Structural adhesives; What are they and why are they so popular?

You will have heard the term ‘structural adhesives’ before but you may be wondering what it actually means.

It is often difficult to find a place with everything you need to know about structural adhesives. As adhesive specialists here at Forgeway, we know how little information there is on the topic.

The aim of this article is to educate you on all things structural adhesives. By the end of this article, you will know what structural adhesives are and will be able to determine if they are a good fit for you.

What is a structural adhesive? 

There are multiple definitions of structural adhesives that you can find on the internet. Some say it is any load-bearing adhesive. Others go into a little bit more depth.

Here at Forgeway, we would define a structural adhesive as any adhesive that has a lap shear strength of at least 15 Mpa. This then means the ‘structural’ in structural adhesives means it physically becomes part of the structure. 

MPa is an SI unit that defines an adhesive’s strength. One megapascal is the equivalent of one Newton per square millimetre (N/mm²). So 15 MPa equals 15 N/mm². 

What are the different types of structural adhesives?

There are three main types of adhesives that we would describe as structural here at Forgeway.

  1. Epoxy adhesives
  2. Methyl methacrylate (MMA) adhesives
  3. Polyurethane adhesives

However, this is our definition of structural adhesives. MS polymer adhesives are semi-structural adhesives as they aren’t able to achieve strengths high enough to become structural. You may also hear cyanoacrylates, anaerobic adhesives or hot melt glues labelled as structural adhesives. For the purpose of this article, we will focus on the three we mentioned above.

Epoxy adhesives (epoxies) 

They can either be one-part or two-part adhesives. Two-part epoxies consist of the epoxy resin on one side and a hardener on the other. When mixed, these two parts will interlink and cure. 

One-part epoxies will have the hardener already mixed in, but require elevated temperatures to start the curing process. 

Advantages of using epoxy adhesives

  • High strength – 30Mpa Lap Shear Strengths can be achieved
  • High chemical and heat resistance
  • High fatigue resistance

Disadvantages of using epoxy adhesives

  • They can be brittle – There are flexible epoxies though
  • Has a gradual cure time that is typically slower than other structural adhesives
  • Some formulations require extensive surface preparation to achieve a strong bond

You can find more information about epoxies by reading our blog about them.

Methyl methacrylate (MMAs) adhesives

Purok adhesive that is an MMA and would require the least amount of surface preparation
10:1 methyl methacrylate adhesive

You may have heard the brand Plexus mentioned in relation to methyl methacrylate adhesives. This is because Plexus has become the generic trademark for methyl methacrylate adhesives(like coke has for cola flavoured soda). But we refer to them as MMAs. 

They usually come as a two-part adhesive. That being said, the two parts can come in different ratios; 10:1 and 1:1.

The 10:1 ratio is for situations where more flexibility is required.

Advantages of using an MMA adhesive

  • Very fast and consistent cure time
  • High strength – 20 Mpa
  • Requires little-to-no surface preparation (will cut-through contamination like oily metals)

Disadvantages of using an MMA adhesive

  • Is usually brittle
  • Most formulations will have a strong smell
  • High exotherm – can’t use it in large quantities

There are a few other things you should be aware of about MMAs though. You can read more about them on our blog.

Polyurethane adhesives

epoxy adhesive
Two-part polyurethane adhesives

Polyurethane-based adhesives are typically two-part, but you can get one-part options too. Adhesive manufacturers can modify these two parts to suit your specific situation. 

If you require the adhesive to have more of one characteristic (such as flexibility), the adhesive manufacturer can modify the polyurethane to suit this.

Advantages of using a polyurethane adhesive

  • Very high flexibility
  • Good strength – 15 Mpa
  • Excellent tear resistance 

Disadvantages of using a polyurethane adhesive

  • Poor UV stability
  • Can have health risks
  • Lower chemical resistance than other structural adhesive chemistries

You can find more information about polyurethane adhesives on our blog.

MS polymer adhesives

MS polymer adhesive
One-part MS polymer

Although MS polymer adhesives aren’t technically ‘structural’, you can still use them in some structural applications. You may hear the term ‘hybrid polymer’ adhesives. MS polymer adhesives are just one type of hybrid polymer adhesives. MS polymers are the most common though.

All MS polymers have sealant properties. But some are specially formulated so you can use them as a sealant rather than as a structural adhesive.

For this article though, we are talking about MS polymers that are formulated to be a structural adhesive.

Advantages of using an MS polymer adhesive

  • No health risks (unless you eat them or are allergic to silanes)
  • Ability to have colour-matched options
  • Highly flexible

Disadvantages of using an MS polymer adhesive

  • Not as high strength – >5 Mpa
  • The cure is slow and inconsistent depending on factors such as heat and moisture
  • Can require surface preparation

As you might expect, there is plenty more you can read about MS polymers on our website.

Where could you use structural adhesives?

You can use a structural adhesive anywhere you need to join two substrates. Sometimes, using a structural adhesive may be slightly over the top if you don’t need the bond to be really high strength.

You can use structural adhesive to bond the vast majority of substrates. Some substrates have been notoriously difficult to bond over the years. However, advances in technology mean that you can find a structural adhesive to bond most substrates.

