Effects of flaming on polypropylene long glass fiber composites for automotive bonding applications with polyurethane

Flaming treatments have been used for many years in the automotive industry to improve the chemical affinity of polymers such as polypropylene with coatings. This study focuses on the adhesion of a specific bonded assembly: a methane-flamed polypropylene long glass fiber (PPGFL) substrate with a two-component polyurethane (PU) adhesive. The effects of the flaming treatment, specifically the equivalence ratio R and the exposure time t, are evaluated. The chemical and surface morphological modifications of the PPGFL caused by the flaming treatment are linked to the mechanical properties of the bonded assembly. Goniometer and X-ray photoelectron spectroscopy analyses reveal that the atomic percentage of grafted oxygen were at a maximum for a slightly flaming reducing mode: R = 1.06 and t = 25 ms. In addition, atomic force microscopy observations show nodular structures formed under these conditions, confirming greater oxidation. These results are consistent with the mechanical characterizations and are compared to those obtained after aging. Based on previous results, a chemical model of polypropylene flaming treatment is proposed. This model, as well as the known reaction mechanisms of the adhesive, allows us to highlight a particular group that plays an essential role in the adhesion of the PPGFL/PU system.