Biomedical Engineering Reference
In-Depth Information
The polyelectrolytes used in GIC can be described as polyalkenoic acids. Originally, the liquid
was based on 40
50% aqueous solution of polyacrylic acid, but the solution was very viscous for
optimal mixing with the powdered glass, unstable and tended to gel over time, and subsequently
lowered the setting rate. Currently manufactured polyacids include the homopolymers or copoly-
mers of unsaturated mono-, di-, or tricarboxylic acids such as maleic acid and itaconic acid to
overcome the problems associated with polyacrylic acid.
During the setting reaction, the surface of glass particles release ions that cross-link the
polymer, and inorganic matrixes form as a result of this reaction. The resultant cement is a highly
complex composite including gel of calcium and aluminum polyacrylates that contains fluoride.
The unreacted portion of the glass powders act as filler for the cement. The glass powder is partly
etched by the polyacid and the outer surface is degraded to siliceous hydrogel that contains fluorite
crystallites. Silicic acid is also released which polymerizes into a silica gel. The set cement contains
many components, a hydrogel matrix, a silica gel matrix, and glass particles containing polysalt
bridges between metallic ions and carboxylate groups
[35]
.
In addition to the chemistry of the basic glass and the polyacid, other factors such as molecular
weight of acids, powder/liquid (P/L) ratio, and particle size and their distribution
[36]
control the
setting and mechanical properties of GICs. Higher molecular weight and P/L ratios increase the
setting rate and mechanical strength
[37]
. Particle size and their uniform distribution substantially
affect the microstructure of the cement and therefore their mechanical properties. It is reported that
small particles corresponded to higher strengths, and an increased proportion of larger particles
corresponded with a decrease in viscosity of the unset cement. The optimization of particle sizing
and distribution can lead to enhanced physical properties and life of restoration
[38]
. The nature of
the filler, its qualitative and quantitative analysis largely decide the mechanical properties of the
restoration material. The shape of the particles is another important aspect influencing the mechani-
cal locking of filler particles to the polymeric matrix; an irregular shape of the filler particle favors
a better physical retention in the polymeric matrix. However, irregular particles possess smaller
packing ability and therefore they cause a heterogeneous stress distribution.
5.5
Modified GIC
The conventional GICs have some clinical limitations such as prolonged setting reaction time, moisture
sensitivity during initial setting, dehydration, and rough surface texture, which can affect mechanical
resistance
[38]
. To overcome these problems, resin-modified GICs (RMGICs) were developed which
contain monomers and photo initiators
[39]
. Setting reactions is based on an acid
base reaction; in
addition, light exposure causes the creation of cross-linking between polymeric chains and polymeriza-
tion of methacrylate. Metal-reinforced GICs were introduced in 1977, and the addition of silver-
amalgam alloy powder to conventional materials increases the physical strength of the cement and pro-
vides radiopacity. They can be used to restore Class II cavities by tunnel preparation, deciduous teeth
(especially Class I), core buildups, and retrograde root filling
[40]
. Several faster-setting, high-viscosity
conventional GICs have been developed with fine glass particles, anhydrous polyacrylic acids of high
molecular weight, and a high powder-to-liquid mixing ratio
[41]
. The setting reaction is the same as the
acid
base reaction of a typical conventional GIC.
