heat curing epoxy...revisited

General Discussion
1

In the Epoxy & EPS Construction Primer there are points which to me seem to be conflicting on the matter of heat curing. A post by Oneula quoting a website by Jim Richardson states:

Quote:

21. We strongly recommend that you postcure your epoxy boards in an oven.
We postcure them for 10 hours at 130 degrees F before we sand.
The epoxy hardens noticeably and is much stronger after the postcure.
They will also take the heat in a hot car without softening."

[=Black][=1]Then later, Greg Loehr says:

Quote:

[=Black][ 3]

[/][/]Heat curing done on an uncured lamination can also cause outgassing from the foam. My best suggestion is to work in an area that is between 60 and 85 degrees. The cure speed of epoxy today, at least our, is much improved with the addition of our new faster curing hardener. This is available now and at 75º you can flip it in 3 hours. The softening John mentioned is a problem with epoxies that have low heat deflection temperatures

[/][/]

This falls right on the heels of talking about heating resin before using it, but it sounds to me like this part is talking about actual curing. So, What is the current thinking? To cook, or not to cook.

My thinking is that with Resin Research, it may not be necessary to postcure. I would love to be told that I’m right, but let me know if I’m wrong. Thanks, as always!

–BCo

2

As a general rule ALL epoxies benefit from a postcure, even the ones that cure happily at ambient temperatures will improve. Having said that I have done a couple of epoxy boards and have been perfectly satisfied with the un-postcured one. It has to be said that the post cured one was tougher, stiffer and stronger.

It depends very much on the type of epoxy. The slow curing ones tend to need a post cure more than the faster ones. It might sound like a cop-out, but follow manufacturers reccomendations, talk to manufacturers they are really helpful.

Post curing is not too hard. A few sheets of 4" insulation foam and a fan heater will often be enough to make a perfectly serviceable oven. Be sure to support the board well as they tend to soften before they toughen, beware slight sagging!

3

The two sources do not conflict.

One talks about post-cure heating. The other says NOT to pre-cure heat treat or the foam will outgas. Obviously, once the epoxy is cured, the foam is sealed with a cured resin and will not outgas.

4

My thinking is that with Resin Research, it may not be necessary to postcure. I would love to be told that I’m right, but let me know if I’m wrong. Thanks, as always!

It depends 11.

Post curing adds more cross-links making the resin tougher and stiffer.

If your boards are built strong…

and if you dont let your boards heat up in the sun or inside a car, post curing is not necessary, but still recommened. ANd you dont need to go crazy with PC…most of my past post cures are done inside my vehicle…all day long.

Std MO for structural (toughened) room cure epoxies is to initial cure around 100F, post cure 175F+. However most surfboard foams wont allow 175F…140 or so is mo bettah.

What’s cool is that Epoxy has been around for decades and there are zillions of variations, and an equal number of performance tests, results and published data.

Only limited by one’s imagination.

5

That does depend a lot on the foam. Certainly outgassing of foam during postcure of epoxy has resulted in delamination of the skin from the core in some well documented boatbuilding cases. It is less of a problem with most modern foams but again I would check with the suppliers.

6

The only out gassing I have occured is when the ambient temp is rising, when glassing. usually I let the blank sit in the sun for a while, then take it back inside to glass. as the blank cools it sucks the resin down to the foam, not vice versa. I do this in all stages of the build, except the gloss.

As for the post cure. The boards that I have post cured have turned out harder, but i don’t think they are any stronger? They just tend to be more pressure ding resistent. Post cure is easy: On a sunny day get a mini van, throw them in the back, throw in a temp gauge. Let them cook all day. If it gets too hot…crack a window, real high tech. Works great. I even have a digital BBQ thermeter so I can remotely check the cooking temp. Like I said Hi TecK.

7

I love the mini-van oven. Living in Wales we only get a few days a year when it will work and that’s usually when I’m in the water! Looking at your address I guess you have it a bit easier.

8

Sealing the board with resin before laminating seems to eliminate any gas issues (with the marko eps anyway). Once it’s laminated, heat curing is much faster, from what we have noticed.

9

I’ll do both top and bottom lams in a day. Bot. first early in the morning and the top early in the evening. The inside of my shed gets close to 90 degrees so I’ve had problems with out gassing during the heat of the day. Next day I’ll do my sanding coats back to back with the 20 min. fast hardender. As soon as it’s not tacky I’ll wrap the board in black shade cloth and lay it out in the sun for the rest of the day. The next day I’ll sand without a single problem. No gumming of the paper and the sand coat is very crispy.

