In-line Monitors of the Degree of Cure after UV and EB Curing using Near-Infrared Spectroscopy

The functional properties of UV- and EB-cured coatings strongly depend on the final conversion of the acrylic double bonds which is achieved during irradiation. For instance, scratch and abrasion resistance depend on the hardness of the layer which is directly related to the degree of cure. Similarly, a high weathering resistance of coatings for outdoor applications can be only achieved if the conversion is largely completed. Insufficient conversion increases the soluble fraction which can be extracted from the coating which is one of the most critical parameters for packaging materials to be used for sensitive goods such as foods, pharmaceuticals etc. A sufficient degree of cure is also indispensable for further processing of the coating, and sufficient wipe resistance is required before stacking or winding. On the other hand, the extent of the conversion depends on numerous influencing variables such as the spectral distribution and the intensity of the incident UV light, the duration of exposure (corresponding to the speed of the web or the conveyor), the composition and the homogeneity of the reactive formulation,
the ambient conditions during irradiation (temperature, inertization etc.) and other factors. Whereas some of them can be easily controlled, this is hardly possible for effects like ageing, pollution or failure of UV lamps, differences between various batches of the lacquer formulation, precipitation of fillers or the photoinitiator, or other unexpected influences. In order to avoid expensive bad batches as a result of curing problems which were not realized, a continuous
process and quality control of the production is necessary. For an efficient control, instant and
continual data on the degree of cure right after UV irradiation are indispensable. However, up to now no practicable analytical method of measurement is available which allows in-line monitoring of the conversion of the acrylic double bonds during operation of a coating machine. In the past, various methods such as ion mobility spectroscopy' or fluorescence probe 2 have been tested for this purpose but none of them was found to be suitable for in-line measurements in a production. An appropriate measuring method must have high sensitivity since the degree of cure has to be determined in thin layers with a thickness in the range of some microns. It must be able to record data at a high sampling rate because of the usually high web speeds used in UV curing. Moreover, a high reproducibility and reliability of the data is required. In this study it will be shown that near-infrared (NIR) spectroscopy which is widely used for process control in chemical 6 complies with these requirements. It provides the proven capabilities of vibrational spectroscopy for the analysis of polymers and an excellent signal-to-noise ratio. In particular, the double bonds in acrylates and methacrylates appear as an isolated band at 1620 nm in the NIR spectrum. Time resolution and sensitivity of NIR spectroscopy are high enough to enable the in-line monitoring of the degree of cure after UV curing under typical production conditions. Finally, instrumentation is cost-effective, rugged and reliable which is essential for use in technical processes.