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Graphical abstract Highlights • Sub- and supercritical CO 2 impregnation of AKD onto paper substrates is conducted. • The development of hydrophobicity was significant 2–3 days after impregnation. • Sticky hydrophobic surfaces was discovered > 140 days after the treatment. • Migration of AKD across the fiber surface causes reduction in the pore sizes. • Hydrogen bonding between AKD and cellulose is observed with FTIR. Abstract Cellulose fibers were impregnated with alkyl ketene dimer (AKD) dissolved in n-heptane and carbon dioxide via sub- and supercritical impregnation techniques. The mechanistic pathways and hydrophobic performance at short and long times were investigated by contact angle (CA) analysis, scanning electron microscopy (SEM) with micrographs analyzed using Image-Pro Premier, and Fourier Transform Infrared (FTIR) analysis. The sizing development was significant after two days of treatment, and hydrophobic performance became uniform after two weeks regardless of the impregnation conditions investigated. Samples prepared at 100 and 200 bar produced more rapid development than those at higher and lower impregnation pressures, with the average CA at 200 bar and 21 °C being 140 ± 5°. 'Sticky' hydrophobicity was observed on surfaces treated at 200 and 250 bar at long times (> 140 days), and adhesive forces between the droplet (>20 μL) and surface were observed at surface tilt angles between 0 – 180°. SEM micrographs of the impregnated samples showed a reduction in substrate pore-size area (PSA) as hydrophobicity developed with time. There was little evidence of reaction-based sizing as the characteristic ketone and ester peaks were not observed in FTIR studies. The lactone ring remained intact. The 200 bar sample showed highest peak intensity for various hydrocarbon bonds observed – suggesting the optimal solubility of AKD in supercritical carbon dioxide (scCO 2). Spreading of AKD across the fiber surfaces appeared to be the main sizing pathway, and identification of hydrogen bonding between AKD and cellulose fibers suggested a possible attachment method. [ABSTRACT FROM AUTHOR]

Alkyl Ketene Dimer (AKD) is a sizing agent commonly introduced into cellulose substrates during papermaking to improve its hydrophobicity. High-pressure impregnation of this solute with supercritical carbon dioxide (scCO 2) into cellulose and other substrates can achieve excellent hydrophobicity of the substrate, and may be used as an alternative technique to traditional coating (sizing) methods. In this study, the solubility of AKD in scCO 2 was modelled using Peng-Robinson equation of state (PR-EOS) with van der Waals mixing rules, and volume-translated PR-EOS (VTPR). Given the unavailability of AKD's experimental critical properties and vapor pressure data, group contribution estimation methods (GCEM) were used to determine these physical constants. Available experimental data from another research group, collected using cloud-point and extraction methods, was used to regress binary interaction parameters and compare the resulting model's capability in predicting solubility. The models for each experimental method predicted crossover pressures within a small range, but fitted the cloud-point experimental data much better than the extraction data, showing good agreement at pressures beyond 15 MPa and all temperatures investigated. The VTPR model showed the least agreement with experimental data. A relative standard deviation of 0.26 was obtained between modelled and experimental data for the cloud-point method, and 0.42 for the extraction method data. The models were confirmed with the Chrastil equation, and strongly demonstrated the linear relationships predicted by this expression. The models additionally predicted a change in temperature dependence around the critical density, with a common linear relationship independent of temperature being observed at densities below the critical value. This newly observed behavior, not predicted by the Chrastil equation, sets the groundwork for further fundamental investigations, while the developed models will assist the design of alternative sizing processes involving AKD and scCO 2. • PR-EOS and van der Waals mixing rules were used to model AKD solubility in scCO 2. • Vapor pressure and critical constants of AKD were estimated by group contributions. • The isotherms show the existence of retrograde vaporization and crossover pressure. • The model was confirmed with the Chrastil equation, showing high accuracy. • Solubility was independent of temperature at regions below the critical density. [ABSTRACT FROM AUTHOR]