Studies in the setting of polyelectrolyte materials

Water-activated versions of zinc polycarboxylate and glass-polyakenoate dental cements have been used in a study of the effect of electrolytes on setting behaviour and properties. In addition to pure water, 1 m solutions of NaCl, NaF, Na₂SO₄ and NaNO₃ have been used to activate setting. The setting reaction of the zinc polycarboxylate was found to be profoundly affected by the presence of such electrolytes, with the working time generally being shorter, and the overall set sharper. The setting reaction of the glass-polyalkenoate was less affected: there was generally no change in the working time, though the set was slightly less sharp. After 24 h at 37°C most of the zinc polycarboxylates had compressive strengths of about 90 MPa, with differences between the different specimens not being statistically significant. The exception was the cement made with aqueous sodium sulphate, which had a compressive strength of 56.8 MPa. By contrast, the strengths of the various glasspolyalkenoates fell into three groups: water- and NaF-activated at 80–85 MPa, Na₂SO₄- and NaNO₃-activated, at 65 MPa, and NaCl-activated, at 53 MPa. These differences were significant at least the 99% confidence level. They are discussed in terms of the effect of the electrolytes on the conformation adopted by the reacting poly(acrylic acid) molecules in solution. It is concluded that the differences in response to the presence of sodium salts between the zinc polycarboxylate and the glass-polyalkenoate confirm the view that these cements set to give substantially different microstructures.