What is the strength of structural adhesives?
Structural adhesives are now a fixed presence in the industrial world and are used in the most varied sectors. For instance, the automotive one, as well as the production of household appliances, companies specialized in the production of windows, and so on. But it is also true that there are still many people and different operators in the field who are absolutely adamant that a bolt, a welding, or a rivet are the better solution in all cases. And this, of course, without truly knowing what the real strength of structural adhesives can be: quite simply, one is inclined to think that a bolt offers a better seal than some glue. This is understandable, because we are used to seeing steel bridges held together by huge bolts, and when we think about glue, we imagine the one we used to use as children to glue together sheets of paper, or the one used in the hobby of model-making. How can liquid adhesives be as strong as mechanical fastening solutions? Here, in this article, we shall illustrate what the real strength of structural adhesives is, to demonstrate that, in many cases, they are both stronger and more resistant than rivets, bolts, and even welds.
The strength of structural glues compared to other fastening methods
How do you calculate the strength of a fastening method? In theory, it’s simple: the fastening methods which lasts longer is the best one. But to translate this instinctive reasoning into certain and precise numbers you need to run some relevant tests. The test to be used as reference is that measuring overlapping shear strength. Here, when comparing the performance of a high-quality epoxy structural double-sided adhesive tape with that of other fastening methods, in many cases the glue turns out to be simply unbeatable.
When fastening large surfaces, the use of structural adhesives is far more effective than rivets and bolts. They, in fact, concentrate on very small areas, leaving the rest of the surface “uncovered”, while the structural adhesive goes on to occupy the entire available surface. That’s why the best structural adhesives can do better than rivets, screws, bolts and yes, even spot welding, positioning themselves together with flux welding as the best method. But specifically, when can one actually call an adhesive structural?
What are structural adhesives?
There are different types of liquid adhesives, epoxy, or acrylic glues. Some of these products are two-component, others one-component. Some are very strong, others less so. When is it possible to call an adhesive structural, without making a mistake and without exaggerating? In general, an industrial double-sided tape can be defined as structural when the bond created is very, very strong. According to the definition used in the United States, a structural a strong double-sided adhesive tape must have a sealing power of at least 7 megapascals. In Europe the bar is raised a lot, as the adjective “structural” can truly only be bestowed onto glues that having a sealing power of about 10 megapascals. Concretely, beyond the numbers, it is right to talk about structural adhesives when they are able to guarantee a bond that is at least as strong as the attached materials.
So, of course, by definition, structural adhesives must have a strong hold to be called as such.
The 3M container test
Over the years, the most different tests have been carried out to demonstrate the strength of various liquid adhesives for professional and industrial uses. Among the most theatrical, but also most effective to explain what the resistance of one adhesive can be, is the test performed – and filmed – by 3M, in Minnesota. Here, during a very cold winter, the 3M laboratories technicians decided to connect the joint of a crane hook with a structural adhesive, without using welds or screws. At that point, a container filled with bricks, with a total weight totalling 8 tons (16000 lb) was lifted with this crane: well, not only did the hook manage to lift the load, but it kept it in place until the next day, overcoming the cold night temperatures of Minnesota without any issues.
When to use structural adhesives?
Once the strength of structural adhesives is understood, it can also be valuable to understand which situations they can be used in. There are so many fastening methods. Let’s consider those we have already mentioned, such as welds, rivets, screws, bolts, nails. But let’s also consider other adhesive products, such as high performance double-sided adhesive tape. When should you use structural adhesives? These products are particularly advantageous – keeping in mind their performance, application times, and costs – especially when aiming for high strength in the face of a narrow fastening area (In other cases, in fact, it could be much more convenient to use double-sided tape, in tape or sheet form): because of this, structural adhesives are excellent alternatives to screws and rivets. It should also be noted that glues for professional uses have high levels of resistance against extreme temperatures, humidity, and ultraviolet rays, which do not compromise the final appearance of the product, which is light and does not require drilling or heavy processing.