Stick With the Right Adhesive

Dec. 12, 2002
Which adhesive where? Knowing the eight basic kinds of adhesives helps in quickly sizing up potential uses.
Light-curing cyanoacrylates fixture almost instantly when exposed to the proper light source.

Patrick J. Courtney
Mike Shannahan
Henkel Loctite Corp.
Rocky Hill, Conn.

It's possible to find hundreds of different assembly adhesives in manufacturing today. But most fall into eight basic categories: anaerobics, cyanoacrylates, light-cured acrylics and cyanoacrylates, hot melts, reactive urethanes, epoxies, polyurethanes, and two-part acrylics.

Anaerobics are one-part adhesives that remain liquid in air but cure into tough thermoset plastics when confined between metal substrates. Anaerobic adhesives typically are used to lock and seal threaded assemblies, retain bearings and bushings on shafts or in housings, and seal metal flanges in place of cut gaskets. Anaerobics can enhance, and in some cases replace, mechanical joining methods, lowering manufacturing costs and prolonging equipment life.

Cyanoacrylates are high-strength, one-part adhesives for bonding plastic, metal, and rubber. Cyanoacrylates cure rapidly at room temperature to form thermoplastic resins when confined between two substrates in the presence of microscopic surface moisture. Cure initiates at the substrate surface, limiting cure-through gap to about 0.01 in.

Cyanoacrylates build fixture strength in seconds and full strength within 24 hr, making them well suited for high-volume, automated production. Polyolefin primers applied to substrates before assembly let cyanoacrylates better adhere to difficult-to-bond plastics such as polypropylene and polyethylene, while accelerators help them cure rapidly in low humidity. So-called "surface insensitive" cyanoacrylates also hasten cure in low humidity as well as on acidic surfaces. And special low-blooming types minimize white haze (blooming or frosting) around bond lines.

Early cyanoacrylate adhesives weren't noted for having good bond strength and chemical resistance, and they didn't work at temperatures above about 180°F. However, newer formulations such as rubber-toughened cyanoacrylates have better peel and impact strength. And recent thermal-resistant types withstand continuous 250°F temperatures.

Cyanoacrylates bond a flexible PVC cushion to a rigid PVC nose portion of these anesthesia masks.

Most adhesive-joint failures are not caused by lack of adhesive strength. More likely culprits include poor design, inadequate surface preparation, inappropriate operating conditions, or simply using the wrong adhesive for the job. Always test designs thoroughly before production begins.

Light-curing acrylics are one-part, solvent-free liquids that turn to thermoset plastics in about 2 to 60 sec when exposed to light of the proper wavelength and irradiance. Cure depths can exceed a half inch. This cure-on-demand approach lets parts be positioned and repositioned before cure. Some hybrid varieties use a secondary cure mechanism such as heat or chemical activators to completely cure adhesive in shadowed areas. Like cyanoacrylates, light-curing acrylics come in thin liquids (about 50 cP) to thixotropic gels. The cured adhesives, depending on formulation, form soft elastomers to glassy plastics that remain clear for improved aesthetics. Light-curing acrylics bond well to a variety of substrates and withstand chemical attack, elevated temperatures, and other environmental assaults.

Light-curing cyanoacrylates are a recent development that combines the benefits of cyanoacrylates and light-curing acrylics while overcoming many of the limitations of each. Light-curing cyanoacrylates fixture almost instantly when exposed to the proper light source and reach about 60% of final strength after just five seconds of exposure. The remaining adhesive in shadowed areas cures by the normal residual-moisture mechanism, eliminating second-step accelerators or activators. Light-curing cyanoacrylates can also bond overlapping, nontransparent parts. These adhesives produce minimal vapors and are insensitive to substrate surface condition. They lower stress cracking on crack-sensitive substrates such as polycarbonate and acrylic. And they bond polyolefin plastics when used with special adhesion promoters compounded into the molded parts or applied to part surfaces before bonding. The use of light-curing cyanoacrylates is growing rapidly in high-volume production of medical devices, cosmetic packaging, speakers, electronic assemblies, and small plastic parts.

Bolted joints secured with anaerobic threadlockers, such as from Henkel Loctite Corp., better retain clamp load under vibration than those fitted with conventional lock washers.

Hot-melt adhesives have been used for decades to assemble industrial and consumer products. Conventional hot melts (basically thermoplastic resins) are heated to reflow onto a bonding surface. Hot-melt adhesives fill large gaps and build high bond strengths upon cooling. Some of the higher performance varieties include ethyl-vinyl acetate (EVA), polyamide, polyolefin, and reactive urethane. EVA hot melts are typically used for low-cost potting applications. Polyamides replace EVAs for more-demanding temperature and environmental conditions. Polyolefins adhere well to polypropylene substrates and resist moisture, polar solvents, acids, bases, and alcohols.

Reactive-urethane adhesives represent the latest in hot-melt technology. Unlike traditional thermoplastic hot-melt resins that can be repeatedly reheated, reactive urethanes form thermoset plastics when fully processed at about 250°F, or roughly 200°F cooler than other hot-melt chemistries. Initial strength develops a bit slower than traditional thermoplastic hot melts, though for structural bonding and difficult-to-bond plastics, reactive urethanes generally get the nod.

Epoxies are one or two-part structural adhesives that bond well to a wide variety of substrates, emit no by-products, fill large volumes and gaps, and shrink minimally when cured. Cured epoxies typically have excellent cohesive strength, exceptional chemical resistance, and good heat resistance. A major disadvantage of epoxies is a longer curing time than other adhesive types. Typical fixture times are between 15 min and 2 hr. Heat accelerates the curing process, though certain substrates such as plastic, limit cure temperatures.

Automatic dispeners speed adhesive application.

Polyurethane adhesives have greater flexibility, better peel strength, and lower modulus than epoxies. These one or two-part systems consist of a soft core for joint flexibility and a rigid skin for cohesive strength, temperature, and chemical resistance. Varying the ratio of hard-to-soft regions makes possible a range of physical properties. Like epoxies, polyurethanes work on several different substrates, though a primer is sometimes required to prepare surfaces. However, polyurethanes must be applied to dry substrates because moisture can compromise bond strength and final appearance. Fixture times are on par with epoxies (15 min to 2 hr) so racks are often needed to hold parts while the adhesive sets. Polyurethanes resist chemicals and elevated temperatures, though long-term exposure to high temperatures degrades them more rapidly than epoxies.

Two-part acrylics easily fill gaps and withstand elevated temperatures and environmental factors similar to epoxies and polyurethanes. However, two-part acrylics can be formulated to fixture faster and adhere better to difficult-to-bond substrates than these other adhesives. And they remain highly flexible for long-term fatigue resistance.

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