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To contribute to the ecological transition, increasing the use of environmentally friendly materials derived from renewable and non-polluting resources is necessary. In particular, bio-based materials such as paper appear to be a relevant alternative to plastic.
In addition to be a multi-scale porous material [1], with pore sizes ranging from several tens of micrometers between fibers down to the nanometer scale within the fiber walls, one distinctive feature of these materials is their high sensitivity to humidity and water. Indeed, when exposed to a humid environment, cellulose fibers swell [2], and their mechanical properties decrease drastically [3]. As a result, water transport within the medium induces gradients of volumetric strain and mechanical properties, which are responsible for deformations at the structural scale, such as the well-known paper curl phenomenon [4]
Moreover, in order to improve the barrier properties of paper, the deposition of a cellulose gel is a widely considered solution [5]. One of the main limitations to the use of such coatings is the deformations induced by gel imbibition and drying. Conversely, these hydromechanical coupling effects associated with the impregnation and subsequent drying of cellulose gels are intentionally exploited in hydromorphing applications, where they are used to shape paper into complex geometries in order to enhance the mechanical properties at structures scale [6], such as sandwich cardboard cores.
In this experimental study, we investigated the imbibition of a suspension of cellulose nanocrystals (CNC) into paper. This process involves a strong coupling between the rheology of the gel, which can transition from a viscoelastic fluid to a viscoelasto-plastic yield-stress gel depending on concentration, the multi-scale porosity of paper, and the deformation of the medium. To this end, imbibition experiments of a CNC gel (Maine University, concentrations ranging from 0% to 14.7% w/w) were carried out on 6 cm-long paper strips made from bleached softwood pulp, and compared with imbibition experiments performed in a non-hygrosensitive paper composed of glass fibers.
A detailed and combined characterization using light transmission imaging, deformation measurements, post-mortem water content analysis, and X-ray tomography enabled us to propose a scenario for water transport within the porous medium.In the absence of CNC, water imbibition occurs over the entire height of the strips. For concentrations between 6 and 8% w/w, a gel impregnation front rapidly propagates through the inter-fiber porosity by capillarity (pore sizes typically > 1 µm), while simultaneously progressing within the intra-fiber microporosity (from about 1 µm down to 1 nm). The front then stops in the inter-fiber porosity, and a second front appears exclusively within the paper fibers, corresponding to water diffusion in the intra-fiber porosity, pumping from the gel. Above a concentration of 10%, only the water intra-fiber diffusion front propagates. In all cases, front propagation is accompanied by swelling of the medium.
These observations should help optimize the development of bio-based materials involving interactions between paper and cellulose gels.
| References | [1] D. Topgaard and O. Söderman, Cellulose, 9, 2002. [2] N. H. Vonk et al., Wood Science and Technology, 58, 2024. [3] S. Lennart, Thesis, Stockholm, Sweden, 1982. [4] E. Reyssat and L. Mahadevan, EPL, 93, 2011. [5] N. V. Padilla Bello et al., Journal of microscopy, 2025. [6] J. Viguié et al., Cellulose, 28, 2021. |
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| Country | France |
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