SHEAR CHAR STRENGTH OF TPUNS

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SHEAR CHAR STRENGTH OF THERMOPLASTIC POLYURETHANE

ELASTOMER NANOCOMPOSITES

Jake A. Lewis, Manuel H. Jaramillo, and Joseph H. Koo*

The University of Texas at Austin, Dept. of Mechanical Engineering, 204 E. Dean Keeton St, C2200,

Austin, TX 78712, USA

*[email protected]

ABSTRACT

Ablative materials, such as thermoplastic elastomer nanocomposites (TPUNs) are used as internal

insulation materials for solid rocket motors (SRMs). These TPUNs are thermoplastic elastomer

reinforced by nanoclays, multi-walled carbon nanotubes, and carbon nanofibers. When these

TPUN materials are exposed to an extreme heat flux, a char layer forms along the surface as it

ablates. This research includes the study of testing procedures for evaluating the shear strength of

this char layer, a characteristic that is important for aerospace and defense applications. The

designated testing procedure will incorporate aspects of previous projects that determined the shear

strengths of several types of TPUNs through the use of our Shear Strength Sensor. This sensor

contains a method for applying traverse loading on a sample, while measuring the reaction force

and the induced strain. This sensor will be used on several types of TPUN test specimens to collect

data until each sample ruptures. In order to determine which ablative exhibited the best

performance, the energy of destruction or the energy of dissipation will be evaluated. The

maximum force will also be accounted for as a secondary quantity for determining the char shear

strength. The proposed test method is fully automated to ensure that each test is repeatable. This

guarantees reliability when collecting the data and eliminates the potential for human error. The

char layer thickness varies depending on the material. Thus, a method for distinguishing the virgin

material from the char will be explored.

1. INTRODUCTION

1.1 Context of Research

Polymeric ablative materials (PAMs) are critical components in the aerospace industry. Ablative

materials are specifically designed for thermal protection against extreme heat fluxes and high

shear forces. The defense mechanism of the ablator stems from the formation of char when subject

to the aforementioned environment. This exposure causes the material to decompose, where it

releases pyrolysis gases and produces a burned carbonaceous residue. This residue is known as the

char layer. The char structure limits the total heat transfer to the remaining virgin material by heat

conduction. With only one mode of heat transfer, the thermal insulation property of the ablative

material significantly improves, which in turn protects the underlying substrate. Therefore, how

well an ablative material protects is directly correlated to the strength of the char layer.

The purpose of this research is to study the feasibility of potential ablative materials for use as

thermal insulation for solid rocket motors (SRMs). The state-of-the-art (SOAT) ablative used for

such application is ethylene propylene diene monomer rubber (EPDM) reinforced with Kevlar®.

With its chemical stability, low thermal conductivity, high ablation resistance, and high physical

adhesion, its performance at protecting against combustion gases is above other ablatives.

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However, previous research has shown potential for using thermoplastic polyurethane elastomer

nanocomposite (TPUN) as an alternative for SRM insulation [1-5]. Not only do these ablatives

form a thick char layer when exposed to a hyperthermal environment, they are also easily

fabricated. A proper method for assessing a material’s potential is evaluating the strength of the

char. Accordingly, samples of TPUN-5 wt% carbon nanofibers (CNF), TPUN-5, 7.5, and 10 wt%,

multi-walled carbon nanotube (MWNT), and EPDM reinforced with Kevlar were exposed to a

well-controlled lab device to simulate the extreme environments [6], then the shear strength tested

to evaluate each material’s char strength.

1.2 Oxy-acetylene Test Bed (OTB)

The OTB was utilized to produce an intense hyperthermal environment for testing ablatives [6].

The device has been used extensively to test ablatives for gathering data of their thermophysical

properties. It is capable of producing heat fluxes above 1,100 W/cm2 and temperatures well above

2000°C. Since the heat flux decreases with the distance from the torch, the OTB was calibrated

using a copper slug. In knowing the slug’s mass, dimension, and specific heat, a calibration curve

was created by testing the sample at multiple distances. The resulting calibration curve allows the

user to place a test sample at the location of the desired heat flux. As for testing TPUN material,

each sample was placed at a torch distance of 12.55mm and a heat flux of 600W/cm2, an

environment quite similar to the conditions of the ablative liner inside of solid rocket motors

(SRMs). Each test was exposed to the flame for a duration of 25 seconds.

