Exploring the relationship between food-grade wax solubility in supercritical carbon dioxide and resulting hydrophobic development of impregnated paper substrates

Abstract

Alkyl ketene dimer (AKD) and vegetable wax (VW) are suitable contenders for supercritical impregnation (SCI) processes, where the waxes may be deposited onto paper fiber surfaces to improve their hydrophobicity for food packaging applications. The solubility of both waxes in supercritical carbon dioxide (scCO2) with n-heptane cosolvent was measured (40 – 60 oC and 10 – 18 MPa). VW was two orders of magnitude more soluble in scCO2 compared with AKD, and both sets of data were validated using the Chrastil empirical equation. SCI of AKD wax into paper matrixes was performed at pre-determined solubility conditions, and the resulting hydrophobicity was measured. Results demonstrated that annealing the samples (4 h, 140, 160, 180 oC) immediately following impregnation generally enabled a more rapid development of the CA over a 2-week period. Crucially, using impregnation temperatures and pressures that maximized AKD solubility in scCO2 resulted in CA up to 130o. Gravimetric analysis of the impregnated samples confirmed the expected deposited wax quantities to within 25% of the solubility quantities. Scanning electron microscopy (SEM) imaging revealed better distribution of AKD in annealed samples and partially confirmed that more intricate structures were responsible for higher measured CA. These results explicitly connect the intuitive relationship that higher solute solubility in scCO2 should lead to bulk property improvements (i.e. higher CA or hydrophobicity) after SCI. Continued research in this area will enable the development of metrics, linking solubility conditions to desired bulk material properties, and is expected to lead to innovative packaging of green materials and processes.