Baffling mystery??

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#1

I have been making small diameter (1" and 2" square) tubes using a wet layup procedure on aluminium mandrels, waxed, and then coated with Rexco Coverall PVA. I am using a 200gsm twill with wide tows. I am using Aeropoxy PR2032 with PH3665 hardener. Being made on male mandrels, the surface has a texture, but the tubes are mainly used in a utility setting, and I am generally satisfied with the quality. Here’s the big problem.

When the tubes are taken off the mandrel, the surface is nice and black, as it should be. I washed the tubes in order to get the release film off of the inside, and the outer surface has turned a milky white. It is entirely unacceptable cosmetically. It looks something like soap scum. I have tried everything to remedy it, including washing with more water, scrubbing, steel wool, acetone etc. I have also tried heat, from a little to melting, and nothing works. The only remedy that works is sanding and rubbing down with a thin layer of black adhesive.

It is obvious that it is a result of washing the tubes, and thus it must have something to do with the release compound. I was presoaking the tubes for about 5 minutes in a tube of PVC, so my conjecture is that the PVA was dissolving and thus dispersing into the bath. But why would a diluted bath of PVA so drastically change the surface? Any insight or ideas for other solutions would be greatly appreciated.

#2

I have found that if the epoxy if not fully cured it can go milky when wet. Maybe try mixing a small amount of resin in a cup (not to thick as it may exotherm and cure more than our parts did), allow to harden in the same way your parts did, then wash. This would allow you to rule out the release or not if it washes fine.

#3

Perhaps you had not fully cured resin that absorbed water. I’ve found laminating and general purpose epoxies tend to do that when they get wet.

#4

I forgot to mention that some of these tubes were as much as two weeks old, they had been post cured at 155F for 4 hours, and that I am also very careful with ratios. I am waiting to call the manufacturer of the epoxy when they open on the west coast.

#5

Curious… I"d say it was water too but, shouldn’t be that… unless your mix ratios were off? Or maybe the mix wasn’t a good mix… like not double cupping or scraping the sides? That’s probably a stretch though… I’ve never had that sort of issue unless I just mixed a bad ratio.

let us know what you find out

#6

The manufacturer had no good answer. I am meticuluous in my procedures. I use one new cup per tube, and even use two scales, one on top of the other, to corroborate mix ratios. Their best guess was water adsorption and some kind of chemical reaction. I salvaged what I could using black leather dye, brushed on and wiped off, followed by a very thin wipe down of epoxy to restore the surface wet look. It is acceptable. I will use a completely different method of cleaning PVA in the future.

#7

How old is the resin and hardener? Sometimes in older resins or if the lid had been left off, some of the solvents/volatiles will have broken down or evaporated. This could throw the ratio off just enough.

Maybe try a small test sample with your new resin to see if it reacts with water before making any parts. Or could be time to look at a new non PVA release.

#8

I’ve have personally seen this twice. 1) Wrong hardener (different manufacturer) 2) Wrong mix ratio.

Just as a reminder, the mix ratio by volume is different than the mix ratio by weight.

#9

Thanks for all the replies. To answer the last few questions, the resin and hardener was very fresh, and I only have the two matching products on hand, so no chance of error in mixing different products. I have now sorted it out in my head. The defect was definitely due to water adsorption during washing, as they were all jet black prior. I say this with certainty because it was not the first time this happened. I do not think the PVA was an issue.

On earlier batches, I also had some tubes with issues and other tubes with none. Clearly the ones with no issue were well cured (being weeks old) and the others not as much (days old.) All tubes received a post cure heat bath, which should have cured them through and through, but it appears that this was not the case. I have enough understanding of the issue to avoid it in the future. Again, I appreciate everyone’s input.