My glasser friend said that he has been using the VF? hardener for the sanding coats and he says he can sand without problems within a few hours at 85 degrees and no post cure.

This is all with Aluzine resin, once it’s done I’m going to try the RR stuff. I’ve heard several comments that it seems to have gotten better. The new stuff will harden pretty quickly and is way easier to sand as long as you use the additive F.

10

why wouldnt someone heat the blank immefiatly b4 laminating it with epoxy. so the blank is cooling/sucking in as its going off?

11

Well, I am officially glad I posted this, even though in so doing I have found out that I was wrong. Glad to know. You all make good points.

Blakestah, thanks for pointing out my misunderstanding. Now I get it.

Rikds, good info. Don’t have a mini-van, but I do have a station wagon.

Craftee, don’t usually let my boards heat up. If they are outside, I’m usually on them. If they are in a car…well, most of my boards don’t fit in a car, but the one that I am going to post cure will, and that’s how I’ll cure it!

Resinhead, pressure ding resistant is good. My board is a bit of a tank in glassing schedule and 3# EPS, but I’ve been known to manhandle boards.

GRBC, this brings up an interesting point. You say heat curing is much faster. Will a non-heat cured board eventually arrive at the same level of curedness as a heat cured board given enough time? In other words, is it the same chemistry, just happening slowly, or do you get a totally different chemistry happening if you heat it up, or is it way mo fastah?

DMP, I’ve used West Systems (high blush, yellow color, no UV protection, kinda thick) for boat applications, and RR for one board. I officially like RR, better than polyester resin, and WAY better than the West Systems stuff.

Many thanks to all,

-BCo

12

Again I’ll generalise…Even if one epoxies at a fairly low temperature as long as the resin reaches its cure temp and stays there long enough, it will cure. We consulted with a couple of suppliers before a major boatbuilding project in my old yard. No heating and a cold Welsh winter. Both said the same, even if it doesn’t cure today or tomorrow, it will cure once it hits the required temperature. We did some tests and mixed epoxy, glued up some samples and put a few in the fridge and some in the freezer and cured the rest at ambient. The cold ones were still rubbery after 3 days in the fridge/freezer. Warmed them up to shop conditions, left them for a week and then did break tests on all samples.

Nothing to choose between them!

Actually this is how pre-preg is made…mixed epoxy impregnated into cloth, partially cured (B-stage cure) and then stored at

-18C. When you want to use it, allow it to come up to ambient lay-up into or onto a mould (it’s slightly tacky so holds its position well), vac bag and cook. Resin softens, flows and cures during the process…brilliant but not cheap nor easy without the right kit.

If I could afford it I’d do a board like that as you could do the whole lamination, top, bottom and rails, in one go, bag and cook, flowcoat and sand, there it is, done at something like 100 times the cost of your usual materials!!

13

The important distinction is how “fast” a hardener you are using.

As a general rule of thumb, the faster the hardener, the less that is gained by post curing.

The RR epoxies that are commononly used for surfboards can be pretty much thought of as fast curing.

The slowest hardener will benefit most from a post cure.

Where a post cure becomes very beneficial, and basically necessary, is when you are using much slower hardeners.

For example, pre-preg and infusion resin pot lifes can be 24hrs or several days!

These slow cures will not reach a very high final “strength” and HDT at room temperature, they must have a post cure to make them worth using. Also, the hotter the post cure (to a point), the more strength that is gained, which causes problems when you are using foam cores that melt!

So a resin is usually chosen that has a pot life that is long enough for you too do the job, but not too long that it requires too hot a post cure!

HDT is Heat Distortion/Deflection Temperature, and is a commonly quoted number which can be roughly taken as an indication of the completeness of the cure.

Post cure is always a good idea, but with the fastest hardeners, I wouldn’t say you have to.

The increase in strength will be relitively small, maybe even negligable.

I wouldn’t loose any sleep about it if you don’t get around to doing it.

But it is good practice, and when you are trying to squeeze every last bit of strength out of a lightweight high performance board, go for it!

14

Has anyone tried surfing their epoxy board for a bit (like say several months) and then post curing? Is there a time limit on how much time can elapse before post curing epoxy? Or can I take my old epoxy board, chuck it in an oven and expect a much stronger board a few hours later?