You may be thinking “the term ‘most substrates’ doesn’t give me a good idea of whether it will suit me.” That’s a fair comment, so I will provide some examples. They should give you an idea of who typically uses structural adhesives and for what purpose.

We supply structural adhesives for use in various situations. You can see some examples below:

  • Construction – such as bonding an aluminium composite panel to the facade of a building. We suggested using a methyl methacrylate adhesive for this situation.
  • Automotive/transportation – such as bonding the GRP rear domes to the back of a bus. The bus manufacturer used an MS polymer for this.
  • Rail – such as a train carriage manufacturer bonding composite flooring to metal. They needed the adhesive to be fire-retardant so they used a polyurethane.
  • Marine – such as a yacht manufacturer bonding plastic shower trays to the wood floor. They used an MS polymer for this bonding.
  • Aerospace – such as an aviation oven manufacturer bonding the steel to a polyether core. They used an epoxy so it could withstand the heat.
  • Solid surface – such as bonding two parts of a countertop/worktop. The solid surface manufacturer required a colour matched option with quick curing, so they used an MMA.
Applying methyl methacrylate to plastic surface

What are the benefits of structural adhesives?

As we have already discussed a few times, welding and mechanical fasteners are the other two popular joining methods.

So when we talk about structural adhesive’s popularity, we mean why people choose them instead of the other two joining methods.

We listed the main advantages to structural adhesives below:

  • They can have equal or greater strength than welding and mechanical fasteners.
  • They are able to bond dissimilar substrates. Welding can only join metal to metal or plastic to plastic. Mechanical fasteners aren’t ideal for joining dissimilar materials as they create a focal point for stress.
  • Structural adhesives are usually the cheapest option. Obviously there are several things that can affect cost. 
  • They will almost always have greater flexibility than the other two joining methods.
  • Usually won’t require as much surface preparation as mechanical fasteners. (Welding doesn’t need any surface preparation, but welding is not always applicable). Mechanical fasteners will require surface preparation such as holes drilled.
  • A lot fewer health and safety regulations to consider when bonding with adhesives as opposed to welding. 

A lot of these advantages are as a result of the sheer amount of structural adhesive options available. Adhesive manufacturers are constantly adjusting the chemical formulations of structural adhesives. Hence, most people are able to find at least one potential structural adhesive formulation that can suit their needs.

In our opinion, it is this variety of options which makes structural adhesives so popular.

What are the disadvantages of using structural adhesives?

Once again, we are comparing structural adhesives against welding and mechanical fasteners as they are the three common methods of joining substrates. 

There are a few things that can put people off using structural adhesives. We created this list of the disadvantages you might want to consider before choosing a structural adhesive:

  • All variations of structural adhesives have a cure time. Some adhesives have a very quick cure time, but it still takes time. 
  • You will need to have a reasonable amount of knowledge on the type of structural adhesive you want, and how to apply it correctly.
  • There is always a caveat involved with structural adhesives. Some higher strength structural adhesives are usually brittle. The temperature can affect the cure times of other structural adhesives. But these are just some examples of caveats. You will need to check the caveats with the structural adhesive before you purchase it. The articles on each structural adhesive will give you a guide of the caveats to look for with each option.
  • The bond is irreversible. You can usually undo mechanical fasteners, but you typically cannot undo the structural adhesive bond. 
  • The bond can fail if not designed correctly.
Health risks can be associated with structural adhesives
One potential caveat with structural adhesives is the health risks involved with some adhesive options (like polyurethanes)

You can mitigate a lot of these disadvantages by becoming the expert and doing your research before purchasing the structural adhesive.

Our Ebook will give you an in-depth guide to everything you need to know about adhesives. Make sure you conduct thorough research before deciding which structural adhesive to use.

There shouldn’t be any problems if you have thoroughly researched and tested the structural adhesive before you purchase and use it.

Where can I buy structural adhesives?

There are masses of structural adhesive suppliers around the world. How can you tell which ones are any good?

It may seem biased seeing as, here at Forgeway, we are one of those companies who manufacture and supply structural adhesives. But we will list a few of the other structural adhesive manufacturers you could consider.

The following list is in no particular order:

This list will give you an idea of the other structural adhesive suppliers who could help you. Or, if you are just looking to get in contact with a structural adhesive manufacturer, a member of our team will be more than happy to help.

Which is the best structural adhesive for you?

This is never an easy question to answer. Traditionally, we would say that methyl methacrylate adhesives would be the best overall structural adhesive for most situations. They have very few drawbacks and are extremely reliable.

Methyl methacrylates are starting to rise in popularity though due to their strength and fast cure. There is definitely an increasing amount of interest in the use of MMAs. 

Epoxies are a good option if you only care about strength. Or, if you want to add some flexibility to that strength, we advise going with a polyurethane.

However, it all comes down to your application. It is difficult for us to say which one is best for you without knowing your bonding application.

An expert can help you determine which structural adhesive is best for you. The expert will give you advice and potentially offer some samples if you know which option you want to explore further. They will also help with some of the testing methods you can carry out to ensure the structural adhesive is right for you.

If you want to try to work it out for yourself, try out our product selector below. It will guide you to the right structural adhesive for you.