The Role of Cold Atmospheric Plasma in Surface Modification and Adhesion Behavior of Polypropylene/Flax Biocomposites

ABSTRACT The emphasis on environmental sustainability has accelerated the adoption of biocomposites as alternatives to synthetic composites. However, their low surface energy may limit adhesion in bonding applications. This study investigates the effects of cold atmospheric plasma (CAP) treatment on the adhesion of flax fiber‐reinforced polypropylene (PP/flax) biocomposites. PP/flax plates were fabricated by sequential hot and cold pressing and subjected to CAP for different durations in atmospheric air. Surface modifications were characterized using contact angle measurements, atomic force microscopy (AFM), X‐ray photoelectron spectroscopy (XPS), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR‐FTIR). Adhesion performance was evaluated by single‐lap shear tests with a structural epoxy adhesive. CAP treatment significantly improved surface wettability, reducing water contact angle from 86° to 36°, while surface free energy reached a maximum after 150 s. Single‐lap shear strength increased from 0.54 MPa to 1.90 MPa at 150 s. Failure mode changed from adhesive failure in reference specimens to substrate failure in CAP‐treated joints. AFM indicated plasma etching increased nanoscale roughness, while XPS and ATR‐FTIR confirmed formation of polar functional groups and oxidation. Improvements beyond 150 s diminished, showing an optimal treatment range. These findings demonstrate that controlled CAP treatment effectively enhances the adhesive bonding of PP/flax.