Fundamental studies on ionomer glasses
Composition-structure-property relationships were studied in glasses of the type used to produce glass lonomer dental cements. These materials are currently being developed as a bone cement in joint replacement surgery. Initially, a simple quaternary (25i02-Al203-CaO-CaF2) glass was produced and was found to undergo minimal fluorine loss. This results in a glass whose composition is reproducible between batches. This composition, however, was too reactive to form a glass ionomer cement. A glass-ceramic approach was pursued to deactivate the glass and following heat treatments, cements were produced whose properties varied depending upon the precise heat treatment used. The heat treatments above the glass transition temperature resulted in amorphous phase separation, probably by a spinodal decomposition mechanism, followed by a two stage crystallization of calcium fluoride (fluorite) and anorthite. The anorthite phase crystallized from fluorite nuclei and, in effect, crystallization occurred by a bulk nucleation mechanism. The glass will also undergo significant crystallization to fluorite below the glass transition temperature, with surface nucleation the dominant mechanism. This explains the deactivation of ionomer glasses used commercially to control the setting properties of dental glass ionomer cements. The structural role of fluorine in these glasses was also investigated. It seems that fluorine is present in the glass network, rather than existing as a discreet entity bonded to a modifying cation. There is evidence that the fluoride ions are bonded only to aluminium sites and act to disrupt network connectivity. A commercial glass was examined and was found to be clearly phase separated. This glass was found to crystallize to apatite and mullite. Apatite is the mineral phase of bone and enamel and this may explain the excellent biocornpatability of glass ionomer cements. Glasses containing sodium and phosphate ions have also been produced. The sodium containing glasses were all found to crystallize from the melt to calcium fluoride. The phosphate containing glasses formed cements without any subsequent modification, and following a simple heat treatment, all of the phosphate containing glasses were found again to crystallize initially to an apatite phase. Selected compositions additionally crystallized to mullite. These glass-ceramics were tougher than the base glasses and this is attributed to the presence of interlocking needles of the two phases. Some of the glasses were easily castable to produce optically clear glasses, and in addition the glasses underwent bulk nucleation via prior amorphous phase separation, to give needles of apatite and mullite. These glass-ceramics thus show tremendous potential for use as a bloactive bone substitute material or as a castable crown.