Nathan Cloeter 499 Final Report

Nathan Cloeter
ENMA 499
Final Report
Due 5-19-2014
Maximizing Ceramic Filler in a Composite with a Polymer Matrix
Abstract
One of the problems that many people face throughout their life is when they go to the
dentist and have to get a cavity filled. There are a wide variety of choices out there today, but
each one of them has a downside to them. Metallic fillings have favorable mechanical qualities,
and can last for as long as fifteen years, but do not have the same color as teeth, and can
therefore be unsightly. Ceramic fillings work as well as metallic fillings, and look the part the as
well. However, ceramic fillings are also highly expensive, and are therefore not a viable option
for anyone who is on a budget. The final option to consider is a composite filling. Composite
fillings, like ceramics, resemble teeth in color and texture. They bond to teeth and are extremely
versatile. Most composite fillings that exist today are expensive and do last for a long period of
time (WebMD.com). However, the versatility of composite samples gives us a chance for further
research. More combinations and materials exist for composites to be tested with, and can give
people the balance between cost, performance, and aesthetics. A good combination of polymer
and matrix already exists from earlier research. However, we cannot get the weight percent of
the filler to be high enough. As a result, the composite still suffers from a lack of durability, and
therefore is not yet a useful solution. This paper looks into two different possibilities to increase
the weight percent of the filler, and some of the early results that have been extracted by these
methods. A change in the traditional polymer matrix, and the introduction of a solvent as a
diluent can help lower the viscosity of the composite before it cures, and theoretically allow us to
increase the weight percent of the ceramic filler and allow us to come one step closer to a dental
filling that can perform just as well as teeth while looking the part.
Introduction
Previous works for composite dental fillings usually revolve around using a polymer
resin as the matrix, and a ceramic nanopowder as the filler. The resin is usually the cause of
failure in the composite samples, and needs to be optimized in order to receive the best results

possible. The reason behind the resin being the point of failure is that the filler particles are
harder than the resin. This means that most of the stress is transmitted through the particles and
into the resin (Chen, Qi et al). This requires the resin to be optimized, and one of the best ways to
do this is by choosing the right filler. A good filler interacts with the matrix and disperses
throughout it in as uniform of a fashion as possible. Previous works by this research group had
alumina being used as the filler, with the matrix being a half and half mixture of Bis-
GMA/TEGDMA (Wang et al). Bis-GMA is a monomer that has been used in resin for dental
fillings for as far back as the 1960’s. It can create a very sturdy resin, and is a commonly formed
monomer (Ferracane). However, Bis-GMA is incredibly viscous, and requires a diluent
monomer like tri (ethylene glycol) dimethacrylate, or TEGDMA, to be added to reduce the
viscosity (Chen, Qi et al). This is especially important when adding the filler to the composite,
because a less viscous solution is easier to add powder into, and can usually result in a higher
amount of powder being allowed to mix in. The alumina filler and Bis-GMA/TEGDMA matrix
created a favorable mechanical interaction, with the samples that contained sixty weight percent
alumina being able to reach a modulus as high as thirteen GPa, and the Hardness values reaching
as high as six hundred MPa (Wang et al). However, one common issue arose from all of the
samples. The composites that resulted from the Bis-GMA/TEGMA matrix and alumina filler
were all colored gray. The resulting color meant that the samples could not be a realistic solution
because they did not have the proper aesthetics that was required for the samples. This meant
that different fillers would need to be selected for the purposes of this project.
Materials Used
Titanium oxide, or titania, is a ceramic that has properties that are similar to alumina.
Titania is commonly used as a dye in white paints, and was therefore thought of as a reliable
means to keep the resulting composite samples white. However, the mechanical properties of
titania are not as favorable as alumina, and therefore, a higher percentage of titania needs to be
added to the composites in order to replicate the results that the alumina composites had attained.
However, after a certain amount of titania is added in, the composite can become difficult to
work with, and as a result can become overly chalky. This means that having too high of a
weight percent of titania in the composite can actually lower the mechanical properties of the
resulting composite. As a result, an additive or a change in the polymer matrix is required in

