The aim of this investigation was to determine the hydrolytic stability of conventional methacrylates in comparison with a new acrylic ether phosphonic acid and a bis(acrylamide) cross-linking monomer under acidic aqueous conditions and to confirm the potential of these new monomers as components in a self-etching enamel-dentin primer, based on shear bond strength measurements. Two new monomers were synthesized, an acrylic ether phosphonic acid and a bis(acrylamide). Characterization of the two monomers was carried out by 1H-NMR, and 13C-NMR spectroscopy and of the phosphonic acid by 31P-NMR spectroscopy. The hydrolytic stability of these monomers was evaluated by 1H-NMR spectroscopy or high performance liquid chromatography (HPLC), and was compared with the hydrolytic stability of conventional adhesive methacrylates, ie, MDP, 4-META, TEGDMA and GDMA. Shear bond strength to enamel and dentin was determined using the acrylic ether phosphonic acid and bis(acrylamide) formulated into what is now being marketed as a self-etching primer (AdheSE, Ivoclar Vivadent), and compared with the total-etch adhesives Prime & Bond NT (Dentsply) and Excite (Ivoclar Vivadent) and the self-etching systems One-Up Bond F (Tokuyama), Touch & Bond (Parkell), i-Bond (Heraeus Kulzer), Prompt-L-Pop (3M ESPE), and Clearfil SE Bond (Kuraray). Initial and 24-h bond strength values of AdheSE were compared with those after 16 weeks of storage at 42°C. Stability studies have shown that conventional methacrylate monomers undergo rapid hydrolysis under acidic aqueous conditions (up to 90% degradation after 16 weeks at 42°C). However, the newly formulated strongly acidic acrylic phosphonic acid and cross-linking bis(acrylamide) are stable under these conditions. Moreover, a self-etching enamel-dentin primer based on these new monomers showed high bond strength values. The shelf life of dental adhesives can be increased by using ether- or amide-linked polymerizable analogues instead of conventional, less stable ester derivatives. This should improve the performance of adhesive bonds made with these more hydrolytically stable comonomers.
Keywords: self-etching primer, hydrolytic stability, methacrylate, phosphonic acid, bis(acrylamide)