Sticky Hydrophobic Behavior of Cellulose Substrates Impregnated with Alkyl Ketene Dimer (AKD) Via Sub- and Supercritical Carbon Dioxide

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 (scCO2). 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.