order to keep the weight percentage of the filler at a higher level without the sample falling apart.
While we are adding a diluent to some of the samples to achieve this goal, we are also changing
up the polymers that are in the matrix to observe the changes this can have in the material as
well. Instead of using a matrix that is Bis-GMA/TEGMA, we are replacing the Bis-GMA with
Benzoyl peroxide, or BPO. BPO is an organic substance that has a multitude of uses, however
most use it as a radical initiator for polymerization, as is the case here as it is combined with
TEGMA. It is also a bleaching agent and is sometimes used to whiten teeth (NIH). One of the
issues with titania is that it does not have the satisfactory mechanical properties that were
exhibited by alumina, and as a result composites require a higher weight percent of titania filler
to be added to reflect the superior properties seen before. However, when mechanically mixing
in any titania filler above fifty percent the composite becomes increasingly difficult to work
with, and the resulting samples become chalky. These chalky bars are not stable and result in the
mechanical properties lowering as the weight percent of filler rises. We are doing two changes so
that we can continue to raise the weight percentage to a higher level. We changed the polymer
matrix, and now have the BisGMA mix with benzoyl peroxide, or BPO. Also, we are adding in a
solvent as a diluent. Adding the solvent and changing the diluent in the polymer matrix are both
efforts at changing the viscosity of the composite samples so that a higher amount of filler can be
added to them. Some of the factors that went into choosing solvents to use in the samples
included their flash points, boiling points, polarities, and what kind of health hazards they
presented. The solvent needs to be removed before the dental filling could be added into their
cavities in order to avoid subjecting any potential patients to hazardous conditions. Also, the
solvent needs to be removed before the sample is subjected to excess light or heat so that it does
not create bubbles in the mold while the resin polymerizes. The solvent is removed by placing
the composite into a vacuum oven at room temperature and blocking all light from entering into
the oven for a lengthy period of time. This gives the solvent a chance to evaporate from the
sample. In order for this to be achieved however, the solvent needs to have a flash point that is
close to or below room temperature. Another property that is important to factor in is the polarity
of the solvent. Titanium oxide is a highly polar powder, and as a result requires a polar solvent to
break it down. These are the two primary factors that go into the choice of the solvent. A
comparison of these can be found at Table 1. Out of the six that are left, the three most suitable
solvents were found to be Acetone, Dimethylformamide (DMF), and Tetrahydrofuran (THF).

These solvents were found to have a good balance of the properties we are looking for, and are
currently being used in the production of composites. However, due to time constraints, we were
limited to only using DMF and Acetone for solvents. If research needs to go into a new direction
in the future, we may expand into THF, Dimethyacetamide (DMAC), Dimethyl sulfoxide
(DMSO), and/or cyclohexane.
Devices Used
One of the best ways to measure the mechanical properties of the composites is to test
them with a three point bend test. The three point bend test gives the flexural modulus of the
samples. The flexural modulus is an important property to measure for these samples because of
the chewing motion that teeth continually undergo. When food is chewed inside of the mouth,
the teeth flex and bend in a cyclical fashion. The samples will be put into the three point bend
test and undergo stress until the point of failure by fracture (Udomphol). From there, samples are
to be tested for hardness. The hardness values can be found by means of a Vickers Hardness
Test. The Vickers Hardness Test is performed by indenting the sample at an angle of 136 degrees
between opposite faces subjected to a load of 1 to 100 kgf for a period of 10 to 15 seconds
(England). The hardness value is an important value to determine because it tells us how well the
filling will react to sudden impacts instead of those that are carried out over an extended period
of time. In the past, the hardness tests were carried out with samples that were broken by the
three point bend test. This is a precedent that is likely to be continued in order to save the amount
of bars that are required to be made for each sample set.
Procedure
The procedure begins with the preparation of the nanopowders. This is done by
taking bulk groups of nanopowder and silinizing them through mechanical stirring. After the
stirring is complete the powder is put through a centrifuge while being suspended in ethanol to
remove any potential contaminants that may be in the powder. After the contaminents and
ethanol from the nanopowder, it is left in a fume hood for any ethanol that remains to evaporate,
and is then mechanically ground to as fine of a size as possible. While the powder is evaporating
the polymer matrix is synthesized. After the matrix is synthesized and we have a prepared
powder, we have the initial opportunity to add a solvent to the matrix to lower the initial

