(181g) High Pressure Impregnation of Alkyl Ketene Dimers (AKD) in Cellulose Substrates, Creating Novel Sticky Hydrophobic Surfaces

Alkyl Ketene Dimers (AKD) are common organic sizing molecules used to impart hydrophobicity on cellulose substrates, making use of their twin organic tails in a ‘stand-up’ orientation. High pressure / supercritical impregnation techniques with carbon dioxide (scCO₂) as the solvent were used to impregnate AKD into cellulose substrates, creating a very thin but uniform film across the cellulose fibers. This technique enabled greater penetration and deposition of small quantities of AKD deep within the substrate, as well as easy removal of the solvent upon completion. Several cellulose substrates were impregnated with AKD in this manner and monitored over time to determine the kinetics of sizing development; the likely sizing mechanisms taking place; and unique surface behavior resulting from the treatment. AKD took up to two weeks to fully develop the ‘stand-up’ orientation and hence sizing effect with an average contact angle (CA) 140 +/- 5^o (scCO₂ at 200 bar and 21^oC). Sizing, however, was already significant after two days of treatment. FTIR traces monitoring the changes in chemical bonding between the AKD and cellulose found no deterioration of the lactone ring in AKD, and evidence of increasing hydrogen bonding between AKD and the cellulose fibers suggested a possible attachment method with time. A spreading-type mechanism was tentatively confirmed with SEM studies, clearly showing a reduction in substrate porosity with time as the AKD continued to orient itself in the stand-up position, using hydrogen-bonding as attachment methods to the cellulose. The near superhydrophobicity achieved on the modified cellulose substrates were identified as ‘sticky hydrophobicity’, a phenomenon observed in nature on rose petals, where water droplets ‘stick’ to the substrate even at very large tilt angles. Contact angle hysteresis (CAH) and sliding angle (SA) measurements confirmed the criteria of sticky hydrophobicity. It is believed that this unique surface behavior has not been previously reported in supercritical impregnation studies with AKD, and this research area is itself rather small. The authors hope to expand on this work to develop highly hydrophobic papers for food-packaging applications using this novel impregnation method. This method is preferable to surface-coating method of thin films in that it treats each fiber within a substrate rather than just the bulk substrate surface; uses significantly less material than traditional methods; and can be easily removed in recycling operations by using the reverse process of high pressure / supercritical extraction.

Reference:

Adenekan, K. and Hutton-Prager, B. (2018). Sticky Hydrophobic Behavior of Cellulose Substrates Impregnated with Alkyl Ketene Dimer (AKD) via Sub- and Supercritical Carbon Dioxide. Journal of Colloids and Surfaces A: Physicochemical and Engineering Aspects, 560: 154-163. https://doi.org/10.1016/j.colsurfa.2018.09.07