1.3 Advanced Diagnostic Equipment

As a way to measure the surface temperature of each sample during ablation testing, the research

team included a two color infra-red (IR) IMPAC ISQ 5 Pyrometer manufactured by LumaSense

Technologies, Germany (Figure 1). To guarantee that the instrument is in fact observing the

sample, a laser pointer affixed to the pyrometer points to the precise location of measurement.

Each measurement is converted into an electric signal, which is then processed and converted into

a temperature value.

Figure 1: (a) IMPAC ISQ 5 Pyrometer; and (b) DALSA Falcon series HD camera used.

To further take advantage of the capabilities of our Polymer Nanocomposite Technology Lab, the

high definition (HD) video camera was used to monitor the surface behavior of a sample during

ablation testing (Figure 1). By doing so, we can optically study the deformation of each sample.

At 15 frames per second, the DALSA Falcon series HD camera captures varies moments of an

ablation test at a pixel resolution of 2048x2048. To account for the brightness of the torch – which

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would oversaturate the image – the HD camera is equipped with an adjustable aperture and an

optical filter.

2. EXPERIMENTAL

2.1 Sample Preparation

This study involves the use of a previously built compressive strength sensor [5] and shear strength

sensor [7] at UT Austin. A sketch of the sensor is presented in figure 2. The majority of the body

of the sensor consists of a linear actuator, the driving force of the machine. The actuator’s motion

is regulated by an H-bridge with a LabVIEW VI, where the user can set Boolean signals, the duty

cycle, and the frequency sent to the H-bridge during testing. By having the speed and the power

of the actuator adjustable, the sensor can test a wide range of ablatives of varying strengths.

Figure 2: Illustration of the set up for the Shear Strength Sensor.

The tester functions by applying a traverse load on the char of the test specimen while the sample

is clamped still. As the probe tip pushes against the sample, the resulting deflection of the ablative

causes a reaction force on the sensor. Throughout this process, the sensor is sending voltage signals

to the labView VI where each signal is converted into a reaction force value (further discussed in

the next section). The reaction forces are measured until the sample fractures. The generated force

and distance values are stored in a text file in the folder of the user’s choice. These values are used

to plot a force versus distance graph, which is used to study the strength of the charred material.

2.2 Calibration of Shear Strength Sensor

In order to collect accurate data from using the sensor, the potentiometer and force sensor must be

calibrated. The potentiometer is calibrated to find the relationship between the actuator’s probe tip

position and the voltage across the potentiometer. This is accomplished by measuring the distance

the probe tip traveled via calipers and measuring the voltage across the potentiometer with a

multimeter. The force sensor - placed inside the probe tip - is calibrated to find the relationship

between the reaction force against the probe and the voltage signal from the sensor. This calibration

is performed with an apparatus consisting of four identical metal springs sandwiched in parallel

between two metal platforms. When the probe presses down on this apparatus, the springs

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compress, and the force sensor sends a voltage signal to the LabVIEW control program. By

knowing the equivalent spring constant of the four springs and the displaced distance of the pressed

metal platform, the reaction force from the springs and the voltage signal from the sensor are

known. This calibration data is used to develop a unique equation for converting each voltage

signal to a reaction force.

2.3 Analysis Method

With the Shear Strength Sensor, we can test and define the shear strength of the char of various

samples. This was accomplished by developing two separate methods for evaluating strength when

given force and distance data. During a shear test, the sample yields - or plastically deforms - when

nearing its breaking point. At this point, the sample’s resistance to the applied load increases to a

peak before significantly decreasing, indicating that the ultimate shear strength has been reached.

It is at this moment that the sample experiences its maximum allowable stress. We ranked the

strength of the different samples based on their ultimate stress as the first method of evaluation.

