Our engineers and experts have come together to answer some frequently asked questions (FAQs) about structural adhesives.
Every situation is different, but our team of expert engineers do get several similar questions over time. Here are some of the most common queries they report hearing from customers. These answers aren’t meant to work precisely for your application. However, they’ll give you a good idea of the various factors to consider as you review design, process and adhesive options to see how 3M Structural Adhesives can help you make things better.
A lot of people think epoxy is just any two-part structural adhesive (it’s almost become a generic term for them) but it’s actually a specific chemistry. When you talk about two-part structural adhesives, there are three main chemistry types – epoxies, acrylics and urethanes – and they all have different characteristics.
Epoxies are the oldest chemistry and still in many cases give the highest performance. Epoxies provide the best fatigue performance and environmental resistance when you’re bonding metals. In addition to metals, epoxies are very good for bonding thermoset composites like CFRP (carbon fiber-reinforced polymer); they also bond glass, ceramic and wood, and even some rubbers and certain thermoplastics. The key to bonding with epoxies is that for room temperature curing you need a well-prepared surface: you have to clean the surface and you may need to abrade to get a good strong bond. Epoxies also don’t cure as quickly as the second type of two-part structural adhesive, which are acrylics.
Acrylics are the newest category of structural adhesive, and they can be described as production-friendly for three reasons. One, for a given amount of open time they cure or build strength very rapidly. Two, they tend to be relatively tolerant of oily surfaces, which can mean less surface prep because you may not have to clean oils off. Three, they are good thermoplastic bonders, so you can get very good strength on thermoplastics. With certain acrylics you can even bond LSE polyolefin materials without having to do any plasma treatment or priming. Some drawbacks: acrylics are not going to have quite the environmental or fatigue performance that an epoxy will have, and they also tend to cure-shrink quite a bit more than epoxies. That can be an issue with certain types of joint designs, particularly constrained joints such as the shaft of a golf club going into the club head.
The third type of structural adhesive that you commonly see is two-part urethanes, and these behave kind of like one-part urethanes. The difference is that because you have two parts the cure is chemical, not moisture-based, so you don’t have depth-of-cure issues or the very slow curing that you get with a one-part urethane sealant. Two-part urethanes tend to be the most flexible and the most rubbery structural adhesives, with the lowest glass transition temperature (Tg), meaning they maintain strength very well at low temperatures but are not very suitable for applications requiring high strength at high temperatures. They usually don’t provide great bonds on unprepared or sometimes even primed metals. Two-part urethanes are typically used for things like plastic, wood, fabric and rubber.
As you can see, each of these three main chemistries has different characteristics, so it’s important to really understand the needs of your application in order to choose the right type. At 3M we know and sell all these types of structural adhesives and we always try to start out by talking about end-use conditions: if it’s not going to last, there’s no sense building it. We’ll work with you to figure out what range of products is appropriate for your end use, then look at production and specify some adhesives you might want to try for validation testing.
Tensile stress is pull perpendicular to the plane and away from the adhesive bond. Force is distributed equally across the entire bond area. (Compression stress is in the opposite direction, where the substrates are pushed together perpendicular to the bond plane.)
Shear stress is pull directed across the adhesive, forcing the substrates to slide past one another. Here the force is in the same plane as the bond and distributed across the entire area.
Cleavage stress is concentrated at one edge of the joint, exerting a prying force on the bond as the substrates separate. While that end of the adhesive joint is experiencing concentrated stress, the other edge of the joint is theoretically under zero stress. Cleavage occurs between two rigid substrates.
Peel is also concentrated at one edge of the joint. At least one of the substrates is flexible, resulting in even more concentration at the leading edge than with cleavage stress.
This is a common question, and it’s very important – for instance, an open-air factory in Georgia could go through seasonal variations from 40°F (4.44 °C) to 104°F (40 °C). Structural adhesives rely on chemical reactions and those reactions are temperature dependent, so the number one consideration is that colder temperatures slow the reaction down and warmer temperatures speed it up.
The Arrhenius equation is a formula for the temperature dependence of a reaction. As a general guideline, for every deviation of 10 degrees Celsius you double or halve the reaction rate. Take as an example an adhesive that has an open time of 20 minutes at 25°C, or room temperature. If you change the temperature to 35°C you would cut that open time in half, to about 10 minutes. In the other direction, if you reduced the temperature to 15°C you would have closer to 40 minutes total open time.
