Effect of Corona Treatment on the Adhesion between a Two-Component Polyurethane Adhesive and Polypropylene

Two-component polyurethane (PU) adhesives possess robust bulk strength and excellent resistance to environmental factors, making them versatile for various applications in construction, transportation, and flexible packaging. However, their adhesion to nonpolar polymers can be suboptimal, potentially limiting their utility. To address this challenge, corona treatment has been employed to enhance the adhesion between nonpolar polymers and PU adhesives. Despite their extensive use, the specific mechanisms behind the adhesion improvement of the two-component PU adhesives on polymer surfaces due to corona treatment remain insufficiently explored at the molecular level, primarily because adhesion involves buried interfaces that are challenging to examine in situ. This study employs sum frequency generation (SFG) vibrational spectroscopy for in situ, nondestructive analysis of the buried interfaces between PU and polypropylene (PP, as a model for nonpolar polymers). Complementary analytical techniques, including Fourier transform infrared (FTIR) spectroscopy and adhesion testing, were also utilized. We conducted time-dependent SFG experiments to observe the molecular interactions at buried interfaces between the PU adhesive and PP during curing. Our findings reveal a significant correlation between adhesion enhancement and the environmental humidity level following corona treatment. Surprisingly, increased adhesion was not observed at low humidity levels (10–15% relative humidity at room temperature), challenging the conventional understanding of the corona treatment effect on adhesion enhancement. SFG measurements indicated that the formation of urea bonds at the PU/PP interface is a key factor in the increase in adhesion strength. This research clarifies the molecular mechanisms by which corona treatment enhances the adhesion of two-component PU adhesives on nonpolar polymer substrates.