Near-field microwave in-process thickness monitoring of coatings undergoing curing

In many applications a relatively thin layer of polymeric coating is sprayed on a metallic substrate in several successive steps, in order to ensure that a desired prescribed final coating thickness is obtained after proper curing: that is, when the coating becomes "dry." Depending on the chemical composition of the coating, the duration between each spray application may vary from a few to tens of minutes. In addition, for most polymeric coatings, curing causes shrinkage and hence reduction in thickness. Consequently, it is of great interest to develop a noncontact and nondestructive testing (NDT) method capable of real-time and in-process measurement of the overall characteristics (thickness and dielectric properties or cure state) of the coating after each spraying, and to correlate the data to the cured (dried) thickness of the coating. This paper presents an in-depth discussion of the problem along with demonstration of the capability of near-field microwave NDT techniques, using open-ended rectangular waveguide probes at X-band (8.2 to 12.4 GHz), to provide a linear correlation between measured data after each spray application to the final thickness for a coating primarily composed of commercially available polyurethane and resin.