Even though each sample has the same dimensions, the distance needed to fracture the char varies

per sample. This is evident in the results shown later in this paper. Accordingly, it is necessary to

evaluate the total energy needed to fracture each sample. For example, if two different materials

showed the same maximum reaction force, the sample that prevented fracture at the furthest

distance has the greater shear strength. As a result, the work needed for the probe to fracture a

sample was the second method used for determining shear strength. Work is defined by the formula

displayed below:

W = F*D [1]

Work can be calculated after each force-distance measurement. After the test, the area under the

force versus distance plot represents the energy needed to fracture the sample. The area can be

calculated by summing up the values of each incremental force multiplied by its respective

distance.

3. RESULTS AND DISCUSSION

The TPUN and Kevlar EPDM samples were cut into 10x10x50 mm and burned using the OTB to

generate char. Three samples of each formulation were tested to ensure and verify correct data. All

the materials were exposed to a heat flux of 600W/cm2 to simulate an SRM combustion chamber

environment.

3.1 Surface Temperature Measurements

Surface temperatures were measured for all the TPUN nanocomposites and the Kevlar-filled

EPDM (control materials) in this study. Figures 3 to 8 show the surface temperatures of the three

samples of TPUNs with 5wt% MMT (nanoclays), TPUNs with 5wt% CNF (carbon nanofibers),

TPUNs with 5wt% MWNT (multi-walled carbon nanotubes), 7.5wt% MWNT, 10wt% MWNT,

and Kevlar-filled EPDM, respectively.

Within each formulation group, the surface temperature of the samples for the most part are

comparable with one another. Examining the surface temperature plots above, the starting surface

temperature for each sample is 1,000 °C. This is because the pyrometer has a set lower limit,

meaning it cannot measure values under the limit. After the test, the samples were pulled away

from the flame and out of sight of the pyrometer, causing the measurements to end immediately.

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Note that in some tests the surface temperature rate decreased - and in some cases became negative

- before leveling out.

For the most part, the surface temperatures of the formulations resulted very similarly. The 5 and

7.5 wt% MWNT and the Kevlar filled EPDM maxed at 2000 °C. The results for these three

formulations were the least noisy and the most trust worthy due to the matched values within each

formulation. Interestingly, the 10 wt% MWNT did have an average max surface temperature that

matched the 5 and 7.5 wt% MWNT formulations. However, the temperature data of samples did

not agree as the samples’ maximum values were above and below 2000°C. With samples of

varying data, more tests are needed to conclude its actual maximum surface temperature during

these conditions. This applies to the 5 wt% MMT formulation as well. It had an average of 1865°C,

but the surface temperature pattern did not repeat between samples. Oddly, the surface

temperatures of the 5 wt% CNF samples were irregularly collected. The pyrometer’s measuring

system failed to continuously measure the surface temperature largely due to the lack of a flat

distinguishable char layer on the CNF surface. Instead, it deformed in such a way that was not

uniform and rough, causing trouble for the pyrometer that needs an even surface to measure the

radiation. Its rough post surface structure can viewed and compared to that of Kevlar EPDM in

Figure 10.

Figure 3: Surface temperatures of 5wt% montmorillonite nanoclay (MMT) thermoplastic

elastomer nanocomposite at an exposure of 600W/cm2.

1000

1200

1400

1600

1800

2000

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (s)

5%MMT1

5%MMT2

5%MMT3

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Figure 4: Surface temperatures of 5wt% carbon nanofibers (CNF) thermoplastic elastomer

nanocomposite at an exposure of 600W/cm2.

Figure 5: Surface temperatures of 5wt% multi-walled carbon nanotubes (MWNT) thermoplastic


Figure 6: Surface temperatures of 7.5wt% multi-walled carbon nanotubes (MWNT) thermoplastic

elastomer nanocomposite.

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (s)

5%MWNT1

5%MWNT2

5%MWNT3

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (s)

5% CNF 1

5% CNF 2

5% CNF 3

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (s)

7.5%MWNT1

7.5%MWNT2

7.5%MWNT3

7

Figure 7: Surface temperatures of 10wt% multi-walled carbon nanotubes (MWNT) thermoplastic


Figure 8: Surface temperatures of Kevlar-filled ethylene propylene diene monomer (EPDM) at an

exposure of 600W/cm2.