It’s not just open time – the total reaction progresses the same way. If at room temperature it took two hours to reach handling strength, at 10°C colder it would take four hours. This isn’t just important for open-air operations and seasonal shifts: it can also be used to affect production. If you want to increase throughput without reducing open time you can assemble parts at room temperature, then move them somewhere 10 or 20 degrees warmer to increase the cure rate. In fact, above about 50°C reactions proceed even faster, so if you look at a technical bulletin on increasing the reaction speed of a heat-curing adhesive, once you get above about 50°C you can fully cure these adhesives that would take days at room temperature in a number of hours.
This is a question we often get from customers that doesn’t have a good, clean answer because it depends. There isn’t a single number to provide, and there are a few things that you should consider when you think about temperature exposure.
How cured is the adhesive?
If the adhesive has just reached handling strength or is still somewhat liquid, temperature exposure will do something different than if it's fully cured three weeks or six months after you assembled it.
What is the absolute magnitude of temperature that will be seen in the application?
How high is the high and how low is the low? This helps understand whether there will be any thermal degradation issues due to the adhesive reaching those extremes.
How long does the assembly see those temperature extremes and every point in between?
If a part sees an absolute high of 150°C, it makes a difference whether it sees that high for five minutes or five weeks, so you have to think about total temperature exposure and any degradation effects based on that. Frequency is also relevant: how often does the part move to the temperature extremes? An outdoor application in the desert that cycles every 24 hours between 40°F (4.44 °C) at night and 115°F (46.11 °C) during the day is very different from something that sees the same extremes but for months at a time with a year-long cycle.
What is the actual load being applied to the adhesive while it’s exposed to the temperature?
This last question may be the most important. Even if the adhesive doesn’t suffer from thermal degradation, it’s still a polymer and will undergo physical changes. Specifically, as temperature increases past a certain point (the Glass Transition Temperature) it will go from a glassy rigid state through a transition to a softer, rubbery state. Physical properties of the adhesive will change as it warms and cools through the transition stage, including rigidity, thermal expansion coefficient and heat capacity, among others, and this can affect the load-bearing ability of the adhesive.
There’s not a straightforward answer for how to prepare any substrate for adhesive without knowing more information. Substrates and adhesives is probably the most complicated question because it depends so dramatically on everything else you need: the overall performance requirements of the adhesive will be chosen based on temperatures, environmental conditions, overall strength needed and process conditions such as how fast you need it to cure. Whether and how to prepare a substrate depends a lot on the type of adhesive you choose, and even within substrates themselves there are different grades: not all ABS is ABS, so it may not be possible to issue a blanket statement on how to prepare that surface.
That said, there are four broad categories of substrates, and even within those there are different adhesive chemistries that bond to each one.
Metals have very high surface energy, so if the surface is clean and dry the adhesive should readily wet out, but all metals are not the same. Take aluminum versus copper. Aluminum is a passivated (inactive) metal and fairly inert, whereas copper is an active metal that will continue to corrode, so even with surface prep considerations you need to consider whether there will be degradation over time from the corrosion.
Traditional materials are things like glass, wood, leather and concrete. They have a middle range of surface energies, but each usually has some unique factor that must be considered. Roughness is one example. Another is natural leather that contains oils from the tanning process – over time these can leach into the adhesive, plasticise it and degrade the bond. Hydrolysis of glass means that moisture penetration is sensitive when you’re bonding glass to ensure it doesn’t degrade.
Engineered plastics are higher surface energy performance plastics like acrylic, polycarbonate, ABS and epoxy-resin composites. These materials are really unique because bonding isn’t just about surface energy – the adhesive may wet out across the surface, but it’s ability to bond will also be dictated by the crystallinity and polarity of the plastic. A material like nylon has a fairly high surface energy, but it’s very crystalline and not very polar. When you look at some of the mechanisms of adhesion, many of the adhesives may bond initially, but then over time the adhesive will fail unless you do more rigorous surface preparation.
Low-Surface-Energy plastics (LSE plastics) are commodity-type plastics like polypropylene and polyethylene, and also things with really low surface energies like fluorinated plastics and silicones. Polyolefins and LSE plastics are kind of a category in themselves because you will need to use primer or some kind of corona treatment, or use a specialty adhesive that’s designed to kind of penetrate into that plastic and create an entangled bond with the polymer of the substrate itself.
All those variables show why there is no easy answer and you typically still need to do testing and prototyping to make sure an adhesive works in your process. A good first place to look at substrate information is the Bonding and Assembly and Material Bonding pages of 3M.com, which have more extensive background on these topics.
