Will PR cure w/o MEKP?

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mecompco

mecompco

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So yeah, I did something stupid today. Filled my silicon mold with pine cones and PR--forgot the MEKP. Will it cure? I can wait a week or two, or I can dump out the whole mess and start over. Advice? Thanks!

Regards,
Michael
 
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It will cure, but not in a week or 2. At least not without some help. You could try using some MEKP and just stirring it as best you can. Or you can put the whole thing in a toaster oven to about 150. It may take several hours (I guessing here). PR will eventually cure on it's own, but now we're talking months. Probably closer to a year.

The fastest way is to do what you suggested, and dump the whole thing and start over.
 
Thanks, Bob. I think I'll take your advice and dump it--I'm using a mold I made with the silicon putty stuff molded around a 5" x .75" rod and there's no way to stir it totally full of cones. Lesson learned!

Regards,
Michael

PS If I had more that one toaster oven, I'd try that--there's no way that stinky stuff is going into the kitchen. ;-)
 
mecompco said:
Thanks, Bob. I think I'll take your advice and dump it--I'm using a mold I made with the silicon putty stuff molded around a 5" x .75" rod and there's no way to stir it totally full of cones. Lesson learned!

Regards,
Michael

PS If I had more that one toaster oven, I'd try that--there's no way that stinky stuff is going into the kitchen. ;-)
Click to expand...
Do you have a separate shop, if so you can typically pick TOs up at thrift shops or Goodwill for very little. I bought a new one for the wife and took the old one for the shop.
 
JD Combs Sr said:
Do you have a separate shop, if so you can typically pick TOs up at thrift shops or Goodwill for very little. I bought a new one for the wife and took the old one for the shop.
Click to expand...

Hmm, yes, an oven for the shop would be handy. Perhaps a trip to Goodwill tomorrow is in order. My wife hasn't complained about pine cones cooking in her kitchen, but they don't stink.

Regards,
Michael
 
It will cure, I have a can of PR sitting on a shelf in the shop totally cured. If you are not in a hurry just set it aside in a year or so it will be cured.
I'd toss the can I have but I keep thinking there must be something I can use a half can of cured clear PR for.
 
I have to say something. Have you read the MSDS on PR? I would not put that stuff in an oven, especially inside a closed room. I know lots of people do it (I even did it once or twice, but never again). It is toxic and has a low flash point.

Rant over.
 
Decided today to just dump out my mess and remind myself NEVER to make that mistake again.

Stuffed my cones back into the mold and mixed up some of the same color (with catalyst this time), we'll see how that works.

Now, what's the best way to get all the uncured PR off the outside of my mold--it is one heck of a sticky mess?

Thanks again for all the good advice and precautions guys!

Regards,
Michael
 
Polyester resin for casting is "pre-promoted" with a catalyst. The MEKP reacts with that catalyst to speed the curing. So, technically, PR is curing in the can.
Do a good turn daily.
Don

mecompco said:
So yeah, I did something stupid today. Filled my silicon mold with pine cones and PR--forgot the MEKP. Will it cure? I can wait a week or two, or I can dump out the whole mess and start over. Advice? Thanks!

Regards,
Michael
Click to expand...
 
Technically, there are three main components used to cast polyester resin:
  1. Resin - uncured polyester macro-molecules dissolved in styrene,
  2. Promoter - without which the resin will take a very long time to set, and
  3. Hardener - which is used to initiate the curing process.
Note that there is no "catalyst". It is a common misnomer to call the hardener a catalyst.

Silmar 41 (the most commonly used PR casting resin for pen blanks) comes pre-promoted with Cobalt Naphthenate. Because of that, it has a limited shelf life and it will turn solid all by itself in time. The crystal it makes, however, is not as strong as one cast with hardener. The most commonly used hardener is Methyl-Ethyl-Ketone-Peroxide (MEKP). It initiates the reaction and provides oxygen molecules that cross-link the polyester chains.

So, to cast the strongest, least brittle blanks, it is important to use fresh resin containing the right type and amount of promoter, and use the right amount of hardener given the ambient temperature, thickness of cast, and other variables. Insufficient hardener will starve the crystal of the oxygen it needs for strength. Too much and the cure will be too hot - resulting in high shrinkage and leading to a brittle blank.

I hope that makes sense,
Eric
 
What the hardner does sure sounds like a catalyst to me. The reaction is started but maybe the MEKP does change into something else. From what I remember a catalyst in a reaction is itself unchanged. You say po-TAY-toe I'll say po-TAH-toe. ;-)
Thanks Eric for the correction.
Do a good turn daily!
Don
 
A catalyst is something that facilitates a chemical reaction, but is not consumed by it. Although the curing of PR is a chain reaction (that is, some of the radicals are consumed but more are produced), the oxygen molecules from the MEKP are consumed. Therefore, MEKP is not a catalyst.
 
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