Rheological and mechanical properties and interfacial stress development of composite cements modified with thio-urethane oligomers

Abstract

Objectives: Thio-urethane oligomers have been shown to reduce stress and increase toughness in highly filled composite materials. This study evaluated the influence of thio-urethane backbone structure on rheological and mechanical properties of resin cements modified with a fixed concentration of the oligomers.

Methods: Thio-urethane oligomers (TU) 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. 20wt% TU were added to BisGMA-UDMA-TEGDMA (5:3:2). 60wt% silanated inorganic fillers were added. Near-IR was used to follow methacrylate conversion and rate of polymerization ( [Formula: see text] ). Mechanical properties were evaluated in three-point bending (ISO 4049) for flexural strength/modulus (FS/FM, and toughness), and notched specimens (ASTM Standard E399-90) for fracture toughness (KIC). PS was measured on the Bioman. Viscosity (V) and gel-points (defined as the crossover between storage and loss shear moduli (G'/G″)) were obtained with rheometry. Glass transition temperature (Tg), cross-link density and homogeneity of the network were obtained with dynamic mechanical analysis. Film-thickness was evaluated according to ISO 4049.

Results: DC and mechanical properties increased and [Formula: see text] and PS decreased with the addition of TUs. Gelation (G'/G″) was delayed and DC at G'/G″ increased in TU groups. Tg and cross-link density dropped in TU groups, while oligomers let to more homogenous networks. An increase in V was observed, with no effect on film-thickness. Significant reductions in PS were achieved at the same time conversion and mechanical properties increased.

Significance: The addition of thio-urethane oligomers proved successful in improving several key properties of resin cements, without disrupting the procedures dentists use to polymerize the material. This approach has potential to be translated to commercial materials very readily.

Keywords: Dynamic mechanical analysis; Mechanical strength; Polymerization stress; Resin cements; Rheological properties; Thio-urethane oligomers.

Figure 1

Degree of conversion as a function of the rate of polymerization for the control and thio-urethane-based groups (PET – tetrafunctional thiol; TMP – trifunctional thiol; AR – aromatic isocyanate; AL – aliphatic isocyanate; CC – cyclic isocyanate). Materials were photoactivated with 550 mW/cm² for 60 s, while vinyl conversion was followed in real time for 5 min.

Figure 2

Polymerization stress in respect of the time for thio-urethane groups in comparison to control (PET – tetrafunctional thiol; TMP – trifunctional thiol; AR – aromatic isocyanate; AL – aliphatic isocyanate; CC – cyclic isocyanate). Materials were photoactivated with 700 mW/cm² for 60 s, while polymerization stress development was followed in real time for 10 min.