Hydrophobic-to-Hydrophilic Transition of Polyethylene Surfaces via Salicylic Acid Grafting

Polyethylene (PE) is one of the most widely used thermoplastics worldwide due to its excellent balance of cost, performance, and sustainability. However, its inherently hydrophobic and chemically inert surface limits its suitability for applications requiring adhesion, such as printing, painting, and lamination—particularly in flexible packaging. These limitations are especially pronounced with water-based paints, which are more environmentally friendly but less compatible with hydrophobic substrates. Plasma or corona treatments are commonly employed to improve surface wettability, but they often lack specificity, are difficult to control, and offer only temporary effects. In this study, we propose a surface functionalization method involving the grafting of salicylic acid (SA)—a polar, aromatic molecule—onto PE films via an aluminum-mediated alkylation approach that is compatible with continuous film processing systems.

Low-density PE films (15 × 15 cm; Dow 203) were surface-softened using infrared heating and then sequentially sprayed with a 155 mg AlCl₃ solution in 25 mL n-heptane and a 500 mg SA solution in 25 mL absolute ethanol using an airbrush (120° fan, 6 mL/min). Unreacted species were removed by ethanol sonication. The modified surfaces were characterized by Fourier Transform Infrared Spectroscopy (Nicolet 520) and optical microscopy (Andostar). Paintability was evaluated using red water-based paint.

Spectroscopic analysis confirmed the formation of aluminum salicylate complexes chemically anchored to the PE surface. Microscopy revealed uniform surface modification, and painting tests demonstrated a significant improvement in the wettability and adhesion of water-based paints. These results indicate that the proposed strategy effectively transforms the PE surface from hydrophobic to hydrophilic, enabling the use of eco-friendly paints.