When is epoxy a bad choice?

Check that PAR..I got the medium fast hardener which with this weather we're havin' should give me some time..yeah..this is my first real amount of epoxy over an 8 oz. tube...should be fun...you guys have taught me alot though...will post pics...

 

I understand that cost and cure time are why so many boat builders are using vinyl-ester.

 

Then you understand more than we builders do! Cost yes, but curing times I can order almost as I want them.

 

well...I'm not sure if my online order for epoxy from US composite went thru or not..I should have waited and phoned the order in this morning instead. If they didnt debit my card..I'll go to St.Pete to that place PAR mentioned..I have been meaning to go check out Don's boat junkyard in Clearwater..maybe hit that too..I hear that place is a south florida institution..

 

I understand what you mean, but indeed jonr might be right. The fun thing about polyester and vinylester is that for 20 minutes (or longer, depends on what you ordered) nothing happens, then BANG!! The stuff is cured. I supply people that pop out 8 products from a single mould in a single shift. Every hour a product (including gelcoat!).

Viewed from that respect, epoxy is slow stuff... (which actually is not too bad)

 

Right,
though, as you know, I can have Ep curing in 15 minutes if I like. Less if quality does´nt matter.

 

Yeah..have to agree...the resin is pretty much the same down the line..but you have like 4 different hardener/catalyst choices from slow set to fast set and in-between ..at least with US composites ..which has a great website..well-explained....and they ship in 2-3 days in Florida...they say the medium speed hardener is their big seller...that's what I ordered and it did go thru...on Tues morning...just not on Monday..but I ordered Sunday night after a holiday so thats to be expected..they were backlogged with online orders.....but when does quality not matter Apex...am I missing something..? I am trying to ensure quality control in my project and then you who I respect as someone who knows something (at least more than I who am a novice with these materials) says that at times quality does not matter...expensive epoxy is involved so it must have to do with a project that matters is my thinking...not trying to bust your chops..well..okay maybe a little... anyways..I'm waiting for warmer weather before I do anything...pics will follow when i do...okay Fanie?..I know your out there some where...

 

Who said quality does´nt matter?:?:

I did not.


Although that statement would be about right. There is in fact not such thing as different "qualities". We know different formulations, read: properties of the different resins and (more important) hardeners. But in terms of quality they are all equal.
For a homebuilt project it is easier to use formulations specially made for the given application, thats true, but one pays a high price, buying the West, SP, Mas or System Three stuff.
We are fine with less sophisticated formulations in professional boatbuilding.
But that has nothing to do with quality.
You could use the cheapest resin on the market and have as good a result as with any of the main suppliers stuff (given you use the right hardener)

I referred clearly to the curing time.

Regards
Richard

 

Okay..I thought that was what you meant...the resin quality..or brand-name used...okay..that is useful to know...and what I suspected...the hardener is where it's at...

 

Let me say I´m not sure you´ve got it...........

 

Wouldn't be the first or last time..but I think I did get what you meant actually...or at I learned something at least...that hardener/activator is where the differences that matter are...that the resin worldwide is pretty much the same...but the catalysts and the speeds of them are quite different..is that right?

 

Basicly, yes.

In epoxy resins, there are basicly 2 different types:

Bisphenol-A based (gives the best mechanical properties, like Tg and stiffness)
Bisphenol-F based (gives more flexibility)

Both tend to crystallise. However, when you mix the 2 together, in a 70-30 to 30-70 blend, the mix will not crystallise. (so crystallisation-free epoxy is always a mix of A and F based materials).
This comes with a slight downside of somewhat reduced properties. (Tg and stiffness suffering somewhat).
There are also blends that are less prone to crystallisation, usually a 90/10 blend.

This is one side of the story.

Second is the thinning down of epoxy. Unmodified epoxy is a thick syrup at best. I have the stuff here, and below 10 degrees C I am not able to get it out of its jerrycan. Below 0 and bring in a knife and fork, please...

So to be nice to the customer, it needs thinning. This can be done in several ways:

-add a solvent. Cheap and quick. However does not improve properties, and makes the epoxy porous. Most popular solvent is benzyl alcohol.

-refraction the epoxy. Expensive process, where the chains of epoxy molecules are broken in smaller pieces, then the not usable stuff filtered out. Not many epoxy producers can do this process, and when the destilation installation of my supplier broke down, it took them 4 months to rebuild, bringing some clients in problems, because these products were hard to come by at other suppliers.

-add a reactive diluent. This is not a solvent, although the small molecule chain gives the same thinning. It differs from a solvent, though, because it does not evaporate, and co-cures with the epoxy. It has one or more epoxy groups (mono-functional of bi-functional) and this does make a difference as well. The mono-functional reactive diluents are cheaper, but as you can guess, will give less mechanical values.

For the fun of it, all above options can be mixed, to tailor the epoxy for a specific situation.
Also foreign additives can be added, to influence impregnation of fibers, toughness, surface quality, etc.

And I am only talking resins here...

For hardeners, there are even more options...

 

Talking hardeners would be harder.

Ähh, why did I say that?

Ahh, yes talking reactive diluents would complicate it.

And talking curing processes would drive a novice crazy.

But for the application given, one must say, there are worst choices than epoxy.

Regards
Richard

 

The bisphenol-F (BPF) resins are actually superior in all respects to the Bisphenol-A (BPA) resins, as certainly as epoxy is superior to polyester. They give outstanding UTS while also maintaining higher flexibility, fracture toughness and elongation. They always yield a higher Tg with a given curing agent. They start with a lower viscosity and are extremely resistant to crystallization, often being blended with the less costly BPA resin for the purpose of lowering viscosity and reducing the tendency toward crystalization.

All this is due to the BPF molecule having more reactive sites, thus allowing a more dense cross-link upon cure. This is reflected in the lower Epoxide Equivalent Weight (EEW) number, usually ~175 vs ~190 for standard BPA resin. As the number of reactive sites per molecule goes up, so does the resin's viscosity. The 'best' (strongest, toughest) epoxy resins of either type are unfortunately very viscous, many even solid at room temperature. So the fact that the BPF resins are substantially thinner while having more reactive sites per molecule is very significant. Another way of looking at it is that for a given reactivity and a given benchmark of cured state properties (UTS, Tg, Elongation, etc.), the formulation that starts with a BPF resin will have a lower viscosity than one that starts with a BPA resin.

The only disadvantage of the BPF resins is, of course, higher cost.

Jimbo

 

Okay...I wouldn't expect you to blind me with all the science..and I'm glad you didn't...but you guys did give me some good info that I can build on...thanks...