Structural Adhesives: A Closer Look at Their Characteristics and Capabilities

You’ve heard about structural adhesives as a potential joining method. Now you want to find out more about structural adhesives to gain a deeper understanding of how they work.

Here at Forgeway, we manufacture industrial adhesives. We have been manufacturing structural adhesives for over 25 years, helping companies solve their joining challenges.

If you’re struggling to find information to help you understand structural adhesives, we can help. Don’t worry, we aren’t trying to sell you on the idea of structural adhesives.

This article will simply give you the definition of structural adhesives, discuss the different types of structural adhesives, and then explain the most common users of structural adhesives.

By the end of this article, you will gain a greater understanding of structural adhesives.

What are structural adhesives?

The definition of a structural adhesive is any adhesive that will cure to become part of the structure it is bonding so that it will maintain the integrity of that structure throughout its lifetime.

This definition basically means the adhesive is strong and durable enough to increase the structural integrity of the structure it is bonding rather than decreasing the strength.

The structural adhesive will go from the liquid state and cure through a chemical reaction to form the bond. 

Several factors can initiate the curing process but it will typically be heat, moisture, or an interaction between two chemicals.

Structural adhesives will usually come in small cartridges (varying sizes available). However, you can also get bulk dispense packaging that will come in larger containers.

Fiberglass bonding with structural adhesive

What characteristics should you look for?

There are so many different applications for these types of adhesives. This makes it difficult to define the characteristics that you should look out for which define the adhesive as ‘structural’.

However, as a rule of thumb, to qualify as ‘structural’ the adhesive should cure to at least a strength of 15 megapascal (MPa). Anything less than 15 MPa might struggle to hold some substrates in place.

An adhesive that has a strength between 1 -15 MPa would qualify as a semi-structural adhesive.

It’s not just about strength though. 

You will have noticed our definition said strength and durability. But durability could mean several different things.

Most companies don’t consider flexibility as important as strength. Nonetheless, we would say that flexibility is just as important as strength. The flexibility of an adhesive ensures it is durable enough to remain structural.

Apart from strength and flexibility, there are other characteristics like chemical and weathering resistance which can increase an adhesive’s durability.

Structural adhesives will need to withstand chemical and weathering resistance
Chemical and weathering resistance is critical from a structural adhesive

Where are the different types of structural adhesives?

There are several different types of adhesives that could qualify as ‘structural’. Nonetheless, three main types of base chemistries are usually capable of meeting the criteria of ‘structural’.

Polyurethane adhesives, acrylic adhesives, and epoxy adhesives are the three most common types classified as ‘structural’.

You should be aware that some formulations of these adhesives would not have the characteristics to qualify as structural. For example, some polyurethane adhesives would only qualify as semi-structural. 

However, if you are looking for a structural adhesive, these three chemistry types would be a good place to start.

Acrylic adhesives (also known as methyl methacrylate adhesives) will only come in two-component packaging. The two parts will create a chemical reaction starting the curing process.

Polyurethane adhesives and epoxy adhesives can either come as two-component (2K) or one-component (1K). 1K or 2K adhesives will make a significant difference to the performance. You should familiarise yourself with the difference between 1K and 2K adhesives.

You should also be aware that some structural adhesives (in particular polyurethanes) carry restrictions around their usage. Make sure you understand any health and safety restrictions before using them.

In most cases, epoxy adhesives and acrylic adhesives will have the highest performance.

Choose an Epoxy or a Methyl Methacrylate adhesive? Find out here

Where can you use structural adhesives?

Most adhesive manufacturers would say ‘you can use structural adhesives anywhere you need to join materials together.’

But we should help you understand what that actually means.

And you should remember that sometimes, there will be applications where alternative joining methods would be a better option. And there are occasions where structural adhesives just might not be a good fit for you. 

With that said, we will provide examples of industries and applications that typically use these adhesives.

Industrial manufacturers – In particular automotive, transportation, and rail manufacturers but other industries like aerospace and marine manufacturers use large quantities of these adhesives.

Construction – The use of structural adhesive to replace welding and mechanical fasteners has been prevalent in the construction industry.

Renewable energy – The use of structural adhesives has helped the renewable energy industry expand rapidly in recent years. 

Structural adhesives are fundamental to helping the renewable energy revolution
Image by wirestock on Freepik

The use of these adhesives isn’t limited to these industries. It’s true, you can use structural adhesives in any application that requires a strong durable bond.

However, you should consider all joining methods to find the right solution for you.

Could structural adhesives be a good method for you?

Without understanding your application details, it’s difficult to know whether structural adhesives could be a good fit for you.

Now that you know a bit more about structural adhesives, you can now assess their suitability against alternative joining methods.

If you are interested to learn more about the alternatives, read our article on welding alternatives. It will help you understand the three main joining methods.

Nonetheless, if you want the help of a joining expert, reach out and a member of our team will help you assess your application. They will help you decide which joining method would be best for you. (Don’t worry, we wouldn’t suggest structural adhesives where they could fail).

However, if you want to learn more about these adhesives yourself, read about the advantages and disadvantages of this joining method by clicking the banner below.

Whilst we have absolute confidence that they are the best joining method, there are still some drawbacks which might be a deal breaker for you.

Read about the advantages and disadvantages by clicking the banner below.

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