I want to focus a little more on epoxy resins. This is because these are the resins that most people associate with “advanced composites”. And also, because they are at the other end of the continuum of the two part, aromatic backbone resin systems. We talked a little about polyester a couple weeks ago, and I even showed the backbone of what is called “unsaturated polyester” which is the liquid form of the resin before it is cured with the strings (fibers) to make fiberglass. There is a resin system that sits right in between polyester and epoxy, called vinyl ester which is intermediate in both cost and properties between polyester and epoxy.
Most epoxies use a Bisphenol-A backbone as a starting place to make the resin. I need to add several chemical diagrams to this post because it takes these diagrams to understand not only the relationships between the resins, but also why each has the properties it has. We showed unsaturated polyester in the previous post to give you an idea of what these chains look like.
The other end of this aromatic, two part resin family that has more of the benzene rings we talked about before is of course epoxy. Most epoxies, like I said before, have a Bisphenol-A backbone, which is really just two benzene rings stuck together with a couple hydrocarbon groups in the middle and OH groups sticking out either end.
So, using this compound to make a resin system, first you have to add what are called “epoxide groups” to each end of this thing. This makes a compound called Bisphenol-A Diglycydyl Ether.
If this looks complicated, please don’t be concerned – it took organic chemists over a hundred years to figure all of this stuff out. The only think I need for folks to take away from this is that there are two benzene rings stuck in the middle of this, and that at each end there is a group that is ready to react with more of this stuff and make long chains of these. This is where the magic of epoxies comes in. The stiffness and strength of epoxies is based on the two benzene rings stuck together. This ring structure is actually flat in real life rather than being a tortuously bent up chain like most hydrocarbons. And when you pull on each end of this molecule it is stiff and very resistant to pulling because the bonds are already straight across the ring. This same ring is what makes cellulose such a strong fiber.
But I digress – I’m talking about resins. The hardener for most of the epoxies, and even some polyesters and vinyl esters, is an amine hardener. Amines in this instance are hydrocarbon with a Nitrogen stuck in the mix. Some of the more common ones are a 4 carbon ring with one nitrogen attached in among the carbons, in sort of a pentagon rather than the benzene hexagon. When they get added in the place of these groups at each end of what you see in the pic above, it makes for a very strong and stiff plastic. And epoxy is very sticky stuff, so it makes a really good resin system especially for carbon fiber composites. These resin systems also need higher temperatures to sure than do your typical polyesters, and even most vinyl esters. There are room temperature curing epoxies, but they don’t use amine hardeners, they are cured with different hardeners and typically don’t make as strong and stiff a plastic as the amine cured epoxies.
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