Effect of water on the physical properties of resin-modified glass ionomer cements

Abstract

Objectives: Resin-modified glass ionomer cements (GIC) are available for clinical use as restorative materials or as liners and bases. This work was conducted to study the effect of water sorption on the physical properties of several resin-modified GIC, by changing the samples’ storage conditions.

Methods: The water sorption, the flexural strength, the flexural elastic modulus, the Vickers hardness and the dimensional changes of five resin-modified GICs were measured using specimens aged for 24 h to 3 mon. The specimens were stored at 37°C, either in a dry environment (A), immersed in water (B), stored in a humid environment (C), stored in a humid environment for 1 h and then immersed in water (D), or immersed in water and subsequently dried (B+A). An analysis of variance was used to compare the results.

Results: The resin-modified GIC absorb during the first 24 h large amounts of water (114–172 mg/cm³) compared to the conventional GIC (30–63 mg/cm³). Water alters the physical properties of resin-modified GICs: With regard to dry specimens, a decrease in the flexural strength of 20 to 80% was observed for samples immersed in water. Decreases in their flexural elastic modulus (50 to 80%) and in their hardness (≈50%) were also observed. Water sorption also provoked an expansion in volume of the immersed specimens, ranging from 3.4 to 11.3% after 24 h.

Significance: The flexural strength and Vickers hardness of the resin-modified GICs are sensitive to the water contained in the tested specimens. A correlation was established between the decrease in their physical properties and the water uptake. However, it should not be concluded that these materials are not adequate for use in applications in direct contact with oral fluids. Probably, resin-modified GIC placed in oral cavities would not be affected to the same extent as in in vitro tests. In an oral environment, the constituents of saliva will certainly decrease the rate of water sorption and will hence delay its effects.

Introduction

Glass ionomer cements (GIC) developed by Wilson and Kent [1] are now widely used in clinical dentistry [2]. GIC are formed by the acid–base reaction of an aqueous polymeric acid and an ion leachable glass [3]. The replacement of the polyacid with a modified polyacid grafted with unsaturated groups, and the incorporation of polymerizable hydrophilic resins [4], [5] are recent modifications of these materials, that led to the resin-modified GIC. The hydrophilic resin, such as HEMA, is added as a co-solvent. It also polymerizes or copolymerizes with the modified polyacid [6]. The resin-modified cements show some advantages over the conventional GIC. They particularly allow a longer working time, a control of the photochemical curing process by the clinician, and a rapid hardening of the surface of the cement. Hence, the photochemical reaction reduces the early sensitivity to moisture and the dehydration associated with the early stage of the acid–base setting reaction of the conventional GIC [7]. Moreover, the resin-modified GICs present higher mechanical properties than the conventional GIC [8], [9], [10].

However, due to the presence of hydrophilic species, the resin-modified GICs are expected to absorb water easily. If water sorption could be beneficial in terms of stress reduction during the setting of the cements [11], it also reduces their compressive strength [12]. Therefore, the present work was aimed at studying the effect of water sorption on the physical properties of five commercially available resin-modified GICs. Thus, flexural strength, flexural elastic modulus and Vickers hardness were evaluated. The specimens used were stored during different time periods in various environments, which differed by the water content.