Functionalized Polypropylene Copolymers as Multisubstrate Hot-Melt Adhesives.

Amine-modified polypropylenes (PP_R, R = NH(Et)₂ (DEA); NH(Et)(CH₂CH₂OH) (EAE); NH(CH₂CH₂OH)₂ (DEOA)) have been prepared via a two-step synthesis and display enhanced adhesive performance with respect to both steel and polypropylene (PP) substrates. PP typically displays poor adhesion to polar substrates, which consequently restricts its utility as a hot-melt adhesive (HMA). Solvent-free, quantitative postmodification of poly(propylene)-co-(11-bromo-1-undecene) (range of comonomer incorporations (3-9 mol %)) with secondary amines yielded amine-modified PPs: PP_DEA, PP_EAE, and PP_DEOA. Rheological and FT-IR characterization identified the presence of a PP-based supramolecular hydrogen bonding network. (Co)polymers were evaluated as HMAs by lap shear strength between steel-steel and steel-plastic substrates. PP_EAE and PP_DEOA both excelled as HMAs between steel, recording the largest mean adhesive forces of 16.8 and 17.4 MPa, respectively; PP_DEOA displayed a 252-fold increase vs PP and comparable adhesive strengths to conventional structural adhesives. The adhesive failure mode in multisubstrate adhesion was found to be a function of interfacial effects, depending on the relative ability of the HMAs to bind to the polar steel surface and diffuse into the plastic substrate. PP_DEA and PP_EAE were found to be optimal in this case with failure by stock break indicative of adhesion greater than the tensile strength of the substrate and consequently appropriate for the application. The unique properties of these bifunctional materials highlight the versatility of the relatively limited application of functionalized PPs to date. This study now allows further sets of functionalized PPs to be readily prepared to meet a diverse array of multisubstrate adhesive requirements.