Biomedical Engineering Reference
In-Depth Information
H
3
O
+
(a)
Ca
2+
O
Si
O
Al
O
Si(OH)
4
silica gel layer
AI
3+
, Ca
2+
cations
for crosslinking
(b)
Ca
2+
Glass
AI
3+
C
PO
4
3-
-
O
O
F
-
Ca
2+
+
H
+
O
O
-
Reacted glass with
silicious gel coating
C
C
Lonically crosslinked poly (salt)
cement matrix containing
anions and cations
HO
O
Poly (acrylic acid)
Figure 12.1.
Schematic illustration of A: acid degradation of an alumino-silicate network and
B: setting reaction in a glass ionomer cement [2].
form is usually a coarse glass frit that results from the quenching of the melt
either onto a metal plate and then into water or directly into water. The glass is
then ground further by dry milling in a ball mill or a gyro mill to a particle size less
than 45
m for a fi ne grained lutting
cement. The main criterion for the design of the glasses is their basic character
that would compensate for the low acidity of polyacrylic acid. Often, the reactiv-
ity of the glasses needs to be reduced by etching of the glass particles with 5%
aqueous acetic acid or annealing of the glass at temperatures in the range of
400 °C to 600 °C depending on the composition. There is a large number of
ionomer glass compositions that have been studied as cement formers. An impor-
tant advantage of the ionomer glasses is that under appropriate heat treatments,
they crystallise into an apatite phase that makes the glass-ceramics biocompatible
and promising for use in restoring and replacing hard tissues in orthopaedic and
dental fi elds. Hill et al. developed a series of glasses that crystallise into needle-
like fl uorapatite and mullite phases that interlock with each other giving rise to
high fracture toughness values [10, 11].
This chapter aims to give an overview of the structure of ionomer glasses
starting from the design of the glasses and fi nishing with the characterisation
μ
m for a fi lling grade cement or less than 15
μ
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