Priming and painting Epoxy

[pauloman]

only epoxies rated for high temp need a post cure. Essentially they are a blend of ambient curing agent and a high temp curing agent that needs elevated temps to cure. using the regular curing agent in the blend allows the resin to become dry to touch, but leaves the high temp curing agent largely un-crosslinked until heated in elevated temps.

note that pretty much all epoxies take about a week for full cure. you would never perform lab tests on any epoxy sample not cured for 10 days or longer

paul oman
progressive epoxy polymers inc

[CatBuilder]

Interesting, Paul.

So do you mean epoxies rated for 120F or more, or is 120F considered high temp? I mean, the surface of a boat in the tropical sun can push those temps sometimes, right?

I had always wondered if there was also a time component to this stuff.

As in many epoxy distributors end up thinking you are making a wind turbine blade in the morning, then trying to spin it up on a tower later that afternoon.

Building boats with less labor than ideal, parts sit for months in 80-100F temps. I notice weeks later, you can't even flex them, even with the infusion epoxy that they suggest to post cure.

As time goes by and hot day after hot day take place, the stuff gets very stiff and hard.

Every component of my boat will sit for at least a month at 100F daily average temps at the end of this build (July in the South). I keep wondering if this is enough. Lots of people say no, but even the infusion epoxy manufacturer said it wasn't absolutely necessary to post cure in this climate, even though it was still recommended.

Confusing stuff.

[pauloman]

the Tg (where epoxies begin to soften) varies a bit with different epoxies. Usually about 160F in dry conditions and about 140F in wet conditions.

special high temp epoxy curing agents don't ever set up in temps under about 100-125 degrees, so they are mixed with 'regular' curing agents, but then need that high temp post initial cure to get the high temp curing agent to set up.

100 plus temps will greatly speed up the cure of regular epoxies which in normal temps take about a week or 10 days for 100% cure. - added solvents and fillers (maybe) can extend this cure time.

progressive epoxy polymers inc.

[AndrewK]

Originally Posted by pauloman

100 plus temps will greatly speed up the cure of regular epoxies which in normal temps take about a week or 10 days for 100% cure.

This can be missleading, regular epoxies in boat building use curing agents that will give a gel time of 20 -200 minutes. Even the 20 minute epoxy will not be 100% cured after 10 days.

CatBuilder; the only way to know how much your resin system will benefit from a post cure is to get the data. But as you know this easier said than done.

[PAR]

Cure rates can be misleading. Most epoxies take about a month to get to 100% cure, but will be 90% - 95% in a week to 10 days. If you're testing your glue lines to 90% of breaking strength, you'd be best advised to wait a month. As to post curing, well it's formulation specific, so contact your vendor for a physical attributes sheet. Most of the time you can just call their tech support line and get all the answers you need.