Post Curing Epoxy

Yes and No,

... not much, when the formulator does a proper job.

Yes,

in general. See Jimbo´s comment:
http://www.boatdesign.net/forums/materials/post-curing-epoxy-27737.html#post291862

Regards
Richard

 

Jimbo,

I am groping for the right words here.

When polymers starts to cure, it is called polymerization.

With epoxy, is it cross linking? I have seen one technical manual that says (for polymer) adding the initiator to start the crosslinking process"".

What is the proper technical term?


Rx

 

Rx, both are cross linking reactions. PE & VE resin cross links with styrene, epoxy resin typically cross links with an amine.

CatBuilder, epoxy curing agents are not extended, in most cases are a single or a blend of amines to provide certain physical properties and variable processing conditions.
You need to know what you want and then select the best value product.
Did you get the data for the Raka system?

 

Hi Everyone,

Post curing of Epoxies is very important and should be carried out as good general practice. It is up to the builder to do the best they can with the tools they have, and heat is available to all!


Its vital that you follow some rules for successful results:

1 If you are not vacuum bagging then all post curing must take place after initial cure at room temp. If you are bagging then you can put heat on after the resin has 'gelled' a bit... this helps with the flow and will remove more resin giving a better fibre/resin ratio for lighter components.

2 Pre cure all tooling to the temperature you want to eventually cure the final product - this avoids shrinkage and you can easily check for distortion before you make your hull etc.

3 Cure all products in the mould and not after demoulding.

4 Once you have assembled the yacht, run another lower temp but for longer as this will cure all the bonding nicely inside the yacht.

5 Use a temperature gauge!! Very important. Tape the sensor to the laminate to get a better idea of the conditions the laminate is being cured under. Just measuring Air temp is not okay.

Small ovens for components can be easily made from thick cardboard boxes and a hairdryer, axial fan heater or a variable heat gun. Generally the more air flowing around the more evenly the heat is distributed. be careful with the heat settings, I regularly cure things up to 100 degrees C like this!
If you want to get flash then you could build a permanent oven under a bench using styrene insulation sheets tape the joins etc to seal it. We call these "Buvens" in our trade....

Cheers, Jim.

 

> Polyamides always yield cured resin that is somewhat flexible, even a bit rubbery

Is there an epoxy flexible enough to be used to build an inflatable boat? Perhaps with Kevlar 29 or Xynole.

 

Catbuilder,

Your wife is quite right. Epoxy once said "will mend everything except the crack of dawn and a broken heart."

Rx

 

I got the information in from Raka.

Tg = "not published"
HDT = "not published"

Specifically, the owner said they do not publish or have these values tested because "all of the epoxy manufacturers make these numbers up."

Further, he said (over the phone only, I've yet to see anything in writing) that the HDT was 130 degrees and Tg "follows from there."

When I asked about what temperature I needed to post cure at, he said, "the boat post cures every day when it sits in the sun all closed up."

Then he talked about ramping up to 212 degrees and that I couldn't achieve that. I agreed. Next he talked about being in business for 25 years. Later, he said I could ramp up to 100 degrees one day, then 110 the next day, then 120 the last day.

All in all, I got no straight answers about any of the data and got no written proof of the epoxy's characteristics. He also was a "talker" and was not really stopping to listen to my questions.

One straight answer I got was that it has a pot life of 2 hours at 25C (77F). At the same temperature, after I spread it thin on my panels, I should get 4 hours. Both times will decrease by 1/2 for every 18 degrees F the temperature is raised. So... I'm looking at a 2-3 hour open time to get all panels done and get the bag going to full vacuum. I think this is possible.

He may be a small shop, but this lack of information has made me hesitate. I was about to place an order for a couple drums. What do you guys think?

PS: I wish Kyle was in charge. I think he'd have all this information and a way to communicate over the computer so you could get questions answered in writing. (In writing not for legal reasons, but so you could keep track of the answers)

 

Why don't you run a test on a small part first?

 

Well, I need to know all of the post curing figures, right? I'm not sure I have the equipment required to make those measurements.

I'm sure it will bond the plywood. My main concern is that if I go and post cure the entire boat at the end, the epoxy will weaken inside the hull layers and slump, gathering in the bilge and leaving voids up topsides.

I don't want to have a bunch of different epoxies with a bunch of different HDT's and Tg's when I go to do a post cure, right? Won't some melt away while others are awaiting their proper post cure temperature? I may be wrong here, but those are things I'm thinking about.

 

What a piece of crap. "All manufacturers make up these numbers, so I do not have them" ???

That is why many manufacturers have a lab the size of a small boat yard. Also, there are labs that can do these measurements for you, here in NL I found one that does a Tg measurement for only a very small amount of money, no fancy reports, just the data you want to know. (2-figure price tag).

In my opinion, a HDT value is of little use, as you can influence that figure while testing, and not every HDT test gives the same number. Tg however, is an exact value (just like melting point of water, for instance), can be determined in various ways, and gives hardly any variation.

 

CatBuilder, based on my comments in some of your earlier threads I think you know what I would do, choose a supplier that does provide the information.
I am biased as my background is laboratory quality control and I have seen how the backyard bucket chemists operate.

Is raka that much cheaper then say system three to warrant taking a risk in not knowing what you are getting?
Raka comment regarding the reactivity is correct for every 10'C rise in temperature the reactivity doubles ie half the gel/open time. The hardeners you select will determine how much of a benefit post curing will be for you, its something to be aware of but not to get bogged down on at this stage.

Your concern about variations in HDT throughout the structure is not an issue, in most builds you will use at least two or three different hardeners to optimise working conditions and possibly required properties.

Out of all of the basic tests that are performed on resins HDT(approximate) is the only one that you can do your self as all it requires is a thermometer, a pot of water and a cooking stove. The resin sample should only be 2-3mm thick, place in the pot and heat the water at ~2'C /min, take out the sample at each degree rise in temp and lightly bend. When it becomes flexible thats your HDT.
Tg is a more repeatable test but then you also need to combine it with the storage and loss modulus by DMA (dynamic mechanical analyser) to get a better understanding on how the resin behaves as it is heated up. You can have 2 curing agents produce the same Tg but one can start to soften more gradually at a lower temperature.

Anyhow you dont need to understand all of the detail about the testing but you do need the basic resin properties; strength, elongation, HDT/Tg, gel time in order to be able to asses 1. if the resin meets your requirements and 2. which one is best value for your $.

andrew

 

On an important project, I wouldn't trust any numbers I got without some rough in-house test that I received the right stuff and that it meets specs - or at least compares well to previous batches.

Engineers don't just trust that they received strong concrete for a bridge - they test each batch.

 

Truly excellent advice, all. Thank you. I am getting System Three and even doing a small test or two with it.

 

That was a shot in your own leg Mr. RAKA!

A pity, Kyle was quite open and helpful.