Objectives: Most of the selfetching enamel/dentin primers are based on the general formulation - acid monomer, water soluble comonomer and water based solvent -. The pH value is in the range of 1.5. Under these conditions hydrolysis of methacrylate ester bonds occurs, the monomers are losing their function, sufficient storage stability can not be guaranteed . 4-META (4-methacryloxyethyl trimellitic anhydride) often is used as acid monomer. In this study the hydrolytical stability of 4-META was compared with the stability of new acrylate etherphosphonic acid monomers, whereby the hydrolysis of the ester bond is suppressed by sterical hindrance with aromatic substituents.
Methods: The acrylate etherphosphonic acid monomers were synthesized by a multi step synthesis and characterized by CH-analysis, IR, 1H-, 13C- and 31P-NMR. The long-term stability was investigated by 1H-NMR, 20 % by weight of each acid monomer was dissolved in a 1:1 mixture of D2O/MeOD. Storage of the samples at 42oC (pH value about 1.5): 6 weeks.
Results: 4-META is immediately converted to 4-MET. After 6 weeks at 42oC one third of the 4-MET was saponified however no chemical change was observed in the case of the new acrylate etherphosphonic acid monomers. The high performance of these acid monomers for selfetching/selfcuring primers could be shown by shear bond strength measurements. Bond strength values in combination with a composite cement were 13.3 ± 2.4 MPa on enamel and 25.5 ± 6.2 MPa on dentin (24h/37oC/H2O, two-way ANOVA significance level p < 0.05). SEM-investigations showed etch pattern formation on enamel and a hybrid layer formation in dentin.
Conclusion: Hydrolytically stable monomers are the key factor for the development of reliable and reproducible selfetching primers.