viscosity. If we choose to add in a solvent, 3 mL of solvent is initially added to the matrix. From
there, the nanopowder is gradually added in in portions and is mechanically stirred as it is added
in. This is a process that repeats until we either run out of powder, or the sample cannot be
stirred any further. If we run into the second scenario, and wish to add in more powder, we have
the choice of adding in additional solvent if we so desire. The solvent is usually added in
portions of 2mL. Additional solvent is added in until we have reached the desired weight percent
of filler, which was calculated beforehand. If solvent is added to the composite, we place the
composite into the vacuum oven, which is set to a pressure of 20 mg of Hg, and sits overnight.
We keep the oven at room temperature to prevent the matrix from hardening the solution into the
final samples, so that any potential harmful solvents are able to completely evaporate. Once the
solvent has evaporate, if there was any to begin with, we fit the samples into molds, and placed
them into an oven so they can undergo their final transformation into the desired composites, and
are removed from the molds. Once the samples are removed from the molds they are polished to
a desired level of smoothness. The three point bend test requires the samples to be completely
smooth, and polishing them takes a significant amount of time. The samples have to be polished
in 400, 600, 6 micron, 3 micron, and then 1 micron grits respectively on all four sides before they
can be tested. The 400 and 600 grits are run for a cycle periods of thirty seconds, with the micron
scale grits running for a minute. After that cycle is complete we observe the sample under a
microscope and check to see how smooth it is. Once the side reached a sufficient level of
smoothness, the sandpaper was switched out to the next grit, and the process repeated. While this
sounds like a menial task, the three point bend test is most likely to fail in areas that aren’t
smooth, so removing these surfaces allows for a truer test to take place, with better datasets.
From there the samples undergo a three point bend test to get their modulus, flexural
strength, and toughness. This is done by placing them under stress until they reach the point of
fracture. We take the stress and strain values that are found by the testing apparatus, and use
them to get the desired values. One final test that we carried out was finding the hardness of the
samples. This was found with a Vickers Hardness Test, and was carried out with the samples that
were broken from the three point bend test. Once this is completed, the samples and their data
are placed into a database that we can use to determine the next step in our research.
Results

