Mechanical and physical properties of contemporary dental luting agents

Statement of problem. New luting agents, particularly with adhesive capability, arc being introduced in an attempt to improve clinical success. Independent studies of basic comparative data are necessary to characterize these materials in relation to mechanical and physical properties. Purpose. The purpose of this study was to compare the flexural strength, modulus of elasticity, and radiopacity and pH of representatives of 5 types (categories) of luting agents. Material and methods. The luting agents included a zinc phosphate, a conventional and a resin-modified glass ionomer, 2 dual-polymerizing resins ("photopolymerized" after mixing and "unphotopolymerized" conditions), and an auto-polymerizing resin. The specimens were prepared and the testing was conducted by 1 person to maximize standardization. Flexural strength (MPa) and modulus of elasticity (GPa) were determined on bar-shaped specimens (2 x 2 x 20 mm) at 24 hours and 3 months (n = 8). Radiopacity (mm Al) was measured by exposing 1 mm thick specimens along with an aluminum step wedge (n = 4). pH was measured using a pH electrode immediately after mixing; at 1, 5, 15, 30 minutes; and at 1, 2, 4, 6, and 24 hours (n = 4). The data were subjected to statistical analyses with analysis of variance and Duncan's multiple range test (P<.05). Results. The resin luting agents (64 to 97 MPa) showed higher flexural strength than all other materials tested (7 to 27 MPa), with the "photopolymerized" (83 to 97 MPa) conditions higher than "unphotopolymerized" (64 to 81 MPa) (P<.0005). Zinc phosphate was the most radiopaque (6.4 mm Al) (P<.0001) and provided the highest rigidity (9.2 GPa) (P<.05). The autopolymerization resin cement was the most radiolucent (1.1 mm Al) (P<.0001). Zinc phosphate and conventional glass ionomer cements were the most acidic immediately after mixing (pH 1.5 to 2.2) but were the least acidic after 24 hours (pH 6.4 to 6.8) (P<.0001). Conclusion. Within the limitations of this study the data showed a wide variation of material properties. The dual-polymerization resin luting agents tested showed the best combination of mechanical and physical properties combined with the highest setting pH. Photopolymerization of these resin-based materials was necessary to maximize strength and rigidity.