Shellac on styrofoam?

This may be a little of topic, but I know that dewaxed shellac is considered as an excellant sealer in between incompatible finishes. But would it seal styrofoam adequetely enough to allow polyester resin to be applied over it? As many of you may know resin will melt styrofoam and even cause it to burst into flames. I can use urethane foam instead but I have a perfectly sized styrofaom box that would save me a lot of build time. Anyone try this? Thanks, Gene
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Some shellac thinners contain MEK. I'd give the traditional check on a piece of scrap, rather than experiment on my project.
Come to think, make it a small scrap to limit the fuel....

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Burst into flames? Have you ever seen that? What information do you have of this?
I've been in the industry for 34 years,have every technical and safety manual, have attended seminars, spoken with the chemists, etc., but have never heard of polyester resin causing foam to burst into flames. You sure have me curious as to where the facts of this come from.
First, you do not have a Styrofoam box. You have molded expandable polystyrene. Styrofoam is Dow Chemical's registered trademark for extruded polystyrene board.
As a barrier, you can get good results with a coat or two of latex paint. Be sure there are no pinholes after the paint dries. I'd use two coats or at least dab in to fill the voids..
Rather than resin, you can use most epoxies on the foam with good results. Try the mixed epoxy on a scrap piece to be sure of the compatibility. Polyester resin contains styrene, the solvent for expanded and extruded polystyrene.
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Edwin, A friend of mine was mixing up a batch of resin in a syrofoam cup and the cup did burst into flames and injure him. It's possible that he used too much catalyst and that type of extreme thermal reaction probaly wouldn't occur on a open surface.
I did consider using the epoxies but as you know they cost considerably more that polyester resin. And pardon my ignorance in not knowing the difference between Styrofoam and molded expandable polystyrene. Thanks, Gene

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wrote:

Well _duh_ ! Yes, That's an excess of catalyst in the resin. Nothing to do with polystyrene foam.
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Smert' spamionam

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Could have been many causes, such as smoking too. The resin itself is a flammable product. Containing mostly styrene monomer, it has the ability to burn. If it was to burn, it would start the foam cup on fire also. To say the resin reacted with the eps foam is quite a stretch.
From: Ask A Scientist http://www.newton.dep.anl.gov/askasci/eng99/eng99263.htm My point is, I once bought a quart can of 100% styrene monomer     liquid. Without inhibitors, it would solidify in 10 seconds to 1 minute, and get so hot     I would have to drop the bottle. "Hazardous polymerization" is even one of the hazards listed on every MSDS chemical safety sheet, right after flammability and such. When you add "catalyst" to casting resin, it is really an oxidizer. It uses up the     inhibitor, then what is left attacks a few double bonds, starting polymerization. Oxygen in air often acts as an accidental inhibitor. I think this is why the surface of some resins remains tacky, incompletely cured.
From a typical MSDS for polyester resin: Health and Safety: Unsaturated PolyEster resin (UPE)
Base resin: polymer chains with reactive sites (50%)
Styrene: reactive diluent (50%)
volatile
flammable:
flash point 31C
explosive limits 1.1-8.0%
NO SMOKING, no naked flames
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I wasn't looking to determine what caused the styrofoam cup to catch fire, but It MAY be because my friend used too much catalyst. I never did ask him how many drops he used. And he was not smoking nor was there any other open flame around at the time. Gene

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wrote in message

OK, that MAY be true. But you made a statement about the resin causing the foam bursting into flames that is unfounded. If people believe that, they may not buy my product and I'm out of a job. Better we should stick to FACTS Ed
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Ed, I'm not trying to beat a dead horse. Having said that, what would have made the cup catch on fire? I have never had a foam cup catch fire on me before, unless I've tossed it into a campfire. So, what can I factually say happened? I'm not trying to be sarcastic.. Gene

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Burning resin would start the cup burning. As would any other burning material. The real question is what started the resin burning? There may have been too much catalyst, an open flame, etc. Once the resin, mostly styrene monomer, starts, any flammable container will go with it. Could have even been an explosion from his dust collector. ;)
My concern is that a mis-statement can cause people to think that the resin in any form will cause eps foam to burst into flames. Styrene will melt at 190 degrees and will burn when contacted by an open flame. Modified grades of material are used in home building and other applications that require a fire retardant. Modified material will only burn in the presence of an open flame and will go out when the flame is removed.
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Gene T wrote:

Just for general reference back when I first started working with fiberglass I made the mistake of activating a quart can of polyester resin with a little too much catalyst, then not using it before it started to cure. It was burning merrily at one point. That was a steel can. My Dad and I were both impressed--we didn't know that it would _do_ that.

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--John
Reply to jclarke at ae tee tee global dot net
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wrote:

Unlike epoxy, curing polyester resin is a catalytic process. It's also exothermic (generates heat). The reaction carries on at a rate dependent on the amount of catalyst, so if you add too much in one go, it can proceed at a furious rate. As polyester resin is also flammable, and may have some other inflammable solvent in there as a thinner, then it's a great fuel source. You know the rest.
I believe this most commonly happens in plants spraying catalysed resin, such as the strand-chopper guns that spray both components onto the inside of a mould. The resin is heavily thinned and heavily catalysed. If you mix up a batch of resin then leave it in the pot for too long (usually because the gun has a fibre jam) , it overheats.
--
Smert' spamionam

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Edwin, You have made some very good points. I suppose that if in my initial post I had said that a foam cup had burst into flames- no one would confuse that with high grade building material foam. I will try to be more careful in the future regarding that type of statement. Gene

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vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email

Perhaps you play with Edwin a little?....
I definitely understood you to say that polyester resin would cause styrene to burst into flames, and because you placed it in the context that you know polyester eats styrene (chemically), I understand Edwin's questioning and think that he stated his case clearly enough. I don't think building material has anything to do with it.
What I can't understand is why anyone would mix polyester in a styrene foam cup. Polyester resin _does_ eat styrene, which is why you asked the question. ***************************************************** Have you noticed that people always run from what they _need_ toward what they want?????
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They don't, they only try to ;)
I'm sure a lot of people just don't know the composition of the material and the cup is cheap and handy so . . . . . . .
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vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email

hmp-hmp. Ok then.....

***************************************************** Have you noticed that people always run from what they _need_ toward what they want?????
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