Exploring the Impact of Primer Adhesives Flow Properties on Orthodontic Bonding

Abstract The aim of this study was to investigate the viscosity and flow behavior of three orthodontic bonding agents (3M, B&E and Morley) in relation to their time-dependent shear bond strength (SBS) in the clinical application. All adhesives exhibited a non-Newtonian, shear-thinning rheology where solvent viscosity reduction occurs and increases in response to shear rate. The attributes of the flows were found to follow the power law model which was applied on the study sites. Chemical structure and functional groups in each adhesive were identified using Fourier-transform infrared spectroscopy (FTIR) analysis. For Morley composite resin, adhesives exhibited the following decreasing viscosity (at low shear rate) order: highest 3M, B&E, then Morley. Nonetheless, at a greater shear rate (60 s −1 ), viscosity trends were reversed, as 3M and B&E maintained their shear-thinning properties, and Morley sintered showing fully shear-thickening behavior. In addition, SBS values were inversely correlated to viscosity, with lower viscosity adhesives exhibiting higher bond strength. The viscosity measurements, as well as the corresponding SBS results, showed good correlations with flow index (n) and viscosity constant (k). These results delineate the importance of rheology in designing the properties of orthodontic adhesives, which may be pivotal for obtaining the desired outcome in terms of bonding and clinical performance, and ultimately viscosities similar to that of the adhesive used in this study should be targeted.