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Considerations For Choosing A Structural Adhesive

Key Highlights:

Structural adhesives provide strong, load bearing joints for industrial and engineering applications. Specifying a structural adhesive involves weighing up technical and process requirements. Adhesives applications are notoriously complex, and it is likely that some trade-offs will be necessary, balancing performance versus processing versus cost.

Kevin Brownsill, head of Technical: Learning and Development, at adhesives specialist Intertronics, shares advice on what to consider when specifying a structural adhesive for a high performance application. 

Often referred to as “load bearing”, structural adhesives hold two substrates together, and should bear the forces involved for the product’s lifetime. The substrates may be metals, plastics, glass, rubbers, ceramics, or composites, and be under constant or intermittent load or shocks. 

One advantage of structural adhesives over alternative bonding methods is their ability to bond dissimilar substrates, such as metals to plastics, and can absorb some of the stress on the joint, such as thermal expansion or impact. The most common structural adhesive chemistries are polyurethanes (PUs), epoxies, and methyl methacrylates (MMAs), as these chemistries offer both adhesive strength and cohesive strength. Cyanoacrylate adhesives (CAs) and single part UV curing acrylic adhesives are also in the mix of candidates you might want to evaluate. 

Be wary of the data sheet 

When narrowing down which adhesive candidates to test, the manufacturer can start with a structural adhesive selector guide that provides overviews of the key features of a number of materials, such as substrate suitability, work time, viscosity, and more. The data sheets can then be used as a guide for further information. However, we advise that manufacturers use data sheets with caution — they are not exhaustive, and many properties will have been obtained under “ideal” laboratory test conditions that do not represent the manufacturer’s individual application.  

While selector guides and data sheets are helpful for shortlisting candidates, they are no substitute for practical trials and tests. For more information on the materials and their properties, it can help to speak to an adhesives specialist directly, who will have experience with the materials and their performance under different conditions. 

Will it bond well? 

Naturally, one of the key considerations when specifying a structural adhesive is how well it will bond to the substrates involved. For the adhesive to wet the substrate (a prerequisite of bonding), the adhesive must have a lower surface energy than the substrate. Many high-performance plastics,  such as polypropylene (PP) and other polyolefins, have low surface energies (with many between 20 and 40 dynes/cm2), and are therefore difficult to bond with adhesives.  

It can help to select substrates that are adhesives friendly. However, manufacturers that wish to specify low surface energy plastics for functional reasons are likely to require an additional process step — surface preparation, such as priming or plasma surface treatment — to allow or improve adhesion.  

To achieve a bond that lasts the lifetime of the product, it is important to consider the environment that the final product will endure, particularly if the adhesive will be exposed to harsh conditions, such as salt water, chemicals, or pressure. If the assembly must withstand changes in temperature, an adhesive that can withstand the relevant temperatures and/or help cope with the difference in substrate CTEs is required.  

Bond line considerations 

The size and shape of the bond line is an important consideration. If it is a large area, this can dictate the gel time or pot life of the material to be specified — it is undesirable for the material to start gelling before dispensing has finished and assembly has begun. The substrate must present a large enough bondline area to reliably bond in a way that can bear the required forces; if not, an alternative joining method may need to be considered. 

Having an understanding of what level of gap filling is required can help inform adhesive specification. If a manufacturer is bonding two moulded parts that are intended to sit flush, is there dimensional variation? It may be that in some places there is no gap to fill, whereas in others, there is 2-3 mm. Without an understanding of this, the manufacturer may find too little, or too much, adhesive is applied, and material may end up squeezing outside of the bond line.  

The rheology of the material can also impact whether the material is appropriate. A highly thixotropic material that stays in place may not be appropriate if the bond line is very thin and the material is required to wick around features.  On the other end of the spectrum, if the material has a low viscosity and is expected to gap fill, the manufacturer may end up losing material to undesired places. 

It can be helpful to involve an adhesives specialist partner before the substrate choice and bond line design is locked in. If the adhesive is treated as the last part of the puzzle, it can be difficult to find a material that meets all the criteria. 

Process considerations  

Naturally, most manufacturers will want an efficient, fast process that limits work in progress and keeps up with demand. However, there are often process trade-offs with material choice — to benefit from the high strength of an epoxy, a manufacturer may need to tolerate a relatively slow cure at room temperature. While some cure reactions can be accelerated with heat, be wary that, if done too quickly, heating can build stress into the component or damage the substrate. 

On the other hand, fast curing two-part materials may mean the adhesive starts to gel in a mixing nozzle, unless it is dispensed continuously or purged regularly. The faster cures of two-part MMAs or single-part CAs will be attractive, but may also limit process flexibility. Single-part UV light curing adhesives offer fast, on demand cure, reducing the length of time parts are fixtured before moving onto the next process. 

There are some newer materials available, such as Born2Bond Structural, designed to address these trade-offs. The adhesive doesn’t offer the same instant tack that most cyanoacrylates do, offering a long mixed open time, but short fixture time once the parts are assembled. 

While it may seem like a small factor, the packaging the material is supplied in can have an important impact on the process, and therefore the selection of a material. If the process is to be automated, selecting a material that only comes in small containers may not be realistic. Similarly, if the material is only supplied in 5 kg pails, but you only need to use 2 g a day, an alternative material will likely be better. 

If a bonding process is being done manually, we can use that as a model to invest in tools that improve the process and add control, such as automated dispensing, a benchtop robot for placement, or full archytas series system. An adhesives specialist can streamline the selection process, advising both on material choice and on how to build a process that is futureproof. This can involve incremental improvements, without redundancy, to improve accuracy, repeatability, throughput, or productivity over time.   

Commercials 

Does the adhesive you have chosen make commercial sense? It might be technically perfect, but so expensive that it damages profitability. We have found that, quite often, the cost per part of the adhesive is less than the cost per part of the application and curing process — maybe quite a lot less. Choosing an inexpensive adhesive may be a false economy if it takes a lot of processing resource or time. Always calculate an overall cost (material plus process, labour, etc) per part, not just a price per gram. 

Specifying a structural adhesive is a complex task. By looking beyond the data sheet, considering the relevant factors, and working with an adhesives specialist, manufacturers can create strong bonds, and straightforward processes. 

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