Chemical surface treatment mechanisms involved in a flamed polypropylene/polyurethane bonded interface: Toward a treatment by nitrogen plasma

Flaming treatments have been used at the industry level since the 1950s to improve the chemical reactivity of polyolefins. This treatment allows the grafting of polar functional groups onto the substrate to improve its chemical affinity with paints and adhesives, which is important in fields such as the automotive industry. Therefore, the primary purpose of this study is to identify which groups grafted by flaming treatments are involved in the adhesion between polypropylene long glass fibers (PPGFL) and polyurethane (PU) adhesives, with or without surrounding water. The effect of an alternative low-pressure nitrogen plasma treatment on the PPGFL/PU interface was then studied. The main conclusions arising from the study of flaming are the following. X-ray photoelectron spectroscopy analyses revealed that the flaming treatment grafted four different chemical groups: hydroxyl, ether, ketone, and acid. Model reactions studied using nuclear magnetic resonance spectroscopy showed that only hydroxyl groups played a role in PPGFL/PU adhesion, as long as the environment was not saturated with water. On the other hand, the low-pressure nitrogen plasma study showed that adhesion could also be obtained through amine groups. Model reactions showed that these groups were very reactive with the isocyanates contained in the adhesive; the reaction was complete and immediate. In addition, this particular reaction pro-duced urea groups that were only slightly parasitized by water, which made the process more robust than flaming. This study primarily aims to link the chemical mechanisms occurring at the interface with a macroscopic adhesion measurement of a bonded system. Further, kinetic monitoring allows the comparison of the reaction speed of each grafted group with the adhesive. Consequently, this makes it possible to determine the functional groups that are most important for adhesion and the most efficient and robust treatment method.