We have made a comparison between the NMR and CRM. To do so we have looked at the front positons as measured by these techniques. In figure 1 the hydrogen signal profiles of a solvent based alkyd sample are shown. About 10 minutes were needed to obtain each profile, because 512 averages were taken. From the figure two stages can clearly be identified. The first profiles show that the film shrinks, this process takes about one hour. At a certain time, after the 7th profile in figure 1, a front forms which moves towards the lower part of the coating.
Figure 1: Time and position dependence of the hydrogen signal obtained from the stain sample. Each profile results from signal integration over 10 min. starting successively at 10, 20, 30, 50, 70, 90 and 120 min after positioning the film immediately after casting. After the 7th profile the profiles are plotted after every 100 min. The left side is the open side of the coating, at the right a cover glass is located.
In figure 3 results of a confocal Raman measurement (CRM) and a NMR measurement is shown. In both figures a front moves down to the bottom of the coating film. This front in the CRM measurement is due to vanishing of double bonds (cross linking). Whereas in the NMR measurement front is observed due to a change in signal decay.
Figure 2 The profiles in a coating as measued by CRM (left) and NMR (right)
Figure 3 shows that in fact both fronts are the same. This shows that vanishing of double bonds directly corresponds to the formation of a cross linked network. This cross linked network has less mobility and causes the faster NMR signal decay, creating the front observed with NMR.
S.J.F. Erich, J. Laven, L. Pel, H.P. Huinink, K. Kopinga, Comparison of NMR and confocal Raman microscopy as coating research tools, Prog. Org. Coat. (2005)
S.J.F. Erich, NMR imaging of curing processes in alkyd coatings, Ph.D. thesis, Eindhoven University of Technology, the Netherlands (2006).