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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"Improving Fracture Toughness of Dental Nanocomposites by Interface Engineering and

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0State%20of%20Art%20Ferracane%202010.pdf>.

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ScienceLab.com, 21 May 2013. Web. 21 Feb. 2014.


ScienceLab.com. "N,N-Dimethylformamide MSDS." N,N-Dimethylformamide MSDS.

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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

Mo

du

lus

(MP

a)

Average Modulus For Each Sample

Modulus

0

10

20

30

40

50

60

70

80

90


Avg

Fle

xura

l Str

en

gth

(M

Pa)

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


Tou

ghn

ess

(kP

a)

Average Toughness For Each Sample

Avg Toughness

0

100

200

300

400

500

600


Har

dn

ess

(M

Pa)

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

Mo

du

lus

(MP

a)

Weight Percent of Titania

Modulus Based on Weight Percent of Titania

Series1

0

100

200

300

400

500

600

0 20 40 60 80

Har

dn

ess

(M

Pa)

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