Many different kinds of coatings on paperare used today to get the necessary surface
and bulk properties of paper for different applications. Coatings are used to enhance
printability, increase wet strength, decrease surface roughness, or change diffusion
rates of gasses or liquids. For example the glossy surface of photographic paper for
inkjet printers is made by coating the paper.
However, easy and accurate methods to measure the thickness and quality of such
coatings are limited. Mostly methods like x-ray absorption and ellipsometry are used.
In some cases even wet chemical methods are employed [1]. Especially the latter
method is very time consuming and cannot be used as an online method. Only x-ray
absorption can be used online, however it can only be used for thick enough films.
In this paper we present two methods based on Fourier Transform Infra Red
Spectroscopy (FTIR) that can do both, measure the thickness of a coating (if a
calibration is available) and provide chemical information at the same time.
The first method is Attenuated Total Reflection Infra Red Spectroscopy (ATR –
IR) using polarized infra red light and the second method is Infra Red Reflection
Absorption Spectroscopy (IRRAS) also utilizing polarized light. The physical principles behind these methods have been described in detail before [1].