Experimental evidence and calculation of the penetration depth of acyl chloride reagent and molecular expansion in a dense layer of PVA coated on a paperboard after chromatogeny grafting

Chromatogeny is a hydrophobization technique with fatty acid chloride without solvent that confers an improved barrier to water and water vapour, thanks to a technology that can be implemented on an industrial scale and adapted to any hydroxylated substrate, including cellulosic materials. In this work, a chromatogenically modified polyvinyl alcohol (PVA) coating layer was used as a high oxygen barrier material and as a model for hydroxylated polymers, including microfibrillated cellulose coating. Multiple passes can be applied to the coated layer to improve grafting densities. However, little is known about the molecular mechanisms and distribution of the reagent in the coated layer or whether it also modifies the board. In this work, we have demonstrated that the modification proceeds from the surface to the interior of the PVA layer by developing an imaging technique based on labelling with osmium tetroxide (OsO₄) of the double bond of an oleyl acyl chloride used as an unsaturated hydrophobizing agent. The result is a brilliant marking of the modified PVA layer strictly limited to the top surface, as revealed by SEM images. Calculations based on simple assumptions about volume expansion due to modification were compared with experimental data, i.e. measurements of the thickness of the grafted layers. The results showed that, under our experimental conditions, the reagent penetrates a zone strictly limited to the upper part of the PVA layer and never reaches the board. Moreover, the second pass does not significantly increase the reagent's penetration depth, but does significantly increase the hydrophobicity of the grafted material, as shown by the Cobb measurements. The cardboard remains intact in all the experimental situations explored on a pilot scale. The techniques developed will be transferred to the emergence of a cellulose-based barrier coating with cellulose microfibril films.