Effect of Polyacrylic Acid Molecular Weight Mixtures on Glass-Ionomer Restoratives

Objectives: The mechanical properties of glass-ionomer (GI) restoratives are increased when the molecular weight (Mw) of polyacrylic acid (PAA) is increased, however, the mechanical property increases are offset by increased viscosity and decreased working characteristics. Replacing a proportion of a high Mw PAA with a low Mw PAA may reduce the viscosity of the GI restorative without reducing the mechanical properties. The aim of the current study was to investigate the effect of using PAA solutions containing a mixture of two Mw PAAs on the performance of a GI restorative. Methods: Series of PAA solutions (at 25, 35 and 45% concentration) were prepared by combining low Mw PAAs (15,000, 25,700 and 40,000) with a high Mw PAA (80,800) at ratios ranging from 0:100 to 100:0 wt.% (in 10 wt.% increments). The PAA solutions were hand-mixed with a commercial GI restorative powder (Ionofil™ Molar; Voco, Cuxhaven, Germany). The 24 h mean compressive fracture strengths and elastic moduli were determined using cylindrical specimens (6 mm height, 4 mm diameter) (n = 10). Data was analysed using regression analysis and a Student's t-test at P<0.05. Results: The slopes of the regression lines were not significant for the compressive fracture strength and elastic modulus data as the ratio of low Mw PAA to high Mw PAA was increased. The Student's t-test (P<0.05) showed significant increases in the mean compressive fracture strength and elastic modulus for Mw combinations of 15,000 and 80,800 at 20:80 wt.%, 25,700 and 80,800 at 30:70 wt.% and 40,000 and 80,800 at 60:40 wt.% compared with the controls. Conclusion: The performance of the GI restorative in terms of mean compressive fracture strength and elastic modulus was improved when the lower Mw PAAs were added to the higher Mw PAA (80,800), however, further work is required to fully optimise PAA Mw mixtures.