EPOXY, ACRYLIC, AND URETHANE ADHESIVES

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 7 MPa in overlap shear at 24°C. (Less than 7 MPa 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 aluminium 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 can effectively bond many loose-fitting parts. Continuous contact between mating surfaces effectively seals against many contaminants.

• Reduce cost and increase production: Several factors combine for savings and productivity: reduced material requirements, weight reduction, fewer assembly and finishing steps, minimal training. With modern adhesives, usually no solvents need to be vented.

Choosing the right adhesive

Taking full advantage of structural adhesives requires consideration of the substrate, the part design, production requirements, and end-use environment. You also need an understanding of the various types of adhesives and their characteristics. The first question to ask is, “ What can structural adhesives do for me?” If you find satisfactory answers listed below, then it will be worth your time to consider the differences between epoxy, acrylic, and urethane, and look into the properties and latest capabilities of each.

When to select an epoxy

Comprising the largest family of structural adhesives, epoxy resin typically provides the highest strength and elevated-temperature resistance of all structural adhesives. You can select pastes or films with a wide range of application options to help match the adhesive to a manufacturing process. For example, epoxy formulations are available to fit into an existing paint/bake cycle or into an ambient temperature multi-station operation.

• Epoxy paste adhesives include one-part heat curing and two-part room-tem