Thio-urethanes improve properties of dual-cured composite cements

Abstract

This study aims at modifying dual-cure composite cements by adding thio-urethane oligomers to improve mechanical properties, especially fracture toughness, and reduce polymerization stress. Thiol-functionalized oligomers were synthesized by combining 1,3-bis(1-isocyanato-1-methylethyl)benzene with trimethylol-tris-3-mercaptopropionate, at 1:2 isocyanate:thiol. Oligomer was added at 0, 10 or 20 wt% to BisGMA-UDMA-TEGDMA (5:3:2, with 25 wt% silanated inorganic fillers) or to one commercial composite cement (Relyx Ultimate, 3M Espe). Near-IR was used to measure methacrylate conversion after photoactivation (700 mW/cm(2) × 60s) and after 72 h. Flexural strength and modulus, toughness, and fracture toughness were evaluated in three-point bending. Polymerization stress was measured with the Bioman. The microtensile bond strength of an indirect composite and a glass ceramic to dentin was also evaluated. Results were analyzed with analysis of variance and Tukey's test (α = 0.05). For BisGMA-UDMA-TEGDMA cements, conversion values were not affected by the addition of thio-urethanes. Flexural strength/modulus increased significantly for both oligomer concentrations, with a 3-fold increase in toughness at 20 wt%. Fracture toughness increased over 2-fold for the thio-urethane modified groups. Contraction stress was reduced by 40% to 50% with the addition of thio-urethanes. The addition of thio-urethane to the commercial cement led to similar flexural strength, toughness, and conversion at 72h compared to the control. Flexural modulus decreased for the 20 wt% group, due to the dilution of the overall filler volume, which also led to decreased stress. However, fracture toughness increased by up to 50%. The microtensile bond strength increased for the experimental composite cement with 20 wt% thio-urethane bonding for both an indirect composite and a glass ceramic. Novel dual-cured composite cements containing thio-urethanes showed increased toughness, fracture toughness and bond strength to dentin while demonstrating reduced contraction stress. All of these benefits are derived without compromising the methacrylate conversion of the resin component. The modification does not require changing the operatory technique.

Keywords: delayed gelation; fracture toughness; kinetics of conversion; polymerization stress; prepolymerized additives; volumetric shrinkage.

Figure 1.

(A) Polymerization stress and (B) fracture toughness values for the 25 wt% filler BisGMA/UDMA/TEGDMA cement and for the commercial cement (RelyX Ultimate) with 0 (control), 10, and 20 wt% of thio-urethane oligomer added.

Figure 2.

Scanning electron microscope images of fractured surfaces of microtensile bond strength ceramic specimens. In general, the modified groups presented rougher surfaces compared to the control materials. However, the majority of failures were mixed adhesive-cohesive regardless of the treatment.

Figure 3.

Scanning electron microscope images of fractured surfaces of microtensile bond strength composite specimens. Both specimens showed a mixed adhesive-cohesive failure pattern. However, exposed dentin tubules can be seen only for the modified groups, indicating that the increased toughness on the cement led the crack to propagate through the interface and not through the bulk of the material.