How FTIR helps to understand curing reaction…

Last time when I started my work on ‘Electroless Plating on Moisture-Curable Polyurethane Undercoating’, I wondered how the moisture-cure reaction(s) took place. Since it was necessary for me to determine the processability of MCPU, the study of MCPU curing profile is definitely a must before I can proceed to the next phase, i.e. electroless plating study.
Note: MCPU = Moisture-curable polyurethane, FTIR = Fourier Transform Infrared
 
In our work, MCPU prepolymer was prepared by mixing chemical A and B in a reactor bottle (I keep it proprietary in here but isocyanate based chemical is one of the ingredient of course). The prepolymer will harden and become solid upon continuous exposure to air moisture, which means the reaction is moisture dependent.
 
Moisture-cure reaction
It is well established that isocyanate reacts with water (moisture) in a two step reaction to form an intermediate carbamic acid that later breakdown into amine and carbon dioxide. Amine will further react with balance isocyanate to form substituted urea. The reaction mechanism is shown below:
[1] R–NCO + H2O [R–NH–C(=O)OOH] R–NH2 + CO2
[2] R–NH2 + R–NCO R–NH–C(=O)NH–R
                                           substituted urea
 
Monitoring the progress of MCPU curing reaction
After studying a few spectroscopy techniques, I agreed that FTIR is one of the best spectroscopic tools for monitoring such reactions. Therefore I used near-real-time FTIR technique, in which FTIR spectrums were recorded at selected time intervals and graphed as a series of FTIR profile spectra.
 
The results were amazing. I managed to observe the depreciation trend of NCO (isocyanate) peak as well as the formation pattern of NH and urea C=O (urea carbonyl) peaks (see Fig. 1 and 2 – click to enlarge).
 
Justification: The consumption of NCO groups that corresponds to isocyanate reaction with moisture was reflected by a significant decreased of NCO peak, while the formation of substituted urea was confirmed through the formation of NH and urea C=O peaks in the FTIR profile spectra. We correlated the reaction(s) as follows:
 
     R–NCO + H2O [R–NH–C(=O)OOH] R–NH2 + CO2
  ν(N=C=O) at 2277 cm-1                 ν(C=O) at 1725–1717 cm-1                           R–NCO
                                                                                              R–NH–C(=O)NH–R
                                                                              ν(NH) at 3334 cm-1 and ν(C=O) at 1659 cm-1
 
For more detail explanation about this topic, please read:
M. N. Mohamad Ibrahim, M. E. Izat & C. S. Sipaut. FTIR characterization of moisture-cured polyurethane. J. Physical Science, 17(1)(2006) 77–89.
 
More chemistries:
  • The effect of hydrogen bonding that caused the shifting of NH group to a lower IR absorption frequency was observed.
  • Both ordered and disordered form of urea C=O were traced during the progress of curing reaction, however the system became totally disordered after completing the curing process. 

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3 Comments (+add yours?)

  1. Jim
    Apr 27, 2007 @ 08:52:30

    Yes. Professional.
    How many years are you working in PU industry?

    Reply

  2. Jim
    Apr 27, 2007 @ 08:53:15

    Oh, can I make a friend with you?

    Reply

  3. Izat
    Apr 30, 2007 @ 22:48:36

    Thanks Jim. I can see that you are really involving yourself in PU business. I worked as a technical personnel for six years in latex gloves industry. I totally involved in the manufacturing of PU gloves for almost two years. Those who really involve in PU will find it very interesting and will hardly put a stop to study PU.
     
    Jim, if you like to be my friend, sure we can be friends. Why should I say no (?).
     
    Best Regards
    << Izat >> 
     
     

    Reply

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