Investigation into the ultrasonic setting of glass ionomer cements Part II Setting times and compressive strengths

Abstract

Glass ionomer cements (GICs) are formed by the reaction of an ion leachable alumino-silicate glass with an aqueous solution of poly (alkenoic acid), PAA. Water is used as the reaction medium [1]. A two-stage setting reaction occurs, resulting in a cement consisting of residual glass particles embedded in a hydrogel polysalt matrix [2]. During the first stage the material is susceptible to water uptake and during the second it is susceptible to dehydration. For example, when GICs are stored in water after an initial set of 15 min, a surface softening occurs, which may be caused by an inhibition of the setting reaction in a superficial layer of the cement [3]. This short-term relationship with water restricts the potential of GICs for healthcare applications. It is for this reason that resin modified GICs (RMGICs) were developed. These materials, conventional GICs into which an organic, photo-polymerisable monomer has been incorporated [4], can be command set by the application of an intense light source. However, RMGICs have drawbacks related to the presence of both non-polymerised monomer and the resin itself [5, 6]. Rapid setting is important both for providing resistance to attack from moisture in the mouth and improving wear resistance, but an alternative method to light-curing which does not require the incorporation of additional chemicals would be beneficial. The application of ultrasonic waves has long been used for setting cement in the building industry, and so studies were performed to utilise the same method for setting dental cements [7]. Most dental clinics have ultrasonic capability as it is used for de-scaling teeth. The effects of applying ultrasonic waves to the surface of the cement include a reduction in both porosity and mean glass particle size (due to breaking up of agglomerates), an improvement in the packing of the glass particles and a decrease in the mean molecular weight of the PAA [8]. Whilst it is not possible to further explain the curing mechanism at this stage, this work will determine the extent to which ultrasound affects the setting times and mechanical properties of a series of hand mixed and mechanically triturated GICs.