Mechanical interlocking by imprinting of undercut micropatterns for improving adhesive strength of polypropylene

To improve adhesive strength, this study investigated a surface modification technique that is inexpensive, semi-permanently effective, and applicable to large areas and inflexible thermoplastics, such as polypropylene (PP). In order to achieve this objective, undercut micropatterns were created on PP plates by a roll-imprinting process to introduce mechanical interlocking. The roll-imprinting molds with various undercut micropatterns were first fabricated using a wire-cut electrical discharge machine. Then, the imprinting process was carried out by controlling the temperature of the PP plate so that it was heated to its crystalline melting temperature just before imprinting and cooled to around the Vicat softening temperature during demolding. The improvement of the adhesive joint strength was then evaluated by performing butt-joint tensile tests on specimens with various undercut angles. The tensile adhesive strength was found to increase with the undercut angle, and the adhesive strength of the 25° undercut angle specimen with the general-purpose epoxy adhesive was 1.7 times higher than that of the specimen without surface modification and with the expensive commercially available cyanoacrylate adhesive specially developed for PP. This result clarified that the mechanical interlocking due to the undercut micropattern could improve the adhesive strength of PP at low material cost.