We all know about heat activated adhesive systems. In terms of specialty adhesives, this concept is one of the most basic out there. But with that said, understanding exactly how heat cured adhesives work is also fundemental to the overall understanding of adhesives so it's always good to get a reminder. There are also a few not so obvious things we'd like to point out about heat curing processes.
How traditional heat activated adhesives work
Heat curing polymer adhesives is all about providing the necessary energy to allow reactive molecules to interact in a reasonable amount of time. At the molecular level, when you combine an epoxy resin with an amine catalyst what you're really doing is putting a bunch of molecules together that have reactive components. For some particular epoxy-amine mixtures, just putting these molecules together at room temperature is enough for them to get started. For example, two-part room temperature curing epoxies can cure to completion in a matter of minutes. One-part systems are a little different, however. They actually already contain all the reactive components needed to create the end polymer. But, the chemical reaction happens extremely slowly at room temperature. So slowly that you can pretty much consider it not happening at all. What it comes down to is energy. By adding energy to these one-part systems (heating them up) we can drastically decrease the amount of time till cure completion from basically never to a few hours.
Take away: heat cure adhesives aren't really changed chemically by the heat in a way that initiates them. The chemicals are already active and are slowly reacting. The heat just speeds up the reaction to a point where the product is actually useful.
Exceptions to the rule
Some adhesives or polymer materials are chemically activated with heat application. But these are much more specialized systems and are rare to come by. An example of such an adhesive would be a cationic UV cure epoxy. Cationic epoxies are able to UV cure because they contain unstable photo-sensitive chemicals. When these chemicals are exposed to light, they initiate a chemical reaction that results in the epoxy resin self-polymerizing. However, these photo-sensitive chemicals are also heat sensitive. So in the same way that the reaction can be activated by UV light, the reaction can be activated by heat as well.
Some not so obvious points about heat activated adhesives
There are a few things that we'd like to point about heat curing adhesives that maybe aren't immediately obvious.
- Room temperature cured adhesives can be further cured with heat - A polymer will never perfectly cure. For example, when a room temperature epoxy cures there might be unreacted components still present in the material. Because the polymer is now a solid substance it is much harder for these unreacted molecules to move around and interact with each other. By adding heat these interactions are able to happen and the polymer is able to further cure. Heating an epoxy that cured at room temperature will normally raise the glass transition temperature, the hardness, and the lap shear strength of the material.
- Curing at the common service temperature is advantageous for many applications - When designing components that will be commonly used at a specific temperature, it is smart to cure the adhesives at this service temperature. As a solid polymer cycles through different temperatures it will expand or shrink which puts stress on bond interface. Curing adhesives at the service temperature will help the application avoid this unnecessary stress.