IONOMER GLASSES: DESIGN AND CHARACTERIZATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 410

12.5 STRUCTURE OF IONOMER GLASSES
The glass compositions that will be described in this section are based mainly on
the system SiO 2 - Al 2 O 3 - P 2 O 5 - CaO - CaF 2 . The properties of the glasses depend
directly on the alkali content as well as the aluminium to silicon ratio and the
fl uorine content. Phosphorus also plays a role in the glass structure as the ratio of
calcium to phosphorus determines the stoichiometry of the glass referenced to
that of apatite. The structure of the glass as well as the composition, are very
important parameters for the setting in glass polyalkenoate cements.
E. De Barra, S.G. Griffi n and R.G. Hill [2, 22, 23] investigated the role of
alkali metal ions, phosphate content and Al/Si ratio on the mechanical properties
of glass polyalkenoate cements. The Al/Si ratio of the studied fl uoro - phospho -
alumino-silicate glass was not found to have a signifi cant infl uence on the proper-
ties of glass polyalkenoate cements. One explanation is that phosphorus charge
balances the aluminium within the glass network reducing the number of Al-O-Si
bonds available for acid hydrolysis. Hill et al. [22] introduced another possible
mechanism for glass hydrolysis that of P-O bonds hydrolysis, especially in the
case of glasses with high phosphorus content or in the case of glasses that have
undergone amorphous phase separation. In the case of simple alumino-silicate
glasses (Figure 12.1a) the removal of charge balancing cations after the acid
attack will lead to hydrolysis of Si-O-Al bonds in the glass network and the sub-
sequent formation of a silica gel layer around the remaining glass particles and
the release of Al 3+ and Ca 2+ available for crosslinking.
In the case of phosphorus containing alumino-silicate ionomer glasses on the
other hand, the lack of alkali earth charge balancing cations because of the pres-
ence of Al-O-P bonds in the glass network will lead only to the release of alu-
minium and phosphorous in the glass network and the formation of a depleted
glass layer instead. Hydrolysis of Si-O-Al bonds does not occur in this case. It is
important to note, that it is likely that phosphate groups would compete with
carboxylate groups for aluminium and calcium ions thereby inhibiting the cross-
linking reaction in the cement matrix (Figure 12.1b). In the case of low phospho-
rus content the setting and working times were extended, whereas at higher
content, the compressive strength and Young's modulus of the cement decreased.
Introducing an alkali ion such as sodium would generally decrease the cement
properties as sodium ions released from the glass disrupted the crosslinking reac-
tion in the polysalt matrix. However, this would also depend on the cement for-
mulation (powder to polymer liquid ratio) as well as the amount of sodium in the
glass composition.
Most interesting however, is the role of fl uorite in the glass structure and the
mechanical properties of the resulting polyalkenoate cements [1, 24, 25]. Figure
12.4 illustrates the role of fl uorine in the glass network. Whereas calcium disrupts
the glass network forming NBO's and charge balancing the charge defi cient AlO 4
tetrahedra, fl uorine replaces BO's with non-bridging fl uorine, thus facilitating
melting of the glass at lower temperature and the acid attack during the setting
reaction in the cement [12, 26, 27]. Furthermore, addition of fl uorite increases the