But when you get older, more skeptical, and less easily impressed, you may start to ask questions. What are those two parts? And why don't they come already mixed together? That would be the sensible way to do it, wouldn't it? I'll answer the first question first. The first part of the two-part epoxy is a low-molecular-weight polymer with epoxy groups at each end. This is what is shown in the model at the top of the page. Can you see the epoxy groups at its ends? Its structure looks like this:
In these prepolymers n can be a high as 25, but the diepoxy in Dad's two-part epoxy adhesive is more likely a small molecule with two epoxy groups, like one of these little fellows:
If Dad's two-part epoxy adhesive is two tubes of liquid, than the diepoxy is probably one of the small molecules. The bigger prepolymers are solids at room temperature. If n is as high as 25, the diepoxy is a hard plastic at room temperature. Diepoxies like that have to be heated up and melted before they can be mixed with that other part of the two-part epoxy mix.
Remember that other part? The second part is a diamine, which looks like this:
When you mix the two parts together, the diepoxy and the diamine, they do something fun. They react, and join together, in such a way that links all the diepoxy and diamine molecules together, like this:
Click here if you'd like to see how this reaction works.
What this gets us isn't a linear polymer. Instead, it's a crosslinked network, that looks something like this:
You see, all the diamines and all the epoxy molecules have become one big molecule. When this happens, the result is a hard substance that can be very strong, but not processable. It can't be molded into shape, or even melted. Think about it. This is why the two parts don't come already mixed together. If they did, they'd be a solid chunk, and not much use as an adhesive.
That still leaves an unanswered question. How do we get that low molecular weight polymer, the one with the epoxy end groups, in the first place? We make that by reacting bisphenol A with epichlorohydrin with some NaOH thrown in as a catalyst. If you want to know how this works, and how we can control how big this pre-polymer is, click here.
Epoxy resins made great adhesives, and are one of the few adhesives that can be used on metals. But they're also used for things like protective coatings, and as materials in things like electronic circuit boards and for patching holes in concrete pavement. What? You want more? Epoxies are also used to make composites. One way we make composites using epoxies is with a nifty process called SCRIMP.
Here are some other polymers used as thermosets: