Hydrolytic stability of self-etching adhesive systems

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 degrees 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 degrees 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.