Resin turning red during post-cure

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#1

After talking to some guys I wanted to check with the forum about what I was told. A few of my parts have been turning redish brown when they are post cured in the oven. I am using a high temp epoxy, mixing well (or so I thought), and using a grams scale to measure weight.

I talked to some composite guys and they told me the reason the parts are turning red is because of improper mixing or improper amount of hardener.

Does this sound right?

I mix the resin for about 5 min in a cup and sometimes heat it with a heat gun (apparently a no-no). Everything is stop on with mixture weights as well. Is there anything wrong with this technique?

#2

Iiiinteresting.

Never heard of epoxy turning colours during post-cure, obviously unless you are cooking too high. Are you free-standing post-curing, or vacuum bag post-curing? Maybe it is reacting with the air?

#3

Only time Ive seen epoxy change colors is when you leave a bunch of excess mixed resin in a cup. Gets hot, turns amber/red.

#4

I would just call the manufacturer and talk to tech support or change resin system

#5

I talked to the distributor and they assured me I was mixing it wrong. the parts are rock solid and stand up to a massive amount of heat (almost directly on a 4-stroke headpipe). I may have turned the heat up a little too quickly but the resin is still rated for higher than I had the temp at. The parts are being post cured about 4 days after de-molding in a household style oven.

#6

Green, red, yellow, orange are common for high temp epoxies. But it shouldn’t really be visible unless you have resin-rich areas.

#7

i’ve seen brown on Cynate ester around the edges of parts. and I know many plastics will brown when heated in the air. But for the whole things to turn redish…i’d say over cooked, or oxidation.
shrugs

#8

it was just sections of the part. I unfortunately dont have any pics either!

So I guess mixing better and heating at a slower rate? How can I prevent oxidation…

#9

which sections? things that might be resin rich (ie: corners where bridging might happen), or just the edges?

to prevent it, vacuum bag it when you cook it. depending on the part, I would just use peel ply as a breather. you don’t need anything else, since it is already cured.

#10

I will have to investigate this.

Few questions…At what point of the bagging/curing of resin should it be put in an oven?..during curing, after curing, a couple days later?

What bagging material will deal with the heat from an oven?

#11

Right after the bagging. Or else it’s curing at room temp, and you’re just post-curing in the oven.

#12

What temp are you post-curing? I know that the cheap stuff airtech sells (green bagging, green peel ply) work up to 400f (though, I’ve had some bagging melt to itself at 375, and tackytape blow at 375 asd well, which prob’ was just bad bagging technique).
Kapton bagging film is up to 700 I think.

As for WHEN, I do not know. Contact your resin maker. I THINK some production places have a cure cycle programed, that drops down to ambient, and then right back up to a post cure stage. So each part might take 12hr to do, but if they know the part well enough not to be inspected pre-post-cure, then they just do it all in one shot.

For VARTM, I would think that you leave it in the bag until it’s cured (i’d say a day or 2 depending on resin), and then just move it over to the oven for post-cure, still in bag. Post curing doesn’t really do much to the part anyway, besides make it stronger. One just trims and finishes after everything.

#13

Hello all. I am in the process of modifying my sport bike to put a muffler in the tail with the tips coming out of the back. I have made some small fiberglass parts before so I am not totally new. I am curious what type of resin I should use and how I would go about post curing it. I have a small electric oven that I use for powdercoating. The muffler would have to withstand about 300 degrees. Thank you in advance.