How to assess the layers' thicknesses in polymer-coated cardboards?

When polymers are coated on the surface of cardboards, they inevitably penetrate through the cardboard pores, resulting in a complex three-layer structure consisting of a free polymer layer, an impregnated layer, and a free cardboard layer. In this article, two quantitative, representative, and reliable methods are proposed to determine the thickness of the individual layers, which is an indispensable prerequisite for deciphering the role of multilayer structures on the final properties of polymer-coated cardboards. The first method involves the analysis of images obtained either by scanning electron microscopy (SEM) or X-ray microtomography. The second method proposes a modeling approach based on experimental measurements of macroscopic physical parameters. The two methods provide congruent information but differ in terms of accessibility and precision. The main challenge with the SEM-based method lies in the cutting technique required to obtain clean cross-sections. X-ray microtomography does not require sample preparation, but its accuracy is lower. The main issue of the modeling approach is the quantitative and representative evaluation of the cardboard's porosity. Finally, both methods provided consistent results, e.g. with impregnated layers thicknesses of [257–270] and [167–247] μm for PHBV- and LLDPE-coated cardboards, respectively, highlighting a total and a partial impregnation. These methods can be used to detect not only an impregnated layer, but also an impregnation gradient in the coated cardboard, which needs to be further studied.