Structural adhesives are used as an alternative to mechanical connections in many applications, from screws and rivets to spot welds. They can offer improved product performance, reliability improvements, greater design freedom, and reductions in manufacturing costs.
Intertronics
Sometimes known as “chemical joining” to contrast with mechanical joining, adhesive bonding involves an adhesive “hardening” to provide a joint capable of bearing the forces it is subjected to over the product’s lifetime while enduring the associated environmental conditions. An adhesive can be cured when a chemical reaction has taken place and its constituent parts (monomers or oligomers) join up and cross-link to become larger molecules (polymers).
Advantages of adhesive bonding
Adhesives bring improved stress distribution. Once cured, the joint’s entire surface carries the load rather than an individual point, avoiding the local stress concentration found in mechanically joined parts.
They also offer other protective benefits, including minimising or eliminating corrosion, boosting impact resistance, avoiding joint fatigue, providing electrical insulation and dampening vibration. In many applications, an adhesive can also act as a sealant, protecting the assembly from water, moisture, dirt, and other environmental contaminants.
Many manufacturers, such as those in automotive or aerospace applications, use adhesives to reduce their assemblies’ weight. Mechanical fastenings are often made of metal, and their weight can add up in applications requiring multiple fastenings. While adhesives are not inherently lightweight, their ability to distribute stress can give design engineers the freedom to reduce part thickness and therefore weight while removing the weight of metal fastenings.
Other manufacturers use adhesives to improve aesthetics, as there is no mechanical fastening or spot weld visible in the final part — an adhesive can provide an almost invisible bond.
Beyond these technical benefits, adhesive bonding can bring operational benefits like reduced component costs, lower labour and assembly costs, and reduced finishing costs.
No perfect solution
Structural adhesives generally have bond strengths in the order of 2,500 to 7,500 psi, and tolerate temperature swings up to 170˚C. Adhesives are polymers, and therefore not as strong as metals, although this is offset by the increased surface contact area of a bonded joint. A downside is that adhesives may be affected by increased temperature, which can reduce the bond strength and chemical resistance.
Where traditional mechanical joints offer instant strength, adhesives reach full strength once the curing reaction is complete. This can happen in seconds, to hours, to days, and the assembled joint must be fixtured for at least part of the time the part is curing. Prior to cure, the adhesive must wet the substrate in order to create a successful bond — surface treatment may be required if bonding a substrate with low surface energy, which adds an additional process.
Choosing a material
When choosing a material, manufacturers need to consider the design, production, and processing considerations, and develop a specification based on the necessary properties and allowable compromises.
There are multiple structural adhesive chemistries, the most common being polyurethanes (PUs), epoxies, and methyl methacrylates (MMAs), but some cyanoacrylate and single part UV curing acrylic materials are in the mix of candidates to evaluate too.
Bonded assemblies must be designed with adhesive bonding in mind. This involves considering bond line geometry, adhesive selection, stress in the joint, mechanical properties of the adhesive and substrates, and the manufacturing process requirements.