When it comes to selecting an adhesive, you'll be met with a vast array of choices. Having a choice is good. But too many choices can create decision fatigue.
You've come across polyurethane adhesives as an option. Now you want to learn more about this adhesive type.
We often get asked about these adhesives here at Forgeway. As an industrial adhesive manufacturer, we have been working with, and alongside, polyurethane adhesives for more than 25 years. In fact, one of our first-ever products was a polyurethane adhesive.
So in this article, we are going to convey our knowledge. You are going to learn the fundamentals of polyurethane adhesives from the typical applications to the pros and cons.
By the end of the article, you'll have a better understanding of one of the most popular adhesive types in the industry. You will be much better placed to make an informed decision about whether they are a good fit for you.
What are polyurethane adhesives?
Polyurethane adhesives (often called PUs) are much the same as other polyurethane products (like foams, sprays, and coatings). They contain a polyol which reacts with an isocyanate to create the final product.
Polyurethane refers to the polymer chains joined by carbamate (urethane) links. The interesting thing about polyurethane products is their ability to be made from a variety of starting materials.
It's the reason you can find polyurethane foams, sprays, and coatings, not just adhesives. It's also the reason you can find different types of polyurethane adhesives.
The characteristics can vary significantly. For example, you can get two-component polyurethane adhesives that have very high strength and rigidity. You can also get single-component polyurethane sealants that are elastic and easy to use.
The use of isocyanates in polyurethane products has recently come under scrutiny due to health concerns. But Polyurethane adhesives continue to be one of the most widely used adhesives worldwide.
Who uses Polyurethane adhesives?
The short answer is 'everyone'. At some point, pretty much everyone will have used a polyurethane adhesive. Whether they are woodworking, repairing something, or sealing up. Polyurethane adhesives are everywhere.
But that's too broad. We want to give you specific applications for polyurethane adhesives:
- Woodworking and DIY - Polyurethanes are a very popular woodworking adhesive due to their ability to expand and form a water-resistant bond.
- Structural vehicle applications - Some vehicle manufacturers use structural polyurethanes to bond critical materials in place on vehicles.
- Industrial sealing applications - Manufacturers of caravans, yachts, and mass transportation vehicles use polyurethane sealants to seal up and ensure the water stays out of their products.
- Glass bonding applications - Single-component structural polyurethane adhesives can bond glass due to their high strength and flexibility. Vehicle windscreens, boat windows, and even construction glazing all use polyurethane glazing adhesives.
- Repair adhesives - Tyre punctures, boat repairs, and bumper repairs will typically use a polyurethane-based adhesive.
As you can see, there are a wide range of applications and uses for polyurethane adhesives/sealants. The versatility and variability of these products ensure people use them the world over.
What are the advantages of Polyurethane Adhesives?
Now that you know a bit more about polyurethane adhesives, it's time to analyse the advantages of polyurethane adhesives. For this section, we'll focus on structural polyurethanes and sealants.
Polyurethane adhesives combine strength and flexibility
There is often the perception that strength is the most important characteristic of an adhesive. When in reality, flexibility is just as important. Sometimes more important.
Some grades of polyurethane adhesives combine strength and flexibility. This means that if you have an application that needs to bond heavy materials and withstand movement, a polyurethane can perform well.
Polyurethane adhesives can bond to a wide range of materials
This characteristic is often referred to as 'adhesion spectrum'. PUs have a wide adhesion spectrum to materials like metals, composites, wood, glass, and some plastics.
The ability to bond wood in particular is a significant benefit. Some other structural adhesives (like acrylics) struggle to form a strong bond to wood due to the moisture content.
This wide adhesion spectrum also means you can form a strong bond with dissimilar materials.
Polyurethane adhesives are low cost and high performance
As we alluded to in a previous point, polyurethane adhesives have varying performance. But one of the most beneficial parts of this performance is the performance-to-cost ratio.
Polyurethane adhesives generally will be among the lower-cost options. Structural polyurethanes may not be as strong as epoxy or acrylic alternatives. But they will be able to compete on price.
And this price to performance ratio is one of the biggest factors behind their popularity.
What are the disadvantages of Polyurethane adhesives?
So now that you have an idea about the advantages, it's time to assess the disadvantages. Polyurethane adhesives come with drawbacks too.
Polyurethane adhesives are sensitive to UV
Sensitivity to UV is generally only a problem when the adhesive has aesthetic requirements such as bonding and sealing applications on vehicles.
Sealant yellowing certainly isn't an issue isolated to polyurethanes. Silicone and Hybrid Polymer-based products can also face the same problem but aren't as likely to.
It also isn't an issue that is isolated to polyurethane adhesives. The yellowing will also occur in polyurethane foams and coatings. Products made from polyurethane are known to turn yellow when exposed to UV.
This yellowing issue can also cause more than just an aesthetic problem. The yellowing can lead to degradation which can lead to failure in sealing applications.
Polyurethane adhesives carry health and safety concerns
As mentioned earlier, polyurethane adhesives have faced recent H+S scrutiny. The EU has implemented restrictions on the use of most polyurethane-based products.
This is because of the isocyanate content. They state that any product containing isocyanates should not be used for industrial applications unless:
- The concentration of isocyanate is less than 0.1%
- The users have received approved training on safe usage
Such restrictions have led manufacturers and industrial users to search for alternative adhesives because the training is not always viable. It's also often undesirable.
Polyurethane adhesives can require extensive surface preparation
All adhesives require surface preparation before bonding. Some will require more preparation than others though. This can lead to extended cycle times impacting production throughput.
Traditional polyurethane adhesives would require abrasion and sometimes priming a surface to ensure a strong bond. Recently developed PUs may not require such extensive preparation to achieve a strong bond.
Nevertheless, polyurethane adhesives for glazing are one of the biggest culprits. They will demand a primed surface to form a strong bond with glass.
On average, polyurethane adhesives will require more surface preparation than other adhesive types to perform. These added steps in the bonding process will increase the likelihood of operator error.
Are polyurethane adhesives a good fit for you
You've read about the applications, the pros, and the cons of polyurethane adhesives. Your understanding of this adhesive chemistry is much greater than it was a few minutes ago. But the question may remain; are polyurethane adhesives the right option for you?
This question often comes to us at Forgeway. Although we supply some polyurethane adhesives, we know that they aren't the best choice for every application.
However, if you're looking for an affordable adhesive with high performance and a broad adhesion spectrum, polyurethanes would likely be a good option to explore. Visit our Pyrok range of PU adhesives by clicking here.
Or, if you're worried about health and safety concerns and need the adhesive to be easy to use and durable in UV-harsh conditions, it would be better to look at alternative adhesives.
Still stuck? You can read more articles comparing polyurethanes against other adhesive types. Or you can get in touch with an adhesive expert to help you solve your exact challenge.
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