How FTIR helps to understand curing reaction...

This article has been published in December 2006 in izatmalezy.spaces.live.com.

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 (click picture 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 & 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.