Why does some epoxy yellow more than others?

[scherzoja]

I've noticed that some brands of epoxy turn more yellow than others and in a shorter period of time. Some epoxy I've used become noticeably yellow after one day, but other brands might take a week or two. I understand that all epoxy will yellow because of exposure to UV light and would like to know if the degree of yellowing and how quickly it turns yellow is any indication of the quality of the epoxy.

Thanks y'all.

[boatman61]

Epoxy should not yellow that quickly..
Are you talking about on wood or some other material.

[scherzoja]

I'm using epoxy to patch polyester deck of a 30 yo production boat. I had to switch suppliers (progressive epoxy). The one I usually used was out because of production issues from last winter storms and COVID. The only temporary alternative (Raka Epoxy) I could find is the one that yellows really quickly. I also have US Composites epoxy, but have not tested that out in the sunlight.

[boatpoker]

Some epoxies produce an amine blush in the curing process, these yellow a little slower than amine free epoxies as the amine slows the UV effect.

[boatman61]

Ahh okay... I'll butt back out now..

[scherzoja]

Boatpoker,

That's interesting because the Progressive Epoxy is advertised as "no blush" and I 'm using the 'no blush' hardener with the Raka Epoxy. I wish I knew more chemistry then I might be able to not rely on vendors' statements. Alas, I can't know everything.

[Chotu]

Raka is actually a great epoxy for general purpose projects like yours. Although I’ve never seen it yellow, I wouldn’t be at all concerned with it being more yellow that what you are used to.

Your project isn’t one that pushes epoxy anywhere near its limits. As long as you UV protect it, it’ll be stronger than the rest of the deck.

[Benz]

My epoxy expert told me that hardener tends to turn yellow on the shelf--his theory was that it was picking up oxidation from the inside of the can. I use a fair bit of West System, and can confirm that the older the hardener, the yellower it is.

[Chotu]

Ok, one area of concern. It’s not this yellow is it??

You didn’t get the UV hardener by mistake, right?

[scherzoja]

It's not the UV hardener because I've used it down below and it cures in the shade, but does not yellow down below: no direct UV down there. The resin and hardener in the bottles are mostly clear, with the hardener having a bit of a slight amber-ish hue.

I've got a partial bottle of slow-cure hardener that is pushing 4years old and it is a much darker orange than it was when new because of the oxidation, but I used new hardener with new resin (well, I bought them at the same time in July, not sure how 'new' they are). Either way, the hardener and resin are almost the same color in the bottles, Raka just yellows must faster and with a darker hue.

The color by itself it not a big deal because non-skid paint is going over the top. I just don't know if the degree of yellowing with one brand over another is indicative of any kind of post-cure structural inferiority or superiority.

[Chotu]

Ok. I’ve used around 50+ gallons of Raka on various parts after the hulls were joined and bulkheads put in and stuff on my boat. I opted for Mas infusion epoxy for fabricating the hulls and bulkheads and beams.

I used raka for fairing, making rudder cassette boxes, holding rudder bushings in place, building steps outside on deck... and I left some in the Florida sun for a year. Part of the steps weren’t painted. It turned yellow but I can’t break it in any way.

Get the paint on soon to protect it (or at least get a tarp or sheet of plywood or something over it), but I’m going to mention again that your application is in no way stressing the epoxy anywhere near its limits. Even Slightly yellowed, the surrounding polyester would mostly likely fail before your repair work if stressed to the breaking point through destructive testing.

[scherzoja]

Thank you for the insight. I'll continue on with the hardware removal and patching with Raka and go worry about something else.

Thanks again.

[Locquatious]

Typically the more "aromatic" "double-bond" epoxies exhibit more yellowing since they absorb UV light closer to the visible range. Bisphenol-A epoxies are very aromatic and lead to yellowing. But in my experience even "hydrogenated" double-bond-removed epoxies eventually turn yellow or brown.

The formulator can add stabilizers that slow or reduce the yellowing. It is not always clear that yellowing affects properties. The yellowing actually increases UV absorption, so once the yellowing starts, the UV is predominately absorbed at the surface and penetrates into the lower regions is exponentially reduced. One could add carbon black, copier toner, or titanium dioxide to reduce the UV penetration. Or just paint the surface.