Polymerization and network formation of UV-curable materials monitored by hyphenated real-time ultrasound reflectometry and near-infrared spectroscopy (RT-US/NIRS)

During cure of coatings a complex interplay of changes in the chemistry and physics of the material occurs. In order to get detailed insight in the mechanisms of cure and network formation it is therefore desired to be able to monitor both simultaneously. In this work a novel technique enabling such analysis is introduced.Hyphenated rapid real-time dynamic ultrasound reflectometry (RT-US) and time-resolved near-infrared spectroscopy (RT-NIRS) have allowed simultaneous monitoring of acrylate photo-polymerizations with regard to both their chemical conversion and mechanical properties. Because of the high frequency at which the ultrasound method operates, it is especially suitable for monitoring the reaction-induced vitrification of a curing resin. Up to 250 NIR spectra and up to 100 ultrasound datapoints can be acquired within a second.We observe that modulus build-up during cure does not linearly follow chemical conversion of acrylate bonds, but changes dramatically at the conversion where vitrification occurs and further cure is retarded or stopped. Experimental data reveal a critical dependence of the development of mechanical properties during the later stage on acrylate conversion, and therefore the importance of final acrylate bond conversion as opposed to the early part of the conversion versus time (maximum rate of polymerisation) when considering the rate of development of mechanical properties (cure speed). A model description of the conversion dependence of the ultrasonic shear modulus illustrates the interpretation.