A second suggestion is to review the technical data pages of adhesives you’re looking at because they show adhesion to a lot of different substrates. These pages typically report two things: a number showing strength under stress in either pounds per square inch / psi or megaPascals / MPa (for overlap shear) or pounds per inch (for peel), and also a failure mode. A failure mode of cohesive failure means that adhesive tested under the conditions listed remained bonded to both substrates after it pulled to failure: the adhesive itself failed rather than the bond. Adhesive failure indicates the adhesive pulled away from one of the substrates. This can provide a rough guide as to whether an adhesive might be suitable and should remain in your consideration group.
The third option, if you have a specific substrate in mind or a question about an additive that might be migrating into the adhesive, is to reach out to 3M. Our technical team can look at what might be happening and do a technical service request to try to help you understand which adhesives over the duration of the part might be a better option.
Everybody loves to download a tech data sheet from the website, look at all the numbers and hope it’s going to explain what’s going on, but it doesn’t quite work like that. Tech data sheets are meant to help compare products against each other, but they don’t guarantee a certain performance in your application.
In order to compare adhesives 3M does very standardized testing. The most common thing you’ll see on a data sheet is usually overlap shear ASTM D-1002. This is a very standardized test that can be done on a wide variety of materials. It helps us look at how well the product adheres to the surface of that material, what surface prep should be done, and even how different temperatures affect the bond. Peel testing is another common set of numbers that look at related information under a different type of stress.
The next thing you’ll see on the data sheet are some inherent characteristics of the product itself, including things like open time, cure time, viscosity characteristics, modulus and elongation. These characteristics help narrow your choices and decide which products to model to see how they work in your application. Again, these are fundamental properties of the adhesive itself, so you can compare, for example, different surface preparation levels with a given adhesive. This does not guarantee performance in your design, so the next thing you need to do is validation testing.
Beyond the data sheet, 3M will also do specific testing for you at no charge. If you have an unusual plastic or paint and need to know what adhesive is best, what surface prep to do or how much cure time is required, we can help you find out. We’ll do that standardized testing at no charge to narrow down the field of adhesives you need to look at for your own validation testing.
Viscosity is a number that you see on data sheets, but it doesn’t really characterize the adhesive – that’s why it can be confusing. These adhesives are non-Newtonian in their behavior, which simply means that their apparent viscosity or how they flow depends on the shear stress that you apply to them. Think about whipping cream: you can stir it easily, but when you put a dollop on pie it stays mounded up. That’s an example of a non-Newtonian behavior, and that’s the way all two-part adhesives behave.
You can’t just look at a viscosity number on a data sheet and fully understand what’s happening, which is why adhesives are characterized using terms like non-sag or self-leveling. A non-sag adhesive is one which, when it’s subject only to the stresses associated with gravity, tends not to slump, drip or run – it stays where you put it like that dollop of whipping cream. Self-leveling product, on the other hand, will smooth and level out to form a nice flat surface, which can be useful for applications like potting.
This comes into play a lot when it comes to bulk dispensing, and you need to understand how both types behave. Typically you need a rheology curve to look at the viscosity behavior vs. shear rate. Another factor to keep in mind is that all of this is temperature-dependent. Another food analogy is to think about honey: it’s very thick when cold, but if you put it in the microwave it will flow a lot better. The same thing applies to adhesives: you can be used to how your adhesive behaves in winter, but then the season changes and your production plant is 20 degrees warmer, so the adhesive may flow a lot more than you expected.
The third factor that affects the apparent viscosity or flow of the adhesive is the reaction. With two-part adhesives, when the two parts meet in the mix nozzle they start reacting. As they do that they gel and build viscosity so it takes a lot more force to make them flow through the mix nozzle. That can become a real issue, particularly if you’re dispensing small quantities per part of fast-cure adhesives.
All those possibilities have to be factored in when you specify which adhesive is going to work best for an application and how you’re going to dispense the material.
To recap, the three things you want to look at are the viscosity vs. shear rate and does that match what you need for your application, what temperatures are you interested in when dispensing the product, and what is the cure rate vs. the volume dispensed per part to ensure the adhesive isn’t curing so quickly that gelled material is constantly building up in the mix nozzle.
Need help finding the right structural adhesive for your project? Contact us if you need product, technical or application advice, or want to collaborate with a 3M technical specialist.
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