Dan Bradshaw,
Industrial Assembly and Components Lord Corp.
Cary, N.C.

Scelzi Enterprises Inc., Fresno, Calif., previously had to hand caulk each custom truck body, inside and out, to prevent leaks, rust, and corrosion, explains Scelzi Shop Foreman Jose Villa. The switch to Lord Engineered Adhesives not only seals the cabs from the environment, it cut 3 hr from the prep and painting process, improved truck appearance by eliminating weld marks, and did away with 100 rivets attaching aluminum diamond plate lifts to the top of the service bodies.

4-Star Trailers, Oklahoma City, extensively tested the strength of bonded joints in its custom horse trailers. 4-Star built two doors. One was a traditional welded frame door that employed double-sided tape and rivets to hold the outer panels in place (a common method of construction in the industry). The second was constructed entirely using Lord Engineered Adhesives. The doors were hung to a heavy steel frame and a hole was drilled in the lower corner of each to simulate where a horse might kick the panel. Then technicians threaded a 3/4-ton winch through the hole. The welded door pulled off the frame under a 40-lb force. At 950 lb of force, the welded door pulled out 5.5 in. and broke apart. In contrast, the bonded door only pulled out 1/2 in. at 1,000 lb of force and came out only 1 in. at a force of 2,500 lb. Additionally, 4-Star halved labor costs. The switch from rivets, adhesive tape, and welding to adhesive bonding also reduced the number of squeaks and rattles.

Compared to mechanical fasteners and welding, structural adhesives evenly distribute stress, act as sealants, reduce noise and vibration, help prevent corrosion, and maintain the original mill finish of exterior surfaces of thin panels. Adhesives join dissimilar materials impossible or costly to join with mechanical fasteners and welding. This gives designers more freedom to choose the best combination of materials for the task.

Although adhesive acceptance has grown, there's still a perception that traditional welds "look" stronger. Unfamiliarity and misinformation about the adhesives and associated assembly process also hampers their use. However, OEMs that have integrated adhesives into their designs have benefited from immediate return on investment, reduced labor and material costs, and more-efficient production. Additionally, OEMs often command a premium for adhesive-bonded assemblies because of their reliability and surface aesthetics that are better than those of their welded or rivetedcounterparts.

But how does a designer switch to an adhesive joint? The key is embracing the conversion process. This includes understanding the chemistry behind adhesives, outlining a proper implementation plan, and partnering with a reputable adhesive supplier.

CHEMISTRY BASICS
Of all the polymers available for sealing and bonding, only five are regarded as structural. Acrylics, cyanoacrylates (CAs), epoxies, urethanes, and anaerobics provide strength and durability comparable to or greater than the materials they bond. CAs, or super glues, have limited structural applications, as do anaerobics, better known as threadlockers. For all practical purposes, therefore, the list of chemistries can be narrowed to acrylics, epoxies, and urethanes.

Understanding the type of chemistry being used is important. In general, acrylics excel at bonding unprepared metals, composites, and thermoplastics. Epoxies give the highest strengths when bonding prepared metals, composites, and natural substrates. Urethanes offer resiliency and flexibility and are candidates for joining composites, thermoplastics, natural materials, and prepared metals.

It is important to remember that structural adhesives are thermoset polymers. They will not melt or change with environmental exposure, temperature, or time. Acrylics and epoxies can withstand temperatures from 40 to 400°F. Most urethanes are good up to 300 or 350°F with a low-end slightly better than the others. Exposure to tap and saltwater, humidity, oil, gasoline, solvents, and other environmental factors won't weaken bond strength in properly designed joints.

Metal bonding: Toughened acrylic adhesives are the best choice for joining similar and dissimilar metals. For painted, primed, or prepared metals, urethanes or epoxies are often specified, while epoxies work best for assemblies exposed to ultrahigh heat or joints that will see exceptionally high shear stress. For impact resistance and toughness, toughened acrylics are tops.

Thermoplastic bonding: Urethanes bond an array of plastics including hard-to-bond polyoelfins (polyethylene and polypropylene) when combined with surface treatments. Epoxies and acrylics are good, but can be too stiff for low modulus materials such as the polyoelfins. Other adhesive chemistries work well as a "modulus bridge" between softer plastics and stiffer substrates when cross-bonding plastics to metals or composites. Composites (including sheetmolding compounds, fiberglass, fiber-reinforced plastics, pultrusion, and carbon-fiber substrates): Urethanes and acrylics are well known for their composite bonding abilities. They need little or no surface prep, are easy to handle, and quickly cure at room temperatures. Toughened epoxies that can be heat accelerated are candidates for giving ultrahigh-strength and demanding performance. Custom epoxies are the best option for highly specialized aerospace applications.

Natural products (wood, stone, plywood, natural/synthetic rubber, and fabrics): Epoxies and urethanes are top candidates because acrylics may not cure on porous surfaces. Urethanes work best if these substrates have residual moisture that they can react with. Rubberlike materials need surface pretreatments while "stone" materials must be free of dust and loose surface materials before bonding.

For cross-bonding one material class to another, it's prudent to collaborate with an adhesive maker having the broadest range of chemistries and products. They will be able to pinpoint a workable fit between competing demands such as material selection, assembly processes, and desired economics.

AN ACTION PLAN
Nothing is more vital to a successful project than a firm commitment to following an implementation plan. First, determine the goals of the project and audit current joining processes to estimate potential savings that will help justify the switch to adhesive bonding. Audits normally require a tour of the manufacturing facility and discussions with designers, manufacturing engineers, and shop-floor personnel. If the audit is promising, then substrate testing to confirm bond performance follows.

Bond trials are the most effective method to demonstrate the ability of a proposed adhesive process. Using the information gathered during the plant tour and audit, designers choose applications suitable for conversion to adhesives based on how they impact costs, process, aesthetics, and throughput. Trials also answer many of the process questions that tend to arise in this kind of change and help fine-tune any manufacturing steps that are unfamiliar to factory personnel. It is important to note that decisionmakers capable of changing the process or design should take part in trials as their buy-in is crucial to the overall switch.

PUTTING ADHESIVES TO THE TEST
Testing procedures vary depending on the overall objective. In some cases, laboratory testing of the substrates is all that is needed. But more often than not, it will be necessary to attempt to break the bonded assemblies. This is sometimes done with brute force using sledgehammers and chisels. Testing, however, is often more sophisticated. It can range from in-house procedures (with specific test parameters) to field trials or, as in the transportation sector, can encompass strict standardized test specifications.

Adhesive suppliers may also add their own tests to verify product performance. They can include both physical and long-term or accelerated environmental testing. And finally, it's best to check the adhesive's track record for similar applications. One common mistake is assuming that a failure of one type of adhesive eliminates adhesives as an option altogether. The failure may be caused by the supplier making an unwise product selection, not understanding process ramifications, or by picking the wrong applications for adhesives in the first place.

MAKE CONTACT
4-Star Trailers, (405) 324-7827, 4startrailers.com
Ellsworth Adhesives, (800) 888-0698, ellsworth.com
Freeman Marine Equipment Inc., (888) 373-3626, freemanmarine.com
Lord Corp., (877) 275 5673, lord.com
Scelzi Enterprises Inc., (800) 858-2883, seinc.com