Chemical Cleaning-Induced Surface Reorganization in Injection-Molded Polypropylene

Injection-molded polypropylene (PP) components are widely used in the automotive industry; however, their surface properties are strongly influenced by post-processing operations such as chemical cleaning, which may induce chemical modification or structural reorganization of the surface layers. Cleaning treatments are commonly applied prior to coating and joining operations to remove mold release agents and processing residues, but reactive chemicals and organic solvents may also modify the surface properties. In this study, the effects of alkaline (NaOH), oxidizing acidic (H₂SO₄), and organic solvent (acetone and decalin) treatments on injection-molded PP were investigated. Surface energy was determined by sessile drop contact angle measurements, surface morphology was analyzed by confocal microscopy, and chemical and crystallization changes by FT-IR spectroscopy and Differential Scanning Calorimetry (DSC). The results showed that the investigated liquids primarily acted as cleaning agents by removing weakly bound surface contaminants. At the same time, the dispersive component of surface energy increased, indicating improved exposure of the underlying PP surface. Decalin treatment caused the largest increase in surface roughness (+43%), while H₂SO₄ treatment resulted in the highest total surface energy (54.3 mJ m⁻²). Overall, the treatments influenced adhesion-related surface properties without causing substantial chemical surface activation. On the other hand, increased crystallinity and an altered height histogram were observed due to the enhanced chain mobility and reorganization in the surface layers.