I’m going to delve just a little bit into the glues (resins) in composites, what they are, how they are sold, and how they are used. In the pics in this post you can see a little bit of how they are used in boat building and also how at least one of the fabrication processes for composites works. And you also see in the pic to the right
how they are sold. Each of the containers in that pic also comes with a hardener that isn’t shown in the pic, but for these resins it is just a small amount, usually sold in a small bottle along with the main resins. Other resins use quite a bit more hardener, but that’s the subject for a future post about curing agents.
These are of course the most common composite reinforcements and resins (Strings and Glues) in use today and make up the bulk of the business of composites. What you see here is a polyester resin, the most common boat building and home hobbyist resin system in use. This type of glue is also used in fiberglass shower stalls, fiberglass bathtubs, and some composite countertops that may be in your home today. This is because it is the least expensive and easiest to use resin system.
I need to delve a little into the chemistry and structure of resins here so that everyone understands why they are as strong as they are. This is also true of things like carbon fiber, but the resin that holds the fibers together has to be pretty strong as well. Here’s a diagram of what is called an “unsaturated” polyester resin.
What you need to take away from this picture are the two 6-sided rings at the top, and the regular repeating pattern of the chemistry. The rings are Benzene rings which is why the resin is so strong. These rings are actually flat and they are very stiff when being pulled or pushed. The rest of the bonds between these rings (Oxygens and Carbons) are at angles to each other so when the resin cures it makes this stiff, tangled structure. And with all of the reaction sites (squiggly lines at each end) they are easy to cure. Most of these resins cure at room temperature which is why they are used so heavily both in industry and for home use. And, this stiff, tangled structure also has lots of surface area where the Oxygen and the CH3 can stick to fibers, so they are very sticky as well.
All composite resins have Benzene rings as a backbone, with epoxies having almost an exclusively Benzene ring chain-like backbone – with a few other elements sprinkled in along the chain to give epoxies their properties. So, since epoxies are so stiff, tangled, and strong, they are the resin of choice for what are called “Advanced Composites”. This typically means carbon fiber with an epoxy resin which is the basis for most of the advanced aircraft manufacturing that goes on today. In fact, the F-22 and F-35 both have carbon epoxy wing skins.
Speaking about that, the airplane that you just took to visit your grandma has quite a bit of composite material on board – mostly in the interiors of the aircraft, but for planes like the 787 composites make up most of the primary structure of the airplane.
But, back to chemistry a little bit. Most epoxies have a Bisphenol-A (BPA) backbone. And while I know that Bisphenol-A has gotten a bad rap and should not be used in food packaging or anything to do with food, or even be involved in human exposure, it is a very useful compound in composites and is perfectly safe once it is bound up in a resin where it can’t get out and do anyone harm.
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