Coating Surface Composition Using ATR-IR Spectroscopy, 2006 TAPPI Advanced Coating Fundamentals Symposium

The composition of a coated paper surface will control many important properties, such as gloss and printability. The ability to determine the relative amounts of the various coating formulation components that are present at the coating surface would be of great potential benefit in problem solving. A scanning electron microscope fitted with an energy dispersive X-ray analyser can perform semi-quantitative elemental analysis, but this cannot differentiate between the organic species present, unless the chemicals have different elements present. Infra-red (IR) spectroscopy can usually identify different organic species present but is not usually very surface sensitive. In this work, we report the application of attenuated total reflectance infra-red (ATR-IR) spectroscopy, where the sample is mounted next to a crystal and the infra-red beam is passed through the crystal and only interacts with a very thin layer of the sample, typically less than 1µm. A range of standard coatings was prepared with various levels of several different common coating components. It is demonstrated that this technique gives good results in determining the relative ratio of pigment to latex or plastic pigment. However, for chemicals that are used at lower levels, typically less than 1%, then it is more difficult to identify separate peaks in the IR spectrum that are not hidden in the noise or obscured by peaks from other components. However, it has been found that if the chemicals are polymers that interact with calcium carbonate surfaces, then the amount of these chemicals at the surface can be inferred from their effect on some of the stronger peaks from the calcium carbonate.