Experimental evidences and calculation of the limited penetration of acyl chloride reagent in a dense layer of PVA coated on a paperboard after chromatogeny grafting

Abstract Paper board have very poor barrier properties to water, water vapour and grease that can be improved by coating a layer, in this work of PolyVinylAlcohol (PVA), which exhibits very good barrier to grease but limited resistance to water and water vapour. Chromatogeny is an hydrophobisation technique that allows to confer better barrier to water and water vapour with a technology implementable at the industrial scale. Several passages can be applied on the coated layer to improve the grafting densities. However, little is known about the molecular mechanisms and the distribution of the reagent among the coated layer or if it modified also the paper board. In this work, we demonstrated that the modification proceeds from the surface to the interior of the PVA layer by developing an imaging technique based on the labelling with Osmium Tetroxide (OsO4) of the double bond of an oleyl acyl chloride used as an unsaturated hydrophobisation agent. The result is a bright tagging of the modified PVA layer strictly limited to the upper surface, as revealed by SEM images. Calculations made on simple assumptions on the volume expansion due to the modification have been confronted to the experimental data, i.e. measurements of the thickness of the grafted layers. The results showed that in our experimental conditions, the reagent penetrates in a zone strictly restricted to the upper part of the PVA layer and never reached the paper board. Moreover, the second passage does not increase significantly the penetration depth of the reagent, but allows increasing notably the hydrophobicity of the grafted material, as shown by Cobb measurements. Paper board remains intact in all the experimental situations explored at pilot scale.