Five sets were sufficiently prepared for testing purposes. These samples differ in several
different categories, such weight percent and diluent, and provide a good roadmap to see how
each change can affect the values of the composite. The samples that were finished were called
RET-27, RET-31, RET-33, RET-34, and RET-35. Their compositions, diluent, and mechanical
properties are listed below as Table 2. RET-27 was a sample that was made before the semester
began, and was good for the purposes of being a control sample. Its weight is comprised of 61%
titania, with the other 39% going to the matrix. Since this was made last semester, the matrix is
still a mixture of BisGMA-TEGDMA, and there is no diluent that was added to it. It had an
average 3PB Modulus of 5.8 MPa, a flexural strength at 48.98 MPa, a toughness of 232 kPa, and
a hardness of 488.8 MPa. RET-31 was a sample that was an attempt at answering the question
about what would happen if we added alumina, titania, and silica together as a filler. The result
did not quite go as well as we would have hoped. All of the mechanical properties were below
the standard set by RET-27, and we could barely get enough of a sample together to test after
polishing. The hardness of the sample was so low that most of it wore away during polishing,
and we had to cut the duration of the cycles short so that we could have some sample left to test.
We used DMF as a diluent, and while the filler was comparable to the rest of the samples at a
little less than 60 weight percent, the amount and types of powder that was added in for each
type affected the composite in a negative way. Out of the fillers that was added, 13.04% of the
filler was titania, 82.46% was alumina, and 4.50% was silica. The flexural modulus was the
lowest out of all five samples, at 5.7 MPa. The flexural strength was also lower than the control
at 47.57 MPa, along with the toughness at 211 kPa. The hardness was by and far the lowest out
of all the samples that were measured at an average value of 257.4 MPa. One of the hopes was
that maybe the color from the titania and silica powders would affect the color of the alumina in
the composite and cause the sample to turn white, but this goal also fell short with RET-31. The
composite came out speckled with different spots and streaks of gray and white throughout the
sample. This sample was more of a guide of what not to do instead of a guide of what to do.
Things started to improve with RET-33 though. RET-33 used the new matrix combination of
TEGMA and BPO, and was comprised of 40 weight percent matrix, and 60 weight percent
titania, with no diluent added to increase the filler percent. It had the highest modulus at 7.1
MPa, and hardness at 568 MPa. However, it suffered in terms of Flexural Strength and
Toughness, bringing in values of 36.22 MPa and 81 kPa respectively, the lowest values out of all

five samples. This makes sense in some senses though. The material that has the highest modulus
will also be the stiffest, and therefore will flex the least amount. The second to last sample that
was tested was RET-34. RET-34, again, had no solvent. However the matrix was also changed
up in a different way. The matrix was comprised of 60.54% BisGMA, 2.95% BPO, and 36.51%
TEGDMA. The weight of the sample was comprised of 45% of the matrix, and 55% titania. The
modulus came in at 6.1 MPa, with the flexural strength having the highest average value at 81.61
MPa. It also had the highest toughness values at 557 kPa, and an above average hardness at
531.7 MPa. The last sample that we were able to measure this semester was RET-35. This has
had the highest percentage of filler that we have been able to fabricate without the sample falling
apart at 72 weight percent titania. This was achieved through using acetone as a diluent and the
TEGDMA/BPO mixture as the polymer matrix. It did have the highest modulus at 6.9 MPa, and
barely lost out on having the highest flexural strength at 81.53 MPa. It also had the second
highest toughness at 503 kPa, but lacked in hardness with an average value of 422.8 MPa.
Unfortunately, we did not have enough time to perform tests with the TGA. The machine needed
to be repaired and was being used by other groups and for previous samples when it was
working. This means that we are missing out on several important groups of data, such as open
porosity and percent composition. Also, the samples that were supposed to be aged are not ready
for testing, so that means that that section of data will also have to wait for a later date.
Analysis
With the exception of RET-31 and the flexural strength/toughness values for RET-33,
adding diluents and changing the matrix each resulted in a general rise in all of the measured
mechanical properties for the solvents. Lowering the viscosity of the composite before it cures
by changing the matrix or adding a diluent allowed us to raise the weight percent of our samples,
and therefore brought about an increase of mechanical properties. Figures I, II, III, and IV
respectively give us the modulus, flexural strength, toughness, and hardness for each of the five
samples that were ready to be tested. Samples 33, 34, and 35 all had their strong points and areas
that need to be addressed. RET-33 had the best modulus and hardness out of all five samples, but
had the worst toughness and flexural strength. This states that it is extremely strong, but brittle at
the same time. This would not be suitable for dental applications due to the continual fatigue that
is placed on teeth from chewing multiple meals a day. While RET-34 did have the best

