Biomedical Engineering Reference
In-Depth Information
the setting of which is based on an acid-base reaction. The acid attacks and
degrades the glass structure, releasing metal cations that cross-link the carboxyl
groups on the polyacrylic acid chains, forming a cement with a microstructure
consisting of reacted and unreacted glass particles that are embedded in a poly-
salt matrix. Although the main application of glass polyalkenoate cements is in
dentistry, these cements may have potential as bone cements eliminating a lot of
problems associated with conventional acrylic cements. The development of
ionomer glasses is therefore an important part of current research in this area as
the structure and the properties of the glass would infl uence the application range
of glass polyalkenoate cements. There are several types of ionomer glasses that
have been developed since the discovery of glass polyalkenoate cements by
Wilson and Kent in the late 1960's. The most common type of ionomer glasses is
fl uorine containing aluminosilicate glasses.
This chapter will give an overview of what has been achieved until today in
this area and will focus on the design and structural characterisation of fl uorine
containing alumino-phospho-silicate ionomer glasses. The chapter starts with
an introduction to the subject, which is followed by a brief description of
Zachariasen's Random Network Theory and Lowenstein's glass formation rules
based on which the ionomer glasses were designed. The chapter continues with a
review on the structural characterisation including original results from the
author's own studies the last four years based on advanced multinuclear solid
state MAS-NMR spectroscopy. The chapter closes with a reference to the crystal-
lisation behaviour of ionomer glasses and recent work on the cation substitution
and its effect on the structure of glasses.
12.2 INTRODUCTION
Glass ionomer cement or glass poly-alkenoate cement is a dental cement pro-
duced by mixing a powder prepared from a calcium alumino-silicate glass with
an aqueous solution of poly-acrylic acid. The calcium alumino-silicate glass that
usually contains fl uoride ions is called an
ionomer
glass. Glasses for this applica-
tion generally contain 20 - 36 wt.% SiO
2
, 15 - 40% Al
2
O
3
, 0 - 35% CaO, 0 - 10%
AlPO
4
, 0 - 40% CaF
2
, 0 - 5% Na
3
AlF
6
and 0 - 6% AlF
3
[1]. During the setting reac-
tion of the cement, the ionomer glass provides the ions that leach from the glass
as a result of glass surface dissolution under the polymeric acid attack that leads
eventually to the polymer chains cross-linking as shown in Figure 12.1 [2].
Glass ionomer cements were fi rst described by Alan Wilson and Brian Kent
in 1971 and presented a natural extension to the zinc polycarboxylate cements
that became available in the late 1960s [3-8]. The fi rst commercial dental cements
were launched in 1975. Their mechanical properties were far more inferior com-
pared to the materials available today [8, 9]. The glass component in dental
cements is of an alumino-silicate type containing Ca and fl uoride ions. The glasses
can be made by mixing the appropriate oxides and fusion of ingredients in the
temperature range of 1200 °C to 1590 °C, depending on the composition. The fi nal
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