Coating research


Introduction
Legislation concerning the use of coatings with Volatile Organic Components (VOC) drives coating manufacturers to focus on and increasingly produce water based coatings. Still, much research is needed to develop water based coatings of the same quality as their organic solvent based counterparts. In addition the performance of coatings should increase further and further. An important aspect for coating performance is  the dynamics of the curing process . Therefore, techniques to probe the spatial and temporal evolution of the coating structure are needed. One technique is Confocal Raman Microscopy (CRM) which can be used to obtain chemical information spatially resolved in a optical  transparent coating film. Recently, a nuclear magnetic resonance (NMR) setup with a high magnetic field gradient has been developed to measure the spatial and temporal evolution of a drying coating film [1]. An advantage of this technique is the ability to perform depth profiling any non transparent coating with a resolution of 5 µm. The NMR set-up probes the mobility of hydrogen nuclei which changes when a coating cures.
 
 

Figure 1. RF coil and film placement in a high magnetic field gradient.

NMR is widely used to image the human body. Its working principle is based on the fact that magnetic nuclei resonate in a magnetic field. A radio frequency pulse can be used to excite these nuclei. The resonance frequency w [rad/s] depends linearly on the magnetic field w =2pigB  where g  is the gyromagnetic ratio (for hydrogen, g=42.58 MHz/T). Spatial information can be obtained by varying the magnetic field with position. Applying a high magnetic field gradient of 36.4 T/m, with home built magnetic pole tips [1,2], a resolution of about 5 µm can be achieved. These pole tips are mounted in an electromagnet having a magnetic field of 1.4 T at the position of the sample, see figure 2. Not only the hydrogen density can be determined using this setup, but also the signal decay can be measured. This signal decay is influenced by the mobility of the hydrogen nuclei, and therefore contains information on the local network structure. Besides hydrogen this the setup can also be used to measure fluorine profiles.
 


Figure 2: A detail of the NMR setup