Eq With Isocyanates

When you are polymerizing a urethane prepolymer, and there is solvent used in the cook plus a significant amount of water in the solvent, do you treat the calc. of water as 18g/mol or 9g/eq. Water has two active hydrogens, so isn't its functionality 2? We are having a big lab battle over this point. Someone out there must have the answer

Pat Hennessey

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Patrick Hennessey
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Yes, the functionality is viewed as 2. In effect the water molecule itself only reacts with 1 NCO group, but then the resultant amine reacts with a further NCO, so 1 mole of water effectively mops up 2 moles of NCO.

Colin

Reply to
Colin Reed

Hmm, I wonder if this is really true. Amino-function contain two acidic protons and the resulting urea is more reactive toward NCO than the urethane linkage.

How are Amines treated in Polyurethane systems? Is the Amine equivalent weight the same as for Epoxies? Or does it depend on post-cure conditions, temperature etc.?

Regards, Oliver

Reply to
Oliver 'Ojo' Bedford

The urea is more reactive towards NCO than the urethane link, and the resultant biuret is more stable than the allophonate that you get from urethane - NCO reaction. However, the rate of formation is orders of magnitude slower than the NCO - OH and even more so the NCO-NH2 reactions until you get to higher temperatures, so normally they are not really considered. Amines are normally treated the same as OH polyols in PUU (polyurethaneurea) systems as far as stoicheiometry is concerned. I've not really worked with epoxies so can't comment on the equivalent weight being "equivalent", so to speak. AFAIK amine terminated polyols are often given an equivalent OH value, although I don't know if this is across the board (I'm only a dumb salesman these days, no longer work in the lab :-)

Colin

Reply to
Colin Reed

A precise answer could only be proposed for a specific system and reaction conditions, and even that would probably be empirical, based on experience with a similar system. For most systems polymerised in bulk, primary amines are usually deemed to only react (once) to the urea stage. But in reality, even minute changes in raw material quality/composition, impurity levels, additives, or processing conditions can change this to very significant extents. Practically, to get around the issue, system performance is often optimised after trials using increasing levels of isocyanate until an optimum is reached. "Colin Reed" a écrit dans le message de news: snipped-for-privacy@individual.net...

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