Interfacial Bond Strength of Resin Cement to Polyetheretherketone (PEEK) Using Experimental Primers Doping Multifunctional Methacrylate and Acrylate Monomers

Polyetheretherketone (PEEK) is a high-performance thermoplastic polymer with low surface energy, chemical inertness, and a highly crystalline structure, which limit durable adhesion to resin-based materials and require improved bonding strategies. This in vitro study evaluated the effect of incorporating multifunctional methacrylate (TMPTMA) and acrylate monomers (A-TMPT and A-DPH) into an experimental primer on the 24 h shear bond strength (SBS) of resin cement to PEEK (n = 6 per group). Experimental primers were prepared by adding varying amounts (100–500 µL) of each multifunctional monomer to a UDMA/MMA-based primer, and SBS was measured after cementation with a dual-cure resin cement. TMPTMA-containing primers produced median SBS values of approximately 10–12 MPa, while acrylate-based primers showed slightly higher values, particularly at higher concentrations, reaching up to ~15.8 MPa; however, no statistically significant improvement over the control group was observed (p > 0.05). Failure modes were predominantly mixed across all conditions. These findings indicate that multifunctional monomers, despite their potential to enhance cross-link density and wetting, do not substantially overcome the intrinsic adhesion-resistant nature of polished PEEK. The results underscore that monomer modification alone is insufficient and that effective adhesion to PEEK likely requires integrated strategies combining chemical activation with mechanical surface conditioning.