Epoxy issues

Hey! I've been working on a few rockets, trying to fly solo to prove that I can handle it on my own, but ran into an issue.

I recently purchased some 4:1 fast setting epoxy from US Composites. (Fantastic service BTW) All was going well with the application of it, until I checked my left over epoxy. I squeezed the container I used to mix it, and it easily cracked. I was concerned, but proceeded to my next batch figuring I didn't wait long enough. It then happened again.

The epoxy can be cracked, and then smashed if you take one of the cracked pieces. I am no expert by an means on this stuff, but I am certain the fact that it is cracking is not good.

Here's what I know:

I am using the right ratio. I use syringes to measure out the hardener and resin, there may be a little more of one or the other here or there, but I am certain the ratio is being very closely held.

The temperature is below optimal. I am using the epoxy at about 55 F, once it won't move around, I bring it to a room where its around 65, and if I'm lucky I put it next to my fire place. The epoxy batches that crack do not undergo this temperature change.

I am mixing the epoxy well. I make sure to spend 5 mins at least mixing. I am using small amounts so it would seem to be a reasonable amount of time.

I really have no idea what I am doing wrong. The epoxy after setting can be crushed. I know that is in no way good. I am no expert, but feel this is important. Please help me out, I want to make sure all rockets I'm flying are not going to fall apart.

Thanks guys!!
(Sorry for any typos, I'll go ahead and fix it up tomorrow, but its late.)

I assume you are using the 150 thick and not the laminating epoxies. It really doesn't matter except the laminating epoxies are not structural epoxies. Epoxy by itself has very little strenght so almost all epoxies I have used from West, Aeropoxy and US composites, Bob Smith will crack as you described. Their strenght is the bond they make between the material ie.. wood, cardboard etc. I am not an epoxy expert but I don't believe you are doing anything wrong. Gather your parts and get to building!!!

Not unusual. If you are making fillets and such with it I would look into adding an amendment. Specifically, Kevlar pulp or chopped glass or carbon fiber.

Or cotton flox...

Epoxy is a matrix into which one puts other things. It holds those other things together. It has some compressive strength, some shear strength, and some tensile strength... But, it typically has perhaps 3% distortion before failure ensues. When one adds fiber or other reinforcement, the fibers prevent the epoxy from using up that 3%... Heck, one of the quick and dirty field tests of adequate epoxy cure is to take a sample and bend it. It should snap, rather than bend!

Adding random fibers into epoxy does not reinforce it very well. Absolutely better than nothing, but not well! A fiber which is 10 degrees off the applied load axis contributes something around 50% of what it would do if fully aligned. In a random mix of fibers in epoxy, only a low percentage of the fibers are going to be oriented to within 10 degrees of the applied load. When one is doing structural work, this isn't how to do it. One uses fabric and orients it to the applied load. It is much more efficient and effective.

If one just needs a fillet but not much reinforcement, I like JBWeld and similar reinforced epoxy. It is stronger and less brittle than typical epoxy with some random chopped fibers added.

Gerald

As a filler I have been using phenolic micro balloons (best attempt at spelling my phone isn't a fan). The "crush" I put the batch under has resulted in the filler being much more difficult to crack. I add the filler to help the epoxy run less, but not enough to make it super thick. I talked to my chemistry teacher and she seemed convinced adding filler would drastically reduce the strength. I could see where she was coming from, but also have read threads where some mix filler into the point of a peanut butter consistency. My understanding is filler does reduce the strength, but not enough to be a cause of concern for most applications as long as you don't plan on doing anything ridiculously forceful on the epoxy. The weather also has warmed up plenty, so the epoxy is bonding at a more nominal temp and I am getting some strong batches. The fillets have been done so there is little need for filler on the rest of the rocket, but thanks for the help guys!!

The application where most people here are using the micro balloons is for external fillets. These are aesthetic features and not very structural. The micro balloons are used to make life easier when you go to sand.

If you are looking to add amendments for structural applications, you'll want to use something specific to that purpose (chopped/milled FG or CF, FG or CF cloth, etc.)