Hydrophobic molded pulp based on partially grafted fibers dispersible in water: part II—orthogonal water penetration model for explaining water resistance of gas grafted dual-layer pulp molds

Gas grafting was applied to paper to produce recycled fibers that were partially hydrophobic yet retained hydrophilicity, which were then used in water-resistant molded pulps. To balance water resistance and mechanical strength, dual-layer molds were fabricated with a gas-grafted hydrophobic top layer and untreated hydrophilic bottom layer. The optimal basis weight of the hydrophobic top layer was investigated, as thin layers reduced water resistance while thick layers lowered tensile strength. High-pressure calendering increased layer density but caused excessive pore collapse, diminishing water resistance. Cobb tests showed effective resistance at hydrophobic top layer weights above 80 g/m 2 with ≤ 400 mL of applied water, but penetration accelerated above 600 mL. To explain the nonlinear penetration behavior, an Orthogonal Water Penetration Model was proposed, suggesting that vertical continuity of hydrophilic patches enables breakthrough once the hydrophobic barrier is breached. This model further revealed that fibers with > 50% residual hydrophilic area cannot form a water-resistant layer. Overall, the proposed dual-layer pulp mold design achieves stable water resistance while minimizing hydrophobic fiber usage.