I am trying to determine what would cause a two component urethane to slow in
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?).
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?
I am not a bug I am a undocumented feature
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
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"
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.
RCO + HOR' -> RCO-R' + H2O
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...
R. 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. :-).
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.
Beng Polymer Engineering
Msc Manufacture & Design with Polymers