Effect of adhesive layer thickness on the bond strength of a zinc polycarboxylate dental cement.

This study reports the effect of varying the thickness of a commercial zinc polycarboxylate dental cement on its adhesion in a metal-to-metal single lap joint. The thickness was controlled externally in a specially designed apparatus, rather than by the inclusion of isodimensional glass spheres, as has been done previously. The results show that both the tensile shear bond strength and the mode of failure of the cement are influenced by the thickness of the cement layer in the bonded joints. Hence, these results confirm previous findings that there is an optimum thickness for the zinc polycarboxylate layer. They also show that failure is more likely to be cohesive with thicker layers. The optimum thickness of the adhesive layer was 205 microns for the particular cement studied, PolyF Plus (De Trey, Dentsply, Weybridge, Surrey, UK), for which the average shear bond strength was found to be 4.03 MPa. Using stress analysis, the maximum shear and tensile stresses, tau 12(max) and sigma 11(max), have also been obtained. These analyses showed that joints failed at the interface between the cement layers and the lap joint at adhesive layer thicknesses of below 100 microns. This resulted from the high stress concentrations in bonded joints with thin adhesive layers. On the other hand, thicker joints failed cohesively within the adhesive layer because of greater contributions from tensile stresses in these joints.