The influence of capsule geometry and cement formulation on the apparent viscosity of dental cements

Objectives This work examines the influence of specific aspects of capsule design and cement formulation on the handling properties of the extruded glass polyalkenoate cement (GPC) pastes. Methods A commercial metal reinforced GPC, HiDense, and experimental GPCs were extruded using a tensometer at loads and rates maintained within end-user limits and the apparent viscosity of the cement paste was determined by applying Poiseuilleʹs law. The influence of the extrusion procedure (mixing time and ram speed), capillary geometry (length and diameter) and cement composition (powder: liquid (P:L) ratio, tartaric acid content and poly(acrylic acid) molar mass) on the apparent viscosity of the cement paste was evaluated. Results The examined GPCs behaved as non-Newtonian, pseudoplastic materials and exhibited a yield stress. Variation of the geometry of the capsule capillary resulted in the apparent viscosity of HiDense increasing by 7% as the length increased from 5 to 15 mm whilst halving the capillary diameter from 2 to 1 mm resulted in a 63% decrease in the apparent viscosity and a 600% increase in the extrusion load. The apparent viscosity of the experimental GPCs was increased by an increase in the P:L ratio and, in general, by the PAA molar mass, whilst the concentration-dependent effect of (+)-tartaric acid (TAA) indicates a working time dependence on TAA content. Conclusions Using this approach optimisation of the rheological properties can be achieved by manipulation of the capsule design and cement formulation due to the dependency of the apparent viscosity on the capillary diameter, TAA content, P:L ratio and poly(acrylic acid) molar mass.