Engineers increasingly rely on adhesives as an alternative to mechanical fasteners for certain applications. Adhesives offer several advantages, including a comparatively lighter weight of the completed assembly, the need for fewer individual components, and faster production. 

Several considerations should go into choosing the ideal adhesive for an application. Knowledge of the substrates, the conditions the joint will experience in use, and the mechanical requirements of the finished part.  

Load-carrying capacity is often the deciding factor in choosing an adhesive product for a particular application, which is defined by the maximum load an adhesive-bonded joint can bear without failing.

When considering load-carrying capacity, adhesives can be classified as:

1. Structural. Structural adhesives demonstrate strengths from 1000 lb-force per square inch (psi) up to 4000 psi. They’re typically reliable enough to bear large loads, offering high shear strength on metals, composites, glass, rigid and flexible thermoplastics, and natural substrates such as wood. They’re durable enough to tolerate environmental exposure and maintain performance integrity (within design limits) when a bonded joint is stressed to its yield point. 

Structural adhesives are most commonly reactive systems, one or two-part systems that undergo a curing reaction. Common systems include epoxies, urethanes, silicones, and acrylics.

2. Non-structural. Non-structural adhesives demonstrate a load-carrying capacity of less than 1000 psi and, therefore, do not bear large loads. They’re used for bonding lighter parts with less stress or for temporary attachments or positioning. For example, non-structural adhesives can be used to hold substrates in place in preparation for an attachment with mechanical fasteners, such as bolts or screws. They can also be used for light-duty packaging, gaskets, veneers, and trim.

Non-structural adhesives are typically one-part, solvent-based polymers that are sticky. Rubbers and acrylics are common, as are hot-melt adhesives. Tape or film formats are also appropriate in some applications. 

Many non-structural adhesives can be re-positioned with varying degrees of tackiness, while others offer drying times as short as one minute. Delivery mechanisms include thin-to-medium liquid or paste formulations. Liquid types can be applied with a brush or roller, via aerosol spray, or as a hot-melt formulation that hardens when cool.

3. Semi-structural. As the name suggests, semi-structural adhesives fall in between structural and non-structural adhesives. 

Semi-structural adhesives are strong, but many cannot maintain their load-bearing support continuously or for long periods without deformation or failure. These adhesives might be used for short-term or temporary attachments, such as keeping substrates in place in preparation for a subsequent process.

It’s worth noting that there’s considerable overlap between the adhesive chemistry used in structural, semi-structural, and non-structural bonding. Epoxies and polyurethanes, both reactive chemistries, produce some of the strongest bonds. Silicones and acrylates can be quite strong but can also be formulated for light-duty applications. 

Aside from the load-carrying capacity, certain variables should be considered when choosing an adhesive, as follows…

• Storage and handling. Some products might have limitations in their shelf life or working life during application. Adhesives might be re-positionable with varying degrees of tackiness, while others offer short setting times, calling for immediate attachment of substrates.

• Adhesion application. Adhesives might require the application of heat to set the bond. Additional time, processes, or equipment might also be required.

• Cost. It’s important to select an adhesive that meets the requirements of the design. However, the expense should account for the labor involved and downtime while the adhesive application sets.

• Aesthetics. Engineers should consider how the applied adhesive might appear in the final assembly. With some products, creep is possible, causing the adhesive to be visible beyond the substrate.

The structural adhesive advantage
Structural adhesives are typically used for long-term, permanent attachment because of their high strength. Automotive, aerospace, consumer appliance, construction, electronics, medical device manufacturing, and other industries rely on structural adhesives — particularly as an alternative to mechanical fasteners.

Features include:

• Reduced assembly size and weight
• Fewer parts per assembly
• Ability to join dissimilar materials
• Easily fill gaps between substrates, sealing out environmental exposure
• Adaptability to automated processes
• Productivity gains with fast-setting formulations
• Improved aesthetic design
• Low cost