Investigation of the post-hardening reaction in glass-ionomer cements based on poly(vinyl phosphonic acid)

In this study the behaviour of two PVPA-based glass-ionomer cements was investigated. The first cement was prepared from PVPA homopolymer and glass, together with a reaction modifier. In the second cement a modified version of the polymer was used instead of the homopolymer. The modification was achieved by the treatment of the polymer with a small quantity of zinc fluoride. The effect of ageing under different conditions on the strength, mass and volume of the cements was determined. The ZnF₂-containing system behaved in a fairly straightforward manner, showing a gradual increase in strength with time (up to 3 months) that was similar to glass-ionomer cements based on poly(acrylic acid). By contrast the unmodified material did not increase in strength with time, a feature that was attributed to extensive crosslinking of the material, causing it to become more brittle and hence more sensitive to defects in the specimens. With regard to the effect on mass and volume, both types of cement behaved like typical set glass-ionomers, displaying a sensitivity to dessication but little, if any, sensitivity to aqueous media.