Powder-liquid ratio and properties of two restorative glass ionomer cements

Changes in the powder-liquid ratio of glass ionomer cements may affect some of its physical properties and acid erosion. The aim of this study was to evaluate the physical properties and acid erosion of two conventional restorative glass ionomer cements against ISO 9917-1:2007 standards after changing the powder-liquid ratio to an adequate consistency for luting indirect restorations. The methodology of ISO Specification 9917-1:2007 was applied to the powder-liquid ratio indicated by the manufacturer and to a modified ratio. Two restorative glass ionomer cements, ChemFil (Ch) (Dentsply) and lonofil Plus (IP) (Voco), were used to evaluate film thickness, compressive strength, net setting time and acid erosion. Thickness was measured three times with a digital micrometer (Digimatic Mitutoyo Corporation). Sample size was five for each cement or condition. Compressive strength (Instron 1011, crosshead speed of 1 mm/min) was evaluated after 24 h immersion in water at 37 degreesC. Sample size was five for each cement or condition. Setting time was evaluated for Ch and IP at 37 degreesC. Sample size was three for each cement or condition. Specimen moulds (30 x 30 x 5 mm) with a central perforation of 5 mm in diameter and 2 mm depth were usedfor acid erosion tests. Erosion depth was measured with a micrometer gauge with a precision of 0. 001 mm, before and after 24-hour immersion in a lactic acid-sodium lactate solution with pH 2.74 at 370C. Sample size wasfivefor each condition. Student's t test was performed with a level of significance ofp< O.05 for each material and condition tested. Arithmetic mean (Standard Deviation). Powder-liquid ratio according to manufacturers: film thickness (in pm): Ch 220 (40), IP: 382 (5); compressive strength (in MPa) at 24 hs: Ch 166.3 (16,6), IP: 100 (10); net setting time (in min.) at 370C: Ch 3.44 (0.3), IP: 5.26 (0.1) ; depth of acid erosion (in mm): Ch 0.15 (0.02), IP: 0.17 (0.02). Modified powder-liquid ratio: film thickness (in pm): Ch 23(1), IP:24(1); compressive strength at 24 hs (in MPa): Ch: 69.3 (14.6), IP: 46.5 (7.4); net setting time (in min.) at 370C: Ch 5.72 (0.1) and IP 9.38 (0.1); depth of acid erosion (in mm): Ch 0.22 (0.02). Data were not recorded for IP because the sample disintegrated in the solution. Student's t test was performed for both materials and conditions with a level of significance of p< 0.05. The difference between each condition tested was statistically significant (p<0.01). While changes in the powder-liquid ratio of a restorative glass ionomer cement can result in some of its properties having values that are not far from those required for luting cements according to ISO specifications, it did not meet the requirements for acid erosion.