The influence of montmorillonite clay reinforcement on the performance of a glass ionomer restorative.

A pristine calcium montmorillonite (Ca-MMT) and an organically modified 12-amino-dodecanoicacid treated montmorillonite (ADA-MMT) clay were evaluated to determine the reinforcement effect on the performance of a glass ionomer (GI) restorative ChemFil Superior (Dentsply DeTrey, Kanstanz, Germany) to assess the potential as a posterior filling material. Mean compressive fracture strengths and standard deviations were determined for groups of 30 cylindrical specimens with 0.5-2.5 wt.% Ca-MMT and ADA-MMT additions to either the powder or the liquid elements of the GI. Working characteristics were assessed using an oscillating rheometer, the interlayer d-spacings (d(001)) of each MMT clay was determined using X-ray diffraction (XRD) and the set cement structure was determined by scanning electron microscopy (SEM). The addition of up to 1.0 wt.% ADA-MMT resulted in a significant increase in the mean compressive fracture strength of the GI utilising the one-way ANOVA and Tukey test comparisons at P<0.05. When Ca-MMT clay was added to the GI a significant reduction in mean strength was recorded (P<0.05). MMT clay addition to the powder and liquid element of the GI in excess of 1.0 wt.% resulted in no significant differences in working characteristics although increases in setting times were evident for MMT addition up to 1.0 wt.%. XRD patterns identified the d(001) peaks for Ca- and ADA-MMT at 2 theta angles of 8.44 degrees and 5.07 degrees and d(001) spacings of 1.04 and 1.74 nm, respectively. The increased interlayer d-spacings recorded for ADA-MMT clay demonstrates that the clay had expanded layers which may have provided an increased opportunity for the polyacrylic acid chains of the GI restorative to diffuse into the MMT galleries. It is postulated that the increased interlayer d-spacings for the ADA-MMT clay had a positive reinforcing effect on the GI compared with the Ca-MMT clay where no increased performance was identified. The study has highlighted the potential for increasing the performance of GI restoratives when using an organically modified clay as a reinforcement which could increase the potential of GIs as posterior filling materials.