Elevated Epoxy Curing

General Composites Discussions Composites Talk
#1

I’ve been working with some room temperature cure epoxies which require post cure in order to achieve their maximum working properties. I’m curious if anyone here is experienced with supported vs. unsupported curing? I was thinking it might be possible to do a supported (in mold) cure to 150F then demold and take the part alone to the 200-220F; this would be done with gentle temperature ramp.

I’m contemplating this because I’m concerned about my mold going to those temperatures, per the mfr spec its possible but I would rather take a more conservative route just to be sure I don’t damage the mold.

I remember reading somewhere that this is what the Woodward guys are doing but can’t seem to locate it. I seem to remember them doing 150F supported then 250F unsupported.

#2

depends on the resin system, everyone is different. Usually an intial low temp cure can be done, then a second post cure unsupported. Read the TDS, if not in there, call tech support.

#3

Typical “room temperature” epoxies are good to about 190 F and needs no more then 150F as post cure , there is no benefit going higher and going to 220 will most likely destroy the part

#4

The one Pro-Set that I’d like to use wants 180F. But the TG is still less then I’d like. Some of the other high temperature epoxies I’m considering need more heat. I’m trying to maximize the TG without going to more exotic tooling.

#5

Check with product stats. We use API’s CCMFCS-2 that has a 200 OR 300f post-cure, but generally needs supported. I have not done a unsupported postcure in awhile (however, I know of one part that was cured unsupported, but I can not say on whether or not surface finish was affected. I’m sure the part sat around for a week after unbagging)

#6

In general, if you leave the part in the mould, you can add heat relatively quickly without destroying the shape or surface quality of the part. (especially when still under vacuum).

Your mould should be able to cope with the heat, however.

If you want to take it further than your mould allows, you will need a free-standing postcure. In that case you need a gentle ramp-up temperature, as not to run over the Tg that is present at that time. So you will need to know how much the TG will rise in an certain amount of time, and you will also need to know the ultimate Tg, and stay away from that at least 20 degrees F or 10 degrees C.
Control and knowledge is everything, but it can be done free standing.

You can take your mould to a higher level using the same procedure, as long as the ultimage Tg has not been reached still. Running a mould higher than its Tg will destroy it, so again, knowledge is everything.

Best suggestion is to do some testing, with sample laminates of the same thickness as the part.

#7

Thanks for your direction Herman, this confirms my suspicion this is a valid method.

As you said the trick is knowing what Tg you achieve with a certain temperature over a certain amount of time. MGS actually shares this data in their spec sheets so I can clearly see that a supported cure at 60C over 15 hours will yield a Tg of 95C. So as a second step I could do an unsupported cure at 80C for another 12-15 hours to achieve the ultimate Tg of the product.

Is it common for the un-supported cure of the part to degrade the surface quality?

#8

Depends on the resin, still. Which MGS resin are you using?

You last question: if not done with care, yes. (remembering the builder of one of my boats, who tossed the boat (not postcured yet) freestanding in a 70 degrees C oven…)

#9

Or even black, cored boats with unsufficient postcure. printtrough all over the place, same process.

(that goes for every dark, cored product. but I’ve seen that boat…)

#10

I have some MGS 285 with 287 hardener that I’ve been testing with; this is all (from MGS) that’s really available in my area and its pretty expensive… I also have some Pro-Set but they don’t provide enough data on Tg to really determine if this is feasible, so I’ll need to give them a call.

Through this process I need to retain the surface quality.

#11

Which Pro-Set?

Momentive Epikote MGS LR285 as it is called now, has an ultimate Tg of some 130 degrees. You can indeed also see the Tg is pushed forward quite much above the cure temp. (a 50 degrees cure gives 75+ values) and also that a 60 degree cure gives even 95 degrees. This indicates that the resin really likes a higher temperature to thoroughly cure.

I suggest curing at least up to 60 inmould, if your mould allows it, then start at 60, and increase the temp 5 degrees/hour untill you reach some 110 degrees. I would not go higher, as the Emodulus drops quite a bit above that.

#12

The Pro-Set combo is 145 w/ 229 - http://www.prosetepoxy.com/pdf/1%20Laminating%20Epoxy/145-229.pdf

I wish these guys had the same style charts as the MGS…

I was thinking of a similar curing schedule but I was only going to go to 80C per their recommendations. Do you really think going to 110C would be beneficial to obtain maximum Tg while not risking the part?