When it comes to selecting a structural adhesive, you’ll face several choices. In fact, the choices often seem endless. Whilst it’s nice to have so much variability, it can make it difficult to decide. 

Here at Forgeway, we manufacture industrial adhesives including structural. We know how difficult it can be to decide the right structural adhesive for your application.

That’s why we like to narrow it down to just three options: 

• Structural Epoxy adhesives
• Structural Acrylic adhesives (aka MMA adhesives)
• Structural Polyurethane adhesives. 

Each option has its advantages and disadvantages. This article will analyse each option to help you understand which structural adhesive type might be best for you. 

By the end of the article, you’ll understand how the options differ. You’ll also understand the pros and cons of each option. The article will help you decide the best adhesive type for you.

Understanding Epoxy, Acrylic, and Polyurethane adhesives

Before diving into the details, it’s essential to grasp why we often compare epoxy, acrylic (MMA), and polyurethane adhesives. As we’ve explained in previous articles, some people offer a broad description of structural adhesives. The range of different adhesive types that could qualify as ‘structural’ is very extensive.

After you boil it down and understand what ‘structural’ means, you’re left with three main adhesive types; Epoxy, Acrylic, and Polyurethane. We’re not saying these are the only type of structural adhesive. We’re merely saying these are the main candidates when identifying structural adhesives.

All three belong to the structural adhesive category. That’s for certain. They all intricately fuse with the structures they bond, creating a bond that typically outlasts the lifetime of the structure being bonded. 

These adhesives are typically available as two-component adhesives. Although you can get single-component variations too. 

The main reason behind their comparison lies in their robust strength. Each chemistry can achieve lap shear strengths exceeding 20 MPa, powerful enough to rival a weld in the right circumstances.

What are the differences between Epoxy, Acrylic, and Polyurethane adhesives?

While they share similarities, epoxy, acrylic (MMA), and polyurethane adhesives all possess distinct differences.

Two-component epoxy adhesives consist of epoxy resin and hardeners (often amine), facilitating curing and bonding. Single-component epoxies rely on an external factor like heat or UV to initiate the curing process.

Two-component Acrylic (MMA) adhesives are similar to epoxies. They employ an acrylic resin combined with hardeners for curing and bonding. It’s rare to find single-component acrylic adhesives.

Polyurethane adhesives, on the other hand, leverage a polyol and isocyanate to trigger curing and bonding. Single-component structural polyurethanes rely on atmospheric moisture to start the curing process.

As you can see, they are similar but have some distinct differences. These different chemistries possess different characteristics. We’ll assess these different characteristics in more detail below. However, you can see a snapshot of the differences in the graphic here.

What are the pros and cons of epoxy adhesives?

The range of different epoxy adhesives available is massive. It’s difficult to describe the pros and cons of epoxies as the variations display different characteristics. 

The below advantages and disadvantages are a holistic view of epoxy adhesives. There may be some variations that don’t possess the following characteristics.

Advantages of epoxy adhesives:

• Very high strength – Epoxy adhesives are the strongest adhesive chemistry. They can often surpass strengths of more than 30 MPa. 
• Good chemical and temperature resistance – Generally, epoxies have the best temperature and chemical resistance compared to other adhesive types.
• Very versatile – You can get many different types of epoxy adhesives. From ultra-high-strength structural adhesives to ultra-low-density void fillers.

Disadvantages of epoxy adhesives:

• Requires surface preparation – Creating a robust bond with epoxy adhesives necessitates meticulous surface preparation. This can involve abrading the substrate.
• Can be brittle – Whilst not all epoxy formulations are brittle, they often display brittleness. The higher the strength, the more rigid and brittle they become. 
• Can have a long cure time – Again, not all epoxy adhesives have a long cure time. Nonetheless, epoxies will usually take several days to reach full strength.

What are the Pros and Cons of Acrylic (MMA) Adhesives?

Similar to epoxy adhesives, there are several different types of acrylic adhesives available. Whilst the characteristics again vary, there are common traits associated with acrylic adhesives.

Pros of Acrylic adhesives:

• Requires minimal surface preparation – Acrylic adhesives do not need extensive surface preparation to ensure a strong bond. Wiping the surface will be sufficient.
• Can bond difficult substrates – Acrylic (MMA) adhesives excel in bonding across a diverse range of substrates. This includes low surface energy plastics like polypropylene and galv. steel.
• Very fast cure time – Acrylic adhesives can have very fast cure times. Sometimes, they can reach handling strength in less than five minutes.

Cons of Acrylic adhesives:

• Heat and chemical resistance – Whilst they still possess some heat and chemical resistance, it will not be as high as epoxy adhesives.
• Create an exothermic reaction – The exothermic reaction from the cure can cause acrylic adhesives to shrink. This leaves witnessing or read-through which can damage aesthetics.
• Can have a strong odour – The odour from acrylic adhesives is completely harmless. However, it can be unpleasant for some users to work with.

What are the Pros and Cons of Polyurethane Adhesives?

Structural polyurethane adhesives are often two-component. Whilst they are still the same chemistry as single-component polyurethanes, the characteristics will differ.

In this section, we are focussing on two-component polyurethane adhesives rather than single-component variations.

Pros of polyurethane adhesives

• Bonds a range of substrates – Polyurethane adhesives are commonly used for bonding a range of substrates. They can create strong bonds with materials such as wood, metals, and polymers.
• Increased flexibility – Structural polyurethanes are well known for their combination of strength and flexibility. This allows for applications with high dynamic load requiring impact resistance.
• Low odour – Acrylics and epoxies can emit a strong odour that is difficult to work with. However, polyurethanes rarely create a strong smell.

Cons of polyurethane adhesives

• Require surface preparation – Polyurethane adhesives often require the use of a primer to ensure a strong bond. At the least, you will need to abrade and extensively clean all surfaces prior to bonding.
• Lengthy cure time – Like some epoxy adhesives, polyurethanes have a longer cure time. A full cure can take up to 7 days in some cases.
• Face restrictions – Most polyurethane-based products contain isocyanates. And as of August 2023, the use of isocyanates will require industrial operators to receive safety training.

Making the choice; Epoxy, Acrylic, or Polyurethane adhesives?

Now you understand the key differences between each structural adhesive type. You also understand the pros and cons of each option. Now it’s time to use that knowledge. But this is where it gets difficult. 

Here at Forgeway, we manufacture each of these adhesive types. We understand that the points above give a brief overview of each chemistry type. 

If you’re looking for more information, we linked articles that will help you understand each adhesive type. If you can’t find the links, we have listed hyperlinks for you:

• An in-depth review of epoxy adhesives
• Diving deeper into acrylic adhesives
• Analysing polyurethane adhesives

Alternatively, if you’re not sure what characteristics you need in an adhesive type, read the article below. It will help you understand what you need to consider when choosing an adhesive.

Or you can reach out to an adhesive expert from Forgeway. They can help you understand your application requirements and suggest a suitable adhesive type to solve your challenge. 

We look forward to helping you!