To reduce the environmental impact of packaging, paper appears to be a credible alternative to plastic, as it is a bio-based, recyclable, and biodegradable material. Various processes have been developed to give paper barrier properties. However, increasing the rigidity of paper structures is a challenge because the manufacturing process does not allow for homogeneous monolayers thicker than 180 µm. In addition, this would reduce the amount of material used and therefore its cost. The solution envisaged in this project to meet this objective is the creation of self-folding structured designs.
Preliminary work has shown that it is possible to create folds and obtain such structures after depositing and drying a hydrogel on paper, based on the water expansion properties of paper fibers and the shrinkage of hydrogels during drying. This promising technique makes it possible to form structures that are difficult to achieve using mechanical processes, while being adaptable, low-cost, and reducing the environmental impact of shaping. The objective of this thesis is therefore to understand and optimize the mechanisms leading to the folding of paper after depositing and drying a bio-based hydrogel. To do this, the project focuses on a detailed characterization of the two main physical phenomena involved: (i) impregnation of the paper substrate with a hydrogel, and (ii) drying and shrinkage of the hydrogel leading to the formation of a fold.
These different phenomena are characterized locally, using model systems, and at the material scale by X-ray microtomography and mechanical testing. To do this, the work draws mainly on the expertise of LGP2, a laboratory specializing in paper, bio-based hydrogels and their shaping, and that of 3SR, for multi-scale mechanical characterization and multiphysical couplings of heterogeneous media, particularly fibrous ones, and 3D imaging.
The results obtained provide fundamental insights into understanding and modeling gel impregnation in fibrous materials and the development of stresses induced by hydrogel shrinkage. They also enable recommendations to be made for transferring this technique to industrial processes.