Figure 9: (a) Post test samples of 5 wt% CNF; and (b) Kevlar-filled ethylene propylene diene

monomer.

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (s)

EPDM-K 1

EPDM-K 2

EPDM-K 3

1000

1200

1400

1600

1800

2000

2200

0 5 10 15 20 25 30

Tem

pe

ratu

re (

°C)

Time (s)

10%MWNT1

10%MWNT2

10%MWNT3

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3.2 High Definition Video Camera Analysis

The HD images (Figures 10 and 11) presented below detail the surface structure of each sample

during ablation testing. Each picture was taken around five seconds of the test.

Figure 10: Samples during ablation testing: (a) 5%MWNT, (b) 5%MMT, and (c) 5%CNF during

ablation testing.

Figure 11: Samples during ablation testing: (a) 7.5w%MWNT; (b) 10%MWNT; and (c) Kevlar

EPDM.

As stated earlier, the 5 wt% CNF formulation deformed unlike the other samples. This is further

supported by observing the HD pictures. The HD picture of CNF is much darker than the others.

This could be due to a number of factors, such as the emissivity and the brightness of the flame as

it touches the surface. Note that in the picture there are several bright spots spotted in the center.

This surface behavior requires further research.

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3.3 Char Shear Strength Results

The proposed method for testing the shear strength was used with the posttest TPUNs and Kevlar

filled EPDM samples. Figures 12 to 17 show the plotted force history of the tested three samples

per formulation.

Figure 12: Shear char force of 5wt% montmorillonite nanoclay (MMT) thermoplastic elastomer

nanocomposite.

Figure 13: Shear char force of 5wt% carbon nanofibers (CNF) thermoplastic elastomer

nanocomposite.

0

1

2

3

4

5

0 2 4 6 8 10 12 14 16

Forc

e (N

)

Distance (mm)

5%MMT1

5%MMT3

5%MMT2

0

10

20

30

40

50

0 2 4 6 8 10 12 14 16

Forc

e (N

)

Distance (mm)

5%CNF1

5%CNF2

5%CNF3

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Figure 14: Shear char force of 5wt% multi-walled carbon nanotubes (MWNT) thermoplastic


Figure 15: Shear char force of 7.5wt% multi-walled carbon nanotubes (MWNT) thermoplastic


Figure 16: Shear char force of 10wt% multi-walled carbon nanotubes (MWNT) thermoplastic

elastomer nanocomposite

-1

0

1

2

3

4

5

6

0 5 10 15 20

Forc

e (N

)

Distance (mm)

Charred 7.5 wt% MWNT1



-2

0

2

4

6

8

0 5 10 15 20 25

Forc

e (N

)

Distance (mm)

10%MWNT1

10%MWNT2

10%MWNT3

0

1

2

3

4

5

6

0 2 4 6 8 10 12 14 16 18

Forc

e (N

)

DIstance (mm)

5%MWNT1

5%MWNT2

5%MWNT3

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Figure 17: Shear char force of Kevlar-filled ethylene propylene diene monomer (EPDM).

3.4 Discussion of Results

The first method for evaluating the shear strengths of the formulations was determining the

maximum force required to fracture the char layer off the specimens. With three samples per

formulation, the maximum char force values were averaged for ease of study. Figure 19 displays

these averages for all of the tested formulations.

Figure 18: Average maximum force of each type of thermoplastic elastomer nanocomposite and

Kevlar-filled EPDM.

After studying the char force data, all formulations except 5wt% CNF exhibited a maximum char

force under 5N. Surprisingly, the weakest formulation was the Kevlar-filled EPDM – the current

material used as insulation for SRMs. Although the experiments resulted in 10 wt% MWNT having

Sample

5 wt% MMT 4.06

5 wt% CNF 37.52

5 wt% MWNT 3.94

7.5 wt% MWNT 3.83

10 wt% MWNT 4.46

Kevlar EPDM 2.67

0.005.00

10.0015.0020.0025.0030.0035.0040.0045.0050.00

Max

Fo

rce

(N)

Average Max Force (N)

-2

-1

0

1

2

3

4

0 5 10 15 20 25

Forc

e (N

)

Distance mm)

Charred Kevlar EPDM1



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the strongest char force, it cannot be determined which formulation is the strongest due to the

overlapping standard deviations of all formulations. Overall, these formulations exhibited a very

weak maximum char force.

