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Structural Adhesives FAQs

  • Every situation is different, but our team of expert engineers do get some 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 kinds of factors to consider as you review design, process and adhesive options to see how 3M Structural Adhesives can help you make things better.

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  • FAQ 1: How does temperature impact the cure profile of my adhesive?

    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 to 104°F. 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...

  • Video of Application Engineer explaining temperature impact the cure profile of adhesives
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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.

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  • Video of Application Engineer explaining temperature resistanceof adhesive for a bonded assemby
  • FAQ 2: How should I think about the temperature resistance of the adhesive for a bonded assembly?

    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.

    If the adhesive has...

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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 at night and 115°F 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.

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  • FAQ 3: How would you prep substrate X?

    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...

  • Video of Application Engineer explaining how to prepare a substrate.
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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, plasticize 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 really low surface energies like fluorinated plastics (polytetrafluoroethylene or Teflon®) 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 psi or megaPascals (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.

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What are the most common stress modes of Structural Adhesives?

  • Tensile

    Tensile

    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

    Shear

    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

    Cleavage

    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

    Peel

    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.

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Structural Adhesive Chemistries

  • Acrylic Adhesives

    These two-part adhesives offer great strength and design flexibility.

  • Epoxy Adhesives

    These adhesives provide excellent durability and resistance to environmental extremes.

    One-Part Epoxy

    Two-Part Epoxy

  • PUR Adhesives

    These one-part products combine the speed of hot melt adhesives with the structural benefits of moisture-curing chemistries.

  • Urethane Adhesives

    These formulations are ideal for creating strong, flexible bonds between dissimilar materials.

  • Anaerobic Adhesives

    These adhesives provide tight fits and seals in threadlocking, pipe sealing and related applications.

  • Instant Adhesives

    These products reach handling strength in 5-10 seconds and achieve extremely high tensile strengths.


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