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Subject: Re: foam experiment
From: (Bruce Hamilton)
Date: Jul 03 1997
Newsgroups: sci.chem

Gerrit <> wrote:

>> I once saw a demonstration that when you mix two chemicals it 
>> produces a foam that comes out of the beaker and  solidifies into
>> a brown mass. Does anyone know what the chemical reaction was.
>> Some kind of polymer forming reaction thet gives off gas in the
>> process.
>This was the reaction between a di-isocyanate and a di-alcohol, forming
>polyurethane.see also :
>Organic Chemstry : page 1252 : ISBN (international) 0-205-08452-4
>Or any other Chemistry Textbook (which deals with polymers)

It was the reaction between a formulated di-isocyanate ( such as
toluene diisocyanate (TDI) or methylene bisphenyl isocyanate (MDI)), 
and either a polyester or polyether polyol with hydroxyl numbers 
between 40 and 600, depending on the desired foam properties 
- along with blowing agents, silicone oils and amine catalysts -
to produce a polyurethane foam. The absence of a blowing agent
would only produce a polyurethane plastic, not a foam. TDI foams
tend to be white, whereas MDI and PAPI ( highly polymerised MDI )
are brown.   

In the old days the blowing agent for rigid foams was Freon 11, 
as it provided foam with a good insulation value. The Freon
boils as the reaction proceeded. These days, they may add water 
- it reacts with the isocyanate to form carbamic acid which is 
unstable in the exotherm generated by the polyol-isocyanate 
reaction, and decomposes to form an amine and carbon dioxide - 
which is the gaseous blowing agent that produces the foam.
The amine reacts with isocyanate to form a urea derivative. 
Another blowing agent alternative to Freon 11 is pentane.
Polyurethane foams are used in the aerosol cans of gap-filling
foams and insulating foams readily available in hardware shops. 
However the best insulating and thermally-stable foams are formed 
with a small amount of overpacking. The growing foam can be 
rapidly destroyed by spraying with a mist of solvent like
trichloroethylene, but once the reaction is complete ( just a 
few minutes ), aggressive solvents like DMF are required, and
they work far slower, even when warmed. Usually unwanted foam 
is cut or burnt away ( warning - toxic fumes ) . 
Note that many formulated products are prepolymerised to 
minimise the amount free isocyanate available - this is because
isocyanates are extremely toxic - they are sensitizers. If a
person is exposed to low levels ( 0.005 ppm is the TLV ), then
subsequent exposures to even lower levels can induce the same
symptoms. The sensitivity can last for years, even decades. 
Don't play with raw isocyanates without learning about all the 
safety precautions first - your quality of life in later years
can suffer dramatically. Commercial polyurethanes are formulated 
and tested to minimise free isocyanate generation. 

          Bruce Hamilton

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