toughness and second best hardness, it did not do as impressively as RET-33 or RET-35 overall.
RET-35, while not having the highest values in any area besides weight percent of filler, had the
best overall performance. It did not come in last place in any category, and while it did not have
any of the highest values, was barely behind first place in modulus, flexural strength, and
toughness. When comparing the samples by weight percent of titania they contained, an almost
linear trend line can be seen when comparing the weight percent to the elastic modulus. This
supports our theory of being able to have higher levels of strength while increasing the weight
percent of filler, or in this case titania. The results for this can be seen in Figure V. Figure VI
make this same comparison with hardness. It is important to see that after sixty weight percent a
drop off starts to occur in hardness. This is concerning, mostly because the modulus increases
while the hardness decreases. Since the hardness tests are performed on the surface of the
samples, while the bend test to find the modulus tests the sample throughout, this means that the
surface of the samples starts to get compromised after sixty weight percent of titania. This could
be explained by the structure becoming too saturated with filler. The amount of filler that is
being added to the matrix could be getting too close to the atomic packing factor of the structure
that the composite forms. As a result the composite cannot hold onto the filler, and some it could
start to come off of the structure to make it more stable. This can be countered by obtaining
smaller nanoparticles. Smaller nanoparticles would allow for us to pack more ceramic into the
crevices of the structure that is made by the polymer matrix. This would allow for us to continue
increasing the size of the weight percent of filler in the composites without worrying about the
structural integrity of the samples.
One of the questions we have wondered is what would happen if we used more than one
ceramic for the filler. If we combined more than one ceramic in the filler, the results could create
a best-case scenario that has a sample with the best mechanical properties while maintaining a
proper color. This was the hope when RET-31 was made, and it unfortunately fell short of our
goals. It did worse than the control in every single category, and had the worst modulus and
hardness out of every single sample that was tested. This was despite changing the matrix and
using a diluent to maximize the amount of filler that was added to the sample. It also did not
have the proper color, having non-uniform speckled dots of gray and white through the sample.
As stated earlier, the best composites would optimize the resin and evenly distribute the loads
throughout the sample. This was not the case, instead of the different ceramics complementing

each other, they had an inverse effect. The ceramics were interfering with one another, and
creating internal stresses on the system. This means that it could not take large stresses, due to
the internal stress it already put on itself, and was doomed to fail from the beginning.
One thing that is important to point out is that testing is not over yet for these samples.
We still have to run TGA and accelerated aging tests on the samples. Thermogravimetric
analysis, or TGA, measures how much weight is either lost or gained as a result of temperature,
atmosphere, and time. These properties can be manipulated to give a variety of properties of a
polymer, and for the composite sample that utilizes a polymer matrix. These properties include
decomposition temperature, thermal/oxidative stability, unbound water/solvent, and inversely
how much water/solvent is bound to the sample. It can also give how much moisture is adsorbed
by the sample (Taipei Tech). This moisture adsorption can also be manipulated to give the
porosity of the sample. The importance of this cannot be understated, especially in terms of how
the sample interacts with moisture. If a sample has too high of a porosity, than it can swell in a
moist environment, such as the inside of a mouth. This swelling can result in a great amount of
pain for the user, and will eventually break down the area that the filling was placed into at a
much faster rate. Another area where swelling can come into place is through accelerated aging.
To mimic the effects that liquids and saliva have on the samples, we place the samples in water
and keep them there for an extended period of time to mimic the aging process that samples
would go through. After this aging process is carried out we would polish the samples and repeat
the tests that we did for the samples that came right out of the oven. This would give us a chance
to see how the environment would affect out samples and see if the samples are still suitable
after this period of time. This is important to carry out after our initial tests, because we don’t
want to waste the time on samples that underperformed on the first tests. Both TGA and
accelerated aging are planned on these samples for the future, and can hopefully answer more
questions than they create.
Conclusion
This semester was focused on lowering the viscosity of the polymer matrix to allow for
more ceramic filler to be added to the solution. These tests were proven to be a success, and give
a roadmap on where to take our research for the future. Increasing the weight percent of titania

filler in a composite allows us to improve the mechanical properties, and takes us one step closer
to designing an affordable composite that lasts a long time while being aesthetically pleasing.
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Figures

