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Structural adhesives offer high strength, creep resistance, and reduced weight as reasons for replacing mechanical or fusion fastening.
From automotive bumpers to disk drive components, bicycle frames to air/fuel breather valves, plastic railings to refrigeration coils, structural adhesives are available for thousands of manufacturing applications. With ongoing advances in adhesive formulation, designers in every industry have diverse options for bonding metal, composites, plastics, rubber, glass, and more. A wide choice of epoxies, acrylics, and urethanes enable design engineers to more easily achieve the right balance of strength, heat resistance, and productivity.
This article describes epoxy, acrylic, and urethane structural adhesives, including their strength, heat properties, creep resistance, and the most appropriate applications.
Advantages of structural adhesives
In general, these structural adhesives have enough cohesive strength and creep resistance to permanently bond high-strength materials, and they have the potential to replace mechanical and fusion fastening in many applications. Practical bond strength is at least 1000 psi in overlap shear at 75[degrees]F (24[degrees]C). (Less than 1000 psi in overlap shear is generally considered too low for structural bonding.)
Beyond having the load-bearing strength to do the job, structural adhesives offer other styling, performance, and production reasons for replacing mechanical or fusion fastening.
* Distribute stress over the entire bonded area: The concentrated stress of rivets, bolts, spot welds, and similar fastening techniques is eliminated. A design engineer can specify lighter, thinner materials without sacrificing strength.
* Bond dissimilar materials: Laminates of dissimilar material can often produce combinations superior in strength and performance to either material alone. Adhesive flexibility compensates for different coefficients of expansion between substrates such as aluminum and glass, for example. Adhesives also provide a film barrier to reduce or prevent bimetallic corrosion between different metals.
* Maintain the integrity of assembled substrates: Mechanical fastener holes are eliminated, as are surface marks from spot welding and brazing. With this virtually invisible fastening, an engineer has greater design latitude, cleaner lines for improved end-user appeal, and less machining or finishing.
* Maximize fatigue resistance: Adhesive flexibility permits high extension and recovery under repeated loading. Energy absorption properties provide up to 20 times the fatigue resistance of riveted or spot-welded assemblies.
* Bond and seal against the environment: Structural adhesives fill voids and gaps and...