Shrinkage / stress reduction and mechanical properties improvement in restorative composites formulated with thio-urethane oligomers

Thio-urethane oligomers (TUs) have been shown to favorably modify methacrylate networks to reduce stress and significantly increase fracture toughness. Since those are very desirable features in dental applications, the objective of this work was to characterize restorative composites formulated with the addition of TUs. TUs were synthesized by combining thiols – pentaerythritol tetra-3-mercaptopropionate (PETMP) or trimethylol-tris-3-mercaptopropionate (TMP) – with isocyanates − 1,6-Hexanediol-diissocyante (HDDI) (aliphatic) or 1,3-bis(1-isocyanato-1-methylethyl)benzene (BDI) (aromatic) or dicyclohexylmethane 4,4'-Diisocyanate (HMDI) (cyclic), at 1:2 isocyanate:thiol, leaving pendant thiols. 20 wt% TU were added to BisGMA-TEGDMA (70-30%). To this organic matrix, 70 wt% silanated inorganic fillers were added. Near-IR was used to follow methacrylate conversion and rate of polymerization (Rp_max). Mechanical properties were evaluated in three-point bending (ISO 4049) for flexural strength/modulus (FS/FM) and toughness (T), and notched specimens (ASTM Standard E399-90) for fracture toughness (K_IC). Polymerization stress (PS) was measured on the Bioman. Volumetric shrinkage (VS) was measured with the bonded disk technique. Glass transition temperature (Tg) and heterogeneity of network were obtained with dynamic mechanical analysis. The addition of TUs led to an increase in mechanical properties (except for Tg and FS). Fracture toughness ranged from 1.6–1.94 MPa m^1/2 for TU-modified groups, an increase of 33–61% in relation to the control (1.21 ± 0.1 MPa m^1/2). Toughness showed a two-fold increase in relation to the control: from 0.91 MPa to values ranging from 1.70–1.95 MPa. Flexural modulus was statistically higher for the TU-modified groups. The Tg, as expected, decreased for all TU groups due to the greater flexibility imparted to the network (which also explains the increase in toughness and fracture toughness). Narrower tan-delta peaks suggest more homogeneous networks for the TU-modified materials, though differences were marked only for TMP_AL. Degree of conversion was not affected by the addition of TUs. VS was similar for all groups, with one exception where VS dropped (PETMP-cyclic). Finally, PS showed a reduction of 23–57% for TU-modified groups (6.7 ± 1.3 to 11.9 ± 1.0 MPa) in relation to the control (15.56 ± 1.4 MPa). The addition of thio-urethane oligomers was able to reduce polymerization stress by up to 57% while increasing fracture toughness by up to 61%.