Mechanism of adhesion of polymers to acid-etched enamel. Effect of acid concentration and washing on bond strength.

The tensile bond strengths of composites to enamel etched with various concentrations of phosphoric acid have been shown to correlate closely with the phase diagram for the system Ca(OH)2/H3PO4/H2O. Bond strengths were independent of the adhesive used and were a maximum when using approximately 20% phosphoric acid as etchant. High bond strengths were obtained with 2% phosphoric acid etchant without 'tag' formation, after short term water immersion. Washing the enamel for 60 s instead of 15 s after etching significantly increased the bond strengths obtained with 16 and 30% acid but not with 2%. Etching enamel with phosphoric acid results in deposition of calcium phosphates, which must be removed to give optimal bonding.