As stated before, the CNF samples did not generate a distinguishable char layer. Thus, when the

samples were sheared, the sensor was shearing the virgin material, a much stronger and more

ductile material than a char layer. As such, the maximum reaction force of the CNF samples

averaged to be 37.52N which roughly occurred around 8 seconds. Without a char layer, the CNF

formulation values are not comparable with the other formulations.

For the most part, the char layers sheared off before the sensor reached 5mm. The data after these

peaked values are when the sensor is both pushing the already sheared char layer off the sample

and rubbing against the virgin material. For some tests, such as the second test with 5 wt% MWNT,

the contact between the sensor and the top of the virgin material produce large forces after fracture.

This needs to be noted when evaluating the energy dissipated as these values do not contribute to

these calculations.

The second method for evaluating the shear strength of char is by determining the energy required

to fracture the char layer. This was accomplished by calculating the area under the force versus

distance plots. Since the char layer of each formulation fractured mid test, the values after fracture

were not used. Figures 19 and 20 display the average energy dissipated for all of the formulations.

Figure 19: Average energy dissipated of 5wt% MMT, 5, 7.5, and 10wt% MWNT, and Kevlar

EPDM.

Sample

5 wt% MMT 3.80

5 wt% MWNT 4.72

7.5 wt% MWNT 2.04

10 wt% MWNT 20.72

Kevlar EPDM 1.83

0.00

5.00

10.00

15.00

20.00

25.00

30.00

Ener

gy D

issi

pat

ed (

N ·

mm

)

Average Energy Dissipated

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Figure 20: Average energy dissipated of 5wt% CNF.

The experimental data display that 5wt% CNF experienced the greatest energy dissipated.

However, since this material did not generate a testable char layer, its results of the virgin material

vary significantly from the other formulations and, thus, were displayed in a separate graph. Within

the formulations worth comparing, 10 wt% MWNT performed well above the other formulations

with an average energy dissipation of 20.72 N·mm. This is mainly due to the fact that it withstood

a much larger strain than the rest of the formulations. It was the only formulation to break in parts

as opposed to the char layer fracturing intact. The next strongest formulation is 5wt% MWNT

(4.72 N·mm), followed by 5wt% MMT (3.80 N·mm), 7.5wt% MWNT (2.04 N·mm), and Kevlar-

filled EPDM (1.83 N·mm).

It can be concluded that the TPUN formulations performed comparably to the current insulator for

SRMs, Kevlar-filled EPDM. With the two methods to evaluate the shear strength, it can be

concluded that the ablator with the greatest strength is 10wt% MWNT.

3.4 Scanning Electron Microscopy (SEM) Analysis

To better understand the nature of the char layer of the TPUN formulations, we performed a

scanning electron microscopy (SEM) analysis on the surfaces of the burnt compositions. The SEM

images displayed on the left are at a magnification of 100X, while the SEM images on the right

are of the pictures on the left displayed at a magnification of 10,000X.

Sample

5 wt% CNF 254.90

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

Ener

gy D

issi

pat

ed (

N ·

mm

)Average Energy Dissipated

a b

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c d

e f

g h

i j

k l

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Figure 21: Scanning electron microscopy (SEM) results of the char layers: (a, b) 5 wt% carbon

nanofibers (CNF), (c, d) 5 wt% montmorillonite nanoclay (MMT), (e, f) 5 wt% multi-walled

carbon nanotubes (MWNT), (g, h) 7.5 wt% MWNT, (i, j) 10 wt% MWNT, and (k, l) Kelvar-filled

EPDM.

Figure 21a displays a visual image of the appearance of the surface of a burnt CNF formulation.

On the micro-scale, it can be seen that there exists a large arrangement of interlocking CNFs. By

examining Figure 21b, we can conclude that the fibers stay entangled due to re-positioning from

the intense exposure. While a char layer may not be visible by the naked eye, it can be confirmed

that CNF does produce char, however, at a much lesser extent than the other formulations.