Figure I: Average Modulus For Each Sample
Figure II: Average Flexural Strength For Each Sample
Figure III: Average Toughness For Each Sample
0
1
2
3
4
5
6
7
8
RET-27 RET-31 RET-33 RET-34 RET-35
Modulus(MPa)
Average Modulus For Each Sample
Modulus
0
10
20
30
40
50
60
70
80
90
AvgFlexuralStrength(MPa)
Average Flexural Strength For Each
Sample
Toughness

Figure IV: Average Hardness For Each Sample
Figure V: Modulus Based on Weight Percent of Titania
0
100
200
300
400
500
600
Toughness(kPa) Average Toughness For Each Sample
Avg Toughness
0
100
200
300
400
500
600
Hardness(MPa)
Average Hardness For Each Sample
Hardness

*RET-31 was not purely titania, and as a result was kept off this chart.
Figure VI: Hardness Based on Weight Percent of Titania
*RET-31 was not purely titania, and as a result was kept off this chart.
Tables
Table 1: Properties of Potential Solvents (ScienceLab.com, LSU)
Solvent Abbreviatio
n
Chemical
Formula
Flash
Point
MP
(Degr
BP
(Degr
Polar
ity
Hazard
ous To
Iritant
To
0
1
2
3
4
5
6
7
8
0 20 40 60 80
Modulus(MPa)
Weight Percent of Titania
Modulus Based on Weight Percent of
Titania
Series1
0
100
200
300
400
500
600
0 20 40 60 80
Hardness(MPa)
Weight Percent of Titania
Hardness Based on Weight Percent of
Titania
Series1

(Degr
ees
C)
ees
C)
ees
C)
Acetone N/A
C3H6O -17 -93 56 5.1
Skin
Contac
t
(Slight
ly)
Skin
Conta
ct, Eye
Conta
ct,
ingesti
on,
inhalat
ion
Cyclohex
ane
N/A
C6-H12 -20 6.47 80.74 0.2
Skin
Contac
t
(Slight
ly),
ingesti
on,
inhalat
ion
eye
contac
t, skin
contac
t
Dimethyl
sulfoxide
DMSO
(CH3)2S
O 89 19 189 7.2
ingesti
on
Eyes,
skin,
inhalat
ion
Dimethyl
acetamid
e
DMAC
CH3C(O)
N(CH3)2 63 -20 165.1 6.5
ingesti
on
Eyes,
skin,
inhalat
ion
Dimethyl
formamid
e
DMF
(CH3)2N
C(O)H 58 -60.5 152 6.4
Skin
contact
,
ingesti
on,
inhalat
ion
eyes,
skin
Tetrahydr
ofuran
THF
(CH2)4O -14 -108 66 4
Ingesti
on,
inhalat
ion
eye
contac
t, skin
contac
t
Water N/A
H20 N/A 0 100 10.2 N/A N/A
Table 2: Average Values for Each of the Samples

Sampl
e
Name
Weigh
t
Percen
t of
Filler
Matrix Solven
t
Averag
e
Modulu
s (MPa)
Averag
e
Flexura
l
Strengt
h
(MPa)
Average
Toughne
ss (kPa)
Averag
e
Hardnes
s (MPa)
RET-
27
61% Bis-
GMA/TEGMA
None 5.83 48.98 232 488.8
RET-
31
60% * BPO/TEGMA DMF 5.73 47.57 210.8 257.4
RET-
33
60% BPO/TEGMA None 7.11 36.22 80.78 568
RET-
34
55% Bis-
GMA/BPO/TEG
MA
None 6.12 81.61 556.7 531.7
RET-
35
72% BPO/TEGMA Aceton
e
6.86 81.53 503.3 422.8
*Filler for RET-31 was made up of 13.04% titania, 4.5% silica, and 82.46% alumina

Nathan Cloeter 499 Final Report

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