Understanding wrinkles appearing on paper functionalized by coating

This doctoral thesis was prepared at the Université Grenoble Alpes under the direction of Raphaël Passas, HDR Lecturer, and of Céline Martin, Associate Professor (Grenoble INP - Pagora, UGA / LGP2).

Florian Le Gallic presented the results of his research entitled Understanding of the wrinkles formation during the coating functionalization of lightweight fibrous material: a multiscale approach.

Since its creation, paper as a material has constantly evolved to meet society's needs. Currently, research focus in the reduction of grammages with constant mechanical characteristics and the functionalization of papers in order to obtain specific surface properties for the reduction of the environmental foot printing of the paper mills. These characteristics can be obtained by coating paper surface with suitable aqueous coating color. The quality of the coating obtained depends on the triplet: paper/coating color/coating process. When coating very light papers, wrinkles may appear at the exit of the coating area. After drying and depending on the nature of the coating, wrinkles can be irreversible and lead to non-conformities.  The aim of this thesis is therefore to understand how and why these wrinkles can appear.

Firstly, the study of water absorption by paper was carried out. This allowed to determine a first order law relating the quantity of water absorbed by the paper as a function of time. In a second step, the wrinkle phenomenon is studied, at different scales, from the point of view of mechanics. This allowed to highlight the impact of Young's modulus on the appearance of wrinkles, particularly in the presence of water. Moreover, on a microscopic scale, a decohesion of the fibrous network was observed. Therefore, wrinkles have a macroscopic and microscopic impact. The last point addressed in this project is the optimization of the coating process. Three process parameters were related to each other: the web speed, the angle of misalignment of the rolls with respect to each other and the web tension. This has enabled the development of a control map that allows the coating to be carried out under the best possible conditions. Finally, a low-weight paper could be coated without wrinkle.