Chemical and structural changes in zinc polycarboxylate cements after immersion in dilute organic acid solutions

The behaviour of zinc polycarboxylate cements in contact with dilute aqueous solutions of organic acids at concentrations close to those existing in buccal medium, was studied. The organic acids were acetic, citric, tartaric and lactic acids, at 0.01 M and 0.001 M. The elution of zinc and magnesium was 10-1000 times greater in acid than in pure water, and correlated with the concentrations and the dissociation constants, pK1, of the acids tested. In all cases, important water losses were observed. In the 0.01 M acids, the cement structure collapsed to form a viscous, compact and homogeneous layer on the cement surface. In this layer, the polymeric carboxylic chains were regenerated from the zinc and magnesium polycarboxylate cement. Comparison with pure water showed that even the smallest concentration of the weak acids greatly modified the cement behaviour. This could explain the well-known differences in erosion processes between theoretical erosion predicted by standard specification tests and the in vivo situation.