The Measurement of Coating Thickness
It is not feasible for the coating operator to depend on laboratory measurements of coating thickness which is made on finished rolls. This can be attributed to variable coating thickness and uniformity due to the dynamic nature of a typical coating process. It is necessary to measure coating thickness across the film as part of the coating sequence, using radiation absorption.
Though, differentiating the coating from the film is a complex challenge. Radiation can serve as a good approach, i.e. a form only absorbed by the coating, like PVDC coatings with a high concentration of chlorine atoms that absorb x-rays. Another example is infrared radiation at wave lengths that are absorbed more by coating in comparison with substrate film. However, the differentiation becomes increasingly difficult with the coating and substrate having similar chemical nature/characteristics. Also, the difference between substrate film and the coated film (measured separately) can be used to infer the coating thickness.