The char of MMT is a hybrid organic/inorganic material and can be viewed in Figure 21c.

Examining the char layer at a higher magnification (Fig. 21d) shows a highly compacted wall of

nanoclay platelets. Such characteristics of a porous char provide good mechanical properties.

Figures 21e, g, and i, display the appearances of char on the surface MWNT compositions. At a

higher magnification (Figs. 21f, h, and j), the compactness of the char is clearly visible. Also, just

as the MMT formulation, the only remaining materials are the nanoparticles. What remains is a

network of protective burnt MWNT, which was reassembled during the ablation test.

The low magnification images of the charred surface of Kevlar-filled EPDM is displayed in Figure

21k. It is clear that the char layer is much smoother and more intact than the other char layers.

When examining at a higher magnification (Fig. 2l), a field of compact spheres and fibers are

visible. Here, unlike the other formulations, the resin remained and helped protect the composition,

highlighting a key reason for its superior protective performance.

After studying the char of each formulation at the microscopic level, it can be concluded that

when the combustion gases penetrate into the porous material, the compositions act in a similar

way. The TPUNs protected in such a way that the polymer burnt at a faster rate than the

nanoparticles, which in turn were the main source of defense against the hypothermal

environment. This was not the case for the Kevlar-filled EPDM, as both the polymer matrix and

the micron-sized Kelvar fibers were protecting against the intense heat flux.

4. CONCLUSION

This study involved an innovative instrument designed to study the unique behaviors of charred

ablators. By using the previously designed shear strength tester, we were able to evaluate the

shear strengths of the char layers from ablated formulations of TPUNs and Kevlar-filled EPDM.

However, it was found that the shear strengths of these insulating materials were very small with

standard deviation values overlapping one another. As for the CNF composition, the tester was

unable to test its minimal char layer. Thus, in order to achieve an effective shear strength test, the

probe of the sensor would need to be remade with a smaller diameter. Although the obtained data

portrays the reliability of the used tester, more tests of these formulations would better verify

their respective shear strengths and minimize the relatively large standard deviations. In doing

so, the shear strengths of these ablators can be more accurately characterized. This shear char

strength sensor and testing method will be adapted to more burnt polymeric ablatives and

nanostructured ablatives developing in our lab.

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5. ACKNOWLEDGEMENT

The authors would like to acknowledge financial support of KAI, LLC for this project.

6. REFERENCES

[1] J. H. Koo, O. A. Ezekoye, J. C. Lee, W. K. Ho, and M. C. Bruns, “Rubber-Clay Nanocomposites

Based on Thermoplastic Elastomers,” in Rubber-Clay Nanocomposites, M. Galimberti, ed., John

Wiley & Sons, Hoboken, NJ (2011), pp.489-521.

[2] P. Ambuken, H. Stretz, J. H. Koo, J. Lee, and R. Trejo, “High Temperature Flammability and

Mechanical Properties of Thermoplastic Polyurethane Nanocomposites,” in Fire and Polymers

VI: New Advances in Flame Retardant Chemistry and Science, C. Wilkie and A. Morgan, eds.,

ACS Books Series, ACS, Washington, DC (2012), pp. 344-360.

[3] E. K. Allcorn, M. Natali, and J. H. Koo, “Ablation performance and characterization of

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[4] P. V. Ambuken, H. A. Stretz, J. H. Koo, J. Messman, and D. Wong, “Effect of addition of

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[5] M. Jaramillo, J. H. Koo, and M. Natali, “Compressive Char Strength of Polyurethane

Elastomer Nanocomposites,” Polymers for Advanced Technology, 25 (77), (2014), 742-751,

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[6] L. Gutierrez, J. Reyes, J. H. Koo et al. “Design of Small-scale Ablative Testing Apparatus with

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[7] D. Forinash, M. Jaramillo, and J. H. Koo et al., “Development of an Apparatus for Measuring

the Shear Strength of Charred Ablatives,” Proc. SAMPE 2012 ISTC, Charleston, SC, Oct. 22-

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SHEAR CHAR STRENGTH OF TPUNS

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