrysler vehicles for firewall sealants, trunk liners, and window sealants.

Epoxy boosts power on the links

At Taylormade Golf, the expression "off with your head" has more than the typical distressing associations. A few years ago, the international golf-club manufacturer found that an adhesive failure was causing the heads and shafts of their clubs to come apart.

"We previously used another epoxy," explains Product Manager Will Priest. "It wasn't the strongest glue, and it wasn't compatible with differences in material-bonding steel and graphite." Taylormade engineers also found that the bonds didn't withstand the thermal cycling of a hot car trunk to a cool basement, for example. "Heads were flying off-and it was a serious liability problem," adds Priest.

To bond their carbon-steel or graphite shafts to heads of stainless steel or titanium, Taylormade called in Ciba-Geigy Corp., Los Angeles, CA. "Ciba-Geigy engineers helped us make changes," says Priest. "We chose two-part epoxy AW136 with catalyst HY994, and a new application process."

One immediate improvement resulted from switching to a very accurate meter-mix system from Sealant Equipment, Oak Park, MI, instead of hand-mixing the epoxy. Next, Taylormade engineers redesigned their application technique. They formerly applied adhesive to the tip of the shaft, installed the head, then twisted it for an even coating of adhesive at the part called the hosel where the bond forms.

But engineers suspected that the technique didn't provide good contact at the hosel. To inspect the wetting surfaces, they used a glass hosel. "Sure enough, you could see air bubbles," says Priest. "Air pockets in the bond area were preventing proper wetting; we were only getting about 50% coverage."

Now, engineers seal the end of the hollow golf shaft with a 0.2-gram weight. Then they dispense about 1 cc of epoxy into the bottom of the hosel instead of applying it to the shaft. An automated press applies about 1034 kPa (150 psi) to press the shaft into the hosel. This forces the high-viscosity epoxy between the shaft and the interior of the hosel. Engineers deliberately apply some excess epoxy, and as the shaft reaches the bottom, the extra squeezes out, explains Priest. "We wipe that excess off, and we know we have 100% coverage."

Continuous flow. The new epoxy didn't mean a sacrifice in manufacturing speed. "We run continuous-flow manufacturing," says Priest. "We need our epoxy to cure fast. Ovens would create a huge bottleneck and we'd have to have a lot of inventory."

Curing stations with heating collars deliver a rapid cure using conduction. The semi-circular elements clamp around the hosel and heat the epoxy to 127 degrees C (260 degrees F) for about two minutes. In addition to being fast, the rapid-cure boosts chemical cross-linking in the bond, which bolsters strength, say Ciba-Geigy engineers.

To quantify the improvements, Taylormade engineers performed pull tests. They cut the club's shaft about eight inches from the head, then mounted it in a machine that tries to pull the head off. "Where we obtained pull strengths of about 1,500 lbf (6,672N) with the old epoxy and the old method, we now obtain strengths of 3,000 lbf (13,345N) minimum, with typical strengths at 4,000 to 5,000 lbf (17,793N - 22,241N)," says Priest. In fact, the steel shaft often breaks before the epoxy fails, he adds.

To simulate real-use dynamics, Taylormade engineers also developed an impact test system: A large hammer on a pendulum delivers 100 impacts, then the club must pass the pull test at 3,000 lbf. "Many of our competitors' clubs withstood only 15 or 20 impacts, then the head would fly off," says Priest.

Now, engineers use the new epoxy at manufacturing sites in Togane, Japan, and Carlsbad, CA. Says Priest: "We haven't had a bond failure in the field since."

Urethane bonds finicky auto substrates

New clear-coats used on auto-body panels provide high-performance protection from hostile environments, and sleek glass suits aerodynamic profiles. The quandary: how to bond glass to surfaces treated expressly to resist just about anything?

"You've got a clear-coat that's designed to resist damage from acid rain and everything else, and you have to bond it directly to glass with adhesive. That's a tough challenge," says Steve Henderson, marketing development manager at Essex Specialty Products, Troy, MI.

At Chrysler's Jefferson plant, Warren, MI, and at General Motors' OPEL facility in Brussels, Belgium, and other European sites, a new urethane adhesive from Essex addresses the problem and improves quality.

For Chrysler's 1995 Jeep Cherokee, the adhesive eliminates a difficult step in the assembly of windshield glass and rear glass. Essex formula 15706 BETASEAL glass adhesive bonds directly to environmentally resistant automotive finishes-without a primer.

Using primers in the process proved difficult for Chrysler because the bond surfaces are surrounded by class-A finishes, explains Henderson. Because assembly required a primer, the sensitive process had to be monitored closely by human operators to ensure that no other surfaces were affected by the solvent-and timing was critical to a sound bond.

Teamwork yields good chemistry. By teaming with paint formulators, glass makers, and Chrysler engineers, Essex developed a primerless-to-paint adhesive for bonding to new top coats that also adheres to glass without primers.

"One of the biggest challenges was to make the change transparent to manufacturing. They can use the new formula with existing equipment to avoid any capital expense," says Henderson. Like the previous formula, the new urethane adhesives are robotically applied. The urethane is extruded in a bead to the stationary glass, then automatically installed in the car. The adhesive also works with existing glass and PVC encapsulation primers.

In addition to eliminating mechanical fasteners and metal weather strips used with gaskets, the polyurethane adhesive also eliminates:

Cost and volatile organic compounds of the primer.

The new adhesives also allow for manufacturing flexibility, say Chrysler engineers. Instead of primer dry-time dictating where in the line the glass must be installed, the installation can be moved in the assembly line to optimize productivity.

The adhesives use a low-conductive formula for vehicles with electronic components such as radio antennae in the windshield or rear glass. And because they add strength and resiliency to the vehicle's structure, they help automakers meet federal motor vehicle safety standards such as roof-crush regulations, add Essex engineers.

By eliminating the need to prime paint, the adhesive offers several other possibilities, say Chrysler engineers. Potential applications: trim bonding and other automotive uses where it could replace welds or fasteners.