E03.
Filler particles, used in dental composites, can be made from different
components. Discuss different filler manufacturing methods, and how these
differences affect the total filler surface per gram filler.
Filler
particles can be made in three different ways. These three methods are:
Grinding
quartz or glass to particle diameters around 1-5 um.
Burning
silicon-tetrachloride in oxygen and hydrogen to form SiO2 particles and HCl.
These particles are around 0.02-0.04 um in diameter.
Use
a sol-gel process to produce a silica gel with particles often around 1 um.
The
smaller and rougher the particles are, the larger filler surface area per gram
filler. This is important to realize, because filler surfaces tie up monomer
molecules. In other words, the larger total filler surface area, the more
immobilized monomer. Since dentists want to work with a certain viscosity, the
only way to decrease the viscosity is thus to use resins with lower viscosity or
larger volume fractions resin. From the above we can conclude that composites
containing the smallest filler particles often contain most resin. When the
particle size exceeds 0.5 um, the effect of filler particle size on the filler
surface area per gram filler is no longer as significant as it is for smaller
filler particles. In other words, the ground or sol-gel processed fillers can
produce composites of similar volume fraction fillers.
Ground
Particles
Grinding
quartz or different glasses makes these particles. The filler particles that
were used in the first dental composites had diameters ranging from 20-40 um and
consisted most often of quartz. The drawback with quartz was its high hardness
(difficult to grind into fine particles) and its lack of x-ray contrast. By
using Ba and/or Sr glass, the filler particles became x-ray opaque and easier to
grind. Today some composites contain ground particles (glass as well as silica)
that are in the one-micron particle size range.
Pyrogenic
Silica Particles
Pyrogenic
silica - used as filler in microfilled composites. The filler is called
pyrogenic silica because burning SiCl4, H2 and O2 makes it. During the reaction
that occurs in the flame, HCl is formed as well as SiO2. The SiO2 that forms
form small silica spheres (200 - 400 Å). One gram of this filler has an
enormous surface area (several hundred square meters).
The
above picture was taken with a transmission electron microscope.
Sol-Gel
Processed Filler Particles
Techniques have also been developed with which porous silica particles are made through a precipitation reaction. These particles are often in the one-micron range. The advantages with these particles are their surface roughness (good mechanical bonding) and their easiness to cut and polish, while their drawbacks include light scattering (poor light penetration) and weakness. Regarding the latter drawbacks, some manufacturers have succeeded to develop sol-gel particles with rather good light penetration and strength
