Urethane cure

I am trying to determine what would cause a two component urethane to slow in
its cure.
It is a two component system. One side is a polyester polyol and the other is
a polymeric MDI.
My first though is mix ratio issues. My question, is there anything else that
can effect the cure rate (slowing it down?).
Michael
Reply to
Miko
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What is abot the temperature? Has anything changed in the production proess of the Polyol or the PMDI? perhaps another catalyst? perhaps something changes with moulds or metal parts?
Michael I am not a bug I am a undocumented feature
Reply to
Michael Erwerle
Since this is a polycondensation reaction, any residual moisture could also slow the cure. Make sure both components are dry.
Larry
Reply to
Larry Effler
NCO +OH is a polyaddition, moiture will produce foam...
it is not crosslinked by water as it was described...
I am not a bug I am a undocumented feature
Reply to
Michael Erwerle
From my fairly limited experience, if you're using dichloromethane as a blowing agent, this through time can degrade some of the mercury catalysts used for PU, so you may need to add some more catalyst to your polyol formulation. If you're not using any dichloromethane and your system has slown and you can't find any temperature changes as Michael pointed out I'd slowly increase the catalyst concentration. Also increasing the temperature of the mixing head and of your mould tools should also help speed up the reaction.
Hope this helps.
"Children are stupid and should go to school"
Reply to
Prov Bloke
I didn't say it was crosslinked by water. Water is a byproduct of the polymerization action. Since it is an equilibrium process any water would retard the reaction.
Larry Effler
RCO + HOR' -> RCO-R' + H2O
Reply to
Larry Effler
The reaction scheme is non-sense.
Oliver
Reply to
Oliver 'Ojo' Bedford
Huh? A polyol and an isocyante don't create byproduct water.
Preferred reaction is; R-N=C=O + R'-O-H => R-(N-H)-(C=O)-O-R'
...no water is evolved. It can be an outside contaminant, though, especially for an opened drum of a hygroscopic polyol...
True, but its not due to an equilibrium reaction condition. The foaming mentioned in the presence of water is a consequence of water reacting with the isocyanate to evolve CO2 (via a carbonic acid intermediate...) and an amine terminus is left behind at the former isocyanate site. The primary amine (vastly preferred over the polyol) can go on to consume yet another isocyanate and form a substituted urea, which chews up yet _another_ isocyanate to form a biuret.
Fortunately, primary alcohols are preferred, even to water, but of course they'll compete to some extent if isocyanate is present, and if the polyol hydroxyls are somehow sterically hindered (high MW, low OH functionality?) the water can become the preferred reactant. Secondary alcohols on the polyol will definitely react after the water is consumed.
So yeah, in short; Water Bad. Measure water content on the suspect polyol side of your two pack by Karl Fisher. If you've got more than 0.25%, you've found a contributing problem.
Another thing which can retard urethane cure is old isocyanate, particularly if it has seen elevated temperatures. Self reaction to form uretidones and cyanurates might slow down its cure rate by the time you get to application. Check the lot number and history on the iso side, and see if it sat in a warehouse for a summer...
Good luck!
Regards R. David Zopf Bomar Specialties 860.379.7748
Reply to
David Zopf
I could have sworn it was a polycondensation reaction, but I stand corrected. This is what happens when an engineer attempts chemisty. Especially when his reference books are at work and not home. :-).
Larry
Reply to
Larry Effler
Michael,
I dont actually think changing the mix will slow down the curing of the reaction if anything it will change the material density after curing! You maybe better contacting your supplier and telling them you want a slow curing mix, you need to know how long a cure you want before doing this.
Good luck
Steve
Beng Polymer Engineering Msc Manufacture & Design with Polymers
Reply to
Steven Woodward

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